Plenarvorträge - DPG-Tagungen

Halbleiterphysik Montag
HL 12 Poster I
Zeit: Montag 16:30–19:00 Raum: Poster A
HL 12.1 Mo 16:30 Poster A
Herstellung und Charakterisierung optisch gepumpter,
oberflächenemittierender Halbleiterlaser — •S. Hövel 1 , N.C.
Gerhardt 1 , M. Hofmann 1 , J. Yang 2 , D. Reuter 2 und A.D.
Wieck 2 — 1 AG Optoelektronische Bauelemente und Werkstoffe, Ruhr-
Universität Bochum — 2 Lehrstuhl für Angewandte Festkörperphysik,
Ruhr-Universität Bochum
Wir charakterisieren die Emission eines optisch gepumpten
oberflächenemittierenden GaInAs/GaAs-Halbleiterlasers (engl. verticalcavity
surface-emitting laser, VCSEL). Die Struktur wurde mittels
Molekularstrahlepitaxie (engl. Molecular beam epitaxy, MBE) gewachsen.
Sie enthält einen λ-Resonator mit drei 6 nm dicken Ga0.80In0.20As
Quantenfilmen, platziert im Maximum des optischen Feldes. Als Spiegel
wurden symmetrische GaAs/AlAs Bragg-Reflektoren (Oben: 16 Paare,
Unten 19.5 Paare) verwendet. Der Laser emittiert bei 1000 nm mit einer
Schwelle von 2.5kW/cm 2 bei Raumtemperatur und Anregung mit einem
gepulsten Ti:Saphir-Laser (100 fs - Pulse bei 800 nm).
Die Abhängigkeit der Laseremission von den Anregungsbedingungen
wird analysiert und diskutiert.
Wir danken der DFG für Unterstützung im GRK384 und SFB491.
HL 12.2 Mo 16:30 Poster A
Deep level transient spectroscopy on copper indium gallium
diselenide thin film solar cells with different gallium content —
•V. Mertens 1 , R. Reineke-Koch 2 , M. Köntges 2 , and J. Parisi 1 —
1 University of Oldenburg, 26111 Oldenburg — 2 Instiute for Solar Engery
Research, 31860 Emmerthal
One approach to increase the open circuit voltage Voc of Cu(In, Ga)Se2
solar cells is to widen the bandgap Eg by using higher gallium contents
than in the standard composition which has a gallium to gallium plus indium
ratio (GGI) of about 0.28. However further investigations on these
materials showed that their Voc is lower than expected from the empirical
relationship Voc = Eg −0.5V which holds for Eg ≤ 1.3eV . To understand
more about the whole alloy system, we focused on the defects and the
change in their characteristics with composition. For our measurements
we use a home build deep level transient spectrometer based on a Boonton
7200B capacitance bridge. We investigated different samples covering the
whole alloy range with different DLTS techniques as majority and minority
carrier DLTS, reverse-bias pulsed DLTS. We found that the transients
are poorly described by a sum of several monoexponential decays. Therefore
other analyses were conducted to determine the underlying physical
mechanism.
HL 12.3 Mo 16:30 Poster A
Five source PVD for the deposition of CIGSS thin films —
•Mario Gossla 1,2 and William N. Shafarman 1 — 1 Institute of Energy
Conversion, University of Delaware, 451 Wyoming Road, Newark,
DE 19716, USA — 2 Institut für Festkörperphysik, Friedrich-Schiller-
Universität Jena, Max-Wien-Platz 1, 07743 Jena, Germany
CuIn1−xGax(Se1−ySy)2 (CIGSS) films are being investigated as absorber
layer materials for the wide-band-gap cell in a tandem or multijunction
thin-film solar cell. To become a viable option for high performance
photovoltaics, tandem or multijunction solar cells based on polycrystalline
thin films will require a wide band gap top cell with at least
15 % efficiency, optical band gap in the range 1.7 ≤ Eg ≤ 1.9 eV, and
small sub-band gap absorption.
A new five-source deposition system was developed for the deposition
of CIGSS thin films. Thin films within the complete compositional range
of 0 ≤ x ≤ 1, and 0 ≤ y ≤ 1 were deposited on Mo-coated soda lime
glass at different temperatures. The films were analyzed by means of IR,
SEM, and XRD. The incorporation of sulfur in the films depends strongly
on the sulfur concentration in the vapor phase, substrate temperature,
and Cu/In ratio. Complete Mo/CIGSS/CdS/-ZnO/NiAl solar cells with
a typical area of 0.5 cm 2 were prepared. Cell performance was monitored
by QE and I-V measurements and is shown to strongly depend on sulfur
content.
