Optical absorption edge broadening in thick InGaN layers: Random alloy atomic disorder and growth mode induced fluctuations

Abstract

To assess the impact of random alloying on the optical properties of the InGaN alloy, high-quality InxGa1−xN (0 < x < 0.18) epilayers grown on c-plane free-standing GaN substrates are characterized both structurally and optically. The thickness (25–100 nm) was adjusted to keep these layers pseudomorphically strained over the whole range of explored indium content as checked by x-ray diffraction measurements. The evolution of the low temperature optical absorption (OA) edge linewidth as a function of absorption energy, and hence the indium content, is analyzed in the framework of the random alloy model. The latter shows that the OA edge linewidth should not markedly increase above an indium content of 4%, varying from 17 meV to 30 meV for 20% indium. The experimental data initially follow the same trend with, however, a deviation from this model for indium contents exceeding only ∼2%. Complementary room temperature near-field photoluminescence measurements carried out using a scanning near-field optical microscope combined with simultaneous surface morphology mappings reveal spatial disorder due to growth meandering. We conclude that for thick high-quality pseudomorphic InGaN layers, a deviation from pure random alloying occurs due to the interplay between indium incorporation and longer range fluctuations induced by the InGaN step-meandering growth mode.

Probing Alloy Formation Using Different Excitonic Species: The Particular Case of InGaN

“…2 continuously increases with rising indium content at the onset of alloy formation (not shown) as frequently reported in the literature [75][76][77][78]. Naturally, with rising x the average trapping potential becomes deeper as larger indium assemblies are formed that ultimately even govern the k distribution of X A at a temperature of 300 K with localization energies already in excess of k B T at x ≈ 3% [22]. In addition, larger indium assemblies can not only lead to exciton localization but also to confinement, which commonly boosts excitonic decay rates.…”

“…However, with increasing x, additional broadening mechanisms beyond a pure alloying-induced broadening also start to affect ΔE X A . As discussed by Butté et al [22], such structural broadening mechanisms could increasingly affect the measured ΔE X A values with rising x.…”

“…Here, KV X is the cation number in the excitonic volume with K ≈ 0.0438 Å −3 for wurtzite GaN in the indium composition range analyzed in this paper [22,59]. An illustration for the probability φ to find, e.g., 0 ≤ n ≤ 7 indium atoms in V X is given for InGaN and InGaAs (K ≈ 0.0221 Å −3 ) [21] in Figs.…”

“…However, such highly localized bound excitons related to isoelectronic centers strongly differ from the wide range of bound excitons known for shallow impurities in semiconductors and cannot be described by an effective mass approach [14][15][16]. The second class of isoelectronic centers (II) evokes the formation of mixed-crystal alloys like, e.g., SiGe [17], GaAsP [18,19], InGaAs [20], AlGaAs [21], InGaN [22], AlGaN [23,24], CdSSe [25,26], ZnSeTe [26], and MgZnO [27]. In these cases, no new electronic levels are formed in the band gap, but rather in the bands themselves, a process often described as hybridization [28].…”

Modeling of a Waveguide‐Based UV–VIS–IR Spectrometer Based on a Lateral (In,Ga)N Alloy Gradient

“…, an energy dependent b has been discussed. In this paper, we use material parameters put forward recently in a study of (In,Ga)N thin films . Various data points from literature are shown in Figure together with our fit curves in order to establish a realistic and analytical dependence of band gap and broadening for our modeling.…”