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Impact of wet treatments on the electrical performance of Ge0.9Sn0.1 based p-MOS capacitors Mohamed Aymen Mahjoub, Thibault HAFFNER, sebastien labau, etienne eustache, joris aubin, Jean-Michel Hartmann, Gerard Ghibaudo, Bernard Pelissier, Franck Bassani, and Bassem Salem ACS Appl. Electron. Mater., Just Accepted Manuscript • DOI: 10.1021/acsaelm.8b00099 • Publication Date (Web): 24 Jan 2019 Downloaded from http://pubs.acs.org on January 28, 2019

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ACS Applied Electronic Materials

Impact of wet treatments on the electrical performance of Ge0.9Sn0.1 based p-MOS capacitors Mohamed Aymen Mahjoub†&, Thibault Haffner†, Sébastien Labau†, Etienne Eustache†, Joris Aubin‡, Jean-Michel Hartmann‡, Gérard Ghibaudo§, Bernard Pelissier†, Franck Bassani†, Bassem Salem†* †

Univ. Grenoble Alpes, CNRS/LTM, 38054 Grenoble Cedex 9, France

‡

Univ. Grenoble Alpes, CEA, LETI, 17 rue des Martyrs, 38054 Grenoble Cedex 9, France

§

Univ. Grenoble Alpes, IMEP-LaHC, 38016 Grenoble, France

ABSTRACT We have investigated the impact of different wet treatments on the electrical performances of Germanium-Tin (GeSn) based p-MOS capacitors with 10 % of Sn. Atomic Force Microscopy (AFM) showed the presence of Sn droplets for the degreased Ge0.9Sn0.1 surface, which were removed by HCl, HF and HF:HCl treatments. On the other hand, (NH4)2S and NH4OH treatments were not fully able to remove these droplets. X-Ray Photoelectron Spectroscopy (XPS) measurements confirmed AFM results and highlighted the efficiency of HF, HCl and HF:HCl treatments in removing Ge and Sn native oxide, which was not the case with (NH4)2S and NH4OH. Nevertheless, XPS showed a re-oxidation of the

Ge0.9Sn0.1 surfaces a few minutes only after HF, HCl and HF:HCl wet treatments. Therefore, another approach was tested. It consisted in using (NH4)2S to protect Ge0.9Sn0.1 surfaces from immediate reoxidation by creating a Ge0.9Sn0.1-S monolayer. Chemical depth profiles of Ge0.9Sn0.1/Al2O3 stacks were investigated using parallel angle resolved XPS (pAR-XPS) indicating a high quality interface when

Ge0.9Sn0.1 surface is cleaned previously by HF then (NH4)2S. There was notably a lack of Sn or Ge diffusion into the Al2O3 layer. C-V characteristics combined with a custom- analytical model yielded a low interface trap density (Dit).

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KEYWORDS Germanium-Tin (GeSn), wet treatments, depth profile, MOS capacitor, C-V measurements, pAR-XPS. INTRODUCTION Germanium-Tin (Ge1-xSnx or GeSn) alloys have recently generated a lot of interest. They are promising candidates in next-generation CMOS devices 1 and in Group-IV photonics 2. By controlling the Sn content

3,4,

GeSn alloys can exhibit superior motilities for both holes and

electrons 5,6. The electron injection velocity is otherwise higher in GeSn than in Ge 7. Moreover, GeSn exhibits a direct and low band gap for Sn contents > 8%, which is desirable for: (i) optoelectronic devices such as lasers 8 (ii) and low power devices such as tunnel-field effect transistors (tunnel FETs)

7,9

and (iii) as embedded source/drain stressors in order to increase

charge carriers mobilities 10. Furthermore, the smaller effective masses of charge carriers makes GeSn a suitable channel material in Metal-Oxide-Semiconductor Field Effect Transistors (pMOSFETs and n-MOSFETs ) 11,12. However, the integration of GeSn requires the fabrication of high-k dielectric/GeSn stacks with a high interface quality. Atomic layer deposition (ALD) which is a low temperature process is well adapted to deposit high-k dielectrics on GeSn layers with an accurate thickness, good uniformity. Surface preparation prior to dielectric deposition plays a crucial role in controlling the high-k/GeSn interface quality, ultimately affecting the electrostatic performance of manufactured devices. For instance, Gupta et al. showed that the combination of an 1:1 HF:HCl wet cleaning followed by a H2O pre-pulsing yielded good C-V characteristics 13. Similar protocols were used by other groups 14–16. G. Han et al. showed interesting electrical performances in Ge0.958Sn0.042 channel p-MOSFETs when a (NH4)2S treatment was used prior to the ALD of high-k dielectrics17. Nevertheless, the impact of wet treatments on the chemical and electrical behavior of interfaces was not systematically investigated on GeSn alloys with a Sn percentage above 8 %. Obtaining 2 ACS Paragon Plus Environment

