Excitonic Effect at Interfaces in Thin Oxide Films · Atomic Layer Deposition technique (ALD) was used to grow, • High quality • Very low roughness crystalline ZnO thin film •

Nuwanjula Samarasingha,1 Cesar Rodriguez,1 Jaime Moya,1 Nalin Fernando,1 Stefan Zollner,1 Patrick

Ponath,2 Kristy J. Kormondy,2 Alex Demkov,2 Dipayan Pal,3 Aakash Mathur,3 Ajaib Singh,3 Surjendu

Dutta,4 Jaya Singhal,5 Sudeshna Chattopadhyay3,4,5

1Department of Physics, New Mexico State University, Las Cruces, NM2Department of Physics, University of Texas at Austin, Austin, Texas3Centre for Materials Science and Engineering, Indian Institute of Technology Indore, Indore 452020, India4Discipline of Physics, Indian Institute of Technology Indore, Indore 452020, India5Biosciences and Biomedical Engineering, Indian Institute of Technology Indore, Indore 452020, India

May 16, 2017AVS New Mexico Symposium

Excitonic Effect at Interfaces in

Thin Oxide Films

1National Science Foundation (DMR-1505172)

Nuwanjula Samarasingha Excitonic Effect at Interfaces in Thin Oxide Films 05/16/2017

Outline

Introduction (Excitons)

Results:

- Excitons at interfaces: ZnO on Si, ZnO on SiO2, SrTiO3 thin films on

semiconductors

- X ray diffraction and X ray reflectance

Tanguy formalism and Type I, Type II quantum well

Conclusions

2

3

The Concept of an Exciton

Valence Band

Conduction BandEnergy

Ef

Ei

Eg

hω

e

Exciton: bound electron – hole pair

Mark Fox, Optical Properties of Solids (Oxford, 2010).

Troy K. Townsend, Inorganic Metal Oxide Nanocrystal Photocatalysts for solar fuel Generation from water (2014).

S.L. Pyshkin, L. Zv. Zifudin, Excitons in highly optically excited gallium phosphide, Journal of luminescence (1974).

Conduction band

Valence band

e

Wannier exciton(typical of inorganic semiconductors)

Semiconductor Picture

Ground State Wannier Exciton

Large radius

Radius is larger than

atomic spacing

Weakly bounded


4

Excitons at interfaces

ZnO on Si, SiO2 and SrTiO3 on Si, Ge, LaAlO3, SrTiO3

Spectroscopic Ellipsometer

What spectroscopic

ellipsometry reveals:

• Thickness (100 Å to 10000 Å)

• Excitonic absorption

• Refractive index

sample

Monochromator

polarizer

analyzer

detector

Φ

X ray reflectivity setup

What x-ray reflectivity

reveals:

• Thickness (5 Å to 1000 Å)

• Surface and interface

roughness

• Electron density profile

Thickness Roughness

X ray diffraction setup

What x-ray diffraction

reveals:

• Distances between atoms

• Lattice mismatch (strain)

• Grain size

X-ray beam

atoms


Outline

Introduction (Excitons)

Results:

- Excitons at interfaces: ZnO on Si, ZnO on SiO2, SrTiO3 thin films on

semiconductors

- X ray diffraction and X ray reflectance

Tanguy formalism and Type I, Type II quantum well

Conclusions

5Nuwanjula Samarasingha Excitonic Effect at Interfaces in Thin Oxide Films 05/16/2017

Film Thickness =𝟐𝝅

𝚫𝑸

X ray

reflectivity

6

ZnO thin film on Si (ALD)

Structural properties of the ZnO thin films

Atomic Layer Deposition technique (ALD) was used to grow,

• High quality

• Very low roughness crystalline ZnO thin film

• Grown at 200CAFM Images


5 nm

38 nm

19 nm

9 nm

7


Three Tauc-Lorentz oscillators.

Thickness confirmed by X ray reflectance and by a uniqueness fit.


8


Do the properties of ZnO layers on Si depend on the ZnO film thickness?

Real and imaginary part of the dielectric function of ZnO layers on Si decrease

monotonically with decreasing thickness.

Small blueshift with decreasing thickness.

Absorb less light with decreasing thickness below ~20 nm.

Kramers–Kronig consistent modeling with Tauc-Lorentz oscillators.

ZnO excitonic

bohr radius =2nm


9

ZnO thin film on SiO2 (ALD)

Do the properties of ZnO layers on SiO2 depend on the ZnO film thickness?

UV-Vis absorption spectra of ZnO films deposited on SiO2 using a UV-

Vis spectrophotometer in transmission mode.

Thinner ZnO films show higher absorption

coefficient at and above band edge.

( ) ( )n

gh B h E

For the direct transition n=1/2Eg - optical band gapB - constanth - incident photon energy


10

X ray diffraction

Peak PDF(2θ)

(100) 31.7651

(002) 34.4179

(101) 36.2497

PDF card number :

01-079-2205

X ray diffraction pattern matches with the standard

diffraction pattern of hexagonal ZnO.

Preferred orientation of the ZnO is along <002>

direction with c axis perpendicular to the substrate

surface.

𝑚𝑒𝑎𝑛 𝑔𝑟𝑎𝑖𝑛 𝑠𝑖𝑧𝑒 =0.9 ∗ 𝜆

𝐹𝑊𝐻𝑀 ∗ cos 𝜃

Scherrer’s equation:

λ: X-ray wavelength

FWHM: full width at half maximum

(in radians)

Grain size for both ZnO/Si and ZnO/SiO2 are

the same (20nm). Crystal structures of

ALD grown ZnO on Si and SiO2 are similar.

