High Frequency Plasma-enhanced Chemical Vapor Research Articles

Current Modulation of a Heterojunction Structure by an Ultra-Thin Graphene Base Electrode.

Graphene has been proposed as the current controlling element of vertical transport in heterojunction transistors, as it could potentially achieve high operation frequencies due to its metallic character and 2D nature. Simulations of graphene acting as a thermionic barrier between the transport of two semiconductor layers have shown cut-off frequencies larger than 1 THz. Furthermore, the use of n-doped amorphous silicon, (n)-a-Si:H, as the semiconductor for this approach could enable flexible electronics with high cutoff frequencies. In this work, we fabricated a vertical structure on a rigid substrate where graphene is embedded between two differently doped (n)-a-Si:H layers deposited by very high frequency (140 MHz) plasma-enhanced chemical vapor deposition. The operation of this heterojunction structure is investigated by the two diode-like interfaces by means of temperature dependent current-voltage characterization, followed by the electrical characterization in a three-terminal configuration. We demonstrate that the vertical current between the (n)-a-Si:H layers is successfully controlled by the ultra-thin graphene base voltage. While current saturation is yet to be achieved, a transconductance of ~230 was obtained, demonstrating a moderate modulation of the collector-emitter current by the ultra-thin graphene base voltage. These results show promising progress towards the application of graphene base heterojunction transistors.

Luminescent Properties of SiC Thin Film Deposited by VHF-PECVD: Effect of Methane Flow Rate

Bulk silicon carbide (SiC) as light emitter is less efficient due to its indirect bandgap. Therefore, nanosized SiC thin film fabrication approach enable emission wavelength shifts due to spatial confinement. The result of luminescent study of SiC thin film deposited via very high frequency plasma-enhanced chemical vapour deposition (VHF-PECVD) are presented. Precursor gasses used were silane and methane. Methane flow rate was varied from 8 sccm to 20 sccm while other parameters were maintained. Raman spectral analysis denotes the quantum confinement effect occurrence in proportion to the methane flow rate increment. The luminescence properties of the deposited SiC thin film ranging from highly green emission (~518 nm) to highly UVB emission (~294 nm) dominant luminescence. Broad blue emission band shifted toward higher wavelength with smaller FWHM as methane flow rate is increased. This results enable the possibility of luminescent SiC thin film applications in photonics and electronic integration as blue light sources.

Spectroscopic ellipsometry investigation of microcrystalline fractions in p-type hydrogenated microcrystalline silicon oxide (p-μc-SiOx:H) ultra-thin films

Spectroscopic ellipsometry (SE) was used to characterize the effect of the CO2/SiH4 ratios on the p-type hydrogenated microcrystalline silicon oxide (p-μc-SiOx:H) thin films. The p-μc-SiOx:H thin films were fabricated by very high frequency plasma-enhanced chemical vapor deposition (VHF-PECVD) method on the soda-lime glass substrates. The CO2/SiH4 ratios were varied from 0.0 to 0.5 with constant H2-dilution during the deposition. The p-μc-SiOx:H samples were analyzed for thickness, microstructure, and optical characteristics by SE. The ellipsometric measurements were performed in the range of 0.75–5.5eV with the incident angle of 50, 60 and 70 degrees. The physical structures were proposed as either the double- and triple-layer models. The optical dispersion was proposed based on Tauc-Lorentz (TL) model in order to extract the information of interest including dielectric dispersion and crystalline fraction. The results from the SE analyses showed that the thin films prepared with pure SiH4 could only be fitted by the triple-layer model, whose effect refractive index was gradually decreased from 3.92 to 2.53 with the increased CO2/SiH4 ratio. With the introduction of the CO2, the obtained films were instead best represented by the double-layer model. The calculated crystalline fractions could be determined from the SE results, which accurately corresponded to those from the Raman spectroscopy and high-resolution transmission electron microscope (HRTEM). In addition, the dielectric dispersion of the thin films was related directly with CO2/SiH4 ratios, but inversely with crystalline fraction with the values of 53 down to 18% with the increased gas mixture ratio.

