Photoinduced Hysteresis of Graphene Field-Effect Transistors Due to

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Photo-induced Hysteresis of Graphene Field Effect Transistor due to Hydrogen-complexed Defects in Silicon Dioxide Guiming Cao, Xiaorong Liu, Yantao Zhang, Weihua Liu, Minming Deng, Guangbing Chen, Guohe Zhang, Quanfu Li, Lemu Girma Beka, Xin Li, and Xiaoli Wang ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.9b02400 • Publication Date (Web): 07 Mar 2019 Downloaded from http://pubs.acs.org on March 15, 2019

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ACS Applied Materials & Interfaces

Photo-induced Hysteresis of Graphene Field Effect Transistor due to Hydrogen-complexed Defects in Silicon Dioxide Guiming Cao♰, Xiaorong Liu♰, Yantao Zhang♰, Weihua Liu*,♰,#, Minming Deng||, Guangbing Chen||, Guohe Zhang♰, Quanfu Li♰, Lemu Girma Beka♰, Xin Li ♰ and Xiaoli Wang♰,§ ♰School

of Microelectronics, School of Electronics and Information Engineering,

Xi’an Jiaotong University, Xi’an, Shaanxi, China, 710049 #Key

Laboratory for Physical Electronics and Devices of the Ministry of Education,

Department of Electronic Science and Technology, School of Electronic and Information Engineering, Xi'an Jiaotong University, Xi'an, Shaanxi, China, 710049 ||Science

and Technology on Analog Integrated Circuit Laboratory, Chongqing,

China, 401332 §School

of Science, Xi’an Jiaotong University, Xi’an, Shaanxi, China, 710049

*Corresponding Author, Email: [email protected] Keywords: graphene, field effect transistor, photo-electric-field-induced doping, proton-hopping, silicon dioxide. Abstract: Photo-induced hysteresis (PIH) of graphene field-effect transistor (G-FET) has attracted many attentions due to its potential in developing photo-electronic or nonvolatile memory devices. In this work, we focused on the role of SiO2 dielectric layer 1

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on PIH, where G-FET has only a SiO2 dielectric layer. Adsorbates are effectively removed before PIH test. The effects of laser wavelength, laser power density and temperature on the PIH are systematically investigated. The PIH is significantly enhanced by increasing hydrogen flow in a hydrogen-atmosphere device thermal annealing. It strongly suggests proton related defects play a key role. Pure electronic process for PIH is further ruled out by the significant dependence of the doping rate on the temperature. A mechanism of PIH based on proton generation after hole trapping at [O3 ≡ Si ― H] is proposed. The proposed mechanism is well supported by our experimental data: 1. The observed threshold photon energy for PIH is between 2.76 and 2.34 eV, which is close to the energy barrier for [O3 ≡ Si ― H] releasing a proton. 2. No obvious carrier mobility degradation after PIH process suggests that the bulk defects in SiO2 are the major contributors rather than graphene/SiO2 interface defects. 3. The dependence of doping rate on the temperature and the laser power density matches a theoretic model based on the random hopping of H+. The results in this work are also valuable for the study of the degradation of other oxide dielectric materials in various field effect transistors.

Introduction The feature of all surface atoms and the finite density of states make graphene very sensitive to its surrounding chemical environment. As a result, hysteretic characteristic generally exists in the transfer curve of graphene channel-based field effect transistor (G-FET). The hysteresis performance is essentially a gate electric field tuned charge transfer between graphene and the adsorbates or substrate.1-8 In addition, photo-induced 2


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hysteresis (PIH) of G-FET is essentially a joint effect of photon excitation and bias voltage over the charge transfer between graphene and its neighbors.9-11 Because photon excitation enables more potential charge transfer processes, the illumination provides an effective knob of tuning the hysteresis performance.10-12 Thus, it has attracted an increasing attention recently towards developing new-type of memory devices. For example, L. Ju et al 13 and Christoph Neumann et al 14 have reported the injection of photon-excited electrons from defects in h-BN substrate into graphene. Most recently, Hyun Ho Choi et al

