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Midterm Review: Important topics • Mobility and Hall effect • Excitonic effects • Photoluminescence for impurity and composition determination • Atomic force microscopy modes – Contact and non-contact modes Slide # Problems 1 • Calculate the bandgaps of AlN and GaN from 2 PL peak positions given for two compositions: x = 0.11, peak at 340 nm; x = 0.45, peak at 290 nm. bind Eex * 13.6 mred 1 2 n m0 2 1 x EGaN xEAlN Eg , AlGaN 1 y EGaN yE AlN Eg , AlGaN Slide # Problems 2 • Calculate the constants α and β in Varshni equation for GaN from the adjacent figure • Calculate the dielectric constant of GaN from the data in the figure. Given: effective electron and hole masses are 0.2 m0 and 0.3m0, respectively. • What is the LO phonon energy in GaN? • Why is excitonic recombination peaks not observed at higher temperature? • Why are the peaks broadened at higher temperature? Slide # Short questions and problems 1 • Explain briefly: – Why is small magnetic field used during Hall mobility measurement? – What are the two major factors that affect mobility? Choose one of these factors and design a device to reduce the scattering factor drastically – Why is optical phonon scattering only at higher temperature but acoustic scattering at lower temperature – Calculate the low field mobility for GaN if the saturation velocity is 2x107 cm/s and the critical electric field is 4x105 V/cm. Why does velocity saturate at higher field? – How does the intensity and peak position of PL peak vary for a quantum well with (a) AlGaAs/GaAs, (b) with AlGaN/GaN Slide # Short questions and problems 2 • In the force vs. distance curve point out the regions where contact mode and non-contact modes are operated. Sketch the AFM topography image of a perfectly square ridge with vertical edges, with a tip that has a conical shape with half angle of 20 degrees. How does the shape looks like if the tip has a parabolic edge? • Mention three major information that you can get from PL. Do you expect PL peak intensity for GaAs and GaN, similar order as for Si and SiC? Explain in detail. • What is quantum confined Stark Effect? Slide #