High Dark Conductivity Research Articles

The role of hydrogen dilution of silane and phosphorus doping on hydrogenated microcrystalline silicon (μc-Si:H) films prepared by hot-wire chemical vapor deposition (HW-CVD) technique

The electrical, structural and optical properties of undoped and phosphorus doped μc-Si:H films prepared by a HW-CVD technique have been studied. The hydrogen (H2) dilution of silane has been varied carefully to produce undoped μc-Si:H films. The amorphous-to-microcrystalline transition was observed for a hydrogen dilution ratio >0.75. The phosphorus doped μc-Si:H films were deposited by varying the phosphine (PH3) gas flow rate. The structural properties of these films have been investigated by Raman spectroscopy, low angle X-ray diffraction spectroscopy and Fourier transform infrared vibrational spectroscopy. Electrical characterization has been carried out by dark conductivity and charge carrier activation energy measurements. The phosphorus doped μc-Si:H films showed that the addition of PH3 to the source gases promotes the growth of crystallinity. The increase in crystallite size and crystalline volume fraction with the addition of PH3 to the source gases indicates that it enhances the crystallization of the μc-Si:H film. Low angle XRD studies shows that the PH3 doped μc-Si:H does not show any preferential orientation crystallites. For optimized deposition conditions PH3 doped μc-Si:H films with high dark conductivity (0.4 S/cm), low activation energy (0.03 eV) and high band gap (1.82 eV) were obtained with a high deposition rate (13 Å/s). However, for these optimized conditions, the hydrogen content was relatively large (8.3 at.%).

Optoelectronic properties of thin film CdS formed by ultraviolet and infrared pulsed-laser deposition

Thin CdS films on glass are formed by ultraviolet (UV) and infrared (IR) pulsed-laser deposition (PLD) at 355 and 1064 nm with a repetition rate of 10 Hz and a pulse width of 10 ns and 150–180 μs, respectively. The applied laser fluence is kept in the range of 2–4 J cm−2. By UV-PLD, the orientation of the c-axis with respect to the glass surface is adjustable via laser fluence, resulting in perpendicular and parallel oriented films at 2 and 4 J cm−2, respectively. The orientation of IR-PLD samples is maintained perpendicularly, independent of the fluence. The optoelectronic properties of the films are studied by photocurrent (PC) and photoluminescence (PL) spectroscopy at 300 K. The PC of the UV-PLD samples reflects the turn of the c-axis, i.e. the absorption anisotropy, of CdS. The IR-PLD films, however, do not show PC because of high dark conductivity. The UV-PLD samples show PL in the range (2.27–2.45 eV). The emission below 2.45 eV is caused by formation of recombination centers. Notably, the film formed at 3 J cm−2 emits the spectral sum of the films formed at 2 and 4 J cm−2. The IR-PLD samples show green emission (2.493 eV) clearly above the CdS bandgap due to band filling. The results pave the way for the creation of smart photonic gratings, which exhibit locally tunable optoelectronic properties.

P-type Window Layers for pin Solar Cells Entirely Fabricated by Hot-Wire CVD

ABSTRACTWe investigate a-SiC:H p-layer deposition for a-Si:H-based solar cells by Hot-Wire CVD using alternatively methane, ethane, and acetylene. Carbon incorporation in the film results from gas-phase reactions and not from direct dissociation at the hot filament for all hydrocarbon gases. Ethane can be dissociated more easily than methane allowing less extreme deposition conditions. With all types of materials the requirements of high dark conductivity and high band gap for the use as window layers in solar cells can be fulfilled. Highest conductivity is observed with ethane indicating a better network structure, which is supported by the IR signatures. A larger band gap (>2 eV) can be obtained at a similar conductivity with the use of acetylene. We compare these results with the utilization of [.proportional]c-Si:H p-layers. All types of p-layers are incorporated into pin solar cells. Methane- and ethane-based a-SiC:H-p-layers yield similar Voc and FF (∼850 mV and 72%). Acetylene-p-layer-based solar cells yield higher current and higher Voc (890 mV) but lower fill factor (∼67%). Microcrystalline p-layers improve Voc and FF up to 900 mV and 72%, respectively, however higher absorption leads to lower short circuit current and prevents an increase of initial efficiency beyond 8%. Using ethane for p-layer deposition, a significant improvement of the stability of all-Hot-Wire CVD pin solar cells is achieved.

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%).

