Increasing Hydrogen Dilution Research Articles

Hydrogen effect on structural and optical properties of in-situ fabricated nc-Si:H prepared by facing targets sputtering

We report high-quality hydrogenated nanocrystalline silicon (nc-Si:H) thin films were prepared by facing targets sputtering (FTS) in the environment of Ar and H2 mixture gases. By increasing hydrogen dilution (RH) of FTS deposition, the structural evolution of as-fabricated Si-based films was observed. Fourier infrared spectroscopy (FTIR) shows that decrease in bonded hydrogen (CH) concentration, especially that of (Si-H2)n with increasing RH, indicating that the H etching effect increased atomic density of these films, thus improving film quality. With the RH increased, the Raman spectra show that the morphological transition from amorphous to crystalline states. We further reveal correlations between the degree of structural order in the amorphous network and the RH by calculating experimental data from the Raman measurements. Moreover, as-fabricated nc-Si:H films show large bandgap and narrow band tail states with increasing RH, which allows potential applications in the window passivation layer for HIT solar cells and Si-based charge-coupled devices (CCD). This work offers a novel route to fabricate high-quality nc-Si:H films for optoelectronic applications.

Ion bombardment measurements and simulations of a low temperature VHF PECVD SiH4–H2 discharge in the a‐Si:H to μc‐Si:H transition regime

We studied ion bombardment during amorphous silicon layer deposition for hydrogen dilutions 5 to 59 with mass resolved IED measurements and simulations. The trends in the peak position of H2+ and SiHy+ IEDs with increasing hydrogen dilution show good agreement between measurements and simulations. A difference in asymmetry of the discharge between simulations and measurements results in a roughly 6 eV lower peak position for the simulations. An increasing SiHy+ ion flux with increasing hydrogen dilution is measured. We hypothesize that this is due to amorphous silicon etching that is enhanced by Hy+ ion bombardment.

Light induced degradation of amorphous silicon containing nanocrystalline silicon

The effect of light soaking (LS) is studied in hydrogenated amorphous silicon (a-Si:H) containing different amounts of nanocrystals, prepared by varying the deposition conditions in a plasma enhanced chemical vapor deposition (PECVD) system. We find that the presence of nanocrystals (nc-Si) stabilizes the a-Si:H against LS. The long range potential fluctuations (LRPF) are measured and we find that they become smaller as the fraction of nanocrystals (χ) in the films increases. Raman spectra show a peak at 490 cm−1, which is taken to be the result of Intermediate Range Order (IRO) in the films. This IRO peak appears just when the hydrogen dilution is sufficiently high for the crystallization to begin, and increases with increasing hydrogen dilution as the crystallization increases. In contrast LRPF have maximum width when χ is zero and it decreases as χ increases. Further, the degradation upon LS is negligible when χ > 25%. We suggest an alternative mechanism for the improved stability, in which the photogenerated carriers moving under the influence of the long range potential fluctuations have a propensity to go to nc-Si, and recombine in that region.

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.

Growth characteristics of amorphous-layer-free nanocrystalline silicon films fabricated by very high frequency PECVD at 250°C

Amorphous-layer-free nanocrystalline silicon films were prepared by a very high frequency plasma enhanced chemical vapor deposition (PECVD) technique using hydrogen-diluted SiH4 at 250°C. The dependence of the crystallinity of the film on the hydrogen dilution ratio and the film thickness was investigated. Raman spectra show that the thickness of the initial amorphous incubation layer on silicon oxide gradually decreases with increasing hydrogen dilution ratio. High-resolution transmission electron microscopy reveals that the initial amorphous incubation layer can be completely eliminated at a hydrogen dilution ratio of 98%, which is lower than that needed for the growth of amorphous-layer-free nanocrystalline silicon using an excitation frequency of 13.56 MHz. More studies on the microstructure evolution of the initial amorphous incubation layer with hydrogen dilution ratios were performed using Fourier-transform infrared spectroscopy. It is suggested that the high hydrogen dilution, as well as the higher plasma excitation frequency, plays an important role in the formation of amorphous-layer-free nanocrystalline silicon films.

