Patent Grant
11101405
The present invention relates to light emitting diode (LED) devices. More particularly, embodiments of the invention relate to LED devices with a confined current injection area.
Light emitting diodes (LEDs) are increasingly being considered as a replacement technology for existing light sources. For example, LEDs are found in signage, traffic signals, automotive tail lights, mobile electronics displays, and televisions. Various benefits of LEDs compared to traditional lighting sources may include increased efficiency, longer lifespan, variable emission spectra, and the ability to be integrated with various form factors.
One type of LED is an organic light emitting diode (OLED) in which the emissive layer of the diode is formed of an organic compound. One advantage of OLEDs is the ability to print the organic emissive layer on flexible substrates. OLEDs have been integrated into thin, flexible displays and are often used to make the displays for portable electronic devices such as cell phones and digital cameras.
Another type of LED is a semiconductor-based LED in which the emissive layer of the diode includes one or more semiconductor-based quantum well layers sandwiched between thicker semiconductor-based cladding layers. Some advantages of semiconductor-based LEDs compared to OLEDs can include increased efficiency and longer lifespan. High luminous efficacy, expressed in lumens per watt (lm/W), is one of the main advantages of semiconductor-based LED lighting, allowing lower energy or power usage compared to other light sources. Luminance (brightness) is the amount of light emitted per unit area of the light source in a given direction and is measured in candela per square meter (cd/m2) and is also commonly referred to as a Nit (nt). Luminance increases with increasing operating current, yet the luminous efficacy is dependent on the current density (A/cm2), increasing initially as current density increases, reaching a maximum and then decreasing due to a phenomenon known as “efficiency droop.” Many factors contribute to the luminous efficacy of an LED device, including the ability to internally generate photons, known as internal quantum efficiency (IQE). Internal quantum efficiency is a function of the quality and structure of the LED device. External quantum efficiency (EQE) is defined as the number of photons emitted divided by the number of electrons injected. EQE is a function of IQE and the light extraction efficiency of the LED device. At low operating current density (also called injection current density, or forward current density) the IQE and EQE of an LED device initially increases as operating current density is increased, then begins to tail off as the operating current density is increased in the phenomenon known as the efficiency droop. At low current density the efficiency is low due to the strong effect of defects or other processes by which electrons and holes recombine without the generation of light, called non-radiative recombination. As those defects become saturated radiative recombination dominates and efficiency increases. An “efficiency droop” or gradual decrease in efficiency begins as the injection-current density surpasses a low value, typically between 1.0 and 10 A/cm2.
Semiconductor-based LEDs are commonly found in a variety of applications, including low-power LEDs used as indicators and signage, medium-power LEDs such as for light panels and automotive tail lights, and high-power LEDs such as for solid-state lighting and liquid crystal display (LCD) backlighting. In one application, high-powered semiconductor-based LED lighting devices may commonly operate at 400-1,500 mA, and may exhibit a luminance of greater than 1,000,000 cd/m2. High-powered semiconductor-based LED lighting devices typically operate at current densities well to the right of peak efficiency on the efficiency curve characteristic of the LED device. Low-powered semiconductor-based LED indicator and signage applications often exhibit a luminance of approximately 100 cd/m2 at operating currents of approximately 20-100 mA. Low-powered semiconductor-based LED lighting devices typically operate at current densities at or to the right of the peak efficiency on the efficiency curve characteristic of the LED device. To provide increased light emission, LED die sizes have been increased, with a 1 mm2 die becoming a fairly common size. Larger LED die sizes can result in reduced current density, which in turn may allow for use of higher currents from hundreds of mA to more than an ampere, thereby lessening the effect of the efficiency droop associated with the LED die at these higher currents.
Thus, the trend in current state-of-the art semiconductor-based LEDs is to increase both the operating current as well as LED size in order to increase efficiency of LEDs since increasing the LED size results in decreased current density and less efficiency droop. At the moment, commercial semiconductor-based LEDs do not get much smaller than 1 mm2.
Embodiments of the invention describe LED devices with a confined current injection area. In an embodiment, an LED device includes an active layer between a first current spreading layer pillar and a second current spreading layer. The first current spreading layer pillar is doped with a first dopant type and the second current spreading layer is doped with a second dopant type opposite the first dopant type. A first cladding layer is between the first current spreading layer pillar and the active layer, and a second cladding layer is between the second current spreading layer and the active layer. The first current spreading layer pillar protrudes away from the first cladding layer, and the first cladding layer is wider than the first current spreading layer pillar. In an embodiment, the first current spreading layer pillar is doped with a p-dopant. In an embodiment, the first current spreading layer pillar comprises GaP, and the first cladding layer includes a material such as AlInP, AlGaInP, or AlGaAs. In an embodiment, the active layer includes less than 10 quantum well layers. In an embodiment the active layer includes a single quantum well layer, and does not include multiple quantum well layers. In an embodiment, the active layer of the LED device has a maximum width of 100 μm or less, and the first current spreading layer pillar has a maximum width of 10 μm or less. In an embodiment the active layer of the LED device has a maximum width of 20 μm or less, and the first current spreading layer pillar has a maximum width of 10 μm or less. In an embodiment, the second current spreading layer is wider than the first current spreading layer pillar.
A passivation layer may span along a surface of the first cladding layer and sidewalls of the first current spreading layer pillar. In an embodiment, an opening is formed in the passivation layer on a surface of the first current spreading layer pillar opposite the first cladding layer. A conductive contact can then be formed within the opening in the passivation layer and in electrical contact with the first current spreading layer pillar without being in direct electrical contact with the first cladding layer.
In an embodiment, the LED device is supported by a post, and a surface area of the top surface of the post is less than the surface area of a bottom surface of the first current spreading layer pillar. In such a configuration, the LED device may be on a carrier substrate. In an embodiment, the LED device is bonded to a display substrate within a display area of the display substrate. For example, the LED device may be bonded to the display substrate and in electric connection with working circuitry within the display substrate, or the LED device may be bonded to a display substrate and in electrical connection with a micro chip also bonded to the display substrate within the display area. In an embodiment, the LED device is incorporated within a display area of a portable electronic device.
