Carrier Dynamics in Group-III Nitride Low-Dimensional Systems: Localization versus Quantum-Confined Stark Effect

Abstract

Continuous-wave and time-resolved optical spectroscopy is used to examine a variety of InGaN/GaN quantum-well and quantum-box samples, grown by molecular beam epitaxy. The results are analyzed in order to clarify the respective influences of electric fields and of carrier localizations on radiative recombinations. The coupling of electron-hole pairs with LO-phonons is also studied in detail, from careful analysis of the size-dependent intensities of LO-phonon replica. From our attempt of modelling the Huang-Rhys factor, S, for excitons in these systems, we conclude that the observed optical recombinations are rather those of electrons and holes separately localized on different potential fluctuations.Introduction All efficient light-emitting devices based on group-III nitride semiconductors involve low-dimensional systems, such as InGaN/GaN [1][2][3] or GaN/AlGaN [4] quantum wells (QWs). An intense research activity has been devoted recently to radiative recombinations in these artificial materials. For InGaN-based nanometric layers, large Stokes shifts have been readily assigned to strong carrier localization, possibly in self-formed quantum dots [5,6]. This idea was supported by observations of In-rich nanoclusters in InGaN layers. Although this clustering may depend on the growth conditions, the random distribution of In atoms induces rather strong potential fluctuations, in the volume of this ternary alloy. These fluctuations, together with large carrier effective masses in group-III nitrides, induce the localization of electron and hole wave-functions on nanometric scales, which prevents their efficient capture by nonradiative centers, related to the high density of threading dislocations. This localization was invoked to interpret (i) the strong Stokes shift between photoluminescence (PL) lines and the absorption onset of InGaN epilayers and QWs and (ii) the rather long PL decay times observed in these systems. Now, for all QWs based on nitrides in their wurtzite phase, including InGaN/GaN ones, electric fields of several hundred kV/cm are present, if the growth axis is (0001). Indeed, this symmetry permits spontaneous and piezoelectric polarizations which are specially strong for group-III nitrides [7,8]. These fields reduce drastically the oscillator strength for the ground-state optical transition, as evidenced [9, 10] by very large, sizedependent, recombination times. They are also partly at the origin of the Stokes shift observed in QWs [11].

Transient photoluminescence of aluminum‐rich (Al,Ga)N low‐dimensional structures

“…As T is increased, the slower decay component becomes faster, while the faster one remains unchanged; the intensity at short delays first increases and remains higher than the 8 K value up to 180 K. One plausible explanation for the slower component of the decay is the slow feeding of the quantum wells by higher‐lying potential fluctuations, in particular – but not only – in the barriers. This has been already observed and commented for (Ga,In)N‐based systems . Slightly raising the temperature results in accelerating the transfer of carriers from those potential fluctuations towards the lower‐lying potential valleys in the QWs.…”

Temperature and well number dependence of exciton localization in InGaN/GaN quantum wells

“…This could only be explained considering the FSLE as a parameter describing a different type of localization from the localization represented by the FWHM of the PL emission peak. Taking into account the idea of two localized centres, one that tightly localizes excitons and another that separately localizes electrons and holes [6], we could conclude that the magnitude of the FWHM of the PL emission peak relates to potential fluctuation that tightly localizes excitons due to the statistical distribution of high InN content sites, while the FSLE worked out from the Huang-Rhys parameters could represent centres that localize excitons separately towards the interfaces of the quantum wells due to the quantum-confined Stark effect (QCSE), resembling a donor-acceptor pair (DAP) coupling [22]. In fact, the piezoelectric field was estimated as 1 MV cm −1 in sample B [8].…”