Ultraviolet irradiation controlled memory effect in graphene field effect
transistors
Zhi-Min Liao*, Han-Chun Wu*, S. Kumar, G. S. Duesberg, G.L.W. Cross, I. V. Shvets, and
Da-Peng Yu
CRANN, School of Physics, Trinity College Dublin, Dublin 2, Ireland
Abstract: Sensitivity of graphene to its surroundings including adsorbates, trapped charges on the
substrate, and photons may open an avenue for photon-controlled graphene memory devices via
control of the charge transfer process. Here, we report that the hysteresis in conductance-gate
voltage dependence of graphene field effect transistors on SiO2 substrate can be enlarged by
ultraviolet irradiation in both air and vacuum. As the irradiation power increases the Fermi energy of
graphene there is a continuous shift from p- to n-doping for the forward gate voltage sweep. The
enhanced charge transfer between graphene and its surroundings induced by ultraviolet illumination
proposed explains the experimental results well. Moreover, the memory effect is demonstrated by
employing a gate voltage pulse to program/erase the memory elements. Our findings improve
understanding of the charge transfer between graphene and its surroundings and also prompt a new
type of graphene based memory device.
Graphene has attracted considerable interest in recent years in view of its novel electronic
structure. In particular, graphene field effect transistors (FETs) are of great potential for applications
as the transistor channel is atomic monolayer thick, it has large surface-to-volume ratio and high
carrier mobility. As graphene is all surface, it can readily receive adsorbates and charges from the
surrounding environment via its ambient facing surface or via underlying substrate. Furthermore, it is
conceivable that these processes can also be altered by illumination. Consequently, the graphene
electronic properties are susceptible to the surrounding environment, which could make graphene
suitable for highly sensitive gas sensors and photon electronics. For example, the doping level, the
mobility, and the asymmetries between electron and hole transport of graphene based FET devices
vary significantly for different samples. For example, the graphene transistors fabricated on SiO2
substrate usually exhibit hysteretic behavior under ambient conditions. The hysteretic behavior is
believed to be caused by the charge transfer between the adsorbates (such as water or O2 molecules)
and graphene or between the trap centers on substrate and graphene. Such a sensitivity of charge
transfer is unique for charge detectors and it could be used for constructing memory cells. To make
progress in this direction it is essential to enhancethe hysteretic behavior and develop means of its
control. Recent experiments showed that the doping in graphene can be controlled by ultraviolet (UV)
irradiation. The UV illumination can generate electron-hole pairs, liberate the adsorbates, activate the
trapped carriers on the SiO2, and thus notably affect the electrical properties of graphene FETs.
Therefore, it would be very interesting to investigate the evolution of the hysteresis with tunable UV
irradiation in both air and vacuum.
11
(a)
UV illumination
10
30
on
20
CNP (V)
Isd (A)
9
8
7
6
off
5
40
-20
0
20
Vg (V)
40
Forward
Backward
10
In air
0
-10
60
(c)
0
10.0
35
20
40
60
25
20
15
10
80
Laser power (%)
(d)
9.0
8.5
8.0
In air
7.5
5
0
20
40
60
80
Laser power (%)
100
100
Forward
Backward
9.5
30
min (e2/h)
Memory window (V)
45
In air
V
4
-40
(b)
0
20
40
60
80
Laser power (%)
100
Figure 1. (a) Isd-Vg curves under 325 nm laser irradiation with a power density of 10 μW/μm2
in air condition. ΔV is defined as the memory window. (b-d) (b) Charge neutral points, (c) memory
window, and (d) minimum conductance as a function of laser power density. 100% of the laser power
density refers to 100 μW/μm2.
Figure 1 (a) shows the Isd - Vg curves measured under laser illumination in air. Here, a 325 nm
laser with maximum power density of 100 μW/μm2 was used. The laser irradiation power was
indicated as the percentage of the maximum power density, which can be tuned by an attenuator. One
can see from figure 1(a) that the width of the hysteresis was greatly enlarged as the graphene FET was
irradiated by laser with a power density of 10 μW/μm2. More interestingly, the width of the hysteresis
can be tuned via UV irradiation. Figure 1(b) shows the CNP for the forward and backward sweeping
as a function of laser power. One can see that with increasing the laser power, the CNP of the forward
branch shifted to the negative direction evidently, while the CNP of the backward branch almost did
not change. The memory window (ΔV), defined as the difference in Vg at the CNP for the forward
and backward sweeping, was first increased rapidly and then tended to saturation, as clearly shown in
Fig. 1(c). The maximum value of the memory window is ~ 40 V. Moreover, the current ratio of
Ion/Ioff at Vg = 0 V is notable, which is ~174% displayed in Figure 1(a). Such a large memory
window and notable current on/off ratio suggest the graphene FETs may have potential applications in
switching cell, which is comparable with the performance of the ferroelectric gated graphene memory.
In addition to the UV irradiation enlarged hysteresis, the minimum conductance (σmin, the
conductance at CNP) was also influenced by the laser power, as shown in Fig. 1(d).
This work is published in: Jie Meng, Han-Chun Wu, Jing-Jing Chen, Ya-Qing Bie, Igor V. Shvets,
Da-Peng Yu, and Zhi-Min Liao, Ultraviolet irradiation controlled memory effect in graphene field
effect transistors, Small 9, 2240 (2013)