Structural characterization of GaAs<Subscript>1−x</Subscript>Bi<Subscript>x</Subscript> alloy by rutherford backscattering spectrometry combined with the channeling technique

Rutherford backscattering spectrometry (RBS) combined with the channeling technique has been applied to a GaAs_1−xBi_x epilayer to investigate concentration and lattice location of Bi atoms and crystalline quality of the epilayer. The metastable GaAs_1−xBi_x alloy layer was grown on a GaAs substrate at a temperature as low as 365°C. The GaBi mole fraction obtained was 2.6 ± 0.2%. Angular scans for [100] and [111] crystal directions reveal that the incorporated Bi atoms exactly occupy substitutional sites in the GaAs crystal lattice. Crystal perfection of the GaAs_1−xBi_x metastable alloy is fairly good in spite of the low growth temperature.     

The preparation and characterization of strained-layer superlattices in the GaAs + GaP System

The technique of metal organic chemical vapor deposi-tion has been used to prepare strained-layer superlattices in the GaAs + GaP system. The superlattices consist of alternating layers of GaP and GaAs_xP_1?x for x = 0.2 to 1.0, which vary in thickness from 30 to 400 Å. The layers were grown by the decomposition of trlmethylgallium and various mixtures of ASH₃ and PH₃ in H₂ at 800δC. The thickness and uniformity of the layers were determined by optical and transmission electron microscopy and x-ray diffraction. The composition of the layers was determined from x-ray diffraction. A new analysis has been developed to determine the layer strain as well as the composition of thick layers (～ 300 Å). Transmission electron microscopy has been used to yield direct evidence that strained-layer superlattices can be used to remove the misfit dislocations generated during the epitaxial growth of a GaAs_xP_1-x alloy on a lattice mismatched GaP substrate. These results are in agreement with the previous work of Matthews and Blakeslee. Optical absorption, photocurrent spectroscopy and photoluminescence have been used to deter-mine the band gap energy as well as the energies for other optical transitions. The values are in excellent agreement with the values predicted by tight binding and effective mass calculations.     

Phosphorus incorporation in GaAsP grown by remote-plasma MOCVD

Epitaxial layers of GaAs and GaAs_xP_1−x were deposited under a variety of conditions using a remote plasma in a metal-organic chemical vapor deposition (MOCVD) reactor. The remote rf plasma dissociated AsH₃ and PH₃ into highly reactive radicals increasing the growth rate at low temperatures and increasing the phosphorus incorporation at temperatures below 850° C. GaAs_xP_1−x with a high P content could be grown at low deposition temperatures by using the plasma to increase the reactivity of PH₃. The growth rate of both GaAs and GaAs_xP_1−x was increased at low temperatures and GaAs_xP_1−x was grown at temperatures down to 550° C.     

100‐period, 1.23‐eV bandgap InGaAs/GaAsP quantum wells for high‐efficiency GaAs solar cells: toward current‐matched Ge‐based tandem cells

Major challenges for InGaAs/GaAsP multiple quantum well (MQW) solar cells include both the difficulty in designing suitable structures and, because of the strain‐balancing requirement, growing high‐quality crystals. The present paper proposes a comprehensive design principle for MQWs that overcomes the trade‐off between light absorption and carrier transport that is based, in particular, on a systematical investigation of GaAsP barrier effects on carrier dynamics that occur for various barrier widths and heights. The fundamental strategies related to structure optimization are as follows: (i) acknowledging that InGaAs wells should be thinner and deeper for a given bandgap to achieve both a higher absorption coefficient for 1e‐1hh transitions and a lower compressive strain accumulation; (ii) understanding that GaAs interlayers with thicknesses of just a few nanometers effectively extend the absorption edge without additional compressive strain and suppress lattice relaxation during growth; and (iii) understanding that GaAsP barriers should be thinner than 3 nm to facilitate tunneling transport and that their phosphorus content should be minimized while avoiding detrimental lattice relaxation. After structural optimization of 1.23‐eV bandgap quantum wells, a cell with 100‐period In_0.30GaAs(3.5 nm)/GaAs(2.7 nm)/GaAsP_0.40(3.0 nm) MQWs exhibited significantly improved performance, showing 16.2% AM 1.5 efficiency without an anti‐reflection coating, and a 70% internal quantum efficiency beyond the GaAs band edge. When compared with the GaAs control cell, the optimized cell showed an absolute enhancement in AM 1.5 efficiency, and 1.22 times higher efficiency with 38% current enhancement with an AM 1.5 cut‐off using a 665‐nm long‐pass filter, thus indicating the strong potential of MQW cells in Ge‐based 3‐J tandem devices. Copyright © 2013 John Wiley & Sons, Ltd.     

