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J. Phys. Chem. C 2008, 112, 18510–18515
Wet Etching of GaAs(100) in Acidic and Basic Solutions: A Synchrotron-Photoemission Spectroscopy Study Mikhail V. Lebedev,*,† Eric Mankel,‡ Thomas Mayer,‡ and Wolfram Jaegermann‡ A. F. Ioffe Physico-Technical Institute, Russian Academy of Sciences, Politekhnicheskaya 26, St. Petersburg, 194021, Russia, and Darmstadt UniVersity of Technology, Institute of Material Science, Petersenstraβe 23, 64287 Darmstadt, Germany ReceiVed: June 24, 2008; ReVised Manuscript ReceiVed: September 17, 2008
The interaction of the oxide-free GaAs(100) surface with acidic (HCl + 2-propanol) and basic (aqueous NH3) solutions is studied by synchrotron-photoemission spectroscopy. It is found that both solutions attack mostly surface gallium atoms and form weakly soluble gallium chlorides and soluble gallium hydroxides, respectively. Thereby, Ga-As bonds at the surface are broken, and elemental arsenic is left behind on the GaAs surface. In addition, adsorbed 2-propanol molecules are observed on etching with HCl + 2-propanol solution, but no adsorbed water molecules are detected on etching with aqueous ammonia solution. 1. Introduction Wet chemical etching procedures are widely applied in device fabrication. Processes occurring at semiconductor/electrolyte interfaces are very complex. Essential steps are adsorption of ions, breaking of semiconductor surface bonds, as well as the formation of surface compounds, which may be soluble in the solution or passivate the surface. GaAs(100) is one of the most intensively studied semiconductor surfaces because of its importance in electronics and optoelectronics applications. Wet chemical etching is often used in preparation and conditioning of the GaAs surface. Both acidic (HCl-based,1-3 H2SO4-based,4 HF-based5) and basic (e.g., NH4OH6-8) etching solutions are applied. It is well-known that all these etching solutions can remove native oxides from the GaAs(100) surface with different etching rate and efficiency concerning left over oxides. As a result, the surface is covered with a rather thick layer of elemental arsenic, as well as with different residual oxides. It was assumed that this elemental arsenic is formed during interaction of the arsenic oxides with the electrolyte ions in the etching solution.9,10 On the other hand, very little data exist so far on the reaction mechanism of the GaAs surface with various etching solutions. It was found that after interaction of the GaAs(110) surface with Br2/H2O solution the surface becomes covered with various Ga and As oxides and hydroxides.11 However, the mechanism of etching with acidic and basic solutions of the pristine GaAs(100) surface is still unclear, though this surface is one of the most studied due to its significance in semiconductor device technology. In this study, the results of detailed investigations of the wet etching of the GaAs(100) surface with acidic (HCl in 2-propanol) and basic (aqueous NH3) solutions are shown. These etching procedures have proven their high efficiency in removal of the native oxide layer.3,8 In particular, etching with a solution of HCl in 2-propanol and subsequent annealing in vacuum at different temperatures enables obtaining the ordered GaAs(100) surfaces with different reconstructions.3 Etching with aqueous ammonia solutions is widely used in semiconductor processing * To whom correspondence should be addressed. E-mail: mleb@ triat.ioffe.ru. Fax: +7(812) 297 10 17. † A. F. Ioffe Physico-Technical Institute. ‡ Darmstadt University of Technology.
