Search

Scanning tunneling microscopy studies of (001) CdTe, a partially ionic II–VI compound, show novel surface structures with step and 2D islands edges aligned along the 〈100〉 crystallographic directions. This behaviour, which is not observed in covalent material, is interpreted in the framework of a model incorporating electrostatic interactions in the calculation of the energy of charged steps that could explain why the free energy of 〈100〉 steps lies below that of the [110] and [110] steps. These results provide new insights about the microscopic growth mode of CdTe especially by atomic layer epitaxy. The energetics of the vicinal surfaces is also strongly influenced by these effects which allows one to organize C-type and A-type surfaces in staircase and checkerboard arrays. These self organized templates are, respectively, used to grow quantum wires and quantum dots.

The irregular short period CdTe/ZnTe superlattice structure is investigated by both stationary and time-resolved optical spectroscopy with and without an external magnetic field as a perturbation. This study is aimed to emphasize the properties of radiative excitonic recombination in a superlattice of this type in comparison with the excitons confined in a single QW structure. The decay time of the excitons is about 400 ps which is deduced from the time-resolved measurements. The g -factors of electrons and holes are obtained by the spin quantum beat measurements combined with Zeeman measurements. The experimental results show that the g -factors of holes in the irregular short period CdTe/ZnTe superlattice become dramatically different in comparison with the single CdTe/CdMgTe quantum wells.

Structures consisting of ZnTe fractional monolayers embedded coherently in a CdTe quantum well are investigated by means of optical measurements. This study is designed to study the influence of the strained ZnTe islands on excitonic recombination and to explore the dependence of the self-organizing effects of the ZnTe island on growth temperatures. The experimental results show that the strained ZnTe islands can strongly modify the excitonic transitions related to the quantization of the excitonic centre-of-mass motion (CM quantization). We have shown that the striking increase of the effective g-factor of the CM quantized excitons is due to the change of the hole g-factor involved since it is found experimentally that the electron g-factor is unchanged. The experimental results also show that a coherent distribution of the ZnTe islands is formed at higher growth temperatures , while a random distribution of the ZnTe islands is formed at lower growth temperatures . A coherent alignment of the islands along the growth direction is concluded.

Single CdTe/CdMgTe quantum well (QW) structure and CdTe/ZnTe irregular superlattice structures grown by molecular beam epitaxy are investigated by stationary and time-resolved photoluminescence in the presence of an applied magnetic field. We study the well width dependence of the electron and hole g-factors by combining Zeeman measurements and the recently developed spin quantum beats technique. The experimental results show that both the electron and hole g-factors have the same sign in wide QWs and increase with decreasing well width. It is also found that the hole g-factor of the excited excitonic states is significantly larger than the hole g-factor of the ground excitonic state in the irregular superlattice structure.

Structures consisting of ZnTe fractional-monolayers inserted coherently in a CdTe quantum well are investigated by absorption measurements and spin-quantum beat measurements. Many sharp exciton transitions are observed. The effective g-factor of the excitons and the electron g-factors involved in those excitons are measured by Zeeman measurements and by spin-quantum beat measurements, respectively. The experimental results show that the strained ZnTe islands can strongly modify the excitonic transitions related to the quantization of the excitonic center-of-mass motion (CM quantization). We will show that the striking increase in the effective g-factor of the CM-quantized excitons is due to the change of the hole g-factor involved, since it is found experimentally that the electron g-factor is unchanged. The experimental results also show that a coherent distribution of the ZnTe islands is formed at higher growth temperature (⩾ 280°C), while a random distribution of the ZnTe islands is present at lower growth temperatures (⩽ 240°C).

