Patent Document

CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application claims priority under 35 U.S.C. §119(e) to U.S. Provisional Patent Application No. 60/220,035, filed Jul. 21, 2000, by Umesh K. Mishra and Stacia Keller, and entitled “METHOD TO REDUCE THE DISLOCATION DENSITY IN GROUP III-NITRIDE FILMS,” which application is incorporated by reference herein. 
    
    
     STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT 
     This invention was made with Government support under Grant No. N00014-96-1024, awarded by the Navy. The Government has certain rights in this invention. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to nitride films, and particularly methods to reduce the dislocation density in group III nitride films for semiconductor devices. 
     2. Description of the Related Art 
     (Note: This application references a number of different publications as indicated throughout the specification by reference numbers enclosed in brackets, e.g., [x]. A list of these different publications ordered according to these reference numbers can be found below at the end of the Detailed Description of the Preferred Embodiment. Each of these publications is incorporated by reference herein.) 
     Gallium Nitride (GaN) epitaxial films are typically obtained through heteroepitaxy on sapphire, silicon carbide, or silicon substrates since single crystalline GaN substrates are still not commercially available. Due to the lattice mismatch and the different chemical nature of the individual substrates, the threading dislocation densities in GaN films are typically on the order of 10 8 -10 9  cm −2  on sapphire and silicon carbide and between 10 9  and 10 11  cm −2  on silicon substrates. These dislocation densities are obtained even after the application of advanced nucleation schemes, where the growth is initiated with the deposition of a very thin (Al)GaN or AlN nucleation layer at growth conditions different from the main GaN bulk layer. [1] 
     To further reduce the dislocation density in GaN films, several forms of epitaxial lateral overgrowth of GaN have been developed, the use of which resulted in virtually dislocation-free material in the overgrown regions. [2,3] Applying this technique, GaN-based laser diodes with a lifetime of more than 10,000 h have been obtained. [4] Furthermore, the leakage current in p-n junctions and the dark current in photodetectors could be significantly reduced. [5,6] However, epitaxial lateral over-growth involves several growth and processing steps, making it a relatively time-consuming and expensive method. Dislocation reduction has also been observed after insertion of multiple GaN or AlN nucleation layers. [7] 
     There is a need for methods of reducing the dislocation density in nitride films, particularly for semiconductor light emitting device applications. There is also a need for such methods which may be performed quickly and inexpensively. The present invention meets these needs. 
     SUMMARY OF THE INVENTION 
     The present invention discloses a semiconductor film having a reduced dislocation density. The film comprises at least one interlayer structure, including a group III-nitride layer, a passivation interlayer disposed on the group III-nitride layer, interrupting the group III-nitride layer, and an island growth interlayer disposed on the passivation interlayer, and interrupting the group III-nitride layer. A method of making a semiconductor film of the present invention comprises producing a semiconductor film including at least one interlayer structure, each interlayer structure produced by the substeps of growing a group III-nitride layer, depositing a passivation interlayer on the group III-nitride layer, depositing an island growth interlayer on the passivation interlayer and continuing growing the group III-nitride layer. 
     In one embodiment of the present invention, dislocation reduction in GaN films grown on sapphire and silicon substrates is achieved by inserting thin InGaN layers grown in a selective island growth mode after passivation of the GaN surface with a submonolayer of silicon nitride. The present invention discloses a method that is most effective at reducing the pure edge dislocation density when it is high, i.e., &gt;10 10  cm −2 . Thus, the structural quality of typically highly dislocated GaN on silicon films is significantly improved. The results are visible in a reduction of the (0002) full width at half maximum (FWHM) from 1300 arcsec for ordinary GaN on silicon to 800 arcsec for GaN films with silicon nitride/InGaN interlayers. In the case of GaN layers grown on sapphire (dislocation density˜10 9  cm −2 ), the method resulted mainly in a reduction of the FWHM of the (10{overscore (1)}2) and (20{overscore (2)}1) diffraction peaks. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Referring now to the drawings in which like reference numbers represent corresponding parts throughout: 
     FIG. 1 illustrates a typical semiconductor film of the invention; 
     FIGS. 2A and 2B are flowcharts illustrating methods of making semiconductor films of the invention; 
     FIG.  3 . is a plot of FWHM of the (0002) and (10{overscore (1)}2) diffraction peaks for samples grown with two silicon nitride/InGaN interlayers of different thickness on silicon (open symbols) and sapphire substrates (closed symbols) following procedures A and B 2 , respectively; 
     FIG. 4 is a table of the (0002) and (10{overscore (1)}2) diffraction peak for GaN films of the present invention; and 
     FIG. 5 is a plot of FWHM of the (0002), (10{overscore (1)}2), and (20{overscore (2)}1) diffraction peaks for GaN grown on sapphire under not fully optimized growth conditions, called “highly dislocated GaN,” standard conditions, and with two Si x N y /InGaN interlayers following procedure B2 [d(Si x N y )=0.14 Å]. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     In a preferred embodiment InGaN inter-layers are grown in a selective island growth mode after partially passivating the GaN surface with silicon nitride, on the structural properties of GaN films grown on silicon and sapphire substrates. The growth of InGaN has shown that InGaN films have a high tendency to grow in a spiral growth mode around threading dislocations with a screw component. InGaN spiral islands were obtained, when the GaN surface was partially passivated with disilane under formation of a submonolayer of silicon nitride (henceforth referred to as “Si x N y ”) prior to deposition of the InGaN layer. [8, 9] It should be understood, however, that the present invention is not limited to embodiments using partial passivation or selective island growth modes. 
