Structural and Electrical Characterization of Si-Modfet Structures Grown at High Rates by Lepecvd
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STRUCTURAL AND ELECTRICAL CHARACTERIZATION OF SiMODFET STRUCTURES GROWN AT HIGH RATES BY LEPECVD C. ROSENBLADa, M. KUMMERa,b, E. MÜLLERc, A. DOMMANNb, H. VON KÄNELa a
Laboratorium für Festkörperphysik, ETH-Zürich, CH-8093 Zürich, Switzerland b Interstaatliche Fachhochschule für Technik Buchs, CH-9471 Buchs, Switzerland c Laboratorium für Mikro- und Nanostrukturen, PSI, CH-5232 Villigen, Switzerland ABSTRACT Strain relaxed graded SiGe buffer layers have been grown by low energy plasma enhanced chemical vapour deposition (LEPECVD). Due to the low ion energies involved in LEPECVD, exceptionally high plasma intensities can be applied without any ion damage of the epitaxial layers. Transmission electron microscopy and X-ray reciprocal space mapping show that relaxed buffer layers grown at rates exceeding 5 nm/s are of a comparable quality to buffer layers grown by standard techniques at much lower rates. Low temperature electric transport measurements on remotely doped tensilely strained Si quantum wells synthesized by LEPECVD show that also good electrical quality is achieved.
INTRODUCTION Although recent attempts to develop alternatives to the strain relaxed graded buffer layers seem promising[1], the graded SiGe buffer layer is still the most successful virtual substrate for strained Si quantum wells. So far molecular beam epitaxy (MBE)[2] or chemical vapour deposition in an ultra clean environment, e.g. UHV-CVD[3] have shown excellent results for the properties of graded SiGe buffer layers. Due to the combination of low growth rates by these conventional techniques of a few Å/s and the need for buffer layers to be several µ m thick, it is economically uninteresting to introduce devices on virtual substrates to the mass market. It thus seems worthwhile to develop new growth techniques optimized for epitaxial growth at high growth rates. Low energy plasma enhanced chemical vapour deposition (LEPECVD) is a technique optimized for high rate deposition of epitaxial semiconductor material. Due to a high current but low voltage arc discharge maintained in the growth environment, significant plasma enhancement of the growth-kinetics is achieved without any accompanying ion damage[4]. The demonstrated growth rates of LEPECVD of more than 5 nm/s[5] applied to the synthesis of the graded buffer layer, would imply a significant reduction of the time required for growing such structures. In the present paper we shall demonstrate that the buffer layers obtained in this way do have a quality comparable to the one obtained by conventional growth techniqes. Also first results on the electrical properties of modulation doped quantum wells (MODQWs) synthesized by LEPECVD will be demonstrated.
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Figure 1 - X-TEM bright field image of a Si-MODQW structure consisting of a Si1-xGex buffer layer linearly graded from x=0 to x=0.35 within 5.2 µm and overgrown by 1.8 µm of Si0.65Ge0.35. A 10 nm remotely doped Si well is grown onto the buffer layer using lower plasma intensity. The Si well is shown in magnification in the inset.
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