TECHNOLOGY LEDs
Figure 4. Unlike a conventional green LED, cranking up the current does not lead to a shift in emission wavelength
Increasing dimensions We are not the pioneers of cubic GaN, and our contribution to this field is to increase the size of this material. We are following in the footsteps of the likes of the groups of Donat As at the University of Paderborn, and Tom Foxon and Sergei Novikov at the University of Nottingham – teams that produced cubic GaN layers and devices, starting mainly with 3C-SiC and GaAs cubic substrates.
Attributes of cubic GaN
One of the great strengths of the cubic phase of GaN is that it has no internal electric fields in the typical growth direction, and thus no quantum confined Stark effect. The benefits of this are twofold: bands no longer move with increasing current, so the emission wavelength is fixed; and the overlap of electrons and holes in the quantum wells increases, so these carriers are more likely to recombine radiatively. Removal of the electric fields also opens up the possibility to turn to wider quantum wells that enable efficient radiative recombination while lowering Auger recombination rates.
Another great virtue of cubic GaN is its bandgap: At 3.2 eV, it is 0.2 eV lower than that of the hexagonal phase. This difference means that cubic GaN has almost a 30 nm head start prior to growth for making long-wavelength emitters, which is a great benefit when aiming for green LEDs.
What’s more, cubic GaN theoretically has better transport properties than the hexagonal variant due to higher crystal symmetry. Holes are a necessary player in producing light from LEDs, but p-type hexagonal GaN is notorious for its low concentration of holes and their poor mobility. Since holes do not move around as freely as electrons, most of the emission from an LED comes from the last quantum well − the one closest to the p-side. Better hole mobility would increase its population in many quantum wells, culminating in more light from the device. At low hole concentrations, hole mobility in cubic GaN has been reported at 350 cm2 less than 200 cm2
V-1 V-1
Figure 5. In a hexagonal InGaN quantum well (left), electron and hole wave- functions are spatially separated due to internal polarization fields. In a cubic InGaN quantum well of the same Indium composition (right), carriers overlap and the emitted photon is of longer wavelength
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www.compoundsemiconductor.net January / February 2014 s-1 s-1 compared to in hexagonal GaN.
The MBE growth technique that both of these groups have used is highly versatile, but limited to growth rates of the order of 100 nm/hr. This means that the deposition of an LED structure can take 10 hours or more, an impractically long time for LED manufacture, which is better served by MOCVD. Groups led by Heber Vilchis from Cinvestav and Shigefusa Chichibu from the University of Tsukuba have shown that this deposition technology can form cubic GaN on GaAs and 3C-SiC, respectively, but crystal size is small and phase purity poor. We are now addressing these shortcomings with an approach that offers production scalability, and begins with the most common form of silicon. By using this form, silicon (001), we have the potential to reach wafer sizes of 300 mm.
To address the well-known mismatch problem, we do not directly deposit a film of cubic GaN on a flat silicon wafer, but instead initiate growth in narrow stripes, which can eventually coalesce (see Figure 1). By employing a micro-patterned substrate, we can ‘trick’ GaN into forming the cubic phase in micro-stripes created from two separate hexagonal crystals.
One of the biggest challenges that we face is that the cubic phase of GaN is metastable – from a thermodynamic perspective, it would rather be hexagonal. However, the energy difference between the two phases is only 10 meV/atom, and if this wide bandgap material forms the cubic phase, it will probably stay that way. The challenge is to get the atoms into the cubic configuration for long enough to repeat the pattern. The difference between these polytypes is merely one of stacking: in the hexagon phase the layers alternate ABABAB, whereas in the cubic phase, every third layer is the same, so the pattern is ABCABC.
Our starting point is a micro-patterned silicon (001) substrate, which is prepared by our collaborator, the group of Steven Brueck at the University of New Mexico. This team uses interference pattern lithography to define long stripes parallel to the (110) direction. These stripes are then etched into the silicon to produce an array of V-shaped grooves with (111)-type sidewalls running the length of the substrate. The grooves have an 800 nm opening and are spaced at a 4 µm pitch.
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