Claus Hermannstädter

Temperature dependent carrier dynamics in telecommunication band InAs quantum dots and dashes grown on InP substrates

InAs quantum dots (QDs) grown on InP substrates can be used as light emitters in the telecommunication bands. In this paper, we present optical characterization of high-density circular quantum dots (QDots) grown on InP(311)B substrates and elongated dots (QDashes) grown on InP(001) substrates. We study the charge carrier transfer and luminescence thermal quenching mechanisms of the QDots and QDashes by investigating the temperature dependence of their time-integrated and time-resolved photoluminescence properties. This results in two different contributions of the thermal activation energies. The larger activation energies are attributed to the carrier escape to the barrier layer and the wetting layer (WL) from QDots and QDashes, respectively. The smaller activation energies are found to be originated from inter-dot/dash carrier transfer via coupled excited states. The variation of the average oscillator strength associated with the carrier re-distribution is discussed. The relation of the two activation...

Enhanced Photon Generation in a Nb/n-InGaAs/p-InP Superconductor/Semiconductor-Diode Light Emitting Device

Research Institute for Electronic Science, Hokkaido University, Sapporo 001-0021, Japan 2CREST, Japan Science and Technology Agency, Kawaguchi 332-0012, Japan 3Graduate School of Information Science Technology, Hokkaido University, Sapporo 060-0814, Japan 4Central Research Laboratory, Hamamatsu Photonics, Hamamatsu 434-8601, Japan 5NTT Basic Research Laboratory, Atsugi 243-0198, Japan 6Japan Science and Technology Agency, Kawaguchi 332-0012, Japan Department of Applied Physics, Tokyo University of Science, Tokyo 162-8601, Japan 8Graduate School of Engineering, Hokkaido University, Sapporo 060-8628, Japan (Dated: October 27, 2009)

Inter-dot coupling and excitation transfer mechanisms of telecommunication band InAs quantum dots at elevated temperatures

We investigate the photoluminescence temperature dependence of individual InAs/InGaAlAs quantum dots emitting in the optical telecommunication bands. The high-density dots are grown on InP substrates and the selection of a smaller dot number is done by the processing of suitable nanometer-sized mesas. Using ensembles of only a few dots inside such mesas, their temperature stability, inter-dot charge transfer, as well as carrier capture and escape mechanisms out of the dots are investigated systematically. This includes the discussion of the dot ensemble and individual dots. Among the single-dot properties, we investigate the transition of emission lines from zero-phonon line to acoustic phonon sideband-dominated line shape with temperature. Moreover, the presence of single recombination lines up to temperatures of about 150?K is demonstrated.

Carrier dynamics and photoluminescence quenching mechanism of strained InGaSb/AlGaSb quantum wells

GaSb based quantum wells (QWs) show promising optical properties in near-infrared spectral range. In this paper, we present photoluminescence (PL) spectroscopies of InxGa1−xSb/AlyGa1−ySb QWs and discuss the possible thermal quenching and non-radiative carrier recombination mechanisms of the QW structures. The In and Al concentrations as well as the QW thicknesses were precisely determined with the X-ray diffraction measurements. Temperature dependent time-integrated and time-resolved PL spectroscopies resulted in the thermal activation energies of ∼45 meV, and the overall self-consistent calculation of the band parameters based on the measured physical values confirmed that the activation energies are due to the hole escape from the QW to the barriers. The relation of the present single carrier escape mechanism with the other escape mechanisms reported with other material systems was discussed based on the estimated band offset. The relation of the present thermal hole escape to the Auger recombination wa...

Cooper-Pair Radiative Recombination in Semiconductor Heterostructures: Impact on Quantum Optics and Optoelectronics

The injection of Cooper pairs into a normal medium such as a semiconductor is known as the proximity effect at the superconductor/normal interface. We confirm this injection as well as the contribution of Cooper pairs to a drastic enhancement of inter-band optical transitions in semiconductor heterostructures. In this paper we investigate and clarify the relation of internal quantum efficiencies and radiative lifetimes in Cooper-pair light emitting diodes (CP-LEDs). A quantitative description of the dynamic photon generation processes is given, and the contribution of the Cooper-pair recombination relative to normal-electron recombination in CP-LEDs is discussed in detail.

Precise slit-width control of niobium apertures for superconducting LEDs

We introduce a novel three-step procedure for precise niobium (Nb)-etching on the nanometer-scale, including the design of high contrast resist patterning and sacrifice layer formation under high radio frequency (RF) power. We present the results of precise slit fabrication using this technique and discuss its application for the production of superconducting devices, such as superconductor-semiconductor-superconductor (S-Sm-S) Josephson junctions. For the reactive ion etching (RIE) of Nb, we selected CF(4) as etchant gas and a positive tone resist to form the etching mask. We found that the combination of resist usage and RIE process allows for etching of thicker Nb layers when utilizing the opposite dependence of the etching rate (ER) on the CF(4) pressure in the case of Nb as compared to the resist. Precise slit-width control of 80 and 200 nm thick Nb apertures was performed with three kinds of ER control, for the resist, the Nb, and the underlying layer. S-Sm-S Josephson junctions were fabricated with lengths as small as 80 nm, which can be considered clean and short and thus exhibit critical currents as high as 50 µA. Moreover, possible further applications, such as for apertures of superconducting light emitting diodes (SC LEDs), are addressed.

Drastic enhancement of interband optical transition probability with electron pairing in semiconductors

Interband optical transition probability (usually given as the B coefficient) is normally fixed for a given semiconductor structure. Here we will show the B coefficient can be drastically enhanced effectively with electron injection in paired states.