Local anodic oxidation of solid GeO films: The nanopatterning possibilities

Surfaces and Interfaces 6 (2017) 56–59

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Local anodic oxidation of solid GeO films: The nanopatterning possibilities K.N. Astankova a,∗, E.B. Gorokhov a, I.A. Azarov a,b, V.A. Volodin a,b, A.V. Latyshev a,b a b

Institute of Semiconductor Physics SB RAS, Novosibirsk 630090, Russia Department of Physics, Novosibirsk State University, Novosibirsk 630090, Russia

a r t i c l e

i n f o

Article history: Received 29 June 2016 Revised 7 November 2016 Accepted 20 November 2016 Available online 7 December 2016 Keywords: Local anodic oxidation Germanium monoxide Germanium dioxide Nanowires

a b s t r a c t Metastable germanium monoxide (GeO) thin-insulating films have been investigated as a new promising material for oxidation scanning probe lithography. Amorphous GeO films were deposited onto cool Si substrates by thermal evaporation of GeO2 heterostructure in vacuum. Properties of GeO films were studied by means of IR spectroscopy, Raman spectroscopy, atomic force microscopy (AFM) and scanning electron microscopy. The nanopatterning of GeO films included three stages. First, AFMinduced local anodic oxidation of GeO layer was used to obtain GeO2 nanowires on Si substrate. After local anodic oxidation in high voltage (≥9 V) regime at 80% relative humidity, the cross-section profile of fabricated GeO2 protrusions contained anomalously high double peaks on a broad base (“two-story shape”). Then, thermal annealing was employed to decompose the GeO film into a GeO2 matrix and Ge nanoclusters. Third, after selective etching of GeO2 from the decomposed GeO film, trenches remained in the porous Ge layer instead of GeO2 nanowires. This may be a potentially useful lithographic approach for fabricating nanoscale structures. © 2016 Elsevier B.V. All rights reserved.

1. Introduction Optical lithography will reach its limit in near future. At the same time, alternative nanopatterning methods are explored. In parallel to the development of beam-based methods using electrons or ions, scanning probe lithography methods are receiving renewed interest because of their flexibility to handle novel materials [1]. Local anodic oxidation (LAO) using scanning probe microscope (SPM) proved to be a versatile method for surface modification. During this process a surface is scanned in air by a negatively biased SPM tip. Ambient humidity forms a water meniscus enfolding the tip and the surface. The application of a voltage between the SPM tip and the substrate ionizes the water molecules, producing oxyanions (OH− or O− ), and transports them to the surface under the tip, where oxidation occurs. The capabilities of the tipinduced local anodic oxidation in nanometer-scale patterning on semiconductors, metals, self-assembled monolayers and polymers have been demonstrated by many researchers [2–5]. The obtained oxide lines and dots allow fabricating various devices, e.g., single electron transistors [6], electron interferometers [7], single-photon detectors [8] or can serve as a resist mask for selective etching and form high packing density Si-nanostructures [9] etc. Previous

∗

Corresponding author. E-mail address: [email protected] (K.N. Astankova).

http://dx.doi.org/10.1016/j.surfin.2016.11.010 2468-0230/© 2016 Elsevier B.V. All rights reserved.

data show that not only the conductive surface but also a thininsulating film grown on a conductive substrate can be modified by this mean [10,11]. Here, we report on the possibilities of GeO films as a new inorganic material for oxidation scanning probe lithography (o-SPL). Due to structural instability, GeO layers have low thresholds for modification and evaporation processes proceeding under local impacts (thermal, electrical or irradiation) [12,13]. In particular, germanium monoxide not only decomposes into Ge and GeO2 at a temperature of 250 °C but also can desorbs as a gasphase (GeO(gas)) at T > 400 °C [14]. 2. Experimental Two processes were used to prepare thin films of homogeneous stoichiometric germanium monoxide (GeO(s)). The first one was the synthesis of heterolayers consisting of a glassy GeO2 matrix with embedded Ge nanoclusters. GeO2 heterolayers were deposited from supersaturated GeO vapor in a low pressure chemical vapor deposition (LP CVD) flow reactor. The growth procedure was described in more details elsewhere [12,14]. It should be noted that the GeO2 :Ge molar ratio in the films thus obtained was always 1:1. Depending on growth conditions, we were able to produce GeO2 heterolayers either with amorphous Ge nanoclusters or with Ge-nanocrystals, but we were not able to deposit non-decomposed and homogeneous in

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Fig. 1. IR absorption spectrum of as-deposited GeO films.

