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Experimental and theoretical results from studies of electron effective masses in 4H SiC are presented. Three principal values of the mass tensor are experimentally resolved by optical detection of cyclotron resonance, and are determined as m(ML)=0.33\ifmmode\pm\else\textpm\fi{}0.01${\mathit{m}}_{0}$, m(M\ensuremath{\Gamma})=0.58\ifmmode\pm\else\textpm\fi{}0.01${\mathit{m}}_{0}$, and m(MK)=0.31\ifmmode\pm\else\textpm\fi{}0.01${\mathit{m}}_{0}$. These values are in good agreement with m(ML)=0.31${\mathit{m}}_{0}$, m(M\ensuremath{\Gamma})=0.57${\mathit{m}}_{0}$, and m(MK)=0.28${\mathit{m}}_{0}$, obtained from band-structure calculations based on the local density approximation to the density-functional theory using the linearized augmented plane-wave method. The conduction-band minimum is found to be at the M point of the Brillouin zone. \textcopyright{} 1996 The American Physical Society.

A strong effect of $p$-type modulation doping on the radiative recombination in $\mathrm{Si}/{\mathrm{Si}}_{1\ensuremath{-}x}{\mathrm{Ge}}_{x}/\mathrm{Si}$ quantum wells is observed by photoluminescence (PL) spectroscopy. The filling of the Si-Ge quantum wells due to the charge transfer of holes from the $B$ modulation doping in the adjacent Si layers causes an appearance of a broad asymmetric PL band with a characteristic sharp high-energy cutoff. A strong enhancement near the Fermi edge is shown, by varying structure parameters and experimental conditions, to correspond to the Fermi-edge singularity. Experimental evidence on the dominant mechanism responsible for the Fermi-edge singularity is given as due to a nearly resonant scattering between the electronic states near the Fermi energy and the next unoccupied subband of the two-dimensional hole gas.

A metastable complex defect in S-doped silicon has been studied with zero-field optically-detected magnetic-resonance (ODMR) spectroscopy. The defect shows two characteristic photoluminescence (PL) spectra S A and S g , corresponding to two different geometric configurations 1 and 2. The PL spectra in both cases originate from excited-state triplet bound excitons in the neutral charge state of the defect. Previous X-band ODMR studies for S A and S g show broad spectra with unresolved hyperfine interaction

Presentation d'un mecanisme de recombinaisons apparaissant dans des semiconducteurs contenant des defauts etendus. Ce modele se fonde sur des donnees experimentales du silicium traite par plasma d'hydrogene contenant des defauts de type plateaux

On etudie deux sortes de mecanismes de chauffage des porteurs de charge via l'absorption hyperfrequence. On demontre experimentalement le transfert d'energie efficace des porteurs de charge chauds au reseau

On montre que l'observation des effets de ce phenomene est due aux interactions microscopiques non disparaissantes se couplant aux etats approchants. De plus, il existe une inequivalence entre ces etats a cause d'un mecanisme d'excitation de l'etat stationnaire. Les interactions hyperfines constituent la principale interaction statique responsable de ces effets

