Ge Interface by Nitrogen Incorporation in a

Sep 16, 2016 - The thermal instability of GeO2/Ge structures lasts as a barrier against the development of Ge-based metal-oxide-semiconductor devices...
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Stabilization of the GeO2/Ge Interface by Nitrogen Incorporation in a One-Step NO Thermal Oxynitridation Gabriela Copetti,† Gabriel V. Soares,† and Cláudio Radtke*,‡ †

Instituto de Física and ‡Instituto de Química, UFRGS, 91509-900 Porto Alegre, Brazil ABSTRACT: The thermal instability of GeO2/Ge structures lasts as a barrier against the development of Ge-based metal-oxide-semiconductor devices. In the present work, stabilization was achieved through the incorporation of nitrogen into the oxide layer by thermally growing GeOxNy films in NO. With this approach, a stable layer is obtained in a single step as opposed to other nitridation techniques (like plasma immersion) which require additional processing. Significant reduction of GeO desorption from the surface and a strong barrier against additional substrate oxidation were obtained by the insertion of a small amount of nitrogen content (N/O ≈ 10%). Nuclear reaction analysis and profiling showed that nitrogen incorporation and removal occur simultaneously during film growth, yielding N to be distributed throughout the whole film, without accumulation in any particular region. Both the oxidation barrier and the lower GeO desorption rate are explained by a reduction of vacancy diffusivity inside the dielectric. This is not caused by the densification of the oxide, but is a consequence of nitrogen blockage of oxygen vacancy diffusion paths. KEYWORDS: germanium, germanium oxynitride, nitric oxide, nuclear reaction analysis, Rutherford backscattering spectrometry

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by several authors.11−18 GeOxNy/Ge structures, with low nitrogen concentration at the interface, have the advantage of exhibiting a lower density of interfacial states (Dit) when compared with those of Ge3N4/Ge structures.11 However, fabrication of GeOxNy films using plasma techniques demands several processing steps: thermal growth of a GeO2 layer, incorporation of nitrogen by plasma nitridation, and postnitridation annealing to heal defects created by ionic bombardment. GeOxNy films have also been fabricated by thermally nitriding GeO2 in ammonia (NH3).18−20 Nevertheless, this technique led to devices with poorer electrical characteristics when compared to plasma-nitrided samples. This is probably due to the formation of a large number of oxygen vacancies at the GeO2/Ge interface during the NH3 treatment at 600 °C. Furthermore, hydrogen incorporation during NH3 annealing may promote GeO desorption from the film surface.21 Though previous works demonstrate the superior stability of GeOxNy films,11 the mechanism through which nitrogen increases thermal stability is not completely understood. While germanium oxynitride properties are scarcely known, silicon oxynitride (SiOxNy) films have already been extensively investigated.22−36 For this reason, comparing Si and Ge oxynitridation can be very useful in order to understand the superior stability of GeOxNy films with respect to GeO2. SiOxNy films form a strong barrier against additional substrate oxidation, even when submitted to O2 atmosphere at high temperatures for several hours.31 This property leads to self-limiting growth, which results

INTRODUCTION Germanium (Ge) is a promising candidate for replacing silicon (Si) as p-channel material in metal-oxide-semiconductor fieldeffect transistors (MOSFET) owing to its high hole mobility.1 However, the lack of a stable passivation layer for Ge surface hinders the development of such technology. Unlike silicon dioxide (SiO2), germanium dioxide (GeO2) is water-soluble and also thermally unstable at temperatures usually employed during device processing.2 This instability is due to the interfacial reaction GeO2 + Ge → 2GeO that occurs at temperatures greater than 400 °C. Oxygen vacancies generated at the GeO2/Ge interface diffuse through the oxide toward the surface, where they promote GeO desorption (as evidenced by thermal desorption spectroscopy3), leading to the deterioration of the device’s electrical properties.4 First-principles calculations predicted that these vacancies are likely to be positively charged, resulting in positive fixed charge at the GeO2/Ge interface.5 The instability of the GeO2/Ge interface also interferes with the development of devices based on high-k metal oxide/Ge gate stacks, since a GeOx interlayer can be formed during high-k deposition and subsequent thermal treatments.6,7 The synthesis of GeOx layers and control of their properties are also relevant in other applications such as resistive switching memory.8,9 Several strategies have been employed to increase the stability of GeO2/Ge structures. Lu and co-workers have demonstrated that water resistance and thermal stability can be significantly enhanced by doping the oxide with hafnium and other metals.10 Another method commonly used to improve thermal stability is the incorporation of nitrogen into GeO2.11 The properties of germanium nitride (Ge3N4) and oxynitride (GeOxNy) films produced by plasma nitridation techniques have been investigated © 2016 American Chemical Society

