Bulk GaSb Research Articles

First-principles study of effects of quantum confinement and strain on the electronic properties of GaSb nanowires

Using first-principles calculations based on density functional theory and projector augmented wave method, we investigate the electronic structures of one-dimensional wurtzite (WZ) and zinc-blende (ZB) GaSb nanowires with different diameters along the [0001] and [111] directions, respectively. The results show that the band gap of the GaSb nanowire increases as the size of the nanowire decreases due to the quantum confinement, and the band structures of the GaSb nanowires display an indirect band structures feature when the diameter of the nanowire is smaller than 3.0 nm, whereas bulk GaSb has a direct gap. Owing to the different responses of the valence band maximum/conduction band minimum energies to strain, the band structures of GaSb nanowires experiences a noticeable indirect-to-direct transition when the nanowires are under the uniaxial strain. For example, an indirect-to-direct band gap transition in the band structure of [111] ZB GaSb nanowires can be realized by applying a uniaxial tensile strain, and this transition in the band structure of [0001] WZ GaSb nanowires can take place by applying both uniaxial tensile and compression strain when the diameter of the nanowire is about 2.0 nm. In addition, it is found that carrier effective mass is dependent on the diameter of the GaSb nanowire, therefore both the electron and hole effective mass values decrease as diameter increases. It is also found that the hole effective mass is smaller than the electron effective mass for GaSb nanowires with the same directions and sizes, indicating that the hole transportation is more prominent than the electron transportation.

Native point defects in GaSb

We have applied positron annihilation spectroscopy to study native point defects in Te-doped n-type and nominally undoped p-type GaSb single crystals. The results show that the dominant vacancy defect trapping positrons in bulk GaSb is the gallium monovacancy. The temperature dependence of the average positron lifetime in both p- and n-type GaSb indicates that negative ion type defects with no associated open volume compete with the Ga vacancies. Based on comparison with theoretical predictions, these negative ions are identified as Ga antisites. The concentrations of these negatively charged defects exceed the Ga vacancy concentrations nearly by an order of magnitude. We conclude that the Ga antisite is the native defect responsible for p-type conductivity in GaSb single crystals.

Electrical characterization of a buried GaSb p‐n junction controlled by native defects

Buried GaSb junctions were induced in Te‐doped GaSb bulk crystals by growing a heavily Zn‐doped GaAs layer on GaSb. However, the p‐n junction resulted to be located much more deeply with respect to the Zn diffusion front and originated by a local rising up of native acceptor density, which controls the p‐type conductivity conversion of the GaSb substrate for about 1 μm beyond the Zn penetration depth. Admittance spectroscopy measurements supported the identification of such defects with the double native acceptor GaSb. Current‐voltage characteristics of the p‐n junction, performed after the removal of the GaAs layer, were here analyzed as a function of the temperature for different Zn doping levels and resulted consistent with the model previously proposed to explain the formation of the junctions.

Surface morphology and Bi incorporation in GaSbBi(As)/GaSb films

Several GaSbBi(As)/GaSb films were grown to investigate the effects of Bi on GaSb surface morphology and bulk composition as a function of growth conditions. Scanning electron microscopy of the surface shows several biphasic droplets consisting of Ga- and Bi-rich phases ≈1μm in diameter form on the surface. Some of these droplets exhibit more unusual features such as facets, sub-droplets, and droplet etching into the underlying film. Bi droplet coverage shows a direct increase with increasing Bi:Ga and Bi:Sb BEP ratios. Rutherford backscatter and X-ray diffraction analyses of these films show Bi concentration of up to 12% and a concurrently increasing unintentional As concentration of up to 9.3%, suggesting the presence of a strain auto-compensation mechanism during film growth. Once Bi concentration reaches 10–12%, Bi incorporation saturates, with excess Bi atoms instead accumulating in the droplets.

