xSbx Layers Research Articles

Tuning structural, optical, electrical and photovoltaic characteristics of n-type CdS1−xSbx layers for optimizing the performance of n-(CdS:Sb)/p-Si solar cells

Tuning structural, optical, electrical and photovoltaic characteristics of n-type CdS1−xSbx layers for optimizing the performance of n-(CdS:Sb)/p-Si solar cells

Antimony segregation in an InAs/InAs1−xSbx superlattice grown by metalorganic chemical vapor deposition

Metal organic chemical vapor deposition (MOCVD) and molecular beam epitaxy (MBE) are two versatile growth techniques that can readily produce multilayer structures with atomic-level precision control, which have found broad applications in technology. However, compared to MBE, MOCVD growth involves the surface reaction of metal-organic precursor compounds, which changes during film deposition. Consequently, a thorough investigation on the chemical profile layer-by-layer is critical for optimizing MOCVD film performance. Here, we examine Sb segregation in an MOCVD-grown InAs/InAs1−xSbx superlattice by analyzing composition and lattice strain at atomic resolution using scanning transmission electron microscopy and compare with the previously reported MBE growth results. Our findings show a different Sb profile along the growth direction in MOCVD, with the segregation coefficient being higher at the InAsSb-on-InAs interface (0.807 ± 0.021) than at the InAs-on-InAsSb interface (0.695 ± 0.009), giving rise to asymmetric composition and lattice strain profiles unlike those obtained with MBE. Furthermore, we obtain direct evidence of Sb clusters with size of ∼1–3 nm and Sb ordering within the InAs1−xSbx layer, which is largely absent in the reported MBE growth. These findings demonstrate the concurrent interplay between surface segregation, surface reconstruction, and surface reaction that is unique to MOCVD growth with broad implications on preparing Sb-containing quantum materials.

Evidence of carrier localization in photoluminescence spectroscopy studies of mid-wavelength infrared InAs/InAs1−xSbx type-II superlattices

The temperature-dependent and excitation-dependent photoluminescence (PL) spectroscopy characterization of mid-wavelength infrared InAs/InAs1−xSbx type-II superlattices reveals evidence of carrier localization. Carrier localization is apparent in the 8meV PL peak position blue shift from 4K to 60K while the peak full-width-at-half-maximum is non-monotonic, peaking at 25K before increasing above 60K. In addition, competition between two recombination processes is evident in the temperature-dependent behavior of the PL peak integrated intensity under low excitation conditions: the intensity decreases from 4K to 80K, increases from 80K to 160K, and decreases above 160K. Excitation-dependent PL studies reveal the dominant recombination mechanism changes from free-to-bound or donor–acceptor-like recombination to excitonic or band-to-band recombination at ~60K. These findings suggest that carrier localization is occurring below 60K, and the confined carriers are holes as these are unintentionally doped n-type superlattices. The localization potentials are due to variations in the InAs1−xSbx composition, the interfaces, and the InAs and InAs1−xSbx layer widths. The width of a Gaussian distribution used to describe the density of states of the band tails due to carrier localization potentials ranges from 2meV–4meV. The larger energy corresponds to the smaller period superlattices, indicating the interface compositional variation is more prominent and creates larger localization potentials than in the longer period superlattices.

Structural and optical properties of GaAsSb QW heterostructures grown by laser deposition

The possibility of using the laser deposition method to grow crystalline light-emitting structures with GaAsSb/GaAs quantum wells (QWs) is experimentally demonstrated for the first time. The growth temperature of the GaAs1 − xSbx layers is varied within the range 450–550°C; according to X-ray diffraction analyses, the content of antimony reaches xSb ≈ 0.37 at a growth temperature of 450°C. Low-temperature (4 K) photoluminescence spectroscopy demonstrates the presence of a peak associated with the GaAsSb/GaAs QW at around 1.3 μm at the minimum laser-light pumping level. The optimal growth temperature Tg = 500°C and arsine flow rate PA = 2.2 × 10−8 mol/s at which the best emission properties of QWs with xSb ∼ 0.17–0.25 are observed at temperatures of 77 and 300 K are determined. It is shown that GaAsSb/GaAs QWs with similar parameters (width and composition) grown by laser deposition at 500°C and metal-organic vapor-phase epitaxy at 580°C have comparable optical quality.

