Pentacene thin-film transistors with HfO2 gate dielectric annealed in NH3 or N2O. Deng, LF; Tang, WM; Leung, CH; Lai, PT; Xu, JP; Che, CM

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1 Title Pentacene thin-film transistors with HfO gate ielectric anneale in NH3 or NO Author(s) Deng, LF; Tang, WM; Leung, CH; Lai, PT; Xu, JP; Che, CM Citation The 008 IEEE International Conference on Electron Devices an Soli-State Circuits (EDSSC), Hong Kong, China, 8-0 December 008. In Conference Proceeings, 008, p. -4 Issue Date 008 URL Rights IEEE Conference on Electron Devices an Soli-State Circuits. Copyright IEEE.

2 Pentacene Thin-Film Transistors with HfO Gate Dielectric Anneale in NH 3 or N O 0BL. F. Deng, W. M. Tang, C. H. Leung, P. T. Lai*, J. P. Xu, an C. M. Che Abstract Pentacene-base Organic Thin-Film Transistor (OTFT) with HfO as gate ielectric is stuie in this work. The HfO ielectric was prepare by RF sputtering at room temperature, an subsequently anneale in N O or NH 3 at 00 o C. The OTFTs were characterize by IV measurement an /f noise measurement. The OTFTs show small threshol voltage an can operate at as low as 3 V. Results inicate that the OTFT anneale in NH 3 shows higher carrier mobility, larger on/off current ratio, smaller sub-threshol swing an smaller Hooge parameter than the OTFT anneale in N O. Therefore, NH 3 -anneale HfO is a promising gate ielectric for the fabrication of high-performance OTFTs. ecisively etermine by the characteristics of the interface between the gate ielectric an organic semiconuctor. In orer to realize high-quality interface, one common metho is to passivate the gate-ielectric surface by nitriation before the evaporation of organic layer. In this work, HfO was prepare by sputtering metho an then anneale in N O or NH 3 respectively to form the gate ielectric of pentacene OTFTs. The IV characteristics of the evices were measure, an use to euce the fiel-effect carrier mobility, threshol voltage, sub-threshol swing an on/off current ratio. Moreover, /f noise was measure to evaluate the interface quality of the OTFTs. I. INTRODUCTION Organic thin-film transistors (OTFT) have got wie attention from researchers ue to their avantages such as low cost an flexible substrate. They are consiere as goo caniates for applications like electronic paper, RFID tags, smart cars an large-size flat-panel isplay [], []. OTFTs with pentacene as active layer an SiO as gate ielectric have achieve a performance which is comparable to amorphous Si thin-film transistors [3]. However, OTFTs with SiO as gate ielectric often work at high operating voltage, which is commonly greater than 5 or 0 V. High operating voltage usually leas to high power consumption an causes inconvenience to the evelopment of portable equipment. One way to ecrease the operating voltage is to reuce the thickness of the gate ielectric [4]. However, too thin gate ielectric will cause large gate leakage. The other way to ecrease the operating voltage is to replace the low-k SiO with high-k ielectric [5], [6]. Hafnium (Hf) -base oxies (e.g. HfO, HfON) are being actively investigate to act as the gate ielectric of inorganic transistors because of their better interface quality with the semiconuctor [7], [8] as well as higher ielectric constant. The carrier transport in the OTFTs is L.F. Deng, W.M. Tang, C.H. Leung an P. T. Lai are with the Electrical an Electronic Department, the University of Hong Kong, Hong Kong. J.P. Xu is with Department of Electronic Science & Technology, Huazhong University of Science an Technology, Wuhan. C.M. Che is with the Department of Chemistry, the University of Hong Kong * Hlaip@eee.hku.hk II. EXPERIMENTAL DETAILS Fig. shows the cross-sectional view of the OTFT to be fabricate as follows. N-type <00> silicon wafers with a resistivity of 0. ~ 0.5 cm were cleane accoring to the stanar RCA metho. Then 5% hyrofluoric aci was use to remove the native oxie of the wafers. Subsequently the wafers were sputtere with a layer of HfO as gate ielectric at room temperature. The sputterer was Denton Vacuum LLC Discovery 635. Before the sputtering, the vacuum in the chamber was kept below 0-6 Torr. The sputterer worke on RF (raio frequency) moe an the power was 30 W. The material of the target was HfO. Argon flowe in the reactive chamber at a rate of 4 sccm. In orer to improve the interfacial characteristics, the samples were ivie into two groups, each anneale in N O an NH 3 respectively with a gas flow rate of 000 ml/min. The annealing process laste for 0 minutes at a temperature of 00 o C. Hyrofluoric aci with 0% concentration was use to remove the back oxie of the silicon substrate which woul be use as the gate electroe. Then pentacene (from Alrich) was eposite on the ielectric in an Ewars Auto 306 evaporator. The substrate was not heate an the vacuum in the chamber was 40-6 Torr. The eposition rate was. nm/min an the final thickness of the pentacene film was 30 nm, which was etecte by a quartz-crystal oscillator. Finally gol was evaporate on the pentacene layer through a shaow mask to form rain an source electroes. The channel length L an with W are 30 m an 00 m respectively. Before the gol /08/$ IEEE

