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.

Epitaxial growth and characterization of SiC on C-plane sapphire substrates by ammonia nitridation

The growth of SiC epilayers on C-face (0 0 0 1) sapphire (alpha-Al2O3) has been performed using CVD method. We found that the quality of SiC epilayers has been improved through the nitridation of substrates by exposing them to ammonia ambient, as compared to growth on bare sapphire substrates. The single crystallinity of these layers was verified by XRD and double crystal XRD measurements. Atomic force microscopy was used to evaluate the surface morphology. Infrared reflectivity and Raman scattering measurement were carried out to investigate the phonon modes in the grown SiC. Detailed Raman analysis identified the 6H nature of the as-grown SiC films. (C) 2002 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.

In situ doping of 3C-SiC grown on (0001) sapphire substrates by LPCVD

The heteroepitaxial growth of n-type and p-type 3C-SiC on (0001) sapphire substrates has been performed with a supply of SiH4+C2H4+H-2 system by introducing ammonia (NH3) and diborane (B2H6) precursors, respectively, into gas mixtures. Intentionally incorporated nitrogen impurity levels were affected by changing the Si/C ratio within the growth reactor. As an acceptor, boron can be added uniformly into the growing 3C-SiC epilayers. Nitrogen-doped 3C-SiC epilayers were n-type conduction, and boron-doped epilayers were p-type and probably heavily compensated.

Influences of initial nitridation process on the optical and structural characterization of GaN layer grown on sapphire (0001) by metalorganic chemical vapor deposition

In this paper. we investigate the influences of the initial nitridation of sapphire substrates on the optical and structural characterizations in GaN films. Two GaN samples with and without 3 min nitridation process were investigated by photoluminescence (PL) spectroscopy in the temperature range of 12-300 K and double-crystal X-ray diffraction (XRD). In the 12 K PL spectra of the GaN sample without nitridation, four dominant peaks at 3.476, 3.409 3.362 and 3.308 eV were observed, which were assigned to donor bound exciton, excitons bound to stacking faults and extended structural defects. In the sample with nitridation, three peaks at 3.453, 3.365. and 3.308 eV were observed at 12 K, no peak related to stacking faults. XRD results at different reflections showed that there are more stacking faults in the samples without nitridation.

Surface acoustic wave velocity and electromechanical coupling coefficient of GaN grown on (0001) sapphire by metal-organic vapour phase epitaxy

High-quality and high-resistivity GaN films were grown on (0001) sapphire face by metal-organic vapour phase epitaxy. To measure the surface acoustic wave properties accurately, we deposited metallized interdigital transducers on the GaN surface. The acoustic surface wave velocity and electromechanical coupling coefficient were measured, respectively, to be 5667 m/s and 1.9% by the pulse method.

Competition between band gap and yellow luminescence in undoped GaN grown by MOVPE on sapphire substrate

Undoped GaN epilayer on c-face (0 0 0 1) sapphire substrate has been grown by metalorganic vapor-phase epitaxy (MOVPE) in a horizontal-type low-pressure two-channel reactor. Photoluminescence (PL) as a function of temperature and excitation intensity have been systematically studied, and the competition between near band gap ultraviolet (UV) and defect-related yellow luminescence (YL) has been extensively investigated, It is revealed that the ratio of the UV-to-YL peak intensities depends strongly on the excitation intensity and the measurement temperature. The obtained results have been analyzed in comparison with the theoretical predications based on a bimolecular model. (C) 2001 Elsevier Science B.V. All rights reserved.

Effect of in situ thermal treatment during growth on crystal quality of GaN epilayer grown on sapphire substrate by MOVPE

GaN epilayers on sapphire substrate grown by metalorganic vapor-phase epitaxy (MOVPE) in a horizontal-type low-pressure two-channel reactor were investigated. Samples were characterized by X-ray diffraction (XRD), Raman scattering, atomic force microscopy (AFM) and photoluminescence (PL) measurements. The influence of the temperature changes between low temperature (LT) deposited GaN buffer and high temperature (WT) grown GaN epilayer on crystal quality of epilayer was extensively studied. The effect of in situ thermal annealing during the growth on improving the GaN layer crystal quality was demonstrated and the possible mechanism involved in such a growth process was discussed. (C) 2001 Elsevier Science B.V. All rights reserved.

