Acid etching for accurate determination of dislocation density in GaN

Hot phosphoric-acid etching and atomic force microscopy (AFM) were used to etch and characterize various GaN materials, including freestanding GaN grown by hydride vapor-phase epitaxy (HVPE), metal-organic chemical-vapor deposition (MOCVD) GaN films on sapphire and silicon carbide, and homoepitaxial GaN films on polished freestanding-GaN wafers. It was found that etching at optimal conditions can accurately reveal the dislocations in GaN; however, the optimal etch conditions were different for samples grown by different techniques. The as-grown HVPE samples were most easily etched, while the MOCVD homoepitaxial films were most difficult to etch. Etch-pit density (EPD) ranging from 4×10⁶ cm⁻² to 5×10⁹ cm⁻² was measured in close agreement with the respective dislocation density determined from transmission electron microscopy (TEM).     

Analysis of reactor geometry and diluent gas flow effects on the metalorganic vapor phase epitaxy of AIN and GaN thin films on α(6H)-SiC substrates

The influence of diluent gas on the metalorganic vapor phase epitaxy of AlN and GaN thin films has been investigated. A computational fluid dynamics model using the finite element method was employed to improve film uniformity and to analyze transport phenomena. The properties of AlN and GaN thin films grown on α(6H)-SiC(0001) substrates in H₂ and N₂ diluent gas environments were evaluated. Thin films of AlN grown in H₂ and N₂ had root mean square (rms) roughness values of 1.5 and 1.8 nm, respectively. The surface and defect microstructures of the GaN thin films, observed by scanning and transmission electron microscopy, respectively, were very similar for both diluents. Low temperature (12K) photoluminescence measurements of GaN films grown in N₂ had peak intensities and full widths at half maximum equal to or better than those films grown in H₂. A room temperature Hall mobility of 275 cm²/V·s was measured on 1 μm thick, Si-doped, n-type (1×10¹⁷ cm⁻³) GaN films grown in N₂. Acceptor-type behavior of Mg-doped GaN films deposited in N₂ was repeatably obtained without post-growth annealing, in contrast to similar films grown in H₂. The GaN growth rates were ∼30% higher when H₂ was used as the diluent. The measured differences in the growth rates of AlN and GaN films in H₂ and N₂ was attributed to the different transport properties of these mixtures, and agreed well with the computer model predictions. Nitrogen is shown to be a feasible alternative diluent to hydrogen for the growth of AlN and GaN thin films.     

Optical properties of thick-film GaN grown by hydride vapor phase epitaxy on MgAl2O4 substrate

A hydride vapor phase epitaxy was employed to grow the 10∼240 μm thick GaN films on a (111) MgAl₂O₄ substrate. The GaN films on a MgAl₂O₄ substrate revealed characteristics of photoluminescence (PL) in impurity doped GaN, which may be due to the out-diffusion and auto-doping of Mg from the MgAl₂O₄ substrate during GaN growth. The PL peak energy of neutral donor bound exciton emission and the frequency of Raman E₂ mode were decreased by increasing the GaN thickness, due to the residual strain relaxation in the epilayers. The dependence of Raman E₂ mode of GaN films on residual strain can be estimated as Δ ω/Δ σ=3.93 (cm⁻¹/GPa).     

Very low resistance ohmic contacts to n-GaN

Ohmic contacts with low resistance are fabricated on n-GaN films using Al/Ti bilayer metallization. GaN films used are 0.3 μm thick layers with carrier concentrations of 1 × 10¹⁹ cm⁻³ grown on the c-plane sapphire by ion-removed electron cyclotron resonance molecular beam epitaxy. The lowest value for the specific contact resistivity (ρ_c) of 1.2×10⁻⁸ Ω·cm² was obtained with furnace annealing at 500°C for 60 min. This result shows the effectiveness of high carrier concentration GaN layers and the low temperature annealing for the realization of low resistance ohmic contacts. Sputtering Auger electron spectroscopy analysis reveals that Al diffuses into Ti layer and comes into contact with the GaN surface.     

