Rapid thermal processing of CuAISe2

CuAl thin film metallic precursors were selenised using a tube furnace or a rapid thermal processor (RTP). A comparison is made between the two processes for slightly Cu rich films and best crystallographic and elemental properties are obtained for films selenised by RTP: it was found that ternary compound could only be formed using the RTP. In both cases a large amount of CuxSey grains are found to develop at the surface of the films. Only samples processed in the RTP showed cathodoluminscence excitation at 2.68 eV characteristics of the electronic bandgap. Al rich samples were used to study the effect of etching the CuxSey phases from the surface in order to reveal the underlying CuAlSe2 material

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

Optical properties of the chalcopyrite semiconductors CuInSe₂, CuInS₂ and CuGaSe₂

CuInSe₂, CuInS₂ and CuGaSe₂ are I-III-VI₂ compound semiconductors with a chalcopyrite structure. These ternary compounds exhibit favourable properties, such as direct band gaps and high absorption coefficients, for application as absorber layers in thin-film solar cells. Recently Cu(In,Ga)Se₂ based photovoltaic devices have demonstrated conversion efficiencies of 20.3 % which is the highest amongst polycrystalline thin-film solar cell technologies. This thesis describes a study of excitonic recombination processes in high quality CuInSe₂, CuInS₂ and CuGaSe₂ single crystals using photoluminescence (PL) spectroscopy as a function of excitation power, temperature and applied magnetic field. Excitation power dependent measurements confirm the identification of the free excitons in the PL spectra of the three chalcopyrite semiconductor compounds. Additional sharp lines in the PL spectra appear to be due to the radiative recombination of excitons bound to shallow hydrogenic defects. PL lines due to excitons bound to more complex defects with a low concentration of defects are also found in CuInSe₂ and CuInS₂. Analysis of the temperature dependent PL spectra lead to activation energies of the free and bound excitons in CuInSe₂, CuInS₂ and CuGaSe₂. In addition, phonon energies have been obtained from the temperature dependence of the free exciton spectral positions and of the full width at half maximum. PL spectra measured in applied magnetic fields allow estimation of the diamagnetic shift rates for CuInSe₂, CuInS₂ and CuGaSe₂. A first-order perturbation model leads to values for the excitonic reduced masses and the effective hole masses can be estimated. For CuInSe₂ a theoretically predicted anisotropy of the effective hole masses is demonstrated. The study of the excitonic states in CuInSe₂, CuInS₂ and CuGaSe₂ provides a deeper understanding of the electronic material properties which can facilitate further improvements in solar cell efficiencies.CuInSe₂, CuInS₂ and CuGaSe₂ are I-III-VI₂ compound semiconductors with a chalcopyrite structure. These ternary compounds exhibit favourable properties, such as direct band gaps and high absorption coefficients, for application as absorber layers in thin-film solar cells. Recently Cu(In,Ga)Se₂ based photovoltaic devices have demonstrated conversion efficiencies of 20.3 % which is the highest amongst polycrystalline thin-film solar cell technologies. This thesis describes a study of excitonic recombination processes in high quality CuInSe₂, CuInS₂ and CuGaSe₂ single crystals using photoluminescence (PL) spectroscopy as a function of excitation power, temperature and applied magnetic field. Excitation power dependent measurements confirm the identification of the free excitons in the PL spectra of the three chalcopyrite semiconductor compounds. Additional sharp lines in the PL spectra appear to be due to the radiative recombination of excitons bound to shallow hydrogenic defects. PL lines due to excitons bound to more complex defects with a low concentration of defects are also found in CuInSe₂ and CuInS₂. Analysis of the temperature dependent PL spectra lead to activation energies of the free and bound excitons in CuInSe₂, CuInS₂ and CuGaSe₂. In addition, phonon energies have been obtained from the temperature dependence of the free exciton spectral positions and of the full width at half maximum. PL spectra measured in applied magnetic fields allow estimation of the diamagnetic shift rates for CuInSe₂, CuInS₂ and CuGaSe₂. A first-order perturbation model leads to values for the excitonic reduced masses and the effective hole masses can be estimated. For CuInSe₂ a theoretically predicted anisotropy of the effective hole masses is demonstrated. The study of the excitonic states in CuInSe₂, CuInS₂ and CuGaSe₂ provides a deeper understanding of the electronic material properties which can facilitate further improvements in solar cell efficiencies

Structural and optical properties of CdS/Cu(In,Ga)Se-2 heterostructures irradiated by high energy electrons

Thin films of Cu(In, Ga)Se-2 (CIGS) with a Ga/(Ga + In) ratio of similar to 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 (E-g) 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, P-1 at similar to 0.91 eV and P-2 at similar to 0.77 eV, have been detected. These bands are tentatively associated with copper atoms substituting indium (Cu-In) and indium vacancies V-In, respectively, as the simplest radiation-induced defects

Optical properties of thin films of Cu2ZnSnSe4 fabricated by sequential deposition and selenisation

The search for new, low-cost semiconductor materials for large-scale production of solar cells has recently resulted in increased interest in the direct band gap quaternary semiconductor compound Cu2ZnSn(S,Se)4. A particular advantage compared to thin-film cells based on CuInGa(S,Se)2 (CIGS) is the absence of indium in the absorber layer. The constituent elements of Cu2ZnSn(S,Se)4 are all abundant in the earth’s crust, the material can be p-type doped and its absorption coefficient exceeds 104 cm–1 [1] in the visible range. An overview of progress on Cu2ZnSn(S,Se)4 thin film solar cell development has recently been published [2]. The conversion efficiency of Cu2ZnSnSe4 (CZTSe) based solar cells is around 4% [3] while that for Cu2ZnSnS4 (CZTS) based solar cells is close to 7% [4]. Recently, Todorov et al. [5] reported a record efficiency of 9.6% for a Cu2ZnSn(Se,S)4 based device. Photoluminescence (PL) is a very effective tool to study the electronic properties of semiconductors. PL spectra are highly sensitive to changes in the elemental composition of compound semiconductors which determines the type and concentrations of defects. Very few PL studies on CZTSe have been reported so far and the band gap in this material still has not been reliably established. Grossberg et al. [6] estimated a band gap value of 1.02 eV at 10 K which is in good agreement with the theoretical predictions of ~1.0 eV reported by Chen et al. [7]. However, an earlier report by Matsushita et al. [8] indicated a quite different value of 1.44 eV using optical absorption measurements at room temperature. In this paper we study the crystalline structure, the morphology, the elemental composition and the defect nature of CZTSe thin films using X-Ray diffraction (XRD), scanning electron microscopy (SEM), wavelength dispersive x-ray spectroscopy (WDX) and PL analysis, respectively

Molecular beam epitaxy as a method for the growth of free-standing bulk zinc-blende GaN and AlGaN crystals

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

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

Growth by molecular beam epitaxy of amorphous and crystalline GaNAs alloys with band gaps from 3.4 to 0.8 eV for solar energy conversion devices

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

Molecular beam epitaxy of GaN1–xBix alloys with high bismuth content

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