Characteristics comparison between GaN epilayers grown on patterned and unpatterned sapphire substrate (0 0 0 1)

We investigated the effects of SiN_x interlayers on the structural and electrical properties of nonpolar <i>a</i>-plane (11-20) GaN grown on <i>r</i>-plane (1-102) sapphire substrates by metal–organic chemical vapor deposition (MOCVD). The Nomarski optical microscope images showed that the deposition conditions of the SiN_x layer could strongly affect the <i>a</i>-plane GaN surface morphology due to the different SiN_x coverage. Basal-plane stacking faults (BSFs) and threading dislocation (TD) densities were reduced in the <i>a</i>-plane GaN samples with high SiN_x coverage and multiple SiN_x-treated GaN interlayers. These results indicate that TD reduction is associated with an increase in the 3D growth step and with the blocking of TD propagation. From on-axis (11-20) X-ray rocking curve (XRC) measurements, the anisotropy of full width at half maximum (FWHM) can be attributed to the crystal mosaicity due to insertion of different SiN_x interlayers. The anisotropy of sheet resistance between the <i>c</i>-and <i>m</i>-axis was also clearly seen in a-plane GaN samples with a high density of defects, which was attributed to the BSFs as scattering centers.

Interfacial engineering for semi-insulating GaN/sapphire template with low dislocation density

The effect of indium surfactant on the stress in GaN films grown on SiC at 950 °C by metalorganic chemical vapor deposition was investigated using a combination of in situ wafer curvature measurements and ex situ high resolution x-ray diffraction (HRXRD). As the molar flow rate of trimethylindium was varied from 0 to 4.5 μmol/min during growth, the real-time stress measurements showed that the mean compressive stress of the GaN films decreased from −0.60 to −0.30 GPa. The lattice constants of the GaN epilayers determined by HRXRD confirmed the stress relaxation promoted by the presence of indium while the rocking curve measurements showed that the threading dislocation (TD) density of GaN films remains nearly unchanged. Atomic force microscopy measurements showed that the indium improved step-flow growth, but simultaneously it drove V-defect formation on the GaN surface, which plays a critical role in stress relaxation of GaN films. Cross-sectional transmission electron microscopy revealed the minor contribution of plastic dislocation motion to stress relaxation by localized TD bending toward V-defects. A nucleation and growth model for the V-defect formation was developed to explain that V-defects are energetically favorable to form at TDs under indium-rich conditions. This model shows that the energy barrier for V-defect formation is significantly reduced when indium is present, which leads to the relaxation of misfit strain energy by increasing the size and density of V-defects. Initiation of V-defects and the role of TDs in V-defect formation are discussed based on the presented model.

Influence of the nucleation layer morphology and epilayer structure on the resistivity of GaN films grown on c-plane sapphire by MOCVD

Effects of N +-implanted sapphire (0 0 0 1) substrate on GaN epilayer

Development of analytical model for strained silicon relaxation on (100) fully relaxed Si 0.8Ge 0.2 pseudo-substrates

The existence of high threading dislocation density (TDD) in GaN-based epilayers is a long unsolved problem, which hinders further applications of defect-sensitive GaN-based devices. Multiple-modulation of epitaxial lateral overgrowth (ELOG) is used to achieve high-quality GaN template on a novel serpentine channel patterned sapphire substrate (SCPSS). The dislocation blocking brought by the serpentine channel patterned mask, coupled with repeated dislocation bending, can reduce the dislocation density to a yet-to-be-optimized level of ∼2 × 10(5) to 2 × 10(6) cm(-2). About 80% area utilization rate of GaN with low TDD and stress relaxation is obtained. The periodical variations of dislocation density, optical properties and residual stress in GaN-based epilayers on SCPSS are analyzed. The quantum efficiency of InGaN/GaN multiple quantum wells (MQWs) on it can be increased by 52% compared with the conventional sapphire substrate. The reduced nonradiative recombination centers, the enhanced carrier localization, and the suppressed quantum confined Stark effect, are the main determinants of improved luminous performance in MQWs on SCPSS. This developed ELOG on serpentine shaped mask needs no interruption and regrowth, which can be a promising candidate for the heteroepitaxy of semipolar/nonpolar GaN and GaAs with high quality.

