Fast and slow transient charging of Oxide Semiconductor Transistors

The comprehension of the governing mechanism which affects device instability is one of the most important requirements for the formation of reliable oxide-thin film transistors (TFTs). However, a quantitative analysis of the dominant mechanism of device instability, which stems from charge trapping induced by defects at the oxide semiconductor interface as well as in its bulk, has not yet been systematically performed. In this study, we examined subgap states, charge-transport dynamics, and various trap characteristics of oxide TFTs by multi-frequency C–V, pulse I–V, and transient current methods to achieve a comprehensive understanding of carrier transport and charge trapping mechanisms. We found that the charge trapping behavior of the tested amorphous InHfZnO (a-IHZO) TFT follows a multi-trapping mechanism, such as temperature-independent fast transient charge trapping by resonant drift of the injected electron and temperature-dependent slow transient charge trapping by charge transport from occupied to unoccupied traps. Understanding fast charging and slow charging described in this study can help to understand the root cause of device instability of oxide TFTs and ultimately improve stability and reliability characteristics.

In recent times, amorphous oxide semiconductor-based TFTs have been attracting enormous attention for display applications [1][2][3][4][5] , owing to their steep subthreshold slope (~0.2 V/decade), high field-effect mobility (5-100 cm 2 / eV·s), and low-temperature fabrication process [6][7][8] . High field-effect mobility enables fast switching, which is especially important for display technology 9,10 . In this research area, various amorphous-oxide semiconductor materials, such as InGaZnO, InHfZnO and InSnZnO have been studied to secure high stability and high reliability while maintaining high mobility [11][12][13][14][15][16][17] . However, the device instability and reliability issues of amorphous oxide semiconductor-based TFTs remain as potential issues faced by their production [18][19][20][21][22][23] . Interfacial and bulk defects in amorphous-oxide semiconductors result in significant charge trapping effects, leading to device instability and reliability issues. Therefore, an accurate understanding and precise management of defects in oxide semiconductors are required for the success of oxide TFTs [24][25][26][27][28][29] . The study of the fast and slow charge trapping were reported by many groups with various methods. Ramon et al., reported that the fast and slow charge trapping were investigated using the charge pumping technique 30 . U. Jung et al., suggested that the quantitative estimation of defects contributing to charge trapping using the discharge current analysis method 31 . These previous studies suggested that charge trapping is affected by both shallow (interface) and deep state (bulk) defects. Despite these studies, however, possible origins and information about locations of charge trapping remains unclear.
In our work, we investigated fast and slow transient charging behaviors in oxide TFTs and their effects on electrical characteristics. To this end, we employed a multi-frequency measurement (MFM) technique to evaluate the subgap density of states (DOS) 32 as well as pulse I-V (PIV) and transient current methods to study time-dependent charge trapping phenomena [33][34][35][36][37][38][39][40][41][42] . In addition, we consider a model for electron charging behavior of oxide TFTs utilizing measurements of transient current with temperature. This shows that transient charge trapping follows two different type processes such as a fast electron charging process where electrons are injected into shallow defects (fast transient charging) and thermally activated electron migration via trap-to-trap conduction (slow transient charging). The former is responsible for mobility degradation and V TH instability, while the latter is responsible for long-time stress V TH instability. As a result of this study, it was found that DOS through MFM measurement technique was exponentially distributed in the shallow energy state region, and fast & slow charge trapping occurred relatively shallow energy in range of less than 0.4 eV below the conduction band minimum through short and long pulse measurement technique. This model enables comprehension of the dynamic charge trapping behavior of oxide TFTs 43 .

