Abstract:
A method for fabricating a light-emitting semiconductor device including a III-Nitride quantum well layer includes selecting a facet orientation of the quantum well layer to control a field strength of a piezoelectric field and/or a field strength of a spontaneous electric field in the quantum well layer, and growing the quantum well layer with the selected facet orientation. The facet orientation may be selected to reduce the magnitude of a piezoelectric field and/or the magnitude of a spontaneous electric field, for example. The facet orientation may also be selected to control or reduce the magnitude of the combined piezoelectric and spontaneous electric field strength. As a result of the reduced magnitude of piezoelectric, spontaneous, or combined piezoelectric and spontaneous electric field strengths in their quantum well layers, light-emitting devices in accordance with the present invention may generate light with increased efficiency compared to prior art devices.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS  
       [0001]    This is a continuation in part of U.S. patent application Ser. No. 09/717,647 filed on Nov. 21, 2000, which is a divisional of U.S. patent application Ser. No. 09/162,708 filed Sep. 29, 1998, now U.S. Pat. No. 6,229,151. U.S. patent application Ser. No. 09/717,647 and U.S. Pat. No. 6,229,151 are incorporated herein by reference in their entirety. 
     
    
     
       FIELD OF THE INVENTION  
         [0002]    The present invention relates to optical semiconductor devices, and particularly, to a structure for improving the efficiency of light emitters and photodetectors fabricated from GaN-based semiconductors.  
         BACKGROUND  
         [0003]    In the following discussion a III-N semiconductor is a semiconductor having a Group III element and nitrogen. III-N semiconductors such as GaN are useful in fabricating light emitting elements that emit in the blue and violet regions of the optical spectrum. These elements include light emitting diodes and laser diodes. Laser diodes that use semiconductor material based on GaN that emit in the blue and violet regions of the spectrum hold the promise of substantially improving the amount of information that can be stored on an optical disk. However, higher efficiencies are needed for both semiconductor light emitters and photodetectors. This is a particularly urgent problem in GaN-based optical semiconductor devices using BN, AlN, GaN, or InN, which are compounds of nitrogen and Group III elements such as B, Al, Ga, and In and their mixed crystal semiconductors (hereinafter, called GaN-based semiconductors ).  
           [0004]    Light emitting elements based on III-N semiconductors are typically fabricated by creating a p-n diode structure having a light generating region between the p-type and n-type layers. The diode is constructed from layers of III-N semiconducting materials. After the appropriate layers are grown, electrodes are formed on the p-type and n-type layers to provide the electrical connections for driving the light-emitting element.  
           [0005]    One class of blue and green light-emitting diodes (LEDs) or short-wavelength laser diodes (LDs) use GaInN/GaN strained quantum wells or GaInN/GaInN strained quantum wells located between the n-type and p-type layers to generate light by the recombination of electrons and holes injected from these layers. In prior art devices, a strained GaN-based semiconductor layer is constructed by growing a {0001} plane of a GaN-based crystal. The resulting layer has a large piezoelectric field. For example, in a Ga 0.9 In 0.1 N strained layer, an extremely large piezoelectric field of around 1 MV/cm is generated.  
           [0006]    In addition, III-Nitride semiconductors having a wurtzite crystal structure exhibit a spontaneous polarization. This spontaneous polarization results in sheets of fixed charge at interfaces between III-Nitride layers of different alloy compositions, such as at the interfaces between a III-Nitride quantum well layer and adjacent III-Nitride layers. These charge sheets produce an electric field in the quantum well layer. This electric field, which may also be extremely large, will be referred to herein as a spontaneous electric field.  
           [0007]    Usually, when an electric field exists in a quantum well, the energy band of the quantum well layer tends to tilt substantially as the electric field increases. As a result, the wave functions of the electrons and holes separate from one another, and the overlap integrals of both wave functions decrease. Since the optical properties such as the light emission and absorption efficiencies depend on these overlap integrals, the efficiency of these devices decreases with increasing electric fields.  
           [0008]    What is needed is a III-Nitride light emitting device in which the problems associated with the internal piezoelectric and spontaneous electric fields have been overcome.  
         SUMMARY  
         [0009]    A method for fabricating a light-emitting semiconductor device including a III-Nitride quantum well layer includes selecting a facet orientation of the quantum well layer to control a field strength of a piezoelectric field and/or a field strength of a spontaneous electric field in the quantum well layer, and growing the quantum well layer with the selected facet orientation. The facet orientation may be selected to reduce the magnitude of a piezoelectric field and/or the magnitude of a spontaneous electric field, for example. The facet orientation may also be selected to control or reduce the magnitude of a combined field strength of a piezoelectric field and a spontaneous electric field in the quantum well layer.  
