Mechanical stress dependence of the Fermi level pinning on an oxidized silicon surface

Abstract

A combination of micro-Raman spectroscopy and micro-XPS (X-ray photo-electron spectroscopy) mapping on statically deflected p-type silicon cantilevers is used to study the mechanical stress dependence of the Fermi level pinning at an oxidized silicon (001) surface. With uniaxial compressive and tensile stress applied parallel to the 110 crystal direction, the observations are relevant to the electronic properties of strain-silicon nano-devices with large surface-to-volume ratios such as nanowires and nanomembranes. The surface Fermi level pinning is found to be even in applied stress, a fact that may be related to the symmetry of the Pb0 silicon/oxide interface defects. For stresses up to 240 MPa, an increase in the pinning energy of 0.16 meV/MPa is observed for compressive stress, while for tensile stress it increases by 0.11 meV/MPa. Using the bulk, valence band deformation potentials the reduction in surface band bending in compression (0.09 meV/MPa) and in tension (0.13 meV/MPa) can be estimated.As silicon devices continue to shrink towards the nanoscale the electronic properties of the silicon/oxide interface, in particular the energy at which the surface Fermi level is pinned, becomes a key factor in the determination of the overall optical and electronic device characteristics [1][2][3][4][5][6]. In parallel with reductions in device size, in-built mechanical stress is widely used to improve the bulk electronic properties of CMOS devices [7], so the question of its effect on surface Fermi level pinning becomes an important one. While there is some evidence from electrical measurements in flash memories [8], MOS capacitors [9, 10] and silicon nano-objects [11][12][13][14], of the way in which stress modifies the surface Fermi level pinning by deep silicon/oxide interface defects, such measurements do not provide direct spectroscopic access to the stress dependence of the pinned Fermi level.The vast literature on electronic spectroscopy of silicon/oxide interface states using surface sensitive techniques [15][16][17][18][19] only includes a small fraction of works which deal with mechanical-stress related effects. Amongst these, experimental studies tend to deal with the consequences of local, bond length strains on the properties of clean, reconstructed surfaces [20,21], while the effect of local strains at the silicon/oxide interface have been studied from a more theoretical perspective [22,23]. In parallel with these photo-emission studies, photo-reflectance has also been used to study strain induced shifts in near-surface electronic energy levels [24][25][26], including a work in which a macroscopic, externally applied stress is used to modify the electronic structure [27]. Although photo-reflectance can be used to estimate surface potential, being an optical technique it is sensitive principally to the mechanical stress dependence of the near-surface, bulk electronic structure.

Piezoresistance in Defect-Engineered Silicon

“…The answer is probably not, given the magnitude of the stressinduced changes in E C − E T estimated here and elsewhere [48][49][50]. The order of magnitude of this change (approximately tens of μV/MPa) has also been directly observed for the intrinsic silicon surface defects [54] that limit lifetimes in high surface-to-volume ratio nano-objects [53]. More generally, deformation potentials of this order of magnitude are typically observed for any electronic state in an inorganic semiconductor [55].…”