HVPE GaN Research Articles

Strain Distribution Across HVPE GaN Layer Grown on Large Square-Patterned Template Studied by Micro-Raman Scattering

A self-separated GaN layer was prepared by hydride vapor phase epitaxy on a large-period square network template. The morphology of the surface formed was consistent with the substrate patterning that comprised terrace and concave. The mixed polarity of the surface was studied by wet etching and micro-Raman scattering. Micro-Raman scattering was performed to investigate the strain distribution of the surface on the front and backside, as well as the cross-section. Differences were observed in the Raman phonon peaks such as the A1(LO), E1(TO), E2(high) phonon, and LO phonon–plasmon mode (LPP−)/A1(TO) peaks between the terrace and concave. The strain distribution varied regularly, which was interpreted based on the facet growth mode.

Temperature-related exciton features on the Ga-/N-Faces of a free-standing HVPE GaN

An apparently different effect of excitonic luminescence has been observed on the Ga-/N-faces of a free-standing HVPE GaN. The neutral donor-bound exciton (D20X) emission of the N-face is only dominant at lower temperature as compared with that of the Ga-face. Moreover, the temperature-related ratio of the peak intensity of D20X to XAn=1 (a free exciton) is found to be about 2.8, which is in coincidence with the ratio of the average dislocation density of the N-face to the Ga-face confirmed by Cathodoluminiscence images. These details could provide useful information for the design of GaN-based and related devices.

GaN boules grown by high rate HVPE

AbstractThe most promising approach for GaN substrate fabrication on the base of HVPE consists in the growth of thick GaN crystals and subsequent cutting of these boules into slices followed by conventional surface preparation. This work focusses on a HVPE GaN boule growth process optimized to highest growth rate and the use of a commercially available vertical HVPE reactor from AIXTRON in order to develop a technology of high productivity to enable low‐cost substrates. To this end, GaN crystals up to thicknesses of 6.3 mm have been grown on 2 inch GaN‐on‐sapphire templates produced by MOVPE. The rate in HVPE growth was 450 µm/h and the material properties of the crystals have been evaluated. No degradation of the material quality was found by comparison of structural, optical and transport properties with published data of the state of the art of bulk GaN. Dislocation densities as low as 6x105 cm‐2, high electron mobility of 960 cm2/Vs for a carrier density of 1x1016 cm‐3, low background impurity incorporation of oxygen and silicon, luminescence with well resolved excitonic emission, and a high value of thermal conductivity of 294±44 W/mK were determined. (© 2011 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

The nucleation of HCl and Cl2‐based HVPE GaN on mis‐oriented sapphire substrates

AbstractThe nucleation of both classic HCl‐based and novel Cl based HVPE GaN on mis‐oriented sapphire substrates was investigated. The use of Cl2in HVPE increases the growth rate by a factor of 4‐5 and strongly reduces the parasitic deposition, allowing for the growth of much thicker wafers than HCl‐based HVPE. Morphological SEM surface studies of the HCl‐based HVPE sample surface show that at 600 °C a nanocrystalline layer is deposited on the sapphire. During the subsequent annealing phase, the morphology changes to a μm‐sized island structure. During overgrowth at 1080 °C, the islands coalesce. Small voids or pinholes are then formed in between the coalescing GaN islands. These pinholes lead to numerous pits on the surface of the GaN at thicknesses of 5 μm. The pits disappear during continued overgrowth and can no longer be found on the surface, when the GaN film reaches a thickness of 45 μm. This particular coalescence mechanism also applies to Cl2‐based HVPE GaN on sapphire (© 2010 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Enhanced growth rates and reduced parasitic deposition by the substitution of Cl 2 for HCl in GaN HVPE