Admittance spectroscopy on buried GaSb junctions due to defect distribution in GaAs/GaSb metalorganic vapor phase epitaxy heterostructures

Admittance spectroscopy was employed to investigate the electrical properties of buried GaSb homojunctions. A strong p-type surface layer was obtained in n-type GaSb bulk crystals through diffusion of Zn atoms. The acceptor impurities were introduced into Te-doped substrates by epitaxially growing a heavily p(Zn)-doped GaAs layer. Current-voltage investigation, after the removal of the GaAs cap layer, demonstrated the formation of the GaSb p-n homojunction. However, the p-n interface of the junctions resulted to be not due to the presence of Zn. In fact, secondary ion mass spectrometry pointed out only a small penetration depth of Zn atoms in the GaSb substrate (∼100 nm), whereas electron beam induced current investigation demonstrated that the p-n junction interface was located more deeply into the substrate (∼1 μm). Admittance spectroscopy and capacitance-voltage investigations led to attribute the change of conduction type from n to p of GaSb beyond the Zn penetration depth to the formation of lattice acceptor defects, influenced by the thermal annealing processes during and after the GaAs growth. An attempt to explain the formation of the buried junction in terms of atomic inter-diffusion is provided, in order to justify, from a microscopic point of view, the low diffusivity of Zn in GaSb, and the depth of the p-n junction interface in the substrate.

Study of Mesa Etching for Infrared Detector Based on InAs/GaSb Superlattice

InAs/GaSb superlattice in infrared detector was grown on GaSb substrates by molecular beam epitaxy technique. Using inductively coupled plasma (ICP) etching technique and Cl2/Ar etching gas, the smooth mesa of the device was formed. The influence of etching time, Cl2 percent and RF power on the etching rate and the surface morphology of InAs bulk, GaSb bulk materials and superlattice were studied. It showed that the etching rate of InAs was lower than that of GaSb and the etching surface was smooth at Cl2 in the range of 20%~40%. The results will benefit to forming ohm contact and decrease surface leakage current in the photovoltaic detector.

Acceptors in undoped GaSb; the role of vacancy defects

The conventional lifetime setup was used to study Czochralski grown unintentionally p- and n- type ([n] ≈ 6 × 1017cm−3) GaSb bulk samples. Several approaches were used to analyze the data. However, it was not possible to successfully analyze the obtained spectrums with the conventional trapping model. From the analyzed data it was derived that the reason for p-type behavior of GaSb was not VGa. Additionally, the role of gallium vacancy was studied and it's effect to lifetime values are shortly discussed.

(Invited) InGaSb MOSFET Channel on Metamorphic Buffer: Materials, Interfaces and Process Options

High quality bulk GaSb and InGaSb quantum wells grown on GaAs substrates were MBE grown using a AlGaSb(As) metamorphic buffer layers to reduce dislocation density down to 107cm-2. Strained In0.36Ga0.64Sb quantum wells with a biaxial compressive strain of 1.8% showed the highest mobility of 1020 cm2/Vs and a low sheet resistance of 3.9 kΩ/sq. at hole sheet at hole density of 1.9x1012 cm-2. Buried channel design with an AlGaSb top barrier improved hole mobility by only 30% compared to surface QW channels. Improving the interface trap density down to 1012 cm-2eV-1 was realized using an amorphous Si layer (in-situ gate oxide) or an InAs layer (ex-situ gate) as a gate passivation. p++-GaSb epitaxial contact layers were developed and showed the leakage current of ~0.1 mA/cm2 not affected by growth defects. Critical elements of "gate-last" InGaSb MOSFET fabrication process were developed utilizing an InAs etch stop layer to avoid high temperature processing of the gate stack.

