Tatsuro Watahiki

High Efficiency Hydrogenated Nanocrystalline Cubic Silicon Carbide/Crystalline Silicon Heterojunction Solar Cells Using an Optimized Buffer Layer

Heterojunction crystalline silicon solar cells using a nanocrystalline cubic silicon carbide (nc-3C-SiC) emitter were optimized by changing the deposition time of a buffer layer. The implied open circuit voltage (implied-Voc) estimated from quasi-steady state photoconductance measurements strongly depended on the buffer deposition time. The implied-Voc of 0.690 V was achieved with a buffer deposition time of 30 s. The optimized solar cell showed an active area efficiency of 19.1% (Voc=0.680 V, Jsc=36.6 mA/cm2, and FF=0.769). The excellent cell performance is a direct evidence of the potential of the nc-3C-SiC:H emitter.

Epitaxial growth and structure of (La1−xLux)2O3 alloys on Si(111)

LaLuO3 layers are epitaxially grown on Si(111) by molecular beam epitaxy using high temperature effusion sources. Samples are prepared by simultaneous as well as alternating growth of La2O3 and Lu2O3. Grazing incidence x-ray diffraction indicates that the resulting crystal structure of the alloys is cubic. Simultaneous and alternating growth with a monolayer period lead to the same distribution of La and Lu with no preferential ordering. In all cases the lattice mismatch to Si is less than 0.6%. The experimental results are analyzed by studying the energetics of hexagonal, bixbyite, and perovskite (La1−xLux)2O3 crystal structures employing density functional theory.

Characterization of Hydrogen in Epitaxial Silicon Films Grown at Very Low Temperatures

Experimental and theoretical analyses of hydrogen atoms incorporated in epitaxial silicon films grown at very low temperatures were investigated using a high resolution X-ray diffractometer (HRXRD) and an ab-initio total energy calculation. We found that the lattice constant of the epitaxial films was expanded by the H atoms and this lattice expansion occurred only in a direction normal to the surface. We proposed the Si–H–Si configuration as a model to explain the lattice expansion phenomenon. The results of the calculation supported this model and also suggested that the microscopic stress was introduced by the H atom in the configuration. In B-doped epitaxial Si films, the B atoms were 100% neutralized by the H atoms and activated by thermal annealing. We increased the growth temperature to overcome these H related problems and succeeded in controlling the H incorporation. The B-doped Si film with a hole concentration of 1.7×1019 cm-3 was obtained at a growth temperature of 240°C.

Improvement of Rear Surface Passivation Quality in p-Type Silicon Heterojunction Solar Cells Using Boron-Doped Microcrystalline Silicon Oxide

Boron-doped microcrystalline silicon oxide (µc-SiOx:H) films for application as a back surface field (BSF) in p-type silicon heterojunction (SHJ) solar cells have been characterized. We found that the µc-SiOx:H(p) film at the optimized condition shows high conductivity and good passivation effect with a low surface recombination velocity of around 102 cm/s. However, too much oxygen atoms in the films increase the defect and the passivation quality degrades. Thus, the control of oxygen content in the films is very important to obtain a high passivation quality. With applying the µc-SiOx:H(p) as a BSF layer leads to improved p-type SHJ solar cells performance, which shows the enchantment of EQE spectra in long wavelengths between 800 and 1200 nm. The highest efficiency of the p-type SHJ solar cell we obtain is 18.5% (active area = 0.88 cm2) with a Voc = 659 mV, Jsc = 34.7 mA/cm2, and FF= 80.9%.

The impact of Ge codoping on the enhancement of photovoltaic characteristics of B-doped Czochralski grown Si crystal

The effect of Ge codoping on minority carrier lifetime in boron (B)-doped Czochralski-silicon (CZ-Si) crystals was investigated. The minority carrier lifetime increased from 110 to 176 µs as Ge concentration was increased from zero to 1 × 1020 cm−3 in B/Ge codoped CZ-Si crystals. Light-induced degradation (LID) experiments showed that B-doped CZ-Si degrades rapidly, while B/Ge codoped CZ-Si degrades more slowly. Moreover, the flow pattern defect (FPD) density of grown-in micro-defects (GMD) in as-grown B/Ge codoped CZ-Si decreased with increasing Ge concentration. From the infrared (IR) absorption studies, it was observed that the interstitial oxygen (Oi) concentration decreased as Ge concentration increased in the crystal. The suppressed LID effect in the B/Ge codoped CZ-Si appears to be related to the low concentration of B-O associated defects, possibly because Ge doping retards the Oi diffusion in addition to the low Oi concentration present (evidenced from IR studies). The mechanism by which the Ge c...

