Towards a Si/SiGe Quantum Cascade Laser for ...

Mater. Res. Soc. Symp. Proc. Vol. 832 © 2005 Materials Research Society

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Towards a Si/SiGe Quantum Cascade Laser for Terahertz Applications D.J. Paul1, S.A. Lynch1, P. Townsend1, Z. Ikonic2, R.W. Kelsall2, P. Harrison2, S.L. Liew3, D.J. Norris3, A.G. Cullis3, J. Zhang4, M. Bain5, H.S. Gamble5, W.R. Tribe6 and D.D. Arnone6 1 Cavendish Laboratory, University of Cambridge, Madingley Road, Cambridge, CB3 0HE, U.K. 2 Institute for Microwaves and Photonics, University of Leeds, Leeds, LS2 9JT, U.K. 3 Department of Electronics and Electrical Engineering, University of Sheffield, Sheffield, S1 3JD, U.K. 4 Blackett Laboratory, Imperial College of Science, technology and Medicine, Prince Consort Road, London, SW7 2BZ, U.K. 5 Electrical and Electronic Engineering, Queens University Belfast, Belfast, BT9 5AH, U.K. 6 TeraView Ltd., 302 to 304 Cambridge Science Park, Milton Road, Cambridge, CB4 0WG, U.K. ABSTRACT While electroluminescence has been demonstrated at terahertz frequencies from Si/SiGe quantum cascade emitters, to date no laser has been achieved due to poor vertical confinement of the optical mode. A method of increasing the vertical confinement of the optical mode for a Si/SiGe quantum cascade laser is demonstrated using silicon-onsilicide technology. Such technology is used with epitaxial growth to demonstrate a strain-symmetrised 600 period Si/SiGe quantum cascade interwell emission and the polarisation is used to demonstrate the optical confinement. Electroluminescence is demonstrated at ~3 THz (~100 µm) from an interwell quantum cascade emitter structure. Calculated model overlap and waveguide losses for ridge waveguides are comparable to values from GaAs quantum cascade lasers demonstrated at terahertz frequencies. The effects of high doping levels in Si/SiGe quantum cascade structures is also investigated with impurity emission demonstrated rather than intersubband emission for the highest doping levels used in the cascade active regions. INTRODUCTION There are a rich number of applications in the terahertz frequency part of the electromagnetic spectrum, many of which are related to the rotational and vibrational modes of molecules [1]. Major applications for terahertz frequencies include security screening [2], explosives and narcotics identification [2], bioweapons detection [3], skin cancer imaging [4], dental imaging [5], proteomics [6] and production monitoring [6]. This part of the electromagnetic spectrum, however, has to date been poorly utilised due to a lack of cheap and practical terahertz sources of radiation. Recently terahertz quantum cascade lasers [7] have been demonstrated using III-V semiconductor heterostructures at low temperatures. More recent developments have allowed operation of these lasers at temperatures up to 150 K [8] but further advances are significantly more difficult due to the polar optical phonon scattering in III-V materials.

