Properties of Hydrogen Terminated Diamond as a Photocathode

Abstract

Electron emission from the negative electron affinity (NEA) surface of hydrogen terminated, boron doped diamond in the [100] orientation is investigated using angle resolved photoemission spectroscopy (ARPES). ARPES measurements using 16 eV synchrotron and 6 eV laser light are compared and found to show a catastrophic failure of the sudden approximation. While the high energy photoemission is found to yield little information regarding the NEA, low energy laser ARPES reveals for the first time that the NEA results from a novel Franck-Condon mechanism coupling electrons in the conduction band to the vacuum. The result opens the door to development of a new class of NEA electron emitters based on this effect.The electron affinity of a semiconductor surface is defined as χ ≡ E vac − E CBM where E vac is the "binding energy" of the vacuum level and E CBM is that of the conduction band minimum. Semiconductors for which the electron affinity is negative, meaning that E vac lies within the band gap, are prized for their unique potential as advanced electron emitters [1]. This is because a negative electron affinity (NEA) surface generally leads to very efficient emission of electrons into the vacuum. The emitted beams also often possess characteristics such as narrow energy spread and low emittance that are highly desirable for use in devices such as electron microscopes, photoinjectors for free electron lasers and the recently demonstrated diamond electron amplifier [2]. Most NEA surfaces are difficult to prepare and maintain, usually requiring the deposition of alkali metals on a specially prepared surface and which exhibit a subsequently short operational lifetime. The [100] surface of hydrogen terminated diamond (H:C[100]) has proven to be a remarkable exception. A χ on the order of -1 eV is repeatedly obtainable by a simple annealing procedure and the surface is robust against exposure to air and other contaminants. H:C[100] achieves this feat through the combination of its large band gap E g = 5.5 eV and robust H terminated surface, stable up to 800 o C, that drives E vac into the band gap.Interest in diamond has been recently rekindled due to the increasing availability of economically viable, devicequality, synthetic crystals as well as the obvious advantages of functionalizing a material possessing such exceptional thermal, electrical and mechanical properties. It is therefore remarkable that no agreed upon mechanism for NEA emission from H:C[100] has been established. In the following Letter we present angle resolved photoemission spectroscopy (ARPES) measurements showing how H:C[100] attains a useful NEA surface by means of strong electron-optical phonon (e-ph) coupling and a breakdown of the sudden approximation. This mechanism is potentially germane to not only NEA emission but also to a range of recently demonstrated diamond based devices [2][3][4].The vast majority of PES experiments performed on H:C[100] have been carried out using photon energies well in excess of E g ([5, 6] for example). A notable e...

Interfacial band configuration and electrical properties of LaAlO3/Al2O3/hydrogenated-diamond metal-oxide-semiconductor field effect transistors

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Spectroscopic signatures of nonpolarons : the case of diamond

“…Large polarons in covalent materials have been historically neglected, and were dubbed "nonpolarons" by Emin 11 . We note that polaronic signatures were found in doped diamond with hydrogen-terminated surface having a negative electron affinity 12 . In the present paper we study polaronic effects in intrinsic diamond, to quantify from first principles the binding and spectral signatures of polarons in non-polar materials.…”

“…The ZPR can be determined experimentally by extrapolating the linear regime at high temperatures to 0 K. Using a Pässler fit 68 to the measurements of Clark et al 69 gives a ZPR of -0.259 eV. Using also the difference in renormalization for isotopes 12 C and 13 C, the obtained ZPR is -0.364 eV 70 . Table 2 shows a range of calculated ZPR that go from -315 meV to -619 meV.…”

Electronic structure of quantum materials studied by angle-resolved photoemission spectroscopy

“…On the contrary, the sudden creation of a hole can be associated with bosonic excitations such as phonons, plasmons, and electron-hole pairs which lead to satellite peaks on the low-energy side of the main photoemission peak ( Åberg, 1967;Brisk and Baker, 1975;Citrin et al, 1977). These are known as intrinsic losses, to be distinguished from extrinsic losses which the photoelectron may suffer during or even after its transit out of the material (Hedin and Lee, 2002;Joynt, 1999;Rameau et al, 2011). 5 The absence of plasmon satellite peaks is generally taken as evidence for violation of the sudden approximation because in the opposite adiabatic limit, the electron is removed slowly enough such that the (N − 1)system remains in a ground state and no intrinsic losses can occur (Gadzuk and Šunjić, 1975;Stöhr et al, 1983).…”