GaN devices for more powerful electronics

The project will focus on developing the next generation of high electron mobility transistors (HEMTs) to be used in power electronics. Power semiconductor devices are used in many different sectors such as automotive applications, military (radar), commercial and high-performance space electronics (base-station transmitters). Gallium nitride is a material that is of interest and is being researched intensively for high electron mobility transistors. What makes GaN appealing is its wide band gap of 3.4eV which means devices can operate at higher temperatures and voltages which in turn makes power electronic modules more efficient. Being able to withstand high temperatures means that the overall cost and size of equipment can be reduced due to the fact that additional components to prevent overheating are not required. The development of these wide band gap semiconductor HEMTs is crucial to reduce power consumption due to their superior efficiency compared to traditionally used silicon devices.
Despite all of these strengths a layer of GaOx on the GaN surface, from reaction with oxygen, can seriously detriment a device. This is due to surface defects caused by the GaOx layer which cause leakage current and current collapse. To mitigate this issue and enhance device performance a gate dielectric is placed on the HEMT in order to minimise gate leakage current and to reduce current collapse. These structures are called metal insulated semiconductor HEMTs (MIS-HEMTs). Recently high-k dielectrics such as ZrO2, HfO2 , La2O3 and Al2O3 have been of interest as gate dielectrics and all entail trade-offs. Alumina has been extensively investigated due to its large band gap (6.5eV) [1] and large breakdown electric field (5-10MV/cm) [2] but has a downfall of a small dielectric (~9) [3] constant but recently doping of alumina has been investigated to increase the dielectric constant. [4]
The novelty of this project will be to tailor the doping of alumina to achieve nanolaminate dielectric of sufficiently high permittivity, band offsets and low leakage current on GaN, so it can be used for threshold voltage control in e-mode MIS-HEMTs. The gate dielectrics will be deposited using atomic layer deposition (ALD). X-ray photoelectron spectroscopy (XPS) and inverse photoemission spectroscopy (IPES) will be used for band alignment characterization, XPS for interface quality by looking for oxides or intermixing between layers, spectroscopic ellipsometry for optical constants, band gap measurements (if they agree with XPS and IPES) and calculate the thickness of layers. CV measurements will be used to find the dielectric constant of deposited dielectric layers and any hysteresis caused by electron trapping and IV measurement to measure the electrical characteristics of the devices.

[1] Bersch E, Rangan S, Bartynski R, Garfunkel E, Vescovo E. Band offsets of ultrathin high-k oxide films with Si. Physical Review B. 2008;78(8).
[2] Lin H, Ye P, Wilk G. Leakage current and breakdown electric-field studies on ultrathin atomic-layer-deposited Al2O3 on GaAs. Applied Physics Letters. 2005;87(18):182904.
[3] Chang Y, Huang M, Chang Y, Lee Y, Chiu H, Kwo J et al. Atomic-layer-deposited Al2O3 and HfO2 on GaN: A comparative study on interfaces and electrical characteristics. Microelectronic Engineering. 2011;88(7):1207-1210.
[4] Partida-Manzanera, T., Roberts, J., Bhat, T., Zhang, Z., Tan, H., Dolmanan, S., Sedghi, N., Tripathy, S. and Potter, R. (2016). Comparative analysis of the effects of tantalum doping and annealing on atomic layer deposited (Ta2O5)x(Al2O3)1-x as potential gate dielectrics for GaN/AlxGa1-xN/GaN high electron mobility transistors. Journal of Applied Physics, 119(2), p.025303.