Photo- and electroluminescence in short-period Si/Ge superlattice structures

Abstract

lnterband optical transitions have been studied in a variety of short-period Si/Ge superlattice structures by means of photocurrent spectroscopy, infrared absorption, photo-and electroluminescence. Furthermore, the bandgap photoluminescence from strain-adjusted Si , Ge, (m = 9, 6, 3; n = 6, 4, 2) adjustment was achieved by a thick, step-graded Si,_,Ge, buffer layer resulting in an improved quality of the superlattice with respect to dislocation density. The hydrostatic pressure dependence was modelled using an approach based on deformation potentials and effective-mass theory. In samples annealed at 500 "C and higher, a systematic shifl of the bandgap was observed which is discussed in terms of a process Involving interdiffusion of the Si and Ge atoms. Bandgap-related electroluminescence was observed in mesa diodes at room temperature, whereas the photoluminescence disappeared at about 40 K. The electroluminescence from samples based on different buffer-layer concepts is compared.Apart from the strain-symmetrized Si/Ge superlattices, another structure that has been proposed to act as an efficient, light-emitting device in the Si-based systems is an ultrathin Ge layer (1-2 monolayers) embedded in bulk Si. We report on the electroluminescence spectra at various temperatures from a sample based on this concept, namely a layer sequence consisting of two periods of Si ,,Ge, grown pseudomorphically on an n+ Si substrate. A very intensive, well resolved electroluminescence was obtained at 55 K from the ow.sgper!a!!ices was studied under applied hydrostatic prees~re, The strain

Spectroscopy of band-to-band optical transitions in Si-Ge alloys and superlattices

“…Both theory and experiment show the same trend, namely, that the band gap of the superlattices decreases as the length of the supercell period increases. This finding confirms earlier observations of Menczigar et al 22 and Olajos et al 23 In Fig. 10, the calculated band gaps correspond to the filled diamonds.…”

“…In the cases, where our measurements can be compared directly to those of Ref. 23, there is agreement within 10 meV for the value of the band gap. In two cases we can also compare the band gaps of samples having the same nominal structure, but grown at different times under different conditions.…”

“…Our measurements are summarized in Table IV along with the results of Olajos et al, 23 who examined some of the same samples, as well as our theoretical calculations of these superlattices. It can be seen that the band gap decreases monotonically as the superlattice period increases in both experiment and theory.…”

Optical properties of Si-Si1−xGex and Si-Ge nanostructures

“…Further studies [13,14] on similar samples by stress-modulated reflectance (or piezoreflectance) have identified that the low energy structures at energies above the Ge direct bandgap are due to quantum confined direct transitions from the Ge spacer region rather than from the pseudo-direct bandgap Sim-Gen superlattices. In addition to modulation spectroscopy, photoluminescence and electroluminescence from both undoped Sir,-Gen superlattices grown on either Ge buffer or graded Sil-xGex buffer and p-i-n diodes were reported [10][11][12][13][14]39,42]. Even an enhanced luminescence has been observed [39] but none of the results can be unambiguously identified as the expected superlattice induced pseudo-direct bandgap transitions.…”

Room temperature electroluminescence of nanofabricated Si–Si1−xGexquantum dot diodes

“…Researchers have recently been investigating the possibility of using Si-SiGe based material and structures for light emitting devices [1,2], particularly the optical properties of ultra-thin Si m -Ge n (m + n = 10) superlattices [3] which were predicted to have a direct bandgap [1,4]. However, there are still no convincing experimental results which unambiguously proved the existence of such a direct bandgap [5,6]. Over the last five years, we have been investigating the optical properties of Si-SiGe quantum wires and dots [7,8] with very promising results, such as the observation of improved light emission efficiency by over two orders of magnitude in dry etched Si-SiGe quantum dots of less than 100 nm or so [9].…”