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Fabrication of SrGe2 thin films on Ge (100), (110), and (111) substrates

  • 1,
  • 1Email author,
  • 1,
  • 2,
  • 2 and
  • 1
Nanoscale Research Letters201813:22

https://doi.org/10.1186/s11671-018-2437-1

Received: 15 December 2017

Accepted: 4 January 2018

Published: 16 January 2018

Abstract

Semiconductor strontium digermanide (SrGe2) has a large absorption coefficient in the near-infrared light region and is expected to be useful for multijunction solar cells. This study firstly demonstrates the formation of SrGe2 thin films via a reactive deposition epitaxy on Ge substrates. The growth morphology of SrGe2 dramatically changed depending on the growth temperature (300−700 °C) and the crystal orientation of the Ge substrate. We succeeded in obtaining single-oriented SrGe2 using a Ge (110) substrate at 500 °C. Development on Si or glass substrates will lead to the application of SrGe2 to high-efficiency thin-film solar cells.

Keywords

  • Germanides
  • Epitaxy
  • Nanostructures
  • Solar cells

Background

Alkaline-earth silicides have been widely investigated because of their useful functions for many technological applications such as solar cells [13], thermoelectrics [46], and optoelectronics [79]. However, the study of germanides has not been active compared to that of silicides even though some studies have predicted interesting electrical and optical properties for germanides [1016].

SrGe2 is one of the alkaline-earth germanides. Theoretical and experimental studies of bulk SrGe2 have revealed the following properties [1216]: (i) a BaSi2-type structure (orthorhombic, space group: \( {D}_{2h}^{16}- Pnma \), no. 62, Z = 8), (ii) an indirect transition semiconductor with a band gap of approximately 0.82 eV, and (iii) an absorption coefficient of 7.8 × 105 cm−1 at 1.5 eV photon, which is higher than that of Ge (4.5 × 105 cm−1 at 1.5 eV photon). These properties mean that SrGe2 is an ideal material for use in the bottom cell of high-efficiency tandem solar cells. Therefore, the fabrication of a SrGe2 thin film on arbitrary substrates would allow thin-film tandem solar cells simultaneously achieving high conversion efficiency and low process cost.

We fabricated thin-film BaSi2, having the same structure as SrGe2, on Si (111) and Si (001) substrates using a two-step method: a BaSi2 template layer was formed via reactive deposition epitaxy (RDE), which is a Ba deposition with heated Si substrates, followed by molecular beam epitaxy (MBE) [17, 18]. This resulted in high-quality (100)-oriented BaSi2 thin films with a long minority carrier life time [19, 20], leading to a large minority carrier diffusion length [21] and a high photoresponsivity at 1.55 eV [22]. The heterojunction solar cell with the p-BaSi2/n-Si structure allowed for a conversion efficiency of 9.9%, the highest value ever reported for semiconducting silicides [23]. These impressive results on the BaSi2 thin films and the attractive properties of bulk SrGe2 strongly motivated us to fabricate SrGe2 thin films.

The two-step method consisting of RDE and MBE to form BaSi2 thin films on Si substrates is applicable to fabricating SrGe2 thin films on Ge substrates because these materials have the same crystal structure [14]. In this study, we tried to form SrGe2 on Ge (100), (110), and (111) substrates using RDE to explore the possibility of SrGe2 thin-film formation.