Structures consisting of fractional-monolayers inserted coherently in a CdTe quantum well are investigated by magneto-optical measurements. It is found that the observed rich structure of excitonic transitions is due to the combined effects of the disorder introduced by the inserted ZnTe layers and of the quantization of the excitonic center-of-mass motion (CM quantization) along the growth direction. This study specially emphasizes the change in nature of the excitonic transitions due to the change of the ZnTe island distribution in the xy-plane. The experimental results show that the coherent distribution of the ZnTe islands is a very critical factor in allowing the optical observation of the CM quantized excitons. It is found that the thickness variation of the ZnTe layer (disordering effects) is responsible for the observation of the confined or localized exciton states.

We have measured the interdiffusion coefficient at CdTeCd1 − xZnxTe interfaces by following the evolution of the X-ray diffraction patterns of superlattices annealed between 360 and 420°C under a Cd overpressure. A linear diffusion model is used to describe the experimental results. The interdiffusion coefficient is given by DCd (cm2/s) = 100 exp[−2.5 eV/kT]. The data agree well with the Cd self-diffusion coefficient measured by other techniques between 500 and 900°C. This shows that only one defect, probably the Cd vacancy, is involved in the diffusion process in bulk material and at CdTeCdZnTe interfaces. At growth temperature generally used for the molecular beam epitaxy of CdTe based superlattices (Tg ⩽ 280°C), the contribution of interdiffusion to the interface morphology can be neglected.

Cd1−xZnxTe layers covering the composition range of 0

Coherent insertions of ZnTe submonolayers and dots in a CdTe matrix have been fabricated by molecular beam epitaxy. The realization of such ultra-small systems is conditioned by monitoring the two-dimensional growth on exactly (001) oriented substrates or vicinal surfaces. High resolution X-ray diffraction data show that the interfaces of the clusters are in perfect register with the surrounding material despite the 6.5% lattice mismatch between ZnTe and CdTe. Finite size effects are proposed to explain the partial relaxation of the strain of the ZnTe insertions. The short range isolectronic potential introduces considerable perturbation in the electronic band structure of the superlattices. Light hole exciton trapping by ZnTe dots is experimentally observed. This effect is well described qualitatively by the envelope function model. As the lateral size of ZnTe islands can be made larger than the exciton Bohr radius by changing the growth conditions, discrete fluctuations of ZnTe thickness are observed. Disorder induced localization resulting from the breaking of the translational symmetry by the dots is proposed to explain the multiple excitonic lines observed in the optical spectra of superlattices incorporating fractional monolayers of ZnTe.

Nanowires are unique objects to explore quasi-one-dimensional electronic systems. In order to obtain quantum-confined excitons in silicon nanowires, we strongly oxidized such nanowires and obtained core-shell silicon-silicon dioxide structures. Low-temperature photoluminescence measurements were performed after each oxidation step to link the band-gap energy to the dimensions of the nanowires. Surprisingly, the electronic band gap first decreases prior to increasing at quantum dimensions (the silicon nanowires, once oxidized, are stressed by the silicon dioxide shell). The controlled nanowire oxidation allowed us to develop an empirical model to estimate the strain-induced gap energy shift. With this model, we extracted the quantum energy as a function of the diameters of smaller nanowires, and its diameter dependence is in agreement with tight-binding calculations.

Experiments have been designed to measure interdiffusion coefficients in CdTe/HgTe (001) superlattices grown by molecular beam epitaxy. The interdiffusion coefficients are deduced from double crystal X-ray diffraction on post-growth-annealed samples. The annealings are realized in the 200-240 degrees C range under Hg saturated pressure. The results indicate that interdiffusion is dependent on concentration. The authors calculate diffused profiles from an expression on D(c,T), and simulate the corresponding X-ray spectra to fit them with the experimental ones.

Doping of CdTe and Cd 1 − x Zn x Te layers and heterostructures with indium donors during their growth by MBE is described. Characterization by SIMS, electrical measurements and photoluminescence is presented with emphasis on: activation efficiency for uniform and planar doping; degree of localization achieved for step-like doping profiles; doped single and multiple quantum wells. Compensation by acceptor impurities and by intrinsic defects occurs at low (≈ 10 16 cm -3 ) and at high ( 18 cm -3 ) In concentrations, respectively. Essentially 100% activation efficiency of the donors is achieved in the intermediate range.

