High-pressure Deposition Research Articles

Parametric optimization of microhardness of diamond-like carbon films prepared by plasma enhanced chemical vapor deposition

Diamond-like amorphous carbon (DLC) thin films may have high hardness and low surface roughness depending on the deposition process parameters. A systematic parametric study of DLC films deposited by plasma enhanced chemical vapor deposition was conducted. Atomic force microscopy was applied to characterize the film microhardness and the surface mean roughness. The complex parametric effects of multiple deposition variables on process responses (microhardness and mean roughness) were examined using a face-centered central composite response surface experimental design. The directions, magnitudes, and significance of various main and interaction effects on the responses were studied. The response contour plots indicated that a combination of relatively high induction power and low deposition pressure at high bias voltage with low Ar dilution produces DLC films with high hardness. The parametric optimization agreed reasonably well with theoretical predictions. The design of experiments is demonstrated to be a powerful tool in exploration of the parameter space of the complex physical process of DLC film deposition.

Importance of Oxides in Carbon/Molecule/Metal Molecular Junctions with Titanium and Copper Top Contacts

Carbon/molecule/metal molecular junctions were fabricated by metal deposition of titanium or copper on monolayers of nitroazobenzene (NAB), biphenyl, and nitrobiphenyl (NBP), and multilayers of NAB and NBP covalently bonded to an carbon substrate. The electronic behavior of Ti junctions was extremely dependent on residual gas pressure during E-beam deposition, due to the formation of a disordered Ti oxyhydroxide deposit. The junction resistance decreased with decreasing residual gas pressure, and the hysteresis and rectification observed previously for relatively high deposition pressure was absent for pressures below . Deletion of the molecular layer resulted in low-resistance junctions for both high and low deposition pressures. Replacement of the Ti with Al with otherwise identical deposition conditions resulted in insulating junctions with much higher resistance and no rectification. Ti junctions made at low residual gas pressure had resistances and current/voltage characteristics similar to those of junctions with Cu top contacts, with the latter exhibiting high yield and good reproducibility. The current/voltage characteristics of both the Ti and Cu junctions fabricated with low residual gas pressure were nonlinear and showed a strong dependence on the molecular layer thickness. The hysteresis and rectification previously observed for junctions fabricated at relatively high residual gas pressure depend on the combination of the NAB layer and the semiconducting film, with the layer conductivity depending strongly on formation conditions. Rectification and hysteresis in NAB/ junctions may result from either redox reactions of the NAB and layers, or from electron injection into the conduction band of Ti oxide.

Design of a gas sensitive transparent heterojunction—The system SrCu 2O 2–ZnO

In this study, transparent oxide p–n heterojunction diodes based on SrCu 2O 2 (SCO) were fabricated by RF magnetron sputtering. A Zn-terminated polar plane (Zn-face), an O-terminated polar plane (O-face), and a non-polar plane (A-face) of highly orientated polycrystalline ZnO plates were used as substrates to clarify the effect of the surface polarity of ZnO upon the p–n heterojunction characteristics. Highly transparent and very electrically conductive SCO films were obtained by the application of low RF power ( Ê < 0.5 W/cm 2) under high deposition pressures (8–10 Pa). Although none of the as-prepared p-SrCu 2O 2/ n-ZnO heterojunctions showed a very good rectifying I– V characteristics, the junction between SCO and the O-terminated ZnO polar surface exhibited clear rectifying I– V characteristics after post-deposition annealing at 923 K in Ar. The origin of the variation in the I– V characteristics depending on the crystal axis orientation of the ZnO substrates is assumed to be due to the surface polarity of the ZnO surface. It would appear that not only the gas-sensing characteristics but also the diode properties depend on the crystal axis orientations of ZnO. A transparent pn junction based on zinc oxide (ZnO) is suggested to have the good gas-sensing properties.

