InN Thin Films Research Articles

Theoretical investigation on structural stability of InN thin films on 3C–SiC(0 0 1)

The structural stability of InN thin films on 3C–SiC(0 0 1) substrate is systematically investigated based on an empirical interatomic potential, which incorporates electrostatic energy due to covalent bond charges and ionic charges. The calculated energy differences among coherently grown 3C–InN(0 0 1), 3C–InN(0 0 1) with misfit dislocations (MDs), and 2H–InN(0 0 0 1) imply that the coherently grown 3C–InN(0 0 1) is stable when the film thickness is less than 7 monolayers (MLs) while 2H–InN(0 0 0 1) is stabilized for the thickness beyond 8 MLs. This is because InN layers in 2H–InN(0 0 0 1) are fully relaxed by one MD. The analysis of atomic configuration at the 3C–InN(0 0 1)/3C–SiC(0 0 1) interfaces reveals that the coordination number of interfacial atoms is quite different from that in the bulk region. Thus, 3C–InN(0 0 1) with MDs on 3C–SiC(0 0 1) is always metastable over entire range of film thickness, consistent with the successful fabrication of 2H–InN(0 0 0 1) on 3C–SiC(0 0 1) by the molecular beam epitaxy. These results suggest that the mismatch in atomic arrangements at the interface crucially affects the structural stability of InN thin films on 3C–SiC(0 0 1) substrate.

Optical Hall Effect in Hexagonal InN

Measurements of the optical Hall effect in naturally doped high-quality wurtzite-structure InN thin films by generalized ellipsometry reveal that both the surface and the interior (bulk) free electron densities decrease with powerlaw dependencies on the film thickness. We discover a significant deviation between the bulk electron and dislocation densities. This difference is attributed here to the existence of surface defects with activation mechanism different from bulk dislocations and identifies the possible origin of the so far persistent natural n-type conductivity in InN. We further quantify the anisotropy of the C-point effective mass.

Auger recombination in InN thin films

Auger recombination is studied in InN thin films using an ultrafast time-resolved photoluminescence apparatus. The decay rates are analyzed with nonlinear dependence of the photoluminescence intensity on the carrier concentration. The fitted radiative recombination coefficients at a temperature of 35K are consistent with the theoretical prediction. The Auger rates are small at low carrier concentrations but increase quadratically with the carrier concentration. The Auger activation energies of 4.3 and 9.0meV obtained from the temperature-dependent Auger coefficient indicate that Auger recombination is weakly dependent on temperature and is a phonon-assisted process.

Determination of Band Structures of InN/GaN Interfaces by Synchrotron Radiation Hard X-ray Photoemission Spectroscopy

We have determined the valence band discontinuities of InN/Mg:GaN and InN/Si:GaN heterostructures by hard x-ray photoemission spectroscopy. InN thin films were grown by pulsed laser deposition method and streaky high-energy electron diffraction pattern was observed, indicating that good crystalline InN films were grown. The value of valence band discontinuity for InN/Mg:GaN is as small as 0.3 eV, while that for InN/Si:GaN is 1.4 eV. The fact that the InN/Mg:GaN interface has the small energy barrier suggests that InN is very promising for the interlayer between transparent conductive layers and Mg:GaN in light emitting diode structures. [DOI: 10.1380/ejssnt.2008.254]

Thin Film InN/Anatase Bilayers as a Substitute Dye/Semiconductor Interface for Solar Cells

AbstractThe electronic properties of an InN/anatse bilayer, proposed as a replacement for the dye/semi-conductor interface in Dye Sensitized Solar Cell[1, 2], are measured. RF sputtered thin films of anatase and InN are used as the “dye” replacement. . Two types of InN film are prepared: polycrystalline samples deposited at high temperature, with an optical band gap of < 1 eV, and as-deposited (at least partially amorphous) samples with an optical band gap >1 eV. Energy Dispersive X-ray fluorescence, X-ray Diffraction, and Raman spectroscopy are used to characterize the samples. The resistance in the dark and under illumination are measured. The samples deposited at high temperature are crystalline and have a sheet resistivity ≈ 4 Ω/⁐, and display no photoconductivity. The partially amorphous samples have sheet resistivity of ≈ 500Ω/⁐. Since both types of InN films, including high quality (based on band gap) polycrystalline InN, do not show increased conductivity with light, we conclude that a solar cell based on an InN/anatase bilayer is not feasible.

