Nanocrystalline Silicon Films Research Articles

Ultrafast terahertz conductivity of photoexcited nanocrystalline silicon

The ultrafast transient ac conductivity of nanocrystalline silicon films is investigated using time-resolved terahertz spectroscopy. While epitaxial silicon on sapphire exhibits a free carrier Drude response, silicon nanocrystals embedded in glass show a response that is best described by a classical Drude–Smith model, suitable for disorder-driven metal–insulator transitions. In this work, we explore the time evolution of the frequency dependent complex conductivity after optical injection of carriers on a picosecond time scale. Furthermore, we show the lifetime of photoconductivity in the silicon nanocrystal films is dominated by trapping at the Si/SiO2 interface states, occurring on a 1–100 ps time scale depending on particle size and hydrogen passivation.

PECVD nanocrystalline silicon films versus porous silicon: Structural and optical properties

A comparative experimental study is conducted of nanocrystalline-and porous-silicon films from the viewpoint of their structural, luminescent, and optical properties, the films being fabricated by PECVD and electrochemical etching, respectively. The x-ray-diffraction data obtained imply that the PECVD films consist of nanocrystals that are uniformly (004) oriented and measure on average 4.8 nm across. It is shown by an AFM investigation that the films have a very rough surface occupied by sharply pointed nanoscale asperities. The films can exhibit room-temperature photoluminescence that is strongest at a photon energy of 1.55 eV. Optical properties of the nanocrystalline-and porous-silicon films are examined in the visible range and the IR. The nanocrystalline-silicon films are found to have similar physical and chemical properties to the porous-silicon films.

The Preparation of Nanocrystalline Silicon by Plasma-Enhanced Hydrogenation for the Fabrication of Light-Emitting Diodes

The fabrication of nanocrystalline silicon light-emitting diodes is reported using a novel plasma-enhanced hydrogenation method. The fabrication process consisted of the deposition of amorphous silicon on a silicon substrate, a hydrogen plasma treatment, and subsequent annealing, and the deposition of TiO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> , indium-tin oxide, and metal contact layers. The entire process was performed at temperatures below 400 degC and is compatible with standard silicon fabrication technologies. The current-voltage (I-V) characteristics of the device showed a rectifying diode behavior where electrons tunneled through the thin TiO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> layer and recombined with the holes injected from the P-type silicon substrate leading to photon generation. The structure of the nanocrystalline silicon films was investigated by scanning electron and transmission electron microscopies, and the spectral distribution of the emitted light was measured by a cathodoluminescence

Charge storage characteristics of hydrogenated nanocrystalline silicon film prepared by rapid thermal annealing

The early stages of hydrogenated nanocrystalline silicon (nc-Si:H) films deposited by plasma-enhanced chemical vapour deposition were characterized by atomic force microscopy. To increase the density of nanocrystals in the nc-Si:H films, the films were annealed by rapid thermal annealing (RTA) at different temperatures and then analysed by Raman spectroscopy. It was found that the recrystallization process of the film was optimal at around 1000°C. The effects of different RTA conditions on charge storage were characterized by capacitance–voltage measurement. Experimental results show that nc-Si:H films obtained by RTA have good charge storage characteristics for nonvolatile memory.

Characterization of doped hydrogenated nanocrystalline silicon films prepared by plasma enhanced chemical vapour deposition

The B- and P-doped hydrogenated nanocrystalline silicon films (nc-Si:H) are prepared by plasma-enhanced chemical vapour deposition (PECVD). The microstructures of doped nc-Si:H films are carefully and systematically characterized by using high resolution electron microscopy (HREM), Raman scattering, x-ray diffraction (XRD), Auger electron spectroscopy (AES), and resonant nucleus reaction (RNR). The results show that as the doping concentration of PH3 increases, the average grain size (d) tends to decrease and the crystalline volume percentage (Xc) increases simultaneously. For the B-doped samples, as the doping concentration of B2H6 increases, no obvious change in the value of d is observed, but the value of Xc is found to decrease. This is especially apparent in the case of heavy B2H6 doped samples, where the films change from nanocrystalline to amorphous.

