SiO2 heterostructures with assistance of swift heavy ions

Author’s Accepted Manuscript Light-emitting defects formed in GeO/SiO2 heterostructures with assistance of swift heavy ions S.G. Cherkova, V.A. Volodin, V.A. Skuratov, M. Stoffel, H. Rinnert, M. Vergnat www.elsevier.com/locate/jlumin

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S0022-2313(18)31053-6 https://doi.org/10.1016/j.jlumin.2018.11.028 LUMIN16098

To appear in: Journal of Luminescence Received date: 14 June 2018 Revised date: 26 October 2018 Accepted date: 14 November 2018 Cite this article as: S.G. Cherkova, V.A. Volodin, V.A. Skuratov, M. Stoffel, H. Rinnert and M. Vergnat, Light-emitting defects formed in GeO/SiO 2 heterostructures with assistance of swift heavy ions, Journal of Luminescence, https://doi.org/10.1016/j.jlumin.2018.11.028 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting galley proof before it is published in its final citable form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

Light-emitting defects formed in GeO/SiO2 heterostructures with assistance of swift heavy ions

S.G. Cherkovaa*, V.A. Volodina,b, V.A. Skuratovc,d,e, M. Stoffelf, H. Rinnertf, M. Vergnatf

a

Rzhanov Institute of Semiconductor Physics, Siberian Branch, Russian Academy of Sciences,

Lavrentiev Ave, 13, Novosibirsk, 630090 Russia b

c

Novosibirsk State University, Pirogova Street, 2, Novosibirsk, 630090 Russia

Joint Institute for Nuclear Research, Joliot-Curie 6, 141980 Dubna, Russia

d

National Research Nuclear University MEPhI, Moscow, Russia

e

Dubna State University, Dubna, Russia

f

Université de Lorraine, CNRS, IJL, F-54000 Nancy, France.

[email protected]

Abstract Germanium suboxide films and GeO/SiO2 multilayer heterostructures deposited onto Si(001) substrates using evaporation in high vacuum were modified using irradiation of 167 MeV Xe+26 ions with fluences varying from 1011 to 1013 cm−2. According to Raman spectroscopy data, the swift heavy ion irradiation does not lead to the expected decomposition of germanium suboxide in germanium nanoclusters and GeO2. Infrared absorption spectroscopy measurements show that under irradiation the GeO/SiO2 layers were intermixed with formation of Ge-O-Si bonds. We report strong photoluminescence in the visible range at room temperature, which is most probably due to Ge-related defect-induced radiative transitions. Moreover, a new infrared

1

luminescence band (~0.8 eV) was observed in irradiated structures, which can be related to defects or defects complexes in GexSiyO2 glass.

Keywords: Luminescence, defects, Ge oxides, GexSiyO2 glasses, swift heavy ions

I.

Introduction Germanium nanocrystals (NCs) and amorphous nanoclusters embedded in dielectric

matrices are still of great interest for researchers both from fundamental and applied points of view [1, 2]. Germanium NCs [3] and defects [4] in germanium oxide films act as traps for charge carriers, which can be exploited in either flash memories or memristors. Moreover, NCs, amorphous nanoclusters and defects in dielectric films may also be of interest for optoelectronics [5]. Bulk germanium is an indirect band gap semiconductor, which results in light emission with very low efficiency. Several approaches have been proposed to overcome this fundamental limitation and to make Ge and GeOx-based light-emitting devices. The main approach is based on band gap engineering using quantum-size effects in NCs [6-12], but defect engineering is also has perspectives [13-17]. It is worth noting that high-efficient light emitting diodes containing either Si, Ge or GexSi1-x nanoparticles have been demonstrated [18-20]. Photoluminescence quantum yields exceeding 60% have been achieved for Si NCs [21-22] emitting light at a wavelength of ~800 nm. In order to obtain infrared (IR) emission, one needs to consider Gebased nanostructures. Defect engineering may also represent a viable route to get light out of Ge. For example, swift heavy ion (SHI) irradiation of Si/SiO2 multilayers is known to lead to the formation of vertically ordered Si nanoclusters due to phase separation in ion tracks [23]. In this paper, we modify the structure of both GeOx films and GeO/SiO2 multilayers using SHI irradiation to form light-emitting Ge-related (oxygen deficient) defects. This leads to the observation of strong photoluminescence in the visible at room temperature. In addition, a 2

photoluminescence band is observed in the infrared range, which can be related to the formation of defects or defect complexes in GexSiyO2 glasses.

II.

