Increase In Hydrogen Dilution Research Articles

Effects of Hydrogen-Dilution on Opto-Structural Properties of Hot-wire CVD Grown a-Si:H/nc-Si:H Multilayer for Photovoltaics

The multilayered thin film structure of amorphous silicon (a-Si:H) and nanocrystalline silicon (nc-Si:H) provides the possibility of bandgap tuning for fabrication of multijunction solar cells. This paper communicates a detailed analysis of optical and structural properties of a-Si:H/nc-Si:H multilayer thin films by hot-wire chemical vapor deposition at low hydrogen (H2) and silane (SiH4) flow rates. A set of multilayer films with 25 bilayers of a-Si:H/nc-Si:H are prepared using different hydrogen-dilution of SiH4 in the alternating nc-Si:H layers. The first and the second order Raman scattering studies reveal the presence of mixed phases of silicon in the nc-Si:H layers. Raman and XRD investigation of the films confirm the presence of different sizes of the silicon nanocrystals. The optical spectroscopic analysis instead of FTIR analysis of multilayer films is utilized uniquely to determine the hydrogen content in the a-Si:H/nc-Si:H multilayers and is related to the amorphous phase of the films. No significant change in hydrogen content is observed and the amorphous phase is found to decrease with increase in hydrogen dilution. Further, no quantum size effect (QSE) was observed due to the large growth time of nc-Si:H layers. Thus the experimental result shows that the bandgap of multilayer films decreases due to reduction in amorphous silicon phase, ineffective QSE and relative loss of hydrogen content.

Analysis of thickness-dependent optical parameters of a-Si:H/nc-Si:H multilayer thin films

Multilayer thin films with alternate hydrogenated amorphous (a-Si:H) and nanocrystalline silicon (nc-Si:H) layers are deposited in hot wire chemical vapor deposition (HWCVD) chamber using hydrogen (H2) dilution of silane (SiH4). Two sets (two samples for each set) have different nc-Si:H layer thickness with the same a-Si:H thickness. The transmittance spectra, refractive index and absorption coefficient are obtained using UV–Vis spectroscopy. An analysis of Tauc plot suggests thickness-dependent optical band gap shift. The films with thick nc-Si:H layers show a decrease in band gap from 1.78 to 1.68 eV, whereas the films with lower nc-Si:H layer thickness have an increased band gap from 1.64 to 1.72 eV. Further, the slope of the Tauc plot (B) and disorder indicator (Urbach energy, Eu) show that the films with thick nc-Si:H layers are poorly ordered as hydrogen dilution increases. These results illustrate that band gap shift can be attributed to the crystal size effect and film disorder. This shifting of optical band gap with the nc-Si:H layer thickness is useful in tuning of the absorber layer band gap which finds an application in photovoltaics.

Influences of hydrogen dilution on the growth of Si-based core–shell nanowires by HWCVD, and their structure and optical properties

Si-based core–shell nanowires were grown on Ni-coated crystal silicon substrates using a hot-wire chemical vapor deposition technique. The NiSi nanoparticles acted as catalysts that facilitated the growth of the core–shell nanowires without any hydrogen dilution as well as that ranging from 20 to 99 %. These nanowires were structured by single-crystalline NiSi cores and amorphous shells with consisting of nanocrystallites embedded within an amorphous matrix. Raman results reveal crystallization of amorphous Si to crystalline Si up to the crystalline volume fraction of 92.3 % for the nanowires grown with hydrogen dilution. An increase in hydrogen dilution enhanced the decomposition rate and the gas-phase reactions for SiC shell formation, while further increases up to 99 % suppressed the growth of the nanowires. Moreover, a phased transition from Si to SiC occurred with increases in hydrogen dilution above 20 %. The nanowires demonstrated superior optical absorption in the visible region, revealing their significant light-trapping ability. This paper discusses the influences of hydrogen dilution on the structure and optical properties of these core–shell nanowires.

Estimation of Optical Properties of Multilayer Silicon Nano Films with Different Hydrogen Dilution

Silicon multilayer thin films with stack of amorphous silicon (a-Si:H) and nanocrystalline silicon (nc-Si:H) of varied hydrogen dilution were prepared by hot wire chemical vapor deposition (HWCVD). The optical properties of the thin films were measured by UV-visible spectroscopy. The smooth transmittance spectra measured in the range of 500-1100nm show films with uniform deposition. The optical constants such as refractive index, absorption coefficient, tauc band gap, extinction coefficient together with thickness were determined using envelope method. It is found that the optical band gap decreases from 1.78eV to 1.6eV as hydrogen dilution increases.