technology.12,13 The surfaces were analyzed by means of highly surface sensitive synchrotron-radiation photoemission spectroscopy. 2. Experimental The experiments have been performed at the undulator beam line U49/2 of the BESSY II storage ring, which provides photons in the energy range between hν ) 90 and 1400 eV. Ga 3d and As 3d core levels were measured with different excitation energies to gain insight into the depth distribution of different species. The spectra were obtained using the Phoibos 150 (SPECS) energy analyzer of the experimental system “SoLiAS” (Solid/Liquid Analysis System)14 permanently operated at BESSY. SoLiAS is equipped with an integrated electrochemistry chamber built from standard glass elements. This glass chamber is purged with inert, dry, carbon-free Ar gas and is directly attached to a special buffer chamber to allow for the transfer of the samples into ultra-high vacuum (UHV) without contact to ambient atmosphere. The GaAs(100) samples used in this study were cut from an n-type wafer with an epitaxially grown top layer having a carrier concentration of about 1018 cm-3. Clean GaAs(100) surfaces were prepared by etching off the native oxide layer with commercial 25% aqueous solution of NH3 in Ar atmosphere8 and subsequent annealing at 500 °C in UHV for removal of the thick elemental arsenic cap-layer that passivates the surface as a result of the etching process.9 The annealing temperature was controlled by a thermocouple positioned in the vicinity of the sample. For the processing, the clean GaAs(100) surface was transferred back into the electrochemistry chamber where some drops of the etching solution were applied to the surface for 2 min. Two etching solutions were considered. As the acidic solution, the mixture of HCl and 2-propanol (1:10) was used. As the basic one, the commercial 25% aqueous solution of NH3 (ammonia hydroxide) was applied. It should be noted that the initial clean GaAs(100) surface prepared by etching with aqueous ammonia solution and subsequent annealing in UHV was hydrophobic and 2-propanol-phobic (rejecting the solution), opposite to the GaAs(100) surface only etched with aqueous ammonia solution, which is hydrophilic.15 Therefore, to keep the contact of the
10.1021/jp805568t CCC: $40.75 2008 American Chemical Society Published on Web 11/04/2008
Etching of GaAs(100) in Acidic and Basic Solutions
Figure 1. Photoemission spectra of As 3d (a) and Ga 3d (b) core levels collected using various excitation energies from the initial GaAs(100) surface obtained by etching with NH3 aqueous solution and subsequent annealing at 500 °C. The dots represent the collected data after background subtraction. The spectra have been deconvoluted in the assigned components by a fitting routine.
etching solution with the surface for an extended time, a continuous flow of the solution onto the surface was maintained. For termination of the reaction, the residual solution was blown off by an Ar jet, and the samples were returned back into UHV for photoemission analysis without any rinsing. 3. Results 3.1. Clean Initial GaAs(100) Surface. Figure 1 shows the As 3d and Ga 3d core-level spectra measured with different excitation energies on the clean GaAs(100) surface as obtained by etching with ammonia hydroxide and annealing in UHV. Fitting of each core-level spectrum was carried out using the Voigt functions with the parameters (Gaussian width for measurement uncertainty, Lorentzian width for lifetime broadening, spin-orbit splitting, branching ratio) similar to those used
J. Phys. Chem. C, Vol. 112, No. 47, 2008 18511 in ref 15. The spectra look very similar to those that we have obtained after the same etching and annealing procedure before.8 The As 3d spectrum of the clean initial GaAs(100) surface measured with the excitation energy of 95 eV can well be fitted with three different components: the bulk As-Ga emission, the surface component on the low-binding energy side of the bulk signal (chemical shift of -0.45 eV), and the small component at the high-binding energy side of the bulk signal (with the chemical shift of +0.55 eV). The low-energy component and the high-energy one can be assigned to As-As surface dimers and excess elemental arsenic As0, respectively.16 The As0 component is not found in the less surface-sensitive spectra, and the dimer component is strongly reduced in the less surfacesensitive spectrum measured with the higher excitation energy of 250 eV and invisible at 650 eV, which clearly indicates surface species. Note that spectra measured with different excitation energies provide information from the surface layer of different thickness due to the difference in the electron inelastic mean free path λ, which for the As 3d level in GaAs has the values of approximately 4.5, 7.3, and 15.1 Å, respectively, for the excitation energies of 95, 250, and 650 eV used in this study.17 The Ga 3d spectrum of the clean initial GaAs(100) surface measured with 95 eV excitation energy can well be fitted with four components (Figure 1b). The most intense is the Ga-As bulk emission component. The second component with the chemical shift of about +0.7 eV can be assigned to gallium suboxides (Ga2Ox),11 though the possibility that this component stems from residual gallium hydroxides cannot be ruled out completely. The other two components are the surface components S1 and S2 with the chemical shifts of -0.35 and +0.35 eV, respectively. The assignment of the S1 and S2 components is still unclear. In ref 15, these components have been related to surface Ga atoms in dimers localized between two neighboring Ga dimers and next to a missing Ga dimer, respectively. However, later this assignment was criticized18 as being in contradiction with subsequent experiments, and now on the basis of first-principle calculations of the surface core-level shifts for the GaAs(100)(2 × 4) surface, it is assumed that these two surface components stem from the second-layer Ga atoms located at different nonequivalent sites.19 In the less surfacesensitive Ga 3d spectra measured with 250 and 650 eV excitation energies, the S1 and S2 components are not visible, and the intensity of the gallium suboxide component is reduced considerably. 3.2. Acidic Solution Etched GaAs(100) Surface. After contact of the clean initial GaAs(100) surface with the HCl in 2-propanol solution, both As 3d and Ga 3d spectra are changed considerably (Figure 2), while their energy position shifts very little (less than 0.1 eV). At the same time, the intense Cl 2p emission appears. In the As 3d spectrum measured with 95 eV excitation energy, the component assigned to As-As dimers on the low binding energy side disappears (Figure 2a). Simultaneously, the intensity of the elemental arsenic component increases considerably, and also a new component appears with the chemical shift of about 1.0 eV. According to the chemistry of the process, this later component can be associated with either As-H or As-Cl bonds. The reported chemical shift for As-H bonds is 0.35 eV,20 and these bonds are probably contained within the elemental arsenic emission. On the other hand, the reported chemical shift for arsenic monochloride AsCl is 1.3 eV,10,21 which is considerably higher than the observed chemical shift of about 1.0 eV. We tentatively assign this component to arsenic monochloride (AsCl*) intermixed with elemental arsenic.
