Abstract: In this work the etching kinetics of (100) III-As is studied at the nanoscale with ICP-MS for various HCl/H 2 O 2 mixtures. It is shown that the etch rate is controlled by both the concentration of the acid and the oxidizing agent. The surface termination during etching has strong impact on the etching kinetics. A similar effect was observed for Ge. The Ge (100) surface is sensitive to surface roughening during etching and reoxidation after native oxide removal.

Abstract: The scaling limit of Si-based complementary metal oxide semiconductor (CMOS) transistors is approaching. The performance enhancement can no longer be guaranteed due to intrinsic mobility issues of silicon. The considerably higher charge carrier mobility of III-V compound semiconductors and Ge has led to renewed interest as these materials are considered for the development of transistors for 7 nm and 5 nm technology nodes [1-3]. Wet-chemical etching for surface and interface processing has proven to be effective for reducing defectivity. Due to the aggressive downscaling of transistors an ultimate etching selectivity and control at the nanometer and atomic-layer scale is required. Therefore, a thorough understanding of the interactions between the substrate and the solution (both chemical and electrochemical) is essential. In this work the etching kinetics of (100) III-As are discussed for various HCl/H 2 O 2 compositions in the low pH range. The etch rates were determined by measuring the total amount of dissolved material with Inductively Coupled Plasma – Mass Spectrometry [4]. Electrochemical impedance measurements were performed to determine the band energetics of InGaAs during etching for various HCl/H 2 O 2 compositions. III-As can be etched in HCl solution in the presence of an oxidizing agent. In a first step the surface bonds are oxidized by H 2 O 2 . The formed III-As (hydr)oxides are  subsequently dissolved by the acid. Fig. 1 shows the influence of the HCl concentration on the etch rate for InGaAs in 0.02 M H 2 O 2 solution. For 〈 0.01M HCl the oxide solubility controls the etch rate. For 〉 0.01M HCl the dissolution kinetics are controlled by the wetting properties (i.e. surface termination): the etch rate decreases with increasing hydrophobicity. It was demonstrated that this is due to Cl-termination. The trend is comparable for GaAs and InAs [5, 6]. Electrochemical impedance measurements for InGaAs suggest that Cl termination results in a significant shift in flat-band potential (E FB ) as shown in Fig. 1. The observed change in the band energetics has an important consequence for the dissolution mechanism of InGaAs. It can be shown that the mechanism for 0.01M HCl is both chemical and electrochemical, while for 1M HCl dissolution is mainly chemical. Such insight can, for instance, be used to influence the surface stoichiometry: suppressing the electrochemical mechanism lowers the build-up of elemental As 0 : InGaAs + 3h + (VB) = 〉 In 3+ + Ga 3+ + As 0       Fig. 2 shows the etch rate of Ge (100) as function of the H 2 O 2 concentration for 1M and 6M HCl. Interestingly, the etching kinetics for Ge are also strongly influenced by the wetting properties. For 1M HCl, the surface ishydrophilic and the etch rate increases from 〈 0.1 nm/min to 21 nm/min in the range 0-50 mM H 2 O 2 . When the HCl concentration is increased to 6M, the surface becomes hydrophobic and the etch rate is lowered. These results suggests that Cl-termination is also important for the dissolution mechanism of Ge. Due to the low solubility of Ge suboxides [7], higher HCl concentrations are required to render the surface hydrophobic during etching.  References [1] J. A. del Alamo, Nature, 479, 317 (2011). [2] R. Pillarisetty, Nature, 479, 324 (2011) [3] M. Heyns and W. Tsai, MRS Bull., 34, 485 (2009). [4] J. Rip, D. Cuypers, S. Arnauts, F. Holsteyns, D.H. van Dorp, S. De Gendt, ECS J. Solid State Sci. Technol., 3, N3064 (2014). [5] D.H. van Dorp, S. Arnauts, D. Cuypers, J. Rip, F. Holsteyns, S. De Gendt and J.J. Kelly, ECS J. Solid State Sci. Technol., 3, P179 (2014). [6] D. H. van Dorp, S. Arnauts, F. Holsteyns and S. De Gendt, ECS J. Solid State Sci. Technol. 4, N5061-N5066 (2015). [7] B. Onsia, T. Conard, S. De Gendt, M. Heyns, I. Hoflijk, P. Mertens, M. Meuris, G. Raskin, S. Sioncke, I. Teerlinck, A. Theuwis, J. Van Steenbergen, C Vinckier, Solid State Phenomena, 103-104, 27 (2005) Figure 1

Abstract: In this work synchrotron radiation photoemission spectroscopy (SRPES) is used to study InP surfaces after different wet chemical treatments. All results are compared to a typical fingerprint of surface components present on an as received InP sample. It is shown that acidified (HCl and H 2 SO 4 ) treatments efficiently remove the native phosphate, although components like P 0 , In 0 and P (2±Δ)+ remain present. In alkaline solution (NH 4 OH) oxide remains present at the surface. As an alternative treatment, the immersion into (NH 4 ) 2 S was studied. This passivation treatment results in fewer surface components which suggests that a higher quality surface is obtained.