In an embodiment, a method of forming an LED device includes patterning a p-n diode layer of an LED substrate to form an array of current spreading layer pillars separated by an array of confinement trenches in a current spreading layer of the p-n diode layer, where the confinement trenches extend through the current spreading layer and expose a cladding layer of the p-n diode layer underneath the current spreading layer. A sacrificial release layer is formed over the array of current spreading layer pillars and the cladding layer. The LED substrate is bonded to a carrier substrate, and a handle substrate is removed from the LED substrate. The p-n diode layer is patterned laterally between the array of current spreading layer pillars to form an array of LED devices, with each LED device including a current spreading layer pillar of the array of current spreading layer pillars. Patterning of the p-n diode layer may include etching through a top current spreading layer, a top cladding layer, one or more quantum well layers, and the cladding layer (e.g. bottom cladding layer) to expose the sacrificial release layer.
An array of bottom electrically conductive contacts may be formed on and in electrical contact with the array of current spreading layer pillars prior to forming the sacrificial release layer over the array of current spreading layer pillars and the cladding layer. The sacrificial release layer may additionally be patterned to form an array of openings in the sacrificial release layer over the array of current spreading layer pillars prior to bonding the LED substrate to the carrier substrate. In such an embodiment, the LED substrate is bonded to the carrier substrate with a bonding material that is located within the array of openings in the sacrificial release layer. Upon forming the array of LED devices, the sacrificial release layer may be removed, and a portion of the array of LED devices is transferred from the carrier substrate to a receiving substrate, for example a display substrate, using an electrostatic transfer head assembly.
In an embodiment, a method of operating a display includes sending a control signal to a driving transistor, and driving a current through an LED device including a confined current injection area in response to the control signal, where the LED device includes a current spreading layer pillar that protrudes away from a cladding layer and the cladding layer is wider than the current spreading layer pillar. For example, the display is a portable electronic device. LED devices in accordance with embodiments of the invention may be driven at injection currents and current densities well below the normal or designed operating conditions for standard LEDs. In an embodiment, the current driven through the LED device is from 1 nA-400 nA. In an embodiment the current is from 1 nA-30 nA. In such an embodiment, the current density flowing the LED device may be from 0.001 A/cm2 to 3 A/cm2. In an embodiment the current is from 200 nA-400 nA. In such an embodiment, the current density flowing the LED device may be from 0.2 A/cm2 to 4 A/cm2. In an embodiment the current is from 100 nA-300 nA. In such an embodiment, the current density flowing the LED device may be from 0.01 A/cm2 to 30 A/cm2.
In an embodiment, an LED device includes an active layer between a first current spreading layer and a second current spreading layer, where the first current spreading layer is doped with a first dopant type and the second current spreading layer is doped with a second dopant type opposite the first dopant type. A first cladding layer is between the first current spreading layer and the active layer, and a second cladding layer is between the second current spreading layer and the active layer. A current confinement region laterally surrounds a current injection region to confine current that flows through the active layer to an interior portion of the LED device and away from sidewalls of the LED device. In an embodiment the LED device does not include a distributed Bragg reflector layer on each side of the active layer. The LED device may be a micro LED device, for example, having a maximum width of 300 μm or less, 100 μm or less, 20 μm or less, or even smaller sizes. The current injection region that confines current that flows through the active layer to an interior portion of the LED device and away from sidewalls of the LED device may have a maximum width less than the LED device, for example, 10 μm or less.
In some configurations the LED device is supported by a post and a sacrificial release layer spans directly beneath the LED device. For example, such a configuration may be on a carrier substrate prior to transferring to a receiving substrate such as a display substrate. In other configurations the LED device is incorporated within a display area of a portable electronic device. In an embodiment the LED device is bonded to a display substrate within a display area of the display substrate and the LED device is in electrical connection with a subpixel driver circuit in the display substrate or a micro chip that is also bonded to the display substrate within the display area where the micro chip includes a subpixel driver circuit for driving the LED device.
A variety of configurations are possible for confining current that flows through the active layer to an interior portion of the LED device and away from sidewalls of the LED device. In an embodiment, the current injection region includes a pillar structure that includes the first current spreading layer, the first cladding layer, and the active layer, and the current confinement region includes a confinement barrier fill that laterally surrounds the pillar structure. The confinement barrier fill may have a larger bandgap than one or more quantum well layers in the active layer. The electrical path through the confinement barrier fill may be characterized by a higher resistance than the electrical path through the pillar structure. For example, the confinement barrier fill may include a material characterized by a higher resistivity than the materials forming the pillar structure, or the confinement barrier fill may include a junction, such as a p-n-p junction. The confinement barrier fill may include multiple layers, for example, a buffer layer and a barrier layer or multiple layers forming a p-n-p junction.
In an embodiment, the current injection region is located within the first current spreading layer and the current confinement region includes a modified confinement barrier region within the first current spreading layer that laterally surrounds the current injection region. For example the modified confinement barrier region may be characterized by a higher resistivity than the current injection region. The modified confinement barrier region may also be doped with a dopant type opposite of the dopant type of the first current spreading layer in the pillar structure. For example, the confinement barrier region within the first current spreading layer may be n-type where the first current spreading layer in the pillar structure is p-type.
In an embodiment, the current injection region is located within the active layer and the current confinement region includes a modified barrier region within the active layer that laterally surrounds the current injection region. The modified confinement barrier region may be characterized by a larger bandgap than the current injection region, for example, by quantum well intermixing in the modified confinement barrier region.
In an embodiment, the current injection region includes a first current injection region located within a first laterally oxidized confinement layer, and the current confinement region includes a first oxidized region of the first laterally oxidized confinement layer that laterally surrounds the first current injection region. The current injection region may additionally include a second current injection region located within a second laterally oxidized confinement layer, and the current confinement region includes a second oxidized region of the second laterally oxidized confinement layer that laterally surrounds the second current injection region. In an embodiment, the one or more laterally oxidized confinement layers may be characterized by higher aluminum concentration than other layers within the LED device, such as the first and second current spreading layers, the first and second cladding layers, and the active layer.
Embodiments of the present invention describe LED devices and manners of forming LED devices with a confined current injection area. In particular, some embodiments of the present invention may relate to micro LED devices and manners of forming micro LED devices with a confined current injection area.
