Structural and optical properties of Ga2(1−x)In2xO3 films prepared on α-Al2O3 (0 0 0 1) by MOCVD

Ga_2(1−x)In_2xO₃ thin films with different indium content x [In/(Ga + In) atomic ratio] were prepared on α-Al₂O₃ (0 0 0 1) substrates by the metal organic chemical vapor deposition (MOCVD). The structural and optical properties of the Ga_2(1−x)In_2xO₃ films were investigated in detail. Microstructure analysis revealed that the film deposited with composition x = 0.2 was polycrystalline structure and the sample prepared with x up to 0.8 exhibited single crystalline structure of In₂O₃. The optical band gap of the films varied with increasing Ga content from 3.72 to 4.58 eV. The average transmittance for the films in the visible range was over 90%.     

Room temperature growth of InxGa1−xN thin films by mixed source modified activated reactive evaporation

Polycrystalline In_xGa_1−xN thin films were prepared by mixed source modified activated reactive evaporation (MARE) technique. The films were deposited at room temperature on glass substrates without any buffer layer. All the films crystallize in the hexagonal wurtzite structure. The indium concentration calculated from XRD peak shift using Vegard's law was found to be varying from 2% to 92%. The band gap varies from 1.72 eV to 3.2 eV for different indium compositions. The indium rich films have higher refractive indices as compared to the gallium rich films. The near infra-red absorption decreases with gallium incorporation into InN lattice which is mainly due to decrease in the free carrier concentration in the alloy system. This fact is further supported from Hall effect measurements. MARE turns out to be a promising technique to grow In_xGa_1−xN films over the entire composition range at room temperature.     

Indium distribution and light emission in wurtzite InGaN alloys: Several-atom In-N clusters

Considering various In distributions, we investigate electronic structures and light emission of wurtzite In_xGa_1 − xN (0?x?1) alloys. We find In_xGa_1 − xN forms a random alloy, in which many several-atom In-N clusters and short In-N- chains can exist. Small In-N clusters, especially in-plane ones, strongly localize valence electrons and dominate light emission in Ga-rich In_xGa_1 − xN alloys, which is consistent with experiments.     

Band gap energy and bowing parameter of In-rich InAlN films grown by magnetron sputtering

The crystal structure, band gap energy and bowing parameter of In-rich In_xAl_1−xN (0.7 < x < 1.0) films grown by magnetron sputtering were investigated. Band gap energies of In_xAl_1−xN films were obtained from absorption spectra. Band gap tailing due to compositional fluctuation in the films was observed. The band gap of the as-grown InN measured by optical absorption method is 1.34 eV, which is larger than the reported 0.7 eV for pure InN prepared by molecular beam epitaxy (MBE) method. This could be explained by the Burstein-Moss effect under carrier concentration of 10²⁰ cm⁻³ of our sputtered films. The bowing parameter of 3.68 eV is obtained for our In_xAl_1−xN film which is consistent with the previous experimental reports and theoretical calculations.     

Electric, thermoelectrical and photoelectric properties of CuGaxIn1−xSe2 thin films

The electrical conductivity σ, Hall effect R_H, and thermoelectric power Q of CuGa_0.25In_0.75Se₂ thin films with different growth conditions have been measured at temperature 300-520 K. These properties were also measured at room temperature for different composition of CuGa_xIn_1−xSe₂ (0.75≥x≥0) deposited at the same evaporation conditions. All investigated films are p-type over the whole temperature range. Electrical conduction was studied in order to establish its mechanism.The room temperature photoelectric response of those films were measured as a function of wavelength (2.5≥λ≥0.3) μm. It is found that the energy gap values follow a second order equation in x giving a downward bowing parameter of about 0.31 eV.     

Experimental investigation of long-wavelength optical lattice vibrations in quaternary AlxInyGa1−x−yN alloys and comparison with results from the pseudo-unit cell model

Raman and Fourier transform infrared (FTIR) spectroscopies have been utilized to measure long-wavelength optical lattice vibrations of high-quality quaternary Al_xIn_yGa_1−x−yN thin films at room temperature. The Al_xIn_yGa_1−x−yN films were grown on c-plane (0 0 0 1) sapphire substrates with AlN as buffer layers using plasma assisted molecular beam epitaxy (PA-MBE) technique with aluminum (Al) mole fraction x ranging from 0.0 to 0.2 and constant indium (In) mole fraction y=0.1. Pseudo unit cell (PUC) model was applied to investigate the phonons frequency, mode number, static dielectric constant, and high frequency dielectric constant of the Al_xIn_yGa_1−x−yN mixed crystals. The theoretical results were compared with the experimental results obtained from the quaternary samples by using Raman and FTIR spectroscopies. The experimental results indicated that the Al_xIn_yGa_1−x−yN alloy had two-mode behavior, which includes A₁(LO), E₁(TO), and E₂(H). Thus, these results are in agreement with the theoretical results of PUC model, which also revealed a two-mode behavior for the quaternary nitride. We also obtained new values of E₁(TO) and E₂(H) for the quaternary nitride samples that have not yet been reported in the literature.     

