Z. Z. Chen

Strain relaxation in InGaN/GaN micro-pillars evidenced by high resolution cathodoluminescence hyperspectral imaging

A size-dependent strain relaxation and its effects on the optical properties of InGaN/GaN multiple quantum wells (QWs) in micro-pillars have been investigated through a combination of high spatial resolution cathodoluminescence (CL) hyperspectral imaging and numerical modeling. The pillars have diameters (d) ranging from 2 to 150 μm and were fabricated from a III-nitride light-emitting diode (LED) structure optimized for yellow-green emission at ∼560 nm. The CL mapping enables us to investigate strain relaxation in these pillars on a sub-micron scale and to confirm for the first time that a narrow (≤2 μm) edge blue-shift occurs even for the large InGaN/GaN pillars (d > 10 μm). The observed maximum blue-shift at the pillar edge exceeds 7 nm with respect to the pillar centre for the pillars with diameters in the 2–16 μm range. For the smallest pillar (d = 2 μm), the total blue-shift at the edge is 17.5 nm including an 8.2 nm “global” blue-shift at the pillar centre in comparison with the unetched wafer. By ...

Study on the morphology and shape control of volcano-shaped patterned sapphire substrates fabricated by imprinting and wet etching

A volcano-shaped patterned sapphire substrate (VPSS) was fabricated by imprinting lithography and wet etching to enhance the light output of LED devices. A hexagonally arranged pattern with different crystal orientations was imprinted onto the sapphire substrate. As the etching time increased, the pattern with a crater in its center was changed from truncated triangular pyramids to truncated hexagonal pyramids with symmetrical sidewall facets. Small craters surrounded by three {108} facets appeared with 3-fold or 6-fold symmetry at the boundaries with neighboring pyramids. The mechanism of sapphire wet etching for VPSS synthesis was correlated to thermodynamics limits and the SiO2 mask pattern. The as-fabricated VPSS with slant angles of 34.3° and 69.9° was considered to enhance the internal quantum efficiency (IQE) and light extraction efficiency (LEE) of GaN-based LEDs.

Silane controlled three dimensional GaN growth and recovery stages on a cone-shape nanoscale patterned sapphire substrate by MOCVD

Three dimensional (3D) growth induced by silane was performed on cone-shape nano-scale patterned sapphire substrates (NPSS) by metal organic chemical vapor deposition (MOCVD). The growth evolution for the silane controlled 3D growth process and the recovery stage were investigated by a series of growth interruptions. The GaN epilayers grown on the templates with different 3D growth conditions were characterized by X-ray diffraction (XRD), Raman scattering, and atomic force microscopy (AFM) measurements. The full width at half maximums (FWHMs) of the (002) and (102) reflections in the XRD rocking curves were 267 and 324 arcsec, respectively, for the sample on NPSS with 600 s of 3D growth. An extremely smooth surface was achieved with an average roughness of 0.10 nm over 3 × 3 μm2. All the above data were superior to those for the planar sample or the NPSS ones without the optimized 3D growth time. The silane addition caused effective 3D growth. The size, homogeneity, and faceted sidewalls of the islands by the 3D growth led to a high crystalline quality, much strain relaxation and a specular surface for the GaN epilayers.

Capability of GaN based micro-light emitting diodes operated at an injection level of kA/cm2

Different size InGaN/GaN based micro-LEDs (μLEDs) are fabricated. An extremely high injection level above 16 kA/cm2 is achieved for 10 μm-diameter LED. The lateral current density and carrier distributions of the μLEDs are simulated by APSYS software. Streak camera time resolved photoluminescence (TRPL) results show clear evidence that the band-gap renormalization (BGR) effect is weakened by strain relaxation in smaller size μLEDs. BGR affects the relaxation of free carriers on the conduction band bottom in multiple quantum wells (MQWs), and then indirectly affects the recombination rate of carriers. An energy band model based on BGR effect is made to explain the high-injection-level phenomenon for μLEDs.