The compositional grading of MOCVD-grown GaAs1−xPx via substrate temperature changes

The first application of MOCVD compositional grading by means of substrate temperature changes is reported for the case of GaAs−P_x (0 ≤ x ≤ 0.30). The method is based upon the higher pyrolysis efficiency ratio of AsH₃ relative to PH₃ at lower growth temperatures. The growth upon GaAs substrates commences at a temperature at which this PH₃/ASH₃ thermal cracking efficiency ratio is extremely high; raising the growth temperature decreases this ratio, and hence the As/P ratio in the solid. GaAs_1−xP_x epilayers grown on GaAs by this method display excellent materials properties and are virtually optically and electrically indistinguishable from conventionally-graded epilayers in which the gas phase mole fractions of transport agents are varied at constant temperature. The method is applicable to systems where temperature-dependent cracking efficiency differentials exist.     

Valence band offsets in strained GaAs1−xPx/GaAs heterojunctions

Valence band offsets at [100]-oriented heterojunctions between tensile-strained GaASj_1−xP_x and unstrained GaAs are studied experimentally and theoretically. Light-hole (LH) and heavy-hole (HH) offsets are first extracted from the well-width dependence of valence subband splittings observed in luminescence spectra of tensile-strained GaAs_1−xP_x/GaAs quantum wells of various compositions (x = 0.06,0.09, and 0.19). This data is then combined with results from two other laboratories, yielding a set of 30 independent experimental offset values for junctions with compositions throughout the range 0.06≤x ≤0.32. The data are found to be highly consistent, with linear fits δE_LH = −140x (meV) and δE_HH= −401x (meV) describing the measured offsets to within less than 5 meV on average. Experimental results are then compared with theoretical predictions for the GaAs_1−x P_x/GaAs system obtained from a tight-binding model for strained heterojunctions. Predictions from the tight-binding calculations are found to lie within experimental scatter for the LH offsets, which define the valence band edge in these heterostructures, while magnitudes of the tight-binding HH offsets exceed measured values by ~20% on average.     

The luminescent properties of nitrogen doped GaAsP light emitting diodes

The properties of GaAsP:N light emitting diodes, which emit throughout the range from 7000 Å to 5500Å (red to green), have been investigated. The diodes were fabricated by Zn diffusion into n-type vapor phase epitaxial layers doped with nitrogen and tellurium. The emission spectra, luminous efficiency, quantum efficiency, brightness, and decay time constant have been investigated as functions of alloy composition. Also, the luminous output of the devices has been studied as a function of drive current density and temperature. In the yellow and green spectral regions the performance of nitrogen doped diodes is more than an order of magnitude better than that of nitrogen free diodes of equivalent peak emission wavelength. The luminous efficiency of diodes in the redamber spectral region, with alloy compositions near GaAs_0.35P_0.65 is equivalent to that obtained in GaP:Zn, O red emitting diodes with 2% quantum efficiency.     

