InN Epilayers Research Articles

Characteristics of InN epilayers grown with H2-assistance

A series of InN films were grown on GaN-on-sapphire template with H2 pulse flow by metal organic vapor phase epitaxy. The scanning electron microscopy and atomic force microscopy observations demonstrate that the smooth surface has been achieved. The X-ray diffraction and Raman spectra measurements indicate that InN layers experience stronger accommodated compressive stress, resulting in a larger fraction of (002) oriented InN grains. On the basics of the first-principles calculations, these features can be understand as competition between N-penetrating effect with the assistance of the H atom and the etching effect of H2. Finally, the absorption spectra in conjunction with simulated results reveal that the band gap energy predominantly increase with increasing compressive strain.

Influence of MBE growth modes and conditions on spontaneous formation of metallic In nanoparticles and electrical properties of InN matrix

Influence of the molecular beam epitaxy (MBE) growth conditions on the electrical properties of the InN epilayers in terms of minimization of the effect of spontaneously formed In nanoparticles was studied. A three-step growth sequence was used, including direct MBE growth of an InN nucleation layer, migration enhanced epitaxy (MEE) of an InN buffer layer, and In-rich MBE growth of the main InN layer, utilizing the droplet elimination by radical-beam irradiation (DERI) technique. The three-step growth regime was found to lead to decreasing the relative amount of In nanoparticles to 4.8% and 3.8% in In-rich and near-stoichiometric conditions, respectively, whereas the transport properties are better for the In-rich growth. Further reduction of the metallic indium inclusions in the InN films, while keeping simultaneously satisfactory transport parameters, is hardly possible due to fundamental processes of InN thermal decomposition and formation of the nitrogen vacancy conglomerates in the InN matrix. The In inclusions are shown to dominate the electrical conductivity of the InN films even at their minimum amount.

Engineering of InN epilayers by repeated deposition of ultrathin layers in pulsed MOCVD growth

Abstract Capabilities of repeated deposition of ultrathin layers by pulsed metalorganic chemical vapor deposition (MOCVD) for improvement of structural and luminescence properties of InN thin films on GaN/sapphire templates were studied by varying the growth temperature and the durations of pulse and pause in the delivery of In precursor. X-ray diffraction, atomic force microscopy, and spatially-resolved photoluminescence (PL) spectroscopy were exploited to characterize the structural quality, surface morphology and luminescence properties. Better structural quality is achieved by using longer trimethylindium pulses. However, it is shown that the luminescence properties of InN epilayers correlate with the pause and pulse ratio rather than with their absolute lengths, and the deposition of 1.5–2 monolayers of InN during one growth cycle is optimal to achieve the highest PL intensity. Moreover, the use of temperature ramping enabled achieving the highest PL intensity and the smallest blue shift of the PL band. The luminescence parameters are linked with the structural properties, and domain-like patterns of InN layers are revealed.

Influence of nitrogen flux on structural and optical properties of InN films fabricated by PAMBE

• InN epilayers were prepared by plasma-assisted MBE at different nitrogen fluxes. • The nitrogen flux is critical to influence the physical properties of the InN layers. • InN samples exhibited streaky cum spotty RHEED patterns at different N 2 fluxes.

Surface and bulk electronic structures of heavily Mg-doped InN epilayer by hard X-ray photoelectron spectroscopy

To evaluate the polarity, energy band diagram, and oxygen (O) distribution of a heavily Mg-doped InN (InN:Mg+) epilayer with a Mg concentration of 1.0 ± 0.5 × 1020 cm−3, the core-level and valence band (VB) photoelectron spectra are investigated by angle-resolved soft and hard X-ray photoelectron spectroscopies. The InN:Mg+ epilayers are grown by radio-frequency plasma-assisted molecular beam epitaxy. In this doping level, the polarity inversion from In-polar to N-polar occurs with the increase in the Mg flow rate under the same growth conditions, and the VB spectrum clearly indicates the direction of polarity of InN:Mg+, which is N-polar. The energy band diagram is considered to exhibit a two-step downward bending structure due to the coexistence of the n+ surface electron accumulation layer and heavily Mg-doped p+ layer formed in the bulk. The O concentration rapidly increases until ∼4 nm with respect to the surface, which is deduced to be one of the reasons of the formation of the anomalous two-step energy band profile.

