Zinc-blende InN 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).

Self‐organized growth of cubic InN dot arrays on cubic GaN using MgO (001) vicinal substrates

Cubic zincblende InN (c‐InN) nanoscale dot arrays are grown on cubic GaN (c‐GaN) by RF‐molecular beam epitaxy using MgO (001) vicinal substrates oriented 3.5° toward [110]. The obtained dot arrays have longer ordering length compared with those grown using conventional on‐axis (just) substrates. The c‐GaN underlayer grown on the vicinal substrate exhibits a single‐domain crystalline structure, while that grown on the just substrate is a mixture of two orthogonal crystalline domains. The change in the c‐GaN domain structure leads to the enlarged domain size and lower density of domain boundaries on the c‐GaN (001) surface in [1–10]. The longer ordering length of the c‐InN dot on the vicinal substrates is reflected by the decrease in the c‐GaN domain boundary that disrupts the lateral ordering of the dot arrays.Atomic force microscope image of c‐InN dots grown on a single‐domain c‐GaN. The sample was fabricated using a MgO (001) vicinal substrate.

The steady-state and transient electron transport within bulk zinc-blende indium nitride: The impact of crystal temperature and doping concentration variations

Within the framework of a semi-classical three-valley Monte Carlo electron transport simulation approach, we analyze the steady-state and transient aspects of the electron transport within bulk zinc-blende indium nitride, with a focus on the response to variations in the crystal temperature and the doping concentration. We find that while the electron transport associated with zinc-blende InN is highly sensitive to the crystal temperature, it is not very sensitive to the doping concentration selection. The device consequences of these results are then explored.

Assessing structural, free-charge carrier, and phonon properties of mixed-phase epitaxial films: The case of InN

We develop and discuss appropriate methods based on x-ray diffraction and generalized infrared spectroscopic ellipsometry to identify wurtizte and zinc-blende polymorphs, and quantify their volume fractions in mixed-phase epitaxial films taking InN as an example. The spectral signatures occurring in the azimuth polarization (M\"uller matrix) maps of mixed-phase epitaxial InN films are discussed and explained in view of polymorphism (zinc-blende versus wurtzite), volume fraction of different polymorphs and their crystallographic orientation, and azimuth angle. A comprehensive study of the structural, phonon and free electron properties of zinc-blende InN films containing inclusions of wurtzite InN is also presented. Thorough analysis on the formation of the zinc-blende and wurtzite phases is given and the structural evolution with film thickness is discussed in detail. The phonon properties of the two phases are determined and discussed together with the determination of the bulk free-charge carrier concentration, and electron accumulation at the mixed-phase InN film surfaces.

Effect of Mg doping on the structural and free-charge carrier properties of InN films

We present a comprehensive study of free-charge carrier and structural properties of two sets of InN films grown by molecular beam epitaxy and systematically doped with Mg from 1.0 × 1018 cm−3 to 3.9 × 1021 cm−3. The free electron and hole concentration, mobility, and plasmon broadening parameters are determined by infrared spectroscopic ellipsometry. The lattice parameters, microstructure, and surface morphology are determined by high-resolution X-ray diffraction and atomic force microscopy. Consistent results on the free-charge carrier type are found in the two sets of InN films and it is inferred that p-type conductivity could be achieved for 1.0 × 1018 cm−3 ≲ [Mg] ≲ 9.0 × 1019 cm−3. The systematic change of free-charge carrier properties with Mg concentration is discussed in relation to the evolution of extended defect density and growth mode. A comparison between the structural characteristics and free electron concentrations in the films provides insights in the role of extended and point defects for the n-type conductivity in InN. It further allows to suggest pathways for achieving compensated InN material with relatively high electron mobility and low defect densities. The critical values of Mg concentration for which polarity inversion and formation of zinc-blende InN occurred are determined. Finally, the effect of Mg doping on the lattice parameters is established and different contributions to the strain in the films are discussed.

Comparative Analysis of Nitrides Band Structures Calculated by the Empirical Pseudopotential Method

The electronic band structures of zinc blende and wurtzite GaN and InN are calculated using the empirical pseudopotential method, with the form factors adjusted to reproduce correctly the most important band features. To this end, a comprehensive analysis and comparison with several experimental and theoretical data reported in the literature is performed. Relevant energy spacings as well as direct and indirect band gaps are then derived from the band structures. The electron effective masses at high symmetry points are also obtained using a parabolic line fit. The calculated parameters are reported together with existing data so that they can be easily compared and used in the interpretation of experiments and for numerical simulation purposes.