In various embodiments, description is made with reference to figures. However, certain embodiments may be practiced without one or more of these specific details, or in combination with other known methods and configurations. In the following description, numerous specific details are set forth, such as specific configurations, dimensions and processes, etc., in order to provide a thorough understanding of the present invention. In other instances, well-known semiconductor processes and manufacturing techniques have not been described in particular detail in order to not unnecessarily obscure the present invention. Reference throughout this specification to “one embodiment” means that a particular feature, structure, configuration, or characteristic described in connection with the embodiment is included in at least one embodiment of the invention. Thus, the appearances of the phrase “in one embodiment” in various places throughout this specification are not necessarily referring to the same embodiment of the invention. Furthermore, the particular features, structures, configurations, or characteristics may be combined in any suitable manner in one or more embodiments.
The terms “spanning”, “over”, “to”, “between” and “on” as used herein may refer to a relative position of one layer with respect to other layers. One layer “spanning,” “over” or “on” another layer or bonded “to” or in “contact” with another layer may be directly in contact with the other layer or may have one or more intervening layers. One layer “between” layers may be directly in contact with the layers or may have one or more intervening layers.
In one aspect, embodiments of the invention describe an LED device integration design in which an LED device is transferred from a carrier substrate and bonded to a receiving substrate using an electrostatic transfer head assembly. In accordance with embodiments of the present invention, a pull-in voltage is applied to an electrostatic transfer head in order to generate a grip pressure on an LED device. It has been observed that it can be difficult to impossible to generate sufficient grip pressure to pick up micro devices with vacuum chucking equipment when micro device sizes are reduced below a specific critical dimension of the vacuum chucking equipment, such as approximately 300 μm or less, or more specifically approximately 100 μm or less. Furthermore, electrostatic transfer heads in accordance with embodiments of the invention can be used to create grip pressures much larger than the 1 atm of pressure associated with vacuum chucking equipment. For example, grip pressures of 2 atm or greater, or even 20 atm or greater may be used in accordance with embodiments of the invention. Accordingly, in one aspect, embodiments of the invention provide the ability to transfer and integrate micro LED devices into applications in which integration is not possible with current vacuum chucking equipment. In some embodiments, the term “micro” LED device or structure as used herein may refer to the descriptive size, e.g. length or width, of certain devices or structures. In some embodiments, “micro” LED devices or structures may be on the scale of 1 μm to approximately 300 μm, or 100 μm or less in many applications. However, it is to be appreciated that embodiments of the present invention are not necessarily so limited, and that certain aspects of the embodiments may be applicable to larger micro LED devices or structures, and possibly smaller size scales.
In one aspect, embodiments of the invention describe LED devices that are poised for pick up and supported by one or more stabilization posts. In accordance with embodiments of the present invention, a pull-in voltage is applied to a transfer head in order to generate a grip pressure on an LED device and pick up the LED device. In accordance with embodiments of the invention, the minimum amount pick up pressure required to pick up an LED device from a stabilization post can be determined by the adhesion strength between the adhesive bonding material from which the stabilization posts are formed and the LED device (or any intermediate layer), as well as the contact area between the top surface of the stabilization post and the LED device. For example, adhesion strength which must be overcome to pick up an LED device is related to the minimum pick up pressure generated by a transfer head as provided in equation (1):
P1A1=P2A2 (1)
where P1 is the minimum grip pressure required to be generated by a transfer head, A1 is the contact area between a transfer head contact surface and LED device contact surface, A2 is the contact area on a top surface of a stabilization post, and P2 is the adhesion strength on the top surface of a stabilization post. In an embodiment, a grip pressure of greater than 1 atmosphere is generated by a transfer head. For example, each transfer head may generate a grip pressure of 2 atmospheres or greater, or even 20 atmospheres or greater without shorting due to dielectric breakdown of the transfer heads. Due to the smaller area, a higher pressure is realized at the top surface of the corresponding stabilization post than the grip pressure generate by a transfer head.
In another aspect, embodiments of the invention describe LED devices, which may be micro LED devices, including a confined current injection area. In an embodiment, an LED device includes a first (e.g. bottom) current spreading layer pillar doped with a first dopant type, a first (e.g. bottom) cladding layer on the bottom current spreading layer, an active layer on the bottom cladding layer, a second (e.g. top) cladding layer on the active layer, and a second (e.g. top) current spreading layer doped with a second dopant type opposite the first dopant type. The bottom current spreading layer pillar protrudes away from the bottom cladding layer, in which the bottom cladding layer is wider than the bottom current spreading layer pillar. In accordance with embodiments of the invention, the active layer is also wider than the bottom current spreading layer pillar. The top cladding layer and top current spreading layer may also be wider than the bottom current spreading layer pillar. In this manner, when a potential is applied across the top current spreading layer and bottom current spreading layer pillar, the current injection area within the active layer is modified by the relationship of the areas of the bottom current spreading layer pillar and top current spreading layer. In operation, the current injection area is reduced as the area of the bottom current spreading layer pillar configuration is reduced. In this manner, the current injection area can be confined internally within the active layer away from external or side surfaces of the active layer.
In other embodiments a current confinement region laterally surrounds a current injection region to confine current that flows through the active layer to an interior portion of the LED device and away from sidewalls of the LED device. A variety of configurations are possible including mesa regrowth techniques, dopant or proton modification of a current distribution layer or cladding layer, quantum well intermixing, and lateral oxidation of a confinement layer. In addition, many of the several current confinement configurations described herein may be combined within a single LED device.
In addition, it is possible to design an LED device in which a top surface area of the top surface of the p-n diode layer is larger than a surface area of the current confinement region within the active layer. This enables larger LED devices to be fabricated, which may be beneficial for transferring the LED devices using an electrostatic transfer head assembly, while also providing a structure in which the confined current injection area results in an increased current density and increased efficiency of the LED device, particularly when operating at injection currents and injection current densities below or near the pre-droop region of the LED device internal quantum efficiency curve.
