Patent Publication Number: US-8530257-B2

Title: Band offset in alingap based light emitters to improve temperature performance

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a divisional of U.S. patent application Ser. No. 10/940,697, filed Sep. 14, 2004, titled IMPROVING BAND OFFSET IN AlInGaP BASED LIGHT EMITTERS TO IMPROVE TEMPERATURE PERFORMANCE, which is incorporated herein by reference in its entirety. 
    
    
     BACKGROUND OF THE INVENTION 
     1. The Field of the Invention 
     The present invention relates to systems and methods for improving band offset in semiconductor light emitters. More particularly, the present invention relates to systems and methods for improving band offset in semiconductor light emitters for improved temperature performance. 
     2. Background and Relevant Art 
     Semiconductor light emitters can take a variety of different forms. Light emitting diodes (LEDs), vertical cavity surface emitting lasers (VCSELs), and edge emitting lasers are examples of semiconductor light emitters. Semiconductor light emitters can also emit light at various wavelengths. However, the ability to emit light at shorter wavelengths (in the visible spectrum, for example) faces several challenges, particularly in VCSELs. 
     One of the system materials currently used to produce VCSELs emitting a wavelength in the visible spectrum is AlInGaP (Aluminum Indium Gallium Phosphide). In fact, AlInGaP materials are often used in lasers that emit light at wavelengths in the red region of the visible spectrum. However, the wavelengths that can be emitted using an AlInGaP system material are typically limited. 
     Some of the factors that limit the range of wavelengths that can be emitted by an AlInGaP device include the inability to obtain sufficiently high p-type doping levels, low hole mobility, and a small conduction band offset. The small conduction band offset can result in poor carrier confinement, which impacts the quality of the device. 
     For example, AlInGaP VCSELs that are designed to be red light emitters (emitting a wavelength on the order of 690-630 nm) face challenges that are related to both temperature and conduction band offset. The thermal distribution of energy can excite carriers out of the quantum wells. If the carriers are not in the quantum wells, then the carriers cannot recombine to produce light. This problem is further complicated by the low conduction band offset. In other words, the quantum wells of AlInGaP devices have shallow wells. The shallowness of the wells combined with temperature leads to poor carrier confinement and carrier leakage. 
     In addition, AlInGaP devices often have a close indirect conduction band in addition to a low conduction band offset. The close indirect conduction band can also lead to carrier leakage from the quantum wells and further requires a phonon to conserve momentum of the photon. Thus, the low conduction band offset, the close indirect conduction band, and higher temperatures result in poor carrier confinement and degrade the high temperature performance of AlInGaP devices. 
     BRIEF SUMMARY OF THE INVENTION 
     These and other limitations are overcome by embodiments of the present invention, which relate to systems and methods for changing band offsets in light emitters, including semiconductor lasers such as VCSELs, to improve the temperature performance of the light emitters. Many VCSELs that emit light in the visible spectrum are AlInGaP based devices. As previously indicated, AlInGaP VCSELs suffer from poor offset in the conduction band as well as a close indirect conduction band. These factors degrade the high temperature performance of AlInGaP based devices. 
     In one embodiment of the invention, nitrogen is added to the quantum well region in small quantities. Typically, nitrogen is added in a range of 0.2 to 2.5 percent, but is often added at a concentration of about 1 percent. The addition of nitrogen to the quantum well region increases the band offset and increases the separation of the indirect conduction band. The addition of nitrogen also impacts the band offset of the valence band. Because the valence band had a discontinuity that was greater than required, the decrease in the discontinuity of the valence band related to the addition of nitrogen does not impact performance of the device. 
     In another embodiment, the addition of nitrogen may change the bandgap of the device and thus change the emission wavelength of the AlInGaP or InGaP based device. The bandgap can be adjusted by increasing the concentration of aluminum and/or decreasing the concentration of indium. This has the effect of keeping the emission wavelength within an acceptable range. The net effect of adding nitrogen, increasing the concentration of aluminum, and/or decreasing the concentration of indium is to increase the conduction band offset, increase the separation of the indirect conduction band, and/or keep the emission wavelength within acceptable bounds. 
     Additional features and advantages of the invention will be set forth in the description which follows, and in part will be obvious from the description, or may be learned by the practice of the invention. The features and advantages of the invention may be realized and obtained by means of the instruments and combinations particularly pointed out in the appended claims. These and other features of the present invention will become more fully apparent from the following description and appended claims, or may be learned by the practice of the invention as set forth hereinafter. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       In order to describe the manner in which the above-recited and other advantages and features of the invention can be obtained, a more particular description of the invention briefly described above will be rendered by reference to specific embodiments thereof which are illustrated in the appended drawings. Understanding that these drawings depict only typical embodiments of the invention and are not therefore to be considered to be limiting of its scope, the invention will be described and explained with additional specificity and detail through the use of the accompanying drawings in which: 
         FIG. 1  illustrates one embodiment of a vertical cavity surface emitting laser that emits light in the visible spectrum; 
         FIG. 2  illustrates one embodiment of an active region for the vertical cavity surface emitting laser shown in  FIG. 1 ; 
         FIG. 3A  illustrates an example of the conduction band and the valence band of a light emitting device; 
         FIG. 3B  illustrates an energy versus momentum diagram with radiative recombination between the direct conduction band and the valence band. 