Received: July 26, 2016 Accepted: September 16, 2016 Published: September 16, 2016 27339

DOI: 10.1021/acsami.6b09244 ACS Appl. Mater. Interfaces 2016, 8, 27339−27345

Research Article

ACS Applied Materials & Interfaces in a film thickness of just a few nanometers.31−33 Despite the pronounced effect of N incorporation in SiO2 films, its role in the formation of such a barrier is still under debate. Nitrogen increases film density, which could hinder the diffusion of several chemical species inside the oxynitride.23 Uematsu and coauthors argue that this is very unlikely in the case of thermally grown SiOxNy films since the barrier effect is observed even with a very low nitrogen content (which implies no substantial density modification).34 Some authors suggest the formation of a layer of silicon nitride (Si3N4) at the SiOxNy/Si interface, which would prevent O2 reaction with the substrate.22 However, low-dose N implantation into SiO2 results in a strong barrier against oxidation that cannot be due to Si3N4 formation.35,36 Another explanation is that nitrogen bonding makes the lattice more rigid on an atomic level, diminishing the diffusion of atoms and molecules.23 It is worth noting that N in nitrided SiO2/Si structures is always bonded to three Si atoms and that N−O bonds may not occur as shown by first-principles calculations.30 Some authors believe that the barrier formation could be related to the SiO2 + Si → 2SiO interfacial reaction, which influences the Si oxidation rate, especially at initial growth stages and at the high temperature regime.36,37 Raider states that nitrogen reaction with SiO or the sites at the interface where SiO is generated could result in a barrier against oxidation even at low nitrogen concentrations.36 Uematsu and co-workers proposed that triply bonded nitrogen would hinder Si−O bond reconstruction, reducing SiO diffusivity to the surface.34,38 The accumulation of SiO at the interface would reduce the rate at which the interfacial reaction occurs and thus inhibit film growth. The fact that nitrogen incorporation significantly lowers boron diffusivity inside SiO2 supports these hypotheses, as the formation of oxygen vacancies related defects at the SiO2/Si interface enhances boron diffusion.38−40 This complex mechanistic scenario taking place at the SiO2/Si system can serve as background for understanding the role of N in GeO2 stabilization. Since the GeO2 + Ge → 2GeO reaction is the source of thermal instability in GeO2/Ge structures, N incorporation must have an influence on this reaction. In the present work, we investigated the incorporation of nitrogen in germanium oxide and its role in the improved stability of the resulting dielectric layer. GeOxNy films were produced in a one-step process by direct thermal oxynitridation of the Ge surface using nitric oxide gas (NO), reducing the eventual damage of other nitridation techniques like plasma immersion. Incorporation of a relatively low nitrogen concentration into the oxide led to significant enhancement of the thermal stability of GeO2/Ge structures. This effect is attributed to the reduction of oxygen vacancy diffusivity in the dielectric film.

