Structure Of Nanocrystalline Silicon Research Articles

Poole-Frenkel emission and defect density in a-Si:H/nc-Si:H multilayer films for “all silicon” third generation photovoltaics

A layered structure of nanocrystalline Silicon (nc-Si) and Si compound dielectric (SiO2, Si3N4, SiC) is becoming popular for the third generation photovoltaics because of its tunable optical bandgap. However, this structure makes poor current conduction, as the barrier potential between Si and Si dielectric is very high. An excellent current conduction can be obtained by replacing the Si dielectric with amorphous Si (a-Si:H), as the barrier potential between a-Si and nc-Si is very low compared to nc-Si/Si dielectric structure. This paper presents the electrical properties of an a-Si:H/nc-Si:H multilayer film fabricated using hot-wire chemical vapour deposition. Hydrogen (H2) dilution in the nc-Si:H layers was considered as a variable parameter in different multilayer films. Formation of a nc-Si:H layer sandwiched between two a-Si:H layers was confirmed by Raman spectroscopy and X-ray diffraction. The current-voltage measurements performed in low temperature reveal different vertical charge transport mechanisms, such as Poole-Frenkel (PF) emission, phonon assisted tunnelling and direct tunnelling. Below room temperature, the PF emission rate increases the current with increase in H2 dilution in the multilayer films. The capacitance-voltage measurement performed near the room temperature explains the increased defects in the multilayer films with an increase in H2 dilution.

Effect of substrate temperature on microstructure and optical properties of hydrogenated nanocrystalline Si thin films grown by plasma enhanced chemical vapor deposition

In this paper is presented the effect of substrate temperature (Ts) on the chemical composition, the morphology, the reflectivity and the carrier life time of hydrogenated nanocrystalline silicon (nc-Si:H) thin films, grown by radio frequency Plasma Enhanced Chemical Vapor Deposition (PECVD) on silicon single crystal (100) using a gas mixture of silane (SiH4) and hydrogen (H2). Substrate temperature was varied from room temperature to 450°C. Characterization of these films by X-ray diffraction (XRD) and Raman spectroscopy revealed that the crystallite size and at the same time the volume fraction of crystallites in the films tends to decrease with increasing Ts. The Fourier transform infrared spectroscopic analysis (FTIR) showed at low temperature the hydrogen is incorporated in the nc-Si:H films in the mono-hydrogen (Si–H) bonding configuration. With increasing Ts the hydrogen bonding in nc-Si:H films shifts from mono-hydrogen (Si–H) to di-hydrogen (Si–H2) and (Si–H2)n. The hydrogen content in the nc-Si:H films decreases with the increase of Ts. From the Atomic Force Microscopy (AFM), it was shown that the increase of Ts tends to increase the porosity and decrease the crystalline grain size. In order to more understand the effect of Ts on this structural change, minority carrier lifetime (MCL) measurement show that only the films with a nanocrystalline silicon structure present an enhancement in MCL which could be related to a quantum size effect and to the SiH-related bonds.

Realization of a Multilayered Nano-Crystalline Silicon Structure by a Novel Low Temperature Process Suitable for Silicon-Based Light Emitting Diodes

A plasma enhanced chemical vapor deposition (PECVD) of amorphous-Si followed by in-situ crystallization has been applied to realize nano-scale crystal grains embedded in amorphous medium. Removal of extra amorphous Si layer is achieved using a sequential reactive ion etching process while the nano-crystalline grains are preserved. The process is carried out all at temperatures below 350o C which is fully compatible with soda-lime glass substrates. Photoluminescence analysis indicates the light-emitting behavior of the processed layer at a wavelength around 550 nm. A correspondence between the photoluminescence peak position and silicon grain size is observed using SEM. Multilayered structures have been employed to realize light emitting diodes on glass substrates. The electroluminescence spectrum of the device has been investigated.

Realization of a Multilayered Nano-Crystalline Silicon Structure by a Novel Low Temperature Process Suitable for Silicon-Based Light Emitting Diodes

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Improvement of amorphous silicon n-i-p solar cells by incorporating double-layer hydrogenated nanocrystalline silicon structure

We develop a double-layer p-type hydrogenated nanocrystalline silicon (p-nc-Si:H) structure consisting of a low hydrogen diluted i/p buffer layer and a high hydrogen diluted p-layer to improve the hydrogenated amorphous silicon (a-Si:H) n-i-p solar cells. The electrical, optical and structural properties of p-nc-Si:H films with different hydrogen dilution ratio ( R H) are investigated. High conductivity, low activation energy and wide band gap are achieved for the thin films. Raman spectroscopy and high-resolution transmission electron microscopy (HRTEM) analyses indicate that the thin films contain nanocrystallites with grain size around 3–5 nm embedded in the amorphous silicon matrix. By inserting a p-nc-Si:H buffer layer at the i/p interface, the overall performance of the solar cell is improved significantly compared to the bufferless cell. The improvement is correlated with the reduction of the density of defect states at the i/p interface.

Electronic Structure of Nanocrystalline Silicon and Oxidized Silicon Surfaces

Electronic Structure of Nanocrystalline Silicon and Oxidized Silicon Surfaces

TEM Study and Hall Measurement of nc-Si Prepared by Controlled Deposition

ABSTRACTWe report the results of a study in which we combined growth experiments with measurements of the nc-structure and of electrical transport Samples were prepared by plasma enhanced-CVD using SiF4 and H2 gases. We also added PH3 and H2 as control parameters for structural change. The microscopic structure was directly observed by TEM. Electron transport in nc-Si was investigated by Hall effect measurements performed at temperatures from 100K to 400K. We produced samples in which the Hall mobility was applied from general transport mechanism of poly crystalline silicon. However, from TEM observation, we conclude that dominant factor on electrical transport strongly depends on the sample structure, and nanocrystalline-silicon structure is so varied as to make it difficult to determine the transport mechanism without the observation of the microscopic structure.

The structure of nanocrystalline silicon formed by laser-induced explosive crystallization

Abstract The structure of nanocrystalline silicon formed by pulsed laser induced explosive crystallization of amorphous silicon has been studied using Raman spectroscopy. It is found that the properties of nanocrystalline silicon are independent of the laser energy density used to initiate explosive crystallization. From the spectrum in the low-wavenumber region it is infrared that the grains in nanocrystalline silicon are highly distorted. The average grain size, as determined from the position of the main peak in the spectrum, is 5–7 nm. Annealing at 500–600°C results in a decrease of the distortion and an increase in average grain size by 1–2 nm.