H. X. Jiang

III-Nitride Blue and Ultraviolet Photonic Crystal Light Emitting Diodes

We present results on enhancement of 460 nm blue and 340 nm UV optical power output in III-nitride light emitting diodes (LEDs) using photonic crystals (PCs) under current injection. Triangular arrays of the PCs with diameter/periodicity of 300/700 nm were patterned using electron-beam lithography and inductively coupled plasma dry etching. The total power at 20 mA of 300×300 μm2 unpackaged LED chips revealed an increase by 63% and 95% for the blue and UV LEDs, respectively, as a result of the PC formation. Possible ways for further improving enhancement of light extraction using PCs are discussed.

InGaN/GaN multiple quantum well solar cells with long operating wavelengths

We report on the fabrication and photovoltaic characteristics of InGaN solar cells by exploiting InGaN/GaN multiple quantum wells (MQWs) with In contents exceeding 0.3, attempting to alleviate to a certain degree the phase separation issue and demonstrate solar cell operation at wavelengths longer than previous attainments (>420 nm). The fabricated solar cells based on In0.3Ga0.7N/GaN MQWs exhibit an open circuit voltage of about 2 V, fill factor of about 60%, and an external efficiency of 40% (10%) at 420 nm (450 nm).

Band structure and fundamental optical transitions in wurtzite AlN

With a recently developed unique deep ultraviolet picoseconds time-resolved photoluminescence (PL) spectroscopy system and improved growth technique, we are able to determine the detailed band structure near the Γ point of wurtzite (WZ) AlN with a direct band gap of 6.12 eV. Combined with first-principles band structure calculations we show that the fundamental optical properties of AlN differ drastically from that of GaN and other WZ semiconductors. The discrepancy in energy band gap values of AlN obtained previously by different methods is explained in terms of the optical selection rules in AlN and is confirmed by measurement of the polarization dependence of the excitonic PL spectra.

Unique optical properties of AlGaN alloys and related ultraviolet emitters

Deep UV photoluminescence spectroscopy has been employed to study the optical properties of AlxGa1−xN alloys (0⩽x⩽1). The emission intensity with polarization of E⊥c and the degree of polarization were found to decrease with increasing x. This is a consequence of the fact that the dominant band edge emission in GaN (AlN) is with polarization of E⊥c(E∥c). Our experimental results suggest that the decreased emission efficiency in AlxGa1−xN alloys and related UV emitters could also be related with their unique polarization property, i.e., the intensity of light emission with polarization of E⊥c decreases with x. It is thus concluded that UV emitters with AlGaN alloys as active layers have very different properties from InGaN and other semiconductor emitters.

Deep Ultraviolet to Near-Infrared Emission and Photoresponse in Layered N-Doped Graphene Quantum Dots

Material that can emit broad spectral wavelengths covering deep ultraviolet, visible, and near-infrared is highly desirable. It can lead to important applications such as broadband modulators, photodetectors, solar cells, bioimaging, and fiber communications. However, there is currently no material that meets such desirable requirement. Here, we report the layered structure of nitrogen-doped graphene quantum dots (N-GQDs) which possess broadband emission ranging from 300 to >1000 nm. The broadband emission is attributed to the layered structure of the N-GQDs that contains a large conjugated system and provides extensive delocalized π electrons. In addition, a broadband photodetector with responsivity as high as 325 V/W is demonstrated by coating N-GQDs onto interdigital gold electrodes. The unusual negative photocurrent is observed which is attributed to the trapping sites induced by the self-passivated surface states in the N-GQDs.

III-nitride photonic crystals

We report the achievement of nanofabrication and characterization of a triangular lattice array of photonic crystals (PCs) with diameter/periodicity as small as 100/180 nm on an InGaN/GaN multiple quantum well using electron-beam lithography and inductively coupled plasma dry etching. Optical measurements of the PCs performed using near-field scanning optical microscopy showed a 60° periodic variation with the angle between the propagation direction of emission light and the PCs lattice. An unprecedented maximum enhancement factor of 20 was obtained for the emission light intensity at wavelengths as short as 475 nm at room temperature with emission light parallel to the Γ–K direction of the PCs lattice. The implications of these results to nitride-based optoelectronic devices, particularly in improving the light extraction efficiency in light-emitting diodes both for blue/green as well as UV emitters, are discussed.

Fundamental optical transitions in GaN

A coherent picture for the band structure near the Γ point and the associated fundamental optical transitions in wurtzite (WZ) GaN, including the electron and hole effective masses and the binding energies of the free excitons associated with different valence bands, has been derived from time‐resolved photoluminescence measurements and a theoretical calculation based on the local density approximation. We also determine the radiative recombination lifetimes of the free excitons and neutral impurity (donor and acceptor) bound excitons in WZ GaN and compare ratios of the radiative lifetimes with calculated values of the ratios obtained with existing theories of free and bound excitons.

InGaN/GaN multiple quantum well concentrator solar cells

We present the growth, fabrication, and photovoltaic characteristics of Inx Ga1−xN/GaN(x∼0.35) multiple quantum well solar cells for concentrator applications. The open circuit voltage, short circuit current density, and solar-energy-to-electricity conversion efficiency were found to increase under concentrated sunlight. The overall efficiency increases from 2.95% to 3.03% when solar concentration increases from 1 to 30 suns and could be enhanced by further improving the material quality.

Nitride deep-ultraviolet light-emitting diodes with microlens array

We report on the fabrication of 280-nm AlGaN-based deep-ultraviolet light-emitting diodes (UV LEDs) on sapphire substrates with an integrated microlens array. Microlenses with a diameter of 12μm were fabricated on the sapphire substrate by resist thermal reflow and plasma dry etching. LED devices were flip-chip bonded on high thermal conductive AlN ceramic submounts to improve the thermal dissipation, and the emitted UV light was extracted through the sapphire substrates. With the integrated microlens array, a 55% enhancement in the output power at 20-mA dc driving was achieved compared with the same LED without microlens. The light extraction enhancement is the result of the reduced internal reflections of the light caused by the microlens surface profile.

III-nitride blue microdisplays

Prototype blue microdisplays have been fabricated from InGaN/GaN quantum wells. The device has a dimension of 0.5×0.5 mm2 and consists of 10×10 pixels 12 μm in diameter. Emission properties such as electroluminescence spectra, output power versus forward current (L–I) characteristic, viewing angle, and uniformity have been measured. Due to the unique properties of III-nitride wide-band-gap semiconductors, microdisplays fabricated from III nitrides can potentially provide unsurpassed performance, including high-brightness/resolution/contrast, high-temperature/high-power operation, high shock resistance, wide viewing angles, full-color spectrum capability, long life, high speed, and low-power consumption, thus providing an enhancement and benefit to the present capabilities of miniature display systems.