Exciton localization in InGaN quantum well devices

Abstract

Emission mechanisms of a device-quality quantum well (QW) structure and bulk three dimensional (3D) InGaN materials grown on sapphire substrates without any epitaxial lateral overgrown GaN base layers were investigated. The InxGa1−xN layers showed various degrees of in-plane spatial potential (band gap) inhomogeneity, which is due to a compositional fluctuation or a few monolayers thickness fluctuation. The degree of fluctuation changed remarkably around a nominal InN molar fraction x=0.2, which changes to nearly 0.08–0.1 for the strained InxGa1−xN. This potential fluctuation induces localized energy states both in the QW and 3D InGaN, showing a large Stokes-like shift. The spontaneous emission from undoped InGaN single QW light-emitting diodes (LEDs), undoped 3D LEDs, and multiple QW (MQW) laser diode (LD) wafers was assigned as being due to the recombination of excitons localized at the potential minima, whose lateral size was determined by cathodoluminescence mapping to vary from less than 60 to 300 nm in QWs. Those structures are referred to as quantum disks (Q disks) or segmented QWs depending on the lateral size. Blueshift of the emission peak by an increase of the driving current was explained to be combined effects of band filling of the localized states by excitons and Coulomb screening of the quantum confined Stark effect induced by the piezoelectric field. The lasing mechanisms of the continuous wave In0.15Ga0.85N MQW LDs having small potential fluctuations can be described by the well-known electron-hole-plasma (EHP) picture. However, the inhomogeneous MQW LDs are considered to lase by EHP in segmented QWs or Q disks. It is desirable to use entire QW planes with small potential inhomogeneity as gain media for higher performance LD operation.

Excitonic field screening and bleaching in InGaN/GaN multiple quantum wells

“…To date, several groups have discussed the blue shift of the photoluminescence (PL) peak energy in InGaN/GaN heterostructures with increasing carrier injection. The observed carrier dynamics has been described using models that attribute the behavior to either the reduction of the quantum-confined Stark effect due to in-well field screening [5,9,10] band-filling of the energy band tail states [11,12] or both [13,14]. The main reason that these studies have not been able to directly elucidate these complicated dynamics is because traditional time integrated PL and time-resolved PL measurements allow only the radiative recombination dynamics to be directly characterized.…”

Ultrafast carrier relaxation in GaN,In0.05Ga0.95N,and anÖzgür1, Everitt2 2003Phys. Rev. B

“…The effect of these In-rich regions on electron-hole recombination through spontaneous emission (SPE) has been studied extensively. 5,12,13 In spite of imperfect material properties, high emission efficiencies are observed not only through enhanced SPE but also through stimulated emission (SE) generated at moderate pump powers. There have been many reports on SE in InGaN epilayers 14,15 and MQW structures.…”

“…Therefore it is important to understand the carrier relaxation and recombination mechanisms in InGaN structures. In addition to many studies on recombination times, 5,6,7 there have been a limited number of reports on ultrafast carrier dynamics in InGaN heterostructures 8,9 and multiple quantum wells (MQW). 10,11 Measurements on heterostructures, single QWs, and MQWs have emphasized different aspects of carrier relaxation in nitrides.…”

Progress in the optical studies of single InGaN/GaN quantum dots

“…The origin of this bright luminescence has been generally attributed to the existence of localization centres or even quantum-dot-like states responsible for the suppression of the diffusion of carriers to the various non-radiative defects. The nature of these centres is somewhat controversial; suggestions include In-rich clusters and monolayer thickness fluctuations within the QW [2][3][4]. Since localization centres in InGaN are believed to play an essential role in enhancing the properties of devices, a great deal of work over the past decade has been devoted to the intentional fabrication and control of the properties of InGaN quantum dots (QDs).…”