Capacitance analyses of hydrogenated nanocrystalline silicon based thin film transistor

The capacitance-voltage (C-V) measurements within 10⁶-10^− 2 Hz frequency range were performed on the hydrogenated nanocrystalline silicon (nc-Si:H) bottom-gate thin film transistor (TFT) and metal-insulator-amorphous silicon (MIAS) structure, mechanically isolated from the same TFT. It was found that the conducting thin layer in nc-Si:H film expands the effective capacitor area beyond the electrode in the TFT structure, which complicates its C-V curves. Considering the TFT capacitance-frequency (C-F) curves, the equivalent circuit of the TFT structure was proposed and mechanism for this area expansion was discussed. On the other hand, the MIAS C-F curves were fitted by the equivalent circuit models to deduce its electrical properties. nc-Si:H neutral bulk effect was revealed by the dependence of the MIAS capacitance on frequency within 10⁶-10³ Hz at both accumulation and depletion regimes. The inversion in MIAS was detected at 10²-10^− 2 Hz for relatively low negative gate bias without any external activation source. The presence of the ac hopping conductivity in the nc-Si:H film was inferred from the fitting. In addition, the density of the interface traps and its energy distribution were determined.     

Preferred growth of nanocrystalline silicon in boron-doped nc-Si:H Films

Preferred growth of nanocrystalline silicon (nc-Si) was first found in boron-doped hydrogenated nanocrystalline (nc-Si:H) films prepared using plasma-enhanced chemical vapor deposition system. The films were characterized by high-resolution transmission electron microscope, X-ray diffraction (XRD) spectrum and Raman Scattering spectrum. The results showed that the diffraction peaks in XRD spectrum were at 2θ≈47° and the exponent of crystalline plane of nc-Si in the film was (2 2 0). A considerable reason was electric field derived from dc bias made the bonds of Si-Si array according to a certain orient. The size and crystalline volume fraction of nc-Si in boron-doped films were intensively depended on the deposited parameters: diborane (B₂H₆) doping ratio in silane (SiH₄), silane dilution ratio in hydrogen (H₂), rf power density, substrate's temperature and reactive pressure, respectively. But preferred growth of nc-Si in the boron-doped nc-Si:H films cannot be obtained by changing these parameters.     

Nanocrystalline silicon fabrication by conventional plasma enhanced chemical vapor deposition for bottom gate thin film transistor

Nanocrystalline silicon was successfully fabricated using conventional plasma enhanced chemical vapor deposition (PECVD) for bottom gate thin film transistor. This was accomplished by promoting nucleation rate in the initial stage of silicon growth by H₂ or SF₆ plasma treatment of the surface of silicon nitride (SiN_x) films. Microstructure of hydrogenated nanocrystalline silicon (nc-Si:H) films confirmed the crystallization of silicon, and nanocrystalline silicon thin film transistor exhibited excellent stability.     

双纳米硅p层优化非晶硅太阳能电池

采用等离子体增强化学气相沉积(Plasma Enhanced Chemical Vapor Deposition,PECVD)技术在高功率密度、高反应气压和低衬底温度下制备出不同氢稀释比RH的硅薄膜.高分辨透射电镜(High-Resolution Transmission Electron Microscopy,HRT...     

Effects of doping concentration on the microstructural and optoelectrical properties of boron doped amorphous and nanocrystalline silicon films

Boron doped hydrogenated amorphous silicon thin films were prepared by plasma-enhanced chemical vapor deposition technique at various flow rate of diborane (F_B). As-deposited samples were thermally annealed at the temperature of 800 °C to obtain the doped nanocrystalline silicon (nc-Si) films. The effect of boron concentration on the microstructural, optical and electrical properties of the films was investigated. X-ray photoelectron spectroscopy (XPS) measurements demonstrated the presence of the substitutional boron in the doped films. It was found that thermal annealing can efficiently activate the dopants in films accompanying with formation of nc-Si grains. Based on the temperature-dependent conductivity measurements, it was shown that the dark conductivity of doped amorphous samples increases monotonously with the increase of doping content. While the dark conductivity of doped nc-Si films is not only determined by the concentration of dopant but also the crystallinity of the films. As increasing the flow rate of diborane, the crystallinity of doped nc-Si films decreases, which causes the decrease of dark conductivity. Finally, the high dark conductivity of 178.68 S cm⁻¹ of the B-doped nc-Si thin films can be obtained.     

