Wurtzite InN Films Research Articles

Phase transition of InN films via trimethylindium flows

Wurtzite (w) and zincblende (zb) InN films have been grown on (011) SrTiO3 (STO) substrates by metal- organic chemical vapor deposition, the epitaxial relation- ships and optical properties are characterized by X-ray diffraction (XRD), absorption and photoluminescence (PL). Based on XRD θ –2θ and Φ scanning results, the epi- taxial relationships between (w- and zb-) InN films and STO substrates are determined, that is, (0001)[1120]w-InN (011)[100]STO and (100)[011]zb-InN //(011)[100]STO. Compared with the w-InN films, the zb-InN films exhibit a red shift in absorption edge and PL spectra, and a much nar- rower and stronger PL spectrum, implying a better optical quality of zb-InN films. The structure transition is supposed to be due to the difference in atom and bond areal density between the crystal plane of w-InN(0001) and zb-InN(100).

Recombination processes in Mg doped wurtzite InN films with p- and n-type conductivity

Obtaining high quality, wurtzite InN films with p-type conductivity is a challenge, and there is limited information about the photoluminescence (PL) characteristics of such films. In this study, we present a comprehensive PL study and discuss in detail the recombination processes in Mg-doped InN films with varying Mg concentrations. We find that at low Mg-doping of 1×1018 cm-3, which yields p-type conductivity, the PL in InN is spatially inhomogeneous. The latter is suggested to be associated with the presence of n-type pockets, displaying photoluminescence at 0.73 eV involving electrons at the Fermi edge above the conduction band edge. Increasing the Mg concentration to 2.9×1019 cm-3 in p-type InN yields strong and spatially uniform photoluminescence at 0.62 eV and 0.68 eV visible all the way to room temperature, indicating homogeneous p-type conductivity. An acceptor binding energy of 64 meV is determined for the Mg acceptor. Further increase of the Mg concentration to 1.8×1020 cm-3 leads to switching conductivity back to n-type. The PL spectra in this highly doped sample reveal only the emission related to the Mg acceptor (at 0.61 eV). In the low-energy tail of the emission, the multiple peaks observed at 0.54 – 0.58 eV are suggested to originate from recombination of carriers localized at stacking faults.

Growth and Characterization of Single Crystalline InN Grown on GaN by RF Sputtering for Robust Schottky Contacts

High-quality, single crystal wurtzite InN films were fabricated by radio-frequency magnetron reactive sputtering on GaN templates. The sputtered InN films in this study were about 100 nm thick. Atomic force microscopy analysis revealed the sputtered InN film had root-mean-square surface roughness of about 0.4 nm, which is comparable to the underlying GaN template. Coupled x-ray diffraction (XRD) measurements confirmed the (0001) preferred growth orientation and ω-rocking curve full-width-half-maximum (FWHM) = 0.85° for the symmetrical (0002) diffraction peak. The present InN film has the best crystal quality in terms of narrower FWHM of XRD rocking curve among reported sputtered InN thin films. In-plane and out-of-plane XRD measurements revealed a relaxed film. Room temperature Hall Effect measurements showed mobility of 110 cm2/V.s and electron concentration of 1–2 × 1020/cm3. The feasibility of utilizing a cost effective and productive method of sputtering to form robust Schottky contacts to GaN using InN, an immiscible and metallic-like semiconductor, was explored.

Effect of deposition conditions on properties of nitrogen rich-InN nanostructures grown on anisotropic Si (110)

Nanocrystalline InN were fabricated on anisotropic silicon [Si(110)] substrates by reactive radio frequency sputtering. The effects of deposition conditions on the InN film characteristics were comprehensively studied. The films were prepared using different argon and nitrogen plasma ratios under 8×10−3mbar at different temperatures and RF powers. X-ray diffraction measurements confirmed that all deposited films are wurtzite nanocrystalline InN films with (101) preferred growth orientation. All of the samples obtained under different deposition conditions were slightly N rich. For optimized deposition conditions, InN film on Si(110) substrate shows smooth surface with root-mean-square roughness around 2nm. The optical properties of InN layers were examined by micro-Raman and FTIR spectroscopy at room temperature. The A1(TO) and E1(TO) modes observed were a consequence of the wurtzite nanocrystalline nature of RF-sputtered films.

