High-quality Epitaxial Layers Research Articles

Properties of Different Room-Temperature Photoluminescence Bands in 4H-SiC Substrates Investigated by Mapping Techniques

The room-temperature photoluminescence (RTPL) was investigated in commercial nitrogen-doped 4H-SiC substrates. In a typical RTPL spectrum of n-type 4H-SiC substrate, the ‘band-edge’ emission was similar to PL signatures that are typically attributed to free-exciton recombination in high-quality thick epitaxial layers. The origin of the deep-defect ‘red’ emission and its influence on recombination properties of SiC remain unclear. In most of the substrates in which the ‘red’ RTPL band was strong, clear reverse correlation between the ‘red’ and ‘band-edge’ RTPL intensities was observed. In contrast, direct correlation was observed between the ‘bandedge’ PL map and distribution of the net free electron concentration. There is a possibility that incorporation of nitrogen donors is influenced by (or influences) incorporation of lifetime-limiting deep defects.

InN Nanorods Grown on Different Planes of Al2O3

Extract Extended abstract of a paper presented at Microscopy and Microanalysis 2007 in Ft. Lauderdale, Florida, USA, August 5 – August 9, 2007

Fully unstrained GaN on sacrificial AlN layers by nano‐heteroepitaxy

AbstractUsually, the fabrication of microelectromechanical systems (MEMS) requires unstrained or tensile strained active layers on a selectively removable sacrificial layer, since compressive strain causes instabilities due to buckling effects. For group III‐nitride based MEMS, AlN is a promising material for sacrificial layers since it can be epitaxially overgrown and etched selectively to GaN. However, due to the larger lattice constants GaN is growing compressively strained on AlN. Nanoheteroepitaxy opens a way to yield fully unstrained, high quality epitaxial GaN layers on nanocrystalline AlN thin film by means of a 3D strain relaxation mechanism. For this purpose sputtered nanocrystalline AlN films were overgrown with single crystalline GaN and AlGaN/GaN layers by metalorganic chemical vapor deposition. The high quality of the layers is proven by an atomically flat surface and a 2D electron gas at the interface of the AlGaN/GaN heterostructure (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

The influence of AlN/GaN superlattice intermediate layer on the properties of GaN grown on Si (111) substrates

AlN/GaN superlattice buffer is inserted between GaN epitaxial layer and Si substrate before epitaxial growth of GaN layer. High-quality and crack-free GaN epitaxial layers can be obtained by inserting AlN/GaN superlattice buffer layer. The influence of AlN/GaN superlattice buffer layer on the properties of GaN films are investigated in this paper. One of the important roles of the superlattice is to release tensile strain between Si substrate and epilayer. Raman spectra show a substantial decrease of in-plane tensile strain in GaN layers by using AlN/GaN superlattice buffer layer. Moreover, TEM cross-sectional images show that the densities of both screw and edge dislocations are significantly reduced. The GaN films grown on Si with the superlattice buffer also have better surface morphology and optical properties.

Morphology Controllable ZnO Growth on Facet-Controlled Epitaxial Lateral Overgrown GaN/Sapphire Templates

High-quality epitaxial ZnO layers grown on facet-controlled epitaxial lateral overgrown (FACELO) GaN/sapphire templates were achieved by an evaporation−physical transport−condensation method. The ZnO/FACELO GaN heterostructures showed a substantial improvement in the crystalline quality with a lower defect density and excellent photoluminescence emission. The effects of the growth temperature and oxygen flow rate on the morphology of ZnO were systematically studied. The determining factors for the formation of different morphologies were found to be gas-phase supersaturation and the surface energy of the growing planes. In addition, the lattice matching between the ZnO and GaN leads to the improvement of the electric and optical properties. With such kind of perfect ZnO/GaN heterostructure interfaces obtained on the semipolar (112̄2) surface, the new opportunities for the fabrication of hybrid ZnO/GaN optoelectronic devices on sapphire are proposed.

