Modulation-doped Heterojunction Research Articles

Key scattering mechanisms limiting the lateral transport in a modulation-doped polar heterojunction

We present a study of the lateral transport of a two-dimensional electron gas (2DEG) in a modulation-doped polar heterojunction (HJ). In contrast to previous studies, we assume that the Coulomb correlation among ionized impurities and among charged dislocations in the HJ is so strong that the 2DEG low-temperature mobility is not limited by impurity and dislocation scattering. The mobility, however, is specified by alloy disorder scattering and combined roughness scattering, which is the total effect induced by both the potential barrier and polarization roughness. The obtained results show that the alloy disorder and combined roughness scattering strongly depend on the alloy content and on the near-interface electron distribution. Our theory is capable of explaining the bell-shaped dependence of the lateral mobility on alloy content observed in AlGaN/GaN and on 2DEG density observed in AlN/GaN, which have not previously been explained.

Investigation of the terahertz emission characteristics of MBE-grown GaAs-based nanostructures

We report experimental work on the terahertz emission characteristics of InAs/GaAs quantum dot (QD) structures and GaAs/AlGaAs modulation-doped heterojunctions (MDH’s), excited by femtosecond laser. Results showed that the terahertz emission from MDH’s can provide information on the GaAs/AlGaAs interface quality while the QD structures have the potential for being intense terahertz emitters; rivaling the emission intensity of p-type bulk InAs.

Spin–orbit interactions in high In-content InGaAs/InAlAs inverted heterojunctions for Rashba spintronic devices

We have studied molecular beam epitaxy growth and magneto-transport of novel In x Ga 1− x As/In x Al 1− x As ( x=0.5 and 0.75) inverted modulation-doped heterojunctions. Large Rashba-type spin–orbit coupling constants, α∼20×10 −12 eV m, as well as high two-dimensional electron mobilities, μ e∼2×10 5 cm 2/V s, have been confirmed at ∼1.5 K. It was also found that larger α’s were confirmed in the higher In-content heterojunctions with thinner InGaAs surface channel. These results are qualitatively explained by the differences in the energy bandgap, the electron effective mass, and the mean electric field strength at the heterojunction interface. The above features of α and μ e, seem to be promising in the applications in the spintronic devices as well as in the mesoscopic structures for novel spin physics based on the Rashba spin–orbit interaction.

Measuring carrier density in parallel conduction layers of quantum Hall systems

An experimental analysis for two parallel conducting layers determines the full resistivity tensor of the parallel layer, at magnetic fields where the other layer is in the quantum Hall regime. In heterostructures which exhibit parallel conduction in the modulation-doped layer, this analysis quantitatively determines the charge density in the doping layer and can be used to estimate the mobility. To illustrate one application, experimental data show magnetic freeze-out of parallel conduction in a modulation-doped heterojunction. As another example, the carrier density of a minimally populated second subband in a two-subband quantum well is determined. A simple formula is derived that can estimate the carrier density in a highly resistive parallel layer from a single Hall measurement of the total system.

Effect of an in-plane magnetic field on the photoluminescence spectrum of modulation-doped quantum wells and heterojunctions

The photoluminescence (PL) spectrum of modulation-doped $\mathrm{GaAs}∕\mathrm{AlGaAs}$ quantum wells and heterojunctions (HJ) is studied under a magnetic field $({B}_{\ensuremath{\Vert}})$ applied parallel to the two-dimensional electron gas (2DEG) layer. The effect of ${B}_{\ensuremath{\Vert}}$ strongly depends on the electron-hole separation, and we revealed remarkable ${B}_{\ensuremath{\Vert}}$-induced modifications of the PL spectra in both types of heterostructures. A model considering the direct optical transitions between the conduction and valence subbands that are shifted in $k$-space under ${B}_{\ensuremath{\Vert}}$, accounts qualitatively for the observed spectral modifications. In the HJs, the 2DEG-hole PL intensity is strongly enhanced relatively to the bulk exciton PL with increasing ${B}_{\ensuremath{\Vert}}$. This means that the distance between the photoholes and the 2DEG decreases with increasing ${B}_{\ensuremath{\Vert}}$, and thus free holes are responsible for the 2DEG-hole PL.