Optically detected magnetic resonance (ODMR) is applied in this work to study the 2.172-eV bound exciton (BE) in GaP, associated with a complex neutral (Cu-Li)-related defect denoted (Cu-Li${)}_{\mathrm{V}}$. The ODMR data are analyzed with a spin-triplet spin Hamiltonian scrH=${\ensuremath{\mu}}_{B}$B\ensuremath{\cdot}g${\ifmmode \tilde{}\else \~{}\fi{}}_{\mathrm{ex}}$\ensuremath{\cdot}S+S\ensuremath{\cdot}D\ifmmode \tilde{}\else \~{}\fi{}\ensuremath{\cdot}S, \ensuremath{\cdot}S, where S is the effective BE spin. The best fit of this Hamiltonian to the ODMR data gives the values ${g}_{x}$=1.98\ifmmode\pm\else\textpm\fi{}0.01, ${g}_{y}$=1.97\ifmmode\pm\else\textpm\fi{}0.01, ${g}_{z}$=2.07\ifmmode\pm\else\textpm\fi{}0.01; ${D}_{x}$=(-0.34\ifmmode\pm\else\textpm\fi{}0.01)\ifmmode\times\else\texttimes\fi{}${10}^{\mathrm{\ensuremath{-}}5}$ eV, ${D}_{y}$=(-1.04 \ifmmode\pm\else\textpm\fi{}0.01)\ifmmode\times\else\texttimes\fi{}${10}^{\mathrm{\ensuremath{-}}5}$ eV, ${D}_{z}$=(1.38\ifmmode\pm\else\textpm\fi{}0.01)\ifmmode\times\else\texttimes\fi{}${10}^{\mathrm{\ensuremath{-}}5}$ eV, where x?[001], y?[11\ifmmode\bar\else\textasciimacron\fi{}0], and z?[110] are the principal axes of the g and D tensors. The determination of these parameters is much more accurate than in previous work, where neither the principal defect axes nor the sign of the components of the D tensor were given. These improved data establish a ${C}_{2v}$ symmetry of the defect, with a different atomic and geometrical arrangement than proposed in previous work. With the combined information from the conditions of creation of the defect, the photoluminescence spectra and the ODMR data, it is suggested to be a three-atomic ${\mathrm{Li}}_{\mathrm{i}}$-${\mathrm{Cu}}_{\mathrm{Ga}}$-${\mathrm{Li}}_{\mathrm{i}}$ chain in a bent configuration in a {110} plane. The components of the g tensor for the BE are all close to g=+2, consistent with a spinlike character of both electron and hole. It is noted that the fit of the ODMR data with the spin-triplet spin Hamiltonian is quite good, and the influence of the nearby singlet state on the magnetic properties of the triplet state can be regarded as negligible.

A comparative study of photoluminescence (PL) quenching of shallow bound excitons (BE's) associated with shallow donors and acceptors and of deep isoelectronic bound excitons (IBE's) in Si in a weak electric field at liquid-He temperatures is reported. In both cases the photoluminescence is quenched due to impact ionization by field-accelerated hot carriers. A comparison between n- and p-type samples showed no significant difference in quenching rate for the shallow donor- and acceptor-related BE's; the quenching starts around 50 V/cm, and both electrons and holes are released as free carriers in the impact process. The IBE's show a completely different behavior and a different impact mechanism. IBE's first show an increase in PL intensity with applied electric field and are difficult to completely quench before the electrical breakdown of the sample around 400 V/cm. In this case it is concluded that only one carrier is released from the center in the impact process. By impact ionizing the shallow donor- and acceptor-related BE's and the free excitons, an efficient recombination channel for free carriers is removed, and the carrier lifetime is drastically increased. This opens up new recombination channels, such as free-to-bound transitions, which are not seen in the absence of an electric field.

A comprehensive discussion of relevant transfer processes for excitons bound to complex defects in GaP is presented on the basis of experimental studies by optically detected magnetic resonance (ODMR). A new experimental procedure is applied for determination of the efficiency of the energy migration processes among complex defects in the GaP lattice. The role of phonon interaction in the transfer transitions is discussed, with the example of excitation-energy transfer from shallow donor-acceptor pairs in GaP to excitons bound at complex isoelectronic defects. It is explained that the sign of the sensitizer ODMR signal may relate to the nature of the phonon interaction. The relevant ODMR signal is positive for ``purely'' electronic transitions or for one-site phonon interaction and it may be observed as negative for two-site phonon interactions in the transfer process. The data presented prove the quenching action of the antisite-related complex defects on the GaP visible emission. The mechanism responsible for photoluminescence quenching in this case is shown to be excitation-energy transfer, probably due to phonon-assisted tunneling of bound excitons.

In this paper we present the first experimental evidence for the cause of the nonresonant background signals commonly observed in optically detected magnetic resonance (ODMR) experiments in silicon. By making a direct comparison of bound-exciton (BE) photoluminescence (PL) intensity changes in an electric field with microwave-field-induced similar PL changes in n-type Si, we show that the main physical mechanism causing the ODMR background signal at lower microwave frequencies is impact ionization of free and bound excitons. We show that the efficiency of the microwave-induced impact ionization of bound excitons (BE's) is very strong, and it causes complete quenching of PL intensity for BE's associated with shallow donors. As a result the PL intensity can be enhanced by more than 150% for isoelectronic BE's. The change in PL intensity with microwave power shows a characteristic threshold, which was typically 20 mW at 9 GHz microwave frequency in our case.

