Low Optical Absorption Coefficient Research Articles

Influence of Surface Morphology of Textured Substrate against Poly-Si Thin Film Solar Cells Performance

Since poly-Si is an indirect band gap material and has low optical absorption coefficient in the visible-infrared region, the light trapping in thin film poly-Si layer by using textured substrate is one of the important technical issue for achievement of high short current. Surface texture of a transparent conductive oxide (TCO) layer on a glass substrate as well as SnO2 with a large grain are usually utilized for the light-trapping technique, i.e., path lengths of the incident light in the poly-Si layer are effectively enhanced by the light-scattering at the textured surface. In this paper, a systematic investigation has been carried out concerning the relationship between poly-Si thin film solar cells performance and surface morphology of substrate texture as a function of root mean square roughness of substrate surface, in order to find the optimum textured substrate and realize the light trapping in the poly-Si solar cells. Furthermore, the influence of textured substrate on optical reflectance, poly-Si microstructure and photovoltaic performance are also discussed.

Core Structures of Dislocations within CdTe Grains

ABSTRACTCdTe is well known as an excellent photovoltaic material for high efficiency solar cell applications because it has a direct band-gap, low fabrication cost and high optical absorption coefficient. However, the nonradiative recombination and low average minority carrier lifetime caused by the defects in CdTe solar cells limit its efficiency. So far, grain boundaries (GB) have been considered to be the major origin of the nonradiative recombination. However, we show that CdTe grains contain many dislocations that could limit device efficiency. Scanning transmission electron microscopy (STEM) was used to determine the atomic structure of intrinsic and extrinsic stacking faults and their terminating partial dislocation cores. Z-contrast images are sensitive to atomic number and are able to distinguish Cd and Te atomic columns. Unpaired Cd and Te atomic columns were found to form the partial dislocation cores, suggesting the presence of dangling bonds. These defects are likely to be electrically active, and may be the origin of the low minority carrier lifetime.

A photoacoustic imager with light illumination through an infrared-transparent silicon CMUT array

A novel hardware design and preliminary experimental results for photoacoustic imaging are reported in this paper. This imaging system makes use of an infrared-transparent capacitive micromachined ultrasonic transducer (CMUT) chip for ultrasound reception and illuminates the image target through the CMUT array. The cascaded arrangement between the light source and transducer array allows for a more compact imager head and results in more uniform illumination. Taking advantage of the low optical absorption coefficient of silicon in the near infrared spectrum as well as the broad acoustic bandwidth that CMUTs provide, an infrared-transparent CMUT array has been developed for ultrasound reception. The center frequency of the polysilicon-membrane CMUT devices used in this photoacoustic system is 3.5 MHz, with a fractional bandwidth of 118% in reception mode. The silicon substrate of the CMUT array has been thinned to 100 μm and an antireflection dielectric layer is coated on the back side to improve the infrared-transmission rate. Initial results show that the transmission rate of a 1.06-μm Nd:Yag laser through this CMUT chip is 12%. This transmission rate can be improved if the thickness of silicon substrate and the thin-film dielectrics in the CMUT structure are properly tailored. Imaging of a metal wire phantom using this cascaded photoacoustic imager is demonstrated.

Optical, mechanical and etch properties of amorphous carbon nitride films grown by plasma enhanced chemical vapor deposition at room temperature

Amorphous carbon nitride (a-CN) films were grown on Si(1 0 0) and SiO 2/Si(1 0 0) substrates by plasma enhanced chemical vapor deposition at room temperature using gas mixtures of CH 4 and N 2. The as-deposited films showed two bond structures of C N and C N, and with increasing the N 2 content the bond structure changed to graphite-like structure. All the samples showed low optical absorption coefficient ( k < 0.15) in the wavelength range of 300–800 nm. The a-CN films exhibited a good resistance to etching (i.e. higher selectivity over SiO 2), which indicates a potential use of a-CN films as a new hard mask material.

AlF 3 film deposited by IAD with end-Hall ion source using SF 6 as working gas

A novel and effective process to fabricate high quality fluoride thin films was presented. Aluminum fluoride films deposited by a conventional thermal evaporation with an ion-assisted deposition (IAD) using SF 6 as a working gas at around room temperature were investigated. In this study, the optimal voltage and current, 50 V and 0.25 A, were found according to the optical properties of the films: high refractive index (1.489 at 193 nm), low optical absorption and extinction coefficient (<10 −4 at 193 nm) in the UV range. The physical properties of the film are high packing density and amorphous without columnar structure. It was proved that using SF 6 working gas in IAD process is a good choice and significantly improves the quality of AlF 3 films.

Enhanced electro-optic effect in GaInAsP–InP three-step quantum wells

We report on the enhanced electro-optic coefficient of GaInAsP three-step quantum wells (3SQW) for high power electrorefraction modulator applications. Measured electro-optic coefficient of the 3SQW is nearly three times higher than the conventional rectangular quantum well (RQW) at λ=1.55 μm. The enhanced electro-optic effect, combined with a low optical absorption coefficient α&amp;lt;1 cm−1 in the 3SQW increases a modulator figure of merit by nearly 36 times, and decreases the power consumption by nearly one order of magnitude compared with a conventional RQW design.

