Jingjian Ren

Electrically pumped waveguide lasing from ZnO nanowires

Ultraviolet semiconductor lasers are widely used for applications in photonics, information storage, biology and medical therapeutics. Although the performance of gallium nitride ultraviolet lasers has improved significantly over the past decade, demand for lower costs, higher powers and shorter wavelengths has motivated interest in zinc oxide (ZnO), which has a wide direct bandgap and a large exciton binding energy. ZnO-based random lasing has been demonstrated with both optical and electrical pumping, but random lasers suffer from reduced output powers, unstable emission spectra and beam divergence. Here, we demonstrate electrically pumped Fabry-Perot type waveguide lasing from laser diodes that consist of Sb-doped p-type ZnO nanowires and n-type ZnO thin films. The diodes exhibit highly stable lasing at room temperature, and can be modelled with finite-difference time-domain methods.

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.

Current self-complianced and self-rectifying resistive switching in Ag-electroded single Na-doped ZnO nanowires.

We demonstrate current self-complianced and self-rectifying bipolar resistive switching in an Ag-electroded Na-doped ZnO nanowire device. The resistive switching is controlled by the formation and rupture of an Ag nanoisland chain on the surface along the Na-doped ZnO nanowire. Na-doping plays important roles in both the self-compliance and self-rectifying properties.

ZnO:Sb/ZnO:Ga Light Emitting Diode on c-Plane Sapphire by Molecular Beam Epitaxy

p-type Sb-doped ZnO (ZnO:Sb)/n-type Ga-doped ZnO (ZnO:Ga) junctions were grown on c-plane sapphire substrates using plasma-assisted molecular-beam epitaxy. Mesa geometry light emitting diodes (LEDs) were fabricated using standard photolithography and lift-off process, with ohmic contacts achieved using Au/Ni and Au/Ti for top ZnO:Sb and bottom ZnO:Ga layers, respectively. Rectifying current–voltage characteristics were achieved. Ultraviolet emission dominates in the electroluminescence spectra of the ZnO LED. An output power of ~32 nW at an applied current of 60 mA was demonstrated. The enhanced output power, as compared to those made on silicon substrates, is attributed to the improved ZnO film quality on sapphire substrates, which is confirmed by X-ray diffraction rocking curve studies.

Noble metal nanodisks epitaxially formed on ZnO nanorods and their effect on photoluminescence

Triangular and hexagonal shaped noble metal (Au, Ag, Pt, Pd) nanodisks were synthesized on the top facets of ZnO nanorods via simple deposition-annealing method. Other metals (Ni, Cu, Cr, Pb, Al) only formed irregular shaped nanostructures on ZnO nanorods. The morphology, elemental composition, as well as growth mechanism of the metal nanodisks/ZnO nanorod composite materials were studied. The localized surface plasmon resonant effects from different metal nanodisks on the photoluminescence of ZnO nanorods were investigated. It was demonstrated that the carriers transfer between the metal nanodisks and ZnO can efficiently manipulate the photoluminescence intensities from the nanorods.

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 Al(2)O(3)-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 Al(2)O(3) 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 10(4) 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.

Nonplanar NiSi Nanocrystal Floating-Gate Memory Based on a Triangular-Shaped Si Nanowire Array for Extending Nanocrystal Memory Scaling Limit

A nonplanar Flash memory architecture with ultrahigh-density (~1.5 × 1012 cm-2) NiSi nanocrystals (NCs) as the floating gate is demonstrated using a triangular-shaped Si nanowire array as the memory transistor channel. The memory device shows good programming, erasing, and retention characteristics. This result suggests that nonplanar devices can extend NC memory scaling limit.

Write-Once–Read-Many-Times Memory Based on ZnO on p-Si for Long-Time Archival Storage

Write-once-read-many-times memory cells were fabricated using ZnO thin film on p-Si (111) substrate. The off- and on-state resistance ratio is over 104 and can be well sustained for more than 100 years and perfectly endure reading cycles of 108 . The conducting filaments consisting of oxygen vacancies are responsible for the switching mechanism.

Strain-less directed self-assembly of Si nanocrystals on patterned SiO2 substrate

Strain induced self-assembled Stranski-Krastanov growth of semiconductor islands on patterned substrate has shown great improvement of island size uniformity and spatial order. Here, we show self-assembled Volmer-Weber (V-W) growth of Si nanocrystals (NCs) on patterned SiO2 substrate via traditional chemical vapor deposition method under certain experimental configurations, induced by surface/interface energy competition without strain. A simplified two-dimensional theoretical model is developed to elucidate V-W island nucleation on the pattern substrate with varied morphologies, which shows good consistency with the experimental results. Our studies provide a general guidance for directing the growth and self-assembly of NCs on non-planar oxide substrates.