Influences of reactor pressure of GaN buffer layers on morphological evolution of GaN grown by MOCVD

The morphological evolution of GaN thin films grown on sapphire by metalorganic chemical vapor deposition was demonstrated to depend strongly on the growth pressure of GaN nucleation layer (NL). For the commonly used two-step growth process, a change in deposition pressure of NL greatly influences the growth mode and morphological evolution of the following GaN epitaxy. By means of atomic force microscopy and scanning electron microscope, it is shown that the initial density and the spacing of nucleation sites on the NL and subsequently the growth mode of FIT GaN epilayer may be directly controlled by tailoring the initial low temperature NL growth pressure. A mode is proposed to explain the TD reduction for NL grown at relatively high reactor pressure. (C) 2003 Elsevier B.V. All rights reserved.

In0.25Ga0.75As films growth on the thin GaAs/AlAs buffer layer on the GaAs(001) substrate

Thin GaAs/AlAs and GaAs/GaAs buffer layer structure have been fabricated on the GaAs(001) substrate. The top GaAs buffer layer is decoupled from the host substrate by introduction of a low temperature thin interlayer (AlAs or GaAs), which was mechanically behaved like the compliant substrate. Four hundred nanometer In0.25Ga0.75As films were grown on these substrates and the traditional substrate directly. Photoluminescence (PL), double-crystal X-ray diffraction (DCXRD) and atomic force microscopy (AFM) measurements were used to estimate the quality of the In0.25Ga0.75As layer and the compliant effects of the low temperature buffer layer. All the measurements shown that the qualities of epilayer have been improved and the substrate have been deteriorated severely. The growth technique of the thin GaAs/AlAs structure was found to be simple but very powerful for heteroepitaxy. (C) 2003 Elsevier Science B.V All rights reserved.

X-ray diffraction analysis of MOCVD grown GaN buffer layers on GaAs(001) substrates

In order to understand the growth feature of GaN on GaAs (0 0 1) substrates grown by metalorganic chemical vapor deposition (MOCVD), the crystallinity of GaN buffer layers with different thicknesses was investigated by using double crystal X-ray diffraction (DCXRD) measurements. The XRD results showed that the buffer layers consist of predominantly hexagonal GaN (h-GaN) and its content increases with buffer layer thickness. The nominal GaN (111) reflections with chi at 54.74degrees can be detected easily, while (0 0 2) reflections are rather weak. The integrated intensity of reflections from (111) planes is 4-6 times that of (0 0 2) reflections. Possible explanations are presented. (C) 2003 Elsevier Science B.V. All rights reserved.

Structural and optical properties of InAlGaN films grown directly on low-temperature buffer layer with (0001)sapphire substrate

Quaternary InAlGaN film has been grown directly on top of low-temperature-deposited GaN buffer layer by low-pressure metalorganic vapor phase epitaxy. High-resolution X-ray diffraction and photoluminescence (PL) results show that the film has good crystal quality and optical property. Temperature-dependent PL and time-resolved PL (TRPL) have been employed to study the carriers recombination dynamics in the film. The TRPL signals can be well fitted as a stretched exponential function exp[-(t/tau)(beta)] from 14 to 250 K, indicating that the emission is attributed to the radiative recombination of excitons localized in disorder quantum nanostructures such as quantum disks originating from indium (In) clusters or In composition fluctuation. The cross-sectional high-resolution electron microscopy measurement further proves that there exist the disorder quantum nanostructures in the quaternary. By investigating the dependence of the exponential parameter beta on the temperature, it is shown that the multiple trapping-detrapping mechanism dominates the diffusion among the localized states. The localized states are considered to have two-dimensional density of states (DOS) at 250 K, since radiative recombination lifetime tau(r) increases linearly with increasing temperature. (C) 2002 Elsevier Science B.V. All rights reserved.

