The effect of patterned sapphire substrate, the growth direction of epitaxial layers, the well-to-well non-uniformity of QWs, the saturated radiative recombination rate and the bending of chips on the carrier recombination dynamics, residual strain, carrier localization and piezoelectric field in th...
The effect of patterned sapphire substrate, the growth direction of epitaxial layers, the well-to-well non-uniformity of QWs, the saturated radiative recombination rate and the bending of chips on the carrier recombination dynamics, residual strain, carrier localization and piezoelectric field in the QWs were investigated by measuring Raman spectroscopy, temperature dependent photoluminescence (TDPL), temperature dependent electroluminescence (TDEL), forward biased-PL, temperature dependent time resolved photoluminescence (TDTRPL), photon energy dependent TRPL (at 11 K) and Electronreflectance spectroscopy (ER). InGaN- based LEDs with different patterned sapphire substrate has different residual strain, localization potential and PL lifetime. The sample with MPSS and NPSS has the improved crystal quality, high radiative recombination rate in the QWs due to smaller residual strain and piezoelectric field in QWs, comparing with sample with planar sapphire substrate. An a-plane non-polar blue-green LED with well-defined crystal orientation was confirmed no piezoelectric field in the QWs due to no blue-shift in the PL spectra applied reverse bias. By comparing the frequency of the E2(high) modes, we demonstrate that the residual compressive strain in an a-plane LED is significantly smaller than in the polar counterpart. Non-polar sample has the deeper localization potential in the QWs confirmed by only red-shift as increasing temperature from TDPL spectrum and the larger E0 (55.13meV) at 11 K from energy dependent TRPL. A polar InGaN/GaN blue LED was measured by TDEL, confirmed that at high injection current, at high temperature, the emission is mainly from p-side QWs, at very low temperature (T<90K), the emission is mainly from n-side QWs, which is attributed to well-to-well non-uniformity, due to the reduced holes capturing-efficiency at p-side QWs and then holes tunneling into n-side QWs. At low injection current, when T<90 K the EL intensity saturates due to the saturated radiative recombination rate, even resulting in the sever efficiency droop. A flexible blue LED with different degree bending was investigated by Raman spectroscopy, ER and TRPL. The PL lifetime of bending sample is longer than that of flat sample, due to the larger compressive strain and piezoelectric field in the QWs, and the more bending, the larger compressive strain and larger piezoelectric field in the QWs. These results demonstrated that the optical characteristics of LEDs are strongly affected by the patterned sapphire substrate, the growth direction of epitaxial layers, the properties of MQWs, and the bending of chips.
The effect of patterned sapphire substrate, the growth direction of epitaxial layers, the well-to-well non-uniformity of QWs, the saturated radiative recombination rate and the bending of chips on the carrier recombination dynamics, residual strain, carrier localization and piezoelectric field in the QWs were investigated by measuring Raman spectroscopy, temperature dependent photoluminescence (TDPL), temperature dependent electroluminescence (TDEL), forward biased-PL, temperature dependent time resolved photoluminescence (TDTRPL), photon energy dependent TRPL (at 11 K) and Electronreflectance spectroscopy (ER). InGaN- based LEDs with different patterned sapphire substrate has different residual strain, localization potential and PL lifetime. The sample with MPSS and NPSS has the improved crystal quality, high radiative recombination rate in the QWs due to smaller residual strain and piezoelectric field in QWs, comparing with sample with planar sapphire substrate. An a-plane non-polar blue-green LED with well-defined crystal orientation was confirmed no piezoelectric field in the QWs due to no blue-shift in the PL spectra applied reverse bias. By comparing the frequency of the E2(high) modes, we demonstrate that the residual compressive strain in an a-plane LED is significantly smaller than in the polar counterpart. Non-polar sample has the deeper localization potential in the QWs confirmed by only red-shift as increasing temperature from TDPL spectrum and the larger E0 (55.13meV) at 11 K from energy dependent TRPL. A polar InGaN/GaN blue LED was measured by TDEL, confirmed that at high injection current, at high temperature, the emission is mainly from p-side QWs, at very low temperature (T<90K), the emission is mainly from n-side QWs, which is attributed to well-to-well non-uniformity, due to the reduced holes capturing-efficiency at p-side QWs and then holes tunneling into n-side QWs. At low injection current, when T<90 K the EL intensity saturates due to the saturated radiative recombination rate, even resulting in the sever efficiency droop. A flexible blue LED with different degree bending was investigated by Raman spectroscopy, ER and TRPL. The PL lifetime of bending sample is longer than that of flat sample, due to the larger compressive strain and piezoelectric field in the QWs, and the more bending, the larger compressive strain and larger piezoelectric field in the QWs. These results demonstrated that the optical characteristics of LEDs are strongly affected by the patterned sapphire substrate, the growth direction of epitaxial layers, the properties of MQWs, and the bending of chips.
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