Quantum transport analysis and narrow-gap heterojunction growth for Rashba-type spintronics devices

Research results of spintronics based on spin-orbit (SO) interaction in non-magnetic semiconductor hetero-junctions obtained recently have been described. Works are based on the two-dimensional electron gases (2DEGs) confined at compound semiconductor narrow bandgap hetero-interface. Due to the electric field originated from the confining potential asymmetry, the 2DEG often yields strong SO interaction which could reveal under no magnetic field. This type of SO interaction (Rashba interaction) can be controlled by the applied gate voltage and hence the field effect transistor (FET) utilizing this principle has so far been proposed and discussed extensively. We describe two recent results in this paper: First is molecular beam epitaxy (MBE) growth of novel narrow-gap modulation-doped heterojunction, InGaSb/InAlSb material system which possibly reveals high quality electronic properties as well as very strong Rashba SO coupling. Recently we indeed obtained the sample with a very large SO coupling constant of ∼40 × 10–12 eVm which is almost comparable to the best value obtained in the former InGaAs/InAlAs systems. Second is relating to the control of Rashba SO interaction in long wires with side gates. Asa result of careful analysis about the dependencies of the SO coupling constant on the gate voltage, we confirmed the side-gate control of the Rashba effect for the first time, which could be a promising result to develop the spin-FET based quantum-bit devices.

Role of Hole Localization in the Magneto-Luminescence of a Two-Dimensional Electron Gas

We investigate the effect of hole localization on the magneto-luminescence emission of a two-dimensional electron system formed in a modulation-doped semiconductor heterojunction. The emission of heterojunctions containing a dilute delta-layer of Be acceptors at a large distance from the interface is compared with that of Be-free samples. A narrow Fermi-edge singularity emission line is observed at low temperature in Be-doped samples, involving electrons in the second confined state. The evolution of this line in a perpendicular magnetic field shows common characteristics with previously reported results in high mobility samples not containing Be. A new emission line appears abruptly at filling factor 2 originating by the recombination of electrons in the lowest Landau level with free valence holes, independent of the presence of Be. The abruptness of this transition, which is also observed in some samples at filling factor 1, reveals a simultaneous change in the electron system over a macroscopic sample area. The new optical emission shows marked deviations with respect to the single-particle behavior, which are tentatively interpreted as the formation of a complex state involving a free photocreated hole and the electron system. This complex unbinds when the Fermi level crosses the mobility edge.

The role of the AlGaAs doping level on the optical gain of two-dimensional electron gas photodetectors

We compare the optical response of two modulation-doped heterojunction photodetectors with identical growth structure, differing only in the doping level of the wide band material. The larger photoresponse of the higher doped device cannot be explained as photocuonductive gain because neither the transit time nor the recombination lifetimes are substantially altered by the change in doping. Alternatively, we propose to explain the results on the basis of an optical gating effect. It is shown that proper control of the two-dimensional electron gas (2-DEG) channel population in dark conditions allows one to optimize optical responsivity and external quantum efficiency.

Study of spin transport in In0.75Ga0.25As/In0.75Al0.25As narrow wires

We systematically studied spin–orbit interaction in narrow wires with different wire widths, where the base material is a normal-type In0.75Ga0.25As/In0.75Al0.25As modulation-doped narrow-gap heterojunction. We used two different methods to make the wires and analyze their spin-dependent transports. As a first method, the wire width was defined by simply mesa-etching the samples on the same substrate. As a second method, we fabricated wire samples with side-gate. The wire width was changed by varying a voltage applied to the side-gate. For the determination of spin–orbit coupling constant, α; we measured magneto-resistance at 1.6 K. In the first method, a remained almost constant in the wires with various widths longer than 0.4 μm. However, in the second method, α increased with decreasing the wire width down to about 1 μm. The increase of α observed in the side-gate structure sample might rather be attributed to the effect of the lateral electric field by the side-gate, which could enhance the effect of the vertical electric field originally existing at the hetero-interface.

