Water-Erasable Memory Device for Security Applications Prepared by

Jan 12, 2018 - We have investigated the atomic layer deposition (ALD) of GeO2 thin films that dissolve in water rapidly and have excellent electrical ...
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Article Cite This: Chem. Mater. 2018, 30, 830−840

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Water-Erasable Memory Device for Security Applications Prepared by the Atomic Layer Deposition of GeO2 Chang Mo Yoon,†,∇ Il-Kwon Oh,†,∇ Yujin Lee,† Jeong-Gyu Song,† Su Jeong Lee,‡ Jae-Min Myoung,‡ Hyun Gu Kim,§,∥ Hyoung-Seok Moon,⊥ Bonggeun Shong,# Han-Bo-Ram Lee,*,§,∥ and Hyungjun Kim*,† †

School of Electrical and Electronic Engineering and ‡Department of Materials Science and Engineering, Yonsei University, Seoul 03722, Korea § Department of Material Science Engineering, Incheon National University, Incheon 22012, Korea ∥ Innovation Center for Chemical Engineering, Incheon National University, Incheon 22012, Korea ⊥ Energy Plant R&D Group, Korea Institute of Industrial Technology (KITECH), Busan 46938, Korea # Department of Chemical Engineering, Hongik University, Seoul 04066, Korea S Supporting Information *

ABSTRACT: We have investigated the atomic layer deposition (ALD) of GeO2 thin films that dissolve in water rapidly and have excellent electrical properties for use in memory devices. The growth characteristics based on surface reactions during the ALD process are discussed by correlation with experimental results and atomistic theoretical calculation. Compared to sputtered GeO2 films, the ALD-grown GeO2 is perfect, pure, and water-soluble at room temperature and has better electrical properties for use as the dielectric layer in memory devices. The superior film properties of ALD GeO2 are attributed to the higher film density, high purity, low roughness, and highly stoichiometric film composition. Finally, we demonstrate the fabrication of charge-trapping memory (CTM) devices with ALD GeO2, and that the electrical information stored in the CTM can be eliminated immediately by exposure to one droplet of water at room temperature. Thus, ALD GeO2 could find widespread application in the fabrication of secure memory devices.

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high magnitude of the reading current for a flash cell in downscaled memory. SiO2 was selected as the first dielectric material for the blocking layer, but the large tunneling current through the ultrathin SiO2 layers is unacceptable with continued scaling down. Therefore, attempts have been made to replace SiO2 by new dielectrics with higher dielectric constants than SiO2 such as Al2O3,4 Gd2O3,5 LaAlOx,6 and YAlOx.7 GeO2 could be a good blocking layer material, similar to SiO2, because of its higher dielectric constant (k ≈ 6)8 than that of SiO2 (k ≈ 3.9) and its reasonable band gap (ca. 6 eV).9 Despite the superior properties of GeO2, it has not been widely used because it is not stable in water-rich environments; in particular, it has a high etching rate in water of 100 nm/s.10 However, if GeO2 were applied as the blocking layer of the SONOS structure, a watererasable device in the bottom level could be fabricated for security purposes. In other words, by etching out the GeO2 blocking layer, the single SONOS structure is destroyed; thus, the stored data is erased in the bottom device level. Germanium oxides have several oxide phases, such as Ge2O, GeO, Ge2O3, and GeO2; however, only GeO2 is etched by water.11 Thus, highly stoichiometric GeO2 films with few defects and vacancies

INTRODUCTION Nowadays, because most information is processed and stored in electronic devices, such as smartphones and computers, the importance of data security has increased. Although software security systems are well-developed, at a device’s end of life, there is no alternative method for data security except for physical destruction.1 The data stored in a hard disk drive (the former generation of data storage devices) can be totally erased by inducing a strong magnetic field. However, the data stored in a flash disk drive memory or a solid-state drive (SSD) memory (the current generation of data storage devices) cannot be erased by magnetic degaussing because they are based on flash memory, which operates by electrical erasure and reprogramming. Thus, a convenient, easy, and quick method to physically erase all the data stored in flash memory devices is required to prevent data recovery by unauthorized people. Currently, flash memory, such as charge-trapping memory (CTM), is the most widely used type of nonvolatile memory (NVM) device for data storage because of its excellent scaling down and high performance.2 Most CTM devices have a silicon−oxide−nitride−oxide−silicon (SONOS) structure,3 and the three ONO layers act as the tunneling oxide layer, nitride charge-trapping layer, and oxide blocking layer, respectively. Of these layers, the blocking layer is the most important because of its strong effect on the programming/erasing speed and the © 2018 American Chemical Society

Received: October 17, 2017 Revised: January 11, 2018 Published: January 12, 2018 830

