Optical Properties of GaN Thin Films on Sapphire Substrates Characterized by Variable-Angle Spectroscopic Ellipsometry

Abstract

The optical properties of GaN thin films grown on sapphire substrates by low-pressure metal organic chemical vapor deposition were investigated by variable-angle spectroscopic ellipsometry. Accurate refractive indices (n, k) below, through, and above the fundamental bandgap of GaN in the spectra range of 1.5 to 3.8 eV (330 to 830 nm) were determined by using a parametric semiconductor model that took the surface roughness effect into account. The fundamental bandgap energy (3.41 eV) of GaN determined by the model was exactly the same as that obtained from an optical transmission experiment using the same sample. The high-frequency dielectric constant ε ∞ (=5.2) of GaN for (E⊥c) obtained from the ellipsometric measurement was also in excellent agreement with the experimental value of 5.2±0.1 (E⊥c) from the IR reflectivity measurements.

Nanofabrication of tunable nanowire lasers via electron and ion-beam based techniques.

“…In addition, mode competition [19] and the spatial hole-burning effect [20] selectively support certain modes and shape the uneven-spaced lasing spectrum. We note that the spectral spacing shown in Figure 9 is much smaller than the predicted FSR of ~1.53 nm for a single transverse-mode operation (using n g =5.6 [21], D=370 nm and L=8 µm for the calculation), which further verifies the multiple transverse-mode operation of the laser. The coupled cavity formed by these two nanowires is shown in Fig.…”

“…Energy transfer through evanescent coupling has been demonstrated 20 and modeled 21 for closely located nanowires. To verify the coupling effect in our nanowire-pairs, we shortened the overlapping length of two nanowires, and then selectively pumped one nanowire and observed the emission from the other one.…”

“…n=2.6 [21]). From this equation, the resonant wavelengths of longitude modes for each subcavity can be calculated.…”

“…In the nanowire pair cavity shown in Figure 8 ( 24 , the resonance condition will be simplified to the simultaneous oscillation of sub-cavity 12 and 14. Since the diameters of the GaN nanowires are much larger than their emission wavelengths, in each sub-cavity many transverse modes exist with different value of group refractive index (n g ) ranging from 5.0-6.0 [21]. For each of these transverse modes, the longitudinal modes can be expressed using…”

Polarization control in GaN nanowire lasers

“…The nanowire passive eigenmodes are determined using a fully vectorial commercial Eigenmode Solver from Lumerical Inc [20]. In the simulation, the operation wavelength is set to 367 nm, the real and imaginary parts of the gold's refractive index are set equal to 1.70 and 1.88 respectively [21], whereas the refractive index of GaN is set to 2.67 [22]. Figure 5 shows two orthogonally polarized HE 11 modes supported by this nanowire/gold-substrate geometry.…”