N. T. Moshegov

A study of the vertical transport of electrons in (GaAs)n(AlAs)m superlattices by Fourier transform infrared spectroscopy

The longitudinal optical vibrational modes have been studied in (GaAs)n(AlAs)m superlattices by means of FTIR spectroscopy. In the undoped samples, confined LO phonons have been observed; the dispersion of LO phonons obtained by FTIR spectroscopy was in good agreement with the Raman data. In the doped (GaAs)n(AlAs)m superlattices, coupling of confined vibrational LO modes with superlattice plasmons has been found. Analysis of the experimental results showed that the frequency of superlattice plasmons in the superlattices studied is mostly determined by the population of the miniband formed by the Gamma -like conduction band states. The fitting of the calculated reflection spectra to the experimental spectra allowed us to measure the population of the minibands, the effective mass and the vertical mobility of electrons.

Transformation of quantum size levels into virtual levels at the boundary between p-GaAs and an AlAs/GaAs superlattice

The transformation of quantum size levels into virtual levels upon a change in the electric field in an AlAs/GaAs superlattice located in the i region of a p-i-n structure is studied experimentally and theoretically. It is shown that an interfacial state at the boundary between the superlattice and the p-GaAs contact layer results in a resonant increase in the probability of photoelectron tunneling from the contact into the superlattice via Wannier-Stark levels.

Photocurrent resonances in short-period AlAs/GaAs superlattices in an electric field

The photocurrent was measured as a function of the external electric field in short-period AlAs/GaAs superlattices for various photon energies. Transport resonances, whose positions do not depend on the photon energy, were observed in these dependences together with optical resonances due to interband transitions in Wannier-Stark levels. It is shown that the transport resonances are due to tunneling of photoelectrons from the p-GaAs contact region into the first level in GaAs wells located 2–5 lattice periods from the contact layer.

Anisotropy of optical phonons in semiconductor superlattices: Raman scattering experiments

Experiments on Raman scattering in the “forward” geometry, permitting observation of anisotropy of the optical phonons, are performed on specially prepared short-period GaAs/AlAs superlattice structures with the substrates removed and the surfaces covered with an antireflective layer. The experimental data agree well with the computational results obtained for the angular dispersion of optical phonons in superlattices on the basis of a modified continuum model.

Fourier‐transform infrared and Raman spectroscopies of plasmon anisotropy in heavily doped GaAs/AlAs superlattices

The plasmon‐longitudinal optical phonon vibrational modes in heavily doped GaAs/AlAs superlattices were analyzed both with Fourier transform infrared and Raman spectroscopies. The modes polarized in the direction normal to the layers were observed in the infrared spectra, while the Raman spectra revealed those modes polarized along the layers. The experimental results allowed us to derive the dispersion relations of the electrons in both directions and these relations were found to be in good agreement with the miniband dispersions calculated in the envelope function approximation when both Γ and X conduction electron states were taken into account. No evidence of the contribution of the two‐dimensional electron states to the vertical transport was found.

Modification of photoelectrical and optical properties of GaAs/AlGaAs quantum-well IR-photodetectors by radiation

Abstract Changes of dark current, photocurrent under black body illumination, IR photoconductivity and IR intersubband absorption spectra caused by 3.5 MeV room-temperature electron irradiation in MBE-grown Si-doped GaAs/AlGaAs multiquantum-well mesa structures (with photoresponse peak at a wavelength of 8.5 gm) are investigated. The following processes were found to occur with a dose increase: (1) decreasing of the dark current in the range of two orders; (2) increasing of photocurrent-to-dark-current ratio by several times; (3) decreasing of intersubband absorption; (4) relatively slight changes in detectivity with nonmonotonical dependence versus bias voltage. However, both photoconductivity and intersubband absorption spectra shapes remained unchanged. A qualitative interpretation of the observed phenomena is given based on the formation of radiation-induced compensating centers. It is proposed to use radiation treatments for local and overall tuning of parameters of quantum well photodetector arrays.

Lasing characteristics of lasers with a vertical cavity based on In0.2Ga0.8As quantum wells

Semiconductor lasers with a vertical cavity with a high external quantum efficiency and high radiation power have been developed and constructed. Powers up to 10 W at T=300 K and 20 W at T=250 K have been obtained for 500 μm aperture lasers operating in the pulsed regime.

Quantum Hall effect in a single-mode wire

The quantum Hall effect in a single-mode wire is studied for the first time. It is established that a well-expressed quantum Hall resistance for filling factors i=1 and 2 is observed in magnetic fields in which the magnetic length becomes less than the width of the wire. Breakdown of proportionality with respect to the magnetic field in the arrangement of the Hall quantization plateau and the dissipative conductivity minima is observed.

Determination of the roughness of heteroboundaries from photocurrent spectra of short-period AlAs/GaAs superlattices

Photocurrent spectroscopy is used to study the nature of the roughness of heteroboundaries in (AlAs)m/(GaAs)n short-period superlattices (m=3−5, n=10−13) grown by molecularbeam epitaxy. The formation of minibands broadens the optical spectra of superlattices in comparison with isolated quantum wells; therefore to analyze the degree of perfection of the boundaries we used the decay of the minibands into a series of discrete Wannier-Stark levels in an electric field parallel to the superlattice axis. Exciton lines were observed in the photocurrent spectra in an electric field corresponding to direct and indirect (in space) transitions between the Wannier-Stark levels. Comparison of experimental data with calculation indicates that even in the better structures, in addition to monotonic variation of the thickness of the layers over area, roughnesses in the heteroboundaries one monolayer in height are present with characteristic lateral dimension not exceeding 10 nm.