A. Raymond

Pressure dependence of the cyclotron mass in n-GaAs-GaAlAs heterojunctions by FIR emission and transport experiments

The pressure dependence of the cyclotron mass of quasi-two-dimensional electrons in GaAs-GaAlAs heterojunctions is studied. The authors use cyclotron emission experiments coupled with transport experiments. An increase of 0.5% per kbar of the cyclotron mass is found in the investigated pressure range 0-8 kbar. Their analysis shows that two effects, which act in opposite directions, must be considered: the first is the increase of the mass due to the increase of the energy gap and the second is the reduction of the non-parabolicity corrections due to the decrease of the two-dimensional electron gas density when pressure is applied. This explains that the observed increase of the cyclotron mass is smaller than the one predicted by the kp perturbation method.

Cyclotron emission study of electron masses in GaAs-GaAlAs heterostructures

We study far-infrared cyclotron emission from GaAs-GaAlAs heterostructures, induced by electric pulses in the presence of a magnetic field and hydrostatic pressure. Cyclotron masses are measured as functions of 2D electron density in the strong electron heating regime at the pressures of P=0 and P=7 kbar. The results are described by an effective two-level k.p theory, which takes consistently into account the effect of band non-parabolicity on electric and magnetic quantization. It is shown that the observed emission spectrum is due to several transitions between Landau levels, since the 2D electron gas is non-degenerate. The same theory is used to describe cyclotron absorption data of other authors, obtained at zero and non-zero pressures under the conditions of degenerate electron gas. It is shown that the cyclotron absorption can be measured in two different regimes, which explains a strong dependence of the data on photon energy. A very good theoretical description of various emission and absorption experiments is achieved with the use of bulk GaAs parameters.

Determination of the basic parameters of pseudomorphic GaInAs quantum wells by means of simultaneous transport and optical investigations

Abstract Combined studies of GaAlAs/GaInAs/GaAs pseudomorphic structures have been performed by means of transport under hydrostatic pressure, luminescence and cyclotron resonance emission experiments. The structures investigated were 130 A quantum wells. They were δ-doped outside of the quantum well, with the spacer thickness ranging between 20 and 60 A. These structures allowed us to study the influence of the carrier density Ns on mobility and on the position of the Fermi level EF above the first electrical sub-hand E0. By using pressure and illumination, we changed Ns from 0.5 to 1.8 × 1012 cm−2. In this range, only the first electric sub-band is populated. A strong decrease of mobility at higher concentrations was shown and depended essentially on correlation effects. High magnetic field transport measurements confirmed that only the lowest sub-band was populated. Moreover, cyclotron resonance emission gave the effective mass as a function of the carrier density in the well. On this basis, the importance of the non parabolicity effect was estimated. All these results were interpreted within the framework of the variational method leading to a determination of the most important physical parameters describing this system.

Anharmonicity and asymmetry of Landau levels for a disordered two-dimensional electron gas

We calculate the density of states of a two-dimensional electron gas located at the interface of a strongly disordered

Oscillations of 2D electron density in GaAs/Ga0.67Al0.33As heterostructures in the QHE regime

\mathrm{Ga}\mathrm{Al}\mathrm{As}∕\mathrm{Ga}\mathrm{As}

Charge transfer and electron mobility in GaAlAs/GaAs modulation-doped heterostructures: the role of interface states

heterojunction. The disorder potential is created by two delta doped layers. The first layer includes the parent donors which provides the well with electrons and creates a smooth disorder potential. The second layer is doped with either acceptor or donor impurities, and is located inside the well, thus creating a strongly disordered potential. We calculate the density of states in the presence of a magnetic field of arbitrary strength by taking into account all perturbative terms in the fifth Klauders approximation. We find an anharmonic spectrum, strongly asymmetric, for the Landau level density of states. At low field, the attractive potential creates the well-known band tails or impurity bands. At higher field, we show that impurity bands are also created by repulsive potentials. We discuss the consequences of the anharmonicity and asymmetry on physical properties in the quantum Hall effect regime.

Conduction electrons in acceptor-doped GaAs/GaAlAs heterostructures: a review

Cyclotron resonance (CR) and quantum transport measurements are performed on three GaAs/Ga0.67Al0.33As heterostructures in the quantum Hall regime at T = 2 K. The relaxation time and the 2D electron density Ns are determined fitting the CR transmission curves by the Drude-type model. In all samples the density Ns exhibits oscillations as a function of magnetic field. The dc quantum transport is also studied on the same structures and, assuming that the Hall resistance determines the electron density at all fields, one obtains density oscillations similar to those measured by CR. This density behaviour is modelled using a reservoir hypothesis.

Cyclotron FIR emission from hot electrons in GaAsGaAlAs heterostructures

We develop a theoretical model of charge equilibrium in modulation-doped GaAlAs/GaAs heterojunctions in which the presence of donor-like interface states is considered. Assuming that the electrons occupy the first and second electronic subbands we calculate the charge transfer in the heterojunction as a function of the temperature, taking into account the metastable character of the Si donors in the GaAlAs-doped layer. We show that interface states, the density of which depends on the growth process and on the sample quality, play an important role in the temperature dependence of the charge equilibrium and allow us to explain the variation of the density of the 2D electrons with the temperature. On this basis we investigate in detail the scattering mechanisms, in order to account for the mobility limitation in the low-temperature range (T<10 K). In this case too the role of interface states is found to be crucial. All the calculations are performed within the generalized variational subband wavefunction model.

Conduction electrons localized by charged magneto-acceptors A 2 in GaAs/GaAlAs quantum wells

We review magneto-optical and magneto-transport effects in GaAs/GaAlAs heterostructures doped in GaAlAs barriers with donors, providing two-dimensional electron gas in GaAs quantum wells, and additionally doped with smaller amounts of acceptors (mostly Be atoms) in the vicinity of 2DEG. One may also deal with residual acceptors (mostly C atoms). The behavior of such systems in the presence of a magnetic field differs appreciably from those doped in the vicinity of 2DEG with donors.

Reservoir model for two-dimensional electron gases in quantizing magnetic fields: A review

Abstract We study far infrared emission from GaAsGaAlAs heterostructures, induced by electric pulses in the presence of a magnetic field and a hydrostatic pressure. Cyclotron masses are measured as functions of 2D electron density in the strong electron heating regime at pressures P = 0 and P = 7 kbar and the detection energy of 4.43 meV. The results are described by an effective two-level k · p theory, which takes consistently into account the effect of bands nonparabolicity in GaAs on electric and magnetic quantization. It is shown that the observed emission spectrum is due to eight transitions between Landau levels (populated up to the optic phonon energy), since under the strong heating conditions the 2D electron gas is nondegenerate. This is independently confirmed by magnetotransport measurements. Very good theoretical description of emission experiments at pressures P = 0 and P = 7 is achieved with the use of bulk GaAs parameters. Theoretical estimations of the heating conditions in crossed magnetic and electric fields indicate that the electric field in our GaAsGaAlAs structures is highly inhomogeneous.