We present an optical (reflectance and photoluminescence) study of metastable excitons in ${\mathrm{Z}\mathrm{n}\mathrm{S}\mathrm{e}/\mathrm{Z}\mathrm{n}}_{1\ensuremath{-}x}{\mathrm{Fe}}_{x}\mathrm{Se}$ quantum-well structures. These states are associated with the type-II $(\ensuremath{-}\frac{1}{2},\ensuremath{-}\frac{3}{2})$ ground-state exciton of the system. The metastable excitons are formed because the Coulomb attraction of the ${m}_{j}=\ensuremath{-}$$\frac{1}{2}$ electron tends to localize the ${m}_{j}=\ensuremath{-}$$\frac{3}{2}$ hole in the vicinity of the electron. This complex breaks apart as the temperature increases, and the system returns to its ground state in which the ${m}_{j}=\ensuremath{-}$$\frac{1}{2}$ electron and the ${m}_{j}=\ensuremath{-}$$\frac{3}{2}$ hole are segregated.

We present a magneto-optical study of ZnSe/Zn1-x-yCdxMnySe quantum-well structures' in which a suitable choice of the Cd composition leads to a system that is type I at zero magnetic field. When a magnetic field is applied perpendicular to the layers of the structure, the band edges split in such a way as to make the upper sigma(-)(+1/2, +3/2) exciton transition type II, while the ground state sigma(+) (-1/2,-3/2) exciton component remains type I at all field values. This alignment reduces the probability for carrier relaxation from the higher-energy exciton component and opens the possibility of hole-spin population inversion via optical pumping.

The large magnetic field-induced band-edge spin splitting that characterizes semimagnetic semiconductors has been used in the past to create tunable quantum confining potentials resulting in spin dependent localization of carriers. Previous work has demonstrated a field-dependent type I/type II transition for the lower energy spin component of the heavy-hole exciton. We report here a magneto-optic study of Zn1−x−yCdxMnySe/Zn1−zMnzSe multiple quantum wells in which judicious selection of composition results in a structure that exhibits a hole and/or electron spin population inversion under conditions of optical pumping. In these structures, the higher energy component of the spin split heavy-hole exciton exhibits a type I to type II transition with applied field (in contrast to previous work in which the lower energy component exhibited such a transition), preventing fast recombination via the normal interband radiative transition. Consequently, the spin-up carriers forming this exciton, at higher energy than their spin-down counterparts, may relax instead via intersubband transitions. In the conduction band, the short spin flip relaxation times lead to rapid decay of the spin-up electron population. The heavy holes, however, have much longer spin relaxation times in these strained structures, resulting in an optically pumped spin population inversion in the Zn1−zMnzSe barriers. Polarization dependent magnetoreflectivity is used to follow the type conversion of the higher energy component and the formation of the hole spin population inversion.

Semimagnetic (or diluted magnetic) semiconductor heterostructures offer many unique opportunities for the study of spin dependent effects, including magnetic field induced quantum confinement and carrier spin lifetimes. In diluted magnetic semiconductors (DMS) such as Zn1 − xFexSe, strong spin exchange interactions significantly modify the band structure in the presence of an applied magnetic field. This allows one to continuously and reversibly modify the band offsets in tailored heterostructures to selectively localize carriers according to their spin state, thereby providing another degree of freedom in tuning the spatial localization and overlap of carrier wave functions after the fact of growth. We review several examples which have been recently demonstrated in Zn1 − xFexSe-based heterostructures grown by molecular beam epitaxy, including field tunable type-I–type-II transitions, the formation of a spin superlattice, and the continuous evolution of band alignment to the realization of a re-entrant type-I structure. Additional control over carrier confinement and lifetimes via carrier spin is possible in structures which consist of alternating layers of Zn1 − xFexSe and Zn1 − yMnySe. In such multiple quantum well systems, the heavy hole exciton simultaneously exhibits both Brillouin and Van Vleck paramagnetic behavior depending upon the spin state probed. Evidence is also observed for the formation of an electron spin population inversion which arises from the spin splitting of the electron states and the simultaneous spin separation of the holes into different layers of the structure.