GaN grown by hydride vapor phase epitaxy on p-type 6H-SiC layers

6H-SiC/GaN pn-heterostructures were grown by subsequent epitaxial growth of p-SiC by low temperature liquid phase epitaxy (LTLPE) and n-GaN by hydride vapor phase epitaxy (HVPE). For the first time, p-type epitaxial layers grown on 6H-SiC wafers were used as substrates for GaN HVPE growth. The GaN layers exhibit high crystal quality which was determined by x-ray diffraction. The full width at a half maximum (FWHM) for the ω-scan rocking curve for (0002) GaN reflection was ∼120 arcsec. The photoluminescence spectra for these films were dominated by band-edge emission. The FWHM of the edge PL peak at 361 nm was about 5 nm (80K).     

MBE growth and properties of GaN and AlxGa1−xN on GaN/SiC substrates

The growth of GaN and AlGaN by molecular beam epitaxy (MBE) has been studied using GaN/SiC substrates. The GaN/SiC substrates consisted of ∼3 μm thick GaN buffer layers grown on 6H-SiC wafers by metalorganic vapor phase epitaxy (MOVPE) at Crée Research, Inc. The MBE-grown GaN films exhibit excellent structural and optical properties—comparable to the best GaN grown by MOVPE. Al_xGa_1−xN films (x ∼ 0.06-0.08) and Al_xGa_1−xN/GaN multi-quantum-well structures which display good optical properties were also grown by MBE on GaN/SiC substrates.     

The impact of nitridation and nucleation layer process conditions on morphology and electron transport in GaN epitaxial films

A systematic study has been performed to determine the characteristics of an optimized nucleation layer for GaN growth on sapphire. The films were grown during GaN process development in a vertical close-spaced showerhead metalorganic chemical vapor deposition reactor. The relationship between growth process parameters and the resultant properties of low temperature GaN nucleation layers and high temperature epitaxial GaN films is detailed. In particular, we discuss the combined influence of nitridation conditions, V/III ratio, temperature and pressure on optimized nucleation layer formation required to achieve reproducible high mobility GaN epitaxy in this reactor geometry. Atomic force microscopy and transmission electron microscopy have been used to study improvements in grain size and orientation of initial epitaxial film growth as a function of varied nitridation and nucleation layer process parameters. Improvements in film morphology and structure are directly related to Hall transport measurements of silicon-doped GaN films. Reproducible growth of silicon-doped GaN films having mobilities of 550 cm²/Vs with electron concentrations of 3 × 10¹⁷ cm⁻³, and defect densities less than 10⁸ cm⁻² is reported. These represent the best reported results to date for GaN growth using a standard two-step process in this reactor geometry.     

Growth of zinc- blende GaN on GaAs (100) substrates at high temperature using low-pressure MOVPE with a Low V/lll molar ratio

Zinc-blende GaN films were grown on GaAs (100) substrates by low-pressure metalorganic vapor phase epitaxy using trimethylgallium or triethylgallium and NH₃. Films grown at lower temperatures contained considerable amounts of carbon, but the carbon concentration was reduced in high temperature growth. When the film was grown at 950°C using triethylgallium and NH₃, its carbon concentration was on the order of 10¹⁷ cm⁻³. The crystalline and optical quality of zinc-blende GaN crystal also improved with high-temperature growth at a low V/III ratio using a thin buffer layer. The films exhibited only one sharp photoluminescence peak at 3.20 eV with a full width at half maximum as low as 70 meV at room temperature.     

Lateral epitaxial overgrowth of GaN films on SiO2 areas via metalorganic vapor phase epitaxy

Lateral epitaxial growth and coalescence of GaN regions over SiO₂ masks previously deposited on GaN/AlN/6H-SiC(0001) substrates and containing 3 μm wide rectangular windows spaced 7 μm apart have been achieved. The extent and microstructural characteristics of these regions of lateral overgrowth were a complex function of stripe orientation, growth temperature, and triethylgallium (TEG) flow rate. The most successful growths were obtained from stripes oriented along 〈1 00〉 at 1100°C and a TEG flow rate of 26 μmol/min. A density of ∼10⁹ cm⁻² threading dislocations, originating from the underlying GaN/AlN interface, were contained in the GaN grown in the window regions. The overgrowth regions, by contrast, contained a very low density of dislocations. The surfaces of the coalesced layers had a terrace structure and an average root mean square roughness of 0.26 nm.     