Results
Inverse staggered TFTs with a-IHZO semiconductor and molybdenum metal electrodes were fabricated. Schematics of a fabricated oxide TFT and a high annular transmission electron microscopy image are presented in Fig. 1(a) and (b), respectively. The semiconductor material composition was verified by analyzing the energy dispersion spectrometry, shown in Fig. 1(c) and the structure was confirmed by a transmission diffraction pattern (see the inset of Fig. 1(c)). After the fabrication of the a-IHZO TFTs, in order to confirm the influence of the Hf content on the IHZO films, the transient current versus charging time curves (the charging time range is from 5 µs to 5 ms.) were measured with various Hf contents, as shown in Fig. 2(a). The charge trapping effect causes a reduction in the drain current with time. The threshold voltage shift (ΔV th ) was determined from the transient current versus charging time curves by using the equation, ΔV th = ΔI DS ·(V GS − V th )/I DS . Where ΔI DS is difference of the drain current during the pulse width (charging time), I DS is the maximum value of the drain current before the pulse width, V GS is the pulse amplitude, and V th is the threshold voltage. From this equation, distributions of ΔV th were plotted with respect to charging time, as shown in Fig. 2(b). Then, the critical charge trapping time (t c ) was determined. We found that t c increased with decreasing Hf content in the a-IHZO (t c values of 4, 7, and 10 mol% Hf content in the a-IHZO TFT are 1.98 μs, 858 ns, and 509 ns, respectively). Therefore, the Hf content in a-IHZO films obviously influences charge trapping mechanism. This experimental results showed that the incorporation of high Hf content in a-IHZO semiconductor increased the defect density, resulting in device instability.
For a comparative study of the effect of defects on the charge trapping phenomena of oxide TFTs, we employed two different gate insulators, Si 3 N 4 and SiO 2 , which significantly influence the interfacial and bulk quality owing to the strain effect, different atomic configuration, and hydrogen content. The left-hand graph of Fig. 3(a) shows the DC I-V and fast I-V (FIV) measurement data for the a-IHZO TFTs with Si 3 N 4 and SiO 2 . The drain current levels and subthreshold slopes (S.S. fast , SiO2 of 0.12 V/dec., S.S. DC , SiO2 of 0.18 V/dec., S.S. fast , Si3N4 of 0.13 V/dec., and S.S. DC , Si3N4 of 0.20 V/dec.) were determined by transfer curve. Those measured by the FIV technique were better than those measured by the DC I-V technique. The S.S. value can be changed to the total trap density (N tot ) using the following equation (1).  Where N total is the total trap density, kT is the thermal energy, q is the elementary charge and C ox is the gate insulator capacitance. The Ntot values of IHZO with SiO 2 (Fast I-V), IHZO with SiO 2 (DC I-V), IHZO with Si 3 N 4 (Fast I-V) and IHZO with Si 3 N 4 (DC I-V) TFTs are 4.37 × 10 10 , 8.72 × 10 10 , 5.1 × 10 10 and 1.02 × 10 11 cm −2 ev −1 , respectively. In addition, we added the values of subthreshold slope to the following sentence.
As presented in the right-hand graph of Fig. 3(a), the mobility values obtained by the FIV technique (μ fast , Si3N4 = 14.79 cm 2 /eV·s and μ fast , SiO2 = 12.1 cm 2 /eV·s) are, as expected, superior to those measured by the DC I-V technique (μ DC , Si3N4 = 11.42 cm 2 /eV·s and μ DC , SiO2 = 9.8 cm 2 /eV·s). The device in which Si 3 N 4 is used as the gate insulating film shows higher drain current than that with SiO 2 gate insulator. The use of a mixed gas of silane (SiH 4 ) and ammonia (NH 3 ) as a source gas during Si 3 N 4 deposition leads to have a high concentration of hydrogen in Si 3 N 4 , acting as donor to the oxide semiconductor, shifting the threshold voltage of TFT toward negative gate bias direction due to high carrier concentration. In accordance with the percolation model, mobility increases in proportion to the carrier concentration 44,45 . Certainly, as shown in Fig. 3(a), Si 3 N 4 devices exhibit high mobility. In agreement with earlier reports regarding metal oxide semiconductor field-effect transistors (MOSFETs), the DC I−V techniques used to extract various device parameters require comparatively long sweep/measurement times, such as a few seconds to tens of seconds, causing fast transient charge trapping and resulting in the underestimation of device performance [33][34][35][36][37][38][39][40][41][42] . Even for an a-IHZO TFT with SiO 2 gate insulator, we observed significant differences in the value of drain current and mobility value measured by the DC I−V and FIV techniques. Moreover, the ratio of the mobilities, μ fast :μ DC , is higher for the a-IHZO TFT with Si 3 N 4 gate insulator (29.5%) than for the a-IHZO TFT with SiO 2 (23.5%), as presented in Fig. 3(a), indicating that the Si 3 N 4 device is susceptible to fast charging. This was verified by the energy-dependent subgap