Download Single Crystal Growth by AHP Method

Solidification and
Crystal Growth Laboratory
Single Crystal Growth by AHP Method
A. Dario (MSc Student), H. Ö. Sicim (MSc Student), and E. Balikci (Assist. Prof.)
Department of Mechanical Engineering
Introduction
Single Crystal Growth Techniques
Crystallization is a process in which a material in the liquid or gas phase
transforms to a crystalline solid. A sample can grow as a single crystal or a
polycrystal. A single crystal material has a continuous crystal lattice
throughout the entire sample (Figure 1.a), whereas a polycrystal material is
composed of grains having different crystal orientations separated by grain
boundaries, (Figure 1.b).
a)
As a result of the aforementioned discussions, producing a
single crystal requires a precisely controlled process.
b)
Figure 1.
Czochralski
Zone Melting
Bridgman
Axial Heat Processing
During crystal growth an interface is formed between the solid and the
second phase, liquid in melt growth. Solute redistribution, fluid flow, and
heat transfer during crystal growth have a significant effect on the
morphology of the solid-liquid interface.
Under certain conditions a planar S/L
interface may be subjected to random
disturbances. If the disturbances develop
into a wavy or cellular surface, the interface
is unstable (Figure 2.a). If the disturbances
disappear with time the interface is said to
be stable (Figure 2.b).
a)
SCG Laboratory
b)
Figure 2.
Instability Analysis
Solid-liquid interface of a pure substance will only
be unstable if the temperature gradient at the
dT
interface, given by G= q obeys the relationship: G<0
Currently, we are conducting a TÜBİTAK funded (1001) research project that focus on the
understanding of melt growth characteristics of germanium-silicon (Ge-Si) alloy crystals, which
are promising candidates in thermopower generation, photovoltaics, cell phones, handsets, GPS
systems, collision warning systems, and data converters. We are using an innovative approach
(Axial Heat Processing -AHP-) in this study. The AHP technique permits close control of
convective effects and thermal field near the growth interface. AHP achieves these by using a
heater immersed in melt near s/l interface. Power and position of the heater can be controlled as
desired to promote planar interface and laminar fluid flow. Such parameters as the fluid flow,
concentration fields in the melt and solid, and interface shape, position, and temperature are being
assessed. All these growth parameters then will be correlated to interface instability, namely the
transition from single to cellular or dendritic growth.
Advantages of AHP method
dz
• Supplies/conducts heat axially to the s/l
interface, and reduces the radial
temperature gradient. Therefore reduces
the radial segregation and promotes a
stable and planar interface.
Constitutional Undercooling:
In alloys, the equilibrium solidification
temperature depends on the solute composition
in front of the interface. The solute piles up in
front of the interface due to the smaller
solubility of the solid when the distribution
coefficient k is smaller than one.
Figure 3.
For pure diffusional mass transport mode, in the steady state situation the
solute concentration decreases from C0 / k to C0 in front of the interface. This
sets a temperature profile before the interface. There may be then a
constitutionally undercooled region in the liquid depending on the temperature
gradient at the interface imposed by an external heating system.
• Minimizes buoyancy driven convection,
but induces a forced flow in the melt.
• Separates the melt into two region with
different concentrations.
How the system operates
20 Thermocouples
Data acquisition system
Chiller
SCR, heater drivers
Software
Vacuum pump
Motor driver
Figure 5.
Figure 4.
Interface stability Criterion: GL  Gc
Output card
 dT 
GL   q 
 dz  z 0
mC0 R
 dC 
 dT 
Gc   L   m L   
DL
 dz  z  0
 dz 
A single crystal sample
 mC 0  T0
mC 0 R
GL  
DL
G L T0

R
DL
mC 0 k  1
T0 
k
GL mC0 k  1

R
kDL
Silicon-Germanium crystals are grown
in a cylindrical shape with a height of
50mm and a diameter of 40mm.
These crystals are then halved along
their length and their crystal structure
are analyzed using SEM, EDS, and
four point probe methods.
Grainy
region
Single crystal
region
Seed