NONDESTRUCTIVE CHARACTERIZATION OF THE NUCLEATION AND
EARLY VERTICAL BRIDGMAN CRYSTAL GROWTH OF Cdt_xZnxTe
B.W. Choi and H.N.O. Wadley
Department of Materials Science and Engineering
University of Virginia
CharlottesviIle, VA 22903
INTRODUCTION
Cdl_xZn xTe aIloys (where 0.03<x<0.05) used as substrates for infrared focal plane
arrays detectors 1,2 are typically grown by the unseeded directional solidification of a melt
via either a vertical or horizontal Bridgman process 1,3,4. In a vertical Bridgman process, the
initiation of the growth process appears to be a crucial step controIling subsequent bulk
ingot quality l,3,4,5. A number of experimental and modeling effort have sought to correlate
the controIlable growth parameters with resulting characteristics of the solidified ingot such
as degree of single crystallinity, dislocation density and solute segregation etc. 6,7. However,
an inadequate understanding of melt undercooling and the detailed thermal conditions at the
ampoule tip have seriously hampered the design of the initial solid nucleation process.
In-situ non-contact eddy current methods sense the electrical conductivity at elevated temperature 8 and provide an approach for monitoring the solidlliquid interface position and shape during the crystal growth for a variety of semiconductors 9,lO. It has recently
been shown that the CdTe system is reasonably well suited for this sensor approach because
the electrical conductivity of the liquid is 4-5 times higher than that of the solid at the melting point 8,11. In this experiment, an eddy current sensor situated in the cone area region of
an ingot has been non-invasively integrated into a commercial scale 17 zone vertical Bridgman furnace and used to detect the formation of the solid phase thereby yielding new
insights about nucleation and early growth characteristics of vertical Bridgman grown
Cdo.955ZnO.045 Te ingots.
FEM SIMULATION AND SENSOR DESIGN
An electromagnetic finite element model can be used to relate the multifrequency
response of eddy current sensor designs to the position of a solidlliquid interface located
near the tip of an ampOUle. The method used here essentially solved for the magnetic vector
potential, A(r, z) where rand z are the radial and the axial coordinates using a commercial
electromagnetic FEM code 12 • The magnetic vector potential obtained could then be directly
used to obtain the sensor's electrical impedance as a function of test frequency. The modeled
problem consisted of a Cdo.955ZnO.045 Te sample contained within a conically shaped non
conducting crucible. Since the model geometry was axisymmetric, a two dimensional FEM
analysis could be used. For modeling purposes, the region of interest (the coil, the sample,
and the intervening space) were divided into elements whose size were determined by skin
Nondestrnctive Characterization ofMaterial VIII
Edited by Robert E. Green Jr., Plenum Press, New York, 1998
371
depth considerations at the highest test frequencies. Figure 1 shows the geometry of the sensor sample in the r-z plane in which a six tum primary coil was used to excite an electromagnetic field and a four tum pickup coil then detected the perturbation of the primary coil's
field resulting from the presence of the conducting sample. Figure 1 also shows the calculated 500 kHz magnetic vector potential field for a flat solid/liquid interface located at a
height, h, of 19.1 rom from the cone tip.
Dknensions in mm.
Figure 1 The eddy current sensor and sample geometry, and the resultant 500 kHz magnetic vector
potential contours near the cone area for an interface height of 19.1mm at 500 kHz.
To simulate the response of the sensor during the onset of nucleation and subsequent
propagation of the solid/liquid interface, a series of FEM calculations were performed with
eleven different locations of a solid/liquid interface in the conical region of the cylindrical
80 mm diameter sample. In this case, input values of the conductivity for a liquid and a solid
phase were 6550 slm and 1400 slm for the sample, respectively 8. Figure 2 shows the calculated normalized imaginary impedance curves at four different frequencies as function of
interface position from the cone tip (h=Omm) to the cone shoulder (h=45rom) for a flat solidi
liquid interface.
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10
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Interface poshJon. h (mm)
Figure 2 The sensors calculated imaginary impedance component variation with interface position at four
frequencies.
In a completely melted condition prior to the onset of nucleation, the imaginary
impedance component has a value determined by the liquid's electrical conductivity. As the
solid volume increases during solidification (i.e. as the interface moves upwards), the imaginary impedance increases at all frequencies and continues to converge asymptotically to the
value of the homogeneous solid.
The largest variations in response to interface position are exhibited at a test frequen-
372
cies between 500kHz and 1.2MHz where both the eddy current density and the skin depth
are large. This frequency range is therefore the most desirable for sensing the nucleation and
early growth of Cd 1_x Zn xTe.