15

have reported the photoelectric memory effect,

essentially a PIH process in G-FET fabricated on a narrow-gap insulator (NGI)/widegap insulator (WGI) dual dielectric substrate. It is attributed to the charge transfer between defects at NGI/WGI interface and graphene. PIH of G-FET with SiO2 substrate is also frequently reported.16-20 However, it shows diverse behaviors. For example, under 220 nm ultra-violet irradiation, p-type doping effect without carrier mobility degradation is observed in atmosphere by M. Z. Iqbal.16,17 While n-type doping effect with significant mobility degradation is observed in vacuum by Gaku Imamura.18 When Luo et al do a similar test under vacuum but with light of wavelength 365 nm, n-type doping is observed without obvious mobility degradation.19 The diversity of PIH lies in the different illumination conditions and the contribution of the adsorbates. The PIH associated with the adsorbates usually results in gas molecular desorption, which is irreversible and causes device instability.19,20 While the PIH associated with substrate is more likely to be reversible and stable. Therefore, it is very necessary to eliminate the contribution of the adsorbates and focus 3



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on PIH associated with only SiO2 substrate. The exact defects in SiO2 contributing to the PIH has not been seriously discussed yet in the literatures.13,14,21 However, the defects in the bulk of SiO2 have been intensively explored in term of reliability of traditional silicon-based metal-oxidesemiconductor (MOS) device.22-24 Hydrogen is a ubiquitous impurity in oxides and strongly affects their electronic and structural properties. In silicon dioxide, hydrogen related defects are responsible for bunch of degradation process of SiO2-dielectriced electronic devices.25-28 For instance, hydrogen-complexed defects are believed to be responsible for the stress-induced leakage current in silica.25,26 Electron spin resonance (ESR) studies of defects in amorphous SiO2 have suggested that hydrogen (in forms of H2 or H2O) plays a key role on initialing intrinsic defects during a thermal treatment or irradiation.27,28 The Si-O bond rupture is closely related to the presence of protonic species formed by hole trapping or by hydrogen ionization at the Si/SiO2 interface.29-31 Hydrogen in different forms (Hydroxyl E’ and Hydrogen bridge) is well known as a defect passivator in amorphous SiO2.32 A photon stimulated tunneling of electrons at the Si/SiO2 and Si/SiC interfaces has been correlated to defects in SiO2

with energy

level about 2.8 eV below the conduction band.33,34 Therefore, it is reasonable to believe that a similar process should also happen at Graphene/SiO2 interface. V. V. Afanas’ev et al have suggested a proton or H+ generation process based on hole trapping at [O3 ≡ Si ― H],29,30 leading to the formation of [O3 ≡ Si ― H] + . The positively charged defects [O3 ≡ Si ― H] + somehow transfer into neutral [O3 ≡ Si ∙ ] defects (E′ centers) in SiO2 by releasing a proton.29 After the proton drifting away, the 4


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neutral [O3 ≡ Si ∙ ] center left behind may trap another H atom thus re-entering the hole trapping process.30 In this picture, a positive charge is always associated with H, either [O3 ≡ Si ― H] + or H+. It is notable [O3 ≡ Si ― H] + is fixed charge while H+ is movable. Most recently, Al-Moatasem El-Sayed et al have theoretically studied the reaction of H with strained Si-O bond, Oxygen vacancy and hydroxyl E′ center.32,35 Their calculation shows that hydrogen passivation of oxygen vacancy generates a pair of [O3 ≡ Si ― H] and the energy barrier for the depassivation of [O3 ≡ Si ― H] into [ O3 ≡ Si ∙ ] and an interstitial H0 is about 2.7 eV. Thus, the illumination with photon energy larger than 2.7 eV should be able to dissociate a proton from such hydrogen passivated defect. Because the general existence of hydrogen-complexed defects in SiO2, it should play an important role on the PIH of G-FET. To focus on PIH of G-FET associate with hydrogen-complexed defects, the device is tested under high vacuum chamber to eliminate the PIH associated with adsorbates in this work. The interesting findings of this work include: (1) An asymmetric PIH behavior at positive and negative back-gate voltage is observed; (2) There is a threshold wavelength for the PIH with photon energy close to the energy barrier for the depassivation of [O3 ≡ Si ― H]; (3) Hydrogen atmosphere thermal annealing significantly enhances the PIH of G-FET. To shed light on the mechanism of PIH of G-FET with SiO2 substrate, the influence of laser power density and temperature on the dynamic process of PIH are also investigated in this work. A model of PIH of G-FET based on the H+ charge traps in SiO2 is proposed and the model perfectly matches the experimental data. This work provides a better 5



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understanding of the PIH of G-FET with SiO2 dielectric layer. More importantly, the experimental characterization of PIH process in this report provides an effective way of investigating the interaction between protons and defects in SiO2 as well as other wide-band-gap oxides.

