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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),

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