Properties of GaN epitaxial layers grown on 6H-SiC(0001) by plasma-assisted molecular beam epitaxy

The structural, electrical, and optical properties of GaN grown on 6H-SiC(0001) substrates by molecular beam epitaxy are studied. Suitable substrate preparation and growth conditions are found to greatly improve the structural quality of the films. Threading dislocation densities of about 3×10⁹ cm⁻² for edge dislocations and <1×10⁶ cm⁻² for screw dislocations are achieved in GaN films of 0.8 μm thickness. Mechanisms of dislocation generation and annihilation are discussed. Increasing the Ga to N flux ratio used during growth is found to improve the surface morphology. An unintentional electron concentration in the films of about 5×10¹⁷ cm⁻³ is observed, and is attributed to excess Si in the films due to a Si-cleaning step used in the substrate preparation. Results from optical characterization are correlated with the structural and electronic studies.     

P- and N-type doping of GaN and AlGaN epitaxial layers grown by metalorganic chemical vapor deposition

Mg- and Si-doped GaN and AlGaN films were grown by metalorganic chemical vapor deposition and characterized by room-temperature photoluminescence and Hall-effect measurements. We show that the p-type carrier concentration resulting from Mg incorporation in GaN:Mg films exhibits a nonlinear dependence both on growth temperature and growth pressure. For GaN and AlGaN, n-type doping due to Si incorporation was found to be a linear function of the silane molar flow. Mg-doped GaN layers with 300K hole concentrations p ∼2×10¹⁸ cm⁻³ and Si-doped GaN films with electron concentrations n∼1×10¹⁹ cm⁻³ have been grown. N-type Al_0.10Ga_0.90N:Si films with resistivities as low as p ∼6.6×10⁻³ Ω-cm have been measured.     

Epitaxial Growth and Characterization of p-Type ZnO

N-doped p-type ZnO thin films were grown on c-sapphire substrates, semi-insulating GaN templates, and n-type ZnO substrates by metal organic chemical vapor deposition (MOCVD). Diethylzinc and oxygen were used as precursors for Zn and O, respectively, while ammonia (NH₃) and nitrous oxide (N₂O) were employed as the nitrogen dopant sources. X-ray diffraction (XRD) studies depicted highly oriented N-doped ZnO thin films. Photoluminescence (PL) measurements showed a main emission line around 380 nm, corresponding to an energy gap of 3.26 eV. Nitrogen concentration in the grown films was analyzed by secondary ion mass spectrometry (SIMS) and was found to be on the order of 10¹⁸ cm⁻³. Electrical properties of N-doped ZnO epilayers grown on semi-insulating GaN:Mg templates were measured by the Hall effect and the results indicated p-type with carrier concentration on the order of 10¹⁷ cm⁻³.     

Acceptor and donor doping of AlxGa1−xN thin film alloys grown on 6H-SiC(0001) substrates via metalorganic vapor phase epitaxy

Thin films of Si-doped Al_xGa_1−xN (0.03≤x≤0.58) having smooth surfaces and strong near-band edge cathodoluminescence were deposited at 0.35–0.5 μm/h on on-axis 6H-SiC(0001) substrates at 1100°C using a 0.1 μm AlN buffer layer for electrical isolation. Alloy films having the compositions of Al_0.08Ga_0.92N and Al_0.48Ga_0.52N exhibited mobilities of 110 and 14 cm²/V·s at carrier concentrations of 9.6×10¹⁸ and 5.0×10¹⁷ cm⁻³, respectively. This marked change was due primarily to charge scattering as a result of the increasing Al concentration in these random alloys. Comparably doped GaN films grown under similar conditions had mobilities between 170 and ∼350 cm²/V·s. Acceptor doping of Al_xGa_1−xN for x≤0.13 was achieved for films deposited at 1100°C. No correlation between the O concentration and p-type electrical behavior was observed.     

The effects of oxygen,nitrogen, and hydrogen annealing on Mg acceptors in GaN as monitored by electron paramagnetic resonance spectroscopy

Electron paramagnetic resonance (EPR) spectroscopy is used to study the unpassivated Mg-related acceptor in GaN films. As expected, the trends observed before and after O₂, N₂, or forming-gas anneals at temperatures <800°C are similar to those typically reported for electrical measurements. However, annealing at temperatures >850°C in O₂ or N₂ permanently removes the signal, contrary to the results of conductivity measurements. Approximately 10¹⁹ cm⁻³ Mg acceptors were detected in some GaN films grown by chemical vapor deposition (CVD) before acceptor activation, suggesting that it is possible to have electrically active Mg in as-grown CVD material.     