           [0010]    In one embodiment, the quantum well layer is grown with a wurtzite crystal structure with the selected facet orientation tilted at least 1°, preferably at least 10°, from the {0001} direction of the wurtzite crystal structure. For example, the selected facet orientation may be advantageously tilted at about 30° to about 50°, about 80° to about 100°, or about 130° to about 150° from {0001}. In another embodiment, the quantum well layer is grown with a zincblende crystal structure with the selected facet orientation tilted at least 1°, preferably at least 10°, from the {111} direction of the zincblende crystal structure.  
           [0011]    In some embodiments the quantum well layer is grown above a nucleation layer grown directly on a substrate surface. The nucleation layer may be grown, for example, by metal-organic vapor deposition at a temperature such that the crystal structure of the nucleation layer substantially replicates the crystal structure of the substrate surface. The substrate surface may be selected to have a lattice mismatch of less than about 10% with the material from which the quantum well layer is formed.  
           [0012]    In one embodiment, a resulting light-emitting semiconductor device includes a III-Nitride quantum well layer having a wurtzite crystal structure and a facet orientation tilted from the {0001} direction of the wurtzite crystal structure at an angle of about 30° to about 50° or about 130° to about 150°. In one implementation, the quantum well layer is formed over a nucleation layer grown directly on a surface of, for example, a SiC, AlN, or GaN substrate. The crystal structure of this nucleation layer substantially replicates the crystal structure of the substrate surface.  
           [0013]    As a result of the reduced magnitude of piezoelectric, spontaneous, or combined piezoelectric and spontaneous electric field strengths in their quantum well layers, light-emitting devices in accordance with the present invention may generate light with increased efficiency compared to prior art devices.  
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0014]    [0014]FIG. 1 illustrates the crystal structure of a WZ-GaN-based semiconductor.  
         [0015]    [0015]FIG. 2 is a graph of the piezoelectric field generated in the quantum well with respect to the growth orientation of the WZ-GaN-based semiconductor quantum well.  
         [0016]    [0016]FIG. 3 illustrates the crystal structure of a ZB-GaN-based semiconductor.  
         [0017]    [0017]FIG. 4 is a graph of the piezoelectric field strength generated in the quantum well with respect to the first path shown in FIG. 3.  
         [0018]    [0018]FIG. 5 is a cross-sectional view of a light-emitting device according to one embodiment of the present invention.  
         [0019]    [0019]FIG. 6 is a graph of the relative light generation efficiency of quantum wells in a semiconductor device of the present invention and a prior art semiconductor device as functions of the well width.  
         [0020]    [0020]FIG. 7 is a cross-sectional view of a light-emitting device according to a second embodiment of the present invention.  
         [0021]    [0021]FIG. 8 is a cross-sectional view of a light-emitting device according to a third embodiment of the present invention. 
     
    
     DETAILED DESCRIPTION  
       [0022]    The present invention is based in part on the observation that the piezoelectric field in a strained quantum well layer depends on the orientation of the crystal structure of the quantum well layer, and hence, by controlling the facet orientation, the piezoelectric field can be minimized. The manner in which this is accomplished may be more easily understood with reference to two types of strained quantum well structures, those based on a wurtzite crystal structure and those based on a zincblende crystal structure.  
         [0023]    Refer now to FIG. 1 which illustrates a wurtzite crystal GaN (WZ-GaN) structure  10 . The piezoelectric field generated in a crystal having a facet orientation along arc  11  in FIG. 1 is shown in FIG. 2 as a function of the angle θ between the {0001} direction and the facet orientation. The data shown in FIG. 2 is for Ga 0.9 In 0.1 N strained quantum well layers. The piezoelectric field reaches maxima in the {0001} direction or the {000-1} direction, and has three orientations at which the piezoelectric field is zero. The same result is obtained for other arcs, e.g., arc  12 . That is, the piezoelectric field is uniquely determined by the difference in the angle between the {0001} direction and the facet orientation of the concerned plane, i.e., the piezoelectric field is independent of φ.  
         [0024]    Hence it is clear from FIG. 2 that there are three sets of planes for which there is no piezoelectric field. For example, the planes at 90° to the c-axis such as, for example, the a-plane, {2-1-10}, the m-plane {0-110}, etc. The planes around 40° and  140 ° to the c-axis also provide planes with a zero piezoelectric field, e.g., the planes {2-1-14}, {01-12}, etc.  
