Supplementary Materials:
The correlation between electric field emission phenomenon
and Schottky contact reverse bias characteristics in
nanostructured systems
J. Yu1*, J. Liu2, M. Breedon1, M. Shafiei1, H. Wen3, Y. X. Li3, W. Wlodarski1, G. Zhang2,
K. Kalantar-zadeh1
1
Department of Electrical and Computer Engineering, RMIT University
GPO Box 2476, Melbourne 3001, Australia
2
Key Laboratory for the Physics and Chemistry of Nanodevices, Department of Electronics
Peking University, Beijing 100871, People's Republic of China
3
Shanghai Institute of Ceramics, Chinese Academy of Sciences
Shanghai 200050, People's Republic of China
PACS: 73.63.Bd, 73.63.Rt, 68.37.Vj
1
I. Experimental
The Schottky diodes were fabricated on 250 m thick research grade, n-type 6H-SiC wafers
(Cree Co. USA). The native oxide layer on the SiC was removed by washing the substrates in
10% HF + H2O for 10 s. The Ohmic contact was formed by the deposition of 40 nm Ti and
100 nm Pt using electron beam evaporation on the backside of the SiC substrates and by the
subsequent annealing (backside face down) at 500C in N2 gas. The metallised wafers were
subsequently cleaned with acetone, isopropanol, DI-water and were then diced into 3×3 mm2.
The Ohmic contacts were formed by annealing the backside metal electrode in a tube furnace
(Altech) at 500°C for duration of 1 hr in Ar gas. The samples were cleaned a second time in
acetone, isopropanol and DI water in preparation for the deposition process.
The deposition of MoO3 nanostructures was performed in a horizontal quartz tube furnace (as
shown in Fig. 1S) by evaporating a known mass of MoO3 powder (Rare Metal Material Co.
China).
FIG. 1S Schematic of MoO3 evaporation process onto SiC substrates.
2
MoO3 powder was weighed at 10 mg and placed on an alumina boat inside a quartz tube at
the centre of a furnace. SiC substrates were placed on alumina boat at a distance of 15 cm
from the heating zone where the temperature was approximately 450°C. Ar was employed as
a gas carrier, in an effort to increase the conductivity of the deposited nanostructured
materials. Previous studies have focused on evaporation using a 10% O2 in 90% Ar carrier
gas.1 The Ar gas was allowed to flow through the quartz tube at a constant rate of
approximately 800 sccm and the evaporation process was performed by increasing the
temperature by heating at rate of 2°C min to 770°C which was then maintained for 30 min.
The system was allowed to cool at a rate of 2°C min and the samples were then removed. The
authors designate this sample as ‘770C’. A different nanostructured MoO3 morphology was
synthesized by repeating the aforementioned process with a deposition temperature of 850C,
designated as ‘850C’. In both cases, a bright-silverish powder was observed on the SiC
substrates after the deposition.
The morphology of the deposited MoO3 nanostructures was characterised by Scanning
Electron Microscopy (SEM) via a FEI Nova NanoSEM. X-Ray diffraction (XRD) was
performed using a Brucker D8 Advance. Transmission Electron Microscopy (TEM) was
performed via a JEOL 1010 to acquire the Selected Area Electron Diffraction (SAED)
pattern.
XRD analysis revealed that both nanostructure types exhibited common crystallographic
planes at (020), (110), (040), (130) and (141) (Fig 2S). Analysis of Sample ‘770C’ shows
the existence of an additional crystallographic plane (021) which is absent in ‘850C’
structures. The ‘850C’ structures revealed the presence of (150), (221) crystallographic
planes which is absent in the ‘770C’ structures. Both ‘770C’ and ‘850C’ crystal phases
indicate a classification of mixed  - MoO3 as deposition at temperatures were conducted
3
above 400ºC.6 The XRD spectrum data is in good agreement with ICDD card file #050508
suggesting the classification of orthorhombic MoO3 with lattice parameters a, b and c as
3.962 Å, 13.85 Å and 3.697 Å, respectively. The crystallographic structure of the MoO3
nanostructures in this work is very similar to MoO3 deposited via thermal evaporation in a
bell shape jar vacuum system.7
FIG. 2S XRD spectra of the MoO3 deposited at (a) ‘770C’ and (b) ‘850C’.
TEM analysis of the crystallographic structure of the synthesized MoO3 nanostructures of
samples ‘770C’ and ‘850C’ are shown in Fig. 3Sa and 3Sb. The selected area electron
diffraction (SAED) patterns of both samples (as shown in the inserts) show that the
synthesised nanostructures are highly crystalline.
4
FIG. 3S TEM images of MoO3 from (a) a nanoplatelet from sample‘770C’ and (b) edge of a
nanoplatelet in sample ‘850C’ (insets show the SAEDs).
For forming the Schottky diodes, the deposited MoO3 nanostructures were coated with
approximately 30 nm Pt using a PECsTM sputtering system to form a circular Schottky
contact of 1 mm in diameter. As a result, the area is approximately, 7.85 × 10-3 cm2.
In the field emission measurements, the MoO3 nanostructures deposited on the SiC substrates
were placed into a lab-built ultrahigh vacuum chamber with a base pressure of 10-7 Pa as
previously reported.2-5 Anode was a glass screen coated with fluorine doped tin-oxide (FTO
glass). The transparent anode allows observation of the two-dimensional distribution of
emission sites on the cathode. The distance between the surface of the anode and the cathode
(sample) during the measurements was kept constant at 526 m.
One sample was fabricated for each case. The field emission measurements were conducted
after the I-V characteristics measurements to avoid any possible degradation of the sample,
when applying high voltages of up to 6000 V.
5
The field emission measurements were performed via two methods and were conducted 20
times each. Results were then averaged. The first method was performed by applying voltage
from a range (2000V to 6000V). The second was done by a forth and backward sweep
(1500V to 6000V and then to 1500V). An emission spread photos (shown below) were also
taken at different voltages to show a uniform spread during the field emission measurements
of the 770C and 850C MoO3 nanostructures and are shown in FIG 4S (a) and 4S(b),
respectively. The ‘850C’ sample was more sensitive requiring less voltage for emission.
FIG. 4S (a) and 4S (b) Photos of the field emission measurements of the (a) ‘770C’ and (b)
‘850C’ MoO3 nanostructures at different applied voltages.
6
The Pt/nanostructured MoO3/SiC Schottky diodes were then placed in a test chamber.
Electrical connections from a Keithley 2602 Sourcemeter were applied via a probe onto the
Pt nanostructured Schottky contact and acquire the I-V characteristics of the nanostructured
MoO3 Schottky diodes similarly to a previous study by the authors.1
II. Comparison of field emission properties
Table I-S. The turn on fields, threshold fields and field enhancement factors of various MoO3
nanomaterials in literature.
Nanomaterial
Turn-on field
Threshold field
Field
(MoO3)
(V/m) at
(V/m) at
enhancement
10A/cm2
1mA/cm2
factor a
Nanoflowers
4.3
-
700
Wei et al. 8
Nanowires
3.5
-
4400
Zhou et al. 7
Nanobelts
13.2
19.5 (10 mA/m2)
-
Li et al. 9
‘770C’
12.4
16.7
590
Current work
‘850C’
11.1
17.3
750
Current work
7
Reference
III. Barrier height calculations from both reverse and forward bias conditions
The forward barrier height is given in Eq. (6) using the extrapolation method:10
B ( FW D) 
kT  A**  T 2 
ln 

