SiH4 H2 Research Articles

Advances in fast 4H–SiC crystal growth and defect reduction by high-temperature gas-source method

This paper reviews recent progress in the development of high-speed 4H–SiC bulk crystal growth technology using the high-temperature gas-source method (high-temperature chemical vapor deposition). Vertical-type reactors capable of growing SiC bulk crystals of 50–75 and 100–150 mm in diameter have been developed, as have reactor configurations and growth conditions to achieve a high growth rate and material quality. Using the reactors, high growth rates up to or exceeding 3 mm/h have been achieved by employing an H2–SiH4–C3H8 gas system with high partial pressures of source gases and high growth temperature of 2500–2550 °C. It is also confirmed that the dislocation densities decline significantly in the direction of growth under proper growth conditions. Key factors for enhancing growth rate and improving crystal quality in the growth process are discussed.

Spherically expanding flame in silane–hydrogen–nitrous oxide–argon mixtures

The effect of silane addition on the laminar flame speed (Su0) of flames propagating in hydrogen–nitrous oxide–argon mixtures has been investigated experimentally for the first time using the spherically expanding flame technique in a constant volume combustion chamber. Replacing hydrogen by silane and maintaining the equivalence ratio constant, much higher flame speeds, explosion peak pressures, and pressure rise coefficients were measured. A previously developed detailed reaction model has been updated based on ab initio thermodynamic properties calculations and collision limit violation analysis. The improved reaction model demonstrates encouraging performance in predicting the flame speed, with a mean absolute error below 11%. To explain the effect of silane addition on the flame dynamics, a number of parameters have been calculated including OH and H rate of production, heat release rate per reaction, and sensitivity coefficient on Su0. The dynamics of freely propagating flames in SiH4–H2–N2O–Ar mixtures is essentially controlled by reactions of the H–O–N chemical system: N2O+H=N2+OH, OH+H2=H2O+H, and N2O(+M)=N2+O(+M). Whereas silane addition does not influence much the rate of production of OH, it significantly modifies that of H with a number of pyrolytic chemical pathways of silicon hydrides, such as SiH+H2=SiH2+H and Si+H2=SiH+H, which act as sink of H atom as they proceed in the backward direction. The reactions forming SiO(s) and SiO2(s), such as SiO+OH=SiO2(s)+H and 2SiO=2SiO(s), are exothermic and significantly contribute to the temperature increase. The adiabatic, constant pressure flame temperature for mixture containing silane is significantly higher, up to several 100’s K. The increase of Su0 induced by silane addition seems to be mostly related to the large increase of the flame temperature which leads to higher energy release rate.

Morphology modulation and structural study of indium assisted silicon nanowires by PECVD

We report a detailed investigation on the growth of In catalyzed SiNWs using SiH4–H2 gas mixture by PECVD. The effect of growth-temperature and timeduration on the structure of grown SiNWs is studied. Morphology obtained from FESEM shows tapered growth of NWs with a distinctively sharp tip. The diameter of NWs is measured at two places (near the base and tip of NW) to evaluate the tapering effect, which decreased with an increase in temperature. An increase in temperature also increases the density of NWs and promotes axial growth. HRTEM, GIXRD spectra and Raman spectroscopy show high crystallinity of the grown NWs. GIXRD spectra show the grown NWs have crystallized along (111), (220) and (311) plane orientation, where (111) being preferred orientation as demonstrated by HRTEM. The surface chemistry analysis performed using XPS shows the presence of Si besides the native oxide as the major element with no trace of In.

Electron energy distribution functions relevant for weakly ionized SiH4–H2 plasma

In the existing fluid and global models for SiH4-containing plasmas, the Maxwellian distribution (MD) of the electron energy distribution function (EEDF) is assumed to determine the reaction coefficients and transport coefficients. However, the EEDFs can exhibit bi-Maxwellian or more complex distributions, which causes large deviations in simulation results. In this paper, the Boltzmann analysis of the EEDF for pure SiH4 and SiH4–H2 plasmas is presented. Calculation results suggest the clear impact of electric field on the behavior of the EEDF. In low reduced electric field (E/N = 5 Td), the shape of the calculated EEDF is close to MD. Due to the vibrational excitation of pure SiH4, the EEDF drops sharply within 4 eV at 100 Td. The difference of EEDF between AC and DC fields is analyzed by the diffusion coefficient of the distribution function. In high-frequency field, the EEDF is more prone to MD. In low-frequency field, the relationship between mean electron energy and E/N has no dependence on the field frequency. Unlike some monoatomic molecular plasmas, the influence of electron–electron collisions on the EEDF is prominent only when the ionization degree is higher than 10−3. Besides, in the SiH4–H2 mixture, it is found that increasing the H2 concentration enhances the electron energy tail. Negative differential conductivity of the drift velocity is observed, and it keeps decreasing with the H2 increasing and finally disappears. Remarkable synergism is observed in the SiH4–H2 mixture. The electron transport coefficients of the pure SiH4 and pure H2 are in satisfactory agreement with available measurements.

