Journal of Crystal Growth 251 (2003) 281–284

Application of multi-step formation during molecularbeam epitaxy for fabricating novel nanomechanical structures

Hiroshi Yamaguchia,*, Yoshiro Hirayamaa,b

aNTT Basic Research Laboratories, Physical Science Laboratories, NTT Corporation, 3-1 Morinosato-Wakamiya, Atsugi,

Kanagawa 243-0198, JapanbCREST, JST, Kawaguchi, Saitama 331-0012, Japan

Abstract

We propose a novel application of ‘‘self-assembling’’ one-dimensional semiconductor nanostructures for nanoscale

electromechanical systems. A sacrificial layer of a GaAs/AlGaAs supperlattice under InAs wires preferentially grown

on bunched steps on misoriented GaAs (1 1 0) surfaces was selectively etched to form semiconductor cantilevers that

have typical lengths, widths, and thicknesses of 50–300, 20–100 and 10–20 nm, respectively. The force constant, as

measured by the force-modulation imaging technique using contact-mode atomic force microscopy, ranges from 0.5 to

10N/m, showing good agreement with that estimated from the elastic constant of InAs.

r 2002 Elsevier Science B.V. All rights reserved.

PACS: 62.20.Dc; 62.25.+g

Keywords: A3. Molecular-beam epitaxy; B2. Semiconducting III–V materials

1. Introduction

The technique of controlling the surface stepdistributions during molecular beam epitaxy(MBE) has been extensively used in the fabricationof low-dimensional semiconductor structures. Inparticular, the multi-step structures formed viastep bunching during growth are frequentlyapplied for the growth of semiconductor quantumwires. This is a clean fabrication technique forsemiconductor nanostructures that does not relyon lithographic processes, which might degradecrystalline quality. Here, we demonstrate the

application of this ‘‘self-assembly’’ technique tothe fabrication of semiconductor nanomechanicalstructures. Nanomechanical structures have thepotential to bring about a revolution in theapplication of semiconductor fine-structure de-vices, such as high-resolution actuators andsensors, high-frequency signal processing compo-nents, and medical diagnostic devices [1,2]. Theyare also important in the study of fundamentalquantum physics. When the resonance frequencyof a nanomechanical resonator becomes suffi-ciently high as the result of size reduction, novelquantum mechanical functions are expected toemerge [1,3–5].

The self-assembly approach is expected toprovide both high crystalline quality and smalldevice sizes [6]. We used the preferential MBE

*Corresponding author. Tel.: +81-46-240-3475; fax: +81-

46-240-4727.

E-mail address: hiroshi@will.brl.ntt.co.jp (H. Yamaguchi).

0022-0248/03/$ - see front matter r 2002 Elsevier Science B.V. All rights reserved.

doi:10.1016/S0022-0248(02)02318-7

growth of InAs on the bunched monomolecularsteps on a GaAs substrate, which was originallyproposed to fabricate semiconductor quantumwires for optoelectric applications [7]. In thispaper, we demonstrate this novel fabricationtechnique and characterize the elastic propertiesof the fabricated nanomechanical cantilevers byforce-modulation imaging using contact-modeatomic force microscopy.

2. Experimental results

Fig. 1 schematically illustrates the fabricationprocesses of the InAs nanoscale cantilevers. Thesubstrates were GaAs (1 1 0) wafers misorientedtoward the (1 1 1)A direction by 51. A 100-nm-thick GaAs buffer layer and a five-period GaAs/Al0.5Ga0.5As (30-nm-thick each) superlattice formthe regular multi-step structures on the growthsurface (Fig. 1(a)). InAs was then grown, resultingin the formation of InAs wires along the bunchedsteps (Fig. 1(b)). Then 5- to 10-mm-wide mesaswere defined by photo-lithographic patterningand following reactive ion etching (RIE) usingBCl3. Selective etching of sacrificial GaAs andAlGaAs layers was then performed using

NH4OH:H2O2:H2O=1:30:300 solution and HF,respectively, to fabricate the InAs nanoscalecantilevers (Figs. 1(c) and (d)).