Towards high frequency heterojunction transistors: Electrical characterization of N-doped amorphous silicon-graphene diodes

N-type doped amorphous hydrogenated silicon (a-Si:H) is deposited on top of graphene (Gr) by means of very high frequency (VHF) and radio frequency plasma-enhanced chemical vapor deposition (PECVD). In order to preserve the structural integrity of the monolayer graphene, a plasma excitation frequency of 140 MHz was successfully applied during the a-Si:H VHF-deposition. Raman spectroscopy results indicate the absence of a defect peak in the graphene spectrum after the VHF-PECVD of (n)-a-Si:H. The diode junction between (n)-a-Si:H and graphene was characterized using temperature dependent current-voltage (IV) and capacitance-voltage measurements, respectively. We demonstrate that the current at the (n)-a-Si:H-graphene interface is dominated by thermionic emission and recombination in the space charge region. The Schottky barrier height (qΦB), derived by temperature dependent IV-characteristics, is about 0.49 eV. The junction properties strongly depend on the applied deposition method of (n)-a-Si:H with a clear advantage of the VHF(140 MHz)-technology. We have demonstrated that (n)-a-Si:H-graphene junctions are a promising technology approach for high frequency heterojunction transistors.

Decoupling high surface recombination velocity and epitaxial growth for silicon passivation layers on crystalline silicon

We have critically evaluated the deposition parameter space of very high frequency plasma-enhanced chemical vapour deposition discharges near the amorphous to crystalline transition for intrinsic a-Si:H passivation layers on Si (1 1 1) wafers. Using a low silane concentration in the SiH4–H2 feedstock gas mixture that created amorphous material just before the transition, we have obtained samples with excellent surface passivation. Also, an a-Si:H matrix was grown with embedded local epitaxial growth of crystalline cones on a Si (1 1 1) substrate, as was revealed with a combined scanning electron and high-resolution transmission electron microscopy study. This local epitaxial growth was introduced by a decrease of the silane concentration in the feedstock gas or an increase in discharge power at low silane concentration. Together with the samples on Si (1 1 1) substrates, layers were co-deposited on Si (1 0 0) substrates. This resulted in void-rich, mono-crystalline epitaxial layers on Si (1 0 0). The epitaxial growth on Si (1 0 0) was compared to the local epitaxial growth on Si (1 1 1). The sparse surface coverage of cones seeded on the Si (1 1 1) substrate is most probably enabled by a combination of nucleation at steps and kinks in the {1 1 1} surface and intense ion bombardment at low silane concentration. The effective carrier lifetime of this sample is low and does not increase upon post-deposition annealing. Thus, sparse local epitaxial growth on Si (1 1 1) is enough to obstruct crystalline silicon surface passivation by amorphous silicon.

Characteristics of silicon nitride deposited by VHF (162 MHz)-plasma enhanced chemical vapor deposition using a multi-tile push–pull plasma source

To prevent moisture and oxygen permeation into flexible organic electronic devices formed on substrates, the deposition of an inorganic diffusion barrier material such as SiNx is important for thin film encapsulation. In this study, by a very high frequency (162 MHz) plasma-enhanced chemical vapor deposition (VHF-PECVD) using a multi-tile push–pull plasma source, SiNx layers were deposited with a gas mixture of NH3/SiH4 with/without N2 and the characteristics of the plasma and the deposited SiNx film as the thin film barrier were investigated. Compared to a lower frequency (60 MHz) plasma, the VHF (162 MHz) multi-tile push–pull plasma showed a lower electron temperature, a higher vibrational temperature, and higher N2 dissociation for an N2 plasma. When a SiNx layer was deposited with a mixture of NH3/SiH4 with N2 at a low temperature of 100 °C, a stoichiometric amorphous Si3N4 layer with very low Si–H bonding could be deposited. The 300 nm thick SiNx film exhibited a low water vapor transmission rate of 1.18 × 10−4 g (m2 · d)−1, in addition to an optical transmittance of higher than 90%.

The role of silane gas flow rate on PECVD-assisted fabrication of silicon nanowires

Silicon (Si) core–shell nanowires (NWs) were successfully prepared by very high frequency plasma-enhanced chemical vapor deposition technique, and the effect of silane (SiH4) gas flow rates on physicochemical properties of silicon NWs was investigated. Field emission scanning electron microscopy (FESEM), energy-dispersive X-ray spectroscopy, transmission electron microscopy (TEM), X-ray diffraction (XRD) and Raman spectroscopy were used to characterize SiNWs. Structural properties and morphology of NWs were studied as a function of SiH4 gas flow rate. Microscopic analysis revealed the formation of SiNWs with average tip and stem diameters ranging from 18 to 30 and 21 to 67 nm, respectively. Furthermore, the average length of Si NWs calculated based on the FESEM images was about 300–1800 nm. We have found that the growth of SiNWs increased with increasing in SiH4 gas flow rate. XRD, Raman spectra in addition to high-resolution TEM, verified the formation of crystalline SiNWs. A possible growth mechanism was suggested based on our observations.