Condensation of bulk excitons on a magnetized two-dimensional electron gas in modulation-doped heterojunctions

The evolution of photoluminescence (PL) spectra with increasing magnetic field $(Bl~7 \mathrm{T})$ was studied in high-mobility, wide $\mathrm{GaAs}/{\mathrm{Al}}_{0.3}{\mathrm{Ga}}_{0.7}\mathrm{As}$ heterojunctions (HJ's) at lattice temperatures ${T}_{L}=1.9\char21{}25 \mathrm{K}.$ The two-dimensional electron-gas (2DEG) density in the studied samples is ${n}_{2D}^{0}=(0.9\char21{}3)\ifmmode\times\else\texttimes\fi{}{10}^{11} {\mathrm{cm}}^{\ensuremath{-}2},$ and it was varied with He-Ne laser illumination by optical depletion. For $B=0$ the PL is completely dominated by exciton recombination in the undoped GaAs layer. As B increases this band shows a typical exciton diamagnetic shift. For a filling factor $\ensuremath{\nu}l~2$ a strong PL transformation is observed: the exciton PL intensity decreases and a new, low-energy PL line abruptly appears and gains intensity at the expense of the exciton PL. We attribute this line to the 2DEG-free hole recombination, and propose that its appearance in the HJ's results from an increased free-exciton dissociation near the 2DEG at $\ensuremath{\nu}l~2.$ Thus, the evolution of the bulk free exciton to 2DEG-free hole PL with increasing B is due to ``condensation'' of the bulk excitons on the magnetized 2DEG layer at $\ensuremath{\nu}l~2.$

Uniaxial-stress dependence of Hall effect in an AlGaAs/GaAs modulation-doped heterojunction

Uniaxial compressive stress was applied to an AlGaAs/GaAs heterojunction. The uniaxial stress coefficients of sheet resistivity, sheet electron concentration, and mobility were obtained. The hydrostatic pressure coefficient of sheet resistivity was also obtained and was used to explain the different magnitude of the uniaxial stress coefficients of sheet electron concentration in the [110] and [11̄0] directions. We obtain a value for the piezoelectric constant e14 of AlAs to be −0.26 C/m2, compared to the value −0.225 C/m2 calculated by K. Hübner [Phys. Status Solidi B 57, 627 (1973)].

Study for realization of spin-polarized field effect transistor in In 0.75Ga 0.25As/In 0.75Al 0.25As heterostructure

For realization of the spin-polarized field effect transistor (spin-FET), we investigated a gated modulation of spin–orbit interaction and a spin-injection from a ferromagnetic metal (FM) to a two-dimensional electron gas (2DEG). Prepared samples were an In 0.75Ga 0.25As/In 0.75Al 0.25As modulation-doped narrow-gap heterojunction grown by molecular beam epitaxy (MBE). For the determination of spin–orbit interaction parameter, α, we measured Shubnikov–de Haas (SdH) oscillations at 1.5 K . We confirmed a modulation of α when a gate voltage was applied. We also carried out spin-injection experiments using a multi-terminal geometry sample, which had two different widths Ni 40Fe 60 electrodes. We observed a spin-valve like effect in a source-drain resistance of ∼0.1% as well as a resistance hysteresis behavior of ∼12% in non-local geometry below 20 K . These results are the first step to realize an active spintronic device, such as spin-FET.

Fabrication of GaAs quantum wires by natural selective doping and its characterization by electrostatic force microscope

We report an in situ fabrication method of self-organized quantum wire structures on patterned GaAs substrates by molecular beam epitaxy. The present method is based upon a selective doping mechanism of amphoteric silicon dopant both on patterned (311)A and on (100) GaAs substrates. The surface electrostatic potential distribution on these patterns was investigated by electrostatic force microscope (EFM) and quantum wire structures were verified. A minimum wire width of ≈80 nm was obtained for samples based on both substrate orientations. Good agreement of the wire width estimation by the EFM estimation and the magnetoresistance measurement on modulation-doped heterojunction single quantum wire transistor confirms the validity of the present approach to fabricate self-assembled quantum wires.