DOS measured by MFM. The subgap DOS extraction of a-IHZO TFTs with two different gate insulators was begun by measuring the frequency-dependent capacitance−voltage (C−V) between source/drain electrodes and gate electrode, using an LCR meter 32 . Subsequently, we adopted the model (MFM) that was proposed by S. Lee and D. H. Kim and finally obtained a frequency-independent C−V and energy-dependent subgap DOS, as shown in Fig. 3(b) 32 . The acceptor-like DOS of the a-IHZO TFT with Si 3 N 4 gate insulator is higher than that of the a-IHZO TFT with SiO 2 gate insulator. Table 1 summarizes the parameters of gate-insulator-dependent subgap DOS from the model g( where N TA is the acceptor like tail state density, kT TA is the acceptor like tail state characteristic energy, N DA is the acceptor like deep state density, and kT DA is the acceptor like deep state characteristic energy. To further understand the defects and transient charging mechanism, the a-IHZO TFTs were subjected to a constant bias, short pulse, and long pulse stresses under measurement temperatures ranging from 25 to 175 °C 43 . To observe the threshold voltage (V TH ) shift, after a constant bias stress duration, the electrical stress was stopped and the DC I-V was measured 43 .
Consistent with previous studies, we confirmed that under low bias stress, electron trapping shows a reversible phenomenon by applying de-trapping (opposite polarity) voltage 43 . As shown in Fig. 4(a), the V TH value can be returned to the value before applying the stress voltage. Also, the dependence of the V TH value under the bias stress (stress voltage of 8 V) hardly changes with each stress cycle. In addition, subthreshold slope (S.S. SiO2 of 0.18 V/dec., and S.S. Si3N4 of 0.20 V/dec.) is almost not affected by the bias stress. Thus, we believe that electron trapping takes place in pre-existing defects. Additionally, the low bias stress applied to the device does not generate a considerable number of defects. Figure 4(a) shows an initial sharp increase in V TH for the first second of constant bias stress. To examine the initial charge trapping for the different quality of major charge transport   Fig. 4(b). A square wave pulse was applied to the gate electrode. The rise and fall times were both 10 µs and pulse width time was 2 ms. The gate voltage pulse was 8 V (Fig. 4(c)). Figure 4(c) displays the PIV (left-hand graph) as well as transient current (right-hand graph) data. During the positive-bias pulse width, the degradation of the source to drain current (ΔI DS ) results from the variation of threshold voltage (ΔV TH ) owing to the trapped charge in the front channel ΔI DS = (W/L)·C ox ·μ·ΔV T , where W and L are the TFT channel width and length, respectively, C ox is the gate oxide capacitance, and μ is the carrier mobility. According to the equation above, ΔV TH can be calculated from ΔI DS : where ΔI DS is the difference in the source to drain current between the end and start of the gate bias pulse, I DS is the maximum source to drain current prior to charge trapping, V GS is the gate voltage pulse amplitude, and V TH is the threshold voltage. The charge trapping in the pre-existing defects through several processes is described in Fig. 5. Electrons drift to the front channel and are charged in acceptor like defects [process P C in Fig. 5]. Then, it follows temperature-dependent electron transfer between the defects (process P T ). In oxide semiconductor channel, when acceptor-like defects are filled with electrons, they became electrically negative, which contributes to ΔV TH and ΔI DS . As will be discussed below, a noticeable feature of acceptor-like defects in the front channel that contribute to fast and slow transient charging is that their energy states are located in a relatively shallow energy range of <0.4 eV below the conduction band minimum. The charging process P C is expected to have a short charging characteristic time because of the very low trap energy and high DOS in the a-IHZO conduction band. Thus, the charging process P C is the major constituent of fast transient charging. Here, we concentrate   Fig. 4(a)) is comparable to the ΔV TH obtained by the PIV measurement method in the μs range, while during the subsequent bias stress of several seconds, ΔV TH gradually increases with stress time, contributing to the observed stress-time-dependent ΔV TH . This indicates that there are two charging processes with different charging characteristic time. We found that fast transient charging (P C ) occurs in the μs range and slow transient charging (temperature-dependent charging, P T ) starts to take place after a few seconds. Thus, fast transient charging (P C ) is significantly responsible for the initial ΔV TH (Fig. 4(a)). Different time scales, such as 10 −6 -10 −4 s (fast component) and 10-10 3 s (slow component), allow the categorization of fast and slow processes, as described in the model.