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While no interband recombination laser has been demonstrated in silicon due to the indirect nature of the band gap, quantum cascade lasers are unipolar, intersubband devices and as such the technique is amenable to indirect materials such as Si and SiGe [9]. A Si/SiGe quantum cascade laser if realised could potentially have a number of significant advantages over III-V quantum cascade lasers. The nature of the Si-Ge bond results in negligible polar optical phonon scattering which produces significantly enhanced intersubband lifetimes with almost no reduction in lifetime up to room temperature [10]. The lifetimes in III-V materials demonstrate polar optical phonon scattering dominating above only 40 K [11] and are one of the major reasons for the low temperature operation of the devices. Silicon also allows mature, cheap and high yield fabrication processes along with the potential of integration with electronic devices. The higher thermal conductivity of the silicon substrate is also advantageous in producing a laser where high currents are required to achieve threshold. Previous work has demonstrated both intra and interwell intersubband emission from Si/SiGe quantum cascade emitters with up to 100 strain-symmetrised periods at frequencies between 1.2 THz (250 µm) and 16.2 THz (18.5 µm) [12][13]. All Si/SiGe quantum cascade emitters demonstrated to date have used hole intersubband transitions mainly due to the significantly lower effective masses of holes compared to electrons in the silicon heterostructure system. Such active regions require to be fabricated into cavities or waveguides if a laser is to be realised. These cavities require the gain from the active region and the cavity to be larger than the waveguide losses if lasing is to occur. To realise a THz laser with wavelengths around 100 µm will require significantly higher numbers of periods than 100 to allow sufficient modal overlap for lasing. In this paper, high modal overlap is demonstrated by the growth of 600 periods of strain-symmetrised Si/SiGe quantum cascade emitter combined with a buried silicide layer to further enhance the modal overlap. WAFER DESIGN AND GROWTH In GaAs THz quantum cascade lasers, a doped plasmon reflector is formed using a heavily doped semiconductor layer beneath the active cascade region to confine the optical mode in the vertical direction [7]. This layer is also used to form the Ohmic contact at the bottom of the active region stack thereby allowing the vertical flow of current through the device. Figure 1 shows the calculated modal overlap for a 4 µm thick Si/SiGe quantum cascade active region [13] formed into a ridge waveguide and with a similar heavily p-type doped region of 3 x 1020 cm–3 directly beneath the active region to act as a plasmon reflector [14]. The top of the sample has a 50 nm thick layer doped at 5 x 1019 cm–3 with aluminium metal on top to form both an Ohmic contact but also which acts as a plasmon reflector. The addition of the metal significantly improves the confinement due to the higher electrical conductivity compared to doped SiGe. For a frequency of 4.8 THz, the confinement of the optical mode in the 4 µm thick Si/SiGe quantum cascade emitter region is only about 18 % even for a 0.3 µm thick buried plasmon reflector. This is far too small to be able to realise a laser using Si/SiGe and is

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Figure 1. The calculated modal overlap for three different THz wavelengths of emission for a TM mode ridge waveguide with a 4 µm thick active SiGe quantum cascade emitter as a function of the thickness of a buried semiconductor layer doped at 3 x 1020 cm–3 directly beneath the active region. A top 50 nm p-SiGe contact layer doped at 5 x 1019 cm–3 with aluminium metal forms a top plasmon reflector. related to the poor electrical conductivity of the doped p-type SiGe resulting in poor vertical confinement of the optical mode. Any solution to this problem requires a buried reflector layer with significantly higher electrical conductivity than heavily doped p-type SiGe. In GaAs this problem was solved by etching the back of the sample away and then coating the bottom of the active cascade region with gold metal to act as the bottom reflector [8]. This can also be attempted in silicon but the thicker wafers require over 600 µm of etching and the ability to stop on a thin 200 nm layer. While technology is

Figure 2. The calculated modal overlap for three different THz wavelengths of emission for a TM mode ridge waveguide with a µm thick active SiGe quantum cascade emitter as a function of the thickness of a buried WSi2 layer directly beneath the active region. A top 50 nm p-SiGe contact layer doped at 5 x 1019 cm–3 with aluminium metal forms a top plasmon reflector.

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available to achieve this type of double metal waveguide, the long etch significantly increases the production cost of such a device. In this paper we demonstrate an alternative technique at the wafer scale which uses a buried silicide layer which has an electrical conductivity significantly higher than doped SiGe and within an order of magnitude of many good metals. WSi2 was chosen as it can stand the high 800 oC chemical vapour deposition (CVD) growth temperatures required to produce a virtual substrate on top of the wafer. Other silicides such as titanium or nickel silicide have higher electrical conductivity and should provide superior performance but suffer from formation temperatures below the CVD growth temperature. The addition of this buried WSi2 beneath the same active region and top plasmon reflector as figure 1 is shown in figure 2. The modal overlap as a function of the silicide thickness is shown and for the three THz wavelengths plotted, the modal overlap has risen to over 90 % for silicide thicknesses above 0.3 µm for all three wavelengths. Two wafers have been produced with 0.2 µm and 0.4 µm thick buried tungsten silicide layers using bond and etch back techniques discussed previously [15]. The top 2 µm silicon box on the silicide was then used as the starting substrate for CVD growth. A 2 µm Si0.8Ge0.2 virtual substrate was grown followed by a 200 nm B-doped bottom Ohmic contact layer of 5 x 1019 cm–3 and a 20 nm graded SiGe injector / collector. 600 periods of 6.5 nm Si0.7Ge0.3 quantum wells with 2 nm Si barriers were grown as the cascade active region. This is an interwell heavy-hole (HH1) to light-hole (LH1) transition which has been demonstrated previously [13]. A graded injector followed by a 40 nm B-doped (3 x 1020 cm–3) Si0.8Ge0.2 cap finished the structure. Modelling ridge waveguide structures produced from this wafer calculates a modal overlap of 58 % with waveguide losses of 30 cm–1 for such a structure [14]. Transmission electron micrographs