     In the present invention, the selective island growth of InGaN can be utilized to reduce the dislocation density in GaN films. In particular, the effect of the thickness of the silicon nitride layer and the number of Si x N y /InGaN interlayers improves the structural quality of the films. This may be evaluated by high resolution x-ray diffraction. 
     FIG. 1 illustrates a typical semiconductor film of the invention. The invention is generally directed to a semiconductor film  100  including at least one interlayer structure  102  including a group III-nitride (such as GaN) layer  104 , a passivation interlayer  106  disposed on the group III-nitride layer  104  and an island growth interlayer  108  (such as InGaN) disposed on the passivation layer  106 . The passivation layer  106  is typically composed of silicon nitride or silicon dioxide, however other passivating materials may also be used. A continued growth of the group III-nitride layer  104  (which is also the beginning of second interlayer structure  102 ) completes the first interlayer structure  102 . Two such interlayer structures  102  are preferred. In addition, the invention is typically formed on a substrate  110  having a nucleation layer  112  disposed thereon and a cap layer  114  (which is also the continued growth of the final group III-nitride layer  104 ) is deposited on the uppermost interlayer structure  102 . 
     It should be noted that, in addition to Si 3 N 4  and SiO 2 , the passivation layer may be formed of any material which will produce a group III-nitride growth perturbation which can be initiated by the deposition of at least one layer to stop the dislocation propagation. Any inorganic dielectric passivating material known to those skilled in the art may be used 
     FIGS. 2A and 2B are flowcharts illustrating methods of making semiconductor films  100  of the invention. FIG. 2A is a flowchart of the steps of making an interlayer structure  102  of the invention. Beginning at block  200 , a group III-nitride layer  104  is grown. Next, a passivation layer is deposited on the group III-nitride layer at block  202 . Then, an island growth layer is deposited on the passivation layer at block  204 . Finally, growing the group III-nitride layer  104  is continued at block  206 . FIG. 2B is a flowchart of the steps of making a semiconductor film of the invention. Beginning at block  208 , a nucleation layer  112  is formed on a substrate  110 . Next, at least one interlayer structure  102  is formed by the substeps  200 - 204  of FIG.  2 A. Substeps  200 - 204  are repeated for each interlayer structure  102  at block  210 . Finally, a cap layer  114  is grown on the last interlayer structure  102  at block  212 . Further details of the steps will be described hereafter. 
     All epitaxial layers in the present invention may be grown by metal-organic chemical vapor deposition using the precursors trimethylgallium (TMGa), trimethyaluminum (TMAl), trimethylindium (TMIn), ammonia, and disilane. Typical embodiments of the invention are grown with two Si x N y /InGaN layers  106 ,  108 , separated by 0.1-5 μm GaN layers  104  to form the interlayer structures  102 . The passivation layers may range from 0.05-5 Å in thickness. 
     For samples grown on silicon substrate  110  (procedure A), the growth may be initiated with the deposition of a 100-nm-thick AlN nucleation layer  112  at 900° C., followed by the growth of a 1.7 μm of GaN layer  104  at a temperature of 1080° C. Following this, the TMGa injection is stopped and 2-10 nmol/min disilane is added for 16-48 seconds to form the passivation layer  106 . The 12-nm-thick In 0.1 Ga 0.9 N layer  108  which follows is deposited at 790° C. using a TMGa and TMIn flow of f TMGa =0.6 μmol/min and f TMIn =12 μmol/min and an ammonia flow of f NH     3   =0.32 mol/min. After deposition of the InGaN layer  108 , the wafer temperature is raised to 1070° C. and a 0.5 μm GaN layer  104  is deposited. Next, the GaN growth is interrupted again and a second passivation layer  106 , a submonolayer of silicon nitride, is grown, followed again by an InGaN layer  108  under the same conditions as the previous one. The structure is completed with the deposition of a 0.8 μm thick GaN cap layer  114 . 
