Influence of surface roughness and internal strain on defect spectrum and intensity of low-temperature photoluminescence of thin Si1−xGex layers

Abstract

Microstructure, surface roughness, morphology, defect spectrum, and low-temperature photoluminescence of thin (10–125 nm) strained Si1−xGex layers (0.1⩽x⩽0.3), deposited by chemical vapor deposition (CVD) at 650 °C on silicon wafers have been studied. Nominally undoped layers with crystalline orientations of 〈100〉 and 〈111〉 have been investigated. Local strain within the layers was estimated from x-ray diffraction data. It decreases with the layer thickness in the 〈100〉-oriented samples, but rises in the 〈111〉-oriented ones. Nanoscale (∼10–30 nm) and microscale (∼0.2–1 μm) morphologies have been found on the surface of the Si1−xGex layers by atomic-force microscopy. The lateral sizes of the morphologies and surface roughness depend on the thickness, germanium concentration x, and crystalline orientation of the layers. The spectrum of defect states N(E) in the band gap of these samples has been experimentally studied by the deep-level-transient-spectroscopy (DLTS) technique. The standard D1(P1), D2, P3, and P4 defect peaks were observed. The N(E) spectrum is strongly influenced by germanium concentration, crystalline orientation, and surface roughness of the films (especially at Ec−E<0.4 eV). Photoluminescence (PL) was excited with argon ion (Ar+) laser at a sample temperature of about 5 K. Both “no-phonon” and phonon-assisted PL peaks around 1.1 eV, as well as a strong peak at 0.80 eV were observed. These peaks originated, respectively, from the no-phonon line from the Si substrate, transverse optical/acoustical phonon replica and dislocation-related Si1−xGex band, D1. Intensities of these PL peaks are influenced by the layer thickness, internal strain, surface roughness, and germanium concentration x. Possible mechanisms of relationship between the local strain, film roughness, the defect spectrum N(E), and the D1 line strength are discussed.

Morphology, Defect Spectrum, and Photoluminescence of Thin Si[sub 0.7]Ge[sub 0.3] Layers

“…This observation disagrees not only with behavior of other dislocation-related defects ͑i.e., placed at E−E v Х 0.51 and 0.63 eV͒ in our nominally undoped samples, but also with results obtained by the same experimental techniques and equipment on the nominally undoped ͗100͘-oriented Si 0.7 Ge 0.3 samples in Ref. 15, where a significant increment in concentrations of the dislocation core states with the sample thickness has been detected ͑see Fig. 7b and c therein͒.…”

“…Characters of dependencies of average surfaces roughness on the thickness of the ͗100͘-oriented Si 0.7 Ge 0.3 samples in this work and in Ref. 15 are quite similar ͑see Fig. 2 in Part I and Fig.…”

“…The N͑E͒ peak, caused by the dislocation core states in Ref. 15 and deeper dislocation-related peaks in our samples, are supposed to be predominantly the bulk defects. Characters of dependencies of average surfaces roughness on the thickness of the ͗100͘-oriented Si 0.7 Ge 0.3 samples in this work and in Ref.…”

“…Changes in distribution of defect states in bandgap of the Si 0.7 Ge 0.3 layers are expected to be the main reason for the observed variation in the PL line strengths. In particular, intensity of these PL lines from Si 1−x Ge x layers normally increases with the density of the relatively deep dislocation-related N͑E͒ peaks [15][16][17][18] but could be suppressed by nonradiative recombination via hole and electron traps.…”