1. X-ray Characterization of Si-doped InAs
nanowires on GaAs(111) substrate
Saqib Muhammad
Prof. Dr Ullrich Pietsch
DESY
19.03.2013

2. Outline
 Introduction
a. Applications and growth of nanowires (NWs)
b. Structure of NWs
Aim or Motivation
a. Doping Influence
b. The effect of Oxide layer on NWs growth
 Results
a. Sample Images
b. Exprimental technique (XRD)
c. Expriment and Results
d. Conclusion
e. Out look

3. Introduction
Why semiconductor nanowires?
-For studying new phenomena at
nanometric one dimensional length.
-Used as building blocks for
electronic and optoelectronic
devices.
Why structural study?
-The electrical and optical
properties of the material changes
with the change in the structural
parameters.
-Therefore the structure of the
nanowirs is more important.

4. Zincblend(ZB):
Stacking ABCABC
Wurtzite(Wz)
Stacking ABABAB
cwz
ZB and WZ have slightly
different lattice parameters!
Crystal Structure InAs(narrow band gap, high e mobility, small effective mass)
cwz > 2/sqrt(3) ac
awz < 1/sqrt(2) ac
1. Structural composition varies among
NWs
2. Strain accommodation at interfaces?
GaAs
InAs
ac=5.653Å
ac=6.085Å
Δa/a=7.1%

5. 1) To determine the efects of etched and non-etched Oxide-layer on NW‘s
growth mechanisam?
Etched Oxide-layer
GaAs (111)B substrates, covered with a thin
layer of Hydrogen Silsesquioxan (HSQ  SiOx);
the HSQ is etched in very diluted
HF to ~ 6 nm thickness.
Aim of the Work
In this talk, we present a X-ray diffraction study of the influences of Si-doping in InAs
NWs grown on GaAs(111) substrate using In-assisted MBE growth.
Unetched
GaAs (111)B substrates, covered with a
thin layer of Hydrogen Silsesquioxan
(HSQ  SiOx), unetched;

6. Samples
1μm1μm
a) Undoped b) Si doped 1x1017 cm-3 c) Si doped 5x1017 cm-3
d) Si doped 1x1018 cm-3 e) Si doped 5x1018 cm-3

7. Experimental technique
AsymmetricalSymmetrical
n λ = 2dhkl sinαf ↔ |q| = 2π/d
qx = 2π/λ (cos(2Ѳ-αi) – cos(αi)
qz= 2π/λ (sin(αi) + sin(2Ѳ-αi)
X-ray experiments have been performed at ESRF
synchrotron-source in Grenoble. XRD was employed
at Id01 beam line using a x-ray wavelength of
λ=1.239 Å and a 2D PILATUS DETECTOR.
ω
Qy
Qx
q

8. Undoped Si- 5x10 17cm-3 Si- 1x10 18cm-3
GaAs(111)
Δa/a=7.1%
0
1
2
3
4
5
6
7
8
9
10
11
12
13
14
Intensity
Symmetric (111) reflection
InAs
NW
(a) (b) (c)
Series one
Qz=19.25nm-1
Qz=17.98nm-1
Qz=18.20nm-1
∆Qz=0.21nm-1
NW
d1
d2
d1
a=5.984Å
a=6.058Å
∆a=1.2%

9. Series one
Lattice constant
6.058Å
5.653Å
4.130Å
5.430Å
InAs
GaAs
As
Si
d2
Using Vegard‘s law
In0.77 Ga0.23 As
a = xaInAS (1-x)aGaAs As
d2 Alloy formation
The alloy formation explain by
surface diffusion of Ga librated
through small holes created
during etching process
Comparison
between symmetric(111) and asymmetric
reflection(331) of undoped and Si-doped
1x1018 cm-3
Undoped
(111)
(331)
(331)
(111)
ZB(NW)
Qz=41.85
Qx=-17.40
ZB(Alloy)
Qz=42.40
Qx=-17.61
Doped
Zb(331)
a=6.044 Å
(-17.31, 41.82)
Zb(111)
a=6.047 Å
qx nm-1
cw /aw =1.658
WZ(105)
qznm-1
Wz(105)
a=4.221 Å
C=7.001 Å
(44.51, -17.48)
∆a=1.2%
a=5.984Å
J. Bauer et al.,
2009ApPhA..96..851B

10. Series two
Undoped Si- 1x10 17cm-3 Si- 5x10 19cm-3
Symmetric (111) reflection
Undoped and Si-doped InAs NWs , cover with a thin layer (HSQ→SiOx), unetched.
GaAS
InAS
No Alloy
formation
Parasitic islands
Asymmetric (331) reflection

11. Conclusion
In Conclusion, the hetroepitexial growth behavior of InAs NWs on GaAs was investegated
Combination of etching and Si-doping produce an alloy with seprate lattice parameter in
Series one
The alloy has zinc-blend structure
After analyzing of above 5 samples by X-ray diffraction we found that the 2nd or unknown
peak can‘t be attribute to Si. Its attribute to an alloying of Substrate (Ga) and wire
material (InAs)
NW‘s have zinc-blend structure with small contribution wurtzite
In case of non-etcehed samples the alloy peak is not observed, for highly doped sample
InAS peaks keep the same shape like for undoped sample
No measurable influence of doping on structure

12. Acknowledgements
Prof. Dr. Ullrich Pietsch University of Siegen
Dr. Andreas Biermanns University of Siegen
Anton Davydok University of Siegen
Dr. Mikhail Lepsa Jülich Forschungszentrum
Dr. Thomas Grap Jülich Forschungszentrum
Thank you for your attention!

13. Focussed Beam Expriment
λ= 1.23 Å
Beam size=300x300nm2
Focal length=129mm
Nanofocus set-up

14.  Low temprature processing
 Precise control dopping
 growth rate precisely control (0.01 to 0.3 μm/min)
 Ultra high vacuum 10-8 to 10-10 torr
Sample preperation (MBE)
Varian GEN-II-MBE
Liquid Au droplet is replaced by a drop
formed group IIIA meterial itself. In, Ga etc.
Controlled supply of As and In in UHV at
elevated substrate temperatures
 NWs growth rate → 0.3-0.4 μm/h
 Substrate Temprature → 530°C

15. Series two
Undoped Si- 1x10 17cm-3
Si- 5x10 19cm-3
Symetric (111) reflection
0
1
2
3
4
5
6
7
8
9
10
11
12
13
14
Undoped and Si-doped InAs NWs , cover with a thin layer (HSQ→SiOx), unetched.
GaAS
InAS
No Alloy formation

16. lattice constants
Peak 1
Peak 2
Peak 3
d1 = 6.085Å
d2
d3 = 5.967Å
different materials involved in the growth process, simplest case:
Substrate (GaAs) and adsorbate (InAs) with identical crystal structure
Series one
d3
Using Vegard‘s law
In0.77 Ga0.23 As
a = xaInAS (1-x)aGaAs As
d3 Alloy formation
The alloy formatin explain
by surface diffusion of Ga
Librated through small holes
Created during etching
process
Lattice constant
6.058Å
5.653Å
4.130Å
5.430Å
InAs
GaAs
As
Si

17. Zincblend(ZB):
Stacking ABCABC
Wurtzite(Wz)
Stacking ABABAB
cwz
ZB and WZ have slightly
different lattice parameters!
Crystal Structure InAs(narrow band gap, high e mobility, small effective mass)
cwz > 2/sqrt(3) ac
awz < 1/sqrt(2) ac
1. Structural composition varies among
NWs
2. Strain accommodation at interfaces?
Si
GaAs
ac= 5.430Å
ac=5.653Å
Δa/a=11%