Investigation of the Thermal Stability of Strained Ge Layers Grown at Low Temperature by Reduced-pressure Chemical Vapou
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1252-I04-06
Investigation of the Thermal Stability of Strained Ge Layers Grown at Low Temperature by Reduced-Pressure Chemical Vapour Deposition on Relaxed Si0.2Ge0.8 Buffers
A. Dobbie, M. Myronov, Xue-Chao Liu, Van H. Nguyen, E. H. C. Parker and D. R. Leadley Nano-Silicon Group, Department of Physics, University of Warwick, Coventry, CV4 7AL, U.K.
ABSTRACT The thermal stability of thin strained germanium (s-Ge) channels on high quality relaxed Si0.2Ge0.8 buffers has been investigated using in-situ H2 annealing at temperatures between 450 °C and 650 °C. The relaxation of the s-Ge epitaxial layers was found to increase with both the s-Ge layer thickness and the annealing temperature, through a combination of surface roughening (rms surface roughness was at least 2 (7) times higher following 550 °C (650 °C) annealing compared to the as-grown s-Ge layers) and misfit dislocation formation at the sGe/Si0.2Ge0.8 interface. Our results suggest that the thermal budget for s-Ge device fabrication should be kept below 550 °C, in order to retain high quality s-Ge layers and minimize degradation of the carrier mobility. INTRODUCTION High quality strained Ge (s-Ge) layers are a promising candidate to achieve high mobility channels in MOSFETs suitable for the 22 nm technology node and beyond, due to the intrinsically higher mobility of Ge compared to Si, and the additional performance enhancements from strain [1, 2]. In order to achieve a s-Ge channel more than a few monolayers thick on a Si substrate it is necessary to engineer a relaxed Si1-xGex buffer with a high Ge content (x > 0.5). We have recently reported high quality s-Ge layers grown by reduced-pressure chemical vapour deposition (RPCVD) at low temperature (T ≤ 450 °C) on a fully relaxed Si0.2Ge0.8 buffer [3]. By using a reverse-grading approach, we achieved a high Ge composition in the buffer, with a smooth surface (rms roughness of ~ 2 nm), low threading dislocation density (~ 4 x 106 cm-2) and much thinner (~ 2.1 m) than can be achieved with conventional linear grading [4]. In order to maximize the performance potential of s-Ge channel transistors, control of both the s-Ge surface passivation prior to gate stack formation and the employment of a low thermal budget to avoid relaxation of the s-Ge channel are critical [5, 6]. In this work we have investigated the thermal stability of s-Ge channels grown on relaxed Si0.2Ge0.8 buffers in H2 ambient as a function of both the s-Ge layer thickness and the annealing temperature. Annealing temperatures in the range of 450 °C – 650 °C were used, as these temperatures are similar to those currently used during the fabrication of advanced CMOS devices.
EXPERIMENT The s-Ge layers investigated in this study were grown by RP-CVD using an ASM Epsilon 2000 reactor on 200 mm reverse-linearly graded relaxed Si0.2Ge0.8 buffers. Figure 1 depicts the full layer structure and the growth details of the buffer are reported elsewhere [4]. The s-Ge layers (up to 80 nm thick) were grown at 400 °C using GeH4 gaseous precursor [3]. Immediat
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