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Electron Emission Properties of Crystalline Diamond and III-Nitride Surfaces

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Electron Emission Properties of Crystalline Diamond and III-Nitride Surfaces Applied Surface Science 130±132Ž. 1998 694±703 Electron emission properties of crystalline diamond and III-nitride surfaces R.J. Nemanich ), P.K. Baumann, M.C. Benjamin, O.-H. Nam, A.T. Sowers, B.L. Ward, H. Ade, R.F. Davis Department of Physics and Department of Materials Science and Engineering, North Carolina State UniÕersity, Raleigh, NC 27695-8202, USA Received 28 October 1997; accepted 20 December 1997 Abstract Wide bandgap semiconductors have the possibility of exhibiting a negative electron affinityŽ. NEA meaning that electrons in the conduction band are not bound by the surface. The surface conditions are shown to be of critical importance in obtaining a negative electron affinity. UV-photoelectron spectroscopy can be used to distinguish and explore the effect. Surface terminations of molecular adsorbates and metals are shown to induce an NEA on diamond. Furthermore, a NEA has been established for epitaxial AlN and AlGaN on 6H±SiC. Field emission measurements from flat surfaces of p-type diamond and AlN are similar, but it is shown that the mechanisms may be quite different. The measurements support the recent suggestions that field emission from p-type diamond originates from the valence band while for AlN on SiC, the field emission results indicate emission from the AlN conduction band. We also report PEEMŽ photo-electron emission microscopy.Ž and FEEM field electron emission microscopy . images of an array of nitride emitters. q 1998 Elsevier Science B.V. All rights reserved. Keywords: Electron emission; Crystalline diamond; III-nitride 1. Introduction energy. The development of CVD diamond film growth processes further stimulated this field of re- The potential of cold cathode emission from dia- search because of the potential of fabricating low mond was recognized after initial photoemission and cost, electron emission structures. More recently, photo-threshold measurements of p-type natural dia- other wide bandgap semiconductors such as BN, wx mond crystals 1,2 . These measurements involved AlN and AlGaN alloys have also shown properties UV light to measure the spectrum of photoelectron indicating a potential for application as cathode emit- energies and the photo-electron yield vs. photon terswx 3±5 . It is evident that most cold cathode emission ) applications could not rely on the presence of UV Corresponding author. Department of Physics, North Carolina light to excite the electron emission. As field emis- State University, Raleigh, NC 27695-8202, USA. Tel.: q1-919- 515-3225; fax: q 1-919-515-7331; e-mail: sion from metals has been, and continues to be, an [email protected]. important area of research, it has been suggested that 0169-4332r98r$19.00 q 1998 Elsevier Science B.V. All rights reserved. PII S0169-4332Ž. 98 00140-8 R.J. Nemanich et al.rApplied Surface Science 130±132() 1998 694±703 695 field emission from a semiconductor with a negative 2. Experimental techniques electron affinity could provide electrons without a barrier to emission. To date, this possibility has not The technique of UV photoemission spectroscopy been realized. Ž.UPS has been employed to characterize the elec- In photoemission, the electrons are excited into tron affinity of a surface. The UPS technique is often the conduction band levels near the surface. Elec- employed for measurement of the valence band and trons near the surface may be emitted directly into surface electronic states of semiconductors. Two as- vacuum, and the spectrum will represent a convolu- pects of a UPS spectrum may be used to indicate a tion of the valence band and conduction band density negative electron affinity of the surface. The two of states. Most electrons, though, will be scattered effects are the appearance of a sharp peak at low and thermalize to the conduction band minimum. For kinetic energy, and an extension of the spectral range a traditional semiconductor with a positive electron to lower energywx 6 . The sharp feature will appear at affinity, these electrons are bound in the semiconduc- the largestŽ. negative binding energy in typical pre- tor and will eventually recombine with the photo-ex- sentations of UPS spectra. This feature is attributed cited holes. For a negative electron affinity surface, to electrons thermalized to the conduction band min- the electrons near the surface at the conduction band imum. For a positive electron affinity, these elec- minimum can also be emitted into vacuum. trons would be bound in the sample. In essence, the field emission process from semi- In addition to the sharp feature that is often conductors is more complex than the photoemission evident in the spectra of a NEA semiconductor, the process. One basic