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Towards a monolithically integrated III–V laser on silicon: optimization of multi-quantum well

growth on InP on Si

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2013 Semicond. Sci. Technol. 28 094008

(http://iopscience.iop.org/0268-1242/28/9/094008)

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IOP PUBLISHING SEMICONDUCTOR SCIENCE AND TECHNOLOGY

Semicond. Sci. Technol. 28 (2013) 094008 (7pp) doi:10.1088/0268-1242/28/9/094008

INVITED PAPER

Towards a monolithically integrated III–Vlaser on silicon: optimization ofmulti-quantum well growth on InP on SiH Kataria1, C Junesand1, Z Wang1,4, W Metaferia1, Y T Sun1,S Lourdudoss1,5, G Patriarche2, A Bazin2, F Raineri2, P Mages3,N Julian3 and J E Bowers3

1 Laboratory of Semiconductor Materials, KTH- Royal Institute of Technology, Electrum 229, 16440,Kista, Sweden2 Laboratoire de Photonique et de Nanostructures (CNRS UPR 20), Route de Nozay, 91460 Marcousis,France3 Department of Electrical and Computer Engineering, University of California, Santa Barbara,CA 93106, USA

E-mail: slo@kth.se

Received 5 April 2013, in final form 15 May 2013Published 21 August 2013Online at stacks.iop.org/SST/28/094008

AbstractHigh-quality InGaAsP/InP multi-quantum wells (MQWs) on the isolated areas of indiumphosphide on silicon necessary for realizing a monolithically integrated silicon laser isachieved. Indium phosphide layer on silicon, the pre-requisite for the growth of quantum wellsis achieved via nano-epitaxial lateral overgrowth (NELOG) technique from a defective seedindium phosphide layer on silicon. This technique makes use of epitaxial lateral overgrowth(ELOG) from closely spaced (1 μm) e-beam lithography-patterned nano-sized openings(∼300 nm) by low-pressure hydride vapor phase epitaxy. A silicon dioxide mask with carefullydesigned opening patterns and thickness with respect to the opening width is used to block thedefects propagating from the indium phosphide seed layer by the so-called necking effect.Growth conditions are optimized to obtain smooth surface morphology even after coalescenceof laterally grown indium phosphide from adjacent openings. Surface morphology and opticalproperties of the NELOG indium phosphide layer are studied using atomic force microscopy,cathodoluminescence and room temperature μ-photoluminescence (μ-PL) measurements.Metal organic vapor phase epitaxial growth of InGaAsP/InP MQWs on the NELOG indiumphosphide is conducted. The mask patterns to avoid loading effect that can cause excessivewell/barrier thickness and composition change with respect to the targeted values is optimized.Cross-sectional transmission electron microscope studies show that the coalesced NELOG InPon Si is defect-free. PL measurement results indicate the good material quality of the grownMQWs. Microdisk (MD) cavities are fabricated from the MQWs on ELOG layer. PL spectrareveal the existence of resonant modes arising out of these MD cavities. A mode solver usingfinite difference method indicates the pertinent steps that should be adopted to realize lasing.

(Some figures may appear in colour only in the online journal)

4 Present Address: Photonics Research Group (INTEC), Ghent University -IMEC, Sint-Pietersnieuwstraat 41, B-9000 Ghent, Belgium.5 Author to whom any correspondence should be addressed.

1. Introduction

To fulfill the requirements of the communication capabilityof the next-generation optical networks and interconnects for

0268-1242/13/094008+07$33.00 1 © 2013 IOP Publishing Ltd Printed in the UK & the USA

