Hydrogenated Nanocrystalline Silicon Thin Films Research Articles

Growth, structural and vibrational properties of hydrogenated nanocrystalline silicon thin films prepared by radiofrequency magnetron sputtering technique at room temperature

The main objective of this paper is to produce the hydrogen-free and hydrogenated nanocrystalline silicon (nc-Si:H(x)) thin films deposited onto the single-crystalline Si(100) wafer by radio-frequency magnetron sputtering (RF-MS) at a substrate temperature of ∼ 300 K. Sputtering was effectuated by a microwave plasma in a mixture of argon and hydrogen (Ar + (H2(x)) gasses taken at different proportions of hydrogen dilution: x = 0, 2.5, and 5 sccm (standard cubic centimeter per minute). The deposition rates of the nc-Si:H were fully controlled, and the thickness of these films was ∼ 200 and 300 Å. Pt-capping layer was deposited in order to protect the nc-Si:H layer from oxidation. The thickness of the capping layer was fixed at ∼ 5 Å.The effects of the hydrogen flow rate and post-growth thermal treatment on the crystalline structure and morphology of as-grown nanocrystalline silicon thin films were systematically investigated by ultraviolet and x-ray photoelectron spectroscopy (UPS and XPS), atomic force microscopy (AFM), grazing incidence x-ray diffraction (GI-XRD) and micro-Raman spectroscopy methods. From the measured data, the average nanocrystallite sizes, surface morphology and other important characteristics of these thin films were analyzed. The grown nc-Si:H thin films may be used as a semiconducting electrode material for electrochemical proton production, which is of great technological importance, especially for solid-state electrochromic device applications.

Effect of boron doping on structural, optical, and electrical properties of hydrogenated nanocrystalline silicon thin films grown by Cat-CVD method

Effect of boron doping on structural, optical, and electrical properties of hydrogenated nanocrystalline silicon thin films grown by Cat-CVD method

Preparation and Crystallization Characteristics of Hydrogenated Nanocrystalline Silicon Thin Films by Plasma-Enhanced Chemical Vapor Deposition

Both intrinsic and doped hydrogenated nanocrystalline silicon (nc-Si:H) thin films were prepared by plasma-enhanced chemical vapor deposition (PECVD) with various process parameters, such as a hydrogen dilution ratio, power, substrate temperature, and doping ratios of phosphorus or boron. The crystallization characteristics of nc-Si:H thin films grown with various process parameters were carefully and systematically investigated by Raman spectroscopy. Generally speaking, the results show that the higher the hydrogen dilution ratio and power or the lower the doping ratio, the higher the average grain size and the crystalline volume fraction of both thin films prepared and investigated here. In addition, a p-i-n type nc-Si:H thin film solar cell, which has an open circuit voltage of 660 mV and a short circuit current intensity of 13.06 mA/cm2, was directly prepared on a flat transparent conductive glass substrate.

Optimizing Thermoelectric Power Factor in p-Type Hydrogenated Nano-crystalline Silicon Thin Films by Varying Carrier Concentration

Most approaches to silicon-based thermoelectrics are focused on reducing the lattice thermal conductivity with minimal deterioration of the thermoelectric power factor. This study investigates the potential of p-type hydrogenated nano-crystalline silicon thin films (μc-Si:H), produced by plasma-enhanced chemical vapor deposition, for thermoelectric applications. We adopt this heterogeneous material structure, known to have a very low thermal conductivity (~ 1 W/m K), in order to obtain an optimized power factor through controlled variation of carrier concentration drawing on stepwise annealing. This approach achieves a best thermoelectric power factor of ~ 3 × 10−4 W/mK2 at a carrier concentration of ~ 4.5 × 1019 cm3 derived from a significant increase of electrical conductivity ~ × 8, alongside a less pronounced reduction of the Seebeck coefficient, while retaining a low thermal conductivity. These thin films have a good thermal and mechanical stability up to 500°C with appropriate adhesion at the film/substrate interface.

