Microcrystalline Silicon Thin Films Research Articles

New two-step growth of microcrystalline silicon thin films without incubation layer

A new two-step growth method was proposed to fabricate microcrystalline silicon (μc-Si:H) thin films by an electron cyclotron resonance plasma enhanced chemical vapor deposition (ECR-PECVD). An ultra thin Si film was first deposited and followed by H 2 plasma treatment for few minutes, and then the μc-Si:H film was deposited on it. High-resolution transmission electron microscope (HRTEM) and Raman spectrometer were used to study the microstructures and the crystalline volume fraction of μc-Si:H films. The HRTEM results show that the amorphous silicon thin film with a thickness of 15 nm can be crystallized by H 2 plasma treatment in 2 min, and then it serves as the seed layer for the subsequent growth of μc-Si:H films. By optimizing the deposition parameters, the μc-Si:H film without amorphous incubation layer can be fabricated by this new two-step method and a proper crystalline volume fraction of 50.6% can be obtained.

The effect of disturbed PECVD electrode surfaces on the homogeneity of microcrystalline silicon films

In this work we present a novel electrode design for the plasma enhanced chemical vapor deposition of microcrystalline silicon thin films that enables optical access to the growing layer under normal incidence. The optical access is realized by piercing the electrode with a conical feed through of 10mm diameter at the electrode side facing the plasma. The influence of the feed through on deposition homogeneity is studied in different pressure regimes from 8mbar to 24mbar on intrinsic layers optimized for state of the art thin-film silicon solar cells. The homogeneity of the layers was determined by spatially resolved thickness measurements and evaluation of the crystalline volume fraction by Raman spectroscopy. With the aim to minimize the influence of the disturbance of the electrode surface the effect of different insets in the feed through on the homogeneity is studied. To find a maximum in optical transmission of the insets at optimal film homogeneity different designs of metallic grids were tested. We show that using the modified electrode it is possible to deposit microcrystalline silicon layers which are comparable in homogeneity to those fabricated with an unchanged standard electrode. This was achieved by covering only 19% of the area of the feed through by a metallic inset.

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 E 04 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/cm 2 with Ag back contacts. By applying highly reflective ZnO/Ag back contacts, j SC = 29.6 mA/cm 2 and η = 9.6% were achieved in a cell with a 2-μm-thick absorber layer.

Influence of ultrathin amorphous silicon layers on the nucleation of microcrystalline silicon films under hydrogen plasma treatment

Microstructures of phosphorus-doped hydrogenated microcrystalline silicon (μc-Si:H) thin films deposited by plasma-enhanced chemical vapor deposition are certainly dependent on the thickness of the H2 plasma-treated amorphous silicon (a-Si:H) layers. An ultrathin H-treated a-Si:H layer is beneficial in obtaining a very thin μc-Si:H film with high conductivity. Experimental results indicate that H2 plasma treatment induces the occurrence of high-pressure H2 in microvoids and causes compressive stress inside the ultrathin a-Si:H layers, thereby enhancing the generation of strained Si–Si bonds and nucleation sites and consequently accelerating the nucleation of μc-Si:H films.

Influence of front and back grating on light trapping in microcrystalline thin-film silicon solar cells

The optics of microcrystalline thin-film silicon solar cells with textured interfaces was investigated. The surface textures lead to scattering and diffraction of the incident light, which increases the effective thickness of the solar cell and results in a higher short circuit current. The aim of this study was to investigate the influence of the frontside and the backside texture on the short circuit current of microcrystalline thin-film silicon solar cells. The interaction of the front and back textures plays a major role in optimizing the overall short circuit current of the solar cell. In this study the front and back textures were approximated by line gratings to simplify the analysis of the wave propagation in the textured solar cell. The influence of the grating period and height on the quantum efficiency and the short circuit current was investigated and optimal grating dimensions were derived. The height of the front and back grating can be used to control the propagation of different diffraction orders in the solar cell. The short circuit current for shorter wavelengths (300-500 nm) is almost independent of the grating dimensions. For intermediate wavelengths (500 nm - 700 nm) the short circuit current is mainly determined by the front grating. For longer wavelength (700 nm to 1100 nm) the short circuit current is a function of the interaction of the front and back grating. An independent adjustment of the grating height of the front and the back grating allows for an increased short circuit current.

