Negative ion production by surface ionization at aluminum-nitride surfaces

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1 Negative ion production by surface ionization at aluminum-nitride surfaces S. Jans, P. Wurz, R. Schletti, T. Fröhlich, E. Hertzberg, and S. Fuselier Citation: Journal of Applied Physics 87, 2587 (2000); doi: / View online: View Table of Contents: Published by the AIP Publishing Articles you may be interested in Negative ion surface production through sputtering in hydrogen plasma Appl. Phys. Lett. 95, (2009); / Relevance of Volume and Surface Plasma Generation of Negative Ions in Gas Discharges AIP Conf. Proc. 763, 122 (2005); / Formation of silicon on plasma synthesized aluminum nitride structure by ion cutting J. Vac. Sci. Technol. B 22, 2748 (2004); / Enhancement of ionization efficiency of surface, electron bombardment and laser ion sources by axial magnetic field application Rev. Sci. Instrum. 75, 1585 (2004); / Dispersion properties of aluminum nitride as measured by an optical waveguide technique Appl. Phys. Lett. 70, 3206 (1997); /

2 JOURNAL OF APPLIED PHYSICS VOLUME 87, NUMBER 5 1 MARCH 2000 Negative ion production by surface ionization at aluminum-nitride surfaces S. Jans, a) P. Wurz, R. Schletti, and T. Fröhlich Physikalisches Institut, University of Berne, Sidlerstrasse 5, CH-3012 Bern, Switzerland E. Hertzberg and S. Fuselier Lockheed Martin Research Laboratories, Space Science Laboratory, 3251 Hanover Street, Palo Alto, California Received 21 June 1999; accepted for publication 2 December 1999 In this article, we report on the observation of the formation of negatively charged ions upon reflection from an aluminum-nitride surface. Positive H 2 and O 2 ions are scattered at small angles of incidence off a single-crystal surface. Charge exchange at the surface yields neutral particles and negative ions in the reflected particle flux. The negative ion fractions are about 15% and 1% for oxygen and hydrogen, respectively. The particle reflection probability is only 2%, which is attributed to the roughness of the surface American Institute of Physics. S I. INTRODUCTION a Electronic mail: sarah.jans@phim.unibe.ch For a mass spectrograph for low energy particles, designed to explore the two-dimensional structure of magnetospheric features on the IMAGE satellite mission 1 and others, surface ionization was identified as the only viable ionization technique to have the potential to meet the requirements concerning ionization efficiency for the energy range of 10 ev to 1 kev within the limitations imposed by the resources space, weight, power, etc. available on a satellite. 2 It was concluded that the detection efficiency should exceed 1%, should be uniform over large areas and should exhibit good long-term stability during the mission duration. Surface ionization introduces new demands on the design of the mass spectrometer and requires the development of new analyzer elements with matched ion optical properties. An instrument meeting these demands has been described recently. 3,4 The main species of interest in magnetospheric research are H and O atoms, which is why we limited our current study to these species. In the past 15 years, surface ionization has been studied extensively for potential application in fusion plasma research. With this technique, ionization efficiencies of up to 67% in the energy range from several ev to about 1 kev 5 7 have been achieved, using low work function surfaces for converting neutral particles or positive ions into negative ions. Actually, one should therefore say electron capture into the negative ion state instead of ionization. Low work function surfaces were obtained by coating a metallic substrate with a monolayer or less of an alkali metal 8 or an alkalineearth metal. 9 The application of this overlayer of metal usually involves a dispenser, which releases defined quantities of the metal upon heating. The alkali metal and to a lesser degree the alkaline-earth metal surfaces are chemically very sensitive and they degrade even in a good vacuum environment with characteristic time constants of hours. Thus, regeneration of the converter surface is necessary, which involves heating of the surface to substantial temperatures 1000 C to evaporate the adsorbates and the alkali or alkaline-earth metal overlayer. 10 Then, one proceeds with the application of a fresh alkali or alkaline-earth metal layer. In addition to surface heating, the handling of a dispenser introduces some complexity such as monitoring the work function of the surface. 11 Despite these experimental challenges, Cs/W 110 Ref. 12 and Ba/W 110 Ref. 13 converter surfaces can in principle be used on a space platform. In order to get away from the rather involved technique using Cs and Ba overlayers, we were looking for alternative converter surfaces, where the regeneration of the converter would be easier or not necessary at all. Our search focused on surfaces, which were known to be good secondary electron emitters, because our conjecture was that these surfaces also might work well for the formation of negative ions. We investigated various insulating surfaces, namely, several polycrystalline diamond surfaces, monocrystalline natural diamond surfaces, two BaZrO 3 surfaces, and an AlN surface, and we found negative ion fractions between a few and almost 30%. 