Realizing a germanium (Ge)-based monolithic light source requires n-type doping with high activation levels, tensilestrain, and an optical cavity. Here, we demonstrate the application of spin-on doping technique, and the use of free-standing structures to induce tensile strain on Ge micro-disks, which act as a simple micro-cavit

Al-Attili, Abdelrahman

Arakawa, Yasuhiko

Arimoto, Hideo

Gardes, Frederic

Higashitarumizu, Naoki

Husain, Muhammad

Ishikawa, Yasuhiko

Iwamoto, Satoshi

Kako, Satoshi

Saito, Shinichi

Realizing a germanium (Ge)-based monolithic light source requires n-type doping with high activation levels, tensile strain, and an optical cavity. Here, we demonstrate the application of spin-on doping technique, and the use of free-standing structures to induce tensile strain on Ge micro-disks, which act as a simple micro-cavity

Southampton (e-Prints Soton)

978-1-4673-8156-7/15/$31.00 c© 2015 IEEEFabrication of Ge micro-disks on free-standing SiO2beams for monolithic light emissionAbdelrahman Al-Attili,Muhammad Husain, Frederic Gardes,Hideo Arimoto, and Shinichi SaitoFaculty of Physical Sciences and EngineeringUniversity of SouthamptonSouthampton, United KingdomEmail: S.Saito@soton.ac.ukNaoki Higashitarumizuand Yasuhiko IshikawaDepartment of Materials EngineeringThe University of TokyoTokyo, JapanSatoshi Kako, Satoshi Iwamoto,and Yasuhiko ArakawaInstitute of Industrial ScienceThe University of TokyoTokyo, JapanAbstract—Realizing a germanium (Ge)-based monolithic lightsource requires high n-type doping, tensile strain, and an opticalcavity. Here, we demonstrate the application of spin-on dopingtechnique, and the use of free-standing structures to induce tensilestrain on Ge micro-disks, which act as a simple micro-cavity.Keywords—Germanium, Photonics, Doping, Strain, CavityI. INTRODUCTIONInnovation in short-distance communications is expected,if we realize a complementary-metal-oxide-semiconductor(CMOS) compatible laser diode on a silicon (Si) chip[1]. Compatibility with standard CMOS processes qualifiesindirect-gap group IV materials for this purpose, among whichGe appears to be the most promising due to its pseudo-directband-gap [2]. The slight difference between Γ and L valleys(136 meV) can be decreased by applying tensile strain andn-type doping [3]. Tensile strain deforms the edges of theconduction band and transforms Ge into a direct gap materialat 2% bi-axial tensile strain, while n-type doping fills the Lvalleys with electrons and reduces the injection requirementsto achieve population inversion [3]–[5]. In fact, lasing fromGe [4], [5], and more recently from GeSn [6], has beendemonstrated. In order to improve the efficiency, strain andn-type doping levels have to be enhanced (Fig. 1).In this paper, we present our work on n-type doping ofGe using spin-on dopants, and the application of tensile strainon a simple Ge micro-disk cavity, by utilizing free-standingstructures using Ge-on-insulator (GOI) wafers.II. SPIN-ON DOPINGFor efficient light emission from Ge, we must achievehigh active carrier concentration, while imposing minimaldamage on crystalline quality upon doping. Direct-gapemission efficiency is sensitive to the crystalline quality, andit was found that the carrier lifetime seems to be limitedby non-radiative recombinations [1], [7]. Consequently,ion-implantation may not be suitable for Ge light emissionapplications. Other diffusion-based techniques, such asin − situ and delta-layer doping, can be implemented [5].Spin-on doping (SOD) is a promising doping technique, inwhich a high-concentration impurity source is span-coated onFig. 