q  J R0 
(1-S)
According to Eq. (4), the reverse current density is proportional to
 q 
 . Considering its first term
 q   B0  , when the magnitude of
q m
B 0 

exp 
exp 

4

s
kT
 kT 



the applied reverse voltage increases the Fermi level is shifted downwards, resulting in the
increase of the barrier height and consequently the decrease of the current density. However,
 component
as the reverse voltage magnitude increases further, the exp  q  qm

4

s 
 kT 

(which is the result of the enhanced electric field) becomes dominant. This term opposes the
barrier height increase and eventually assists the thermionic emission of free carriers over the
barrier in reverse bias.11 The reverse barrier height requires taking into account this enhanced
electric field phenomenon and is given by:
B ( REV )
 a 2  q3  N D 
kT  A**  T 2 
kT 
4
 VR  bi 


ln 

2
q  J R0 
8   s 
q 
(2-S)
We assume the temperature of 300 K and the value for ND as 1020 cm-3. The results presented
in Table II-S show that reverse barrier heights are approximately less than half of that of the
forward barrier height in a nanostructured Schottky contact. The calculations also indicate
that by using the I-V curve fitting measurements for estimating the enhancement factor, the
reverse barrier height might be slightly underestimated since the values are lower than the
reverse barrier heights, which were obtained using the field emission measurements.
8
Table II-S. Calculated nanostructured MoO3 Schottky barrier heights in forward and the
reverse bias.
Nanomaterial
 B ( FW D)
B ( REV ) (meV) - no
B ( REV ) (meV) -
B ( REV ) (meV) -
(meV)
enhancement
using field
Schottky
factor used
emission
measurement
enhancement
enhancement
factor (a)
factor (Schottky-a)
770C
645
698
301
259
850C
640
650
277
238
9
References
1
J. Yu, S. J. Ippolito, M. Shafiei, D. Dhawan, W. Wlodarski, and K. Kalantar-zadeh, Applied
Physics Letters 94, 013504 (2009).
2
W. W. Wang, G. M. Zhang, L. G. Yu, X. Bai, Z. X. Zhang, and X. Y. Zhao, Physica E-LowDimensional Systems & Nanostructures 36, 86-91 (2007).
3
L. Chen, G. M. Zhang, M. S. Wang, and Q. F. Zhang, Chinese Physics 14, 181-185 (2005).
4
J. Xiao, G. M. Zhang, X. Bai, L. G. Yu, X. Y. Zhao, and D. Z. Guo, Vacuum 83, 265-272
(2008).
5
L. G. Yu, G. M. Zhang, Y. Wu, X. Bai, and D. Z. Guo, Journal of Crystal Growth 310, 31253130 (2008).
6
O. M. Hussain and K. S. Rao, Materials Chemistry and Physics 80, 638-646 (2003).
7
J. Zhou, N. S. Xu, S. Z. Deng, J. Chen, J. C. She, and Z. L. Wang, Advanced Materials 15,
1835-1840 (2003).
8
G. D. Wei, W. P. Qin, D. S. Zhang, G. F. Wang, R. J. Kim, K. Z. Zheng, and L. L. Wang,
Journal of Alloys and Compounds 481, 417-421 (2009).
9
Y. B. Li, Y. Bando, D. Golberg, and K. Kurashima, Applied Physics Letters 81, 5048-5050
(2002).
10
S. Sze and K. Ng, Physics of semiconductor devices (Wiley, 2008).
11
J. Yu, S. J. Ippolito, W. Wlodarski, M. Strano, and K. Kalantar-Zadeh, Nanotechnology 21, 8
(2010).
10