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The future generation of modern illumination should not only be cheap and highly efficient, but also demonstrate high quality of light, light which allows better color differentiation and fidelity. Here we are presenting a novel approach to create a white solid-state light source providing ultimate color rendition necessary for a number of applications. The proposed semi-hybrid device combines a monolithic blue-cyan light emitting diode (MBC LED) with a green-red phosphor mixture. It has shown a superior color rendering index (CRI), 98.6, at correlated color temperature of around 3400 K. The MBC LED epi-structure did not suffer from the efficiency reduction typical for monolithic multi-color emitters and was implemented in the two most popular chip designs: “epi-up” and “flip-chip”. Redistribution of the blue and cyan band amplitudes in the white-light emission spectrum, using the operating current, is found to be an effective tool for fine tuning the color characteristics. (Figure presented.).

We demonstrate a phosphor free, dichromatic GaN-based monolithic white LED with vertically stacked green and blue emitting multiple quantum wells. The optimal thickness of GaN barrier layer between green and blue quantum wells used is 8 nm. This device can be tuned over a wide range of correlated color temperature (CCT) to achieve warm white (CCT = 3600 K) to cool white (CCT = 13,000 K) emission by current modulation from 2.3 A/cm2 to 12.9 A/cm2. It is also demonstrated for the first time that a color rendering index (CRI) as high as 67 can be achieved with such a dichromatic source. The observed CCT and CRI tunability is associated with the spectral power evolution due to the pumping-induced carrier redistribution.

External quantum efficiency of industrial-grade green InGaN light-emitting diodes (LEDs) has been measured in a wide range of operating currents at various temperatures from 13 K to 300 K. Unlike blue LEDs, the efficiency as a function of current is found to have a multi-peak character, which could not be fitted by a simple ABC-model. This observation correlated with splitting of LED emission spectra into two peaks at certain currents. The characterization data are interpreted in terms of non-uniformity of the LED active region, which is tentatively attributed to extended defects like V-pits. We suggest a new approach to evaluation of temperature-dependent light extraction and internal quantum efficiencies taking into account the active region non-uniformity. As a result, the temperature dependence of light extraction and internal quantum efficiencies have been evaluated in the temperature range mentioned above and compared with those of blue LEDs.

External quantum efficiency of industrial-grade green InGaN light-emitting diodes (LEDs) has been measured in a wide range of operating currents at various temperatures from 13 K to 300 K. Unlike blue LEDs, the efficiency as a function of current is found to have a multi-peak character, which could not be fitted by a simple ABC-model. This observation correlated with splitting of LED emission spectra into two peaks at certain currents. The characterization data are interpreted in terms of non-uniformity of the LED active region, which is tentatively attributed to extended defects like V-pits. We suggest a new approach to evaluation of temperature-dependent light extraction and internal quantum efficiencies taking into account the active region non-uniformity. As a result, the temperature dependence of light extraction and internal quantum efficiencies have been evaluated in the temperature range mentioned above and compared with those of blue LEDs.

Internal quantum efficiency (IQE) of a blue high-brightness InGaN/GaN light-emitting diode (LED) was evaluated from the external quantum efficiency measured as a function of current at various temperatures ranged between 13 and 440 K. Processing the data with a novel evaluation procedure based on the ABC-model, we have determined the temperature-dependent IQE of the LED structure and light extraction efficiency of the LED chip. Separate evaluation of these parameters is helpful for further optimization of the heterostructure and chip designs. The data obtained enable making a guess on the temperature dependence of the radiative and Auger recombination coefficients, which may be important for identification of dominant mechanisms responsible for the efficiency droop in III-nitride LEDs. Thermal degradation of the LED performance in terms of the emission efficiency is also considered.