DOI: 10.1021/acs.chemmater.7b04371 Chem. Mater. 2018, 30, 830−840

Article

Chemistry of Materials

chamber by Ar carrier gas, and the flow rate (50 sccm) was controlled by a mass flow controller (MFC). Ar gas at the same flow rate was also used to purge excess gas molecules and byproducts between each precursor and reactant exposure step. O3 was used as a counter reactant for O3-ALD, and its concentration (8%) was controlled by an ozone generator and ozone sensor (MKS AX 8200, Applied Materials). The substrate temperature was maintained at 200 °C for both Ge precursors. A p-type Si(100) wafer with low B doping concentration was used as the substrate. The substrate was cleaned in Radio Corporation of America (RCA) solution (1:1:5 = NH4OH/ H2O2/H2O by volume) at 70 °C for 10 min, followed by dipping in a buffered oxide etchant solution (100:1 = H2O/HF) for 30 s to remove the native oxide. The growth characteristics of the ALD GeO2 on Si substrates were systematically investigated by changing precursor exposure time (ts), cycle number, and growth temperature (Ts) to find the optimal conditions for ALD growth based on the fixed oxidant exposure time (tr) and purging time (tp). For a comparative study, ALD SiO2 and sputtered germanium oxide films were also deposited. For the deposition of ALD SiO2, diisopropylamino-silane (DIPAS) was used as a Si precursor, and O2 plasma was used as reactant gas by using the same tool as in the ALD GeO2 case. O2 plasma was generated by using a 100 W radio frequency (RF) plasma power source. In the ALD SiO2 process, ts = 2 s with tp = 5 s and tr = 2 s. The substrate temperature was maintained at 200 °C for all ALD SiO2 experiments. For the deposition of sputtered germanium oxides, a commercial sputter chamber (RSP5003, SNTek Co.) was used, and GeO2 films were deposited by a direct current (dc) magnetron sputterer with 50 W of plasma power. Density functional theory (DFT) calculations were carried out using Orca 3.0.3 software.23 Geometry optimizations were performed using B3LYP-D3 functional24 with def2-SVP basis set.24,25 Then, singlepoint energies of the optimized structures were obtained with DSDBLYP-D3 functional26 and def2-TZVP basis set. All calculations employed the RIJCOSX approximation for efficiency.27 The germanol group of the hydroxylated GeO2 surface was modeled as the GeH3OH molecule because it is known that the adsorption energies are little dependent on the choice of the oxide cluster model.28 Indeed, the calculated adsorption energies on a larger edingtonite-like Ge5O8H7OH cluster were within 3 kcal/mol of those on GeH3OH (see the Supporting Information, Table S2), validating the use of a minimal cluster. The thicknesses of the GeO2 films were measured by ellipsometry (Elli-SE-F, Ellipso Technology). The chemical compositions of the GeO2 films were analyzed by X-ray photoelectron spectroscopy (XPS; K-Alpha model, Thermo Scientific Co.) with a 1486.6 eV Al Kα monochromatic source. Surface cleaning was performed by using Ar sputtering for 20 s to remove surface contaminants before XPS analysis. The film density and roughness of the GeO2 films on Si were analyzed by X-ray reflectivity (XRR) with Cu Kα radiation (Smart Lab, Rigaku). XRR simulation was performed using GlobalFit software. We obtained the best-fit results by using a two-layer model (consisting of GeO2/Si substrate) instead of a three-layer model (consisting of GeO2/interlayer/Si substrate). The goodness of fit for the two-layer model of all GeO2 films was 0.999, which is higher than those of the other models. To analyze the etching properties, we deposited 50 nm thick ALD GeO2 layers using Ge(OnBu)4 or Ge(NMe2)4 on transparent glass samples. The 50 nm thick GeO2 films were etched by deionized (DI) water at room temperature (RT) for 1 s. Then, we dried the samples in the ambient air. The optical properties of the etched films were evaluated by an ultraviolet−visible spectrophotometer (UV−vis−NIR, V-670, JASCO). For analysis of the chemical composition of the etched residue, the samples were analyzed by an energy-dispersive X-ray spectroscopy (EDS) device equipped to a field emission scanning electron microscope (FE-SEM; JEOL JSM-7001F, JEOL Ltd.). The morphologies and roughness of the GeO2 films were characterized using atomic force microscopy (AFM; VEECO Co., Multimode model). The interlayer and crystallinity of the GeO2 film were measured by transmission electron microscopy (TEM; JEMARM200F of JEOL).

are essential for its application as a water-soluble layer, as well as a blocking layer. There are several techniques to form GeO2 thin films, such as sputtering,12 thermal oxidation,13−16 and chemical vapor deposition (CVD).17 However, these methods are not suitable for the fabrication of down-scaled CTM devices because of their low conformality and poor thickness controllability and uniformity. Moreover, most reported germanium oxides fabricated by these methods have a mixed chemical composition of GeO and GeO2. In contrast, atomic layer deposition (ALD) has many benefits for nanoscale device fabrication, such as atomic scale thickness control, excellent conformality, and low impurity contamination, which are significant characteristics for its use as a blocking layer, as well as a sacrificial layer.18,19 Moreover, the ALD process produces highly stoichiometric GeO2 films; for example, GeO2 thin films grown by ALD with 1,2-bis[(2,6-diisopropylphenyl)imino]-acenaphthene-germanium and O3 show excellent stoichiometry.20 To date, although there have been several reports on the preparation of GeO2 by ALD,20,21 ALD GeO2 has been investigated only as a passivation layer for logic device applications. In this article, we report the systematic investigation of the growth characteristics, films properties, and electrical properties of ALD GeO2 for water-soluble memory device applications. ALD GeO2 processes were developed by using two new precursors, tetrakis(dimethylamino)-germanium (Ge(N(Me)2)4) and germanium-n-butoxide (Ge(OnBu)4). The fundamental electrical properties of ALD GeO2 in metal oxide semiconductor (MOS) capacitors were evaluated, and the disposal of the stored data in the CTM device structure using ALD GeO2 was investigated after exposure to water. We believe that the water solubility of memory devices based on ALD GeO2 dielectrics could be utilized to enhance device security and play a significant role in the field of transient electronics.

