UC Santa Barbara

While most commercial gallium nitride (GaN) devices are grown in the (0001) Ga-polar orientation, the N-polar (0001 ̅) orientation is advantageous for heterostructures which can benefit from reversed polarization fields including transistors, photodetectors, solar cells, and optoelectronic devices. One particularly attractive application is tunnel junctions, since GaN/(In,Ga)N/GaN tunnel junction devices rely on the piezoelectric polarization in the N-polar orientation. In these devices the tunneling probability is proportional to the indium composition in the InGaN layer, motivating the investigation of the upper limit of indium incorporation in N-polar InGaN layers embedded into GaN. Devices with high indium mole fraction active regions (>0.25) typically suffer from high defect densities and low quantum efficiency, primarily due to 1) the large 10% lattice mismatch between InN and GaN, and 2) the low thermal stability of InN, requiring significantly lower deposition temperatures compared to GaN layers. The work presented in this thesis addresses the above challenges in multiple ways.

First, the deposition of N-polar InGaN layers for tunnel devices will be detailed including the optical, structural, and electrical properties of layers with indium compositions up to 0.46. The electrical performance of N-polar GaN/In0.35Ga0.65N/GaN tunnel diodes will be discussed, including temperature-dependent measurements which confirmed tunneling behavior under reverse bias.

Second, the growth of N-polar InN quantum dots and thin films on vicinal GaN base layers will be presented. For thin layers, quantum dot-like features were spontaneously formed to relieve the strain between the InN and GaN layers. For thicker layers above 10 nm, high electron mobilities up to 706 cm2/Vs were measured using Hall effect measurements indicating high quality layers. The properties of GaN/InN/GaN double heterostructures will be presented as well.

Next, the use of lattice-engineered InGaN pseudo-substrates (PSs) as base layers for InGaN deposition will be discussed as a strategy to mitigate the lattice mismatch when growing layers with high indium content. Relaxed N-polar InGaN films were grown by MOCVD on N-polar InGaN PSs using a novel digital approach which enabled the deposition of thick layers while maintaining smooth surfaces, and InGaN/GaN multiple quantum wells were deposited on both N-polar and Ga-polar InGaN PSs. The use of the InGaN PSs resulted in InGaN layers with about 50% higher In compositions and enhanced optical properties compared to those grown on traditional GaN templates.

Finally, the impurity incorporation behavior of N-polar layers grown at reduced temperatures in the InGaN growth regime will be presented along with the growth and process optimization for the fabrication of reduced temperature p-GaN layers. Although the majority of this thesis focuses on the growth of indium-containing layers, these studies are useful for the fabrication of N-polar devices grown at low temperatures. By minimizing the impurity incorporation and optimizing the p-GaN growth process, p-contact resistances as low as 2.77 mΩcm2 were achieved.