Abstract:
Selective layer disordering in a doped III-nitride superlattice can be achieved by depositing a dielectric capping layer on a portion of the surface of the superlattice and annealing the superlattice to induce disorder of the layer interfaces under the uncapped portion and suppress disorder of the interfaces under the capped portion. The method can be used to create devices, such as optical waveguides, light-emitting diodes, photodetectors, solar cells, modulators, laser, and amplifiers.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a continuation-in-part of U.S. application Ser. No. 13/558,516, filed Jul. 26, 2012, which claimed the benefit of U.S. Provisional Application No. 61/513,924, filed Aug. 1, 2011, both of which are incorporated herein by reference. 
    
    
     STATEMENT OF GOVERNMENT INTEREST 
     This invention was made with Government support under contract no. DE-AC04-94AL85000 awarded by the U.S. Department of Energy to Sandia Corporation. The Government has certain rights in the invention. 
    
    
     FIELD OF THE INVENTION 
     The present invention relates to compound semiconductors and, in particular, to a method for selective layer disordering in III-nitrides using a capping layer. 
     BACKGROUND OF THE INVENTION 
     AlGaN-based heterostructures have large conduction band offsets to support intersubband transitions at telecommunication wavelengths (1.3 μm and 1.55 μm). See C. Gmachl et al.,  Appl. Phys. Lett.  77, 3722 (2000); N. Suzuki and N. Iizuka,  Jpn. J. Appl. Phys.  2 38, L363 (1999); D. Hofstetter et al.,  Proc. IEEE  98, 1234 (2010); and M. Beeler et al.,  Semicond. Sci. Tech.  28, 074022 (2013). Various III-nitride intersubband structures have been demonstrated, such as optical modulators, quantum cascade lasers, and photodetectors. See N. Suzuki and N. Iizuka,  Jpn. J. Appl. Phys.  2 36, L1006 (1997); N. Iizuka et al.,  Electron. Lett.  40, 962 (2004); G. Sun et al.,  Superlattice Microst.  37, 107 (2005); D. Hofstetter et al.,  Appl. Phys. Lett.  83, 572 (2003); and D. Hofstetter et al.,  J. Phys .- Condens. Mat.  21, 174208 (2009). These demonstrations are limited to discrete devices, indicating they are farther behind other III-V compound semiconductor devices that have processing methods to integrate multiple devices and create photonic integrated circuits. For III-nitride devices to compete with existing technologies, new integration techniques are required. 
     SUMMARY OF THE INVENTION 
     The present invention is directed to a method for selective layer disordering in III-nitrides, comprising growing a III-nitride heterostructure on a substrate at a growth temperature chosen to prevent layer disordering and doping one or more of the heterostructure layers with an impurity during growth, depositing a dielectric capping layer on a portion of the surface of the heterostructure to provide a capped portion and an uncapped portion, and annealing the heterostructure at an annealing temperature and time sufficient to induce disorder of the heterostructure layer interfaces under the uncapped portion and suppress disorder of the interfaces under the capped portion. For example, the heterostructure can be a superlattice comprising a periodic structure of AlGaN/AlN, GaN/AlN, GaN/AlGaN, InGaN/GaN, InGaN/AlN, InGaN/AlGaN, AlInN/GaN, AlInN/AlN, or AlInN/InGaN layers. For example, the impurity can comprise silicon, magnesium, selenium, or tellurium, preferably with a dopant concentration of greater than 5×10 18  cm −3 . The superlattice can be grown by metal-organic vapor phase epitaxy, molecular beam epitaxy, and vapor phase epitaxy, preferably at a growth temperature of less than 885° C. The annealing temperature is preferably greater than 700° C. and, more preferably, greater than 1000° C. For example, the substrate can comprise sapphire or SiC, preferably with an intervening low dislocation density template layer. Alternatively, the substrate can comprise a material having an inherently low dislocation density, such as AlN, GaN, or alloys thereof. For example, the dielectric capping layer can comprise SiN, SiON, or SiO 2 . 
     As an example of the invention, selective layer disordering in an intersubband Al 0.028 Ga 0.972 N/AlN superlattice with a silicon nitride (SiN x ) capping layer was demonstrated. When the Al 0.028 Ga 0.972 N/AlN superlattice is subject to a high temperature anneal the SiNx capped portions suppress layer disordering compared to the uncapped portions. Additionally, the rate of layer disordering is reduced as the thickness of the SiN x  is increased. The SiN x  layer inhibits vacancy formation at the epitaxial layer surface, slowing down Si diffusion from the doped quantum wells, and the movement of Al and Ga atoms within the lattice and across the heterointerfaces. Patterning of the SiN x  layer results in selective layer disordering, an attractive process for integrating active III-nitride based intersubband devices with passive low-loss waveguides. 
