Sheng Chu

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.

Electrically pumped ultraviolet ZnO diode lasers on Si

Electrically pumped ZnO quantum well diode lasers are reported. Sb-doped p-type ZnO/Ga-doped n-type ZnO with an MgZnO/ZnO/MgZnO quantum well embedded in the junction was grown on Si by molecular beam epitaxy. The diodes emit lasing at room temperature with a very low threshold injection current density of 10 A/cm2. The lasing mechanism is exciton-related recombination and the feedback is provided by close-loop scattering from closely packed nanocolumnar ZnO grains formed on Si.

Sb-doped p-ZnO∕Ga-doped n-ZnO homojunction ultraviolet light emitting diodes

ZnO p-n homojunction light emitting diodes were fabricated based on p-type Sb-doped ZnO∕n-type Ga-doped ZnO thin films. Low resistivity Au∕NiO and Au∕Ti contacts were formed on top of p-type and n-type ZnO layers, respectively. Au∕NiO contacts on p-type ZnO exhibited a low specific resistivity of 7.4×10−4Ωcm2. The light emitting diodes yielded strong near-band-edge emissions in temperature-dependent and injection current-dependent electroluminescence measurements.

Ultraviolet emission from Sb-doped p -type ZnO based heterojunction light-emitting diodes

Heterojunction light emitting diodes (LEDs) were fabricated by making Au∕Ni top Ohmic contacts on Sb-doped p-type ZnO film with low specific contact resistivity and Al∕Ti back Ohmic contacts on n-type Si substrate. Near-band edge and deep-level emissions were observed from the LED devices at both low temperatures and room temperature, which is due to band-to-band and band-to-deep level radiative recombinations in ZnO, respectively. The electroluminescence emissions precisely match those of photoluminescence spectra from Sb-doped p-type ZnO, indicating that the ZnO layer acts as the active region for the radiative recombinations of electrons and holes in the diode operation.

ZnO homojunction photodiodes based on Sb-doped p-type nanowire array and n-type film for ultraviolet detection

ZnO p-n homojunctions based on Sb-doped p-type nanowire array and n-type film were grown by combining chemical vapor deposition (for nanowires) with molecular-beam epitaxy (for film). Indium tin oxide and Ti/Au were used as contacts to the ZnO nanowires and film, respectively. Characteristics of field-effect transistors using ZnO nanowires as channels indicate p-type conductivity of the nanowires. Electron beam induced current profiling confirmed the existence of ZnO p-n homojunction. Rectifying I-V characteristic showed a turn-on voltage of around 3 V. Very good response to ultraviolet light illumination was observed from photocurrent measurements.

Dominant ultraviolet light emissions in packed ZnO columnar homojunction diodes

The growth of Sb-doped p-type ZnO∕Ga-doped n-type ZnO homojunction on Si (100) substrate by molecular beam epitaxy led to closely packed vertical ZnO columns with lateral diameters ranging from 100to400nm. Mesa structures were defined and Ohmic contact of both n-type ZnO and p-type ZnO was realized with Au∕Ti and Au∕NiO, respectively. I-V and C-V curves present typical electrical properties of a diode, indicating that reliable p-type ZnO was formed. Electroluminescence shows dominant ultraviolet emissions with insignificant deep-level related yellow/green band emissions at different drive currents from 60to100mA at room temperature.

Enhanced output power using MgZnO/ZnO/MgZnO double heterostructure in ZnO homojunction light emitting diode

A diode with Sb-doped p-type ZnO, MgZnO/ZnO/MgZnO double heterostructure, and undoped n-type ZnO layers was grown on c-plane sapphire substrate by plasma-assisted molecular-beam epitaxy. Hall effect measurement showed that the top p-type Sb-doped ZnO layer has a hole concentration of 1 × 1017cm−3. Mesa geometry light emitting diodes were fabricated with Au/Ni and Au/Ti Ohmic contacts on top of the p-type and n-type layers, respectively. Ultraviolet emission was achieved, which yielded an output power of 457 nW at 140 mA. The enhancement of the output power is attributed to carrier confinement in the good-quality intrinsic layer of the double heterostructure. The spatial distribution of light emission was characterized.

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.