SEM observation and analysis of InGaN/GaN multiple quantum well structure using obliquely polished sample.

The InGaN/GaN multiple quantum well structures on sapphire substrates were observed by scanning electron microscope (SEM) using obliquely polished samples. From the contrast change across the p-n junction, piezoelectric fields were deduced. The direction of the piezoelectric field was consistent with the theoretical prediction, but the strength was smaller. Strain-stress calculation showed that strain relief along the surface is not significant; therefore, it is not responsible for the smaller value of the obtained field strength. As one cause, carrier distribution in the wells under the steady-state condition was pointed out. Potential profile along the surface was different for the samples polished from the surface and from the substrate. This was confirmed by the SEM contrast profile.

Observation of the potential distribution in GaN-based devices by a scanning electron microscope.

Mapping of the potential distribution using a scanning electron microscope (SEM) has been reported in recent years [1,2] for semiconductors such as Si, GaAs and InP. But, there are no such studies on GaN-based devices, to our knowledge. In this study, we observed two types of GaN-based devices by SEM to see if there is a condition that the contrast matches the potential distribution of the devices. The first device we studied was GaN p-n junction (p, n ∼5 × 10(17) cm(-3)). The device was cut, and polished from the cross-section to a flat surface. The cross-section was observed by SEM. Fig. 1(a) shows an SEM image taken at 3 kV. The p-region appears bright and the n-region appears dark. The image intensity changes at the position of p-n junction, for which we used electron beam induced current (EBIC) technique to determine the p-n junction position. Fig. 1(b) is a line profile across the p-n junction (broken line) of the SEM image together with a calculated potential distribution (solid line) using p and n concentrations. It can be seen that the contrast profile matches the potential distribution very well. The SEM observations were carried out for several accelerating voltages. But, best result was obtained at 3 kV. For lower accelerating voltages, the image seemed to reflect the surface potential. On the other hand, higher accelerating voltages resulted in blurred images. The second sample was a light emitting diode structure based on AlN where a multiple quantum well (MQW) structure was sandwiched by p- and n-AlGaN materials. In this case, the sample was obliquely polished from the surface (∼10°) to improve the lateral resolution. The SEM image could reveal the structure of MQW.jmicro;63/suppl_1/i22/DFU051F1F1DFU051F1Fig. 1.(a) SEM image of p-n GaN. (b) Comparison of line profile across the p-n junction (broken line) and a calculated potential distribution (solid line). AcknowledgementWe thank professor H. Amano (Nagoya University) for providing the samples.