3 evaporation, the vacuum in the chamber was about 80-6 Torr. L I WC ox V g (3) Fig.. Cross-sectional view of top-contact OTFT After calculation, the carrier mobility is cm /Vs for the N O-anneale evice an cm /Vs for the NH 3 -anneale one. By extrapolating the I f( Vg) curve to the X-axis, the threshol voltage can be calculate. Besies, sub-threshol swing is also of great significance to analyze the switching characteristics of OTFTs. By plotting the transfer characteristics in the semi-logarithmic representation as in the inset of Fig. 4, the sub-threshol swing (SS) is foun to be 0.36 V/ec an 0.9 V/ec for the N O-anneale an NH 3 -anneale OTFTs respectively. The evices were characterize by HP445B Semiconuctor Parameter Analyzer, Berkeley Technology Associates FET Noise Analyzer Moel 9603 an HP 35665A Dynamic Signal Analyzer to measure the I-V curve an /f noise characteristics of the organic thin-film transistors. The measurements were taken by a probe station in the ambient atmosphere. SS Log( I ) V g (4) III. RESULTS AND DISCUSSION Fig. an Fig. 3 reveal the output characteristics of the OTFTs with HfO as the gate ielectric anneale in N O an NH 3 respectively. The OTFTs can work at very low operating voltage (less than 3.0 V), which is much lower than that of OTFT with SiO as gate ielectric. The gate leakage is low when the former works at low rain-source voltage. Fig. 4 isplays the comparison of the transfer characteristic of the two transistors. The on/off current ratio is about for the former an.50 4 for the latter. Carrier mobility an threshol voltage V th are two of the most important parameters to evaluate the performance of OTFTs. To euce their values, the transfer characteristics curve of the OTFTs is plotte as I f( Vg), as shown in Fig. 4. Accoring to the stanar formula when OTFT operates in the saturation regime we can get W I C V V L ox( g th) () W I Cox( Vg Vth) L () where W an L are the with an the length of the channel respectively; Cox is the oxie capacitance per unit area. From (), I(A) Vg=-3V Vg=-.5V Vg=-.5V Vg=-0.75V V(V) Fig.. Output characteristic of the OTFT anneale in N O I(A) Vg=-3V Vg=-.5V Vg=-.5V Vg=-0.75V V(V) Fig. 3. Output characteristic of the OTFT anneale in NH 3