     For the first set of samples grown on c-plane sapphire substrate  110  (procedure B 1 ), the growth is initiated with a 20-nm-thick GaN nucleation layer  112 , followed by the growth of a 0.5 μm GaN layer  104  at 1070° C. Afterwards 0.15 Å passivation layer  106  of silicon nitride followed by a 12 nm InGaN layer  108  is deposited in the same manner as described for the growth on silicon substrates  110 . The growth is continued with the deposition of a 0.5 μm GaN layer  104 , a second 0.15-Å-thick passivation layer  106  of silicon nitride, a 12 nm InGaN layer  108 , and completed with 2 μm cap layer  114  of GaN. The effect of the number of Si x N y /InGaN interlayers is shown in a second set of samples with four Si x N y /InGaN interlayers, all separated by 0.5 μm GaN to form interlayer structures  102 . The thickness of the cap layer  114  may be reduced to 1 μm to keep the total thickness of the epitaxial layer constant. 
     In the case of the second set of samples grown on sapphire substrate  110  (procedure B 2 ), the thickness of the GaN layer  102  prior to deposition of each of the two InGaN layers  108  of the interlayer structure  102  is increased to 3 μm, and the thickness of the GaN cap layer  114  to 2.5 μm. 
     The average thickness of the passivation layers  106 , the Si 3 N 4  submonolayers (Si x N y ), may be calculated from the results obtained for the deposition of thick Si 3 N 4  layers on silicon under similar conditions, and assuming a homogeneous distribution of silicon over the surface. [10] 
     The structural quality of the invention may be evaluated by high resolution x-ray diffraction using a PHILIPS MATERIALS RESEARCH diffractometer equipped with a four crystal [Ga(220)] monochromator utilizing the Cu K/α line of λ=0.15406 nm. All rocking curves may be obtained in the symmetric geometry (ω=θ). For the off-axis scans, the wafer is tilted about the ψ axis (commonly referred to as χ on many four-circle diffractometers) by the appropriate angle. The full width at half maximums (FWHMs) of the on- and off-axis diffraction peaks are a measure of the mosaic in the epitaxial layer and can each be related to specific types of threading dislocations (TDs). The FWHM of the symmetric (0002) diffraction peak is related to the tilt of the subgrains with respect to the substrate, and thus to the density of pure screw and mixed TDs. The off-axis (10{overscore (1)}2) and (20{overscore (2)}1) FWHMs result from a combination of the tilt and the twist of the subgrains and are related to the density of mixed and pure edge TDs. Thereby, the sensitivity toward pure edge TDs increases with increasing asymmetry (increasing ψ). [11] 
     FIG. 3 shows the dependence of the FWHM of the (0002) and (10{overscore (1)}2) rocking curves on the average thickness of the silicon nitride passivation layers  106  found for samples grown on silicon substrates  110  following procedure A and samples grown on sapphire substrates  110  via procedure B 2 . Since the island growth of InGaN layer  108  depends on the predeposition of silicon nitride, first the influence of the silicon nitride layer  106  thickness was investigated. In the case of the samples grown on silicon  110 , the films grown in a regular manner without Si x N y /InGaN interlayers were of poor crystalline quality (TD density ˜10 11  cm −2 ), as visible in the wide FWHM of the (0002) diffraction peak of about 1300 arcsec. (The poor quality was mainly related to the fact that the growth conditions of the AlN nucleation layer  112  had not been fully optimized.) However, the crystalline quality of the GaN-on-silicon layers  104  is significantly improved by inserting the two Si x N y /InGaN interlayer structures  102 . The FWHM of the (0002) diffraction peak decreased from 1280 to 795 arcsec for a silicon nitride layer  106  thickness of 0.14 Å. A further increase in the silicon nitride layer  106  thickness causes the layer quality to degrade again, and the FWHM increases to 920 arcsec. 
     FIG. 4 is a table of the (0002) and (10{overscore (1)}2) diffraction peak for GaN films  100  of the present invention. For the samples grown on sapphire substrates  110 , which were of higher crystalline quality when grown in a regular manner (TD density&lt;10 9  cm −2 ) the insertion of the Si x N y /InGaN interlayers results in a decrease of the FWHM of the asymmetric (10{overscore (1)}2) diffraction peak but did not affect the FWHM of the symmetric (0002) diffraction. 
