STR Group Soft Impact Ltd.

Saint Petersburg, Russia

STR Group Soft Impact Ltd.

Saint Petersburg, Russia
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Karpov S.Y.,STR GroupSoft Impact Ltd
Photonics Research | Year: 2017

A simple semi-empirical model for radiative and Auger recombination constants is suggested, accounting for hole localization by composition fluctuations in InGaN alloys. Strengthening of fluctuation with the indium molar fraction in InGaN is found to be largely responsible for decreases in both the radiative and Auger recombination constants with emission wavelength. The model provides good fitting of the experimental spectral dependencies of the recombination constants, thus demonstrating implication of the carrier localization to light-emitting diode efficiency reduction in the “green gap. © 2017 Chinese Laser Press.


Zakheim D.A.,RAS Ioffe Physical - Technical Institute | Pavluchenko A.S.,Epi Center | Bauman D.A.,JSC Svetlana Optoelectronics | Bulashevich K.A.,STR Group Soft Impact Ltd. | And 2 more authors.
Physica Status Solidi (A) Applications and Materials Science | Year: 2012

In this paper, we report on the results of experimental and theoretical study of a promising way for suppression of the efficiency droop with current in InGaN-based light emitting diodes. Simulations carried out using a drift-diffusion approach with quantum-mechanical corrections clearly show that non-radiative Auger recombination is the principal mechanism limiting the device performance at high-injection level. New design of LED heterostructure with short-period superlattice in the active region is proposed and assessed theoretically. Experimentally, the implementation of the structure design in high-power devices has resulted in substantial suppression of the efficiency droop compared to conventional multiquantum-well InGaN LEDs. Copyright © 2012 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.


Durnev M.V.,RAS Ioffe Physical - Technical Institute | Karpov S.Y.,STR Group Soft Impact Ltd.
Physica Status Solidi (B) Basic Research | Year: 2013

The polarization degree and emission pattern anisotropy of light emitted from strained (0001)Al(In)GaN heterostructures are studied by modelling. The model considers accurately the nonparabolicity of heavy, light, and split-off hole bands, the relevant modification of their densities of states, as well as the optical matrix element dispersion in the k-space. Accounting for all the above factors provides good agreement of the theoretical predictions with the data reported for both bulk AlGaN alloys grown on sapphire substrates and UV LED structures with Al(In)GaN quantum wells serving as the active regions of the devices. The band-structure modification in Al(In)GaN materials under variation of their composition and elastic strain are found to be the primary factors controlling the polarization degree and emission patterns of TE- and TM-polarized light. The anisotropy of the TM-polarized emission pattern (dependence of the emission intensity on the direction of photon release) and polarization degree of the emitted light are found to be remarkably influenced by the nonparabolicity of the hole bands. Comparison of the experimental polarization degrees (symbols) with the theoretical ones (lines) calculated for different in-plane strains in light-emitting Al(In)GaN heterostructures. © 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.


Durnev M.V.,Russian Academy of Sciences | Omelchenko A.V.,Russian Academy of Sciences | Yakovlev E.V.,STR Group Soft Impact Ltd. | Evstratov I.Yu.,STR Group Soft Impact Ltd. | Karpov S.Yu.,STR Group Soft Impact Ltd.
Applied Physics Letters | Year: 2010

Indium incorporation into strained InGaN coherently grown on a GaN substrate with arbitrary polarity is simulated using a simplified epitaxy model. The InGaN composition is predicted as a function of C-axis inclination angle. Effect of strain originated from the lattice mismatch on optical transitions in the bulk InGaN and quantum wells is examined with account of both complex valence band structure and polarization charges induced at the InGaN/GaN interfaces. A higher indium incorporation on nonpolar and semipolar planes, as compared to the ordinary C-plane, is found to not necessarily result in a longer emission wavelength. © 2010 American Institute of Physics.


Chernyakov A.E.,Russian Academy of Sciences | Bulashevich K.A.,STR Group Soft Impact Ltd. | Karpov S.Y.,STR Group Soft Impact Ltd. | Zakgeim A.L.,Russian Academy of Sciences
Physica Status Solidi (A) Applications and Materials Science | Year: 2013

Comprehensive analysis of current spreading, temperature distribution, and near-field electroluminescence (EL) of high-power flip-chip InGaN/GaN light-emitting diodes (LEDs) has been performed by combination of experimental and theoretical methods. High-resolution mapping of EL and thermal radiation was obtained by optical and infra-red (IR) microscopes. Thermal resistance of the chip was measured by forward-voltage relaxation technique. 3D coupled electrical, thermal, and optical simulations were carried out using hybrid 1D/3D model. The theoretical predictions agree well with available observations. The lateral distributions of the near-field EL intensity and temperature are found to be qualitatively similar to that of the current density but to have substantially lower degree of non-uniformity. Therefore, it becomes incorrect to judge the current crowding in the LED chip by the measured contrast of the EL image. The role of contact resistances in stabilization of the current spreading pattern is revealed by modeling. Copyright © 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.


