High-resolution X-ray diffraction analysis on HVPE-grown thick GaN layers

Effect of the V/III ratio during buffer layer growth on the yellow and blue luminescence in undoped GaN epilayer has been studied by means of photoluminescence spectroscopy and high resolution X-ray diffraction. It is found that the densities of screw and edge threading dislocations increase with the V/III ratio of the buffer layer, and the intensities of the yellow luminescence (YL) and blue luminescence (BL) emissions also increase dramatically. However, the density ratio of the edge threading dislocation to the screw threading dislocation remains invariant, as well as the intensity ratio of YL emission to BL emission. It can be concluded from these phenomena that the edge threading dislocation and screw threading dislocation can enhance the YL and BL emissions, respectively.

We report on the reduction of the threading edge dislocation density of Al 0.15 Ga 0.85 N buffer layers for efficient 350nm light-emitting diodes (LEDs). Structures were grown by metalorganic vapor phase epitaxy (MOVPE) on (0001) sapphire substrates using three-dimensional (3D) facetted GaN nucleation islands. The flattening of the overgrowing AlGaN buffer layers could be controlled by choosing appropriate growth conditions resulting in smooth surfaces. High-resolution X-ray diffraction (HRXRD) ω-scans show that a prolonged 3D growth phase leads to a narrowing of the asymmetric diffraction peaks and hence to an effective reduction of the density of edge-type threading dislocations. Photoluminescence (PL) and electroluminescence (EL) measurements show directly the beneficial effect of the improved crystal quality on the optical emission properties. The output power of LED structures grown on an optimized buffer was increased by a factor of 6 compared to structures grown on a two-dimensional (2D) low Al content AlGaN nucleation layer.

ABSTRACTWe have studied the possibilities of reducing the threading dislocation (TD) density in GaAs on Si using free side wall growth on patterned Si substrates and/or using a soft ZnSe interlayer in combination with post growth annealing procedures. TD densities were accurately determined using large area plan view TEM and were found to be unaffected by proximity to free side walls and by the ZnSe interlayer. Post growth heat treatments led to a factor of ∼2 reduction in TD density and to bunching of dislocations throughout the thickness of the film.

Epitaxial lateral overgrowth (ELO) has been used to reduce threading dislocation (TD) densities in GaN. This paper reports transmission and scanning electron microscopy studies of TDs in GaN films grown by ELO on GaN nanocolumns produced either during growth by MBE (method 1), or by self‐organised patterning on a uniform layer grown by MOCVD (method 2). In method 1, isolated nanocolumns grew without TDs, but some TDs formed when nanocolumns coalesced. Although some of these TDs extended through the overlayer, extensive lateral migration of others, depending on the dislocation type, led to dislocation annihilation, lowering the overall TD density. In method 2, a similar process of lateral migration of TDs was observed during ELO, resulting in a continuous GaN overlayer with TD densities less than 108 cm–2. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

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.

We have grown blue light-emitting diodes (LEDs) with low threading dislocation densities (TDDs) by using SiN interlayers on Si (111) substrates. Our growth technique using SiN layers makes it possible to decrease twist components (edge-type threading dislocation components). The edge-type TDDs are almost the same values as those of LEDs grown on Al₂O₃ (0001) substrates. EQE of LEDs grown on Si (111) substrates increases with decreasing edge-type dislocation in the low-current-density region, and the EQE of the sample with low TDD is almost as high as that of the LED grown on an Al₂O₃ (0001) substrate at room temperature. It is found that the hot/cold factors (HCFs) of LEDs grown on Si (111) substrates increase with decreasing edge-type dislocations in the low-current-density region, but are less than those of an LED grown on an Al₂O₃ (0001) substrate. Time-resolved photoluminescence (TRPL) shows that the dominant origin of the thermal quenching is edge-type dislocations in our samples, but other defects such as screw-type dislocations also contribute to it. We also found the fluctuated emission patterns consisting of bright and dark areas originated from the difference of Shockley–Read–Hall (SRH) type defect densities in the multi-quantum wells (MQWs) grown on Si (111) substrates. The bright areas spread, and the configurations of the bright areas change into ring-like patterns with reducing edge-type TDDs. We suggest that the internal quantum efficiency (IQE) of dark areas should be promoted to improve the performance of the MQWs grown on Si (111) substrates.

