Low-frequency noise properties of GaN Schottky barriers deposited on intermediate temperature buffer layers

Metal-semiconductor-metal (MSM) structures were fabricated by RF-plasma-assisted MBE using different buffer layer structures. One type of buffer structure consists of an AlN high-temperature buffer layer (HTBL) and a GaN intermediate temperature buffer layer (ITBL), another buffer structure consists of just a single A IN HTBL. Systematic measurements in the flicker noise and deep level transient Fourier spectroscopy (DLTFS) measurements were used to characterize the defect properties in the films. Both the noise and DLTFS measurements indicate improved properties for devices fabricated with the use of ITBL and is attributed to the relaxation of residue strain in the epitaxial layer during growth process. (C) 2003 Elsevier Ltd. All rights reserved.

Indium-doping enhanced two-dimensional-electron-gas performance in AlGaN/GaN heterostructures

Indium (In)-doping was applied in GaN layers during growth of AlGaN/GaN heterostructure with unintentionally doped or modulation Si-doped AlGaN layers. It was found that In-doping was effective in improving electron sheet density of two-dimensional-electron-gas (2DEG) in the heterostructures. Furthermore, In-doping also improved mobility in heterostructures with Si modulation-doped in AlGaN layers. The possible reasons were discussed. X-ray diffraction (XRD) and wet chemical etching revealed that crystalline quality of GaN was improved by In-doping. It was proposed that In-doping modified growth kinetics of GaN.

Growth and characterization of GaN on LiGaO2 and LiAlO2

The nearly lattice-matched LiGaO2 and LiAlO2 substrates have been used for the growth of GaN by LP-MOVPE. GaN epilayers have been grown on the two substrates at very low input partial pressure of hydrogen and relatively low growth temperature. The difference in the growth rate, crystal and optical qualities of hexagonal GaN epilayers grown on LiAlO2 and LiGaO2 substrate with two polar domains are investigated. LiAlO2 and LiGaO2 single crystal with a single domain structure and an adequate surface plane are promising substrates for the growth of high quality of hexagonal GaN thin films.

Effects of residual C and O impurities on photoluminescence in undoped GaN epilayers

Photoluminescence (PL) was investigated in undoped GaN from 4.8 K to room temperature. The 4.8 K spectra exhibited recombinations of free exciton, donor-acceptor pair (DAP), blue and yellow bands (Ybs). The blue band (BB) was also identified to be a DAP recombination. The YB was assigned to a recombination from deep levels. The energy-dispersive X-ray spectroscopy show that C and O are the main residual impurities in undoped GaN and that C concentration is lower in the epilayers with the stronger BB. The electronic structures of native defects, C and O impurities, and their complexes were calculated using ab initio local-density-functional (LDF) methods with linear muffin-tin-orbital and 72-atomic supercell. The theoretical analyses suggest that the electron transitions from O-N states to C-N and to V-Ga states are responsible for DAP and the BB, respectively, and the electron transitions between the inner levels of the C-N-O-N complex may be responsible for the YB in our samples. (C) 2002 Elsevier Science B.V. All rights reserved.

High-quality GaN grown by gas-source MBE

High-quality GaN epilayers were consistently obtained using a home-made gas-sourer MBE system on sapphire substrates. Room-temperature electron mobility of the grown GaN film is 300 cm(2)/V s with a background electron concentration as low as 2 x 10(17) cm(-3) The full-width at half-maximum of the GaN (0 0 0 2) double-crystal X-ray rocking curve is 6 arcmin. At low temperature (3.5 K), the FWHM of the: near-band-edge photoluminescence emission line is 10 meV. Furthermore, using piezoelectric effect alone with the high-quality films, two-dimensional electron gas was formed in a GaN/AlN/GaN/sapphire structure. Its room-temperature and low-temperature (77 K) electron mobility is 680 cm(2)/V s and 1700 cm(2)/V s, and the corresponding sheet electron density is 3.2 x 10(13) and 2.6 x 10(13) cm(-2), respectively. (C) 2001 Published by Elsevier Science.

