Photoluminescence efficiency of zincblende InGaN/GaN quantum wells

Abstract

Growing green and amber emitting InGaN/GaN quantum wells in the zincblende, rather than the wurtzite, crystal phase has the potential to improve efficiency. However, optimization of the emission efficiency of these heterostructures is still required to compete with more conventional alternatives. Photoluminescence time decays were used to assess how the quantum well width and number of quantum wells affect the recombination rates, and temperature dependent photoluminescence was used to determine the factors affecting recombination efficiency. The radiative recombination lifetime was found to be approximately 600 ps and to increase weakly with well width, consistent with a change in the exciton binding energy. The relative efficiency at room temperature was found to increase by a factor of five when the number of wells was increased from one to five. Furthermore, the efficiency increased by factor 2.2 when the width was increased from 2.5 to 7.5 nm. These results indicate that thermionic emission is the most important process reducing efficiency at temperatures in excess of 100 K. Moreover, the weak dependence of the rate of radiative recombination on well width means that increasing well thickness is an effective way of suppressing thermionic emission and thereby increasing efficiency in zincblende InGaN/GaN quantum wells, in contrast to those grown in the wurtzite phase.

Reduced Auger Coefficient through Efficient Carrier Capture and Improved Radiative Efficiency from the Broadband Optical Cavity: A Mechanism for Potential Droop Mitigation in InGaN/GaN LEDs

“…BTB (b 4 ) and SRH (b 5 ) dominate at a very long time (∼ t 3 ), by which the residual carriers decay from the QW. It may be noted that these assignments of processes at various time scales t 1 , t 2 , and t 3 (obtained values of these time constants are given in Figure ) are in congruence with the quantum mechanically estimated time constants. − The time constant associated with the combined effect of the transitions (mostly) a 1 and (partly) b 1 is obtained by the single exponential fit to the initial carrier kinetics, where Δα reaches its peak negative value. The fit for the control sample near the initial kinetics at the InGaN band edge is shown in the inset of Figure a.…”

Cathodoluminescence studies of the optical properties of a zincblende InGaN/GaN single quantum well

“…As discussed earlier, in the panchromatic CL images the regions associated with the pale stripes in the SE images appear dark. This can be explained by the optical properties of SFs in zb-GaN, which are significantly different from SFs in wz-GaN [31,36,40]. As a simplified picture, one can consider a SF in wz-GaN as a monolayer-wide zb insertion and vice versa.…”

Efficiency droop in zincblende InGaN/GaN quantum wells

“…It is important to note there is evidence of a significant amount of thermionic emission from these samples, with a steep drop in efficiency reported previously as the temperature is increased from 10 K to room temperature for the SQW sample that is significantly ameliorated for the otherwise similar MQW sample. 22,25 This suggests that carriers are escaping the QW at higher temperatures and are more likely to be recaptured in a MQW structure. Thermionic emission from these samples is likely exacerbated by the broad range of energy states in the zb-QWs that extends close to the band edge of the barriers, cap and UL, as demonstrated by the emission spectrum shown in Fig.…”

“…28 Although the SED experiments did reveal very occasional regions of hexagonal material a few tens of nanometres in size in the QW, cathodoluminescence (CL) imaging confirmed that these regions have little influence on the overall emission spectrum. 28 As detailed previously, 25 the presence of stacking faults in these samples leads to the QW layers having a non-planar geometry, thus an accurate value for the indium content within the QW layers cannot be obtained from X-ray diffraction measurements. Instead, the indium composition was estimated from the position of the photoluminescence (PL) peak at low excitation.…”