Microstructure and electronic properties of microcrystalline silicon carbide thin films prepared by hot-wire CVD

An overview on microstructural and electronic properties of stoichiometric microcrystalline silicon carbide (μc-SiC) prepared by Hot-Wire Chemical Vapor Deposition (HWCVD) at low substrate temperatures will be given. The electronic properties are strongly dependent on crystalline phase, local bonding, strain, defects, impurities, etc. Therefore these quantities need to be carefully investigated in order to evaluate their influence and to develop strategies for material improvement. We will particularly address the validity of different experimental methods like Raman spectroscopy and IR spectroscopy to provide information on the crystalline volume fraction by comparing the results with Transmission Electron Microscopy (TEM) and X-Ray diffraction data. Finally the electronic properties as derived from optical absorption and transport measurements will be related to the microstructure.     

Structural changes in tungsten wire and their effect on the properties of hydrogenated nanocrystalline cubic silicon carbide thin films

We investigated the structural changes in tungsten wire heated to 1800 °C in SiH₄/CH₄/H₂/N₂ atmosphere and the effect of the aging tungsten wire on the properties of N-doped hydrogenated nanocrystalline cubic silicon carbide (nc-3C-SiC:H) thin films. The aged tungsten wire had two parts: hot parts of the middle of the wire and relatively cold parts connected to clamps. Tungsten carbide (W₂C) layer formed in the wire of the hot parts, while crystalline silicon and cubic silicon carbide (c-Si/3C-SiC) layer deposited on the wire of the cold parts. N-doped nc-3C-SiC:H thin films were deposited for 5 min (thickness: ~ 30 nm) after the tungsten wire was heated under the same condition as during the film deposition for given times (exposure time). No changes in the structural, electrical and optical properties of the nc-3C-SiC:H thin films were observed for the exposure time up to 450 min.     

Aluminum doped silicon carbide thin films prepared by hot-wire CVD: Influence of the substrate temperature on material properties

Successful p-type doping of μc-SiC:H with Al introduced from trimethylaluminum has been already demonstrated. In this work we focus on the influence of substrate temperature (T_S = 300-390 °C) on the Al-doping. As T_S is reduced from 390 °C to 300 °C, the crystallinity decreases from 75% to 55% and the dark conductivity σ_D decreases first by about three orders of magnitude before increasing again at T_S = 300 °C. Both microstructure, as determined from Raman spectroscopy, and optical absorption are little affected by the change in T_S. Upon annealing at 450 °C in vacuum, σ_D increases typically by two orders of magnitude up to 10⁻⁴ S/cm, which is explained by dopant activation as a result of hydrogen desorption. It is concluded that a process temperature > 350 °C is needed to obtain effective Al-doping for p-type μc-SiC:H thin films.     

Development of microcrystalline silicon carbide window layers by hot-wire CVD and their applications in microcrystalline silicon thin film solar cells

Microcrystalline silicon carbide (μc-SiC:H) thin films in stoichiometric form were deposited from the gas mixture of monomethylsilane (MMS) and hydrogen by Hot-Wire Chemical Vapor Deposition (HWCVD). These films are highly conductive n-type. The optical gap E₀₄ is about 3.0-3.2 eV. Such μc-SiC:H window layers were successfully applied in n-side illuminated n-i-p microcrystalline silicon thin film solar cells. By increasing the absorber layer thickness from 1 to 2.5 μm, the short circuit current density (j_SC) increases from 23 to 26 mA/cm² with Ag back contacts. By applying highly reflective ZnO/Ag back contacts, j_SC = 29.6 mA/cm² and η = 9.6% were achieved in a cell with a 2-μm-thick absorber layer.     