         FIG. 4A  illustrates another example of the conduction band and the valence band of a light emitting device; and 
         FIG. 4B  illustrates an energy versus momentum diagram with radiative recombination between the direct conduction band and the valence band but with a larger offset between the direct conduction band and the indirect band compared to the offset between the direct conduction band and the indirect band illustrated in  FIG. 3B . 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The visible spectrum includes wavelengths of approximately 400 nm to 750 nm. At these wavelengths, the system materials used to make light emitting devices such as vertical cavity surface emitting lasers (VCSELs) include AlInGaP based devices, AlGaAs based devices, InGaP based devices, and other system materials known to one of skill in the art. AlInGaP is often selected for these types of light emitting devices including those that emit in the red region of the visible spectrum (690 to 630 nm, for example). 
     The performance of an AlInGaP based device is often related to the band offsets and carrier confinement. For example, AlInGaP devices tend to have a direct bandgap that is greater than the direct bandgap of AlGaAs devices, but the conduction band offset is insufficient in AlInGaP devices. The effect of the low conduction band offset in AlInGaP devices has an impact on the wavelengths that can be emitted and further has an impact on the performance of the device, particularly at higher temperatures because the low conduction band offset causes poor carrier confinement. 
     In addition to the poor conduction band offset, the indirect band in AlInGaP is low enough that electrons from a heavily populated direct band can populate the higher density of states in the indirect band, and not take part in radiative recombination. 
     The present invention relates to systems and methods for improving the band offset in AlInGaP based devices, for example, in the visible spectrum. Although embodiments of the invention are discussed with reference to AlInGaP based devices, one of skill in the art can appreciate that the invention also applies to other system materials including, but not limited to, AlGaAs and InGaP and combinations thereof. 
     Embodiments of the present invention also improve the temperature performance of AlInGaP based devices. In one embodiment, small quantities of N (Nitrogen) are added to the quantum well layers of AlInGaP based light emitting devices. The nitrogen increases the conduction band offset and also improves the separation of the indirect conduction band. In some embodiments, the wavelength emitted by this type of device can be maintained by changing the composition of the system material. For example, the concentration of aluminum may be increased and/or the concentration of indium may be decreased to keep the emission wavelength within acceptable bounds. 
       FIG. 1  illustrates generally the epitaxial structure of a vertical cavity surface emitting laser (VCSEL) that emits light in the visible spectrum. The VCSEL  100  includes a substrate  102 . In this example, n-type DBR (Distributed Bragg Reflector) layers  104  are formed on the substrate  102 . An active region  110  is formed on the n-type DBR layers  104 . Next, p-type DBR layers  106  are formed on the active region  110 . Then, a cap  112  is typically formed on the p-type DBR layers  106 . One of skill in the art can appreciate that other layers may be included in a VCSEL structure. 
     For example, VCSELs may include insulating regions that are formed, for instance, by implanting ions or by forming an oxide layer. The insulating region is often used to form a conductive annular central opening in the VCSEL. VCSELs may also include tunnel junctions. One of skill in the art can also appreciate that some of the layers may be doped and form either n type or p type materials. 
     In this example, the DBR layers  104 ,  106  typically include n pairs of two materials. The two materials in each pair have different refractive indices. The change in refractive index from one material to the other enables DBR layers to have high reflectivity. Each layer in the DBR layers  104 ,  106  is approximately a quarter wavelength thick. Each DBR layer  104 ,  106  can include, by way of example and not limitation, from 30 to 50 layer pairs. The DBR layers  104 ,  106  provide the reflectivity that is needed for the VCSEL  100  to lase. 
     In this example, the substrate  102  and the cap  112  may both be formed from GaAs. Each pair of layers in the n-DBR layer  104  and the p-DBR layer  106  may include a layer of AlGaAs and a layer of AlAs. 
     The active region  110 , with reference to both  FIG. 1  and  FIG. 2 , may include a quantum well structure. The active region  110  illustrates quantum wells  202  that are separated by barrier layers  204 . The quantum wells  202  and the barrier layers  204  are bounded on either side by spacer layers  206 . 
     The quantum wells  202  have a lower bandgap and is the location where carrier recombination preferably occurs for light emission. As previously stated, however, carrier confinement is problematic because of the low conduction band offset. In addition, conventional devices have a close indirect conduction band that results in carrier leakage. The indirect conduction bandgap is typically inefficient for light emitting purposes. As a result, a close indirect conduction band is typically undesirable in light emitting devices. 
