Thick GaN Films Research Articles

Strain-reduced GaN thick-film grown by hydride vapor phase epitaxy utilizing dot air-bridged structure

A 300 μm GaN thick-film, in diameter 1.5 in, was demonstrated without any crack by hydride vapor phase epitaxy (HVPE) growth. The technique used in relaxing the residual stress caused by differences of thermal expansion coefficients (TEC) and lattice constants between GaN and sapphire substrate to prevent GaN film from crack is called a dot air-bridged structure. After the laser lift-off process, 300-μm-thick freestanding GaN wafer, in diameter 1.5 in, could be fabricated. The compressive stress in the dot air-bridged structure was measured by micro-Raman spectroscopy with the E 2(high) phonon mode. The compressive stress could be reduced to as small as 0.04 GPa, which could prevent the crack during the epitaxial process for GaN growth by HVPE. It is important to obtain a large-area crack-free GaN thick-film, which can be used for fabricating freestanding GaN wafer.

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.

A Novel Technique for Growing Crack-Free GaN Thick Film by Hydride Vapor Phase Epitaxy

To prevent the cracking of GaN thick films grown on a sapphire substrate by hydride vapor phase epitaxy (HVPE), a novel technique without complex processes is developed. By adding a temperature ramping step in the HVPE GaN epitaxy process, more than 300-µm-thick high-quality crack-free GaN thick films on sapphire substrate can be obtained by this technique. After separation by a conventional laser-induced lift-off process, a 1.5 in. 300 µm freestanding GaN wafer with a dislocation density of approximately 1 ×107 cm-2 could be fabricated without any cracks. No additional designed-patterned or stress-reduced structures were applied in these samples to reduce the dislocation density and thermal stress.

High-Quality Thick GaN Overgrown on an Array of SiO2 Nanomasks by HVPE

We report nanoepitaxy of thick GaN films by hydride vapor-phase epitaxy (HVPE) using a uniform array of nanomasks that were prepared by electron-beam evaporation and anodic aluminum oxide membranes. The controllable size and density of the nanoisland mask were about and , respectively. Subsequent overgrowth takes place selectively on the exposed GaN surface without the masks to the formation of the continuous layer. High-resolution X-ray diffraction shows reduction of threading dislocations in the overgrown layers. Improved optical property and significant stress relaxation in the epilayer are demonstrated by photoluminescence and micro-Raman spectra. The deposition method is a more promising technique for obtaining freestanding GaN substrates with high-quality and uniform features.

Generation of misfit dislocations in highly mismatched GaN/AlN layers

A grid of regularly-distributed misfit dislocation (MD) arrays is observed in GaN films grown on AlN buffer layers by plan-view and cross-sectional transmission electron microscopy (TEM). It is found that the spacing of the MD arrays decreases with increasing the GaN film thickness. Before the formation of grids in very thin GaN films, short MDs originated around and bordered on long dislocations probably in order to achieve an isotropic strain relaxation, which indicate that the MD grid is formed by a progressive introduction of MDs with the GaN growth. Furthermore, high resolution TEM displays that, due to large misfit strain between GaN and AlN, a part of the MDs is produced by interfacial migration of pre-existing threading dislocations (TDs) in AlN layers at the early stage of the GaN growth. The MDs in GaN are thus proposed to be introduced first by interfacial migration of TDs in AlN and then gradually form into a grid around the misfit segments of TDs. This is supported by a good agreement between experimental stain values of the GaN films and a theoretical prediction based on the TD migration.

Triangular Extended Microtunnels in GaN Prepared by Selective Crystallographic Wet Chemical Etching

Extended microtunnels with triangular cross sections in thick GaN films were demonstrated using wet chemical etching on specially designed epitaxial lateral overgrowth structures. For tunnels along the and directions of GaN, the and facets are the etch stop planes with activation energies of 23 kcal/mol determined by wet chemical etching. The axial etching rate of the tunnels in the direction is twice as large than that along the direction. The highest etching rate of the tunnels in the axial direction is .

