Laser spectroscopy of semiconductor quantum wells Optical pumping and optically detected nuclear magnetic resonance

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Spin relaxation in optically excited quantum wells

Interband excitation of GaAs with circularly polarised light produces a spin-polarised population of photoexcited carriers. The equilibrium degree of spin polarisation under cw excitation depends on the balance of spin relaxation and recombination rates, and can be monitored directly through the degree of polarisation of recombination radiation. We have measured the polarisation at 2 K of recombination radiation following band edge excitation for a variety of GaAs/AlGaAs quantum well samples of carrier content ranging from ≈ 108 cm -2 up to ≈ 2×1011 cm-2 in which values of recombination time have been measured, and have thereby determined the spin relaxation times. Two remarkable results emerge; (i) the spin relaxation time increases from 30 ps to > 8 ns as the carrier concentration is increased; and (ii) for degenerate n-type quantum wells the recombination radiation is close to 100% circularly polarised, there being strong evidence for a long spin memory in the electron Fermi sea. The first result may be explained in terms of a spin relaxation mechanism involving electron-hole exchange; the second has, at present, no clear theoretical interpretation

Optically detected nuclear magnetic resonance of spatially selected lattice nuclei within a GaAs quantum well

The spectra of nuclear resonances provide a powerful microscopic probe of condensed matter, yet nuclear magnetic resonance (NMR) techniques have not so far been used to study semiconductor heterostructures. That is because only a tiny fraction of all the nuclei lie In the region of Interest, and conventional NMR cannot distinguish these. We report the first observation of optically detected NMR (ODNMR) of spatially selected nuclei In a semiconductor quantum well using an optical pumping technique which has the sensitivity and selectivity to overcome the limitations of conventional NMR. The sample is a 200 Angstrom thick single quantum well made p-type by applied electrical bias with a degenerate hole population of 1011 cm-2. The spectrum contains resonances corresponding to the three isotopes which make up the lattice: 69Ga, 71Ga, and 75As. It differs strikingly from that reported for bulk GaAs in that there is a 60(7)-kHz quadrupole splitting of the 75As resonance, shown In Fig. 1, which we attribute to the residual strain present in the GaAs layer

Optically detected nuclear magnetic resonance of nuclei within a quantum well

Observations are reported of nuclear magnetic resonance (NMR) for nuclei in single GaAs/AlGaAs quantum wells, using an optical pumping and detection scheme. The technique gives signals only from nuclei in contact with the quantum-confined electron wavefunction and therefore discriminates against substrate and barrier nuclei. Some general factors which affect the strength of signals are discussed and preliminary results are presented which show the potential of NMR for measuring strain in pseudomorphic systems and for detecting and identifying impurities

Optically detected nuclear magnetic resonance in semiconductor quantum wells

We demonstrate use of a spatially selective optical pumping technique to investigate NMR in GaAs quantum wells. The method has previously been used to study bulk semiconductors but not lower dimensional systems. For this its sensitivity (<1011 spins) and optical selectively offer unique advantages which allow discrimination of quantum well nuclei from those in the substrate and buffer layers. Circularly polarised interband optical excitation and recombination are used to generate and monitor spin-polarised conduction electrons. The nuclear spins of the lattice are strongly polarised by contact hyperfine interaction (AI.S) with electrons, and NMR is detected as a dip in circular polarisation of the electron recombination radiation at resonance. The technique has a variety of possible applications including study of confined electron distributions, measurement of strain in stained layer systems via quadrupole splitting of NMR, and detection and identification of impurities

Magnetic g-factor of electrons in GaAs/AlGaAs quantum wells

The magnitude and sign of the effective magnetic splitting factor g* for conduction electrons in GaAs/AlxGa(1-x)As quantum wells have been determined as a function of well width down to 5 nm. The experimental method is based on combined measurements of the decay time of photoluminescence and of the suppression of its circular polarization under polarized optical pumping in a magnetic field perpendicular to the growth axis (Hanle effect). Measurements as a function of hole sheet density in the wells reveal a transition from excitonic behavior with very small apparent g value for low density, to larger absolute values characteristic of free electrons at higher densities. For 20-nm wells g* for electrons is close to the bulk value (-0.44), and increases for narrower wells passing through zero for well width close to 5.5 nm. A theoretical analysis based on three-band k.p theory, including allowance for conduction-band nonparabolicity and for wave-function penetration into the barriers, gives a reasonable representation of the data, leading to the conclusion that g* in quantum wells has a value close to that of electrons in the bulk at the confinement energy above the band minimum