EXPERIMENTAL DETAILS The GeOx films and GeO/SiO2 multilayers were obtained by evaporation of GeO2 powder

or by alternating evaporations of GeO2 and SiO2 powders in high vacuum (10-8 Torr) onto Si(001) substrates heated up to 100°C. The deposition rate (about 0.1 nm/s) was controlled by a quartz microbalance. The 100 nm thick GeOx film was capped by a 100 nm thick SiO2 layer. The multilayer sample contains 20 periods of GeOx(5nm)/SiO2(5nm) layers. The multilayers were capped by a 40 nm thick SiO2 layer. The samples were irradiated with 167 MeV Xe+26 ions at various fluences between 1011 cm−2 and 1013 cm−2, using the cyclotron at FLNR JINR, Dubna. Within the studied layers, stopping power of the ions was nearly completely (~99%) due to the ionization losses, which in the SiO2 layers were ~14.5 keV nm−1, according to the SRIM (The Stopping and Range of Ions in Matter) code (www.srim.org) calculations. The projected range ofthe ions was more than 20 microns. The stoichiometry and structure of the samples was determined using Fourier transform infrared (FTIR) absorption spectroscopy, the spectrometer having a spectral resolution of about 4 cm-1. The Raman spectra were recorded in the backscattering geometry and the 514.5 nm Ar+ laser line was used as excitation source. The optical properties of the as-deposited and annealed samples were investigated by photoluminescence (PL) spectroscopy. A He-Cd laser (325 nm line) was used to excite the PL. A cryostat with temperature stability ±0.5K was used for the low-temperature PL measurements. The PL spectra were measured using a monochromator equipped with a 600 lines/mm grating and a liquid nitrogen cooled InGaAs-detector having a long wavelength cutoff at ~1610 nm. For the study of the PL in the visible range, a photomultiplier tube was used as detector. All PL spectra were corrected from the response of the detector. 3

III.

RESULTS AND DISCUSSION Figure 1 shows the Raman spectra of both GeOx films (a) and GeO/SiO2 multilayers (b)

measured before and after SHI irradiation with various fluences between 2x1011 cm-2 and 5x1013 cm-2. Since the multilayers are semitransparent, the Raman signal originating from the Si substrate can be observed in the investigated spectral region. As in our previous works, [11, 12, 24], we have subtracted the Raman spectrum originating from the Si substrate from the Raman spectra of the samples to suppress the background. The Raman spectra of the as-deposited samples do not show any peak originating from Ge-Ge (~275-300 cm-1), Ge-Si (~400-420 cm-1) or Si-Si (~480-500 cm-1) local bond vibrations. We can thus conclude that the as-deposited samples contain neither Ge (or Si) NCs nor amorphous Ge (or Si) clusters. For the SHI irradiated samples, one cannot either identify a peak related to scattering from Ge-Ge vibrations. Thus, SHI irradiation does not lead to the expected decomposition of GeOx in Ge and GeO2. This is quite surprising because, even in more stable SiO2 layers, that kind of irradiation leads to the formation of oxygen depleted SiOx precipitates [25-26]. We can indeed see a peak at ~480 cm-1 originating from local Si-Si vibrations which increases with the ion fluence. However, it is worth noting that this peak can also arise from disordered regions along the ion tracks in the silicon substrate, since the projected range of the ions is about 20 microns. The low-frequency shoulder (up to 400 cm-1) may be due to scattering of LO-type local vibrations of Si-Si bonds. As it was reported by Saikran et al. [27], crystalline GeO2 is characterized by a Raman peak centered at 440 cm-1, related to optical phonon vibrations. However, in our case the GeOx films are amorphous. The same authors [27] also reported that SHI irradiation of GeO2 layers leads to the appearance in the Raman spectra of a peak at ~300 cm-1, related to Ge-Ge bond vibrations after irradiation at fluences higher than 1013 cm-2. It should be noted, however, that in their case the

