Optical constants of epitaxial AlGaN films and their temperature dependence

Abstract

We have studied the dependence of the absorption edge and the refractive index of wurtzite AlxGa1−xN films on temperature and composition using transmission and photothermal deflection spectroscopy. The Al molar fraction of the AlxGa1−xN films grown by plasma induced molecular beam epitaxy was varied through the entire range of composition (0⩽x⩽1). We determined the absorption edges of AlxGa1−xN films and a bowing parameter of 1.3±0.2 eV. The refractive index in the photon energy range between 1 and 5.5 eV and temperatures between 7 and 295 K was deduced from the interference fringes. The static refractive index n(0) changed from 2.29 for GaN to 1.96 for AlN at room temperature. A variation of temperature from 295 to 7 K resulted in a decrease of refractive index (at photon energies close to the band gap) by 0.05±0.01 and in an energy shift of the absorption edge of about 64±5 meV independent of the Al content of the films. Using the Kramers–Kronig dispersion relation and an approximation for the dispersion coefficient for photon energies near the band gap, the refractive index could be described as a function of photon energy, Al content, and temperature.

Si doping of high-Al-mole fraction AlxGa1−xN alloys with rf plasma-induced molecular-beam-epitaxy

“…The bowing parameter used for AlGaN band gap calculation was 1.3 eV. 17 The optical absorption data are in good agreement with the theoretical band gap expectation and the absorption spectra in the inset shows that band edge absorption with heavy Si doping does not have any added significant features. The Burstein-Moss shift observed in a heavily doped semiconductor is not noticeable in the AlGaN:Si and no deep levels or other absorption centers are introduced, which would absorb UV light.…”

AlGaN-Based Bragg Reflectors

“…The effective bandgap of the Al x Ga 1-x N films was defined as the photon energy E 4.8 at which the absorption coefficient equals a value of 10 4.8 cm -1 . The physical justification for this definition is given by photoluminescence and reflection measurements of GaN, showing that E 4.8 lies between the photon energy of the free excitons FX A and FX B in the temperature range between 5 K and 300 K [8]. Moreover, the effective bandgap E 4.8 (GaN) follows the temperature dependence of the free excitons and increases from 3.420 eV to 3.480 eV with decreasing temperature from 300 K to 5 K. The energy offset E 4.8 (GaN)-FX A and E 4.8 (GaN)-FX B is approximately 2 and -5 meV.…”

“…For alloys with x > 0.8, the substrate temperature was changed from 810 to 1000 °C, which resulted in improved structural properties of the films and a FWHM rocking curve of 0.2° [7]. The thickness (≈1 µm) and the growth rate (between 0.5 and 0.6 µm/h) of the films were determined by scanning electron microscopy with an accuracy of ±10 nm [8]. The Al x Ga 1-x N films had rms surface roughnesses between 5 and 15 nm, as obtained by atomic force microscopy.…”

“…The reflectivity between the epitaxial AlGaN film and the sapphire substrate was determined using the results of Malitson, who developed a Sellmeir equation to permit calculation of the index of refraction of sapphire between 200 and 6000 nm. The index of refraction, n, of the Group-III nitrides was obtained from examination of interference fringe minima and maxima in the transmission spectrum [8].…”

Picosecond Acoustics in Single Quantum Wells of Cubic GaN/(Al,Ga)N

“…This means that with the increase of λ, the probe signal becomes more sensitive to the layers where the strain pulse appears earlier and leaves later, which are the ones located further from the sample surface. This observation cannot be explained by the longer penetration depth of probe light with the increase of λ, since the Al 0.8 Ga 0.2 N barriers are transparent for light with λ > 300 nm [16], and only a small fraction of the probe light is absorbed by the 10-nm-thick QW [17]. To understand the strong λ dependence of SðtÞ, we discuss the two main contributions by which the acoustic pulse interacts with the probe light.…”