Total Ionizing Dose Radiation Effects in the P-Type Polycrystalline Silicon Thin Film Transistors

Yuan Liu(刘远)$^{1,2\hyperlink{s}{**}}$, Kai Liu(刘凯)$^{1,2}$, Rong-Sheng Chen(陈荣盛)$^{2}$, Yu-Rong Liu(刘玉荣)$^{2}$,\\ Yun-Fei En(恩云飞)$^{1}$, Bin Li(李斌)$^{2}$, Wen-Xiao Fang(方文啸)$^{1\hyperlink{s}{**}}$

doi:10.1088/0256-307X/34/1/018501

⇦Chinese Physics Letters, 2017, Vol. 34, No. 1, Article code 018501⇨ Total Ionizing Dose Radiation Effects in the P-Type Polycrystalline Silicon Thin Film Transistors * Yuan Liu(刘远)1,2**, Kai Liu(刘凯)1,2, Rong-Sheng Chen(陈荣盛)2, Yu-Rong Liu(刘玉荣)2, Yun-Fei En(恩云飞)1, Bin Li(李斌)2, Wen-Xiao Fang(方文啸)1** Affiliations 1Science and Technology on Reliability Physics and Application of Electronic Component Laboratory, China Electronic Produce Reliability and Environmental Testing Research Institute, Guangzhou 510610 2School of Electronic and Information Engineering, South China University of Technology, Guangzhou 510641 Received 22 June 2016 ^*Supported by the National Natural Science Foundation of China under Grant Nos 61574048 and 61204112, the Science and Technology Research Project of Guangdong Province under Grant Nos 2015B090912002 and 2014A030313656, and the Pearl River S$\&$T Nova Program of Guangzhou.
^**Corresponding author. Email: liuyuan@ceprei.com; fangwx@ceprei.com Citation Text: Liu Y, Liu K, Chen R S, Liu Y R and En Y F et al 2017 Chin. Phys. Lett. 34 018501 Abstract The total ionizing dose radiation effects in the polycrystalline silicon thin film transistors are studied. Transfer characteristics, high-frequency capacitance-voltage curves and low-frequency noises (LFN) are measured before and after radiation. The experimental results show that threshold voltage and hole-field-effect mobility decrease, while sub-threshold swing and low-frequency noise increase with the increase of the total dose. The contributions of radiation induced interface states and oxide trapped charges to the shift of threshold voltage are also estimated. Furthermore, spatial distributions of oxide trapped charges before and after radiation are extracted based on the LFN measurements. DOI:10.1088/0256-307X/34/1/018501 PACS:85.30.Tv, 61.80.Ed, 73.40.Qv, 85.40.Qx © 2017 Chinese Physics Society Article Text Polycrystalline silicon thin film transistors (poly-Si TFTs) are key pixel switching elements for active matrix flat panel displays. Recent advances in TFT fabrication have improved their performance so as to allow their use in harsh radiation environments (e.g., digital x-ray imaging devices in the medical applications).[1-3] Thus examination of their operational stability when they are exposed to radiation environments is urgently needed. Recently, some works have focused on the degradation of n-type poly-Si TFTs that are exposed in the x-ray, gamma, alpha particle and high-energy electron radiation environments.[4-7] There is a significant degradation in the threshold voltage, leakage current and mobility. However, fewer studies have concentrated on the radiation effects on p-type poly-Si TFTs till now. In addition, the contributions of radiation-induced interface states and oxide-trapped charges to the degradation of electrical parameters have still not been discerned. There is room to investigate the degradation mechanism in the p-type poly-Si TFTs under radiation environments. This study gives an in-depth analysis of degradation induced by total ionizing dose (TID) radiation in the p-type poly-Si TFTs. The variations of threshold voltage, hole-field-effect mobility, sub-threshold swing, flat-band voltage and low-frequency noise (LFN) are extracted. Thus the evolution and mechanisms of radiation effects in the poly-Si TFTs are then discussed. The devices under test (DUTs) were p-type poly-Si TFTs fabricated by use of the excimer laser annealing (ELA) technique. The thickness of gate oxide was 50 nm while the thickness of the poly-Si film was 50 nm. The experiments were performed by using Co$^{60}$ source with a dose rate of 87.85 rad(Si)/s. The $I$–$V$ and $C$–$V$ characteristic measurements were carried out by the Keithley 4200. Low-frequency noises (LFN) were measured by use of an Agilent 35670 dynamic signal analyzer, while filters and amplifier units were provided by the Agilent E4725A. According to the reported worst case bias in the MOSFET,[8,9] radiation-induced holes in the gate oxide should drift to the Si/SiO$_2$ interface during irradiation. Thereby, p-type poly-Si TFTs are irradiated with OFF bias in this work. In the OFF bias, the gate was connected to the ground while drain and source were connected to $V_{\rm DD}$ ($-$8 V). The transfer characteristics of typical p-type poly-Si thin film transistors measured at drain voltage $V_{\rm ds}=-0.1$ V, before and after irradiation are shown in Fig. 1. By using the linear extrapolation method, the threshold voltage was extracted from the $I_{\rm DS}$–$V_{\rm GS}$ curves. As shown in Fig. 1, the negative shift of transfer characteristics has been observed after irradiation. The threshold voltage ($V_{\rm th}$) reduced and then on-current decreased with the increase of the total dose. Extracted variation of threshold voltage ($\Delta V_{\rm th}$) is presented in the inset of Fig. 1. The threshold voltage is reduced from $-$7.33 V to $-$11.52 V after 100 krad(Si) radiation. These results were primarily caused by the trapping of radiation-induced holes in the gate oxide. Electron–hole pairs are generated in the gate oxide during radiation, and some fraction will be trapped and formed positive oxide trapped charges. Thus the accumulation of positive trapped charges results in a negative shift of threshold voltage.[8-11]