Correlation Between Sheet Carrier Density-Mobility Product and Persistent Photoconductivity in AlGaN/GaN Modulation Doped Heterostructures

High quality Al0.25Ga0.75N/GaN modulation-doped heterojunction field-effect transistor (MOD-HFET) structures grown on sapphire substrates with high sheet carrier density and mobility products (nsμ > 1016/Vs at room temperature) have been grown by metal organic chemical vapor deposition (MOCVD). The optimized structures were achieved by varying structural parameters, including the AlGaN spacer layer thickness, the Si-doped AlGaN barrier layer thickness, the Si-doping concentration, and the growth pressure. In these structures, the persistent photoconductivity (PPC) effect associated with the two-dimensional electron gas (2DEG) system was invariantly observed. As a consequence, the characteristic parameters of the 2DEG were sensitive to light and the sensitivity was associated with permanent photoinduced increases in the 2DEG carrier mobility (μ) and sheet carrier density (ns). However, we observed that the magnitude of the PPC and hence the photoinduced instability associated with these heterostructures were a strong function of only one parameter, the product of ns and μ, which is the most important parameter for the HFET device design. For a fixed excitation photon dose, the ratio of the low temperature PPC to the dark conductivity level was observed to decrease from 200% to 3% as the nsμ (300 K) product was increased from 0.048 × 1016/Vs to 1.4 × 1016/Vs. Based on our studies, we suggest that the magnitude of the low temperature PPC can be used as a sensitive probe for monitoring the electronic quality of the AlGaN/GaN HFET structures.

Energy behavior with magnetic field of negatively charged magnetoexcitons in quantum wells and heterojunctions

AbstractWe present the results of the magneto-photoluminescence measurements performed on modulation doped GaAs/AlGaAs heterostructures in high magnetic fields (up to 60T) and low temperatures (0.37−1.5K). With increasing magnetic field we observe the formation of the triplet and singlet states of negatively charged magneto-excitons (X−) in addition to the neutral exciton (X0). Their behavior with field strongly depends on the sample geometry. In the case of a modulation doped quantum well (QW) with a well-width of 200A, the and states cross at a magnetic field of about 40T, whereas for a modulation-doped single heterojunction (SHJ) these states show no crossing over the whole range of available fields.

HEMT modelling using semi-physical expressions for the equilibrium space-charge parameters of the modulation-doped heterojunction

Simple expressions are derived for the 2-DEG concentration in the lightly doped side and the partially depleted space-charge width on the heavily doped side of a modulation-doped heterojunction under equilibrium. The derivation employs physical reasoning to determine the mathematical form of the expressions and fixes the constants in the form empirically so that the results of the expressions agree with those of numerical calculations. It is shown that the expressions derived enable an easy estimation of the maximum gate voltage swing, and can be used to construct a mathematical representation of the nonlinear gradual saturation segment of the 2-DEG concentration against gate voltage characteristics, for circuit simulation purposes.

Correlation between Sheet Carrier Density-Mobility Product and Persistent Photoconductivity in ALGAN/GAN Modulation Doped Heterostructures

