Structural-relaxation-driven electron doping of amorphous oxide semiconductors by increasing the concentration of oxygen vacancies in shallow-donor states

Abstract

The electronic states of oxygen vacancies (V O s) in amorphous oxide semiconductors are shallow donors, deep donors or electron traps; these are determined by the local atomic structure. Because the amorphous phase is metastable compared with the crystalline phase, the degree of structural disorder is likely to decrease, which is referred to as structural relaxation (SR). Thus SR can affect the V O electronic state by changing the local atomic conditions. In this study, we demonstrated that electron doping is possible through the SR of amorphous oxides without redox reactions using a novel device structure that prevents extrinsic reactions with electrodes and ambient atmosphere during annealing. The concentration of V O s in the shallow-donor state in amorphous In-Ga-Zn-O (a-IGZO) increases from~10 16 to~10 19 cm − 3 with increasing annealing temperatures between 300 and 450°C. The SR-driven doping effect is strongly dependent on the annealing temperature but not on the annealing time. The Arrhenius activation energy of the SR-driven doping effect is 1.76 eV, which is similar to the bonding energies in a-IGZO. Our findings suggest that the free volume in a-IGZO decreases during SR, and the V O s in either deep-donor or electrontrap states are consequently transformed into shallow-donor states. NPG Asia Materials (2016) 8, e250; doi:10.1038/am.2016.11; published online 25 March 2016 INTRODUCTION Amorphous metal oxides, such as amorphous indium gallium zinc oxide (a-IGZO), have become mainstream materials for large-area and flexible electronics because the long-range structural disorder enhances the uniformity of the electrical properties and mechanical flexibility compared with crystalline metal oxides. 1-5 A notable characteristic of amorphous oxides, relative to their crystalline counterparts, is that these materials contain structural disorder-related defects (for example, free volume) in addition to non-stoichiometric defects (for example, oxygen vacancies), which significantly affect the corresponding electrical properties. 3,[6][7][8] Moreover, the degree of structural disorder in amorphous metal oxides is always likely to decrease to form more stable structures because internal atomic rearrangement occurs even below the glass transition temperature (T g ). This process is known as structural relaxation (SR) 9-11 and results in continuous changes in the electrical properties. Thus understanding the effects of SR on the electrical properties of amorphous oxides is important for applications in future electronics.In addition to uniformity and flexibility, amorphous metal oxides exhibit tunable electrical conductivity through redox reaction control

Cu Diffusion‐Driven Dynamic Modulation of the Electrical Properties of Amorphous Oxide Semiconductors

“…Schottky diode parameters, Φ B , ideality factor ( n ), and series resistance ( R S ) were extracted based on thermionic theory. Details of the estimation are provided in Figure S1 of the Supporting Information . The Φ B and n values of the W devices are similar to the values of the Cu devices, whereas the R S of the former devices is smaller.…”

“…Ideally, the difference in Φ B between both interfaces should be negligible because the W devices are symmetric in structure or the work function difference between W (4.55 eV) and Cu (4.65 eV) is small for the Cu devices . These phenomena would be induced that the defects at the TE/a‐IGZO interfaces, such as reduction in the a‐IGZO surface (i.e., increase in the V O concentration) in the vacuum chamber before TE deposition and/or TE material bombardment‐induced changes in the chemical states of the a‐IGZO elements (Figure S1c, Supporting Information), are generated during TE deposition . These interface states enhances the barrier tunneling conduction and consequently decrease the effective Φ B at the TE interface .…”

“…These phenomena would be induced that the defects at the TE/a‐IGZO interfaces, such as reduction in the a‐IGZO surface (i.e., increase in the V O concentration) in the vacuum chamber before TE deposition and/or TE material bombardment‐induced changes in the chemical states of the a‐IGZO elements (Figure S1c, Supporting Information), are generated during TE deposition . These interface states enhances the barrier tunneling conduction and consequently decrease the effective Φ B at the TE interface . Because the initial free electron concentration in a‐IGZO in the devices is below 10 16 cm −3 , the dominant conduction mechanism of the devices is Schottky thermionic emission .…”

“…These interface states enhances the barrier tunneling conduction and consequently decrease the effective Φ B at the TE interface . Because the initial free electron concentration in a‐IGZO in the devices is below 10 16 cm −3 , the dominant conduction mechanism of the devices is Schottky thermionic emission . Schottky diode parameters, Φ B , ideality factor ( n ), and series resistance ( R S ) were extracted based on thermionic theory.…”

“…We previously reported the increase in the free electron concentration in a‐IGZO in W/a‐IGZO/W devices (W devices) after annealing due to structural relaxation (SR)‐driven doping. During annealing at below the glass transition temperature, the interactions between W and a‐IGZO are negligible, and the free volume in a‐IGZO is reduced (i.e., SR), resulting in an increase in the doping concentration ( N D ) . By comparing the changes in the electrical properties of the Cu devices with the changes in the electrical properties of the W devices, the extrinsic Cu diffusion effect and intrinsic SR‐driven doping effect on the electrical properties of a‐IGZO could be separated.…”