Investigation on high mobility nanocrystalline Si with crystalline Si heterostructure

Abstract A heterostructure of n-type hydrogenated nanocrystalline silicon (nc-Si:H) film with p-type crystalline silicon, i.e., (n)nc-Si:H/(p)C-Si, was fabricated to investigate carrier characteristics and transport. After electrical experiments, carrier information, such as hole and electron as well as 2-dimension electronic gas in the studied system, was identified respectively. The forward current conduction was analyzed while the reverse current transport was distinguished as different mechanisms within the different range of negative applied voltage. The performed study also leads us to ascribe the main origin of short transient times on the produced structure to a tunneling mechanism.

Optical properties of nanocrystalline silicon films deposited by plasma-enhanced chemical vapor deposition

Nanocrystalline silicon (nc-Si) films were deposited by a plasma-enhanced chemical vapor deposition technique using SiF4/SiH4/H2 gas mixtures. The optical properties of the nc-Si films were examined by varying the deposition temperature (Td) under two different hydrogen flow rate ([H2]) conditions. For these films, we found two photoluminescence (PL) spectra at around 1.7–1.75eV and 2.2–2.3eV. The peak energy, EPL, of the 1.7–1.75-eV PL band was found to shift as Td or [H2] changes. It was found that the decrease in Td acts to decrease the average grain size, 〈δ〉, and to increase both the optical band gap, Egopt, and the EPL values. By contrast, the increase in [H2] decreased the 〈δ〉 value, while increased the values of Egopt, and EPL. Thus, as either Td decreases or [H2] increases, it is found that a decrease in 〈δ〉 corresponds well with increases in Egopt and EPL. As a consequence, it was suggested that an increase of EPL of the 1.7–1.75-eV PL band can be connected with an increase in Egopt, through a decrease in 〈δ〉. However, the PL process can not be connected with the transition between both the bands, related to formation of nanocrystals. Based on these results, it was proposed that the use of both low Td and high [H2] conditions would allow to grow nc-Si films with small grains.

Evolution of Structural and Electronic Properties in Boron-doped Nanocrystalline Silicon Thin Films

AbstractWe report on the boron-doping dependence of the structural and electronic properties in nanocrystalline silicon (nc-Si:H) films directly deposited by plasma- enhanced chemical vapor deposition (PECVD). The crystallinity, micro-structure, and dark conductivity of the films were investigated by gradually varying the ratio of trimethylboron [B(CH3)3 or TMB] to silane (SiH4) from 0.1 to 2 %. It was found that the low level of boron doping (&lt; 0.2 %) first compensated the nc-Si:H material which demonstrates slightly n-type properties. As the doping increased up to 0.5 %, the maximum dark conductivity (ód) of 1.11 S/cm was obtained while high crystalline fraction (Xc) of the films (over 70 %) was maintained. However, further increase in a TMB-to-SiH4 ratio reduced ód to the order of 10-7 S/cm due to a phase transition of the films from nanocrystalline to amorphous, which was indicated by Raman spectra measurements.P-channel nc-Si:H thin film transistors (TFTs) with top gate and staggered source/drain contacts were fabricated using the developed p+ nc-Si:H layer. The fabricated TFT exhibits a threshold voltage (VTp) of -26.2 V and field effective mobility of holes (μp) of 0.24 cm2/V·s.

Breakdown physics of low-temperature silicon epitaxy grown from silane radicals

We grow epitaxial silicon films on (100) Si wafers at low temperature $(l400\phantom{\rule{0.2em}{0ex}}\ifmmode^\circ\else\textdegree\fi{}\mathrm{C})$, from silane radicals, to understand the mechanisms of sudden epitaxy breakdown and simultaneous growth of amorphous and crystalline silicon phases. Surface roughness is below $0.7\phantom{\rule{0.3em}{0ex}}\mathrm{nm}$ before breakdown and below $1\phantom{\rule{0.3em}{0ex}}\mathrm{nm}$ in epitaxial regions persisting after breakdown, in contrast to the roughness-induced breakdown observed in molecular beam epitaxy from atomic Si. Spherical caps of hydrogenated amorphous Si (a-Si:H) breakdown cones protrude above the crystal surface, with each sphere centered on its cone apex. This means that the a-Si:H grows isotropically from impinging radicals with low surface mobility and that the a-Si:H growth rate is higher than the epitaxial growth rate. Similar physical mechanisms likely apply to nanocrystalline silicon film growth.

