1. Some Key Materials Issues in Semiconductor
Quantum Dots Based Nano-electronics
Prabhu Balasubramanian
PhD Graduate Student
Department of Materials Science and Engineering
Rensselaer Polytechnic Institute, Troy, NY, USA
2013 Rensselaer Nanotechnology Center Research Symposium
Wednesday, November 6, 2013
Advisor: Prof. Robert Hull
Collaborators: J. A. Floro, UVa; J. F. Graham, FEI; J. L. Gray, PSU
NYSTAR: SUNY/INDEX & RPI FOCUS CENTER

2. Background
750 nm
AFMQDM: A nanostructure comprising
of four island ridges (QDs) and a
central pyramidal pit; observed in epi-
SixGe(1-x)/Si(100) under conditions of
limited adatom mobility.
QDM Height Profile
-15
-10
-5
0
5
0 100 200 300
Height(nm) Position (nm)
QD: Semiconductor nanostructures that
form during strained epitaxial growth, here
in Ge/Si(100) system.
<510> <311>
Quantum Dot
(Pyramid)
Quantum Dot
(Dome)
<15 3 23>
Substrate
Epitaxial Ge Film
We have expertise in self-assembled
nanostructures that form during strained
epitaxial growth, here in 𝑺𝑺𝑺𝑺𝒙𝒙 𝑮𝑮𝑮𝑮(𝟏𝟏−𝒙𝒙)films on
Si

3. Potential Nanologic Applications
Two Key Requirements:
1) Favorable band structure alignments within QDs and QDMs
for carrier localization.
2) Doping of QDs and QDMs for the creation of holes.
Spin Exchange Switch (SES)
V V
V VV
Tang et al, PRL 97, 119903 (2006)
Coherent Spin Switch (CSS)
Potential Nanologic Applications Based on Charge and Spin
Localizations Inside QDs
Majority Gate that implements QDMs
C.S. Lent , P.D. Tougaw, Proc. IEEE, vol 85, pp. 541-557, APRIL 1997
Possible Charge Configurations (logic
states) of a QDM
“0” “1”

4. 1) Local Nano-chemistry
To investigate composition distribution across QDMs
Nanochemistry Electronic
Band-Gap
Charge Localization
Property
2) FIB Damage
To study structural damage recovery
of FIB implanted Si
Key Materials Issues
Modified
surface
FIB Column
Sample
Ion Beam
FIB Beam Diameter: Few tens of nm
Ion Current Density: 0.1-10 𝐴𝐴 𝑐𝑐𝑐𝑐−2
(at least three
orders of magnitude higher relative to conventional,
broad ion beams)

5. Goal I: QDM Nano-chemistry

6. Experimental Measurements
Auger Signal Strength Dependent on:
1. Local surface angle
2. Geometry of the
feature
Characterization Technique: Auger Electron
Spectroscopy (Resolution: less than 10 nm
under optimum conditions);PHI 700
M. Prutton et. al JAP 54 (1), 374 (1983)
Topography Corrected Auger Signals (TCAS)1 used to determine
variation of c-Si and c-Ge within QDMs.
The TCAS were then suitably converted to composition.
Incident
electrons
Auger
electrons
To Detector
Self-shadowing
Sample
To Detector
Neighbor-shadowing
To Detector
Beam obstruction
X-sectional View

7. : Number of Primary Electrons;σ1 : Primary Beam; σ2 : Backscattered Electrons;
R : Backscattered Electron Correction Factor
eN
Approach
2
2
2
)1(2
2
2Pr
2
2
1
1
σ
πσ
σ
πσ
x
ee
NR
x
ee
N
BSE
j
imary
j
Total
j
−







 −
+
−








=+=
Primary
J. Cazaux, Surf. Sci. 125 (2), 335(1983); M. P. Seah, Surf. Interface
Anal. 33 (12), 950 (2002)
Sequence
Construction of an AED Function
Convolution of AED with Model Composition
Profiles
Total
j
Comparison of Simulated Profiles with the
Experimental Profile
Spatial distribution of emitted Auger electrons depends on
spatial distributions of primary and backscattered electrons

8. Auger Spatial Resolution
Surface Angle
(°)
σ1
(nm)
σ2
(nm)
R do, Effective Spatial
Resolution , 80%-20%
metric (nm)
0 11 420 1.33 27
17 12 440 1.37 31
28 13 480 1.41 35
39 14 510 1.45 41
BSEs do not significantly degrade the resolution
FWHM, Primary Beam = 2.35*σ1 ~ do
Measured on thin sample edges CASINOv2.42
NIST
Backscattering
Correction
Factor
2
2
2
)1(2
2
2Pr
2
2
1
1
σ
πσ
σ
πσ
x
ee
NR
x
ee
N
BSE
j
imary
j
Total
j
−







 −
+
−








=+= Primary

9. Experimental Results
Pit Bases at 17±9 % Si (Leite et al.)
Pit Bases at 58±4 % Si (Our Measurement)
Pits are the most Ge enriched regions within QDMs
M. S. Leite et. al, PRB 73 (12), 4 (2006)
(b)
-15
-10
-5
0
5
0 100 200 300 400 500
Height(nm)
Position (nm)
2.0µm
(a)
AFM
Remnant topography effects and
potential desorption effects have
increased measurement errors inside pits
This Work
Leite et al.
Leite et al.
(Convoluted)
0
20
40
60
80
100
0 100 200 300 400 500
Position (nm)
Silicon(%)
%Si
(c)
%GePit
QDQD
0
20
40
60
80
100
0 100 200 300 400 500
Composition(%)
Position (nm)

