1. Field of the Invention
The invention is related to semiconductor micro-cavity light emitting diodes (MCLEDs).
2. Description of the Related Art
(Note: This application references a number of different publications as indicated throughout the specification by one or more reference numbers within parentheses, e.g., (Ref. x). A list of these different publications ordered according to these reference numbers can be found below in the section entitled “References.” Each of these publications is incorporated by reference herein.)
The current state-of-the-art of nitride-based light emitters comprises two distinct families of devices: high brightness light emitting diodes (LEDs) and laser diodes (LDs). Gallium nitride-based LEDs hold the promise of revolutionizing the lighting industry, thanks to their high efficiency and versatility. Similarly, gallium nitride-based LDs have already had a significant impact on high-density data storage.
Standard nitride-based LEDs are typically fabricated from epitaxial material grown by metal organic chemical vapor deposition (MOCVD).
This standard LED is typically converted into a high brightness LED by one of two techniques. The first technique is chip shaping, wherein the substrate material is shaped to maximize light extraction (Ref. 4). By shaping the substrate that the epitaxial material is grown on, more of the generated light that is typically lost to total internal reflection is directed out of the LED, thereby improving extraction efficiency.
The second technique to improve brightness of a standard LED is to deposit a highly reflective mirror material on the surface and collect the light from the substrate side of the LED, if the substrate is transparent (Ref. 5). With this technique, all of the light emitted downward, which is typically lost, can be reflected back towards the surface and extracted from the LED.
These performance enhancing features may be used together to maximize extraction efficiency. While these techniques enhance extraction efficiency they do not confine the light to discrete cavity modes.
Laser diodes operate on an entirely different principle than LEDs. While the light in LEDs is generated through spontaneous emission, lasers use stimulated emission to efficiently generate light that is extremely spectrally pure and directional (Ref. 3).
Laser diodes have an epitaxial structure similar to the standard LED structure, but have additional layers inserted to confine the light to the emitting region in order to maximize the stimulated emission. These additional layers greatly increase the complexity of the epitaxial growth for laser diodes. The epitaxial material also requires substantially more processing in order to fabricate the laser diodes.
Another characteristic of laser diodes is that the light is emitted parallel to the substrate, which can cause difficulties with implementation of these devices. In order to direct the light vertically, the chip must be placed on its side which can further complicate the design of an optical system. The additional complexity of the material growth and processing of the laser diodes results in these devices having a higher cost.
One application of these devices is for lighting. The light emitted by these devices may be used to excite a phosphor or combination of phosphors to produce secondary emission in the green/red/yellow range of the color spectrum, as in a “white” light emitting diode.
Another very important application for gallium nitride based light sources is as excitation sources for materials that have an absorption edge close to the GaN or [Al, In, Ga]N band-edge. For example, a GaN ultraviolet light emitting diode could be used as an excitation source to detect biological agents that would absorb the light and then re-emit light at a characteristic wavelength. This absorption and re-emission would thus mark the presence of a specific biological agent.
Another recent application of GaN based light sources is as light sources for plastic optical fiber communication systems.
As described in U.S. Pat. No. 5,226,053 (Ref. 1), when an LED is placed within a Fabry-Perot optical cavity defined by a highly reflective mirror on one side and a less reflective mirror on the other, the emission of light from the active region is restricted to specific modes of the optical cavity. This leads to enhancements in directionality and light extraction efficiency, as well as spectral narrowing, and, in specific cases, improvements in quantum efficiency. Optical devices of this type are known as a resonant cavity light emitting diodes (RCLEDs).
Nitride-based RCLEDs (Ref. 6) have been demonstrated, but are not generally produced. The limited benefits of the RCLED are offset by the complicated processing necessary to fabricate these devices.
The present invention describes a device that may be considered a type of RCLED, and is known as a Micro-Cavity Light Emitting Diode (MCLED). The advantages of the MCLED stem from the confinement of the spontaneously emitted light into cavity modes. All of the benefits of RCLEDs are further enhanced when the length of the cavity, defined by the mirrors, is reduced, and, at a certain point, the device enters a “micro-cavity” regime (Ref. 2).
The “micro-cavity” regime is defined as the critical thickness below which the cavity order, mc, is less than 2×n2, where n is the index of refraction of the emitting material. The cavity order, mc, is defined as [2nLc/λ] rounded to the closest integer, where Lc is the length of the cavity and λ is the wavelength of emitted light in air. The fundamental premise of the present invention is a nitride LED that is in a cavity with a cavity length short enough for the device to be in the micro-cavity regime for the nitride material system.
