UC Santa Barbara

Tunnel junctions (TJs) offer alternative designs and promise in some cases improved performances for nitride-based light-emitting diode (LEDs) and laser diodes (LDs). To achieve the high p-type doping in the TJ, Mg-doped GaN with controllable doping was desired. For visible wavelength range applications, two TJ techniques, hybrid TJ and all MBE TJ, were investigated. The study targeted a low voltage penalty compared to the conventional ITO contact. For ultraviolet (UV) applications, a hybrid UV TJ was enabled by the development of low resistivity n-AlGaN. In recent years, GaN-based power electronics have attracted great interests due to the attractive physical properties of the III-N material system. Vertical GaN p-n diodes with low leakage and high breakdown field was demonstrated.

With a valved Mg source and indium as surfactant, high quality p-GaN with controllable and reproducible doping levels are grown by NH3 MBE. The high doping levels achievable enables TJ applications. Details of doing Hall measurements on p-GaN were also discussed.

The voltage penalty of the TJ LEDs and LDs, in comparison with standard contact technologies, has been a major concern especially for commercial applications. In this study, methods to achieve low excess voltage were investigated. Using NH3 MBE, hybrid GaN TJs were grown on commercial metalorganic chemical vapor deposition (MOCVD) grown blue LED wafers. Atom probe tomography (APT) and secondary ion mass spectrometry (SIMS) indicate 1 min buffered HF (BHF) clean of the regrowth interface reduced Mg and impurity incorporation into the n++ regrown TJ layers. The wafers were processed and measured in parallel to reference wafers using both university and industry cleanroom and measurement setups. At 20 A·cm-2, TJ LEDs grown with Si δ-doping at the junction interface processed in the university cleanroom had a forward voltage of 3.10 V in comparison to 2.87 V for LEDs processed with a standard ITO contact. Unencapsulated TJ LEDs processed by industrial process without indium tin oxide (ITO) or current blocking layer (CBL) had about 0.3 V excess voltage compared to reference LEDs. The TJ LEDs also had more uniform light emission profile. The low excess voltage and consistent results acquired in both settings suggest that tunnel junction can be scaled for industrial processes. Similar studies were done to for all MBE TJs. Systematic studies examined the effect of InGaN interlayers.

Highly doped n-Al0.6Ga0.4N can be used to form tunnel junctions (TJs) on deep ultraviolet (UVC) LEDs and potentially double the light extraction efficiency (LEE) compared to the use of p-GaN/p-AlGaN. High quality Al0.6Ga0.4N was grown by NH3-assisted molecular beam epitaxy (NH3 MBE) on top of AlN on SiC substrate. The film is crack free under scanning electron microscope (SEM) for the thickness investigated (up to 1 µm). X-ray diffraction reciprocal space map scan was used to determine the Al composition and the result is in close agreement with APT measurement result. By varying the growth parameters including growth rate, and Si cell temperature, n-Al0.6Ga0.4N with a doping level of 4×1019 /cm3 and a resistivity of 3 mΩ·cm was achieved. SIMS measurement shows that a high Si doping level up to 2×1020 /cm3. Using a vanadium-based annealed contact, ohmic contact with a specific resistance of 10-6 Ω·cm2 as determined by circular transmission line measurement (CTLM) was achieved. Finally, the n-AlGaN regrowth was done on MOCVD grown UVC LEDs to form UVC TJ LED. The sample was processed into thin film flip chip (TFFC) configuration. The emission wavelength is around 278 nm and the excess voltage of processed UV LED is around 4.1 V.

In the last part of the study, growth development of low leakage, high reverse breakdown field GaN p-n diodes was shown. Efforts were made to optimize morphology and achieve low impurity un-intentionally doped (UID) GaN in the drift region. Secondary ion mass spectrometry (SIMS) and capacitance-voltage (CV) measurements showed oxygen and carbon concentrations in the low 1016 cm-3 and ND-NA level of 3×1015 cm-3. This was combined with the use of high-quality p-GaN to make GaN p-n diodes with on/off ratio>1010, ideality factor of 1.33, and a minimum specific on resistance of 0.29 mΩ·cm2.