HgCdTe growth on (552) oriented CdZnTe by metalorganic vapor phase epitaxy

We report the growth of HgCdTe on (552)B CdZnTe by metalorganic vapor phase epitaxy (MOVPE). The (552) plane is obtained by 180 rotation of the (211) plane about the [111] twist axis. Both are 19.47 degrees from (111), but in opposite directions. HgCdTe grown on the (552)B-oriented CdZnTe has a growth rate similar to the (211)B, but the surface morphology is very different. The (552)B films exhibit no void defects, but do exhibit ∼40 μm size hillocks at densities of 10–50 cm⁻². The hillocks, however, are significantly flatter and shorter than those observed on (100) metalorganic vapor phase epitaxy (MOVPE) HgCdTe films. For a 12–14 μm thick film the height of the highest point on the hillock is less than 0.75 μm. No twinning was observed by back-reflection Laue x-ray diffraction for (552)B HgCdTe films and the x-ray double crystal rocking curve widths are comparable to those obtained on (211)B films grown side-by-side and with similar alloy composition. Etch pit density (EPD) measurements show EPD values in the range of (0.6–5)×10⁵ cm⁻², again very similar to those currently observed in (211)B MOVPE HgCdTe. The transport properties and ease of dopant incorporation and activation are all comparable to those obtained in (211)B HgCdTe. Mid-wave infrared (MWIR) photodiode detector arrays were fabricated on (552)B HgCdTe films grown in the P-n-N device configuration (upper case denotes layers with wider bandgaps). Radiometric characterization at T=120–160 K show that the detectors have classical spectral response with a cutoff wavelength of 5.22 μm at 120 K, quantum efficiency ∼78%, and diffusion current is the dominant dark current mechanism near zero bias voltage. Overall, the results suggest that (552)B may be the preferred orientation for MOVPE growth of HgCdTe on CdZnTe to achieve improved operability in focal plane arrays.     

MBE growth of HgCdTe on silicon substrates for large format MWIR focal plane arrays

HgCdTe p-on-n double layer heterojunctions (DLHJs) for mid-wave infrared (MWIR) detector applications have been grown on 100 mm (4 inch) diameter (211) silicon substrates by molecular beam epitaxy (MBE). The structural quality of these films is excellent, as demonstrated by x-ray rocking curves with full widths at half maximum (FWHMs) of 80–100 arcsec, and etch pit densities from 1 10⁶ to 7 10⁶ cm⁻². Morphological defect densities for these layers are generally less than 1000 cm⁻². Improving Hg flux coverage of the wafer during growth can reduce void defects near the edges of the wafers. Improved tellurium source designs have resulted in better temporal flux stability and a reduction of the center to edge x-value variation from 9% to only 2%. Photovoltaic MWIR detectors have been fabricated from some of these 100mm wafers, and the devices show performance at 140 K which is comparable to other MWIR detectors grown on bulk CdZnTe substrates by MBE and by liquid phase epitaxy.     

Residual impurities in GaN/Al2O3 grown by metalorganic vapor phase epitaxy

Residual impurities in GaN films on sapphire (A1₂O₃) substrates grown by two-step metalorganic vapor phase epitaxy (MOVPE) have been investigated. We have mainly investigated the incorporation of carbon into the GaN films with GaN buffer layers on A1₂O₃ during MOVPE growth, comparing trimethygallium (TMGa) and triethygallium (TEGa) as the typical gallium precursors. The films were characterized by secondary ion mass spectroscopy analysis, photolu-minescence, and Hall measurements. The carbon, hydrogen, and oxygen concentrations increase with decreasing growth temperature in using TMGa. Especially the carbon concentration increases with decreasing a V/III ratio, for both TMGa and TEGa. There is about two times more carbon in the GaN films grown using TEGa than those using TMGa. The carbon from TMGa mainly enhances the D-A pair emission (∼378 nm), which shows the carbon makes an acceptor level at nitrogen sites in GaN. On the other hand, the carbon from TEGa enhances a deep emission (∼550 nm), which shows the carbon makes not only an acceptor level but deep levels at interstitial sites in GaN. The carbon impurities originate from methyl radicals for TMGa, or ethyl radicals for TEGa. It is supposed that, in the case of TEGa, the carbon impurities are not always located at nitrogen sites, but are also located at interstitial sites because of the C-C bonding in ethyl radicals.     

