Probing deep level centers in GaN epilayers with variable-frequency capacitance-voltage characteristics of Au/GaN Schottky contacts

Under identical preparation conditions, Au/GaN Schottky contacts were prepared on two kinds of GaN epilayers with significantly different background electron concentrations and mobility as well as yellow emission intensities. Current-voltage (I-V) and variable-frequency capacitance-voltage (C-V) characteristics show that the Schottky contacts on the GaN epilayer with a higher background carrier concentration and strong yellow emission exhibit anomalous reverse-bias I-V and C-V characteristics. This is attributed to the presence of deep level centers. Theoretical simulation of the low-frequency C-V curves leads to a determination of the density and energy level position of the deep centers. (c) 2006 American Institute of Physics.

Growth and magnetic properties of zincblende CrSb epilayers on relaxed and strained (In, Ga)As buffers

Zincblende CrSb (zb-CrSb) layers with room-temperature ferromagnetism have been grown on relaxed and strained (In,Ga)As buffer layers epitaxially prepared on (001) GaAs substrates by molecular-beam epitaxy. The structural characterizations of CrSb layers fabricated under the two cases are studied by using synchrotron grazing incidence x-ray diffraction (GID). The results of GID experiments indicate that no sign of second phase exists in all the zb-CrSb layers. Superconducting quantum interference device measurements demonstrate that the thickness of zb-CrSb layers grown on both relaxed and strained (In,Ga)As buffer layers can be increased to similar to 12 monolayers (similar to 3.6nm), compared to similar to 3 monolayers (similar to 1nm) on GaAs directly.

Stress and its effect on optical properties of GaN epilayers grown on Si(111), 6H-SiC(0001), and c-plane sapphire

The stress states in unintentionally doped GaN epilayers grown on Si(111), 6H-SiC(0001), and c-plane sapphire, and their effects on optical properties of GaN films were investigated by means of room-temperature confocal micro-Raman scattering and photoluminescence techniques. Relatively large tensile stress exists in GaN epilayers grown on Si and 6H-SiC while a small compressive stress appears in the film grown on sapphire. The latter indicates effective strain relaxation in the GaN buffer layer inserted in the GaN/sapphire sample, while the 50-nm-thick AlN buffer adopted in the GaN/Si sample remains highly strained. The analysis shows that the thermal mismatch between the epilayers and the substrates plays a major role in determining the residual strain in the films. Finally, a linear coefficient of 21.1+/-3.2 meV/GPa characterizing the relationship between the luminescent bandgap and the biaxial stress of the GaN films is obtained. (C) 2003 American Institute of Physics.

Effects of reactor pressure on GaN nucleation layers and subsequent GaN epilayers grown on sapphire substrate

The influence of reactor pressure on GaN nucleation layer (NL) and the quality of subsequent GaN on sapphire is studied. The layers were grown by low-pressure metalorganic chemical vapor deposition (MOCVD) on c-plane sapphire substrates and investigated by in situ laser reflectometry, atomic force microscope, scanning electron microscope, X-ray diffraction and photoluminescence. With the increase of reactor pressure prior to high-temperature GaN growth, the size of GaN nuclei formed after annealing decreases, the spacing between nucleation sites increases and the coalescence of GaN nuclei is deferred. The optical and crystalline qualities of GaN epilayer were improved when NLs were deposited at high pressure. The elongated lateral overgrowth of GaN islands is responsible for the quality improvement. (C) 2003 Elsevier Science B.V. All rights reserved.

High-resolution X-ray diffraction analysis of the Bragg peak integrated intensity in highly mismatched III-N epilayers

A new method of measuring the thickness of GaN epilayers on sapphire (0 0 0 1) substrates by using double crystal X-ray diffraction was proposed. The ratio of the integrated intensity between the GaN epilayer and the sapphire substrate showed a linear relationship with the GaN epilayer thickness up to 2.12 mum. It is practical and convenient to measure the GaN epilayer thickness using this ratio, and can mostly eliminate the effect of the reabsorption, the extinction and other scattering factors of the GaN epilayers. (C) 2003 Elsevier Science B.V. All rights reserved.