Fig. 1. Transfer characteristics of poly-Si TFTs before and after irradiation.

Fig. 2. Variations of normalized hole-field-effect mobility and sub-threshold swing in the poly-Si TFT before and after irradiation.

Shifts of normalized hole-field-effect mobility ($\mu_{\rm eff}$) and sub-threshold swing (SS) are presented in Fig. 2. During irradiation, hydrogen ions (protons) are released as radiation induced holes 'hop' through the oxide or as they are trapped near the SiO$_2$/Si interface. The protons can also drift to the SiO$_2$/Si interface where they can remove hydrogen atoms from H-passivated dangling bonds (D) and form H$_2$, via the simple reaction[9,10] $$\begin{align} {\rm SiH}+{\rm H}^+ \Rightarrow {\rm D}^++{\rm H}_2.~~ \tag {1} \end{align} $$ Thus dangling bond defects (D$^+$) are formed located at the interface.[9,10] One of the principle effects of interface traps buildup is an increase in the sub-threshold swing of poly-silicon TFTs. The relationship between SS and the interface trap density ($D_{\rm it}$) can be calculated by[11,12] $$\begin{align} \Delta SS=2.3KT/q (1+q\Delta D_{\rm it}/C_{\rm ox}),~~ \tag {2} \end{align} $$ where $C_{\rm ox}$ is the gate oxide capacitance per unit area. With the increase of total dose, the amount of interface trap increases with $4.71\times10^{11}$ cm$^{-2}$eV$^{-1}$ after 100 krad(Si) radiation. The bulk doping concentration ($N_{\rm b}$) is about $10^{16}$ cm$^{-3}$ and thus the bulk potential ($\phi_{\rm f}=-KT/q \cdot \ln(N_{\rm b}/n_{\rm i})$) is about $-$0.35 eV. Similar to the midgap technique,[13] the extraction technique based on sub-threshold curves measures the number of radiation-induced interface traps from weak inversion condition to strong inversion condition. By considering grain boundary traps, surface potential in the poly-Si TFTs can be calculated based on 1D Poisson's equation.[14,15] To simplify the calculation, the surface potential $\phi_{\rm SI}$ corresponding to strong inversion is approximated to be 2$\phi_{\rm f}$ while the surface potential $\phi_{\rm WI}$ corresponding to weak inversion is approximated to be $\phi_{\rm f}$. Thus the band gap range ($\Delta \phi_{\rm p}$) over which the interface traps contribute to the shift of threshold voltage is about $\Delta \phi_{\rm p} \approx \phi_{\rm SI}-\phi_{\rm WI} \approx \phi_{\rm f}$.[16] Therefore, the variation of interface trap densities ($\Delta N_{\rm it}=\phi_{\rm f} \Delta D_{\rm it}$, per unit area) can be then estimated,[17,18] as graphed in Fig. 3.

Fig. 3. Radiation-induced interface states and oxide-trapped charge versus total dose.

