In this decade, light-emitting diode (LED) has become a highly developing industry both in Taiwan and mainland China. The world market share of Taiwan-made component semiconductor epitaxy wafers is about 70%. Among them, GaN/InGaN grown on sapphire wafers for white LED application are dominant.
The follow figure is the TEM image of the GaN/InGaN multiple quantum well (MQW) structure. Its thickness normally falls to 2-3 nm/6-7 nm. At a specific crystallographic axis (Fig.b), optimal contrast can be obtained. For a low magnification (Fig.a), diffraction contrast seems to be good for the thickness measurement. By contrast, the thickness measurement cannot be easily obtained from the phase contrast in the atom-resolved image (Fig.c)
MQW structure of the white LED
(a) Low magnification TEM image (b) Diffraction pattern along [11-20] axis. |
(c) High-resolution electron microscope image |
Because the thickness of the LED MQW is 2-3 nm, composition identification cannot be obtained with the traditional SEM/EDX. For TEM/EDX, however, the probe size can be focused down to 1 nm, so it will be much easier for the composition identification for such thin films.
After 2010, EDX detection efficiency has been dramatically improved in three aspects：
- Better emission efficiency of the FE-gun
- Bigger EDX detection area, 100 mm2
- Four detectors around the TEM.
Take MQW and superlattice (SL) as an example, The figure below exhibits the results taken by the new system. The fig.a is the TEM/STEM image. Fig.b exhibits the composition analysis of the MQW, where the main compositions, Ga and N, have a stoichiometric ratio of nearly 50 to 50. The minor compositions, Al and In, the content for both elements is close to 1-2 %, which is the lowest detection limit for the traditional EDX. In Fig.c, the content of In has been further determined at SL. In this case, smaller than 0.5 % of In is detected, which is comparable with the AES.
TEM/EDX analysis of white LED MQW and SL
(a) TEM/STEM image (b) Content ratio of Al, In, Ga, and N at MQW (c) Content ratio of In, Ga, and N at SL. |
In the past, the detection limit of TEM/EDX cannot compare with that of SIMS. With progression of instrument and data processing development, two different technologies now have a nice correlation in LED component analysis. We use them on MQW analysis, as shown in figures below, where the variation of Al and In contents is highly coherent.
The resolution on periodic MQW is nearly the same. For Al determination at deeper layers, the thickness measurement by TEM/EDX cross-section is even more accurate than that by SIMS depth profile due to the sidewall effect near craters.
Elementary distribution by different technologies
(a) TEM/EDX line scan (b) SIMS depth profile |
Data after PC software output comparison between TEM/EDX and SIMS, where highly correlation is found between them. (a) TEM/EDX line scan and SIMS depth profile for Al. (b) High magnification bright field TEM image. (c) TEM/EDX line scan and SIMS depth profile for In. Highly correlation is observed between two technologies.
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