HVPE-grown GaN Research Articles

Fine structure of the red luminescence band in undoped GaN

Many point defects in GaN responsible for broad photoluminescence (PL) bands remain unidentified. Their presence in thick GaN layers grown by hydride vapor phase epitaxy (HVPE) detrimentally affects the material quality and may hinder the use of GaN in high-power electronic devices. One of the main PL bands in HVPE-grown GaN is the red luminescence (RL) band with a maximum at 1.8 eV. We observed the fine structure of this band with a zero-phonon line (ZPL) at 2.36 eV, which may help to identify the related defect. The shift of the ZPL with excitation intensity and the temperature-related transformation of the RL band fine structure indicate that the RL band is caused by transitions from a shallow donor (at low temperature) or from the conduction band (above 50 K) to an unknown deep acceptor having an energy level 1.130 eV above the valence band.

Reduction of Dislocation Density in HVPE-Grown GaN Epilayers by Using In Situ-Etched Porous Templates

In this paper, a method was demonstrated to reduce the dislocation density of GaN film grown by hydride vapor phase epitaxy (HVPE) on an in situ selective hydrogen-etched GaN/sapphire template. The dislocations regions were etched by hydrogen to form cavities. The porous structure was formed on the GaN template grown by metal organic chemical vapor deposition after in situ hydrogen etching. The etching condition was optimized by modulating the etching temperature, pressure, and etching time. Two-step buffer layer growth and high temperature GaN film deposition were carried on the porous template. The growth parameters were optimized to keep the porous structure unfilled. The dislocations originally located in etched cavities could not propagate to the next layer grown by HVPE. Therefore, the dislocation density could be significantly reduced. High crystal quality of GaN is obtained with a low dislocation density. The full width at half-maximum FWHM of (002) is 35 arcs, and the FWHM of (102) is 48 arcs.

Accumulation of Background Impurities in Hydride Vapor Phase Epitaxy Grown GaN Layers

We report on accumulation of background Si and O impurities measured by secondary ion mass spectrometry (SIMS) at the sub-interfaces in undoped, Zn- and Mg-doped multi-layer GaN structures grown by hydride vapor phase epitaxy (HVPE) on sapphire substrates with growth interruptions. The impurities accumulation is attributed to reaction of ammonia with the rector quartz ware during the growth interruptions. Because of this effect, HVPE-grown GaN layers had excessive Si and O concentration on the surface that may hamper forming of ohmic contacts especially in the case of p-type layers and may complicate homo-epitaxial growth of a device structure.

Hardness control for improvement of dislocation reduction in HVPE-grown freestanding GaN substrates

Nano-indentation measurements on freestanding GaN substrates clearly showed, for the first time, that the hardness of the GaN crystal can be controlled by changing the growth conditions for hydride vapor phase epitaxy (HVPE). The hardness of the GaN crystal is probably governed by heterogeneous nucleation of dislocations through a nitrogen vacancy-related mechanism. The observed changes in the nano-indentation hardness can be explained in terms of the dependence on growth condition of the concentration of nitrogen vacancies in the GaN crystal. This control of the crystal hardness has a significant effect on the dislocation-reducing process during the HVPE-growth of freestanding GaN substrates. According to the theory, the threading dislocation density (TDD) should decrease continuously with increasing growth thickness. However, as a result of the accumulation of growth-induced stress, the reduction of TDD for a freestanding GaN substrate with a less-hard crystal stopped at a certain critical thickness and became saturated at around the mid-106/cm2 range. This saturation behavior of TDDs can be overcome by making the GaN crystal harder by changing the HVPE conditions, giving freestanding GaN substrates with extremely low TDDs in the range 105/cm2.

