Photoreflectance spectra of highly-oriented Mg2Si(111)//Si(111) films

Y. Terai1*, H. Hoshida1, R. Kinoshita1, A. Shevlyagin2, I. Chernev2 and A. Gouralnik2

1 Department of Computer Science and Electronics, Kyushu Institute of Technology, 680-

4 Kawazu, Iizuka, 820-8502, Japan 2 Institute of Automation and Control Processes FEB RAS, Vladivostok, Russia

E-mail: terai@cse.kyutech.ac.jp

(Received January 18, 2020) Direct transition energies of Mg2Si were obtained by photoreflectance (PR) spectra of a highly-

oriented Mg2Si(111)//Si(111) film. In the PR spectra at 9 K, direct transition energies of E1 = 2.38

eV, E2 = 2.58 eV, E3 = 2.69 eV and E4 = 2.82 eV were observed. In the temperature dependence of

PR spectra, E1 and E2 shifted to lower energy at high temperatures, but there was no temperature

dependence of transition energies in E3 and E4. These results showed that the temperature

dependences of band structure in Mg2Si differ at direct transition points.

1. Introduction

Magnesium half silicide Mg2Si is an interesting material for Si-based infrared (IR) detectors and

thermoelectric devices operating at 500−800 K. In the band structure, the valence-band maximum is

at Γ point, and the conduction-band minimum is at X point. The fundamental energy gap is an indirect

with an energy gap (Eind) of 0.6−0.8 eV [1]. There are a few reports about the direct transition energy

(Ed) in Mg2Si [2, 3], and temperature-dependence of Ed has not been reported. In this report,

photoreflectance (PR) spectra were measured in a highly-oriented Mg2Si(111)//Si(111) film to

investigate the temperature dependence of Ed.

2. Experiments

The highly-oriented Mg2Si(111) film with a thickness of 90 nm was grown on a Si(111) substrate by

solid phase epitaxy [4]. The optical reflectance spectrum was recorded using a double-beam

spectrophotometer (Hitachi, U-4000) at room temperature (RT). The Raman spectrum was measured

at a semi-backscatter geometry using a frequency-doubled Nd:YAG laser (532 nm) and a

spectrometer (Jobin Yvon, TRIAX-550) with a CCD detector (Spex-Jobin Yvon, Spectra One). In PR

measurements, a halogen lamp in conjunction with a single grating monochromator was used as a

probe source. The pump source was a 785 nm laser mechanically chopped at a frequency of 140 Hz.

The modulated reflection signal (ΔR/R) was detected by a Si photodiode.

3. Results and Discussion

Figures 1 and 2 show the reflectance and Raman spectra of the Mg2Si(111)//Si(111) film, respectively.

These spectra were almost consistent with those of Mg2Si single crystal [3, 5]. In the Raman spectra,

the wavenumbers of Raman lines originating from Mg2Si (A, LO, B, C, 2LO) were shifted to lower

wavenumber in comparison with the Mg2Si single crystal. The shifts indicate that a compressive

strain is included in the thin film due to the lattice mismatch between Mg2Si and Si. Figure 3 shows

the PR spectrum measured at 9 K. The clear modulation signal (black line) was observed at 2.2−3.0

JJAP Conf. Proc. , 011004 (2020) https://doi.org/10.7567/JJAPCP.8.01100485th Asia-Pacific Conference on Semiconducting Silicides and Related Materials (APAC-Silicide 2019)

© 2020 The Author(s). Content from this work may be used under the terms of the .Any further distribution of this work must maintain attribution to the author(s) and the title of the work, journal citation and DOI.

Creative Commons Attribution 4.0 license

eV. This is the first observation of a PR spectrum in Mg2Si. The ΔR/R originates from direct

transitions in the band structure of Mg2Si. By the fitting using the Aspnes third derivative functional

form [6], four direct transition energies of E1 = 2.38 eV, E2 = 2.58 eV, E3 = 2.69 eV and E4 = 2.82 eV

were obtained in the spectrum. The colored lines in Fig. 3 are the fitting results. Figure 4 shows

temperature dependence of PR spectra at 9−195 K. The red arrows in Fig. 4 are the obtained direct

transition energies by the fitting at each temperature. As seen in the figure, the E1 and E2 shifted to

lower energy at high temperatures, but clear temperature dependence was not observed in E3 and E4.

The temperature dependences of E1, E2, E3 and E4 are plotted in Fig. 5(a)-(d). In E1 and E2, the

temperature coefficient α (−dE/dT) was obtained to be α = 11×10-4 in E1, α = 7×10-4 in E2 by the

fitting using Varshni low of [Eg(T) = E0 – αT2/(T + β)]. The solid lines in Fig. 5(a), (b) are the fitting

results. So far, the temperature dependence of indirect energy gap Eind was measured in a Mg2Si

single crystal [7]. These α values of E1 and E2 are almost the same as that of Eind (α = 8.4×10-4).

