Properties Of InN Thin Films Research Articles

Correlation of Threading Dislocations with the Electron Concentration and Mobility in InN Heteroepitaxial Layers Grown by MBE

The quantitative interdependencies of growth conditions, crystal defects and electrical/electronic properties of InN thin films, grown by plasma-assisted molecular beam epitaxy on GaN (0001) buffer layers have been investigated. InN epilayers with thickness near 700 nm, grown under different substrate temperature and/or growth rate, have been analyzed. Bulk electron concentration (Nbulk) and mobility values were extracted for each InN film using the inverted version of the multilayer Petritz model, subtracting the conductivity of a corresponding 120 nm InN film. The results indicate a significant reduction of the threading dislocation density by enhancing the diffusion length of indium adatoms during InN growth, through increase of substrate temperature and reduction of growth rate. The electrical characteristics deteriorate with increasing threading dislocation density. Assuming threading dislocations as exclusive sources of donors in InN, their charge state could be between +1 and +2 per c lattice constant length of dislocation line for Nbulk ≈ 4.0–5.7×1017 cm−3, and approximately +2 or larger for Nbulk ≈ 1.5–1.8×1018 cm−3. The scattering effect of threading dislocations is significantly weaker compared to reported theoretical calculations, i.e. it would correspond to an order of magnitude lower threading dislocation density than the experimentally observed density in the range of 1010 cm−2.

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.

Morphology and Optical Properties of RF Sputtering Deposited Indium Nitride Layers Under Different N₂/Ar Ratio.

InN thin films were prepared using reactive radio frequency sputtering on (111) Si substrates under different N₂/Ar ratio. The surface morphology and optical properties of InN thin films were characterized by X-ray diffraction, atomic force microscopy, scanning electron microscope, energy dispersive spectrometer, and UV-visible-NIR spectrophotometer at room temperature. It was found that the growth rate and surface RMS roughness of InN layers all significantly decreased with the increasing of N₂ content in the sputtering gas. All the InN films were wurtzite structure with the proportion of N₂ increasing from 40% to 100% in the mixture gas. The highly c-axis orientation InN films could only be obtained in the composition ratios of N₂ higher than 90%. The atomic percentage of N is much higher than that of In at high N₂ content films, which mainly due to the chemical reaction on the surface of Si substrate. The apparent optical band gap energy is estimated at 1.83 eV. However, the InN absorption band tail is strongly influenced by the sputtering due to a change in the species of the plasma.

Influence of fabrication steps on optical and electrical properties of InN thin films

This paper reports on a case study of the impact of fabrication steps on InN material properties. We discuss the influence of annealing time and sequence of device processing steps. Photoluminescence (PL), surface morphology and electrical transport (electrical resistivity and low frequency noise) properties have been studied as responses to the adopted fabrication steps. Surface morphology has a strong correlation with annealing times, while sequences of fabrication steps do not appear to be influential. In contrast, the optical and electrical properties demonstrate correlation with both etching and thermal annealing. For all the studied samples PL peaks were in the vicinity of 0.7 eV, but the intensity and full width at half maximum (FWHM) demonstrate a dependence on the technological steps followed. Sheet resistance and electrical resistivity seem to be lower in the case of high defect introduction due to both etching and thermal treatments. The same effect is revealed through 1/f noise level measurements. A reduction of electrical resistivity is connected to an increase in 1/f noise level.

Influence of GaN underlayer thickness on structural, electrical and optical properties of InN films grown by PAMBE

We present an extensive study on the structural, electrical and optical properties of InN thin films grown on c-Al2O3, GaN(130nm)/Al2O3, GaN(200nm)/Al2O3 and GaN(4μm)/Al2O3 by using plasma-assisted molecular beam epitaxy. The high resolution X-ray diffraction study reveals better crystalline quality for the film grown on GaN(4μm)/Al2O3 as compared to others. The electronic and optical properties seem to be greatly influenced by the structural quality of the films, as can be evidenced from Hall measurement and optical absorption spectroscopy. Kane's k.p model was used to describe the dependence of optical absorption edge of InN films on carrier concentration by considering the non-parabolic dispersion relation for carrier in the conduction band. Room temperature Raman spectra for the InN films grown on GaN show the signature of residual tensile stress in contrast to the compressive stress observed for the films grown directly on c-Al2O3.

