P. A. Stolk

On the nanostructure of pure amorphous silicon

New features of the nanoscale structure of amorphous (a)‐Si produced by ion‐implantation‐induced amorphization of crystalline (c)‐Si have been determined by the technique of small‐angle x‐ray scattering (SAXS). Si ion energies up to 17 MeV were used to generate a thick amorphous layer (8 μm) on a c‐Si wafer to enable the SAXS measurements. As‐implanted and thermally annealed (up to 540 °C) a‐Si were studied. No nanovoids were detected within a sensitivity of 0.1 vol %, but the atomic‐scale structure produced a measurable diffuse scattering signal that decreased with increasing anneal temperatures. These measurements show that the known density deficit of 1.8% in a‐Si relative to c‐Si cannot be due to voids and that a‐Si is homogeneous on nm length scale.

Explosive crystallization of amorphous silicon: triggering and propagation

Abstract Amorphous silicon may be transformed into crystalline silicon via a self-sustained process driven by the latent heat released upon crystallization. This is called explosive crystallization and is found to occur under rapid-heating conditions such as laser annealing. In this paper we compare different kinds of explosive crystallization, with emphasis on pulsed-laser induced explosive crystallization of ion-implanted amorphous silicon. It is shown that explosive crystallization of amorphous surface layers yields randomly oriented fine-grain polycrystalline silicon while explosive crystallization of amorphous layers buried beneath a crystalline top layer results in epitaxially aligned single-crystal silicon. The results are used to discuss ultra-rapid crystal nucleation and growth.

Structural and electrical defects in amorphous silicon probed by positrons and electrons

The structure of pure amorphous Si, prepared by ion implantation, has been investigated by variable‐energy positron annihilation spectroscopy (PAS) and lifetime measurements of optically generated free carriers. In general, PAS measurements are thought to be sensitive to vacancy‐type defects while the carrier lifetime depends on the density of band‐gap states (e.g., dangling bonds). The PAS measurements indicate that the density of positron‐trapping defects can be reduced by thermal annealing at 500 °C. Concurrent with the removal of structural defects the density of band gap states is reduced as indicated by an increased photocarrier lifetime by a factor of 10. Some material has been implanted with H+ and annealed at a low temperature (150 °C). The hydrogen is expected to passivate electrical defects associated with strained and dangling bonds and indeed the photocarrier lifetime is increased in this material. Moreover, the PAS measurements cannot distinguish this material from 500 °C annealed amorphous ...

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.

Triggering explosive crystallization of amorphous silicon

Abstract The initial stages of explosive crystallization (EC) of amorphous Si (a-Si) are investigated. A thin layer of liquid Si (l-Si), highly undercooled with respect to crystalline Si (c-Si) is formed by nanosecond pulsed ruby laser irradiation of a-Si prepared by ion implantation. Time-resolved reflectivity measurements are used to determine the time delay in the onset of EC for different surface structures. If a thin single crystal layer of Si covers the a-Si, EC proceeds immediately. In the absence of a seed for EC, a maximum time delay of 11±2 ns is observed. Intermediate delay times are found if the surface layer contains small c-Si clusters.

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.

Picosecond Photocarrier Lifetimes in Ion-Irradiated Amorphous and Crystalline Silicon

Crystalline silicon (c-Si) and structurally relaxed amorphous silicon (a-Si) were implanted with 1 MeV Si + at liquid nitrogen temperature. The photocarrier lifetime τ in the implanted samples was determined with sub-picosecond resolution through pump-probe reflectivity measurements. At low damage levels (i.e. 14 ions/cm 2 ), τ decreases with increasing ion dose in both materials, indicating a build up of trapping and recombination centers. The dominant centers in c-Si appear to be related to simple defects. The generation rate of electrically active defects is found to be the same in relaxed a-Si and c-Si, which suggests that the structural defects formed in a-Si strongly resemble the simple defects in c-Si. For ion doses > 10 14 /cm 2 , τ saturates at a level of 0.8 ps for both materials. Strikingly, the saturation sets in far below the dose needed to amorphize (>10 15 /cm 2 ). The defect density in a-Si at saturation is estimated to be ≈1.6 at.%.