Defect Luminescence in Some Layered Binary Chalcogenide Semiconductors

Abstract

A number of semiconductors such as GaS, GaSe, GaSSe show layered structure where intralayer bonding is strong and interlayer bonding quite weak. With bandgaps mostly in the visible and the near infrared and high crystal structure anisotropy, such semiconductors olfer interesting possibilities for optoelectronic applications In this review, we will summarize the recent developments on the photoluminescent properties of these materials such as luminescence due to donor-acceptor pair recombination. As these materials are undoped, the observed photoluminescence is attributed mostly to defect states present in these materials. InroductionLayered semiconductors have attracted much attention due to their structural properties and possible optical applications [1]. Elements of the group III-VI allow for a number of binary and ternary compounds such as GaS, GaSe, TlInSz, GaSexSI.x that crystallize in a layered structure. The layered structure contains a number of alternating layers whose number within the unit cell can be more than one. A good example is GaS where each layer is composed of four atoms stacked along the c axis of a hexagonal structure with a repeating unit of S-Ga-Ga-S and there are two layers in the unit cell [2]. GaS crystallizes in the so called ,B-polytype and carries the symmetry properties of the D2,, space group. Intralayer forces are primarily ionic-covalent, while the interlayer interactions are dominated by van der Waals forces. Due to weak interlayer interaction these materials are easily cleaved along these layers. Likewise, GaSe crystallizes in a hexagonal structure [3] however, depending on the stacking sequences exhibit four modifications labeled; 8, B, y, and 6-polytypes. The e and B are 2H hexagonal type while y and 6 are 3R trigonal type. The 3prevalence of a particular polytype is a fimction of growth techniques. Solid solutions of A -B6X-C61.x have typically been observed to display layered structures with the polytype of the binary parent compound dominating that of the mixed crystal for values of x close to that of the binary compound . Mixed polytypes have also been observed for a narrow range of x values For example, in the case of GaSe1.x, polytype a has been observed for x<0.01, a mixture of polytypes e and y for 0.0l<x<0.03, polytgype y for 0.05<x<0.4 and polytype B for O.5<x<1, [2]. Finally, ternary compounds of the A3B C62 type in which binary parents A3C6 and BSC6 have the ratio of 1:1 also display layered structures [4].Energy band structures of these layered semiconductors show both indirect and direct band gaps [1,5] mostly in the visible and the near infrared. Much work has been done with near band edge emission in both binary and ternary mixed crystals identifying both direct and indirect, free and bound excitonic states [6][7][8][9][10][11]. Particularly notable is the observation of resonant exciton in GaSe [12] where the free exciton associated with the direct band gap and states near the indirect conduction band minimum are involved. Free and bound excitons have also b...