4 (-I) / ((A) / ) NO Anneale -I(A) E-3 E Vg(V) NO Anneale Vg(V) V=-3V Fig. 4. Transfer characteristic of the OTFTs anneale in NO or NH3 (Inset shows the I-Vg curve in semi-logarithmic scale) Si(A /Hz) E-7 E-8 E-9 E-0 E- E- E-3 V=-3V NO Anneale Logf(Hz) Fig. 5. /f noise characteristics of the OTFTs anneale in N O or NH 3 To further stuy the interface quality of the organic thin-film transistors, their /f noise spectrum was measure. The /f noise was teste in the frequency (f) range of 3.5 Hz an.6 khz. Fig. 5 shows the comparison of the /f noise for the two transistors. The noise level of the transistor anneale in the NH 3 ambient is about orers lower than that of its counterpart anneale in the N O ambient. It reveals that the evice gain better interfacial quality through the NH 3 annealing than the N O annealing. In orer to analyze the /f noise characteristic further, the Hooge parameter is extracte accoring to the Hooge s empirical formula for the /f noise [9] S ( f) i I (5) Nf where S i is spectral ensity of the /f noise of rain current I ; an N is the total number of carriers in the channel. For the case of organic thin-film transistors, (5) can be approximate as [0], fsi ( f) L (6) evsi where e is the electron charge. In this work, f 30Hz is use for the calculation of. In the case of transistor which experience the N O annealing at 00 o C, the Hooge parameter is 50.. By contrast, the Hooge parameter is 0.74 for the transistor anneale in NH 3 at 00 o C. This means that the current fluctuation ue to the /f noise in the former is more severe than the latter. In terms of sub-threshol swing, the transistor anneale in NH 3 is 0.9, which is better than 0.36 of the transistor anneale in N O. As a result, organic thin-film transistor anneale in NH 3 shows better switching characteristics than its counterpart anneale in N O. Besies, the former has higher carrier mobility than the latter. All these avantages of the transistor anneale in NH 3 over its counterpart anneale in N O can be attribute to its more nitrogen incorporation at the ielectric surface, an hence better interface quality between the gate ielectric an organic semiconuctor. In other wors, higher carrier mobility means weaker trap-relate scattering for the carriers. Smaller /f noise an sub-threshol swing mean less traps in the channel / at the interface. On the other han, the transistor anneale in NH 3 has a larger threshol voltage than the transistor anneale in N O, possibly because more nitrogen incorporation results in more positive oxie charges in the former. More quantitative analysis lies in better unerstaning of the carrier transport mechanism in the interface between the organic layer an the high-k ielectric layer of the organic thin-film transistors. TABLE I Device parameters of the OTFTs anneale in N O or NH 3 Annealing gas N O NH 3 C ( / ) ox F cm t ( ) ox nm. 0.9 k ( cm / Vs) V ( ) th V SS (V/ecae) on/off ratio (0 4 ) I ( A) V Vg 3 V IV. CONCLUSION The application of HfO as gate ielectric to pentacene thin-film transistors is stuie in this paper. OTFTs with HfO as gate ielectric were realize by RF sputtering an subsequent annealing in N O or NH 3 at 00 o C. Both can operate uner a supply voltage of as low as 3 V. The OTFT anneale in NH 3 isplays higher carrier mobility, larger

5 on/off current ratio, smaller Hooge parameter an smaller sub-threshol swing than its counterpart anneale in N O. All these avantages make the organic thin-film transistor suitable in low-voltage an low-power applications. To conclue, HfO anneale in NH 3 is very promising to act as the gate ielectric of high-performance OTFTs. ACKNOWLEDGEMENT This work is supporte by the RGC of HKSAR, China (Project No. HKU 733/07E), the URC for See Fun for Strategic Research Theme of HKU on Molecular Materials, an the University Development Fun (Nanotechnology Research Institute, ) of the University of Hong Kong. REFERENCES [] C. D. Dimitrakopoulos, an P. R. L. Malenfant, Organic thin film transistors for large area electronics, Av. Mater., vol.4, p.99, 00. [] G. Horowitz, Oganic fiel-effect transistors, Av. Mater., vol.0, p.365, 998. [3] S. F. Nelson, Y. Y. Lin, D. J. Guniach an T. N. Jackson, Temperature-inepenent transport in high-mobility pentacene transistors, Appl. Phys. Lett., vol.7, p.854, 998. [4] L. A. Majewski, R. Schroeer, M. Grell, Organic fiel-effect transistors with ultrathin gate insulator, Synth. Met. Vol.44, p.97, 004. [5] J. Tary, M. Erouel, A.L. Deman, A. Gagnaire, V. Teoorescu, M.G. Blanchin, B. Canut, A. Barau an M. Zaharescu, Organic thin film transistors with HfO high-k gate ielectric grown by anoic oxiation or eposite by sol-gel, Microelectronics Reliability, vol.47, p.37, 007. [6] C. D. Dimitrakopoulos, S. Purushothaman, J. Kymissis, A. Callegari an J. M. Shaw, Low-voltage organic transistors on plastic comprising high-ielectric constant gate insulators, Science, vol.83, p.8, 999. [7] T. J. Park, S. K. Kim, an J. H. Kim, Electrical properties of high-k HfO films on Si-xGex Substrates, Microelectronic Engineering, vol.80, p., 005. [8] N. Wu, Q. C. Zhang, C.X. Zhu, D. S. H. Chan, M. F. Li, N. Balasubramanian, Albert Chin an Dim-Lee Kwong, Alternative surface passivation on germanium for metal-oxie-semiconuctor applications with high-k gate ielectric, Appl. Phys. Lett., vol.85, p.47, 004. [9] F. N. Hooge, /f noise source, IEEE TRANSACTIONS ON ELECTRON DEVICES. Vol.4, p.96, 994. [0] M. J. Deen, O. Marinov, J. F. Yu, S. Holcroft, an W. Woos, Low-Frequency Noise in Polymer Transistors, IEEE TRANSACTIONS ON ELECTRON DEVICES, vol.48, p.688,00.