         [0025]    The strength of the piezoelectric field depends on the strain in and the composition of the InGaN strained quantum well layer. However, the 90° facet orientation measured from the {0001} direction where the piezoelectric field becomes 0 does not strongly depend on the ratio of Ga to In. In addition, for typical InGaN quantum well LEDs the plane orientations corresponding to the 40° and 140° orientations discussed above typically change by no more than about of 5° from the 40° and 140° values determined for the composition shown in FIG. 2.  
         [0026]    The present invention is also based in part on the observation that the strength of the spontaneous electric field in a wurtzite crystal structure III-Nitride quantum well layer depends on the facet orientation of the quantum well layer, and hence the spontaneous electric field can also be minimized by controlling the facet orientation. For example, the spontaneous electric field approaches zero for a III-Nitride quantum well layer having approximately an a-plane or approximately an m-plane facet orientation. Such facet orientations are tilted at angles of, for example, about 80° to about 90° with respect to the {0001} direction of the wurtzite crystal structure.  
         [0027]    The strength of the spontaneous electric field also depends on the composition of the quantum well layer and on the composition of its adjacent layers. For the case of Ga 0.9 In 0.1 N quantum well layers between GaN layers (as considered in FIG. 2, for example), the piezoelectric field is typically much larger than the spontaneous electric field. Hence, in this case the combination of the piezoelectric and spontaneous electric fields is dominated by the piezoelectric field, and it may be advantageous to select a facet orientation that minimizes the piezoelectric field. For quantum well layers or adjacent layers formed from other III-Nitride compositions, such as materials including aluminum, for example, the spontaneous electric field may be comparable to or even dominate the piezoelectric field. If the spontaneous electric field dominates the piezoelectric field, it may be advantageous to select a facet orientation that minimizes the spontaneous electric field. If the piezoelectric and spontaneous electric fields are comparable, it may be advantageous to select a facet orientation that minimizes the combined field but does not necessarily minimize either the spontaneous or piezoelectric fields separately.  
         [0028]    A similar analysis can be applied to other crystal structures. Consider a zincblende crystal structure GaN-based semiconductor layer, referred to as ZB-GaN in the following discussion. A ZB-Ga 0.9 In 0.1 N strained quantum well layer can be formed on GaN in a manner analogous to the WZ-GaN-based semiconductor strained quantum well layer discussed above. FIG. 3 shows the crystal structure  20  of the ZB-GaN-based semiconductor. To simplify the discussion, the spherical coordinate system used with reference to FIG. 1 will also be used here. The radius vector has a polar angle θ measured from the {001} direction and a cone angle, Φ about the {001} direction. First and second paths having a constant azimuth angle are shown at  21  and  22 .  
         [0029]    Refer now to FIG. 4, which is a plot of the piezoelectric field in the strained quantum well layer with respect to the polar angle θ for various orientations of the strained quantum well layer on path  21 . In FIG. 4, Φ=45° and the {001} direction corresponds to θ=0°. The {111} direction corresponds to θ=54.7°, the {110} direction corresponds to θ=90°, and the {11-1} direction corresponds to θ=125.3°. It is clear from FIG. 4, that the piezoelectric field has maxima in the {111} direction (θ around 55°) and the {11-1} direction (θ around 125°). More importantly, the piezoelectric field goes to zero for θ=0, 90°, and 180°.  
         [0030]    A similar analysis with respect to path  22  shows that the piezoelectric field is essentially 0 for all points along this path. Path  22  corresponds to a Ga 0.9 In 0.1 N strained quantum well layer in which the growth orientation corresponds to θ and Φ=90°. Hence, in a strained quantum well crystal of ZB-GaN-based semiconductor, almost no piezoelectric field is generated in the strained quantum well layer that has growth planes beginning in the {001} plane or {011} plane and a facet orientation angle θ on path  22 . A similar result holds for planes that are equivalent to these.  
         [0031]    The manner in which the above-described observations are used in the fabrication of a light emitter will now be explained with the aid of FIG. 5 which is a cross-sectional view of a light-emitting device  30  according to the present invention. Light-emitting device  30  may be, e.g., a light-emitting diode or a laser diode. If the crystal growth orientation is excluded, the composition of each deposited layer is essentially that used in a conventional III-Nitride light-emitting device.  
