Magnetic Resonance Studies of Recombination Processes in GaN-Based Light Emitting Diodes
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Mat. Res. Soc. Symp. Proc. Vol. 395 01996 Materials Research Society
because it forms deeper levels in GaN than Mg, thereby shifting the luminescence to somewhat lower energies and minimizing the In mole fraction (and the accompanying strain) in the active layer. Two variations of the device were studied, differing only in the InN mole fraction in the active InGaN layer comprised either of Ino.0 6Gao.94N (referred to as blue diodes) or In 0.23Ga 0.77N (referred to as green diodes). The c-axis of the thin-film LED was always perpendicular to the magnetic field and light was extracted using a quartz light pipe and detected by a UV-enhanced Si photodiode. Electroluminescence (EL) spectra were measured using a 0.22 m double grating spectrometer. The magnetic resonance apparatus is based on a Varian E9 ESR X-band (9.25 GHz.) spectrometer and a p-i-n modulator for microwave power modulation. Samples were cooled to approximately 2 K using a liquid helium flow cryostat and biased with a constant current. ELDMR signals were detected as changes in the EL in-phase with the microwave power modulation and EDMR signals were obtained by detecting the AC voltage across the sample in-phase with magnetic field modulation. LOW TEMPERATURE ELECTROLUMINESCENCE The intense room temperature blue and green EL from these devices has , T=2 K attracted considerable interest recently. S Blue-' Our magnetic resonance measurements require cryogenic temperatures and rela" .l4 " "'Greennn Ii tively low biases; therefore, an examination * 2.7 2.9 3.1 of the low temperature, low current EL is n (eV) Energy important in interpreting our results. EL spectra taken at T-2 K for typical blue and oe~ L s T Blue Diode Green Diode green diodes are shown in Fig. 1. Our low temperature EL spectra both show two distinct peaks. To analyze the two peak lecroluinecene Ilecr Figue 1The structures further we have fit each spectrum to a pair of gaussian lines. We believe the peaks at -2.7 eV (blue LEDs) and 2.0 2.5 3.0 3.5 -2.4 eV (green LEDs) are due to a deep Energy (eV) donor to Zn-acceptor recombination.` In Figure 1 The electroluminescence spectra both diodes there is a second line at -2.9 of blue and green LEDs. The dashed lines eV that, other than differences in relative are the results of fitting each spectrum to intensity, is essentially the same for the two two gaussians. In the inset the fits to the diodes, as illustrated in the inset to Fig.l. feature at -2.9 eV are compared. We also note that the relative intensity of this line increases with increasing bias for both diodes. While it could be argued that this band in the green diodes is due a near bandedge emission (energy gap of lno.23Gao.7N = 3.00 eV), this would require a separate explanation for the existence such a band at the same energy in the blue diode and the lack of a similar band, presumably shifted down in energy, in the green diode. Since this EL line is independent of In fraction, we suggest that this line is due to a transition outside the InGaN layer, most probably in one of the AIGa
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