Temperature stress on a thin film transistor with a novel BaZnSnO semiconductor using a solution process

Abstract

We have fabricated novel BaZnSnO-TFT using a solution process and investigated the electrical performance and temperature stability. BaZnSnO-TFT shows an improved field-effect mobility of 3.2 cm² V⁻¹ s⁻¹, a subthreshold swing of 0.61 V per decade and an on/off current ratio of 2 × 10⁷ compared to those of ZnSnO-TFT. Density of state distribution of BaZnSnO and ZnSnO semiconductor has been extracted from electrical measurements. BaZnSnO-TFT shows an improved electrical performance and temperature stability due to smaller oxygen vacancies, less bulk trap density and interface state density.

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1. Introduction

Recently, thin film transistors (TFTs) based on oxide semiconductors have been reported as an alternative to amorphous silicon (a-Si) TFTs due to high mobility, high stability and transparency in visible light region.^1,2 Many deposition methods such as radio frequency magnetron sputtering, pulse laser deposition, and solution processes, have been reported to prepare oxide thin films. Solution-processed oxide thin films as a channel layer of TFTs are paid more attention due to simple, low cost, large area uniformity and various compositions of oxide thin films.

For realization of high performance of oxide TFTs, In-based semiconductors such as InZnO, InGaZnO, and HfInZnO, have been frequently reported.^3,4 In³⁺ is favorable to achieve high mobility due to its special electron configuration of 4d¹⁰5s⁰.⁵ However, a rare-earth indium element is expensive, toxic, and unstable in hydrogen plasma, which will enhance the fabrication cost of transistor. Sn-based oxide semiconductor has become a competitive alternative for indium because Sn⁴⁺ has a similar electron configuration with In³⁺ and amorphous ZnSnO-TFTs have achieved high performances comparable with InGaZnO-TFTs. In order to reduce oxygen vacancies of ZnSnO film, Si, Zr, Al element have been doped into ZnSnO film to suppress oxygen vacancies.^6–8 To find more effective carrier suppressors, we have considered three important factors consisting of low standard electrode potential (SEP), larger bandgap, and low electro-negativity. For this reason, Ba is also an attractive material for carrier suppressors due to its low SEP (−2.90 V), wide bandgap (4.8 eV) and lower electro-negativity (0.89). Here, solution-processed BaZnSnO film as an active layer of TFTs was firstly reported.

The field effect mobility of oxide TFTs is acceptable for next-generation flat panel displays. Thin film transistors are affected by ambient temperature and self-heating in the work process. Thus, the device reliability including temperature instability has already limited the development of oxide TFTs into commercial electron electronic products. For commercial application, precise knowledge of the mechanism of temperature instability is necessary. Density of states (DOS) including the interfacial trap density and bulk trap density has a strong influence on temperature instability. In this work, we have fabricated a novel BaZnSnO-TFT and firstly studied the temperature instability. DOS distribution of BaZnSnO semiconductor has been extracted from electrical measurements. It provides a quantitative analysis that correlates the temperature instability of BaZnSnO-TFT to DOS parameters.

2. Experimental details

BaZnSnO thin film was prepared using zinc acetate dehydrate (Zn(CH₃COO)₂·2H₂O), tin chloride dehydrate (SnCl₂·2H₂O) and barium nitrate (Ba(NO₃)₂). These precursors were dissolved in 2-methoxyethanol. A 0.1 M monoethanolamine (MEA) was then added in the precursor solution as a sol–gel stabilizer. The concentration of metal precursors was 0.25 M and the molar ratio was 1 : 2 : 0.15 for Zn : Sn : Ba. The solution was stirred at 50 °C for 3 h.

A bottom-gate-type TFT was fabricated. Highly doped n-type silicon wafer with 0.01–0.025 Ω cm was used as substrate and gate electrode. A 300 nm thick SiO₂ used as a gate dielectric was fabricated by thermal oxidation on the high-doped n-type silicon wafer. Prior to processing, the wafer was cleaned with a standard wet-cleaning procedure. The BaZnSnO film were prepared by spin-coating at 3000 rpm for 30 s and prebaked at 200 °C for 20 min. Subsequently, the wafer was annealed at 500 °C for 80 min in ambient air. Finally, a 60 nm thick Al film was sequentially vacuum deposited onto active layer using a metal mask to define transistor with channel width W = 1000 μm and channel length L = 200 μm.

