Negative electron affinity and electron emission at cesiated GaN and AlN surfaces

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1 Ž. Applied Surface Science Negative electron affinity and electron emission at cesiated GaN and AlN surfaces C.I. Wu, A. Kahn ) Department of Electrical Engineering, Engineering Quadrangle, Princeton Materials Institute, Princeton UniÕersity, Princeton, NJ , USA Abstract The electronic structure of GaN and AlN Ž surfaces and modification by cesium Ž Cs. adsorption are investigated via ultra-violet and X-ray photoemission spectroscopy Ž UPS, XPS. and total yield spectroscopy. The electron affinity Ž EA. of the clean and ordered 1=1 surfaces is found to be equal to 3.3 and 1.9 ev for GaN and AlN, respectively. Cs adsorption Ž with the help of oxygen pre-treatment in the case of GaN. reduces EA on both surfaces by about ev, leading to true negative electron affinity Ž NEA. in the case of AlN and effective NEA in the case of GaN. Total yield spectroscopy confirms NEA on both surfaces. q 2000 Elsevier Science B.V. All rights reserved. PACS: At; Da Keywords: Negative electron affinity; Gallium nitride; Aluminum nitride; Photoemission spectroscopy 1. Introduction Nitride semiconductors have attracted a great deal of attention over the past few years for their applications in light emitting devices and high power and high temperature electronics. Given their wide band gap and low natural electron affinity Ž EA., AlN and GaN have also been considered as possible negative electron affinity Ž NEA. materials w1,2 x. NEA has multiple applications in photon detectors or cold cathode emitters wx 3 and has been extensively investiw4,5 x. NEA results from the combination of a relatively gated for a number of semiconductor surfaces low natural EA, generally associated with a large ) Corresponding author. Tel.: q ;fax: q address: kahn@ee.princeton.edu Ž A. Kahn.. band gap, and a surface dipole which lowers further the vacuum level Ž E. VAC. Two different types of NEA can occur at a semiconductor surface. True NEA describes a situation whereby EVAC is below the surface conduction band minimum Ž E. Ž Fig. 1a. C. This requires a small initial EA and a substantial lowering of EVAC via surface dipole. True NEA is independent of band bending at the semiconductor surface. EffectiÕe NEA is more frequently encountered. This situation corresponds to a small but positive EA on a p-doped material. Downward band bending at the semiconductor surface lowers E below the bulk E Ž Fig. 1b. VAC C. In either case, an assessment of NEA using surface spectroscopy demands a careful determination of the position of EVAC relative to the valence and conduction band edges of the solid. This task has been met with various degrees of success over the years, in particular for AlN wx 2, leading to some confusion r00r$ - see front matter q 2000 Elsevier Science B.V. All rights reserved. Ž. PII: S

2 ( ) C.I. Wu, A. KahnrApplied Surface Science Fig. 1. Schematic energy diagram for a surface exhibiting Ž. a true NEA and Ž. b effective NEA. regarding the conditions necessary to achieve NEA. The present paper dispels some of these uncertainties by presenting our systematic investigation of the electronic affinity and secondary electron yield at clean and cesiated surfaces of p-type GaN and Ž nominally n-type. AlN using ultra-violet and X-ray photoemission spectroscopy Ž UPS, XPS. and total yield spectroscopy. We show that the deposition of cesium Ž Cs. leads to true and effectiõe NEA on AlN and p-gan, respectively. We use total electron yield spectroscopy to confirm the occurrence of NEA, and we demonstrate an order of magnitude increase in the yield following cesiation of the GaN surface. 2. Experiments Ž 17 P-type wurzite GaN films Mg doped; 5=10 cm y3 were grown by metal-organic chemical vapor deposition on sapphire at Emcore, and AlN films Ž unintentionally doped n-type by oxygen inclusion. were grown by molecular beam epitaxy on SiŽ 111. at Lucent Technologies. The samples were loaded in an ultrahigh vacuum analysis chamber Žbase pressure of 4=10 y11 Torr equipped with low energy electron diffraction Ž LEED., Auger electron spectroscopy Ž AES., XPS and UPS. The samples were mounted in tantalum foil that could be heated resistively up to 14008C. The sample temperature was measured by thermocouple and infrared pyrometer. The GaN and AlN surfaces were prepared by multiple cycles of nitrogen ion sputtering Ž 0.5 kev. and annealing Ž 9008C for GaN; 11008C for AlN. w6 8 x. The sputtered and annealed surfaces contained less than 2% of a monolayer of oxygen and exhibited sharp Ž 1=1. LEED patterns with low diffuse background. The GaN and AlN valence band spectra were recorded using the He II photon line Ž 40.8 ev. from a gas discharge lamp. The onset of photoemission was measured with y5 V applied to the sample to clear the detector work function. The overall resolution of the UPS measurement was 0.15 ev. XPS was performed with the Zr M photon line Ž ev. or the Al K photon line Ž ev. of our X-ray source. The absolute resolution of the XPS measurement was 0.7 ev, but accuracy on peak energy displacement was evaluated at 0.2 ev. The position of the Fermi level Ž E. F was measured by UPS on clean gold films. All the measurements were done at room temperature. Cs was evaporated on room temperature GaN and AlN from a standard SAES Getters dispenser. The source was extensively outgassed such that the pressure increase during evaporation did not exceed 2 = 10 y11 Torr. The Cs coverage was monitored from the intensity of the Cs 3d 3r2 core level measured by XPS. We define one monolayer Ž 1 ML. as the coverage corresponding to the saturation intensity of the Cs 3d core level. Finally, total yield spectroscopy was performed by irradiating the sample with a ev electron beam. The primary electron current, I, was 0 measured with a q100 V applied to the sample to insure complete electron collection. The emitted current, I, was measured with y100 V applied to the S sample. The total yield coefficient, h, is defined as the ratio I ri. S 0 3. Results and discussion The EA of the AlN and GaN surfaces was measured in a standard way, using the secondary electron cut-off to derive the position of E VAC, the top of the valence band Ž E. V and the band gap. The bottom spectra of Figs. 2 and 3 correspond to clean AlN and GaN surfaces. Previous investigations by our group w9,10x and others w11,12x have conclusively shown that these surfaces exhibit large densities of occupied states which overlap with the bottom part of the band gap. These states must be taken into consideration to determine EV in order to correctly measure the surface ionization energy Ž IE. and EA. We have shown that these states can be eliminated either