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a high interface quality between GeSn alloys with a high Sn percentage > 8 % and high-k is more challenging. In the following, we present a systematic study of the impact of various wet treatments on the electrical performance of Ge0.9Sn0.1 p-MOS capacitors. Nine different wet protocols calling upon acid and basic solutions used in industrial and academic environments were tested. After these wet treatments, the surface roughness of the Ge0.9Sn0.1 will be determined by atomic force microscopy (AFM). The chemical nature of Ge0.9Sn0.1 surface will be intensively studied by Xray photoelectron spectroscopy (XPS). Then, Al2O3 layers were deposited at 250°C by ALD on Ge0.9Sn0.1 layers after various surface preparations. A combination of maximum entropy algorithms and parallel angle-resolved X-ray photoelectron spectroscopy (pAR-XPS) will give us access to chemical depth profiling in the resulting stacks 18. Finally, capacitance-voltage (CV) measurements combined with theoretical calculations will be performed to extract the interface trap density, i.e. the Dit, in each Al2O3/ Ge0.9Sn0.1 p-MOS capacitor. In other words, we will quantify the influence of the various wet treatments on the electrical performances of the resulting device. EXPERIMENTAL SECTION Growth procedures. The 50 nm thick Ge0.9Sn0.1 layers used in this work were grown on top of 200 mm Ge-buffered Si(001) substrates in a single wafer Epi Centura 5200 Reduced PressureChemical Vapor Deposition (RP-CVD) cluster tool from Applied Materials

19.

Ge Strain

Relaxed Buffer layer of 2.5 µm thick was grown on top of a slightly p-type doped Si(001) substrate. Then a fully Ge0.9Sn0.1 compressively-strained epitaxial layer was grown at 325°C, 100 Torr with Ge2H6 and SnCl4 precursors. More details can be found elsewhere 4. Al2O3 layers were deposited at 250°C using as precursors trimethylaluminum (TMA) (Epichem, 99.9%) and de-ionized (DI-H2O) (Cambridge Isotope Labs, 99.7%) in a Fiji 200 tool from ULTRATECH. 3 ACS Paragon Plus Environment


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Sample preparation. In this work, 10 samples were investigated. Each sample received a distinctive wet treatment. Table 1 summarizes the different cleaning procedures and the resulting samples. Table 1. Description of the various cleaning procedures used in this work. Name C1 = degrease

Cleaning procedures 1. Acetone for 2 min

Sample S1

2. Ethanol for 2 min 3. Isopropanol for 2 min 4. Blow dry with N2 C2

1. Degrease

S2

2. HCl for 1 min 3. Blow dry with N2 C3

1. Degrease

S3

2. HF for 1 min 3. Blow dry with N2 C4

1. Degrease

S4

2. HF/HCl, 1:1 for 1 min 3. Blow dry with N2 C5

1. Degrease

S5

2. NH4OH for 1 min 3. Blow dry with N2 C6

1. Degrease

S6

2. (NH4)2S for 30 min 3. Blow dry with N2 C7

1. Degrease

S7

2. HCl for 1 min then


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3. (NH4)2S for 30 min 4. Blow dry with N2 C8

