rus
Issue #1/2014
N.Kachalova, V.Voitsekhovich, A.Mamuta, V.Khomenko, V.Osinsky, N.Lyakhova, A.Sushiy, N.Sukhoviy
Photoluminescence of GaN / InGaN MQW multiple quantum wells formed on the GaN nanorods
Photoluminescence of GaN / InGaN MQW multiple quantum wells formed on the GaN nanorods
Optical properties of GaN/InGaN MQWs were investigated by PL spectrum. Emission intensity of GaN/InGaN MQWs on “planar” GaN nanotemplates was an order of magnitude lower in compared to the InGaN MQWs grown on GaN nanorods.
Multiple InGaN/GaN quantum wells (MQWs) are often used as the active layers of GaN light-emitting diodes (LEDs) and lasers due to their efficient radi-ative recombination and radiating capacity within the wide spectral range (visi-ble and ultraviolet spectral regions). However, attainment of high efficiency of optical-electronic devices on the basis of GaN remains the critical task which depends on the parameters of internal and external quantum efficiencies. In-crease of the parameter of internal quantum efficiency is attained by decreasing the dislocations density in GaN layers, at the same time in order to increase the external quantum efficiency the templates with certain pattern and surface roughness as well as nanostructuring are used.
High level of disregistry of lattices and thermal-expansion coefficients be-tween GaN layer and template upon the epitaxial film growth results in large amount of dislocations (~109-1010 cm-2) which are the centers of nonradiative recombination and decrease the internal quantum efficiency of LEDs. Besides, the threading dislocations in the light emitters operating with high-density cur-rents cause the devices loss of life. Thus, the task to remove the threading dislocations is important stage on the way for increase of the internal quantum efficiency. Considerable decrease of the threading dislocations density (down to ~107 cm-2) on GaN hetero-epitaxial structures grown on SiO2 can be provided using the processes of lateral silting of epitaxial layers. Only regions above the mask can be used as the materials with low density of defects. In order to attain the low density of defects along the whole area of plate, two-stage process which is quite complex, expensive and time-consuming is used.
Experimental and theoretical studies showed that use of low-dimensional structures favors the decrease of defects density and decreases the number of nonradiative recombination centers and increases the internal quantum efficien-cy of GaN LEDs. Also stress relief upon 3-dimensional nanostructuring de-creases the stress caused by piezoelectric effect and thus provides the increase of internal quantum efficiency. Besides, the potential of nanophotonics draws attention with the capability of increase of the external quantum efficiency via light reflection from periodic nanostructures [1].
Experimental Part
Samples of the LED structure with active layers based on 4-periodic InGaN/GaN quantum wells were grown by the method of gas-phase deposition using the metal-organic compounds upon the lateral silting of oriented mask of nanoporous SiO2. Herewith, the arrays of GaN and InGaN nanodots and na-norings on GaN nanorods were formed. The growth conditions were selected on the basis of study of anode aluminum oxide nanotemplates including the estimation of diffuse length of epitaxial GaN structures (calculated effective dif-fuse length was Ld ≈ 3.2 µm which corresponded to the dislocations density of ~3∙106 cm-2) [2–5].
When studying the GaN nanorods arrays using the method of transmission electron microscopy (TEM) it was determined that nanorods have vertical side walls in the pores of SiO2 mask. Herewith, their height is determined by the mask thickness. Deposition of gallium nitride above the mask was not observed due to low coefficient of its attachment to SiO2. 4-periodic InGaN/GaN-MQW on "two-dimensional" samples and on GaN nanorods were grown on GaN nanotemplates (Fig. 1). In this research the photoluminescent characteristics of obtained epitaxial structures were studied. Measurements were performed at the site of Femtosecond Laser Complex Common Use Center under the Institute of Physics of the National Academy of Sciences of Ukraine. Flow-diagram of the experimental facility is given in Fig. 2.
The photoluminescence spectrums of epitaxial structures with 4-periodic InGaN/GaN-MQW were obtained at the temperature 20˚С (Fig. 3). Radiation of the third harmonic (TH) of Mira Optima 900-F femtosecond laser was used as the source of excitation which corresponded to the wavelength of 302 nm. Mira Optima 900-F femtosecond laser generated the pulses on the wavelength λ = 906 nm with duration of 80 fs and recurrence rate of 76 MHz, mean power of 500 mW. Peak intensity of TH excitation pulses was 25 kW/cm2 upon the duration of pulses of more than 0.5 ps and mean power of less than 1 mW and the pulse recurrence rate remained constant. Luminescence registration was carried out by Acton Spectra Pro 2500 spectrometer with the exposition of 5s on 25 nm.
As is seen from Fig. 1a the radiation efficiency of the MQWs of "two-dimensional" samples is one order lower than the intensity of structures on GaN nanorods (Fig. 1b); it is connected with the reduction of number of nonradiative recombination centers at the expense of dislocations density decrease and de-crease of the internal reflection of light and reduction of piezoelectric field. Be-sides, maximum of radiation from MQWs grown on nanorods is shifted by 67 nm (Fig. 3) in comparison with the check "two-dimensional" sample due to larger concentration of In in MQW. Apparently, it is connected with the fact that less strained structures can contain more indium during the process of InGaN growth because the compressive deformations prevent from the incorporation of In atoms into InGaN lattice.
Conclusion
The capability of effective growth of high-quality GaN layers is demon-strated through the method of gas-phase deposition using the metal-organic compounds upon the lateral silting of oriented mask of nanoporous SiO2 as well as formation of dense arrays of GaN and InGaN nanodots and nanorings on GaN using the nanoporous SiO2 masks.
