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Surface & Coatings Technology

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Biaxially strained germanium micro-dot array by hydrogen ion implantation

Pengfei Jiaa,b, Miao Zhanga, Jun Maa,b, Linxi Dongc, Gaofeng Wangc, Paul K. Chud,Zhongying Xuea,⁎, Zengfeng Dia,⁎, Xi Wanga

a State Key Laboratory of Functional Materials for Informatics, Shanghai Institute of Microsystem and Information Technology, Chinese Academy of Sciences, Shanghai200050, ChinabUniversity of Chinese Academy of Sciences, Beijing 100049, Chinac Key Laboratory of RF Circuits and System of Ministry of Education, Electronic and Information College of Hangzhou Dianzi University, Hangzhou 310018, Chinad Department of Physics and Department of Materials Science and Engineering, City University of Hong Kong, Tat Chee Avenue, Kowloon, Hong Kong, China

A R T I C L E I N F O

Keywords:GermaniumStrainIon implantationMicro-dots

A B S T R A C T

Although strain engineering is an effective method to modify the bandgap of germanium for germanium-basedmicroelectronic, the introduction of biaxial tensile strain with a particular pattern to germanium is challenging.Herein, a facile approach to produce biaxially strained germanium micro-dot arrays by hydrogen ion im-plantation is described. By changing the ion implantation and annealing conditions, the morphology of themicro-dots can be optimized and the biaxially tensile strain can be tuned to a maximum value of 0.6%. Thismethod which is compatible with mainstream complementary metal-oxide-semiconductor processing can beextended to strain engineering in wafer scale.

Germanium (Ge) has been widely explored for optical and electronicapplications because of the high hole mobility [1] and compatibilitywith mainstream integrated circuit technology. Ge has an indirectbandgap of 0.664 eV at the overall minimum L valley and a directbandgap of 0.8 eV at the local minimum Г valley [2]. The indirect en-ergy bandgap results in exceedingly low radiative recombination effi-ciency thereby limiting its application to optical detection [3] and la-sers [4]. Strain engineering can reduce the difference between the twovalleys and under biaxial tensile strain, germanium can become a directbandgap semiconductor [5–9], thus increasing the light emission effi-ciency [10–11]. Various methods have been proposed to induce tensilestrain in germanium, for instance, epitaxial growing strained germa-nium membrane on Si substrate [12–14], utilization of plane stressorlayers [8,15–17] and mechanical introduction of tensile strain [18–23].Nevertheless, these methods require a complicated design of stressdistribution or matched stressor or large dislocation density, and theformation of the biaxial tensile strain with a predesired distributionpattern is challenging.

In this letter, we report a facile approach to prepare an array ofbiaxially tensile strained micro-dots in germanium by hydrogen (H) ionimplantation. In this technique, H ions are implanted into the Ge sub-strate through a window defined by lithography and the implanted Hions form H2 bubbles during subsequent annealing. Owing to localoutward expansion of the H2 bubbles, biaxially tensile strained micro-

dots are formed on the surface. By changing the ion implantationconditions, the morphology of the micro-dots and strain can be variedand optimized.

A 100 nm Si3N4 layer was deposited on Ge substrate by inductively-coupled plasma chemical vapor deposition (ICPCVD) as a protectivelayer to suppress ion sputtering. After a layer of photoresist was spin-coated, an array of circle windows was fabricated by lithography. H ionimplantation was conducted at 30 keV and the ion fluence was5×1016 cm−2. Afterwards, the photoresist and Si3N4 layer werestripped with acetone and 5% HF, respectively. The samples were thenannealed under Ar at different temperature for 1 h to form the arrays ofmicro-dots.

Fig. 1 shows the atomic force microscopy (AFM) 3D images of thestrained micro-dots array formed annealing at 400 °C, 500 °C and 600 °Crevealing the formation of ordered micro-dots (Fig. 1(a), (c) and (e)).All the micro-dots have a symmetric spherical structure and the dia-meter-height ratio is about 20:1, which makes it easy to integrate withother functional structures. The average height of the dots changes from45 to 110 nm when the annealing temperature is increased from 400 to600 °C as shown in Fig. 1(b), (d), and (f). The H-induced blisteringmechanism in Ge is qualitatively similar to that in silicon [24]. As Hions are implanted into Ge, the implantation-induced defects trap H inthe Ge substrate creating nucleation sites for the formation of micro-cracks. After annealing at a low temperature, poorly hydrogenated

https://doi.org/10.1016/j.surfcoat.2018.07.077Received 8 April 2018; Received in revised form 10 July 2018; Accepted 26 July 2018

⁎ Corresponding authors.E-mail addresses: simsnow@mail.sim.ac.cn (Z. Xue), zfdi@mail.sim.ac.cn (Z. Di).

