GaAs Material Research Articles

Temperature dependence of the photoluminescence from InP/GaAs type-II ultrathin quantum wells

Temperature dependence of the photoluminescence (PL) spectra has been investigated for InP/GaAs type-II ultrathin quantum wells (QWs). Room temperature PL has been observed for 1.43 monolayer thick ultrathin QW. Fitting parameters of the Bose–Einstein empirical relation for ultrathin QWs show that the temperature dependence of PL peak energy is similar to the temperature dependence of the band gap for InP and GaAs materials. In addition, we have also determined the PL quenching mechanism from the Arrhenius-like plot of integrated PL intensity. Thermal escape of carriers from these ultrathin QWs into the GaAs barrier is mainly responsible for the PL quenching with temperature. This is also supported by the observation that the PL intensity related to the GaAs barrier increases with increasing temperature.

Efficient III–V tunneling diodes with ErAs recombination centers

We report on recombination diodes containing monolayer depositions of ErAs between highly doped n- and p-layers in the (Al)GaAs and In(Al)GaAs material system. The ErAs material provides metallic states across the band gap of the host semiconductor that act as efficient recombination centers. Both electrons and holes can tunnel into the ErAs, resulting in extremely high tunneling current densities in the tens of kA cm−2 range. A device resistance area product of 1–2 × 10−5 Ω cm2 at low bias (±0.2 V) has been measured. Measurements on In(Al)GaAs and (Al)GaAs diodes will be compared to a simple theoretical model. Low band gap and high (p-) doping levels are identified as key parameters for achieving highest recombination currents. Compared to ErAs-free diodes, ErAs-enhanced recombination diodes provide orders of magnitude higher current densities at both moderate and low forward and reverse bias. This is attributed to the smaller and narrower tunnel barriers from the n- and p-layers into the ErAs compared to tunneling from the n- to the p-side through the whole depletion region.

Spectrum and Binding Energy of an Off-Center Hydrogenic Donor in a Spherical Quantum Dot

Off-center impurity effects in a spherical quantum dot are theoretically studied by degenerate perturbation method in strong confinement. The energy levels and binding energies are computed for the typical GaAs material as function of the donor position. The numerical results show the quantum size effect. We note that the energy levels and binding energies are not only related to the position of donor and the strength of confinement, but also related to the fold of degenerate states. We can see obviously that gaps will appear among the degenerate states and the splitting of energy levels and binding energies will appear as the position of the impurity is shifted away off the center.

Two-photon interference of the emission from electrically tunable remote quantum dots

Single semiconductor quantum dots have been widely studied within devices that can apply an electric field. In the most common system, the low energy offset between the InGaAs quantum dot and the surrounding GaAs material limits the magnitude of field that can be applied to tens of kVcm^-1, before carriers tunnel out of the dot. The Stark shift experienced by the emission line is typically 1 meV. We report that by embedding the quantum dots in a quantum well heterostructure the vertical field that can be applied is increased by over an order of magnitude whilst preserving the narrow linewidths, high internal quantum efficiencies and familiar emission spectra. Individual dots can then be continuously tuned to the same energy allowing for two-photon interference between remote, independent, quantum dots.

The correlation energies and nonlinear optical absorptions of an exciton in a disc-like quantum dot

Using exact diagonalization techniques, the low-lying states of an exciton, and the linear and nonlinear optical absorptions in a disc-like quantum dot are theoretically studied. The numerical results for the typical GaAs material show the so-called quantum size effect. Also, our study is restricted on the transition between the S state ( L = 0) and the P state ( L = 1). The optical absorption coefficients are greatly enhanced because of the induced size confinement. Meantime, we find that the total optical absorption coefficient is about two times bigger than that obtained by without considering exciton effects. Additionally, the optical absorption saturation intensity can be controlled by the incident optical intensity I.

