J.C. van den Heuvel

The relation between the optical properties and the hydrogen concentration in a-Si:H

Abstract The relation between the optical properties of hydrogenated amorphous silicon (a-Si:H) and the hydrogen concentration in the material was studied. A quantitative model is presented that describes the relation between the optical gap, the refractive index, and the hydrogen content of a-Si:H. This model is based on a model for the incorporation of hydrogen in the a-Si:H network, the concept of bond polarizabilities, and the harmonic oscillator approximation. We determined the optical gap and refractive index from reflection and transmission measurements. The hydrogen content was determined by nuclear reaction analysis (NRA). The values obtained from the theoretical model correspond well with our experimental results and correspond also with experimental results from literature.

Diffusion length measurements of thin amorphous silicon layers

Abstract A new method for the analysis of diffusion length measurements by the Surface Photovoltage (SPV) method is presented. It takes into account the effect of the reflection of light from the back contact in thin layers and the effect of a finite bandwidth of the used interference filters. The model was found to agree with experiments on thin amorphous silicon (a-Si:H) layers. It is shown that in the region were these effects are negligible this method is equivalent to the standard method.

A PDS study of hydrogenated amorphous silicon films deposited by RF glow-discharge from silane-hydrogen mixtures

Abstract The sub-bandgap absorption of intrinsic hydrogenated amorphous silicon films deposited by rf glow-discharge from different silane-hydrogen mixtures has been measured by means of PDS. Using a mixture of 55 vol.% silane, 45 vol.% hydrogen and a relatively high rf power level of 36 mW/cm 2 to obtain a high growth rate, yielded films with an improved structural order, as deduced from the slope of the Urbach edge, compared to films grown under the conditions of using pure silane and an rf power just sufficient to maintain the plasma. These conditions are known to yield good quality films, but with the disadvantage of having a low growth rate.

Growth model for microcrystalline silicon

Abstract Hydrogenated amorphous and microcrystalline silicon films were deposited using the RF glow discharge method. Silane-hydrogen mixtures were used. The conditions under which amorphous and microcrystalline silicon are formed have been investigated with regard to the deposition rate. The deposition rate of good quality amorphous and microcrystalline silicon appeared to be nearly equal. This indicates that the description of the deposition conditions for microcrystalline material, commonly mentioned as a near chemical equilibrium situation, is not complete. Both types of films can be etched in a hydrogen plasma. The etch rate of the amorphous film was about ten times faster compared to the etch rate of microcrystalline films. Based on the results a growth model for the formation of microcrystalline films has been developed.

The ambipolar diffusion length measured by the surface photovoltage technique

The surface photovoltage (SPV) technique is often used to determine the diffusion length in hydrogenated amorphous silicon (a‐Si:H). The use of this technique for a‐Si:H is disputed because it has been shown by computer simulations that the drift component is not negligible compared to the diffusion current in SPV measurements on a‐Si:H. For crystalline semiconductors the SPV technique gives the minority‐carrier diffusion length while the drift current is negligible. We found from the solution of the transport equations that the SPV technique can be used in the case of a‐Si:H, but gives the ambipolar diffusion length which consists of the mobility and lifetime of both holes and electrons. However, the term diffusion length is misleading since the hole drift current and the hole diffusion current are of the same order of magnitude for a‐Si:H. This result is in accordance with the results of the computer simulations mentioned earlier. The drift current assists the diffusion current and increases the measure...

Improved Stability of RF Glow Discharge Deposited a-Si:H Achieved by Hydrogen Dilution of Silane.

The magnitude of metastable light-Induced degradation of intrinsic hydrogenated amorphous silicon films deposited by rf glow discharge from different selene-hydrogen mixtures wan studied. By stone of spacecharge-limited current measurements the Fermi-lava1 density of states was determined for as-grown and light-soaked n + -t-n + devices. Films deposited using a mixture of 55 vol.% silane and 45 vol.% hydrogen and a relatively high rf power level of 62 mW/cm 2 to obtain a high growth rate exhibited an improved stability with regard to light-Induced degradation of the film, compared with films grown under the conditions of using pure etlane and an rf power Just sufficient to seintaln the plasma, conditions known to ylald good quality films. The results are explained in terms of an improvement in the structural oraer of the film.

The effect of silane dilution with hydrogen on the deposition of glow-discharge-deposited silicon films

The effect of silane dilution with hydrogen at different power levels on the growth rate of intrinsic hydrogenated amorphous and microcrystalline silicon films has been studied. The results are explained from the point of chemical equilibrium. Good quality microcrystalline and amorphous layers are grown with nearly the same deposition rate. Above a deposition rate of 1 As−1 only amorphous layers are formed Hydrogen dilution appears to be necessary to synthesise glow-discharge microcrystalline hydrogenated silicon.