Quantum confinement in rare-earth doped semiconductor systems

Current Opinion in Solid State and Materials Science 7 (2003) 143–149

Quantum confinement in rare-earth doped semiconductor systems A.J. Kenyon* Department of Electronic and Electrical Engineering, University College London, Torrington Place, London WC1 E 7 JE, UK Received 11 March 2002; accepted 5 June 2003

Abstract Rare-earth doped materials are widely deployed in optoelectronics and optical telecommunications technology, despite the difficulties associated with doping silicon with optically active rare-earth ions. Nanotechnology promises new classes of materials and devices that exploit the unique optical and electronic properties of low-dimensional structures; in particular, quantum confinement can modify the electronic structure of semiconductor hosts in such a way as to greatly increase the radiative efficiency of the rare-earth dopant. This article reviews some of the recent developments in the field and highlights possible future directions.  2003 Elsevier Ltd. All rights reserved. Keywords: Rare-earth doped materials; Optoelectronics; Erbium; Nanotechnology; Semiconductors PAC Numbers: 76.30.Kg

1. Introduction The ever-increasing demand for optical sources compatible with fibre communications technology has led to a growing requirement for novel materials exhibiting emission at telecommunications wavelengths. In existing optical fibres the coincidence between the 1535-nm intra-4f 4 I 13 / 2 → 4 I 15 / 2 transition of the Er 31 ion and the principal low loss window in silica is widely exploited to produce erbium-doped gain elements and sources [*1], such as the erbium-doped fibre amplifier, developed in the 1980s [2,3] and now widely deployed in long-haul telecommunications links. The technology continues to develop as ever-larger bandwidths are demanded. However, whilst fibre-based optical amplification is largely a solved problem, the production of a silicon-based optical source operating around 1.5 mm has proved far more problematic. Having an indirect band gap with an energy of 1.1 eV, bulk silicon cannot produce emission in this wavelength region, and therefore must either be structurally modified or doped with an appropriate luminescent ion, the most obvious candidate being erbium. Despite much work in this area [4–6], efficient roomtemperature luminescence from erbium-doped silicon remains elusive, principally due to the low solubility of *Tel.: 144-207-679-3270; fax: 144-207-387-4350. E-mail address: [email protected] (A.J. Kenyon). 1359-0286 / 03 / $ – see front matter  2003 Elsevier Ltd. All rights reserved. doi:10.1016 / S1359-0286(03)00043-3

erbium in bulk silicon combined with very strong nonradiative coupling between the erbium ion and the silicon host [4,**5,*7]. The upper limit of rare-earth ion solubility in silicon is of the order of 10 19 cm 23 , arising from a combination of the mismatch in ionic radii between the Er 31 ion and silicon and the predominantly sp 3 bonding of the semiconductor host. As a result, erbium ions tend to cluster together in precipitates that provide further nonradiative de-excitation pathways. Langer and Sokolov [**8] have presented a compelling argument that spontaneous emission from erbium-doped bulk semiconductors is limited to total emitted powers less than 1 mW for realistic device sizes, an observation that highlights the necessity of increasing the radiative decay probability of erbium in silicon. The optical excitation of erbium in silicon is a complex process, involving carrier generation or injection, trapping at an erbium-related trap level that lies 0.15 eV below the silicon conduction band, and Auger transfer [**5,*9]. The steps involved are illustrated schematically in Fig. 1. The very low room temperature luminescence yield from erbium-doped silicon is due primarily to a combination of Auger quenching and backtransfer to the silicon host. The former (process V) is important at temperatures above 30 K, and because Auger interactions are related to the concentration of free carriers in the silicon host, this process is proportional to the doping level of the silicon host. Backtransfer (process VI) becomes significant for

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also produce long carrier lifetimes and a high degree of localisation in real space. This may both suppress the Auger backtransfer processes that limit luminescence efficiency in bulk silicon, and increase the interaction probability between the confined carriers and the luminescent rare-earth ions.

