Morphological switching in synchrotron-radiation-excited Ge homoepitaxy: Transitionfrom kinetic roughening to smoothingHousei Akazawa Citation: Journal of Applied Physics 97, 106105 (2005); doi: 10.1063/1.1900295 View online: http://dx.doi.org/10.1063/1.1900295 View Table of Contents: http://scitation.aip.org/content/aip/journal/jap/97/10?ver=pdfcov Published by the AIP Publishing Articles you may be interested in Chains of quantum dot molecules grown on Si surface pre-patterned by ion-assisted nanoimprint lithography Appl. Phys. Lett. 105, 153106 (2014); 10.1063/1.4898579 Enhanced Ge/Si(001) island areal density and self-organization due to P predeposition J. Appl. Phys. 109, 093526 (2011); 10.1063/1.3587226 Hydrogen induced roughening and smoothing in surface morphology during synchrotron-radiation-excited Ge H4 -source homoepitaxy on Ge(001) J. Appl. Phys. 99, 103505 (2006); 10.1063/1.2194232 On the formation mechanism of epitaxial Ge islands on partially relaxed SiGe buffer layers J. Vac. Sci. Technol. B 22, 2257 (2004); 10.1116/1.1775188 Effects of atomic hydrogen on the selective area growth of Si and Si 1−x Ge x thin films on Si and SiO 2surfaces: Inhibition, nucleation, and growth J. Vac. Sci. Technol. A 22, 578 (2004); 10.1116/1.1699336

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Morphological switching in synchrotron-radiation-excited Ge homoepitaxy:Transition from kinetic roughening to smoothing

Housei Akazawaa!

Nippon Telegraph and Telephone Corporation (NTTC) Microsystem Integration Laboratories 3-1Morinosato Wakamiya, Atsugi-shi, Kanagawa 243-0198, Japan

sReceived 22 November 2004; accepted 10 March 2005; published online 9 May 2005d

The surface morphology of Ge films during GeH4-source molecular beam epitaxy on a Ges100dsubstrate is dramatically changed through irradiation with synchrotron radiationshn.100 eVd. Attemperatures below 300 °C, a two-dimensionally flat film continues to grow with the surface tightlypassivated by a GeHx hydride layer. At temperatures above 350 °C, dangling bonds are partiallyterminated with hydrogen atoms while Ge adatoms are highly mobile, resulting in a nonuniformnucleation that creates the islands. Small islands merge into a few-hundred-nanometer-tall islandssurrounded by thes113d-face sidewalls. This kinetic roughening continues until the nucleation onthe s001d plane ceases due to the buildup of the GeHx layer. Once the surface free energy isminimized by the hydrogen termination, flattening of the islands proceeds to reduce the chemicalpotential of the system. The valleys between the islands are preferentially filled, and the film iseventually converted into a smooth epilayer with no memory of the initial roughening. ©2005American Institute of Physics. fDOI: 10.1063/1.1900295g

Germanium clean surfaces roughen more easily than Sisurfaces. The order-disorder transition fromps238d- to s131d- reconstructed structures of the Ges001d surface is awell-known surface phenomenon. During Ge film growth,atomic-scale roughness results in islands. The representativesystem is the Stranski–Krastanov growth in Ge heteroepitaxyon Sis001d driven by the relaxation of the lattice-mismatchedstrain.1–3 In Ge homoepitaxy, we naturally expect rougheningat low temperatures4 and smoothing at high temperatures.5

However, Bramblettet al.6 observed large islands beingformed during the Ge2H6-gas source molecular beam epitaxysGSMBEd on Ges001d above 400 °C. Hydrogen atoms cer-tainly triggered the kinetic roughening in this case althoughthe role of hydrogen was opposite to the so-called surfactanteffect,7 i.e., maintaining an atomically flat surface. This pa-per reports that illumination with synchrotron radiationsSRdduring chemical vapor depositionsCVDd of Ge on Ges001dproduces submicrometer-scale islands. This large-scale ki-netic roughening changes to smoothing at a critical point,and the smoothing continues until the film is eventually con-verted into a two-dimensional defect-free epilayer.

We grew Ge crystalline films by SR-excited CVDsSR-CVDd in an ultrahigh vacuumsUHVd chamber con-nected to the “Super-ALIS” electron storage ring.8 The pho-tons incident to the substrate surface had an energy of 10–1500 eV with the maximum photon flux at 100 eV. Storagecurrent during growth ranged from 390 to 440 mA, corre-sponding to a photon flux density at the substrate of0.88–131016 s−1 mm−2. A 5-m-long differential pumpingsection in the middle of the beamline enabled the 99.99%pure GeH4 gas to be introduced into the growth chamber at1310−4–3310−2 Torr for the SR-CVD. Mirror-polished Gewafers were pretreated in an HCl:H2O2:H2O=1:1:10solu-

tion for 30 s, and in an NH4OH:H2O2:H2O=1:1:10solu-tion for 30 s, with a final rinse in pure water. The substratesurface was cleaned by UHV annealing at 600 °C. We usedspectroscopic ellipsometry for real-time monitoring. Ellipso-metric anglessC ,Dd were derived from the Fresnel reflec-tion coefficient ratio betweenp- ands-polarized lights inci-dent to the solid surface through the relationr=Rp/Rs

=tanC ·expsiDd. The time evolution of thesC ,Dd pairs at3.4 eV were recorded.

