Chemical and Thin-Film Strategies for New Transparent Conducting Oxides

Abstract

Transparent conducting oxides (TCOs) have been known and employed technologically for more than 50 years, primarily in the form of doped single-cation oxides such as In2O3 and SnO2. Beginning in the 1990s, however, multi-cation oxide TCOs began to be developed in Japan (see the article by Minami in this issue and the references therein) and at the former Bell Laboratories. Since then, new TCO phases are being reported with increasing frequency as technological interest in this area heightens. At the same time, our fundamental understanding of the chemical and structural origins of transparent conductivity continues to expand and promises a pathway to dramatically improved materials for a host of applications. This article describes a collaborative, multi-investigator bulk an d thin-film research effort at Northwestern University aimed at the synthesis, characterization, and enhanced understanding of multi-cation (compound and solidsolution) TCOs, and provides a brief account of what we are discovering about this important class of materials.

Structural, optical, and electrical properties of indium-doped cadmium oxide films prepared by pulsed filtered cathodic arc deposition

“…Fortunately, the bandgap of CdO can be considerably widened via the Burstein-Moss shift by increasing the electron concentration [4][5][6]. Therefore, dopants including indium (In), tin (Sn), titanium (Ti), zinc (Zn), aluminum (Al), yttrium (Y), and fluorine (F) have been introduced into CdO in order to improve the conductivity and widen the bandgap [4,[6][7][8][9][10][11][12]. Aside from environmental issues, doped CdO materials are nearly ideal for photoelectrical and other possible applications, such as solar energy harvesting, optical communications, gas sensors, thin-film resistors, IR heat mirrors, etc.…”

Transparent and conductive indium doped cadmium oxide thin films prepared by pulsed filtered cathodic arc deposition

“…It is known that the optical absorption edge can be considerably widened via a Burstein-Moss shift by increasing the electron concentration [8,9]. Therefore, various dopants, including indium (In), tin (Sn), titanium (Ti), zinc (Zn), aluminum (Al), and fluorine (F), have been introduced into CdO to simultaneously increase the conductivity and widen the bandgap [10][11][12][13][14][15][16][17][18]. Among these doping elements, indium has been investigated in CdO since the two elements have very close ionic radii and both show excellent photoelectric properties [15,19].…”

“…Therefore, various dopants, including indium (In), tin (Sn), titanium (Ti), zinc (Zn), aluminum (Al), and fluorine (F), have been introduced into CdO to simultaneously increase the conductivity and widen the bandgap [10][11][12][13][14][15][16][17][18]. Among these doping elements, indium has been investigated in CdO since the two elements have very close ionic radii and both show excellent photoelectric properties [15,19]. Furthermore, In-doping can favorably alter the CdO band structure by extensive mixing of In 5s and Cd 5s states, which lowers optical absorption by weakening the intraband transitions [12].…”

Recent Advances in Understanding the Structure and Properties of Amorphous Oxide Semiconductors

“…[1] Following the discovery of SnO 2 with a similar unique combination of properties, [2] several patents were filed in the 1940s to employ TCOs as antistatic coatings and transparent heaters-long before the discovery of the now well-known Sn-doped In 2 O 3 (ITO) and Al-doped ZnO, [3] widely employed as flat panel display electrodes in the past decades. Despite great technological demand for TCOs [4][5][6][7][8][9][10][11][12][13][14][15][16][17][18][19][20] and extensive experimental efforts to improve the conductivity via impurity doping, [21,22] to tune the work function and carrier concentration via cation composition, [23][24][25][26][27][28] to achieve two-dimensional transport via heterointerfaces, [29] and to p-dope the oxides toward active layers of transparent electronics, [30][31][32] theoretical understanding of these fascinating materials has lagged behind significantly. The first electronic band structure of ITO was calculated in 2001; [33] the role of native defects in prototype TCOs was understood after 2002; [34][35][36][37] the properties of multi-cation TCOs were first considered in 2004 [37][38][39][40][41]…”