L. Calvo-Barrio

In-depth resolved Raman scattering analysis for the identification of secondary phases: Characterization of Cu2ZnSnS4 layers for solar cell applications

This work reports the in-depth resolved Raman scattering analysis with different excitation wavelengths of Cu2ZnSnS4 layers. Secondary phases constitute a central problem in this material, particularly since they cannot be distinguished by x-ray diffraction. Raman spectra measured with 325 nm excitation light after sputtering the layers to different depths show peaks that are not detectable by excitation in the visible. These are identified with Cu3SnS4 modes at the surface region while spectra measured close to the back region show peaks from ZnS and MoS2. Observation of ZnS is enhanced by resonant excitation conditions achieved when working with UV excitation.

ZnSe Etching of Zn‐Rich Cu2ZnSnSe4: An Oxidation Route for Improved Solar‐Cell Efficiency

Cu2ZnSnSe4 kesterite compounds are some of the most promising materials for low-cost thin-film photovoltaics. However, the synthesis of absorbers for high-performing devices is still a complex issue. So far, the best devices rely on absorbers grown in a Zn-rich and Cu-poor environment. These off-stoichiometric conditions favor the presence of a ZnSe secondary phase, which has been proved to be highly detrimental for device performance. Therefore, an effective method for the selective removal of this phase is important. Previous attempts to remove this phase by using acidic etching or highly toxic organic compounds have been reported but so far with moderate impact on device performance. Herein, a new oxidizing route to ensure efficient removal of ZnSe is presented based on treatment with a mixture of an oxidizing agent and a mineral acid followed by treatment in an aqueous Na2S solution. Three different oxidizing agents were tested: H2O2, KMnO4, and K2Cr2O7, combined with different concentrations of H2SO4. With all of these agents Se(2-) from the ZnSe surface phase is selectively oxidized to Se(0), forming an elemental Se phase, which is removed with the subsequent etching in Na2S. Using KMnO4 in a H2SO4-based medium, a large improvement on the conversion efficiency of the devices is observed, related to an improvement of all the optoelectronic parameters of the cells. Improvement of short-circuit current density (J(sc)) and series resistance is directly related to the selective etching of the ZnSe surface phase, which has a demonstrated current-blocking effect. In addition, a significant improvement of open-circuit voltage (V(oc)), shunt resistance (R(sh)), and fill factor (FF) are attributed to a passivation effect of the kesterite absorber surface resulting from the chemical processes, an effect that likely leads to a reduction of nonradiative-recombination states density and a subsequent improvement of the p-n junction.

Ion‐beam synthesis of amorphous SiC films: Structural analysis and recrystallization

The analysis of SiC films obtained by carbon ion implantation into amorphous Si (preamorphized by Ge ion implantation) has been performed by infrared and Raman scattering spectroscopies, transmission electron microscopy, Rutherford backscattering, and x‐ray photoelectron spectroscopy (XPS). The data obtained show the formation of an amorphous Si1−xCx layer on top of the amorphous Si one by successive Ge and C implantations. The fitting of the XPS spectra indicates the presence of about 70% of Si–C bonds in addition to the Si–Si and C–C ones in the implanted region, with a composition in the range 0.35<x<0.6. This points out the existence of a partial chemical order in the layer, in between the cases of perfect mixing and complete chemical order. Recrystallization of the layers has been achieved by ion‐beam induced epitaxial crystallization (IBIEC), which gives rise to a nanocrystalline SiC layer. However, recrystallization is not complete, observing still the presence of Si–Si and C–C bonds in an amorphou...

Raman microprobe characterization of electrodeposited S-rich CuIn(S,Se)2 for photovoltaic applications: Microstructural analysis

This article reports a detailed Raman scattering and microstructural characterization of S-rich CuIn(S,Se)2 absorbers produced by electrodeposition of nanocrystalline CuInSe2 precursors and subsequent reactive annealing under sulfurizing conditions. Surface and in-depth resolved Raman microprobe measurements have been correlated with the analysis of the layers by optical and scanning electron microscopy, x-ray diffraction, and in-depth Auger electron spectroscopy. This has allowed corroboration of the high crystalline quality of the sulfurized layers. The sulfurizing conditions used also lead to the formation of a relatively thick MoS2 intermediate layer between the absorber and the Mo back contact. The analysis of the absorbers has also allowed identification of the presence of In-rich secondary phases, which are likely related to the coexistence in the electrodeposited precursors of ordered vacancy compound domains with the main chalcopyrite phase, in spite of the Cu-rich conditions used in the growth. ...

