Containerless methods are sought for bulk crystal growth and epitaxy which thus far are a less visible component of materials science in space efforts. In the opinion of the author, this is an anomaly which ought to be corrected, because container interactions are a major problem in earth bound materials processing, including crystal growth, and can be avoided or at least significantly reduced in space. The space environment is unique in solving some of these problems, e.g., memory effects in the integration of different classes of materials in high resolution multilayer heterostructures by molecular beam epitaxy or organometallic molecular beam epitaxy. Spectroscopic method of noncontact temperature measurements exist that could be developed in this context. The error in the absolute temperature measurement achieved by these techniques decreases with decreasing substrate temperature and supplements pyrometric measurements that are better suited for high temperature measurements

Bachmann, Klaus J.

English

NASA Technical Reports Server

N90-17911
COMMENTS ON CONTAINERLESS BULK CRYSTAL GROWTH
AND EPITAXY IN SPACE AND ON THEIR IMPLICATIONS
REGARDING NON-CONTACT TEMPERATURE MEASUREMENTS
Klaus J. Bachmann
Department of Materials Science and Engineering
and
Department of Chemical Engineering
North Carolina State University
Raleigh, NC 27695-7907
1. Introduction
Variations in the diffusion layer thickness during the growth of
single crystals from a supersaturated nutrient phase (liquid or va-
por) cause striations in the concentrations of the segregating
components. These striations follow the contours of the solid-nutrient
interface and can be revealed for post-growth analysis by staining
techniques or x-ray topography. Wafers cut perpendicular to the
growth direction of a crystal, produced in a thermal geometry that
results in a non-planar interface, thus exhibit a variation of the local
concentrations of the segregating components (impurities, alloy
components) 1. This is particularly detrimental in applications which
require uniform local properties over large distances on the wafer
surface as for example in the fabrication of planar integrated circuits.
Therefore, the control of the diffusion layer thickness during crystal
growth is a technologically important topic. Since one of the causes
for the modulation of the diffusion layer thickness is the uncontrol-
led variation in buoyancy driven convective flow of the nutrient,
experimentation in the microgravity environment of space can
contribute to the understanding and control of fluid dynamics
problems in crystal growth. It is a well established component of
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NASA's materials science in space program which is reviewed in
more detail in the paper by Archibald Fripp in these proceedings 2.
In this paper, the aim is at containerless methods of bulk
crystal growth and epitaxy which thus far are a less visible
component of materials science in space efforts. In the opinion of the
author, this is an anomaly which ought to be corrected, because
container interactions are a major problem in earth bound materials
processing, including crystal growth, and can be avoided or at least
significantly reduced in space. Clearly non-contact methods of
temperature measurements are essential for containerless processes.
In processes requiring an ultra high vacuum they are also easier to
implement in space than on the ground because of the absence of
windows in the path of light beams that may change their intensity
and polarization due to the clouding of the windows and strain,
respectively. In view of the restrictions on the length of this
presentation we focus on one example each from the areas of bulk
crystal growth and epitaxy to discuss more specific requirements.
2.Containerless Bulk Crystal Growth
There exist two ways of implementing the directional
solidification of contained congruently melting materials: 1. The
motion of the container with the charge relative to a stationary
position of the melting point isotherm (Bridgman method). 2. The
motion of the position of the melting point isotherm relative to a
stationary charge by slow cooling (gradient freezing method).
Although appearing primitive at first glance, the gradient freezing
method has been developed into a viable method for the industrial
fabrication of large single crystals of III-V compounds with
exceptional control of the interface shape 3. Since this control is
achieved by careful design of passive elements, and the method
requires only minimum operator attention for reproducible results, it
is well worth being considered in the context of remotely controlled
use in space.
