Improving ultraviolet LEDs with ITO
The cost of ultraviolet LED is compromised by complex device processing, such as flip-chip packaging or substrate lift-off, that is needed to address the low hole density of p-type AlGaN. But the problems with p-type GaN can be overcome by introducing an MOCVD-grown, transparent electrode of ITO, according to a team from Sun Yat-Sen University, China.
"We believe that our MOCVD-ITO technology can achieve high transmittance in the UVA and UVB range and make the fabrication of UV optoelectronic devices cheap and easy," remarks team spokesperson Gang Wang.
Another strength of the team's technology is that it has enabled UVA LEDs to operate at a lower voltage for a given current density (see Figure).
ITO is often used as a transparent conductive oxide in blue LEDs, but transferring it to UV variants is not easy, because the high work function of p-type nitrides hampers the formation of an ohmic contact between the ITO electrode and the p-type upper layer of the LED.
"Using interface treatment, we modify the interface property, so that the Schottky barrier at the ITO/LED interface is lowered," explains Wang.
The team has tried a variety of treatments, purging the interface with oxygen, trimethylindium (TMI) and tetrakis(dimethylamino)tin. Using the later, known as TDMASn, turned out to be the most effective treatment for improving the performance of the UVA LED.
"We want to emphasise that in-situ treatment may only be realised with MOCVD," argues Wang. "If ITO is grown by other methods, such as magnetron sputtering or thermal evaporation, in-situ treatment seems impossible."
Purging with tetrakis (dimethylamino) tin improvers the interface between ITO and GaN, leading to UV LEDs with lower forward voltages.
The ITO films were added in a home-built MOCVD tool.
"We built our lab-made MOCVD system because we believe that an MOCVD system for oxides could be cheaper," explains Wang, who points out that similar tools for growing nitrides are more complicated. "The designer has to isolate the growth chamber from atmosphere in order to suppress oxygen contaminants."
For oxygen, there are obviously no issues associated with oxygen contamination. Thus, the design of the MOCVD tool is much easier, and its cost far lower.
The researchers uncovered the benefits of purging with TDMASn by taking AlGaN-based LED structures and depositing 85 nm-thick layers of ITO by MOCVD. For the control, they deposited ITO directly on the device; they also produced two alternatives, involving intermediary 6s purges with either TMI or TDMASn.
As well as making 250 μm by 760 μm LEDs from these epiwafers, the engineers produced isolated ITO films with the three different treatments, to determine the impact of these processes on transmission through the oxide. All three samples had a transmission in excess of 90 percent at 368 nm, with the variant formed with TDMASn producing the highest value, 95.9 percent.
Additional benefits of the TDMASn purge included a reduction in surface roughness, a lower sheet resistance, and a lower forward voltage. At a 100 mA drive current, this LED operated at 3.83 V, more than 0.1 V lower than the cousins formed by the other processes.
"We are now using our MOCVD-ITO as electrodes in flip-chip UVA LEDs," explains Wang. In a conventional flip-chip LED, a metal mirror has a dual role, acting as a reflector and an electrode. However, the refectance of the mirror is inferior to that of a distributed Bragg reflector, so Wang and his co-workers are trying to increase light extraction efficiency by combining the distributed Bragg reflector with ITO.
"We are also focusing on large-size, one watt, highpower UVA-LEDs and their application in UV curing."
