title: Reliability Characterisation of III-Nitrides Based Devices for Technology Development
Creators: name: Duffy, S
Publisher: name: Liverpool John Moores University
Year: 2019

description: III-nitrides based devices are considered as outstanding options for a range of extremely relevant applications. These devices can significantly improve the efficiency of high-power switching appliations. They are predicted to dominate applications in the low carbon economy. In recent years, these devices have been steadily improved and each year new record performances have been reported. Regardless of the superior performance of III-nitrides based devices, and particularly AlGaN/GaN high electron mobility transistors (HEMTs), achieving reliability at the same time as the high performance that the device boasts is a factor that is holding back widespread commercial and industrial development. Recoverable degradation (e.g. current collapse and on-resistance) and unrecoverable degradation (e.g. access resistance of contacts, and gate leakage current) persist to be limiting reliability factors. The mechanisms contributing towards performance and reliability degradation of AlGaN/GaN HEMTs, namely self-heating, charge trapping and strain, are required to be minimised; an important step before large-scale deployment can be attained. The strong coupling of these degradation mechanisms, under normal device operation, makes the quantitative contribution of each mechanism indistinct due to the lack of standard characterisation techniques. In this Thesis, the impact of the source/drain (S/D) and gate terminals of an AlGaN/GaN HEMT on its thermal management was investigated. Using Infrascope measurements, a substantial increase in temperature and resistance at the inner ends of the S/D contacts was observed. High-resolution X-ray diffraction technique combined with drift-diffusion (DD) simulations showed that strain reduction at the vicinity of S/D contacts is the origin of temperature rise. The strain reduction was also observed below the metal gate. Through electro-thermal simulations, the electrical stress on Ohmic contacts was shown to reduce the strain; leading to the inverse/converse piezoelectric effect. A new parametric technique was developed to decouple the mechanisms constituting device degradation in AlGaN/GaN HEMTs under normal device operation, namely self-heating and charge trapping. Both source (IS) and drain (ID) transient currents were used under various biasing conditions to analyse charge trapping behaviour. Two types of charge trapping mechanisms have been identified: (i) bulk trapping occurring on a time scale of &lt;1 ms, followed by (ii) surface trapping and redistribution &gt;1 ms. Through monitoring the difference between I_S and I_D, bulk trapping time constant is shown to be independent of V_DS and V_GS. Also, V_GS is found to have no effect on the bulk trap density. Surface trapping is found to have a much greater impact on slow degradation when compared to self-heating and bulk trapping. At a short time scale (&lt;1ms), the RF performance is restricted by both bulk trapping and self-heating effects. At a longer time scale (&gt;1ms), the dynamic ON resistance degradation is limited mainly by surface trapping accumulation and redistribution. Using the understanding of the degradation mechanism behaviour and origins, optimisations to the Ohmic and Schottky contacts as well as a new AlGaN/GaN HEMT architecture were proposed. In an attempt to improve the thermal management of S/D contacts, an Ohmic contact recess process is proposed to reduce the access resistance and enhance DC/RF performance of AlGaN/GaN HEMTs with a high Al concentration. A contact resistance (RC) of ~0.3 Ω.mm was achieved via optimal recess conditions. Small RC was found to lead to a higher current density at the inner edges of the contact, which resulted in a large increase of channel temperature beneath the S/D contacts. A highly n-doped AlGaN overgrowth layer was proposed to reduce the current density, and thus channel temperature at the Ohmic contacts. Titanium Nitride (TiN) Schottky processing was implemented to minimise the observed strain reduction beneath the gate metal. The optimal Schottky contact is obtained for TiN thicknesses of &lt; 10 nm, which preserves the strain within the AlGaN barrier layer. As a result, Schottky barrier of 1.06 eV, a leakage current of 6 nA and improved linearity of 1.6 was achieved. In addition, C – V and I – V characterisations revealed very low trapping density within the optimised device. Lastly, a new device architecture was proposed to increase the 2-dimentional electron gas (2DEG) density and mobility, without compromising the enhancements of our proposed S/D and gate optimisations. This structure consists of (i) step-graded AlGaN barrier layer to increase strain and (ii) implementing AlN as the interfacial spacer layer.