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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
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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
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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?
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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
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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?
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