High Dark Conductivity Research Articles

Inert gas dilution: An alternative technique to make layers for a-Si:H solar cells

The optoelectronic properties of p and n layers strongly influence the performance of glass/TCO/p-i-n/metal amorphous silicon solar cells. High dark conductivity and transparency to visible light are normally required to achieve good quality p and n layers. The use of microcrystalline films has recently been promoted since both dark conductivity and sunlight transparency are higher for μc-Si:H than for a-Si:H. A widely used procedure to obtain them consists of using high dilution levels in hydrogen. Degradation of the transparent conducting oxide (TCO) layer results, however, from the use of hydrogen, even at moderate substrate temperatures of about 180°C. Helium gas dilution is presented as an alternative. a-Si:H films have been prepared by r.f.-glow discharge of SiH 4-helium gas mixtures. The dependence of the optical and photoelectronic properties on the process parameters, i.e. r.f.-power density and gas phase composition is studied. The dilution optimization process yields high transparencies and conductivities.

Low-power deposition of fluorinated microcrystalline silicon hydrogen alloy films

Fluorinated microcrystalline silicon hydrogen alloy films have been prepared by the radio frequency glow-discharge decomposition of silicon tetrafluoride/hydrogen mixtures. Thereby, μc-Si:F:H films with high dark conductivity (∼10−3 Ω−1 cm−1), high photoconductivity (∼10−4 Ω−1 cm−1), showing crystalline structure in selected-area transmission electron microscope diffraction patterns as well as sharp infrared absorption and Raman shift spectra have been obtained under conditions of low-power density (∼0.15 W cm−2) and hydrogen dilution (∼20%).

Formation and properties of In-doped high-conductivity CdS film

High dark-conductivity CdS films have been prepared by coevaporation of CdS and In and the physical properties of the films were investigated. The dark conductivity of the films prepared at room ranged from 10−1 to 103 S cm−1. Analyzing the film structure by x-ray analysis, it was found that the In atoms were doped substitutionally into the Cd site of a CdS crystallite at a low concentration In doping stage and then doped interstitially into the CdS crystallite at a high concentration In doping stage. In the carrier density versus the doped In concentration relation, the n-type characteristic was found. This was explained by the two In doping processes described above. Further, the negative temperature dependence of the carrier density was detected in the very high-concentration In-doped samples. We used an explanation similar to the one given by Hung and Gliessman [Phys. Rev. 96, 1226 (1954)], that is, by using a tentative model in which the substitutionally doped low concentration In atoms form a shallow discrete donor level and the interstitially doped high-concentration In atoms form an impurity band in the forbidden band.

Formation and properties of vacuum-evaporated high conductivity n-type CdTe films

High dark conductivity CdTe films have been prepared by co-evaporating CdTe and cadmium. The structural, electrical and optical properties were investigated. The dark conductivity of the film increased monotonically with an increase in the amount of co-evaporated cadmium. The highest dark conductivity of the films obtained in this experiment was 1.4x10 -2Ω -1 cm -1. The film structure was of the zinc blende type with a preferential orientation of the (111) planes parallel to the substrate and fibrous. The crystallinity of the films was similar to that of films without cadmium doping. The dark conductivity vs. the reciprocal temperature characteristics showed regular aspects. High dark conductivity films will be useful for CdTe thin film device applications.

Formation and properties of very high-conductivity CdTe film made by evaporation

Very high-conductivity n-type CdTe films with large crystallite size were prepared by vacuum coevaporation of CdTe and Cd and the electrical properties and the structure were investigated. The highest dark conductivity at room temperature of the film obtained was 6.5×103 S cm−1. The conductivity decreased with the increase of the ambient temperature. The electron concentration increased and the mobility decreased with the temperature. The film was a polycrystalline hexagonal phase and the crystallite size was a few tens of μm.

Enhancement of open circuit voltage in high efficiency amorphous silicon alloy solar cells

We have developed a microcrystalline fluorinated p+ silicon alloy which has high dark conductivity and low optical loss. Incorporation of this material in single and tandem amorphous silicon alloy based solar cells has resulted in increased open circuit voltage and conversion efficiency.

Formation and properties of vacuum‐evaporated high conductivity n‐type CdTe film

AbstractHigh dark conductivity CdTe films have been prepared by a co‐evaporated process of CdTe and Cd. The structural, electrical and optical properties were investigated. Dark conductivity of the film increased monotonically with an increase of the co‐evaporated Cd. The highest dark conductivity of the film obtained in this experiment was 0.54 ohm‐1 cm‐1. The film structure was of the zinc‐blend type with a preferential orientation of the (111) planes parallel to the substrate and fibers. The crystallinity of the dark films was similar to films without Cd doping. This high dark conductivity film will be useful for thin film CdTe solar cells.

Highly Conductive and Wide Band GaP Microcrystalline Silicon Films Prepared by Photochemical Vapor Deposition and Applications to Devices

AbstractDoped hydrogenated microcrystalline silicon (μc-Si:H) and fluorinated hydrogenated microcrystalline (μc-Si:F:H) films were prepared by the mercury photosensitized decomposition of a disilane-hydrogen or a difluorosilane-hydrogen gas mixture, respectively. The maximum dark conductivity and optical band gap of μc-Si:H films were respectively 20 S•cm−1 and ∼2.0 eV for n-type and 1 S•cm−1 and 2.3 eV for p-type. A higher dark conductivity as much as 50 S•cm−1 and a wide gap of 2.0 eV were obtained for n-type μc-Si:F:H. It is most significant that the gaseous ratio of hydrogen to disilane should be enhanced to obtain such a highly conductive and wide gap film. The crystallinity of the photo-deposited μc-Si:H films appeared to be improved in comparison with that of films by the conventional plasma glow discharge technique.