A shallow complex acceptor in CdTe with a binding energy of 56.4\ifmmode\pm\else\textpm\fi{}0.5 meV is reported. A bound-exciton (BE) line at 1.5901 eV is found to be associated with this acceptor, the most shallow acceptor-related bound exciton discovered so far in CdTe. Two-hole photoluminescence satellites recorded with resonant excitation in the 1.5901-eV bound-exciton line reveal several s-like excited states of the acceptor hole, of which the 2S replica interacts strongly with phonons in the range of the two-${\mathrm{LO}}^{\mathrm{\ensuremath{\Gamma}}}$-phonon mode. A complicated phonon spectrum in the optical-phonon range is found for this bound exciton. Further, the two-hole spectrum contains additional electronic lines, not seen for substitutional acceptors in tetrahedral symmetry. Two such lines, corresponding to hole energies 7.3 and 33.3 meV above the acceptor ground state, are interpreted as excited states associated with the component of the ${\ensuremath{\Gamma}}_{8}$ hole state split by the local strain field, and the 2${P}_{3/2}$ excited hole state, respectively. Dye-laser-excited photoluminescence excitation spectra similarly reveal excited states to the bound exciton, particularly a doublet at \ensuremath{\approxeq}2.6 meV above the 1.5901-eV lowest BE state. These excited BE states are interpreted as states split off from the lowest at 1.5901 eV by the local axial strain field at the defect.

Optical detection of magnetic resonance (ODMR) via photoluminescence is demonstrated to be a powerful technique for the study of magnetic properties of shallow bound electron states in GaP. Two interstitial shallow Li-related donors, ${\mathrm{Li}}_{\mathrm{A}}$ and ${\mathrm{Li}}_{\mathrm{B}}$, are studied in this work with the ODMR technique. The ${\mathrm{Li}}_{\mathrm{A}}$ donor has a binding energy of \ensuremath{\approxeq}59 meV and an isotropic g factor of ${g}_{e}$=1.99\ifmmode\pm\else\textpm\fi{}0.01. The ODMR spectrum is easily observed without external stress, indicating that the corresponding interstitial site is distorted from the Ga-like tetrahedral site to a symmetry lower than trigonal. The ${\mathrm{Li}}_{\mathrm{B}}$ donor (binding energy \ensuremath{\approxeq}91 meV) has a quite similar isotropic g value ${g}_{e}$\ensuremath{\approxeq}1.99\ifmmode\pm\else\textpm\fi{}0.01, as expected in a P-like tetrahedral interstitial site, slightly distorted along the trigonal axis. Excitation transfer effects are conveniently detected via the spectral behavior of the ODMR signal, and are found to occur frequently in GaP. An interesting observation is an anisotropic donorlike resonance in (Cu-Li)-doped GaP, believed to be associated with an electron bound as a secondary particle at the hole-attractive neutral (Cu-Li${)}_{\mathrm{III}}$ complex defect at 2.242 eV. Transfer effects are found to cause linewidth narrowing in ODMR donor signals, which usually are considerably broader than those obtained in the conventional EPR technique.

MECHANISMS FOR THE OPTICALLY DETECTED MAGNETIC-RESONANCE BACKGROUND SIGNAL IN EPITAXIAL GAAS

The infrared optical absorption peak at 0.34 eV in silicon, which is usually associated with the singly negative charge state of the divacancy, has been investigated in electron-irradiated samples with use of different optical excitation conditions. It is proposed that a strong Jahn-Teller distortion makes it possible to populate this charge state, when the defect initially is in the neutral charge state, either by the capture of a photoexcited free electron from the conduction band, or by the direct photoexcitation of an electron from the valence band to a defect orbital. Experimental evidence for the existence of these reactions is presented. A defect level at ${E}_{c}$-0.54 eV, frequently associated with the singly negative charge state of the divacancy, is identified as one of the levels from which these photoexcited free electrons originate. The 0.34-eV peak is attributed to an internal transition in the singly negative charge state of the divacancy center, implying the existence of a shallow defect state at approximately ${E}_{c}$-0.07 eV for this charge state. Experimental support is given for the existence of this shallow state. A tentative explanation, based on the strong Jahn-Teller distortion of the singly negative charge state, is suggested for the fact that the doubly negative charge state is not observed at temperatures below 90 K.

