In-rich Material Research Articles

The effect of annealing on the surface morphology of strained and unstrained In xAl 1− xN thin films

In x Al 1− x N is a particularly useful group-III nitride alloy because by adjusting its composition it can be lattice matched to GaN. Such lattice-matched layers may find application in distributed Bragg reflectors (DBRs) and high electron mobility transistors (HEMTs). However, compared with other semiconducting nitride alloys, In x Al 1- x N has not been researched extensively. In this study, thin In x Al 1− x N epilayers were grown by metal-organic vapour phase epitaxy (MOVPE) on GaN and Al y Ga 1− y N layers. Samples were subjected to annealing at their growth temperature of 790 °C for varying lengths of time, or alternatively to a temperature ramp to 1000 °C. Their subsequent surface morphologies were analysed by atomic force microscopy (AFM). For both unstrained In x Al 1− x N epilayers grown on GaN and compressively strained epilayers grown on Al y Ga 1− y N, surface features and fissures were seen to develop as a consequence of thermal treatment, resulting in surface roughening. It is possible that these features are caused by the loss of In-rich material formed on spinodal decomposition. Additionally, trends seen in the strained In x Al 1− x N layers may suggest that the presence of biaxial strain stabilises the alloy by suppressing the spinode and shifting it to higher indium compositions.

Ab Initio Calculations of Crystalline and Amorphous In2Se3 Compounds for Chalcogenide Phase Change Memory

AbstractAb Initio calculations of various configurations of In2Se3 compounds are used to gain insight into the transition from crystalline to amorphous phase. The structures considered are based on wurzite structures with 1/3 of indium sites vacant as observed experimentally. From extensive calculations for possible vacancy configurations in In2Se3 compounds, predictions based on the local coordination of In/Se atoms are made for the energetically favorable vacancy ordering structures. Results indicate that in the most stable In vacancy configurations, Se atoms have coordination of either 2 or 3 (In atoms have coordination of 4). Other coordinations lead to significantly higher formation energies. Results from analyzing the total energy and electronic structure of a range of off-stoichiometry, including vacancies, interstitials and anti-site, configurations, suggest that the energetically most favorable way to form In-rich material is via incorporation of Se vacancies, while Se occupying a vacant site is the most favorable for formation of Se-rich phase. Based on these calculations, predictions are made on how stoichiometry deviations impact structural evolution during phase change.

Low‐temperature grown compositionally graded InGaN films

AbstractA new thin film growth technique known as energetic neutral atomic‐beam lithography/epitaxy (ENABLE) provides a large energetic N atom flux and eliminates the need for high substrate temperatures as compared to molecular beam epitaxy, making isothermal growth over the entire InGaN alloy composition range possible without phase separation. 500–800 nm thick compositionally graded InGaN films were grown by ENABLE at ∼450 °C with the following structure types: (1) with the Ga‐rich material on the surface and (2) with the Inrich material on the surface. Rutherford backscattering spectrometry, transmission electron microscopy, X‐ray diffraction, absorption spectroscopy, photoluminescence, and Hall effect measurements were used to assess the thickness, composition, crystalline quality, and optical and electrical properties of the films. The results establish the new ENABLE method as uniquely capable of growing compositionally graded InGaN films and, in the future, InN/GaN heterostructures. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Effect of native defects on optical properties of InxGa1−xN alloys

The energy position of the optical-absorption edge and the free-carrier populations in InxGa1−xN ternary alloys can be controlled using high-energy He+4 irradiation. The blueshift of the absorption edge after irradiation in In-rich material (x>0.34) is attributed to the band-filling effect (Burstein-Moss shift) due to the native donors introduced by the irradiation. In Ga-rich material, optical-absorption measurements show that the irradiation-introduced native defects are inside the band gap, where they are incorporated as acceptors. The observed irradiation-produced changes in the optical-absorption edge and the carrier populations in InxGa1−xN are in excellent agreement with the predictions of the amphoteric defect model.

Characterization of CuInS2 thin films prepared by electrodeposition and sulfurization with photoluminescence spectroscopy

Abstract Potentiostatic electrodeposition and sulfurization techniques were used to prepare polycrystalline CuInS 2 thin films. X-ray diffraction and photoresponse measurements in a photoelectrochemical cell (PEC) revealed that photoactive polycrystalline CuInS 2 films can be deposited on Ti substrate. Photoluminescence (PL) spectroscopy was used to investigate the prepared thin films and optically characterize them. PL spectra revealed the defect structure of the samples with an acceptor energy level at 109 meV above the valance band and a donor energy level at 71 meV below the conduction band. The CuInS 2 thin films prepared in this investigation are observed to be In-rich material with n-type electrical conductivity.

