Self-Assembled InGaAs/GaAs Quantum Dot ...

Invited Paper

Self-Assembled InGaAs/GaAs Quantum Dot Microtube Coherent Light Sources on GaAs and Silicon Z. Mia*, S. Vicknesha, F. Lia, and P. Bhattacharyab Department of Electrical and Computer Engineering, McGill University, 3480 University Street Montreal, Quebec H3A 2A7, Canada * : [email protected] b Solid State Electronics Laboratory, Department of Electrical Engineering and Computer Science University of Michigan, Ann Arbor, MI 48109-2122, USA a

ABSTRACT We have investigated the fabrication and emission characteristics of InGaAs/GaAs quantum dot microtubebased coherent light sources on GaAs and Si, which are formed by self-rolling of pseudomorphically strained semiconductor bilayers through controlled release from the substrate. Tailoring of the optical modes is achieved by engineering the shape of the microtube ring resonators. Using substrate-to-substrate transfer method, we have also achieved, for the first time, three-dimensionally confined quantum dot microtube optical ring resonators on Si, that are relatively free of dislocations. Sharp polarized and regularly spaced optical modes, with an intrinsic Q-factor of ~ 3,000, were measured at 77 K. Keywords: Quantum dot, semiconductor tube, monolithic integration, silicon photonics, and optical interconnect

1. INTRODUCTION Future high-speed circuits and computing systems will require, with all probability, on-chip optical interconnects, due to the extremely low noise, low power consumption, and virtually unlimited bandwidth.1,2 A critical component for inter- and intra-chip optical interconnects is a high performance semiconductor laser on Si, that can be monolithically integrated with CMOS-electronics. Due to its indirect bandgap, Si is a poor light emitter by nature. By using quantum confinement and Raman scattering effects,3,5-7 an optically pumped Si Raman laser and impressive optical gain have been achieved. Additionally, by placing nanocrystal quantum dots in a Si photonic crystal membrane microcavity, enhancement of spontaneous emission at room temperature has been realized.8 Significant progress has also been made in the hybrid integration of GaAs and InP based semiconductor lasers on Si.9-12 To overcome issues associated with the material incompatibility between III-V materials and Si, various dislocation reduction techniques, including strained-layer superlattices,13 relaxed and graded SiGe layers,12 AlSb buffer layers,9 compliant substrates,14 and hybrid evanescent devices,10 have been developed. However, the achievement of a low threshold and highly reliable Si-based laser has remained elusive. Recently, rolled-up semiconductor micro/nano-tubes have emerged as a promising technique for the realization of high performance Si-based nanoscale lasers. Such microtubes are formed by self-rolling of pseudomorphically strained semiconductor heterostructures through controlled release from the substrate using a selective etching procedure.15-17 Since its discovery nearly a decade ago,15 semiconductor microtubes have been intensively investigated and exploited as nanoreactors,16 nanopipelines,17 and injection needles.18 Rolled-up microtubes offer exceptional design flexibility for achieving high-Q optical microcavities. The tube diameter is determined by the incorporated strain of the pseudomorphic bilayer. By controlling the thicknesses and/or compositions of the bilayer, tubes with various diameters, ranging from ~ 10 nm to ~ 100 µm, have been demonstrated.17,28 The number of revolutions, on the other hand, is controlled by the etching time. Consequently, the wall thickness of the tubes, as well as their positions, can be precisely determined. Recently, waveguiding has been demonstrated in rolled-up microtubes.19 Compared to photonic crystal, microdisk, and micropillar based optical microcavities, such rolled-up semiconductor micro/nano-tubes exhibit a number of distinct advantages, including atomically smooth surface and a near-perfect overlap between the active medium and the maximum optical field intensity. Recent studies also suggest that the confined optical modes preferentially emit from the inside edge of the tube, thereby leading to nanoscale lasers with directional emission.20,21 Moreover, in situ tailored

Silicon Photonics IV, edited by Joel A. Kubby, Graham T. Reed, Proc. of SPIE Vol. 7220, 72200S · © 2009 SPIE · CCC code: 0277-786X/09/$18 · doi: 10.1117/12.810117

