Nitride semiconductor-based light emitters (LEDs and laser diodes) are influenced by magnesium (Mg) acceptors, limiting conductivity and operational temperature due to high ionization energy. Mg also causes strong optical absorption, reducing laser diode efficiency. Dielectric polarization engineering using wurtzite nitride lattice symmetry (polarization doping) has been proposed to manipulate electrical properties. Our study demonstrates low threshold current density (2.5 kA/cm2), low internal losses (around 5 cm-1), and good thermal stability in fabricated laser diodes, enabling operation at cryogenic temperatures. Notably, polarization-doped p-layers yield lower voltage than Mg-doped ones. Understanding hole injection from polarization-doped layers remains a challenge.
Although the polarization doping is a break-through technology for deep UV emitters, it is also very useful for enhancing p-type conductivity and lower the resistance in the classical InGaN laser diodes operating in the visible part of the spectrum. We were able, additionally, to show that these devices can operate in broader temperature range, especially at low (cryogenic) temperatures. We also show the drastic reduction in hydrogen content in top layers of the laser diodes, which may influence their reliability.
In this work, we study the optoelectrical properties of nitride LED structures employing polarization doping for the p-type layers. We compare standard Mg-doped, partially doped, and undoped AlGaN p-type layers. The electrical properties of these samples are similar, proving the successful use of polarization doping. The optical measurements suggest that doping of the electron blocking layer is required for preserving good light emission efficiency. We also studied our samples at lowered temperatures and observed no freeze-out region down to 77K. For top metal contact, sub contact doping is indispensable because the intrinsic top layer causes the Schottky barrier.
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