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EXPERIMENTAL SECTION

For this study, the Ge(N(Me)2)4 and Ge(OnBu)4 precursors were provided by Air Liquide and used as Ge precursors. For identification of the appropriate evaporation temperatures, simultaneous thermogravimetric analysis (TGA, Mettler Toledo) and differential scanning calorimetry (DSC, 1 STARe System) measurements were carried out. The measurements were performed in an inert atmosphere (O2 and H2O < 0.5 ppm), and the temperature ramp was set to 10 °C/min with an internal N2 flow of 180 standard cubic centimeters per minute (sccm) from 25 to 500 °C. The analyses were performed in an open cup at 20 Torr. The pump was directly plugged to the exhaust of the TG and DSC devices to reduce the pressure, and stability was maintained using a needle valve. A negligible residual mass was observed for both precursors, indicating clean evaporation without the generation of any nonvolatile residues over a wide range of temperatures (see the Supporting Information, Figure S1a). Faster evaporation was observed for Ge(NMe2)4 as compared to the Ge(OnBu)4 precursor. Thus, to achieve the same vapor pressure (ca. 30 mTorr) of the Ge precursors, which is required for the ALD reaction, the heating temperatures of the bubbler canisters of Ge(NMe2)4 and Ge(OnBu)4 were maintained at 50 and 105 °C, respectively. In addition, sharp endothermic peaks were observed at 420 °C for Ge(OnBu)4 and 470 °C for Ge(NMe2)4, where the thermal decomposition of both precursors occurred (see the Supporting Information, Figure S1b). We used a commercial ALD chamber (Lucida M100-PL, NCD Co.) with a double showerhead for good uniformity.22 Ge(N(Me)2)4 and Ge(OnBu)4 were evaporated at 50 and 105 °C, respectively, in stainless-steel bubblers to obtain sufficient vapor pressure (∼30 mTorr). The vaporized precursors were transported into the reaction 831

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Figure 1. O3-ALD GeO2 growth characteristics using Ge(OnBu)4 and Ge(NMe2)4 at Ts = 200 and 275 °C, respectively. (a) Function of ts under the condition of tr = 2 s and tp = 5 s. (b) Function of the number of ALD cycles and (c) function of growth temperature under the condition of ts = 2 s, tr = 2 s, and tp = 5 s. (d) DFT-calculated adsorption energies of the precursors on the hydroxylated GeO2 surface: −1LH, removal of one hydrogenated ligand; −2LH, removal of two hydrogenated ligands. The electrical properties based on capacitive−voltage (C−V) and current−voltage (I−V) characteristics were evaluated using a 590 C−V analyzer (Keithley) and a 4155C semiconductor parameter analyzer (Agilent), respectively. The C−V characteristics were measured at 1 MHz by sweeping the gate voltage from −5 to 5 V, and I−V characteristics were measured at a range of gate voltages from −5 to 5 V. For the MOS capacitor fabrication, the ALD and sputtered dielectric films were deposited on a p-type Si (100) substrate. The Ru top electrode was deposited by dc magnetron sputtering with 30 W of plasma power (RSP5003, SNTek Co.). The thickness of the Ru top electrode was 100 nm, and a patterned shadow mask was used to define a contact area of 100 μm in diameter. The interface trap density (Dit) was calculated via a conductance method. More detailed descriptions of the electrical evaluation can be found in previous reports.29,30 For the analysis of the charging characteristics, an MOS structure with ALD-GeO2-based SONO layers was fabricated. By using plasma-enhanced CVD with SiH4 and O2 plasma, 10 nm thick SiO2 layers were deposited on the Si substrate, followed by 15 nm thick SiNx layers using the same method with SiH4 and NH3 plasma (HiDep PE-CVD model, BMR Technology). On top of the SiNx/SiO2/Si substrate, we deposited a 40 nm thick GeO2 layer using the developed ALD process and Ge(OnBu)4. A 100 nm thick and 100 μm long Ru gate was fabricated on the ALD GeO2 by sputtering with a shadow mask (RSP5003, SNTek Co.). We demonstrated light emitting diode (LED) electronic circuits composed of 100 nm thick patterned Ru conducting lines on 40 nm thick GeO2 layers, which were deposited on the Si substrate. For the analysis of the water-soluble properties of the CTM devices and LED electronic circuits, one droplet of water was dropped onto the circuit for 1 s at RT.

of Ge(OnBu)4 (2 s), and the saturated GPC of Ge(NMe2)4 (0.45 Å/cycle) is higher than that of Ge(OnBu)4 (0.3 Å/cycle). Figure 1b shows the film thicknesses measured as a function of the growth cycle for both precursors. The plots of thickness versus the number of ALD cycles are well-fitted by a linear fitting model passing through the origin. This indicates that there was no nucleation incubation, which is advantageous for ALD precursors. Figure 1c shows the temperature-dependence curves of ALD GeO2 for both precursors; the curves have similar shapes. A temperature region in which the GPC was almost unchanged was observed from 180 to 230 °C for the Ge(NMe2)4 precursor and from 200 to 330 °C for the Ge(OnBu)4 precursor; this is typical of ALD growth because of the insensitivity of the surface adsorption sites to temperature. These temperature regions are usually called the ALD window, where Ge(NMe2)4 has an ALD window at temperatures lower than that of Ge(OnBu)4. For temperatures below the minimum ALD window, the GPCs of both ALD techniques decreased with decreasing Ts because there was insufficient thermal energy for surface adsorption and reaction.32 In the hightemperature region over 250 °C, the GPC of Ge(NMe2)4 increased with increasing Ts. In this region, the precursor molecules are thermally decomposed because of the high thermal energy of the substrate, increasing the GPC relative to that in the ALD window region.32 Although the thermal decomposition temperatures of Ge(NMe2)4 and Ge(OnBu)4 are 420 and 470 °C, respectively, as shown in the Supporting Information, Figure S1b, they can be lowered by the addition of surface hydroxyl groups, which allows the decomposition of molecules on the substrate at lower temperatures than that in the canister. The adsorption energetics of the two Ge precursors were investigated by DFT calculations, as shown in Figure 1d. The calculated energies of adsorption for each reactant are

