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.     

Nanomechanical characterizations of InGaN thin films

In_xGa_1−xN thin films with In concentration ranging from 25 to 34 at.% were deposited on sapphire substrate by metal-organic chemical vapor deposition (MOCVD). Crystalline structure and surface morphology of the deposited films were studied by using X-ray diffraction (XRD) and atomic force microscopy (AFM). Hardness, Young's modulus and creep resistance were measured using a nanoindenter. Among the deposited films, In_0.25Ga_0.75N film exhibits a larger grain size and a higher surface roughness. Results indicate that hardness decreases slightly with increasing In concentration in the In_xGa_1−xN films ranged from 16.6 ± 1.1 to 16.1 ± 0.7 GPa and, Young's modulus for the In_0.25Ga_0.75N, In_0.3Ga_0.7N and In_0.34Ga_0.66N films are 375.8 ± 23.1, 322.4 ± 13.5 and 373.9 ± 28.6 GPa, respectively. In addition, the time-dependent nanoindentation creep experiments are presented in this article.     

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%.     

The influence of chemical treatment and thermal annealing on AlxGa1−xN surfaces: An XPS study

The influences of chemical treatment and thermal annealing of Al_xGa_1−xN (x = 0.20) have been investigated by X-ray photoelectron spectroscopy (XPS). XPS analysis showed that successive chemical treatments and annealing produced changes in the stoichiometry of the Al_xGa_1−xN surface, with the surface concentration of N increasing and Al and Ga decreasing with increasing temperature. Band bending occurred at the Al_xGa_1−xN surface, in parallel with the observed changes in stoichiometry. These results are discussed in the context of the creation of surface states via the activation of vacancies and induced by defects. These findings point towards the possibility of selecting and/or engineering the band structure at Al_xGa_1−xN surfaces through a combination of surface preparation and annealing.     

Effect of hydrogen on modulation-doped AlGaAs/InGaAs/GaAs heterostructures: a photoluminescence study

The effect of hydrogen on donors and interface defects in silicon modulation doped Al_xGa_1−xAs/In_yGa_1−yAs/GaAs heterostructures has been investigated by photoluminescence (PL). Hydrogenation was carried out on two sets of samples, one set consists of high quality pseudomorphic heterostructures and another set having partially lattice relaxed structures prone to the defects. On exposure of high quality pseudomorphic structures to hydrogen plasma above 150 °C, a significant blue shift in the PL peak positions as well as bandwidth narrowing is observed. This indicates, the reduction in two-dimensional electron gas in the In_yGa_1−yAs quantum well due to hydrogen passivation of silicon donors in the Al_xGa_1−xAs supply layer. The reactivation of the donors is observed upon annealing the hydrogenated sample for 1 h at 250 °C under hydrogen ambient. Another interesting feature is a significant improvement in the PL of lattice-relaxed structures upon hydrogenation of the samples above 250 °C, which is attributed to the hydrogen passivation of interface defects due to the misfit dislocations.     

Dependence of the interband transitions on the in mole fraction and the applied electric field in InxGa1−xAs/In0.52Al0.48As multiple quantum wells

Transmission electron microscopy (TEM) and photocurrent (PC) measurements were carried out to investigate the microstructural properties and excitonic transitions in In_xGa_1−xAs/In_0.52Al_0.48As multiple quantum wells (MQWs) for x = 0.54, 0.57 and 0.60. TEM images showed that high-quality 11-period In_xGa_1−xAs/In_0.52Al_0.48As MQWs had high-quality heterointerfaces. The results for the PC spectra at 300 K showed that the peaks corresponding to the excitonic transitions from the ground state electronic sub-band to the ground state heavy-hole band (E₁-HH₁) and the ground state electronic sub-band to the ground state light-hole band (E₁-LH₁) became closer to each other with decreasing In mole fraction and that E₁-HH₁ and E₁-LH₁ excitonic peaks shifted to longer wavelength with increasing applied electric field. The calculated values of the E₁-HH₁ interband transition energies were in qualitative agreement with those obtained form the PC measurements with and without applied electric field. These results can be helpful in understanding potential applications of In_xGa_1−xAs/In_yAl_1−yAs MQWs dependent on In mole fraction and applied electric field in long-wavelength optoelectronic devices.     

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.     

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.     

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.     