Semicond. Sci. Technol. 28 (2013) 094008 Invited Paper

supercomputing and cloud computing, an integrated platformthat can effectively bridge electronics and photonics has beenidentified as the key enabling technology. This thought hasdriven the idea of integrating the most commercial electronicmaterial silicon with most efficient optical materials III–Vslike InP, GaAs and GaSb. Due to the versatile wavelength(1.55 μm) of the InP-based quaternary/ternary alloys forlong haul communication, there is even a stronger interest tointegrate InP on Si. Related efforts started more than 25 yearsago. They started with direct growth of InP on Si [1, 2], butthe large lattice mismatch (8%) and the difference in thermalexpansion coefficient between InP and Si resulted in an InPlayer full of defects, with defect densities reaching as high as109 cm−2. A few techniques such as cleaning the Si surfacewith buffered HF [3], low temperature growth [4], thermalcycle growth with alternate growth and annealing steps [5],helped in reducing the defect densities by a few orders ofmagnitude. However, the layers were too thick to be consideredfor integration on silicon. Another technique called hybridintegration based on non-epitaxial wafer bonding of InP tosilicon using molecular bonding [6] or adhesive bonding [7]has shown promising results. Successful integration of hybridevanescent lasers, amplifiers, photodetectors and modulatorshas been demonstrated [8, 9]. Nevertheless, considering thecomplexity, cost and yield of this process, there is a constantsearch among the industries for reliable monolithic integrationfeasibility in a CMOS fab [10].

A promising technique for monolithic integration ofthe highly mismatched material system is epitaxial lateralovergrowth (ELOG). It has been used to grow GaAs onSi by liquid phase epitaxy (LPE) [11, 12], InP on Si byhydride vapor phase epitaxy (HVPE) [13] and GaN on sapphireby metal organic vapor phase epitaxy (MOVPE) [14]. InHVPE, a near equilibrium process, the growth rate is directlycontrolled by input mass transport, hence it results in highgrowth rates; besides, it is extremely difficult for group IIIchlorides to get adsorbed on a dielectric mask with respect tothe semiconductor and hence it is extremely selective, whichis ideal for ELOG. ELOG through the high aspect ratio (>2)dielectric mask openings on precoated InP on Si blocks all thethreading dislocations originating from the InP seed layer onSi, and defect-free InP can be obtained not only on the layerjust above the dielectric mask as the previous studies haveshown [11–14], but even above the openings, as the studies inour laboratory have indicated [15, 16].

In this work, to finally achieve a monolithically integratedIII–V laser on silicon, we utilize nano-epitaxial lateralovergrowth (NELOG) of InP on InP-coated Si wafers onisolated areas through nano-sized (300 nm) openings, andon top of which quantum wells (QWs) are grown. NELOGon isolated areas is chosen since such a situation can arise ina truly monolithic integration. NELOG creates opportunityto grow thin enough layers of InP on Si suitable forintegration of active and passive devices on InP and Si,respectively. Different mask designs are tested to optimizethe morphology of NELOG InP surface and to reduce theloading effect during the subsequent multi-quantum well(MQW) growth on non-planar sample surface containing

isolated areas of planarized NELOG area. Surface morphologyand optical properties of these templates are studied usingatomic force microscopy (AFM), cathodoluminescence (CL)and micro-photoluminescence (μ-PL) measurements. Afterthe optimization of the mask surface for MQW growth,InGaAsP MQWs with separate confinement heterostructure(SCH) layers with a targeted emission wavelength of 1.55 μmare grown on this NELOG InP on Si template. Cross-sectionaltransmission electron microscope (TEM) analysis of theselayers reveals that the coalesced NELOG InP on Si is defect-free, and the subsequently grown MQWs are quite uniformacross the NELOG InP layer. Microdisk (MD) structures oftwo sizes 12 and 8 μm are fabricated using e-beam lithography(EBL) and inductively coupled plasma (ICP) etching of devicestructures. Under room temperature optical pumping, resonantmodes were observed in the PL spectra. A mode solver usingfinite difference method indicates the pertinent steps thatshould be adopted to realize lasing.

2. Growth and analysis of NELOG InP/Si andMQWs

Commercially available Si (0 0 1) wafers 4◦ off orientedtowards 〈1 1 1〉 with precoated InP seed layer (∼1.5 μmthick) bought from Spire Corporation, USA, are used in thisexperiment. Since a thick enough seed layer was grown on 4 ◦

off oriented substrate, it is less likely that antiphase domainsexist [17]. The defect density of the seed layer however wasestimated to be on the order of 1 × 109 cm−2 [18]. Prior toany fabrication and NELOG, InP on Si samples are polished tohave a smoother surface morphology for further processing andsubsequent epitaxial growth. Four polished InP on Si samples(A, B, C and D) with different mask patterns as shown infigure 1 are used to optimize the NELOG of InP on Si surfacefor subsequent MQW growth and device fabrication. Theirdescriptions are given later.