Pressure based first-principles study of the electronic, elastic, optic and phonon properties of zincblende InN

Generalized gradient approximation proposed by Perdew–Burke–Ernzerhof (GGA-PBE) is used to determine the effect of pressure on electronic, elastic, acoustic, optical and vibrational properties of zincblende InN along with the Ultra soft pseudopotential method. The structural properties show good consistency and stability at elevated pressures. The zincblende InN displays zero band gap and its metallicity maintains even at high pressures. The density of states appear in a quarterly divided region, where the contribution of different states have been discussed, and it is found that the peak positions are consistent with experimental L1, L3, K absorption and emission edges. The effect of pressure appears in strong hybridization due to which DOS above and below the Fermi level are shifting to the corresponding higher and lower energies due to p–d hybridization. The calculated elastic constants agree well with the literature. Except C44 and Cs, all others show an increasing trend with the pressure. Acoustic wave speeds have been calculated in [100], [110] and [111] directions with the help of elastic constants for the first time. For the optical properties, the main peaks of the imaginary part of dielectric function lie in close vicinity of experiment and shift to higher energies with a reduction in peak intensities when the pressure effects come into play. Similarly the absorption peaks are red shifted with respect to hydro-static pressure. The refractive index is maximum at lower energies and its magnitude reduces with pressure and the maximum value of energy loss function is obtained corresponding to minimum dielectric function. Phonon frequencies in high symmetry directions agree well with the only available first principle study. Except XTA, WTA, and LTA, all the other modes show an increase in phonon frequencies when pressure is exerted, this is further confirmed by Gruneisen parameters calculated for the first time.

Hybrid functional study of the elastic and structural properties of wurtzite and zinc-blende group-III nitrides

We present a complete set of elastic and structural parameters for wurtzite and zinc-blende group-III nitrides, GaN, InN and AlN, calculated ab initio. A high degree of accuracy is ensured using the Heyd-Scuseria-Ernzerhof (HSE) hybrid functional approach. Both lattice and elastic constants are obtained by means of the stresses acting on the crystal, rather than by a minimization of the lattice energy. Our results show very good agreement with available experimental data. Furthermore, all the independent internal strain parameters of the wurtzite nitrides are reported.

Monte Carlo transport simulation of velocity undershoot in zinc blende and wurtzite InN

AbstractVelocity undershoot in zinc blende (ZB) and wurtzite (WZ) InN is investigated by ensemble Monte Carlo (EMC) calculation. The results show that velocity undershoot arises from the relatively long energy relaxation time compared with momentum. Monte Carlo transport simulations over wide range of electric fields is presented in the paper. The results show that velocity undershoot impacts the electron transport greatly, compared with velocity overshoot, when the electric field changes quickly with time and space. A comparison study between WZ and ZB InN shows that WZ InN has more advantages in device applications due to its excellent electron transport properties.

Effect of Phase Purity on Dislocation Density of Pressurized-Reactor Metalorganic Vapor Phase Epitaxy Grown InN

The effect of the metastable zincblende (ZB) InN inclusion in the stable wurtzite (WZ) InN on the threading dislocation densities (TDDs) of an InN film grown by pressurized-reactor metalorganic vapor phase epitaxy has been studied by X-ray diffraction measurements. InN films are directly grown on c-plane sapphire substrates with nitrided surfaces at 1600 Torr with the different growth temperature from 500 to 700 °C. Films including ZB-InN show the correlation between the ZB volume fraction and the edge component of TDDs, not the screw component of TDDs. This result can be crystallographically understood by a simple model explaining how the ZB structure is included, i.e., ZB domains existing side-by-side with WZ domains and twined ZB domains. This can be clearly observed by electron backscatter diffraction.

