Thin films of LaF3 were e-gun and thermally deposited on several substrates. The e-gun deposited films are fluorine deficient, have high ionic conductivities that persist to 77 K, and high effective dielectric constants. The thermally deposited material tends to be closer to stoichiometric, and have higher effective breakdown field strengths. Thermally deposited LaF3 films with resistivities in excess of 10 to the 12th power ohms - cm were deposited on metal coated glass substrates. The LaF3 films were shown to adhere well to PbSnTe, surviving repeated cycles between room temperature and 77 K. The LaF3 films on GaAs were also studied

Sher, A.

Tsuo, Y. H.

English

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Produced by the NASA Center for Aerospace Information (CASI) 
https://ntrs.nasa.gov/search.jsp?R=19810012614 2020-03-21T13:27:48+00:00Z
Final Report
NACA Grant NSG-1653
College of i;iliiam and Mary
April 20, 1981
INSULATORS FOR Fbl-XSnXTe
(NASA-C$-164175) INSOLATOAS FOR
	 181-21143
Pb (1-x) Sn (x) Te Final deport (College of
Yillia• and teary) 5 p HC A02/ffF A01
CSCL 07A	 unclas
G3/25 42010
Principal Investigatcrs:
*
Yuan-Huai Tsuo
Arden Shert
Current Addresses
*SERI
	 ti ^p(^^^'^1
1617 Cole Boulevard
	
c4	 ^610
Golden, Colorado 80401
tSRI International
333 Ravenswood Avenue
Menlo Park, CA 94025
	 /44911. k^'^
ABSTRACT
Thin films of LaF3 have been a-gun and thermally deposited on
several substrates. The a-gun deposited films are flurrine deficient,
have high ionic conductivities that persist to 77°K, and high effective
dielectric constants. The thermally deposited material tends to be
closer to stoichiametric, and have higher effective breakdown field
strengths. Thermally deposited LaF 3 films with resistivities in excess
of 10127-cm were deposited on metal coated glass substrates. The LaF3
films were shown to adhere well to PbSnTe, surviving repeated cycles
between room temperature and 77°K. LaF 3 films on GaAs were also studied.
3
i
A lengthy raid-year report was submitted in which details of the
sample preparation methods and some results obtained on LaF 3
 insulators
on PbSnTe samples and other substrates were presented. The main results
were that this insulator could be made with resistivities in excess of
101206-cm and was thermally compatible with the PbSnTe material. LaF3
coated PbSnTe samples could be cycled between room temperature and 77°K
repeatedly without seperating from the substrate. These preliminary
results were obtained on highly doped samples, and the goal of the second
period was to make some deposits on low carrier concentration samples so
their space charge and interface state characteristics could be measured.
Two PbSnTe samples designated 4TI and 5TI were prepared and tested.
The sample preparation procedure that was used follows:
1) Back aluminum contacts were deposited.
2) The front surfaces were mechanically polished
3) The samples along with Cd(99.9998%) metal were sealed
under vacuum in a quartz tube.
4) The tube was heated to 400% for 120 minutes.
5) The front surfaces of the samples were then electrochemically
polished.
6) LaF 3
 thin films were thermally deposited.
7) Finally, aluminum contacts were deposited.
The first sample prepared in this fashion was 4TI. It had a 1500A
layer of LaF 3 . There were two dots deposited on the front surface. One
of the dots was shorted from the outset. The second was the best Sample
in this series. Capacitance and disapation factor measurements at i'WOk6_
were made on it at room temperature with bias voltages ranging from
-4 volts to +4 volts. Initially it exhibited no discernible leakage
current. Unfortunately it broke down and shorted before it was tested
at 77'K. Because at room temperature the carrier concentration was high,
the 100 kHz capacitance and disapation factor were as expected essentially
constant at all bias voltages.
The second sample 5TI had a 12001 LaF 3
 layer. Six dots were
deposited.on it. All of these dots leaked,so only zero hiss measurements
were made.
1
The sole positive result from these two samples was the demonstration
that it is possible to deposit a blocking LaF 3 contact on PbSnTe. Unfor-
tunately this experiment exhausted our supply of PbSeTe. Clearlsy addi-
tional work is needed to bring the LaF 3 deposition process under better
control so the yield is improved.
Since we exhausted our supply of PbSnTe we carried on our studies
of LaF3 deposition methods on a few good samples of GaAs that we had.
The GaAs was chosen for two reasons. It, like PbSnTe, is a material on
which it is difficult to form good blocking insulator layers. The second
reason is that GaAs is well known to have a large interface state density
peak in the band gap that prevents it from being inverted. If GaAs could
be inverted then inhancement mode MOSFET devices could be built from it
with their well known advantages over the current depletion mode GaAs
MESFET devices. In another study there were indications that LaF 3 on the
GaAs surface reduced the troublesome interface density.
An n-type GaAs sample was coated with 1000A of LaF 3 . The upper gate
electrod area was a rather large 0.11 cm 2 . No attempt was trade in the
vacuum station to remove the thin wide layer that is always present on
GaAs. Thus the LaF 3 deposit was placed on ton of this thin layer. This
is a circumstance that will need to be remedied in future experiments.
The vacuum used in the deposition was poor ^-10 -6 torr. Thus some oxygen
contamination is present in the LaF 3 . Once again this is a feature that
should be eliminated. Finally the GaAs substrate temperature was not
controlled during the deposition, and this is a variable that should be
optimall y chosen. Thus there are significant improvements that can
be made.
Despite this the LaF 3 film did not break down for gate voltages
ranging between +1 and -10 volts. The leakage current density observed
at +1 volt, where the sample is in accumulation so all the voltage is
across the insulator,was, 200 nAJcm2 which corresponds to a resistivity
of 5 x 1011il-cm. While this is a high resistivity, particularly for the
size of the area being used, it is still not large enough to make standard
quasistatic capacitance vs. voltage measurements easy to interpret. The
2
e•
results are inconc'_jsive, but the quasistatic capacitance showed a
tendency to increase as the bias voltage was lowered. If this behavior
is verified it would support the conclusion that the Fermi energy had
passed over the interface state density peak and the sample was beginning
to invert.
We plan one further measurement on this sample. We have access
to a Solartron frequency response analyzer. This device measures the
complex impedance of a circuit from 10-4 Hz to 104 Hz. It can be used Zo
make reliable impedance measurements at very low frequencies even in the
presence of small leakage currents. A preliminary measurement on this
sample displayed a totally unexpected feature. The sample evidently
exhibited a multiple resonance at a few mHz. The Nyquist plot has three
cycles. The preliminary measurement was made with an improper impedance
matching circuit between the sample and the Solartron. The defects in the
matching circuit should not have caused the resonances, but it does distort
them so they aren't accurately measured. A new matching circuit is cur-
tently under construction, and when it is completed another data set will
be taken and analyzed.
Finallv, we conclude that it is possible to make useful insulators
for electronic devices from LaF 3 , however we still have not optimized the
deposition processes. LaF 3 will adhere to PbSnTe and we expect it will be
a useful insulator. Since PbSnTe has a high dielectric constant, insulators
for it must also have high effective dielectric constants. This can be
best accompl'-shed with LaF 3 by e-gun depositing the material. E-gun depos-
ited LaF 3 is fluorine defficient. This material bas a high ionic :: , nduc-
tivity, and thin dipole layers even at 77°K. Thus it responds as though it
were a high dielectric constant media, which when made properly has a
large breakdown field strength.
3


Insulators for Pb(1-x)Sn(x)Te

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