Fig. 2(a) shows a SEM image of the successfullyfabricated InAs cantilevers. They are typically 50-to 300-nm long and 20- to 100-nm wide. Thespacing between adjacent wires is 400–800 nm forthese 51 misoriented samples and can be controlledby adjusting the misorientation angle. The SEMobservation from the side shows that the fabri-cated cantilevers are not deflected showing littleplastic deformation during the process. The latticerelaxation mechanism of this InAs growth onmulti-stepped GaAs (0 0 1) has not yet clarified indetail but the in situ STM characterization onsingular GaAs (0 0 1) surfaces suggests that thelarge portion of lattice mismatch is relaxed byconfining dislocations at the InAs/GaAs interface[8,9]. With further selective etching, the InAscantilevers stuck to the surface (Fig. 2(b)). Weexpect that the applying a critical point dryerwould prevent the cantilever sticking [10]. Thethickness of the InAs nano-cantilever was mea-sured by means of SEM observation and rangedfrom 10 to 20 nm depending on the amount ofdeposited InAs. One of the main advantages ofusing InAs for the electromechanical structures is

Fig. 1. Schematic illustration of the fabrication process of InAs nanoscale cantilevers: (a) after the growth of GaAs/AlGaAs SL. A

regular multi-step structure is formed; (b) after the deposition of InAs. InAs wires are preferentially grown at the multi-steps; (c) after

the lithographic patterning of mesa stripes; (d) after the selective etching of GaAs/AlGaAs SL to form the InAs nanoscale cantilevers.

H. Yamaguchi, Y. Hirayama / Journal of Crystal Growth 251 (2003) 281–284282

the native accumulation of electrons in the near-surface region due to surface Fermi level pinningin the conduction band [11–14]. In other single-crystal semiconductors, where the surface Fermi

level is pinned in the band gap, carriers aredepleted when the membrane thickness is reducedto nanometer scale. In contrast, electrons accu-mulate in InAs nano-cantilevers when the lowestquantum level formed in the InAs nano-cantileveris lower than the pinning position of the surfaceFermi level. This condition is satisfied for all thefabricated cantilevers [8].

The elastic properties of the InAs nano-canti-levers were then characterized by force-modula-tion imaging by contact-mode atomic forcemicroscopy (AFM) [15,16]. We detected thechange in the deflection of the AFM cantileverscanned over the sample surface while applyingsample-height modulation with the z-piezo-trans-lator. Fig. 3 shows a typical topographic image (a)and stiffness map (b) of two InAs nano-cantile-vers. The signal voltage from the photodetector,which is proportional to the AFM cantileverdeflection dzc, is mapped in Fig 3(b) with thesample height modulation dzs of 0.2 nmrms. Thecomplicated images beside the two InAs nano-cantilevers are due to the finite curvature of theAFM cantilever tip. The dark contrast corre-sponding to the small stiffness induced by theelastic deflection of InAs nano-cantilever is clearlyvisible. The contrast becomes darker at largerdistances from the InAs nano-cantilever support.The force constant were evaluated for 14 InAsnano-cantilevers by comparing these images withclassical elasticity theory and are plotted as a

Fig. 2. Typical SEM images of fabricated InAs nano-cantile-

vers: (a) top view of successfully fabricated 300-nm-long

cantilevers; (b) side view of unsuccessfully fabricated cantilevers

more than 1-mm long. The apex of the cantilevers stuck to the

etched surface.

Fig. 3. (a) A topographic image and (b) stiffness map for two fabricated InAs InAs nano-cantilevers obtained by contact-mode AFM

characterization.

H. Yamaguchi, Y. Hirayama / Journal of Crystal Growth 251 (2003) 281–284 283

function of measured InAs nano-cantilever widthin Fig. 4. For the comparison among thesedifferent InAs nano-cantilevers, the force constantwas normalized using the standard InAs nano-cantilever length of 100 nm. It ranges from 0.5 to10N/m depending on the size of InAs nano-cantilever. The dashed lines show the forceconstants calculated from classical elasticity theoryfor four different values of InAs nano-cantileverthickness. The thickness range shows good agree-ment with the SEM observation, justifying ourforce constant measurement technique. The datashows that the thickness tends to slowly increasewith InAs nano-cantilever width as indicated bythe dotted line. The dotted line corresponds to therelation tBw0.25, which might indicate somescaling nature of InAs wire growth on the bunchedsteps.

3. Conclusion

We have successfully fabricated an InAs nanos-cale cantilever array by using a ‘‘bottom-up’’ self-organization growth technique. The force constant

of fabricated cantilevers was determined by aforce-modulation imaging technique using con-tact-mode AFM. The obtained force constantshows good agreement with that estimation fromthe bulk InAs elastic constant.

Acknowledgements

The authors are grateful to Dr. Sunao Ishiharaand Dr. Takaaki Mukai for their encouragementthroughout this work. This work was partlysupported by the NEDO International JointResearch Program ‘‘Nano-elasticity’’.

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Fig. 4. Force constants 100 nm from the clamped position

evaluated from the fitting of InAs nano-cantilever deflection-

mapping for 14 different cantilevers. The dashed line shows

force constants calculated from the InAs nano-cantilever

dimensions and elastic constant of bulk InAs.

H. Yamaguchi, Y. Hirayama / Journal of Crystal Growth 251 (2003) 281–284284