Metal-organic Vapor Phase Epitaxy Growth and Characterization of InAlGaN Epilayers

In order to improve crystal quality for growth of quaternary InAlGaN, a series of InAlGaN films were grown on GaN buffer layer under different growth temperatures and carrier gases by low-pressure metal-organic vapor phase epitaxy. Energy dispersive spectroscopy (EDS) was employed to measure the chemical composition of the quaternary, high resolution X-ray diffraction (HRXRD) and photoluminescence (PL) technique were used to characterize structural and optical properties of the epilayers, respectively. The PL spectra of InAlGaN show with and without the broad-deep level emission when only N2 and a N2+H2 mixture were used as carrier gas, respectively. At pressure of 1.01×104 Pa and with mixed gases of nitrogen and hydrogen as carrier gas, different alloy compositions of the films were obtained by changing the growth temperature while keeping the fluxes of precursors of indium (In), aluminum (Al), gallium (Ga) and nitrogen (N2) constant. A combination of HRXRD and PL measurements enable us to explore the relative optimum growth parameters-growth temperature between 850℃ and 870℃,using mixed gas of N2+H2 as carrier gas.

Growth and photoluminescence of InAlGaN films

Two quaternary InAlGaN films were grown by metal-organic chemical-vapor deposition (MOCVD) on sapphire (0001) substrates with and without high-temperature GaN interlayer, respectively. The structural and optical properties of the quaternary films were investigated by high-resolution X-ray diffraction (HRXRD), high-resolution electron microscopy (HREM), temperature-dependent photoluminescence (PL) spectroscopy and time-resolved photoluminescence (TRPL) spectroscopy. According to the HRXRD and PL results, it is demonstrated that two samples have the same crystal quality. The TRPL signals of both samples were fitted well as a stretched exponential decay from 14 K to 250 K, indicating significant disorder in the materials, which is attributed to recombination of excitons localized in disorder quantum nanostructures such as quantum dots or quantum disks originating from indium (In) clusters or In composition fluctuation. The cross-section HREM measurement further proves that there exist disorder quantum nanostructures in the quaternary. By investigating the temperature dependence of the dispersive exponent beta, it is shown that the stretched exponential decays of the two samples originate from different mechanisms. (C) 2003 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

Reduction in crystallographic tilt of lateral epitaxial overgrown GaN by using new patterned shape mask

We proposed a new method to suppress the crystallographic tilt in the lateral epitaxial overgrowth of GaN by using an oxide mask with a newly designed pattern. A rhombus mask with edges oriented in the direction of <10 - 10>(GaN) was used instead of the traditional stripe mask. The morphology evolution during the LEO GaN with the rhombus mask was investigated by SEM, and the crystallographic tilt in the LEO GaN was measured by DC-XRD. It is found that using the new rhombus mask can decrease the crystallographic tilt in the LEO GaN. In addition, this method makes the ELO GaN stripes easy to coalesce. (C) 2003 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

The influence of nitrogen clustering effect on optical transitions in GaInNAs/GaAs quantum wells

Photoluminescence (PL) spectra of GaInNAs/GaAs multiple quantum wells and GaInNAs epilayers grown on GaAs substrate show an apparent "S-shape" temperature-dependence of the of dominant luminescence peak. At low temperature and weak excitation conditions, a PL peak related to nitrogen cluster-induced bound states can be well resolved in the PL spectra. It displays a remarkable red shift of up to 60 meV and is thermally quenched below 100 K with increasing temperature, being attributed to N-cluster induced bound states. The indium incorporation exhibits significant effect on the cluster formation. The rapid thermal annealing treatment at 750 C can essentially remove the bound states-induced peak.

Optical transitions in GaNAs/GaAs single quantum well

We have investigated GaNAs/GaAs single quantum wells (SQWs) grown by molecular beam epitaxy (MBE) using photoluminescence (PL), time-resolved PL (TRPL) and photovoltaic (PV) techniques. The low temperature PL is dominated by spatially direct transitions involving electrons confined in GaNAs well and holes localized in the same GaNAs layer. This assignment was supported by PL decay time measurements and absorption line-shape analysis derived from the PV measurements. By fitting the experimental data with a simple calculation, the band offset of the GaN0.015As0.985/GaAS heterostructure was estimated, and a type II band lineup in GaN0.015As0.985/GaAs QWs was suggested. Moreover, DeltaE(C), the discontinuity of conductor band, is found to be a nonlinear function of the nitrogen (N) composition (x), and the average variation of DeltaE(C) is about 0.110eV per %N, The measured band bowing coefficient shows a strong function of x, giving an experimental support to the theoretic calculation of Wei et al [Ref.2].