The main limitation in the application of hydride vapor phase epitaxy for the large scale production of thick free-standing GaN substrates is the so-called parasitic deposition, which limits the growth time and wafer thickness by blocking the gallium precursor inlet. By utilizing Cl 2 instead of the usual HCl gas for the production of the gallium chlorine precursor, we found a rapid increase in growth rate from ∼80 to ∼400 μm/h for an equally large flow of 25 sccm. This allowed us to grow, without any additional optimization, 1.2 mm thick high quality GaN wafers, which spontaneously lifted off from their 0.3° mis-oriented GaN on sapphire HCl-based HVPE templates. These layers exhibited clear transparencies, indicating a high purity, dislocation densities in the order of 10 6 cm −2, and narrow rocking curve XRD FWHMs of 54 and 166 arcsec in for the 0002 and 101−5 directions, respectively. In view of these findings, the possibility of depositing Cl2-based HVPE GaN directly on sapphire via the standard low temperature nucleation-high temperature overgrowth process of classic HCl-based HVPE was investigated. By varying the Cl 2 flow during the 3 min long 615 °C nucleation phase, the nucleation conditions yielding smooth 0.7∼mm thick GaN on sapphire films were determined. The nucleation and coalescence mechanism of Cl 2-based HVPE GaN was found to proceed similar to that of HCl-based HVPE.

Optical nonlinearities and carrier dynamics in semi‐insulating crystals

AbstractWe analyze conditions when the deep‐impurity governed carrier generation and transport in semi‐insulating crystals create the coexisting free carrier, photorefractive, and absorptive nonlinearities. By monitoring the nonequilibrium carrier and space charge field dynamics via the probe beam diffraction on transient gratings, number of electrical parameters in bulk semi‐insulating crystal have been determined, as carrier lifetime, diffusion coefficient, diffusion length, and sign of carriers, photogenerated from deep traps. We show a novel possibility to record a transient reflection grating via spatial modulation of the deep trap occupation and applicability of this technique to evaluate the compensation ratio of these traps as well as their recombination properties. Experimentally measured diffraction kinetics on free carrier, photorefractive and absorptive gratings in few nanosecond time interval provided the values of diffusion coefficient and carrier lifetime in bulk CdTe and ZnTe crystals, doped by V, Ge, Al, Sc as well in InP:Fe. Carrier transport peculiarities in these crystals revealed the type (n or p) of dominant carriers, generated from the deep impurity levels. In n‐type HVPE GaN crystals, we observed impact of light‐induced space‐charge field to carrier diffusion. The transient reflection gratings in semi‐insulating CdTe:V allowed discrimination of deep trap governed carrier recombination rate from the other recombination channels. (© 2009 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

On the nucleation, coalescence, and overgrowth of HVPE GaN on misoriented sapphire substrates and the origin of pinholes

The nucleation of HVPE GaN on misoriented sapphire substrates and the transition from the nucleation layer to an epitaxial film were investigated. After a KOH/NaOH eutectic etch of the approximately 45 μ m thick GaN layer, grown on sapphire using a low temperature nucleation, high temperature epitaxy process, the cross-sections revealed columnar structures, up to roughly 1 μ m above the sapphire substrate. Photoetching of the thick GaN layers revealed inhomogeneous defect distributions along the cross-sections, which appeared to be related to the numerous pinholes originating at the GaN/sapphire interface. We present a model explaining the formation of pinholes by the coalescence of the GaN nuclei during the epitaxial overgrowth.

Atomic and electronic structure of the nonpolarGaN(11¯00)surface

We present a cross-section scanning tunneling microscopy (STM), scanning tunneling spectroscopy (STS) and ab initio density-functional theory simulations study of the cleaved nonpolar $(1\overline{1}00)$ surface ($m$-plane) of $n$-type HVPE GaN free-standing quasisubstrates. Atomically resolved empty and filled states STM topographies show that no reconstruction occurs upon cleavage, as predicted by theory. STS measurements on clean and atomically flat cleaved surfaces (defect concentration ${\ensuremath{\sigma}}_{d}\ensuremath{\le}2\ifmmode\times\else\texttimes\fi{}{10}^{12}\text{ }{\text{cm}}^{\ensuremath{-}2}$) show that the Fermi energy is not pinned and the tunneling current flows through Ga-like electronic states lying outside the fundamental band gap. On surface areas with defect concentration ${\ensuremath{\sigma}}_{d}\ensuremath{\ge}3\ifmmode\times\else\texttimes\fi{}{10}^{13}\text{ }{\text{cm}}^{\ensuremath{-}2}$, the Fermi energy is pinned inside the band gap in defect-derived surface states and tunneling through filled (empty) N-like (Ga-like) states takes place.

