Light-emitting Heterostructures Research Articles

Multifunctional application of planar 2D molecule for light-emitting heterostructures

The object of research is the donor-acceptor compound, organic and hybrid heterostructures based on it. The paper is focused on a comprehensive approach to solving the problem of the efficiency of light-emitting devices, finding new technological and design solutions for the use of organic compounds as multifunctional materials for various types of light-emitting devices. The paper presents the multifunctional application of a planar 2D molecule as an emission layer for typical and inverted types of light-emitting heterostructures, as well as a matrix for a host-guest system using inorganic quantum dots. The developed light-emitting structures are characterized by external quantum efficiency typical for fluorescent devices, but good stability over the entire length of the consumption voltage. QLED brightness is 1600 cd·m-2 and EQE 1.4 %, which are good parameters for use in display technology. Organic LEDs based on planar molecules are promising candidates for use in modern lighting systems. A separate advantage of these light-emitting structures is the multifunctionality of using one compound for different types of light-emitting structures, including inverted heterostructures. Special attention is paid to the technological and design implementation of invert structures, since their geometry allows direct connection to the back board of the n-channel transistor on the substrate. In addition, organic LEDs have low energy consumption and are environmentally friendly due to the absence of toxic substances in their architecture, which creates the prerequisites for saving energy resources and reducing the industrial burden on the environment.

Creation of effective sources of white radiation based on GaP(As,N) on silicon substrates

The article is devoted to the study of light-emitting heterostructures based on GaP(As,N) dilute nitrides, monolithically grown on silicon substrates by plasma-assisted molecular beam epitaxy. Current-voltage characteristics and electroluminescence spectra of the grown heterostructures are obtained. For the first time, a unique effect is observed in GaP(As,N) dilute nitrides - the appearance of white electroluminescence when a reverse bias is applied. The result was obtained due to the original design of the light-emitting heterostructure and the unique properties of dilute nitride solid solutions.

The impact of ZnO nanotube on the performance of hybrid inorganic/organic light-emitting diode as a single-mode ring-core UV waveguide

After a systematic survey in hybrid inorganic/organic light-emitting heterostructure devices based on ZnO in the last decay, in this novel work, the impact of the single-mode ring-core waveguide based on ZnO nanotube (NT)/MEH-PPV for ultraviolet organic light-emitting diode (UV-OLED) application has been carefully scrutinized for the first time. The proposed structure has been fabricated, simulated and compared with conventional ZnO nanorod (NR)/MEH-PPV structure. To synthesize ZnO NTs, the as-grown chemical bath deposited ZnO NRs have been etched in KCL solution in various molar (M) concentration, etching time and etching temperature. The optimized etching condition is obtained in 1 M concentration of KCL solution, 4 h etching time and 90 °C temperature. The structural properties (such as strain, stress and texture coefficient), electrical properties (such as band gap energy) and optical properties (such as Urbach energy, absorbance and photoluminescence spectra) of ZnO NTs have been investigated, systematically. In continue, hybrid UV-OLEDs have been fabricated based on ZnO NRs and ZnO NTs. According to the results, ZnO NT-based OLED depicts superior electrical and optical results including lower turn-on voltage (11.2 V < 15 V) and higher UV peak in electroluminescence spectra with respect to ZnO NR-based device. To acquire more enlightenment about UV emission mechanism, the proposed devices have been simulated through Silvaco TCAD and Lumerical FDTD software. The results from simulations illustrate great agreement with experimental results. Higher radiative recombination rate, higher Purcell factor and single-mode waveguiding effect of ring-core ZnO NT lead to major superiority of the ZnO NT-based UV-OLED fabricated and simulated in this work.

РОЗРОБКА НЕДОПОВАНОЇ ОРГАНІЧНОЇ СВІТЛОВИПРОМІНЮВАЛЬНОЇ СТРУКТУРИ ІЗ ЕМІСІЄЮ У ТЕМНО-СИНЬОМУ ДІАПАЗОНІ НА ОСНОВІ ДОНОРНО-АКЦЕПТОРНОГО МАТЕРІАЛУ КАРБАЗОЛОТЕТРАХЛОРОПІРИДИНУ

The development of efficient organic light emitting diodes (OLED) based on the phenomenon of intramolecular thermally activated delayed fluorescence (TADF), in the design of which there are no blue phosphorescent emitters based on rare earth metals, still remains a challenge in the development of new lighting systems and OLED displays. The article proposes a technological approach to the formation of new type of OLED, where the emitter is an organic donor-acceptor molecular material 9- (2,3,5,6-tetrachloropyridin-4-yl) -9H-carbazole (4-CzPyCl4), in which electronic interaction between the donor and acceptor fragment plays a key role in the mechanism of delayed fluorescence. The design of the developed light-emitting heterostructure uses layer-by-layer formation of functional nanosized organic films, in contrast to traditional OLED designs of dark blue color radiation, which uses a guest-host systemThe external quantum efficiency of the developed OLED is 2.8%. The maximum brightness of 3,000 cd/m2 is reached at a voltage of 15 V. The chromaticity coordinates CIE (x, y) 1931 are (0.15, 0.13), which corresponds to the “dark blue” emitting spectral zone.

