In polycrystalline silicon (pc-Si) thin-film solar cells, defect passivation is critical to device performance. Isoelectronic or covalently bonded impurities, hydrogenic, extended defects and defects with localized levels in the bandgap (deep level defects) are typically introduced during the fabrication of, and/or are inherent to, pc-Si thin-film solar cells. These defects dramatically affect minority carrier lifetimes. Removing and/or passivating these defects is required to maximize minority carrier lifetimes and is typically done through thermal annealing and passivation techniques. For pc-Si thin-film solar cells on low temperature glass superstrates, rapid thermal annealing (RTA) and hydrogen plasma passivation (hydrogenation) are powerful techniques to achieve effective removal and passivation of these defects. In this thesis, three silicon thin-film solar cells structures on low-temperature glass are subjected to variations in RTA high-temperature plateaus, RTA plateau times, and hydrogen plasma passivation parameters. These solar cells are referred to as ALICIA, EVA and PLASMA. By varying the RTA plateau temperature and time at plateau, the trade-off between extensive dopant diffusion and maximum defect removal is optimized. To reduce the density of point defects and to electrically activate the majority of dopants, an RTA process is shown to be essential. For all three of the thin-film solar cell structures investigated in this thesis, a shorter, higher-temperature RTA process provides the best open-circuit voltage (Voc). Extensive RTA plateau times cause excessive dopant smearing, increasing n = 2 recombination and shunt resistance losses. Hydrogenation is shown to be an essential step to achieve maximum device performance by `healing' the defects inherent to pc-Si thin-film solar cells. If the hydrogen concentration is about 1-2 times the density of oxygen in the cells as measured by secondary ion mass spectroscopy (SIMS), the cells seem to respond best to hydrogenation, with good resultant Voc and short-circuit for all cells investigated in this thesis. The effect of hydrogen passivation on the Voc is spectacular, typically increasing it by a factor of 2 to 3.5. Hydrogen de-bonding from repeated thermal treatments at increasing temperature provides a deeper understanding of what defects exist and the nature of the defects that limit the cell voltage. The variation in RTA and hydrogenation process parameters produces significant empirical insight into the effectiveness of RTA processes for point defect removal, dopant activation, point defect and grain boundary passivation, and impurity passivation. SIMS measurements are used to determine the impurities present in the cells' bulk and the amount of hydrogen available to passivate defects. From the results presented it appears that pc-Si thin-film solar cells on low-temperature glass are a promising, and potentially lower-cost, alternative to Si wafer based cells.

University, Diss., 2003--Freiburg (Breisgau).