Characterization of dislocation etch pits in HVPE-grown GaN using different wet chemical etching methods

The hydride vapor phase epitaxy (HVPE) grown GaN samples were etched by H 3PO 4, mixed solution of H 2SO 4 and H 3PO 4 (HH) and molten eutectic of KOH–NaOH ( E), respectively. The etching characteristics and surface morphologies were studied by scanning electron microscopy (SEM) and atomic force microscopy (AFM). The density of etch pits obtained by E etching under optimum etching condition was around 3.0 × 10 8 cm −2, which was consistent with the results obtained by the cathodoluminescence (CL) investigation. Two types of etch pits with different sizes were all revealed on the GaN surface using different etching methods. The large etch pits were formed on screw or mixed dislocations, while small etch pits were formed on edge dislocations. The difference in the size of etch pits was interpreted by Cabrera's thermodynamic theory. Prolonging etching time, the morphology changes of etch pits were different by using H 3PO 4, HH and E. The etching rate by H 3PO 4 etching was the fastest and that by E etching was the slowest. According to these etching results, it could be concluded that E etching was a better method for evaluation of dislocations.

Revealing extended defects in HVPE-grown GaN

In this communication we will summarize the results of a complementary study of structural and chemical non-homogeneities that are present in thick HVPE-grown GaN layers. It will be shown that complex extended defects are formed during HVPE growth, and are clearly visible after photo-etching on both Ga-polar surface and on any non-polar cleavage or section planes. Large chemical (electrically active) defects, such as growth striations, overgrown or empty pits (pinholes) and clustered irregular inclusions, are accompanied by non-uniform distribution of crystallographic defects (dislocations). Possible reasons of formation of these complex structures are discussed. The nature of defects revealed by selective etching was subsequently confirmed using TEM, orthodox etching and compared with the CL method. The non-homogeneities were studied in GaN crystals grown in different laboratories showing markedly different morphological characteristics.

Reduction of the dislocation density in HVPE-grown GaN epi-layers by an in situ SiN x treatment

A new method has been approached to reduce the dislocation density in hydride vapor phase epitaxy (HVPE)-grown GaN epi-layers. By this method, state of the art quality, GaN layers were grown on sapphire substrates with an in situ treatment of SiN x in HVPE process. It was found out that the in situ treatment worked most effectively when introduced after the deposition of the nucleation layer (NL). Amorphous SiN x served as a mask allowing only selective growth of GaN seeds as confirmed by scanning electron microscopy (SEM) analysis and thus a reduction of the dislocation density (down to 10 6 cm −2) with one order of magnitude lower compared to the standard samples (grown without the SiN x treatment) was obtained. Transmission electron microscopy (TEM) studies revealed that dislocations with a mixed (both screw and edge component) character mostly thread into the GaN layer. Low-temperature photoluminescence showed good optical quality of the layers grown with optimal position of SiN x treatment.

Reduction of dislocation density in epitaxial GaN layers by overgrowth of defect-related etch pits

GaN templates grown by the metal organic chemical vapor deposition method were etched in a defect-selective molten salts eutectic and were subsequently overgrown by a GaN layer using the hydride vapor phase epitaxy (HVPE) method. Optimized conditions of etching and of HVPE growth processes resulted in a significant reduction of the dislocations density (DD). Local areas virtually free of dislocations were obtained on ∼50% of the surface, while the average DD was reduced from 3×109 cm−2 in the template to about 2×107 cm−2 in the HVPE-grown GaN layer. A model has been developed to explain the mechanism of reduction of the DD during the overgrowth process. The model was confirmed by the photoetching of cleaved layers.

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

The threading dislocation density of hydride vapor phase epitaxy (HVPE)-grown thick GaN layers was measured by high-resolution X-ray diffraction (HR-XRD). Three models were compared, namely mosaic model, Kaganer model and modified Kaganer model. X-ray rocking curves (XRC) of (0 0 0 2), (1 0 1¯ 5), (1 0 1¯ 4), (1 0 1¯ 3), (1 0 1¯ 2), (1 0 1¯ 1) and (1 0 1¯ 0) planes were recorded for quantitative analysis. The screw-, edge-, and mixed-type threading dislocation densities were simulated from the XRD line profile by using the three models. The dislocation density was also measured by atomic force microscopy (AFM), wet chemical etching and cathodoluminescence (CL). The results showed that the Kaganer model was more physically precise and well explained the rocking curve broadening for HVPE-grown high-quality GaN compared with the mosaic model. Assuming a randomly distributed threading dislocation configuration, we modified the Kaganer model. Based on the modified Kaganer model, the edge and screw threading dislocation densities in HVPE-grown GaN thick films ranging from 20 μm up to 700 μm were analyzed. It was shown that screw-type dislocation density decreased more rapidly than edge-type dislocation with increase in film thickness.

