The interest in rare earth (RE) elements as luminescent centers is due to the narrowness and stability of the luminescent transitions. In this paper we review the mechanisms that can be used to electrically excite rare earth impurities in semiconductors: pair recombination energy exchange and impact excitation. The different means of providing energetic electrons for impact excitation are also discussed. We also propose a possible explanation for the temperature dependence of photoluminescence in silicon.

Time-resolved excitation and relaxation processes of the impact excitation emission (EL) at 1.54μm of Er3+ ions doped in InP were investigated in the temperature range from 77K to 330K. The decay process was almost exponential in all the measured temperature range and showed little thermal quenching. The decay time of 2ms at 77K decreased only to tms at 330K. This result contrasted with the large thermal quenching and nonexponential characteristics of the photoluminescence (PL) time decay at higher temperatures, suggesting different Er3+ centers excited between EL and PL. A two-emission-center model is proposed and the different behaviors of thermal quenching and time decay between EL and PL emissions are consistently explained.

We report the first study of impact excitation of Er ions in GaAs. The MOCVDgrown, p+-n structured EL devices were fabricated by growing, at different temperatures, GaAs:Er layers on top of the n+ GaAs substrates. P+ layers were made by Zn diffusion from the top surfaces. When we forward biased these diodes, their EL spectra were similar to their respective PL spectra for each sample but different from each other's. However, when we reverse biased these diodes, EL spectra obtained from all samples are the same, which were different from their PL spectra. These results indicate that the Er center(s) excited by direct impact is different from the Er center(s) excited through electron-hole recombination and subsequent energy transfer. By using RBS channeling, we found that most of the Er ions, in our MOCVD-grown GaAs:Er samples, occupy a displaced tetrahedral interstitial site. From these PL, EL and RBS results, we conclude that only a small amount of Er ions emit luminescence when they are indirectly excited through energy transfer.

Heterostructures consisting of III-V semiconductors and epitaxial layers of the rare earth monoarsenides can be grown by molecular beam epitaxy. By alloying ErAs with ScAs, lattice match can be achieved with (Al,Ga)As. Using magneto-transport measurements, we show that these layers are semi-metallic with equal electron and hole concentrations, 3.0 ×1020 cm−3. Shubnikov-de Haas oscillations are used to confirm the predicted Fermi surface geometry and measure the electron effective mass and the coupling to the 4f spin on the Er3+ ion. Remarkably, the material shows no transition from semimetal to semiconductor as the film thickness is reduced to three monolayers. Below three monolayers the films are not uniform and are believed to consist of islands three monolayers high.

This system provides a unique opportunity to explore novel electronics based on controlled transport through semimetal/semiconductor heterostructures. Lateral transport through semimetal islands immersed in a δ-doped layer may provide a fast non-linear material for THz electronics. Vertical transport through thin epitaxial layers may enable resonant tunneling hot electron transistors with a semi-metal base. Preliminary experiments on transistor like test structures measure some transfer through a 10 monolayer thick semimetal base. They also identify overgrowth of the III-V semiconductor on the semimetal layer as the key materials issue.

Erbium-substituted Srl-xErxF2+x strontium fluoride layers were grown on InP(100) substrates by sublimation under ultra-high vacuum of high purity solid solution powders.

Their composition and the spatial distribution of the basic components were deduced from Rutherford backscattering (RBS), using a nuclear microprobe, and secondary ion mass spectrometry (SIMS). For x lower than 20% it is shown that the film composition is analogous with the initial sintered material and quite homogeneous. The ageing in air of the films was followed by infrared absorption studies. No significant water adsorption was revealed.

The texture of the layers was studied by RBS in channelling condition. For x as high as 15% it is shown that the films are well crystallized and well epitaxially grown on InP (100) substrates.

Infrared emission due to the 4I13/2-4I15/2 electronic transition on the 4f11 inner shells of Er3+ ions (1.53μm), which presents potential applications for optical communications, was studied at room temperature as a function of x. It is demonstrated that the highest luminescence efficiency of these layers corresponds to x upper than 20%.

By thermally evaporating CaF2 and NdF3, we have grown Nd-doped CaF2 films on Si(111), A1/Si(111) and quarter-wavelength Ta2O5/SiO2 multilayer Bragg reflectors. The optical and structural properties of the CaF2:Nd films are characterized by photoluminescence spectroscopy (PL) and x-ray diffraction. The effects of different Nd concentration, growth temperatures and post-annealing were studied. Regardless of the substrates, the as-grown films show emission lines at wavelengths similar to bulk CaF2:Nd. Annealing the films at 700°C in forming gas results in a new emission pattern. Little difference between the PL spectra of polycrystalline and single crystal CaF2:Nd films is observed, indicating that the luminescence efficiency is insensitive to the crystalline quality of the films.

With the recent success of using rare-earth doped fluoride crystals as high power visible upconversion lasers, we have explored the use of MBE grown fluoride layers for a possible waveguide laser. By confining the pumped light in a waveguide with dimensions on the order of a few micron, the pump power density will increase promoting higher efficiencies at room temperatures. Initially, we have grown planar waveguides of erbium doped ZnF2 on MgF2 substrates using molecular beam epitaxy and have formed channel waveguides by ion milling. By exciting individual channels with an 800 nm pump, we have generated strong upconversion fluorescence at 410 nm, 550 nm and 670 nm and at numerous weaker peaks. The fabrication techniques can be adapted to semiconductor substrates for making compact diode-pumped visible and infrared lasers. A number of fluoride materials that are useful as laser host crystals are lattice matched to GaAs (100) and GaAs (111) offering the possibility of integrating the channel waveguide laser with the semiconductor diode laser pump source. For example SrF2 may be grown on GaAs (100) as a cladding layer followed by PbF2 doped with a rare-earth ion. Also LaF3 may be grown on GaAs (111) followed by CeF3 doped with a rare-earth ion. Both PbF2 and CeF3 have low phonon energies and a higher index of refraction than their respective lattice matched cladding layers and should be capable of provide an attractive upconversion laser waveguide system. Our initial upconversion luminescence results on erbium doped PbF2 on GaAs (100) using a intervening SrF2 cladding layer are 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.