Hydrogenated Nano-Crystalline Silicon Thin Films in SiO2 Matrix for Next Generation Solar Cells Using Glow Discharged Decomposition

Hydrogenated Nanocrystalline Silicon (nc-Si:H) thin films using SiH4/H2 mixture by glow discharged decomposition were investigated on c-Si and glass substrates. The effects of substrate temperature on the Structural, Optical and Electrical properties of the films were investigated by X-ray diffraction, Raman scattering, FT/IR, Optical transmission and Atomic Force Microscopy (AFM). Substrate temperatures ([TSB]) of the films were changed from 100oC to 250oC. It has been revealed the strong dependence on the film’s properties with the substrate temperatures. XRD and Raman measurements were shown that the higher substrate temperature (250oC) exhibits the highest crystalline volume fraction ([ρ] = 95%) and the lowest crystalline size ([Ω] = 3.5 nm) as well, having the highest H-content and the lowest O-content. At 250oC, the lowest mobility and the highest resistivity were also found to be ~37.5 cm2/v.s and 7.35 Ω-cm. Refractive index and the optical energy gap (Eg) were estimated by 3.8 and 1.9 eV having the growth rate of 4.2 nm/min. At 250oC, it was resulted in a blue shift of the absorption edge having uniform grain distributions. Results indicate that in situ hydrogen cleaning effects is prominent and localized orderly high density Si-Si bonds are exhibiting quantum size effects at highest substrate temperature.

Hydrogenated nanocrystalline silicon thin films with promising thermoelectric properties

The search for materials with suitable thermoelectric properties that are environmentally friendly and abundant led us to investigate p- and n-type hydrogenated nanocrystalline silicon (nc-Si:H) thin films, produced by plasma-enhanced chemical vapor deposition. The Seebeck coefficient and power factor were measured at room temperature showing optimized values of 512 µV K−1 and 3.6 × 10−5 W m−1 K−2, for p-type, and −188 µV K−1 and 2.2 × 10−4 W m−1 K−2, for n-type thin films. The thermoelectric output power of one nc-Si:H pair of both n- and p-type materials is ~91 µW per material cm3, for a thermal gradient of 8 K. The output voltage and current values show a linear dependence with the number of pairs interconnected in series and/or parallel and show good integration performance.

Effects of substrate temperature on structural, optical and morphological properties of hydrogenated nanocrystalline silicon thin films prepared by inductively coupled plasma chemical vapor deposition

Hydrogenated nanocrystalline silicon (nc-Si:H) thin films were deposited on glass substrate by inductively coupled plasma chemical vapor deposition technique in this paper. The structural, optical, and morphological properties of samples were investigated and found to be strongly dependent on the substrate temperature. X-ray diffraction patterns show silicon film deposited at 25 °C is almost entirely amorphous, however, as the substrate temperature increased from 100 to 300 °C, the grain size of samples increases from 4.1 to 14.6 nm indicates improved crystalline quality. Raman spectroscopy revealed the nc-Si:H films contain crystalline, amorphous and an intermediate structure, and the crystallinity of sample deposited at 300 °C reaching a maximum value of 56 %. The optical absorption spectra of nc-Si:H films show hydrogen content and quantum confinement effect have strong influence on the optical band gap. Atomic force microscopy images show good coincidence with above results.

Influence of annealing on microstructure and piezoresistive properties of boron-doped hydrogenated nanocrystalline silicon thin films prepared by PECVD

Boron-doped hydrogenated nanocrystalline silicon (nc-Si:H) thin films were deposited by plasma enhanced chemical vapor deposition technique. Annealing treatment was performed on the deposited films at 400 °C for 60 min in nitrogen atmosphere. Microstructure of the as-deposited and annealed films was characterized by X-ray diffraction (XRD) and Raman scatter spectra, surface morphology of these films was analyzed with atomic force microscopy (AFM), and piezoresistive properties of these films were evaluated by a four-point bending-based measurement system. The influence of annealing treatment on microstructure and piezoresistive properties of boron-doped nc-Si:H thin films was comparatively studied. The Raman scatter spectra and XRD results together with AFM analysis results revealed that annealing treatment can increase the average grain size and crystalline volume fraction of boron-doped nc-Si:H thin films, and can alter grains distribution and concentration of the films. The piezoresistive property evaluation results showed that annealing treatment can increase the gauge factor of boron-doped nc-Si:H thin films from 29.9 to 42.3. These results indicated that annealing treatment can act as an effective way to improve piezoresistive sensitivity of boron-doped nc-Si:H thin films. In this paper, the correlation between boron-doped nc-Si:H thin films’ piezoresistive properties and microstructure changes induced by annealing was discussed in detail.

