p-Type doping of II-VI heterostructures from surface states: application to ferromagnetic Cd1−x_{1-x}Mnx_xTe quantum wells

We present a study of p-type doping of CdTe and Cd$_{1-x}$Mn$_x$Te 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 $2 \times 10^{11}$ cm$^{-2}$. Surface doping was successfully applied to obtain carrier-induced ferromagnetism in a Cd$_{1-x}$Mn$_x$Te quantum well. The observed temperature dependence of photoluminescence spectra, and the critical temperature, correspond well to those previously reported for ferromagnetic quantum wells doped with nitrogen.Comment: 4 figure

Femtosecond study of the interplay between excitons, trions, and carriers in (Cd,Mn)Te quantum wells

We present an absorption study of the neutral and positively charged exciton (trion) under the influence of a femtosecond, circularly polarized, resonant pump pulse. Three populations are involved: free holes, excitons, and trions, all exhibiting transient spin polarization. In particular, a polarization of the hole gas is created by the formation of trions. The evolution of these populations is studied, including the spin flip and trion formation processes. The contributions of several mechanisms to intensity changes are evaluated, including phase space filling and spin-dependent screening. We propose a new explanation of the oscillator strength stealing phenomena observed in p-doped quantum wells, based on the screening of neutral excitons by charge carriers. We have also found that binding heavy holes into charged excitons excludes them from the interaction with the rest of the system, so that oscillator strength stealing is partially blockedComment: 4 pages, 4 figure

Neutral and charged excitons in a CdTe-based quantum well

We present a summary of our spectroscopic studies of the oscillator strength of transitions related to the formation of neutral and positively charged excitons in modulation p-doped CdTe-based quantum wells. The hole concentration was controlled in the range from 10¹⁰ cm⁻² to 10¹¹ cm⁻². Continuous-wave and time-resolved femtosecond pump–probe absorption measurements were performed. They allowed us to study the interacting system of excitons, trions, and free holes. Characteristic times of the system were determined, such as the trion formation time. A new explanation of so-called oscillator strength «stealing» has been proposed, in terms of spin-dependent screening. Experimental evidence is presented for optical creation of transient spin polarization in the quantum well

Femtosecond Dynamics of Neutral and Charged Exciton Absorption in Cd1−x\text{}_{1-x}Mnx\text{}_{x}Te Quantum Well

We present a study of time-dependent transmission spectra of a modulation-doped Cd$\text{}_{1-x}$Mn$\text{}_{x}$Te/Cd$\text{}_{1-y-z}$Zn$\text{}_{y}$Mg$\text{}_{z}$Te quantum well with variable hole gas concentration. We study the influence of pump pulses on excitonic absorption in subpicosecond time scale. A spectrally broad probe pulse of duration of 40 femtoseconds was used to record the absorption spectra at controlled delay. Studies of temporal evolution of exciton energies revealed coherence decay of linearly polarized excitons and thermalization of non-equilibrium exciton states. We found that a characteristic timescale for thermalization of non-equilibrium populations of photocreated excitons is between 0.8 and 3.6 ps. The timescale of this process depends on the hole concentration in quantum well: for higher hole concentration the decay is faster. Long-lived photo-induced magnetization accompanied by heating of the magnetic system was also observed

Many-Body Interactions in the CdTe-Based Quantum Well under Strong Optical Excitation

We performed pump-probe time-resolved absorption measurements on a modulation p-doped (Cd,Mn)Te quantum well. The density of the 2D hole gas was controlled, in the 10$\text{}^{10}$ cm$\text{}^{-2}$ range, by additional cw illumination. Interactions between photocreated neutral (X) and charged (X$\text{}^{+}$) excitons and the 2D hole gas were analyzed. We found that the strongest effect is the influence of the carriers on both X and X$\text{}^{+}$ optical resonances. Neutral and charged excitons are screened by holes, which results in the decrease in their intensities. We conclude from polarization resolved experiments that this screening is spin-dependent: it is more efficient between holes with opposite spins. Binding holes into the charged excitons reduces the screening of the neutral excitons and leads to an enhancement of the neutral exciton intensity. We also analyzed weaker effects, due to exciton-exciton interaction, at a constant hole density. We found that the reduction of the neutral exciton intensity due to two different mechanisms (phase-space filling and biexciton formation) is almost equal. We observed a spin-dependent blue shift of the neutral exciton line in the presence of a population of neutral excitons created by the pump pulse. Due to the attractive interaction between excitons with opposite spins, the corresponding shift of the neutral exciton is smaller than that observed due to the interaction between excitons with the same spins

