Plasma-enhanced Chemical Vapor Deposition System Research Articles

Reduction of leakage current by O2 plasma treatment for device isolation of AlGaN/GaN heterojunction field-effect transistors

The influence of O2 plasma treatment on the mesa-isolated region of AlGaN/GaN heterojunction field-effect transistors (HFETs) was studied. The etched surface of the undoped GaN layer was exposed to O2 plasma generated by a plasma-enhanced chemical vapor deposition system. The current–voltage characteristics indicated that the current of the mesa-isolated region was strongly dependent on the treatment temperature. Treatment with O2 plasma at 300°C and 250W for 15min was confirmed to be the optimal condition, under which isolation current was reduced by four orders of magnitude and photovoltaic response was suppressed. The photoluminescence spectrum showed a decrease in the density of defects related to the yellow luminescence band and the occurrence of defects related to the blue luminescence band. X-ray photoelectron spectroscopy results showed the formation of Ga2O3 and a possible defect of substitutional oxygen on the nitrogen site ON. AlGaN/GaN HFETs with an on/off drain current ratio of 1.73×107 were obtained, and the breakdown voltage of the mesa-isolated region increased from 171.5 to 467.2V.

Practical silicon deposition rules derived from silane monitoring during plasma-enhanced chemical vapor deposition

We clarify the difference between the SiH4 consumption efficiency η and the SiH4 depletion fraction D, as measured in the pumping line and the actual reactor of an industrial plasma-enhanced chemical vapor deposition system. In the absence of significant polysilane and powder formation, η is proportional to the film growth rate. Above a certain powder formation threshold, any additional amount of SiH4 consumed translates into increased powder formation rather than into a faster growing Si film. In order to discuss a zero-dimensional analytical model and a two-dimensional numerical model, we measure η as a function of the radio frequency (RF) power density coupled into the plasma, the total gas flow rate, the input SiH4 concentration, and the reactor pressure. The adjunction of a small trimethylboron flow rate increases η and reduces the formation of powder, while the adjunction of a small disilane flow rate decreases η and favors the formation of powder. Unlike η, D is a location-dependent quantity. It is related to the SiH4 concentration in the plasma cp, and to the phase of the growing Si film, whether the substrate is glass or a c-Si wafer. In order to investigate transient effects due to the RF matching, the precoating of reactor walls, or the introduction of a purifier in the gas line, we measure the gas residence time and acquire time-resolved SiH4 density measurements throughout the ignition and the termination of a plasma.

Characteristics on Silicon Oxynitride Stack Layer of ALD-Al2O2 Passivation Layer for c-Si Solar Cell

Silicon oxynitride that can be deposited two times faster than general SiNx:H layer was applied to fabricate the passivation protection layer of atomic layer deposition (ALD) Al2O3. The protection layer is deposited by plasma-enhanced chemical vapor deposition to protect Al2O3 passivation layer from a high temperature metallization process for contact firing in screen-printed silicon solar cell. In this study, we studied passivation performance of ALD Al2O3 film as functions of process temperature and RF plasma effect in plasma-enhanced chemical vapor deposition system. Al2O3/SiON stacks coated at 400 oC showed higher lifetime values in the as-stacked state. In contrast, a high quality Al2O3/SiON stack was obtained with a plasma power of 400 W and a capping-deposition temperature of 200 oC after the firing process. The best lifetime was achieved with stack films fired at 850 oC. These results demonstrated the potential of the Al2O3/SiON passivated layer for crystalline silicon solar cells.

Defining a parameter of plasma-enhanced CVD to characterize the effect of silicon-surface passivation in heterojunction solar cells

The hydrogen content (CH) and microstructure of a hydrogenated amorphous-silicon (a-Si:H) layer fabricated by plasma-enhanced chemical vapor deposition (PECVD) were analyzed to determine the effect of surface passivation on crystalline silicon (c-Si). The ratio of radio-frequency power to gas pressure (Cpp in W·Pa−1) of the PECVD system is defined as a characterization parameter. It was found that CH and the passivation of a-Si:H layers were sensitively affected by Cpp. However, CH remained almost constant at the same Cpp even though the PECVD power and pressure were widely varied. We determined that an optimal region exists in the range of 0.75 < Cpp < 1.25, where a high implied open-circuit voltage of 732 mV was obtained from a passivated a-Si:H/c-Si/a-Si:H structure, indicating that Cpp was a useful parameter for characterizing the surface passivation effect of a a-Si:H layer on c-Si.