Synthesis of highly conductive boron-doped p-type hydrogenated microcrystalline silicon (μc-Si:H) by a hot-wire chemical vapor deposition (HWCVD) technique

Boron-doped hydrogenated microcrystalline silicon (μc-Si:H) films were prepared using hot-wire chemical vapor deposition (HWCVD) technique. Structural, electrical and optical properties of these thin films were systematically studied as a function of B 2H 6 gas (diborane) phase ratio (Variation in B 2H 6 gas phase ratio, dopant gas being diluted in hydrogen, affected the film properties through variation in doping level and hydrogen dilution). Characterization of these films from low angle X-ray diffraction and Raman spectroscopy revealed that the high conductive film consists of mixed phase of microcrystalline silicon embedded in an amorphous network. Even a small increase in hydrogen dilution showed marked effect on film microstructure. At the optimized deposition conditions, films with high dark conductivity (0.08 (Ω cm) −1) with low charge carrier activation energy (0.025 eV) and low optical absorption coefficient with high optical band gap (∼2.0 eV) were obtained. At these deposition conditions, however, the growth rate was small (6 Å/s) and hydrogen content was large (9 at%).

The roles of deposition pressure and rf power in opto-electronic properties of a-SiO:H films

Hydrogenated amorphous silicon oxide (a-SiO:H) films have been prepared in a conventional single-chamber radio frequency (13.56 MHz) plasma enhanced chemical vapour deposition system with a carbon dioxide , silane and hydrogen gas mixture. Variation in optical gap from 1.96 to 2.12 eV has been obtained by controlling the deposition pressure and radio frequency (rf) power. The photoconductivities of films in each set are similar to each other for similar optical gaps. High rf power does not have any remarkable deteriorative effect on material but the rate of deposition of film and optical gap are enhanced. At rf power density the and are and 2.11 eV respectively whereas under similar other conditions but with rf power density the corresponding and are and 1.96 eV respectively. By lowering the deposition pressure the optical gap of films has also been enhanced. At 1.5 Torr pressure, the optical gap is 1.96 eV and at 0.13 Torr it becomes 2.12 eV. Here variation of the dilution of hydrogen is adopted in order to look into the role of hydrogen atoms in plasma and characteristics of the deposited films. A model in which incorporation of oxygen is described through plasma kinetics is proposed in this regard. With a higher optical gap the photoconductivity reduces but this reduction is mostly due to there being a lower optical absorption coefficient , partly due to there being a lower value of the product of the mobility , lifetime and quantum efficiency and also partly due to the mid-gap defect density . For the power variation set of samples, with optical gap up to about 1.99 eV, and the photoconductivity remain around and respectively and, with this optical gap, the dark conductivity is . When the material is degraded by soaking in light, a modification of mid-gap states and also of the valence band tail state takes place and falls, together with the photoconductivity. For material of 1.95 eV optical gap the initial and are reduced to and respectively after 3 h of soaking in light.

Influence of substrate in photothermal measurements of thin film absorption

In photothermal measurements of low optical absorption coefficients, as in the case of thin films of amorphous semiconductors, the substrate absorption can play a non-negligible role. In this paper, the substrate influence is discussed and generalized on the basis of a theoretical model of a two-layer absorbing sample, and the interaction of the thermal and optical parameters which control the photothermal process in the sample is shown. Some experimental results obtained on thin films of amorphous silicon carbide deposited on different substrates are presented and discussed.

The optical properties and laser irradiation of some common glasses

The reflectance spectra of four common glasses have been measured from 300 to 2000 cm-1. The glasses examined were Vitreosil, Tempax, Sodalime, and BK7. A Kramers-Kronig analysis of the reflectance data was used to calculate the real and imaginary parts of the refractive index. These were used to predict how the glasses would react to high-power carbon dioxide laser irradiation. The results indicated that the lower optical absorption coefficient and the lower thermal expansion coefficient of the Vitreosil glass should make it less susceptible to laser induced damage.

Ca-SUBSTITUTED POLYCRYSTALLINE GARNETS WITH VERY LOW OPTICAL ABSORPTION

Ca-substituted polycrystalline garnets with low optical absorption coefficient, below 2 cm-1, can be reproducibly prepared by reducing the cause of dispersion and absorption. The lowest optical absorption coefficient which has been measured so far is 0.5 cm-1 at the wavelength of 1.55 μm. Reduction of porosity can be accomplished by simultaneous substitution of Y3+ ions by Ca2+ ions in combination with V5+ ions for charge compensation and by sintering near the temperature at which the anomalous grain growth originates. Also, heat treatment at the temperature in the normal grain growth region reduces optical absorption of CaV-substituted polycrystalline garnets. It was found that the dispersion effect on optical absorption caused by grains is small in comparison with the dispersion caused by pores.

Translucent Y3Al5O12 ceramics

Abstract The sintering of Y 3 Al 5 O 12 to full density and translucency using either SiO 2 or MgO as a dopant is described. The resulting ceramics have a regular microstructures and low optical absorption coefficients.

Pulsed photoacoustic technique to measure the absorption coefficients of the highly transparent optical materials

A new technique of measuring the low optical absorption coefficient is described by using pulsed photoacoustic calorimetry. The frequency dependence of the photoacoustic signal is different owing to its main origin, either the bulk absorption or the surface absorption. The Fourier transform of the pulsed photoacoustic signal, generated by a pulsed CO 2 laser, is obtained as the frequency response of the photoacoustic signal. The ratio of the bulk absorption coefficient and surface absorption coefficient is acquired as 1:1.5.

Measurement Of Very Low Optical Absorption Coefficients In Bulk Materials

The measurement of very low optical absorption coefficients in bulk materials is described with emphasis on adiabatic laser calorimetry with long rod samples. Bulk absorption coefficients lower than 10-5 cm-1 have been achieved in both crystals and glasses using this technique. At extremely low levels, both sensitivity and discrimination between surface and bulk sources of absorption is required. The technique described is perhaps the best way at the present time of studying such low level absorptions.