Supplementary Information
Influence of surface null potential on nonvolatile bistable resistive
switching memory behavior of dilutely aluminum doped ZnO thin
film
1
EFM analysis
FIG. SI1 EFM height mode image (0.5 × 0.5 µm2) at an applied bias of +0.5 V. The bias induced coalescence of
grains is represented with arrows.
In EFM, when a voltage is applied between tip and sample, there will be a charge – charge
interactions. Hence, for the sample, the total electrostatic force acting on the tip under an
applied dc bias is due to the charge – charge interaction and change in the capacitance energy
due to tip – sample capacitance. When a dc bias is applied to the metal tip, the total force on
the tip is given by [1, 2]
F
q s qt
1 dC
VA  VC 2 ,

2
2 dz
40 z
(1)
where C is the capacitance between the tip and sample, Z ≈ 50 nm is the tip – sample
separation, qs is the surface charge, qt is the charge induced on the tip, VA is applied dc bias,
VC is the difference between Fermi levels of the sample and tip or the contact potential
between the tip and the sample surface with VC  m   AZO  E f   . In this formula m is
work function of metal,  AZO  4.5 eV is the electron affinity of AZO [3],  is band
bending caused by surface states and E f is the Fermi level position in the AZO with respect
to the bottom of the conduction band. Moreover, entire cancellation of the signal is not
expected due to the existence of the first term in Equation 1, leaving some residual image due
to electrostatic forces.
2
FIG. SI2. Plot showing relation between tip voltage and work function difference between tip
metallization and AZO surface.
According to the Sez model [4], the measured Vn is a function of difference between
the metal work function and electron affinity of AZO ( m   AZO ). Hence,
 Eg

  E   
 q

 B  am   AZO   1  a 
 am   AZO   b ,
(2)
with  B is the barrier energy between metal and semiconductor, E g is band gap of AZO (in
the present case it is ≈ 3.7 eV),  E is the energy level at surface relative to the valance band
and the quantities a and b represent the slope and intercept in Equation (2). Figure S1
represent a plot of measured Vn  and m   AZO . The linear variation between functions was
observed. The least square fitting to the experimental data gives a slope a  of 0.52 ± 0.008
and an intercept b  of -0.027 ± 0.009. The value of the slope was observed less than 1,
which indicate the presence of surface states. Using the above values of slope, intercept as
well as the dielectric constant  i  and the thickness   of the metal – semiconductor
interface, the density of surface states Ds  can be calculated with following expression [2],
Ds 
1  a  i
aq
,
(3)
and  E can be calculated using the expression,
3
E 
Eg
q

b  
.
1 a
(4)
Substituting the values of the slope and intercept in the Equations (3) and (4) for air gap as
dielectric interface, the calculated density of surface state is 4.8 ± 0.4 × 1011 cm-2 eV-1 with
E  2.4  0.2 eV . These results reveal that considerable surface states exist for the given
surface state energy and AZO surface remains unpinned from the charged defects, since
defect density required to pin the Fermi level is about 1014 cm-2 [5].
The EFM images were produced because as the voltage increased, the probe began to
repel in the grain boundary regions. The observed result qualitatively suggests the existence
of positive temperature coefficient in AZO and is associated with the barrier produced at the
grain boundaries due to segregation of acceptor and donor dopants. Moreover, to confirm this
point can be the point of future experiments
The EFM images clearly show the raising of potential barriers with increase in the
electric field gradient. The bright regions begin to appear in Figure 1(b) as shown in the
manuscript are related with semiconductor material, signifying grains are current conductors,
while dark regions are insulating regions associated with grain boundaries.
Using EFM studies, the percentage effective barriers neb  can be calculated from the
following relationship: neb  100nab ngb , where nab and n gb represent the active potential
barriers and total number of grain boundaries, which are 12 and 90 respectively, considered
over two different samples. The relation gives neb ≈ 13% for the AZO system. It is worth
noting that the observed effective potential barriers are lower than observed in ZnO (≈ 15 to
33%). Hence, it is possible to obtain low voltage electronic components such as Varistors
using AZO. The observed results can be explained as follows. Since, the electrical charge
located at grain boundaries is due to trapping of majority carriers by the electron states. It
creates potential barrier free carriers and increases the resistance of the grain-boundary
4
regions. Doping increases the amount of the trapped charge and influences the height of the
potential barrier. In the present case, it can be suggested that doping eventually filled all the
grain boundary states with trapped carriers and the charge cannot increase any more. This
increases the carrier concentration and the space-charge density in the depletion region,
leading to a reduction in the percentage potential barriers as well as height and width of the
potential barrier and making it transparent to tunnel the carriers. Furthermore, at a very high
doping level, the potential barrier is well below the Fermi level and the electrons can pass
over the barrier without great perturbation.
Conductive filament formation and spatial extension regions
FIG. SI3. Schematic representing the formation and spatial extension regions of CF within AZO particle and grain
boundaries during set and reset states. The white colored solid lines represent grain boundaries. The blue colored circles
represent intersection points of different grain regions. (a) The red and yellow colored solid lines represent CF at grain
boundaries and within the grain regions respectively, during forming or set process of the device. The pink colored
lines highlight the formation and extension of CF at interparticle boundaries, within the particle, grain boundaries and
within grain regions. (b) The dotted lines represent the rupturing of CF during reset process at interparticle boundaries,
within the particle, grain boundaries and within grain regions.
5
Numerical analysis of I – V data
FIG. SI4. Linear fitting of I – V data with different models. (a) log- log plot of the fitting of LRS region represent Ohmic
type conduction. (b) Semi log with V1/2 fitting for a HRS region of the curve. (c) Represent log – log plot of fitted using
power law near high voltage regime.
6
References
1
P. Girard, Nanotechnology 12, 485 (2001).
2
P. M. Bridger, Z. Z. Bandić, E. C. Piquette, and T. C. Mcgill, Appl. Phys. Lett. 74, 3522
(1999).
3
J. J. Robbins, and C. A. Wolden, Appl. Phys. Lett. 83, 3933 (2003).
4
S. M. Sez, Physics of Semiconductor Devices (Wiley, New York, 1981). PP 270 – 275.
5
A. Zur, T. C. Mcgill, and D. L. Smith, Phys. Rev. B 28, 2060 (1983).
7