3. Porous silicon

Fig. 1. Processes involved in the excitation of erbium in bulk silicon. (I) Generation of electron–hole pairs by electrical injection or optical excitation. (II) Trapping of carriers at an erbium-related trap centre in the silicon band gap. (III) Recombination of carriers, leading to excitation of Er 31 from ground state to 4 I 13 / 2 level. (IV) Radiative recombination of Er 31 with emission of 1.5 mm photon. (V) Non-radiative recombination of excited Er 31 by Auger transfer to free carriers. (VI) De-excitation of Er 31 , generating bound exciton at the trap centre (‘backtransfer’: the reverse of III).

temperatures above 130 K, and can lead either to reexcitation of the Er 31 ion, or nonradiative recombination of the carrier pair via phonon coupling. The 0.15 eV required to promote carriers from the trap centre to the conduction band is provided easily by phonons, hence the thermal activation of this mechanism, which is therefore that that most severely limits the room temperature luminescence yield.

2. Confined semiconductor systems Semiconductor quantum structures with dimensions less than the excitonic Bohr radius (|5 nm for silicon) exhibit interesting electronic and optical properties due to the confinement of electrons in one, two, or three dimensions. The principal consequences of quantum confinement are an increase in the band gap energy, and an associated increased probability of radiative transitions. Confinement of carriers in real space causes their wavefunctions to spread out in momentum space, increasing the probability of radiative processes due to greater wavefunction overlap. Because of the modification of their band structure, doping such confined systems with rare-earth ions can also help to overcome some of the nonradiative de-excitation problems associated with bulk silicon. Quantum confinement effects

For some years now, porous silicon has attracted much attention as a material for silicon-based optoelectronics. Confinement of carriers within the nanometre-scale silicon pillars formed by the etching process greatly increases the efficiency of photo- and electroluminescence from silicon. The band gap of porous silicon may readily be controlled by varying the etch conditions, and therefore the porosity of the material. The emission wavelength of the porous silicon may thus be varied by controlling the etch step, and as a result a number of electroluminescent devices have been made that emit in the visible region. Several groups have developed this further and have incorporated erbium into porous silicon by ion implantation, thermal diffusion, or by electrochemical methods, to produce 1.5 mm sources based on silicon [10–13]. Photoluminescence excitation measurements have demonstrated a combination of direct optical excitation and carrier-mediated indirect excitation pathways from Er-doped porous silicon [14]. Erbium-doped porous silicon may most readily be prepared by introducing the rare-earth dopant by diffusion. After anodic etching, the porous silicon is immersed in a solution of ErCl 3 and alcohol. Erbium diffuses into the silicon pores, and a subsequent anneal in an oxygen-rich ambient at 1000 8C activates the Er 31 1.5 mm luminescence band. However, the fragile nature of the host can make activation of erbium in porous silicon problematic, as the use of high annealing temperatures can damage the porous silicon matrix. Capping the porous silicon surface with a layer of silicon nitride can greatly reduce the detrimental effect of high temperatures on the fragile host [15]. Other rare earths, including terbium and europium have been incorporated into porous silicon by similar diffusion techniques, though in these cases no evidence has been reported of carrier-mediated excitation of either Tb 31 or Eu 31 luminescence [16,17]. The photoluminescence intensity from erbium in porous silicon increases with post-diffusion activation temperature, reaching a maximum for anneals around 1100 8C. However, the conductivity of porous silicon falls exponentially with annealing temperature. Likewise, increasing both the thickness and porosity of the porous layer (and therefore the degree of quantum confinement) reduces the conductivity. Care must therefore be taken when designing electroluminescent devices from erbium-doped porous silicon; an activation temperature of around 800 8C repre-