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defects disappear in favor of highly hydrogenated defects and a smallamount of molecular H2 is formed to create pressure in the vicinity ofthe micro-cracks. During high-temperature annealing, most of the hy-drogenated defects dissociate forming more molecular H2 which istrapped by the micro-cracks and exerts pressure to the internal surfacecausing expansion and coalescence of the micro-cracks. The observedmicro-dots are actually the exterior of the micro-cracks formed in-ternally and therefore, the height of the micro-dots can be adjusted bythe annealing temperature [25]. There is an optimal temperaturewindow for the formation of micro-dots. When the annealing tem-perature is below 350 °C, the thermal activation energy is lower thanthe threshold energy of hydrogenated defect desorption and conse-quently, few micro-dots are formed. On the other hand, when thetemperature is higher than 700 °C, the micro-dots are prone to rupturedue to the excessive H2 aggregation and internal pressure, as mani-fested by ruptured blisters like craters appearing (results not shownhere).

The morphology of the micro-dots can be varied by other experi-mental parameters such as the ion implantation energy and fluence.Fig. 2(a), (c) and (e) show the micro-dots formed at 500 °C under dif-ferent conditions. The height of the micro-dots formed by ion

implantation at a high energy is less than that at a low energy for thesame ion fluence as shown in Fig. 2(a) and (c). According to SRIM(Stopping and Range of Ions in Materials) software simulation, theaverage penetration depth of 30 keV H ions is 173 nm for Ge coatedwith a 100 nm thick Si3N4 and 277 nm for 46 keV H ions. For high-energy H ion implantation, because of rigidity, the thicker Ge mem-brane splits from the host substrate thereby resulting in less deforma-tion and a more gentle morphology. In addition, the implantation flu-ence regulates the morphology of the micro-dots as shown in Fig. 2(c)and (e). As the fluence decreases from 5×1016 to 4× 1016 cm−2, theheight of the micro-dots changes from 56 to 50 nm. Our experimentsalso disclose that micro-dots cannot form when the fluence is less than3×1016 cm−2 regardless of the annealing temperature. It implies thatwhen the implantation fluence is lower than the threshold, the limitedH2 pressure due to the low implantation dose will not induce cracks inthe germanium crystal.

The size of the micro-dots can be controlled by the size of the im-plantation window as shown in Fig. 3(a–c). For window diameters of2 μm and 3 μm, only single micro-dots are formed and the micro-dotsize correlates with the window size as shown by the dashed orangecircles. However, if the window size is larger than 4 μm, the dimension

Fig. 1. AFM images of the micro-dots on germanium fabricated by 30 keV H ion implantation with a fluence of 5×1016 cm−2 and annealing at different tem-perature: (a) 400 °C; (c) 500 °C; (e) 600 °C. (b), (d), (f) height profiles along the dotted lines in (a), (c), (e), respectively.

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controllability becomes worse. In addition to single micro-dots, doubleand ill-defined micro-dots are formed sporadically as marked by thedashed white circle in Fig. 3(c). To obtain sub-micrometer biaxiallystrained germanium dot arrays, a small window is preferred.

Fig. 4 shows the cross-sectional transmission electron microscopy(TEM) micrographs of the micro-dots after annealing at 600 °C. Acontinuous (100) oriented crack with a thin tail parallel to the topsurface is shown in Fig. 4(a). The length of the crack is coincident with

the diameter of implantation window of 3 μm and the crack exhibits anupward arch configuration with the height h* marked by the yellowdash line. The TEM image suggests that the micro-dot originates fromthe formation of internal crack which serves as the reservoir for ag-gregation of H2 and provides sufficient internal pressure to form bulgeon the surface, as discussed above. In fact, a similar H blistering phe-nomenon is utilized in the smart-cut technology to fabricate silicon-on-insulator (SOI) [24,26–28]. The protuberant micro-dot with a height of

Fig. 2. (a) AFM image of the micro-dots formed by 30 keV H implantation with a fluence of 5× 1016 cm−2; (c) AFM image of the micro-dots formed by 46 keV Himplantation with a fluence of 5× 1016 cm−2; (e) AFM image of the micro-dots formed by 46 keV H implantation with a fluence of 4× 1016 cm−2; (b), (d), (f) heightprofiles along the dotted lines in (a), (c), (e), respectively.

