GaAs Quantum Wells Research Articles

Mobility of near surface MOVPE grown InGaAs/InP quantum wells

In this work, we study the electron mobility of near surface metal organic vapor phase epitaxy-grown InGaAs quantum wells. We utilize Hall mobility measurements in conjunction with simulations to quantify the surface charge defect density. Buried quantum wells are limited by polar optical phonon scattering at room temperature. In contrast, the quantum wells directly at the surface are limited by remote charge impurity scattering from defects situated at the III–V/oxide interface or inside the oxide, showing a mobility of 1500 cm2/V s. Passivating the InGaAs surface by depositing an oxide reduces the amount of defects at the interface, increasing the mobility to 2600 cm2/V s.

Transfer function replacement of phenomenological single-mode equations in semiconductor microcavity modeling.

Semiconductor microcavities are frequently studied in the context of semiconductor lasers and in application-oriented fundamental research on topics such as linear and nonlinear polariton systems, polariton lasers, polariton pattern formation, and polaritonic Bose-Einstein condensates. A commonly used approach to describe theoretical properties includes a phenomenological single-mode equation that complements the equation for the nonlinear optical response (interband polarization) of the semiconductor. Here, we show how to replace the single-mode equation by a fully predictive transfer function method that, in contrast to the single-mode equation, accounts for propagation, retardation, and pulse-filtering effects of the incident light field traversing the distributed Bragg reflector (DBR) mirrors, without substantially increasing the numerical complexity of the solution. As examples, we use cavities containing GaAs quantum wells and transition-metal dichalcogenides (TMDs).

Effects of 1-MeV Electron Irradiation on the Photoluminescence of GaInNAs|GaAs Single Quantum Well Structure

Minimizing the impact of radiation-induced degradation in dilute nitride based optoelectronic devices is crucial in its applications. The effects of 1-MeV electron irradiation (of 1 × 1014–1 × 1016 e/cm2 range) on undoped GaInNAs|GaAs single quantum-well (QW) structure has been studied by low-temperature photoluminescence (PL). PL spectra of GaInNAs|GaAs QW are measured before and after electron irradiation. The results show a slight enhancement of the PL intensity in relatively low electron fluence, and then subsequent deterioration of PL with the increase of cumulative electron fluences. The enhancement in PL intensity at low electron doses is explained by recombination-enhanced defect reaction model, and the degradation at high electron doses is explained by irradiation-induced defects in the lattice.

Spatio-temporal coherence in vertically emitting GaAs-based electrically driven polariton lasers

We report on the implementation of a GaAs-based, vertically emitting electrically pumped polariton laser operated at cryogenic temperatures. The structure consists of a high quality factor AlGaAs/AlAs microcavity (Q=15 000) with two stacks of four GaAs quantum wells and features a Rabi splitting of 11 meV. Polariton lasing manifests by a clear threshold in the input–output characteristics of our device with a sharp drop in the emission linewidth and a continuous blueshift of 0.7 meV above threshold with increasing injection current. We measure spatial and temporal coherence of our device in the condensed phase by utilizing interference spectroscopy. Our results clearly demonstrate that electrically driven polariton lasers have promise as monolithic polaritonic sources of coherent light.