In another aspect, it has been observed that non-radiative recombination may occur along exterior surfaces of the active layer (e.g. along sidewalls of the LED devices). It is believed that such non-radiative recombination may be the result of defects, for example, that may be the result of forming mesa trenches through the p-n diode layer to form an array of LED devices or a result of surface states from dangling bonds at the terminated surface that can enable current flow and non-radiative recombination. This non-radiative recombination can also be a result of band bending at the surface leading to a density of states were electrons and holes can be confined until they combine non-radiatively. Such non-radiative recombination may have a significant effect on LED device efficiency, particularly at low current densities in the pre-droop region of the IQE curve where the LED device is driven at currents that are unable to saturate the defects. In accordance with embodiments of the invention, the current injection area can be confined internally within the active layer, so that the current does not spread laterally to the exterior or side surfaces of the active layer where a larger amount of defects may be present. As a result, the amount of non-radiative recombination near the exterior or side surfaces of the active layer can be reduced and efficiency of the LED device increased.
The LED devices in accordance with embodiments of the invention are highly efficient at light emission and may consume very little power compared to LCD or OLED display technologies. For example, a conventional display panel may achieve a full white screen luminance of 100-750 cd/m2. It is understood that a luminance of greater than 686 cd/m2 may be required for sunlight readable screens. In accordance with some embodiments of the invention, an LED device may be transferred and bonded to a display backplane such as a thin film transistor (TFT) substrate backplane used for OLED display panels, where the semiconductor-based LED device replaces the organic LED film of the OLED display. In this manner, a highly efficient semiconductor-based LED device replaces a less efficient organic LED film. Furthermore, the width/length of the semiconductor-based LED device may be much less than the allocated subpixel area of the display panel, which is typically filled with the organic LED film. In other embodiments, the LED devices are integrated with a substrate including a plurality of micro chips that replace the working circuitry (e.g. subpixel driver circuits) that are typically formed within a TFT substrate backplane.
LED devices in accordance with embodiments of the invention may operate well below the normal or designed operating conditions for standard LEDs. The LED devices may also be fundamentally different than lasers, and operate at significantly lower currents than lasers. For example, the principle of emission for LED devices in accordance with embodiments of the invention may be spontaneous, non-directional photon emission, compared to stimulated, coherent light that is characteristic of lasers. Lasers typically include distributed Bragg reflector (DBR) layers on opposite sides of the active layer for stimulating coherent light emission, also known as lasing. Lasing is not necessary for operation of LED devices in accordance with embodiments of the invention. As a result, the LED devices may be thinner than typical lasers, and do not require reflector layers on opposite sides of the active layer for stimulating coherent light emission.
For illustrative purposes, in accordance with embodiments of the invention it is contemplated that the LED devices may be driven using a similar driving circuitry as a conventional OLED display panel, for example a thin film transistor (TFT) backplane. However, embodiments are not so limited. For example, in another embodiment the LED devices are driven by micro chips that are also electrostatically transferred to a receiving substrate. Assuming subpixel operating characteristics of 25 nA injection current, an exemplary LED device having a 1 μm2 confined current injection area roughly corresponds to a current density of 2.5 A/cm2, an exemplary LED device having a 25 μm2 confined current injection area roughly corresponds to a current density of 0.1 A/cm2, and an exemplary LED device having a 100 μm2 confined current injection area roughly corresponds to a current density of 0.025 A/cm2. Referring to
In the following description exemplary processing sequences are described for forming an array of LED devices, which may be micro LED devices. Referring now to
Specifically, exemplary primary processing sequences are described for forming an array of red emitting LED devices. While the primary processing sequences are described for red emitting LED devices, it is to be understood that the exemplary processing sequences can be used for LED devices with different emission spectra, and that certain modifications are contemplated, particularly when processing different materials. Additionally, in different materials the shape of the IQE curve may differ, specifically the peak may occur at current densities other than that shown in
An active layer 108 is formed on the cladding layer 106. The active layer 108 may include a multi-quantum-well (MQW) configuration or a single-quantum-well (SQW) configuration. In accordance with embodiments of the invention, a reduced number of quantum wells may offer more resistance to lateral current spreading, higher carrier density, and aid in confining current internally within the completed LED device. In an embodiment, the active layer 108 includes a SQW. In an embodiment, active layer 108 includes a MQW configuration with less than 10 quantum well layers. Additional layers may also be included in the active layer 108, such as one or more barrier layers. For example, a MQW configuration may include multiple quantum well layers separated by barrier layers.
Referring again to
In an embodiment, bulk LED substrate 100 includes a 250-500 μm thick growth substrate 102, a 0.1-1.0 μm thick current spreading layer 104, a 0.05-0.5 μm thick cladding layer 106, an active layer 108, a 0.05-5 μm thick cladding layer 110, and a 0.1-1.5 μm thick current spreading layer 114. These thicknesses are exemplary, and embodiments of the invention are not limited to these exemplary thicknesses.
Referring now to
As will become more apparent in the following description, the width of the current spreading layer pillars 118 at least partly determines the ability to increase current density within the LED device as well as the ability to confine current internally within the LED devices and away from the external sidewalls where non-radiative recombination may occur. While some lateral current spreading occurs within the device, embodiments of the invention generally refer to the confined current area as the area of the quantum well directly above the current spreading layer pillars 118. Width of the current spreading layer pillars 118 may also be related to width of the LED devices. In some embodiments, current spreading layer pillars 118 have a width between 1 and 10 μm. In an embodiment, current spreading layer pillars 118 have a width or diameter of approximately 2.5 μm.
Referring now to
A sacrificial release layer 126 may then be formed over the array of current spreading layer pillars 118 as illustrated in
Still referring to
Referring now to
In an embodiment, stabilization layer 130 is spin coated or spray coated over the patterned sacrificial release layer 126, though other application techniques may be used. Following application of the stabilization layer 130, the stabilization layer may be pre-baked to remove the solvents. After pre-baking the stabilization layer 130 the patterned bulk substrate 100 is bonded to the carrier substrate 140 with the stabilization layer 130. In an embodiment, bonding includes curing the stabilization layer 130. Where the stabilization layer 130 is formed of BCB, curing temperatures should not exceed approximately 350° C., which represents the temperature at which BCB begins to degrade. Achieving a 100% full cure of the stabilization layer may not be required in accordance with embodiments of the invention. In an embodiment, stabilization layer 130 is cured to a sufficient curing percentage (e.g. 70% or greater for BCB) at which point the stabilization layer 130 will no longer reflow. Moreover, it has been observed that partially cured BCB may possess sufficient adhesion strengths with carrier substrate 140 and the patterned sacrificial release layer 126. In an embodiment, stabilization layer may be sufficiently cured to sufficiently resist the sacrificial release layer release operation.