Fast transient charging.
To investigate fast and slow charging, we applied a simplified model proposed by G. Bersuker 43 . In this model, we assume that channel electrons drift to the defects and can be negatively charged when they occupy defects whose energies are in quasi-resonance. Following the assumption above, equation (3) shows the kinetics of fast charging. And the solution of this equation can be represented by Eq. (4):

O pt
Where, n is the density of the occupied traps, t is the time, p is the probability of a electron-trapping event, N 0 is the total density of available traps.
To evaluate the influence of temperature on fast transient charging, the transient current versus time characteristics of the a-IHZO TFTs with SiO 2 and Si 3 N 4 gate insulators were measured by applying a short pulse (100 µs) at 25 to 125 °C ( Fig. 6(a) and (b)). As such in previous study 46 , as seen in Figs 6(a) and 5(b), the source to drain current tends to increase with increasing measurement temperature. According to the model, more electrons escape from the localized state at relatively high measurement temperature 46 . On the other hand, ΔI DS for the initial 20 μs of the pulse width is almost the same regardless of measurement temperature, as seen in Fig. 6(c). A representative example of fitting Equation (4) to the measured source to drain current with time for Si 3 N 4 and SiO 2 gate-insulator-stacked a-IHZO devices is presented in Fig. 6(c), where the fitting was performed to ΔI DS for the initial 20 μs of the pulse width in Fig. 6(a) and (b), which is much shorter than the time it takes for the ΔI DS to saturate. We should notice that time dependence of the drain current does not depend on temperature, representing that fast charging is not a temperature-activated process. The obtained N 0 is of the order of 10 13 cm −2 (Si 3 N 4 gate insulator) and 10 12 cm −2 (SiO 2 gate insulator). Coulomb repulsion, which prevents charge trapping at most available defect sites, acts between trapped charges, the final density seems to be low.

Slow transient charging.
It has been demonstrated in previous studies that slow transient charging is as effectively de-trapped as fast transient charging, under the same detrapping voltage 47,48 . This shows that traps with fast and slow transient charging have similar detrapping kinetics, suggesting that the traps of the two types of charging have similar physical characteristics. The model we apply and study begins with the assumption that the fast and slow transient charging occur in the same traps 43 .
In accordance with the slow transient charging model, slow transient charging might be attributed to the capture of secondary electrons, which stem from the traps, charged by the fast drift process (P C ) (Fig. 5) 43 . Thus, when some trapped electrons are activated from the traps by thermal energy and are hopping over the conduction band minima, they can be re-trapped in the surrounding empty traps before these electrons gain sufficient kinetic energy. This is the trap-conduction band-trap process (P T ) (Fig. 5).
The fast (microsecond scale) and slow (second scale) transient charging have a very large difference in charging characteristic time scale, so a fast drift charging process occurs immediately in the traps where thermally activated detrapping occurs and this continuously provides activated electrons to continue the slow transient charging.
where N is the trap density, N S is the density of the secondary traps positioned close the filled traps, and p s (i) = σ s J s (i), where σ s is the capture cross-section for the electrons, and J s expressed in Eq. (6) is defined by electrons that are thermally activated and emitted from the traps (Fig. 5).