First-principles hyperfine tensors for electrons and holes in GaAs and silicon

Understanding (and controlling) hyperfine interactions in semiconductor nanostructures is important for fundamental studies of material properties as well as for quantum information processing with electron, hole, and nuclear-spin states. Through a combination of first-principles density-functional theory (DFT) and $\mathbf{k}\cdot\mathbf{p}$ theory, we have calculated hyperfine tensors for electrons and holes in GaAs and crystalline silicon. Accounting for relativistic effects near the nuclear core, we find contact hyperfine interactions for electrons in GaAs that are consistent with Knight-shift measurements performed on GaAs quantum wells and are roughly consistent with prior estimates extrapolated from measurements on InSb. We find that a combination of DFT and $\mathbf{k}\cdot\mathbf{p}$ theory is necessary to accurately determine the contact hyperfine interaction for electrons at a conduction-band minimum in silicon that is consistent with bulk Knight-shift measurements. For hole spins in GaAs, the overall magnitude of the hyperfine couplings we find from DFT is consistent with previous theory based on free-atom properties, and with heavy-hole Overhauser shifts measured in GaAs (and InGaAs) quantum dots. In addition, we theoretically predict that the heavy-hole hyperfine coupling to the As nuclear spins is stronger and almost purely Ising-like, while the (weaker) coupling to the Ga nuclear spins has significant non-Ising corrections. In the case of hole spins in silicon, we find (surprisingly) that the strength of the hyperfine interaction in the valence band is comparable to that in the conduction band and that the hyperfine tensors are highly anisotropic (Ising-like) in the heavy-hole subspace. These results suggest that the hyperfine coupling cannot be ruled out as a limiting mechanism for coherence ($T_2^{\ast}$) times recently measured for heavy holes in silicon quantum dots.

Estimation of the magnitude of the Ruderman-Kittel interaction in 3d and 2d GaAs crystals

In this paper, estimates are made of the Ruderman-Kittel interaction in a gallium arsenide crystal and in GaAs quantum wells of varying widths. The calculation results indicate that this interaction may be weaker than the magnetic dipole-dipole interaction by an order of magnitude. Also discovered was a relatively strong dependence of this interaction on the level of doping.

Symmetry breaking of the persistent spin helix in quantum transport

We exploit the high-symmetry spin state obtained for equal Rashba and linear Dresselhaus interactions to derive a closed-form expression for the weak localization magnetoconductivity -- the paradigmatic signature of spin-orbit coupling in quantum transport. The small parameter of the theory is the deviation from the symmetry state introduced by the mismatch of the linear terms and by the cubic Dresselhaus term. In this regime, we perform quantum transport experiments in GaAs quantum wells. Top and back gates allow independent tuning of the Rashba and Dresselhaus terms in order to explore the broken-symmetry regime where the formula applies. We present a reliable two-step method to extract all parameters from fits to the new expression, obtaining excellent agreement with recent experiments. This provides experimental confirmation of the new theory, and advances spin-orbit coupling towards a powerful resource in emerging quantum technologies.

Study on Optical Properties and Waveguide Structure Optimization of Semiconductor Lasers with InGaAs Quantum Well

InGaAs quantum wells (QWs) are grown on GaAs substrates by metal-organic chemieal vapor deposition (MOCVD). Sample grown on substrates (100) oriented 2° off towards <111> exhibits relatively high photoluminescence (PL) intensity and narrow full-width at half-maximum (FWHM). Increasing growth rate leads to the enhancement of PL intensity and decrease of FWHM. Asymmetric waveguide layers are applied to reduce the confinement factor of base mode and increase the loss of higher-order mode.

Особенности когерентной спиновой динамики двумерного электронного газа в режиме холловского ферромагнетика

By means of the time-resolved Kerr rotation technique, a spin-depolarized electron system at a filling factor ν = 1 in a GaAs quantum well was studied. At temperatures above 5 K, a transition from a new spin-correlated state to a state with low spin stiffness characteristic of a single-particle electron system was found. A nonlinear contribution to the decay of Larmor oscillations arising at low temperatures, when spin-spin correlations determine the ground state of a two-dimensional electron system, is highlighted. The parameters of the fluctuating magnetic field acting on individual electron spins are estimated.

Active region designs and mounting schemes of THz quantum-cascade laser

THz quantum cascade-lasers (QCLs) are the best sources for generating efficient THz radiations, which are fabricated by the semiconductor heterostructure. The layers of GaAs quantum wells and AlGaAs barriers are formed with different thicknesses to design the heterostructure. In the present paper, we review different designs of THz QCLs active regions which are followed by a comparison between epitaxy-up and down processing techniques of the fabricated wafers. The present review should be helpful for the scientific community working in the scientific area of the THz radiation field and, in particular, for the researchers interested to switch from the epitaxy-up processing of the wafers to epitaxy-down mounting to make possible an increase of the operating temperature of the THz QCLs in the continuous wave operation mode.