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.

Arsenic atomic layer doping in Si using AsH3

Atomic layer doping of arsenic (As-ALD) in Si is investigated using a single wafer reduced pressure chemical vapor deposition tool. Hydrogen-free and hydrogen-terminated Si surfaces are exposed to AsH3 in the temperature range between 300°C and 700°C followed by Si capping using H2–SiH4 or H2–Si2H6 gas mixture at 450–700°C. After exposing to AsH3 below 400°C, no As spike formation is observed on both hydrogen-free and hydrogen-terminated Si surface. The incorporated As dose is increasing with increasing AsH3 exposure time and saturates. Before saturation, a higher As dose is observed at higher AsH3 partial pressure for the same AsH3 exposure time. As incorporation is not influenced by the precursor (SiH4 or Si2H6) used for Si buffer deposition. By increasing SiH4 or Si2H6 partial pressure, a higher amount of As is incorporated at AsH3 exposed surface and a higher background doping of segregated As in the Si cap is observed. The slope of the segregated As is not influenced by the growth rate at 600°C, which means that the As segregation seems to be under thermal equilibrium. Lower segregation of As in the Si cap is observed at lower Si cap growth temperature. The results shown here may offer a precise location and dose control of As into Si.

Pressure induced metallization of SiH4(H2)2 via first-principles calculations

Structure of SiH4–H2, which has attracted much theoretical and experimental effort, but there remains many unsolved problems, for example, behavior of H2 under compression, as well as the metallization mechanism induced by pressure. In this work, two competitive structures of SiH4(H2)2 were studied to find the stable structure in the pressure range of 6.8–35GPa. Our results indicate that tI18 structure is the possible structure, which can also be viewed as a distorted face-centered-cubic lattice. The absence of imaginary frequency modes for phonon dispersion curves indicates that tI18 structure is dynamically stable in the pressure range of 6.8–35GPa. The optimized structure is also in good agreement with experimental structure, supported by X-ray diffraction data. Specially, the calculated low-frequency phonon modes and electronic band structure show that this crystal becomes metallic at 190GPa and 0K, and the potential superconductivity in this compound can be attributed to a conventional electron–phonon mechanism that couples the low-frequency phonon mode to the sp-like conduction electrons. The electron density difference is also analyzed to have a further comprehension on pressure-induced metallization mechanism of this compound. Contrasting with the general view in the previous studies that the metallization may arise from the interaction of SiH4 and H2, it is found the interaction between H atoms in SiH4 and H2 molecules is the main factor, that induces the metallization or even superconductivity of SiH4(H2)2 under high pressure.

High‐efficiency thin‐film silicon solar cells with improved light‐soaking stability

ABSTRACTHydrogenated amorphous silicon (a‐Si : H) films are prepared by plasma‐enhanced chemical vapor deposition (PECVD) with a triode electrode configuration in which a SiH4–H2 glow‐discharge plasma is confined spatially away from the substrate. Although the deposition rate (0.1–0.5 Å/s) is lower than that of the conventional diode PECVD process (2.5 Å/s), the light‐induced degradation in conversion efficiency (Δη/ηini) of a single‐junction solar cell is substantially reduced (e.g., Δη/ηini ~ 10% at an absorber thickness of ti = 250 nm), and efficiencies after light soaking (LS) maintain >9% for ti = 180–390 nm. By applying the improved a‐Si : H layers as top cell absorbers in a‐Si : H/hydrogenated microcrystalline silicon (µc‐Si : H) tandem solar cells, the light‐induced degradation can be reduced further (e.g., Δη/ηini ~ 5% at ti = 250 nm). As a result, we obtain confirmed stabilized efficiencies of 9.6% (LS condition: 100 mW/cm2, 50 °C, 1000 h) and 11.9% (LS condition: 125 mW/cm2, 48 °C, 310 h) for a‐Si : H single‐junction and a‐Si : H/µc‐Si : H tandem solar cells, respectively. Copyright © 2012 John Wiley & Sons, Ltd.

