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.     

Microstructure and enhanced morphology of planar nonpolar m-plane GaN grown by hydride vapor phase epitaxy

Nonpolar ( ) m-plane gallium nitride has been grown heteroepitaxially on (100) γ-LiAlO₂ by several groups. Previous attempts to grow m-plane GaN by hydride vapor phase epitaxy (HVPE) yielded films unsuitable for subsequent device regrowth because of the high densities of faceted voids intersecting the films’ free surfaces. We report here on the growth of planar m-plane GaN films on (100) γ-LiAlO₂ and elimination of bulk and surface defects. The morphology achieved is smooth enough to allow for fabrication of m-plane GaN templates and free-standing substrates for nonpolar device regrowth. The GaN films were grown in a horizontal HVPE reactor at 860–890°C. Growth rates ranged from 30 μm/h to 240 μm/h, yielding free-standing films up to 250-μm thickness. The m-plane GaN films were optically specular and mirror-like, with undulations having 50–200-nm peak-to-valley heights over millimeter length scales. Atomic force microscopy revealed a striated surface morphology, similar to that observed in m-plane GaN films grown by molecular beam epitaxy (MBE). Root-mean-square (RMS) roughness was 0.636 nm over 25-μm² areas. Transmission electron microscopy (TEM) was performed on the m-plane GaN films to quantify microstructural defect densities. Basal-plane stacking faults of 1×10⁵ cm⁻¹ were observed, while 4×10⁹ cm⁻² threading dislocations were observed in the g=0002 diffraction condition.     

MBE growth and properties of ZnO on sapphire and SiC substrates

Molecular beam epitaxy (MBE) of ZnO on both sapphire and SiC substrates has been demonstrated. ZnO was used as a buffer layer for the epitaxial growth of GaN. ZnO is a würtzite crystal with a close lattice match (<2% mismatch) to GaN, an energy gap of 3.3 eV at room temperature, a low predicted conduction band offset to both GaN and SiC, and high electron conductivity. ZnO is relatively soft compared to the nitride semiconductors and is expected to act as a compliant buffer layer. Inductively coupled radio frequency plasma sources were used to generate active beams of nitrogen and oxygen for MBE growth. Characterization of the oxygen plasma by optical emission spectroscopy clearly indicated significant dissociation of O₂ into atomic oxygen. Reflected high energy electron diffraction (RHEED) of the ZnO growth surface showed a two-dimensional growth. ZnO layers had n-type carrier concentration of 9 × 10¹⁸ cm⁻³ with an electron mobility of 260 cm²/V-s. Initial I-V measurements displayed ohmic behavior across the SiC/ZnO and the ZnO/GaN heterointerfaces. RHEED of GaN growth by MBE on the ZnO buffer layers also exhibited a two-dimensional growth. We have demonstrated the viability of using ZnO as a buffer layer for the MBE growth of GaN.     

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.     

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.     

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.     

TEM structural studies of undoped and Si-doped GaN grown on Al2O3 substrate

Transmission electron microscopy was used to study the microstructure of GaN films undoped or Si-doped to 10¹⁷ or 10¹⁸ cm^?3 and grown by molecular-beam epitaxy on (0001) Al₂O₃ substrate without nitridation or a buffer layer. Defect structures including inversion domains, nanopipes, and (0001) stacking faults were studied. The influence of Si doping on the threading dislocation density and the dimensions of GaN grains bounded by inversion domains was assessed. Smoothing of the steplike morphology of the GaN film surface occurs at a Si concentration of 10¹⁷ cm^?3.     

Growth and characterization of indium arsenide thin films

The growth and characterization of indium arsenide films grown on indium phosphide substrates by the metal organic chemical vapor deposition (MOCVD) process is reported. Either ethyl dimethyl indium or trimethyl indium were found to be suitable in combination with arsine as source compounds. The highest electron mobilities were observed in films nucleated at reduced growth temperature. Scanning electron microscopy studies show that film nucleation at low temperature prevents thermal etch pits from forming on the InP surface before growth proceeds at an elevated temperature. Electron mobilities as high as 21,000 cm²V⁻¹ sec⁻¹ at 300 K were thus obtained for a film only 3.4 μm thick. This mobility is significantly higher than was previously observed in InAs films grown by MOCVD. From the depth dependence of transport properties, we find that in our films electrons are accumulated near the air interface of the film, presumably by positive ions in the native oxide. The mobility is limited by electrons scattering predominantly from ionized impurities at low temperature and from lattice vibrations and dislocations at high temperature. However, scattering from dislocations is greatly reduced in the surface accumulation layer due to screening by a high density of electrons. These dislocations arise from lattice mismatch and interface disorder at the film-substrate interface, preventing these films from obtaining mobility values of bulk indium arsenide.     