1. Band-gap of (a) bulk Ge, and (b) tensile-strained n-doped Ge. Tensilestrain deforms conduction band edges and splits the heavy-hole and light-holebands. While n-type doping fills the L valley with electrons, and consequentlyincreases the probability of electrons injection into the Γ valleytop of Ge and annealed. SOD imposes minimal damage oncrystalline quality because it is based on diffusion of dopants,and potentially can achieve high activation levels because theimpurity concentration in the SOD solution can exceed thesolid solubility in Ge [8], [9].Many processing obstacles have to be overcome toestablish a SOD recipe for heavy doping of Ge. Theseobstacles are due to thermal stresses during annealing, whichcause the Ge layer to crack and peel off in the case of GOIwafers. Moreover, Ge oxidizes at temperatures exceeding450 ◦C forming gaseous Ge mono-oxides (GeO), and thiscomplicates the annealing process during which the diffusionof dopants occur [9], [10]. We have used two types of spin-ondopants, a water-based and an alcohol-based solutions, todope GOI wafers with 70 nm Ge [9]. After optimizing thedoping process for both solutions, mainly through dilutionand patterning the Ge layer before SOD, doping of 10µm Ge disks was possible with an activation level of (1 -2)×1019 cm−3. Photoluminescence (PL) measurements wereconducted using a laser with 740 nm wavelength. PL spectrashowed an increase in direct-gap emission intensity, whichcan be attributed to indirect L valleys filling by doping, andthis increases the probability of electrons injection into thedirect Γ valleys. Observing the power dependence of PLFig. 2. Photoluminescence measurements showing the effect of doping ondirect-gap emission from 10-µm-diameter Ge disk. Different colors indicateddifferent pumping powers of 5, 7.5, and 10 mWintensity for intrinsic and doped samples, we found a changein power-dependence behavior from quadratic to linear [9].This indicates a faster increase in direct-gap emission intensitywith increased excitation power for the doped Ge.III. TENSILE STRAIN BY FREE-STANDING SIO2 BEAMSTensile strain application by fabricating free-standing Gebeams is known for high achievable strain values [11], [12].The dependence of this strain on beam dimensions providesan additional advantage of strain tunability. Embedding anoptical cavity within free-standing structures however is notstraightforward.In order to embed the functionality of a micro-cavitywithin a free-standing structure, we fabricated Ge micro-diskson top of suspended SiO2 beams, using GOI wafers (Fig.3). Ge disks with diameters varying from 1 to 10 µm weredry-etched. Then the surface was passivated with 100 nm(a) (b) (c)(d) (e) (f)GeSiO2SiFig. 3. Fabrication process of Ge disks on suspended SiO2 beams: (a) GOIwafer cleaning, (b) dry-etching Ge disks, (c) dry-etching BOX layer, (d) wet-etching the remaining BOX using HF, (e) and suspension by etching the Sisubstrate using TMAH. (f) Shows the final structure after the beam bendingFig. 4. Scanning electron microscopy images of a Ge micro-disk on: (a) asuspended SiO2 beam with curved edges, and (b) a suspended SiO2 beam withstraight edges. Insets show optical microscope images of similar structuresSiO2 deposited using Plasma-enhanced CVD at 350 ◦C.Afterwards, hole openings were dry-etched in the buried-oxide (BOX) layer centered around the Ge disks to definethe beams. To be suspended, the beams have to be alignedwith the 〈001〉 direction, as shown in Fig. 4. Due to thesensitivity of alkali wet-etching that will be used afterwardsto the surface roughness of Si, dry-etching was tuned suchthat 20 nm BOX remains un-etched, and then removedusing diluted Hydrofluoric (HF) acid. Then immediately thebulk Si substrate was etched using Tetra-methyl-ammoniumhydroxide (TMAH) to suspend the structures. Several beamdesigns were included in our chip, such as beams withcurved edges (Fig. 4 (a)) to enhance beam stability uponsuspension, and conventional rectangular beams. To reducethe effect of the boundaries on the beam, it is possible toutilize wet-etching anisotropy to limit the under-etching atthe beam edges, as shown in Fig. 4 (b). Direction of bendingwas unpredictable, and beams with curved edges had bettersurvivability compared to conventional rectangular beams.SiO2 beams bend upon suspension due to residualstresses within the thermally-grown SiO2 film. BOX layerhas built-in compressive stresses that originate during thehigh-temperature growth process, due to the difference inthermal expansion coefficient between Si and SiO2 [13].Releasing such a