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RESULTS AND DISCUSSION Figure 1a shows the growth per cycle (GPC) of O3-ALD GeO2 films for both Ge precursors, Ge(NMe2)4 and Ge(OnBu)4, as a function of ts at a Ts of 200 °C. The saturation behavior of GPC is a typical growth characteristic of ALD, indicating that both precursors are suitable for the ALD process.31 The exposure time for the saturation of Ge(NMe2)4 (1 s) is shorter than that 832

DOI: 10.1021/acs.chemmater.7b04371 Chem. Mater. 2018, 30, 830−840

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GPC because excess Ge precursors cannot react with the preadsorbed Ge precursors on the surface. The process steps before and after the saturation of oxidant exposure are also similar. In addition, we observed different exposure times of the precursors for GPC saturation, as shown in Figure 1a. The exposure time for saturation could be affected by vapor pressure, mobility, and reactivity of the precursors.31 Because the same vapor pressure was used for both Ge precursors, their different mobilities and reactivities can result in different exposure times until GPC saturation. The mobility of the precursors affects the precursor exposure times for saturation because the precursors require time to arrive from the canister to the surface of samples, and mobility affects the number of impingements on the surface of Si substrates. The mobility of the molecules is inversely proportional to the molecular weight, which is 365.05 and 248.94 g/mol for Ge(OnBu)4 and Ge(NMe2)4, respectively. Therefore, the Ge(OnBu)4 precursor requires longer time to arrive at the surface of the substrate than the Ge(NMe2)4 precursor. Moreover, we observed that the reactivity of Ge(NMe2)4 to the substrate is much greater than that of Ge(OnBu)4; thus, the Ge(NMe2)4 molecules can react more rapidly with surface hydroxyl groups compared to Ge(OnBu)4 until saturation. Additionally, the wider range of process temperatures of Ge(NMe2)4, especially below 180 °C, as compared to that of Ge(OnBu)4 can also be explained by the higher reactivity. Therefore, based on the correlation of experimental results and DFT calculation, we concluded that the higher GPC results from the smaller projected area, whereas the lower exposure times for saturation and the wider range of process temperatures of Ge(NMe2)4 than those of Ge(OnBu)4 are caused by the higher reactivity and mobility. To investigate the etching properties, we deposited 50 nm thick GeO2 films using Ge(OnBu)4 and Ge(NMe2)4 on transparent glass substrates. In addition, as a control sample, we deposited 50 nm thick germanium oxides by using the sputtering technique. To fully etch the GeO2 films, we carried out the etching for 1 s at RT because the etching rate of GeO2 films by water is more than 60 nm/s (see the Supporting Information, Figure S2). Figure 2a,b shows images of the glass substrates after etching the ALD GeO2 films prepared from Ge(OnBu)4 and the germanium oxide films prepared by sputtering, respectively. In contrast to the clear transparent glass image seen in the ALD GeO2 case, a blurred image of the glass substrate was observed for the sputtered germanium oxides. Figure 2c shows the transmittance of the etched GeO2 films on glass. The transmittance of the bare glass substrate is ca. 91% from 350 to 675 nm, where the same values were observed at five points over the glass substrate. The transmittance values of the etched ALD GeO2 films for both Ge precursors on glass are similar (90−91%) to that of bare glass. However, lower transmittance values of less than 80% were observed for the sputtered germanium oxide thin films compared to those for ALD GeO2. To quantitatively analyze the etching residue, we measured the atomic percentage of Ge on the glass substrates after etching the ALD GeO2 films prepared from Ge(OnBu)4 and Ge(NMe2)4, and sputtered germanium oxide films by using EDS line spectra over 10 mm, as shown in Figure 2d. We observed almost no Ge signal for the whole region after etching both ALD GeO2 films. In contrast, we observed significant quantities of Ge on the glass, consistent with the blurred image of Figure 2b. Because germanium oxide phases other than GeO2 are not completely etched by water,