Metamorphic InAsSb/AlInAsSb heterostructures for optoelectronic applications

Metamorphic heterostructures containing bulk InAs1−xSbx layers and AlInAsSb barriers were grown on GaSb substrates. The lattice mismatch (up to 2.1%) between the GaSb substrates and the InAsSb layers was accommodated by the growth of GaInSb linearly graded buffers. The 1 μm thick InAsSb0.44 layer with an absorption edge above 9 μm exhibited an in-plane residual strain of about 0.08%. InAs1−xSbx structures with x = 0.2 and x = 0.44 operated as light emitting diodes at 80 K demonstrated output powers of 90 μW and 8 μW at 5 μm and 8 μm, respectively.

Conduction- and Valence-Band Energies in Bulk InAs1−x Sb x and Type II InAs1−x Sb x /InAs Strained-Layer Superlattices

The energy gaps were studied in two types of structures: unrelaxed bulk InAs1−xSbx layers with x = 0.2 to 0.46 grown on metamorphic buffers and type II InAs1−xSbx/InAs strained-layer superlattices (SLS) with x = 0.225 to 0.296 in the temperature range from T = 13 K to 300 K. All structures were grown on GaSb substrates. The longest wavelength of photoluminescence (PL) at low temperatures was observed from bulk InAs0.56Sb0.44 with a peak at 10.3 μm and full-width at half-maximum (FWHM) of 11 meV. The PL data for the bulk InAs1−xSbx materials of various compositions imply an energy gap bowing parameter of 0.87 eV. A low-temperature PL peak at 9.1 μm with FWHM of 13 meV was observed for InAs0.704Sb0.296/InAs SLS. The PL spectrum of InAs0.775Sb0.225/InAs SLS under pulsed excitation revealed a second peak associated with recombination of electrons in the three-dimensional (3D) continuum with holes in the InAs0.775Sb0.225. This experiment determined the conduction-band offset in the InAs0.775Sb0.225/InAs SLS. The energies of the conduction and valence bands in unstrained InAs1−xSbx and their bowing with respect to the Sb composition are discussed.

Analysis of the modified optical properties and band structure of GaAs1−xSbx-capped InAs/GaAs quantum dots

The origin of the modified optical properties of InAs/GaAs quantum dots (QD) capped with a thin GaAs1−xSbx layer is analyzed in terms of the band structure. To do so, the size, shape, and composition of the QDs and capping layer are determined through cross-sectional scanning tunnelling microscopy and used as input parameters in an 8 × 8 k·p model. As the Sb content is increased, there are two competing effects determining carrier confinement and the oscillator strength: the increased QD height and reduced strain on one side and the reduced QD-capping layer valence band offset on the other. Nevertheless, the observed evolution of the photoluminescence (PL) intensity with Sb cannot be explained in terms of the oscillator strength between ground states, which decreases dramatically for Sb > 16%, where the band alignment becomes type II with the hole wavefunction localized outside the QD in the capping layer. Contrary to this behaviour, the PL intensity in the type II QDs is similar (at 15 K) or even larger (at room temperature) than in the type I Sb-free reference QDs. This indicates that the PL efficiency is dominated by carrier dynamics, which is altered by the presence of the GaAsSb capping layer. In particular, the presence of Sb leads to an enhanced PL thermal stability. From the comparison between the activation energies for thermal quenching of the PL and the modelled band structure, the main carrier escape mechanisms are suggested. In standard GaAs-capped QDs, escape of both electrons and holes to the GaAs barrier is the main PL quenching mechanism. For small-moderate Sb (<16%) for which the type I band alignment is kept, electrons escape to the GaAs barrier and holes escape to the GaAsSb capping layer, where redistribution and retraping processes can take place. For Sb contents above 16% (type-II region), holes remain in the GaAsSb layer and the escape of electrons from the QD to the GaAs barrier is most likely the dominant PL quenching mechanism. This means that electrons and holes behave dynamically as uncorrelated pairs in both the type-I and type-II structures.

