This paper reports the incorporation of erbium into MBE Si and Si/Ge alloys with substrate temperatures of 500°C and 700°C. Using a solid source MBE system, concentrations of erbium between 1018 and 1022 cm−3 have been studied by photoluminescence, electrical measurements, SIMS and TEM. We find no shallow donors or acceptors attributable to erbium but we observe a high concentration of deep states with an activation energy of ~360 meV. The photoluminescence output is of greatest magnitude when [Er] =2 × 1018 cm−3. Above this concentration the onset of erbium precipitates can just be observed using TEM and at even higher concentrations structured growths of erbium suicide are apparent. The effect on the optical activity of Si:Er that has subsequently been implanted with oxygen is also reported.

The growth of erbium doped gallium arsenide by MBE at normal substrate temperatures (~580°C) is constrained by a solubility limit of 8×1017 cm−3. This is much less than is desirable for optical emitters using the forbidden 4f transitions of Er3+ to produce radiation at 1.54µm. We have developed an MBE technique where it is possible to produce spherical mesoscopic precipitates containing erbium as a matrix element within the gallium arsenide. Structural and analytical studies indicate that the precipitate is cubic (rock salt) erbium arsenide. The physical size of the precipitates is self limiting as a result of surface migration occurring during MBE growth. By adjusting the growth conditions it is possible to produce an array of uniform erbium arsenide quantum dots of a size chosen from the range 10–20Å. The dot density can be varied by changing the erbium flux.

Two-color optical spectroscopy is used to directly manipulate shallow levels available in Si:Er samples of different parameters. It is observed that the excitation energy provided by a laser operating in the visible can be temporarily stored by trapping photogenerated carriers at shallow centers available in the material. Subsequently, this energy can be transferred to the 4f-electron: core of the Er3+ ion in a trap ionization process induced by a mid-infrared pulse from a free-electron laser. In that way, Er-related luminescence at 1.5 mu m can be generated by an infrared pulse applied within several milliseconds after the band-to-band excitation pulse. By scanning the wavelength of the free-electron laser, ionization-spectra of the shallow centers participating in the energy transfer are obtained, allowing for their identification. On that basis, the involvement of thermal donors is suggested. The results demonstrate that excitation of Er ions in Si is a multichannel energy transfer;,process where shallow centers play an important role.

Two-color optical spectroscopy is used to directly manipulate shallow levels available in Si:Er samples of different parameters. It is shown that Er photoluminescence can be quenched with a free-electron laser pulse applied shortly after the band-to-band excitation. For longer delay times between the visible and the infrared pulses an enhancenment of Er photoluminescence is observed. The effect is explained by the energy storage due to trapping of the photo generated carriers at shallow centers. These are subsequently ionized by the infrared beam and their recombination energy is transferred to the 4f-electron core of the Er3+ ion. In that way Er-related luminescence at 1.5μm can be generated by an infrared pulse applied within several milliseconds after the band-to-band excitation. By scanning the wavelength of the free-electron laser ionization spectra of the shallow centers participating in the energy storage are obtained providing their fingerprint. Presented results convincingly demonstrate power of the free-electron laser as a novel tool for spectroscopic applications in materials science of silicon.

Positron annihilation experiments reveal an open-volume defect in the deep state atomic configurations of bistable donors In and Ga in CdF2. The size of the open volume is at least half of a monovacancy. The results are similar to those obtained previously for the DX centers in the covalent system AlxGa1-xAs. It is therefore likely that the bond-breaking mechanism (substitutional to interstitial atomic motion) responsible for metastability of point defects in covalent semiconductors is more universal and its validity extends to highly ionic compounds, similar to CdF2. [S0031-9007(99)08958-9].

