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Investigating carrier localization and transfer in InGaN/GaN quantum wells with V-pits using near-field scanning optical microscopy and correlation analysis

View Article: PubMed Central - PubMed

ABSTRACT

The V-pits and potential fluctuations in InGaN/GaN multiple quantum wells (MQWs) are key factors for understanding the performance of InGaN/GaN-based light-emitting diodes (LEDs). However, photoluminescence (PL) measurements using conventional optical microscopy only provide ensemble information due to the spatial resolution limit, known as the diffraction barrier, which hinders the analysis of dislocations and potential fluctuations. Here, in order to investigate the influence of the V-pits and potential fluctuations on local optical properties, we performed nanoscopic luminescence mapping for standard and V-pit InGaN/GaN MQWs samples with different sized V-pits using near-field scanning optical microscopy (NSOM) with illumination mode (I-mode) at various laser excitation powers. From the nanoscopic PL mapping data, we could clearly observe luminescence features associated with dislocations and potential fluctuations in the InGaN/GaN MQWs. We also employed correlation analysis to quantitatively analyze the nanoscopic PL mapping data for the different MQWs samples. Based on the results of NSOM PL with I-mode and correlation analysis, we could demonstrate that carrier transfer in the MQWs sample with large sized V-pits is suppressed by deeper potential fluctuations and higher energy barriers compared to the standard sample.

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NSOM PL mapping images obtained from standard and V-pit samples at 100 μW laser power.Monochromatic NSOM PL images of the standard and V-pit samples, only obtained from main MQWs wavelength regime, are shown in (a) and (d), respectively. Using double-peak Gaussian fitting, mappings of peak wavelength and FWHM were also obtained. (b,c) Peak wavelength and FWHM mapping images of the standard sample. (e,f) Peak wavelength and FWHM mapping images of the V-pit sample. Inset values (λave, λstd, FWHMave, FWHMstd) show average and standard deviation values of the peak wavelength and FWHM. The values of Δ in the color bars of the peak wavelength and peak FWHM images indicate the differences between the maximum and minimum wavelengths and FWHM, respectively. In the monochromatic and peak FWHM images, we used white and black in the scale bars of the maximum intensity value, respectively, for better comparison with each other.
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f3: NSOM PL mapping images obtained from standard and V-pit samples at 100 μW laser power.Monochromatic NSOM PL images of the standard and V-pit samples, only obtained from main MQWs wavelength regime, are shown in (a) and (d), respectively. Using double-peak Gaussian fitting, mappings of peak wavelength and FWHM were also obtained. (b,c) Peak wavelength and FWHM mapping images of the standard sample. (e,f) Peak wavelength and FWHM mapping images of the V-pit sample. Inset values (λave, λstd, FWHMave, FWHMstd) show average and standard deviation values of the peak wavelength and FWHM. The values of Δ in the color bars of the peak wavelength and peak FWHM images indicate the differences between the maximum and minimum wavelengths and FWHM, respectively. In the monochromatic and peak FWHM images, we used white and black in the scale bars of the maximum intensity value, respectively, for better comparison with each other.