Structural defects in GaN crystals grown by HVPE on needle-shaped GaN seeds obtained under high N 2 pressure

A bulk crystal of GaN grown by double hydride vapor phase epitaxy (HVPE) on a needle-shaped GaN single-crystalline seed, obtained by the high nitrogen pressure solution (HNPS) method, was studied by transmission electron microscopy (TEM). Structural defects at characteristic places, such as the seed–HVPE GaN interface, the first HVPE/second HVPE interface, and at morphological imperfections of the seed were characterized. The TEM studies have shown that despite high structural quality of the pressure-grown seed, structural defects are present in the HVPE-grown GaN crystal. A possible reason for strain and structural defects generation could be small lattice mismatch between highly oxygen-doped seed crystal and the HVPE GaN layer as well as between particular sectors of the HVPE GaN layer growing in different crystallographic directions. It was also observed that morphological imperfections of the seed surface may lead to the introduction of the high number of defects extending into the overgrown layer.

Advantages of the time-resolved four-wave mixing technique for studies of non-equilibrium carrier dynamics in bulk semiconductors and structures

We demonstrate versatility of the four-wave mixing technique in investigating non-equilibrium carrier dynamics in semiconductor crystals and epitaxial structures. Measurements of the effective diffusion coefficient in the presence of a light-induced space charge field have allowed us to determine the photoconductivity type of photorefractive CdTe and InP crystals doped with deep impurities. Hole mobility values of μh=(96±22)cm2/Vs in CdTe and μh=(152±16)cm2/Vs in InP have been found from the ambipolar diffusion coefficient. Studies of a number of differently grown GaN layers with dislocation density ranging from 106 to 1010cm−2 have revealed a correlation of the longer carrier lifetimes with the smaller dislocation density. Competition between radiative and non-radiative recombination channels has been observed in low dislocation density HVPE-grown bulk-like GaN.

Orthodox etching of HVPE-grown GaN

Orthodox etching of HVPE-grown GaN in molten eutectic of KOH+NaOH (E etch) and in hot sulfuric and phosphoric acids (HH etch) is discussed in detail. Three size grades of pits are formed by the preferential E etching at the outcrops of threading dislocations on the Ga-polar surface of GaN. Using transmission electron microscopy (TEM) as the calibration tool, it is shown that the largest pits are formed on screw, intermediate on mixed and the smallest on edge dislocations. This sequence of size does not follow the sequence of the Burgers values (and thus the magnitude of the elastic energy) of corresponding dislocations. This discrepancy is explained taking into account the effect of decoration of dislocations, the degree of which is expected to be different depending on the lattice deformation around the dislocations, i.e. on the edge component of the Burgers vector. It is argued that the large scatter of optimal etching temperatures required for revealing all three types of dislocations in HVPE-grown samples from different sources also depends upon the energetic status of dislocations. The role of kinetics for reliability of etching in both etches is discussed and the way of optimization of the etching parameters is shown.

Depth-resolved X-ray determination of surface strain in free-standing films of HVPE-grown GaN and 71Ga NMR characterization

Free-standing GaN films grown by hydride vapor phase epitaxy (HVPE) on c-plane sapphire have been studied for in-plane anisotropic strain. Lattice parameters are obtained from high-resolution X-ray diffraction data and the film quality is determined by measuring the rocking curves and by 71Ga nuclear magnetic resonance (NMR). The in-plane strain was determined using grazing incidence X-ray diffraction and conventional X-ray measurements. It is found that the in-plane lattice parameter varies with depth and has estimated surface strain anisotropy of 4.0791×10-3 up to a thickness of 0.3 μm. The 71Ga NMR experiments reveal different degrees of inhomogeneity amongst the three samples. This is shown by the appearance of an additional broad central-transition peak shifted to higher frequency by a Knight shift from conduction electrons in sample regions having high carrier concentrations.