The current–voltage characteristics of the devices were measured using an Agilent 4155C semiconductor analyzer. The capacitance measurements were conducted with an Agilent E4980A Precision LCR meter. The thickness of thin film was determined by a surface profiler (Alpha-Step IQ). X-ray diffraction (XRD) scans were measured with a Rigaku D\Max-2200 X-ray diffractometer using Cu Kα radiation. Optical properties of thin films were measured using a UV-vis spectrophotometer. The chemical bonding states of films was carried out with the X-ray photoelectron spectroscopy (XPS) (Thermo-ESCALAB 250Xl). All measurements were carried out at room temperature.

3. Results and discussion

Fig. 1 (a) shows the X-ray diffraction pattern of the ZnSnO and BaZnSnO (0.15) film prepared on SiO₂/Si substrate and annealed at 500 °C for 80 min. The ZnSnO film and BaZnSnO film show amorphous phase with no prominent peak. The amorphous oxide thin films have more uniform structures and smoother surfaces which could yield smooth channel/gate insulator interface. The optical transmittance spectra of ZnSnO and BaZnSnO (0.15) film on the glass substrate were characterized as shown in Fig. 1(b). The average transmittances in the range of 400–700 nm are over 87% for both films. The energy bandgap (E_g) can be obtained by using the following equation:

(αhγ)² = β(hγ − E_g) (1)

where h is Planck's constant, γ is the frequency of incident photon, and β is energy independent constant. E_g can be obtained by extrapolating the linear portion of the (αhγ)² and hγ plots to the energy axis, as shown in the inset of Fig. 1(b). The band gap of BaZnSnO (0.15) film is obtained to be 3.51 eV. The bandgap of undoped ZnSnO film is 3.40 eV. The broadening of band gap of BaZnSnO leads to a reduction in the electron charge carrier and an increase in the activation energy. On the other hand, the broadening of band gap of BaZnSnO film makes the climb of carrier from the valence band to the conduction band more difficult.^9,10

Fig. 1 (a) The X-ray diffraction pattern of the ZnSnO and BaZnSnO (0.15) film. (b) The optical transmittance spectra of ZnSnO and BaZnSnO (0.15) film on the glass substrate.

Fig. 2 shows O1s peaks of XPS spectra of ZnSnO and BaZnSnO (0.15) film. The O1s region of the XPS spectrum can be divided into three regions: low binding energy (O_I, 530.7 eV), media binding energy (O_II, 531.5 eV) and high binding energy (O_III, 532.5 eV). The O_I peak indicates a metal–oxygen lattice peak without oxygen deficiency. Similarly, the O_II and O_III are related to the O²⁻ ions located in the oxygen deficient region in the film and bonded oxygen such as H₂O, O₂ on the film surface or inside thin films, respectively. The change of V_o concentration can be depicted by the peak area variation of O_II component. The integral areas of O_I, O_II and O_III can be obtained by fitting the XPS spectra of O1s. S₁, S₂, and S₃ represent the integral areas of O_I, O_II and O_III peak, respectively. S_tot is the sum of the integral areas of O_I, O_II and O_III peak. Interestingly, S₂/S_tot value of BaZnSnO film is about 25.9%, which is smaller than that of the ZnSnO film (34.2%). It indicates that Ba acted as a carrier suppressor in the ZnSnO film. Ba ions are easily to form stronger chemical bonds with oxygen than Zn and Sn ions, which can attributed to the relatively high bond strength of Ba–O (562 kJ mol⁻¹) comparing with Zn–O (395 kJ mol⁻¹) and Sn–O (531.8 kJ mol⁻¹). Thus, Ba can effectively suppress the formation of oxygen vacancies and lower oxygen vacancies are favorable to improve stability of thin film transistor.

Fig. 2 O1s peaks of XPS spectra of ZnSnO and BaZnSnO film.

Fig. 3(a) and (b) shows the output characteristics of the BaZnSnO (0.15)-TFT tested at room temperature, respectively. The BaZnSnO-TFT shows a normally off type n-channel enhanced-mode behavior. No current crowding is observed from Fig. 3(a) indicating good ohmic contacts between BaZnSnO channel layer and Al source/drain electrode. Fig. 3(b) shows the electrical hysteresis of source-to-drain current (I_DS) as a function of V_GS with the fixed drain voltage (V_DS) at 20 V for BaZnSnO (0.15)-TFT. The V_GS is swept by 1 V increments along a forward sweep from −20 V to 30 V and along a reverse sweep from 30 V to −20 V. The clockwise hysteresis suggests that negative charge carriers are trapped at the interface between the channel and the gate insulator or injected into the gate insulator from the channel.¹¹ The electrical hysteresis of BaZnSnO (0.15)-TFT is obviously better than that of ZnSnO-TFT. The results clearly indicate the poor stability of ZnSnO-TFTs, which could be attributed to charge trapping at the bulk and the ZnSnO/SiO₂ interface. BaZnSnO (0.15)-TFT shows no obvious hysteresis, indicating that less negative charge carriers are trapped at the interface between the channel and the gate insulator or injected into the gate insulator. The result is also verified by the smaller subthreshold swing. The mobility of the BaZnSnO-TFT in the saturation region was extracted from the following equation:

