J. P. Teixeira

Comparison of fluctuating potentials and donor-acceptor pair transitions in a Cu-poor Cu2ZnSnS4 based solar cell

The structure of the electronic energy levels of a single phase Cu2ZnSnS4 film, as confirmed by Raman Scattering and x-ray diffraction, is investigated through a dependence on the excitation power of the photoluminescence (PL). The behavior of the observed asymmetric band, with a peak energy at ∼1.22 eV, is compared with two theoretical models: (i) fluctuating potentials and (ii) donor-acceptor pair transitions. It is shown that the radiative recombination channels in the Cu-poor film are strongly influenced by tail states in the bandgap as a consequence of a heavy doping and compensation levels. The contribution of the PL for the evaluation of secondary phases is also highlighted.

Cd and Cu Interdiffusion in Cu(In, Ga)Se 2 /CdS Hetero-Interfaces

We report a detailed characterization of an industry-like prepared Cu(In, Ga)Se2 (CIGS)/CdS heterojunction by scanning transmission electron microscopy and photoluminescence (PL). Energy dispersive X-ray spectroscopy shows the presence of several regions in the CIGS layer that are Cu deprived and Cd enriched, suggesting the segregation of Cd–Se. Concurrently, the CdS layer shows Cd-deprived regions with the presence of Cu, suggesting a segregation of Cu–S. The two types of segregations are always found together, which, to the best of our knowledge, is observed for the first time. The results indicate that there is a diffusion process that replaces Cu with Cd in the CIGS layer and Cd with Cu in the CdS layer. Using a combinatorial approach, we identified that this effect is independent of focused-ion beam sample preparation and of the transmission electron microscopy grid. Furthermore, PL measurements before and after an HCl etch indicate a lower degree of defects in the postetch sample, compatible with the segregates removal. We hypothesize that Cu

Synthesis and formation mechanism of CuInSe2 nanowires by one-step self-catalysed evaporation growth

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Influence of CdS and Zn x Sn 1−x O y buffer layers on the photoluminescence of Cu(In, Ga)Se 2 thin films

Se nanodomains react during the chemical bath process to form these segregates since the chemical reaction that dominates this process is thermodynamically favorable. These results provide important additional information about the formation of the CIGS/CdS interface.

Muonium states in Cu2ZnSnS4 solar cell material

High-quality CuInSe2 (CISe) nanowires have been prepared by a one-step evaporation process. The presented growth process results in a composite material consisting of CISe NWs on top of a polycrystalline CISe base layer. The nanowires were extensively characterized by transmission electron microscopy, confirming their composition and atomic-scale crystal structure with a very low number of structural defects. From these analyses, we infer that the growth axis is along the [111] direction. The polycrystalline base layer has a tetragonal chalcopyrite structure and is optically active as confirmed by X-ray diffraction and photoluminescence (PL) analysis, respectively. Potential applications of this composite CISe NW/base-layer material for photovoltaic energy conversion are supported by the reduced reflectivity of the material and its strong PL intensity. The presented growth method is based on elemental evaporation under vacuum conditions, which makes the process compatible with the fabrication of photovoltaic devices.

Fluctuating potentials in GaAs:Si nanowires: critical reduction of the influence of polytypism on the electronic structure

In this work, an optical study by photoluminescence on the influence of different buffer layers on a Cu(In,Ga)Se2 (CIGS) thin film is presented. Chemical bath deposited CdS was compared with atomic layer deposited ZnxSn1- xOy (ZnSnO). The CIGS bulk and CIGS/buffer interface in both samples are strongly influenced by fluctuating potentials, being less pronounced for the sample with the ZnSnO buffer layer. This study emphasizes the potential application of the ZnSnO semiconductor in CIGS based solar cells.

Substrate and Mg doping effects in GaAs nanowires

We investigated bulk and thin-film samples of the quaternary p-type semiconductor Cu2ZnSnS4 (CZTS) by μSR, in order to characterize the existing muonium signals. We find that the majority of the implanted muons form a diamagnetic state broadened by an interaction with the Cu nuclear moments, which we interpret as Mu+ bound to sulphur. A paramagnetic fraction is also present at low temperatures and the ratio between the two muon charge states, Mu+ and Mu0, varies between 20 and 40% prior to the onset of muon diffusion, which occurs at around 150 K. The fraction of Mu0 is found to be sensitive to the defect content of the sample. The paramagnetic fraction has two different contributions and their origin is discussed and related with the muon role as a probe for charge carriers in the material.

