Optical Properties of Ingan/GaN Multi Quantum Well Structures
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ABSTRACT A set of GaN/InGaN multiple quantum wells (QWs) with well thickness 30 A and barrier thickness 60 A were grown by MOCVD on sapphire substrates. The n-type Si doping of the InGaN QWs was varied, in order to produce a different electron concentration in the QWs for the different samples. Optical spectra were obtained by time resolved photoluminescence spectroscopy. The data show weak excitonic spectra from the QWs as well as a broad deeper emission with a much stronger intensity. The spectral shape becomes narrower and the energy position shifts to higher energies with increasing doping. The two different emissions are not easily separated in CW or time integrated spectra, but are clearly observed in a time resolved spectral measurement due to their different recombination rates. The deeper emission has a long and non-exponential decay, with an average decay time in the order of several hundred nanoseconds. The higher energy exciton emission has a much faster decay of about 1 ns. The lower energy band is tentatively explained as due to separately localized electron-hole (e-h) pairs in the QW.
INTRODUCTION The technological interest of nitrides has been focused on optoelectronic devices such as light emitting diodes (LEDs), and lasers. Most of these devices are realized in GaN/InGaN confined structures [1,2]. Very few detailed studies of the recombination processes of carriers in confined InGaN layers have been reported, however, and the recombination mechanisms are not yet well understood. The photoluminescence (PL) emission from both epitaxial InGaN layers and InGaN quantum wells (QWs), are characterized by a relatively broad emission. In epitaxial layers it has been concluded that the observed emission is due to localized excitons [3]. Other authors have described the spectra as related to interband recombination of carriers localized at potential fluctuations in the alloy [4]. In GaN/InGaN QW structures the observed main PL emission is shifted down in energy with several hundreds of meV compared to the QW bandgap, as confirmed by transmittance and electroreflectance measurements [5]. The observed emission is generally described as due to excitons localized at deep traps. It has been proposed that these traps originate from In-rich regions acting as quantum dots, formed from compositional disorder with a separation into an In-rich alloy phase [6]. 631
Mat. Res. Soc. Symp. Proc. Vol. 482 0 1998 Materials Research Society
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Figure 1. a) Time integrated photoluminescence spectra at 2K for three different GaN/InGaN QW samples, with pulsed excitation at 3.15 eV. The spectral shape becomes narrower and the energy position shifts towards higher energies, due to screening of the piezoelectric field, with increasing doping in the sample. b) Photoluminescence excitation spectra for the same samples. A peak at 3.5 eV due to excitonic absorption in the GaN barrier is seen for all three sa
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