Fabrication and characterization of metal-semiconductor-metal (MSM) ultraviolet photodetectors on undoped GaN/sapphire grown by MBE

Interdigital metal-semiconductor-metal (MSM) ultraviolet photoconductive detectors have been fabricated on undoped GaN films grown by molecular beam epitaxy (MBE), Response dependence on wavelength, applied current, excitation powers and chopper frequency has been extensively investigated. It is shown that the photodetector's spectral response remained nearly constant for wavelengths above the band gap and dropped sharply by almost three orders of magnitude for wavelengths longer than the band gap. It increases linearly with the applied constant current, but very nonlinearly with illuminating power. The photodetectors showed high photoconductor gains resulting from trapping of minority carriers (holes) at acceptor impurities or defects. The results demonstrated the high quality of the GaN crystal used to fabricate these devices. (C) 2000 Elsevier Science B.V. All rights reserved.

Improvement of thin silicon on sapphire (SOS) film materials and device performances by solid phase epitaxy

The increased emphasis on sub-micron CMOS/SOS devices has placed a demand for high quality thin silicon on sapphire (SOS) films with thickness of the order 100-200 nm. It is demonstrated that the crystalline quality of as-grown thin SOS films by the CVD method can be greatly improved by solid phase epitaxy (SPE) process: implantation of self-silicon ions and subsequent thermal annealing. Subsequent regrowth of this amorphous layer leads to a greater improvement in silicon layer crystallinity and channel carrier mobility, evidenced, respectively, by double crystal X-ray diffraction and electrical measurements. We concluded that the thin SPE SOS films are suitable for application to high-performance CMOS circuitry. (C) 2000 Elsevier Science S.A. All rights reserved.

Electrical properties of GaN deposited on nitridated sapphire by molecular beam epitaxy using NH3 cracked on the growing surface

We have found that GaN epilayers grown by NH3-source molecular beam epitaxy (MBE) contain hydrogen. Dependent on the hydrogen concentration, GaN on (0001) sapphire can be either under biaxially compressive strain or under biaxially tensile strain. Furthermore, we notice that background electrons in GaN increase with hydrogen incorporation. X-ray photoelectron spectroscopy (XPS) measurements of the N1s region indicate that hydrogen is bound to nitrogen. So, the microdefect Ga...H-N is an effective nitrogen vacancy in GaN, and it may be a donor partly answering for the background electrons. (C) 1999 Elsevier Science B.V. All rights reserved.

The effect of buried AlxGa1-xN isolating layers on the transport properties of GaN deposited on sapphire substrate by molecular beam epitaxy using NH3

It is believed that the highly dislocated region near the GaN/sapphire interface is a degenerate layer. In this paper a direct evidence for such a proposal is presented. By inserting a buried AlxGa1-xN (x > 0.5) isolating layer to separate the interface region from the bulk region, the background electron concentration can be significantly reduced, while care must be taken to guarantee that there is no degrading of Hall mobility when choosing the thickness of the isolating layer. (C) 1998 Elsevier Science B.V. All rights reserved.

Electronic and vibrational Raman scattering in resonance with yellow luminescence transitions in GaN on sapphire substrate

We analyze low-temperature Raman and photoluminescence spectra of MBE-grown GaN layers on sapphire. Strong and sharp Raman peaks are observed in the low frequency region. These peaks, which are enhanced by excitation in resonance with yellow luminescence transitions, are attributed to electronic transitions related to shallow donor levels in hexagonal GaN. It is proposed that a low frequency Raman peak at 11.7 meV is caused by a pseudo-local vibration mode related to defects involved in yellow luminescence transitions. The dependence of the photoluminescence spectra on temperature gives additional information about the residual impurities in these GaN layers.

Rapid thermal annealing processing of GaN epilayer on sapphire(0 0 0 1)

The effect of rapid thermal annealing (RTA) in a Nz ambient up to 900 degrees C has been investigated for GaN films grown on sapphire(0 0 0 1) substrates. Raman spectra, X-ray diffractometry and Hall-effect studies were performed for this purpose. The Raman spectra show the presence of the E-2 (high) mode and a shift in the wave number of this mode with respect to the annealing processing. This result suggests the presence and relaxation of residual stress due to thermal expansion misfit in the films which are confirmed by X-ray measurements and the structure quality of GaN epilayer was improved. Furthermore, the electron mobility increased at room temperature with respect to decrease of background electron concentration after RTA. (C) 1998 Elsevier Science B.V. All rights reserved.