Stefan Schmult

Midinfrared intersubband electroluminescence of Si/SiGe quantum cascade structures

Unipolar intersubband lasers like quantum cascade laser structures might be realized not only in III–V semiconductors but also in Si/SiGe multiple layer structures since no optical transitions across the indirect band gap are involved. We report on well-defined intersubband electroluminescence emission of Si/SiGe quantum cascade structures with different active quantum wells parameters. The complex valence band structure and a nonradiative relaxation rate of about 400 fs were calculated by multiband k·p formalism including Si/Ge segregation effects. The observed spectral shift of the electroluminescence peak from 146 to 159 meV is described well by quantum confinement of the two lowest heavy hole subbands. The electroluminescence observed reveals transverse magnetic polarization, a spectral line shape that changes with the direction of the current, and low-energy line broadening with an increase in temperature and current. All these features are described well by the k·p model calculation.

Midinfrared intraband electroluminescence from AlInAs quantum dots

Midinfrared electroluminescence from a cascade of coupled AlInAs quantum dots and GaAs quantum wells at low temperature (80 K) is demonstrated. At low injection currents, the spectra show a clear peak at 158 meV with a luminescence width of 15 meV which is associated with transitions from the s shells of a resonant subensemble of quantum dots. A Stark shift to 143 meV and spectral broadening is observed at higher injection currents which is associated with luminescence from the inhomogeneously broadened quantum-dot ensemble. The reported design is a possible solution to obtain population inversion in unipolar quantum-dot-cascade structures.

Carbon-doped symmetric GaAs∕AlGaAs quantum wells with hole mobilities beyond 106cm2∕Vs

Utilizing a carbon filament doping source, we prepared two-dimensional hole gases in a symmetric quantum-well structure in the GaAs∕AlGaAs heterosystem. Low-temperature hole mobilities up to 1.2×106cm2∕Vs at a density of 2.3×1011cm−2 were achieved on GaAs (001) substrates. In contrast to electron systems, the hole mobility sensitively depends on variations of the quantum-well width and the spacer thickness. In particular, an increase of the quantum-well width from an optimal value of 15 nm to 18 nm is accompanied by a 35% reduction of the hole mobility. The quality of ultrahigh-mobility electron systems is not affected by the employed carbon-doping source.Utilizing a carbon filament doping source, we prepared two-dimensional hole gases in a symmetric quantum-well structure in the GaAs∕AlGaAs heterosystem. Low-temperature hole mobilities up to 1.2×106cm2∕Vs at a density of 2.3×1011cm−2 were achieved on GaAs (001) substrates. In contrast to electron systems, the hole mobility sensitively depends on variations of the quantum-well width and the spacer thickness. In particular, an increase of the quantum-well width from an optimal value of 15 nm to 18 nm is accompanied by a 35% reduction of the hole mobility. The quality of ultrahigh-mobility electron systems is not affected by the employed carbon-doping source.

Nonradiative relaxation times in diagonal transition Si/SiGe quantum cascade structures

Here, we explore experimentally and theoretically the possibility to prolong the upper hole state nonradiative lifetime of Si/SiGe quantum cascade (QC) structures by using a spatially indirect diagonal transition between two SiGe quantum well ground states. With the recent observation of well resolved midinfrared electroluminescence from heavy hole intersubband transitions in Si/SiGe valence-band QC structures, a Si-based QC laser seems no longer to be out of reach. A long carrier lifetime and maybe population inversion, however, appear to be impossible for structure designs with a vertical intersubband transition studied so far. This is due to the nonresonant behavior of deformation potential scattering dominant in unipolar SiGe. We report on calculations of the band structure using a six-band k⋅p model and of hole deformation potential scattering that predict significantly increased nonradiative lifetimes for large barrier thickness, reaching about 20 ps for 35 A Si barrier layer width. Electroluminesen...

