GaAs Band Gap Research Articles

Description of excitonic absorption using the Sommerfeld enhancement factor and band-fluctuations

Abstract One of the challenges of excitonic materials is the accurate determination of the exciton binding energy and bandgap from optical measurements. The difficulty arises from the overlap of the discrete and continuous excitonic absorption at the band edge. Many researches have modeled the shape of the absorption edge of such materials on the seminal formulation proposed by Elliott in 1957 (Phys. Rev. 108 1384–9) and its several modifications such as non-parabolic bands, magnetic potentials and electron–hole-polaron interactions. However, exciton binding energies obtained from optical absorption often vary strongly depending on the chosen ‘Elliott formula’. Here, we propose an alternative and rather simple approach, which has previously been successful in the determination of the optical bandgap of amorphous, direct and indirect semiconductors, based on the band-fluctuations (BFs) model. In this model, the fluctuations due to disorder, temperature or lattice vibrations give rise to the well known exponential shape of band tail states. The formulation results in an analytic equation for the fundamental absorption with 6 parameters only. To test it, the binding energy and optical bandgap of GaAs and the family of tri-halide perovskites ( MAPbX 3 ), X = Br , I , Cl , over a wide range of temperatures, are obtained by fitting the modified Elliott model. The results for the bandgap, linewidth and exciton binding energy are in good agreement with reports based on non-optical measurements. Moreover, due to the polar nature of perovskites, the retrieved binding energies can be compared with those computed with a model proposed by Kane (1978 Phys. Rev. B 18 6849). In the latter model, the exciton is surrounded by a cloud of virtual phonons interacting via the Frölich interaction. As a consequence, the upper bound for the binding energy of the exciton-polaron system can be estimated. These results are in good agreement with the optical parameters obtained with the proposed Elliott equation including BFs.

Experimental demonstration of frequency- downconverted arm-length stabilization for a future upgraded gravitational wave detector.

Ground-based laser interferometric gravitational wave detectors (GWDs) consist of multiple optical cavity systems whose lengths need to be interferometrically controlled. An arm-length stabilization (ALS) system has played an important role in bringing these interferometers into an operational state and enhancing their duty cycle. The sensitivity of these detectors can be improved if the thermal noise of their test mass mirror coatings is reduced. Crystalline AlGaAs coatings are a promising candidate for this. However, the current ALS system with a frequency-doubled 532 nm light is no longer an option with AlGaAs coatings because the 532 nm light is absorbed by AlGaAs coatings due to the narrow bandgap of GaAs. Therefore, alternative locking schemes must be developed. In this Letter, we describe an experimental demonstration of a novel ALS scheme, to the best of our knowledge, which is compatible with AlGaAs coatings. This ALS scheme will enable the use of AlGaAs coatings in current and future terrestrial gravitational wave detectors.

III-V material-based junction-free L-shaped gate normal line tunneling FET for improved performance

In this paper, we introduce a novel III–V compound material-based junction-free (JF) L-shaped gate normal line tunneling field-effect transistor (III–V JF L GNLTFET) for improved output performance at 0.5 V operation. The key design metric, i.e. JF or junctionless design, in our device eliminates issues like random dopant fluctuations (RDF) and high thermal budgets and streamlines the fabrication. The implementation of III–V compound material, i.e. low bandgap compound GaSb, in the source region, combined with the larger area gate normal line tunneling, improves the ON current for our proposed III–V JF L GNLTFET device. Additionally, the utilization of large bandgap GaAs compounds on the drain and channel sides eliminates ambipolarity and further enhances the performance of our proposed device. Meaning that the proposed device simultaneously improves the ON current and suppresses the ambipolarity. Our proposed III–V JF L GNLTFET exhibits enhanced output performance with an ON current of 23.2 μA μm−1 and a minimum and average subthreshold swing of 3.7 mV dec−1 and 15.82 mV dec−1 respectively. Furthermore, the proposed III–V JF L GNLTFET also gives superior RF/analog performance with transconductance (168.65 μS), cut-off frequency (33.52 GHz), gain-bandwidth product (5.11 GHz), and transconductance-frequency product (243.7 GHz).