A new buffer layer for MOCVD growth of GaN on sapphire

High quality GaN films have been grown on sapphire substrates (C face and A face) by atmospheric pressure metalorganic chemical vapor deposition (MOCVD) using a new buffer layer. With our reactor configuration and growth parameters, a GaN film grown on a single GaN buffer layer appears opaque with high density of hexagonal pits. Using a single A1N buffer layer results in extremely nonuniform morphology with mirror-like areas near the edge of the substrates and opaque areas in the center. The double buffer layer we report here, with GaN as the first layer and A1N as the second, each with an optimized thickness, leads to mirror-like films across the entire substrate. Scanning electron microscopy, photoluminescence, x-ray diffraction, and van der Pauw geometry Hall measurement data are presented to establish the quality of our films. The mechanism for this new buffer layer is also discussed.     

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.     

Integration of GaN thin films with dissimilar substrate materials by Pd-In metal bonding and laser lift-off

Gallium nitride (GaN) thin films grown on sapphire substrates were successfully bonded and transferred onto GaAs, Si, and polymer “receptor” substrates using a low-temperature Pd-In bond followed by a laser lift-off (LLO) process to remove the sapphire growth substrate. The GaN/sapphire structures were joined to the receptor substrate by pressure bonding a Pd-In bilayer coated GaN surface onto a Pd coated receptor substrate at a temperature of 200°C. X-ray diffraction showed that the intermetallic compound PdIn₃ had formed during the bonding process. LLO, using a single 600 mJ/cm², 38 ns KrF (248 nm) excimer laser pulse directed through the transparent sapphire substrate, followed by a low-temperature heat treatment, completed the transfer of the GaN onto the “receptor” substrate. Cross-sectional scanning electron microscopy and x-ray rocking curves showed that the film quality did not degrade significantly during the bonding and LLO process.     

Investigation of microstructure and V-defect formation in In<Subscript>x</Subscript>Ga<Subscript>1−x</Subscript>N/GaN MQW grown using temperature-gradient metalorganic chemical vapor deposition

Temperature-gradient metalorganic chemical vapor deposition (MOCVD) was used to deposit In_xGa_1−xN/GaN multiple quantum well (MQW) structures with a concentration gradient of indium across the wafer. These MQW structures were deposited on low defect density (2×10⁸ cm⁻²) GaN template layers for investigation of microstructural properties and V-defect (pinhole) formation. Room temperature (RT) photoluminescence (PL) and photomodulated transmission (PT) were used for optical characterization, which show a systematic decrease in emission energy for a decrease in growth temperature. Triple-axis x-ray diffraction (XRD), scanning electron microscopy, and cross-sectional transmission electron microscopy were used to obtain microstructural properties of different regions across the wafer. Results show that there is a decrease in crystal quality and an increase in V-defect formation with increasing indium concentration. A direct correlation was found between V-defect density and growth temperature due to increased strain and indium segregation for increasing indium concentration.     

The role of the low temperature buffer layer and layer thickness in the optimization of OMVPE growth of GaN on sapphire

In agreement with previous work,¹² a thin, low temperature GaN buffer layer, that is used to initiate OMVPE growth of GaN growth on sapphire, is shown to play a critical role in determining the surface morphology of the main GaN epilayer. X-ray analysis shows that the mosaicity of the main GaN epilayer continues to improve even after several μm of epitaxy. This continuing improvement in crystal perfection correlates with an improvement in Hall mobility for thicker samples. So far, we have obtained a maximum mobility of 600 cm²/V-s in a 6 μm GaN epilayer. Atomic force microscopy (AFM) analysis of the buffer layer and x-ray analysis of the main epilayer lead us to conclude that the both of these effects reflect the degree of coherence in the main GaN epitaxial layer. These results are consistent with the growth model presented by Hiramatsu et al., however, our AFM data indicates that for GaN buffer layers partial coherence can be achieved during the low temperature growth stage.     