     The method can be used to create devices, such as optical waveguides, light-emitting diodes, photodetectors, solar cells, modulators, laser, and amplifiers. For example, selective disordering can be useful in III-nitride photonic integrated circuits creating active and passive regions of the chip. With this method, passive waveguides can be integrated with an intersubband optical modulator using only one epitaxial material growth run. An absorbing modulator structure can be grown and selective layer disordering can be used to create passive, non-absorbing, portions that can behave as waveguides. Likewise, higher bandgap regions of a laser diode can also be created either near the output facets, to prevent optical damage, or outside the active stripe region to help confine the laser mode. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The detailed description will refer to the following drawings, wherein like elements are referred to by like numbers. 
         FIG. 1  is a schematic illustration of a superlattice comprising an uncapped portion and a portion capped with a dielectric layer. 
         FIGS. 2( a ) and 2( b )  are graphs of the transverse magnetic (TM) light transmission versus wavelength of Si-doped Al 0.028 Ga 0.972 N/AlN superlattices grown at 800° C. and annealed at 1000° C. for 0-20 minutes.  FIG. 2( a )  shows the transmission under the uncapped portion.  FIG. 2( b )  shows the transmission of the portion capped with 150 nm SiN x . 
         FIG. 3( a )  is a graph of the change in energy of the absorption peak as a function of annealing time for samples capped with 0 to 250 nm of SiN x . FIG.  3 ( b ) is a graph of change in absorption of the peak as a function of annealing time for samples capped with 0 to 250 nm of SiN x . 
         FIG. 4  is a schematic illustration of a possible model for selective layer disordering in III-nitride superlattices. 
         FIG. 5  is a graph of absorption spectra for superlattices grown on high- and low-dislocation substrates. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     A method used in conventional III-V compound semiconductors for device integration is layer disordering. Impurity-induced layer disordering (IILD) or layer intermixing is a well-studied phenomena in the III-arsenide and III-phosphide material systems. See W. D. Ladig et al.,  Appl. Phys. Lett.  38, 776 (1981); D. G. Deppe and N. Holoyak, Jr.,  J. Appl. Phys.  64, R93 (1988); and D. G. Deppe et al.,  Appl. Phys. Lett.  52, 1413 (1988). Normally an unintentionally doped (UID) heterointerface, such as an AlAs/GaAs, is stable (no Al—Ga interdiffusion) up to high temperatures (˜900° C.). See L. L. Chang and A. Koma,  Appl. Phys. Lett.  29, 138 (1976). The introduction of impurities by diffusion, implantation, or during growth lowers the temperature where layer disordering can occur. The impurity diffuses at these lower temperatures, and as it travels through the semiconductor layers the crystal atoms can reorganize, leading to intermixing of the heterointerfaces. In particular, movement of vacancies or impurities within the semiconductor lattice promotes congruent movement of the column III atoms within the semiconductor lattice. The rearrangement of the column III atoms causes the heterointerfaces to become less abrupt and shifts the electronic states within the quantum wells. For interband (bandgap) transitions this causes a blue-shift in the wavelength, and in intersubband transitions it causes a red-shift in wavelength. If this method is selective (site specific), one can produce portions of the superlattice that are not absorbing. 
     III-nitrides have been thought to be stable under standard growth and processing temperatures because of their high crystal bond strength and low diffusivity of impurities. However, layer disordering has recently been found to be also possible in III-nitride semiconductors. See J. J. Wierer et al.,  Appl. Phys. Lett.  97 (2010); and U.S. application Ser. No. 13/558,516. In particular, highly Si-doped quantum wells (QWs) used for intersubband absorption have been shown to layer disorder when subject to high growth temperatures or subject to post-growth high temperature annealing. At high temperatures, the Si diffuses out of the QWs of the superlattice, promoting layer disordering of the heterointerfaces, thereby decreasing and red-shifting the absorption. Layer disordering is prevented (mitigated) at growth temperatures lower than 885° C., and is a fundamental material phenomena regardless of the method used to grow the sample. Layer disordering with the as-grown doping is not selective, however, requiring an alternative process to make selective layer disordering possible. 
