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)

The first- and second-order Raman scattering and IR reflection have been studied for hexagonal InN layers grown on (0001) and (11̄02) sapphire substrates. All six Raman-active optical phonons were observed and assigned: E2(low) at 87 cm−1, E2(high) at 488 cm−1, A1(TO) at 447 cm−1, E1(TO) at 476 cm−1, A1(LO) at 586 cm−1, and E1(LO) at 593 cm−1. The ratio between the InN static dielectric constants for the ordinary and extraordinary directions was found to be ε⊥0/ε∥0=0.91. The phonon dispersion curves, phonon density-of-state function, and lattice specific heat were calculated. The Debye temperature at 0 K for hexagonal InN was estimated to be 370 K.

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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Articles you may be interested inElectron and hole deep levels related to Sb-mediated Ge quantum dots embedded in n-type Si, studied by deep level transient spectroscopy Appl. Phys. Lett. 102, 232106 (2013); 10.1063/1.4809595 Improved calculation of vacancy properties in Ge using the Heyd-Scuseria-Ernzerhof range-separated hybrid functional Schottky barriers formed by depositing Au on n-type Ge have been used to study the antimony-vacancy complex ͑E center͒. Both hole and electron transitions have been observed because the formation of an inversion layer at the semiconductor surface enables minority carriers to be injected when the Schottky barrier is forward biased. It is argued that the E center in Ge has three charge states: double negative, single negative, and neutral. The free energy of electron ionization for the double acceptor level of the complex has been found to be ⌬G(ϭ/Ϫ)ϭ0.294 Ϫ4.2 kT (eV), where k is Boltzmann's constant. Consequently, the position of the double acceptor level of the E center ͕E(ϭ/Ϫ)ϭE c Ϫ⌬G(ϭ/Ϫ)͖ is temperature dependent. In moderately Sb-doped (N Sb ϭ10 13 -10 15 cm Ϫ3 ) Ge crystals at equilibrium conditions half-occupancy of the double acceptor state of the Sb-V complex occurs when the Fermi level is at about E c Ϫ0.20 eV. The single acceptor level of the E center is in the lower part of the band gap. The activation energy of hole emission from the E(Ϫ/0) level has been determined as 0.307 eV. The introduction of one Sb-V defect results in the removal of three free electrons in Sb-doped Ge. It is proposed that this is one of the main reasons for the fast free carrier removal and n→p conversion of the conductivity type in Ge:Sb upon electron-or gamma-irradiation at room temperature.

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