Dopant Evaporation Sources For Molecular-beam Epitaxy

Two simple dopant evaporation sources are described and characterized for use in a III-V molecular beam epitaxy (MBE) system. The first is for the evaporation of silicon donors and utilizes a strip of silicon from a commercial doped wafer. The advantages are its cleanliness as shown by the significantly lower compensation ratio, speed of response to current changes allowing abrupt doping changes, efficiency and reliability. Following the design considerations of the silicon strip source we constructed a beryllium source. Instabilities in the heating pattern showed that beryllium could not be reliably used until we redesigned the source to allow the beryllium to be held by a tantalum foil. This redesign was successful, with high efficiency and fast response, but secondary ion mass spectrometry (SIMS) results show oxygen incorporation similar to that of conventionally evaporated beryllium. The same SIMS results show abrupt ( less-than-or-equal-to 50 angstrom) doping profiles made by changing the current through the source (i.e. without the use of the shutter).