Epitaxial CuInSe2 thin films grown by molecular beam epitaxy and migration enhanced epitaxy

While CuInSe2 chalcopyrite materials are mainly used in their polycrystalline form to prepare thin film solar cells, epitaxial layers have been used for the characterization of defects. Typically, epitaxial layers are grown by metal-organic vapor phase epitaxy or molecular beam epitaxy (MBE). Here we present epitaxial layers grown by migration enhanced epitaxy (MEE) and compare the materials quality to MBE grown layers. CuInSe2 layers were grown on GaAs (001) substrates by co-evaporation of Cu, In, and Se using substrate temperatures of 450 ºC, 530 ºC, and 620 ºC. The layers were characterized by high resolution X-ray diffraction (HR-XRD), high-resolution transmission electron microscopy (HRTEM), Raman spectroscopy, and atomic force microscopy (AFM). HR-XRD and HR-TEM show a better crystalline quality of the MEE grown layers, and Raman scattering measurements confirm single phase CuInSe2. AFM shows the previously observed faceting of the (001) surface into {112} facets with trenches formed along the [110] direction. The surface of MEE-grown samples appears smoother compared to MBE-grown samples, a similar trend is observed with increasing growth temperature.The authors would like to acknowledge the CAPES (CAPES-INL 04/14), CNPq, and FAPEMIG funding agencies for financial support. We acknowledge the collaboration project with IMMCSIC (AIC-B-2011-0806). P.M.P.S. acknowledges financial support from EU through the FP7 Marie Curie IEF 2012 Action No. 327367.info:eu-repo/semantics/publishedVersio

Passivation of interfaces in thin film solar cells: understanding the effects of a nanostructured rear point contact layer

Thin film solar cells based in Cu(In,Ga)Se2 (CIGS) are among the most efficient polycrystalline solar cells, surpassing CdTe and even polycrystalline silicon solar cells. For further developments, the CIGS technology has to start incorporating different solar cell architectures and strategies that allow for very low interface recombination. In this work, we study and characterize ultrathin 350 nm CIGS solar cells with a rear interface passivation strategy. The rear passivation was achieved using an Al2O3 nanopatterned point structure. Using the cell results, photoluminescence measurements and detailed optical simulations based on the experimental results, we show that by including the nanopatterned point contact structure, the interface defect concentration lowers, which ultimately leads to an increase of solar cell electrical performance mostly by increase of the open circuit voltage. Gains to the short circuit current are distributed between an increased rear optical reflection and also due to electrical effects. Our approach of mixing several techniques allowed us to make a discussion considering the different passivation gains which has not been done in detail in previous works. A solar cell with a nanopatterned rear contact and a 350 nm thick CIGS absorber provided an average power conversion efficiency close to 10%.publishe

Caracterização óptica e elétrica de filmes finos de GaAs dopados com mg crescidos pela técnica de MBE