ZnO/SiO2

ZnO/Si

No structural difference between ZnO thin film grown on Si and SiO2.


11

The real and imaginary part of the dielectric

function of thin SrTiO3 layers on Si or Ge are

much smaller than the bulk.

SrTiO3 on Si/Ge: Absorb less light.

Spectroscopic Ellipsometry comparison of SrTiO3

on Si, Ge and bulk SrTiO3

Do the properties of SrTiO3 layers depend on the substrates?

Pranjal Kumar Gogoi, Daniel Schmidt, Temperature dependent dielectric function of bulk

SrTiO3:Urbach tail, band edges and excitonic effects, (2016).

X ray reflectivity


12

SrTiO3 on LaAlO3: Absorb more light

Spectroscopic Ellipsometry comparison of SrTiO3

on LaAlO3 and bulk SrTiO3

Do the properties of SrTiO3 layers depend on the substrates?

X ray reflectivity

Pranjal Kumar Gogoi, Daniel Schmidt, Temperature dependent dielectric function of bulk SrTiO3:Urbach tail,

band edges and excitonic effects, (2016).


X ray diffraction

𝛚

SrTiO3(200) 𝐚Ʇ (Å)

Vertical

Strain

(𝜺Ʇ)

In Plane

Strain

(𝛆‖

)

STO-Si 23.27° 3.899 -0.15% 0.25%

STO-Ge 23.28° 3.898 -0.18% 0.30%

STO-LAO 22.87° 3.964 1.51% -2.50%

13

20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40

101

102

103

104

105

-2 ()

Inte

nsity (

cp

s)

SrTiO3 on LaAlO

3

STO (200)

LAO (024)LAO (036)

STO (300)

Experimental DataModel

F

100

101

102

103

104

Inte

nsity (

cp

s)

SrTiO3 on Si Si (400)

STO (200)

STO (300)

100

101

102

103

104

Inte

nsity (

cp

s)

SrTiO3 on Ge

Ge (400)

STO (200)

STO (300)

Is strain on SrTiO3 responsible for this

fluctuations of the dielectric function?

ω-2θ (°)

SrTiO3/Si,Ge SrTiO3/LaAlO3


14

Influence of excitonic absorption on the

dielectric function and Tanguy formalism

R.J. Elliott, Phys. Rev. 108, 1384 (1957).C. Tanguy, Phys. Rev. Lett. 75, 4090 (1995).

ln ( )( )

d zz

dz

( )g

Rz

E z

2 223/2

2 2

2( )

2 o o

eA P

m

P = < 𝑒 ҧ𝑝 h > dipole matrix element (overlap of electron and hole)

Free-

carrier

absorption

Sommerfeld

enhancement

(excitonic

effects)

2( ) { [ ( )] [ ( )] 2 [ (0)]}

( )a a a

A RE g E i g E i g

E i

Digamma

function

Amplitude

pre-factor

𝑔 𝑎 𝜉 = 2ln(𝜉) − 2𝜋cot(𝜋𝜉) − 2𝜓(𝜉) −1

𝜉


Type-II quantum well and excitonic effect

15

Type-II quantum well

Do the properties of thin oxide films depend on the film thickness and

substrate?

Amplitude pre-factor

𝐴 =ћ2𝑒2

2𝜋𝜀0𝑚02

2𝜇

ћ2

3

2𝑃 2

P = < 𝑒 ҧ𝑝 h > dipole matrix element

Either the electron is confined, or the hole, but not both.

overlap dipole matrix element (and thus the excitonic absorption)

is strongly reduced.

deconfinement

(spatially indirect)

e

SrTiO3Si


16

Do the properties of thin oxide films depend on the film thickness and

substrate?

Amplitude pre-factor

𝐴 =ћ2𝑒2

2𝜋𝜀0𝑚02

2𝜇

ћ2

3

2𝑃 2

P = < 𝑒 ҧ𝑝 h > dipole matrix element

Type-I quantum well

confinement

Absorption peak at 4.2 eV is larger in a 20 nm thick SrTiO3 layer on a

LaAlO3 substrate than in bulk SrTiO3.

Both the electron and the hole are confined which cause an increase

in the dipole overlap matrix element.

Type-I quantum well and excitonic effect

SrTiO3 LaAlO3

Ec

Ec

Ev

Ev

Eg = 3.2 eV Eg = 5.6 eV

e


17

Conclusions

Crystal structures of ALD grown ZnO on Si and SiO2 substrates are similar.

Wide band gap thin film on narrow band gap substrate (SrTiO3 on Si or Ge

and ZnO on Si; all of which are type II quantum well ) experience separation

of electron and hole (excitonic deconfinement) which leads to decrease in

the excitonic absorption.

In a type I quantum well (with a narrow band gap thin film on a wide band

gap substrate like SrTiO3 on LaAlO3 or ZnO on SiO2), both the electron and

hole are confined, resulting in enhancement of the excitonic absorption.


Thank You

18Nuwanjula Samarasingha Excitonic Effect at Interfaces in Thin Oxide Films 05/16/2017

Documents

Excitonic Effect at Interfaces in Thin Oxide Films · Atomic Layer Deposition technique (ALD) was used to grow, • High quality • Very low roughness crystalline ZnO thin film •