Evolution of Threading Dislocation Density and Stress in GaN Films Grown on (111) Si Substrates by Metalorganic Chemical Vapor Deposition

We have studied the evolution of threading dislocations (TDs), stress, and cracking of GaN films grown on (111) Si substrates using a variety of buffer layers including thin AlN, compositionally graded Al _x Ga_1-x N (0 ≤ x ≤ 1), and AlN/Al _y Ga_1-y N/Al _x Ga_1-x N (0 ≤ x ≤ 1, y = 0 and 0.25) multilayer buffers. We find a reduction in TD density in GaN films grown on graded Al _x Ga_1-x N buffer layers, in comparison with those grown directly on a thin AlN buffer layer. Threading dislocation bending and annihilation occurs in the region in the graded Al _x Ga_1-x N grown under a compressive stress, which leads to a decrease of TD density in the overgrown GaN films. In addition, growing a thin AlN/Al _y Ga_1-y N bilayer prior to growing the compositionally graded Al _x Ga_1-x N buffer layer significantly reduces the initial TD density in the Al _x Ga_1-x N buffer layer, which subsequently further reduces the TD density in the overgrown GaN film. In-situ stress measurements reveal a delayed compressive-to-tensile stress transition for GaN films grown on graded Al _x Ga_1-x N buffer layers or multilayer buffers, in comparison to the film grown on a thin AlN buffer layer, which subsequently reduces the crack densities in the films.     

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).     

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.     

Correlation Between Threading Dislocations and Nonradiative Recombination Centers in InN Observed by IR Cathodoluminescence

The correlation between threading dislocations (TDs) and nonradiative recombination centers in InN films was investigated by infrared cathodoluminescence (CL). Samples were grown on nitridated (0001) sapphire substrates with a low-temperature-grown InN buffer layer by radio frequency molecular-beam epitaxy (RF-MBE). Panchromatic CL images of the InN films showed a high density of dark spots in a range of 10⁸ cm⁻² to 10⁹ cm⁻². The sample with a higher density of TDs had a higher density of CL dark spots. A depth-dependent CL measurement confirmed that CL dark spots aligned almost vertically in the film like TDs. Reasonable correlation between TDs and the nonradiative regions was also observed by a cross-sectional CL image of the InN film regrown on a microfaceted InN template, in which the TD density was dramatically reduced in part. These results suggest that threading dislocations act as nonradiative recombination centers in InN.     

The effect of built-in electric field in GaN/AlGaN quantum wells with high AIN mole fraction

Spontaneous and piezoelectric polarization in hexagonal GaN/AlGaN heterostructures give rise to large built-in electric fields. The effect of the builtin electric field in GaN/Al_xGa_1−xN quantum wells was investigated for x=0.2 to 0.8 by photoluminescence studies. The quantum well structures were grown by molecular beam epitaxy on (0001) sapphire substrates. Cross-sectional transmission electron microscopy performed on the samples revealed abrupt interfaces and uniform layer thicknesses. The low temperature (4 K) photoluminescence peaks were progressively red-shifted due to the quantum confined Stark effect depending on the AlN mole fraction in the barriers and the thickness of the GaN quantum well. Our results verify the existence of very large built-in electric fields of up to 5 MV/cm in GaN/Al_0.8Ga_0.2N quantum wells.     

锥形图形衬底上氮化镓薄膜生长和表征

GaN films are grown on cone-shaped patterned sapphire substrates(CPSSs)by metal-organic chemical vapor deposition,and the influence of the temperature during the middle stage of GaN growth on the threading dislocation(TD)density of GaN is investigated.High-resolution X-ray diffraction(XRD)and cathodeluminescence(CL)wereusedtocharacterizetheGaNfilms.TheXRDresultsshowedthattheedge-typedislocation density of GaN grown on CPSS is remarkably reduced compared to that of GaN grown on conventional sapphire substrates(CSSs).Furthermore,whenthegrowthtemperatureinthemiddlestageofGaNgrownonCPSSdecreases,the full width at half maximum of the asymmetry(102)plane of GaN is reduced.This reduction is attributed to the enhancement of vertical growth in the middle stage with a more triangular-like shape and the bending of TDs.The CL intensity spatial mapping results also showed the superior optical properties of GaN grown on CPSS to those of GaN on CSS,and that the density of dark spots of GaN grown on CPSS induced by nonradiative recombination is reduced when the growth temperature in the middle stage decreases.     