Modulation magnesium-doping in AlGaN/GaN superlattices

Low resistivity of p-type Mg-doped AlGaN/GaN superlattices (SLs) is demonstrated. The resistivity of the SLs is less than 0.6 Omega .cm. and the measured hole concentration is higher than 1x10(18)cm(-3). The resistivity of SLs is much lower, and the hole concentration of SLs is much higher, than that of bulk GaN and AlGaN, The electrical properties of the SLs are less sensitive than the conventional bulk lavers.

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.

Stoichiometry in GaAs grown in outer space measured nondestructively

A semi-insulating (SI) GaAs single crystal ingot was successfully grown in a recoverable satellite. The two-dimensional distribution of stoichiometry in space-grown SI-GaAs single crystal wafer was studied nondestructively based upon x-ray Band diffraction. The avenge stoichiometry in the space-grown crystal is 0.50007 with mean square deviation of 6 x 10(-6), and shows a better stoichiametric property than the ground-grown SI-GaAs. The average etch pit density (EPD) of dislocations in the crystal revealed by molten KOH is 2.0 x 10(4) cm(-2), and the highest EPD is 3.1 x 10(4) cm(-2). This result indicates that the structural properly of the crystal is quite good.

The effect of low temperature GaAs nucleation on the growth of GaN on Silicon (001) during MOVPE process

High quality cubic GaN was grown on Silicon (001) by metalorganic vapor phase epitaxy (MOVPE) using a GaAs nucleation layer grown at low temperature. The influence of various nucleation conditions on the GaN epilayers' quality was investigated. We found that the GaAs nucleation layer grown by atomic layer epitaxy (ALE) could improve the quality of GaN films by depressing the formation of mixed phase. Photoluminescence (PL) and X-ray diffraction were used to characterize the properties of GaN epilayers. High quality GaN epilayers with PL full width at half maximum (FWHM) of 130meV at room temperature and X-ray FWHM of 70 arc-min were obtained by using 10-20nm GaAs nucleation layer grown by ALE.

Study of infrared luminescence from Er-implanted GaN films

In this study, we report the dependences of infrared luminescence properties of Er-implanted GaN thin films (GaN:Er) on the kinds of substrates used to grow GaN, the growth techniques of GaN, the implantation parameters and annealing procedures. The experimental results showed that the photoluminescence (PL) intensity at 1.54 mum was severely influenced by different kinds of substrates. The integrated PL peak intensity from GaN:Er /Al2O3 (00001) was three and five times stronger than that from GaN:Er /Si (111) grown by molecular beam epitaxy (MBE) and by metalorganic chemical vapor deposition (MOCVD), respectively. The PL spectra observed from GaN:Er/Al2O3 (0001) grown by MOCVD and by MBE displayed a similar feature, but those samples grown by MOCVD exhibited a stronger 1.54 mum PL. It was also found that there was a strong correlation between the PL intensity with ion implantation parameters and annealing procedures. Ion implantation induced damage in host material could be only partly recovered by an appropriate annealing temperature procedure. The thermal quenching of PL from 15 to 300 K was also estimated. In comparison with the integrated PL intensity at 15 K, it is reduced by only about 30 % when going up to 300 K for GaN:Er/Al2O3 sample grown by MOCVD. Our results also show that the strongest PL intensity comes from GaN:Er grown on Al2O3 substrate by MOCVD. (C) 2004 Elsevier B.V. All rights reserved.