Comparison between extended microtunnels along different crystal orientations in GaN

AbstractTriangular prism‐like extended microtunnels (EMTs) can be easily formed in specially designed epitaxial lateral overgrowth GaN thick films by selective wet chemical etching in molten potassium hydroxide (KOH). In this study, extended microtunnels along the 〈1$ \bar 1 $00〉 and the 〈1$ \bar 2 $00〉 directions were fabricated and compared. If tunnels along the 〈1$ \bar 1 $00〉 direction, the {11$ {\bar 2} {\bar 2} $} planes would be the etch stop plane. While the tunnels along 〈1$ \bar 2 $00〉 direction, the etch stop plane would become to {10$ {\bar 1} {\bar 1} $} planes. The activation energies of wet chemical etch for the {11$ {\bar 2} {\bar 2} $} planes and the {10$ {\bar 1} {\bar 1} $} planes were determined to be 23.6 kcal/mol (1.02 eV) and 22.8 kcal/mol (0.99 eV) using Arrhenius plot. On the other hand, the etching depths of the tunnels along the 〈1$ \bar 1 $00〉 direction were more than twice the depths of the tunnels along the 〈1$ \bar 2 $00〉 direction during the same etching time. The highest etch rate of the tunnels along the axial direction can reach 1000 μm/hr in 〈1$ \bar 1 $00〉 direction, which is believed to be the highest etch rate of GaN ever reported. Additional crystal facet of m‐plane in GaN was also observed in the sample with extended microtunnels along the 〈1$ \bar 2 $00〉 direction. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Growth of free-standing highly luminescent undoped and Mg-doped GaN thick films with a columnar structure

Undoped and Mg-doped GaN thick films with a columnar structure have been grown on gold-covered fused silica substrates in a three-zone horizontal quartz-tube reactor. The undoped films were grown by a two-step chemical vapor deposition method using gallium and ammonium chloride as starting reagents and ammonia as carrier gas. The Mg-doped films were deposited by the same method, but with an initial Ga–Mg alloying step. These GaN thick films have a wurtzite structure and exhibit strong room-temperature luminescence with the characteristic GaN band-edge, and the Mg-related emissions for the doped films. Their optoelectronic properties are comparable to GaN films grown epitaxially with other more elaborate techniques. This growth technique can be used for inexpensive, large-area electroluminescent devices.

Structural characterization of non-polar (1 1 2 0) and semi-polar (1 1 2 6) GaN films grown on r-plane sapphire

Thick GaN films, with (1 1 2 0) or (1 1 2 6) planes parallel to the r-plane of sapphire, were grown by molecular beam epitaxy using AlN or GaN buffer layers. Characterization by transmission electron microscopy revealed a high density of basal-plane stacking faults (BSFs) in the (1 1 2 0) non-polar GaN ( a-GaN) films. {1 1 2 0} and {1 0 1 0} prismatic-plane and {1 1 0 2} pyramidal-plane stacking faults (SFs) were observed to terminate some BSFs. The SFs on {1 0 1 0} and {1 1 0 2} planes generally formed closed domains. For (1 1 2 6) semi-polar GaN ( s-GaN) films, most threading dislocations were located at small-angle grain boundaries. Many BSFs were observed close to the AlN/GaN interface but, in comparison with a-GaN, the s-GaN films had much lower BSF density at the top surface. Inversion domain boundaries (IDBs) on {1 0 1 0} planes were observed to form closed domains. GaN/AlGaN multiple quantum wells (MQWs) grown on s-GaN or a-GaN followed the morphology of the GaN surface. Some IDBs in the s-GaN propagated through the GaN/AlGaN MQWs to the top surface.

Thickness measurement of GaN films by X-ray diffraction

Precise measurement and control of GaN-film thickness is very important for GaN-based material epitaxy and device fabrication. However, GaN-films heteroepitaxially grown on large-mismatch substrates, such as sapphire, SiC and Si, etc., usually show a mosaic structure, which causes great difficulty to the GaN-film thickness measurement. Combining the advantages of Williamson-Hall plot method and diffraction profile shape analysis method, a new strategy was presented to effectively distinguish the X-ray diffraction broadening factors of finite crystallite size and inhomogeneous strain, which can be used to precisely and reliably determine the thickness of epitaxial films. The thickness of a series of GaN films grown on sapphire substrates in the range of 0.7—4.2 μm were measured by this method. Comparing with the thickness obtained from spectroscopic ellipsometry measurements, the difference was found to be within 4%, which shows the excellent performance of this method.

