Realization of Cu-Doped p‑Type ZnO Thin Films by Molecular Beam Epitaxy

Cu-doped p-type ZnO films are grown on c-sapphire substrates by plasma-assisted molecular beam epitaxy. Photoluminescence (PL) experiments reveal a shallow acceptor state at 0.15 eV above the valence band edge. Hall effect results indicate that a growth condition window is found for the formation of p-type ZnO thin films, and the best conductivity is achieved with a high hole concentration of 1.54 × 10¹⁸ cm^–3, a low resistivity of 0.6 Ω cm, and a moderate mobility of 6.65 cm² V^–1 s^–1 at room temperature. Metal oxide semiconductor capacitor devices have been fabricated on the Cu-doped ZnO films, and the characteristics of capacitance–voltage measurements demonstrate that the Cu-doped ZnO thin films under proper growth conditions are p-type. Seebeck measurements on these Cu-doped ZnO samples lead to positive Seebeck coefficients and further confirm the p-type conductivity. Other measurements such as X-ray diffraction, X-ray photoelectron, Raman, and absorption spectroscopies are also performed to elucidate the structural and optical characteristics of the Cu-doped p-type ZnO films. The p-type conductivity is explained to originate from Cu substitution of Zn with a valency of +1 state. However, all p-type samples are converted to n-type over time, which is mostly due to the carrier compensation from extrinsic defects of ZnO

Phase Transition and Bandgap Engineering of MgSnO Thin Films for Solar-Blind Ultraviolet Photodetector Applications

Tin oxide (SnO2) is one of the transparent conducting oxide semiconductors that have been widely used in optoelectronic devices. To extend SnO2-based optoelectronic applications into the deep ultraviolet solar-blind wavelength range, in this research, MgSnO alloy thin films were grown on c-sapphire using plasma-assisted molecular beam epitaxy. As Mg composition is between 0 and ∼24.4 at. %, MgSnO films exhibit rutile structure. The lattice constants increase as the Mg composition increases. MgSnO films become amorphous as Mg composition exceeds 24.4 at. % and eventually become rock-salt structures as Mg composition exceeds 45.9 at. %. The optical bandgap of MgSnO increases with the increase in Mg composition. Metal–semiconductor–metal (MSM) photodetector devices were fabricated and characterized. When Mg composition increases, both the dark current and photocurrent of the devices decrease. High responsivities were observed for all MgSnO MSM devices

Resistive Switching in Single Epitaxial ZnO Nanoislands

Resistive memory is one of the most promising candidates for next-generation nonvolatile memory technology due to its variety of advantages, such as simple structure and low-power consumption. Bipolar resistive switching behavior was observed in epitaxial ZnO nanoislands with base diameters and heights ranging around 30 and 40 nm, respectively. All four different states (initial, electroformed, ON, and OFF) of the nanoscale resistive memories were measured by conductive atomic force microscopy immediately after the voltage sweeping was performed. Auger electron spectroscopy and other experiments were also carried out to investigate the switching mechanism. The formation and rupture of conducting filaments induced by oxygen vacancy migration are responsible for the resistive switching behaviors of ZnO resistive memories at the nanoscale

Carbon Nanotube Memory by the Self-Assembly of Silicon Nanocrystals as Charge Storage Nodes

A memory structure based on self-aligned silicon nanocrystals (Si NCs) grown over Al₂O₃-covered parallel-aligned carbon nanotubes (CNTs) by gas source molecular beam epitaxy is reported. Electrostatic force microscopy characterizations directly prove the charging and discharging of discrete NCs through the Al₂O₃ layer covering the CNTs. A CNT field effect transistor based on the NC/CNT structure is fabricated and characterized, demonstrating evident memory characteristics. Direct tunneling and Fowler–Nordheim tunneling phenomena are observed at different programming/erasing voltages. Retention is demonstrated to be on the order of 10⁴ s. Although there is still plenty of room to enhance the performance, the results suggest that CNT-based NC memory with diminutive CNTs and NCs could be an alternative structure to replace traditional floating gate memory