Investigation of ICPECVD Silicon Nitride Films for HgCdTe Surface Passivation

In this paper, we report results of a study of SiN x thin films for surface passivation of HgCdTe epitaxial layers. The hydrogenated amorphous SiN x films under study were deposited by a SENTECH SI500D inductively coupled plasma-enhanced chemical vapor deposition (ICPECVD) system with a high-density and low-ion-energy plasma source at relatively low substrate temperatures (80°C to 100°C). A series of SiN x films were first deposited on CdTe/GaAs and Si substrates under different deposition conditions to examine the influence of ICP power, deposition temperature, and NH3/SiH4 ratio on properties of the SiN x films. To investigate SiN x deposition conditions suitable for surface passivation of HgCdTe, the SiN x /n-Hg0.68Cd0.32Te interface characteristics were investigated employing capacitance–voltage measurements, and the corresponding interface trap densities D it were extracted from the high-frequency and low-frequency characteristics. Analysis of SiN x /n-Hg0.68Cd0.32Te metal–insulator–semiconductor (MIS) structures indicated that Si-rich SiN x films deposited at 100°C by ICPECVD exhibit electrical characteristics suitable for surface passivation of HgCdTe-based devices, that is, interface trap densities in the range of mid-1010 cm−2 eV−1 and fixed negative interface charge densities of ∼1011 cm−2. In addition, the relationship between bond concentration and surface passivation performance has been explored based on infrared (IR) absorbance spectra. The Si–H and N–H bond concentrations were found to be directly correlated with passivation performance, such that SiN x films with a combination of high [Si–H] and low [N–H] bond concentrations were found to be suitable as electrical passivation layers on HgCdTe.

Investigation on the passivated Si/Al2O3 interface fabricated by non-vacuum spatial atomic layer deposition system.

Currently, aluminum oxide stacked with silicon nitride (Al2O3/SiNx:H) is a promising rear passivation material for high-efficiency P-type passivated emitter and rear cell (PERC). It has been indicated that atomic layer deposition system (ALD) is much more suitable to prepare high-quality Al2O3 films than plasma-enhanced chemical vapor deposition system and other process techniques. In this study, an ultrafast, non-vacuum spatial ALD with the deposition rate of around 10 nm/min, developed by our group, is hired to deposit Al2O3 films. Upon post-annealing for the Al2O3 films, the unwanted delamination, regarded as blisters, was found by an optical microscope. This may lead to a worse contact within the Si/Al2O3 interface, deteriorating the passivation quality. Thin stoichiometric silicon dioxide films prepared on the Si surface prior to Al2O3 fabrication effectively reduce a considerable amount of blisters. The residual blisters can be further out-gassed when the Al2O3 films are thinned to 8 nm and annealed above 650°C. Eventually, the entire PERC with the improved triple-layer SiO2/Al2O3/SiNx:H stacked passivation film has an obvious gain in open-circuit voltage (Voc) and short-circuit current (Jsc) because of the increased minority carrier lifetime and internal rear-side reflectance, respectively. The electrical performance of the optimized PERC with the Voc of 0.647 V, Jsc of 38.2 mA/cm2, fill factor of 0.776, and the efficiency of 19.18% can be achieved.

Improved performance of AlGaN/GaN HEMT by N2O plasma pre-treatment**Project supported by the National Natural Science Foundation of China (Grant No. 61334002) and the Opening Project of Science and Technology on Reliability Physics and Application Technology of Electronic Component Laboratory, China (Grant No. ZHD201206).

The influence of an N2O plasma pre-treatment technique on characteristics of AlGaN/GaN high electron mobility transistor (HEMT) prepared by using a plasma-enhanced chemical vapor deposition (PECVD) system is presented. After the plasma treatment, the peak transconductance (gm) increases from 209 mS/mm to 293 mS/mm. Moreover, it is observed that the reverse gate leakage current is lowered by one order of magnitude and the drain current dispersion is improved in the plasma-treated device. From the analysis of frequency-dependent conductance, it can be seen that the trap state density (DT) and time constant (τT) of the N2O-treated device are smaller than those of a non-treated device. The results indicate that the N2O plasma pre-pretreatment before the gate metal deposition could be a promising approach to enhancing the performance of the device.

Enhanced photovoltaic property by forming p-i-n structures containing Si quantum dots/SiC multilayers.