14,15 Recently, a new theoretical approach to explain resonant transfer processes in d-electron metals, semiconductors, and insulators has been undertaken. 16 According to this theory, the negative ion yield depends strongly on the parallel velocity of the moving ion. The authors could qualitatively explain experimental results on LiF. We are going to compare our new experimental results on the AlN surface to this and other theories. II. EXPERIMENTAL SETUP The experiment consists of an ion source, a beam guiding system, a sample stage with housing and with an alkali dispenser unit not used for this study, and a detection unit. All these units are contained in a single vacuum chamber pumped by a turbomolecular pump. Ions are formed in an electron impact ion source Nier type, with the intensity of the primary ion beam of 10 fa, and are accelerated to /2000/87(5)/2587/6/$ American Institute of Physics

3 2588 J. Appl. Phys., Vol. 87, No. 5, 1 March 2000 Jans et al. energies from 300 to 2000 ev. These ions are then deflected in a 90 cylindrical analyzer with an energy width of the ion beam at the sample of E/E 1% FWHM. The energy analyzer focuses the ion beam on the entrance aperture of the sample housing. Two diaphragms limit the beam size to 1 mm and the beam divergence to 1. The impact angle of the ion beam on the conversion surface can be chosen between 90 and 0 with respect to the surface normal. The reflected beam is recorded on a two-dimensional position-sensitive microchannel plate MCP detector with a viewing angle of 12.5 in azimuthal and 12.5 in polar direction. The detector floats on an adjustable high voltage. A retarding potential analyzer RPA consisting of three grids in mounted in front of the MCP detector. The detector unit, including the RPA, is shielded electrostatically and can be rotated independently from the converter surface around the same axis. The outer grids of the RPA are grounded to shield the inner grid which has a permanent bias to supress positive ions. An additional grid in front of the MCP detector at negative potential with respect to the MCP detector serves to reject secondary electrons originating from the preceding grids and the converter surface. Using the two-dimensional information from the detector, an angularly resolved conversion efficiency is derived. In addition, angular scattering in the polar and azimuthal directions is measured. The ionization efficiency, that is, the fraction of negative ions scattered off the surface, is determined by measuring the total reflected beam positive ions are always hindered from reaching the detector as explained above and its neutral component. To sweep out negatively charged particles from the reflected beam, the MCP detector is floated on a high negative voltage with respect to the converter surface. The difference between the total reflected beam and its neutral component is the flux of reflected negative ions. The ionization efficiency is then the flux of reflected negative ions devided by the total reflected beam. Although we want eventually to use surface ionization to negatively ionize neutral atoms, for this experiment, we used positive ions. Furthermore, we used hydrogen and oxygen molecular ions because molecular ions can be produced far more efficiently in our system. Recently, we verified experimentally for oxygen that the negative ion yield does not change using positive ions or neutrals as primary particles. 10,17 Ions approaching the surface are effectively neutralized upon dissociation. From the total current on the sample and the count rate on the detector, after correction for the detection efficiency, 13 we determine the particle reflection efficiency, that is the number of particles ions and neutrals scattered inside the detection area of the detector a cone of 25 in diameter divided by the incoming particle flux. To calculate the incoming particle flux, one has to consider the secondary electron emission of the surface because these electrons also contribute to the measured current. This was done by measuring the primary current twice, once with the sample being floated on a voltage of about 17 V and once with the sample at 0 V. With the floating voltage, most secondary electrons were being retained at the surface. From the two values we calculated the current fraction caused by the secondary electrons and subtracted it from the total current. The product of the ionization efficiency and the reflection efficiency is the detection efficiency, which is the crucial number for our application. III. AIN SAMPLE Bulk AIN has a hexagonal structure wurtzite with almost the same a-axis lattice constant as SiC 3.11 Å, SiC: 3.08 Å and a direct optical band gap of 6.2 ev. The investigated sample was a thin AIN film deposited on a 6H SiC 0001 substrate by molecular-beam epitaxy techniques, where refers to the family of hexagonal polytypes and 6H specifies the six layer repetition. The AIN layer also exhibits a hexagonal structure like bulk material. The surface roughness is 17.7 Å as measured with an atomic force microscope. For the AIN surface, a negative electron affinity NEA was reported. 