Neste trabalho foram crescidos filmes finos de oxinitreto de silício (SiOxNy) por processamento térmico rápido (RTP) utilizando um forno térmico convencional adaptado, objetivando fabricar células solares MOS com baixo custo agregado e bom rendimento de conversão de baixas intensidades luminosas em energia elétrica de forma reprodutível. A receita de oxinitretação otimizada foi desenvolvida em ambiente misto de 5N2:1O2 na temperatura de 850°C para tempos de processo, na faixa de 10 a 80s seguido por uma passivação em 2L/min de N2 por 80s. Os dielétricos crescidos foram caracterizados fisicamente quanto à espessura (entre 1,50 e 2,95nm), à microrugosidade (<0,95nmRMS) e à concentração de nitrogênio (1,0-2,1%atm). As características de tunelamento foram investigadas em capacitores MOS e apontaram para a existência de armadilhas interfaciais do tipo K capazes de armazenar cargas positivas. Nas células solares MOS, a corrente de fundo foi característica para todos os processos de oxinitretação empregados (~0,5-2µA/cm2) e apresentaram níveis de resposta à luz incidente na faixa de 1 a 8mA/cm2 compatível com aplicações de conversão de energia em ambientes internos e externos (energy harvesting). A característica densidade de corrente x tensão de porta (JxVG) das células solares apresentou um comportamento aproximadamente linear desde a densidade de corrente de curto-circuito (JSC) até a tensão de curto-circuito (VOC) implicando em potência gerada máximas (PGmáx) de até centenas de µA/cm2 para VG ? VOC/2 para uma ampla faixa de intensidade radiante incidente (11,8 - 105,7mW/cm2) alcançando rendimentos de conversão de até 5,5%. / In this work, silicon oxynitrides (SiOxNy) were grown by means of a homemade Rapid Thermal Processing (RTP). The goal was to manufacture MOS solar cells with a reduced price and reasonable light conversion efficiency for low light intensity. The optimized oxidation recipe consisted of using an environment with gas mixture of 5N2:1O2 at a temperature of 850°C and different processing times in the range of 10 to 80s followed by a passivation step in ultrapure N2 (2L/min) at the same temperature of 850oC for 80s. The oxynitrides were grown with thickness in the range of 1.50 to 2.95nm with surface microroughness lower than 0.95nmRMS and nitrogen concentration in the range of 1.0 to 2.1%atm. The tunneling characteristics were studied with the aid of MOS capacitor and K-type interfacial traps related to Si(p)/Si?N structure were detected positively charged for VG > 0. The background current in the MOS solar cells (~0.5-2µA/cm2) were similar for all samples and the current response to the incident light was in the range of 1 to 8mA/cm2, which is compatible with energy conversion for indoor and outdoor environments (energy harvesting). The current density x gate voltage (JxVG) characteristics of the MOS solar cells presented a nearly linear behavior since the short-circuit current density (JSC) till to the open circuit voltage (VOC) so that the maximum generated power was of hundreds of µA/cm2 for VG ? VOC/2 for a large range of radiant intensities (11.8 - 105.7 mW/cm2) and achieving efficiency conversion up to 5.5%.

The original work presented in this thesis describes research into Si-based luminescent materials, prepared specifically by ion implantation and rapid thermal processing of thermal oxide films. An in-depth optical characterisation, employing photoluminescence (PL) and Raman spectroscopy was complimented with electron microscopy, revealing the source of efficient room temperature PL as nano-scale silicon inclusions (Si-NCs). The evolution of the Si-NC size and density with isothermal and isochronal annealing may be described using classical thermodynamics according to a diffusion limited, Ostwald ripening process. Values for the coarsening rate and activation energy, extracted from the evolution of the Si-NC size with annealing indicate that the transport of Si atoms and precipitate formation are enhanced in ion implanted films, attributable to the presence of vacancy and interstitial defects generated during ion irradiation. The PL and Raman spectra are well correlated with the evolving Si-NC size and density according to the quantum confinement (QC) model in which samples containing larger clusters emit at longer wavelengths. However, the formation of bound exciton states within the band gap of small clusters (< 2nm), as a result of specific surface chemistries, suppresses higher energy emissions. The increase in PL intensity with annealing was exactly correlated with the increase in PL lifetime, characteristic of the removal of non-radiative defects. A dependence of the PL dynamics on emission energy, with higher energies exhibiting shorter lifetimes, further evidences the QC effect. Blue shifted emission at high excitation flux and/or low temperature is correspondent with the slower PL dynamics and preferential saturation at longer wavelengths. Raman spectra were fit using a phonon confinement model, from which Si-NC size distributions were extracted and found to compare favourably with those obtained from TEM images. Stresses in the films, determined from the Raman peak position, were used as an independent method for calculating the Si surface energy, which is very close to the literature values. A single, high temperature anneal of Si and erbium (Er) co-doped films revealed a preferential aggregation of Er at the Si-NC formation site, which is of particular importance for the photo-sensitization of Er PL around 1.5μm. The Er PL was enhanced in the presence of Si-NCs by several orders of magnitude compared with a reference SiO2:Er. Whilst broadband pumping of the Er via Si-NCs evidences a non-resonant energy transfer mechanism with an efficiency which depends on the Si-NC size, the process is limited at high excitation flux by a combination of low sensitizer (Si-NC) density and non-radiative losses. Finally the Si-NC PL intensity in phosphorus (P) co-doped films was studied and found to depend strongly on the annealing conditions and P concentration. For lower temperature treatments, a factor 2 PL enhancement, relative to an un-doped reference was obtained, attributed to the passivation of Si-NC surface defects. Higher temperature treatments resulted in the monotonic quenching of the PL with increasing P concentration, attributed to the introduction of an efficient Augerre combination channel as a result of the ionization of P-donors inside large Si-NCs. A simple statistical model predicts this behaviour and provides an incidental estimate of the Si-NC size.