The photoluminescence of CdTe/Cd1-xZnxTe (x approximately=0.1) strained-layer heterostructures has been studied at 1.8 K as a function of hydrostatic pressure up to 2.7 GPa. The pressure dependence of transitions related to the substrate has been measured separately, and this enables a direct determination of the hydrostatic deformation potential a=-3.55 eV for bulk CdTe. The pressure coefficients of the heavy hole direct and of the light hole indirect transitions are found to be similar and slightly lower than those of bulk CdTe. The weak influence of the confinement effect on the structure could explain the results.

We review in this paper some of the optical studies performed on CdTe/CdZnTe heterostructures which demonstrate interesting specific band structure effects in this strain-layer system. Due to the order of magnitude of the three physical parameters which determine the optical properties of such structure (bandgap discontinuity, lattice mismatch induced strain, electron-hole interaction), we observe (i) two types of quantization in CdTe/CdZnTe single quantum well: the first one, for well thickness smaller than 200 A, is the usual description of a 2D-exciton, whereas the second one, for wider wells, corresponds to the quantization of the motion of the exciton centre-of-mass. (ii) The mixed type nature of the CdTe/CdZnTe superlattice: type I for heavy-hole excitons and type II for light-hole excitons with the possibility of changing the relative energy positions of the type I and type II transitions with the built-in strains. (iii) A dominant piezoelectric effect in the (111) oriented strain-layer superlattices which generates an internal electric field of the order of 104 V/cm and modifies the optical properties of these Te-based heterostructures.

Single CdTe/CdMgTe quantum well (QW) structures are investigated by stationary and time‐resolved photoluminescence in the presence of an applied magnetic field. We study the dependence of the electron and the hole g factors on the well width by combining Zeeman measurements and the recently developed spin quantum beats technique. The experimental results show that both the electron and the hole g factors have the same sign in wide QWs and increase with decreasing well width. The electron spin phase relaxation time is about 250 ps in our QW structures.

We present single shot transmission experiments with 20 ps laser pulses. When a 10/sup 5/ V/cm static electric field is applied on a CdTe/Cd/sub 0.62/Zn/sub 0.38/Te multiple quantum well, the optical transmission increases. At a 804 nm wavelength the enhancement factor is equal to 1.6. For a pulse intensity higher than 0.7 fJ /spl mu/m/sup -2/, screening of the applied electric field occurs, restoring a zero-field transmission. These experiments imply that the screening rise time is shorter than 10 ps which results from weak confinement of the holes within the quantum wells.

Separate confinement heterostructure (SCH) lasers with different active layer were grown by molecular beam epitaxy for photopumped emission around 3.4 mu m. They show that a multiquantum well gain medium has a superior threshold variation characteristic temperature, though low-temperature thresholds are higher than with a thick active layer. A SCH with a single quantum well as active layer does not show any stimulated emission, because of poor carrier transfer from the barriers to the well.

The low temperature, LO-phonon Raman scattering is studied in two types of CdTe/Cd 0.92 Zn 0.08 Te multiple quantum wells: types I and II. The resonance profile has peaks at the energy for which the incoming beam is at the lowest exciton bands, and much stronger peaks at the outgoing beam resonance. This observation is explained by the admixture of exciton states with large in-plane K -vectors with the K ∼0 exciton state and by the small dynamic damping of the excitons. The electronic structure of the system suggests that the admixture is due to hole scattering by alloy fluctuations in the CdZnTe barriers.

