High speed, low driving voltage vertical cavity germanium-silicon modulators for optical

3.2 Growth Techniques 
3.2.1 Deposition Systems 
3.2.1.1 Molecular Beam Epitaxy 
Molecular beam epitaxy (MBE) was the original technique used for the growth of 
epitaxial SiGe layers and is the growth technique that commonly used in III–V 
heterostructure growth research. In MBE a molecular or atomic beam of the growth 
species is created by heating the raw material above its melting point using an effusion 
cell. Shutters are then used to select which materials may impinge on the heated 
substrate on which nonequilibrium thermodynamic processes compete to produce an 
epitaxial layer. 
Since Si and Ge melting points are quite different, it is not very efficient to use 
MBE to grow SiGe alloys. Also, it is not very easy to get high quality material and 
well controlled doping profiles in the MBE system available to this work. 
3.2.1.2 Chemical Vapor Deposition 
Standard silicon CVD is typically carried out at atmospheric pressure or reduced 
pressure (~5-100 Torr) and involves the pyrolysis or reaction at an elevated 
temperature of a precursor gas of Si, such as silane, disilane or a silicon halide. Radio 
frequency coils or high-intensity lamps are used to heat the system to temperatures 
ranging from 900ºC to greater than 1100ºC to volatilize nominally contaminating 
species, such as water, oxygen or carbon and then to grow the desired layer. 
CVD has the advantage over MBE of lower background contamination and 
fewer defects. It is also significantly more uniform mainly due to the higher 
development budgets and, consequently, to the development of high throughput 
production tools. The biggest advantage of CVD growth is that the growth rate and Ge 

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composition are dependent on the pressure, gas flow rate and substrate temperature, 
allowing multiple choice of variables to control the layer characteristics. However, if 
any of these parameters change, the growth rate and composition need to be 
recalibrated.