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
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.