IIIB- Nitride Semiconductors for High Temperature Electronic Applications
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alloys for high temperature applications. We report the results of vacuum evaporation of scandium metal in an atomic N ambient for producing thin films of ScN, and diode structures made from n-type ScN and p-type Si. YN nitride films decomposed in humid air in a matter of hours to days. EXPERIMENT The vacuum system is based on a stainless steel 6-way cross with 150 mm outside diameter copper sealed flanges. The vertical ports are used for sample insertion (top), the evaporator feedthroughs (bottom). The four horizontal ports are used for the SVTA Inc. radio frequency atomic nitrogen source, viewports, shutter, main chamber pump and quartz crystal thin film thickness monitor. The system has a 100 l/sec turbo pump roughing system, and a 1500 I/sec cryopump for use during deposition, because the atomic nitrogen caused corrosion and oil decomposition problems with our conventional pumping system. Base pressure in the unbaked system was in the 10-7 Torr range in several hours. The substrates were clamped to a pyrolytic boron nitride-coated graphite heater, the substrate temperature was monitored with an optical pyrometer. A four-conductor (6mm diameter copper conductors) feedthrough was used to support and make contact to two tungsten
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Mat. Res. Soc. Symp. Proc. Vol. 572 © 1999 Materials Research Society
boats (6mm wide tungsten foil 0.25mm thick with a central dimple) for evaporation of two metals at one time. Typical deposition currents were in the 100-125 Ampere range. The nitrogen source was operated on pure nitrogen at about 2 x10-4 Torr. Sc deposition rates were controlled manually in the 0.05-0.3 nm/sec range. Substrates were washed in alcohol and preheated in the chamber to degas the substrates for about 1 hr. Small (2-3 mm diameter) chips of Sc or Y metal (99.99%) were placed in the boats for evaporation. Hall effect measurements were made with a commercial Hall effect set-up (Keithly Instruments). A schematic of the experimental set-up is shown in the figure 1.
Boats
Seso
View from top
Evaporators
Figure 1. Schematic of experimental set-up.
RESULTS Several dozens films were grown on various substrates: quartz microscope slides (Quartz Scientific, Inc. Fairport Harbor OH) and silicon (100). Typically, a low metal evaporation rate was used, because higher rates were very difficult to control, and resulted in Sc- rich ScN or metal capped films. Films where the deposition resulted in a Sc metal overlayers were usually discarded immediately, and no further ScN growth was attempted. ScN and YN "skin" or "crust" formed on the metal in the evaporation boat. Well-formed bulk cubic crystals were observed in SEM observation of the evaporation remnants. While still in the chamber, YN films showed a deep red color, ScN films were yellow to greenish-yellow. Upon Removal from the vacuum system, the YN films became transparent in a matter of hours. YN capped with AIN was more stable, but decomposed from the edges of the film inward with the same eventual outcome. Sc metal films could be evaporated in this system,
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