     Samples with different numbers of Si x N y /InGaN interlayers may be grown on sapphire substrates  110  following procedure B 1 , using the optimum Si x N y  layer  106  thickness of 0.14 Å, as determined in experiments. The lower crystalline quality of these samples [TD density˜(5-8)×10 9  cm −2 ] compared to those grown following procedure B 2  is related to the low thickness of the GaN layer  104  prior to deposition of the first Si x N y /InGaN layer. For samples without, with two, and with four interlayers (10{overscore (1)}2) FWHM, values of 870, 720 and 795 arcsec, respectively, have been measured (not shown). The corresponding values of the (0002) FWHMs were 390, 390 and 420 arcsec. The results demonstrate that the increase of the number of Si x N y /InGaN interlayers from two to four caused the crystalline quality to degrade again. Obviously, the Si x N y /InGaN interlayers can also create new defects if too high in number. For this reason, embodiments using two Si x N y /InGaN interlayers may be preferred, however the invention is not limited to two interlayer structures. 
     FIG. 5 displays the dependence of the FWHM of the x-ray diffraction peak on increasing inclination angle ψ during the measurement for three different GaN film  100  samples. Since the sensitivity of the diffraction measurement with respect to pure edge TDs increases with increasing ψ, the FWHM of the (20{overscore (2)}1) diffraction peak reflects their density even more strongly than the (10{overscore (1)}2) diffraction peak. Furthermore, the mosaic due to pure edge dislocations may be estimated through extrapolation of the data toward ψ=90°. [11] The graph illustrates the improved crystalline quality of the sample with two Si x N y /InGaN interlayers using the optimum silicon nitride layer  106  thickness of 0.14 Å, exhibiting a FWHM of the (20{overscore (2)}1) diffraction peak of 500 arcsec in comparison to the standard GaN film (720 arcsec). The highly dislocated GaN film (TD density of 5×10 9  cm −2 ), which was grown under growth conditions not fully optimized, shows an even broader (20{overscore (2)}1) FWHM of 910 arcsec. The results demonstrate that the crystalline quality of GaN films  100  can be significantly improved through the insertion of the Si x N y /InGaN interlayers. Although a homogeneous distribution of Si atoms on the GaN surface may be assumed in calculating the Si x N y  layer  106  thickness, the Si atoms accumulate in surface areas with a high density of N-dangling bonds and specifically at the surface sites created by the intersection of the edge dislocations with the GaN layer  104  surface. The thin silicon nitride layer  106  masks the dislocation, preventing the adsorption of Ga and N species. In contrast, the intersections of threading dislocations with screw character, which create an additional surface step, act as nucleation sites for the InGaN layer  108  growth resulting in the formation of the InGaN islands. The islands then overgrow the passivated areas of the GaN layer  104  surface. This mechanism is more effective the higher the TD density in the starting layer due to the closer dislocation distance. 
     CONCLUSION 
     In conclusion, dislocation reduction in GaN films grown on silicon and sapphire substrates may be observed through a passivation of the GaN surface with silicon nitride. The subsequently grown InGaN islands overgrow areas with pure edge dislocations. The present invention is most effective at reducing the pure edge dislocation density when it is high, i.e., &gt;10 10  cm −2 . For highly dislocated GaN on silicon films (TD density 10 11  cm −2 ), the FWHM of the (0002) diffraction peak decreases from approximately 1300 to 800 arcsec after insertion of two silicon nitride/InGaN interlayers. In the case of GaN layers grown on sapphire (dislocation density˜10 9  cm −2 ), the method results mainly in a reduction of the FWHM of the (10{overscore (1)}2) and (20{overscore (2)}1) diffraction peaks. The described method is most effective for applications which do not require a complete elimination of dislocations. 
     Although the present invention has been detailed with respect to GaN, equivalently, the present invention may be extended to apply to all group III-nitrides, aluminum-, gallium-, indium- and boron- (AlN, GaN, InN, BN) and their alloys with phosphorous (P), arsenic (As) and antimony (Sb). In addition, as previously discussed, the passivation layer may be any material which produces a growth perturbation in the group III-nitride and thereby halts the dislocation propagation. The invention may also be applied to the growth of group-III nitrides on any compatible simple or complex oxide substrate known to those skilled in the art. Some examples include silicon, sapphire, silicon carbide, zinc oxide, lithium gallate, lithium aluminate and aluminum nitride. 
     This concludes the description including the preferred embodiments of the present invention. The foregoing description of the preferred embodiment 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. The above specification, examples and data provide a complete description of the use of the invention. Since many embodiments of the invention can be made without departing from the spirit and scope of the invention, the invention resides in the claims hereinafter appended. 
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Technology Category: 5