Model of n-type quadruple δ-doped GaAs quantum wells

Thomas Fermi (TF) approach was used to represent a model for the n-type quadruple δ-doped (QDD) system in GaAs. This is the first physical model in the literature for the QDD system. The energies, the wave functions and the Fermi energy (EF) were calculated. These calculations were performed for different density of impurities in the doped layer (N2D) and for different separation distances (d) between the doped layers. Preliminary results show that the value of the Fermi level for the first proposed model is well below its expected value. This observation is exploited for a second model consistent with the expected values of the EF. Our results indicate that there is a limit at the bottom of the potential profile for a multilayered doped delta system. This has a great importance for calculations in optoelectronic and transport properties.

Transverse Magnetosonic Waves and Viscoelastic Resonance in a Two-Dimensional Highly Viscous Electron Fluid

In high-mobility materials, conduction electrons can form a viscous fluid at low temperatures. We demonstrate that in a high-frequency flow of a two-dimensional electron fluid in a magnetic field the two types of excitations can coexist: those of the shear stress (previously unknown transverse magnetosound) and those associated with the charge density (conventional magnetoplasmons). The dispersion law and the damping coefficient of transverse magnetosound originate from the time dispersion of the viscosity of the fluid. Both the viscoelastic and the plasmonic components of the flow exhibit the recently proposed viscoelastic resonance that is related to the own dynamics of shear stress of charged fluids in a magnetic field. We argue that the generation of transverse magnetosound, manifesting itself by the viscoelastic resonance, is apparently responsible for the peak in photoresistance and peculiarities in photovoltage observed in ultrahigh-mobility GaAs quantum wells.

Optical Gain in Laser Heterostructures with an Active Area Based on an InGaAs/InGaAlAs Superlattice

An active area based on InGaAs/InGaAlAs superlattice for laser diodes operating in the spectral range between 1535 and 1565 nm is proposed and realized practically. It is demonstrated that using a superlattice increases the mode gain at the same values of the pump-diode current density relative to a typical active-area design based on an array of InGaAs quantum wells.

Interlayer distance effects on absorption coefficient and refraction index change in p-type double-δ-doped GaAs quantum wells**Project supported by PISA 2016–1 UJAT and PRODEP Folio UJAT-245 of México.

In the framework of the Thomas–Fermi (TF) approach, a model for the p-type double-δ-doped (DDD) system in GaAs is presented. This model, unlike other works in the literature, takes into account that the Poisson equation associated with the system is nonlinear. The electronic structure is calculated for heavy and light holes. The changes in the electronic structure result of the distance d between the doped layers are studied. In particular, the relative absorption coefficient as well as the relative refractive index change is calculated as a function of the incident photon energy for heavy holes. The effect of the interlayer distance exhibits, in the absorption coefficient, a red shift of the peak position and a decrease in amplitude when the distance increases. In addition, the relative refractive index change node has a red shift as well as the interlayer distance increases. The calculations show that the effect of the separation between layers has a greater influence on the linear terms. These results are very important for theoretical calculations and engineering of optical and electronic devices based in δ-doped GaAs.

Ultrabroad tuning range (100 nm) of external-cavity continuous-wave high-power semiconductor lasers based on a single InGaAs quantum well.

An external cavity in Littrow configuration based on a reflective diffraction grating and a high-power semiconductor laser based on an asymmetric heterostructure with low optical loss was studied. A continuous-wave optical output power of 13 W with a linewidth of 0.15 nm was achieved for an external-cavity laser. It was shown that a decrease in the length of the laser chip to 1500 µm makes it possible to expand the tuning range of the lasing spectrum to 100 nm, while the maximum side-mode suppression reaches 45 dB, and at the edges of the spectral tuning range it is no worse than 30 dB. It was shown that the main factors in reducing the optical power when tuning the laser spectrum are an increase in the threshold current and internal optical loss of the laser chip.

