The Surface Decontamination Chamber

This system allows the decontamination and etching of wafers with in-situ characterisation of the surface using SIMS.

It consists of a stainless steel chamber connected to the MBE growth system by UHV transfer tubes, the chamber uses two turbomolecular pumps and achieves a base pressure of 1E^-10mbar. The 2 inch wafer holders are moved into the chamber using a cassetrac transfer system and are loaded into a heater assembly. A hydrogen radical source (supplied by Oxford Applied Research) is then used to remove oxygen and hydrocarbons from the surface. It is found that to do this effectively the GaAs samples generally have to be heated to 400-500 degrees Centrigrade. The surface decontamination levels are monitored by an in-situ VG SIMS system and the optimum cleaning recipe determined. In addition we are studying the effectiveness of using low energy electron stimulated desorption to remove contaminants from the surface.

The aim of this work is to enable the highly precise MBE technique to control the motion of electrons in more than one dimension. This is achieved by growing a series of layers, removing them from the growth system, forming patterns in the surface of the epilayer using conventional optical lithography and wet etching and replacing the wafer in the UHV system. The surface of the wafer is then decontaminated using the Hydrogen radical flux and the wafer moved under UHV conditions to the MBE growth chamber where further epilayers are deposited.

The effect of this technique is to use the strata exposed by the ex-situ etching to control the motion of electrons in the layers deposited by the second growth sequence. This allows the formation of one-dimensional wires, size variable tunnelling structures and a host of other devices. A major advantage of this technique is that it allows for the wafer scale processing of devices.

This work is partly supported by a collaboration with the Toshiba Cambridge Research Laboratory.