Absorption and Emission of Hexagonal InN. Evidence of Narrow Fundamental Band Gap

Abstract

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)

How Indium Nitride Senses Water

“…for a summary of the valence state energetic offsets), resulting in a direct band gap, E g , of 0.89 eV, in line with calculations, 27,28 but larger than the two-particle optical gap of ~0.70 eV. 9,29 The Fermi surface as a function of the out-of-plane momentum k z , measured over an X-ray energy range of 32-800 eV, is shown in Fig. 2 (f).…”

“…[1][2][3][4] Innovations in molecular beam epitaxy 5,6 and metal-organic chemical vapor deposition 7,8 have made large scale production of very high quality InN feasible, and necessitated a revision of the fundamental optical gap from 1.9 eV to ~0.69 eV. 9 In the wake of these developments, alloys of InN with other III-Ns allow for precise control of the band gap over a wide energy range, paving the way for applications such as high-brightness white LEDs, high frequency electronics and solar cells accepting infrared to UV light 10,11 In the infrared, InN based diodes even offer an environmentally benign alternative to GaAs based diodes.…”

“…3 (c)). Due to its bulk-like nature, it only shows diffuse spectral weight 9 close to the Fermi level in Figs. 3 ((a), (d) and (e)).…”

Ab‐initio theory of semiconductor band structures: New developments and progress

“…(zinc blende) or 0.71 eV (wurtzite) in extremely good agreement with measured values [53,57,[68][69][70][71]. In general, the improvement results from the good performance of the HSE03 starting point for materials that comprise d-electrons such as GaAs, CdS, GaN, ZnO, and ZnS, for which the mean absolute relative error of the HSE03 + 0 0 G W gaps is calculated to be 7.9%, while it is about 12.2% in the GGA + 0 0 G W approach.…”

Understanding the role of Si doping on surface charge and optical properties: Photoluminescence study of intrinsic and Si-doped InN nanowires

“…InN, after being discovered of a narrow bandgap (E g ∼ 0.65 − 1 eV) [1][2][3][4][5][6][7][8][9][10][11][12][13][14] and predicted to possess the largest electron mobility among group-III nitrides (∼ 4400 cm 2 ·V −1 ·s −1 at 300 K), 15 has emerged as a highly promising material for infrared photodetectors and lasers, solar cells, ultrahigh-speed transistors, and sensors. [16][17][18] To date, however, the practical device applications of InN-based materials have been severely limited by the presence of extremely large residual electron density and the uncontrolled surface charge properties, as well as the difficulty in achieving p-type conductivity.…”