Closed cycle chiller as a low cost alternative to liquid nitrogen in molecular beam epitaxy

Abstract

The high cost of cooling the cryoshroud in a molecular beam epitaxy system has been greatly reduced by replacing liquid nitrogen (LN2) as a coolant with a silicone polymer heat transfer fluid cooled to as low as −80 °C by a closed cycle chiller. Gallium arsenide epitaxial layers have been grown with two different cooling configurations of the shroud: conventional LN2 cooling and cooling to −70 °C with the chiller. The partial pressure of water in the chamber is a factor of about 2.5 higher with the closed cycle chiller operating at −70 °C than with liquid nitrogen in the shroud. No significant difference is observed in the density of deep levels in the GaAs, as determined by deep level transient spectroscopy.

Defect energy levels in p-type GaAsBi and GaAs grown by MBE at low temperatures

“…A second candidate, the double donor level of the As Ga -As i complex, also known as EL2, was previously observed in p-type bulk GaAs as a hole trap having parameters similar to the Fe acceptor [22]. Since the single donor level of EL2 was not observed in the DLTS spectra of n-type GaAs layers grown at standard conditions in this MBE chamber [19,34], we can rule out the As Ga -As i complex as the identity of the hole trap present in these samples. The concentration of the Fe acceptor trap is the same in both samples A and B, whereas the net acceptor concentration, taken to be the carbon doping concentration in these standard GaAs samples, differs by a factor of 4.…”

Bandgap and optical absorption edge of GaAs1−xBix alloys with 0 < x < 17.8%

“…During operation, the MBE shroud was cooled to $À80 C with a polysiloxane heat transfer fluid, instead of conventional liquid nitrogen cooling. 12,13 Each sample consists of a 300-500 nm GaAs buffer layer, a)…”

Closed-cycle cooling of cryopanels in molecular beam epitaxy

“…5 During growth, the chiller is maintained at its lowest achievable temperature of $À78 C, and the TSP reservoir is filled with LN 2 . Single filament effusion cells were used for Al, Ga, and Si, and a two-zone valved cracker for As 2 .…”

“…5 Given that oxygen and carbon containing gas species react more strongly with and produce more detrimental effects on Al-containing is cooled in steps of 20 C from þ20 C to the lowest achievable temperature of À78 C and then warmed back to þ20 C. The steps in the H 2 O partial pressure correspond to changes in the shroud temperature. 5 Given that oxygen and carbon containing gas species react more strongly with and produce more detrimental effects on Al-containing is cooled in steps of 20 C from þ20 C to the lowest achievable temperature of À78 C and then warmed back to þ20 C. The steps in the H 2 O partial pressure correspond to changes in the shroud temperature.…”

“…5 This reduces LN 2 consumption and the cost associated with cooling the MBE cryopanels by an order of magnitude. This leads to high LN 2 consumption and contributes to the high cost of operating an MBE system.…”