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Nano-engineered silicon light emitting diodes and optically active waveguides
Optical Materials 32 (2010) 1601–1605
Contents lists available at ScienceDirect
Optical Materials journal homepage: www.elsevier.com/locate/optmat
Nano-engineered silicon light emitting diodes and optically active waveguides K.P. Homewood *, M.A. Lourenço, R.M. Gwilliam Advanced Technology Institute, Faculty of Engineering and Physical Sciences, University of Surrey, Guildford, Surrey GU2 7XH, United Kingdom
a r t i c l e
i n f o
Article history: Received 11 January 2010 Received in revised form 27 May 2010 Accepted 27 May 2010 Available online 26 June 2010 Keywords: Light emitting diodes Silicon Erbium Gain Optical amplifier Laser
a b s t r a c t In this paper, we first introduce and discuss the current state-of-the-art in integrated silicon photonic technology. We argue that the only missing link to the incorporation of this technology into mainstream high end silicon chips and systems are the availability of fully integrated silicon light sources and amplifiers. We go onto describe how dislocation engineering can be used to nano-engineer, locally, the strain in optically active silicon devices to enable high operating temperatures. We show how, by combining this approach with the incorporation of rare earths, that have optical levels in the near infra-red below the silicon band-gap, a potential route exists to meet these needs. In particular, we show that the use of erbium, together with dislocation engineering, could provide useful optical emission and gain at the important 1.5 lm wavelength that dominates optical data transfer. Ó 2010 Elsevier B.V. All rights reserved.
1. Introduction Integrated silicon photonics technology has advanced rapidly in the last few years [1–11] to the point that the only remaining components needed for the final implementation of full silicon photonic systems is to develop electrically pumped optical amplifiers, light emitting diodes (LEDs) and lasers in silicon also using a complementary metal oxide semiconductor (CMOS) compatible technology. The preferred wavelength for such systems is the 1.5 lm band. Here, we report on silicon LEDs emitting at 1.5 lm and measurements of optical gain at this wavelength capable of electrical excitation. Emission and gain at 1.5 lm are obtained by incorporating erbium in silicon. Nano-engineering the devices using dislocation loop technology can extend this functionality to high temperature operation. The gain values obtained are significantly greater than previously supposed. Consequently, this approach now offers a realistic route to the Holy Grail of silicon photonics – electrically pumped silicon optical amplifier and laser devices using standard silicon process technology. Optical gain in silicon has been demonstrated utilising the Raman effect [4,6,7] and, more recently, phase matched four wave mixing [10]. However, neither is capable of being electrically driven. There is a report of low temperature optically pumped lasing in crystalline silicon [11]. The emission occurs at a wavelength of 1.27 lm, subsequently attributed to the G-centre introduced using surface texturing. Again this is a significant result but it is unclear whether this centre can be incorporated in sufficient amounts con* Corresponding author. Tel.: +44 1483 689285; fax: +44 1483 686091. E-mail address: [email protected] (K.P. Homewood). 0925-3467/$ - see front matter Ó 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.optmat.2010.05.025
trollably using CMOS compatible processes or operated at high temperatures. The gain curves shown in all these approaches are very narrow, at around 1 nm, making them rather impractical for optical amplifiers where a broad band gain spectrum is needed to increase tolerance to the input laser wavelength and to support wavelength division multiplexing [10]. Here, we show that gain can be achieved over a wavelength range of around 150 nm, centred at a wavelength of 1550 nm. 2. Experimental details Silicon LEDs have been produced by the incorporation of rare earths to provide optical emission at specific wavelengths associated with the internal rare earth transitions. Tuning is achieved by implantation of rare earths into the active region of a bulk silicon diode where dislocation loops have also been introduced to nano-engineer the local strain field in the device. Optically active waveguides have been produced using a similar approach but into silicon on insulator (SOI) substrates. The nano-engineered strain field enables us to control the movement of the injected carriers. Perhaps the most significant aspect of the dislocation loop engineering (DE) is that not only can light emission in silicon be enabled but also the quenching of light and optical activity with increased operating temperature can be completely eliminated and even reversed [2,12]. In this paper we limit ourselves to LEDs and waveguides doped with erbium in the trivalent 3+ state. In general, semiconductor light emitting diodes suffer from the efficiency dropping significantly as the operating temperature increases. This is due to competition between the light emitting centres and non-radiative recombination through defects – the