对GaN薄膜不同腐蚀方法的腐蚀坑的研究

用Thomas Swan公司的MOCVD系统在蓝宝石(0001)面上生长了高质量的GaN薄膜.采用多种化学腐蚀方法,如熔融KOH,H3PO4与H2SO4混合酸和HCl气相腐蚀法,利用SEM及TEM技术对GaN薄膜中的位错进行了研究.SEM显示在GaN薄膜相同位置处,不同腐蚀法所得的腐蚀坑的形态和密度有明显差别.结果表明HCl气相腐蚀可以显示纯螺位错、纯刃位错和混合位错;H3PO4与H2SO4混合酸腐蚀可以显示纯螺位错和混合位错;而熔融KOH腐蚀仅能显示纯螺位错.     

Observation of localized centers with anomalous behavior in light-emitting heterostructures with multiple InGaN/GaN quantum wells

For the first time, using a complex of admittance spectroscopy, light-emitting heterostructures with InGaN/GaN multiple quantum wells were studied in a wide temperature range of 6–300 K. Three peaks are found in the conductance spectra; these peaks correspond to emission of charge carriers from the quantum wells and point defects distributed in the semiconductor bulk. Two low-temperature peaks possess an anomalous behavior, specifically, the peak with a low value of apparent activation energy (17 meV) is shifted to higher temperatures compared with the higher-energy peak (30 meV). The latter is attributed to a bulk defect having anomalously large capture cross section σ _n = 1.5 × 10⁻¹¹ cm².     

Investigation of Ga oxide films directly grown on n-type GaN by photoelectrochemical oxidation using He-Cd laser

By using a He-Cd laser in a chemical solution of H₃PO₄ with a pH value of 3.5, Ga oxide films were directly grown on n-type GaN. From the energy-dispersive spectrometer (EDS) measurement and x-ray diffraction (XRD) measurement, the grown Ga oxide film was identified as (104) α-Ga₂O₃ structure. A small amount of phosphors existed and bonded with oxygen on the grown films. The as-grown films were amorphous. From the XRD analysis, it is evident that annealing of the α-Ga₂O₃ films led to a change in the microstructure from an amorphous to a polycrystalline phase. In addition, the as-grown low-density films gradually became dense films during the annealing process. Furthermore, the surface roughness of the annealed films also gradually decreased. Hexagonal pinholes on the grown films were observed. The density of the hexagonal pinholes was similar to the defect density of the n-type GaN. From the cross-sectional transmission electron microscopy (TEM) micrographs, it is evident that the hexagonal pinholes originated from defects in the n-type GaN.     

GaN and AlGaN high-voltage rectifiers grown by metal-organic chemical-vapor deposition

In this paper, we report the study of the electrical characteristics of GaN and AlGaN vertical p-i-n junctions and Schottky rectifiers grown on both sapphire and SiC substrates by metal-organic chemical-vapor deposition. For GaN p-i-n rectifiers grown on SiC with a relatively thin “i” region of 2 μm, a breakdown voltage over 400 V, and forward voltage as low as 4.5 V at 100 A/cm² are exhibited for a 60-μm-diameter device. A GaN Schottky diode with a 2-μm-thick undoped layer exhibits a blocking voltage in excess of ∼230 V at a reverse-leakage current density below 1 mA/cm², and a forward-voltage drop of 3.5 V at a current density of 100 A/cm². It has been found that with the same device structure and process approach, the leakage current of a device grown on a SiC substrate is much lower than a device grown on a sapphire substrate. The use of Mg ion implantation for p-guard rings as planar-edge terminations in mesageometry GaN Schottky rectifiers has also been studied.     

在蓝宝石图形衬底上生长低穿透性位错的GaN薄膜

使用MOCVD外延系统,采用3D-2D生长模式在圆锥图形蓝宝石衬底上生长GaN薄膜。研究发现3D-2D生长模式能够有效的减少GaN薄膜的穿透性位错,其中3D GaN层的生长条件是关键：低V/III比,低温和高生长压力。为了进一步减少TD,3D GaN层的厚度应该与图形衬底上的图形高度接近。当3D GaN层生长结束时,3D GaN层把图形衬底的图形围在其中,具有倾斜的侧壁和（0001）向的上表面,而图形上基本没有沉积物。在接下来的2D生长过程里,GaN沿倾斜侧面快速生长,使得侧面上的穿透性位错产生弯曲,从而减少GaN薄膜的穿透性位错。经过对3D条件的优化,GaN薄膜的穿透性位错降低到1×108cm-2,XRD测试得到的（002）,（102）半宽分别达到211弧秒和219弧秒。     

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.     

