酸化物半導体MgIn2O4を用いた新規な電界効果型トランジス タと置換ドーピングの効果 Novel Thin Film Transistors with Oxide Semiconductor MgIn2O4 with and without Substitutional Doping 平野 由希子* 曽根 雄司* 松本 真二* 安部 美樹子* 中村 有希* Yukiko HIRANO Yuji SONE Shinji MATSUMOTO Mikiko ABE Yuki NAKAMURA 植田 尚之 Naoyuki UEDA * 山田 勝幸 * Katsuyuki YAMADA 要 旨 酸化物半導体MgIn2O4を活性層に用いた新規な電界効果型トランジスタを作製した．更に，活 性層に対する置換ドーピングの効果を検証し，活性層中のMgをAlで置換することによってキャリ アの生成を効果的に制御できることを見出した．活性層にAlをドープしたTFTでは，ノーマリー オフの特性と高い移動度が両立する．また，従来の酸化物半導体TFTでは，半導体成膜時の酸素 量に依存してTFT特性が敏感に変化することが問題となっているが，ドーピングによるキャリア 制御を行うことで酸素量依存性が緩和され，広いプロセスマージンで高性能なTFTの作製が可能 となることを示した． ABSTRACT Novel thin film transisters (TFT) with oxide semiconductor MgIn2O4 as an active layer were presented. We succeeded in controlling carrier generation in the active layer by substituting Al on Mg site. The doped TFT operated in normally-off mode with relatively high field-effect mobility. Furthermore, the characteristics of the doped TFT were less sensitive to the oxygen concentration during sputtering of the active layer. The doping enlarged the process margin and stabilized the TFT characteristics at high levels. * 研究開発本部 先端技術研究センター Advanced Technology R&D Center, Reseach and Development Group Ricoh Technical Report No.37 38 DECEMBER, 2011 high mobility. In addition, a process window may be too 1. Background narrow to achieve a normally-off characteristic. In recent years, liquid crystal displays (LCD), organic The control of the carrier generation is the key to electro-luminescent (EL) displays, electronic paper, and achieve high performance TFTs. Here we present a novel the like have been made into practical use as flat panel TFT with MgIn2O4 (IMO) as an active layer, and a highly displays (FPDs). efficient method of controlling carrier generation: n-type substitutional doping. FPDs are driven by a driver circuit including a thin film transistor (TFT) having an active layer of amorphous silicon (a-Si) or polycrystalline silicon. These 2. Experimental Details and Results a-Si TFTs and polycrystalline silicon TFTs (particularly low temperature poly-crystalline silicon (LTPS) TFTs) 2-1 have advantages and disadvantages. For example, A Novel IMO-TFT although a-Si TFTs have good uniformity, they have For a channel material of TFT, we chose a compound disadvantages of insufficient mobility for driving a large MgIn2O4 (IMO) that was previously reported as a screen LCD at a high speed, and a large shift of a transparent conducting oxide.2) The IMO crystal has a threshold voltage in continuous driving. Although LTPS- cubic inverse-spinel structure in which the two TFTs have high mobility, they have a disadvantage in tetrahedral sites in the unit cell are occupied by In3+ and that threshold voltages largely vary due to a process for four octahedral sites are randomly occupied by Mg2+ and crystallizing an active layer by annealing using an In3+. A one-dimensional chain (a rutile chain) of edge- excimer laser. sharing InO6 and MgO6 octahedrons runs in various Thus, there is a demand for a novel TFT technology three-dimensional directions, and an InO4 tetrahedron having combined advantages of a-Si TFT and LTPS-TFT. functions to connect the rutile chains. Since bottom of To satisfy these demands, a TFT with an oxide the conduction band is constituted by an isotropic 5s semiconductor, in which higher carrier mobility than orbital of indium and lies at the Γ point, moving amorphous silicon (a-Si) is expected, has been actively directions of the electrons are isotropic. The transporting developed. characteristics of the carriers do not depend on the Specifically, after Nomura et al. disclosed a TFT using orientation of the film. Therefore, there is no amorphous InGaZnO4 (a-IGZO) that is capable of being disadvantage caused by the anisotropic property of the deposited at room temperature and exhibits higher crystal structure, such as the case of ZnO with a wurtz 1) carrier mobility than a-Si, numerous studies on type crystal structure. amorphous oxide semiconductors having high carrier VDS mobility have been extensively carried out. In such amorphous oxide semiconductors, carrier VGS G electrons are generated by oxygen vacancy. Thus, IMO SiO2 Al Al doped Si oxygen concentration in a deposition process needs to be rigorously controlled. The characteristics of TFTs Fig.1 Schematic illustration of IMO-TFT. with the amorphous oxide semiconductor may easily result in a depletion mode when attempting to achieve Ricoh Technical Report No.37 39 DECEMBER, 2011 For the TFT fabrication, we used a bottom gate and The typical output and transfer characteristics