Abstract: The goal towards miniaturization of silicon (Si) - based complementary metal-oxide semiconductor (CMOS) devices has challenged the semiconductor industry in improving the device performance while sustaining the scale requirement. [1-2] In this perspective, Ge appears to be a suitable candidate to replace Si due to its higher electron and hole mobility than Si. To date, a defect-free GeO x /Ge interface has never been achieved so far in terms of uniformity, composition, and reliability [3-5]. Hence, insight in Ge surface chemistry and morphology is crucial for novel device applications. Here, we report on the (electro)chemical etching behavior, surface morphology and composition of n-type Ge (100) in acidic halide solutions using various analytical and spectroscopic techniques. The use of an integrated (electro)chemical etching chamber connected to X-ray photoelectron spectroscopy (XPS) instrument to exclude the effect of oxygen from the atmosphere is highlighted. Photoelectrochemical current-voltage (j-V) characteristics of n-doped Ge (100) samples were studied in aqueous HCl solutions. Fig. 1A shows j-V plots measured at different light intensities for 1M HCl at a scan rate of 10 mV/s. Region (I) shows that electron-hole recombination dominates due to the weak electric field at the surface. At more positive potential, a plateau feature (region II) can be seen from the curves. In this region of strong band bending, photoexcited holes generated within and near the depletion layer migrate and diffuse to the solution interface where they react to give rise to oxide formation. The photocurrent in this region is independent of the applied potential. The photocurrent in region II (at 0.8V) is plotted as a function of the light intensity for various HCl concentrations (Fig. 1A inset). In this plateau region, the photocurrent increases linearly with increasing light intensity. These results, indicative for a high oxide solubility, show that no passivating oxide layer is formed during electrochemical etching for photocurrent densities up to ~6.0 mA cm -2 . Unexpectedly, random pyramids (Fig. 1B), as evidenced by the formation of characteristic (111) facets, were observed for concentrated HCl solutions and were accompanied by an increase in photocurrent and a decrease in reflectance. By patterning the pyramids, a high structure density could be achieved. The resulting lowering of the reflectance shows that light coupling was improved. XPS data reveal (Fig. 1C) that at low HCl concentration, surface chemistry is dominated by Ge-OH. At high HCl concentrations, Ge-OH is effectively converted into Ge-Cl. Upon surface chlorination, back bonds are being stabilized and as a result the etch rate is lowered. Ge (111) planes serve as an etch stop evidenced by the facetted surface. On the other hand, in Ge chemical etching, HBr effectively removes (vs HCl) GeO 2 and suboxides providing an air stable surface as confirmed by the XPS spectra. IPA rinsing maintains the electrostatics and chemical composition after Br-passivation (Figure 1D). The brominated Ge surface reoxidizes during H 2 O rinsing in Ar atmosphere, resulting in a strong upward shift of the surface Fermi level indicating an e- donating behavior of H 2 O. [6] In summary, this research displays new outlooks for various applications like batteries [7], biomedicine [8] , photodetectors [9] and for light absorbing and harvesting of solar energy [10] . In addition, it serves as an eye-opener for the potential of semiconductor surface chemical studies using UHV-integrated electrochemical cell connected to XPS to study etching systems. References [1] R. Pillarisetty, Nature 479, 324-328 (2011). [2] M. Kobayashi, G. Thareja, M. Ishibashi, Y. Sun, P. Griffin, J. McVittie, P. Pianetta, K. Saraswat and Y. Nishi, J. Appl. Phys. 106, 104117-7 (2009). [3] L. Tsetseris and S.T. Pantelides, Appl. Phys. Lett. 95, 262107-3 (2009). [4] L. Tsetseris and S.T. Pantelides, Microelectron. Eng. 88, 395-398 (2011). [5] M. Yang, R.Q. Wu, Q. Chen, W.S. Deng, Y.P. Feng, J.W. Chai, J.S. Pan and S.J. Wang, Appl. Phys. Lett. 94, 142903-3 (2009). [6] G. H. A. Abrenica, M. V. Lebedev, G. Okorn, D. H. van Dorp and M. Fingerle, Appl. Phys. Lett. 113, 062104 (2018). [7] J. D. Ocon, J. W. Kim, G. H. A. Abrenica, J-K. Lee and J. Lee, Phys. Chem. Chem. Phys. 16, 22487-22494 (2014). [8] H. Geng, J. Dai, J. Li, Z. Di and X. Liu, Sci. Rep . 6, 37474 (2016). [9] J. Michel, J. Liu and L. C. Kimerling, Nat. Photonics 4, 527534 (2010). [10] S. L. Shinde, T. D. Dao, S. Ishii, L-W. Nien, K. K. Nanda and T. Nagao, ACS Photonics 4, 1722-1729 (2017). Figure 1