Carbon-doped high-mobility two-dimensional hole gases on (110) faced GaAs

Carbon-doped high-mobility two-dimensional hole gases grown on (110) oriented GaAs substrates have been grown with hole mobilities exceeding 106cm2∕Vs in single heterojunction GaAs∕AlGaAs structures. At these high mobilities, a pronounced mobility anisotropy has been observed. Rashba induced spin-splitting in these asymmetric structures has been found to be independent on the transport direction.

Field-effect-induced midinfrared electroluminescence of a quantum-wire-cascade structure by remote δ-doping

We present a quantum-cascade emitter in the galliumarsenide/aluminum–galliumarsenide (GaAs/AlGaAs) heterosystem whose emission properties are controlled by an additional electric field perpendicular to the transport direction. In our case, the additional field is established by remote δ-silicon doping, which is also responsible for charge carrier supply. The field originating from the δ-doping gives rise to an in-plane confinement creating a quantum-wire cascade. This field-effect quantum-cascade emitter is realized using the cleaved edge overgrowth method. Radiative electronic transitions between discrete energy levels in coupled quantum wires were calculated for such a structure. Without an additional electric field, no significant transport is observed. With a field applied, midinfrared emission is observed at a peak wave number of 1200 cm−1 with a full width at half maximum of 300 cm−1 for a heat-sink temperature of 20 K. The presented sample is an experimental proposal for a unipolar quantum-wire int...

Terahertz tunnel ionization of DX-centers in AlGaAs : Te

Ionization of DX-centers in AlGaAs : Te has been investigated in strong terahertz electric fields of FIR-laser radiation with photon energies much smaller than the impurity binding energy. Detachment of electrons from DX-centers is caused by phonon-assisted tunneling being independent of the field frequency as long as the tunneling time is smaller than the field period. In the opposite case an enhancement of the emission probability with rising frequency has been observed. At very high-field strengths direct tunneling without involving phonons dominates and finally emission rates get frequency independent.

Carbon-doped high-mobility hole gases on (001) and (110) GaAs

Since Stormer and Tsang have introduced the two-dimensional hole gas (2DHG) in the GaAs∕AlGaAs heterosystem, the choice of suitable dopants was limited to beryllium and silicon over the last 20years. Both acceptor atoms have significant disadvantages, i.e., either high diffusion rates or a limitation to specific growth directions. Utilizing a carbon filament doping source we prepared high-quality 2DHGs in the (001) and the nonpolar (110) crystal plane with carrier mobilies beyond 106cm2∕Vs in quantum-well and single-interface structures. Low temperature magnetoresistance measurements recover a large number of fractional quantum Hall effect states and show a pronounced beating pattern from which the Rashba-induced spin splitting has been determined. In addition, 2DHGs have been grown on cleaved edges of (110) and (001) wafers with transport features in qualitative agreement with our findings on (110) substrates.

Field‐effect induced mid‐infrared intersubband electroluminescence of quantum wire cascade structures

Employing the Cleaved Edge Overgrowth technique, GaAs/AlGaAs quantum wire cascade structures have been fabricated. The quantum wire states are formed by the perpendicular overlap of two confinement potentials, one resulting from a strong potential modulation generated by quantum wells along the [001]‐crystal direction, and a second resulting from an additional in‐plane confinement generated by a silicon‐δ‐doping along the [110]‐crystal direction. Radiative electronic transitions between discrete energy levels in coupled quantum wires are predicted in these samples. Above a threshold of 200 mA, mid‐infrared electroluminescence is observed at an energy of 150 meV. The devices were temperature controlled at 20 K. The emission intensity is clearly current dependent and rises linearly with a slope efficiency of 0.1 nW/mA up to a maximum output power of 17 nW.

Electrically tunable mid-infrared electroluminescence from graded cascade structures

Mid-infrared electroluminescence (EL) is observed from multiperiod bilayer type-II InAs/AlGaSb structures with the effective interlayer separation controlled by bias. The emission with powers in the microwatt range is characterized by a linear dependence of the photon energy on the bias. By measuring the temperature and current dependence of EL, we find evidence that the EL emission results from recombination of holes in AlGaSb quantum wells (QWs) with electrons occupying two different quantum states in InAs QWs.