Semiconductors doped with rare-earth (RE) elements have attracted a lot of attention as alternative materials for producing electrically pumpe d semiconductor lasers whose emission wavelength is very weakly dependent on temperature. This prospect is especially attractive in the case of indirect-gap Silicon, whose photonic applications as the material for light emitters still remain more of a hope than a reality. In view of a desirable emission wavelength at 1.5 μm, a lot of research has concentrated on Si:Er (see Coffa et al. for a recent review). It is generally recognized that doping with Er ions presents one of the most promising approaches to Silicon photonics. However, despiteintensive investigations, stimulated emission has not been conclusively demonstrated for Si.Er or for any other RE-doped semiconductor. This is in striking contrast to optical amplifiers and lasers based on various erbium-doped glasses. In this article, which builds on recent articles in MRS Bulletin on Silicon photonics, we will address the issues relevant to efficient light generation by semiconductors doped with RE elements in general, and specifically by Si:Er-based structures. The intraimpurity electronic structure of RE ions is dominate d by electron-electron and spin-orbit interactions within the 4 f shell. In the case of Er 3+ , they produce separated J-multiplets with 4 I 15/2 and 4 I 13/2 as the ground and the lowest-lying excited states, respectively. Due to the effective Screening of 4 f electrons by the outer electron Shells, the host has a very limited influence and changes only slightly the relative positions of the levels. Depending on a particular site symmetry, the even terms of the crystal field split the free-ion J-multiplets into the Stark components typically by several meV for the ground State. The energy-level diagram of an Er 3+ ion in a cubic crystal field is shown in Figure 1, where the energy transfer paths relevant for Si:Er are also schematically indicated. The odd terms of the crystal field potential admix the states of opposite parity to the 4 f 11 configuration of the Er 3+ ion, thereby introducing a certain degree of electric-dipole strength into the otherwise forbidden intra-4 f -shell transitions. This effect enhance s slightly the magnetic-dipole strength of the 4 I 15/2 ↔ 4 I 13/2 transition and is host- and site-dependent. There-fore, Er-related center s of different microstructure can be fairly easily identified.

Positron annihilation experiments reveal an open-volume defect in the deep state atomic configurations of bistable donors In and Ga in CdF2. The size of the open volume is at least half of a monovacancy. The results are similar to those obtained previously for the DX centers in the covalent system AlxGa1-xAs. It is therefore likely that the bond-breaking mechanism (substitutional to interstitial atomic motion) responsible for metastability of point defects in covalent semiconductors is more universal and its validity extends to even some highly ionic compounds, like CdF2. (C) 1999 Elsevier Science B.V. All rights reserved.

Results of a two-color spectroscopy in the visible and the mid-infrared on erbium-doped silicon (Si:Er) are presented. In the experiments, pulsed beam provided by a free-electron laser is directed on a sample under primary above-band-gap excitation with another laser. It is shown that the powerful infrared beam can be ionize carriers localized at shallow traps. Liberation of these carriers makes them available for excitation of erbium and thereby enhances the luminescence intensity. Identification of shallow levels responsible for the effect is discussed.

An enhancement of 1.5μm Si:Er photoluminescence by a mid-infrared pulse from a free-electron laser is investigated in detail. It is concluded that the effect is a consequence of defect-related energy storage in Si:Er samples. Carriers generated by a band-to-band excitation are participating not only in the excitation of Er luminescence via the excitonic mechanism, but are also trapped at various defect states. The infrared pulse photoionizes them, thus promoting extra carriers into the excitation channel of the Er3+ ion and leading to an additional luminescence. By scanning the wavelength of the free-electron laser ionization spectra of shallow centers participating in the energy transfer are obtained. The results also elucidate a special role of oxygen in Si:Er luminescence.

The recording of an image hologram in CdF2:Ga crystals at room temperature is reported. Writing the phase holographic pattern utilizes a unique property of Ga and In doped CdF2 crystals, namely, the bistability of these dopants. In contrast to CdF2:In crystals, in which there is only one bistable center, the holographic experiment reveals two different bistable centers in CdF2:Ga. Room temperature bistability is related to Ga, while the low temperature center resembles very much the properties of In, likely a non-intentional In dopant of the CdF2:Ga crystals. The holographic sensitivity of doped CdF2 crystals compares favorably with sensitivity of standard photorefractive materials. The main advantage for some applications of using the novel mechanism is the inherently local nature of the light-induced refractive index changes.

Participation of shallow states in the energy transfer processes between the rare-earth ions and a host semiconductor matrix is discussed. It is argued that shallow levels link the atomic-like states of the inner core of the rare-earth ion with the band structure of the host crystal. For the two most investigated systems InP:Yb and Si:Er the important role of these states at various stages of excitation and de-excitation mechanisms is shown. Models of the energy transfer process with a formation of a shallow intermediate state are discussed. It is shown that the formation of such a state is essential for the RE ion core activation. Experimental results supporting the involvement of shallow states are reviewed. Temperature dependencies of the photoluminescence activation and decay time constants are presented; these show an important role of shallow doping in the activation of Auger processes relevant to the mechanism of rare-earth ion photoluminescence. For the Si:Er system direct evidence for the two-stage excitation mechanism is discussed.

