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The systematic features of the formation of Ge nanocrystals in SiO2 thin films implanted with Ge ions and then subjected to high-temperature annealing (1130°C) are studied in relation to hydrostatic pressure. It is established that annealing at atmospheric pressure is accompanied by the diffusion of Ge atoms from the implantation region to the Si substrate and does not induce the formation of Ge nanocrystals. An increase in pressure during annealing yields a deceleration in the diffusion of germanium into silicon and is accompanied by the formation of twinned lamellae at the Si/SiO2 interface (at pressures of ~103 bar) or by the nucleation and growth of Ge nanocrystals (at pressures of ~104 bar) in the SiO2 film. The results are discussed on the basis of the concept of a change in the activation volume of the formation and migration of point defects under conditions of compression.

Si nanocrystals (NCs) have widely been used as sensitizers in Er-doped SiO2 layers where following the recombination of excitons (electron-hole pairs) in optically excited Si NCs energy is transferred to the higher energy levels of the 4f shell of Er3+ ions and subsequently decay radiatively via 4I13/2 → 4I15/2 transition. This as a result emits light at ~1.53 m, which coincides well the silica-based optical fibers. However, the temperature and concentration quenching of Er3+ ions in Si-rich SiO2 matrices are the basic obstacles for practical application. In the quest for an alternative host material, Ge-rich SiO2 layers have presently attracted a considerable interest due to strong quantum confinement effect and a better control of the surface oxidation of Ge NCs with respect to Si-NCs. In fact, the growth of Er-doped Ge nanoperticles embedded in a SiO2 layer and the photoluminescence (PL) properties have recently been studied. Despite progress in PL response, electrically pumped light-emitting devices (LEDs) are highly required from the optoelectronic standpoints. We have investigated Ge NCs embedded in a metal-oxide semiconductor (MOS) structure co-doped with Er3+ ions and the corresponding electroluminescence (EL). Here we follow the two step implantation approach: (i) First, 130 keV Ge ions have been implanted (dose of 2–6E16 ions/cm2) into a thermally grown 200 nm thick SiO2 layer followed by furnace annealing (FA) at 950 C for 60 min, which is followed by (ii) 250 keV Er implantation (dose of 1-5E15 ions/cm2) combined with FA in the range of 800-1100oC for 30 min in nitrogen ambience. The MOS structures have been fabricated by depositing indium-tin-oxide (ITO) and aluminium in the front and rare sides of the samples, respectively, and patterning the ITO layer using photolithography. Interestingly, instead of electrically driven pumping of Er3+ ions by Ge NCs, we found that an EL band ~407 nm associated to the Ge-related oxygen-deficiency centres (GeODCs) have been pumped by Er3+ ions and discuss the observed phenomenon on the ground of an inverse energy transfer process. Concentration and temperature dependent modification of microstructure and the subsequent impact on EL response will also be discussed.

For a comparative study of the energy transfer mechanism during room temperature electroluminescence (EL), two sets of samples have been prepared by implanting either Ge and Er ions (type-I) or Si and Er ions (type-II) into a 200 nm SiO2 layer combined with rapid-thermal-annealing. Three reference samples have been prepared by implanting only Er, Ge or Si ions in SiO2 followed by post-implantation annealing. The formation of Si and Ge nanocrystals (NCs) was confirmed by transmission electron microscopy. The metal-oxide-semiconductor dot structure was prepared by depositing indium-tin-oxide and Al films on the front and rear sides of the structure. Quantum-confinement mediated recombination of carriers in Si-nanocrystals (Si-NCs) as well as the triplet (T1) → singlet (S0) transition in the O3-Si-Si-O3 oxygen deficient centre (ODC) provides the 750 and 475 nm bands, respectively, in the only Si-implanted sample. Conversely, the 525, 550, 660 and 1532 nm emissions corresponding to the deexcitation of the 2H11/2, 4S3/2, 4F9/2 and 4I13/2 states to the ground state 4I15/2 of the Er3+, respectively, were recorded for the Er-doped SiO2 layer. A blue-violet light at ~400 nm, representing the T1→S0 transition in the O3-Si-Ge-O3 ODC, was observed in the Ge-NCs embedded SiO2 layer. Although the intensity of the Er-related visible/infrared EL signals in type-I sample was found disappearing/decreasing with a concomitant enhancement of the 400 nm band, a sharp rise in intensity of the 1532 nm Er EL at the expense of the Si-NC related 750 nm band was evidenced in type-II sample. While the later result confirms the energy transfer mechanism from Si-NCs to the nearest Er3+, the former finding can be explained in terms of the energy transfer from Er3+ to the Ge-related ODC.