Results and Discussions

Figure 1. (a) The schematic diagram of the graphene FET on Si/SiO2 substrate. (b) Raman test result of the graphene used as the channel. Inset is the optical image of the device, where the width and length of device are 10 m and 32 m, respectively. (c) Hysteresis of transfer curves in air before and after near ultra-violet (NUV, =405 nm) illumination. (d) Transfer characteristics of the device in vacuum before and after half an hour NUV illumination. VDS is 0.1 V for the transfer curve tests.

Figure 1a shows the schematic of a G-FET under PIH test. Figure 1b shows the 6


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Raman Spectra of the graphene channel. The I2D (2670 cm-1)/ IG (1580 cm-1) ratio ~2 and low D (1350 cm-1) band confirm the good quality of graphene used in this work.36 Inset in Figure1b is the optical image of the device, where the width (W) and length (L) of graphene channel are 10 m and 32 m, respectively. Figure 1c exhibits the hysteresis behavior of the transfer curves of G-FET in air before and after near ultra-violet illumination (NUV, =405 nm). As-fabricated G-FET is heavily p-type doped with a charge neutral point voltage (VCNP) around 45 to 60 V and there is significant hysteresis behavior due to the adsorbed molecules such as O2 or H2O.1,5 After about half an hour NUV illumination in air, the hysteresis behavior still exists without any observable reduction. To focus on the PIH associated with only SiO2 substrate, the device is put into a vacuum chamber (3×10-4 Pa) and illuminated with near ultra-violet laser to remove the adsorbates. Figure 1d shows the transfer curves in vacuum before and after NUV illumination. When the device is putted into the vacuum chamber for over one hour, the VCNP left shifts to around 3 V and the hysteresis behavior is barely visible. It indicates that most of the adsorbates on graphene from air have been successfully removed.37,38 Half-hour NUV illumination in vacuum at zero gate voltage (refer as NUV annealing hereafter) pushes VCNP further left shifts to around -13 V and further reduces the hysteresis characteristic. This is opposite to what happens in air. It suggests that NUV annealing in vacuum induces n-type doping. This is in consistent with Young Duck Kim’s report, where n-type doping is also observed in G-FET without gate voltage after 442 nm laser irradiation.21 They have suggested that the photo-induced n-type doping 7



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is due to the charge-carrier transfer between the trap sites in SiO2 and graphene. In brief, the NUV annealing introduces positive charges to the trap sites at the near surface of SiO2 and those positive trapping charges cause n-type doping of graphene channel through gate-effect. Young Duck Kim et al have obtained the energy level of the trap sites relative to the bottom of the SiO2 conduction band.21 It is ~2.6 eV (Eb) below the conduction band. It is notable that such n-type doping effect after NUV annealing in this work as well as Young Duck Kim’s report happens under zero gate voltage. No space charge movement involves. Only those trap sites in the near surface of SiO2 contribute to the charge-carrier transfer.21,39 After NUV annealing, the G-FET is then subjected to further PIH measurements in this report.

Figure 2. PIH measurements under 405 nm NUV laser illumination in vacuum. (a) Time evolution of the transfer characteristic of device at VBGH = -40 V. (b) Time evolution of the VCNP at 8


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various VBGH. (c) Time evolution of VCNP during the recovery process. The VCNP has been initially shifted to about -40 V after a PIH process. (d) The response of source-and-drain current (IDS) to NUV laser illumination at VBG =-40 V. The power density of laser is 200 mW/mm2.

The PIH behavior after NUV annealing is shown in Figure 2a. The PIH measurement condition is as following: the back-gate voltage is held at -40 V (referred as holding gate voltage: VBGH) except those interval periods of transfer curve measurement.40 A laser beam (405 nm, 200 mW/mm2) is focused on the device during the whole PIH process. The transfer curve left shifts until its VCNP close to VBGH = -40 V and the shift is denoted by the red arrow in Figure 2a. It is notable that the shape of the transfer curves during PIH process shows no visible change. The mobility of holes and electrons (μ = L/(W𝐶𝑜𝑥𝑉𝐷𝑆)∂I𝐷𝑆/∂𝑉𝐵𝐺, where Cox is the capacity of gate oxides with a typical value 11.5 nF/cm2 ) are ~1420 cm2/(V ∙ s) and ~1360 cm2/(V ∙ s), respectively. The mobility of holes and electrons keep almost unchanged during the PIH process, as shown in Figure S2 in Supporting Information. It indicates that the defects or dopants, which normally result in mobility degradation, introduced into the graphene channel during PIH is negligible (also see Figure S3 in Supporting Information). This is significantly different from Gaku Imamura’s report, where significant mobility degradation is observed when the device in vacuum is illuminated with 220 nm laser.18 Therefore the shift of VCNP in our case is more likely a gating effect of further accumulated trapping charges in the SiO2 during the PIH process. The shifts of VCNP during the PIH processes with various VBGH ranging from -60 to 40 V are plotted in Figure 2b. At negative VBGH (-20 and -40 V), the VCNP gradually 9