Many-Body Interactions in the CdTe-Based Quantum Well under Strong Optical Excitation

We performed pump-probe time-resolved absorption measurements on a modulation p-doped (Cd,Mn)Te quantum well. The density of the 2D hole gas was controlled, in the 10$\text{}^{10}$ cm$\text{}^{-2}$ range, by additional cw illumination. Interactions between photocreated neutral (X) and charged (X$\text{}^{+}$) excitons and the 2D hole gas were analyzed. We found that the strongest effect is the influence of the carriers on both X and X$\text{}^{+}$ optical resonances. Neutral and charged excitons are screened by holes, which results in the decrease in their intensities. We conclude from polarization resolved experiments that this screening is spin-dependent: it is more efficient between holes with opposite spins. Binding holes into the charged excitons reduces the screening of the neutral excitons and leads to an enhancement of the neutral exciton intensity. We also analyzed weaker effects, due to exciton-exciton interaction, at a constant hole density. We found that the reduction of the neutral exciton intensity due to two different mechanisms (phase-space filling and biexciton formation) is almost equal. We observed a spin-dependent blue shift of the neutral exciton line in the presence of a population of neutral excitons created by the pump pulse. Due to the attractive interaction between excitons with opposite spins, the corresponding shift of the neutral exciton is smaller than that observed due to the interaction between excitons with the same spins

Carrier Density Control by Illumination in Surface Doped, p-Type (Cd,Mn)Te Quantum Wells

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 1×10$\text{}^{10}$ cm$\text{}^{-2}$) to 45×10$\text{}^{10}$ cm$\text{}^{-2}$. The illumination with 425 nm wavelength almost doubled the hole gas density from the initial 24×10$\text{}^{10}$ cm$\text{}^{-2}$. The depletion mechanism was most effective for illumination with the orange (575 nm) light

Optical Properties of 2D Systems

We performed pump-probe time-resolved absorption measurements on a modulation p-doped (Cd,Mn)Te quantum well. The density of the 2D hole gas was controlled, in the 10 10 cm −2 range, by additional cw illumination. Interactions between photocreated neutral (X) and charged (X + ) excitons and the 2D hole gas were analyzed. We found that the strongest effect is the influence of the carriers on both X and X + optical resonances. Neutral and charged excitons are screened by holes, which results in the decrease in their intensities. We conclude from polarization resolved experiments that this screening is spin-dependent: it is more efficient between holes with opposite spins. Binding holes into the charged excitons reduces the screening of the neutral excitons and leads to an enhancement of the neutral exciton intensity. We also analyzed weaker effects, due to exciton-exciton interaction, at a constant hole density. We found that the reduction of the neutral exciton intensity due to two different mechanisms (phase-space filling and biexciton formation) is almost equal. We observed a spin-dependent blue shift of the neutral exciton line in the presence of a population of neutral excitons created by the pump pulse. Due to the attractive interaction between excitons with opposite spins, the corresponding shift of the neutral exciton is smaller than that observed due to the interaction between excitons with the same spins

Control of Ferromagnetism in Cd1−x\text{}_{1-x}Mnx\text{}_{x}Te Based Quantum Wells

New structures aiming at controlling the ferromagnetic properties of diluted magnetic semiconductor quantum wells are presented. The carrier density is controlled by applying a voltage across a p-i-n diode. A new method, creating a 2D hole gas by adjusting the distance between the quantum well and surface, offers opportunities for a broader range of structures

Neutral and charged excitons in a CdTe-based quantum well

We present a summary of our spectroscopic studies of the oscillator strength of transitions related to the formation of neutral and positively charged excitons in modulation p-doped CdTe-based quantum wells. The hole concentration was controlled in the range from 10¹⁰ cm⁻² to 10¹¹ cm⁻². Continuous-wave and time-resolved femtosecond pump–probe absorption measurements were performed. They allowed us to study the interacting system of excitons, trions, and free holes. Characteristic times of the system were determined, such as the trion formation time. A new explanation of so-called oscillator strength «stealing» has been proposed, in terms of spin-dependent screening. Experimental evidence is presented for optical creation of transient spin polarization in the quantum well