Growth of CuInSe 2 nanowires without external catalyst by molecular beam epitaxy

In this work, the effects of Si doping in GaAs nanowires (NWs) grown on GaAs (111)B by molecular beam epitaxy with different Si doping levels (nominal free carrier concentrations of 1 × 1016, 8 × 1016, 1 × 1018 and 5 × 1018 cm-3) are deeply investigated using scanning electron microscopy (SEM), transmission electron microscopy (TEM), grazing incidence X-ray diffraction (GID), photoluminescence (PL) and cathadoluminescence (CL). TEM results reveal a mixture of wurtzite (WZ) and zinc-blende (ZB) segments along the NW axis independently of the Si doping levels. GID measurements suggest a slight increase of the ZB fraction with the Si doping. Low temperature PL and CL spectra exhibit sharp lines in the energy range 1.41-1.48 eV, for the samples with lower Si doping levels. However, the emission intensity increases and is accompanied by a clear broadening of the observed lines for the samples with higher Si doping levels. The staggered type-II band alignment only determines the optical properties of the lower doping levels in GaAs:Si NWs. For the higher Si doping levels, the electronic energy level structure of the NWs is determined by electrostatic fluctuating potentials intimately related to the amphoteric behavior of the Si dopant in GaAs. For the heavily doped NWs, the estimated depth of the potential wells is ∼96-117 meV. Our results reveal that the occurrence of the fluctuating potentials is not dependent on the crystalline phase and shows that the limitation imposed by the polytypism can be overcome.

Large amplitude Spin Transfer Torque Nano-Oscillators implemented with intermediate thickness MgO barriers in the 10–30 Ωμm 2 range

Mg doping of GaAs nanowires has been established as a viable alternative to Be doping in order to achieve p-type electrical conductivity. Although reports on the optical properties are available, few reports exist about the physical properties of intermediate-to-high Mg doping in GaAs nanowires grown by molecular beam epitaxy (MBE) on GaAs(111)B and Si(111) substrates. In this work, we address this topic and present further understanding on the fundamental aspects. As the Mg doping was increased, structural and optical investigations revealed: i) a lower influence of the polytypic nature of the GaAs nanowires on their electronic structure; ii) a considerable reduction of the density of vertical nanowires, which is almost null for growth on Si(111); iii) the occurrence of a higher WZ phase fraction, in particular for growth on Si(111); iv) an increase of the activation energy to release the less bound carrier in the radiative state from nanowires grown on GaAs(111)B; and v) a higher influence of defects on the activation of nonradiative de-excitation channels in the case of nanowires only grown on Si(111). Back-gate field effect transistors were fabricated with individual nanowires and the p-type electrical conductivity was measured with free hole concentration ranging from 2.7 × 1016 cm−3 to 1.4 × 1017 cm−3. The estimated electrical mobility was in the range ≈0.3–39 cm2 /Vs and the dominant scattering mechanism is ascribed to the WZ/ZB interfaces. Electrical and optical measurements showed a lower influence of the polytypic structure of the nanowires on their electronic structure. The involvement of Mg in one of the radiative transitions observed for growth on the Si(111) substrate is suggested.

Secondary crystalline phases identification in Cu_2ZnSnSe_4 thin films: contributions from Raman scattering and photoluminescence

We report the growth of CuInSe2 nanowires (NWs) using an evaporation process. The NWs grow on top of an underlying CuInSe2 polycrystalline layer that initially forms on Si(100) substrates with native SiO2. Reference samples grown after removal of the native oxide do not exhibit the formation of NWs. The polycrystalline base layer has a tetragonal chalcopyrite structure and is optically active as confirmed by x-ray diffraction and photoluminescence, respectively. Transmission electron microscopy confirms the composition and atomic structure of the CuInSe2 NWs. Samples with NWs exhibit a reduced reflectance compared with reference samples, making them interesting for photovoltaic applications.