Absorption and Raman scattering processes in InN films and dots
Introduction
We begin by reviewing the values of the phonon frequencies that are reported in the literature for indium nitride (InN), films. They are correlated to the strain state of InN and these values are, within the experimental uncertainty, consistent with each other. This we show by metonymy: due to the lack of information concerning the experimental values of the deformation potentials in InN, we use the value of the high energy E2 mode to calibrate the strain state in the epilayer. Next, by combining Raman spectroscopy measurements and X-ray investigations of large size InN quantum dots deposited by Metal-organic vapour phase epitaxy (MOVPE) on sapphire, we show these frequencies to experience a blue shift with increasing compression. The usual shape of the dots corresponds to truncated pyramids with a hexagonal base as revealed by atomic force microscopy (AFM) measurements. The aspect ratio of these dots is about 0.2. The E2 phonon frequency shift detected in the micro-Raman spectra, recorded from single dots of sizes ranging from 480 nm down to 30 nm in height, allowed an approximate evaluation of the residual strain field. Careful analysis of these data makes it clear that the islands are weakly strained, likely due to the formation of dislocations at the InN/GaN interface. Nevertheless, the reduction in the size of InN islands leads to an increasing strain field. Finally, a slight strain increase was evident when capping the dots with a thin GaN top surface layer, thus reinforcing the major role played by plastic strain relaxation. Finally, we report the observation of a broad absorption in the 1.25 eV region that is typical of thin InN films. Such a feature we attribute to light absorption at the energy of the fundamental direct band gap of InN, while we attribute the low energy 650–800 meV photoluminescence (PL) to an extrinsic recombination process analogous to the processes that produce the blue band in AlN and the yellow band in GaN. Substantial broadening of the PL peak is attributed to both the residual doping, and strain.
Section snippets
Phonon frequencies in InN films
As shown in Table 1, the values of the phonon frequencies in InN are currently under debate [1], [2], [3], [4], [5], [6], [7], [8], [9]. There is a large scatter of available values, which is not correlated to the growth technique used to grow InN. This scatter is dominantly related to a combination of residual stress effects and residual doping. This is not surprising: previous investigations of the phonon frequencies in GaN have shown for instance that the influence of the value of the V/III
Phonon frequencies in quantum dots grown by MOVPE
A 1 μm GaN buffer layer was deposited onto a sapphire substrate, followed by the growth of InN islands using TMIn and NH3 as precursors [21]. Only one plane of InN islands were grown. The fabrication parameters investigated during this study were: the growth temperature (Tg) in the range 400–700°C, the V/III molar ratio in the range of 30000–7000, and the growth time from 3600 down to 10 s. For large InN dots, material quality assessment was achieved by X-ray diffraction, with a full width at
Optical properties of thin InN films
Fig. 8 illustrates the 300 K absorption spectra of InN films taken with a Bruker Fourier Transform Spectrometer. The bottom spectrum corresponds to a very thin film (t=300 nm for sample 2), while the spectrum in the middle of the figure corresponds to a thicker one (t=600 nm for sample 1). Sample 1 clearly indicates a very weak onset of absorption starting near 800 meV and a clear absorption feature at 1.25 eV. For the sake of completeness we also report the reflectivity spectrum of sample 1 (top
Conclusions
By using an ad hoc representation where the E2 (high) phonon frequency calibrates the strain experienced by the epilayers we have shown that phonon energies of InN films are correlated. A biaxial strain model could currently account for this correlation. We determined by AFM, the morphology of single InN dots grown on GaN, and we correlated it to the estimated magnitude of the in-plane strain. Dots exhibit a weak inhomogeneous compressive strain that increases slightly for smaller islands.
References (35)
- et al.
Solid State Communications
(1999) - et al.
J. Appl. Phys.
(1975) - et al.
Phys. Stat. Solidi (b)
(2001)et al.Appl. Phys. Lett.
(2002) - et al.
Appl. Phys. Lett.
(1999)et al.Phys. Stat. Sol. (b).
(1999) - et al.
Solid State Communi.
(2000) - et al.
Appl. Phys. Lett.
(1996) - et al.
Appl. Phys. Lett.
(1998) - et al.
Solid State Communi.
(2000) - et al.
H. Riechert Appl. Phys. Lett.
(2000) - et al.
Solid State Communi.
(1996)
Mat. Res. Society Proc.
J. Electron. Mater.
J. Appl. Phys.
Phys. Rev. B
Inst. Phys. Conf. Series.
Mat. Res. Society Proc.
J. Electron. Mater.
Appl. Phys.
J. Appl. Phys.
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