Increasing Ultraviolet Efficiency With Nano-patterned Sapphire
Switching from conventional patterning of sapphire to a nano-scale variant trims epitaxial growth times and materials costs while boosting extraction efficiency.
By JIANCHANG YAN, PENG DONG, AND JUNXI WANG from the INSTITUTE OF SEMICONDUCTORS AT THE CHINESE ACADEMY OF SCIENCES.
Deep ultraviolet(DUV) sources can serve many applications, including air and water purification, disinfection, bioagent detection (see image above), curing and non-line-of-sight communication. Traditionally, the mercury lamp has served this spectral range, but it is bulky, fragile and unsuitable for providing a modulated light source.
All of these weaknesses can be addressed with a DUV LED. Following several years of research into this class of solid-state emitter, chips now span 365 nm to 210 nm, with some devices producing milliwatt levels of light output while operating for thousands of hours. It is a level of performance that is adequate for some applications, but far higher levels of performance will drive significant market penetration for the DUV LED. Today it is held back by a modest internal quantum efficiency and a low light extraction efficiency, which combine to limit this chip’s external quantum efficiency and output power.
The low internal quantum efficiency stems from the high density of crystal defects – they are typically of the order of 1010-1011 cm-2. DUV LEDs are riddled with these defects because there is a large lattice mismatch and a significant thermal expansion mismatch between the sapphire substrate and the AlN and AlGaN epilayers that form the LED (see Figure 1). Meanwhile, light extraction is poor, due to the combination of low internal total reflection at the epi-layers’ flat interfaces and absorption of the emission from the active region by the top p-GaN.
Figure 1. The device structure of the DUV LED on flat sapphire substrate with a 1-µm-thick AlN template layer (a) delivers an inferior performance compared to that built on a nano-patterned sapphire substrate with a 4-µm-thick AlN template layer (b)
One option for increasing light extraction in these devices is to follow a widely adopted approach for boosting the brightness of blue LEDs. Forming these blue-emitting chips on micro-cone-shaped, patterned-sapphire has not only increased light extraction, but also enhanced crystal quality, leading to a substantial hike in luminous efficiency. However, care must be taken when attempting to replicate this type of approach with DUV LEDs, because AlN does not completely coalesce to form a smooth surface on conventional, micrometre-scale patterned sapphire. Why? Because the aluminium species have a low surface mobility.
Some research groups have suppressed crystal defects, both in AlN and in the upper epilayers, by turning to epitaxial lateral overgrowth techniques on either micro-striped patterned sapphire, or on templates formed by the growth of AlN on a flat sapphire substrate. AlN can then coalesce on unetched flat regions, leading to enhanced light output and external quantum efficiency for the DUV LEDs. However, this approach is not without its penalties. Due to the large space between the micro-patterns, coalescence thicknesses for AlN can be up to 10 µm, and this leads to far greater epitaxial time and cost.
From micro to nano
To trim the coalescence thickness while maintaining a high output power for the DUV LED, our team at the Institute of Semiconductors at the Chinese Academy of Sciences has developed novel nano-patterned sapphire substrates. They form the foundation for nanoscale epitaxial layer overgrowth of an AlN template layer for DUV LEDs (see Figure 1).
Turning to these nano-patterned substrates slashes the coalescence thickness of the AlN film to just 3 µm. This is not at the expense of material quality − according to high-resolution X-ray diffraction and cross-sectional transmission electron microscope analysis of AlN and upper epilayers − and it enables the fabrication of LEDs with an impressive level of performance. Driven at 20 mA, a 282 nm LED formed on this foundation produces nearly twice the output power of an equivalent device formed on a conventional AlN-on-sapphire template.
We formed our patterned sapphire with a nanosphere lithography technique that involves wet-etching (see Figure 2). We begin by depositing a 200-nm-thick SiO2 film onto 2-inch (0001) sapphire by plasma-enhanced CVD. A positive photoresist is then spin-coated on this oxide, before a highly ordered self-assembled monolayer of polystyrene nanospheres is added via dip-coating. The wafer is then exposed to UV light using conventional photolithography, before the nanospheres are removed with deionised water and the photoresist developed to form nano-sized holes. Subsequent inductively coupled plasma etching transfers this pattern to the SiO2 film, before the sapphire substrate is etched for 10 minutes in a mixture of H2SO4 and H3PO4 solution and the SiO2 mask removed by HF.
Figure 2. The process flow for fabricating the nano-patterned sapphire
This process creates a sapphire substrate that is patterned with concave triangular cones, which have dimensions defined by the anisotropic etching of the sapphire crystal (see Figure 3 for scanning electron microscopy images of pattern sapphire with periods of 900 nm and 600 nm).