Modification of GaAs(001)-β2(2x4) surface by Pd-decoration: A DFT study

Photoelectric modification mechanisms of Pd-decoration on GaAs(001)-β2(2 × 4) surface were examined through first-principles calculations based on density functional theory (DFT). Among all purposed Pd-decorated GaAs(001)-β2(2 × 4) models, seven demonstrated have good stability. Due to the adsorption of Pd atoms, band gap and work function of GaAs(001)-β2(2 × 4) surface decreased and further enhanced the photoemission capability of the structure. The evidence showed that the reduced transmission resistance and transmission loss of Pd-decorated GaAs(001)-β2(2 × 4) structure were due to the formation of the localized internal polarization field resulted from charge transfer between Pd and GaAs. When electron come across the Pd/GaAs interface, the internal electric field could accelerate electron passing though in just one dimension, which could reduce the interface recombination and enhance interface electron transmission capability. Additionally, Pd-decoration led to a red-shift in the absorption edge and enhanced light adsorption capability in the range of 0.7 to 1.7 eV. The optoelectronic modification effect could be utilized to address the interface transmission losses in multi-junction solar cells, and further optimize the solar cell performance.

Separated Electronic and Strain Interfaces in Core/Dual‐Shell Nanowires: Unlocking the Potential of Strained GaAs for Applications Across Near‐Infrared

AbstractSemiconductor nanowires have inspired plenty of novel nanotechnology device concepts in photonics, electronics, and sensing, owing to their unique functionalities and integrability in heterogeneous platforms. Lattice‐mismatched core/shell heterostructures, in particular, open new avenues for strain engineering and material properties modification. A notable case is the widely tunable tensile strain in the core of GaAs/InxAl1‐xAs core/shell nanowires, which can be used to tailor the GaAs bandgap for applications across near‐infrared, like optical fiber telecommunication, imaging, photovoltaics, etc. As it is shown here, though, the bandgap narrowing under high tensile strain in the GaAs core is accompanied by fast non‐radiative recombination, which is undesirable for any device application. The limiting role of the lattice‐mismatched core/shell interface is revealed, and a novel core/dual‐shell heterostructure that employs an intermediate AlyGa1‐yAs shell (spacer) is proposed. This spacer decouples the GaAs/AlyGa1‐yAs interface, which confines electrons and holes into GaAs, from the lattice‐mismatched AlyGa1‐yAs/InxAl1‐xAs one, whereas the strain in GaAs is unaffected. Choosing the optimal spacer thickness, the photoluminescence yield increases significantly, with longer emission decay lifetimes and slower carrier cooling rates. Besides unlocking the potential of GaAs for photonic applications across near‐infrared, the proposed heterostructure concept can also be adopted for other material systems.

Temperature Dependency of Photoelectronic Properties of Group III-V Arsenide Solar Cell

This study explores the effect of temperature on different characteristics of Solar Cells (SC) composed of a structured III-V arsenide group. The temperature dependence of the SC characteristics was investigated numerically and by simulation. In both approaches, each characteristic was compared with a conventional Si SC. InAs showed superior stability and lower temperature sensitivity, as it has a negligible decrease of 0.098 eV in the energy bandgap, while the energy bandgaps of Si, AlAs, and GaAs are 0.129, 0.186, and 0.200 eV, respectively. Moreover, with a decay rate of 81.911 mV/°K, InAs exhibited the lowest temperature sensitivity in open-circuit voltage. InAs additionally demonstrated the least increase in degradation rate, while the SC power output is still a cause of concern. AlAs, Si, and GaAs had a total accumulative gradient change of 0.162, 0.136, and 0.034% in the degradation rate, respectively, while InAs showcased the highest stability by displaying a change of only 0.008%. A comparative analysis illustrated that among these III-V arsenide compounds, InAs had a rock-bottom sensitivity to temperature changes and better temperature stability in both numerical and simulation approaches.

