High-temperature Vapor Phase Epitaxy Research Articles

Growth and impact of intrinsic interlayers in high temperature vapor phase epitaxy of GaN

Structural quality of GaN layers grown on foreign substrates is usually improved by applying masks or interlayers, which interact with the crystal structure defects, mainly with threading dislocations, and consequently reduce their density. In this work, GaN interlayers with intrinsic defect structures were employed as barriers impeding the propagation of threading dislocations. The GaN samples were produced in a multistage deposition process by high-temperature vapor phase epitaxy (HTVPE). They consisted of GaN seed layers deposited on 15 × 15 mm2 (001)-oriented sapphire substrates, structured GaN interlayers and GaN top layers. The structuring of the interlayers was facilitated by changing the substrate temperature and the V/III ratio during the growth. The optical and scanning electron microscopies revealed that the structured interlayers contain pyramidal facets, vertical trenches and elongated voids. The results of X-ray diffraction and Raman spectroscopy have shown that these microstructure features are able to reduce the density of threading dislocations in the top layer and the tensile residual stress that develops in the top layer during the deposition process. These results demonstrate the potential of the intrinsic interlayer concept for the in-line defect and stress engineering in HTVPE GaN layer stacks.

Influence of sapphire substrate with miscut angles on hexagonal boron nitride films grown by halide vapor phase epitaxy

(002) hexagonal boron nitride (h-BN) micron films were epitaxially grown on c-plane sapphire substrates with miscut angles to m-plane (m-miscut angle) by high-temperature and low-pressure halide vapor phase epitaxy.

Structure and Optical Properties of AlN Crystals Grown by Metal Nitride Vapor Phase Epitaxy with Different V/III Ratios.

The epitaxial aluminum nitride (AlN) crystals were grown on c-plane sapphire using high-temperature metal nitride vapor phase epitaxy at the source materials’ different molar flow ratios (V/III ratios). The effects of various V/III ratios on the surface morphology, crystalline quality, material straining, and optical properties of heteroepitaxial AlN thin films were studied using X-ray diffraction, scanning electron microscopy, Raman spectroscopy, and photoluminescence (PL). With the increase in the V/III ratio from 1473 to 7367, the substrate surface underwent changes that vary from whiskers to three-dimensional island structures, two-dimensional layered stack structures, and stacked sheet structures. Additionally, due to the presence of nanoscale pits on the substrate surface, almost all samples were tensile stressers. The PL spectra demonstrated the defect luminescence of the epitaxial films, indicating that nitrogen vacancies and oxygen impurities were the samples’ main defects.

Hexagonal boron nitride film on sapphire substrate grown by low-pressure and high-temperature halide vapor phase epitaxy

(002) hexagonal boron nitride (h-BN) films are epitaxially grown on c-plane sapphire substrates by low-pressure and high-temperature halide vapor phase epitaxy, using boron trichloride and ammonia as the boron source and nitrogen source. The influence of the gas phase composition has been investigated to improve the crystalline quality. For the deposition at a temperature of 1650 °C, the optimal nitrogen to boron ratio is identified as 6 and the corresponding deposition rate is around 26 nm/min. Under this condition, the c-lattice parameter of the h-BN layer is 6.66 Å. The h-BN film exhibits high transmittance larger than 70% in the visible range and its optical bandgap is determined to be 5.2 eV.

Growth of Semi-Polar (101¯3) AlN Film on M-Plane Sapphire with High-Temperature Nitridation by HVPE.