Microcrystalline silicon solar cells fabricated by VHF plasma CVD method

A series of systematic investigations on microcrystalline silicon (μc-Si:H) solar cells at high deposition rates has been studied. The effect of high deposition pressure and narrow cathode-substrate (CS) distance on the deposition rate and quality of microcrystalline silicon is discussed. The microcrystalline silicon solar cell is adopted as middle cell and bottom cell in a three-stacked junction solar cell. The characteristics of large area three-stacked junction solar cells, whose area is 801.6 cm2 including grid electrode areas, are studied in various deposition rates from 1 to 3 nm/s of microcrystalline silicon. An initial efficiency of 13.1% is demonstrated in the three-stacked junction solar cell with microcrystalline silicon deposited at 3 nm/s.

Mechanical Properties of Thin GDP Shells Used as Cryogenic Direct Drive Targets at OMEGA

Thin glow discharge polymer (GDP) shells are currently used as the targets for cryogenic direct drive laser fusion experiments. These shells need to be filled with nearly 1000 atm of D2 and cooled to cryogenic temperatures without failing due to buckling and bursting pressures they experience in this process. Therefore, the mechanical and permeation properties of these shells are of utmost importance in successful and rapid filling with D2. In this paper, we present an overview of buckle and burst pressures of several different types of GDP shells. These include those made using traditional GDP deposition parameters (“standard GDP”) using a high deposition pressure and using modified parameters (“strong GDP”) of low deposition pressure that leads to more robust shells.

Recent developments in the high growth rate technique of device-grade microcrystalline silicon thin film

A novel approach for deposition of device-quality microcrystalline silicon at a high rate using plasma-enhanced chemical vapour deposition has been developed under high-pressure depletion conditions. This method enables us to obtain materials with good crystallinity up to a high deposition rate of 3.8 nm s−1 at around 300°C. Further improvements in the crystallinity as well as electrical properties have been achieved using a mesh electrode with a hollow-like discharge around the mesh. This hollow mesh method results in a low defect density of 2.6 × 1016 cm−3 and a volume fraction over 70% at a growth rate of 5.8 nm s−1. Another beneficial effect of this method was complete suppression of powder formation even at the high deposition pressure. These methods have been applied to solar cells, and an efficiency of 8.1% has been obtained at 1.2 nm s−1.

Microcrystalline materials and cells deposited by RF glow discharge

Abstract Recent developments in depositing high quality intrinsic and doped microcrystalline Si at low temperatures, 100–140°C, and high rates of ∼6 nm/s are reviewed. A new high-pressure depletion deposition method is described that suppresses ion bombardment and yields low defect densities. Passivation of oxygen related donors by hydrogen is discussed as well as the dissociation of passivated B–H dopant atoms by annealing at 200°C. Interface damage effects on superstrate and substrate cells are identified. The influence of the microcrystalline texture formed spontaneously during growth on optimizing optical confinement has been studied.

Silicon–on–insulator waveguide photodetector with Ge/Si self-assembled islands

We have investigated a silicon-based near-infrared photodetector using a waveguide with strong optical confinement. The high-difference index waveguide is obtained with a silicon–on–insulator substrate. The optically active region consists of self-assembled Ge/Si islands embedded in a p-i-n junction. The Ge/Si islands grown by high-pressure chemical-vapor deposition exhibit a broad photoluminescence and electroluminescence which are resonant around 1.5 μm. The photoluminescence and electroluminescence energies are correlated to the island size and to the island composition using a six-band k⋅p calculation. The spectral responsivity of the detectors is measured in a front facet coupling geometry with a broadband source and with semiconductor laser diodes. For a 0 V applied bias, responsivities of 25 and 0.25 mA/W are measured at room temperature at 1.3 and 1.55 μm, respectively.

