As-grown InN Research Articles

Preparation of InN films at different substrate temperatures and the effect of operating temperatures on the carrier transmission characteristics of p-NiO/n-InN heterojunction

The good properties of ITO substrate endow the as-grown InN thin films especially attractive for various optoelectronic applications. Herein, the InN thin films were prepared on the ITO substrates by magnetron sputtering. The effects of substrate temperature (Ts) on the structural, morphological, and optical properties of InN films were assessed by X-ray diffractometer, scanning electron microscope and ultraviolet–visible spectrophotometer. The combination of structural and morphological investigation suggested that the better InN thin film was obtained at 200 °C. Then, the variation of the optical band gap and transmittance with Ts was discussed. What's more, the p-NiO/n-InN heterojunction was trial-produced, and the carrier transport characteristics were studied in the range of 30–120 °C. The heterojunction exhibited excellent I–V characteristics at all operating temperatures. This work enriches the description of InN materials and devices, and provides a useful reference for the practical application of InN.

Impact of the Deposition Temperature on the Structural and Electrical Properties of InN Films Grown on Self-Standing Diamond Substrates by Low-Temperature ECR-MOCVD

The progress of InN semiconductors is still in its infancy compared to GaN-based devices and materials. Herein, InN thin films were grown on self-standing diamond substrates using low-temperature electron cyclotron resonance plasma-enhanced metal organic chemical vapor deposition (ECR-PEMOCVD) with inert N2 used as a nitrogen source. The thermal conductivity of diamond substrates makes the as-grown InN films especially attractive for various optoelectronic applications. Structural and electrical properties which depend on deposition temperature were systematically investigated by reflection high-energy electron diffraction (RHEED), X-ray diffraction (XRD), scanning electron microscopy (SEM), atomic force microscopy (AFM), and Hall effect measurement. The results indicated that the quality and properties of InN films were significantly influenced by the deposition temperature, and InN films with highly c-axis preferential orientation and surface morphology were obtained at optimized temperatures of 400 °C. Moreover, their electrical properties with deposition temperature were studied, and their tendency was correlated with the dependence on micro- structure and morphology.

Growth of nonpolar InN nanocrystal films by RF plasma-assisted evaporation technique

Nonpolar InN nanocrystal films were grown by RF plasma-assisted evaporation technique on bare Si and quartz substrates, simultaneously. The as-grown InN films were characterized by X-ray diffraction, scanning electron microscopy, and Raman spectroscopy. The synergistic cooperation among the evaporation temperature, chamber pressure and substrate temperature is critical for the formation of nonpolar InN films. The absence of the (0 0 2) peak in the X-ray diffraction patterns indicates that as-grown InN films grew preferentially along the nonpolar plane. In addition, the InN films grown on Si and quartz substrates exhibit different crystallinity, morphology and stress. The In-rich growth condition can be responsible for the formation of nonpolar InN films.

Effect of Sputtering Power on the Structural and Optical Properties of Inn Nanodots on Al2O3 by Magnetron Sputtering

In this work, we reported the effects of sputtering power on the structure, optical and electrical properties of InN nanodots prepared on Al2O3 substrate by magnetron sputtering.The results showed that the as-grown InN films exhibited uniform nanodot morphology and the size of the InN nano grains increased with the sputtering power was increased. The InN nanodot exhibited highly c-axis prefered orientation with mainly InN (002) diffraction. The optical band gap of InN samples showed an decreasing trend with the increase in sputteirng power. Moreover, the electrical properties of the InN samples were discussed in detail by hall effect and the carrier concentration and mobility could be adjusted from 3.233×1019 to 1.655×1020 cm-3 and 1.151 to 10.101 cm2/v•s, respectively. These results will lay a good fundation for the applicaion of InN material in the field of gas sensers and light emitting diodes.

Growth of well-oriented InN nanodots by magnetron sputtering with varying sputtering temperature

Herein, indium nitride nanodots were deposited on sapphire substrates by radio-frequency magnetron sputtering under different sputtering temperatures. The structure and morphology results revealed that the as-grown InN films exhibited a c-axis preferred oriented growth and a well-oriented InN nanodot morphology. The x-ray photoelectron spectra results indicated that the In atoms existed only as combined InN and not in the form of In2O3 in consequence of the fact that the atomic ratio of In/N was approximately 1:1.06. The energy bandgap of the InN nanodots varied from 1.68 to 2.01 eV as the sputtering temperature increased. Moreover, the electrical properties of the InN nanodots were also discussed in detail using Hall effect results.