In an embodiment, the stabilization layer 130 is thicker than the height of the current spreading layer pillars 118 and openings 128 in the patterned sacrificial release layer 126. In this manner, the thickness of the stabilization layer filling openings 128 will become stabilization posts 132, and the remainder of the thickness of the stabilization layer 130 over the filled openings 128 can function to adhesively bond the patterned bulk LED substrate 100 to a carrier substrate 140.
In the embodiment illustrated in
Following bonding of the patterned bulk LED substrate 100 to the carrier substrate 140, the handle substrate 102 is removed as illustrated in
Referring now to
In an embodiment, prior to forming the top conductive contact layer 152 an ohmic contact layer 150 can optionally be formed to make ohmic contact with the current spreading layer 104. In an embodiment, ohmic contact layer 150 may be a metallic layer. In an embodiment, ohmic contact layer 150 is a thin GeAu layer. For example, the ohmic contact layer 150 may be 50 angstroms thick. In the particular embodiment illustrated, the ohmic contact layer 150 is not formed directly over the current spreading layer pillars 118, corresponding to the current confinement area within the LED devices, so as to not reflect light back into the LED device and potentially reduce light emission. In some embodiments, ohmic contact layer 150 forms a ring around the current spreading layer pillars 118.
Referring now to
Still referring to
Following the formation of discrete and laterally separate LED devices 156, the sacrificial release layer 126 may be removed.
Still referring to
In accordance with embodiments of the invention the LED devices 156 may be micro LED devices. In an embodiment, an LED device 156 has a maximum width or length at the top surface 162 of top current spreading layer 104 of 300 μm or less, or more specifically approximately 100 μm or less. The active area within the LED device 156 may be smaller than the top surface 162 due to location of the bottom current spreading layer pillars 118. In an embodiment, the top surface 162 has a maximum dimension of 1 to 100 μm, 1 to 50 μm, or more specifically 3 to 20 μm. In an embodiment, a pitch of the array of LED devices 156 on the carrier substrate may be (1 to 300 μm) by (1 to 300 μm), or more specifically (1 to 100 μm) by (1 to 100 μm), for example, 20 μm by 20 μm, 10 μm by 10 μm, or 5 μm by 5 μm. In an exemplary embodiment, a pitch of the array of LED devices 156 on the carrier substrate is 11 μm by 11 μm. In such an exemplary embodiment, the width/length of the top surface 162 is approximately 9-10 μm, and spacing between adjacent LED devices 156 is approximately 1-2 μm. Sizing of the bottom current spreading layer pillars 118 may be dependent upon the width of the LED devices 156 and the desired efficiency of the LED devices 156.
In the above exemplary embodiments, manners for forming LED devices 156 including current spreading layer pillars are described. In the above embodiments, the current spreading layer pillars are formed from current spreading layer 114 using a one-sided process in which the pillars are formed prior to transferring the p-n diode layer from the handle substrate to the carrier substrate. In other embodiments, the current spreading layer pillars may be formed from current spreading layer 104 using a two-sided process in which the pillars are formed after transferring the p-n diode layer from the handle substrate to the carrier substrate. Accordingly, in some embodiments the LED device pillar structure may be inverted. Though an inverted LED device pillar structure may not provide a larger contact area for a transfer operation to a receiving substrate, such as described with regard to
Referring now to
It is believed that such non-radiative recombination may be the result of defects, for example, that may be the result of forming mesa trenches through the p-n diode layer to form an array of LED devices or a result of surface states from dangling bonds at the terminated surface that can enable current flow and non-radiative recombination. Such non-radiative recombination may have a significant effect on LED device efficiency, particularly at low current densities in the pre-droop region of the IQE curve where the LED device is driven at currents that are unable to saturate the defects. As illustrated in the above simulation data, it is expected that for LED devices without internally confined current injection areas, as the LED device width (and active layer width) is increased above 10-20 μm the radiative recombination (resulting in light emission) in the center of the device increases as the width increases until the peak value approaches the theoretical value for surface recombination. In accordance with embodiments of the invention, the current injection area can be confined internally within the active layer using a variety of different structures so that the current does not spread laterally to the exterior or side surfaces of the active layer where a larger amount of defects may be present. As a result, the amount of non-radiative recombination due to edge effects in the non-radiative zone near the exterior sidewall surfaces of the active layer can be reduced or eliminated and efficiency of the LED device increased.
In the embodiment illustrated in
As described above, it is believed that non-radiative recombination may be the result of defects, for example, that may be the result of etching through the p-n diode layer or a result of surface states from dangling bonds at the terminated surface that can enable current flow and non-radiative recombination. Such non-radiative recombination may have a significant effect on LED device efficiency, particularly at low current densities in the pre-droop region of the IQE curve where the LED device is driven at currents that are unable to saturate the defects. In an embodiment, confinement barrier fill 172 is formed using an epitaxial growth technique such as MBE or MOCVD in order to occupy the available surface states along the pillar structure 170, particularly along the active layer 108. In this manner, a continuous crystal structure possessing a larger bandgap and/or higher resistivity than the layers forming the pillar structures 170 can be formed laterally around the pillar structures 170, and discrete sidewalls are not formed around the active layer 108 forming the pillar structure 170. In accordance with some embodiments of the invention, the confinement barrier fill 172 forms a current confinement region laterally surrounding the pillar structures forming the current injection region in order to confine current that flows through the active layer 108 to an interior portion of the LED device and away from sidewalls of the LED device.