compressively-stressed beam will partiallyrelief the stresses by beam elongation, as shown in Fig. 5 (a)and (b), while residual stresses work on deflecting the beam[13], [14]. Consequently, upward bending, for example, willcause the upper side of the beam to be tensile-strained, whilethe bottom side gets compressed [14], as shown in Fig. 5 (c).We expect that this tensile strain can be transferred to a Gemicro-disk on top of the beam, if there is a good adhesionbetween the disk and the beam. Laser microscopy heightmapping (Fig. 5 (d)) shows the bending behavior of a Gemicro-disk on a free-standing SiO2 beam with curved edges.This bending with a maximum deflection of about 0.8 µmcontributed to an enhancement in tensile strain within the Gedisk, as confirmed by Raman spectroscopy.Raman spectroscopy measurements were conducted usinga laser with 532 nm wavelength and 2 µm spot size. Dueto sensitivity of suspended structures to heating by laserFig. 5. Tensile strain by free-standing SiO2 beams: (a) thermally-grown SiO2with built-in compressive stresses, (b) partial relief of stresses by elongationafter suspension, and (c) bending due to residual stresses. (d) Laser microscopyheight map of a Ge micro-disk on a free-standing SiO2 beam. Upward bendingof the beam is responsible for tensile strain accumulation within the diskexcitation, we measured the Raman shift dependence onthe excitation power. Excitation power was increased insteps from 55 µW to 1 mW, and data points were fit witha Lorentzian to extract the Raman peak position. Powerdependence of the Raman peak position had a linear trend,and by extrapolating this linear relation we could estimatethe Raman peak position without the effect of heating. Thereference un-patterned GOI wafer had a Raman shift of -0.8cm−1 relative to bulk Ge, which corresponds to approximately0.2% bi-axial tensile strain. This strain is presumably dueto Ge growth on Si during the GOI manufacturing process.After dry etching the Ge disks, it was found that tensile strainrelaxes and the shift in Raman peak position reduces. Infact, in small disks with diameters around 2 µm, the Ramanshift after dry etching is nearly zero compared to bulk Ge.However, when the disks are suspended by releasing the SiO2beams, a slight enhancement in tensile strain is observed.For instance, Raman peak of the 3 µm Ge disk shown inFig. 4 (a) is shifted by -1.11 cm−1 relative to bulk Ge.The orientation of this tensile strain, being either uni-axialor bi-axial, is not obvious. Accordingly, the tensile straincan be estimated to be 0.73% (0.28%) uni-axial (bi-axial) [15].PL measurements of Ge micro-disks on free-standing SiO2beams were performed using a laser with 730 nm wavelength(Fig. 6). We investigated the dependence of free-standing Gedisks’ spectra on the excitation power, and the broadeningof the PL spectra has been observed upon increasing theexcitation power. This was attributed to the sensitivity ofsuspended structures to heating by laser excitation. It wasfound that heating induced a splitting between the light-hole(LH) and heavy-hole (HH) bands, and the splitting energyincreased upon heating.Fig. 6. Photoluminescence intensity of a free-standing Ge micro-disk fordifferent excitation powers. Splitting of heavy-hole and light-hole bands isevident as assigned with the arrowsIV. A SIMPLE MICRO-CAVITYGe micro-disks are being investigated as a temptingstructure for realizing an on-chip laser diode, due to theirsmall footprint and the high quality (Q) factors of theWhispering Gallery Modes (WGM) that are confined in suchstructures [16], [17]. Before suspension, we could not detectany WGM resonances because of the thin Ge film and themode leakage path through the Si substrate. On the other hand,sharp peak resonances were observed at room temperaturefrom a 3 µm Ge disk on a free-standing SiO2 beam, as shownin Fig. 7. A maximum quality factor of 192 was calculatedby fitting the resonant peak with a Lorentzian function, anddividing the center frequency by the full-width half-maximum.Q-factors are degraded with increased pumping power due toheating effects