Ishiang Shih

Breaking the Carrier Injection Bottleneck of Phosphor-Free Nanowire White Light-Emitting Diodes

We have examined the carrier injection process of axial nanowire light-emitting diode (LED) structures and identified that poor carrier injection efficiency, due to the large surface recombination, is the primary cause for the extremely low output power of phosphor-free nanowire white LEDs. We have further developed InGaN/GaN/AlGaN dot-in-a-wire core-shell white LEDs on Si substrate, which can break the carrier injection efficiency bottleneck, leading to a massive enhancement in the output power. At room temperature, the devices can exhibit an output power of ~1.5 mW, which is more than 2 orders of magnitude stronger than nanowire LEDs without shell coverage. Additionally, such phosphor-free nanowire white LEDs can deliver an unprecedentedly high color rendering index of ~92-98 in both the warm and cool white regions, with the color rendering capability approaching that of an ideal light source, i.e. a blackbody.

Aluminum nitride nanowire light emitting diodes: Breaking the fundamental bottleneck of deep ultraviolet light sources

Despite broad interest in aluminum gallium nitride (AlGaN) optoelectronic devices for deep ultraviolet (DUV) applications, the performance of conventional Al(Ga)N planar devices drastically decays when approaching the AlN end, including low internal quantum efficiencies (IQEs) and high device operation voltages. Here we show that these challenges can be addressed by utilizing nitrogen (N) polar Al(Ga)N nanowires grown directly on Si substrate. By carefully tuning the synthesis conditions, a record IQE of 80% can be realized with N-polar AlN nanowires, which is nearly ten times higher compared to high quality planar AlN. The first 210 nm emitting AlN nanowire light emitting diodes (LEDs) were achieved, with a turn on voltage of about 6 V, which is significantly lower than the commonly observed 20 – 40 V. This can be ascribed to both efficient Mg doping by controlling the nanowire growth rate and N-polarity induced internal electrical field that favors hole injection. In the end, high performance N-polar AlGaN nanowire LEDs with emission wavelengths covering the UV-B/C bands were also demonstrated.

Optically Pumped Two-Dimensional MoS2 Lasers Operating at Room-Temperature

The discovery of direct bandgap semiconducting two-dimensional (2D) transition metal dichalcogenides (TMDCs) has opened a new era in flexible optoelectronic devices. Critical to this development is the realization of a semiconductor laser using the emerging 2D TMDCs. Here, by embedding 2D MoS2 at the interface between a free-standing microdisk and microsphere, we have demonstrated, for the first time, room-temperature lasing from 2D TMDCs. The devices exhibit multiple lasing peaks in the wavelength range of ∼600 to 800 nm. The threshold is measured to be ∼5 μW under continuous wave operation at room temperature. No saturation in the output power is measured for pump powers more than 2 orders of magnitude larger than the threshold. The superior performance is attributed to the large gain of 2D TMDCs and the strong coupling between the 2D MoS2 gain medium and optical modes in the unique optical cavity.

InN p-i-n Nanowire Solar Cells on Si

In this paper, we report the first experimental demonstration of InN nanowire solar cells. By employing an in situ deposited In seeding layer, we have achieved electronically pure, nearly intrinsic InN nanowires directly on Si(1 1 1) substrates by molecular beam epitaxy. The growth and characterization of Si- and Mg-doped InN nanowires is also investigated, which can exhibit superior structural and optical properties. We have further studied the epitaxial growth, fabrication, and characterization of InN:Si/i-InN and InN:Mg/i-InN/InN:Si axial nanowire structures on p-type and n-type Si(1 1 1) substrates, respectively. With the use of a CdS surface passivation, InN:Mg/i-InN/InN:Si nanowire homojunction solar cells exhibit a promising short-circuit current density of ~14.4 mA/cm2 and power-conversion efficiency of ~0.68% under simulated one-sun (AM 1.5G) illumination. This work suggests the first successful demonstration of p-type doping in InN nanowires and also constitutes important progress for the development of InGaN-based, full-solar-spectrum photovoltaics.

Exciton Kinetics, Quantum Efficiency, and Efficiency Droop of Monolayer MoS2 Light-Emitting Devices

We have investigated the quantum efficiency of monolayer MoS2 light-emitting devices through detailed temperature and power-dependent photoluminescence studies and rate equation analysis. The internal quantum efficiency can reach 45 and 8.3% at 83 and 300 K, respectively. However, efficiency droop is clearly measured with increasing carrier injection due to the unusually large Auger recombination coefficient, which is found to be ∼10(-24) cm(6)/s at room temperature, nearly 6 orders of magnitude higher than that of conventional bulk semiconductors. The significantly elevated Auger recombination in the emerging two-dimensional (2D) semiconductors is primarily an indirect process and is attributed to the abrupt bounding surfaces and the enhanced correlation, mediated by magnified Coulomb interactions, between electrons and holes confined in a 2D structure.