Mentions: Monochromatic NSOM PL mapping images of the standard and V-pit samples obtained only from the main MQWs wavelength regime with 100 μW laser power are shown in Fig. 3a and d, respectively. The NSOM PL mapping size was 5 μm × 5 μm for each sample. Non-radiative recombination centers (NRCs) related to dislocations are clearly observed as the low intensity areas in each monochromatic NSOM PL image. Large spatial intensity fluctuations were also observed in the V-pit sample, as shown in Fig. 3d. The emission spectrum of each sample consists of two peaks which come from the main MQWs and the InGaN/GaN SLs. Each near-field double peak spectrum was fitted using double Gaussian fitting in order to completely separate and obtain spectrum information of main MQWs without emission of InGaN/GaN SLs. The peak wavelength and FWHM mappings of each sample were obtained, and are shown in Fig. 3b,c and e,f, respectively. By comparing mapping images, we determined that there were obvious correlations between the monochromatic and peak wavelength mappings, and between the monochromatic and peak FWHM mappings. For the monochromatic and peak wavelength mappings, the stronger intensity regions exhibited a longer peak wavelength tendency. Moreover, based on the comparison between the monochromatic and peak FWHM mappings, it was found that the stronger intensity regions exhibited a narrower FWHM tendency. These correlations not only suggest that radiative carrier recombination processes occur more often in the In-rich or thick QW areas, but also that the low intensity regions are related to NRCs or the spatial variations of the radiative recombination rate which may occur due to separate localization of electrons and holes at different regions51. However, the V-pit sample obviously shows stronger fluctuations in intensity, wavelength and FWHM compared to the standard sample. These fluctuations are more clearly measured from color bar and the standard deviation values in inset of each sample mapping. Moreover, peak wavelength mapping of the V-pit sample shows longer wavelength in almost all areas compared to the standard sample, which is also consistent with the macroscopic PL result. Based on these results, we believe that deep spatial potential fluctuations exist in the V-pit sample because of strain relaxation caused by the large sized V-pits. Although large-scale localization potential is observed from NSOM PL mapping, small-scale localization potential is difficult to be observed by NSOM PL due to the limited spatial resolution of NSOM. Small-scale localization potential strongly affects the carrier transport and recombination compared to large-scale localization potential. Therefore, small-scale localization potential should be considered for estimation of potential fluctuations in each sample. Even though it is difficult to be observed directly from NSOM PL mapping image, it can be indirectly estimated from the broadening of the near-field spectra of each sample394245. Average FWHM values of the standard and V-pit sample at 100 μW are 13.8 nm (83 meV) and 23.3 nm (133 meV), respectively. Because the total broadening is expressed by the sum of the homogeneous and inhomogeneous components, homogeneous broadening should be considered before comparing two average FWHM values of each sample. We note that homogeneous broadening from the semipolar plane of V-pit was not considered in the calculation since the semipolar plane of each sample occupies small portion of the surface. By considering the reported homogeneous broadening value in c-plane InGaN QWs at room temperature (~29 meV)55, inhomogeneous broadening values of the standard and V-pit samples are ~54 meV and ~104 meV, respectively. From this result, we can estimate that the V-pit sample has larger inhomogeneous broadening compared to the standard sample due to small-scale localization potential.


