V. S. Sizov

A monolithic white LED with an active region based on InGaN QWs separated by short-period InGaN/GaN superlattices

A new approach to development of effective monolithic white-light emitters is described based on using a short-period InGaN/GaN superlattice as a barrier layer in the active region of LED structures between InGaN quantum wells emitting in the blue and yellow-green spectral ranges. The optical properties of structures of this kind have been studied, and it is demonstrated that the use of such a superlattice makes it possible to obtain effective emission from the active region.

Effect of strain relaxation on active-region formation in InGaN/(Al)GaN heterostructures for green LEDs

Processes of active-region formation for green LEDs on the basis of multilayer strained InGaN/GaN nanoheterostructures have been studied. It is shown that the formation of structures of this kind is highly affected by elastic stress relaxation leading to a larger amount of indium incorporated into InGaN layers. For structures emitting in the blue spectral range, an increase in the number of quantum wells (QWs) from 1 to 10 does not lead to stress relaxation or to a shift of the emission wavelength, whereas for structures emitting in the green spectral range, raising the number of QWs from one to five causes a monotonic increase in the emission wavelength.

Influence of hydrogen on local phase separation in InGaN thin layers and properties of light-emitting structures based on them

Results of studies of hydrogen addition during the growth of thin (∼2–3 nm) InGaN layers on their structural properties and properties of light-emitting structures that contain InGaN/GaN heterostructures in the active region are reported. It is shown that, with the known effect of a decrease in the average content of In, hydrogen addition leads to varying the local phase separation in the InGaN layers. Hydrogen addition during the growth of the InGaN layers initially causes suppression of the local phase separation, while hydrogen addition during interruptions of the growth after deposition of the InGaN films leads to a decrease in the size of the formed local In-enriched regions and to a certain increase in the local content of the In atoms.

High-efficiency InGaN/GaN/AlGaN light-emitting diodes with short-period InGaN/GaN superlattice for 530–560 nm range

A new method of forming the active region in high-efficiency InGaN/GaN/AlGaN light-emitting diode (LED) structure for long-wave green range is described. The introduction of a short-period InGaN/GaN superlattice situated immediately under the emitting quantum well and overgrown with GaN layer at reduced temperature leads to a more than tenfold increase in the efficiency of emission. For the proposed LEDs, the maximum quantum efficiency was 12% at 552 nm and 8% at 560 nm.

Study of tunneling transport of carriers in structures with an InGaN/GaN active region

Properties of light-emitting structures with an InGaN/GaN active region emitting in a range of 500–550 nm are studied. Photoluminescence of the structures is studied at various values of external bias and temperature as well as with time resolution. With the reverse bias, a decrease in the carrier lifetime associated with tunneling exit of the carriers from the active region is found. The mechanism of tunneling leakage is simulated allowing for the Boltzmann distribution of carriers by energy; it is shown that the calculated and experimental dependences agree well. It is shown that the tunneling transport exerts a considerable effect on the characteristics of structures with an InGaN/GaN active region.

The use of short-period InGaN/GaN superlattices in blue-region light-emitting diodes

Optical and light-emitting diode structures with an active InGaN region containing short-period InGaN/GaN superlattices are studied. It is shown that short-period superlattices are thin two-dimensional layers with a relatively low In content that contain inclusions with a high In content 1–3 nm thick. Inclusions manifest themselves from the point of view of optical properties as a nonuniform array of quantum dots involved in a residual quantum well. The use of short-period superlattices in light-emitting diode structures allows one to decrease the concentration of nonradiative centers, as well as to increase the injection of carriers in the active region due to an increase in the effective height of the AlGaN barrier, which in general leads to an increase in the quantum efficiency of light-emitting diodes.

Nonequilibrium population of charge carriers in structures with InGaN deep quantum dots

Electronic and optical properties of ensembles of quantum dots with various energies of activation from the ground-state level to the continuous-spectrum region were studied theoretically and experimentally with the InGaN quantum dots as an example. It is shown that, depending on the activation energy, both the quasi-equilibrium statistic of charge carriers at the levels of quantum dots and nonequilibrium statistic at room temperature are possible. In the latter case, the position of the maximum in the emission spectrum is governed by the value of the demarcation transition: the quantum dots with the transition energy higher than this value feature the quasi-equilibrium population of charge carriers, while the quantum dots with the transition energy lower than the demarcation-transition energy feature the nonequilibrium population. A model based on kinetic equations was used in the theoretical analysis. The key parameters determining the statistic are the parameters of thermal ejection of charge carriers; these parameters depend exponentially on the activation energy. It is shown experimentally that the use of stimulated phase decomposition makes it possible to appreciably increase the activation energy. In this case, the thermal-activation time is found to be much longer than the recombination time for an electron-hole pair, which suppresses the redistribution of charge carriers between the quantum dots and gives rise to the nonequilibrium population. The effect of nonequilibrium population on the luminescent properties of the structures with quantum dots is studied in detail.

Phase separation and nonradiative carrier recombination in active regions of light-emitting devices based on InGaN quantum dots in a GaN or AlGaN matrix

Structures with InGaN nanolayers within GaN and AlGaN matrices, which constitute active regions of light-emitting devices, have been studied. Spectra and relative intensities of photoluminescence (PL) in the temperature range 20–300 K and the dependence of the position of the PL peak on the energy of the excitation photons have been measured. It is shown that, to account for the temperature dependence of the PL intensity, it is necessary to take into consideration, in addition to the nonradiative recombination via defects in the matrix and in the residual quantum well (RQW), an additional recombination channel with low activation energy, which is presumably associated with defects located close to quantum dots. It is demonstrated that structures with the AlGaN matrix show a larger decrease in the PL intensity upon an increase in temperature from 50 to ∼200 K, compared with structures with the GaN matrix. Analysis of the temperature dependence of the PL intensity in terms of the model that considers these three channels of nonradiative recombination shows that this dependence is associated with a decrease in the carrier’s localization energy relative to the ground state in the RQW. Such a decrease is due to suppression of the phase separation in InGaN layers grown within the AlGaN matrix. This behavior is confirmed by PL measurements at different excitation photon’s energies and leads, in addition to the lower localization energy, a decrease in the concentration of recombination centers is observed for these samples.

Kinetics and inhomogeneous carrier injection in InGaN nanolayers

The electrical and optical properties of light-emitting devices with an active region containing several layers of InGaN/GaN quantum dots (QDs) separated by GaN spacers are studied. It is shown that the overgrowth of the QD layer with an InGaN layer that has a reduced In content at higher temperatures raises the confinement energy of carriers in QDs. Furthermore, inhomogeneous carrier injection, predominantly into regions with higher confinement energy, is observed. The electrical and optical properties of p-n junctions and the effect of the inhomogeneities on these properties are studied in detail. It is shown that the shifts of photoluminescence and electroluminescence lines, which are observed when changing the experimental conditions, are related to these properties of the inhomogeneities in the p-n junction.

Indium-rich island structures formed by in-situ nanomasking technology

A new method of forming InGaN/GaN nanostructures emitting in the long-wavelength green spectral range is proposed and implemented. The method is based on growing a thin InGaN layer over nanosized pits formed in situ by etching a locally AlN-masked GaN layer in a hydrogen-containing atmosphere.