- Email: [email protected]
Deposition and in-situ characterization of ultra-thin films
Thin Solid Films 459 (2004) 17–22
Deposition and in-situ characterization of ultra-thin films ˇ ´ ˇ ´ ´ Miroslav Kolıbal, Stanislav Voborny, Jindrich Mach, Jan Cechal, Petr Babor, Stanislav Prusa, ˚ˇ ˇ ˇ´ Spousta, Tomas ´ˇ Sikola* Jirı Institute of Physical Engineering, Brno University of Technology, Technicka´ 2, 616 69 Brno, Czech Republic Available Online February 26 2004
Abstract In the paper the application of a complex UHV apparatus for in-situ investigation of low-temperature Ga–N ultra-thin film synthesis by thermal Ga atoms and hyperthermal nitrogen ions is presented. It was found by XPS that the highest content of Ga–N bonds was present in the layers grown on a ‘nitrogen-rich’ substrate at enhanced substrate temperature (400 8C). ‘Nitrogenrich’ Si surfaces grew from hyperthermal nitrogen ions on substrates contaminated by oxygen (‘native’ and chemically etched Si). The ‘gallium-rich’ substrate possessed smaller number of Ga–N bonds. Higher content of Ga–O bonds was found on Si substrates covered by native oxide and chemically etched silicon. Due to low nitrogen ion currents the thickness of the films was small (-4 ML). 䊚 2003 Elsevier B.V. All rights reserved. PACS: 81.15.Ef Keywords: Direct ion beam deposition; Ultra-thin films; GaN; X-Ray photoelectron spectroscopy (XPS)
1. Introduction As the dimensions of electronic and optoelectronic devices are shrinking, the ultra-thin filmsymultilayers have become an important issue in deposition technologies w1x. There are various methods how to prepare these films with typical dimensions below 10 nm, however, to analyse them makes the task more difficult. In addition to a need to use surface sensitive methods, ex-situ analyses and measurements are not optimal as the surface contamination during the transfer of the samples through ambient atmospheric conditions makes their study almost impossible. To increase reliability of the analysis, it is essential to analyze the films in-situ during or after their deposition without a significant delay. The similar problem arises when the early periods of thin film growth are studied. In the contribution, in-situ investigation of low-temperature synthesis of Ga–N ultra-thin films (-600 8C) by thermal Ga atoms and hyperthermal nitrogen ions is presented. The ‘conventional’ synthesis takes part at *Corresponding author. Tel.: q420-5-4114-2707; fax: q420-54114-2842. ˇ E-mail addresses: [email protected] (T. Sikola), ´ [email protected] (S. Voborny).
higher temperatures (e.g. )1000 8C for MOCVD) due to higher activation energies for GaN growth. The results obtained in such a study contribute to finding the basic principles and mechanisms of the early periods in the low-temperature growth of thicker GaN layers based on application of hyperthermal ions. The films were prepared at different deposition parameters (ion energy, ion-to-atom arrival ratio, substrate temperature) on silicon substrates pre-treated in several distinct ways and preferentially analyzed in-situ by Xray photoelectron spectroscopy (XPS). Additionally, to quickly monitor the technological steps already in the deposition chamber, qualitative methods as time-offlight low energy ion scattering (TOF-LEIS), and static secondary ion mass spectroscopy (SIMS) were applied. Direct ion beam deposition, called shortly IBD (ion beam deposition), is an advanced method for the preparation of clean ultra-thin films. It is characterized by the application of hyperthermal primary ions (-100 eV preferentially) as a film forming material from which the layer is constituted w2,3x. There are several advantages of this method: direct mass and energy control of the ion beam and its incidence angle, UHV conditions making in-situ analysis possible and an energy excess of ions compared to average atom energies in evapora-
0040-6090/04/$ - see front matter 䊚 2003 Elsevier B.V. All rights reserved. doi:10.1016/j.tsf.2003.12.076
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Fig. 1. Complex UHV apparatus for deposition and in-situ analysis of thin films.
tion or sputtering methods (E-10 eV) w2x. This energy excess results in a subsurface growth improving the layer adhesion and in formation of metastable compounds. 2. System design A schematic view of the complex UHV apparatus for deposition and in-situ analysis of ultra-thin films is shown in Fig. 1. The apparatus consists of three interlinked major chambers. The first, called the deposition chamber, contains a home-built ion gun for direct ion beam deposition consisting of a plasma ion source, Wien filter and beam transport optics. Further, an ebeam effusion cell (Omicron) for evaporation of solid elements (e.g. metals) has been installed there. Additionally, to direct ion beam deposition, this configuration enables to carry out an UHV-atomic beam evaporation assisted by a hyperthermal ion beam. A home-built experimental set-up consisting of an electron-impact ion beam source (ISE 100, Omicron) together with a drift tube with an MCP detector (Hamamatsu) at its end is used for TOF-LEIS. The same ion beam source along with a quadrupole mass spectrometer equipped with a three-lens-extractionion optics (QMG 421 Balzers) makes SIMS analysis possible.