Effect of sapphire-substrate thickness on the curvature of thick GaN films grown by hydride vapor phase epitaxy

The effect of sapphire-substrate thickness on the curvature and stress in thick hydride vapor phase epitaxial GaN films was studied by high-resolution x-ray diffraction at variable temperatures. The curvature was found to have the maximum value for comparable thicknesses of the film and the substrate, while the stress at the film surface decreases with increasing film thickness and increases with increasing substrate thickness, which is in very good agreement with the simulation results. The curvature at the growth temperature was found to be strongly influenced by the value of the intrinsic tensile strain, which is determined by the film/substrate thickness ratio.

The roles of low-temperature buffer layer for thick GaN growth on sapphire

Thick GaN films were grown on two different low-temperature GaN (LT-GaN)-buffer layers, i.e., one- and two-step LT-GaN buffer, by hydride vapor-phase epitaxy (HVPE). NH 4Cl layer was included in two-step LT-GaN, which was evaluated by X-ray diffraction patterns. Many voids were observed at the interface for both samples. We will discuss the mechanism of void formation for both samples. The full-width at half-maximum (FWHM) values of (0 0 0 2) ω-rocking curves were 390 and 440 arcsec for 200-μm-thick GaN on one- and two-step LT-GaN, respectively. The 200-μm-thick high-temperature GaN (HT-GaN) film on two-step LT-GaN was self-separated without any cracks after cooling down. NH 4Cl layer included into two-step LT-GaN buffer was effectively contributed to the voids formation and self-separation in realizing a stress-free free-standing GaN (FS-GaN) substrate.

Self-separated freestanding GaN using a NH4Cl interlayer

Thick GaN films were grown on void buffer layer by hydride vapor phase epitaxy. The void-buffer layers were consisted of NH4Cl, GaN dots, and low-temperature (LT) GaN buffer layer. Instead of GaN, NH4Cl was easily synthesized in NH3 and HCl atmospheres by simply lowering the growth temperature to 500°C, and LT GaN buffer growth was followed during increasing substrate temperature to 600°C. The LT GaN buffer acted as a protecting layer against evaporation of the NH4Cl and a seeding layer for the high temperature (HT) GaN. The NH4Cl layer between a sapphire substrate with GaN dots and the LT GaN buffer were fully evaporated during the HT GaN growth at 1040°C. Many voids were formed at interface caused by evaporation of the NH4Cl layer, which strongly assisted self-separation of thick HT GaN during cooldown after the growth resulting in a 200μm thick freestanding (FS) GaN. The FS GaN showed smooth surface morphology and absence of any crack. The a-axis and c-axis lattice constants of FS GaN were 3.189 and 5.185Å, respectively, which well agrees with those of strain-free bulk GaN. The observed donor-bound exciton emission peak at 3.4718eV agreed with the peak position of bulk GaN. All these features indicate that the obtained FS GaN through the self-separation process is nearly strain-free.

Impact of V/III ratio on electrical properties of GaN thick films grown by hydride vapor-phase epitaxy

Impact of V/III ratio on electrical properties of GaN thick films are investigated, which are grown by hydride vapor-phase epitaxy. The authors note that the electron concentration of GaN films decreases with the increase of V/III ratio, while their electrical resistivity and electron mobility increase simultaneously. These indicate that enhancing V/III ratio suppresses electron-feeding sources in GaN films, which is not by generating electron-trapping centers but by reducing donor-type defects. On the other hand, it is shown that the linewidth of x-ray rocking curves in GaN films decreases and the near-band edge emission intensity of 10K photoluminescence spectra increases as V/III ratio increases. These mean that higher V/III ratio condition helps for reducing crystalline point defects in GaN films. In terms of theoretical fitting into the temperature-dependence curves of electron mobilities, it is found that the electron transport of GaN films grown in lower V/III ratio condition is more hampered by defect scatterings. Consequently, it is suggested that the generation of donor-type defects in the GaN thick films is more suppressed by higher V/III ratios, which induces lower background electron concentration and higher electron mobility.

