Infrared absorption spectra of impurities and radiation damage in silicon semiconductor

Gilmer, T. E., Jr.

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Produced by the NASA Center for Aerospace Information (CASI) 
https://ntrs.nasa.gov/search.jsp?R=19690020982 2020-03-12T03:39:50+00:00Z
Semi-Annual Progress Report No. 8
to the
NATIONAL SCIENCE AND SPACE ADMINISTRATION
in connection with
NASA Grant NGR 47-004-006
Final Report
V.P.I. Project 313176
INFRARED ABSORPTION DUE TO IMPURITIES AND
RADIATION DAMAGE IN SEMICONDUCTORS
by
T. E. Gilmer, Jr.
Virginia Polytechnic Institute
Blacksburg, Virginia
March, 1969
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(ACCESSION NUMBER) 1TNRY1[
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- 1 -
Introduction
Shallow donor and acceptor states in silicon and germanium have been the
subject of considerable interest and effort, both experimental and theoretical.
The first work in this area was devoted to the study of states due to substi-
tutional impurities (of atoms from groups III and V of the periodic table),
and both experiment and theory have been reviewed by Kohn (1). The experiments
have consisted of infrared absorption measurements on samples containing small
amounts of the impurity (usually 10 15 to 1017 atoms/cm 3 ). The small concentration
is necessary to avoid overlapping wave functions due to states of neighboring
impurities. Also the sample must be refrigerated (usually to liquid helium temp-
erature) so that a large fraction of the donors or acceptors will be un-ionized.
The absorption spectra thus obtained give the energies of the excited states of
the impurities, as well as transition probabilities and ionization energies.
More recent work has extended this type of measurement to other impurities.
At this laboratory we have investigated L.Lthium and lithium-oxygen complexes in
silicon (2), the most recent of this work being supported by NASA and described
below. These optical measurements have also been used in studies of defects
introduced in radiation damage of semiconductors. To date, the defects which have
been observed by this method have all involved an impurity as an integral part of
the defect (3). The efforts and results of our study in this area are described below.
This report, then, describes the optical measurements conducted over the
last three years and consisting of the study of several types of defects in silicon.
The work can be conveniently divided into the following parts:
(I) The lithium-oxygen interaction in silicon has been the object of considerable
further examination, extending the work previously done at this lab-
oratory (2).
(II) Electron-irradiated silicon has been studfd. Several samples and
different approaches have been used including helium temperature irradiation.
- 2 -
(III) The L-center, presumably a lithium-damage-center complex in silicon,
has been noted in electron paramagnetic resonance measurements (4).
The third phase of our work has sought possible optical absorptions
due to this L-center.
I. Lithium in Silicon
The previous work at this laboratory (2) on lithium in silicon had been
carried out on silicon crystals which were either float-zone (FZ) prepared or
pulled from quartz crucibles (QC). The former have low oxygen concentrations
while the latter have oxygen concentrations of the order of 10 17 to 10 18 cm-3
and higher. Just how low the oxygen concentration is in FZ samples, (and how
much it varies) has not been directly determined due to several experimental
difficulties. Use of the nine micron absorption below concentrations of 10 17 cm-3
is difficult if not impossible. A direct determination by mass spectroscopy for
these levels is complicated by surface oxide layers.
Not withstanding these difficulties it is important for several reasons to
know the behavior of oxygen in silicon when the concentrations might lie in
the range from 10 1" to 1017 cm 3. Accordingly, one direction of the present
research as prei:iously indicated has been the study of the Li-0 interaction in
this oxygen concentration range, i.e., in the range between "typical" float-
zone and "typical" Czochralski quartz crucible silicon. Since oxygen concen-
tration determination is so difficult, it was determined to use the experiment
to give an indication of oxygen concentration, i.e., to look for samples
which gave both the typical FZ infrared spectrum for lithium (interpreted as
due to the isolated lithium atom), and the QC lithium spectrum, which is apparently
due to LiO.
Three equations may be written down which involve the lithium concentration,
LLi+j, the lithium oxide concentration [LiOt], and the oxygen concentration,
- 3 -
[0].
[LiO+)C = [Li+][0],
[0] + [LiO+.J = No,
[Li+] + [LiO+] = NL.