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.

Origin of photoluminescence in nanocrystalline Si:H films

We have studied the origin of photoluminescence (PL) from hydrogenated nanocrystalline silicon (nc-Si:H) films produced by a plasma-enhanced chemical vapor deposition technique using SiF 4/SiH 4/H 2 gas mixtures. The nc-Si:H films were characterized using X-ray diffraction, infrared, Raman spectroscopy, optical absorption and stress, and were examined for PL by varying the deposition temperature ( T d) under two different hydrogen flow rate ([H 2]) conditions. The PL exhibited two peaks at around 1.7–1.75 and 2.2–2.3 eV. The peak energy, E PL, of the 1.7–1.75-eV PL band was found to shift as T d or [H 2] changes. It was found that the decrease in T d acts to decrease the average grain size, 〈 δ〉, and to increase both the optical band gap, E g opt , and the E PL values. By contrast, the increase in [H 2] decreased the 〈 δ〉 value, while increased the values of E g opt and E PL. Thus, as either T d decreases or [H 2] increases, it is found that a decrease in 〈 δ〉 corresponds well with increases in E g opt and E PL. As a consequence, it was suggested that an increase in E PL of the 1.7–1.75-eV PL band can be connected with an increase in E g opt , through a decrease in 〈 δ〉. However, the PL process cannot be connected with the transition between both the bands, related to formation of nanocrystals. Based on these results, it was proposed that the use of both low T d and high [H 2] conditions would allow to grow nc-Si:H films with small grains.

Raman spectra of intrinsic and doped hydrogenated nanocrystalline silicon films

Raman scattering characteristics of intrinsic and doped hydrogenated nanocrystalline silicon films which prepared by a plasma-enhanced chemical vapor deposition system are investigated. Results indicate that Raman spectra depend intensively on microstructure and impurity in the films. Taking into account phonon confinement effect and tensile strain effect in Si nanocrystals, peak redshift of measured transverse optical modes in Raman spectra of intrinsic films can be well interpreted. With respect to Raman scattering from doped samples, besides phonon confinement effect, the peak of experimental transverse optical mode further downshifts with heightening doping level, which can be primarily assigned to impurity effect from doping. In addition, the increase in relative integral intensity ratio of transverse acoustic branch to transverse optical mode and that of longitudinal acoustic branch to transverse optical mode with decreasing mean dimension of nanocrystals and heightening doping ratio, respectively, can be ascribed to disorder. Furthermore, at the same doping level, incorporation of boron can induce higher disorder than incorporation of phosphorus in nc-Si:H films.

Thin film silicon MEMS microresonators fabricated by hot-wire chemical vapor deposition

Thin film silicon MEMS electrostatic microresonators are fabricated on glass substrates by hot-wire chemical vapor deposition with the silicon structural layer spanning the amorphous-to-nanocrystalline transition. The amorphous-to-nanocrystalline transition is induced by increasing the hydrogen dilution of the reaction gases during the thin film silicon deposition. All processing steps are carried out at temperatures ⩽110 °C. Hot-wire deposition allows significantly faster deposition rates of thin-silicon films than standard RF plasma-enhanced chemical vapor deposition. In addition, the lower stress present in hot-wire films due to the absence of ion bombardment during growth allows the fabrication of thin film nanocrystalline silicon microresonators. The microresonators are electrostatically actuated and the resulting deflection is measured optically. The crystallinity of the structural layer does not have an observable effect in the rigidity of the resonators. The quality factor of the resonators shows a maximum at 85% H2 dilution, corresponding to a material with a structure intermediate between amorphous and nanocrystalline. A sharp decrease in quality factors is observed for higher dilutions which correspond to nanocrystalline silicon films.

