In–Ga–Zn oxide (IGZO) is a next-generation semiconductor material seen as an alternative to silicon. Despite the importance of the controllability of characteristics and the reliability of devices, defects in IGZO have not been fully understood. We investigated defects in IGZO thin films using electron spin resonance (ESR) spectroscopy. In as-sputtered IGZO thin films, we observed an ESR signal which had a g-value of g = 2.010, and the signal was found to disappear under thermal treatment. Annealing in a reductive atmosphere, such as N2 atmosphere, generated an ESR signal with g = 1.932 in IGZO thin films. The temperature dependence of the latter signal suggests that the signal is induced by delocalized unpaired electrons (i.e., conduction electrons). In fact, a comparison between the conductivity and ESR signal intensity revealed that the signal's intensity is related to the number of conduction electrons in the IGZO thin film. The signal's intensity did not increase with oxygen vacancy alone but also with increases in both oxygen vacancy and hydrogen concentration. In addition, first-principle calculation suggests that the conduction electrons in IGZO may be generated by defects that occur when hydrogen atoms are inserted into oxygen vacancies.
Formation of source and drain regions in a top-gate self-aligned field-effect transistor and its mechanism were examined. A stack structure of silicon nitride and an oxide semiconductor (OS) is effective, and hydrogen trapped in oxygen vacancies in the OS and the presence of metallic indium at the interface are important.
Author KeywordsTGSA; OS FET; short channel length; formation method of S-D regions; CAAC.
-We have reported that the transistors having the c-axis-aligned crystalline (CAAC) In-Ga-Zn oxide (IGZO) show good performance. Recently, In-Sn-Zn Oxide (ITZO) has attracted much attention because of its high electron mobility, as well as IGZO. However, it has been reported that ITZO field effect transistors (FET) tend to have positive Vth (normally-on characteristics) and poor reliability compared with IGZO-FETs. We have reported that high-performance and high-reliability OS-FETs can be fabricated by using CAAC-IGZO, which has high crystallinity and has no clear grain boundaries, as an active layer. Therefore, we have fabricated CAAC-ITZO thin films to improve performance of ITZO-FETs by using CAAC-ITZO as an active layer. In addition, FETs employing CAAC-ITZO have better characteristics and reliability than FETs using nano-crystal ITZO. Furthermore, constant photocurrent method (CPM) measurement was carried out in order to estimate density of deep-level defect states caused by oxygen vacancies in oxide semiconductors. The results show that CAAC-ITZO has lower density of deep-level defect states than nano-crystal ITZO. We attribute the improvement in reliability of ITZO-FETs to a decrease in deep-level defect states of an ITZO active layer, as is the case with IGZO.
We have reported that, in semiconductor devices based on In-Ga-Zn-O (IGZO), the crystallinity of IGZO is associated with the reliability of the devices. Therefore, evaluation and classification of the crystallinity is important. In this study, we evaluated nano-scale crystallinity of IGZO films deposited by various conditions, and found the relationship between nano-scale crystallinity and physical properties related to the reliability, so that we propose a new classification of the crystallinity of IGZO.
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