Near infrared electroluminescence from p-NiO/n-InN/n-GaN light-emitting diode fabricated by PAMBE

Near infrared light-emitting diode (LED) based on p-NiO/n-InN/n-GaN was realized by plasma-assisted molecular beam epitaxy (PAMBE) combined with radio frequency (rf) magnetron sputtering. The device exhibited diode-like rectifying current–voltage characteristics with a turn-on voltage of 1.0V. Under forward bias, prominent near infrared (NIR) emissions were observed at peak around 1570nm at room temperature. The NIR emission was ascribed to the band-edge emission of InN epilayer based on the photoluminescence spectrum and band diagram of the heterojunction. Moreover, the study of the LED in terms of the stability was also discussed.

Structural and electronic properties of InN epitaxial layer grown on c-plane sapphire by chemical vapor deposition technique

Growth of InN epilayers on c-plane sapphire substrate by chemical vapor deposition technique using pure indium metal and ammonia as precursors has been systematically explored. It has been found that [0001] oriented indium nitride epitaxial layers with smooth surface morphology can be grown on c-plane sapphire substrates by optimizing the growth conditions. Bandgap of the film is observed to be Burstein–Moss shifted likely to be due to high background electron concentration. It has been found that the concentration of this unintentional doping decreases with the increase in the growth temperature and the ammonia flux. Epitaxial quality on the other hand deteriorates as the growth temperature increases. Moreover, the morphology of the deposited layer has been found to change from flat top islands to faceted mounds as the flow rate of ammonia increases. This phenomenon is expected to be related to the difference in surface termination character at low and high ammonia flow rates.

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.

Influence of InN epilayers on structural, electrical and optical properties of NiO films grown by magnetron sputtering

The NiO films were prepared on GaN substrate by depositing an InN epilayer on GaN by plasma-assisted molecular beam epitaxy (PAMBE) and then growing NiO films on InN epilayer by radio frequency (rf) magnetron sputtering. We studied the effect of InN epilayers on structure, surface morphology, electrical and optical properties of NiO films using X-ray diffraction (XRD), scanning electron microscopy (SEM), Hall and optical absorption measurement. It was found that the NiO films grown with InN epilayer exhibited better crystalline qualities with more coalescent surface morphologies and an enhancement of the electrical properties. Moreover, the effect of vacuum annealing on the crystalline qualities of the NiO films was also investigated.

InN nanocolumns grown by molecular beam epitaxy and their luminescence properties

InN nanocolumns (NCs) without vertical diameter variation have been grown on GaN templates by molecular beam epitaxy. Growth temperature is the key parameter to obtain such shaped InN NCs. The density and surface distribution can be controlled by the V/III ratio and initial nucleation process. Intense photoluminescence (PL) is observed for the optimal InN NCs, about two orders of magnitude stronger than the InN epilayers. Superior excitation power and temperature dependence of the PL indicate significantly reduced defect density and residual electron density, non-degenerate property, and possibly suggest that there is a small or negligible level of electron accumulation at the smooth non-polar lateral surfaces.

Angular-dependent Raman study of a- and s-plane InN

Angular-dependent polarized Raman spectroscopy was utilized to study nonpolar a-plane (11¯20) and semipolar s-plane (101¯1) InN epilayers. The intensity dependence of the Raman peaks assigned to the vibrational modes A1(TO), E1(TO), and E2h on the angle ψ that corresponds to rotation around the growth axis, is very well reproduced by using expressions taking into account the corresponding Raman tensors and the experimental geometry, providing thus a reliable technique towards assessing the sample quality. The s- and a-plane InN epilayers grown on nitridated r-plane sapphire (Al2O3) exhibit good crystalline quality as deduced from the excellent fitting of the experimental angle-dependent peak intensities to the theoretical expressions as well as from the small width of the Raman peaks. On the contrary, in the case of the s-plane epilayer grown on non-nitridated r-plane sapphire, fitting of the angular dependence is much worse and can be modeled only by considering the presence of two structural modifications, rotated so as their c-axes are almost perpendicular to each other. Although the presence of the second variant is verified by transmission electron and atomic force microscopies, angular dependent Raman spectroscopy offers a non-destructive and quick way for its quantification. Rapid thermal annealing of this sample did not affect the angular dependence of the peak intensities. The shift of the E1(TO) and E2h Raman peaks was used for the estimation of the strain state of the samples.