The base pressure achievable in the chamber pumped by 3 ion pumps (280 lys in total) is within an order of 10y7 Pa. The samples can be in-situ transported to an adjacent analytical chamber for XPS (base pressure f10y7 Pa) or to another chamber containing surface analytical methods, such as scanning tunnelling microscopyyatomic force microscopy (STMyAFM) and low energy electron diffraction (LEED). The base pressure in this chamber is in an order of 10y8 Pa. In addition to a ‘low-energy’ mode (generation of hyperthermal ions of 20–300 eV), the IBD ion beam gun can be run also in a ‘high-energy’ mode (ion energy up to 3000 eV) to clean sample surfaces by ion beam bombardment and to analyse surface. A three-stage differentially pumped system enables to maintain ultrahigh vacuum in the deposition chamber (f1=10y6 Pa) while the ion source is operated at pressures 1=100 Pa. The typical ion beam currents in the low energy mode are in the order 10y1 mA, the spot size of the beam is approximately 8–10 mm (FWHM). Hence, the corresponding low ion current densities determine the applications to the growth of ultra-thin films. 3. Experimental The apparatus was applied to test the ability to synthesize Ga–N layers at lower substrate temperatures
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(-600 8C). In addition to silicon substrates covered with native silicon oxide, hydrogen terminated Si (111), reconstructed Si (111)-7=7, Ga-covered Si (111) and Si (111) pre-treated by hyperthermal nitrogen ion beam were used as substrates. The substrates with native silicon oxide were in-situ thermally treated for several hours at 700 8C, hydrogen terminated silicon substrates were prepared by etching of Si(111) samples in a 2% HF solution for 2 min. The removal of native oxide and reconstruction of Si (111) surface was made by thermal flashing according to w4x. A Si (111) substrate with native oxide was annealed for several hours at 700 8C first, and then thermally flashed for a total time of 2 min slightly above 1200 8C by electric current going directly through it. Flashing was always interrupted when the pressure tended to exceed a range of 10y6 Pa to avoid the ‘hazy’ appearance of the substrate w4x. The Ga-covered Si (111) substrate was prepared by evaporation of Ga layers (f5 ML) on a Si (111) substrate from the effusion cell. The modification of Si (111) substrates by hyperthermal nitrogen ion beam (50 eV) was carried out for a period of 1 h for every sample and the results will be shown below. The work pressure in the chamber during all deposition experiments was kept in a range of 10y6 Pa. Evaporation rate of the effusion cell was calibrated by the crystal quartz monitor (Sycon Instruments). During Ga–N growth experiments both the IBD ion gun and ebeam effusion cell were run for 1 h simultaneously. The energy of the ion gun was changed within the interval of 50–150 eV and the nitrogen ion current varied from 200 to 250 nA. All experiments were done without a mass separation to keep the ion current at a reasonable value and, therefore, the concurrent impact of Nq and Nq 2 ions on samples took place. To maintain the sample contamination by oxygen or hydrocarbons sufficiently low, the 30-min rinsing of the ion gun by nitrogen gas was made before each deposition. The Ga-evaporation rate from the effusion cell was matched to this presently maximum achievable ion beam current and thus the growth rate was very low (typically 4 ML per 1 h). Further, the corresponding Ga-atomic flux will be called a ‘standard flux’. Each technological step (substrate preparation and modification, Ga–N growth) was monitored for different operational conditions in detail by XPS. Fast complementary monitoring was carried out by TOF-LEIS, and SIMS, as well. XPS analysis of the Ga–N layers was performed by a commercial spectrometer (EA 125, DAR 400, Omicron) at X-ray energy of 1253.6 eV (Mg Ka) in the mode of constant path energy (15 and 25 eV). Photoelectrons were collected at two distinct detection angles of 418 (measured from the sample surface). The operational pressure was kept below 2=10y6 Pa. The quantitative evaluation of element concentrations and chemical bonds from peak intensities and chemical
19
shifts, respectively, was carried out by means of software as SDP40 (spectral data processor) and XPS multiquant w5 x . In TOF-LEIS experiments 3 keV helium ions were used. After acceleration the ion beam was chopped over a removable screen-plate slit to produce ion pulses of a time width G120 ns (FWHM). The time-of-flight measurements were controlled by an Ortec electronics consisting of a time-to-amplitude convertor (TAC) and a multichannel-pulse-analyzer card integrated into a computer. SIMS was run in the static mode (current density jf102 nAycm2) using argon primary ions impinging the surface under 458. The electronics available made the detection of only positive ions possible. 