Relaxed SiGe Layers with High Ge Content by Compliant Substrates
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Relaxed SiGe Layers with High Ge Content by Compliant Substrates H. Yin1, R.L. Peterson1, K.D. Hobart2, S.R. Shieh3, T.S. Duffy3, and J.C. Sturm1 Center for Photonics and Optoelectronic Materials and Department of Electrical Engineering, Princeton University, Princeton, NJ 08544 2 Naval Research Laboratory, Washington, DC 20357 3 Department of Geosciences, Princeton University, Princeton, NJ 08544 1
ABSTRACT Relaxed, high Ge content SiGe layers have been realized using stress balance on a compliant borophosphorosilicate glass (BPSG). A 30-nm fully-strained Si0.7Ge0.3 layer was transferred onto a 1 µm BPSG film by wafer-bonding and Smart-cut processes, after which the continuous Si0.7Ge0.3 film was patterned into small islands to allow for lateral expansion. After the strain in Si0.7Ge0.3 islands was released by the lateral expansion resulting from the flow of the BPSG, a Si0.4Ge0.6 layer was commensurately deposited under compression. Upon equilibrium after an annealing, stress balance was formed between the SiGe films, resulting in a larger inplane lattice constant than that of relaxed Si0.7Ge0.3. With a thiner (6 nm) Si0.7Ge0.3 starting film, an in-plane lattice constant equivalent to fully-relaxed Si0.45Ge0.55 has been obtained. INTRODUCTION Relaxed SiGe has recently drawn attention for its wide application in advanced electronics and opto-electronics. The carrier mobility can be considerably enhanced in strainedSi and strained-Ge MOSFETs fabricated on relaxed SiGe [1,2]. It also allows the integration of III-V semiconductors with Si substrates [3]. Conventionally, relaxed SiGe is made by thick compositionally-graded SiGe buffers grown on silicon ( ~ 1 µm per 10% Ge change). Because of the interaction of misfit dislocations to cause threading dislocations, relaxed SiGe fabricated by this approach suffers from a high density of threading dislocations (about 106 cm-3) [4]. The recent application of viscous BPSG as a compliant substrate has yielded highquality, fully-relaxed Si0.7Ge0.3 films through a process which does not fundamentally require misfit dislocations [5,6]. The process is described as follows: a 30-nm Si0.7Ge0.3 film commensurately strained to bulk Si(100) is transferred to a 200-nm BPSG film (4.4% B and 4.1% P by weight) on a silicon wafer by wafer-bonding, Smart-cut and etch-back processes, and then patterned to islands. No relaxation takes place up to this point, since the highest process temperature is 550oC and therefore the BPSG film remains rigid. When the anneal temperature is elevated to 800oC, the BPSG film softens (a viscosity of 1.2 x 1011Poise at 800oC) and the asbonded, compressively-strained Si0.7Ge0.3 film starts to relax by macroscopic expansion. Transmission electron microscopy (TEM) examination has indicated that the dislocation density is low and therefore suggested that the underlying relaxation mechanism is indeed the lateral expansion of SiGe layers on viscous BPSG films when the island dimension is small enough to prevent buckling [6]. Our previous work has sh
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