In an embodiment, the confinement barrier fill 172 has a larger bandgap and/or larger resistivity than the materials forming the active layer 108. In an embodiment, the confinement barrier fill 172 has a larger bandgap and/or larger resistivity than the current spreading layer 114. The confinement barrier fill 172 may also have a larger bandgap and/or larger resistivity than the cladding layer 110. The inclusion of a confinement barrier fill 172 with a larger bandgap than the active region may have two effects. One is that a larger bandgap may be transparent to the light emitted from the active layer. Another effect is that the larger bandgap and/or larger resistivity will create a hetero-barrier that inhibits current from leaking through the regrown confinement barrier fill 172. In addition to bandgap and/or resistivity, other considerations such as lattice matching factor into suitability of particular regrowth materials are taken for the confinement barrier fill 172. Exemplary materials, in order of suitability, for the exemplary red emitting LED devices described herein include GaP, AlP, AlGaP, AlAs, AlGaAs, AlInGaP, AlGaAsP, and any As—P—Al—Ga—In may allow a larger bandgap than the material(s) forming the active layer 108. Additional potentially suitable materials include GaN, InN, InGaN, AlN, AlGaN, and any nitride alloy with a larger bandgap than the material(s) forming the active layer 108. The confinement barrier fill 172 for red emitting LED devices may additionally be doped (e.g. in-situ doped) with a dopant material to increase resistivity or render the confinement barrier fill 172 semi-insulating. For example, the red emitting LED devices described herein may be doped with a material such as Cr, Ni, or Fe. Exemplary materials for the exemplary blue or green emitting LED devices described herein include GaN, AlGan, InGaN, AlN, InAlN, AlInGaN. The confinement barrier fill 172 for blue or green emitting LED devices may additionally be doped (e.g. in-situ doped) with a material such as Fe or C.
After forming the confinement barrier fill 172, the p-n diode layer may be transferred from the handle substrate 102 to a carrier substrate 140.
The structures illustrated in
Referring now to
In an exemplary red emitting LED device structure, growth substrate 102 is formed of GaAs, buffer layer 173 is a graded layer that is graded from GaAs to GaP or is GaP, and barrier layer 175 is formed of GaP. In an embodiment, barrier layer 175 has a larger bandgap and/or resistivity than the material(s) forming the active layer 108. As previously described, barrier layer 175 may be doped, for example with a Cr, Ni, or Fe dopant to increase resistivity or render the barrier layer 175 semi-insulating.
After forming the confinement barrier fill 172, the p-n diode layer may be transferred from the handle substrate 102 to a carrier substrate 140.
After forming the confinement barrier fill 172, the p-n diode layer may be transferred from the handle substrate 102 to a carrier substrate 140.
In the particular embodiment illustrated in
Referring now to
Referring to
A variety of species may be implanted into the current spreading layer 114. In one embodiment, a neutral species is implanted into the current spreading layer 114 to create defects to current spreading. For example, He or H can be implanted, also known as proton bombardment or proton implantation. The damage created by proton bombardment in turn increases the resistivity of the implanted material. In an embodiment, implantation extends through the active layer 108. In such an embodiment, the amount of damage is significant enough to increase resistivity for current confinement while not too much damage to act as a significant source for non-radiative recombination.
In an embodiment, a dopant is implanted into the current spreading layer 114 to increase the resistivity of the current spreading layer, render the current spreading layer semi-insulating, or change the overriding dopant type of the layer (e.g. from p-type to n-type). For example, Si may be implanted into a p-doped current spreading layer 114, and Zn or Mg may be implanted into an n-doped current spreading layer 114. Or Fe, Cr, Ni, or some other such dopant can be added to make the layer semi-insulting.
Referring to
Following the implantation or diffusion operations to form the modified confinement barrier region 178, the p-n diode layer may be transferred from the handle substrate 102 to a carrier substrate 140 using a processing sequence similar to the one previously described above with regard to
Referring to
Intermixing of the quantum wells may result in the transformation of multiple quantum wells separated by barrier layers to a single intermixed layer with a larger bandgap than the original quantum wells.
Referring now to
Following lateral oxidation of the one or more oxidizable confinement layers 185 the sacrificial release layer 126 spanning between and directly underneath the array of LED devices may be removed similarly as described above with regard to
In an embodiment a sidewall passivation layer may be formed along sidewalls 168 of the LED devices. For example, a sidewall passivation layer may be used to protect the oxidized regions 186 from etching during removal of the sacrificial release layer 126. A sidewall passivation layer can serve other purposes, such as protecting the active layer from shorting when forming a top contact layer upon transfer to a receiving substrate, and shorting between adjacent LED devices during an electrostatic transfer operation. A sidewall passivation layer can be formed with the one-sided process as previously described. In an embodiment, a sidewall passivation layer is formed using a two-sided process as described with regard to
Referring to
Referring now to
In accordance with embodiments of the invention, heat may be applied to the carrier substrate, transfer head assembly, or receiving substrate during the pickup, transfer, and bonding operations. For example, heat can be applied through the transfer head assembly during the pick up and transfer operations, in which the heat may or may not liquefy LED device bonding layers. The transfer head assembly may additionally apply heat during the bonding operation on the receiving substrate that may or may not liquefy one of the bonding layers on the LED device or receiving substrate to cause diffusion between the bonding layers.
The operation of applying the voltage to create a grip pressure on the array of LED devices can be performed in various orders. For example, the voltage can be applied prior to contacting the array of LED devices with the array of transfer heads, while contacting the LED devices with the array of transfer heads, or after contacting the LED devices with the array of transfer heads. The voltage may also be applied prior to, while, or after applying heat to the bonding layers.
Where the transfer heads 204 include bipolar electrodes, an alternating voltage may be applied across a pair of electrodes in each transfer head 204 so that at a particular point in time when a negative voltage is applied to one electrode, a positive voltage is applied to the other electrode in the pair, and vice versa to create the pickup pressure. Releasing the array of LED devices from the transfer heads 204 may be accomplished with a varied of methods including turning off the voltage sources, lowering the voltage across the pair of electrodes, changing a waveform of the AC voltage, and grounding the voltage sources.