of IMO- top contact configuration [see Fig.1] where the heavily TFT are shown in Fig.2. The IMO-TFT showed good doped Si served as the gate and the thermally oxidized characteristics with a field-effect mobility of 10.2 cm2/Vs, SiO2 layer (200 nm thick) served as the gate insulator. an on-off current ratio over 108 and S-value of 0.11 V/dec. IMO was deposited to form an active layer with a 2-2 thickness of 55 nm by DC magnetron sputtering via a Carrier Generation by Substitutional Doping metal mask using a MgIn2O4 sintered body target (Mitsui Mining & Smelting CO., LTD). An argon gas and an In order to control carrier generation in the active oxygen gas were introduced as a sputtering gas. The layer, we implemented the n-type substitutional doping. total pressure was fixed at 1.1 Pa, and the oxygen The carrier electrons are supposed to be generated when concentration was 2.0%. The first annealing was an n-type substituting cation having a larger valence is performed in air for one hour at 400°C to increase introduced into a substituted cation site. Here we chose crystallinity. Next, Al was deposited by vacuum Al as a dopant. The substitution of Al on Mg site forms a evaporation via a metal mask to form source and drain donor. electrodes with thicknesses of 100 nm. A channel length For the doping to work effectively, a host material was 50 μm, and a channel width was 400 μm. Finally, the needs to have a rigid structure (such as a spinel second annealing was carried out in air at 300°C for an structure) and at least a short-range order of the crystal hour in order to improve adhesiveness and electric has to be maintained. In view of this aspect, IMO is a contact in an interface between the source and drain particularly preferable host material. electrodes and the active layer. The TFTs discussed in this section were fabricated by 0.5 0.4 IDS (mA) the process similar to the one stated in section 2-1. The VGS : 0-28V, step 4V first annealing was not performed, so the highest temperature in the fabrication process was 300°C. The 0.3 oxygen concentration during the sputtering process was 0.2 2.0%. For Al-doped IMO-TFT, a Mg0.99Al0.01In2O4 sintered 0.1 body target (Mitsui Mining & Smelting CO., LTD) was 0 0 10 20 used when sputtering an active layer. 30 In order to confirm the carrier generation by doping, IDS (A) VDS (V) 0.001 transfer characteristics are compared in Fig.3 for the 10-5 non-doped IMO-TFT and Al-doped TFT whose active layers were sputtered at the same oxygen concentration. 10-7 It is well known that a TFT with higher carrier 10-9 concentration has smaller Von, the turn-on voltage at 10-11 10-13 -20 Fig.2 which the drain current starts to increase in the transfer VDS=20V -10 0 10 VGS (V) Typical (a) output and characteristics of IMO-TFT. Ricoh Technical Report No.37 20 curve. It is apparent from Fig.3 that the Al-doped active layer contained more carriers than the non-doped active 30 layer did. Since the number of carriers generated by (b) oxygen vacancy was assumed to be equivalent in the transfer active layers sputtered at the same oxygen concentration, 40 DECEMBER, 2011 the excess carriers in the doped layer must be originated 2-3 from the substitution of Al on Mg site. Improved IDS (A) Al(1%)-doped IMO-TFT 10-9 the process similar to the one stated in section 2-1. The first annealing was not performed, so the highest temperature in the fabrication process was 300°C. The non-doped IMO-TFT 10-11 Fig.3 by The TFTs discussed in this section were fabricated by 10-5 10-13 -20 Characteristics Substitutional Doping 0.001 10-7 TFT oxygen concentration during the sputtering process was VDS=20V -10 0 VGS (V) 10 varied as a parameter. The oxygen concentration dependence of the transfer 20 characteristics of non-doped IMO-TFTs and Al-doped IMO-TFTs are shown in Fig.5 and Fig.6, respectively. A Transfer curves of non-doped IMO-TFT and Al(1%)-doped IMO-TFT. The active layers of both TFTs were sputtered at the oxygen concentration of 2.0%. relationship between the oxygen concentration and the field-effect mobility is illustrated in Fig.7. 0.001 For comparison, transfer curves of non-doped a-IGZO- 10-5 IDS (A) TFT and Sn-doped a-IGZO-TFT fabricated by the similar process as IMO-TFT are shown in Fig.4. On the contrary to the IMO case, doped IGZO-TFT had larger Von meaning that the doping did not generate carriers. In non-doped IMO-TFT 10-7 oxygen concentration 10-9 1.2% 4.4% 8.0% 10-11 cases of highly amorphous compounds such as IGZO, 10-13 -20 doping induces the local structural change and a stable -10 local structure may be formed. Thus, carriers can not be generated effectively. In this case of IGZO, the affinity for Fig.5 oxygen of Sn resulted in larger Von compared to the nondoped TFT. 0 VGS (V) 10 20 (VDS=20V) Transfer curves of non-doped IMO-TFTs. The active layers were sputtered at the oxygen concentration of 1.2%, 4.4% and 8.0%. 