Evaluation of a new high capacity, all-tantalum molecular-beam-epitaxy arsenic cracker furnace

Abstract

A novel effusion furnace for the production of As2 from As4 by combined thermal and catalytic cracking has been evaluated in a Varian GEN II molecular-beam epitaxy (MBE) system. The furnace consists of two temperature zones with a one-piece all-tantalum combined crucible, delivery tube, and cracking zone. Ta baffles are inserted into the cracking zone. Thermal isolation of the ‘‘warm’’ sublimation zone and the ‘‘hot’’ cracking zone is achieved by water or liquid nitrogen cooling of the sublimation zone. With the cracking zone at 1500 °C and no power to the 200 cc capacity sublimator, the sublimation zone rises to only 75 °C. Cracking of As4 begins at about 750 °C and saturates at about 900 °C. The As flux stabilizes in about 1.25 h. Installation of the cracker furnace in a position previously used for the old As effusion furnace resulted in considerable outgassing due to the previous condensation of As and trapped impurities on the cryoshroud around the mouth of the old furnace. Soon after installation, growth of nominally undoped GaAs in the As4 mode (by holding the cracking zone at 400 °C) gave background doping concentrations of p=2×1014/cm3, but the first attempts at growth of nominally undoped GaAs with As2 gave high n-type background doping levels due to the outgassing mentioned above. A combination of secondary ion mass spectroscopy (SIMS) and photoluminescence analysis showed decreased C and increased S in the As2 grown layers. Following mechanical scraping and extended thermal cleaning of the shroud area around the cracking zone, the residual As2 background doping dropped dramatically (to high 1013/cm3) while still remaining n type. Material and device benefits we have observed for As2 grown GaAs and (Al,Ga)As are: slightly improved surface morphology, decreased C incorporation, increased doping efficiency in (Al,Ga)As:Si, greatly improved low-temperature photoluminescence, and excellent heterojunction acoustic charge transport (HACT) device results.