IR Spectroscopic Study of Silicon Nitride Films Grown at a Low Substrate Temperature Using Very High Frequency Plasma-Enhanced Chemical Vapor Deposition

Hydrogenated amorphous silicon nitride (a-SiNx:H) films have been grown from a SiH4–N2 gas mixture through very high frequency (VHF) plasma-enhanced chemical vapor deposition (PECVD) at 50℃. The films are dense and transparent in the visible region. The peak frequency of the Si–N stretching mode in the IR absorption spectrum increases with increasing N–H bond density, which is similar to the behavior of a-SiNx:H films grown from SiH4–NH3 gas. During storage in a dry air atmosphere, the Si–O absorption increases. A large shift in the peak frequency of the Si–N stretching mode in the initial stage of oxidation, which is higher than the shift expected from the increase in the N–H bond density, is mainly caused by the change in the sum of electronegativity of nearest neighbors around the Si–N bond due to the increase in the Si–O bond density.

Hydrogen dilution on an undoped silicon oxide layer and Its application to amorphous silicon thin-film solar cells

This paper proposes the use of undoped hydrogenated microcrystalline silicon oxide (μc-SiOx:H) deposited on an n-μc-Si:H layer of amorphous silicon single-junction superstrate-configuration thin-film solar cells produced using 40MHz very high frequency plasma-enhanced chemical vapor deposition. We found that undoped μc-SiOx:H thin film under optimized hydrogen dilution conditions had high crystallinity, high conductivity, a wide optical band gap, and a high refractive index, which are advantageous properties in solar cells. However, deposition at higher hydrogen dilutions degraded the quality and optoelectronic properties of the films, because the morphology of the films changed from microcrystalline to amorphous. These results suggest that the use of an optimized undoped μc-SiOx:H layer improves a-Si:H thin-film solar cell performance through enhancement of the short-circuit current density Jsc. The increased Jsc can be attributed to an improved light-trapping capability in the long wavelength range, between 620 and 680nm, as demonstrated by the external quantum efficiency. This technique also allows optimal conversion efficiency to be achieved. The results demonstrated that hydrogen dilution plays a dominant role in the improvement of film quality and solar cell performance; however, the tradeoff between refractive index and conductivity must be considered.

Extraction of the defect density of states in microcrystalline silicon from experimental results and simulation studies

The constant photocurrent method in the ac-mode (ac-CPM) is used to determine the defect density of states (DOS) in hydrogenated microcrystalline silicon (μc-Si:H) prepared by very high frequency plasma-enhanced chemical vapor deposition (VHF-PECVD). The absorption coefficient spectrum (ac-α(hv)), is measured under ac-CPM conditions at 60 Hz. The measured ac-α(hv) is converted by the CPM spectroscopy into a DOS distribution covering a portion in the lower energy range of occupied states. We have found that the density of valence band-tail states falls exponentially towards the gap with a typical band-tail width of 63 meV. Independently, computer simulations of the ac-CPM are developed using a DOS model that is consistent with the measured ac-α(hv) in the present work and a previously measured transient photocurrent (TPC) for the same material. The DOS distribution model suggested by the measurements in the lower and in the upper part of the energy-gap, as well as by the numerical modelling in the middle part of the energy-gap, coincide reasonably well with the real DOS distribution in hydrogenated microcrystalline silicon because the computed ac-α(hv) is found to agree satisfactorily with the measured ac-α(hv).

Effect of thermal annealing on the blue luminescence of amorphous silicon oxycarbide films

Amorphous silicon oxycarbide (a-SiCxOy) films that displayed blue luminescence were fabricated using very high frequency plasma-enhanced chemical vapor deposition (VHF-PECVD) at a temperature of 250°C. The effects of thermal annealing on the photoluminescence (PL) were investigated. Thermal annealing at 600°C resulted in a remarkable enhancement in the blue PL, which was clearly observed with the naked eyes in a bright room. The blue PL featured an excitation wavelength independent recombination dynamic on a nanosecond timescale. Neither Si nor SiC quantum dots were present in the annealed films as revealed by transmission electron microscopy. The PL results, combined with an analysis of the chemical bonds present in the films, revealed that the origin of the blue PL was largely due to electron–hole pair recombination through Si-related neutral oxygen vacancy defect centers.