Oscillations of the surface lattice parameter were observed by RHEED during the homoepitaxial growth of (001) CdTe by Molecular Beam Epitaxy and Atomic Layer Epitaxy. The oscillations are associated to a deformation, induced by the surface reconstructions, at the free edges of the small 2D islands formed during the growth. In the same way, a lateral relaxation is measured during the layer by layer `de-growth' of (001) CdTe. Experiments using a CCD X-ray sensitive camera combined with the very bright X-ray beam offered by the European Synchrotron Radiation Facility allowed us to investigate the two layer behavior of the CdTe surface in real time during the ablation by ion sputtering. The results show a relaxation mechanism which is effective only when islands are present on the surface. A correlation has been found between the size of the islands, their distribution, and the surface reconstruction. Particularly, a long- distance correlation between islands along the [1,-1,0] direction has been observed.© (1999) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.

The CdxHg1−xTe compounds are well suited to the design of resonant microcavity devices. Indeed these compounds display a wide variation of bandgap and refractive index with composition, while the lattice parameter remains practically unchanged. Microcavities resonating in the 3–5 µm range have been prepared by molecular beam epitaxy. Light emitting diodes (LEDs) are obtained by stacking a lower Bragg mirror (10.5 periods) which is doped n-type and a nominally undoped cavity medium containing a 50 nm active layer (CdTe-HgTe pseudo-alloy). The upper mirror is a gold layer deposited on the cavity, which is partly p-type doped. The diode emission is observed under direct bias, up to room temperature, in coincidence with the cavity resonance mode (linewidth 8 meV). It is much narrower than the inhomogeneous linewidth of the active layer (60 meV at 300K). The directivity is also much better. The diode properties are only very slightly dependent on temperature. A similar device can also be designed to make an infrared detector whose active layer thickness is reduced with respect to conventional detectors. The detector efficiency at the resonance wavelength may be increased by a factor close to the cavity finesse. With 16.5 periods in the lower mirror and a dielectric mirror as upper mirror (seven periods of ZnS/YF3), it has been possible to make a cavity resonating at 3.06 µm whose quality factor reaches 350. By photopumping the cavity across the dielectric mirror with a YAG microlaser, a laser emission occurred at the cavity resonance. At 10K, the laser threshold is 45 kW/cm2 and the linewidth is only 1.7 meV. These results demonstrate the usefulness of the microcavity concept for designing new devices such as LED or lasers which could be the basis for new applications of CdHgTe compounds.

A CdHgTe resonant cavity light emitting diode (RCLED) is proposed as a new infrared emitter. The device is prepared by molecular beam epitaxy on a CdZnTe substrate. A 10.5 periods Bragg mirror is first deposited. The cavity material is made of Cd0.75Hg0.25Te and contains a wide well (50 nm) designed to emit at 3.2 µm. The last three periods of the mirror are n-type doped while the cavity material is covered by a thin p-type CdZnTe layer. A gold layer closes the cavity, serving as the second mirror of a Fabry-Perot cavity tuned around 3.18 urn. It also provides an ohmic contact to the p-region. Under forward bias, the emission spectrum displays a narrow peak (8 meV full width at half maximum) corresponding to the cavity resonance. The position and linewidth of this line are independent of temperature. The directivity of the diode is also improved with respect to a conventional emitter, in agreement with theoretical expectations. Taking advantage of the spectral properties of the RCLED a new multispectral device has been fabricated.

The optimal growth conditions to grow by molecular beam epitaxy CdxHg1−xTe/CdyHg1−yTe heterostructures (0.3

The spectroscopic properties of CdHgTe/CdTe heterostructures are studied using photoluminescence, absorption and, for the first time, Fourier transform photoluminescence excitation spectroscopy. The analysis of these data shows that the line position can be well predicted from the confined carrier levels. The absorption spectrum is fitted with a theoretical calculation up to the second transition energy. The linewidth is dominated, in thin wells, by a well width fluctuation of the order of plusminus1 monolayer, while in wide wells it is due to alloy fluctuations and defect-induced broadening mechanisms.

