GaAs Layers Research Articles

Influence of an ultra-thin buffer layer on the growth and properties of pseudomorphic GaAsBi layers

A series of 100 nm-thick pseudomorphic GaAsBi layers with the Bi content varying from 0.97 to 11.2% have been grown by molecular beam epitaxy (MBE) on the semi-insulating GaAs(100) substrates buffered with an ultra-thin up to 20 nm thick GaAs layer. The main attention in this work was focused on the investigation of relaxation in the Bi induced compressively-strained GaAsBi layers containing a various content of Bi. The lattice parameters of GaAs- Bi compound and the Bi concentration have been evaluated from high resolution X-ray diffraction measurements. The relaxation values of GaAsBi layers ranging from 0.4 to 3.5% were obtained analyzing the symmetric and asymmetric reciprocal space maps of (004) and (115) planes, respectively. Also, the complex study was performed to clarify the relaxation effect on structural, morphological and optical properties of bismide layers. Optical measurements revealed a significant reduction of the energy band gap from 1.34 to 0.92 eV for the layers containing 0.97–8.6% of Bi in the GaAs lattice.

MOCVD growth and thermal stability analysis of 1.2 µm InGaAs/GaAs multi quantum well structure

InGaAs/GaAs multiple quantum well (MQW) is desirable for many optoelectronic devices with the wavelength around 1.2 µm. However, the high defect density and instability caused by high strain have always been the problems. In this paper, an InGaAs/GaAs MQW structure was grown on GaAs substrate at low temperature by metal-organic chemical vapor deposition (MOCVD) technology. The InGaAs layers were grown with triethylgallium (TEGa) and trimethylgallium (TMGa), respectively. We studied the stability of InGaAs/GaAs MQW by evaluating the photoluminescence (PL) peak shift after annealing. It was found that the peaks of MQW grown by TEGa and TMGa were both blue-shifted after annealing. A smaller shift (<4 nm) in the MQW grown with TEGa exhibits better thermal stability compared to that of MQW grown with TMGa. This could be due to the weaker indium (In) and gallium (Ga) interdiffusion at the InGaAs/GaAs interface for the sample grown with TEGa. Furthermore, we fabricated a vertical external cavity surface emitting laser (VECSEL) based on the 6-period InGaAs/GaAs MQW grown with TEGa with an emission wavelength of ~1.2 µm.

Pseudo-dielectric function spectra of the near surface layer of GaAs implanted with various fluence of Xe+ ions

Monocrystalline (100) semi-insulating gallium arsenide (GaAs) samples were irradiated with a 250-keV Xe+ ion beam at room temperature. To investigate the effect of ion irradiation on GaAs samples, the ion fluence was varied from 1 × 1012 to 3 × 1016 ions/cm2. The spectroscopic ellipsometry (SE) method and the Rutherford backscattering spectrometry (RBS) with nuclear reaction (NR) method were used to determine the properties of the virgin and implanted samples. The SE approach reveals that the pseudo-dielectric function of implanted GaAs samples varies with ion fluence. The RBS approach, which exposes the depth-profile of As, Ga, and Xe in the implanted samples, allows us to correlate changes in the pseudo-dielectric function with structural modifications caused by the ion irradiation. The NR approach points out the existence of an oxygen-enriched layer on the surface of implanted GaAs samples. Lastly, the optical study involving the Cauchy and Urbach dispersion model measures the refractive index n and extinction coefficient k of the native oxide layer on the surface of GaAs samples.

Determination of the far-infrared dielectric function of a thin InGaAs layer using a detuned Salisbury screen

We present a method to determine the far-infrared dielectric function parameters of a thin In0.53Ga0.47As layer. We use a detuned Salisbury screen configuration to enhance the interaction of far infrared light with optical phonons in the InGaAs layer. From polarized angle-resolved reflectance spectrum and Raman spectroscopy, we obtain experimental data that we adjust using a dielectric function model fulfilling causality. We provide a complete set of parameters for an analytic expression of In0.53Ga0.47As dielectric function in the optical phonon frequency range and deduce a value for the static dielectric constant.