Development of the Transparent Conductive Oxide Layer for Nanocrystalline Cubic Silicon Carbide/Silicon Heterojunction Solar Cells with Aluminum Oxide Passivation Layers

We developed an In2O3:H/indium–tin oxide (ITO) stack as the front transparent conductive oxide (TCO) layer of nanocrystalline cubic silicon carbide/crystalline silicon heterojunction solar cells with Al2O3 passivation layers. We investigated the solar cell performance and optical and electrical properties of this layer with various annealing temperatures. The solar cells with In2O3:H and In2O3:H/ITO layers show a higher short circuit current density (Jsc) than that with an ITO layer owing to their lower surface reflection and lower free carrier absorption. The solar cell with the In2O3:H/ITO stack shows a higher fill factor (FF) than that with the In2O3:H layer. The solar cell with the In2O3:H/ITO stack shows an aperture area efficiency of 16.8% (Voc = 0.638 V, Jsc = 34.5 mA/cm2, and FF= 0.762). These results indicate that the In2O3:H/ITO stack has good optical and electrical properties after annealing.

Effects of Annealing and Atomic Hydrogen Treatment on Aluminum Oxide Passivation Layers for Crystalline Silicon Solar Cells

of trimethylaluminum (TMA), hydrogen (H2), and carbon dioxide (CO2) was used for the reactant gas. The TMA flow rate was controlled by a vaporizer system equipped with a liquid mass flow controller. The flow rate of TMA used in this study is 1 mg/min, which is equivalent to 0.3 sccm. The CO2/TMA ratio was kept constant at 37. The details of the deposition condition were reported elsewhere. 3) To evaluate the surface passivation quality, identical a-Al1� xOx:H films with thicknesses of 6–28 nm were deposited on both sides of the wafers. The substrates were cleaned by ultrasonic cleaning in acetone and ethanol solvent. A 1-min HF (5%) dip was performed to remove native oxides after ultrasonic cleaning. Between the front and back side deposition, no chemical treatment was carried out. The thicknesses of a-Al1� xOx:H films were determined by spectroscopic ellipsometry using Cauchy model. To investigate the effect of hydrogen in the films, thermal annealing and AHT were carried out. The thermal annealing was carried out at 475 � C in forming gas (3% H2 in N2) for 1 min. The AHT was carried out using a hot-wire technique. 18) The front sides of the samples were exposed to atomic hydrogen produced from H2 gas using heated tungsten wires. The atomic hydrogen density, the pressure, the substrate temperature, the tungsten wire temperature, and the exposure time were 1 � 10 12 cm � 3 , 80 Pa, 185 � C, 1390 � C, and 24 min, respectively. The tungsten wire temperature was measured using a radiation thermometer. The atomic hydrogen density was estimated from the transmittance of the tungsten phosphate glass. 19) The atomic composition and bonding conditions for each state were investigated using Rutherford backscattering (RBS), elastic recoil detection analysis (ERDA), and attenuated total reflection Fourier transform infrared spectroscopy (ATRFTIR) measurement. The effective lifetime (� eff) was measured by microwave photoconductance decay (MW-PCD) and quasi-steady-state

Growth and structural characterization of epitaxial (La1−xLux)2O3 layers grown on Si(111)

The authors study the growth and structure of epitaxial (La1−xLux)2O3 alloy layers on Si(111) in either homogeneous or digital alloy form. Layer-by-layer growth is achieved by thermal evaporation from La and Lu oxides at a substrate temperature of 700 °C. The grown structures have an abrupt Si/oxide interface as observed by grazing incidence x-ray diffraction and transmission electron microscopy. The in-plane lattice parameter of the (La1−xLux)2O3 reaches values within 0.2% of Si. In-plane lattice constants determined by density functional theory are only slightly off Vegard’s law whereas larger deviations are found for the out-of-plane direction. This may explain the different in-plane lattice constants they measure for digitally or randomly grown (La1−xLux)2O3.

New approach to low-temperature Si epitaxy by using hot wire cell method

Abstract Hot wire (HW) cell method was applied to Si epitaxy and high-quality epitaxial Si films were obtained at a pressure of approximately 0.05 Torr. The growth rate increased linearly with increasing SiH 4 flow rate. It was found that Si epitaxy was possible for substrate temperatures ranging from 200 to 400°C. However, there was a critical thickness for epitaxy below 600°C.

Heterojunction p-Cu2O/n-Ga2O3 diode with high breakdown voltage

Heterojunction p-Cu2O/n-β-Ga2O3 diodes were fabricated on an epitaxially grown β-Ga2O3(001) layer. The reverse breakdown voltage of these p-n diodes reached 1.49 kV with a specific on-resistance of 8.2 mΩ cm2. The leakage current of the p-n diodes was lower than that of the Schottky barrier diode due to the higher barrier height against the electron. The ideality factor of the p-n diode was 1.31. It indicated that some portion of the recombination current at the interface contributed to the forward current, but the diffusion current was the dominant. The forward current more than 100 A/cm2 indicated the lower conduction band offset at the hetero-interface between Cu2O and Ga2O3 layers than that predicted from the bulk properties, resulting in such a high forward current without limitation. These results open the possibility of advanced device structures for wide bandgap Ga2O3 to achieve higher breakdown voltage and lower on-resistance.