Plenarvorträge - DPG-Tagungen
Plenarvorträge - DPG-Tagungen
Plenarvorträge - DPG-Tagungen
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Halbleiterphysik Montag<br />
HL 4.6 Mo 11:30 H13<br />
Ladungsträger-Dynamik in Stickstoff-implantiertem GaAs —<br />
•S. Sinning, T. Dekorsy und M. Helm — Forschungszentrum Rossendorf,<br />
Postfach 51 01 19, 01314 Dresden<br />
III-V-Halbleitern mit geringen Stickstoffkonzentrationen gelten<br />
sowohl als Materialsystem als auch für Anwendungen seit einiger<br />
Zeit größeres Interesse. Neben epitaktischen Methoden bietet die<br />
Ionen-Implantation einen effektiven Weg zur Einbringung des Stickstoffs<br />
in das Substrat. Nachteil dieses Verfahrens ist der dem Gitter durch die<br />
Ionen-Implantation zugefügte Schaden.<br />
Wir untersuchen die Effektivität des Einbaus von aktivem Stickstoff<br />
nach Implantation und thermischer Ausheilung (RTA). Bei RTA-<br />
Bedingungen von 650 ◦ C/30s ist der Einbau des Stickstoffs in die Matrix<br />
optimal. Eine Verbesserung der Gitterqualität kann durch Implantation<br />
bei erhöhten Temperaturen (T > 200 ◦ C) erreicht werden. Die Ladungsträger-Dynamik<br />
im sub-Pikosekunden-Bereich für Implantationen<br />
bei Raumtemperatur und bei erhöhten Temperaturen wird mit der von<br />
nicht-implantiertem GaAs verglichen. Es werden signifikante Unterschiede<br />
beobachtet, die auf eine starke Modifizierung der Bandstruktur des<br />
stickstoffhaltigen GaAs zurückzuführen sind.<br />
HL 4.7 Mo 11:45 H13<br />
Stimulated bosonic scattering in the exciton-biexciton system<br />
of a ZnSe single quantum well — Daniel Hägele, •Stefan<br />
Pfalz, and Michael Oestreich — Universität Hannover, Institut<br />
für Festkörperphysik, Abteilung Nanostrukturen, Appelstraße 2, 30167<br />
Hannover<br />
Although it has been long known that semiconductor excitons and<br />
biexcitons are quasiparticles of bosonic nature, all experiments claiming<br />
bosonic effects are discussed controversially. Here, we give direct experimental<br />
evidence that the decay of a spin-zero biexciton into an exciton<br />
and a photon with opposite spins (±1) is subject to bosonic enhancement<br />
caused by the presence of other excitons. In an optically created<br />
spin polarized gas of excitons, biexcitons decay preferentially into that<br />
final state whose resulting exciton has the same spin orientation as the<br />
majority of excitons. We measure in a 10 nm ZnSe quantum well a photoluminescence<br />
polarization degree of up to 8 % at the biexciton emission<br />
line due to stimulated bosonic scattering. The biexcitonic PL polarization<br />
is opposite to that of the excitons which exhibit a polarization degree of<br />
−50 %. While the polarization of the exciton line can be fully understood<br />
by the usual optical spin selection rules, the biexcitonic polarization can<br />
only be explained by the bosonic nature of excitons.<br />
HL 4.8 Mo 12:00 H13<br />
How fast is the insulator-to-metal transition in VO2? — •T.<br />
Dekorsy 1 , A. Cavalleri 2 , H.H. Chong 2 , J.C. Kiefer 3 , and R.W.<br />
Schoenlein 2 — 1 Forschungszentrum Rossendorf, 01314 Dresden —<br />
2 Materials Sciences Division, Lawrence Berkeley National Laboratory,<br />
Berkeley, CA — 3 Universite du Quebec, INRS energie et materiaux,<br />
Varennes, Quebec<br />
We investigate the photo-induced insulator-to-metal transition in VO2,<br />
a strongly correlated semiconductor with monoclinic structure. The<br />
insulator-to-metal transition (Tc= 341 K) is initiated by photo-doping<br />
of holes into the correlated valence band with high-power femtosecond<br />
laser pulses. Experiments with time-resolutions ranging from the 100fs<br />
regime down to 10-fs reveal a minimum time constant for the phase<br />
transition to be accomplished of approximately 75 fs. This observation<br />
gives evidence for a temporal bottleneck of the phase transition since an<br />
atomic rearrangement and symmetry increase for the formation of the<br />
high-temperature metal phase has to take place. Femtosecond excitation<br />
of VO2 with low power laser pulses reveals the excitation of low-energy coherent<br />
phonons which have displacements along the coordinates relevant<br />
for the phase transition. Interestingly the observed temporal bottleneck<br />
coincides with half the period of these phonons. Our findings demonstrate<br />
a new approach for the investigation of the interplay between atomic and<br />
electronic structure on ultrashort time scales.<br />
HL 4.9 Mo 12:15 H13<br />
Optically induced realspace-transfer between mesoscopic quantum<br />
dots — •Claus Metzner and Dominik Stehr — Technische<br />
Physik I, Universität Erlangen Nürnberg, Erwin-Rommel-Str. 1, 91052<br />
Erlangen<br />
We theoretically investigate a physical scenario in which packets of<br />
electrons (less than 100) are moving coherently over mesoscopic distances<br />
in a two-dimensional potential landscape, created by laterally selective<br />