1. Degrease

S8

2. HF for 1 min then 3. (NH4)2S for 30 min 4. Blow dry with N2 C9

1. Degrease

S9

2. HF/HCl, 1:1 for 1 min 3. (NH4)2S for 30 min 4. Blow dry with N2 C10

1. Degrease

S10

2. H2O2:H2SO4, 1:5 (Piranha) for 1 min 3. Three rinses in DI-H2O 4. Blow dry with N2

Characterization Techniques. XPS and pAR-XPS measurements were performed in a customized Thermo Fisher Scientific Theta 300 pAR-XPS tool. The X-ray source was a monochromatic aluminum anode source emitting at 1486.6 eV. Analysis was performed in ultra-high vacuum conditions (P8%). This effect could be explained by the strain field in Ge0.9Sn0.1, as described by Flynn et al. 22 and the oxidation of the Ge0.9Sn0.1 which can increase the diffusion kinetics of the Sn. However, the presence of Sn droplets is not well understood yet and need more investigation. For samples S3 and S8 (Figure 1 b and Figure 1 e), particles were completely removed and the RMS roughness reduced to values around 0.26 nm. We had the same behavior for samples S2, S4, S7 and S9 (AFM images not shown here) which explains the low RMS roughness obtained (Figure 1g). However, very small particles were still present on the surface of samples S5 (Figure 1c) and S6 (Figure 1e), which explain why RMS roughness was 0.4 and 0.35 nm for S5 and S6, respectively. This means that protocols C5 and C6 are powerless to remove those particles. 6 ACS Paragon Plus Environment

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Figure (1f) shows a damaged surface, with a RMS roughness of 6.5 nm. H2SO4 had a deleterious impact on the GeSn surface, with material loss and surface roughening.

7.0

(g) 6.50

6.5

RMS (nm)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


6.0

0.5

0.43

0.40

0.4

0.35 0.26

0.3 0.2

C1

C2

0.28

C3

0.30

C4

0.26 0.27 0.26

C5

C6

C7

C8

C9

C10

Cleaning procedures

Figure 1. 5×5 µm AFM images of the Ge0.9Sn0.1surfaces after various wet treatments: (a) after a degrease (b)-(e) surfaces treated with HF (C3), NH4OH (C5) and (NH4)2S (C6). (f) Optical image of Ge0.9Sn0.1 surface treated with Piranha (C10). (g) Surface RMS roughness as a function of the various cleaning procedures. Native oxide removal and germanium-Tin dangling bond passivation were studied on samples S1-S10 by XPS measurements. The binding energies of XPS spectra were corrected according to the C1s peak. Their reference binding energy of 285 eV was used for the calibration of the



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spectrometer 23. Recorded peaks were fitted using a Shirley background. The relative amounts of carbon contamination, Ge oxide (Geox) and Sn oxide (Snox) were deduced from the recorded XPS spectra with the following equations:

𝑅𝐺𝑒𝑜𝑥(%) =

𝐼𝐺𝑒3𝑑𝑜𝑥 𝐼𝐺𝑒3𝑑 + 𝐼𝐺𝑒3𝑑𝑜𝑥

𝑅𝑆𝑛𝑜𝑥(%) =

× 100

𝐼𝑆𝑛3𝑑5/2𝑜𝑥 𝐼𝑆𝑛3𝑑5/2 + 𝐼𝑆𝑛3𝑑5/2𝑜𝑥

𝑅𝐶(%) = 𝐼𝐺𝑒3𝑑

𝐼𝐶1𝑠 + 𝐼𝐺𝑒3𝑑𝑜𝑥

× 100

× 100

(1)

(2)

(3)

𝑅𝐺𝑒𝑜𝑥(%) and 𝑅𝑆𝑛𝑜𝑥(%) are the oxidized fractions of Ge and Sn. 𝑅𝐶(%) is the ratio of the C1s peak intensity divided by that of both Ge peaks. 𝐼𝐺𝑒3𝑑, 𝐼𝐺𝑒3𝑑𝑜𝑥, 𝐼𝑆𝑛3𝑑5/2, 𝐼𝑆𝑛3𝑑5/2𝑜𝑥 and 𝐼𝐶1𝑠 are the intensities of the Ge3d, Ge3dox, Sn3d5/2, Sn3d5/2ox and C1s peaks, respectively. Figure 2 shows the fitted Ge3d (figure 2a) and Sn3d5/2 (figure 2b) of samples S1-S10. Curves are normalized to the height of the bulk peak in order to emphasize peak shape differences. Figure 2c shows the calculated values of 𝑅𝐺𝑒𝑜𝑥(%), 𝑅𝑆𝑛𝑜𝑥(%) and 𝑅𝐶(%) for samples S1-S10. Regardless of the surface probed, Ge3d peak fitting (Figure 2a) reveals the presence of Ge3d3/2 and Ge3d5/2 peaks located at 28.8 and 29.23 eV, respectively. These peaks are a signatures of Ge in the Ge0.9Sn0.1 layer. Except for sample S8, there are two additional broad peaks, shifted from the Ge3d5/2 peak by 1 eV and 3.1 eV toward higher binding energies. They are attributed to the Ge+1 and Ge+3 states respectively, i.e. to native oxide 24–26. For sample S8, just a very low intensity Ge+1 peak was observed. 𝑅𝐺𝑒𝑜𝑥(%) calculations highlight a decrease of the Geox amount in samples S2-S10 compared to sample S1 (i.e. the degreased sample) even more for