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left shifts until it reaches VBGH (see Figure S4 in Supporting Information). It indicates that the number of positive trapping charges in SiO2 increases or those positive trapping charges move into the depth area until the back-gate voltage is fully screened off. However, when VBGH decreases to -60 V, the VCNP eventually reaches only -50 V, implying a partial screen off the back-gate within 35 minutes. In contrast, at positive VBGH (20 and 40 V), the VCNP shifts only slightly to around 0 V. The PIH shows a highly asymmetric characteristic for positive and negative VBGH. It indicates that there should be a limited space for positive charges to move from the bulk towards the SiO2/graphene interface or a much larger energy barrier for negative charge trapping in SiO2. Figure 2c illustrates the recovery of VCNP as the VBGH is switched from -40 V to zero and 40 V, respectively. The recovery happens only when the illumination condition maintains. When the VBGH is switched back to zero volt, the VCNP back shifts but does not reach its initial value, -13 V as denoted by a red-dash line, within 35 minutes. However, when VBGH is switched to 40 V, the VCNP backshifts quickly close to its initial value. The right shift of VCNP during the recovery process should be attributed to the decrease of the positive trapping charges or the movement of those positive charges towards the surface of SiO2. The PIH process is also demonstrated by monitoring source-and-drain current (IDS), as shown in Figure 2d. The laser is turned on and off repeatedly to depict the effect of illumination. When VBG is just switched from 0 to -40 V, after a sudden increase from 8 to 25 A, IDS starts a gentle decrease due to the background hysteresis behavior and 10


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soon stabilizes at 23 A. When the laser is turned on, IDS quickly decreases to 13 A within 10 minutes. The decrease of IDS is following the black dash line as indicated by black arrow in Figure 2a. Once the laser is turned off, the decrease of IDS stops immediately. The sharp contrast of IDS behavior between the laser-off and laser-on period indicates that the hysteresis observed here only associates with illumination of 405 nm laser. The background hysteresis in the absence of NUV illumination has been well suppressed (see also Figure S5 in Supporting Information).

Figure 3. Laser wavelength and power density dependence of PIH process. (a) The shifts of the transfer curve when the device is illuminated by lasers of different wavelength for 30 min at VBGH = -40 V. The initial VCNP for all processes are around at -13 V. (b) The shifts of transfer curve during the PIH recovery process when the device is illuminated by lasers of different wavelength 11



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for 30 min at VBGH = 0 V. The initial VCNP for all recovery processes are around -40 V. (c) The plots of IDS during the PIH process stimulated by laser with different power density. (d) The change of VCNP (VCNP) versus time during the PIH process. The solid lines are the fitting curves. The wavelength of laser in (c) and (d) is 405 nm.

Since the PIH is essentially an illumination associated charge transfer process, the laser wavelength and power density should have importance effect. Figure 3a elucidates the left shifts of the transfer curve when the device is illuminated by lasers of different wavelength. The vertical dash line indicates VBGH = -40 V and the illumination time is 30 minutes. When the laser wavelengths are 532 and 650 nm, the changes of VCNP (VCNP) are only -2 and -4 V, respectively. Those values are at the same level of the VCNP when the device is not illuminated (dark). It indicates that the PIH is negligible for the lasers with these two wavelengths. However, when the laser wavelength is changed to 405 and 450 nm, VCNP significantly increase to -30 and -26 V, respectively. It suggests that there is a critical wavelength between 450 and 532 nm (corresponding to a critical photon energy between 2.76 eV and 2.34 eV) for the PIH process. The PIH recovery process is depicted in Figure 3b. When the laser wavelengths are 532 and 650 nm, the changes of VCNP are also as small as VCNP when the device is not illuminated (dark). However, when the wavelength is 405 or 450 nm, the VCNP are either around 17 V. It suggests a similar critical wavelength exists between 450 and 532 nm for the recovery process. The critical photon energy is consistent with the energy level of the defects below the conduction band of SiO2 proposed by V. V. Afanas’ev et al 33,34 as well as the energy barrier for the depassivation of [O3 ≡ Si ― H].32 12


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Figure 3c exhibits the effect of laser power density on the PIH. The changing rate of IDS increases with the increase of laser power density. After 30 minutes, IDS stabilizes at different level, which indicates a saturation of the PIH. It is notable that when the laser power density increases to 800 mW/mm2, the IDS drops quickly to a minimum value and then increases again. This is because the VCNP left shifts over VBGH to a more negative voltage, as presented in Figure S6 in Supporting Information. It implies a further increase of positive trapping charges even after the gate bias electric field is fully screened off. VCNP versus time is plotted in Figure 3d. The changing rate of VCNP increases with the increase of laser power density.