4. Results and discussions In this section the results achieved by XPS in individual steps necessary for running experiments on Ga–N layer deposition are discussed in detail. Our TOF-LEIS and SIMS experiments failed to detect not only carbon and oxygen impurities in the films, but also a nitrogen component of these films itself. As for TOF-LEIS the peaks of these elements were surpassed by the broad high-intensity peak of ions scattered from the silicon substrate w6x. Due to the restriction to application of argon ions and detection of positive ions only, the sensitivity of our SIMS experimental set-up towards nitrogen, carbon and oxygen was too low as these elements possess very low secondary ion yields for positive ions (in case of nitrogen by a factor of 10 000 lower than for Ga). However, TOF-LEIS and SIMS provided a quick check of the presence of gallium at the substrate after deposition. Here, particularly SIMS was a useful technique capable to sense small amounts of Ga at the substrates after depositions at 400 8C substrate temperature. TOF-LEIS was of assistance in estimation of a film thickness as it was found the width of the Ga peak depends on the thickness of Ga-based ultra-thin films w6x. 4.1. Substrate cleaning In Fig. 2 the typical XPS spectra of etched and thermally flashed Si substrates are shown. The peaks corresponding to carbon and oxygen impurities are lower in case of the latter spectrum (C – 5 atomic%, O – 2 atomic%). In the figure inset on the left the detail of the Si 2p peak of the thermally flashed substrate is shown. The peak Si 2p (q4) corresponding to the Si bond with four oxygen atoms (f103.5 eV) in silicon covered with native oxide (see the dotted line) disappeared and only the peak (known as Si 2p (q0)) related to Si–Si bond remained. In the inset on the right the detail of the Si 2p peak of the etched substrate shows
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small broadening on the left side of the peak due to the chemical shift caused by the single Si–O bond (Si 2p (q1)). 4.2. Substrate modification To prepare ‘N-rich’ substrates, the silicon substrates were bombarded by hyperthermal nitrogen ions. In Fig. 3 the detailed scans of Si 2p and N 1s peaks taken by XPS at these substrates are shown. The Si 2p peaks drawn in the thick solid line and dotted line correspond to thermally flashed Si (111) substrates bombarded by nitrogen ions at room and enhanced (400 8C) substrate temperatures, respectively. The peaks do not show any significant broadening typical for the formation of chemical bonds of Si with nitrogen or oxygen. Therefore, nitrogen giving N 1s peaks (Fig. 3, right) was most likely just weakly trapped in near-surface layers. The concentration of nitrogen in these samples was 2.4% and 5.3% for RT and 400 8C, respectively. Mild broadening on the left side of the peak is observable for the experiment run on the etched substrate (dash-dotted line) and quite remarkable for the ion beam bombardment of a substrate with a native oxide (thin solid line). Processing the peaks revealed the broadening was caused by bonds of Si to oxygen and nitrogen (oxinitrides) and the thickness of these modified layers was approximately 1 nm. The ratio of the atomic concentrations of Si bound to N and Si bound to Si was 0.15 for the etched substrate (0.3 for Si–OySi–Si). The same ratio for the substrate with native oxide was 2.2
Fig. 3. Detailed scans of Si 2p and N 1s peaks taken by XPS at Si(111) substrates bombarded by hyperthermal nitrogen ions (50 eV): (a) thermally flashed substrate bombarded at room temperature (thick solid line), (b) thermally flashed substrate bombarded at 400 8C (dotted line), (c) substrate etched in 2% HF solution and bombarded at room temperature, (RT) (dash-dotted line), (d) substrate covered with native oxide and bombarded at RT (thin solid line).
(0.5). The big N 1s peak on the right in the figure also confirms an increase of nitrogen in the substrate top layers (oxygen peak in the XPS spectrum was significantly reduced after nitrogen ion bombardment). Using the results of the XPS substrate analysis in Fig. 2 we can deduce that nitrogen was bound to silicon
Fig. 2. Typical XPS spectra of etched (upper spectrum) and thermally flashed (lower spectrum) Si(111) substrates. The insets: detailed scans of Si 2p peaks, (a) thermally flashed Si (left)-for comparison a double Si 2p peak typical for Si covered by native oxide is shown (dotted line), (b) Si etched in 2% HF solution (right).
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Fig. 4. Detailed scans of Ga 2p3y2 and N 1s peaks taken by XPS on layers deposited by simultaneous Ga evaporation and bombardment by hyperthermal nitrogen ions: (a) 50 eV nitrogen ions, RT, thermally flashed Si substrate (thick solid line), (b) 50 eV nitrogen ions, RT, silicon surface (containing native oxide) covered by Ga layers (‘Ga-rich’ surface), (dotted line) (c) 50 eV nitrogen ions, 400 8C, thermally flashed and nitrogen ion beam bombarded (400 8C) Si substrate (‘N-rich surface’), (dash-dotted line), (d) 50 eV nitrogen ions, RT, substrate etched in 2% HF solution (thin solid line). The Ga 2p3y2 peaks were normalized to the same height to make peak shifts and shapes better comparable.
only in the case of presence of oxygen atoms in the surface layers. Taking into account the energy of the Si–O bond (3.4 eV) is higher than the Si–N one (2.0 eV), the Si–O bond is generally preferred (exactly valid under equilibrium conditions only). Therefore, one can suppose that physical mechanisms could be more responsible for the formation of Si–N bonds under presence of oxygen atoms. One of the reasons for that might be higher energy transfer from nitrogen atoms to oxygen atoms than to silicon atoms as the mass of nitrogen is closer to oxygen than to silicon. However, the collision of molecular nitrogen ions with a surface, which were prevailing in the ion beam, is a complex phenomena (e.g. dissociative adsorption might occur) and, therefore, the separate roles of hyperthermal nitrogen atomic and molecular ions in interaction with the silicon substrate should be experimentally studied by the mass filtered ion beams in the future.