No description available.

A rapid thermal processing (RTP) reactor for the preparation of graded CuIn1-xGaxSe2-ySy (CIGSeS) thin-film solar cells has been designed, assembled and is being used at the Photovoltaic Materials Laboratory of the Florida Solar Energy Center. CIGSeS films having the optimum composition, morphology, and semiconducting properties were prepared using RTP. Initially films having various Cu/(In+Ga) ratios were prepared. In the next step selenium incorporation in these films was optimized, followed by sulfur incorporation in the surface to increase the bandgap at the surface. The compositional gradient of sulfur was fine-tuned so as to increase the conversion efficiency. Materials properties of these films were characterized by optical microscopy, SEM, AFM, EDS, XRD, GIXRD, AES, and EPMA. The completed cells were extensively studied by electrical characterization. Current-voltage (I-V), external and internal quantum efficiency (EQE and IQE), capacitance-voltage (C-V), and light beam induced current (LBIC) analysis were carried out. Current Density (J)-Voltage (V) curves were obtained at different temperatures. The temperature dependence of the open circuit voltage and fill factor has been estimated. The bandgap value calculated from the intercept of the linear extrapolation was ~1.1-1.2 eV. Capacitance-voltage analysis gave a carrier density of ~4.0 x 1015 cm-3. Semiconductor properties analysis of CuIn1-xGaxSe2-ySy (CIGSeS) thin-film solar cells has been carried out. The values of various PV parameters determined using this analysis were as follows: shunt resistance (Rp) of ~510 Ohms-cm2 under illumination and ~1300 Ohms-cm2 in dark, series resistance (Rs) of ~0.8 Ohms-cm2 under illumination and ~1.7 Ohms-cm2 in dark, diode quality factor (A) of 1.87, and reverse saturation current density (Jo) of 1.5 x 10-7A cm-2. The efficiency of 12.78% obtained during this research is the highest efficiency obtained by any University or National Lab for copper chalcopyrite solar cells prepared by RTP. CIGS2 cells have a better match to the solar spectrum due to their comparatively higher band-gap as compared to CIGS cells. However, they are presently limited to efficiencies below 13% which is considerably lower than that of CIGS cells of 19.9%. One of the reasons for this lower efficiency is the conduction band offset between the CIGS2 absorber layer and the CdS heterojunction partner layer. The band offset value between CIGS2 and CdS was estimated by a combination of ultraviolet photoelectron spectroscopy (UPS) and Inverse Photoemission Spectroscopy (IPES) to be -0.45 eV, i.e. a cliff is present between these two layers, enhancing the recombination at the junction, this limits the efficiency of CIGS2 wide-gap chalcopyrite solar cells.