Fabrication and hard X-ray photoemission analysis of photocathodes with sharp solar-blind sensitivity using AlGaN films grown on Si substrates

Photocathode devices operating in reflection-mode, where the photoemission is detected on the same side as the light irradiation, were developed for the detection of deep ultraviolet light by using p-Al_xGa_1−xN films grown on Si(1 1 1) substrates. The external quantum efficiencies were as high as 20-15% at 200 nm and 280 nm, while the value was as low as 10⁻²% at 310 nm. The on-off ratio was more than four orders of magnitude, which represents high solar-blind sensitivity. The escape probability of Al_xGa_1−xN photocathode was decreased with increase of AlN mole fraction. The effective barrier potential against the photoelectron emission near the surface was reduced due to the upward shift of conduction band of Al_xGa_1−xN. The photoemission from the Al_xGa_1−xN films terminated with Cs-O adatoms will be discussed in terms of band diagrams that were evaluated by hard X-ray photoelectron spectroscopy.     

Photoluminescence scanning on InAs/InGaAs quantum dot structures

The photoluminescence spectra of InAs quantum dots (QDs) embedded into four types of In_xGa_1−xAs/GaAs (x = 0.10, 0.15, 0.20 and 0.25) multi quantum well MBE structures have been investigated at 300 K in dependence on the QD position on the wafer. PL mapping was performed with 325 nm HeCd laser (35 mW) focused down to 200 μm (110 W/cm²) as the excitation source. The structures with x = 0.15 In/Ga composition in the In_xGa_1−xAs capping layer exhibited the maximum photoluminescence intensity. Strong inhomogeneity of the PL intensity is observed by mapping samples with the In/Ga composition of x ≥ 0.20-0.25. The reduction of the PL intensity is accompanied by a gradual “blue” shift of the luminescence maximum at 300 K as follows from the quantum dot PL mapping. The mechanism of this effect has been analyzed. PL peak shifts versus capping layer composition are discussed as well.     

Mechanical, tribological, and electrochemical behavior of Cr1−xAlxN coatings deposited by r.f. reactive magnetron co-sputtering method

Chromium aluminum nitride (Cr_1−xAl_xN) coatings were deposited onto AISI H13 steel and silicon substrates by r.f. reactive magnetron co-sputtering in (Ar/N₂) gas mixture from chromium and aluminum targets. Properties of deposited Cr_1−xAl_xN coatings such as compositional, structural, morphological, electrochemical, mechanical and tribological, were investigated as functions of aluminum content. X-ray diffraction patterns of Cr_1−xAl_xN coatings with different atomic concentrations of aluminum (0.51 < x < 0.69) showed the presence and evolution of (1 1 1), (2 0 0), and (1 0 2) crystallographic orientations associated to the Cr_1−xAl_xN cubic and w-AlN phases, respectively. The rate of corrosion of the steel coated with Cr_1−xAl_xN varied with the applied power; however, always being clearly lower when compared to the uncoated substrate. The behavior of the protective effect of the Cr_1−xAl_xN coatings is based on the substitution of Cr for Al, when the power applied to the aluminum target increases. The mechanical properties were also sensitive to the power applied, leading to a maximum in hardness and a reduced elastic modulus of 30 and 303 GPa at 350 W and a monotonic decrease to 11 and 212 GPa at 450 W, respectively. Finally, the friction coefficient measured by pin-on disk revealed values between 0.45 and 0.70 in humid atmosphere.     

Synthesis of homogeneous pentenary chalcopyrite alloys with a classical two-step growth process

Homogeneous single-phase Cu(In_0.75Ga_0.25)(Se_1−yS_y)₂ chalcopyrite alloys were prepared by a novel two-step growth process. CuIn_0.75Ga_0.25 precursors were deposited by DC magnetron sputtering and the subsequent reaction processes in a reactive H₂Se/Ar/H₂S atmosphere was optimized to prevent the formation and separation of stable ternary phases. X-ray diffraction (XRD) analysis of these films revealed characteristic chalcopyrite peaks with a high degree of symmetry, indicative of homogeneous rather than compositionally graded material. The lattice parameters of the single-phase Cu(In_0.75Ga_0.25)(Se_1−yS_y)₂ pentenary alloys decreased linearly with an increase in the S/(S+Se) ratio in accordance with Vegard's law. X-ray photoelectron spectroscopy (XPS) depth profiling confirmed the in-depth compositional uniformity of the pentenary alloys, prepared under optimized selenization/sulfurization conditions.     