Detached phenomenon: Its effect on the crystal quality of Ga(1−x)InxSb bulk crystal grown by the VDS technique

Vertical directional solidification (VDS) technique is used on the combined growth principals of the conventional methods since 1994, which leads to the detached growth. For evaluation of the detached growth, five bulk ingots of indium doped gallium–antimonide GaSb:In (In=0.5, 0.25, 0.15) have been grown – without the seed, without contact to the ampoule wall, without coating and without external pressure. The gap is attributed to compensate the differential thermal dilatation that is grown with the reduced diameter than the diameter of the ampoule. VDS experiments have been proved that the sum of the contact angle and growth angle is large enough to allow detachment without any additional pressure difference under the melt to offset hydrostatic pressure. A meniscus forms at the bottom of the melt, the capillarity effect establishes due to which spontaneous gap could be created by the melt free surface, thus no thermal shear stress and thermo-mechanical stresses at the interface. Detached grown bulk GaSb:In crystals showed superior crystal quality with the highest physical properties and mobility than the crystals grown ever. The axial and radial composition profile of the grown GaSb:In ingots showed variation ≤10%. From the conical region, dislocation density decreases in the growth direction and reaches less than 103cm−2.

MBE growth of GaSb-based photodetectors on 6-inch diameter GaAs substrates via select buffers

GaSb-based, 6.1 Å lattice-constant, infrared photodetector materials were grown on large diameter, 6-in. GaAs substrates by molecular beam epitaxy. Multiple metamorphic buffer architectures, including bulk GaSb nucleation, AlAsSb superlattices, and graded GaAsSb ternary alloys, were investigated to bridge the 7.8% mismatch gap between the GaAs substrates and the GaSb-based epitaxial layers. Unique surface morphologies and crystal structure properties, as revealed by atomic force microscopy and cross-section transmission electron microscopy, pointed to different relaxation mechanisms for different buffer architectures. GaSb nucleation results in a more island-like surface morphology with a mix of 90° misfit and 60°-type threading dislocations, while the graded ternary buffer results in a cross-hatch surface morphology with effective filtering of the threading dislocations. Low root-mean-square roughness values of 5–20 Å were obtained for this type of metamorphic epilayer growth. A generic InAsSb/AlAsSb nBn photodiode structure, where the first “n” is the contact layer, the “B” is the wide-bandgap barrier layer, and the second “n” is the n-type narrow bandgap absorber layer, with ∼4 μm cutoff wavelength was grown on 6 in. GaAs substrates using the different metamorphic buffers. Cross-wafer optical and structural measurements showed excellent epitaxial layer uniformity, with PL wavelength variation <0.1 μm. Dark currents of 2–3 × 10−6 A/cm2 were measured on devices fabricated from the photodiode material grown on GaAs. This was about 5-times higher than measured on the same structure grown on GaSb substrates. This work demonstrates a promising path to satisfy the increasing demand for even larger area focal plane array detectors in a commercial production environment.

Electrical and interfacial properties of GaAs/GaSb metal-organic vapour phase epitaxy heterostructures

The electrical properties of GaAs/GaSb heterojunctions grown by metal-organic vapour phase epitaxy were carefully investigated. The structures were formed by heavily p(Zn)-doped GaAs layers deposited on n(Te)-doped GaSb bulk crystal used as substrates. The current-voltage characteristics showed the formation of a GaSb p-n homojunction, which was expected to be induced by Zn diffusion into GaSb. Nevertheless, secondary ion mass spectrometry pointed out a small penetration depth of Zn atoms in the GaSb substrate, resulting unaffected by post-growth annealing processes. Electron beam induced current analysis demonstrated that the p-n junction interface was located more deeply into the substrate (∼1 μm). This result was confirmed by capacitance-voltage (C-V) and electrochemical C-V characterizations. Admittance spectroscopy led to attribute the change of conduction type from n to p in GaSb to the formation of additional shallow acceptor levels, activated by GaAs growth and post-growth thermal annealing processes. An attempt to explain the formation of the buried junction in terms of atomic interdiffusion is provided, in order to justify, from the microscopic point of view, the low diffusivity of Zn in GaSb, and the apparently uncorrelated depth of the p-n junction interface in the substrate.