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radiative recombination route is largely temperature independent. This is the cause of the poor light emission seen in indirect gap materials where the radiative transition is forbidden or, in practice, has a very long radiative recombination lifetime and emission through the fast non-radiative centres completely dominates. However by using DE the thermal quenching is largely reduced or eliminated. Dislocation engineering technology has been described in detail elsewhere [2,12–18] but for completeness we will outline the underlying principles here. The DE approach uses controlled incorporation of dislocation loops to form arrays at critical parts of the device. The individual loops are monolayer ‘‘islands” tens of nanometres across of additional silicon atoms inserted and surrounded by a simple closed edge-dislocation. The loops distort the surroundings and introduce local strain. Strain occurs at the dislocation edge producing a maximum negative pressure just outside the loop and a maximum positive pressure just inside the loop. The pressures are considerable, at around 50 GPa. The strain decays inversely with distance away from the loop edge. The net pressure remains zero in the lattice as a whole. The pressure and its spatial dependence can be calculated using conventional elastic theory of dislocations [19] requiring only Poisson’s ratio and Young’s modulus of silicon [2]. The total strain field is the superposition of the strain fields due to the individual loops. A typical loop array is shown in Fig. 1. The band structure and energies of semiconductors are strain dependent. Consequently, if strain within a semiconductor material or device can be modified so can the band gap energies, enabling potential variations to be introduced in the bands to control carrier migration. This is the principle underlying the field of strain layer semiconductors and devices, now widely applied in devices such as lasers and microprocessors. Strain is normally introduced by combining semiconductors of differing lattice constants using hetero-epitaxy. The DE approach is powerful in that strain is introduced only by manipulation of the host atoms. For silicon the pressure dependence of the lowest conduction band has a negative pressure coefficient so outside the loop the band gap is increased and inside is reduced. Consequently, a loop repels carriers and can be used to corral the carriers in parts of the device. The
pressure dependence of the band gap is 1.4 meV/kbar [20] giving loop induced modifications of the band gap by up to 0.75 eV. Loops are formed by introducing excess silicon atoms using ion implantation. Implants can be a dopant, such as arsenic or boron or, alternatively, the host atom silicon itself can be used. Loops are formed after a suitable anneal after which the dopant atoms sit substitutionally, generating excess interstitials that can agglomerate eventually into the dislocation loops. In the case of dopant atoms, they will also be electrically activated. The depth, density, size and distribution of the loop array can be engineered by adjusting the implant and annealing parameters [14,17,18]. Using silicon as the implant species enables the loop engineering and the device doping to be independently controlled and optimized. This technology has been employed to form dislocation engineered silicon LEDs (DELEDs). A device and the band diagram are shown in Fig. 2. Standard processing forms ohmic contacts to the top and back sides and a window has been left in the back contact. The loops form a two dimensional array near and parallel with the depletion region edge in the upper p-side. Also shown are extrinsic, optically active centres, between the depletion region edge and the loops introduced using a separate implant step. Importantly, unlike other silicon based light technologies, DELEDs are operated under forward bias as normal LEDs and typically have turn on voltages of around 1 V [2]. In a conventional p–n junction, placed under forward bias, carriers are injected across the barrier as the potential is lowered. In silicon, due to its indirect nature, intrinsic recombination lifetimes are very long and diffusion lengths of the order of microns. Consequently, most injected carriers reach and recombine at efficient non-radiative centres in bulk regions or in particular at the surface. This is the reason for the lack of electroluminescence (EL) in silicon diodes. In DELEDs strain due to the dislocation array in the plane of the junction increases the band gap energy introducing a potential barrier (Fig. 2b). As the loops lie in the (1 1 1) planes and the silicon (1 0 0) planes are parallel to the junction the strain field of the loop array zigzags and an injected carrier always only sees a potential barrier in the band. The barrier prevents onward diffusion of the carriers to defects and the surface which instead recombine radiatively. An LED technology is a prerequisite for laser development but in addition a gain mechanism has to be introduced. Optical gain in semiconductors is normally achieved using population inversion of carriers in a p–n junction under strong forward bias. However,
(a)
(b)
Top contact
Energy
Loops P
RE
Depletion region
N
Window
EV
EC
Bottom contact Fig. 1. Cross sectional and plan view transmission electron micrographs showing a typical dislocation loop array in silicon. Note that because the dislocation loops are in the (1 1 1) planes we are seeing their projection onto the (1 0 0) plane. This sample was implanted with boron at a dose of 1015 cm2, at an energy of 40 keV, and annealed at 950 °C for 1 min.
Fig. 2. (a) A schematic cross-section of a typical dislocation engineered light emitting diode with a rare earth implant incorporated between the dislocation loop array and the upper edge of the depletion region. Ohmic contacts are placed on the top and lower side of the device. A window has been left in the lower contact to allow light to escape. (b) An illustration of the energy band diagram through the device indicating the main optical transitions.