Free standing GaN layers with GaN nanorod buffer layer

Thick GaN films were grown by HVPE (Hydride Vapor Phase Epitaxy) on c-sapphire substrates with GaN nano-rod buffer layers. Lateral epitaxial growth mode was adapted to grow GaN thick films on nano-structure buffer. Thick GaN films were self-separated during cooling down by thermal stress caused by the difference of thermal expansion coefficient (TEC) between GaN and sapphire. Since the nano-rod buffer consists of nano-rods and voids, it is mechanically weaker than planar GaN layers and contributes to the self-separation of GaN thick films. 200 μm-thick free-standing GaN substrates show smooth surface morphology without any microcracks. The full width at half maximum (FWHM) of (0002) X-ray rocking curve is 619 [arcsec]. Donor-bound exciton emission is observed at 3.4718 eV in low-temperature photoluminescence spectra, which is nearly the same peak position as bulk GaN. (© 2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Characterization of free‐standing GaN substrates prepared by self lift‐off

Free-standing GaN layers were successfully prepared by self lift-off process. Single crystalline ZnO buffer layer and GaN layer were successively grown on sapphire substrate by plasma assisted molecular beam epitaxy. Thick GaN film was grown on this template substrate for the realization of stress-free free-standing substrate by hydride vapor phase epitaxy. The a-axis and c-axis lattice constants of free-standing GaN were 3.189A and 5.185A, respectively. Peak positions of photoluminescence spectrum were D0X of 3.4715 eV and FXA of 3.4791 eV. These results suggest that the stress-free GaN layers were successfully prepared by self lift-off process. (© 2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Growth of thick a‐plane GaN on r‐plane sapphire by direct synthesis method

Thick film growth of nonpolar a-plane GaN was carried out on r-plane sapphire by direct synthesis method for a substrate of homoepitaxy of a-plane GaN. A 30 μm thick a-plane GaN film was obtained at growth temperature at 1050 °C with a low-temperature buffer layer deposited at 700 °C. The surface was smooth but has some pits. Without a buffer layer, 20 μm thick a-plane GaN with much fewer pits was obtained. The best FWHM of XRD peak in ω-mode obtained was less than 700 arcsec. This film can be used as a substrate for a-plane III-nitrides growth.

Bending in HVPE grown GaN films: origin and reduction possibilities

AbstractWe have studied the effects of film and substrate thicknesses on the bending and strain of thick GaN films grown by HVPE. Both experimental and simulation data, being in a very good agreement, show the highest bending at a critical film thickness slightly smaller than the substrate thickness, while the strain remains decreasing with only a small increase at much higher film thicknesses. Temperature dependent measurements allows a separation of the thermally induced component and an extrapolation of the bending to the growth temperature. The latter was found to be dependent on both the film and substrate thickness and to have a strong effect on the remaining bending of freestanding films. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Growth of highly resistive Ga‐polar GaN by LP‐MOVPE

Electrical characteristics of 1.3 µm thick GaN films were investigated with respect to the polar type. Unintentionally doped N-polar GaN grown on nitrided sapphire substrates had a hexagonal facetted morphology and consistently exhibited a high n-type conductivity with a sheet resistance of ∼10 Ω/sq, corresponding to a carrier concentration on the order of 1019 cm–3. In contrast, Ga-polar GaN films grown on low temperature AlN buffer layers showed a high sheet resistance of ∼109 Ω/sq. GaN films of mixed polarity exhibited sheet resistance in-between the two extreme values obtained for polarity pure films. The results of this work show that the existence of N-polar domains (even on the nano-scale) in an otherwise Ga-polar matrix significantly decreases the insulating properties of unintentionally doped GaN films. (© 2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Properties of Fe-doped semi-insulating GaN substrates for high-frequency device fabrication

Thick freestanding GaN films were grown by hydride vapor-phase epitaxial method on both pattern-masked and unmasked GaAs substrates. Both approaches resulted on films characterized by a large excess of free carriers at room temperature. The use of a GaAs back surface NiTi substrate protective layer increased the concentration of incorporated iron impurities and yield semi-insulating films. The morphology and crystalline quality of these films show a strong dependence on nucleation layer growth approach and Fe doping concentration. Near infrared photoluminescence and Raman scattering verified the incorporation and activation of the Fe impurities, and its effect on the free carrier concentration.