introduction
An
acoustoelectric effect on light been generated interface in high resistivity
GaAs semiconductor has been shown [1] to be an effective method to study deep
traps centers that can play an important role in a substrate materials used for
opto- and micro-electronic devices. This new method in connection with acoustic
transient spectroscopy (A-DLTS) can extend a number techniques used to study
deep centers in high resistivity semiconductors [2-4].
In the present work we describe
the technique of acoustic transient spectroscopy based on the utilization of
space charge inhomogenity in high resistivity GaAs produced by non-uniform
illumination. The space charge inhomogenity can generate in proper conditions
both the longitudinal and transversal or surface acoustic wave by applying a
high frequency electric field [5].
The
basic idea of the used acoustic transient technique consists in the analysis of
the amplitude time development of such generated acoustic wave after the light
is turned off that is detected by the receiving transducer. The trap states
inside the semiconductor band gap are either filed or emptied by an external
stimulus, such as an optical illumination. After the external stimulus is
removed, the traps either emit or capture charges to move forwards their
equilibrium state.
The
acoustic transient spectroscopy (A-DLTS) technique was applied to study deep
centers for different acoustic waves with various polarization and/or
propagation direction and obtained A-DLTS spectra are compared.
Experimental
principle
Illuminating the part of high
resistivity semiconductor by weakly absorbed light of proper energy, free
carriers are produced. These photo-excited free carriers that alter the
population of trapping centers by filling or emptying the trap levels for
electron and/or holes, can immigrate from the illuminated to dark part of
semiconductor producing the space charge region followed by internal arising of
the internal electric field. When the illumination is turned off the
photo-excited carriers recombine thereby establishing their thermal
equilibrium.
The release of carriers from
deep trap levels that leads to the thermal equilibrium on a new steady state
has the dependence on time [6]
D n ( t ) = n t0 exp ( -t / t ) , (1)
where Dnt0 represents the
variation in trap occupancy due to the acoustoelectric field and t is the time constant associated with the
release of the carriers from deep centers when illumination is turned off.
Since the amplitude of interface acoustic wave is proportional to the
nonequilibrium carrier density at the semiconductor interface the decay time
constant associated with the relaxation of the acoustic wave amplitude is a
direct measure of t.
Using the well known relation
expressing the temperature dependence of the relaxation time characterizing the
acoustoelectric transient [6], the activation energies, trap density, and
corresponding capture cross-sections can be determined.
Experimental
results
The experimental arrangement and
the principle of A-DLTS technique based on the computer evaluation of
isothermal acoustoelectric transients used in this work for the determination
of deep centers parameters have been already described [6, 7]. The light pulses
of 200 ms width filling centers completely were applied to the investigated
high resistivity Cr-doped GaAs
sample through the metal screen using the IR-LED with the maximum in spectral
characteristic of 900 nm.
The fast and slow mode of
transverse acoustic wave (T1 and T2) and longitudinal
acoustic wave (L) of frequency 13 MHz were generated through the
acoustoelectric effect on space charge inhomogenity by applying hf electric
field pulses in <110> direction and detected by
transverse and longitudinal quartz transducers, respectively. The surface
acoustic wave (SAW) of frequency 5 MHz was generated in <001> direction and detected by IDT.
Using a method of computer
evaluation of isothermal acoustoelectric transients by applying a data
compression algorithm [6] the activation energies and corresponding capture
cross-sections were be determined from transient measurements of acoustic waves
amplitudes.
Fig. 1 represents A-DLTS spectra obtained for
various acoustic waves and calculated from the isothermal the acoustoelectric
transients.
Figure 1 A-DLTS spectra of Cr-doped GaAs for transversal (T1),
longitudinal (L) and surface acoustic wave (SAW)
The activation energies and
corresponding capture cross-sections determined from the Arrhenius plots are
summarized in Table 1.
Propagation
|
Polarization
|
Deep Centers
|
|
|
Direction |
|
E [eV] |
s[cm2] |
|
[110] [110] [001] |
[001] (T1) [110] (L) (SAW) |
1.28 0.72 0.47 0.31 0.27 0.35 0.22 0.74 0.32 0.77 |
3.1x10-4 2.4x10-13 3.0x10-15 4.8x10-18 2.1x10-18 1.7x10-16 4.1x10-18 5.1x10-12 2.0x10-7 2.5x10-17 |
Table 1. Summary of the deep centers parameter detected in high resistivity GaAs
for various acoustic waved
Most of the obtained energy
levels of deep centers are in good agreement with the values found by the other
techniques [8-10] that have been already compared and discussed [1].
Comparing the results obtained
for different acoustic waves (T1, T2, L, SAW) generated
on the same sample and mostly for the same screen indicates that the
interaction of acoustic wave and deep centers depends both on the propagation
direction and polarization of generated acoustic wave. Further study of this
effect can give more complete information about the deep centers
In conclusion, we have
demonstrated that the acoustic waves generated on light produced inhomogenity
can be used in acoustic transient spectroscopy to study deep centers in high
resistivity semiconductors and can give the useful information concerning the
deep centers character.
Acknowledgements
The author would like to thank
Dr. I. Turek for sample providing and Mr. F. Černobila for technical
assistance. This work was partly supported by Grant No. 1/8308/01 of Slovak
Ministry of Education.
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