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.     

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.     

Low temperature growth and planar doping of ZnSe in a plasma-stimulated LP-MOVPE system

In a low-pressure metalorganic vapor phase epitaxy process, we used dc-plasma activated nitrogen to dope ZnSe, grown with ditertiarybutylselenide and dimethylzinc-triethylamine. The nitrogen concentration of up to 2 × 10¹⁸ cm⁻³ in the doped layers can be adjusted by the growth temperature, the dc-plasma power, and the N₂ dopant flow. Due to the high n-type background carrier concentration of the order of 10¹⁷ cm⁻³ in undoped samples, the doped layers show n-type conductivity or were semi-insulating because of an additional compensation by hydrogen incorporated with a concentration of the order of 10¹⁸ cm⁻³. A planar doping scheme was applied to reduce this hydrogen incorporation by one order of magnitude, although H₂ was used as carrier gas.     

Correlation of biaxial strains, bound exciton energies, and defect microstructures in gan films grown on AlN/6H-SiC(0001) substrates

Biaxial strains resulting from mismatches in thermal expansion coefficients and lattice parameters in 22 GaN films grown on A1N buffer layers previously deposited on vicinal and on-axis 6H-SiC(0001) substrates were measured via changes in the c-axis lattice parameter. A Poisson’s ratio of ν = 0.18 was calculated. The bound exciton energy (E_BX) was a linear function of these strains. The shift in E_BX with film stress was 23 meV/GPa. Threading dislocations densities of ~10¹⁰/cm² and ~10⁸/em² were determined for GaN films grown on vicinal and on-axis SiC, respectively. A 0.9% residual compressive strain at the GaN/AIN interface was observed by high resolution transmission electron microscopy (HRTEM).     

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.     

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.     

Activation of silicon ion-implanted gallium nitride by furnace annealing

Ion implantation into III–V nitride materials is animportant technology for high-power and high-temperature digital and monolithic microwave integrated circuits. We report the results of the electrical, optical, and surface morphology of Si ion-implanted GaN films using furnace annealing. We demonstrate high sheet-carrier densities for relatively low-dose (n_atoms=5×10¹⁴ cm⁻²) Si implants into AlN/GaN/sapphire heteroepitaxial films. The samples that were annealed at 1150°C in N₂ for 5 min exhibited a smooth surface morphology and a sheet electron concentration n_s ∼9.0×10¹³ cm⁻², corresponding to an estimated 19% electrical activation and a 38% Si donor activation in GaN films grown on sapphire substrates. Variable-temperature Hall-effect measurem entsindicate a Si donor ionization energy ∼15 meV.     

GaN Power Schottky Diodes with Drift Layers Grown on Four Substrates

We have examined the performance of gallium nitride (GaN) high-power Schottky diodes fabricated on unintentionally doped (UID) metalorganic chemical vapor deposition (MOCVD) films grown simultaneously on four substrates ranging in threading dislocation density from 5 × 10³ cm ^{- 2} to 10¹⁰ cm ^{- 2}. The substrates were an intentionally doped and a UID freestanding hydride vapor phase epitaxy substrate, an MOCVD GaN template grown on a sapphire wafer, and a bulk GaN substrate grown via an ammonothermal method. Capacitance–voltage (C–V) results showed the carrier concentration was ～2 × 10¹⁶ cm^?3 for films grown on each of the four substrates. With that doping level, the theoretical breakdown voltage (V _b) is ～1600 V. However, measured V _b for the devices tested on each of the four substrates fell short of this value. Also, the breakdown voltages across each of the four substrates were not substantially different. This result was especially surprising for films grown on bulk GaN substrates, because of their superior crystal quality, as determined from their x-ray rocking curve widths. Simple probability calculations showed that most of the diodes tested on the bulk substrate did not cover a single threading dislocation. Although optimization of edge-termination schemes is likely to improve V _b, we believe that point defects, not threading dislocations, are the main reason for the reduced performance of these devices.     

Carrier gas and VI/II ratio effects on carbon clusters incorporation into ZnO films grown by MOCVD