where J is the current density, n is the density of the occupied fast traps, τ is the de-trapping time constant, E i is the trap energy, k is the Boltzmann constant, and T is the temperature. The transient current was measured by applying a long pulse stress (pulse width: 1000 sec.) to the a-IHZO TFTs with SiO 2 and Si 3 N 4 gate insulators at a temperature from 25 to 125 °C to verify the temperature dependent slow transient charging model described above. Representatively, the measured data for the a-IHZO TFT with SiO 2 device is shown in Fig. 7(a). The results of fitting the ΔV TH values for the whole temperature range using Eq. (5) are shown in Fig. 7(b). The fitting was repeatedly carried out by increasing the terms one by one, starting with a single term in the sum of Eq. (5). Fitting the Eq. (5) in Fig. 7(b) requires two terms, which implies that slow charging occurs in more than one type of defects. The fitting of equation (6) for all temperature data requires two terms, i = 1, 2. The fitting process to obtain parameters p i (T) and N s was repeated for various stress temperatures (25,75,125, and 175 °C) (Fig. 5(b)). The measured ΔI DS data of 25 °C and the theoretical line of equation (5) are plotted with different p i (Fig. 7(c)). Among various fitting values, the measurement data can best be described as p 1 is 0.01 and p 2 is 0.0005. The slope of [ln(p i ) vs 1/kT] presents the energy barriers height E i for the detrapping electrons influencing on the charge flux J s (i) (i = 1, 2) in Eq. (6): E 1 = 0.17 eV, E 2 = 0.23 eV (Si 3 N 4 gate insulator), and E 1 = 0.18 eV, E 2 = 0.27 eV (SiO 2 gate insulator) (Fig. 7(d)). The available trap density (N S ) which can recapture the secondary electrons, are of the order of 10 9 cm −2 (Si 3 N 4 gate insulator) and 10 8 cm −2 (SiO 2 gate insulator), implying that most of the secondary electrons from the initial traps are not re-trapping. The N 0 and N s values of the a-IHZO TFT with Si 3 N 4 gate insulator are each approximately one order of magnitude higher than those of the a-IHZO TFT with SiO 2 gate insulator.

Discussion
In this study, we proposed a quantitative analysis of the dominant mechanism of device instability stemming from the charge trapping behavior and defect characteristics of oxide TFTs. To systematically analyze the charge trapping mechanism, we probed the subgap DOS by MFM measurements, as well as examining fast/slow transient charging and trap energy by short pulse stress and long pulse stress. These analyses provide a foundation for an instability model of oxide TFTs.
The obtained results show that the a-IHZO TFT with Si 3 N 4 gate insulator is vulnerable to fast/slow charge trapping, causing significant device instability, high acceptor-like defect density, and shallow trap energy compared with those of the a-IHZO TFT with SiO 2 gate insulator. We believe that this method will help to understand the defects of oxide semiconductor transistor and will guide the direction of defect control.

Methods
Device Fabrication. Molybdenum metal used as a gate/source/drain electrode was deposited by DC sputtering. For the preparation of both the Si 3 N 4 and SiO 2 gate insulators, we used plasma-enhanced chemical vapor deposition (PECVD) in the gas chemistry of SiH 4 /NH 3 and SiH 4 /N 2 O, respectively. Then, a-IHZO oxide semiconductor films of thickness 40 nm were deposited by means of a radio-frequency plasma sputtering method. The IHZO targets were composed of a mixture of HfO 2 , In 2 O 3 , and ZnO powders. For example, the IHZO (7 mol% -Hf content) target was composed of HfO 2 : In 2 O 3 : ZnO = 0.07: 1: 1. The a-IHZO (7 mol% -Hf content) film composition was verified by analyzing the energy dispersion spectrometry (see the Fig. 1(c)). For the comparison purpose of effect of IHZO semiconductor regarding Hf contents, the Hf content of the IHZO targets were prepared in contents of 4, 7, and 10 mol%. After the formation of the a-IHZO layer, the etch stopper layer was formed of SiO 2 using PECVD. Subsequently, the TFT devices were passivated with PECVD SiO 2 followed by contact etching.
Device Characterization. The DC I-V measurements were performed using a semiconductor parameter analyzer (Agilent 4156 C), and the fast I-V (FIV) and pulse I-V (PIV) measurements were carried out using a pulse-generation unit (Agilent B1104A) and a digital oscilloscope (Agilent MSO6052A). For the DC I-V measurements, the voltage sweep rate was 1 V/s, whereas for the FIV and PIV measurements, the voltage scan rate was 1 V/µs. The multi-frequency measurements (MFM) were performed using an LCR meter (Agilent 4284 A).