Research on Nanocrystalline Silicon Film Solar Cells

Nanocrystalline silicon film has become the research hit of today’ s P-V solar technology. It’s optical band gap was controlled through changing the grain size and crystalline volume fraction for the quanta dimension effect. The crystalline volume fraction in nc-Si:H is modulated by varying the hydrogen concentration in the silane plasma. Also, the crystallinity of the material increases with increasing hydrogen dilution ratio, the band tail energy width of the nc-Si:H concurrently decreases. Two sets of nc-Si:H solar cells were made with different layer thicknesss, their electronic and photonic bandgap, absorption coefficient, optical band gap, nanocrystalline grain size(D), and etc have been stuied. In addition, we discussed the relationship between the stress of nc-Si thin films and H2 ratio. At last nc-Si:H solar cells have been designed and prepared successfully in the optimized processing parameters.

Hydrogenated Amorphous Silicon Oxide Solar Cells Fabricated near the Phase Transition between Amorphous and Microcrystalline Structures

We investigated the properties of hydrogenated amorphous silicon oxide (a-Si1-xOx:H) deposited near the phase transition between amorphous and microcrystalline structures. a-Si1-xOx:H films were prepared by plasma-enhanced chemical vapor deposition using a gas mixture of silane, hydrogen, and carbon dioxide. The film structure was changed from amorphous to microcrystalline phase by increasing hydrogen dilution. Optical and electrical characterizations revealed that wide-gap a-Si1-xOx:H films were deposited under phase transition conditions. We also fabricated a-Si1-xOx:H single-junction p–i–n solar cells by varying the hydrogen dilution for the i-layer. The solar cells showed a maximum open circuit voltage of 1.04 V (Jsc=7.92 mA/cm2, FF=0.64, Eff=5.2%) when the i-layer was deposited under phase transition conditions.

Surface morphology evolution of microcrystalline silicon p-layer prepared by RFPECVD

Surface morphology and crystallinity of silicon p-layer have been investigated on the effect of various deposition conditions of RFPECVD in order to clarify crystalline nuclei evolution. Crystallinity increased with increasing hydrogen dilution, but decreased with increasing both RF input power and diborane doping. Microcrystalline nuclei size increased with increasing working pressure, RF input power and substrate temperature. However, surface morphology was not correlated with crystallinity or deposition rate. A film with relatively large nuclei but having rather low crystallinity can be prepared for some condition, and vice versa. In order to get a dense microcrystalline silicon intrinsic layer subsequently deposited, surface morphology control of microcrystalline p-layer seems to be important as well as crystallinity.

Experimental and theoretical study of the evolution of surface roughness in amorphous silicon films grown by low-temperature plasma-enhanced chemical vapor deposition

Using real-time spectroscopic ellipsometry the evolution of the surface roughness of amorphous silicon thin films grown by low-temperature $(200\text{ }\ifmmode^\circ\else\textdegree\fi{}\text{C})$ plasma-enhanced chemical vapor deposition (PECVD) at high process gas pressure (3 mTorr) has been studied as a function of the hydrogen dilution gas-flow ratio ${R}_{H}=[{\text{H}}_{2}]/[{\text{SiH}}_{4}]$ with $15\ensuremath{\le}{R}_{H}\ensuremath{\le}60$. To describe the roughness evolution, we have used a $3\mathrm{D}$ linearized continuum equation which includes a negative surface-tension term to take into account the destabilizing effects of short-range attraction and/or shadowing, as well as a smoothing term corresponding to surface diffusion. Using this model we have obtained very good agreement with experimental results for the evolution of the surface roughness in the case of large dilution ratio. However, our results indicate that for small dilution ratio the surface slopes are significantly larger and as a result additional nonlinear terms need to be included at large thicknesses. Our results also indicate that surface diffusion plays an important role during PECVD film growth while the diffusion rate increases with increasing hydrogen dilution ratio. We also find that the early stages of island nucleation play an important role in determining the subsequent roughness evolution. In particular, the assumption of a large wetting angle $({\ensuremath{\theta}}_{W}\ensuremath{\simeq}90\ifmmode^\circ\else\textdegree\fi{})$ for the $3\mathrm{D}$ islands formed in the initial stages leads to significantly better agreement with experiments than a smaller wetting angle $({\ensuremath{\theta}}_{W}\ensuremath{\simeq}45\ifmmode^\circ\else\textdegree\fi{})$. This is consistent with recent experiments on liquid Si droplets on ${\text{SiO}}_{2}$ [H. Kanai et al., J. Mater. Sci. 42, 9529 (2007)] substrates in which a wetting angle of $90\ifmmode^\circ\else\textdegree\fi{}$ was observed.

