Ilya Titkov - Academia.edu (original) (raw)

Papers by Ilya Titkov

Materials, Nov 18, 2017

External quantum efficiency of industrial-grade green InGaN light-emitting diodes (LEDs) has been... more 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.

VCSEL arrays can play an important role in the increasing the data throughput of VCSEL-based opti... more VCSEL arrays can play an important role in the increasing the data throughput of VCSEL-based optical interconnects both due to the need to increase the channel density and due to new emerging technologies like optical wireless. In this work we show the progress in the development of high-speed VCSEL arrays suitable for multicore fiber transmission leading to an increase of the total throughput through single fiber to 600 Gbps. We also discuss a novel type of compact VCSEL mini-arrays capable of high-speed modulation and coherent emission at the same time. Photon-photon resonance and coherent effects can help increase the resonant frequency and the bandwidth of the VCSELs and enable devices capable of 100 GHz operation.

Proceedings of SPIE, Mar 8, 2014

Internal Quantum Efficiency (IQE) of two-colour monolithic white light emitting diode (LED) was m... more 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.

ABSTRACT The recent advancement in the growth technology of InGaN/GaN has decently positioned InG... more ABSTRACT The recent advancement in the growth technology of InGaN/GaN has decently positioned InGaN based white LEDs to leap into the area of general or daily lighting. Monolithic white LEDs with multiple QWs were previously demonstrated by Damilano et al. [1] in 2001. However, there are several challenges yet to be overcome for InGaN based monolithic white LEDs to establish themselves as an alternative to other day-to-day lighting sources [2,3]. Alongside the key characteristics of luminous efficacy and EQE, colour rendering index (CRI) and correlated colour temperature (CCT) are important characteristics for these structures [2,4]. Investigated monolithic white structures were similar to that described in [5] and contained blue and green InGaN multiple QWs without short-period superlattice between them and emitting at 440 nm and 530 nm, respectively. The electroluminescence (EL) measurements were done in the CW and pulse current modes. An integration sphere (Labsphere “CDS 600” spectrometer) and a pulse generator (Agilent 8114A) were used to perform the measurements. The CCT and Green/Blue radiant flux ratio were investigated at extended operation currents from 100mA to 2A using current pulses from 100ns to 100µs with a duty cycle varying from 1% to 95%. The strong dependence of the CCT on the duty cycle value, with the CCT value decreasing by more than three times at high duty cycle values (shown at the 300 mA pulse operation current) was demonstrated. The pulse width variation seems to have a negligible effect on the CCT. To account for the joule heating, a duty cycle more than 1% was considered as an overheated mode. For the 1% duty cycle it was demonstrated that the CCT was tuneable in three times by modulating input current and pulse width. It has also been demonstrated that there is a possibility of keeping luminous flux independent of pulse width variation for a constant value of current pulse. [1] B. Damilano, N. Grandjean, C. Pernot, and J. Massies, “Monolithic white light emitting diodes based on InGaN/GaN multiple-quantum wells,” Jpn. J. Appl. Phys. 40, L918–L920 (2001). [2] I. E. Titkov, A. Yadav, V. L. Zerova, M. Zulonas, A. F. Tsatsulnikov, W. V. Lundin, A. V. Sakharov, and E. U. Rafailov, "Internal quantum efficiency and tunable colour temperature in monolithic white InGaN/GaN LED," Proc. SPIE 8986, Gallium Nitride Materials and Devices IX, 89862A (March 8, 2014). doi:10.1117/12.2040086. [3] M. H. Crawford, "LEDs for Solid-State lighting: Performance Challenges and recent advances,” IEEE Journal of Selected Topics in Quantum Electronics 15(4), 1028-1040 (2009). [4] B. Damilano, N. Trad, J. Brault, P. Demolon, F. Natali, and J. Massies, “Colour control in monolithic white light emitting diodes using a (Ga,In)N/GaN multiple quantum well light converter,” Phys. Status Solidi A 209(3), 465-468 (2012). [5] A. F. Tsatsulnikov, W. V. Lundin, A. V. Sakharov, E. E. Zavarin, S. O. Usov, A. E. Nikolaev, N. V. Kryzhanovskaya, M. A. Synitsin, V. S. Sizov, A. L. Zakgeim, and M. N. Mizerov, “A monolithic white LED with an active region based on InGaN QWs separated by short-period InGaN/GaN superlattices,” Semiconductors 44(6), 808–811 (2010).