Extending group‐III nitrides to the infrared: Recent advances in InN

In this article, we provide an overview on the recent advances made in InN, including both planar and nanowire structures. With the improved epitaxial growth process, the background electron concentration can be reduced to low 1017 and 1013 cm−3 ranges in planar and nanowire structures, respectively, with the latter approaching the intrinsic limit of InN at room temperature. Extensive studies have shown that the measurement of p‐type conduction in InN epilayers has been fundamentally limited by the presence of electron accumulation on the c‐plane surfaces. In contrast, the nonpolar grown surfaces of InN are found to be completely free of electron accumulation. Moreover, through direct Mg dopant incorporation, the surfaces (nonpolar m‐plane) of InN nanowires can be transformed from intrinsic to nearly p‐type degenerate, which has led to the first direct measurement of p‐type conduction in InN.

Growth temperature induced physical property variation of InN films grown on nitrided sapphire substrate by PAMBE

InN epilayers were deposited on nitrided sapphire substrates using plasma-assisted molecular beam epitaxy (PAMBE) system. The physical properties of InN films under different growth temperatures were thoroughly studied. The XRD results indicated that as-prepared InN films without Indium droplets were preferentially oriented in the c-axis direction. The SEM results showed that the InN films grown in a two-dimensional mode with a thickness of ∼200 nm under the growth temperature of 460 °C. In addition, the optical absorption measurement exhibited that the energy bandgap of InN films was around 0.75–0.81eV and the PL spectra of the epilayers revealed an infrared emission. Moreover, the electrical properties of the films were discussed by Hall effect in detail.

Effect of nitridation on structure, electrical and optical properties of InN epilayers grown on sapphire by PAMBE

InN epilayers were prepared on c-plane (0001) sapphire substrates by plasma-assisted molecular beam epitaxy (PAMBE). We studied the effect of nitridation on structure, surface morphology, electrical and optical properties of InN films using X-ray diffraction (XRD), Reflection high-energy electron diffraction (RHEED), scanning electron microscopy (SEM), atomic force microscopy (AFM), Hall and photoluminescence (PL) measurement. The results showed a significant improvement of the crystalline qualities and surface morphologies and enhancement of the electrical and optical properties for InN films grown after nitridation. Moreover, the energy band-gap of InN films were also determined by optical absorption and photoluminescence (PL) measurement.

Is the Fermi‐level pinned on InN grown surfaces?

AbstractIn this paper, we discuss the Fermi‐level pinning issue on InN grown surfaces, in particular the nonpolar m ‐planes. Detailed angle‐resolved X‐ray photoelectron spectroscopy studies show that for intrinsic InN nanowires the Fermi‐level on the nonpolar grown surfaces is located 0.1 to 0.2 eV below the conduction band edge, i.e., not pinned in the conduction band. This indicates that the commonly measured Fermi‐level pinning in the conduction band on nonpolar grown surfaces is not an intrinsic property of InN. Furthermore, our studies also suggest that surface states are positioned near the band edge and therefore do not contribute to the Fermi‐level pinning. This is consistent with recent theoretical and experimental studies. In the end, with the absence of the Fermi‐level pinning in the conduction band and surface electron accumulation, the unique electrical and optical properties of intrinsic InN nanowires are discussed, and compared with previously reported InN epilayers and n ‐type InN nanowires. (© 2014 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Selective growth of N-polar InN through an in situ AlN mask on a sapphire substrate

Selective growth of N-polar InN by exploiting the ∼100° gap between the upper limits of the growth temperatures of In- and N- polarities has been introduced. An InN epilayer grown at a temperature in this gap on a sapphire substrate covered with an ultrathin AlN layer has demonstrated N-polarity. The dislocation density of such-grown InN layer has been significantly reduced (one order of magnitude lower than the conventional InN epilayers). The results have demonstrated great potential for improving the crystalline quality of hetero-epitaxial InN films. This concept can be easily adopted for other substrates.