Mechanical properties of hydrogenated nanocrystalline silicon thin film studied by finite element method

In this research, hydrogenated nanocrystalline silicon (nc-Si:H) thin films were fabricated by using plasma enhanced chemical vapour deposition system. The finite element model of nanoindentation was established to simulate the mechanical properties of a nc-Si:H thin film. By comparing numerical simulation data with experimental data, the correctness of the model was validated. The mechanical properties of nc-Si:H thin films obtained were: Young’s modulus of 45 GPa, yield stress of 4·5 MPa, Poisson’s ratio of 0·3 and the hardness was calculated to be 2·5 GPa.

Experimental Investigation of Microstructure and Piezoresistive Properties of Phosphorus-Doped Hydrogenated Nanocrystalline Silicon Thin Films Prepared by PECVD

This paper presents an experimental investigation of microstructure and piezoresistive properties of phosphorus-doped hydrogenated nanocrystalline silicon (nc-Si:H) thin films. The phosphorus-doped nc-Si:H thin films (5% doping ratio of PH3 to SiH4) were deposited by plasma enhanced chemical vapor deposition (PECVD) technique. The microstructure and surface morphology of the deposited thin films was characterized and analyzed with Raman spectroscopy and atomic force microscopy (AFM), respectively. The piezoresistive properties of the deposited thin films were investigated with a designed four-point bending-based evaluation system. In addition, the influence of temperature on the piezoresistive properties of these thin films was evaluated with the temperature coefficient of resistance (TCR) measurements from room temperature up to 80°C. The experimental results show that phosphorus-doped nc-Si:H thin films prepared by PECVD technique are a two-phase material that constitutes of nanocrystalline silicon and amorphous silicon, and they present a granular structure composed of homogeneously scattered nanoclusters formed by nanocrystalline silicon grains (6nm). Moreover, phosphorus-doped nc-Si:H thin films exhibit negative GF at room temperature and show good thermal stability from room temperature up to 80°C, and the value of GF and TCR is about-31 and-509ppm/°C, respectively. These features could make phosphorus-doped nc-Si:H thin films act as a promising material for piezoresistive-based MEMS sensor.

Structural analysis of hydrogenated nanocrystalline silicon thin films as a function of substrate temperature during deposition

In this contribution, the micro- and macro-structure of plasma grown hydrogenated nanocrystalline silicon (nc-Si:H) thin films were followed with respect to substrate temperature ( T s ) ranging from 80 °C to 200 °C. nc-Si:H films were deposited by plasma enhanced chemical vapor deposition technique using silane gas highly diluted by hydrogen and high RF power density. Micro-structure analysis was performed with grazing angle X-ray diffraction (GAXRD) and dispersive Raman spectroscopies. Parallelly, morphological properties of the films were investigated via field emission scanning electron microscopy (FE-SEM) by taking both surface and cross-sectional micrographs. GAXRD results suggest the presence of 4–5 nm nanocrystallites with the size almost independent of T s . The detailed analysis of the Raman spectra reveals one- and two-phonon modes due to amorphous and crystalline silicon (c-Si), where all c-Si related peaks shifted to lower frequencies. Raman nanocrystalline volume fraction is found to be greater than 50% for all T s ; it increases together with the short-range order at elevated T s . This micro-structural improvement with T s is considered to be slight when compared to the FE-SEM-observed drastic changes of morphology, particularly the size of large conglomerates (35–250 nm). The behavior of micro- and macro-structure with T s is correlated well with the previously determined bonding and lateral conductivity measurements. It seems to be the macro-structure that accounts for bonding and electrical properties of nc-Si:H thin films.

Design and Investigation of SST/nc-Si:H/M (M = Ag, Au, Ni) and M/nc-Si:H/M Multifunctional Devices

Hydrogenated nanocrystalline Silicon thin films prepared by the very high frequency chemical vapor deposition technique (VHF-CVD) on stainless steel (SST) substrates are used to design Schottky point contact barriers for the purpose of solar energy conversion and passive electronic component applications. In this process, the contact performance between SST and M (M = Ag, Au, and Ni) and between Ag, Au, and Ni electrodes was characterized by means of current-voltage, capacitance-voltage, and light intensity dependence of short circuit (Isc) current and open circuit voltage (Voc) of the contacts. Particularly, the devices ideality factors, barrier heights were evaluated by the Schottky method and compared to the Cheung's. Best Schottky device performance with lowest ideality factor suitable for electronic applications was observed in the SST/nc-Si:H/Ag structure. This device reflects a Voc of 229 mV with an Isc of 1.6 mA/cm2 under an illumination intensity of ~40 klux. On the other hand, the highest Isc being 9.0 mA/cm2 and the Voc of 53.1 mV were observed for Ni/nc-Si:H/Au structure. As these voltages represent the maximum biasing voltage for some of the designed devices, the SST/nc-Si:H/M and M/nc-Si:H/M can be regarded as multifunctional self-energy that provided electronic devices suitable for active or passive applications.