High quality thin GaN templates grown by hydride vapor phase epitaxy on sapphire substrates

The growth by hydride vapor phase epitaxy (HVPE) of high quality thin GaN layers (d=8μm) on c-plane sapphire substrates with dislocation densities lower than 2×108cm−2 is demonstrated using a two-step process similar to that of metal organic vapor phase epitaxy (MOVPE). Ex situ surface preparation and nucleation layer thickness are shown to be critical factors in achieving these high quality epilayers as they allow controlling the polarity and the dislocation density, respectively. Furthermore, we demonstrate that in situ reflectivity monitoring applied to HVPE is a powerful technique for rapidly optimizing the growth parameters. As a result, thin HVPE-grown GaN layers with state of the art MOVPE GaN quality are obtained as demonstrated through structural and optical characterizations.

Catalyst-free growth of GaN nanowires

We have grown GaN and AlGaN nanowires on Si (111) substrates with gassource molecular beam epitaxy (MBE). No metal catalysts were used. The nanowires displayed a number of interesting materials properties, including room-temperature luminescence intensity greater than that of free-standing HVPE-grown GaN, relaxed lattice parameters, and the tendency of nanowires dispersed in solvents to align in response to electric fields. The wires were well separated, 50–250 nm in diameter, and grew to lengths ranging from 2 µm to 7 µm. Transmission electron microscopy indicated that the wires were free of defects, unlike the surrounding matrix layer.

Etching, Raman and PL study of thick HVPE-grown GaN

HVPE-grown homo-epitaxial 2 mm thick GaN single crystals were examined using different techniques, including orthodox defect-selective etching in molten bases and electroless photo-etching (PEC method) in aqueous KOH solution, micro-Raman spectroscopy and photoluminescence (PL). Examination was performed on the (1 0 −1 0) longitudinal cross-section, which was parallel to the macroscopic [0 0 0 1] growth direction of the crystal. Surface features formed during etching were examined using DIC optical microscopy, scanning electron microscopy (SEM) and atomic force microscopy (AFM). Different types of crystallographic and electrically active defects were recognized and analyzed. These are: individual and aligned dislocations, large areas of remarkably different carrier concentration and local growth striation patterns. Both etching methods revealed the presence of individual dislocations and low-angle grain boundaries with dislocation lines parallel and perpendicular to the [0 0 0 1] growth directions. Photo-etching showed remarkable difference of etching rate and defect type/density in the central part of the GaN crystals and in so-called wings. Very low rate of PEC etching of wings resulted from remarkably increased carrier concentration in this part of the crystal, as was established by micro-Raman spectroscopy. In the wings other peculiar structural features were revealed, namely areas with clear, but randomly oriented growth striation patterns. Micro-Raman measurements showed large local differences in carrier concentration inside and outside these areas. PL examination remained in good agreement with the results of etching and Raman spectroscopy: the spectra taken from the central part of the crystal were typical for low carrier concentration GaN (clear DBA pick at 3.47 eV), while spectra from the wings were very broad, typical for high carrier concentration material.

Structural and optical characterization of GaN epilayers grown on Si(1 1 1) substrates by hydride vapor-phase epitaxy

The GaN epitaxial layer was grown by hydride vapor-phase epitaxy (HVPE) on Si(1 1 1) substrate with AlN buffer layer. Structural and optical properties of GaN layer were characterized by high-resolution X-ray diffraction, transmission electron microscopy (TEM) and photoluminescence (PL) measurements. TEM images showed that the dislocation density in the GaN layer was ∼9×10 9/cm 2. The full-width at half-maximum (FWHM) values of the X-ray rocking curve for GaN (0 0 0 2) and (1 0 1¯ 2) were 13.7 and 35.3 arcmin, respectively. The PL spectra of our sample exhibited a predominant band-edge emission of the wurzite GaN epilayer near 3.36 eV with FWHM=64.2 meV at 50 K and 76.5 meV at room temperature. The experimental data indicated that the HVPE-grown GaN layers exhibited superior quality compared to GaN layers grown by metal organic chemical vapor deposition in prior literatures.

