Topic #1: MBE Growth of Magnetic III-Nitrides
Molecular beam epitaxy (MBE) is a well-known technique to prepare ideal smooth, atomically-layered, thin films of metals, semiconductors, and even oxides. This is accomplished mainly by thermal evaporation of the growth species from “effusion cells” and other elemental sources onto a substrate which is typically held at high temperatures (typically anywhere from room temperature up to 1000 ºC). Growth of nitrides by MBE has been a topic of considerable interest for many years, with much progress in the area of GaN epitaxial growth since 1995. In particular, notable progress in nitride growth by MBE was enabled by the commercialization and availability to researchers of nitrogen “plasma” sources. Although the N2 molecule is chemically very stable and therefore non-reactive for MBE purposes, N plasma sources operate typically by passing N2 gas through a small tube and applying radio-frequency or microwave electric and magnetic fields to ionize the gas and produce an out-going beam consisting of N atoms and ions. In general, a complex series of ionized and excited molecules can also be produced. Simultaneous deposition of gallium can lead to the growth of GaN at rates of up to several hundred nanometers per hour. The substrates are typically sapphire(0001). Using N plasma sources also enables the growth of other transition metal nitrides (TM nitrides) such as scandium nitride and manganese nitride.
Considerable interest has arisen in the last few years to grow magnetic-doped GaN in order to produce a magnetic semiconductor. This was partially motivated by a theoretical prediction that MnGaN would be ferromagnetic (FM) above room temperature [T. Dietl, H. Ohno, F. Matsukura, et al., “Zener model description of ferromagnetism in zinc-blende magnetic semiconductors,” Science 287 (5455), 1019 (2000)].We are exploring the surface science of this field using a combination of MBE and scanning tunneling microscopy (STM). The basic approach is to co-deposit magnetic atoms, such as Mn or Cr, together with the Ga and N to result in a material such as MnGaN. Usually, researchers want the Mn(Cr) concentration to be about 5% of the Ga concentration, but we are exploring a wide range of concentrations within our approach.
Besides the growth of magnetic-doped semiconductors, we are also exploring the growth of magnetic binary TM nitrides such as manganese nitride and chromium nitride. These novel materials have interesting magnetic properties ranging from antiferromagnetic (aFM) to FM. The type of magnetism often depends on the ratio of transition metal to nitrogen composition ratio (TM:N ratio) in the compound. Using MBE, it is possible to control that ratio and thus control the type of magnetism in the resulting film. Such films are then also suitable for magnetic investigations using spin-polarized STM (SP-STM).
The MBE growth and STM investigation of these novel materials is enabled through the use of a dedicated MBE-STM vacuum “system” which includes separate vacuum chambers for MBE and STM which are connected by a gate valve and allow direct sample transfer between the chambers without exposure of the sample to air. A picture of this MBE-STM system is shown in the “Labs” section of this website. |