Investigation of the dynamic parameters of electroluminescence in different parts of the spectrum in local regions of the light-emitting heterostructure

A method for measuring the distribution of the differential charge carriers lifetime over energy levels in the local regions of a light-emitting heterostructure is presented. The method has been tested on commercial green InGaN-based LEDs. It has been determined that with an increase in the energy level (with decreasing wavelength), the charge carriers lifetime decreases. It is shown that the relative inhomogeneity of the distribution of the charge carriers lifetimes in local regions of the die in the long-wavelength part of the electroluminescence spectrum is higher than in the short-wavelength part of the spectrum.

Measurement of the Distribution Profile of Electroluminescence Cutoff Frequencies over the Area of a Light-Emitting Heterostructure

A hardware–software complex that has no analogues in Russia and abroad and that is designed to measure the distribution of the cutoff frequency of electroluminescence over the area of a light-emitting heterostructure (LHS) is described. The complex provides a spatial resolution of 0.65 µm, an upper measurement limit of 40 MHz, and a relative error of 2%. The electroluminescence cutoff frequency in local areas of an LHS is determined from the decay of the brightness of pixels of LHS images by 1.19 times obtained using a digital CMOS camera with a step-by-step increase in the pulse repetition rate of the LHS-supplying current with an off-duty factor of 2. The complex and the measurement method were tested on commercial green light-emitting diodes. The results of measuring the cutoff-frequency distribution can be used to assess the homogeneity of LHSs.

Change of the defects density distribution profile over the area of the InGaN/GaN light-emitting heterostructures during current tests

A method for measuring of the defects density distribution profile over the area of the LED chip is presented. Using the blue commercial LEDs as an example, it is shown that the emission power decreases when LEDs testing under the increased density pulsed current for 190 h. The decrease of the LEDs emission power is accompanied by a change in the defects density distribution profile. It was determined that during the degradation process a non-uniform increase in the defects density occurs in various regions of the heterostructure: in regions with a higher defect density, the increment in the density of defects is larger.

Naturally Degradable Photonic Devices with Transient Function by Heterostructured Waxy-Sublimating and Water-Soluble Materials.

Combined dry–wet transient materials and devices are introduced, which are based on water‐dissolvable dye‐doped polymers layered onto nonpolar cyclic hydrocarbon sublimating substrates. Light‐emitting heterostructures showing amplified spontaneous emission are obtained on transient elements and used as illumination sources for speckle‐free, full‐field imaging, and transient optical labels are realized that incorporate QR‐codes with stably encoded information. The transient behavior is also studied at the microscopic scale, highlighting the real‐time evolution of material domains in the sublimating compound. Finally, the exhausted components are fully soluble in water thus being naturally degradable. This technology opens new and versatile routes for environmental sensing, storage conditions monitoring, and organic photonics.

Measurement and analysis of the recombination coefficients distribution on the area of light-emitting InGaN/GaN heterostructures

A method and an installation for evaluation the profile of distribution of recombination coefficients and current density over the area of light-emitting heterostructures at low injection levels based on local values of the 3 dB frequency are described. Using the example of measuring the parameters of commercial blue InGaN/GaN-based LEDs, it is shown that the current density is higher in those areas of the chip where the 3 dB frequency is lower.

Hardware-Software Complex for the Investigation of the Dynamic Characteristics of Separate Spectral Components of the Emission Spectrum of Leds

We present the description of an automated hardware-software complex intended for measuring the 3 dB frequency of modulation and evaluation of the quantum efficiency of separate spectral components of the full spectrum of electroluminescence of light-emitting diodes, including the range of low levels of excitation. The principle of operation of this complex is based on measuring the spectrum of radiation of a light-emitting diode with the help of an OceanOptics USB2000+ spectrometer in the signal-accumulation mode in the course of transmission of current pulses with given amplitude and repetition frequency through the light-emitting diode with subsequent selection of separate spectral components from the full spectrum of radiation. We present the results of testing the hardware-software complex performed by measuring, as an example, the frequency characteristics of separate spectral components of the full electroluminescence spectrum of commercial InGaN green light-emitting diodes. The analyzed hardware-software complex can be used for the diagnostics of the quality of light-emitting heterostructures with quantum wells.