3 252 ( ) C.I. Wu, A. KahnrApplied Surface Science by comparing valence band edges measured with surface and bulk sensitive XPS w9,10 x, or by adsorpw9 11 x. The results of these measure- tion of oxygen ments have been published elsewhere w6 10x and are reproduced in Table 1. The deposition of Cs reduces IE and EA by an amount equal to the shift of the secondary electron cut-off corrected for the change in band bending. On AlN, the latter is obtained from the shift of the Al 2p core level measured by XPS. Fig. 2 shows the AlN valence band as a function of increasing Cs coverage w13 x. The Cs 5p doublet at ; ev below E F is best resolved at intermediate coverage Ži.e ML. when Cs Cs interaction is weak and adsorption sites are equivalent. The left-hand side of the spectra shows the progressive shift of the photoemission cut-off that indicates a lowering of the vacuum level. The maximum shift corresponds to a 2.6"0.15 ev decrease of EA with 0.9 ML Cs. Given the initial 1.9 ev EA of the clean surface, this decrease results in true NEA Ž Fig. 4a.. It is accompanied by a four-fold Fig. 3. He II valence band spectra and secondary electron cut-off for clean p-gan Ž bottom. and CsrO rp-gan Ž top. surfaces w10 x. increase in the intensity of the secondary electron peak. The reduction in EA results from the formation of a surface dipole induced by the polarization of the Cs-substrate bond. Note that the 2.6 ev decrease in EA, which brings EVAC below the conduction band minimum at the AlN surface, is actually directly measured in photoemission spectroscopy. This is highly unusual because the observable shift of E VAC, and thus of the secondary electron cut-off, is generally limited by the bottom edge of the conduction band, which is the lowest energy a secondary electron can have. In this particular case, however, the large density of unoccupied surface states extending below the conduction band minimum and overlap- ping with the top of the band gap wx 9 allows electrons at the surface to have lower energies than the conduction band minimum. In this case, EVAC can be detected via photoemission spectroscopy even though EA is truly negative. A similar situation was reported for LiF w14 x. The deposition of Cs on p-gan reduces EA by 2.2 ev, in good agreement with the value of 2.3" 0.15 ev previously reported wx 1. Added to the 1.2 ev downward band bending, 1 this reduction brings the vacuum level just below the bulk conduction band 2 Fig. 2. He II valence band and secondary electron cut-off as a function of Cs coverage on AlN w13 x. 1 E ye is equal to 1.5 ev on this surface Ž Table 1. F V. The doping parameter Ž E y E in the bulk. F V is estimated at 0.3 ev from the Mg density and the acceptor ionization level, leading to a 1.2-eV band bending.

4 ( ) C.I. Wu, A. KahnrApplied Surface Science Table 1 Position of the top of the valence band below the Fermi level, IE and EA of AlN and GaN Ž =1 clean and cesiated surfaces. The numbers for cesiated GaN correspond to the surface pre-exposed to oxygen. True or effective NEA is specified. The numbers are taken from previous investigations w6 10x Clean surfaces Cesiated surfaces E ye Ž ev. IE Ž ev. EA Ž ev. E ye Ž ev. IE Ž ev. EA Ž ev. F V F V AlN Ž n-type.; E s6.2 ev y0.7 Ž true. G GaN Ž p-type.; E s3.4 ev y0.7 Ž eff.. G minimum Ž Fig. 4b., leading to a situation of weak effective NEA. The vacuum level can be pulled down further by exposing the clean surface to oxyw10 x, following a well- gen prior to Cs deposition known procedure developed for GaAs. The highly polar Cs`O bond increases the magnitude of the surface dipole. Fig. 3 shows that this procedure shifts the secondary electron cut-off by 2.8 ev while keeping the band bending unchanged Žas deduced from the Ga 3d core level.. EVAC is therefore 0.7 ev below the bulk conduction band minimum, indicating effective NEA. The IE and EA of the two surfaces following Cs adsorption are summarized in Table 1. Verification of NEA is obtained via total yield spectroscopy. Fig. 5a shows the total yield coefficient, h, as a function of incident electron energy on the clean and cesiated AlN surfaces. h reaches a Fig. 4. Schematic band diagram for cesiated Ž. a AlN w13x and Ž. b p-ganž w10 x. Fig. 5. Total electron yield as a function of incident electron energy for Ž. a clean and cesiated AlN; and Ž. b clean, oxygen-exposed, cesiated, and oxygen exposedqcesiated p-gan.

