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Different planes To produce green-emitting devices based on the nitrides, engineers have to increase the indium content in the quantum well. In conventional structures grown on the c-plane of GaN, this leads to high internal electric fi elds that pull apart the electrons and holes and hamper light emission.

These unwanted electric fi elds can be reduced, and even eliminated, by growing devices on different planes of GaN, known as semi-polar and non-polar planes. Ammono-GaN substrates with these orientations are available commercially from us.

Our partners have started to investigate the potential of devices grown on this classes of substrate, fabricating LEDs and laser diodes on the semi-polar (2021) plane. Using plasma-assisted MBE, LEDs were formed on this substrate and also on the (0001) plane. Peak emission wavelength varied considerably between the two forms of device, with emission at 387 nm for the semi-polar LED and 462 nm for the non-polar, due to considerably lower indium incorporation for growth on the semi-polar plane.

Characterisation of the semi-polar epitaxial LED structure reveals a smooth surface morphology with atomic steps, according to atomic force microscopy, and high photoluminescence intensity. Inspection of the material with a transmission electron microscope indicates that the interfaces are abrupt and there is good structural quality. Ultraviolet laser diodes emitting at 388 nm have also been formed on ammonothermal (2021) substrates. Devices with a ridge waveguide along [1210] direction exhibited a threshold current density and voltage of 13.2 kA/cm2

and 10.8 V. These values

are quite high, but should be reduced by processing the ridge along the [1014] direction, because this should lead to higher material gain, thanks to in-plane anisotropy.

Electronic devices Native substrates should also aid GaN power devices. Currently, these are manufactured on either silicon or SiC, but if GaN were used, this would allow a switch from a lateral to a vertical device architecture. With lateral devices, there are many problems associated with lateral current fl ow near the buffer layers and overlying dielectric layers, including current-collapse, high dynamic on-resistance and an inability to support avalanche

Programs with Ammono participation

In 2013, Ammono started to work with the European Space Agency. This partnership began because GaN is now viewed as a key strategic material for the development of space electronics.

Devices made from this material are perfect for space applications: they have a high resistivity to ionizing radiation, a low energy loss, and when they are grown on Ammono-GaN they can deliver a higher electrical effi ciency than when they are formed on other materials. Today, Ammono

develops wafers optimized for the development of RF electronics for space communications. The exceptional quality of the substrate leads to devices with a longer lifetime, which is highly valued, due to the very high costs associated with getting every kilogram into space.

Another program launched in 2013 is the Strategies for Wide-Bandgap, Inexpensive Transistors for Controlling High-Effi ciency Systems (SWITCHES) initiative run by the US Advanced Research Projects Agency-Energy. This $27 million programme aims to

stimulate the development of power electronics on GaN and related materials. Ammono is involved in two projects within the programme. They have the goals of conceiving transistors and Schottky barrier diodes that will allow more effi cient energy transmission.

Last but not least, one of the share- holders of Ammono is Nichia – the world leader in optoelectronics. The access to top of the class substrates allows them to keep at the forefront of optoelectronic innovation.

64 October 2014 Copyright Compound Semiconductor

breakdown. Further downsides arise from stress in the buffer layers that cause wafers to bow, limit epilayer thickness and ultimately place a ceiling on the maximum blocking voltage, which is typically no more than 1.2 kV.

Turning to a native substrate allows GaN power devices to be constructed with a vertical architecture. Current can then fl ow through the epitaxial layers, leading to devices that are immune from current collapse and dynamic on-resistance.

In such devices, which exhibit true avalanche breakdown, charge trapping effects and barriers to heat removal are absent, and it is possible to grow very thick layers of GaN that produce very high blocking voltages.

Our partners have started to investigate the potential of such devices grown on native substrates, evaluating the performance of GaN Schottky barrier diodes and HEMTs. At the IWN conference held this year, we co-authored a paper detailing a 6 GHz RF power transistor formed on Ammono-GaN, following our collaboration with the Institute of Electron Technology from Warsaw. In HEMTs and GaN Schottky barrier diodes, the low defect density in the ammonothermal substrate is retained in the epilayers, which have an atomically smooth surface and fl at interfaces.

Comparisons between devices built on different forms of GaN substrate are still to be carried out. However, some very impressive results were realized with devices formed on Ammono-GaN (see Figure 2). Schottky diodes produced at University of Notre Dame produce a leakage current density of the order of 10-11

A cm-2

, according to publications by this team.

The electric fi eld at breakdown voltage in these devices is 3.3 MV/cm, so close to the theoretically predicted critical fi eld of 3.5-3.8 MV/cm. The HEMTs on Ammono-GaN have a two-dimensional electron gas density of 8 x 1012 with a mobility of 1600 cm2 77 K.

cm-2 V-1 s-1 at 300 K and 8000 cm2 V-1 s-1 at

These results, plus those of the LEDs and lasers, highlight the promise of Ammono-GaN, and indicate that better substrates lead to better device results. Availability of this material will steadily increase, and in time there should be more devices based on homoepitaxial GaN on the market delivering top of the class levels of performance.

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