Determination of the heterojunction type in structures with GaAsSb/GaAs quantum wells with various antimony fractions by optical methods

The type of heterojunction in the GaAs_{1 ? x} Sb _x /GaAs heterostructure at x = 0.36 is studied by photoluminescence spectroscopy and time-resolved photoluminescence. A GaAsSb/GaAs heterostructure with an Sb fraction of 15%, for which we can confidently state is a type-I heterojunction, was studied for comparison. It was established from the blue shift of the photoluminescence line depending on the excitation power and relaxation time of the photoluminescence signal from the GaAs_{1 ? x} Sb _x /GaAs quantum well, which was ??11 ns, that the GaAs_{1 ? x} Sb _x /GaAs structure at an Sb content of 36% clearly constitutes a type II heterojunction. This was additionally evidenced by the data obtained for structures with an Sb content of 15%, in which case no shift of the location of the photoluminescence line on the pump power was observed, while the relaxation time of photoluminescence in the region of the signal from the quantum well was ??1.5 ns.     

Phosphorus diffusion in gallium arsenide

The formation of monocrystalline GaAs_1?xP_x by the solid state diffusion of phosphorus into monocrystalline GaAs at a phosphorus pressure of 35 atm has been confirmed by X-rays and reflectivity measurements. Phosphorus distribution in GaAs specimens diffused in the temperature range 800–1100°C has been determined by reflectance studies. The diffusion coefficient and the activation energy computed from the known phosphorus distributions in the diffused specimens are found to depend on the content of phosphorus in the diffused region. The variation of the depth of GaAs_1?xP_x-GaAs junction with time (at a fixed temperature, 1000°C) and temperature (for a fixed time, 24.5 hr) has been studied.     

Emission properties of heterostructures with a (GaAsSb-InGaAs)/GaAs bilayer quantum well

The specific features of the emission characteristics of GaAs-based heterostructures with a GaAs_{1 ? x} Sb _x -In _y Ga_{1 ? y} As bilayer quantum well are studied. The heterostructures are grown by metal-organic chemical vapor deposition (MOCVD). With an analysis of previously reported data on the MOCVD growth process taken into account, the temperature range (560–580°C), the relation between the fluxes emitted by the sources of Group-V and ?III elements (?1), and the order of layer growth for the production of the active region of a GaAs/InGaP laser heterostructure are determined experimentally. The active region is a GaAs_0.75Sb_0.25-In_0.2Ga_0.8As bilayer quantum well. For the structure, a 1075-nm electroluminescence signal attributed to indirect transitions between the valence band of the GaAs_0.75Sb_0.25 layer and the conduction band of the In_0.2Ga_0.8As layer is observed. An increase in the continuous-wave pump current yields a decrease in the 1075-nm emission intensity and initiates stable lasing at a wavelength of 1022 nm at a threshold current density of 1.4 kA cm^?2 at room temperature. Lasing occurs at transitions direct in coordinate space.     

Radiative recombination via direct optical transitions in In1 −x GaxAs (0≤x≤0.16) solid solutions

Photoluminescence from In_1?x Ga_xAs (0≤x≤0.16) solid solution epilayers LPE-grown on (111)InAs substrates and electroluminescence from p-n junctions on their bases have been studied in the temperature range 77–450 K. Despite the negative lattice mismatch between epilayer and substrate, radiative recombination in epilayers occurs via direct optical transitions ensuring a high internal quantum efficiency of luminescence (6% at 295 K).     