Experimental

A molecular beam epitaxy system (base pressure, 5 × 10−7 Pa) equipped with a standard Knudsen cell for Sr and an electron-beam evaporation source for Si were used in this investigation. Sr was deposited on Ge (100), (110), and (111) substrates where the substrate temperature (Tsub) ranged from 300 to 700 °C. Before the deposition, the Ge substrate was cleaned using a 1.5% HF solution for 2 min and a 7% HCl solution for 5 min. The deposition rate and time of Sr were, respectively, 0.7 nm/min and 120 min for Ge (001), 1.4 nm/min and 30 min for Ge (011), and 1.3 nm/min and 60 min for Ge (111). The deposition rate varied depending on the amount of the Sr source because the Knudsen cell temperature was fixed at 380 °C. After that, 5-nm-thick amorphous Si was deposited at room temperature to protect the RDE layer from oxidation because Sr−Ge compounds are easily oxidized by air. The crystallinity of the sample was evaluated using reflection high-energy electron diffraction (RHEED) and X-ray diffraction (XRD; Rigaku Smart Lab) with Cu Kα radiation. In addition, the surface morphology was observed using scanning electron microscopy (SEM; Hitachi SU-8020) and transmission electron microscopy (TEM; FEI Tecnai Osiris) operated at 200 kV, equipped with an energy-dispersive X-ray spectrometer (EDX), and a high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) system with a probe diameter of ~ 1 nm.

Results and Discussion

Figure 1 shows the RHEED and θ–2θ XRD patterns of the samples after the Sr deposition. For all samples, streaky or spotted RHEED patterns were observed after the Sr deposition, implying the epitaxial growth of Sr−Ge compounds. For the samples with a Ge (100) substrate, peaks from Sr5Ge3 appear for all Tsub (Fig. 1a−e). In addition, peaks from SrGe appear for Tsub = 600 and 700 °C (Fig. 1d, e). Only the sample with Tsub = 300 °C exhibits the peak from SrGe2 (Fig. 1a), the target material in this study. Figure 1a shows that the sample with Tsub = 300 °C contains preferentially [100]-oriented SrGe2 and [220]-oriented Sr5Ge3. The peak derived from the substrate, Ge (200), is more noticeable for higher Tsub. This behavior is related to the surface coverage of Sr–Ge compounds on the substrate as revealed in Fig. 2. For the samples with a Ge (110) substrate, no peaks other than those from SrGe2 (411) and the Ge substrate are observed for Tsub = 300−600 °C (Fig. 1f−i). The peak from SrGe2 (411) exhibits the highest intensity for Tsub = 500 °C (Fig. 1h), suggesting that the sample with Tsub = 500 °C contains single-composition SrGe2 with high [411] orientation. For the samples with a Ge (111) substrate, the peaks from SrGe2 appear for all Tsub (Fig. 1k−o). The samples with Tsub = 300, 400, 500, and 700 °C exhibit [110]-oriented SrGe2 (Fig. 1k–m, o), while the SrGe2 peaks for Tsub = 300 and 400 °C are quite broad. The samples with Tsub = 500 and 600 °C exhibit multi-oriented SrGe2 (Fig. 1m, n). In addition, the small peak from Sr5Ge3 (220) appears for Tsub = 400, 500, and 700 °C (Fig. 1l, m, o). Therefore, the growth morphology of Sr–Ge compounds on a Ge substrate dramatically changes depending on the growth temperature and the crystal orientation of the substrate. This behavior is likely related to the surface energy of the Ge substrate depending on the crystal orientation [24] and the balance of the supply rate of Ge atoms from the substrate and the evaporation rates of Sr atoms from the sample surface.
Figure 1
Fig. 1

RHEED and θ–2θ XRD patterns of the samples after the Sr deposition. The crystal orientation of the Ge substrate is ae (100), fj (110), and ko (111). Tsub is ranged from 300 to 700 °C for each substrate. The peaks corresponding to SrGe2 are highlighted in red

Figure 2
Fig. 2

SEM images of the samples after the Sr deposition. The crystal orientation of the Ge substrate is ae (100), fj, (110), and ko (111). Tsub is ranged from 300 to 700 °C for each substrate. The arrows in each image show the crystal directions of the Ge substrates