         [0032]    Light-emitting device  30  is constructed from a number of layers. An n-type GaN contact layer  33 , an n-type AlGaN cladding layer  34 , a strained multiple quantum well layer  35 , a p-type AlGaN cladding layer  36 , and a p-type GaN contact layer  37  are successively deposited on a substrate  31  which is typically, AlN, SiC, or GaN. An n-electrode  38  and a p-electrode  39  are deposited as shown.  
         [0033]    The strained multiple quantum well layer  35  is typically constructed from GaInN/GaN or GaInN/GaInN. In a light-emitting device according to the present invention, the layers of the quantum well may be caused to grow such that the piezoelectric field, the spontaneous electric field, or the combination of the piezoelectric field and the spontaneous electric field in the quantum well layer is negligible.  
         [0034]    As noted above, there are a number of planes for which the piezoelectric field is substantially zero. One of these may be utilized in a light-emitting device according to the present invention. The particular plane will depend on the type of crystal. For example, in the case of a WZ-GaN light emitter, the {2-1-10} plane of the strained quantum layer material can be caused to grow by selecting the appropriate growing surface of substrate  31 . If the substrate is SiC, for example, the SiC may be cut such that the {2-1-10} plane is used for growing layer  33 . The choice of an a-plane (e.g., {2-1-10}) or an m-plane facet orientation also minimizes the spontaneous electric field and thus the combination of the piezoelectric and spontaneous electric fields.  
         [0035]    The relative efficiency of a light-emitting device according to the present invention (in which the combination of the piezoelectric and spontaneous electric fields is negligible) and a conventional light-emitting device including a quantum well having a {0001} facet orientation (such as a conventional device grown on the {0001} plane of a sapphire substrate, for example) is shown in FIG. 6 as a function of the width of the quantum well. Curve A is the efficiency for the device discussed above with reference to FIG. 5, and curve B is the efficiency of the conventional device. It will be appreciated from this figure that the present invention provides a substantial improvement in the efficiency of light generation.  
         [0036]    Conventional III-Nitride light-emitting devices frequently include a III-Nitride nucleation layer grown on the substrate by metal-organic chemical vapor deposition at a low temperature (typically less than about 800° C.) to accommodate lattice mismatch between the substrate and device layers grown above the nucleation layer. This is typically the case for devices grown on sapphire substrates, for example. Unfortunately, low temperature III-Nitride nucleation layers and III-Nitride device layers grown above them typically adopt a wurtzite crystal structure with a {0001} facet orientation regardless of the facet orientation of the substrate surface on which the layers are grown. As noted above, this leads to a high piezoelectric field and poor efficiency.  
         [0037]    Referring to FIG. 8, in one embodiment of the present invention light-emitting device  30  includes a high temperature III-Nitride nucleation layer  32  in addition to the layers shown in FIG. 5. High temperature III-Nitride nucleation layer  32  is grown directly on substrate  31  by, e.g., metal-organic chemical vapor deposition at a temperature sufficiently high that the crystal structure of layer  32  substantially replicates that of the surface of substrate  31  on which layer  32  is grown. In particular, the facet orientation of high temperature nucleation layer  32  and of subsequently grown layers is substantially the same as that of the surface of substrate  31 . Consequently, the facet orientation of quantum wells in multiple quantum well layer  35  can be controlled by selecting the facet orientation of the surface of substrate  31  on which high temperature III-Nitride nucleation layer  32  is grown.  
         [0038]    Nucleation layer  32  is typically grown at a temperature greater than about 800° C. The composition of nucleation layer  32  may include, for example, nitrogen and any element or combination of elements from group III of the periodic table. Typically, the composition of nucleation layer  32  is selected to have a lattice mismatch of less than about 10% with the surface of substrate  31  on which layer  32  is grown and with subsequently grown device layers.  
         [0039]    In this embodiment, substrate  31  is typically a non-{0001} orientation substrate chosen to have a lattice mismatch of less than about 10% with nucleation layer  32 . Suitable substrates in this embodiment include but are not limited to non-{0001} planes of SiC, AlN, and GaN as described above.  
         [0040]    Although FIG. 8 shows n-type GaN contact layer  33  grown directly on high temperature nucleation layer  32 , other implementations may include additional layers disposed between nucleation layer  32  and contact layer  33 . Such additional layers may include, for example, buffer layers and defect reduction layers.  
         [0041]    In one implementation, substrate  31  is a SiC substrate having a growth surface with a facet orientation at an angle of about 40° from {0001}, and high temperature nucleation layer  32  is an Al x Ga 1-x N layer with 0&lt;x≦1. In this implementation, nucleation layer  32  is typically grown at a temperature of about 1000° C. to about 1200° C., more typically about 1100° C. The mole fraction x of Aluminum in nucleation layer  32  is typically about 0.04 to about 1.0. The thickness of nucleation layer  32  in this implementation is typically about 100 Angstroms (Å) to about 1 micron (μm).  