Energy distributions of H and H2 in a Ar–SiH4–H2 plasma during transition from a-Si to nc-Si film deposition

The assessment of H and H2 densities by mass spectrometry methods in silane–hydrogen (SiH4–H2) plasma-assisted CVD processes requires estimation of the depletion factor for the parent molecules (SiH4 and H2). Such an estimation is non-trivial as H2 is both depleted and generated in SiH4–H2 plasmas, and the usual depletion factor becomes a ‘depletion-generation’ factor. A method for calculating the depletion-generation factor in SiH4–H2 plasmas is presented and applied to de-convolute energy distributions of H and H2 from energy-resolved residual gas analysis (ERRGA) performed using a HIDEN EQP300 mass-energy analyser. The ERRGA method is described and the energy distribution of the H radicals and H2 molecules is obtained for an LEPECVD plasma during the transition from a-Si to nc-Si deposition at constant discharge operation parameters for gas input flow rates of 50 sccm Ar and 10 sccm SiH4, with the addition of H2 (0–50 sccm) at reactor pressures of 1–3 Pa. The H and H2 energy distributions show two main groups, one below 0.7 eV and another at 1.2–1.5 eV, with a strong variation in total density with increase in H2 input. Above 30 sccm H2 input, a step increase in the lower energy component is observed. Energy distributions are discussed in relation to H and H2 generation reactions and gas heating in PECVD processes.

Hydrogenated amorphous silicon deposited under accurately controlled ion bombardment using pulse-shaped substrate biasing

We have applied pulse-shaped biasing to the expanding thermal plasma deposition of hydrogenated amorphous silicon at substrate temperatures ∼200 °C and growth rates around 1 nm/s. Substrate voltage measurements and measurements with a retarding field energy analyzer demonstrate the achieved control over the ion energy distribution for deposition on conductive substrates and for deposition of conductive materials on nonconductive substrates. Presence of negative ions/particles in the Ar–H2–SiH4 plasma is deduced from a voltage offset during biasing. Densification of the material at low Urbach energies is observed at a deposited energy <4.8 eV/Si atom and attributed to an increase in surface mobility of mobile species as well as well as surface atom displacement. The subsequent increase in Urbach energy >4.8 eV/Si atom is attributed to bulk atom displacement in subsurface layers. We make the unique experimental abservation of a decreasing Tauc band gap at increasing total hydrogen concentration—this allows to directly relate the band gap of amorphous silicon to the presence of nanovoids in the material.

Measurement of rotational temperature using SiH(A2Δ-X2Π) emission spectrum in SiH4–H2 plasmas

Based on the synthetic spectrum method of comparing with the experimental SiH(A2Δ-X2Π) diatomic molecular emission spectrum, the rotational temperatures of SiH4–H2 plasmas were investigated for various operating conditions. The plasma was generated between parallel plate electrodes biased at 40.68 MHz for thin film silicon deposition. Operating conditions of the gas pressure and the input power were varied from 2 to 8 Torr and from 200 to 600 W, respectively. Also, the total gas flow rate and H2/SiH4 gas flow ratio were changed. By increasing the input power, the rotational temperature was increased up to 865 K by more energetic electrons collisions. The magnitude of rotational temperature was reduced by 200 K due to particle cooling effects, with increasing the total flow rates and gas pressure. The experimental results were discussed further using Si/SiH emission intensity ratio to show the characteristics of electron temperature.

Powder formation in SiH4–H2 discharge in large area capacitively coupled reactors: A study of the combined effect of interelectrode distance and pressure

One of the main challenges for silicon thin film deposition for solar cell applications is to achieve high rate deposition in order to reduce the manufacturing costs. However, when silane and hydrogen are used as precursor gas in parallel plate plasma-enhanced chemical vapor deposition, high rate deposition is generally synonymous of powdery discharge. In this work, time- and space-resolved light scattering experiments are presented. These were performed in an industrial-type large area reactor with a variable interelectrode distance. Results show that with a standard 25 mm interelectrode distance, the fraction of silane transformed into powder can be as high as 50% and that reducing the interelectrode distance shifts to higher pressure the appearance of powder in the discharge. From a standard 25 mm interelectrode distance to a 10 mm narrow gap reactor, the threshold pressure was increased from 2 to 7 mbars. More generally, it is proposed that the onset of powder formation depends mainly on the product of the interelectrode distance and the gas residence time in the discharge.

