Abstract The scanning electron microscopy techniques of electron backscatter diffraction (EBSD), electron channelling contrast imaging (ECCI) and cathodoluminescence (CL) hyperspectral imaging provide complementary information on the structural and luminescence properties of materials rapidly and non-destructively, with a spatial resolution of tens of nanometres. EBSD provides crystal orientation, crystal phase and strain analysis, whilst ECCI is used to determine the planar distribution of extended defects over a large area of a given sample. CL reveals the influence of crystal structure, composition and strain on intrinsic luminescence and/or reveals defect-related luminescence. Dark features are also observed in CL images where carrier recombination at defects is non-radiative. The combination of these techniques is a powerful approach to clarifying the role of crystallography and extended defects on a material’s light emission properties. Here we describe the EBSD, ECCI and CL techniques and illustrate their use for investigating the structural and light emitting properties of UV-emitting nitride semiconductor structures. We discuss our investigations of the type, density and distribution of defects in GaN, AlN and AlGaN thin films and also discuss the determination of the polarity of GaN nanowires.
We investigated the effect of bow reduction of AlInGaN-based deep-ultraviolet light-emitting diode (DUV LED) wafers using internally focused laser patterning. The laser-induced stress inside of the sapphire substrate was found to increase with increasing density of laser lines and decreasing distance of the laser lines to the back surface of the sapphire substrate. By adjusting the laser process, the convex bow of DUV LED wafers could be almost completely eliminated. This bow reduction resulted in an improved uniformity of the LED forward voltage across the wafer compared to the reference wafer without laser patterning.
In the framework of k·p-theory, semipolar (202¯1) and (202¯1¯) InGaN/GaN quantum wells (QWs) have equivalent band structures and are expected to have identical optical polarization properties. However, (202¯1) QWs consistently exhibit a lower degree of linear polarization (DLP) than (202¯1¯) QWs. To understand this peculiarity, we investigate the optical properties of (202¯1) and (202¯1¯) InGaN/GaN single QW light-emitting diodes (LEDs) via resonant polarization-resolved photoluminescence microscopy. LEDs were grown on bulk substrates by metal organic vapor phase epitaxy with different indium concentrations resulting in emission wavelengths between 442 nm and 491 nm. We discuss the origin of their DLP via k·p band structure calculations. An analytical expression to estimate the DLP in the Boltzmann-regime is proposed. Measurements of the DLP at 10 K and 300 K are compared to m-plane LEDs and highlight several discrepancies with calculations. We observe a strong correlation between DLPs and spectral widths, which indicates that inhomogeneous broadening affects the optical polarization properties. Considering indium content fluctuations, QW thickness fluctuations, and the localization length of charge carriers, we argue that different broadenings apply to each subband and introduce a formalism using effective masses to account for inhomogeneous broadening in the calculation of the DLP. We conclude that the different DLP of (202¯1) and (202¯1¯) QWs might be related to different effective broadenings of their valence subbands induced by the rougher upper QW interface in (202¯1), by the larger sensitivity of holes to this upper interface due to the polarization field in (202¯1), and/or by the different degrees of localization of holes.
We daily rely upon vertical-cavity surface-emitting lasers (VCSELs) for facial recognition and data communication. These lasers are now experiencing exponential growth and serves in other applications as well such as oxygen monitoring in combustion processes and in anesthetized patients in hospitals and as a source of heating in industry in the form of a large-sized array. The large interest for this laser class is linked to its beneficial qualities such as low threshold current, circular-symmetric low-divergent output beam, high efficiency, compactness, and low fabrication cost due to on-wafer testing. Due to these advantages, there is a strong push to realize VCSELs in other wavelength regimes, beyond the commercially available infrared and red. This would open completely new markets such as flood lights, projectors, sterilization, and medical diagnosis and treatment.
