Advanced characterisation of novel III-nitride semiconductor based photonics and electronics on polar and non-polar substrates

Advanced characterisation has been carried out on a number of novel III-nitride based photonics and electronics, including micro-LED arrays achieved by a direct epitaxy approach, high performance c-plane HEMTs structure achieved by a novel growth method and non-polar GaN/AlGaN HEMTs. In this work, a systematic study has been conducted to understand the electrical properties of these novel devices, demonstrating their excellent properties. Furthermore, the electrical properties are directly related to epitaxial growth, which provides useful information for further improving device performance, such as 2D growth mode for GaN on a large lattice-mismatched substrate which plays an important in obtaining high breakdown and minimised leakage current for HEMTs.
Micro-LEDs are the key elements for a microdisplay system, where electrical properties are extremely important. Potentially, any leakage current can trigger to turn on any neighbouring microLEDs which are supposed to be off. Instead of using conventional fabrication methods which normally enhances leakage current, our team developed a direct epitaxy approach to achieving microLED arrays. In this work, detailed I-V characteristic and capacitance measurements have been conducted on these novel microLED devices, demonstrating leakage currents as low as 14.1 nA per LED and a smooth negative capacitance curve instead of odd positive capacitance performances. Furthermore, a comparison study between our microLEDs and the microLEDs prepared using the conventional method indicates our device shows a large reduction of size-dependent inefficiency while such a behaviour is never observed on the microLEDs fabricated by the conventional methods.
Unlike the classic two-step method for GaN growth on large lattice-matched sapphire, our team developed a high-temperature AlN buffer technology, where a 2D growth mode, instead of an initial 2D and then 3D growth mode that typically happens for the growth of conventional GaN growth, takes place through the whole growth process. This method allows us to achieve a breakdown electric field strength of 2.5 MV/cm, a leakage current of as low as 41.7 pA at 20 V and saturation current densities as high as 1.1 A/mm. In this work a systematic study has conducted in order to establish a relationship between the excellent device performance and material properties, where a very low screw dislocation density plays a critical role, while our 2D growth method can provide an excellent opportunity for achieving such a low screw dislocation density. This demonstrates the major advantage over the classic two-step method in the growth of power and RF devices. In our case, we have obtained an unintentional doping as low as 2×10^14 cm-3 and screw dislocation densities of 2.3×10^7 cm-2.
Compared with c-plane GaN based HEMTs due to its intrinsic polarisation, non-polar GaN/AlGaN HEMTs on r-plane sapphire yields potential advantages in terms of the fabrication of normal-off devices which are particularly important for practical applications. However, it is a great challenge to achieve high quality non-polar GaN on sapphire. Some initial work has been conducted, where the detailed characterisation indicates an electron mobility of 43 cm2 V-1 s-1 has been initially obtained. Furthermore, instead of using an AlGaN/GaN heterostructure with a modulation doping, we deliberately use a quantum well structure as an electron channel, leading to a mobility of 76 cm2 V-1 s-1. Our simulations as well as measurements also provide a guideline for optimising the general epitaxial structure.