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Figure 2. Photoemission spectra of As 3d (a) and Ga 3d (b) core levels collected using various excitation energies from the GaAs(100) surface after contact with HCl + 2-propanol solution. The dots represent the collected data after background subtraction. The spectra have been deconvoluted in the assigned components by a fitting routine.
In the less surface-sensitive As 3d spectra measured with higher excitation energies, the intensity of the elemental arsenic component is considerably smaller. The intensity of the component with the chemical shift of 1.0 eV decreases as well. In the Ga 3d spectrum measured with the excitation energy of 95 eV, five distinct components are resolved (Figure 2b). The first one is the main component associated with bulk Ga-As photoemission. The other ones are the components with the chemical shifts of 0.35, 1.1, 1.6, and 2.0 eV. The first chemically shifted component appears at the position of the surface S2 component visible in the spectrum of the initial clean surface. The other chemically shifted components can be associated with the surface gallium species to which, respectively, 1, 2, and 3 chlorides are attached.10 In the spectra measured with the higher excitation energy, the component at the S2 position disappears, and the intensities of all gallium-chloride-related components
Lebedev et al.
Figure 3. Photoemission spectra of As 3d (a) and Ga 3d (b) core levels collected using various excitation energies from the GaAs(100) surface after contact with HCl + 2-propanol solution and annealing at 480 °C. The dots represent the collected data after background subtraction. The spectra have been deconvoluted in the assigned components by a fitting routine.
decrease considerably; however, the intensities of GaCl3- and GaCl2-related components always exceed the intensity of the GaCl-related component (Figure 2b). These Ga 3d spectra look very similar to the ones obtained after heating to room temperature a sample with a thin layer of HCl-2-propanol solution frozen-in on the GaAs(100) surface after etching off the native oxide layer.10 After annealing at 480 °C of the GaAs(100) surface treated with HCl-2-propanol solution, the Cl 2p peak survives, though its intensity decreases dramatically. At the same time, the shapes of both As 3d and Ga 3d emissions are changed (Figure 3). In the As 3d spectrum measured with 95 eV excitation energy, the intensities of the component assigned to elemental arsenic and of the component with the chemical shift of about 1.0 eV are reduced dramatically. Simultaneously, the low-binding energy component with the chemical shift of -0.45 eV appears, which can be assigned to restoration of the As-As surface dimers. On the other hand, the Ga 3d spectra measured with
Etching of GaAs(100) in Acidic and Basic Solutions
J. Phys. Chem. C, Vol. 112, No. 47, 2008 18513
Figure 5. Evolution of the core levels O 1s (a) and C 1s (b) during contact of the initial GaAs(100) surface with HCl + 2-propanol solution and subsequent annealing.
Figure 4. Photoemission spectra of As 3d (a) and Ga 3d (b) core levels collected using various excitation energies from the GaAs(100) surface after contact with aqueous NH3 solution. The dots represent the collected data after background subtraction. The spectra have been deconvoluted in the assigned components by a fitting routine.
different excitation energies restore to the spectra of the initial clean surface: all GaCl-related components disappeared; the gallium suboxide component is restored; and in the spectrum measured with 95 eV excitation energy, the surface-related components S1 and S2 reappear. Since after annealing a small amount of chlorine is still present on the surface, the tentative assignment of the As component with a chemical shift of 1.0 eV to arsenic monochloride intermixed with elemental arsenic is supported. 3.3. Basic Solution Etched GaAs(100) Surface. After contact of the clean initial GaAs(100) surface with the ammonia aqueous solution, both Ga 3d and As 3d emissions are shifted to higher energies by more than 0.2 eV. At the same time, only the shape of the As 3d spectrum is changed (Figure 4a), whereas the Ga 3d spectrum remains essentially unchanged (Figure 4b). In addition, a small N 1s peak appears at a binding energy of about 401 eV.