EXPERIMENTAL PROCEDURES
Sensor Installation and Measurement Methodology
A two-coil "encircling" eddy current sensor, with a design similar to that analyzed
above, was placed near the ampoule tip region of a commercial scale 17 zone vertical Bridgman furnace, as shown in Figure 3. To minimize disturbances to the crystal growth thermal
environment, the sensors coils were wound on the pre-existing concentric alumina tubes
which normally used to support the conically shaped ampoule inside the furnace. A 1.02mm
platinum wire was then used to wind a 38.1 mm long primary coil while a 0.25mm platinum
wire was used to wind a 12.7 mm long secondary coil.
For the measurement of two coil multifrequency eddy current sensors 8 ,9,1O, a continuous signal was supplied to the primary coil by the variable frequency oscillator with
Impedance GainlPhase Analyzer (HP4194A). Frequency dependent gain (g) measurements
for the two coil system were obtained by recording the ratios of the voltage induced across
the secondary coil with the voltage drop across a 1 ohm low inductance precision resistor in
the primary circuit. The phase difference (<1» between these two voltage signals was also
monitored. The sensor's gain/phase response was normalized by that of the empty sensor.
These empty sensor measurements were made at the growth temperature and gave reference
empty coil gain (go) and phase (<1>0) measurements at each test frequency. During a growth
experiment, gain/phase data was collected at test frequencies between 50 kHz and 5 MHz.
Since the translation rate of the furnace was less than 2mmlhour, the data was collected and
downlQaded to a personal computer once every 5 to 10 minutes throughout growth.
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Figure 3 Schematic diagrams of 17 zone vertical Bridgman furnace and eddy current sensor located at the
ampoule tip.
The real and imaginary components of the normalized impedance (Z) in the presence
of a sample are given by:
Re(Z) = (:Jsin(<1>-<1>o)
Im(Z)
373
Growth Experiments
The axial temperature profile of a stationary empty ampoule in the cone area was
measured prior to two growth runs. The temperature gradient where solidification was initiated was about 8.6°Clcm.
Two growth runs were monitored to observe melting, nucleation and initial growth
with the nucleation sensor. The charge of about 3.3 kg was placed in a 80mm inner diameter
conical pyrolitic boron nitride crucible and then sealed in an 85mm diameter quartz ampoule
under 10--6 torr pressure. A first growth run compounded the constituents and thus allowed
the monitoring of precompounding as well as subsequent solidification of the fully melted
charge. The position of the furnace with respect to the stationary ampoule was conveniently
referenced by a pointer attached to the furnace, Figure 3. The pointer position for the start of
the first run was 7.4 cm. The same charge was used in a second run, but with a raised furnace at a pointer position of 9.3 cm to start. Normally, growth of previously compounded
material is accomplished from a cooler region of the furnace and the follow-up second run
explored the consequence of this using the remelted charge.
RESULTS AND INTERPRETATION
For the first run, the ampoule tip temperature was about 10°C below the melting
point and about 20°C below for the second run. This resulted in significantly different melting behaviors in the tip region. In the first run, complete melting of the charge occurred
before the main growth run, whereas in the second run, the eddy current sensor clearly indicated that melting was incomplete.
Compounding I Complete Melting (first) Run
Figure 4(a) shows the normalized imaginary impedance as a function of time during
the first experiment beginning with furnace heating. The shaded areas in Figure 4 represent
periods during which the furnace was held stationary.
(a)
(b)
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30
Time (hrs)
Figure 4 (a) Measured impedance response at three frequencies as functions of time during the CZT0322
run and the estimated cone tip temperature, and (b) The solidlliquid interface position, h, as a function of
time during initial nucleation and growth of solid.
A sharp drop in impedance occurred at 1.1 hours, and appears to be melting of the
elemental Zn and Cd as the temperature approached 700°C. These elements were added to
precompounded equiatomic CdTe to bring the target composition to CdO.955ZnO.045Te. As
each element dissolved to eventually form solid with the CdO.955ZnO.045Te composition, the
impedance returned towards its "empty" value as the metallic components were compounded. This solid has a very low conductivity at temperatures less than 700°C. The solid
state mixing process proceeded as the furnace temperature was increased to a set point of
374
1150 °C (at the end of segment #3). Towards the end of this stage, the impedance began to
drop sharply, consistent with the beginning of melting (and the associated rise in conductivity) near the ampoule tip. On entering segment #4, the cone tip approached a temperature of
1090°C, while the cone shoulder approached 1105°C. The melting temperature for this
composition ingot is 1098 0c. The continued drop in impedance indicates that it required
approximately 2 hours during segment #4 for the charged material to melt completely to the
cone tip. The normalized impedance curve at the end of segment #4 compared well with the
FEM results for an entirely liquid sample. After this melting transient, a small increase in
the imaginary component was observed during the remainder of segment #4.