Figure 2 shows SEM images of the sample surfaces. It is seen that the substrates are mostly covered by Sr−Ge compounds for Tsub = 300 °C (Fig. 2a, f,k). For Tsub = 400, 500, and 600 °C, we can observe the unique patterns reflecting the crystal orientation of the substrates, that is, twofold symmetry for Ge (100) (Fig. 2b−d), onefold symmetry for Ge (110) (Fig. 2g−i), and threefold symmetry for Ge (111) (Fig. 2l−n). These patterns can also be seen for silicides on Si substrates [1, 25] and ensure the epitaxial growth of Sr−Ge compounds on the Ge substrates. The samples with Tsub = 700 °C exhibit dot patterns, suggesting that the Sr atoms migrated rapidly and/or evaporated due to the high Tsub. These SEM results account for the streaky or spotted RHEED patterns in Fig. 1. Therefore, we succeeded in obtaining single-oriented SrGe2 using a Ge (110) substrate with Tsub = 500 °C, while for Ge (100) and Ge (111) substrates, multiple-oriented SrGe2 or other Sr–Ge compounds were obtained.

We evaluated the detailed cross-sectional structure of the sample with a Ge (110) substrate and Tsub = 500 °C. To prevent oxidation of the SrGe2, a 100-nm-thick amorphous Si layer was deposited on the sample surface. The HAADF-STEM image in Fig. 3a and the EDX mapping in Fig. 3b show that the Sr–Ge compound is formed on nearly the entire surface of the Ge substrate. The magnified HAADF-STEM image in Fig. 3c shows that the Sr–Ge compound digs into the Ge substrate, which is a typical feature of RDE growth [17, 18]. The elemental composition profile in Fig. 3d shows that Sr and Ge exist with a composition of 1:2. The results in Figs. 1 and 3 confirm the formation of SrGe2 crystals.
Figure 3
Fig. 3

HAADF-STEM and EDX characterization of the SrGe2 thin film grown on the Ge (110) substrate at 500 °C. a HAADF-STEM image. b EDX elemental map from the region shown in panel a. c Magnified HAADF-STEM image. d Elemental composition profile obtained by a STEM-EDX line scan measurement along the arrow in panel (c)

The bright-field TEM image in Fig. 4a and the dark-field TEM images in Fig. 4b, c show that while SrGe2 is epitaxially grown on the Ge substrate, it has two orientations in the in-plane direction. The lattice image in Fig. 4d clearly shows two SrGe2 crystals (A and B) and a grain boundary between them. The selected area diffraction pattern (SAED) in Fig. 4e shows diffraction patterns corresponding to two SrGe2 crystals (A and B). Figure 4d, e also shows that the Ge (111) plane and the SrGe2 (220) plane are parallel in each crystal. These results suggest that the SrGe2 crystals A and B epitaxially grew from the Ge (111) plane of the substrate and then collided with each other. No defects, such as dislocations or stacking faults, were found in the SrGe2 besides the grain boundary. Therefore, high-quality SrGe2 crystals were successfully obtained via RDE growth on a Ge(110) substrate.
Figure 4
Fig. 4

TEM characterization of the SrGe2 thin film grown on the Ge (110) substrate at 500 °C. a Bright-field TEM image. b, c Dark-field TEM images using the SrGe2 {220} plane reflection shown in each diffraction pattern. d High-resolution lattice image showing SrGe2 crystals. e SAED pattern showing the SrGe2 〈113〉 zone axis, taken from the region including SrGe2 crystals and the Ge substrate

Conclusions

We successfully formed thin films of SrGe2 via RDE growth on Ge substrates. The growth morphology of SrGe2 dramatically changed depending on the growth temperature and the crystal orientation of the Ge substrate. Even though multiple-oriented SrGe2 or other Sr–Ge compounds were obtained for Ge (100) and Ge (111) substrates, we succeeded in obtaining single-oriented SrGe2 by using a Ge (110) substrate at a growth temperature of 500 °C. Transmission electron microscopy revealed that the SrGe2 thin film on the Ge (110) substrate had no dislocation at the substrate interface. Therefore, we demonstrated that high-quality SrGe2 thin films can be produced. At present, we are investigating the characterization of the SrGe2 thin films and their development on Si and glass substrates for the application of SrGe2 to near infrared light absorption layers of multijunction solar cells.

