Optical properties of thin films and quantum wells of In_xGa_1-xN/GaN and their dependence on laser irradiation

Abstract

We have grown bulk In x Ga 1−x N films and In x Ga 1−x N/GaN (x 0.1) multiple quantum well structures on Al 2 O 3 substrates by electron cyclotron resonance assisted molecular beam epitaxy. We have not found any signs of phase separation in bulk films with low indium content, x 0.1. However, we observed inhomogeneous spatial distribution of indium with x varying from 0.1 to 0.2. In multiple quantum well structures with thin In x Ga 1−x N layers non-homogeneity was absent under certain growth conditions. Exposure of In x Ga 1−x N/GaN multiple quantum well structures to pulses of a nitrogen laser with power density I 10 6 W cm −2 led to irreversible changes in both luminescence and reflection spectra. These results, as well as chemical analysis of irradiated surface layers performed using Auger electron spectroscopy, indicate that intensive laser irradiation decreases nitrogen content in the crystal lattice of GaN and gives rise to surface metallization.

Nanoscale Measurement of Laser-Induced Temperature Rise and Field Evaporation Effects in CdTe and GaN

“…With this differential field around the specimen apex, it is not possible to measure a uniform composition across the specimen, a conclusion also reached by Mancini et al 31 Additional support for an athermal process can be found in estimating the temperature increase due to the laser pulse using an adiabatic approximation, ΔT = Iτβ/C p , where ΔT is the temperature change, I is the laser intensity, τ is the laser pulse length, β is the absorption coefficient, and C p is the heat capacity. 62 Taking τ = 10 ps, β = 1.5 × 10 5 cm −1 , 63 C p = 1 J/ cm 3 •K, 64 and I = 1 × 10 6 W/cm 2 for 0.02 pJ (which gave the expected GaN stoichiometry as shown in Figure 8a) gives a ΔT ≈ 2 K. Even with the absorption coefficient enhanced to be fully metal-like due to the high fields, 6 this laser-induced temperature change would be only 20 K, which was shown by the measurements where the base temperature was changed to be insufficient to affect a significant change in the evaporation behavior. At relatively high laser energies (>1 pJ), previous research has shown evaporation displaying an apparent lack of nitrogen on the side of the specimen where the laser is incident.…”

“…1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 22 specimen apex, it is not possible to measure a uniform composition across the specimen; a conclusion also reached by Mancini et al 31 Additional support for an athermal process can be found in estimating the temperature increase due to the laser pulse using an adiabatic approximation, ∆T = Iτβ/C p , where ∆T is the temperature change, I is the laser intensity, τ is the laser pulse length, β is the absorption coefficient, and C p is the heat capacity. 62 T...…”