HL 12.4 Mo 16:30 Poster A
Surface-heat-emittance optimization of CIS-based flexible solar
cells — •Carsten Bundesmann 1 , Ferdinand Dürr 1,2 , Mathias
Schubert 1 , and Karsten Otte 2 — 1 Fakultät für Physik und
Geowissenschaften, Institut für Experimentelle Physik II, Universität
Leipzig, Linnéstraße 5, 04103 Leipzig — 2 Solarion GmbH, Ostende 5,
04288 Leipzig
CuInSe2 based flexible thin film solar cells on polyimide substrates are
possible alternatives for crystalline silicon or GaAs based solar cells in
space applications due to their low weight. Still their feasibility has to be
proven. In order to achieve high specific power (W/kg), important optimization
parameters parameters are, for instance, efficiency and surface
heat emittance. The later is studied here for CuInSe2 based thin film solar
cells grown on flexible polyimide substrates in a web-coater based process.
It is shown that the maximum values of the surface heat emittance
depends strongly on the layer structure and the layer optical properties.
The dielectric functions of each single layer is extracted by infrared spectroscopic
ellipsometry. Using these data a layer model is established, the
reflectivity and the surface heat emittance is calculated. Upon varying
the model parameters the maximum of the surface heat emittance is determined.
The simulated results are compared with measured values of
real solar cell structures.
HL 12.5 Mo 16:30 Poster A
Stray current contributions in photo-assisted tunneling spectroscopy
on CIGSe absorbers: an aid or a nuisance? — •U.
Herber 1 , C. Heske 1 , E. Umbach 1 , F. Karg 2 , T. P. Niesen 2 ,
G. Hanna 3 , and U. Rau 3 — 1 Experimentelle Physik II, Universität
Würzburg, 97074 Würzburg — 2 Shell Solar, München — 3 Institut für
Physikalische Elektronik, Universität Stuttgart, 70569 Stuttgart
Polycrystalline Cu(In,Ga)Se2 solar cell absorbers are investigated with
photo-modulated scanning tunneling spectroscopy. For RTP (rapid thermal
processing) absorbers, we find pronounced lateral inhomogeneities of
the photo-induced tunneling current (PITC), as reported earlier [1]. In
addition, similar inhomogeneities are also found on co-evaporated CIGSe
absorbers.
However, every modulated process in scanning probe microscopies is
accompanied by stray effects, in particular by an additional current contribution
via the capacitance between sample and tip. The complex phase
and amplitude of this stray signal can be measured separately, and thus
its contribution can be subtracted from the PITC scan data. On the other
hand, the stray signal is strongly determined by the (chemical) condition
of the surface. The situation within a series of sputter-and-heat cycles
is compared with corresponding photo-emission data and interpreted in
the framework of the equivalent circuit approach.
[1] D. Eich et al., Thin Solid Films 361, 258 (2000)
HL 12.6 Mo 16:30 Poster A
Epitaxial growth of CuGaS2 on Si substrates investigated by
high resolution X-ray diffraction — •J. Cieslak 1 , A. Dietz 1 , J.
Eberhardt 1 , M. Gossla 1 , Th. Hahn 1 , J. Kräusslich 2 , H. Metzner
1 , U. Reislöhner 1 , W. Witthuhn 1 , and F. Wunderlich 2 —
1 Friedrich-Schiller-Universität Jena, Institut für Festkörperphysik, Max-
Wien-Platz 1, 07743 Jena — 2 Friedrich-Schiller-Universität Jena, Institut
für Optik und Quantenelektronik Max-Wien-Platz 1, 07743 Jena
The wide-band gap chalcopyrite semiconductor CuGaS2 (CGS) has recently
attracted some attention since its direct band gap of Egap=2.5eV
(T=5K) makes it suitable for applications in tandem solar cells. We reported
the epitaxial growth of high-quality CGS layers on Si(111) [1].
In this work we elucidate the epitaxial growth and structural properties
of these heteroepitaxial layers. High resolution X-ray diffraction
(HRXRD) was performed at the European Synchrotron Radiation Facility
(ESRF) in Grenoble (France). High-precision ω/2θ-scans with different
in-plane diffraction vectors were carried out.
It was found, that the CGS layers show lattice parameters resembling
those reported in literature for bulk material and that any existence of
metastable ordering, prominent in epitaxial layers of CuInS2, can be ruled
out. This behaviour is discussed within the context of the observed epitaxial
relations between CGS and Si(111) as well as Si(100) substrates.
[1] H. Metzner et al., Appl. Phys. Lett., 81(1), 156 (2002).