High efficiency light emitting devices in silicon

Materials Science and Engineering B105 (2003) 83–90

High efficiency light emitting devices in silicon Maria Eloisa Castagna∗ , Salvatore Coffa, Mariantonietta Monaco, Anna Muscara, Liliana Caristia, Simona Lorenti, Alberto Messina STMicroelectronics, Corporate R&D, Stradale Primosole 50, 95121 Catania, Italy

Abstract We report on the fabrication and performances of highly efficient Si-based light sources. The devices consist of MOS structures with erbium (Er) implanted in the thin gate oxide. They exhibit strong 1.54 ␮m electro-luminescence (EL) at 300 K with a 10% external quantum efficiency, comparable to that of standard light emitting diodes using III–V semiconductors. Emission at different wavelengths has been achieved incorporating different rare earths (terbium (Tb) and ytterbium (Yb)) in the gate dielectric. The external quantum efficiency depends on the rare-earth ions incorporated and ranges from 10% (for a Tb doped MOS) to 0.1% (for an Yb doped MOS). RE excitation is caused by hot electrons impact and oxide wearout limits the reliability of the devices. Much more stable light emitting MOS devices have been fabricated using Er-doped silicon rich oxide (SRO) films as gate dielectric. These devices show a high stability, with an external quantum efficiency reduced to 0.2%. In these devices, Er pumping occurs partially by hot electrons and partially by energy transfer from the Si nanostructures to the rare-earth ions, depending on Si excess in the film. Si/SiO2 Fabry–Perot microcavities have been fabricated to enhance the external quantum emission along the cavity axis and the spectral purity of emission from the films that are used as active media to fabricate a Si-based resonant cavity light emitting diode (RCLED). These structures are fabricated by chemical vapour deposition on a silicon substrate. The microcavities are tuned at different wavelengths (nm): 540, 980 and 1540 (characteristic emission wavelengths, respectively, for Tb, Yb and Er). The reflectivity of the microcavities is about 97% and the quality factor ranges from 60 (for the cavity tuned at 980 nm) to 95 (for the cavities tuned at 540 and 1540 nm). © 2003 Elsevier B.V. All rights reserved. Keywords: MOS; Erbium; Silicon rich oxide (SRO); Rare earths; Implantation

1. Introduction Si is the semiconductor of choice for the fabrication of advanced electronic devices. Hence, implementation of efficient optical functions in Si would allow us to use the mature and low cost Si Ultra Large-Scale Integration (ULSI) technology for the fabrication of integrated optoelectronic circuits [1,2]. Si indirect band gap makes it unsuitable for efficient light emission and modulation at room temperature. The implementation of efficient optical functions in Si would greatly simplify the integration of electronic and photonic devices. The first goal is to fabricate efficient and electrically-driven room temperature light emission sources in Si. For application in the telecommunication field emission at 1.54 ␮m is requested. This target has been pursued by ∗ Corresponding author. Tel.: +39-095-7407725; fax: +39-095-7407099. E-mail address: [email protected] (M.E. Castagna).

0921-5107/$ – see front matter © 2003 Elsevier B.V. All rights reserved. doi:10.1016/j.mseb.2003.08.021

several approaches: among them, rare-earth doping of semiconductor materials has been one of the most explored [3–9]. In this paper, we propose a Si-based light source consisting of a MOS structure with different rare earths (erbium (Er), ytterbium (Yb) and terbium (Tb)) implanted in the thin gate oxide, in order to obtain emission at different wavelengths. Some of these devices (Er- and Tb-doped) show a 10% external quantum efficiency at room temperature, comparable to that of standard light emitting diodes using III–V semiconductors. Er ions are excited by impact through hot electrons. Oxide wearout limit the reliability of the devices. Moreover, we will show that reliability of these devices can be improved by the presence of a Si excess in the gate dielectric. The devices, using a SRO film as gate dielectric, show a high stability, with an external quantum efficiency reduced to 0.2%. In these devices, Er pumping occurs partially by hot electron impact and partially by energy transfer from the Si nanostructures to the rare-earth ions, depending on the Si excess in the film.