Development of a hollow cathode plasma source for microcrystalline silicon thin films deposition

The development of a hollow cathode plasma source and its implementation in an already existing plasma reactor used for the deposition of μc-Si:H thin films is presented. Electrical measurements of hydrogen discharges at 13.56 MHz were carried for two different in geometry hollow electrodes and the electrical parameters were compared with a previous Capacitively Coupled source installed in the same reactor. The results show a significant enhancement of discharge current and higher power dissipation for the same applied voltage. At reduced gas pressures that are necessary to minimize the dust formation, the hollow electrode with the larger cavity geometry showed better performance, achieving high electron densities of above 109 cm−3. Microcrystalline silicon thin films were deposited from SiH4/H2 discharges and an enhancement of the growth rate was found for the hollow cathode source as compared with the CCP source.

High-Efficiency Microcrystalline Silicon and Microcrystalline Silicon-Germanium Alloy Solar Cells

ABSTRACTThis paper presents our material studies on hydrogenated microcrystalline silicon (μc-Si:H) and microcrystalline silicon-germanium alloy (μc-Si1-xGex:H) thin films for the development of high efficiency p-i-n junction solar cells. In μc-Si:H solar cells, we have evaluated the structural properties of the intrinsic μc-Si:H layers grown by plasma-enhanced chemical vapor deposition at high deposition rates (>2 nm/s). Several design criteria for the device grade μc-Si:H are proposed in terms of crystallographic orientation, grain size and grain boundary passivation. Meanwhile, in μc-Si1-xGex:H solar cells, we have succeeded in boosting the infrared response of solar cell upon Ge incorporation up to x∼0.2. Nevertheless, a degradation of solar cell parameters is observed for large Ge contents (x>0.2) and thick i-layers (> 1 μm), which is attributed to the influence of the Ge dangling bonds that act as acceptorlike states in undoped μc-Si1-xGex:H. To improve the device performance, we introduce an oxygen doping technique to compensate the native defect acceptors in μc-Si1-xGex:H p-i-n solar cells.

Rf excited optical emission spectrum of radicals generated during hot wire chemical vapour deposition for the preparation of microcrystalline silicon thin film

To study the radicals behavior in the hot wire chemical vapour deposition (HWCVD) process for the preparation of microcrystalline Si (μc-Si: H) thin film, a weak radio frequency (rf) power was introduced to excite the radicals generated in HWCVD chamber. The spectrum of rf-excited HWCVD (rf-HWCVD) was obtained by subtracting the emission of hot wires from the spectrum measured by OES. The influence of the rf power on the rf-HWCVD spectrum can be neglected as the rf power density was less than 0.1 W/cm2. Under the same deposition parameters,the emission spectra for rf-HWCVD and plasma enhanced CVD (PECVD) processes are different. Under the low deposition pressure (7.5 Pa), the intensities of SiH* and Hα vary with the hot wire temperature reversely, which is characteristic of HWCVD with high gas dissociation rate and high concentration of atomic H. The ratio of intensity of Hα to SiH* in the emission spectrum of rf-HWCVD varying with deposition pressure is consistent with the crystalline fraction of μc-Si: H film. The results indicate that the optical emission spectroscopy measurement is a suitable method for the investigation of the HWCVD process excited by a weak rf-power.

Growth kinetics of plasma deposited microcrystalline silicon thin films

A continuum model for the nucleation and growth of microcrystalline silicon thin films from SiH4/H2 discharges is presented. The simulation considers mass balances and surface coverage with adspecies and silicon clusters up to the size where they can be considered as thermodynamically stable. The model is combined to a plasma gas phase simulator and a simulator of thin film morphology and is used for studying the growth differences in two different regions, the center and the edge of a 30×30cm2 substrate. The predictions for the film growth rate, film crystallinity and surface roughness in both regions are presented and discussed together with the main processes governing nucleation and growth and the slow step for stable nanocrystal formation.

On the oxidation mechanism of microcrystalline silicon thin films studied by Fourier transform infrared spectroscopy

Insight into the oxidation mechanism of microcrystalline silicon thin films has been obtained by means of Fourier transform infrared spectroscopy. The films were deposited by using the expanding thermal plasma and their oxidation upon air exposure was followed in time. Transmission spectra were recorded directly after deposition and at regular intervals up to 8 months after deposition. The interpretation of the spectra is focused on the Si–H x stretching (2000–2100 cm −1), Si–O–Si (1000–1200 cm −1), and O x Si–H y modes (2130–2250 cm −1). A short time scale (< 3 months) oxidation of the crystalline grain boundaries is observed, while at longer time scales, the oxidation of the amorphous tissue and the formation of O–H groups on the grain boundary surfaces play a role. The implications of this study on the quality of microcrystalline silicon exhibiting no post-deposition oxidation are discussed: it is not sufficient to merely passivate the surface of the crystalline grains and fill the gap between the grains with amorphous silicon. Instead, the quality of the amorphous silicon tissue should also be taken into account, since this oxidation can affect the passivating properties of the amorphous tissue on the surface of the crystalline silicon grains.

