P-type doping in III-Nitrides has long presented a challenge in the development of wide bandgap optoelectronic devices. To date, magnesium is the only commercially viable acceptor in III-Nitrides. Beryllium has been considered a potential alternative to magnesium, and initial theoretical calculations as well as photoluminescence studies suggested that it is shallower than magnesium in GaN. However, to date, there have been no reliable or repeatable examples of p-type GaN:Be in literature. Here, we present a systematic study of MOCVD-grown GaN:Be with varied doping conditions. All samples show prominent UV and yellow luminescence, characteristic of beryllium acceptor in GaN.
We report here our latest results of achieving high quantum efficiency (QE) N-polar GaN photocathodes through engineering of bulk and interfacial impurity concentrations. The photocathode structures studied consist of an N-polar p-GaN absorbing layer with 10 nm u-GaN cap layer. An increase in QE from 0.27% to 10.1% is observed for photocathodes grown without a regrowth interface between p-GaN and u-GaN cap layers compared to an identical structure with a regrown interface. SIMS is used to identify impurities at the regrowth interface, which cause lower QE due to scattering and modification of the electric field. The QE is further increased to 26.6% through a 2x increase in hole concentration when the structure is grown on a high hillock density template. This work demonstrates a combined approach that has the promise of advancing toward and surpassing performance of current state-of-the-art photocathodes in the UV range.
Polarization engineering of III-Nitride materials in the nitrogen polarity has allowed for improved 2DEG confinement in GaN/AlGaN HEMTs and reduction in efficiency droop in MQW LEDs. Achieving p-type material through Mg doping continues to be a challenge in the nitrogen polarity compared to the gallium polarity with lower stability for substitutional replacement of Mg for Ga under nitrogen-rich conditions as calculated by density functional theory [1] and shown experimentally [2]. High conductivity p-type material in the nitrogen polar orientation is needed for improved efficiency of a variety of III-nitride emitters, detectors and high-hole-mobility transistors (HHMTs).
In this work, GaN:Mg was overgrown on N-polar GaN templates on on-axis sapphire substrates by MOCVD. The underlying GaN template growths were optimized to achieve control over hillock density. Low and high hillock density templates were then used for overgrowth of GaN:Mg to study dependency of Mg incorporation efficiency on hillock density. Similar N-polar uGaN/GaN:Mg were epitaxially grown on high hillock density and low hillock density N-polar GaN/sapphire to form Cs-free photocathode detector devices. Photocathodes with high hillock density produced a quantum efficiency of 24.4% whereas low hillock density device produced an efficiency of only 0.8%, larger than 30X difference. To understand the origin of this difference, material quality and composition of these devices were studied. Secondary ion mass spectroscopy (SIMS) showed Mg concentration of 4x10^22 atoms/cm3 and 5x10^19 atoms/cm3 for high and low hillock density devices respectively. Atom probe tomography was performed to study incorporation and distribution of Mg within and at sidewalls of hillock structures.
[1] Q. Sun et al., Phys. Rev. B 73, 155337 (2006).
[2] J. Marini et al., J. Elec. Mat. 26, 5820 (2017).
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