Nano-layer structure of silicon-on-insulator materials

Silicon-on-insulator (SOI) has been recognized as a promising semiconductor starting material for ICs where high speed and low power consumption are desirable, in addition to its unique applications in radiation-hardened circuits. In the present paper, three novel SOI nano-layer structures have been demonstrated. ULTRA-THIN SOI has been fabricated by separation by implantation of oxygen (SIMOX) technique at low oxygen ion energy of 45 keV and implantation dosage of 1.81017/cm2. The formed SOI layer is uniform with thickness of only 60 nm. This layer is of crystalline quality. and the interface between this layer and the buried oxide layer is very sharp, PATTERNED SOI nanostructure is illustrated by source and drain on insulator (DSOI) MOSFETs. The DSOI structure has been formed by selective oxygen ion implantation in SIMOX process. With the patterned SOI technology, the floating-body effect and self-heating effect, which occur in the conventional SOI devices, are significantly suppressed. In order to improve the total-dose irradiation hardness of SOI devices, SILICON ON INSULATING MULTILAYERS (SOIM) nano-structure is proposed. The buried insulating multilayers, which are composed of SiOx and SiNy layers, have been realized by implantation of nitride and oxygen ions into silicon in turn at different ion energies, followed by two steps of high temperature annealing process, respectively, Electric property investigation shows that the hardness to the total-dose irradiation of SOIM is remarkably superior to those of the conventional SIMOX SOI and the Bond-and-Etch-Back SOI.

Current-induced migration of surface adatoms during GaN growth by molecular beam epitaxy

The influence of electric fields on surface migration of Gallium (Ga) and Nitrogen (N) adatoms is studied during GaN growth by molecular beam epitaxy (MBE). When a direct current (DC) is used to heat the sample, long distance migration of Ga adatoms and diffusion asymmetry of N adatoms at steps are observed. On the other hand, if an alternating current (AC) is used, no such preferential adatom migration is found. This effect is attributed to the effective positive charges of surface adatoms. representing an effect of electro-migration. The implications of such current-induced surface migration to GaN epitaxy are subsequently investigated. It is seen to firstly change the distribution of Ga adatoms on a growing surface, and thus make the growth to be Ga-limited at one side of the sample but N-limited at the other side. This leads to different optical qualities of the film and different morphologies of the surface. (C) 2001 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.

Modification of emission wavelength of self-assembled In(Ga)As/GaAs quantum dots covered by InxGa1-xAs(0 <= x <= 0.3) layer

Red shifts of emission wavelength of self-organized In(Cla)As/GaAs quantum dots (QDs) covered by 3 nm thick InxGa1-xAs layer with three different In mole fractions (x = 0.1, 0.2 and 0.3, respectively) have been observed. Transmission electron microscopy images demonstrate that the stress along growth direction in the InAs dots was reduced due to introducing the InxGa1-xAs (x = 0.1, 0.2 and 0.3) covering layer instead of GaAs layer. Atomic force microscopy pictures show a smoother surface of InAs islands covered by an In0.2Ga0.8As layer. It is explained by the calculations that the redshifts of the photoluminescence (PL) spectra from the QDs covered by the InxGa1-xAs (x greater than or equal to 0.1) layers were mainly due to the reducing of the strain other than the InAs/GaAs intermixing in the InAs QDs. The temperature dependent PL spectra further confirm that the InGaAs covering layer can effectively suppress the temperature sensitivity of PL emissions. 1.3 mum emission wavelength with a very narrow linewidth of 19.2 mcV at room temperature has been obtained successfully from In,In0.5Ga0.5As/GaAs self-assembled QDs covered by a 3-nm In0.2Ga0.2As strain reducing layer. (C) 2001 Elsevier Science B.V. All rights reserved.

Structural properties of SI-GaAs grown in space

The structural properties of Semi-insulating gallium arsenide (SI-GaAs) crystal grown with power-travelling technique in space have been studied by double-crystal x-ray diffractometry and chemical etching. The quality of the crystal was first evaluated by x-ray rocking-curve method. The full width at half maximum of x-ray rocking curve in space-grown SI-GaAs is 9.4+/-0.08 are seconds. The average density of dislocations revealed by molten KOH is 2.0 X 10(4) cm(-2), and the highest density is 3.1 X 10(4) cm(-2). The stoichiometry in the single crystal grown in space is improved as well. Unfortunately, the rear of the ingot grown in space is polycrystalline owing to being out of control of power. (C) 1999 COSPAR. Published by Elsevier Science Ltd.