172
To relate this to containerless processing, consider a molten
charge held in a stationary position in a microgravity environment
by a variable levitation force, e.g. electrostatic, electromagnetic or
acoustic levitation, as illustrated in figure 1. The preferred choice of
the levitation method depends on the requirements regarding the
surrounding atmosphere and melt composition. Providing for an
appropriate axial temperature gradient and attaching a seed crystal
to this melt at the point of minimum temperature on the surface of
the melt, in principle, should permit one to initiate crystal growth in a
controlled manner. For shaping of the crystal and the interface it
may be advantageous to provide for a relative motion of the position
- Fs = 1 0-6 g + Flevitatio n
.......± 7_/___
j, ill
eod
Figure 1. Schematic representation of seeded directional solidification of a
melt confined to a stationary position by levitation forces
173
of the energy well and the seed, but not to the extent of the
conditions of Czochralski pulling at lg since slow cooling at nearly
stationary position of the molten charge is the most desirable mode
of crystal growth. The potential of the above method must be
compared to alternative methods available on the ground, in
particular float zoning and skull melting. In the opinion of the author,
the above described floating drop method provides probably for
greater flexibility in the control of the interface shape than the
alternatives and benefits from the absence of density gradient
driven convection. Therefore, it deserves evaluation by modeling and
by initial experiments concerning the directional solidification of
levitated melts on the ground. The method can be altered slightly for
solution growth initiating nucleation by a pulse of a cold Helium jet
at the point of lowest temperature on the surface of the solution
followed by slow cooling.
Since the temperature distribution in the interior of the melt
can be unequivocally derived from the temperature at the surface if
the thermal conductivity and emissivities of the solid and liquid are
known in the temperature range of interest, thermal imaging reading
out a sufficient number of pixels for interface modeling will be
important for the optimization of the control of the heat input into
the melt, its position and the heat extraction at the seed. Fiberoptics
guided sensors will be least intrusive and will permit the design of a
thermal geometry that minimizes radial temperature gradients in the
melt. The axial temperature gradient will depend on the material.
The most interesting materials to be studied would be highly
reactive compounds and elements that melt at high temperatures
and that have thus container problems without solutions on earth. At
present the restrictions to the power available for space experimen-
tation make the pursuit of such crystal growth tasks unrealistic, but
should not deter an _v_!uation of more manageable model systems,
e.g. directional solidification of low melting point/low density metals
and solution growth of materials that are currently in particular
demand for research tasks which can be done with small crystals,
but that require standards of perfection and purity unattainable by
conventional methods. An example, is the growth of pure untwinned
174
single crystals in the systems Cu3Ba2YO7-8 where Y stands for one of
the rare earth elements. Regardless of the technological potential of
these compounds, in the opinion of the author, studies of their
fundamental properties would benefit from access to more perfect
single crystals and warrant the effort to design appropriate crystal
growth methods. Of course, if these methods address general
problems of crystal growth technology which are bound to remain a
challenge for future advanced materials developments in general,
they deserve attention irrespective of special materials needs.
3. Containerless Epitaxial Crystal Growth
In view of the increasing demands for advanced ICs incorporating
optoelectronic devices, sensors and novel switching elements, e.g.
based on resonant tunneling, the combination of different classes of
materials on a monolithic chip is presently under intense investi-
gation. The methods that provide for optimum resolution are mole-
cular beam epitaxy (MBE) and (OMCVD). Currently organometallic
molecular beam epitaxy (OMMBE) is being explored in several
laboratories which aim at combining the best features of the two
methods, i.e high uniformity (OMCVD), atomic resolution and
flexibility in the control of the surface kinetics by the independent
manipulation of individual ballistic source beams and UHV in-situ
diagnostics (MBE). Figure 2 shows a schematic crossection of the
process chamber of an OMMBE system built by the author at NCSU.
Fluid dynamics problems are absent under the conditions of
these methods, but memory effects are a serious impediment to the
combination of several important classes of materials in high quality
heteroepitaxial structures. These memory effects are caused by the
accumulation of precursor molecules and products of the surface
reaction at the substrate on the walls of the vacuum chamber and
pumps. If these materials have sufficient vapor pressure at operating
conditions to introduce in an uncontrolled manner dopants into
subsequent epilayers clearly the achievement of high quality
hetrostructures is foiled. For example, the group liB elements are
175
93
2:
14 12
13
9
Figure 2. Schematic representation of a cross section of the process
chamber of an OMMBE system 4. 1. Thermocouple for tempera-
ture control of heaterblock 8; 2. Linear motion feedthrough;
3. Top Flange; 4. Heat shields; 5. Heater elements; 6. Substrate
mounted with indium on Mo substrate carrier; 7. Key on substrate
carrier engaging into pick-up tool on magnetic transfer rod 10;
9. Gate valve; I1. Tube connection to load-lock; 12.-14. Multiple OM
source beam ports; 15. Cryo-shroud; 16. Tube connection to by-
pass pump.