High efficiency a-Si:H/a-SiGe:H tandem solar cells fabricated with the combination of V- and U-shaped band gap profiling techniques

Hydrogenated amorphous silicon germanium (a-SiGe:H) films prepared by very high frequency plasma-enhanced chemical vapor deposition (VHF-PECVD) using a mixture of SiH4, H2, and GeH4 were investigated for their use as the bottom cell of amorphous silicon/amorphous silicon germanium (a-Si:H/a-SiGe:H) tandem solar cell structures. Narrow optical band gaps (Eopt) in the range of 1.5 to 1.6 eV were obtained by varying the GeH4/(SiH4 + GeH4) gas flow rate ratio in low-temperature deposition. The a-SiGe:H films deposited with various GeH4/(SiH4 + GeH4) gas flow rate ratios were used as intrinsic layers for the a-Si:H/a-SiGe:H tandem solar cells with different graded band gaps: V-, VU-, and U-shapes. It was found that using the VU-shape improves the solar cell efficiency owing to a higher Jsc when compared with using V-shape. The VU-shape’s Voc and FF are also improved when compared with the U-shape’s Voc and FF. As a result, a high efficiency of 11.0% (Voc = 1.74 V, Jsc = 9.07 mA/cm2, and FF = 0.70) was successfully achieved with the solar cells fabricated using the VU-shape graded band gap technique.

High-efficiency microcrystalline silicon solar cells on honeycomb textured substrates grown with high-rate VHF plasma-enhanced chemical vapor deposition

The potential of high-rate growth of high-quality microcrystalline silicon (µc-Si:H) films for solar cell applications is investigated by very high frequency plasma-enhanced chemical vapor deposition (VHF-PECVD) under a high-pressure SiH4 depletion condition. It is found that the morphology of textured substrates plays an important role in not only light trapping but also µc-Si:H film growth. A high conversion efficiency of 11.1% is attained in a substrate-type µc-Si:H cell on a substrate with honeycomb textures, which has rounded concaves in a honeycomb arrangement with an appropriate period. It is also clarified that ZnO:B films grown by metal organic chemical vapor deposition (MOCVD) are beneficial in terms of carrier collection compared with the standard In2O3:Sn (ITO) film grown by sputtering. On the basis of these findings, a new world-record µc-Si:H cell with a certified conversion efficiency of 11.8% is developed with a relatively high deposition rate of 1 nm/s.

Improved photovoltaic properties of amorphous silicon thin-film solar cells with an un-doped silicon oxide layer

This paper proposes the use of undoped hydrogenated microcrystalline silicon oxide (μc-SiOx:H) deposited on the n-μc-Si:H layer of amorphous silicon single-junction superstrate configuration thin-film solar cells produced through 40MHz very high frequency plasma-enhanced chemical vapor deposition. Raman spectroscopy and optoelectronic analyses of the undoped μc-SiOx:H thin film revealed that adding a small amount of oxygen into a μc-network results in a low optical absorption, wide band gap, high optical band gap E04, high refractive index, reasonable conductivity, and crystalline volume fraction, which are advantageous properties in solar cells. Compared with a standard cell, the current density–voltage (J–V) characteristics of the cell with an undoped μc-SiOx:H/n-μc-Si:H structure showed an enhancement in short-circuit current density Jsc from 13.32 to 13.60mA/cm2, and in conversion efficiency from 8.53% to 8.61%. The increased Jsc mechanism can be attributed to an improved light-trapping capability in the long wavelength range between 510 and 660nm, as demonstrated by the external quantum efficiency.

Optimization of an i-a-SiOx:H absorber layer for thin film silicon solar cell applications

Wide gap, high quality hydrogenated intrinsic amorphous silicon (i-a-Si:H) alloy films are essentially required for multi-junction solar cells fabrication to enhance the open-circuit voltage (Voc) of the top cell. In this paper, intrinsic hydrogenated amorphous silicon oxide films (i-a-SiOx:H) have been prepared using a very high frequency plasma-enhanced chemical vapor deposition technique for use as an absorber layer of the top cell for double-junction amorphous silicon thin film solar cells. We studied the effects of the carbon dioxide (CO2) to silane (SiH4) ratio on the properties of the i-a-SiOx:H films and also optimized these films for application in a-SiOx:H/a-Si:H double-junction solar cells. The Voc of the a-SiOx:H/a-Si:H double-junction solar cells was higher than that of conventional a-Si:H/a-Si:H double-junction solar cells. Using an optimized deposition condition, a-SiOx:H/a-Si:H double-junction solar cell with an initial cell efficiency (η) of 10.2% was produced.

Modeling and experiments of microcrystalline silicon film deposited via VHF-PECVD

A 1-D plasma model coupled with a well-mixed reactor model was used to simulate the growth properties of hydrogenated microcrystalline silicon film deposited by very high frequency plasma-enhanced chemical vapor deposition from SiH4 and H2 gas mixtures. Plasma parameters of the former, such as electron density and electron temperature, were determined and used as input values for the latter, in which concentrations of gas phase species, crystalline orientation, hydrogen content, surface fraction of dangling bonds, and deposition rate were calculated. Simultaneously, a series of in situ optical emission spectroscopy measurements and film depositions were carried out to investigate the correlation between the model and the experiments. Desired agreements between both were achieved.