Referring now to
In the particular embodiment illustrated, the TFT substrate 300 includes a switching transistor T1 connected to a data line from the driver circuit 310 and a driving transistor T2 connected to a power line connected to the power supply line 314. The gate of the switching transistor T1 may also be connected to a scan line from the scan driver circuit 312. A patterned bank layer 326 including bank openings 327 is formed over the substrate 300. In an embodiment, bank openings 327 correspond to subpixels 328. Bank layer 326 may be formed by a variety of techniques such as ink jet printing, screen printing, lamination, spin coating, CVD, PVD and may be formed of opaque, transparent, or semitransparent materials. In an embodiment, bank layer 326 is formed of an insulating material. In an embodiment, bank layer is formed of a black matrix material to absorb emitted or ambient light. Thickness of the bank layer 326 and width of the bank openings 327 may depend upon the height of the LED devices 156 transferred to and bonded within the openings, height of the electrostatic transfer heads, and resolution of the display panel. In an embodiment, exemplary thickness of the bank layer 326 is between 1 μm-50 μm.
Electrically conductive bottom electrodes 342, ground tie lines 344 and ground ring 316 may optionally be formed over the display substrate 300. In the embodiments illustrated an arrangement of ground tie lines 344 run between bank openings 327 in the pixel area 304 of the display panel 3700. Ground tie lines 344 may be formed on the bank layer 326 or alternative, openings 332 may be formed in the bank layer 326 to expose ground tie lines 344 beneath bank layer 326. In an embodiment, ground tie liens 344 are formed between the bank openings 327 in the pixel area and are electrically connected to the ground ring 316 or a ground line in the non-display area. In this manner, the Vss signal may be more uniformly applied to the matrix of subpixels resulting in more uniform brightness across the display panel 3700.
A passivation layer 348 formed around the LED devices 156 within the bank openings 327 may perform functions such as preventing electrical shorting between the top and bottom electrode layers 318, 342 and providing for adequate step coverage of top electrode layer 318 between the top conductive contacts 152 and ground tie lines 344. The passivation layer 348 may also cover any portions of the bottom electrode layer 342 to prevent possible shorting with the top electrode layer 318. In accordance with embodiments of the invention, the passivation layer 348 may be formed of a variety of materials such as, but not limited to epoxy, acrylic (polyacrylate) such as poly(methyl methacrylate) (PMMA), benzocyclobutene (BCB), polymide, and polyester. In an embodiment, passivation layer 348 is formed by ink jet printing or screen printing around the LED devices 156 to fill the subpixel areas defined by bank openings 327.
Top electrode layer 318 may be opaque, reflective, transparent, or semi-transparent depending upon the particular application. In top emission display panels the top electrode layer 318 may be a transparent conductive material such as amorphous silicon, transparent conductive polymer, or transparent conductive oxide. Following the formation of top electrode layer 318 an encapsulation layer 346 is formed over substrate 300. For example, encapsulation layer 346 may be a flexible encapsulation layer or rigid layer. In accordance with some embodiments of the invention, a circular polarizer may not be required to suppress ambient light reflection. As a result, display panels 3700 in accordance with embodiments of the invention may be packaged without a circular polarizer, resulting in increased luminance of the display panel.
In an embodiment, one or more LED devices 156 are arranged in a subpixel circuit. A first terminal (e.g. bottom conductive contact) of the LED device 156 is coupled with a driving transistor. For example, the LED device 156 can be bonded to a bonding pad coupled with the driving transistor. In an embodiment, a redundant pair of LED devices 156 are bonded to the bottom electrode 342 that is coupled with the driving transistor T2. The one or more LED devices 156 may be any of the LED devices described herein including a confined current injection area. A ground line is electrically coupled with a second terminal (e.g. top conductive contact) for the one or more LED devices.
A current can be driven through the one or more LED devices, for example, from the driving transistor T2. In a high side drive configuration the one or more LED devices may be on the drain side of a PMOS driver transistor or a source side of an NMOS driver transistor so that the subpixel circuit pushes current through the p-terminal of the LED device. Alternatively, the subpixel circuit can be arranged in a low side drive configuration in which case the ground line becomes the power line and current is pulled through the n-terminal of the LED device.
In accordance with embodiments of the invention, the subpixel circuit may operate at comparatively low currents or current densities in the pre-droop range of the characteristic efficiency curve of the LED devices, or near a maximum efficiency value past the pre-droop range. Thus, rather than increasing the size of the LED devices to increase efficiency, the effective size of the current injection area is confined in order to increase the current density within the LED device. In embodiments where the LED devices are utilized in display applications, as opposed to high-powered applications, the LED devices can operate at comparatively lower current ranges, where a slight increase in current density may result in a significant improvement in IQE and EQE of the LED devices.
In an embodiment, a subpixel circuit comprises a driving transistor, a first terminal (e.g. bottom electrically conductive contact) of an LED device with confined current injection area is coupled with the driving transistor, and a ground line is coupled with a second terminal (e.g. top electrically conductive contact) of the LED device. In an embodiment, the LED device is operated by driving a current through the LED device in response to sending a control signal to the driving transistor. In some embodiments, the current may range from 1 nA-400 nA. In an embodiment, the current ranges from 1 nA-30 nA. In an embodiment, an LED device is operated with a current from 1 nA-30 nA in a display having a 400 pixel per inch (PPI) resolution. In an embodiment, the current ranges from 200 nA-400 nA. In an embodiment, an LED device is operated with a current from 200 nA-400 nA in a display having a 100 PPI resolution. In some embodiments, an LED device is operated with a confined current density from 0.001 A/cm2 to 40 A/cm2. In an embodiment, the current density ranges from 0.001 A/cm2 to 3 A/cm2. In an embodiment, such a current density range may be applicable to a display having a 400 PPI resolution. In an embodiment, the current density ranges from 0.2 A/cm2 to 4 A/cm2. In an embodiment, such a current density range may be applicable to a display having a 100 PPI resolution.
The following examples are provided to illustrate the effect of current confinement, and the relationship of efficiency, current and current density for LED devices in accordance with embodiments of the invention. In accordance with embodiments of the invention, a designer may select a desired efficiency and luminance of an LED device with a characteristic efficiency curve, such as the exemplary efficiency curve illustrated in
In one embodiment, a display panel is a 5.5 inch full high definition display with 1920×1800 resolution, and 400 pixels per inch (PPI) including a 63.5 μm RGB pixel size. To achieve a 300 Nit output (white) with LED devices having a 10% EQE, the display panel uses approximately 10 nA-30 nA of current per LED, assuming one LED per subpixel. For an LED device with a 10×10 μm confined current injection area this corresponds to a current density of 0.01 A/cm2-0.03 A/cm2. This is well below the normal or designed operating conditions for standard LEDs.