Strain relaxation mechanism in a reverse compositionally graded SiGe heterostructure

A concept of compositional reverse-grading (RG) in SiGe∕Si heteroepitaxy has been proposed, in which the graded layer lattice mismatch starts at the highest value at the RG/Si interface and decreases to a final mismatch at the SiGe/RG interface. Using various characterization techniques, the authors show that this low-dislocation-density strain relaxation mechanism relies on the large nucleation rates of misfit dislocations at the abrupt RG/Si interface and the reduction of threading dislocations at the SiGe/RG interface by facilitating glide. The RG concept enables the growth of high-quality relaxed epitaxial layer on a thin buffer layer, suitable as a substrate for many microelectronic and optoelectronic applications.

High-purity GaN epitaxial layers for power devices on low-dislocation-density GaN substrates

GaN epitaxial layers at low-doping concentrations around 1×10 16 cm −3 for power devices have been grown on low-dislocation-density GaN substrates. The epitaxial layers have shown much less-compensated behavior in net doping concentration, which arises from their much high-purity nature than those on sapphire substrates. Schottky barrier diode fabricated on the layer have shown extremely low leakage current and high breakdown voltage of 580 V, and low specific on-resistance of 1.3 mΩ cm 2, as expected from its high-quality epitaxial layer.

In-situ spectroscopic study of the As and Te on the Si (112) surface for high-quality epitaxial layers

A detailed analysis of the As-exposed Si (112) and subsequent Te exposure was performed. X-ray photoelectron spectroscopy shows that the Te- and As-exposed Si (112) surface had 70% As and 27% Te coverage, respectively. Direct surface coverage measurement with ion scattering spectroscopy (ISS) shows that the Si (111) surface is completely covered by As, and that of the Si (112) had about 78% and 20% coverage of As and Te, respectively. Finally, using ISS shadowing effects, it was found that the Te atoms were positioned mainly on the step edges.

P-down ZnSTeSe/ZnSe/GaAs heterostructure photodiodes

High-quality quaternary ZnSTeSe epitaxial layers with uniform carrier concentration of 1/spl times/10/sup 17/ cm/sup -3/ were successfully grown on p-GaAs substrates by molecular beam epitaxy. P-down ZnSTeSe/ZnSe/GaAs heterostructure photodiodes were also fabricated. It was found that the maximum quantum efficiency of the fabricated ZnSTeSe photodiodes was around 75% with a large spectral width of 500 nm.

Molecular beam epitaxy growth of In0.52Al0.48As∕In0.53Ga0.47As metamorphic high electron mobility transistor employing growth interruption and in situ rapid thermal annealing

We investigated the effects of high temperature (∼700°C) in situ rapid thermal annealing (RTA) carried out during growth interruption between spacer and δ-doping layers of an In0.52Al0.48As∕In0.53Ga0.47As metamorphic high electron mobility transistor (MHEMT) grown on a compositionally graded InGaAlAs buffer layer. The in situ RTA improved optical and structural properties of the MHEMT without degradation of transport property, while postgrowth RTA improved the structural property of the MHEMT but significantly degraded mobility due to the defect-assisted Si diffusion. The results indicate the potential of the in situ RTA for use in the growth of high-quality metamorphic epitaxial layers for optoelectronic applications requiring improved optical and electrical properties.

Morphological defects and uniformity issues of 4H-SiC homoepitaxial layers grown on off-oriented (0 0 0 1) Si faces

The morphological defects and uniformity of 4H-SiC epilayers grown by hot wall CVD at 1500 °C on off-oriented (0 0 0 1) Si faces are characterized by atomic force microscope, Nomarski optical microscopy, and Micro-Raman spectroscopy. Typical morphological defects including triangular defects, wavy steps, round pits, and groove defects are observed in mirror-like SiC epilayers. The preparation of the substrate surface is necessary for the growth of high-quality 4H-SiC epitaxial layers with low-surface defect density under optimized growth conditions.