Abstract: The Gate-All-Around (GAA) architecture constructed of vertically stacked horizontal Silicon Nano-Wires (Si NWs) are a promising candidate to replace FinFET for device scaling at sub 5-nm technology nodes. In this paper, Si NWs’ release, which is the sacrificial film etching of SiGe 25% (Silicon 0.75 Germanium 0.25 ) selective to Si, will be presented. It is known that the boundary layer between Si and SiGe 25% in the multistack is very sharp, since Ge diffusion depth into Si is generally limited up to 1nm. However, the thermal annealing process is known to cause intermixing of SiGe/Si at the boundary layer with an intermixing depth ranging from 1 to 2 nm [1]. In other words, there is a possibility of Ge residue remaining at the Si NWs’ surface after the selective etch of the sacrificial SiGe 25% film using a formulated chemistry [2] . The presence of impurities on the Si NWs channel surface is expected to cause an increase in leakage current causing a degradation in device performance. Therefore, the motivation in this study is to investigate the Ge residue in Si NWs after SiGe:Si selective etching and means of removal them from the Si NWs’ surface. To investigate the surface clean, blanket wafers of 50-nm SiGe 25% on Si were prepared by epitaxial growth. The SiGe 25% layer was then removed using the formulated chemical. These wafers were analyzed by dynamic SIMS and confirmed the diffusion of Ge into Si, which indicates the need of a subsequent surface clean to remaining Ge. At first, various commodity chemicals like HF, HCl and HPM (a mixture of HCl/H 2 O 2 /H 2 O) followed by DIW rinse were evaluated; however, none of these reduced the level of diffused Ge. Similarly, there was no further Ge reduction even with additional process time with the formulated chemical. Finally, APM (a mixture of NH4OH/H 2 O 2 /H 2 O) followed by DIW rinse was investigated, and the Ge concentration on the Si surface was reduced. The H 2 O 2 oxidized Ge to Ge (OH) 2 , which subsequently dissolved in H 2 O [3][4] . Furthermore, the H 2 O 2 oxidized the Si surface to SiO 2 , which was etched by NH 4 OH. As a result, it is proposed that the intermixing layer of SiGe/Si of Si surface was etched with concomitant reduction of the Ge concentration on the Si surface [5]. The intermixing layer of SiGe/Si has a much lower Ge concentration than SiGe 25%. Therefore, the chemicals that are effective for Si etching should also be effective for removing the intermixing layer of SiGe/Si [6] [7]. The result of etching the intermixing layer of SiGe/Si with these chemicals will be presented. In addition, the surface roughness compared to before post process was improved, which is also beneficial for enhancing device performance. Furthermore, the impact of the thermal budget during the annealing process on the removal performance of Ge residue and the difference of intermixing depth of SiGe/Si will be shown. Finally, the most efficient post cleaning for Si NWs will be proposed. In summary, the Ge residue remaining at the Si NWs channel surface, which could not be removed by the formulated chemistry, will be efficiently removed by etching this intermixing layer. It will be shown that an optimized clean after the Si NWs’ release can be effective in removing Ge residue from this Si channel surface. [1] H. Mertens et al., ECS Transactions, 77 (5) 19-30 (2017) [2] K. Komori et al., UCPSS.1662-9787, 282,107-112(2018) [3] K. Komori et al., ECS Transactions, 80(2) 141-146 (2017) [4] N. Cerniglia et al., J. Electrochem. Soc, 109(6) 508-125(1962) [5] G. K. Celler et al., Electrochemical and Solid-State Letters, 3 (1) 47-49 (2000) [6] J. Phys. Chem. C, 118, 4, 2044-2051(2014) [7] O. Tabata et al., Sensors and Actuators A, 34(1) 51-57(1992)