Structural characterization of InxGa1−xAs/Inp layers under different stresses

In_xGa_1−xAs layers on InP substrate can be subjected to compressive or tensile strain due to lattice parameter differences depending on the alloy composition. In order to examine in details the strain of InGaAs/InP epiatxial layers and its evolution after subjecting the layers to annealing at high pressure, X-ray synchrotron topography, high resolution X-ray diffraction and atomic force microscopy have been employed. The data show that the changes of structural properties of the InGaAs layers subjected to high temperature-high pressure treatment at 670 K-1.2 Gpa, strongly depend on initial strain state and defect structure. The annealing of samples under high pressure results in change of strain in tensile layers only. The behaviour of observed defects is discussed.     

AlN, GaN, AlxGa1 − xN nanotubes and GaN/AlxGa1 − xN nanotube heterojunctions

Semiconductor optoelectronic devices based on GaN and on InGaN or AlGaN alloys and superlattices can operate in a wide range of wavelengths, from far infrared to near ultraviolet region. The efficiency of these devices could be enhanced by shrinking the size and increasing the density of the semiconductor components. Nanostructured materials are natural candidates to fulfill these requirements. Here we use the density functional theory to study the electronic and structural properties of (10,0) GaN, AlN, Al_xGa_1 − xN nanotubes and GaN/Al_xGa_1 − xN heterojunctions, 0<x<1. The Al_xGa_1 − xN nanotubes exhibit direct band gaps for the whole range of Al compositions, with band gaps varying from 3.45 to 4.85 eV, and a negative band gap bowing coefficient of −0.14 eV. The GaN/Al_xGa_1 − xN nanotube heterojunctions show a type-I band alignment, with the valence band offsets showing a non-linear dependence with the Al content in the nanotube alloy. The results show the possibility of engineering the band gaps and band offsets of these III-nitrides nanotubes by alloying on the cation sites.     

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.     

Nano-plasmon enhancement effect on MWIR light emitting diode performance

We observed a significant increase in electro luminescence from GaSb based mid-wave infrared (MWIR) LED device through coupling with localized surface plasmon of a single layer Au nano-particles. We fabricated an interband cascade (IC) LED device with nine cascade active/injection layers with InAs/Ga_1−x In_xSb/InAs quantum well (QW) active region. Thin Au plasmon layer of 20 nm thickness is deposited on top anode electrode by e-beam technique, which resulted in 100% increase in light output for 50 μm square mesa device. We also observed a reduction in the device turn on voltage and increase in the apparent black body emission temperature due to nano-structure surface plasmon layer.     

Structural properties of CuInSe2 films prepared by selenization of metallic precursors on MoNx film substrates

The aim of this work was to study the effect of MoN_x film substrates on the structural properties of CuInSe₂ films prepared by selenization of metallic Cu-In alloy precursors. MoN_x films were prepared by reactive dc-magnetron sputtering. All the CuInSe₂ films exhibit single phase chalcopyrite structure with (1 1 2) preferred orientation, which can be explained by the reduction of lattice mismatch between CuInSe₂ and MoN_x. The bulk composition of selenized CuInSe₂ films are near stoichiometric, but the surface composition analysis suggests Cu deficiency on surface area. Furthermore, ordered defect compound, CuIn₂Se_3.5 is found on the surface of CuInSe₂ films. The results will be helpful for fabricating Cd-free ZnO buffer layer CuInSe₂ and Cu(In_1−xGa_x)Se₂ based thin film solar cells.     

Optimization of growth parameters for MOVPE-grown GaSb and Ga1−xInxSb

The triethylgallium/trimethylantimony (TEGa/TMSb) precursor combination was used for the metal-organic vapour phase epitaxial growth of GaSb at a growth temperature of 520 °C at atmospheric pressure. Trimethylindium was added in the case of Ga_1−xIn_xSb growth. The effects of group V flux to group III flux ratio (V/III ratio) on the crystallinity and optical properties of GaSb layers are reported. It has been observed from the crystalline quality and optical properties that nominal V/III ratios of values greater than unity are required for GaSb epitaxial layers grown at this temperature. It has also been shown that Ga_1−xIn_xSb can be grown using TEGa as a source of gallium species at atmospheric pressure. The relationship between Ga_1−xIn_xSb vapour composition and solid composition has been studied at a V/III ratio of 0.78.     

