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## Midterm Review: Important topics

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**Mobility and Hall effect**Excitonic effects Photoluminescence for impurity and composition determination Atomic force microscopy modes Contact and non-contact modes Midterm Review: Important topics**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.**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?**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 Short questions and problems 1**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?