Structural properties of InAs/InAs1–xSbx type-II superlattices grown by molecular beam epitaxy

Strain-balanced InAs/InAs1−xSbx type-II superlattices (SLs) have been proposed for possible long-wavelength infrared applications. This paper reports a detailed structural characterization study of InAs/InAs1−xSbx SLs with varied Sb composition grown on GaSb (001) substrates by modulated and conventional molecular beam epitaxy (MBE). X-ray diffraction was used to determine the SL periods and the average composition of the InAs1−xSbx alloy layers. Cross-section transmission electron micrographs revealed the separate In(As)Sb/InAs(Sb) ordered-alloy layers within individual InAs1−xSbx layers for SLs grown by modulated MBE. For the SLs grown by conventional MBE, examination by high-resolution electron microscopy revealed that interfaces for InAs1−xSbx deposited on InAs were more abrupt, relative to InAs deposited on InAs1−xSbx: this feature was attributed to Sb surfactant segregation occurring during the SL growth. Overall, these results establish that strain-balanced SL structures with excellent crystallinity can be achieved with proper design (well thickness versus Sb composition) and suitably optimized growth conditions.

Properties of unrelaxed InAs1−XSbX alloys grown on compositionally graded buffers

Unrelaxed InAs1−xSbx layers with lattice constants up to 2.1% larger than that of GaSb substrates were grown by molecular beam epitaxy on GaInSb and AlGaInSb compositionally graded buffer layers. The topmost section of the buffers was unrelaxed but strained. The in-plane lattice constant of the top buffer layer was grown to be equal to the lattice constant of unrelaxed and unstrained InAs1−xSbx with given X. The InAs0.56Sb0.44 layers demonstrate photoluminescence peak at 9.4 μm at 150 K. The minority carrier lifetime measured at 77 K for InAs0.8Sb0.2 was τ = 250 ns.

Time‐resolved photoluminescence of type‐II InAs/GaAs quantum dots covered by a thin GaAs1–xSbx layer

AbstractWe investigate carrier lifetimes of InAs/GaAs quantum dots (QDs) covered by a thin GaAs1–xSbx layer by time‐resolved photoluminescence (PL). Both the power dependent PL peak shift and the longer decay time confirm the type‐II band alignments. Different recombination paths have been identified by temperature dependent measurements. At low temperatures, the long‐range recombination with holes trapped in the GaAsSb layer is significant, resulting in non‐single‐exponential decays. The short‐range recombination with holes confined in the band‐bending region surrounding the InAs QDs is important at higher temperatures. The variation in decay time across the ground‐state and the temporal PL peak redshift further confirm the localization of holes in the GaAsSb layer. (© 2009 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Effects of thermal annealing on the emission properties of type-II InAs/GaAsSb quantum dots

We report the effects of thermal annealing on the emission properties of type-II InAs quantum dots (QDs) covered by a thin GaAs1−xSbx layer. Apart from large blueshifts and a pronounced narrowing of the QD emission peak, the annealing induced alloy intermixing also leads to enhanced radiative recombination rates and reduced localized states in the GaAsSb layer. Evidences of the evolution from type-II to type-I band alignments are obtained from time-resolved and power-dependent photoluminescence measurements. We demonstrate that postgrowth thermal annealing can be used to tailor the band alignment, the wave function overlaps, and hence the recombination dynamics in the InAs/GaAsSb type-II QDs.