Fully Unstrained GaN on Thick AlN Layers for MEMS Application

ABSTRACTUsually, the fabrication of microelectromechanical systems (MEMS) requires unstrained or tensile strained active layers on a selectively removable sacrificial layer. Compressive strain would lead to instabilities due to buckling effects. For group III-nitride based MEMS, AlN is a promising material for sacrificial layers since it can be epitaxially overgrown and etched selectively to GaN. However, due to the larger lattice constants GaN is growing compressively strained on AlN. Nanoheteroepitaxy opens a way to yield unstrained, high quality epitaxial GaN layers on nanocrystalline AlN thin film by means of a 3D strain relaxation mechanism. For this purpose sputtered nanocrystalline AlN films were overgrown with single crystalline GaN and AlGaN/GaN layers by metalorganic chemical vapor deposition. The high quality of the layers is proven by an atomically flat surface and a 2D electron gas at the interface of the AlGaN/GaN heterostructure.

Homoepitaxial growth on 4H-SiC Substrates by Chemical Vapor Deposition

AbstractLow dislocation density and even micropipe free substrates open up a great opportunity to produce high quality epitaxial layers. The epi-layer will become more sensitive to the epitaxial process itself and less dependent on varying substrate quality. Results presented here indicate a strong dependence of the epitaxial layers' growth mode and surface roughness on the substrates' dislocations and domain structure. Thick epitaxial layers grown on near on-axis wafers, both C-face and Si-face, show very similar growth behaviors as for physical vapor transport (PVT) grown crystals. The mix between step-flow and dislocation driven growth on near on-axis C-face chemical vapor deposition (CVD) grown epi-layers (30 μm thick) is resulting in a as smooth, and measured surface roughness is as low, as for growth on 8o off-axis substrates.Surface roughness, Ra, measured on a 50 μm thick epitaxial layer, grown on a low EPD (1E+4 cm-2) 8o off-axis micropipe free substrate, was as low as 1.0 nm (2.98x2.32 mm area).

Direct growth of high-quality CdTe epilayers on Si(2 1 1) substrates by metalorganic vapor-phase epitaxy

High-quality epitaxial CdTe layers were grown on Si(2 1 1) substrates in a metalorganic vapor-phase epitaxy. In order to obtain homo-orientation growth on Si substrates, pre-treatment of the substrates was carried out in a separate chamber by annealing them together with pieces of GaAs at 800–900 °C in a hydrogen environment. Grown epilayers had very good substrate adhesion. The full-width at half-maximum value of the X-ray double crystal rocking curve from the CdTe(4 2 2) plane decreased rapidly with increasing layer thickness, and remained between 140–200 arcsec for layers thicker than 18 μm. Photoluminescence measurement at 4.2 K showed high-intensity bound excitonic emission and very small defect-related deep emissions indicating the high crystalline quality of the grown layers.

Electrical transport properties of aluminum-implanted 4H–SiC

The free hole density and low-field mobility of aluminum-doped 4H–SiC were investigated in the temperature range of 100–900K, both, experimentally and theoretically. Experimental data for implanted p-type 4H–SiC were compared with theoretical calculations using parameters determined for high-quality epitaxial layers. The deformation potential for intra- and intervalley scattering by acoustic phonons and the effective coupling constant for intra- and intervalley scattering by nonpolar optical phonons were determined. The detailed analysis of the implanted layers with aluminum-targeted concentration ranging from 3.33×1018to1021cm−3 shows that (i) about half of the implanted atoms are electrically active in the SiC lattice, (ii) a systematic compensation of about 10% of the doping level is induced by the implantation process, (iii) two different ionization energies for the aluminum atoms have to be used. Their origin is discussed in terms of inequivalent hexagonal and cubic lattice sites. Finally, the doping dependence of the ionization ratio and Hall mobility are given for non- and weakly (10%) compensated material at 292K. The maximum achievable mobility for low-doped material in p-type 4H–SiC is shown to be 93cm2∕Vs at room temperature.