Observation of new critical point in InxAl1−xAs alloy using spectroscopic ellipsometry

Using a spectroscopic ellipsometry, pseudodielectric functions 〈?〉 of In_xAl_1−xAs ternary alloy films (x = 0.43, 0.62, 0.75, and 1.00) from 0.74 to 6.48 eV were determined. Fast in-situ chemical etching to effectively remove surface overlayers using charge-coupled device detector and to avoid the reoxidation of the surface of films prior to the ellipsometric spectrum measurement was performed. At the high energy region, an additional critical point structure which is interpreted as the E′₁ transition from the band structure calculation of the linear augmented Slater-type orbital method was reported.     

The role of strain in controlling the surface morphology of AlxGa1−xN following in situ treatment with SiH4 and NH3

Treatment of GaN with SiH₄ and NH₃ increases the size of surface pits associated with threading dislocations, allowing them to be easily imaged by atomic force microscopy. Here, we assess the effect of a similar treatment on Al_xGa_1−xN surfaces for x ≤ 0.4. For relaxed Al_xGa_1−xN epilayers, an increase in the observed size and density of threading dislocation pits is observed. However, if the Al_xGa_1−xN is under tensile strain, the treatment results in the appearance of nanometre-scale surface hillocks. These hillocks may prevent observation of the dislocation pits. The hillocks are found to consist of crystalline Al_xGa_1−xN, and hence are suggested to be formed by strain driven etching or transformation of the surface by SiH₄ and NH₃.     

Analysis of Raman scattering of Ga1−xMnxAs dilute magnetic semiconductor

Ferromagnetic Ga_1−xMn_xAs layers (where x=1.4-3.0%) grown on (1 0 0) GaAs substrates by molecular beam epitaxy were characterized using Raman spectroscopy. As Mn is introduced into GaAs, a marked increase in disorder in the material occurs, as indicated by the growth of the disorder-allowed transverse-optical Raman line. Another important result is that as the Mn concentration in Ga_1−xMn_xAs increases further beyond ca. 2%, Raman-active coupled-plasmon-longitudinal-optical phonon modes arise, which signals the increasing presence of holes, and thus provides a useful tool for determining their concentration. Using the depletion-layer approach from the Raman spectroscopy data, we determined the carrier concentration for samples with x=2.2% and 3.0% was to be 7.2×10¹⁹ and 8.3×10²⁰ cm⁻³, respectively.     

Excitation intensity dependence of lateral photocurrent in InGaAs/GaAs dot-chain structures

Optical properties of multi-layer In_xGa_1 − xAs/GaAs dot-chain heterostructures are studied by means of lateral photoconductivity (PC). Effects of carrier generation and recombination on the photoresponsivity of deep defects and quantum dot arrays are considered. It is shown that the radiative recombination significantly affects the lateral PC spectra thus leading to a nonlinear dependence of photocurrent on excitation intensity. For ground state excitation of the quantum dots the photocurrent nonlinearities are determined by a competition of both generation and recombination processes which include thermal activation.     

A Hall coefficient investigation of ferromagnetic Ga1−xMnxAs layers on (100) GaAs substrates

Ferromagnetic Ga_1−xMn_xAs epilayers with Mn mole fraction in the range of x≈2.2-4.4% were grown on semi-insulating (100) GaAs substrates using the molecular beam epitaxy technique. The transport properties of these epilayers were investigated through Hall effect measurements. The measured hole concentration of Ga_1−xMn_xAs layers varied from 4.4×10¹⁹ to 3.4×10¹⁹ cm⁻³ in the range of x≈2.2-4.4% at room temperature. From temperature dependent resisitivity data, the sample with x≈4.4% shows typical behavior for insulator Ga_1−xMn_xAs and the samples with x≈2.2 and 3.7% show typical behavior for metallic Ga_1−xMn_xAs. The Hall coefficient for the samples with x≈2.2 and 4.4% was fitted assuming a magnetic susceptibility given by Curie-Weiss law in a paramagnetic region. This model provides good fits to the measured data up to and the Curie temperature T_c was estimated to be 65, 83 K and hole concentration p was estimated to be 5.1×10¹⁹, 4.6×10¹⁹ cm⁻³ for the samples with x≈2.2 and 4.4%, respectively, confirming the existence of an anomalous Hall effect for metallic and insulating samples.