The yellow Luminescence in GaN centered at 2.2 eV has been studied in various epitaxial layers grown by MOVPE on sapphire and by the sandwich sublimation methode on 6H-SiC substrates. The photoluminescence and optically detected magnetic resonance results can be consistently explained by a recombination model involving shallow donors and deep donors.

Free and bound exciton luminescences as well as donor-acceptor pair recombination of GaN epitaxial layers on 6H-SiC and sapphire substrates were investigated using time integrated and time resolved photoluminescence measurements at low temperatures. Lifetimes are determined for the donor bound exciton at 3.4722eV and for two acceptor bound excitons with energies of 3.4672eV and 3.459eV. Luminescences between 3.29eV and 3.37eV are identified as due to excitons deeply bound to centers located near the substrate-epilayer interface.

Bandstructure and transport properties of 4H- and 6H-SiC : optically detected cyclotron resonance investigations

We present experimental data on the band-structure and high-mobility transport properties of 6H and 4H-SiC epitaxial films based on optically detected cyclotron resonance investigations. From the orientational dependence of the electron effective mass in 6H-SiC we obtain direct evidence for the camels back nature of the conduction band between the M and L points. The broadening of the resonance signal in 4H-SIC as a function of temperature is used to extract information on electron mobilities and to conclude on the role of the different scattering mechanisms. Under high microwave powers an enhancement of the electron effective mass is found which is explained by a coupling of the electrons with longitudinal optical phonons.

The effects of microwave fields on recombination processes, which are responsible for the optical detection of cyclotron resonance (ODCR) in 4H and 6H SiC epitaxial layers, have been investigated. We present experimental evidence indicating that the dominant mechanism of ODCR in SiC, at low temperatures and in a common range of microwave power (<200 mW), is microwave-induced lattice heating under the cyclotron resonance conditions. The results also show that at low temperatures and low microwave power the dominant scattering mechanism is impurity scattering, while carrier scattering by lattice phonons dominates under high microwave power conditions.

We investigate epitaxial layers of GaN on c-plane sapphire by photoluminescence, optical density and x-ray diffraction. Besides the well known luminescence from hexagonal GaN we have identified two emission bands from cubic GaN. We observe the emission of the donor bound exciton in cubic GaN at 3.279 eV. The luminescence at 3.15 - 3.21 eV is explained as the cubic donor - acceptor recombination. The corresponding acceptor binding energy is found to be low as . For layers of 400 nm thickness, the optical density yield values for the average contents of cubic GaN between 10 and 25%. Proper growth conditions minimize the cubic contents in the upper regions of such layers.

Photoluminescence (PL) of GaN is commonly dominated by the annihilation of excitons bound to a 34.5 meV deep donor appearing at 3.472 eV (for strain free GaN at 4 K). With PL we were able to resolve two additional donor bound exciton transitions. The excitons have localization energies of 3.7 ± 0.3 and 11.3 ± 05 meV, respectively. Using Haynes rule the respective donors lie 56.5 and 18.5 meV below the conduction band. The applicability of Haynes rule could nicely be confirmed by IR absorption where the 1s-2p transition of the 34.5 and 58 meV donor are observed. The issue of residual donors in GaN will be discussed in this context.

A comprehensive study of the yellow photoluminescence (YL) in GaN epitaxial films grown by hydrid vapor phase epitaxy and by metal organic vapor phase epitaxy is presented including time-integrated and time-resolved photoluminescence (PL), PL excitation (PLE) and optically detected magnetic resonance (ODMR) experiments. ODMR reveals the participation of shallow and deep double donors based on the analysis of the g-values. This recombination model is supported by time-resolved investigations. PLE spectra show a close connection between the excitation processes of the YL band and of the inner transition of Fe3+ at 1.293 eV. Two-color stimulation experiments prove energy transfer between YL and the Fe3+ center by hole transfer, strongly confirming the YL recombination model involving a deep level 1.2 eV above the valence band.

We were able to significantly improve the quality of the heteroepitaxial films of the cubic modification of silicon carbide, 3C–SiC, by the use of organosilicon precursors and a careful control of the initial stage of the growth. The films were prepared by chemical vapor deposition from an organometallic precursor, methyltrichlorosilane. It is shown that the interfacial stress, which originates from the large lattice mismatch between the silicon substrate and the SiC film, efficiently relaxes with increasing distance between the interface yielding thick films of high quality for a thickness of ⩾20 μm.

We report on highly resolved exciton spectra of GaN films grown by hydride vapor phase epitaxy. Using calorimetric absorption and calorimetric reflection spectroscopy the excitonic transitions originating from the A-, B-, and C-valence bands are precisely determined and the crystal-field and spin-orbit-splitting energies are calculated. We also present the first magneto-optical experiments on the neutral-donor-bound exciton. Electron as well as the hole g-values are obtained by analyzing the Zeeman splittings at 12 T.

Photoluminescence (PL) has been used to study GaN layers, which were grown hy the sublimation sandwich technique on 6H-SiC substrates. The promising quality of the samples, which have a lattice mismatch of 3.5%, is demonstrated by the high intensity and the small half width (2 meV) of the donor bound exciton which is comparable to that for metalorganic vapour phase epitaxy grown layers. Fourier transform infrared photoluminescence spectroscopy was used to search for 3d elements (deep defects) which are incorporated as natural contaminants during the epitaxial growth. An emission with a no phonon line at 1.3 eV was observed, which is proposed in the literature to be caused by an internal 3d transition Fe3+, and also two additional strong emissions with sharp no phonon lines at 1.19 and 1.04 eV. The PL measurements show, that the emission at 1.19 eV is due to an internal 3d transition of a d(2) system. The no phonon line at 1.04 eV belongs to an internal 3d transition of a d(1), d(2), or d(7) system, possibly Ti2+, Ti3+, V3+, or Co2+. (C) 1996 The Institute of Materials.