Optical experiments, performed at 1.7 K, on three CdTe/(Cd,Zn)Te quantum-well separate-confinement heterostructures, reveal a type-I behavior of the 1s light-hole excitonic recombination line, although the band-to-band potential could be type II due to the strains. This behavior is explained in terms of the Coulombic interaction between the localized electrons and the light holes. Comparison between experimental and theoretical results provides a value for the strain-free relative valence-band offset: ${\mathit{q}}_{\mathit{v}}^{0}$\ensuremath{\approxeq}20%.

The surface and bulk contributions have been distinguished in the angle-resolved X-ray photoemission spectra of Te(3d 5 2 ) electronic state from the CdTe(111) surface with the binding energies Eb1 = 574.0 eV (±0.1 eV) and Eb2 = 572.8 eV (± 0.1 eV), respectively. Several reconstructions, namely (1 × 1), (2 × 2), (2√3 × 2√3)R30° and c(8 × 4) have been observed for the CdTe(111) MBE-grown surfaces for different tellurium fluxes as a function of the substrate temperature. The growth of subsequent CdTe(111) layers on the reconstructed surfaces was attempted under different conditions such as different tellurium pressure and sample temperature. A correlation between the reconstruction type and the phase of the corresponding RHEED oscillations observed during the growth was found. Bilayer surface representations derived from the “hexagonal ring” model recently proposed for the √19 structure observed on the GaAs(111) surface [1] are suggested for the (2 × 2), c(8 × 4) and (2√3 × 2√3)R30° CdTe(111) structure

We report luminescence spectra with spatial and spectral resolution which show the predominant effect of lithium on optical and electrical properties of annealed ZnTe. Lithium is identified in photoluminescence spectra at 5 K whereas SEM visualizes the diffusion of this impurity. We can also observe the effect of high impurity concentration of luminescence with the apparition of excitons bound to LiLi pairs.

Infrared measurements are made on ZnTe doped with Ag, Cu, and Au acceptor impurities. Observed transitions are discussed in the framework of the theory of Baldereschi. The binding energies Ea are (123.8 ± 0.5), (150.3 ± 0.5), and (271.6 ± 0.5) meV for Ag, Cu, and Au impurity, respectively, These results are in good agreement with that deduced from luminescence experiments. The average number of emitted phonons N is deduced for all known acceptors in ZnTe (Li, P, As, Ag, Cu, and Au) using luminescence and IR measurements. An empirical relation N equals; 62E2a is found. The cross-section for Au in ZnTe is found to be (3.5 ± 0.7) × 10−16 cm2 with an effective field ratio which ranges between 2.3 and 4. Des mesures de transmission infrarouge ont ete effectuees sur des echantillons de ZnTe dopes avec les accepteurs Ag, Cu et Au. La discussion, a partir de la theorie de Baldereschi, a permis de definir les energies d'ionisation Ea. Les valeurs de Ea sont (123,8 ± 0,5); (150,3 ± 0,5) et (271,6 ± ± 0,5) me V pour Ag, Cu et Au respectivement, en excellent accord avec celles deduites des experiences de luminescence. Les mesures de transmission IR et de luminescence ont aussi permis de determiner le nombre moyen de phonons emis N, pour tous les accepteurs connus dans ZnTe (Li, P, As, Ag, Cu, Au), et la relation empirique N equals; 62E2a a pu ětre deduite. Enfin, la section de capture optique de l'accepteur Au dans ZnTe a ete deduite des mesures de transmission IR. On a trouve σ equals; (3,5 ± 0,7) × 10−16 cm2 avec un coefficient de champ effectif se situant entre 2, 3 et 4.

