Ahm Arno Smets

Vacancies and voids in hydrogenated amorphous silicon

The hydride configurations in the hydrogenated amorphous silicon (a-Si:H) network have been studied by means of infrared absorption spectroscopy. The results on the film mass density of a-Si:H deposited by means of an expanding thermal plasma reveal the presence of two distinct regions in terms of hydrogen content and microstructure: below approximately 14 at. % H a-Si:H contains predominantly divacancies decorated by hydrogen, above 14 at. % H a-Si:H contains microscopic voids. These two distinct regions provide additional information on the origin of the low and high hydride stretching modes at 1980–2010 and 2070–2100 cm−1, respectively.

High-rate deposition of microcrystalline silicon p-i-n solar cells in the high pressure depletion regime

Hydrogenated microcrystalline silicon films (μc-Si:H) deposited at high deposition rates (∼2 nm/s) by means of the very-high-frequency (VHF) deposition technique in the high pressure depletion regime have been integrated into single junction p-i-n solar cells. It is demonstrated that μc-Si:H solar cells can be optimized using a twofold approach. First the bulk properties, deposited under steady-state plasma conditions, are optimized by monitoring the presence of crystalline grain boundaries in μc-Si:H. These hydrogenated crystalline grain boundaries can easily be detected via the crystalline surface hydrides contribution to the narrow high stretching modes by infrared transmission spectroscopy. The crystalline grain boundaries suffer from postdeposition oxidation which results in a reduced red response of the solar cell. The absence of these crystalline surfaces in an as-deposited μc-Si:H matrix reflects the device grade microcrystalline bulk material. Second, the prevention of silane backdiffusion from t...

Hydrogenated amorphous silicon deposited at very high growth rates by an expanding Ar–H2–SiH4 plasma

The properties of hydrogenated amorphous silicon (a-Si:H) deposited at very high growth rates (6–80 nm/s) by means of a remote Ar–H2–SiH4 plasma have been investigated as a function of the H2 flow in the Ar–H2 operated plasma source. Both the structural and optoelectronic properties of the films improve with increasing H2 flow, and a-Si:H suitable for the application in solar cells has been obtained at deposition rates of 10 nm/s for high H2 flows and a substrate temperature of 400 °C. The “optimized” material has a hole drift mobility which is about a factor of 10 higher than for standard a-Si:H. The electron drift mobility, however, is slightly lower than for standard a-Si:H. Furthermore, preliminary results on solar cells with intrinsic a-Si:H deposited at 7 nm/s are presented. Relating the film properties to the SiH4 dissociation reactions reveals that optimum film quality is obtained for conditions where H from the plasma source governs SiH4 dissociation and where SiH3 contributes dominantly to film ...

On the growth mechanism of a-Si:H

Abstract The kinetic growth model for hydrogenated amorphous silicon (a-Si:H) from SiH 3 radicals in SiH 4 plasmas is reviewed on the basis of recently obtained experimental and computational data. New surface reactions are considered and their implications for the a-Si:H film growth mechanism are discussed. Furthermore, from the experimentally observed substrate temperature-dependence of the bulk hydrogen content and the composition of the a-Si:H surface hydrides, it is concluded that surface processes play an important role in hydrogen elimination from the film during growth.

Infrared analysis of the bulk silicon-hydrogen bonds as an optimization tool for high-rate deposition of microcrystalline silicon solar cells

It is demonstrated that the signature of bulk hydrogen stretching modes in the infrared of microcrystalline silicon (μc-Si:H) deposited at high deposition rates can be used for solar cell optimization in the high pressure depletion regime. A relation between the performance of a p-i-n solar cell and the hydride stretching modes corresponding to hydrogenated crystalline grain boundaries is observed. These crystalline surfaces show postdeposition oxidation and the absence of these surfaces in the μc-Si:H matrix reflects device grade microcrystalline material.

Temperature dependence of the surface roughness evolution during hydrogenated amorphous silicon film growth

The scaling behavior of the surface morphology of hydrogenated amorphous silicon deposited from a SiH3 dominated plasma has been studied using atomic force microscopy and in situ ellipsometry. The observed substrate temperature dependence of growth exponent β reflects a crossover behavior from random deposition at 100 °C to a surface diffusion controlled smoothening around 250 °C to full surface relaxation around 500 °C. This crossover behavior has been reproduced by Monte Carlo simulations assuming a site dependent surface diffusion process, revealing an activation energy of ∼1.0 eV for the ruling surface smoothening mechanism. The implications for a-Si:H growth are discussed.

High hole drift mobility in a-Si:H deposited at high growth rates for solar cell application

Time-of-flight measurements on hydrogenated amorphous silicon deposited with a remote expanding thermal plasma at growth rates up to 12 nm/s have revealed a 7 to 10 times larger hole mobility than for films deposited with conventional rf-PECVD. The electron mobility on the other hand is up to 3 times less. Based on a determination of the density of states by post-transit photo-current analysis we suggest a comparable defect density at mid-gap as for films deposited with rf-PECVD. These material properties have been obtained at a substrate temperature of 400°C, which is needed to obtain solar grade material at these growth rates. Possible causes of these particular material properties, which may have application in thin film solar cells, are discussed. Furthermore we show that the high substrate temperature is still a drawback in solar cell preparation when using the standard p-i-n configuration.

Direct and highly sensitive measurement of defect-related absorption in amorphous silicon thin films by cavity ringdown spectroscopy

Cavity ringdown spectroscopy has been applied to hydrogenated amorphous silicon (a-Si:H) showing that this fully optical method is suited for the detection of defect-related absorption in thin films with a minimal detectable absorption of 1×10−6 per laser pulse and without the need for a calibration procedure. Absolute absorption coefficient spectra for photon energies between 0.7 and 1.7 eV have been obtained for thin a-Si:H films (4–98 nm) revealing a different spectral dependence for defects located in the bulk and in the surface/interface region of a-Si:H.

The effect of ion-surface and ion-bulk interactions during hydrogenated amorphous silicon deposition

The ion-bombardment induced surface and bulk processes during hydrogenated amorphous silicon (a‐Si:H) deposition have been studied by employing an external rf substrate bias (ERFSB) in a remote Ar–H2–SiH4 expanding thermal plasma (ETP). The comparison of the ETP chemical vapor deposition without and with ERFSB enables us to identify some important ion-surface and ion-bulk interactions responsible for film property modifications. Employing ERFSB creates an additional growth flux and the low energetic ions deliver an extra 5–10eV per Si atom deposited at typical deposition rates of 10–42A∕s which is a sufficient ion dose to modify the film growth. It is demonstrated that the extra surface and bulk process during a‐Si:H growth, induced by the additional ion bombardment, provide an extra degree of freedom to manipulate the a‐Si:H microstructure. An ion-film interaction diagram is introduced, which is used to discriminate ion-surface interactions from ion-bulk interactions. According to this ion-film interacti...

Highly Efficient Hybrid Polymer and Amorphous Silicon Multijunction Solar Cells with Effective Optical Management

Highly efficient hybrid multijunction solar cells are constructed with a wide-bandgap amorphous silicon for the front subcell and a low-bandgap polymer for the back subcell. Power conversion efficiencies of 11.6% and 13.2% are achieved in tandem and triple-junction configurations, respectively. The high efficiencies are enabled by deploying effective optical management and by using photoactive materials with complementary absorption.