Temperature dependence of infrared reflectance spectra of InN

To investigate both the optical and electrical properties of InN, we have measured the infrared reflectance spectra of InN thin films and performed the fitting analyses of the infrared spectra to obtain not only phonon frequencies and the damping factors but also the carrier concentration of InN. In this paper, we extend the aim of those analyses to the electron mobility and demonstrate that the temperature dependence of the electron mobility can be discussed using the infrared reflectance spectra analyses.

Determination of the third- and fifth-order nonlinear refractive indices in InN thin films

We have combined the reflection and transmission Z-scan (RZ- and TZ-scans) techniques under femtosecond laser at 800nm to extract both the third- and fifth-order nonlinear refractive indices (n2 and n4) in InN thin films. The observation of the nonlinear refractive index saturation in the intensity-dependent RZ-scan measurement indicates the existence of the fifth-order effect. By the aid of the TZ-scan, the fifth-order nonlinear effect has been enhanced by enlarging the cascaded contribution from the increased laser interaction length, where large n2 of −2.5×10−11cm2∕W and n4 of 2.1×10−19cm4∕W2 have been determined.

Raman scattering by longitudinal optical phonons in InN nanocolumns grown on Si(1 1 1) and Si(0 0 1) substrates

Raman measurements in high-quality InN nanocolumns and thin films grown on both Si(1 1 1) and Si(1 0 0) substrates display a low-energy coupled LO phonon–plasmon mode together with uncoupled longitudinal optical (LO) phonons. The coupled mode is attributed to the spontaneous accumulation of electrons on the lateral surfaces of the nanocolumns, while the uncoupled ones originates from the inner part of the nanocolumns. The LO mode in the columnar samples appears close to the E 1(LO) frequency. This indicates that most of the incident light is entering through the lateral surfaces of the nanocolumns, resulting in pure longitudinal–optical mode with quasi-E 1 symmetry. For increasing growth temperature, the electron density decreases as the growth rate increases. The present results indicate that electron accumulation layers do not only form on polar surfaces of InN, but also occur on non-polar ones. According to recent calculations, we attribute the electron surface accumulation to the temperature dependent In-rich surface reconstruction on the nanocolumns sidewalls.

In2O3/Si heterojunction solar cells fabricated by InN oxidation

The present paper reports on the fabrication of In 2 O 3 layers by oxidation of InN thin films deposited on n -type silicon substrate by magnetron reactive sputtering. The subsequent solar cells using the resulting In 2 O 3 /Si heterojunctions exhibit an ideality factor, deduced from current-voltage ( I – V ) characteristics of around 2.52 at a forward bias of 0.5 V. Other measured parameters were the short-circuit current ( I sc ), the open-circuit voltage ( V oc ), the maximum output power ( P m ), the fill factor ( FF ) and the efficiency ( η ), which had values of were 3.17 mA, 0.75 V, 0.869 mW, 0.365 and 9.66 %, respectively, under AM 1.5 illumination. The value of series resistance was around 107 Ω. In 2 O 3 films formed by the oxidation of InN have a higher open-circuit voltage than In 2 O 3 -based solar cells formed by the oxidation of indium.

Energy relaxation of InN thin films

The energy relaxation of InN thin films has been studied by ultrafast time-resolved photoluminescence technique. The obtained carrier cooling curves can be explained by carriers releasing excessive energy through the carrier–LO-phonon interaction. The extracted effective phonon emission times decrease as the photoexcited carrier concentration reduces and come close to the theoretical prediction of 23fs at small carrier concentration. The reduction of energy loss rate at high photoexcited carrier density is attributed to the hot phonon effect.