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sents a compromise between luminescence efficiency and conductivity, and produces the most intense emission [*18]. Electroluminescence studies of erbium-doped porous silicon indicate the existence of different luminescence mechanisms for the two biasing conditions. Different temperature quenching effects are observed in forward and reverse bias, with very different activation energies. The implication is that there are at least two erbium-related trap levels in porous silicon [18]. Nevertheless, temperature quenching of erbium emission in porous silicon is less pronounced than in crystalline silicon, possibly due to the increased band gap and the lack of an extended crystalline lattice, though luminescence bandwidths are narrow: around 10 nm at room temperature. The luminescence lifetime of the 1.5 mm emission depends on temperature, being around 1 ms at 300 K; such temperature behaviour suggests that thermal quenching is a result of the thermalisation of carriers localised at erbium-related trap levels [19]. Work by Wang et al. [20] has characterised the optical efficiency of different erbium sites in porous silicon. Comparing samples doped by immersion to those implanted with erbium, temperature quenching was much stronger in the latter, implying that the preferred site for efficient luminescence from erbium in porous silicon is at the surface of the pores rather than deeper in the bulk material. This may be due to the influence of oxide layers around the porous silicon nanowires.

4. Nanocrystalline silicon Nanocrystals of silicon in an amorphous silicon host may readily be produced by, for example, laser annealing [21]. In this system, confinement is produced by the crystalline–amorphous boundary [22], and doping this material with rare-earth ions leads to enhanced rare-earth emission due to the same quantum confinement effects seen in porous silicon. Luminescence at 1.5 mm has been obtained from erbium incorporated into a ridge waveguide of nanocrystalline silicon, and stimulated emission with a pumping threshold of 10 MW cm 22 was reported. In common with porous silicon, erbium-doped microcrystalline silicon films exhibit much weaker temperature quenching effects than is the case for bulk silicon [23]. However, in this case it is thought that the erbium ions lie within the amorphous silicon, which, having a larger band gap energy than bulk silicon (1.6 eV compared to 1.1 eV at 300 K), suffers from backtransfer effects at higher temperatures than bulk silicon [*7]. Studies of Stark splitting showing eight sharp lines in the emission spectrum of erbium in amorphous / nanocrystalline silicon films suggests that the optically active species is an octahedrally coordinated erbium ion in an ErO 6 complex [24].

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5. Silicon resonating quantum structures By placing the erbium-doped silicon active layer within a microcavity, radiative transition probabilities may be greatly increased, and to exploit this effect devices have been constructed that consist of Fabry–Perot microcavities incorporating the active layer. This layer may be either amorphous or porous silicon [*18,25–28], and confinement effects produced by resonating structures produce short luminescent lifetimes, allowing the high modulation rates required of sources based on erbium-doped silicon active layers. One approach has been to fabricate structures of alternating layers of silicon and silica, thus forming Bragg reflector stacks surrounding a quarter-wavelength thick active erbium-doped layer, which may be erbium-doped silicon or erbium oxide [29]. Such systems are often referred to as silicon–silica superlattices [30–32]. Strong confinement produced by high-Q cavities can modify both the spectral linewidth and luminescence lifetime of the erbium emission as well as enhancing emission intensity and directivity. Careful design of the microcavity can result in nearly 100% reflectivity in the stop band with a reflectivity minimum at the erbium emission wavelength of 1.54 mm. In the case of porous silicon, multilayer structures can be fabricated by modulating the etching current during sample production. In this way, layers of alternating refractive index may be produced such that Bragg mirrors may be formed either side of a chosen active layer [**27,33]. Erbium can be introduced into the porous layers using electrochemical doping, or the layer may be implanted with erbium. As is the case for ‘bulk’ porous silicon, annealing in oxygen or nitrogen [*18] activates the erbium luminescence. Such a structure produces enhancement and linewidth narrowing of erbium emission, though suffers from the drawback that emission from erbium incorporated in porous layers outside the active region (i.e. within the Bragg mirrors) cannot be suppressed. Care must be taken in the design of the microcavity, as the oxidation / annealing step changes the density, and therefore the refractive index, of the porous layers. This results in a wavelength shift in the reflectivity spectrum of the cavity, which must be allowed for in order for the reflectivity minimum to overlap the erbium emission band after annealing. The periodic nature of the Bragg cavity constitutes a photonic band gap structure, and therefore the erbium emission from within the cavity is both highly directional and enhanced. Emission through the Bragg stack can be up to 38 times more intense than that in-plane from the side of the sample and is concentrated into a 208 cone around the surface normal [**27]. The ability to tailor the cavity structure also enables the emission peak to be centred in a wavelength region within which erbium emits only weakly. The luminescence enhancement produced by the cavity,