Here N0 is the total number of oxygen atoms per cubic centimeter and N  is the
total number of lithium atoms per cubic centimeter as determined by room
temperature resistivity measurements. The room temperature equilibrium
concentration values are found using the room temperature values of C which
equals 1015 cm-3
 (5). These concentration values are assumed to be tLe values
seen during a low temperature experiment (i.e., the room temperature values are
"frozen in").
From the above considerations it is inferred that FZ crystals have an
oxygen concentration of 10 14
 cm-3 or less since no LiO spectra .cave yet been
positively detected in FZ crystals. That is, it is inferred that with the
range of [Li+) used, an N0
 of 10 14 cm 3 is probably detectable, and an
N0 = 10 15
 cm-3 would give an Li0 spectrum which would be clearly discernible.
Two samples which we have studied in this program came from a crystal which
was first grown by the Czochralski method from a quartz crucible and then had one
pass made through it by a float zone in vacuum. The sample was n-type before
lithium diffusion, having been doped with phosphorus to a concentration of 10 15 cm-3
as indicated by room temperature resistivity. The nine micron absorption showed
no detectable oxygen. The absorption spectrum is given in Figure 1. The
second of the samples had a somewhat high donor concentration, but showed a
similar absorption spectrum. The spectrum indicated a split in the phos-
phorus peaks normally occurring at 34.4 mev (2P 0 ), and 39.2 mev (2P- ). The
split in these peaks was observed to be 0.8 ± 0.1 mev. These peaks do exhibit
a similar splitting when subjected to uniaxial stress. Thus further experiments
- 4 -
were performed to determine if a new effect was being observed. Samples
containing phosphorus in this same concentration range were obtained from
another supplier. Similar absorption spectra were run, and no splitting
was found. A re-run on the first sample was then made, and this time no
splitting was observed for the phosphorus lines in it. It is thus con-
cluded that the first holder exerted a stress on this sample the first time,
and that the splitting was due to this stress.
In order to observe unambiguous spectra from both isolated lithium and
lithium oxide in the same sample, it would be necessary to obtain silicon with
an oxygen concentration of about 10 15/cc. Single crystal silicon with this
oxygen concentration is not readily available. It could probably be prepared
by introducing oxygen into the furnace while a silicon boule is being zone
refined. To develop the technique needed for this process would no doubt require
extensive experimentation on the part of the supplier. It has not been possible
to conduct such a study and the sought for sample showing both Li and Li0
absorptions has not been obtained.
The results of this phase of the research have served to provide further
information on the behavior of oxygen in silicon. In the sample referred to
above, a single zone pass of a crystal first pulled from a quartz crucible has
shown no evidence of the Li0 spectrum and thus suggests that the oxygen has
diffused from the crystal. It is, of course, possible that the oxygen has not
left the crystal but formed some sort of aggregates which do not show. This
is thought to be less likely, however, since the results of reference 2 indicate
that lithium-oxygen complexes of higher oxygen ratios do show an infrared
absorption spectrum in the range covered. When taken with Pell's results (5),
the indication is that the single zone pass has reduced the concentration to
something of the order of 10 14 cm-3.
- 5 -
II. Low Temperature Eiectron Irradiation and Infrared Study
In spite of the large amount of information which has been obtained on
defects in irradiated silicon there is lithe of these data which do not depend
fundamei:tally on a particular impunity which is an essential part of the defect
(see Ref. 3). For example, electrons paramagnetic resonance (epr) spectra have
bean attributed to vacancy + phosphorus, vacancy + aluminum, vacancy + oxygen,
interstitial aluminum + substitutional aluminum, etc. In the only instance of
the observation of an isolated primary defect two epr spectra have been attributed
to the isolated vacancy in two different charge states. In the case of the
intersLitial silicon atom which must be formed in each primary radiation damage
event, no data is observed which can be attributed to the isolated defect. Indeed,
the fate of the interstitial atoms was a complete mystery until recent work by
Watkins (see Ref. 3, pg. 78). This work has served to emphasize the extremely
mobile nature of these interstitials even at temperatures below 80 0K, and has
involved an impurity atom (aluminum in this case) in the observable defect.
Thus it was determined to attempt observation of some of these mobile defects by
irradiation in a cryostat which allows for the irradiation to be carried out
at low temperature, and the study (in this case infrared absorption) to be
carried out without breaking vacuum or allowing the samples to warm up.