Si quantum dots (Si QDs)/SiC multilayers were fabricated by annealing hydrogenated amorphous Si/SiC multilayers prepared in a plasma-enhanced chemical vapor deposition system. The thickness of amorphous Si layer was designed to be 4 nm, and the thickness of amorphous SiC layer was kept at 2 nm. Transmission electron microscopy observation revealed the formation of Si QDs after 900°C annealing. The optical properties of the Si QDs/SiC multilayers were studied, and the optical band gap deduced from the optical absorption coefficient result is 1.48 eV. Moreover, the p-i-n structure with n-a-Si/i-(Si QDs/SiC multilayers)/p-Si was fabricated, and the carrier transportation mechanism was investigated. The p-i-n structure was used in a solar cell device. The cell had the open circuit voltage of 532 mV and the power conversion efficiency (PCE) of 6.28%.PACS81.07.Ta; 78.67.Pt; 88.40.jj

Conformal encapsulation of three-dimensional, bioresorbable polymeric scaffolds using plasma-enhanced chemical vapor deposition.

Bioresorbable polymers such as poly(ε-caprolactone) (PCL) have a multitude of potential biomaterial applications such as controlled-release drug delivery and regenerative tissue engineering. For such biological applications, the fabrication of porous three-dimensional bioresorbable materials with tunable surface chemistry is critical to maximize their surface-to-volume ratio, mimic the extracellular matrix, and increase drug-loading capacity. Here, two different fluorocarbon (FC) precursors (octofluoropropane (C3F8) and hexafluoropropylene oxide (HFPO)) were used to deposit FC films on PCL scaffolds using plasma-enhanced chemical vapor deposition (PECVD). These two coating systems were chosen with the intent of modifying the scaffold surfaces to be bio-nonreactive while maintaining desirable bulk properties of the scaffold. X-ray photoelectron spectroscopy showed high-CF2 content films were deposited on both the exterior and interior of PCL scaffolds and that deposition behavior is PECVD system specific. Scanning electron microscopy data confirmed that FC film deposition yielded conformal rather than blanket coatings as the porous scaffold structure was maintained after plasma treatment. Treated scaffolds seeded with human dermal fibroblasts (HDF) demonstrate that the cells do not attach after 72 h and that the scaffolds are noncytotoxic to HDF. This work demonstrates conformal FC coatings can be deposited on 3D polymeric scaffolds using PECVD to fabricate 3D bio-nonreactive materials.

Synthesis and photoluminescence property of silicon carbide nanowires thin film by HF-PECVD system

A sample and scalable synthetic strategy was developed for the fabrication of nanocrystalline SiC (nc-SiC) thin film. Thin sheet of nanocrystalline diamond was deposited on Si substrate by hot filament-assisted plasma-enhanced chemical vapour deposition system (HF-PECVD). Further, the resulting carbon-based sheet was heated at 1200 °C to allow a solid state reaction between C and Si substrate to form the SiC thin films. The synthesized films mainly consist of β-SiC nanowires with diameters of about 50 nm and tens of micrometers long. The nanowires axes lie along the [1 1 1] direction and possess a high density of planar defects. The β-SiC nanowires thin films exhibit the strong photoluminescence (PL) peak at a wavelength of 400 nm, which is significantly shifted to the blue compared with the reported PL results of SiC materials. The blue shift may be ascribed to morphology, quantum size confinement effects of the nanomaterials and abundant structure defects that existed in the nanowires.

Effect of Radio-Frequency and Low-Frequency Bias Voltage on the Formation of Amorphous Carbon Films Deposited by Plasma Enhanced Chemical Vapor Deposition

The effect of radio-frequency (RF) or low-frequency (LF) bias voltage on the formation of amorphous hydrogenated carbon (a-C:H) films was studied on silicon substrates with a low methane (CH4) concentration (2–10 vol.%) in CH4+Ar mixtures. The bias substrate was applied either by RF (13.56 MHz) or by LF (150 kHz) power supply. The highest hardness values (∼18–22 GPa) with lower hydrogen content in the films (∼20 at.%) deposited at 10 vol.% CH4, was achieved by using the RF bias. However, the films deposited using the LF bias, under similar RF plasma generation power and CH4 concentration (50 W and 10 vol.%, respectively), displayed lower hardness (∼6–12 GPa) with high hydrogen content (∼40 at.%). The structures analyzed by Fourier Transform Infrared (FTIR) and Raman scattering measurements provide an indication of trans-polyacetylene structure formation. However, its excessive formation in the films deposited by the LF bias method is consistent with its higher bonded hydrogen concentration and low level of hardness, as compared to the film prepared by the RF bias method. It was found that the effect of RF bias on the film structure and properties is stronger than the effect of the low-frequency (LF) bias under identical radio-frequency (RF) powered electrode and identical PECVD (plasma enhanced chemical vapor deposition) system configuration.