Figure 1 depicts the typicalC-D trajectories during thetwo-dimensional epitaxy by SR-CVD at 300 °C and GSMBEat 450 °C. While the Ges001d surface was being illuminatedwith an SR beam normal to the surface, a Ge-hydridesGeHxdlayer instantaneously covered the surface immediately afterthe surface had been exposed to a GeH4 flux. The quick shiftin the C direction strace O→Ad was caused by a slightlydifferent optical constant for Ge–H bonds than for Ge–Ge

adElectronic mail: akazawa@aecl.ntt.co.jp

FIG. 1. C-D trajectories during the two cycless1 and 2d of SR-CVD at300 °Cstraces O→A →B→C and C→B→D→Ed and GSMBE at 450 °Cstrace P→Qd at a GeH4 pressure of 1310−2 Torr. Points O and P are thestarting points. Data points were taken every 5 s.

JOURNAL OF APPLIED PHYSICS97, 106105s2005d

0021-8979/2005/97~10!/106105/3/$22.50 © 2005 American Institute of Physics97, 106105-1

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bonds. The following trace, A→B, corresponds to thesteady-state conversion of the GeHx layer into a Ge epilayers2.2 Å s−1d. Because the epilayer cannot be optically dis-criminated from the crystalline substrate, the shift toward ahigher D strace A→Bd indicates improved surface flatnesscompared with the initial Ge surface. When the supply ofGeH4 gas was terminated, the GeHx overlayer quickly de-composed through the thermal desorption of H2. This wasrevealed by the quick backshift along the trace B→C. Thesame processes were repeated in the second exposure-and-evacuation cyclestrace C→B→D→Ed; using Si2H6 duringSR-CVD produces similar formation and decomposition inthe SiHx layer.9 During GSMBE at 450 °C, dissociativelysticking GeH3 was soon converted into Ge atoms, and mostof the surface became terminated with dangling bonds. Theshift upward and to the right in trace P→Q is interpreted asthe smoothing of the Ge epilayer surface from the as-cleanedGe surface.

When the temperature of the SR-CVD was increased to350 °C, theC-D trajectory became a large-closed loop, asshown in Fig. 2. From the beginning of the growth, thesC ,Dd point was directed straight at the lowerD region,suggesting the Volmer–Weber-type growth. In this pure-Gesystem, a lowerD corresponds to the presence of a surfaceroughness layer, i.e., the formation of three-dimensional is-lands. The roughness reached the maximum at the bottom ofthe loopspoint Ad, then the trajectory returned to the origin.A similar trajectory was previously seen during SR-CVD ofa Si1−xGex alloy film with a high proportion of Ge onSis001d.10 End point E is located close to the origin, indicat-ing that the surface roughness of the epilayer was compa-rable to that of the initial Ge substrate. Corresponding to thechanges inD at 3.4 eV, the imaginary part of the pseudodi-electric function between 1.5 and 5 eV was attenuated to thelowest level whenD reached the minimum. The amplitudethen slowly recovered to the initial level. This observationshows that theD at 3.4 eV reflected the thickness of thesurface roughness layer, i.e., the height of the islands. Thelower the GeH4 pressure and the higher the temperature, thelarger the loops are. Similar spectroellipsometric monitoringof the roughening/smoothing behavior was reported by Liet al.11 but only at the initial stage of Si growth on Sis100d

achieved by electron cyclotron resonance plasma-enhancedCVD.

The cross-sectional transmission electron microscopysXTEMd image in Fig. 3sad shows the typical Ge islands inmidroughening. Each island consists ofs113d facet planeswith s001d top planes. Thiss113d-face island structure re-sembles that observed in Ge2H6-GSMBE on Ges100d.6 Theislands are typically 200-nm high and 400-nm wide. Figure3sbd is an XTEM image of the final epitaxial film at destina-tion point E in Fig. 2. Even after the long journey along thetrace O→A →B→E, the interface was very smooth, and theepilayer was single crystalline without any antiphase grainboundaries or dislocations. The memory of the roughening/smoothing had disappeared from the crystal image.