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Ge samples were oxynitrided in 200 mbar of NO gas enriched to 98% in the 15N rare isotope (15N16O). This allows us to quantify nitrogen content and determine its distribution in the GeOxNy film using nuclear reactions. The choice of NO in place of N2O is to avoid the complex interplay of the decomposition products of the latter during thermal annealings.22 15N concentration profiles were obtained by nuclear reaction profiling (NRP) using the resonance at 429 keV of the crosssectional curve of 15N(p,γα)12C nuclear reaction. To optimize depth resolution, a 45° angle between incident proton beam and sample surface normal was used. Nuclear reaction analysis (NRA) at 900 keV of the plateau region of the cross-sectional curve of the 15N(p,α)12C was employed to determine 15N areal densities. 16O areal densities were obtained using the 16O(α,α)16O resonant elastic scattering at 3.03 MeV. X-ray reflectometry (XRR) measurements using a PANalytical X’Pert diffractometer with a Cu Kα source were used to determine film thickness with 3% accuracy. GeO2 films were deposited on Ge (100) and Si (100) substrates by reactive magnetron sputtering in an AJA Orion-8 UHV system. The deposition was performed at room temperature, using a Ge target and a dc pulsed plasma source (55W). Chamber pressure was 6 mTorr and O2 and Ar flux were set 4.5 and 20 sccm, respectively. Following deposition, samples were submitted to treatments in 15N16O gas. For evaluating film resistance to additional oxidation, GeO2 and GeOxNy samples were submitted to thermal treatment in O2 gas enriched to 97% in the 18O rare isotope (18O2). The use of an isotopically enriched gas allows the distinction between oxygen atoms incorporated during this treatment and previously existent oxygen atoms. 18O concentration profiles and areal densities were obtained by NRP and NRA measurements using the resonance at 151 keV and the plateau at 730 keV of the 18O(p,α)15N nuclear reaction, respectively.

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RESULTS AND DISCUSSION Figure 1 shows Ge areal densities adsorbed on Si samples during the thermal growth of GeO2 and GeOxNy films. The amount of

Figure 1. Ge areal densities adsorbed on Si samples placed above Ge substrates during GeO2 and GeOxNy thermal growth for 1 h in 200 mbar of O2 and NO, respectively, as a function of oxidation/oxynitridation temperature.

Ge desorbed from the oxide surface during oxidation in O2 increases with temperature, as seen in ref 41. Following NO treatments, Ge areal densities were below RBS sensitivity for temperatures under 600 °C. At 600 °C, the amount of Ge detected was 2 orders of magnitude lower than on the analogous sample prepared in O2, evidencing that GeO desorption was greatly suppressed. To determine the amount of nitrogen incorporated during the oxynitridation process, a set of samples was fabricated in 15N16O gas, using two different temperatures: 500 and 550 °C. These temperatures are close to the onset of high vacancy production at the GeO2/Ge interface as evidenced by the results of Figure 1. Thus, one could better probe the effect of nitrogen incorporation in the growth of the dielectric film.

EXPERIMENTAL DETAILS

p-type Ge (100) wafers were cleaned in an ultrasonic acetone bath, followed by a cyclic cleaning procedure using sequential 30 s dips in H2O/HCl(37%) (4:1) and H2O2(30%). Oxynitridation was performed in a conventional resistively heated furnace, using NO gas at different temperatures. To compare the stability of the resulting GeOxNy films with the stability of thermally grown GeO2, oxidations in O2 were also accomplished. Si (100) samples, with native oxide, were placed above the Ge substrates to track GeO desorption during thermal growth of GeOxNy and GeO2 films. Thus, part of the GeO desorbed from the surface of these films was adsorbed onto Si. Areal densities of Ge atoms adsorbed on Si were obtained by Rutherford backscattering spectrometry (RBS) using He2+ ions of 2 MeV. 27340

DOI: 10.1021/acsami.6b09244 ACS Appl. Mater. Interfaces 2016, 8, 27339−27345

Research Article

ACS Applied Materials & Interfaces

Table 1. 16O and 15N Areal Densities (Obtained by NRA), Nitrogen Percentage, and Thickness (Obtained by XRR) for GeOxNy Films Grown in 200 mbar of 15N16O Gas oxynitridation time (min)