Phase diagram on phase purity of InN grown pressurized‐reactor MOVPE

AbstractInN epitaxial films are directly grown on sapphire (0001) substrates by using Pressurised‐Reactor metalorganic vapour phase epitaxy (PR‐MOVPE) system originally developed by ourselves. This system makes it possible to control the side wall of islands perpendicular to the substrate surface, which is important in the heteroepitaxial growth with large lattice‐mismatch from the analogy of the GaN growth on sapphire substrates. This high pressure also leads to the growth at the higher temperature than that at the atmospheric pressure. In this paper, InN films are grown by PR‐MOVPE at 2,400 Torr. Its polarity is nitrogen one, which has the superior efficiency to indium polarity in the capture of nitrogen at the growth front. To crystallographically identify films using the diffractions from sapphire (006), wurtzite (WZ) InN (002), zincblende (ZB) InN (111) and metallic indium (101), 2θ–ω scans and the pole figure measurements in X‐ray diffraction are measured. From the pole figure measurements, the orientation and crystalline phases of grains in crystals are determined. The phase diagram between trimethylindium as an indium source flow rate and the growth temperature is drawn. The regions with pure WZ InN and with the incorporation of ZB phase and the metallic indium into WZ phase and the metal‐indium are observed. It is also observed that the grains with WZ phase have c‐axis normal to the (111) plane at the side wall of the ZB grains. Finally, the growth conditions for pure WZ InN make it clear by using PR‐MOVPE. (© 2012 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Piezoelectric properties of zinc blende quantum dots

AbstractPiezoelectric coefficients of zinc blende (ZB) InN, GaN, and AlN have been estimated from the piezoelectric coefficients of the wurtzite (WZ) system. This procedure is based on a rotation of the first‐order piezoelectric tensor of a (001)‐oriented ZB structure to a (111)‐oriented ZB structure, which is similar to a WZ structure. The derived expressions for the piezoelectric coefficients in a (111)‐oriented ZB system are benchmarked against literature coefficients of different WZ materials, showing a very good agreement. To perform the desired opposite operation, a least square fitting procedure was used to find the ZB piezoelectric coefficient e14 that provides the closest reverse transformation to the known WZ constants. Using e14, the piezoelectric potential in a GaN/AlN QD is calculated and compared to a InAs/GaAs QD, revealing a much larger potential in the nitride system, even though the lattice mismatch is much smaller in this system. The result is related to the large piezoelectric coefficient e14 in ZB nitride materials.

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).

Two types of quantum-confinement characters for the bound states in the InGaN/GaN quantum wells

The ( Ga 1 − x In x N ) Nw ( GaN ) Nb single and multiple quantum wells (SQWs and MQWs) are investigated theoretically using the sp 3 s ⁎ tight-binding (TB) method with inclusion of spin–orbit interaction. This study explores the effects of barrier thickness L b , well width L w , indium content x and valence-band offset (VBO) on the quantum confinement (QC) characteristics of the bound states in the well and on the electronic transitions. The calculations are based on the validity of two assumptions: the virtual crystal approximation (VCA) for the structure of the alloyed Ga 1 − x In x N wells, and the macroscopic theory of elasticity (MTE) for the structure of the computational supercell as a whole. The results demonstrate the following main trends: (1) the existence of two types of QC characteristics for the bound states in the GaInN alloyed wells. The nitrogen p-level ( E p N = 2.71 eV , which is associated with InN TB parametrization), displays a threshold/edge that divides the bound states into two types: (i) block-like localized states (in the energy range E p N < E < E g GaN , where E g GaN = 3.3 eV is the energy gap of zinc-blende GaN) and (ii) singlet-like localized states (in the energy range E g InN < E < E p N , where E g InN = 0.71 eV is the energy gap of zinc-blende InN). The confinement energy versus well width L w is found to follow an exponential rule in the former energy region and a power-law rule in the latter one. A stronger localization should be expected as the level becomes deeper in the quantum well. (2) The TB results of E g were compared with the available photoluminescence (PL) data of 1-ML and 2-ML thick SQWs. Taking into account the error bars due to the lattice relaxation and interface specific effects, the TB results provide evidence that the high-energy emissions ( E 0 1 W = 3.125 eV and E 0 2 W = 2.845 eV , for 1-ML- and 2-ML-thick wells, respectively) must have originated from wells with fractional filling (i.e., low indium contents). (3) The TB results predict that the indium content would rise as the well width increases. Unfortunately, overcoming this limitation of fractional monolayers is likely to remain beyond the capability of the currently existing growth techniques. The indium content being kept low is a natural authenticity which is the compromise to make in growing ultrathin GaInN/GaN quantum wells free of misfit dislocations.