Figure 4. The mechanism of H+ migration induced graphene doping. (a) The VCNP left shift behaviors of device annealed at different hydrogen atmosphere at VBGH = -80 V. The power density 13



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of NUV laser is about 450 mW/mm2. (b) The process of H+ releasing from [O3 ≡ Si ― H] + during laser on period and the process of H+ capturing by [O3 ≡ Si ∙ ] during laser off period. (c)-(e) The schematic of dynamic process of protons hopping in SiO2, where the green circles ‘+’ and black ‘-’ represent H+ and electrons, respectively. (f)-(h) are the corresponding band diagram of (c)-(e), where E𝑓0is Dirac point energy of graphene. The energy difference between the conduction-band bottom of SiO2 and graphene is Eg=3.66, n-type Si is ESi=3.2 eV. LH+ is the width of H+ distribution, denoted by the green area.

The critical photon energy for the PIH process provides an important clue for us. The hydrogen-complexed defects like hydroxyl E’ and hydrogen bridge in SiO2 are the most likely candidates account for the PIH according to our knowledge about the hydrogen-complexed defects.27,28 The hydrogen in the thermal grown SiO2 originates from either the thermal annealing or chemical vapor deposition in the H-containing (H2 and water) environments. To verify whether hydrogen plays a key role on the PIH process, we changed the hydrogen flow during a thermal annealing of the device (see the details from the device fabrication process). The PIH performances of the devices thermal annealed at different hydrogen atmosphere are shown in Figure 4a. When the flow of H2 increases from 0 to 20 sccm, both the changing rate and amplitude of VCNP significantly increase. This is a clear evidence that higher H2 flow induces more defects, which contribute to the PIH, in SiO2. It is also confirmed by the significantly increased signal of [O3 ≡ Si ― H] defects in the Raman Spectra of SiO2, as shown in Figure S7a in Supporting Information. Based on the trapping charges of the fixed defects [O3 ≡ Si ― H] + and movable 14


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protons (H+) in SiO2,29,30 a microscopic mechanism for the PIH is proposed. Figure 4b depicts the photon-induced transformation process between the fixed and movable charge. Figure 4c depicts the charge transfer process during the NUV annealing process. When the device is annealed by NUV illumination, the [O3 ≡ Si ― H] defects near graphene are excited by photons and emit electrons into graphene. Those [O3 ≡ Si ― H] defects near surface of SiO2 are then positive charged. After a long time NUV annealing, the number of positive charges [O3 ≡ Si ― H] + saturates. It results in a saturated initial negative VCNP (as shown in Figure 1d and Figure S1). After that, when a negative VBGH and NUV illumination are applied simultaneously,

PIH process happens: (1)

When the device is upon NUV illumination, H+ is released from [O3 ≡ Si ― H] + as depicted in Figure 4b. (2) Driven by external electric field, those H+ would migrate forward to SiO2/Si interface, as shown in Figure 4d. The redistribution of H+ results in the shift of VCNP through gating effect. (3) Once the laser is turned off, H+ would rapidly be captured by neutral [O3 ≡ Si ∙ ] defects, forming the fixed positive charges

[O3 ≡ Si ― H] + again and so the PIH process stops as observed in Figure 2d. (4) Movable ions of H+ drift towards the SiO2/Si interface continuously until the negative VBGH is completely screened off, as demonstrated in Figure 4e. Figure 4f to 4h are the corresponding band diagram of Figure 4c to 4e, respectively. This model provides a perfect explanation for all main features of the PIH performance reported here. Firstly, the critical photon energy is close to the energy needed for hydroxyl E’ emitting electron to the conduction band of SiO2 as well as the energy needed for the depassivation of [O3 ≡ Si ― H]. Secondly, it well explains the 15