In an embodiment, the parameters of Example 1 are the same, with a smaller 1 μm×1 μm confined current injection area. With this reduced current injection area the corresponding current density increases to 1 A/cm2-3 A/cm2. Thus, Example 2 illustrates that at operating currents of 10 nA-30 nA, small changes in current injection area from 10 μm×10 μm to 1 μm×1 μm can have a significant effect on current density. In turn, the change in current density may affect efficiency of the LED device.
In one embodiment, a display panel is a 5.5 inch full high definition display with 1920×1800 resolution, and 400 pixels per inch (PPI) including a 63.5 μm RGB pixel size. Each subpixel includes an LED device with a 10 μm×10 μm confined current injection area. Luminance is maintained at 300 Nit output (white). In this example, it is desired to achieve a 40% EQE. With this increased efficiency, lower operating currents may be used. In an embodiment, an operating current of 3 nA-6 nA per LED is selected. With these parameters an LED device with a 10 μm×10 μm confined current injection area operates at 0.003 A/cm2-0.006 A/cm2, and an LED device with a 1 μm×1 μm confined current injection area operates at 0.3 A/cm2-0.6 A/cm2.
In one embodiment, a display panel is a 5.5 inch display with a lower resolution of 100 PPI including a 254 μm RGB pixel size. To achieve a 300 Nit output (white) with LED devices having a 10% EQE, the display panel uses a higher operating current of approximately 200 nA-400 nA of current per LED, assuming one LED per subpixel. For an LED device with a 10 μm×10 μm confined current injection area this corresponds to a current density of 0.2 A/cm2-0.4 A/cm2. A 1 μm×1 μm confined current injection area corresponds to a current density of 20 A/cm2-40 A/cm2, and a 3 μm×3 μm confined current injection area corresponds to a current density of 2 A/cm2-4 A/cm2. Thus, Example 4 illustrates that with lower resolution displays, there is a smaller density of LED devices, and higher operating currents are used to achieve a similar brightness (300 Nit) as higher resolution displays.
In one embodiment, a display panel has 716 PPI including a 35 μm RGB pixel size. To achieve a 300 Nit output (white) with LED devices having a 10% EQE, the display panel uses an operating current of approximately 4-7 nA. With these parameters an LED device with a 10 μm×10 μm confined current injection area operates at 0.004 A/cm2-0.007 A/cm2, and an LED device with a 1 μm×1 μm confined current injection area operates at 0.4 A/cm2-0.7 A/cm2.
In another embodiment the required brightness of the display is increased to 3000 Nit. In all examples above the required current would increase about 10× if the same EQE is targeted. Subsequently, the current density would also increase 10× for the above examples. In one embodiment the required operating brightness is a range from 300 Nit to 3000 Nit. The current and subsequently the current density would span a range of 1-10× the 300 Nit range. In the case of Examples 1 and 2 (above) where now 300 Nit to 3000 Nit is required, an LED device with a 10 μm×10 μm confined current injection area operates at a current density of 0.01 A/cm2-0.3 A/cm2 and an LED device with a 1 μm×1 μm confined current injection area operates at 1 A/cm2-30 A/cm2.
In each of the above exemplary embodiments, the brightness of the display is such that the LED devices are operating at very low current densities that are not typical of standard LEDs. The typical performance of standard LEDs show low IQEs at current densities below 1 A/cm2. In accordance with embodiments of the invention, the current injection area is confined such that the current density can be increased to allow operation of the LED devices in a current density regime where IQE, and EQE, are optimized.
In an embodiment, the LED devices are bonded to a display substrate in a display area of the display substrate. For example, the display substrate may have a pixel configuration, in which the LED devices described above are incorporated into one or more subpixel arrays. The size of the LED devices may also be scalable with the available area of the subpixels. In some embodiments, the LED devices are bonded to a display substrate having a resolution of 100 PPI or more. In the Examples provided above, exemplary red-green-blue (RGB) pixel sizes of 35 μm were described for a display having 716 PPI, RGB pixels sizes of 63.5 μm were described for a display having 400 PPI, and RGB pixels sizes of 254 μm were described for a display having 100 PPI. In some embodiments, the LED devices have a maximum width of 100 μm or less. As display resolution increases, the available space for LED devices decreases. In some embodiments, the LED devices have a maximum width of 20 μm or less, 10 μm or less, or even 5 μm or less. Referring back to the above discussion with regard to
In some embodiments, the display 3830 includes one or more LED devices 156 that are formed in accordance with embodiments of the invention described above. Depending on its applications, the display system 3800 may include other components. These other components include, but are not limited to, memory, a touch-screen controller, and a battery. In various implementations, the display system 3800 may be a television, tablet, phone, laptop, computer monitor, kiosk, digital camera, handheld game console, media display, ebook display, or large area signage display.
In utilizing the various aspects of this invention, it would become apparent to one skilled in the art that combinations or variations of the above embodiments are possible for forming an LED device including any one of a confined current injection area. Although the present invention has been described in language specific to structural features and/or methodological acts, it is to be understood that the invention defined in the appended claims is not necessarily limited to the specific features or acts described. The specific features and acts disclosed are instead to be understood as particularly graceful implementations of the claimed invention useful for illustrating the present invention.
This application is a continuation of U.S. patent application Ser. No. 15/223,900, filed Jul. 29, 2016, which is a divisional of U.S. patent application Ser. No. 14/194,509, filed Feb. 28, 2014, now U.S. Pat. No. 9,450,147, which is a continuation-in-part of U.S. patent application Ser. No. 14/141,735 filed on Dec. 27, 2013, now U.S. Pat. No. 9,583,466. The full disclosures of U.S. patent application Ser. No. 14/194,509 and U.S. patent application Ser. No. 14/141,735 are incorporated herein by reference.
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