M. Mäenpää

Phase transformations in laser‐irradiated Au‐Si thin films

Laser‐pulse‐induced melting, interdiffusion, and rapid resolidification are applied to deposited Au‐Si thin films of various compositions. It is found that, if 30‐ns pulses are used, amorphous Au‐Si films can be produced over a compositional range 9–91 at.% Au. The stability of the amorphous phases varies with their composition. Thermal decomposition involves the formation of a single‐metastable silicide with a hexagonal structrue. Application of 300‐μs laser pulses directly leads to formation of the same compound.

Electrical characteristics of thin Ni2Si, NiSi, and NiSi2 layers grown on silicon

We have determined the resistivity, carrier concentration, and Hall mobility as a function of thickness (700–3000 Å) of Ni2Si, NiSi, and NiSi2 layers formed by vacuum annealing at 270÷v300°C, ≈ 400°C, and ≈ 800°C, respectively, of nickel films vacuum-deposited on a silicon substrate (111 n-type and 100 p-type Si ρ ≈ 1KΩ). The layer thicknesses were measured by 2 MeV4He+ backscattering spectrometry. The silicide phase was confirmed by x-ray measurements. The electrical measurements were carried out using van der Pauw configuration. We found the electrical transport parameters to be independent of the film thickness within the experimental uncertainty. The Hall factors were assumed to be unity. The majority carriers are electrons in NiSi and holes in Ni2Si and NiSi2. The resistivity values are 24±2, 14±1, and 34±2 μΩcm, the electron concentrations are 9±3, 10 and 7±1, and ≈ 2 × 1022 cm−3, and the Hall mobilities are 3±1, ≈ 4.5 and 6, and ≈ 9 cm2/Vs for Ni2Si, NiSi (〈100〉 and 〈111〉), and NiSi2, respectively. The systematic error in the measured values caused by currents in the high resistivity substrate is estimated to be less than 6% for the Hall coefficient. The results show that Ni2Si, NiSi, and NiSi2 layers formed by a thin film reaction are electrically metallic conductors, a result which concurs with those reported previously (1) for refractory metal silicides. The Hall mobility increases with the Si content in the silicide. The electron concentration is lowest for NiSi2 leading to the highest resistivity for the epitaxial phase of NiSi2.

Epitaxial regrowth of thin amorphous GaAs layers

Channeling and transmission electron microscopy have been used to investigate the parameters that govern the crystal quality following capless funace annealing at low temperature (∼ 400 °C) in ion‐implanted GaAs. From the results obtained, we concluded that the crystal quality after annealing depends strongly on the thickness of the amorphous layer generated by ion implantation and the number of residual defects increases linearly with the thickness of the implanted layer. Single‐crystal regrowth free of defects detectable by megaelectron volt He+ channeling was achieved for a very thin amorphous layer (≲ 400 A).

Epitaxial growth of amorphous Ge films deposited on single‐crystal Ge

The epitaxial growth of amorphous Ge films deposited onto 110 Ge substrate is demonstrated. Substrate cleaning prior to deposition involves only conventional chemical procedures. The growth appears to be a strong function of the interface cleanliness. Two different growth mechanisms are observed: (a) a direct transition from amorphous to single-crystalline layer and (b) the growth involving the transition of amorphous to polycrystals to single crystal.

Epitaxial reordering of ion-irradiated NiSi2 layers

Abstract Thermally grown (at about 800°C) epitaxial NiSi2 layers on silicon single-crystal substrates were ion irradiated at liquid nitrogen temperature so that the surface region was nearly amorphous but with a crystalline NiSi2 seed near the NiSi2Si interface. The epitaxial reordering of the highly defective region occurs in a layer-by-layer manner and at relatively low temperatures (about 80°C). The activation energy for regrowth was found to be 1.2 ± 0.2 eV. These results indicate that the growth characteristics of NiSi2 without mass transport are drastically different from those where mass transport is required such as in the case of thermally formed NiSi2 from Ni/c-Si couples (where c-Si denotes crystalline silicon).

Saturation of Si activation at high doping levels in GaAs

Abstract Ion implantation of Si is extensively employed in the fabrication of GaAs integrated circuits as an n-type dopant. We have investigated the electrical activation of Si in GaAs with high dose (1017–1019cm−3) room temperature Si implantations in semi-insulating GaAS. A co-implantation of As with Si was used to study the influence of local stoichiometry and substrate morphology on the electrical activation of Si. A van der Pauw method was employed for electrical characterization. Our results show that the previously reported saturation in the free electron concentration at 2 × 1018 cm−3 is not altered by co-implanted As. We compile our results with those of several publications to obtain the electrical activation after ~850°C annealing for implanted Si in the concentration range 1016–1020 Si cm−3. A quantitative description of the saturation effect is presented and discussed in terms of a simple saturation mechanism.

Interdiffusion of thin bilayers of copper and nickel

Abstract The interdiffusion of vacuum-deposited Cu/Ni bilayers is investigated for annealing at temperatures from 500 to 800°C. Backscattering spectra, electrical sheet resistance data and X-ray analysis confirm independently that strong interdiffusion sets in at 400°C after annealing for 15 min. We conclude that thin nickel films will not perform as reliable diffusion barrier for copper in metallization schemes with silicon.

GeSi heterostructures by crystallization of amorphous layers

Abstract Heterostructure of Ge x Si 100− x layers on Si〈100〉 substrates were fabricated by solid phase growth. The samples were analyzed by X-ray diffraction, transmission electron microscopy, megaelectronvolt backscattering spectrometry and four-point probe measurements before and after thermal annealing. The conditions at the interface between the amorphous layer and the substrate play an important role in controlling the crystalline structure of the grown layer. With increasing content of germanium in the alloy, the onset time (delay time) for crystallization is observed to decrease. A model is developed to rationalize the crystallization behavior observed on both single-crystal silicon and SiO 2 substrates.

Contact resistivities of sputtered TiN and TiTiN metallizations on solar-cell-type-silicon

Abstract Contact resistivities of TiN and TiTiN contacts on a shallow junction solar-cell-type silicon substrate have been investigated. The contact materials were sputter-deposited. The method of the transmission line model was applied for contact resistivity measurements. The contact resistivity of the n + SiTiN contact system was 2 × 10 −3 Ωcm 2 ± 50 per cent and remained constant after annealing up to 700°C for 30 min. For the n + SiTiTiN system, the contact resistivity of 9 × 10 −4 Ωcm 2 ± 50 per cent was measured. A heat treatment of 700°C. 30 min decreases this value by one order of magnitude and the interposed Ti fully reacts with Si and forms a TiSi 2 layer. The voltage drop caused by the n + SiTiN contact system in a standard non-concentrator solar cell is negligible. The n + SiTiSi 2 TiN contact system should be acceptable for Si solar cells used at up to 100 times solar concentration.

Crystallization investigation of NiSi2 thin films

Recrystallizations of ion-irradiated single crystalline and polycrystalline NiSi2 films are investigated. In the single crystalline case, the irradiated surface portion of the NiSi2 film reorders epitaxially in a layer-by-layer manner, initiating from the remaining undamaged single crystalline NiSi2 seed near the NiSi2/Si interface. The irradiated polycrystal-line NiSi2 layer recrystallizes via growth from a fixed number of existing nuclei in the layer. In both cases, the recrystallization occurs with a relatively high velocity at very low temperatures (~ 100°C) and the activation energy of the growth rate is similar (1.2 to 1.4 eV). These results reflect the metallic bonding nature of NiSi2 and the fact that nucleation and mass transport are not required for growth from an amorphous mixture with a stoichiometric atomic composition and existing nuclei.