In0.25Ga0.75As growth on low-temperature thin buffer layers formed on GaAs (001) substrate

High-quality In0.25Ga0.75As films were grown on low-temperature (LT) ultra-thin GaAs buffer layers formed on GaAs (0 0 1) substrate by molecular beam epitaxy. The epilayers were studied by atomic force microscopy (AFM), photo luminescence (PL) and double crystal X-ray diffraction (DCXRD), All the measurements indicated that LT thin buffer layer technique is a simple but powerful growth technique for heteroepitaxy. (C) 2002 Elsevier Science B.V. All rights reserved.

SiO2/Ta interface reaction in magnetic multilayers and its influence on Ta buffer layers

Ta is often used as a buffer layer in magnetic multilayers. In this study, Ta/Ni81Fe19/Ta multilayers were deposited by magnetron sputtering on sing-crystal Si with a 300-nm-thick SiO2 film. The composition and chemical states at the interface region of SiO2/Ta were studied using the X-ray photoelectron spectroscopy (XPS) and peak decomposition technique. The results show that there is an 'inter-mixing layer" at the SiO2/Ta interface due to a thermodynamically favorable reaction: 15 SiO2 + 37 Ta = 6 Ta2O5 + 5 Ta5Si3. Therefore, the Ta buffer layer thickness used to induce NiFe (111) texture increases.

Characterization of deep levels in Pt-GaN Schottky diodes deposited on intermediate-temperature buffer layers

Gallium nitride (GaN)-based Schottky junctions were fabricated by RF-plasma-assisted molecular beam epitaxy (MBE). The GaN epitaxial layers were deposited on novel double buffer layers that consist of a conventional low-temperature buffer layer (LTBL) grown at 500 degreesC and an intermediate-temperature buffer layer (ITBL) deposited at 690 degreesC. Low-frequency excess noise and deep level transient Fourier spectroscopy (DLTFS) were measured from the devices. The results demonstrate a significant reduction in the density of deep levels in the devices fabricated with the GaN films grown with an ITBL. Compared to the control sample, which was grown with just a conventional LTBL, a three-order-of-magnitude reduction in the deep levels 0.4 eV below the conduction band minimum (Ec) is observed in the bulk of the thin films using DLTFS measurements.

Rapid thermal annealing effects on step-graded InAlAs buffer layer and In0.52Al0.48As/In0.53Ga0.47As metamorphic high electron mobility transistor structures on GaAs substrates

A step-graded InAlAs buffer layer and an In0.52Al0.48As/In0.53Ga0.47As metamorphic high electron mobility transistor (MM-HEMT) structures were grown by molecular beam epitaxy on GaAs (001) substrates, and rapid thermal annealing was performed on them in the temperature range 500-800 degreesC for 30 s. The as-grown and annealed samples were investigated with Hall measurements, and 77 K photoluminescence. After rapid thermal annealing, the resistivities of step-graded InAlAs buffer layer structures became high. This can avoid leaky characteristics and parasitic capacitance for MM-HEMT devices. The highest sheet carrier density n(s) and mobility mu for MM-HEMT structures were achieved by annealing at 600 and 650degreesC, respectively. The relative intensities of the transitions between the second electron subband to the first heavy-hole subband and the first electron subband to the first heavy-hole subband in the MM-HEMT InGaAs well layer were compared under different annealing temperatures. (C) 2002 American Institute of Physics.

Study of GaN thin films grown on intermediate-temperature buffer layers by molecular beam epitaxy

A detailed characterisation study of GaN thin films grown by rf-plasma molecular beam epitaxy on intermediate-temperature buffer layers (ITBL) was carried out with Hall, photoluminescence (PL) and deep-level transient Fourier spectroscopy (DLTFS) techniques. The unique feature of our GaN thin films is that the GaN epitaxial layers are grown on top of a double layer that consists of an ITBL, which is grown at 690 degreesC, and a conventional low-temperature buffer layer deposited at 500 degreesC. It is observed that the electron mobility increases steadily with the thickness of the ITBL, which peaks at 377 cm(2)V(-1)S(-1) for an ITBL thickness of 800 nm. The PL also demonstrated systematic improvements with the thickness of the ITBL. The DLTFS results suggest a three-order-of-magnitude reduction in the deep level at E-c-0.40 eV in the device fabricated with the GaN films grown on an ITBL thickness of 1.25 mum in comparison with the control device without an ITBL. Our analyses indicate that the utilization of an ITBL in addition to the conventional low-temperature buffer layer leads to the relaxation of residual strain within the material, resulting in an improvement in the optoelectronic properties of the films. (C) 2002 Elsevier Science BN. All rights reserved.

