Room temperature fabricated flexible NiO/IGZO pn diode under mechanical strain
Introduction
Electronics fabricated on flexible plastic substrates promise to enable a variety of new applications, like rollable displays or woven electronics. Additionally, the use of roll to roll techniques allows cost-effective, large scale processing on flexible substrates [1]. Besides thin-film transistors, thin-film diodes are important components for the realization of flexible electronic devices. In particular, flexible power convertors for solar cells on plastic substrates call for flexible diodes.
The limited temperature resistance of plastic foils requires low temperature fabrication processes. Oxide semiconductors, mainly n-type Indium–Gallium–Zinc-Oxide (IGZO), have attracted much attention since they provide good electrical properties even when they are deposited at room temperature [2]. At the same time, an oxidic p-type semiconductor with a comparable performance is not available nowadays [3].
Besides materials like ZnORh2O3 [4], ZnCo2O4 [5], SnO [6], CuAlO2 [7], Cu2O [8], or SrCu2O2 [9], nickel oxide (NiO) is a possible candidate for the fabrication of oxidic p-type semiconductor layers on flexible substrates, whereas NiO has the advantage that it can be deposited by reactive sputtering using a simple metallic nickel target. In the past, NiO in combination with ZnO or IZO/IGZO has been used to fabricate pn diodes on rigid glass substrates [10], [11].
This paper describes the room temperature deposition of p-type semiconducting NiO on plastic foil. Subsequently, NiO is combined with n-type IGZO to fabricate flexible pn diodes directly on free standing plastic substrates. Bendability of the manufactured diodes is investigated by operating and characterizing the devices while bent to a radius of 10 mm in tensile as well as in compressive direction. Finally, the diodes are used to realize a rectifier circuit, which shows improved performance when the diodes are bent.
Section snippets
Room temperature oxide semiconductor deposition
NiO was deposited by DC magnetron sputtering from a metallic Ni [11] target using a PVD products sputtering system. Ni was oxidized during the sputter process in a 50% Ar, 50% O2 atmosphere at a pressure of 6 mTorr (Ar flow = 10 sccm, O2 flow = 10 sccm), while a DC power of 200 W was applied. The angle between the substrate and the magnetron was 45°, and the substrate was rotating with 12 rpm. Due to self-heating during the sputter process, the temperature of the substrate increases to values up to 35
Diode characteristic
I–V characteristics for flexible diodes with different diode areas AD, measured using an Agilent B1500 parameter analyzer are given in Fig. 4. The diodes exhibited a rectifying characteristic with an effective threshold voltage VTH,eff between 1.02 V and 1.32 V (depending on AD). The on–off current ratios depend on AD and varied between ≈103 and ≈104 (V = ±2.5 V). The reverse breakdown voltage was ≈−5 V. The capacitance of the diodes, measured at a DC voltage of 0 V and at a frequency of 100 kHz, was
Bending
To investigate and demonstrate the flexibility of the fabricated NiO/IGZO pn diodes, bending tests were performed as follows: Diodes were measured before bending, then attached to double sided tape and attached to a curved surface, in the way that tensile or compressive strain was applied to the NiO/IGZO pn diodes. The radius of the curvature was 10 mm, corresponding to mechanical strain ε of ≈0.25% [17]. The bent diodes were then contacted with probe needles and measured while bent.
Tensile
Flexible rectifier
The rectifying abilities of the presented NiO/IGZO diodes under AC conditions were tested by connecting a flexible diode (AD = 0.09 mm2) to an off the shelf 10 kΩ resistor in series. Then, a 50 Hz sinusoidal 5 V peak-to-peak AC signal was applied to the series connection of the flexible diode and the resistor. A digital oscilloscope was used to monitor the input voltage, and the output voltage between diode and resistor. The input signal, as well as the output signal, measured using a flat and bent
Conclusion
Room temperature fabricated NiO (np = +1.6 × 1017 cm−3) was introduced as p-type oxide semiconductor suitable for the deposition on flexible plastic substrates. NiO was combined with n-type IGZO (ne = −2.2 × 1019 cm−3) to realize a flexible, low temperature fabricated pn diode. The fabricated diodes exhibited characteristic diode behavior with an ideality factor of ≈3.2. The NiO/IGZO pn diodes stayed fully operational when exposed to tensile or compressive mechanical strain of 0.25%, induced by bending
Acknowledgements
This work has been scientifically evaluated by the Swiss National Scientific Foundation (SNSF), and financed by the Swiss Confederation and Nano-Tera.ch.
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