GaAs Base Research Articles

An Exploration of All‐Inorganic Perovskite/Gallium Arsenide Tandem Solar Cells

All‐inorganic perovskite/gallium arsenide (GaAs) tandem solar cells are of great interest for potential space applications. Herein, planar all‐inorganic four‐terminal (4‐T) and two‐terminal (2‐T) perovskite/GaAs tandem solar cells are simulated and optimized, respectively. To achieve higher absorption in the 4‐T configuration, the reflection and parasitic absorption have to be reduced through optimizing the thickness of the perovskite and GaAs base, reducing the thickness of SnO2 and the organic hole transport layer (HTL), and introducing an antireflection coating, respectively. To balance the short‐circuit current and open‐circuit voltage of the GaAs bottom cell, the doping concentration on GaAs is optimized to 1018 cm−3 that has resulted in a high power conversion efficiency (PCE) of 30.97%. Based on the results of 4‐T configuration, all‐inorganic perovskites with different halide compositions are used for current matching to achieve a high‐efficiency 2‐T configuration. After studying the effects of defect density and optimizing the doping concentration of the GaAs base, an extremely high PCE of 30.67% is achieved based on 2‐T CsPbIBr2/GaAs tandem solar cells. Furthermore, the current mismatching under the AM0 spectrum is eliminated for potential high altitude/space application, and the device offers a very competitive PCE of 27.23% compared with that of traditional GaAs double‐junction tandem solar cells.

Performance prediction of AlGaAs/GaAs betavoltaic cells irradiated by nickel-63 radioisotope

The performance of a n/p betavoltaic heterostructure, i.e. aluminum gallium arsenide (AlxGa1-xAs) on gallium arsenide (GaAs) substrate has been evaluated by nickel-63 (Ni63) beta-particles irradiation with anaverage kinetic energy of 17.1 keV. The thickness of AlxGa1-xAs emitter layer was set to 1.2 μm with an aluminum molar fraction of 0.1. The thickness of GaAs base region was fixed to be 3 μm. During the calculations, the reflection from the front surface, the metallurgical interface, and the limits of the depletion region were carefully taken into account. Moreover, the equivalent circuit accounts for the ohmic losses. The simulation results reveal that by using a radioactivity density of 10 mCi/cm2, the conversion efficiency (ɳ) of an optimized device structure increases up to 33%. The other cell electrical parameters, such as the short-circuit current density (Jsc), the open-circuit voltage (Voc), and the maximum electrical power density (Pmax) are observed to be 438.09 nA/cm2, 0.97 V, and 337.35 nW/cm2, respectively.

Fabrication of AlGaAs/GaAs/diamond heterojunctions for diamond-collector HBTs

Diamond is a highly attractive ultrawide bandgap semiconductor for next-generation high-power switching devices and RF devices for its superior physical and electrical properties. However, the lack of effective n-type dopants in diamond has limited the material to only unipolar p-type device applications. Heterostructure bipolar devices that use better n-type semiconductors together with p-type diamond is an approach to get high performance devices. In this work, p–n–p AlGaAs/GaAs/diamond heterojunction bipolar transistors (HBTs) are proposed and fabricated using a grafting technique. The double-heterojunction is formed by transferring an AlGaAs(p-type)/GaAs(n-type) membrane onto single-crystalline p-type doped diamond with an electron affinity of 0.32 eV. The epitaxial AlGaAs/GaAs emitter-base p–n junction shows an ideality factor of 1.09 with an Ion/Ioff of 1.53 × 107 at ± 1.5 V. The grafted GaAs/diamond n–p junction shows an ideality factor of 3.67 with an Ion/Ioff of 3.74 × 1010 at ± 5.2 V. Due to the valence-band energy barrier of 0.3 eV between the GaAs base and the diamond collector, the measured current gain for the HBT is slightly below unity. Simulations show that by reducing the electron affinity value of the p-type diamond, the base-collector energy barrier height can be correspondingly reduced, and high current gain can be expected.

