10. Appendix

A4.1 A Prototypical Deep Center in N-Type Zincblende-Type Semiconductors 675
A detailed description of these techniques is beyond the scope of this book.
Interested readers are referred to many reviews found in the literature
[Lang74, Sah75, Lang84, Li94]. The basic idea behind these two techniques
involves the use of a semiconductor containing deep centers as the insulating
layer between two conductors to form a parallel-plate capacitor. For example,
a reverse-biased pn-junction forms such a capacitor [see, for example,
Wang89a]. The highly doped p-type and n-type regions form the conductors
while the depletion layer between them forms the insulator. Unlike a standard
insulator whose thickness is fixed, the thickness of the semiconductor
space-charge layer can be varied by applying an electric field to populate or
depopulate the deep centers. This change in the thickness of the space-charge
layer can be monitored accurately by measuring the corresponding change in
capacitance. A Schottky Barrier formed at a metal/semiconductor junction as
a result of Fermi Level pinning (see 8.3.3) is another example of a variablecapacitance
condenser [see, for example, Wang89b]. In both cases the charge
state of the deep centers in study (assumed to be the only kind in the sample)
affects the junction capacitance. For example, the deep centers can be filled
by applying an appropriate electrical pulse (sometimes referred to as a filling
pulse). Suppose this deep level is normally above the Fermi Level in the depletion
layer. A filling pulse will change the band bending so as to lower the
deep level towards the Fermi level. Whenever a deep level is below the Fermi
level it becomes filled with electrons. Since these electrons have to come from
the filled shallow donors in the n-type region, the depletion layer expands.
As a result, the junction width increases and the capacitance decreases. When
the applied field is removed the deep centers return to their equilibrium occupancy
by emission of carriers and, correspondingly, the junction capacitance
increases. However, in many deep centers the rate of this emission is usually
thermally activated and strongly dependent on temperature. The emission
process can be monitored by measuring the junction capacitance either as a
function of time at fixed temperature or for a fixed time interval as a function
of temperatures.
In the case of DLTS experiments one fixes the time interval (known as the
time window) defined by two times t1 and t2 and then measures the difference
in capacitance C given by ¢C C(t2) C(t1) while sweeping the temperature.
The resulting ¢C (labeled as the DLTS signal in Fig. A4.1) versus temperature
curves are known as the DLTS spectra. As seen in Fig. A4.1 dips and peaks
can appear in these spectra. A dip in capacitance indicates a sudden decrease
in the capacitance which results when the emission rate of carriers from the
deep centers falls within the time window. Typically by selecting several time
windows appropriately and measuring the corresponding temperature of the
dip in the DLTS spectra one can construct an Arrhenius plot (see 7.1.2 for
definition) for the emission rate of carriers from the deep centers. A variation
of the DLTS technique can be used to measure the capture rate of carriers by
deep centers. Figure A4.2 shows Arrhenius plots for the emission and capture
rates of the deep centers in AlGaAs:Te obtained by Lang et al.. The corresponding
activation energies for emission and capture of electrons determined