We have investigated the carrier spin segregation process in a ZnSe/${\mathrm{Zn}}_{0.96}$${\mathrm{Fe}}_{0.04}$Se quantum-well structure by studying the energy and relative intensity of the ground-state exciton components as a function of applied magnetic field. The ${\mathit{e}}_{1}$${\mathit{h}}_{1}$ heavy-hole exciton initially tends towards type-II behavior as the field is applied but the trend reverses itself at higher fields as the conduction-band splitting of the magnetic barriers becomes significant. The interplay between the electron-hole Coulomb attraction and the tunable type-II confining potentials is described in the context of a variational model.

We describe the results of a magneto-optical study of a class of semiconductor heterostructures in which one of the constituent layers is a diluted magnetic semiconductor (DMS). The large magnetic band splittings of DMS materials result in spin-dependent confining potentials for electrons and holes which can be changed externally by varying the applied magnetic field. The modifications in the band alignment result in changes, sometimes dramatic, of the optical properties in the vicinity of the band gap.

Line-shape analysis of magnetoreflectance spectra from ZnSe/Zn0.99Fe0.01Se quantum well structures was performed using a classical dielectric function model. This model explains the spectral dependence on the sample geometry, as well as providing additional evidence of spin superlattice formation in ZnSe/ZnFeSe heterostructures.

We have studied a new type of semiconductor quantum well system in which the carrier-ion exchange interactions are determined by the spin of the carrier. The samples consist of alternating layers of Zn1−xMnxSe and Zn1−yFeySe grown by molecular-beam epitaxy (MBE). At zero field, the carriers initially interact randomly with both transition metal species. When a magnetic field is applied, the excitonic wave functions are increasingly localized in one or the other of the magnetic layers according to their spin state as the competing spin exchange interactions define the confining potential. The spin components of the heavy hole exciton are subsequently dominated by different exchange interactions as revealed by their temperature and field dependence: the behavior of the spin-down component (−3/2,−1/2) is described by exchange interactions of the carriers with the Mn2+ ions and exhibits Brillouin paramagnetic behavior, while the spin-up component (+3/2,+1/2) is dominated by interactions with Fe2+ ions and exhibits Van Vleck paramagnetism. These structures are thus characterized by an initial competition and eventual coexistence of Brillouin- and Van Vleck-like paramagnetic behavior for the exciton.

We present a magnetoreflectivity study of a ZnSe/Zn-0.85/Mn0.15Se multiple-quantum-well structure. The spin splittings of the two confined excitons e(1)h(1) and e(2)h(2) are studied as a function of applied magnetic field. Differences in the energies and intensities between the two excitonic transitions are discussed and interpreted using a variational calculation. We show that these diluted-magnetic-semiconductor-based structures may be interesting hosts for the study of heavy-hole/light-hole band mixing, as the energies of the confinement hole states change with the application of an external magnetic field.

We studied strain-induced band splittings of ZnSe/GaAs and Zn1−xMnxSe/GaAs epilayers of 0.064–3 μm thickness by reflectance and polarized photoluminescence. Polarized photolumi- nescence was found particularly useful in optical transition identification. The spectacular difference in magnetic field sensitivity of heavy hole and light hole exciton in ZnMnSe is also very helpful in transition identification. The evaluated heavy-light hole band splitting is in general accordance with previous data. An exceptionally strong variation of the strain with epilayer thickness is observed in the thickness range 0.5–2 μm.

We report the study of a mixed-spin multiple-quantum-well system consisting of alternating ${\mathrm{Zn}}_{1\mathrm{\ensuremath{-}}\mathit{x}}$${\mathrm{Mn}}_{\mathit{x}}$Se and ${\mathrm{Zn}}_{1\mathrm{\ensuremath{-}}\mathit{y}}$${\mathrm{Fe}}_{\mathit{y}}$Se layers, grown by molecular-beam epitaxy. The two spin components of the heavy-hole exciton are each dominated by different exchange interactions as revealed by their temperature and magnetic-field dependence. We find that the energy of the spin-down ${\mathrm{\ensuremath{\sigma}}}_{+}$ component (-3/2,-1/2) is well described by exchange interactions of the electrons and holes with ${\mathrm{Mn}}^{2+}$ ions (Brillouin paramagnet), while the energy of the spin-up ${\mathrm{\ensuremath{\sigma}}}_{\mathrm{\ensuremath{-}}}$ component (+3/2,+1/2) is determined by the interactions of carriers with ${\mathrm{Fe}}^{2+}$ ions (Van Vleck paramagnet). Thus, these tailored heterostructures permit simultaneous study of Brillouin and Van Vleck paramagnetism as well as associated carrier dynamics in a single structure.

