Radiation-hardened property of single-walled carbon nanotube film-based field-effect transistors under low-energy proton irradiation

Xiaorui Zhang, Huiping Zhu, Song’ang Peng,, Guodong Xiong,3, Chaoyi Zhu, Xinnan Huang,Shurui Cao, Junjun Zhang,3, Yunpeng Yan, Yao Yao, Dayong Zhang, Jingyuan Shi,Lei Wang, Bo Li, and Zhi Jin,†

1High-Frequency High-Voltage Device and Integrated Circuits R&D Center, Institute of Microelectronics, Chinese Academy of Sciences,Beijing 100029, China

2Key Laboratory of Science and Technology on Silicon Devices, Institute of Microelectronics, Chinese Academy of Sciences,

Beijing 100029, China

3University of Chinese Academy of Sciences, Beijing 100049, China

4Department of Chemistry, City University of Hong Kong, Hong Kong 999077, China

Abstract: Strong C-C bonds, nanoscale cross-section and low atomic number make single-walled carbon nanotubes (SWCNTs)a potential candidate material for integrated circuits (ICs) applied in outer space. However, very little work combines the simulation calculations with the electrical measurements of SWCNT field-effect transistors (FETs), which limits further understanding on the mechanisms of radiation effects. Here, SWCNT film-based FETs were fabricated to explore the total ionizing dose (TID)and displacement damage effect on the electrical performance under low-energy proton irradiation with different fluences up to 1 × 1015 p/cm2. Large negative shift of the threshold voltage and obvious decrease of the on-state current verified the TID effect caused in the oxide layer. The stability of the subthreshold swing and the off-state current reveals that the displacement damage caused in the CNT layer is not serious, which proves that the CNT film is radiation-hardened. Specially, according to the simulation, we found the displacement damage caused by protons is different in the source/drain contact area and channel area, leading to varying degrees of change for the contact resistance and sheet resistance. Having analyzed the simulation results and electrical measurements, we explained the low-energy proton irradiation mechanism of the CNT FETs, which is essential for the construction of radiation-hardened CNT film-based ICs for aircrafts.

Key words: SWCNT FETs; low-energy proton irradiation; radiation effects; electrical performance; TID effect; displacement damage effect; simulation

1. Introduction

The development of modern integrated circuits (ICs) has been hindered by further downscaling the physical size of transistors. Single-walled carbon nanotubes (SWCNTs) are promising to replace silicon as a new generation semiconductor material to continue Moore’s Law due to the ultrathin body and excellent electrical properties[1-5]. In particular, possessing strong C-C bonds, nanoscale cross-section and low atomic number, SWCNTs have great potential in fabricating radiation-hardened field-effect transistors (FETs), which are required in aerospace applications[6-11].

With the advancement of space exploration, severe challenges have been put forward for the radiation resistance of ICs, which directly affects the lifetime of spacecrafts[12,13].Space radiation sources mainly come from the particles in the geomagnetic trapping radiation belt, solar cosmic rays and galactic cosmic rays, all of which contain protons. Compared with high energy proton radiation, low energy proton(< 1 MeV) radiation often means higher displacement damage, which plays an important role in the degradation of transistors performance[14]. Additionally, protons with energies ranging from 50 to 200 keV have uncertain effects on the transistors[15-17]. Therefore, it is of great significance for the reliability of spacecrafts to study the effect of low energy proton radiation on transistors[18].

However, studies are mostly carried out under high energy proton irradiation, where the resistivity of CNTs decreased (under 8-12 MeV)[19], the structure of CNTs was considered unchanged (under 3.3 MeV)[20]and the tensile strength of CNTs was strengthened (under 2 MeV with the fluence of 1 × 1013p/cm2)[21]. The lack of study on low energy proton radiation effect limits further understanding of radiation effects on CNTs. Thus, it is necessary to explore the radiation effects on CNTs under low energy proton irradiation.

