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Building Non-polar And Semi-polar LEDs On Silicon

Patterned silicon provides a promising foundation for non-polar and semi-polar GaN LEDs that could sport unprecedented modulation speeds and bright emission in the green and yellow by Kai Ding, Natalia Izyumskaya, Ümit Özguer, Hadis Morkoç and Vitaliy Avrutin from Virginia Commonwealth University and Sebastian Metzner, Frank Bertram and Juergen Christen from the Otto-Von-Guericke University

Of the billions of GaN LEDs made every year, the vast majority are produced by growing epilayers of nitrides on c-plane sapphire. It is an approach that's now relatively easy to take, but the LEDs that result suffer from two fundamental flaws: a reduced overlap of the electron and hole wavefunctions, due to a strong polarization field in the active region; and a low efficiency for the incorporation of indium in the InGaN quantum wells. Both these issues contribute to the "˜green-gap', a low efficiency for LEDs emitting in the green and yellow; and they hamper the development of full colour displays, laser diodes, and highbrightness sources for general lighting.

What's more, the strong polarization fields in the active region "“ a phenomenon that goes by the name of the quantum confined Stark effect "“ imposes serious limitations on Li-Fi, an emerging communication technology that uses light to transmit data over free space. Spatial separation of electron and hole wave functions that results from these fields leads to long carrier lifetimes: for blue LEDs it is tens of nanoseconds, and for green variants it is an order of magnitude higher. The lengthy lifetimes limit LED bandwidth to the megahertz range.

Figure 1. Select low-index crystallographic planes in group-III nitrides.

One promising approach to enhancing the modulation speed of the LEDs, and also increasing their efficiency in the green and yellow, is to switch growth from the polar orientation to one that is either semi-polar or non-polar. The former reduces the polarization fields, while the latter eliminates them completely (see Figure 1 for common non-polar and semi-polar planes).

For semi-polar planes, the angle made with the polar c-plane governs the extent that the electric field is reduced. When devices are made on semipolar (1101) and (1122), thanks to inclinations close to 60° with respect to the c-plane, the polarization discontinuity at InGaN/GaN interfaces is sufficiently small, aiding the fabrication of green devices with tolerable compositions of InGaN. Note that in theory, the weakening of the fields, which push emission to shorter wavelengths, is not the only benefit of semi-polar planes "“ they also promise to enhance the efficiency of indium incorporation.

What platform?

The ideal platforms for non-polar and semi-polar LEDs are native substrates with identical orientations. But natural limitations are making such structures prohibitively expensive for mass production. Today, HVPE and ammonothermal methods can produce relatively large GaN crystals, but these approaches are hindered by the strong tendency of GaN to grow in the polar (0001) direction. With HVPE it's possible to produce (0001)-oriented disks that are a few millimetres thick, limited by large internal stress; while the ammonothermal method yields hexagonal prisms that are a few centimetres thick, with a (0001) base plane (see Figure 2). In this case, crystal thickness is limited by the low growth rate. Due to these limitations, the non-polar and semi-polar substrates that are diced from (0001)-oriented boules are elongated rectangles with a width of no more than a few centimetres.

Figure 2. Comparison of the available GaN and silicon substrate sizes for growth of non-polar and semi-polar GaN (taking non-polar (1120) GaN as an example): (a) ammonothermal GaN, and (b) patterned silicon (110) substrate.

One promising alternative, which we are pursuing at Virginia Commonwealth University, VA, and the Otto-von-Guericke University in Germany, is to grow semi-polar and non-polar LEDs on patterned silicon. This approach has much to recommend, as silicon substrates are low in cost, available in large sizes, and allow the use of processes that are compatible with mature silicon technology. For example, a silicon platform could allow the combination of drive circuitry and LEDs on a single platform.

Many groups, both in academia and industry, have developed c-plane LEDs on silicon. A great deal of their efforts have focused on the development of stress compensating buffer layers, which mitigate a large tensile strain induced by the large thermal mismatch between silicon and GaN. Left unchecked, this strain causes the material to crack.

Unfortunately, none of the orientations of silicon can be directly employed to grow non-polar or semi-polar GaN LEDs. However, due to its epitaxial relationship with GaN, patterned silicon can provide a platform for growing GaN with various non-polar and semi-polar planes.

The key to producing non-polar and semi-polar orientations is the exposure of the {111} facets of a properly oriented silicon substrate for GaN nucleation (see Figure 3). As GaN grows epitaxially on silicon (111) in the c-direction, thanks to the epitaxial relationships GaN <0001>||silicon <111> and GaN <2110>||silicon <011>, it is possible to form: nonpolar (1100) (m-plane) and (1120) (a-plane) GaN on patterned silicon (112) and silicon (110) substrates, respectively; and semi-polar (1101), (1122), and (2021) GaN on patterned silicon substrates of different orientations.

We have undertaken experimental studies that confirm the theoretical predictions for complete elimination of the polarization field at the interfaces in nonpolar InGaN/GaN and GaN/AlGaN heterostructures, along with the substantial reduction in the semipolar varieties. Our photoluminescence spectra from (1100)-oriented 6 nm InGaN/GaN double heterostructures show no blue-shift with increasing excitation, suggesting an absence of the polarization fields and thus the elimination of the quantumconfined Stark effect. For semi-polar (1101) 6 nm In0.15Ga0.85N/GaN double heterostructures emitting in the blue, the shift to shorter wavelengths is just one-fifth of that for c-plane counterparts (see Figure 4). Note that similar results have been reported for other semi-polar orientations.

Recent experiments indicate that the benefits of the weaker fields are not limited to shorter radiative carrier recombination lifetimes, but extend to the dominance of radiative recombination by excitons "“ this is particularly attractive for high-efficiency LEDs and lasers. We have shown this to be the case in non-polar and semi-polar GaN/AlGaN systems, while teams at Cambridge and the University of Brunswick "“ Institute of Technology have identified this behaviour in InGaN/GaN heterostructures.