Composition and optical properties of dilute-Sb GaN1−xSbx highly mismatched alloys grown by MBE-2
3. Experimental results
The GaSb-incorporation in the samples grown under N-rich conditions was studied using RBS and WDX. Figure 2 shows the Sb profile measured by RBS from the sample with an Sb flux of 3.4 × 10−8 Torr and also the Monte–Carlo simulation of x-ray generation under 8 kV electron beam excitation.
Figure 2. RBS measured depth profile of the GaSb mole fraction for a typical N-rich GaN1 − xSbx sample (solid line) and Monte–Carlo Sb x-ray generation with depth (dashed line).
Due to the WDX surface sensitivity, Monte–Carlo simulations were used to estimate the x-ray generation rate with depth. A weighted average was then performed between the Monte–Carlo x-ray intensity with depth curve, and the measured RBS GaSb depth profile, shown in figure 2. This allowed a weighted average GaSb percentage to be determined for direct comparison of the RBS and WDX results. Figure 3 shows the WDX and RBS measurements of GaSb mole % incorporation with Sb growth flux for N-rich samples. WDX shows the lowest measured GaSb mole% to be (0.27 ± 0.01)% and the highest measurement to be (0.66 ± 0.02)%, assuming a systematic error of 1% of the measured value.
Figure 3. Plot of WDX and RBS GaSb mole percentage against Sb growth flux for N-rich GaN1 − xSbx layers.
An 8 keV, 40 nA electron beam was used to search for room temperature CL from these N-rich GaN1 − xSbx samples. No GaN1 − xSbx related luminescence peaks were observed in the range 330–850 nm.
The GaSb incorporation was found to be much lower in the Ga-rich GaN1 − xSbx samples. Due to the very small amounts of Sb extra care and analysis were required to quantify the GaSb content using WDX. To maximise the signal to noise a 7 kV electron beam, large counting times (240 s for the Sb L peak) and high currents (150 nA) were used. For each sample 10 random points were probed across the surface using a 10 µm defocused electron beam. In some cases the measured Sb x-ray counts were below the measured background for some of the data points and a negative value was then used in the calculation of the average GaSb atomic percentage. The resulting GaSb mole percentages are plotted against Sb flux in figure 4 which shows the lowest non-zero measurement to be (0.004 ± 0.002)% and the highest measured GaSb mole% to be (0.017 ± 0.001)%, assuming a systematic error of 5% of the measured value due to the large composition difference between the standards and the sample. At such low concentrations there may be additional uncertainties due to the correction procedures applied in the analysis of the WDX data.