Electroluminescent shockley diodes of GaAs and GaAsl−x,Px

GaAs and GaAs_1−xP_x p-n-p-n Shockley diode structures have teen prepared by an epitaxial vapor-phase growth technique. The two-terminal devices exhibit a negative-resistance I-V characteristic, which allows them to be used as an electroluminescent diode with a built-in latching capability. The GaAs diodes have breakover voltages between 2 and 30 V at room temperature, and switching characteristics which depend appreciably on the impurity concentrations and thicknesses of the two inner layers. After breakdown, the diodes emit near-bandgap infrared radiation with external quantum efficiencies as high as 0.39%. The emission spectra have been found to consist of two peaks, one corresponding to recombination in the innermost n-layer, and the other corresponding to recombination in one of the two p-type layers. The emission for the GaAs diodes occurs uniformly over the entire surface area. For GaAs_1−x P_x, the type of compositional grading used between the GaAs substrate and the p-n junction regions has been found to dramatically affect the I-V characteristics and switching capabilities of the Shockley diodes. Most importantly, the emission from the GaAs_1−xP_x devices occurs in small filaments for alloys with x ≃ 0.18. At high currents, the filaments form an orthogonal grid pattern parallel to {110} planes, thereby demonstrating a clear relationship to lattice-misfit dislocations generated during compositional grading. This research was supported in part by the Office of Naval Research, Arlington, Va., and in part by RCA Laboratories, Princeton, New Jersey.     

A novel pseudomorphic (GaAs1−xSbx-InyGa1−yAs)/GaAs bilayer-quantum-well structure lattice-matched to GaAs for long-wavelength optoelectronics

Two types of quantum well (QW) structures grown lattice matched on (100) GaAs have been studied. The first type of structure consists of pseudomorphic GaAs_xSb_1-x/GaAs (x≤0.3) SQWs which show emission wavelengths longer than those reported for pseudomorphic In_yGa_1−yAs/GaAs QWs. However, the attractive emission wavelength of 1.3 μm has not been achieved. To reach this goal, a novel type of bilayer QW (BQW) has been grown consisting of a stack of two adjacent pseudomorphic layers of GaAs_xSb_1−x and In Ga_1-y As embedded between GaAs confinement layers. In this BQW, a type-II heterojunction is formed between GaAs_xSb_1−x and In_yGa_1−yAs, resulting in a spatially indirect radiative recombination of electrons and holes at emission wavelengths longer than those achieved in the GaAs_xSb_1−x/GaAs and Ii_yGa_1−yAs/GaAs SQWs. The longest 300K emission wavelength observed so far was 1.332 μm.     

Electrical transport in n-type ZnMgSSe grown by molecular beam epitaxy on GaAs

Significant progress in improving the performance of blue-green II-VI semiconductor injection lasers has come about from advances in the epitaxial growth and doping of ZnMgSSe on GaAs substrates. This paper investigates electrical transport and its relation to structural quality in n-type Zn_1-yMg_y S_xSe_1-x epilayers doped with Cl, grown by molecular beam epitaxy. The composition parameters x and y vary from about 0.12-0.18 and 0.08-0.15, respectively. The quaternary epilayers studied are lattice-matched (or nearly so) to the GaAs substrate. Temperature-dependent Hall-effect measurements are performed on seven n-type ZnMgSSe:Cl epilayers, and a technique is presented whereby the resulting mobility-vs-temperature data is compared with data for ZnSe to obtain a structural figure of merit that is useful in characterizing the quaternary epilayer.     

Investigation of GaAs]-x_x P crystals using the pseudo-kossel X-ray and photoluminescence techniques

Two indirect methods of determining composition in GaAs_{1-x x}P films grown epitaxially on GaAs substrates are discussed in this paper. In the first method precision lattice parameters, obtained using the pseudo-Kossel technique in back reflection, are compared to compositions determined with the electron microprobe. A plot of corrected lattice parameter vs. %GaP (microprobe) over the range of composition investigated (0.10 < x < 0.55) shows that Vegard’s law holds very closely in this system. The slope of the line is -0.002042 Å/%GaP while the intercept is within 0.028% of the established lattice parameter for GaAs. No point lies farther than 0.003 Å from the line describing Vegard’s law.     

Photoluminescence characterization of UHV/CVD grown multi-quantum well structures

Ultra-high vacuum chemical vapor deposition is particularly suitable for growth of band gap engineered Ge_xSi_1?x structures, since abrupt epilayers with a very high degree of uniformity can be obtained. We have grown multi-quantum well structures over a range of well widths and have characterized them by photoluminescence spectroscopy. We have observed prominent quantum well-related features that shift to higher energy in samples with narrower well widths. Spectra taken from various points on a 75 mm wafer show a maximum variation of ±3 meV in position of the no phonon peak in the 35Å well multi-quantum well sample.     

