Optical study of diffusion limitation in MBE growth of SiGe quantum wells

Abstract

We report on detailed studies of the bandgap of Si/Si,Ger-, quantum well structures grown on (001) Si by molecular beam epitaxy. Photocurrent and photoluminescence spectroscopy are used t t determine the bandgap of the SiGe alloy up to x = 0.67. We found that interdiffusion of the SiGe layers limited the maximum Ge content in the alloy layers at a high growth temperature (720°C). At a lower growth temperature (500'C) diffusion is negligible. This is verified by pi-n structures and p-type modulation-doped quantum wells. In the modulation-doped samples the bandgap could be reduced to 1.5 wm while still showing intense bandgap related photoluminescence. As well as an alloy-related onset the p i -n diodes reveal a low-energy threshold, which is defect related. Low growth temperatures lead to defects located in the SiGe layers. Raising the number of quantum wells and Ge content up to almost critical thickness we found a maximum external responsivity of 4 x mesa-type pin photodiodes.

Strained Si1−xGex multi-quantum well waveguide structures on (110) Si

“…It is shown in Ref. 15, that in consideration of all these contributions the absorption coefficient ␣ of the QWs can approximately be described by ␣ϳ͑hϪE g ͒. 3 As can be seen from the inset in Fig.…”

“…Furthermore, interdiffusion limits the maximally obtainable Ge content at high growth temperatures. 15 For these reasons the layers have been grown at a temperature of only 510°C. Both mesa diodes ͑400 m ϫ300 m͒ for photocurrent spectroscopy ͑inset We have used photocurrent spectroscopy on the mesa diodes to measure the optical absorption spectra of the Si 1Ϫx Ge x /Si MQW structures.…”

“…In order to determine quantitatively the effective band-gap energy E g , a curve-fitting technique has been employed. 15 Each photocurrent spectrum is a superposition of the absorption of the Si 1Ϫx Ge x MQWs and a defect related onset. The defect related thresholds fit quite well to a quadratic energy dependence of ␣, valid for indirect bulk semiconductors, 17 and have been subtracted from the data.…”

Photoluminescence study of initial interdiffusion of SiGe/Si quantum wells grown by ultrahigh vacuum-chemical vapor deposition

“…7 In the most mature device, the HBT, a very significant improvement of device speed is obtained, [3][4][5] while preserving compatibility with current very large scale integrated processing technology. 4,5 However, even moderate temperature processing (TϽ900°C) can cause interdiffusion [8][9][10][11][12][13][14][15][16][17][18][19] of Si/Ge at the interface of the heterostructures. Therefore, fundamental understanding of interdiffusion mechanisms is an important issue in silicongermanium device processing.…”

“…[15][16][17][18][19][20] This latter technique is sensitive to small variations in the shape of thin quantum wells, since it gives information on the band gap and confinement energy rather than on structural properties of the material. An initial enhancement of interdiffusion was observed in many of these studies, [11][12][13][14] rendering interdiffusion difficult to model using Fick's relations.…”