MCLEDs possess superior light extraction efficiency, improved spectral purity, and improved directionality of emission, as compared to conventional LEDs and RCLEDs. The enhanced performance of the MCLED should make the nitride-based MCLED much more promising for widespread use. The utility of nitride based MCLEDs will be in applications where performance requirements are too strict for LEDs but where the performance and cost of laser diodes are not necessary.
What is needed are improved methods for the fabrication of nitride based MCLEDs through innovative device design and material growth, as well as advanced nitride processing and fabrication steps. The present invention satisfies these needs.
The present invention discloses a micro-cavity light emitting diode (MCLED), comprising a spontaneously light emitting nitride-based active region placed within a micro-cavity bounded by a first mirror and a second mirror, wherein the micro-cavity has been thinned to a resonant thickness within a micro-cavity regime. The resonant thickness may be a micro-cavity length Lc satisfying a resonance requirement approximately equal to an integral multiple of half-wavelengths of light emitted by the nitride based active region, such that: Lc≈mc×λ/2n. The micro-cavity length can be slightly reduced from the resonance requirement in order to detune the cavity so the micro-cavity will be in resonance with emitted light only when the light is emitted at an angle off of the normal (90°) and the angle is within a critical angle for light emission for the MCLED.
The nitride based active region may be placed, with respect to the first or second mirror, at an anti-node of an optical wave within the micro-cavity. The active region may be less than λ/4n thick to allow for the entire active region to be located at an anti-node of a standing wave within the micro-cavity. The nitride-based active region may be a compound active region consisting of localized active regions located on adjacent anti-nodes of a standing optical wave.
The MCLED may further comprise n-type gallium nitride (GaN) and p-type GaN bounded by the first and second mirrors. The p-type GaN thickness may locate the active region on an anti-node of the standing optical wave.
The first mirror may comprise a highly reflective metal mirror at one end of the micro-cavity that makes good electrical contact with the n-GaN and the second mirror may comprise an interfacial mirror at another end of the cavity. Alternatively, a single distributed Bragg reflector may be used for the first or second mirror.
The MCLED may further comprise a current confinement layer which may be an ion implantation layer.
Referring now to the drawings in which like reference numbers represent corresponding parts throughout:
In the following description of the preferred embodiment, reference is made to the accompanying drawings which form a part hereof, and in which is shown by way of illustration a specific embodiment in which the invention may be practiced. It is to be understood that other embodiments may be utilized and structural changes may be made without departing from the scope of the present invention.
Overview
The general purpose of the nitride-based MCLED described herein is as a high efficiency, high spectral purity, and high directionality light emitter of wavelengths from UV to green. The light emitted by a MCLED is more spectrally pure and is more directional than the light from a standard LED. Also, a MCLED has higher efficiency than a standard LED.
With all of these advantages, MCLEDs are the natural choice for applications where superior performance than a LED is required, but the high cost and extreme performance of a laser diode are not necessary. Some examples of suitable applications have been mentioned previously, high efficiency light emitters, optical excitation sources, and light sources for fiber optic data transmission.
Technical Description
FIGS. 2(a), 2(b), 2(c) and
In the design outlined herein, the mirrors used are a highly reflective metal mirror (201), (301) at one end of the cavity and the GaN/air interface at the other end (202), (302). In a typical micro-cavity LED, the metal mirror may have a reflectivity of 50-100%, while the interfacial mirror has a reflectivity of approximately 20%, in the case of a GaN/air interface. Light (203), (303) may be extracted at the end with the interfacial mirror.
The micro-cavity may be considered within the micro-cavity regime if the resonant thickness (204), (304) has a dimension Lc such that mc<2×n2, where mc is the cavity order defined as an integer multiplied by [2nLc/λ], n is the index of refraction for the nitride-based material at that wavelength λ, and Lc is the micro-cavity length. For λ=490 nm, the index of refraction of GaN is approximately n=2.4. The micro-cavity regime for this device would be when the cavity is 1176 nm or smaller. Similarly, for a different nitride device that emits at 420 nm and has an index of refraction at that wavelength of approximately n=2.49, the cavity length would need to be less than 1046 nm in order for the device to operate in the micro-cavity regime.