Tuning the linear and nonlinear optical properties in double and triple δ− doped GaAs semiconductor: Impact of electric and magnetic fields

Abstract We report the optical absorption coefficients of double and triple δ − doped layers inserted symmetrically and asymmetrically in GaAs quantum wells under the influence of electric and magnetic fields within the effective mass approximation. Double and triple δ − doped layers have the same total amount of impurities and facilitate reasonable comparisons. The peak position of the optical absorption coefficients (OACs) only exhibit a blue shift when we increase the magnitude of the electric field for the double δ − doped layers. However, in the case of the triple δ − doped layers, the peak positions of the OACs first red shift and then blue shift with increasing electric field. Another important result is that the forbidden red shift in the variation of the OACs becomes possible by applying a magnetic field when we apply an electric field to the double δ − doped layers). We hope that these new findings will be helpful for the fabrication of a new generation of optoelectronic devices.

Giant Stark effect in coupled quantum wells: Analytical model

Coupled quantum wells have been proposed as candidates for highly polarizable structures due to their near-degenerate and dipole-coupled electronic states. Hence, many interesting applications in linear and nonlinear optics can be envisioned. We analyze this proposal considering a simple structure with a delta-function barrier separating the wells. While very substantial Stark shifts are certainly predicted for this geometry, perturbative estimates based on polarizabilities (and hyperpolarizabilities) fail beyond a critical field strength that depends inversely on the barrier. Hence, a giant Stark effect due to near-degenerate states is invariably limited by a small critical field. Our analytical (hyper) polarizability expressions are applied to find quantitative Stark shifts for GaAs quantum wells and transition-metal dichalcogenide bilayers. The predicted Stark shifts and critical fields agree with the field dependence observed in a range of available experiments.

Magnetosonic Waves in a Two-Dimensional Electron Fermi Liquid

The properties of highly viscous fluids at high frequencies become similar to those of amorphous solids. In particular, the propagation of not only longitudinal acoustic waves (plasmons in the case of an electron fluid), but also transverse acoustic waves associated with shear deformations becomes possible. In this work, the formation of transverse acoustic waves at high frequencies in a two-dimensional electron fluid in a magnetic field is studied. Consideration is performed within Landau’s Fermi-liquid model. It is shown that the dynamics of Fermi-liquid excitations is described by hydrodynamic equations at a rather strong quasiparticle interaction. The Navier–Stokes equation and expressions for high-frequency shear viscosity coefficients are derived. Based on the equations obtained, dispersion laws are calculated for transverse and longitudinal magnetosonic waves. It is shown that the cyclotron frequency entering the viscosity coefficients and the dispersion relation of transverse magnetosonic waves is renormalized and typically becomes lower than the ordinary cyclotron frequency determining cyclotron resonance. The latter fact was apparently observed in the photoresistance of high-mobility GaAs quantum wells in which two-dimensional electrons form a viscous fluid.

Hall effect in a ballistic flow of two-dimensional interacting particles

In high-quality solid-state systems at low temperatures, the hydrodynamic or the ballistic regimes of heat and charge transport are realized in the electron and the phonon systems. In these regimes, the thermal and the electric conductance of the sample can reach abnormally large magnitudes. In this paper, we study the Hall effect in a system of interacting two-dimensional charged particles in a ballistic regime. We demonstrated that the Hall electric field is caused by a change in the densities of particles due to the effect of external fields on their free motions between the sample edges. In one-component (electron or hole) systems the Hall coefficient turns out to one half compared with the one in conventional disordered Ohmic samples. This result is consistent with the recent experiment on measuring of the Hall resistance in ultra-high-mobility GaAs quantum wells. In two-component electron-hole systems the Hall electric field depends linearly on the difference between the concentrations of electrons and holes near the charge neutrality point (the equilibrium electron and hole densities coincide) and saturates to the Hall field of a one-component system far from the charge neutrality point. We also studied the corrections to magnetoresistance and the Hall electric field due to inter-particle scattering being a precursor of forming a viscous flow. For the samples shorter than the inter-particle scattering length, the obtained corrections govern the dependencies of magnetoresistance and the Hall field on temperature.