Excited state Faraday anomalous dispersion optical filters based on indirect laser pumping.

The direct pump method now used in excited state Faraday anomalous dispersion optical filters (ES-FADOFs) requires that the transition between the target and the ground state is an electric dipole allowed transition and that a laser that operates at the exact pump wavelength is available. This is not always satisfied in practice. An indirect laser pump method for ES-FADOF is proposed and experimentally realized. Compared with the commonly used direct pump method, this indirect pump method can reach the same performance using lasers at very different wavelengths. This method can greatly extend the wavelength range of FADOF and provide a novel scheme for ES-FADOF design.

Parity-time symmetric Bragg structure in atomic vapor.

We propose an efficient scheme in helium or alkaline earth atomic vapor to achieve a parity-time symmetric Bragg structure using coherent lights. Unidirectional invisibility can be realized in this scheme, i.e., the atomic vapor shows total transparency for probe light incident from one particular direction, but exhibits enhanced Bragg reflection for probe from the opposite side. By changing the relative phase between the coherent lights, this direction can easily be manipulated, providing a convenient way for investigating special properties of 𝒫𝒯 -symmetric Bragg structures.

Absorption-free Bragg reflector using Zeeman sublevels in atomic vapor.

Absorption-free Bragg reflector has been studied in ions doped in crystals. We propose a new scheme using Zeeman sublevels of atoms to construct an absorption-free Bragg reflector with practical laser power. Its spatial period of refractive index equals half of the wavelength of the incident standing-wave coupling light. The proposal is simulated in a helium atom scheme, and can be extended to alkali earth atoms.

Study on GaN nucleation and coalescence in the initial growth stages on nanoscale patterned sapphire substrates by MOCVD

GaN growth on cone-shaped nanoscale patterned sapphire substrates (NPSS) in the initial stages was investigated via a series of growth interruptions. The thickness of the nucleation layer (NL) was optimized as 15 nm. A high-quality GaN film was achieved with an average surface roughness of 0.13 nm over 5 × 5 μm2, a threading dislocation density (TDD) of 1.80 × 108 cm−2 and a residual stress of 0.40 GPa. X-ray diffraction (XRD) ω/φ-scans, cathodoluminescence (CL) mapping and transmission electron microscopy (TEM) measurements were carried out to study the GaN coalescence process on NPSS. Nano-nucleating, lateral growth and the submerging of the large tilt grains were confirmed as the primary factors that reduced the TDD and residual stress. The mechanism for GaN nucleation on NPSS and its effect on the subsequent growth stages were elucidated.

Modification of far-field radiation pattern by shaping InGaN/GaN nanorods

In this work, we report on the fabrication of “golftee,” “castle,” and “pillar” shaped InGaN/GaN nanorod light-emitting diode (LED) arrays with a typical rod diameter of 200 nm based on nanoimprint lithography, dry etching, and wet etching. The photoluminescence (PL) integral intensities per active region area for “golftee,” “castle,” and “pillar” shaped nanorod samples were found to be 2.6, 1.9, and 2.2 times stronger than that of a conventional planar LED. Additionally, the far-field radiation patterns of the three different shaped nanorod samples were investigated based on angular resolved PL (ARPL) measurements. It was found that the sharp lobes appeared at certain angles in the ARPL curve of the “golftee” sample, while broad lobes were observed in the ARPL curves of the “castle” and “pillar” samples. Further analysis suggests that the shorter PL lifetime and smaller spectral width of the “golftee” sample were due to the coupling of photon modes with excitons, which also led to the observed high effic...

Excited state atomic optical filters

In this paper, we introduce by far the most promising schemes to realize excited state atomic filter. Excited state Faraday anomalous dispersion optical filter (ES-FADOF) with direct pumping, as the traditional method, provides a very direct way, while ES-FADOF with indirect pumping, as proposed very recently, provides much more choices. Moreover, induced dichroism excited state atomic line filter can provide much narrower bandwidth but lower transmittance. Based on the requirement of specific applications, different methods should be used.