Efficiency of commercial 620 nm InAlGaP Golden Dragon-cased high-power LEDs has been studied under extremely high pump current density up to 4.5 kA/cm2 and pulse duration from microsecond down to sub-nanosecond range. No efficiency decrease and negligible red shift of the emission wavelength is observed in the whole range of drive currents at nanosecond-range pulses with duty cycles well below 1%. Analysis of the pulse-duration dependence of the LED efficiency and emission spectrum suggests the active region overheating to be the major mechanism of the LED efficiency reduction at higher pumping, dominating over the electron overflow and Auger recombination.

Two blue (450 nm) light–emitting diodes (LED), which only differ in top p-GaN layer growth conditions, were comparatively investigated. I-V, C-V, TLM, Electroluminescence (EL) and Photoluminescence (PL) techniques were applied to clarify a correlation between MOCVD carrier gas and internal properties. The A-structure grown in the pure N2 environment demonstrated better parameters than the B-structure grown in the N2/H2 (1:1) gas mixture. The mixed growth atmosphere leaded to an increase of sheet resistances of p-GaN layer. EL and PL measurements confirmed the advantage of the pure N2 utilization, and C(VR) measurement pointed the increase of static charge concentration near the p-GaN interface in the B structure.

In this paper, current-dependent emission spectra and efficiency measured on the same AlGaInP red light-emitting diode (LED) pumped with the current pulses of very different durations are recorded. This enabled for the first time distinguishing between high-carrier concentration and self-heating effects on the efficiency decline at high current magnitudes. The electron leakage to the p-side of the LED structure, which is the major mechanism of the efficiency reduction, is found to rise substantially when the device self-heating starts to develop. As a result, in comparison to continuous-wave excitation, driving the LED with sub-microsecond current pulses allows suppressing the device self-heating and, eventually, increasing the operating current by an order of magnitude without noticeable efficiency losses. Based on the reduced ABC-model, neglecting Auger recombination, the light extraction efficiency, injection efficiency, and internal quantum efficiency of the LED are estimated, suggesting light extraction to be the most critical factor for the overall efficiency of the LED. The coupled spectral/power LED characterization using the variable-duration current pulse pumping is found to be an effective approach for analyzing mechanisms of the device operation.

White electroluminescence (EL) from ZnO/GaN structures fabricated by pulsed laser deposition of Zn:In onto GaN:Mg/GaN structures MOCVD-grown on Al2O3 substrates has been observed. The white light is produced by superposition of two strongest emission lines, narrow blue and broad yellow, peaked at 440 and 550 nm, respectively. The intensity ratio of different EL lines from ZnO/GaN/Al2O3 structures depends on the ZnO film quality and drive current. The white EL is due to the high density of structural defects at the n-ZnO/p-GaN interface. A band diagram of the n-ZnO/p-GaN/n-GaN is constructed and a qualitative explanation of the EL is given. Conditions of ZnO deposition strongly affects the properties of the recombination emission and predetermines the EL spectrum of the LED structure if it does not have high quantum efficiency (more than 1%) such as in commercial LEDs.

We studied a Pb x Zr 1− x TiO 3 /SnO 2 /Al 2 O 3 heterostructure as a base for transparent ferroelectric field-effect transistor. Single-crystal SnO 2 /Al 2 O 3 epitaxial films with the electron mobility of 25 cm 2 /V were grown by pulsed laser deposition using two YAG:Nd lasers. Depletion mode transistor Au/PZT/SnO 2 /Al 2 O 3 was produced by laser ablation and RF sputtering. All the samples demonstrate clockwise hysteresis of the source–drain characteristic. The energy distribution of traps at the PZT/SnO 2 interface was determined using a modified version of a transient current method. The effect of PZT intergrain boundaries on the retention time was taken into account for experimental data discussion.