2.1. Polishing of InP on Si seed layer

Prior to the deposition of SiO2 mask, the InP seed layer onSi is polished using a two-step polishing process involvingcommercially available Al2O3-based slurry manufactured byLogitech R© and in-house prepared NaOCl- and citric acid-based slurry. It has been shown [19] that to improve the surfacemorphology of NELOG of InP, polishing of InP seed on Si iscritical. Due to the high polishing rate (1 μm min−1) and a verythin seed layer, a thicker layer of InP (6 μm) is overgrown onthe seed layer to gain ample margin for controlled polishing.After chemical mechanical polishing (CMP) process, the totalthickness of the resulting seed layer is 2 μm and its surfaceroughness ranges from 1 to 5 nm.

2.2. Pattern design

Four polished InP on Si seed samples A, B, C and D are usedin this experiment. Pattern A contains two types of patternswith single and double openings, each of width 300 nm andlength 50 μm. Pattern B contains ten evenly spaced (1 μm)

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Semicond. Sci. Technol. 28 (2013) 094008 Invited Paper

(a) (b) (c)

Figure 1. Mask designs for sample A (a), sample B (b) and samples C and D (c).

openings, each of width 300 nm and length 50 μm. PatternsC and D contain ten evenly spaced (1 μm) openings, eachof width 300 nm and length 250 μm; in addition, these alsocontained two SiO2 barriers separated apart by 15 μm and ofwidth 5 μm and length 50 μm. The SiO2 barriers are stripesdesigned to obtain high-quality NELOG InP on the isolatedpattern fields. Thus, the distance between the barriers wouldalso be the approximate width of the coalesced NELOG InPon Si layer. Sample descriptions and mask designs for eachsample are given in figure 1. All samples are approximatelyof 1 × 1 cm2 size. Separation between two parallel openingson all the samples is also kept constant at 1 μm to optimizethe growth time. SiO2 mask thickness is kept at 700 nm for allthe samples to maintain mask-to-opening aspect ratio greaterthan 2 to take advantage of the so-called necking effect thatconfines the threading dislocations to the side wall of the mask.Although originally, the necking effect with an aspect ratio ofgreater than 1 was proposed in a perfectly etched vertical walls[20], more recent results of growth of InP from Si trenches takeadvantage of the extended defect filtering with an aspect ratiogreater than 2 [21]. Hence, we adopted this ratio.

2.3. Pattern fabrication, epitaxial growth andcharacterization

As described in the previous section, these polished templatesare deposited with 700 nm thick SiO2 mask using plasmaenhanced chemical vapor deposition (PECVD). 700 nmthickness is essential in this case to facilitate the defect neckingeffect through mask openings of size 300 nm, because anaspect ratio of greater than 2 is required to block all threadingdislocations originating from the InP seed layer on Si. Lineopenings that are 30◦ off (1 1 0) are defined using EBL, andCHF3-based reactive ion etching (RIE) with EBL resist asan etch mask is utilized for pattern transfer. Different patterndesigns as described in figure 1 are fabricated using the sameprocess flow.

2.3.1. Sample A. In the first experiment, InP doped withsulfur with a nominal doping ∼1 × 1018 cm−3 is grown onsample A, using LP-HVPE with a growth time of 2 min, V/IIIratio of 8 and growth temperature of 610 ◦C. Sulfur-dopedgrowth is chosen due to two specific reasons: first, sulfur-doped InP ELOG layer would serve as n-contact layer in caseelectrical contacts are to be made, and secondly sulfur doping

during the growth is understood to prevent the movement ofthreading dislocations due to the so-called impurity hardeningeffect [22, 23]. Growth from double openings on sample Aresulted in completely coalesced NELOG of InP with smoothand flat surface morphology. A planar sulfur-doped InP (InP:S)sample is used as reference during HVPE growth. Opticaland surface quality of these layers is studied using CL andAFM measurements. Experiments on these coalesced layersof NELOG InP are continued with a subsequent InGaAsPMQW growth with the targeted wavelength of 1.55 μm usingMOVPE. Massive loading effect is observed after the MQWgrowth.