4.3. Ga–N deposition Simultaneous deposition of Ga atoms and nitrogen ions on Si substrates treated or modified in the ways described above was carried out. In Fig. 4 the detailed scans of Ga 2p3y2 and N 1s peaks taken by XPS at sample surfaces are shown. The Ga 2p3y2 peaks were normalized to the same height to make peak shifts and shapes better comparable. Table 1 shows the relative content of Ga–N, Ga–O and Ga–Ga bonds found by this technique in the deposited layers. The Ga 2p3y2 peak drawn in the thick solid line corresponds to an experiment made for the standard Ga-atomic flux and 50 eV nitrogen ion beam on a thermally flashed silicon substrate kept at room temperature. The peak does not show any significant broadening typical for the formation of chemical bonds of Ga with nitrogen or oxygen. The similar peak was obtained for the layers deposited
Table 1 The relative content of Ga–N, Ga–O and Ga–Ga bonds in the films Ga bondsysubstrate
Ga–N Ga–O Ga–Ga
Concentration (%) Ga-rich, 50 eV
N-rich, 400 8C, 50 eV
Etched, 50 eV
25 17 58
47 13 40
20 35 45
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on a thermally flashed Si substrate kept at room temperature when 150 eV nitrogen ion beam was used. The same result was also achieved when Ga-atomic flux was decreased by 50% and the 50 eV nitrogen ion beam was applied on the thermally flashed silicon substrate maintained at room temperature. The Ga 2p3y2 peak drawn in dotted line is widened towards higher binding energies due to the bonds created by Ga atoms to N (25%) and O (17%) (i.e. 58% belongs to the bonds associated with the non-shifted Ga peak, i.e. to Ga–Ga bonds). This peak corresponds to an experiment made for the standard Ga-atomic flux and 50 eV nitrogen ion beam on a silicon surface (containing native oxide) covered by Ga layers (‘Ga-rich’ surface) and kept at room temperature. The peak shift due to the Ga–N bond is smaller (0.9 eV) in comparison with the shift made by Ga–O bond (1.8 eV). The dash-dotted peak has a relatively narrow maximum shifted in the way characteristic for Ga–N bonds. The peak fitting procedure confirmed the highest content of Ga–N bonds in the layers prepared by our experiments at all (47% Ga–N, 13% Ga–O). The peak belongs to the layers deposited on a thermally flashed ‘N-rich’ Si(111) substrate heated to 400 8C using the standard Ga-atomic flux and 50 eV nitrogen ion beam. Before deposition the substrate was bombarded at an enhanced substrate temperature (400 8C) by nitrogen hyperthermal ions. As shown in Fig. 3, XPS analysis of such a bombarded substrate did not reveal a presence of Si–N bonds (dotted line). However, an increased amount of nitrogen was found at the surface (see corresponding N 1s peak in this figure). The layer thickness estimated by XPS was only 0.4 ML, which means that the nonbonded Ga was almost completely evaporated from the heated surface. The thin solid line belongs to the layers deposited on a silicon substrate etched in 2% HF solution and kept at room temperature, the energy of nitrogen ions was 50 eV. The broad peak indicates relatively high content of Si–O bonds (35% Ga–O, 20% Ga–N). The results show that the low-temperature synthesis of Ga–N layers by Ga atoms and hyperthermal nitrogen
ions is promoted by enhanced substrate temperature and probably by the presence of N atoms on the surface. However, due to the low nitrogen ion current the thickness of the films is small. 5. Conclusions The experiments described in this paper demonstrate the suitability of our complex UHV apparatus for insitu investigation of ‘low temperature’ Ga–N ultra-thin film synthesis. The synthesis of Ga–N layers was most successful on a ‘nitrogen-rich’ substrate at enhanced substrate temperature (400 8C). The ‘gallium-rich’ substrate possessed smaller number of Ga–N bonds. Higher content of Ga–O bonds was found on Si substrates covered by native oxide and chemically etched silicon. ‘Nitrogen-rich’ Si surfaces grew on substrates contaminated by oxygen (‘native’ and chemically etched Si). In this study, XPS appeared to be the most valuable method for the layer analysis. Acknowledgments This work was supported by the Czech Grant Agency (Project No. 202y02y0767 and 102y02y0506yA), the Ministry of Education CR (SEZ:J22y98:262100002) and the Austrian-Czech project Kontakt (No. 2003-18). References w1x B. Yuwono, T. Schloesser, A. Gschwardtner, G. Innertsberger, A. Grassi, A. Olbrich, W.H. Krautschneider, Microelectron. Eng. 48 (1–4) (1999) 51. w2x D. Marton, Film deposition from low-energy ion beams, in: J.W. Rabalais (Ed.), Low Energy Ion-Surface Interactions, John Wiley and Sons, Chichester, 1994, p. 511, ISBN 0 471 93891 2. w3x W.M. Lau, X. Feng, I. Bello, S. Sant, K.K. Foo, R.P.W. Lawson, Nucl. Instrum. Methods Phys. Res. B 59–60 (1991) 316. w4x B.S. Swartzentruber, Y.W. Mo, M.B. Webb, M.G. Lagally, J. Vac. Sci. Technol. A 7 (4) (1989) 2901. w5x M. Mohai, http:yywww.chemres.huyAKKL. ˇ w6x M. Kolıbal, ´ ´ S. Prusa, T. Sikola, Surf. Sci., in print. ˚ ˇ P. Babor,
