GeSn Nanocrystals in GeSnSiO2 by Magnetron Sputtering for Short

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GeSn Nanocrystals in GeSnSiO by Magnetron Sputtering for Short-Wave Infrared Detection Adrian Slav, Catalin Palade, Constantin Logofatu, Ioana Dascalescu, Ana Maria Lepadatu, Ionel Stavarache, Florin Comanescu, Sorina Iftimie, Stefan Antohe, Sorina Lazanu, Valentin S Teodorescu, Dan Buca, Magdalena Lidia Ciurea, Mariana Braic, and Toma Stoica ACS Appl. Nano Mater., Just Accepted Manuscript • DOI: 10.1021/acsanm.9b00571 • Publication Date (Web): 10 May 2019 Downloaded from http://pubs.acs.org on May 13, 2019

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ACS Applied Nano Materials

GeSn Nanocrystals in GeSnSiO2 by Magnetron Sputtering for Short-Wave Infrared Detection

Adrian Slav,1 Catalin Palade,1 Constantin Logofatu,1 Ioana Dascalescu,1 Ana M. Lepadatu,1 Ionel Stavarache,1 Florin Comanescu,5 Sorina Iftimie,3 Stefan Antohe,3 Sorina Lazanu,1 Valentin S. Teodorescu,1 Dan Buca4, Magdalena L. Ciurea,1,6 Mariana Braic,2 Toma Stoica1* 1National

Institute of Materials Physics, 405A Atomistilor St., 077125 Magurele, Romania.

2National

Institute for Optoelectronics, 409 Atomistilor St., 077125 Magurele, Romania.

3University

of Bucharest, Faculty of Physics, 405 Atomistilor St., 077125 Magurele,

Romania. 4Peter

Grünberg Institut (PGI 9) and JARA Fundamentals of Future Information

Technologies, Forschungszentrum Jülich, 52425 Jülich, Germany. 5National

Institute for Research and Development in Microtechnologies, 077190 Voluntari,

Romania. 6Academy

of Romanian Scientists, 050094 Bucharest, Romania.

1 ACS Paragon Plus Environment

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ABSTRACT: Detection in short-wave infrared (SWIR) has become a very stringent technology requirement for developing fields like hyperspectral imaging or climate changes. In a market dominated by III-V materials, GeSn, a Si compatible semiconductor, has the advantage of cost efficiency and inerrability by using the mature Si technology. Despite the recent progress in material growth, the easy fabrication of crystalline GeSn still remains a major challenge and different methods are under investigation. We present the formation of GeSn nanocrystals (NCs) embedded in oxide matrix and their SWIR characterization. The simple and costeffective fabrication method is based on thermal treatment of amorphous (Ge1-xSnx)y(SiO2)1-y layers deposited by magnetron sputtering. The nanocrystallization for Ge1-xSnx with 9 – 22 at.% Sn composition in SiO2 matrix with 9% to 15% mole percent, was studied under low thermal budget annealing in the 350 – 450 oC temperature range. While the Sn at.% content is the main parameter influencing the band-structure of the NCs, the SWIR sensitivity can be optimized by SiO2 content and H2 gas component in the deposition atmosphere. Their role is not only changing the crystallization parameters, but also reduce the carrier recombination by passivation of NCs defects. The experiments indicate a limited composition dependent temperature range for GeSn NCs formation before β-Sn phase formation occurs. NCs with an average size of 6 nm are uniformly distributed in the film, except surface region where larger GeSn NCs are formed. Spectral photovoltaic current measured on SiO2 embedded GeSn NCs


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deposited on p-Si substrate shows extended SWIR sensitivity up to 2.4 µm for 15 at.% Sn in GeSn NCs. The large extension of the SWIR detection is a result of many factors related to the growth parameters, but also to the in-situ or ex-situ annealing procedures that influence the uniformity and size distribution of NCs.

KEYWORDS:

GeSn

alloy;

nanocrystals;

nanocomposite,

magnetron

sputtering;

heterojunction; spectral photocurrent.