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Fig. 1. (a) Schematic cross-section and (b) picture of the rare-earth doped light emitting MOS device.

Finally, in order to enhance the external quantum emission of the devices we have fabricated Si/SiO2 microcavities tuned at different wavelengths.

2. Device fabrication The devices consist of MOS capacitors fabricated on a lightly doped p-type epitaxial layer (22 ␮m thick) grown on thick p+ Si substrate. A standard LOCOS process defines the active area of the devices. The gate dielectric of the device consists of either: (a) a thermally grown stoichiometric oxide with thickness ranging from 240 to 620 Å; or (b) a 750 Å thick SRO film, with a refractive index (n) ranging from 1.5 to 1.8, deposited by plasma enhanced chemical vapour deposition (PECVD). The gate dielectric layers were implanted with rare-earth ions (Er, Tb and Yb) at different energy (ranging from 16 to 50 keV) and to a total fluence ranging from 1×1014 to 1×1015 ions/cm2 . The implantation energy has been chosen to have the projected range of the ion distribution roughly in the middle of the gate dielectric. After implantation annealing at 800 or 1000 ◦ C for 30 min was performed under a nitrogen flux to eliminate implantation defects and to obtain the agglomeration of the Si excess in the SRO films. The structure of the device was completed by the CVD deposition of a 3000 Å thick n+ poly(silicon) layer. Finally, a metal ring consisting on an Al–Si–Cu layer, 3 ␮m thick, completes the device structure, and allows device bonding on a standard TO3 package. A schematic (a) cross-section and (b) a picture of the device are reported in Fig. 1. In a single die three MOS devices are present with different active areas: 1500 ␮m × 1500 ␮m; 350 ␮m × 350 ␮m; and 100 ␮m × 100 ␮m. In the scheme, the dimensions are not realistic, but we have clearly indicated the gate dielectric that corresponds to the emitting area of the device.

measured on MOS devices having a RE-doped stoichiometric gate oxide, are reported. The active area of the device is 2.5 mm2 and the oxide thickness is 620 Å. The shape of the spectra is typical for RE emission in an amorphous matrix. The emission at 540 nm for Tb-doped MOS is due to the transition from 5 D4 to 7 F5 level, the emission at 980 nm in Yb-doped MOS is due to transition from 2 F5/2 to 2 F7/2 level, while the emission at 1540 nm for Er-doped MOS is due to transition from 4 I13/2 to 4 I15/2 level. In Fig. 3, we show the emitted power versus current for devices embodying different rare-earth ions in the gate

Fig. 2. Normalised room temperature electro-luminescence spectra measured on devices having Er-doped SiO2 (solid line), Yb-doped SiO2 (dashed line) and Tb-doped SiO2 (dotted line) as gate dielectric.

3. Electrical characterisation and electro-luminescence (EL) spectra of the RE-doped MOS devices The large band gap of SiO2 results in hot electrons with sufficient energy to pump most of the rare-earth ions. Hence, devices with different wavelengths of emission can be achieved. In Fig. 2, the strong 300 K EL spectra,

Fig. 3. Comparison of emitted power vs. current among devices with Er-doped SiO2 (squares), Tb-doped SiO2 (diamonds), and Yb-doped SiO2 (triangles).

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oxide and annealed at 1000 ◦ C. For Tb, at lower current, the emitted power increases linearly with current until saturation is achieved at current above 500 ␮A when all optically active Tb ions have been pumped. For Er the saturation is achieved at 50 ␮A. For Yb the emitted power increases linearly with current and is about 2 orders of magnitude lower than that measured in Tb-doped devices. The emitted power from the LED was measured using an integrating sphere positioned immediately above the device in order to collect most of the emitted light. It should be noted that for Er and Tb the emitted power is comparable with that of III–V emitting devices commonly used. We can calculate the external quantum efficiency using the equation: η=

POPT / hν I/q

(1)

that describes the ratio of the photons emitted (POPT /hν) on the electrons flux (I/q). We can use this equation in the linear regime. This demonstrates that we have fabricated Si-based light emitting devices with external quantum efficiency (∼10%) identical to that of state-of-the-art III–V devices. Only the maximum output is limited by the finite density of RE ions and by the large value of radiative lifetime. However, in many applications in integrated devices, the use of high power is not needed while a high efficiency is essential. We have fitted the data of emitted power versus current, reported in Fig. 3, with the equation: EL = ELmax