Investigating carrier localization and transfer in InGaN/GaN quantum wells with V-pits using near-field scanning optical microscopy and correlation analysis
NSOM PL mapping images obtained from standard and V-pit samples at 100 μW laser power.Monochromatic NSOM PL images of the standard and V-pit samples, only obtained from main MQWs wavelength regime, are shown in (a) and (d), respectively. Using double-peak Gaussian fitting, mappings of peak wavelength and FWHM were also obtained. (b,c) Peak wavelength and FWHM mapping images of the standard sample. (e,f) Peak wavelength and FWHM mapping images of the V-pit sample. Inset values (λave, λstd, FWHMave, FWHMstd) show average and standard deviation values of the peak wavelength and FWHM. The values of Δ in the color bars of the peak wavelength and peak FWHM images indicate the differences between the maximum and minimum wavelengths and FWHM, respectively. In the monochromatic and peak FWHM images, we used white and black in the scale bars of the maximum intensity value, respectively, for better comparison with each other.
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f3: NSOM PL mapping images obtained from standard and V-pit samples at 100 μW laser power.Monochromatic NSOM PL images of the standard and V-pit samples, only obtained from main MQWs wavelength regime, are shown in (a) and (d), respectively. Using double-peak Gaussian fitting, mappings of peak wavelength and FWHM were also obtained. (b,c) Peak wavelength and FWHM mapping images of the standard sample. (e,f) Peak wavelength and FWHM mapping images of the V-pit sample. Inset values (λave, λstd, FWHMave, FWHMstd) show average and standard deviation values of the peak wavelength and FWHM. The values of Δ in the color bars of the peak wavelength and peak FWHM images indicate the differences between the maximum and minimum wavelengths and FWHM, respectively. In the monochromatic and peak FWHM images, we used white and black in the scale bars of the maximum intensity value, respectively, for better comparison with each other.
Mentions: Monochromatic NSOM PL mapping images of the standard and V-pit samples obtained only from the main MQWs wavelength regime with 100 μW laser power are shown in Fig. 3a and d, respectively. The NSOM PL mapping size was 5 μm × 5 μm for each sample. Non-radiative recombination centers (NRCs) related to dislocations are clearly observed as the low intensity areas in each monochromatic NSOM PL image. Large spatial intensity fluctuations were also observed in the V-pit sample, as shown in Fig. 3d. The emission spectrum of each sample consists of two peaks which come from the main MQWs and the InGaN/GaN SLs. Each near-field double peak spectrum was fitted using double Gaussian fitting in order to completely separate and obtain spectrum information of main MQWs without emission of InGaN/GaN SLs. The peak wavelength and FWHM mappings of each sample were obtained, and are shown in Fig. 3b,c and e,f, respectively. By comparing mapping images, we determined that there were obvious correlations between the monochromatic and peak wavelength mappings, and between the monochromatic and peak FWHM mappings. For the monochromatic and peak wavelength mappings, the stronger intensity regions exhibited a longer peak wavelength tendency. Moreover, based on the comparison between the monochromatic and peak FWHM mappings, it was found that the stronger intensity regions exhibited a narrower FWHM tendency. These correlations not only suggest that radiative carrier recombination processes occur more often in the In-rich or thick QW areas, but also that the low intensity regions are related to NRCs or the spatial variations of the radiative recombination rate which may occur due to separate localization of electrons and holes at different regions51. However, the V-pit sample obviously shows stronger fluctuations in intensity, wavelength and FWHM compared to the standard sample. These fluctuations are more clearly measured from color bar and the standard deviation values in inset of each sample mapping. Moreover, peak wavelength mapping of the V-pit sample shows longer wavelength in almost all areas compared to the standard sample, which is also consistent with the macroscopic PL result. Based on these results, we believe that deep spatial potential fluctuations exist in the V-pit sample because of strain relaxation caused by the large sized V-pits. Although large-scale localization potential is observed from NSOM PL mapping, small-scale localization potential is difficult to be observed by NSOM PL due to the limited spatial resolution of NSOM. Small-scale localization potential strongly affects the carrier transport and recombination compared to large-scale localization potential. Therefore, small-scale localization potential should be considered for estimation of potential fluctuations in each sample. Even though it is difficult to be observed directly from NSOM PL mapping image, it can be indirectly estimated from the broadening of the near-field spectra of each sample394245. Average FWHM values of the standard and V-pit sample at 100 μW are 13.8 nm (83 meV) and 23.3 nm (133 meV), respectively. Because the total broadening is expressed by the sum of the homogeneous and inhomogeneous components, homogeneous broadening should be considered before comparing two average FWHM values of each sample. We note that homogeneous broadening from the semipolar plane of V-pit was not considered in the calculation since the semipolar plane of each sample occupies small portion of the surface. By considering the reported homogeneous broadening value in c-plane InGaN QWs at room temperature (~29 meV)55, inhomogeneous broadening values of the standard and V-pit samples are ~54 meV and ~104 meV, respectively. From this result, we can estimate that the V-pit sample has larger inhomogeneous broadening compared to the standard sample due to small-scale localization potential.

View Article: PubMed Central - PubMed

ABSTRACT

The V-pits and potential fluctuations in InGaN/GaN multiple quantum wells (MQWs) are key factors for understanding the performance of InGaN/GaN-based light-emitting diodes (LEDs). However, photoluminescence (PL) measurements using conventional optical microscopy only provide ensemble information due to the spatial resolution limit, known as the diffraction barrier, which hinders the analysis of dislocations and potential fluctuations. Here, in order to investigate the influence of the V-pits and potential fluctuations on local optical properties, we performed nanoscopic luminescence mapping for standard and V-pit InGaN/GaN MQWs samples with different sized V-pits using near-field scanning optical microscopy (NSOM) with illumination mode (I-mode) at various laser excitation powers. From the nanoscopic PL mapping data, we could clearly observe luminescence features associated with dislocations and potential fluctuations in the InGaN/GaN MQWs. We also employed correlation analysis to quantitatively analyze the nanoscopic PL mapping data for the different MQWs samples. Based on the results of NSOM PL with I-mode and correlation analysis, we could demonstrate that carrier transfer in the MQWs sample with large sized V-pits is suppressed by deeper potential fluctuations and higher energy barriers compared to the standard sample.

No MeSH data available.


Related in: MedlinePlus