Internal quantum efficiency of III-nitride quantum dot superlattices grown by plasma-assisted molecular-beam epitaxy

Abstract

We present a study of the optical properties of GaN/AlN and InGaN/GaN quantum dot (QD) superlattices grown via plasma-assisted molecular-beam epitaxy, as compared to their quantum well (QW) counterparts. The three-dimensional/two-dimensional nature of the structures has been verified using atomic force microscopy and transmission electron microscopy. The QD superlattices present higher internal quantum efficiency as compared to the respective QWs as a result of the three-dimensional carrier localization in the islands. In the QW samples, photoluminescence (PL) measurements point out a certain degree of carrier localization due to structural defects or thickness fluctuations, which is more pronounced in InGaN/GaN QWs due to alloy inhomogeneity. In the case of the QD stacks, carrier localization on potential fluctuations with a spatial extension smaller than the QD size is observed only for the InGaN QD-sample with the highest In content (peak emission around 2.76 eV). These results confirm the efficiency of the QD three-dimensional confinement in circumventing the potential fluctuations related to structural defects or alloy inhomogeneity. PL excitation measurements demonstrate efficient carrier transfer from the wetting layer to the QDs in the GaN/AlN system, even for low QD densities ($10 10 cm À3). In the case of InGaN/GaN QDs, transport losses in the GaN barriers cannot be discarded, but an upper limit to these losses of 15% is deduced from PL measurements as a function of the excitation wavelength.

AlGaN/AlN quantum dots for UV light emitters

“…The flux of active nitrogen was fixed at Φ N = 0.32 ML/s, and the substrate temperature (T S ) was controlled to be in the range of T S = 720-745 °C. The growth of AlGaN SK-QDs was performed under N-rich conditions, since the nitrogen excess reduces the mobility of adsorbed species during growth, resulting in a high density of small QDs [8][9][10]. The Al-to-metal flux ratio, Φ Al /(Φ Al + Φ Ga ), was varied from 0.12 to 0.42.…”

Carrier localization in InN/InGaN multiple-quantum wells with high In-content

“…The thermal stability of the luminescence is thus strongly improved by the lowdimensional carrier confinement, as already reported in InN/ GaN and InGaN/GaN QD and QW structures. 6,12 The thermal evolution of the PL emission is characterized by the intensity quenching due to the activation of nonradiative recombination processes, and also by the energy shift and broadening of the PL. For the analyzed samples, the measured thermal extinction of the PL intensity, I(T), can be reproduced considering two nonradiative recombination channels: 13 IðTÞ…”

Ultrafast Nonlinear Optical Response in GaN/AlN Quantum Dots for Optical Switching Applications at 1.55 μm

“…9(b) should correspond directly to the internal quantum efficiency (IQE) at different temperatures. These results confirm the improved thermal stability of QDs over QWs, as a result of the 3D carrier confinement (Adelmann et al, 2000;Damilano et al, 1999;Gacevic et al, 2011;Guillot et al, 2006;Sénés et al, 2007) To probe the competition between radiative and nonradiative recombination processes, we now focus on the PL decay time (Renard et al, 2009). Time-resolved PL experiments were performed using a frequency-tripled Ti-Sapphire laser (λ = 270 nm) with a pulse width around 200 fs.…”

“…AFM image of GaN/AlN QDs synthesized by deposition of 4 MLs of GaN under (a) N-rich and (b) Ga-rich conditions. Note that N-rich conditions lead to higher density of smaller QDs whereas Ga-rich conditions lead to lower density of bigger QDs (after Gacevic et al (Gacevic et al, 2011)). …”