ABSTRACT Currently, the manufacture of thin films of GaAs doped with Mn (GaAsMn) on Si (100), is an object of great interest, due to its possible integration with silicon technology, generating a significant development in the functionality of the devices optoelectronics and spintronics. In this work, we present a systematic study of the structural, optical, morphological and magnetic characterization of GaAsMn thin films prepared by R.F sputtering on a Si (100) substrate, for growth temperatures of 100 and 200 oC, respectively. From Raman spectra, the transverse optical (TO) and longitudinal optical (LO) vibrational modes, located at 290 cm-1 and 265 cm-1 of GaAs were identified. Additionally, vibrational modes of MnAs were identified, due to the replacement of Ga atoms with Mn atoms in high concentrations. The segregation of Mn was bear out by X-ray diffraction, where crystalline planes are evidenced in the directions (400) and (200) of polycrystalline GaAs, and crystallographic planes belonging to the Mn1+xAs phases. The morphology and growth mode of the thin films of GaAsMn / Si (100), was carried out by means of atomic force microscopy (AFM) and scanning electron microscopy (SEM) images, taken on the surface and in cross-section, respectively. Finally, an analysis of the magnetic properties of the GaAs thin films made from magnetic force microscopy (MFM) images, evidence surface magnetic domains formation, which depending on growth conditions. We concluded that, all properties of GaAsMn thin films are depending on growth conditions.

RESUMEN Actualmente, la fabricación de películas delgadas de GaAs dopadas con Mn (GaAsMn) sobre Si (100), es un objeto de gran interés debido a su posible integración con la tecnología del silicio, generando un desarrollo significativo en la funcionalidad de los dispositivos optoelectrónicos y espintrónicos. En este trabajo, presentamos un estudio sistemático de la caracterización estructural, morfológica, óptica, y magnética de películas delgadas de GaAsMn preparadas por pulverización catódica R.F sobre un substrato de silicio (100), para temperaturas del crecimiento de 100 y 200 oC, respectivamente. A partir de los espectros Raman se identificaron los modos vibracionales, trasversal óptico (TO) y longitudinal óptico (LO) de GaAs, localizados en 290 cm-1 y 265 cm-1, respectivamente. Adicionalmente, se identificaron modos vibracionales de MnAs, debido a la substitucion de átomos de Ga por átomos de Mn en altas concentraciones. La segregación de Mn, fue corroborada mediante difracción de rayos-X, en donde se evidencian planos cristalinos en las direcciones (400) y (200) de GaAs policristalino, y planos cristalográficos pertenecientes a fases de Mn1+xAs. La morfología y el modo de crecimiento de las películas delgadas de GaAsMn/Si (100), se llevó a cabo mediante imágenes de microscopía de fuerza atómica (AFM) y microscopía electrónica de barrido (SEM) tomadas sobre la superficie y en sección transversal, respectivamente. Finalmente, un análisis de las propiedades magnéticas de las películas delgadas de GaAsMn a partir de imágenes de microscopía de fuerza magnética (MFM), revelan la presencia de dominios magnéticos superficiales provenientes de MnAs. Concluimos que las propiedades físicas de las películas de GaAsMn dependen de las condiciones de crecimiento.

Abstract Polycrystalline thin film solar cells made with absorber materials such as CdTe, CIGS, CZTS and metalorganic halides (perovskites) are currently important alternatives for the silicon solar cell technology, which still dominates the photovoltaic market. Then, it is important to have tools which can be used to design this kind of solar cells. For this purpose, we have developed a unified simple analytical model that can be applied to thin film solar cells. The model is based on the basic physics of hetero-junction devices, but it takes into consideration that the space charge region can extend along the major part of the cell length, particularly for very thin cells, causing important effects that typically are not observed in conventional junction devices. Photo-generated carriers are collected by electric field-drift instead of diffusion, and simultaneously strong recombination at this region may dominate the electrical I-V characteristic of the cell. Since the space-charge region width varies with the applied voltage, the illumination current density and the saturation dark current density are no longer independent of the voltage as is assumed for conventional solar cells. When the model is applied to CIS and CdTe solar cells as examples, it is found that it is possible to design very thin film solar cells (absorber less than 1 μm thick) with high efficiencies, whenever the recombination velocity at the back surface becomes small (102 cm/s), instead of the high recombination velocities present at ohmic contacts (107 cm/s). This fact implies the cost reduction of thin film solar cells by reducing absorber material thickness, and therefore it poses a challenge to develop deposition methods for very thin CdTe and CIGS absorber materials without pinholes, so that improved efficiencies are obtained when the surface recombination velocity is made small at the back by having a p+ or an electron blocking region before the ohmic contact. This result also explains the high efficiencies achieved by very thin perovskite solar cells.

Resumen En este trabajo, reportamos la caracterización de monocristales de Si(100) y GaSb(111) utilizando espectroscopia Raman y fotoacóstica (principal interés) en configuración abierta y cerrada, para compuestos semiconductores III-V con estructura tipo zinc blenda. Los espectros Raman generalmente muestran dos picos, un pico a baja frecuencia correspondiente a modos fononicos TO y un pico en alta frecuencia correspondiente a modos fononicos LO. Un fuerte pico es mostrado en la position 226 cm-1 y uno un poco más débil en 237 cm-1, que son los modos TO y LO respectivamente, debido a la orientación cristalina del material. Con el fin de determinar la difusividad térmica de los materiales se utilizaron láseres con longitudes de onda de 650 nm y 535 nm. Los resultados de la difusividad térmica de los monocristales de Si y GaSb, obtenidos a partir del modelo de Rosencwaig y Gersho (RG) se analizaron en función de la orientación cristalográfica. Discutimos la recombinación no-radiactiva que se origina en la superficie y en el volumen del cristal, que contribuye a la señal fotoacústica, en términos del tipo de celda y de la línea de excitación.

Abstract In this work, we report the characterization of Si (100) and GaSb (111) single crystals by using Raman spectroscopy and a photoacoustic cell in open and closed configuration. For III-V compound semiconductors of the zinc-blende crystal structure, Raman spectra generally show two peaks. The lower-frequency peak corresponds to TO phonons, and the higher frequency peak corresponds to LO phonons. A strong peak was found at 226 cm-1 and a weak peak at 237 cm-1, which are the TO and LO modes, respectively. Raman spectrum of GaSb (111) show that the integrate intensity of TO mode is greater than LO mode due to disoriented of the single crystal. In order to obtain the thermal diffuivity, the crystals were exciting by means of solid-state lasers of wavelengths of 650 nm and 535 nm, respectively. The results of the thermal diffusivity of single crystals, obtained from the Rosencwaig and Gersho (RG) model were analyzed according to the crystallographic orientation. We have also discussed the contribution of the non-radiative recombination to the photoacustic signal from surface and bulk depending on the type of cell and excitation of the laser line.