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Black phosphorus (BP) is a narrow bandgap (∼0.3 eV) semiconductor with a great potential for optoelectronic devices in the mid-infrared wavelength. However, it has been challenging to achieve a high-quality scalable BP thin film. Here we present the successful synthesis of optically active BP films on a centimeter scale. We utilize the pulsed laser deposition of amorphous red phosphorus, another allotrope of phosphorus, followed by a high-pressure treatment at ∼8 GPa to induce a phase conversion into BP crystals. The crystalline quality was improved through thermal annealing, resulting in the observation of photoluminescence emission at mid-infrared wavelengths. We demonstrate high-pressure conversion on a centimeter scale with a continuous film with a thickness of ∼18 nm using a flat-belt-type high-pressure apparatus. This synthesis procedure presents a promising route to obtain optical-quality BP films, enabling the exploration of integrated optoelectronic device applications such as light-emitting devices and mid-infrared cameras on a chip scale.

Oxide-based transparent p–n homojunctions are desirable for the development of transparent electronics. However, most oxide semiconductors are intrinsically n-type and to achieve p-type wide-gap oxide is still challenging. Previously, we have demonstrated that alloying a high-mobility n-type material such as CdO with p-type NiO can provide an avenue for electronic band engineering and consequently achieve bipolar conductivity in the midalloy composition. In this study, we synthesized O-rich NixCd1–xO alloys (NixCd1–xO1+δ) over the entire composition with Li and Cu doping using radio frequency (RF) magnetron sputtering at room temperature. We show that by Li and Cu doping the conductivity in the p-type regime of these alloys is improved, which also leads to a wider composition window for bipolar doping in NixCd1–xO. Specifically, by Li doping, the p-type alloy composition can be extended to x≥ 0.3 so that the bipolar doping window is expanded to 0.30 ≤ x≤ 0.52. Detailed measurements on electrical, structural, optical, and electronic properties suggest that Li is an effective acceptor, offering a promising way to improve the p-type conduction in Ni-rich NixCd1–xO alloys as well as to regulate the bipolar conductivity in this alloy system. A p-Ni0.7Cd0.3O:Li/n-Ni0.45Cd0.55O quasi-homojunction was fabricated and a rectification ratio ∼102with an ideality factor of ∼2.9 was obtained. The demonstrated quasi-homojunction structure also showed >60% transmittance in the visible spectrum.

In this work, we have synthesized Cd1–xGaxO1+δalloy thin films at room temperature over the entire composition range by radio frequency magnetron sputtering. We found that alloy films with high Ga contents of x> 0.3 are amorphous. Amorphous Cd1–xGaxO1+δalloys in the composition range of 0.3 < x< 0.5 exhibit a high electron mobility of 10–20 cm2V–1s–1with a resistivity in the range of 10–2to high 10–4Ω cm range. The resistivity of the amorphous alloys can also be controlled over 5 orders of magnitude from 7 × 10–4to 77 Ω cm by controlling the oxygen stoichiometry. Over the entire composition range, these crystalline and amorphous alloys have a large tunable intrinsic band gap range of 2.2–4.8 eV as well as a conduction band minimum range of 5.8–4.5 eV below the vacuum level. Our results suggest that amorphous Cd1–xGaxO1+δalloy films with 0.3 < x< 0.4 have favorable optoelectronic properties as transparent conductors on flexible and/or organic substrates, whereas the band edges and electrical conductivity of films with 0.3 < x< 0.7 can be manipulated for transparent thin-film transistors as well as electron transport layers.