where I_DS is the drain–source current, W is the width of channel, L is the length of channel, C_i is the capacitance per unit area of the insulator layer (C_i = 10.0 nF cm⁻²), V_GS is the gate voltage, μ is the saturation mobility and V_th is the threshold voltage. The V_th of the devices was determined by extrapolating the I_DS^1/2 vs. V_GS plot to I_DS = 0. In addition, the subthreshold swing (SS) is calculated from the subthreshold region using the following equationSS is given by the maximum slope in the transfer curve. The BaZnSnO-TFT shows a μ, V_th, I_on/off, and SS of 3.2 cm² V⁻¹ s⁻¹, 7.1 V, 2 × 10⁷, and 0.61 V per decade, respectively. In addition, the maximum interfacial traps density (N_it) can be estimated from the following equation:where k is Boltzmann constant, q is the elementary electron charge and T is the absolution temperature. The N_it,max value of BaZnSnO-TFT is estimated to be 5.7 × 10¹¹ cm⁻², which is comparable with the published value for sputtered InGaZnO-TFT fabricated on a SiO₂-based substrate.^12,13 The low N_itmeans that BaZnSnO-TFT has a low charge trapping at the BaZnSnO/SiO₂ interface. For comparison, ZnSnO-TFT is fabricated and electrical performance is investigated, as shown in Fig. 3(b).

Fig. 3 The (a) output and (b) the electrical hysteresis of the BaZnSnO (0.15)-TFT tested at room temperature. the effect of (c) Ba ratio and (d) annealing temperature on the electrical performance of BaZnSnO-TFT.

The effect of Ba ratio and annealing temperature on the electrical performance of BaZnSnO-TFT is investigated. The electrical performance of BaZnSnO-TFT at a V_DS of 20 V for various Ba content levels is shown in Fig. 3(c). The 15 mol% Ba-doped ZnSnO-TFT has the highest saturation mobility value of 3.2 cm² V⁻¹ s⁻¹. When the Ba content is 25 mol%, the saturation mobility decreases from 3.2 to 0.86 cm² V⁻¹ s⁻¹. As the Ba content increases, the threshold voltage increases monotonically from −2.5 to 15.6 V. The result is attributed to the excess Ba addition results in the lower carrier concentration and the decreased conductivity of the channel layer. The saturation mobility and the subthreshold swing as a function of the annealing temperature are shown in Fig. 3(d) when the annealing temperature is increased to 300 to 600 °C, the saturation mobility increases from 0.2 to 3.4 cm² V⁻¹ s⁻¹. The result is due to the increase in the number of native defects especially oxygen vacancies with the annealing temperature increasing. Unfortunately, the excess oxygen deficiencies induced by high temperature will lead to a high SS value. It is easily seen that BaZnSnO (0.15)-TFT shows a superior electrical performance. BaZnSnO (0.15)-TFT shows an improved field-effect mobility of 3.2 cm² V⁻¹ s⁻¹, a subthreshold swing of 0.61 V per decade and an on/off current ratio of 2 × 10⁷. It is concluded that Ba doping can suppress oxygen vacancies and reduce the interface trap state, and thus improve the device characteristics. In short, the BaZnSnO (0.15)-TFT exhibits an acceptable electrical performance for next-generation flat panel displays.