Abbreviations

EDX: 

Energy-dispersive X-ray spectrometer

HAADF-STEM: 

High-angle annular dark-field scanning transmission electron microscopy

MBE: 

Molecular beam epitaxy

RDE: 

Reactive deposition epitaxy

RHEED: 

Reflection high-energy electron diffraction

SEM: 

Scanning electron microscopy

TEM: 

Transmission electron microscopy

T sub

Substrate temperature

XRD: 

X-ray diffraction

Declarations

Acknowledgements

Some experiments were performed at the Nanotechnology Platform in the University of Tsukuba.

Funding

This work was financially supported by the Nanotech CUPAL.

Authors’ Contributions

KT and TI conceived and designed the experiments. TI fabricated all samples. TI, RT, NS, and NY conducted the sample evaluations and data analyses. KT and TS managed the research and supervised the project. All the authors discussed the results and commented on the manuscript. All authors read and approved the final manuscript.

Competing Interests

The authors declare that they have no competing interests.

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Authors’ Affiliations

(1)
Institute of Applied Physics, University of Tsukuba, Tsukuba, Japan
(2)
Electron Microscope Facility, TIA, AIST, Tsukuba, Japan

References

  1. Suemasu T, Usami N (2017) Exploring the potential of semiconducting BaSi2 for thin-film solar cell applications. J Phys D Appl Phys 50:23001View ArticleGoogle Scholar
  2. Vismara R, Isabella O, Zeman M (2017) Back-contacted BaSi2 solar cells: an optical study. Opt Express 25:A402View ArticleGoogle Scholar
  3. Kumar M, Umezawa N, Imai M (2014) BaSi2 as a promising low-cost, earth-abundant material with large optical activity for thin-film solar cells: a hybrid density functional study. Appl Phys Express 7:71203View ArticleGoogle Scholar
  4. Hashimoto K, Kurosaki K, Imamura Y, Muta H, Yamanaka S (2007) Thermoelectric properties of BaSi2, SrSi2, and LaSi. J Appl Phys 102:63703View ArticleGoogle Scholar
  5. Akasaka M, Iida T, Matsumoto A, Yamanaka K, Takanashi Y, Imai T, Hamada N (2008) The thermoelectric properties of bulk crystalline n- and p-type Mg2Si prepared by the vertical Bridgman method. J Appl Phys 104:13703View ArticleGoogle Scholar
  6. Sales BC, Delaire O, McGuire MA, May AF (2011) Thermoelectric properties of Co-, Ir-, and Os-doped FeSi alloys: evidence for strong electron-phonon coupling. Phys Rev B 83:125209View ArticleGoogle Scholar
  7. Leong D, Harry M, Reeson KJ, KPA H (1997) Silicon/iron-disilicide light-emitting diode operating at a wavelength of 1.5 μm. Nature 387:686–688View ArticleGoogle Scholar
  8. Suemasu T, Negishi Y, Takakura K, Hasegawa F (2000) Room temperature 1.6 μm electroluminescence from a Si-based light emitting diode with β-FeSi2 active region. Jpn J Appl Phys 39:L1013–L1015View ArticleGoogle Scholar
  9. Terai Y, Maeda Y (2004) Enhancement of 1.54 μm photoluminescence observed in al-doped β-FeSi2. Appl Phys Lett 84:903–905View ArticleGoogle Scholar
  10. Peng H, Wang CL, Li JC, Zhang RZ, Wang MX, Wang HC, Sun Y, Sheng M (2010) Lattice dynamic properties of BaSi2 and BaGe2 from first principle calculations. Phys Lett A 374:3797–3800View ArticleGoogle Scholar
