Electron spin relaxation times in excess of the localized limit have been measured in MBE n-GaAs layers, with the times depending on the doping concentration. We have optically oriented the electrons in the samples, and measured spin lifetimes via luminescence depolarization in a transverse magnetic field (Hanle effect). The lifetimes thus obtained were 14 and 26 ns for samples nominally doped at 1 Γ 10 15 and 3 Γ 10 15 cm --3 , respectively. The dominant dephasing mechanism, which is the hyperfine interaction of localized electrons with lattice nuclei, is discussed. Our results are presented in the context of our larger goal, which is to use resonance techniques for spin measurements and control. In this context, the Hanle spin lifetime measurement is a necessary step to be followed by optically detected magnetic resonance in a longitudinal magnetic field.
Electronic spin in semiconductors is a well-known candidate for quantum computation [1]. In this work, we focus on low temperature spin lifetime measurements in n-type GaAs, with electron localization at donor sites being analogous to the electron localization present in quantum dots.
Long spin lifetimes have been reported for electrons in n-type GaAs. Dzhioev et al.
[2] reported a Hanle effect spin lifetime measurement of 42 ns in a 35 mm GaAs layer grown by liquid phase epitaxy, doped with n ΒΌ 10 15 cm --3 . Kikkawa and Awschalom [3] used time-resolved Faraday rotation to measure a spin lifetime of 130 ns in a bulk GaAs crystal (mechanically thinned to 50 mm), doped with n ΒΌ 10 16 cm --3 .
Our objective is to measure and manipulate electron spins in n-GaAs. We orient the electronic spin optically; that is, we use polarized light to inject spin-polarized electrons. Comparing optical orientation of electrons in semiconductors to optical orientation of atoms in a gas, one may say that working with the spin of the electrons in n-type semiconductors is analogous to optically pumping the gas atoms in the ground state [4], whereas orienting the spin of minority electrons in p-type semiconductors is analogous to the orientation of excited atoms in a gas. Thus, in n-type semiconductors the doped electrons provide a means for extending the spin lifetime well beyond the radiative recombination time.
The main focus of this paper is the results of optical orientation and optical detection of spins. Spin lifetimes of 26 and 14 ns were observed. Spin manipulation through microwave techniques is also being explored, and some comments on those efforts will be given at the end of the paper.
We report measurements on GaAs samples grown by molecular beam epitaxy (MBE) at the Naval Research Laboratory in Washington, D.C. The sample epilayer structure is as follows: (starting with a semi-insulating GaAs substrate) a 100 A GaAs layer, a 100 A