Photoreflectance spectroscopy of low-dimensional GaAs/AlGaAs structures

Results of room-temperature photoreflectance measurements on three GaAs/Al_0.33Ga_0.67As multiquantum well (MQW) structures with three different widths of wells and on two GaAs/Al_0.33Ga_0.67As high-electron-mobility transistor (HEMT) structures are presented. Energy-gap-related transitions in GaAs and AlGaAs were observed. The Al content in AlGaAs was determined. MQW transition energies were determined using the first derivative of a Gaussian profile of the measured resonances. In order to identify the transitions in the MQS, the experimentally observed energies were compared with results of the envelope function calculation method for a rectangular quantum well. The Franz–Keldysh oscillation (FKO) model was also used to determine the built-in electric field in various parts of the investigated structures. The values of the electric fields allow us to hypothesise about the origin of these fields. © 1997 John Wiley & Sons, Ltd.     

OMVPE regrowth of CH3I-vapor-etched GaAs

In experiments on the interrupted growth of GaAs by organometallic vapor phase epitaxy (OMVPE), we have compared the properties of two types of epilayers: those grown on CH₃I-vapor-etched first epilayers and those grown on first epilayers that were either untreated or etched with H₂SO₄. The OMVPE growth and CH₃I etching were performed in two different reactors, and each sample was briefly exposed to air immediately before being placed in the OMVPE reactor for growth of the second epilayer. For CH₃I etch temperatures below 500° C, the two types of second epilayers are comparable in surface morphology, as characterized by Nomarski interference microscopy, and in optical quality, as characterized by low-temperature photoluminescence measurements. Electrical characterization by C-V depth profiling shows that electron accumulation at the regrowth interface is increased very little by CH₃I etching. Such etching prior to regrowth eliminates most of the electron accumulation resulting from H₂SO₄ etching of the first epilayer.     

Characterization of Double-Junction GaAsP Two-Color LED Structure

The structural, optical, electrical and electrical–optical properties of a double-junction GaAsP light-emitting diode (LED) structure grown on a GaP (100) substrate by using a molecular beam epitaxy technique were investigated. The p–n junction layers of GaAs_1?xP_x and GaAs_1?yP_y, which form the double-junction LED structure, were grown with two different P/As ratios. High-resolution x-ray diffraction (HRXRD), photoluminescence (PL), and current–voltage (I–V) measurements were used to investigate the structural, optical and electrical properties of the sample. Alloy composition values (x, y) and some crystal structure parameters were determined using HRXRD measurements. The phosphorus compositions of the first and second junctions were found to be 63.120% and 82.040%, respectively. Using PL emission peak positions at room temperature, the band gap energies (E_g) of the first and second junctions were found to be 1.867 eV and 2.098 eV, respectively. In addition, the alloy compositions were calculated by Vegard’s law using PL measurements. The turn-on voltage (V_on) and series resistance (R_s) of the device were obtained from the I–V measurements to be 4.548 V and 119 Ω, respectively. It was observed that the LED device emitted in the red (664.020 nm) and yellow (591.325 nm) color regions.     

Dislocation-free undoped semi-insulating GaAs epilayers prepared by chloride chemical vapor deposition and successive wafer annealing

Dislocation-free (DF) undoped semi-insulating GaAs epilayers have been realized by chloride chemical vapor deposition and successive wafer annealing. It was found that undoped conductive DF GaAs epilayers grown on Si-doped n-type DF GaAs substrates can be converted to semi-insulating by wafer annealing at temperatures higher than 950°C. The resistivity of these semi-insulating epilayers was higher than 10⁷ Ωcm. The outdiffusion of Si from the substrate to the epilayer was analyzed by secondary ion mass spectrometry and it was found that the thickness of the outdiffusion region was only 1μm.