Arturas Zukauskas

Lattice and energy band engineering in AlInGaN/GaN heterostructures

We report on structural, optical, and electrical properties of AlxInyGa1−x−yNGaN heterostructures grown on sapphire and 6H–SiC substrates. Our results demonstrate that incorporation of In reduces the lattice mismatch, Δa, between AlInGaN and GaN, and that an In to Al ratio of close to 1:5 results in nearly strain-free heterostructures. The observed reduction in band gap, ΔEg, determined from photoluminescence measurements, is more than 1.5 times higher than estimated from the linear dependencies of Δa and ΔEg on the In molar fraction. The incorporation of In and resulting changes in the built-in strain in AlInGaN/GaN heterostructures strongly affect the transport properties of the two-dimensional electron gas at the heterointerface. The obtained results demonstrate the potential of strain energy band engineering for GaN-based electronic applications.

Double-Scaled Potential Profile in a Group-III Nitride Alloy Revealed by Monte Carlo Simulation of Exciton Hopping

The temperature dependences of the peak position and width of the photoluminescence band in Al0.1In0.01Ga0.89N layers were explained by Monte Carlo simulation of exciton localization and hopping. The introduction of a doubled-scaled potential profile due to inhomogeneous distribution of indium allowed obtaining a good quantitative fit of the experimental data. Hopping of excitons was assumed to occur through localized states distributed on a 16 meV energy scale within the In-rich clusters with the average energy in these clusters dispersed on a larger (42 meV) scale.

Quadrichromatic white solid-state lamp with digital feedback

White light with high color rendering indices can be produced by additive color mixing of emissions from several light-emitting diodes (LEDs) having different primary colors. White Versatile Solid-State Lamps (VSSLs) with variable color temperature, constant-chromaticity dimming, and efficiency/color-rendering trade-off can be developed using pulse-width modulation (PWM) driving technique. However, such lamps exhibit chromaticity shifts caused by different temperature and aging coefficients of the optical output for primary LEDs of different colors. To overcome this drawback, we developed a polychromatic white solid-state lamp with an internal digital feedback. The lamp features a quadrichromatic (red-amber-green-blue) design based on commercially available high-power LEDs. The design is optimized to achieve high values of the general color rendering index (69 to 79 points) in the color-temperature range of 2856 to 6504 K. A computer-controlled driving circuit contains a pulse-width modulator and a photodiode-based meter. The software performs periodical measurement of the radiant flux from primary LEDs of each color and adjusts the widths of the driving pulses. These VSSLs with feedback found application in phototherapy of seasonal affective disorder (SAD).

Luminescence decay in highly excited GaN grown by hydride vapor-phase epitaxy

Carrier recombination dynamics in GaN grown by hydride vapor-phase epitaxy has been studied by means of transient photoluminescence under high photoexcitation conditions that are close to stimulated emission regime. The luminescence transient featured an exponential decay with the time constant of 205 ps at room temperature. The transient was shown to be in good agreement with a model of saturated centers of nonradiative recombination with the trap density of ∼1017 cm−3 and carrier recombination coefficients of ∼10−8 cm3/s. In such a regime, the lifetimes of electrons and holes have a common value of 410 ps.

Cultural Preferences to Color Quality of Illumination of Different Artwork Objects Revealed by a Color Rendition Engine

The preferences to color quality of illumination were investigated for American and Chinese subjects using a solid-state source of white light with the continuously tunable color saturation ability and correlated color temperature of quadrichromatic blends. Subjects were asked to identify both “most natural” and preferred blends. For very familiar objects, cultural differences did not affect the average of the selected blends. For less familiar objects (various paintings), cultural differences in the average selected blends depended on the level of the familiarity of the content. An unfamiliar painting also showed preferences to color temperature being dependent on the cultural background. In all cases, the American subjects exhibited noticeably wider distributions of selection rates.

Optimization of multichip white solid-state lighting source with four or more LEDs

Polychromatic solid-state lamps that produce white light by additive mixing of the emissions from primary colored light emitting diodes (LEDs) should have a higher luminous efficiency that those using phosphors. These lamps require emission spectra that feature an optimal trade-off between luminous efficacy and color rendering. We developed a mathematical technique that allows us to maximize the luminous efficacy and general color rendering index (CRI) for the white solid-state lamp composed of an arbitrary number of primary LEDs with given spectra. We use this method in order to compare the optimal efficacy and general CRI for 4 and 5 primary LEDs with that for 2 and 3 LEDs. For a particular color temperature, the required number of primary LEDs depends on the trade-off between efficacy and general CRI. The quadrichromatic lamp is shown to meet requirements for most practical applications. Quintichromatic lamps and lamps with a higher number of primary LEDs yield negligible benefit in improving color rendering. However, quintichromatic LED lamps are capable of producing quasi-continuous spectra that might meet special lighting needs.

