Download Investigation into Germanium thin films formed by a recrystallisation method Acknowledgements:

Investigation into Germanium thin films formed by a recrystallisation method
Acknowledgements: Vishesh Sharma, Dr Peter Gammon, Chunwa Chan, Dr Vishal
Shah, Dr Mark Crouch, Dr David Walker, Corrine Maltby
Motivation
Thin films of germanium on silicon carbide have previously been formed using a conventional
epitaxial method, which resulted in imperfect layers of high surface roughness and
1
Polycrystallinity.
2
However, it showed lower turn on voltage.
Germanium on silicon carbide can be used to produce hybrid devices, which can have control and
power devices on one chip.
Photonics, low band gap of Germanium and wide band gap of Silicon carbide means one photodiode can be used to cover the whole solar spectrum.
Source
N+
Gate
Drain
N+
Ge P+ Substrate
Power device
School of Engineering
SiC Substrate
Fig1: A sketch of a power device in
SiC controlled by a CMOS in Ge
Ge
100
Ge
111
Ge
120
Why are we using Silicon instead of SiC in this project?
Silicon carbide is very expensive.
Several new techniques are being investigated including Ge evaporation and thermal annealing,
so cheap Si is used to learn these processes.
Furthermore, analysis equipment like the Profile-o-meter, interferometer and XRD are being
used.
SiC Substrate
Germanium Peak
Fig2: A Sketch of Polycrytsalline
Germanium on Silicon Carbide
Results XRD
X-ray diffraction(XRD) tells us how crystalline the layer is
and its orientation.
Silicon Peak
500nm 900°C (Blue) have a very well defined Germanium
peak.
Decreasing the annealing temperature results in a less
crystalline layer, reducing peak intensity. This can be seen
in the 800°C film (Red) of the same thickness with a
reduced intensity, doublet peak.
A thinner film also produces a less defined Germanium
peak because it is thinner. This is shown by the 100nm
film (Green) annealed at 900°C.
Fig3. A rocking curve obtained via X-Ray Diffract-o-meter
Ge film was amorphous for all thin films annealed at or
below 700°C.
Results Interferometer
Interferometer can scan and show the
texture of the surface and give us a
RMS Surface roughness reading.
Thickest layer of 900°C annealed
sample was too dull to take a
roughness reading.
Samples annealed at 800°C or below
produced a very consistent Ge thin
film. However at 900°C Ge thin film
got more rough with increasing film
thickness.
Log of Ge film thickness (nm)
Fig4: A plot of surface roughness of Ge thin film
Fabrication/Experimental
25 Ge thin films on Si substrate were deposited.
5 samples per thickness were made.
Thicknesses were 500nm, 200nm, 100nm, 50nm, 20nm.
Samples from each thickness were annealed at 5 different temperatures.
Annealing temperatures and dwell times were 900°C, 800°C, 700°C with dwell
time of 1 minute, 500°C with 5 minutes dwell time and finally 300°C with one
hours dwell time.
Ge thin films were deposited using Electron Beam Evaporator.
Afterwards, they were annealed in a Rapid Thermal Annealer. However for 1
hour dwell time a Tube Furnace was used instead.
To verify thickness of Ge film a Profile-o-meter was used.
Interferometer was used to measure surface roughness
Finally, Crystallinity and crystal orientation of the Ge thin film were analysed
using X-Ray Difractometer.
Future work
Annealing the sample past the melting point
of Ge to analyse the effects on crystallinity,
which is at 928°C.
Samples can be annealed at higher
temperatures for longer period of time in a
tube furnace.
References
1. P. M. Gammon, Development of SiC Heterojunction Power Devices, May 2011,
Warwick University.Page 83
2. P. M. Gammon, Development of SiC Heterojunction Power Devices, May 2011,
Warwick University.Page 122