Molecular Beam Epitaxy (MBE)
MBE (sometimes unkindly referred to as "Mega Buck Evaporator" or "Mostly Broken Equipment") is a semiconductor growth technique with the advantages that
- There is stoichiometric and layer thickness control down to about the 1% level,
- Abrupt interfaces are possible and
- Since it is an ultra-high vacuum (UHV) technique it is compatible with other UHV based techniques such as electron diffraction (low-energy (LEED), reflection high-energy (RHEED) and Auger (AES)), scanning tunnelling microscopy (STM), secondary ion mass spectrometry (SIMS) and x-ray photoelectron spectroscopy (XPS).
Figure 1: Schematic overview of the MBE system at La Trobe university.
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There are a number of MBE variants such as MOMBE (Metallo-Organic Molecular Beam Epitaxy) however they are all characterised by the shuttered interruption of directed thermal-energy atomic or molecular beams. The MBE system at La Trobe University is a highly modified first-generation Varian MBE-360. It comprises a fast-entry load lock (see Figure 1) through which up to four 50mm wafers, either Indium bonded to a Molybdenum block or supported on a Molybdenum wafer support, and mounted together on a stainless steel carousel (see Figure 2) can be passed.
Figure 2: Stainless steel carousel for
wafer transportation.
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An intermediate chamber, from which single wafers can be picked up and passed into the growth chamber, buffers the load lock from the growth chamber and helps to preserve the integrity of the growth chamber vacuum (see Figure 1). The MBE system has provision for up to eight sources (see Figure 3) that are presently loaded with the group III elements Gallium, Indium and Aluminium and the group V elements Arsenic and Nitrogen (delivered by a microwave plasma source). Silicon (n-type) and Beryllium (p-type) dopant sources allow the growth of pn structures and the last source holds an atomic Hydrogen source. The substrates are heated by four tungsten filament heaters positioned behind the sample currently in the growth chamber (see Figure 4).
Figure 3: Schematic of the MBE source arrangement.
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The substrate temperature is measured by a thermocouple positioned centrally behind but not touching the substrate however because this thermocouple does not measure the sample surface temperature it must be calibrated. This is done by monitoring the temperature at which the Arsenic rich surface reconstruction, as monitored by RHEED, changes from 4x2 to 2x2 at 520 degrees C (see Figure 5).The growth chamber is pumped by a 400 litre/second ion pump and a cryopump and achieves a residual gas pressure of the order or 10-10 Torr. The intermediate chamber is also pumped by an ion pump and following sample loading and overnight pumping achieves a residual gas pressure of the order or 10-9 Torr. The load lock is pumped firstly by a turbo-molecular pump backed by a diaphragm pump and then by a small ion pump prior to passing samples into the intermediate chamber. The pressure in the load lock is typically 10-6 or 10-7 Torr immediately before the transfer is made.
Figure 4: Schematic of the substrate heating assembly.
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Furnace temperatures are measured by thermocouples and controlled by a computer-based system (see Figure 6) capable of holding source temperatures stable at the 0.1 degree C level although this error is probably smaller than the error associated with the electronic ice-points. High level PC software (written in-house) readily allows complex multi-layered structures to be grown under full computer control. Up to four levels of nested loops can be programmed giving excellent flexibility to the grower. Group III flux rates can be measured using RHEED although our preferred technique for determining growth rates to high precision is to grow a standard multi-layered structure which can be x-rayed to yield the layer thicknesses and chemical composition to at least the 1% level.
Figure 5: Surface reconstruction changes from the
4x2 to 2x2 as determined by RHEED
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Approaches to simplify the task of analysing such structures have been reported in the literature ("Thickness and Composition Determination of MBE-grown Strained Multiple Quantum Well Structures by X-ray Diffraction", B.F. Usher and D. Zhou, Proceedings of the Fourth International Conference on Thin Film Physics and Applications, SPIE Volume 4086, pp 76-81). The Arsenic flux is monitored by an ion gauge measuring beam-equivalent pressure and the nitrogen flux is presently being calibrated post-growth and will probably be correlated with the Nitrogen gas pressure in the chamber as measured from the ion pump controller.
Figure 6: Control Electronics for the
MBE System.
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- Fig 1: schematic overview
- Fig 2: Pic of carrousel
- Fig 3: schematic sources
- Fig 4 : Schematic Substrate Holder
- Fig 5 : Surface reconstruction changes from the 4x2 to 2x2 as determined by RHEED
- Fig 6 : Pic of Electronics Rack
