Probing buried interfaces with Non-linear optical probes: Si/CoSi₂.

L.J. Richter, K.W. Kolasinski and J.C. Stephenson (844)

Objectives: Characterize non-linear optical techniques as probes of Schottky barrier geometric and electronic structure.

Problem: As semiconductor device design rules shrink, performance is increasingly dependent upon interface quality. However, few diagnostics exist which can characterize buried interfaces; traditional charged particle probes can only access model interfaces between substrates and ultrathin films.

Approach: We have undertaken second harmonic generation (SHG) and sum frequency generation (SFG) studies of Schottky barriers formed between Si(111) and (100) oriented substrates and CoSi₂. SHG and SFG have the traditional advantages of optical probes: they are nondestructive and can be used in-situ/on line; additionally, as nonlinear techniques they have the added advantages of being uniquely interface sensitive (bulk second order mixing is forbidden in centrosymmetric media such as Si and CoSi₂) and having greater sensitivity to interface structure. The Si/CoSi₂ interface is ideal for studies of Schottky barriers as it is atomically abrupt and epitaxial and has been extensively characterized both experimentally and theoretically.

Results and Future Plans: Si/CoSi₂ interfaces were prepared by ion beam mesotaxy. The resulting samples consisted of nominally 20 nm of crystalline Si on top of 60 nm of CoSi₂ on a crystalline Si substrate. For (111) oriented substrates, two interface structures can be formed, aligned or A-type and twined or B-type. The mesotaxy could be optimized to produce interfaces predominantly of each type. Shown in the Figure is a polar representation of the p-polarized SHG intensity at 532 nm, as a function of the angle between the plane of incidence and the indicated Si crystal axes for both A and B-type (111) interfaces.

The 1064 nm fundamental was p-polarized and the angle-of-incidence was 45°. One can clearly identify the twined interfaces by the 180° rotation of the SHG patterns.

Spectroscopic studies of all 3 interfaces: (111) A, (111) B and (100), have been performed via SFG between a fixed 1064 nm probe and a tunable, 1400 nm-2200 nm probe. Previous SFG studies of disordered Au/GaAs Schottky barriers have observed resonant SFG response due to localized defect states in the GaAs band gap. These states have been associated with Fermi-level pinning. Our studies of the well ordered CoSi₂/Si interfaces did not observe localized states in the Si gap. For all 3 interfaces, the SFG response was observed to smoothly increase for tunable probe photon energies below the Schottky barrier height, possibly due to density-of-states effects at the metal/semiconductor interface. Theoretical models for the observed SFG response are being sought.