Enhanced Hydrogen Detection Based on Mg-Doped InN Epilayer.

It is a fact that surface electron accumulation layer with sheet electron density in the magnitude of ~1013 cm−2 on InN, either as-grown or Mg-doped, makes InN an excellent candidate for sensing application. In this paper, the response of hydrogen sensors based on Mg-doped InN films (InN:Mg) grown by molecular beam epitaxy has been investigated. The sensor exhibits a resistance variation ratio of 16.8% with response/recovery times of less than 2 min under exposure to 2000 ppm H2/air at 125 °C, which is 60% higher in the magnitude of response than the one based on the as-grown InN film. Hall-effect measurement shows that the InN:Mg with suitable Mg doping level exhibits larger sheet resistance, which accords with buried p-type conduction in the InN bulk. This work shows the advantage of InN:Mg and signifies its potential for sensing application.

RHEED에 의한 GaN, InN 핵생성층의 열처리 효과 분석

GaN and InN epilayers with nucleation layer (LT-buffer) were grown on (0001) sapphire substrates by radio-frequency plasma-assisted molecular beam epitaxy (RF-MBE). As-grown and annealed GaN and InN nucleation layers grown at various growth condition were observed by reflection high-energy electron diffraction (RHEED). When temperature of effusion cell for III source was very low, diffraction pattern with cubic symmetry was observed and zincblende nucleation layer was flattened easily by annealing. As cell temperature increased, LT-GaN and LT-InN showed typical diffraction pattern from wurtzite structure, and FWHM of (10-12) plane decreased remarkably which means much improved crystalline quality. Diffraction pattern was changed to be from streaky to spotty when plasma power was raised from 160 to 220 W because higher plasma power makes more nitrogen adatoms on the surface and suppressed surface mobility of III species. Therefore, though wurtzite nucleation layer was a little hard to be flattened compared to zincblende, higher cell temperature led to easier movement of III surface adatoms and resulted in better crystalline quality of GaN and InN epilayers.

Hydrogen adsorbed at N-polar InN: Significant changes in the surface electronic properties

The interaction of atomic hydrogen and ammonia with as-grown N-polar InN surfaces is investigated using in situ photoelectron spectroscopy. Changes in the surface electronic properties, including the band alignment and work function, as well as the chemical bonding states of the substrate and adsorbates are characterized. Ammonia molecules are dissociating at the InN surface, resulting in adsorption of hydrogen species. Consequently, the considerable changes of the chemical and electronic properties of the InN surface during ammonia interaction are almost identical to those found for adsorption of atomic hydrogen. In both cases, hydrogen atoms preferentially bond to surface nitrogen atoms, resulting in the disappearance of the nitrogen dangling-bond-related occupied surface state close to the valence band edge at $\ensuremath{\sim}1.6$ eV binding energy and the formation of new occupied electron states at the conduction band edge. Furthermore, a decrease in work function during adsorption from 4.7 to 3.7--3.8 eV, as well as an increase in the surface downward band bending by 0.3 eV, confirm that hydrogen is acting as electron donor at InN surfaces and therefore has to be considered as one main reason for the surface electron accumulation observed at N-polar InN samples exposed to ambient conditions, for example as the dissociation product of molecules. The measured formation and occupation of electronic states above the conduction band minimum occur in conjunction with the observed increase in surface electron concentration and underline the relationship between the energy position of occupied electron states and surface band alignment for InN as a small-band-gap semiconductor.

Growth of a-Plane InN Film and Its THz Emission

We report the growth of a-plane InN on an r-plane sapphire substrate by plasma-assisted molecular-beam epitaxy. It is found that the a-plane InN is successfully grown by using a GaN buffer layer, which has been confirmed by reflection high-energy electron diffraction, x-ray diffraction and Raman scattering measurements. The Hall effect measurement shows that the electron mobility of the as-grown a-plane InN is about 406 cm2 /V·s with a residual electron concentration of 5.7 × 10 cm−3. THz emission from the a-plane InN film is also studied, where it is found that the emission amplitude is inversely proportional to the conductivity.