(Invited) Tunneling FET Device Technology for Ultra-Low Power Integrated Circuits

A tunnel FET (TFET) [1-3] is regarded as one of the most promising steep slope devices, which are effective in reducing the power consumption of LSIs. Here, low Ioff and high ratio of Ion/Ioff are also needed for practical logic devices, in addition to low S.S. However, S.S. values of reported TFETs, averaged over a wide sub-threshold region, are still too high [3]. Also, Si-based TFETs are known to have difficulties in high Ion because of the low tunneling probability due to high and indirect bandgap. Thus, TFETs using Ge/III-V semiconductors and the type-II hetero-structures are expected to boost the TFET performance [1-3]. We are currently working for TFETs using the following material systems, (1) InGaAs TFET [4-9] (2) GaAsSb (source)/InGaAs (channel) TFET [10] (3) Ge TFET [11-13] (4) Ge (source)/strained Si (channel) TFET [4-6, 14] (5) (Si, Ge) (source)/oxide semiconductor (channel) TFET [15]. In this presentation, we address the critical issues of high performance TFETs and the device engineering to solve these issues. After that, we introduce the recent results of the TFETs using the above materials. For InGaAs TFETs, we introduced solid-phase Zn diffusion from Zn-doped SOG films [4-7], in order to realize the source junction with steep impurity profiles. Also, we have proposed an ultrathin quantum well (QW) channel InGaAs TFET [6, 8, 9], where high Ion due to electron confinement into QW and low S.S due to suppression of the junction leakage current are expecxted. In addition, thin EOT and the low interface defect density are also critical to TFETs. We introduced ZrO2/InGaAs gate stacks [9]. However, TFETs without Al2O3 passivation exhibited higher Ioff, higher S.S. and lower Ion, bcause of the higher interface state density at ZrO2/InGaAs interfaces. Insertion of only 1-to-2-cycle Al2O3 can provide the optimal TFET performance. As a result, InGaAs QW TFETs with In0.75Ga0.25As and the ZrO2/Al2O3 gate stack shows the minimum S.S. of 50 mV/dec. at room temperature. On the other hand, one of the ideal TFET structures to realize small S.S. and high Ion can be a bi-layer TFET with vertical tunneling [15]. The small S.S. is attributed to direct modulation of the DOS overlap between the source and the channel over the entire channel region by Vg. Also, high Ion is due to the large tunneling area and the short tunneling distance determined by the channel thickness. In addition, introduction of type-II hetero-structures into bi-layers is quite effective. Thus, we have proposed a novel bi-layer TFETs using a column IV semiconductor (Si, SiGe, Ge)/oxide semiconductor type-II hetero-structure. It has been found from the results of device simulation that extremely-steep minimum S.S. of 1~2 mV/dec. and high Ion of ~100 μA/μm at Vdd can be realized in comparison with the pn-junction-based Ge TFETs. We have experimentally demonstrated the Si/ZnO and Ge/ZnO TFET operation [15]. Here, ZnO films were deposited by PLD. It is confirmed that the ZnO channel layer with the columnar-shaped poly-crystalline structure with the grain size of ~20 nm and the TiN/Al2O3 gate stack [16] have the relatively smooth interfaces. The higher Ion is obtained for the Ge/ZnO TFETs. The minimum S.S. for the p-Si/n-ZnO TFET is 71 mV/dec, which is still higher than expected. Further improvements in the quality, the defect density and the thickness uniformity of oxide semiconductors are expected to provide superior electrical characteristics. This work was supported by JST-CREST Grant Number JPMJCR1332, Japan, and a Grant-in-Aid for Scientific Research (17H06148) from MEXT. The authors would like to thank Drs. M. Yokoyama and T. Yamamoto in Sumitomo Chemical Corporation, and Dr. M. Mitsuhara in NTT for their collaborations.