Modeling of Ga1−xInxAs1−y−zNySbz/GaAs quantum well properties for near-infrared lasers

The aim of this work is to model the properties of GaInAsNSb/GaAs compressively strained structures. Indeed, Ga_1?xIn_xAs_1?y?zN_ySb_z has been found to be a potentially superior material to GaInAsN for long wavelength laser dedicated to optical fiber communications. Furthermore, this material can be grown on GaAs substrate while having a bandgap smaller than that of GaInNAs. The influence of nitrogen and antimony on the bandgap and the transition energy is explored. Also, the effect of these two elements on the optical gain and threshold current density is investigated. For example, a structure composed of one 7.5 nm thick quantum well of material with In=30%, N=3.5%, Sb=1% composition exhibits a threshold current density of 339.8 A/cm² and an emission wavelength of 1.5365 μm (at T=300 K). It can be shown that increasing the concentration of indium to 35% with a concentration of nitrogen and antimony, of 2.5% and 1%, respectively, results in a decrease of the threshold current density down to 253.7 A/cm² for a two well structure. Same structure incorporating five wells shows a threshold current density as low as 221.4 A/cm² for T=300 K, which agrees well with the reported experimental results.     

Transparent and Flexible Mn1−x−y(CexLay)O2−δ Ultrathin-Film Device for Highly-Stable Pseudocapacitance Application

Control over the fabrication of state-of-the-art portable pseudocapacitors with the desired transparency, mechanical flexibility, capacitance, and durability is challenging, but if resolved will have fundamental implications. Here, defect-rich Mn_1−x−y(Ce_xLa_y)O_2−δ ultrathin films with controllable thicknesses (5–627 nm) and transmittance (≈29–100%) are fabricated via an electrochemical chronoamperometric deposition using a aqueous precursor derived from end-of-life nickel-metal hydride batteries. Due to percolation impacts on the optoelectronic properties of ultrathin films, a representative Mn_1−x−y(Ce_xLa_y)O_2−δ film with 86% transmittance exhibits an outstanding areal capacitance of 3.4 mF cm⁻², mainly attributed to the intercalation/de-intercalation of anionic O²⁻ through the atomic tunnels of the stratified Mn_1−x−y(Ce_xLa_y)O_2−δ crystallites. Furthermore, the Mn_1−x−y(Ce_xLa_y)O_2−δ thin-film device exhibits excellent capacitance retention of ≈90% after 16 000 cycles. Such stability is associated with intervalence charge transfer occurring among interstitial Ce/La cations and Mn oxidation states within the Mn_1−x−y(Ce_xLa_y)O_2−δ structure. The energy and power densities of the transparent flexible Mn_1−x−y(Ce_xLa_y)O_2−δ full-cell pseudocapacitor device, is measured to be 0.088 μWh cm⁻² and 843 µW cm⁻², respectively. These values show insignificant changes under vigorous twisting and bending to 45–180° confirming these value-added materials are intriguing alternatives for size-sensitive energy storage devices.     

Effective masses and evidence for alloy disorder effects in GaxIn1−xAsyP1−y investigated by shallow donor photoconductivity

An investigation is reported of shallow donor photoconductivity for four compositions in the alloy series Ga_xIn_1−xAs_yP_1−y (y = 0.22, 0.56, 0.83, 1.0). The sample peak mobilities exceed 1.5 m² V⁻¹ s⁻¹. A linear effective mass variation through the alloy range is found to be most consistent with the experimental data. This is at variance with the predictions of K.P theory: possible reasons for this are discussed.     

Fundamentals of Ge1−xSnx and SiyGe1−x-ySnx RPCVD epitaxy

We have studied epitaxial growth of Ge_1−xSn_x and Si_yGe_1−x-ySn_x materials in 200 mm and 300 mm industrial CVD reactors using industry standard precursors. The growth kinetics of undoped GeSn were firstly studied via varying growth parameters including growth temperatures, GeH₄and SnCl₄precursor flows, which indicated that the material growth is highly dependent on surface kinetic limitations involving the SnCl₄reaction pathway. Secondly, the growth kinetics of doped layer growth by varying the growth temperatures and the PH₃and B₂H₆dopants flows were investigated. It was shown that B₂H₆had the effect of increasing the growth rate and decreasing the Sn incorporation whereas PH₃had no effect on the growth rate but increased the Sn incorporation. Thirdly, the SiGeSn growth kinetics using SiH₄, GeH₄, and SnCl₄as precursors were discussed, which revealed that the careful control of the growth rate was required to produce compositionally homogenous SiGeSn alloy. Moreover, the material and optical characterizations have been conducted to examine the material quality. Finally, the GeSn quantum well structure was grown to exhibit the precise control of the growth parameters.     