Deposition and properties of highly c-oriented of InN films on sapphire substrates with ECR-plasma-enhanced MOCVD

InN films with highly c-axis preferred orientation were deposited on sapphire substrate by low-temperature electron cyclotron resonance plasma-enhanced metal organic chemical vapor deposition (ECR-PEMOCVD). Trimethyl indium (TMIn) and N2 were applied as precursors of In and N, respectively. The quality of as-grown InN films were systematically investigated as a function of TMIn fluxes by means of reflection high-energy electron diffraction (RHEED), X-ray diffraction analysis (XRD), and atomic force microscopy (AFM). The results show that the dense and uniform InN films with highly c-axis preferred orientation are successfully achieved on sapphire substrates under optimized TMIn flux of 0.8 ml·min−1. The InN films reported here will provide various opportunities for the development of high efficiency and high-performance semiconductor devices based on InN material.

Identifying vacancy complexes in compound semiconductors with positron annihilation spectroscopy: A case study of InN

We present a comprehensive study of vacancy and vacancy-impurity complexes in InN combining positron annihilation spectroscopy and ab initio calculations. Positron densities and annihilation characteristics of common vacancy-type defects are calculated using density functional theory, and the feasibility of their experimental detection and distinction with positron annihilation methods is discussed. The computational results are compared to positron lifetime and conventional as well as coincidence Doppler broadening measurements of several representative InN samples. The particular dominant vacancy-type positron traps are identified and their characteristic positron lifetimes, Doppler ratio curves, and line-shape parameters determined.We find that indium vacancies (${V}_{\text{In}}$) and their complexes with nitrogen vacancies (${V}_{\text{N}}$) or impurities act as efficient positron traps, inducing distinct changes in the annihilation parameters compared to the InN lattice. Neutral or positively charged ${V}_{\text{N}}$ and pure ${V}_{\text{N}}$ complexes, on the other hand, do not trap positrons. The predominantly introduced positron trap in irradiated InN is identified as the isolated ${V}_{\text{In}}$, while in as-grown InN layers ${V}_{\text{In}}$ do not occur isolated but complexed with one or more ${V}_{\text{N}}$. The number of ${V}_{\text{N}}$ per ${V}_{\text{In}}$ in these complexes is found to increase from the near-surface region toward the layer-substrate interface.

Epitaxial Growth of High Quality Nonpolar InN Films on LiGaO2 Substrates

A new system for obtaining high quality nonpolar InN films has been demonstrated. It is found that a-plane InN epitaxially grows on LiGaO2 (001) with high phase purity and smooth surface. The in-plane epitaxial relationships are InN [0001]// LiGaO2 [100], and InN [11̅00]// LiGaO2 [010]. The interface between the substrate and a-plane InN is atomically abrupt. The threading dislocation density drops dramatically from 2 × 1010 cm−2 of its initial growth stage to 3 × 109 cm−2 when the InN film thickness reaches 30 nm, which we believe is driven by dislocation merging or annihilation near the nucleation layer. The band edge emission from its photoluminescence spectra of as-grown a-plane InN is located at 0.72 eV. This novel high quality nonpolar a-plane InN on a LiGaO2 (001) system opens up a new possibility for high-efficiency nonpolar InN devices.

Influence of N2 Flux on InN Film Deposition on Sapphire (0001) Substrates by ECR-PEMOCVD

Highly preferred InN films are deposited on sapphire (0001) substrates by electron cyclotron resonance plasma enhanced metal organic chemical vapor deposition (ECR-PEMOCVD) without using a buffer layer. The structure, surface morphological and electrical characteristics of InN are investigated by in-situ reflection high energy electron diffraction, x-ray diffraction, x-ray photoelectron spectroscopy, atomic force microscopy and Hall effect measurement. The quality of the as-grown InN films is markedly improved at the optimized N2 flux of 100 sccm. The results show that the properties of the films are strongly dependent on N2 flux.

Is electron accumulation universal at InN polar surfaces?

Recent experiments indicate the universality of electron accumulation and downward surface band bending at as-grown InN surfaces with polar or nonpolar orientations. Here, we demonstrate the possibility to prepare flatband InN (0001¯) surfaces. We have also measured the surface stoichiometry of InN surfaces by using core-level photoelectron spectroscopy. The flatband InN (0001¯) surface is stoichiometric and free of In adlayer. It implies that the removal of In adlayer at the InN (0001¯) surface leads to the absence of downward surface band bending. On the other hand, the stoichiometric InN (0001) surface still exhibits surface band bending due to the noncentrosymmetry in the wurtzite structure.

