Molecular Beam Epitaxy
Nanolab has a Riber-32 MBE system as shown in the figure. Since it was installed in 1996, more than 700 samples of GaN have been grown so far. It consists of several major parts such as growth chamber, load lock, cells for molecular sources and control units for growth and gas flow.
The growth chamber consists of a sample manipulator, a cell panel for various molecular sources including N2 plasma source, analysis tools (RHEED, RGA & ion gauges), liquid nitrogen (LN2) cryoshrouds, and UHV pumping. The manipulator is xyz-controllable, possible to rotate and has heating capability up to ~ 1000 ° C with W filament. UHV is maintained by a cryopump and titanium sublimation pump (TSP). Pressure can go down below 10-10 Torr and even lower (~ 10-11 range) with LN2 cooling. Chamber pressure ranges 3 ~ 6 ´ 10-5 Torr during growth according to different N2 flow settings.
The load lock is used as a buffer (or preparation) chamber. It has a four-sample storage with one heating station, which enables outgassing of a substrate, for example, ~ 500 ° C for Si and sapphire substrates. Its pressure is ~ 10-8 Torr range and maintained by 2 pumps, ion pump and TSP.
The cell panel consists of 6 cells for 6 solid sources and an injector for a gas source. 5 solid sources are currently used: Ga, Al, Er, Eu and Tm. Ga is for the growth of GaN films and Al is both for AlN buffer layers and AlGaN films. Er, Eu and Tm are dopants for visible emission. They can be replaced with other materials if it is necessary. Knudsen cells filled with these materials are usually heated at high temperature to obtain molecular beam flux for growth. Typical temperature settings for those are like followings: Ga at ~ 900 ° C, Al at ~ 1100 ° C, Er at ~ 860 ° C, Eu at ~ 400 ° C and Tm at ~ 600 ° C.