Ultrabroadband terahertz time-domain spectroscopy using III-V photoconductive membranes on silicon

Electromagnetic waves in the terahertz (THz) frequency range are widely used in spectroscopy, imaging and sensing. However, commercial, table-top systems covering the entire frequency range from 100 GHz to 10 THz are not available today. Fiber-coupled spectrometers, which employ photoconductive antennas as emitters and receivers, show a bandwidth limited to 6.5 THz and some suffer from spectral artifacts above 4 THz. For these systems, we identify THz absorption in the polar substrate of the photoconductive antenna as the main reason for these limitations. To overcome them, we developed photoconductive membrane (PCM) antennas, which consist of a 1.2 µm-thin InGaAs layer bonded on a Si substrate. These antennas combine efficient THz generation and detection in InGaAs with absorption-free THz transmission through a Si substrate. With these devices, we demonstrate a fiber-coupled THz spectrometer with a total bandwidth of 10 THz and an artifact-free spectrum up to 6 THz. The PCM antennas present a promising path toward fiber-coupled, ultrabroadband THz spectrometers.

Perfect selective metamaterial absorber with thin-film of GaAs layer in the visible region for solar cell applications

In this paper, we have presented a new design of a metamaterial perfect absorber (MPA) consisting of three layers of metal-dielectric-metal in which the top layer is considered of special kind square patches at different places in a unit cell. This MPA exhibits wideband, wide-angle, and polarization-independent absorption performance in the visible region. The proposed MPA structure is composed of a resonator of metal and spacer of a dielectric layer. The interactions between the resonator and the coupling of metal-dielectric create the plasmonics effects which are responsible for the perfect absorption. Under a specific condition, this simulated absorber structure exhibits an extremely high broadband absorption between 591.54 and 704.40 nm wavelength range with near-unity absorption, and a single peak observed at 385.33 nm with the absorption of 94.16%. We extracted the impedance of the absorber and matched it with free space, and also demonstrated the effective permittivity and permeability. Moreover, the parametric study of the resonators, dielectric layer, and multi-band topology has also been investigated. The polarization-insensitive-based metamaterial may be utilized to improve the efficiency of different devices in the visible range. Furthermore, we have calculated the absorption of the proposed MPA under solar radiation (AM1.5) for different structural parameters. The proposed absorber greatly enhances the conversion efficiency which is highly useful for solar cells. We also determined the short circuit current density of this absorber for different thicknesses of the GaAs layer. The meta-surface of Al metal provides a good performance in comparison to other costly metals and the proposed structure may be used for different devices.

Evidence of Delta Phase of Fe in MBE-Grown Thin Epitaxial Films on GaAs

Fe/GaAs is an important system for the study of spin injection behavior that can vary with the nature and interfaces of Fe films. Here, we investigate the effect of interfacial strain on the microstructure, interfaces and phase-formation behavior in epitaxially grown Fe films. To vary the strain, we have characterized Fe films of various thicknesses ranging from 10 to 1000 nm which were grown using molecular beam epitaxy on GaAs (011) and AlGaAs (001) substrates. High resolution X-ray diffraction studies revealed that films with higher thicknesses exhibited an equilibrium α-Fe phase, while the films with less than 10 nm thicknesses indicated the presence of δ-Fe. Transmission electron microscopy revealed the interface for 10-nm-thick films had strain lobes with no interfacial phase formation for films deposited at room temperature. At a higher deposition temperature of 175 °C, similar strain lobes were observed for a 10-nm-thick film. Extended annealing at 200 °C transformed the metastable δ-Fe phase to an equilibrium α-Fe. However, at higher temperature, the interface contained an intermixing layer of (FeAl)GaAs. We demonstrate that the interfacial strain plays a major role in stabilizing the metastable δ-Fe on GaAs.

Formation of superhydrophobic porous GaAs layer: effect of substrate doping type

Formation of superhydrophobic porous GaAs layer: effect of substrate doping type

Ultrafast enhancement and optical control of magnetization in ferromagnet/semiconductor layered structures via superdiffusive spin transports

While several mechanisms of ultrafast demagnetization were still being debated, a new superdiffusive spin transport (SST) was proposed as a new mechanism of ultrafast demagnetization in layered structures, and was demonstrated in ferromagnet/normal metal and ferromagnet/normal metal/ferromagnet layered structures. Transient magnetization enhancement in Fe layer was observed as the evidence of SST in a Ni/Ru/Fe layered structure when the Ni layer was photo-excited. However, it was not observed repeatedly elsewhere. Here, we explore possible SST in a new ferromagnet/semiconductor layered structure without magnetic couplings, and observe transient magnetization enhancement in directly photo-excited L10-MnGa ferromagnetic layer of a L10-MnGa/GaAs layered structure, being very convincing evidence of SST because the magnetization enhancement occurred impossibly in a sole photo-excited L10-MnGa layer if there were no SST from GaAs layer. It is more fascinating that we fulfill ultrafast optical manipulations of transient magnetization in L10-MnGa layer using circularly polarized pump pulses (CPPP). Right-handed CPPP can enhance transient magnetization extremely over linearly polarized pump pulses, while left-handed CPPP can weaken one vastly. A modified three-temperature model is developed to take account of SST effect and is used to reproduce our experimental results well. Our results exhibit high efficient and ultrafast bidirectional magnetic modulations in ferromagnet/semiconductor hybrid structures which have potential applications in ultrafast magneto-optical modulations and optical manipulations of ferromagnet/semiconductor hybrid spintronic devices.

