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In this thesis,we present a detailed study of persistent photoconductivityef
fects in SiGe/Si quantum wells as well as AlGaSb/InAs quantum wells.I. SiGe/Si
quantum wells Persistent Photoconductivity (PPC) has been observed in boron-
doped SiGe/Siquantum wells. The decay kinetics of the PPC effect can be well d
escribed by a stretched-exponential function,I(t)=I(0)exp[-(t/τ)^β](0<β<1),
which is usually observed in many disorder materials. Through the studies of t
he PPCeffect under various conditions, such as different temperature, differen
t photon energy of photoexcitation, and different Ge content, we identify that
the alloy potential fluctuations induced by compositional disorder are theorig
in of the PPC effect in SiGe/Si quantum wells.II. AlGaSb/InAs quantum wells T
he "positive" persistent photoconductivity (PPC) and "negative" persistentphot
oconductivity (NPPC) have been studied in AlGaSb/InAs quantum wells. Comp-arin
g with the published reports, several new results have been observed,andthey c
an not be explained by previously proposed models. Through the studiesof the N
PPC and PPC effects under various conditions, such as different photonenergy o
f excitation,different temperature, and different Al composition x, wesuggest
that the NPPC and PPC effects are caused by two competing processes.At low tem
perature, electrons in InAs layer are photoexcited into the valence band on A
lGaSb layer. The return of the photoexcited electrons are prohibitedby the bar
rier due to interface band bending,and thus the NPPC occurs. At high temperat
ure, the photoconduction is dominated by the photogenerated electrons in the I
nAs well, in which the relaxation of the excess electrons is prohibitedby an e
nergy barrier due to the trapping of photoexcited holes by deep defectsin the
InAs well.
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