ZnO thin films were deposited on a R-plane sapphire substrate. The effects of the thermal energy and the kinetic energy of the sputtered species on the growth of ZnO thin films were investigated. By varying the substrate temperature, chamber pressure, and radio frequency power, the structure of ZnO thin films was transformed from polycrystalline to epitaxial on R-plane sapphire substrates. High quality ZnO epitaxial thin films were grown at the condition of 400 °C, 250 W, and 5 mTorr. According to reflection high energy electron diffraction and reflection electron microscopy observations, there were no double diffraction distortion and any other patterns. Its surface roughness observed by atomic force microscopy was about 27 nm.

A 1.5 GHz range low insertion loss surface acoustic wave (SAW) filter has been developed using a ZnO/sapphire substrate and IIDT-type electrodes with two external reflectors. This filter had an insertion loss of 1.3 dB, stop-band attenuation of greater than 30 dB and matching impedance of 50 Ω pure resistivity without external matching networks. This filter has the smallest insertion loss of all transverse-type GHz-range SAW filters reported previously and it is suitable for use as a RF-stage filter for future 1.5 GHz range Japanese digital cellular systems.


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High-quality AlGaN/GaN undoped single heterostructures (SH) with different Al contents have been grown on sapphire substrates. The magnetotransport investigation was performed on these samples at a low temperature. The observation of Shubnikov–de Hass oscillations in the magnetic fields below 3 T and the integer quantum Hall effect confirmed the existence of the two-dimensional electron gas (2DEG) at the AlGaN/GaN interface. The Al0.18Ga0.82N/GaN SH shows a Hall mobility of 10300 cm²/V s at a carrier sheet density of 6.19×1012/cm² measured at 1.5 K. To the best of our knowledge, this is the highest carrier mobility ever measured in GaN-based semiconductors grown on sapphire substrates. The Al composition dependence of the mobility and carrier sheet density were also investigated. Based on the piezoelectric field effect, the Al composition dependence of the 2DEG sheet density was calculated, which agreed well with the experimental result. The negative magnetoresistance with parabolic magnetic-field dependence in the low magnetic field was also observed in the sample with the highest 2DEG sheet density.

A high light-extraction efficiency was demonstrated in the flip-chip light-emitting diode (FCLED) with a textured sapphire substrate. The bottom side of a sapphire substrate was patterned using a dry etching process to increase the light-extraction efficiency. Light output power measurements indicated that the scattering of photons emitted in the active layer was considerably enhanced at the textured sapphire substrate resulting in an increase in the probability of escaping from the FCLED. The light-output power of the FCLED was increased by 40.2% for a 0.4-mm deep FCLED with a periodic distance of 13-mm mesh-type texture on the bottom side of the sapphire substrate.


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Characterization of a Zn0.7Mg0.3O/ZnO heterostructure field-effect transistor (HFET) is reported. The HFET was based on a Zn0.7Mg0.3O/ZnO/Zn0.7Mg0.3O single quantum well(SQW)grown on an a-plane sapphire substrate by molecular-beam epitaxy, and was fabricated by a conventional photolithography technique combined with dry etching. Room-temperature characteristic of the HFET was a n-channel depletion type with a transconductance of 0.70mS/mm and a field-effect mobility of 140cm2/Vs, in good agreement with the electron Hall mobility in SQW of 130cm2/Vs. The on∕off ratio at VDS=3V was ∼800, which was limited by an insufficiently suppressed leakage current through the bottom Zn0.7Mg0.3O barrier.

In this paper we describe recent experimental efforts to produce high quality thick (300 μm) GaN layers on sapphire, the removal of such a layer from the sapphire substrate, and the properties of the so obtained free-standing GaN material. The growth process is described in some detail in the vertical reactor geometry used in this work. Defects like dislocations, micro-cracks and pits produced during growth are discussed, along with procedures to minimize their concentration on the growing surface. The laser lift-off technique is shown to be a feasible technology, in particular if a powerful laser with a large spot size can be used. A major problem with the free-standing material is the typically large bowing of such a wafer, due to the built in defect concentrations near the former GaN-sapphire interface. This bowing typically causes a rather large width of the XRD rocking curve of the free-standing material, while optical data confirm virtually strain free material of excellent quality at the top surface.


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The dependence of polar direction of GaN film on growth conditions has been investigated by changing either the group-V/group-III ratio (V/III ratio) in supplying the source gas or the deposition rate. GaN films were deposited on a nitrided sapphire by two-step metalorganic chemical vapor deposition. The surface morphology changed from flat hexagonal to pyramidal hexagonal facet with the increase of V/III ratio. However, the polar direction of GaN on an optimized buffer layer of 20 nm thickness was N-face (-c) polarity, independent of both the V/III ratio and the deposition rate.

The polarity of the GaN epitaxtial layer can be determined by that of an interface (nitrided sapphire, annealed buffer layer or GaN substrate) at the deposition of GaN epitaxial layer. The higher V/III ratio enhanced the nucleation density, and reduced the size of hexagonal facets. The nuclei, forming the favorable hexagonal facets of wurtzite GaN, should grow laterally along the directions to cover a room among the facets until coalescence. After coalescence, -c GaN growth on a flat hexagonal facet results in a pyramidal hexagonal facet. The growth mode for -c GaN has been discussed with respect to surface structure and migration length of adsorbing precursors, in comparison with Ga-face (+c) GaN.


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