Growth of silicon carbide thin films by hot-wire chemical vapor deposition from SiH4/CH4/H2

Silicon carbide (SiC) thin films were prepared by hot-wire chemical vapor deposition from SiH₄/CH₄/H₂ and their structural properties were investigated by X-ray diffraction, Fourier transform infrared absorption and Raman scattering spectroscopies. At 2 Torr, Si-crystallite-embedded amorphous SiC (a-Si_1 − xC_x:H) grew at filament temperatures (T_f) below 1600 °C and nanocrystalline cubic SiC (nc-3C-SiC:H) grew above T_f = 1700 °C. On the other hand, At 4 Torr, a-Si_1 − xC_x:H grew at T_f = 1400 °C and nc-3C-SiC grew above T_f = 1600 °C. When the intakes of Si and C atoms into the film per unit time are almost the same and H radicals with a high density are generated, which takes place at high T_f, nc-3C-SiC grows. On the other hand, at low T_f the intake of Si atoms is larger than that of C atoms and, consequently, Si-rich a-Si_1 − xC_x:H or Si-crystallite-embedded a-Si_1 − xC_x:H grow.     

Aluminum doped silicon carbide thin films prepared by hot-wire CVD: Investigation of defects with electron spin resonance

Al-doped p-type μc-SiC:H is prepared in a wide range of HWCVD preparation parameters like Al-doping ratio, deposition pressure, substrate and filament temperatures. We investigate the structural and electrical properties, and focus on identification of paramagnetic defect states by electron spin resonance (ESR). Nominally undoped μc-SiC:H is of a high n-type conductivity (σ_D = 10^− 6-10^− 1 S/cm) and shows a narrow central ESR line (g ≈ 2.003, peak-to-peak linewidth ?H_pp ≈ 4 G) with two pairs of satellites and a spin density N_S = 10¹⁹ cm^− 3. Al-doping results in the compensation of dark conductivity to as low as σ_D = 10^− 11 S/cm and at higher doping concentrations to effective p-type material. Increase of Al-doping results in reduction of crystallinity (I_C^IR), ESR line shifts to g ≈ 2.01 and becomes as broad as ?H_pp ≈ 30 G, not unlike to the resonance of singly occupied paramagnetic valence band tail states in a-Si:H. ESR spectrum of highly crystalline Al-doped μc-SiC:H however has a g-value very close to undoped μc-SiC:H. Electron spin density in compensated material decreases to 5 × 10¹⁷ cm^− 3 before it increases again for the highly doped material.     

Structural and optical properties of four-hexagonal polytype nanocrystalline silicon carbide films deposited by plasma enhanced chemical vapor deposition technique

Four-hexagonal polytype films of nanocrystalline silicon carbide (4H-nc-SiC) were deposited by plasma enhanced chemical vapor deposition method with more than 3×10⁴ W m⁻² threshold of power density, high hydrogen dilution ratio, and bias pretreatment. The source gases were silane, methane and hydrogen. Our work showed that under conditions similar to those used for the growth of μc-SiC—except a higher power densities over a threshold, a bigger bias pretreatment on substrates, and a moderate bias deposition—nc-SiC films could indeed be achieved. The Raman spectra and transmission electron microscopy diffraction patterns demonstrated that the as-grown films from the H₂-CH₄-SiH₄ plasma consist of amorphous network and phase-pure crystalline silicon carbide which has the 4H polytype structure. The microcolumnar 4H-SiC nanocrystallites of a mean size of approximately 1.6×10⁻⁸ m in diameter are encapsulated by amorphous SiC networks. The photoluminescence spectra of 4H-SiC at room temperature, peaking at 8.10×10⁻⁷ m using a wavelength of 5.145×10⁻⁷ m of argon ion laser, were obtained at room temperature; the luminescence mechanism is thought to be related to transitions in the energy band gap which could be ascribed to the surface states and defects in the structure of 4H-SiC nanocrystalline in these films due to its small size. The as-grown films showed an optical transmittance of 89% at 6.58×10⁻⁷ m. This higher transmittance is believed to be from the small size and amorphous matrix.     