1. INTRODUCTION The fast development of new applications in sensing, process-monitoring applications or enhancement of machine vision systems,1 e.g. satellite Earth observations, push the scientific research community to look for new materials for light emitters and detectors in the 1.5 µm 3 µm wavelength region, also called short-wave infrared (SWIR). The detection/monitoring capabilities, impossible with other technologies, are related to the unique absorption properties in this wavelength range of the constituent molecules/ elements. If SWIR region up to 1.7 µm can be covered by III-V materials, e.g. InGaAs, the window up to 3 µm requires novel materials as well as simple and cost effective production technologies. The best approach to realize Si based sensors that can fully use the maturity of the Si technology is to develop direct bandgap semiconductors based solely on IV group elements. For Si photonics, 2,3 the major limitation is the indirect bandgap character of the commonly 3 ACS Paragon Plus Environment


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used IV-group semiconductors, Si, Ge, C elements and their alloys. The electroluminescence efficiency in SiGe based on alloys is only about 0.1% at room temperature.4 The confinement of electrons in Si5 and Ge6,7 quantum dots (QDs), or in more complex defect engineered QDs in Ge8 were studied as solutions to improve their photonic properties. The quantum confinement results in local increase of the optical transition probability, but also in lower absorption in the assemble of QDs, as well as blue-shift of the optical bandgap.9-11 Recent progress in epitaxy of GeSn alloys has led to the proof of fundamental direct bandgap in GeSn alloys for Sn contents above 8 at.%.12-14 Moreover, low temperature lasing action in GeSn alloys and GeSn/SiGeSn heterostructures opens a promising path for integrated Si photonics.13,15-17 The main challenge towards efficient room temperature operation, as required for most sensing applications, is the ability to grow crystalline GeSn alloys with high Sn content necessary for a large Γ to L- valleys energy difference.18 Crystalline GeSn binaries or SiGeSn ternaries have the advantage of increased radiative recombination probability and, additionally, bandgap tuning by composition, dimension (quantization) and strain variation, similar to that of quantum wells (QWs) and QDs structures.16,19-21 The strain in epitaxially grown GeSn can be exploited to obtain optical microcavities.22 Nanocrystals (NCs) with high Sn concentration up to 42% were obtained using colloidal technique and iodide reaction,23-25 by Sn precipitation in molecular beam epitaxy (MBE) grown films, or by magneton-sputtering (MS) followed by rapid thermal annealing (RTA).26,27 However, the low miscibility of Ge and Sn (80%) and the higher affinity of oxygen for Ge than for Sn. In amorphous as-deposited layer, the uniformly distributed Sn atoms have, with a high probability, Ge atoms as neighbors. This can explain the negligible Sn oxidation, neither at surface nor in the bulk. Upon annealing, due to Sn diffusion and segregation, surface Sn-reach layer oxidizes by ambient exposure. The Si2p XPS spectra (SI, Figure F4) indicate that Si atoms are mainly bonded to oxygen both in the “bulk” and at the surface. The surface roughness of GeSnSiO2 layers increases with the nanocrystallization temperature, as shown by AFM images in Figures 4e – 4h. The average roughness over all surface topography, Ra, increases from 1.4 nm for the as-deposited layer at RT to 2.0 nm for as-deposited layer at substrate temperature of 200 oC. For annealing driven nanocrystallization the roughness increases to 2.8 nm for 400°C, and to 22.6 nm at 450°C where strong -Sn segregation takes place. In summary, the GeSn NCs formation by ex-situ annealing in GeSnSiO2 layers occurs in a limited temperature regime. It starts at 350°C and is degraded at 450°C with the segregation of -Sn. The Sn concentration in GeSn NCs increases compared with the mean value in the as-deposited sample and decreases later on when Sn segregation occurs.



