TECHNOLOGY LEDs
There is a lattice mismatch of about 20 percent between hexagonal GaN and silicon (111). So, if thick layers are required, complicated strain management structures are needed to prevent cracking. However, our application only requires a thin layer, so we avoid such complexities.
To prevent alloying of silicon and gallium, we begin by growing an AlN/AlGaN buffer layer. This is followed by deposition of hexagonal GaN: Two (0001) planes of hexagonal GaN grow from the (111) sidewalls of the grooves towards one another (see Figure 2). It is this growth that is the key to our approach, because, atomically, the (0001) plane of hexagonal GaN looks exactly like that of the (111) face of cubic GaN. The two (0001) planes are offset by the natural angle between (111) faces, due to the geometry of the substrate. So, when the next adatom wanders down into the groove, it doesn’t see two hexagonal GaN crystals growing out at one another – it sees the cubic phase, and settles in a cubic lattice position.
Following this nucleation of the cubic phase, growth proceeds rapidly upward, filling the groove in the new (001) direction. The result is a long, 2-3 µm wide triangular prism of single-crystal cubic GaN that is nestled in the stripe. It features a hexagonal GaN cladding on two of its sides.
With the cubic phase present, it is now possible to grow LED structures, just as one would for the hexagonal case. Forming a high-quality green LED is much easier with this phase of GaN, however, because: less indium in needed in the wells, thanks to the smaller bandgap of cubic GaN; and there is less strain in the active region, because the InGaN layer is only grown in long, thin stripes. A more relaxed layer is beneficial, because it is less likely to form dislocations and more likely to incorporate more indium.
Scrutinising our sample with electron-backscatter diffraction confirms the presence of cubic GaN in the centre of the stripe, and hexagonal material at the edges. Meanwhile, cathodoluminescence reveals that our cubic material has a band-gap of 3.23 eV. According to cross-sectional transmission electron microscopy, the cubic regions are virtually free of line defects and without wurtzite inclusions. This is a breakthrough, because other groups have reported wurtzite inclusions within their cubic GaN. These imperfections may have a detrimental effect if present in the active region of an LED.
We have fabricated a fully functioning, green- emitting cubic GaN LED on our engineered substrate (see Figure 3). It’s electroluminescence spectra are stable, even when the driving current is changed by an order of magnitude (see Figures 4 and 5). In comparison, subjecting a similar hexagonal GaN LED to changes of drive current of this order can induce a blue-shift in emission wavelength of up to 20 nm. We have also carried
out preliminary electrical measurements with nano-probes, finding good electrical performance for p-doped cubic GaN (see Figure 6).
Our proof-of-concept devices built of cubic GaN are promising, wavelength-stable green LEDs with good electrical properties. But there is more work to do. First, we want to propel the LED to longer wavelengths: Why should we stop at green, when we could reach yellow and even red?
In addition to reaching deeper into the colour spectrum, we will continue our study of the widely unexplored electrical properties of the cubic phase. We also plan to fabricate higher- performance cubic LEDs by removing the silicon substrate, so light extraction is increased through the underside of the stripes. A longer-term goal of ours is to expand and coalesce the separate cubic GaN stripes into a single, large epitaxial film. Ultimately, this technology has the potential to be scaled up to larger silicon wafers. Handling and processing equipment is already in place for this, thanks to the silicon IC industry.
If we could bring this technology to 300 mm or larger wafers, this could drastically increase the number of LEDs that could be formed from a single growth run. What’s more, these cubic GaN templates don’t just have to be the ideal platform for making green LEDs – they could be a great foundation for blue and red emitters too. If they could arm the makers of colour-tuneable lighting systems with efficient LEDs spanning the entire visible spectrum, it would enable them to offer products that set a new benchmark for performance.
£ This work was supported primarily by the Engineering Research Centers Program of the National Science Foundation under NSF Cooperative Agreement No. EEC-0812056. This work was also supported by New York State under NYSTAR contract C090145. Any opinions, findings and conclusions or recommendations expressed in this material are those of the author(s) and do not necessarily reflect those of the National Science Foundation or New York State.
© 2014 Angel Business Communications. Permission required.
Figure 6. Nano four-point- probe and transmission line experiments indicate that resistivity is about 0.28 Ωcm
Cubic GaN grown on micro-patterned silcion (001). Direct GaN on silicon technology could lead to fully integrated LED displays
Further Reading J. M. Stark, et al. Appl. Phys. Lett. 103 232107 (2013)
January / February 2014
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