Low-temperature integration of lead-based ferroelectric capacitors on Si with diffusion barrier layer

Ferroelectric lead zirconate titanate thin films have been integrated on silicon substrates using Ti–Al-based conducting diffusion barriers produced by sputter deposition. The microstructure of the Ti–Al barrier layer was systematically altered through changes in the sputtering conditions, specifically the power density and deposition pressure. We find that the crystallinity of the Ti–Al film strongly correlates with sputtering power density and ambient i.e., it is amorphous at low power density and/or high deposition pressure, and polycrystalline at high power density and/or low deposition pressure. Electron energy loss spectroscopy studies demonstrate that the amorphous Ti–Al (a-Ti–Al) films contain a higher concentration of dissolved oxygen than crystalline Ti–Al. A low temperature sol–gel process has been used to prepare Pb(Zr,Ti)O3 PZT films at 450 °C on conducting Si wafers with a-Ti–Al conducting barrier layer and La–Sr–Co–O top and bottom electrodes. The excellent ferroelectric properties obtained with the a-Ti–Al barrier provide a promising approach for integration of PZT-based capacitors with silicon transistor technology for the fabrication of nonvolatile ferroelectric memories.

Composition, structure and annealing-induced phase separation in SiO x films produced by thermal evaporation of SiO in vacuum

SiO x thin films with various oxygen contents (1.1⩽ x⩽1.7) have been prepared by thermal evaporation of silicon monoxide in vacuum. The film composition is controlled by changing both deposition rate (between 0.2 and 6 nm/s) and depositing the films at two different residual pressures, (2×10 −4 or 1×10 −3 Pa). Long-term stability of all films is ensured by a post-deposition anneal at 520 K for 30 min and this is explained with reference to the annealing-induced decrease of the volume fraction occupied by pores in the film. The oxygen content, x, of each film is determined using both infrared absorption and Rutherford backscattering measurements and the x-values obtained in each case are quite different. The observed difference is related to the low density of the films, particularly those deposited at high residual pressure and high deposition rates, which, in addition to the oxygen deficiency, decreases the frequency of the TO stretching mode. An increase of the oxygen content of the films is observed upon annealing at 970 K accompanied by formation of amorphous silicon particles. Phase separation is completed after annealing at 1300 K and Si nanocrystals embedded in SiO 2 matrix are produced, but the infrared absorption results show that the level of chemical disorder in the matrix is appreciably higher than that in ‘pure’ SiO 2 thin films.

Surface evolution of ultrahigh vacuum magnetron sputter deposited amorphous SiO2 thin films

The surface morphology evolution of amorphous SiO2 thin films deposited by ultrahigh vacuum radio-frequency magnetron sputtering was studied by atomic force microscopy. The results show that: (1) the surface roughness of the deposited films reduces with increased substrate temperatures; (2) the surface roughness increases with higher deposition pressures; and (3) there is a roughening transition at the critical thickness of ∼ 90 nm for the substrate temperature of 713 K. The results also show that the surface roughness at the early stages of growth evolves according to a power law. Further growth beyond the critical thickness leads to a sharp increase in roughness. The experimental results are compared with the previous theoretical and experimental studies on surface evolution during sputter deposition, and discussed in terms of the competition between surface diffusion and shadowing.

Comprehensive study of microcrystalline silicon solar cells deposited at high rate using 13.56 MHz plasma-enhanced chemical vapor deposition

In this article we present a comprehensive study of microcrystalline silicon (μc-Si:H) p-i-n solar cells prepared by using plasma-enhanced chemical vapor deposition (PECVD) at 13.56 MHz excitation frequency. In the first step the cell development was performed in a small area PECVD reactor showing the relationship between the deposition process parameters and the resulting solar cell performance. Subsequent up-scaling to a substrate area of 30×30 cm2 confirmed the scalability of optimized deposition parameters to large area reactors. We investigated the deposition regime of high rf power Prf (0.25–0.7 W/cm2) and high deposition pressure pdep (1–11 Torr) for the μc-Si:H i layer. Furthermore, the influence of silane concentration and deposition temperature was studied. A transition between amorphous and microcrystalline growth could be achieved by a variation of either deposition pressure, plasma power, or silane concentration. The best microcrystalline silicon solar cells were prepared close to the transition to amorphous growth. A high deposition pressure was a prerequisite for obtaining high quality material at a high growth rate. The best solar cell efficiencies achieved so far are 8.1% and 6.6% at i-layer growth rates of 5 and 10 Å/s, respectively, for μc-Si:H single junction cells. Applied in a-Si:H/μc-Si:H tandem cells a stabilized efficiency of 10.0% was achieved.