Fig. 1. (Color online) Structure and properties of the SWCNT-film-based FET. (a) AFM morphologic image showing the SWCNT film deposited on the Si/SiO2 substrate. The inset is the optical image of a SWCNT FET, of which the channel length is 10 μm and the width is 20 μm. (b) Typical transfer charateristics curves of the SWCNT FET before irradiation at VDS = -10 V. (c) Schematic diagram of the low-energy charged particle irradiation simulation test device composed of an ionization chamber, accelerator and irradiation chamber. (d) Schematic showing the total ionizing dose(TID) and displacement damage effect in the SWCNT FET induced by proton irradiation.

In this article, semiconducting SWCNT FETs with a backgate structure were prepared by the solution-deposited method, and the impact of 150 keV protons with different fluences up to 1 × 1015p/cm2on them were investigated. The electrical behaviors of SWCNT FETs were studied before and after irradiation. Especially, Stopping and Ranges of Ions in Matter (SRIM)[22]and Geometry and Tracking (GEANT 4)[23,24]software toolkits were used to simulate the radiation effects of 150 keV proton irradiation on metal/CNT contact and the exposed channel region, for the first time to our knowledge.By combing simulation results with the electrical measurements, the mechanism of SWCNT FET degradation by proton radiation was thoroughly revealed, which provides new insight for the radiation-harden technology of SWCNT filmbased FETs.

2. Experimental

2.1. SWCNT film preparation

Shown in the inset of Fig. 1(a) is the SWCNT FET applied in the irradiation experiment. The n-doped silicon substrate was employed as a back-gate. The semiconducting SWCNTs channel was fabricated from 99.9% arc-discharged CNTs dispersions purchased from Suzhou CINK Nano Materials Co. Ltd.The diameters of SWCNTs we used range from 1.3 to 1.7 nm,the lengths were between 0.8-2.5μm, and the initial concentration was more than 0.2 mg/mL. The silicon substrate with 300 nm silica was ultrasonically treated with acetone, isopropanol and deionized water for 10 min, and after that the substrate was baked at 120 °C for 30 min. Then the SWCNTs were diluted for 20 times by o-xylene. Followed by 24 h immersion in diluted SWCNTs dispersions, the Si/SiO2substrate was rinsed with o-xylene, purged with N2and baked at 150 °C for 30 min at atmosphere.

2.2. Device fabrication

In addition to the conventional FETs, the transmission line model (TLM) test structures were fabricated on the SWCNT film to study the radiation effects on the contact resistance (RC) and the sheet resistance (RSH) of SWCNTs. Active regions were etched by oxygen plasma to isolate the devices.Electrodes of Ti/Pd/Au (2/30/50 nm) were fabricated with electron beam lithography (EBL), electron beam evaporation and lift-off.

2.3. Characterization

Surface morphology and quality of SWCNT film were characterized by atomic force microscopy (AFM) and Raman spectroscopy (LabRAM HR Raman system with a laser wavelength of 473 nm). Electrical properties were measured using the Keithley 4200 electrometer system.

2.4. Radiation experiment

The proton radiation experiment was carried out at the low-energy charged particle irradiation simulation test device of Harbin Institute of Technology. The irradiation ion energy is 150 keV, the beam current is 80 nA/cm2, the proton fluences are 5 × 1012, 5 × 1013, 5 × 1014and 1 × 1015p/cm2, respectively, and the vacuum degree of sample chamber is 10-4Pa during the irradiation.

Fig. 2. (Color online) Simulation results of (a) the distribution of protons in the source/drain region (Au/Pd/Ti/SWCNT/SiO2/Si), (b) the number of vacancies in the source/drain region (Au/Pd/Ti/SWCNT/SiO2/Si), (c) distribution of protons in the channel region (SWCNT/SiO2/Si), and (d) the number of vacancies in the channel region (SWCNT/SiO2/Si) by SRIM. The energy of the protons is 150 keV. The inset is the illustration of the simulation region, including the source/drain contact region and the SWCNT channel region.