MOVPE grown Ga1−xInxAs solar cells for GaInP/GaInAs tandem applications

Lattice-mismatched Ga_1−xIn_xAs solar cells with an absorption edge between 900 and 1150 nm have been grown on GaAs substrates. Different graded Ga_1−xIn_xAs buffer layers and solar cell structures were evaluated to achieve a good electrical performance of the device. External quantum efficiencies comparable to our best GaAs solar cells were measured. The best 1 cm² cell with a bandgap energy of 1.18 eV has an efficiency of 22.6% at AM1.5g and a short circuit current density of 36.4 mA/cm². To our knowledge, this is the highest reported efficiency for a Ga_0.83In_0.17As solar cell.     

A comparison of TMG with TEG for the growth of InxGa1−xAs

MOCVD growth of In_xGa_1?xAs from trimethylgallium (TMG), triethylgallium (TEG), trimethylindium (TMI) and arsine was studied over a wide range of growth conditions. It was found that the In_xGa_1?xAs strain with respect to InP is strongly influenced by the arsine concentration when grown with TEG and TMI. In contrast the In_xGa_1?xAs strain was independent of arsine concentration when grown with TMG and TMI. It was also observed that the growth rate of In_xGa_1?xAs is higher for TMG than for TEG with the same TMI and arsine flow. In addition an interaction between TEG and dimethylzinc (DMZ) was also observed. We show that MOCVD growth process involves many complex reactions and cannot be considered as a simple decomposition of each precursor. The interactions between precursors, which takes place in the gas phase or on the growing surface, has to be considered. We have utilized the TEG/arsine interaction for the growth of strain compensated superlattices by modulating the arsine flow into the reactor chamber while keeping the TEG and TMI constant. Structures with up to 100 periods of 100 Å of +1% In_0.6Ga_0.4As and 200 Å of ?0.5% In_0.5Ga_0.5As were grown with excellent characteristics.     

Vacancies in Hg1−xCdxTe

Measurements have been performed of the carrier concentrations in vacancy-doped Hg_1−xCd_xTe with x=0.22, 0.29, 0.45, and 0.5. Anneals to establish the carrier concentrations were performed on both the mercury- and tellurium-rich sides of the phase field. When these results were added to earlier data for x=0.2 and 0.4, and assuming that all vacancies are doubly ionized, then vacancy concentrations for all values of x and anneal temperature can be represented by simple equations. On the mercury side of the phase field, the vacancy concentrations varied as 2.50×10²³(1−x) exp[−1.00/kT] for low concentrations, and as 3.97×10⁷(1−x)^1/3n _i ^2/3 exp[−0.33/kT] for high concentrations, where n_i is the intrinsic carrier concentration. On the tellurium rich side, the vacancy concentrations varied as 2.81 × 10²²(1−x) exp[−0.65/kT] for low concentrations and as 1.92×10⁷(1−x)^1/3n _i ^2/3 exp[−0.22/kT] for high concentrations.     

Optical band gap of Cd1−x MnxTe and Zn1−x MnxTe semiconductors

The dependence of the optical band gap for Zn_1?x Mn_xTe and Cd_1?x Mn_xTe semiconductor compounds was investigated by the methods of cathodoluminescence and optical reflection. It was found that, for Zn_1?x Mn_xTe compounds in the region x?0.2, the band gap is additionally broadened by a magnitude of about 0.08 eV, which is related to the high density of interstitial-type defects in single crystals. For x?0.3, the probability of the existence of these defects decreases substantially, which is related to the distortion of tetrahedra of the crystal lattice of Zn_1?x Mn_xTe by Mn atoms, which are incorporated into each tetrahedron.     