     The present invention is directed to a method of selective impurity-induced layer disordering in III-nitride materials and devices made therewith. As shown in  FIG. 1 , the method for selective impurity-induced layer disordering in III-nitride materials comprises growing an III-nitride heterostructure  11  on a substrate  12 . The heterostructure  11  can be a superlattice comprising a periodic structure of (nano-)layers of two or more III-nitride semiconductor materials with different band gaps at least some of which are doped with an impurity during growth. For example, the superlattice  11  can comprise a plurality of alternating quantum well  13  and barrier  14  layers wherein the quantum well layers  13  are doped during growth. The superlattice is grown at a sufficiently low growth temperature to prevent layer disordering during growth. A dielectric capping layer  15  is deposited on a portion  16  of the surface of the superlattice  11  prior to the post-growth annealing. The superlattice  11  can then be selectively disordered when the post-growth annealing conditions promote layer disordering in the uncapped portion  17  and suppress layer disordering in the capped portion  16  of the superlattice. 
     The superlattice can be grown by, for example, metal-organic vapor phase epitaxy, molecular beam epitaxy, or vapor phase epitaxy. The superlattice is preferably grown on a low dislocation substrate, such as AlN, GaN, or alloys thereof. Alternatively, the superlattice can be grown on a sapphire or SiC substrate, preferably with an intervening template layer of AlN, GaN, InGaN, or AlGaN. The III-nitride superlattice can comprise a plurality of AlGaN/AlN, GaN/AlN, GaN/AlGaN, InGaN/GaN, InGaN/AlN, InGaN/AlGaN, AlInN/GaN, AlInN/AlN, or AlInN/InGaN layers, preferably of less than 10 nm thickness. Although many impurities can be used, the impurity can comprise, for example, silicon, magnesium, selenium, or tellurium. The preferred dopant concentration will also depend on the particular III-nitride superlattice system and impurity, but can typically be greater than 5×10 18  cm −3 . For example, the dielectric capping layer can comprise SiN, SiON, or SiO 2 . The growth and post-growth annealing temperatures will depend on the particular III-nitride superlattice system. However, the growth temperature can typically be less than 885° C. The post-growth annealing temperature can typically be greater than 700° C. and, more typically, can be greater than 1000° C. A carrier gas, for example, N 2 , NH 3 , H 2 , or He, can be used during the post-growth anneal to further control layer disordering in the uncapped portions 
     As an example of the invention, a silicon nitride (SiN x ) cap layer was shown to inhibit layer disordering of the heterointerfaces in Si-doped AlGaN/AlN intersubband structures. Portions of the superlattice capped with SiN x  layer disorder at a slower rate compared to uncapped portions. Additionally, the thicker the SiN x  layer, the slower the layer disordering. It is suggested that the SiN x  layer prevents vacancies formation at the wafer surface, suppressing Si diffusion and layer disordering. 
     The exemplary intersubband superlattice structures were grown by metal-organic vapor phase epitaxy (MOVPE) on (0001) sapphire substrates misoriented 0.2° toward the m-plane. First, ˜2 μm thick AlN template layers with dislocation densities of ˜3×10 9  cm −2  and ˜10 10  cm −2  were grown on the substrate. All data presented below is with the superlattices grown on the low dislocation density template layers, except those for which the rate of layer disordering was compared versus dislocation density. Next, a superlattice was grown on the template layer at 800° C., consisting of 20 periods of Al 0.028 Ga 0.972 N quantum wells (QWs) and AlN barrier layers that were ˜14.5 Å and ˜45 Å thick, respectively. The Al 0.028 Ga 0.972 N QWs were Si-doped during growth with an electron concentration of ˜7×10 19  cm −3 . The superlattice has an intersubband absorption centered near ˜1.56 μm. A ˜20 nm AlN top layer was grown on the superlattice structure. 
     After growth, the intersubband samples were formed into waveguides by cutting out ˜8-10 mm long pieces and polishing 45° facets, thereby creating a multipass waveguide. This waveguide was used to measure the intersubband absorption in a Fourier transform infrared spectrometer with a near-infrared polarizer to measure only transverse magnetic (TM) light. This measurement was performed on both as-grown and annealed samples with and without SiN x  capping layers. 
     Following growth, a SiN x  capping layer was deposited at a temperature of 250° C. on a portion of each sample via plasma-enhanced chemical vapor deposition (PECVD) using silane and nitrogen as sources. Capping layer thicknesses of 50 nm, 150 nm, 200 nm, and 250 nm were deposited to determine the layer disordering rate as a function of SiN x  thickness. After deposition, the absorption spectra of the samples were measured without annealing. Then the samples were subject to 5 min. anneals in a rapid thermal annealer (RTA) at 1000° C. in N 2  with the sample oriented epitaxial side up. The annealing occurred for multiple times up to a cumulative total of 20 min., and the absorption measurements were performed after each 5 min. interval. 