thickness GeO films. To obtain thin GeO film, a second process was used. A GeO2 heterolayer (400 – 500 nm) on Si substrate was thermally evaporated in a high vacuum (10−7 Torr). The heated GeO2 film sublimates in accordance with the reaction GeO2 (solid) + Ge(solid) → 2GeO(gas) resulting GeO vapor condense onto a Si substrate kept at room temperature with formation of GeO layers. The thickness of the GeO layers thus formed was controlled in situ by a laser scanning ellipsometer (He-Ne, λ = 632.8 nm). The composition and structure of GeO layers were examined by means of IR spectroscopy and Raman scattering spectroscopy. Normal incidence IR absorption measurements were carried out with a spectral resolution of 4 cm−1 . Raman spectra were registered by a triple spectrometer in quasi-back-scattering geometry, with the 514.5 nm Ar+ laser line being used as the pumping source. The surface morphology of modified GeO films was analyzed with atomic force microscopy and scanning electron microscopy (SEM). The local anodic oxidation was performed using an atomic force microscope operating in contact mode and a conductive cantilever with a heavily doped diamond-like coating (tip curvature radius ∼50 nm, constant force ∼11.5 N/m, resonance frequency 190 – 325 kHz). The sample voltage was varied from +6 to + 10 V. The relative humidity (RH) was kept 80% during the experiment. After LAO process, the GeO layer was annealed in a diffusion oven with inert atmosphere. 3. Results and discussion First, one should confirm that the metastable GeO films obtained in the second process were not decomposed into Ge and GeO2 . Fig. 1 presents the IR absorption spectrum of as-deposited thick GeO layer (55 nm) in the range of 400 – 1200 cm−1 . Two intense Ge–O bands are observed at 530 cm−1 and 820 cm−1 . They are assigned to the Ge–O–Ge bending vibration mode and to the stretching vibration mode of the same group, respectively [13,15]. Raman scattering spectroscopy is a very efficient method to observe the presence of Ge in amorphous or crystalline phase. Bulk crystalline Ge is characterized by a narrow intense band at 301 cm−1 , which corresponds to the scattering of photons by optical phonons. In the case of amorphous Ge, the disorder changes the vibrational density of states and the Raman spectrum is characterized by a broad band at 275 – 280 cm−1 [16]. The Raman spectrum of the as-deposited sample showed no peaks (amorphous or crystalline) caused by the Raman scattering at Ge–Ge bond vibrations (Fig. 2, curve 1). This means that the as-deposited GeO films did not contain pure Ge clusters. These clusters (amorphous or crystalline) appear in metastable GeO films as a result of heatinduced decomposition: 2GeO(solid) → GeO2 (solid) + Ge(solid) (see curves 2 and 3 in Fig. 2).

Fig. 2. Raman spectra of GeO films: 1 − as-deposited; 2 − annealed at 300 °C during 5 min.; 3 − annealed at 530 °C during 5 min.

The nanopatterning of GeO layers included three stages: (1) AFM-tip-induced local anodic oxidation of GeO layer resulting GeO2 nanowires formation: GeO(solid) + H2 O → GeO2 (solid) + H2 ; (2) Decomposition of GeO layer into a GeO2 matrix and Ge nanoclusters by thermal annealing: 2GeO(solid) → GeO2 (solid) + Ge(solid); (3) Selective etching of GeO2 from the GeO2 heterolayer leading to the formation of trenches in porous Ge layer (instead GeO2 nanowires). For o-SPL homogeneous non-decomposed GeO layer of about 9 nm thick on Si substrate was prepared. According to AFM data, the average roughness of the layer did not exceed ∼0.4 nm. When a certain voltage is applied to the Si sample with the thin-insulating GeO layer as keeping a conductive cantilever at a position in contact with the surface, a GeO2 pattern was formed by anodic oxidation: GeO + H2 O → GeO2 + H2 . Fig. 3(a) shows the variation in height (0.7 – 15 nm) and width (90 – 500 nm) of the oxide lines fabricated by applying various sample voltages (6 – 10 V) and pulse duration (9 – 130 ms). The written lines are topographically protruded since the molar volume of GeO2 was larger than that of GeO. At low sample voltage (<9 V) and RH = 80%, the crosssection of oxide lines had domelike shape with low aspect ratio height/width (Fig. 3(b)). When the sample voltage increases to ≥9 V and RH > 60%, the form of oxide pattern was reported to exhibit a narrow peak on a broad base, called a “two-story shape” [17]. Due to a large electric field under the tip apex and a high concentration of oxyanions, the growth rate of the oxide is large enough to form a narrow peak at the center of oxide line. The increase in the oxide width is caused by lateral diffusion of oxyanions in the oxide/water bridge interface [18]. In our case, at a high sample voltage (≥9 V) and RH = 80%, “two-story shaped” GeO2 protrusions were observed too, but there appeared double peak on a broad base. We suppose that, at high voltage (≥9 V), the high electrostatic field in the vicinity of the tip induced desorption of GeO film in the central part of the oxide line: GeO(solid) → GeO(gas). Furthermore, many studies of SPM oxidation of thin-insulating films performed by different groups have demonstrated that the oxide height usually did not exceed the initial thickness of the layer under oxidation in similar conditions [10,11,17,18]. In our case the oxide height exceeded 10 nm due to peaks and reached 18 nm in some places, although the initial thickness of the GeO layer was 9 nm (Fig. 3(b), (c)). The growth mechanism of GeO2 nanowires with high peaks remains unclear.

