Emerging Thin Film Structured Devices for Post-CMOS Technology

Abstract

2-Dimensional Electron Gas(2DEG) at the III-V semiconductors interface based on GaAs, GaN heterostructure has been spotlighted because of a high carrier mobility and quantum confinement effect at the interface. Electrons were confined along the direction perpendicular to the interface which became quantized and only discrete level of energy was possible, while the electrons can move freely parallel to the interface direction, denoting the high conductivity characteristics. However, despite having excellent electrical properties, its application is limited because of single crystal sapphire substrate and multiple epitaxial layers which have severe restriction for compatibility to the state-of-the-art silicon technology. Since then, it has been reported that highly conductive 2DEG characteristics can be implemented even at the interface between oxides with perovskite structures. In particular, 2DEG at the LaAlO3/SrTiO3 (LAO/STO) heterostructure interface using single crystal STO substrate and epitaxial LAO top layer shows high conductivity based on the high carrier density of interface (1013~1014 /cm2), where 100 times higher than that of typical Si semiconductors. So far, 2DEG at oxide heterostructure was realized through ‘polar catastrophe’ mechanism at the interface of single crystalline (or epitaxial) oxide heterostructure. Since, the sharp interface between single crystal materials is essential for this mechanism, it is still difficult to apply it to a conventional semiconductor process. Recently, it was demonstrated that the formation of 2DEG was viable using non-stoichiometric amorphous LAO or Al2O3 overlayer on the single crystal STO substrate. Since, there are no the sharp interface or charge discontinuity for ‘polar catastrophe’ mechanism, another 2DEG formation mechanism was introduced based on oxygen vacancies(V¬O). The VO-based mechanism explains that the VO generated at the surface of the bottom layer during the heterostructure deposition process forming the high density of free electrons at the interface. However, the single crystalline substrate was still required for the creation of 2DEG, which has been unsolved issue yet. This research describes the first implementation of highly conductive 2DEG at the non-single crystal binary oxide ultra-thin heterostructure using atomic layer deposition process (ALD). This new channel material system can overcome the structural and process limitations of conventional Si-based semiconductors and can be easily applied to conventional semiconductor processes, since ALD is a mature process in the present semiconductor industry enabling an extraordinarily uniform and conformal growth of functional thin films with atomic level accuracy. In this results, by applying the VO-based 2DEG formation mechanism, a 2DEG formation mechanism based on the surface reduction reaction of strong reducing precursors during ALD processes was proposed and verified through various analysis of physical/chemical properties. First, an Al2O3/TiO2 (3/15 nm) heterostructure was formed using ALD on SiO2 substrate. As the result of analyzing the interfacial sheet resistance of the Al2O3/TiO2 heterostructure by Hall measurement as the function of Al2O3 overlayer thickness, the typical sheet resistance changing behavior of the oxide heterostructure 2DEG system was confirmed. The electrical properties are comparable to those from the epitaxial LAO/single-crystal STO heterostructures 2DEG. The chemical analysis (XPS, Angle Resolved XPS (ARXPS) and STEM, EELS) indicated consistent results that a large amount of VO ware generated on the TiO2 surface during ALD of Al2O3 overlayer. The estimated thickness of confined 2DEG was ~2.2 nm within TiO2 side from the Al2O3/TiO2 interface. Through the above analysis results, a new ALD-based 2DEG formation mechanism is proposed; the bottom layer surface is reduced by strong reducing precursor (Trimethylaluminium, TMA) injected during the Al2O3 ALD process and a large amount of VO are generated at the interface of oxide heterostructure, causes confinement of high density of free electrons. i.e. 2DEG. To verify the proposed 2DEG formation mechanism, a special in-situ ALD chamber was designed for direct observation of sheet resistance of heterostructure interface during the ALD of Al2O3 over-layer. By analyzing the correlation between crystallinity and the activation energy of the VO, the optimized ALD process conditions for 2DEG formation were suggested. Furthermore, using the proposed Al2O3/TiO2 heterostructure as a channel, we implemented a normally-on type field effect transistor (FETs) on a 4-inch SiO2 wafer. Fabricated 2DEG-FET shows excellent electrical properties with a high drain current (Ion > ~ 11.6 μA/μm), high on/off-current ratio (Ion/Ioff >108), and low SS of ~100 mV/dec. which outperforms TFTs using single crystalline LAO/STO heterostructures reported thus far. With the oxide heterostructure using ultrathin bottom layer (<10 nm), low Ioff can be achieved due to facile full-depletion of ultrathin bottom layer while maintaining high Ion through 2DEG channel (thereby high Ion/Ioff ratio) which cannot be realized using thick single crystalline substrates such as STO. Details of Al2O3/TiO2 2DEG are given in chapter 2. By applying the proposed ALD-based 2DEG formation mechanism above, ZnO was introduced into the bottom layer of the heterostructure. Similar to TiO2, ZnO has semiconductor characteristics based on VO while improving 2DEG characteristics based on high intrinsic mobility. It was confirmed that the 2DEG layer was formed at Al2O3/ZnO (3/5 nm) heterostructure by showing the sheet resistance behavior of the typical 2DEG system, which is the same as the single crystal or TiO2 2DEG system. Since, the sheet carrier density was similar to those of the previous 2DEG systems, mobility was about three times higher mobility than TiO2-2DEG denoting lower sheet resistance. In addition, 2DEG could be implemented at a much lower temperature and heterostructure thickness due to the high re-oxidation resistance of and VO in ZnO. However, it is widely known that a highly conductive thin film can be easily formed through doping with external elements with ZnO. To verify that high conductivity of Al2O3/ZnO heterostructure interface was not due to the doping of the overlayer element, an oxide layer of Al, Hf, and Ti, which is known to have high conductivity characteristics through doping with ZnO, was applied to the overlayer of a heterostructure. Except TiO2 overlayer, the sheet resistance of Al2O3 and HfO2 overlayered heterostructure shows typical characteristics of 2DEG system. In addition, the sheet resistance decreased in proportion to the reducing power of the precursor used for the deposition of each overlayer, which is expected tendency for the proposed ALD-based 2DEG formation mechanism. It was also confirmed that VO was present on the surface of the ZnO in the result of chemical analysis through XPS and STEM/EELS. Through the in-situ sheet resistance analysis during Al2O3 ALD process, the change of sheet resistance with the injection of the reducing precursor and the oxidizing reactant gas was directly observed, respectively to verify the ALD-based 2DEG formation mechanism. The role of the overlayer is and the origin of the critical thickness of overlayer were also analyzed. It was confirmed that the role of the overlayer was an oxidation barrier to prevent re-oxidation of VO, and the critical thickness shown in the ex-situ analysis was the minimum thickness which could act as an effective oxygen barrier. By applying the Al2O3/ZnO heterojunction to the channel of FETs, normally-off type 2DEG-FETs implemented which can be the basis of low power devices. Compared to TiO2-2DEG FETs, ZnO-2DEG FETs shows much improved electrical properties for mobility (8 cm2/V·s), leakage current (~109 μA/μm), subthreshold swing (~90 mV/dec.) and hysteresis characteristics. Details of ZnO-based 2DEG system are given in chapter 3.