P. S. S. Guimaraes

Broadening of the Si doping layer in planar-doped GaAs in the limit of high concentrations

Abstract A series of samples of GaAs planar doped with Si, grown by MBE at low substrate temperatures and with different doping concentrations, is investigated. A comparison of Shubnikov-de Haas measurements and self-consistent numerical calculations shows that a broadening of the doped region occurs in spite of the low growth temperature. The broadening occurs via segregation of the Si impurities with the growth surface when the solid solubility limit of Si in GaAs is exceeded. For the growth conditions used this limit is determined to be (2.1 ± 0.2) × 1019 cm−3. At high doping densities an intrinsic compensation mechanism becomes active, limiting the concentration of conduction electrons.

Evidence for negatively charged DX-center in Si-doped AlGaAs from persistent photoconductivity measurements

Abstract Persistent photoconductivity experiments were carried out in Si-doped AlxGa1-xAs at 77 K. The density of persistent free electrons was changed by a factor of 4 to 8. The electron mobilities micro were observed to increase for increasing electron densities n and from the behavior of micro(n) the variation of the ionized scattering impurity densities Ni with n was evaluated. The densities Ni show little change as compared to n. It is concluded that negatively charged centers are the only or at the least the dominant deep centers related to silicon impurities in AlGaAs.

Impurity pairing in the optical absorption of doped semiconductors

Abstract We developed a theoretical approach based on the Alternant Molecular Orbital Model for the hydrogen molecule to predict the optical absorption spectrum of donor pairs in doped semiconductors. The fundamental absorption line has been obtained through the density of states in the pair approximation while the interaction between the lattice and the electron excitation has been considered in terms of the Franck-Condon model.

P337 The failure of a rhythm…not always cortical…

Objective To present two cases illustrating non-epileptic paroxysms only detected in a long-term video-electroencephalography (LTVEEG). Methods Case report. Results Case 1: 40xa0years-old, female, followed in an epilepsy clinic. She described a stereotyped ascending epigastric sensation, palpitations and blurred vision with a simultaneous dreaming state “as watching movie frames”. These sensations were brief, without subsequent confusion. It began at 12xa0years-old with a variable frequency, without remission although antiepileptic drugs. Routine electroencephalogram (EEG) revealed a doubtful right focal epileptiform temporal activity. MRI showed a cystic lesion in the right mesial temporal region. Electrocardiography (ECG) Holter was normal. During LTVEEG it was recorded two usual episodes with concomitant complete atrioventricular block with syncopal EEG features. Case 2: 20xa0years-old, female, with generalized tonic-clonic seizures since 9xa0years-old. At 19xa0years-old, she developed paroxysms described as a fainting sensation accompanied by a lower limbs tremor. LTVEEG showed a generalized epileptiform activity and a variable RR interval. It was recorded a fainting episode while collecting blood samples. Simultaneous ECG revealed a progressive increment of the RR interval ending in a ten seconds asystolia and EEG showed alpha depression, followed by a low voltage fast activity and subsequent generalized high amplitude delta. Discussion These two cases emphasize the usefulness of the LTVEEG in the differential diagnosis of epilepsy mimics. Cardiac dysrhythmias are probably underestimated mimic and it is essential to search carefully ECG abnormalities. Conclusion In case of doubtful phenomenology or drug-resistant epilepsy, LTVEEG with ECG recording is crucial to a definitive diagnosis.

Photo-induced transition in the conduction regime of a two-dimensional system

Abstract We report on experimental evidence of a transition in the conduction regime of a two-dimensional electron gas in GaAs with a single layer of silicon planar doping. The magnetic field and temperature dependence of the electrical conductivity show a gradual change from activated to metallic type conduction under illumination with below band-gap radiation. We propose that the mechanism for the change in the conduction regime occurs via an EL2-like defect. The electrons photo-excited to the conduction band from this deep level are spatially separated from their parent defects due to the high intrinsic electric field. At low temperatures the return tunneling probability of these electrons is low and the concentration of carriers in the potential well in the region of the doping layer is adjusted by the total light dose that the sample has been submitted to.

Hot electrons in delta-doped GaAs(Si) layers

Abstract Low temperature (2K) photoluminescence measurements have been performed in delta-doped GaAs(Si) layers, with carrier sheet concentrations Ns ranging from 1012 to 1013cm−2. Under the same excitation intensity, the luminescence spectra of all samples show a high energy tail which becomes more pronounced as Ns increases. Well defined electron temperatures, higher than the lattice temperature, are obtained, characterizing the electron heating effect in this system. The architecture of the delta-doping allows to observe hot electrons even at relatively low laser pumping intensities.

A method to estimate the self-consistency effects on electronic structure calculations

Abstract A simple method to estimate the effects of the self- -consistency on the calculation of the electronic structure of molecule and crystals is proposed. This procedure is based on the charge transfer between the constituent atoms and is intended to avoid much computational effort. Numerical results are presented for GaAs.