Aluminum nitride (AlN) films were grown on the m-plane sapphire by high-temperature hydride vapor phase epitaxy (HVPE). The effect of high-temperature nitridation on the quality of AlN film was studied. The high-temperature nitridation is favorable for the formation of semi-polar single (103) orientation AlN film, the quality of which shows strong dependence on the nitridation temperature. The full width at half maximum of X-ray diffraction for (103) AlN film was only 0.343° at the optimum nitridation temperature of 1300 °C. It is found that the nano-holes were formed on the surface of substrates by the decomposition of sapphire in the process of high-temperature nitridation, which is closely related to the quality improvement of AlN. At the critical nitridation temperature of 1300 °C, the average size of the nano-holes is about 70 nm, which is in favor of promoting the rapid coalescence of AlN micro-grains in the early stages. However, the size of nano-holes will be enlarged with the further increase of nitridation temperature, which begins to play a negative role in the coalescence of AlN grains. As a result, the grain size will be increased and extended to the epilayer, leading to the deterioration of the AlN film.

Effect of flux rate on AlN epilayers grown by hydride vapor phase epitaxy

The surface morphology and crystalline quality of AlN epilayers grown by high-temperature hydride vapor phase epitaxy (HVPE) with various growth rate were investigated by AFM, XRD and cross-sectional TEM analyses. The growth rate of AlN films were controlled by regulating the flux rates of HCl and NH3 while keeping V/III ratio constant. The surface morphology of as-grown AlN films revealed that the control of the appropriate growth rate is beneficial to obtain uniform step-flow morphology. And the crystalline quality of AlN layers has been improved at an appropriate growth rate, since the growth mode of AlN under such condition is beneficial to the bending of dislocations. The dislocation bending and reduction mechanism has also been demonstrated.

Influence comparison of N2 and NH3 nitrogen sources on AlN films grown by halide vapor phase epitaxy**Project supported by the National Key R&D Program of China (Grant No. 2017YFB0404100).

A comparison of the nitrogen sources (N2 and NH3) influence on AlN films grown by high-temperature halide vapor phase epitaxy (HVPE) is reported. The x-ray rocking curves (XRCs) indicate that the full width at half maximum (FWHM) of (0002) plane for AlN films using N2 as nitrogen source is generally smaller than that using NH3. Optical microscope and atomic force microscope (AFM) results show that it is presently still more difficult to control the crack and surface morphology of AlN films with thicknesses of 5-10 μm using N2 as the nitrogen source compared to that using NH3. Compared with one-step growth, two-step growth strategy has been proved more effective in stress control and reducing the density of threading dislocations for AlN epilayers using N2 as the nitrogen source. These investigations reveal that using N2 as nitrogen source in HVPE growth of AlN is immature at present, but exhibits great potential.

Single-phase high-quality semipolar (10–13) AlN epilayers on m-plane (10–10) sapphire substrates

Single-phase flat semipolar (10–13) AlN epilayers on m-plane (10–10) sapphire substrates grown by ammonia-free high-temperature metalorganic vapor phase epitaxy are firstly demonstrated. It is found that twins and basal-plane stacking faults are undetectable by X-ray diffraction and cross-sectional characterization. An X-ray rocking curve (XRC) shows the full widths at half maximum (FWHM) of the (10–13) and the (0002) diffraction peaks from a ∼2.3 μm thick AlN film as narrow as 322 and 373 arcsec, respectively, indicating a high structural quality. Semipolar AlN epilayers hold great promise for high performance deep-ultraviolet (DUV) optoelectronic device applications.

Recent progress of high temperature vapor phase epitaxy for the growth of GaN layers – Controlled coalescence of nucleation layers

Abstract This paper describes recent developments of the high temperature vapor phase epitaxy (HTVPE) for the growth of GaN seed layers on sapphire substrates. The concept of a controlled coalescence of nucleation layers adopted from metal organic vapor phase epitaxy is used for the first time in HTVPE to reduce the density of threading dislocations of the seed layers. This concept implies a delay of coalescence by reducing the ammonia flow and increasing the admixture of hydrogen in the carrier gas. A series of nucleation and seed layers grown under different ammonia flows and hydrogen concentrations is presented. The layers are analyzed by X-ray diffraction as well as by optical and scanning electron microscopy. Microscopic investigations reveal that nucleation layers with large facetted growth islands are formed during the delayed coalescence. The measured rocking curves and estimated dislocation densities from the mosaicity model indicate a significant improvement of the crystal quality of the corresponding seed layers. The lowest dislocation densities are found to be in the range of 4–5 × 108 cm−2, compared with a dislocation density of about 1 × 109 cm−2 in a reference layer grown without an intentional delay of coalescence. A possible explanation is that the existence of large growth islands with extended pyramidal facets causes a preferred bunching and annihilation of dislocations.