and free-carrier absorption (FCA) losses [15].Finite-domain time-difference (FDTD) simulations weredone to determine the nature of these sharp resonances. Anelectric field was used to excite a similar structure of a 3 µmGe disk on a free-standing SiO2 beam. FDTD simulationsresulted in sharp-peak WGMs centered on 1508, 1561, 1626,1708, and 1811 nm. Assuming that only transverse-electric(TE) modes can be guided through (70 - 100) nm Ge, theseTEm,n WGMs can be referred to as TE13,1, TE12,1, TE11,1,TE10,1, and TE9,1, respectively [15]. Where m is the numberof full wavelengths of the confined mode around the innercircumference of the disk, and n is the number of fieldmaxima along the radius of the disk. Accordingly, we canrefer to the sharp peaks in Fig. 7 as TE10,1, and TE9,1, withthe corresponding field profiles shown in the figure.Broadening of the resonant peaks with increased pumpingindicates that losses due to absorption, because of the shiftin Ge absorption edge by heating, and FCA losses, are highand also increase with pumping [15]. These losses can beovercome by further device optimization. Higher tensilestrain reduces the required pumping levels for populationFig. 7. Photoluminescence spectrum of a 3-µm-diameter Ge disk on a free-standing SiO2 beam. Sharp peaks are whispering-gallery-mode resonanceswithin the direct-gap of Ge. Mode profiles are found using finite-domain time-difference simulations, and the corresponding resonances can be assigned tobe TE10,1 and TE9,1 at 1708, and 1811 nm, respectivelyinversion, and consequently avoid excessive heating effects.We expect that enhanced adhesion between the Ge and BOXlayers, in addition to optimization of the beam design, wouldimprove the applied strain. n-type doping fills the L valleysin the conduction band with electrons, which minimizes therequired injection of electron-hole pairs and consequentlyreduces the free hole absorption losses [2]. Spin-on dopingcan potentially be applied to achieve high activation levels inGe. Additionally, further enhancement of Ge crystalline andinterface quality with the substrate are important to increasethe direct-gap emission efficiency [1], [7].V. CONCLUSIONSpin-on doping is a valuable technique to achieve highactivation levels in Ge, however there are many processingobstacles that originate from thermal stresses during annealingand oxidation of Ge, that have to be overcome. Suspension ofGe structures on top of SiO2 beams, is capable of deliveringtensile strain to Ge due to beams’ bending, and confiningoptical modes in the case of Ge micro-disks. Sensitivity toheating effects is pronounced in free-standing structures, thiseffect was found to induce additional separation betweenthe LH and HH bands. We believe that combining previoustechniques of doping and tensile-strain is promising for Gelight emission purposes and future realization of a Si-basedon-chip optical communication link.ACKNOWLEDGMENTParts of the studies discussed here were supported byJapan Society for the Promotion of Science (JSPS) throughits “Funding Program for World-Leading Innovation R&Don Science and Technology (FIRST)”, the Project for Devel-oping Innovation Systems, and Kakenhi 216860312, MEXT,Japan. This work is also supported by EPSRC StandardGrant (EP/M009416/1), EPSRC Manufacturing Fellowship(EP/M008975/1), EU FP7 Marie-Curie Carrier-Integration-Grant (PCIG13-GA-2013-618116), University of SouthamptonZepler Institute Research Collaboration Stimulus Fund, andHitachi. The raw data of this paper can be obtained from theUniversity of Southampton ePrints research repository, DOI:10.5258/SOTON/382885.REFERENCES[1] S. Saito, F. Y. Gardes, A. Z. Al-Attili, K. Tani, K. Oda, Y. Suwa, T. Ido,Y. Ishikawa, S. 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Fabrication of Ge micro-disks on free-standing SiO<sub>2</sub> beams for monolithic light emission

Fabrication of Ge micro-disks on free-standing SiO2
beams for monolithic light emission

Fabrication of Ge micro-disks on free-standing SiO2 beams for monolithic light emission

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