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therefore, compensates for the low yield at these wavelengths. Extending the concept of placing the optically active rare-earth ions into a resonant cavity, Polman’s group have investigated the effect of modifying the radiative transition rate of Er 31 in silica microcavities [34]. Cavities were formed either by producing erbium-doped silica thin films, or a highly monodisperse size-selected set of erbium-doped silica colloidal spheres with diameters in the range of 150–350 nm. In both cases, the radiative transition rate of the Er 31 1.5 mm transition could be modified greatly by changing the refractive index of the ambient by placing the films or microspheres in contact with liquids having a range of refractive indices [*35]. Modification of the decay rate occurs as a direct result of changes in the local density of states function surrounding the erbium ions.

6. Silicon nanoclusters Silicon nanoclusters may readily be formed in silica either by ion implantation, or during deposition or epitaxy [36,37,*38]. As a result of quantum confinement effects, the embedded nanoclusters emit light in the visible and near-infra-red region due to excitonic recombination; the wavelength of emission can be tuned by changing the cluster dimensions [*39], though obtaining a monodisperse size distribution of nanoclusters remains problematic [40,**41]. Absorption cross-sections of the nanoclusters are large, in the 10 215 –10 216 cm 2 range [**42]. However, although embedded nanoclusters have been studied for some time as promising candidates for light emission from silicon [*39,**43,**44], photoluminescence efficiencies are low [45], as it is generally thought that even for nanoclusters as small as 2 nm in diameter the silicon remains predominantly indirect-gap. Nevertheless, when this material is co-doped with erbium ions intense 1.5 mm emission can be obtained [**46,47]. Indirect excitation of erbium photoluminescence as a result of coupling between the absorption bands of the silicon nanoclusters and erbium excited states has been demonstrated by a number of groups [**46,48,49,**50]. Photoluminescence data suggest that the optically active erbium ions are located within the amorphous silica matrix and that excitons generated within the nanoclusters transfer excitation to erbium ions [51]. Luminescence from the silicon nanoclusters is in competition to that from the rare-earth ion, though by using a sufficiently high rareearth concentration silicon nanocluster luminescence can be effectively quenched. It is likely that the transfer of excitation takes place via a dipole–dipole interaction populating higher exited states of the erbium ion that feed into the rare-earth metastable state via rapid phononassisted decay (Fig. 2), though some work has suggested that excitation transfer between nanoclusters and erbium ions occurs for those clusters for which the exciton

Fig. 2. Schematic representation of excitation transfer between silicon nanoclusters and erbium ions in a silica matrix. Typical cluster sizes are around 2–3 nm diameter. Production of an exciton by absorption of a photon with energy .Eg is followed by dipole–dipole transfer of excitation to a nearby erbium ion. Non-radiative decay to the 4 I 13 / 2 metastable state is followed by emission of a photon with a wavelength of 1.5 mm.

energies are resonant with electronic transitions of the rare-earth ion [**52]. An effective absorption cross-section for the indirect exciton-mediated excitation of erbium ions can be defined and measured to be of the order of 7.333 10 217 cm 2 [**53]. This represents an increase in the effective absorption cross-section of erbium in silica by four orders of magnitude, and as such suggests the possibility of producing compact planar sources and amplifiers. Initial results from a silicon nanocrystal / erbium codoped waveguide produced a net gain of 0.6 dB cm 21 [*54], though more recent results have demonstrated net gain of up to 7 dB cm 21 in an erbium-doped silicon-rich silica waveguide device fabricated by PECVD [**55,**56]. Excitons were generated in the silicon nanoclusters using the 476 nm line from an argon-ion laser (ensuring that excitation of the erbium ions was predominantly by excitation exchange from the silicon nanoclusters). This is an extremely significant result, as it opens up the possibility of a broad-band pumped erbium-doped gain elements, and possibly a silicon-based laser operating at telecommunications wavelengths. Some work has been performed on silicon-rich silica doped with a range of rare-earth ions other than erbium, including Pr, Nd, Tm and Eu [**50,*57]. Similar excitation exchange mechanisms were found to operate in each case. An additional benefit of the exciton-mediated pumping scheme is that constraints on pump wavelength are relaxed because the initial absorption of pump photons is by the broad-band absorbing silicon nanoclusters. Fig. 3 shows a