High power, longevity gallium arsenide photoconductive semiconductor switches

A GaAs Photoconductive Semiconductor Switch (PCSS) with a 3-mm gap between the two opposed contact electrodes was developed with carefully chosen GaAs material, mechanical structure design, contact fabrication techniques and its insulation protection. It is charged by a pulse power supply under a bias of 15 kV, illuminated by laser pulses of 1064 nm in wavelength, 56.12 μJ in optical energy per shot and 1 kHz pulse repetition rate (PRR). The GaAs PCSS can last for more than 3.6×106 shots and produce output pulses of 2 MW in peak power, 2 ns pulse duration and 65 ps time jitter from a 50-Ω load of the oscilloscope. When an electric field of 100 kV/cm bias was applied, the peak power of the load was measured at 10 MW. A series of measurements on the voltage conversion rates (VCR) and time jitters have been carried out as the bias voltage increases. In particular, taking into account the dependence of optical absorption coefficient on the bias voltages, the curve of the VCR changes with the bias voltages were analyzed quantitatively.

Optomechanical zipper cavity lasers: theoretical analysis of tuning range and stability

The design of highly wavelength tunable semiconductor laser structures is presented. The system is based on a one dimensional photonic crystal cavity consisting of two patterned, doubly-clamped nanobeams, otherwise known as a "zipper" cavity. Zipper cavities are highly dispersive with respect to the gap between nanobeams in which extremely strong radiation pressure forces exist. Schemes for controlling the zipper cavity wavelength both optically and electrically are presented. Tuning ranges as high as 75 nm are achieved for a nominal design wavelength of lambda = 1.3 microm. Sensitivity of the mechanically compliant laser structure to thermal noise is considered, and it is found that dynamic back-action of radiation pressure in the form of an optical or electrical spring can be used to stabilize the laser frequency. Fabrication of zipper cavity laser structures in GaAs material with embedded self-assembled InAs quantum dots is presented, along with measurements of photoluminescence spectroscopy of the zipper cavity modes.

The effects of the magnetic field and dielectric screening on the diamagnetic susceptibility of a donor in a quantum well with anisotropic effective mass

The diamagnetic susceptibility and the binding energy of a hydrogenic donor in a quantum well with different mass anisotropy parameters γ= m ⊥/ m || are investigated in the presence of a magnetic field using a trial wave function with two parameters in the framework of the effective mass approximation. It is observed that the effects of magnetic field on diamagnetic susceptibility of the donor in two-dimensional semiconductors with various effective mass anisotropy parameters are quite different. Also it has been found that for Si there is an obvious increase in the binding energy for spatially dependent screening compared with that for constant screening in the range of considered well widths, whereas the spatially dependent screening effect is small for Ge and GaAs materials.

Tunneling MOSFETs Based on III-V Staggered Heterojunctions

AbstractA critical problem for the progression of CMOS electronics to the nanoscale is the reduction of power density, while at the same time preserving high speed performance. One of the most promising approaches is to aggressively reduce the power supply voltage by using a novel device, the tunneling MOSFET (TMOSFET), which is a MOSFET that operates by tunnel-injection of carriers from source to channel, rather than by conventional thermionic emission. TMOSFETs benefit from steep (sub-60mV/dec) gate turn-on characteristics. In this paper we show that TMOSFET designs based on staggered heterojunctions are particularly promising, since the choice of materials for the injector (source) and channel allows optimization of the tunneling probability at the heterojunction. Analysis and simulation of MOSFETs based on the GaAlSb / InGaAs material system are presented. The energy offset between the valence band of the injector and the conduction band of the channel at the heterojunction can be tailored over a wide range, from negative values (“offset” band lineup) to values in excess of 1eV. We find by simulation that for optimal values of effective heterojunction bandgap near 0.2eV, the resulting MOSFETs are capable of delivering >0.5mA/mm while maintaining on-off ratio greater than four orders of magnitude over voltage swing of 0.3V. We also discuss a variety of materials-related challenges that must be overcome to realize the predicted performance. Among these are the need to provide near ideal heterojunctions between the materials, employ high K dielectrics with very low interface state density, and achieve good alignment between the gate and the heterojunction. Different configurations for the tunneling MOSFETs are presented.