Atomic-scale insights of indium segregation and its suppression by GaAs insertion layer in InGaAs/AlGaAs multiple quantum wells

The In segregation and its suppression in InGaAs/AlGaAs quantum well are investigated by using high-resolution x-ray diffraction (XRD) and photoluminescence (PL), combined with the state-of-the-art aberration corrected scanning transmission electron microscopy (Cs-STEM) techniques. To facility our study, we grow two multiple quantum wells (MQWs) samples, which are almost identical except that in sample B a thin GaAs layer is inserted in each of the InGaAs well and AlGaAs barrier layer comparing to pristine InGaAs/AlGaAs MQWs (sample A). Our study indeed shows the direct evidences that In segregation occurs in the InGaAs/AlGaAs interface, and the effect of the GaAs insertion layer on suppressing the segregation of In atoms is also demonstrated on the atomic-scale. Therefore, the atomic-scale insights are provided to understand the segregation behavior of In atoms and to unravel the underlying mechanism of the effect of GaAs insertion layer on the improvement of crystallinity, interface roughness, and further an enhanced optical performance of InGaAs/AlGaAs QWs.

Design and optimization of mid-infrared hot electron detector based on Al/GaAs fishnet nanostructure for CO2 sensing.

Hot electron detectors (HEDs) based on plasmon resonance can circumvent a semiconductor's bandgap limitation and have high sensitivity, suitable for infrared gas detectors. Unfortunately, there are few literature reports on research in the mid-infrared (MIR) region. Herein, we design and optimize a HED based on Al/GaAs fishnet nanostructure for MIR CO2 sensing, and its optical-electrical properties are numerically studied. Surface plasmons not only achieve strong absorptance at CO2 emission wavelength but also greatly improve the photoelectric responsivity over a plane structure detector (∼42times). By changing the thickness of the GaAs layer, the detection wavelength can also be actively adjusted, achieving a larger range of multi-gas detection. The effect of external voltage is also considered. This work highlights a potential engineering application value and offers a path toward more compact and efficient MIR gas detectors.

Spin-Current Oscillations in Diluted Magnetic Semiconductor Multibarrier GaMnAs/GaAs: Role of Temperature and Bias Voltage

This paper has studied the electronic properties of multi-diluted magnetic semiconductor (DMS) layers Ga(1 − x)MnxAs interposed between nonmagnetic GaAs layers. The asymmetry of confining potential on the transmission coefficient by tuning the temperature and the size of the (DMS) layers was discussed. The diluted magnetic layers Ga(1 − x)MnxAs behave as barriers for spin-up holes and quantum wells for spin-down holes. Furthermore, we have addressed the impact of an applied bias voltage and the temperature on the variation of the spin-polarization and spin current densities. Our findings reveal that the transmission coefficients present an oscillating behavior due to the resonant states and strongly depend on the temperature of the system and the number of magnetic layers. Furthermore, the obtained results demonstrated that the number of these states is multiplied by augmenting the magnetic layers. Moreover, we demonstrate that the asymmetric structure presents a completely different transmission of holes than the symmetric structure. Furthermore, the negative differential resistance (NDR) is demonstrated in the current density variations. Especially, this (NDR) was more intense for spin-up holes than spin-down holes. The findings in the present paper can be useful in manufacturing spin-filters by adjusting the values of the temperature and the external voltages.