Theoretical investigation of electronic structure and optical response in relation to the transport properties of Ga1−xInxN (x = 0, 0.25, 0.50, 0.75)

The state-of-the-art all-electron FLPAW method and the BoltzTrap software package based on semi-classical theory were adopted to explore the electronic structure and the optical and thermoelectric properties of Ga_1−xIn_xN. Ga_1−xIn_xN is predicted to be a direct band gap material for all values of x. Moreover, the band gap varies between 2.99 eV and 1.95 eV as x changes. Optical parameters such as the dielectric constant, absorption coefficient, reflectivity and refractive index are calculated and discussed in detail. The doping of In plays an important role in the modulation of the optical constants. The static dielectric constant ɛ(0) of Ga_1−xIn_xN was calculated as 3.95, 3.99, 3.99 and 4.03 at x = 0.00, 0.25, 0.50 and 0.75, respectively. The static refractive index is 2.0 for pure Ga_1−xIn_xN at x = 0.00. The thermal properties varied greatly as x fluctuated. The ternary alloy has large values for the Seebeck coefficient and figure of merit at high temperatures and is thus suitable for thermoelectric applications. Pure Ga_1−xIn_xN at x = 0 exhibited ZT = 0.80 at room temperature, and at higher temperatures, the thermal conductivity decreased with increased In doping.     

Band-edge exciton transitions in (Ga1−xMnx)N diluted magnetic semiconductor films with above room temperature ferromagnetic transition

(Ga_1−xMn_x)N thin films grown on GaN buffer layers by using molecular beam epitaxy were investigated with the goal of producing diluted magnetic semiconductors (DMSs) with band-edge exciton transitions for applications in optomagnetic devices. The magnetization curve as a function of the magnetic field at 5 K indicated that ferromagnetism existed in the (Ga_1−xMn_x)N thin films, and the magnetization curve as a function of the temperature showed that the ferromagnetic transition temperature of the (Ga_1−xMn_x)N thin film was above room temperature. Photoluminescence and photoluminescence excitation spectra showed that band-edge exciton transitions in (Ga_1−xMn_x)N thin films appeared. These results indicate that the (Ga_1−xMn_x)N DMSs with a magnetic single phase hold promise for potential applications in spin optoelectronic devices in the blue region of the spectrum.     

Influence of indium content on the optical, electrical and crystallization kinetics of Se100−xInxthin films deposited by flash evaporation technique

Thin films of Se _100−xIn_x (x=10, 20 and 30 at%) have been prepared by the flash evaporation technique. The effect of the indium content on optical band gap of the Se_100−x In_x films has been investigated by the optical characterization. The optical band gap values of the Se_100−x In_x thin films were determined and are found to decrease with increasing indium content. This indium content changes the width of localized states in the optical band gaps of the thin films. It was found that the optical band gap, E_g, of the Se_100−x In_x films changes from 1.78 to 1.37 eV with increasing indium content from 10 to 30 at%, while the width of localized states in optical band gap changes from 375 to 342 meV. The temperature dependence of the dark electrical conductivity were studied in the temperature range 303-433 K and revealed two activation energies providing two electrical conduction mechanisms. The activation energy of the Se_100−x In_x films in the high temperature region changes from 0.49 to 0.32 eV with increasing indium content from 10 to 30 at%, while the hopping activation energy in the lower temperature region changes from 0.17 to 0.22 meV. The change in the electrical conductivity with time during the amorphous-to-crystalline transformation is recorded for amorphous Se_100−xIn_x films at two points of isothermal temperatures 370 and 400 K. The formal crystallization theory of Avrami has been used to calculate the kinetic parameters of crystallization.     

Electrical and optical properties of Ga doped zinc oxide thin films deposited at room temperature by continuous composition spread

Ga doped ZnO (GZO) thin films were deposited on glass substrates at room temperature by continuous composition spread (CCS) method. CCS is thin films growth method of various Ga_xZn_1−xO(GZO) thin film compositions on a substrate, and evaluating critical properties as a function position, which is directly related to material composition. Various compositions of Ga doped ZnO deposited at room temperature were explored to find excellent electrical and optical properties. Optimized GZO thin films with a low resistivity of 1.46 × 10⁻³ Ω cm and an average transmittance above 90% in the 550 nm wavelength region were able to be formed at an Ar pressure of 2.66 Pa and a room temperature. Also, optimized composition of the GZO thin film which had the lowest resistivity and high transmittance was found at 0.8 wt.% Ga₂O₃ doped in ZnO.     