Deposition and optical studies of silicon carbide nitride thin films

Thin films of silicon carbide nitride (SiCN) have been prepared by reactive radioactive frequency (r.f.) sputtering using SiC target and nitrogen as the reactant gas. Deposition rates are studied as a function of deposition pressures and argon-nitrogen flow ratios. The optical absorption studies indicated the band edge shifting of the films when the nitrogen ratios are increased during deposition. Fourier transform infrared spectroscopy (FTIR) analysis on the films indicated several stretching modes corresponding to SiC, SiN and CN compositions.     

Microstructure of highly crystalline silicon carbide thin films grown by HWCVD technique

Highly crystalline silicon carbide films were synthesised by HWCVD technique. Raman spectroscopic studies show that the SiC films contain crystalline SiC and also carbon phases. Carbon is graphitic at higher chamber pressures (≥ 50 Pa) and resembles diamond-like carbon at low pressure (5 Pa). Cross-section TEM results show a columnar morphology of the crystallites with typical column diameters up to ∼ 50 nm. Transmission electron diffraction patterns reveal SiC in its cubic and hexagonal SiC phases and the C diamond phase at low pressure. Annealing at 1000 °C for 1 h results in enhancement of crystallite size without nucleation of new phases.     

Growth and characterization of silicon carbide thin films on silicon using a hollow cathode pulse sputtering technique

Investigations of thin film depositions of silicon carbide (SiC) from pulse sputtering a hollow cathode SiC target are presented. The unique feature of the hollow cathode technique is that germanium can be added to the films. This changes the properties of the SiC. Such changes include evidence of GeC bonds, lowering of the resistivity, and lowering of the bandgap. The analysis includes crystallographic and morphological studies of the deposited films and their quality using X-ray diffraction, transmission electron microscopy, scanning electron microscopy, and Raman spectroscopy data. Basic electrical properties are also presented along with optical bandgap information gathered from spectroscopic ellipsometry data.     

Silicon surface passivation by hot-wire CVD Si thin films studied by in situ surface spectroscopy

Silicon thin films can provide an excellent surface passivation of crystalline silicon (c-Si) which is of importance for high efficiency heterojunction solar cells or diffused emitter solar cells with well-passivated rear surfaces. Hot-wire chemical vapor deposition (hot-wire CVD) is an attractive method to synthesize Si thin films for these applications as the method is ion-bombardment free yielding good quality films over a wide range of deposition rates. The properties of the interface between hot-wire CVD Si thin films and H-terminated c-Si substrates have been studied during film growth by three complementary in situ techniques. Spectroscopic ellipsometry has been used to determine the optical properties and thickness of the films, whereas information on the H-bonding modes and H-depth profile has been obtained by attenuated total reflection infrared spectroscopy. Second-harmonic generation (SHG), a nonlinear optical technique sensitive to surface and interface states, has been used to probe two-photon resonances related to modified Si-Si bonds at the interface. By correlating the observations with ex situ lifetime spectroscopy experiments the growth and surface passivation mechanism of the Si films are discussed.     

Amorphous silicon carbide heterojunction solar cells on p-type substrates

The performance of silicon heterojunction (SHJ) solar cells is discussed in this paper in regard to their dependence on the applied amorphous silicon layers, their thicknesses and surface morphology. The emitter system investigated in this work consists of an n-doped, hydrogenized, amorphous silicon carbide a-SiC:H(n) layer with or without a pure, hydrogenized, intrinsic, amorphous silicon a-Si:H(i) intermediate layer. All solar cells were fabricated on p-type FZ-silicon and feature a high-efficiency backside consisting of a SiO₂ passivation layer and a diffused local boron back surface field, allowing us to focus only on the effects of the front side emitter system. The highest solar cell efficiency achieved within this work is 18.5%, which is one of the highest values for SHJ-solar cells using p-type substrates. A dependence of the passivation quality on the surface morphology was only observed for solar cells including an a-Si:H(i) layer. It could be shown that the fill factor suffers from a reduction due to a reduced pseudo fill factor for emitter thicknesses below 11 nm due to a lower passivation quality and/or a higher potential for shunting thorough the a-Si emitter to the crystalline wafer with the conductive indium tin oxide layer. Furthermore, the influence of a variation of the doping gas flow (PH₃) during the plasma enhanced chemical vapor deposition of the doped amorphous silicon carbide a-SiC:H(n) on the solar cell current-voltage characteristic-parameter has been investigated. We could demonstrate that a-SiC:H(n) shows in principle the same dependence on PH₃-flow as pure a-Si:H(n).     

