Material characteristics and applications of transparent amorphous oxide semiconductors

Abstract

A morphous semiconductors have created a new area of electronics known as 'giant microelectronics' typifi ed by devices such as solar cells and active-matrix (AM) fl at-panel displays. Single-crystalline semiconductor technology, typifi ed by crystal silicon electronics, is unsuitable for such applications, whereas amorphous or polycrystalline fi lms can be easily formed over large areas of greater than 1 m 2 at low temperature (e.g. < 400 °C) on both glass and plastic substrates, facilitating these new applications. Due to carrier scattering and trapping at defects on grain boundaries in polycrystalline materials (the grain boundary problem), hydrogenated amorphous silicon (a-Si:H) has been used more widely than polycrystalline silicon (p-Si) for practical large-size applications. However, the critical obstacles to be overcome to realize a-Si:H in future applications are low mobility (< 1 cm 2 /V s) and instability against electric stress and photo-illumination.In late 2004, we reported that an amorphous oxide semiconductor (AOS) with the composition a-InGaZnO 4 (a-IGZO) can be applied in the fabrication of fl exible, transparent thin-fi lm transistors (TFTs) having much improved performance compared to conventional TFTs based on a-Si:H and organic materials [1]. AOS materials have superior characteristics to conventional semiconductors, and therefore AOS TFT technology has grown very rapidly toward the realization of TFT backplanes for next-generation flat-panel displays. As summarized in Figure 1, a variety of prototype AM displays have been demonstrated by several companies within the last fi ve years since the fi rst report of AOS TFTs. Herein, we review the unique characteristics of AOS materials in relation to their TFT characteristics, and discuss why AOSs have such attractive electrical properties related to the electronic structures specifi c to transparent oxides. Active-matrix displays and circuits based on AOS TFT arraysTh e fi rst AOS-TFT was reported in late 2004 (see Figure 1). Just one year later, Toppan Printing quickly followed with the fi rst AM display based on AOS as a fl exible black-and-white electronic paper (e-paper) using a-IGZO TFTs

Flexible indium–gallium–zinc–oxide Schottky diode operating beyond 2.45 GHz

“…Based on Supplementary Note 1, (E C À E F ) is found to be 0.308 eV and hence the barrier height from the C-V plot is 0.864 eV. It is higher than the barrier height from J-V characteristics, which is most likely because of the imperfect uniformity of the Schottky junction 22 . In Fig.…”

“…An aluminium (Al) electrode is used here as the ohmic contact because the work function of Al is only 4.2 eV and the contact resistance between Al and IGZO is known to be quite low 32 . A platinum (Pt) electrode is chosen as the Schottky contact due to its high work function of 5.4 eV 22 . The overlapping region between the two electrodes determines the effective area of the diode.…”

“…This shows that the series resistance has a sensitive dependence on the thickness of IGZO layer. Given that the effective Richardson constant of IGZO is 41 A cm 2 K 2 and based on the thermionic emission theory 22,34,35 , the extracted barrier heights are 0.79 eV, 0.73 eV and 0.65 eV for the diodes with 100 nm, 80 nm, and 50 nm IGZO layer, respectively (fitting is shown in Supplementary Fig. 3).…”

“…According to the study conducted by Olziersky et al 36 , the moderate In/Ga atom ratio of 0.95 should yield a reasonably high carrier mobility. We have therefore assumed that the mobility of IGZO is 10 cm 2 V À 1 s À 1 (roughly the mobility value obtained from TFTs based on similar IGZO material 22,28,30 ). Figure 1d shows that the barrier height can be obtained from the sum of V bi and E C À E F .…”

“…Metal-oxide semiconductors, on the other hand, have already been incorporated into flexible electronics in both academia and industry 7,8,21,22 . To date, most research on metaloxide semiconductors has focused on thin-film transistors (TFTs) 7,8,[21][22][23] , and little has been reported on diodes [24][25][26][27][28][29][30] 27), but these were all made on glass substrate using high-temperature annealing processes and hence not applicable to flexible substrates. To the best of our knowledge, the highest frequency obtained on plastic substrates was 27 MHz by IGZOCu 2 O diodes 10 .…”