An AFM profile image of NELOG InP on Si from doubleopenings is shown in figure 2(a) which indicates that itstop surface is flat and smooth with the surface roughness ofaround 1 nm. An aspect ratio (lateral to vertical growth rate)of over 3 is observed. Lateral growth of almost 5 μm on eachside of the nano-openings (300 nm) is observed. Figure 2(b)shows the panchromatic CL images of the NELOG InP onSi the brightness of which indicates low defect densities inthe NELOG InP layer and high PL efficiency. A magnifiedview of the panchromatic CL of NELOG InP from one of thedouble openings shows that no defect is seen even above thenano-openings, which demonstrates the defect necking effect.However, some defects aligned in the middle of NELOG InPlayer are faintly visible, which are probably created duringcoalescence of different growth fronts. Since the antiphasedomains would have been non-existent in the ELOG layer(see section 2), the above defects can be point defects orstacking faults. Figure 2(c) shows the optical microscopeimages of sample A after NELOG by HVPE and subsequentMQW growth by MOVPE. The growth rate of the MQWswas estimated to be greater than 100 times as high as onthe reference sample indicating enormous loading effect. Inaddition, MQW growth was not selective. This suggests that achange in mask design and stripping of SiO2 mask are neededprior to MQW growth.

2.3.2. Sample B. Experiments on sample A show thatcoalescence of different growth fronts from two adjacentopenings is optimized and resulted in a smooth and flatNELOG surface, but due to huge loading effect during theMQW growth, a new mask is designed to overcome thisloading effect. This time the number of opening is increased to10, to obtain a large coalesced area of NELOG InP for device

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Semicond. Sci. Technol. 28 (2013) 094008 Invited Paper

(a)(b) (c)

Figure 2. Data for sample A. (a) AFM cross section of NELOG InP on Si from double openings. (b) Panchromatic CL images of NELOGInP on Si with some visible defects due to coalescence. (c) Microscope image of NELOG InP templates after first MQW growth revealing aconsiderable loading effect and non-selective growth.

800 850 900 950 10000

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InP Reference N-ELOG InP on Si

(b)(a)

Figure 3. (a) AFM image of coaleseced NELOG InP on Si of sample B. (b) Averaged PL spectra from different spots of the same comparedwith that of the planar InP reference.

demonstration. Taking these observations into consideration,sample B is prepared using the same polishing and fabricationprocess, but this time an InP seed window is left open nextto potential NELOG area, to reduce the loading effect duringMQW growth, as shown in figure 1(b). NELOG of sulfurdoped InP is conducted using LP-HVPE with the same growthconditions as used for sample A, but with an increased V/IIIratio of 10 and increased growth time of 2 min and 20 s. InP:Slayer (0.5 μm thick) grown at the same time on InP:S substrateis used as a reference. Completely coalesced NELOG InP on Siis obtained. Optical and surface quality of the layer is studiedusing μ-PL and AFM measurements. The template is used forthe growth of MQW using MOVPE, and prior to MQW growth,SiO2 mask is stripped off by means of HF for 5 min. Removalof SiO2 mask prior to MQW growth has helped in furtherreducing the loading effect [24]. After the HF treatment eventhough all the SiO2 is stripped off, the NELOG InP regions iskept intact.

NELOG of InP from multiple nano-openings also resultedin a completely coalesced layer. However, as the AFM imageshown in figure 3(a) indicates, the surface morphology ofcoalesced layer is not uniform and found to have an RMSroughness of approximately 35 nm, which is not suitable fordevice fabrication. The average PL intensity of NELOG InPon Si is almost 20% of the reference sample (figure 3(b)). Onepossible reason for this decrease in PL intensity is the fact

that the NELOG InP layer is in contact with the grown InPin the adjacent InP seed window, where no defect-blockingmechanism is present. Due to absence of defect blockingmechanisms in the direct growth above the seed window, thereis a greater chance of stacking faults and gliding dislocationsgetting introduced in the adjoining NELOG InP layer. This alsoraises the question whether the material quality in the vicinityof the junction is deteriorated or the whole NELOG layer isaffected by this adjacency. The stacking faults that propagatealong 〈1 1 1〉 direction should be restricted in the vicinity ofthe junction due to the low thickness of the NELOG layer.However, dislocations arising from the region directly abovethe seed layer can bend and penetrate/glide into the adjacentNELOG layer. The resulting deteriorated surface morphology(figure 3(a)) is also attributed to this pathway. A subsequentMQW growth on this sample resulted in very broad PL spectra.Even though stripping of SiO2 mask prior to MQW growthhelped to some extent in the loading effect reduction, the finalsurface morphology and broad line width of PL spectra of thetemplate make it unsuitable for device fabrication.