16

O 1015 atoms/cm2

15 23 60 120 240

10.1 ± 0.5 10.4 ± 0.5 9.9 ± 0.5 10.5 ± 0.5 14.3 ± 0.7

30 60 120 240

29.0 ± 1.5 34.2 ± 1.7

15

82.4 ± 4.1

N 1015 atoms/cm2

500 °C 1.0 ± 0.1 0.9 ± 0.1 1.1 ± 0.1 1.5 ± 0.1 1.6 ± 0.1 550 °C 3.5 ± 0.2 3.9 ± 0.2 5.1 ± 0.3 4.2 ± 0.2

16 O and 15N areal densities, nitrogen percentage, and thickness obtained for these samples are listed in Table 1. It was observed that nitrogen areal density increases with temperature but is approximately constant with time, where samples grown at 500 °C had 15N areal densities of about 1 × 1015 atoms/cm2. For 550 °C samples, 15N areal densities had a mean value of 4 × 1015 atoms/cm2. Temperature increase also led to higher 16 O areal densities. Nitrogen percentages, in respect to the total sum of 15N and 16O areal densities, presented a mean value of 10%. Neither time nor temperature appears to have any significant influence in nitrogen percentage. Despite the relative low N percentage incorporated in the films, GeO desorption during film growth is strongly suppressed, as shown in Figure 1. Interestingly, Murarka and co-workers observed that SiOxNy films have a greater resistance to additional oxidation when nitrogen percentage is between 10 and 20%.31 It is important to note that previous works show that both Ge and Si oxynitrides are susceptible to N exchange for O at film surface during air exposure which can deplete the N amount originally incorporated by the NO annealing.12,42 Air humidity was shown to accelerate this exchange process in GeOxNy samples.43 X-ray reflectivity measurements performed on samples grown at 500 °C for different times show that after an initial rapid film growth, thickness increases slowly, varying from 4.3 to 6.2 nm in a period of 23−120 min. At 550 °C, thickness saturation is observed after 2 h. In Figure 2, the reflectivity curves of two

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N/(15N + 16O) (%) 9.0 8.0 10.0 12.5 10.0 10.8 10.2 4.9

thickness (nm)

4.3 ± 0.1 5.2 ± 0.2 6.2 ± 0.2

10.3 ± 0.5 18.6 ± 0.6 18.5 ± 0.6

incorporation of both nitrogen and oxygen, maintaining a low nitrogen percentage but increasing thickness. Figure 3 shows the experimental excitation curves of the 15 N(p,γα)12C nuclear reaction of two samples grown at 550 °C

Figure 3. Experimental excitation curves of the 15N(p,γα)12C nuclear reaction for samples grown in 200 mbar of 15NO at 550 °C for 1 and 4 h. Proton energy corresponding to the surface is indicated by the black dashed line. Colored dashed lines indicate the energies corresponding to the GeOxNy/Ge interface in each sample, according to thickness values obtained by XRR.

for two time periods: 1 and 4 h. Measured film thicknesses were 10.3 and 19.9 nm, respectively. Since the depth in the film scales with proton energy and the detected gamma yield is proportional to 15N concentration, the curves represent the concentration versus depth profiles convoluted with instrument and energy loss functions. Assuming GeO2 bulk density (3.6 g/cm3) and stopping power (227.4 keVcm2/mg for 429 keV protons), it was estimated that nitrogen is distributed in a 9.8 nm thick region of the 1 h sample and in 19.6 nm of the 4 h sample. The comparison of these distributions with film thickness leads to the conclusion that nitrogen is spread throughout the whole GeOxNy film (or at least a great extent of it) and not accumulated in a certain region. As in the case of Si oxynitridation, GeO2 structural defects, such as oxygen vacancies produced at the GeO2/Ge interface, should act as nitrogen incorporation sites. To investigate substrate influence in nitrogen incorporation, 18 nm thick GeO2 films deposited over Si and Ge substrates were submitted to thermal treatment in 200 mbar of 15NO at 500 °C for 2 h. The resulting nitrogen distributions are shown in Figure 4. As expected, the amount of 15N incorporated in the GeO2/Ge sample (1.1 × 1015 atoms/cm2) was larger than in the GeO2/Si sample (0.8 × 1015 atoms/cm2). Most of the 15N atoms were

Figure 2. X-ray reflectivity curves of GeOxNy films grown in 200 mbar of 15 16 N O gas for 1 h at 500 and 550 °C. a.u. stands for arbitrary units.