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this requires high gain because the high doping required leads to very high free carrier and Auger losses, and is only achievable in direct gap semiconductors with fast radiative transitions. An alternative is to incorporate a dopant that has an electronic structure enabling internal gain. Although such systems are often low gain they can also be incorporated into low-loss devices, in principle enabling net optical gain. Erbium is excited electrically in silicon via band gap recombination [21–25]. Elimination of thermal de-excitation and room temperature EL under forward bias has been demonstrated in boron co-doped samples [2,12]. Er3+ possesses intrinsic gain. Erbium has been suggested as a route to gain in silicon, but optical cross sections were supposed too small (1020 cm2) [25] for sufficient gain, requiring incorporation of unrealistic concentrations (1020 cm3) of the active ion. The samples for the gain measurements are planar waveguides formed in silicon on SOI substrates. The top silicon layer forms the waveguide and erbium has been implanted into this region. Some of the waveguides have had dislocation loops incorporated by implanting boron. The waveguide structure and its energy band diagram are shown in Fig. 3. Samples have either had a single erbium implant or multiple energy and dose implants to give a more uniform distribution of erbium. Details of all the samples are given in Table 1. The experimental set-up developed to measure these low gain samples is shown in Fig. 4. The main features that distinguish this approach and enable measurement of such low gain samples are the use of photoluminescence (PL) generated within the sample as the probe and two lock-in amplifiers in series. The first gives the PL and the second the change in PL, DPL, due to the pump. The pump laser is operated at the wavelength of 514 nm focused to a stripe on the sample 250 lm wide and 1 cm long. The sensitivity of this novel technique [26] is remarkable, enabling the measurement of gain/loss coefficients as small as 0.001 cm1. The broad band PL probe allows measurement of the full loss/gain spectrum as opposed to the usual single wavelength measurement. It also avoids alignment issues with external light probes allowing
variable temperature measurements. Measurements have been made between 80 K and room temperature. 3. Results and discussion In Fig. 5a we show the EL from a typical erbium DELED at 300 K, corresponding to the well known internal transitions from the 4I13/2 to 4I15/2 levels in the 4f shell. This sample was implanted with 1015 B cm2 at 30 keV and furnace annealed at 950 °C for 20 min, subsequently implanted with erbium at 5 1013 cm2 at 0.4 MeV and annealed at 950 °C for 1 min. The device operates under forward bias of 2.8 V at a current of 25 mA. In Fig. 5b is shown a typical temperature dependence of the erbium emission, taken at a constant current of 25 mA. It can be seen that the usual temperature quenching of erbium emission in silicon has been reversed allowing 1.5 lm wavelength EL in silicon at room temperature. It is this emission that provides the basis for design of the waveguides for the gain measurements shown in Table 1. In Table 2 implant details are summarised and the effective concentration calculated by averaging the dose over the waveguide volume; the waveguide length is 1 cm for all samples. Fig. 6 shows a typical PL spectrum from one of the waveguide samples. The samples have been selected to have broad emission allowing full spectral dependence measurements. In general all samples have emission bands peaking around 1.1, 1.3 and 1.5 lm. The broad feature at 1.3 lm, between the usual silicon peak at 1.1 lm and the erbium emission between 1.5 and 1.6 lm, is an implant induced defect band. Fig. 7a shows PL and DPL (magnified by 100) from the sample S4. Note the DPL drops in the 1–1.5 lm wavelength region but increases in the region of the erbium transitions. The structure
Probe laser
Pump laser
Chopper 2
Lock-in 2
ΔPL
Lock-in 1
PL
Lens Chopper 1
(a)
(b)
Surface
Energy
Detector
Loops
Spectrometer
RE
Sample
Silicon over-layer EV
Cryostat
EC
Oxide Silicon substrate Fig. 3. (a) A schematic cross-section of the silicon on insulator waveguide samples used for the gain measurements. Some of the samples are controls and do not contain the dislocation loops shown here. (b) An illustration of the energy band diagram through the silicon over-layer showing the main optical transitions.
Fig. 4. Schematic of the new experimental gain system developed for these measurements. The probe laser excites photoluminescence in the sample which is then propagated through the waveguide region which is excited by the pump laser. The two lock-ins connected in series allows independent monitoring of the PL and change in PL from which loss or gain can be calculated. The two series lock-in method gives a greatly enhanced sensitivity compared with more conventional systems.
Table 1 Here we list the silicon over-layer doping and the implant and anneal conditions for all the samples. For the multiple erbium implant samples the dose of the various implants are listed first and then their respective implant energies. Sample S1 S2 S3 S4
Over-layer doping (type/cm3) 17
n/2 10 p/3 1016 n/1 1014 n/1 1014
B implant (cm2/keV)
B anneal (°C/min)
Er implants (cm2/MeV)
Er anneal (°C/min)
– – 1 1015/30 1 1015/30
– – 950/20 950/20
(1.6, 1.0, 0.75, 0.5, 0.33) 1013/1.5, 1.0, 0.65, 0.4, 0.25 (1.6, 1.0, 0.75, 0.5, 0.33) 1013/1.5, 1.0, 0.65, 0.4, 0.25 2 1013/0.4 2 1013/0.4
850/1 850/1 950/1 950/1
K.P. Homewood et al. / Optical Materials 32 (2010) 1601–1605
EL response (a.u.)
40
EL integrated intensity (a.u.)
1604
(a)
20
0 1300
1400 1500 1600 Wavelength (nm)
1700
0.14
(b) 0.10
0.06
0.02 50
150 250 Temperature (K)
Fig. 5. (a) Electroluminescence intensity against wavelength for an erbium doped dislocation engineered silicon LED operating at room temperature at a forward bias of 2.8 V and forward current of 25 mA. This transition corresponds to the Er internal transition from the 4I13/2 to 4I15/2 levels in the 4f shell. (b) Operating temperature dependence of the erbium integrated EL intensity of a typical device.