We present the first true voltage-mode VCSEL driver achieving 60 Gbit/s at peak efficiency of 0.3... more We present the first true voltage-mode VCSEL driver achieving 60 Gbit/s at peak efficiency of 0.36pJ/bit with BER < 1e-12 improving the state-of-the-art by a factor of 2. Transmission experiments showcase error-free 56 Gbit/s transmission up to 100 m fiber. Advanced demonstration enables even 224 Gbit/s when using SWDM at a total efficiency of 0.4 pJ/bit.

Proceedings of SPIE, Mar 8, 2016

Two blue (450 nm) light-emitting diodes (LED), which only differ in top p-GaN layer growth condit... more 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 N 2 environment demonstrated better parameters than the B-structure grown in the N 2 /H 2 (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 N 2 utilization, and C(V R) measurement pointed the increase of static charge concentration near the p-GaN interface in the B structure.

Summary form only. At present the better part of coordinate-sensitive photodetectors is based on ... more 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

$Research paper thumbnail of Light absorption and refraction due to intersubband transitions of hot electrons in coupled GaAs/AlGaAs quantum wells$

Semiconductors, Jul 1, 1998

Variation of the absorption coefficient and refractive index of a system of tunnel-coupled GaAs/A... more 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

We present a new hyperchromatic laser-based multifocal display. In the proposed design multiple f... more We present a new hyperchromatic laser-based multifocal display. In the proposed design multiple full-colour virtual image planes can be displayed simultaneously at different depths in front of the observer through wavelength (de-)multiplexing. Each depth plane is displayed through its own combination of red, green and blue lasers at specific wavelengths. Hyperchromatic displays can be useful for augmented (AR) and mixed reality (MR) applications where real and virtual objects are located at different depths in front of the observer because they allow the human eye to focus on virtual objects and reduce the vergence-accommodation conflict (VAC). We present a laboratory demonstrator where the images generated by two red lasers (630 nm and 960 nm) were separated by more than 3 meters. We also discuss the applicability of the technology to the automotive head-up-display (HUD) systems and present an HUD system based on low-cost off-the-shelf components.

European Quantum Electronics Conference, Jun 21, 2015

Sapphire substrates are widely used for the manufacturing of blue and green LEDs and diode lasers... more Sapphire substrates are widely used for the manufacturing of blue and green LEDs and diode lasers. Dicing of sapphire wafers with deposited GaN device structures is therefore an important technological challenge. During the last ten years the dicing efficiency has been investigated for a wide range of wavelengths from DUV (157nm) to UV (355nm); pulse widths (in nanosecond regime); beam shaping; and power levels [1]. Traditionally, some industrial sapphire dicers use nanosecond pulses with a wavelength of 355 nm. In this configuration, the laser cutting is performed through the back side of the sapphire wafer. Furthermore, nanosecond pulses with a wavelength of 266 nm are used for cutting from the front (the side with the structure) of the wafer. A few attempts to dice with a green laser, also in Q-switched regime, emitting pulses with a width of a few tenths of a nanosecond have been reported in the literature [1, 2]. Recently the first Q-switched infrared pulsed lasers generating ten picosecond pulses at a wavelength of 1060/532 nm were presented on the market.

Applied sciences, Jul 17, 2018

We demonstrate a phosphor free, dichromatic GaN-based monolithic white LED with vertically stacke... more 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/cm 2 to 12.9 A/cm 2. 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.