4

GeO2 films were not capped by a SiO2 layer. Thus, irradiation causes oxygen removal from the films, which eventually leads to the formation of Ge clusters. Let us discuss the possible reasons for the absence of GeOx decomposition in our case. It is known that, inside the tracks, the temperature can reach 5000 K during 10−11–10−10 s [28]. This is due to very high ionization losses. The concentration of excited electrons inside the track can reach values up to 1022 cm-3 [28]. The GeOx area near the track can be evaporated with formation of GeO gas, but due to very fast cooling (~1013 K/s) [28], it may possibly condensate into GeO, not Ge and GeO2. This situation is very similar to laser annealing with femtosecond pulses. These fast treatments also did not lead to decomposition of GeO films into Ge and GeO2 [29]. Figure 2 shows the IR absorption spectra of both GeOx films and GeO/SiO2 multilayers measured before and after SHI irradiation with various fluences ranging from 2x1011 cm-2 to 5x1013 cm-2. It should be noted that a virgin Si substrate was used as a reference when measuring FTIR transmission spectra. The spectra are dominated by a main line at about 1070-1078 cm-1 which is definitely ascribed to the Si-O-Si stretching mode in the SiO2 layers [26, 30]. It can be seen that with increasing the radiation fluence, the position of this peak shifts to the long-wave region and its intensity becomes lower. A similar behavior, observed in Si/SiO2 multilayer structures under SHI irradiation, is caused by radiation defects and by the formation of oxygendepleted areas in the SiO2 matrix. For both as-deposited samples, one can observe a peak at 810 cm-1. It can be due to Si-OSi bending vibration mode [30] or due to the Ge-O-Ge stretching vibration mode [8, 9, 11, 31, 32]. For the latter case, the peak is known to shift approximately linearly with the stoichiometry parameter x. In the case of GeOx films, Jishiashvili et al. [32] have established a relation between the vibration frequency ω and the stoichiometry parameter x in GeOx layers which is given by: ω(cm-1)=72.4·x+743

(1)

5

As it was mentioned, the experimental peak position for both as-deposited samples is ~810 cm-1. We can thus conclude that the GeOx layers in the multilayer structure deposited at 100°C have a stoichiometry close to germanium monoxide (according to formula 1, x≈1). One can see that SHI irradiation does not practically change the vibration properties of the 100 nm thick GeOx film (Fig. 2a). The peak position corresponding to the Ge-O-Ge stretching vibration mode remains approximately the same, only a small shift (from 810 up to 813 cm-1) is seen for the sample irradiated with the highest fluence (curve 5). Nevertheless, some changes are seen in the 100 nm thick SiO2 cap layer. The peak corresponding to the Si-O-Si stretching mode shifts from 1078 to 1069 cm-1. This may be due to the abovementioned formation of oxygendepleted SiOx precipitates caused by irradiation. One can also see a weak low-frequency shoulder (~1000 cm-1) of the Si-O-Si peak for the curve 5, which can be due to some intermixing at the GeO/SiO2 interface. Similar but somewhat more pronounced effects can be seen in the irradiated multilayered sample (Fig. 2b). In this case, the Si-O-Si stretching mode peak is shifted from 1070 to 1058 cm-1. More interesting changes occur with the Ge-O-Ge peak, especially for the highest ion fluence. The peak at 810 cm-1 almost disappears while two new features – a peak at ~870-890 cm-1 and a low-frequency shoulder (~1000 cm-1) of the Si-O-Si peak appear. The first feature can be related to stretching vibrations of Ge-O-Ge bonds in GeOx areas with x close to 2. The second feature can possibly be due to vibrations of Si-O-Ge bonds [33]. This assignment is further strengthened by the observation reported in Figure 2 (c). We have deposited a 300 nm thick solid alloy of both GeO and SiO2. In this case, one can observe a broad peak at 810-890 cm-1, a more pronounced peak at ~1000 cm-1 and the Si-O-Si stretching mode peak at 1070 cm-1.Thus, SHI irradiation with the highest fluence (i.e. 1013 cm−2) leads to intermixing of GeO/SiO2 layers and to the formation of a GeO-SiO2 solid alloy. Moreover, some Ge atoms probably replace Si atoms

