Transverse Electric Lasing at a Record Short Wavelength 244.63 nm from GaN Quantum Wells with Weak Exciton Localization

Abstract

We have demonstrated a record short wavelength lasing at 244.63 nm with TE dominant polarization from GaN quantum wells (QWs) at room temperature (RT). The optical threshold of 310 kW/cm 2 is comparable to state-of-the-art AlGaN QW lasers at similar wavelengths. The sample was grown on the AlN/sapphire template pesudomorphically. X-ray diffraction (XRD) shows unambiguous higherorder satellite peaks indicating a sharp interface amid the active region. The excitonic localization was revealed and studied by the photoluminescence (PL) and timeresolved PL (TRPL) spectroscopy at temperatures ranging from 15 K to RT. At 15 K, the multiple-component PL decay curves with the decay time varying from 62.6 to 2.77 ns at different energies confirmed the localized excitons. The peak energy of the temperature-dependent PL spectra exhibited the "S-shape" behavior; and the weak exciton localization with a small localization energy of 14.3 meV was observed. Therefore, even in the low temperature region, the escape possibility of excitons increased as the temperature rose. As a result, the fwhm of the emission spectra changed significantly when the temperature was below 150 K. Above 150 K, the PL decay shape changed from the two-component exponential decay to the single exponential decay, indicating complete delocalization of excitons. The work demonstrates the weak localization and thus smooth interface in the GaN/AlN active region, which are desirable for DUV lasers operating at RT.

Exciton Effects in Low-Barrier GaN/AlGaN Quantum Wells

“…The exciton localisation energies, obtained as the maximum of the difference between the value predicted by Varshni's formula, and the corresponding experimental points recorded at a low temperature, are of the same order of magnitude of those observed in the similar structures [8,10,22]. Additionally, they clearly show an increase with increasing Al content (31 meV, 37 meV and 59 meV for x = 0.15, x = 0.23 and x = 0.37, respectively), as well as the maximum temperature value for observing exciton delocalisation (100 K, 150 K and 200 K, for x = 0.15, x = 0.23 and x = 0.37 respectively).…”

“…The former analysis leads to attributing this emission feature to the excitonic emission, which is also expected to have a large oscillator strength . The exciton localization energy values, estimated as the difference between the exciton PL resonance at low temperatures, in which the exciton localization is effective, and the value extrapolated at 10 K by Varshni’s law related to the free exciton emission (experimentally observed at a higher temperature), are of the same order of magnitude as those observed in similar structures. ,, …”

“…Continuous lines represent a fit carried out on the high temperature experimental data for the main bands, by using the Varshni empirical formula, extrapolated to 10 K. Finally, the dashed lines represent the AlGaN barrier energy gap (as retrieved from Vurgaftman et al, [19]) for the different Al concentrations. The temperature behaviour of the main emission bands observed up to room temperature, present the "Sshaped" trend associated to exciton localisation effects [8,22], expected in these structures because of wellwidth fluctuations and/or well/barrier interface quality, rather than barrier composition fluctuations [10].…”

“…As an alternative, in the last few years, attention has been focused on gallium nitride layers with extremely reduced dimensionality, that is, ultrathin or quasi-2D GaN QWs, that can exhibit high quantum confinement, enhanced carrier localization, and a blue shift of their bulk optical emission when inserted in AlGaN matrix. These active regions have recently been demonstrated to be beneficial for efficient UV emission in LEDs , and laser devices . Interesting features of excitonic properties have been theoretically anticipated and experimentally observed in these systems, such as high exciton binding energy, , high radiative recombination rate, , and large energy splitting between dark and bright excitons. , Additionally, exciton localization has also been widely reported in the literature, typically related to interface roughness, ,, suggesting that a detailed study on the relationship between structural properties and excitonic effects deserves further investigation.…”

“…As an alternative, in the last years, attention has focused on gallium nitride layers with extremely reduced dimensionality, that is, ultrathin or quasi 2D GaN quantum wells, that can exhibit high quantum confinement, enhanced carrier localisation and blue-shift of their bulk optical emission if inserted in AlGaN matrix. Those active regions have recently demonstrated to be beneficial for efficient UV light emission in LEDs [6,7] and laser devices [8]. Interesting features of excitonic properties have been theoretically anticipated and experimentally observed in these systems, such as high exciton binding energy [9,10], high radiative recombination rate [8,9], high energy splitting between dark and bright exciton [9,11].…”