Solid-Phase Epitaxial Crystallisation of Ge x Si 1-x Alloy Layers
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ROBERT G. ELLIMAN, WAH-CHUNG WONG and PER KRINGHOJ Electronic Materials Engineering Department, Research School of Physical Sciences and Engineering, Australian National University, Canberra, ACT 0200, AUSTRALIA.
ABSTRACT Thermally-induced solid-phase epitaxial crystallisation (SPEC) and ion-beam-induced epitaxial crystallisation (IBIEC) of amorphous GexSil_x alloy layers is examined for three different starting structures: a) strain-relaxed alloy layers of uniform composition, b) strained alloy layers of uniform composition, and c) Ge implanted Si layers. Thermal annealing experiments show that the activation energy for strain-relaxed alloys is higher than that expected from a simple extrapolation between the activation energies of Si and Ge, and exceeds that of Si for x _•0.3. Experiments on thin strained layers show that MBE grown strained layers which are stable during annealing at 1100°C for 60 s are also fully strained after SPEC, whereas layers which relax during annealing at 11 00°C also relax during SPEC. Experiments on ion-implanted GexSi I _x structures show that fully strained Si/GexSiIx /Si heterostructures can be fabricated for ion fluences below a critical fluence, and as for uniform alloy layers that this critical fluence is accurately predicted by equilibrium theory. Strain relaxation during SPEC of uniform alloys and implanted structures is shown to be correlated with a sudden reduction in crystallisation velocity which is believed to be caused by stress-induced roughening or faceting of the crystalline/amorphous interface. IBIEC of thick (800 nm) implanted layers is shown to be limited by competition from ion-beam induced random crystallisation, while thin (120 nm) uniform alloys and implanted structures are shown to crystallise epitaxially and to exhibit similar behaviour to thermally annealed samples under certain conditions. INTRODUCTION Solid-phase epitaxial crystallisation (SPEC) of amorphous semiconductor layers has been extensively studied over the past few decades [1-5]. The crystallisation rate has been measured over a wide temperature range and shown to be a thermally activated process characterised by an activation energy of 2.68 eV [2]. The crystallisation rate has also been shown to depend on crystallographic orientation, being fastest in the (100) orientation and slowest in the (111) orientation, and on the presence of impurities [1]. Low concentrations (_solid solubility limit) most impurities are observed to retard SPEC, with some impurities being segregated at the crystalline/amorphous interface during SPEC [1]. An impurity of particular interest in this regard is H which has also been shown [3] to segregate at the crystalline amorphous interface and to retard SPEC. In addition, recent experiments have also shown that hydrostatic pressure and biaxial stress can increase the rate of SPEC [4,5]. The available data suggests that SPEC is activated by a bond-breaking event at the crystalline/amorphous interface [5] and that this event occurs at a specific 'defect' site which can exis
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