Abstract
GaN-based light emitting diodes (LEDs) have been fabricated on sapphire substrates with different thicknesses of GaN buffer layer grown by a combination of hydride vapor phase epitaxy and metalorganic chemical vapor deposition. We analyzed the LED efficiency and modulation characteristics with buffer thicknesses of 12 μm and 30 μm. With the buffer thickness increase, cathodoluminescence hyperspectral imaging shows that the dislocation density in the buffer layer decreases from ∼1.3 × 108 cm−2 to ∼1.0 × 108 cm−2, and Raman spectra suggest that the compressive stress in the quantum wells is partly relaxed, which leads to a large blue shift in the peak emission wavelength of the photoluminescence and electroluminescent spectra. The combined effects of the low dislocation density and stress relaxation lead to improvements in the efficiency of LEDs with the 30 μm GaN buffer, but the electrical-to-optical modulation bandwidth is higher for the LEDs with the 12 μm GaN buffer. A rate equation analysis suggests that defect-related nonradiative recombination can help increase the modulation bandwidth but reduce the LED efficiency at low currents, suggesting that a compromise should be made in the choice of defect density.
Highlights
Johnson et al reported that light emitting diodes (LEDs) on sapphire substrates with a 15 μm GaN buffer layer grown by metalorganic chemical vapor deposition (MOCVD) had a higher indium concentration than those with a 5 μm buffer layer [11]
Compared with MOCVD growth, a hydride vapor phase epitaxy (HVPE) system could accelerate growth of unintentionally doped GaN (u-GaN)/ n-GaN on sapphire substrates and reduce precursor usage [12], allowing the cost of the LED growth to be reduced through combining the HVPE and MOCVD growth technologies
An extra step of MOCVD growth was required in our work, our method offers the potential advantages of growing high quality GaN template or inserting a special GaN layer, e.g. a heavily doped GaN layer
Summary
GaN-based light emitting diodes (LEDs) are already widely used in solid state lighting (SSL) [1], and additional applications which are emerging including visible light communication (VLC). Homogeneous growth of LED epitaxial layers on bulk GaN substrates is expected to alleviate these problems, but reducing the cost of thick GaN substrates is still challenging for mass production In this case, thick GaN buffer layers (e.g. tens of microns of GaN in this work) on sapphire substrates may offer an alternative approach to alleviating these problems, as this can help reduce the dislocation density and relax the compressive strain in the MQWs. Johnson et al reported that LEDs on sapphire substrates with a 15 μm GaN buffer layer grown by metalorganic chemical vapor deposition (MOCVD) had a higher indium concentration than those with a 5 μm buffer layer [11]. Raman spectra were measured to demonstrate the relaxation of compressive stress within the LEDs which explains the large blue peak shift in the electroluminescence (EL) and photoluminescence (PL) spectra with increased GaN buffer thickness Based on these characteristics, the external quantum efficiency (EQE) and electrical-optical modulation bandwidth were analyzed using a carrier recombination model. Based on the experimental and theoretical analyses in this study, the intentional introduction of defects could be an effective approach to improving the modulation bandwidth of GaN-based LEDs for applications in VLC
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