Effects of doping and indium inclusions on the structural and optical properties of InN thin films grown by MOCVD

High-resolution x-ray diffraction (HRXRD), Raman scattering and photoluminescence (PL) are used to investigate the structural and optical properties of InN thin films grown by metalorganic chemical vapour deposition (MOCVD). They reveal that the phonon frequency ϖ (in cm−1) of varies with residual biaxial strains εxx (in %) induced by Mg doping, Zn doping and In inclusions as ϖ = 488.9–43.9 × εxx, relating to a Raman factor of 10.8 cm−1 GPa−1. They also reveal a strong correlation between PL energies and electron concentrations due to bandgap renormalization and band-filling effects. The evolution of HRXRD, due to chemical etching, provides evidence that the In inclusions not only appear on the surface and/or at grain boundaries but are also embedded in the InN crystals. Optical absorptions and PL spectra exhibit no apparent changes upon chemical etching, confirming the absence or minor effect of coupling on In-related plasmonic resonance and/or interband transitions, especially for those In inclusions present on the surface and/or at the grain boundaries of InN.

Direct current magnetron sputtering deposition of InN thin films

In this paper, InN thin films were deposited on Si (1 0 0) and K9 glass by reactive direct current magnetron sputtering. The target was In metal with the purity of 99.999% and the gases were Ar (99.999%) and N 2 (99.999%). The properties of InN thin films were studied. Scanning electron microscopy (SEM) shows that the film surface is very rough and energy dispersive X-ray spectroscopy (EDX) shows that the film contains In, N and very little O. X-ray diffraction (XRD) and Raman scattering reveal that the film mainly contains hexagonal InN. The four-probe measurement shows that InN film is conductive. The transmission measurement demonstrates that the transmission of InN deposited on K9 glass is as low as 0.5% from 400 nm to 800 nm.

The characterization and properties of InN grown by MOCVD

The characterization and properties of InN thin films grown on GaN templates by metalorganic chemical vapor deposition (MOCVD) at various growth temperatures were investigated. Their carrier concentrations ranged from 4.6 × 10 18 to 4 × 10 19 cm −3 and mobility valued from 150 to 1300 cm 2 /V s. The variation of the growth temperature brought about different growth rates. It was also found that growth rate is increased with the increasing growth temperature and reached 470 nm/h for the InN epitaxial layer grown at 675 °C. The surface roughness of InN layers was obtained from AFM measurement. The structural quality of the InN layers was determined by TEM. The surface and cross-sectional morphologies of these films are evaluated by SEM. The layer crystalline quality was investigated by means of X-ray diffraction in the rocking curves. Photoluminescence measurements performed at 7 K and room temperature gave emission peaks at around 0.7 eV.

Effect of island coalescence on structural and electrical properties of InN thin films

In this work, coalescence aspects of InN epitaxy are addressed. The coalescence phenomena have been studied in thin InN epilayers by means of electron microscopy and X-ray diffraction. Coalescence time and the corresponding diffusion coefficients at elevated temperatures were estimated for InN deposition. The substrate temperature was found to impact drastically the coalescence of the epilayer, and consequently, the electrical and transport properties of hexagonal InN material. Additionally, a simple growth model was suggested to explain the formation of domain boundaries and (0 0 0 1) stacking faults formed during the coalescence. In particular, it is shown that two adjacent and tilted, hexagonal-shaped InN domains may form a non-coherent boundary along a { 1 1 ¯ 0 0 } plane. We also suggest that the interaction between tilted domains induces formation of basal dislocations. This interaction has two consequences: a localized lateral growth of the most epitaxially oriented domain (forming a basal (0 0 0 1) stacking fault) followed by the formation of a surface step, and consequently the termination of a threading dislocation by its dissociation and propagation under the formed (0 0 0 1) stacking fault.

Effect of dislocations on electrical and electron transport properties of InN thin films. II. Density and mobility of the carriers

The influence of dislocations on electron transport properties of undoped InN thin films grown by molecular-beam epitaxy on AlN(0001) pseudosubstrates is reported. The microstructure and the electron transport in InN(0001) films of varying thickness were analyzed by transmission electron microscopy and variable temperature Hall-effect measurements. It was found that crystal defects have strong effects on the electron concentration and mobility of the carriers in the films. In particular, the combined analysis of microscopy and Hall data showed a direct dependence between free carrier and dislocation densities in InN. It was demonstrated that threading dislocations are active suppliers of the electrons and an exponential decay of their density with the thickness implies the corresponding decay in the carrier density. The analysis of the electron transport yields also a temperature-independent carrier concentration, which indicates degenerate donor levels in the narrow band-gap InN material. The relative insensitivity of the mobility with respect to the temperature suggests that a temperature-independent dislocation strain field scattering dominates over ionized impurity∕defect and phonon scattering causing the increase of the mobility with rising layer thickness due to the reducing dislocation density. Room temperature mobilities in excess of 1500cm2V−1s−1 were obtained for ∼800nm thick InN layers with the dislocation densities of ∼3×109cm−2.