Internal Resistive Barriers Related to Zinc Diffusion During the Growth of Inverted Metamorphic Multijunction Solar Cells

Majority carrier barriers at heterointerfaces are a common source of non-linear resistance that hinders concentrator solar cell performance. The source of a particular barrier is often unclear in a multijunction device with numerous heterointerfaces. We demonstrate Zn-dopant diffusion during inverted metamorphic multijunction (IMM) device growth to be one key cause of internal barrier formation. Using an inverted GaAs/GaAs tandem solar cell with a high temperature annealing layer grown in between each subcell, we simulate the annealing conditions of a multijunction growth in a simplified structure. Through analysis of the device by secondary ion mass spectrometry (SIMS) and electrochemical capacitance–voltage profiling, we show that annealing causes Zn to diffuse out of the top cell Ga0.5In0.5P back surface field (BSF) and accumulate in the GaAs base. Through equilibrium band modeling, we show that the resultant doping profile forms an energetic barrier to hole flow in the valence band, which correlates with fill factor losses in the current–voltage curves measured under concentration. When we, instead, employ a C-doped Al0.2Ga0.8As BSF layer in the top cell, we do not observe evidence of a heterojunction barrier. We attribute this difference to the reduced diffusivity of carbon, confirmed by SIMS, as well as more favorable valence band offsets between GaAs and Al0.2Ga0.8As. Finally, we compare 5-junction IMM cells with Al0.2Ga0.8As:C and Ga0.5In0.5P:Zn BSF layers in the GaAs third junction, respectively, and show a significantly improved device performance under concentration when Al0.2Ga0.8As:C is employed. We demonstrate the importance of designing annealing tolerance into multijunction structures that are subjected to extended annealing during growth.

MOCVD growth of InGaP/GaAs heterojunction bipolar transistors on 200 mm Si wafers for heterogeneous integration with Si CMOS

Heterojunction bipolar transistors (HBTs) with an In0.49Ga0.51P emitter and a GaAs base and collector were fabricated on epitaxial films grown directly onto Si substrates using a thin germanium (100%) buffer layer. All the materials (Ge, GaAs, In0.49Ga0.51P) were grown epitaxially using metal–organic chemical vapor deposition. The thin germanium buffer layer has a threading dislocation density (TDD) of ∼3 × 107 cm−2. The TDD of the active device layers does not worsen and is maintained at ∼2 × 107 cm−2, based on the density of dark spots detected in an electron-beam-induced current plan view image. Although the TDD is high, a DC current gain of 95 can be achieved by the In0.49Ga0.51P/GaAs HBT on a Si substrate. In addition, collector current and base current ideality factors (nc and nb) of 1.07 and 1.23, respectively, and average breakdown voltage (BVcbo) of 14.2 V can be realized. These results enable applications such as power amplifiers for mobile phone handsets and monolithic integration of HBTs with standard Si CMOS transistors on a common Si platform.

Calculated Characteristics of a Transistor Laser Using Alloys of Gr‐IV Elements

The performance of a transistor laser (TL) structure consisting of SiGeSn emitter, SiGeSn base incorporating a GeSn quantum well (QW) and a GeSn collector is studied in the present work. The Sn concentration is chosen to yield direct band gap for all the layers. An earlier model developed for InGaAs QW in GaAs base, that could satisfactorily explain most of the experimental data, is employed to calculate terminal currents, threshold current, and light power output. The model solves continuity equation, considers virtual states above QW, Fermi Golden rule to calculate gain, Fermi statistics for occupancy of subbands and broadening of states. A low value of threshold base current is predicted.

AlGaAs/GaAs HBTs with C-doped base and undoped emitter-base spacer layer

Effect of undoped GaAs spacer layer between AlGaAs emitter layer and C-doped GaAs base layer on DC characteristics of AlGaAs/GaAs HBTs with ledge structure was investigated. Devices with spacer layer thickness (∼3-5 nm) and optimal ledge geometry demonstrate stable high current gain (βmax = 120-140) combined with collector-emitter breakdown voltage of 23-25V.