A good fitting of the As 3d spectra measured with any excitation energy is achieved with three components. Besides the bulk As-Ga component, the elemental arsenic As0 component with the chemical shift of 0.55 eV and a new component with the chemical shift of about 3.0 eV can be observed. The energy position of the latter component is similar to that obtained for the arsenic oxide As2O3 (3.2 eV).22,23 However, taking into account the occurrence of a large amount of OH- ions in the solution, we assign this component to arsenic hydroxides like As(OH)3. Besides, this component can contain some amount of complex oxides like (NH4)3AsO4 since some nitrogen is visible at the surface. No low-energy component related to As-As surface dimers was found in the spectra. After annealing at 500 °C, all spectra restore the original shape and energy position as found for the clean initial GaAs(100) surface. 3.4. Evolution of Surface Contamination. The intensity of the C 1s and O 1s emissions from the clean initial oxide-free GaAs(100) surface (each measured highly surface sensitive with the excitation energies of 350 and 650 eV, respectively) was rather little, in accordance with ref 8. After contact of the clean initial GaAs(100) surface with the HCl in 2-propanol solution, the intensity of C 1s and O 1s emissions increases considerably probably due to adsorption of 2-propanol molecules (Figure 5). After annealing this surface, the intensities of O 1s and C 1s emissions decrease essentially, which is the evidence of the removal of the adsorbed 2-propanol molecules. On the other hand, after contact of the clean initial GaAs(100) surface with the ammonia aqueous solution, the intensity of the O 1s peak does not change too much, which may be an indication that no adsorbed solvent (water) molecules are present in UHV (Figure 6a). The observed broadening of the O 1s peak can be related to the occurrence of arsenic hydroxides at the surface. The intensity of the C 1s emission increases considerably (Figure 6b). We relate the increase in the intensity of the C 1s emission to residual hydrocarbon impurities existing in the solution. After annealing, this extra hydrocarbon contamina-
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Figure 6. Evolution of the core levels O 1s (a) and C 1s (b) during contact of the initial GaAs(100) surface with aqueous NH3 solution and subsequent annealing.
Figure 7. As 3d/Ga 3d intensity ratios for photoemission lines measured using excitation energy of 95 eV at different stages of the experiment.
tion disappears completely, and the broadening of the O 1s emission reduces (Figure 6). 4. Discussion Considering the core-level spectra shown in Figures 2 and 4, as well as the ratio of integrated As 3d to Ga 3d emissions at different stages of the experiment (Figure 7), some conclusions about the mechanism of GaAs(100) etching can be drawn. In acidic HCl + 2-propanol solution, the Cl- ions attack mostly surface gallium atoms forming different gallium chlorides. Formation of gallium chlorides causes breaking of the Ga-As bonds and formation of elemental arsenic at the surface with a small amount of arsenic monochloride intermixed. On the other hand, after the contact with HCl + 2-propanol solution, the Asto-Ga ratio reduces slightly; this might indicate the formation of some arsenic chlorides that dissolve into the solution. This is in line with the experiments with frozen HCl + 2-propanol solution on a GaAs(100) surface,10 where arsenic chlorides were found on the cooled sample (at liquid-nitrogen temperature) only after prolonged exposure to synchrotron radiation, while these chlorides have desorbed after heating to room temperature. The OH- ions in aqueous ammonia solution also attack mostly Ga sites at the GaAs(100) surface. Hydroxide and ammonium ions react with Ga atoms to form NH4Ga(OH)4 complexes that are soluble in water.24 Therefore, no Ga-related
Lebedev et al. hydroxide exists on the surface. This is also supported by the considerable increase of the As-to-Ga ratio observed after contact of the GaAs(100) surface with aqueous ammonia solution (Figure 7). The change in surface stoichiometry could be the reason for the observed shift of the surface Fermi level by 0.2 eV. Transfer of surface gallium atoms to solution after breaking of the surface Ga-As bonds causes the formation of elemental arsenic. The arsenic hydroxide species found in the As 3d spectra (Figure 4) can be formed through the direct interaction of hydroxide ions with the surface As atoms at the pristine GaAs(100) surface. Formation of As(OH)3 through interaction of OH- ions with the elemental arsenic can be probably ruled