     In the example of  FIG. 2 , the quantum wells  202  include InGaP. The barrier layers  204  may include AlInGaP. In one embodiment of the invention, N is added to the quantum wells  202  and/or the barrier layers in various quantities. For example, N can be added in a range of 0.2 to 2.5 percent. In one embodiment, N is added to the quantum wells  202  at a concentration of 1 percent. The addition of N has an impact on both the conduction band offset as well as the valence band offset or valence band discontinuity. 
     The impact of nitrogen in the active region  110  or in the quantum wells  202  is further illustrated by  FIGS. 3A ,  3 B,  4 A, and  4 B, which illustrate relationships between conduction bands and valence bands.  FIG. 3A , for example, illustrates the conduction band  302  and the valence band  304  for the active region  300  before N is added to the quantum wells. The energy levels  312  correspond to the conduction band of the quantum wells and the energy levels  314  correspond to energy levels of the valence band of the quantum wells. The depth  306  of the wells in the valence band  304  are relatively deep in this example. The depth  318  of the conduction band is shallow and suffers from poor carrier confinement as previously described. The bandgap  310  represents the energy difference between the valence band  304  and the conduction band  302 . The bandgap  310  is also related to the emission wavelength. 
       FIG. 3B  illustrates the energy (E) versus momentum (K) relationship between the valence band  316 , the direct conduction band  326  and the indirect conduction band  328 . The line  322  indicates the recombination of an electron and a hole and the arrow  319  represents an emitted photon. In this example, the bandgap  320  is relatively close to the bandgap  310 . The separation  324  between the direct conduction band  326  and the indirect conduction band  328 , however, is low. As a result, carriers may be lost from the direct conduction band  326  to the indirect conduction band  328  as previously described. In other words, the high density of states and the small separation  324  causes electrons to parasitically populate the indirect conduction band  328 . 
       FIG. 4A  illustrates a bandgap diagram for a light emitting device (AlInGaP or InGaP based device) with N added in concentrations as described above. Adding N to the quantum wells and/or to the barrier layers increases the depth  418  of the quantum wells in the conduction band  402  compared to the depth  318  of the quantum wells in the conduction band  302 . In other words, the conduction band offset is increased by the addition of N, thereby reducing carrier leakage at higher temperatures. In other words, a deeper quantum well in the conduction band improves carrier confinement at higher temperatures and ensures that carrier recombination can occur in the quantum wells. 
     In one embodiment, however, the addition of N also has the effect of decreasing the depth  406  of the quantum wells in the valence band  404  compared to the depth  306  of the quantum wells in the valence band  304 . However, the valence band  404  had more of a discontinuity or band offset that was needed. Thus, the depth  406  in the valence band  404  is not too shallow and the depth  418  of the conduction band  402  controls. In one embodiment, the depth  418  of the conduction band  402  controls the effectiveness of carrier recombination at higher temperatures. 
     The addition of N to the quantum wells may also have an impact on the bandgap  410 . In this example, the bandgap  410  may no longer be substantially equal to the bandgap  310 . Thus, the emission wavelength associated with the active region  400  is different that the emission wavelength associated with the active region  300 . 
     Embodiments of the invention, in addition to increasing the conduction band offset, also keep the emission wavelength (and thus the bandgap) within acceptable bounds. By way of example and not limitation, embodiments of the invention may increase the concentration of aluminum in the quantum wells and/or decrease the concentration of indium in the quantum wells to keep the emission wavelength within acceptable bounds. 
       FIG. 4B  illustrates an effect of adding N to the quantum wells on the indirect conduction band  428 . The indirect conduction band  428  is still present, but the separation  424  in energy between the direct conduction band  426  and the indirect conduction band  428  has increased (in comparison to the separation  324  shown in  FIG. 3B ), thereby decreasing the parasitic population in the indirect conduction band  428 . In other words, the separation of the indirect conduction band  428  from the direct conduction band  426  increases, which leads a higher portion of the electrons residing in the direct conduction band  426  where optical emission is the dominant recombination mechanism. 
     The net effect of adding N (in a range of 0.5 to 2 percent as previously stated), increasing the concentration of Al, and/or decreasing the concentration of In, is an increase in the conduction band offset and increased separation of the indirect band from the direct band while keeping the emission wavelength within acceptable bounds. One of skill in the art can appreciate that embodiments of the invention are not limited to the preferred range of 0.5 to 2 percent N, but extend to concentrations outside of this range. 
     The present invention may be embodied in other specific forms without departing from its spirit or essential characteristics. The described embodiments are to be considered in all respects only as illustrative and not restrictive. The scope of the invention is, therefore, indicated by the appended claims rather than by the foregoing description. All changes which come within the meaning and range of equivalency of the claims are to be embraced within their scope.