Optimization of inductively coupled plasma etching for low nanometer scale air-hole arrays in two-dimensional GaAs-based photonic crystals

This paper mainly describes fabrication of two-dimensional GaAs-based photonic crystals with low nanometer scale air-hole arrays using an inductively coupled plasma (ICP) etching system. The sidewall profile and surface characteristics of the photonic crystals are systematically investigated as a function of process parameters including ICP power, RF power and pressure. Various ICP powers have no significant effect on the verticality of air-hole sidewall and surface smoothness. In contrast, RF power and chamber pressure play a remarkable role in improving sidewall verticality and surface characteristics of photonic crystals indicating different etching mechanisms for low nanometer scale photonic crystals. The desired photonic crystals have been achieved with hole diameters as low as 130 nm with smooth and vertical profiles by developing a suitable ICP processes. The influence of the ICP parameters on this device system are analyzed mainly by scanning electron microscopy. This fabrication approach is not limited to GaAs material, and may be efficiently applied to the development of most two-dimensional photonic crystal slabs.

Spin polarization of the neutral exciton in a single InAs quantum dot at zero magnetic field

The work presented in this thesis deals with optical studies of semiconductor quantum dots (QDs) in the InGaAs material system. It is shown that for self-assembled InAs QDs, the interaction with the surrounding GaAs barrier and the InAs wetting layer (WL) in particular, has a very large impact on their optical properties. The ability to control the charge state of individual QDs is demonstrated and attributed to a modulation in the carrier transport dynamics in the WL. After photo-excitation of carriers (electrons and holes) in the barrier, they will migrate in the sample and with a certain probability become captured into a QD. During this migration, the carriers can be affected by exerting them to an external magnetic field or by altering the temperature. An external magnetic field applied perpendicular to the carrier transport direction will lead to a decrease in the carrier drift velocity since their trajectories are bent, and at sufficiently high field strength become circular. In turn, this decreases the probability for the carriers to reach the QD since the probability for the carriers to get trapped in WL localizing potentials increases. An elevated temperature leads to an increased escape rate out of these potentials and again increases the flow of carriers towards the QD. These effects have significantly different strengths for electrons and holes due to the large difference in their respective masses and therefore it constitutes a way to control the supply of charges to the QD. Another effect of the different capture probabilities for electrons and holes into a QD that is explored is the ability to achieve spin polarization of the neutral exciton (X0). It has been concluded frequently in the literature that X0 cannot maintain its spin without application of an external magnetic field, due to the anisotropic electron – hole exchange interaction (AEI). In our studies, we show that at certain excitation conditions, the AEI can be by-passed since an electron is captured faster than a hole into a QD. The result is that the electron will populate the QD solely for a certain time window, before the hole is captured. During this time window and at polarized excitation, which creates spin polarized carriers, the electron can polarize the QD nuclei. In this way, a nuclear magnetic field is built up with a magnitude as high as ~ 1.5 T. This field will stabilize the X0 spin in a similar manner as an external magnetic field would. The build-up time for this nuclear field was determined to be ~ 10 ms and the polarization degree achieved for X0 is ~ 60 %. In contrast to the case of X0, the AEI is naturally cancelled for the negatively charged exciton (X-) and the positively charged exciton (X+) complexes. This is due to the fact that the electron (hole) spin is paired off in case of X- (X+). Accordingly, an even higher polarization degree (~ 73 %) is measured for the positively charged exciton. In a different study, pyramidal QD structures were employed. In contrast to fabrication of self-assembled QDs, the position of QDs can be controlled in these samples as they are grown in inverted pyramids that are etched into a substrate. After sample processing, the result is free-standing AlGaAs pyramids with InGaAs QDs inside. Due to the pyramidal shape of these structures, the light extraction is considerably enhanced which opens up possibilities to study processes un-resolvable in self-assembled QDs. This has allowed studies of Auger-like shake-up processes of holes in single QDs. Normally, after radiative recombination of X+, the QD is populated with a ground state hole. However, at recombination, a fraction of the energy can be transferred to the hole so that it afterwards occupies an excited state instead. This process is detected experimentally as a red-shifted luminescence satellite peak with an intensity on the order of ~ 1/1000 of the main X+ peak intensity. The identification of the satellite peak is based on its intensity correlation with the X+ peak, photoluminescence excitation measurements and on magnetic field measurements.