AbstractHigh quality Al0.25Ga0.75N/GaN modulation-doped heterojunction field-effect transistor (MOD-HFET) structures grown on sapphire substrates with high sheet carrier density and mobility products (nsμ > 1016/Vs at room temperature) have been grown by metal organic chemical vapor deposition (MOCVD). The optimized structures were achieved by varying structural parameters, including the AlGaN spacer layer thickness, the Si-doped AlGaN barrier layer thickness, the Si-doping concentration, and the growth pressure. In these structures, the persistent photoconductivity (PPC) effect associated with the two-dimensional electron gas (2DEG) system was invariantly observed. As a consequence, the characteristic parameters of the 2DEG were sensitive to light and the sensitivity was associated with permanent photoinduced increases in the 2DEG carrier mobility (μ) and sheet carrier density (ns). However, we observed that the magnitude of the PPC and hence the photoinduced instability associated with these heterostructures were a strong function of only one parameter, the product of ns and μ, which is the most important parameter for the HFET device design. For a fixed excitation photon dose, the ratio of the low temperature PPC to the dark conductivity level was observed to decrease from 200% to 3% as the nsμ (300 K) product was increased from 0.048 × 1016/Vs to 1.4 ‘times; 1016/Vs. Based on our studies, we suggest that the magnitude of the low temperature PPC can be used as a sensitive probe for monitoring the electronic quality of the AlGaN/GaN HFET structures.

Path-integral approach to the electron density of states at the interface of a single modulation-doped heterojunction

The electronic density of states (DOS) at the interface of a single modulation-doped heterojunction is calculated both in the fluctuation band tail and in the semiclassical limit using the path-integral method. Due to charge density inhomogeneities in the heavily doped barrier region, random potential fluctuations are generated in whose minima carriers are localized, resulting in a band-tail density spectrum. The screening of the long-range potential fluctuations, which are important for the problem considered, is accounted for by using the two-dimensional Thomas-Fermi model. The statistical properties of the random impurity charge distribution are taken into account using the binary correlation function of the random potential for the specific geometry of the problem in two limiting cases of the general correlation function. Analytical expressions for the dimensionless functions of the exponential and preexponential of the band-tail DOS, as a function of the energy, are obtained in the weak and strong screening limit for the quasi-2D case under consideration and are compared to the respective functions for the general d-dimensional case. The dependence of the band-tail DOS behavior on the relevant parameters of the system, namely, spacer layer thickness, doping layer thickness, 2D EG thickness, and 3D-impurity concentration is studied numerically for ${\mathrm{Al}}_{x}{\mathrm{Ga}}_{1\ensuremath{-}x}\mathrm{A}\mathrm{s}\ensuremath{-}\mathrm{G}\mathrm{a}\mathrm{A}\mathrm{s}$ modulation doped heterostructures and numerical results for the 2D-DOS are presented. The band-tail results for the 2D-DOS are compared with the Kane approximation derived by taking the limit $\stackrel{\ensuremath{\rightarrow}}{t}0$ in order to determine $\ensuremath{\rho}(E)$ in the whole energy range. We compare the results from the two computed cases of the path-integral expression for the DOS corresponding to the two limits of the general correlation function with each other. On the other hand, we compare the semiclassical limit and the white Gaussian noise limit of the above results with many other theoretical methods such as the generalized semiclassical method, multiple scattering method and the simulations resulting from the tight-binding model.

Investigation of delta-doped quantum wells for power FET applications

We report the use of strain-balanced quantum-well structures to generate high carrier density, high mobility layers suitable for power field effect transistor (FET) applications. Standard designs of modulation-doped heterojunctions have a sheet carrier density limited to a maximum of ∼3× 1012cm−2, while doped channel devices allow higher densities, but with degraded mobility. By combining the technique of delta-doping with the use of a compositionally graded InGaAs quantum well, grown strain balanced on InP, high mobilities and excellent saturation drift velocities have been obtained for sheet densities of 4–5× 1012cm−2. This paper describes the structure and electrical properties of the layers and assesses their potential for FETs.

Collective excitations of electron disks in laterally patterned Si/SiGe modulation-doped heterojunctions

Square lattices of isolated electron disks of diameters 0.2–2 μm were obtained by laterally patterning n-type modulation-doped Si/SiGe heterojunctions grown by ultra high vacuum chemical vapor deposition. The disk lattices were fabricated by electron-beam lithography and reactive ion etching. The collective excitations in the disks were studied by microwave magnetotransmission experiments. The resonance dispersion as well as the disk plasma frequencies are theoretically analysed assuming a nearly parabolic confining potential.