W. W. Rühle

Spin injection into semiconductors

The injection of spin-polarized electrons is presently one of the major challenges in semiconductor spin electronics. We propose and demonstrate a most efficient spin injection using diluted magnetic semiconductors as spin aligners. Time-resolved photoluminescence with a Cd0.98Mn0.02Te/CdTe structure proves the feasibility of the spin-alignment mechanism.

Spin transport in GaAs

We present a spectroscopic method for studying spin transport in semiconductors. Our time-resolved experiments have an important implication for spin electronics as they show that spin-polarized electron drift is possible in semiconductors over typical device lengths in high electric fields. We demonstrate an almost complete conservation of the orientation of the electron spin during transport in GaAs over a distance as long as 4 μm and fields up to 6 kV/cm.

Auger recombination in intrinsic GaAs

The recombination kinetics of the electron‐hole plasma in strongly excited, intrinsic GaAs is investigated at room temperature by time‐resolved photoluminescence using a line‐shape analysis of transient spectra. Special structuring of the samples prevents stimulated emission and diffusion. Population of higher energetic subsidiary conduction‐band valleys must be taken into account for densities ≳1.5×1019 cm−3. A significant influence of Auger recombination is detected for densities ≳2.5×1019 cm−3. The bimolecular recombination coefficient and an effective Auger coefficient are found to be B=(1.7±0.2)×10−10 cm3 s−1 and Ceff=(7±4)×10−30 cm6 s−1, respectively.

Ultrafast (GaIn)(NAs)/GaAs vertical-cavity surface-emitting laser for the 1.3 μm wavelength regime

(GaIn)(NAs) vertical-cavity surface-emitting lasers for room-temperature emission at 1.3 μm wavelength are designed and grown by metal-organic vapor-phase epitaxy using dimethylhydrazine and tertiarybutylarsine. Room-temperature operation at wavelengths up to 1.285 μm is achieved with low optical pumping thresholds between 1.6 and 2.0 kW/cm2. Stimulated emission dynamics after femtosecond optical pumping are measured and compare favorably with results on (GaIn)As/Ga(PAs)-based structures.

Free‐carrier lifetime and deep‐level luminescence in semi‐insulating GaAs: The influence of indium doping and growth in a magnetic field

The influence of In doping and/or of growth in a magnetic field on the properties of Czochralski‐grown semi‐insulating GaAs wafers is investigated. We determine the spatial distribution of the free‐carrier lifetime by time‐resolved luminescence in the ps regime. The results are compared with the spatial distribution of the near‐band‐edge and deep‐level luminescence. The macroscopical and microscopical homogeneity of the carrier lifetime and the luminescence intensities are improved by the growth in a magnetic field. Indium doping leads to similar improvements and additionally to an increase of the absolute value of the lifetime. The combination of In doping and growth in a magnetic field gives the best results.

Spin injection, spin transport and spin coherence

We discuss advances, advantages and problems of spintronics through the example of a semiconductor laser whose emission intensity and polarization are modulated by the spin orientation of electrons. We show that spin transport should be feasible at room temperature and present possible concepts and first results concerning spin injection at high temperatures. Finally, we describe the coherent dynamics of coupled electron and hole spins in a quantum mechanical picture and measure the magnetic field-induced dynamics of localized excitons in a 3 nm GaAs quantum well. The system is capable of performing a quantum controlled not operation (CNOT), which realizes a basic two-qubit operation of quantum information processing in a semiconductor nanostructure.

Ultrabroadband chirped mirrors for femtosecond lasers

We report on the performance of widely tunable femtosecond and continuous-wave Ti:sapphire lasers that use a newly developed ultrabroadband mirror set. The mirrors exhibit high reflectivity (R>99%) and smooth variation of group delay versus frequency over a wavelength range from 660 to 1060 nm. Mode-locked operation with pulse durations of 85 fs was achieved from 693 to 978 nm with only one set of ultrabroadband mirrors.

(GaIn)(NAs)/GaAs vertical-cavity surface-emitting laser with ultrabroad temperature operation range

The temperature dependence of the emission of a (GaIn)(NAs)/GaAs vertical-cavity surface-emitting laser is investigated. We find laser emission over an extremely broad temperature range from 30 K up to 388 K. The laser threshold varies from 5 kW/cm2 at 373 K down to a minimum of 1 kW/cm2 at 180 K and increases again to 4 kW/cm2 at 30 K. Picosecond emission dynamics after femtosecond optical excitation is obtained with peak delays below 33 ps and pulse widths below 20 ps over the entire operation range.

Nonresonant electron and hole tunneling times in GaAs/Al0.35Ga0.65As asymmetric double quantum wells

Nonresonant carrier tunneling is investigated by time‐resolved and time‐averaged optical methods for a series of samples with various barrier thicknesses. The electron tunneling times decrease exponentially with the decrease of barrier thickness from 8 to 3 nm, and the trend is well described by a semiclassical model. Additional efficient hole tunneling is observed in the 3 nm barrier sample, and the time constant is of the order of 50 ps.

Femtosecond carrier kinetics in low‐temperature‐grown GaAs

The carrier energy distribution and recombination kinetics in low‐temperature‐grown and annealed GaAs (with 1% excess arsenic occurring as microclusters/point defects) have been time resolved at 295 K through luminescence upconversion and correlation spectroscopy using an 80 fs, 720 nm Ti:sapphire laser. We infer that the radiative recombination coefficient is the same in GaAs:As as in normal GaAs and that both electron and hole trapping times are about 1 ps. Luminescence is detected at energies as much as 300 meV below the band gap and is identified with delocalized states induced by excess As. We observe that the presence of excess As gives photoexcited carriers a higher initial temperature than in GaAs. Finally, since the carrier density decays faster than the emitted optical phonons, energy relaxation is inhibited more effectively than in normal GaAs.