M. Eckardt

Enhanced recombination tunneling in GaAs pn junctions containing low-temperature-grown-gaas and ErAs layers

We report electrical conductivity studies of highly-doped GaAs pn diodes containing a strongly n-doped low-temperature-grown (LT)–GaAs layer and pn junctions containing an approximately one monolayer thick ErAs layer. At room temperature, current densities of 1 kA/cm2 for the n-LT–GaAs samples and 6u2002kA/cm2 for the ErAs samples at 1 V forward bias have been measured. The I–V characteristics under forward bias for the n-LT–GaAs and ErAs samples exhibit significantly different behavior. At low temperatures, the n-LT–GaAs samples reveal a shoulder in the I–V characteristics, which can be explained by a model taking into account tunneling of carriers into LT midgap states. A similar model was able to explain the current transport in the ErAs diodes as tunneling of carriers into metallic regions inside the pn junction.

Novel concept for efficient THz-emitters based on quasi-ballistic transport in an asymmetric superlattice

We report on a novel concept for a THz-photomixer, which is not (as usually) limited by the carrier lifetime in GaAs and allows impedance matching to the attached antenna. The concept is based on quasi-ballistic electron transport in a doping superlattice.

Transient high-field transport and electro-optical properties of AlxGa1−xAs-grading-structures

Abstract Highfield transport phenomena in specially designed Al x Ga 1− x As heterostructures are investigated. A semiclassical Monte Carlo simulation provides the theoretical carrier evolution in space and time. Transient absorption spectra for the theoretically obtained charge distributions are calculated to extract information from femto-second pump and probe experiments.

Ultrafast spectroscopy of impact ionization and avalanche multiplication in GaAs

Femtosecond carrier dynamics in biased AlxGa1−xAs heterostructure diodes is investigated tracing transient modifications of the Franz-Keldysh absorption spectrum. The nonlinear optical response is sensitive to the number of electron-hole pairs in the high-field region of the sample. As a result, the dynamical buildup of a nonequilibrium carrier avalanche due to impact ionization for electric fields F⩾350kV∕cm is directly analyzed in the time domain. The time scale of the carrier multiplication is found to be in the order of 10ps depending on the number of photoinjected carriers. Monte Carlo simulations in a simplified band structure agree well with the experiment.

Femtosecond spectroscopy of unipolar nanometer-scale high-field transport of holes in Al0.08Ga0.92As

High-field transport in GaAs is investigated tracing ultrafast modifications of the Franz–Keldysh absorption spectrum of a AlxGa1−xAs heterostructure diode. A sophisticated sample design allows us to isolate the unipolar tranport properties of holes in combination with a nanometer scale definition of layers for both photoexcitation and detection of the propagating carrier distribution. Transient velocities and spatial broadening of the hole ensemble are directly measured for electric fields between 15 and 200 kV∕cm comparing room temperature operation to results for TL=4K. Even at low temperatures, the transient hole velocities are found not to exceed a value of 1.2×107cm∕s which is a result of ultrafast optical phonon emission with a scattering time below 25 fs.

High-field electron transport in GaAs/AlxGa1−xAs p–i–n–i–p-structures investigated by ultrafast absorption changes

Abstract High-field electron transport is studied in a GaAs/AlxGa1−xAs p–i–n–i–p-structure with a specially designed composition profile. Electron transfer in real space and energy relaxation is investigated via time-resolved measurements using various probe energies. We observe a transfer time of ∼10 ps for the transport of electrons over 600 nm . This time is found to be nearly independent of the reverse bias voltage. The experimental findings are in good agreement with Monte Carlo simulations.

THz carrier oscillations in GaAs heterostructures detected via two color femtosecond pump probe spectroscopy

Realistic Monte Carlo simulations as well as first experimental data show that classical, quasi ballistic, phase-coherent electron oscillations are possible in relative wide suitably designed confining potentials. Three different confining potentials have been investigated and the oscillation frequencies expected from the calculations were observed in the pump & probe experiments.

High-field electron transport in AlxGa1−x as diodes investigated tracing ultrafast absorption changes

Abstract The topic of this contribution is the investigation of transient, nonequilibrium high-field electron transport in GaAs. Our two key ingredients are a unique laser system, which provides fs pump and probe pulses of independently tunable wavelengths and an AlGaAs hetero p–i–n diode with sophisticated design. This combination allows us to obtain time- and spatially resolved information about the temporal evolution of a photo-generated electron ensemble from ballistic to side-valley dominated transport, without disturbing hole contributions. Fields ranging from 7 to 180xa0kV/cm are investigated. Results are compared successfully to Monte Carlo Simulations.

THz Collective Real-Space Oscillations of Ballistic Electrons in Wide Parabolic Potential Wells: an Exotic Transport Regime

After femtosecond photoinjection of carriers near the boundary of a parabolic shaped 250 nm wide potential well, electrons are found to coherently oscillate across the well instead of performing the intuitively expected unidirectional relaxation towards the bottom of the well. Surprisingly, the coherence of the periodic electron motion is maintained despite multiple phonon scattering events. This novel transport regime is predicted by Monte Carlo simulations and verified by analyzing the femtosecond transmission of the heterostructure.

Ballistic high-field transport in mesoscopic confining potentials—observation of THz oscillations in AlxGa1−xAs heterostructures

Abstract Realistic Monte-Carlo simulations as well as first experimental data show that classical, quasi-ballistic, phase-coherent electron oscillations are possible in relatively wide and suitably designed confining potentials. Two different confining potentials have been investigated and the oscillation frequencies expected from the calculations were observed in the pump and probe experiments.