White electroluminescence (EL) from ZnO/GaN structures fabricated by pulsed laser deposition of ZnO:In onto GaN:Mg/GaN structures MOCVD-grown on Al2O3 substrates has been observed. The white light is produced by superposition of the two strongest emission lines, narrow blue and broad yellow, peaked at 440 and 550 nm, respectively. The intensity ratio of different EL lines from ZnO/GaN/Al2O3 structures depends on the ZnO film quality and drive current. The white EL is due to the high density of structural defects at the n-ZnO/p-GaN interface. A band diagram of the n-ZnO/p-GaN/n-GaN structure is constructed and a qualitative explanation of the EL is suggested.

Thin-film Pt/PZT/Ir(Ti/SiO/Si) and Ir/PZT/PTO/Ir(Ti/SiO2/Si) capacitors, as-grown and after ageing, are investigated. A comprehensive study based on the Auger spectroscopy and measurement of electrical and ferroelectrical characteristics is performed. Ageing results in increasing of the oxygen content in PZT, broadening of interfaces owing to formation of metal oxides, diffusion of Pb and Ti towards the top electrode along the grain boundaries, and diffusion of Ti into PZT grain. These processes induce decreasing of the structure capacitance, healing of the interface traps, enhance leakage and dielectric loss.

In this work carrier lifetime investigations by applying photoluminescence frequency domain technique to investigate blue and green light emitting diode (LED) structures, thus representing wide spectra for the InGaN LEDs were performed. Similar measurements have been carried out in GaN at extremely low excited carrier densities [1], as low as 1 mW/cm2 by using UV LED as an excitation source. For these measurements we applied laser diode (LD), to excite resonantly the MQW's in the sample investigated with modulated light. This allowed us to cover a broad range of excitation power density (1 – 500 mW/cm2). To analyze the activation mechanisms measurements down to 10 K temperatures were performed. The non-equilibrium charge carrier lifetime dynamics was investigated by applying a model with a superposition of exponential and stretch exponential decay depended on the sample and on the measurement condition. The uniqueness of this method is the possibility to study the transient processes in structures under very low to low non-equilibrium charge carrier densities at an un-saturated recombination channel condition.

The possibility of fabrication of a transparent ferroelectric field effect transistor (FFET) with a high electron mobility channel was demonstrated. The highest obtained electron mobility in the channel SnO2/c-Al2O3 is 25 cm2/V·s and SnO2/r-Al2O3 is 40 cm2/V·s at the electron density of 1019–1020 cm−3. A transparent FFET structure Au/PZT/SnO2/Al2O3 has been fabricated. The best sample had 94% conductivity modulation at the gate voltage from −5 V to 2 V. The difference in the channel current at positive and negative remnant polarizations of the undergate ferroelectric is 37%.

The possibility of fabricating a ferroelectric FET based on a Pb(ZrxTi1−x)O3/SnO2 (PZT/SnO2) heterostructure is investigated. Sb-doped epitaxial SnO2/Al2O3 thin film deposited by YAG laser ablation from a metal target is used as the FET channel. The highest obtained electron mobility in the channel is 25 cm2/(V s) at an electron density of 8 × 1019 cm−3. The possibility of growing PZT film directly on SnO2 film using two different techniques, laser ablation and magnetron sputtering, is demonstrated. Both methods have been used in the fabrication of Au/PZT/SnO2 capacitor heterostructures, whose top Au electrode is 250×250 μm2 in size. These cells demonstrate a capacitance of 1000 pF at a 10-V bias and remnant polarization up to 16 μC/cm2. A Au/PZT/SnO2/Al2O3 transistor structure with 94% modulation of the channel current is fabricated. The difference in the channel current under the effect of positive and negative remnant polarization of the undergate ferroelectric is 37%.