Precise measurements of the dispersion of the index of refraction for Cd1−xZnxTe alloys

By using a prism coupler technique in conjunction with reflectivity measurements, we have obtained highly accurate relations for the dispersion of the indices of refraction n for a series of MBE-grown Cd_1−xZn_xTe alloys. Initially, the prism coupler technique was used to determine n at discrete wavelengths with an accuracy of at least 0.1%, and also to concurrently determine the epilayer thicknesses with an uncertainty of less than 0.5%. Having obtained precise values for both n (at discrete wavelengths) and the thicknesses of the Cd_1−xZn_xTe epilayers, we were then able to correctly decipher the values for n at the maxima and minima of the reflectivity spectra observed on the above epilayers, and thereby generate a continuous variation of the indices of refraction as a function of wavelength. Fitting the dispersion of n in each alloy to a Sellmeier-type dispersion relation, we have obtained the dependence of the constants appearing in this relation on the alloy concentration. This enables one to predict n not only as a function of wavelength, but also as a function of alloy composition.     

Optimization of PP− junction termination for new power devices

In this paper PP⁻-type junction termination is investigated. Based on 2D simulations, we have optimized this termination in the 1200–1400 V range under non-punch-through (NPT) and in the 400–600 V range under punch-through (PT) conditions. We have analysed the influence of the doping and the length of the lowly-doped P⁻ region on the efficiency of the termination and the influence of charges in the field oxide. These simulation results were then validated using test devices.     

Long-wavelength photodiodes based on Ga1−x InxAsySb1−y wit composition near the miscibility boundary

The possibility of using liquid-phase epitaxy to obtain Ga_1−x In_xAs_ySb_1−y solid solutions isoperiodic with GaSb near the miscibility boundary is investigated. The effect of crystallographic orientation of the substrate on the composition of the solid solutions grown in this way is examined, and the indium concentration is observed to grow from 0.215 to 0.238 in the Ga_1−x In_xAs_ySb_1−y solid phase in the series of substrate orientations (100), (111)A, (111)B. A change in the composition of the solid solution leads to a shift of the long-wavelength edge of the spectral distribution of the photosensitivity. The use of a GaSb (111)B substrate made it possible, without lowering the epitaxy temperature, to increase the indium content in the solid phase to 23.8% and to create long-wavelength photodiodes with spectral photosensitivity threshold λ _th=2.55 μm. The primary characteristics of such photodiodes are described, along with aspects of their fabrication. The proposed fabrication technique shows potential for building optoelectronic devices (lasers, LED’s, photodiodes) based on Ga_1−x In_xAs_ySb_1−y solid solutions with red boundary as high as 2.7 μm. Fiz. Tekh. Poluprovodn. 33, 249–253 (February 1999)     

Electrodeposition of CuIn1−xGaxSe2 solar cells with a periodically-textured surface for efficient light collection

The photoresponse of CuIn_1?xGa_xSe₂ (CIGS) solar cells is improved using a periodically-textured structure as an antireflection layer. The CIGS absorber layers were prepared by one-step electrodeposition from an aqueous solution containing 12 mM CuSO₄, 25 mM In₂(SO₄)₃, 28 mM Ga₂(SO₄)₃, and 25 mM SeO₂. The electrodeposited CIGS films exhibit the (112)-preferred orientation of the chalcopyrite structures and feature improved film stoichiometry after the selenization process. In addition, the lower bandgap value of 0.97 eV is caused by the discrepancy of the reduction potentials for each constituent, resulting in insufficient Ga content in the deposited films. Using self-assembled silica nanoparticles as the etching mask, periodically-textured structures can be easily formed on an indium tin oxide (ITO)-coated soda lime glass to achieve a low average reflection (<10.5%) in a wide wavelength and incident angle range. With the periodic textured structures suppressing light reflections from the front surface, the photogenerated current in the semi-transparent CIGS solar cells made with transparent conducting electrodes is 1.82 times higher than they otherwise would be.     

Structure and optical properties of heterostructures based on MOCVD (Al x Ga1 − x As1 − y P y )1 − z Si z alloys

Epitaxial heterostructures produced by MOCVD on the basis of Al _x Ga_{1 ? x} As ternary alloys with the composition parameter x ≈ 0.20–0.50 and doped to a high Si and P atomic content are studied. Using the high-resolution X-ray diffraction technique, scanning electron microscopy, X-ray microanalysis, Raman spectroscopy, and photoluminescence spectroscopy, it is shown that the epitaxial films grown by MOCVD are formed of five-component (Al _x Ga_{1 ? x} As_{1 ? y} P _y )_{1 ? z} Si _z alloys.     