Deposition and in-situ characterization of ultra-thin films ˇ ´ ˇ ´ ´ Miroslav Kolıbal, Stanislav Voborny, Jindrich Mach, Jan Cechal, Petr Babor, Stanislav Prusa, ˚ˇ ˇ ˇ´ Spousta, Tomas ´ˇ Sikola* Jirı Institute of Physical Engineering, Brno University of Technology, Technicka´ 2, 616 69 Brno, Czech Republic Available Online February 26 2004
Abstract In the paper the application of a complex UHV apparatus for in-situ investigation of low-temperature Ga–N ultra-thin film synthesis by thermal Ga atoms and hyperthermal nitrogen ions is presented. It was found by XPS that the highest content of Ga–N bonds was present in the layers grown on a ‘nitrogen-rich’ substrate at enhanced substrate temperature (400 8C). ‘Nitrogenrich’ Si surfaces grew from hyperthermal nitrogen ions on substrates contaminated by oxygen (‘native’ and chemically etched Si). The ‘gallium-rich’ substrate possessed smaller number of Ga–N bonds. Higher content of Ga–O bonds was found on Si substrates covered by native oxide and chemically etched silicon. Due to low nitrogen ion currents the thickness of the films was small (-4 ML). 䊚 2003 Elsevier B.V. All rights reserved. PACS: 81.15.Ef Keywords: Direct ion beam deposition; Ultra-thin films; GaN; X-Ray photoelectron spectroscopy (XPS)
1. Introduction As the dimensions of electronic and optoelectronic devices are shrinking, the ultra-thin filmsymultilayers have become an important issue in deposition technologies w1x. There are various methods how to prepare these films with typical dimensions below 10 nm, however, to analyse them makes the task more difficult. In addition to a need to use surface sensitive methods, ex-situ analyses and measurements are not optimal as the surface contamination during the transfer of the samples through ambient atmospheric conditions makes their study almost impossible. To increase reliability of the analysis, it is essential to analyze the films in-situ during or after their deposition without a significant delay. The similar problem arises when the early periods of thin film growth are studied. In the contribution, in-situ investigation of low-temperature synthesis of Ga–N ultra-thin films (-600 8C) by thermal Ga atoms and hyperthermal nitrogen ions is presented. The ‘conventional’ synthesis takes part at *Corresponding author. Tel.: q420-5-4114-2707; fax: q420-54114-2842. ˇ E-mail addresses: [email protected] (T. Sikola), ´ [email protected] (S. Voborny).
higher temperatures (e.g. )1000 8C for MOCVD) due to higher activation energies for GaN growth. The results obtained in such a study contribute to finding the basic principles and mechanisms of the early periods in the low-temperature growth of thicker GaN layers based on application of hyperthermal ions. The films were prepared at different deposition parameters (ion energy, ion-to-atom arrival ratio, substrate temperature) on silicon substrates pre-treated in several distinct ways and preferentially analyzed in-situ by Xray photoelectron spectroscopy (XPS). Additionally, to quickly monitor the technological steps already in the deposition chamber, qualitative methods as time-offlight low energy ion scattering (TOF-LEIS), and static secondary ion mass spectroscopy (SIMS) were applied. Direct ion beam deposition, called shortly IBD (ion beam deposition), is an advanced method for the preparation of clean ultra-thin films. It is characterized by the application of hyperthermal primary ions (-100 eV preferentially) as a film forming material from which the layer is constituted w2,3x. There are several advantages of this method: direct mass and energy control of the ion beam and its incidence angle, UHV conditions making in-situ analysis possible and an energy excess of ions compared to average atom energies in evapora-
0040-6090/04/$ - see front matter 䊚 2003 Elsevier B.V. All rights reserved. doi:10.1016/j.tsf.2003.12.076
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Fig. 1. Complex UHV apparatus for deposition and in-situ analysis of thin films.
tion or sputtering methods (E-10 eV) w2x. This energy excess results in a subsurface growth improving the layer adhesion and in formation of metastable compounds. 2. System design A schematic view of the complex UHV apparatus for deposition and in-situ analysis of ultra-thin films is shown in Fig. 1. The apparatus consists of three interlinked major chambers. The first, called the deposition chamber, contains a home-built ion gun for direct ion beam deposition consisting of a plasma ion source, Wien filter and beam transport optics. Further, an ebeam effusion cell (Omicron) for evaporation of solid elements (e.g. metals) has been installed there. Additionally, to direct ion beam deposition, this configuration enables to carry out an UHV-atomic beam evaporation assisted by a hyperthermal ion beam. A home-built experimental set-up consisting of an electron-impact ion beam source (ISE 100, Omicron) together with a drift tube with an MCP detector (Hamamatsu) at its end is used for TOF-LEIS. The same ion beam source along with a quadrupole mass spectrometer equipped with a three-lens-extractionion optics (QMG 421 Balzers) makes SIMS analysis possible.