The growth of spin superlattice structures in which spin-up and spin-down carriers occupy alternating layers in the structure has recently been reported. We report here measurements of electron and hole spin lifetimes and spin relaxation processes. In dc photoluminescence spectra, both the higher-energy (+1/2,+3/2) and lower-energy (−1/2,−3/2) heavy-hole exciton interband transitions are observed, even though it would be energetically favorable for the spin-up carriers to first relax to the spin-down state before radiative recombination. From the field dependence of the intensity ratios of these components and a rate equation model, we determine the heavy hole τhs and electron τes spin lifetimes relative to the radiative lifetime τr, with τhs/τr≊4.5 and τes/τr≊0.08, so that the heavy-hole spin lifetime is ≊50 times longer than that of the electron. This is attributed to the strain-induced splitting of the heavy- and light-hole bands, which prevents fast spin relaxation of the holes.

We report the study of a mixed-spin multiple-quantum-well system consisting of alternating Zn1-xMnxSe and Zn1-yFeySe layers, grown by molecular-beam epitaxy. The two spin components of the heavy-hole exciton are each dominated by different exchange interactions as revealed by their temperature and magnetic-field dependence. We find that the energy of the spin-down sigma(+) component (-3/2, - 1/2) is well described by exchange interactions of the electron and holes with Mn2+ ions (Brillouin paramagnet), while the energy of the spin-up sigma(-) component (+3/2, +1/2) is determined by the interactions of carriers with Fe2+ ions (Van Vleck paramagnet). Thus, these tailored heterostructures permit simultaneous study of Brillouin and Van Vleck paramagnetism as well as associated carrier dynamics in a single structure.

We report the observation of long spin lifetimes for heavy holes in (Zn,Mn)Se and (Zn,Fe)Se based heterostructures. These long spin relaxation times are observed in both simple strained epilayers, as well as in complex spin superlattice structures in which the spin up and spin down carriers occupy alternating layers of the superlattice. In photoluminescence spectra, both the higher energy (+1/2, +3/2) and lower energy (−1/2, −3/2)heavy hole exciton interband transitions are observed, even though it would be energetically favorable for the spin up carriers to first relax to the spin down state before radiative recombination. From the magnetic field dependence of the intensity ratio of these components and a rate equation model, we find that the heavy hole spin lifetime τhs is substantially longer than the electron spin lifetime τes, with τhs /τr ≈ 4 and τes /τr ≈ 0.06, where τr is the radiative recombination time. This is attributed to the strain induced splitting of the heavy and light hole bands, which inhibits mixing of the hole spin states and subsequent dipole-allowed transitions producing fast spin relaxation.

We have studied carrier spin relaxation in diluted magnetic semiconductor (DMS) epilayer and spin superlattice samples using polarization and magnetic field dependent photoluminescence. The giant Zeeman splitting of the DMS materials permits unambiguous identification of electron and hole excitonic spin relaxation times. We find that the heavy hole spin lifetime τhs is comparable to the radiative recombination time τr, and substantially longer than the electron spin lifetime τes, with τhs/τr≊4 and τes/ τr≊0.06. We attribute these long spin lifetimes to the strain induced splitting of the bulk degenerate light and heavy hole bands, which inhibits mixing of the hole spin states and subsequent heavy hole spin flip relaxation processes. In the regime of weak confinement, the spin relaxation times are independent of the degree of quantum confinement, the number of interfaces, the magnetic ion concentration, and the magnetic species. We conclude that spin flip via exchange scattering between the carriers and magnetic ions is not the dominant excitonic spin relaxation mechanism.

We have studied the spin-relaxation process in spin superlattice structures, both at zero field (no confining potential) and as a function of applied field (variable confining potential). Evidence of an unexpectedly long hole spin-relaxation time associated with the strain splitting of the valence band has been found. In addition, excitonic spin-relaxation times which are unaffected by the strength of the spin-dependent confining potential were observed. We demonstrate that for excitons, spin flip via the magnetic ion-carrier exchange interaction is not the dominant spin-relaxation mechanism, although it may play a more important role in the case of energetic hot carriers.