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We present a study of the exciton dynamics in modulation doped ${\mathrm{G}\mathrm{a}\mathrm{A}\mathrm{s}/\mathrm{G}\mathrm{a}}_{1\ensuremath{-}x}{\mathrm{Al}}_{x}\mathrm{As}$ heterojunctions and GaAs epilayers. The comparison permits to identify the features characteristic for the heterojunctions. In particular, we analyze the rise time of the transient photoluminescence (PL) intensity. In general, we find a longer PL rise time for the lower-energy excitons indicating that the time required for the energy relaxation process increases with increasing binding energy of the excitons. Moreover, the rise time of the free excitons turns out to be conspicuously longer in heterojunctions than in epilayers although the time integrated PL spectra of the two systems are similar. From our analysis we conclude that the long rise time observed in the heterojunctions is closely connected with the vertical drift of the photoexcited carriers driven by the interface potential. In fact, we find that the carriers (i.e., holes in n-doped samples) photoexcited in the heterojunctions drift vertically from the heterointerface to the flat band region, where they finally recombine after forming excitons (bimolecular formation) with oppositely charged and locally excited carriers (i.e., electrons in n-doped samples).

We have characterized the gap states from photoinduced absorption and photoinduced ESR studies on one-dimensional (1D) halogen-bridged metal complexes with degenerate and nondegenerate charge-density wave (CDW) ground state. A comparison of excitation profiles of self-trapped exciton (STE) luminescence with those of the gap states demonstrates that excitons are relaxed to spin-soliton pairs. From an analysis of the temperature dependence of luminescence decay time, conversion from the STE to the solitonic state is found to occur through a finite potential barrier, the magnitude of which strongly depends on the degeneracy of the CDW.

From low-field measurements of the susceptibility at temperatures up to 385 K, fitted to the Curie-Weiss expression, the exchange constant J/${\mathrm{k}}_{\mathrm{B}}$ of ${\mathrm{Pb}}_{1\mathrm{\ensuremath{-}}\mathrm{x}}$${\mathrm{Eu}}_{\mathrm{x}}$Te has been obtained for different values of x. It has been observed that J/${\mathrm{k}}_{\mathrm{B}}$ decreases with increasing x. Reasons for this decrease are proposed. Measurements of the low-temperature magnetization at fields up to 23 T yield a value of the pair exchange constant ${\mathrm{J}}_{\mathrm{p}}$/${\mathrm{k}}_{\mathrm{B}}$, which is somewhat larger than the values from the susceptibility measurements and is nearly independent of x. This result seems to be due to the clustering of Eu atoms.

The electronic structure of the one-dimensional Ni complexes, [Ni(chxn${)}_{2}$X]${\mathit{X}}_{2}$(X=Cl, Br; (chxn)=1R,2R-cyclohexanediamine), is studied together with the discrete Ni complexes, [Ni${\mathit{X}}_{2}$([14]${\mathrm{aneN}}_{4}$)]${\mathrm{ClO}}_{4}$(X=Cl; Br;([14]${\mathrm{aneN}}_{4}$)=1,4,8,11-tetraazacyclote tradecane),using optical spectroscopy, x-ray photoelectron spectroscopy (XPS) and Auger electron spectroscopy. The optical spectra show that the Br compounds have a smaller gap as compared with the Cl compounds. An analysis using a simple X-Ni-X trimer model on the optical spectra and the Ni 2p XP spectra yields quantitative estimates for the charge transfer (CT) energy \ensuremath{\Delta} and the transfer energy T for discrete and one-dimensional Ni complexes. The analysis on the Ni LVV Auger spectra in conjunction with the valence XP spectra indicates that the average on-side d-d Coulomb energy U in the one-dimensional and discrete Ni complexes is about 5 eV, quite similar to the case of the Ni dihalides. The obtained results demonstrate that the one-dimensional Ni complexes are CT insulators. We discuss the differences in the electronic structures of the one-dimensional Ni complexes compared with the Ni dihalides and the one-dimensional Pt complexes on the basis of the estimated parameter values of \ensuremath{\Delta}, T, and U. \textcopyright{} 1996 The American Physical Society.

