V. G. Mokerov

Electron transport and optical properties of shallow GaAs/InGaAs/GaAs quantum wells with a thin central AlAs barrier

Shallow GaAs/InGaAs/GaAs quantum well structures with and without a three-monolayer thick AlAs central barrier have been investigated for different well widths and Si doping levels. The transport parameters are determined by resistivity measurements in the temperature range 4–300 K and magnetotransport in magnetic fields up to 12 T. The (subband) carrier concentrations and mobilities are extracted from the Hall data and Shubnikov–de Haas oscillations. We find that the transport parameters are strongly affected by the insertion of the AlAs central barrier. Photoluminescence spectra, measured at 77 K, show an increase of the transition energies upon insertion of the barrier. The transport and optical data are analysed with the help of self-consistent calculations of the subband structure and envelope wavefunctions. Insertion of the AlAs central barrier changes the spatial distribution of the electron wavefunctions and leads to the formation of hybrid states, i.e., states which extend over the InGaAs and the delta-doped layer quantum wells.

Photoluminescence and electron subband population in modulation doped AlGaAs/GaAs/AlGaAs heterostructures

The anomalies in photoluminescence spectra of modulation doped AlGaAs/GaAs/AlGaAs double barrier heterostructures with various GaAs quantum well (QW) widths are observed. The intensity of the photoluminescence peak excited by Ar laser and associated with electron optical transitions from the second QW subband to the valence band becomes higher than for the case of the first (lowest) subband in the GaAs QW width range L=22.5–30 nm. The calculations of electron-polar optical phonon scattering rate show that the well-width dependence of the intersubband electron scattering rate due to emission of confined and interface optical phonons is responsible for the photoexcited electron subband population redistribution and anomalies in the photoluminescence spectra. The transition rate for the second QW subband electron binding to the exciton state is evaluated as 25×1010 s−1 at L<22.5 nm and 4×1010 s −1 at 22.5

Ionizing-Radiation Response of the GaAs/(Al, Ga)As PHEMT: A Comparison of Gamma- and X-ray Results

An experiment is reported on the effect of 60Co gamma rays or 45-keV x-ray photons on the GaAs/(Al, Ga)As PHEMT. It is shown that x-ray treatment can improve the dc performance of the device in some cases. This finding is attributed in part to the annealing or modification of DX centers.

Electron transport in unipolar heterostructure transistors with quantum dots in strong electric fields

A model for the explaining specific features of the electron transport in strong electric fields in the quantum-dot unipolar heterostructure transistor (AlGaAs/GaAs/InAs/GaAs/InAs) is presented. It is shown that the two-step shape of the output current-voltage characteristic ID(VD) and the anomalous dependence of the drain current ID on the gate voltage VG are caused by the ionization of quantum dots in the strong electric field at the drain gate edge. The ionization of quantum dots sets in at the drain voltage VD that exceeds the VD1 value, at which the ID(VD) dependence is saturated (the first step of the I-V characteristic). With the subsequent increase in VD, i.e., for VD>VD1, the ID(VD) dependence has a second abrupt rise due to the ionization of quantum dots, and then, for VD=VD2>VD1, the current ID is saturated for the second time (the second step in the current-voltage characteristic). It is suggested to use this phenomenon for the determining the population of quantum dots with electrons. The model presented also describes the twice-repeated variation in the sign of transconductance gm=dID/dVG as a function of VG.

Electron mobility in a AlGaAs/GaAs/AlGaAs quantum well

An oscillatory dependence of the electron mobility on the quantum well (QW) thickness in a AlGaAs/GaAs/AlGaAs heterostructure with double-sided modulation doping has been observed experimentally. A steep decrease in mobility with increasing electron concentration in the QW is established. The conditions for an increase in mobility on introducing a thin barrier into the QW are determined. The first experimental observation of an increase in mobility by a factor of 1.3 in a QW of thickness L=26 nm upon introducing a thin (1–1.5 nm) AlAs barrier is reported.

Molecular beam epitaxy growth of a planar p?n junction on a (111)A GaAs substrate, using the amphoteric property of silicon dopant

Molecular beam epitaxy has been applied to grow planar and only Si-doped epitaxial n- and p-type layers on GaAs substrates with (111)A orientation. The morphology of the n-layers is significantly better than that of the p-layers. However, in both cases, the photoluminescence spectra and carrier mobility show no significant difference from the same characteristics of (100) crystal samples. Planar p–n junctions have also been grown. Depending on the structure of the layers, the I–V characteristics have a form which is typical of conventional or inverted diodes.

Characterization of Selectively Doped InAs-Quantum-Dot GaAs-Based Multilayer Heterostructures by High-Resolution X-ray Diffraction

GaAs-based heterostructures with one, two, or three InAs quantum-dot layers are examined by high-resolution x-ray diffraction. The quality and structure of the specimens are characterized from rocking curves. In the case of three quantum-dot layers, two different superlattices alternating in the lateral direction are discovered in the specimen. The superlattices differ in the InAs content of the GaAs matrix. It is shown that the existence of two superlattices is due to the vertical correlated growth of quantum dots and is related to an InAs wetting layer.

Optical and transport properties of short period InAs/GaAs superlattices near quantum dot formation

We have investigated the optical and transport properties of short-period superlattices of InAs/GaAs, grown by molecular beam epitaxy, with different numbers of periods (3 ≤ N ≤ 24) and a total thickness of 14 nm. Band structure calculations show that these superlattices represent a quantum well with average composition In0.16Ga0.84As. The electron wavefunctions are only slightly modulated by the superlattice potential as compared to a single quantum well with the same composition, which was grown as a reference sample. The photoluminescence, the resistance, the Shubnikov–de Haas effect and the Hall effect have been measured as a function of the InAs layer thickness Q in the range of 0.33 ≤ Q ≤ 2.7 monolayers (ML). The electron densities range from 6.8 to 11.5 × 1011 cm−2 for Q ≤ 2.0 ML. The photoluminescence and magneto-transport data show that only one sub-band is occupied. When Q ≥ 2.7 ML, quantum dots are formed and the metallic type of conductivity changes to variable range hopping conductivity.

Decrease of channel conductivity with increasing sheet electron concentration in modulation-doped heterostructures

The great decrease of electron mobility with increasing sheet electron concentration nS>5×1015 m−2 in modulation-doped Al0.25Ga0.75As/GaAs/Al0.25Ga0.75As and Al0.25Ga0.75As/In0.19Ga0.81As/GaAs quantum wells is observed experimentally. At nS>1016 m−2 a conductivity decreases with increasing the sheet electron concentration. The calculations of electron mobility limited by polar optical (PO) phonon scattering show that the great increase of electron intrasubband scattering by emission of PO phonons when nS exceeds 1015 m−2 is responsible for the mobility and conductivity decrease. When nS changes in the range of 1015−1017 m−2, the alternate increase and decrease of channel conductivity is observed.

Study of the structural perfection and distribution/redistribution of silicon in epitaxial GaAs films grown by molecular beam epitaxy on (100), (111)A, and (111)B substrates

Silicon distribution before and after thermal annealing in thin doped GaAs layers grown by molecular beam epitaxy on (100)-, (111)A-, (111)B-oriented substrates is studied by X-ray diffraction and SIMS. The surface morphology of the epitaxial films inside and outside an ion etch crater that arises during SIMS measurements is studied by atomic force microscopy. Distinctions in the surface relief inside the crater for different orientations have been revealed. Observed differences in the doping profiles are explained by features of the surface relief developing in the course of ion etching in SIMS measurements and by enhanced Si diffusion via growth defects.