硼掺杂对热丝CVD法制备纳米晶硅薄膜微结构与光电性能的影响

采用热丝化学气相沉积法制备了不同B2H6掺杂比例(B2H6/SiH4为2%~15%)的p型纳米晶硅薄膜,通过探索B2H6掺杂比例、晶化率、光学带隙和电学性能(电导率、载流子浓度、霍尔迁移率)之间的关系以及薄膜掺杂机理来研究B2H6掺杂比例对薄膜微结构和光电性能的影响。在掺杂比例为11%时成功获得了电导率为32 S/cm的高电导率硼掺杂nc-Si∶H薄膜。     

Size modulation of nanocrystalline silicon embedded in amorphous silicon oxide by Cat-CVD

Different issues related to controlling size of nanocrystalline silicon (nc-Si) embedded in hydrogenated amorphous silicon oxide (a-SiO_x:H) deposited by catalytic chemical vapor deposition (Cat-CVD) have been reported. Films were deposited using tantalum (Ta) and tungsten (W) filaments and it is observed that films deposited using tantalum filament resulted in good control on the properties. The parameters which can affect the size of nc-Si domains have been studied which include hydrogen flow rate, catalyst and substrate temperatures. The deposited samples are characterized by X-ray diffraction, HRTEM and micro-Raman spectroscopy, for determining the size of the deposited nc-Si. The crystallite formation starts for Ta-catalyst around the temperature of 1700 °C.     

Effect of hydrogen plasma treatment on the surface morphology, microstructure and electronic transport properties of nc-Si:H

Hydrogenated nanocrystalline silicon (nc-Si:H) films, deposited by reactive radio-frequency sputtering with 33% hydrogen dilution in argon at 200 °C, were treated with low-power hydrogen plasma at room temperature at various power densities (0.1-0.5 W/cm²) and durations (10 s-10 min). Plasma treatment reduced the surface root mean square roughness and increased the average grain size. This was attributed to the mass transport of Si atoms on the surface by surface and grain boundary diffusion. Plasma treatment under low power density (0.1 W/cm²) for short duration (10 s) caused a significant enhancement of crystalline volume fraction and electrical conductivity, compared to as-deposited film. While higher power (0.5 W/cm²) hydrogen plasma treatment for longer durations (up to 10 min) caused moderate improvement in crystalline fraction and electrical properties; however, the magnitude of improvement is not significant compared to low-power (0.1 W/cm²)/short-duration (10 s) plasma exposure. The results indicate that low-power hydrogen plasma treatment at room temperature can be an effective tool to improve the structural and electrical properties of nc-Si:H.     

One- and two-phonon Raman scattering from hydrogenated nanocrystalline silicon films

One- and two-phonon Raman scattering from intrinsic and boron as well as phosphorus doped hydrogenated nanocrystalline silicon films prepared by plasma enhanced chemical vapor deposition technique were investigated. With regard to one-phonon Raman measurements of intrinsic films, redshifts attending by asymmetrical broadening of one-phonon transverse optical (TO) mode with diminishing mean dimension of Si nanocrystals can be ascribed to incorporating effects of phonon confinement and tensile strain. Photoluminescence behavior of these intrinsic specimens can be interpreted by a consistent way when recombination of quantum-confined excitons in Si nanocrystals is assumed. As to one-phonon Raman signals of doped nc-Si:H materials, besides joint effects of phonon confinement and tensile strain, additional redshifts accompanying with asymmetrical broadening of one-phonon TO band with increasing doping level can be assigned to carrier effect and disorder from doping. With diminishing average size of Si nanocrystals or increasing dopants, the decay of two-phonon Raman amplitudes of intrinsic and doped samples can be attributed to disorder. Raman experiments indicate that all the energies of the two-phonon TO branches are different from twice the energies of the one-phonon TO active bands, which reveal that the two-phonon TO modes do not come from the Raman active phonons at wavevector k=0. The peak shift of two-phonon transverse optical (2TO) modes relates to phonon confinement and disorder. Negligible peak shift in TO (2TO) modes of intrinsic and doped films on temperature indicates that the interface strain in nc-Si:H/c-Si can be ignored.     