  11. Ud Din H, Reshak AH, Murtaza G, Amin B, Ali R, Alahmed ZA, Chyský J, Bila J, Kamarudin H (2015) Structural, elastic, thermal and electronic properties of M2X (M = Sr, Ba and X = Si, Ge, Sn) compounds in anti-fluorite structure: first principle calculations . Indian J Phys 89:369–375Google Scholar
  12. Palenzona A, Pani M (2005) The phase diagram of the Sr–Ge system. J Alloys Compd 402:136–140View ArticleGoogle Scholar
  13. Migas DB, Shaposhnikov VL, Borisenko VE (2007) Isostructural BaSi2, BaGe2 and SrGe2: electronic and optical properties. Phys Status Solidi (B) Basic Res 244:2611–2618View ArticleGoogle Scholar
  14. Kumar M, Umezawa N, Imai M (2014) (Sr,Ba)(Si,Ge)2 for thin-film solar-cell applications: first-principles study. J Appl Phys 115:203718View ArticleGoogle Scholar
  15. Wang J-T, Chen C, Kawazoe Y (2015) Phase stability and transition of BaSi2-type disilicides and digermanides. Phys Rev B 91:54107View ArticleGoogle Scholar
  16. Kumar M, Umezawa N, Imai M (2015) Structural, electronic and optical characteristics of SrGe2 and BaGe2: a combined experimental and computational study. J Alloys Compd 630:126–132View ArticleGoogle Scholar
  17. Inomata Y, Nakamura T, Suemasu T, Hasegawa F (2004) Epitaxial growth of semiconducting BaSi2 films on Si(111) substrates by molecular beam epitaxy. Jpn J Appl Phys 43:L478–L481View ArticleGoogle Scholar
  18. Toh K, Hara KO, Usami N, Saito N, Yoshizawa N, Toko K, Suemasu T (2012) Molecular beam epitaxy of BaSi2 thin films on Si(001) substrates. J Cryst Growth 345:16–21View ArticleGoogle Scholar
  19. Hara KO, Usami N, Toh K, Baba M, Toko K, Suemasu T (2012) Investigation of the recombination mechanism of excess carriers in undoped BaSi2 films on silicon. J Appl Phys 112:83108View ArticleGoogle Scholar
  20. Takabe R, Hara KO, Baba M, Du W, Shimada N, Toko K, Usami N, Suemasu T (2014) Influence of grain size and surface condition on minority-carrier lifetime in undoped n-BaSi2 on Si(111). J Appl Phys 115:193510View ArticleGoogle Scholar
  21. Baba M, Toh K, Toko K, Saito N, Yoshizawa N, Jiptner K, Sekiguchi T, Hara KO, Usami N, Suemasu T (2012) Investigation of grain boundaries in BaSi2 epitaxial films on Si(111) substrates using transmission electron microscopy and electron-beam-induced current technique. J Cryst Growth 348:75–79View ArticleGoogle Scholar
  22. Du W, Suzuno M, Ajmal Khan M, Toh K, Baba M, Nakamura K, Toko K, Usami N, Suemasu T (2012) Improved photoresponsivity of semiconducting BaSi2 epitaxial films grown on a tunnel junction for thin-film solar cells. Appl Phys Lett 100:152114View ArticleGoogle Scholar
  23. Yachi S, Takabe R, Takeuchi H, Toko K, Suemasu T (2016) Effect of amorphous Si capping layer on the hole transport properties of BaSi2 and improved conversion efficiency approaching 10% in p-BaSi2/n-Si solar cells. Appl Phys Lett 109:72103View ArticleGoogle Scholar
  24. Stekolnikov AA, Furthmüller J, Bechstedt F (2002) Absolute surface energies of group-IV semiconductors: dependence on orientation and reconstruction. Phys Rev B 65:115318View ArticleGoogle Scholar
  25. Wang H, Wu T (2012) A general lithography-free method of microscale/nanoscale fabrication and patterning on Si and Ge surfaces. Nanoscale Res Lett 7:110View ArticleGoogle Scholar

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© The Author(s). 2018

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