In-situ study of InAs quantum dots encapsulated in asymmetric (Al)GaAs confinement barriers

In this work the self-assembling of InAs quantum dots (QDs) within asymmetric barriers of (Al)GaAs is studied via reflection high energy electron diffraction (RHEED). A comparative study between the AlGaAs/InAs/GaAs interfaces and its mirror-like heterostructure GaAs/InAs/AlGaAs showed significant differences in the self-assembling and capping of the QDs. The critical thickness of InAs QDs results was proven to be larger when it is grown on AlGaAs alloys, compared with the deposition on GaAs layers. This change is explained by the reduced mobility of In atoms on the Al-containing surfaces, for which the QDs density is increased due to the strain relieve. Through the in-situ analysis of diffusion parameters, it is concluded that the mobility of In atoms decreases the mass transport of 2D and 3D precursors that conduces to the self-assembling of the QDs nanoislands, modifying the rate at which the QDs are formed. Further, during the first stages of QDs capping it is observed that the III-V materials intermixing plays a predominant role. The nanoislands are less affected when are covered by AlGaAs in comparison with the GaAs capping, preserving the QDs morphology and avoiding materials alloying. By following the RHEED intensity behavior during the QDs capping, a model was proposed to obtain quantitative parameters for the smoothing process. High resolution x ray diffraction (HRXRD) measurements show the composition of sharp interfaces for the AlGaAs/InAs/GaAs heterostructure. Lastly, numerical simulations were performed to evaluate the strain changes using the experimental information as input data.

Influences of Native Oxide on the Properties of Ultrathin Al2O3‐Interfaced Si/GaAs Heterojunctions

AbstractThe structure property of non‐ideal Si/GaAs heterostructures that are integrated with the ultra‐thin oxide (UO) tunneling interfacial layer is systematically investigated. Si nanomembranes (NMs) are oxidized in different time periods prior to the hetero‐integration process to create the non‐ideal single‐side passivated Si/GaAs heterostructure. The atomic‐level oxygen distribution and the degree of oxygen content in Si NM and GaAs are carefully investigated using atom probe tomography (APT) and X‐ray photoelectron spectroscopy (XPS) to trace changes in the chemical composition and reactional mechanism across the UO interface when the surface of Si NM is exposed to air for different periods of time. The negatively induced charges at the UO layer cause oxygen diffusion to the GaAs layer and form the unwanted GaAs oxide layer. This native oxide stack noticeably degrades the thermal properties of the Si/GaAs heterostructure as Si NMs become more oxidized. This study reveals that the poor surface passivation on one side of the heterointerface leads to a both‐side oxidation, thus severely deteriorating the transport properties across the heterojunction formed with the UO layer.

Theoretical Achievement of THz Gain-Bandwidth Product of Wafer-Bonded InGaAs/Si Avalanche Photodiodes With Poly-Si Bonding Layer

The combination of high-absorption indium gallium arsenide (InGaAs) with Si material (excellent avalanche characteristic) is an ideal solution for the achievement of high-gain low-noise near-infrared avalanche photodetector (APD). Traditional Si-based epitaxial InGaAs thin film suffers from high threading dislocations (TDs), leading to deterioration of noise and gain-bandwidth product (GBP) of APD. InGaAs/Si APD with GBP exceeding terahertz (THz) is reported for the first time theoretically in this work. The calculated APD is based on the semiconductor interlayer bonding in which a thin polycrystalline Si (poly-Si) bonding layer is inserted at the bonded interface to isolate the mismatched lattices of InGaAs and Si single crystals. Extreme low dark current of 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">−12</sup> A of APD at room temperature is achieved due to the absence of TDs in InGaAs layer and it is located at 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">−5</sup> A near breakdown voltage ( <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">${V}_{\text{br}}$ </tex-math></inline-formula> ). More importantly, a deviation of the dark and optical currents near <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">${V}_{\text{br}}$ </tex-math></inline-formula> due to the trapping effect of holes at InGaAs/Si bonded interface is identified. The change in gain with bias is inconsistent with the 3-dB bandwidth (BW). The 3-dB BW becomes linear versus bias with the increase in poly-Si thickness, leading to high 3-dB BW (8–10 GHz) near <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">${V}_{\text{br}}$ </tex-math></inline-formula> . Finally, ultrahigh GBPs of 1.2 and 1.3 THz are achieved for APD with 3- and 5-nm-thick poly-Si bonding layer, respectively. This may give guidance for the fabrication of semiconductor APDs which can meet next-generation high bit-rate applications.