Influence of semi-insulating InP substrates on InAlAs epilayers grown by molecular beam epitaxy

Tensile-strained InAlAs layers have been grown by solid-source molecular beam epitaxy on as-grown Fe-doped semi-insulating (SI) InP substrates and undoped SI InP substrates obtained by annealing undoped conductive InP wafers (wafer-annealed InP). The effect of the two substrates on InAlAs epilayers and InAlAs/InP type II heterostructures has been studied by using a variety of characterization techniques. Our calculation data proved that the out-diffusion of Fe atoms in InP substrate may not take place due to their low diffusion, coefficient. Double-crystal X-ray diffraction measurements show that the lattice mismatch between the InAlAs layers and the two substrates is different, which is originated from their different Fe concentrations. Furthermore, photoluminescence results indicate that the type II heterostructure grown on the wafer-annealed InP substrate exhibits better optical and interface properties than that grown on the as-grown Fe-doped substrate. We have also given a physically coherent explanation on the basis of these investigations. (C) 2003 Elsevier Science B.V. All rights reserved.

Growth temperature effect on the optical and material properties of AlxInyGa1-x-yN epilayers grown by MOCVD

AlxInyGa1-x-yN epilayers have been grown by metalorganic chemical vapor deposition (MOCVD) at different temperatures from 800 to 870degreesC. The incorporation of indium is found to increase with decreasing growth temperature, while the incorporation of Al remains nearly constant. The optical properties of the samples have been investigated by photoluminescence (PL) and time-resolved photoluminescence (TRPL) at different temperatures. The results show that the sample grown at 820 C exhibits the best optical quality for its large PL intensity and the absence of the yellow luminescence. Furthermore the temperature-dependent PL and TRPL of the sample reveals its less exciton localization effect caused by alloy fluctuations. In the scanning electron microscopy measurement, much uniform surface morphology is found for the sample grown at 820degreesC, in good agreement with the PL results, The improvement of AlxInyGa1-x-yN quality is well correlated with the incorporation of indium into AlGaN and the possible mechanism is discussed. (C) 2002 Elsevier Science B.V. All rights reserved.

Comparative study on the broadening of exciton luminescence linewidth due to phonon in zinc-blende and wurtzite GaN epilayers

Linewidth broadening of exciton luminescence in wurtzite and zinc-blende GaN epilayers was investigated as a function of temperature with photoluminescence. A widely accepted theoretical model was used to fit the experimental data, so that the coupling parameters between exciton and acoustic and longitudinal optical phonons were obtained for both structures. It was found that the coupling constants of both exciton-acoustic optical phonon coupling and exciton-longitudinal optical phonon coupling for zinc-blende GaN are almost twice as much as the corresponding values of wurtzite GaN. These results show that the relatively strong exciton-phonon scattering seems to be characteristic to zinc-blende GaN film. (C) 2002 American Institute of Physics.

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.

Improved purity of long-wavelength InAsSb epilayers grown by melt epitaxy in fused silica boats

In this study, we first present the process of the melt epitaxial (ME) growth method, and the improvement of low-temperature electron mobility of the long-wavelength InAsSb epilayers grown by ME in a fused silica boat. The electrical properties were investigated by van der Pauw measurement at 300 and 77 K. It is seen that the electron mobility of the InAsSb samples grown by graphite boat decreased from 55,700 to 26,600 cm(2)/V s when the temperature was reduced from 300 to 77 K, while for the samples grown by fused silica boat, the electron mobility increased from 52,600 at 300 K to 54,400 cm(2)/V s at 77 K. The electron mobility of 54,400cm(2)/Vs is the best result, so far, for the InAsSb materials with cutoff wavelength of 8-12 mum at 77 K. This may be attributed to the reduction of the carbon contamination by using a fused silica boat instead of a graphite boat. (C) 2002 Elsevier Science B.V. All rights reserved.

Investigation of {111}A and {111} planes of c-GaN epilayers grown on GaAs(001) by MOCVD

A determination of {1 1 1}A and {1 1 1}B in cubic GaN(c-GaN) was investigated by X-ray diffraction technique in detail. The c-GaN films are grown on GaAs(0 0 1) substrates by metalorganic chemical vapor deposition(MOCVD). The difference of integrated intensities measured by omega scan for the different order diffractions from {1 1 1}A and {1 1 1}B planes in the four-circle diffractometer gives convincing evidence as to which is the {1 1 1}A and which is the {1 1 1}B planes. The lesser deviation between the ratios of /F-h k l/(2)//F-(h) over bar (k) over bar (l) over bar/(2) and the calculated values after dispersion correction for atomic scattering factor shows that the content of parasitic hexagonal GaN(h-GaN) grown on c-GaN{1 1 1}A planes is higher than that on {1 1 1}B planes. The reciprocal space mappings provide additional proof that the h-GaN inclusions in c-GaN films appear as lamellar structure. (C) 2001 Published by Elsevier Science B.V.