2.3.3. Samples C and D. Experimental observations fromsamples A and B helped in further optimization of maskdesign. This time mask design with SiO2 barriers as shownin figure 1(c) is applied on samples C and D. The SiO2 barriers

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Semicond. Sci. Technol. 28 (2013) 094008 Invited Paper

Figure 4. (a) AFM image of NELOG InP of sample C after MQW growth; the dip in the AFM image is the area of stripped off SiO2 barriersprior to MQWs growth. (b) Top view after MQW growth with different regions I, II and III where PL spectra were measured, which arepresented in (c).

next to the nano-sized openings would act as the hinder forthe passage of the stacking faults and dislocation from theovergrown InP in open seed window into the NELOG InPlayer. The same fabrication steps and growth conditions asfor sample B are applied. A planar InP:S sample is used asa reference during HVPE growth. NELOG of InP resulted incompletely coalesced layer and its quality drastically improveswith respect to that of sample B. Samples C and D areused to grow MQW structure in two separate runs. PlanarInP:S substrates are used as reference during MQW growthin MOVPE. Like with sample B, prior to MQW growth,SiO2 masks on both samples C and D are stripped off using5 min HF treatment.

2.3.4. MQW growth. In the first MOVPE run, five QWswithout any SCH layers are grown on sample C. The surfacemorphology and the optical properties of these layers arestudied using AFM and μ-PL. A very smooth surface andenhanced PL intensity of MQWs are found promising fordevice fabrication. Considering the similar mask design andNELOG of InP on sample D, a device structure with SCHlayers for optical pumping is grown using MOVPE. Thestructure consists of 50 nm of unintentionally doped InP layer,three In0.76Ga0.24As0.83P0.17 wells of thickness 8 nm surroundedby In0.485Ga0.515As0.83P0.17 barriers of thickness 7 nm, two SCH

layers In0.78Ga0.22As0.479P0.521 of thickness 120 nm bracketingthe MQW stack for better mode confinement and 50 nmzinc doped InP as p-type cladding layer. Apart from thep-type cladding layer the whole structure is unintentionallydoped. Smooth surface morphology and enhanced PL intensitystudied using AFM and PL measurements make thesetemplates suitable for any device fabrication. Figure 4(a)shows the AFM cross section image of the MQWs on sampleC which reveals a smooth and planar surface morphology.Similarly, sample D exhibited surface roughness of 8.7 nmafter MQW growth which is again of the same order ofmagnitude (∼ 4 nm) as that of the planar InP reference sample.In addition, it also proves that stripping off these barriersprior to MQW growth helps in eliminating the loading effectobserved earlier (figure 2(c)). Figure 4(b) is the top view of theNELOG and nearby regions of sample C after MQW growth.The PL spectra of the MQWs grown at different regions I(NELOG between the SiO2 barriers), II (NELOG away fromthe barriers) and III (InP directly on the seed) are summarizedin figure 4(c). The PL intensity is the highest in the region Ibetween the barriers and the lowest in the region III, just abovethe seed. All these demonstrate the advantage of the additionalSiO2 barrier stripes at the boundaries of the NELOG patterns.We would like to emphasize that the PL intensity of the MQWon NELOG InP/Si on site I is also comparable to the referencesample as shown in figure 4(c).

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Semicond. Sci. Technol. 28 (2013) 094008 Invited Paper

Figure 5. SEM images of etched MDs on sample D.

Figure 6. PL spectra of optical pumped MDs of MQWs on sampleD and the reference sample. Resonant modes are clearly visible.

3. Device fabrication and characterization

3.1. Fabrication of devices

The MDs are defined on sample D provided with MQWs, usingEBL and subsequent RIE etching of 300 nm thick SiO2 onMQWs and ICP etching of MQWs. ICP etching is done using

Cl2/CH4/H2/Ar-based chemistry. MDs are etched down to adepth of 700 nm. Figure 5 shows the SEM images of the etchedMDs. The disks are aligned within the regions of I and II shownin figure 4(c). The planar reference sample was also providedwith similar MDs.