Effect of interfacial lattice mismatch on bulk carrier concentration and band gap of InN

The issue of ambiguous values of the band gap (0.6 to 2 eV) of InN thin film in literature has been addressed by a careful experiment. We have grown wurtzite InN films by PA-MBE simultaneously on differently modified c-plane sapphire substrates and characterized by complementary structural and chemical probes. Our studies discount Mie resonances caused by metallic In segregation at grain boundaries as the reason for low band gap values (≈ 0.6 eV) and also the formation of Indium oxides and oxynitrides as the cause for high band gap value (≈ 2.0 eV). It is observed that polycrystallinity arising from azimuthal miss-orientation of c-oriented wurtzite InN crystals increases the carrier concentration and the band gap values. We have reviewed the band gap, carrier concentration, and effective mass of InN in literature and our own measurements, which show that the Moss-Burstein relation with a non-parabolic conduction band accounts for the observed variation of band gap with carrier concentration.

Tailoring the in-plane epitaxial relationship of InN films on (1 1 1)SrTiO3 substrates by substrate pretreatment

Wurtzite InN films of c-axis orientation have been grown on as-received and water-immersed (111)SrTiO3 (STO) substrates by metal–organic chemical vapor deposition. The epitaxial relationships between InN films and STO substrates are investigated by X-ray diffraction. Two kinds of in-plane alignment of [112¯0]InN∥[11¯0]STO and [11¯00]InN∥[11¯0]STO with a 30° rotation relative to each other have been realized on as-received and water-immersed (111)STO substrates, respectively. The in-plane orientation of InN relative to STO is strongly dependent on substrate pretreatment, but not sensitive to growth temperature and trimethylindium flow rates. The difference in terminated surfaces of as-received and water-immersed STO substrates are postulated to be responsible for the change in epitaxial relationships. Compared with each other, the in-plane epitaxial relationships of [11¯00]InN∥[11¯0]STO and [112¯0]InN∥[11¯0]STO are respectively energetically preferred due to a higher bonded density and thus a lower interface energy on SrO3- and Ti-terminated STO substrates.

Wurtzite to zincblende transition of InN films on (011) SrTiO3 by decreasing trimethylindium flows

Wurtzite (w) and zincblende (zb) InN films have been grown on (011) SrTiO3 (STO) substrates by metal-organic chemical vapor deposition, the epitaxial relationships and optical properties are characterized by X-ray diffraction (XRD), absorption and photoluminescence (PL). Based on XRD θ–2θ and Φ scanning results, the epitaxial relationships between (w- and zb-) InN films and STO substrates are determined, that is, (0001)[1120]w-InN//(011)[100]STO and \((100)[011]_{\mathit{zb}\mbox{-}\mathrm{InN}}//(011)[100]_{\mathrm{STO}}\). Compared with the w-InN films, the zb-InN films exhibit a red shift in absorption edge and PL spectra, and a much narrower and stronger PL spectrum, implying a better optical quality of zb-InN films. The structure transition is supposed to be due to the difference in atom and bond areal density between the crystal plane of w-InN(0001) and zb-InN(100).

Unintentional incorporation of hydrogen in wurtzite InN with different surface orientations

We have studied hydrogen impurities and related structural properties in state-of-the-art wurtzite InN films with polar, nonpolar, and semipolar surface orientations. The effects of thermal annealing and chemical treatment on the incorporation and stability of H are also discussed. The near-surface and bulk hydrogen concentrations in the as-grown films increase when changing the surface orientation from (0001) to (0001¯) to (11¯01) and to (112¯0), which may be associated with a decrease in the grain size and change of the growth mode from 2D to 3D. Thermal annealing at 350oC in N2 leads to a reduction of H concentrations and the intrinsic levels of bulk H are found to correlate with the structural quality and defects in the annealed films.

Direct growth of hexagonal InN films on 6H-SiC by radio-frequency metal-organic molecular-beam epitaxy

Wurtzite InN films were prepared on a 6H-SiC substrate by a self-designed plasma-assisted metal-organic molecular-beam epitaxy system without a buffer layer. In this article, the authors investigate the structural and optical properties of InN films grown on a 6H-SiC substrate. The crystallinity and microstructure of the thin film were further characterized by x-ray diffraction (XRD), field-emission scanning-electron microscopy, and transmission-electron microscopy. Electrical and optical properties were evaluated by Hall and photoluminescence (PL) measurements. XRD results indicate that InN film grown at 500 °C is epitaxially grown along the c-axis orientation. The two-dimensional growth mode is clearly shown in scanning-electron microscope images. Room-temperature PL spectra show that the emission peak is located at ∼0.83 eV due to the Burstein–Moss effect. In addition, the crystalline InN samples crack and peel away from the substrate at elevated growth temperature. This phenomenon may be attributed to lattice mismatch and grain coalescence while increasing the growth temperature. The narrow window of the growth temperature plays an important role in engineering the InN epitaxial growth.

