Pulsed UV Laser Irradiation of ZNS Films on SI and GAAS
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678 In the case of films as has previously been reported in the literature [6]. were employed. films sputtered onto Si two types of substrate cleaning procedure 0 Torr. The first consisted of a chemical clean followed by a 400 C bake at 10 Alternatively in-situ argon-ion etching just prior to deposition was utilised. GaAs substrates were subjected only to the former cleaning procedure. During sputtering the Ar gas pressure was ,v10-2 Torr and the substrates were maintained either at room temperature or \ 200°C, with deposition rates lying in the range 20-60 R min-1 . All the sputtered films were polycrystalline. The film thicknesses employed ranged between 11v 0.2 - 1.0 1m. Selected specimens 2 were implanted with 300 KeV Mn ions at a dose of "ý 3-5 x 101i ions cm- at A.E.R.E. Harwell. In view of the large band-gap of ZnS, the laser anneals were carried out with an electron beam-pumped rare-gas halide laser emitting in the UV, as The 308 nm XeCl line was used predominantly previously described [3,4]. although some anneals were carried out using the 351 nm radiation from XeF. Typical pulse widths were in the range 30-40 ns and energy densities of up to 112.5 J cm- 2 could be obtained by focussing the beam. No beam homogenising apparatus was used; however beam uniformity over the annealed areas was estimated to be no worse than '\± 10%. During annealing the specimens were in an Ar environment in a specially constructed cell capable of being pressurised to '• 1000 psig. It has been shown that significant vaporisation of ZnS occurs In the present if the anneals are carried out at atmospheric pressure [3,4]. studies annealing pressures of '\ 100-200 psig were used as no significant improvements have been noticed at higher pressures. Reflection high energy electron diffraction (RHEED) was utilised to The data was determine film structure before and after laser annealing. obtained in a Philips EM 200 electron microscope operating at 80 kV. Specimen surfaces were also studied optically using Nomarski interference microscopy. The SIMS profiles were determined at Imperial College, London. RESULTS Figure 1 shows characteristic RHEED patterns from 'v 0.8 pm thick single The data for the virgin crystal films on Si subjected to various treatments. film, Figure la shows a spot pattern corresponding to cubic ZnS but additional reflections are observed. These can be accounted for by invoking the presence of twinning and double diffraction effects but the presence of a small amount of Results obtained after Mn hexagonal material cannot be ruled out [7]. implantation and a 1 hour, 500 0 C thermal anneal of the implanted film are shown in Figures lb and lc respectively. From the re-emergence of the spots in Figure lc, thermal annealing appears to be effective in restoring some structural order. It is also interesting to note that there is no hint of the additional satellite spots of Figure la in Figure 1c, the latter being characteristic of purely cubic material. The data shown in Figure ld was TABLE 1 Thermal expansion coeffici
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