Plenarvorträge - DPG-Tagungen

Halbleiterphysik Freitag
HL 52.2 Fr 12:30 H17
Störbandleitung in Al-dotiertem 6H-SiC — •Michael Krieger,
Kurt Semmelroth und Gerhard Pensl — Lehrstuhl für Angewandte
Physik, Universität Erlangen-Nürnberg, Staudtstr. 7/A3, D-91058 Erlangen
Al-dotiertes 6H-SiC wurde mittels Kapazitäts-Spannungs- (C-
V), Admittanzspektroskopie- (AS), Leitfähigkeits- und Hall-Effekt-
Messungen untersucht. Für C-V und AS wurden Schottky-Kontakte
auf Wafer-Bereichen mit unterschiedlicher Al-Konzentration präpariert
([Al] = 1.8 × 10 17 cm −3 . . . 3.4 × 10 18 cm −3 ). C(T)- bzw. G/ω(T)-Kurven,
aufgenommen an Kontakten mit niedriger [Al], zeigen jeweils eine Stufe
bzw. einen Peak, die dem Al-Akzeptor zugeordnet wird. Eine zweite Stufe
bzw. ein zweiter Peak wurde an Kontakten mit [Al] ≥ 7 × 10 17 cm −3
beobachtet. Der Arrhenius-Plot der G/ω(T)-Peak-Maxima ergibt eine
Aktivierungsenergie von 17meV, die als thermische Aktivierungsenergie
für Hopping-Leitung in einem Störband interpretiert wird. Die gleiche
Energie wurde auch aus Leitfähigkeitsmessungen an dem gleichen Wafer-
Bereich wie für AS ermittelt. Für T > 160K entspricht die Steigung der
ρ(1/T)-Kurve der Aktivierungsenergie des Al-Akzeptors; für T < 160K
wurde aus der Steigung eine Aktivierungsenergie von 17meV ermittelt.
Bei T ≈ 75K ändert der Hall-Effekt, gemessen an der gleichen Probe,
sein Vorzeichen von plus nach minus. Die Ergebnisse werden im Rahmen
der Arbeiten über Störbandleitung von Mott und Twose bzw. Shklovskii
und Efros diskutiert.
HL 53 Elektronentheorie
HL 52.3 Fr 12:45 H17
Low Density of Interface States in 4H-SiC MOS Capacitors
Generated by Nitrogen Implantation — •Florin Ciobanu 1 ,
Gerhard Pensl 1 , Valeri Afanas’ev 2 , and Günter Wagner 3
— 1 Lehrstuhl für Angewandte Physik — 2 Department of Physics,
University of Leuven, Belgium — 3 Institut für Kristallzüchtung, Berlin
Nitrogen (N)-doped 4H-SiC epilayers (thickness = 8µm, [N] =
2.2·10 16 cm −3 ) were, in addition, implanted with N at concentrations varying
between 1·10 17 cm −3 and 3·10 19 cm −3 (Gaussian profile with maximum
located 30nm apart from the wafer surface). MOS capacitors were fabricated
on the N-implanted surface by nominally dry oxidation at 1120 ◦ C
for 24 h (oxide thickness = 110nm) followed by a post-oxidation anneal at
the same temperature in Ar for 60 min. High-frequency conductance measurements
reveal an interface state density DIT in the low 10 10 eV −1 cm −2
- range close to the conduction band edge. This value is about 3 orders of
magnitude lower than corresponding values for standard-processed 4H-
SiC MOS capacitors. However, the N implantation causes a negative
flatband shift of the C-V characteristics, which probably originates from
a fixed charge at the interface (≈ 10 12 elementary charges/cm 2 ). The effect
of the implanted N concentration on the density of interface states
and on the flatband shift is investigated and discussed.
Zeit: Freitag 12:00–12:30 Raum: H13
HL 53.1 Fr 12:00 H13
A renormalization approach for the Anderson model in d = 1
and d = 2 at the band edge: Scaling of the localization length
— •Stefanie Russ — Institut für Theoretische Physik III, Universität
Giessen, D-35392 Giessen, Germany
A renormalization procedure for the electronic wave functions of the
one- and two-dimensional Anderson model with diagonal disorder is developed
that applies close to the band edges. This theory leads in d = 1 to
a scaling form of the localization length λ ∼ 〈ǫ 2 〉 −1/3 f((2 − |E|)/〈ǫ 2 〉 2/3 ),
in agreement with former works. In d = 2 a similar scaling form applies
for the localization volume V (participation ratio), i.e. V 1/2 ∼
〈ǫ 2 〉 −1/2 g((4 − |E|)/〈ǫ 2 〉). Here, 〈ǫ 2 〉 is the variance of the site potentials,
E is the energy and f(x) and g(x) are the scaling functions
(f(0) = g(0) = 1).
λ and V close to the band edges are studied by numerical simulations
and confirm this scaling ansatz. The scaling functions f(x) and g(x) show
a crossover between the two regimes x ≪ 1 and x ≫ 1. It is shown that
these two regimes refer to Λ ≫ λ0 and Λ ≪ λ0, respectively where Λ is
the wavelength, λ0 in d = 1 is the localization length at the band edge
and λ0 in d = 2 is the effective localization length V 1/2 at the band edge.
HL 53.2 Fr 12:15 H13
A Ground State based Exact-Exchange Formalism: Implementation
and Applications — •Matthias Wahn 1 und Jörg Neugebauer
2 — 1 Fritz-Haber-Institut der Max-Planck-Gesellschaft, Faradayweg
4-6, D-14195 Berlin-Dahlem — 2 Universität Paderborn, Fakultät
für Naturwissenschaften, FB 6 Physik, Warburger Straße, D-33095 Paderborn
Recent studies have shown that the exact treatment of the exchangepotential
(EXX-method) within the Kohn-Sham formalism significantly
improves the description of bandgaps within density-functional theory [1].
The original formulation is based on a Greens function approach requiring
the calculation of a large number of unoccupied states thus making
the approach computationally rather expensive. We have therefore developed
an alternative formulation [2], recently also proposed by [3], which
is solely based on ground-state quantities and implemented it into our
plane wave pseudopotential program SFHIngX. Compared to the original
formulation it speeds up the calculations significantly and requires
also much less memory. The new approach has been successfully applied
on various III-V and II-VI semiconductors. Also, it turned out that it
provides a direct way to analyze and visualize electronic self-interaction
effects.
[1] M. Städele, M. Moukara, J.A. Majewski, P. Vogl, and A. Görling,
Phys. Rev. B 59, 10031 (1999).
[2] Matthias Wahn, Diplomarbeit, TU Berlin (2002)
[3] S. Kümmel and J.P. Perdew, Phys. Rev. Lett. 90, 043004 (2003)