The observation of 1.54 μm luminescence in erbium-doped-GaAs (GaAs:Er) and AlxGal-xAs, has stimulated research efforts because of their potential application to light sources in optical communications. In this paper, we study a different aspect of the photoresponse behavior of this material. The dependence of the carrier lifetime on the doping concentration of Erbium is investigated in GaAs:Er grown by MBE. A reduction in the carrier lifetime down to ∼1 ps is observed for the highest doping (∼5×1019 cm−3) investigated. Together with the high-resistivity observed for the higher doping values, this material serves as a novel photoconductor material for high-speed optoelectronics.

The heteroepitaxial growth of EuTe on PbTe (111) by molecular beam epitaxy (MBE) was investigated using in situ reflection high energy electron diffraction (RHEED). As a function of substrate temperature and Te2 flux rate, the resulting EuTe (111) surfaces exhibit several different surface reconstructions corresponding to Te-stabilized or Eu-stabilized surfaces. The Eustabilized surface shows a (2√3 × 2√3)R30° surface reconstruction. Because of the strain induced tendency for 3D islanding, only in a narrow window of MBE growth parameters perfect 2D layer-by-layer heteroepitaxial growth exists. Using such optimized MBE growth conditions, we have fabricated a series of PbTe/EuTe superlattices. In such superlattices the wide band gap EuTe layers act as barriers and the narrow band gap PbTe as quantum wells. The superlattices were investigated by high resolution x-ray diffraction, showing their high structural perfection. Modulated low temperature Fourier transform infrared reflection measurements were performed in order to determine the confined energy levels in the PbTe quantum wells. The measurements indicate that mini-subbands are formed in the PbTe quantum wells with a mini-band width of 22 meV in agreement with envelope function calculations.

The microstructure and its effects on the photoluminescence properties of SrS:Eu2+,Sm3+ thin films grown with different conditions were studied by transmission electron microscopy, x-ray diffraction and photoluminescence techniques. The SrS:Eu2+,Sm3+ thin films were prepared by e-beam evaporation at different substrate temperatures and growth rates. Both of these growth conditions affect the crystallinity of the thin films. The Sm3+ emission is stronger in the films grown at higher growth rates and at an optimum substrate temperature. We believe that the stronger Sm3+ emission is due to the higher population of Sm trivalent charge states in the films. Further increase of the substrate temperature increases the grain size in the films, but has no significant effect on the PL emission properties. In contrast, the Eu2+ emission is less sensitive to growth conditions.

The report submitted contributes to the series of research works in the spheres of rare-earth (RE) application to electronic industry, and namely, to the production of such devices as surface-barrier structures, solar cells’ elements.

The review of results of KPR, ODMR, RBS and PL investigations of the III-V semiconductors doped with rare-earth elements is represented. The possible sites of RE related centers in hostare discussed. The possible mechanism of excitation of intrashell f-f transitions is considered. The simple theoretical model of the substitutional RE center in a binary semiconductors is discussed.

The chemical trends of rare-earth impurities in semiconductors are analyzed on the basis of a selfconsistent tight-binding Green's function technique. The 4f states are treated as a frozen core and we get essentially no 5d derived states in the gap. We calculate the 4f ionization levels in the solid and conclude that the stable state is mostly 3+. We then describe the results of a LDA calculation for Er in silicon which we find to be stable in the interstitial position. Finally we consider in some detail the case of Yb in InP where we discusss the possible origin of the shallow gap levels and the mechanisms for luminescence excitation and quenching.

In this work we have developed a model for the kinetics of the energy transfer from the host lattice to the localized core excited states of rare earth isoelectronic structured traps (REI-trap). We have derive a set of differential equations for semi-insulating semiconductor governing the kinetics of rare earth luminescence. The numerically simulated rise and decay times of luminescence show a good quantitative agreement with the experimental data obtained for InP:Yb, over a wide range of generation rates.

The electronic structure of Yb3+-doped Si is elucidated in terms of level repulsion between the Si vacancy's deep levels (and spectral density) and the Yb3+ levels, both for bulk Si and for small clusters. The 2F5/2 level of Yb3+ splits into a Γ8 level and a Γ6 level, with the Γ6 repelled most, by the nearby Γ6 (A1) level of the Si vacancy. The level-repulsion is either upwards or downwards in energy, depending on whether the Al-like vacancy level lies below or above this Yb3+ level. The 2F7/2 Yb 3+ level is split into Γ6, Γ7, and Γ8 sub-levels, all moving downwards in energy, with Γ6 moving most, again due to strong level repulsion from the nearby Al-like vacancy level, while the more-distant, higher-energy T2-like (Γ7 and Γ8) vacancy level produces a weaker repulsion. In small clusters, the Si-vacancy's wavefunctions and deep level energies are sensitive to cluster-size, and changes in them alter the level repulsion experienced by the Yb 3+ levels, even though the 4f electrons are localized.

A model of the ground state for a rare-earth element in a binary semiconductor is suggested and illustrated for ytterbium in InP. It is shown that the charge state of Yb3+ with 4f13 shell is more stable than the state of Yb2+ with f 14 shell.