The base pressure achievable in the chamber pumped by 3 ion pumps (280 lys in total) is within an order of 10y7 Pa. The samples can be in-situ transported to an adjacent analytical chamber for XPS (base pressure f10y7 Pa) or to another chamber containing surface analytical methods, such as scanning tunnelling microscopyyatomic force microscopy (STMyAFM) and low energy electron diffraction (LEED). The base pressure in this chamber is in an order of 10y8 Pa. In addition to a ‘low-energy’ mode (generation of hyperthermal ions of 20–300 eV), the IBD ion beam gun can be run also in a ‘high-energy’ mode (ion energy up to 3000 eV) to clean sample surfaces by ion beam bombardment and to analyse surface. A three-stage differentially pumped system enables to maintain ultrahigh vacuum in the deposition chamber (f1=10y6 Pa) while the ion source is operated at pressures 1=100 Pa. The typical ion beam currents in the low energy mode are in the order 10y1 mA, the spot size of the beam is approximately 8–10 mm (FWHM). Hence, the corresponding low ion current densities determine the applications to the growth of ultra-thin films. 3. Experimental The apparatus was applied to test the ability to synthesize Ga–N layers at lower substrate temperatures
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(-600 8C). In addition to silicon substrates covered with native silicon oxide, hydrogen terminated Si (111), reconstructed Si (111)-7=7, Ga-covered Si (111) and Si (111) pre-treated by hyperthermal nitrogen ion beam were used as substrates. The substrates with native silicon oxide were in-situ thermally treated for several hours at 700 8C, hydrogen terminated silicon substrates were prepared by etching of Si(111) samples in a 2% HF solution for 2 min. The removal of native oxide and reconstruction of Si (111) surface was made by thermal flashing according to w4x. A Si (111) substrate with native oxide was annealed for several hours at 700 8C first, and then thermally flashed for a total time of 2 min slightly above 1200 8C by electric current going directly through it. Flashing was always interrupted when the pressure tended to exceed a range of 10y6 Pa to avoid the ‘hazy’ appearance of the substrate w4x. The Ga-covered Si (111) substrate was prepared by evaporation of Ga layers (f5 ML) on a Si (111) substrate from the effusion cell. The modification of Si (111) substrates by hyperthermal nitrogen ion beam (50 eV) was carried out for a period of 1 h for every sample and the results will be shown below. The work pressure in the chamber during all deposition experiments was kept in a range of 10y6 Pa. Evaporation rate of the effusion cell was calibrated by the crystal quartz monitor (Sycon Instruments). During Ga–N growth experiments both the IBD ion gun and ebeam effusion cell were run for 1 h simultaneously. The energy of the ion gun was changed within the interval of 50–150 eV and the nitrogen ion current varied from 200 to 250 nA. All experiments were done without a mass separation to keep the ion current at a reasonable value and, therefore, the concurrent impact of Nq and Nq 2 ions on samples took place. To maintain the sample contamination by oxygen or hydrocarbons sufficiently low, the 30-min rinsing of the ion gun by nitrogen gas was made before each deposition. The Ga-evaporation rate from the effusion cell was matched to this presently maximum achievable ion beam current and thus the growth rate was very low (typically 4 ML per 1 h). Further, the corresponding Ga-atomic flux will be called a ‘standard flux’. Each technological step (substrate preparation and modification, Ga–N growth) was monitored for different operational conditions in detail by XPS. Fast complementary monitoring was carried out by TOF-LEIS, and SIMS, as well. XPS analysis of the Ga–N layers was performed by a commercial spectrometer (EA 125, DAR 400, Omicron) at X-ray energy of 1253.6 eV (Mg Ka) in the mode of constant path energy (15 and 25 eV). Photoelectrons were collected at two distinct detection angles of 418 (measured from the sample surface). The operational pressure was kept below 2=10y6 Pa. The quantitative evaluation of element concentrations and chemical bonds from peak intensities and chemical
19
shifts, respectively, was carried out by means of software as SDP40 (spectral data processor) and XPS multiquant w5 x . In TOF-LEIS experiments 3 keV helium ions were used. After acceleration the ion beam was chopped over a removable screen-plate slit to produce ion pulses of a time width G120 ns (FWHM). The time-of-flight measurements were controlled by an Ortec electronics consisting of a time-to-amplitude convertor (TAC) and a multichannel-pulse-analyzer card integrated into a computer. SIMS was run in the static mode (current density jf102 nAycm2) using argon primary ions impinging the surface under 458. The electronics available made the detection of only positive ions possible. 