Stabilization of ZnO(0001) Surface

and Self-assembled

Nanostructures studied by

Scanning Probe Microscopy

Ju Hong Lai (賴柜宏),

J. C. A. Huang (黃榮俊 教授)

Physics Department, National Cheng Kung

University, Tainan, Taiwan

SPM investigation

Scanning Tunneling Microscopy (STM) & Scanning Force Microscopy (SFM)

Topographic images Electronic properties

(iii) X-sectonal STM study of

ZnO LDOS

Surface

potential

(i) Stabilization of ZnO polar

surface with charged

surface Nano-defects

(ii) Nucleation & growth

of Co on ZnO

Part I.

Stabilization of ZnO(0001) Polar

plane with surface Nano-defects

Wurtzite ZnO

)0211(

c

uc

a

OZn

)0011(

)0001(

)0211(

Direct band-gap of 3.37 eV, near-UV

emission, a large excitation binding

energy (60 meV), the transparent

conductivity and catalysis properties.

Wurtzite ZnO is easy to fabricate as

crystalline structure along c-axis.

There are polarity problems on the

(0001)ZnO surfaces.

Light-emitting diode based on ZnO

Polar surface of ZnO(0001)—Zn surface (0001)—O surface

E

[0001]ZnO

+σ

-σ

+σ

-σ

R1

R2

Divergent surface energy !

VE4πσ0 4πσR1

…

The (0001)-Zn and (0001)-O polar surfaces need to be stabilized

by quenching the internal electrostatic dipolar field.

Zn

O

Stabilizing mechanism for polar surface

ZZuZRR

R

RRNeReZN

AucRAucc

Roo

4

121

)(

99.1,65.0)2

(

21

1

211

21

charge compensation

R1R2

N·Zn-O double-layers

creating surface state for charge transfer

δe- δh+

by Tasker, J. Phys. C 12, 4977 (1979)

by PRL (86) 3811 (2001), PRB (67) 035403 (2003),

PRB (68) 245409 (2003), PRB(77) 035332 (2008)

2D metallic surface state

However, the 2D metallic surface state has

been not observed on the (0001)-Zn surface

by ARPES measurements.

by PRB (78) 155414 (2008), PRB (79) 075314 (2009)

(0001)ZnO polar surfaces are stabilized

by other pathways.

Polar surface Stabilization by Zn vacanciesO

Zn

Zn vacancy compensation charge δ

ZZRR

R

4

1

21

1

Zn

O

nonstoichiometry ZnO

triangular island

21 Zn : 28 O

+ 7

VZncompensation

charge δ

by PRB (68) 245409 (2003), PRB (78) 155424 (2008)

by PRL (90) 016102 (2003), PRB (77) 041405 (2008)

STM

AFM

Motivation

Evidence of charged surface defects ?

Stabilizing mechanism of ZnO polar surface via surface defects?

STM

AFM

Non-stoichiometry ZnO

triangular island

21 Zn : 28 O

+ 7

VZnCompensation

charge δ

How to create and control surface defects on ZnO surface?

Experiment

1. Ion sputtering at 1 keV for 10 min.

2. Annealing at 750oC for 15 min.

The clean Zn-terminated surface can be

achieved by 3 - 4 cycles.

The ZnO single crystal was hydrothermally grown in a high

pressure autoclaves with 20 – 70 MPa by means of the direct-

temperature drop in the aqueous solution of KOH + LiOH at

the crystallization temperature of 320 – 400oC. The samples

were 7 mm × 2 mm × 0.3 mm slabs as polished by two surfaces

of (0001) and (000-1).

Sample fabrication

Surface cleaning Ion irradiationAfter cleaning process, the Zn-(0001)

terminated surface was irradiated by Ar+

ion beams with Eion at 850oC.

Eion=0.5 - 2.5 keV

The surface structure, morphology and electron properties of samples

were in situ investigated by RHEED, AFM, SKPM, STM and STS,

respectively.

EquipmentAFM & STM chamber: 1 × 10-10 torr

variable temperature: 25K ~ 800K

RHEED E-Gun

Ar+ Ion-sputtering Gun Cooling tank:

LN2 or LHe2

Thermal

evaporator

Preparation chamber: 8 × 10-10 torr

variable temperature: RT ~ 1400K

SPM stage

RHEED Screen

]0011[

]0101[

]0101[

-5 0 5 10 15 200.00.30.60.91.2

distance (nm)heig

ht (n

m)

Step height ~

2.6 Å

25 nm

(a) (b)

6 nm

]0001[

]0101[

]1012[

ZnO

Initial cleaning surface of (0001)-Zn

6.22

cÅ

After Ion-bombardment

5nm

(c)h

eig

ht (n

m)

distance (nm)

0 2 4 6 8 10 12

0.6

0.8

1.0

2.6 Å~1.1 Åpits

20nm

(a)

]0011[

]0101[

]0101[

(b)

Hexagonal cavities

pits

0 2 4 6 8 10 12

0.6

0.8

1.0

heig

ht (n

m)

2.6 Å~1.1 Å

distance (nm)

pits

O-terminated Hexagonal Cavities

(a) 5nm

The depth of hexagonal cavities with a diameter of 4 – 5 nm is 1.1 ± 0.2 Å which is much less

than one Zn-O double-layer height. The exposed O-terminated surface shows a downward

relaxation because of the removal of above Zn atoms. The formation of O-terminated

hexagonal cavities results from the collective motion of Zn deficiencies at high temperature.