3. Results and discussion

The AFM morphologic image of SWCNT films deposited on a silicon substrate with 300 nm silica is shown in Fig. 1(a),where the inset is the optical image of SWCNT-film-based FET with a channel length of 10μm and a width of 20μm. The electrical properties of the SWCNT FETs were measured before irradiation. The transfer characteristics of the SWCNT FET shown in Fig. 1(b) indicates a typical p-type FET behavior, mainly because the absorption of water and oxygen molecules results in p-doping of the CNTs when exposed to air[25]. The on/off ratio (Ion/Ioff) of the SWCNT FETs before irradiation is more than 104, which is suitable for a logic circuit. The mobility is about 10 cm2/(V·s). The low-energy proton irradiation device with an ionization chamber, accelerator and irradiation chamber is sketched in Fig. 1(c), in which the samples were irradiated with four different proton fluences of 5 × 1012, 5 × 1013, 5 ×1014and 1 × 1015p/cm2. Proton irradiation results in two main effects: total ionizing dose (TID) and displacement damage effect. TID effect accumulates positive oxide-trapped charges in the back-gate oxide (SiO2), which will introduce negative induced charges into the CNTs, while displacement damage effect causes irreversible damage to the CNTs,thereby affecting the interface state (Fig. 1(d)).

SRIM software toolkit was employed to simulate the penetration depth and displacement damage of the CNT FET after 150 keV proton irradiation[15,22]. The simulation was divided into the source/drain region (inset of Fig. 2(a):Au/Pd/Ti/CNT/SiO2/Si) and the CNT channel region (inset of Fig. 2(c): CNT/SiO2/Si). The simulation model was based on the thickness and density of each layer, including Au (50 nm,19.32 g/cm3), Pd (30 nm, 12.02 g/cm3), Ti (2 nm, 4.54 g/cm3),CNT (1.5 nm (the deposited CNT film can be considered as a single layer according to the linear density of 20 lines/μm),1.20 g/cm3), SiO2(300 nm, 2.15 g/cm3) and Si (500μm,2.32 g/cm3). The proton distribution in the metal-CNT contact region and the channel region is concentrated in the depth ranging from 1000 to 1500 nm, which corresponds to the Si substrate layer, indicating that the 150 keV protons penetrate the contact and the channel region during the irradiation process, and finally stopped in the Si substrate(Figs. 2(a) and 2(c)). Although most of the protons eventually stopped in the Si substrate, vacancies are generated in each layer along the trace during the proton irradiation, which is known as displacement damage. The number of vacancies in each layer is simulated, showing that the displacement damage caused by the proton irradiation is different in the CNT-layer covered by metal and that exposed to air (Figs. 2(b) and 2(d)). It can be seen from the enlarged image in Fig. 2(b) that the vacancy number of the CNT-layer covered by metal (red marking line:x= 82 nm) is much larger than that in the CNTlayer exposed to air (Fig. 2(d):x= 0 nm), which is of great significance for studying the properties of CNTs before and after irradiation.

Fig. 3. Simulation result of the energy loss in the metal/CNT contact and channel region of the CNT-layer performed by GEANT 4 with four different proton irradiation fluences of 5 × 1012, 5 × 1013, 5 × 1014 and 1 × 1015 p/cm2.

GEANT 4 software was used to further explore the effect of proton irradiation on the contact and channel region of the CNT-layer[23], computing the energy deposition of 150 keV protons passing through the CNT-layer with different fluences of 5 × 1012, 5 × 1013, 5 × 1014and 1 ×1015p/cm2. As shown in Fig. 3, the energy loss increases with irradiation fluence in both the source/drain and channel region of the CNT-layer. In addition, the deposited energy in the source/drain region of the CNT-layer is greater than that in the channel region at each fluence, revealing that the displacement damage of the metal-covered CNT is more serious.