18 The presence of the NEA depends on the surface structure and determination. Materials exhibiting a NEA are good secondary electron emitters as any valence band electrons promoted into the conduction band may escape from the surface uninhibited. They can possibly be used to build cold cathode emitters. The sample was stored in a container in air for several years before introduction into the vacuum chamber. It was washed in methanol immediately before being mounted. After pump-down, the entire vacuum chamber was baked out at 80 C for 20 h, after which a residual gas pressure of mbar was achieved. The sample could be heated to 300 C in order to remove adsorbates from the surface. During operation, the pressure may rise into the low 10 7 mbar range as a result of leaking the test gas into the ion source chamber. IV. RESULTS The measured angular scattering for an O 2 beam impinging on the AIN surface at an incident angle of 82 and 85 is displayed in Fig. 1 and Fig. 2, respectively. The full width half maximum FWHM of the angular distribution is 12 in polar and azimuthal direction at 82 incidence angle. This is wider than we observed for other samples 15 and is attributed to the large surface roughness. The observed pedestal is caused by nonspecular scattering of the particles into the hemisphere due to surface roughness on an atomic scale. It is reduced for smaller angles of incident, but remains very large. Assuming this nonspecular scattering to be isotropic, we can calculate that only 2% of all reflected particles contribute to the specular peak. The remaining particles are lost for analysis. This is in good agreement with our results of the reflection efficiency measurements. For the particle reflection efficiency at an impact angle of 82 and a beam energy of 780 ev, we measured a value of 1.8% 0.8%. This is about an order of magnitude smaller than for other samples. 15 The secondary electron emission value was 39% under these conditions. Figures 3 and 4 show the negative ion yield for scattered oxygen. The results were corrected by considering the different detection efficiencies for neutral particles and ions. The

4 J. Appl. Phys., Vol. 87, No. 5, 1 March 2000 Jans et al FIG. 1. Measured angular scattering in polar and azimuthal directions for an O 2 ion beam with an energy of 780 ev impinging on an AlN surface at an incident angle of 82. total flux contains also fast secondary electrons. The corresponding counts were estimated by scattering Ne particles from the surface and an appropriate correction was applied. Neon has a similar mass as oxygen but does not form negative ions. The dependence on the energy component parallel to the surface was measured for primary energies ranging from 450 to 2000 ev for the two constant perpendicular energy values 10 and 22 ev. Most molecules dissociate upon neutralization before they hit the surface. The mean energy of single atoms is about half of the primary energy. For this reason, any given energy values refer to the energy of single atoms in the following and so do the values given in the figures. The negative ion fraction obtained at 390 ev is 14.8% 1.3% this is consistent with the earlier results 15 and there is a small decrease as the parallel energy increases as can be seen in Fig. 3. There is no sign of a kinetic resonance in the measured energy range. Both curves have the same shape. The dependence on the energy component perpendicular to the surface is depicted in Fig. 4. The charge fraction takes a maximum value of 16.6% at 10.5 ev. We consider the shape of the curve to be more reliable than it may seem by considering the large error bars for the uncertainties which are mostly due to the following systematic errors. The energy distribution of the scattered ions overlaps with the one of the FIG. 2. Measured angular scattering in polar and azimuthal directions for an O 2 ion beam with an energy of 780 ev impinging on an AlN surface at an incident angle of 85. secondary electrons emitted from the grids in front of the MCP detector. Thus, by setting the last grid in front of the MCP onto a negative potential in order to eliminate most secondary electrons, one also loses part of the ions to be measured. On the other hand, there are still some secondary electrons of higher energies that are not sweeped out. Besides that, the uncertainty of the detection efficiency is con- FIG. 3. Dependence of the ion yield on the energy component parallel to the AlN surface for an O 2 primary ion beam. The energy values correspond to the parallel energy components of the single atoms after dissociation of the molecules in front of the surface. The energy component perpendicular to the surface E was held constant, at E 11 ev squares and at E 5 ev diamonds, again these values refer to dissociated particles.