difference is that there may be an width of the photoemission spectrum Ž.W can be insufficient supply of electrons to the conduction related to the electron affinity Ž.x . The spectral band. In comparison to metals, the supply of elec- width is obtained from a linear extrapolation of the trons is not a problem for moderate emission cur- emission onset edge to zero intensity at both the low rents. For the wide bandgap semiconductors of dia- kinetic energy cutoff and at the high kinetic energy mond and the group III-nitrides, successful n-type endŽ. reflecting the valence band maximum . The doping has proved to be an illusive property. Thus, following relations apply for the two cases: field emission measurements may reflect aspects of xshnyE yW for a positive electron affinity, Ž.i the supply of electrons to the semiconductor, Ž ii . g s y y the transport of electrons to the surface, and finally 0 hn Eg W for a negative electron affinity Ž. iii the emission from the surface. Ž.1 In this study, we summarize recent results for methods to obtain a negative electron affinity of where Eg is the bandgap and hy is the excitation diamond, show that the III-nitrides are also potential energy. We stress that the photoemission measure- materials for cold cathode emitter structures, and ments cannot be used to determine the energy posi- address the relationship of photoemission and field tion of the electron affinity for the NEA surface. emission. The materials of this study will be con- Careful measurements of the width of the spectra are fined to either single crystals or high quality epitax- helpful in distinguishing whether the effect is direct ial films on single crystal substrates. In this way, emission of the electrons from conduction band states issues related to the microstructure of the film may or whether excitons are involved in the emission be minimized. More specifically, the studies focus process. The effects of excitons have recently been on the surfaces of type IIB p-type diamond crystals reported by Bandis and Patewx 7 . and epitaxial films of AlN, GaN and their alloys on In addition to measurement of the electron affin- 6H±SiC substrates. While diamond is a cubic mate- ity, the position of the surface Fermi level can be rial, the nitrides form in the hexagonal wurtzite obtained. For a grounded sample, the Fermi level of structure but are still characterized with tetrahedral the sample will be the same as that of the metal sp3 type bonding. The Al±Ga-nitrides form a contin- holder. The Fermi level of the metal can easily be uous solid solution with a band gap that ranges from determined, and the energy difference of the valence 3.4 eV for GaN to 6.2 eV for AlN. band maximum and the metal Fermi level yields the 696 R.J. Nemanich et al.rApplied Surface Science 130±132() 1998 694±703 position of the surface Fermi level of the semicon- represent the average field at the surface as the ductor. In some instances, charging or photovoltage applied voltage divided by the distanceŽ in these affects will complicate the analysis. To avoid these studies units of Vrmm are often used. affects, we have employed p-type diamond and thin One example where this effect would not hold has nitride layers grown on n-type SiC. Charging was been reported by Geis et al.wx 9 for field emission not observed in any samples. Photovoltage effects from nitrogen doped diamond crystals. In this case, it are more difficult to discern. For the thin metals on was suggested that the field was applied at the diamond, emission was observed at the Fermi level metal±diamond interfaceŽ as opposed to the dia- indicating no photovoltage band bending. For the mond±vacuum interface. The applied field then nitride layers, films are effectively undoped and the changes the width of depletion region in the dia- n-type SiC substrates will preclude substantial ef- mond, and there should be little potential drop be- fects. We note that neither charging or photovoltage tween the emitting surface to the anode. For this affects will change the width of the photoemission case, the I±V curves would not depend on the spectrum, so the analysis of the electron affinity is average field but would look nearly identical for a less complicated. wide range of anode to surface distances. The experi- The field emission measurements in this study ments of Geis et al. on type Ib diamond support this were completed using a moving probe system. With modelwx 8 . In this case, the applied field was sug- this method, an anode is positioned above the sample gested to assist in supplying the electrons to the and the I vs. V is measured. The anode is stepped diamond conduction band, and the diamond surface toward the sample surface, and the measurement is was suggested to not affect the emission process. repeated. The I±V curves may be fit to the Fowler± One of the difficulties of field emission measure- NordheimŽ.
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