Gas Phase Conditions for Obtaining Device Quality Amorphous Silicon at Low Temperature and High Deposition Rate

AbstractThe aim of this paper is to find a parameter space for deposition of amorphous silicon films at low substrate temperature by VHF PECVD process for application in solar cell fabrication on cheap plastics. Our studies show that at lower substrate temperature, keeping the pressure constant, the ion energy flux reaching the growth surface decreases, which we partly attribute to increasing gas phase collisions arising from an increase in gas density. The role of hydrogen is two fold: (1) higher hydrogen dilution increases the ion energy and restores it to its required value at low temperatures; (2) a normal to dusty plasma transition occurs at lower hydrogen to silane flow ratio and this transition regime shifts to higher dilution ratios for lower substrate temperatures. Thus the role of high hydrogen dilution at low temperature is to avoid the dusty regime. Thus the role of high hydrogen dilution at low temperature is to avoid the dusty regime. The ion energy flux at low substrate temperature can also be restored to the value obtained at high substrate temperature, without increasing hydrogen dilution, by simply lowering the chamber pressure or increasing the delivered plasma power, though the IEDFs in these cases differ substantially from the IEDF at high temperature conditions. We propose that a low pressure or high power in combination with a modest hydrogen dilution (high enough to avoid dusty regime) will deliver silicon films at low temperature without sacrificing deposition rate.

Effect of hydrogen dilution on crystalline properties of nano-crystalline silicon thin films in fast growth

Nano-crystalline silicon films were prepared from SiH4 diluted with hydrogen by plasma enhanced chemical vapor deposition at a pressure of 230 Pa. The effect of hydrogen dilution on their growth rate and crystalline properties were investigated. The experimental results indicate that the crystalline fraction and grain size increase with increasing hydrogen dilution ratio, and when the hydrogen dilution ratio increases to 99%, the crystalline fraction reaches 70%. The deposition rate decreases with increasing hydrogen dilution ratio, when the hydrogen dilution ratio decreases from 99% to 95%, the deposition rate of thin film increases from 0.3nm/s to 0.8nm/s.

Optoelectronic properties of hot-wire silicon layers deposited at 100 °C

Hot-wire chemical vapor deposition is employed for the deposition of amorphous and microcrystalline silicon layers at substrate temperature kept below 100°C with the aid of active cooling of the substrate holder. The hydrogen dilution is varied in order to investigate films at the amorphous-to-microcrystalline transition. While the amorphous layers can be produced with a reasonably low defect density as deduced from subgap optical absorption spectra and a good photosensitivity, the microcrystalline layers are of a lesser quality, most probably due to a decrease of crystallinity during the film growth. In the amorphous growth regime, the Urbach energy values decrease with increasing hydrogen dilution, reaching a minimum of 67meV just before the microcrystalline threshold. By varying the total gas pressure, the growth rate of the films is changed. The lowest deposition rate of this study (0.16nm/s) produced the amorphous sample with the highest photoresponse (1×106).