Defect analysis of thin film Si-based alloys deposited by hot-wire CVD using junction capacitance methods

We evaluate and compare the electronic properties of hot-wire CVD deposited a-Si:H and a-Si,Ge:H films with those produced by the glow discharge (PECVD) method. A good indicator of film quality with respect to solar cell applications is the narrowness of the band tail widths determined by transient photocapacitance (TPC) spectroscopy. We focus on the excellent electronic properties of hot-wire CVD a-Si,Ge:H alloys that have recently been produced by a 1800  °C filament temperature process. These alloy samples were compared to a-Si,Ge:H films of the same optical gaps deposited by PECVD. Light-induced degradation was examined in a few samples and compared to the behavior PECVD a-Si,Ge:H alloys of similar optical gap. The effects of intentional oxygen contamination were also studied on a series of HWCVD a-Si,Ge:H samples containing 29at.% Ge.     

High-rate deposition of amorphous silicon films using hot-wire CVD with a coil-shaped filament

To reduce the manufacturing cost of amorphous silicon (a-Si:H)-based photovoltaic devices, it is important to deposit high-quality a-Si:H and related materials at a high deposition rate. To this end, we designed and constructed a hot-wire deposition chamber with a coiled filament design and with multiple gas inlets. The process gas could be directed into the chamber through the filament coil and have maximum exposure to the high-temperature filament surface. Using such a chamber design, we deposited a-Si:H films at high deposition rates up to 800 Å s⁻¹ and dense, low-void a-Si:H at rates up to 240 Å s⁻¹.     

Effect of thermal annealing on the structural and mechanical properties of amorphous silicon carbide films prepared by polymer-source chemical vapor deposition

We report on the effect of thermal annealing on the structural and mechanical properties of amorphous SiC thin films prepared by means of a polymer-source chemical vapor deposition process. The chemical bondings of the a-SiC:H films were systematically examined by means of Fourier transform infrared spectroscopy (FTIR). The film composition was measured by X-ray photoelectron spectroscopy, while X-ray reflectivity measurements were used to account for the film density variations caused by the post-annealing treatments over the 750-1200 °C range. In addition, their mechanical properties (hardness and Young's modulus) were investigated by using the nano-indentation technique. FTIR measurements revealed that not only the intensity of a-SiC absorption band linearly increases but also its position is found to shift to a higher wave number as a result of annealing. In addition, the bond density of Si―C is found to increase from (101.6-224.5) × 10²¹ bond·cm^− 3 accompanied by a decrease of Si―H bond density from (2.58-0.46)× 10²¹ bond·cm^− 3 as a result of increasing the annealing temperature (T_a) from 750 to 1200 °C. Annealing-induced film densification is confirmed, as the a-SiC film density is found to increase from 2.36 to ∼ 2.75 g/cm^− 3 when T_a is raised from 750 to 1200 °C. In addition, as T_a is increased from 750 to 1200 °C, both hardness and Young's modulus are found to increase from 15.5 to 17.6 GPa and 155 to 178 GPa, respectively. Our results confirm the previously established linear correlation between the mechanical properties of the a-SiC films and their bond densities.     