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Influence of deposition conditions on nanocrystallization process The GeSn NCs formation was investigated for a large set of (Ge1-xSnx)1-y(SiO2)y samples with different fabrication and processing parameters, some of them summarized in Table I. The Sn (at.%) and SiO2 (mole %) nominal concentration values of the as-deposited samples were obtained from EDX-SEM measurements on layers deposited on metallic-Ti6Al4V substrates (see Experimental Section). XRD 2θ and FWHM are the peak position and width of the corresponding GeSn (111) crystallographic planes. The Sn concentration in GeSn NCs is computed from 2 peak values by linear interpolation between Ge and -Sn, supposing negligible strain in NCs caused by the amorphous matrix.37 This is justified by the low temperatures used for the formation of GeSn NCs, lower than 500 oC. It is supported by published data on single elemental Ge and Si NCs embedded in oxide matrix that can experience tensile or compressive strain increasing with annealing temperature, but almost negligible for temperature of 700 oC.41 Table 1. Investigated set of (Ge1-xSnx)1-y(SiO2)y samples with different mean compositions (x= Sn (%) and y= SiO2 (%)) in layer, sputtering atmosphere (gas), substrate (Tsub) and annealing (TRTA) temperatures. The Sn concentration in GeSn NCs (NCs-Sn(%) is evaluated based on XRD (111) GeSn data. Where is the case, the annealing time was 30s. The symbols * and **


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denote nanocrystalline GeSn and -Sn segregation, respectively. Their absence means poor crystallization degree (amorphous). sampl e

EDX - SEM Sn (%)

Gas

SiO2 (%)

XRD (111) GeSn

Tsub (oC)

TRTA (oC)

2

FWH M

NCs-Sn (%)

S1

12.80.32 10.80.21 Ar

RT

400

26.73

4.5

19.2±0.56

S2

12.30.30 11.00.20 Ar

200

400

26.74

3.85

18.9±0.53

S3

14.20.36 11.70.23

200

400

26.94

1.77*

13.6±0.47

S4

14.40.35 13.80.25

200

400

26.88

1.79*

15.2±0.46

S5

17.90.41 14.90.26

200

400

26.87

1.60*

15.4±0.42

2.20

25.6±0.42

S6

22.40.47 14.30.24

S7

13.70.32 11.20.20

S8

12.90.31 13.30.25

S9

13.30.29 12.10.24

S10

13.80.32 2.70.06

S11

14.00.35 1.60.05

Ar+5%H 2

Ar+5%H 2

Ar+5%H 2

Ar+5%H 2

Ar+5%H 2

Ar+5%H 2

Ar+5%H 2

Ar+5%H 2

Ar+5%H 2

as-dep 26.49 200

300

26.66

1.31*

21.0±0.45

400

26.76

1.09**

18.4±0.48

300

as-dep 26.87

1.60*

15.4±0.51

340

as-dep 26.86

1.40*

15.7±0.49

370

as-dep 26.98

1.36*

12.5±0.48

300

as-dep 26.93

0.43**

13.8±0.29

350

as-dep 26.84

0.47**

16.2±0.31

The elevated substrate temperature, Tsub, during deposition is equivalent with a dynamic annealing. It increases the surface mobility of the adatoms and results in formation of pre-clusters of GeSn. The later RTA annealing formed the final NCs. The influence of Tsub on crystallization of sample S1 and S2, annealed at 400°C is presented in Figure 5a. The c-Si 17 ACS Paragon Plus Environment


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XRD peak at 54.6o can be canceled by appropriate orientation of the substrate, as is the case in Figure 3a. By analogy with the hydrogenation of a-Si, two effects of adding hydrogen to the deposition atmosphere (Ar+5%H2), are expected: photosensitivity increase by H “curing” of the defect bonds and reduction of the nanocrystallization thermal budget. 42,43 The hydrogen effect is demonstrated by the enhancement of the crystallization observed in Figure 5a, sample S3. The change in crystallization is better described by the dependence of the XRD peak width (FWHM) of the GeSn (111) crystallographic plane, as exemplified in Figure 5b. FWHM decreases (from above 4o in the sample as-deposited at RT to less than 2o for the sample deposited at 200 oC substrate temperature and with 5% H2 gas mixture.