Monolithic integration of AlGaAs distributed Bragg reflectors on virtual Ge substrates via aspect ratio trapping

High quality AlxGa1-xAs distributed Bragg reflectors (DBRs) were successfully monolithically grown on on-axis Si (100) substrates via a Ge layer formed by aspect ratio trapping (ART) technique. The GaAs/ART-Ge/Si-based DBRs have reflectivity spectra comparable to those grown on conventional bulk off-cut GaAs substrates and have smooth morphology and reasonable periodicity and uniformity. Antiphase domain formation is significantly reduced in GaAs on ART-Ge/Si substrates, and etch pit density of the GaAs base layer on the ART-Ge substrates ranges from 105 to 6 × 106 cm−2. These results paved the way for future VCSEL growth and fabrication on these ART-Ge substrates and also confirm that virtual Ge substrates via ART technique are effective Si platforms for optoelectronic integrated circuits.

Axial InAs/GaAs heterostructures on silicon in a nanowire geometry

InAs segments were grown on top of GaAs islands, initially created by droplet epitaxy on silicon substrate. We systematically explored the growth-parameter space for the deposition of InAs, identifying the conditions for the selective growth on GaAs and for purely axial growth. The axial InAs segments were formed with their sidewalls rotated by 30 compared to the GaAs base islands underneath. Synchrotron X-ray diffraction experiments revealed that the InAs segments are grown relaxed on top of GaAs, with a predominantly zincblende crystal structure and stacking faults.

Self-assisted GaAs nanowires with selectable number density on Silicon without oxide layer

We present the growth of self-assisted GaAs nanowires (NWs) with selectable number density on bare Si(1 1 1), not covered by the silicon oxide. We determine the number density of the NWs by initially self-assembling GaAs islands on whose top a single NW is nucleated. The number density of the initial GaAs base islands can be tuned by droplet epitaxy and the same degree of control is then transferred to the NWs. This procedure is completely performed during a single growth in an ultra-high vacuum environment and requires neither an oxide layer covering the substrate, nor any pre-patterning technique.

Control over the Number Density and Diameter of GaAs Nanowires on Si(111) Mediated by Droplet Epitaxy

We present a novel approach for the growth of GaAs nanowires (NWs) with controllable number density and diameter, which consists of the combination between droplet epitaxy (DE) and self-assisted NW growth. In our method, GaAs islands are initially formed on Si(111) by DE and, subsequently, GaAs NWs are selectively grown on their top facet, which acts as a nucleation site. By DE, we can successfully tailor the number density and diameter of the template of initial GaAs islands and the same degree of control is transferred to the final GaAs NWs. We show how, by a suitable choice of V/III flux ratio, a single NW can be accommodated on top of each GaAs base island. By transmission electron microscopy, as well as cathodo- and photoluminescence spectroscopy, we confirmed the high structural and optical quality of GaAs NWs grown by our method. We believe that this combined approach can be more generally applied to the fabrication of different homo- or heteroepitaxial NWs, nucleated on the top of predefined islands obtained by DE.

Analytical model for threshold‐base current of a transistor laser with multiple quantum wells in the base

The authors investigate in this study the effect of incorporating multiple quantum wells (QWs) in the base of a heterojunction bipolar transistor laser on its performance. The expression for the terminal current is obtained by solving continuity equation in the base relating the bulk carrier density with the two-dimensional (2D) carrier density via virtual states. The gain in the QW is obtained by considering strain, 2D density-of-states, polarisation dependent momentum matrix element, Fermi statistics and Lorentzian broadening. The authors have taken as the basis for comparison the calculated as well as the experimental threshold-base current of 21.5 mA for a 16 nm wide InGaAs QW in GaAs base placed at 59 nm away from the emitter junction. Calculated value of 7.06 mA for three 16 nm wide QWs placed at 29, 59 and 79 nms in the base indicate substantial reduction in threshold-base current. Calculations are also made for three QWs of different widths having variable barrier widths. Reduction of threshold-base current by different amounts is observed in all cases.