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S7-S9 samples. 𝑅𝐺𝑒𝑜𝑥 indeed decreases from 24.2% for samples S1 down to 6.7, 3.0 and 5.9% for samples S7, S8 and S9, respectively. Sn3d5/2 peak fitting for samples S1-S9, shows the presence of a peak at 485 eV. This peak is attributed to Sn in the Ge0.9Sn0.1 layer. In the S1 case, two additional peaks were observed at 486.5 and 487.2 eV. They are likely due to the two stable oxidation states Sn2+ and Sn4+, respectively, in metallic Sn environments

27,28.

This is an indication that the particles showed

on the AFM image are most probably Sn droplets. Sn2+ and Sn4+ peaks are removed in samples S2-S4, with the emergence of another peak at 486 eV, denoted Sn2+GeSn which is probably due to the unavoidable oxidation of Sn in air (after wet treatment and prior to XPS analysis). The shift between the Sn2+ and Sn2+GeSn can be explained by the “structure induced chemical shift” concept

29,30.

Actually, strains in the Sn droplets and in the Ge0.9Sn0.1 layer are different,

probably resulting in a modification of the Sn-O binding energy and, then, a different shift. Furthermore, an intense Sn2+GeSn peak is observed in the S2 sample compared to the S3 and S4 samples. This is likely due to the HF treatment, which yields a hydrogen-terminated surface in the S3-S4 case. Meanwhile, a dip in HCl results in a chlorine terminated S2 surface with more oxygen31. In the S5 and S6 cases, the Sn2+ and Sn4+ peaks are still present and the Sn2+GeSn peak is more intense. This is due to the inability of (NH4)2S and NH4OH chemicals to completely remove Sn droplets, in line with AFM observations (Figures 1c and 1d). For samples S7-S9, the same behavior as Ge oxides is observed. Indeed, only a very low intensity Sn2+GeSn peak is detected. 𝑅𝑆𝑛𝑜𝑥(%) is therefore very low and is around 8.4%, 3.4% and 7.1% for S7, S8 and S9 samples, respectively. (NH4)2S is thus efficient in preventing the very fast re-oxidation of the Ge0.9Sn0.1 surface (in-between the wet treatments and the loading of samples in the inert environment of the XPS tool). For sample S10, the Sn3d5/2 peak was not detected, which is likely due to a complete removal of the Ge0.9Sn0.1 layer by the piranha-based wet treatment C10, in agreement with the very high surface roughness of sample S10 (Figure 1f). 9 ACS Paragon Plus Environment


Furthermore, 𝑅𝐶(%) calculations (figure 2c) showed that carbon contamination was the lowest for S7, S9 and especially S8 samples. Indeed, a dip in HF is known to generate hydrophobic surfaces and helps to reduce contamination from aqueous solutions

31,32.

(NH4)2S effectively

protected the Ge0.9Sn0.1 surface from immediate re-oxidation by creating a Ge0.9Sn0.1 -S bond that can be removed by annealing. XPS measurements indeed showed the presence of a S2s peak (figure 2d) coming from S atoms on the Ge0.9Sn0.1 surface after (NH4)2S wet treatments. This peak disappeared after an annealing at 250 °C under vacuum which evidence that the bonding between the sulfur and Ge0.9Sn0.1 is sufficiently weak. To conclude, AFM and XPS results showed that wet treatments C7, C9 and especially C8 yielded the best compromises in terms of surface roughness and chemistry. Indeed HF/HCl, HCl and especially HF treatments were efficient to remove the Ge0.9Sn0.1 native oxide (Geox and Snox) and to improve the surface smoothness. Although (NH4)2S treatment did not remove oxide, this latter is necessary to protect the surface after HF, HCl or HF/HCl treatments.

a)

0.8

Ge3d3/2

0.6

0.4 1+

Ge3d

3+

Ge3d

0.2

Intensity (normalized)

1.0

Ge3d5/2

S1 S2 S3

S4 S5 S6 S7

Sa m pl e

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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S8

S9 S10 26

28

30

Binding Energy (eV)

32

34


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2+

Sn - S

0.8

4+

Sn - S 0.6

Sn3d5/22+

0.4

0.2

Intensity (normalized)