summarized in the Supporting Information, Table S1. The attachment of the Ge precursor to surface germanol, accompanied by the removal of hydrogenated ligands [(GeO)xGeL4−x + GeOH → (GeO)x+1GeL3−x + LH; x = 0, 1], was considered the model reaction during the precursor pulse because reactive surface hydroxyl moieties can be generated by oxidative ozone pulses.33 We found that the exothermicity in the reaction of Ge(NMe2)4 is significantly larger than that of Ge(OnBu)4 by 7−8 kcal/mol per ligand removed, paralleling the higher reactivity of Ge(NMe2)4. Such a difference between the two precursors can be explained by the bond dissociation energies (BDEs) of Ge−N and Ge−O. During the surface reaction described above, the types of individual bonds do not change for Ge(OnBu)4, while the N− Ge and O−H bonds are substituted by O−Ge and N−H bonds in the reaction of Ge(NMe2)4. The BDE of O−Ge is significantly larger than that of N−Ge (ΔE ≈ 8 kcal/mol), and the BDEs of N−H and O−H are similar.34 Therefore, the formation of more stable bonds stabilizes the Ge(NMe2)4 adduct, while a small exothermicity was observed for Ge(OnBu)4. Moreover, because the reactions of both precursors would involve structurally similar proton transfer transition states, it can be assumed that their activation barriers would also follow similar trends, per the Brønsted−Evans−Polanyi relationship.35 On the other hand, the saturation coverage of each precursor would be inversely related to the projected surface area of the adsorbed GeL3 species, assuming there is a sufficient number density of hydroxyl groups present on the surface. The van der Waals radius of Ge(OnBu)3 of r ≈ 8.5 Å is larger than r ≈ 4.7 Å of Ge(NMe2)3, corresponding to the lower GPC of Ge(OnBu)4. Therefore, we conclude that the reactivity of Ge(NMe2)4 is higher than that of Ge(OnBu)4 because of the different types of Ge−ligand bonds in the two precursors. Generally, the growth characteristics, such as GPC and the time for saturation during the ALD process, are affected by the amounts of adsorbed precursors and the reactivity of the precursors. The amounts of adsorbed precursors are mainly determined by the steric hindrance of the ligands of the precursor compounds.36 Because the ALD reaction is based on the self-saturation of the adsorbed precursor, the other factors such as the reactivity and mobility of the precursors rarely affect the amount of adsorbed precursor under the same thermodynamic conditions (process temperature and pressure). Precursor molecules that are already adsorbed on the surface add steric hindrance, preventing the occupation of nearby adsorption sites of other precursor molecules; thus, the GPC differs depending on the size of ligand and the projected area of the molecule.36 Because the van der Waals radius of Ge(OnBu)3 and the projected area of Ge(OnBu)4 are, respectively, much larger than those of Ge(NMe2)3 and Ge(NMe2)4 (see the Supporting Information, Table S2), the steric hindrance effects of Ge(OnBu)4 are more significant than those of Ge(NMe2)4. Thus, the GPC of Ge(NMe2)4 is much greater than that of Ge(OnBu)4. Therefore, the ratio of the GPC between Ge(NMe2)4 and Ge(OnBu)4 is almost the same as the average reciprocal ratio of maximum and minimum projected area of the Ge precursors (see the Supporting Information, Table S2). In contrast, until saturation, when all available reactive groups are occupied by Ge precursors, the reactivity of the Ge precursors only governs the ALD elimination and exchange reactions of the ligands,37 affecting the GPC little. After saturation, the reactivity does not affect the 833

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and Ge(NMe2)4 and sputtered germanium oxides. The XPS spectrum of the Ge 3d core level in Figure 3a shows two main peaks at 30 and 31.7 eV corresponding to Ge2+ in GeO and Ge4+ in GeO2, respectively.29 Smaller ratios of Ge2+ bonding to Ge4+ bonding were observed in the GeO2 prepared by ALD compared to GeO2 prepared by sputtering (0.09 for both ALD GeO2 films with Ge(OnBu)4 and Ge(NMe2)4 and 0.13 for sputtered GeO2 films). Figure 3b shows the O 1s XPS data of the ALD and sputter GeO2 films. In the O 1s core-level spectrum shown in Figure 3b, two peaks were observed at 529.4 and 531 eV, which correspond to GeO and GeO2 bonds, respectively.29 Similar to the Ge 3d spectra shown in Figure 3a and the O 1s spectra, the ALD GeO2 films have a much smaller ratio of GeO bonding to GeO2 bonding compared to the sputtered germanium oxide films (0.04 and 0.05 for ALD with Ge(OnBu)4 and Ge(NMe2)4, respectively, and 0.12 for sputter). Both ALD processes using Ge(OnBu)4 and Ge(NMe2)4 produced highly stoichiometric GeO2 films, where the Ge to O ratio was 1:1.96 for Ge(OnBu)4 and 1:1.95 for Ge(NMe2)4. In contrast, sputtering yielded less stoichiometric germanium oxide films (Ge/O = 1:1.87). Although GeO2 dissolves well in water at room temperature, GeO does not dissolve in water at all. Thus, the formation of GeO bonding is undesirable for the clean etching of the GeO2 films. Because the ALD GeO2 films have fewer GeO bonds compared to the sputtered GeO2 films, the etching properties were improved, as shown in Figure 2. The poorly stoichiometric germanium oxide films grown by sputtering probably result from the growth mechanism. Previously reported GeO2 films grown by thermal oxidation at a process temperature of greater than 500 °C showed poor stoichiometry because the GeO2 films easily transform to GeO above 400 °C.12 In other words, GeO2 bonds can be changed to GeO bonds by external energy sources, such as thermal energy and kinetic energy by momentum transfer. In the sputtering process, highly energetic particles, such as radicals and ions, are produced by plasma generation, and films are deposited by the collision of energetic particles onto the surface. Thus, GeO2 films are easily transformed to the GeO phase during the sputtering process. In contrast, during the ALD process, the surface reaction takes place under the mild conditions of layer-by-layer deposition20,22 and the range of ALD process temperatures (200−275 °C) was much lower than 400 °C, at which GeO2 is thermally unstable. Thus, we can conclude that the external energy that induces phase transformation from GeO2 to GeO during ALD is much lower than that during sputtering, leading to the formation of highly stoichiometric GeO2 by ALD. Additionally, we observed pure films with no carbon and nitrogen impurities in the ALD and sputtered germanium oxide films (see the Supporting Information, Figure S3). The film densities of the germanium oxides prepared by ALD using Ge(OnBu)4 and Ge(NMe2)4 and by sputtering were analyzed by XRR. Figure 3c,d shows the XRR curves with simulation results, and film densities calculated from XRR data, respectively. The XRR results, such as film thickness, roughness, and film density, are summarized in Table S3 of the Supporting Information. The total film thicknesses of ALD GeO2 prepared from Ge(OnBu)4 and Ge(NMe2)4 and the sputtered germanium oxides obtained from XRR simulations were 10.0, 10.5, and 10.4 nm, respectively, which were almost the same values as compared to those determined by ellipsometry measurements. From the additional TEM analysis, we observed an interlayer with a thickness of approximately 1

