Conduction and Injection in Off-Stoichiometry Oxides
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Conduction and Injection in Off-Stoichiolnetry Oxides K.-T. Chang, C. Lam, and K. Rose Center for Integrated Electronics Rensselaer Polytechnic Institute Troy, New York 12181 ABSTRACT Slightly silicon rich oxides or off-stoichiometry oxides (OSO) have been prepared by LPCVD. When R, the nitrous oxide/silane ratio, equals or exceeds 30 these films have the refractive index and CV characteristics of stoichiometric CVD oxide. Injection into these oxides by oxides richer in silicon has been compared with injection into thermal oxides. We find that silicon rich oxides with higher silicon content having the same R values provide the same enhancement of electron injection into thermal oxides and off-stoichiometry oxides. Conduction in off-stoichiometry oxides depends on preparation, but can be accounted for by tunneling between islands. INTRODUCTION We have previously examined enhanced injection at the interface between silicon-rich-oxide (SRO) and thermal oxide. [1) The enhanced injection was accounted for in terms of the field enhancement associated with the curvature of silicon crystallites in an Si0 2 matrix. In practical devices such as EAROMs we are interested in using the enhanced injection to transfer electrons between a control gate and a floating gate. Hence, we are interested in transport through a deposited oxide sandwiched between SRO injecting layers on polysilicon layers - a dual-electron-injector structure (DEIS) [2]. This paper reports studies of injection and conduction in oxides deposited in an LPCVD reactor. By increasing R, the nitrous oxide silane ratio, one can increase the concentration of oxygen in these films. For R>10 these slightly silicon-rich films have refractive indexes and CV characteristics which approach stoichiometric CVD oxide. These materials were termed off-stoichiometry oxides (OSO) by DiMaria and co-workers [31who deposited them at atmospheric pressure. EXPERIMENTAL METHOD Our LPCVD hot-wall quartz-tube reactor uses nitrous oxide and 10% silane in nitrogen as reactant gases. Films were generally grown at 0.4 torr and 650°C. However, ellipsometric measurements indicate little change in refractive index when films are deposited between 610 and 680°C for R>10. By varying R, one can grow a sequence of SRO, OSO, and SRO without venting the reactor. A gas ratio R=3 was used to grow SRO injectors. To study injection into thermal oxides, 8ohm-cm p-type (100) silicon wafers were thermally oxidized to 40 nm. A 21.5 nm SRO film was then deposited on top of the thermal oxide. The sample was then annealed in nitrogen for 30 min, at 1000°C. OSO films for direct measurements were deposited on 0.13 ohm-cm ntype (100) wafers and annezled in nitrogen for 30 min. at 9500C. 20 mil aluminum dots were used to form contacts to the SRO; the back of the silicon wafer was also coated with aluminum. This was followed by a post-metallization annealing in forming gas for 20 minutes at 450*C. DEIS structures were formed on top of a 500 nm thermal oxide on 0.1 ohm-cm n-type wafers. 300nm of in-situ doped n+ poly
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