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INDUSTRY GaN SUBSTRATES


When nitride lasers can deliver an output of a Watt or more, they can be used in laser displays delivering exceptional brightness and colour resolution. What’s more, these sources can be used to construct high-power laser arrays that form the heart of efficient white-light sources involving the pumping of a phosphor. This technology is being pursued by BMW for use in car headlights.


Working in partnership with TopGaN Lasers and the Institute of High Pressure Physics in Warsaw, Poland, powerful arrays of InGaN laser diodes have been fabricated. By forming several stripe emitters on a single chip, the power density per emitter is reduced. This avoids catastrophic optical damage that leads to facet melting – which can occur above a threshold current density of around 50 MW/cm2 with long lifetimes.


– and ultimately creates devices


Violet-emitting chips formed in this manner with three and 16 emitters have delivered a maximum output power of 2.5 W and 4 W, respectively (see Figure 1). These laser, which deliver a performance that is ‘top of the class’, typically operate at 408 nm - 412 nm. Threshold current for three-stripe emitter is 1.2 A, corresponding to a current density of 5 kA/cm2


, and threshold


voltage is 6.5 V. Meanwhile, slope efficiency is 0.76 A/W, and for a 10 μm stripe-width device the line-width is very narrow – it is just 0.25 nm. Another impressive attribute of these chips is that their light-current characteristics for individual stripes are fairly similar, with emission comparable within an experimental error of ±0.4 nm. This means that the emission wavelength is very uniform within one chip.


As explained previously, this multi-emitter chip design avoids catastrophic optical damage at the facet, leading to a long lifetime. For our 16-emitter device, which delivers an output of 4 W at a 6A drive current, the operating current density is just 18 kA/cm2 6000 hours.


. This enables a reasonable lifetime that exceeds


Figure 1. UV Laser Matrix emission spectrum (a) and the dependence between the optical power and current (b)(courtesy of TopGaN).


Scrutinizing our material with various common semiconductor characterisation techniques, such as X-ray diffraction, microscopy and optical methods has already proven that this form of GaN is superior to its HVPE-grown cousin (see Bulk GaN: Ammonothermal trumps HVPE from Compound Semiconductor March 2010, p.12). Also, intrinsically, for bigger wafers sizes the production cost for an Ammono-GaN wafer is lower than for HVPE GaN. To what extent this leads to superior device performance is still to be fully established, but we are making efforts to determine this by teaming up with various partners, together producing and evaluating devices built on our substrates.


In the majority cases, results obtained by these partners are confidential and covered by non-disclosure agreements. But there are some instances where we can reveal findings, because participants in the project have received external funding and there is an obligation to report results in the public domain.


Better lasers


One such effort has focused on the fabrication and testing of laser diodes. This has showcased the virtues of working with GaN substrates formed from ammonothermal crystal growth.


Figure 2. Epistructure of the GaN Schottky barrier diode and a corresponding scanning tunneling electron microscopy image. The growth interface between bulk substrate and epitaxial layer is invisible. (Courtesy of University Notre Dame)


62 www.compoundsemiconductor.net October 2014 Copyright Compound Semiconductor


We attribute the world-class performance of these lasers to the exceptional quality of the Ammono-GaN substrates. If the defect density in a laser is high, or non-uniform, it is well established that this impairs performance, shortens lifetime, drags down efficiency and hampers reliability. These issues do not exist in our partner’s lasers − one of the great strengths of our substrates is their very low, homogeneous dislocation density that is replicated in the epitaxial structure.


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