Correlation between microstructure and residual stress of nanocrystalline silicon films

AbstractThe nanocrystalline silicon films have been prepared using very high frequency PECVD technique. The deposition power density and hydrogen dilution of Silane were varied. The sizes of the grains varies from 6 nm to 32 nm for films deposited under different conditions. The stress of the nanocrystalline films have been estimated from (220) line. It is observed that compressive stress decreases and become tensile stress as Xc of the films increases with increase in power density. Whereas compressive stress increases as crystalline volume fraction increases with increase in hydrogen dilution. Maximum tensile stress and compressive stress obtained are 0.23 GPa and 5.1 GPa respectively. It is also observed that compressive stress in the film increases as grain size increases. (© 2010 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Transparent polymer and diamond-like hydrogenated amorphous carbon thin films by PECVD technique

Optical, electrical and structural properties of hydrogenated amorphous carbon films deposited by radio frequency plasma enhanced chemical vapour deposition technique (RF-PECVED) have been investigated thoroughly. The films were deposited using a mixture of hydrogen and acetylene gases at a low substrate temperature of 200 °C. The electrical resistivities of the films are ⩾ 109 Ω cm. The ratio of hydrogenated sp3 to sp2 bonded carbon atoms in the films is obtained by deconvoluting the infrared spectra over the 2800 cm−1–3100 cm−1 range. Polymer-like hydrogenated amorphous carbon films are grown at a low power (50 mW cm−2) having a high hydrogen content (30.3–46.1 at%). Increase in hydrogen dilution increases the total hydrogen content as well as the ratio of hydrogenated sp3 to sp2 carbon bonding and it becomes a maximum at an optimum hydrogen dilution. Similarly the grain density also increases with the increase in hydrogen dilution. On the other hand, at a higher RF power of 150 mW cm−2 much stronger C–C sp3 and C–C sp2 bonds are formed and the total hydrogen content drastically reduces to a very low value of 1.1–1.8 at%. This indicates the growth of hydrogenated diamond-like carbon (DLC) structure. The stronger C–C sp3 bonds present in the DLC film make the films harder as well as Raman active.

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.

Thin film silicon solar cells grown near the edge of amorphous to microcrystalline transition

Thin films of silicon can be grown by glow-discharge decomposition of silane and hydrogen. As the hydrogen dilution increases, a transition from amorphous to microcrystalline phase takes place. The regime below and above this edge has been found to be most suitable for making high efficiency amorphous and microcrystalline solar cells. In this paper I outline the status of material research, cell performance and manufacturing of thin film silicon grown near the edge of amorphous to microcrystalline transition.

Synthesis of highly conductive boron-doped p-type hydrogenated microcrystalline silicon (μc-Si:H) by a hot-wire chemical vapor deposition (HWCVD) technique

Boron-doped hydrogenated microcrystalline silicon (μc-Si:H) films were prepared using hot-wire chemical vapor deposition (HWCVD) technique. Structural, electrical and optical properties of these thin films were systematically studied as a function of B 2H 6 gas (diborane) phase ratio (Variation in B 2H 6 gas phase ratio, dopant gas being diluted in hydrogen, affected the film properties through variation in doping level and hydrogen dilution). Characterization of these films from low angle X-ray diffraction and Raman spectroscopy revealed that the high conductive film consists of mixed phase of microcrystalline silicon embedded in an amorphous network. Even a small increase in hydrogen dilution showed marked effect on film microstructure. At the optimized deposition conditions, films with high dark conductivity (0.08 (Ω cm) −1) with low charge carrier activation energy (0.025 eV) and low optical absorption coefficient with high optical band gap (∼2.0 eV) were obtained. At these deposition conditions, however, the growth rate was small (6 Å/s) and hydrogen content was large (9 at%).

Amorphous Silicon Alloy Materials and Solar Cells Near the Threshold of Microcrystallinity

AbstractOne of the most effective techniques used to obtain high quality amorphous silicon alloys is the use of hydrogen dilution during film growth. The resultant material exhibits a more ordered microstructure and gives rise to high efficiency solar cells. As the hydrogen dilution increases, however, a threshold is reached, beyond which microcrystallites begin to form rapidly. In this paper, we review some of the interesting features associated with the thin film materials obtained from various hydrogen dilutions. They include the observation of linear-like objects in the TEM micrograph, a shift of the principal Si TO band in the Raman spectrum, a sharp, low temperature peak in the H2 evolution spectrum, a shift of the wagging mode in the IR spectrum, and a narrowing of the Si (111) peak in the X-ray diffraction pattern. These spectroscopic tools have allowed us to optimize deposition conditions to near the threshold of microcrystallinity and obtain desired high quality materials. Incorporation of the improved materials into device configuration has significantly enhanced the solar cell performance. Using a spectral-splitting, triple-junction configuration, the spectral response of a typical high efficiency device spans from below 350 nm to beyond 950 nm with a peak quantum efficiency exceeding 90%; the triple stack generates a photocurrent of 27 mA/cm2. This paper describes the effect of the improved materials on various solar cell structures, including a 13% active-area, stable triple-junction device.