Luminescence of Localised Excitons in InGaN/GaN Multiple Quantum Wells

Abstract

. Although optical properties of InGaN/GaN quantum well structures are being widely investigated, the emission mechanisms in this system are still not completely understood [1][2][3][4][5][6][7]. Localised excitons is the most probable mechanism responsible for emission from InGaN/GaN MQWs, however the emission efficiency is highly sensitive to impurity and interface states, inhomogeneities and compositional separation of the InGaN alloy as well as to built-in electric field [3][4][5][6][7].Here we report on a study of InGaN/GaN MQW structures with various well thickness, d, by temperature-dependent, excitation power-dependent and time-resolved photoluminescence techniques. The samples were grown on sapphire substrates by metalorganic chemical vapour deposition. The undoped MQW structures were grown at temperatures of 1020 and 720 C for GaN and InGaN, respectively, and consisted of five periods of 10 nm thick GaN barriers and In x Ga 1--x N (x 0.15) wells. A series of samples with d ¼ 2, 2.5, 3, 3.5, and 4 nm were investigated.Luminescence spectra of structures studied show two main bands originated from the QW and barrier/buffer layers (Fig. 1). These two bands dominate in the emission over a broad range of excitation intensities. The QW luminescence is Stokes shifted by about 220 meV in respect to the estimated bandgap. It is attributed to the localised excitons. The barrier/buffer related band is situated at the bandgap energy of GaN and is attributed to the free-carrier emission. The QW luminescence blueshifts with increasing exci-

Stimulated Emission in InGaN/GaN Multiple Quantum Wells with Different Indium Content

“…The emission properties of highly excited InGaN/GaN MQWs can be interpreted by recombination of partly localized carriers under screened built-in field [5,6]. Thus, high-power excitation spectroscopy can reveal the impact of fluctuating potential on both spontaneous and stimulated emission of InGaN/GaN MQWs with the different mean In content.…”

“…The higher-energy band can be attributed to band--to-band recombination in the electron-hole system [7]. The QW related band is attributed to radiative recombination of partially localized carriers within the randomly distributed energy potential of the QW [5]. The QW related band steadily redshifts with increase in the In content, while the well/buffer related band is positioned at the same photon energy in the vicinity of the GaN band gap.…”

Quantum‐well thickness dependence of stimulated emission in InGaN/GaN structures

“…Also, a blue shift can be distinguished with increasing the excitation density. Therefore, the lower-energy band can be attributed to localised-excitation emission from the QW region with the band-filling effect [3,4,10,11]. The higher-energy band keeps a well-width−independent peak position and is typical of band-to-band transitions in GaN (buffer-layer emission) [11].…”

“…Therefore, the lower-energy band can be attributed to localised-excitation emission from the QW region with the band-filling effect [3,4,10,11]. The higher-energy band keeps a well-width−independent peak position and is typical of band-to-band transitions in GaN (buffer-layer emission) [11]. With increasing the excitation density (I > 0.1 mJ/cm 2 ), laterally amplified luminescence occurs on the high-energy wing of the QW band what is typical of stimulated emission from band-tail states.…”

“…Variation in well width results not only in alteration of electronic states by quantum size effect, but also in the quantum-confined Stark effect, as well as in intricate changes of thermodynamics of In segregation that is due to lattice mismatch between InN and GaN. Our previous study on optical and structural properties of InGaN/GaN MQWs has evidenced that an increased well thickness invokes formation of spatially separated In-rich and In-poor regions in the quantum well layers [9][10][11]. Here we report on a study of excitation-density dependent and time-resolved lateral emission in InGaN/GaN MQW structures with various well thickness, d. The high-excitation regime applied enabled us to minimise the built-in electric field effect on emission from localised states.…”