Fabrication and interface properties of amorphous Ga2O3/GaAs heterojunction

Heterojunctions based on the combination of wide and narrow bandgap materials provide a basis for realizing high-performance electronic and optoelectronic devices. So, it is crucial to study the interface properties of heterojunctions. Inspired by this idea, an ultra-wide bandgap Ga2O3 film was grown on a narrow bandgap GaAs by atomic layer deposition (ALD) in this work. The morphology was analyzed by atomic force microscope (AFM), showing an excellent uniformity of the film. The thickness of Ga2O3 film was estimated to be 80 nm by Ellipsometry, and its unique adsorption in the solar blind wavelength range was also investigated. X‐ray photoelectron spectroscopy (XPS) was used to characterize the band alignment, and elememtal composition. The value of the valence band offset was 3.15 ± 0.04 eV, and the conduction band offset was 0.25 ± 0.02 eV. The work functions of Ga2O3, and GaAs were calculated by first principles based on density functional theory (DFT), building the carrier immigration mode at the interface of Ga2O3/GaAs heterojunction combined with the band alignment. The band configurations of Ga2O3/GaAs heterojunction illustrate the possibility of realizing high-speed photodetectors. Furthermore, the band offset provides the reference value of barrier height, facilitating the design of electronic and optoelectronic devices based on this heterojunction.

Numerical Study of High Spectral Efficiency and High-Temperature Energy Harvesting Metamaterial Emitter to Improve Thermophotovoltaic Performance

Design work was done on a broadband and wide-angle selective pyramid metamaterial emitter. COMSOL Multiphysics software was used to study the emitter, which was made of tungsten and aluminum nitride. The width of the unit cell and the tungsten ground thickness were fixed while other geometric parameters, such as the base length of the pyramid, the height of the pyramid, and the thickness of the dielectric, were tuned to produce the emitter’s desired broadband emission. A high average emissivity over 0.96 below the cutoff wavelength (0.1 μm-2.2 μm) was seen in the numerical simulation. The developed metamaterial emitter also had good emissivity across a broad range of incidence angles, from 0° to 60°, and was polarization independent. In addition, the planned emitter for the InGaAs cell has a better spectral efficiency than the blackbody other designed emitter. In general, the planned selective nanopyramid emitter was realized with 75% spectrum efficiency for InGaAs band gap energy (0.55 eV) at 1200 K, which was greater than blackbody and prior papers.

Conduction characteristics of n-InAs/n-GaAs heterojunctions with misfit dislocations

We investigated the transport properties of an n-InAs/n-GaAs heterojunction, where misfit dislocations are confined at the hetero-interface by forming a misfit dislocation network. The electric current I across the interface from n-InAs to n-GaAs was measured as a function of applied voltage V a. I is strongly suppressed at up to V a ∼ 1.0 V which is larger than the intrinsic conduction band offset between InAs and GaAs.I increases exponentially at low and high V a (=0.0–0.5 and 1.0–1.2 V), while the increase of I is relatively moderate at intermediate voltage V a (=0.5–1.0 V). We theoretically evaluated the I–V a characteristics of the n-InAs/n-GaAs heterojunction by using the thermionic-field emission model and examined the effects of the interface states due to the misfit dislocations. The comparison of the calculated results with the experimental data indicates the existence of acceptor-like interface states in the band gap of GaAs.

Tuning spin-orbit coupling and realizing inverse persistent spin helix by an extra above-barrier radiation in a GaAs/Al0.3Ga0.7As heterostructure.

A persistent spin helix with equal strength of the Rashba and Dresselhaus spin-orbit coupling (SOC) is expected for future spintronic devices due to the suppression of spin relaxation. In this work we investigate the optical tuning of the Rashba and Dresselhaus SOC by monitoring the spin-galvanic effect (SGE) in a GaAs/Al0.3Ga0.7As two dimensional electron gas. An extra control light above the bandgap of the barrier is introduced to tune the SGE excited by a circularly polarized light below the bandgap of GaAs. We observe different tunability of the Rashba- and Dresselhaus-related SGE currents and extract the ratio of the Rashba and Dresselhaus coefficients. It decreases monotonously with the power of the control light and reaches a particular value of ∼-1, implying the formation of the inverse persistent spin helix state. By analyzing the optical tuning process phenomenologically and microscopically, we reveal greater optical tunability of the Rashba SOC than that of the Dresselhaus SOC.