Effect of dislocations on electrical and electron transport properties of InN thin films. I. Strain relief and formation of a dislocation network

The strain-relaxation phenomena and the formation of a dislocation network in 2H-InN epilayers during molecular beam epitaxy are reported. Plastic and elastic strain relaxations were studied by reflection high-energy electron diffraction, transmission electron microscopy, and high resolution x-ray diffraction. Characterization of the surface properties has been performed using atomic force microscopy and photoelectron spectroscopy. In the framework of the growth model the following stages of the strain relief have been proposed: plastic relaxation of strain by the introduction of geometric misfit dislocations, elastic strain relief during island growth, formation of threading dislocations induced by the coalescence of the islands, and relaxation of elastic strain by the introduction of secondary misfit dislocations. The model emphasizes the determining role of the coalescence process in the formation of a dislocation network in heteroepitaxially grown 2H-InN. Edge-type threading dislocations and dislocations of mixed character have been found to be dominating defects in the wurtzite InN layers. It has been shown that the threading dislocation density decreases exponentially during the film growth due to recombination and, hence, annihilation of dislocations, reaching ∼109cm−2 for ∼2200nm thick InN films.

Correlation between structural and electrical properties of InN thin films prepared by molecular beam epitaxy

The strain-relaxation phenomena and the formation of a dislocation network in 2 H -InN epilayers during molecular beam epitaxy are reported. The proposed growth model emphasizes the dominant role of the coalescence process in the formation of a dislocation network in 2 H -InN. Edge type threading dislocations and dislocations of mixed character have been found to be the dominating defects in wurtzite InN layers. It is demonstrated that these dislocations are active suppliers of electrons and an exponential decay of their density with the thickness implies a corresponding decay in the carrier density. Room temperature mobility in excess of 1500 cm 2 V −1 s −1 was obtained for ∼800 nm thick InN layers with dislocation densities of ∼3×10 9 cm −2 .

Microstructural assessment of InN-on-GaN films grown by plasma-assisted MBE

The structural properties of InN thin films, grown by rf plasma-assisted molecular beam epitaxy on Ga-face GaN/Al 2O 3(0001) substrates, were investigated by means of conventional and high resolution electron microscopy. Our observations showed that a uniform InN film of total thickness up to 1 μm could be readily grown on GaN without any indication of columnar growth. A clear epitaxial orientation relationship of [ 0001 ] GaN ∥ [ 0001 ] InN , ( 10 1 ̄ 0 ) GaN ∥ ( 10 1 ̄ 0 ) InN was determined. The quality of the InN film was rather good, having threading dislocations as the dominant structural defect with a density in the range of 10 9–10 10 cm −2. The crystal lattice parameters of wurtzite InN were estimated by electron diffraction analysis to be a = 0.354 nm and c = 0.569 nm, using Al 2O 3 as the reference crystal. Heteroepitaxial growth of InN on GaN was accomplished by the introduction of a network of three regularly spaced misfit dislocation arrays at the atomically flat interface plane. The experimentally measured distance of misfit dislocations was 2.72 nm. This is in good agreement with the theoretical value derived from the in-plane lattice mismatch of InN and GaN, which indicated that nearly full relaxation of the interfacial strain between the two crystal lattices was achieved.

Temperature effects on optical properties of InN thin films

Transmission measurements have been carried out on InN thin films grown by radio frequency magnetron sputtering on a sapphire (0001) substrate at 10–300 K. With the aid of a novel procedure developed for analyzing the transmission spectra, the effect of temperature on optical properties, such as the absorption coefficient, band-gap, Urbach bandtail characteristics, refractive index and extinction coefficient, of InN thin films has been determined. The wavelength and temperature dependence of the absorption coefficient in both the Urbach and intrinsic absorption regions has been described by a series of empirical formulae. The temperature dependence of the refractive index dispersion below the band-gap is also found to follow a Sellmeier equation. These formulae are very useful for the characterization and device design of InN films. The free-electron concentration in the InN thin film determined here is also found to be in good agreement with that obtained from infrared reflection measurements.