Photothermal deflection investigation of bulk Si and GaSb transport properties

The photo-thermal deflection technique (PTD) is used to study the transport properties such as non-radiative lifetime of minority carriers (τnr), electronic diffusivity (D) and surface recombination velocity (S) in bulk silicon (Si) and gallium antimonide (GaSb) semiconductors. A generalized one-dimensional theoretical model has been also developed, and the coincidence between experimental curves giving the normalized amplitude and phase variations versus square root modulation frequency and the corresponding theoretical curves makes possible to deduce the electronic parameters cited above.

Photothermal Investigation of Te-Doped Bulk Gasb: Correlation between Non-Radiative Lifetime and Thermal Diffusivity

Photothermal deflection PTD is used in order to reveal the influence of Te doping on non radiative carrier lifetime for GaSb bulk sample when illuminated by a modulated and monochromatic light beam. Theoretical simulations are obtained from an adapted theoretical model, based on the resolution of both heat and carrier diffusion equations. Auger recombination coefficient obtained from the linear relation between lifetime and the inverse of squared concentration is in agreement with literature. Moreover, it is found that low n-doping reduces the surface recombination velocity S because of dangling bonds neutralisation at the surface. However for higher doping concentration, S is enhanced by Te doping.

Thermal Conductivity of InAs/GaSb Type II Superlattice

The cross-plane thermal conductivity of a type II InAs/GaSb superlattice (T2SL) is measured from 13 K to 300 K using the 3{\omega} method. Thermal conductivity is reduced by up to 2 orders of magnitude relative to the GaSb bulk substrate. The low thermal conductivity of around 1-8 W/m\cdotK may serve as an advantage for thermoelectric applications at low temperatures, while presenting a challenge for T2SL quantum cascade lasers and high power light emitting diodes. We introduce a power-law approximation to model non-linearities in the thermal conductivity, resulting in increased or decreased peak temperature for negative or positive exponents, respectively.

Evaluation of the bond polarizabilities of zincblende‐type semiconductors: Application to the Raman spectra of disordered GaSb/AlSb (001) superlattices

AbstractWe derive expressions for the bond polarizability (BP) parameters involving only directly measurable quantities, such as dielectric and elasto‐optic constants of the bulk crystal and apply them to zincblende‐type crystals. The vibrational modes are calculated on the basis of an eleven (11) parameter rigid‐ion model approximation (RIM). Having estimated the BP parameters for GaSb and AlSb bulk crystals, we calculate the Raman spectra away of resonance conditions for the vibrations of perfect and disordered (GaSb)1/(AlSb)1 (001) superlattices (SL). The disordered SL is approximated with primitive cells much larger than the primitive cell of the perfect 1 × 1 SL. Furthermore, we show that disorder modifies the Raman spectra of the perfect SL by introducing asymmetry as well as by the formation of additional peaks in both the acoustic and optical range of the Raman spectra. Puzzlingly, even a small degree of disorder, results in a blueshift of all modes frequencies, especially the strongest optical ones.

Photopyroelectric back detection configuration for thermal diffusivity measurement of undoped and doped GaSb semiconductors

Photopyroelectric technique in back detection configuration (BPPE) was used to investigate thermal diffusivity measurement of undoped and doped GaSb bulk samples. A theoretical model was developed and specific design of pyroelectric sensor based on PVDF film of 25μm thickness was constructed. The methodology is based on a designed back photopyroelectric detection cell and thermal diffusivity value is obtained directly as a result of a scan of the BPPE phase and amplitude signal as a function of square root modulation frequency.This method involves the thermally thick limit of the pyroelectric signal in the standard BPPE. To avoid the conventional requirement for set-up transfer function a signal normalization procedure is implemented.Obtained thermal diffusivity coefficients by means of this technique are 16.7 ± 0.5*10−6 m2s−1, 6.3 ±0.3 *10−6 m2s−1, and 7.3 ± 0.2 *10−6 m2s−1 respectively for undoped GaSb, Te- doped GaSb and Zn-doped GaSb semiconductors.By studying samples of GaSb with different types of doping (doping with Te and with Zn) we notice a change in the thermal diffusivity function of doping.