High mechanical and electrical reliability of bottom-gate microcrystalline silicon thin film transistors on polyimide substrate

Bottom-gate microcrystalline silicon thin film transistors (μc-Si:H TFTs) were fabricated by conventional 13.56 MHz RF plasma-enhanced chemical vapor deposition at 200 °C. In the high pressure depletion regime, the deposition rate of the μc-Si:H film is 24 nm/min and the amorphous incubation layer near the μc-Si:H/silicon nitride interface is unobvious. The crystalline fraction of μc-Si:H film with the thickness of 50 nm is 71%. From nano beam electron diffraction, the μc-Si:H film has a better crystalline order within a short-range lattice structure. The lattice parameter was measured to be 3.1 Å, which reflecting the lattice plane has a (111) direction. Finally, the μc-Si:H film was used as the active layer in TFTs structure. The field effect mobility, subthreshold swing and the threshold voltage are 0.95 cm 2/V, 0.85 V/dec. and 2.05 V, respectively. The output characteristic also shows no evidence of current crowing at low drain-source voltage ( V ds), implying good contact properties achieved with the n + a-Si:H source-drain ohmic contact layer. After 70 h 1 μA constant current stress, the threshold voltage shifts are 4.44 V and 0.42 V for the a-Si:H TFT and μc-Si:H TFT, respectively. The μc-Si:H thin film transistors show a better electrical stability than the amorphous silicon thin film transistors because of the lower defect density in the μc-Si:H film.

Carrier mobility, band tails and defects in microcrystalline silicon

The development of microcrystalline silicon thin films and devices is briefly reviewed. Transport mechanisms, and the attendant key parameters of carrier mobility, band-tail width and defect density, are linked to film structure and composition. In particular we discuss the wide (but systematic) variations in time-of-flight mobility and its unusual field-dependence. While microcrystalline silicon remains an inferior semiconductor to single-crystal silicon, we propose, and support by means of a computer model, that present device-grade material may be of sufficient quality to justify re-examining whether useful thin-film bipolar devices might be developed. These could find application as more sensitive photo-detectors, and as current drivers in organic LED displays and logic circuits.

Initial growth of intrinsic microcrystalline silicon thin film: Dependence on pre-hydrogen glow discharge and substrate surface morphology

We found the decreases of amorphous incubation volume from Raman spectra and surface roughness from AFM in hydrogenated microcrystalline silicon (μc-Si:H) films deposited with a pre-hydrogen glow discharge. The above phenomena are attributed to the increase in the nuclei density as observed by AFM measurements. Substrate surface morphology of eagle2000 glass modified by wet etching also has a positive effect on the nucleation and crystalline formation. In addition, μc-Si:H doped layer is also beneficial for decreasing the amorphous incubation layer thickness because of surface roughness and crystallinity in the μc-Si:H doped layer.

Investigations on Microcrystalline Silicon Films for Solar Cell Application

Hydrogenated microcrystalline silicon (µc-Si:H) thin film for solar cells is prepared by plasma-enhanced chemical vapor deposition and physical properties of the µc-Si:H p-layer has been investigated. With respect to stable efficiency, this film is expected to surpass the performance of conventional amorphous silicon based solar cells and very soon be a close competitor to other thin film photovoltaic materials. Silicon in various structural forms has a direct effect on the efficiency of solar cell devices with different electron mobility and photon conversion. A Raman microscope is adopted to study the degree of crystallinity of Si film by analyzing the integrated intensity peaks at 480, 510 and 520 cm ‒1 , which corresponds to the amorphous phase (a-Si:H), microcrystalline (µc-Si:H) and large crystals (c-Si), respectively. The crystal volume fraction is calculated from the ratio of the crystalline and the amorphous phase. The results are compared with high-resolution transmission electron microscopy (HR-TEM) for the determination of crystallinity factor. Optical properties such as refractive index, extinction coefficient, and band gap are studied with reflectance spectra.

Impact of front and rear texture of thin-film microcrystalline silicon solar cells on their light trapping properties

The effect of front and rear texture of thin-film microcrystalline silicon solar cells on light trapping is evaluated by characterizing solar cell specimens with both superstrate (p-i-n) and substrate (n-i-p) configurations that have a variety of surface morphologies including intentionally polished flat surfaces. It is demonstrated that the front texture enhances light absorption and external quantum efficiency from the visible region to the near-infrared region, while the rear texture increases these properties only at wavelengths longer than around 600 nm. The photocurrent enhancement by the rear texture is comparable or superior to that by the front texture, especially in n-i-p solar cells with a thin transparent conductive oxide (TCO) layer on the front surface. Irrespective of the cell configuration, parasitic absorption loss in solar cells is increased by the textures. Loss analyses show that the absorption loss at textured back-surface reflectors (BSRs) plays a dominant role in n-i-p solar cells and is obviously affected by the localized surface plasmon absorption induced by the Ag reflector with microroughness on its surface. In p-i-n solar cells, additional absorption loss due to the thick front TCO layers is superimposed on that induced by the textured BSR and becomes dominant with increasing wavelengths.