Universal bandgap bowing in group-III nitride alloys

The energy gaps of molecular-beam-epitaxy grown wurtzite-structure In_1−xAl_xN alloys with x≤0.25 have been measured by absorption and photoluminescence experiments. The results are consistent with the recent discovery of a narrow bandgap of ∼0.7 eV for InN. A bowing parameter of 3 eV was determined from the composition dependence of these bandgaps. Combined with previously reported data of InGaN and GaAlN, these results show a universal relationship between the bandgap variations of group-III nitride alloys and their compositions.     

Surface reconstructions of InGaAs alloys

The surface reconstructions of In_xGa_1−xAs alloys grown by molecular beam epitaxy on the (0 0 1) surfaces of GaAs and InAs have been studied by reflection high-energy electron diffraction and scanning tunnelling microscopy. A surface phase diagram is presented for the nominally strain-free alloy as a function of substrate temperature and alloy composition, and structural models for the commonly observed 3× reconstructions are discussed. Two new, electronically stable structural models are described that account for the transition of the In_xGa_1−xAs surface alloy from a c(4 × 4) to an asymmetric 3× reconstruction and that are fully consistent with all current experimental evidence.     

Characterization of Zr-Si-N films deposited by cathodic vacuum arc with different N2/SiH4 flow rates

Zr-Si-N films were deposited on silicon and steel substrates by cathodic vacuum arc with different N₂/SiH₄ flow rates. The N₂/SiH₄ flow rates were adjusted at the range from 0 to 12 sccm. The films were characterized by X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), scanning electron microscope (SEM), hardness and wear tests. The structure and the mechanical properties of Zr-Si-N films were compared to those of ZrN films. The results of XRD and XPS showed that Zr-Si-N films consisted of ZrN crystallites and SiN_x amorphous phase. With increasing N₂/SiH₄ flow rates, the orientation of Zr-Si-N films became to a mixture of (1 1 1) and (2 0 0). The column width became smaller, and then appeared to vanish with the increase in N₂/SiH₄ flow rates. The hardness and Young's modulus of Zr-Si-N films increased with the N₂/SiH₄ flow rates, reached a maximum value of 36 GPa and 320 GPa at 9 sccm, and then decreased 32 GPa and 305 GPa at 12 sccm, respectively. A low and stable of friction coefficient was obtained for the Zr-Si-N films. Friction coefficient was about 0.1.     

Numerical optimization of In-mole fractions and layer thicknesses in AlxGa1−xN/AlN/GaN high electron mobility transistors with InGaN back barriers

The effects of the In-mole fraction (x) of an In_xGa_1−xN back barrier layer and the thicknesses of different layers in pseudomorphic Al_yGa_1−yN/AlN/GaN/In_xGa_1−xN/GaN heterostructures on band structures and carrier densities were investigated with the help of one-dimensional self-consistent solutions of non-linear Schrödinger-Poisson equations. Strain relaxation limits were also calculated for the investigated Al_yGa_1−yN barrier layer and In_xGa_1−xN back barriers. From an experimental point of view, two different optimized structures are suggested, and the possible effects on carrier density and mobility are discussed.     

Photoluminescence investigation of ferromagnetic Ga1−xMnxN layers with GaN templates grown on sapphire (0 0 0 1) substrates

We have investigated the temperature and composition dependent photoluminescence (PL) spectra in Ga_1−xMn_xN layers (where x ≈ 0.1-0.8%) grown on sapphire (0 0 0 1) substrates using the plasma-enhanced molecular beam epitaxy technique. The efficient PL is peaked in the red (1.86 eV), yellow (2.34 eV), and blue (3.29 eV) spectral range. The band-gap energy of the Ga_1−xMn_xN layers decreased with increasing temperature and manganese composition. The band-gap energy of the Ga_1−xMn_xN layers was modeled by the Varshni equation and the parameters were determined to be α = 2.3 × 10⁻⁴, 2.7 × 10⁻⁴, 3.4 × 10⁻⁴ eV/K and β = 210, 210, and 230 K for the manganese composition x = 0.1%, 0.2%, and 0.8%, respectively. As the Mn concentration in the Ga_1−xMn_xN layers increased, the temperature dependence of the band-gap energy was clearly reduced.