Dr Richard Moug
Professor Gerald S Buller
At Heriot-Watt a major programme is being supported by EPSRC for the growth of high quality layers of II-VI semiconductors grown by molecular beam epitaxy (MBE) on semiconductor substrates. The group also carries out its own sample characterisation (structural and electrical). Optical characterisation is carried out in conjunction with the Heriot-Watt Nano-optics group, and other characterisation techniques are available through the European SANDiE network. The MBE growth facility at Heriot-watt is unique within the UK in that it only grows II-VI semiconductor materials. These can be divided into two groups: the conventional II-VI semiconductors such as ZnSe, ZnMgSSe, ZnCdSe and CdSe which we have produced for over 15 years, and the novel metastable semiconductors MgS, MnS and CrS. We are the only laboratory worldwide which can produce this range of materials.
Molecular Beam Epitaxy (MBE) is carried out in an Ultra High Vacuum environment. After cleaning, samples are transported through the system through three progressively lower pressure chambers until they arrive in the growth chamber.
Heriot-Watt MBE system with the new growth chamber in the foreground.
Here semiconductor layers are deposited by evaporating elements from heated sources (Knudsen Cells or K-cells). The flux of material which is produced by the cell is controlled by altering the temperature of the cell, and the flux can be turned on or off by using a mechanical shutter. For most materials grown by MBE, the growth rate is typically one monolayer per second or less. With shutter opening and closing times of better than 1 second, this means that the thickness of layers can be controlled to better than one monolayer.
The Heriot-Watt growth system
Our research programme is centred on a twin growth chamber Vacuum Generators (now Oxford Instruments) MBE V80H system (shown above). We have recently been awarded an EPSRC grant to completely replace one of the growth chambers. The new growth chamber is fully automated and allows us to grow far more complex multilayer structures than was previously possible.
The MBE group carries out its own characterisation of samples in-house using a variety of techniques for structural and electrical measurements. Electrical characterisation We have pioneered the electrochemical C-V profiling technique for determining the carrier concentration distributions both in single layers and multilayer devices of ZnSe based materials . More recently, we have extended this to zinc oxide layers .
We have access to the Heriot-Watt Dimension TM 3100 AFM system which is used to produce the surface topography scans shown here. We also have a Bede 200 X-ray diffractometer which is used primarily to assess the structure of layers by double crystal X-ray diffraction. In the case of the novel metastable sulphides we have used this technique to show that thick layers have the zinc blende structure , and have also shown that double crystal x-ray diffraction can be used to extract structural information from II-VI layers as thin as 1nm using a method know as X-ray interference .
1. S.Y. Wang et al., Appl. Phys. Lett. 60(1992) 344
2. X. Tang et al., Appl. Phys. Lett., 84 (2004) 3043
3. C. Bradford et al., Phys. Rev. B 64 (2001) 195309
4. K.A. Prior et al., J. Crystal Growth, 251 (2003) 565