Quantification of Mg Dopants in GaN Nanowires

The characterization of doping is crucial for understanding and improving the electrical and optical properties of semiconductors to produce reliable and powerful electronic and optical devices.

While the size of semiconductor structures has steadily decreased with technical development, including high-performance nanowires (Fig. 1), the exact determination of ever lower concentrations of dopants is of great importance for the precise characterization of these materials. This poses a challenge for conventional analytical techniques, as low element contents must be quantified from very small sample quantities.

Here we have investigated single nanowires with two different layers of known composition with unknown thickness on a substrate of indium gallium nitride (Fig. 2b) via SEM EDS and SEM WDS. Applying an accelerating voltage of 10 kV leads to an X-ray excitation depth up to 500 nm in the present material, as suggested by the equation of Anderson & Hasler (Fig. 2a).

Fig. 1: SEM images of semiconducting nanowire structures in side and top view. Since nanowires have unique mechanical, electrical, thermal and optical properties, they are valuable for various applications, e.g. in electronics, photonics, computing, sensing, manufacturing, healthcare, data storage and many more.

Fig. 2a: The diagram shows the relationship between the spatial resolution of the X-rays and the acceleration voltage applied for analysis. Z, the diameter of the excitation volume, is a function of the primary energy and the material composition (Anderson & Hasler, 1966). At 10 kV, the minimum spatial resolution for X-ray microanalysis in GaN is 500 nm, while at 4 kV even smaller objects down to approx. 100 nm can be analyzed.

Fig. 2b: The investigated nanowires consist of two different layers of known composition but unknown thickness on a substrate of indium gallium nitride. The top layer consists of Mg-doped gallium nitride and is at least 50 nm thick. The second, presumably thin layer consists of aluminum gallium nitride. The analytical task is to determine the exact thickness of the layers by varying the kV during the analysis.

In addition to Ga and N, the EDS spectrum shows a peak for Al, which indicates that the second layer was captured by the excitation volume (Fig. 3). Due to the strong overlaps, the EDS spectrum cannot be used to make any statement about possible proportions of In and Mg in the analysis. The WDS spectrum, on the other hand, shows a peak for both Mg and In (Fig. 3) due to the high spectral resolution and thus documents that the substrate of the nanowires is penetrated with the analysis at 10 kV.

When the accelerating voltage is decreased to 4 kV, the X-ray excitation depth is restricted to 100 nm (Fig. 4). In and Al disappear from the spectra, which indicates that the excitation volume is limited to the top layer (Fig. 2b). By slowly increasing the voltage until the elements In and Al reappear in the spectra, the thicknesses of the layers of the nanostructure can be determined. If the excitation volume is limited to the top layer only, it is possible to reliably determine the Mg dopant concentration with a value of 0.08 wt% in GaN.

Fig. 3: Combined EDS and WDS spectra recorded on GaN nanowires at an acceleration voltage of 10 kV. The EDS spectrum clearly shows the peaks for Ga and N as well as a small peak for Al. However, the presence of In and Mg can neither be confirmed nor rejected by the EDS analysis, as there are strong overlaps with the peaks of the main elements. The WDS analysis, on the other hand, can resolve the peaks better and shows a small peak for In Mz next to the N K line and a small peak for Mg Ka on the high-energy side of the Ga L lines.

Fig. 4: This time, the combined EDS and WDS spectra were recorded on the GaN nanowires at an acceleration voltage of only 4 kV. Again, the EDS spectrum clearly shows the peaks for Ga and N, but cannot give any indication of the presence or absence of In and Mg due to the strong overlaps. The WDS spectrum, however, clearly shows the absence of a peak for In and a significant peak for Mg Ka.

Samples & References:

Data and sample curtesy of Dr. Eric Robin (CEA-IRIG, Université Grenoble Alpes, France)

Further Resources:

Webinar: Advanced Materials Microanalysis using QUANTAX WDS with a Grazing Incidence X-ray Optic

QUANTAX WDS

QUANTAX WDS is a parallel-beam wavelength-dispersive X-ray spectrometer that is optimized for the determination of low X-ray energies. The system is fully integrated in the ESPRIT software, allowing simultaneous acquisition and combined quantification with EDS.

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