IT-11-P-3140 Electron Beam induced Currents in GaN p-n Junctions

Off-axis electron holography (EH) is a unique approach for analyzing potential variation down to nanometer scale. In absence of magnetic fields and in kinematical diffraction condition, the phase modulation is proportional to the potential (sum of mean inner potential and built-in potential V_pn) and the specimen thickness. However, quantitative measurement of V_pn in GaN shows a huge deviation with the theoretical value. Additionally, it seems that the hypothetical “dead layer” on both surfaces affected by focused ion beam (FIB) preparation is not the only reason for the discrepancy since the measured V_pn from the bulk (n-GaN)/shell (p-GaN) geometry of the GaN p-n junction is not affected by the FIB preparation [1].
In this study, we assess the influence of the electron beam on the measured V_pn at GaN p-n junctions. Two GaN p-n junctions of comparable dopant concentration were prepared by FIB. Thereby, a needle-like shape geometry is deliberately chosen to ensure that the whole specimen is illuminated during the experiment [2]. Moreover, a defined path of the induced current arising from electron-hole pair generation and secondary electron emission is provided. Low voltage (5 kV) is applied for the final polishing step to mitigate the surface damaging by FIB.
Fig. 1 shows the FIB milled needle with a sketch of the layer structure. In order to enhance the signal to noise ratio of the phase, series of holograms separately for each illumination intensity were recorded and subsequently averaged. The reconstructed phase depicts a clear phase jump at the p-n junction (Fig. 1 (c)).
Two electron beam induced effects are observed. Firstly, in Fig. 2 the measured V_pn increases with reducing the electron dose rate. This illumination dependency of V_pn can be quantitatively explained by a solar-cell model. Hence, V_pn = V_{pn, expected} – V_bias is fitted on the data set, where V_{pn, expected} is determined by the doping concentration (sample A: 3.41 V, sample B: 3.43 V), V_bias is the voltage drop across the p-n junction, which is affected by illumination induced currents owing to secondary electron emission and e-h pair generation [3]. Additionally, fluctuation of V_pn is depicted by two needles, which are prepared from a same wafer and with the same FIB preparation steps. However, both needles (I and II) show the same behaviour of V_pn over the electron dose rate. Secondly, beam damage of the specimen is observed in Fig. 3. The V_pn diminishes over a period of time and might converge to a constant value, which is about the half of the initially measured V_pn.

1. S. Yazdi et al., journal of physics: Conf. Ser. 471 (2013), p.012041.

2. A. Lenk, Dissertation, Dresden (2008).

3. S.M. Sze in “Physics of Semiconductor Devices”, 2. Edition, John Wiley & Sons (1981).