Undoped ZnO films were deposited by atmospheric metal-organic chemical vapor deposition (MOCVD) on (0001) ZnO substrate. The films were grown at various partial pressure ratios of oxygen and zinc precursors (VI/II) using either N₂ or H₂ as carrier gas. Micro-Raman scattering was employed to study the effects of carrier gas, VI/II ratio and annealing on carbon impurity incorporation into the ZnO films. Besides the well known phonon modes of ZnO, Raman spectra of the samples grown with N₂ carrier gas show two additional broad peaks, which are ascribed to carbon sp² clusters related modes, spreading in the frequency range 1300–1600 cm^?1 and dominate the Raman spectrum of the sample grown under oxygen deficiency (VI/II=0.25). In addition, a band centered at ～520 cm^?1, considered as some defects related local vibrations, appears in the samples grown with N₂ as carrier gas and its intensity increases when the VI/II ratio decreases. The average cluster size, estimated from the intensity ratio of D over G bands of the carbon sp² clusters, ranges from 16.5 to 19.4 Å. However, in all the samples grown with H₂ as carrier gas, the bands related to carbon sp² clusters and defects, are largely suppressed and the second-order-Raman scattering band (1050–1200 cm^?1) is clearly observed in addition to the bulk ZnO lattice modes. After annealing the samples at 900 °C in oxygen ambient, the crystal quality has been improved for all the samples but the carbon related bands, formed in the as-deposited films grown with the N₂ carrier gas, were only weakened.     

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).     

(AlGa)As grown by low pressure metalorganic vapor phase epitaxy using a N2 carrier

We have studied the growth of Al_xGa_1−xAs (0.24<x<0.34) using a N₂ carrier in low pressure metalorganic vapor phase epitaxy. Growth temperature, gas velocity, and V/III ratio were varied to achieve optimum growth conditions. Layers with excellent morphology and electrical and optical properties comparable to samples grown using standard conditions (with a H₂ carrier) can be deposited in a nitrogen ambient. Al_0.24Ga_0.76As bulk material grown on an AlAs buffer layer with a background doping of 1.3×10¹⁶ cm⁻³ showed Hall mobilities of 4500 and 2300 cm²/Vs at 77 and 300K. Photoluminescence studies at 2K revealed strong bound exciton transitions with a full width at half maximum of 5.2 meV for Al_0.29Ga_0.71AS.     

The influence of OMVPE growth pressure on the morphology, compensation, and doping of GaN and related alloys

Growth pressure has a dramatic influence on the grain size, transport characteristics, optical recombination processes, and alloy composition of GaN and AlGaN films. We report on systematic studies which have been performed in a close spaced showerhead reactor and a vertical quartz tube reactor, which demonstrate increased grain size with increased growth pressure. Data suggesting the compensating nature of grain boundaries in GaN films is presented, and the impact of grain size on high mobility silicon-doped GaN and highly resistive unintentionally doped GaN films is discussed. We detail the influence of pressure on AlGaN film growth, and show how AlGaN must be grown at pressures which are lower than those used for the growth of optimized GaN films. By controlling growth pressure, we have grown high electron mobility transistor (HEMT) device structures having highly resistive (10⁵ Ω-cm) isolation layers, room temperature sheet carrier concentrations of 1.2×10¹³ cm⁻² and mobilities of 1500 cm²/Vs, and reduced trapping effects in fabricated devices.     

Characterization of vapor phase epitaxial ZnSe films

Properties of ZnSe films doped with donor impurities were investigated. The ZnSe films were grown at 350°C by using metallic zinc and selenium as the source materials; their vapors were transported separately by H₂ gas under atmospheric pressure. Iodine-doped ZnSe films were grown using CH₃I (1000 ppm, diluted in helium) as a dopant source. However, it was necessary to stop this dopant flow during the film growth to obtain epitaxial films. HC1 gas etching and evacuation of the reaction apparatus before the film growth began were employed to obtain epitaxial films and to avoid redistribution of impurities without heat-treatment at higher temperature. Secondary ion mass spectroscopy analysis indicated that both chlorine and gallium were included in the layers, as well as iodine, because of residual HC1 gas. Optically high-quality and rather highly conductive n-type ZnSe films were obtained. Maximum electron concentration was 3.3 × 10¹⁷ cm⁻³.     

Realization of intrinsic p-type ZnO thin films by metal organic chemical vapor deposition

P-type ZnO thin films were grown on sapphire substrates with and without nitrous oxide (N₂O) by metal organic chemical vapor deposition (MOCVD). The intrinsic p-type ZnO films were achieved by controlling the Zn:O ratio in the range of 0.05–0.2 without N₂O flow. Secondary ion mass spectroscopy (SIMS) showed that the films contained little or no nitrogen (N) impurities for all samples. The p-type behavior of the samples should be due to the intrinsic acceptor-like defects V_Zn, for ZnO film grown without nitrous oxide, and N, occupying O sites as acceptors for ZnO film grown with nitrous oxide. The best p-type ZnO film has low resistivity of 0.369 Ω-cm, high carrier density of 1.62×10¹⁹ cm⁻³, and mobility of 3.14 cm²/V-s. The obtained p-type ZnO films possess a transmittance of nearly 100% in the visible region and strong near-band-edge emission.     