6

in SiO2 and Si-rich precipitates can be formed (see also the peak originating from Si-Si bonds in the Raman spectra shown in figure 1). Figure 3 displays the room temperature PL spectra of both GeOx and GeO/SiO2 multilayers measured before and after SHI irradiation with a fluence of 1013 cm2. We report the observation of an intense PL band in the visible range (Figure 3). In the as-deposited samples, broad PL bands appear with a long-wavelength shoulder. SHI irradiation leads to an enhancement of the PL signal for both samples (Figures 3a and 3b), which can be even seen by the naked eyes. For the irradiated samples, in the case of the multilayered structure, the PL signal is more intense and its maximum is shifted from 425 nm (GeOx film) to 500 nm (GeO/SiO2 alloy). These bands may be related to Ge (or Si) induced oxygen-vacancy defects (two neighboring Ge (or Si) atoms in the SiO2 matrix). Similar bands were observed in Ge-implanted and annealed SiO2 films [34, 35], but the position of the maximum was about 420 nm. According to the calculations in ref. [36], only the non-bridging oxygen (NBO) defect (- O – Ge ) can give rise to a weak PL band peaked at 425-460 nm (2.9–2.7 eV). Such defects in GeO2 are therefore unlikely to be responsible for the bright blue PL bands. The origin of the high-brightness light in the range from 2 to 3 eV needs to be further clarified. Batra et al. [37] assigned the luminescence at around 575 nm (2.2 eV) to Ge nanoparticles evolving from a sub-stoichiometric Ge oxide after annealing. This may possibly explain the long wavelength shoulder observed in our PL spectra. Figure 4 shows the low-temperature PL spectra of both GeOx and GeO/SiO2 samples. The common feature for the as-deposited samples is the presence of the Si transverse optical (TO) line at 1124 nm originating from the Si substrate. SHI irradiation creates defects in Si substrates that quench the bulk-Si related PL. It also leads to the decrease of the PL intensity related to some earlier reported defects in GeOx layers [15]. Moreover, we note the appearance of a new broad peak with a maximum at 1550 nm (i.e. 0.8 eV) for both samples. It is worth to be mention that the long-wavelength cutoff of our detector is about 1610 nm. Consequently a possible 7

contribution at longer wavelength is cutted and one can see only noise at the long-wavelength edge. The presence of a narrow peak at 1566 nm in the PL spectra of the as-deposited samples is not clear and will be the topic of future studies. Ardyanian et al. [15] observed in GeOx films a PL peak at ~ 800 nm, which was attributed to defects. This interpretation is supported by the fact that the peak disappears with either annealing or hydrogenation. The same authors also observed a weak long-wavelength luminescence in the 1200-1600 nm spectral region after annealing above 400 oC, which was explained by the formation of Ge aggregates. However, we did not observe the appearance of Ge-Ge bonds in the Raman spectra (Figure 1). By using calculations, Zyubin et al. [38], have shown that the di-vacancy of oxygen is a light-emitting center having a lower radiation energy compared with a single vacancy. Therefore, vacancy complexes can possibly be responsible for light emission in the IR range. We believe that the observation of a new infrared PL broad band (~0.8 eV) can be related to defects in GexSiyO2 glass and not to defects located in the Si substrate. In the latter case, the peaks should be equal for both samples, because both films are equally semi-transparent.

CONCLUSIONS By using Raman spectroscopy, we have shown that SHI irradiation with 167 MeV Xe+ ions and fluences as high as 1013 cm−2 does not lead to the formation of a noticeable amount of Ge-Ge bonds in 100 nm thick GeO films and GeO(5nm)/SiO2(5nm) multilayered heterostructures. According to FTIR spectroscopy data, SHI irradiation leads to intermixing of GeO and SiO2 layers in GeO/SiO2 multilayers. Strong photoluminescence was observed in the visible range, with a significant increase of the photoluminescence intensity for SHI irradiated samples. Visible light emission may be due to either Ge (or Si) related defects in the films. For irradiated samples, infrared photoluminescence was observed at low temperatures, presumably caused by a new type of defect in GexSiyO2 glass. 8

ACKNOWLEDGMENTS The work was carried out according to the state research program of ISP SB RAS project number 0306-2016-0015. V.A.V. is thankful to administration of Université de Lorraine for visit grant.

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of

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Fig. 1. Raman spectra of a GeOx layer (a) and of GeO/SiO2 multilayers (b) measured before (curves 1) and after Xe ion irradiation. Fluences, (cm-2): 2) 1011, 3) 4x1011, 4) 1012, 5) 1013. Fig. 2. FTIR absorption spectra of a GeOx layer (a) and of GeO/SiO2 multilayers measured (b) before (curves 1) and after Xe ion irradiation. Fluences, (cm-2): 2) 1011, 3) 4x1011, 4) 1012, 5) 1013. (c) - FTIR-spectrum of an as-deposited GeO-SiO2 solid alloy. Fig. 3. Room temperature PL spectra of a GeOx layer (a) and of a GeO/SiO2 multilayer (b) measured in the visible range before (curves 1) and after irradiation with Xe ions for a fluence of 1013 cm-2 (curves 2). Fig. 4. PL spectra measured at T=9 K of a GeOx layer (a) and of a GeO/SiO2 multilayer (b) in the infrared before (curves 1) and after irradiation with Xe ions for a fluence of 1013 cm-2 (curves 2).

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