Suitability of Fe/GaAs and (Co,Ni)Mn(Ga,Ge) for Spintronics Applications: An Ab Initio Study

In this paper, we use density functional theory to compare the usability of Fe(110)/GaAs(110) and new materials like the magnetic Heusler compound Co <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> MnGe or Ni <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> MnGa for spintronics applications. One quantity, which can easily be obtained and which is of major interest, is the degree of spin polarization at the Fermi level for the different systems under consideration. Therefore, we compare the spin polarization at the Fermi level for different configurations. For a Fe-GaAs multilayer system, a spin polarization of 19.6% is achieved at the interface, which increases towards the bulk polarization of Fe within a few monolayers. But, if only 1/4 monolayer of Fe is covering the GaAs surface, a strong ionic relaxation modifies the density of states, and thus, destroys the spin polarization. The Co <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> MnGe/MgO multilayer system shows an even larger polarization of 60.7% at the interface. Although a martensitic transformation shifts the pseudogap towards the Fermi level for the Ni <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> MnGa system, Co states close this gap.

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.

Structure, magnetization, and low-temperature spin dynamic behavior of zincblende Mn-rich Mn(Ga)As nanoclusters embedded in GaAs

We have fabricated a set of samples of zincblende Mn-rich Mn(Ga)As clusters embedded in GaAs matrices by annealing (Ga,Mn)As films with different nominal Mn content at 650 °C. For the samples with Mn content no more than 4.5%, the Curie temperature reaches nearly 360 K. However, when Mn content is higher than 5.4%, the samples exhibit a spin-glass-like behavior. We suggest that these different magnetic properties are caused by the competing result of dipolar and Ruderman–Kittel–Kasuya–Yosida interaction among clusters. The low-temperature spin dynamic behavior, especially the relaxation effect, shows the extreme creeping effect which is reflected by the time constant τ of ∼1011 s at 10 K. We explain this phenomenon by the hierarchical model based on the mean-field approach. We also explain the memory effect by the relationship between the correlation function and the susceptibility.

Excitons in coupled quantum dots: hydrostatic pressure and electric field effects

AbstractThe variational procedure, in the effective‐mass and parabolic‐band approximations, is used in order to investigate the effects of hydrostatic pressure and in‐growth direction applied electric field on the exciton states in vertically GaAs–Ga1–x Alx As coupled quantum dots. We have found that when the symmetrical lengths and radius of each QD are comparable with the Bohr radius of the GaAs material and for a finite value of the applied electric field, the binding energy always diminishes with the length of the central barrier because the two carriers in the exciton are localized in two well defined different regions of the system. However, for the zero electric field the binding energy decreases with the barrier width from the limit value corresponding to the exciton confined in one quantum dot of volume V up to reach a minimum and then increases to a value which corresponds to that of the exciton confined in an isolated quantum dot of volume V /2. Additionally, we have found that the applied electric field caninduce that the lowest structure in the photoluminescence‐peak energy transitions be associated to spatially indirect excitons, situation which in the zero limit of the electric field and independent of the dimensions of the two coupled dots, always corresponds to spatially direct excitons. The main hydrostatic pressure effect reveals an increasing in the exciton binding energy, without modifying the direct or indirect exciton regime, and a well defined rigid blue‐shift in the photoluminescence peak energy transitions in the presence of an applied electric field. (© 2009 WILEY‐VCH Verlag GmbH &amp; Co. KGaA, Weinheim)

Simultaneous generation and detection of ultrashort voltage pulses in low-temperature grown GaAs with below-bandgap laser pulses

We present a method that allows for the simultaneous generation and detection of ultrashort voltage pulses, which propagate on a planar transmission line, in the same material with laser pulses of the same wavelength. The generation is accomplished by exciting band-tail states below the fundamental bandgap of a low-temperature grown GaAs layer, while the detection takes advantage of the electro-optic effect in the GaAs material. This simple scheme considerably enhances previous measurement techniques and is capable of generating and measuring frequencies exceeding 1 THz. The optimum wavelength for the combined generation and detection technique is found at ∼900 nm.