notorious in this regard making the combimation of II-VI and III-V
compounds in monolithic structures extremely difficult. Similar
difficulties are encountered in the growth of high resolution II-IV-
V2/III-V structures5,6 and other classes of materials combinations
176
for the fabrication of multilayer structures where sample transfers
through gate valves prevent the maintaining reproducible growth
conditions and almost certainly assure cross contamination.
Therefore, the utilization of the infinite pumping rate of the space
vacuum eliminating pumps and a major part of the enclosing
structure of ground based UHV systems offers unique possibilities.
Non-contact temperature measurements are of particular in-
terest in this context for the monitoring of the absolute temperature
of the surface of the substrate which is a general problem in UHV.
Given the drive towards low temperature processing, which is a ne-
cessity to limit reactions and interdiffusion processes at the inter-
faces of high resolution heterostructures, spectroscopic methods of
non-contact temperature measurements become increasingly feasi-
ble. For example, photoreflectance spectroscopy is feasible in the cur-
rently preferred range of substrate temperatures, 500K < T _< 800K 7.
It is based on the modulation of the field at the surface of a semicon-
ductor due to the photogeneration of carriers in a modulated laser
beam that strikes the surface at the same location as the unmodu-
lated reflected probe beam of frequency co. Structure in AR(co)/R(co) is
observed in the vicinity of singularities in the dielectric function
which occur at wave vectors k where Vk(Ec-Ev) -- 0. This is illus-
trated in figure 3 for GaAs 8. The opportunity for utilizing the method
for non-contact temperature measurements arises because the
bands shift with temperature. The associated shift in the positions of
the various transitions is for GaAs 10-4< _AE/3T_<10-3 eV/KT,8
The accuracy of the method is limited by the line shape which
depends on the defect density in the material. Figure 4 shows the
photoreflectance spectra for the Eo transition of a series of CdxMn 1-x
Te crystals of different alloy composition 0 ___x ___ 0.59. Note the
abrupt change in the line broadening at x ___0.2 which is due to a
steep increase in the microtwinning and line defect density in the
alloys with larger Mn concentrations. Also, note that the position of
the photoreflectance signal shifts with the alloy concentration so that
the error in the determination of the alloy composition adds to the
error in the temperature measurement. Generally the alloy composi-
tion is well controlled in heterostructures employing semiconductor
177
L_c
LI
_'+"' I_, '1 c°l'l_°+'° It I
I _ l _ 1! _,+,I_q
"'/ -_! \ _ Iit
OZ Oi 0 02 04
_[,,] {,o<:>}--
33
32
30
29
19
Ill
l?
16
15
14
log 200 3,00 400
"K
Figure 3. PosiliOnS of Ihe inierband Iran_iiions of GaAs where Vk(Ec-l-vj---O
(top) and lcmperature de.pendcnce of these l.rtinsitions determin-
ed hv modulation spectroscopy (bottom) 8.
178
e
m
q
7,
o
o
n.
L
m
I,
-'NdLs
i
_ t A
| L ,
tO
X0.43
X; 0'I,
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X: I0'1
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1)z ,.i, ,.o__.i
1.7 1.8
j_ )1:401,
?'f
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PHOTON ENERGYsoV
Figure 4. Photoreflectance spectra at room temperature of Cdl-xMnxTe
crystals obtained at room temperature for various compositions 9
alloys so that this error is small. It can be measured on-site without
removing the wafer from the substrate stage by a variety of surface
analysis methods.
