In-situ monitoring of surface stoichiometry and growth kinetics study of GaN (0001) in MOVPE by spectroscopic ellipsometry

GaN surface stoichiometry and growth kinetics in MOVPE were studied by in-situ spectroscopic ellipsometry. The effect of MOVPE conditions on both the surface stoichiometry and growth kinetics was investigated. The surface stoichiometry, such as N-rich, Ga-rich and Ga-excess surfaces, was monitored, and was drastically changed by the variation of the NH₃ partial pressure. When the TMG supply was interrupted during the growth, the layer-by-layer decomposition/revaporation was observed in H₂/NH₃ ambient. The decomposition rate was measured as a function of the NH₃ flow rate at the conventional epilayer growth temperatures (1050–1140 C). The decomposition rate was decreased with the increase in the N coverage on the GaN surface. it was found that the surface stoichiometry is a very important parameter for the control of the MOVPE growth kinetics.     

Basic studies of gallium nitride growth on sapphire by metalorganic chemical vapor deposition and optical properties of deposited layers

We have studied the growth of gallium nitride on c-plane sapphire substrates. The layers were grown in a horizontal metalorganic chemical vapor deposition reactor at atmospheric pressure using trimethylgallium (TMG) and ammonia (NH₃). Variation of the V/III ratio (150–2500) shows a distinct effect on the growth rate. With decreasing V/III ratio, we find an increasing growth rate. Variation of the growth temperature (700–1000°C) shows a weak increase in growth rate with temperature. Furthermore, we performed secondary ion mass spectroscopy measurements and find an increasing carbon incorporation in the GaN films with decreasing ammonia partial pressure and a growing accumulation of carbon at the substrate interface. Photoluminescence measurements show that samples with high carbon content show a strong yellow luminescence peaking at 2.2 eV and a near band gap emission at 3.31 eV. With increasing carbon content, the intensity of the 3.31 eV line increases suggesting that a carbon related center is involved.     

Selective area growth of GaN using gas source molecular beam epitaxy

Ammonia cracking efficiencies on various surfaces were examined. The following is an ordering of surfaces according to their ammonia cracking efficiencies: GaN (highest), Si₃N₄, SiO₂ (lowest). Selective area growth of GaN was performed over SiO₂ masks deposited on GaN previously grown on sapphire substrates using ammonia-based molecular beam epitaxy. GaN growth on patterned SiO₂/GaN is very selective at a growth temperature of 800°C. Good quality growth occurs in the window region with no deposits on the mask surface when growth is performed at 800°C, whereas some deposits on the SiO₂ masks accumulate when growth is performed at 700°C. The ratio of lateral growth rate to vertical growth rate is ≤1.     

Growth of AlxGa1−x as by metalorganic chemical vapor deposition using trimethylgallium and trimethylamine alane

High-quality Al_xGa_1−xAs layers with aluminum arsenide contentx up to 0.34 have been grown in a low pressure metalorganic chemical vapor deposition (MOCVD) system using trimethylgallium (TMG), trimethylamine alane (TMAA) and arsine. The carbon content in these films depended on growth conditions but was in general lower than in those obtained with trimethylaluminum (TMA) instead of TMAA in the same reactor under similar conditions. Unlike TMA grown layers, the TMAA grown Al_xGa_1−xAs layers, (grown at much lower temperature—down to 650° C), exhibited room temperature photolu-minescence (PL). Low temperature (25 K) PL from these films showed sharp bound exciton peaks with a line width of 5.1 meV for Al_0.25Ga_0.75As. A 39 period Al_0.28Ga_0.72As (5.5 nm)/GaAs (8.0 nm) superlattice grown at 650° C showed a strong PL peak at 25 K with a line width of 5.5 meV attesting to the high quality of these layers.     

Au/Ge/Ni ohmic contacts to n-Type InP

The relationship between the electrical properties and microstructure for annealed Au/Ge/Ni contacts to n-type InP, with an initial doping level of 10¹⁷ cm^-3, have been studied. Metal layers were deposited by electron beam evaporation in the following sequence: 25 nm Ni, 50 nm Ge, and 40 nm Au. Annealing was done in a nitrogen atmosphere at 250-400‡C. The onset of ohmic behavior at 325‡C corresponded to the decomposition of a ternary Ni-In-P phase at the InP surface and the subsequent formation of Ni₂P plus Au₁₀In₃, producing a lower barrier height at the InP interface. This reaction was driven by the inward diffusion of Au and outward diffusion of In. Further annealing, up to 400‡C, resulted in a decrease in contact resistance, which corresponded to the formation of NiP and Au₉ln₄ from Ni₂P and Au₁₀In₃,respectively, with some Ge doping of InP also likely. A minimum contact resistance of 10^-7 Ω-cm² was achieved with a 10 s anneal at 400‡C.     