Influence of defects on the absorption edge of InN thin films: The band gap value

We investigate the optical-absorption spectra of InN thin films whose electron density varies from $\ensuremath{\sim}{10}^{17}\phantom{\rule{0.3em}{0ex}}\text{to}\phantom{\rule{0.3em}{0ex}}\ensuremath{\sim}{10}^{21}\phantom{\rule{0.3em}{0ex}}{\mathrm{cm}}^{\ensuremath{-}3}$. The low-density films are grown by molecular-beam-epitaxy deposition while highly degenerate films are grown by plasma-source molecular-beam epitaxy. The optical-absorption edge is found to increase from $0.61\phantom{\rule{0.3em}{0ex}}\text{to}\phantom{\rule{0.3em}{0ex}}1.90\phantom{\rule{0.3em}{0ex}}\mathrm{eV}$ as the carrier density of the films is increased from low to high density. Since films are polycrystalline and contain various types of defects, we discuss the band gap values by studying the influence of electron degeneracy, electron-electron, electron-ionized impurities, and electron-LO-phonon interaction self-energies on the spectral absorption coefficients of these films. The quasiparticle self-energies of the valence and conduction bands are calculated using dielectric screening within the random-phase approximation. Using one-particle Green's function analysis, we self-consistently determine the chemical potential for films by coupling equations for the chemical potential and the single-particle scattering rate calculated within the effective-mass approximation for the electron scatterings from ionized impurities and LO phonons. By subtracting the influence of self-energies and chemical potential from the optical-absorption edge energy, we estimate the intrinsic band gap values for the films. We also determine the variations in the calculated band gap values due to the variations in the electron effective mass and static dielectric constant. For the lowest-density film, the estimated band gap energy is $\ensuremath{\sim}0.59\phantom{\rule{0.3em}{0ex}}\mathrm{eV}$, while for the highest-density film, it varies from $\ensuremath{\sim}0.60\phantom{\rule{0.3em}{0ex}}\text{to}\phantom{\rule{0.3em}{0ex}}\ensuremath{\sim}0.68\phantom{\rule{0.3em}{0ex}}\mathrm{eV}$ depending on the values of electron effective mass and dielectric constant.

Effect of surface oxidation on electron transport in InN thin films

The chemical and electron transport properties of oxidized indium nitride epilayers and indium oxide/indium nitride heterostructures are reported. It is shown that the accumulation of electrons at the InN surface can be manipulated by the formation of a thin surface oxide layer using an ozone-assisted oxidation processing. It results in improved transport properties and in a reduction of the electron sheet concentration of the InN epilayer caused by a passivation of the surface donors and a shift of the electron density distribution peak from the surface toward the bulk InN. Using the ensemble Monte Carlo simulation method, the electron mobility for different dislocation densities and surface band bending values has been calculated. The theoretical results correlate well with our experimental data. In opposition to the ozone treatment, in epitaxial oxide/nitride heterojunctions the electron sheet concentration of InN raises due to the increasing band bending at the heterointerface affecting adversely the electron transport properties.

The nature of nitrogen related point defects in common forms of InN

The role of point defects related to the presence of excess nitrogen is elucidated for InN thin films grown by different techniques. Elastic recoil detection analysis has shown the presence of excess nitrogen in state-of-the-art InN films. Using x-ray photoelectron spectroscopy and x-ray diffraction it is shown that two distinct forms of point defects can be distinguished; one of these appears to be an interstitial form of nitrogen, common in some forms of polycrystalline InN. The other is associated with a combined biaxial and hydrostatic strain observed for molecular beam epitaxy (MBE) and chemical vapor deposition (CVD) grown films, and may be a mixture of the nitrogen-on-metal antisite defect and lower densities of indium vacancies and interstitial nitrogen. The high density of defects present in all the InN samples examined suggests that stoichiometry related point defects dominate the electrical and optical properties of the material. The difference in the type of point defect observed for polycrystalline (rf sputtered) and epitaxial (MBE and CVD) InN supports existing evidence that the Moss-Burstein effect is not an adequate description of the apparent band-gap difference between InN samples grown by different techniques.