1.0

b)

0.0 S1

S2 S3

S6 S7

Sa m pl e

S4 S5

S8

S9

4+

Sn - GeSn 482

483

484

485

486

S10

487

Binding Energy (eV)

488

489

4400

60

RGeox(%) RSnox(%) RC(%)

50

40

30

20

3850

Intensity (u.a)

c)

Ratio (%)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


490

d)

Without anneal With anneal

S2S

3300

2750 10

0

S1

S2

S3

S4

S5

S6

S7

S8

S9

S10

2200

232

Sample

230

228

226

224

Binding energy (eV)

Figure 2. XPS spectra: (a) of Ge3d and (b) Sn3d5/2 for S1-S10 surfaces after one of the C1-C10 cleaning procedures. (c) 𝑅𝐺𝑒𝑜𝑥(%), 𝑅𝑆𝑛𝑜𝑥(%) and 𝑅𝐶(%) values for samples S1-S10. (d) S2s spectra of Ge0.9Sn0.1 surface after a dip in HF followed by a dip in (NH4)2S (blue line) and after a 250°C, 10 min. anneal under vacuum (red line). A proper determination of the chemical composition and morphology of the Al2O3/ Ge0.9Sn0.1 interface was a priority after this detailed study of the Ge0.9Sn0.1 surfaces after different wet sequences. As a first step, XPS was used to check the Sn presence and no traces were detected for all samples treated with the (NH4)2S. Then, pAR-XPS experiments were performed on S1S9 samples with 2 nm thick Al2O3 dielectric. Ge3d, Sn3d5/2, Al2p and O1s spectra were 11 ACS Paragon Plus Environment


recorded at emission angles from 23.5° to 75.25° with respect to the normal of the sample. Using these spectra, the composition depth profiles of the different samples were calculated using the Avantage® software based on the maximum entropy method 33,34. Figure 3 shows the chemical depth profiles of samples S1 (Figure 3a), S7 (Figure 3b), S8 (Figure 3c) and S9 (Figure 3d). Regardless of the sample, depth profiles show the expected stoichiometry in the top part of the Al2O3 layer and in the bottom part of the Ge0.9Sn0.1 layer. The depth profile of the Al2O3/S1 sample shows the presence of Geox and Snox oxides at the interface. Snox represents 27 % of the total oxide and is closer to the surface than Geox, which support our hypothesis (based on AFM and XPS data) that the “particles” are tin droplets. Snox was not detected at the S7, S8 or S9 /Al2O3 interface. Meanwhile, a Geox trace is observed. These profiles otherwise did not show a diffusion of Sn or Ge into the Al2O3 layer. Two major issues are thus solved: (i) the presence of Snox at the interface, which is known to have a metallic behavior and deteriorate the electrical performances of the Ge0.9Sn0.1/Al2O3 stack and (ii) the degradation of the high-k dielectric layer because of Ge or/and Sn diffusion. Depth profiles were almost the same in the other samples, with more Geox and Snox, however.

O(AL2O3)

Ge Geox

Sn Snox

80 6

Composition (%)

60 40

100

(a) - Al2O3/S1

5 4 3 2 1 0

20

0

1

2

3

4

Depth (nm)

5

Al

O(AL2O3)

Ge Geox

Sn Snox

(b) - Al2O3/S7

6

60

Composition (%)

80

Al

Composition (%)

100

Composition (%)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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40

5 4 3 2 1 0

0

1

2

20

3

4

5

Depth (nm)

0

0 0

1

2

3

4

5

0

1

2

3

4

5

Depth (nm)

Depth (nm)


Page 13 of 27

100

Al

O(AL2O3)

Ge Geox

Sn Snox

100

(c) - Al2O3/S8

Al Ge Geox

40

5 4 3 2 1 0

0

1

2

3

4

Depth (nm)

20 0

5

(d) -Al2O3/S9

O Sn Snox

6 Composition (%)

60

Composition (%)

80 6 Composition (%)

80

Composition (%)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


60 40

5 4 3 2 1 0

0

1

2

3

4

5

Depth (nm)

20 0

0

1

2

3

4

5

0

1

Depth (nm)

2

3

4

5

Depth (nm)