5 254 ( ) C.I. Wu, A. KahnrApplied Surface Science maximum which is 2.5 times larger on the NEA surface than on the clean surface. h peaks at ev and decreases rapidly to about half its maximum value as the incident electron energy increases. On cesiated p-gan, however, reaches a much higher maximum Ž ; 16. and decreases more slowly with increasing energy than in the case of AlN Ž Fig. 5b.. h is essentially independent of energy above 1000 ev for the GaN surface exposed to oxygen prior to cesiation. These differences are explained in the following way. Three processes affect secondary electron emission: Ž. 1 electron generation in the material; Ž. 2 electron transport to the surface; and Ž. 3 electron emission in the vacuum. The size of the gap affects Ž. 1, but should have little impact on the ratio. Doping and EA, on the other hand, affect both Ž. 2 and Ž. 3. AlN is unintentionally doped n-type and upward band bending occurs near the surface ŽFig. 4a.. The secondary electrons generated close the surface can escape from the solid given the true NEA condition of the surface. However, as the primary electron energy increases, the majority of secondary electrons are excited deeper into the solid and have more time to thermalize to the bottom of the band. Regardless of whether they are generated in or beyond the depletion region, diffusion toward the surface is impeded by the depletion field which pushes the carriers back toward the bulk. As a result, the total yield decreases rapidly with increasing primary energy. In p-type GaN, on the other hand, the bands bend downward near the surface Ž Fig. 4b. and the electrons that thermalize to the bottom of the conduction band are accelerated toward the surface by the field of the depletion region. Ballistic transport across the region allows them to escape to the vacuum. Above a certain threshold, the secondary electron emission should therefore loose its dependence on the incident electron energy, in agreement with the data presented in Fig. 5b. The relatively small difference between the CsrGaN and CsrO rgan cases is accounted for by the increase 2 in effective NEA in the latter case. 4. Summary The EA and electron emission properties of clean and cesiated GaN and AlN Ž = 1 surfaces were investigated via photoemission spectroscopy and secondary electron emission measurement. The EA is found equal to 3.3"0.2 and 1.9"0.2 ev for GaN and AlN surfaces, respectively w9,10 x. The deposition of Cs reduces EAŽ AlN. by 2.6" 0.3 ev, leading to true NEA. With the assistance of a 1.2-eV initial downward band bending, effectiõe NEA is achieved on p-gan following the sequential adsorption of oxygen and deposition of Cs, which lowers EAŽ GaN. by 2.8"0.3 ev. The total electron yield is strongly affected by the direction of band bending near the surface. For CsrAlN, the upward band bending limits the total yield. h reaches a maximum of 8 for incident electron energies of ev and then decreases rapidly with increasing primary energy because secondary electrons excited deep in the solid are pushed back into the bulk by the field of the depletion region. On the other hand, CsrGaN gives a maximum yield of 16 at higher incident electron energy Ž ev.. This maximum is preserved up to much higher incident energies because the field of depletion region helps secondary electrons escape from the NEA solid. Acknowledgements Support of this work by the National Science Foundation Ž DMR is gratefully acknowledged. The authors also thank Drs. E.S. Hellman and D.N.E. Buchanan for providing AlN, Dr. I. Ferguson for GaN samples, and Prof. P. Soukiassian for useful advice on Cs experiments. References wx 1 M. Eyckeler, W. Monch, T.U. Kampen, R. Dimitrov, O. Ambacher, M. Stutzmann, J. Vac. Sci. Technol., B 16 Ž wx 2 M.C. Benjamin, M.D. Bremser, T.W. Weeks Jr., S.W. King, R.F. Davis, R.J. Nemanich, Appl. Surf. Sci. 104 Ž wx 3 R.L. Bell, Negative Electron Affinity Devices, Clarendon Press, Oxford, wx 4 F.J. Himpsel, J.A. Knapp, J.A. VanVechten, D.E. Eastman, Phys. Rev. B 20 Ž wx 5 L.W. James, J.L. Moll, Phys. Rev. 183 Ž wx 6 C.I. Wu, A. Kahn, N. Taskar, D. Dorman, D. Galapher, J. Appl. Phys. 83 Ž wx 7 C.I. Wu, A. Kahn, J. Vac. Sci. Technol., B 16 Ž

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