Design, growth and characterisation of resonant cavity light emitting diodes (RCLEDs) for mid-infrared applications

Al-Saymari, Furat and Krier, Anthony (2020) Design, growth and characterisation of resonant cavity light emitting diodes (RCLEDs) for mid-infrared applications. PhD thesis, Lancaster University.

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Abstract

There is a growing requirement for high brightness light emitting diodes which can operate in the technologically important mid-infrared spectral range (3-5 µm), for applications such as environmental monitoring, industrial process control and spectroscopy. To date, mid-infrared (λ> 4 µm) LEDs exhibit a poor 300 K external efficiency with broadband emission spectra, resulting in low available output power at the target wavelength. The resonant cavity structure is an attractive solution to improve the performance of these LEDs by locating the active region inside a Fabry-Perot cavity which is formed between two distributed Bragg reflectors (DBRs). In this thesis, we demonstrated four novel mid-infrared resonant cavity LED (RCLED) structures operating near 4.0 μm, 4.2 μm, 4.5 μm, and 4.6 μm at room temperature. Three samples were grown on GaSb substrate and one on InAs substrate using molecular beam epitaxy (MBE). Different III-V semiconductor materials; bulk InAsSb, AlInAs/InAsSb strained multi quantum wells (MQWs), and InAs/GaAsSb superlattices (SLs) were used as active regions, positioned at the antinode of the electric field inside the micro-cavity. The 1λ-thick cavities, containing the active regions, were sandwiched between two high contrast latticematched AlAs0.08Sb0.92/GaSb DBR mirrors for GaSb-based RC structures and lattice-matched AlAs0.16Sb0.84/GaAs.08Sb0.92 DBR mirrors for the InAs-based RC structure respectively. The RC structures were first designed theoretically, where the thickness of the micro-cavity layers and the DBR layers were evaluated corresponding to the target wavelength of RC emission spectra. Then, the reflectivity of the top and bottom mirror were investigated to achieve high emission enhancement as determined by the number of layers in the DBR. The simulated results show that 13.5 pairs in the bottom DBR mirror with reflectivity (R>98%) and 5 pairs in the top DBR mirror with reflectivity (R>83%) are sufficient to achieve very high enhancement factors. The temperature dependence of the transmission spectra of the RC samples was measured over the range from 77 K to 300 K. It was found that the position of the optical cavity mode and the DBR stopband centre shift towards longer wavelength very slowly as the temperature increases at a rate of <0.4 nm/K and <0.3 nm/K, respectively. At room temperature the optical modes exhibit a narrow linewidth in the transmission spectrum (Δλ<100 nm). The room temperature quality factors were found to be in the range from ~60 to ~100, predicting that the emission of the RCLEDs should have a strong enhancement. Although all the RC samples exhibit some detuning, between the wavelength of the DBR stopband centre and the position of the optical cavity mode, the simulated results show that the spectral linewidth remains narrow and the emission enhancement factors are still high. Two entirely new GaSb-based mid-infrared RCLEDs were fabricated, (i) using bulk InAsSb operating at ~4.2 µm and (ii) with AlInAs/InAsSb MQWs operating at ~4.5 µm. The temperature dependence of the electroluminescence spectra and I-V characterization of both these RCLEDs and their reference LEDs were measured experimentally over the range from 20 K to 300 K. The RCLEDs show significantly better temperature stability, narrower emission linewidth, and high enhancement factors. The bulk InAsSb RCLED exhibits a significantly narrower (10x) spectral linewidth, (6x) superior temperature stability, (70x) higher peak intensity, (33x) higher integrated output power compared to that of the reference without a resonant cavity. For the MQWs RCLED, the peak intensity and the integrated emission were enhanced by a factor of ~85 and ~13, respectively. It also shows a superior temperature stability of ~0.35 nm/K and a narrow emission linewidth of ~70 nm (which are less than that of the reference LED by 7x and 16x, respectively). The optical and electrical properties of the RCLEDs without top DBR mirror were also investigated. The results show that the intensity and integrated enhancement are still achieved, but somewhat less than that of the full RCLED structures, due to low reflectivity (~33%) of the interface semiconductor/air top mirror. The high brightness, better spectral purity, narrow linewidth and superior temperature stability, of the RCLEDs developed in this work are rather attractive features, enabling these devices to be readily implemented in the next generation of optical gas sensor instrumentation.