         [0042]    It may be particularly advantageous to grow III-Nitride light-emitting devices on SiC substrates having growth surfaces with facet orientations at about 40° or about 140° from {0001}. The crystal structures of these facets are more like the {0001) facet than are the a-planes and m-planes. Consequently, it may be easier to grow high quality III-Nitride crystals on SiC substrate facets oriented at about 40° or about 140° from {0001} than on SiC a-plane or m-plane facets. Moreover, it is difficult and wasteful of material to prepare a-plane or m-plane SiC substrates from standard commercially available SiC ingots. It may be more cost-effective to prepare SiC substrates with facet orientations of about 40° or about 140° from {0001}.  
         [0043]    In another implementation of the embodiment illustrated in FIG. 8, substrate  31  is an AlN substrate having a growth surface with a facet orientation at an angle of about 90° from {0001}. The growth surface may be an a-plane or an m-plane, for example. In this implementation also, nucleation layer  32  may be formed from Al x Ga 1-x N (0&lt;x≦1) typically grown at a temperature of about 1000° C. to about 1200° C., more typically about 1100° C. The mole fraction x of Aluminum in nucleation layer  32  and the thickness of layer  32  may be as described in the above implementation, for example.  
         [0044]    The present invention may also be utilized to provide improved performance from photodetectors. Photodetectors fabricated by growing the device on the {0001} plane of a sapphire substrate exhibit an efficiency and absorption band that depend on light intensity. In particular, the efficiency of conversion increases with light intensity while the useful wavelength range decreases.  
         [0045]    In a photodetector according to the present invention, the device is grown on a substrate that results in, for example, little or no piezoelectric field in the strained quantum well layer. Hence, the dependence of the efficiency and the absorption band on light intensity are substantially reduced or eliminated. In general, the growing technique for a photodetector is the same as that used to construct a light emitter, however, thicker strained quantum well layers are utilized to improve the absorption of the incident light.  
         [0046]    It would be advantageous in many circumstances to utilize a sapphire or SiC substrate in which the layers, except for strained quantum wells, are grown on the {0001} plane, since substrates cut to provide growth on a {0001} plane are commercially available. Refer now to FIG. 7 which is a cross-sectional view of the optical semiconductor device  50  according to another embodiment of the present invention in which only the layers related solely to light emission and absorption have the desired facet orientation. Device  50  is constructed by growing an n-type GaN contact layer  53  and an n-type AlGaN cladding layer  54  on the {0001} plane orientation on the substrate  51  such as SiC or GaN based on conventional technology. Next, by selective growing or selective etching, the {2-1-14) plane or {01-12} plane, for example, is formed. The GaInN/GaN or GaInN/GaInN strained multiple quantum well layer  55  is then formed by repeating the crystal growth.  
         [0047]    Next, the remaining p-type Al GaN cladding layer  56  and the p-type GaN contact layer  57  are successively deposited and formed. The p-type Al GaN cladding layer  56  and the p-type GaN contact layer  57  change the crystal structure back to that corresponding to the {0001} plane from the facet orientation of the well layer  55  and become layers with specific thicknesses. The n-electrode  58  and the p-electrode  59  are formed as the electrodes on the n-type GaN contact layer  53  and the p-type GaN contact layer  57 , respectively. The growing surfaces  55 A,  55 B on both sides of the GaInN strained multiple quantum well layer  55  are, for example, the {01-12} plane or the {2-1-14} plane. The p-type AlGaN cladding layer  56  and the p-type GaN contact layer  57  become flat growing surfaces. To simplify the next process, it is advisable that they be several microns thick. In the preferred embodiment of the present invention, an AlN buffer layer  52  is grown on the substrate  51 .  
         [0048]    As noted above, the specific plane selected for growing the quantum well layer depends on the crystal type. For devices based on compound semiconductors other than GaN, such as AlN, it can be shown, for example, that the piezoelectric field as a function of the facet orientation behaves similarly to that described above if the crystal type is the same. The orientation inclination, θ, for which the piezoelectric field is 0 may, however, change by as much as 10°.  
         [0049]    Various modifications to the present invention will become apparent to those skilled in the art from the foregoing description and accompanying drawings. Accordingly, the present invention is to be limited solely by the scope of the following claims.