Significant progress has been achieved in the past two decades in III-nitride light emitters. It has captured the attention of the scientific community due to applications in solid-state lighting, sterilization, gas sensing, and polymer curing. The success in p-type GaN revolutionized the lighting industry. However, hole injection still remains as a bottleneck in the nitride semiconductor LEDs and LDs. The low hole density poses significantly more challenges for emission wavelength in the deeper ultraviolet regime. Recently, tunnel junction based GaN LEDs were explored to reduce the contact resistance to p-type layer and enhance the current spreading. The first topic is the theoretical design and experimental demonstration of tunnel junctions using polarization engineering. A theoretically predicted design is put to test using blue LED heterostructures grown by molecular beam epitaxy. The results show that polarization engineering enhances hole injection by tunneling, and identifies the operation regime in which is the enhancement can be leveraged. To enable efficient electrically injected deep-UV photonic devices, a detailed study of Mg doping in AlGaInN is performed. A significant enhancement of Mg incorporation efficiency is observed for InGaN and AlGaN alloys compared to GaN. Optimal Mg doping concentration in InGaN results in a very high hole concentration of 1019 cm-3 at room temperature, significantly higher than what is achieved in p-type GaN. Incorporating such p-InGaN layer in the p-type contacts of UV-C LEDs significantly reduces the p-contact resistance, enabling continuous wave operation compared to pulsed operation p-GaN contact device at 243nm. Similar Mg doping optimization in graded p-AlGaN layers also exhibit ~1000 times improvement over constant p-AlGaN to a resistivity of 104 Ω_cm. Homoepitaxy of AlN on newly available bulk AlN crystals by MBE is a critical enabler for quantum heterostructures for electronic and photonic devices. To enable homoepitaxy of AlN, a new surface preparation technique using plasma-assisted molecular beam epitaxy is presented. A growth window to achieve smooth parallel atomic steps on single crystal bulk AlN substrate is identified. High Al-content AlGaN layers are then grown pseudomorphically on the bulk AlN without relaxation. AlGaN and AlN epitaxial layers with very low impurity concentrations such as Si, O, H, and C are measured by secondary ion mass spectrometry. These findings are the critical enablers of MBE-grown UV lasers and high performance power electronic devices on low dislocation density bulk AlN substrates.