A systematic study of acceptors incorporated by diffusion is carried out on refined melt-grown ZnTe. The properties of five substitutional acceptors are investigated by low-temperature photoluminescence. Since the binding energy of excitons on neutral acceptors is almost unaffected by the central cell potential, the neutral acceptors are identified by the two-hole transitions which give accurate values of acceptor ionization energies. The two dominant acceptors “b” at 61 meV and “a” at 149 meV are identified as LiZn and CuZn, respectively, while the hole binding energies on NaZn, AgZn, and AuZn are established as 62.8, 123, and 272 meV, respectively. These experiments show that all the relatively shallow acceptors involve impurities and not stoichiometric defects. Une etude systematique des accepteurs introduits par diffusion a ete faite sur ZnTe. Les proprietes de cinq accepteurs sont etudiees en photoluminescence. Comme l'energie de liaison des excitons sur les accepteurs neutres n'est pas affectee par le potentiel de cellule centrale, les accepteurs sont identifies grǎce aux transitions deux-trous qui donnent une valeur tres precise de la profondeur de chaque niveau. Les deux accepteurs dominants: “a” a 149 meV et “b” a 61 meV sont attribues au lithium et au cuivre en site zinc alors que les niveaux accepteurs introduits par Na, Ag et Au sont respectivement a 62.8, 123 et 272 meV. L'ensemble de ces experiences montre que tous les accepteurs peu profonds mettent en jeu des impuretes et non des defauts de stoechiometrie.

The stoichiometry of compound semiconductors interacts with the solubility of impurities. Oxygen impurity in ZnTe is a good element for solubility studies because oxygen doped ZnTe exhibits a well characterized luminescence spectrum. It is shown that Zinc implantation in nominally undoped ZnTe brings out an oxygen-related luminescence spectrum, revealing some previously inactive oxygen impurities. Furthermore oxygen can be introduced in ZnTe by ion implantation but oxygen dissolution in Te sites is much more complete if a dual implantation (O + Zn) is carried out.

The admittance spectroscopy technique is shown to be a very convenient tool for analyzing majority carrier traps: energy level, capture cross section and concentrations are easily obtained without complicated mathematical treatment. The series resistance in the material underlying the Schottky or pn junction is also detected when the free carriers are freezing out. It allows to get the shallowest level energy and its compensation ratio. The method has been applied to ZnTe material analysis and the effect on the admittance of six different acceptors is demonstrated. When comparing the electrically determined ionization energy of a given impurity with its optical value the former appears as systematically lower but most of the difference can be ascribed to Poole Frenkel or impurity concentration effects.

The authors report the first clearly identified neutral double acceptor in any II-VI semiconductor, observed in Zn-annealed ZnTe. Optical properties of the associated bound exciton (BE) clearly identify the number of holes present in the exciton and the acceptor ground state and establish Td site symmetry. Zeeman and uniaxial stress perturbation spectroscopies were both required to establish the character of the double acceptor ground state in which the crystal-field ( Gamma 3- Gamma 5) splitting of the J=2 hole-hole state ( approximately 4.8 meV) is unexpectedly large compared to the J=0-J=2 splitting, ( approximately 0.9 meV), so that the Gamma 3(J=2) state of the neutral double acceptor lies lowest and is approximately 2 meV below the Gamma 1(J=0) state. A fourfold splitting is seen in the (2s1,1s1) excited state of the double acceptor when this is observed in the satellites of the main BE line. This is attributed to the presence of states with J=0, 1, 2 and 3 due to the coupling of the spins of the two holes, although residual crystal-field splittings may also be significant.

The electrical and optical properties of ZnTe or CdTe crystals are generally inhomogeneous after annealing. This is shown to be due to the redistribution of preexisting impurities. Indeed, after growth, these crystals contain some excess tellurium which must precipitate since the solidus line has a retrograde shape. The liquid Te droplets are then able to purify the surrounding materials by a solid‐liquid segregation mechanism. On annealing, the tellurium excess can disappear more or less rapidly: the Te precipitates and the small inclusions shrink progressively while the impurities are released into the crystal. This mechanism explains the bright dots observed in the scanning electron microscope (cathodoluminescence mode) and due to the high lithium concentration. Under other conditions, dark dots are obtained and the role of copper is suspected. This model is discussed on the basis of the available data (segregation and diffusion coefficients). Further confirmation is obtained from direct chemical analysis of the crystal and of the inclusions itself. The consequences of the segregation of impurities on the interpretation of the self‐compensation problem are analyzed. A fully satisfactory interpretation of conductivity variation with zinc pressure previously attributed to the zinc vacancy, can be derived with the single hypothesis that acceptor and donor impurities in sufficient amount are stored in the crystal. The dominant role of residual impurities in the close control of the II‐VI materials is particularly emphasized.