In addition, interface traps can have a significant effect on carrier mobility and recombination rates of carriers at the surface. As shown in Fig. 2, with the buildup of interface traps, the mean free time of hole transports decreases and then results in the decrease of the hole-field-effect mobility. Similar to C-Si MOSFETs, the mobility degradation can be expressed as[19] $$\begin{align} \mu_{\rm eff}=\frac{\mu_0}{1+\eta \Delta N_{\rm it}},~~ \tag {3} \end{align} $$ where $\mu_0$ is the mobility prior to irradiation, $\eta$ is a fitting parameter which is about $6.56\times10^{-13}$ cm$^2$ in this work. The variation of threshold voltage results from the formation of interface traps and oxide trapped charges ($N_{\rm ox}$), which can be calculated by[18,20] $$\begin{align} \Delta V_{\rm th}=-|q\Delta N_{\rm ox}/C_{\rm ox} |- |q\Delta N_{\rm it}/C_{\rm ox}|.~~ \tag {4} \end{align} $$ Using Eq. (4), the contributions of radiation-induced oxide-trapped charges ($\Delta V_{\rm ox}$) and interface traps ($\Delta V_{\rm it}$) to threshold voltage shifts can be simply separated. Thus the variation of oxide-trapped charges ($\Delta N_{\rm ox}$) can then be estimated, as illustrated in Fig. 3. With the increase of total dose, the concentration of radiation-induced oxide-trapped charges is larger than the formation of radiation-induced interface traps, which indicates that radiation-induced oxide-trapped charges may dominate the degradation of p-type poly-Si TFTs under total dose radiation environments.

Fig. 4. The $C$–$V$ characteristics of poly-Si TFTs before and after irradiation.

The capacitance–voltage curves before and after irradiation at a fixed frequency (500 kHz) are presented in Fig. 4. Similar to the variation of transfer characteristics, negative shifts of $C$–$V$ curves are observed after irradiation. The capacitance at the flat band is[19] $$\begin{align} C_{\rm fb}=\Big(\frac{1}{C_{\rm ox}}+\frac{L_{\rm d}}{\epsilon_0 \epsilon_{\rm si}}\Big)^{-1}.~~ \tag {5} \end{align} $$ The Debye length $L_{\rm d}$ is defined as $$\begin{align} L_{\rm d}= \sqrt{\epsilon_0 \epsilon_{\rm si} KT/q^2N_{\rm b}}.~~ \tag {6} \end{align} $$ Using Eqs. (5) and (6), we can obtain $L_{\rm d}$ to be about 41.4 nm and $C_{\rm fb}$ of about $0.83C_{\rm ox}$. Therefore, the variation of flat-band voltage can then be extracted and presented in the inset of Fig. 4. The flat-band voltage is reduced from $-$6.4 V to $-$10.1 V, which is similar to the estimated value of $q\Delta N_{\rm ox}/C_{\rm ox}$ in Fig. 3. The $C$–$V$ measurement results validate the accuracy of extracted $\Delta N_{\rm ox}$ results from the $I$–$V$ curves. Low-frequency noises of poly-Si TFT before and after irradiation have been measured as graphed in Fig. 5. When p-type poly-Si TFTs were operated in the sub-threshold region, the flicker noise is not only determined by the tunnel-assisted charge exchange between the channel and defects in the near-interfacial SiO$_2$, but also affected by the carrier trapping/detrapping processes between the channel inversion carriers and the intragrain traps within the grain boundary depletion region.[21-23] Some studies proposed that these noise characteristics can be described by the carrier number with the correlated mobility fluctuation model.[22,23] For traps that are distributed uniformly in space (throughout the oxide) and in energy (in the silicon band gap), the fluctuating trapped charges produce a fluctuation of drain source current and effective gate voltage. Therefore, the normalized drain current spectral density can be expressed as[11,22,23] $$\begin{align} \frac{S_{\rm ID}}{I_{\rm DS}^2}=\Big(\frac{g_{\rm m}}{I_{\rm DS}}\Big)^2 S_{\rm vfb} =\Big(\frac{g_{\rm m}}{I_{\rm DS}}\Big)^2 \frac{q^2KT \lambda N_{\rm t}}{WLC_{\rm ox}^2f},~~ \tag {7} \end{align} $$ where $g_{\rm m}$ is the transconductance, $S_{\rm vfb}$ is the flatband spectral density, $N_{\rm t}$ is the number of traps per unit energy per unit area at the Fermi level $E_{\rm f}$ which includes traps in the grain boundaries, interface traps, oxide traps and border traps,[17,23] and $\lambda$ is the tunneling attenuation coefficient of the electron wave function in the gate oxide, which is about 0.1 nm for SiO$_2$.[23]

Fig. 5. Normalized drain current noise power spectral density ($S_{\rm ID}/I_{\rm DS}^2$) versus drain current ($I_{\rm DS}$) ($f=25$ Hz). The continuous lines are fit to (a) $S_{\rm vfb}=1.81\times10^{-8}$ V$^2$Hz$^{-1}$ (before radiation), and (b) $S_{\rm vfb}$=$2.01\times10^{-8}$ V$^2$Hz$^{-1}$ (after 100 K radiation).

