B.M. Monroy

Influence of the surrounding host in obtaining tunable and strong visible photoluminescence from silicon nanoparticles

We have investigated the influence of the microstructure and chemistry of the surrounding host on the strong visible photoluminescence (PL) from silicon nanoclusters (nc-Si) embedded in three different silicon-based dielectric compounds: SixNy:H,Cl, SixNyOz:H,Cl, and SixOz:H,Cl, obtained from silicon nitride films deposited by SiH2Cl2∕NH3∕H2 plasma-enhanced chemical vapor deposition at different growth pressures. A blueshift is found in the PL coming from the nc-Si as the content of oxygen in the surrounding host is increased, and a significant improvement in PL intensity is achieved when the nc-Si are well passivated with O instead of H. We discuss the PL behavior in terms of the quantum confinement model and passivation state of the nc-Si surface.

Polymorphous silicon thin films obtained by plasma-enhanced chemical vapor deposition using dichlorosilane as silicon precursor.

Polymorphous silicon thin films (pm-Si) have been deposited from mixtures of dichlorosilane and hydrogen, using argon as the diluting gas by plasma-enhanced chemical vapor deposition. The deposition conditions were chosen to simultaneously obtain both Si nanocrystallites and an amorphous silicon matrix in the as-grown samples. High resolution transmission electron microscopy studies show the crystallinity of Si domains whose dimensions are in the interval of 2-14 nm. The surface passivation state of the silicon nanocrystals was inferred from Fourier transform infrared spectroscopy analysis. Two optical absorption edges, corresponding to the amorphous matrix and the Si nanocrystals, were observed for all the pm-Si thin films. Intense visible photoluminescence was observed for the as-grown samples. The possibility of using these thin films for the down-conversion effect in silicon solar cells is discussed.

Photoluminescence Study of Gallium Nitride Thin Films Obtained by Infrared Close Space Vapor Transport

Photoluminescence (PL) studies in GaN thin films grown by infrared close space vapor transport (CSVT-IR) in vacuum are presented in this work. The growth of GaN thin films was done on a variety of substrates like silicon, sapphire and fused silica. Room temperature PL spectra of all the GaN films show near band-edge emission (NBE) and a broad blue and green luminescence (BL, GL), which can be seen with the naked eye in a bright room. The sample grown by infrared CSVT on the silicon substrate shows several emission peaks from 2.4 to 3.22 eV with a pronounced red shift with respect to the band gap energy. The sample grown on sapphire shows strong and broad ultraviolet emission peaks (UVL) centered at 3.19 eV and it exhibits a red shift of NBE. The PL spectrum of GaN films deposited on fused silica exhibited a unique and strong blue-green emission peak centered at 2.38 eV. The presence of yellow and green luminescence in all samples is related to native defects in the structure such as dislocations in GaN and/or the presence of amorphous phases. We analyze the material quality that can be obtained by CSVT-IR in vacuum, which is a high yield technique with simple equipment set-up, in terms of the PL results obtained in each case.

Structure and optical properties of silicon nanocrystals embedded in amorphous silicon thin films obtained by PECVD

Silicon nanocrystals embedded in amorphous silicon matrix were obtained by plasma enhanced chemical vapor deposition using dichlorosilane as silicon precursor. The RF power and dichlorosilane to hydrogen flow rate ratio were varied to obtain different crystalline fractions and average sizes of silicon nanocrystals. High-resolution transmission electron microscopy images and RAMAN measurements confirmed the existence of nanocrystals embedded in the amorphous matrix with average sizes between 2 and 6nm. Different crystalline fractions (from 12% to 54%) can be achieved in these films by regulating the selected growth parameters. The global optical constants of the films were obtained by UV-visible transmittance measurements. Effective band gap variations from 1.78 to 2.3 eV were confirmed by Tauc plot method. Absorption coefficients higher than standard amorphous siliconwere obtained in these thin films for specific growth parameters. The relationship between the optical properties is discussed in terms of the different internal nanostructures of the samples.

Study of prolonged light-soaking (∼400 Hrs) effect on Pm-Si:H solar cell structures

Different pm-Si:H structures were grown using PECVD technique in order to study the effect of light exposition on solar cell structures based on these materials. PIN and NIP structures were analyzed during 400 hrs of light-soaking exposition. The evolution of the structural and optical properties was observed and characterized by Raman spectroscopy, spectroscopic ellipsometry and exodiffusion experiments. The effect observed is related to defects creation due to induced hydrogen diffusion, break of Si-H bonds and the generation of dangling bonds that causes less passivated films. The film microstructure, hydrogen stability, configuration and therefore the optical properties varied with the exposition time. The crystalline fraction of these structures is between 12 to 18% and increase with the exposition time. The optical gap decreases from 1.76 to 1.6 eV for the PIN structure while for the NIP decreases from 1.62 to 1.54 eV. The hydrogen stability and its amount bonded in these films are dependent of the device architecture. Hydrogen diffusion induces structural crystallization and generates a decrease on the absorption properties of the films which in turn is expected to reduce the device efficiency during operation. In this work we show that long range motion of hydrogen during light soaking causes a hydrogen rearrangement on the film and microstructure changes along with a shift on the exodiffusion peaks. Hydrogen diffusion is very different during light-soaking for both structures. We determined that the total hydrogen that effuses from PIN structure is lower than for the NIP, which is expected to cause less degradation of its optoelectronic properties under illumination, and a more stable device during operation.

Study of the previous light-soaking effect on annealed hydrogenated polymorphous silicon solar cells structures

This work describes a study performed on pm-Si:H PIN and NIP structures, deposited by PECVD. We show the effect of light-soaking (AM1.5) on pm-Si:H thin films structures optical properties. We propose a model for long range diffusion of hydrogen through the whole device. From the evolution of Raman spectra after annealing, we show that this long range motion of hydrogen causes a hydrogen rearrangement on the film along with a shift on the exodiffusion peaks explaining the degradation process in this type of devices. As a result we determined that for the PIN structure a hydrogen diffusion barrier is generated by defects, which causes a less degradation of its optoelectronic properties under illumination, and a more stable device during operation is expected.

Photoluminescence spectroscopy as a tool for quality control of GaN thin film to be used in solar cell devices

The use of III-V and semiconductor nitrides in solar cells has been of interest in the PV-community due to the wide variation range of the band gap in these materials. Particularly, the processing of hetero-junction structures of AlGaN/GaN and Si(p)/GaN(n) has been of great interest recently. In this work, the quality of GaN and InGaN thin films grown by Molecular Beam Epitaxy (MBE) on different substrate and buffer layers has been studied by photoluminescence spectroscopy (PL). The PL measurements were processed as function of sample temperature. In the PL spectra it is possible to observe a strong near band-gap-edge emission and a broad blue, green and yellow luminescence (BL, GL, YL), which can be assigned to the presence of Ga and N vacancies, amorphous phases, deep level impurities and structural defects. The relative intensity between the different peaks of the bands related to defects or impurities were studied as a tool for quality control of the films.