MS-8-P-5804 Electroluminescence imaging of defects in GaN HEMT structures

This contribution deals with the detection and analysis of electroluminescence (EL) emitted by depletion mode (normally-on) InAlN/GaN high electron mobility transistors (HEMTs) at room temperature and high drain-source voltages. Contacting of examined transistor structures was performed under an optical microscope equipped with color CCD camera and thermoelectric cooling system; for further details about the developed low-light imaging setup see [1]. It has been observed that EL signal comes predominantly from the area along the gate fingers, which is consistent with observations made by other groups [2], [3]. EL maps captured at gate-source voltage V_GS = 0 V and at drain-source voltages V_DS in the range from 20 to 30 V revealed inhomogeneities in intensity of emitted EL signal (Fig. 1). This is clearly evident for V_DS below 25 V and particularly for right gate electrode, where the area with weaker luminescence is marked in Fig. 1f by dotted line. Such EL quenching is directly linked to that part of transistor, where the intensity of electric field is suppressed due to the local electric breakdown or due to leakage currents in the vicinity of the gate terminal. It has been found, that some electroluminescence arises also from the area near the edge of the drain contact pad (marked by arrow in Fig. 1f). In this region, a large amount of EL signal intensity varies significantly over time, which is most likely caused by local charging and consequent discharging of the traps represented by metastable states in the energy gap of the semiconductor. Such time-instability of EL intensity has been observed also in the vicinity of the gate fingers, which is easily noticeable in Fig. 1e in the form of a bright spot highlighted by arrow; the rest of captured maps do not show any evidence of increased luminescence in this area. Similar results have been observed also by mapping of currents induced by electron beam (EBIC) in scanning electron microscope [4]. Obtained results will be discussed in context of the quality assessment of such advanced semiconductor devices.

References
[1] Priesol, J. et al, In Proceedings of ADEPT, 2^nd International Conference on Advances in Electronic and Photonic Technologies, June 1-4, 2014, Tatranská Lomnica, Slovakia; accepted, in print
[2] Lossy, R. et al, Phys. Stat. Sol. C 6, No. 6, 1382–1385 (2009)
[3] Meneghini, M. et al, IEEE Trans. Device Mater. Rel. 13 (2), 357–361 (2013)
[4] Kováč, J. et al, Microelectronics Reliability 52 (7), 1323–1327 (2012)