Figure 5. (a) The influence of the substrate temperature, Tsub, and the mix of H2 in Ar atmosphere on the XRD diffractograms (b) FWHM of the (111)GeSn XRD peak for samples S1–S6 given in Table I. (111)GeSn peak position in comparison to c-Ge shown as inset. (c) XRD


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spectra as a function of the annealing temperature for sample S6. The layers are deposited on c-Si substrate. The increase of Sn concentration from 14 at.% (sample S3, S4) up to about 18 at.% Sn in samples S5, does not significantly change the XRD spectra, neither the FWHM nor the peak position (Figure 5b). This weak variation in Sn concentration in NCs might be related to the hindering effect of SiO2 on nanocrystallization, its content being unintentionally increased from 11.7 at.% in S3 to 14.3 at.% in S5. Further increase of the Sn content to 22.4% (sample S6, Table 1) strongly improve the sample crystallinity, the FWHM of the XRD GeSn (111) peak decreases to ~1.0° while its position shifts to smaller angle indicate increased Sn content in the NCs. Sample S6, even in as-deposited conditions, shows a large network of GeSn NCs with well-defined XRD peak and FWHM of about 2°. The later decreases to 1.3o after annealing at 300 oC, reaching to 1.09° after 400 oC annealing. However, -Sn segregation occurred at this temperature (Figure 5c). The Sn concentration in GeSn NCs was evaluated to be 25.6 % in asdeposited sample, and about 21 % and 18.4 %, after 300 °C and 400 oC annealing, respectively. As observed on samples S1 and S2, the increasing of the substrate temperature improved the crystallization of GeSn NCs in GeSnSiO2. This dynamic in-situ annealing effect may result in avoiding post-deposition annealing and increased homogeneity of the NCs distribution in the sample. In Figure 6a, the XRD diffractograms of samples deposited at substrate temperatures of 300 °C, 320 °C and 370 °C are plotted in comparison to that of S3 as-deposited sample (Tsub = 200 oC). The as-deposited sample S3 presents a broad XRD peak being almost 19 ACS Paragon Plus Environment


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amorphous. The samples S7 – S9 clearly show the formation of GeSn NCs with a constant increase of the crystallization degree with the deposition temperature (see Fig 6a and FWHM in Table 1). Similar with ex-situ annealing, the increase in thermal budget reduces the Sn concentration in NCs, from about 15 at.% for Tsub = 300°C (S7) to about 12 at.% at Tsub = 370°C (S9).

Figure 6. Influence of the substrate temperature, Tsub, during deposition and of the SiO2 concentration on nanocrystallization. XRD diffractograms measured on as-deposited samples: (a) for different Tsub. (b) samples deposited at 300 oC, with different SiO2 content. The layers are deposited on c-Si substrate. As indicated above, the nanocrystallization and -Sn segregation can be influenced by the SiO2 matrix concentration. This is emphasized now in Figure 6b by the comparison of XRD spectra measured on samples with 11% (S7) and 2.7%. (S10) SiO2 mole percent, both deposited at substrate temperature of 300°C. In sample S7, with high SiO2 content, the formation of GeSn


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NCs is not accompanied by -Sn segregation. On the other hand, for sample S10 with low SiO2 content, the crystallization is substantially enhanced, offering a very sharp GeSn (111) XRD peak with FWHM of 0.43° in comparison to 1.6° for S7 (Table 1). However, strong -Sn segregation is observed. One can conclude that SiO2 component induces a delay of both nanocrystallization and -Sn segregation, increasing the annealing and deposition temperatures at which these phenomena occur (see also the discussion regarding Figure 3a). We can conclude that the annealing and the increase of deposition temperature have similar effects on nanocrystallization and -Sn segregation. By annealing, Ge and Sn segregate to form GeSn NCs embedded in the rest of the GeSnSiO2 of less GeSn content. The SiO2 around GeSn NCs has a role of surface passivation, but must not isolate electrically the NCs for a high photoconduction. The segregation of Ge and Sn is accompanied by their diffusion and oxidation at the film surface during exposure to air, as revealed by HRTEM and XPS experiments. Because Sn seems to be faster segregated than Ge, the GeSn particles have a higher Sn concentration in the first stage of this process, such as this effect enhances the crystallization. Annealing at higher temperatures results in phase separation of Sn from GeSn crystals due to low miscibility of Ge and Sn. Consequently, the crystallization is increased but GeSn NCs have lower Sn concentration.