Proc. of SPIE Vol. 7220 72200S-1

optical modes can be achieved when the microtube is elastically deformed by applying an external force.22 To date, however, optical resonant modes of InGaAs/GaAs microtubes have only been observed at 5 K.20,23-25 It has been demonstrated that quantum emitters, such as quantum wells and dots incorporated in rolled-up microtubes exhibit significantly improved optical quality, compared to the as grown well or dot layers.19,26 Free-standing microtubes have also been demonstrated using photolithography and wet etching techniques.23 With the complete removal of the underlying AlAs layer, microtubes can be controllably released from the handling GaAs substrates.27 Such microtubes may be subsequently transferred to Si substrates using various techniques, including solution-based dispersion and transfer printing,28,29 thereby leading to III-V based optical microcavity devices on Si. The lattice and thermal coefficient mismatches, that typically lead to the generation of dislocations, are essentially eliminated in freestanding microtubes on Si. Therefore, compared to the conventional III-V based materials and devices on Si made by direct epitaxial growth or wafer bonding, free-standing microtube-based devices on Si may exhibit negligible dislocations. It may also be noted that such microtubes can be fabricated using a single step photolithography, and both the fabrication and transfer processes are scalable, mass-producible, and CMOS-compatible. In spite of these distinct advantages, free-standing semiconductor microtube optical ring resonators on Si have not been realized. In this context, we have performed a detailed investigation of the fabrication and characterization of semiconductor microtube-based optical microcavity devices on GaAs and Si substrates, wherein self-assembled InGaAs/GaAs quantum dots are incorporated as the gain medium. Tailoring of the optical modes is achieved through engineering the shape of the microtube ring resonators. Using substrate-on-substrate transfer method, we have also achieved, for the first time, three-dimensionally confined quantum dot microtube optical ring resonators on Si, which are relatively free of dislocations, with optical properties identical to those formed directly on GaAs. The diameters of the tubes are in the range of 5 – 7 µm, and the wall thicknesses vary from ~ 50 – 400 nm. Sharp polarized and regularly spaced optical modes, with an intrinsic Q-factor of ~ 3,000, were measured at 77 K. In what follows, Sec. 2 describes the epitaxial growth and fabrication of quantum dot microtube ring resonators on GaAs and Si. Their emission characteristics are presented in Sec. 3. Conclusions are made in Sec. 4.

2. MOLECULAR BEAM EPITAXIAL GROWTH AND FABRICATION OF QUANTUM DOT MICROTUBE OPTICAL RING RESONATORS ON GaAs and Si SUBSTRATES 2.1 Molecular beam epitaxial growth of InGaAs/GaAs quantum dot heterostructures on GaAs and Si substrates The molecular beam epitaxial growth of InGaAs/GaAs bilayers for the formation of rolled-up microtubes on GaAs is first described. After oxide desorption, a 0.5 µm GaAs buffer layer was grown on GaAs substrates at ~ 600 °C. The InGaAs/GaAs bilayer, illustrated in Fig. 1(a), was grown on a 50 nm AlAs layer on the GaAs buffer. The heterostructure consists of a 20 nm In0.18Ga0.82As and 30 nm Gas layer as well as two In0.5Ga0.5As quantum dot layers embedded in the GaAs matrix. The use of quantum dots can substantially reduce nonradiative recombination associated with the presence of any surface defects, due to the 3-dimensional localization of carriers in the dots. To achieve high quality InGaAs/GaAs quantum dot heterostructures on Si substrates, a GaAs buffer layer (~ 2 µm) was first grown on Si by metal organic vapor phase epitaxy. The surface dislocation densities are ~ 5×107 cm-2. To minimize the formation of antiphase domains and stacking faults at the GaAs/Si misfit interface, (001)-oriented Si substrates misoriented 4° towards [111] were utilized. The InGaAs/GaAs quantum dot heterostructures grown on Si is illustrated in Fig. 1(b). The InGaAs/GaAs quantum dot heterostructures grown on both GaAs and Si substrates exhibit strong photoluminescence emission at room temperature. GaAs (30 nm)

GaAs (30 nm) In0.18Ga0.82As (20 nm)

InGaAs QDs

In0.18Ga0.82As (20 nm)

InGaAs QDs

AlAs (50 nm)

AlAs (50 nm)

GaAs buffer (2 µm)

GaAs Substrate

Si Substrate

(a)

(b)

Figure 1: Schematics of the InGaAs/GaAs quantum dot heterostructures grown on (a) GaAs and (b) Si substrates.


