H. Sigg

Magnetotransport and cyclotron resonance investigations of the 2D electron gas in e−-beam irradiated AlGaAs/GaAs heterojunctions

Abstract A 2D electron gas (2DEG) with a controlled density of repulsive scatters at the interface is obtained by irradiation of AlGaAs/GaAs heterojunctions with 1 MeV electrons. Measurements of the magnetotransport yield that the plateaus in the Hall resistance are at their quantized values R xy = h / e 2 v ( v is the integer filling factor) but show a significant broadening. The lineposition h ω c of the cyclotron resonance is found to be shifted to higher energies such that ( h ω rmc ) 2 = ( h ω rmc 0 ) 2 + ( h ω) 2 with h ω c 0 the energy before irradiation and h ω≊30 cm − . From the investigation on partially up to fully annealed samples we found that ( h ω) 2 is proportional to n sc the density of scatters of the SDEG.

Cyclotron resonance of two-dimensional electron systems under the influence of ionized impurity-scattering

Abstract Two-dimensional electron gases in AlGaAs/GaAs heterostructures with well defined additional impurity doping at the interface have been investigated by means of cyclotron resonance. Additional δ-doping in various concentrations within the spread of the wavefunction enabled the investigation of the specific influence of impurities on the CR in the two-dimensional electron gas. Donor (Si)-doped systems exhibit a pronounced, filling factor correlated oscillation of the CR linewidth and of the effective mass. Acceptor (Be)-doped systems exhibit a second resonance over a wide range of magnetic fields and a strong linewidth-narrowing in the quantum limit (ν ⩽ 1), when only a single spin Landau level is occupied. The origin of these doping-specific effects is discussed.

Effects of the filling factor on the cyclotron resonance in gated AlGaAs/GaAs heterostructures

Abstract AlGaAs/GaAs heterostructures have been prepared with large area semi-transparent front gates. This gave us the possibility to vary the charge density Ns over a wide regime from Ns = 1 × 1010 to 5 × 1011 cm−2 and to study in detail filling factor (v) dependent effects on the cyclotron resonance (CR) mass m ∗ , linewidth and amplitude. We found abrupt jumps for m ∗ near v = 1 and v =2 and explain our results by interacting combined CR from different Landau and spin levels and by energy renormalization near integer v.

A novel FIR spectrometer for cyclotron resonance emission studies in semiconductor research

Abstract In this work we describe a simple but powerful set-up to detect very sensitively cyclotron resonance emission from the two-dimensional electron gas system in AlGaAs/GaAs heterostructures. Both, the emitting sample and the magnetically tuned detector are placed at different positions, in the same magnet. This assures a very stable and reliable operation in a standard magnet system. The performance of the system is demonstrated by the first investigation of the cyclotron resonance emission of an e−-beam irradiated AlGaAs/GaAs heterostructure. We compare the emission data with the results of cyclotron resonance transmission experiments.

Percolation at v=1 in the 2D electron system oF e−beam irradiated AlGaAs/GaAs heterostructures

Abstract We report on the low temperature AC and DC magneto transport properties of e − -beam irradiated AlGaAs/GaAs heterojunctions. This work has been motivated by earlier investigations of the cyclotron resonance (CR) which show a spectacular difference between the AC and DC mobility. While the DC transport in such irradiated samples reflects a very low mobility of the carriers, the CR linewidth is very small for high frequencies and at magnetic fields corresponding to the extreme quantum limit of single Landau level occupation, suggesting a high mobility of the carriers. We show that at low temperature the longitudinal resistance strongly increases when the system is in this quantum limit of an apparent high mobility CR. This discrepancy between AC and DC behaviour is attributed to a percolation of the 2D electron system.

Magneto-Capacitance in Two-Dimensional Electronic Systems in AlxGa1−xAs/GaAs Heterostructures Under the Influence of Ionized Impurities

We have studied the magneto-capacitance of impurity-influenced two-dimensional electronic gases(2DEG’s) at the interface of AlGaAs/GaAs-heteTostTuctmes. The 2D-carrier density as evaluated from the Shubnikov de Haas-type oscillations indicates an impurity-specific filling of the lowest spin-split Landau level. We interpret this behavior with the bound states of impurities, which have to be taken into account when considering the degeneracy of the 2DEG in the magnetic quantum limit.

Far-IR emission spectroscopy on electron-beam-irradiated A1GaAs/GaAs heterostructures

The electrically excited cyclotron resonance (CR) emission is investigated for the 2-dimensional electrongas in AlGaAs/GaAs heterostructures. The effect of sample inhomogeneities and impurities has been inves-tigated by degrading samples by e-beam irradiation and by growing samples with an impurity scatteringlayer close to the interface. The CR absorption has been studied for comparison. A good correspondanceof the two methods has been found: peak shifts are exactly reproduced and Landau level filling factordependent linewidth oscillations occur in both cases. However, the absolute value and the oscillation am-plitude of the linewidth are larger for the emission. This effect is explained by electrical heating and themagnetic field dependent current distribution, and forms a strong limitation for such current driven CRemission sources.

Mobility enhancement by regular positioning of donor impurities

Abstract We discuss the cyclotron resonance in epitaxially grown n-GaAs, for which the Si impurities are positioned in a regular sequence of layers, instead of a random distribution in the volume. Using this as a starting point, we consider the possibility of ordered planar arrays.

Influence of Repulsive Scatterers on the Cyclotron Resonance in Two-Dimensional Electron Systems with Controlled Acceptor Impurity Concentration

We review experimental results of the influence of impurities on the cyclotron resonance (CR) in two-dimensional electron systems (2DES). Using A1GaAs/GaAs samples with well-defined species and concentration of acceptor impurities we can analyze in detail several effects on the CR, e.g., filling factor dependent linewidth, strong linewidth narrowing, impurity concentration dependent shifts of the CR line position etc. We show that these effects are determined by collective properties of the random distribution of repulsive scatterers, leading to a reinterpretation of the harmonic potential model by Mikeska and Schmidt. In the light of our experiments on samples with well-defined impurities we can show, that much of the experimental CR data obtained so far even in so-called high mobility samples, are still essentially governed by the influence of impurities.

Cyclotron Resonance of 2D Electron Systems in Intentionally Doped AlGaAs/GaAs Quantum Wells and Heterostructures

An introduction is given to various aspects of the electron-impurity interaction that can be realized and studied in intentionally and selectively doped two-dimensional (2D) electron systems. We exemplify this by presenting transport and far infrared (FIR) measurements in differently doped A1GaAs/GaAs heterostructures and quantum well structures. The FIR response is studied for electrons bound to 2D confined impurities of donors in a quantum well. In the same system we have studied the transition from an insulator to metal behavior. We contrast the results on donor doped systems with our experiments on acceptor doped AlGaAs/GaAs heterostructures. The experimental findings for doping induced phenomena on the cyclotron resonance of 2D-electron systems lead us to discuss the validity of the single particle model of bound states.