Optical Emission Spectroscopy Measurement of Ar\H&lt;sub&gt;2&lt;/sub&gt;\CH&lt;sub&gt;4&lt;/sub&gt; RF Plasma for Nano-Crystal Diamond Film Deposition

Ar\H2\CH4 gas mixture was utilized to grow nanocrystal diamond films in a RF plasma enhanced CVD system. CH4\ H2 ratios were changed to study the effect of plasma radicals on the deposit, in which optical emission spectroscopy (OES) was applied to analyze the plasma radicals. It was found that Hα, Hβ, Hγ, CH, C2 were the main radicals in the plasma. Among them, the CH intensity of OES was usually quite strong and increased sharply when the ratio of CH4/H2 was greater than 3%. The intensity of C2 was weak and basically unchanged with the addition of methane. This study can provide a new possible technical application for depositing NCD films.

Architecture of superthick diamond-like carbon films with excellent high temperature wear resistance

A plasma-enhanced chemical vapor deposition system was used to deposit super thick diamond-like carbon (DLC)-based films ((Six-DLC/Siy-DLC)n). The aim of this work is to investigate the properties of super thick films to verify that increasing the thickness of DLC films offers the possibility of improving their properties at high temperatures. The investigation revealed that superthick (Six-DLC/Siy-DLC)n film exhibited excellent tribological property up to 500°C. One reason is that a thin layer that consists of nanocrystals SiC is formed on the top of wear track. Another is that the stress mostly concentrates near the top surface.

Structural Properties of P-Type ZnO Thin Film Post-Treated by NH&lt;sub&gt;3&lt;/sub&gt; Plasma Method

In this paper, a simple method is reported to obtain nitrogen-doped p-ZnO film. In this method NH3 plasma, generated in a plasma-enhanced chemical vapor deposition system, was employed to treat ZnO thin film. By hall-effect measurement a p type conductivity was observed for the treated film with the hole density of 3.6×1016. XPS result confirmed nitrogen was incorporated into ZnO film during the treatment process to occupy the oxygen positions.

Improved amorphous/crystalline silicon interface passivation for heterojunction solar cells by low-temperature chemical vapor deposition and post-annealing treatment

In this study, hydrogenated amorphous silicon (a-Si:H) thin films are deposited using a radio-frequency plasma-enhanced chemical vapor deposition (RF-PECVD) system. The Si-H configuration of the a-Si:H/c-Si interface is regulated by optimizing the deposition temperature and post-annealing duration to improve the minority carrier lifetime (τeff) of a commercial Czochralski (Cz) silicon wafer. The mechanism of this improvement involves saturation of the microstructural defects with hydrogen evolved within the a-Si:H films due to the transformation from SiH2 into SiH during the annealing process. The post-annealing temperature is controlled to ∼180 °C so that silicon heterojunction solar cells (SHJ) could be prepared without an additional annealing step. To achieve better performance of the SHJ solar cells, we also optimize the thickness of the a-Si:H passivation layer. Finally, complete SHJ solar cells are fabricated using different temperatures for the a-Si:H film deposition to study the influence of the deposition temperature on the solar cell parameters. For the optimized a-Si:H deposition conditions, an efficiency of 18.41% is achieved on a textured Cz silicon wafer.

Increase in the DSSC efficiency when using metal-coated carbon nanowall counter electrodes

This research was conducted to improve the efficiency of dye-sensitized solar cells (DSSCs) using metal-layer-coated carbon nanowalls (CNWs) as counter electrodes. The CNWs were synthesized on a fluorine-doped tin-oxide (FTO) glass substrate in a microwave plasma-enhanced chemical vapor deposition (PECVD) system using methane (CH4), and the CNWS were sputter-coated with metal films several times by using an RF magnetron sputtering system and four-inch metal targets (Cu, W and Ni, separately). Then, the metal-layer-coated CNWs were used as counter electrodes for manufacturing the DSCCs. The vertical and the surface conditions of the metal-coated CNWs used as the DSCC electrodes were characterized by their electrical variations through field-emission scanning electron microscopy (FE-SEM) and Hall measurements. Their optical characteristics were analyzed using UV-Vis equipment, and the energy conversion efficiencies of the DSSCs manufactured using the metal-layer-coated CNWs as the counter electrodes were measured. The results confirmed that the efficiency improved when the W-coated CNW was used as the counter electrode.

