Single crystal (xtal) specimens maintain translational symmetry over macroscopic distances (crystal dimensions are typically 0.1 mm – 10 cm).
Structure determination and intrinsic property measure-ments are preferably, sometimes exclusively, carried out on single crystals.
For certain applications, most notably those which rely on optical and/or electronic properties (laser crystals, semiconductors, etc.), single crystals are necessary.
Crystal growth consists of two steps :
Nucleation and Growth
If nucleation rates are slow and growth is rapid large crystals will result.
If nucleation is rapid, relative to growth, small crystals or even polycrystalline samples will result.
For rapid growth rates diffusion coefficients must be large, hence crystal growth typically occurs via formation of a solid from another state of matter :
(a) Liquid (Melt) ® Solid (Freezing)
(b) Gas (Vapor) ® Solid (Condensation)
(c) Solution ® Solid (Precipitation)
Several techniques are used separately or in combination
to induce nucleation of the solid phase at a slow and controlled rate :
(a) Slow Cooling
of Melts
(b) Temperature
Gradients
(c) Introduction
of Seed Crystals
Crystal Growth Methods
(1) Slow cooling of the melt
With congruently melting materials (those which maintain the same composition on melting) one simply melts a mixture of the desired composition then cools slowly (typically 2-10° C/hr) through the melting point.
More difficult with incongruently melting materials, knowledge of the phase diagram is needed (see pp 19-21 in West).
Often times the phase diagram is not known, consequently there is no guarantee that crystals will have the intended stoichiometry.
Molten salt fluxes are often used to facilitate crystal growth in systems where melting points are very high and/or incongruent melting occurs.
Crystals grown in this way are often rather small, thus this method is frequently used in research, but usually not appropriate for applications where large xtals are needed.
(2) Czochralski Method
A seed crystal is attached to a rod, which is rotated slowly.
The seed crystal is dipped into a melt held at a temperature slightly above the melting point.
A temperature gradient is set up by cooling the rod and slowly withdrawing it from the melt (the surrounding atmosphere is cooler than the melt)
Decreasing the speed with which the crystal is pulled from the melt, increases the quality of the crystals (fewer defects) but decreases the growth rate.
The advantage of the Czochralski method is that large single crystals can be grown, thus it used extensively in the semiconductor industry.
In general this method is not suitable for incongruently melting compounds, and of course the need for a seed crystal of the same composition limits its use as tool for exploratory synthetic research.
(3) Zone Melting
A polycrystalline specimen is prepared, typically in the shape of a cylinder and placed into a crucible, with a seed crystal near the top of the crucible.
The sample cylinder is placed in a furnace with a very narrow hot zone (sometimes this is done using halogen lamps as heat sources).
The portion of the cylinder containing the seed crystal is heated to the melting point, and the rest of the cylinder is slowly pulled through the hot zone.
Zone melting setups are modifications of either the Bridgman or Stockbarger methods of crystal growth.
Bridgman ® Hot zone moves,
crucible stationary
Stockbarger ® Crucible
moves, hot zone stationary
An advantage of the zone melting
technique is that impurities tend to be concentrated in the melted portion
of the sample. Consequently, this process sweeps them out of the sample
and concentrates them at the end of the crystal boule, which is then cut
off and discarded. Thus this method is sometimes used to purify semiconductor
crystals.
(4) Chemical Vapor Transport
A polycrystalline sample, A, and a transporting species, B, are sealed together inside a tube.
Upon heating the transporting species reacts with the sample to produce a gaseous species AB.
When AB reaches the other end of the tube, which is held at a different temperature, it decomposes and redeposits A.
If formation of AB is endothermic (rxn ¬ , as T ¯ )
A (powder) + B (g) ®
AB (g) (hot end)
AB (g) ®
A (xtal) + B (g) (cold end)
If formation of AB is exothermic (rxn ® , as T ¯ )
A (powder) + B (g) ®
AB (g) (cold end)
AB (g) ®
A (xtal) + B (g) (hot end)
Typical transporting agents include:
I2, Br2, Cl2, HCl, NH4Cl, H2, H2O, TeCl4, AlCl3, CO, S2
Temperature gradient is typically created and controlled using a two-zone furnace.
Tubes are usually SiO2, unless reactive, in which case metal tubes (Pt, Au, Nb, Ta, W) are used.
Examples :
Fe3O4 (s) + 8HCl (g) ®
FeCl2 (g) + FeCl3 (g) + 4H2O
(Endothermic)
ZrNCl (s) + 3HCl (g) ® ZrCl4 (g) + NH3 (g) (Exothermic)
SnO2 (s) + CO ®
SnO (g) + CO2 (g)
SnO (g) + CO2 (g) + 2CaO (s) ®
Ca2SnO4 (s) + CO (g)
Chemical Vapor Transport is a good
method of growing high quality crystals from powders. However, growth rates
are usually quite slow (mg/hr) which makes this approach more attractive
for research than for industrial applications.
Thin Film Deposition
All deposition methods involve growing a film whose thickness can vary from tens of angstroms to several millimeters, on a preexisting substrate.
They vary in their methods of delivering the reactants/product to the substrate.
Films can be single crystals, epitaxial, polycrystalline
or amorphous.
(1) Chemical Vapor Deposition
Similar to chemical vapor transport, involves one or more gas phase species which react on a solid surface (substrate) to deposit a solid flim.
