Preparation of Freestanding GaN and GaN Template by Hydride Vapor Phase Epitaxy

Freestanding GaN were fabricated by conventional HVPE method. Scanning electron microscopy (SEM) measurements of etched GaN thick film show that the dislocation density is about low 10⁶ cm^-2. The full-width at half maximum (FWHM) of ω mode scan for the freestanding GaN (002) and (102) plane were 72 and 85 arcsec, respectively. Atomic force microscopy (AFM) measurements of thin GaN templates show that the dislocation density is about 10⁸ cm^-2. FWHM of the ω mode scan for the thin GaN template (002) plane were 209 and 299 arcsec, respectively.

KEYWORDS: GaN, HVPE, freestanding, dislocation, roughness

1. Introduction

GaN is a promising material for optoelectronic devices such as laser diode and light emitting diode in the blue and ultra-violet wavelength regions as well as electronic devices,operating at high temperature due to its wide direct band gap and high thermal conductivity. ¹⁾

During past decade, there have been a numerous progress in the GaN-based nitride semiconductors such as high brightness blue light emitting diodes (LEDs) and InGaN/GaN multi-quantum-well (MQW) blue laser diodes (LDs). The preparation of device quality nitrides has usually been carried out by metalorganic vapor phase epitaxy (MOVPE) on sapphire or SiC substrates with AlN or GaN buffer layers deposited at relatively low temperatures. ^{2, 3)}

Such heteroepitaxy causes high threading dislocation density and bending, due to the lattice mismatch and differences in the thermal expansion coefficient between GaN and substrate. Currently, for the commercialization of nitride based blue laser diodes and ultra-violet LEDs, a high quality freestanding GaN with low dislocation density is in strong demand.^{4, 5)}

Extensive research to reduce dislocations in GaN crystal is currently in progress. Epitaxial lateral overgrowth (ELO) of GaN on a patterned mask has been studied to reduce the dislocation density in the grown layer.6-8)

In this paper, we present the properties of freestanding GaN substrates prepared by conventional hydride vapor phase epitaxy (HVPE) system without using ELO or buffer layer technologies.

2. Experiment

GaN thick films with thickness more than 300 μm and thin GaN templates with thickness range from 5 to 15 μm have been successfully grown on sapphire substrates by conventional HVPE system (Fig. 1).⁹⁾

Two-inch c-plane sapphire substrate was placed in a hotwall HVPE reactor. Ga metal and HCl are pre-reacted to form GaCl gas, which is transported by nitrogen carrier gas to the hot growth-zone where it reacts with NH3 and deposits GaN on the (0001) sapphire substrate. For a V/III ratio from 20 to 35, a growth rate about 50 μm/h can be reproducibly achieved.

X-ray rocking curve measurements were performed on a high-resolution double-crystal diffractometer using Cu K_α1 radiation. A Si (100) crystal was used as the beam conditioner.

To measure the dislocation density of freestanding GaN thick films, etched surface was observed by scanning electron microscope after H₃PO₄ etching at 220ºC.

3. Results and Discussion

For the preparation of freestanding GaN thick films,GaN thick layers were removed from the sapphire substrates by laser-assisted lift-off method (248 nm line of KrF laser, with 20 ns pulse width and 50 Hz pulse rate). A laser beam energy density of 0.2 to 0.3 J/cm² was enough to release the nitrogen from the film forming a thin layer of liquid Ga. To prevent fractures induced by the wafer bow

ing during the laser liftoff process, the GaN/sapphire templates were kept hot at a temperature below the decomposition temperature.¹⁰⁾ The grown surfaces of the freestanding GaN are inadequate for homoepitaxial growth due to the existence of hillocks. Flat and smooth surfaces are obtained after mechanical polishing, which introduces subsurface damage extending up to 4000Å below the surface. The polished growth surfaces (Ga-face) were reactive ion etched to remove the damaged layer.¹¹⁾

Figure 2 shows double crystal XRD profiles of the GaN (002) and (102) plane in ω-scan. The full width at half maximum (FWHM) is 72 and 85 arcsec, respectively. Crystal quality of the GaN substrate was also evaluated by etch pit density (EPD). After the etching the number of etch pits was counted by SEM observation. The EPD is counted about 2.4×10⁶ cm^-2 (Fig. 3). Transmission electron microscopy (TEM) has been the general method to measure the dislocation density, despite of the extensive and skillful sample preparation process.12) However, in case of GaN films having low defect densities such as below ~10⁷cm^-2, TEM method may be uncertain and has difficulties observing the number of threading dislocations due to the small measurement areas.11) Oshima et al. reported that the dislocation density of Void-Assisted Separation (VAS) GaN was 5×10⁶ cm^-2 by EPD measurement.¹³⁾ Motoki et al. reported that the measured dislocation values were 5×10⁵, 2×10⁵ and 4×10⁴ cm-² by EPD, TEM and cathodeluminescence measurements, respectively.¹⁴⁾ This result shows that there are some differences in measured dislocation density by the measurement method. Considering the uncertainty of measured EPD of about 2.4×10⁶ cm^-2, this result reveals that conventional HVPE system could be used for the low dislocation density GaN growth without ELO technologies.

Figure 4 shows the double crystal XRD profiles of the 13 µm thin GaN template in ω-scan. The FWHM of (002) and (102) plane is 224 and 299 arcsec, respectively. These values are compatible with that of metalorganic chemical vapor deposition (MOCVD) GaN films of 2 µm thick. Despite rather thick GaN templates by HVPE compared to

MOCVD GaN, GaN templates could be used instead of MOCVD GaN due to its rather low dislocation density of middle of 10⁸ cm^-2 of dislocation density (Fig. 5). The issues of rough surface of HVPE GaN template so could be solved by precise process control. Figure 6 shows the roughness of GaN template compared with MOCVD GaN measured by optical 3D surface profiler. Although HVPE GaN template has about 2 times higher roughness than that of MOCVD GaN, the optical microscope shows that the roughness of homoepitaxial GaN on HVPE GaN could be used instead of MOCVD GaN.

4. Summary

Free standing GaN thick films with low defect density which is suitable for the high power blue laser diode manufacturing were grown on sapphire by HVPE. Using the same HVPE growth technology, GaN templates with moderate surface roughness and defect density (~5×10⁸ cm^-2), which is suitable for high power LED manufacturing were grown on sapphire by HVPE.

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