Mapping of strain and electric fields inGaAs/Alx

Abstract

The usefulness of semiconductor heterostructures derives from the possibility to engineer their electronic and optical properties to match the requirements of many different applications. Optically detected nuclear magnetic resonance provides the possibility to map microscopic properties of such samples with a high spatial resolution through the splitting of resonance lines. In a multiple quantum-well sample, we measure the distortion of the crystal lattice and find variations of the order of 10 Ϫ5 over distances of a few mm. Internal electric fields also cause resonance line splittings. Comparing the electric field-induced resonance line splittings in different quantum wells, we mapped the vertical variation of the electric field from a Schottky contact with a spatial resolution of some 40 nm.

Pulsed optically detected NMR of single GaAs/AlGaAs quantum wells

“…While bulk GaAs does not show quadrupole splittings, they are common in heterostructures, where mechanical strain and electric fields lift the equality of the three Dm I nuclear spin-flip transition energies [10]. Apart from the splitting, the satellites show inhomogeneous broadening; the width of the satellite lines is 4 kHz, compared to 2 kHz for the central transition, which is not affected by the quadrupole coupling in first order.…”

“…The NMR spectrum is then recorded by saturating one nuclear spin species with a cw radiofrequency (rf) field, either by sweeping the frequency or the strength of the external magnetic field. This approach has made possible, e.g., measurements of the Knight-shift [8] and quadrupole splittings [9,10]. Such optically detected NMR (ODNMR) experiments provide sufficient sensitivity to measure spectra of individual quantum wells [11][12][13].…”

Light-induced Knight shifts inGaAs∕AlxGa1−xAsquantum wells

“…Figure 1 shows a typical spectrum. The splitting is due to quadrupole interaction of the I =3/2 nuclear spin, which would be absent in an ideal GaAs crystal 23 and indicates a distortion of the quantum-well sample. 24 Since the Knight shifts are relatively small, we measured them in a two-dimensional NMR experiment, using different electron-spin densities during the evolution and detection periods.…”

An optical NMR spectrometer for Larmor-beat detection and high-resolution POWER NMR

“…Exemplary results have detailed single-carrier distributions of electron spin and charge, 9 local electric fields that give rise to band bending at material interfaces, 9 fewelectron cooperative behavior that minimizes spin and Coulombic energies in a two-dimensional electron gas, 10 and lateral distributions of strain in GaAs quantum wells. 11 The key to ONMR is the coupling of photoexcited electrons to lattice nuclei via the contact hyperfine interaction. In III-V materials, circularly polarized light excites spinpolarized electrons whose relaxation induces fluctuations in the contact hyperfine interaction, thus transferring spin order to lattice nuclei.…”