Incubation Layer Thickness Research Articles

The microstructure evolution of hydrogenated microcrystalline germanium promoted by power gradient method

This paper studies the microstructure evolution of hydrogenated microcrystalline germanium (μc-Ge:H) thin films deposited by plasma enhanced chemical vapor deposition (PECVD). There is an amorphous incubation layer formed in the initial deposition stage of μc-Ge:H thin film. It is demonstrated that the thickness of incubation layer can be reduced by high hydrogen dilution and high discharge power method. However, at high hydrogen dilution, the deposition rate of μc-Ge:H appears a sharply decrease. Using a high discharge power can compensate the deposition rate decrease but lead to decrease of average grain size and appearance of micro-void in the μc-Ge:H thin film. In addition, by comparing two thickness groups of μc-Ge:H thin films deposited at different discharge powers, it is noticed that the evolution process relates to the formation of crystal nucleuses. Thus, a power gradient method is proposed to understand the mechanism of nucleation and crystal growth in the initial deposition process of μc-Ge:H films. Finally, by power gradient method, the incubation layer thickness of μc-Ge:H thin films has been decreased to less than 6nm. Moreover, Raman scattering spectra shows a 38nm μc-Ge:H film has a crystal fraction (XC) of 62.4%. Meanwhile, the mobility of TFT devices shows the improved electrical property of μc-Ge:H film deposited by power gradient method.

Nucleation of microcrystalline silicon: on the effect of the substrate surface nature and nano-imprint topography

The nucleation of microcrystalline silicon thin-films has been investigated for various substrate natures and topographies. An earlier nucleation onset on aluminium-doped zinc oxide compared to glass substrates has been revealed, associated with a microstructure enhancement and reduced surface energy. Both aspects resulted in a larger crystallite density, following classical nucleation theory. Additionally, the nucleation onset was (plasma deposition) condition-dependent. Therefore, surface chemistry and its interplay with the plasma have been proposed as key factors affecting nucleation and growth. As such, preliminary proof of the substrate nature’s role in microcrystalline silicon growth has been provided. Subsequently, the impact of nano-imprint lithography prepared surfaces on the initial microcrystalline silicon growth has been explored. Strong topographies, with a 5-fold surface area enhancement, led to a reduction in crystalline volume fraction of ~20%. However, no correlation between topography and microstructure has been found. Instead, the suppressed crystallization has been partially ascribed to a reduced growth flux, limited surface diffusion and increased incubation layer thickness, originating from the surface area enhancement when transiting from flat to nanostructured surfaces. Furthermore, fundamental plasma parameters have been reviewed in relation with surface topography. Strong topographies are not expected to affect the ion-to-growth flux ratio. However, the reduced ion flux (due to increasing surface area) further limited the already weak ion energy transfer to surface processes. Additionally, the atomic hydrogen flux, i.e. the driving force for microcrystalline growth, has been found to decrease by a factor of 10 when transiting from flat to nanostructured topography. This resulted in an almost 6-fold reduction of the hydrogen-to-growth flux ratio, a much stronger effect than the ion-to-growth flux ratio. Since previous studies regarding crystalline growth stated the necessity for enhanced ion- and atomic hydrogen-to-growth flux ratios, reduction of the latter is suggested to be the main cause of suppressed crystallization kinetics.

Nanocrystalline Silicon Oxide Emitters for Silicon Hetero Junction Solar Cells

In this study we developed a hydrogenated nanocrystalline silicon oxide (p)nc-SiOx:H as an emitter window layer for silicon heterojunction solar cells. We investigated the variation of refractive index and crystalline volume fraction at different growth conditions by Plasma Enhanced Chemical Vapor Deposition (PECVD) and we show that combining a low refractive index (n ∼ 2.65) and low parasitic absorption the (p)nc-SiOx:H emitter can replace the standard (p)a-Si:H, which leads to a short circuit current increase of up to 4%. We also show a method to reduce the incubation layer thickness in the initial stage of growth using a CO2 plasma treatment of the intrinsic amorphous layer surface prior to the emitter deposition. Lifetime measurements prove that the plasma treatment and the emitter layer deposition did not compromise the passivation layer quality. Moreover, in order to improve the p-emitter/n-type TCO contact, a highly doped nc-Si:H layer is implemented on top of the emitter, which leads to lower series resistance (Rs,light) and higher fill factor (FF) without affecting the open circuit voltage (Voc).

Enhanced Performance of Flexible Nanocrystalline Silicon Thin-Film Solar Cells Using Seed Layers with High Hydrogen Dilution

Flexible hydrogenated nanocrystalline (nc-Si:H) thin-film solar cells were prepared by very high frequency plasma enhanced chemical vapor deposition (VHF-PECVD), and the effect of highly crystalline intrinsic Si seed layers at the initial growth stage of i nc-Si:H absorbers on their structural and electrical properties and on the performance of solar cells was investigated. The crystallization of i nc-Si:H absorbers was significantly enforced by the introduction of highly crystalline seed layers, resulting in the reduction of defect-dense a-Si:H grain boundary and incubation layer thickness. The open circuit voltage of the nc-Si:H solar cells with the seed layers was improved by the decrease of charged defect density in the defect-rich amorphous region.