A GeSi-buffer structure for growth of high-quality GaAs epitaxial layers on a Si substrate

A SiGe-buffer structure for growth of high-quality GaAs layers on a Si (100) substrate is proposed. For the growth of this SiGe-buffer structure, a 0.8-µm Si0.1 Ge0.9 layer was first grown. Because of the large mismatch between this layer and the Si substrate, many dislocations formed near the interface and in the low part of the Si0.1Ge0.9 layer. A 0.8-µm Si0.05Ge0.95 layer and a 1-µm top Ge layer were subsequently grown. The strained Si0.05Ge0.95/Si0.1Ge0.9 and Ge/Si0.05Ge0.95 interfaces formed can bend and terminate the upward-propagated dislocations very effectively. An in-situ annealing process is also performed for each individual layer. Finally, a 1–3-µm GaAs film was grown by metal-organic chemical vapor deposition (MOCVD) at 600°C. The experimental results show that the dislocation density in the top Ge and GaAs layers can be greatly reduced, and the surface was kept very smooth after growth, while the total thickness of the structure was only 5.1 µm (2.6-µm SiGe-buffer structure +2.5-µm GaAs layer).

Coherent longitudinal optical phonon and plasmon coupling in the near-surface region of InN

Coherent phonon spectroscopy of a high-quality InN epitaxial layer is carried out using time-resolved second-harmonic generation. A coherent longitudinal optical phonon and plasmon coupling mode only at 447cm−1 can be resolved in the spectrum. Its frequency shows no dependence on the photoinjected carrier density up to 1.5×1019cm−3. This phenomenon is attributed to the hybridization of a coherent A1(LO) phonon with the intrinsic cold plasma accumulated in the near-surface region of InN, where the plasma density could reach on the order of 1020cm−3, much higher than the bulk carrier concentration 1×1018cm−3 determined by Hall effect measurement.

Room-temperature photoluminescence of Ga0.96In0.04As0.11Sb0.89 lattice matched to InAs

Photoluminescence (PL) has been observed at room temperature from a Ga0.96In0.04As0.11Sb0.89 quaternary solid solution for the first time. High-quality epitaxial layers of n-type (Te-doped) Ga0.96In0.04As0.11Sb0.89 with low In content were grown by liquid phase epitaxy (LPE) lattice-matched to InAs(100) substrates from a Ga-rich melt. The PL properties of the material were investigated over a wide temperature range, and the principal radiative transitions were identified. In the temperature range <150 K, donor-acceptor recombination involving the first and second ionization state of native antisite defects was the dominant radiative-recombination process, whereas interband recombination was found to dominate at room temperature.

High-quality ZnSSeTe epitaxial layers grown by MBE

High-quality quaternary ZnSSeTe epitaxial layers were successively grown on GaAs substrates by molecular beam epitaxy. It was found that a ZnSSe layer was automatically formed between a ZnSe buffer layer and ZnSSeTe epitaxial layer when we increased the ZnS cell temperature. It was also found that the photoluminescence (PL) activation energy of the sample was 263 meV, which was much larger than those observed from ZnSe and/or ZnSSe epitaxial layers. The much larger PL activation energy was attributed to the fact that excitons in ZnSSeTe were still bounded at the localized Te or Te n cluster states.

Ion beam characterization of GaAs 1− x− yN xBi y epitaxial layers

Incorporation of Bi in GaAs 1− x N x epitaxial layers represents a significant interest as Bi compensates the lattice parameter reduction caused by the N incorporation while contributing to the reduction of the band gap energy. GaAs 1− x− y N x Bi y epitaxial layers were grown on GaAs wafers by molecular beam epitaxy. The quality of the films as well as the concentration and lattice location of Bi and N were characterized by channeling Rutherford backscattering spectrometry and nuclear reaction analysis using 2 and 3.72 MeV He beams, respectively. The amount of nitrogen in the film was measured by means of the 14N(α, p) 17O endothermic nuclear reaction and elastic recoil detection. The results indicate that high quality epitaxial layers were obtained, with y=1.8% Bi incorporated into the layer. Angular scan along the main axes showed no strain in the film and indicated that most of Bi atoms are located at substitutional sites. Nitrogen lattice incorporation is more difficult to establish because of the presence of Bi in the layer, but we estimate the substitutional fraction to be 71 ± 6%.