Structural defects at the interface between Pb0.95La0.05Ti0.8Zr0.2O3 and La1.85Sr0.15CuO4 were studied using the method of isothermal current relaxation. Two cases were considered: (a) the width of the defect-containing layer is much smaller than the width of the space-charge region and (b) the width of the defect-containing layer is greater than the width of the space-charge region. It is shown that, for the samples studied, the width of the defect-containing layer exceeds 50–100 A, and the density of states for deep-level centers in the energy interval Ev+(0.55–0.65) eV is about 3×1020 cm−3 eV−1; this value corresponds to a surface state density of about 2×1014 cm−2 eV−1. It is shown that the density of states for deep-level centers increases from the interface into the depth of the semiconductor.

The analysis of transient currents in Au/PLZT/LSCO gate-capacitor structure has been used to determine the distribution of surface traps at the F/S interface. The total charge associated with the surface states recharge under the bias variation was found to be Qt ∼3 μC/cm2, which is far less than the remnant polarization at the operating bias values.

Internal quantum efficiency (IQE) of a high-brightness blue LED has been evaluated from the external quantum efficiency measured as a function of current at room temperature. Processing the data with a novel evaluation procedure based on the ABC-model, we have determined separately IQE of the LED structure and light extraction efficiency (LEE) of UX:3 chip. Full text Nowadays, understanding of LED efficiency behavior at high currents is quite critical to find ways for further improve­ment of III-nitride LED performance [1]. External quantum ef­ficiency ηe (EQE) provides integral information on the recom­bination and photon emission processes in LEDs. Meanwhile EQE is the product of IQE ηi and LEE ηext at negligible car­rier leakage from the active region. Separate determination of IQE and LEE would be much more helpful, providing correla­tion between these parameters and specific epi-structure and chip design. In this paper, we extend the approach of [2,3] to the whole range of the current/optical power variation, provid­ing an express tool for separate evaluation of IQE and LEE. We studied an InGaN-based LED fabricated by Osram OS. LED structure grown by MOCVD on sapphire substrate was processed as UX:3 chip and mounted into the Golden Dragon package without molding. EQE was measured with Labsphere CDS-600 spectrometer. Plotting EQE versus output power P and finding the power Pm corresponding to EQE maximum ηm enables comparing the measurements with the analytical rela­tionships ηi = Q/(Q+p1/2+p-1/2) ,p = P/Pm , and Q = B/(AC) 1/2 where A, Band C are recombination constants [4]. As a result, maximum IQE value equal to QI(Q+2) can be found from the ratio ηm/ηe plotted as a function of p1/2 +p1-1/2 (see Fig.la) and then LEE calculated as ηext = ηm (Q+2)/Q . Experimental EQE as a function of normalized optical power p is shown in Fig. 1 b along with the analytical approximation based on the ABC­model. The approximation fits perfectly the measurements in the range of the optical power (or operating current) variation by eight orders of magnitude. In conclusion, new express method for separate evaluation of IQE and LEE of III-nitride LEDs is suggested and applied to characterization of a high-brightness blue LED. With this method, we obtained LEE from the free chip surface to the air as 69.8% and IQE as 85.7% at the maximum and 65.2% at the operation current 350 rnA. [I] G. Verzellesi, D. Saguatti, M. Meneghini, F. Bertazzi, M. Goano, G. Meneghesso, and E. Zanoni, "Efficiency droop in InGaN/GaN blue light-emitting diodes: Physical mechanisms and remedies," 1. AppL Phys., vol. 114, no. 7, pp. 071101, Aug., 2013. [2] C. van Opdorp and G. W. 't Hooft, "Method for determining effective non radiative lifetime and leakage losses in double-heterostructure las­ers," 1. AppL Phys., vol. 52, no. 6, pp. 3827-3839, Feb., 1981. [3] M. Meneghini, N. Trivellin, G. Meneghesso, E. Zanoni, U. Zehnder, and B. Hahn, "A combined electro-optical method for the determination of the recombination parameters in InGaN-based light-emitting diodes," 1. AppL Phys., vol. 106, no. II, pp. 114508, Dec., 2009. [4] Qi Dai, Qifeng Shan, ling Wang, S. Chhajed, laehee Cho, E. F. Schubert, M. H. Crawford, D. D. Koleske, Min-Ho Kim, and Yongjo Park, "Carrier recombination mechanisms and efficiency droop in GalnN/GaN light-emitting diodes," App/. Phys. Leu., vol. 97, no. 13, pp. 133507, Sept., 2010. © 2014 IEEE.