στJ/q στJ/q + 1

(2)

where ELmax is the maximum EL signal at saturation, σ the effective cross-section of excitation, τ the overall lifetime (taking into account radiative and non-radiative processes) and J/q the electrons flux. In Table 1, the emission wavelength, the cross-section and external quantum efficiency of

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Table 1 Emission wavelength, cross-section and external quantum efficiency of MOS devices with rare-earth doped gate dielectric Rare earth Erbium Terbium Ytterbium

λ emission (nm) 1540 540 980

σ (cm2 ) 10−14

1× 4 × 10−15 1 × 10−15

ηext (%) 10 10 0.1

MOS devices with rare-earth doped gate dielectric are reported. Our previous investigations [3] demonstrated that carrier transport in these devices can be fully explained by Fowler–Nordheim tunnelling for both undoped and rare-earth doped MOS. The results so far shown suggest that rare-earth pumping occurs through impact excitation of hot electrons [4] that are injected in the conduction band of the oxide from the n+ doped poly-Si layer. These electrons can reach a high average energy (up to 5 eV) and can hence pump the rare-earth ions. This is confirmed by the large measured value for the excitation cross-section. We have measured a value of charge to breakdown equal to 13 C/cm2 at a current density of 0.04 A/cm2 for an undoped gate oxide. Instead, for an Er-doped gate stoichiometric oxide we achieved an enhancement of charge to breakdown that is equal to 30 C/cm2 for the same current density. When hot electrons impact Er ions, they lose a part of their energy, and are not able to create defects into oxide that are eventually responsible for the oxide breakdown. The annealing temperature has a very strong effect on the electro-luminescence intensity achieved in these devices. As an example, in Fig. 4a we compare the emitted power versus current for two Er-doped MOS annealed at (◦ C): 1000 (circles) and 800 (squares), respectively. A much higher value of the saturation power is achieved from the 800 ◦ C annealed device, suggesting that the fraction of excitable Er ions is higher than in the 1000 ◦ C annealed device. This behaviour

Fig. 4. (a) Comparison of emitted power vs. current among devices with Er-doped SiO2 annealed at 800 ◦ C (squares) and at 1000 ◦ C (circles); and (b) bright field cross-section TEM image of the Er-doped SiO2 annealed at 1000 ◦ C.

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Fig. 5. Comparison of electrical characteristics for MOS devices with Tb-doped SiO2 annealed at 800 ◦ C with different oxide thickness: 620 Å (circles), 340 Å (triangles), and 260 Å (squares); (a) J–V plot and (b) Fowler–Nordheim (F–N) plot.

is attributed to a significant clustering of the Er ions upon annealing at 1000 ◦ C, as demonstrated by the TEM image, reported in Fig. 4b, where Er precipitates are clearly evident. In order to reduce the voltage at which these devices can be operated we have fabricated MOS structures with a reduced oxide thickness. In Fig. 5, we report the (a) J–V curves and the (b) Fowler–Nordheim plots for Tb-doped MOS with three different gate oxide thicknesses (Å): 620 (diamonds), 340 (triangles) and 260 (squares). All current–voltage measurements on these devices show the current flow in accumulation conditions (poly layer is negatively biased with respect to the substrate). Once more, as shown in Fig. 5b, the behaviour can be fully attributed to a Fowler–Nordheim tunneling. In Fig. 6, we report the emitted power versus current density for the three different devices. Data clearly show that, in spite of the fact that the effective cross-section (reported in Table 2) decreases from 5 × 10−15 to 4 × 10−16 cm2 when the oxide thickness is reduced from 620 to 240 Å, a large power emission can still be achieved even for the lower oxide thickness.

Fig. 6. Comparison of emitted power vs. current for devices with Tb-doped SiO2 annealed at 800 ◦ C with different thickness: 620 Å (circles), 340 Å (triangles), and 260 Å (squares).