A 2D Bismuth-Induced Honeycomb Surface Structure on GaAs(111).

Two-dimensional (2D) topological insulators have fascinating physical properties which are promising for applications within spintronics. In order to realize spintronic devices working at room temperature, materials with a large nontrivial gap are needed. Bismuthene, a 2D layer of Bi atoms in a honeycomb structure, has recently attracted strong attention because of its record-large nontrivial gap, which is due to the strong spin-orbit coupling of Bi and the unusually strong interaction of the Bi atoms with the surface atoms of the substrate underneath. It would be a significant step forward to be able to form 2D materials with properties such as bismuthene on semiconductors such as GaAs, which has a band gap size relevant for electronics and a direct band gap for optical applications. Here, we present the successful formation of a 2D Bi honeycomb structure on GaAs, which fulfills these conditions. Bi atoms have been incorporated into a clean GaAs(111) surface, with As termination, based on Bi deposition under optimized growth conditions. Low-temperature scanning tunneling microscopy and spectroscopy (LT-STM/S) demonstrates a well-ordered large-scale honeycomb structure, consisting of Bi atoms in a √3 × √3 30° reconstruction on GaAs(111). X-ray photoelectron spectroscopy shows that the Bi atoms of the honeycomb structure only bond to the underlying As atoms. This is supported by calculations based on density functional theory that confirm the honeycomb structure with a large Bi-As binding energy and predict Bi-induced electronic bands within the GaAs band gap that open up a gap of nontrivial topological nature. STS results support the existence of Bi-induced states within the GaAs band gap. The GaAs:Bi honeycomb layer found here has a similar structure as previously published bismuthene on SiC or on Ag, though with a significantly larger lattice constant and only weak Bi-Bi bonding. It can therefore be considered as an extreme case of bismuthene, which is fundamentally interesting. Furthermore, it has the same exciting electronic properties, opening a large nontrivial gap, which is the requirement for room-temperature spintronic applications, and it is directly integrated in GaAs, a direct band gap semiconductor with a large range of (opto)electronic devices.

Analysis of mixed optical transitions in dilute magnetic AlAs/GaAs/GaMnAs quantum wells grown on high substrate index by molecular beam epitaxy

In this study, we report the influence of high-index substrate orientation on the optical properties of dilute Mn containing a single quantum well (QW) structure grown by molecular beam epitaxy (MBE). The electronic band structure of the samples is calculated with finite element methods (FEM). Photomodulated reflectance (PR) spectroscopy is employed to determine the optical transition energies at room temperature. PR experimental results are analyzed with the Third Derivative Functional Form (TDFF) signal function. TDFF fitting shows that the optical transition starts below the fundamental GaAs bandgap energy for the samples grown on (311) B and (411) B-oriented substrates. Comparing TDFF fitting and calculated optical transition energies with FEM show that the optical transition occurs in type-I and type-II as direct and indirect transitions, respectively. Type-I and -II optical transitions are addressed concerning the electronic band structure of the sample grown on oriented substrates.

Пленение излучения и субзонный пик в спектрах квантового выхода фотоэмиссии из p-GaAs(Cs,O)

In the spectra of the photoemission quantum yield of p-GaAs(Cs,O) measured in the reflection geometry, a peak was found at a photon energy lower than the band gap of GaAs. It is shown that the appearance of the peak is due to the trapping of weakly absorbed radiation, which scatters diffusely on the rough back face of the epitaxial structure. Possible microscopic mechanisms of the appearance of the peak are discussed: the Franz-Keldysh effect in the surface electric field and adsorption-modified optical transitions in cesium adatoms.