Deposition of Diamond-Like Carbon Film in a Closed-Space CH4 RF Plasma

Deposition of diamond-like carbon (DLC) film and mass spectrometry measurements were carried out in a closed-space CH4 rf (13.56 MHz) plasma (without both gas injection and vacuum pumping during the process). At pressures less than 0.6 Torr, the thickness of the DLC film deposited increased with increasing elapsed deposition time, and reached a maximum value, but after this the film thickness started to decrease, which was considered to be caused predominantly by ion-induced sputter etching. The maximum film thickness appeared at larger elapsed time for higher deposition pressure. The mass concentrations of hydrocarbon ions indicated anomalous behavior at early deposition times, but those of higher hydrocarbon ions are clearly increased at the point where the film thickness started to decrease. These results suggested that the ratio of precursor CH3al density to the total hydrocarbon ion density (ΦCH3/Φion in the CH4a was an important factor for the carbon film formation, and when this ratio reduced to a certain critical value with increasing elapsed deposition time, the deposited film was then re-etched predominately by the secondary higher hydrocarbon ions. At 1.0 Torr where a polymer-like soft carbon film was deposited, such re-etching of the deposited film was not observed.

Defects in CVD diamond: Raman and XRD studies

Our earlier studies on diamond films grown by the hot filament CVD (HFCVD) technique have clearly shown the presence of the non-diamond phase in case of samples grown at higher deposition pressures. Results of more detailed investigations of samples grown at higher as well as lower deposition pressures by Raman and XRD techniques are reported in this paper. Appreciable broadening of the features in the Raman spectra of films grown at higher deposition pressures is attributed to the presence of the anisotropic stress in the samples. XRD studies have provided information regarding the domain size and the strain. The strain in the case of samples grown at higher deposition pressures is found to be five times higher than in samples grown at lower deposition pressures. The observed strain is correlated with the growth pressure.

Nanopolar reorientation in ferroelectric thin films

The influence of varying oxygen pressure P(O2) during the growth of Ba0.4Sr0.6TiO3 thin films is investigated using dielectric and local optical probes. A transition from in-plane to out-of-plane ferroelectricity is observed with increasing P(O2). Signatures of in-plane and out-of-plane ferroelectricity are identified using dielectric response and time-resolved confocal scanning optical microscopy (TRCSOM). At the crossover pressure between in-plane and out-of-plane polarization (Pc=85 mTorr), TRCSOM measurements reveal a soft, highly dispersive out-of-plane polarization that reorients in plane under modest applied electric fields. At higher deposition pressures, the out-of-plane polarization is hardened and is less dispersive at microwave frequencies, and the dielectric tuning is suppressed. Nanopolar reorientation is believed to be responsible for the marked increase in dielectric tuning at P(O2)=Pc.

Thin film poly-Si formation by Cat-CVD method and its application for solar cells

Poly-Si thin film which has an ESR spin density of 6.9 E16/cm3 was obtained by Cat-CVD with a deposition rate of 5.4 Å/s at a relatively high deposition pressure (approx. 30 Pa) with a high hydrogen dilution (SiH4/H2=0.05). The low ESR spin density films were applied to solar cells as a photo-active layer. Although the dark V–I curves of the cell showed diode characteristics, the photo V–I curves showed very small photo-current densities, lower than 0.5 mA/cm2. A large amount of higher order silane molecules at the present high deposition pressure might be one of the causes of this small photo-current density, as well as grain boundary recombination and an incubation layer at the first stage of film deposition.

