Experimental and theoretical study of substrate modes in (Al,In)GaN laser diodes

Abstract

In semiconductor laser diodes, high refractive index epitaxial layers can act as parasitic waveguides and cause severe losses to the mode propagating in the laser waveguide. For (Al,In)GaN laser diodes the parasitic modes are typically caused by the SiC or GaN substrate or buffer layers. Substrate modes have an impact on near-field and far-field of the waveguide mode, on laser threshold, and on gain spectra. We present scanning near-field measurements of the substrate mode and demonstrate that the period of the standing wave in the GaN substrate varies over a wide range and critically depends on the effective refractive index of the waveguide mode.1 Introduction For (Al,In)GaN laser diodes (LD) on bulk GaN substrates the refractive index of the substrate is higher than that of the effective refractive index of the waveguide. Thus any part of the waveguide mode tunneling as evanescent wave through the n-side AlGaN cladding can propagate in the substrate. Reflected under a small angle of incidence from the bottom surface of the substrate it will form a standing wave. This so-called substrate mode causes a periodic modulation of the total loss of the laser mode. We observe this as periodic modulation in the gain spectra measured by the Hakki-Paoli method and by a dependency of the threshold current on the n-cladding layer thickness [1]. The far-field shows side-lobes rising from the substrate mode travelling at an angle to the waveguide which is determined by the effective refractive index of the mode and the refractive index of the substrate. We also observe the standing wave in the substrate by a measurement of the near-field intensity on the cleaved surface of the substrate just below the laser ridge waveguide. This is the most direct observation of the substrate mode.For a quantitative analysis we establish a two-dimensional finite element electromagnetic solver, to simulate periodic variations in the cavity loss that reproduce measurements of the modal gain in terms of absolute value and oscillations amplitude. Details and results of these simulations can be found in [1]. The two-dimensional simulation model shows a periodic interference pattern in the substrate [1], very similar to what can be found in measurements (see Fig. 1). An oscillation period of about 1 µm can be extracted from this pattern. These simulations can be used to optimize the vertical structure (within the constraints given by growth conditions) in order to minimize the impact of the substrate modes [1].Here we concentrate on near-field measurements of the substrate mode intensity distribution. We demonstrate that the spatial period of the substrate mode varies over a wide range and critically depends on the effective refractive index of the waveguide.

Beam quality of blue InGaN laser for projection

“…Parasitic laser modes guided within the substrate are well known [1]. However, in this paper we will show that waveguide structures that may be sufficient for 405 nm operation give rise to leaky modes at 440 nm.…”

Spectral and time resolved scanning near‐field microscopy of broad area 405 nm InGaN laser diode dynamics

“…In this paper we focus on lateral line-scans along the active layer, that means parallel to the epitaxial layers at the vertical center of the laser mode. Nevertheless the waveguide confinement in vertical transverse direction is also a critical point and was investigated elsewhere [8,9], but has no immediate impact on the features described here. For the detection of the signal we are using different devices.…”

Al-free cladding-layer blue laser diodes with a low aspect ratio in far-field beam pattern

“…Normally, the FFP aspect ratio of GaN-based LDs is larger than 3 [2,7] , which is nearly twice of that for conventional AlGaInP-based red LDs [8] . The reported θ ⊥ for GaN-based LDs was usually larger than 20°[ 4,5,7,9] , while a smaller radiation angle of 16.5° was reported for conventional AlGaInP-based red LDs [8] . In order to reduce the aspect ratio of GaN-based LDs, it is necessary to decrease the radiation angle of the laser beam in the direction perpendicular to the junction plane (θ ⊥ ) and increase the beam divergence angle parallel to the junction plane (θ // ).…”