Study of Nanocrystalline Silicon Films Synthesized Below 100 °C by Catalytic Chemical Vapor Deposition

Nanocrystalline silicon (nc-Si) films were synthesized by catalytic chemical vapor deposition at a low substrate temperature (100 degrees C) for use as an active layer in bottom-gate thin-film transistors. The hydrogen-dilution technique was employed to increase the crystalline volume fraction of the synthesized films. The incubation layer thickness was estimated to be 5.1 nm for a hydrogen-dilution ratio, R(H) (= [H2]/[SiH4]), of 54. When R(H) was increased from 64 to 74, the deposition rate decreased from 20 to 0.5 nm/min. In order to achieve a high deposition rate and high crystallinity near the interface region, we modulated R(H) through the film thickness. We also fabricated metal-insulator-semiconductor-insulator-semiconductor diodes from multilayer structures consisting of an nc-Si layer sandwiched between two silicon nitride layers. By analyzing the capacitance-voltage characteristics of these diodes, we found that the hysteresis and rectifying behavior of these diodes were affected by the the nc-Si layer thickness.

Characteristiscs of Nanocrystalline Silicon Films Deposited by Cat-CVD below 100 °C

Nanocrystalline Silicon (nc-Si) films were deposited by catalytic chemical vapor deposition (Cat-CVD) at a low substrate temperature (100 °C) for use as an active layer in bottom-gate thin-film transistors (TFTs). Hydrogen dilution technique was attempted to increase the crystalline volume fraction (Xc). The hydrogen dilution ratio, RH = [H2]/[SiH4], was varied from 44 to 74. In order to obtain nc-Si film with a high XC, a thin incubation layer and a fast deposition rate simultaneously, we varied RH and pressure as the deposition proceeded. When the pressure was set to 50 mTorr during the initial stage and changed to 60 mTorr for the rest of the deposition, the incubation layer thickness was suppressed to 4 nm.

Characteristiscs of Nanocrystalline Silicon Films Deposited by Cat-CVD Below 100 ℃

Nanocrystalline Silicon (nc-Si) films were deposited by catalytic chemical vapor deposition (Cat-CVD) at a low substrate temperature (100 {degree sign}C) for use as an active layer in bottom-gate thin-film transistors (TFTs). Hydrogen dilution technique was attempted to increase the crystalline volume fraction (Xc). The hydrogen dilution ratio, RH=[H2]/[SiH4], was varied from 2 to 74. Xc of the nc-Si films was estimated by Raman spectrum. The incubation-layer thickness was estimated by transmission electron spectroscopy (TEM). Incubation layer was limited up to 5.26 nm and crystallinity was 71 %.

Structural investigation of nC-Si/SiOx:H thin films from He diluted (SiH4 + CO2) plasma at low temperature

Detail structural characterizations of the nC-Si/SiOx:H films prepared from low temperature (300°C) SiH4 plasma, have been performed using various spectroscopic and microscopic probes, e.g., IR spectroscopy, ellipsometry, scanning electron microscopy and atomic force microscopy. The growth structure has been probed by Bruggeman effective medium approximation fitting to the ellipsometry data, considering a three-layer growth model, which has been identified by FESEM studies. It has been observed that with the reduction in pressure (p) the overall crystallinity improves along with the lowering in the incubation layer thickness, and the reduction of void fraction in the bulk as well as in the growth zone and surface layer. The maximum crystallinity in the bulk has been identified at p=0.6Torr, corresponding to the lowest roughness on the surface. Oxygen incorporation has been found to be favored at lower gas pressure in the plasma, along with simultaneous dehydrogenation of the silicon network which remains the key criteria for attaining enhanced nanocrystallinity. Plausible formation mechanism of the nC-Si/SiOx:H structure, activated by chemical reactions occurring in the He diluted (SiH4+CO2) plasma has been investigated.

Key issues for high-efficiency silicon thin film solar cells prepared by RF-PECVD under high-pressure-depletion conditions

Our recent work on deposition and characterization of hydrogenated microcrystalline silicon (μ c-Si:H) thin films and silicon thin film solar cells prepared by RF-PECVD under high-pressure-depletion conditions is summarized in this paper. Several key issues are studied in detail: 1) process windows for device-quality μ c-Si:H thin films, 2) formation mechanism of amorphous silicon incubation layer and the effective methods to reduce the incubation layer thickness, 3) modification of crystalline fraction volume of intrinsic μ c-Si:H layers and its influence on the device performance of μ c-Si:H solar cells, 4) deposition of high conductive p-type μ c-Si:H window layers with high crystalline fraction volume, and the influence of p-layer on the device performance. After solving the above key issues, a high efficiency of 8.16% is obtained for μ c-Si:H sing-junction solar cell with intrinsic layer prepared by RF-PECVD under high-pressure-depletion conditions. When it is used as bottom cell in a-Si:H/μ c-Si:H tandem solar cell, the efficiency of tandem cell reaches 11.61%.

