Plenarvorträge - DPG-Tagungen

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