GeOxNy films grown in 15N16O for 1 h are presented. The film grown at 500 °C has a thickness of 5.2 nm. Increasing the temperature to 550 °C led to the formation of a thicker film (10.3 nm). Therefore, a higher temperature results in a higher 27341

DOI: 10.1021/acsami.6b09244 ACS Appl. Mater. Interfaces 2016, 8, 27339−27345

Research Article

ACS Applied Materials & Interfaces

consumption, showing that film growth is saturated. This demonstrates that during film growth there is a competition between N incorporation and removal, resulting in N exchange at the surface. Nitrogen removal has also been reported in the thermal oxynitridation of Si using NO.32 In a mechanism similar to what is proposed for Si, the reaction of NO with the oxynitride structure would result in the release of previously incorporated nitrogen as an inert molecule, such as N2, yielding a dangling bond. This dangling bond acts as dissociation site for incoming NO molecules, leading to the additional incorporation of N and O atoms. Since Ge−O bond (157.3 kcal/mol) is more stable than Ge−N (65 kcal/mol),44,45 oxygen content in the film ends up being much higher than nitrogen. In order to test the resistance to additional oxidation, Ge16O2 and Ge16Ox15Ny films thermally grown on Ge were exposed to 1 atm of 18O2 at 550 °C for 30 min. Thickness prior to 18O2 treatment was approximately 9 nm for both dielectric films. 18O areal density in the GeOxNy sample after 18O2 annealing was 31.2 × 1015 atoms/cm2, almost three times lower than that in the GeO2 sample, which incorporated 90.2 × 1015 18O atoms/cm2. The excitation curves of the 18O(p,α)15N nuclear reaction are displayed in Figure 6. While 18O incorporates in the GeOxNy/Ge structure only to a depth of 11.6 nm from film surface, in GeO2/ Ge sample it was spread to 27.4 nm. Also, in the GeOxNy/Ge sample, most of 18O atoms were incorporated into the film structure, with little substrate consumption. For comparison, the excitation curve of a 9 nm Ge18O2 film grown in 18O2 is shown. The lack of 18O atoms at its surface is due to isotopic exchange during air exposure, as observed in several samples. These results demonstrate that nitrogen incorporation into germanium oxide structure significantly reduces Ge substrate oxidation. Therefore, just as with SiOxNy, GeOxNy films form an oxidation barrier. Additionally, excitation curves of the 15N(p,γα)12C reaction of the Ge16Ox15Ny sample were obtained before and after 18O2 annealing (Figure 6B). After reoxidation, the film maintained 70% of its original nitrogen content. The remaining 15N atoms migrated toward the interface, most likely occupying newly created oxygen vacancies. These results evidence that despite the different mobilities of N and O nitrogen is indeed mobile inside the oxynitride during thermal treatments.

Figure 4. Experimental excitation curves of 15N(p,γα)12C reaction of 18 nm thick GeO2 films, deposited over Ge and Si substrates, after exposure to 200 mbar of 15NO at 500 °C for 2 h. The shaded area indicates energy window corresponding to the deposited 18 nm thick GeO2 layer.

incorporated at defects formed during film deposition on both substrates. However, additional incorporation sites are present in the GeO2/Ge structure due to the interfacial reaction, which produces oxygen vacancies during thermal annealing. Aiming at understanding how nitrogen is incorporated during oxynitridation, clean Ge substrates were submitted to a two-step treatment as depicted in Figure 5. Sample A was oxynitrided in 200 mbar of NO enriched in 15N (15N16O) at 550 °C for 2 h. Sample B was prepared in the same way as sample A followed by an oxynitridation for additional 2 h in natural NO (14N16O) under the same conditions. Sample C was prepared with the same procedure as sample B inverting the gas sequence (first 14N16O, followed by 15N16O). In this way, we could follow incorporation, removal, and transport of nitrogen during Ge oxynitridation by comparing 15N concentration profiles. NRP measurements performed on these samples are shown in Figure 5a. 15N is accumulated at the near surface region in the sample C, annealed last in 15N16O. In the sample B, annealed first in 15N16O, 15N is located at a deeper region and is absent from the surface. For comparison, the excitation curve of sample A, with an estimated thickness of 18.6 nm, is also shown. After the initial 2 h of oxynitridation, there is very little additional substrate