Figure 3. Chemical depth profiles of (a) Al2O3/S1, (b) Al2O3/S7, (c) Al2O3/S8 and (d) Al2O3/S9 samples. The relative concentration of Germanium (Ge), Tin (Sn), Germanium oxide (Geox), Tin oxide (Snox), Aluminum (Al) and Oxygen (O) are plotted as function of depth. C-V measurements were carried out to validate the previous results and assess the impact of the various wet treatments on the electrical properties of Al2O3/ Ge0.9Sn0.1 stacks. Therefore, 10 nm of Al2O3 was deposited on the various types of samples (S1 to S9). Then, MOS capacitors were fabricated using conventional photolithography and lift-off steps (Figure 4) in back-to-back capacitor configuration in order to avoid ohmic contact on Ge0.9Sn0.1 which simplify the process (Figure 4). The gate metals were deposited in a MEB550 tool from PLASSYS (5nm of titanium and 150 nm of gold, respectively). In the following, the p-MOS capacitors based on S1-S9 samples will be denoted D1-D9. i.e. D1 for a device fabricated with a S1 surface preparation, D2 with S2 and so on.



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Figure 4. 3D/Cross sectional views of the Ge0.9Sn0.1 -based MOS devices designed as circular capacitors in back to back configurations. To eliminate the impact of series resistances on C-V curves, a correction of the measured capacitance (Cm) has been applied using the following formula proposed by Nicollian et al 35:

Cc =

(G2m + ω2C2m)Cm

(4)

a2 + ω2C2m

Where Cc and Gm are the corrected capacitance and the measured conductance, respectively. 𝜔 = 2𝜋𝑓, 𝑎 = ―(𝐺2𝑚 + 𝜔2𝐶2𝑚)𝑅𝑠 and 𝑅𝑆 = 𝐺𝑚 ― 𝑠𝑎 ―(𝐺2𝑚 ― 𝑠𝑎 + 𝜔2𝐶2𝑚 ― 𝑠𝑎). Gm ― sa and Cm ― sa are the measured conductance and capacitance in strong accumulation, respectively. Figure 5a and Figure 5b show C-V curves acquired in the 1 kHz-1 MHz frequency range on samples D1 and D8, respectively. In the D1 case, (figure 5a) C-V curves are nearly flat, the dispersion in accumulation is high (12.3%) between 1 kHz and 1 MHz and the flat band voltage shift is large, indicating a poor interface quality with the Al2O3 layer

36.

Conversely, sample

D8’s C-V curves show a low dispersion of 3.3 % in accumulation between 1 kHz and 1 MHz


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and a small frequency-dependent flat band voltage shift, indicating a very good Al2O3/ Ge0.9Sn0.1 interface quality 37,38 and highlighting the efficiency of the C8 cleaning procedure. In order to identify the best wet treatment, a comparison between the various C-V characteristics was performed. Figure 5c and Figure 5d show the normalized C-V curves (C/Cox) of different samples D1-D9 at 1 kHz and 1 MHz, respectively. Compared to D1’s C-V curves, D7, D9 and especially D8 C-V curves are noticeably shifted in the negative voltage direction, which indicates a decrease of Dit. These variations are in agreement with AFM, XPS and pAR-XPS results. Definitely, Ge0.9Sn0.1 surfaces used in the fabrication of these devices show the lowest GeOX and SnOx and the best interface quality with Al2O3 layer. Moreover, at 1 kHz (figure 5c) a clear change of the minimum capacitance (Cmin) in the depletion region is observed. These effects can be explained by the presence of traps. Indeed, as extra charges have to fill traps, the applied voltage has to be higher to have the same surface potential (ψS). Moreover, charge consumption by traps reduces the remaining charge in the depletion region and reduces the surface potential 𝜓𝑆, resulting in an increase of Cmin 39. Moreover, hysteretic behaviors in C-V measurements (figure available in the supporting information) of D3, D6 and D8 capacitors showed that D8 seems to have the best Al2O3/Ge0.9Sn0.1 interface. Indeed, D8 presents the lowest ∆Vfb meaning a low trap charge density. Based on those observations, the choice of chemical wet treatment plays a crucial role on the electrical performance of MOS capacitors. It is nevertheless important to determine the Dit of each device to have a better vision of the electrical reliability of the fabricated devices and the efficiency of the associated wet treatment procedures.