Fabrication of nanocrystalline silicon carbide thin film by helicon wave plasma enhanced chemical vapour deposition

Nanocrystalline cubic silicon carbide thin films have been fabricated by helicon wave plasma enhanced chemical vapour deposition on Si substrates using the mixture of SiH₄, CH₄, and H₂ at a low substrate temperature of 300 °C. The infrared absorption spectroscopy analyses and microstructural characteristics of the samples deposited at various magnetic fields indicate that the high plasma intensity in helicon wave mode is a key factor to the success of growing nanocrystalline silicon carbide thin films at a relative low substrate temperature. Transmission electron microscopy measurements reveal that the films consist of silicon carbide nanoparticles with an average grain size of several nanometers, and the light emission measurements show a strong blue photoluminescence at room temperature, which is considered to be caused by the quantum confine effect of small size silicon carbide nanoparticles.     

Piezoresistive properties of nanocrystalline silicon thin films deposited on plastic substrates by hot-wire chemical vapor deposition

The piezoresistive property of n-type and p-type nanocrystalline silicon thin films deposited on plastic (PEN) at a substrate temperature of 150 °C by hot-wire chemical vapor deposition, is studied. The crystalline fraction decreased from 80% to 65% in p-type and from 84% to 62% in n-type films, as the dopant gas-to-silane flow rate ratio was increased from 0.18% to 3-3.5%. N-type films have negative gauge factor (− 11 to − 16) and p-type films have positive gauge factor (9 to 25). In n-type films the higher gauge factors (in absolute value) were obtained by increasing the doping level whereas in p-type films higher gauge factors were obtained by increasing the crystalline fraction.     

Applications of microcrystalline hydrogenated cubic silicon carbide for amorphous silicon thin film solar cells

We demonstrated the fabrication of n-i-p type amorphous silicon (a-Si:H) thin film solar cells using phosphorus doped microcrystalline cubic silicon carbide (μc-3C-SiC:H) films as a window layer. The Hot-wire CVD method and a covering technique of titanium dioxide TiO₂ on TCO was utilized for the cell fabrication. The cell configuration is TCO/TiO₂/n-type μc-3C-SiC:H/intrinsic a-Si:H/p-type μc- SiC_x (a-SiC_x:H including μc-Si:H phase)/Al. Approximately 4.5% efficiency with a V_oc of 0.953 V was obtained for AM-1.5 light irradiation. We also prepared a cell with the undoped a-Si_1−xC_x:H film as a buffer layer to improve the n/i interface. A maximum V_oc of 0.966 V was obtained.     

Deposition of silicon nitride thin films by hot-wire CVD at 100 °C and 250 °C

Silicon nitride thin films for use as passivation layers in solar cells and organic electronics or as gate dielectrics in thin-film transistors were deposited by the Hot-wire chemical vapor deposition technique at a high deposition rate (1-3 ?/s) and at low substrate temperature. Films were deposited using NH₃/SiH₄ flow rate ratios between 1 and 70 and substrate temperatures of 100 °C and 250 °C. For NH₃/SiH₄ ratios between 40 and 70, highly transparent (T ~ 90%), dense films (2.56-2.74 g/cm³) with good dielectric properties and refractive index between 1.93 and 2.08 were deposited on glass substrates. Etch rates in BHF of 2.7 ?/s and < 0.5 ?/s were obtained for films deposited at 100 °C and 250 °C, respectively. Films deposited at both substrate temperatures showed electrical conductivity ~ 10^− 14 Ω^− 1 cm^− 1 and breakdown fields > 10 MV cm^− 1.     

Synthesis and characterization of nanostructured silicon carbide

Equiatomic nanostructured silicon carbide was successfully prepared by milling elemental Si and C powders, using a planetary ball mill. The synthesis of this carbide proceeded at milling conditions corresponding to 5.19 W/g shock power. The reaction was gradual and completed after 15 h. After 20 h of alloying duration and towards the end of the process, the SiC diffraction crystallite size (DCS) reached a critical value of 4 nm. At this same alloying duration, SEM characterization revealed that the powders exhibit homogeneous distribution of the particles with 0.3 µm in size.