Figure 5. (a) Experimental excitation curves of the 15N(p,γα)12C reaction for samples submitted to sequential oxynitridations: 15N16O followed by 14 16 N O (sample B) and 14N16O followed by 15N16O (sample C). Each step was performed in 200 mbar at 550 °C for 2 h. Sample A was submitted to a single 2 h step in 15N16O. Dashed line indicates the energy corresponding to the surface. (b) Preparation procedure of samples A−C. During film growth, nitrogen is incorporated into structural defects such as oxygen vacancies. Nitrogen removal also occurs, leading to the formation of dangling bonds, which act as additional incorporation sites for N and O atoms. 27342

DOI: 10.1021/acsami.6b09244 ACS Appl. Mater. Interfaces 2016, 8, 27339−27345

Research Article

ACS Applied Materials & Interfaces

Figure 6. (a) Experimental excitation curves of the 18O(p,α)15N reaction for 9 nm Ge16O2/Ge and Ge16Ox15Ny/Ge samples submitted to 1 atm of 18O2 at 550 °C for 30 min. The excitation curve of a 9 nm thick Ge18O2 grown in 18O2 is shown for comparison. Dashed line indicates the energy corresponding to the surface. (b) Experimental excitation curves of 15N(p,γα)12C reaction of the Ge16Ox15Ny sample obtained before and after thermal treatment in 18O2.

Figure 7. Oxygen vacancy diffusivity reduction due to nitrogen insertion in germanium oxide structure results in the lowering of GeO desorption rate from film surface and in vacancy annihilation.

During oxynitride growth, oxygen vacancies produced at the dielectric/Ge interface diffuse toward the film surface and constitute incorporation sites for oxygen and nitrogen (as evidenced in Figure 5). The hindered diffusion of vacancies intensifies vacancy annihilation. Thus, the rate at which the substrate is consumed is lowered, and saturation of film thickness is achieved. A higher temperature results in a higher vacancy flux toward the surface, since vacancy diffusivity increases with temperature. This promotes greater nitrogen and oxygen incorporation and a higher substrate consumption rate as evidenced by our oxynitridation kinetics data. Consequently, the resulting film thickness increases, as observed. Growth tends to saturate when the film becomes thick enough to efficiently block vacancy diffusion to the surface and to suppress GeO desorption. This mechanism is similar to the one proposed by Uematsu and coauthors for the thermal growth of SiOxNy films using N2O gas (in which NO is the nitridating agent), where incorporated nitrogen prevents SiO diffusion to the surface, leading to SiO accumulation at the interface and self-limited film growth.34 One could suggest that the low oxidation rate of the oxynitride films is caused by the densification of the GeO2 structure due nitrogen incorporation, which would hinder the diffusion of interstitial molecules and atoms inside the dielectric. This is very unlikely since nitrogen content is low. In fact, GeOxNy film density obtained by X-ray reflectometry measurements varied between 3.0−3.6 g/cm3. Since GeO2 has 3.6 g/cm3, there was no overall density increase due to nitrogen. Also, since nitrogen is spread in a great extent of the film, the formation of a dense barrier due to its accumulation in some particular region

The suppression of GeO desorption caused by nitrogen insertion into the oxide can be explained by a reduction in oxygen vacancy diffusivity. When the GeO2/Ge structure is heated, oxygen vacancy flux F from the interface to the surface is determined by Fick’s law

F = −D∇C in which ∇C is the concentration gradient and D, the vacancy diffusivity. Under the assumption of a steady state and that vacancy concentration at the interface Cint is much larger than at the surface, GeO desorption rate R, during vacuum annealing, can be written as R=F≈D

