Carrier-induced nonlinearities in InGaN/GaN quantum wells with V-pits
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esearch Letters
Carrier-induced nonlinearities in InGaN/GaN quantum wells with V-pits Meg Mahat and Antonio Llopis, Department of Physics, University of North Texas, Denton, Texas 76203 Richard D. Schaller, Argonne National Laboratory, Center for Nanoscale Materials, Argonne, Illinois 60439 Ian Watson, SUPA, Institute of Photonics, University of Strathclyde, Glasgow, UK Sergio Periera, CICECO, Universidade de Aveiro, 3810-193 Aveiro, Portugal Arup Neogi, Department of Physics, University of North Texas, Denton, Texas 76203 Address all correspondence to A. Neogi at [email protected] (Received 6 March 2012; accepted 22 May 2012)
Abstract Ultrafast differential transmission spectroscopy was employed to study the carrier dynamics in InGaN/GaN multiple quantum wells with high inverted hexagonal pits density due to threading dislocation. By monitoring the temporal evolution of the excitonic absorption spectrum, a reduction of the quantum-confinement Stark shift was observed due to the photo-induced in-well field screening at low carrier densities and excitonic absorption quenching at high carrier densities. By comparing the differential absorption spectra at various injected carrier densities, the in-well field screening effect was distinguished from excitonic bleaching.
It has been observed that despite high dislocation densities compared with lattice-matched semiconductors, InGaN/ GaN-based systems can exhibit internal quantum efficiencies for light emission in excess of 60%.[1–3] Strained multiple quantum wells (MQWs) on sapphire substrates have large intrinsic piezoelectric fields (PEFs) along the growth direction resulting in inverted hexagonal pit (IHP) or “V-shaped” pits.[2,3] These pits are normally formed during the incorporation of In at relatively lower temperatures (∼800 °C) compared with GaN growth temperatures due to small surface perturbations. The narrowing of the quantum wells (QWs) as they fall into the pits increases the band gap and results in a potential barrier that effectively shields carriers from the nonradiative recombination center at the center of the IHP.[1] As the thickness of the QWs are reduced within the folded IHP nanostructure, the built-in strain field, localization and interface effects are also modified compared with the wells grown on normal c-plane facets. The carrier transport is strongly affected by the change in the potential distribution within the InGaN layers around the V-shaped pits.[2] The potential ridges prevents the carrier from diffusing outside them, whereas the potential peaks cause carriers to travel a roundabout route around them.[3] The carriers anisotropically diffuse for several hundred nanometers along a specific direction, which results in localized domains with strong photoluminescence (PL) or modified absorption characteristics.[4] Our recent study of the PL process in V-pit InGaN/GaN MQWs suggest that the carrier recombination process in c-plane QWs is significantly longer (∼5 ns) compared with that (∼1.5 ns) in V-shaped pit QWs due to the larger PEFs in wider wells.[5] T
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