Optical and Photorefractive Properties of Strongly Reduced BaTiO3

Strong reduction of BaTiO3 induced the appearance of an absorption peak around 600–610 nm and a high dark conductivity, both of which increase with increasing reduction level. The carriers change from holes to electrons after reduction. Because of the influence of the high dark conductivity, beam coupling gain becomes highly dependent on incident light intensity, and response time changes little with intensity. For the sample reduced at 10-15 atm oxygen partial pressure and a temperature of 900°C, the dark conductivity is estimated to be 1.6×10-10 ( Ω·cm)-1, which increases by about four orders of magnitude after reduction.

Photorefractive effect in hydrogen-reduced BaTiO 3

Optical and photorefractive properties of hydrogen-reduced BaTiO 3 are investigated. Hydrogen-reduction induced a broad optical absorption around 620 nm. From two beam coupling, the electrooptic gain is highly dependent on intensity, with electrons being the major carriers. When the annealing temperature increases, the electrooptic gain decreases, though the trap density increases. From light-induced erasure decay measurement, the response time has a little change with intensity at low intensity, though it is much faster than that of the as-grown sample. These properties can be attributed to high dark conductivity of the reduced sample. The dark conductivity increases about three orders after hydrogen-reduction. It is about 6.6 × 10 −11 1/( cm Ω) for the reduced sample, compared with 2.3 × 10 −14 1/( cm Ω) for the as-grown.

Near-Intrinsic Microcrystalline Silicon for Use in Thin Film Transistors

ABSTRACTInverted-staggered thin film transistors (TFTs) incorporating hydrogenated microcrystalline silicon for both contact and channel regions have been fabricated by plasma enhanced chemical vapour deposition (PECVD) using the high hydrogen-dilution method. The deposition parameters for the channel region were chosen to yield near-intrinsic material with a dark conductivity activation energy of 0.7 eV and a Tauc gap of 1.98 eV, while the doped contact layer was optimised to produce a high dark conductivity of 10 S/cm.These devices exhibit a low off-current but the field effect mobility is found to be lower than that of similar devices incorporating an optimised amorphous silicon channel region. The mobility activation energy in these devices is similar to those incorporating an amorphous channel, but the mobility pre-factor is reduced. We propose that this is due to inhomogeneous conduction through a microcrystalline region with a smaller grain size at the dielectric/channel interface.

Photovoltaic effects in porphyrin polymer films and heterojunctions

Electropolymerized porphyrin films on indium–tin–oxide substrates have been characterized using Rutherford backscattering spectrometry, absorption spectroscopy, electrical characterization methods and with step profiling. With these methods the density of the films (ρ=1.35 g/cm3) and the absorption coefficients α(λ) have been determined. For film thicknesses exceeding 40 nm, silver electrical contacts without shunts are achieved by evaporation. The dark conductivity of the films amounts to 10−13–10−12 Ω−1 cm−1. When applying a band model for the conduction in the films, the dark space charge limited current and the exponent in the relation between photoconductivity and illumination intensity (σ∼Iγ, γ=0.65±0.05) indicate an exponential trap distribution in the band gap of the films. From the action spectra, filter effects of the photoconductance and low mobilities are inferred. Spin coating of acceptor layers on top of the polymer films results in the formation of heterojunctions showing photovoltaic behavior, with an open-circuit voltage 0.4–0.6 V. The short-circuit current is controlled by electron transfer at the donor/acceptor interface only and is limited by filter effects in the bulk and by the low conductivity of the materials. The optoelectrical properties of the layers are different if analyzed using a mercury contact (higher dark conductivity, no photoconductivity) which is attributed to the intro- duction of dopants from ambient air in this case.

Amorphous and microcrystalline silicon films deposited by hot-wire chemical vapor deposition at filament temperatures between 1500 and 1900 °C

The optical, electronic and structural properties of thin films deposited by Hot-wire chemical vapor deposition with filament temperatures, Tfil, between 1500 and 1900 °C from silane and hydrogen are studied. The substrate temperature, Tsub, was kept constant at 220 °C. Amorphous silicon films (a-Si:H) are obtained at high filament temperatures, low deposition pressures and low hydrogen-to-silane flow rate ratio (Tfil∼1900 °C, p<30 mTorr and FH2/FSiH4≤1). At these deposition conditions, high growth rates are observed (rd≥10 Ås−1) both with and without hydrogen dilution, and no silicon deposition was observed on the filaments. However, if a lower filament temperature is used (Tfil∼1500 °C) a transition from a-Si:H to microcrystalline silicon (μc-Si:H) occurs as the pressure is decreased from above 0.3 Torr to below 0.1 Torr. The highest dark conductivity and lowest activation energy, of ∼1 Scm−1 and <0.1 eV, respectively, were observed for μc-Si:H deposited at p∼50 mTorr. In this Tfil regime, μc-Si:H growth is achieved without hydrogen dilution, for substrate temperatures as low as ∼150 °C, and for very thin films (∼0.05 μm). Silicon growth on the filaments is observed. For both Tfil regimes, an amorphous to microcrystalline transition is also observed when the hydrogen dilution is increased (FH2/FSiH4≳4). A kinetic growth model is developed, which suggests that the transition from amorphous to microcrystalline can be explained by considering a balance between the concentration of atomic hydrogen and the concentration of the precursor to silicon deposition (SixHz with z≤3x) near the sample. This concentration ratio is shown to be controlled both by the deposition pressure, p, and the filament temperature, Tfil.