Proceedings of SPIE, Jan 25, 2000

We are presenting a simple non-destructive method for characterizing SiC samples (Lely-crystals, ... more We are presenting a simple non-destructive method for characterizing SiC samples (Lely-crystals, CREE-substrates, and epitaxial films). With our method we observed ultraviolet differential reflection spectra of SiC samples and compared with pure Lely-crystal to estimate their structural quality. Our optical differential method is based on the experimental fact that doping of a crystal leads to appreciable changes of the optical fundamental absorption spectrum, which we interpreted as a uniform broadening and a shift of differential spectra. The broadening of absorption peaks can be caused not only by doping, but also by any defects of the crystal lattice (neutral impurities, clusters, micro-pipes and others), that destroy its periodicity. The shifts of these peaks inform us about the free carrier concentration. The experiment has shown we can detect minimum free carriers concentration up to nmin equals (ND-NA) equals 6 (DOT) 1015 cm-3. Besides we can detect minimal frequency of impacts with lattice defects as vmin equals 3 (DOT) 1012 s-1. Converting to charged centers concentration it equals (ND + NA) equals 5 (DOT) 1016 cm-3. Considering the small depth of light probe (less than 0.1 micrometers ) and delicacy of thin films, our contactless method is mostly applicable for its testing.

Proceedings of SPIE, Feb 1, 2001

ABSTRACT In this paper we present a simple non-destructive method for testing SiC plate single-cr... more ABSTRACT 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 (Nd-Na) 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 (Ohm(DOT)cm)-1 (that is corresponding to (Nd-Na) approximately 2 (DOT) 1016 cm-3 for 6H-SiC:N Lely crystals).

Summary form only given. A simple non-destructive method for characterising SiC samples (Lely-cry... more 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 this work carrier lifetime investigations by applying photoluminescence frequency domain techn... more 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.

Materials Science in Semiconductor Processing, Feb 1, 2001

In this paper, we present a simple non-destructive method for testing SiC plate single crystals o... more 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 (Nd–Na) for 6H–SiC Lely crystals. The conductivity measurements were carried out with alternating current of 747kHz frequency. The sensitivity of the method is limited by minimal conductivity 1Ω−1cm−1 (that is corresponding to (Nd–Na)∼2×1016cm−3 for 6H–SiC:N Lely crystals).

Vertical-Cavity Surface-Emitting Lasers XXVI, 2022

Vertical-cavity surface-emitting lasers (VCSELs) are of utmost importance as key components for h... more Vertical-cavity surface-emitting lasers (VCSELs) are of utmost importance as key components for high-speed datacom, sensor and free-space applications. Therefore, for a successful further optimization of their performance, understanding their aging behavior is of crucial importance. Photocurrent spectroscopy (PCS) is a powerful, nondestructive technique which can be used to analyze semiconductor materials. Applying it on VCSELs makes it a powerful tool to investigate these tiny devices. In this work, we present room temperature high-resolution PCS analyses of fresh vs. aged 850 nm VCSELs. These VCSELs are characterized before and after aging by means of PCS, which measures essentially the convolution of the top mirror and intrinsic region absorption spectra. Heavy hole and light hole quantum well transitions are revealed and the related quantum-confined Stark effect is studied. The VCSELs used in this study are mounted on a standard V-connector and were intentionally aged at extreme conditions to accelerate their degradation till reaching optical damage. It was found that in these VCSELs, a reduced PCS current is observed, which is possibly caused by nonradiative recombination centers generated by the aging-related processes. Moreover, we observe that aging of the devices at very high current densities results in the evolution of defect related states, which modify the IV-curve under reverse bias. Degraded devices also show a systematic shift in breakdown voltage towards lower values, indicating a possible shrinkage of the undoped region by impurity electromigration and diffusion. Interestingly, these changes are minimal in stable devices that were aged under normal conditions.

Journal of Applied Physics, Jul 7, 2018

In this paper, current-dependent emission spectra and efficiency measured on the same AlGaInP red... more 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 selfheating effects on the efficiency decline at high current magnitudes. The electron leakage to the pside 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 variableduration current pulse pumping is found to be an effective approach for analyzing mechanisms of the device operation.