An Al x In1−x As/GaAs heterojunction ultra-thin film solar cell with 20% efficiency

An ultra-thin film photovoltaic cell, which incorporates an Al x In1−x As/GaAs heterojunction, is simulated using Adept 1D simulation tool, and it is with an energy conversion efficiency of 20.06% (under 1 sun, AM1.5G illumination) for 604 nm cell thickness (excluding the substrate thickness), and optimized layer thickness and doping concentration for each layer of the device. The device has an n-type AlAs window layer (highly doped), an n-type Al x In1−x As emitter layer and a p-type GaAs base layer. Germanium (Ge) substrate is used for the structure. The device parameters are optimized separately for each layer. Based on these optimizations, the ultra-thin film solar cell design is proposed after careful consideration of lattice mismatch between two adjacent layers of the device.

Modeling Resonance-Free Modulation Response in Transistor Lasers With Single and Multiple Quantum Wells in the Base

We have developed the expressions for terminal currents in transistor lasers (TLs) having a single quantum well (SQW) as well as multiple quantum wells (MQWs) of different well and barrier widths in the base by solving a continuity equation relating the bulk carrier density with the 2-D carrier density via virtual states (VS). The gain in the quantum well (QW) is obtained by considering strain, 2-D density-of-states, polarization-dependent momentum matrix element, Fermi statistics, and Lorentzian broadening. A calculated value of 7.06 mA of threshold base current for three 16-nm-wide QWs in the base indicates a substantial reduction from the calculated and experimental value of 21.5 mA for a 16-nm-wide InGaAs QW in GaAs base. A similar reduction is also obtained for three QWs of different widths having variable barrier widths. The estimated modulation bandwidths (BWs) are higher in the case of MQW structures than in the SQW TL. Above threshold, the effective base recombination time, including spontaneous and stimulated processes, gives rise to a fast recombination process in the base, which leads to resonance-free modulation response. The estimated recombination time compares favorably with the value reported from the analysis of experimental data.

Analytical theory of a small signal modulation response of a transistor laser with dots-in-well in the base

We have developed the theory for the threshold base current, light power output and small signal modulation response of a transistor laser, the base of which contains a quantum well (QW) with a layer of quantum dots (QDs) inserted within it. Our theory involves the solution of the continuity equation for an arbitrary location of the QW in the base, and the solution of coupled rate equations for carriers and photons in the QD. The excitonic recombination model is assumed to be valid in the QD. The estimated threshold base current for InAs QDs embedded in the InGaAs QW is found to be smaller than the value for the InGaAs QW in the GaAs base. The values of the modulation bandwidths for different values of the base current however do not reach the high values obtained for the QW system.

Estimated threshold base current and light power output of a transistor laser with InGaAs quantum well in GaAs base

We have solved the continuity equation for electrons in the base of an InGaP–GaAs–GaAs heterojunction bipolar transistor laser (TL) in which the position of an InGaAs quantum well (QW) in GaAs base is variable. The injected minority carrier is related to the two-dimensional carrier in QW via virtual states (VSs). The values for optical gain in the QW are obtained by considering subband energies and envelope functions in presence of strain, polarization dependent momentum matrix element and Lorentzian lineshape. Relating the gain with threshold current and the latter with base current via VS current, the threshold base current and power output from the TL are estimated. Good agreement between the calculated and the experimental threshold base currents is obtained and the match for light output power is satisfactory within experimental uncertainty. Our calculated charge distribution in the base shows similar behaviour as in the charge control analysis of the experimental data.