Figure 2. Etching property of 50 nm thick sputtered and O3-ALD GeO2 prepared using Ge(NMe2)4 and Ge(OnBu)4 on a transparent glass substrate. (a) Captured photo after etching of (a) ALD GeO2 by Ge(NMe2)4 and (b) sputter GeO2. (c) Transmittance of etched GeO2 films on transparent glass and (d) EDS line spectra.

the other phases would remain as a noncontinuous morphology.11 Consequently, the residue from other germanium oxide phases scatters and reflects light, resulting in the vague image of the sputtered sample. Thus, the observation of the vague image indicates that the sputtered germanium oxide is a nonhomogeneous phase, but the ALD germanium oxide is homogeneous. In addition, the dissolution process of GeO2 by water molecules should be accompanied by the generation of Ge3+ and OH− ions in the water solvent, such that the gasphase water molecules under conditions of high humidity cannot etch out the ALD GeO2 layer. Consistently, in a previous report, it was demonstrated that GeO2 films were not etched in ∼45% relative humidity because of the formation of a blocking layer against the reaction of water molecules.38 The chemical compositions of the ALD GeO2 films were investigated by XPS. For comparison, the GeO2 thin films were also deposited by sputtering. Figure 3a shows Ge 3d XPS data of 50 nm thick ALD GeO2 samples prepared from Ge(OnBu)4 834

DOI: 10.1021/acs.chemmater.7b04371 Chem. Mater. 2018, 30, 830−840

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Figure 3. Chemical composition of the 10 nm thick sputtered and the ALD GeO2 films prepared from Ge(OnBu)4 and Ge(NMe2)4. (a) Ge 3d corelevel spectra. (b) O 1s core-level spectra. (c) XRR data and simulation of 10 nm thick sputtered and ALD GeO2 by using Ge(OnBu)4 and Ge(NMe2)4. (d) Film density values of the GeO2 films measured from XRR analysis shown in part c. AFM analysis of the sputtered and ALD GeO2 films prepared using Ge(OnBu)4 and Ge(NMe2)4. The right image in part g is a z-scale bar for three samples in parts e−g. The root-mean-square surface roughness values of the GeO2 films are 0.1, 0.2, and 2.9 nm for ALD GeO2 using Ge(OnBu)4 and Ge(NMe2)4, and sputtered GeO2, respectively.

nm between ALD GeO2 film and Si substrate (shown in Figure S4); however, it is hard to distinguish the layer from the GeO2 layer, probably because of a lack of significant difference in stoichiometry and microstructure. Similar film densities for both ALD GeO2 films were observed, although the film density of ALD GeO2 was much higher compared to those of the sputtered germanium oxides. Generally, highly stoichiometric films of metal dioxide systems have higher film densities because monoxide bonding, defects, and oxygen vacancies,

which result in a lower film density, are incorporated to a greater extent in less stoichiometric films.39 Thus, from our results, compared to those of the sputtered germanium oxides, the higher densities of the ALD GeO2 films were attributed to the higher stoichiometry, which was mainly caused by the lesser amount of GeO in the films. Other factors that can affect the film density, such as impurity content (see the Supporting Information, Figure S3) and crystallinity (see the Supporting Information, Figure S5), are the same for the ALD and 835