According to Eq. (7), $S_{\rm vfb}$ and $N_{\rm t}$ can be extracted. The extracted $N_{\rm t}$ is about $1.81\times10^{20}$ cm$^{-3}$eV$^{-1}$ before radiation and about $2.01\times10^{20}$ cm$^{-3}$eV$^{-1}$ after radiation. The increase of the noise spectra after radiation indicates the increase of radiation-induced traps near the SiO$_2$/Si interface. Furthermore, the degradation of sub-threshold swing indicates that the generation of radiation-induced interface traps $\Delta N_{\rm it}$ may result in the increase of $N_{\rm t}(E_{\rm f})$ located toward mid-gap, which is similar to the reported variation of localized traps in the band gap of MOS and LTPS TFTs after stress.[24,25] The distribution of oxide trap density in space and energy near and below the valence band edge of silicon can also be calculated by using the $1/f$ noise.[18,26,27] As reported in Refs. [18,26,27], trapping and de-trapping of carriers tunneling from the inversion layer to the oxide traps near the interface cause $1/f$ noise, with each tunneling depth ($x$) corresponding to a specific time constant $\tau=1/2\pi f$. To have a qualitative spatial distribution of traps in the gate oxide, the frequency is converted to the tunneling depth as follows:[18,26,27] $$\begin{align} 1/2\pi f=\tau_0 \exp(x/\lambda),~~ \tag {8} \end{align} $$ where $\tau_0$ is the time constant at the interface, and $x$ is the distance into the gate dielectric from the interface. The value of $\tau_0$ is typically taken equal to $10^{-10}$ s for traps distributed up to 5 nm. Based on Eq. (8), the frequency is converted to the tunneling depth. Therefore, $S_{\rm ID}/I_{\rm DS}^2$ versus frequency curves can be transformed into the extracted trap density profiles for poly-Si TFT before and after radiation, as presented in Fig. 6. The results directly show that the density of traps filled with holes is increased after radiation, in agreement with the extracted results of $\Delta N_{\rm ox}$ from $I$–$V$ curves. In addition, the increase of oxide-trapped charges are located more in the region near Si/SiO$_2$ interface. This phenomenon is mainly determined by the direction of the vertical electrical field in the gate oxide during radiation.

Fig. 6. Extracted trap spatial distribution in the gate oxide of poly-Si TFT before and after radiation.

In summary, total dose effects in the poly-Si TFTs have been investigated. It is found that transfer characteristics and $C$–$V$ curves are shifted negatively after irradiation. These phenomena are caused by the creation of oxide-trapped charges and interface traps. Therefore, negative threshold voltage shifts, the decrease of hole-field-effect mobility, the increase of sub-threshold swing and low-frequency noise are observed. Furthermore, spatial distributions of oxide-trapped charges are extracted. References N-channel polysilicon thin film transistors as gamma-ray detectorsPerformance of in-pixel circuits for photon counting arrays (PCAs) based on polycrystalline silicon TFTsSilicon X-Ray Detector With Integrated Thin-Film Transistor for Biomedical ApplicationsEffects of gamma-ray irradiation on polycrystalline silicon thin-film transistorsEffects of x-ray irradiation on polycrystalline silicon, thin-film transistorsDegradation of polycrystalline silicon TFTs due to alpha particles irradiation stressPositive bias temperature instability of irradiated n-channel thin film transistorsTotal-Ionizing-Dose Effects in Modern CMOS TechnologiesRadiation Effects in MOS OxidesTotal Dose Ionizing Radiation Effects in the Indium–Zinc Oxide Thin-Film TransistorsGamma radiation effects on indium-zinc oxide thin-film transistorsSimple technique for separating the effects of interface traps and trapped-oxide charge in metal-oxide-semiconductor transistorsAn Explicit Analytical Solution to the Grain Boundary Barrier Height in Undoped Polycrystalline Semiconductor Thin-Film TransistorsSurface-Potential-Based Drain Current Model of Polysilicon TFTs With Gaussian and Exponential DOS DistributionA critical comparison of charge-pumping, dual-transistor, and midgap measurement techniques (MOS transistors)Estimating oxide-trap, interface-trap, and border-trap charge densities in metal-oxide-semiconductor transistorsLow frequency noise and radiation response in the partially depleted SOI MOSFETs with ion implanted buried oxideBorder traps: Issues for MOS radiation response and long-term reliabilityA New Model for the 1/f Noise of Polycrystalline Silicon Thin-Film TransistorsFlicker noise in gate overlapped polycrystalline silicon thin-film transistorsModel of low frequency noise in polycrystalline silicon thin-film transistorsShort Channel Effects on LTPS TFT DegradationEffects of Total Dose Irradiation on the Gate-Voltage Dependence of the $\hbox{1}/f$ Noise of nMOS and pMOS TransistorsA 1/f noise technique to extract the oxide trap density near the conduction band edge of siliconCharacterization of High-Current Stress-Induced Instability in Amorphous InGaZnO Thin-Film Transistors by Low-Frequency Noise Measurements

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