M. S. Kagan

Electroluminescence at 7 terahertz from phosphorus donors in silicon

Terahertz (THz) emissions corresponding to intracenter transitions of phosphorus impurities in silicon have been observed up to 30K. Electrical pulses (250ns) with a repetition rate of 413Hz were used for excitation, and the peak power was calculated to be ∼20μW∕facet for a 190×120μm2 device with a peak pumping current of 400mA at 12K. THz emission intensity increased linearly with pumping current and quenched when the sample temperature was above 30K. The current–voltage characteristics suggested a conduction and excitation mechanism by injection of electrons from a Schottky barrier followed by impact ionization of the neutral impurities.

Terahertz emission from electrically pumped gallium doped silicon devices

Current pumped terahertz (THz) emitting devices have been fabricated from gallium doped silicon. The time resolved peak power was 12μW per facet at a peak pumping current of 400mA, and the emission was observed up to temperatures near 30K. The spectra occurred in two distinct series at 7.9–8.5THz, and at 13.2–13.8THz. The emission was attributed to the radiative transitions of holes from the split sublevels of the 1Γ8 excited state to the sublevels of the 1Γ8+ ground state and the 1Γ7+ ground state, yielding an energy separation of 22±0.07meV between the two ground states. These results indicated that emitters based on Ga impurity transitions open up a range of THz frequencies, and the properties of their spectra can improve the understanding of impurity level physics.

Resonant acceptor states and terahertz stimulated emission of uniaxially strained germanium

The stimulated emission spectrum of uniaxially strained p-Ge is presented. The energy spectrum of the states of a shallow acceptor in Ge under uniaxial compression is calculated. The threshold pressure at which the acceptor state split off from the ground state becomes resonant is found. The pressure dependence of the width of this resonant level is calculated. The stimulated emission lines are identified. In particular, it is shown that the principal emission peak corresponds to the transition of holes from the resonant 1s (1sr) state to the local p±1 state. The probabilities of optical transitions are calculated. A mechanism of population inversion due to the intense resonant scattering of hot holes with an energy corresponding to the position of the 1sr level is proposed.

Electrical passivation of Si∕SiGe∕Si structures by 1-octadecene monolayers

The passivating effects of organic monolayers of 1-octadecene deposited onto the silicon surfaces of both n and p conductivities were studied for Si∕SiGe∕Si structures grown by molecular beam epitaxy and chemical vapor deposition. Measurements of the capacitance versus voltage and current versus voltage were made on structures covered with the organic monolayer and compared with unpassivated structures covered with native silicon dioxide. The results demonstrate that the organic passivation provides a decrease of surface charge and an increase of carrier concentration in the near-surface layers and/or the SiGe quantum wells.

Electron localization in a nondegenerate semiconductor with a random potential due to charged impurities

The saturation of the electron mobility, as determined according to the magnetoresistance, was observed in a semiconductor with a large-scale potential due to charged impurities. It was shown that the saturation is due to the existence of a quantum mobility threshold. A negative magnetoresistance of nondegenerate electrons, which is due to the suppression of quantum interferences corrections to the conductivity by the magnetic field, was found. The magnitude of these effects near the mobility threshold was explained by the absence of short, closed, electronic trajectories in the large-scale potential. A relation was established between the amplitude of the random potential and the saturated values of the mobility and the quantum corrections to the conductivity.

The design and operation of TeraHertz sources based on silicon germanium alloys

During the past few years, vigorous studies have begun on semiconductor devices that generate and detect frequencies from 0.3 - 10 TeraHertz (1000 30 /spl mu/m). Previous THz sources were based on electrical methods using transistor oscillators (to 0.5 THz), diode frequency multipliers (to 2.5 THz), and femtosecond optical pulse switches. Infrared emitters such as the Quantum Cascade Laser in the III-V semiconductors have been difficult to extend to THz frequencies due to reststrahlen absorption by polar phonons. In contrast, Si has lower absorption and devices may be able to operate over a broader THz range than the III-V semiconductors. This report describes the fabrication and characterization of THz sources based on three different design approaches: intersubband transitions in Silicon Germanium quantum wells, resonant state transitions in boron-doped strained SiGe quantum wells, and dopant impurity transitions in doped Si layers.

Effect of a terahertz cavity on the conductivity of short-period GaAs/AlAs superlattices

The miniband conductivity in short-period GaAs/AlAs superlattices with a terahertz cavity has been studied. A stepwise decrease in the current caused by the formation of the electrical domains has been observed in current–voltage characteristics at a certain threshold voltage. It has been found that this threshold voltage changes considerably under the variation of the cavity parameters. The shift of the threshold has been explained by the excitation of high-amplitude oscillations in the cavity.

Deep-level spectroscopy studies of confinement levels in SiGe quantum wells

The recharging of quantum confinement levels in SiGe quantum wells (QWs) was studied by charge deep-level transient spectroscopy (Q-DLTS) for Si/SiGe/Si structures with different Ge contents in the SiGe layer. The set of levels were observed as the different slopes in the Arrhenius plots for the same Q-DLTS peak in different temperature ranges. These activation energies were compared to the energies of quantum confinement levels in the QW calculated in frames of six-band model taking into account spin-orbit interaction and attributed to a thermally activated tunneling of holes from the SiGe QW.

Capacitance study of selectively doped SiGe/Si heterostructures

Capacitance studies of Si/Si1?xGex/Si heterostructures with three Si1?xGex layers selectively doped with boron are presented. A surface charge and carrier distributions in Si1?xGex/Si heterostructures were found as a function of Ge content. The large positive charge observed at the surface of p-Si/SiGe/Si heterostructures was shown to result from the surface segregation of Ge during the epitaxial growth. The effects of native oxide and additional annealing in hydrogen ambient on this charge were also found. It was found that population of quantum wells strongly increased after exposure of the SiGe/Si structures under voltage.

Shallow Acceptor States in SiGe Quantum Wells

The temperature dependences of conductivity and hole mobility in boron doped SiGe quantum well structures were studied. The conductivity at low temperatures is shown to be due to hopping over neutral B centers while at higher temperatures, it is due to two-stage excitation including thermal activation of holes from the ground to strain-split B states following by hole tunneling into the valence band.