X-ray Characterization of Si-doped InAs nanowires on GaAs(111) substrate

Saqib Muhammad Prof. Dr Ullrich Pietsch

DESY 19.03.2013

Outline Introduction a. Applications and growth of nanowires (NWs)b. Structure of NWs

Aim or Motivationa. Doping Influenceb. The effect of Oxide layer on NWs growth

Resultsa. Sample Imagesb. Exprimental technique (XRD)c. Expriment and Resultsd. Conclusion e. Out look

IntroductionWhy 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.

Zincblend(ZB):Stacking ABCABC

Wurtzite(Wz)Stacking ABABAB

accwz awz

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

%

1) To determine the efects of etched and non-etched Oxide-layer on NW‘s growth mechanisam?

Etched Oxide-layerGaAs (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 WorkIn 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.

UnetchedGaAs (111)B substrates, covered with a thin layer of Hydrogen Silsesquioxan (HSQ SiOx), unetched;

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

1.2μ

m

100nm

Experimental technique

d

Asymmetrical Symmetrical

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.

ωQy

Qx

q

Undoped Si- 5x10 17cm-3 Si- 1x10 18cm-3

GaAs(111)

Δa/a

=7.1

%

01234567891011121314

Inte

nsity

Symmetric (111) reflection

InAsNW

(a) (b) (c)Series one

Qz=19.25nm-1

Qz=17.98nm-1

Qz=18.20nm-1

∆Qz=0.21nm-1

NWd1

d2

d1

a=5.984Åa=6.058Å

∆a=1.2%

Series one

Lattice constant6.058Å5.653Å4.130Å5.430Å

InAsGaAs AsSi

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=41.85Qx=-17.40

ZB(Alloy)Qz=42.40Qx=-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)

q z nm

-1

Wz(105)a=4.221 ÅC=7.001 Å(44.51, -17.48)

∆a=1.2%

a=5.984Å

J. Bauer et al., 2009ApPhA..96..851B

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

NW(331)

Asymmetric (331) reflection

ConclusionIn 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

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!

Focussed Beam Expriment

λ= 1.23 Å Beam size=300x300nm2 Focal length=129mm

Nanofocus set-up

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

Series two

Undoped Si- 1x10 17cm-3 Si- 5x10 19cm-3

Symetric (111) reflection

01234567891011121314

Undoped and Si-doped InAs NWs , cover with a thin layer (HSQ→SiOx), unetched.

GaAS

InAS

No Alloy formation

lattice constants Peak 1 Peak 2Peak 3

d1 = 6.085Åd2

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 formationThe alloy formatin explainby surface diffusion of GaLibrated through small holesCreated during etching process

Lattice constant

6.058Å5.653Å4.130Å5.430Å

InAsGaAs AsSi

Zincblend(ZB):Stacking ABCABC

Wurtzite(Wz)Stacking ABABAB

accwz awz

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%