Fig. 4 shows the evolution of the transfer characteristic of ZnSnO-TFT and BaZnSnO-TFT as a function of the temperature. For both devices, the transfer curves are negatively shifted with increasing temperature. As the temperature increases from 293 K to 353 K, V_th of ZnSnO-TFT is shifted by approximately −13.6 V from −2.5 to −16.1 V. However, the corresponding negative V_th shift in BaZnSnO-TFT is relatively small. V_th of BaZnSnO-TFT is shifted by approximately −4.7 V from 7.1 to 2.4 V for BaZnSnO-TFT. BaZnSnO-TFT shows a smaller V_th shift than that of ZnSnO-TFT. At higher temperatures, more electrons can escape from the localized states and contribute to the free carriers, which cause a smaller threshold voltage. The negative V_th shift with increasing temperature may be related to the shift of the Fermi level E_F. The shift of E_F can be expressed by the variation of the conduction activation energy (E_a). E_a is the energy difference between E_F and the edge of the conduction band (E_C), which equates to the average energy that a trapped electron escapes from the localized state. In case of general thin film transistors, the temperature dependence of the V_th shift is closely related with the increase in thermally active electrons. It is necessary to investigate the impact of thermally activated process on the change in free electrons. In generally, the generated free electrons in oxide semiconductors are mainly attributed to oxygen vacancies.¹⁴ It is reported that thermally activated electrons are exponentially increased with increasing temperature.¹⁵ The drain current (I_DS) is thermally activated and can be described by:^16,17

I_DS = I_DS0 exp(−E_a/kT) (5)

where I_DS0 is the prefactor, k is the Boltzmann constant, T is the temperature, E_a can be estimated by fitting of temperature-dependent log(I_DS) versus 1/T curve as shown in Fig. 5. It is assumed that thermally activated electron from deep level trap sites into the conduction band move quickly toward the drain electrode under a electrical field. Thus, the rate-limiting process would involve thermal excitation of the trapped charge, and E_a can be extracted as a function of V_GS in the forbidden bandgap (as shown in Fig. 6(a)). The maximum E_a value is obtained to be 1.14 eV at a V_GS of −12 V. For BaZnSnO-TFT, the maximum E_a value is about 1.09 eV at a V_GS of 0 V, which is similar with the reported value by H. Godo et al.¹⁸ E_a value decreases with increasing V_GS. The decreasing rate of E_a are estimated to be 0.23 eV V⁻¹ for BaZnSnO-TFT and 0.11 eV V⁻¹ for ZnSnO-TFT, indicating that BaZnSnO semiconductor shows a fast moving E_F level of with respective to V_GS. The rate of change in E_F level of with respective to V_GS is inversely proportional to the magnitude of total trap density (N_tot) including the density of state (DOS) of bulk semiconductor film and interface trap density between active layer and SiO₂ film. The result suggests that BaZnSnO-TFT exhibits a relatively low bulk and interface trap density.

Fig. 4 The evolution of the transfer characteristic of ZnSnO-TFT and BaZnSnO (0.15)-TFT as a function of the temperature.

Fig. 5 Relationship between log(I_DS) and 1/T as a function of V_GS for (a) ZnSnO-TFT and (b) BaZnSnO (0.15)-TFT.

Fig. 6 (a) The relation between V_GS and E_a. (b) Calculated DOS distribution as a function of E_a.

The density of states (DOS) of the BaZnSnO-TFT and ZnSnO-TFT can be extracted from the relationship between E_a and V_GS by assuming the current in the active layer distributed uniformly in several hundred angstroms of active layer. The DOS can be calculated using the following equation.¹⁹

where ε_i and d_i are respectively the permittivity and the thickness of the gate dielectric, t is the thickness of the active layer, and q is the electron charge. Fig. 6(b) shows the relationship between density of state and activation energy of BaZnSnO-TFT and ZnSnO-TFT. For BaZnSnO-TFT, the magnitude of density decreases from 8.9 × 10¹⁸ to 2.8 × 10¹⁷ cm⁻³ as the activation energy increases. The calculated DOS distribution is comparable to the reported solution-processed HfInZnO-TFT.²⁰ The total DOS for BaZnSnO-TFT is obviously smaller than that for ZnSnO-TFT at a specific energy level. The total DOS value at a specific energy level is the summation of bulk trap density and interface state density. Compared with ZnSnO-TFT, BaZnSnO TFTs have a lower density of trap states in the subgap DOS, which leads to superior electrical performance and better temperature stability. Thus, BaZnSnO film could be a useful active layer for high performance thin film transistors.

4. Conclusions

In summary, we have fabricated BaZnSnO-TFT by solution process. Compared to the ZnSnO-TFT, BaZnSnO (0.15)-TFT exhibits a better electrical performance and temperature stability. Extraction of the DOS of transistor using temperature-dependent field effect measurement is used to understand the electrical performance and stability. It is attributed that much smaller oxygen vacancies and DOS distribution compared to the ZnSnO-TFT.

Acknowledgements

The authors would like to acknowledge the financial support given by the Natural Science Foundation of China (51302165, 61274082, 61077013), Shanghai Municipal Education Commission (ZZSD13047), Innovation Fund of shanghai University and China Postdoctoral Science Special Fund (2012T50387).

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