Reactive particle beam based deposition process of nano-crystalline silicon thin film at low temperature for the flexible AM-OLED backplane

A novel deposition process for depositing nano-crystalline silicon (nc-Si) thin films at low temperature was developed using reactive particle beam assisted chemical vapor deposition (RPB-CVD) for applications to the thin film transistor (TFT) backplane of flexible active matrix-OLEDs with plastic substrates. During the formation of nc-Si thin films by the RPB-CVD process with a silicon reflector electrode at low temperatures or room temperature, energetic particles could induce the formation of a crystalline phase in polymorphous Si thin films without additional substrate heating. The effects of the incident RPB energy controlled by the reflector bias were confirmed by Raman spectroscopy. The dark conductivity of polymorphous Si thin films increased with increasing reflector bias, whereas the ratio of photo and dark conductivity decreased monotonically. The optical band gap of the Si thin films also could be changed from amorphous to nano-crystalline by controlling the reflector bias. The first results of a primitive nc-Si TFT by RPB-CVD at room temperature demonstrate the technical potential of RPB-based processes as flexible TFT backplanes.     

Nanocrystalline silicon films with high conductivity and the application for PIN solar cells

Intrinsic nanocrystalline silicon films (nc-Si:H) were prepared by plasma enhanced chemical vapor deposition (PECVD) method. Films’ microstructures and characteristics were studied with Raman spectroscopy and Atom Force Microscope (AFM). The electronic conductivity of nc-Si:H films was found to be 4.9×10⁰Ω⁻¹ cm⁻¹, which was one order of magnitude higher thanthe reported 10⁻³-10⁻¹ Ω⁻¹ cm⁻¹. And PIN solar cells with nc-Si:H film as intrinsic thin-layer (ITO/n⁺-nc-Si:H/i-nc-Si:H/p-c-Si/Ag) were researched. The cell's performances were measured, the open-circuit voltage V_oc was 534.7 mV, short-circuit current I_sc was 49.24 mA (3 cm²) and fill factor FF was 0.4228.     

PECVD法生长的nc－Si：H膜电导率的研究

使用ＰＥＣＶＤ方法生长了ｎｃ－Ｓｉ：Ｈ膜，Ｘ射线衍射、Ｒａｍａｎ光谱和电镜观测表明样品具备了纳米结构特征。测量了样品在７７Ｋ～４００Ｋ温度范围的电导率，并使用二相随机分布有效介质理论，计算了ｎｃ－Ｓｉ：Ｈ膜中晶粒部分和晶界部分的电导率。对计算结果进行了理论分析，初步探讨了ｎｃ－Ｓｉ结构对其导电性能的影响，提出ｎｃ－Ｓｉ：Ｈ的高电导率来源于膜中纳米晶粒的小尺寸效应。     

Role of argon in hot wire chemical vapor deposition of hydrogenated nanocrystalline silicon thin films

Structural, optical and electrical properties of hydrogenated nanocrystalline silicon (nc-Si:H) films, deposited from silane (SiH₄) and argon (Ar) gas mixture without hydrogen by hot wire chemical vapor deposition (HW-CVD) method were investigated. Film properties are carefully and systematically studied as a function of argon dilution of silane (R_Ar). We observed that the deposition rate is much higher (4-23 Å/s) compared to conventional plasma enhanced chemical vapor deposited nc-Si:H films using Ar dilution of silane (0.5-0.83 Å/s). Characterization of these films with Raman spectroscopy revealed that Ar dilution of silane in HW-CVD endorses the growth of crystallinity and structural order in the nc-Si:H films. The Fourier transform infrared spectroscopic analysis showed that with increasing Ar dilution, the hydrogen bonding in the films shifts from di-hydrogen (Si-H₂) and (Si-H₂)_n complexes to mono-hydrogen (Si-H) bounded species. The hydrogen content in the films increases with increasing Ar dilution and was found to be < 4 at.% over the entire range of Ar dilutions of silane studied. However, the band gap shows decreasing trend with increase in Ar dilution of silane and it has been attributed to the decrease in the percentage of the amorphous phase in the film. The microstructure parameter was found to be > 0.4 for the films deposited at low Ar dilution of silane and ~ 0.1 or even less for the films deposited at higher Ar dilution, suggesting that there is an enhancement of structural order and homogeneity in the film. From the present study it has been concluded that the Ar dilution of silane is a key process parameter to induce the crystallinity and to improve the structural ordering in the nc-Si:H films deposited by the HW-CVD method.     