Table 2 Here we summarise the erbium implant in the waveguide showing the total dose, the peak concentrations and the final effective concentration. Total Er dose (cm2)
Peak Er concentration (cm3)
Over-layer thickness (lm)
Effective concentration (cm3)
S1 S2 S3 S4
4.1 1013 4.1 1013 2 1013 2 1013
2 1017 1.7 1017 3 1018 3 10 18
2.1 2.5 1.5 5.0
2 1017 1.7 1017 1.3 1017 4 1016
PL Lock-in response (a.u.)
Sample
12
ΔPL 6
0
(a)
160
120
1200
1400
1600
Wavelength (nm)
0.015 80
0.010 40 -1
Δg (cm )
PL response (a.u.)
-6 1000
0 1000
1200
1400
1600
S3 S4
0.005
0.000
Wavelength (nm) Fig. 6. Photoluminescence spectrum from sample S3. The spectrum consists of three main regions: the silicon emission at around 1.1 lm and its phonon replica at 1.2 lm, the erbium emission between 1.5 and 1.6 lm, and a broad defect related emission peaking at around 1.3 lm.
expected from the dominant transitions is clear in both plots. The photoluminescence intensity decays with distance x, for the pump laser on and off, IP and I0, respectively, can be written as IP = PL + DPL = exp(g–aP)x and I0 = PL = exp–a0x, where g is the gain due to the pump, and aP and a0 are the respective loss coefficients. We can also write
IP PL þ DPL ¼ expðg aP Þx exp a0 x ¼ expðg aÞx ¼ eDgx ¼ PL I0 ð1Þ where a = aP–a0 is the optical loss and Dg = g–a is the change in the net material gain. Fig. 7b shows the data in Fig. 7a converted to the change in the net material gain, in units of cm1. We used Eq. (1) to obtain
-0.005
(b) -0.010 1000
1200
1400
1600
Wavelength (nm) Fig. 7. (a) The photoluminescence and change in photoluminescence spectra from lock-in 1 and lock-in 2, respectively, for sample S4. (b) The gain spectrum calculated from the data in (a) for sample S4 and also, for comparison, the data for sample S3.
Dg = [ln (1 + DPL/PL)]/L) where L is the cavity length (cm). Also shown for comparison is data for sample S3. In both cases gain is seen where anticipated, over the erbium region from 1.5 to 1.6 lm. Fig. 8 summarises the final data for all samples. It shows the maximum change in net gain Dgmax against the effective erbium doses for all the samples. The experimental parameters were the same for all measurements to provide a direct comparison
K.P. Homewood et al. / Optical Materials 32 (2010) 1601–1605
temperatures. Light emission and gain at 1.5 lm at room temperature has been demonstrated by combining this technology with the incorporation, using ion implantation, of erbium. The gain values already demonstrated suggest that the development of optical amplifiers and lasers for full integration into silicon photonics platforms is now a realistic possibility.
0.16
0.12
Δg max (cm-1)
1605
Acknowledgements
0.08
We thank the European Research Council for financial support, under FP7, for the award of ERC Advanced Investigator Grant – SILAMPS, Grant No. 226470.
0.04
References 0.00 0
1
2 17
3
-3
Er dose (10 cm ) Fig. 8. The maximum gain for all four samples plotted as a function of the effective dose: S1 (4), S2 (N), S3 (s) and S4 (d). Samples S3 and S4 have dislocation loops but samples S1 and S2 have not. The bottom solid line shows the expected gain given the previously supposed optical emission cross section for erbium in silicon.