It is well established that Si nanocrystals (NCs) can act as sensitizers in Er-doped SiO2 during optical pumping [1-4]. In fact, following the recombination of excitons (electron-hole pairs) in optically excited Si NCs transfer their energy to the higher energy levels of the nearest Er3+ ions and subsequently decay to the ground state by intra-4f transitions. Among those, the 4I13/2 → 4I15/2 radiative transition has attracted substantial interest since the respective luminescence at ~ 1.53 m corresponds to the maximum transparency of silica-based optical fibers [2]. Ge is another group-IV element with similar electronic properties to that of Si. We have examined the impact of Ge NCs in Er-doped SiO2 layers by investigating electroluminescence (EL) of the metal-oxide semiconductor (MOS) structures, where the Er-doped Ge-rich SiO2 layers have been prepared by ion implantation technique combined with rapid thermal annealing (RTA). The samples have been prepared by two steps: (i) 130 keV Ge ions have been implanted with a dose of 2  1016 ions/cm2 in a 200 nm thick thermally grown SiO2 layers followed by RTA at 1050 oC for 180 s, and subsequently (ii) 250 keV Er ions have been implanted with a dose of 1  1015 ions/cm2 followed by RTA in the range of 850-1050 oC for 6-150 s in nitrogen ambience. Transmission electron microscopy experiments reveal formation of randomly oriented Ge NCs with average size ~4 nm. The MOS structures have been fabricated by depositing indium-tin-oxide (ITO) and aluminium in the front and rare sides of the samples, respectively, and patterning the ITO layer using photolithography. During EL measurements, in absence of the visible range band correlated to the quantum confinement in Ge NCs a band appears at ~400 nm in Ge-rich SiO2 layer as a consequence of hot electron mediated impact excitation in Ge-related oxygen-deficiency centres (GeODCs) during electrical pumping [5]. We find an increase of the 400 nm EL intensity with a concomitant reduction of the Er-related emission, and discuss the observed phenomenon on the ground of an inverse energy transfer process [4] from excited Er3+ to the GeODCs. [1] A. Polman, J. Appl. Phys. 82, 1 (1997). [2] O. Savchyn, F. R. Ruhge, P. G. Kik, R. M. Todi, K. R. Coffey, H. Nukala, and H. Heinrich, Phys. Rev. B 76, 195419 (2007). [3] M. Fujii, K. Imakita, K. Watanabe, and S. Hayashi, J. Appl. Phys. 95, 272 (2004). [4] I. Izeddin, A. S. Moskalenko, I. N. Yassievich, M. Fujii, and T. Gregorkiewicz, Phys. Rev. Lett. 97, 207401 (2006). [5] L. Rebohle, J. von Borany, R. A. Yankov, W. Skorupa, I. E. Tyschenko, H. Fröb, and K. Leo, Appl. Phys. Lett. 71, 2809 (1997).

Optically active Er3+ ions dispersed in a SiO2 layer are often used for amplifying signals at the wavelength of ~1.5 m that coincides with the maximum transparency window of silica-based optical fibers. In order to enhance the 1.5 m Er luminescence, a sufficient number of Er ions must be in the excited state, and it is only possible by employing a high-power pumped laser. Si nanocrystals (NCs) embedded in an Er-doped SiO2 layer show a new avenue to excite Er3+ ions indirectly, where Si-NC behaves like a sensitizer [1]. The basic mechanism of the above mentioned system relies on three facts: (i) optical excitation of Si-NC, (ii) nonradiative transfer of energy from Si-NC to the neighbouring Er3+, and (iii) subsequent relaxation from the first excited state to the ground state of Er3+ by emitting light at 1.5 m. However, to integrate these photonic systems into current microelectronics, it is essential to fabricate metal-oxide-semiconductor based light-emitting devices. Based on electroluminescence (EL) experiments, some groups have shown the importance of Si-NCs in enhancing the 1.5 µm Er luminescence. Here we use an experimental approach for evaluating the influence of Ge NCs on the Er EL in Er-doped SiO2 layers. In particular, we demonstrate an increase in intensity of the 400 nm band, characteristic of the Ge-NC related oxygen-deficiency centres (GeODC), at the expense of the Er-related signals with maxima at 522, 550, 660 and 1532 nm during electrical pumping at room-temperature  indicating an energy transfer process from the excited Er ions to the confined carriers in the GeODC, which is just opposite to the concept commonly used for the Er-doped SiO2 layers containing Si-NCs. Formation of Ge-NCs was verified by transmission electron microscopy. [1] Polman A, Nature Materials 2002;1:10.