1T’ RexMo1−xS2–2H MoS2 Lateral Heterojunction for Enhanced Hydrogen Evolution Reaction Performance

The imperfect interfaces between 2D transition metal dichalcogenides (TMDs) are suitable for boosting the hydrogen evolution reaction (HER) during water electrolysis. Here, the improved catalytic activity at the spatial heterojunction between 1T’ Re_xMo_1−xS₂ and 2H MoS₂ is reported. Atomic-scale electron microscopy confirms that the heterojunction is constructed by an in-situ two-step growth process through chemical vapor deposition. Electrochemical microcell measurements demonstrate that the 1T’ Re_xMo_1−xS₂–2H MoS₂ lateral heterojunction exhibits the best HER catalytic performance among all TMD catalysts with an overpotential of ≈84 mV at 10 mA cm⁻² current density and 58 mV dec⁻¹ Tafel slope. Kelvin probe force microscopy shows ≈40 meV as the work function difference between 2H MoS₂ and 1T’ Re_xMo_1−xS₂, facilitating the electron transfer from 2H MoS₂ to 1T’ Re_xMo_1−xS₂ at the heterojunction. First-principles calculations reveal that Mo-rich heterojunctions with high structural stability are formed, and the HER performance is improved with the combination of increased density of states near the Fermi level and optimal ΔG_H* as low as 0.07 eV. Those synergetic effects with many electrons and active sites with optimal ΔG_H* improve HER performance at the heterojunction. These results provide new insights into understanding the role of the heterojunction for HER.     

Superstructured ordering in Al x Ga1 − x As and Ga x In1 − x P alloys

Epitaxial heterostructures produced on the basis of Al _x Ga_{1 ? x} As and Ga _x In_{1 ? x} P ternary alloys by metal-organic chemical vapor deposition are studied. The composition parameter x of the alloys was ～0.50. By X-ray diffraction studies, scanning electron microscopy, atomic force microscopy, and photoluminescence spectroscopy, it is shown that superstructured ordered phases with the stoichiometry composition III_{1 ? η}III_{1 + η}V₂ can be formed. As a consequence of this effect, not only does the cubic crystal symmetry change to the tetragonal type in the new compound, but also the optical properties become different from those of disordered alloy with the same composition.     

Hole scattering mechanisms in Hg1−xCdxTe

In this paper, we analyze and discuss the roles of nine different scattering mechanisms—ionized impurity, polar and nonpolar optical, acoustic, dislocation, strain field, alloy disorder, neutral impurity, and piezoelectric—in limiting the hole mobilities in p-type Hg_1−xCd_xTe crystals. The analysis is based on obtaining a good fit between theory and experiment for the light and heavy hole drift mobilities by optimizing certain unknown (or at the most vaguely known) material parameters such as the heavy hole mobility effective mass, degree of compensation, and the dislocation and strain field scattering strengths. For theoretical calculations, we have adopted the relaxation time approach, keeping in view its inadequacy for the polar scattering. The energy dispersive hole relaxation times have been drawn from the published literature that take into account the p-symmetry of valence band wave functions. The temperature dependencies of multiple charge states of impurities and of Debye screening length have been taken into account through a numerical calculation for the Fermi energy. Mobility data for the present analysis have been selected from the HgCdTe literature to represent a wide range of material characteristics (x=0.2–0.4, p=3×10¹⁵–1×10¹⁷ cm⁻³ at 77K, μ_peak≅200-1000cm²V⁻¹s⁻¹). While analyzing the light hole mobility, the acoustic deformation and neutral impurity potentials were also treated as adjustable. We conclude that