Opto-electronic properties of GeSnSiO2 with GeSn NCs. 21 ACS Paragon Plus Environment


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The spectral dependence of the absorption coefficient was studied by transmittance/ reflectance measurements on samples deposited on transparent FQ substrates. The absorption coefficient was evaluated using the formula α=ln((1-R)/T)/d, where R, T and d are reflectance, transmitance and film thickness, respectively. The optical bandgap using Tauc plot of the spectral absorption was found in general to be in the wavelength range 2 – 3 µm. The methodology is exemplified in Fig 7 which compares the samples S10 and S11 (see Table 1). These samples were chosen also to present the effect of -Sn segregation on optical properties of layers. Sample S11 shows specific XRD peaks corresponding to -Sn while in the sample S10 Sn segregation does not occurs (Figure 7a).

Figure 7. -Sn effect on optical absorption in samples deposited on FQ: (a) XRD curves. (b) transmittance and reflectance curves. (c) Spectral dependence of the absorption coefficient  in Tauc representation.


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The measured transmittance and reflectance data are shown in Figure 7b. The sharp peak in Figure 7c is due to an absorption line in FQ substrate. The fundamental gap in S10 obtained by linear extrapolation to zero of 2 is in SWIR, at about 3 µm. Similar spectral dependence is observed for S11 but shifted to higher absorption coefficient due to additional absorption induced by segregated metallic -Sn. Photocurrent measurements were performed on ITO/GeSn NCs in SiO2/p-Si substrate/Al heterostructure diodes. Indium-tin-oxide (ITO) top electrode with thickness of about 90 nm was deposited by magnetron sputtering at a temperature of 160 oC. Its transmittance is higher than 80% up to a wavelength of 3 µm and has a conductance of about 800 Ω/. The diode structure is schematically shown in Figure 8a. The rectifying character of the diode, fabricated on sample S8, is revealed by the I-V characteristics in the temperature range 100 K to 300 K (Figure 8b). The spectral photocurrent was measured in photovoltaic regime at zero voltage bias, using chopped monochromatic light. The sample names correspond to those of the corresponding GeSn NCs embedded in SiO2 layers in Table 1. Example of spectral dependence of the photovoltaic current in VIS-SWIR range measured on sample S8 at 200 K is shown in Figure 8c. The response at wavelengths below 1.1 µm is mainly given by the photo-generation of carriers in the Si substrate. The infra-red (IR) part of the photocurrent spectrum between 1.2 – 2.2 µm, represents the collection of photo-carriers generated in GeSnSiO2 layer. The interest here is in the SWIR part of the spectrum extended by Sn alloying of Ge to longer IR 23 ACS Paragon Plus Environment


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wavelengths. The SWIR sensitivity is almost constant at temperatures lower than 200 K and decreases above it with a deactivation energy of about 0.36 eV, as shown by the temperature dependence of the photocurrent, under excitation of =1.3 µm (inset of Figure 8c). The spectral dependence of the normalized photo-current at different temperatures for a large set of samples is shown in Figure 8d. The SWIR spectral photocurrent has been measured in the range 1.2 µm to 2.4 µm and normalized to the spectrum of the monochromatic light source in order to obtain the spectral dependence of the quantum efficiency, i.e. the ratio Iph/eph (), where Iph is the photocurrent, ph the incident photon flux and e the electron charge. The curves in Figure 8d are measured at 100 K on diodes with different GeSnSiO2 layers containing NCs (see Table 1 for details). For comparison with GeSn the light detection limits in Ge is indicated by vertical short lines marking the position of the indirect and direct bandgap in Ge at 100 K. Sample S5 shows the largest SWIR extension of the sensitivity up to 2.4 µm. This sample was deposited at substrate temperature of 200°C and after 400°C annealing has 15% Sn in the formed GeSn NCs (Table 1). No -Sn segregation was observed in the diode.