Heavily doped CdTe films grown by close-spaced vapor transport technique combined with free evaporation

Very heavily doped n-type polycrystalline CdTe films doped with metallic cadmium were prepared by using close-spaced vapor transport technique combined with free evaporation and the electrical, structural, and morphological properties were investigated. Cadmium was introduced as a dopant by evaporation during films preparation. The highest dark conductivity at room temperature of the films obtained was 1.18×104 S cm−1. The dark conductivity decreased with the increase of the ambient temperature. The highest dark electron concentration obtained was 1.59×1022 cm−3 and increased with the temperature. The mobility decreased with the temperature. The film was a polycrystalline cubic phase. We show the surface topography of the film using scanning electron microscope and scanning tunneling microscopy techniques, in order to see the growth patterns. The crystallite size was very uniform of a few μm, and the growth patterns on the grain surface were in form of terraces.

Effect of Indium Doping on the Properties of Sprayed Cd_{0.8} Zn_{0.2}S Films

Indium doped mixed sulphide films were prepared by the spray pyrolysis technique on glass substrates by spraying aqueous solutions of Cadmium chloride, Zinc Chloride, Indium Trichloride and Thiourea. The Indium doping concentration was varied from 0.025\% to 2.00\% by weight. It is found that the film thickness increases with increase in doping concentration. The electrical and optical properties of the films such as dark conductivity thermoelectric power and optical absorption were studied. The films prepared with 1\% by weight doping concentration show high thermoelectric power and dark conductivity.

Photorefractive gratings in planar optical waveguides in cubic crystals

The space-charge field of the photorefractive grating in planar waveguides in cubic crystals is considered. Analytical expressions are derived from three cases: the initial stage of photorefractive grating formation, the steady state, and the photorefractive grating formation in waveguides with a short diffusion length and high dark conductivity and without trap saturation. The orientation dependencies of the waveguiding two-beam coupling factor for the (110), the (111), and the (100) cuts of the substrate are considered. The overlap factors of the cross distributions of the space-charge field and the light field of the waveguide modes are calculated for waveguides with a step-shaped profile of the refractive index.

Microcrystalline silicon carbide — New useful material for improvement of solar cell performance

Abstract A new kind of semiconductor — doped microcrystalline silicon carbide (μc−Si(1−X)CX) — which is useful for improvement of photovoltaic performance has been developed. A series of experimental verifications on some unique properties such as high dark conductivity with good optical transparency in the solar spectrum is introduced. Then, the usefulness of this new functional material is demonstrated as the cases of wide gap window, front interface modifier, BSF (Back Surface Field) and RSOC (Rear Side Ohmic Contact) effects in the fabrication processes of a-Si and poly-Si sosed solar cells. The mechanisms of the performance improvement are studied and dicuussed. It could be expected that there exists a quite wide variety of availability for not only amorphous solar cells but also single crystalline and polycrystalline solar cells and photo imaging sensors.

Conversion of chemically deposited ZnS films to photoconducting ZnO films

The formation of photoconducting ZnO films from chemically deposited ZnS films by air annealing is reported. The ZnS to ZnO conversion was confirmed by x-ray diffraction (XRD) and optical studies of the films. As-deposited films exhibited very little dark conductivity and no photoconductivity. Air annealing of the films at about 400 degrees C for at least 15 min converted them to ZnO films with higher dark conductivity and photoconductivity. Repeated deposition improved the dark conductivity of the as-deposited films and dark conductivity and photoconductivity of the annealed films. The value of the photo- to dark conductivity ratio depends on the annealing time and temperature. These characteristics of ZnO films may be exploited for their potential application in low-cost solar cells and photodetectors.

Hydrogenated microcrystalline silicon films produced at low temperature by the hot wire deposition method

In this letter we report the synthesis of hydrogenated microcrystalline silicon at low temperature and without hydrogen dilution of the silane gas by the hot wire method. These films are characterized by higher dark conductivity and larger band gap compared to hydrogenated amorphous silicon. Microcrystallinity in these films is clearly established from the sharp crystalline TO-like peak in the first-order Raman spectra. The crystallite size and its volume fraction show a critical dependence on the silane flow rate.

