Abstract
Obtaining high quality, wurtzite InN films with p-type conductivity is a challenge, and there is limited information about the photoluminescence (PL) characteristics of such films. In this study, we present a comprehensive PL study and discuss in detail the recombination processes in Mg-doped InN films with varying Mg concentrations. We find that at low Mg-doping of 1×1018 cm-3, which yields p-type conductivity, the PL in InN is spatially inhomogeneous. The latter is suggested to be associated with the presence of n-type pockets, displaying photoluminescence at 0.73 eV involving electrons at the Fermi edge above the conduction band edge. Increasing the Mg concentration to 2.9×1019 cm-3 in p-type InN yields strong and spatially uniform photoluminescence at 0.62 eV and 0.68 eV visible all the way to room temperature, indicating homogeneous p-type conductivity. An acceptor binding energy of 64 meV is determined for the Mg acceptor. Further increase of the Mg concentration to 1.8×1020 cm-3 leads to switching conductivity back to n-type. The PL spectra in this highly doped sample reveal only the emission related to the Mg acceptor (at 0.61 eV). In the low-energy tail of the emission, the multiple peaks observed at 0.54 – 0.58 eV are suggested to originate from recombination of carriers localized at stacking faults.
Highlights
InN has attracted a considerable research interest due to the prospects of using it in generation optoelectronics,1–3 and high-frequency electronic devices.4 In 2002, the low temperature (2 K) bandgap of wurtzite InN with a relatively low free electron concentration of 2×1018 cm-3 was revised to 0.7 eV,5,6 being much lower than the previously reported value close to 2 eV
The highly Mg doped sample (n-type) with a Mg concentration of 1.8×1020 cm-3 shows emission centered at approximately 0.61 eV
Mg doped InN films grown by Plasma Assisted Molecular Beam Epitaxy (PA-MBE) have been studied by means of photoluminescence spectroscopy
Summary
InN has attracted a considerable research interest due to the prospects of using it in generation optoelectronics, and high-frequency electronic devices. In 2002, the low temperature (2 K) bandgap of wurtzite InN with a relatively low free electron concentration of 2×1018 cm-3 was revised to 0.7 eV, being much lower than the previously reported value close to 2 eV. Obtaining p-type conductivity in InN is difficult due to the high intrinsic residual electron concentration in this material. Even undoped InN samples tend to have a high concentration of donors incorporated, lifting the Fermi level into the conduction band, and making the material degenerate. In partially compensated n-type InN films doped with Mg, PL peaks at 0.67 eV and 0.60 eV, attributed to band-toband and band-to-acceptor transitions, respectively, were reported.. Mg-doped InN nanowires, which are believed to not suffer from strain and structural defects, have been shown to emit at 0.67 eV and 0.61 eV (at 7 K).32 These emissions have been attributed to band-toband transitions and donor-acceptor pair transitions, respectively. We have reported a comprehensive study on free charge carrier and structural properties of nominally undoped and a series of Mg-doped (both n- and p-type) InN films.. The PL properties of highly Mg-doped InN are investigated and discussed in view of the presence of structural defects
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