GaAs/GaP Quantum-Well Heterostructures Grown on Si Substrates

Molecular-beam epitaxy is used to produce GaP/Si hybrid substrates that allow the growth of highly efficient light-emitting heterostructures with GaAs/GaP quantum wells. Despite the relatively high concentration of nonradiative-recombination centers in GaP/Si layers, GaAs/GaP quantum-well heterostructures grown on GaP/Si hybrid substrates are highly competitive in terms of efficiency and temperature stability of luminescence to similar heterostructures grown on lattice-matched GaP substrates.

Optically invariant InGaN nanowire light-emitting diodes on flexible substrates under mechanical manipulation

The integration of GaN-based light-emitting diodes (LEDs) onto flexible platforms provides opportunities for conformal lighting, wearable electronics, and bendable displays. While this technology may enhance the functionality of the light source, the development of flexible GaN LEDs suffers from performance degradation, when mechanical bending is applied during operation. A unique approach to eliminate the degradation employs dot-in-wire structures, using cylindrical light-emitting heterostructures that protrude above the flexible platform, separating the active light-emitting region from the bending substrate. Here, we demonstrate the optical enhancement of nanowire light emitters by changing the geometric orientation within a 1 × 1 mm2 array of nanowires on a flexible platform through bending of the substrate platform. The flexible structures were achieved by transferring GaN nanowire LEDs from sapphire substrates onto flexible polyethylene terephthalate (PET) using a “paste-and-cut” integration process. The I–V characteristics of the nanowire LEDs showed negligible change after integration onto the PET, with a turn-on voltage of 2.5 V and a forward current of 400 μA at 4 V. A significant advantage for the nanowire devices on PET was demonstrated by tilting the LEDs through substrate bending that increased the electroluminescence (EL) intensity, while the I–V characteristics and the EL peak position remained constant. Through finite-element analysis and three-dimensional finite-difference time-domain modeling, it was determined that tilting the protruding devices changed the effective distance between the structures, enhancing their electromagnetic coupling to increase light output without affecting the electrical properties or peak emission wavelength of the LEDs.

XXIII Международный симпозиум Нанофизика и наноэлектроника", Нижний Новгород, 11-14 марта 2019 г. Гетероструктуры с GaAs/GaP-квантовыми ямами, выращенные на Si-подложках

AbstractMolecular-beam epitaxy is used to produce GaP/Si hybrid substrates that allow the growth of highly efficient light-emitting heterostructures with GaAs/GaP quantum wells. Despite the relatively high concentration of nonradiative-recombination centers in GaP/Si layers, GaAs/GaP quantum-well heterostructures grown on GaP/Si hybrid substrates are highly competitive in terms of efficiency and temperature stability of luminescence to similar heterostructures grown on lattice-matched GaP substrates.

Measuring Complex for the Diagnostics of the Quality of Light–Emitting Heterostructures According to Photoelectric and Optical Responses Under Local Photoexcitation

We propose an automated measuring complex for the diagnostics of the lateral homogeneity of semiconductor heterostructures by measurements and analysis of the photoelectric and optic responses to their local photoexcitation in static and dynamic modes. The electronic-mechanical and optical positioning systems guarantee the illumination of local region of the investigated structure with a minimum diameter of the spot of 14 μm and a positioning accuracy of 10 μm. It is shown that the measuring complex can be used for the diagnostics of the quality of light-emitting heterostructures, transistors, solar cells, and photodiodes.

Quantum Efficiency of Gallium Nitride–Based Heterostructures with GaInN Quantum Wells

An important parameter of light-emitting heterostructures is their external quantum efficiency. However, another strict requirement for the structures used in fabricating blue and white light-emitting diodes is that the wavelength in the emission spectrum peak and its spread over the entire structure must be 460 ± 5 nm. This is explained primarily by the most frequently used white light-emitting diode design being based on crystals coated with a luminescent layer of a certain composition excited by blue emissions. Deviating from the specified spectral parameters of heterostructures strongly degrades the light and color characteristics of light-emitting diodes. In this work, we solve the problem of optimizing the design and way of growing the active region of an emitting structure consisting of a set of Ga1 – xInxN quantum wells and wider-gap GaN barriers with a certain wavelength in the emission spectrum peak. The variation in the critical thickness of the pseudomorphic layer in the Poisson ratio ranges of 0 to 0.2 for GaAlN and 0 to 0.4 for GaInN is calculated. The emission wavelength is determined by both the bulk Ga1 – xInxN band gap, which depends on the molar fraction of In in quantum wells, and the quantum well thickness in quantum-sized layers. It is found from the obtained dependences that to obtain the required wavelength of 460 nm in the emission spectrum peak, the Ga1 – xInxN layers must be around 10.3% indium and have a quantum well thickness of about 2.5 nm. The effect the In distribution profile in quantum wells has on the external quantum efficiency, the uniformity of the emission wavelength distribution in the spectral peak, and the emission power distribution over the structure is considered. The best results are obtained for a trapezoidal In distribution, since it ensures the narrowest emission wavelength spread in the spectral peak and the most uniform emission power distribution over the structure. Studying the effect the number of quantum wells has on the properties of a heterostructure shows that the maximum external quantum efficiency corresponds to 4 to 5 quantum wells. The highest emission wavelength uniformity in the spectral peak over the structure is obtained at 5 to 7 quantum wells. The optimum number of quantum wells in the active region of the heterostructure is found to be 5.