DOI: 10.1021/acs.chemmater.7b04371 Chem. Mater. 2018, 30, 830−840

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Chemistry of Materials sputtered germanium oxide thin films. Additionally, the surface roughness obtained from the XRR measurements shows similar results (around 0.40 nm) for both ALD GeO2 films, while a rougher surface morphology of 2.80 nm was observed for the sputtered germanium oxides. Figure 3(e−g) shows the images of the surface morphologies of ALD GeO2 prepared from Ge(OnBu)4 and Ge(NMe2)4 and sputtered germanium oxides measured by AFM, respectively. The root-mean-square (RMS) roughness values from images are 0.1, 0.2, and 2.9 nm, respectively, corresponding to the results of the XRR simulations. Wet etching is a chemical process that acts on a material’s surface. As such, materials with higher surface areas (i.e., with rougher surfaces) show faster etching rates. In addition, a rough surface consists of uneven morphologies with high surface energies and thermodynamic instabilities, implying that chemical reactions are more favorable on these uneven features.40 This consequently leads to high etching rate. In this work, compared to sputtered GeO2, ALD GeO2 showed much smoother surface. However, the etching rates of ALD GeO2 and sputtered GeO2 were similar (Figure S2), because both ALD GeO2 and sputtered GeO2 are etched very quickly at room temperature (100 nm/s), such that the difference in etching rate on nanoscale thin films was not recognizable in our time scale. However, the ALD GeO2 showed much better etching uniformity due to its better stoichiometry compared to that of the sputtered GeO2. Film roughness also influences electrical properties. Rough films have a larger variation in thickness between different positions than smooth films, and thus probably induce a larger variation in electrical potential, leading to a high leakage current.36 To evaluate the fundamental electrical properties of the 10 nm thick ALD and 30 nm thick sputtered germanium oxides, we fabricated MOS capacitors using GeO2 as the gate insulator and measured the C−V and I−V characteristics, as shown in Figure 4a,b, respectively. For comparison, the MOS capacitor fabricated with a 10 nm thick ALD SiO2 dielectric was also measured; the SiO2 dielectric has been widely used in semiconductor applications for gate dielectrics because of its good electrical properties.41 The electrical properties such as a dielectric constant, hysteresis, interface trap density, and leakage currents are summarized in the Supporting Information, Table S4. As shown in Figure 4a, the C−V characteristics of both ALD GeO2 films show similar and typical behavior of an MOS capacitor with p-type Si substrate. The dielectric constants extracted from the maximum capacitance value are 5.94 for Ge(OnBu)4 and 5.77 for Ge(NMe2)4, which is a range similar to those reported for GeO2 films, 5.2−5.842 and about 6.8 In addition, although the hysteresis properties are similar, the interface trap densities of both Ge(OnBu)4 and Ge(NMe2)4 show little difference and are comparable to commonly used gate dielectrics.43 In contrast, the sputtered germanium oxide samples had a degraded C−V curve and could not function as a dielectric layer. The extracted dielectric constant was 1.05, which is much lower than that of ALD GeO2 films, which is caused by the imperfect accumulation behavior of the MOS capacitor. In addition, degraded device performance was also observed in the significant large hysteresis and Dit values. The MOS capacitor containing ALD SiO2 shows typical C−V characteristics, and the parameters and electrical properties of ALD SiO2 are comparable to those of ALD GeO2. Thus, we conclude that ALD GeO2 thin films show superior electrical properties and are suitable for use in memory devices.

Figure 4. Electrical properties of the 10 nm thick sputtered film and the ALD GeO2 films prepared using Ge(OnBu)4 and Ge(NMe2)4 with 10 nm thick ALD SiO2. (a) C−V curve and (b) I−V curve of MOS capacitors using 10 nm thick dielectric films.

Figure 4b shows the I−V characteristics of the 10 nm thick ALD GeO2 and SiO2 films, and 30 nm thick sputtered germanium oxide films. The leakage current densities for the ALD GeO2 films are similar, whereas the leakage current density of the sputtered germanium oxides is significantly higher (105 times), making it unsuitable as an insulating layer. We observed a higher ratio of GeO bonding to GeO2 bonding inside the films. Thus, the etching ability was degraded, and the electrical properties were negatively affected. Among germanium suboxides (Ge1+, Ge2+, Ge3+, and Ge4+), the Ge−O bonds (Ge2+) are known to be poor components regarding electrical properties because the Ge−O bonds inside GeO2 films act as trap sites and oxygen vacancies.44 Trap sites and oxygen vacancies in the film generate gap states in Eg that contribute to the leakage current by trap-assisted tunneling; thus, the fewer trap charges in ALD GeO2 result in lower leakage currents in the MOS capacitor compared to sputter GeO2. In addition, a much rougher surface morphology was observed in ALD GeO2 compared to that of the sputter GeO2, as shown in Figure 3e− g. The rough surface of the sputtered GeO2 films forms a rough interface with the Ru top electrode. Because the electric field is increased where the thickness of films is relatively thin, significant electron emissions at the rough interface result in a high leakage current in the sputtered GeO2.36,45 Additionally, considering the similar electrical properties of ALD GeO2 with ALD SiO2, ALD GeO2 would be suitable for the gate dielectric for memory applications. We fabricated CTM devices composed of oxide−nitride− oxide (ONO) nanolaminate structures with a 10 nm thick CVD SiO2 tunneling insulator, 15 nm thick CVD SiNx chargetrapping layer, 40 nm thick ALD GeO2 control gate oxide, and 836

DOI: 10.1021/acs.chemmater.7b04371 Chem. Mater. 2018, 30, 830−840

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Chemistry of Materials

Figure 5. (a) Schematic drawing of the CTM devices with an ONO structure including the ALD GeO2 layers. (b) C−V curves of CTM at varying sweeping voltage range. C−V curve at a voltage range from −10 to 10 V (c) before and (d) after etching by a droplet of water at RT for 1 s. The insets of parts c and d show the captured images of the memory device before and after etching, respectively. The operation of the LED device (e) before and (f) after etching the GeO2.

