Jérôme Perrin

Temperature dependence of the sticking and loss probabilities of silyl radicals on hydrogenated amorphous silicon

Abstract The sticking probability ( s ) and the total loss probability (β) of silyl (SiH 3 ) radicals on the growing surface of hydrogenated amorphous silicon (a-Si : H) were determined from (1) the deposition of a-Si : H, (2) the step-coverage in a trench structure on a c-Si wafer, and (3) the determination of β using a grid system, performed in the temperature range from room temperature to 480 ° C. A sticking probability ( s ) of 0.09 ± 0.02 and a total loss probability (β) of 0.26 ± 0.05 were determined for the SiH 3 radicals in the surface temperature range below 300 ° C. From 350 ° C, the sticking probability increases whereas β remains constant up to 460 ° C where s ≈ β and reaches saturation. These results are interpreted in terms of surface diffusion of SiH 3 depending on the hydrogen coverage of the a-Si : H surface.

Sticking and recombination of the SiH3 radical on hydrogenated amorphous silicon: The catalytic effect of diborane

The deposition rate of hydrogenated amorphous silicon films in SiH4 glow-discharge is drastically enhanced upon addition of B2H6 when the gas-phase concentration exceeds 10−4. This cannot be attributed to gas-phase reactions and must be interpreted as an increase of the sticking probability of the dominant SiH3 radical. However, the total surface loss probability (β) of SiH3 which includes both sticking (s) and recombination (γ) increases only above 10−2 B2H6 concentration, which reveals that between 10−4 and 10−2 the ratio sβ increases. A precursor-state model is proposed in which SiH3 first physisorbs on the H-covered surface and migrates until it recombines, or chemisorbs on a free dangling bond site. At a typical deposition temperature of 200° C, the only mechanism of creation of dangling bonds in the absence of B2H6 is precisely the recombination of SiH3 as SiH4 by H abstraction, which limits the sticking probability to a fraction of β. This restriction is overcome with the help of hydroboron radicals, presumably BH3, which catalyze H2 desorption.

Dissociation cross sections of silane and disilane by electron impact

Abstract The total dissociation cross sections for silane and disilane are reported for electron energies above their ionization thresholds up to 110 eV. The measurements are derived from a kinetic analysis of silane and disilane dissociation in a constant-flow multipole dc plasma reactor. The methane dissociation cross section was also measured and found in agreement with published data. Maxima for silane and disilane, occurring around 60 eV, are respectively (1.2±3)×10 −15 cm 2 and (2.6±0.6)× 10 −15 cm 2 . Total ionization cross sections are also measured and above 50 eV the ratios of dissociative ionization to dissociation cross sections are 0.5±0.1 and 0.25±0.10 respectively for silane and disilane. The probability for silane elimination in the disilane fragmentation reaches a maximum of 0.8 at 19 eV and decreases down to 0.5 at 100 eV. Dissociation processes of silane and disilane are discussed in comparison with methane and ethane

NUMERICAL MODELING OF THE OPTICAL PROPERTIES OF HYDROGENATED AMORPHOUS-SILICON-BASED P-I-N SOLAR CELLS DEPOSITED ON ROUGH TRANSPARENT CONDUCTING OXIDE SUBSTRATES

Hydrogenated amorphous‐silicon (a‐Si:H) ‐based solar cells consist of two electrodes and a p‐i‐n structure, deposited on glass substrates. Depositing the p‐i‐n layers and the back metallic electrode on an optically rough transparent conducting oxide (TCO) electrode enhances the absorption of the incident light in the active i layer: Light is scattered at the rough front interface and is partially trapped in the high refraction index layer, as in a waveguide. In addition TCO roughness increases the front transmission coefficient, increasing the amount of light in the active layer. TCO texture yields a relative increase of the conversion efficiency up to 30%. A semiempirical model of thin‐film solar‐cell optics is presented, taking into account the interface roughness by introducing experimentally derived scattering coefficients and treating the propagation of specular light in a rigorous way. Numerically simulated spectral response and total reflectance of standard solar cells deposited on different TCO te...

a-Si:H Deposition from SiH4 and Si2H6 rf-Discharges: Pressure and Temperature Dependence of Film Growth in Relation to α-γ Discharge Transition