179
An added advantage of the utilization of spectroscopic methods
of non-contact temperature measurement, such as phoptoreflectance,
is the simultaneous provision of valuable information on the success
of the process and on radiation damage events that may occur by the
collision of energetic particles with the semiconductor surface. Part of
this problem is alleviated by the sweeping action of the wake shield
in a space ultrahigh vacuum facility (SURF). However, light mass/high
velocity particles could enter into the volume behind the wake shield
and hit the substrate. This is not necessarily a problem since the
growth proceeds under self annealing conditions. Both the damage
and annealing could be followed by the signature provided by the
photoreflectance signal. Figure 5 shows the line broadening and
annealing of the radiation damage introduced into crystals of CdTe
and Cdl_xMnxTe by the same dose of 3 KeV Ar+10,11 Even more
detailed information on the type of damage and sophisticated quality
control regarding the finished heterostructures can be provided
without need for retrieval of the wafer by optical spectroscopies with
high spatial resolution at space ambient temperature.
In summary, there exists a need for containerless processing
both with regard to bulk crystal growth and epitaxy. The space
environment is unique in solving some of these problems, e.g.
memory effects in the integration of different classes of materials in
high resolution multilayer heterostructures by MBE or OMMBE.
Spectroscopic methods of non-contact temperature measurements
exist that could be developed in this context. The error in the
absolute temperature measurement achieved by these techniques
decreases with decreasing substrate temperature and supplements
thus pyrometric measurements that are better suited for high
temperature measurements. The spectroscopic methods cover a
continuous range of temperatures from space ambient to a few
hundred K. They provide, in addition to temperature data, valuable
information on the quality of heteroepitaxial growth and on radiation
damage and annealing processes without need for the retrieval of the
wafer. The justification for developing these methods for use in space
is predicated by the seriousness of the commitment to the
18o
development of the SURF without which no meaningful R&D program
is possible.
"E
.d
e,/-
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0
p.
0
UJ
Ii
UJ
n-
O
p-
O
:z:
D.
CdosMnQ=Te
cleaved
1.8
Ar ÷ Ion -
bomb.
I0 min
anneal
-'_ 200 "C
1.8
II --I.I
\/anne.,
,L • _ * ' A J. • •
1.7 13 1.9
PHOTON ENERGY/eV
W
C
=1
tll
E
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.5 1.E
Ar + Ion-
bomb.
10 min
anneal
200C
L5
0 min
1.4 1.5 1.6
PHOTON ENERGY/eV
Figure 5. Line broadcning and recovery of the photoreflcctancc signal
after radiation damage and annealing of CdTe and Cdl_,_Mn×Tc 10
181
REFERENCES
1. J.R. Carruthers and A.F. Wilt, Crystal Growth and Chanlcterization, Proc.
ICCG Spring School Japan 1974, R. Ueda and J.B. Mullin, cds., Norlh Holland
Publishing Company, Amsterdam, 1975, p.107
2. A. Fripp, This Volume p.
3. W. Gault, E.M. Monberg and J.E. Clemans, J. Crystal Growth 74, 491 (1986)
4. K.J. Bachmann, unpublished design
5. G.C. Xing, J.B. Posthill, G. Solomon, M. Timmons and K.J. Bachmann, J.
Crystal Growth 1988, in print
6. G.C. Xing, K.J. Bachmann, J.B. Posthill, G.L. Solomon and M. Timmons, in
Heteroepitaxial Approaches in Semiconductors: Lattice Mismatch and Its
Consequences, A. Macrander, ed., The Electrochemical Society,
Pennington, N.J., 1989, in print
7. H. Shen, Z. Hang, SH. Pan, F. H. Pollack and J.M. Woodall, Appl. Phys. Lett.
52, 2058 (1988)
8. B.O. Seraphim, J. Appl. Phys. 37, 721 (1966); J.I. Pankove, Optical Processes in
Semiconductors, Prentice Hall, Englewood Cliffs, NJ, 1971
9. K.Y. Lay, H. Neff and K.J. Bachmann, phys. stat. sol. (a) 92, 567 (1985)
10. H. Neff, K. Y. Lay, B. Abid, P. Lange and K. J. Bachmann, J. Appl. Phys., 60,
151 (1986).
11. H. Neff, K.Y. Lay and K.J. Bachmann, Surface Science 189/190, 661, (1987)
182


Comments on containerless bulk crystal growth and epitaxy in space and on their implications regarding non-contact temperature measurements

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