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Figure 8. Photosensitivity of GeSn NCs in SiO2. (a) Schematic view of the diode structure; (b) temperature dependence of dark I-V characteristics of a diode using sample S8. (c) Spectral photovoltaic current of diode S8 at 200 K in VIS-SWIR range. (inset) photocurrent dependence under  = 1.3 µm illumination as a function of temperature. (d) Spectral dependence of the quantum efficiency of the photovoltaic current measured on different samples at 100 K in SWIR range. (inset) shows the temperature dependence of spectral sensitivity of sample S8. For the as-deposited sample S8 deposited at 340 oC, the photocurrent is detected up to 2.2 µm, shorter wavelength than 2.4 µm for S5, in spite of the fact that from XRD measurements almost the same Sn concentration in NCs was found (Table 1). This suggest that the mean Sn concentration in NCs is not straightforward correlated to the photocurrent generated in the heterojunction region. The two samples differ by the concentration of Sn in the layer, 18% in S5 and 13% in S8, as well as in the nanocrystallization procedure, ex-situ annealed at 400 oC for S5 deposited at 200 oC and in-situ annealing for S8 deposited at 340 oC. Thus, different nanocrystallization parameters can induce differences in film uniformity and NCs size distribution, that can explain the small variation in spectral responses of S5 and S8 layers. The effect is stronger in samples deposited at higher substrate temperature as found from HRTEM and XPS analyses. The reduction of the detection wavelength limit is more pronounced for the in-situ annealed sample S9, deposited at 370 oC, where the spectral photocurrent has the sensitivity limit at 1.8 µm behaving like almost pure Ge NCs (Figure 8d). In another in-situ annealed sample S7, deposited at lower 300 °C, having almost same Sn 25 ACS Paragon Plus Environment


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concentration in NCs, the reduced photocurrent in comparison to samples S5 and S8 can be related to lower quality or/and weak passivation of GeSn NCs due to lower annealing temperature and SiO2 content (see Table 1). The photocurrent is also reduced in samples containing very low amount of SiO2 (samples S10 and S11 with about 2% SiO2), that also show strong -Sn segregation. The reduced photosensitivity in these samples can be explained considering that: the low concentration of SiO2 reduces the passivation of NCs surface, while the segregation of metallic Sn reduces the penetration of light to the junction region. We can conclude that SWIR photosensitivity of the heterojunction of GeSn NCs embedded in SiO2 with Si substrate is well correlated with the fabrication condition and nanocrystallization behavior. The SWIR extension of the sensitivity up to 2.4 µm was found in a (Ge1-xSnx)y(SiO2)1-y (x~18% Sn and y~15% SiO2) layer deposited at 200oC that formed GeSn NCs of about 15% Sn after RTA at 400 oC. Similar spectral photosensitivity detected up to 2.2 µm was obtained by in-situ annealing, sample deposited at higher temperature, 340 oC, with same Sn concentration of about 15% in NCs, but lower Sn(%) in the layer, x~13%. For higher sensitivity, the films should have moderate SiO2 content in order to passivate GeSn NCs, but low enough to not electrically isolate the NCs and make possible the collection of photocarriers. However, reducing the SiO2 content to approximately 2% caused a drastic reduction in photosensitivity. The extension of the photosensitivity to 2.4 µm for NCs with 15% Sn corresponds to an energy cut-off of 0.51 eV, that is more than 100 meV higher than the theoretical bandgap18,44,45 of the relaxed bulk GeSn with same Sn concentration. The bandgap increase by quantum confinement effect21,23-25 can be the origin of this discrepancy, 26 ACS Paragon Plus Environment

