The paper is devoted to the study of the features of CdTe surface treatment under laser irradiation with both different wavelengths (lambda = 300-800 nm) and pulse durations (tau(p) = 7 ns-1 ms). The thermal conductivity of the semi-insulating p-like CdTe semiconductor crystals was evaluated using the photoacoustic gas-microphone method. Simulations of the melting threshold were performed based on the three stage model of the laser induced excitation and relaxation. In particular, the following processes were considered in frames of the model: (i) rapid interband thermalization, (ii) nonradiative interband and (iii) nonradiative surface recombination. It was revealed that in the range of pulse durations from 7 ns to 1 mu s, the melting threshold of the CdTe mainly depended on the absorption coefficient alpha(lambda). For pulse durations longer than 1 mu s the threshold started to depend also on the spectra of the reflectivity coefficient R(lambda). The obtained results have been used for optimization of the laser-assisted techniques of surface processing and stimulated doping of CdTe crystals.

The melting threshold of CdTe I-th depending on the laser radiation wavelength (lambda = 300 ... 800 nm) and pulse duration (tau(p) = 7 ns ... 1 ms) have been calculated. Three fractions of the energy released during thermalization of excess carriers were considered: immediately after excitation (i), during non-radiative bulk (ii) and non-radiative surface (iii) recombination, which together determine the depth of heat penetration into the crystal and, therefore, the melting threshold of the CdTe surface region. It has been shown that in the range of laser pulse durations from 7 ns up to 1 mu s, the CdTe melting threshold mainly depends on the absorption coefficient alpha(lambda). For pulse durations longer than 1 mu s, it started to depend also on the spectra of the reflectivity coefficient R(lambda). It has been found that changes in the non-equilibrium excess carriers parameters - surface recombination velocity, lifetime and diffusion depth - can vary the CdTe melting threshold at least by 25 percent.

For InGaN/AlGaN/GaN ultraviolet (lambda = 365 nm) light emitting diodes, the S-shaped current-voltage characteristic was observed at liquid nitrogen temperature due to the transition from single injection (electrons) to double (electrons and holes) injection. Initially only electron injection into the active region takes place and as the current is increased, the injection of holes starts. It has been found that at the voltage of the minimum of the negative differential resistance region, oscillations of the current, accompanied by increasing electroluminescence intensity arise, and the repetition rate of the oscillations increases with the direct current rise. The intensity of the main ultraviolet and yellow electroluminescence bands sharply increases with increasing current at negative differential resistance region due to the rise of holes injection.

Електричні та люмінесцентні характеристики світлодіодів ультрафіолетового випромінювання 365 нм при Т = 77 К / В.П. Велещук, О.І. Власенко, З.К. Власенко, І.В. Петренко, Є.В Малий, М.П. Киселюк, О.В. Шефер, В.В. Борщ // Збірник тез конференції молодих вчених з фізики напівпровідників «Лашкарьовські читання» з міжнародною участю, Київ, 3-5 квітня 2019. – К. : ІФН НАНУ, 2019. – C. 44.

Electrical and Luminescence Properties of Ultraviolet LEDs 365-400 nm / V.P. Veleschuk, A.I. Vlasenko, Z.K. Vlasenko, V.V. Shynkarenko, Ya.Ya. Kudryk, V.V. Borshch, O.B. Borshch, A.V. Shefer, M.P. Kisselyuk // XVII International Freik Conference Physics and Technology of Thin Films and Nanosystems : abstract book. – Ivano-Frankivsk : Vasyl Stefanyk Precarpatian national University, 2019. – P. 294.

Current-voltage characteristic and electroluminescence of UV LEDs 365 nm at liquid nitrogen temperature / V. Veleschuk, A. Vlasenko, Z. Vlasenko, . Petrenko, Y. Malyi, V.V. Borshch, O.B. Borshch, A.V. Shefer // Optica Applicata. - 2019. - Vol. 49, № 1. - P. 125-133. - URL: http://opticaapplicata.pwr.edu.pl/article.php?id=2019100125

Visible electro- and photoluminescence of the ultraviolet LEDs (lambda = 365 nm) was studied. It is established that at nitrogen temperature yellow electroluminescence is absent and occurs when the current or temperature increases. The light output power and luminous efficiency of the yellow electroluminescence in the 450-740 nm range were measured. At temperature decreasing an increase of the luminescence intensity in the spectrum range 393-460...490 nm is observed.

The dependences of the melting threshold of CdTe under the pulsed laser irradiation on the radiation wavelength л and the laser pulse duration тp are calculated with regard for the non-equilibrium character of charge carriers. Three components of the energy released at the thermalization of excited carriers under the nanosecond laser irradiation of CdTe in the fundamental absorption region are considered: the component that dominates immediately after the excitation, and the components released at the nonradiative bulk and nonradiative surface recombinations. Together, they determine the depth of heat penetration into the crystal and, therefore, its melting threshold. It is shown that the CdTe melting threshold grows from 2.6 to 4.75 MW/cm2, when л changes from 300 to 800 nm at тp = 20 ns. The changes in the non-equilibrium charge carrier parameters (the surface recombination rate, lifetime, and diffusion depth) are found to vary the CdTe melting threshold by at least 30%.