We suspected that the atomic-scale roughness on a ther-mally cleaned Ges001d surface could be amplified, so weexamined the effect of a Ge buffer layersGSMBE at450 °Cd. However, we could not identify any difference inthe roughening behavior with SR-CVD between the as-cleaned and the buffer-layer-covered Ge surfaces. To clarifythe relationship between hydrogen termination and morpho-logical evolution, 80-s exposures of GeH4 gas at 3310−3 Torr followed by a 10-min evacuation to an UHVwere repeated while continuing the SR-beam illumination at350 °C. Figure 4 plots theC-D trajectories during the sevencycles of the intermittent growth. Small U-shaped trajecto-ries straces O→A, B→C, D→E, F→G, H→ I, J→K, andL →Md line up along the virtual envelope loop that would

FIG. 2. C-D trajectory during SR-CVD at 350 °C at a GeH4 pressure of1310−2 Torr. Data points were taken every 3 s.

FIG. 3. sad XTEM images of the Ge islands before smoothing andsbd of theGe epilayer after smoothing;scd model of the kinetic roughening/smoothing.

106105-2 H. Akazawa J. Appl. Phys. 97, 106105 ~2005!

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have been traced if GeH4 had been continuously fed. Notethat when GeH4 exposure is turned off, hydrogen atoms de-sorb and dangling bonds are regenerated because the tem-perature exceeds the threshold of the H2 desorption. Nucle-ation on the hydrogen-frees001d and s113d planes occurredduring the following GeH4 exposure. The series of U-shapedtrajectories indicate that whatever the morphology of the Geoverlayer, the roughness initially increases with GeH4 expo-suresD decreasesd, and when the surface is densely coveredwith Ge hydrides, nucleation switches to preferential embed-ding of the Ge atoms in the valleys, flattening the surfacesDincreasesd.

As shown in Fig. 3scd, the initial strong islanding withthe SR-beam illumination can be explained by the followingscenario. When a virgin Ges100d surface is exposed to pho-tolysis productssGeHx and Hd, the surface starts to be cov-ered with hydrides. Internal reflection infraredspectroscopy12 and photoelectron spectroscopy13 of such sur-faces as well as the dependence of the Ge growth rate ontemperature14 revealed that only a monohydride phase canexist at 150 °C and that all H atoms disappear with isother-mal annealing at 300 °C. This means that H atom coverage iscontrolled by the competition between hydrogenation anddesorption. As SR-CVD continues, H atom coverage gradu-ally increases with time.

The migration of Ge adatoms is well activated at350 °C,14 and the Ge atoms preferentially nucleate at thedangling-bond-terminated sites. A limited number of dan-gling bonds facilitate the cohesion of Ge atoms, which leadsto initial roughening. Islanding is enhanced by “coarsening”,which means that small islands merge into larger ones whenthey contact one another. In a low-energy electron micros-copy study of Ge island formation on Sis100d, Rosset al.15

observed coarsening in the shape transition from pyramids to

domes, which reduced the chemical potential. As long as theGe atoms are highly mobile when the growing surface iselectronically excited, coarsening is strongly promoted, pro-ducing large islands with as001d top plane ands113d side-walls fstep 1 in Fig. 3scdg.

The s001d surface with two dangling bonds for each top-layer atom is the crystal growth surface. Thes231d-H phaseof the s001d surface is still reactive in terms of the SiHx

sticking, analogous to Si surfaces.16,17Furthermore, the crosssection of the photon-stimulated H desorption is greater on as001d surface than on as113d surface; the glancing directionof the incident photon beam to the latter reduces the photondensity per unit area. Therefore, faster nucleation on thes001d top surface than on thes113d facet planes promotestaller islandsfstep 2 in Fig. 3scdg.

As islands become taller, thes001d top surface area isreduced. Also, thes001d surfaces become saturated with hy-drides so that nucleation on thes001d surface is considerablydeactivated. The surface free energy is minimized when dan-gling bonds are fully terminated with hydrogen atoms. Con-sequently, flattening of the three-dimensional islands reducesthe chemical potential of the system. Embedding of GeHx

species at the bottoms of the valleys then becomes the domi-nant growth channelfstep 3 in Fig. 3scdg. The structure isconverted to a sinusoidal-like undulation structure as growthproceeds. Antiphase boundaries between the neighboringsides of the adjacent islands diminish as the islands mergetogether. In summary, we found that SR irradiation changesthe growth morphology from planar to a Volmer–Weber-type, even though the epilayer and substrate consist of thesame material.

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FIG. 4. C-D trajectories during the seven cycles of the intermittent Ge layergrowth at 350 °C and a GeH4 pressure of 3310−3 Torr. Trajectories corre-sponding to odd cycle numbers are indicated by solid lines and those cor-responding to even cycle numbers are indicated by dotted lines.

106105-3 H. Akazawa J. Appl. Phys. 97, 106105 ~2005!

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