Effect of radio frequency power and total mass-flow rate on the properties of microcrystalline silicon films prepared by helium-diluted-silane glow discharge

Hydrogenated microcrystalline silicon thin films have been prepared by plasma-enhanced chemical vapor deposition at relatively low deposition temperatures (180 °C). Helium dilution of silane, instead of the more commonly approach of hydrogen dilution, has been used to promote microcrystalline growth. The effect of the applied radio frequency power (RFP) and the total gas flow on the structural, optical and electrical characteristics have been studied. As observed from the structural measurements, microcrystalline growth is favored as the applied RFP is increased and/or the total gas flow is decreased. Increasing the RFP however, brings associated an increase in the defect density in the amorphous tissue surrounding the crystalline grains and/or an increase in intra-grain defects as deduced from the structural, optical and electrical measurements. Microcrystalline growth and defect formation is rationalize in terms of the He* deexcitation process and high energy He + ions bombardment.

The light stability of microcrystalline silicon thin films deposited by VHF–PECVD method

Microcrystalline silicon thin film is deposited under different conditions by plasma enhanced chemical vapor deposition. The light stability with different crystallinity and grain size is studied, and the growth mechanism is analyzed using the scaling behavior of roughening surface evolution. Degradation of photoconductivity mainly depends on crystallinity and grain size, but fundamentally, on the growth mechanism. Materials with high crystallinity and large grain size are more stable under light soaking. With the increasing of deposition pressure and input power, growth process transfers to zero diffusion limit growth mechanism, and films deposited present less grain size and poor light stability.

Film-thickness dependence of structural and electrical properties of boron-doped hydrogenated microcrystalline silicon prepared by radiofrequency magnetron sputtering

Abstract Boron-doped hydrogenated microcrystalline silicon (μc-Si:H) thin films were prepared by radiofrequency magnetron sputtering and the film-thickness dependence of the structural and electrical properties was investigated. The target was a boron-doped silicon wafer on which boron grains were put or not on top of it. At room temperature, the dark conductivities of the μc-Si:H thin films prepared without boron grains were below 10 − 6 S cm − 1 . On the other hand, for the films prepared with boron grains having thickness above 50 nm, the conductivities were higher than 6 × 10 − 1 S cm − 1 and their activation energies were about 0.05 eV. As the film thickness was decreased, the dark conductivity decreased: ∼ 10 − 1 S cm − 1 for the 20 nm film and ∼ 10 − 6 S cm − 1 for the 10 nm film. This decrease was caused by the decrease in the crystalline volume fraction.

The effect of initial discharge conditions on the properties of microcrystalline silicon thin films and solar cells

This paper studies the effects of silane back diffusion in the initial plasma ignition stage on the properties of microcrystalline silicon (μc-Si:H) films by Raman spectroscopy and spectroscopic ellipsometry, through delaying the injection of SiH4 gas to the reactor before plasma ignition. Compared with standard discharge condition, delayed SiH4 gas condition could prevent the back diffusion of SiH4 from the reactor to the deposition region effectively, which induced the formation of a thick amorphous incubation layer in the interface between bulk film and glass substrate. Applying this method, it obtains the improvement of spectral response in the middle and long wavelength region by combining this method with solar cell fabrication. Finally, results are explained by modifying zero-order analytical model, and a good agreement is found between the model and experiments concerning the optimum delayed injection time.

Enhanced Nucleation of Microcrystalline Silicon Thin Films Deposited by Inductively Coupled Plasma Chemical Vapor Deposition with Low-Frequency Pulse Substrate Bias

Microcrystalline silicon (µc-Si) films were deposited by inductively coupled plasma chemical vapor deposition with a low-frequency and low-duty pulse substrate bias (PSB). The crystallinity of the films was significantly improved by the PSB. In the case of the low-frequency and low-duty PSB, the duty ratio affected the crystallinity more than the negative peak voltage. Cross-sectional transmission electron microscopy measurements revealed that the nucleation density at the µc-Si/glass interface was increased by the PSB. This technique will be useful in fabricating high-performance bottom-gate µc-Si thin-film transistors for large-area electronics.