4. Results and discussions In this section the results achieved by XPS in individual steps necessary for running experiments on Ga–N layer deposition are discussed in detail. Our TOF-LEIS and SIMS experiments failed to detect not only carbon and oxygen impurities in the films, but also a nitrogen component of these films itself. As for TOF-LEIS the peaks of these elements were surpassed by the broad high-intensity peak of ions scattered from the silicon substrate w6x. Due to the restriction to application of argon ions and detection of positive ions only, the sensitivity of our SIMS experimental set-up towards nitrogen, carbon and oxygen was too low as these elements possess very low secondary ion yields for positive ions (in case of nitrogen by a factor of 10 000 lower than for Ga). However, TOF-LEIS and SIMS provided a quick check of the presence of gallium at the substrate after deposition. Here, particularly SIMS was a useful technique capable to sense small amounts of Ga at the substrates after depositions at 400 8C substrate temperature. TOF-LEIS was of assistance in estimation of a film thickness as it was found the width of the Ga peak depends on the thickness of Ga-based ultra-thin films w6x. 4.1. Substrate cleaning In Fig. 2 the typical XPS spectra of etched and thermally flashed Si substrates are shown. The peaks corresponding to carbon and oxygen impurities are lower in case of the latter spectrum (C – 5 atomic%, O – 2 atomic%). In the figure inset on the left the detail of the Si 2p peak of the thermally flashed substrate is shown. The peak Si 2p (q4) corresponding to the Si bond with four oxygen atoms (f103.5 eV) in silicon covered with native oxide (see the dotted line) disappeared and only the peak (known as Si 2p (q0)) related to Si–Si bond remained. In the inset on the right the detail of the Si 2p peak of the etched substrate shows
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small broadening on the left side of the peak due to the chemical shift caused by the single Si–O bond (Si 2p (q1)). 4.2. Substrate modification To prepare ‘N-rich’ substrates, the silicon substrates were bombarded by hyperthermal nitrogen ions. In Fig. 3 the detailed scans of Si 2p and N 1s peaks taken by XPS at these substrates are shown. The Si 2p peaks drawn in the thick solid line and dotted line correspond to thermally flashed Si (111) substrates bombarded by nitrogen ions at room and enhanced (400 8C) substrate temperatures, respectively. The peaks do not show any significant broadening typical for the formation of chemical bonds of Si with nitrogen or oxygen. Therefore, nitrogen giving N 1s peaks (Fig. 3, right) was most likely just weakly trapped in near-surface layers. The concentration of nitrogen in these samples was 2.4% and 5.3% for RT and 400 8C, respectively. Mild broadening on the left side of the peak is observable for the experiment run on the etched substrate (dash-dotted line) and quite remarkable for the ion beam bombardment of a substrate with a native oxide (thin solid line). Processing the peaks revealed the broadening was caused by bonds of Si to oxygen and nitrogen (oxinitrides) and the thickness of these modified layers was approximately 1 nm. The ratio of the atomic concentrations of Si bound to N and Si bound to Si was 0.15 for the etched substrate (0.3 for Si–OySi–Si). The same ratio for the substrate with native oxide was 2.2
Fig. 3. Detailed scans of Si 2p and N 1s peaks taken by XPS at Si(111) substrates bombarded by hyperthermal nitrogen ions (50 eV): (a) thermally flashed substrate bombarded at room temperature (thick solid line), (b) thermally flashed substrate bombarded at 400 8C (dotted line), (c) substrate etched in 2% HF solution and bombarded at room temperature, (RT) (dash-dotted line), (d) substrate covered with native oxide and bombarded at RT (thin solid line).
(0.5). The big N 1s peak on the right in the figure also confirms an increase of nitrogen in the substrate top layers (oxygen peak in the XPS spectrum was significantly reduced after nitrogen ion bombardment). Using the results of the XPS substrate analysis in Fig. 2 we can deduce that nitrogen was bound to silicon
Fig. 2. Typical XPS spectra of etched (upper spectrum) and thermally flashed (lower spectrum) Si(111) substrates. The insets: detailed scans of Si 2p peaks, (a) thermally flashed Si (left)-for comparison a double Si 2p peak typical for Si covered by native oxide is shown (dotted line), (b) Si etched in 2% HF solution (right).
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Fig. 4. Detailed scans of Ga 2p3y2 and N 1s peaks taken by XPS on layers deposited by simultaneous Ga evaporation and bombardment by hyperthermal nitrogen ions: (a) 50 eV nitrogen ions, RT, thermally flashed Si substrate (thick solid line), (b) 50 eV nitrogen ions, RT, silicon surface (containing native oxide) covered by Ga layers (‘Ga-rich’ surface), (dotted line) (c) 50 eV nitrogen ions, 400 8C, thermally flashed and nitrogen ion beam bombarded (400 8C) Si substrate (‘N-rich surface’), (dash-dotted line), (d) 50 eV nitrogen ions, RT, substrate etched in 2% HF solution (thin solid line). The Ga 2p3y2 peaks were normalized to the same height to make peak shifts and shapes better comparable.
only in the case of presence of oxygen atoms in the surface layers. Taking into account the energy of the Si–O bond (3.4 eV) is higher than the Si–N one (2.0 eV), the Si–O bond is generally preferred (exactly valid under equilibrium conditions only). Therefore, one can suppose that physical mechanisms could be more responsible for the formation of Si–N bonds under presence of oxygen atoms. One of the reasons for that might be higher energy transfer from nitrogen atoms to oxygen atoms than to silicon atoms as the mass of nitrogen is closer to oxygen than to silicon. However, the collision of molecular nitrogen ions with a surface, which were prevailing in the ion beam, is a complex phenomena (e.g. dissociative adsorption might occur) and, therefore, the separate roles of hyperthermal nitrogen atomic and molecular ions in interaction with the silicon substrate should be experimentally studied by the mass filtered ion beams in the future.