Growth of thick gan films on rf sputtered ain buffer layer by hydride vapor phase epitaxy

High crystalline quality thick GaN films were grown by vapor phase epitaxy using GaCl₃ and NH₃. The growth rate was in the range of 10~15 Μm/h. GaN films grown at higher temperatures (960~ 1020?C) were single crystalline with smooth surface morphologies. No chlorine impurity was incorporated in these films during growth. The best crystalline quality and surface morphology of grown films was achieved by sputtering a thin A1N buffer layer, prior to growth. According to reflection high energy electron diffraction and atomic force microscopy measurements, as-sputtered A1N buffer layer was amorphous with root means square roughness of 0.395 nm and then crystallized during the GaN growth. This improved the GaN growth due to more uniform distribution of GaN nucleation. Rutherford backscattering channeling experiments produced the lowest value from the GaN film grown on a-Al₂O₃ with a 500å A1N buffer layer at 1020?C.     

Structures and physical properties of sputtered amorphous SiC films

Optical and electrical properties have been measured for amorphous SiC films prepared by rf sputtering in a pure Ar atmosphere with a sintered 6H-SiC target. The absorption edge E₀ determined from the relation of αhΝ = B(hΝ-E₀)² ranged from 1.45 to 1.80 eV depending on the film thickness and the substrate temperature. The room temperature electrical conductivity is in the range of 5.4×10⁻¹¹ and 1.4×10⁻⁵ Ω⁻¹cm⁻¹. The absorption edge decreases and the conductivity increases with increasing film thickness. The absorption edge shifts to shorter wavelengths (blue shift) and the conductivity decreases during annealing below 400‡C for 60 min, whereas the absorption edge shifts to the longer wavelength side (red shift) and the conductivity increases during annealing at 800‡C It is proposed that the two annealing processes cause structural changes in amorphous SiC films, one of which involves removal of defects or voids while the other involves rearrangement or rebonding of the component atoms.     

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.     

Studies on nucleation kinetics of InxGa1−xN/GaN heterostructures

Nucleation kinetics during the growth of In_xGa_1−xN on a GaN substrate have been studied. The behavior of nonequilibrium between the In_xGa_1−xN and the GaN substrate has been analyzed, and hence, the expression derived for the stress-induced supercooling/superheating has been numerically evaluated. The maximum amount of stress-induced supercooling is found to be 1.017 K at x=0.12. These values are incorporated in the classical heterogeneous nucleation theory. Using the regular solution model, the interfacial tension between the nucleus and substrate and, hence, the interfacial tension between nucleus and mother phase and thermodynamical potential of the compounds have been calculated. The amount of driving force available for the nucleation has been determined for different compositions and degrees of supercooling. It has been shown that the value of the interaction parameter of InN-GaN plays a dominant role in nucleation and growth kinetics of In_xGa_1−xN on a GaN substrate. These values have been used to evaluate the nucleation parameters. It is shown that the nucleation barrier for the formation of a In_xGa_1−xN nucleus on a GaN substrate is minimum in the range of x=0.12 to x=0.17, and it has been qualitatively proved that good quality In_xGa_1−xN on GaN can be grown only in the range 0<x≤0.2.     

Miscibility gap in the GaAsy Sb1−y system

Measurements have been made to determine accurately the solidus curve of the pseudobinary section GaAs_ySb_1−y, by annealing samples to equilibrium and determining compositions by x-ray powder photography. It is found that the equilibriui diagram shows a peritectic form with a peritectic temperature of 745 ± 1‡C and a miscibility gap at that temperature extending from y = 0.38 to y = 0.68. It is also shown that as the temperature is lowered the miscibility gap widens rapidly, being from y = 0.30 to y = 0.95 at 700‡C. The form of these phase boundaries is important when growth of GaAs_ySb_1−y alloys by liquid phase epitaxy or similar techniques is considered.     

The excellent spontaneous ultraviolet emission of GaN nanostructures grown on silicon substrates by thermal vapor deposition

In this study, GaN nanostructures were grown on p-Si (111) substrate by thermal chemical vapor deposition (TCVD). Ga vapor directly reacted with NH₃ solution in N₂ carrier gas flow of 2 L/min at different temperatures (950–1050 °C). The influence of NH₃ solution and growth temperature on the morphology, structure, optical and photoresponse properties of GaN nanostructures was investigated. Scanning electron microscopy images showed that the densities of the NWs varied with increasing temperature. The use of NH₃ solution and increased growth temperature improved the crystalline quality of GaN nanostructures. The photoluminescence (PL) spectra of nanostructures displayed a near band-edge (NBE) emission at around 363–367 nm. Higher growth temperature (1050 °C) resulted in a strong NBE emission with no yellow emission peak. With +5 V applied bias, the NWs metal–semiconductor–metal UV photodetector exhibited a high photocurrent of 1.6×10⁻³ A. The photocurrent to dark current contrast ratio was 120.     

