K. Köhler

Observation of Bloch oscillations in a semiconductor superlattice

We report unambiguous experimental evidence for electron Bloch oscillations: The transient four-wave mixing signal of a biased semiconductor superlattice shows a periodic modulation with a time constant expected from Bloch oscillation theory. The oscillation frequency can be tuned over 400% with the applied electric field. The electron performs up to four oscillation cycles before the coherence is lost.

Quantum Cascade Detectors

This paper gives an overview on the design, fabrication, and characterization of quantum cascade detectors. They are tailorable infrared photodetectors based on intersubband transitions in semiconductor quantum wells that do not require an external bias voltage due to their asymmetric conduction band profile. They thus profit from favorable noise behavior, reduced thermal load, and simpler readout circuits. This was demonstrated at wavelengths from the near infrared at 2 mum to THz radiation at 87 mum using different semiconductor material systems.

Quantum beats of light hole and heavy hole excitons in quantum wells

We report the first observation of quantum beats due to the interference of the polarization decay of heavy hole and light hole excitons in semiconductor quantum wells.

Femtosecond time-resolved optical pump-probe spectroscopy at kilohertz-scan-rates over nanosecond-time-delays without mechanical delay line

We demonstrate a technique for femtosecond time-resolved optical pump-probe spectroscopy that allows to scan over a nanosecond time delay at a kilohertz scan rate without mechanical delay line. Two mode-locked femtosecond lasers with approximately 1 GHz repetition rate are linked at a fixed difference frequency of ΔfR=11kHz. One laser delivers the pump pulses, the other provides the probe pulses. The relative time delay is linearly ramped between zero and the inverse laser repetition frequency at a rate ΔfR, enabling high-speed scanning over a 1 ns time delay. The advantages of this method for all-optical pump-probe experiments become evident in an observation of coherent acoustic phonons in a semiconductor superlattice via transient reflectivity changes. A detection shot-noise limited signal resolution of 7×10−8 is obtained with a total measurement time of 250 s. The time resolution is 230 fs.

Control of the Mg doping profile in III-N light-emitting diodes and its effect on the electroluminescence efficiency

The influence of the Mg doping profile on the electroluminescence (EL) efficiency of (AlGaIn)N quantum well (QW) light-emitting diodes, grown by low-pressure metal-organic vapor-phase epitaxy on sapphire, has been investigated. The actual Mg profile close to the active region was found to be influenced by segregation as well as by diffusion during growth. In a first experiment, diffusion of the Mg dopants towards the QW region through a not intentionally doped narrow GaN spacer layer, separating the topmost GaInN quantum well from the AlGaN:Mg electron-blocking barrier, was controlled by the growth temperature of the AlGaN:Mg barrier and GaN:Mg contact layer. Starting from low growth temperatures, an increase in Mg concentration close to the active region results in an improved hole injection and thus increased EL efficiency. However, for a too high growth temperature, an excessive spread of the Mg atoms into the active region leads to nonradiative recombination in the QW active region and thus a reductio...

GaN-based micro-LED arrays on flexible substrates for optical cochlear implants

Currently available cochlear implants are based on electrical stimulation of the spiral ganglion neurons. Optical stimulation with arrays of micro-sized light-emitting diodes (µLEDs) promises to increase the number of distinguishable frequencies. Here, the development of a flexible GaN-based micro-LED array as an optical cochlear implant is reported for application in a mouse model. The fabrication of 15 µm thin and highly flexible devices is enabled by a laser-based layer transfer process of the GaN-LEDs from sapphire to a polyimide-on-silicon carrier wafer. The fabricated 50 × 50 µm2 LEDs are contacted via conducting paths on both p- and n-sides of the LEDs. Up to three separate channels could be addressed. The probes, composed of a linear array of the said µLEDs bonded to the flexible polyimide substrate, are peeled off the carrier wafer and attached to flexible printed circuit boards. Probes with four µLEDs and a width of 230 µm are successfully implanted in the mouse cochlea both in vitro and in vivo. The LEDs emit 60 µW at 1 mA after peel-off, corresponding to a radiant emittance of 6 mW mm−2.

N-induced vibrational modes in GaAsN and GaInAsN studied by resonant Raman scattering

Vibrational modes introduced by the incorporation of N into GaAs and GaInAs have been studied by Raman spectroscopy on samples grown by molecular-beam epitaxy using a rf nitrogen plasma source. When proceeding from GaAs1−xNx to Ga1−yInyAs1−xNx with x⩽0.04 and y⩽0.12, the nitrogen-induced vibrational mode near 470 cm−1 observed in GaAsN was found to broaden and to split into up to three components with one component at a frequency higher than that of the Ga–N mode in GaAsN. This observation shows that the incorporation of In into GaAsN strongly affects the local bonding of the N atoms by changing the local strain distributions as well as the formation of a significant fraction of In–N bonds. The resonant enhancement in the scattering cross section of the Ga–N vibrational mode, observed in low N-content GaAs1−xNx (x≈0.01) for incident photon energies matching the mostly N-related E+ transition at around 1.8 eV, was found to broaden significantly upon increasing N content as well as upon the addition of In t...

Reduced nonthermal rollover of wide-well GaInN light-emitting diodes

Nonthermal rollover (or efficiency droop) of the electroluminescence (EL) efficiency has been investigated for near-UV-emitting (AlGaIn)N single-well light-emitting diodes (LED) with varying GaInN well widths grown on substrates with different dislocation densities (DDs). For each DD the well width of the mesa-LEDs has been optimized for maximum EL efficiency at high operating currents. LEDs on freestanding GaN (DD 4×107 cm−2) with an 18 nm thick GaInN wide-well active region show the highest efficiency, and the output power-versus-current characteristic remains linear up to the highest pulsed current density of 750 A/cm2. In contrast, LEDs on sapphire grown with conventional low-temperature nucleation (DD 109 cm−2) exhibit the optimum well width at 3 nm and show significant nonthermal rollover.

Coherent control of acoustic phonons in semiconductor superlattices

Coherent acoustic phonons are generated in GaAs/AlAs superlattices by excitation with femtosecond laser pulses. Several modes of the acoustic phonon spectrum are observed, in agreement with the effect of zone folding in the mini-Brillouin zone of the superlattice. By applying successive pump pulses we are able to silence the first back-folded mode near q=0, while selectively enhancing the coherent amplitude of higher order backfolded modes. This increase in the spectroscopic sensitivity opens the way to detailed time resolved studies of higher order acoustic modes in superlattices.

Single Chip White LEDs

The operation principles of the various single chip luminescence conversion (LUCO).white LEDs, that can be realized with blue or ultraviolet (UV) LED chips, are briefly reviewed. In order to demonstrate the feasibility of the so far almost unexplored case of UV pumped tricolor LUCO white LEDs, In x Ga 1-x N single quantum well LED layer sequences were grown by MOCVD and processed into near-UV LED pump chips. Europium doped oxide and sulfide phosphors emitting in the blue, the green and the red spectral range were used for UV to visible conversion. With white blends prepared from these phosphors, housed white LEDs with high color quality were realized.