Visible-Blind UV Photodetector Based on Single-Walled Carbon Nanotube Thin Film/ZnO Vertical Heterostructures

Ultraviolet (UV) photodetectors based on heterojunctions of conventional (Ge, Si, and GaAs) and wide bandgap semiconductors have been recently demonstrated, but achieving high UV sensitivity and visible-blind photodetection still remains a challenge. Here, we utilized a semitransparent film of p-type semiconducting single-walled carbon nanotubes (SC-SWNTs) with an energy gap of 0.68 ± 0.07 eV in combination with a molecular beam epitaxy grown n-ZnO layer to build a vertical p-SC-SWNT/n-ZnO heterojunction-based UV photodetector. The resulting device shows a current rectification ratio of 10³, a current photoresponsivity up to 400 A/W in the UV spectral range from 370 to 230 nm, and a low dark current. The detector is practically visible-blind with the UV-to-visible photoresponsivity ratio of 10⁵ due to extremely short photocarrier lifetimes in the one-dimensional SWNTs because of strong electron–phonon interactions leading to exciton formation. In this vertical configuration, UV radiation penetrates the top semitransparent SC-SWNT layer with low losses (10–20%) and excites photocarriers within the n-ZnO layer in close proximity to the p-SC-SWNT/n-ZnO interface, where electron–hole pairs are efficiently separated by a high built-in electric field associated with the heterojunction

Investigation of Phase Transition and Ultrawide Band Gap Engineering in MgGaO Semiconductor Thin Films

Magnesium gallium oxide (MgGaO) ternary alloys with band gap energy larger than ∼5.0 eV can provide opportunities for optoelectronics in the deep ultraviolet spectral range and power electronics with extremely high critical field strength. It is important to grow high-quality MgGaO alloys with varied Mg compositions and understand their structural and optical properties. From this perspective, 20 MgGaO samples with Mg atomic percentages from 0 to 100% were grown by using oxygen plasma-assisted molecular beam epitaxy. Band gap tuning from 5.03 to 5.89 eV was achieved for the ternary alloys, and all samples had a transmittance of over ∼90% in the visible spectral range. The lattice structures were confirmed to transform from the β phase in Ga-rich materials to the β and rocksalt mixture phase in high-Ga high-Mg alloys and to the pure rocksalt phase in Mg-rich alloys. How lattice parameters change with the increase of Mg atom % and the epitaxy relationship between MgGaO films and c-sapphire substrates were revealed

Role of Carbon Interstitials in Transition Metal Substrates on Controllable Synthesis of High-Quality Large-Area Two-Dimensional Hexagonal Boron Nitride Layers

Reliable and controllable synthesis of two-dimensional (2D) hexagonal boron nitride (h-BN) layers is highly desirable for their applications as 2D dielectric and wide bandgap semiconductors. In this work, we demonstrate that the dissolution of carbon into cobalt (Co) and nickel (Ni) substrates can facilitate the growth of h-BN and attain large-area 2D homogeneity. The morphology of the h-BN film can be controlled from 2D layer-plus-3D islands to homogeneous 2D few-layers by tuning the carbon interstitial concentration in the Co substrate through a carburization process prior to the h-BN growth step. Comprehensive characterizations were performed to evaluate structural, electrical, optical, and dielectric properties of these samples. Single-crystal h-BN flakes with an edge length of ∼600 μm were demonstrated on carburized Ni. An average breakdown electric field of 9 MV/cm was achieved for an as-grown continuous 3-layer h-BN on carburized Co. Density functional theory calculations reveal that the interstitial carbon atoms can increase the adsorption energy of B and N atoms on the Co(111) surface and decrease the diffusion activation energy and, in turn, promote the nucleation and growth of 2D h-BN