C int tox

with tox being the oxide thickness.3 With reduced vacancy diffusivity due to nitrogen, less oxygen vacancies are able to reach the surface and promote GeO desorption. Wang and co-workers demonstrated that when a material that blocks GeO desorption is deposited on top of GeO2 oxygen vacancies tend to be annihilated according to the disproportionation reaction 2GeO → GeO2 + Ge.46 Germanium oxidation is strongly influenced by the formation of oxygen vacancies because they act as incorporation sites for incoming oxygen.41 Since the annihilation process leaves a smaller number of sites available for oxygen incorporation, this explains the greater resistance to additional oxidation that was observed in GeOxNy films when compared to GeO2 films. The consequences of nitrogen incorporation in GeO2 are depicted in Figure 7. 27343

DOI: 10.1021/acsami.6b09244 ACS Appl. Mater. Interfaces 2016, 8, 27339−27345

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ACS Applied Materials & Interfaces

through Selection of High-K Materials and Suppression of GeO Volatilization. Appl. Surf. Sci. 2008, 254, 6100−6105. (8) Prakash, A.; Maikap, S.; Rahaman, S. Z.; Majumdar, S.; Manna, S.; Ray, S. K. Resistive Switching Memory Characteristics of Ge/GeOx Nanowires and Evidence of Oxygen Ion Migration. Nanoscale Res. Lett. 2013, 8, 220. (9) Rahaman, S. Z.; Maikap, S.; Chen, W. S.; Lee, H. Y.; Chen, F. T.; Kao, M. J.; Tsai, M. J. Repeatable Unipolar/Bipolar Resistive Memory Characteristics and Switching Mechanism using a Cu Nanofilament in a GeOx Film. Appl. Phys. Lett. 2012, 101, 073106. (10) Lu, C.; Lee, C. H.; Zhang, W.; Nishimura, T.; Nagashio, K.; Toriumi, A. Structural and Thermodynamic Consideration of Metal Oxide Doped GeO2 for Gate Stack Formation on Germanium. J. Appl. Phys. 2014, 116, 174103. (11) Watanabe, H.; Kutsuki, K.; Kasuya, A.; Hideshima, I.; Okamoto, G.; Saito, S.; Ono, T.; Hosoi, T.; Shimura, T. Gate Stack Technology for Advanced High-Mobility Ge-Channel Metal-Oxide-Semiconductor Devices − Fundamental Aspects of Germanium Oxides and Application of Plasma Nitridation Technique for Fabrication of Scalable Oxynitride Dielectrics. Curr. Appl. Phys. 2012, 12, S10−S19. (12) Maeda, T.; Yasuda, T.; Nishizawa, M.; Miyata, N.; Morita, Y.; Takagi, S. Pure Germanium Nitride Formation by Atomic Nitrogen Radicals for Application to Ge Metal-Insulator-Semiconductor Structures. J. Appl. Phys. 2006, 100, 014101. (13) Hayakawa, R.; Yoshida, M.; Ide, K.; Yamashita, Y.; Yoshikawa, H.; Kobayashi, K.; Kunugi, S.; Uehara, T.; Fujimura, N. Structural Analysis and Electrical Properties of Pure Ge3N4 Dielectric Layers Formed by an Atmospheric-Pressure Nitrogen Plasma. J. Appl. Phys. 2011, 110, 064103. (14) Maeda, T.; Yasuda, T.; Nishizawa, M.; Miyata, N.; Morita, Y.; Takagi, S. Ge Metal-Insulator-Semiconductor Structures with Ge3N4 Dielectrics by Direct Nitridation of Ge Substrates. Appl. Phys. Lett. 2004, 85, 3181−3183. (15) Minoura, Y.; Kasuya, A.; Hosoi, T.; Shimura, T.; Watanabe, H. Design and Control of Ge-based Metal-Oxide-Semiconductor Interfaces for High-Mobility Field-Effect Transistors with Ultrathin Oxynitride Gate Dielectrics. Appl. Phys. Lett. 2013, 103, 033502. (16) Kutsuki, K.; Okamoto, G.; Hosoi, T.; Shimura, T.; Watanabe, H. Germanium Oxynitride Gate Dielectrics Formed by Plasma Nitridation of Ultrathin Thermal Oxides on Ge(100). Appl. Phys. Lett. 2009, 95, 022102. (17) Kutsuki, K.; Hideshima, I.; Okamoto, G.; Hosoi, T.; Shimura, T.; Watanabe, H. Thermal Robustness and Improved Electrical Properties of Ultrathin Germanium Oxynitride Gate Dielectric. Jpn. J. Appl. Phys. 2011, 50, 010106. (18) Bhatt, P.; Chaudhuri, K.; Kothari, S.; Nainani, A.; Lodha, S. Germanium Oxynitride Gate Interlayer Dielectric Formed on Ge(100) using Decoupled Plasma Nitridation. Appl. Phys. Lett. 2013, 103, 172107. (19) Chui, C. O.; Ito, F.; Saraswat, K. C. Nanoscale Germanium MOS Dielectrics - Part I: Germanium Oxynitrides. IEEE Trans. Electron Devices 2006, 53, 1501−1508. (20) Wu, N.; Zhang, Q.; Zhu, C.; Yeo, C. C.; Whang, S. J.; Chan, D. S. H.; Li, M. F.; Cho, B. J.; Chin, A.; Kwong, D. L.; Du, A. Y.; Tung, C. H.; Balasubramanian, N. Effect of Surface NH3 Anneal on the Physical and Electrical Properties of HfO2 Films on Ge Substrate. Appl. Phys. Lett. 2004, 84, 3741−3743. (21) Bom, N. M.; Soares, G. V.; Hartmann, S.; Bordin, A.; Radtke, C. GeO2/Ge Structure Submitted to Annealing in Deuterium: Incorporation Pathways and Associated Oxide Modifications. Appl. Phys. Lett. 2014, 105, 141605. (22) Baumvol, I. J. R. Atomic Transport during Growth of Ultrathin Dielectrics on Silicon. Surf. Sci. Rep. 1999, 36, 1−166. (23) Gusev, E. P.; Lu, H. C.; Garfunkel, E. L.; Gustafsson, T.; Green, M. L. Growth and Characterization of Ultrathin Nitrided Silicon Oxide Films. IBM J. Res. Dev. 1999, 43, 265−286. (24) Lu, H. C.; Gusev, E. P.; Gustafsson, T.; Garfunkel, E. Effect of Near-Interfacial Nitrogen on the Oxidation Behavior of Ultrathin Silicon Oxynitrides. J. Appl. Phys. 1997, 81, 6992−6995.