2.2 Fabrication of InGaAs/GaAs quantum dot microtube optical ring resonators on GaAs substrates Both non-free-standing and free-standing InGaAs/GaAs quantum dot microtubes have been achieved on GaAs substrates. To realize non-free-standing microtubes, strained InGaAs/GaAs mesas, with various sizes, were first defined using photolithography and wet etching techniques. Illustrated in Figs. 2(a) and (b), by selectively etching the AlAs sacrificial layer using hydrofluoric (HF) based solutions,24,30 the InGaAs/GaAs thin film can roll-up into nano or microtubes, due to the relaxation of strain. The preferred rolling direction is along the direction on GaAs.17,24 Fig. 3(a) shows the optical microscopy image of an InGaAs/GaAs quantum dot microtube. The scanning electron microscopy image of the microtube is also illustrated in Fig. 3(b), wherein the etched GaAs, formed microtube, and unetched region are identified. The cross-sectional scanning electron microscopy image of the microtube is shown in the inset of Fig. 3(b). The microtube exhibits an inner diameter of ~ 5.2 µm, which agrees well with the calculated values using continuum mechanical models.31 It is important to note that the tube diameter is directly related to the strain of the InGaAs/GaAs heterostructure, while the number of revolutions is controlled by the etching time. Consequently, both the positions and wall thicknesses of such rolled-up microtubes can be precisely determined.

c

InGaAs QDs

-

Sacrificial bin

Sacrificial etching

aAs Substrate

(a)

(b)

Figure 2: (a) and (b) Illustrations of the formation of InGaAs/GaAs quantum dot microtube ring resonators on GaAs substrates.

Unetched area

Rolling direction

GaAs buffer 10 µm

(a)

Etched area

S-47005.OkV 12.Ommx9.O1k(U

10 µm (b)

Figure 3: (a) Optical microscopy image of an InGaAs/GaAs quantum dot microtube formed on GaAs substrates; (b) Scanning electron microscopy image of the quantum dot microtube, wherein the etched GaAs, unetched (as-grown) InGaAs/GaAs quantum dot layer, and rolled-up InGaAs/GaAs quantum dot microbube are also indentified. The cross-sectional scanning electron microscopy image of the microtube is shown in the inset.


To achieve free-standing quantum dot microtubes, a U-shaped mesa was first defined on the sample by etching to the InGaAs layer, illustrated in Fig. 4(a). At one edge of the mesa, the AlAs sacrificial layer was also etched through, which was used to define the starting edge of the rolled-up microtube. The self-rolling process was initiated with the selective removal of the AlAs sacrificial layer using HF based solutions.32 After a certain distance, the middle part of the tube was separated from the substrate. As a result, with continuous rolling of the tube on the side pieces, free-standing microtubes can be achieved, as illustrated in Fig. 4(b). The scanning electron microscopy image of a free-standing InGaAs/GaAs quantum dot microtube formed on GaAs substrate is shown in Fig. 5(a), wherein the free-standing region has 1.75 revolutions and a diameter of approximately 5.2 µm. The air-gap between the microtube and GaAs substrate is estimated to be about 0.2 – 0.3 µm. It is also evident that the positions of the free-standing microtubes, as well as their separation from the substrate, can be precisely controlled by the U-shaped mesa and the etching time. The shape of the resulting microtube ring resonators can also be engineered, shown in Fig. 5(b), in order to achieve precisely tailored optical modes.

Free-standing microtube

(a)

(b)

Figure 4: (a) and (b) Illustrations of the formation of free-standing quantum dot microtubes on GaAs substrates.

15.0kV 5,5 30.0kV 10.4mm x3.00k SE(M)

x4.50k SE(M)

ib.duri

10.Ourn

(a)

(b)

Figure 5: Scanning electron microscopy image of (a) a free-standing InGaAs/GaAs quantum dot microtube and (b) a quantum dot microtube with an engineered shape on GaAs substrates.

2.3 Fabrication of free-standing InGaAs/GaAs quantum dot microtube optical ring resonators on Si substrates We have recently developed two approaches, including direct epitaxial growth and substrate-on-substrate transfer, for the fabrication of free-standing InGaAs/GaAs quantum dot microtube optical ring resonators on Si substrates. As discussed in Sec. 2.1, high quality InGaAs/GaAs quantum dot heterostructures can be grown directly on Si substrates. Therefore, free-standing InGaAs/GaAs quantum dot microtubes can be fabricated on Si using the process described in Sec. 2.2. The scanning electron microscopy image of a free-standing microtube fabricated on Si using this


method is shown in Fig. 6 (a). Such quantum dot microtubes exhibit strong photoluminescence emission at room temperature, shown in Fig. 6(b). However, III-V materials grown on Si generally exhibit relatively large surface roughness, significantly limiting the maximum achievable Q factors of the resulting optical microcavities.

