4) a) The Deal Grove model for deposition determines the growth rate of a CVD film as kạng Cry kg + hg N v is the growth rate ks is the

surface reaction rate v= hg is the mass transfer coefficient CT is the total concentration of the gas in the reactor N is the number of atoms per unit volume in the film Y is the mole fraction of the reactant Y= _CG CT i) Simplify the expression for growth rate assuming ks << hg. Is this the surface reaction-controlled case or the mass transfer controlled case? Why? ii) Simplify the expression for growth rate assuming kç » hg. Is this the surface reaction-controlled case or the mass transfer controlled case? Why? iii) Is the growth rate of a film dependent on temperature if it is mass transfer controlled? Why? iv) Is the growth rate of a film dependent on temperature if it is surface reaction controlled? Why? v) Is the growth rate of a film dependent on reactor geometry if it is mass transfer controlled? Why? vi) Is the growth rate of a film dependent on reactor geometry if it is surface reaction controlled? Why? [18 1/3 marks]/n[18 1/3 marks] b) Discuss concisely the differences between Chemical Vapour Deposition (CVD) and Molecular Beam Epitaxy (MBE). An engineer has designed a GaAs/InGaAs quantum well laser. She has specified that control of the layer thicknesses within the device is required to within a single molecular layer. Would CVD or MBE be an appropriate technology to realise her device? [10 marks] 2.0 1.5 2. 1.0 0.0 -0.5 -1.00 1.00 2.0 1.5 1.0 2. Jedn 0.0 -0.5 -1.00 1.00 Page 8 of 17 2.0 1.5 1.0 0.0 -0.5 0.0 0.0 microns microns (a.) (b.) (c.) Figure 3. Simulation of trench filling with LPCVD oxide with different sticking coefficients. -1.00 0.0 microns 1.00/nc) The sequence of diagrams in Figure 3 shows the results of a SPEEDIE simulation of trench filling using Low Pressure Chemical Vapour Deposition (LPCVD) of SiO₂ in a narrow trench. The same isotropic arrival angle distribution (n=1) is used for all 3 simulations but the sticking coefficient, Sc,is varied. i) Identify the plot with the highest value of the sticking coefficient. ii) Identify the plot with the lowest value of the sticking coefficient. iii) Figure 3 (b) indicates that void formation may be a problem with this process. Why are voids in trenches problematic? [5 marks]