does not occur. However, the reduced diffusivity can be explained by percolation theory. Nitrogen atoms block percolation paths for vacancies and thus lower their mobility. Uematsu et al. propose that SiO diffusion inside SiOxNy is hindered because N bonding with three Si atoms fixes the SiO2 framework and blocks Si−O bonds reconstruction in a large area.38 The reduction of vacancy diffusivity in GeOxNy, due to an analogous mechanism, allows GeO desorption from the surface to be efficiently suppressed by the incorporation of a low nitrogen concentration, as observed.

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CONCLUSIONS An oxynitride layer was synthesized on Ge by a one-step thermal treatment with NO. N incorporation strongly suppresses GeO desorption, improving significantly the dielectric stability. N depth profiling evidenced that this element is spread throughout the whole oxynitride film, with its incorporation being intervened by the production of vacancies at the dielectric/Ge interface. Besides O transport during thermal growth of the oxynitride layer, N was also confirmed as a mobile species. However, the lower mobility of the later results in the blockage of vacancy diffusion paths. This reduced vacancy diffusivity leads to a greater thermal stability and oxidation resistance of the overall oxynitride lattice in comparison with pure oxide. The final N content in the formed layer is the net result of its incorporation and removal, mechanisms that take place simultaneously during exposure of Ge to NO at high temperatures. Depending on time and temperature of the NO treatment, oxynitride layers with different thicknesses can be obtained. This procedure paves the way to synthesize interlayers with specified characteristics aiming at stable dielectric stacks prepared on Ge.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Tel.: +55 51 33086204. Fax: +55 51 33087304. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We are grateful for the financial support of INCT Namitec, INCT INES, MCT/CNPq, CAPES, and FAPERGS.

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REFERENCES

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DOI: 10.1021/acsami.6b09244 ACS Appl. Mater. Interfaces 2016, 8, 27339−27345