Band gap energy and bowing parameter of In-rich InAlN films grown by magnetron sputtering

The crystal structure, band gap energy and bowing parameter of In-rich In x Al 1− x N (0.7 < x < 1.0) films grown by magnetron sputtering were investigated. Band gap energies of In x Al 1− x N films were obtained from absorption spectra. Band gap tailing due to compositional fluctuation in the films was observed. The band gap of the as-grown InN measured by optical absorption method is 1.34 eV, which is larger than the reported 0.7 eV for pure InN prepared by molecular beam epitaxy (MBE) method. This could be explained by the Burstein–Moss effect under carrier concentration of 10 20 cm −3 of our sputtered films. The bowing parameter of 3.68 eV is obtained for our In x Al 1− x N film which is consistent with the previous experimental reports and theoretical calculations.

The role of threading dislocations and unintentionally incorporated impurities on the bulk electron conductivity of In-face InN

The origin of bulk electrons in In-face InN has been studied by considering the effects of both unintentionally incorporated impurities and threading dislocation densities on electron transport properties. The concentration of unintentionally incorporated oxygen and hydrogen scaled with the bulk electron concentration while threading dislocations had no discernable effect on the electron concentration. We conclude that unintentional impurities were the significant source of electrons and threading dislocations acted only as scattering centers limiting the electron mobility in as-grown InN films. Further, we present In-face InN growth techniques controlling the incorporation of oxygen and hydrogen and reducing threading dislocation densities.

Depletion of surface accumulation charge in InN by anodic oxidation

Si-doped InN layer by molecular beam epitaxy was subjected to anodic oxidation in 0.1 M potassium hydroxide (KOH) electrolyte and characterized by electrochemical methods to derive carrier profile at the InN surface. The obtained results were compared to the characteristics of a planar resistor structure and vertical metal-oxide-semiconductor (MOS) diodes with Ni-metal contacts on the oxidized InN. Both measurements in electrolyte and in air confirmed the formation of a surface oxide layer after the anodic treatment and depletion of the surface accumulation charge of the as-grown InN. The upward band bending of InN at the oxide interface was also concluded from the analysis of capacitance-voltage characteristics of the MOS diodes. Transmission electron microscopy revealed a nonuniform oxide layer containing porelike structures of a few nanometers in diameter.

Terahertz emission from silicon and magnesium doped indium nitride

We report an experimental study of femtosecond optically excited emission of terahertz frequency electromagnetic radiation from as-grown n-type InN, silicon doped InN, and magnesium doped InN. We have measured the terahertz emission from these materials as function of dc Hall mobility and carrier concentrations. Terahertz emission from InN:Si and native n-type InN increases with mobility as expected for transient photocurrents as primary mechanism of terahertz emission from InN. InN:Mg exhibits enhanced terahertz emission compared to InN:Si. This is experimental evidence for Mg being electrically active as an acceptor in InN. Terahertz emission from InN:Si is less strong than terahertz emission from native n-type InN because of an increased electron concentration due to silicon being an electrically active donor in InN.

Sources of unintentional conductivity in InN

Using first-principles methods, we investigate the effects of monatomic hydrogen in InN. We find that hydrogen can occupy interstitial and substitutional sites. Interstitial hydrogen is stable in the bond-center configuration and acts exclusively as a shallow donor, with a H–N stretching vibration at 3050cm−1. Hydrogen can also substitute for nitrogen in InN, bonding equally to the four In nearest neighbors in a multicenter-bond configuration. Substitutional hydrogen has low formation energy and, counterintuitively, is a double donor. Our results suggest that monatomic hydrogen is a plausible cause of the unintentional n-type conductivity that is often observed in as-grown InN.

TEM studies of as-grown, irradiated and annealed InN films

Transmission electron microscopy was applied to study structural changes of InN films grown by molecular beam epitaxy on c-sapphire substrates with a GaN buffer layer. The films were studied as-grown and also following by rapid thermal annealing, irradiation with 2 MeV He + ions, and annealing after irradiation. Defects formed after each procedure were discussed. The results of these studies show that randomly distributed extended defects, formed upon irradiation, can come to uniform distribution throughout the samples upon annealing. An irradiation by 2 MeV He + ions followed by thermal annealing at 425 and 475 °C leads to the unusual increase of the electron mobility of these films. Annealing at 500 °C led to the formation of In clusters and delamination of the film from the substrates.