High-Voltage GaAs Photovoltaic Laser Power Converters

The main drawback of photovoltaic (PV) laser power converters based on GaAs material is the low output voltage, which is often insufficient to power electronic circuits directly. Aside from the use of a dc-dc converter in combination with a single PV converter, it is possible to boost the voltage by the monolithic serial interconnection of several converter segments on a single chip, often called multiple converters. Another novel approach introduced in this paper is a multijunction PV cell, where several subcells of the same material are stacked onto a substrate and interconnected by interband tunnel diodes. This paper explores a tandem GaAs-GaAs converter and compares the results with a 2-V multiple PV converter under monochromatic laser illumination. In addition, 4-V multiple PV converters with maximum monochromatic efficiencies (810 nm) of up to 50.1% at 51.6 W/cm2 and 6-V converters are investigated. Thereby, the fabrication technology of the devices is outlined and the influence of inhomogeneous illumination on the performance of these energy converters is discussed.

Investigation of C60 Epitaxial Growth Mechanism on GaAs Substrates

Intensity oscillations of reflection high-energy electron diffraction are observed during epitaxial growth of a C60 layer on GaAs (111)B, (111)A, and (001) substrates. The frequencies of the oscillations coincide well with the growth rates of C60 layers, suggesting that C60 layers grow by repeating nucleation and step-flow growth mode, as is the case with GaAs and other semiconductor materials. Anomalous oscillations are observed in the initial stage of C60 layer growth on a GaAs (111)B surface with (2×2) structure. These oscillations indicate that the growth of the first C60 layer is completed at the point of approximately 0.5 monolayer coverage by C60 molecules. This phenomenon is explained by a model in which C60 adsorption sites are limited by As trimers adsorbed on the GaAs (111)B surface. A similar relationship between C60 adsorption and surface reconstruction is observed on (001) GaAs substrates, i.e., the first C60 layer on the (2×4) surface is terminated at approximately 0.5 monolayer coverage, while full coverage is needed of the c(4×4) surface. These observed results are strongly supported by the reported results of scanning tunnel microscopy.

An off-center donor and nonlinear absorption spectra of spherical quantum dots

In the present work, off-center donor impurity, the linear and the third-order nonlinear absorption coefficients have been studied by using perturbation method. Numerical results for typical GaAs material show the quantum size effect. Also, we find that the size of the dot, and the position of impurity have a great effect on the linear, the nonlinear and the total absorption coefficients. Additionally, the total optical absorption saturation intensity can be controlled by the size of dot or the incident optical intensity.

Negative Uniaxial Crystal Behavior Of Circular Photonic Crystal

<para xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> In this paper, we introduce an unconventional photonic crystal (PhC) geometry which defines two resonance frequencies. The presented circular PhC structure behaves as a negative uniaxial crystal and admits two preferred propagation directions defined by an extraordinary and an ordinary refractive index representing two field polarizations. The circular grating profile splits the electromagnetic field into a radial (extraordinary field) and a tangential (ordinary field) component, which represent two modes of the periodic structure. The total field in the PhC slab is generated by the superposition effect of the ordinary and extraordinary field produced in the 2-D periodic plane. This field configuration is obtained by the analogy with the dielectric multilayer structure. The presented PhC circular lattice pattern consists of air holes in a GaAs material <formula formulatype="inline"><tex Notation="TeX">$({n}=3.408)$</tex> </formula> along circular concentric lines which have the same distance defined as the PhC <emphasis emphasistype="bold"><emphasis emphasistype="italic">a</emphasis></emphasis> period. We validate the birefringence theory by the comparison between analytical and numerical results and then between the numerical and experimental ones. </para>