     The transmissions for the uncapped portion of the sample compared with the portions capped with 150 nm of SiN x  are shown in  FIGS. 2( a ) and 2( b ) . With increased annealing time both the uncapped and capped portions exhibit a redshift in wavelength and a simultaneous decrease in absorption. However, the SiN x  capped portions redshift at a much slower rate compared to the uncapped portions. Clearly the SiN x  cap is affecting the layer disordering occurring within the underlying Al 0.028 Ga 0.972 N/AlN superlattice. Since the layer disordering is triggered by Si impurity diffusion, the SiN x  capping layer is most likely changing the rate of Si diffusion within the sample. 
     To explore this further, the SiN x  thickness was varied using otherwise similar samples. The resulting change in absorption energy and absorption per pass are plotted in  FIGS. 3( a ) and 3( b ) .  FIG. 3( a )  is a graph of the change in energy of the absorption peak as a function of annealing time for samples capped with 0 to 250 nm of SiN x .  FIG. 3( b )  is a graph of change in absorption of the peak as a function of annealing time for samples capped with 0 to 250 nm of SiN x . The data for SiN x  thickness of 0 nm is an average of all the uncapped areas of the samples. There is a redshift in wavelength and a decrease in absorption for all samples. However, as the SiN x  thickness increases there is a decrease in the redshift and decrease in absorption. This dependence of layer disordering on the SiN x  thickness is another indication that the SiN x  cap has an effect on the Si diffusion within the samples. 
     The dependence of layer disordering on the presence and thickness of the SiN x  layer suggests the following layer disordering mechanism. A schematic illustration of this mechanism is shown in  FIG. 4 . The Si impurity is necessary for layer disordering as demonstrated previously. See J. J. Wierer et al.,  Appl Phys Lett  97, (2010), and U.S. application Ser. No. 13/558,516. Column III vacancies (V III ) can be created at the surface, or from a threading dislocation (1). This vacancy allows the Si to diffuse (2) as a substitutional mechanism via the column III vacancies. The movement of the Si allows for the Al or Ga to backfill (3) the column III vacancy sites, promoting intermixing of Al-rich and Ga-rich AlGaN QWs and barrier layers. Vacancies can be more easily created at the uncapped surface compared to the SiN x  capped surface. Therefore, SiN x  suppresses (X) vacancy formation and hence layer disordering. Furthermore, as the SiN x  thickness increases the air interface at the top surface becomes farther from the superlattice, in turn further suppressing vacancy formation and layer disordering. Thus, the air interface is a source for vacancy formation, whilst the SiN x  suppresses vacancy formation. 
     The SiN x  layer cannot completely prevent layer disordering, indicating that there is a finite amount of vacancies in the as-grown superlattice and that other sources for vacancy creation are possible. This may be a reason why differences in layer disordering are found with different superlattice growth techniques. The deposition method and resulting crystallinity of the SiN x  may also affect layer disordering. 
     Another possible source of vacancies is threading dislocations that are prevalent in III-nitride semiconductors grown on sapphire substrates. Therefore, a change in the number of threading dislocations can result in a change in the rate of layer disordering. To test this hypothesis, superlattices were grown on substrates having different dislocation densities (˜3×10 9  cm −2  and ˜10 10  cm −2 ). The TM transmission of these two samples is shown in  FIG. 5 . The absorption in the higher dislocation density (DD) sample is much smaller compared to the low dislocation density sample. The threading dislocations present the superlattice with a large amount of surface area for addition vacancy formation and subsequent layer disordering. For samples with a high dislocation density substrate, layer disordering occurs during growth. These dislocations are also available for vacancy formation in post-growth annealing and can also contribute to the layer disordering during annealing. Therefore, threading dislocations can be an additional source of vacancy formation, even if a SiN x  capping layer is used. Therefore, SiN x  capping of intersubband samples to prevent layer disordering can become more effective as the threading dislocation density in the substrate is reduced. This can be accomplished by using a sapphire or SiC substrate with an intervening template layer, as described above. Alternatively, the superlattice can be grown on a low dislocation bulk substrate, such as AlN or GaN, with or without an intervening template layer. 
     The present invention has been described as a method for selective layer disordering in III-nitrides with a capping layer. It will be understood that the above description is merely illustrative of the applications of the principles of the present invention, the scope of which is to be determined by the claims viewed in light of the specification. Other variants and modifications of the invention will be apparent to those of skill in the art.