Atomic layer deposition of conductive and semiconductive oxides

Abstract

Conductive and semiconductive oxides constitute a class of materials of which the electrical conductivity and optical transparency can be modulated through material design (e.g., doping and alloying) and external influences (e.g., gating in a transistor or gas exposure in a gas sensor). These (semi)conductive oxides, often categorized as amorphous oxide semiconductors or transparent conductive oxides, have, therefore, been commonplace in, for example, solar cells and displays, as well as in an increasing variety of other applications including memory, logic, photonics, and sensing. Among the various deposition techniques, the use of atomic layer deposition (ALD) has been gaining in popularity in recent years. Specifically since the early 2000s, many ALD processes for doped and compound conductive metal oxides have been developed. The interest in such oxides prepared by ALD can most likely be attributed to the distinct merits of ALD, such as low-temperature processing, excellent uniformity and conformality, and accurate control over the doping level and composition. Moreover, as device dimensions shrink the need for high-quality, ultrathin materials becomes ever more important. These merits of ALD stem directly from the self-limiting nature of the surface chemistry that drives the ALD growth. On the other hand, the strong role that surface chemistry has in the growth mechanism brings in many intricacies, and detailed understanding of these aspects has been vital for the development of high-quality doped and compound oxides by ALD. Examples of growth effects that can occur during ALD of compound oxides include growth delays, clustering of dopants, and interruption of grain growth by doping. Such effects often need to be accounted for or mitigated, while on the other hand, there are also clear cases where such growth effects can be leveraged to achieve enhanced or new functionality. In this review paper, an overview of the library of ALD processes that has emerged is presented. Available precursor chemistries, dopants as well as achieved film properties—most notably the carrier densities and (field-effect) mobilities of the films—are presented. A selection of important ALD effects that can occur during the deposition of doped and compound conductive oxides is showcased, and their effect on the optical and electrical properties are highlighted. Mitigation and improvement strategies for negative growth effects are presented. This is done through case studies that clearly illustrate these effects, drawing both from literature and from our own recent work.