Fig. 3. AFM images of micro-dots formed with different pre-designed implantation window: (a) 2 μm; (b) 3 μm; (c) 4 μm. The dotted orange circles indicate the sizesof the pre-designed implantation windows. The dotted white circle in (c) indicates that two micro-dots are formed in single implantation window.

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Fig. 4. (a) Cross-section TEM image of Ge micro-dot; (b) AFM 3D image of the micro-dot shown in (a). (c) High-resolution TEM image of the buckled Ge membranenear the surface.

Fig. 5. (a) Raman shift of the micro-dots on germanium fabricated by 30 keV H ion implantation with a fluence of 5×1016 cm−2 and annealing at differenttemperature from 400 to 600 °C; (b–d) distribution of biaxial tensile strain in the array of micro-dots fabricated by 30 keV H ion implantation with a fluence of5×1016 cm−2 and annealing at 400, 500 and 600 °C, respectively.

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h marked by the white dash line has a maximum height of 110 nmconsistent with the AFM data in Fig. 4(b). The high-resolution TEMimage acquired near the top surface of the buckled Ge membrane inFig. 4(c) shows good Ge crystallinity after annealing at 600 °C. Thearray of strained germanium micro-dots with good crystalline qualityhas large potential for microelectronic applications.

The biaxial strain distribution in the micro-dot is evaluated bymicro-Raman scattering spectroscopy using the 325 nm He-Cd laser. Toeliminate the possible laser heating effect which may induce artificialRaman peak shifts, the excitation laser power is reduced to 50 μW for aspot size of 1 μm. Since the penetration depth of the 325 nm laser in Geis about 8 nm [29], the Raman data provide local strain informationfrom the membrane close to the surface. As shown in Fig. 5(a), thefingerprint peak of bulk Ge is at 300 cm−1 and the Raman shift ofbiaxially strained Ge |ω|(Δω=ωstrain−ω0) increases from 0.9 cm−1 to2.4 cm−1 when the annealing temperature is increased from 400 to600 °C. The red shift of the Raman spectra proves that the micro-dotareas undergo tensile strain. On account of the large diameter-heightratio and relatively free expansion in the direction perpendicular to thesurface, the strain εzz should be close to 0, while the biaxially tensilestrain is mainly formed in the horizontal plane. The equivalent in-planebiaxial strain εbiaxial= εxx= εyy can be determined by the followingequation [16,30]:

= − −ε ω ω( )/ 390biaxial strain 0 (1)

where ω0 and ωstrain represent the peak positions of the GeeGe vibra-tion mode of bulk Ge (at 300 cm−1) and strained micro-dot of Ge, re-spectively. For the strained micro-dot obtained by annealing at 400 °C,the biaxial tensile strain is estimated to be 0.23% and it increases to0.62% after annealing at 600 °C. The control of biaxial tensile strain inthe Ge micro-dot is confirmed by strain mapping as shown inFig. 5(b–d). Regular strain patterns are observed from the Ge micro-dotsannealing at different temperature. Moreover, the strain distributioncoincides with the array of micro-dots, indicating that the biaxial strainis due to mechanical deformation of the buckled Ge membrane in eachmicro-dot. More serious mechanical deformation occurring during an-nealing at higher temperature (lower than 700 °C) produces largerbiaxial tensile strain as expected. It is noted that Raman scattering onlyreveals the average strain distribution of each micro-dot due to thelimitation of the laser size but in fact, the maximum biaxial tensilestrain across the micro-dot may be larger than that estimated by Ramanscattering.

In summary, an array of biaxially tensile strained Ge micro-dots isproduced by H ion implantation. For a certain micro-dot diameter, theheight can be adjusted by the ion implantation fluence and energy aswell as annealing temperature to optimize the corresponding biaxialstrain. In addition, the diameter of the micro-dot is also determined bythe predesigned ion implantation window. The results provide insightsinto strain engineering of semiconductors and the methodology can beextended to production of desirable strain patterns on the wafer scale.

Acknowledgements

The authors acknowledge the Program of Shanghai Academic/Technology Research Leader (16XD1404200), Key Research Project ofFrontier Science, Chinese Academy of Sciences (QYZDB-SSW-JSC021),Hong Kong Research Grants Council (RGC) General Research Funds(GRF) No. CityU 1120561, National Science and Technology MajorProject (2016ZX02301003-004-001), and the National Natural ScienceFoundation of China (grant no. 61774163 and 61674159).

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