Keith Leong

Photonic crystal intermediate reflectors for micromorph solar cells: a comparative study

Wave-optics analysis is performed to investigate the benefits of utilizing Bragg-reflectors and inverted ZnO opals as intermediate reflectors in micromorph cells. The Bragg-reflector and the inverted ZnO opal intermediate reflector increase the current generated in a 100 nm thick upper a-Si:H cell within a micromorph cell by as much as 20% and 13%, respectively. The current generated in the bottom muc-Si:H cell within the micromorph is also greater when the Bragg-reflector is used as the intermediate reflector. The Bragg-reflector outperforms the ZnO inverted opal because it has a larger stop-gap, is optically thin, and due to greater absorption losses that occur in the opaline intermediate reflectors.

Selectively transparent and conducting photonic crystal rear-contacts for thin-film silicon-based building integrated photovoltaics

Wave-optics analysis is performed to show that selectively transparent and conducting photonic crystals (STCPCs) can be utilized as rear contacts to enhance the performance of building-integrated photovoltaics (BIPV). For instance, the current generated in an a-Si:H cell with an STCPC functioning as its rear contact is comparable to that of a similar cell with an optimized ZnO/Ag rear contact. However, the solar lumens (~3.5 klm/m2) and power (~430W/m2) transmitted through the cell with the STCPC rear contact can potentially provide indoor heating and lighting, respectively. Moreover, experimental results show that STCPC rear contacts could be used to control the color temperature of light transmitted through BIPV panels.

Influence of Hydrogen Dilution on Properties of Silicon Films Prepared by D.C. Saddle-Field Glow-Discharge: Observation of Microcrystallinity

ABSTRACT In the D.C. saddle field glow discharge deposition the transition from amorphous to microcrystalline silicon thin films occurs when the silane concentration in the gas phase drops below 10%. We report here the results of Raman spectroscopy, SEM, TEM, and HRTEM studies of the film morphology. We estimate the average crystallite size to be in the range of 5 to 7 nm and the crystalline volume fraction of 25 to 35%. INTRODUCTION Amorphous Si is widely used for large area photovoltaic and microelectronic applications [1]. The use of microcrystalline Si is expected to improve stability against light-induced degradation and provide more efficient doping over that offered by amorphous silicon. Recently, we reported on the growth of mixed phase amorphous-microcrystalline silicon using the D.C. saddle field glow discharge deposition method [2]. The films were grown using hydrogen dilution of silane during the deposition. We were able to identify the growth conditions and the types of substrates that promote microcrystallinity. In this work we present the structural properties of saddle field glow discharge deposited microcrystalline Si films as a function of hydrogen dilution. The films were studied using Raman spectroscopy, SEM, TEM and high-resolution TEM.

Erbium-Doped Amorphous Carbon-Based Thin Films: A Photonic Material Prepared by Low-Temperature RF-PEMOCVD

The integration of photonic materials into CMOS processing involves the use of new materials. A simple one-step metal-organic radio frequency plasma enhanced chemical vapor deposition system (RF-PEMOCVD) was deployed to grow erbium-doped amorphous carbon thin films (a-C:(Er)) on Si substrates at low temperatures (<200 °C). A partially fluorinated metal-organic compound, tris(6,6,7,7,8,8,8-heptafluoro-2,2-dimethyl-3,5-octanedionate) Erbium(+III) or abbreviated Er(fod)3, was incorporated in situ into a-C based host. Six-fold enhancement of Er room-temperature photoluminescence at 1.54 μm was demonstrated by deuteration of the a-C host. Furthermore, the effect of RF power and substrate temperature on the photoluminescence of a-C:D(Er) films was investigated and analyzed in terms of the film structure. Photoluminescence signal increases with increasing RF power, which is the result of an increase in [O]/[Er] ratio and the respective erbium-oxygen coordination number. Moreover, photoluminescence intensity decreases with increasing substrate temperature, which is attributed to an increased desorption rate or a lower sticking coefficient of the fluorinated fragments during film growth and hence [Er] decreases. In addition, it is observed that Er concentration quenching begins at ~2.2 at% and continues to increase until 5.5 at% in the studied a-C:D(Er) matrix. This technique provides the capability of doping Er in a vertically uniform profile.

Reduction of Photoluminescence Quenching by Deuteration of Ytterbium-Doped Amorphous Carbon-Based Photonic Materials

In situ Yb-doped amorphous carbon thin films were grown on Si substrates at low temperatures (<200 °C) by a simple one-step RF-PEMOCVD system as a potential photonic material for direct integration with Si CMOS back end-of-line processing. Room temperature photoluminescence around 1 µm was observed via direct incorporation of optically active Yb3+ ions from the selected Yb(fod)3 metal-organic compound. The partially fluorinated Yb(fod)3 compound assists the suppression of photoluminescence quenching by substitution of C–H with C–F bonds. A four-fold enhancement of Yb photoluminescence was demonstrated via deuteration of the a-C host. The substrate temperature greatly influences the relative deposition rate of the plasma dissociated metal-organic species, and hence the concentration of the various elements. Yb and F incorporation are promoted at lower substrate temperatures, and suppressed at higher substrate temperatures. O concentration is slightly elevated at higher substrate temperatures. Photoluminescence was limited by the concentration of Yb within the film, the concentration of Yb ions in the +3 state, and the relative amount of quenching due to the various de-excitation pathways associated with the vibrational modes of the host a-C network. The observed wide full-width-at-half-maximum photoluminescence signal is a result of the variety of local bonding environments due to the a-C matrix, and the bonding of the Yb3+ ions to O and/or F ions as observed in the X-ray photoelectron spectroscopy analyses.