Silicon films deposited from SiCl 4 by an r.f. cold plasma technique: X-ray photoelectron spectroscopy and electrical conductivity studies

Chemical and electrical characteristics of microcrystalline silicon films deposited from SiCl 4 by an r.f. glow discharge were determined using X-ray photoelectron spectroscopy (XPS) and conductivity measurements. The chlorine content in the bulk was found to be about 2–3 at.%, forming partially ionic SiCl bonds as reflected by the XPS Cl2p chemical shift. The apparent shift in the Si 2p line is attributed to displacement of the Fermi level induced by the chlorine, which acts as a p-type dopant. This is confirmed by the electrical conductivity measurements carried out for undoped, B 2H 6- and PH 3-doped films. The rather high dark conductivity σ D ≈ 10 -3 Ω -1 cm -1 of the undoped film seems to be associated with the microcrystalline structure. The degree and extent of silicon oxidation across the surface region were studied in detail for two differently air-exposed samples and are discussed in terms of the microcrystalline structure of the film. Compositional variations observed across the silicon-substrate interface are attributed to reactions with the stainless steel substrate during the initial deposition process.

Conductive polymer: reticulate doping with charge-transfer complex

Additives which can form charge-transfer (CT) complexes enhance the photoconductivity of polymers. In contrast, the dark conductivity of polymers containing this type of additive increases only slightly. Several crystalline CT complexes exhibit high dark conductivity. When introduced into polymer matrix these smuld therefore significantly increase its conductivity provided that the CT complexes are ordered, because their high conductivity is related to the strong interactions between donor and acceptor stacks in the crystal. We describe here a new method of introducing microcrystalline CT complexes into polymers. It consists of growing microcrystals during film casting. The system, poly (bisphenol A carbonate)/(PC) with microcrystals of the CT complex of 7,78,8-tetra-cyanoquinodimethane (TCNQ) with tetrathiotetracene (TTT), reaches the dark conductivity of 3 × 10−2Ω−1 cm−1 at room temperature at 1 wt% of the complex. This conductivity is only slightly temperature dependent.

高周波スパッタ法による水素化非晶質シリコン（ａ‐Ｓｉ：Ｈ）へのＩＩＩ族元素添加効果

Doping effects of group III elements to hydrogenated amorphous silicon (a-Si : H) have been investigated using co-sputtering of poly-crystalline Si and dopant elements. Compared with gaseous doping of boron by glow discharge (GD) method, doping efficiency of Al and Ga by RF sputtering is smaller at lower doping concentrations (less than 0.5%), but at higher concentrations dark conductivity changes drastically more than 10 orders of magnitude, and high dark conductivity (10-1 ohm-1 cm-1) with low activation energy (0.02 eV) can be achieved. Optical band gap decreases with increasing doping concentration.

A ’’photocurrent-inversion effect’’ of sintered AgBr microcrystals at room temperature

The photocurrent of sintered AgBr microcrystals of high dark conductivity has been measured at room temperature by using a polarization technique of mobile silver ions. Under the condition in which mobile silver ions were polarized, first, a negative photocurrent was observed and, consequently, this negative photocurrent was reversed to a positive one; that is, a ’’photocurrent-inversion effect’’ has been found. This negative photocurrent and the inversion effect of the current were explained by the theory of photographic sensitivity and the experimental estimation of the range of photoelectrons (?5×10−4 cm) in this sintered AgBr. It has been concluded that excess silver ions could be regarded not only as being significant for the electron traps in the surface region, but also as being the electron traps whose distribution could be varied by external voltage.

Interface effects and high conductivity in oxides grown from polycrystalline silicon

Several dark-current and photocurrent techniques have been used to determine the nature of high dark conductivity observed in oxides grown from polycrystalline silicon. Photocurrent measurements on polycrystalline Si-SiO2-Al MOS structures give idential interface energy barrier heights to those on single-crystal Si-SiO2-Al MOS structures, and do not show the presence of any measurable oxide charge. Dark-current measurements on polycrystalline Si MOS structures oxidized to varying degrees show an abrupt conductivity decrease when the polycrystalline Si is completely oxidized to the underlying single-crystal Si substrate. It is concluded from these experiments that the high dark conductivity observed is an interface phenomenon that is due to localized field enhancement near the injecting contact. This field enhancement, not being due to positive oxide charge, is speculated to be caused by surface asperities.

Oxide Charge Trapping Induced by Ion Implantation in SiO2

Experimental measurements directed at understanding oxide modifications induced by Na+ and Al ion implantation are described. Both implanted species created high dark conductivity in unannealed specimens. This had a temperature dependence independent of ion type suggesting that the conductivity is related to displacement damape. Internal photoemission studies showed that Na+ implantation had produced a barrier lowering at both interfaces and also permitted sub-threshold photoemission starting with photons of about 1 eV. Thermal annealing returned the oxide to its insulating state and removed the sub-threshold photoemission. Photo-depopulation measurements indicated that the oxide still contained electron traps not present in the unimplanted samples.