Microcrystalline Silicon Solar Cells Deposited at High Rates in a Single Chamber

Abstract Hydrogenated microcrystalline silicon(μc-Si:H) thin films with deposition rate of over 2 nm/s were prepared by very high frequency plasma-enhanced chemical vapor deposition under high working pressure and high power, using a shower-head cathode. The effects of working pressure and electrode distance on the microcrystalline silicon growth rate and crystallization fraction were studied. We found that thin films with high deposition rate (Rd) and simultaneously proper crystallization fraction could be obtained only when these two parameters have good matches. After that, the intrinsic layer was further optimized by modifying silane concentration. The performance of solar cells was improved by controlling the silane input time which reduced the incubation layer thickness and then improved the quality of the p/i interface. After this initial optimization, a μc-Si:H singlejunction p-i-n solar cell with the conversion efficiency of 5.3% was deposited at the Rd of 2 nm/s in a single chamber.

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.

Structural evolution optimization at p/i interface and in the bulk intrinsic‐layer for high efficiency microcrystalline silicon solar cells

AbstractThe structural evolution in high‐rate deposited hydrogenated microcrystalline silicon (μc‐Si:H) films and solar cells has been investigated. In order to reduce the incubation layer thickness, a high‐quality initial seed layer at p/i interface has been developed by lowering silane concentration and very high frequency (VHF) power simultaneously. This initial seed layer not only acts as a seed layer for the high‐rate deposited i ‐layer, but also it can reduce the ion bombardment on the p/i interface. Then, a novel VHF power profiling technique, which is designed by dynamically decreasing the VHF power step by step during the deposition of μc‐Si:H, has been developed to control the structural evolution along the growth direction in the bulk i ‐layer. Another advantage of this VHF power profiling technique is the reduced ion bombardments on growth surface because of decreasing the VHF power. At last, a high conversion efficiency of 9.4% has been obtained for a single junction μc‐Si:H p‐i‐n solar cell with i ‐layer deposited at average deposition rate over 10 Å/s (© 2010 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

A new method used to control the structure of high rate microcrystalline silicon thin films

AbstractWe report a systematic study of plasma heating effect on microcrystalline silicon (μc‐Si:H) deposition. Normally, substrate surface temperature increases with time during a high rate deposition of μc‐Si:H thin film, especially under a high power and high pressure condition. We deposited μc‐Si:H films using a very high frequency discharge under the high pressure and high power condition at a fixed heater temperature or a profiled heater temperature. Raman spectra with different wavelength excitations showed that a proper heater temperature profiling during μc‐Si:H deposition is an effective method to modify the structure of μc‐Si:H films, which can control the structure evolution to form a uniform crystallinity along the growth direction and reduce the amorphous incubation layer thickness (© 2010 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Influence of n-type layer structure on performance and light-induced degradation of n-i-p microcrystalline silicon solar cells

A series of n-i-p microcrystalline silicon thin film solar cells with different values of crystalline volume fraction Xc of n-type layers are prepared by radio frequency plasma enhanced chemical vapor deposition. It is found that the structure of intrinsic layer is strongly dependent on the structure of n-type layer, especially the incubation layer thickness at n/i interface and Xc of intrinsic layer. This series of solar cells were light-soaked under 100 mW/cm2 for 400 h. The experiment results demonstrate that the solar cell with the highest Xc of intrinsic layer (Xc(i)=65%) has the lowest light-induced degradation ratio. Then the solar cell with n-type layer deposited in an amorphous silicon/microcrystalline silicon transition region (Xc(i) =54%) is light-soaked under the irradiations of white light, red light and blue light with the same light intensities, separately. After 400 h light-soaking, the light degradation ratio is only 2% for the red light irradiation, while it is 8% for the blue light irradiation.

The Characterization of Bottom-gate Thin Film Transistors adapted Nanocrystalline Silicon as Active Layer by Catalytic CVD at Low Temperature

The nanocrystalline silicon bottom-gate thin film transistor (nc-si TFT) should improve on formed amorphous incubation layers at the onset of deposition because these layers deteriorate the performance of the transistor. We attempted modulation of hydrogen dilution ratio to achieve both the minimal incubation layer and high deposition rate. The incubation layer thickness was estimated by transmission electron microscopy (TEM) and crystallization fraction was measured by Raman spectroscopy.