However,
In
By placing the LED within a cavity, light can only be emitted into certain available modes that are resonant with the cavity. The number of available modes is an equivalent definition of the cavity order. The number of these modes that are within the critical angle extraction cone of the material divided by the total number of modes will be approximately equal to the extraction efficiency of the device, ηext≈mext/mc, where ηext is the extraction efficiency of the device, mext is the number of modes within the extraction cone, and mc is the cavity order. When the cavity is thin enough so that there is only one mode within the extraction angle, mext==1, then the device is operating within the micro-cavity regime and the extraction efficiency will be η≈1/mc. Proper design of the cavity thickness and the placement of the active region within the cavity will ensure the optimum performance of the MCLED device.
There are several benefits to operating a device within the micro-cavity regime. A GaN-based micro-cavity LED will have improved extraction efficiency compared to a standard LED or RCLED and the extraction efficiency will have an inverse relation to the cavity length, the shorter the cavity the higher the extraction efficiency. MCLEDs will also exhibit enhanced directionality due to the confinement of the emitted light into available cavity modes. Furthermore, the micro-cavity LED emission is generally more spectrally pure than the emission from a standard LED. All of these benefits can be realized with a suitably designed and properly fabricated GaN-based MCLED device.
Growth and Fabrication
A typical growth and fabrication sequence that yields the micro-cavity light emitting diode that is proposed in this disclosure is described below. It should be noted that variations in the growth technique, order of the processing steps, deposition techniques, and etch techniques can be made without affecting the design of the MCLED device.
1. The epitaxial crystal material grown by MOCVD on a c-plane sapphire substrate (501). In alternative embodiments, the epitaxial crystal may be grown by any crystal growth method such as MBE and HVPE, and on any substrate, for example, GaN, in any crystal orientation.
2. Unintentionally doped GaN layer (502). The first layer is unintentionally doped (UID) GaN that is approximately 2 microns thick. In alternative embodiments, this layer may be omitted.
3. Highly doped n-type layer (503). The next MOCVD grown layer is 2 microns of highly Si-doped GaN which acts as the n-type layer in the diode. The density of charge carriers is roughly ˜5×1018 per cubic centimeter. In alternative embodiments, this layer may be thicker or thinner.
4. Active region of the MCLED (504). The active region is comprised of 3 quantum wells 4 nm thick separated by GaN barriers 8 nm thick. The quantum wells are comprised of InGaN with an indium composition chosen for 480 nm light emission. The barriers are undoped GaN. In alternative embodiments, single quantum well or multiple quantum well active region with any thickness quantum wells and barriers may be grown as long as the active region can be localized to an anti-node of the standing optical wave. Additionally, the quantum wells may be grown to emit at any wavelength within the nitride wavelength range for alternative embodiments.
5. Mg doped AlGaN electron blocking layer (505). This layer is 20 nm thick.
6. Thin Mg-doped GaN layer (506). This layer acts as the p-type layer in the diode. The thickness of this layer is approximately 168 nm so that the active region (504) will be located on anti-node of the optical standing wave. Alternatively, any p-type GaN thickness that locates the active region on an anti-node of the standing optical wave may be used as long as the thickness of the cavity is such that the device is operating in the micro-cavity regime.
Block 602 represents the step of depositing and patterning current apertures in SiO2 to create a current confinement layer. The current can only enter the GaN through the aperture in the SiO2, which limits the emission to the aperture region. The SiO2 is deposited by plasma enhanced chemical vapor deposition (PECVD) at 250° C. on the p-GaN layer. A current aperture region may then be patterned in the SiO2. An alternative current confinement scheme may utilize ion implantation of the p-GaN, n-GaN or active layer, or an alternative insulator instead of SiO2. Alternative embodiments may use no current confinement layer or other current confinement configurations.
Block 603 represents the step of mirror and p-contact deposition. A silver mirror is deposited by electron beam deposition in the aperture region created in Block 602 to define one side of the cavity and act as the electrical contact to the p-type GaN. In alternative embodiments, any highly reflective metal mirror that makes a good contact with the p-doped GaN may be used for the mirror contact.
Block 604 represents the step of solder metal deposition. A blanket layer of solder metal consisting of 200 nm Sn and 1 μm Au is deposited by thermal evaporation on the current confinement layer of Block 602, as an adhesive layer that will attach the sub-mount carrier wafer to the sample.