Ap

m x3.00k SE(U)

PL Intensity (a.u.)

Various techniques, including transfer printing and solution casting methods have been developed to transfer microstructures directly from their host substrates to foreign ones.33 Recently, Chun et al. reported the transfer of nonfree-standing In0.2Ga0.8As/GaAs semiconductor microtubes from GaAs to Si substrates using solution casting.28 However, during this transfer process, microtubes can be easily damaged. As a result, the achievement of high quality free-standing-microtubes on Si has not been demonstrated. We have recently developed a special transfer method, solution assisted substrate-on-substrate transfer. In this approach, free-standing quantum dot microtubes are first released from the handling GaAs substrates by selectively etching the AlAs sacrificial layer. By placing the GaAs wafer on Si substrates in an appropriate solvent solution, such microtubes can then be transferred directly onto Si substrates and are attached to Si by van der Waals force through surface tension. During this transfer process, excellent structural properties can be maintained. Fig. 7(a) shows the scanning electron microscopy image of an InGaAs/GaAs quantum dot microtube transferred onto Si substrate. Room temperature photoluminescence emission spectrum of one transferred microtube on Si is illustrated in Fig. 7(b). Detailed studies also confirm that such free-standing microtubes on Si are relatively free of dislocations, with optical properties identical to those formed directly on GaAs substrates.

T=295K

1000 1100 1200 Wavelength (nm)

iO.Oum

(a)

(b)

PL Intensity (a.u.)

Figure 6: (a) Scanning electron microscopy image of a free-standing InGaAs/GaAs quantum dot microtube fabricated directly on Si substrates; (b) Photoluminescence emission spectrum measured at room temperature.

10 0kV 7.1mm x7.00k SE(M)

5.00urn

(a)

T=295K

1100 1200 Wavelength (nm)

1300

(b)

Figure 7: (a) Scanning electron microscopy image of a free-standing InGaAs/GaAs quantum dot microtube transferred from GaAs onto Si substrates; (b) Room temperature photoluminescence emission spectrum of the non-free-standing region of the InGaAs/GaAs quantum dot microtube on Si.


3. RESULTS AND DISCUSSIONS 3.1 Emission characteristics of non-free-standing InGaAs/GaAs quantum dot microtubes on GaAs The emission characteristic of non-free-standing InGaAs/GaAs quantum dot microtubes was studied using cathodoluminescence (CL) measurements. Fig. 8(a) shows the 100 K, 10 kV CL monochromatic micrograph of an InGaAs/GaAs quantum dot microtube and the surrounding etched GaAs and unetched InGaAs/GaAs quantum dot layers, acquired at the peak emission wavelength (approximately 1.04 µm) of the unetched dot layer. The dark GaAs region is due to the absence of any quantum dots. Compared to the unetched quantum dot layer, the InGaAs/GaAs quantum dot microtube is highly uniform and bright, suggesting a significant improvement in the quantum dot optical quality, due to the reduced strain distribution in the microtube structure. CL spectra measured directly from the quantum dot microtube resonator at 100 K is illustrated in Fig. 8 (b) and is also compared with that of the unetched quantum dot layer (inset of Fig. 8 (b)). It may be noted that, with the reduced strain distribution in the microtubes, there is a small red shift (ΔE ~ 10 meV) in the quantum dot signal. An optical resonant mode at approximately 1.02 µm is clearly observed. The emission linewidth is ~ 5 nm, corresponding to a cavity Q factor of 204. Radiative losses through the substrate are identified to be the primary cause for the observed small Q factor.

GaAs

QD tube

- Unetched

Unetched area

area

100 K

- rube

5 nm C

1000

1050

1075

Wavelength (flm)

C

0)

1 µm

08

0.9

1.0

1.1

12

Wavelength (tm) (a)

(b)

Figure 8: (a) 100K, 10kV CL monochromatic micrograph acquired from an InGaAs/GaAs microtube at a peak emission wavelength of 1040 nm; (b) CL spectra measured directly from the InGaAs/GaAs quantum dot microtube structure at 100K. Comparison between CL spectra measured on the microtube and as grown area is shown in the inset.