The electronic structures of ZnTe and ${\mathrm{Zn}}_{0.7}$${\mathrm{Mn}}_{0.3}$Te have been investigated by means of synchrotron-radiation photoemission experiments. Te 4d core-yield spectra of ZnTe and ${\mathrm{Zn}}_{0.7}$${\mathrm{Mn}}_{0.3}$Te reflect the density of states (DOS) of the conduction bands. The two DOS peaks in the ZnTe spectrum observed at 0.6 and 4.3 eV above the conduction-band minimum are ascribed to relatively flat regions of the conduction bands near the ${\mathit{L}}_{1}$ and ${\mathit{X}}_{1}$, and the ${\mathrm{\ensuremath{\Gamma}}}_{15}$ and ${\mathit{L}}_{3}$, symmetry points, respectively. The contribution of the Mn 3d states to the valence bands of ${\mathrm{Zn}}_{0.7}$${\mathrm{Mn}}_{0.3}$Te has been evaluated by means of resonant enhancement of Mn 3d photoionization cross section near the Mn 3p-3d core excitation. The Mn 3d\char21{}derived partial DOS of ${\mathrm{Zn}}_{0.7}$${\mathrm{Mn}}_{0.3}$Te is found to be very similar to that of ${\mathrm{Cd}}_{0.8}$${\mathrm{Mn}}_{0.2}$Te. This suggests nearly equal Mn-Te bond lengths in these ternary alloys.

This thesis work aims at a better understanding of magneto-optical properties of dilute nitrides and II-VI diluted magnetic semiconductor quantum structures. The thesis is divided into two parts. The first part gives an introduction of the research fields, together with a brief summary of the scientific results included in the thesis. The second part consists of seven scientific articles that present the main findings of the thesis work. Below is a short summary of the thesis. Dilute nitrides have been of great scientific interest since their development in the early 1990s, because of their unusual fundamental physical properties as well as their potential for device applications. Incorporation of a small amount of N in conventional Ga(In)As or Ga(In)P semiconductors leads to dramatic modifications in both electronic and optical properties of the materials. This makes the dilute nitrides ideally suited for novel optoelectronic devices such as light emitting devices for fiber-optic communications, highly efficient visible light emitting devices, multi-junction solar cells, etc. In addition, diluted nitrides open a window for combining Si-based electronics with III-V compounds-based optoelectronics on Si wafers, promising for novel optoelectronic integrated circuits. Full exploration and optimization of this new material system in device applications requires a detailed understanding of their physical properties. Papers I and II report detailed studies of effects of post-growth rapid thermal annealing (RTA) and growth conditions (i.e. presence of N ions, N2 flow, growth temperature and In alloying) on the formation of grown-in defects in Ga(In)NP. High N2 flow and bombardment of impinging N ions on grown sample surface is found to facilitate formation of defects, such as Ga interstitial (Gai) related defects, revealed by optically detected magnetic resonance (ODMR). These defects act as competing carrier recombination centers, which efficiently decrease photoluminescence (PL) intensity. Incorporation of a small amount of In (e.g. 5.1%) in GaNP seems to play a minor role in the formation of the defects. In GaInNP with 45% of In, on the other hand, the defects were found to be abundant. Effect of RTA on the defects is found to depend on initial configurations of Gai related defects formed during the growth. In Paper III, the first identification of an interfacial defect at a heterojunction between two semiconductors (i.e. GaP/GaNP) is presented. The interface nature of the defect is clearly manifested by the observation of ODMR lines originating from only two out of four equivalent orientations. Based on its resolved hyperfine interaction between an unpaired electronic spin (S=1/2) and a nuclear spin (I=1/2), the defect is concluded to involve a P atom at its core with a defect/impurity partner along a direction. Defect formation is shown to be facilitated by N ion bombardment. In Paper IV, the effects of post-growth hydrogenation on the efficiency of the nonradiative (NR) recombination centers in GaNP are studied. Based on the ODMR results, incorporation of H is found to increase the efficiency of the NR recombination via defects such as Ga interstitials. In Paper V, we report on our results from a systematic study of layered structures containing an InGaNAs/GaAs quantum well, by the optically detected cyclotron resonance (ODCR) technique. By monitoring PL emissions from various layers, the predominant ODCR peak is shown to be related to electrons in GaAs/AlAs superlattices. This demonstrates the role of the SL as an escape route for the carriers confined within the InGaNAs/GaAs single quantum well. The last two papers are within a relatively new field of spintronics which utilizes not only the charge (as in conventional electronics) but also the quantum mechanical property of spin of the electron. Spintronics offers a pathway towards integration of information storage, processing and communications into a single technology. Spintronics also promises advantages over conventional charge-based electronics since spin can be manipulated on a much shorter time scale and at lower cost of energy. Success of semiconductor-based spintronics relies on our ability to inject spin polarized electrons or holes into semiconductors, spin transport with minimum loss and reliable spin detection. In Papers VI and VII, we study the efficiency and mechanism for carrier/exciton and spin injection from a diluted magnetic semiconductor (DMS) ZnMnSe quantum well into nonmagnetic CdSe quantum dots (QD’s) by means of spin-polarized magneto PL combined with tunable laser spectroscopy. By means of a detailed rate equation analysis presented in Paper VI, the injected spin polarization is deduced to be about 32%, decreasing from 100% before the injection. The observed spin loss is shown to occur during the spin injection process. In Paper VII, we present evidence that energy transfer is the dominant mechanism for carrier/exciton injection from the DMS to the QD’s. This is based on the fact that carrier/exciton injection efficiency is independent of the width of the ZnSe tunneling barrier inserted between the DMS and QD’s. In sharp contrast, spin injection efficiency is found to be largely suppressed in the structures with wide barriers, pointing towards increasing spin loss.