Passivation of oxygen-related donors in microcrystalline silicon by low temperature deposition

Low-temperature processing for high-performance solar cells based on hydrogenated microcrystalline silicon (μc-Si:H) has been developed using a conventional rf plasma-enhanced chemical vapor deposition (PECVD) technique at an excitation frequency of 13.56 MHz under a high deposition pressure condition. Among pin type solar cells, it is found that deposition temperature of i-layer at 140 °C is effective particularly for improving open circuit voltage (Voc), surprisingly without deteriorating short circuit current or fill factor. Carrier density of undoped μc-Si abruptly decreases for deposition temperatures lower than 180 °C, and the improvement of Voc is ascribed to a decrease of shunt leakage current arising from the oxygen-related donors. This implies that oxygen-related donors can be passivated at low deposition temperatures and that hydrogen plays an important role for the passivation. We propose a simple model for the hydrogen passivation of oxygen related donors. We apply this passivation technique to solar cells, and consequently a conversion efficiency of 8.9% (Voc=0.51 V, Jsc=25 mA/cm−1, FF=0.70) has been obtained in spite of an oxygen concentration of 2×1019 cm−3 in combination with device optimization such as a p-layer. Effect of deposition temperature of i-layer upon other solar cell parameter, short circuit current, and fill factor is also discussed.

Chemical vapour deposition of κ-Al 2O 3

The influence of experimental variables in combination with catalysis on the growth and microstructure of chemically vapour deposited κ-Al 2O 3 was investigated. The Al 2O 3 coatings were deposited in the temperature range of 800–1000°C and at pressures of 50–400 mbar. Hydrogen sulfide (H 2S) was used as a doping/catalysing agent. General deposition characteristics of κ-Al 2O 3 as a function of temperature, pressure and H 2S concentration are reported. In addition to scanning electron microscopy and X-ray diffraction, the microstructure and chemistry of the κ-Al 2O 3 layers were analysed using transmission electron microscopy. The growth rate of κ-Al 2O 3 could be strongly enhanced by applying H 2S doping and reasonable deposition rates for κ-Al 2O 3 could be obtained at 800°C. Relatively very high growth rates could be obtained by increasing the total pressure simultaneously with H 2S doping. Higher deposition pressure, however, increased the contribution of the homogenous gas phase reaction resulting in unacceptable thickness variations. In general, κ-Al 2O 3 deposited at different process conditions did not exhibit any pronounced microstructural or morphological differences, except at the deposition temperature of 800°C together with at higher doping levels (H 2S>0.8%). Under these experimental conditions γ-Al 2O 3 was obtained. Enrichment of sulfur could be confirmed to occur in γ-Al 2O 3 while no sulfur was found in κ-Al 2O 3. Production-scale aspects concerning deposition of κ-Al 2O 3 and α-Al 2O 3 will be dealt with.

Microcrystalline silicon solar cells prepared by 13.56 MHz PECVD at high growth rates: Solar cell and material properties

ABSTRACTMicrocrystalline silicon (μc-Si:H) solar cells were prepared in a wide range of deposition parameters using 13.56 MHz plasma-enhanced chemical vapour deposition (PECVD). The best μc-Si:H solar cells were prepared close to the transition to amorphous silicon (a-Si:H) growth at very high deposition pressures (∼10 Torr) showing solar cell efficiencies up to 8.0 % at a deposition rate of 5ÊÅ/s. Investigations of the solar cells were performed by Raman spectroscopy and transmission electron microscopy (TEM). TEM measurements revealed similar structural properties with similar high crystalline volume fractions for these cells although they showed distinctly different efficiencies. However, an increased amorphous volume fraction was detected by Raman spectroscopy for the low efficiency cells prepared at low deposition pressures. This result is attributed to an increased ion bombardment at low pressures.

Nanostructured silicon films produced by PECVD

ABSTRACTThis paper presents the process conditions that lead to the production of nanostructured silicon films grown by plasma enhanced chemical vapour deposition close to the so-called gamma regime (powder formation), highly dense and with low density of bulk states. Thus, the powder management is one important issue to be addressed in this paper. As a general rule we observed that high quality films (low density of states and high μτ products) are obtained when films are grown under low ion bombardment at high hydrogen dilution and deposition pressure conditions, to allow the proper surface passivation and surface activation.