Bulashevich K.A.,STR Group Soft Impact Ltd. | Khokhlev O.V.,STR Group Soft Impact Ltd. | Evstratov I.Yu.,STR Group Soft Impact Ltd. | Karpov S.Yu.,STR Group Soft Impact Ltd.
Proceedings of SPIE - The International Society for Optical Engineering | Year: 2012

The paper discusses various factors affecting internal quantum efficiency (IQE) of state-of-the-art III-nitride lightemitting diodes (LEDs). A general figure of merit for LED heterostructures, namely the quality factor, is proposed on the basis of a simple recombination model, which enables comparison of overall performances of the structures either emitting light at different wavelengths or having substantially different designs. The relationships between the quality factor, maximum value of IQE, and IQE droop with the current density are revealed. Some ways for IQE improvement and reducing its droop are considered. Among them, the use of short-period superlattice (SPSL) active regions is found to be quite promising. The operation of such structures and their properties are examined in detail by simulations accounting for quantum corrections to electron and hole transport within the quantum-potential concept. © 2012 Copyright Society of Photo-Optical Instrumentation Engineers (SPIE).


Karpov S.,STR Group Soft Impact Ltd.
Proceedings of the International Conference on Numerical Simulation of Optoelectronic Devices, NUSOD | Year: 2014

The paper reviews applications of ABC-model to interpret internal quantum efficiency and its droop in III-nitride light-emitting diodes (LEDs). Advantages of the model, its intrinsic limitations, and tentative mechanisms responsible for deviation of the model predictions from available observations are discussed. New experimental information on recombination processes in the LED active regions coming from the ABC-model is considered along with still open questions and tasks for further experimental and theoretical research. © 2014 IEEE.


Khokhlev O.V.,STR Group Soft Impact Ltd. | Bulashevich K.A.,STR Group Soft Impact Ltd. | Karpov S.Y.,STR Group Soft Impact Ltd.
Physica Status Solidi (A) Applications and Materials Science | Year: 2013

The paper considers new opportunities for design of various optoelectronic devices opened by the use of polarization doping in III-nitride heterostructures, including distributed polarization doping (DPD) in graded-composition alloys. The polarization doping is routinely exploiting in high-electron mobility transistors for a long time. In contrast, this concept has not yet been applied to light-emitting diodes (LEDs), laser diodes (LDs), and solar cells (SCs) until recently, in spite of its great potential advantage - capability of producing holes with high concentrations that can never be achieved by conventional impurity doping. Using simulations, we apply the concept of polarization doping to find efficient ways for solution of practically important problems: (i) improvement of p-doping in deep-UV LED structures, (ii) development of p-type Ohmic contact to AlGaN alloys with high aluminum content, and (iii) development of III-nitride tunnel junctions (TJs) capable of forward bias operation. Band diagram and distribution of carrier concentrations in the prototype of acceptor-free deep-UV LED structure considered in the paper. © 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.


Karpov S.,STR Group Soft Impact Ltd.
Optical and Quantum Electronics | Year: 2015

The paper reviews applications of ABC-model to interpret internal quantum efficiency and its droop in III-nitride light-emitting diodes. Advantages of the model, its intrinsic limitations, and tentative mechanisms responsible for deviation of the model predictions from available observations are discussed. New experimental information on recombination processes in the LED active regions coming in terms of the ABC-model is considered along with still open questions and tasks for further experimental and theoretical studies. © 2014, Springer Science+Business Media New York.


Karpov S.Y.,STR Group Soft Impact Ltd.
Physica Status Solidi - Rapid Research Letters | Year: 2010

A model is suggested accounting for effects of localized electron and hole states formed by composition fluctuations in the InGaN active region of a III-nitride LED on non-radiative carrier recombination at threading dislocations. The model enables explanation of the abnormal temperature dependence of internal quantum efficiency (IQE) of a green LED structure recently observed at low current densities. The theoretical predictions are in quantitative agreement with experiment in the temperature range between 200 K and 453 K. © 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

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