2.6Å

~1.1Å

]0001[

]0101[

]1012[

(b)

Electronic & Electrostatic

properties of surface defects?

5nm

Surface charge ?

Scanning tunneling spectra (STS) for O-

and Zn- terminated ZnO(0001) planes

The surface band is modified by surface dipolar field.

~ 3.2 eV

STS for O-terminated cavity and Zn-

terminated islands(a)

-σ+σ

-σ

d23d12

Op

+σ

Znp

(b)vacuum

CE

VE

FE

Op

CE

VE

FE

vacuum

Znp

-2 -1 0 1 2 3

(c)

Sample bias (V)

dI/

dV

(A

rb. U

nit.) Zn-terminated

surface

O-terminated

cavity

dI/dVSTM

7nm7nm

Inversion Surface Dipole Domain

shiftEg~ 3.2 eV

shift

Surface potential distribution of (0001)-Zn

by Kelvin probe microscopy (KPM)

kKPM

What is the electric charge of the surface defect (pits) ?

Surface dipolar filed effect !!

fZnZnOm

Zn

S EV

fOZnOm

O

S EV

Surface contact potential, Vs

m

Zn

SV

CE

VE

FE

Vacuum

ZnO

fE

Vacuum

ZnO

fEZn

O

SV

CE

VE

FE

Vacuum

O

m

surface

surfacemSV

OZn

O

S

Zn

S VVV

σ, surface charge density

(/cm2)d12+d23 , (Å )

ε, dielectric

constant of

ZnO

ΔV, surface potential

difference (V)

1.5E+13 2.6 8.5 (Buk) 0.082951147

N(r) = # of pair-distance ri between VP

ri

)('

)()(

rN

rNrg

N’(r): Distribution of ri in absence of

VP-VP interaction

g(r): Pair-correlation function

Interaction between P vacancies in

InP(110)

)exp(1

4)(

0 sR

r

rεπε

qrV

)(ln)( rgkTrW

)('

)()(

rN

rNrg

Pair-correlation function

Mean force potential

Screened Coulomb Potential

W(r

)

V(r)

Rs = 1.2 nm, and q = + 1e

4 nm

7 nm

(a)

(b)r

Pair-distribution of pits with charge

0 1 2 3 4 5 6 7 8 9 100

50

100

150

200

250

300

N'(r), random distribution

N(r

), c

ounts

r (nm)

(c)

Pair correlation function g(r):

g(r) = N(r)/N'(r)

0 1 2 3 4 5 6 7 8 9 10

0.0

0.5

1.0

1.5

0 1 2 3 4 5 6 7 8 9 10-0.02

0.00

0.02

0.04

0.06

W(r

), In

tera

ction e

nerg

y (

eV

)

-e

-2e

r (nm)

r (nm)

g(r

)

Mean force potential energy:

W(r) = -kT ln [g(r)]

Pair-distribution of pits with charge

sR

r

rεπε

qrV exp

1

4)(

0

kTEFkTπme

εεπR

Fe

s

21

21232

3

0

2

2

2

~ 0.8 nm

Screened Coulomb potential energy:

charge state of VZn ~ -1e ± 0.2 e

Stabilizing mechanism with

charged surface defects

Non-stoichiometry ZnO

triangular island

21 Zn : 28 O = 3 : 4

+ 7

VZncompensation

charge δ

(Review) Model of Compensation Charge:

(d)

Ratio of Zn to O for hexagonal cavity on

triangular island

The ratio of Zn and O atoms is 135:191.

The Zn removal is about 30% which departs from the prediction (25%) by Tasker’s

approach. It implies that there is other pathway to achieve polarity compensation.

Zn O

]0011[

]0101[

]0101[

Surface relaxation & Inversion surface dipoles

Zn removal percentage

Assumption:

12

23

Zn

O

O

Zn

23O12Zn

d

d

p

p

A

A

zdσp,zdσp

%100

2

%100%100

12

12

1223

12

ddc

d

dd

d

AA

AR

OOZn

OO

Relaxation of O-terminated plane

-σ+σ

-σ

d23d12

Op

+σ

Znp

DO 2

cOp

Znp

ZnA

OA

O

Zn]0001[

0ApApεε

eOOZnZn

o

E

Statistic analysis of O-terminated cavities

& model fitting parameter

d12=0.6024Å , c = 5.181Å

%100

2

%100

12

12

dDc

d

AA

AR

O

ZnO

OO

d12 = 0.65Å , c = 5.207Å

by PRB (67) 035403

fitting parameters

The surface defect concentration depended on defect structure can be fitted by

this model established by ISDD involving the polar surface stability.

Eq.

Conclusions

The surface Zn defects on (0001)-Zn surface can be introducedusing annealing & ion-bombardment.

The surface defects at sub-monolayer depth, including thehexagonal cavities (4 – 5 nm) and small pits (0.5 – 1.2 nm),are the O-terminated surfaces which possess negative chargecompared to the Zn-terminated surface.

The average charge of each surface nanodefect (pit) estimatedat (-1e) using the pair-distribution analysis.

We establish a new model “inversion surface dipole domain”that can be used to interpret the stabilization of polar surface.