Relaxation dynamics and residual strain in metamorphic AlSb on GaAs

We have observed the evolution of the accumulated stress during heteroepitaxial growth of highly lattice mismatched AlSb on GaAs by measuring the deformation of the substrate as a function of time. High resolution transmission electron microscopy images show almost all of the plastic relaxation is accommodated by an array of 90° misfit dislocations at the interface. The in-plane lattice parameter of the resulting metamorphic AlSb is slightly smaller (0.3%) than the bulk value and perfectly matches the lattice parameter of bulk GaSb. It is, therefore, possible to grow nearly stress-free GaSb on GaAs using a metamorphic AlSb buffer layer.

First-principles study of (InAs)1/(GaSb)1 superlattice nanowires

As the active areas and the connection part, semiconductor nanowires have ideal shapes which are beneficial to restricting the electron motion and atomic periodicity to one one-dimensional structure. The effective selection of material components in nanowires can enhance the advantages of low-dimensional structures by analyzing the features of bulk materials. Furthermore, the nanowire properties can also be tailored by controlling the internal or intrinsic characteristics such as diameters, crystallographic growth direction, structural phase, surface crystallographic plane or saturation degree, and by applying external influences such as electric, magnetic, thermal and force fields. The bulk InAs and GaSb have approximate lattice constants, thereby resulting in small lattice mismatch for InAs/GaSb heterostructures that can finally be grown into excellent infrared optoelectronic materials. Moreover, the bulk InAs has the lowest electron effective mass in binary III-V compound semiconductors, leading to high transport features for electrons distributing most in InAs layers of InAs/GaSb superlattices. In the present work, the zinc-blend (InAs)1/(GaSb)1 superlattice nanowires (subscript denotes the number of molecular or double-atomic layers) with [001] and [111] crystallographic wire-axes have been studied by first-principles calculations for their structural, electronic and mechanical properties together with the rule of different nanowire diameters (from ～0.5 to ～2.0 nm). We also analyze the stress effects from external forces and examine the electronic band-edge changes with strain in wire-axis direction to determine the deformation potentials.

Optical Properties of GaSb Nanofibers.

Amorphous GaSb nanofibers were obtained by ion beam irradiation of bulk GaSb single-crystal wafers, resulting in fibers with diameters of ~20 nm. The Raman spectra and photoluminescence (PL) of the ion irradiation-induced nanofibers before and after annealing were studied. Results show that the Raman intensity of the GaSb LO phonon mode decreased after ion beam irradiation as a result of the formation of the amorphous nanofibers. A new mode is observed at ~155 cm-1 both from the unannealed and annealed GaSb nanofiber samples related to the A1g mode of Sb–Sb bond vibration. Room temperature PL measurements of the annealed nanofibers present a wide feature band at ~1.4–1.6 eV. The room temperature PL properties of the irradiated samples presents a large blue shift compared to bulk GaSb. Annealed nanofibers and annealed nanofibers with Au nanodots present two different PL peaks (400 and 540 nm), both of which may originate from Ga or O vacancies in GaO. The enhanced PL and new band characteristics in nanostructured GaSb suggest that the nanostructured fibers may have unique applications in optoelectronic devices.

Minority carrier lifetime in type-2 InAs–GaSb strained-layer superlattices and bulk HgCdTe materials

Minority carrier lifetime, τ, in type-2 strained-layer superlattices (SLSs) and in long-wave Hg0.78Cd0.22Te (MCT) was measured by optical modulation response technique. It was shown that at 77 K radiative recombination can contribute to the measured τ values. The Shockley–Read–Hall (SRH) lifetimes were attained as 100 ns, 31 ns, and more than 1 μs for midwave infrared superlattices, long-wave infrared (LWIR) superlattices, and MCT correspondingly. The nature of the difference between the SRH lifetimes in LWIR superlattice and MCT is discussed.