0.001 non-doped IGZO-TFT 0.001 10-5 10-7 Sn(1%)-doped IGZO-TFT 10-9 10-11 IDS (A) IDS (A) 10-5 VDS=20V Fig.4 -10 0 VGS (V) 10 20 oxygen concentration 10-9 10-13 -20 Transfer curves of non-doped IGZO-TFT and Sn(1%)-doped IGZO-TFT. The active layers of both TFTs were sputtered at the oxygen concentration of 2.0%. Ricoh Technical Report No.37 10-7 2.8% 5.2% 6.0% 8.0% 10-11 10-13 -20 Al(1%)-doped IMO-TFT Fig.6 41 -10 0 VGS (V) 10 20 (VDS=20V) Transfer curves of Al(1%)-doped IMO-TFTs. The active layers were sputtered at the oxygen concentration of 2.8%, 5.2%, 6.0% and 8.0%. DECEMBER, 2011 Field effect mobility (cm2/Vs) 8 characteristics of high field-effect mobility and normally- Al(1%)-doped IMO-TFT 7 off operation can be achieved in the wider range of 6 oxygen concentration. The precise oxygen amount 5 control is no longer necessary. non-doped IMO-TFT 4 3. Conclusion 3 2 0 2 4 6 8 The novel IMO-TFT with a field-effect mobility of 10.2 10 cm2/Vs and an on-off current ratio over 108 was oxygen concentration (%) Fig.7 presented. Since IMO has a cubic spinel structure and Relationship between oxygen concentration during sputtering and field effect mobility of non-doped and Al(1%)-doped TFTs. isotropy in the bottom of the conduction band, IMOTFTs may have minimum characteristic variations and can be used in a large-size active matrix panel. For non-doped IMO-TFTs, the transfer characteristics Furthermore, we introduced the n-type substituional strongly depended on the oxygen concentration. When doping to control carrier generation in the IMO active the oxygen concentration increased, the oxygen vacancy layer. The Al-doped IMO-TFTs showed improved in the active layer decreased, and so did the carrier characteristics of high field-effect mobility and normally- concentration. Accordingly Von increased and the off operation in a wider process range compared to the normally-off operation was achieved with the oxygen non-doped TFT. concentration of 4.4% and higher. However, the fieldeffect mobility deteriorated for higher Other methods of controlling the carrier generation in oxygen oxides reported previously such as doping (not concentration as shown in Fig.7. In order to obtain a non- substitutional)3, doped TFT with desired characteristics such as result in a reduction of carrier mobility. With the normally-off substitutional operation and maximum field-effect 4) and composition change5) tend to doping method, we succeeded in mobility, precise oxygen flow control during sputtering controlling the carrier generation without any such process is indispensable. drawbacks. The substitutional doping is essential to enlarge the process margin and stabilize the TFT The Al-doped IMO-TFT showed excellent normally-off 2 operation with a field-effect mobility of 6.0 cm /Vs and characteristics at high levels. 8 on-off current ratio over 10 at the oxygen concentration of 5.2%. Further, this excellent transfer characteristic References was oxygen 1) K. Nomura et al.: Room-temperature fabrication of concentration. The Von scarcely shifted. The field-effect transparent flexible thin-film transistors using mobility amorphous oxide semiconductors, Nature, Vol.432, relatively was maintained almost for constant higher in the oxygen concentration range of 1.6 to 6.0%, and slightly (2004), pp.488-492. decreased at 8% and more. 2) N. Ueda et al.: New oxide phase with wide band gap and high electroconductivity, MgIn2O4, Appl. Phys. The characteristics of the doped-TFTs are less Lett., Vol.61, (1992), pp.1954-1955. sensitive to the oxygen concentration, because the carriers are generated by Al substitution regardless of the oxygen concentration. Consequently, the excellent Ricoh Technical Report No.37 42 DECEMBER, 2011 3) D.-H. Cho et al.: Al and Sn-doped zinc indium oxide thin film transistors for AMOLED back-plane, SID Symposium Digest, Vol.40, (2009), pp.280-283. 4) C.-J. Kim et al.: Amorphous hafnium-indium-zinc oxide semiconductor thin film transistors, Appl. Phys. Lett., Vol.95, (2009), pp.252103-252105. 5) K. Nomura et al.: Amorphous oxide semiconductors for high-performance flexible thin-film transistors, Jpn J. Appl. Phys., Vol.45, (2006), pp.4303-4308. Ricoh Technical Report No.37 43 DECEMBER, 2011