Formation mechanisms of embedded wurtzite and zincblende indium nitride nanocrystals

We have examined the formation of InN nanocrystals embedded in InAs. Low temperature (77K) N ion implantation into InAs leads to the formation of an amorphous layer with crystalline InAs remnants. Rapid thermal annealing up to 550 °C leads to the nucleation of zincblende InN nanocrystals with a maximum likelihood radius of 1.3 ± 0.2 nm. Rapid thermal annealing at 600 °C leads to nucleation of zincblende and wurtzite InN, with an increase in maximum likelihood radius to 2.6 ± 0.4 nm. These results are consistent with the predictions of a thermodynamic model for the nanoscale-size-dependence of zincblende and wurtzite InN nucleation.

Structural anisotropy of nonpolar and semipolar InN epitaxial layers

We present a detailed study of the structural characteristics of molecular beam epitaxy grown nonpolar InN films with a- and m-plane surface orientations on r-plane sapphire and (100) γ-LiAlO2, respectively, and semipolar (101¯1) InN grown on r-plane sapphire. The on-axis rocking curve (RC) widths were found to exhibit anisotropic dependence on the azimuth angle with minima at InN [0001] for the a-plane films, and maxima at InN [0001] for the m-plane and semipolar films. The different contributions to the RC broadening are analyzed and discussed. The finite size of the crystallites and extended defects are suggested to be the dominant factors determining the RC anisotropy in a-plane InN, while surface roughness and curvature could not play a major role. Furthermore, strategy to reduce the anisotropy and magnitude of the tilt and minimize defect densities in a-plane InN films is suggested. In contrast to the nonpolar films, the semipolar InN was found to contain two domains nucleating on zinc-blende InN(111)A and InN(111)B faces. These two wurtzite domains develop with different growth rates, which was suggested to be a consequence of their different polarity. Both, a- and m-plane InN films have basal stacking fault densities similar or even lower compared to nonpolar InN grown on free-standing GaN substrates, indicating good prospects of heteroepitaxy on foreign substrates for the growth of InN-based devices.

Low-field and high-field electron transport in zinc blende InN

We report on the electron transport in zinc blende InN simulated by the ensemble Monte Carlo method. To obtain the relevant band structure parameters, ab initio calculations have been carried out. Then, Monte Carlo transport simulations at room temperature and over a wide range of carrier concentrations have been performed. We obtain a steady-state peak drift velocity around 3.3×107 cm/s at an electric field of 55 kV/cm. For low-doped material, a room-temperature low-field mobility of about 6000 cm2/V s is calculated. A comparison with wurtzite InN does not reveal an advantage for the zinc blende InN phase regarding the electron transport.

Structural changes during the natural aging process of InN quantum dots

The natural aging process of InN nanostructures by the formation of indium oxides is examined by transmission electron microscopy related techniques. Uncapped and GaN-capped InN quantum dots (QDs) on GaN/sapphire substrates were grown under the same conditions and kept at room temperature/pressure conditions. The GaN capping layer is found to preserve the InN QDs in the wurtzite phase, avoiding the formation of group-III oxides, while in the uncapped sample, a thin layer of cubic phases are formed that envelops the nucleus of wurtzite InN. These cubic phases are shown to be mainly bcc-In2O3 for long aged samples where the nitrogen atoms in the InN surface layers have been substituted by atmospheric oxygen. This process implies the gradual transformation of the In sublattice from hcp to a quasi-fcc structure. Metastable zinc-blende InN phases rich in oxygen atoms are proposed to act as intermediate phases and they are evinced in samples less aged. The large concurrence of interplanar spaces, the twin formation, and the existence of a free surface that facilitates the transformation support this mechanism and would explain the high instability of the InN nanostructures at ambient conditions.

Unusual electronic properties of InN

Electronic structure of zinc-blende and wurtzite InN using linear combination of atomic orbitals and the latest approach of generalised gradient approximation within full potential linearised augmented plane wave schemes is reported. An unusual small band gap and real space analysis of our first ever experimental Compton profile are discussed.

Synthesis of [100] Wurtzite InN Nanowires and [011] Zinc-Blende InN Nanorods

One-dimensional wurtzite InN nanowires and zincblende InN nanorods are prepared by chemical vapour deposition (CVD) method on natural cleavage plane (110) of GaAs. The growth direction of InN nanowires is [100], with wurtzite structure. The stable crystal structure of InN is wurtzite (w-InN), zincblende structure (z-InN) is only reported for 2D InN crystals before. However, in this work, the zincblende InN nanorods [011] are synthesized and characterized. The SEM and TEM images show that every nanorod shapes a conical tip, which can be explained by the anisotropy of growth process and the theory of Ehrlich–Schwoebel barrier.