99/03790 Estimation of solar radiation from the number of sunshine hours: Amprawum, D. B. and Dorvlo, A. S. S. Applied Energy, 1999, 63, (3), 161–167

In this study device quality CuInSe2 films were grown using a relatively simple, tolerant and reproducible two-stage technique. This process involved the selenization of various CuIn precursors in a H2Se/Ar atmosphere. The variation in the microstructure (grain size and lattice defects) and optical properties of the CuInSe2 thin films on gradually changing the stoichiometry from Cu rich to In rich was investigated. Scanning electron microscopy (SEM) and transmission electron microscopy (TEM) studies revealed that Cu-rich and In-rich films were clearly distinct in their defect structure. In general, In-rich material exhibited relatively small grains (0.2–0.8 μm), which were highly defected. In contrast to In-rich films, Cu-rich films were characterized by the presence of large faceted grains (1–4 μm) with relatively low defect density. These material properties were clearly reflected in low temperature photoluminescence (PL) studies, indicating sharp transitions for Cu-rich films compared to broad emission lines for In-rich materials. Cu-rich materials were characterized by four relatively sharp emission lines at 1.036 eV, 0.993 eV, 0.971 eV and 0.942 eV at 6 K. As the composition was gradually changed from Cu rich to stoichiometric compositions only one broad emission line could be observed at 0.964 eV, which corresponds to a donor–acceptor pair transition. In the case of In-rich material (Cu/In atomic ratio=0.3–0.6), three dominant transitions were observed at 1.10 eV, 0.975 eV, and 0.89 eV. The observed spectra are explained by considering the formation energies of the defects and the composition of the specific film.

Structural and optical characterization of polycrystalline CuInSe 2

In this study device quality CuInSe 2 films were grown using a relatively simple, tolerant and reproducible two-stage technique. This process involved the selenization of various CuIn precursors in a H 2Se/Ar atmosphere. The variation in the microstructure (grain size and lattice defects) and optical properties of the CuInSe 2 thin films on gradually changing the stoichiometry from Cu rich to In rich was investigated. Scanning electron microscopy (SEM) and transmission electron microscopy (TEM) studies revealed that Cu-rich and In-rich films were clearly distinct in their defect structure. In general, In-rich material exhibited relatively small grains (0.2–0.8 μm), which were highly defected. In contrast to In-rich films, Cu-rich films were characterized by the presence of large faceted grains (1–4 μm) with relatively low defect density. These material properties were clearly reflected in low temperature photoluminescence (PL) studies, indicating sharp transitions for Cu-rich films compared to broad emission lines for In-rich materials. Cu-rich materials were characterized by four relatively sharp emission lines at 1.036 eV, 0.993 eV, 0.971 eV and 0.942 eV at 6 K. As the composition was gradually changed from Cu rich to stoichiometric compositions only one broad emission line could be observed at 0.964 eV, which corresponds to a donor–acceptor pair transition. In the case of In-rich material (Cu/In atomic ratio=0.3–0.6), three dominant transitions were observed at 1.10 eV, 0.975 eV, and 0.89 eV. The observed spectra are explained by considering the formation energies of the defects and the composition of the specific film.

Type-II-->type-I transition in (GaX)n/(InX)n (001) superlattices (X=P, Sb) as a function of period n.

Coherently strained GaX/InX interfaces (X=P, Sb) lattice matched to a (001)-oriented substrate are predicted to have a type-I band-gap alignment, with both the valence-band maximum and the conduction-band minimum (CBM) located on the In-rich material. At the same time, the CBM wave function of short-period (GaX${)}_{\mathit{n}}$/(InX${)}_{\mathit{n}}$ superlattices is predicted to have larger amplitude on the GaX layers, leading to a type-II alignment. We show that (i) a type-II\ensuremath{\rightarrow}type-I transition occurs around the period n=4; (ii) this transition has a different origin with respect to the well-known case of GaAs/AlAs superlattices; (iii) the band structure of ultrathin superlattices cannot be explained in terms of a simple effective-mass theory; (iv) the wave-function localization in short-period superlattices is determined by the atomic orbital energies.

Defect chemistry of CuInS 2, investigated by electrical measurements and Mössbauer spectroscopy

Electrical measurements are performed on CuInS 2 single crystals (grown via chemical vapour transport), which were annealed under well defined conditions of molecularity and stoichiometry on four different positions of the CuInS 2 phase boundary. In this way two acceptor levels are detected, viz. at 0.10 eV in In-rich material and at 0.15 eV in Cu-rich material. The first is ascribed to V cu, the latter to V In or Cu In. Also a donor level at 0.035 eV is observed, which is assigned to V s or In cu as most likely defects. The incorporation of iron, which is by far the most important impurity as revealed by spectrochemical analysis (concentration about 5 × 10 18 cm −3), was studied by means of Mössbauer spectroscopy. Irrespective of the molecularity and the stoichiometry iron appeared to be incorporated as Fe 2+ on both Cu and In sites in a concentration ratio of roughly 3:1.

Luminescence of CuInS 2: I. The broad band emission and its dependence on the defect chemistry

The emission of CuInS 2 is studied as a function of the exact composition in terms of deviations from molecularity and stoichiometry. By means of temperature-dependent (4.2–220 K) and excitation-intensity-dependent measurements, the two broad emission bands usually present are correlated with donor-acceptor transitions. In In-rich material the acceptor is located at 0.10 eV above the valence band. This acceptor is ascribed to V Cu. In Cu-rich CuInS 2 the acceptor level is located at 0.15 eV above the valence band, which is attributed to either V In or Cu In. Two donor levels are identified, their ionization energies being about 35 and 72 meV. These donors are present in both In and Cu-rich samples and may originate from either intrinsic defects or impurities (Fe). Also, the influence of quenching of the crystals as well as the effect of different crystal growth methods (melt-growth and vapour-growth) on the emission is studied.