Preparation of μc-SiC and its application for light emitting diodes

We have investigated the preparation conditions of n-type μc-SiC films with high dark-conductivity (10 -3-1 S/cm) and wide optical band gap (2.1–2.4 eV) deposited by electron cyclotron resonance (ECR) plasma chemical vapor deposition (CVD) from two kinds of gas sources, SiH 4 + CH 4 + H 2 + PH 3 and SiH 4 + C 2H 2 + H 2 + PH 3. It was found that the deposition of the CH 4-based μc-SiC films required a higher gas pressure (above 2.6 m Torr) than that used for conventional ECR CVD to suppress hydrogen ions impinging on an underlying layer. For the C 2H 2-based μc-SiC films, we found that besides the suppression of the hydrogen ions a large supply of the atomic hydrogen and the reduction of the C 2H 2/SiH 4 were necessary. We have applied n-type μc-SiC to an electron injector of two kinds of Si-based light emitting diodes whose structures were Al/n-type μc-SiC/i-type a-SiC/a-SiN/p-type a-SiC/SnO 2/glass, and indium tin oxide/n-type μc-SiC/porous Si/p-type c-Si/Au. In both diodes, we clearly observed visible light emission.

Undoped and Phosphorus Doped a-Sic:H Films: Investigation of Electrical Properties and Hall Effect

We have deposited by Plasma Enhanced Chemical Vapour Deposition (PECVD), under particular conditions of high H2 dilution and high power density, undoped and phosphorus doped (µc‐SiC:H films. We have obtained n‐doped films with a high band‐gap (E04 up to 2.1 eV) and a high dark conductivity (σ4 up to 10 S cm‐1) which are promising materials for window layers in solar cells. Thermo Electric Power (TEP) measurements allowed to identify the type of majority carriers. The dark conductivity and the Hall mobility have been obtained as a function of temperature in the range 80‐480 K.

Doping of amorphous silicon by potassium ion implantation

Abstract Highly efficient n-type doping of hydrogenated amorphous silicon films has been achieved by ion implantation of potassium. A high dark conductivity, after implantation and annealing at the optimal temperature, could be obtained over the entire range of concentrations investigated (1018−4 × 1020cm−3). Post-hydrogenation with a Kaufmann ion source produced an increase in conductivity by a factor of up to four. The effect is opposite to that found in phosphorus-implanted samples, in which a decrease in dark conductivity after post-hydrogenation was observed. The highest conductivity obtained without microcry-stallization of the film was 1·6 × 10−l Ω−1 cm−1. The conductivities of potassium-doped samples were higher than those of phosphorus-implanted samples by two orders of magnitude at high concentrations and four orders of magnitude at Iow concentrations. Thermal equilibration experiments were carried out on both annealed and post-hydrogenated samples which confirmed that the eauilibration behaviou...

Light-Induced Effects in a-Si:H Films Fabricated by Intense Light-Flash Assisted Plasma CVD, and Its Application to the Stabilisation of a-Si:H Solar Cells

ABSTRACTWe describe a new technique for the deposition of a-Si:H, in which the growing film surface is periodically irradiated with high-intensity xenon light pulses. The films show high stability of the photoconductivity under illumination, but also exhibit a high dark conductivity, so that the photosensitivity is reduced. Infrared and Raman scattering measurements indicate little difference in the Si-H and Si-Si bonding structures compared to reference films; however, there is a large increase in the sub-bandgap absorption in addition to a shift of the Fermi level, implying some subtle modification of the film growth process caused by the irradiation. Results on the preparation and stability testing of solar cells fabricated using this technique are also described.

Highly conductive and wide optical band gap n-type μc-SiC prepared by electron cyclotron resonance plasma-enhanced chemical vapor deposition

We have investigated the gas pressure dependence of electron cyclotron resonance (ECR) plasma-enhanced chemical vapor deposition (PECVD) and prepared n-type μc-SiC:H with wide optical band gap (2.1–2.5 eV) and high dark conductivity (10−3– 1 S/cm). It has been suggested from plasma diagnoses of the ECR plasma that at low gas pressure a strong etching effect of hydrogen radicals and/or ions dominates the film growth process and the hydrogen ions impinging on the growing surface make the formation of μc-SiC:H difficult, and that at high gas pressure, for the formation of μc-SiC:H, there are nonemissive radicals contributing to the surface coverage or a nucleus formation mechanism which has not been taken into consideration in conventional rf-PECVD.