5 2590 J. Appl. Phys., Vol. 87, No. 5, 1 March 2000 Jans et al. FIG. 4. Dependence of the ion yield on the energy component perpendicular to the AlN surface for an O 2 primary ion beam. The energy component parallel to the surface is held constant at E 390 ev. The horizontal error bars originate from the setting of the impact angle, the uncertainty of the total energy is less than the symbol size. FIG. 6. Dependence of the ion yield on the energy component parallel to the AlN surface for an H 2 primary ion beam. The energy component perpendicular to the surface E was held at a constant value of 5 ev. siderably large. Within the data set presented in Fig. 4 the total energy of the incoming ions changes only slightly and the relative efficiencies are therefore more accurate. We were restricted to this small range of perpendicular energy because for steeper incoming angles, the fraction of particles scattered outside the detection region became too large. Also the range of energies of our ion beam system is limited. From the results above, it follows that the detection efficiency, that is the product of the ionization efficiency and the reflection efficiency, for oxygen amounts to 0.3%. Measurement results of a smoother AIN surface than the one we used would certainly be more satisfying for the small value in the detection efficiency we obtain is mainly due to the wide angular scattering of the particles. The reflected particle flux consists mainly of atomic particles. According to Ref. 17, we estimated the fraction of atomic particles to be 70%. The ion yield for a pure atomic ion beam would not be very different from our value under the assumption the ionization probabilities for O 2 and O are similar. Actually, the affinity level of O 2 is 0.44 ev whereas the one of O is 1.45 ev, which means that the ion yield for O is somewhat higher than the measured negative ion yield for the mixture of O 2 and O. The negative ion fraction for an incoming O 2 beam with a primary energy of 780 ev at an angle of incidence of 82 as a function of time is depicted in Fig. 5. The AIN sample was left in the vacuum chamber for three months. On the 74th day, it was heated moderately for a few minutes to remove possible adsorbates from the surface. Given the uncertainties of the measurement no substantial change with time can be argued, that is, no significant degrading was observed over this extended time period in spite of the moderate vacuum environment (10 7 mbar). The results for hydrogen are shown in Figs. 6 and 7 for the same energy ranges. The corrections for fast electrons were done using the results of scattered helium. The obtained ionization efficiency is about 1% at 390 ev at an angle of incidence of 83 and is thus much lower than the one obtained for oxygen. At larger parallel energies we cannot even be sure anymore if we obtained any ions. However, the ion yield does not seem to change as the normal velocity of the incoming hydrogen ion changes, Fig. 7. FIG. 5. Dependence of the negative ion yield on time for an O 2 primary ion beam with an energy of 780 ev. The probe was heated for a few minutes on the 74th day. FIG. 7. Dependence of the negative ion yield on the energy component perpendicular to the AlN surface for an H 2 primary ion beam. Energy component parallel to the surface: 390 ev. The horizontal error bars originate from the setting of the impact angle, the error of the total energy is less than the symbol size.