Albedo‐Enabled Enhanced Energy Harvesting via GaAs Bifacial Thin‐Film Solar Cells

In addition to direct solar illumination to the photovoltaic cell, the albedo effect provides a unique opportunity to enhance energy harvesting. However, conventional solar cells are inefficient albedo energy harvesters because the rear side of the cell is usually blocked by a thick substrate or metal contact. In this study, structurally thin active layers of GaAs thin‐film solar cells covered with transparent Ag nanowires that enable bifacial solar cell operations are fabricated. The GaAs bifacial thin‐film solar cell is fabricated by bonding the active regions of the solar cell on colorless polyimide sheets with highly transparent epoxy adhesives. The fabricated bifacial solar cells without any anti‐reflection coatings demonstrated power conversion efficiencies of 8.54 and 6.78% at the front and rear sides, respectively, under AM 1.5 G irradiance conditions. The increase in energy harvesting by the GaAs bifacial thin‐film solar cells was estimated, which showed approximately a 72% increase in short‐circuit current density (JSC) when the solar cells operated on a high‐reflection‐coated ground. Positioning the solar cells on snow or sand (deserts and beaches) increasedJSCby ≈44 and 31%, respectively, compared to monofacial solar cells.

Structural Properties of Superlattices {LTG-GaAs/GaAs : Si} on GaAs (100) and (111)A Substrates

The proposed new structure for photoconductive antennas is a multilayer epitaxial film grown on a GaAs (111) A substrate and consisting of alternating layers of undoped GaAs grown at low temperature (LTG-GaAs) and GaAs layers formed in the standard high-temperature regime and doped with silicon (GaAs : Si). The ratio of As4 and Ga fluxes (γ) was chosen such that the GaAs : Si layers hadp-type conductivity. LTG-GaAs layers were grown at an increased γ value. A similar structure was grown on a (100) substrate as a reference sample. Phase composition was investigated using high-resolution X-ray diffractometry, surface relief — by mean of atomic force microscopy and distribution of Si impurity over thickness — using secondary ion mass spectrometry.

Imaging the electrostatic landscape of unstrained self-assemble GaAs quantum dots

Unstrained GaAs quantum dots are promising candidates for quantum information devices due to their optical properties, but their electronic properties have remained relatively unexplored until now. In this work, we systematically investigate the electronic structure and natural charging of GaAs quantum dots at room temperature using Kelvin probe force microscopy (KPFM). We observe a clear electrical signal from these structures demonstrating a lower surface potential in the middle of the dot. We ascribe this to charge accumulation and confinement inside these structures. Our systematical investigation reveals that the change in surface potential is larger for a nominal dot filling of 2 nm and then starts to decrease for thicker GaAs layers. Using k · p calculation, we show that the confinement comes from the band bending due to the surface Fermi level pinning. We find a correlation between the calculated charge density and the KPFM signal indicating that k · p calculations could be used to estimate the KPFM signal for a given structure. Our results suggest that these self-assembled structures could be used to study physical phenomena connected to charged quantum dots like Coulomb blockade or Kondo effect.

Hyperbolic-tangent composition-graded InxGa1-xAs/GaAs (100) structures grown by molecular beam epitaxy

The molecular beam epitaxial growth and physical properties of In graded-composition layers of InGaAs on GaAs (100) substrates varying nominal In concentration from 12 to 97% and vice versa is analyzed along the growth front. The substrate temperature was varied as the In concentration was changed in order to propitiate the near-optimal growth conditions for the InGaAs alloy. Hyperbolic-tangent compositional gradients, from In-rich to Ga-rich alloys and vice versa, observed RHEED patterns characteristic of a flat surface during the growth. The HRXRD patterns showed clear diffraction peaks associated with the In concentration alloy at which the graded sample started, followed by a characteristic plateau of graded layers. The occurrence of dislocations in hyperbolic tangent gradients that relieve the strain up to a certain thickness is explored, reaching a nominal or an averaged lattice constant.

Capacitance and conductance oscillations from electron tunneling into high energy levels of a quantum well in a p−i−n diode

Two-dimensional electron and hole gases separated by a few nm from each other are produced in p-i-n diodes based upon MBE-grown GaAs/AlGaAs heterostructures. At such interlayer distances, the exciton formation and possibly Bose-Einstein condensation (BEC) is expected. We measure the capacitance between the layers and find it to oscillate as a function of the bias voltage. The peak values exceed the geometric capacitance by up to a factor of two. The surprisingly regular periods of the oscillations are of the order of 10 to 30 mV and scale linearly with the inverse of the thickness, between 60 and 150 nm, of the GaAs layer placed between the barrier and the p-doped AlGaAs. The oscillations are related to the resonant electron tunneling into high energy levels of this GaAs layer acting as a quantum well. We suggest that long lifetimes of the electrons in these levels are the origin of the large peak values of the capacitance. The possible relation of the capacitance oscillations with BEC is discussed.