Internal Quantum Efficiency (IQE) of two-colour monolithic white light emitting diode (LED) was measured by temperature dependant electro-luminescence (TDEL) and analysed with modified rate equation based on ABC model. External, internal and injection efficiencies of blue and green quantum wells were analysed separately. Monolithic white LED contained one green InGaN QW and two blue QWs being separated by GaN barrier. This paper reports also the tunable behaviour of correlated colour temperature (CCT) in pulsed operation mode and effect of self-heating on device performance.

Structures of the transparent ferroelectric field-effect transistor PZT/SnO2/Al2O3 with “normal” and “anomalous” conductivity hystereses of the channel are prepared and investigated. The “normal” modulation loop for these structures is obtained for the first time. Antimony-doped SnO2/Al2O3 epitaxial films evaporated by a YAG laser from a metal target are used as a channel of the field-effect transistor. Ferroelectric PZT films are deposited using magnetron sputtering. The distribution of deep levels at the PZT/SnO2 interface is measured by the modified relaxation current method. It is established that the ratio between the polarization charge and the trap charge at the PZT/SnO2 interface critically affects the direction of the conductivity hysteresis of the channel.

Variation of the absorption coefficient and refractive index of a system of tunnel-coupled GaAs/AlGaAs quantum wells in a longitudinal electric field is discovered and investigated in the spectral region corresponding to intersubband electron transitions. The phenomena observed are explained by electron heating in the electric field and electron transfer in physical space. The equilibrium absorption spectra at lattice temperatures of 80 and 295 K are presented.

In this paper, we present a simple non-destructive method for testing SiC plate single crystals of any size and shape. The method is based on measuring the impedance changes of an inductive ferrite-cored coil due to placing the sample into the core gap. The method is valid for any SiC polytypes, though we used 6H one. Using this method we have obtained and discussed a conductivity as a function of doping level (N d –N a ) for 6H–SiC Lely crystals. The conductivity measurements were carried out with alternating current of 747 kHz frequency. The sensitivity of the method is limited by minimal conductivity 1 Ω −1 cm −1 (that is corresponding to (N d –N a )∼2×10 16 cm −3 for 6H–SiC : N Lely crystals).

A photocurrent directed opposite to ferroelectric (FE) polarization is observed in short-circuit thin-film polycrystalline Pt/PZT/Ir structures. The direction and magnitude of photocurrent are defined by the sign and magnitude of the FE polarization. A model based on a photovoltaic effect with characteristics determined by polarization of PZT grains is proposed. The model considers the field interaction of FE polarization charge with the charge carriers in intergranular PbO channel. Thin-film FE capacitor is considered as a photosensitive heterogeneous medium, where the conduction of PbO channels along PZT grain boundaries is controlled by FE polarization.

Summary form only. At present the better part of coordinate-sensitive photodetectors is based on longitudinal photo-voltage effect in p-n junction. The linear range of their sensitivity via coordinate of illuminated region is limited by the diffusion length of non-equilibrium carriers for the semiconductor sensitive cell and does not exceed 2 mm. We present a new coordinate-sensitive photodetector design. Our device is based on detecting a magnetic part of the electromagnetic radiation from the semiconductor surface illuminated by interband light. The electromagnetic radiation is produced by currents of non-equilibrium carriers moving outside the region of the light excitation and separated by a surface barrier.

