Improving the electrical properties of transparent ZnO-based thin- film transistors using MgO gate dielectric with various oxygen concentrations.

Zinc oxide (ZnO)-based thin-film transistors (TFTs) have attracted increasing attention towards flat-panel displays as alternatives to silicon-based TFTs due to their transparency to visible light. Magnesium oxide (MgO) has a wide bandgap (7.8 eV) and high dielectric constant (k). This leads to the development of TFTs using MgO as a gate oxide layer, which can significantly reduce the operating voltage. However, the electrical properties and dielectric constant of MgO are determined from the percentage of oxygen in MgO. In this study, a MgO gate-oxide was deposited on ZnO by magnetron sputtering at various oxygen concentrations (0%, 66%, and 100%) to fabricate TFTs. With an increase in the oxygen concentration, the oxygen vacancies of MgO were compensated, thereby improving the crystallinity and enhancing the dielectric constant from 6.53 to 12.9 for the oxygen concentrations of 0% and 100%. No pinch-off (saturation) behavior was observed in the TFTs with 0% oxygen; however, the pinch-off voltages were significantly reduced to 17 and 2 V in the TFTs with 66% and 100% oxygen, respectively; hence, the TFT-100 could be operated at a low operating voltage (2 V). With an increase in oxygen from 0% to 100%, the threshold voltage and trap-state density significantly decreased from -159 V and 1.6 × 1018cm-3to -31.4 V and 6.5 × 1016cm-3, respectively. The TFTs with 0% oxygen exhibited a higher field-effect mobility of 12 cm2V-1s-1due to the uncompensated oxygen vacancy in ZnO, which had a higher electron concentration. After introducing oxygen atoms, the field-effect mobility decreased to 0.16 cm2V-1s-1in the TFTs with 66% oxygen, which can be attributed to the compensated oxygen vacancy and lower electron concentration. In contrast, the field-effect mobility increased to 1.88 cm2V-1s-1for the TFTs with 100% oxygen due to the enhanced dielectric constant and crystallinity of MgO.

Separate absorption and multiplication solar-blind photodiodes based on p-NiO/MgO/n-ZnO heterostructure.

High-performance solar-blind separate absorption and multiplication avalanche photodiodes (SAM-APDs) were fabricated based on a p-NiO/MgO/n-ZnO dual heterojunction structure. The prepared SAM-APDs exhibited a separated absorption and multiplication structure that used NiO and ZnO as absorption layers, and ultrawide-bandgap MgO as a multiplication layer. When the reverse-bias voltage exceeded 6 V, carrier avalanche multiplication occurred, and the avalanche gain reached a high value of 2.7 × 103, corresponding to a 1120% quantum efficiency, at a reverse-bias voltage of 10 V. These solar-blind SAM-APDs had an ultraviolet (UV) (310 nm)/visible (500 nm) rejection ratio as high as 563.6 at a 2 V reverse-bias voltage. These features render the SAM-APDs highly suitable for practical applications as UV solar-blind photodetectors.

Base-width modulation effects on the optoelectronic characteristics of n-ITO/p-NiO/n-ZnO heterojunction bipolar phototransistors.

ITO/NiO/ZnO npn heterojunction bipolar phototransistors (HBPTs) with various base widths are fabricated using a radio-frequency sputtering system. The effects of base-width modulation on the optoelectronic characteristics of the prepared HBPTs are studied. The dark current of HBPTs decreases with increasing base width because the injected electrons from the emitter are recombined in the wide base region. The photocurrent increases with decreasing base width, which is attributed to higher emitter-base injection efficiency. The responsivity increases with the collector-emitter voltage (VCE) in the HBPTs with a 100 nm base width, whereas the responsivity sharply decreases atVCE> 4 V for the HBPTs with a thinner base width (80 nm) due to the punch-through effect. In contrast, the responsivity approaches saturation at largeVCEfor HBPTs with a thicker base width (120 nm). The responsivity and detectivity decrease with increasing incident light intensity, which is caused by an increase in the base recombination loss. The HBPTs with a base width of 100 nm exhibits the largest responsivity and detectivity; their detectivity is higher than that of HBPTs with base widths of 80 and 120 nm by approximately two and three orders, respectively.

Effect of nickel diffusion and oxygen behavior on heterojunction Schottky diodes of Au/NiO/ZnO with a NiO interlayer prepared by radio-frequency magnetron sputtering.

The rectifying characteristic of Au/ZnO Schottky diodes (SDs) was remarkably improved by introducing a NiO layer in-between the Au and ZnO layers. Compared with the Au/ZnO SDs, the introduction of the NiO layer significantly enhanced the rectification ratio from 1.38 to 1300, and reduced the ideality factor from 5.78 to 2.14. The NiO and ZnO layers were deposited on an indium-tin-oxide/glass substrate by radio-frequency magnetron sputtering. Secondary ion mass spectroscopy showed that Ni atoms diffused from NiO to ZnO, leading to a graded distribution of Ni in ZnO. X-ray diffraction demonstrated that the diffusion of Ni atoms increased the grain size and electron concentration of ZnO. X-ray photoelectron spectroscopy showed that the interstitial oxygen (Oi) atoms in NiO and ZnO compensated the oxygen vacancies (OV) at the NiO/ZnO interface; the amount of OV was significantly reduced, while Oi vanished at the interface. The band diagram revealed a potential drop in the bulk ZnO, owing to the graded distribution of Ni in ZnO, which accelerated the carriers, collected by the outer circuit. The carriers at the NiO/ZnO interface easily crossed over the barrier height, instead of being recombined by OV, owing to the lower amount of OV at the interface.

Barrier thickness dependence of Mg x Zn1-x O/ZnO quantum well (QW) on the performance of a p-NiO/QW/n-ZnO photodiode.

An Mg x Zn1-x O/ZnO quantum well (QW) structure, with various barrier (Mg x Zn1-x O layer) thicknesses, was inserted into p-NiO/n-ZnO heterojunction photodiodes (HPDs) by using a radio-frequency magnetron sputtering system. The effect of various barrier thicknesses on the performance of QW-PDs was investigated. A band diagram shows that the QW-PD with 10 nm barrier layer presents a tunneling carrier transport mechanism, the UV- and visible-generated carriers tunnel through the thin barrier layer. Whereas the QW-PDs with thicker (≧25 nm) barrier layers show recombination-tunneling carrier transport. The visible-generated carriers are effectively confined within the well layer in the QW structure, causing the visible-response to be greatly reduced by more than 3 orders compared to that in the QW-PD with a 10 nm barrier layer. However, on further increasing the barrier thickness beyond 25 nm, the visible-response will no longer be reduced. In contrast, with decreasing the barrier thickness from 60 to 25 nm, the UV-response increases due to the overlap increase of the fundamental electron and hole wave function in the quantum well. Such a result drastically enhances the rejection ratio (320 nm/500 nm) from 264 for QW-PDs with a 10 nm barrier to 2986 for QW-PDs with a 25 nm barrier layer by a 11.3 ratio.