Polar and nonpolar HVPE GaN substrates: impact of doping on the structural, electrical and optical characteristics

AbstractPolar and nonpolar bulk GaN substrates with low defect density and high structural and optical quality are demonstrated. The effect of doping by silicon, oxygen and iron within moderate doping levels on the properties of the polar GaN substrates was found uncompromised, as confirmed by high resolution X‐ray diffraction and low temperature photoluminescence spectroscopy. In contrast, the lattice parameters were affected significantly, which has to be considered in the subsequent homoepitaxial device growth. The boule growth and respectively the nonpolar substrate homogeneity were found to be hampered by the doping, due to surface microcracking and higher impurity incorporation, while n‐type undoped nonpolar substrates were demonstrated of superior quality. (© 2009 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Optical Characterization of Defect-Related Carrier Recombination and Transport Features in GaN Substrates and CVD Diamonds

Defect related carrier recombination and transport properties have been investigated in differently doped HVPE GaN substrates and CVD diamond layers. Carrier generation by interband transitions or by deep-defect photoexcitation were realized for studies of GaN samples by using picosecond pulses at 351 nm or 527 nm. This allowed to create favorable conditions for radiative and nonradiative recombination in the crystals and reveal peculiarities of photoelectrical properties of high and low density plasma in undoped, doped, and compensated GaN. In CVD diamonds, carrier diffusion length was found equal to ~ 0.5 μm and non-dependent on nitrogen density, while the carrier lifetime varied from 0.2 to 0.6 ns.

The structure and dynamics of GaN(0001) surface during HVPE GaN growth –ab initiostudy

The structure and dynamics of GaN(0001) surface during HVPE GaN growth –<i>ab initio</i>study

High rate photoelectrochemical etching of GaN and the use of patterned substrates for HVPE regrowth

High etching rates of HVPE GaN, up to 2 μm/min, have been achieved by photoelectrochemical etching to produce stripes which are 70–100 μm high. These wafers have been used as substrates for subsequent HVPE growth. The surface after the second growth was irregular and needed to be polished prior to defect density evaluation. The dislocation densities measured by the defect-selective etching were 2×10 7 cm −2 and 3×10 5 cm −2 in the stripe region and the low-dislocation-density lateral growth region, respectively. The low-dislocation-density regions formed stripes of about 10 μm.

Circular Transmission Line Model (CTLM) Analysis for Non-Linear VI Characteristics on Mg doped GaN

In this paper, we present a model that can be used to determine semiconductor material parameters for metal-semiconductor contacts with non-linear voltage-current (V-I) characteristics. The model uses the Circular Transmission Line Model (CTLM) approach to calculate sheet resistance (RSH) and resistivity (ρ) of Mg doped HVPE and MBE GaN epilayers giving values close to those achieved by Hall measurements.

Identification of subsurface damage in freestanding HVPE GaN substrates and its influence on epitaxial growth of GaN epilayers

Freestanding hydride vapor phase epitaxy grown GaN substrates subjected to various polishing methods were characterized for their surface and subsurface conditions. Although different polishing processes led to very smooth surfaces as seen in atomic force microscopy and optical microscopy, they resulted in drastically different optical properties measured by photoluminescence (PL) and cathodoluminescence (CL). PL measurements showed a large increase in defect luminescence. There was also subsurface damage evidenced in CL imaging for one polishing process. Morphological and optical characteristics of an overgrown GaN thin film indicate great differences in the initial nucleation on the substrates subjected to different polishing processes even though the starting surface roughness was similar. This difference continued for the growth of a 405 nm light emitting diode (LED) structure. A PL lifetime reduction by factors of two and four, respectively, was seen for the band edge peak of the GaN layer and the quantum well peak of the LED structure as measured by time-resolved PL. This variability in device performance emphasizes the need for a greater understanding of the effects of starting surfaces, and subsurface characteristics, on the epitaxial growth.

Cathodoluminescence and TEM studies of HVPE GaN layers grown on porous SiC substrates

Plain-view bright-field transmission electron microscopy and cathodoluminescence are used to study the defect structure of GaN films grown by hydride vapor-phase epitaxy on porous and nonporous SiC substrates. It is shown that the use of porous substrate reduces the mosaic structure of the films. This finding supports the compliance of porous SiC substrates, which was proposed by the authors earlier.

