GaN VCSELs designed with higher conductivity
Researchers at Meijo University and Nagoya University in Japan able to design GaN-based vertical-cavity surface-emitting lasers (VCSELs) with good electrical conductivity and can be readily grown.
GaN-based vertical-cavity surface-emitting lasers (VCSELs) are used in retinal scanning displays, adaptive headlights, and high-speed visible-light communication systems.
GaN-based vertical-cavity surface-emitting lasers (VCSELs) face issues such as poor conductivity. It is difficult to make these devices in mass-volumes using existing approaches to improve the conductivity . Research headed by Takeuchi and colleagues has now demonstrated a design that provides good conduction and is readily grown.
Bragg reflectors inside GaN VCSELs provide the necessary reflectivity for an effective cavity to emitt laser light . Bragg reflectors are alternating layers of materials with different refractive index to ensure high reflectivity. Intracavity contacts can increase conductivity of GaN VCSELs but also increase the cavity size leading to poor optical confinement, complex fabrication processes, high threshold current densities and a low output-versus-input power efficiency .
The low conductivity in DBR structures is due to polarization charges between the layers of different materials' AlInN and GaN. To overcome the effects of polarization charges, Takeuchi and colleagues 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.
Meijo and Nagoya University researchers have also found a method to expedite the AlInN growth rate to over 0.5 micro m/h. The result is a 1.5 (Lambda)-cavity GaN-based VCSEL with an n-type conducting AlInN/GaN distributed Bragg reflector that has a peak reflectivity of over 99.9% , threshold current of 2.6 mA, corresponding to a threshold current density of 5.2 kA/cm2, and an operating voltage was 4.7V, shared in the release.