We study the dynamics of exciton-spin injection, transfer, and relaxation in self-assembled quantum dots (QDs) of CdSe coupled with a diluted magnetic semiconductor (DMS) layer of ${\mathrm{Zn}}_{0.80}{\mathrm{Mn}}_{0.20}\mathrm{Se}$, where spin-polarized excitons can be injected from the DMS into the QDs. The degree of circular polarization $P$ of excitonic photoluminescence (PL) at $5\phantom{\rule{0.3em}{0ex}}\mathrm{T}$ in the coupled QDs exhibits a rapid increase with increasing delay time, up to $+0.3$ at $25\phantom{\rule{0.3em}{0ex}}\mathrm{ps}$ after the pulse excitation of the DMS by a linearly polarized light. This development of a positive $P$ value directly reflects the spin-injection dynamics from the DMS, since the intrinsic polarization of the QD excitons due to Zeeman splitting is $P\ensuremath{\sim}\ensuremath{-}0.1$ when only the QDs are selectively excited. The $P$ value gradually decays with time after reaching its maximum, as a result of the exciton-spin relaxation with a time constant of $800\phantom{\rule{0.3em}{0ex}}\mathrm{ps}$ in the QDs. Time-resolved circularly polarized PL spectra immediately after the pulse excitation directly show the exciton-energy dependence of the spin-injection dynamics in the QD ensemble, where two-dimensional-like QDs with higher exciton energies show higher receptivity to the spin-polarized excitons than three-dimensional-like dots with lower exciton energies. A rate equation analysis reveals all time constants responsible for the spin-injection dynamics. We deduce a time constant of $10\phantom{\rule{0.3em}{0ex}}\mathrm{ps}$ for the spin injection. The spin-injection efficiency of 0.94 is also obtained, which corresponds to the ratio between the number of the spin-polarized excitons responsible for the rise of the positive $P$ value in the QD emission and the total number of the excitons injected from the DMS. Moreover, we observe that interdot exciton transfer significantly affects the $P$ value within the QD emission band after the fast spin injection, in addition to the spin relaxation within the QDs.

We report on time-resolved photoluminescence studies of exciton dynamics in a ZnMnSe/ZnSSe type-II quantum well. Strong separation of the electrons, confined to the nonmagnetic semiconductor ZnSSe, ...

We provide compelling experimental evidence for a spin-conserving energy-transfer process via localized excitations as being responsible for the observed optical spin injection in $\mathrm{Zn}\mathrm{Mn}\mathrm{Se}∕\mathrm{Zn}\mathrm{Cd}\mathrm{Se}$ quantum structures. We show that such a process is common and inherent to a diluted magnetic semiconductor because of strong potential fluctuations.

cw hot photoluminescence (PL) complemented by transient PL measurements is employed to evaluate momentum and spin relaxation of heavy hole (HH) excitons in ZnMnSe CdSe superlattices. The rate of ac ...