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Fig. 3. (a) Topographic 2D AFM image of GeO2 line array. (b) Cross-section profile along A-B line. (c) 3D AFM image of GeO2 line array. Oxidation conditions were varied for each line, the parameters being written in the image.

Fig. 4. SEM image of porous Ge layer (100 nm) formed after selective etching of the GeO2 matrix in the GeO2 heterolayer: (a) – on the surface, (b) – in the bulk.

After the local anodic oxidation, the GeO layer with the GeO2 line array was annealed in a diffusion oven with inert atmosphere at 300 °C during 2 min. Thermal annealing leads to the decomposition of the GeO layer into a GeO2 matrix and Ge nanoclusters embedded in this matrix (GeO2 heterolayer): 2GeO(solid) → GeO2 (solid) + Ge(solid). Finally, the GeO2 heterolayer with the GeO2 line array was immersed into deionized water to form the patterns. After selective etching of the GeO2 matrix, the Ge nanoclusters agglomerated to form a thin layer (∼6 – 7 nm) on the Si substrate. SEM images shown in Fig. 4(a) and (b) demonstrate the surface morphology and the volume structure of a similar 100 nm-thick Ge layer. We have found that layer was continuous spongy film with a skeletal structure formed from amor-

phous Ge nanoclusters. Annealing (550 °C) in an inert atmosphere enhances the cohesion and causes crystallization of Ge nanoclusters. In places where GeO2 nanowires were removed from the GeO film, trenches remained in porous Ge layer (Fig. 5). The lateral resolution of trenches varied between 10 and 300 nm. This result supports the previous suggestion that a germanium monoxide film can be converted into dioxide by AFM-induced oxidation, because among the various dielectrics (GeO, GeO2 , SiO2 , Si3 N4 , Ge3 N4 ) only GeO2 is soluble in water and can be easily removed. The heights (h) of protruded parts and the depth (d) of the buried parts of GeO2 lines depending on the sample voltage and pulse duration are given in Table 1. The volume of the buried oxide can be con-

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Acknowledgments This research was partly supported by the Russian Foundation for Basic Research (grant No. 16-07-00975) and by the Russian Science Foundation (grant No. 14-22-00143, experiments on GeO deposition). One of the authors (K.N.A.) is grateful to Dr. T.A. Gavrilova for the SEM study of GeO layers and to K.V. Bulah (MIPT) for AFM modifications of GeO films. The authors thank Dr. V.A. Stuchinsky for the proofreading the article. References

Fig. 5. Topographic AFM image of trenches in a porous Ge layer formed after selective etching of GeO2 in the GeO2 heterolayer. Table 1 The heights (h) of the protruded parts and the depth (d) of the buried parts of GeO2 lines and the conversion ratio [(h + d)/d] depending on the sample voltage and pulse duration. The data were measured from the line profiles (5 point per lines) and then averaged. Voltage (V),

6.5

7

7.5

8

9

10

10

pulse duration (ms) h (nm) d (nm) (h + d)/d

9 0.7 0.5 2.4

14 1.2 1 2.2

20 1.4 1.2 2.2

26 1.8 1.3 2.4

40 2.8 2.4 2.2

65 8 3 3.7

130 15 5.5 3.7

sidered as the volume of consumed GeO which was converted into GeO2 . The sum of the protruded and buried parts can be assumed as the volume of the oxide after conversion. The conversion ratios of GeO2 volume to consumed GeO volume [(h + d)/d] at low voltage (<9 V) were found to be a constant value of 2.3, indicating a complete conversion of monoxide. The developed surface relief of the Ge layer is probably due to the contaminations emerged during the GeO2 etching process. Further optimization of the o-SPL described is possible by varying the process parameters (tip curvature radius of the cantilever, humidity, voltage, pulse duration etc.). 4. Conclusions It was demonstrated that AFM-induced local anodic oxidation is a very effective way for converting GeO into GeO2 . When a high sample voltage (≥9 V) was applied at high RH (80%), the crosssection profile of GeO2 protrusions exhibited a “two-story shape” with an anomalously high double peak. Such a configuration of the peak may be associated with field-induced local desorption of GeO layer at high sample voltage. After decomposition of GeO film and dissolution of GeO2 , the oxide lines converted into trenches formed in the porous Ge layer. The nanopatterning described offers lithographic approach potentially useful for fabrication of the nanoscale structures. Since GeO2 is a blue luminescent material, then the formation of GeO2 nanowires can be of interest for future optical applications.

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