Block 605 represents the step of sub-mounting by wafer bonding. A silicon wafer is bonded to the surface of the processed epitaxial material at the solder metal layer of Block 604 to provide mechanical support after the sapphire wafer is removed by laser lift-off. The bonding occurs under manual pressure at 285° C. In an alternative embodiment, the MCLED could be fabricated with a different sub-mount wafer in place of the silicon wafer.
Block 606 represents the step of laser lift-off of the substrate comprising the sapphire substrate being removed by dissociation of the gallium and nitrogen by laser excitation. The laser used is a KrF excimer laser with a pulse energy of 400 mJ. In alternative embodiments, the removal of the MCLED substrate may be achieved if the substrate is etched away with a reactive ion etch, etched away with a laser assisted wet etch, or removed by mechanical polishing, or other removal techniques.
Block 607 represents the step of cavity thinning using an etch. The cavity, comprising n-GaN, p-GaN and the active layer, is reduced from its original thickness of approximately 4 microns to 800 nm, corresponding to a cavity order of 8, by reactive ion etching. The reactive ion etching may be performed in a chlorine environment at a power of 150 watts for 45 minutes. In alternative embodiments, the cavity thinning step may use other techniques such as inductively coupled plasma etching, photo-electrochemical etching, chemical etching, or mechanical polishing. In alternative embodiments, the cavity maybe thinned to different thicknesses.
Block 608 represents the step of chemical mechanical polishing. The surface of the micro-cavity sample comprising the GaN/Air interface acting as a mirror is smoothed by polishing with silica slurry. The surface is smoothed to a roughness of 0.5-2.0 nm (RMS), as determined by atomic force microscopy. For polishing, a 0.02 μm colloidal silica slurry is used for 10 minutes at 50 rpm.
Block 609 represents the step of mesa formation. Mesas are patterned and defined on the surface of the sample to electrically isolate individual devices. The mesas are patterned by standard photolithography and etched by reactive ion etching. The sample is etched in a chlorine environment at a power 150 watts for 25 minutes. In alternative embodiments, other isolating techniques may be used.
Block 610 represents the step of n-contact deposition. The electrical contact to the n-doped GaN is defined by photolithography and then deposited by electron beam deposition on the n-GaN layer. The contact is typically comprised of Ti/Al/Ni/Au. A ring shaped contact is used to maximize the electrical efficiency of the device. In alternative embodiments, other metals and metal deposition techniques may be used.
The MCLED device of
The current confining scheme may comprise, for example, a current confining layer (707) made from an insulator such as silicon dioxide (SiO2), an aperture (708) in the current confining layer (707) and a highly reflective metal/conductor mirror deposited inside the aperture (708). The highly reflective metal mirror, for example silver, acts as a first mirror (704) at one end of the micro-cavity and also provides good electrical contact to the p-GaN (702). Alternatively, the current confinement scheme may comprise ion implanted p-GaN (702), n-GaN (701) or nitride-based active region (703). The current confinement scheme limits light emission (709) to the aperture region. The MCLED may further comprise solder metals (710) for wafer bonding the micro-cavity to a mechanical support (711), wherein the mechanical support may be, for example, a silicon wafer.
The MCLED further comprises an interfacial mirror acting as the second mirror (705) for the MCLED micro-cavity. The second mirror (705) may also comprise a GaN/air interface, GaN/epoxy interface or any interfacial material which would expand the extraction cone of the MCLED and enable even higher extraction efficiency, at the second mirror end of the cavity. The n-type contact (706) may be a highly reflective metal, such as silver, acting as the second mirror (705).
The p-GaN (702) thickness locates the active region on an anti-node of the standing optical wave. The nitride based active region (703) is less than λ/4n (where n is refractive index of the active region) thick to allow for the entire active region to be located at an anti-node of a standing wave within the micro-cavity.
The MCLED of
The key feature of this invention is the ability to fabricate and operate a current injected MCLED that has been thinned to a “resonant” thickness within the micro-cavity regime. In an exemplary device, the emission wavelength was 498 nm and the index of refraction of GaN at 498 nm is approximately n=2.4. These values yield a maximum cavity length of approximately 1175 nm in order to be in the micro-cavity regime. The cavity length, or resonant thickness (712) of this exemplary device is approximately 800 nm, verified by profilometry, scanning electron microscopy, and computer modeling of the emission pattern. At this wavelength and cavity length, the MCLED is determined to be roughly in resonance, and is slightly detuned. For this example, zAR (713) is approximately 250 nm.