Microtwins and twin inclusions in the 3C-SiC epilayers grown on Si(001) by APCVD

Microtwins in the 3C-SiC films grown on Si(001) by APCVD were analyzed in detail using an X-ray four-circle diffractometer. The empty set scan shows that 3C-SiC films can grow on Si substrates epitaxially and the epitaxial relationship is revealed as (001)(3C-SiC)//(001)(Si), [111](3C-SiC)//[111](Si). Other diffractions emerged in the pole figures of the (111) 3C-SiC. We performed the (10 (1) over bar0) h-SiC and the reciprocal space mapping of the (002) plane of twins for the first time, finding that the diffractions at chi = 15.8 degrees result from not hexagonal SiC but microtwins of 3C-SiC, and twin inclusions are estimated to be 1%.

Surface roughness and high density of cubic twins and hexagonal inclusions in cubic GaN epilayers

Surface roughness and its correlation with the polarity of internal hexagonal inclusions and cubic twins have been investigated by atomic force microscopy (AFM), scanning electron microscopy (SEM), transmission electron microscopy (TEM) and X-ray diffraction (XRD). The surface roughness resulted from large amount of strips, which prolonged in [1 (1) over bar0] direction with small size in [110] or [110] direction. The sidestep of each strip is just the top of high density of hexagonal inclusions or cubic microtwins. Moreover, XRD shows that the amount of hexagonal inclusions and cubic microtwins measured in [110] direction are twice or more as much as in [110] direction. Therefore, it is hexagonal inclusions, cubic twins and their distributive polarity that is responsible to the surface characteristics of cubic GaN epilayers.

Hydrogen behavior in GaN epilayers grown by NH3-MBE

Hydrogen behavior in unintentionally doped GaN epilayers on sapphire substrates grown by NH3-MBE is investigated. Firstly, we find by using nuclear reaction analysis (NRA) that with increasing hydrogen concentration the background electron concentration increases, which suggests that there exists a hydrogen-related donor in undoped GaN, Secondly, Fourier transform infrared (FTIR) absorption and X-ray photoelectron spectroscopy (XPS) reveal Further that hydrogen atom is bound to nitrogen atom in GaN with a local vibrational mode at about 3211 cm(-1) Hence, it is presumed that the hydrogen-related complex Ga. . .H-N is a hydrogen-related donor candidate partly responsible for high n-type background commonly observed in GaN films. Finally, Raman spectroscopy results of the epilayers show that ill addition to the expected compressive biaxial strain, in some cases GaN films suffer from serious tensile biaxial strain. This anomalous behavior has been well interpreted in terms of interstitial hydrogen lattice dilation. (C) 2001 Elsevier Science B.V. All rights reserved.

Polarity dependence of hexagonal inclusions and cubic twins in GaN/GaAs(001) epilayers measured by conventional X-ray pole figure and grazing incident diffraction pole figure

The distribution of mixed phases and its dependence on the polarity of cubic GaN epilayers are investigated by conventional X-ray pole figure and grazing incident diffraction (GID) pole figure. The hexagonal inclusions and cubic twins can be classified into two portions: one is formed with strict crystalline orientations, the other with crystalline misorientations. The former can be measured by conventional pole figures which reveal that the density of lamellate hexagonal grains and cubic twins located on (1 1 1)(Ga) and ((1) over bar (1) over bar1)(Ga) along [1 (1) over bar 0] direction are higher than those on ((1) over bar 1 1), and (1 (1) over bar 1)(N) along [110] direction. However, the low signals from tiny mixed phases with crystalline misorientations, detected by GID pole figures, distribute in a larger phi region near the [1 1 0] and [(1) over bar (1) over bar 0] directions with much weaker intensity, and in a smaller phi region near the [1 (1) over bar 0] and [(1) over bar 1 0] directions with slightly stronger intensity. (C) 2001 Elsevier Science B.V. All rights reserved.