Carrier dynamics of type-II InAs∕GaAs quantum dots covered by a thin GaAs1−xSbx layer

Carrier dynamics of InAs∕GaAs quantum dots (QDs) covered by a thin GaAs1−xSbx layer were investigated by time-resolved photoluminescence (PL). Both the power dependence of PL peak shift and the long decay time constants confirm the type-II band alignment at the GaAsSb–InAs interface. Different recombination paths have been clarified by temperature dependent measurements. At lower temperatures, the long-range recombination between the QD electrons and the holes trapped by localized states in the GaAsSb layer is important, resulting in a non-single-exponential decay. At higher temperatures, optical transitions are dominated by the short-range recombination with the holes confined to the band-bending region surrounding the QDs.

Growth and characterization of Sn-doped GaAsSb and GaAs epilayers on GaAs (001)

We investigated the growth of GaAs1−xSbx(x⩽0.4) and GaAs epilayers with heavy Sn doping using molecular-beam epitaxy, and characterized the doping concentration as a function of Sn K-cell temperature. The carrier concentration was found to be expressed as a function of the reciprocal Sn effusion cell temperature with respect to GaAs1−xSbx(x=0.15) layers. At cell temperatures of 670–800°C, carrier concentrations ranging from 5×1016to8×1018cm−3 showed a simple exponential dependency. When the ratios of TO-mode to LO-mode intensities in GaAs1−xSbx(x=0.15) layers were measured as a function of Sn-cell temperature by micro-Raman scattering, the peak intensity ratio was linearly proportional to the Sn-cell temperature in the range of 700–900°C.

Cross-sectional scanning tunneling microscopy of GaAsSb/GaAs quantum well structures

We have used cross-sectional scanning tunneling microscopy (STM) to perform nanometer-scale characterization of compositional structure and interfacial properties within GaAs1−xSbx/GaAs double-quantum well structures. An algorithm has been devised based on analysis of strain effects in STM data to obtain detailed, quantitative compositional profiles within alloy layers. Using this and other analysis techniques, we have assessed the influence of group V anion soaks at each heterojunction interface on interface roughness and abruptness. An As soak at the GaAs–on–GaAs1−xSbx interfaces reduces interface roughness but leads to a slight loss of abruptness at the interface, while an As+Sb soak at GaAs1−xSbx–on–GaAs interfaces improves abruptness while leaving interface roughness largely unaffected. Significant compositional grading at the nanometer scale is observed within the GaAs1−xSbx layers.

Raman and dielectric function spectra of strained GaAs1−xSbx layers on InP

Raman spectroscopy and variable angle spectroscopic ellipsometry (SE) were used to study pseudomorphically strained GaAs1−xSbx layers (0.22⩽x⩽0.65) grown on InP by metal-organic chemical vapor deposition. From the Raman spectra the composition dependence of the GaAs-like LO phonon mode was deduced. For comparison with literature data for unstrained GaAs1−xSbx, the strain-induced frequency shift of the GaAs-like LO phonon was calculated and subtracted from the present experimental data. When corrected for strain effects the composition dependent GaAs-like LO phonon frequency could be fitted by the linear relation ωLO=292.3−51x(cm−1) for the present range of alloy compositions. The pseudodielectric function spectra, deduced from SE measurements covering the range of photon energies from 0.8 to 5.0 eV, were fitted by a multilayer model employing a set of parametric oscillators to describe the GaAs1−xSbx dielectric function. From the resulting parametric dielectric functions the composition dependence of the E1 and E1+Δ1 critical point energies was deduced for pseudomorphically strained GaAs1−xSbx on InP and, after correction for strain effects on the interband transitions, also for unstrained GaAs1−xSbx.