A Raman spectrum was conducted to characterize the displacement damage of the CNT-film-based channel after irradiation[9,26]. The D peak intensity of the CNT channel increases with the increase of the irradiation fluence as plotted in Fig. 4(a), in which the G peak was normalized. The statistics result of the ratio of the D peak intensity to the G peak intensity (ID/IG) by Raman mapping is shown in Fig. 4(b), of which the test area is 5 × 5μm2with a step of 0.5μm.ID/IGincreases from 0.10 (before irradiation) to 0.12 (5 × 1012p/cm2),0.16 (5 × 1013p/cm2), 0.22 (5 × 1014p/cm2) and 0.30 (1 ×1015p/cm2), indicating that displacement damage of the CNT-film-based channel increases with the increase of the irradiation fluence[27], which is consistent with the simulation results in Fig. 3.

The electrical properties of the radiation-hardened CNT FET, represented by the transfer characteristics, were measured at room temperature in ambient air and found to be in line with the simulation results. The back-gate FET with 300 nm oxide is more sensitive to TID effect when compared to the top-gate FET with a thin oxide[8,28]. Fig. 5(a) shows the typical transfer characteristics of the CNT FET before and after irradiation with four different fluences in linear (left) and logarithmic (right) coordinates atVDS= -10 V, from which the threshold voltage (Vth), subthreshold swing (SS), mobility, onstate current (Ion), off-state current (Ioff) andIon/Ioffwith four different proton irradiation fluences were calculated. The simplifiedY-function method, expressed as[29,30]

Fig. 4. Raman spectra of SWCNT FETs before and after proton irradiation with four different proton fluences of 5 × 1012, 5 × 1013, 5 × 1014 and 1 × 1015 p/cm2. (a) Single point Raman spectra of SWCNT FETs with different proton fluences, in which the G peaks are normalized.(b) Statistical study on the ratio of the D peak intensity to the G peak intensity (ID/IG) with different proton fluences. Each sample has 121 test points, which are obtained by Raman mapping. The test area is 5 × 5 μm2 and the spacing is 0.5 μm.

In terms of the energy band, the p-type FET doped by water and oxygen has a Schottky barrier (SB) with metal, which plays a decisive role in the performance of the CNT FET[32,33].The negative charges induced in the CNT channel cause the Fermi level to move toward the conduction band. Due to the pinning effect between CNT and metal[34], the energy band of CNT bends downward and the SB will be thicker (Fig. 5(c)).In case of the on-state, holes are blocked by the thicker SB,and thusIondecreases with the increasing fluence (Fig. 6(b)).Due to the obvious decrease ofIon,Ion/Ioffdecreases a little with the increasing irradiation fluence (Fig. 6(d)). In order to ensure the reliability of the change evaluation, the measurement of the device performance change was carried out in situ.

Fig. 5. (Color online) (a) Typical transfer characteristics curves of the SWCNT FETs at VDS = -10 V with different proton fluences. (b) Threshold voltage (Vth) values extracted by the Y-function method with different proton fluences. (c) Band structure of SWCNT with metal before and after proton irradiation at the on- (left) and off-state (right). (d) Hysteresis in typical transfer characteristics curves with different proton fluences at VDS = -10 V.

According to the energy band diagram in Fig. 5(c), the variation ofIoffcharacteristics should be the same as that ofIon,but the result of the rate ofIoffchange in Fig. 6(c) is smoothly fluctuating, which results from the interface traps between CNT and SiO2[35,36].After irradiation, a small amount of displacement damage was produced in the CNT and SiO2, forming new interface states. As plotted in Fig. 5(d), the hysteresis voltage (H), which is generally defined as the absolute value of the difference between gate voltages (of positive and negative sweep) at halfIon, is relatively stable under the increasing irradiation fluence (His 70, 71, 68, 73,70 and 69 V, respectively), indicating that the interface state changed little.

The trend of the rate of mobility change is similar to that ofIonunder the four irradiation fluences (Fig. 6(e)), indicating that holes in the CNT channel were affected by the Coulomb scattering of the positive oxide-trapped charges in SiO2[37]. Besides, the degradation of mobility is also related to the displacement damage caused in the CNT layer.