The impact of rapid thermal annealing (RTA) in producing samples by sequential implantation of Si and Er ions into a 200 nm SiO2 layer combined with different annealing cycles as well as the corresponding room-temperature visible and infrared photoluminescence (PL) have been studied. The Er-related PL intensity at 1533 nm for the samples prepared by implanting Si with subsequent annealing, followed by Er implantation and final annealing (type-I) was found to be stronger than the one produced similarly but without the first annealing step (type-II). In fact, the 1533 nm peak intensity in the optimized RTA processed sample is comparable to the PL yield of the furnace-annealed sample. Moreover, the excitation wavelength (405 nm) was found to be suitable for exciting the Si=O related point defects in the SiO2 layer, and can provide a PL band with a maximum at ~580 nm. While this band was further intensified in presence of Si nanocrystals (Si NCs), it became weaker by introducing additional Er3+ ions with a concomitant rise of the 1533 nm Er PL, confirming the visible range pumping of Er3+. The detailed spectral analyses suggest that the 580 nm band is the result of the excitation/deexcitation mechanism in molecule like states in Si=O or the Si=O state mediated recombination of carriers in Si NCs according to the model proposed by Wolkin et al. [Phys. Rev. Lett. 82, 197 (1999)]. The samples were further characterized by transmission electron microscopy and Fourier-transform infrared-spectroscopy. The time resolved PL measurements and a modelling by rate equations were also performed to determine and justify the energy migration mechanism from Si NC to the neighbouring Er3+.

The Er-doped SiO2 layers containing Si nanocrystals (Si-ncs) have attracted considerable interest for more than a decade in realizing efficient light sources at 1.54 um [1,2], owing to the coincidence of the luminescence wavelength to the absorption minimum of the silicon based optical fibers. Although about two orders of magnitude Er luminescence has been noticed in presence of Si-ncs in a long time annealed sputtering deposited samples [3], observation of such high efficiency in ion implantation processed samples is scarce. Recently, we have succeeded in producing such a system by a combination of sequential Si and Er implantations and rapid thermal annealing (RTA). The processing conditions have been optimized for achieving maximum Er photoluminescence (PL) at 1533 nm at the expense of a broad luminescence band peaking at ~580 nm. Spectral analyses suggest that the appearance of such visible range PL band can be explained in the light of the interfacial state mediated recombination of carriers in the Si-ncs according to the quantum-confinement model [4]. The energy migration from Si-ncs to the nearby Er ions has further been manifested using time-resolved PL measurements. [1] A. Polman, J. Appl. Phys. 82, 1 (1997). [2] G. Franzò, S. Boninelli, D. Pacifici, F. Priolo, F. Iacona, and C. Bongiorno, Appl. Phys. Lett. 82, 3871 (2003). [3] M. Fujii, M. Yoshida, Y. Kanzawa, S. Hayashi, and K. Yamamoto, Appl. Phys. Lett. 71, 1198 (1997). [4] M. V. Wolkin, J. Jorne, P. M. Fauchet, G. Allan, and Delerue, Phys. Rev. Lett. 82, 197 (1999).

The Er-doped SiO2 layers containing Si nanocrystals (Si-ncs) have attracted considerable interest for more than a decade in realizing efficient light sources at 1.54 um, owing to the coincidence of the luminescence wavelength to the absorption minimum of the silicon based optical fibres.1 Although about two orders of magnitude Er luminescence has been noticed in long time annealed sputtered deposited samples,2 observation of such high efficiency in ion implantation processed samples is scarce. Recently, we have succeeded in producing such a system by a combination of sequential Si and Er implantations and rapid thermal annealing (RTA). The processing conditions have been optimized for achieving maximum Er photoluminescence (PL) at 1533 nm at the expense of a broad luminescence band peaking at ~580 nm. Spectral analyses suggest that the appearance of such visible range PL band can be explained in the light of the interfacial state mediated recombination of carriers in the Si-ncs according to the model proposed by Wolkin et al.3. The energy migration from Si-ncs to the nearby Er ions has further been manifested using time-resolved PL measurements. [1] A. Polman, J. Appl. Phys. 82, 1 (1997). [2] M. Fujii, M. Yoshida, Y. Kanzawa, S. Hayashi, and K. Yamamoto, Appl. Phys. Lett. 71, 1198 (1997). [3] M. V. Wolkin, J. Jorne, P. M. Fauchet, G. Allan, and Delerue, Phys. Rev. Lett. 82, 197 (1999).

Endotoxial growth of InSb nanocrystals