between the different waveguide structures. Dgmax is calculated using the standard method by plotting the change in gain as a function of pump power density, fitting the data using the usual rate equations and extracting the maximum change in gain from the high power limit at gain saturation for each sample [26,27]. The lowest fitted line in Fig. 8 is the expected gain using the previously presumed value of 1.8 1020 cm2 for the optical cross section [25]; the middle line is the fit for samples that have no dislocation loops and the upper line is the fit obtained for samples nano-engineered with dislocation loops. We obtain a value for the erbium optical emission cross section, r, (where r = Dgmax/N, N is the effective erbium concentration) for the samples with the dislocation loops of (5.0 ± 0.1) 1019 cm2. This is around 30 times higher than previously anticipated. It is worth noting that a similarly high value of r of (2.0 ± 0.1) 1019 cm2 has been previously observed for erbium in SiO2 where silicon nano-crystals have been introduced [27] suggesting that the presence of silicon may be the key factor in the observed enhancement. The significance of the new results is that we should now be able to obtain enough gain, with achievable levels of erbium incorporation, to make this a feasible route to amplification and lasing in silicon. 4. Conclusions We have described how nano-engineering of the local strain field in silicon devices can be used to modify their functionality and efficiency. In particular we have shown that dislocation engineering – the controlled introduction of dislocation loops – can enable light emission and optical activity in silicon at high operating
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Contents lists available at ScienceDirect
Optical Materials journal homepage: www.elsevier.com/locate/optmat
Nano-engineered silicon light emitting diodes and optically active waveguides K.P. Homewood *, M.A. Lourenço, R.M. Gwilliam Advanced Technology Institute, Faculty of Engineering and Physical Sciences, University of Surrey, Guildford, Surrey GU2 7XH, United Kingdom
a r t i c l e
i n f o
Article history: Received 11 January 2010 Received in revised form 27 May 2010 Accepted 27 May 2010 Available online 26 June 2010 Keywords: Light emitting diodes Silicon Erbium Gain Optical amplifier Laser
a b s t r a c t In this paper, we first introduce and discuss the current state-of-the-art in integrated silicon photonic technology. We argue that the only missing link to the incorporation of this technology into mainstream high end silicon chips and systems are the availability of fully integrated silicon light sources and amplifiers. We go onto describe how dislocation engineering can be used to nano-engineer, locally, the strain in optically active silicon devices to enable high operating temperatures. We show how, by combining this approach with the incorporation of rare earths, that have optical levels in the near infra-red below the silicon band-gap, a potential route exists to meet these needs. In particular, we show that the use of erbium, together with dislocation engineering, could provide useful optical emission and gain at the important 1.5 lm wavelength that dominates optical data transfer. Ó 2010 Elsevier B.V. All rights reserved.
1. Introduction Integrated silicon photonics technology has advanced rapidly in the last few years [1–11] to the point that the only remaining components needed for the final implementation of full silicon photonic systems is to develop electrically pumped optical amplifiers, light emitting diodes (LEDs) and lasers in silicon also using a complementary metal oxide semiconductor (CMOS) compatible technology. The preferred wavelength for such systems is the 1.5 lm band. Here, we report on silicon LEDs emitting at 1.5 lm and measurements of optical gain at this wavelength capable of electrical excitation. Emission and gain at 1.5 lm are obtained by incorporating erbium in silicon. Nano-engineering the devices using dislocation loop technology can extend this functionality to high temperature operation. The gain values obtained are significantly greater than previously supposed. Consequently, this approach now offers a realistic route to the Holy Grail of silicon photonics – electrically pumped silicon optical amplifier and laser devices using standard silicon process technology. Optical gain in silicon has been demonstrated utilising the Raman effect [4,6,7] and, more recently, phase matched four wave mixing [10]. However, neither is capable of being electrically driven. There is a report of low temperature optically pumped lasing in crystalline silicon [11]. The emission occurs at a wavelength of 1.27 lm, subsequently attributed to the G-centre introduced using surface texturing. Again this is a significant result but it is unclear whether this centre can be incorporated in sufficient amounts con* Corresponding author. Tel.: +44 1483 689285; fax: +44 1483 686091. E-mail address: [email protected] (K.P. Homewood). 0925-3467/$ - see front matter Ó 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.optmat.2010.05.025
trollably using CMOS compatible processes or operated at high temperatures. The gain curves shown in all these approaches are very narrow, at around 1 nm, making them rather impractical for optical amplifiers where a broad band gain spectrum is needed to increase tolerance to the input laser wavelength and to support wavelength division multiplexing [10]. Here, we show that gain can be achieved over a wavelength range of around 150 nm, centred at a wavelength of 1550 nm. 2. Experimental details Silicon LEDs have been produced by the incorporation of rare earths to provide optical emission at specific wavelengths associated with the internal rare earth transitions. Tuning is achieved by implantation of rare earths into the active region of a bulk silicon diode where dislocation loops have also been introduced to nano-engineer the local strain field in the device. Optically active waveguides have been produced using a similar approach but into silicon on insulator (SOI) substrates. The nano-engineered strain field enables us to control the movement of the injected carriers. Perhaps the most significant aspect of the dislocation loop engineering (DE) is that not only can light emission in silicon be enabled but also the quenching of light and optical activity with increased operating temperature can be completely eliminated and even reversed [2,12]. In this paper we limit ourselves to LEDs and waveguides doped with erbium in the trivalent 3+ state. In general, semiconductor light emitting diodes suffer from the efficiency dropping significantly as the operating temperature increases. This is due to competition between the light emitting centres and non-radiative recombination through defects – the