We present an interpretation of the pressure and temperature dependence of the growth kinetics of hydrogenated amorphous silicon (a-Si:H) in SiH4 and Si2H6 rf-glow discharges. At constant rf power, the a-Si:H deposition rate increases drastically at a given pressure depending on the nature of the gas and on the wall temperature. The threshold nature of this transition is attributed to the onset of an electron avalanche due to ion-induced secondary electron emission and ionization in the plasma sheaths close to the electrodes. We analyze the effect of various plasma parameters governing this α-γ discharge transition in terms of equivalent circuit of the discharge and power dissipation mechanisms. The ratio of a-Si:H deposition rates from SiH4 and Si2H6 rf discharges at the same rf power and flow rate is strongly dependent on the pressure because the α-γ transition occurs at a lower pressure for Si2H6 than for SiH4. The transition is shifted to higher pressures as the temperature increases primarily because of the reduction of gas density, which explains contradicting results in the literature on the influence of temperature on a-Si:H deposition rate. At a given rf power and substrate temperature, the optical, structural and electrical film properties are correlated with the variation of deposition rate as a function of SiH4 or Si2H6 pressure.

Production mechanism and reactivity of the SiH radical in a silane plasma

Abstract SiH free radicals in a dc multipole post-discharge plasma were analyzed using laser-induced fluorescence. The cross section for SiH formation from 40–70 eV electron impact induced dissociation of SiH 4 was measured to be (10 ± 5) × 10 −17 cm 2 . At the instant of its formation in its X 2 Π ground state, the rotational temperature of the SiH radical is 950 K. This temperature subsequently relaxes by collisions with SiH at a collisional relaxation rate k = (9 ± 2) × 10 13 cm 3 /mole s. Quenching of the A 2 Δ electronic state of SiH was found to be negligible below 0.3 Torr. The primary reaction path for destruction of the SiH radical was observed to be via: SiH + SiH 4 → products, k = 2 × 10 12 cm 3 /mole s.

Dissociative excitation of SiH4, SiD4, Si2H6 and GeH4 by 0–100 eV electron impact

Abstract We have measured emission cross sections of various electronically excited fragments produced by electron-impact dissociation of SiH 4 , SiD 4 , Si 2 H 6 and GeH 4 . At low impact energy (10–20 eV), the measured appearance potentials are correlated to specific dissociation processes. Below 22 eV superexcited states of SiH 4 play a dominant role in the formation of neutral excited fragments. In agreement with the results obtained on alkanes, the cross sections for fragment emission from Si 2 H 6 are lower than those for SiH 4 . On the other hand, the comparison of cross sections at 100 eV for fragment emission, dissociation and ionization on going from CH 4 to SiH 4 and GeH 4 shows an increase of the probability for production of neutral ground-state fragments at the cost of excited or ionic fragments. Both effects can be explained by a growing probability for internal conversion among the decay channels of superexcited states with increasing number of atoms or electrons in the parent molecule. For each molecule, the H Balmer-emission cross sections at 100 eV are proportional to n − b , where n ⩾ 3 is the principal quantum number of the upper state of H and 3 b 4 and SiD 4 on both fragment emission and ionization cross sections from 0 to 100 eV gives strong evidence of the competition between dissociation and autoionization in the decay of superexcited states.

Emission cross sections from fragments produced by electron impact on silane

Abstract Optical emission spectroscopy in the visible and near UV of a silane plasma was performed in a low pressure hot cathode glow discharge bounded into a magnetized multipolar wall. Emissions from Si, Si+, SiH, SiH+ and H are shown to originate from the dissociative excitation of silane molecules by electron impact. The absolute cross sections for the various photoemissive processes were measured in the 17–68 eV range. The relevance of optical emission spectroscopy to silane plasma diagnostics is discussed.

Surface reaction and recombination of the SiH3 radical on hydrogenated amorphous silicon

Mercury photosensitized decomposition of SiH4 is used to study surface reactions of SiH3 on hydrogenated amorphous silicon (a‐Si:H). The method involves modeling of gas phase production, reaction and diffusion to the walls of reactive species, in a parallel plate reactor, combined with measurements of surface reflection coefficient of SiH3, spatial density profile of SiH3, and a‐Si:H deposition rate. The reaction probability of SiH3 on a‐Si:H varies from 0.1 up to 0.2 in the 40–350 °C temperature domain. However, a large fraction (≥60%) of adsorbed SiH3 recombine on the surface, instead of being incorporated in the film.

Growth of hydrogenated amorphous silicon due to controlled ion bombardment from a pure silane plasma

The growth process, optical and structural properties of a‐Si:H films deposited from a silane multipole dc discharge are analyzed by real time and spectroscopic ellipsometry, and ir absorption spectroscopy. Films deposited mainly from neutral polymerized species are systematically compared to films deposited from monomeric ionic species at various ion incident energy up to 100‐eV. An increase of ion bombardment energy is shown to favor the formation of high density homogeneous and isotropic films.