Influence of bias voltage on the optical and structural properties of nc-Si:H films grown by layer-by-layer (LBL) deposition technique

The effects of applying a positive bias of 25 to 100 V on the optical, structural and photoluminescence (PL) properties of hydrogenated nanocrystalline silicon (nc-Si:H) films produced by layer-by-layer (LBL) deposition technique has been studied. Optical characterization of the films has been obtained from UV-VIS-NIR spectroscopy measurements. Structural characterization has been performed using X-ray diffraction, micro-Raman spectroscopy and field emission scanning electron microscope (FESEM). PL spectroscopy technique has been used to investigate the PL properties of the films. In general, the films formed shows a mixed phase of silicon (Si) nanocrystallites embedded within an amorphous phase of the Si matrix. The crystalline volume fraction and grain size of the Si nanocrystallites have been shown to be strongly dependent on the applied bias voltage. High applied bias voltage enhances the growth rate of the films but reduces the refractive index and the optical energy gap of the films. Higher crystalline volume fraction of the films prepared at low bias voltages exhibits room temperature PL at around 1.8 eV (700 nm).     

Study on variable capacitance diode of (p)nc-Si:H/(n)c-Si heterojunction

Hydrogenated nanocrystalline silicon (nc-Si:H) layers of boron-doped increasing step by step was deposited on n-type crystalline silicon substrate using Plasma Enhanced Chemical Vapor Deposition (PECVD) system. After evaporating Ohm contact electrode on the side of substrate and on the side of nc-Si:H film, a structure of electrode/(p)nc-Si:H/(n)c-Si/electrode was obtained. It is confirmed by electrical measurement such as I-V curve, C-V curve and DLTS that this is a variable capacitance diode.     

Highly conductive p-type nanocrystalline silicon films deposited by RF-PECVD using silane and trimethylboron mixtures at high pressure

In this paper we present a study of boron-doped nc-Si:H films prepared by PECVD at high deposition pressure (≥4 mbar), high plasma power and low substrate temperature (≤200 °C) using trimethylboron (TMB) as a dopant gas. The influence of deposition parameters on electrical, structural and optical properties is investigated. We determine the deposition conditions that lead to the formation of p-type nanocrystalline silicon thin films with very high crystallinity, high value of dark conductivity (>7 (Ω cm)⁻¹) and high optical band gap (≥1.7 eV). Modeling of ellipsometry spectra reveals that the film growth mechanism should proceed through a sub-surface layer mechanism that leads to silicon crystallization.The obtained films are very good candidates for application in amorphous and nanocrystalline silicon solar cells as a p-type window layer.     

Influence of RF power on structural optical and electrical properties of hydrogenated nano-crystalline silicon (<Emphasis Type="Italic">nc</Emphasis>-Si:H) thin films deposited by PE-CVD