Thermal properties and thermoelectric microdevices with InN thin films

We report the load characteristics of miniature thermoelectric power devices using InN thin films prepared by reactive radio-frequency (RF) sputtering. The devices are composed of 25-pair and 12-pair InN-chromel films on a SiO2 glass substrate. The open output voltage and the maximum output power were 0.11V and 2.9×10-8W at ΔT=262K for the 25-pair device, respectively, and 0.05V and 1.3×10-8W at ΔT=234K for the 12-pair device, respectively. The open output voltage and the maximum output power were proportional to (ΔT)n (n=1 and 2, respectively).

Resonant terahertz generation from InN thin films

Highly efficient conversion from ultrafast optical pulses to their terahertz (THz) counterparts has been achieved with InN thin films. An average THz output power as high as 0.931 microW has been obtained for an average pump power of 1 W, corresponding to a normalized conversion efficiency of 190% mm(-2). Based on our measured dependences of the THz output power on pump polarization, incident angle, pump power, and InN film thickness, resonance-enhanced optical rectification is one of the most plausible mechanisms for the THz generation in the InN films.

Interaction of hydrogen with InN thin films elaborated on InP(1 0 0)

III–V semiconductor compound structures are widely applied in technology of advanced microelectronics, optoelectronics, and gas sensors. In this paper, we report on the use of XPS to characterize in situ the interaction of thermally activated hydrogen atoms and hydrogen molecules with InP(1 0 0) surfaces covered by thin InN overlayers. XPS spectra were taken with an ESCALAB-210 spectrometer after repeated hydrogenation cycles at temperatures up to 350 °C. The evolution of the In 3d, In 4d, P 2p, N 1s, O 1s and C 1s photoelectron spectra was carefully monitored. The XPS spectra of the hydrogen exposed surface revealed significant differences compared to those from the non-hydrogenated surface. InN films were found to be weakly reactive to hydrogen under experimental conditions explored. The behavior of P atoms at the hydrogenated surface was dependent on the parameters characterizing each hydrogenation (exposure, hydrogen species used, annealing temperature). Moreover, the heavily hydrogenated surface exhibited a phosphorus enrichment.

Reduced surface electron accumulation at InN films by ozone induced oxidation

A room temperature ozone induced oxidation of thin InN films is proposed to improve the electric transport properties. The sheet carrier density is reduced upon oxidation by a value which is in the order of the electron concentration of an untreated InN surface. Thus, ozone effectively passivates the surface defect states on InN and might be an effective method to prepare InN films for electronic applications. A model for the improved electron transport properties is proposed taking into account the decreased surface band bending and the decreased influence of surface electrons on the net mobility of InN layers.

Growth and Characterization of InN Thin Films on Sapphire byMOCVD

Indium nitride thin films are grown on sapphire substrates bymetal-organic chemical vapour deposition (MOCVD). By employing three-step layer buffers, the mirror-like layers on two-inch sapphirewafers have been obtained. The structural, optical and electricalcharacteristics of InN are investigated by x-ray diffraction, scanningelectron microscopy, atomic force microscopy, photoluminescence andinfrared optical absorption. The photoluminescence and the absorptionstudies of the materials reveal a marked energy bandgap structurearound 0.70 eV at room temperature. The room-temperature Hall mobilityand carrier concentration of the film are typically 939 cm2/Vs,and 3.9×1018cm−3, respectively.

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.

Strain distribution of thin InN epilayers grown on (0001) GaN templates by molecular beam epitaxy

A structural characterization of thin InN films is performed to determine the post-growth strain distribution, using electron microscopy techniques. A 60° misfit dislocation network at the InN∕GaN interface effectively accommodates the lattice mismatch. The InN in-plane lattice parameter, which remained practically constant throughout the epilayer thickness, was precisely determined by electron diffraction analysis, and cross-section and plan-view lattice images. Image analysis using the geometric phase and projection methods revealed a uniform distribution of the residual tensile strain along the growth and lateral directions. The in-plane strain is primarily attributed to InN island coalescence during the initial stages of growth.