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despite the isolation of NC sites intentionally weak in our samples, which corresponds to a diminished blue shift. Stronger blue shift with bandgap close to 1.0 eV is reported in well isolated GeSn NCs obtained by colloidal techniques. 23-25 Another contribution to the blue shift of the sensitivity limit could be given by the Burstein–Moss effect in n-type GeSn due to low density of states in conduction band of the direct bandgap, as pointed out in Ref. 45. In literature, there are some reports about the photosensitivity extension to longer wavelengths by using MBE, CVD or MS epitaxial growth for alloying of Ge with Sn.46 For epitaxial layers the bandgap value is found much closer to the theoretical value, especially for strain-relaxed layers. Thus, photoconduction spectra extended up to 2.4 were reported for CVD layer with 10 – 11 at.% Sn.47,48 In GeSn prepared by MBE and sputtering epitaxy, cut-off wavelengths extended in the 1.8 – 2.0 µm range have been obtained for even lower Sn concentration of 6%,46 probably with the contribution of strain-induced bandgap narrowing. For GeSn NCs based SWIR photosensors we envisage future improvement by a better control of NCs size in GeSn/SiO2 multilayers, by formation of p-i-n GeSnSiO2 diodes instead of GeSnSiO2/Si heterostructures, or by field-effect increasing of photosensitivity similarly to the case of Ge-NCs/TiO2 in Ref. 58. 4. CONCLUSIONS We presented the deposition and thermal evolution of SWIR photosensitive layers based on GeSn NCs by using MS deposition of (Ge1-xSnx)y(SiO2)1-y alloys. GeSn NCs embedded in oxide are formed either by post-deposition annealing at temperatures in the range 350 oC – 27 ACS Paragon Plus Environment


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450 oC, or during the growth by deposition at high temperatures 300 oC – 370 oC. In the first stage of nanocrystallization, the segregation of Sn results in formation of GeSn NCs of higher Sn concentration than the mean value in the layer. By annealing at higher temperatures, beside improving the crystallization degree of GeSn NCs, -Sn segregation and its diffusion to the layer surface have consequences in diminishing the Sn concentration in NCs and generating non-uniformity from top to down in the film and increasing the roughness of the layer. The Sn and SiO2 concentrations were varied in the ranges 9 – 22 at% and 11 % – 15 at.%, respectively. Lowering of SiO2 concentration to 2 – 3 % increases the crystallization and -Sn segregation, but decreases the photosensitivity, demonstrating the important role of SiO2 in passivation of NCs boundaries. Hydrogen 5% added to Ar sputtering gas enhances the nanocrystallization and increases the photosensitivity in SWIR range of GeSn. Extension of the IR detection up to 2.4 µm was demonstrated in samples containing GeSn NCs with 15 at.% Sn by measuring the photovoltaic current in SiO2 embedded GeSn NCs / p-Si heterojunctions diodes. The cut-off wavelength is not straightforward related to the Sn composition of NCs, but influenced by other growth parameters, as well as by in-situ or ex-situ annealing procedures that can change the uniformity and size distribution of NCs. Further investigations are necessary in order to fabricate p-i-n diodes with optimized GeSn NCs uniformity. ASSOCIATED CONTENT Supporting Information.


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Additional experimental details on deposition-annealing process and measurement errors; TEM-EDX measurements showing the increased Sn concentration at surface; XRD comparison of nanocrystallization of GeSnSiO2 deposited on FQ and c-Si; XPS showing 2p peak of oxidized Si. AUTHOR INFORMATION Corresponding Author *E-mails: [email protected] Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Notes The authors declare no competing financial interest. ACKNOWLEDGMENT This work was supported by UEFISCDI project M-ERA.NET GESNAPHOTO Contract no. 58/2016 and project PCE contract no. 122/2017, and by Romanian Ministry of National Education through NIMP Core Program PN19-03 no. 21N/2019 and INOE Core Project 33N/2018. REFERENCES 29 ACS Paragon Plus Environment


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Moto, K.; Sugino, T.; Matsumura, R.; Ikenoue, H.; Miyao, M.; Sadoh, T. Low-

Temperature (

GeSn Nanocrystals in GeSnSiO2 by Magnetron Sputtering for Short

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