The invention relates to semiconductor light emission devices, such as, for example, light emitting diodes (LEDs) and lasers. More particularly, the invention relates to a semiconductor light emission device having an improved current confinement structure.
Semiconductor light emission devices, such as LEDs and lasers, for example, have an active light emission layer in which electrons and holes are converted into photons to produce optical emissions.
When the positive voltage bias is removed, or a negative bias voltage is applied, the holes and electrons move in opposite directions away from the active 13 and toward the p-type and n-type contacts 9 and 11, respectively. In this case, the active region 13 becomes depleted of holes and electrons. In LEDs, the photons that are emitted from the device are not all in phase, and so the light that is emitted from the device is said to be non-coherent light. In laser diodes, mirrors are included in the device to cause some of the photons to be reflected within the active region 13 to produce a pumping action. This pumping action results in the photons that are emitted from the device being in phase. In this case, the light emitted from the device is said to be coherent light, i.e., laser light.
The semiconductor device 2 typically also includes a current confinement structure 15 that serves to channel the current to a limited area in the active region 13 where the electrical current will be converted into light. The use of a current confinement structure in semiconductor light emission devices can increase light conversion efficiency by channeling the current only to an area in the active region from which light can escape the device. This prevents current from being injected into an area in the active region where the resulting light produced in the active region might be blocked (e.g., by an opaque metal contact) and thereby prevented from escaping the device. Current confinement is also essential for applications that require high current density at the p/n junction, such as in high speed LED and laser diode device applications.
There are many existing current confinement techniques that are used in semiconductor light emission devices. In the device 2 shown in
Another technique for providing current confinement in semiconductor light emission devices involves selectively oxidizing portions of a buried semiconductor layer to confine the current.
Accordingly, a need exists for a semiconductor light emission device having an improved current confinement structure as compared to existing current confinement structures and that provides better thermal performance and device reliability than that provided by existing current confinement structures.
The invention provides a semiconductor light emission device comprising a substrate of n-type material, at least a first layer of n-type material disposed on an upper surface of the substrate, at least one diffusion accommodation layer of n-type material disposed on the first layer of n-type material, at least one active layer of material disposed on the diffusion accommodation layer to provide an active region in the semiconductor light emission device for conversion of electrons and holes into photons, at least one layer of p-type material disposed on the active layer, at least one p contact disposed on the first layer of p-type material, at least one n contact disposed on a bottom surface of the substrate, and at least first and second diffusion areas in which p dopants have been diffused into the semiconductor light emission device. The material of the diffusion accommodation layer has a bandgap that is higher than the bandgap of the material of the active layer. The first and second diffusion areas pass through the first layer of p-type material and the active region and terminate in the diffusion accommodation layer. First and second p+/n junctions exist in the diffusion accommodation layer where the first and second diffusion areas terminate in the diffusion accommodation layer. A p/n junction exists in the active region. The existence of the first and second p+/n junctions in the higher bandgap material of the diffusion accommodation layer operate as a current confinement structure by causing electrical current to be channeled away from the p+/n junctions and into the p/n junction.
The invention provides a method for performing current confinement in a semiconductor light emission device. The method comprises providing a semiconductor light emission device comprising at least an active region disposed between first and second layers, and forming first and second diffusion areas in the semiconductor device. The active region comprises at least one layer of p-n material. The first layer comprises at least one diffusion accommodation layer of n-type material. The second layer comprises at least one layer of p-type material. The first and second diffusion areas are formed by diffusing p dopants into the semiconductor light emission device such that the first and second diffusion areas pass through the first layer and the active region and terminate in the diffusion accommodation layer. The material of the diffusion accommodation layer has a bandgap that is higher than the bandgap of the material of the active layer. A first p+/n junction exists where the first diffusion area terminates in the diffusion accommodation layer. A second p+/n junction exists where the second diffusion area terminates in the diffusion accommodation layer. A p/n junction exists in the active region. The existence of the first and second p+/n junctions in the higher bandgap material of the diffusion accommodation layer operates as a current confinement structure by causing electrical current to be channeled away from the p+/n junction and into the p/n junction.
These and other features and advantages of the invention will become apparent from the following description, drawings and claims.
The invention provides a semiconductor light emission device having a current confinement region that comprises a diffusion accommodation layer located adjacent the active region. The diffusion accommodation layer comprises a material that has a higher bandgap than the bandgap of the material in the active region. Diffusion of dopants into portions of the diffusion accommodation layer causes p+/n junctions to exist in the diffusion accommodation layer on either side of the p/n junction that exists in the active region. Due to the material of the diffusion accommodation layer having a bandgap that is higher than the bandgap of the material of the active region, the p+/n junctions turn on at a threshold forward bias voltage level that is higher than the threshold forward bias voltage level at which the p/n junction turns on. Because of this, the p+/n junctions are effectively turned off while the p/n junction is turned on, which causes electrical current to be channeled away from the p+/n junctions and into the p/n junction, thereby confining the current to a particular area of the active region.