Optical Band Gap Measurements of InN Films in the Strong Degeneracy Limit

ABSTRACTThe optical properties of InN thin films (0.5 μm thick) grown on sapphire substrates by plasma source molecular beam epitaxy deposition have been measured in order to study the effect of electron degeneracy on the band gap measurement. X-ray diffraction measurements show that the films are wurtzite polycrystalline at a growth temperature of 325 °C, whereas a completely c-axis textured growth at a temperature of 475°C. The Raman bands A1 (LO) and E2 are rather broad indicating the presence of a large number of structural defects. Hall effect measurements show that both the films are n-type with carrier concentrations of (8.0 ± 1.6) × 1020 cm−3 and (3 ± 0.6) × 1020cm−3, respectively. The optical absorption data on these samples show n dependent band gap edge and a peak corresponding to plasmon due to strong electron degeneracy. The band gap absorption data were analyzed assuming a direct band gap and incorporating the Moss-Burstein shift effect. By taking into account the non-parabolic dispersion and the band-renormalization effects for the conduction band of InN, the calculated true band gap (0.7 eV) agrees with other recent measurements on high quality InN films.

Influence of growth rates on properties of InN thin films

InN thin films are grown at extremely low growth rates by reactive evaporation using a N2 radio-frequency plasma, and the dependencies of their properties on film growth rates are investigated. The carrier concentration of InN films shows a significant decrease, from 3.2 × 1020 to 4.3 × 1018 cm−3, by lowering the growth rate from 0.23 to 0.05 μm/h. However, their resistivity increases from 5.6 × 10−4 to 2.5 Ω · cm. Crystallinity, surface morphology and other results are also discussed in connection with these properties.

He/N 2 reactive gas composition and the structural and electrical properties of InN thin films grown on A1N-nucleated (00.1) sapphire by planar magnetron sputtering

The effects of He/N 2 reactive gas composition on the structural and electrical properties of InN thin films deposited by magnetron sputtering on A1N-nucleated (00.1) sapphire substrates have been investigated. X-ray scattering studies demonstrate that the films are an aggregate of strained heteroepitaxial domains and highly relaxed textured domains and that their apportionment is strongly tied to the reactive gas composition. The sputtering rate, as evidenced by film thickness, nominally increases with increasing volume fraction of N 2, but enhanced growth and epitaxial volume are observed at concentrations of N 2 near 60%. The variation in electrical transport properties with increasing N 2 fraction in the gas mixture is smoother, with the n-type Hall mobility rising monotonically from 35 cm 2 V −1 s −1 for films deposited in a 30% N 2 volume fraction to 55 cm 2 V −1 s −1 for growth in pure N 2, largel y due to an increase in the volume of heteroepitaxial domains, the consequent reduction in the density of high-angle grain boundaries, and the resulting decrease in intergrain resistance and film resistivity.

Buffer Layer Thickness and the Properties of InN Thin Films on AIN- Seeded (00.1) Sapphire And (111) Silicon

ABSTRACTIntroduction of a buffer layer to facilitate heteroepitaxy in thin films of the Group IIIA nitrides has had a tremendous impact on growth morphology and electrical transport. While AIN- and self-seeded growth of GaN has captured the majority of attention, the use of AIN-buffered substrates for InN thin films has also had considerable success. Herein, the properties of InN thin films grown by reactive magnetron sputtering on AIN-buffered (00.1) sapphire and (111) silicon are presented and, in particular, the evolution of the structural and electrical transport properties as a function of buffer layer sputter time (corresponding to thicknesses from ∼50Å to ∼0.64 μm) described. Pertinent results include: (a) for the InN overlayer, structural coherence and homogeneous strain normal to the (00.1) growth plane are highly dependent on the thickness of the AIN-buffer layer; (b) the homogeneous strain in the AIN-buffer layer is virtually nonexistent from a thickness of 200Å (where a significant X-ray intensity for (00.2)AIN is observed); and (c) the n-type electrical mobility for films on AIN-nucleated (00.1) sapphire is independent of AIN-buffer layer thickness, owing to divergent variations in carrier concentration and film resistivity. These effects are in the main interpreted as arising from a competition between the lattice mismatch of the InN overlayer with the substrate and with the AIN-buffer layer.

Electrical and Optical Properties of InN Thin Films Prepared by Reactive Evaporation Method

InN thin films, which are less studied III-V compound semiconductors, are prepared by a rf excited reactive evaporation method on glass or semi-insulating GaAs (100) substrates. The deposition conditions of the films are discussed and structures and electrical and optical properties of them are investigated. Relatively high mobility is obtained when the substrate temperature is high and the nitrogen gas pressure is low. On the other hand, relatively low carrier density is obtained when the substrate temperature is low and the nitrogen gas pressure is high. Color of the InN thin film is dark red and its band gap energy is 1.92 eV. Crystallinity of the InN thin films are improved by using the GaAs substrates.