Highly Uniform and Stable n‑Type Carbon Nanotube Transistors by Using Positively Charged Silicon Nitride Thin Films

Air-stable n-doping of carbon nanotubes is presented by utilizing SiN_<i>x</i> thin films deposited by plasma-enhanced chemical vapor deposition. The fixed positive charges in SiN_<i>x</i>, arising from ⁺SiN₃ dangling bonds induce strong field-effect doping of underlying nanotubes. Specifically, an electron doping density of ∼10²⁰ cm^–3 is estimated from capacitance voltage measurements of the fixed charge within the SiN_<i>x</i>. This high doping concentration results in thinning of the Schottky barrier widths at the nanotube/metal contacts, thus allowing for efficient injection of electrons by tunnelling. As a proof-of-concept, n-type thin-film transistors using random networks of semiconductor-enriched nanotubes are presented with an electron mobility of ∼10 cm²/V s, which is comparable to the hole mobility of as-made p-type devices. The devices are highly stable without any noticeable change in the electrical properties upon exposure to ambient air for 30 days. Furthermore, the devices exhibit high uniformity over large areas, which is an important requirement for use in practical applications. The work presents a robust approach for physicochemical doping of carbon nanotubes by relying on field-effect rather than a charge transfer mechanism

Formation of Nanoscale Composites of Compound Semiconductors Driven by Charge Transfer

Composites are a class of materials that are formed by mixing two or more components. These materials often have new functional properties compared to their constituent materials. Traditionally composites are formed by self-assembly due to structural dissimilarities or by engineering different layers or structures in the material. Here we report the synthesis of a uniform and stoichiometric composite of CdO and SnTe with a novel nanocomposite structure stabilized by the dissimilarity of the electronic band structure of the constituent materials. The composite has interesting electronic properties which range from highly n-type in CdO to semi-insulating in the intermediate composition range to highly p-type in SnTe. This can be explained by the overlap of the conduction and valence band of the constituent compounds. Ultimately, our work identifies a new class of composite semiconductors in which nanoscale self-organization is driven and stabilized by charge transfer between constituent materials

Room-Temperature-Synthesized High-Mobility Transparent Amorphous CdO–Ga₂O₃ Alloys with Widely Tunable Electronic Bands

In this work, we have synthesized Cd_1–<i>x</i>Ga_<i>x</i>O_1+δ alloy 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 <i>x</i> > 0.3 are amorphous. Amorphous Cd_1–<i>x</i>Ga_<i>x</i>O_1+δ alloys in the composition range of 0.3 < <i>x</i> < 0.5 exhibit a high electron mobility of 10–20 cm² V^–1 s^–1 with a resistivity in the range of 10^–2 to high 10^–4 Ω cm range. The resistivity of the amorphous alloys can also be controlled over 5 orders of magnitude from 7 × 10^–4 to 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 Cd_1–<i>x</i>Ga_<i>x</i>O_1+δ alloy films with 0.3 < <i>x</i> < 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 < <i>x</i> < 0.7 can be manipulated for transparent thin-film transistors as well as electron transport layers

Band Gap Engineering of Oxide Photoelectrodes: Characterization of ZnO_1–<i>x</i>Se_<i>x</i>

No single material or materials system today is a clear choice for photoelectrochemical electrode applications. Generally, materials with narrow, well-aligned band gaps are unstable in solution and stable materials have band gaps that are too wide to efficiently absorb sunlight. Here, we demonstrate the narrowing of the ZnO band gap and combine a variety of electrical, spectroscopic, and photoelectrochemical methods to explore the opportunities for this so-called highly mismatched alloy in photoelectrochemical water splitting applications. We find that the conduction band edge of ZnO_1–<i>x</i>Se_<i>x</i> is located at 4.95 eV below the vacuum level (0.5 V below the hydrogen evolution potential). Soft X-ray emission and absorption spectroscopies confirm that the previously observed ∼1 eV reduction in the ZnO band gap with the addition of selenium result from the formation of a narrow Se-derived band. We observe that this narrow band contributes to photocurrent production using applied bias incident photon to current efficiency measurements at an electrochemical junction. Electrical measurements, electrochemical flat band, and photocurrent measurements as a function of <i>x</i> in ZnO_1–<i>x</i>Se_<i>x</i> alloys indicate that this alloy is a good candidate for an oxide/silicon tandem photoelectrochemical device because of the natural band alignment between the silicon valence band and the ZnO_1–<i>x</i>Se_<i>x</i> conduction band. We observe that the photocurrent onset in preliminary ZnO_{1–<u><i>x</i></u>}Se_<i>x</i>/silicon diode tandem devices is shifted toward spontaneous hydrogen production compared to ZnO_1–<i>x</i>Se_<i>x</i> films grown on sapphire. With these findings, we hope that our method of band gap engineering oxides for photoelectrodes can be extended to devise better materials systems for spontaneous solar water splitting