Several complex defects in ZnTe created by Ag diffusion at a rather high doping level are studied by optical spectroscopy. In addition to the usual substitutional ${\mathrm{Ag}}_{\mathrm{Zn}}$ acceptor bound exciton (BE) at 2.3737 eV, a new prominent BE, ${S}_{1}$, with a lowest energy of 2.3149 eV appears. This transition has a strong phonon coupling and corresponds to a neutral isoelectronic defect. Two electronic states at zero field are revealed by transmission data; a doublet at the lowest state (2.3149 eV) and a singlet at slightly higher energy (2.3155 eV). The electronic properties of the ${S}_{1}$ BE are revealed by optical data, including the magneto-optical Zeeman spectra. The electronic structure can be understood as a consequence of a strong compressional axial local crystal field in combination with an electron-hole exchange interaction. It is further concluded that both carriers are bound to the complex defect by an attractive central-cell potential. The identity of the ${S}_{1}$ defect as a pair of substitutional ${\mathrm{Ag}}_{\mathrm{Zn}}$ and interstitial ${\mathrm{Ag}}_{\mathrm{i}}$ in the 〈111〉 direction is consistent with the trigonal symmetry observed in magneto-optical data. In addition to the ${S}_{1}$ defect, several acceptorlike complex defects are created, of which ${S}_{2}$ is a BE with its lowest electronic line at 2.3486 eV, and ${S}_{3}$ similarly at 2.365 eV. These BE excitations give rise to several electronic levels both in the ground state (the acceptor hole) and in the excited state (the BE state). The complicated electronic structure can be explained by a combination of a low-symmetry crystal field and an exchange interaction. An identification of the defects ${S}_{2}$ and ${S}_{3}$ as composed of three atoms is suggested.

Etude des spectres de photoluminescence avec excitation par un laser a colorant accordable. A partir d'une comparaison avec des donnees de transmission optique, on conclut que la structure ondulatoire n'est pas reliee a la densite d'etats des excitons lies, et qu'elle est une manifestation spectrale directe du transfert d'exciton aniste par phonon, entre accepteurs neutres

An extensive study of complex defects induced by Cu diffusion in ZnTe is reported. About thirty-five different bound excitons are observed in optical spectra at low temperatures in the range 2.0–2.4 eV, all related to different neutral (“isoelectronic”) or acceptor-like Cu-related complexes. The electronic structure of these bound excitons is studied in detail with a combination of optical emission and absorption spectra, together with Zeeman data and FTIR transmission. In most cases the electron is the loosely bound particle in the bound exciton for both acceptor complexes (“pseudo-donor model”) and neutral complexes (HTL model). The hole is more localized, presumably due to the hole-attractive CuZn pseudopotential. For acceptor-like complexes usually only one electronic bound exciton line is seen, while other electronic states are lifted above the bandgap energy by the action of the local stress field. Similarly neutral complexes usually have a single bound hole state in the gap, derived from a CuZn central cell. Further, novel electronic satellite spectra in photoluminescence and resonant Raman scattering, seem to be common for Cu-related complex defects at high doping levels. Another interesting effect is a strongly enhanced phonon coupling observed in the absorption spectrum of these complex bound excitons, compared to the coupling strength in the emission case. The possible identities of these Cu-related complex defects are discussed, considering their observed symmetries and the available information on chemical impurities present in the starting material. Several of these complex defects are thermally unstable at room temperature, presumably typical for the cases where an interstitial Cui is part of the complex.