The study of defects and strain in GaN substrates and epilayers for vertical device development using synchrotron X-ray topography techniques is presented. Synchrotron monochromatic beam X-ray topography (SMBXT) studies show that ammonothermal-grown GaN substrate wafers have the lowest dislocation densities while patterned hydride vapor phase epitaxy (HVPE) GaN reveal a starkly heterogeneous distribution of dislocations with large areas containing low threading dislocation densities and HVPE GaN substrates contain high dislocation densities. Epitaxial growth does not nucleate new defects but interactions of dislocations with point defects can cause changes in dislocation configurations. Rocking curve Analysis by Dynamical Simulation (RADS) is shown to be effective in analysis of implantation by correlating the depth profile of strain with the doping profile. Removal of lattice damage induced by implantation requires annealing with proper capping or under high pressures to limit loss of material and lattice distortion. Inductively coupled plasma (ICP) etching may introduce strain in GaN material that can be partially recovered by TBCl etching. Diffusion doping does not introduce new strains in GaN material while neutron transmutation doping at low levels can lead to curing of defects while higher levels lead to irradiation damage and nucleation of new defects.

GaN epilayer was grown on a new polyhedral patterned sapphire substrate (new PSS) by metal–organic chemical vapor deposition. The new PSS was prepared by combining the dry etching technique and wet etching technique. The X-ray diffraction indicated that the full width at half maximum values of (0002) and (\(10\overline{1}2\)) diffraction peaks in the GaN epilayer grown on the new PSS were evidently smaller than that in the GaN epilayer grown on the normal treated PSS. The improvement of GaN quality was attributed to the reduction of threading dislocations (TDs) in GaN epilayer, and the mechanism of the reduction of TDs was analyzed. The influence of the new PSS on the optical properties as well as the residual stress in GaN epilayer was also discussed.

Heterojunction bipolar transistors (HBTs) with GaAs0.825P0.175 bases and collectors and In0.40Ga0.60P emitters were integrated monolithically onto Si substrates. The HBT structures were grown epitaxially on Si via metalorganic chemical vapor deposition, using SiGe compositionally graded buffers to accommodate the lattice mismatch while maintaining threading dislocation density at an acceptable level (∼3 × 106 cm−2). GaAs0.825P0.175 is used as an active material instead of GaAs because of its higher bandgap (increased breakdown voltage) and closer lattice constant to Si. Misfit dislocation density in the active device layers, measured by electron-beam-induced current, was reduced by making iterative changes to the epitaxial structure. This optimized process culminated in a GaAs0.825P0.175/In0.40Ga0.60P HBT grown on Si with a DC current gain of 156. By considering the various GaAsP/InGaP HBTs grown on Si substrates alongside several control devices grown on GaAs substrates, a wide range of threading dislocation densities and misfit dislocation densities in the active layers could be correlated with HBT current gain. The effect of threading dislocations on current gain was moderated by the reduction in minority carrier lifetime in the base region, in agreement with existing models for GaAs light-emitting diodes and photovoltaic cells. Current gain was shown to be extremely sensitive to misfit dislocations in the active layers of the HBT—much more sensitive than to threading dislocations. We develop a model for this relationship where increased base current is mediated by Fermi level pinning near misfit dislocations.

It is widely believed that the lack of high-quality GaN wafers severely hinders the progress in GaN-based devices, especially for defect-sensitive devices. Here, low-cost AlN buffer layers were sputtered on cone-shaped patterned sapphire substrates (PSSs) to obtain high-quality GaN epilayers. Without any mask or regrowth, facet-controlled epitaxial lateral overgrowth was realized by metal-organic chemical vapor deposition. The uniform coating of the sputtered AlN buffer layer and the optimized multiple modulation guaranteed high growth selectivity and uniformity of the GaN epilayer. As a result, an extremely smooth surface was achieved with an average roughness of 0.17 nm over 3 × 3 μm2. It was found that the sputtered AlN buffer layer could significantly suppress dislocations on the cones. Moreover, the optimized three-dimensional growth process could effectively promote dislocation bending. Therefore, the threading dislocation density (TDD) of the GaN epilayer was reduced to 4.6 × 107 cm-2, which is about an order of magnitude lower than the case of two-step GaN on the PSS. In addition, contamination and crack in the light-emitting diode fabricated on the obtained GaN were also effectively suppressed by using the sputtered AlN buffer layer. All of these advantages led to a high output power of 116 mW at 500 mA with an emission wavelength of 375 nm. This simple, yet effective growth technique is believed to have great application prospects in high-performance TDD-sensitive optoelectronic and electronic devices.