We present a detailed photoluminescence excitation study of the optical transitions in GaN. This technique is employed to distinguish between band-to-band excitation and exciton contribution to the formation of the free exciton, bound exciton, violet and yellow photoluminescence bands. We show the dominant role of the Fröhlich polar intraband scattering in the formation of the free exciton states. We demonstrate that bound exciton states in a large extent are created by the capture of the free excitons by shallow impurities as well as by phononassisted resonant excitation of the bound exciton states. The capture of the free carriers excited in the band continuum is a main excitation source for the violet and yellow bands. However, distinct A- and C-exciton resonances are detected in the excitation spectra of the violet and yellow emission bands.

We studied ZnSe single quantum wells with well widths from 4 to 1 nm by optically detected cyclotron resonance (ODCR). In the samples grown by metal organic vapour phase epitaxy, where the residual shallow donor concentration is still of the order of mid 1016 cm−3, we find maximum electron mobilities of 46 000 cm2/V·s at low temperatures. The mechanism causing the cyclotron resonances to be observed in the luminescence is dominantly an energy transfer by heating the crystal lattice.

We present experimental evidence demonstrating that the dominant mechanism responsible for the optical detection of cyclotron resonance (ODCR) in SiC, at low temperatures and a common range of microwave power (<200 mW), is mainly due to microwave-induced lattice heating under the cyclotron resonance conditions. The results also show that at low temperatures and low microwave powers, the dominant scattering mechanism is impurity scattering, while carrier scattering by lattice phonons dominates under the high microwave power conditions.

Epitaxial layers of GaN on c-plane sapphire are analyzed by continuous-wave and time-resolved photoluminescence at 4K and by X-ray diffraction. Besides the well-known emissions from hexagonal GaN we observe luminescence bands at 3.279 and 3.15 to 3.21 eV which are identified as the transition of the donor bound exciton and the donor-acceptor pair recombination in cubic GaN, respectively. Measurements of the luminescence decay times are essential for the clarification of the emission processes. Due to the probing depth of about 200 nm in PL we find that the fraction of cubic phase typically decreases with layer thickness. In our best samples, however, we do not detect the cubic phase at all.

We report on photoluminescence and magneto-optical experiments of the neutral donor bound exciton in GaN epitaxial films. At low temperatures we observe free and bound exciton recombination with a linewidth as narrow as 1.4 meV. For the magnetic field applied perpendicularly to the c-axis of the crystal the donor bound exciton line significantly broadens and, at a field of 12 T a partially resolved splitting into two lines is visible. For the magnetic field parallel to the c-axis, only a very small broadening occurs. We analyze this behaviour in analogy to CdS and present a first estimate of the heavy hole g-value in GaN.

Results from optically detected cyclotron resonance (ODCR) studies of electron effective masses in 4H SiC are reported. ODCR measurements were performed on high-purity n-type 4H SiC epitaxial layers grown by chemical vapor deposition at both X band (9.23 GHz) and Q band (35.05 GHz) microwave frequencies. Electron effective masses in 4H SiC were directly determined as m⊥*=0.42m0 and m∥*=0.29m0. A scattering time in the basal plane τ⊥≊4.3×10−11 s, and hence, the corresponding electron mobility μ⊥≊1.8×105 cm2/V s, was obtained from a fit of the ODCR line shape.

We report on conductance and cyclotron resonance (CR) experiments on GaN epitaxial films grown by the OMVPE and HVPE techniques. From a precise determination of the electron effective mass the donor binding energy in the effective mass approximation (EMT) is calculated. We obtain 31.7 meV. The transport experiments on the HVPE films show that the conductance is thermally activated with an activation energy of 15 meV in contrast to the OMVPE films which showed temperature independent conductivity for temperatures between 4 and 100 K.

Fourier transform infrared absorption spectroscopy has been performed on a GaN epitaxial film grown by the hydride vapor phase epitaxy on sapphire substrate. We observe a transition at 215 cm(-1) with a half width of 2 cm(-1) which we attribute to an electronic transition on the basis of temperature dependent measurements. We assign it to the is to 2p transition of the residual shallow donors. Using effective mass theory neglecting anisotropies in the effective mass and dielectric constant the binding energy of the shallow donors is calculated to be 35.5 +/- 0.5 meV. This compares favourably well with a direct determination of the electron effective mass from which 33 meV is deduced.

Fourier transform infrared absorption spectroscopy has been performed on a GaN epitaxial film grown by the hydride vapor phase epitaxy on sapphire substrate. We observe a transition at 215 cm−1 with a half width of 2 cm−1, which we attribute to an electronic transition on the basis of temperature dependent measurements. We assign it to the 1s-2p transition of the residual shallow donors. Using effective mass theory neglecting anisotropies in the effective mass and dielectric constant the binding energy of the shallow donors is calculated to be (35.5 ± 0.5) meV. The electron effective mass is (0.236 ± 0.005) mo. Based on this knowledge, the energy separation between the donor-acceptor and the band-acceptor transitions as seen in photoluminescence at 45 K can be used to evaluate the donor concentration in the epitaxial films.