Photoluminescence Characteristics of GaN Layers Grown on SOI Substrates and Relation to Material Properties
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Downloaded from https://www.cambridge.org/core. Southern Cross University, on 08 Dec 2019 at 22:47:19, subject to the Cambridge Core terms of use, available at https://www.cambridge.org/core/terms. https://doi.org/10.1557/PROC-482-307
were 53nm and 81nm respectively. GaN buffer growth temperature was 500'C and layer growth temperature was 940'C. No intentional doping was performed during growth. Different thicknesses of GaN layers were grown under different growth conditions. Table I summarizes layer thickness, V/IlI ratio and growth rate for each of the samples. Table I: GaN on SOI sample thickness and growth parameters. Sample GaN thickness (gim) V/IR ratio Growth rate (jtm/h) 1.2 1.20 3000 A 1.2 1500 B 2.42 750 2.0 C 2.00 Photoluminescence experiments were performed using an Argon ion laser emitting at a wavelength of 334nm as selected by an intra-cavity prism. Measurements were performed at low temperature (7K) and also as a function of temperature in a helium cooled cryostat. The photoluminescence signal was detected by a Hamamatsu GaAs photomultiplier cooled at -20'C. Reflectivity measurements were performed at near normal incidence using a Xenon lamp dispersed through a monochromator. Detection was made through a Silicon photodiode operating in a photovoltafc mode. PHOTOLUMINESCENCE RESULTS PL spectra recorded at 7K from the GaN/SOI samples are reported in Fig. 1 together with the spectrum of a reference sample (silicon doped GaN layer grown on a sapphire substrate), because of similar lineshapes in these samples. 800
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T =7K
-A 600.
........ GaN/sapphlre
Cd CA
400
C 200
3.35
. ,
3.45
3.40
3.50
Energy (eV) Figure 1: 7K PL spectra of GaN/SOI samples, and of a reference sample (Si doped GaN layer grown on sapphire). The PL spectra of GaN/SOI samples exhibit mainly two transitions: one intense near bandgap transition around 3.47eV and another one arising at a lower energy around 3.41eV. The PL spectra were studied as a function of temperature, and the evolution of the PL spectrum in sample A is reported
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Downloaded from https://www.cambridge.org/core. Southern Cross University, on 08 Dec 2019 at 22:47:19, subject to the Cambridge Core terms of use, available at https://www.cambridge.org/core/terms. https://doi.org/10.1557/PROC-482-307
as a function of temperature in Fig. 2. As can be seen in the insert of Fig. 2, the main energy peak transition is decreasing with increasing temperature as is commonly observed for bandgap related transitions in GaN [7]. However, the 3.41eV is shown to blue-shift when the temperature is increased. Such a behavior may be attributed to some defect level thermally emptied as the temperature is increased, thus involving a defect level in the bandgap. Low-temperature PL spectra with 3.41eV recombination band are often observed in GaN [810], but its origin is still the subject of some controversy: either a free to bound transition Doh involving an oxygen donor level [8] or due to the formation of bound excitons on c-axis screw dislocations. Other authors [10]
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