Orientation relationship between hexagonal inclusions and cubic GaN grown on GaAs(001) substrates

Cubic GaN/GaAs(0 0 1) epilayers and hexagonal inclusions are characterized by X-ray diffraction (XRD), Photoluminescence (PL), Raman spectroscopy, and transmission electron microscopy (TEM). The X-ray {0 0 0 2} and (1 0 (1) over bar 0) pole figures show that the orientation relationships between cubic GaN and hexagonal inclusions are (1 1 1)//(0 0 0 1), <1 1 2 >//<1 0 (1) over bar 0 >. The distribution of hexagonal inclusions mainly results from the interfacial bonding disorder in the grain boundaries parallel to hexagonal <0 0 0 1 > directions and the lattice mismatch in <0 0 0 1 > directions on {1 0 (1) over bar 0} planes. In order to reduce the energy increase in cubic epilayers, hexagonal lamellas with smaller sizes in <0 0 0 1 > directions often nucleate inside the buffer layer or near the interface between the buffer layer and the epitaxial layer, and penetrate through the whole epitaxial layer with this orientation relationship. (C) 2001 Elsevier Science B.V. All rights reserved.

Influences of initial buffer layer deposition on electrical and optical properties in cubic GaN grown on GaAs(100) by metalorganic chemical vapor deposition

We present some results on the effect of initial buffer layer on the crystalline quality of Cubic GaN epitaxial layers grown on GaAs(100) substrates by metalorganic chemical vapor deposition. Photoluminescence and Hall measurements were performed to characterize the electrical and optical properties of cubic GaN. The crystalline quality subsequently grown high-temperature (HT) cubic GaN layers strongly depended on thermal effects during the temperature ramping process after low temperature (LT) growth of the buffer layers. Atomic force microscope (AFM) and reflection high-energy electron diffraction (RHEED) were employed to investigate this temperature ramping process. Furthermore, the role of thermal treatment during the temperature ramping process was identified. Using the optimum buffer layer, the full width at half maxim (FWHM) at room temperature photoluminescence 5.6 nm was achieved. To our knowledge, this is the best FWHM value for cubic GaN to date. The background carrier concentration was as low as 3 x 10(13) cm(-3). (C) 2000 Published by Elsevier Science S.A. All rights reserved.

Influence of precipitates on GaN epilayer quality

GaN epilayers grown on pre-nitridated (0001) sapphire substrates by metallorganic vapor phase epitaxy were investigated by wavelength dispersive X-ray spectroscopy and energy dispersive S-ray spectroscopy. Precipitates were observed to mainly consist of O impurity whose strengths were weaker than surrounding matrix. The precipitates were larger in size and distributed more sparsely and inhomogeneously in < 11-20 > directions of the epilayers grown on substrates pre-nitridated for longer periods. The larger precipitates often joined to cracks in the TEM specimens. The crack formation seems to be attributed to the compressive stress concentration at edge angles of the larger precipitates. Yellow luminescence of the epilayers was imaged by cathodoluminescence. The distribution similarity between the cathodoluminescence and the precipitates suggested that the precipitates were responsible for the yellow luminescence band. (C) 2000 Elsevier Science S.A, All rights reserved.

Growth and characterization of GaN on LiGaO2

The nearly lattice-matched (0 0 1)LiGaO2 substrates have been used for the growth of GaN by LP-MOVPE, GaN epilayers have been grown on both domains at very low input partial pressure of hydrogen and relatively low growth temperature. The differences in the growth rate, crystal and optical qualities of hexagonal GaN epilayers grown on LiGaO2 substrate with two polar domains are investigated. LiGaO2 single crystal with a single domain structure and an adequate surface plane is a promising substrate for the growth of high quality of hexagonal GaN thin films. (C) 1998 Published by Elsevier Science B.V. All rights reserved.

High-concentration hydrogen in unintentionally doped GaN

We use nuclear reaction analysis to study hydrogen in unintentionally doped GaN, and high-concentration hydrogen, nearly 10(21) cm(-3), is detected. Accordingly, a broad but intense infrared absorption zone with a peak at 2962 cm(-1) is reported, which is tentatively assigned to the stretch mode of NH: Ga complex. The complex is assumed to be one candidate answering for background electrons in unintentionally doped GaN. (C) 1998 Elsevier Science B.V. All rights reserved.

Catalytic Activation of Mg-Doped GaN by Hydrogen Desorption Using Different Metal Thin Layers

The annealing of Mg-doped GaN with Pt and Mo layers has been found to effectively improve the hole concentration of such material by more than 2 times as high as those in the same material without metal. Compared with the Ni and Mo catalysts, Pt showed good activation effect for hydrogen desorption and ohmic contact to the Ni/Au electrode. Despite the weak hydrogen desorption, Mo did not diffuse into the GaNepilayer in the annealing process, thus suppressing the carrier compensation phenomenon with respect to Ni and Pt depositions, which resulted in the high activation of Mg acceptors. For the GaN activated with the Ni, Pt, and Mo layers, the blue emission became dominant, followed by a clear peak redshift and the degradation of photoluminescence signal when compared with that of GaN without metal.