A novel technique for measuring differential lattice dilation by scanning tunneling microscope is proposed. The method was used to measure the metastable lattice dilation caused by a deep-shallow phototransformation of bistable In impurities in CdF2 crystals. A total linear crystal shrinkage of 1.8×10−6 results from a partial counterbalancing of the lattice contraction associated with the photoionized deep localized In state by a lattice expansion caused by weakly bound electrons at the shallower hydrogenlike state of the bistable In donors.

Lattice relaxation accompanying phototransformation of In bistable centers from the ground, deep state to the shallow state in CdF2 crystal has been measured with the use of scanning tunnelling microscope.It is shown that relatively small macroscopic changes of the crystal length in the order of 1.8 x 10 -6 accompany the phototransformation of In ions.Lattice expansion upon the influence of population of shallow donor levels in CdF2 explains the observed small changes of lattice constant during the process.

We report measurements of photoinduced magnetisation of CdF2 bulk crystals doped with In or Ga. At temperatures at which metastable photo transformation of donors occurs, magnetic susceptibility increases and it follows a Curie law. Low magnetic field data are dominated by spin-spin ferromagnetic coupling of d(5) electrons of Mn codopants with donor electrons of In or Ga metastably phototransferred from a localised to the hydrogenic shallow donor state. Enhancement of magnetisation is due to effective spin increase resulting from this coupling. At high magnetic fields (above 2 T) and temperatures below 5K, saturation of the photoinduced magnetisation is observed. This provides first direct proof of a negative-U character of the donors in CdF2.

It is shown that metastable centers in various semiconductors can be used for efficient hologram recording. This type of holographic materials has a very high dynamic range and sensitivity. Their major drawback for possible applications is low metastability temperature. This problem can be overcome by using CdF2:Ga crystals, which exhibit metastability below 240 K. This material is suitable for writing thick and multiple holograms both in static and dynamic regimes.

We demonstrate that semiconducting CdF2 crystals doped with gallium provide an efficient medium for optical storage of information in static and dynamic regimes in a temperature range close to 300 K. Ga is a bistable center in CdF2 crystals. Illumination by visible and UV light below 500 nm causes phototransformation of these centers from a deep-localized to a shallow-hydrogenic state. They are separated by a vibronic barrier that causes metastability below 250 K. The phototransformation changes the local polarizability, and thus, the local refractive coefficient. This, in turn, allows writing a phase hologram with a diffraction efficiency and decay time being temperature dependent with the activation energy Eact=0.65±0.1 eV. A spontaneous decay of the grating is caused by a thermal recovery of the Ga impurity from the metastable hydrogenic state to the localized ground state. The writing is a local process.

Contraction of CdF2 crystals during phototransformation of In bistable centres has been measured with use of scanning tunnelling microscope. The observed change of the lattice constant (-1.8x10(-6)) is metastable at liquid helium temperature. This result confirms that the large lattice relaxation is responsible for the metastability of In dopant in CdF2.

We show that DX-like centers in Cd1-xMnxTe1-ySex:In crystal can be used in writing volume holographic gratings. The scattering efficiency is higher than 10% which proves the dispersive character of the light-induced gratings. Two different metastable centers with different lattice relaxation were found ill the crystal. Measurements of the power dependence of the degenerate four-wave mixing scattering efficiency testify that both of these centers have negative U properties.PACS numbers: 74.62.Dh, 61.72.Ji, 42.65.Hw

The cause of carrier removal in GaAs due to plasma processing is found to consist of two mechanisms from a photoreflectance depth-profiling study. In the close vicinity of the surface, the Fermi level is pinned by the defects created by ion bombardment. In a much deeper region, electrical passivation of doped impurities themselves is playing the dominant role. Secondary-ion mass analysis confirmed that hydrogen diffusion is indeed responsible for the latter process. We suggest that the physically damaged surface layer acts as a sponge for even a trace amount of hydrogen in the processing environment.