Improved carrier confinement and stimulated recombination rate in GaN-based vertical-cavity surface-emitting lasers with buried p-AlGaN inversion layer
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 AlxGa1-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-Al0.10Ga0.90N 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.
Section snippets
Simulation models and device structure
Optoelectronic properties and carrier dynamics of GaN based VCSEL with buried p-Al0.10Ga0.90N layer are numerically investigated by PICS3D (Photonic Integrated Circuit Simulator in 3D) of Crosslight™. Poisson's equation, current continuity equations, carrier transport equations, complex wave equations, and rate equations of VCSEL devices are solved in cylindrical coordinates. Free carrier model including the wurtzite energy band structure was used to calculate stimulated recombination rate of
The effect of buried p-AlGaN layer on lateral current confinement
To investigate the influence of buried p-AlGaN layer on the performance of VCSEL, two structures are first studied for comparison. Device A is a conventional VCSEL without p-AlGaN layer and Device B is VCSEL with p-AlGaN buried in n-GaN contact layer. For Device B, the thickness of buried p-AlGaN layer (t) is 20 nm and the distance between p-AlGaN layer and MQW (d) is 40 nm. All other parameters remain the same for both Devices A and B, including the cavity length. The light output powers and
Conclusions
In this work, a ring-shape buried p-AlGaN inversion layer inside n-GaN of VCSEL was proposed to laterally confine the electrons for efficient optical gain. It is demonstrated that the p-AlGaN layer introduces a local potential barrier to force the electrons converging into the center aperture before entering MQWs, contributing to effective stimulated recombination. The influences of thickness and position of buried p-AlGaN layer on the performance of VCSEL are also discussed in detail. The best
CRediT authorship contribution statement
Mei Cui: Data curation, Writing - original draft. Yuanbin Gao: Conceptualization, Methodology, Software. Sheng Hang: Conceptualization, Methodology, Software. Xuejiao Qiu: Conceptualization, Methodology, Software. Yonghui Zhang: Supervision. Zi-Hui Zhang: Supervision, Writing - review & editing. Wei Guo: Supervision, Writing - review & editing. Jichun Ye: Supervision, Writing - review & editing.
Declaration of competing interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgments
This work was supported by National Key Research and Development Program of China (2016YFB0400800), National Natural Science Foundation of China (61975051, 61974149) and Program for 100-Talent-Plan of Hebei Province (E2016100010).
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