Mohammad Suja

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 × 10(18) cm(-3), a low resistivity of 0.6 Ω cm, and a moderate mobility of 6.65 cm(2) 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.

An Sb-doped p-type ZnO nanowire based random laser diode.

An electrically pumped Sb-doped ZnO nanowire/Ga-doped ZnO p-n homojunction random laser is demonstrated. Catalyst-free Sb-doped ZnO nanowires were grown on a Ga-doped ZnO thin film on a Si substrate by chemical vapor deposition. The morphology of the as-grown titled nanowires was observed by scanning electron microscopy. X-ray photoelectron spectroscopy results indicated the incorporation of Sb dopants. Shallow acceptor states of Sb-doped nanowires were confirmed by photoluminescence measurements. Current-voltage measurements of ZnO nanowire structures assembled from p- and n-type materials showed a typical p-n diode characteristic with a threshold voltage of about 7.5 V. Very good photoresponse was observed in the UV region operated at 0 V and different reverse biases. Random lasing behavior with a low-threshold current of around 10 mA was demonstrated at room temperature. The output power was 170 nW at 30 mA.

Ultraviolet random lasing from asymmetrically contacted MgZnO metal-semiconductor-metal device

Nitrogen-doped Mg0.12Zn0.88O nanocrystalline thin film was grown on c-plane sapphire substrate. Asymmetric Ni/Au and Ti/Au Schottky contacts and symmetric Ni/Au contacts were deposited on the thin film to form metal-semiconductor-metal (MSM) laser devices. Current-voltage, photocurrent, and electroluminescence characterizations were performed. Evident random lasing with a threshold current of ∼36 mA is demonstrated only from the asymmetric MSM device. Random lasing peaks are mostly distributed between 340 and 360 nm and an output power of 15 nW is measured at 43 mA injection current. The electron affinity difference between the contact metal and Mg0.12Zn0.88O:N layer plays an important role for electron and hole injection and subsequent stimulated random lasing.

Large-area growth of multi-layer hexagonal boron nitride on polished cobalt foils by plasma-assisted molecular beam epitaxy

Two-dimensional (2D) hexagonal boron nitride (h-BN), which has a similar honeycomb lattice structure to graphene, is promising as a dielectric material for a wide variety of potential applications based on 2D materials. Synthesis of high-quality, large-size and single-crystalline h-BN domains is of vital importance for fundamental research as well as practical applications. In this work, we report the growth of h-BN films on mechanically polished cobalt (Co) foils using plasma-assisted molecular beam epitaxy. Under appropriate growth conditions, the coverage of h-BN layers can be readily controlled by growth time. A large-area, multi-layer h-BN film with a thickness of 5~6 nm is confirmed by Raman spectroscopy, scanning electron microscopy, X-ray photoelectron spectroscopy and transmission electron microscopy. In addition, the size of h-BN single domains is 20~100 μm. Dielectric property of as-grown h-BN film is evaluated by characterization of Co(foil)/h-BN/Co(contact) capacitor devices. Breakdown electric field is in the range of 3.0~3.3 MV/cm, which indicates that the epitaxial h-BN film has good insulating characteristics. In addition, the effect of substrate morphology on h-BN growth is discussed regarding different domain density, lateral size, and thickness of the h-BN films grown on unpolished and polished Co foils.

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.

Direct growth of hexagonal boron nitride/graphene heterostructures on cobalt foil substrates by plasma-assisted molecular beam epitaxy

Graphene/hexagonal boron nitride (G/h-BN) heterostructures have attracted a great deal of attention because of their exceptional properties and wide variety of potential applications in nanoelectronics. However, direct growth of large-area, high-quality, and stacked structures in a controllable and scalable way remains challenging. In this work, we demonstrate the synthesis of h-BN/graphene (h-BN/G) heterostructures on cobalt (Co) foil by sequential deposition of graphene and h-BN layers using plasma-assisted molecular beam epitaxy. It is found that the coverage of h-BN layers can be readily controlled on the epitaxial graphene by growth time. Large-area, uniform-quality, and multi-layer h-BN films on thin graphite layers were achieved. Based on an h-BN (5–6 nm)/G (26–27 nm) heterostructure, capacitor devices with Co(foil)/G/h-BN/Co(contact) configuration were fabricated to evaluate the dielectric properties of h-BN. The measured breakdown electric field showed a high value of ∼2.5–3.2 MV/cm. Both I-V and...

Electrically driven deep ultraviolet MgZnO lasers at room temperature

Semiconductor lasers in the deep ultraviolet (UV) range have numerous potential applications ranging from water purification and medical diagnosis to high-density data storage and flexible displays. Nevertheless, very little success was achieved in the realization of electrically driven deep UV semiconductor lasers to date. In this paper, we report the fabrication and characterization of deep UV MgZnO semiconductor lasers. These lasers are operated with continuous current mode at room temperature and the shortest wavelength reaches 284 nm. The wide bandgap MgZnO thin films with various Mg mole fractions were grown on c-sapphire substrate using radio-frequency plasma assisted molecular beam epitaxy. Metal-semiconductor-metal (MSM) random laser devices were fabricated using lithography and metallization processes. Besides the demonstration of scalable emission wavelength, very low threshold current densities of 29~33 A/cm2 are achieved. Numerical modeling reveals that impact ionization process is responsible for the generation of hole carriers in the MgZnO MSM devices. The interaction of electrons and holes leads to radiative excitonic recombination and subsequent coherent random lasing.

Enhanced random lasing from distributed Bragg reflector assisted Au-ZnO nanowire Schottky diode

An electrically pumped ultraviolet random laser based on an Au-ZnO nanowire Schottky junction on top of a SiO2/SiNx distributed Bragg reflector (DBR) has been fabricated. Electrical characterization shows typical Schottky diode current-voltage characteristics. Evident random lasing behavior is observed from electroluminescence measurement at room temperature. In comparison with a reference device having similar nanowire morphology but no DBR, this laser demonstrates almost 1.8 times reduction in threshold current and 4 times enhancement in output power. The performance enhancement originates from the incorporation of the DBR structure, which provides high reflectivity in the designed wavelength range.