A solid-state NMR and DFT study of compositional modulations in Al(x)Ga(1-x)As

Area of Science:

• Materials Science
• Solid-State Physics
• Spectroscopy

Background:

• Aluminum Gallium Arsenide (AlGaAs) is a crucial semiconductor alloy with tunable electronic and optical properties.
• Understanding the atomic-scale order and disorder in AlGaAs is essential for controlling its material properties and device performance.
• Nuclear Magnetic Resonance (NMR) spectroscopy is a powerful technique for probing local atomic environments in solid materials.

Purpose of the Study:

• To investigate the degree of occupational order and disorder in Al(x)Ga(1-x)As lift-off films.
• To identify and characterize the different arsenic (As) atomic sites based on their Aluminum (Al) and Gallium (Ga) nearest neighbors.
• To correlate experimental NMR data with theoretical calculations to understand the distribution of Electric Field Gradients (EFGs).

Main Methods:

• Performed (75)As and (69)Ga Nuclear Magnetic Resonance (NMR) experiments, including spin-echo and 3QMAS techniques, at high magnetic fields (18.8 Tesla).
• Utilized high radiofrequency (rf) field strengths and a specialized NMR probe for enhanced spectral resolution.
• Employed spectral deconvolution with an evolutionary algorithm and Density Functional Theory (DFT) calculations for data analysis and interpretation.

Main Results:

• Successfully identified all possible As(Al(n)Ga(4-n)) sites (n=0-4) in (75)As NMR spectra, indicating a lack of long-range order.
• Observed a distribution of Electric Field Gradients (EFGs) at arsenic sites, reflecting occupational disorder in the Al/Ga sub-lattice.
• Experimental quadrupolar interactions showed excellent agreement with DFT calculations, which accurately modeled EFG distributions.

Conclusions:

• The occupational disorder in AlGaAs films significantly influences the local atomic environments and is detectable via NMR.
• (75)As quadrupolar interactions are highly sensitive to compositional variations, extending up to the 7th coordination shell.
• The combination of advanced NMR techniques and DFT calculations provides a robust method for characterizing disorder in semiconductor alloys.