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.

Effect of buffer layer growth conditions on the secondary hexagonal phase content in cubic GaN films on GaAs(001) substrates

In this letter, we investigated the effect of the buffer layer growth conditions on the secondary hexagonal phase content in cubic GaN films on GaAs(0 0 1) substrate. The reflection high-energy electron diffraction (RHEED) pattern of the low-temperature GaN buffer layers shows that both the deposition temperature and time are important in obtaining a smooth surface. Four-circle X-ray double-crystal diffraction (XRDCD) reciprocal space mapping was used to study the hexagonal phase inclusions in the cubic GaN (c-GaN) films grown on the buffer layers. The calculation of the volume contents of the hexagonal phase shows that higher temperature and longer time deposition of the buffer layer is not preferable for growing pure c-GaN film. Under optimized condition, 47 meV FWHM of near band gap emission of the c-GaN film was achieved. (C) 2000 Elsevier Science B.V. All rights reserved.

Sputtering of ZnO buffer layer on Si for GaN blue light emitting materials

The preparation of high quality ZnO/Si substrates for the growth of GaN blue light emitting materials is considered. ZnO thin films have been deposited on Si(100) and Si(lll) substrates by conventional magnetron sputtering. Morphology, crystallinity and c-axis preferred orientation of ZnO thin films have been investigated by transmitting electron microscopy (TEM), X-ray diffraction (XRD) and X-ray rocking curve (XRC). It is proved that the ZnO thin films have perfect structure. The full-width-at-half-maximum (FWHM) of the ZnO(002) XRC of these films is about 1 degrees, while the minimum is 0.353 degrees. This result is better than the minimum FWHM (about 2 degrees) reported by other research groups. Moreover, comparison and discussion are given on film structure of ZnO/Si(100) and ZnO/Si(lll).

The effects of carbonized buffer layer on the growth of SiC on Si

Carbonized buffer layers were formed with C2H4 on Si(100) and Si(111) substrates using different methods and SIC epilayers were grown on each buffer layer at 1050 degrees C with simultaneous supply of C2H4 and Si2H6. The structure of carbonized and epitaxy layers was analyzed with in situ RHEED. The buffer layers formed at 800 degrees C were polycrystalline on both Si(100) and Si(111) substrates whereas they were single crystals, with twins on Si(100) and without tu ins on Si(111)substrates. when formed with a gradual rise in substrate temperature from 300 degrees C to growth temperature. Raising the substrate temperature slowly results in the formation of more twins. Epilayers grown on carbonized polycrystalline lavers are polycrystalline. Single crystal epilayers without twins grow on single crystalline buffer layers without twins or with a few twins. (C) 1999 Elsevier Science B.V. All rights reserved.

Microstructure evolution of GaN buffer layer on MgAl2O4 substrate

Microstructure of GaN buffer layer grown on (111)MgAl2O4 substrate by metalorganic vapor phase epitaxy (MOVPE) was studied by transmission electron microscopy (TEM). It has been observed that the early deposition of GaN buffer layer on the substrate at a relatively low temperature formed a continual island-sublayer (5 nm thick) with hexagonal crystallographic structure, and the subsequent GaN buffer deposition led to crystal columns which are composed of nano-crystal slices with mixed cubic and hexagonal phases. After high-temperature annealing, the crystallinity of nano-crystal slices and island-sublayer in the buffer layer have been improved. The formation of threading dislocations in the GaN him is attributed not only to the lattice mismatch of GaN/MgAl2O4 interface, but also to the stacking mismatches at the crystal column boundaries. (C) 1998 Published by Elsevier Science B.V. All rights reserved.