Sputtered Gallium Nitride Tunnel Junction Contacts
Gallium nitride LEDs produced for commercialization currently use indium tin oxide (ITO) as both a current spreading layer (CSL) and a contact to p-GaN. ITO is known to absorb wavelengths in the UV and visible light regions, the primary spectrum of GaN devices . Tunnel junctions (TJ) have been proposed as an alternative p-contact and CSL which would transmit more light from the active region  while generating holes, from the electron tunneling, to reach the quantum wells of the LEDs for additional radiative recombination -. The fabrication process would also be simplified because the top layer of both the p- and n-contacts would be n-GaN and the metal contacts to both could be deposited at the same time. However, until now all GaN TJs have been deposited by MOCVD or MBE systems despite sputtering machines offering a lower cost and easier to use method of depositing GaN  . This thesis explores the application of two sputtering techniques, the electron cyclotron resonance (ECR) and radio frequency (RF) magnetron, to the creation of TJs on blue GaN LEDs.
Both sputtering methods utilized silicon doped n-GaN targets with the intention of depositing the n-GaN layer of the TJs. The films were examined for their transmissivity of 440 nm light as well as their resistivity. The ECR system was observed to produce GaN that lost around 5% of the emitted light when N2 in conjunction with argon was used in the sputtering gas. Substrate heating did not meaningfully affect the transmissive property of the deposited GaN. Despite the good optical quality, the GaN remained resistive. Secondary ion mass spectrometry (SIMS) of the n-GaN target found it contain many impurities. High amounts of carbon, hydrogen, oxygen, magnesium, and calcium among other elements were discovered to be within the target. This stopped the research to produce a tunnel junction with the ECR system, but with a cleaner target it could still be a viable option.
The RF magnetron machine also relied upon nitrogen within the sputtering gas mixture to produce transparent GaN films. In general, the higher the rate of N2 the less absorption of the tested 440 nm light. Substrate heating did improve the transparency of the GaN when sputtered with the RF magnetron machine. For a gas mixture of 38:25 sccm of N2:Ar and a temperature of 800 ⁰C, the GaN layer absorbed less than one percent of the blue wavelength. Hall-effect measurements showed greater substrate heating also increased the carrier concentration with films reaching the high 1019 and low 1020 cm-3 ranges.
During the course of this work the RF magnetron system was modified and the maximum substrate temperature was lowered to 650 ⁰C. Some previous films had been grown at 600 ⁰C and this temperature was kept for continuity, but a silicon target was co-sputtered with the n-GaN target to raise the carrier concentration of the GaN. Hall measurements of these samples presented them to have higher mobilities compared to the GaN sputtered with only the GaN target. Sputtering the silicon target at 35 W of power was determined to give the highest carrier concentration of 1.137x1018 cm-3.
A tunnel junction was created utilizing the RF magnetron sputtering system on a UCSB blue LED structure deposited on a sapphire substrate with a MOCVD machine. Silicon was co-sputtered at 35 W and the nitrogen and substrate temperature were set to 38 sccm and 650 ⁰C, respectively, to create the n-GaN layer of the TJ. LEDs with an area of 0.1 mm2 were fabricated and required only two photolithography steps compared to the three steps necessary for ITO GaN LEDs. After a chlorine etch performed by a RIE machine to create the LED mesas, the MOCVD n-GaN around the mesas was noted to be rough in some areas of the wafer. When metal contacts were deposited on these areas, the metal did not stick to the n-GaN. Many LEDs were created in the smoother regions and were observed to emit blue light. These were tested for their current-voltage relationships and were found to have turn-on voltages of approximately 6.5 V with the least resistive devices reaching almost 5 mA at 10 V. These are the first reported GaN tunnel junctions deposited onto LEDs with a sputtering system.