Capacitance, photoluminescence and infra-red absorption experiments have been performed on copper doped ZnTe single crystals. Comparative measurements on samples treated at various temperatures show clearly that Cu introduces a single acceptor level with high solubility. Spectroscopic results allow to identify the persistent acceptor “a” at 149 meV as copper on substitutional zinc site confirming that native defects are not responsible for the p-type conductivity of undoped ZnTe.

Novel recombination lines are observed as electronic satellites to bound excitons (BE:s) in photoluminescence (PL) spectra of Cu-doped ZnTe, when the doping is sufficiently-strong (1017-1018 cm−3) that complex Cu-related defects dominate the BE spectra. These replica are observed at discrete energies in the range 90-170 meV below the PL lines caused by the direct recombination of the BE state. The strength of these satellites is typically 5-10% of the parent BE line, and has a similar width (~0,2 - 0,4 meV). Such satellites occur with BE recombination for acceptor BE:s (ABEis) as well as isoelectronic complex BE:s (IBE:s). They also occur in a similar way in resonant Raman scattering (RRS). Magnetooptical data at 10T reveal an extremely complicated Zeeman splitting pattern of these satellites. The collected experimental data from selective dyelaser excited luminescence (SPL) spectra and excitation spectra (PLE) together with RRS and Zeeman results, suggest a recombination model where the BE state is interacting with a neutral acceptor state on a different site,, In the recombination process final state electronic shake up excitations of the type 1s-2s are observed on acceptors which are interacting with the defect site carrying the initial state BE excitation.

Stress and Zeeman experiments performed on the luminescence of the A X 1 bound exciton allowed unambigous identification of a shallow double acceptor in Zn Te.

High energy electron irradiation of ZnTe crystals at 4 K gives rise to a new luminescence line at 2.361 eV and to a broad band at 1.578 eV. These features disappear on annealing at T⪖ 180 K. The defect responsible for these radiative transitions is tentatively identified with Frenkel type close pair. Another broad line at 2.065 eV appears after a 77 irradiation. Its annealing temperature is near 300 K.

Characterization of microplasmas has been performed on InP‐In0.53Ga0.47As avalanche photodiodes using electron induced current and low‐temperature cathodoluminescence measurements. Microplasmas are shown to be associated with a local increase of the electric field resulting from doping fluctuations which are tentatively associated with impurity segregation at crystal imperfections.

A new method for the analysis of heterostructures is described and applied to the characterization of InP‐InGaAs and AlInAs‐InGaAs interfaces. The technique’s principle relies on the selective quenching of the cathodoluminescence of each layer induced by application of an electric field. The cathodoluminescence quenching curves as a function of reverse bias voltage give the position of the interfaces in good agreement with the capacitance versus voltage profile, and hysteresis in the luminescence behavior is ascribed to deep levels at or near the interfaces.

Efficient radiative defects are introduced in silicon by annealing at 450 °C. They are associated with very sharp and intense transitions at 0.926 eV (H line) and at 0.767 eV (P line). A close correlation has been established between the intensity of these lines and the oxygen content of the sample. The carbon content correlates also with the line H. It is shown that the radiation induced lines at 0.79 eV (C line) and at 0.97 eV (G line) are similarly influenced by the oxygen and carbon content. Although the detailed nature of the centers responsible for H and P lines is not known it can be put forward that their formation is controlled by Oi migration, and that the center leading to H must be similar to the G center (CSi–Sii−CSi).

Most defects in semiconductors are able to bind excitons1. In the case of low doping levels the bound excitons (BE:s) may be treated as non-inter- acting localized excitations, even though the BE wave function may be quite extended in space. A higher doping level may influence BE spectra in several ways, as briefly discussed in this contribution. A trivial effect is that high doping usually leads to the formation of complex associates involving the dopant atoms, in addition to the simple substitutional defects dominating at low doping levels. Often a large number of such complex BE spectra are present simultaneously in optical spectra.