In the last decade, we have seen very rapid and significant developments in Raman scattering experiments on GaN and related nitride compounds: the Γ-point phonon frequencies have been identified for both cubic and hexagonal structures of binary compounds of GaN, AlN, and InN. The phonon spectra of their ternary alloys, InGaN and AlGaN, were also intensively studied. On the basis of these studies, characterizations of strain, compositional fluctuation, defects, impurities, etc, are now being intensively conducted. Besides such pure lattice properties, coupled modes between a lattice vibration (LO phonon) and a collective excitation of free carriers (plasmon) in GaN have been thoroughly studied, and the results are now widely applied to characterize carrier-transport properties. Low-dimensional structures of nitrides such as quantum dots and superlattices will soon enter the most active field of Raman scattering characterization. This article briefly reviews the present status of Raman scattering experiments on GaN and related nitride compounds and presents some future prospects.

It has been recognized that Raman scattering spectroscopy is a powerful tool to characterize SiC crystals non‐destructively. We review recent significant developments in the use of Raman scattering to study structural and electronic properties of SiC crystals. The areas to be discussed in the first part include polytype identification, evaluation of stacking disorder and ion‐implantation damages, and stress evaluation. The Raman scattering by electronic transitions is discussed in the second part of this article. We concentrate on the plasmon LO‐phonon coupled modes whose spectral profiles are used to evaluate the carrier concentration and mobility. Anisotropic electronic properties of α‐SiC and characteristics of heavily doped crystals are discussed. Semiconductor‐to‐metal transition and Fano interference effect are also treated.

Wurtzite InN films were grown on a thick GaN layer by metalorganic vapor phase epitaxy. Growth of a (0001)-oriented single crystalline layer was confirmed by Raman scattering, x-ray diffraction, and reflection high energy electron diffraction. We observed at room temperature strong photoluminescence (PL) at 0.76 eV as well as a clear absorption edge at 0.7–1.0 eV. In contrast, no PL was observed, even by high power excitation, at ∼1.9 eV, which had been reported as the band gap in absorption experiments on polycrystalline films. Careful inspection strongly suggests that a wurtzite InN single crystal has a true bandgap of 0.7–1.0 eV, and the discrepancy could be attributed to the difference in crystallinity.

The physical properties of InN crystals are known rather poorly, since the existing growth techniques have not produced epitaxial layers of good quality [1,2]. Even a key parameter of InN -the band gap E g -has not been firmly established so far. E g values of 1.8 eV to 2.1 eV have usually been estimated from the absorption spectra obtained on polycrystalline and nanocrystalline hexagonal InN [3][4][5][6]. No data on the band-to-band photoluminescence (PL) of InN are available in the literature. Recently an improved growth technique has made it possible to obtain single-crystalline InN layers [7]. Optical measurements on these InN layers have shown some strong differences from absorption data reported earlier [8]. In the present work the electronic structure of singlecrystalline InN layers was carefully studied by means of optical absorption, PL, and photoluminescence excitation (PLE) spectroscopy as well as by ab initio calculations. Our results revealed for hexagonal InN a band gap of about 0.9 eV, which is much smaller than the values of 1.8 eV to 2.1 eV reported previously.Single-crystalline InN epilayers were grown on (0001) sapphire substrates either by plasma-assisted molecular-beam epitaxy (PAMBE) [7] or metalorganic molecular-beam epitaxy (MOMBE) [9] and were characterized by many techniques. Only hexagonal symmetry, with no traces of other polymorphs, was established by X-ray analysis in all the samples. For characterization the symmetric (0002) and asymmetric ð11 2 24Þ Bragg reflexes were used. From these data the lattice constants in the InN layers were found to be c ¼ 5.7039 A and a ¼ 3.5365 A. The narrow profiles of q and q-2q scans at the (0002) reflex (250-300 arcsec and 50-60 arcsec, respectively) indicate a good crystalline quality. Polarized Raman spectra of InN show agreement with the selection rules for the hexagonal symmetry. The Raman phonon line widths correspond to a well-ordered crystal lattice [9,10]. Atomic force microscopy measurements did not reveal any columnar structure in the samples studied. According to the Auger data, the oxygen concentration did not exceed 0.1%. The Hall concentration of electrons n ranged from 9 Â 10 18 to 1.2 Â 10 19 cm -3 in the best samples, and their mobility was found to be as high as m $ 1900 cm 2 V -1 s -1 .The absorption coefficient a(w) for PAMBE-and MOMBE-grown InN samples at 300 K is shown in Fig. 1. The layer thickness was measured by means of scanning electron microscopy. The aðwÞ spectra were calculated from the transmission spectra with corrections for multiple reflections. It can be seen that the edge absorption rapidly reaches values of a(w) > 5 Â 10 4 cm À1 , which is typical of direct band-gap crystals. The inset in Fig. 1 shows that the absorption coefficient can be described by the relation a(w) $ ( hw -E g ) 1/2 usually applicable to allowed direct interband transitions. From the measurement of the absorption edges it can be concluded that the E g phys. stat. sol. (b) 229, No. 3, R1-R3 (2002)

Interference of impulsively excited coherent phonons in semimetals has been studied by using a double-pulse pump–probe technique. Enhancement of the oscillation amplitude of an A1g mode is observed when the separation time of the double-pulse is matched to the period of the phonon oscillation, and a cancellation is observed when the separation time is adjusted to half the period of the phonon oscillation. The amplitude after the second pulse shows a sinusoidal dependence as a function of the separation time, and this dependence is explained in terms of a superposition of two coherent phonon oscillations. In addition, not only the A1g mode but also an Eg mode have been observed by electro-optic sampling.

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A survey of most recent studies of optical absorption, photoluminescence, photoluminescence excitation, and photomodulated reflectance spectra of single-crystalline hexagonal InN layers is presented. The samples studied were undoped n-type InN with electron concentrations between 6 Â 10 18 and 4 Â 10 19 cm --3 . It has been found that hexagonal InN is a narrow-gap semiconductor with a band gap of about 0.7 eV, which is much lower than the band gap cited in the literature. We also describe optical investigations of In-rich In x Ga 1--x N alloy layers (0.36 < x < 1) which have shown that the bowing parameter of b $ 2.5 eV allows one to reconcile our results and the literature data for the band gap of In x Ga 1--x N alloys over the entire composition region. Special attention is paid to the effects of post-growth treatment of InN crystals. It is shown that annealing in vacuum leads to a decrease in electron concentration and considerable homogenization of the optical characteristics of InN samples. At the same time, annealing in an oxygen atmosphere leads to formation of optically transparent alloys of InN-In 2 O 3 type, the band gap of which reaches approximately 2 eV at an oxygen concentration of about 20%. It is evident from photoluminescence spectra that the samples saturated partially by oxygen still contain fragments of InN of mesoscopic size.

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