Anomalous Optical Properties of InN Nanobelts: Evidence of Surface Band Bending and Photoelastic Effects

Abstract

In the past few years InN, the least understood Group III nitride compound, has attracted a great deal of interest because of its unique properties, making it suitable for applications in various electronic and optoelectronic devices. [1][2][3][4][5][6][7][8] In addition, InN is considered as a potential material for lowcost, low-power-consumption, high-sensitivity detection in gas, vapors, and liquids by virtue of its intrinsic surface charge accumulation. [9][10][11][12] Consequently, the preparation and study of InN nanomaterials are of particular importance in the aforementioned devices owing to the high surface-to-volume ratio as compared to the bulk, a property which may largely enhance the operation efficiency. One-dimensional (1D) nanostructures are excellent systems in which one can investigate the dependence of their unique fundamental properties on dimensionality and their potential applications. [13][14][15][16][17] Recently, it has been demonstrated that wurtzite nanobelts have the potential of converting biological mechanical energy, vibration energy, and biofluid hydraulic energy into electricity, raising the tantalizing prospect of the self-powering of wireless nanodevices and nanosystems for optoelectronics, biosensors, and resonators.[18] Therefore, InN nanobelts are promising candidates for incorporation into the above-mentioned devices. Despite all these potential advantages of InN nanobelts studies on this material remain scarce, most probably because of the difficulties associated with the growth of belt-like nanostructures. In this Communication, we have discovered several unique behaviors based on the study of photoluminescence (PL) and Raman scattering spectra of high-quality InN nanobelts fabricated by the metal-organic chemical vapor deposition (MOCVD) technique. The PL spectra of these InN nanobelts exhibit an extraordinarily large blue-shift with increasing excitation intensity, as compared with their thin-film counterparts. In addition, we found that the phonon frequencies of InN nanobelts decrease with increasing excitation intensity. Surface band bending, piezoelectricity, and photoelastic effects were employed to explain these anomalous behaviors. Besides, temperature-dependent PL spectra are also consistent with the prediction of our proposed model. Structural and morphological information of the as-synthesized InN nanobelts was obtained from scanning electron microscopy (SEM) images. Apparently, high-density belt-like structures with length of several tens of nanometers cover the substrate, as shown in Figure 1. A high-magnification SEM image (inset to Fig. 1) revealed that the morphologies of the nanobelts are well-faceted with smooth surfaces. A detailed examination of the nanobelts along their length showed that they were uniform in both width and thickness, ranging from 40 to 250 nm and 10 to 35 nm, respectively.Typical PL spectra of the nanobelts measured at 20 K under different excitation intensities are shown in Figure 2a. A strong PL emission at 0.82 eV and blue-shifts o...

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

“…8,[19][20][21][22][23][24][25][26][27][28][29][30] For example, in general, the currently reported nominally undoped InN is n-type degenerate, with the residual electron densities in the range of ∼ 1 × 10 18 cm −3 , or higher. 8,11,25,[31][32][33][34] Moreover, it has been generally observed that there exists a very high electron concentration (∼ 1 × 10 13−14 cm −2 ) at both the polar and nonpolar grown surfaces of InN films, 19,35 and the Fermi-level (E F ) is pinned deep into the conduction band at the surfaces; 19,20,29,30 similar electron accumulation profile has also been measured at the lateral nonpolar grown surfaces of [0001]-oriented wurtzite InN nanowires. 8,11,21,22,25,36 In this regard, significant efforts have been devoted to understanding the fundamental surface charge properties of InN.…”

“…In terms of nonpolar InN surface, recent studies suggest that the surface electron accumulation may depend critically on the surface states, impurities, stoichiometry, and polarity; 27,37 and the absence of electron accumulation at nonpolar surface has been predicted. 23,27 In experiments, however, only recent cross-sectional scanning photoelectron microscopy and spectroscopy studies at nonpolar cleaved InN surface exhibits the unpinned E F , 38,39 while in general the electron accumulation is prevalently observed at nonpolar InN surface; 22,34,35 the electron accumulation issue at nonpolar InN surface had remained elusive.…”

Correlation of Morphology Evolution with Carrier Dynamics in InN Films Heteroepitaxially Grown by MOMBE

“…Most InN samples reported recently are n-type degenerate semiconductors, with an electron concentration of approximately 1 × 10 18 cm −3 or higher. Much research has been dedicated to understanding the fundamental properties of InN, especially its photoluminescence (PL) properties [8][9][10][11]. The optical properties of semiconductors are very sensitive to the increase in carrier concentration and are reflected in the blueshift of peak energy and the expansion of the peak width.…”

Energy-Dependent Time-Resolved Photoluminescence of Self-Catalyzed InN Nanocolumns

“…Most InN samples reported are degenerate n-type materials with an electron concentration of about 1 × 10 18 cm −3 or even higher. In order to understand the fundamental properties of InN, much research has been conducted, especially on the photoluminescent (PL) properties of InN, which has been investigated intensively [8][9][10][11]. In semiconductor materials, the optical properties are extremely sensitive to carrier concentration, i.e., blueshift of peak energy and the broadening of full-with-half-maximum (FWHM) with increasing carrier concentration.…”