Typically, the reaction is initiated by heating the substrate. Other mechanisms of supplying the activation energy necessary to initiate reactions include: laser CVD, photo CVD, and plasma enhanced CVD.
Typical Parameters
Examples
WCl6 (g) + 3H2 (g) ® W (s) + 6HCl (g)
SiH4 (g) + O2 (g) ® SiO2 (s) + 2H2 (g)
6TiCl4 (g) + 8NH3 (g) ®
6TiN (s) + 24HCl (g) + N2 (g)
Advantages
Disadvantages
(2) Vacuum Evaporation
Vacuum evaporation consists of three steps
(a) Transition of the solid/liquid source to the gas phase
(resistive heating, flash evap., e-beam).
(b) Transport of the vapor to the substrate
(c) Condensation of the vapor on the substrate (deposition)
Typical parameters
Pressure ® 10-5 torr
Source to Substrate ® 10-50 cm
Deposition Rates ® ~ 60 Å/min
Examples :
Resisitive Heating
Cr (1400° C), In (950° C)
Electron Beam Heating
ZrO2 (2400° C), W (3230° C), Ta2O5 (2000° C)
Advantages
Disadvantages
(3) Laser Ablation/Pulsed Laser Evaporation
This technique is very similar to vacuum evaporation, but instead of a heated source, you now have a ceramic target which is bombarded by laser pulses.
Because the heating is very rapid and localized, even complex materials can be evaporated congruently. Thus this method has been used on many occasions to grow films of the High TC superconductors, such as YBa2Cu3O7-x.
Typical parameters
Pressure ® 10-7 – 0.2 torr
Target to Substrate ® 3-70 cm
Deposition Rates ® 1-200 Å/min
Examples :
Example #1
Example #2
Advantages
Disadvantages
(4) Sputtering
Sputtering is similar in several respects to laser ablation. The primary difference is that ion bombardment is used instead of a laser pulse to displace atoms or clusters of atoms from the target.
The ions which bombard the target are produced in a plasma (gaseous collection of ionized and neutral species) discharge. The plasma is created by application of a large electric field, which partially ionizes the neutral gas (Ar, He, N2, etc.) present in the chamber.
Plasma – Neutral species, electrons, positive ions
DC Electric Field – Metallic (Conductive) Targets
RF Electric Field – Insulating and conductive Targets
Typical parameters
Pressure ® 1 – 100 mtorr
Target to Substrate ® 4-10 cm
Deposition rate ® 40-400 Å/min
Examples :
Example 1
Example 2
Advantages
Disadvantages
(5) Molecular Beam Epitaxy (MBE)
Beams of atoms and/or molecules are directed onto a substrate where layer by layer growth (epitaxy) of a crystalline film occurs.
The atomic/molecular beams are created by evaporation from solid/liquid sources just as in vacuum evaporation, however the timing and flux of these beams is carefully controlled through the use of shutters which separate the source(s) from the substrate.
The MBE growth chamber must be maintained at Ultra High Vacuum (background vacuum ~ 10-11 torr)
Examples :
GaAs, Al1-xGaxAs, InP, Si1-xGex, YBa2Cu3O7-x
Advantages
Disadvantages
(6) Liquid Phase Epitaxy (LPE)
A substrate is brought into contact with a saturated
solution of the film material at an appropriate temperature. The substrate
is then cooled at a suitable rate to lead to film growth.
Examples :
Typically compounds and alloys of III-V
Semiconductors (similar to MBE)
Advantages
Disadvantages
Summary Crystal Growth and Thin Film Deposition
Crystal Growth
(1) Slow Cooling of the Melt
Simple method for growing crystals
of congruently melting compounds, when used in conjunction with low melting
fluxes can be used to grow crystals at low temperature.
(2) Czochralski Method
Useful for growing large crystals
of congruently melting compounds from a seed crystal
(3) Zone Melting
Excellent for producing high quality
pure crystals, or purifying crystals grown via another technique.
(4) Chemical Vapor Transport
Useful (primarily research) tool
for growing crystals from powders, using a reactive transport gas.
Thin Film Deposition
(1) Chemical Vapor Deposition
Rapid growth rates, good film quality
make this method of choice for producing films of relatively simple, thermodynamically
stable compounds.
(2) Vacuum Evaporation
Simple (relatively inexpensive)
and widely used method for applying coatings. Not so good for refractories
(materials with a high melting point) and complex materials.
(3) Laser Ablation/Pulsed Laser
Evaporation
Primary use is deposition of films
which are more complex (i.e. ternary, quaternary and higher) due to retention
of targe stoichiometry in film. Plagued by non-uniform coverage.
(4) Sputtering
Versatile method, reasonable growth
rates, uniform coverage, can be applied to materials with complex stoichiometry,
potentially capable of producing new materials and metastable phases.
(5) Molecular Beam Epitaxy (MBE)
Ultimate control over the deposition
process allows one to create complex film architectures. Slow growth
rates and expensive instrumentation limit its use primarily to research.
(6) Liquid Phase Epitaxy (LPE)
Primary use is the deposition of
high purity III-V semiconductors, much simpler and less expensive than
MBE but applicable to a narrower class of materials. Furthermore,
control over layer thickness is also sacrificed.