Johnson Wong

Interaction between BRCA2 and replication protein A is compromised by a cancer-predisposing mutation in BRCA2

Mutations in the BRCA1 and BRCA2 genes predispose women to familial, early-onset breast cancer. Both the BRCA1 and BRCA2 proteins appear to function in the homologous recombination pathway of DNA double-strand break repair. Both BRCA1 and BRCA2 have also been implicated in transcription by RNA polymerase II, for both proteins have domains which, when tethered adjacent to a promoter, can activate transcription. In experiments reported here, we have used protein affinity chromatography and coimmunoprecipitation techniques to show that the putative N-terminal acidic transcriptional activation domain of BRCA2 interacts with replication protein A (RPA), a protein essential for DNA repair, replication and recombination. This interaction was not mediated by DNA and was specific for human RPA but not yeast RPA. Since the cancer-predisposing mutation Y42C in BRCA2 significantly compromised the interaction between RPA and BRCA2, this interaction may be biologically important. That BRCA2 protein in HeLa cell extract also coimmunoprecipitated with RPA suggested that this interaction occurs in vivo. Therefore, the transcriptional activation domains within BRCA2, and perhaps BRCA1, may provide links to RPA and DNA repair processes rather than transcription.

Assessing the requirements for nucleotide excision repair proteins of Saccharomyces cerevisiae in an in vitro system.

Nucleotide excision repair (NER) is the primary mechanism by which both Saccharomyces cerevisiae and human cells remove the DNA lesions caused by ultraviolet light and other mutagens. This complex process involves the coordinated actions of more than 20 polypeptides. To facilitate biochemical studies of NER in yeast, we have established a simple protocol for preparing whole cell extracts which perform NER in vitro. As expected, this assay of in vitro repair was dependent on the products of RAD genes such as RAD14, RAD4, and RAD2. Interestingly, it was also dependent upon proteins encoded by the RAD7, RAD16, and RAD23 genes whose precise roles in NER are uncertain, but not the RAD26 gene whose product is believed to participate in coupling NER to transcription. Replication protein A (RPA/Rpa), known to be required for NER in human cell extracts, was also shown by antibody inhibition and immunodepletion experiments to be required for NER in our yeast cell extracts. Moreover, yeast cells with temperature-sensitive mutations in the RFA2 gene, which encodes the 34-kDa subunit of Rpa, had increased sensitivity to UV and yielded extracts defective in NER in vitro. These data indicate that Rpa is an essential component of the NER machinery in S. cerevisiae as it is in mammalian cells.

A Systematic Loss Analysis Method for Rear-Passivated Silicon Solar Cells

By combining commonly available solar cell characterization methods with easy-to-prepare test structures and partially processed rear-passivated solar cells from the production line, we show that various cell loss mechanisms can be quantified in exquisite detail to generate process-related diagnostics. An example monocrystalline silicon localized back surface field solar cell type is examined using a systematic routine that breaks down the factors limiting open-circuit voltage, short-circuit current, and fill factor (FF) to identify the cell structures headroom for improvement.

Analysis of intrinsic hydrogenated amorphous silicon passivation layer growth for use in heterojunction silicon wafer solar cells by optical emission spectroscopy

The structure and quality of intrinsic hydrogenated amorphous silicon thin films are studied with intended use as passivation layer in heterojunction silicon wafer solar cells. The thin film layers are formed by radio-frequency parallel-plate plasma-enhanced chemical vapor deposition. While the passivation quality of the films is found to improve steadily with increasing deposition temperature, a very narrow process window in terms of pressure variation is observed. The best effective lifetime is obtained at a hydrogen to silane dilution ratio of 1 and a pressure of 66.7 Pa for the used tool configuration. Raman crystallinity and Urbach energy obtained from fitting ellipsometry data confirm that the degradation of the passivation quality outside the process window is due to a phase change into microcrystalline silicon with different growth mechanisms and an increase in bonding related defects. Film growth mechanisms are proposed to account for the observed narrow process window, which are verified by opti...

Excellent Silicon Surface Passivation Achieved by Industrial Inductively Coupled Plasma Deposited Hydrogenated Intrinsic Amorphous Silicon Suboxide

We present an alternative method of depositing a high-quality passivation film for heterojunction silicon wafer solar cells, in this paper. The deposition of hydrogenated intrinsic amorphous silicon suboxide is accomplished by decomposing hydrogen, silane, and carbon dioxide in an industrial remote inductively coupled plasma platform. Through the investigation on CO2 partial pressure and process temperature, excellent surface passivation quality and optical properties are achieved. It is found that the hydrogen content in the film is much higher than what is commonly reported in intrinsic amorphous silicon due to oxygen incorporation. The observed slow depletion of hydrogen with increasing temperature greatly enhances its process window as well. The effective lifetime of symmetrically passivated samples under the optimal condition exceeds 4.7 ms on planar -type Czochralski silicon wafers with a resistivity of 1 Ωcm, which is equivalent to an effective surface recombination velocity of less than 1.7 cms−1 and an implied open-circuit voltage () of 741 mV. A comparison with several high quality passivation schemes for solar cells reveals that the developed inductively coupled plasma deposited films show excellent passivation quality. The excellent optical property and resistance to degradation make it an excellent substitute for industrial heterojunction silicon solar cell production.