Epitaxial EuTe layers exhibit narrow luminescence peaks with a large Stokes shift and a large magnetic-field dependence. At low temperatures, the band-gap energy as well as the energy of the narrow luminescence peaks decreases with increasing field. The Stokes shift, in contrast, increases with magnetic field from $340\phantom{\rule{0.3em}{0ex}}\mathrm{meV}$ up to $500\phantom{\rule{0.3em}{0ex}}\mathrm{meV}$. From a comparison of the magnetic-field dependence of the band gap, the luminescence peaks, and the Stokes shift with density plots of the quasiparticle bandstructure of EuTe, it is concluded that the luminescence originates from a deeper conduction band than that causing the absorption edge. The existence of this deeper band is confirmed by an analysis of the dispersion of the refraction index and by a faint absorption onset. Time-resolved photoluminescence experiments and the continuous-wave photoluminescence intensity as a function of magnetic field and temperature indicate a change of the selection rule for the luminescence transition from optically allowed to optically forbidden, either when the magnetic field exceeds the critical field or when the temperature increases considerably above the phase transition temperature. This change is attributed to the presence of magnetic polarons, which breaks the selection rules at low temperatures and below the critical field and thus enables the luminescence. In the parameter range where no magnetic polarons exist, at high temperatures and above the critical field, almost no luminescence appears.

A measure of the Fe 3d partial density of states (DOS) in the valence bands of ${\mathrm{Cd}}_{0.89}$${\mathrm{Fe}}_{0.11}$Se single-phase single crystals has been obtained using synchrotron-radiation photoemission. Based on an analysis of the Fe 3d DOS spectrum in terms of a configuration-interaction calculation on a ${\mathrm{Fe}}^{2+}$(${\mathrm{Se}}^{2\mathrm{\ensuremath{-}}}$${)}_{4}$ model cluster, we find that the spectral intensities in the top 6 eV are predominantly due to transitions to ${\mathit{d}}^{6}$L final-state configuration, where L represents a ligand hole. On the other hand, those at 7--15 eV originate mainly from transitions to the ${\mathit{d}}^{5}$ configuration. The model calculation provides the Fe 3d Coulomb correlation energy of 4.5 eV.

We investigated the dynamics of free and bound excitonic magnetic polarons in ${\mathrm{Cd}}_{1\ensuremath{-}x}{\mathrm{Mn}}_{x}\mathrm{Te}$ $(x=0.03)$ single crystals by time-resolved photoluminescence experiments. We found that the magnetic polarons formed from free excitons are more stable than those formed from bound excitons. As a consequence, the peak energies of the optical emissions of free excitons and donor-bound excitons cross in the time-resolved photoluminescence spectra. Furthermore, we observed that nonmagnetic localization is not required to form free magnetic polarons.

We have studied the dispersion of polaritons in CuBr by means of resonant Brillouin scattering. As a result of $k$-linear terms and exchange coupling the polariton dispersion relations are highly structured. From a theoretical fit we have derived the pertinent exciton parameters and from them the corresponding Luttinger band parameters. The scattering mechanisms invoked for the interpretation of the experiments are piezoelectric scattering and deformation potential scattering. On the basis of independently determined deformation-potential constants it is shown that the deformation-potential scattering suffices to account for the observed scattering efficiencies in piezoelectric forbidden cases. From an analysis of the scattering intensities it is possible, in principle, to determine independently the absolute deformation potentials of the conduction and the valence bands.

By using a tunable laser various resonant aspects of spin-flip Raman scattering for electrons and holes bound to donor and acceptor states in ZnTe have been studied. $g$ values as well as diamagnetic coefficients for these states have been determined with high accuracy. The acceptor $g$ factors and their anisotropy are interpreted on the basis of the calculations of Cho et al. The resonance effect of the spin-flip scattering is discussed in relationship to the bound exciton luminescence. Multiple spin-flip scattering observed for donor electrons is explained with a model involving exchange coupling between the donor and the donor-bound exciton.