Table 2 Implantation fluence, energy, Tb peak concentration and cross-section for MOS devices with Tb-doped gate dielectric, with thickness ranging from 240 to 620 Å, annealed at 800 ◦ C SiO2 thickness (Å) Fluence (cm−2 ) Energy (keV) Tb (%) σ (cm2 ) 260 340 620

1 × 1014 2 × 1014 1 × 1015

16 25 50

0.6 0.9 2.7

4 × 10−16 6 × 10−16 5 × 10−15

It should be noted that, in the case of Tb, a very large excitation energy (∼2.5 eV) is needed to pump the ions to the first excited state: hence, the observed reduction in the excitation cross-section can be attributed to a reduction in the average energy of the hot electrons in the thinner devices.

4. Erbium emission in SRO films The rare-earth doped MOS devices with a stoichiometric SiO2 exhibit strong quantum efficiency, but reliability, although improved with respect to undoped oxide, is still a severe issue. To improve the reliability we have fabricated MOS devices in which the stoichiometric oxide is replaced by a SRO film. The presence of the Si excess has in fact a two-fold effect. First of all, the Si nanostructures arising from agglomeration of the Si excess, are known [3–5] to act as sensitisers for the Er ions: excitation in the nanocrystals (produced by either optical or electrical means) is rapidly transferred to the rare-earth ions. Moreover, the presence of the Si excess can reduce the wear out of the dielectric by providing the excess charge accumulated in the oxide matrix with a conductive path to the electrodes. Recently, it has been demonstrated that by using silicon rich oxide (SRO), which consists of Si nanoclusters embedded in a SiO2 matrix, the cross-section for optical excitation of Er3+ ions can be enhanced by more than 4 orders of magnitude (∼10−16 ÷ 10−17 ) [5]. Si nanoclusters act as

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Fig. 7. (a) PL Intensity at 1540 nm vs. pump photon flux for an SRO film with a refractive index of 1.6 annealed at three different temperatures; (b) excitation cross-section at 1540 nm vs. annealing temperature for SRO films with a refractive index of 1.6 (squares) and 1.7 (triangles).

classical sensitiser atoms that absorb incident photons and that transfer the energy to luminescent Er3+ ions. Optical excitation of nanoclusters and energy transfer from nano-clusters to Er3+ ions have been largely investigated, even if some mechanisms are not clear at present [5–7]. Our goal is to create e–h pairs into nanocrystals through electrical excitation of the SRO film. SRO (SiOx , x < 2) films have been deposited by rf glow-discharge decomposition of SiH4 /N2 O gas mixture in a plasma enhanced chemical vapour deposition (PECVD) chamber. The thickness of the deposited film is 750 Å. The films are characterised by measuring the refractive index (n) that ranges from 1.5 to 1.8 and depends on the silicon excess against a stoichiometric SiO2 . The film was successively Er-doped by implantation, as previously reported. To obtain the nucleation, growth and crystallisation of Si nanoclusters a thermal annealing post-deposition is necessary. Our films have been annealed at temperatures ranging from 800 to 1100 ◦ C for 30 min under a nitrogen flux to evaluate the contribution of nanocrystal formation on erbium excitation. The crystallites are found to be a few nanometers in size with a spherical shape and are uniformly distributed through the whole film, after an annealing at 1000 ◦ C [10]. Fig. 7a shows the PL intensity at 1540 nm versus pump photon flux for a film with a refractive index of 1.6, annealed at three different temperatures. We have fitted the data with the expression: PL = PLmax

στφ στφ + 1

Table 3 Photoluminescence cross-section and radiative decay time of Er-doped SRO films with different refractive index and annealed at different temperatures for 30 min Annealing (◦ C)

n = 1.6 σ

800 1000 1100

(cm2 ) 10−19

8× 2 × 10−18 5 × 10−18

n = 1.7 τ (ms) 5.3 4.8 4.6

σ (cm2 ) 10−18

4× 9 × 10−18 2 × 10−17

τ (ms) 2.5 2.3 2

cross-section ranges from 8 × 10−19 cm2 (n = 1.6, 800 ◦ C) to 1.2 × 10−17 cm2 (n = 1.7, 1100 ◦ C). It is observed that the cross-section increases when the Si excess is increased and/or when the annealing temperature is increased. We have fabricated MOS structures that use these films as gate dielectric. Current–voltage measurements on these devices, plotted in Fig. 8, reveals that the behaviour cannot be simply explained by a Fowler–Nordheim tunneling and that a more complex mechanism is operative. The presence of the Si