Langmuir probe plasma parameters and kinetic rates in a Ar–SiH4–H2 plasma during nc-Si films deposition for photovoltaic applications

An assessment of main electron-impact and secondary (homogeneous) gas-phase reaction rates of silane in an argon–silane–hydrogen plasma during nano-crystalline silicon deposition is presented.Radially resolved Langmuir probe plasma parameters (electron temperature and density) and electron energy distribution functions (eedfs) have been evaluated for Ar, Ar–H2 and Ar–SiH4–H2 plasma in a low-energy plasma-enhanced chemical vapour deposition reactor. Input flow rates of 50 sccm Ar, 10 sccm SiH4 and 0–50 sccm H2 have been used for a reactor pressure range 1–4 Pa.The eedfs are used to evaluate kinetic rate constants for electron-impact dissociative processes of SiH4 and H2 and to infer the amount of atomic H available for the silane–hydrogen gas-phase reaction, observing trends with an increase in H2 input flow.The evolution of silane kinetic rates with an increase in H2 input indicates that conditions corresponding to nc-Si deposition are characterized by a dominance of silane–hydrogen gas-phase rates over electron-impact dissociation rates up to about two orders of magnitude.

Ammonia-free deposition of silicon nitride films using pulsed-plasma chemical vapor deposition under near atmospheric pressure

Ammonia-free deposition of silicon nitride (SiNX) films have been achieved on Si(100) substrate at low temperature (200°C) by using plasma enhanced chemical vapor deposition operated at near atmospheric pressure. A pulsed power supply enables a stable discharge of a SiH4–H2–N2 system at near atmospheric pressures without using any inert gases such as He. Characterization by x-ray photoelectron spectroscopy and Fourier transform infrared spectroscopy indicates that the grown film is a SiNx film with its N∕Si ratio varying for 0.85–0.90, depending on the N2∕SiH4 flow ratio (1000–4000). Despite the use of N2 instead of NH3, a high rate growth (10–70nm∕min) is enabled, which would be beneficial in forming protection/passivation layers in solar cells. The breakdown field of 7.4MV∕cm seems also promising for its use as a gate insulator in amorphous-silicon-based thin film transistors (TFTs).

Formation of SiH3 Radicals and Nanoparticles in SiH4–H2 Plasmas Observed by Time-Resolved Cavity Ringdown Spectroscopy

The growth of nanoparticles between electrodes in a pulsed very high frequency (VHF) silane–hydrogen (SiH4–H2) plasma under hydrogenated microcrystalline silicon (µc-Si:H) growth conditions has been studied by time-resolved cavity ringdown (τ-CRD) spectroscopy. The light absorption of silyl (SiH3) radicals and the light scattering and absorption of nanoparticles have been measured in the UV spectral range (220 and 280 nm). The contribution of the SiH3 radicals and nanoparticles to the measured cavity loss could be distinguished by 1) varying the wavelength of the probe laser pulse, 2) using time-resolved information of the SiH3 radicals and the nanoparticle density, 3) the measured spatial distribution of the species between the electrodes, and 4) the dependence of these distributions on the electrode temperature. From the time evolution of cavity loss related to the nanoparticles growing in plasma, we can determine whether the light losses of the nanoparticles are in the Rayleigh or Mie regime. Subsequently, these measurements also provide information on typical particle size. Additional scanning electron microscopy (SEM) analyses, which reveal the evolution of the nanoparticle size distribution with time, corroborate the results obtained by τ-CRD. Finally, the spatial distribution of the SiH3 radical density and the electrode temperature dependence of the nanoparticles between the electrodes have been studied. These results are discussed in terms of the dominant forces acting on nanoparticles in plasma and the nanoparticle growth mechanism.

Measured Gas Concentrations and Flow Properties in SiH4–H2 Mixtures

Each gas concentration in silane–hydrogen (SiH4–H2) gas mixtures was measured using a diaphragm gauge and a quartz friction pressure gauge (D- and Q-gauges). At constant pressures of 0.13–1.3 kPa, the Q-gauge pressure readings depended on the densities of SiH4 and H2. Therefore, the dependence of the Q-gauge pressure readings on the partial pressures of SiH4 could be used to measure the concentrations of these gases in the gas mixtures. The relative SiH4 density in the flow of gas mixtures was higher than the SiH4 flow rate ratio, indicating that the flow of the mixtures was intermediate between molecular and viscous. The viscosities of the gas mixtures estimated from the pressure-normalized Q-gauge pressures were compared with the calculations by Wilke's equation, with the finding that the viscosity of the gas mixture can be derived using the present measurement.