10. Simulations Vs. Experiment
Profile ID Δ%Si
(RMS )
Profile I
(Convoluted)
0.7
Profile
II(Convoluted)
2
Compositional contrast across
the QDM = 12%
Si Composition continually
decreases towards pit cusps
starting from exterior edges
of QDMs
Profile I(Before Conv.)
Profile I(After Conv.)
Experimental
(a)
Pit
QDQD
50
55
60
65
70
75
0 100 200 300 400 500
Silicon(%)
Position (nm)Position (nm)
Profile II(Before Conv.)
Profile II(After Conv.)
Experimental
(b)
50
55
60
65
70
75
0 100 200 300 400 500
Silicon(%)
Position (nm)
Pit
QDQD
Position (nm)

11. Goal II: Structural Damage
Recovery of FIB Implanted Silicon

12. Background
Target Surface
Target Atoms
Implanted Ion
Implanted Ion
Trajectory
Damage Volume
KEYEnergy Loss Mechanisms:
Electron energy loss (EEL) and
nuclear energy loss (NEL)
NEL results in interstitials through
binary collisions ~13eV needed for
lattice atom displacement
Collision cascade, 10-13
sec
Fast Relaxation of collision
cascade, 10-11 to 10-12 sec
Slower Relaxation
(Thermal)
Time

13. Fabrication Technique
MS-FIB Column
Schematic
LMIS
Extraction electrodes
Condenser lens (L1)
Ion current selection
apertures
Wien filter
Mass selection apertures
Blanking plates
Faraday cup
Scanning and stigmation
octupoles
Objective lens (L2)
Sample
Broad Beam
Ion
Implanter
Mass Selecting
Focused Ion Beam
(MS-FIB)
Ion Energy 60keV Si+ 30keV Si++ (MS-FIB);
30keV Ge++ (MS-FIB);
and 30keV Ga+ (FIB)
Dose Range to to
Ion Current 1-10 µA 22 pA (Si++);
28 pA (Ge++);
and 53 pA (Ga+)
Beam
Diameter
1/2 to 5/8 in. 10s of nm
Ion Current
Density
6-60 0.1 - 10

14.  Raman Spectroscopy (RS) is an optical spectroscopy technique
to probe vibrational states in a material
At sufficiently high implant doses the vibrational density of states
would be affected significantly by damage which would produce a
noticeable change in Raman signals
The acquired Raman signals are averaged over the probe depth of
the incident laser
Characterization Technique
From SRIM
(Simulations)
Type of Implant
60 keV Si 60 keV Ge 30 keV Ga
Ion Range (nm) 88 48 28
Straggle (nm) 33 17 11
Laser Probe Depth (nm)
In
Crystalline Si
In
Amorphous Si
140 15
For 405 nm Laser

15. Measurement Technique
Peak Height,
H
Peak
Position
Non-amorphizing implants: Fitted by a Voigt function
( a mix of Gaussian and Lorentzian functions)
Partially amorphizing implants: Fitted using a combination
of 100% Gaussian and Voigt functions
Fully amorphizing implants: Fitted using Gaussian alone
RS was used for measurements of
structural damage produced by
implantation
Structural Damage= 1 −
𝐻𝐻
𝐻𝐻0
Stress inferred from peak shifts
From Implant
From
unimplanted Si

16. Implant Energy: 60keV
Ion Dose Rates: 10-100 �µA
cm2 (Broad Beam) and 0.1 �A
cm2 (FIB)
Si-FIB implants produce a relatively higher damage.
Raman Spectroscopy: Ion Dose Rate Effect
As-implanted Post-annealing
Annealing Conditions: 730 °C, 10 minutes in nitrogen ambient
Implantation at a higher dose rate (FIB Vs. broad beam) leads to a greater structural
damage in the as-implanted state, but produces a greater structural damage recovery
upon annealing.

17. In the as-implanted state Ge implants lead
to a greater structural damage.
Raman Spectroscopy: Ion Species Effect
Implant Energy: 30keV, Ge2+; 30keV Ga+
Ion Dose Rates: 0.1-10 �A
cm2 (FIB)
Annealing Conditions: 730 °C, 10 minutes in nitrogen ambient
As-implanted Post-annealing
Ge implants recover structural damage
better upon annealing.
Presumably a higher solid solubility of Ge in Si when compared to that of Ga leads to a
higher structural damage recovery during the annealing.

18. Summary
QDM Nano-chemistry
• QDM pit bases are regions lowest in Si (highest in Ge) with a
composition of 58±4 % Si. Holes would localize at pit bases.
• Composition increases monotonically towards pit bases
starting from the exterior edges of QDMs.
FIB Damage Recovery in Si
• Relative to broad beam Si implantation, Si-FIB implantation
causes a higher structural damage, but, leads to a better
damage recovery upon annealing.
• Presumably a higher solid solubility of Ge than Ga in Si aids
in the better (100% vs. 63% for the highest dose,
5𝑥𝑥𝑥𝑥15 𝑐𝑐𝑐𝑐−2) structural damage recovery for the former
implant upon annealing.

19. Acknowledgements
• Prof. Dan Lewis, RPI for discussions on spatial distribution
of backscattered electrons
• M. David Frey, RPI for AFM instrumentation
• Denny Paul, PHI and Rob Planty, RPI for instrumentation of
Auger electron spectroscopy
• Ray Dove, RPI for Raman instrumentation
• Dr. Jeremy Graham, FEI and Hamed Parvaneh, RPI for
FIB implants
• Hamed Parvaneh and Kiran Sasikumar at RPI for various
discussions