7

(a)- D1

Capacitance ×10-7 (F/Cm2)

Capacitance ×10-7 (F/Cm2)

7 6 5

1 kHz 5 kHz 10 kHz 50 kHz 100 kHz 200 kHz 400 kHz 600 kHz 800 kHz 1 MHz

4 3 2 1

-3

-2

-1

0

1

2

(b)- D8

6 5

1 kHz 5 kHz 10 kHz 50 kHz 100 kHz 200 kHz 400 kHz 600 kHz 800 kHz 1 MHz

4 3 2 1 0

3

-3

-2

Voltage (V)

1.05

-1

0

1

2

3

1

2

3

Voltage (V)

1.2

(c)- 1 kHz

1.00

(d)- 1 MHz

1.0

0.95

0.8

0.90

D1 D2 D3 D4 D5 D6 D7 D8 D9

0.85 0.80 0.75 0.70 0.65 -3

C/Cox

C/Cox

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 16 of 27

D1 D2 D3 D4 D5 D6 D7 D8 D9

0.6 0.4 0.2 0.0

-2

-1

0

1

2

3

-3

-2

-1

0

Voltage (V)

Voltage (V)

Figure 5. C-V characteristics of Au/Ti/ Al2O3/ Ge0.9Sn0.1 MOS capacitors: (a) degreased surface (S1) and (b) surface after a C8 wet treatment (HF/(NH4)2S). Superposition of C-V characteristics of D1-D9 device capacitances at 1 kHz (c) and 1MHz (d). The conductance method is usually used for Dit determination. Nevertheless, in the case of low bandgap semiconductors, this method is not reliable because of minority carrier’s responses. In the following, calculations will be performed with a custom- analytical model that enables to assess the impact of Dit on the capacitance response. The total capacitance (Ct) of the Al2O3/ Ge0.9Sn0.1 system is modeled in the quasi-static regime by the following formula 40 :

(

)

1 1 Ct = + Cox Cp + Cn + Css

―1

(5)


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Where Cox, Css, Cp and Cn are the capacitances of the gate oxide, the interface traps, the majority and minority carriers semiconductor space charges, respectively. Due to the low Ge0.9Sn0.1 bandgap, the interface trap levels were considered as a quasi-continuum of potential widthdΦt. They consist of many levels, which are so closely spaced in energy that they cannot be distinguished as separate levels. Then, Ct can be expressed as:

Ct(ψs, Nit, Φt, dΦt,f) =

[

1 + Cox

]

1 Cp(ψs) +

Cn(ψs) 1 + ω2τ2g

+

―1

(6)

Css(ψs, Dit, Φt, dΦt) 1 + ω2τ2it

Where ψs is the surface potential, Φt is the trap potential, 𝜏𝑔 is the carrier generation time, and 𝜏𝑖𝑡 is the trap response time 39. The minority and majority carriers capacitances Cn and Cp are obtained by the derivation of semiconductor charge (Qsc) given by the following formula: ni2 Na [exp (β ⋅ ψs) ― 1] + [exp ( ―β ⋅ ψs) ― 1] + Na ⋅ ψs Qsc(ψs) = 2 ⋅ q ⋅ εsc β ⋅ Na β

[

]

(7)

where Na and ni are the concentration of acceptor atoms and the intrinsic carrier density, respectively. β = 1 kT with k and T are the Boltzmann constant and the Temperature, respectively. Cn and Cp are thus given by:

[

sign(ψs) ⋅ q ⋅ εsc Cn(ψs) =

Cp(ψs) =

ni2 ⋅ exp(β ⋅ ψs) Na

] (8)

Qsc(ψs) sign(ψs) ⋅ q ⋅ εsc[Na ― Na ⋅ exp( ― β ⋅ ψs)] Qsc(ψs)

{

(9)

1, ψs > 0

With Sign(ψs) = 0, ψs = 0

―1, ψs < 0 17 ACS Paragon Plus Environment


Page 18 of 27

and Css is given by the following formula: Css(ψs, Dit, Φt, dΦt,f) =

dQss dψs

= qβ

Dit dΦt

∫

Φt + Φt ―

dΦt 2 ( ) dΦtFt ψs,Φt

(1 ― Ft(ψs,Φt))dΦ

2

(10) with Qss(ψs, Dit, Φt, dΦt,f) Dit

= ―q dΦt

∫

Φt + Φt ―

dΦt 2 ( ) dΦtFt ψs,Φt dΦt

(11)