Effects of H2/NH3 flow-rate ratio on the luminescent, structural, and electrical properties of GaN epitaxial layers grown by MOCVD

GaN epitaxial layers were grown on sapphire substrates in a separate-flow reactor by metalorganic chemical vapor deposition. The flow-rate ratio of H₂ on the upper stream to NH₃ on the bottom stream is varied from 0.5 to 2. The growth condition and characterization of the GaN epitaxial layers are investigated in detail. The H₂ flow rate of the upper stream strongly affects the reactant gas flow pattern near the substrate surface and thus influences the quality of epitaxial layers. At the optimum H₂/NH₃ flow ratio of 1.0, we can obtain a good quality of GaN epitaxial layers which exhibit a strong near band-edge emis-sion in the 20 K photoluminescence (PL), a full width at half maximum of 66 meV for the 300 K PL, an electron mobility of 266 cm²/V-s and concentration of 1 × 10¹⁸ cm⁻³ at 300 K.     

Resistivity changes of heavily-boron-doped CVD-prepared polycrystalline silicon caused by thermal annealing

Heavily-boron-doped polycrystalline Si films were deposited at 600°C on thermally grown SiO₂ by the thermal decomposition of SiH₄-BCl₃-H₂ mixture. Resistivity changes with isochronal or sequential annealing were systematically examined. Temperature dependence of equilibrium saturation carrier concentration was determined at 800 ～ 1100°C. Since as-deposited polycrystalline Si is in the super-saturated state, carrier concentration decreases from the super-saturated to equilibrium saturation value by annealings over 700°C for poly Si doped with over 2 × 10²⁰ cm^?3 resulting in anomalous resistivity change. Carrier concentration changes reversibly between saturation values with sequential annealing and is determined by the last annealing temperature when the annealing time is long enough. Mobility increases with annealing temperature, however, less increase is found for heavily doped poly Si, which is attributed to the suppression of grain growth caused by electrically inactive Si-B compounds.     

Optical properties and recombination mechanisms in GaN and GaN:Mg grown by metalorganic vapor phase epitaxy

Photoluminescence (PL) and reflection spectra of undoped and Mg-doped GaN single layers grown on sapphire substrates by metalorganic vapor phase epitaxy (MOVPE) were investigated in a wide range of temperatures, excitation intensities, and doping levels. The undoped layers show n-type conductivity (μ=400 cm²/Vs, n=3×10¹⁷ cm⁻³). After annealing at T=600–700°C, the Mg-doped layers showed p-type conductivity determined by the potential-profiling technique. A small value of the full width at half maximum (FWHM=2.8 meV) of the excitonic emission and a high ratio between excitonic and deep level emission (≈5300) are evidences of the high layer quality. Two donor centers with activation energies of 35 and 22 meV were observed in undoped layers. A fine structure of the PL band with two narrow lines in the spectral range of the donor-acceptor pair (DAP) recombination was found in undoped layers. An anomaly was established in the temperature behavior of two groups of PL lines in the acceptor-bound exciton and in donor-acceptor pair regions in Mg doped layers. The lower energy line quenched with increasing temperature appreciably faster than the high energy ones. Our data does not agree with the DAP recombination model. It suggests that new approaches are required to explain the recombination mechanisms in undoped and Mg-doped GaN epitaxial layers.     

The effects of carrier gas and substrate temperature on ZnO films prepared by ultrasonic spray pyrolysis

ZnO films were deposited on glass substrates in the temperature range of 350–470 °C under an atmosphere of compressed air or nitrogen (N₂) by using ultrasonic spray pyrolysis technique. Structural, electrical and optical properties of the ZnO films were investigated using X-ray diffraction (XRD), scanning electron microscopy (SEM), electrical two-probe and optical transmittance measurements. The ZnO films deposited in the range of 350–430 °C were polycrystalline with the wurtzite hexagonal structure having preferred orientation depending on the substrate temperature. The ZnO films deposited below 400 °C had a preferred (100) orientation while those deposited above 400 °C mostly had a preferred (002) orientation. The resistivity values of ZnO films depended on the types of carrier gas. The ZnO thin films deposited under N₂ atmosphere in the range of 370–410 °C showed dense surface morphologies and resistivity values of 0.6–1.1 Ω-cm, a few orders of magnitude lower than those deposited under compressed air. Hydrogen substition in ZnO possibly contributed to decreasing resistivity in ZnO thin films deposited under N₂ gas. The Hall measurements showed that the behavior of ZnO films deposited at 410 °C under the N₂ atmosphere was n-type with a carrier density of 8.9–9.2×10¹⁶ cm^-3 and mobility of ～70 cm²/Vs. ZnO thin films showed transmission values at 550 nm wavelength in a range of 70–80%. The values of band gaps extrapolated from the transmission results showed bandgap shrinkage in an order of milli electron volts in ZnO films deposited under N₂ compared to those deposited under compressed air. The calculation showed that the bandgap reduction was possibly a result of carrier–carrier interactions.     

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.