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recovery process of PIH. According to the mechanism proposed by Young Duck Kim et al,21 the electrons are excited to the conduction band and swept to graphene by the gate voltage. In such case, the positive trapping charges would always increase and thus the VCNP should always left shift regardless of the gate voltage polarity. However, others’ reports 41 as well as our results in this report clearly demonstrate a two-way shift behavior of VCNP depending on the gate voltage polarity. For a full understanding of the two-way shift of VCNP, it is reasonable to believe that there is a redistribution of the trapping charges when illumination and electric field coexists. Thirdly, it well explains the asymmetric characteristic of PIH for positive and negative VBGH. Because the initial positive charges mostly locate within a few nanometers (denoted as LH+~8 nm 26) near the SiO2 surface,21,39 the allowed migration space for protons towards to the interface of graphene/SiO2 at positive VBGH is obviously much smaller than that towards to the interface of Si/SiO2 at negative VBGH (see Figure S8 in Supporting Information). To verify the role of [O3 ≡ Si ― H] on PIH process, the in-situ Raman Spectra characterize has been carried out in this report (see Figure S7b and Figure S7c in Supporting Information). At negative VBGH, it is found that the normalized intensity of Si-H band, corresponding to the concentration of [O3 ≡ Si ― H], gradually decreases within 30 minutes. This is because the migration of H+ towards to SiO2/Si interface causes the decrease of hydrogen concentration near the surface of SiO2. While the normalized intensity of Si-H band at a positive VBGH almost keeps unchanged. It demonstrates the hydrogen concentration near the surface of SiO2 is nearly not affected by the laser illumination, which is in good agreement with previous discussions. 16


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The migration of H+ in SiO2 is a random hopping process with a prevailing direction along the electric field. The H+ hopping process can be well described by a dispersive transport theory.42-45 This model provides an excellent fitting with experimental data at different laser power density, as plotted in Figure 3d. For the initial VCNP = -13 V, the density of protons estimated in SiO2 within a thin layer

is about ρ0

= 8.75 × 1019𝑐𝑚 ―3. It is in good agreement with the hydrogen concentration obtained by nuclear reaction analysis method.26 The detailed calculations are discussed in the supporting information. The change of VCNP can be estimated by equation S4 in the supporting information. The model would naturally give a high asymmetrical PIH behavior for positive and negative VBGH, as observed in Figure 2b. For the recovery process, in principle, the change of VCNP can also be well described by assuming a proton distribution with an improved trial function

43,46

rather than within a thin layer. At VBGH = 0 V, without

external electric field, it is hard for those protons to migrate back to the original thin layer near the SiO2 surface. Thus, VCNP shows a quite slow response and does not reach the original value. While at a positive VBGH, the external electric field compels protons back to the near surface of SiO2 and so results in VCNP quickly back shifting to initial value, as shown in Figure 2c.

17



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Figure 5. The dependence of PIH on the temperature. (a) The change of VCNP (∆VCNP) versus time at different temperature at VBGH = -40 V. (b) The plot of the proton average displacement per hop μ𝐻 versus 1000/T. (c) the initial VCNP of device after NUV annealing at different temperature.

Because the proton migration is involved in our proposed mechanism for the PIH, the dynamic process should be temperature dependent. On the contrary, if the accumulation and redistribution of trapping charges are all through electron or hole transport, the dynamic process should be much less temperature dependent. Because electron/hole hopping is a pure quantum tunneling process, which is weakly coupled with phonons. Therefore, PIH measurements at different temperature provides an effective verification for the mechanism. The experimental results are shown in Figure 5a, where the temperature ranges from 150 K to 400 K. The change rate of VCNP and the amplitude of VCNP significantly increase with the temperature. This is a clear evidence that the charge transport corresponding to the PIH process is mainly through proton migration rather than electron/hole hopping or tunneling. The average displacement per hop μ𝐻 is a measure of the proton moving speed. It is plotted versus 1000/T in Figure 5b. According to Equation S3 in Supporting Information, the hopping activation energy of H+ in SiO2 extracted from the slope of the linear fitting is about 0.12 eV. It is well consistent with the reported theoretic value, which is in the range of 0.1 to 0.2 eV.47,48 Moreover, in our proposed mechanism, the initial VCNP after NUV annealing is determined by the electron tunneling from the trapping sites at the near surface of SiO2 to graphene with the help of photon. Thus, the initial VCNP after NUV annealing should 18


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be temperature independent. This is well supported by the result of the measurements of VCNP after NUV annealing at different temperature, as plotted in Figure 5c. The initial VCNP keeps almost unchanged at around -13 V as the temperature increase from 300 to 420 K.