Highly efficient, spectrally pure 340 nm ultraviolet emission from AlxGa1−xN nanowire based light emitting diodes

High crystal quality, vertically aligned AlxGa1-xN nanowire based double heterojunction light emitting diodes (LEDs) are grown on Si substrate by molecular beam epitaxy. Such AlxGa1-xN nanowires exhibit unique core-shell structures, which can significantly suppress surface nonradiative recombination. We successfully demonstrate highly efficient AlxGa1-xN nanowire array based LEDs operating at ∼340 nm. Such nanowire devices exhibit superior electrical and optical performance, including an internal quantum efficiency of ∼59% at room temperature, a relatively small series resistance, highly stable emission characteristics, and the absence of efficiency droop under pulsed biasing conditions.

Engineering the Carrier Dynamics of InGaN Nanowire White Light-Emitting Diodes by Distributed p-AlGaN Electron Blocking Layers

We report on the demonstration of a new type of axial nanowire LED heterostructures, with the use of self-organized InGaN/AlGaN dot-in-a-wire core-shell nanowire arrays. The large bandgap AlGaN shell is spontaneously formed on the sidewall of the nanowire during the growth of AlGaN barrier of the quantum dot active region. As such, nonradiative surface recombination, that dominates the carrier dynamics of conventional axial nanowire LED structures, can be largely eliminated, leading to significantly increased carrier lifetime from ~0.3 ns to 4.5 ns. The luminescence emission is also enhanced by orders of magnitude. Moreover, the p-doped AlGaN barrier layers can function as distributed electron blocking layers (EBLs), which is found to be more effective in reducing electron overflow, compared to the conventional AlGaN EBL. The device displays strong white-light emission, with a color rendering index of ~95. An output power of >5 mW is measured for a 1 mm × 1 mm device, which is more than 500 times stronger than the conventional InGaN axial nanowire LEDs without AlGaN distributed EBLs.

A study of the size effect on the temperature-dependent resistivity of bismuth nanowires with rectangular cross-sections

Rectangular cross-section bismuth nanowires with dimensions of 50 nm by 70–200 nm (thickness by width) were fabricated using an electron beam writing technique. Individual nanowire measurement is possible using this method. The resistivities of the 50 nm thick nanowires were dependent on line width. The measured resistivity of 70, 120 and 200 nm wide nanowires was 4.05 × 10−3, 2.87 × 10−3 and 2.30 × 10−3 Ω cm at 300 K respectively. Temperature-dependent resistance measurements indicated that the electrical conductivity of the Bi nanowires was carrier dependent, and the carrier density decreased at low temperature, showing that the all the Bi nanowires exhibited semiconductor behaviour. The size-dependent resistivity of the Bi nanowires was an indication of the ordinary size effect in the one-dimensional nanowire, where the carrier mobility was grain boundary scattering dominated.

Tin- and indium-doped zinc oxide films prepared by RF magnetron sputtering

Abstract Doping effects of zinc oxide by indium and tin have been studied by an rf magnetron sputtering technique. It was found that both indium and tin were effective in producing low resistivity zinc oxide thin films. The film resistivity was observed to decrease by about 3 orders of magnitude as the SnO 2 content was increased from 0 to 2 wt% (in targets) and was essentially unchanged as the SnO 2 content was further increased to 10 wt%. For the indium doped films, the resistivity decreased continuously by about 4 orders of magnitude as the In 2 O 3 content was increased from 0 to 10 wt%. From X-ray diffraction, lattice constants were found to increase of indium and tin content in the films.

Color-tunable, phosphor-free InGaN nanowire light-emitting diode arrays monolithically integrated on silicon

We demonstrate controllable and tunable full color light generation through the monolithic integration of blue, green/yellow, and orange/red InGaN nanowire light-emitting diodes (LEDs). Such multi-color nanowire LED arrays are fabricated directly on Si substrate using a three-step selective area molecular beam epitaxy growth process. The lateral-arranged multi-color subpixels enable controlled light mixing at the chip-level and yield color-tunable light emission with CCT values in the range from 1900 K to 6800 K, while maintaining excellent color rendering capability. This work provides a viable approach for achieving micron and nanoscale tunable full-color LED arrays without the compromise between the device efficiency and light quality associated with conventional phosphor-based LEDs.