Investigation of reactive ion etching of dielectrics and Si in CHF₃/O₂ or CHF₃/Ar for photovoltaic applications

Using a combination of etch rate, photoconductance, and deep level transient spectroscopy(DLTS) measurements, the authors have investigated the use of reactive ion etching (RIE) of dielectrics and Si in CHF₃∕O₂ and CHF₃∕Arplasmas for photovoltaic applications. The radio frequency power (rf-power) and gas flow rate dependencies have shown that the addition of either O₂ or Ar to CHF₃ can be used effectively to change the etch selectivity between SiO₂ and Si₃N₄. The effective carrier lifetime of samples degraded upon exposure to a CHF₃-based plasma, reflecting the introduction of recombination centers in the near-surface region. The extent of minority carrier lifetime degradation was similar in both types of plasmas, suggesting that the same defects were responsible for the increased recombination. However, the rf-power dependence of lifetime degradation in n- and p-type Si was different. Moreover, the lifetime degradation did not exhibit a linear rf-power dependence, suggesting that primary defects were not the dominant recombination centers responsible for the decrease in lifetime. Indeed, DLTS measurements have shown that secondary defects were formed in samples exposed to the plasma after annealing at 400°C, the temperature at which a SiN:H layer is deposited on samples to passivate their surfaces. The minority carrier lifetime degradation in RIE processed samples could be partially avoided using post-RIE chemical treatments.The authors would like to thank the Australian Research Council for financial support

Influence of reactive ion etching on the minority carrier lifetime in P-type Si

Quasi-steady-state photoconductance (QSSPC) and deep level transient spectroscopy (DLTS) were used to characterize the recombination properties of reactive ion etched p-type Si. The effective lifetime of the plasma-processed samples degraded after etching, with the densities of recombination centers increasing linearly with etch time, before reaching a plateau. Evidence is provided for the long-range (> 2 µm) migration of defects in the samples plasma-etched at room temperature. The relationship between rf power and lifetime degradation is also discussed. A defect with energy position at (0.31 ± 0.02) eV was detected by DLTS in RIE p-Si, whereas no defect level was measured in n-type Si. We demonstrate that this energy level could be used to adequately model the injection-dependence of the measured carrier lifetimes using the Shockley-Read-Hall model

Immunological Change in a Parasite-Impoverished Environment: Divergent Signals from Four Island Taxa

Dramatic declines of native Hawaiian avifauna due to the human-mediated emergence of avian malaria and pox prompted an examination of whether island taxa share a common altered immunological signature, potentially driven by reduced genetic diversity and reduced exposure to parasites. We tested this hypothesis by characterizing parasite prevalence, genetic diversity and three measures of immune response in two recently-introduced species (Neochmia temporalis and Zosterops lateralis) and two island endemics (Acrocephalus aequinoctialis and A. rimitarae) and then comparing the results to those observed in closely-related mainland counterparts. The prevalence of blood parasites was significantly lower in 3 of 4 island taxa, due in part to the absence of certain parasite lineages represented in mainland populations. Indices of genetic diversity were unchanged in the island population of N. temporalis; however, allelic richness was significantly lower in the island population of Z. lateralis while both allelic richness and heterozygosity were significantly reduced in the two island-endemic species examined. Although parasite prevalence and genetic diversity generally conformed to expectations for an island system, we did not find evidence for a pattern of uniformly altered immune responses in island taxa, even amongst endemic taxa with the longest residence times. The island population of Z. lateralis exhibited a significantly reduced inflammatory cell-mediated response while levels of natural antibodies remained unchanged for this and the other recently introduced island taxon. In contrast, the island endemic A. rimitarae exhibited a significantly increased inflammatory response as well as higher levels of natural antibodies and complement. These measures were unchanged or lower in A. aequinoctialis. We suggest that small differences in the pathogenic landscape and the stochastic history of mutation and genetic drift are likely to be important in shaping the unique immunological profiles of small isolated populations. Consequently, predicting the impact of introduced disease on the many other endemic faunas of the remote Pacific will remain a challenge

A heat transfer model for concentrating silicon solar cells in a spectrally splitting hybrid receiver

Photovoltaic solar cells under concentrated illumination require passive or active cooling. This paper aims at analyzing the heat transfer in concentrating back contact silicon solar cells for a hybrid photovoltaic/thermal receiver, using a combined electro-thermal numerical model. The concentration ratio on the cells has been assumed to be up to 100 suns (for linear concentrator applications). It is shown that the presence of voids in the solder joints has a significant effect on the performance of the heat transfer circuit especially if the solder layer doesn't cover the whole face of the cell contacts (i.e. using point solders). The cell-to-heat-sink temperature difference can reach above 50°C because of the internal thermal resistance of the cell mounting structure (excluding the convective heat transfer resistance of the heat sink). Reducing the void content and increasing the solder area have significant effect on reducing this temperature difference. A heat sink convection heat transfer coefficient has been added to the model to simulate a typical water cooling channel. It is shown that spectral beam splitting can be employed to keep the cell temperature below 60°C for concentration ratios of up to 100 suns