Figure 6. The measurement results of device response to the pulsed NUV illumination. (a) The response of IDS to the pulsed NUV illumination at VBGH = -40 V. (b) One cycle of the photo response, including a photocurrent response (process I) and a hysteresis process (process II)s. (c) The linear fitting of the decrease of IDS in process II. (d) The changing rate of carrier concentration in each cycle as a function of the accumulated illumination time. The solid lines are the theoretic fitting curves following the model. (e) The parameter μ𝐻 versus the applied electric field Eox.

To further verify the proposed microscopic mechanism and the theoretic model, the pulsed laser is used to reduce the possible heating effect and more precisely capture the dynamic process of PIH. The response of IDS to the pulsed laser illumination is plotted in Figure 6a, where VBGH is -40 V. A plot of IDS in one cycle of the laser onand-off is exhibited in Figure 6b. During the laser-on period, the IDS curve clearly shows 19



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two distinguished response processes: a sharp decline process (I) and a slow decline process (II). The sharp decline of IDS in process I is due to the photocurrent, which origins from the bolometric effect and photovoltaic effect at the graphene/metal junction (see the analysis in Supporting Information). The change of IDS in process I versus gate voltage is plotted in Figure S9, and it is in agreement with the photocurrent of the biased graphene.49 The decrease of IDS in process II accumulates after cycle to cycle. As a result, IDS continuously drops as the cycle number increases. It implies that the decrease of IDS in process II is an accumulated doping effect of the trapping charges in SiO2. The process II is corresponding to the PIH process we are discussing here. Because the carrier mobility almost keeps a constant during the PIH process, the changing rate of carrier concentration 𝑑𝑛/𝑑𝑡 could be directly extracted from the changing rate of IDS through the relationship d𝐼𝐷𝑆/𝑑𝑡 = ― 𝑑𝑛/𝑑𝑡 ∙

(

𝑒 𝑉𝐷𝑆𝑊 𝐿

). The linear

fitting of the IDS curve in process II in a cycle is plotted in Figure 6c. And the extracted changing rates of carrier concentration (𝑑𝑛/𝑑𝑡) at various VBGH are plotted versus time in Figure 6d. The solid lines are the theoretic fitting curves following the model described in the supporting information. It perfectly fits with the experiment results. Figure 6e illustrates the extracted 𝜇𝐻 as a function of electric field Eox (Eox = VBGH/dox). By fitting with the Equation S3, the parameter m equals 0.77, which agrees well with M. Houssa et al’s report.42 For the graphene with more defects, the changing rate of IDS decreases because of the lower carrier mobility (see Figure S10 in Supporting Information). But it is notable that the changing rate of carrier concentration 𝑑𝑛/𝑑𝑡 during PIH process is 20


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determined by the change of the trapping charges in SiO2 through gating effect. Thus, the defects in graphene have no influence on the changing rate of carrier concentration and the amplitude of VCNP in PIH process. As shown in Figure S10b and Figure S10d, the VCNP shift rates are similar for the two graphene channels with different defect concentration.

Conclusions In conclusion, photo-induced hysteresis (PIH) of G-FET with mono SiO2 dielectric layer is experimentally investigated. It is found there is threshold photon energy between 2.76 eV and 2.34 eV for PIH. This is very close to the theoretic value of energy barrier for depassivation of [O3 ≡ Si ― H] into [O3 ≡ Si ∙ ] and an interstitial H0. The increase of H2 flow during a device thermal annealing process significantly enhances PIH. It provides a direct evidence for the proton-related microscopic mechanism of PIH. The proton-related microscopic mechanism of PIH is as following: 1. Positive charged defects [O3 ≡ Si ― H] + release movable H+ when the device is illuminated. 2. Those H+ subsequently move forward to SiO2/Si interface driven by external electric field and cause a shift of VCNP. 3. Once the laser is turned off, H+ would rapidly be captured by neutral [O3 ≡ Si ∙ ] defects and forming fixed positive charges [O3 ≡ Si ― H] + again. The proposed mechanism based on the migration of H+ is further verified by temperature dependence test and a pulsed laser stimulated PIH experiment. The activation energy EA of H+ hopping in SiO2 extracted from the experimental data is 0.12 eV, which is in good agreement with the theoretic value. The microscopic mechanism revealed in this work provides a new yet deeper understanding for photo 21



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induced hysteresis or memory effect of G-FET. The results are also valuable for designing graphene and other two-dimensional material based opt-electronic devices as well as non-volatile memories.