Measurement of second order susceptibilities of GaN and AlGaN

Rotational Maker fringes, scaled with respect to χ11(2) of crystalline quartz, were used to determine the second order susceptibilities χ31(2) and χ33(2) for samples of thin AlxGa1−xN films, a thicker GaN film, and a free-standing GaN platelets. The pump wavelength was 1064nm. The AlxGa1−xN samples, ranging in thickness from roughly 0.5to4.4μm, were grown by metalorganic chemical vapor deposition (MOCVD) and hydride vapor-phase epitaxy (HVPE) on (0001) sapphire substrates. The Al mole fractions x were 0, 0.419, 0.507, 0.618, 0.660, and 0.666, for the MOCVD-grown samples, and x=0, 0.279, 0.363, and 0.593 for the HVPE-grown samples. An additional HVPE-grown GaN sample ∼70μm thick was also examined. The free-standing bulk GaN platelets consisted of an HVPE grown film ∼226μm thick removed from its growth substrate, and a crystal ∼160μm thick grown by high-pressure techniques. For the AlxGa1−xN samples, the magnitudes of χ31(2) and χ33(2) decrease roughly linearly with increasing x and extrapolate to ∼0 for x=1. Furthermore, the constraint expected for a perfect wurtzite structure, namely χ33(2)=−2χ31(2), was seldom observed, and the samples with x=0.660 and x=0.666 showed χ31(2) and χ33(2) having the same sign. These results are consistent with the theoretical studies of nonlinear susceptibilities for AlN and GaN performed by Chen et al. [Appl. Phys. Lett. 66, 1129 (1995)]. The thicker bulk GaN samples displayed a complex superposition of high- and low-frequency Maker fringes due to the multiple-pass interference of the pump and second-harmonic generation beams, and the nonlinear coefficients were approximately consistent with those measured for the thin-film GaN sample.

Depth-dependent optical properties of a HVPE-grown freestanding Si-doped GaN single crystal

Results of the cathodoluminescence (CL) and micro-Raman measurements in a freestanding 300 μm-thick Si-doped GaN single crystal grown by hydride vapor phase epitaxy (HVPE) are presented. The depth-dependent CL results along the c-axis on the cross-section show that the integrated CL intensities of the near-band-gap emission gradually increase with the CL linewidths broadened from the N face to the Ga face. The physical origin of increase in the CL intensity has been studied by the micro-Raman measurements along the c-axis, which revealed the carrier concentration gradually increases from 2.3×10 17 (the N face) to 9.3×10 17 cm −3 (the Ga face). Our experimental evidence indicates that the broadening of the CL linewidth is due to the increased fluctuation of the local bandgap caused by the heavier concentration of the donor impurities.

Micro-Raman scattering profiling studies on HVPE-grown free-standing GaN

Free-standing GaN of ∼330 μm thickness with low defect density was prepared by hydride vapor-phase epitaxy (HVPE) on sapphire in a vertical atmospheric-pressure reactor and a subsequent laser-induced lift-off process. The structural and optical properties of the material were assessed by various characterization techniques, like X-ray diffraction, photo- and cathodoluminescence, spectroscopic ellipsometry, positron annihilation spectroscopy, and transmission electron microscopy. Here, we focus on μ-Raman scattering profiling studies providing the vertical strain distribution and the evolution of the crystalline quality with increasing layer thickness. Profiles of the free-carrier concentration are obtained from monitoring the LO-phonon plasmon coupled mode. Comparative investigations are performed on the material before and after separation of the sapphire substrate. The GaN material presented here is well capable of serving as a substrate for further homoepitaxial strain-relaxed and crack-free growth needed for fabrication of high-quality III-nitride device heterostructures. (© 2004 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)