CONTROLLED POLYTYPISM IN A VARIABLE BANDGAP SEMICONDUCTOR

LEAD IODIDE (PBI2) IS A LAYERED SEMICONDUCTOR COMPOUND THAT POSSESSES A NUMBER OF STRUCTURAL MODIFICATIONS KNOWN AS POLYTYPES. THE FORMATION OF THESE STRUCTURES IS KNOWN TO BE CAUSED BY IMPURITIES. RECENTLY METHODS FOR GROWING THIS MATERIAL IN VERY HIGH PURITY FORM HAVE BEEN REPORTED IN THE LITERATURE USING METHODS SIMILAR TO THOSE USED BY RADIATION MONITORING DEVICES, INC. TO PRODUCE CDTE FOR COMMERCIAL PRODUCTS. WHEN PREPARED BY THESE TECHNIQUES THE PBI2 WAS OBSERVED TO BE OF A SINGLE POLYTYPE. TO BETTER UNDERSTAND THE NATURE OF THE POLYTYPES, RADIATION MONITORING DEVICES, INC. PROPOSES TO GROW HIGH PURITY PBI2 USING A MODIFICATION OF THE TECHNIQUES REPORTED IN THE LITERATURE AND THEN DELIBERATELY INDUCE THE FORMATION OF POLYTYPES. IT IS ALREADY KNOWN THAT THE VARIOUS POLYTYPES OF PBI2 HAVE WIDELY DIFFERING ELECTRICAL AND OPTICAL PROPERTIES, FOR EXAMPLE THE SEMICONDUCTOR BANDGAP CAN VARY FROM ABOUT 1 EV TO ABOUT 2.5EV. THE ABILITY TO CONTROL THE FORMATION OF THESE STRUCTURES ALLOWS THE PRODUCTION OF SEMICONDUCTORS WITH TAILORED PROPERTIES. PBI2 HAS SOME IMPORTANT COMMERCIAL APPLICATIONS, MOST NOTABLY AS A MATERIAL FOR USE IN SOLID STATE NUCLEAR SPECTROMETERS, AND THE ABILITY TO CONTROL THE CRYSTAL STRUCTURE OF THIS MATERIAL SHOULD SIGNIFICANTLY INCREASE IT'S PERFORMANCE IN SUCH APPLICATIONS.