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The intensity of the electroluminescence was found to be strongly dependent on the thickness of the nanometre-scale silicon layer. A novel approach to exploiting the excitation exchange effect between erbium and silicon nanostructures in silicon-rich silica layers has been to form a very thin (2 nm) layer of buried suboxide within an epitaxially grown silicon sample [62,*63]. The blocking potential produced by the thin oxide layers raises the kinetic energy of electrons to increase the efficiency of impact excitation of erbium within the silicon layers. Once again, temperature quenching effects were reported to be much reduced in these devices.

7. Future prospects Fig. 3. Photoluminescence spectra of erbium-doped stoichiometric silica (lower line) and erbium-doped silica containing 10 at% excess silicon in the form of 2–3 nm diameter silicon nanocrystals (upper line). Both spectra were obtained using an excitation wavelength of 476 nm, away from Er 31 absorption bands.

photoluminescence spectrum from an erbium-doped silica sample containing silicon nanoclusters for which the excitation wavelength was 476 nm; in this case, excitation of the erbium luminescence is primarily indirect. Broadband pump sources can be used, and as a result excitation of Er 31 emission in silicon-rich silica using a commercial camera flashgun has been demonstrated [58]. Cheap flashlamp-pumped gain elements and sources thus become a real possibility. As well as photoluminescence, electroluminescence has now been achieved from both erbium-doped silicon-rich silica [*59] and silicon / silica superlattices [*60]. In Ref. [*59], thin layers of erbium-doped SiO x were deposited by magnetron sputtering onto silicon substrates. Ohmic contacts were diffused into the bottom of the substrate and gold contacts evaporated onto the top surface. Electroluminescence results showed lower threshold and increased efficiency than a similarly produced erbium-doped silicon device. Similar MOS devices have also been produced by ion implantation; both erbium and terbium have been employed as the optically active rare-earth ion and have been implanted into 50 nm thick SiO 2 layers. Erbium ions were excited by hot electrons as a result of Fowler–Nordheim tunnelling, which produced electrons with average energies in excess of 5 eV. In the case of the erbium-doped device, an enhancement of |7-fold was seen in the EL from the MOS structure when compared to that from an erbium-doped pn diode. Internal efficiencies of .4310 25 and impact excitation cross-sections of 13 10 215 cm 2 were reported [61]. In the case of the superlattices, a thin layer of silicon (d,4.0 nm) was deposited by magnetron sputtering onto a film of erbium-doped silica grown on a silicon substrate (both n1 and p were used) and a top contact of gold used for electrical connection.

Despite the wide range of semiconductor materials that have been used to produce erbium-doped LEDs, no device has yet achieved sufficient efficiency to be competitive with existing III–V based sources. Work therefore continues on pushing quantum efficiency figures up by exploiting quantum confinement and photonic nanostructures. Confined systems remain of great interest thanks to the ability to tune the optical response of the host and to utilise quantum confinement effects to optimise both optical and electrical host-rare earth interactions. Erbiumdoped silicon microcavities show much promise; likewise nanostructured and nanocrystalline materials, which also exhibit interesting nonlinear optical properties. In conclusion, therefore, the field of rare-earth doped quantum structures is a rapidly developing one. Recent advances have demonstrated the real possibility of producing low-cost planar gain elements that exploit the unique optical and electronic properties of quantum confined semiconductors. The nonlinear properties of such confined systems are proving to be of great interest outside the immediate field of optoelectronics, and are likely to find further application in all-optical memories and optical computing [*1].

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