Time-resolved cavity ringdown spectroscopy on nanoparticle generation in a SiH4–H2 VHF plasma

Time-resolved cavity ringdown (τ-CRD) spectroscopy has been applied to monitor the silyl (SiH3) radicals and nanoparticles in a pulsed very high frequency (VHF) silane-hydrogen plasma under microcrystalline silicon (μc-Si:H) deposition conditions. The measured cavity loss reveals four time intervals (I up to VI) in the first 4s of the plasma pulse. By variation of the laser wavelength, it is demonstrated that the small cavity loss at 220nm reflects the SiH3 absorption in interval I. In intervals II and III, an additional cavity loss appears. This additional cavity loss corresponds to Rayleigh and Mie scattering by growing nanoparticles. Interval IV reflects the loss of nanoparticles between the electrodes during the afterglow of the plasma pulse. The evolution of the nanoparticle generation determined from the τ-CRD measurements are further confirmed by additional scanning electron microscopy analyses on the nanoparticles created in the plasma pulse.

Plasma diagnostics as a tool for process optimization: the case of microcrystalline silicon deposition

Properties of thin films such as the crystallinity of silicon deposited from SiH4–H2 discharges are governed by the plasma composition. Therefore, it is crucial to measure the plasma composition in order to understand and optimize the deposition rate, deposition efficiency and material quality of thin film deposition, such as for microcrystalline silicon, which is a key material for silicon thin film photovoltaic solar cells. This task can be performed by using Fourier transform infrared absorption spectroscopy or optical emission spectroscopy. This work compares these two techniques and shows their range of applicability and their limitations.

The effect of ion-surface and ion-bulk interactions during hydrogenated amorphous silicon deposition

The ion-bombardment induced surface and bulk processes during hydrogenated amorphous silicon (a-Si:H) deposition have been studied by employing an external rf substrate bias (ERFSB) in a remote Ar–H2–SiH4 expanding thermal plasma (ETP). The comparison of the ETP chemical vapor deposition without and with ERFSB enables us to identify some important ion-surface and ion-bulk interactions responsible for film property modifications. Employing ERFSB creates an additional growth flux and the low energetic ions deliver an extra 5–10eV per Si atom deposited at typical deposition rates of 10–42Å∕s which is a sufficient ion dose to modify the film growth. It is demonstrated that the extra surface and bulk process during a-Si:H growth, induced by the additional ion bombardment, provide an extra degree of freedom to manipulate the a-Si:H microstructure. An ion-film interaction diagram is introduced, which is used to discriminate ion-surface interactions from ion-bulk interactions. According to this ion-film interaction diagram, the a-Si:H grown with ERFSB can be roughly classified in three phases. In phase I the only ion-surface process activated is Si surface atom displacement. In phase II also ion-induced Si bulk atom displacement is sufficiently activated, whereas in phase III ion-induced Si atom sputtering is significant. Phase I is characterized by a reduction in the nanosized void density, a reduction in defect density, and an improvement of the photoresponse. We find that the Si surface displacement is the process responsible for various improvements of the material properties via the enhanced surface migration. Phase II is characterized by an enhancement of vacancy incorporation. In accordance with the introduced ion-film interaction diagram, the Si atom bulk displacement process is responsible for the incorporation of additional vacancies. Phase III is characterized by the decrease in growth flux and the increase in void density. The significant contribution of ion-sputtering processes is responsible for the effects observed in phase III.

Toward the fast deposition of highly crystallized microcrystalline silicon films with low defect density for Si thin-film solar cells

In this study, employing a high-density, low-temperature SiH4–H2 mixture microwave plasma, we investigate the influence of source gas supply configuration on deposition rate and structural properties of microcrystalline silicon (μc-Si) films, and demonstrate the plasma parameters for fast deposition of highly crystallized μc-Si films with low defect density. A fast deposition rate of 65Å/s has been achieved for a SiH4 concentration of 67% diluted in H2 with a high Raman crystallinity of Xc>65% and a low defect density of (1–2)×1016cm−3 by adjusting source gas supply configuration and plasma conditions. A sufficient supply of deposition precursors, such as SiH3, as well as atomic hydrogen H on film growing surface is effective for the high-rate synthesis of highly crystallized μc-Si films, for the reduction in defect density, and for the improvement in film homogeneity and compactability. A preliminary result of p–i–n structure μc-Si thin-film solar cells using the resulting μc-Si films as an intrinsic absorption layer is presented.