Atmospheric aging and light-induced degradation of amorphous and nanostructured silicon using photoconductivity and electron spin resonance

Previous studies indicate that the dark conductivity in amorphous and microcrystalline silicon may increase or decrease with exposure to deionized water (DIW) or pure oxygen at 80 °C but always decreases with light exposure. While the light-induced effect is linked to paramagnetic dangling bonds (Do), the origin of metastability in microcrystalline silicon remains unclear. In this study, we use steady-state photoconductivity (SSPC), dual-beam photoconductivity (DBP), and electron spin resonance (ESR), to study the behaviors under soaking in DIW and (or) pure oxygen at 80 °C and light-exposure of amorphous (a-Si:H) and nanostructured (nc-Si:H) silicon samples deposited in a capacitively coupled plasma-enhanced chemical vapor deposition system. Powders from thick samples of low and high crystallinity (Xc) peeling off large substrates were collected in quartz tubes for ESR measurements. Dark conductivity decreases upon exposure to pure oxygen at 80 °C for nc-Si:H but remains unchanged for a-Si:H. The ESR signal attributed to Do decreases with soaking in DIW for high and low crystallinity nc-Si:H but the effect is more significant for higher Xc. Changes in SSPC, DBP, and ESR are used to compare the degradation mechanisms because of O2 exposure and light for amorphous and nanostructured silicon.

High performance and high stability mechanisms of microcrystalline silicon-based thin-film solar cells deposited by laser-assisted plasma-enhancement chemical vapor deposition system

The laser-assisted plasma-enhanced chemical vapor deposition (LAPECVD) system was proposed to deposit high performance and high stability Si-based thin-film solar cells. In the LAPECVD system, the CO2 laser and plasma were simultaneously utilized to effectively decompose the SiH4 reaction gas. Consequently, the hydrogen concentration in the i-Si absorption film was reduced with an increase of CO2 laser power. Furthermore, the microcrystalline i-Si film could be formed due to the formation of more Si nucleation seeds. Si-nanoclusters were formed on the microcrystalline i-Si films deposited in the LAPECVD system. The associated carrier mobility was increased with increasing the CO2 laser power. The XRD measurements demonstrated that a gradual transformation from amorphous to crystalline as guiding the assisting laser. According to the FTIR measurement, the estimated hydrogen content reduction ratio of the light-soaked i-Si films decreased from 16.5% to 5% as the assisting laser power increased from 0W to 80W. The corresponding conversion efficiency degradation ratio of 20.20% and 5.74% was obtained, the high performance and high stability of the resulting Si-based p–i–n thin film solar cells were obtained.

Photoacoustic spectroscopy and thermal diffusivity measurement on hydrogenated amorphous carbon thin films deposited by plasma-enhanced chemical vapor deposition

In this work, the photoacoustic spectroscopy and the open photoacoustic cell method are employed in the thermo-optical characterization of hydrogenated amorphous carbon thin films. The samples were fabricated in a modified plasma-enhanced chemical vapor deposition system with varying deposition time from 2 to 30min. The optical absorption spectrum was determined for wavelengths of 300 to 2200nm. The optical transmission spectrum was also recorded from photoacoustic spectroscopy. The energy gap was determined from the optical absorption spectra. The thermal diffusivity was found using open photoacoustic cell technique. The thickness of the films was confirmed to be an important parameter for the optimization of the fabrication process and the improvement of thermo-optical performances of the thin films. Spectroscopic results showed lower absorption and UV transparency for the thinner film. In the infrared region the −OH band decreased with increasing film thickness. On the other hand, the thermal diffusivity measurement presented an increase of this variable with decreasing film thickness.

Substrate temperature control in a system fordeposition of carbon nanostructures at atmospheric pressure

The paper presents a plasma-enhanced chemical vapor deposition system for synthesis of carbon nanostructures on a metal substrate at atmospheric pressure with a system for substrate temperature control. It is realized by using a miniature microwave plasma source with a surface-wave discharge. An important part of the system is the movable target holder which allows adjustment of the distance between the plasma flame and the substrate. The substrate temperature increases during the deposition process due to the heating by the plasma flame. A constant value of the substrate temperature (400 – 700 °C) is essential for the synthesis of various carbon structures. An electronic system with sensors positioned in the holder is designed for measurement and control of the substrate temperature. The substrate temperature is measured in continuous and pulsed modes of the discharge, with the temperature value being controlled by the amplitude, repetition rate and duration of the microwave pulses.