Figure 3. A transmission electron micrograph of the complete virtual substrate and 600 period active region superlattice grown on top of the silicon-on-silicide wafer. The silcide is the dark poly-crystalline layer 0.4 µm thick near the bottom of the image.

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Figure 4. A transmission electron micrograph of the bottom of the 600 period active region superlattice. of the structure demonstrate the successful growth of the heterolayers on the buried silicide wafer (figure 3) along with the excellent planarity and uniformity of the active periods (figure 4). There is, however, a higher defect density than growth on blank silicon wafers, the source of which has not yet been identified. EXPERIMENTAL MEASUREMENTS Fourier transform infrared (FTIR) spectroscopy was performed using a Bruker 66V stepscan spectrometer with a liquid-He cooled Si bolometer for detection. The sample was placed in a continuous flow cryostat providing measurement temperatures between 4.2 and 300 K. Voltage was applied to the sample in the form of a 50 kHz pulsestream with a variable duty cycle. This was gated at 413 Hz to permit measurements using a lock-in amplifier to be performed using the gating pulse as a reference. The bolometer has an optimum frequency of operation around 410 Hz. The voltage was applied vertically across the 600 quantum wells and all parts of the system in which THz radiation propagated were purged with N2 to eliminated absorption due to water vapour. Samples were fabricated into 240 µm by 240 µm square mesas to investigate the electroluminescent behaviour of the quantum cascade structures. Silicon tetrachloride reactive ion etching was used to etch the mesa, stopping on the 200 nm buried bottom Ohmic contact region. Aluminium (1 % silicon) was then lifted-off to form Ohmic contacts to the top and bottom of the quantum cascade emitter before being annealled at 400 oC. Care was taken to make sure the aluminium does not spike through the active quantum wells of the device. The electroluminescent emission was measured from the side of the sample. The current voltage characterisitics for a 240 µm mesa device is shown in figure 5 for 3 different duty cycles. The power versus current is shown in figure 6 for the same duty cycles at 4.2 K. Comparing both graphs indicates that the correct alignment of the HH1 and LH1 subbands for interwell emission occurs at around 60 mA or approximately 13 V. At the highest duty cycles and currents, however, strong heating is observed in the

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Figure 5. The current-voltage characteristics of a cascade emitter at 4.2 K for 3 different duty cycles. devices. Electroluminescence spectra were therefore taken at a low enough duty cycle and current to minimise heating effects. The edge emitted spectrum from a Fourier transform infrared spectrometer is demonstrated in figure 7 at 4.2 K for a duty cycle of 5 % with the centre of the peak at 14 meV (~3 THz or 100 µm). As the voltage on the device is increased the electroluminescent peak shifts to higher energies as expected from the band structure of the system [13]. The wafer structure was designed to support a TM cavity (ridge waveguide) and therefore the polarisation has been measured to see if any confinement

Figure 6. The output power as a function of current for different duty cycles at 4.2 K. Between 0.06 mA and 0.08 mA for 5% and 10% duty cycles the electroluminescence is dominated by intersubband emission while for higher currents and the 20% duty cycle there is substantial Joule heating.

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Figure 7. The FTIR spectrum of a 240 µm x 240 µm edge emitting device at 4.2 K, 10% duty cycle and 15 V. effects are observed. At the lowest currents the emission power was too low to get any significant signal with a polarizer in the beam path. Polarised emission could only be measured for currents above 140 mA where some Joule heating is also present. The TE mode if scaled to lower currents approaches zero emission power at around 140 mA. Therefore below approximately 100 mA where only the designed HH1 to LH1 interwell transition is present and with reduced Joule heating, only the TM mode is supported (figure 8) as expected from theory for this type of waveguide structure.