Micro-cavity operation was evidenced by variations in the emission spectrum with respect to angle and spectral narrowing of the quantum well emission. By comparing the separation of the cavity modes in the emission spectra at different angles a cavity length of approximately 800 nm can be calculated and micro-cavity operation can be verified.
Evidence of spectral narrowing of the emission by the MCLED device was also collected, as shown in
Possible Modifications and Variations
Several modifications and variations that incorporate the essential elements of the present invention are outlined below. Additionally, several alternative materials, growth conditions and techniques may be used in practice of this invention, as shall be enumerated below. Any modifications or variations could be used in combination.
1. The growth structure of
2. The epitaxial crystal material of
3. The epitaxial crystal material of
4. The growth structure of
5. The growth structure of
6. The epitaxial crystal material of
7. The growth structure of
8. The growth structure of
9. The processing method of
10. The processing method of
11. The processing method of
12. The MCLED of
13. The growth structure of
14. The MCLED of
15. The MCLED of
16. The MCLED of
17. The contacts of
18. The MCLED fabricated using the method of
19. The growth structure of
20. The cavity thinning/etch process of
21. Silver mirrors were used to define the one side of the cavity in
22. Laser lift off for removal of the sapphire is used in
23. In
24. The MCLED of
25.
Advantages and Improvements
The advantages and improvements over existing practice, and the features believed to be new include the following:
1. This is the first time that a micro-cavity LED has been fabricated in the GaN material system.
2. The GaN based micro-cavity LED exhibits superior efficiency over a standard LEDs and resonant cavity LEDs that are not within the micro-cavity regime. The gain in efficiency stem from gains in extraction efficiency.
3. The GaN based micro-cavity LED exhibits superior directionality compared to standard LEDs and resonant cavity LEDs that are not within the micro-cavity regime.
4. Chemical mechanical polishing of the surface of the MCLED device permits well defined cavity modes in the micro-cavity regime which allows for enhanced extraction efficiency.
5. Thin active regions (less than λ/4n, where n is refractive index at wavelength λ) allow for the entire active region to be located at an anti-node (peak) of the standing wave within the cavity, thus ensuring that all of the quantum wells of the device are contributing to the light emission and thereby enhancing the efficiency of the device.
6. Locating the thin active region onto a peak of the standing optical wave ensures that the emission from the active region is resonant with the cavity.
The following references are incorporated by reference herein:
This concludes the description of the preferred embodiment of the present invention. The foregoing description of one or more embodiments of the invention has been presented for the purposes of illustration and description. It is not intended to be exhaustive or to limit the invention to the precise form disclosed. Many modifications and variations are possible in light of the above teaching. It is intended that the scope of the invention be limited not by this detailed description, but rather by the claims appended hereto.
This application claims the benefit under 35 U.S.C. Section 119(e) of the following co-pending and commonly-assigned U.S. patent application: U.S. Provisional Patent Application Ser. No. 60/711,940, filed on Aug. 26, 2005, by P. Morgan Pattison, Rajat Sharma, Steven P. DenBaars and Shuji Nakamura, entitled “SEMICONDUCTOR MICRO-CAVITY LIGHT EMITTING DIODE”, attorneys docket number 30794.146-US-P1 (2006-017-1); which application is incorporated by reference herein. This application is related to the following co-pending and commonly-assigned applications: U.S. Utility application Ser. No. 11/067,956, filed on Feb. 28, 2005, by Claude C. A. Weisbuch, Aurelien J. F. David, and Steven P. DenBaars, entitled “HIGH EFFICIENCY LIGHT EMITTING DIODE (LED) WITH OPTIMIZED PHOTONIC CRYSTAL EXTRACTOR,” attorneys' docket number 30794.126-US-01 (2005-198-1); U.S. Utility application Ser. No. 11/403,288, filed on Apr. 13, 2005, by James S. Speck, Benjamin A. Haskell, P. Morgan Pattison and Troy J. Baker entitled “ETCHING TECHNIQUE FOR THE FABRICATION OF THIN (Al, In, Ga)N LAYERS” attorneys' docket number 30794.132-US-U1 (2005-509); which applications are incorporated by reference herein.
Number | Date | Country | |
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60711940 | Aug 2005 | US |