X-ray photoelectron spectroscopy study on GaN crystal irradiated by slow highly charged ions

We utilize slow highly charged ions of Xeq+ and Pbq+ to irradiate GaN crystal films grown on sapphire substrate, and use X-ray photoelectron spectroscopy to analyze its surface chemical composition and chemical state of the elements. The results show that highly charged ions can etch the sample surface obviously, and the GaN sample irradiated by highly charged ions has N depletion or is Ga rich on its surface. Besides, the relative content of Ga-Ga bond increases as the dose and charge state of the incident ions increase. In addition, the binding energy of Ga 3d(5/2) electrons corresponding to Ga-Ga bond of the irradiated GaN sample is smaller compared with that of the Ga bulk material. This can be attributed to the lattice damage, which shifts the binding energy of inner orbital electrons to the lower end.

Properties of InAlN/GaN Heterostructures Prepared by Molecular Beam Epitaxy

InAlN thin films and InAlN/GaN heterostructures have been intensively studied over recent years due to their applications in a variety of devices, including high electron mobility transistors (HEMTs). However, the quality of InAlN remains relatively poor with basic material and structural characteristics remain unclear.

Molecular beam epitaxy (MBE) is used to synthesize the materials for this research, as MBE is a widely used tool for semiconductor growth but has rarely been explored for InAlN growth. X-ray photoelectron spectroscopy (XPS) is used to determine the electronic and chemical characteristics of InAlN surfaces. This tool is used for the first time in application to MBE-grown InAlN and heterostructures for the characterization of surface oxides, the bare surface barrier height (BSBH), and valence band offsets (VBOs).

The surface properties of InAlN are studied in relation to surface oxide characteristics and formation. First, the native oxide compositions are studied. Then, methods enabling the effective removal of the native oxides are found. Finally, annealing is explored for the reliable growth of surface thermal oxides.

The bulk properties of InAlN films are studied. The unintentional compositional grading in InAlN during MBE growth is discovered and found to be affected by strain and relaxation. The optical characterization of InAlN using spectroscopy ellipsometry (SE) is also developed and reveals that a two-phase InAlN model applies to MBE-grown InAlN due to its natural formation of a nanocolumnar microstructure. The insertion of an AlN interlayer is found to mitigate the formation of this microstructure and increases mobility of whole structure by fivefold.

Finally, the synthesis and characterization of InAlN/GaN HEMT device structures are explored. The density and energy distribution of surface states are studied with relationships to surface chemical composition and surface oxide. The determination of the VBOs of InAlN/GaN structures with different In compositions are discussed at last.

Characterization of ZnO Nanorods Grown on GaN Using Aqueous Solution Method

Uniformly distributed ZnO nanorods with diameter 70-100 nm and 1-2μm long have been successfully grown at low temperatures on GaN by using the inexpensive aqueous solution method. The formation of the ZnO nanorods and the growth parameters are controlled by reactant concentration, temperature and pH. No catalyst is required. The XRD studies show that the ZnO nanorods are single crystals and that they grow along the c axis of the crystal plane. The room temperature photoluminescence measurements have shown ultraviolet peaks at 388nm with high intensity, which are comparable to those found in high quality ZnO films. The mechanism of the nanorod growth in the aqueous solution is proposed. The dependence of the ZnO nanorods on the growth parameters was also investigated. While changing the growth temperature from 60°C to 150°C, the morphology of the ZnO nanorods changed from sharp tip (needle shape) to flat tip (rod shape). These kinds of structure are useful in laser and field emission application.

Growth of ZnO Nanorods on GaN Using Aqueous Solution Method

Uniformly distributed ZnO nanorods with diameter 80-120 nm and 1-2µm long have been successfully grown at low temperatures on GaN by using the inexpensive aqueous solution method. The formation of the ZnO nanorods and the growth parameters are controlled by reactant concentration, temperature and pH. No catalyst is required. The XRD studies show that the ZnO nanorods are single crystals and that they grow along the c axis of the crystal plane. The room temperature photoluminescence measurements have shown ultraviolet peaks at 388nm with high intensity, which are comparable to those found in high quality ZnO films. The mechanism of the nanorod growth in the aqueous solution is proposed. The dependence of the ZnO nanorods on the growth parameters was also investigated. While changing the growth temperature from 60°C to 150°C, the morphology of the ZnO nanorods changed from sharp tip with high aspect ratio to flat tip with smaller aspect ratio. These kinds of structure are useful in laser and field emission application.