Metastability of Interstitial Clusters in Ion-damaged Silicon Studied by Isothermal Capacitance Transient Spectroscopy P. K. Giri Department of Physics, Indian Institute of Technology Guwahati, North Guwahati, Guwahati 781039, India. Keywords : Metastable defects, Interstitial clusters, DLTS, ion implantation, silicon Abstract. We review our results on trapping kinetics studies at defect clusters in ion damaged silicon studied by depletion layer capacitance transient spectroscopic techniques. Conventional deep level transient spectroscopy (DLTS) studies on as-implanted and low temperature annealed Si show two major peaks corresponding to a divacancy trap and an interstitial cluster related trap. Kinetics of trapping at the clusters has been monitored over several orders of magnitude in time using an isothermal transient spectroscopic technique called time analyzed transient spectroscopy (TATS). Two distinct effects have been observed regarding the metastability of defect clusters in as-implanted and partially annealed samples. Firstly, with the help of higher order TATS used for monitoring the trap occupancy as a function of filling time, we show that charge redistribution among multiple traps occurs at low temperature in as-implanted samples. A detailed analysis of the relative trap occupancies reveals that the interstitial cluster related major trap exists in two metastable configurations, perhaps with negative U (Hubbard correlation energy), and the stable configuration of the defect is a midgap compensating trap. Secondly, in partially annealed samples, we observe a novel metastability of the defect clusters near room temperature where the trap energy progressively deepens with increasing filling time, finally stabilizing for large filling times at a fixed temperature, and the emission rate of carrier from any relaxed state is nearly temperature independent. From the athermal nature of the associated TATS spectra obtained at different temperatures, it is argued that the defect metastability is driven by change in configurational entropy associated with multiple trapping/detrapping process. These results constitute direct experimental evidence of metastability for small interstitial clusters in silicon and opens up opportunities for further studies on this new class of defect. The necessity of using a time domain relaxation spectroscopy such as TATS in the study of metastabilty is demonstrated. Introduction At present there is a concerted effort towards understanding the behavior of microscopic defect clusters that are produced by ion implantation of crystalline silicon. The supersaturation of Si self- interstitials produced by ion implantation is responsible for several important processes such as transient enhanced diffusion (TED) of dopants [1], formation of extended defects [2]. Due to the high diffusivity and low equilibrium concentration, it is expected that the extra interstitials (I) will coalesce into more complex defect structure and these complexes are believed to evolve into extended {311} platelets upon annealing [3]. The remarkable observation of TED even in the absence of {311} defects in Si has led to the conclusion that microscopic interstitial clusters are responsible for TED in Si [4]. This has triggered a widespread interest in the investigation of small clusters of interstitials in Si. In particular, theoretical studies by several groups attempt to provide valuable insight about the energetics of clustering and their various electronic, optical and vibration properties [5-12]. However, very few experiments have been performed to verify such predictions. While extended defects are well monitored and characterized by transmission electron microscopy (TEM) analyses, the structure and behavior of nanometer sized I-cluster is undetectable with this technique. Experimental assessment on I agglomeration has been attempted using inverse modeling of the supersaturation derived from Boron TED and magic numbers for stable cluster sizes were determined [12]. Among the stable clusters, the four-interstitial ( 4 I ) cluster was predicted to have no electronic states in the bandgap of Si. More recent studies show that first magic number for a stable aggregate is 3 I [7] and it has been argued that the 3 I cluster is responsible for photoluminescence W-band in ion-