Modification of Thin Film Growth using Glancing-Angle Ions
- PDF / 2,165,001 Bytes
- 11 Pages / 417.6 x 639 pts Page_size
- 11 Downloads / 209 Views
91 Mat. Res. Soc. Symp. Proc. Vol. 585 ©2000 Materials Research Society
This paper describes applications of glancing-angle ions for modifying semiconductor surfaces, including sputter cleaning, smoothening, enhancing surface diffusivity, and producing in-plane orientation of polycrystalline films. EXPERIMENTAL PROCEDURES The experiments were carried out in a III-V semiconductor molecular beam epitaxy (MBE) chamber with a 25 keV reflection high-energy electron diffraction (RHEED) system and a specular ion current measurement (SICM) system that has been described elsewhere.[2] In the specular ion current measurements, the ion beam is focused on the substrate surface at a glancing angle and the specularly scattered ion current is measured. In the present experiments, the Ar or He ion beam was incident at 4)= 3 to 15' relative to the nominal substrate surface plane. The beam energy was I to 4 keV, with typical current densities of = 10 ptA-cm2 . For the SICM measurements, the scattered ion beam was detected using a movable Faraday cup. The substrates for the growth experiments were semi-insulating, double-polished vicinal GaAs(001) wafers, misoriented by 2.50±0.50 towards (110), as measured using RHEED.[5]. For polycrystalline GaAs film growth, the substrates were Si(001) that had been thermally oxidized to produce an amorphous SiO 2 surface. For the static surface experiments, singular GaAs(001) and Si(001) surfaces were used. The surface cleaning procedures are described below. GaAs growth was carried out from Ga and As 4 beams. RESULTS AND DISCUSSION Surface Smoothening Ion bombardment often promotes surface roughening and ripple formation. [6] On the other hand, experimental data suggests that glancing angle ions can have a smoothening effect. Early work in this area was carried out by Spiller [7 ], who used a 300 eV Ar ion beam incident at 4) = 10"to smoothen the layers in Rh/C multilayer x-ray mirrors. By using ion bombardment between layer depositions, the reflectivities were increased by as much as a factor of 2. We have investigated the effect of a 1 keV Ar ion beam incident at 4)= 15" on the GaAs(001) surface. In order to demonstrate a smoothening effect, a surface was initially roughened by heating to 650 0 C at a low As 4 overpressure. Atomic force microscopy (AFM), using a Digital Instruments Nanoscope III AFM with Si 3N 4 tips operated in contact mode in air, was used to measure surface morphology after growth. AFM images of the surface before (Fig. I a) and after (Fig. Ib) ion bombardment with a dose of 2.3x101 ions/cm2 showed a dramatic reduction in roughness from a RMS value of 8 nm to 0.5 nm. The pronounced smoothening effect of glancing-angle ions can be explained based on the combination of two factors. First, for small 4) the sputter yield increases rapidly with increasing 4).The local 4value is higher at surface protrusions than at flat portions of a surface, leading to a larger sputtering yield for protrusions. Second, protrusions intercept a larger ion current than the same area of flat surface
Data Loading...