Electronic states in hydrogenated nanocrystalline silicon thin films detected by photocurrent technique

Electronic states of hydrogenated nanocrystalline silicon (nc-Si:H) thin films had been investigated by temperature-dependent photocurrent measurements. It was found that the photocurrent in weak absorption region is dominated by a thermal-assisted transport due to the interfacial barrier. In strong absorption region, the direct transition is observed at the electronic state above the interfacial barrier, where the photocurrent abnormally increases with temperature decreasing due to the reduction of phonon scattering in the extended state transport. The temperature-dependent photocurrent is explained well by a simple coupled-rate equation model for both the weak and strong absorption regions, demonstrating the extended state in nc-Si:H.

On the origin of deep oxygen defects in hydrogenated nanocrystalline silicon thin films used in photovoltaic applications

To effectively mitigate oxygen induced degradation of electronic properties in hydrogenated nanocrystalline silicon (nc-Si:H) the microscopic origins of the associated defects must be determined. This work elucidates the origin of a 0.7eV photoluminescence (PL) band observed in thermally annealed, oxygen contaminated nc-Si:H. Raman and PL spectroscopy data demonstrate the deep defects responsible for the 0.7eV PL reside in the crystalline phase. The electronic levels enabling the emission exist 0.4eV below the conduction band edge, or 0.4eV above the valence band edge, or both, with transitions into or out of extended states as opposed to band tail recombination. The defect formation energy is 0.6eV, which suggests that the formation involves a local reconfiguration of oxygen and hydrogen as opposed to bulk diffusion. Hydrogen plays multiple roles in the microstructural evolution and electronic activity of these centers. TEM micrographs show dislocations in the nanocrystalline regions, which suggests that oxygen gettering occurs. It seems in the absence of hydrogen dislocations in Si nanocrystals scavenge oxygen impurities from grain boundaries to become optically active, enabling emission at 0.7eV.

Effect of Hydrogen Dilution on the Nanostructural and Electrooptical Characteristics of Hydrogenated Nanocrystalline Silicon Thin Films Prepared by Plasma Enhanced Chemical Vapor Deposition

Nanocrystalline hydrogenated amorphous silicon (nc-Si:H) thin films were deposited on silicon wafers and glass by plasma-enhanced chemical vapor deposition. The hydrogen dilution in the precursor gases, [SiH4/H2], were varied from 1 to 0.01 with the other deposition factors kept constant. The nanocrystallite size and volume fraction increased steadily with increasing hydrogen dilution ratio in the gas from 1 to 0.01. The mean size of the nanocrystallites ranged from ∼1 to ∼7 nm. The band gap of the films varied according to the hydrogen dilution, indicating the nanostructural features of the films. Film resistivity was dependent on the crystallite size and volume fraction in the films. In particular, the resistivity of a simple P–I–N type device decreased with increasing nanocrystallite size. The increased crystallinity can be explained by the predominance of Si–H bonds in the films.

Low temperature plasma production of hydrogenated nanocrystalline silicon thin films

The hydrogenated nanocrystalline silicon (nc-Si:H) thin films were produced by capacitively-coupled plasma enhanced chemical vapor deposition (PECVD) technique at low substrate temperatures (Ts ≈ 40–200 °C). Firstly, for particular growth parameters, the lowest stable Ts was determined to avoid temperature fluctuations during the film deposition. The influence of the Ts on the structural and optical properties of the films was investigated by the Fourier transform infrared (FTIR), UV–visible transmittance/reflectance and X-ray diffraction (XRD) spectroscopies. Also, the films deposited at the center of the PECVD electrode and those around the edge of the PECVD electrode were compared within each deposition cycle. The XRD and UV–visible reflectance analyses reveal the nanocrystalline phase for the films grown at the edge at all Ts and for the center films only at 200 °C. The crystallinity fraction and lateral dark conductivity decrease with lowered Ts. FTIR analyses were used to track the hydrogen content, void fraction and amorphous matrix volume fraction within the films. The optical constants obtained from the UV–visible transmittance spectroscopy were correlated well with the FTIR results. Finally, the optimal Ts was concluded for the application of the produced nc-Si:H in silicon-based thin film devices on plastic substrates.

