Recently, a class of shallow acceptor levels in silicon, called

Recently, a class of shallow acceptor levels in silicon, called X-levels, have been discovered which are associated with all the Group III A acceptors. The indium related X-level at 0.113eV appears to be ideally suited to match the 8-12)Jm atmospheric window. This defect has been shown to arise from a substitutional In and C nearest neighbor pair. We have performed a variety of measurements to determine the feasibility of utilizing the In-X defect in a photoconductor . In particular, silicon crystals heavily doped with both in­ dium and carbon have been grown by thermal gradient transport from indium solutions. Annealing studies have been done to determine whether In-X concentrations in excess of 1-10^'cm""^ can be produced in reasonable an­ nealing times. Such a concentration would be necessary to produce detectors with adequate quantum efficiency. An ideal dopant in silicon from an infrared systems point of view would be one with an ionization energy at about .leV, so that detector response would be matched to the 8-14ym atmospheric transmission window. Several years ago, work on the development of indium-doped silicon for 3-8ym photoconductive detectors led to the discovery of a new acceptor level related to the presence of indium and having an ionization energy of O.lleV, considerably shallower than substitutional indium (.156ev) and an almost ideal match to the 8-14ym window. The level, termed In-X, first manifested itself in low temperature Hall data! in the photoconductive spectrum of Si (In)^ and in optical absorption measurements . ^ Subsequent measurements on other group IIIA acceptors in silicon have revealed similar levels. ^»^ Early investigations of the In-X level were carried out with the expressed intent of eliminating it from the doped silicon material since the X center acted as a source of thermal noise in the Si(In) photodetectors. Because of its ionization energy, however, it would be interesting to determine the feasibility of utilizing this particular defect in a photoconductor and to understand in greater detail the origin of this class of defects. Basic properties5 of the In-X center The X-center in indium doped silicon has been found to exist in concentrations of 10~^ to 10"^ times that of the indium concentration in pulled crystals. The data (see Figure 7 of reference 5) shows no In-X center dependence on oxygen, boron or more than one indium per complex. There is a linear relationship seen between the X/In ratio and the carbon concentration. The slope of the line through the data is one, implying the X center depends on only a single carbon atom per complex. This carbon dependence was checked by growing Si(In) which was essentially free of carbon. No measurable X-centers (<5* lO-^X/cm^) were detected.