3.2. Characterization of devices

The etched MDs are optically pumped using a 1.18 μm laserdiode. As figure 6 shows, similar PL intensity is observed onboth MDs arising out of sample D and the reference supportingthe good quality of MQWs grown on NELOG InP on Si.Resonant modes are visible in PL spectra of MDs on bothsample D and the reference.

TEM analysis of these MD cavities is done, to assess theuniformity of the NELOG InP layer on Si and MQW grownatop. In figures 7(a) and (b), there are no apparent signs ofdefects in the ELOG layer. In figure 7(c) the good uniformityof MQW shows that it is indeed possible to have defect-freeInP surface optimized for high-quality MQW growth on Si.

3.3. Analysis of mode profiles of devices

Figures 8(a) and (b) show the simulated optical mode of theMD obtained from a mode solver based on finite differencemethod from which the leakage loss through the unetchedNELOG layer due to the shallow etching of MD cavitites canbe easily identified. The mode solver is a Matlab code based onthe method described in [25]. Figure 8(c) shows the calculatedmode loss for different etch depths.

We can see from the trend that leakage loss increasesexponentially with decreasing etch depth. In the currentexperiment the depth of the etched MDs is around 700 nm,which resulted in a tremendous leakage loss (∼100 cm−1). Thetotal optimal gain that is expected from the designed MQWsis about 2500 cm−1. With an optical confinement factor ofaround 5% in the active region, the modal gain is expected tobe 125 cm−1. Since, the optical mode gain is of the same orderas the leakage loss and there are additional losses due to thesidewall scattering, it is understood that this would not allowthe disk to lase. In order to reduce the leakage loss, deep etching

(a) (b) (c)

Figure 7. TEM images of etched MD on sample D. Plot (a) shows the lamella taken out from the MD along with the openings, showing thatdefects are blocked by the SiO2 mask. (b) A magnified view of NELOG InP showing no visible defects in the NELOG region. (c) Magnifiedview of the MQWs shows their uniformity.

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Semicond. Sci. Technol. 28 (2013) 094008 Invited Paper

(a) (b) (c)

Figure 8. Mode profiles of MDs etched with; (a) 0.8 μm depth; (b) 1 μm depth and (c) plot of leakage loss versus etch depth.

of MD is needed. In addition the possible scattering loss causeddue to sidewall roughness of MDs induced during ICP etchingis also identified as a reason. Nevertheless, the growth resultssupport the idea that NELOG is indeed a promising technologyto integrate InP on Si monolithically.

4. Conclusions

Different mask designs to obtain high-quality NELOG InPon Si on the isolated pattern fields have been optimized.It is found that introducing additional SiO2 barrier stripesadjacent to the pattern fields bordering the defective seed layersuppresses the defects in the NELOG InP as supported byoptical, morphological and structural studies by μ-PL, AFMand TEM, respectively. MOVPE growth of MQWs on thesetemplates has also been optimized by designing the patternfields to avoid excessive loading effect. Thus, depending onthe size of the pattern fields, a large area of smoothly coalescedNELOG of InP on Si and subsequent growth of MQWs for anypossible device fabrication can be achieved. MDs of differentdimensions in the NELOG region were fabricated using EBLand ICP etching. PL intensity of the MDs containing MQWson NELOG InP is comparable to that of the reference MDs onplanar substrate. PL spectra of both exhibit resonant modes butthe lasing action is understood to be suppressed presumablydue to sidewall roughness induced due to ICP etching ofMQWs causing large scattering losses and in turn lack ofsufficient feedback mechanism in the cavities. Simulationsconfirm the mode leakage due to shallow etching of theMDs. Thus, this study addressed the issues that are relevantfor growing active devices in isolated areas for a large-scaleintegration platform.

Acknowledgments

The work was supported by Swedish Research Council (VR),Swedish Foundation for Strategic Research (SSF), SwedishGovernmental Agency for Innovation Systems (Vinnova) andIntel Corporation through the URO program. HK would liketo thank India4EU, an EU project within Erasmus MundusExternal Cooperation Window, for receiving the doctoralfellowship.

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