The V.P.I. 1.5 Mev Van de Graaff accelerator was modified to produce electrons,
and a number of experiments have been carried out. High resistivity (500 ohm-cm)
P-type silicon was first bombarded at liquid air and liquid helium temperatures
with about Z x 10 16 electrons cm 2 at an energy of 0.7 Mev. The absorptions which
were observed and reported in the last two semi-annual reports have now been
determined to be spurious. They do not appear where the sample chamber is
separates' from the beam tube by a 1 mil aluminum foil, and are apparently due
to the deposit on the cold sample of organic fragments produced by the ionizing
radiation.
,-
- 6 -
With the aluminum foil in place, irradiations with as many as 0.4 x 1017
electrons cm 2 at an energy of 0.7 Mev have been carried out at liquid helium
temperatures (T about 120 K.). The entire wavelength range from 2 - 35 microns
was then covered, and no new absorptions found in any part of the range. This
higher irradiation thus gives the same results as those carried out errlier
and reported in the last two semi-annual reports when allowance is made for
the spurious absorptions.
The results which have been obtained in this phase of the research constitute
important corroborative evidence for the mobility in silicon of interstitials
at very low temperatures. Electron spin resonance experiments had indicated by
indirect results that interstitials were mobile at temperatures as low as 4 0 K.
But there are defec-s in silicon (e.g., the isolated lithium impurity) which do
not give an esr signal and do give rise to infrared absorption. The results
of this phase of our observations, with irradiation at temperatures as low as
120 K, give further evidence that the interstitial does not remain even at this
very low temperature. One possibility, however, should be pointed out: it is
distinctly possible that the interstitial, or the vacancy-interstitial pair,
may undergo radiation annealing due to the infrared radiations falling on the
sample in the absorption determination. There has been considerable evidence
recently of radiation annealing, and this possibility should not be overlooked.
III. Attempt To Observe Optical Absorption By the L-Center
A number of attempts to induce an optical absorption which would be
attributable to the lithium-damage-center complex (4) in silico:i have all been
unsuccessful. Initial resistivity and irradiation times were first chosen to
correspond closely with those used by Goldstein, as this appeared to give the
best probability of producing an observable absorption. Then variations were
made in these parameters, but still no optical absorptions were found.
- 7 -
The first sample had been doped with lithium by the usual lithium diffusion
technique, i.e. from a lithium-tin bath. The resultant resistivity (0.15 ohm-cm)
indicates about 5 x 10 16
 lithium atoms per cubic centimeter. The sample was
irradiated with a total flux of 3.4 x 1016
 electrons of 0.5 to 0.6 Mev energy.
This corresponds to 0.7 x 1015
 electrons per square centimeter. Another sample
was lithium.
-doped to a concentration of about 5 x 10 15
 atoms/cm3
 and irradiated
with 1.0 x 1016
 electrons/cm2 . In this case the electron energy was 0.7 to 0.8
Mev.
This last sample was again irradiated with approximately the same total flux as
before so that the total amount received was 2.1 x 10 16 electrons/cm2 . In all
of these cases, as was indicated above, no indication of induced optical activity
could be found.
It is perhaps appropriate to report that subsequent experiments on electron
_Z _U_-_,_-..__a__- .3 ilicon by the same electron source have produced
s. These centers have been studied by resistivity
ve also given rise to optical absorption.
References
1. Kohn, W., "Shallow Impurity States in Silicon and Germanium", Solid State
Physics 5, 257, Academic Press, Inc., New York (1957).
2. Gilmer, T. E., Jr., Franks, R. K. and Bell, R. J., "An Optical Study of
Lithium and Lithium-Oxygen Complexes as Donor Impurities in Silicon", J.
Phys. Chem.Solids 26, 1195 (1965).
3. Corbett, J. W. Solid State Physics Supplement 7, Academic Press, Inc.,
New York (1966).
4. Goldstein, B. "Direct Observation of Lithium - Defect Interaction in Silicon
by Electron Paramagnetic Resonance Measurements" Phys. Rev. Lett. 17, 428 (1966).
5. Pell, E. M. "Interaction Between Li and 0 in Silicon", Solid State Physics
in Electronics and Telecommunication, 1, 261 (1960).
W^) 1N31J1 l13Q3 NOI1JbOSUV


Infrared absorption due to impurities and radiation damage in semiconductors Final report

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