The emission uniformity of LED chips in the entire range of brightness and colors is the problem in LED displays manufacture process. It was approved that at lowering brightness gradations appearing the radiation nonuniformity between LED chips, and the higher disorders will be seen on the lesser emission levels. The RGB LED chips, observed by us, were based on AlGaInP (red), In0.3Ga0.7N/GaN (green), and In0.2Ga0.8N/GaN (blue) and had nominal working current 20 mA. Analysis of the current-voltage characteristics and capacitance-voltage characteristics showed the presence of inhomogeneous semiconductor junctions and ohmic contacts in blue LEDs structures that are the source of possible irregularities in the final emission of LED displays. The variation of voltages (at current 10 mA) was 2.81—2.98 V for blue structures, and 1.9—2.0 V for red ones. Some of the blue structures had additional parasite current-flow mechanism at low current. Rise time and fall time of electroluminescence pulse of blue structures were measured. The shortest LED pulse time in LED displays is limited by duration of 6—8 ns. The quality of LED displays may be improved by implementing the preliminary control of LED chips in a narrower range of voltage variation at operating currents, or more narrow interval of I—V variation. This additional LEDs sorting will of course reduce the percentage of the total amount of light-emitting diodes, but may improve the image (video, photo) quality at translations by using a lower brightness gradations.

In this work, we studied the controlled microplasma breakdown of InGaN/GaN heterostructures in power light-emitting diodes (LEDs) prepared on various substrates (SiC, AuSn/Si, Al2O3). It was ascertained that LED microplasma characteristics are related to their functional parameters. It was shown that nondestructive control of the critical defects and diagnostics of the InGaN/GaN power LEDs are possible because they are based on the following parameters of microplasmas: (1) voltage of the first microplasma; (2) current of the first microplasma; (3) amount of the microplasmas; (4) integrated luminescence intensity of the microplasmas; (5) ratio of the blue and yellow band luminescence peak intensities for the microplasmas; (6) displacement between electroluminescence peaks for forward and reverse biases. It was ascertained that the majority of microplasmas arise near the faces of the crystallites’ grain boundaries.

The photothermoacoustic method has been used for diagnostics of thermobarodynamic processes in the metal In(400 nm)/semiconductor (CdTe) thin-film system under nanosecond laser irradiation (7 ns, λ = 532 nm) in natural conditions (in air) and in a liquid medium (water). From the analysis of the data obtained, the dependence of the pressure induced in the energy-release region on the irradiation energy density has been established and the melting threshold of In film has been determined. Under irradiation of In/CdTe in water, the pressure is higher than in air: 17 times higher at the melting threshold of In film and 30 times higher at twice the temperature. It has been found that the laser pulse treatment of In/CdTe/Au samples in water makes it possible to obtain diode structures with better parameters: smaller leak currents and a steeper current-voltage characteristic under the forward bias of the p - n junction.

The shift between the maxima of the electroluminescence spectra of In x Ga 1 − x N/GaN structures is measured at forward and reverse bias depending on the indium content x in the quantum well and on the substrate material (SiC, AuSn/Si, and Al 2 O 3 ). It is established that this shift increases as the indium concentration in the In x Ga 1 − x N layer and mechanical stresses from the substrate increase.

Пінковська 2 1 Інститут фізики напівпровідників ім.В. Є. Лашкарьова НАН України, Київ 2 Інститут ядерних досліджень НАН України, Київ ВПЛИВ НЕЙТРОННОГО ОПРОМІНЕННЯ НА ХАРАКТЕРИСТИКИ ПОТУЖНИХ InGaN/GaN СВІТЛОДІОДІВ Вивчено вплив потоку швидких нейтронів реактора (Е = 2 МeВ, Ф = 2•10 14 н/см 2 ) на вольт-амперні та вольтфарадні характеристики, інтенсивність електролюмінесценції потужних ІnGaN/GaN світлодіодів на кремнієвовуглецевій підкладинці та на кремнієвій підкладинці із золото-олов'яним контактом.Виявлено, що величина та знак зміни тунельних струмів після радіаційного опромінення у світловипромінюючих ІnGaN/GaN гетероструктурах суттєво залежить від підкладинки

Фототермоакустическим методом проведена диагностика термобародинамических процессов, происходящих при наносекундном лазерном облучении (7 ns, λ = 532 nm) в естественных условиях (на воздухе) и в жидкой среде (вода), тонкопленочной системы металл In(400 nm)−полупроводник CdTe. На основе анализа полученных данных установлена зависимость давления, возникающего в области энерговыделения, от плотности энергии облучения системы и определен порог плавления плeнки In. При облучении In/CdTe в воде давление больше, чем на воздухе — в 17 раз на пороге плавления плeнки In и в 30 раз при его двухкратном превышении. Выяснено, что импульсная лазерная обработка образцов In/CdTe/Au в воде позволяет получать диодные структуры с лучшими параметрами — меньшими токами утечки и более крутой вольтамперной характеристикой при прямом смещении p−n-перехода.

The electroluminescence spectra at reverse biases in LED InGaN/GaN heterostructures at liquid nitrogen temperatures were studied. At the reverse bias and T = 77 K, avalanche microplasmas breakdowns were observed. Electroluminescence spectra demonstrate two peaks caused by the recombination of carriers in different parts of the structure (quantum well and p-GaN layer). The temperature narrowing the half-width and the shift of electroluminescence spectra peaks inherent to microplasmas were observed.

It was shown that in the GaAsP/GaP and InGaN/GaN heterostructures during current passage redistribution of electroluminescence intensity on the structure surface takes place simultaneously with radiation of acoustic emission. Local (on surface area) fluctuations of electroluminescence intensity are observed together with general degradation of electro-physical parameters.

Microscopic studies, composition analyses, and studies of microhardness have shown that the epitaxial CdTe/CdHgTe variband heterosystem contains a developed system of dislocation ensembles of various nature (in-grown, mismatching, generated on the joint boundaries of three-dimensional islands, etc.) with segments inclined or parallel to the interface. It has been established that there is a correlation between an increase (inhomogeneous through the thickness) in the microhardness of the heterosystem and the distribution of the dislocation density in the epitaxial layer.