Abstract: Recent developments in device technology resulted in an upsurge of interest in III-V compound semiconductors. These include improved fabrication techniques for flexible electronic devices,[1,2] and epitaxial integration of various III-V channel materials on Si-based platform wafers enabling the development of highly scaled CMOS transistors [3–5] and nanophotonic devices [6]. While fabrication strategies for traditional III-V optoelectronic devices such as light-emitting diodes, lasers and solar cells are well established, the very small dimensions of these new nanodevices pose new problems. In such applications wet-chemical etching remains an essential step. The ever decreasing size of III-V devices requires ultimately atomic-layer-scale control of surfaces in terms of etching selectivity, stoichiometry and morphology.[7,8] Considerable expertise and literature is currently available on III-V semiconductor etching[9,10]. However, much of the reported work is empirical, etch rates are high, and mechanistic insight into (electro)chemical processes occurring at the semiconductor-solution interface is often lacking. In this work an overview will be given of wet-chemical approaches for nanoscale and atomic-layer-scale etching of Ga(In)As and InP. Two types of etching systems will be discussed. The first is based on the use of acidic H 2 O 2 solution. Inductively coupled plasma mass spectrometry (ICP-MS) measurements, used to determine etching kinetics, showed that under similar conditions the etch rate of Ga(In)As in H 2 SO 4 /H 2 O 2 solution is more than an order of magnitude higher than that of InP. Another striking observation is the influence of the acid, H 2 SO 4 and HCl, on etching kinetics. An increase in HCl concentration leads to an increase in the etch rate of InP while the dissolution rate of Ga(In)As is markedly lowered for the active etching range. Previous work suggested that the surface oxide or hydroxide may be important.[11,12] We have used X-ray photoemission spectroscopy (XPS) and time-of-flight elastic recoil detection analysis (ToF-ERDA) to obtain information about (hydr)oxide formation on the etched surfaces. ToF-ERDA measurements also allowed us the detect surface chlorine in the case of HCl-based etchants. The results indicate that, while the initial step (the breaking of the III-V surface bond) is the same for both semiconductors, the ease with which the resulting group V hydroxide entity at the surface can be deprotonated determines whether the etch rate w ill be high (Ga(In)As) or low (InP). The mechanism can also help to explain the contrasting role of HCl in the dissolution of the two semiconductors. An alternative digital type of etching approach for InGaAs and InAs involves self-limiting surface oxidation in O 3 /H 2 O followed by an oxide removal step in HCl solution[12]. ICP-MS quantification of the dissolved surface oxide species indicates that a high stoichiometry of etching is obtained and that the number of equivalent oxidized atomic layers can be controlled by adjusting the dissolved O 3 concentration. For 〉 4 cycles some surface roughening is observed, most likely due to due to the high solubility of As oxides in water. An important advantage of the 2-step approach is that defect selective etching can be effectively suppressed. Examples of applications of the two etching systems will be highlighted. [1] C.-W. Cheng, K.-T. Shiu, N. Li, S.-J. Han, L. Shi, D.K. Sadana, Nat. Commun. 4 (2013) 1577. doi:10.1038/ncomms2583 [2] N.J. Smeenk, J. Engel, P. Mulder, G.J. Bauhuis, G. Bissels, J.J. Schermer, E. Vlieg, J.J. Kelly, ECS J. Solid State Sci. Technol. 2 (2013) P58–P65. [3] J.A. del Alamo, Nature. 479 (2011) 317–323. doi:10.1038/nature10677 [4] M. Paladugu, C. Merckling, R. Loo, O. Richard, H. Bender, J. Dekoster, W. Vandervorst, M. Caymax, M. Heyns, Cryst. Growth Des. 12 (2012) 4696–4702. doi:10.1021/cg300779v [5] N. Waldron, C. Merckling, L. Teugels, P. Ong, S.A.U. Ibrahim, F. Sebaai, A. Pourghaderi, K. Barla, N. Collaert, A.V.-Y. Thean, IEEE Electron Device Lett. 35 (2014) 1097–1099. doi:10.1109/LED.2014.2359579 [6] Y. Shi, Z. Wang, J. Van Campenhout, M. Pantouvaki, W. Guo, B. Kunert, and D. Van Thourhout, Optica 4 , 1468-1473 (2017). doi:10.1364/OPTICA.4.001468 [7] K.J. Kanarik, T. Lill, E.A. Hudson, S. Sriraman, S. Tan, J. Marks, V. Vahedi, R.A. Gottscho, J. Vac. Sci. Technol. Vac. Surf. Films. 33 (2015) 020802. doi:10.1116/1.4913379 [8] G.S. Oehrlein, D. Metzler, C. Li, ECS J. Solid State Sci. Technol. 4 (2015) N5041–N5053. doi:10.1149/2.0061506jss [9] P.H.L. Notten, J.E.A.M. Meerakker, J.J. Kelly, Elsevier Advanced Technology, 199. [10] A.R. Clawson, Mater. Sci. Eng. R Rep. 31 (2001) 1–438 [11] D.H. van Dorp, S. Arnauts, D. Cuypers, J. Rip, F. Holsteyns, S.D. Gendt, J.J. Kelly, ECS J. Solid State Sci. Technol. 3 (2014) P179–P184. doi:10.1149/2.021405jss [12] D.H. van Dorp, S. Arnauts, F. Holsteyns, S.D. Gendt, ECS J. Solid State Sci. Technol. 4 (2015) N5061–N5066. doi:10.1149/2.0081506jss