This study investigated the influence of H2 preconditioning on the nucleation and growth of self-assembled germanium islands on silicon (001). For standard samples, atomic force microscopy and Raman spectroscopy showed a steady evolution of a bimodal island size distribution as small islands ripened into large ones through strain relief via intermixing and defect formation. For preconditioned samples, H2 exposure during cooldown led to an accelerated initial evolution, doubling the density of slightly larger islands with a narrow size distribution and few unusually large islands, as strain relief occurred via increased intermixing during nucleation and growth. However, evolution then saturated earlier due to higher compressive strain inhibiting further nucleation and growth.

1. University of
Southampton
University of
Southampton
Influence of HInfluence of H22 Preconditioning on thePreconditioning on the
Nucleation and Growth ofNucleation and Growth of
Self-Assembled Germanium IslandsSelf-Assembled Germanium Islands
on Silicon (001)on Silicon (001)
G. D. M. Dilliway, D. M. Bagnall, P. Ashburn
University of Southampton
N. E. B. Cowern, C. Jeynes, E. Mendoza
University of Surrey
Lu Xu, P. J. McNally
Dublin City University

2. University of
Southampton
University of
Southampton
OutlineOutline
 Introduction:
 Ge self-assembled nanostructures on Si (001)
 The role of hydrogen
 Experimental set-up:
 Deposition method
 Characterisation techniques
 Results
 Summary
 Conclusions

3. University of
Southampton
University of
Southampton
Self-assembled Ge islands on SiSelf-assembled Ge islands on Si
Self-assembly of Ge nanostructures compatible with Si-
based technology
 Si and Ge crystallise in a diamond cubic structure
 Ge lattice constant > Si lattice constant by 4.2%
Stranski-Krastanov growth mode
 After the coherent growth of ~3ML of Ge (wetting
layer), 3D islands form to relieve the misfit strain
Understanding the influence of the growth conditions on
the self-assembly process controlling the size
distribution

4. University of
Southampton
University of
Southampton
The role of hydrogenThe role of hydrogen
During pre-deposition:
 Ex- or in-situ for hydrogen-terminated surfaces
 In-situ for desorbing SiO2
During deposition:
 H2 is used as a carrier-gas
 For growth from hydrides, as a by-product
The need to understand the influence of hydrogen
on the nucleation and evolution of Ge self-assembled
nanostructures
 Annealing in H2 at high temperatures the
formation of steps and terraces on the (001) Si surface
 The higher the H2 pressure the higher the density
of defects
T. Komeda, Y. Kumagai, Phys. Rev. B 53(3), 1385 (1997)
In our study, H2 is present :
1. during pre-deposition
2. during deposition

5. University of
Southampton
University of
Southampton
Growth sequenceGrowth sequence
‘RCA’ oxide
desorption in
H2 100 sccm 1Torr
950ºC
Pump-down
chamber
to 10-3
Torr
Si high T buffer
from pure SiH4
100 sccm 1Torr
5 min
2 min 30s
40 s 40 s
650ºC
4 min 3 different durations
Time
Temperature
Pump-down
chamber
to 10-3
Torr
Lower temperature
to 650ºC at
10-3
Torr
(standard set)
Lower temperature
to 650ºC in H2
100 sccm 1Torr
(preconditioned set)
Ge self-assembled
islands from GeH4
(10.2% in H2) 30 sccm
0.5 Torr

6. University of
Southampton
University of
Southampton
Characterisation techniquesCharacterisation techniques
 Rutherford backscattering spectrometry
(RBS) in ‘random’ orientation
 Atomic force microscopy (AFM) tapping
mode
 Micro-Raman spectroscopy ( Raman) inμ
backscattering configuration

7. University of
Southampton
University of
Southampton
RBS resultsRBS results
0
2
4
6
8
10
0 2 4 6 8 10 12 14
Growth duration (s)
Gesurfacecoverage
(ML)
Standard Preconditioned
wetting layer

8. University of
Southampton
University of
Southampton
AFM results (shortest duration)AFM results (shortest duration)
Mixed islands: 7 mμ -2
Small height: 5-10 nm
(97%) width: 30-50 nm
Large height: 70 nm
width: 300 nm
PreconditionedStandard
Small islands: 4 mμ -2
Narrow size
distribution
Height: 3-8 nm
Width: 20-40 nm

9. University of
Southampton
University of
Southampton
AFM results (intermediate duration)AFM results (intermediate duration)
Preconditioned
Mixed islands: 10 mμ -2
Bimodal size distribution
Small height: 8-15 nm
(62 %) width: 50-100 nm
Large height: 20-30 nm
width: 120-200
nm
Standard
Mixed islands: 3 mμ -2
Bimodal size distribution
Small height: 5-8 nm
(70%) width: 30-50 nm
Large height: 20-40 nm
width: 100-180
nm

10. University of
Southampton
University of
Southampton
AFM results (longest duration)AFM results (longest duration)
Preconditioned
Mixed islands: 11 mμ -2
Bimodal size distribution
Small height: 8-20 nm
(54%) width: 50-100 nm
Large height: 25-40 nm
width: 150-200
nm
Standard
Mixed islands: 7 mμ -2
Bimodal size distribution
Small height: 10-20 nm
width: 100-120
nm
Large height: 25-40 nm
(60%) width: 200-250
nm

11. University of
Southampton
University of
Southampton
ISi-Ge/IGe-Ge= 2(1-x)/Bx ISi-Ge , IGe-Ge intensities of the Si-Ge and Ge-Ge
peaks
B = 2.218
wGe-Ge = 282.5 + 16x - 384ε wGe-Ge shift of the Ge-Ge peak
x Ge concentration (%)
ε strain
 Diffusion activation barrier for Ge adatoms on a strained Ge (001)
surface increases with compressive strain
A. Van de Walle, M. Asta, P.W. Voorhees, Phys. Rev. B 67, 041308 (2003)
 The periphery of the island is the location of the highest strain
P. Raiteri, L. Miglio, F. Valentinotti, M. Celino, Appl. Phys. Lett. 80(20), 3736
(2002)
Raman resultsμRaman resultsμ
Sample Ge
conc.
(%)
Strain (%) d[Ge]/dt
(%/s)
QD2H 53 -1.649
QD3H 47 -1.264
-3.0
Sample Ge
conc.
(%)
Strain (%) d[Ge]/dt
(%/s)
QD2 52 -1.045
QD3 55 -0.928
+1.5
Standard Preconditioned

12. University of
Southampton
University of
Southampton
Summary and conclusionsSummary and conclusions
standard samplesstandard samples
RBS, AFM and Raman results on Ge self-assembledμRaman results on Ge self-assembledμ
islands on two types of Si surfaces:islands on two types of Si surfaces:
1. Standard well-known steady evolution:
- low density of small islands, narrow size distribution
- bimodal size distribution resulted from ripening:
- small islands still dominate
- strain in the large islands is relived trough
intermixing
- predominantly large islands in which strain is relieved
through both intermixing and defect formation

13. University of
Southampton
University of
Southampton Summary and conclusionsSummary and conclusions
preconditioned samplespreconditioned samples
2. Preconditioned very accelerated initial evolution, which
then saturates:
 The addition of H2 during the ramp-down preserves a non-
equilibrium surface morphology:
- double the density of slightly larger islands (narrow size
distribution) + a very low density of unusually large island
(still under study)
- increase in the density and size through nucleation and
growth; misfit strain relaxes through intermixing
 During the final stages, evolution slows down because of
the high compressive strain in the deposited layer :
- saturation of the nucleation and growth; misfit strain
increases intermixing increases