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radiative recombination route is largely temperature independent. This is the cause of the poor light emission seen in indirect gap materials where the radiative transition is forbidden or, in practice, has a very long radiative recombination lifetime and emission through the fast non-radiative centres completely dominates. However by using DE the thermal quenching is largely reduced or eliminated. Dislocation engineering technology has been described in detail elsewhere [2,12–18] but for completeness we will outline the underlying principles here. The DE approach uses controlled incorporation of dislocation loops to form arrays at critical parts of the device. The individual loops are monolayer ‘‘islands” tens of nanometres across of additional silicon atoms inserted and surrounded by a simple closed edge-dislocation. The loops distort the surroundings and introduce local strain. Strain occurs at the dislocation edge producing a maximum negative pressure just outside the loop and a maximum positive pressure just inside the loop. The pressures are considerable, at around 50 GPa. The strain decays inversely with distance away from the loop edge. The net pressure remains zero in the lattice as a whole. The pressure and its spatial dependence can be calculated using conventional elastic theory of dislocations [19] requiring only Poisson’s ratio and Young’s modulus of silicon [2]. The total strain field is the superposition of the strain fields due to the individual loops. A typical loop array is shown in Fig. 1. The band structure and energies of semiconductors are strain dependent. Consequently, if strain within a semiconductor material or device can be modified so can the band gap energies, enabling potential variations to be introduced in the bands to control carrier migration. This is the principle underlying the field of strain layer semiconductors and devices, now widely applied in devices such as lasers and microprocessors. Strain is normally introduced by combining semiconductors of differing lattice constants using hetero-epitaxy. The DE approach is powerful in that strain is introduced only by manipulation of the host atoms. For silicon the pressure dependence of the lowest conduction band has a negative pressure coefficient so outside the loop the band gap is increased and inside is reduced. Consequently, a loop repels carriers and can be used to corral the carriers in parts of the device. The
pressure dependence of the band gap is 1.4 meV/kbar [20] giving loop induced modifications of the band gap by up to 0.75 eV. Loops are formed by introducing excess silicon atoms using ion implantation. Implants can be a dopant, such as arsenic or boron or, alternatively, the host atom silicon itself can be used. Loops are formed after a suitable anneal after which the dopant atoms sit substitutionally, generating excess interstitials that can agglomerate eventually into the dislocation loops. In the case of dopant atoms, they will also be electrically activated. The depth, density, size and distribution of the loop array can be engineered by adjusting the implant and annealing parameters [14,17,18]. Using silicon as the implant species enables the loop engineering and the device doping to be independently controlled and optimized. This technology has been employed to form dislocation engineered silicon LEDs (DELEDs). A device and the band diagram are shown in Fig. 2. Standard processing forms ohmic contacts to the top and back sides and a window has been left in the back contact. The loops form a two dimensional array near and parallel with the depletion region edge in the upper p-side. Also shown are extrinsic, optically active centres, between the depletion region edge and the loops introduced using a separate implant step. Importantly, unlike other silicon based light technologies, DELEDs are operated under forward bias as normal LEDs and typically have turn on voltages of around 1 V [2]. In a conventional p–n junction, placed under forward bias, carriers are injected across the barrier as the potential is lowered. In silicon, due to its indirect nature, intrinsic recombination lifetimes are very long and diffusion lengths of the order of microns. Consequently, most injected carriers reach and recombine at efficient non-radiative centres in bulk regions or in particular at the surface. This is the reason for the lack of electroluminescence (EL) in silicon diodes. In DELEDs strain due to the dislocation array in the plane of the junction increases the band gap energy introducing a potential barrier (Fig. 2b). As the loops lie in the (1 1 1) planes and the silicon (1 0 0) planes are parallel to the junction the strain field of the loop array zigzags and an injected carrier always only sees a potential barrier in the band. The barrier prevents onward diffusion of the carriers to defects and the surface which instead recombine radiatively. An LED technology is a prerequisite for laser development but in addition a gain mechanism has to be introduced. Optical gain in semiconductors is normally achieved using population inversion of carriers in a p–n junction under strong forward bias. However,
(a)
(b)
Top contact
Energy
Loops P
RE
Depletion region
N
Window
EV
EC
Bottom contact Fig. 1. Cross sectional and plan view transmission electron micrographs showing a typical dislocation loop array in silicon. Note that because the dislocation loops are in the (1 1 1) planes we are seeing their projection onto the (1 0 0) plane. This sample was implanted with boron at a dose of 1015 cm2, at an energy of 40 keV, and annealed at 950 °C for 1 min.
Fig. 2. (a) A schematic cross-section of a typical dislocation engineered light emitting diode with a rare earth implant incorporated between the dislocation loop array and the upper edge of the depletion region. Ohmic contacts are placed on the top and lower side of the device. A window has been left in the lower contact to allow light to escape. (b) An illustration of the energy band diagram through the device indicating the main optical transitions.