Defect-rich GaN interlayer facilitating the annihilation of threading dislocations in polar GaN crystals grown on (0001)-oriented sapphire substrates

The interaction of microstructure defects is regarded as a possible tool for the reduction of the defect density and improvement of the crystal quality. In this study, this general approach is applied to reduce the density of threading dislocations in GaN crystals grown using high-temperature vapor phase epitaxy directly on (0001)-oriented sapphire substrates. The GaN crystals under study were deposited in three steps with different process temperatures, growth rates, and ammonia flows. The first GaN layer accommodates the lattice misfit between sapphire and gallium nitride. Thus, it contains a high number of randomly distributed threading dislocations. The next GaN layer, which is internally structured and defect-rich, bends and bunches these dislocations and facilitates their annihilation. The uppermost GaN layer mainly contains bunched threading dislocations terminating large areas of almost defect-free GaN. In order to be able to visualize and to quantify the microstructure changes in individual parts of the sandwich-like structure, the samples were investigated using nanofocused synchrotron diffraction, confocal micro-Raman spectroscopy, and transmission electron microscopy. The transmission electron microscopy provided information about the kind of microstructure defects and their mutual interaction. The synchrotron diffraction and the micro-Raman spectroscopy revealed the depth profiles of dislocation density and lattice parameters.

Heteroepitaxial growth of GaN on sapphire substrates by high temperature vapor phase epitaxy

This paper presents recent progress in the development of the high temperature vapor phase epitaxy (HTVPE) for a direct deposition of GaN layers on sapphire substrates and the impact of the improved deposition technique on the quality of the HTVPE GaN layers. This technique uses an improved HTVPE reactor for a better control of the growth process. The reactor contains a newly designed Ga evaporation cell made of refractory metals, which significantly reduces the concentration of process-induced impurities. As a result, the impurity concentrations in the HTVPE layers were reduced to or below 1016 cm−3. For a direct deposition of GaN on (0001)-oriented sapphire, a multi-step process was developed that includes the growth of a nucleation layer and a seed layer, which can be used as a template for further overgrowth with an increased growth rate to produce several ten µm thick GaN layers. The properties of the HTVPE layers are characterized by optical and scanning electron microscopy, glow discharge mass spectrometry, Raman spectroscopy, X-ray diffraction, and photoluminescence. The residual stresses and dislocation densities are presented for seed and overgrown layers, and compared with the properties of layers grown by established methods of chemical vapor phase epitaxy. In this context, challenges and prospects of the HTVPE method are discussed.

Microstructure and influence of buffer layer on threading dislocations in (0 0 0 1) AlN/sapphire grown by hydride vapor phase epitaxy

The microstructures of AlN films grown by high-temperature hydride vapor phase epitaxy (HT-HVPE) on (0 0 0 1) sapphires with three different kind of buffer layers (BLs) were investigated by plan-view and cross-sectional transmission electron microscopy (TEM). The buffer layers were grown by HVPE at 1350 °C, MOCVD 1050 °C and HVPE at 1050 °C. The microstructures and in-plane misorientation of buffer layers are related to the crystal quality of the AlN films and dislocation evolution. The misorientation angles and average grain sizes of buffer layers were analyzed quantitatively using Moire fringe method. The buffer layer grown at 1350 °C by HVPE was recognized to be single crystalline, but buffer layers with mosaic-structure are formed at 1050 °C either by MOCVD or HVPE. High growth temperature of 1350 °C significantly reduced the in-plane misorientation of the buffer layer and the dislocation density was greatly reduced. AlN films on high-temperature growth buffer layers (∼1350 °C) have the lowest treading dislocation density ∼3 × 108 cm−2 determined using a combination of X-ray rocking curve measurement and plan-view transmission electron microscopy.

