A. J. M. Berntsen

Contribution of defects to electronic, structural, and thermodynamic properties of amorphous silicon

The structure of pure, nonhydrogenated amorphous silicon (a‐Si) was modified by means of ion implantation, furnace annealing, and pulsed laser annealing. Defects in a‐Si were probed by measuring the photocarrier lifetime τ at low carrier densities (1018/cm3) with subpicosecond resolution using pump‐probe reflectivity measurements. The average cross section of defect‐related midgap states for free‐carrier capture is found to be 6×10−16 cm2. In addition, the average bond‐angle distortion Δθ in a‐Si was derived from Raman spectroscopy. Annealing as‐implanted a‐Si for 1 h at T≤500 °C induces defect annihilation as well as network relaxation. In contrast, 32 ns pulsed laser heating of a‐Si just below the melting threshold leads to relaxation of Δθ without significant defect annihilation. This annealing behavior can be understood on the basis of defect diffusion kinetics. Implanting fully relaxed a‐Si with 1 MeV B+, Si+, and Xe+ up to damage levels of 0.004 displacements per atom raises the defect density witho...

Local structure and bonding states in a‐Si1−xCx:H

Infrared spectra of a‐Si1−xCx:H deposited in a glow discharge of a silane/methane mixture have been measured. Comparison with elastic recoil detection and Rutherford backscattering spectrometry shows that the mean number of hydrogen atoms attached to silicon per silicon atom ([HSi]/[Si]) increases with higher carbon content and that more Si—H2 bonding configurations are formed. Hydrogen is preferentially bonded in (Si—H2)n clusters, which partly explains the observed apparent shift of the Si—H stretching mode to higher energy. The remaining contribution to this shift is believed to result from Si—H on surfaces of voids instead of an inductive effect. From composition measurements we observe that for each carbon atom, three hydrogen atoms are incorporated in the material, suggesting that during deposition carbon is initially incorporated in CH3 groups. However, the mean number of C—H bonds per carbon atom decreases from about 2.2±0.4 to 1.4±0.3 with increasing carbon content, indicating that the majority o...

Structural, compositional and optical properties of hydrogenated amorphous silicon-carbon alloys

Abstract Steady-state optical modulation spectroscopy (OMS) measurements have been carried out on well characterized hydrogenated amorphous silicon-carbon alloy layers. A series of samples with methane-to-silane gas flow ratios varying from 0.1 to 0.7 was deposited at 250°C using the conventional r.f. glow-discharge deposition method. In order to characterize the material, we investigated the structural, compositional and optical properties of these alloys by means of Fourier-transform infrared spectroscopy, Raman spectroscopy, elastic recoil detection, Rutherford back-scattering spectrometry and optical transmission and reflection spectroscopy. OMS measurements were performed at room temperature and at T 50 K. From room-temperature data the energy positions of the dangling-bond states were deduced. Transition energies involving D° states are found to shift to higher values with increasing carbon content, while transition energies involving D− states remain almost constant. At lower temperatures, band-ta...

Defects in amorphous silicon probed by subpicosecond photocarrier dynamics

The photocarrier dynamics in pure nonhydrogenated amorphous silicon (a‐Si) have been studied with subpicosecond resolution using pump‐probe reflectivity measurements. The photocarrier lifetime increases with the annealing temperature from 1 ps for as‐implanted a‐Si to 11 ps for a‐Si annealed at 500 °C. The lifetime in annealed a‐Si can be returned to the as‐implanted level by ion irradiation. These observations indicate that a‐Si can accommodate a variable number of defect‐related trapping and recombination centers. The saturated defect density in as‐implanted a‐Si is estimated to be ≊1.6 at. %. Comparison with Raman spectroscopy suggests that various kinds of structural defects are present in a‐Si.

Characterization of amorphous silicon

S‐parameter positron beam measurements have been done on several kinds of a‐Si: Kr‐sputtered a‐Si, PECVD a‐Si, MeV ion beam amorphized Si and a‐Si grown in an MBE‐system at a low deposition temperature. Kr sputtered a‐Si becomes denser for higher Kr concentration. PECVD a‐Si:H contains micro‐cavities with a size depending on growth temperature. MeV ion beam amorphized Si contains 1.2 at. % small vacancies, which decreases upon annealing (relaxation) to 0.4 at. %. This effect can be mimicked by H‐implantation and subsequent annealing, showing that at least some of the dangling bonds in a‐Si are located at these vacancy‐type defects. Finally positron measurements show that MBE‐system grown a‐Si contains large open‐volume defects. The positron annihilation data are supplemented by data from some other techniques.

Ion implantation into amorphous solids

We describe the effects of implantation of silicon into pure amorphous silicon and hydrogenated amorphous silicon and of erbium into soda-lime silicate glass and hydrogenated amorphous silicon. The former implantations and subsequent annealing treatments are used as a means of controlled defect creation. The experiments lead to new insights into the relation between defect density and bond-angle variation in both types of amorphous silicon. Erbium implantation is used to produce planar optical waveguide materials. It is demonstrated that erbium implantation into glass and amorphous silicon leads to efficient photoluminescence. It appears that these materials are attractive candidates for Er-doped waveguide amplifiers.

Structural order in thin a‐Si:H films

Hydrogenated amorphous silicon (a‐Si:H) films were grown in an rf‐plasma deposition system on various substrates. The thickness of the films ranged from 11 to 579 nm. The structural properties of the films were studied by means of ex situ Raman spectroscopy. The width of the transverse‐optic peak in the Raman spectrum was used as a measure for the amount of bond‐angle variation in the films. In contrast to earlier reports, it is found that bond‐angle variation in glow‐discharge‐deposited a‐Si:H does not depend on the film thickness, nor on the type of substrate material.

KR INCORPORATION IN SPUTTERED AMORPHOUS SI LAYERS

Amorphous Si layers were grown by krypton plasma sputter deposition at 310 °C. By pulsation of the substrate potential between 0 and 50 eV, the Kr concentration in the layers could be varied to a maximum of 5.5 at. %. A model which describes trapping of inert gas atoms in the sputtered layer in terms of implantation and trapping, diffusion, growth, resputtering, and gas sputtering is presented. High‐resolution electron microscopy, electrode‐probe (x‐ray) microanalysis, positron annihilation, Raman spectroscopy, Mossbauer spectroscopy, and bending and hardness measurements were performed on the deposited layers. It turns out that the ion assisted growth leads to a strong reduction of open volume defects. The experiments point to the presence of very small Kr agglomerates. From the Mossbauer experiments a lower limit of 250 K for the Debye temperature of the Kr agglomerates is derived. Molecular‐dynamic simulations from which the Debye temperatures of Kr mono‐, di‐, and trimers in amorphous Si can be derive...

Separating the Contributions of Hydrogen and Structural Relaxation to Damage Annealing in a-Si:H

This paper investigates the effects of ion implantation and annealing for pure (a-Si) and hydrogenated amorphous silicon (a-Si:H). The photocarrier lifetime in as-deposited a-Si:H decreases from ≥200 to 3 ps after 1 MeV Si + implantation to doses exceeding 10 14 /cm 2 . A comparison with relaxed a- Si suggests that damage generation in a-Si:H merely arises from displacements in the silicon network. Annealing of ion-damaged a-Si:H at 200-500 °C recovers the carrier lifetime to 60-100 ps as a result of hydrogen passivation of electrical defects. However, Raman spectroscopy shows that hydrogen does not significantly enhance long-range network relaxations during annealing. This implies that thermal treatments of ion-implanted a-Si:H can not fully recover the as-deposited state.