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this requires high gain because the high doping required leads to very high free carrier and Auger losses, and is only achievable in direct gap semiconductors with fast radiative transitions. An alternative is to incorporate a dopant that has an electronic structure enabling internal gain. Although such systems are often low gain they can also be incorporated into low-loss devices, in principle enabling net optical gain. Erbium is excited electrically in silicon via band gap recombination [21–25]. Elimination of thermal de-excitation and room temperature EL under forward bias has been demonstrated in boron co-doped samples [2,12]. Er3+ possesses intrinsic gain. Erbium has been suggested as a route to gain in silicon, but optical cross sections were supposed too small (1020 cm2) [25] for sufficient gain, requiring incorporation of unrealistic concentrations (1020 cm3) of the active ion. The samples for the gain measurements are planar waveguides formed in silicon on SOI substrates. The top silicon layer forms the waveguide and erbium has been implanted into this region. Some of the waveguides have had dislocation loops incorporated by implanting boron. The waveguide structure and its energy band diagram are shown in Fig. 3. Samples have either had a single erbium implant or multiple energy and dose implants to give a more uniform distribution of erbium. Details of all the samples are given in Table 1. The experimental set-up developed to measure these low gain samples is shown in Fig. 4. The main features that distinguish this approach and enable measurement of such low gain samples are the use of photoluminescence (PL) generated within the sample as the probe and two lock-in amplifiers in series. The first gives the PL and the second the change in PL, DPL, due to the pump. The pump laser is operated at the wavelength of 514 nm focused to a stripe on the sample 250 lm wide and 1 cm long. The sensitivity of this novel technique [26] is remarkable, enabling the measurement of gain/loss coefficients as small as 0.001 cm1. The broad band PL probe allows measurement of the full loss/gain spectrum as opposed to the usual single wavelength measurement. It also avoids alignment issues with external light probes allowing
variable temperature measurements. Measurements have been made between 80 K and room temperature. 3. Results and discussion In Fig. 5a we show the EL from a typical erbium DELED at 300 K, corresponding to the well known internal transitions from the 4I13/2 to 4I15/2 levels in the 4f shell. This sample was implanted with 1015 B cm2 at 30 keV and furnace annealed at 950 °C for 20 min, subsequently implanted with erbium at 5 1013 cm2 at 0.4 MeV and annealed at 950 °C for 1 min. The device operates under forward bias of 2.8 V at a current of 25 mA. In Fig. 5b is shown a typical temperature dependence of the erbium emission, taken at a constant current of 25 mA. It can be seen that the usual temperature quenching of erbium emission in silicon has been reversed allowing 1.5 lm wavelength EL in silicon at room temperature. It is this emission that provides the basis for design of the waveguides for the gain measurements shown in Table 1. In Table 2 implant details are summarised and the effective concentration calculated by averaging the dose over the waveguide volume; the waveguide length is 1 cm for all samples. Fig. 6 shows a typical PL spectrum from one of the waveguide samples. The samples have been selected to have broad emission allowing full spectral dependence measurements. In general all samples have emission bands peaking around 1.1, 1.3 and 1.5 lm. The broad feature at 1.3 lm, between the usual silicon peak at 1.1 lm and the erbium emission between 1.5 and 1.6 lm, is an implant induced defect band. Fig. 7a shows PL and DPL (magnified by 100) from the sample S4. Note the DPL drops in the 1–1.5 lm wavelength region but increases in the region of the erbium transitions. The structure
Probe laser
Pump laser
Chopper 2
Lock-in 2
ΔPL
Lock-in 1
PL
Lens Chopper 1
(a)
(b)
Surface
Energy
Detector
Loops
Spectrometer
RE
Sample
Silicon over-layer EV
Cryostat
EC
Oxide Silicon substrate Fig. 3. (a) A schematic cross-section of the silicon on insulator waveguide samples used for the gain measurements. Some of the samples are controls and do not contain the dislocation loops shown here. (b) An illustration of the energy band diagram through the silicon over-layer showing the main optical transitions.
Fig. 4. Schematic of the new experimental gain system developed for these measurements. The probe laser excites photoluminescence in the sample which is then propagated through the waveguide region which is excited by the pump laser. The two lock-ins connected in series allows independent monitoring of the PL and change in PL from which loss or gain can be calculated. The two series lock-in method gives a greatly enhanced sensitivity compared with more conventional systems.
Table 1 Here we list the silicon over-layer doping and the implant and anneal conditions for all the samples. For the multiple erbium implant samples the dose of the various implants are listed first and then their respective implant energies. Sample S1 S2 S3 S4
Over-layer doping (type/cm3) 17
n/2 10 p/3 1016 n/1 1014 n/1 1014
B implant (cm2/keV)
B anneal (°C/min)
Er implants (cm2/MeV)
Er anneal (°C/min)
– – 1 1015/30 1 1015/30
– – 950/20 950/20
(1.6, 1.0, 0.75, 0.5, 0.33) 1013/1.5, 1.0, 0.65, 0.4, 0.25 (1.6, 1.0, 0.75, 0.5, 0.33) 1013/1.5, 1.0, 0.65, 0.4, 0.25 2 1013/0.4 2 1013/0.4
850/1 850/1 950/1 950/1
K.P. Homewood et al. / Optical Materials 32 (2010) 1601–1605
EL response (a.u.)
40
EL integrated intensity (a.u.)
1604
(a)
20
0 1300
1400 1500 1600 Wavelength (nm)
1700
0.14
(b) 0.10
0.06
0.02 50
150 250 Temperature (K)
Fig. 5. (a) Electroluminescence intensity against wavelength for an erbium doped dislocation engineered silicon LED operating at room temperature at a forward bias of 2.8 V and forward current of 25 mA. This transition corresponds to the Er internal transition from the 4I13/2 to 4I15/2 levels in the 4f shell. (b) Operating temperature dependence of the erbium integrated EL intensity of a typical device.