Methods The graphene synthesized by typical chemical vapor deposition (CVD) method 36, was transferred to highly n-doped Si wafer with 300 nm SiO2, using PMMA as transfer vehicle. Then the photolithography followed by O2 plasma etching was performed to define the graphene pattern. The source-drain electrodes were deposited by electronbeam evaporation of Ti (10 nm) and Au (100 nm). After the fabrication of the device, annealing at 300 ℃ in a mixed gas ambient of Ar/H2 (97 sccm/3 sccm) for 1 hour was carried out to remove the organic residues (PMMA and photoresist during fabrication) and improve the quality of the graphene-metal contact. As a comparison, a mixed gas ambient of Ar/H2 with concentration of 100 sccm/0 sccm and 80 sccm/20 sccm were also adapted to anneal the device. Before the PIH measurement, the G-FET was annealed by near-ultra-violet (NUV) illumination with a laser (=405 nm, 200 mW/mm2) in a vacuum chamber (with vacuum level better than 310-4 Pa) for half hour to remove most of the possible adsorbed molecules. The transfer characteristic of the G-FET was monitored through fast transfer curve measurement during the PFID process. The same measurement method could be found in our previous report.40 A series of lasers (MTO, Inc, 100 mW) with wavelength ranging from 405 to 650 nm were employed in the experiment. The spot size is controllable to tune the power density of illumination. The electrical 22


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measurements were carried out by Agilent 4155C and Keysight B2901A in a high vacuum chamber, and the Raman Spectroscopy (LabRAM HR Evolution) with 532 nm exciting wavelength was performed to characterize the graphene in atmospheric condition. ASSOCIATED CONTENT Supporting Information Available: Description of the holding gate voltage and NUV power density dependent left-shift of transfer characteristic of graphene FET, in-situ Raman characteristic of SiO2, the dispersive transport model of protons in SiO2, and the bolometric effect and photo voltaic effect jointly caused photocurrent in graphene FET. This material is available free of charge via the Internet at http://pubs.acs.org. AUTHOR INFORMATION Corresponding Author *Email: [email protected] Notes The authors declare no competing financial interest. ACKNOWLEDGMENT This work was financially supported by the National Natural Science Foundation of China (Grant Nos. 61671368, 61172041, 91123018 and 61404103), Science and Technology on Analog Integrated Circuit Laboratory (614280205040617), Science and Technology Planning Project of Guangdong Province, China (2017A010103004), Basic

Public

Welfare

Research

Planning

Project

of

Zhejiang

Province

(LGG19F040002) and the Fundamental Research Funds for the Central Universities. 23



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We thank Dr. Wang of Instrumental Analysis Center of Xi’an Jiaotong University, for the RAMAN measurement. REFERENCES 1. Wang, H. M.; Wu, Y. H.; Cong, C. X.; Shang, J. Z.; Yu, T. Hysteresis of Electronic Transport in Graphene Transistors. ACS Nano 2010, 4, 7221-7228. 2. Cho, C.; Lee, Y. G.; Jung, U.; Kang, C. G.; Lim, S.; Hwang, H. J.; Choi, H.; Lee, B. H. Correlation Between the Hysteresis and the Initial Defect Density of Graphene. Appl Phys Lett 2013, 103, 083110. 3. Liao, Z. M.; Han, B. H.; Zhou, Y. B.; Yu, D. P. Hysteresis Reversion in Graphene Field-Effect Transistors. J Chem Phys 2010, 133, 044703. 4. Sun, P. Z.; Zhu, M.; Wang, K. L.; Zhong, M. L.; Wei, J. Q.; Wu, D. H.; Cheng, Y.; Zhu, H. W. Photoinduced Molecular Desorption from Graphene Films. Appl Phys Lett 2012, 101, 053107. 5. Xu, H.; Chen, Y. B.; Zhang, J.; Zhang, H. L. Investigating the Mechanism of Hysteresis Effect in Graphene Electrical Field Device Fabricated on SiO2 Substrates using Raman Spectroscopy. Small 2012, 8, 2833-2840. 6. Lee, Y. G.; Kang, C. G.; Cho, C.; Kim, Y.; Hwang, H. J.; Lee, B. H. Quantitative Analysis of Hysteretic Reactions at the Interface of Graphene and SiO2 Using the Short Pulse I-V Method. Carbon 2013, 60, 453-460. 7. Joshi, P.; Romero, H. E.; Neal, A. T.; Toutam, V. K.; Tadigadapa, S. A. Intrinsic Doping and Gate Hysteresis in Graphene Field Effect Devices Fabricated on SiO2 Substrates. J Phys-Condens Mat 2010, 22, 334214. 24


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114x60mm (300 x 300 DPI)


Photoinduced Hysteresis of Graphene Field-Effect Transistors Due to

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