Room-temperature infrared photoluminescence from sputter-deposited InN films

Wurtzite InN films are deposited on Si(100) and glass substrates by reactive rf-magnetron sputtering at a substrate temperature of 300 °C. The InN films have high electron concentrations of n∼4×1020 cm−3 at 300 K. The optical band-gap energy is determined to be ∼1.4 eV at 300 K. This value is considerably smaller than the previously reported value of ∼2 eV in sputter-deposited InN films. Strong infrared photoluminescence (PL) with a peak at ∼1.3 eV is also observed at room temperature. Thermal annealing in dry N2 atmosphere at 600 °C changes the deposited film from wurtzite InN to a cubic In2O3 that still exhibits PL emission but in the visible spectral region (∼2 eV).

Optical properties of InN grown on Si(111) substrate

AbstractA comprehensive characterization of the optical properties of wurtzite InN films grown by molecular beam epitaxy on Si(111) substrates is presented. Two types of films are investigated in this work: InN on AlN/Si(111) and InN on GaN/AlN/Si(111). Their properties are compared to a layer deposited on GaN/sapphire substrate. The dielectric function (DF) is obtained from spectroscopic ellipsometry (SE). The infrared studies yield the plasma frequency and thus the electron density, while the interband absorption is probed between 0.56 and 9.8 eV. For InN grown on Si(111) substrate, the absorption onset is slightly shifted to higher energies with respect to the InN film grown on GaN/sapphire which can be attributed to higher electron concentrations. Despite this, strongly pronounced optical transitions due to critical points of the band structure are found in the high‐energy part of the DF. It emphasizes the already promising quality of the InN films on silicon. Band‐gap renormalization (BGR), band filling, and strain are taken into account in order to estimate the intrinsic band gap of wurtzite InN. For the InN layers on silicon, we get a band gap between 0.66 and 0.685 eV.

Electron accumulation at nonpolar and semipolar surfaces of wurtzite InN from generalized infrared ellipsometry

The free electron properties of nonpolar (112¯0)-oriented and semipolar (101¯1)-oriented wurtzite InN films are studied by generalized infrared ellipsometry (GIRSE). We demonstrate the sensitivity of GIRSE to the surface charge accumulation layer and find a distinct surface electron accumulation to occur at all surfaces. The obtained surface electron sheet densities are found to vary from 0.9×1013 to 2.3×1014 cm−2 depending on the surface orientation and bulk electron concentration. The upper limits of the surface electron mobility parameters of 417–644 cm2/V s are determined and discussed in the light of electron confinement at the surface.

Influence of strain on the band gap energy of wurtzite InN

AbstractDegenerate wurtzite InN films with electron concentrations in the range of 1018 cm–3 were studied by using spectroscopic ellipsometry (SE) in two spectral ranges. The analysis of the frequencies of the coupled phonon‐plasmon modes in the mid‐infrared region allows the determination of the plasma frequency and thus of the carrier density in the bulk of the samples. SE from the near‐infrared to the visible range provides the dielectric function (DF) around the absorption edge. The electron concentration as an important input parameter enables accurate determination of Burstein–Moss shift and band‐gap renormalization for the analysis of the imaginary part of the DF. Taking into account the in‐plane strain of the films, which is caused by lattice mismatch between InN and buffer/substrate, we obtain a zero‐density strain‐free band gap of 0.675 eV at room temperature. (© 2009 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

The influence of the LT-InN buffer growth conditions on the quality of InN films grown on Si(1 1 1) substrate by MBE

This work aims to explore the effect of the growth conditions of low-temperature InN (LT-InN) buffer layers on the quality of wurtzite InN films grown on Si(1 1 1) substrates by plasma-assisted molecular beam epitaxy (PA-MBE). Experimental results indicated that higher growth rate and lower thickness of LT-InN buffer layer guarantee better crystalline quality and optical properties for InN films grown at the same temperature. The crystalline quality of the InN epilayers was investigated by high-resolution X-ray diffraction (XRD) performed on the reflection plans of various inclination angles. XRD study confirmed that the dominating threading dislocation was edge type. Surface morphology was measured by atomic force microscope (AFM), and optical property was characterized by photoluminescence (PL). The best InN film was obtained from the sample with a 20 nm LT-InN buffer layer, 140 nm/h growth rate, and 500 °C HT-InN growth temperature. The characterization results showed 2.785×10 10 cm –2 edge-type dislocation density estimated by XRD, 11.57 nm AFM RMS roughness, and 0.666 eV near band-edge PL transition at 15 K with 24 meV broadening.