Effect of Sb composition on lateral oxidation rates in AlAs1−xSbx

We demonstrate the effect of antimony (Sb) composition on the oxidation mechanism of AlAs1−xSbx (x<0.21) layers on GaAs substrates. It has been demonstrated that addition of a group-III element like Ga to AlAs slows the rate of oxidation. In contrast, addition of a group-V element like Sb to AlAs changes the oxidation mechanism in more than one way. The oxidation rate increases with Sb addition, and the oxidation reaction changes from a diffusion-limited mechanism to a reaction-rate-limited mechanism at higher oxidation temperatures. This is attributed to the increase in the permeability of the oxide. Nonuniform segregation of Sb is observed upon oxidation. The activation energy of the oxidation reaction-rate constant initially decreases with the Sb composition upto 10%, further Sb addition increases the activation energy.

Interband optical properties of molecular-beam epitaxially grown GaAs1−xSbx on GaAs substrates

Spectroscopic ellipsometry (SE) from 0.8 to 5 eV and photoreflectance (PR) from 0.7 to 1.5 eV were used to study the interband optical response at room temperature of epitaxial GaAs1−xSbx layers with 0<x<0.5. The samples were grown by molecular-beam epitaxy at 520 °C on (001)-GaAs substrates and characterized by low-temperature photoluminescence and x-ray diffraction. The complex dielectric function ε̃(ω) of GaAs1−xSbx vs x was derived from the ellipsometric spectra after mathematically removing the oxide overlayer effects. The SE and PR spectra were analyzed with their energy derivatives in terms of standard analytical line shapes: in particular the E0, E1, E1+Δ1. E0′, and E2 critical point energies were derived as a function of x. On this basis the energy-shift model is appropriate to interpolate ε̃x(ω) for any x<0.5, thus allowing a nondestructive optical diagnostic of epitaxial heterostructures based on GaAs1−xSbx.

GaAsSb/GaAs quantum well growth by MOCVD hydride epitaxy with laser sputtering of antimony

A new method of obtaining quantum-size GaAs1−x Sbx (x⩽0.45) layers is proposed. The method consists in laser vaporization of solid metallic antimony near the substrate directly in the reactor. The antimony concentration is set by the antimony sputtering time with the arsine flux shut off. The polarization of the photoluminescence of the obtained layers indicates the formation of quantum wires. The heterostructures obtained are used to fabricate laser diodes.

In situ composition control of III-As1 − xSbx alloys during molecular beam epitaxy using line-of-sight mass spectrometry

The interaction of a Sb2 molecular beam with the film surface during molecular beam epitaxy (MBE) of GaAs1 − xSbx layers was monitored by a quadrupole mass spectrometer (QMS) positioned in line-of-sight of the substrate. It was observed that the QMS signal from the monomeric Sb-species (massq = 121 or 123 amute) is proportional to the total Sb-flux leaving the film surface within a wide range of temperatures and surface composition. This enabled the accurate determination of the antimony incorporation coefficient, σ(Sb), and consequently the Sb-mole fraction during growth. Due to its applicability in growth conditions where σ(Sb) is less than unity, this compositional monitoring technique expands the practical range of MBE growth parameters for III-As1 − xSbx alloy growth. Real-time in situ control of alloy composition was demonstrated by implementing a feedback control scheme based on the QMS Sb signal. GaAs1 − xSbx alloys in which the Sb-fraction varied in a controlled manner in the direction of growth were deposited.

Deep levels caused by misfit dislocations in GaAsSb/GaAs heterostructures

Two deep electron traps induced by lattice mismatch in relaxed GaAs1−xSbx layers (x=0% to 3%) grown by liquid phase epitaxy (LPE) on GaAs substrates have been revealed by means of deep-level transient spectroscopy. One of the traps, that shows nonstandard, logarithmic capture kinetics and whose energy level is tied to the valence-band edge, has been related to electron states associated with α dislocations. The other trap has been attributed to the EL2 defect and possible reasons of its unexpected formation in the LPE-grown layers are briefly discussed.