Discussion of ways to achieve mid and far IR intraband lasing just by lateral electric field carrier (electron or hole) heating in multiple quantum well (MQW) structures is given. It is argued that the Gunn diodes are low frequency indirect transition lasers based on hot electron population inversion arising under electron intervalley transfer. In the MQW structures direct optical transitions exist while hot carrier population inversion can be achieved due to inter-valley/real space transfer. The two MQW structures are considered in this work: GaAs/AlAs and GaAs/InGaAs systems. In the first the hot electron (Gamma) -X intervalley/real space transfer from GaAs layers to AlAs layers provides population inversion while in the second the inversion can arise due to interlevel/interlayer transfer. Evaluations via the Monte-Carlo simulation of the hot electron phenomena in some of the structures are given and observation of the hot carrier phenomena of the type (including far and mid IR emission and absorption) are presented. Consideration of the appropriate laser design which provides also a way to cope with the low frequency (Gunn type) current oscillations is given.© (2001) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.

In this paper we present a simple non-destructive method for testing SiC plate single-crystals of any size and shape. Themethod is based on measuring the impedance changes of an inductive ferrite-cored coil due to placing the sample into thecore gap. The method is valid for any SiC polytypes, though we used 6H one. Using this method we have obtained anddiscussed a conductivity as a function of doping level (NdNa) for 6H-SiC Lely crystals. The conductivity measurements were carried out with alternating current of 747 kHz frequency. The sensitivity of the method is limited by minimal conductivity 1 (Ohnrcm)1 (that is corresponding to (NdNa) 2•1016cnf3 for 6H-SiC:N Lely crystals).Keywords: high frequency; non-destructive, special conductivity, silicon carbide. 1. INTRODUCTION The problem we consider is the non-destructive measurement of the conductivity of small irregular SiC crystals, which are commonly used in scientific laboratories. Note that the contact measurement of the conductivity of wide band gapsemiconductors is possible only with specially treated contacts. There is a good way to solve the problem

Summary form only given. A simple non-destructive method for characterising SiC samples (Lely-crystals, CREE-substrates, and epitaxial films) is presented. The observed ultraviolet differential reflection spectra of SiC samples were compared with a pure Lely-crystal (model sample) to estimate the structural quality of the sample. The method presented is based on a differential study of ultraviolet reflection spectra of SiC films in comparison with a perfect Lely-crystal.

Insulating c-oriented hexagonal epitaxial gallium nitride (GaN) films have been obtained by means of pulsed laser sputtering of a gallium target in nonactivated nitrogen atmosphere. The GaN films were deposited onto (0001)-oriented sapphire substrates either directly or above a ZnO buffer layer. The laser-deposited films exhibit edge photoluminescence at 370 nm.

A simple experimental method of light emitting diode (LED) injection efficiency (IE) determination was suggested. IE and internal quantum efficiency (IQE) calculation is an actual and difficult problem in LED science. In this paper IE and IQE of blue LEDs were determined separately. The method is based on electroluminescence data fitting by the modified rate equation model. Efficiency droop caused by Auger recombination and poor injection were taken into account. Only one reasonable assumption was accepted during the calculations: IE tends to 1 at low current densities.

Electro-optic modulation in transverse (perpendicular to the layers) and longitudinal (parallel to the layers) electric fields was studied in simple rectangular and asymmetric tunnel-coupled GaAs/AlGaAs quantum wells (QW). The investigated phenomena can be used for fast amplitude and phase light modulation.

Birefringence and absorption modulation under electron heating in a longitudinal electric field in the tunnel-coupled GaAs/AlGaAs quantum wells have been found and investigated in the spectral region corresponding to intersubband electron transitions. The observed phenomena are explained by electron heating in electric field and electron transfer in real space. The equilibrium absorption spectra at different lattice temperatures are analyzed.

Optical absorption and birefringence due to intersubband electron transitions were investigated in GaAs/AlAs MQW structures. These structures are intended for creation of a mid-infrared laser of a new type. Experimental results on electron redistribution between size-quantization levels under electron heating were obtained up to an electric field of 3500?V?cm-1.