Supplementary Information
Physical device modeling of carbon nanotube/GaAs photovoltaic cells
Hong Li, Wan Khai Loke, Qing Zhang*, S. F. Yoon
Microelectronics Center, School of Electrical and Electronics Engineering, Nanyang Technological
University, S1-B2c-20, Singapore 639798
I. FABRICATION METHOD
The schematic of fabrication process is shown in Fig. S1. An n-type GaAs substrate (doping level
of 2  1018 cm-3) was used as the substrate. A 100-nm-thick Si3N4 layer was deposited onto the
substrate with a plasma-enhanced chemical vapor deposition system at a deposition rate of 18 nm/min
using a mixed flow gases of 100-sccm 10% silane gas (diluted by nitrigen gas), 20-sccm 99%
ammonia gas and 800-sccm nitrogen gas. The deposition temperature and RF power were 300 ºC and
40 W, respectively. The substrate was soaked in diluted HCl solution (10% acid by volume) for 5
mins to etch away possible native Ga2O2 layer, and then the back-contact G (5 nm Ni/25 nm Ge/100
nm Au/20 nm Ni/200 nm Au) was deposited in an E-beam evaporator. Next, the devices were baked
in a chamber, in which the temperature was increased from 200 to 400 °C and held at 400 °C for 1
*Author
to whom correspondence should be addressed. Electronic address: eqzhang@ntu.edu.sg.
1
min in nitrogen ambient, to form an Ohmic contact to the n-type GaAs. The electrode pair S and D (5
nm Ti /40 nm Au) was patterned on the surface of Si3N4 using optical lithography and lift-off
processes. A photoresist window was defined between S and D. Then, the Si3N4 layer in the window
was etched away via reactive ion etching (RIE) process using 10-sccm carbon tetrafluoride, 5-sccm
oxygen at room temperature. The pressure and RF power were 30 mTorr and 50 W, respectively.
Finally, single-walled carbon nanotubes (SWNTs) were bridged between the S and D electrodes.
Thus, the center segment of the SWNTs was in contact with the n-type GaAs substrate through a thin
layer of Ga2O3 formed in the RIE process.
FIG. S1. The flow chart of fabrication process. The blue, cyan, orange, wine and magenta colors
delineate the GaAs, Si3N4, Au electrodes, photoresist and SWNTs, respectively.
II. FITTING OF THE IDG-VDG CHARACTERISTICS AT LOW BIAS
2
Upon fitting the observed results at low FB with Equation 1, we obtained n and A listed in Table I.
Figure S2 depicts a relation3 between n and temperature T, i.e., n(T )  2.3 
530
. Such a temperature
T
dependence could be attributed to the tunneling current component.3,4 In an ideal case, where the
current is dominated by recombination of minority carriers injected into the bulk semiconductors, the
ideality factor is 1.5 Recombination of carriers in the space charge region could result in an ideality
factor around 2.6 While high ideality factors (n>>2) was attributed to deep-level-assisted tunneling.7
In our case, n ~ 4.1 at 300 K suggests the tunneling current through the SWNT/GaAs interband states.
A is exponentially dependent on the temperature, because it is related to the portion of electrons
thermally emitted from the GaAs to the SWNT.8
T (K)
50
100
125
175
225
250
275
300
n
13.4
7.4
6.6
5.343
4.778
4.36
4.418
4.2
A (  10-15)
0.7
12
80
550
2400
3800
10000
19500
Table I: n and A obtained by fitting the observed curves with Equation 1 at different temperatures.
16
14
ideality factor, n
fitting data
12
n
10
8
6
4
2
50
100
150
200
250
300
T (K)
Fig. S2. Fitting the ideality factor n with n(T )  2.3 
530
. The open symbols are data from Table I.
T
3
III.
FITTING OF THE IDG-VDG CHARACTERISTICS AT LOW BIAS
The heterojunction shows a PV effect under the illumination of a beam of white light (50 mW/cm2).
According to Prince,9 the PV effect could be modeled by I DG 
( I J  I sc ) Rsh  VDG
, where IJ is the FB
Rct  Rsh
heterojunction current described by Equation 1 (in manuscript), Isc is the measured short-circuit
current, Rsh is a shunt resistor and Rct is the resistance at SWNT/Au contact. The fitting parameters are
displayed in Table II. Rsh is in GΩ range, implying that the surface leakage current through adsorbents
on SWNT is negligible.
T (K)
Rsh (GΩ)
50
9
125
2
200
1.3
300
1.1
Table II. Fitting parameters of the IDG-VDG characteristics with illumination.
4
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