The Effect of Ge Content in MBE Si (1-x) Ge (x) on the Evolution of {311} Defects
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The Effect of Ge Content in MBE Si(1-x) Ge(x) on the Evolution of {311} Defects Robert Crosby, Jackie Frazer, Dr. K. S. Jones, Dr. M. E. Law, A. Nylandsted Larsen1, and J. Lundsgaard Hansen1 University of Florida, SWAMP Group Gainesville, Fl. 1 Institute of Physics and Astronomy, University of Aarhus, DK-8000, Aarhus, Denmark ABSTRACT Molecular beam epitaxial Si1-xGex layers of various Ge concentrations ranging from 0% to 50% were grown on top of a Si substrate. The wafers were then implanted with a 40 keV, 1 x 1014cm-2 Si+. To study the development of {311} defects, the samples were annealed at 750oC for times ranging from 0 to 20 minutes. TEM was utilized to observe both the formation and dissolution of the defects. The Si1-xGex samples with ≤ 5% Ge exhibit {311} defect formation and dissolution; however, samples with the Ge content lying between 15% and 50% showed only dislocation loop formation. It is suggested that the decrease in bond strength with increasing Ge content is the reason for the lack of {311} defect formation. INTRODUCTION It has been discovered that SiGe alloys have a higher mobility (than that of Si), and its band gap can be engineered for certain applications [1]. Because of these features, Si1-xGex alloys have been used in avalanche photodiode devices and heterojunction bipolar transistors [2-4]. The narrowing of the band gap is related to the germanium concentration [5] and the strain of the layers as well [6]. Moreover, the strain and layer critical thickness is dependent upon Ge content [5]. These thin Si1-xGex films can be grown pseudomorphically (and are therefore strained) onto a Si substrate [7]. When the layer thickness grows larger than a concentration-dependent critical thickness, the structure will introduce dislocations with misfits dislocations parallel to the interface and threading defects propagating through the epitaxial layer to reach a relaxed state. To circumvent the issue of threading of dislocations in a layer above the pseudomorphic thickness, compositionally graded buffer layers have been used. This allows the formation of low defect density SiGe layers on Si with reasonable thicknesses [8]. These low defect density SiGe layers are ideal for studies of the effect of Ge content on the point defect evolution. Annealing of ion implanted dopants into semiconductors can illustrate anomalous diffusion [9] such as transient enhanced diffusion (TED) resulting from excess interstitials injected into the damaged region [10]. During annealing, excess interstitials can migrate and form {311} defects for given conditions. The time-temperature profile for {311} defect formation ranges from hours at 670oC anneals to minutes at 815oC anneals [10]. These rod-like defects visible by transmission electron microscopy reside on the {311} plane as a single monolayer of hexagonal Si (or Ge) and grow in the direction [11]. {311} defects extend uniaxially allowing for the incorporation of selfinterstitials into the lattice while minimizing the number of dangling bonds. Upon annealing these {311
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