Exportado OPUSMade available in DSpace on 2019-08-13T00:33:48Z (GMT). No. of bitstreams: 1 disserta__o_henrique_limborco.pdf: 4317668 bytes, checksum: e11c9abda92184df879ed749453b3af3 (MD5) Previous issue date: 5O crescimento de filmes finos de GaAs dopados com Magnésio crescidos em substratos de GaAs(100) pela técnica de Epitaxia por Feixe Molecular (MBE) foram muito pouco estudados[1]. O Magnésio é uma opção não cancerígena e não tóxica ao Berílio como dopante tipo p em materiais como GaAs, GaN e outras ligas[1-3], formando um defeito aceitador raso no GaAs, quando incorporado num sítio de Ga[1, 4] (defeito substitucional Mg Ga). Até onde sabemos, nunca foi realizado um estudo das propriedades elétricas e ópticas de filmes finos de GaAs dopados com Mg crescidos pela técnica de MBE em substratos de GaAs com orientação (111)B. Para esse fim foram crescidas 79 amostras em substratos semi-isolantes de GaAs não dopados, com orientações (100) e (111)B, com variadas condições de crescimento. As propriedades de transporte eletrônico, como mobilidade Hall e concentração de portadores Hall, foram estudadas através de medidas de efeito Hall padrão usando o método de Van der Pauw de quatro pontos. Medidas de efeito Hall a temperatura ambiente foram realizadas para as amostras com concentração de portadores na faixa de 1016 ~ 1019cm -3. A energia de ativação para o processo de dessorção do Mg na superfície em crescimento do filme foi determinada para estas duas orientações. Foi encontrado o valor de para as amostras crescidas em substratos de GaAs (100) e para (111)B. Nossos resultados confirmam os valores encontrados anteriormente para a orientação (100) e indicam, conforme sugerido por Makoto et al.[5], que o sítio disponível para incorporação do Mg nas amostras com a orientação (111)B é mais estável que o sítio ligante para a orientação (100). As propriedades ópticas das amostras foram analisadas através de medidas de fotoluminescência em diversas temperaturas. A posição do pico e-A foi usada para determinar a energia de ativação do defeito na estrutura eletrônica do GaAs e revelou uma pequena diferença entre o valor para os filmes finos crescidos em substratos de GaAs (100) e (111)B, Ea100= 27meV e Ea11b= 33meV, respectivamente. Um estudo do comportamento da PL em função da temperatura também foi realizado para adquirir algum conhecimento dos mecanismos de recombinação não radiativos presentes na amostra.Magnesium doped GaAs thin films grown in (100) and (111)B Semi-Insulating GaAs substrates by MBE weren't deeply investigated until today. Mg is a promising option (non-carcinogenic and non-toxic) to replace Beryllium as a p-type dopant in III -V materials, such as GaAs, GaN and alloys[1-3]. Mg forms a shallow defect in the GaAs electronic structure, being incorporated in a Ga site (substitucional defect MgGa) [1, 4]. As far as we know, no report was made concerning the electrical and optical properties of thin films of Mg doped GaAs grown in GaAs substrates with crystal orientation (111)B by the MBE technique. For this purpose several GaAs:Mg samples were grown, with different growth conditions using two substrate crystalline orientations. The transport properties were investigated by the standard Hall measurements techniques, by the four point probe Van der Pauw method. This measurements were carried out at room temperature for samples with Hall carrier concentrations ranging from 1016cm-3 to 1019cm-3. The activation energy for the Mg desorption process from the GaAs surface was found to be different for the two substrate orientations. The activation energy Ea= 1,5 eV, found for the (100) orientation confirms already reported values for samples grown in the (100) orientation. For the (111)B substrate orientation Ea=2,5eV, a result that confirm s the suggestion of Makoto et al.[5], which expected that the available site for the Mg incorporation in this substrate orientation is more stable than the one in the (100) orientation. The thin films optical properties were investigated by photoluminescence at several temperatures. The photoluminescence peak position of the e-A transition was used to find the activation energy of the defect in the GaAs electronic band. The results have shown an activation energy of Ea100= 27meV for the (100) samples Ea111b= 33meV and for the (111)B samples. The temperature dependent PL spectrum was investigated in order to understand the role of the nonradiative recombination mechanisms

Structural and ferroelectric properties of Sr1−xBaxBi2Nb2O9 thin films obtained by dip-coating

The paper presents the structural and ferroelectric results for Sr1−xBaxBi2Nb2O9(x=0.30; 0.85) thin films, which were obtained by using dip-coating. The solutions containing the desirable ions were prepared from the powders of the previous studied ceramic samples. The films were deposited at room temperature on Fluorine-doped Tin Oxide (FTO) substrates and submitted to a heat treatment for crystallization. The films were characterized by using scanning microscopy electronic, energy dispersive spectroscopy and ellipsometry. Hysteresis ferroelectric loops were obtained, at room temperature, by using a Sawyer-Tower circuit at several frequencies. A well-defined grain structure was observed for both compositions. The energy dispersive spectroscopy (EDS) measurements revealed the presence of the corresponding elements from the chemical composition of the ceramic systems. The band-gap energy was around 3.3eV for both samples. Typical hysteresis loops for normal and relaxor ferroelectrics were obtained for x=0.30 and 0.85, respectively