The performance and degradation characteristics of continuous-wave (cw) InGaN multiple-quantum-well laser diodes are reported. A cw threshold current as low as 62 mA was obtained for ridge-waveguide laser diodes on epitaxially laterally overgrown GaN on sapphire substrates grown by metalorganic chemical vapor deposition. Transmission electron microscopy reveals a defect density <5×107 cm−2 in the active region. The emission wavelength was near 400 nm with output powers greater than 20 mW per facet. Under cw conditions, laser oscillation was observed up to 70 °C. The room-temperature cw operation lifetimes, measured at a constant output power of 2 mW, exceeded 15 h. From the temperature dependence of the laser diode lifetimes, an activation energy of 0.50 eV±0.05 eV was determined.
This presentation will provide an overview of the state-of-the-art in the development of AlGaN-based far-UVC-LED technologies. We will explore origins for the observed decline in the external quantum efficiency (EQE) with decreasing emission wavelength and present different approaches to improve the RRE, CIE, and LEE of UV light emitters. We will also discuss design aspects for far-UVC irradiation systems and provide an outlook of future prospects of far-UVC-LED device technology as well as the potential for a wider use of far-UVC sources in applications like room air disinfection.
Vertically injected thin-film ultraviolet light-emitting diodes operating at 325 and 280nm are demonstrated. Low-temperature AlN interlayers allow crack-free growth of AlxGa1−xN with compositions up to x=0.53 on GaN-on-sapphire templates. The GaN layer allows laser-induced separation of the highly strained epi stack from the sapphire substrate with high yield. Cathode contacts are formed on nitrogen-face AlxGa1−xN (up to x=0.53) and allow vertical injection of current into the active region. Controlled roughening of the nitrogen-face AlxGa1−xN is also demonstrated through photoelectrochemical etching and results in >2.5× light extraction gain for 325 and 280nm devices.
Q -switching is demonstrated in a two-section InGaN multiple-quantum-well (MQW) laser diode consisting of an electroabsorption modulator and amplifier (gain) section. The modulator and gain sections are optically coupled and share the same InGaN MQW active region, but they are electrically separated by a narrow dry-etched trench. Applying a reverse bias voltage to the modulator section controls the absorption in the modulator portion of the device by compensating the piezoelectric field in the InGaN quantum wells. Changes in the absorption coefficient as large as 5000 cm−1 were realized with a moderate reverse bias of 7.2 V. By forward biasing, the amplifier section at a constant current of 225 mA and by controlling the reverse bias modulator voltage, the output power of the two-section laser diode could be switched between <0.5 mW (off state) and more than 3 mW (on state) with a laser emission wavelength near 401 nm.
The recombination kinetics of the free exciton (FX) and basal plane stacking fault (BSF) emission in a-plane GaN epitaxial lateral overgrowth structures is analyzed by ps-time-resolved cathodoluminescence microscopy in the temperature range from 5K to 300K. The capture of FX by donors and the thermionic emission of holes from the BSF Quantum Well is analyzed.
We have studied the interface formation between cyclopentene and $(2\ifmmode\times\else\texttimes\fi{}4)$ reconstructed InP (001) surfaces by soft x-ray photoemission spectroscopy, reflectance anisotropy spectroscopy (RAS), and ab initio theory. After preparation of an uncontaminated $(2\ifmmode\times\else\texttimes\fi{}4)$ reconstruction under ultrahigh vacuum conditions, the surface was exposed to cyclopentene as monitored by RAS. The changes in the $\text{In}\text{ }4d$, $\text{P}\text{ }2p$, and $\text{C}\text{ }1s$ core-level emission lines upon molecule adsorption indicate a covalent bonding of cyclopentene to the topmost atoms of the surface at two different bonding sites. Based on these results, a structure model is suggested, which is supported by ab initio calculations of the total-energy, the RAS signature, and the $\text{In}\text{ }4d$ and $\text{P}\text{ }2p$ core-level shifts. Our results suggest that the cyclopentene adsorption is a two-step process: first cyclopentene adsorbs on the ``mixed dimer'' and second the changes in the surface structure enable the additional adsorption on the second layer In--In surface bond.