(3)

where PLmax is the maximum PL signal at saturation, σ the effective cross-section of excitation, τ the overall lifetime (taking into account radiative and non-radiative processes) and φ the pump photon flux. Fitting the data with Eq. (3) we obtain στ and, measuring the decay time, we can calculate the cross-section. The data are summarised in Table 3. In Fig. 7b the trend of the cross-section versus annealing temperature is reported for a film with n = 1.6 and for a film with n = 1.7. The

Fig. 8. Comparison of electrical characteristic for a MOS with Er-doped SiO2 (circles), Er-doped SRO n = 1.7 (triangles), Er-doped SRO n = 1.64 (squares), Er-doped SRO n = 1.61 (diamonds) and the theoretical curve for an undoped thermal oxide as gate dielectric (solid line).

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Si excess in the film and on its detailed distribution, which is significantly affected by the annealing temperature. Upon decreasing the Si excess in the film the contribution of excitation due to direct hot electron impact increases: hence, a higher emitted power is achieved since the cross-section increases. The nanoclusters improve the conductivity of the SRO with respect to a stoichiometric SiO2 . For this reason, the current that flows in the gate dielectric does not create damage in the SRO: in fact no defects related to hot electrons impact are created. This device is then stable, even if the efficiency, as shown in Fig. 9, is of 0.2%. Fig. 9. Comparison of emitted power vs. current for devices with Er-doped SiO2 (circles), Er-doped SRO n = 1.7 (squares), Er-doped SRO n = 1.64 (triangles) and Er-doped SRO n = 1.61 (diamonds).

excess strongly improves the conductivity of the SRO films compared to stoichiometric oxide: in particular, large current density is achieved even at low operating voltage. As a result of the larger conductivity the wear out of the devices is significantly reduced and charge to breakdown values well above 1000 C/cm2 , at a current density of 0.04 A/cm2 , are achieved even with low excess. Electro-luminescence spectra (not shown) once more reveal the typical shape of Er emission in an amorphous matrix. Data are shown in Fig. 9 and summarised in Table 4. In Fig. 9, the emitted power versus current is reported for a MOS with Er-doped SiO2 annealed at 800 ◦ C and a MOS with Er-doped SRO with different amounts Si excess. The emitted power decreases when the Si excess in the film increases: for a film with n = 1.7 the trend of emitted power is linear and, at the same current, the emitted power reveals a difference of about four orders of magnitude in the linear regime for the MOS with Er-doped SiO2 as gate dielectric, while for a film with n = 1.61, a saturation of the signal is achieved and the emitted power reveals a difference of one order of magnitude. These data can be explained assuming that in these devices different pumping mechanisms for the Er ions are simultaneously operating. In particular, Er can be excited by direct hot electron impact (like in stoichiometric oxide MOS) and by energy transfer from excited Si nanostructures. The relative contribution of these two mechanisms depends on the Table 4 Electro-luminescence cross-section and radiative decay time of MOS devices with Er-doped SRO as gate dielectric, annealed at different temperatures for 30 min and with two different refractive indexes Refractive index

Annealing (◦ C)

τ (ms)

σ (cm2 )

