M. Bertolini

Light and Electric Field Control of Ferromagnetism in Magnetic Quantum Structures

A strong influence of illumination and electric bias on the Curie temperature and saturation value of the magnetization is demonstrated for semiconductor structures containing a modulation-doped p-type Cd(0.96)Mn(0.04)Te quantum well placed in various built-in electric fields. It is shown that both light beam and bias voltage generate an isothermal and reversible crossover between the paramagnetic and ferromagnetic phases, in the way that is predetermined by the structure design. The observed behavior is in quantitative agreement with the expectations for systems, in which ferromagnetic interactions are mediated by the weakly disordered two-dimensional hole liquid.

p-type doping of II–VI heterostructures from surface states: Application to ferromagnetic Cd1−xMnxTe quantum wells

We present a study of p-type doping of CdTe and Cd1−xMnxTe quantum wells from surface states. We show that this method is as efficient as usual modulation doping with nitrogen acceptors, and leads to hole densities exceeding 2×1011 cm−2. Surface doping was applied to obtain samples with Cd1−xMnxTe quantum well with up to x=9.3% containing hole gas. We could also increase the growth temperature up to 280 °C, which results in sharper photoluminescence lines, when compared to the similar nitrogen doped samples. Carrier-induced ferromagnetism was observed in surface doped samples.

Light and electric-field control of ferromagnetism in Cd1-xMnxTe based quantum wells

Light and electric field control of ferromagnetism in Cd 0.96 Mn 0.04 Te quantum wells is demonstrated. Ferromagnetic transition is observed at a constant temperature as a function of sheet concentration of 2D free carrier gas in the quantum well. The carrier concentration is controlled either by light with photon energy above the band gap of the barriers or by applying a voltage in a p-i-n diode structure.

Microphotoluminescence study of p-type (Cd,Mn)Te quantum wells

The carrier density and the spin density were measured locally in p-doped quantum wells made of the diluted magnetic semiconductor (Cd,Mn)Te. Both densities were derived from microphotoluminescence maps recorded under a magnetic field. The presence of the hole gas was achieved either by nitrogen doping, or by employing surface acceptor states. The authors found that the correlation length of the carrier density fluctuations is larger (3μm) for surface doping than for nitrogen doping (<1μm), with no effect of the disorder introduced by the Mn ions. The spin density fluctuates on a smaller scale.

Spectroscopy and Characteristic Energies of a Spin‐Polarized Hole Gas

The giant Zeeman splitting present in a quantum well made of a diluted magnetic semiconductor allows us not only to completely polarize the hole gas it contains, but also to destabilize the charged exciton singlet state (which emits at small spin splitting) in favor of an electron‐hole state involving a majority‐spin hole. At low carrier density, this is the neutral exciton, and at higher carrier density, a weakly correlated electron‐hole pair. By comparing spectra recorder under different conditions, we access different characteristic energies such as the true binding energy of the charged exciton, the spin splitting necessary to fully polarize the hole gas, and the energy of excited states of the hole gas which are involved in optical transitions.