Site-Selective Electron Nuclear Double Resonance in the Exchange- Narrowed Regime of the ESR in Conducting Solids G. Denninger,* ,1 K. Baldenhofer,* R. Geßler,* L. Binet,² and D. Gourier² *2. Physikalisches Institut, Universita ¨t Stuttgart, Pfaffenwaldring 57, D-70550 Stuttgart, Germany; and ² Ecole Nationale Supe ´rieure de Chimie de Paris, Laboratoire de Chimie Applique ´e de l’Etat Solide, UMR CNRS 7574, 11 rue Pierre et Marie Curie, 75231 Paris cedex 05, France Received March 14, 2000; revised August 2, 2000 Electron nuclear magnetic double resonance on conduction elec- trons reveals the hyperfine interaction hidden by the fast electron spin exchange. We used the Overhauser shift technique to inves- tigate the electron spin density of the conduction band of gallium oxide, -Ga 2 O 3 . Due to the monoclinic structure, the conduction band of -Ga 2 O 3 is anisotropic and it is dominated by contribu- tions from the two nonequivalent Ga sites. The large quadrupole couplings of the two gallium isotopes 69 Ga and 71 Ga (both with I  3/2) are completely resolved in ourdouble-resonance experiments. This resolved quadrupole interaction allows the determination of the electric field gradients at both gallium sites with high precision and high sensitivity. The resolved quadrupole splitting is the key to the site-selected determination of the hyperfine interaction. The concepts behind these double-resonance techniques are rather gen- eral and should be applicable in similar semiconductor systems. © 2001 Academic Press Key Words: ESR/NMR double resonance; conduction electron spin resonance; hyperfine interaction; electric field gradient deter- mination; site-selective spin density determination. INTRODUCTION Magnetic Resonance in Semiconductors Recent developments in semiconductor physics have brought up several families of materials such as SiC (1), GaN and related alloys (2), transparent oxide conductors (TCO) such as ZnO (3), and more recently TCOs with rutile type chains (edge-sharing octahedra chains) such as spinels (4) or -Ga 2 O 3 (5, 6). A better understanding of the relation between the electronic and optical properties of these compounds and their structure requires probing their electronic structure and the atomic environments, especially in the case of TCOs. For these compounds, which contain both unpaired electron spins and nonzero nuclear spins, nuclear magnetic resonance (NMR) and electron spin resonance (ESR) should be natural tech- niques for such structural investigations. NMR is a high- resolution technique providing geometrical information about atomic sites through the analysis of the electric field gradients (EFGs) but suffers from a low sensitivity. ESR has a much higher sensitivity and detects the conduction electrons, which are the active components of semiconductors. However, the drawback is a low resolution and the hyperfine interaction providing information about the electronic structure and the atomic environments can rarely be analyzed. Electron nuclear double resonance (ENDOR) overcomes these shortcomings. Originally devised by Feher (7), ENDOR consists in detecting nuclear spin transitions through a change in the ESR absorption of the electrons interacting with the observed nuclei. This spectroscopy combines the advantages of the high resolution of NMR and the high sensitivity of ESR, along with a great selectivity since only nuclei interacting with unpaired electron spins are detected (8). A review of the structural analysis of point defects in solids by ENDOR spectroscopy is given in Ref. (9). As long as the correlation time of an unpaired electron at the nuclear positions is sufficiently long, ENDOR spectroscopy is a powerful technique for determining the hyperfine interactions and the spin density distributions in paramagnetic systems. The spin–spin correlation time  c has to be long compared to the inverse hyperfine frequency,  c  h/ A i , where A i denotes the hyperfine coupling energy of the nucleus i . This condition is usually fulfilled for localized electronic spins and for large hyperfine couplings A i . If on the other hand the electronic spin density is distributed over many nuclei, e.g., for a shallow donor electron in semiconductors, the hyperfine couplings A i become small. By increasing the density n e of these extended paramagnetic electron states, the exchange interaction of the electronic spins will dominate the paramagnetic properties and the electronic spins will be delocalized. As a consequence, the correlation time  c will become fairly short ( c  10 -13 s for exchange interactions in the meV range) and the condition  c  h/ A i prevails even for large coupling A i . The ESR line is exchange narrowed with no remaining traces of the hyperfine splitting and ENDOR in the normal way is no longer possible. In fact, the ENDOR lines would be broadened in frequency to something like f  1/  c  10 4 GHz and are unobservable. The hyperfine interaction I i  A i  S between the nuclear spin 1 To whom correspondence should be addressed. E-mail: g.denninger@physik.uni-stuttgart.de. Fax: 0049-711-685-5285. Journal of Magnetic Resonance 148, 248 –256 (2001) doi:10.1006/jmre.2000.2232, available online at http://www.idealibrary.com on 248 1090-7807/01 $35.00 Copyright © 2001 by Academic Press All rights of reproduction in any form reserved.