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)
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.
We demonstrate that vertically aligned InN nanorods can be grown on Si͑111͒ by plasma-assisted molecular-beam epitaxy. Detailed structural characterization indicates that individual nanorods are wurtzite InN single crystals with the growth direction along the c axis. Near-infrared photoluminescence ͑PL͒ from InN nanorods can be clearly observed at room temperature. However, in comparison to the InN epitaxial films, the PL efficiency is significantly lower. Moreover, the variable-temperature PL measurements of InN nanorods exhibit anomalous temperature effects. We propose that these unusual PL properties are results of considerable structural disorder ͑especially for the low-temperature grown InN nanorods͒ and strong surface electron accumulation effects.
We have studied the temperature dependence of the photoluminescence ͑PL͒ spectra of molecular beam epitaxy grown ultrathin Zn 1Ϫx Cd x Se/ZnSe quantum wells with random and inhomogeneous Cd distributions over cation sublattice within the temperature interval 2-300 K. Depending on the Cd concentration, the PL band maximum position E max PL (T) follows either a ''normal'' or an ''anomalous'' ͑known as ''S-shaped''͒ temperature dependence. We have analyzed both dependences in detail for a model of an island ensemble which can be characterized by a single-mode distribution of the most important parameters governing the optical properties of the quantum well. We demonstrate that the anomalous behavior arises due to the strong temperature dependence of the lifetimes of a family of metastable states participating in formation of the PL band at low temperatures. The metastablility of some island states is ascribed to a complex topological structure of the islands. The mechanism of the exciton-phonon interaction responsible for the fast decrease of the lifetime of these states with the increase of temperature has the same origin as the mechanism leading to the vanishing of narrow lines in -PL. We also present results of time-resolved experiments which yield the shift of the PL band for hot excitons cooling in a cold lattice.
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