The current confinement method and structure used in accordance with the invention have several advantages over the known current confinement structures and methods described above with reference to
In contrast to the etched mesa (
The term “n-type material”, as that term is used herein, means that the material has more negative carriers (electrons) than positive carriers (holes). The term “p-type material”, as that term is used herein, means that the material has more positive carriers (holes) than negative carriers (electrons). The term “p-n material”, as that term is used herein, means that the material has an equal number of holes and electrons such that it is neither positively nor negatively charged, but rather, is electrically at equilibrium.
The invention is not limited with respect to the materials that are used for the substrate 101 or for the layers 102, 110, 111, and 112. An example of a suitable materials system for the device 100 is the gallium arsenide/aluminum gallium arsenide (GaAs/AlGaAs) materials system. If this materials system is chosen for the device 100, the active layer 111 will typically be made of GaAs and the n-type diffusion accommodation layer 110 will typically be made of AlAs. Because GaAs has a bandgap of 1.42 electron-volts (eV) and AlAs has a bandgap of 2.17 eV, the diffusion accommodation layer 110 has a bandgap that is higher than the bandgap of the active layer 111, which is needed in order to provide a p+/n junction that has a threshold voltage that is higher than the threshold voltage of the p/n junction of the active layer 111. If this material system is used, the substrate 101 will typically be an n-type GaAs wafer, and the layers 102 and 112 will typically be made of n-type and p-type AlGaAs, respectively.
Another example of a suitable materials system for the device 100 is the gallium arsenide/aluminum indium gallium phosphide (GaAs/AlInGaP) materials system. If this system is chosen for the device 100, the active layer 111 will typically be made of InGaP and the diffusion accommodation layer 110 will typically be made of AlInP. Because AlInP has a bandgap of 2.35 eV and InGaP has a bandgap of 1.9 eV, the diffusion accommodation layer 110 has a bandgap that is higher than the bandgap of the active layer 111. If this material system is used, the substrate 101 will typically be an n-type GaAs wafer, and layers 102 and 112 will typically be made of n-type and p-type AlInGaP, respectively.
With reference to
This diffusion process is referred to herein as a “deep” diffusion process due to the fact that the diffusion front, which is represented by dashed lines 130A and 130B, penetrates through the active region 111 and into the n-type diffusion accommodation layer 110. Thus, the diffusion front terminates in the diffusion accommodation layer 110. This deep diffusion process is in contrast to known “shallow” diffusion processes in which the diffusion front stops before reaching the active region. In accordance with the invention, at the locations where the diffusion front terminates in the diffusion accommodation layer 110, p+/n junctions are formed. Wherever the diffusion front did not pass through the active region 111, a continuous p/n junction exists in the active region as originally grown. The location of this p/n junction is indicated by the bracket labeled 132. As indicated above, the p+/n junctions have higher threshold voltages than that of the p/n junction, which ensures that all current will be confined to the portion of the active region 111 indicated by the bracket labeled 132.
Through this selective diffusion process, the contact regions 133 in layer 112 where the metal for the p contacts (not shown) will subsequently be placed can be selectively doped to have a higher p doping than adjacent areas in layer 112. This increase in p doping at these locations reduces series electrical resistance associated with the p metal contacts. In addition, in the area 134 in between the contact regions 133, which generally corresponds to the optical emission window, a lower p doping can be provided to reduce free carrier absorption in this area. This is particularly beneficial when the device 140 is implemented as a vertical cavity surface light emitting laser (VCSEL) having mirror structures on both sides of the active region for reflecting photons back into the active region. In such cases, the free carrier absorption loss in the emission window 134 will be reduced. In addition, in single mode laser applications, the higher p doping outside of emission window 134 can be used to suppress higher order modes that might otherwise occur due to free carrier absorption loss being so large that the higher order lasing mode is not sustainable.
With reference to
It should be noted that the invention has been described with reference to a few illustrative embodiments for the purposes of demonstrating the principles and concepts of the invention. The invention, however, is not limited to these embodiments, as will be understood by persons of skill in the art in view of the disclosure provided herein. Many modifications can be made to the embodiments described herein, and all such modifications are within the scope of the invention. For example, additional layers other than those shown in