Analysis of ion beam induced damage and amorphization of 6H-SiC by Raman scattering

Raman scattering analysis of damaged SiC layers obtained by 200 keV Ge+ ion implantation into 6H-SiC has been performed as a function of the implanted dose (up to 1015 cm−2) and annealing temperature (up to 1500°C). The results obtained show the presence of three different damage levels: low damage level (doses ≤3 × 1012 cm−2), medium to high damage level (doses between 1013 and 1014 cm-2), and formation of a continuous amorphous layer for doses higher than the amorphization threshold of 2–3 × 1014 cm−2.Moreover, at doses of about 1014 cm−2 (below the amorphization threshold) amorphous domains are already observed. The Raman spectra indicate the existence of structural differences between the amorphous phase at doses below and above the threshold. After annealing, there is a residual damage which cannot be removed even at the highest annealing temperature of 1500°C. Differences in residual damage between the samples implanted at doses of 1014 and 1015 cm-2 and annealed at the highest temperatures are observed from the peaks in the 1000–1850 cm-1 spectral region. Finally, annealing at the highest temperature is required to observe the complete disappearance of the amorphous bands.

In-depth resolved Raman scattering analysis of secondary phases in Cu-poor CuInSe2 based thin films

Raman scattering analysis of Cu-poor CuInSe2 layers shows the coexistence of the ordered vacancy compound (OVC), CuAu–CuInSe2 and chalcopyrite (CH) CuInSe2 phases as function of the Cu/In content ratio x. In-depth resolved measurements from layers with x≤0.57 show a strong inhibition in the relative intensity of the CH-CuInSe2 mode at the back region. Micro-Raman spectra directly measured at different regions from the layers with 0.66≤x≤0.71 also suggest a higher content of the OVC phase at this back region. These data suggest an enhancement in the formation of OVC at this region in the layers.

Investigation of compositional inhomogeneities in complex polycrystalline Cu(In,Ga)Se2 layers for solar cells

In-depth resolved composition inhomogeneities of polycrystalline Cu(In,Ga)Se2 (CIGS) complex layers for high efficiency solar cells were investigated with Raman scattering measurements. In-depth resolved analysis of the frequency of the main CIGS Raman mode in the spectra measured after sputtering of the layers at different depths lead to identification of different compositions across the layer thickness. These data are in good agreement at both qualitative and quantitative levels with the in-depth resolved composition analysis of the samples by sputtered neutral mass spectroscopy. In addition, Raman measurements also allow detection of additional phases as ordered vacancy compounds.

Combined in-depth scanning Auger microscopy and Raman scattering characterisation of CuInS2 polycrystalline films

Abstract In this work, the combination of in-depth scanning Auger microscopy with Raman microprobe spectroscopy is applied for the detailed microstructural characterisation of CuInS 2 (CIS) thin films. CIS films are used for the fabrication of high efficiency solar cell devices. These films are obtained by sequential sputtering of Cu and In layers on a Mo-coated glass substrate, followed by a sulphurisation step at 500°C in a rapid thermal processing furnace. In order to study this process, samples obtained at intermediate steps are investigated. The obtained data show the formation of the CIS phase already at the first stages of the sulphurisation process, although with a highly disordered structure. Moreover, segregation of CuS towards the surface is observed before sulphurisation is completed. This fact is accompanied by a significant increase of the structural quality of the CIS film, which allows for the fabrication of high efficiency solar cell devices. The performed analysis corroborates the strong complementarity between the used techniques for the detailed microstructural analysis of complex multilayer systems.

Analysis of S-rich CuIn(S,Se)2 layers for photovoltaic applications: Influence of the sulfurization temperature on the crystalline properties of electrodeposited and sulfurized CuInSe2 precursors

This paper reports the microstructural analysis of S-rich CuIn(S,Se)2 layers produced by electrodeposition of CuInSe2 precursors and annealing under sulfurizing conditions as a function of the temperature of sulfurization. The characterization of the layers by Raman scattering, scanning electron microscopy, Auger electron spectroscopy, and XRD techniques has allowed observation of the strong dependence of the crystalline quality of these layers on the sulfurization temperature: Higher sulfurization temperatures lead to films with improved crystallinity, larger average grain size, and lower density of structural defects. However, it also favors the formation of a thicker MoS2 interphase layer between the CuInS2 absorber layer and the Mo back contact. Decreasing the temperature of sulfurization leads to a significant decrease in the thickness of this intermediate layer and is also accompanied by significant changes in the composition of the interface region between the absorber and the MoS2 layer, which bec...

Synthesis of SiC Microstructures in Si Technology by High Dose Carbon Implantation: Etch‐Stop Properties

The use of high dose carbon ion implantation in Si for the production of membranes and microstructures is investigated. Si wafers were implanted with carbon doses of 10{sup 17} and 5 {times} 10{sup 17} cm{sup {minus}2}, at an energy of 300 keV and a temperature of 500 C. The structural analysis of these samples revealed the formation of a highly stable buried layer of crystalline {beta}-SiC precipitates aligned with the Si matrix. The etch-stop properties of this layer have been investigated using tetramethyl-ammonium hydroxide as etchant solution. Secondary ion mass spectrometry measurements performed on the etched samples have allowed an estimate of the minimum dose needed for obtaining an etch-stop layer to a value in the range 2 to 3 {times} 10{sup 17} ions/cm{sup 2}. This behavior has been explained assuming the existence of a percolation process in a SiC/Si binary system. Finally, very thin crystalline membranes and self-standing structures with average surface roughness in the range 6 to 7 nm have been obtained.