doping of a quantum well. The motion of the charge packets is controlled<br />
and monitored in a purely optical way by using ultrashort light (or THz)<br />
pulses, acting as time-dependent electric driving forces. In the strong field<br />
regime, the interacting few-particle system shows a complex, nonlinear,<br />
collective dynamics, partially corrupted by stochastic phonon scattering<br />
events. Nevertheless, a controlled transfer between metastable states<br />
(corresponding to spatially separated potential minima, or ‘meso-dots’)<br />
can be achieved with proper light pulses. The pulse parameters can be<br />
found automatically by evolutionary optimization.<br />
HL 4.10 Mo 12:30 H13<br />
THz photomixer based on quasi-ballistic transport in AlGaAs<br />
nipnip-superlattices — •Frank H. Renner 1 , O. Klar 1 , S.<br />
Malzer 1 , M. Eckardt 1 , A. Schwanhäußer 1 , G. Loata 2 ,<br />
T. Löffler 2 , H. Roskos 2 , D. Driscoll 3 , M. Hanson 3 , A.C.<br />
Gossard 3 , and G.H. Döhler 1 — 1 Insitut für Technische Physik I,<br />
Universität Erlangen-Nürnberg , Germany — 2 Lehrstuhl für Ultrakurzzeitphysik,<br />
Universität Frankfurt a.M., Germany — 3 Materials<br />
Departement, UC Santa Barbara, U.S.A.<br />
Conventional THz-photomixers are antenna-structures on a LT-GaAslayer<br />
and are based on the photoconductivity of LT-GaAs, characterized<br />
by an extremely short carrier-lifetime . We have developed a novel concept<br />
for photomixing based on the quasi-ballisitic transport of electrons<br />
in AlGaAs-i-layers. In this concept, the emitter is not limited by the lifetime<br />
of the photogenerated carriers and its efficiency proves to be higher<br />
than in conventional LT-photomixers.<br />
The emitter consists of a stack of nano-pin-diodes. The lengths and<br />
Al-contents of the i-layers in this nipnip-superlattice are optimized for<br />
the transport of the carriers. Recombination of the photogenerated carriers<br />
takes place inside the recombination-enhanced np-diodes between the<br />
nano-pin-diodes. The THz-modulated current is fed into an attached planar<br />
antenna, which is either a resonant dipole-antenna or a non-resonant<br />
spiral- or log. periodic-antenna.<br />
In this contribution we present the concept of the nipnip-emitter and<br />
its realization in the AlGaAs material system, including experimental<br />
results on the THz-output and frequency response of the emitter.<br />
HL 4.11 Mo 12:45 H13<br />
Unipolar impact ionization in GaAs/AlGaAs heterostructures<br />
— •O. Schmidt 1 , M. Eckardt 1 , A. Schwanhäusser 1 , G.H.<br />
Döhler 1 , S. Trumm 2 , M. Betz 2 , F. Sotier 2 , M. Hanson 3 , and<br />
A.C. Gossard 3 — 1 Technische Physik I, Universität Erlangen-<br />
Nürnberg — 2 Physik-Department E11, Technische Universität München<br />
— 3 Materials Department, UCSB Santa Barbara, USA<br />
Impact ionization and the resulting avalanche multiplication in pindiodes<br />
represent one of the technically most feasible methods for detecting<br />
and amplifying small optical signals. The ratio of the electron and<br />
hole ionization coefficient α/β determines the ultimate multiplication and<br />
noise performance of Avalanche Photodiodes. This study indicated, that<br />
the ratio α/β is much higher in a specially designed heterostructure than<br />
in a comparable homogeneous AlGaAs-diode.<br />
Our bandgap engineered AlGaAs pin heterostructure allows for spatially<br />
selective carrier injection by a wavelength adapted pump pulse. This<br />
makes it possible to measure multiplication factors for unipolar initiated<br />
transport and deduce the ionization coefficient for electrons and holes, respectively.<br />
From steady state experiments we found that in graded band<br />
structures the electron ionization coefficient is similar to that of homogeneous<br />
structures, whereas the hole coefficient is much smaller.<br />
Our Monte-Carlo calculations are supporting these findings. To gain more<br />
information about differences of electron and hole impact ionization with<br />
a resolution of fs in time and nm in space, in addition, we are performing<br />
two color fs pump and probe experiments.<br />
HL 4.12 Mo 13:00 H13<br />
Phasenaufgelöste Pulsreflexion an gepumpten ZnSe-Schichten<br />
— •Matthias Seemann, Frank Kieseling, Heinrich Stolz,<br />
Günter Manzke und Klaus Henneberger — Universität Rostock,<br />
Fachbereich Physik, D-18051 Rostock<br />
Wir berichten über Reflexionsexperimente eines schwachen 150 fs Testpulses<br />
an einer ZnSe-Epitaxieschicht. Die Reflexion kann durch die komplexe<br />
dielektrische Funktion (DF) beschrieben werden. Diese kann im<br />
linearen Fall mit Hilfe der spektralen Interferometrie ohne Kenntnis der<br />
Phase des Testpulses bestimmt werden. Der Halbleiter wird durch einen<br />
nichtkollinearen resonanten Anregungspuls optisch gepumpt. Dieser generiert<br />
Ladungsträger (Exzitonengas oder ein Elektron-Loch-Plasma),