3.2 Emission characteristics of free-standing InGaAs/GaAs quantum dot microtube optical ring resonators on GaAs The emission characteristic of free-standing InGaAs/GaAs quantum dot microtube optical ring resonators on GaAs was studied using micro-photoluminescence measurements. As illustrated in Fig. 9(a), the resulting microtube has approximately 2.5 revolutions, with wall thicknesses varying from ~ 100 – 150 nm. Both the inside and outside edges are also identified. The sample was mounted on a cryostat with continuous liquid nitrogen flow and cooled to 77 K. A semiconductor laser, with an emission wavelength of 641nm was focused onto the free-standing region of the microtube using an objective (100×, NA=0.7). The emitted light was collected by the same objective, analyzed by a spectrometer, and detected by a liquid nitrogen cooled InGaAs detector and lock-in amplifier. In this measurement scheme, both the excitation and signal collection were located at the same spot. The emission characteristic measured at a pump power of 22 µW is shown in Fig. 9(b). Four sharp optical resonant modes, spaced apart by approximately 14 meV, can be clearly observed. It may also be noted that the presence of the inside and outside edges leads to non-degenerate optical modes, illustrated in the inset of Fig. 9(b).21 An intrinsic Q factor of ~ 3,000 was derived. Analysis using finite-difference timedomain (FDTD) methods further confirms that the observed optical modes are TM polarized, with an electric field parallel to the tube wall.21 Detailed measurements and analysis of the emission characteristics of such microtube optical ring resonators are currently in progress.


77 K

Outside edge

\J.

1015

1020

1025

Wavelength (nm)

1000 1020 1040 1060 Wavelength (nm) (a)

(b)

Figure 9: (a) Cross-sectional sketch of a quantum dot microtube optical ring resonator on GaAs; (b) Photoluminescence emission spectrum of a free-standing InGaAs/GaAs quantum dot microtube on GaAs measured at a pump power of 22 µW at 77 K. The presence of the inside and outside edges around the tube also leads to nondegenerate optical modes (inset).

3.3 Emission characteristics of InGaAs/GaAs quantum dot microtube optical ring resonators on Si substrates We have also studied the emission characteristic of 3-dimensionally confined InGaAs/GaAs quantum dot microtube optical ring resonators transferred onto Si substrates. The measurement scheme was identical to that described in Sec. 3.2. Photoluminescence spectrum measured from the free-standing quantum dot microtube is shown in Fig. 10, which exhibits five dominant optical eigenmodes. Photoluminescence emission directly from the as grown quantum dot ensemble is also shown for comparison. Axial optical modes can also be observed, detailed in the inset of Fig. 10, which are believed to be related to the engineered shape of the microtube optical ring resonator.24 Detailed measurements and analysis of the 3-dimensionally confined quantum dot microtube optical ring resonators on Si are currently in progress.

I

Non-degenerate radial modes

77 K

V

1085

Emission directly from quantum dots

1000

1050

1100

1100

1115

Axial field distribution

1130

Wavelength (nm)

1150

Wavelength (nm) Figure 10: Emission spectrum of a 3-dimensionally confined InGaAs/GaAs quantum dot microtube ring resonator transferred onto Si substrate measured at a pump power of 15 µW at 77K. Both the axial and radial modes are illustrated in the inset.


4. CONCLUSIONS In summary, we have investigated the fabrication and emission characteristics of rolled-up InGaAs/GaAs quantum dot microtube-based coherent light sources on GaAs and Si substrates. Controlled emission from microtube ring resonators can be achieved by engineering the shape of the resonators. Using substrate-on-substrate transfer method, we have also demonstrated, for the first time, three-dimensionally confined quantum dot microtube optical ring resonators on Si, that are relatively free of dislocations. Sharp polarized and regularly spaced optical modes, with an intrinsic Q-factor of ~ 3,000, were measured at 77 K.

ACKNOWLEDGEMENTS The authors wish to thank W. Guo, D. Basu, and G. Huang at the University of Michigan for their assistance with the growth of quantum dot samples, Prof. D. Drouin at Université de Sherbrooke for the cathodoluminescence measurements on our devices, and A. Fatehi at McGill University for assistance with the scanning electron microscopy studies. This work is being supported by the Canada Foundation for Innovation, Natural Sciences and Engineering Research Council of Canada, Fonds de recherché sur la nature et les technologies, and Canadian Institute for Photonic Innovations. REFERENCES 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15. 16.

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