Molecular-beam epitaxial growth of HgCdTe infrared focal-plane arrays on silicon substrates for midwave infrared applications

Molecular beam epitaxy has been employed to deposit HgCdTe infrared detector structures on Si(112) substrates with performance at 125K that is equivalent to detectors grown on conventional CdZnTe substrates. The detector structures are grown on Si via CdTe(112)B buffer layers, whose structural properties include x-ray rocking curve full width at half maximum of 63 arc-sec and near-surface etch pit density of 3–5 × 10⁵ cm⁻² for 9 μm thick CdTe films. HgCdTe p⁺-on-n device structures were grown by molecular beam epitaxy (MBE) on both bulk CdZnTe and Si with 125K cutoff wavelengths ranging from 3.5 to 5 μm. External quantum efficiencies of 70%, limited only by reflection loss at the uncoated Si-vacuum interface, were achieved for detectors on Si. The current-voltage (I-V) characteristics of MBE-grown detectors on CdZnTe and Si were found to be equivalent, with reverse breakdown voltages well in excess of 700 mV. The temperature dependences of the I-V characteristics of MBE-grown diodes on CdZnTe and Si were found to be essentially identical and in agreement with a diffusion-limited current model for temperatures down to 110K. The performance of MBE-grown diodes on Si is also equivalent to that of typical liquid phase epitaxy-grown devices on CdZnTe with R₀A products in the 10⁶–10⁷ Θ-cm² range for 3.6 μm cutoff at 125K and R₀A products in the 10⁴–10⁵ Θ-cm² range for 4.7 μm cutoff at 125K.     

Growth optimization and doping with Si and Be of high quality GaN on Si(111) by molecular beam epitaxy

GaN layers have been grown by plasma-assisted molecular beam epitaxy on AlN-buffered Si(111) substrates. An initial Al coverage of the Si substrate of aproximately 3 nm lead to the best AlN layers in terms of x-ray diffraction data, with values of full-width at half-maximum down to 10 arcmin. A (2×2) surface reconstruction of the AlN layer can be observed when growing under stoichiometry conditions and for substrate temperatures up to 850°C. Atomic force microscopy reveals that an optimal roughness of 4.6 nm is obtained for AlN layers grown at 850°C. Optimization in the subsequent growth of the GaN determined that a reduced growth rate at the beginning of the growth favors the coalescence of the grains on the surface and improves the optical quality of the film. Following this procedure, an optimum x-ray full-width at half-maximum value of 8.5 arcmin for the GaN layer was obtained. Si-doped GaN layers were grown with doping concentrations up to 1.7×10¹⁹ cm⁻³ and mobilities approximately 100 cm²/V s. Secondary ion mass spectroscopy measurements of Be-doped GaN films indicate that Be is incorporated in the film covering more than two orders of magnitude by increasing the Be-cell temperature. Optical activation energy of Be acceptors between 90 and 100 meV was derived from photoluminescence experiments.     

Influence of growth conditions and surface reaction byproducts on GaN grown via metal organic molecular beam epitaxy: Toward an understanding of surface reaction chemistry

The surface reaction byproducts during the growth of GaN films via metal organic molecular beam epitaxy (MOMBE) were investigated as a means to optimize material properties. Ethylene and ethane were identified as the dominant surface reaction hydrocarbon byproducts, averaging 27.63% and 7.15% of the total gas content present during growth. Intense ultraviolet (UV) photoexcitation during growth was found to significantly increase the abundance of ethylene and ethane while reducing the presence of H₂ and N₂. At 920°C, UV excitation was shown to enhance growth rate and crystalline quality while reducing carbon incorporation. Over a limited growth condition range, a 4.5×10¹⁹−3.4×10²⁰ cm⁻³ variation in carbon incorporation was achieved at constant high vacuum. Coupled with growth rate gains, UV excitation yielded films with ∼58% less integrated carbon content. Structural material property variations are reported for various ammonia flows and growth temperatures. The results suggest that high carbon incorporation can be achieved and regulated during MOMBE growth and that in-situ optimization through hydrocarbon analysis may provide further enhancement in the allowable carbon concentration range.