The Influence of Hafnium Doping on Density of States in Zinc Oxide Thin-Film Transistors Deposited via Atomic Layer Deposition
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Thin-film transistors (TFTs) with atomic layer deposition (ALD) HfZnO (HZO) as channel layer and Al2O3 as gate insulator were successfully fabricated. Compared with ZnO-TFT, the stability of HZO-TFT was obviously improved as Hf doping can suppress the generation of oxygen related defects. The transfer characteristics of TFTs at different temperatures were also investigated, and temperature stability enhancement was observed for the TFT with Hf doping. The density of states (DOS) was calculated based on the experimentally obtained E a, which can explain the experimental observation. A high-field effect mobility of 9.4 cm2/Vs, a suitable turn-on voltage of 0.26 V, a high on/off ratio of over 107 and a steep sub-threshold swing of 0.3 V/decade were obtained in HZO-TFT. The results showed that temperature stability enhancement in HfZnO thin-film transistors are attributed to the smaller DOS.
KeywordsAtomic Layer Deposition Channel Layer Threshold Voltage Shift Interface Trap Density Atomic Layer Deposition Method
ZnO-based thin-film transistors (TFT) have recently attracted a great deal of attention owing to their potential application in active matrix liquate crystal display (AMLCD) and active matrix organic light emitting diodes (AMOLED) [1, 2, 3]. As an important part of TFTs, channel layers play a crucial role in TFT performance. ZnO-based single channel layer or double channel layer, such as HZO , IGZO , and IZO/IGZO , has been investigated for use in TFTs because of their high mobility. Among those ZnO-based TFTs, amorphous indium-gallium-tin oxide (IGZO) TFTs are regarded as the most promising devices as indium and gallium have excellent lattice matching with ZnO and gallium could suppress carrier generation via oxygen vacancy formation in IGZO. Recently, LG Display has released a 55 in, full high-definition (FHD) OLED television which utilizes an IGZO TFT active matrix . However, materials such as indium (In) and gallium (Ga) have some disadvantages, including toxicity, element scarcity, and indium extraction in hydrogen plasma . Specifically, the instability of ZnO-based TFTs is still a problem hard to solve, as ZnO-based TFTs usually contain defects in the active channel layer and deep-level defects in the channel/insulator interface . To obtain highly stable Zn-based oxide TFTs, many studies have been devoted to the development of robust active layers by the incorporation of metal cations (such as Ga, Al, Hf, and Si) based on sputtering method or spin-coating method [10, 11, 12, 13, 14], with the expectation to reduce the density of defects such as oxygen vacancies. There are few reports on adopting ALD method to realize doping in Zn-based oxide TFTs. As it is known, ALD offers various advantages including accurate thickness and composition control, excellent uniformity and step coverage, low defect density, low deposition temperatures, and good reproducibility [15, 16, 17]. Furthermore, due to the self-limiting and layer-by-layer (or “digital”) growth, ALD exhibits the unique in situ atomic layer doping capability of achieving precise doping with atomic level control during the deposition process.
Up to now, to our limited knowledge, the deposition of channel layers using ALD has been focused on almost pure ZnO layers. Given its low standard electrode potential, Hf is a more efficient suppressor of the generation of oxygen vacancies. Adding Hf element could suppress the growth of columnar structure, and therefore drastically decrease the carrier concentration and hall mobility in HZO films [18, 19, 20]. Hence, in this paper, Hf doped ZnO has been deposited via ALD and applied in TFTs as channel layer. The related parameters such as the Hf contents, growth temperature, channel thickness, and width/length ratio (W = L) were evaluated and optimized. The influence of Hf doping on performance of ZnO-TFT was studied. Especially, the detailed effect of Hf doping on the DOS has never been addressed. In the present letter, the DOS was calculated based on the experimentally obtained E a, which can explain the experimental observation.
Results and Discussion
Comparison of the electrical properties of the devices
V on (V)
where V 0 = V GS−V th,0, V th,0 is the threshold voltage at the start of the stress measurement, β is the stretched exponential exponent, and τ reflects the characteristics of carrier trapping time. The obtained τ, β values are 1.58 × 106 s, 0.39 for ZnO-TFT, and 2.32 × 106 s, 0.56 for HZO-TFT, respectively. It demonstrates that the degradation of HZO-TFT is slower than that of ZnO-TFT under a long-time operation.
In summary, we have successfully fabricated HZO-TFT via ALD, and the effect of Hf-doped ZnO on the stability of device was studied. Significantly improved on/off ratio and temperature stress stability were obtained in the HZO-TFT due to the reduced DOS, to be specific, a high field-effect mobility of 9.4 cm2/Vs, a suitable turn-on voltage of 0.26 V, a high on/off ratio of over 107, a steep sub-threshold swing of 0.3 V/decade. The proposed HZO-TFT in this paper can act as driving devices in the next-generation flat panel displays.
This work is supported by the National Key Basic Research Program of China (2015CB655005), Science and Technology Commission of Shanghai Municipality Program (14DZ228090), Project of National Post-Doctor Fund (2015M580315) and National Natural Science Foundation of China (61077013, 61274082, 51072111, and 51302165).
XD carried out the experiments and drafted the manuscript. XD and JZ participated in the design of the study and performed the analysis. CQ, JS participated in the measurements. XJ conceived the study and participated in its design. JZ and ZZ supervised the overall study and polished the manuscript. All authors read and approved the final manuscript
The authors declare that they have no competing interests.
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