Low-temperature preparation of flexible a-Si:H solar cells with hydrogenated nanocrystalline silicon p layer

In this paper we report on flexible a-Si:H solar cells prepared on polyethylene naphthalate (PEN) substrates using p-type hydrogenated nanocrystalline silicon thin films (p-nc-Si:H) as the window layer. The p-nc-Si:H films were prepared at low temperature (150 °C) using trimethylboron (TMB) as a dopant gas. The influence of the silane concentration (SC) on the electrical and structural properties of ultra-thin p-nc-Si:H as well as the performance of solar cells on PEN was investigated. The results show that the crystalline fraction and conductivity of p-nc-Si:H thin films diminished, while the deposition rate and RMS roughness of films increased, when the SC increases from 0.53% to 0.8%. For the a-Si:H solar cells on PEN with the non-textured electrodes, the best efficiency of 6.3% was achieved with the p-nc-Si:H thin films deposited at SC = 0.67%.

New Numerical Method to Calculate the True Optical Absorption of Hydrogenated Nanocrystalline Silicon Thin Films

The enhanced optical absorption measured by Constant Photocurrent Method (CPM) of hydrogenated nanocrystalline silicon thin films is due mainly to bulk and/or surface light scattering effects. A new numerical method is presented to calculate both true optical absorption and scattering coefficient from CPM absorption spectra of nanotextured nano-crystalline silicon films. Bulk and surface light scattering contributions can be unified through the correlation obtained between the scattering coefficient and surface roughness obtained using our method.

Low Temperature Growth of Hydrogenated Silicon Prepared by PECVD from Argon Diluted Silane Plasma

In order to contribute to the understanding of the optoelectronics properties of hydrogenated nanocrystalline silicon thin films, a detailed study has been conducted. The samples were deposited by 13.56 MHz PECVD (Plasma-Enhanced Chemical Vapor Deposition) of silane argon mixture. The argon dilution of silane for all samples studied was 96% by volume. The substrate temperature was fixed at 200oC. The influence of depositions parameters on optical proprieties of samples was studied by UV-Vis-NIR spectroscopy. The structural evolution was studied by Raman spectroscopy and X-ray diffraction (XRD). Intrinsic-layer samples depositions were made in this experiment in order to obtain the transition from the amorphous to crystalline phase materials. The deposition pressure varied from 400 mTorr to 1400 mTorr and the rf power from 50 to 250 W. The structural evolution studies show that beyond 200 W, we observed an amorphous-nanocrystalline transition, with an increase in crystalline fraction by increasing rf power and working pressure. Films near the amorphous to nanocrystalline transition region are grown at reasonably high deposition rates (~10 /s), which are highly desirable for the fabrication of cost effective devices. The deposition rate increases with increasing rf power and process pressure. Different crystalline fractions (21% to 95%) and crystallite size (6 - 16 nm) can be achieved by controlling the process pressure and rf power. These structural changes are well correlated to the variation of optical proprieties of the thin films.

Effects of high hydrogen dilution ratio on optical properties of hydrogenated nanocrystalline silicon thin films

Hydrogenated nanocrystalline silicon thin films were prepared by plasma enhanced vapor deposition technique. In our experiment, hydrogen dilution ratio RH was changed mainly, while the other parameters, such as the radio frequency power, the direct current bias value, the chamber pressure, the total gas flow and the substrate temperature were kept constant. The film's surface topography was gained by AFM. The chemical bond was confirmed by Fourier transform infrared spectra. The optical properties were characterized by transmission spectra. To consider absorption peak of stretching vibration mode of SiH3 at 2140cm−1 and to reduce the calculation error, a hydrogen content calculation method was proposed. Effects of hydrogen dilution ratio on the deposition rate v and hydrogen content CH were investigated. The bonding mode and the force constants k of chemical bond, the structural factor f in films were changed by high hydrogen dilution ratio, which gave rise to the shift of absorption peak of infrared stretching mode and the decrease of optical band gap Eg.