HVPE GaN growth on porous SiC with closed surface porosity

Crystal quality of GaN films grown by hydride vapor-phase epitaxy on porous and non-porous SiC substrates is studied. The effect of reduction of threading dislocation density in the films grown on porous substrates is discussed. We suggest that at the early stages of the growth, the porous substrate is capable to re-distribute the mismatch strain in the heterostructure. This results in specific mechanism of strain relaxation in the epitaxial film, which occurs through self-organization of a ‘superlattice’ of planar defects. At the later stages of the growth, these defects hinder the propagation of threading dislocations.

Development of Homoepitaxially Grown GaN Thin Film Layers on Freestanding Bulk m-plane Substrates by Metalorganic Chemical Vapor Deposition (MOCVD)

AbstractHigh quality homoepitaxial growth of m-plane GaN films on freestanding m-plane HVPE GaN substrates has been performed using metalorganic chemical vapor deposition. For this a large growth space was studied. Large areas of no-nucleation along with presence of high density of defects were observed when layers were grown under growth conditions for c-plane GaN. It is believed that these structural defects were in large part due to the low lateral growth rates as well as unequal lateral growth rates in a- and c- crystallographic directions. To achieve high quality, fully coalesced epitaxial layers, growth conditions were optimized with respect to growth temperature, V/III ratios and reactor pressure. Higher growth temperatures led to smoother surfaces due to increased surface diffusion of adatoms. Overall, growth at higher temperature and lower V/III ratio decreased the surface roughness and resulted in better optical properties as observed by photoluminescence. Although optimization resulted in highly smooth layers, some macroscopic defects were still observed on the epi-surface as a result of contamination and subsurface damage remaining on bulk substrates possibly due to polishing. Addition of a step involving annealing of the bulk substrate under H2: N2 environment, prior to growth, drastically reduced such macroscopic defects.

DLTS study of n-type GaN grown by MOCVD on GaN substrates

Abstract Electron traps in n-type GaN layers grown homoepitaxially by MOCVD on free-standing GaN substrates have been characterized using DLTS for vertical Schottky diodes. Two free-standing HVPE GaN substrates (A and B), obtained from two different sources, are used. The Si-doped GaN layers with the thickness of 5 μm are grown on an area of 0.9×0.9 cm2 of substrate A and on an area of 1×1 cm2 of substrate B. Two traps labeled B1 ( E c – 0.23 eV ) and B2 ( E c – 0.58 eV ) are observed with trap B2 dominant in GaN on both substrates. There exist no dislocation-related traps which have been previously observed in MOCVD GaN on sapphire. This might be correlated to the reduction in dislocation density due to the homoepitaxial growth. However, it is found that there is a large variation, more than an order of magnitude, in trap B2 concentration and that the B2 spatial distributions are different between the two substrates used.

Thermal conductivity, dislocation density and GaN device design

Abstract The performance of high power transistor devices is intimately connected to the substrate thermal conductivity. In this study, the relationship between thermal conductivity and dislocation density is examined using the 3 omega technique and free standing HVPE GaN substrates. Dislocation density is measured using imaging cathodoluminescence. In a low dislocation density regime below 105 cm−2, the thermal conductivity appears to plateau out near 230 W/K m and can be altered by the presence of isotopic defects and point defects. For high dislocation densities the thermal conductivity is severely degraded due to phonon scattering from dislocations. These results are applied to the design of homoepitaxially and heteroepitaxially grown HEMT devices and the efficiency of heat extraction and the influence of lateral heat spreading on device performance are compared.

Bending in HVPE GaN free‐standing films: effects of laser lift‐off, polishing and high‐pressure annealing

AbstractWe have studied the effects of laser lift‐off and polishing processes on the bending of free‐standing HVPE grown GaN thick films. Their structural characteristics were accessed by reciprocal space mapping and lattice parameters measurements as well as by Raman scattering and photoluminescence. The in‐plane strain difference between the two faces was found to have determining effect on the bending of the free‐standing films. Removing the high‐defect‐density near‐interface region either by melting caused by laser lift‐off, or by polishing, or by point defects dissociation caused by high‐pressure annealing was found to lead to flattening of the strain distribution along the film thickness and a significant reduction of the bending of the free‐standing films. (© 2006 WILEY‐VCH Verlag GmbH &amp; Co. KGaA, Weinheim)