6 J. Appl. Phys., Vol. 87, No. 5, 1 March 2000 Jans et al V. DISCUSSION Several theories exist for surface ionization. The most advanced theory covers negative ion formation upon reflection from low work function metal surfaces 7 and explains the experimental results for low work function surfaces pretty well: When approaching a metal surface, the affinity level of an atom undergoes a downshift and broadening close to the surface because of the interaction with the induced image charge in the metal. Once the position of the affinity level gets close to the Fermi level of the metal, resonant oneelectron tunneling between the metal and the atom state will populate the negative ionic state. Since affinity levels of atoms are of the order of 1 ev, the work function of a metal usually around 5 ev has to be lowered by the application of a thin overlayer of an alkali metal or an alkali-earth metal to obtain appreciable yields of negative ions. 5,9 Aluminum nitride has a wide band gap of 6.2 ev. For our surface, the lower conduction-band edge is considered to be situated above the vacuum level, while the surface Fermi level should be near the center of the gap. 18 However, the electrons for the formation of a negative ion have to come from the upper edge of the valence band. With the broadening and downshifting of the affinity level of the atom, the overlap between the populated states in AIN and the empty projectile state will be small and so the ion yield according to this model. AlN being an insulator will help with the ion yield, since reneutralization of a formed ion in front of the surface is inhibited by the wide band gap. Moreover, some important ingredients of the theory like the free electron metal assumption are not fulfilled for an insulator. In conclusion, we do not think we can apply this theory here. In Ref. 19, a simple theoretical model for grazing scattering of oxygen atoms from an LiF surface was proposed. According to that approach, the formation of negative ions is due to binary collisions with F ions. In grazing scattering, a projectile interacts with a large number of target atoms along its path. This number grows bigger as the velocity parallel to the surface increases and so does the ionization efficiency. The authors could reproduce the sharp onset of the ion yield they measured, but their model fails to explain the decrease at higher velocities. There is an even more important argument. In our experiment the incoming particle could interact with any surface atom because of the covalent bonding of the AIN crystal. But our measurements were all made at incidence angles of about 77 to 87, therefore the particles can only interact with one or few surface atoms and the expected negative ion fraction is small. Since we do not find a sharp onset in our data, and the model fails to reproduce the decrease at higher energies, it is not considered suitable to explain our results. As mentioned above, the same results 19 have recently been explained qualitatively very well with a new theoretical approach reported in Ref. 16. The theory is developed from a Newns Anderson Hamiltonian, whose parameters are chosen according to the surface and projectile. There is an effective band broadening of the solid s surface density of states as seen from the moving particle. When the parallel velocity is large enough, so that the broadened and shifted affinity state overlaps with the extending valence band, the capture probability becomes different of zero. The model predicts a strong dependence of the ionization probability on the parallel velocity of the moving ion. The probability increases with parallel velocity until the band is very broadened. It falls as soon as the electrons of the ion can populate the conduction band, which is also extending as seen from the moving ion. The normal velocity plays a secondary role. As it increases the incoming particle gets closer to the surface and it therefore has an influence on the charge transfer probability, but the shape of the curve ion capture probability versus parallel velocity does not change significantly. In contrast to the ionic LiF, the bonding of AIN is covalent. If, nevertheless, the same approach holds for scattering oxygen and hydrogen atoms from an AIN surface, the measured ion yields must be located within the range where the capture probability is slightly falling. According to our lower energy limit of 200 ev, the onset of the probability should occur at low parallel velocities. It must then remain constant over a wide range. Above all, this should hold for hydrogen as the electron affinity of H 0.75 ev is much lower than the one for O 1.45 ev. Hopefully, future theoretical calculations will explain the results presented in this article. VI. CONCLUSIONS We have observed the formation of negative ions upon reflection of H 2 and O 2 from an aluminum-nitride surface. The negative ion yield is about 15% for oxygen and 1% for hydrogen at a primary beam energy of 780 ev energy of dissociated particles: 390 ev at an incidence angle of 82. It remains nearly constant with the primary energy components parallel and perpendicular to the surface of the dissociated particles ranging from 200 to 950 ev and 1.5 to 23 ev, respectively. We find no kinetic resonance as the parallel velocity changes. The ionization efficiency does not change significantly over a long period of time three months in spite of the moderate vacuum environment (10 7 mbar). The angular width of the scattered beam could be improved by reducing the surface roughness. Therefore, AIN meets the most important requirements for application on a space platform. ACKNOWLEGDMENTS The authors are grateful to H. Hofstetter and R. Liniger for their contributions in the areas of fabrication and electronics, respectively and to Professor P. Bochsler for continous support of this project. This work is supported by the Swiss National Science Foundation. 1 Imager for Magnetopause-to-Aurora Global Exploration IMAGE, NASA MIDEX program, Principal investigation Dr. James Burch, Southwest Research Institute, San Antonio, TX, P. Wurz, P. Bochsler, A. G. Ghielmetti, E. G. Shelley, F. Herrero, and M. F. Smith, in Proceedings of the Symposium on Surface Science, edited by P. Varga and G. Betz Kaprun, Austria, 1993, pp A. G. Ghielmetti, E. G. Shelley, S. Fuselier, P. Wurz, P. Bochsler, F. Herrero, M. F. Smith, and T. Stephen, Opt. Eng. Bellingham 33, P. Wurz, M. R. Aellig, P. Bochsler, A. G. Ghielmetti, E. G. Shelley, S. A.

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