Phase relations and homogeneity region of the hightemperature superconducting phase (Bi,Pb)2Sr2Ca2Cu3O10+d

For the nominal composition of Bi_2.27−xPb_xSr₂Ca₂Cu₃O_10+d, the lead content was varied from x < 0.05 to 0.45. The compositions were examined between 800 and 890‡C which is supposed to be the temperatue range over which the so-called 2223 phase (Bi₂Sr₂Ca₂Cu₃O_10+d) is stable. Only compositions between x < 0.18 to 0.36 could be synthezised in a single phase state. For x <0.36, a lead-containing phase with a stoichiometry of Pb₄(Sr,Ca)₅CuO_d with a small solubiliy of Bi is formed, for x > 0.18 mainly Bi₂Sr₂CaCu₂O_8+d and cuprates are the equilibrium phases. The temperature range for the 2223 phase was found to be 800 to 890‡C but the 2223 phase has extremely varying cation ratios over this temperaure range. Former single phase 2223 samples turn to multiphase samples when annealed at slightly higher or lower temperaures. A decrease in the Pb solubility with increasing as well as decreasing temperature with a maximum at about 850‡C was found for the 2223 phase.     

Properties of GaN epitaxial layers grown at high growth rates by metalorganic chemical vapor deposition

GaN epitaxial layers were grown at high growth rates by increasing the input trimethylgallium (TMG) flow rate while keeping the NH₃ flow rate constant in metalorganic chemical vapor deposition. The electrical and optical properties of the grown layers have been investigated. With the increasing TMG flow rate, the electron concentration tends to decrease gradually and the Hall mobility decreases significantly. Considering the temperature dependence of the Hall mobility and the correlation between the Hall mobility and the electron concentration, it has been indicated that the more acceptors are incorporated and consequently the compensation ratio becomes higher with increasing the TMG flow rate. Photoluminescence measurements have revealed that the intensity ratio of the bound exciton emission to the 2.2 eV band emission, which is assumed to correlate to carbon or Ga vacancies, was decreased with increasing the TMG flow rate. It might be reasonable to take a lot of acceptor incorporation to explain the degradation of the electrical and optical properties in the samples grown at high growth rates by increasing the TMG flow rate.     

Pulsed laser deposition and processing of wide band gap semiconductors and related materials

The present work describes the novel, relatively simple, and efficient technique of pulsed laser deposition for rapid prototyping of thin films and multi-layer heterostructures of wide band gap semiconductors and related materials. In this method, a KrF pulsed excimer laser is used for ablation of polycrystalline, stoichiometric targets of wide band gap materials. Upon laser absorption by the target surface, a strong plasm a plume is produced which then condenses onto the substrate, kept at a suitable distance from the target surface. We have optimized the processing parameters such as laser fluence, substrate temperature, background gas pressure, target to substrate distance, and pulse repetition rate for the growth of high quality crstalline thin films and heterostructures. The films have been characterized by x-ray diffraction, Rutherford backscattering and ion channeling spectrometry, high resolution transmission electron microscopy, atomic force microscopy, ultraviolet (UV)-visible spectroscopy, cathodoluminescence, and electrical transport measurements. We show that high quality AlN and GaN thin films can be grown by pulsed laser deposition at relatively lower substrate temperatures (750–800°C) than those employed in metal organic chemical vapor deposition (MOCVD), (1000–1100°C), an alternative growth method. The pulsed laser deposited GaN films (∼0.5 μm thick), grown on AlN buffered sapphire (0001), shows an x-ray diffraction rocking curve full width at half maximum (FWHM) of 5–7 arc-min. The ion channeling minimum yield in the surface region for AlN and GaN is ∼3%, indicating a high degree of crystallinity. The optical band gap for AlN and GaN is found to be 6.2 and 3.4 eV, respectively. These epitaxial films are shiny, and the surface root mean square roughness is ∼5–15 nm. The electrical resistivity of the GaN films is in the range of 10⁻²–10² Θ-cm with a mobility in excess of 80 cm²V⁻¹s⁻¹ and a carrier concentration of 10¹⁷–10¹⁹ cm⁻³, depending upon the buffer layers and growth conditions. We have also demonstrated the application of the pulsed laser deposition technique for integration of technologically important materials with the III–V nitrides. The examples include pulsed laser deposition of ZnO/GaN heterostructures for UV-blue lasers and epitaxial growth of TiN on GaN and SiC for low resistance ohmic contact metallization. Employing the pulsed laser, we also demonstrate a dry etching process for GaN and AlN films.     

