Improved carrier confinement and stimulated recombination rate in GaN-based vertical-cavity surface-emitting lasers with buried p-AlGaN inversion layer

Highlights

• A buried ring-shape p-AlGaN layer is introduced in VCSEL to , confine electrons.

• By numerical calculation, total current density in the aperture of VCSEL is significantly increased.

• The output power increases by 57.0% for VCSEL with buried p-AlGaN compared to conventional VCSEL.

• The effects of thickness and position of buried p-AlGaN on the performance of VCSEL were theoretically investigated.

Abstract

A GaN-based vertical cavity surface emitting laser (VCSEL) featuring a buried ring-shape p-Al_0.10Ga_0.90N inside n-GaN contact layer for lateral electron confinement is proposed. The p-AlGaN layer inserted in n-GaN forms an n-p-n structure, acting as a potential barrier to prevent vertical electron migration outside the aperture of the VCSEL, where optical gain is accumulated. By adjusting the thickness and position of the p-AlGaN layer, electron concentration and stimulated recombination rate in the aperture of the VCSEL increased significantly. Consequently, the output power of VCSEL with buried p-AlGaN layer increases by 57% compared to the conventional VCSEL at an injection current of 10 mA. The detailed mechanism responsible for this enhancement is further explored. This work suggests that the introduction of the buried p-AlGaN layer in VCSEL can provide new line of thought in achieving effective current confinement in the development of high-efficient, low-threshold solid-state lasers.

Introduction

III nitride-based optoelectronic devices especially light emitting diodes and laser diodes have been intensively investigated over the past decades due to their unique properties of compact size, tunable wavelength and environment friendly property. Among these devices, vertical-cavity surface-emitting-lasers (VCSELs) manifest huge advantages over conventional edge-emitting laser diodes including single-mode lasing, circular shape far-field emission profile and small beam divergence emitted from the surface of a monolithic structure [1,2]. Moreover, the small cavity volume in VCSEL allows high modulation frequency which can be widely used in plastic optic fiber due to the ease of beam coupling into the waveguide [3,4]. As a result, VCSELs have great potential in applications such as optical storage, bio-sensing, head-up-displays, near-eye displays and so on [5]. Even though electrically pumped GaN-based VCSELs have already been successfully fabricated, they have not been commercialized in a large scale so far. One of the reasons is that, due to the large difference in lattice constants and thermal expansion coefficients between Al_xGa_1-xN, GaN and foreign substrate like sapphire and silicon, threading dislocations and even cracks are generated inside epitaxial thin films, degrading the performance of VCSEL [6]. However, this problem can be mitigated via thin film growth optimization [[7], [8], [9]]. A bigger challenge is the lack of lateral current confinement which reduces carrier flow into the active region and thus low stimulated emission rate [10]. From this point of view, improvement of lateral current confinement in nitride-based VCSEL is critically important. To solve this problem, various methods have been attempted. For instance, Kuramoto and Hang et al. reported that a thin layer of SiO₂ with a small dielectric constant can effectively block vertical current injection to undesirable regions [11,12]. Lai et al. proposed a model with AlN buried layer to achieve additional lateral current confinement while indium-tin-oxide (ITO) transparent conductive oxide is located above the aperture to enable good contact property [13]. Additionally, boron ion implantation [14] and silicon diffusion [15] in the peripheral of the aperture were also used for lateral current confinement. Nevertheless, it is noted that above mentioned techniques are all focused on current confinement in the p-type region. VCSEL devices still suffer from poor lateral current confinement in n-type region as electrons exhibit higher mobility than holes, and thus being the dominant factor limiting carrier transportation and light amplification. So far, very few studies have been conducted on carrier flow modulation in n-type region, which deserves in-depth explorations.

In this work, we design a GaN-based VCSEL with a buried ring-shape p-Al_0.10Ga_0.90N electron confinement layer inside n-GaN contact layer, providing a local potential barrier that reduces the migration of electrons to undesirable areas of the device, leading to electron confinement into the aperture for light amplification. We discovered that, the thickness and position of p-AlGaN plays a significant role, which can remarkably affect the device performance.