X-ray diffraction and high resolution transmission electron microscopy of 3C-SiC/AlN/6H-SiC(0001)

The structure and crystal quality of epitaxial films of SiC/AlN/6H-SiC(0001) prepared by chemical vapor deposition were evaluated by high resolution transmission electron microscopy (HRTEM) and x-ray diffraction techniques. Cross-sectional HRTEM revealed an abrupt AlN layer-6H-SiC substrate junction, but the transition between the AlN and SiC layers was much rougher, leading to the formation of a highly disordered SiC region adjacent to the interface. The AlN layer was relatively defect free, while the SiC layer contained many microtwins and stacking faults originating at the top SiC/AlN interface. The SiC layer was the 3C-polytype, as determined by double crystal x-ray rocking curves. The SiC layers were under in-plane compressive stress, with calculated defect density between 2–4×10⁷ defects/cm⁻².     

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.     

Improvements of the SiC homoepitaxy process in a horizontal cold-wall CVD reactor

In this research effort, we investigate the influence of the cold-wall reactor geometry on the chemical vapor deposition (CVD) growth process of 4H-SiC and the quality of lightly doped epitaxial layers. Stable growth conditions with respect to growth rate and C/Si ratio of the gas-phase can be achieved by the appropriate choice of the distance between susceptor and walls of the inner quartz tube. A background doping concentration in the range of 10¹⁴ cm⁻³ is realized by employing a high temperature stable and hydrogen etch resistant coating of the graphite susceptor. Doping and thickness homogeneity of epitaxial layers on 35 mm diam. 4H-SiC substrates, expressed by σ/mean, are as low as 6.9 and 7.7%, respectively. From deep level transient spectroscopy measurements, the concentration of the frequently reported intrinsic Z₁-center in 4H-SiC is determined to be below the detection limit of 10¹² cm⁻³.     

额外HCl和氮化对HVPE GaN生长的影响

在氢化物气相外延(HVPE)生长Ga N过程中,发现了一种在成核阶段向生长区添加额外HCl来改善Ga N外延薄膜质量的方法,并且讨论了额外HCl和氮化对Ga N形貌和质量的影响.两种方法都可以大幅度地改善Ga N的晶体质量和性质,但机理不同.氮化是通过在衬底表面形成Al N小岛,促进了衬底表面的成核和薄膜的融合;而添加额外HCl则被认为是通过改变生长表面的过饱和度引起快速成核从而促进薄膜的生长而改善晶体质量和性质的     

Metalorganic chemical vapor deposition growth of high optical quality and high mobility GaN

Hall mobilities as high as 702 and 1230 cm²/Vs at 300 and 160K along with low dislocation densities of 4.0 × 10⁸ cm^-2 have been achieved in GaN films grown on sapphire by metalorganic chemical vapor deposition. High growth temperatures have been established to be crucial for optimal GaN film quality. Photoluminescence measurements revealed a low intensity of the deep defect band around 550 nm in films grown under optimized conditions.     

Growth Optimization of an Electron Confining InN/GaN Quantum Well Heterostructure

The molecular beam epitaxy of In-face InN (0001) epilayers with optimized surface morphology, structural quality, and electrical properties was investigated. Namely, compact InN epilayers with atomically flat surfaces, grown in a step-flow mode, were obtained using stoichiometric fluxes of In and N and substrate temperatures in the range from 400°C to 435°C. Typical values for the electron concentration and the Hall mobility at 300 K were 4.3 × 10¹⁸ cm⁻³ and 1210 cm²/Vs, respectively. The growth mode of InN during the very first stage of the nucleation was investigated analytically, and it was found that the growth proceeds through nucleation and fast coalescence of two-dimensional (2-D)–like InN islands. The preceding conditions were used to grow an InN/GaN quantum well (QW) heterostructure, which exhibited well-defined interfaces. Schottky contacts were successfully fabricated using a 15-nm GaN barrier enhancement cap layer. Capacitance-voltage measurements revealed the confinement of electrons within the InN QW and demonstrated the capability to modulate the electron density within an InN channel. The sheet concentration of the confined electrons (1.5 × 10¹³ cm⁻²) is similar to the calculated sheet polarization charge concentration (1.3 × 10¹³ cm⁻²) at the InN/GaN interface. However, electrons may also originate from ionized donors with a density of 8 × 10¹⁸ cm⁻³ within the InN layer.     

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.