out since no As(OH)3-related component was found after etching off the native oxide layer from GaAs(100) when the obtained amount of elemental arsenic was considerably higher.8 After annealing of the sample treated with HCl + 2-propanol solution, the corresponding As-to-Ga ratio increases considerably (Figure 7), which testifies that the gallium atoms are removed from the surface in the form of gallium chlorides. On the contrary, after annealing of the sample treated with aqueous ammonium solution, only elemental arsenic and arsenic hydroxides (Figure 4a) disappear from the surface, which is accompanied by the decrease in the As-to-Ga ratio (Figure 7). The composition of the acidic and basic solution etched GaAs(100) surface exhibits some differences as compared to the composition of the GaAs(100) surface obtained after etching off the native-oxide layer with the solutions considered in this study.8,10 In particular, the amount of the elemental arsenic found on the surface after etching off the oxide is considerably larger than the amount of elemental arsenic found on the clean GaAs(100) surface after its interaction with acidic and basic solution. Therefore, it can be concluded that the majority of elemental arsenic found after etching off the native oxide layer from the GaAs(100) surface stems from the oxide layer and not from bulk GaAs.10 In addition, after interaction of the clean GaAs(100) surface with aqueous basic ammonia solution, some amount of arsenic hydroxide was found, as opposed to the interaction of the same solution with the native oxide covered surface.8 Adsorption of 2-propanol molecules and the lack of the adsorption of water molecules can be explained by the difference in reactivity of alcohol and water molecules solvating anions in the solutions.25 According to the results of quantum-chemical calculations, water molecules hydrating anions are essentially nonreactive with respect to both electronic donation and acceptance and thus hardly adsorb on the surface. On the contrary, the molecules of various alcohols solvating the anions in the solutions can participate in the reactions involving acceptance of the electrons, and therefore, in some cases they can adsorb at the surface. It should be noted that the existence of 2-propanol on a GaAs(100) surface after etching with HCl + 2-propanol solution at room temperature and even at 400 K was observed in ref 10, where it was related to stabilization of dipolar 2-propanol molecules by Cl- ions bound to back-bonded surface Ga atoms. 5. Summary Interaction of the oxide-free GaAs(100) surface with acidic (HCl + 2-propanol) and basic (aqueous NH3) solutions has been studied by photoemission induced by synchrotron radiation. It was found that after contact of HCl + 2-propanol solution the GaAs(100) surface is covered by various gallium chlorides and elemental arsenic. On the other hand, after contact with aqueous
Etching of GaAs(100) in Acidic and Basic Solutions ammonia solution, Ga is removed from the surface, and the surface becomes covered with elemental arsenic and arsenic hydroxide. These results evidence that the acidic, as well as the basic solutions, preferentially attack gallium sites forming weakly soluble gallium chlorides and soluble gallium hydroxides, respectively. Thereby, the Ga-As bonds are broken, and elemental arsenic is left behind on the GaAs surface. In addition, adsorption of 2-propanol molecules is detected on etching with HCl + 2-propanol solution, but no adsorption of water molecules is found on etching with aqueous ammonia solution. References and Notes (1) Suzuki, T.; Ogawa, M. Appl. Phys. Lett. 1977, 31, 473. (2) Vasquez, R. P.; Lewis, B. F.; Grunthaner, F. J. J. Vac. Sci. Technol. B 1983, 1, 791. (3) Tereshchenko, O. E.; Chikichev, S. I.; Terekhov, A. S. J. Vac. Sci. Technol. A 1999, 17, 2655. (4) Liu, Z.; Sun, Y.; Machuca, F.; Pianetta, P.; Spicer, W. E.; Pease, R. F. W. J. Vac. Sci. Technol. A 2003, 21, 212. (5) Adachi, S.; Kikuchi, D. J. Electrochem. Soc. 2000, 147, 4618. (6) Chang, C. C.; Citrin, P. H.; Schwartz, B. J. Vac. Sci. Technol. 1977, 14, 943. (7) DeSalvo, G. C.; Bozada, C. A.; Ebel, J. L.; Look, D. C.; Barrette, J. P.; Cerny, C. L. A.; Dettmer, R. W.; Gillespie, J. K.; Havasy, C. K.; Jenkins, T. J.; Nakano, K.; Pettiford, C. I.; Quach, T. K.; Sewell, J. S.; Via, G. D. J. Electrochem. Soc. 1996, 143, 3652. (8) Lebedev, M. V.; Ensling, D.; Hunger, R.; Mayer, T.; Jaegermann, W. Appl. Surf. Sci. 2004, 229, 226. (9) Alperovich, V. L.; Tereshchenko, O. E.; Rudaya, N. S.; Sheglov, D. V.; Latyshev, A. V.; Terekhov, A. S. Appl. Surf. Sci. 2004, 235, 249.
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