F. Luckert

Optical properties of high quality Cu2ZnSnSe4 thin films

Cu2ZnSnSe4 thin films, fabricated on bare or molybdenum coated glass substrates by magnetron sputtering and selenisation, were studied by a range of techniques. Photoluminescence spectra reveal an excitonic peak and two phonon replicas of a donor-acceptor pair (DAP) recombination. Its acceptor and donor ionisation energies are 27 and 7 meV, respectively. This demonstrates that high-quality Cu2ZnSnSe4 thin films can be fabricated. An experimental value for the longitudinal optical phonon energy of 28 meV was estimated. The band gap energy of 1.01 eV at room temperature was determined using optical absorption spectra.

Highly mismatched crystalline and amorphous GaN1−xAsx alloys in the whole composition range

Alloying is a commonly accepted method to tailor properties of semiconductor materials for specific applications. Only a limited number of semiconductor alloys can be easily synthesized in the full composition range. Such alloys are, in general, formed of component elements that are well matched in terms of ionicity, atom size, and electronegativity. In contrast there is a broad class of potential semiconductor alloys formed of component materials with distinctly different properties. In most instances these mismatched alloys are immiscible under standard growth conditions. Here we report on the properties of GaN1−xAsx, a highly mismatched, immiscible alloy system that was successfully synthesized in the whole composition range using a nonequilibrium low temperature molecular beam epitaxy technique. The alloys are amorphous in the composition range of 0.17<x<0.75 and crystalline outside this region. The amorphous films have smooth morphology, homogeneous composition, and sharp, well defined optical absorp...

Excited states of the free excitons in CuInSe2 single crystals

High-quality CuInSe2 single crystals were studied using polarization resolved photoluminescence (PL) and magnetophotoluminescence (MPL). The emission lines related to the first (n=2) excited states for the A and B free excitons were observed in the PL and MPL spectra at 1.0481 meV and 1.0516 meV, respectively. The spectral positions of these lines were used to estimate accurate values for the A and B exciton binding energies (8.5 meV and 8.4 meV, respectively), Bohr radii (7.5 nm), band gaps (EgA=1.050 eV and EgB=1.054 eV), and the static dielectric constant (11.3) assuming the hydrogenic model.

Excitation power and temperature dependence of excitons in CuInSe2

Excitonic recombination processes in high quality CuInSe2 single crystals have been studied by photoluminescence (PL) and reflectance spectroscopy as a function of excitation powers and temperature. Excitation power dependent measurements confirm the identification of well-resolved A and B free excitons in the PL spectra and analysis of the temperature quenching of these lines provides values for activation energies. These are found to vary from sample to sample, with values of 12.5 and 18.4meV for the A and B excitons, respectively, in the one showing the highest quality spectra. Analysis of the temperature and power dependent PL spectra from the bound excitonic lines, labelled M1, M2, and M3 appearing in multiplets points to a likely assignment of the hole involved in each case. The M1 excitons appear to involve a conduction band electron and a hole from the B valence band hole. In contrast, an A valence band hole appears to be involved for the M2 and M3 excitons. In addition, the M1 exciton multiplet seems to be due to the radiative recombination of excitons bound to shallow hydrogenic defects, whereas the excitons involved in M2 and M3 are bound to more complex defects. In contrast to the M1 exciton multiplet, the excitonic lines of M2 and M3 saturate at high excitation powers suggesting that the concentration of the defects involved is low

Diamagnetic shift of the A free exciton in CuGaSe2 single crystals

Single crystals of CuGaSe2 were studied using magnetophotoluminescence in magnetic fields up to 20 T at 4.2 K. The rate of the diamagnetic shift in the A free exciton peak was determined to be 9.82×10−6 eV/T2. This rate was used to calculate the reduced mass as 0.115m0, the binding energy as 12.9 meV, the Bohr radius as 5.1 nm and an effective hole mass of 0.64m0 (m0 is the free electron mass) of the free A exciton using a low-field perturbation approach and the hydrogenic model.

Carrier localization and related photoluminescence in cubic AlGaN epilayers

The steady state and time-resolved photoluminescence (PL) spectra of cubic AlxGa1-xN have been measured for 0 < x < 1. The intensity of the room temperature PL increases by an order of magnitude when the AlN content increases from x = 0 to x = 0.95. Additionally, the PL decay slows down with the decrease of temperature and increase of x. These results show that strong localization of carriers on alloy composition fluctuations plays a large role in determining the intensity and temporal evolution of the PL. The activation energy for the localized carriers increases with the increase of x and reaches the value of 55 meV at x = 0.95.

Anisotropy of effective masses in CuInSe2

Anisotropy of the valence band is experimentally demonstrated in CuInSe2, a key component of the absorber layer in one of the leading thin-film solar cell technology. By changing the orientation of applied magnetic fields with respect to the crystal lattice, we measure considerable differences in the diamagnetic shifts and effective g-factors for the A and B free excitons. The resulting free exciton reduced masses are combined with a perturbation model for non-degenerate independent excitons and theoretical dielectric constants to provide the anisotropic effective hole masses, revealing anisotropies of 5.5 (4.2) for the A (B) valence bands.

Excited States of the A and B Free Excitons in CuInSe2

CuInSe2 single crystals, grown by the vertical Bridgman technique were studied using polarisation resolved photoluminescence (PL) at cryogenic temperatures. The emission lines related to the first (n = 2) excited states for the A and B free excitons were observed in the PL spectra at 1.0481 and 1.0516 eV, respectively. The spectral positions of these lines were used to estimate accurate values for the A and B exciton binding energies (8.5 and 8.4 meV, respectively), Bohr radii (7.5 nm), band gaps (EgA = 1.050 eV and EgB = 1.054 eV), and the static dielectric constant (11.3) assuming the hydrogenic model.