Electronic band structure of wurtzite InN around the fundamental gap in the presence of biaxial strain

AbstractWe present results of a theoretical study of the electronic band structure of wurtzite InN films under biaxial strain in the C ‐plane (0001) and in planes that correspond to non‐polar orientations such as the A ‐plane (11$ \bar 2 $0) and the M ‐plane (1$ \bar 1 $00). The calculations are performed under the k · p perturbation theory approach using the Bir–Pikus Hamiltonian. The results show that the fundamental bandgap of InN shifts by 30 meV (14 meV) for isotropic tensile (compressive) strain in the C ‐plane with out‐of‐plane contraction (dilation) of 0.2%. For films of non‐polar orientations, the c‐axis lies in the film plane and the strain is expected to be different between directions parallel and perpendicular to c. Such anisotropic strain give rise to valence band mixing which results in dramatic changes in the optical polarization properties as evidenced by the calculated oscillator strengths of the interband transitions. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Polarized photoluminescence and absorption in A-plane InN films

The authors report the observation of strong polarization anisotropy in the photoluminescence (PL) and the absorption spectra of [112¯0] oriented A-plane wurtzite InN films grown on R-plane (11¯02) sapphire substrates using molecular beam epitaxy. For A-plane films the c axis lies in the film plane. The PL signal collected along [112¯0] with electric vector E⊥c is more than three times larger than for E‖c. Both PL signals peak around 0.67eV at 10K. The absorption edge for E‖c is shifted to higher energy by 20meV relative to E⊥c. Optical polarization anisotropy in wurtzite nitrides originates from their valence band structure which can be significantly modified by strain in the film. The authors explain the observed polarization anisotropy by comparison with electronic band structure calculations that take into account anisotropic in-plane strain in the films. The results suggest that wurtzite InN has a narrow band gap close to 0.7eV at 10K.

InN layers grown on silicon substrates: effect of substrate temperature and buffer layers

This work reports on the effect of the substrate temperature and buffer layers on wurtzite InN films grown by plasma-assisted molecular beam epitaxy (PA-MBE) on Si(1 1 1) substrates. Growth temperature is found to be the most critical parameter during the growth of InN layers. Growths performed above 500 °C lead to the formation of metallic indium droplets on the surface. Once the temperature has been fixed below 500 °C, the morphology can be controlled by the effective indium to nitrogen molecular flux ratio, from N-rich conditions that lead to columnar InN layers, to stoichiometric conditions leading to coalesced InN films. The effect of different types of buffer layers has also been investigated determining the crystal quality of the samples. X-ray diffraction (XRD) rocking curves around the InN (0 0 0 2) reflection yield a minimum full-width at half-maximum (FWHM) value of 682 arcsec for a compact InN layer grown on a high-temperature AlN buffer layer.

Heteroepitaxial growth of wurtzite InN films on Si(111) exhibiting strong near-infrared photoluminescence at room temperature

High-quality InN epitaxial films have been grown by nitrogen-plasma-assisted molecular-beam epitaxy on Si(111) substrates using a double-buffer technique. Growth of a (0001)-oriented single crystalline wurtzite–InN layer was confirmed by reflection high-energy electron diffraction, x-ray diffraction, and Raman scattering. At room temperature, these films exhibited strong near-infrared (0.6–0.9 eV) photoluminescence (PL). In addition to the optical absorption measurement of absorption edge and direct band nature, the PL signal was found to depend linearly on the excitation laser intensity over a wide intensity range. These results indicate that the observed PL is due to the emission of direct band-to-band recombination rather than the band-to-defect (or impurity) deep emission.

Experimental Consideration of Optical Band-Gap Energy of Wurtzite InN

Wurtzite InN films were grown on a thick GaN layer by metalorganic vapor phase epitaxy (MOVPE). Growth of a (0001)-oriented single crystalline layer was confirmed by Raman scattering with polarized geometry, X-ray diffraction, and reflection high-energy-electron diffraction. In the growth, an extremely high V/III ratio of around 700000 was used to suppress inclusion of metal-indium. We observed at room temperature strong photoluminescence (PL) at 0.75 eV as well as a clear absorption edge at 0.7–1.0 eV. In contrast, no PL was observed, even at high power excitation of 0.6 MW/cm2 at ∼1.9 eV, which had been reported as the band-gap in absorption experiments on single crystalline films grown by microwave-excited MOVPE and poly-crystalline ones. Careful inspection strongly suggests that a wurtzite InN single crystal has a true band-gap energy of 0.7–1.0 eV, and the discrepancy between this and previous data could be due to the difference in the crystallinity of the materials measured.

Effects of the narrow band gap on the properties of InN

Infrared reflection experiments were performed on wurtzite InN films with a range of free-electron concentrations grown by molecular-beam epitaxy. Measurements of the plasma edge frequencies were used to determine electron effective masses. The results show a pronounced increase in the electron effective mass with increasing electron concentration, indicating a nonparabolic conduction band in InN. We have also found a large Burstein-Moss shift of the fundamental band gap. The observed effects are quantitatively described by the kip interaction within the two-band Kane model of narrow-gap semiconductors.