Transparent Ohmic Contacts to p-GaN by AZO with Subcontact Layers

Thanks to its wide band gap engineered by alloying with Mg and In and its built-in piezoelectric field, gallium nitride (GaN) is the material widely used in optoelectronics, for the fabrication of light emitting diodes (LEDs) and laser diodes (LDs) as well as in high frequency electronics for the preparation of high electron mobility transistors (HEMTs). The steady improvement in the already mature epitaxial technology has enabled interesting device developments, however there are also processing concerns which should be taken into account when optimizing a device. One of such is the design of appropriate contact layers for current biasing of the devices. In the case of LEDs and LDs, it is now being regarded that optically active materials are preferred over metals due to the possibility of their usage for enhancing light confinement or extraction.<br/>In this work we analyze the application of a transparent conducting oxide (TCO) ZnO:Al, or AZO deposited by sputtering as the top contact to p-GaN epitaxial layers and if it is possible to improve its performance by introducing ultrathin subcontact layers at the interface and appropriate surface processing. We see that AZO forms an ohmic contact after annealing at 800°C. What is relevant, the application of subcontact layers of Au and Ni lowers the contact forming temperatures to 600°C and 650°C, respectively and substantially increases the currents under the same bias. These metals were applied since the Ni/Au bilayer is the standard low-resistivity ohmic contact to p-GaN. We analyze the interfacial layer using high resolution TEM, XRD and SIMS to try to understand the mechanism of contact formation. For the Ni we see the formation of NiO spots in cross-section TEM and NiO peaks in the XRD suggesting the presence of NiO is needed for the contact to form. However when we fabricate a sample with NiO directly at the interface instead of Ni, we don’t observe ohmic I-V characteristics. This might mean that a more intimate reaction at the interface might take place, which we try to deconvolute using cross-sectional scanning tunnelling microscope. The scheme with Au also yields ohmic behavior however no clear mechanism for this has been yet found. We analyze the influence of the thickness of the Ni and Au layers on the contact formation and also apply an ultrathin Ni/Au bilayer to try to undestand the role of each element in contact formation. What is specific, we anneal the contacts in nitrogen instead of the commonly used air, as the oxygen source for the contacts to form is the AZO film itself, as seen from the NiO formation. We also assess the length of the temperature formation step and its influence on the performance.<br/>Finally, we demonstrate light emission from LD structures with the AZO-based contacts and determine their influence on the emission of the light from the diode structure.<br/><br/>This work was supported by the National Centre for Research and Development, Poland, project 'OxyGaN' - M-ERA.NET2/2019/6/2020, by the Hungarian NRDI Fund, grant number 2019-2.1.7-ERA-NET-2020-00002 and by the Israel Ministry of Science and Technology in the frames of the M-era.net Programme.