Nan Guan

Flexible White Light Emitting Diodes Based on Nitride Nanowires and Nanophosphors

We report the first demonstration of flexible white phosphor-converted light emitting diodes (LEDs) based on p–n junction core/shell nitride nanowires. GaN nanowires containing seven radial In0.2Ga0.8N/GaN quantum wells were grown by metal–organic chemical vapor deposition on a sapphire substrate by a catalyst-free approach. To fabricate the flexible LED, the nanowires are embedded into a phosphor-doped polymer matrix, peeled off from the growth substrate, and contacted using a flexible and transparent silver nanowire mesh. The electroluminescence of a flexible device presents a cool-white color with a spectral distribution covering a broad spectral range from 400 to 700 nm. Mechanical bending stress down to a curvature radius of 5 mm does not yield any degradation of the LED performance. The maximal measured external quantum efficiency of the white LED is 9.3%, and the wall plug efficiency is 2.4%.

Flexible Photodiodes Based on Nitride Core/Shell p–n Junction Nanowires

A flexible nitride p-n photodiode is demonstrated. The device consists of a composite nanowire/polymer membrane transferred onto a flexible substrate. The active element for light sensing is a vertical array of core/shell p–n junction nanowires containing InGaN/GaN quantum wells grown by MOVPE. Electron/hole generation and transport in core/shell nanowires are modeled within nonequilibrium Green function formalism showing a good agreement with experimental results. Fully flexible transparent contacts based on a silver nanowire network are used for device fabrication, which allows bending the detector to a few millimeter curvature radius without damage. The detector shows a photoresponse at wavelengths shorter than 430 nm with a peak responsivity of 0.096 A/W at 370 nm under zero bias. The operation speed for a 0.3 × 0.3 cm2 detector patch was tested between 4 Hz and 2 kHz. The −3 dB cutoff was found to be ∼35 Hz, which is faster than the operation speed for typical photoconductive detectors and which is compatible with UV monitoring applications.

Optimization of the optical coupling in nanowire-based integrated photonic platforms by FDTD simulation

The optimized design of a photonic platform based on a nanowire light emitting diode (LED) and a nanowire photodetector connected with a waveguide is proposed. The light coupling efficiency from the LED to the detector is optimized as a function of the geometrical parameters of the system using the finite difference time domain simulation tool Lumerical. Starting from a design reported in the literature with a coupling efficiency of only 8.7%, we propose an optimized photonic platform with efficiency reaching 65.5%.

InGaN/GaN nanowire flexible light emitting diodes and photodetectors

In this paper, we present our recent progress towards flexible nitride nanowire devices: we propose a method to combine high flexibility of polymer films with high quantum efficiency provided by nitride nanowires. The lift-off and transfer procedure allows to assemble free-standing layers of nanowire materials with different bandgaps without any constraint related to lattice-matching or growth condition compatibility. Following this method, we demonstrate blue, green, two-colour and white light emitting diodes as well as p-n photodiodes for integrable UVA sensors.

Nitride-nanowire-based flexible LEDs

Nitride LEDs are coming to replace other light sources in almost all general lighting, as well as in displays and life-science applications. Inorganic semiconductor devices, however, are naturally mechanically rigid and cannot be used in applications that require mechanical flexibility. Flexible LEDs are therefore currently a topic of intense research, as they are desirable for use in many applications, including rollable displays, wearable intelligent optoelectronics, bendable or implantable light sources, and biomedical devices. At present, flexible devices are mainly fabricated from organic materials. For example, organic LEDs (OLEDs) are already being used commercially in curved TV and smartphone screens. However, OLEDs have worse temporal stability and lower luminescence (especially in the blue spectral range) than nitride semiconductor LEDs. Substantial research efforts are thus being made to fabricate flexible inorganic LEDs.1

Optical properties of photodetectors based on single GaN nanowires with a transparent graphene contact

We report the fabrication and optical and electrical characterization of photodetectors for the UV spectral range based on single p–n junction nanowires with a transparent contact of a new type. The contact is based on CVD-grown (chemical-vapor deposition) graphene. The active region of the nitride nanowires contains a set of 30 radial In0.18Ga0.82N/GaN quantum wells. The structure is grown by metal-organic vaporphase epitaxy. The photodetectors are fabricated using electron-beam lithography. The current–voltage characteristics exhibit a rectifying behavior. The spectral sensitivity of the photodetector is recorded starting from 3 eV and extending far in the UV range. The maximal photoresponse is observed at a wavelength of 367 nm (sensitivity 1.9 mA/W). The response switching time of the photodetector is less than 0.1 s.

Nitride nanowire light emitting diodes

Here we present our recent results on nitride nanowire light emitting diodes. In particular, we discuss the single wire light emitting diode fabrication and coupling of single nanowire emitters with waveguides in order to form a functional photonic platform. We also discuss our recent advances towards flexible nitride nanowire devices. We propose a method to combine high flexibility of polymer films with high quantum efficiency provided by nitride nanowires to achieve flexible inorganic light emitting diodes.