1.70

800 1000

0.62 0.86

5.6 × 10−18 4.4 × 10−16

1.64

800 1000

0.87 0.99

8.8 × 10−17 2.7 × 10−16

1.60

800 1000

1.10 1.12

4.9 × 10−16 2.4 × 10−16

5. Microcavities fabrication In order to enhance the external quantum efficiency of our devices we have fabricated passive microcavities (MCs) [11], where the dielectric mirrors are composed of Si/SiO2 multilayers. The term passive means that these MCs do not contain the active media, but are simple interference filters. Their fabrication is important to establish the fabrication processes. Microcavities can enhance light intensity by orders of magnitude at predetermined locations within the structure [12,13]. When the cavity is in resonance with the electronic rare ion transition embedded in the region of maximal electromagnetic field, the light interacts with these sites [14]. This interaction was shown to enhance the optical emission in the microcavity growth direction. This is due to radiative decay into redistributed modes in space. Our aim is to fabricate microcavities tuned at different wavelength and then characterise the films, and the densification after an annealing process. Fig. 10 shows the TEM images of three different microcavities tuned at three different wavelengths. The Bragg reflectors of the microcavity are made of alternating layers of Si and SiO2 , with quarter wavelength thickness, depending on the tuning wavelength [14]. A SiO2 film constitutes the half wavelength thick microcavity spacer. This layer is not an active medium, but is used just to investigate the formation of a defect in the photonic band gap created by the Si/SiO2 multilayers in the microcavity growth direction. The wavelengths of microcavities resonance were selected to coincide with the maximum of the emission spectrum for the treated rare earths (Er, Yb and Tb). Because of the high refractive index difference between Si (nSi ≈ 3.7) and SiO2 (nSiO2 ≈ 1.46), high reflecting mirrors are obtained by using only a few periods. We used three Si/SiO2 pairs for each mirror. In Table 5 are reported the microcavities layer thicknesses. Si layers have been deposited by LPCVD and SiO2 layers by PECVD. The microcavities have been annealed at 1000 ◦ C for 30 min: we did not observe densification for Si layers, instead we observed a densification of 1% for SiO2 due to H2 loss during the thermal annealing.

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Fig. 10. Bright field cross-section TEM images of three different Si/SiO2 microcavities tuned at three different wavelengths: (a) 1540 nm; (b) 980 nm; and (c) 540 nm. The images have been acquired at the same magnification.

Table 5 Thickness of films (nm) at λ/4 and λ/2 constituting passive microcavities tuned at different wavelengths Layer

λ/4 SiO2 λ/4 Si λ/2 SiO2

Passive microcavities tuned at different wavelengths can be fabricated with a good uniformity on the wafer.

Wavelength (λ) 540 nm

980 nm

1540 nm

93 40 186

170 72 340

264 113 528

Fig. 11 shows the reflectance spectra of the fabricated microcavities. The resonance wavelengths are 550, 985 and 1535 nm and the stop bands are, respectively, 450, 780 and 1200 nm. The maximum of reflectivity for all cavities is about 97%. A microcavity is characterised through the quality factor Q defined as the ratio of the width (full width at half maximum) of the resonance (λ) to λ: Q = λ/λ. The quality factors obtained for the cavities are 60, for the cavity tuned at 980 nm, and 96 for the others. By increasing the number of Si/SiO2 pairs in the DBR the peak becomes narrower and the quality factor increases, but the poly-Si absorption, in the visible range, increases at the expense of reflectivity. Reflectance spectra do not show significant differences for different points on the wafer.

6. Conclusions We have fabricated Si-based light sources consisting of a MOS structure having RE-doped SiO2 or Er-doped SRO as gate dielectrics. The first structure with RE-doped SiO2 shows a high quantum external efficiency (10% for Tb and Er and 0.1% for Yb) and emission at different wavelengths depending on the rare-earth implanted, but its reliability is limited by the charge to breakdown and the threshold voltage. It’s possible to enhance the charge to breakdown and decrease the threshold voltage by decreasing the film thickness; the cross-section changes of one order of magnitude. The device with Er-doped SRO as gate dielectric shows a lower quantum external efficiency (0.2%), but is much more stable than the first one. In order to enhance the external quantum efficiency of our devices we have fabricated passive microcavity tuned at three different wavelengths corresponding at the wavelengths of emission of terbium, ytterbium and erbium.

Acknowledgements We would like to express our thanks to Giuseppe Faro for SiO2 deposition, Salvatore Nicotra for Si deposition, Rosario Mangano for rare earths implantation and Corrado Bongiorno for TEM analysis.

References

Fig. 11. Reflectance spectra for three different microcavities: 1540 nm (solid line), 980 nm (dashed line), 540 nm (dotted line).

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