4.3. Ga–N deposition Simultaneous deposition of Ga atoms and nitrogen ions on Si substrates treated or modified in the ways described above was carried out. In Fig. 4 the detailed scans of Ga 2p3y2 and N 1s peaks taken by XPS at sample surfaces are shown. The Ga 2p3y2 peaks were normalized to the same height to make peak shifts and shapes better comparable. Table 1 shows the relative content of Ga–N, Ga–O and Ga–Ga bonds found by this technique in the deposited layers. The Ga 2p3y2 peak drawn in the thick solid line corresponds to an experiment made for the standard Ga-atomic flux and 50 eV nitrogen ion beam on a thermally flashed silicon substrate kept at room temperature. The peak does not show any significant broadening typical for the formation of chemical bonds of Ga with nitrogen or oxygen. The similar peak was obtained for the layers deposited
Table 1 The relative content of Ga–N, Ga–O and Ga–Ga bonds in the films Ga bondsysubstrate
Ga–N Ga–O Ga–Ga
Concentration (%) Ga-rich, 50 eV
N-rich, 400 8C, 50 eV
Etched, 50 eV
25 17 58
47 13 40
20 35 45
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on a thermally flashed Si substrate kept at room temperature when 150 eV nitrogen ion beam was used. The same result was also achieved when Ga-atomic flux was decreased by 50% and the 50 eV nitrogen ion beam was applied on the thermally flashed silicon substrate maintained at room temperature. The Ga 2p3y2 peak drawn in dotted line is widened towards higher binding energies due to the bonds created by Ga atoms to N (25%) and O (17%) (i.e. 58% belongs to the bonds associated with the non-shifted Ga peak, i.e. to Ga–Ga bonds). This peak corresponds to an experiment made for the standard Ga-atomic flux and 50 eV nitrogen ion beam on a silicon surface (containing native oxide) covered by Ga layers (‘Ga-rich’ surface) and kept at room temperature. The peak shift due to the Ga–N bond is smaller (0.9 eV) in comparison with the shift made by Ga–O bond (1.8 eV). The dash-dotted peak has a relatively narrow maximum shifted in the way characteristic for Ga–N bonds. The peak fitting procedure confirmed the highest content of Ga–N bonds in the layers prepared by our experiments at all (47% Ga–N, 13% Ga–O). The peak belongs to the layers deposited on a thermally flashed ‘N-rich’ Si(111) substrate heated to 400 8C using the standard Ga-atomic flux and 50 eV nitrogen ion beam. Before deposition the substrate was bombarded at an enhanced substrate temperature (400 8C) by nitrogen hyperthermal ions. As shown in Fig. 3, XPS analysis of such a bombarded substrate did not reveal a presence of Si–N bonds (dotted line). However, an increased amount of nitrogen was found at the surface (see corresponding N 1s peak in this figure). The layer thickness estimated by XPS was only 0.4 ML, which means that the nonbonded Ga was almost completely evaporated from the heated surface. The thin solid line belongs to the layers deposited on a silicon substrate etched in 2% HF solution and kept at room temperature, the energy of nitrogen ions was 50 eV. The broad peak indicates relatively high content of Si–O bonds (35% Ga–O, 20% Ga–N). The results show that the low-temperature synthesis of Ga–N layers by Ga atoms and hyperthermal nitrogen
ions is promoted by enhanced substrate temperature and probably by the presence of N atoms on the surface. However, due to the low nitrogen ion current the thickness of the films is small. 5. Conclusions The experiments described in this paper demonstrate the suitability of our complex UHV apparatus for insitu investigation of ‘low temperature’ Ga–N ultra-thin film synthesis. The synthesis of Ga–N layers was most successful on a ‘nitrogen-rich’ substrate at enhanced substrate temperature (400 8C). The ‘gallium-rich’ substrate possessed smaller number of Ga–N bonds. Higher content of Ga–O bonds was found on Si substrates covered by native oxide and chemically etched silicon. ‘Nitrogen-rich’ Si surfaces grew on substrates contaminated by oxygen (‘native’ and chemically etched Si). In this study, XPS appeared to be the most valuable method for the layer analysis. Acknowledgments This work was supported by the Czech Grant Agency (Project No. 202y02y0767 and 102y02y0506yA), the Ministry of Education CR (SEZ:J22y98:262100002) and the Austrian-Czech project Kontakt (No. 2003-18). References w1x B. Yuwono, T. Schloesser, A. Gschwardtner, G. Innertsberger, A. Grassi, A. Olbrich, W.H. Krautschneider, Microelectron. Eng. 48 (1–4) (1999) 51. w2x D. Marton, Film deposition from low-energy ion beams, in: J.W. Rabalais (Ed.), Low Energy Ion-Surface Interactions, John Wiley and Sons, Chichester, 1994, p. 511, ISBN 0 471 93891 2. w3x W.M. Lau, X. Feng, I. Bello, S. Sant, K.K. Foo, R.P.W. Lawson, Nucl. Instrum. Methods Phys. Res. B 59–60 (1991) 316. w4x B.S. Swartzentruber, Y.W. Mo, M.B. Webb, M.G. Lagally, J. Vac. Sci. Technol. A 7 (4) (1989) 2901. w5x M. Mohai, http:yywww.chemres.huyAKKL. ˇ w6x M. Kolıbal, ´ ´ S. Prusa, T. Sikola, Surf. Sci., in print. ˚ ˇ P. Babor,