
- Journal of Semiconductors
- Vol. 42, Issue 11, 112002 (2021)
Abstract
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[
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[
However, studies are mostly carried out under high energy proton irradiation, where the resistivity of CNTs decreased (under 8–12 MeV)[
In this article, semiconducting SWCNT FETs with a back-gate structure were prepared by the solution-deposited method, and the impact of 150 keV protons with different fluences up to 1 × 1015 p/cm2 on them were investigated. The electrical behaviors of SWCNT FETs were studied before and after irradiation. Especially, Stopping and Ranges of Ions in Matter (SRIM)[
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 0.8–2.5 μm, 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/SiO2 substrate was rinsed with o-xylene, purged with N2 and baked at 150 °C for 30 min at atmosphere.
Figure 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
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 × 1014 and 1 × 1015 p/cm2, respectively, and the vacuum degree of sample chamber is 10–4 Pa during the irradiation.
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 10 μm and a width of 20 μm. 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[
SRIM software toolkit was employed to simulate the penetration depth and displacement damage of the CNT FET after 150 keV proton irradiation[
Figure 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.
GEANT 4 software was used to further explore the effect of proton irradiation on the contact and channel region of the CNT-layer[
Figure 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.
A Raman spectrum was conducted to characterize the displacement damage of the CNT-film-based channel after irradiation[
Figure 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 (
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[
Figure 5.(Color online) (a) Typical transfer characteristics curves of the SWCNT FETs at
was used to extract Vth (Fig. 5(b)), where gm is the transconductance defined as
Figure 6.Statistics measurements of (a) the threshold voltage (
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[
According to the energy band diagram in Fig. 5(c), the variation of Ioff characteristics should be the same as that of Ion, but the result of the rate of Ioff change in Fig. 6(c) is smoothly fluctuating, which results from the interface traps between CNT and SiO2[
The trend of the rate of mobility change is similar to that of Ion under 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[
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 × 1015 p/cm2 (Fig. 6(f) showing the statistics measurements of SS). SS can be expressed as[
where k is the Boltzmann’s constant, T is the temperature, q is the elementary charge, Cox is the capacitance density of 300 nm silica, Cit is the interface states capacitance, and Dit is 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 = 7 μm) and different channel length (L = 4, 6, 10, 14, 18, 22, 28 and 35 μm, respectively) were fabricated to measure RC and RSH (the inset of Fig. 7(a) showing the optical image of a TLM structure). The total resistance (RT) in a CNT FET consists of RC and RSH as
Figure 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
where RT can be extracted from the I–V characteristics of a complete CNT TLM test structure as shown in Fig. 7(a)[
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 of Vth and the decrease of Ion, while other electrical parameters such as SS, Ioff would 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 of RC and RSH. 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
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)
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