100 μm thick Ru control gate, as shown in Figure 5a. We used ALD GeO2 for a control gate oxide, instead of SiO2. Since the ALD GeO2 showed superior electrical properties to SiO2 in MOS capacitor structures (as shown in Figure 4), the ALD GeO2 was expected to show better gate oxide properties in the CTM devices than SiO2. In addition, thin interlayers composed of GeOaNb and SiOcNd that have superior properties over GeO246,47 were observed in the TEM and EDS analysis (see Figure S6 of the Supporting Information). The charge-trapping effect and water solubility of this device was evaluated for secure memory applications. Figure 5b shows the C−V characteristics of an MOS capacitor made with ALD GeO2 by Ge(OnBu)4 precursor, which was chosen because GeO2 prepared from Ge(OnBu)4 shows better film properties such as higher stoichiometry, film density, etching properties, and better electrical properties as compared to GeO2 prepared from

Ge(NMe2)4. The C−V curves measured between −5 and 5 V show almost no hysteresis, indicating that this voltage range is insufficient for the operation of the memory device. On increasing the sweeping voltage range to between −10 and 10 V, the hysteresis increased to 0.786 V, indicating the chargetrapping effects of SiNx. On increasing the voltage range to between −12 and 12 V, and −14 and 14 V, we observed further increased hysteresis up to 1.626 and 3.488 V, respectively. The amount of charge stored in the devices increased as the sweeping voltage range increased; that is, 0 for −5 and 5 V, −9.17 × 1011 cm−2 for −10 and 10 V, −1.92 × 1012 cm−2 for −12 and 12 V, and −4.03 × 1012 cm−2 for −14 and 14 V, which were calculated using the midgap charge separation method.29 These results indicate that charges were stored inside SiNx layers, and the 40 nm thick GeO2 layers show stability in the 837

DOI: 10.1021/acs.chemmater.7b04371 Chem. Mater. 2018, 30, 830−840

Article

Chemistry of Materials applied field up to an operating voltage range between −14 and 14 V. To investigate the water solubility of the memory devices, we measured their electrical properties before and after dropping one droplet of water onto the memory device, and the results are shown in Figure 5c,d, respectively. A sweeping voltage range from −10 to 10 V was used for stable charge-trapping operation. In contrast to the good memory device behavior before the addition of water (shown in Figure 5c), after the addition of water, we immediately observed significant degradation in the electrical properties, resulting in the loss of electrical information in memory devices. The insets of Figure 5c,d show photos of memory devices before and after water addition. A region of GeO2 film solubilized by water can be seen. Finally, we demonstrated an LED device and electronic circuits that convert electrical information to visible light. Before the addition of water to the devices, we observed normal behavior, and the electrical information was converted to optical signals (Figure 5e). However, after the addition of water, the LED sign was turned off because of the dissolved GeO2 gate oxide layers (Figure 5f). Thus, we conclude that ALD-GeO2-based memory devices show good electrical properties for use in memory devices with high-quality GeO2 gate oxide layers, as well as security, immediately losing electrical information after the addition of a single drop of water for 1 s. Although we have demonstrated the prototype of water-soluble memory devices by using the gate oxide layers of ALD GeO2, the tunneling insulator layers of CVD SiO2, and charge-trapping layers of CVD SiNx, more control of the electrical properties or better device performance can be achieved for the separate preparation of tunneling insulator using ALD. To apply the water-erasable memory device to integrated circuit systems, however, we need to deliver water to each tiny cell where water can remove a stacked GeO2 layer which is covered by various layers. By using a microfluidic system, water can be delivered to each tiny cell (see the Supporting Information, Figure S7 and Movie S1).48 In addition, a stacked GeO2 layer of organic and inorganic layers can be well etched out by water, as shown in Figure S7. Thus, these results show the feasibility of applying our water-erasable memory device to integrated circuit systems. In any case, the simple process scheme together with the newly developed GeO2 ALD process technology would give a further degree of freedom in the fabrication of future secure memory devices.

GeO2, a disposable memory device to store data securely can be implemented in a flash memory card that can be discarded after reading.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.chemmater.7b04371. Technical information on TG and DSC curves; DFTcalculated adsorption energies (E); maximum and minimum projected areas; GeO2 etching test; XPS analysis; film thickness, roughness, density, and goodness of fit determined by XRR; TEM analysis; XRD analysis; and a summary of electrical properties (PDF) Movie S1: water being delivered using a microfluidic system (AVI)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. ORCID

Jae-Min Myoung: 0000-0002-9895-4915 Han-Bo-Ram Lee: 0000-0002-0097-6738 Hyungjun Kim: 0000-0003-2256-8046 Author Contributions ∇

C.M.Y. and I.-K.O. equally contributed as 1st authors.

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The Ge precursors used in this work were provided by Air Liquide Korea Company. This work was conducted under the support of the Korea Institute of Industrial Technology as fundamental technology development project (KITECH EO17-0042), and private Industry cooperation R&D promotion project (KITECH EE-17-0019), and this research was supported by the MOTIE (Ministry of Trade, Industry & Energy) (10080643) and KSRC (Korea Semiconductor Research Consortium) support program for the development of the future semiconductor device.

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CONCLUSIONS In summary, we have reported ALD GeO2 processes to fabricate water-soluble memory devices. We developed a GeO2 ALD process by using two Ge precursors and correlated the experimental results with theoretical calculations. In addition, we investigated the etching and electrical properties of ALD GeO2 together with those of sputtered GeO2. Both Ge precursors were usable for the growth of GeO2 with an O3. They show typical growth characteristics of ALD, indicating they are suitable for the ALD process. In addition, ALD GeO2 films show excellent etching properties, which are attributed to the excellent film quality, such as low impurity level, high stoichiometry, high film density, and low roughness. We evaluated the electrical properties of ALD GeO2, and based on those results, we have concluded that ALD GeO2 can be used as an alternative to ALD SiO2. Finally, we demonstrated CTM devices by using ALD GeO2 and observed the erasure of the stored data under the exposure of water. By utilizing ALD

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