Positively charged manganese acceptor disclosed by photoluminescence spectra in an n-i-p-i-n heterostructure with a Mn-doped GaAs base

A type of manganese impurity center with two holes bound in it is disclosed in a lightly Mn-doped base layer of an n-i-p-i-n heterostructure. In addition to the intensively investigated (e, AMn0) peak, a photoluminescence (PL) peak appears at 820.3 nm under zero bias and is continuously shifted to 822.5 nm by negatively biasing the structure. Its circular polarization has the same polarity as PL (e, AMn0), and has a magnetic dependence fitted by a paramagnetic Brillouin function with S = 1/2 and g = 2.09. These observations are consistent with the physical picture that in the AMn+ center both the − 3/2 and the − 1/2 holes occupy the lowest 1S hydrogenic orbit with their spins parallel to each other and antiparallel to the 5/2 local spin of the 3d shell. This spin coupling picture in the AMn+ center has been confirmed by a modified numerical calculation based upon effective mass theory. The feature of the ability to tune the impurity level of the AMn+ center makes it attractive for optically and electrically manipulating local magnetic spins in semiconductors.

Deep traps study of radiation-damaged semi-insulating GaAs detectors introduced by neutrons

Abstract Neutron detectors based on bulk semi-insulating (SI) GaAs represent an interesting and perspective solution due to its relatively high resistance against neutrons. Therefore SI GaAs material seems to be an excellent candidate for fabrication of a neutron imaging detector. Set of detectors based on SI GaAs material was bombarded with various integral neutron fluencies (10 11 –10 15 n cm −2 ). The performance of detectors was evaluated via measured spectra of 241 Am radionuclide sources. From the results a significant worsening in detection ability was observed at fluencies exceeding (over) ∼10 13 n cm −2 . These observations are explained by lattice defects in the base SI GaAs material or in the overall detector structure (electrode interfaces, surface) introduced by neutrons. Photo-induced current transient spectroscopy was used to determine changes in deep level states of bulk SI GaAs base material.

AlGaAs/GaAs heterojunction phototransistor with a double delta-doped base grown by AP MOVPE

Heterojunction bipolar phototransistors (HPTs), based on GaAs technology, are widely used in the optoelectronic-integrated circuit (OEIC) [H.T. Lin, D.H. Rich, A. Larsson, J. Appl. Phys. 79 (1996) 8015 [1]; Lin Chien-chung, M. Wayne, J.S. Harris Jr., F. Sugihwo, Appl. Phys. Lett. 76 (2000) 1188 [2]]. They exhibit a large optical gain and a negligible noise level at low bias voltages. The presented work describes the fabrication and device characteristics of n–p–n AlGaAs/GaAs heterojunction phototransistor with a double Zn delta-doped GaAs base region (2δ-Zn-AlGaAs/GaAs HPT). Such construction of the transistor allows to obtain higher current gain and lower power consumption. The electrical and optical properties of the epitaxial HPT structure were examined using capacitance–voltage (EC– V) measurements, photovoltage, photocurrent and micro-Raman spectroscopies. The current–voltage (dc I– V) characteristics as well as the results of time response measurements are also presented.

Characteristics of an InGaP/Al[sub X]Ga[sub 1−X]As/GaAs composite-emitter heterojunction bipolar transistor (CEHBT)

The characteristics of an interesting InGaP/AlXGa1−XAs/GaAs composite-emitter heterojunction bipolar transistor (CEHBT) are studied and demonstrated. By employing an AlXGa1−XAs graded layer between an InGaP emitter and GaAs base layer, a nearly zero conduction-band discontinuity can be obtained. Owing to the complete disappearance of the potential spike at the emitter/base hetero-interface, our CEHBT with AlXGa1−XAs graded layer exhibited good dc performance with current gains of 280 and 11 at collector currents of IC=12.6 mA and 0.2 nA, respectively. Furthermore, our CEHBT also shows a lower offset voltage and collector and base current ideality factors. In microwave characteristics, for a 3×20 μm2 device, the unit current gain cutoff frequency fT and maximum oscillation frequency fmax are 41.3 and 35.4 GHz, respectively. Consequently, this work provides promise for high-performance analog, digital, and microwave device applications.