Optically-pumped ⁷⁵As NMR Reveals an Electric Field Gradient at an Al₂O₃-GaAs Interface and Very Low Nuclear Spin Temperatures

We have investigated the interface between bulk GaAs and an 11nm thick layer of atomic layer deposition (ALD) Al₂O₃. Comparing GaAs with a native oxide and that with alumina results in different spectra. Using optical pumping of conduction electrons, nuclear spins enhancements localized near the interface is achieved by tuning the laser to bandedge states that result in a shallow penetration depth.

The ⁷⁵As OPNMR spectra that are recorded in this regime show evidence of unusual quadrupolar splitting for GaAs. The splitting arises in part from biaxial strain, introduced by the different thermal expansion coefficients of the two materials (OPNMR is conducted at temperatures of 6 -10 K). However, strain alone does not account for the quadrupolar satellite shapes. We will discuss contributions to the electric field gradient at the interface that lead to the quadrupolar satellites.

In addition, the satellites themselves are asymmetric and reveal that these depend strongly on the photon energy used for optical pumping. Our models of spin temperature suggest a surprising cooling of the spin system to between 1.5 to 3 mK.