During segment #5, the furnace was translated upwards for 10 hours (from a starting
pointer position of 7.4 cm) at a rate of 1.87mmlhour. The temperature of the cone tip
decreased during this segment while the eddy current data showed a very slight rise consistent with the retention of a supercooled liquid whose conductivity was slightly decreasing.
At the end of segment #5, the supercooling was estimated to be about 18°C while the melt
conductivity was approximately 6,100 mhos/m. Continued cooling in the cone area occurred
during segment #6 as the furnace was translated at a rate of 1.49 mmlhr. At a process time of
27.8 hours, an abrupt return of the imaginary impedance towards its null value was seen at
all test frequencies. The furnace position at this moment was 9.9cm and the estimated supercooling at the tip of the ampoule was estimated to be about 20°C for this composition alloy.
Examination of the normalized impedance curve immediately after the sharp rise in impedance indicated an interface shift that increased with test frequency and so this abrupt impedance change corresponded to nucleation of solid at the cone tip. For the location of the solidi
liquid interface, the imaginary impedance data was compared with the FEM result shown in
Figure 2. Figure 4(b) clearly shows that nucleation from a homogeneous liquid abruptly
occurred at a process time of 27.8 hours. The unstable solidification event caused the interface position to move upwards about 19 mm from the cone tip over a period of 20 minutes.
The solidification rate during this nucleation was therefore estimated to be 57 mmlh, in contrast to a furnace translation rate of only 1.49 mmlh.
Remelted (second) Run
The second run were conducted using the sample from first run but with a furnace
starting position closer to that normally used for precompounded growth runs. Figure 5(a)
shows that the variation of the imaginary impedance component with process time is quite
different to that of the first run.
(b)
,1070
'000
--
E
I
S
e
{~
i
-It
!
I
Figure 5 (a) Measured imaginary component impedance response for run CZT0405. The estimated cone tip
temperature is also shown for comparison. (b) Interface position area during the early remelting!
solidification stage of run CZT0405.
During the first 20 hours of the process period, no abrupt variations in imaginary
component were observed. Instead, the imaginary component gradually decreased with process time as the samples electrical conductivity gradually increased. Note that the imaginary
375
impedance component was still decreasing at the end of segment #4, indicating that the sample had not reached a quiescent state prior to the start of furnace translation. Melt back was
seen to have occurred progressively with process time during segment #4. However, complete melting was not achieved. Using the FEM result of Figure 2, the interface position, h,
was determined from the measured imaginary impedance data in Figure 5(b). It appeared to
have arrested 9.6 mm above the cone tip at the beginning of the segment #5. As the furnace
was raised (in segment #5), the interface propagated upwards away from the cone tip. The
initial solidification velocity was estimated to be 1.1mmlhr compared to a furnace translation rate was 1.49 mmlh. As the process continued beyond 25 hrs, the rate of interface
movement appeared to increase to about that of the furnace. It is interesting to note that this
process sequence has usually been thought to have caused complete melting and to have
induced nucleation from an undercooled melt. The results shown in Figure 5 show clearly
that this does not occur.
SUMMARY
A two coil "encircling" eddy current sensor for monitoring the initial stages of crystal growth has been designed and integrated into a 17 zone commercial vertical Bridgman
furnace. A electromagnetic finite element model was used to convert sensor impedance data
to a solid/liquid interface's position so that solid nucleation and the growth velocity could
both be measured. The sensor was used to determine the degree of melting, the extent of liquid supercooling, the moment of solid nucleation and the initial growth velocity from fully
melted and partially remelted ingots. It reveals large supercoolings and unstable nucleation!
growth from insitu compounded melts. The normal growth process conditions used for precompounded charges are found to result in incomplete melt back, no supercooling and a
growth velocity slightly less than that of the fuma.:::e translation rate. These observations
indicate that this two coil "encircling" eddy current sensor may be helpful for optimizing the
growth process conditions of current processes as well as initiating the seeded solidification,
and introducing new feedback control strategies into vertical Bridgman growth processes.
ACKNOWLEDGMENTS
This work has been performed as a part of the research of the Infrared Materials Producibility Program conducted by a consortium that includes Johnson Matthey Electronics,
Texas Instruments, II-VI Inc., Loral, the University of Minnesota and the University of Virginia. We are grateful for the many helpful discussions with our colleagues in these organizations and inparticular to Kumar Dharmasena at UVa, Pok-kai Lio at Texas Instruments,
and to Brent Bollong, Art Socha, Daniel Bakken and the staff of JME for their assistance in
preparing the samples. The consortium work has been supported by ARPNCMO under contract MDA972-91-C-0046 monitored by Raymond Balcerak.
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