Table 2 Here we summarise the erbium implant in the waveguide showing the total dose, the peak concentrations and the final effective concentration. Total Er dose (cm2)
Peak Er concentration (cm3)
Over-layer thickness (lm)
Effective concentration (cm3)
S1 S2 S3 S4
4.1 1013 4.1 1013 2 1013 2 1013
2 1017 1.7 1017 3 1018 3 10 18
2.1 2.5 1.5 5.0
2 1017 1.7 1017 1.3 1017 4 1016
PL Lock-in response (a.u.)
Sample
12
ΔPL 6
0
(a)
160
120
1200
1400
1600
Wavelength (nm)
0.015 80
0.010 40 -1
Δg (cm )
PL response (a.u.)
-6 1000
0 1000
1200
1400
1600
S3 S4
0.005
0.000
Wavelength (nm) Fig. 6. Photoluminescence spectrum from sample S3. The spectrum consists of three main regions: the silicon emission at around 1.1 lm and its phonon replica at 1.2 lm, the erbium emission between 1.5 and 1.6 lm, and a broad defect related emission peaking at around 1.3 lm.
expected from the dominant transitions is clear in both plots. The photoluminescence intensity decays with distance x, for the pump laser on and off, IP and I0, respectively, can be written as IP = PL + DPL = exp(g–aP)x and I0 = PL = exp–a0x, where g is the gain due to the pump, and aP and a0 are the respective loss coefficients. We can also write
IP PL þ DPL ¼ expðg aP Þx exp a0 x ¼ expðg aÞx ¼ eDgx ¼ PL I0 ð1Þ where a = aP–a0 is the optical loss and Dg = g–a is the change in the net material gain. Fig. 7b shows the data in Fig. 7a converted to the change in the net material gain, in units of cm1. We used Eq. (1) to obtain
-0.005
(b) -0.010 1000
1200
1400
1600
Wavelength (nm) Fig. 7. (a) The photoluminescence and change in photoluminescence spectra from lock-in 1 and lock-in 2, respectively, for sample S4. (b) The gain spectrum calculated from the data in (a) for sample S4 and also, for comparison, the data for sample S3.
Dg = [ln (1 + DPL/PL)]/L) where L is the cavity length (cm). Also shown for comparison is data for sample S3. In both cases gain is seen where anticipated, over the erbium region from 1.5 to 1.6 lm. Fig. 8 summarises the final data for all samples. It shows the maximum change in net gain Dgmax against the effective erbium doses for all the samples. The experimental parameters were the same for all measurements to provide a direct comparison
K.P. Homewood et al. / Optical Materials 32 (2010) 1601–1605
temperatures. Light emission and gain at 1.5 lm at room temperature has been demonstrated by combining this technology with the incorporation, using ion implantation, of erbium. The gain values already demonstrated suggest that the development of optical amplifiers and lasers for full integration into silicon photonics platforms is now a realistic possibility.
0.16
0.12
Δg max (cm-1)
1605
Acknowledgements
0.08
We thank the European Research Council for financial support, under FP7, for the award of ERC Advanced Investigator Grant – SILAMPS, Grant No. 226470.
0.04
References 0.00 0
1
2 17
3
-3
Er dose (10 cm ) Fig. 8. The maximum gain for all four samples plotted as a function of the effective dose: S1 (4), S2 (N), S3 (s) and S4 (d). Samples S3 and S4 have dislocation loops but samples S1 and S2 have not. The bottom solid line shows the expected gain given the previously supposed optical emission cross section for erbium in silicon.
between the different waveguide structures. Dgmax is calculated using the standard method by plotting the change in gain as a function of pump power density, fitting the data using the usual rate equations and extracting the maximum change in gain from the high power limit at gain saturation for each sample [26,27]. The lowest fitted line in Fig. 8 is the expected gain using the previously presumed value of 1.8 1020 cm2 for the optical cross section [25]; the middle line is the fit for samples that have no dislocation loops and the upper line is the fit obtained for samples nano-engineered with dislocation loops. We obtain a value for the erbium optical emission cross section, r, (where r = Dgmax/N, N is the effective erbium concentration) for the samples with the dislocation loops of (5.0 ± 0.1) 1019 cm2. This is around 30 times higher than previously anticipated. It is worth noting that a similarly high value of r of (2.0 ± 0.1) 1019 cm2 has been previously observed for erbium in SiO2 where silicon nano-crystals have been introduced [27] suggesting that the presence of silicon may be the key factor in the observed enhancement. The significance of the new results is that we should now be able to obtain enough gain, with achievable levels of erbium incorporation, to make this a feasible route to amplification and lasing in silicon. 4. Conclusions We have described how nano-engineering of the local strain field in silicon devices can be used to modify their functionality and efficiency. In particular we have shown that dislocation engineering – the controlled introduction of dislocation loops – can enable light emission and optical activity in silicon at high operating
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