Fabrication and Optimization of a-Si:H n-i-p Single-junction Solar Cells with 8 Å/s Intrinsic Layers of Protocrystalline Si:H Materials

AbstractAt the University of Toledo (UT), we have investigated hydrogenated amorphous silicon (a-Si:H) n-i-p solar cells with intrinsic layers deposited at high rates, ~ 8 Å/s, using our UT multi-chamber load-locked PECVD system. a-Si:H i-layers were grown with a VHF plasma density of ~ 0.2 W/cm2 and a frequency of 70 MHz using various hydrogen dilution levels. It is observed from the current-voltage (I-V) device performance characteristics that the open-circuit voltage (Voc) increases with increasing hydrogen dilution reaching a maximum and then decreasing. This drop in Voc can be attributed to the transition region (or protocrystalline regime) from an amorphous phase into a mixed amorphous+nanocrystalline (a + nc) phase for the i-layer. An initial efficiency of 9.99% (Voc = 0.986 V, Jsc = 13.98 mA/cm2, FF = 72.5%) was obtained. Quantum efficiency (QE) measurement has shown that the blue light response increases as the hydrogen dilution increases. Very good blue light spectral response with QE values over 0.7 at the wavelength of 400 nm have been obtained for a-Si:H cells made under specific deposition conditions in which tailored protocrystalline silicon materials were incorporated at the i/p interface region.

Analysis of structure and defects in thin silicon films deposited from hydrogen diluted silane

Thin silicon layers have been deposited from silane diluted with hydrogen. The dilution ratio R ( R = [H 2]/[SiH 4]) has been varied between R = 0 and R = 40. The structural properties of Si:H films have been studied using transmission electron microscopy imaging and Raman spectroscopy. The phase evolution from the amorphous phase into the mixed and eventually microcrystalline phase strongly depends on the hydrogen dilution. The initiation of the microcrystalline growth occurs between R = 20 and R = 25. The phase transition becomes more abrupt with increasing hydrogen dilution. Optoelectronic properties of the layers have been determined. Increasing hydrogen dilution results in films with increasing effective defect density and Urbach energy, which is related to inhomogeneous growth. The charge deep-level transient spectroscopy technique (Q-DLTS) was applied for the first time on hydrogen diluted thin silicon films in order to investigate the energy distribution of the defect states in these layers as a function of the dilution ratio R. The Q-DLTS spectra indicate a difference in defect-state distribution when the films evolve from the amorphous phase into the microcrystalline phase.

The change of photoluminescence characteristics of amorphous carbon films due to hydrogen dilution

A series of hydrogenated amorphous carbon films were prepared using the hydrogen dilution method in a plasma enhanced chemical vapor deposition system. The luminescence characteristics were studied as a function of hydrogen dilution ratio. A broad PL band centered at 2.45 eV was found in the sample without hydrogen dilution. With increasing hydrogen dilution ratio, both the optical band gap and the photoluminescence peak were shifted to high energy as a consequent of the size decrease of π bonding states of the sp 2 hybridized carbon phases. A change of the PL characteristics was found on further increasing the hydrogen dilution ratio to 1.5. Besides the PL band around 2.45 eV, a new PL band at 3.05 eV appeared as the excitation photon energy exceeded 3.31 eV. Based on the measurements of structural and bonding configurations, the PL band can be attributed to the radiative recombination of geminate electron–hole pairs in small sp 2 clusters containing CC and C–H units.

Roles of hydrogen dilution on the microstructural and optoelectronic properties of B-doped nanocrystalline Si:H thin films

We report on the roles of hydrogen dilution on the microstructural and optoelectronic properties of boron-doped nanocrystalline silicon thin films grown by plasma-enhanced chemical vapor deposition on glass substrates, through x-ray diffraction, scanning electron microscope, Raman scattering, optical transmission, temperature-dependent dark conductivity, and elastic recoil detection analysis (ERDA) measurements. The grain size, crystallinity, absorption coefficient, refractive index, and conductivity are found to decrease basically with increasing hydrogen dilution ratio ΔH. The Tauc’s optical band gap is found to increase with ΔH due to the quantum size effect. By the aid of hydrogen ion bombardment effect [J. Appl. Phys. 93, 1262 (2003)] as well as the hydrogen-induced annealing effect, the clear increase of hydrogen content with ΔH and the different hydrogen distribution from ERDA help us to elucidate the correlation of hydrogen dilution with the crystallinity and grain size. The argument has been further supported by a simple absorption model and the yielded refractive index and absorption coefficient.

