Closed-cycle cooling of cryopanels in molecular beam epitaxy

Closed-cycle cooling of the cryoshroud in a molecular beam epitaxy (MBE) system with a dimethyl polysiloxane heat transfer fluid has reduced liquid nitrogen consumption by an order of magnitude, significantly lowering operating costs. The temperature dependence of cryopanel pumping efficacy in the MBE system has been investigated. H2O, CO, CO2, and As4 are all pumped effectively by liquid nitrogen cooled cryopanels (−196 °C) in the MBE. At −78 °C, the operating temperature of the closed-cycle chiller, H2O and As4 are pumped effectively, while CO and CO2 are not. The pumping speed for H2O is found to increase exponentially with decreasing temperature. Below ∼−40 °C and ∼−95 °C, the pumping speeds for As4 and H2O saturate, respectively. AlGaAs layers grown with the closed-cycle-cooled shroud show strong photoluminescence, expected room temperature electron mobility, and background doping levels less than 4 × 1015 cm−3.