Figure 3. Scanning electron microscopy images of the fabricated nano-patterned sapphire with periods of 900 nm (a) and 600 nm (b)
Growth of our epistructure is undertaken with our homebuilt low-pressure MOCVD tool. It is fitted with trimethylaluminum, trimethylgallium and ammonia for providing aluminium, gallium and nitrogen sources, respectively.
Starting with nano-patterned sapphire with a 900 nm period, we deposit a 25-nm low-temperature AlN buffer layer at 550 °C, before ramping the temperature to 1200 °C to grow a 4-µm AlN template. Growth of this layer occurs under continuous flow, with a relatively low V/III ratio of below 1000 and a low chamber pressure of 50 torr. We have scrutinised the quality of this AlN film with a variety of techniques. Cross-sectional scanning electron microscopy reveals that the AlN completely coalesced after 3-µm of growth, thanks to the nano-scale substrate patterns and the AlN lateral growth. Meanwhile, atomic force microscopy shows that the AlN is smooth – it has a root-mean-square roughness of 0.15 nm. The crystal quality of this 4 µm-thick AlN film is better than that produced on conventional sapphire, according to X-ray rocking curves that provide figures for full-width at half maximum of 86.4 arcsec and 320.4 arcsec for the (002) and (102) directions, respectively. Patterning sapphire also cuts defects, with threading dislocations reduced due to bending that results from lateral overgrowth.
On the 4-µm-thick AlN template formed on our nano-patterned sapphire we deposited: a 20-pair AlN/AlGaN superlattice; a 3.5-µm-thick, silicon-doped n-Al0.55Ga0.45N layer; five 3-nm-thick, un-doped Al0.4Ga0.6N quantum wells sandwiched by 12-nm-thick, silicon-doped Al0.5Ga0.5N barriers; a magnesium-doped Al0.65Ga0.35N electron-blocking layer; a 50-nm-thick,p-AlGaN cladding layer; and a 150-nm-thick, highly doped p-GaN contact layer. For comparison, we also formed an identical structure on conventional sapphire, using a 1 µm-thick AlN template.
Better, brighter material
Superior material quality in the structure formed on nano-patterned sapphire is revealed with transmission electron microscopy (see Figure 4). Dislocation density in the n-AlGaN layer formed on patterned sapphire is 1.6 × 109 cm-2,compared to 3.4 × 109 cm-2 for the same epilayer on conventional sapphire. Cutting the dislocation density increases the internal quantum efficiency from 28 percent to 45 percent, according to temperature-dependent photoluminescence measurements on ‘top-GaN-less’ multiple quantum well structures.
Figure 4. Cross-sectional scanning electron microscopy reveals the high quality of the AlN grown on nano-patterned sapphire
Epiwafers were used to form 380 µm by 380 µm devices. Contact photolithography and inductively coupled plasma etching defined the p-njunction mesa, prior to the deposition of a Ti/Al/Ti/Au (20 nm /120 nm/20 nm/100 nm) metal stack onto the n-AlGaN using an electron-beam evaporator.
This was annealed in nitrogen gas at 850 °C. The ohmic contact for p-GaN employed a Ni/Au (5 nm/10 nm) metal stack, annealed at 550 °C in air. Chips were then flip-chip bonded with gold bumps to silicon sub-mounts, which were attached to metal-core printed circuit boards with sliver paste for device testing. This improved heat dissipation from the DUV LED.
Our devices built on patterned sapphire emit a peak wavelength of 282 nm and, when operated at 20 mA, produce an output power and external quantum efficiency of 3.03 mW and 3.45 percent, respectively. Output saturates at 60 mA, hitting 6.56 mW. In comparison, the equivalent LED formed on conventional sapphire is limited to 2.53 mW at 50 mA, primarily due to inferior heat dissipation.
Driven at 20 mA, our DUV LED grown on the novel platform delivers an external quantum efficiency that is 98 percent higher than that of the control sample (see Figures 5 and 6). Since external quantum efficiency is a product of internal quantum efficiency, light extraction efficiency and carrier injection efficiency – and the measurements for ‘top-GaN-less’ multiple quantum wells show a 60 percent gain in internal quantum efficiency with the patterned sapphire – it is highly likely that the patterning has boosted extraction efficiency. This is to be expected, because light scattering at the interface between AlN and nano-patterned sapphire should decrease total internal reflection and absorption in the p-GaN layer, while increasing the photon’s opportunity for escaping from the sapphire backside.
We will now try to build even more impressive devices. The DUV LED’s efficiency and output power can be increased with a combination of superior heat dissipation, optimisation of the dimensions of the nano-patterned sapphire, and improvements to the growth processes used to deposit AlN onto the substrate. In addition, we will investigate the impact of nano-patterning on the reliability of our devices.
Figure. 5 The patterning of sapphire leads to an increase in electroluminescence intensit
Figure 6. Nano-patterning increases the output power and the efficiency of DUV LEDs
Measuring ultraviolet LED output
Forming nano-patterned sapphire involves dip-coating and cleaning processes