Energy Band/Lattice Mismatch Engineering in Quaternary AlInGaN/GaN Heterostructure

We discuss structural, optical and electrical properties of AlxInyGa1—x—yN/GaN heterostructures grown on sapphire and 6H-SiC substrates. The incorporation of In reduces the lattice mismatch in a much stronger way than the energy gap discontinuity. An In to Al ratio close to 1:5 should result in nearly strain-free heterostructures. The incorporation of In and resulting changes in the built-in strain in AlInGaN/GaN heterostructures strongly affect the transport properties of the two-dimensional (2D) electron gas at the heterointerface. Using the incorporation of In in order to increase the Al molar fraction preserving the same value of strain should allow us to greatly enhance the sheet carrier density at the AlGaInN/GaN heterointerface. Further work on improving In incorporation techniques is needed in order to meet these expectations.

Pulsed Atomic Layer Epitaxy of Quaternary AlInGaN Layers for Ultraviolet Light Emitters

We report on a pulsed atomic layer epitaxy (PALE) technique for quaternary AlInGaN growth. PALE allows for the deposition of high-quality AlInGaN layers at much lower temperatures than those required for conventional low-pressure metalorganic chemical vapor deposition (MOCVD). The low growth temperature leads to an efficient incorporation of indium and, as a result, to a dramatic improvement in emission properties of the material. A deep ultraviolet optical gain of 300 cm -1 peaked at 330 nm is obtained in quaternary AlInGaN MQWs. Also, in PALE AlInGaN material, the optical emission properties do not degrade with increasing Al mole fraction. These results establish PALE as a promising technique for deep UV emitters.

Firelight LED Source: Toward a Balanced Approach to the Performance of Solid-State Lighting for Outdoor Environments

We report on a blue-amber (“firelight”) cluster of light-emitting diodes (LEDs) with extra-low correlated color temperature (~1860 K) optimized for outdoor lighting under mesopic conditions. When compared with common white LEDs, the firelight LED cluster shows considerably reduced indexes of melatonin suppression and skyglow, increased retinal illuminance for elderly people, but a reduced performance of perceiving colors, which, however, can be tolerated at mesopic luminance. In comparison with an almost metameric high-pressure sodium lamp, the cluster exhibits a potentially higher luminous efficacy, similar reaction time and detection threshold of luminance contrasts for achromatic targets, and noticeably improved color discrimination characteristics.

High-power LEDs for plant cultivation

We report on high-power solid-state lighting facility for cultivation of greenhouse vegetables and on the results of the study of control of photosynthetic activity and growth morphology of radish and lettuce imposed by variation of the spectral composition of illumination. Experimental lighting modules (useful area of 0.22 m2) were designed based on 4 types of high-power light-emitting diodes (LEDs) with emission peaked in red at the wavelengths of 660 nm and 640 nm (predominantly absorbed by chlorophyll a and b for photosynthesis, respectively), in blue at 455 nm (phototropic function), and in far-red at 735 nm (important for photomorphology). Morphological characteristics, chlorophyll and phytohormone concentrations in radish and lettuce grown in phytotron chambers under lighting with different spectral composition of the LED-based illuminator and under illumination by high pressure sodium lamps with an equivalent photosynthetic photon flux density were compared. A well-balanced solid-state lighting was found to enhance production of green mass and to ensure healthy morphogenesis of plants compared to those grown using conventional lighting. We observed that the plant morphology and concentrations of morphologically active phytohormones is strongly affected by the spectral composition of light in the red region. Commercial application of the LED-based illumination for large-scale plant cultivation is discussed. This technology is favorable from the point of view of energy consumption, controllable growth, and food safety but is hindered by high cost of the LEDs. Large scale manufacturing of high-power red AlInGaP-based LEDs emitting at 650 nm and a further decrease of the photon price for the LEDs emitting in the vicinity of the absorption peak of chlorophylls have to be achieved to promote horticulture applications.