Large VLWIR Hg1−xCdxTe photovoltaic detectors

Very long wavelength infrared (VLWIR; 15 to 17 μm) detectors are required for remote sensing sounding applications. Infrared sounders provide temperature, pressure and moisture profiles of the atmosphere used in weather prediction models that track storms, predict levels of precipitation etc. Traditionally, photoconductive VLWIR (λ_c >15 μm) detectors have been used for sounding applications. However, photoconductive detectors suffer from performance issues, such as non-linearity that is 10X – 100X that of photovoltaic detectors. Radiometric calibration for remote sensing interferometry requires detectors with low non-linearity. Photoconductive detectors also suffer from non-uniform spatial optical response. Advances in molecular beam epitaxy (MBE) growth of mercury cadmium telluride (HgCdTe) and detector architectures have resulted in high performance detectors fabricated in the 15 μm to 17 μmm spectral range. Recently, VLWIR (λ_c ∼ 17 μm at 78 K) photovoltaic large (1000 μm diameter) detectors have been fabricated and measured at flux values targeting remote sensing interferometry applications. The operating temperature is near 78 K, permitting the use of passive radiators in spacecraft to cool the detectors. Detector non-AR coated quantum efficiency >60% was measured in these large detectors. A linear response was measured, while varying the spot size incident on the 1000 μm detectors. This excellent response uniformity, measured as a function of spot size, implies that low frequency spatial response variations are absent. The 1000 μm diameter, λ_c ∼ 17 μm at 78 K detectors have dark currents ∼160 μA at a −100 mV bias and at 78 K. Interfacing with the low (comparable to the contact and series resistance) junction impedance detectors is not feasible. Therefore a custom pre-amplifier was designed to interface with the large VLWIR detectors operating in reverse bias. A breadboard was fabricated incorporating the custom designed preamplifier interfacing with the 1000 μm diameter VLWIR detectors. Response versus flux measurements were made on the large VLWIR detectors and non-linearity <0.15% was measured at high flux values in the 2.5×10¹⁷ to 3.5×10¹⁷ ph-cm⁻²sec⁻¹ range. This non-linearity is an order of magnitude better than for photoconductive detectors.     

Organometallic VPE growth of AlxGa1−xAs

The " hybrid" organometallic VPE process for the growth of Al_xGa_l-xAs has been explored. Six growth parameters have been considered; substrate orientation, substrate temperature during growth, total flow rate and ratios of Al, HC1 and As to the total group III flow rate. The effects of these six growth parameters on the growth process (growth rate, composition and surface morphology) and the materials properties (carrier concentration, photoluminescence intensity and spectrum) have been systematically studied. A technique has been developed for the growth of heterostructures by changing only the H₂ diluent flow rate. This results in high quality heterostructures with x changing from 0.05 to 0.30 in > 100 å.     

High C-doping of MOVPE grown thin AlxGa1−xAs layers for AlGaAs/GaAs interband tunneling devices

High hole concentrations in LP-MOVPE grown GaAs and AlGaAs layers can be achieved by intrinsic C-doping using TMGa and TMAl as carbon sources. Free carrier concentrations exceeding 10²⁰ cm⁻³ were realised at low growth temperatures between 520–540°C and V/III ratios <1.2. The C-concentration increases significantly with the Al-content in Al_xGa_1−xAs layers. We observed an increase in the atom- and free carrier concentration from 5·10¹⁹ cm⁻³ in GaAs to 1.5·10²⁰ cm⁻³ in Al_0.2Ga_0.8As for the same growth conditions. Interband tunneling devices with n-type Si and p-type C-doped AlGaAs layers and barriers made of Al_0.25Ga_0.26In_0.49P have been investigated.     

−31 dBm sensitivity high efficiency rectifier for energy scavenging

RF energy scavenging is capable in converting RF signals into electricity and has become a promising solution to power energy-constrained wireless networks. However, it has low power conversion efficiency especially when the harvested RF power is small. For this reason, we propose an enhanced differential-drive rectifier to improve the efficiency and the sensitivity of rectifier for energy scavenging applications. The proposed rectifier achieves dynamically controlled threshold voltage and reduces leakage current in the transistors through DTMOS transistor in differential-drive topology. The voltage boosting circuit further increases the sensitivity through step up the input signal before the signal enters the rectifier. The decoupling capacitor shunts the noise of the input signal before the signal is injected into the cross-connected gate reducing the voltage drop and maintaining the PCE of the rectifier. The rectifier is designed based on the 0.18 µm Silterra CMOS process technology. Effects of decoupling capacitors, voltage boosting circuit and output load on PCE of the rectifier have been evaluated. Technology scaling and parasitic effects to the rectifier have also been presented. Performance of N-stages proposed rectifier has been compared with the conventional BTMOS rectifier. The proposed method achieves the highest sensitivity of −31 dBm for 1, 3 and 5 stages rectifiers without the need of off-chip load capacitor.