High‐Temperature Ionic Epitaxy of Halide Perovskite Thin Film and the Hidden Carrier Dynamics

High-temperature vapor phase epitaxy (VPE) has been proved ubiquitously powerful in enabling high-performance electro-optic devices in III-V semiconductor field. A typical example is the successful growth of p-type GaN by VPE for blue light-emitting diodes. VPE excels as it controls film defects such as point/interface defects and grain boundary, thanks to its high-temperature processing condition and controllable deposition rate. For the first time, single-crystalline high-temperature VPE halide perovskite thin film has been demonstrated-a unique platform on unveiling previously uncovered carrier dynamics in inorganic halide perovskites. Toward wafer-scale epitaxial and grain boundary-free film is grown with alkali halides as substrates. It is shown the metal alkali halides could be used as universal substrates for VPE growth of perovskite due to their similar material chemistry and lattice constant. With VPE, hot photoluminescence and nanosecond photo-Dember effect are revealed in inorganic halide perovskite. These two phenomena suggest that inorganic halide perovskite could be as compelling as its organic-inorganic counterpart regarding optoelectronic properties and help explain the long carrier lifetime in halide perovskite. The findings suggest a new avenue on developing high-quality large-scale single-crystalline halide perovskite films requiring precise control of defects and morphology.

Modified high temperature vapor phase epitaxy for growth of GaN films

The high temperature vapor phase epitaxy (HTVPE) uses ammonia and thermally evaporated, elemental gallium as precursors for the growth of GaN films. This paper investigates the HTVPE using numerical modeling of the gallium vapor transport and systematical growth experiments. The presented growth setup allows a flexible control of the growth process and variation of growth parameters in a wide range. Based on simulation results, the impact of the gallium melt temperature, the gas flow in the reactor and the reactor pressure on the gallium vapor transport is studied in detail. Experimental growth rates as function of the reactor pressure and the melt temperature are compared with the numerical simulation. The influence of hydrogen in the carrier gas on the properties of the HTVPE GaN layers is discussed.

Effect of the Ammonia Flow on the Formation of Microstructure Defects in GaN Layers Grown by High-Temperature Vapor Phase Epitaxy

High-temperature vapor phase epitaxy (HTVPE) is a physical vapor transport technology for a deposition of gallium nitride (GaN) layers. However, little is known about the influence of the deposition parameters on the microstructure of the layers. In order to fill this gap, the influence of the ammonia (NH3) flow applied during the HTVPE growth on the microstructure of the deposited GaN layers is investigated in this work. Although the HTVPE technology is intended to grow GaN layers on foreign substrates, the GaN layers under study were grown on GaN templates produced by metal organic vapor phase epitaxy in order to be able to separate the growth defects from the defects induced by the lattice misfit between the foreign substrate and the GaN layer. The microstructure of the layers is characterized by means of high-resolution x-ray diffraction (XRD), transmission electron microscopy and photoluminescence. In samples deposited at low ammonia flow, planar defects were detected, along which the nitrogen atoms are found to be substituted by impurity atoms. The interplay between these planar defects and the threading dislocations is discussed. A combination of XRD and micro-Raman spectroscopy reveals the presence of compressive residual stress in the samples.

