Bulk properties of InN films determined by experiments and theory
Introduction
The optical and electronic properties of InN have drawn intensive research, mostly because of their potential applications for optoelectronic and electronic devices [1]. Fundamental properties of the material, such as its band-gap energy, are still not well established [2]. To date, there is an increasing interest in developing deposition techniques to yield high quality InN films for the fabrication of III-nitride based high-efficiency multi-junction solar cells, and high-frequency and high-power devices. However, there are major roadblocks preventing the full realization of these envisioned devices. In addition to the high concentration of extended defects, typical for heteroepitaxial deposition, challenges still remain due to the low In dissociation temperature, high incorporation of oxygen, and lack of conductivity type control [3]. Therefore, alternative deposition techniques to reduce these technical difficulties are highly desirable.
Theoretical studies of InN are usually based on density functional theory (DFT) [4], [5], [6], but there is uncertainty regarding the band gap values. It is well known that local density approximations (LDA) or generalized gradient approximations (GGA) severely underestimates the fundamental band gaps of semiconductors. For example, the reported band gap energies of InN with DFT/LDA or DFT/GGA are between 0.0 and −0.3 eV [4], [5]. Quasi-particle corrections to the LDA change the bandgap to values between 0.6 and 0.8 eV [5], [6].
We used the projector augmented wave (PAW) method within the DFT and in conjunction with Green׳s function and the screened Coulomb interaction approximation (GW0) to obtain accurate band gap energies. In addition, the Bethe–Salpeter equation (BSE) is employed to analyze the two-particle exciton effect of the optical spectra. Combined investigations of experimental and theoretical studies reveal that the fundamental band-gap energy is ~0.8 eV. However, since the density of states (DOS) at the conduction band edge is small, the absorption is small in the energy region 0.8−1.2 eV and a clear onset to absorption occurs at ~1.2 eV. The BSE calculation reveals a strong excitonic peak just below the absorption edge of InN. Our theoretical results are compared with photoluminescence (PL) and absorption spectra for InN deposited on GaN templates by a modified ion beam assisted deposition (IBAD) technique.
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Experimental details
Thin InN films were deposited at room temperature, 250 °C, or 395 °C by the modified ion beam assisted deposition (IBAD) on GaN/sapphire template substrates. The IBAD system consists of an electron-beam evaporator and a Kaufmann-type nitrogen ion gun in a vacuum chamber evacuated by a cryopump at base pressures below 10−4 Pa. The nitrogen ion energy of 100 eV and the evaporated 99.99% In metal reach simultaneously the substrate with an arrival rate ratio ARR(N/In) of 1.0, 1.5, 2.0, 2.5, or 3.0.
Computational details
All the calculations are performed using DFT by employing the PAW method as implemented in the Vienna ab initio simulation package (VASP) [7]. The atomic structure of InN is modeled by a fully relaxed wurtzite structure of 4 atoms (: space group number 186). The In-4d electrons are treated as valence electrons. The structure is relaxed using the local density approximation (LDA) with a 10×10×8 k-mesh, until forces on the atoms were less than 12 meV Å−1. The energy cut-off was taken to
Photoluminescence
Luminescence measurements were carried out at 4.2 K using the 532 nm line of a doubled frequency Nd:Yag laser. The sample emission was dispersed and analyzed by a double spectrometer fitted with 1 µm blazed gratings and a LN2-cooled Ge detector and/or with an InSb detector. Intense and sharp single emission bands were observed from all measured samples. Fig. 1 shows the PL spectra of three InN films, with thicknesses of 50 and 80 nm, deposited at 250 and 395 °C. One can see that the full width at
Summary
In conclusion, we present optical properties of InN within the framework of experimental measurements and theoretical calculations. The modified ion beam assisted deposition technique was used to grow high quality thin InN film. The DOS, absorption coefficients, and the real and imaginary parts of the dielectric functions of InN are studied theoretically by employing the projector augmented wave (PAW) method within the scGW0 approximation. Combined investigations of experimental and theoretical
Acknowledgments
The authors acknowledge the financial support of the Brazilian agencies FAPESB/PRONEX and CNPq, the Swedish Research Council (VR), and Norwegian Research Council (NFR). We acknowledge access to HPC centers NSC and USIT for high-performance computing resources via SNIC/SNAC, Matter network and NOTUR. The experimental work was part of an ONR/IF-USP NICOP program initiated by the Brazilian PI Prof. José Roberto Leite.
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Current affiliation: Environmental Remediation Materials Unit, National Institute for Materials Science, Ibaraki 305-0047, Japan.