Effect of Growth Pressure and Gas-Phase Chemistry on the Optical Quality of InGaN/GaN Multi-Quantum Wells

Abstract

Blue light-emitting diodes (LED's), utilizing InGaN-based multi-quantum well (MQW) active regions deposited by organometallic chemical vapor epitaxy (OMVPE), are one of the fundamental building-blocks for current solid-state lighting applications. Studies [1,2] have previously been conducted to explore the optical and physical properties of the active MQW's over a variety of different OMVPE growth conditions. However, the conclusions of these papers have often been contradictory, possibly due to a limited data set or lack of understanding of the fundamental fluid dynamics and gas-phase chemistry that occurs during the deposition process.Multi-quantum well structures grown over a range of pressures from typical low-pressure production processes at 200 Torr, up to near-atmospheric growth conditions at 700 Torr, have been investigated in this study. At all growth pressures, clear trends of gas-phase chemical reactions are observed for increased gas residence times (lower gas speeds from the injector flange and lower rotation rates) and increased V/III ratios (higher NH 3 flows).Confocal microscopy, excitation-dependent PL (PLE), and time-resolved photoluminescence (TRPL) have been employed on these MQW structures to investigate the carrier lifetime characteristics. Confocal emission images show spatially-separated bright and dark regions. The bright regions are red-shifted in wavelength relative to the dark regions, suggesting microscopic spatial localization of high indium content regions. As the growth pressure and gas residence times are reduced, a larger difference in band-gap between bright and dark regions, longer lifetimes, and higher average PL intensities can be obtained, indicating that higher optical quality material can be realized. Optimized MQW's grown at high pressure exhibit higher PLE slope intensities and IQE characteristics than lower pressure samples. Results on simple LED structures indicate that the improvement in MQW optical quality at high pressures translates to higher output power at a 110 A/cm 2 injection current density. 505

Optical Characterization of Carrier Localization, Carrier Transportation and Carrier Recombination in Blue-Emitting InGaN/GaN MQWs

“…Since the carrier life-time (τ) is calculated by 1/τ = 1/τ r + 1/τ nr , [5] it is predominantly affected by the smaller life-time. Consequently, as the temperature increases from 10 K to 70 K, the radiative recombination life-time increases, leading to larger carrier life-time.…”

“…Detailed growth conditions are described elsewhere. 5 The growth temperature was also tuned to achieve a target PL peak wavelength of ∼449 nm, corresponding to an indium composition of ∼14%. Figure 1 shows the system diagram of low-temperature PL system.…”

PL Spatial Variation in InGaN/GaN MQWs Studied by Confocal Microscopy and TRPL Spectroscopy

“…Detailed growth conditions are described elsewhere. 15 In this case, the MQW structures were close to exactly the same such that QCSE and strain-related internal electric fields were considered not to be a major influence in the run-to-run comparison. The growth temperature was also tuned to achieve a target PL peak wavelength of ∼449 nm, corresponding to the indium composition of ∼14%.…”

“…It is conceivable that samples grown at higher pressure may have fewer point defect and vacancies, and as a result, better crystal quality. 15 The bright region average intensity was calculated over 10 × 10 pixels in the bright region of the CLSM image. Figure 2d shows the bright region average PL intensity as a function of the PL peak energy difference.…”

(Invited) Study of Carrier Localization, Carrier Transportation and Carrier Recombination Processes in Blue-Emitting InGaN/GaN MQWs

“…Four-period MQWs were then grown on the GaN template, consisting of 2.7-nm-thick InGaN quantum well layers and 12.5 nm-thick GaN barrier layers, as was confirmed by XRD. Detailed growth conditions are described elsewhere (5). The growth temperature was also tuned to achieve a target PL peak wavelength of ~449 nm, corresponding to an indium composition of ~14%.…”