Light trapping and subbandgap maximum in photoemission quantum yield spectra of p-GaAs(Cs,O)

In the spectra of the photoemission quantum yield of p-GaAs(Cs,O) measured in the reflection geometry, a peak was found at a photon energy lower than the band gap of GaAs. It is shown that the appearance of the peak is due to the trapping of weakly absorbed radiation, which scatters diffusely on the rough back face of the epitaxial structure. Possible microscopic mechanisms of the appearance of the peak are discussed: the Franz-Keldysh effect in the surface electric field and adsorption-modified optical transitions in cesium adatoms. Keywords: photoemission, GaAs, negative electron affinity, quantum yield spectra.

Machine learning-aided band gap prediction of semiconductors with low concentration doping.

The important physical quantities of materials, such as band gap, have been predicted efficiently with the help of machine learning (ML), in addition to the widely used experimental or computational methods. Under this scheme, by combining density functional theory (DFT) calculations and ML predictive models, the band gaps of doped semiconductors with normal doping concentrations are predicted successfully. Our present work provides a solution to the problem of how to obtain the band gaps of semiconductors doped with extremely low concentrations, which are important cases for some device designs. The structures were constructed by configuration screening with a symmetric criterion, and three-dimensional spatial structural variation was mapped to one-dimensional features, which are the key steps for the ML predictive model. The biggest error in the predicted band gaps of dilute nitride-doped GaAs by ML models is not more than 10%, compared with values obtained from DFT. Considering the limitation of material data, a few-shot learning method was further adopted to check the performance of the predictive models. The performance of the ML models was validated using data out of training and testing datasets. Our method will efficiently accelerate the prediction of physical properties of semiconductors with extremely low-concentration doping.

Temperature-resilient anapole modes associated with TE polarization in semiconductor nanowires

Polarization-dependent scattering anisotropy of cylindrical nanowires has numerous potential applications in, for example, nanoantennas, photothermal therapy, thermophotovoltaics, catalysis, sensing, optical filters and switches. In all these applications, temperature-dependent material properties play an important role and often adversely impact performance depending on the dominance of either radiative or dissipative damping. Here, we employ numerical modeling based on Mie scattering theory to investigate and compare the temperature and polarization-dependent optical anisotropy of metallic (gold, Au) nanowires with indirect (silicon, Si) and direct (gallium arsenide, GaAs) bandgap semiconducting nanowires. Results indicate that plasmonic scattering resonances in semiconductors, within the absorption band, deteriorate with an increase in temperature whereas those occurring away from the absorption band strengthen as a result of the increase in phononic contribution. Indirect-bandgap thin ([Formula: see text]) Si nanowires present low absorption efficiencies for both the transverse electric (TE, [Formula: see text]) and magnetic (TM, [Formula: see text]) modes, and high scattering efficiencies for the TM mode at shorter wavelengths making them suitable as highly efficient scatterers. Temperature-resilient higher-order anapole modes with their characteristic high absorption and low scattering efficiencies are also observed in the semiconductor nanowires ([Formula: see text] nm) for the TE polarization. Herein, the GaAs nanowires present [Formula: see text] times greater absorption efficiencies compared to the Si nanowires making them especially suitable for temperature-resilient applications such as scanning near-field optical microscopy (SNOM), localized heating, non-invasive sensing or detection that require strong localization of energy in the near field.

Design and analysis of a dual gate tunnel FET with InGaAs source pockets for improved performance

In this work, we propose a novel dual gate tunnel FET (TFET) with an improved ON current. The source pockets of the proposed TFET are designed with a narrow bandgap InGaAs material. As a result, the tunnel width is minimized and a large number of charge carriers will tunnel via the source-channel junction. The DC parameters, as well as RF figures of merit, are analyzed for the proposed TFET. The results have shown that the proposed TFET exhibits a 3.8x increment in ON current, a 51.75% decrement in threshold voltage, and a 50.67% reduction in the subthreshold swing as compared to the existing device. The proposed device has reported a transconductance of 15.4 mS/μm, an output conductance of 8.4 mS/μm, a cut-off frequency of 22.35 THz, and a gain-bandwidth-product of 5.11 THz at a supply voltage of 1 V. Further, temperature stability analysis is carried out to study its impact on DC parameters of the proposed TFET.

