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There is a significant difference in the lattice parameters of GaN and AlN and for many device applications AlxGa1-xN substrates would be preferable to either GaN or AlN. We have studied the growth of free-standing zinc-blende and wurtzite AlxGa1-xN bulk crystals by plasma-assisted molecular beam epitaxy (PA-MBE). Thick (similar to 10 mu m) zinc-blende and wurtzite AlxGa1-xN films were grown by PA-MBE on 2-in. GaAs (0 0 1) and GaAs (1 1 1)B substrates respectively and were removed from the GaAs substrate after the growth. We demonstrate that free-standing zinc-blende and wurtzite AlxGa1-xN wafers can be achieved by PA-MBE for a wide range of Al compositions. (C) 2011 Elsevier B.V. All rights reserved.

We have analysed bismuth incorporation into GaN layers using plasma-assisted molecular beam epitaxy (PA-MBE) at extremely low growth temperatures of less than ∼100 °C under both Ga-rich and N-rich growth conditions. The formation of amorphous GaN1−xBix alloys is promoted by growth under Ga-rich conditions. The amorphous matrix has a short-range order resembling random crystalline GaN1−xBix alloys. We have observed the formation of small crystalline clusters embedded into amorphous GaN1−xBix alloys. Despite the fact that the films are pseudo-amorphous we observe a well defined optical absorption edges that rapidly shift to very low energy of ∼1 eV.

Solar cells with the structure ZnO:Al/i-ZnO/CdS/Cu(In,Ga)Se-2/Mo/polyimide were examined using a range of techniques. The elemental composition of the Cu(InGa)Se-2 (CIGS) layers, their crystalline structure and optical properties were studied. Photoluminescence (PL) spectra of the CIGS absorber layers were studied as functions of temperature (4.2-240 K) and excitation power density. The band gap energy E-g of the CIGS layers was determined by employing photoluminescence excitation (PLE) spectroscopy. The influence of sodium incorporation on the PL properties of CIGS was analysed. Correlations of the optical properties of the CIGS absorber layers and the photovoltaic parameters of the solar cells were revealed.

We have studied the growth of zinc-blende GaN and AlxGa1-xN layers, structures and bulk crystals by molecular beam epitaxy (MBE). MBE is normally regarded as an epitaxial technique for growth of very thin layers with monolayer control of their thickness. However, we have used the MBE technique for bulk crystal growth and have produced GaN layers up to 100 mu m in thickness. Thick, undoped, cubic GaN films were grown on semi-insulating GaAs (0 0 1) substrates by a modified plasma-assisted molecular beam epitaxy (PA-MBE) method and were removed from the GaAs substrate after the growth. The resulting free-standing GaN wafers may be used as substrates for further epitaxy of cubic GaN-based structures and devices. We have demonstrated that the PA-MBE process, we had developed, also allows us to achieve free-standing zinc-blende AlxGa1-xN wafers.

We have studied the growth of wurtzite GaN and AlxGa1−xN layers and bulk crystals by molecular beam epitaxy (MBE). MBE is normally regarded as an epitaxial technique for the growth of very thin layers with monolayer control of their thickness. However, we have used the MBE technique for bulk crystal growth and have produced 2 in diameter wurtzite AlxGa1−xN layers up to 10 μm in thickness. Undoped wurtzite AlxGa1−xN films were grown on GaAs (1 1 1)B substrates by a plasma-assisted molecular beam epitaxy (PA-MBE) method and were removed from the GaAs substrate after the growth. The fact that free-standing ternary AlxGa1−xN wafers can be grown is very significant for the potential future production of wurtzite AlxGa1−xN substrates optimized for AlGaN-based device structures.