Monte Carlo modeling of the dc saddle field plasma: Discharge characteristics of N2 and SiH4

Enhancements in rates of ionization, dissociation, and current in the dc saddle field (DCSF) glow discharge are studied using the direct Monte Carlo method (DMCM). The DCSF consists of a planar semitransparent anode positioned between two planar cathodes, producing a symmetric electric field that serves to lengthen the path of electrons. Simulations of current versus anode transparency of a N2 discharge agree very well with experimental results reported previously. Numerical results of the DCSF SiH4 discharge are also presented. At typical operating conditions (600V, 5cm cathode-anode spacing, anode transparency of 0.8), DMCM predicts a significant increase in the dissociation rate at pressures below 100mTorr in comparison to the dc diode. In consideration of its use for plasma enhanced chemical vapor deposition, the efficiency with which the DCSF operates at low pressures makes it attractive for the production of thin films whose qualities are sensitive to gas phase reactions.

Quantifying Edge and Peripheral Recombination Losses in Industrial Silicon Solar Cells

A finite-element model is constructed to represent a silicon solar cell as a vast network of diodes with different saturation current densities, with focus on the definition of three recombination parameters to describe the vicinity of the wafer edges. By simulating the voltage distribution across the cell plane, as well as the cell current-voltage characteristics at different illumination intensities, these peripheral and edge recombination parameters are extracted for various cell types and processes by comparison with corresponding measurement data. It is noted that the monocrystalline silicon PERC cells studied have significantly lower peripheral and second diode edge recombination compared with Al-BSF cells. For the Al-BSF cells studied, there can be ~0.25%-0.6% absolute efficiency gain if the peripheral and edge recombination sources are eliminated.

The augmented saddle field discharge characteristics and its applications for plasma enhanced chemical vapour deposition

A high ion flux parallel electrode plasma is proposed and studied in its DC configuration. By cascading a diode source region which supplies electrons and a saddle field region where these seed electrons are energized and amplified, the energy of ion bombardment on the substrate can be decoupled from the plasma density. The sufficiently large density of electrons and holes in the vicinity of the substrate raises the possibility to perform plasma enhanced chemical vapour deposition on insulating materials, at low sheath voltages (around 40 V in the configuration studied), at low temperatures in which the surface mobility of film growth species may be provided by the bombardment of moderate energy ions. As a benchmarking exercise, experiments are carried out on silane discharge characteristics and deposition of hydrogenated amorphous silicon (a-Si:H) on both silicon wafer and glass. The films grown at low anode voltages have excellent microstructures with predominantly monohydride bonds, sharp band tails, b...

Differential electroluminescence imaging and the current transport efficiency of silicon wafer solar cells

The current transport efficiency of a solar cell, defined as the fraction of local light-induced current that leads to terminal current increase, is experimentally determined for industrially made crystalline silicon wafer solar cells. Fast mapping is possible by simple differential electroluminescence imaging and the application of a reciprocity relationship. The sensitivity of the current transport efficiency to different components of series resistance is studied through computer finite element analysis. A measurement routine that can quantify the current division to the different metal busbars on the solar cell is proposed to qualify screen printed metal fingers.

Biomedical Applications of Semiconductor Quantum Dots

In recent decades‚ the exquisite sensitivity and versatility of optical technologies have led to numerous breakthroughs in biological research‚ including real-time imaging of live cells‚ gene expression profiling‚ cell sorting‚ and clinical diagnostics. A key component in optical detection schemes is the probe design. These probes are constructed from organic fluorophores‚ such as fluorescein and tetramethylrhodamine (TMR)‚ and recognition molecules. The optical emission of fluorophores is used to visualize the activities of biomolecules‚ while the recognition molecules direct the fluorophores to specific cells‚ tissues‚ or organs. Although optical probes are widely used‚ most organic fluorophores exhibit unfavourable properties that have hampered their applications in single-protein tracking in living cells‚ molecular pathology‚ and other research areas. These properties include photobleaching‚ sensitivity to environmental conditions‚ and inability to excite multiple fluorophores using a single wavelength. A new generation of probes has emerged in the last five years that overcomes many of the limitations associated with organic fluorophores. These probes employ fluorophores that are sub-100 nm in size and composed of inorganic atoms. Unlike organic-only fluorophores‚ the optical and electronic properties of inorganic fluorophores can be tuned during the synthesis process by changing their size‚ shape‚ or composition. In this chapter‚ we will describe the use of one type of inorganic fluorophore‚ semiconductor nanocrystals‚ for the development of “custom-designed” probes for biomedical detection. Semiconductor nanocrystals‚ also known as “quantum dots” (qdots)‚ are typically composed of atoms from groups II-VI (CdSe‚ CdS‚ ZnSe) and III-V (InP and InAs)‚ and are defined as particles with physical dimensions smaller than the Bohr exciton radius. The Bohr exciton radius of prototypical CdSe qdots‚ as illustrated in Fig. 1‚ is ~10 nm. The unique optical and electronic properties of qdots have spurred a great deal of research into their potential applications in the design of novel biological probes‚ light emitting diodes‚ photovoltaic cells‚ among other devices.