The low-temperature photoluminescence of bound excitons and electron-hole droplets in $n$-type Si is studied in a weak electric field for the first time, using simultaneous pulsed excitation for both the laser excitation and the applied electric field. The luminescence is quenched because of impact ionization by field-accelerated electrons and holes. The quenching starts around 50 V/cm, depending on sample purity and excitation intensity.

Cu diffusion in ZnTe in the temperature range 300--500 \ifmmode^\circ\else\textdegree\fi{}C gives rise to a neutral (isoelectronic) complex, which has been studied in this work via its bound-exciton spectrum at low temperatures. The bound exciton has a peculiar electronic structure with two electronic lines, B at 2.260 eV and A at 2.261 eV. The lower-energy B line is only strong in photoluminescence at low temperatures. In absorption the B line is not detected, while the A line with its phonon replicas dominates the spectrum. Further, the two lines have quite different phonon coupling. Surprisingly enough the A and B lines show exactly the same behavior in Zeeman measurements up to 10 T, which indicates a similar electronic structure for both lines. A detailed analysis of Zeeman data for magnetic fields up to 10 T is performed, with a perturbation Hamiltonian containing the major effects of both electron-hole exchange interaction and the trigonal local strain field. An electronic excited state is observed at 2.363 eV, believed to be associated with ${m}_{J}$=1/2 hole states, while the Zeeman data for the lower bound-exciton lines are consistent with ${m}_{J}$=(3/2) holes. It is concluded that the hole in the bound exciton is bound as a primary particle to the neutral defect potential, while the electron is rather delocalized like a shallow donor electron. The observed unusual doubling of the electronic structure may be explained in terms of two different static distortions of the defect in the excited state, i.e., when an exciton is bound at the defect. An identity of the complex as an [${X}_{\mathrm{Zn}}$-${\mathrm{Cu}}_{\mathrm{i}}$] pair is suggested, where ${X}_{\mathrm{Zn}}$ is an unknown acceptor, possibly ${V}_{\mathrm{Zn}}$ or ${\mathrm{Cu}}_{\mathrm{Zn}}$.

A complex defect with a bound exciton (BE) at 2.3296 eV (at 2 K) in Cu-diffused ZnTe is studied in detail with laser spectroscopy, including magneto-optical measurements in fields up to 10 T. The defect is found to be axial and (110)-oriented, suggesting a substitutional pair ${\mathrm{Cu}}_{\mathrm{Zn}}$-${D}_{\mathrm{Zn}}$ as the identity, where ${D}_{\mathrm{Zn}}$ is a shallow donor on a Zn site. This suggestion is consistent with the fact that the defect is neutral, from the absence of thermalization in Zeeman transmission data. The Zeeman data can be fitted with a spin Hamiltonian dominated by a rather strong tensional local strain field, with a small electron-hole exchange interaction (\ensuremath{\approxeq}0.02 meV). The defect has a moderately strong hole-attractive local potential, presumably due to ${\mathrm{Cu}}_{\mathrm{Zn}}$, and the angular momentum for the bound hole is not quenched. The tensional sign of the local strain field leaves a \ensuremath{\Vert}J,${M}_{J}$〉=\ensuremath{\Vert}(3/2,\ifmmode\pm\else\textpm\fi{} 3) / 2 〉 doublet as the lowest hole state, with a hole g factor given by K=1.25\ifmmode\pm\else\textpm\fi{}0.02, L=-0.21\ifmmode\pm\else\textpm\fi{}0.01. The electron in this BE is loosely bound as a secondary particle, with an isotropic g value ${g}_{e}$=-0.38\ifmmode\pm\else\textpm\fi{}0.02, quite similar to the value ${g}_{e}$=-0.40 for shallow donors in ZnTe.