Modulated silicon doping improves GaN-based vertical-cavity surface-emitting lasers
The silicon doping is used to form the distributed Bragg reflector mirrors, improving conductivity in the VCSEL.
Due to the nature of nitride compounds, blue-emitting gallium nitride (GaN)-based vertical-cavity surface-emitting lasers (VCSELs) have been tough to make; the structures designed for commercializing these devices have poor conducting properties, and existing approaches to improve the conductivity introduce fabrication complexities while performance is inhibited.
Now, researchers at Meijo University and Nagoya University in Japan demonstrated a design of GaN-based vertical-cavity surface-emitting lasers (VCSELs) that provides good electrical conductivity and is readily grown.1
VCSELs generally use distributed Bragg-reflector (DBR) mirrors to form a laser cavity. Intracavity contacts can help improve the poor conductivity of GaN VCSELs, but these increase the cavity size, leading to poor optical confinement, complex fabrication processes, high threshold current densities, and a low the slope efficiency.
The low conductivity in DBR structures is the result of polarization charges between its layers of aluminum indium nitride (AlInN) and GaN. To overcome the effects of polarization charges, the researchers used silicon-doped nitrides and introduced modulation doping into the layers of the structure. The increased silicon dopant concentrations at the interfaces help to neutralize the polarization effects.
The researchers have also devised a method to expedite the AlInN growth rate to greater than 0.5 μm/h. The result is a 1.5λ-cavity GaN-based VCSEL with an n-type conducting AlInN/GaN DBR that has a peak reflectivity of more than 99.9%, a threshold current of 2.6 mA corresponding to a threshold current density of 5.2 kA/cm2, and an operating voltage of 4.7 V.
"GaN-based vertical-cavity surface-emitting lasers (VCSELs) are expected to be adopted in various applications, such as retinal scanning displays, adaptive headlights, and high-speed visible-light communication systems," says researcher Tetsuya Takeuchi and colleagues.
1. Kazuki Ikeyama et al., Applied Physics Express (2016); http://iopscience.iop.org/1882-0786/9/10/102101