Growth mechanism and microstructure of low defect density InN (0001) In-face thin films on Si (111) substrates

Transmission electron microscopy has been employed to analyze the direct nucleation and growth, by plasma-assisted molecular beam epitaxy, of high quality InN (0001) In-face thin films on (111) Si substrates. Critical steps of the heteroepitaxial growth process are InN nucleation at low substrate temperature under excessively high N-flux conditions and subsequent growth of the main InN epilayer at the optimum conditions, namely, substrate temperature 400–450 °C and In/N flux ratio close to 1. InN nucleation occurs in the form of a very high density of three dimensional (3D) islands, which coalesce very fast into a low surface roughness InN film. The reduced reactivity of Si at low temperature and its fast coverage by InN limit the amount of unintentional Si nitridation by the excessively high nitrogen flux and good bonding/adhesion of the InN film directly on the Si substrate is achieved. The subsequent overgrowth of the main InN epilayer, in a layer-by-layer growth mode that enhances the lateral growth of InN, reduces significantly the crystal mosaicity and the density of threading dislocations is about an order of magnitude less compared to InN films grown using an AlN/GaN intermediate nucleation/buffer layer on Si. The InN films exhibit the In-face polarity and very smooth atomically stepped surfaces.

Impact of Mg concentration on energy-band-depth profile of Mg-doped InN epilayers analyzed by hard X-ray photoelectron spectroscopy

The electronic structures of Mg-doped InN (Mg-InN) epilayers with the Mg concentration, [Mg], ranging from 1 × 1019 to 5 × 1019 cm−3 were systematically investigated by soft and hard X-ray photoelectron spectroscopies. The angle-resolved results on the core-level and valence band photoelectron spectra as a function of [Mg] revealed that the energy band of Mg-InN showed downward bending due to the n+ surface electron accumulation and p type layers formed in the bulk. With an increase in [Mg], the energy-band changed from monotonic to two-step n+p homojunction structures. The oxygen concentration rapidly increased at the middle-bulk region (∼4.5 to ∼7.5 nm) from the surface, which was one of the reasons of the transformation of two-step energy band.

High-pressure lattice dynamics in wurtzite and rocksalt indium nitride investigated by means of Raman spectroscopy

We present an experimental and theoretical lattice-dynamical study of InN at high hydrostatic pressures. We perform Raman scattering measurements on five InN epilayers, with different residual strain and free electron concentrations. The experimental results are analyzed in terms of ab initio lattice-dynamical calculations on both wurtzite InN (w-InN) and rocksalt InN (rs-InN) as a function of pressure. Experimental and theoretical pressure coefficients of the optical modes in w-InN are compared, and the role of residual strain on the measured pressure coefficients is analyzed. In the case of the LO band, we analyze and discuss its pressure behavior considering the double-resonance mechanism responsible for the selective excitation of LO phonons with large wave vectors in w-InN. The pressure behavior of the L − coupled mode observed in a heavily doped n-type sample allows us to estimate the pressure dependence of the electron effective mass in w-InN. The results thus obtained are in good agreement with k · p theory. The wurtzite-to-rocksalt phase transition on the upstroke cycle and the rocksalt-to-wurtzite backtransition on the downstroke cycle are investigated, and the Raman spectra of both phases are interpreted in terms of DFT lattice-dynamical calculations.

Systematic investigation of surface and bulk electronic structure of undoped In-polar InN epilayers by hard X-ray photoelectron spectroscopy

The surface and bulk electronic structure of undoped In-polar InN (u-InN) epilayers with surface electron accumulation (SEA) layer was investigated by soft and hard X-ray photoelectron spectroscopies (SX-PES and HX-PES, respectively). The potential-energy profile was obtained by fitting the N 1s core-level spectra accounting for the probing depth of both SX-PES and HX-PES and the surface downward band bending. In this study, we found that a significant potential-energy bending as large as 1.2 ± 0.05 eV occurred from surface to the depth of ∼20 nm. Taking into account such a large downward bending, the valence band maximum (VBM) with respect to Fermi energy (EF) level in the bulk was determined to be 0.75 ± 0.05 eV. A weak signal with a peak position at 0.3 ± 0.05 eV was reproducibly observed from the VBM to EF level in the HX-PES spectrum. The peak position was in agreement with the calculated energy of the E1 sub-band in the surface quantum well. HX-PES is powerful tool for revealing the intrinsic bulk electronic structure of materials with large potential-energy bending. This work provides important information for the future investigation of electronic structures of undoped and Mg-doped InGaN, as well as Mg-doped InN epilayers with a SEA layer similar to u-InN.