Electrochemical capacitance-voltage profiling of nonuniformly doped GaAs heterostructures with SQWs and MQWs for LED applications

Light-emitting heterostructures with single and multiple GaAs/InGaAs quantum wells have been investigated by means of electrochemical capacitance-voltage (ECV) profiling. Capacitance-voltage characteristics were measured; concentration profiles of free charge carriers over the heterostructure depth as well as the intensity of quantum well filling by charge carriers were obtained. In heterostructures with a single quantum well (QW), we considered limitations of capacitance techniques for undoped QW profiling, which are situated near the metallurgic border of the p-n junction. We made a detailed consideration of phenomena related to Debye smearing and we developed and analyzed the dependence of the space charge region width on the doping. Special attention was paid to investigation of "the blind" area. This was inspired by the practical problem from capacitance spectroscopy of semiconductors, when the researcher poses the task of obtaining a free charge carrier depth distribution profile as deep as possible in the space charge region, i.e. where the intensity of the electric field is maximum. Generally, the active QW of a LED heterostructure is placed deep in the space charge region, so reaching these regions is extremely important for practical problems. We present an evolution of capacitance-voltage characteristics during ECV profiling of nonuniformly doped p-n-heterostructures. For a heterostructure with multiple quantum wells, we registered a response from 6 QWs.

Измерительный комплекс для диагностики качества светоизлучающих гетероструктур по фотоэлектрическому и оптическому отклику при локальном фотовозбуждении

Измерительный комплекс для диагностики качества светоизлучающих гетероструктур по фотоэлектрическому и оптическому отклику при локальном фотовозбуждении

Photoelectric diagnostics of InGaN-based LEDs in static and dynamic modes

The method and measuring installation for diagnostics of light-emitting heterostructures (HS) with the quantum well (QW) by registration of the photocurrent arising in case of radiation of HS the narrow-band optical radiation of different wavelength in the static and dynamic modes are described. On the example of commercial InGaN-based LEDs it is shown that offset of a maximum of photocurrent spectrum concerning a maximum of electroluminescence spectrum is defined by QW depth. The steepness of photocurrent spectrum on the section corresponding to absorption in QW is defined by structural perfection of heteroboundaries. In case of harmonic modulation of a luminous flux on all probed wave-lengths amplitude of a photocurrent of blue LEDs monotonically falls down with growth of modulation frequency with the steepness 0,2 dB for a decade, and at green LEDs to the frequency of 200 kHz the plateau and then recession with the steepness 8 dB for a decade is watched.

The morphology of high-current electron-beam-initiated fractures in InGaN/GaN heterostructures

The morphology of fractures in InGaN/GaN heterostructures on sapphire substrates under multipulse irradiation by a high-current electron beam is experimentally investigated. It is established that excitation of samples by an electron beam with the threshold density from the heterostructure side leads to the formation of microfractures the number, size, and shape of which change during irradiation and are determined by individual properties of investigated samples. In heterostructures with stimulated luminescence, at the instant of the excitation pulse, bright microregions were detected against a background of homogeneous cathodoluminescence. The spatial positions of these microregions coincide with those of residual microfractures. Possible mechanisms of the electron-beam-initiated fracture of light-emitting heterostructures are analyzed.

Light-Emitting n-ZnO Nanotube/n&lt;sup&gt;&amp;#x002B;&lt;/sup&gt;-GaAs Heterostructures Processed at Low Temperatures

In this letter, a light-emitting heterostructure is designed and manufactured using vertically aligned n-type zinc oxide nanorods (ZnO NRs) on n + -GaAs wafers. The chemically grown ZnO NRs were also converted to nanotubes (NTs) using 2 molar KCl solution and the device fabrication was repeated using them. Both types exhibited a significant visible light at a forward bias of 1 V but with different brightness. Photoluminescence ~390 nm from NRs and NTs was obtained and the current injection in these devices is explained by a tunneling phenomenon.