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.     

Neutron-induced trapping levels in aluminum gallium arsenide

Deep level transient spectroscopy (DLTS) measurements have been performed on a variety of Al_xGa_1-xAs p-n junctions prior to and following a series of fast neutron irradiations at room temperature and subsequent isochronal anneals. In contrast with electron and proton irradiated GaAs, neutron irradiation produces a single, broad featureless DLTS band which is a majority carrier trap in both n and p type material. The characteristics of this neutron-induced trap are relatively independent of growth method, dopant type and concentration. In GaAs, the thermal emission energies of the trap are 0.58 to 0.68 eV depending on the particular junction. These energies increase with Al content to 0.94 eV at 20% Al. The trap introduction rate, which also increases with Al content, is 0.7 cm_-1 in GaAs. Isochronal annealing to temperatures as high as 400‡C results in a smaller FWHM of the DLTS band, a shift in the peak to higher temperatures, and a modest decrease in magnitude. Above 400‡C the magnitude decreases rapidly, suggesting a similarity with the antisite defect, As_Ga, which has been observed to anneal in this range.     

Electrically active defects in shallow pre-amorphisedp + n junctions in silicon

An examination of shallow pre-amorphisedp ⁺ n junctions in silicon has revealed three distinct defect related phenomena determined largely by the annealing temperature and relative location of the junction and the amorphous-crystalline (α-c) boundary. For temperatures below 800‡ C all samples displayed leakage currents of ∼10⁻³ A/cm² irrespective of the amorphising atom (Si⁺, Ge⁺ or Sn⁺). The generation centres responsible were identified to be near mid-gap deep level donors lying beyond the α-c interface. For samples annealed above 800‡ C, the leakage current was determined by the interstitial dislocation loops at the α-c boundary. If these were deeper than the junction, a leakage current density of ∼10⁻⁵ A/cm² resulted. From the growth of these loops during furnace annealing it was concluded that the growth was supported by the influx of recoil implanted silicon interstitials initially positioned beyond the α-c boundary. In the case where the as-implanted junction was deeper than the α-c boundary, annealing above 800° C resulted in a transient enhancement in the boron diffusion coefficient. As with the dislocation loop growth, this was attributed to the presence of the recoil implanted silicon interstitials.     

Growth of GaBN ternary solutions by organometallic vapor phase epitaxy

Layers of Ga_1-xB_xN with compositions from x = 0 to x = 0.07 were grown by organometallic vapor phase epitaxy on sapphire substrates using trimethylgallium, triethylboron (TEB) and NH₃ as precursors. Growth was done in the temperature range from 450 to 1000°C. The presence of boron was detected by the shift in the (0002) peak position in x-ray diffraction, by x-ray photoelectron spectroscopy, secondary ion mass spectrometry measurements, and by the changes in the band gap as measured by optical transmission. It was found that for the studied range of compositions the band gap varied from 3.4 eV for x = 0 to 3.63 eVfor x = 0.05. At certain TEB concentrations in the gas phase, the growth rate decreased abruptly, most likely because of a growth poisoning by the onset of growth of a very slow growing B-rich phase. The threshold TEB concentration for this growth poisoning decreased with increasing growth temperature; and at 1000°C, less than 1% of B could be incorporated as a result. GaBN alloys with about 7% substitutional boron were also produced by implantation of 5 × 10¹⁶ cm⁻² B ions at 60 keV into GaN, as evidenced by the shift of the band edge emission in cathodoluminescence spectra from 3.4 eV for GaN to 3.85 eV for GaBN.     

Growth and characterization of lattice-matched epitaxial films of GaxIn1−xAs/InP by liquid-phase epitaxy

We determined the conditions for successful lattice-matched growth by liquid-phase epitaxy near T = 620‡ C of Ga_XIn_1−XAs on [111B] InP substrates. We have used the results of the growth of both lattice-matched and intentionally lattice-mismatched epitaxial layers, (0.4 ≤ X ≤ 0.7) to calculate a phase diagram which gives the correct liquidus temperature, (T_L ± 1‡ C), and the correct solid composition, (± 5 % of the nominal composition), for the entire range of growth solutions considered for this important ternary semi-conductor system. The parameters appropriate to this calculation are significantly different from those used to describe the growth of Ga_XIn_1−XAs on GaAs. The results of this calculation play an important part in the better understanding of the quaternary alloy Ga_XIn_1−XAs_yP_1−y. Our measurements show that the ternary alloy lattice-matched to InP is Ga_0.47In_0.53As, semiconductor with a direct band gap about 0.75 eV at room temperature. We have grown p-n junction homostructures and double-heterostructures on InP substrates. These wafers have been used to make detectors in the 1.0 – 1.7/um range of the optical spectrum.     

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.