The ability to fabricate semiconductor wafers with spatially selected regions of different bandgap material is required for the fabrication of monolithic photonic integrated circuits (PIC's). Although this subject has been studied for three decades and many semiconductor engineering approaches have been proposed, the problem of achieving reproducible results has constantly challenged scientists and engineers. This concerns not only the techniques relaying on multiple sequential epitaxial growth and selective area epitaxy, but also the conventional quantum well intermixing (QWI) technique that has been investigated as a post-growth approach for bandgap engineering. Among different QWI techniques, those based on the use of different lasers appear to be attractive in the context of high-precision and the potential for cost-effective bandgap engineering. For instance, a tightly focused beam of the infrared (IR) laser could be used for the annealing of small regions of a semiconductor wafer comprising different quantum well (QW) or quantum dot (QD) microstructures. The precision of such an approach in delivering wafers with well defined regions of different bandgap material will depend on the ability to control the laser-induced temperature, dynamics of the heating-cooling process and the ability to take advantage of the bandgap engineering diagnostics. In the frame of this thesis, I have investigated IR laser-induced QWI processes in QW wafers comprising GaAs/A1GaAs and InP/InGaAsP microstructures and in InAs QD microstructures grown on InP substrates. For that purpose, I have designed and set up a 2-laser system for selective area rapid thermal annealing (Laser-RTA) of semiconductor wafers. The advantage of such an approach is that it allows carrying out annealing with heating-cooling rates unattainable with conventional RTA techniques, while a tightly focused beam of one of the IR lasers is used for `spot annealing'. These features have enabled me to introduce a new method for iterative bandgap engineering at selected areas (IBESA) of semiconductor wafers. The method proves the ability to deliver both GaAs and InP based QW/QD wafers with regions of different bandgap energy controlled to better than « 1nm of the spectral emission wavelength. The IBESA technique could be used for tuning the optical characteristics of particular regions of a QW wafer prepared for the fabrication of a PIC. Also, this approach has the potential for tuning the emission wavelength of individual QDs in wafers designed, e.g., for the fabrication of single photon emitters. In the 1st Chapter of the thesis, I provide a short review of the literature on QWI techniques and I introduce the Laser - RTA method. The 2nd Chapter is devoted to the description of the fundamental processes related to the absorption of laser light in semiconductors. I also discuss the results of the finite element method applied for modeling and semi-quantitative description of the Laser - RTA process. Details of the experimental setup and developed procedures are provided in the 3rd Chapter. The results concerning direct bandgap engineering and iterative bandgap engineering are discussed in the 4th and 5th Chapters, respectively.

With the MOSFET scaling practice, the performance of IC devices is improved tremendously as we experienced in the last decades. However, the small semiconductor devices also bring some drawbacks among which the high gate leakage current is becoming increasingly serious. This thesis work is focused on the of gate leakage current reduction in thin oxide semiconductor devices. The method being studied is the Phonon Energy Coupling Enhancement (PECE) effect induced by Rapid Thermal Processing (RTP). The basic MOS capacitors are used to check improvements of leakage current reduction after appropriate RTP process. Through sets of experiments, it is found that after RTP in Helium environment could bring about four orders reduction in gate leakage current of MOS capacitors.

In this thesis, multiple activities have been carried out in order to improve the process of CIGSS solar cell fabrication on a 4" x 4" substrate. The process of CIGSS solar cell fabrication at FSEC's PV Materials Lab involves a series of steps that were all carried out manually in the past. A LABVIEW program has been written to carry out automated sputter deposition of Mo back contact, CuGa, In metallic precursors on a soda lime glass substrate using a stepper motor control for better uniformity. Further, selenization/ sulfurization of these precursors was carried out using rapid thermal processing (RTP). CIGS films were sulfurized using chemical bath deposition (CBD). ZnO:Al was deposited on the CIGSS films using RF sputtering. A separate LABVIEW program was written to automate the process of ZnO:Al deposition. Ni/Al contact fingers were deposited on the ZnO:Al layer using the e-beam evaporation technique. Further, in order to test these solar cells in-house, a simple current-voltage (IV) tracer was fabricated using LABVIEW. A quantum efficiency (QE) measurement setup was built with guidance from the National Renewable Energy Laboratory (NREL). Lastly, analysis of data from photovoltaic (PV) modules installed on the FSEC test site has been carried out using a LABVIEW program in order to find out their rate of degradation as time progresses. A 'C' program has also been written as an aid for keeping a daily log of errors in data and for troubleshooting of the same.

No description available.