Studies on high temperature vapor phase epitaxy of GaN

This paper deals with technological development and investigation of the high temperature vapor phase epitaxy (HTVPE) for the growth of GaN films. A novel Ga evaporation cell designed for an optimal local temperature field and gas flow is described. The influence of the reactor pressure and the temperature of the Ga melt on the growth rate in HTVPE is studied. The experiments demonstrated growth rates up to 165µm/h, which could be of potential interest for the deposition of thick GaN layers. Optical properties of the HTVPE layers, as well as typical process impurities are studied by PL, SIMS and GDMS, and discussed in detail. First experiments on a deposition of GaN films on sapphire substrates by HTVPE are presented.

Investigation of GaN layers grown by high temperature vapor phase epitaxy

AbstractFirst experimental results on the growth of GaN films by a modified high temperature vapor phase epitaxy method (HTVPE) are presented. The method bases on a novel growth reactor, which was developed to overcome the drawbacks of conventional HTVPE. GaN films were deposited either on (0001) sapphire substrates or on MOVPE‐grown GaN templates of 15×15 mm2 with growth rates in the range of 2–7 µm/h. The influence of the substrate temperature and ammonia flow on the structural properties of the deposited films was systematically investigated in experiments on MOVPE templates. It is shown that the structural quality of the templates can be well reproduced in the overgrown layers. The layer quality and the stress relations are found to depend very sensitively on the ammonia flow in the carrier gas. Growth experiments with a direct, high‐temperature deposition of GaN films on sapphire demonstrated a strong dependence of the nucleation process on the substrate temperature and the carrier gas composition. The potential of HTVPE for the heteroepitaxial growth of GaN films on sapphire is discussed. (© 2014 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Epitaxial lateral overgrowth of a-AlN layer on patterned a-AlN template by HT-MOVPE

Epitaxial lateral overgrowth (ELO) of a-plane AlN (a-AlN) has been carried out on patterned a-AlN by high-temperature metal-organic vapor phase epitaxy (HT-MOVPE). The a-AlN templates were grown on +0.5° off r-plane sapphire. The ELO a-AlN layers were found to have coalesced well on the a-AlN templates with trenches formed along 〈 1 0 1 ¯ 0 〉 , while the ELO a-AlN layers were not coalesced on the a-AlN templates with trenches formed along 〈 0 0 0 1 〉 . Both the a-AlN template and the coalesced ELO a-AlN layer were investigated by X-ray diffraction (XRD), atomic force microscopy (AFM), scanning electron microscopy (SEM), cathodoluminescence (CL) and transmission electron microscopy (TEM). Consequently, it was found that the ELO technique is useful for improving the crystalline quality of a-AlN layer by HT-MOVPE.

Annihilation mechanism of threading dislocations in AlN grown by growth form modification method using V/III ratio

High-quality AlN layers with atomically flat surface were grown on a c-plane sapphire substrate by high-temperature metal-organic vapor phase epitaxy (HT-MOVPE). Controlling V/III ratio during growth led to change the growth rate for each facet resulting in the change of the macroscopic form of grain at the transition V/III ratio. The threading dislocations were annihilated with the formation of dislocation loops at the changing of grain form. AlN crystallinity was improved due to the reason that small AlN grains were incorporated by the big AlN grains during growth. These phenomena were confirmed by transmission electron microscopy (TEM) and atomic force microscopy (AFM).

Epitaxial lateral overgrowth of AlN on trench-patterned AlN layers

Epitaxial lateral overgrowth (ELO) of high-quality AlN layers on trench-patterned AlN underlying layers has been demonstrated by high-temperature metal-organic vapor phase epitaxy. Under optimized conditions, a high growth rate and an atomically flat surface of thick ELO-AlN without cracks were obtained. Fully coalesced ELO-AlN epilayers were obtained on AlN templates having 〈 1 0 1 ¯ 0 〉 trenches. AlN grown above the groove had low dislocation density due to filtration caused by the grooves. In addition, most of the dislocations threading from the AlN template were annihilated by the formation of a loop structure during growth with increasing thickness of AlN. The average dislocation density of the ELO-AlN was calculated to be 4×10 7 cm −2.