Using low temperature MBE, we have shown that it is possible to grow amorphous GaN1-xAsx layers with a variable As content (0 < x < 0.8) on both crystalline (sapphire and silicon) and amorphous (glass and Pyrex glass) substrates. Despite the fact that the samples with high As content are amorphous, we observe a gradual continuous decrease of bandgap from similar to 3.4 to similar to 0.8 eV with increase in As content. To the best of our knowledge this is the first demonstration of homogeneous amorphous GaN-based alloys over a wide composition range. The large band gap range of the amorphous phase of GaNAs covers much of the solar spectrum. The amorphous nature of the GaNAs alloys is particularly advantageous since low cost substrates such as glass and Pyrex glass can be used for solar cell fabrication.

Thin films of Cu(In, Ga)Se2 (CIGS) with a Ga/(Ga + In) ratio of ~0.27 corresponding to the standard elemental composition for solar-energy transducers were grown on Mo-coated glass substrates by the Cu, In, Ga, and Se co-evaporation technique from different sources. Transmission (T), photoluminescence (PL), and photoluminescence excitation (PLE) spectra at 4.2 K were used to analyze electronic properties in the asgrown and electron-irradiated CIGS films. The band-gap energy (Eg) of the CIGS films measured using both transmission and PLE methods was found to be about 1.28 eV at 4.2 K. Two deep bands in the PL spectra of the irradiated CIGS films, P1 at ~0.91 eV and P2 at ~0.77 eV, have been detected. These bands are tentatively associated with copper atoms substituting indium (CuIn) and indium vacancies VIn, respectively, as the simplest radiation-induced defects.

The chemical composition of Cu(In,Ga)Se-2 (CIGS) semiconductor compounds is analyzed by local x-ray spectral microanalysis and scanning Auger electron spectroscopy. X-ray diffraction analysis reveals a difference in the predominant orientation of CIGS films depending on the technological conditions under which they are grown. The chemical composition is found to have a strong effect on the shift in the self-absorption edge of CIGS compounds. It is shown that a change in the relative proportion of Ga and In in CIGS semiconducting compounds leads to a change in the band gap E-g for this material in the 1.05-1.72 eV spectral range at 4.2 K.

A new alloy system, the GaN(sub 1-x)As(sub x) alloys in the whole composition range was successfully synthesized using the non-equilibrium low temperature molecular beam epitaxy method. 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 absorption edges. The bandgap energy varies in a broad energy range from 3.4 eV in GaN to0.8 eV at x0.85. The reduction of the band gap can be attributed primarily to the downward movement of the conduction band for alloys with x 0.2, and to the upward movement of the valence band for alloys with x 0.2.

In this paper we have studied the unintentional incorporation of arsenic into thick (up to ∼75 μm) zinc-blende (cubic) GaN and AlGaN layers grown by a plasma-assisted molecular beam epitaxy (PA-MBE) on (001) GaAs substrates. We present detailed studies of As incorporation into free-standing zinc-blende (Al)GaN by secondary ion mass spectroscopy (SIMS) and electron probe microanalysis (EPMA). We demonstrate that there is no significant arsenic contamination of zinc-blende GaN layers and substrates due to diffusion of arsenic during the prolonged growths by PA-MBE. Arsenic incorporation is limited to the first few hundred nanometers of the layer and it is due to initiation phase of the growth of zinc-blende material, which involves use of arsenic as a surfactant to promote the growth of the cubic phase. These data have allowed us to determine the thickness of the unintentionally arsenic contaminated GaN region, which needs to be removed by polishing during production of free-standing zinc-blende GaN substrates. (© 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

The steady state and time-resolved photoluminescence (PL) spectra of cubic AlxGa1-xN have been measured for 0

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 (E-g(A)=1.050 eV and E-g(B)=1.054 eV), and the static dielectric constant (11.3) assuming the hydrogenic model.

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.

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. (C) 2012 American Institute of Physics. [http://dx.doi.org/10.1063/1.4709448]

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 (E g A = 1.050 eV and E g B = 1.054 eV), and the static dielectric constant (11.3) assuming the hydrogenic model.

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 0.2, and to the upward movement of the valence band for alloys with x