The study of the substrate temperature depended growth properties of microcrystalline silicon films deposited by VHF-PECVD method

In this paper, we have measured the temperature depended growth properties of hydrogenated microcrystalline silicon (μc-Si:H) films, prepared by very high frequency plasma-enhanced chemical vapor deposition (VHF-PECVD) from SiH4 and H2 gas mixtures. And, a 1D plasma model coupled with a well-mixed reactor model is used to simulate the growth process, in which concentrations of gas phase species, the crystalline orientation, the hydrogen content and the deposition rate are calculated. It suggests that the increasing surface fraction of the dangling bonds with the increase of substrate temperatures is responsible for the increase in the grain sizes. At the same time, the observed variations of the X-ray-diffraction intensities and the deposition rates of the films with temperature result from the differences in the growth rates of the facets.

Design of cooling system for large-area and high-rate deposition of hydrogenated microcrystalline silicon

We have developed a high frequency (40 MHz) plasma-enhanced chemical vapor deposition (PECVD) system for large-area and high-rate deposition of hydrogenated microcrystalline silicon (μc-Si:H). The large-area and high-rate deposition is known to be the key for mass production of silicon thin film solar cells, and the high-power operation made it possible to obtain deposition rates up to 1.8 nm/s. However, it was reported that there were several drawbacks, and temperature was typically concerned by many researchers. The temperature is one of the most important factors in μc-Si:H deposition processes using PECVD for the reason that temperature is in charge of the surface reaction and the growth kinetics. Without cooling, the temperature was observed to increase considerably during deposition of a thick intrinsic layer with thickness of 2 μm. In order to operate PECVD processes with constant temperature, it is required to design an efficient temperature control system. We have developed a cooling system which is designed for a susceptor and a showerhead. Susceptor cooling is performed in a direct cooling mode where a coolant is circulated through a tube inside a susceptor, and appropriate flow rates and temperatures of the coolant were calculated. Showerhead cooling system is designed to make use of an indirect cooling mode where there is efficient thermal radiation heat exchange between a showerhead and chamber wall. The efficiency was substantially improved by anodizing surface of a showerhead. This cooling system allows stable processes of large-area μc-Si:H deposition at high power.

Microcrystalline Silicon Solar Cells Deposited at High Rates in a Single Chamber

Abstract Hydrogenated microcrystalline silicon(μc-Si:H) thin films with deposition rate of over 2 nm/s were prepared by very high frequency plasma-enhanced chemical vapor deposition under high working pressure and high power, using a shower-head cathode. The effects of working pressure and electrode distance on the microcrystalline silicon growth rate and crystallization fraction were studied. We found that thin films with high deposition rate (Rd) and simultaneously proper crystallization fraction could be obtained only when these two parameters have good matches. After that, the intrinsic layer was further optimized by modifying silane concentration. The performance of solar cells was improved by controlling the silane input time which reduced the incubation layer thickness and then improved the quality of the p/i interface. After this initial optimization, a μc-Si:H singlejunction p-i-n solar cell with the conversion efficiency of 5.3% was deposited at the Rd of 2 nm/s in a single chamber.

Effect of Hydrogen Dilution on the Metastability of Hydrogenated Amorphous Silicon Oxide Solar Cells

We investigated the effect of H2 dilution on the stability of hydrogenated amorphous silicon oxide (a-Si1-xOx:H) films and solar cells. a-Si1-xOx:H films were prepared by very high frequency plasma-enhanced chemical vapor deposition (VHF-PECVD) using a gas mixture of silane (SiH4), hydrogen (H2), and carbon dioxide (CO2). The degradation results after light soaking for 310 h under AM1.5, 125 mW/cm2 at 48 °C clearly showed that the film and device stabilities were improved substantially with increasing H2 dilution. Our best stable result with a high open-circuit voltage (Voc) of 1.03 V for a-Si1-xOx:H single-junction solar cells showed a degradation of 16.6%. This result indicates that the stability against the light soaking of the a-Si1-xOx:H material is not inferior to that of a conventional hydrogenated amorphous silicon (a-Si:H) material. The temperature dependences of current–voltage (I–V) characteristics observed before and after light soaking showed that a-Si1-xOx:H solar cells had a similar temperature dependence to conventional a-Si:H solar cells.