Fig. 4 Temperature dependence of PR spectra. Fig. 3 PR spectrum of Mg2Si film at 9 K.

Fig. 1 Reflectance spectrum of Mg2Si film Fig. 2 Raman spectrum of Mg2Si film

011004-2JJAP Conf. Proc. , 011004 (2020) 8

In the report of the electroreflectance spectrum in Mg2Si [2], the peaks at (1) 2.27, (2) 2.51, (3) 2.61,

and (4) 2.78 eV were observed at RT. It was concluded that the energy of the peak (1) was the direct

transition energy for Γ15 → Γ1 transition in the band structure of Mg2Si. Then, it was proposed that

the peaks (2) and (3) were considered as the transitions at L’3 → L1 and Λ3 → Λ1. Based on this

report, E1, E2 and E3 obtained in Fig. 3 are assigned to Γ15 → Γ1, L’3 → L1 and Λ3 → Λ1 transition

energies. In the band structure calculated by empirical pseudopotential method [8], the direct

transition of fourth highest energy is at X’5 → X1. So, we assigned that E4 in Fig. 3 was the direct

transition energy at X’5 → X1. In the band structure of Mg2Si under an isotropic compressive strain

[9], it was reported that transition energies at Γ and L points become large with the contraction of the

lattice of Mg2Si. When the temperature decreases, the lattice of Mg2Si shrinks. So, it can be assumed

that the compressive strain in the Mg2Si film increase at lower temperatures. The increase of E1 and

E2 at low temperatures in Fig. 5 is qualitatively understood by the increase of a compressive strain

in the film. While, the bottom energy of the conduction band at X point increases and decreases

depending on the compressive strain [9]. As a result, it is interpreted that E4 did not show a clear

temperature dependence. Although the detailed band structure change at Λ point is unknown at

present, it may be that E3 also did not show temperature dependence due to a complex temperature

dependence of band structure.

Fig. 5 Temperature dependence of (a) E1, (b) E2, (c) E3 and (d) E4.

0 20 40 60 80 100 120 140 160 180

2.34

2.36

2.38

2.40

E1 (

eV

)

Temperature (K)

(a)

0 20 40 60 80 100 120 140 160 180

2.54

2.56

2.58

2.60(b)

E2 (

eV

)Temperature (K)

0 20 40 60 80 100 120 140 160 180

2.68

2.70

2.72

2.74(c)

E3 (

eV

)

Temperature (K)

0 20 40 60 80 100 120 140 160 180

2.78

2.80

2.82

2.84

E

4 (

eV

)

Temperature (K)

(d)

011004-3JJAP Conf. Proc. , 011004 (2020) 8

4. Conclusion

In a highly-oriented Mg2Si(111)//Si(111) film grown by solid phase epitaxy, direct transition energies

were measured by photoreflectance (PR) spectra. In the PR spectra at 9 K, four transition energies of

E1 = 2.38 eV, E2 = 2.58 eV, E3 = 2.69 eV and E4 = 2.82 eV were obtained. The energy of E1, E2, E3

and E4 are assigned to the direct transition energy at Γ15 → Γ1, L’3 → L1, Λ3 → Λ1 and X’5 → X1 in

the band structure of Mg2Si. In the temperature dependence of these transition energies, E1 and E2

shifted to lower energy at high temperatures, but there was no clear temperature dependence in E3

and E4.

Acknowledgment

This work was supported in part by JSPS KAKENHI Grant Numbers 18H01477. References

[1] V.E. Brisenko (Ed.), Semiconducting Silicides, Springer-Verla, Berlin Heidelberg, 2000. [2] F. Vazquez, Richard A. Forman, and M. Cardona, Phys. Rev. 176, 905 (1968). [3] W. J. Scouler, Phys. Rev. 178, 1353 (1969). [4] N.G. Galkin et al., Thin Solid Films 515, 8230 (2007). [5] S. Onari and M. Cardona, Phys. Rev. B 14, 8 (1976). [6] D. E. Aspnes, in Handbook on Semiconductors, edited by T. S. Moss (North-Holland, Amsterdam), 2,

109, (1980). [7] H. Udono, H. Tajima, M. Uchikoshi, and M. Itakura, Jpn. J. Appl. Phys. 54, 07JB06 (2015). [8] M. Y. Au-Yang et al., Phys. Rev. 186, 1358 (1969). [9] C. Qian,X. Quan, X. QingQuan, and Z. JinMin, Sci. China-Rhys. Mech. Astron. 56, 701 (2013).

011004-4JJAP Conf. Proc. , 011004 (2020) 8