Energy-Efficient III-V Tunnel FET-Based Synaptic Device with Enhanced Charge Trapping Ability Utilizing Both Hot Hole and Hot Electron Injections for Analog Neuromorphic Computing.

A charge trap device based on field-effect transistors (FET) is a promising candidate for artificial synapses because of its high reliability and mature fabrication technology. However, conventional MOSFET-based charge trap synapses require a strong stimulus for synaptic update because of their inefficient hot-carrier injection into the charge trapping layer, consequently causing a slow speed operation and large power consumption. Here, we propose a highly efficient charge trap synapse using III-V materials-based tunnel field-effect transistor (TFET). Our synaptic TFETs present superior subthreshold swing and improved charge trapping ability utilizing both carriers as charge trapping sources: hot holes created by impact ionization in the narrow bandgap InGaAs after being provided from the p+-source, and band-to-band tunneling hot electrons (BBHEs) generated at the abrupt p+n junctions in the TFETs. Thanks to these advances, our devices achieved outstanding efficiency in synaptic characteristics with a 5750 times faster synaptic update speed and 51 times lower sub-fJ/um2 energy consumption per single synaptic update in comparison to the MOSFET-based synapse. An artificial neural network (ANN) simulation also confirmed a high recognition accuracy of handwritten digits up to ∼90% in a multilayer perceptron neural network based on our synaptic devices.

Nanoscale Impact Ionization and Electroluminescence in a Biased Scanning-Tunneling-Microscope Junction

Electroluminescence from a p-type GaAs(110) surface was induced by tunneling electrons in a scanning tunneling microscope under both polarities of bias voltage. The optical spectra exhibit a polarity-independent luminescence peak at 1.47 eV resulting from the exciton recombination. However, the quantum yield of photon emission at negative bias voltage is two orders of magnitude weaker than that at positive bias voltage. Moreover, the luminescence at negative bias voltage shows the linear dependence of bias voltage, distinct from the rapid rise due to resonant electron injection at positive bias. Furthermore, the threshold bias voltage for electroluminescence at negative bias is nearly twice the bandgap of GaAs, not simply satisfying the energy conservation for the creation of an electron-hole pair. Through theoretical calculation, we propose an impact ionization model to nicely explain the newly observed electroluminescence at negative bias voltage. We believe that this mechanism of impact ionization could be readily applied to other nanoscale optoelectronics including 2D semiconductors and 1D nanostructures.

Modeling free-carrier absorption in ultrathin III-V solar cells with light management.

Ultrathin III-V solar cells with proper light management have become more attractive than their optically thick counterparts as they are less expensive and lightweight, can maintain photon absorption, and have high radiation tolerance for space-related applications. Comprehensive optical modeling efforts have provided pathways to improve device efficiency in ultrathin GaAs solar cells with light trapping structures. Usually, the absorption mechanism known as free-carrier absorption (FCA) is ignored in these models due to the ultrathin layers and the direct bandgap of GaAs. This manuscript reports the significance of considering FCA as a parasitic loss caused by the optical enhancement in highly doped non-active layers between the ultrathin solar cell and backside light trapping structures. We model FCA based on Drude theory in a p-type AlGaAs layer behind ultrathin GaAs solar cells with a planar mirror and cylindrical gratings. Our results show that, depending on the AlGaAs thickness and doping concentration, free carriers will absorb transmitted photons and reduce the backside reflectance, degrading the current and voltage output from ideal conditions. One example shows that for a 300 nm-thick GaAs solar cell, the Ag mirror's peak reflectance decreases nearly 12% when the AlGaAs back layer is 800 nm-thick at a doping concentration of 4x1019 cm-3. Notably, the cylindrical grating designs with 38.5%, 46.5%, and 64.9% AlGaAs coverage resulted in an absolute efficiency reduction of 0.6%, 1.8%, and 2.9% at a doping concentration of 4x1019 cm-3, respectively. This novel study demonstrates that FCA in non-active layers must be properly addressed in the device design to progress the efficiency of ultrathin III-V solar cells with light trapping structures.