ABSTRACT
Ultra-violet light emitting diodes (UV LEDs) and lasers based on the III-Nitride material system are very promising since they can enable compact, safe, and efficient solid-state sources of UV light for a range of applications. The primary challenges for UV LEDs are related to the poor conductivity of the p-AlGaN layers, hole injection (p-contact), and the low light extraction efficiency of the LED structures. In this abstract, we discuss the exploration of metal-semiconductor junctions that enhance both light extraction efficiency and hole injection.
Tunnel junction (TJ)-based UV LEDs provide a distinct and unique pathway to eliminate several challenges associated with UV LEDs [1-4]. Metal semiconductor tunnel junctions (MS TJs) have been demonstrated earlier with efficient injection into p-AlGaN [5]. The negative polarization charges at the GaN (InGaN)/AlGaN interface create the necessary band bending to facilitate carrier tunneling. This concept is similar to a traditional tunnel junction, but with metal replacing the top n-AlGaN layer. Eliminating the top n-AlGaN layer offers advantages for MOCVD growth, as surface activation is now possible. Efficient tunneling between the metal and p-AlGaN can be achieved using a polarization-engineered layer, such as GaN (InGaN), with an optimized thickness, and the use of UV-reflective metals like Aluminum are particularly promising since they offer a method to achieve highly reflective UV LEDs.
In this work, we discuss the design and demonstration of all-MOCVD grown metal-semiconductor tunnel junction UV LEDs. We then show how the design of the metal can greatly impact the hole injection and light extraction/reflectivity at the metal/semiconductor interface. Al-based metal/semiconductor junctions have excellent reflectivity, but show poor hole injection/p-contact. However, Ni-based metal/semiconductor junctions have poor reflectivity but have excellent hole injection. We show that hybrid ultra-thin Ni/Al layers can give excellent reflectivity while reducing the voltage drop significantly. Using this approach, we show a 43% improvement in the peak external quantum efficiency, and low on-resistance and operating voltage for ultrathin Ni/Al/GaN/AlGaN metal-semiconductor junctions.
The epitaxial structure was grown at TNSC with a Taiyo Nippon Sanso SR4000HT MOCVD reactor. It consists of an active region with three pairs of 1.7 nm Al0.42Ga0.58 N quantum wells (QWs) separated by 1.25 nm Al0.5Ga0.5N quantum barriers. The tunnel junction is composed of a 6 nm p++ Al0.5Ga0.5N layer (doped with Mg at which is capped with a 4 nm GaN layer. A control sample was grown with a similar structure, except for the tunnel junction which was replaced with a 50 nm thick p-GaN layer to represent a standard LED. The p-type layers were activated through rapid thermal annealing at for 28 minutes in N2. Three different metal contacts were deposited to the MS TJ sample to make top contact: Ni(20 nm)/Au, Ni(1 nm)/Al(100 nm)/Ni/Au and Al(100nm)/Ni/Au. It has been shown that 1 nm Ni contact improves the reflectivity from 30% to 60% at 290 nm[6]. The improved reflectivity in addition to the thinner GaN layer leads to less absorption and better light extraction.
The composition and thickness of each layer was extracted from the profile for both the control sample and MS tunnel junction sample. Current-voltage characteristics show a voltage drop of 8.3 V,11.8,15.3 V at 20 A/cm2 for the Ni, hybrid Ni/Al and Al contacts respectively. The increase in voltage drop is due to the different work function Wm of the metals. Previous work has shown that thin 1nm Ni has a lower Wm than thicker Ni [7]. Hence it has a slighter higher schottky barrier height than the Ni based contact. All of the samples showed a low on-resistance of 5×10-3 ohm.cm2 indicating efficient tunneling in the MS TJ structures. Replacing Ni with the reflective hybrid metal contacts results in a 43-57% increase in the peak EQE. The electroluminescence spectra of all samples show a peak wavelength emission shifting from 296 nm to 292 nm as the current density increases from 2 A/cm2 to 200 A/cm2.
In summary, we have successfully demonstrated metal semiconductor tunnel junctions for MOCVD-grown UV LEDs. By depositing hybrid Ni/Al contacts, low voltage drop, resistance and higher EQE is achieved. The results of this work provide an alternative method for reducing the absorbing layer thickness while improving the external quantum efficiency. Future works involve optimizing the polarization engineered interlayer and doping/thickness of the p++ AlGaN.
This work was funded by ARO DEVCOM Grant No. W911NF2220163 (UWBG RF Center, program manager Dr. Tom Oder)
References:
1. Zhang, Y., et al.,Applied Physics Letters, 2016. 109(12).
2. Pandey, A., et al., Photonics Research, 2020. 8(3).
3. Fan Arcara, V., et al., Journal of Applied Physics, 2019. 126(22): p. 224503.
4. Nagata, K., et al., Applied Physics Express, 2021. 14(8): p. 084001.
5. Zhang, Y., et al., Applied Physics Letters, 2017. 111(5).
6. Kneissl, M. and J. Rass, 2016, Springer.
7. Grodzicki, M., et al., Applied surface science, 2014. 304: p. 24-28.
