We report both decrease and increase in the 2D carrier gas density in a simple (Cd,Mn)Te/(Cd,Mg)Te heterostructure with (Cd,Mn)Te quantum well. The two effects were achieved by light with different photon energies. The quantum wells were 10 nm wide with 2D hole gas supplied by,surface states. For the sample with 25 nm cap layer thickness, it was possible to tune the hole gas concentration from almost empty well (hole density below 1 x 10(10) cm(-2)) to 45 x 10(10) cm(-2). The illumination with 425 nm wavelength almost doubled the hole gas density from the initial 24 x 10(10) cm(-2). The depletion mechanism was most effective for illumination with the orange (575 nm) light.

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.

We present a study of static fluctuations in p‐doped (Cd,Mn)Te quantum wells, exploiting the high sensitivity of photoluminescence spectra to local magnetization and carrier density. We find that carrier density fluctuations have a much stronger influence on the magnetically ordered phase than fluctuations of the Mn concentration.

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.

One main interest of carrier‐induced ferromagnetism in diluted magnetic semiconductors, is that it allows an active and sometimes reversible control of the magnetic properties, simply by manipulating the carrier density or carrier distribution using the usual techniques of semiconductor technology. We shall give two examples realised in (Cd,Mn)Te quantum wells: (i) control of the magnitude of the magnetisation by changing the carrier density; (ii) control of the magnetic anisotropy by changing the light‐hole/heavy‐hole splitting by strain and confinement in a quantum well. We will also shortly discuss the peculiarities of the II–VI quantum wells with respect to systems based on other diluted magnetic semiconductors. (© 2004 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Properties of carrier-induced ferromagnetism in modulation-doped quantum wells of p-type (Cd,Mn)Te are studied by photoluminescence and reflectivity in magnetic field. Valence-band engineering can strongly modify the magnetic properties of a system of localized spins interacting via a 2D hole gas. This is obtained through a strain control of the light-hole (lh)/ heavy-hole (hh) splitting. The axis of easy magnetization can be turned from the growth direction (with a hh gas) to be in-plane (lh gas). It is also found that the strong alloy fluctuations in (Cd,Mn)Te quantum wells containing about 10% of Mn ions are strong enough to significantly modify the magnetic state of the coupled ion-carrier system. A similar effect is observed at moderate Mn content, by restoring the lh/hh degeneracy.

Photoluminescence of p-type modulation doped (Cd,Mn)Te quantum wells is studied with carrier density up to 5 X 10(11) cm(-2) at various spin splittings. This splitting can be made larger than the characteristic energies of the system thanks to the giant Zeeman effect. At small spin splitting and regardless of the carrier density, the photoluminescence exhibits a single line, which corresponds to the charged exciton in the singlet state. Above a certain spin splitting, the charged exciton is destabilized in favor of the exciton at vanishing hole density, and in favor of a double line at higher carrier density. It is found here that the charged exciton destabilization energy hardly depends on the carrier density. The double line is found to be band-to-band like, with the same initial state - where the holes have the same spin orientation - and final states that differ by some excitation of the 2D hole gas. In addition, the spin splitting needed to fully polarize the hole gas is twice smaller than expected from the single particle image and gives a unique insight into many-body effects in the hole gas.

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.

Using photoluminescence and photoluminescence excitation, weinvestigate the magnetic properties of (Cd,Mn)Te quantum wells inserted in pin diodes. Such structures give us the opportunity to tune the carrier density by applying a bias of the order of 1 V. Results obtained in a small magnetic field show the presence of magnetic domains, which persist as long as the quantum well is populated.

New structures aiming at controlling ferromagnetic properties of Diluted Magnetic Semiconductors quantum wells are presented. The carrier density is monitored by applying voltage in p-i-n diode or adjusting a distance between quantum well and surface. Surface doping was successfully applied to obtain samples with CdMnTe quantum well with up to 9.3% Mn concentration.

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.

The ability to control the properties of magnetic materials would be highly desirable from fundamental and technological viewpoints, particularly in view of recent developments in magneto-electronics and spintronics. Diluted magnetic semiconductors (D.M.S), where ferromagnetic interactions are mediated through the carriers, are particularly attracting materials since they offer the possibility to modulate the magnetic properties by modulating the carrier density. The first demonstration has been performed with III-V D.M.S layers inserted in a Schottky diode.