Figure 8. The TE and TM polarisations of the edge-emitted output power for 15 V and 10% duty cycle.

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The use of the buried-silicide layers in placed of a heavily doped semiconductor layer as the bottom reflector increased the modal overlap for ridge waveguide cavities from ~18 % to 58 %. Calculations of the waveguide losses for this structure produce a value of 30 cm–1 excluding mirror losses from the etched facets at the ends of the waveguide [14]. These figures are comparable to values from the first GaAs THz quantum cascade laser which had a modal overlap of 47 % and waveguide losses of 26 cm–1 for a 1.24 mm long ridge-waveguide stripe [7]. This suggests that the Si/SiGe material structure developed in this paper is close to that required to demonstrate a quantum cascade laser operating at terahertz frequencies.

Figure 9. A spectrum from a 100 µm diameter microdisk device taken at 10 % dutycycle, 35 V and 405 mA at 4.2 K. HEAVY DOPING IN WAVEGUIDE STRUCTURES Once successful intersubband electroluminescence had been demonstrated with the silicide substrates, a new set of interwell cascade samples was grown by CVD with an identical design of Si and SiGe heterolayers to those demonstrated previously in the paper but with boron doping in the quantum wells of ~ 1018 cm–3. These designs are aimed at increasing the current through the samples. The wafer had a number of different devices fabricated including ridge waveguides and square microdisk and circular microdisks. Figure 9 shows a typical spectrum that all these devices demonstrate. No intersubband electroluminescence was observed but instead electroluminescence from impurity states. Previously electroluminescence was observed from B impurity states in modulation doped SiGe quantum wells [16][17] which corresponded to absorption measurements in bulk silicon [18][19]. At higher temperatures the impurity state electroluminescence was quenched and intersubband electroluminescence dominated the measured spectra. In figure 9, B impurity states are observed at 31 meV, 34.4 meV and 39.8 meV. These states correspond to the absoption lines in bulk, unstrained Si and therefore have not been

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shifted by strain. The mechanism for electrical injection into the impurities which produce the electroluminescence is not understood. The other states at 22.4 meV, 26.2 meV, 27.7 meV and 42 meV are related to P impurities which have not previously been observed. Secondary ion mass spectrometry measurements demonstrate that the only P in these samples was in the 1.2 µm silicon left on top of the silicide before growth of the SiGe heterolayers at a level of 5 x 1015 cm–3. As the P is not in the Si0.7Ge0.3 quantum wells and there is no shift due to strain in the peak positions with energy this strongly suggests that the electrons populating the P states are the result of the p-i-n diode formed between the bottom B Ohmic contact layer and the P doped layer close to the silicide. CONCLUSIONS The first SiGe growth on top of silicon on buried-silicide wafers has been demonstrated. A 600 period strain-symmetrised Si/SiGe quantum cascade emitter was grown on top of a virtual substrate using CVD without any significant problems related to the threading segments of the dislocations from the virtual substrate. Electroluminescence from an interwell quantum cascade emitter was demonstrated at 4.2 K and 15 V with a polarisation corresponding only to a TM mode at low currents. The use of the buriedsilicide layer in placed of a heavily doped semiconductor layer as the bottom reflector increased the modal overlap for ridge waveguide cavities from ~18 % to 58 %. These modal overlap figures combined with the calculated losses for the ridge waveguide system are comparable to values from GaAs quantum cascade lasers operating at terahertz frequencies. Electroluminescence from both boron and phosphorous impurities were also demonstrated in a second wafer which had identical heterolayer growth but included higher boron doping densities in all the Si0.7Ge0.3 quantum wells. It is not completely clear the electrical injection mechanism into these impurity states as the states are not shifted in energy as one would expect with strain. ACKNOWLEDGEMENTS The authors would like to thank DARPA, EC and EPSRC for financial support for the work in this paper. REFERENCES 1. 2. 3. 4. 5.

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