Influence of the deposition pressure on the preparation of μc-Si:H thin films in hot-wire-assisted MWECR-CVD system

Considering the important influence of the deposition pressure on the growth of thin films, such as deposition rate, crystalline volume fraction and density, etc., and based on the analysis of the advantages and disadvantages on the mono-pressure method, we proposed a new method of high- and low-pressure combination to prepare hydrogenated microcrystalline silicon (μc-Si:H) films, i.e. at first we used high pressure to deposit film in 2 min in order to minish the thickness of incubation layer from the amorphous phase transition to crystalline phase, and then used low pressure to deposit film in 18 min to improve the density and decrease the oxidation of the film. The experimental results showed that using this new method the thin film with high crystalline volume fraction of 61% and low light-induced degradation ratio of 5.6% at 210 min was obtained, and meanwhile, it also possessed higher density and better photoelectronic properties than mono-pressure method.

The influence of n-layer on structural properties of i-layer in n-i-p μc-Si∶H thin film solar cells

The intrinsic layers with different thickness were deposited on amorphous n-type layers and microcrystalline n-type layers respectively, and all samples were prepared by radio frequency plasma enhanced chemical vapor deposition (RF-PECVD). The effect of different n-type layers on structural properties of i-layer was studied. It is concluded that the highly microcrystalline n-layer can reduce the thickness of incubation layer, promote structural uniformity of i-layer and improve the performance of n-i-p μc-Si:H thin film solar cells.

Formation mechanism of incubation layers in the initial stage of microcrystalline silicon growth by PECVD

The incubation layers in microcrystalline silicon films (μc-Si:H) are studied in detail. The incubation layers in μc-Si:H films are investigated by bifacial Raman spectra, and the results indicate that either decreasing silane concentration (SC) or increasing plasma power can reduce the thickness of incubation layer. The analysis of the in-situ diagnosis by plasma optical emission spectrum (OES) shows that the emission intensities of the SiH*(412 nm) and Hα (656 nm) lines are time-dependent, thus SiH*/Hα ratio is of temporal evolution. The variation of SiH*/Hα ratio can indicate the variation in relative concentration of precursor and atomic hydrogen in the plasma. And the atomic hydrogen plays a crucial role in the formation of μc-Si:H; thus, with the plasma excited, the temporal-evolution SiH*/Hα ratio has a great influence on the formation of an incubation layer in the initial growth stage. The fact that decreasing the SC or increasing the plasma power can decrease the SiH*/Hα ratio is used to explain why the thickness of incubation layer can reduce with decreasing the SC or increasing the plasma power.

Micro-Raman and ultraviolet ellipsometry studies on μc-Si:H films prepared by H2 dilution to the Ar-assisted SiH4 plasma in radio frequency glow discharge

Micro-Raman and ultraviolet ellipsometry studies have been performed on μc-Si:H films prepared by increasing the H2 dilution to the Ar-assisted SiH4 plasma in rf glow discharge. Combining the results obtained from the Bruggeman effective medium approximation fitting to the ellipsometry data and the Gaussian deconvolution of Raman spectra, it has been observed that the overall crystallinity improves along with the lowering in the incubation layer thickness, elimination of the amorphous component from the bulk, and the reduction of void fraction in the bulk as well as in the growth zone and surface layer. However, at very high H2/Ar ratio in the plasma a lowering in the crystallinity has been recorded along with an associated increase in the voids and an appearance of a small amorphous component in the bulk of the material. An increase in the voids arising at the grain boundary zone causes the hindrance to the crystallization in the network and is the result of higher H2 dilution, beyond a certain level, to the Ar-assisted SiH4 plasma, in the formation of a Si:H network. A correlation has been established between the data obtained from micro-Raman and ellipsometry in the structural characterization of a silicon–hydrogen system.

Growth and characterization of microcrystalline silicon–germanium films

Microcrystalline (μc) thin films of Si 1− x Ge x :H (0.16⩽ x⩽0.89) were prepared on quartz, Cr-evaporated Si(1 0 0) and H-terminated Si(1 0 0) by plasma-enhanced chemical vapor deposition (CVD) from SiH 4+GeH 4+H 2 gas mixtures with various GeH 4/SiH 4 molar fractions and substrate temperatures. The crystallinity of the films was characterized by Raman scattering spectroscopy and transmission electron microscopy (TEM). We have demonstrated the formation of a highly crystallized and structurally relaxed Si 1− x Ge x network with nearly homogeneous composition in the temperature range 200–300 °C. However, for the deposition on quartz and Cr-evaporated Si(1 0 0), the crystallite nuclei emerge after the growth of the amorphous layer as thick as 25 nm which is markedly larger than the incubation layer thickness for μc-Si:H (typically ∼5 nm under a similar deposition condition).