The transfer characteristics in the logarithmic coordinate in Fig. 5(a) illustrate the radiation hardness of the CNT FET,where the subthreshold swing (SS) is almost the same with the fluences up to 1 × 1015p/cm2(Fig. 6(f) showing the statistics measurements of SS). SS can be expressed as[38]

wherekis the Boltzmann’s constant,Tis the temperature,qis the elementary charge,Coxis the capacitance density of 300 nm silica,Citis the interface states capacitance, andDitis the interface states density. The stability of SS further verifies that the interface states density (Dit) did not change significantly.

To go a further step, the contact resistance (RC) and the sheet resistance (RSH) were explored under four different irradiation fluences. The transmission line model (TLM) test structures with the same channel width (W= 7μm) and different channel length (L= 4, 6, 10, 14, 18, 22, 28 and 35μm, respectively) were fabricated to measureRCandRSH(the inset of Fig. 7(a) showing the optical image of a TLM structure). The total resistance (RT) in a CNT FET consists ofRCandRSHas

Fig. 6. Statistics measurements of (a) the threshold voltage (Vth), (b) rate of on-current (Ion) change, (c) rate of off-current (Ioff) change, (d) rate of on/off ratio change, (e) rate of mobility change, and (f) subthreshold swing (SS) with four different proton irradiation fluences of 5 × 1012, 5 ×1013, 5 × 1014 and 1 × 1015 p/cm2.

whereRTcan be extracted from theI-Vcharacteristics of a complete CNT TLM test structure as shown in Fig. 7(a)[39-45].The measured total resistances before and after irradiation with four fluences are plotted as a function of the channel length between two electrodes (Fig. 7(b)). 2RCandRSH/Ware determined by the intercept and slope of the data fitting line in Fig. 7(b), respectively. On one hand, bothRSHandRCincrease with the increasing fluence, which is due to the thicker SB and the decreased mobility. On the other hand, the results show thatRSHis 1.1 (5 × 1012p/cm2), 1.1 (5 × 1013p/cm2),1.7 (5 × 1014p/cm2) and 6.3 (1 × 1015p/cm2) times of that before irradiation, andRCis 2.2 (5 × 1012p/cm2), 6.9 (5 ×1013p/cm2), 15 (5 × 1014p/cm2) and 22 (1 × 1015p/cm2)times of that before irradiation, indicating thatRCis more severely affected by proton irradiation, which agrees well with the simulation result that the displacement damage caused in the source/drain region of the CNT layer is more serious than that in the channel region.

4. Conclusions

In conclusion, we have fabricated SWCNT film-based FETs with back-gate structure, of which the electrical behavior was explored under low-energy proton irradiation. It is found that the TID effect caused the negative shift ofVthand the decrease ofIon, while other electrical parameters such as SS,Ioffwould not change obviously with the increasing fluence, revealing that the displacement damage caused in the SWCNT FET is not serious. More interestingly, the displacement damage in the metal/CNT and channel region was simulated and found to be different, explaining the various changes ofRCandRSH. Combining the simulation results and electrical measurements, we have analyzed the low-energy proton irradiation mechanism of the CNT FETs, which provides meaningful guidelines for the radiation hardness technology of CNT film-based ICs for aircraft application in outer space.

Acknowledgments

Fig. 7. (Color online) TLM measurements of SWCNT FETs before and after proton irradiation with four different proton fluences of 5 × 1012, 5 ×1013, 5 × 1014 and 1 × 1015 p/cm2. (a) Typical current-voltage curves of a complete SWCNT TLM test structure before proton irradiation at VBG =-10 V. Inset is the optical image of a SWCNT TLM test structure consisting of several FETs, which have the same channel width (7 μm) and different channel length (4, 6, 10, 14, 18, 22, 28 and 35 μm, respectively). (b) Length-dependent total resistances of SWCNT FETs with different proton fluences.

This work was financially supported by the National Natural Science Foundation of China (No. 61704189), the Common Information System Equipment Pre-Research Special Technology Project (31513020404-2), Youth Innovation Promotion Association of Chinese Academy of Sciences and the Opening Project of Key Laboratory of Microelectronic Devices & Integrated Technology, and the Key Research Program of Frontier Sciences, CAS (Grant ZDBS-LY-JSC015)