We report synthesis of hydrogenated nanocrystalline silicon (nc-Si:H) thin films by using conventional plasma enhanced chemical vapor deposition (PE-CVD) system from gas mixture of pure silane (SiH₄) and hydrogen (H₂). We investigated the effect of RF power on structural, optical and electrical properties using various characterization techniques including Raman spectroscopy, FTIR spectroscopy, UV–visible spectroscopy etc. Low angle XRD and Raman spectroscopy analysis revealed that the RF power in PE-CVD is a critical process parameter to induce nanocrystallization in Si:H films. The FTIR spectroscopy analysis results indicate that with increase in RF power the predominant hydrogen bonding in films shifts from Si–H to Si–H₂ and (Si–H₂)_n bonded species bonded species. However, the bonded hydrogen content didn’t show particular trend with change in RF power. The UV–visible spectroscopy analysis shows that the band tail width (E₀₄–E_Tauc) with increase in RF power. The defect density and Urbach energy also increases with increase in RF power. The highest dark conductivity (and lowest charge carrier activation energy) was obtained for the film deposited at RF power of 125 W indicating that 125 W is optimized RF power of our PE-CVD unit. At this optimized RF power nc-Si:H films with crystallite size ~3.7 nm having good degree of crystallinity (~86.7 %) and high band gap (E_Tauc ~ 2.01 eV and E₀₄ ~ 2.58 eV) were obtained with a low hydrogen content (6.2 at.%) at moderately high deposition rate (0.24 nm/s).     

Effects of hydrogen atmosphere on pulsed-DC sputtered nanocrystalline Si:H films

Hydrogenated nanocrystalline silicon (nc-Si:H) films were prepared by a pulsed-DC magnetron sputtering method under an atmosphere of hydrogen/argon mixture. The effects of hydrogen concentration on the structural and electrical properties of the films were systematically investigated using grazing incidence X-ray diffraction (GIXRD), Raman spectroscopy, and conductivity measurement. A threshold hydrogen concentration of about 70% was found necessary before any crystallinity was detectable. The deposition rate decreased monotonically with increasing hydrogen concentration, while the conductivity varied with crystallite size. The abnormally low conductivity level of these nc-Si:H films was due to the extraordinarily high defect density, which was attributed both to the enhanced ion bombardment from the pulsed-DC plasma and to the oxygen contamination from the target.     

氢化碳化硅(SixC1-x∶H)薄膜发光特性和纳米线发光增强研究

氢化碳化硅薄膜作为一种宽带隙的半导体材料,具有优越的物理特性,其在光电子器件上的潜在应用引起了人们的兴趣。利用等离子增强化学气相沉积(PECVD)系统制备了一系列氢化碳化硅薄膜,通过改变反应前驱物及流量比调节薄膜的室温光致发光性质。实验发现在一定范围内随着流量比R(CH_4/SiH_4)的提高,氢化碳化硅薄膜的光致发光峰位蓝移且发光强度增强;同时反应前驱物中的氢会极大影响氢化碳化硅薄膜的发光强度。通过椭偏仪(Ellipsometer)测量了薄膜的光学常数,发现薄膜沉积速率随着流量比R的增加而降低;傅里叶红外光谱仪(FTIR)测试表明Si-C有序度随着流量比的增加而增大。同时研究了三维纳米线结构对多态碳化硅薄膜发光性质的影响。光致发光测试结果表明三维纳米线结构可以有效提高薄膜的光致发光强度。     

Thermal stability of Al/nanocrystalline-Si bilayers investigated by in situ heating energy-filtered transmission electron microscopy

In situ heating energy-filtered transmission electron microscopy was employed to investigate the interfacial intermixing/reactions during thermal annealing of Al/nanocrystalline-Si (nc-Si) bilayers in the temperature range of 150–500 °C. In comparison with the Al/amorphous-Si (a-Si) bilayer, the Al/nc-Si bilayers were found to be much more stable against thermal annealing. Wetting and c-Si growth processes along Al grain boundaries, which take place during annealing of Al/a-Si bilayers, do not occur in Al/nc-Si bilayers, because of the lack of thermodynamic driving forces in the latter case. As a consequence, also in contrast with Al/a-Si bilayers, no layer exchange occurs in Al/nc-Si bilayers, not even after annealing at 500 °C. Instead, intermixing of Al/nc-Si is realized at the Al/nc-Si interface by the formation of Al spikes growing into the nc-Si sublayers at temperatures higher than 300 °C. The relatively low Al-spike formation temperature in Al/nc-Si systems, as compared with that for Al/single-crystalline Si systems, is ascribed to the higher Gibbs energy of nanocrystalline Si as compared to single-crystalline Si.