Morphological improvements in nanostructured nitride photodiodes
Detection technologies for the UV and visible range are well established, but many are limited by either performance or size, weight, and power. The ubiquitous silicon-based photodiodes are cheap and reliable but suffer in UV applications, while photomultiplier tubes offer high performance but can be unreliable and power hungry. To solve these issues we developed a novel morphology and device architecture using gallium nitride as the functional material. Here, we outline our efforts to develop high-efficiency nitride optoelectronic devices, the properties of which are particularly dependent on morphology.
Previously1 our team demonstrated processes for creating vertical gallium nitride (GaN) pillars based on low-cost GaN-on-silicon epitaxy (deposition of crystalline layers upon substrates). It is possible to accurately control the morphologies of these structures by varying the parameters for inductively coupled plasma (ICP) etching (see Figure 2). The resultant structures have unique optical characteristics and can be used as nanostructured optical cavities. Furthermore, when we use these n-type GaN pillars as a template for epitaxial growth of p-type GaN shells, we can achieve significantly improved optical and structural properties.2 We can further control characteristics of the novel core-shell structure using different growth methodologies: hydride vapor phase epitaxy and metallorganic chemical vapor deposition. Figure 3 reveals the effect of epitaxial growth conditions and methods on the resultant shell morphology, where variations in sidewall inclination represent the range of achievable polarities.
Realizing the promise of 3D structured detectors requires careful consideration of morphologically derived device characteristics. Using this work as the underpinning of future developments in vertically oriented core-shell diodes will enable significant improvements in the operational characteristics of scintillator-based radiation detectors.
By developing a growth process for non-polar and semi-polar GaN facets, PIN diodes fabricated using this template will benefit from reduced defect densities, enhanced ionization coefficients, and lower voltage operations.3 Future work will include transitioning large-area radial GaN PIN structures into detector arrays.
The authors gratefully acknowledge the support of Dave Peterson and the Defense Threat Reduction Agency. This work was supported by the DTRA Basic Research Award HDTRA1-10-1-0107.
Matthew King is a senior engineer in the Advanced Technology Laboratory. His primary focus is on advances in multifunctional materials and devices, particularly nitrides for electro-optical and radio-frequency applications. He is PI in a variety of multidisciplinary efforts, holds five patents, and has more than 30 trade secrets.