Impacts of hydrogen dilution on growth and optical properties of a-SiC:H films

Hydrogenated amorphous silicon-carbon (a-SiC:H) films were deposited by plasma enhanced chemical vapor deposition (PECVD) with a fixed methane to silane ratio ([CH4]/[SiH4]) of 1.2 and a wide range of hydrogen dilution (R-H=[H-2]/[SiH4 + CH4]) values of 12, 22, 33, 102 and 135. The impacts of RH on the structural and optical properties of the films were investigated by using UV-VIS transmission, Fourier transform infrared (FTIR) absorption, Raman scattering and photoluminescence (PL) measurements. The effects of high temperature annealing on the films were also probed. It is found that with increasing hydrogen dilution, the optical band gap increases, and the PL peak blueshifts from similar to1.43 to 1.62 eV. In annealed state, the room temperature PL peak for the low R-H samples disappears, while the PL peak for the high R-H samples appears at similar to 2.08 eV, which is attributed to nanocrystalline Si particles confined by Si-C and Si-O bonds.

High quality amorphous silicon materials and cells grown with hydrogen dilution

Hydrogen dilution of the active gas during deposition has been found to be a very effective way to improve the quality of amorphous silicon-based materials and solar cells. With increasing hydrogen dilution, the material is characterized by an improved order, and at a certain threshold dilution, the amorphous to microcrystalline transition takes place. The best material is grown just below the threshold and is heterogeneous consisting of tiny crystallites embedded in an amorphous matrix of improved order. In this paper, we discuss the effects of hydrogen dilution on the material and cell properties of amorphous silicon-based alloys and provide an explanation for their improved stability against light-induced degradation. We also discuss some special properties of the on-the-edge materials that are not seen in the conventional amorphous or microcrystalline alloys.

Correlation between the improved stability and low temperature hydrogen effusion in hydrogenated amorphous silicon films grown from hydrogen dilution of silane

Correlation between stability and hydrogen effusion at low temperature has been studied in a series of hydrogenated amorphous silicon (a-Si:H) films grown from different hydrogen (H 2) dilution of silane. The hydrogen effusion spectra are decomposed into two components: low and high temperature peaks located near 395 and 500 °C, respectively. The volume fraction of the integrated intensity of the low temperature peak increases with increasing hydrogen dilution. The results indicate that the stability significantly improves when the volume fraction of the low temperature peak is larger than 0.5. For the highly H 2-diluted a-Si:H films with the volume fraction more than 0.8, in particular, no or little significant change in the photoconductivity is observed. Possible explanations for the results are discussed.

Low-temperature deposition of amorphous silicon solar cells

We develop amorphous silicon (a-Si:H)-based solar cells by plasma-enhanced chemical vapor deposition (PECVD) at deposition temperatures of T s=75°C and 100°C, compatible with low-cost plastic substrates. The structural and electronic properties of low-temperature standard PECVD a-Si:H, both doped and undoped, prevent the photovoltaic application of this material. In this paper, we demonstrate how to achieve device-quality a-Si:H even at low deposition temperatures. In the first part, we show the dependence of structural and carrier transport properties on the deposition temperature. The sub-band gap absorption coefficient and the Urbach energy increase when the deposition temperature declines from T s=150°C to 50°C, the conductivity of doped layers and mobility-lifetime product of intrinsic a-Si:H drop drastically. Therefore, in the second part we investigate the impact of increasing hydrogen dilution of the feedstock gases on the properties of low-temperature a-Si:H. We restore n-type a-Si : H device-quality conductivity while the p-type a-Si:H conductivity is still inferior. For undoped layers, we depict the hole diffusion length, the mobility-lifetime product for electrons, the Urbach energy, and sub-band gap absorption coefficient as a function of the hydrogen dilution ratio. We incorporate these optimized materials in solar cell structures of single and multilayer design and record initial efficiencies of η=6.0% at a deposition temperature of T s=100°C, and η=3.8% at T s=75°C. For prospective opaque polymer substrates we develop, in addition to our conventional pin cells, devices in nip design with similar performance.