GaN and its related alloys have emerged as eminently useful materials for the fabrication of green-to-ultraviolet light-emitting diodes (LEDs), laser diodes (LD) and also high-power, high-efficiency transistor devices due to their widely tunable bandgap. The key technological challenge in the manufacturing of these high performance optoelectronic and electronic devices is the absence of GaN substrates of practical size. As a result, all epitaxial GaN films are grown on the top of a substrate with different lattice constant, such as sapphire, Si and SiC, leading to the inevitable introduction of dislocations and residual strain in the epilayers, which is known to degrade the performance and reliability of the devices. Moreover, the popularly used sapphire substrate is an insulator with low thermal conductivity and difficult to cleave beneath the GaN based LD wafers. So there is a high demand for both low dislocation GaN epilayers and freestanding substrate. In this work, we present two approaches to meet this demand. (1). The Innovative One-step Lateral Epitaxy with Serpentine Mask Structures. There are two key features to this technology, one being a unique mask structure that blocks threading dislocations using amorphous films, and the other being associated with the controlled selective nucleation directly on sapphire surface. The direction of the growth front of GaN makes two 90° turns before emerging from the serpentine channel. The growth direction at the beginning is along the substrate surface norm that subsequently turns to become parallel to the surface and finally turns towards the surface norm again when emerging from the serpentine channel. It achieves the blockage of threading dislocations from reaching the active device region. The second key feature is the selective nucleation of GaN on sapphire surfaces but not on the amorphous mask surfaces, which is the enabling factor that allows for the entire fabrication process being carried out with only one single epitaxial step, leading to significant reduction in the fabrication cost. The microstructural and optical properties of the GaN epilayer grown on serpentine masked structures will be described. Furthermore, we will summarize that this novel technique is a promising candidate for the growth of high quality III-nitride and the subsequent high-performance device fabrication including high brightness LED, laser diodes, and high-power, high-efficiency transistors. (2) Free-Standing (FS) GaN Substrate by HVPE. Recently GaN substrate has been developed to improve qualities of III-nitride devices, thermal conductivity, dislocation density, thermal and lattice mismatch, etc. due to home-epitaxial growth through various growth methods including ammonothermal, High-pressure solution growth, Na-flux and Hydride vapor phase epitaxy (HVPE). In spite of advantage of HVPE method such as high growth rate, capability of wide area wafer and already commercialized growth methods for GaN substrate with comparing other growth methods, the problems of large dislocation density, parasitic nucleation and stress controls in HVPE- GaN substrate were still remained. We will provide one simple HVPE growth method, named as the pulsed NH3-flow modulation method, to realize a crack-free GaN thick layer with high crystal quality and nearly no residual strain. A few simple self-separation methods are developed for removal of hetero-substrates to obtain FS-GaN. 2-inch FS-GaN substrates are obtained with high quality. The FWHM values are about 108 and 81 arcsec for (002) and (102) reflections. The corresponding dislocation density of the freestanding GaN is low to 3-7×106 cm-2. We will also discuss self-separation mechanisms for FS-GaN substrates grown by HVPE.

Time-resolved photoluminescence spectroscopy in GaN-based semiconductors with micron spatial resolution

In-doped GaN thin films were grown by metalorganic chemical vapor deposition (MOCVD) with different trimethylindium (TMIn) flow rates and those structural and electrical characteristics were investigated by X-ray diffraction (XRD), atomic force microscopy (AFM), Hall effect measurement and deep level transient spectroscopy (DLTS). The full width at half maximum (FWHM) of X-ray rocking curves and the pit density, which is estimated by AFM images, decrease with increasing TMIn flow rate. It seems that In doping has definite effects of reducing dislocation density in In-doped GaN films. Hall mobility increases with increasing TMIn flow rate. As an explanation, from an analysis of temperature-dependent Hall effect measurements based on a two-layer mixed conduction model, the mobility contribution of the epilayer compared to that of the interface layer between GaN and sapphire is increased due to reduction of dislocation density in In-doped GaN thin films. It is also observed that the reduction of deep level E2 is related to In-doping.

High-quality GaN epilayers were grown on patterned Si(111) substrates by metalorganic chemical vapor deposition. Crack-free GaN epilayers were obtained by growing the GaN on the plateau of the patterned Si(111) substrate. A high-temperature AlN buffer layer improved the crystalline quality of the GaN epilayer. When the growth temperature of the AlN buffer layer was 1000 °C, the GaN epitaxial layer showed a smooth surface and high crystalline quality. The dislocation density of the subsequently grown GaN layer was 2×109 cm-2, which is comparable to those of conventional GaN epilayers grown on sapphire substrate.