Free-standing GaN substrates are of great importance for short wavelength Laser Diodes (LDs) and high performance Light Emitting Diodes (LEDs) applications. One of the technical barriers for the applications is relatively high threading dislocation density (TDD) in GaN substrates known as non-radiative centers. In this paper, we analyzed the influence of TDD to Hall effects in Si-doped GaN free-standing substrates grown by Hydride Vapor Phase Epitaxy(HVPE). To verify the role of threading dislocations as deep acceptor level and carrier scattering lines, we compared active carrier concentration and Si doping concentration[nSi] of each GaN substrate with different DD. In addition, we measured O doping concentration[nO] of unintentionally doped(UID) GaN substrate that had similar DD with the Si-doped GaN substrate to consider the influence of doped O known as main background impurity, and compared the Hall characteristics of UID GaN with that of the Si-doped GaN. From the results, it was experimentally postulated that threading dislocations function as trap centers disturbing the movement of n-carriers in GaN substrates. (© 2005 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Low threading dislocation Ge on Si by combining deposition and etching

An m-plane freestanding GaN substrate satisfying both low resistivity (ρ = 8.5 × 10−3 Ω·cm) and a low point-defect concentration, being applicable to vertically conducting power-switching devices, was grown by hydride vapor phase epitaxy on a nearly bowing-free bulk GaN seed wafer synthesized by the ammonothermal method in supercritical ammonia using an acidic mineralizer. Its threading dislocation and basal-plane staking-fault densities were approximately 104 cm−2 and lower than 100 cm−1, respectively. A record-long fast-component photoluminescence lifetime of 2.07 ns at room temperature was obtained for the near-band-edge emission, reflecting a significantly low concentration of nonradiative recombination centers composed of Ga vacancies.

This article reports on the study of microstructure and dislocation of InN films using high resolution x-ray diffraction grown on sapphire (0001) both by metalorganic chemical vapor deposition (MOCVD) and molecular beam epitaxy (MBE). The mosaic tilt, twist, and correlation lengths of InN films are determined by using symmetrical and asymmetrical reflections as well as reciprocal space mapping. Deducing from these results, MBE-grown InN film exhibits the edge-type dislocations of 4.0×109 cm−2, which is about ten times higher than the density of screw-type dislocations. In MOCVD-grown InN sample, the edge-type dislocations density is as high as 2.1×1010 cm−2, and the screw-type dislocations density is 1.3×109 cm−2. They indicate that edge type is the predominant dislocation type in the InN films. By comparing the reported transmission electron microscopy results, the accuracy of evaluation for the dislocation density using the mosaic model is proved.

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.

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.

GaN/sapphire layers have been grown by Metal Organic Vapour Phase Epitaxy (MOVPE). An amorphous silicon nitride layer is deposited using a SiH4/NH3 mixture prior to the growth of the low temperature GaN buffer layer. Such a process induces a 3D nucleation at the early beginning of the growth, resulting in a kind of maskless ELO process with random opening sizes. This produces a significant decrease of the threading dislocation (TD) density compared to the best GaN/sapphire templates. Ultra Low Dislocation density (ULD) GaN layers were obtained with TD density as low as 7×107cm−2 as measured by atomic force microscopy (AFM), cathodoluminescence and transmission electron microscopy (TEM). Time-resolved photoluminescence experiments show that the lifetime of the A free exciton is principally limited by capture onto residual donors, similar to the situation for nearly dislocation-free homoepitaxial layers.

The effect of varying threading dislocation densities on the internal quantum efficiencies (IQEs) of InGaN quantum wells (QWs), with and without intentionally created “V-pits,” is reported here. InGaN QW samples grown on GaN-on-sapphire templates with threading dislocation densities of &amp;lt;1 × 108 and &amp;lt;1 × 109 cm−2 are compared, with and without GaN/InGaN superlattice (SL) layers incorporated to intentionally open up the threading dislocation cores and form large-size “V-pits.” The formation of “V-pits” is confirmed by cross-sectional transmission electron microscopy to initiate from threading dislocations in the SL layers. The densities of the pits are confirmed by plan-view SEM to agree with the substrate threading dislocation densities. The experimental room temperature IQEs of the “V-pit” QW samples are enhanced to 15% ± 1% compared to 6% ± 2% for conventional QW samples. Both conventional and “V-pit” samples show insensitivity to the magnitude of the dislocation densities with respect to IQE performance, while the “V-pit” samples show shifts in the peak emission wavelengths compared to the conventional samples, attributed to strain modulation. This study provides additional understanding of the causes of the observed insensitivity of the IQEs to different threading dislocation densities.

M‐plane non‐polar bulk GaN substrates were regrown by hydride vapour phase epitaxy (HVPE), aiming to enlarge their lateral size. The non‐polar GaN substrates were synthesized by growing thick GaN boules by HVPE along the c‐axis, and wafering transversely to expose non‐polar (m‐ or a‐plane) surfaces. Non‐polar GaN substrates obtained in this manner contains lower defect density comparing to those produced by heteroepitaxial growth. After the regrowth, substantial lateral expansion in [0001], [1120] and [1120] was observed. Threading dislocation densities were estimated by cathodoluminescence measured at the cross‐section areas. The dislocation densities in the substrate and the regrown areas were approximately 5×106 cm–2 and 1×106 cm–2, respectively, indicating a comparable structural quality achieved by the regrowth. Non‐polar GaN substrates prepared by this process provide a promising method to fabricate devices for next generation. (© 2008 WILEY‐VCH Verlag GmbH &amp; Co. KGaA, Weinheim)