Carrier conduction in heterojunction of hydrogenated nanocrystalline silicon with crystal silicon

Phosphorus-doped hydrogenated nanocrystalline silicon (n-type nc-Si:H) film was deposited by plasma enhanced chemical vapor deposition technique on heavy doped p-type crystalline silicon ((p +)c-Si) substrate to form heterojunction of (n)nc-Si:H/(p +)c-Si. In electrical experiments, both negative resistance in forward current–voltage ( I–V) measurements and current staircases in reverse I–V experimental data were observed from this structure of (n)nc-Si:H/(p +)c-Si at 77 K, which reveals it as a semiconductor heterojunction tunnel diode. The forward current is assigned to interband tunneling, excess and thermionic emission component, respectively. Within reverse bias voltage range from 0 to around − 13 V, the reverse current can be ascribed to minority carriers instead of majority carriers tunneling across the depletion layer in heterojunction. Cross reverse applied voltage range from − 13 to about − 37 V, the reverse current can be attributed to injection of electrons via sequent resonant tunneling through Si nanocrystals into substrate. As further increasing reverse applied voltage, the reverse current can be allocated to carrier avalanche multiplication within amorphous buffer layer region to enhance electron resonant tunneling in nc-Si:H layer.

Investigation on the initial growth of nanocrystalline silicon prepared from hydrogen-diluted SiCl4 at low temperatures

Ultrathin nanocrystalline silicon films have been fabricated by plasma enhanced chemical vapour deposition from hydrogen-diluted SiCl4 at a low temperature of 250 °C. The Raman spectra measurements and high resolution transmission electron microscopy observations reveal that nanocrystalline silicon can be directly grown on an insulator glass substrate at the initial growth of the deposition process. The crystallinity and the grain size increase with the increase in the deposition period. Furthermore, the crystalline fraction as well as the grain size also increase with the decrease in the hydrogen dilution ratio. The I–V characteristic curves measured by a tuned Langmuir probe in the SiCl4/H2 plasma region at a substrate temperature of 100 °C show that silicon films with higher conductivity were deposited on the probe surface in the deposition process. This behaviour is completely different from that observed in the SiH4/H2 system. It is considered that the formation of nanometre size grains occurs in the gas phase reaction process at the initial stage of film growth, while the growth of grains is mainly governed by the surface reaction process where chlorine and hydrogen play an important role. The direct growth of nanocrystalline Si on the insulator surface at low temperatures using SiCl4/H2 opens up the possibility of directly fabricating nc-Si/SiO2 or nc-Si/a-SiNx systems at low temperatures.

The average size of Si nanoparticles prepared by pulsed laser ablation in the gas mixture of He/Ar, Ne/Ar or He/Ne

The nanocrystalline silicon films were prepared by pulsed laser ablation in the gas mixture of high-purity inert gases (He/Ar, Ne/Ar or He/Ne) under the deposition pressure of 10 Pa. The microstructural properties of the films produced under various mixed ratios were characterized by scanning electron microscopy, X-ray diffraction and Raman. The result shows that the closer to the Si atomic mass the average atomic mass of the ambient gas, the smaller the average size of the Si nanoparticles, which may be attributed to more effective energy transfer among the ambient atoms and Si atoms.

Nanocrystalline silicon films as multifunctional material for optoelectronic and photovoltaic applications

Nanocrystalline silicon (nc-Si) already attracted a considerable attention as a promising material for photovoltaic applications, while its full optoelectronic potentiality is still under investigation, due to the relatively poor knowledge of the correlations between growth conditions, microstructure and physical properties. This paper aims at the illustration of the preliminary results of a 3 years project, addressed at the demonstration of the multifunctionality of nanocrystalline silicon, via the understanding of the quantitative correlations between growth process parameters and the structural, optical, electrical and physico-chemical properties of nc-silicon. The main topics foreseen and actively pursued were: • The development of a gas phase kinetic model to be used for the simulations of the plasma enhanced chemical deposition (PECVD) process, the development of a kinetic model relative to the interaction of radicals present in the plasma phase at the substrate interface, and last but not least, the development of models relative to the subsequent nc-Si growth. • The growth of undoped and doped nc-Si layers on convenient substrates by the low energy plasma enhanced chemical deposition (LEPECVD) process using silane, phosphine, diborane and hydrogen mixtures. • The quantitative experimental determination of the correlation among the crystallinity fraction, the film microstructure, the grain size/shape/orientation, the hydrogen content, the density of the recombination centres, the optical absorption coefficient the strain/stress state and the carrier mobility and diffusion length in undoped and n-type and p-type films, in view of the optimization of the minority carrier generation and carrier collection. The preliminary results already demonstrate that a proper modelling of the growth process is of great help in the selection of optimised growth procedures for nc-Si films and that a full range experimental characterisation is needed to get the proper inputs to the modelling activity.