Amorphous-crystalline silicon heterojunction solar cells formed by the DC saddle field PECVD system: A deposition parameter optimization

The DC Saddle Field PECVD system was used to deposit hydrogenated amorphous silicon (a-Si:H) layers for high efficiency amorphous-crystalline silicon heterojunction (ACSHJ) solar cells. The plasma controlling parameters; including the chamber pressure, gas phase dopant concentration for the p-type a-Si:H (a-Si:H(p+)) emitter, and substrate temperature were varied. The substrate temperature was found to be a critical parameter for the deposition of intrinsic a-Si:H as epitaxial formation can occur with just a temperature increase of 10°C. The processing capabilities have been developed to construct ACSHJ solar cells with 15.5% conversion efficiency for a 4.2 cm2 area.

PECVD deposition and characterization of silicon oxynitride for optical applications

Silicon oxynitride (SiON) films have been found to possess extremely useful properties for optical applications. In optoelectronics, a major advantage of this material is the ability to tune the refractive index from 1.45 to 2.00, allowing designers the flexibility to custom tailor and optimize the refractive index value in the targeted optical device. In addition, its minimum allowable bending radius is much lower compared to other silica materials. This opens up the possibility of miniaturizing integrated photonic systems. Moreover, silicon oxynitride prepared using Plasma Enhanced Chemical Vapor Deposition (PECVD) can be deposited at high growth rates while exhibiting good homogeneity with wide refractive index tuning range making it a well-suited core layer for planar waveguide technologies and microphotonic devices. In this research work, the deposition process and the properties of SiON are discussed. The obtained refractive index as well as the X-ray photoelectron spectroscopy (XPS) analysis are highlighted. Furthermore, FTIR results as a function of the process parameters are presented and their influence on the film properties is discussed.

Photocarrier Radiometric Lifetime Measurements of Intrinsic Amorphous-Crystalline Silicon Heterostructure

Intrinsic hydrogenated amorphous silicon films were deposited by the DC saddle field system on crystalline silicon wafers. The substrate temperature of the amorphous film, crystalline silicon surface cleaning schemes, and the native oxide etchant were varied. The transport parameters of the amorphous-crystalline silicon heterostructures were evaluated by Photocarrier Radiometric (PCR) lifetime measurements. PCR bulk lifetime estimates were obtained using the quinhydrone in methanol solution to passivate the crystalline silicon surface. We present the effectiveness of the PCR system in evaluating different surface passivation schemes.

Self-irradiation enhanced tritium solubility in hydrogenated amorphous and crystalline silicon

Experimental results on tritium effusion, along with the tritium depth profiles, from hydrogenated amorphous silicon (a-Si:H) and crystalline silicon (c-Si) tritiated in tritium (T2) gas at various temperatures and pressures are presented. The results indicate that tritium incorporation is a function of the material microstructure of the as-grown films, rather than the tritium exposure condition. The highest tritium concentration obtained is for a-Si:H deposited at a substrate temperature of 200°C. The tritium content is about 20 at. % on average with a penetration depth of about 50 nm. In contrast, tritium occluded in the c-Si is about 4 at. % with penetration depth of about 10 nm. The tritium concentration observed in a-Si:H and c-Si is much higher than the reported results for the post-hydrogenation process. β irradiation appears to catalyze the tritiation process and enhance tritium dissolution in the silicon matrix. The combination of tritium decay and β-induced ionizations results in formation of re...

Self-catalyzed Tritium Incorporation in Amorphous and Crystalline

Tritiated amorphous and crystalline silicon is prepared by exposing silicon samples to tritium gas (T 2 ) at various pressures and temperatures. Total tritium content and tritium concentration depth profiles in the tritiated samples are obtained using thermal effusion and Secondary Ion Mass Spectroscopy (SIMS) measurements. The results indicate that tritium incorporation is a function of the material microstructure rather than the tritium exposure condition. The highest tritium concentration attained in the amorphous silicon is about 20 at.% on average with a penetration depth of about 50 nm. In contrast, the tritium occluded in the c-Si is about 4 at.% with a penetration depth of about 10 nm. The tritium concentration observed in a-Si:H and c-Si is higher than reported results from post-hydrogenation experiments. The beta irradiation appears to catalyze the tritiation process and enhance the tritium dissolution in silicon material.