2

Where 𝐷𝑖𝑡 is the real trap density and Ft(Φt, ψs) is the Fermi-Dirac distribution function. Figure 6.a shows the fitting of the experimental C-V characteristics of device D8 (fitting for other device are shown in the supporting information) by the model described previously. The best C-V fitting is obtained by adjusting the Dit value. The Dit values provided in the following from the best fitting of the experimental data. Figure 6.b depicts the calculated Dit values for different devices D1-D9. Important differences in the Dit values among the evaluated devices have been revealed. The origin of this difference is probably due to the cleaning procedure since it is the only parameter that changed during the devices fabrication. The Dit values obtained confirm AFM and XPS findings which highlighted the superiority of HF cleaning procedure terminated with (NH4)2S, especially the C8 one. Calculations give for sample D8 a Dit value as low as 9× 1011 cm-²eV-1, which is very promising for Ge0.9Sn0.1 CMOS integration. Indeed, according to the literature the lowest Dit reported until now for Al2O3/GeSn interface is in the 2-5x 1012 cm-²eV-1 range 13,14,36.


Page 19 of 27

6.5

(a)- D8 at 1 kHz

10

6.0

Dit×1012(cm-2eV-1)

Capacitance ×10-7 (F/cm2)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Experiment Simulation

5.5

5.0

4.5

4.0

-4

-3

-2

-1

0

1

2

3

4

(b) 9.2E12

8

9E12

8.3E12

8E12 7E12

6

6E12

4

4E12

2

0

2.2E12 9E11

D1

D2

Voltage (V)

D3

D4

D5

D6

D7

D8

D9

Device

Figure 6. C-V fitting: (a) comparison between experimental C-V data (blue squares) and theoretical simulation (red curve). (b) Calculated Dit values for D1-D9 devices, i.e. Al2O3/ Ge0.9Sn0.1 stacks with various surface preparations. CONCLUSION In summary, the impact of nine wet treatments prior to Al2O3 deposition on Ge0.9Sn0.1 layers was rigorously investigated. AFM showed the presence of Sn droplets on the degreased Ge0.9Sn0.1 surface. HCl and HF were found to be very efficient in removing Sn droplets and remarkably improving the surface roughness. NH4OH and (NH4)2S were not be able to completely remove Sn precipitates. Meanwhile, Piranha etched Ge0.9Sn0.1 resulted in really rough surfaces. Compared to the degreased Ge0.9Sn0.1, XPS analysis showed a reduction of the oxidized Ge and Sn amount, i.e. Geox and Snox, after wet treatments, irrespectively of the cleaning procedure used. However, they show that an important amount of Geox and Snox was still present on the surface after most treatments, save those terminated with a (NH4)2S dip. These oxides were most likely formed during the air exposure between wet treatments and XPS analysis. Additional Sn oxide peaks were present when dipping the samples in NH4OH and (NH4)2S baths only. They were attributed to the Sn droplets still present on the surface. (NH4)2S, after a dip in HCl and HF, was shown to effectively protect the Ge0.9Sn0.1 surface from immediate re-oxidation by creating a Ge0.9Sn0.1-S monolayer. Chemical depth-profiles obtained



Page 20 of 27

by pAR-XPS did not reveal the presence of any Sn or Ge pile-ups at the Al2O3/Ge0.9Sn0.1 interface and did not show a diffusion of those elements in the Al2O3 layer. A custom- analytical model was developed and used to extract Dit from the C-V characteristics of the Al2O3/ Ge0.9Sn0.1 capacitors performed with various wet treatments. It was found that device having treated with HF following by a (NH4)2S termination yielded a very low Dit of 9 × 1011 cm-²eV-1. Those results will be very useful for Ge0.9Sn0.1 surface preparation prior to high-k dielectrics deposition, which will be crucial for the fabrication of CMOS and photonics devices.

AUTHOR INFORMATION Coresponding author: Dr Bassem Salem Tel.: + 33 4 38 78 24 55; fax: +33 4 38 78 58 92 *E-mail address: [email protected] &Present

address: Dr Mohamed Aymen MAHJOUB, Luxembourg Institute of Science and

Technology (LIST), 5 Avenue des Hauts-Fourneaux, 4362 Esch-sur-Alzette.

Note The authors declare no competing financial interest

Supporting Information: additional AFM image of Ge0.9Sn0.1 surface before wet treatment; forward and reverse bias C-V characteristics measured at 1 MHz of Au/Ti/Al2O3/Ge0.9Sn0.1 MOS capacitors using different wet treatments. 20 ACS Paragon Plus Environment

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ACKNOWLEDGMENTS The authors thank the members of the technical staff of the PTA-Grenoble for their technical support. REFERENCES (1)

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