The study of photoluminescence spectrums showed that the nanorod ar-rays have a number of benefits in comparison with the common "two-dimensional" films:
Low dislocations density (which causes the increase of internal quan-tum efficiency);
Capability of larger concentration of indium in solid solution;
Extension of the visible range of photoluminescence radiation spectrum into the long-wavelength region; and
Light extraction increase.
It should be noted that use of femtosecond laser in combination with the harmonics generator gives the opportunity to observe the spectrums of photolu-minescence of semiconductor structures within the wide spectral range.
This paper was prepared upon the support of Target-Oriented Integrated Research Program of the National Academy of Sciences of Ukraine "Fundamental Problems of Nanostructural Systems, Nanomaterials, Nanotechnologies" for 2010-2014.
High level of disregistry of lattices and thermal-expansion coefficients be-tween GaN layer and template upon the epitaxial film growth results in large amount of dislocations (~109-1010 cm-2) which are the centers of nonradiative recombination and decrease the internal quantum efficiency of LEDs. Besides, the threading dislocations in the light emitters operating with high-density cur-rents cause the devices loss of life. Thus, the task to remove the threading dislocations is important stage on the way for increase of the internal quantum efficiency. Considerable decrease of the threading dislocations density (down to ~107 cm-2) on GaN hetero-epitaxial structures grown on SiO2 can be provided using the processes of lateral silting of epitaxial layers. Only regions above the mask can be used as the materials with low density of defects. In order to attain the low density of defects along the whole area of plate, two-stage process which is quite complex, expensive and time-consuming is used.
Experimental and theoretical studies showed that use of low-dimensional structures favors the decrease of defects density and decreases the number of nonradiative recombination centers and increases the internal quantum efficien-cy of GaN LEDs. Also stress relief upon 3-dimensional nanostructuring de-creases the stress caused by piezoelectric effect and thus provides the increase of internal quantum efficiency. Besides, the potential of nanophotonics draws attention with the capability of increase of the external quantum efficiency via light reflection from periodic nanostructures [1].
Experimental Part
Samples of the LED structure with active layers based on 4-periodic InGaN/GaN quantum wells were grown by the method of gas-phase deposition using the metal-organic compounds upon the lateral silting of oriented mask of nanoporous SiO2. Herewith, the arrays of GaN and InGaN nanodots and na-norings on GaN nanorods were formed. The growth conditions were selected on the basis of study of anode aluminum oxide nanotemplates including the estimation of diffuse length of epitaxial GaN structures (calculated effective dif-fuse length was Ld ≈ 3.2 µm which corresponded to the dislocations density of ~3∙106 cm-2) [2–5].
When studying the GaN nanorods arrays using the method of transmission electron microscopy (TEM) it was determined that nanorods have vertical side walls in the pores of SiO2 mask. Herewith, their height is determined by the mask thickness. Deposition of gallium nitride above the mask was not observed due to low coefficient of its attachment to SiO2. 4-periodic InGaN/GaN-MQW on "two-dimensional" samples and on GaN nanorods were grown on GaN nanotemplates (Fig. 1). In this research the photoluminescent characteristics of obtained epitaxial structures were studied. Measurements were performed at the site of Femtosecond Laser Complex Common Use Center under the Institute of Physics of the National Academy of Sciences of Ukraine. Flow-diagram of the experimental facility is given in Fig. 2.
The photoluminescence spectrums of epitaxial structures with 4-periodic InGaN/GaN-MQW were obtained at the temperature 20˚С (Fig. 3). Radiation of the third harmonic (TH) of Mira Optima 900-F femtosecond laser was used as the source of excitation which corresponded to the wavelength of 302 nm. Mira Optima 900-F femtosecond laser generated the pulses on the wavelength λ = 906 nm with duration of 80 fs and recurrence rate of 76 MHz, mean power of 500 mW. Peak intensity of TH excitation pulses was 25 kW/cm2 upon the duration of pulses of more than 0.5 ps and mean power of less than 1 mW and the pulse recurrence rate remained constant. Luminescence registration was carried out by Acton Spectra Pro 2500 spectrometer with the exposition of 5s on 25 nm.
As is seen from Fig. 1a the radiation efficiency of the MQWs of "two-dimensional" samples is one order lower than the intensity of structures on GaN nanorods (Fig. 1b); it is connected with the reduction of number of nonradiative recombination centers at the expense of dislocations density decrease and de-crease of the internal reflection of light and reduction of piezoelectric field. Be-sides, maximum of radiation from MQWs grown on nanorods is shifted by 67 nm (Fig. 3) in comparison with the check "two-dimensional" sample due to larger concentration of In in MQW. Apparently, it is connected with the fact that less strained structures can contain more indium during the process of InGaN growth because the compressive deformations prevent from the incorporation of In atoms into InGaN lattice.
Conclusion
The capability of effective growth of high-quality GaN layers is demon-strated through the method of gas-phase deposition using the metal-organic compounds upon the lateral silting of oriented mask of nanoporous SiO2 as well as formation of dense arrays of GaN and InGaN nanodots and nanorings on GaN using the nanoporous SiO2 masks.
The study of photoluminescence spectrums showed that the nanorod ar-rays have a number of benefits in comparison with the common "two-dimensional" films:
Low dislocations density (which causes the increase of internal quan-tum efficiency);
Capability of larger concentration of indium in solid solution;
Extension of the visible range of photoluminescence radiation spectrum into the long-wavelength region; and
Light extraction increase.
It should be noted that use of femtosecond laser in combination with the harmonics generator gives the opportunity to observe the spectrums of photolu-minescence of semiconductor structures within the wide spectral range.
This paper was prepared upon the support of Target-Oriented Integrated Research Program of the National Academy of Sciences of Ukraine "Fundamental Problems of Nanostructural Systems, Nanomaterials, Nanotechnologies" for 2010-2014.
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