III-N UV LEDs are currently of major interest for applications such as sterilisation (UVC) and phototherapy (UVB). Most work to date has focussed on the AlGaN system which is relatively well understood, but this limits the possibility for bandgap engineering in the active region structure. In this presentation we will outline the demonstration of a UV Light emitting diode operating around 340 nm using boron containing (BAlGaN) quantum wells. The device exhibited a higher output intensity compare to the boron free reference sample prepared at the same time. The potential for using such alloys for UVB and UVC devices will be discussed.
As we reduce the emission wavelength of III-Nitride materials, and delve deeper into the UV region, the struggle to keep the material at a high internal quantum efficiency escalates. A reduction in the quantum confined Stark effect and an improvement in strain engineering are just two of the challenges that wurtzite boron nitride (wz-BN) could play a key role in. In this presentation, we investigate the possibility of incorporating wz-BN into ternary and quaternary multiple quantum wells serving as the active region for UV emitters. This work was funded by Science Foundation Ireland (IPIC and PIADs.)
The spectra of the optically pumped ZnMgSSe/ZnSe multiple quantum well separate confinement heterostructure lasers and the PL spectra of the heterostructures were investigated as functions of temperature (T=80-500 K), excitation intensity (Iexc=0.001-1000 kW/cm2) and number of excitation pulses (N=106). The laser threshold temperature dependence was investigated and explained by alternation of the spontaneous emission efficiency and spectral broadening with temperature rise. It was shown that at room temperature the active region overheating value does not exceed 10 K at high Iexc. It was established that the degradation of the heterostructures under high levels of excitation is due to the inherent laser emission interaction with the heterostructure leading to the appearance of intrinsic defects and to the inherent light stimulated interdiffusion at ZnMgSSe/ZnSe interfaces.
Bernd Schineller, Harry Protzmann, Markus Luenenbuerger, Georg Gerstenbrandt, Michael Heuken, Evgenii Lutsenko, Vitaly Zubialevich, Vyacheslav Pavlovskii, Alexander Gurskii, Gennadii Yablonskii
Laser and optical properties of InGaN/GaN multiple quantum well heterostructures were investigated as functions of temperature (T=80-450 K) and excitation intensity (Iexc=10-1100 kW/cm2) of the N2 laser radiation. Laser action was achieved in all types of the MQWs from the violet up to the blue spectral region (λlas=405-470 nm). The laser threshold at room temperature was 35-100 kW/cm2 and 70-150 kW/cm2 for the 'violet' and 'blue' lasers, correspondingly. The characteristic temperature in the temperature range of 80-220 K was T0=180 K for the 'violet' and T0=530 K for the 'blue' lasers. The T0 was lower for all types of lasers at T>250 K. It was shown that the overheating of the active region under high excitation intensities can reach 40-100 K and it is due to inherent laser radiation.
GaN-based optoelectronics allow covering the spectral range from green to UV. Silicon (Si) is an alternative substrate to the commonly used sapphire and silicon carbide (SiC) but requires sophisticated buffer structures. In this work, two high-temperature (HT) layer stacks and two low-temperature (LT) AlN layers were used for the growth of GaN buffers for optoelectronic devices on (111)-oriented Si substrates using AIXTRON metalorganic vapor phase epitaxy (MOVPE) reactors. AlN, AlGaN and GaN were grown as HT layer stack to form stress reduction layers. GaInN MQW (multi quantum wells), electroluminescence test structures (ELT) and AlN/GaN DBR (distributed Bragg reflectors) were deposited on these buffer structures on Si. The growth process was monitored by in-situ reflectivity measurements. Photoluminescence (PL), electroluminescence and the luminescence under high optical excitation of the samples on Si have been studied. Laser action at optical excitation was obtained in the MQW with a room temperature (RT) laser threshold of Ithr = 40 - 80 kW/cm2. Laser action was achieved up to 350°C. Electroluminescence emission from the ELT InGaN/GaN heterostructures was observed and measured under a minimal DC voltage of about 4 V. AlN/GaN DBR with ten periods showed reflectivities of 60% for wavelengths of 436 nm and 537 nm, respectively.
Evgenii Lutsenko, Vitalii Zubialevich, Vyacheslav Pavlovskii, Igor Marko, Alexander Gurskii, Gennadii Yablonskii, Holger Kalisch, Thomas Walther, Oliver Schoen, Harry Protzmann, Markus Luenenbuerger, Bernd Schineller, Michael Heuken
ZnSe/ZnMgSSe and InGaN/GaN heterostructure based lasers under optical transverse pumping by pulsed N2-laser radiation were investigated in a wide spectral, temperature and excitation intensity range for various types of heterostructures which differed in the epitaxial layer composition, layer sequence and thickness. The spectral- angular distribution of the laser emission of the ZnSe/ZnMgSSe separate confinement heterostructures and the influence of the excitaion intensity and the cavity length on the laser mode structure were investigated. It was found that the main degradation mechanism of the ZnSe/ZnMgSSe multiple quantum well heterostructures at temperatures higher than 400 K is the diffusion of S atoms from the barriers into the quantum wells which leads to increasing point defect concentration in the active layers of the laser. The recovery of the laser threshold of the ZnSe/ZnMgSSe MQW-SCHs degraded during thermal annealing after the action of the inherent laser radiation is attributed to a significant decrease of the point defects in the active layers. Laser action of InGaN/GaN multiple quantum well heterostructures in the blue spectral region has been obtained for the first time and investigated. Temperature tuning of InGaN/GaN MQW laser emission from the violet to blue has been realized. The influence of the photoluminescence characteristics on the laser parameters of the InGaN/GaN MQW optically pumped lasers operating in the blue spectral region is investigated.
Gennadii Yablonskii, Evgenii Lutsenko, Vitalii Zubialevich, Vyacheslav Pavlovskii, Igor Marko, Alexander Gurskii, Assadullah Alam, Harry Protzmann, Markus Luenenbuerger, Bernd Schineller, Michael Heuken
The influence of layer thickness, heterostructure design, optical confinement factor and spontaneous emission efficiency on laser parameters of GaN based quantum well optically pumped lasers is studied in wide spectral (373 - 470 nm), temperature (77 - 600 K) and excitation intensity (102 - 3 106 W/cm2) regions. The laser threshold enhancement from 70 kW/cm2 for the 421 nm operating laser to 900 kW/cm2 for the 469.5 nm laser leads to the reduction of highest operation temperature of the laser from 585 K for the 421 nm laser to 295 K for the 469.5 nm laser with increasing operating wavelength. As a rule the far field pattern of the laser emission consists of two light spots localized at positive and negative angles of 30 - 50 degree(s). The laser spectra structure in the far-field of the SQWs and MQWs with low thickness of the active layers depended on the registration angle. The spatial distribution of the laser light in the far-field consisting of transverse and leaky modes was calculated and compared with the experimental results. Calculations of the optical confinement factor and the electromagnetic field distribution inside and outside of the heterostructures showed that the MQW lasers operate in the high order transverse mode regime. The spectral-angular distribution of the emission of the SQW and MQW lasers with low active layer thickness is due to the leaky mode formation.
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