Effects of hydrogen dilution on deposition process of nano-crystalline silicon film by SiCl4/H2 plasma

Growth of nano-crystalline silicon (nano-Si) films from hydrogen-diluted SiCl4 by the plasma enhanced chemical vapour deposition technique at 250 °C has been studied through mass spectrometry, Langmuir probe diagnostic techniques and Raman spectra measurements. The effects of the hydrogen dilution ratio on the relative densities of SiCln (n = 0–2) in SiCl4/H2 plasma are investigated. The average electron energy (Ee) and electron density (Ne) in SiCl4/H2 plasma increase with the hydrogen dilution ratio till their maxima at 9.25 eV and 3.7 × 109 cm−3, respectively. A suitable hydrogen dilution ratio R (0.4–0.67) is beneficial for the formation of SiCln (n = 0–2) radicals because Ee and Ne both have maxima in this range. More SiCln (n = 0–2) radicals will improve the deposition rate and film quality. In addition, hydrogen radicals play an important role in the surface reaction process. The reaction of H and SiCln (n = 0–2) on the growing surface is beneficial for creating dangling bond sites and prompting the film growth, and the exothermic reaction exothermic energy of H with Cl on the film-growing surface results in the increase of the effective temperature of the film growth.

Modelling of grain boundary effects in nanocrystalline/multicrystalline silicon heterojunction solar cells

Heterojunction solar cells formed by nanocrystalline silicon films on fine-grained multicrystalline silicon substrates are simulated in the presence of grain boundaries. The effects of grain boundaries on the dark and illuminated current–voltage (I–V) characteristics and spectral response (SR) of heterojunction (HJ) solar cells are assessed using 1D and 2D device simulations. The grain boundary in fine-grained multicrystalline silicon is modelled in two ways: as a defective surface with continuous defect distribution throughout the bandgap, and as a hypothetical sheet with a certain recombination velocity for electrons and holes. The SR and I–V characteristics of HJs are exploited to characterize grain boundary effects on the photovoltaic properties of the solar cells and photodetectors. Simulation results show noticeable differences on the dark I–V and SR of on- and off-grain boundary HJs. Grain boundary effects become important when fine-grained multicrystalline substrates are used. Measurement results of tiny test structures fabricated on the grain boundary show consistently inferior dark I–V and SR characteristics compared to those fabricated away from the grain and allow us to quantify the recombination at the grain boundary.

Mechanisms of the growth of nanocrystalline Si:H films deposited by PECVD

Hydrogenated nanocrystalline silicon (nc-Si:H) films were deposited using plasma-enhanced chemical vapor deposition from a SiF4/SiH4/H2 gas mixtures. Properties were examined of nc-Si:H films produced by decreasing the deposition temperature (Td) under two different hydrogen dilution ([H2]) conditions. For these films, the X-ray diffraction, the Raman scattering, the Fourier transform infrared absorption and the stress were investigated. Our results show that the decrease in Td has significant effects in the decrease of the average grain size (〈δ〉), the crystalline volume fraction (ρ) values, and an increase in the density of SiH-related bonds (NSiH) values. On the contrary, increases in [H2] decreased the 〈δ〉 and the NSiH, while the ρ were increased. Our experiments also confirmed that the increase in ρ corresponds with the decrease in NSiH. In view of these results, it may be concluded that the use of both low Td and high [H2] conditions might lead to growth of nc-Si:H films with small grains and high crystallinity. In this context, the surface processes (such as diffusion and etching) for the growth of nc-Si:H films were extensively discussed in this current work.