H. Jia

Hydrogen Diffusion in Undoped and B-Doped a-Si 1-x C x :H

A deuterium secondary ion mass spectrometry (SIMS) study of hydrogen diffusion in undoped and boron-doped a-Si 0.86 C 0.4 :H deposited by an electron cyclotron resonance plasma is described. The undoped films deposited at 250°C clearly indicated deuterium-hydrogen interdiffusion at T ≥ 350°C. The dispersion parameter a of the power-law time dependent diffusion constant D = D 00 (ω t ) −α decreased from ∼0.3 at T = 350 and 400°C to ∼0.1 at 450°C, and the activation energy for a diffusion length of 1000 A was ∼1.0 eV. These results are discussed in relation to previous studies of a-Si:H. The diffusion in ∼0.2 and ∼0.6 at.% B-doped a-Si 0.86 C 0.14 :H sharply differs from that in B-doped a-Si:H, where an enhancement of up to ∼10 3 was previously observed. In doped a-Si 0.86 C 0.14 :H, the diffusion of most of the H atoms is strongly suppressed, but a small fraction undergoes fast diffusion. IR Measurements indicate that the B-doping reduces the bulk-like Si-H stretch vibration at ∼2000 cm 1 . Upon annealing, the Si-CH n and C-H wag modes at ∼780 and ∼1000 cm −1 , resp., increase, while the 640 and ∼2000 cm 1 Si-H wag and stretch modes, resp., weaken, indicating transfer of hydrogen from Si- to C-bonds, in which the H atoms are apparently deeply trapped. As in a-Si:H, the fast diffusion component is apparently due to carrier recombination-enhanced weak Si-Si bond breaking. The results suggest that B-doping also induces microvoids and enhances the rate of breaking of weak Si-C bonds, leading to enhanced trapping of H.

Mobility-Lifetime Products in Glow Discharge and RF Sputter Deposited A-Si:H

A comparative study of the deposition temperature (T s ) dependence of the Mobility-lifetime (μτ) products of the charge carriers in glow-discharge and rf sputter-deposited a-Si:H is described and discussed. The T s -dependence of the μτs the majority carrier light-intensity exponents of the two types of films are strikingly similar. These observations lead to the conclusion that the structure of the recombination levels as well as the recombination processes are in accord with the “defect pool” Model, in contrast to previous suggestions. The differences between the two types of films thus appear to be limited to the differences in the concentrations of dangling bonds.

Small Angle X-Ray Scattering (Saxs) and IR Study of Microvoid Dynamics in Annealed RF Sputter-Deposited A-SI:H

A SAXS and IR study of microvoid distribution and dynamics in a-Si:H deposited by rf sputtering at 200 – 600 W on nominally unheated substrates is described and discussed. The 640 cm -1 band of the 200 W film yielded a total Si-bonded H content C H =21 at. %; the 840 – 890 cm -1 band yielded a dihydride content CH 2 3.4 at. %. The SAXS measurements yielded a microvoid volume fraction vf=8.2 vol.%, and tilting SAXS data indicated elongated voids consistent with a columnar microstructure. In the other films, 9 H 2 was negligible, vf was -2 vol. %. Annealing from 250°C to 310°C for 6 hrs resulted basically in no changes of C H and vf. However C H decreased and vf increased significantly after annealing at 350°C and above. The results showed a strong correlation between the IR determined C H and C H2 and the SAXS determined vf.

On the Nature of the Native Defect ESR in Thin Diamond Films

The X-band ESR of thin diamond films deposited from 99.5% H 2 /0.5% CH 4 is compared to that of films similarly prepared from D 2 CD 4 and H 2 / 13 CH 4 . The main line and the satellites at ±7.2 G are unaffected by annealing at T 13 C spin-lattice relaxation rate, which indicates that the distribution of paramagnetic centers is homogeneous to within ∼0.04 μm. However, they may be nonuniformly distributed on a finer scale, consistent with the concentrations in m ulti vacancies or stacking faults recently suggested by Fanciulli and Moustakas.

Study of Short-Range Motion of Atomic Hydrogen in Amorphous Silicon by Neutron Reflectometry

Preliminary results of neutron reflectometry (NR) measurements on rf sputter-deposited a-Si:H/a-Si:D bilayers indicate that this technique may be used to monitor H and D motions over distances of {approx} 10 to 200 {Angstrom} with a nominal resolution of 5--10 {Angstrom}. In studying rf sputter-deposited thin films containing a high density of microvoids annealed at 270 C, we found that the hydrogen diffused a distance of only {approx} 100 {Angstrom}. Further annealing at 270 and 280 C produced no additional motion. This result is consistent with a model of this system in which the hydrogen is trapped in microvoids after moving a relatively short distance.