Influence of Mg Doping on the Morphological, Optical, and Structural Properties of InGaN/GaN Multiple Quantum Wells

In this report, the influence of magnesium doping on the characteristics of InGaN/GaN multiple quantum wells (MQWs) was investigated by means of atomic force microscopy (AFM), photoluminescence (PL), and X-ray diffraction (XRD). Five-period InGaN/GaN MQWs with different magnesium doping levels were grown by metalorganic chemical vapor deposition. The AFM measurements indicated that magnesium doping led to a smoother surface morphology. The V-defect density was observed to decrease with increasing magnesium doping concentration from ∼10⁹ cm⁻² (no doping) to ∼10⁶ cm⁻² (Cp₂Mg: 0.04 sccm) and further to 0 (Cp₂ Mg: 0.2 sccm). The PL measurements showed that magnesium doping resulted in stronger emission, which can be attributed to the screening of the polarization-induced band bending. XRD revealed that magnesium doping had no measurable effect on the indium composition and growth rate of the MQWs. These results suggest that magnesium doping in MQWs might improve the optical properties of GaN photonic devices.     

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.     

GaN MOS Capacitors and FETs on Plasma-Etched GaN Surfaces

The electrical characteristics of gallium nitride (GaN) metal-oxide-semiconductor (MOS) capacitors and field-effect transistors (FETs) made on as-grown surfaces, dry-etched surfaces using reactive-ion etching (RIE), and wet-etch treated surfaces after the dry etch were measured. Capacitance and conductance techniques were used to obtain the MOS properties for capacitors. Devices with only an RIE plasma dry-etch process have poor yield and noisy capacitance in the low-frequency accumulation region. Those on dry/wet-etch treated samples have more negative ultraviolet (UV) assistant capacitance-voltage (CV) shift, and higher interface-state densities than those on as-grown samples, but have similar surface potential fluctuation. Threshold voltages of 2 V for an as-grown GaN MOSFET and 1 V for a dry/wet-etched MOSFET were measured. Maximum field-effect mobility for long-channel (L _ch = 100 μm) MOSFETs on the as-grown GaN wafer and the dry/wet-etched GaN wafer were obtained as 167 cm² V⁻¹ s⁻¹ and 119 cm² V⁻¹ s⁻¹, respectively. The higher interface trap density and lower field-effect mobility indicate that post-plasma-etch wet etching can only partially remove the damages from RIE.     

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.     

Junction Temperature Measurements and Thermal Modeling of GaInN/GaN Quantum Well Light-Emitting Diodes

We present a junction temperature analysis of GaInN/GaN quantum well (QW) light-emitting diodes (LEDs) grown on sapphire and bulk GaN substrate by micro-Raman spectroscopy. The temperature was measured up to a drive current of 250 mA (357 A/cm²). We find better cooling efficiency in dies grown on GaN substrates with a thermal resistance of 75 K/W. For dies on sapphire substrates we find values as high as 425 K/W. Poor thermal performance in the latter is attributed to the low thermal conductivity of the sapphire. Three-dimensional finite-element simulations show good agreement with the experimental results, validating our thermal model for the design of better cooled structures.     

Electrical transport properties of single GaN and InN nanowires

The transport properties of single GaN and InN nanowires grown by thermal catalytic chemical vapor deposition were measured as a function of temperature, annealing condition (for GaN) and length/square of radius ratio (for InN). The as-grown GaN nanowires were insulating and exhibited n-type conductivity (n ≈ 2×10¹⁷ cm⁻³, mobility of 30 cm²/V s) after annealing at 700°C. A simple fabrication process for GaN nanowire field-effect transistors on Si substrates was employed to measure the temperature dependence of resistance. The transport was dominated by tunneling in these annealed nanowires. InN nanowires showed resistivity on the order of 4×10⁻⁴ Ω cm and the specific contact resistivity for unalloyed Pd/Ti/Pt/Au ohmic contacts was near 1.09×10⁻⁷ Ω cm². For In N nanowires with diameters <100 nm, the total resistance did not increase linearly with length/square of radius ratio but decreased exponentially, presumably due to more pronounced surface effect. The temperature dependence of resistance showed a positive temperature coefficient and a functional form characteristic of metallic conduction in the InN nanowires.     

Effects of TiN Buffer Layer Thickness on GaN Growth

Smooth GaN layers were successfully grown on metallic TiN buffer layers by metalorganic chemical vapor deposition (MOCVD). One important factor in controlling GaN layer smoothness was the TiN layer thickness. We investigated systematically the effects of this thickness, and found an optimal thickness of 5 nm, at which the smallest average grain size (20 nm) and smoothest surface were obtained. The TiN layers increased surface coverage with GaN hexagons at an early stage of GaN growth, indicating that enhancing the GaN nucleation is essential for smooth GaN layer growth, and small grain size and smooth surface are needed to enhance GaN nucleation. Further reduction in TiN layer thickness to 2 nm decreased the surface coverage with GaN hexagons, and a high density of grooves and holes were observed in the surface of the 2-μm-thick GaN layers. Defect structures in the GaN layers grown on the TiN layers were remarkably changed on reduction of TiN layer thickness from 5 nm to 2 nm. GaN growth was found to be sensitive to the TiN layer thickness between 2 nm and 5 nm.     

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.