Dehydrogenation and the surface phase transition on diamond (111): Kinetics and electronic structure

Abstract

The (1ϫ1) to (2ϫ1) surface phase transition of the hydrogen-covered diamond ͑111͒ surface is investigated by core level spectroscopy, low-energy electron diffraction, and measurements of the electron affinity. The latter method is shown to be a reliable measure of the hydrogen coverage. Prolonged annealing of the surface at 1000 K converts the hydrogen-terminated (1ϫ1) structure with an electron affinity of Ϫ1.27 eV to a hydrogen-free (2ϫ1) reconstruction, increases the separation of valence-band maximum from the Fermi level E F from 0.68 to 0.88 eV, and results in a positive electron affinity of ϩ0.38 eV. Annealing the surface at high temperature ͑up to 1400 K͒ yields the same (2ϫ1) surface structure albeit with an increase in the separation of the valence-band maximum from E F to 1.42 eV and a positive electron affinity of 0.8 eV which is associated with a partial surface graphitization. An analysis of the kinetics of the thermally induced hydrogen desorption yields an activation energy of 1.25Ϯ0.2 eV. It was found that hydrogen desorption and reconstruction are surface phase transitions which are not directly linked. Instead, an intermediate phase with a high concentration of dangling bonds ͑up to 70%͒ is observed. The (1ϫ1) to (2ϫ1) phase transition is phenomenologically well described by a first-order transition provided a critical density of dangling bonds of about 70% is included in the analysis in such a way that the rate constant for reconstruction vanishes below that value. ͓S0163-1829͑99͒10407-7͔

High temperature photoelectron emission and surface photovoltage in semiconducting diamond

“…During a second, 1200 K annealing cycle (Figure 2(b)), the C 1s binding energy reversed its direction of change at 1000 K and at 1110 K. There was no return to the original binding energy when this surface was cooled to 300 K. The temperature of 1000 K corresponds to the onset of the (2 Â 1) reconstruction, in agreement with Cui et al 9 During the final 960 K heating cycle of the (2 Â 1) surface (Figure 2(c)), the binding energy showed only a small (0.07 eV) reversible shift.…”

“…Our lowest measured value was 283.87 eV and this is taken as the flat-band binding energy for our diamond (111) sample. The binding energy of the main core level component for the (2 Â 1) surface (285.1 eV) is larger than that measured at room temperature by Pate (284.8 eV) 14 but close to that measured by Cui et al 9 (285.0 eV). Cui et al also report a higher binding energy (2 Â 1) surface (285.9 eV) when the diamond is heated to 1400 K.…”

“…16 Low electron energy LEED patterns recorded at 20 eV showing the second order spots (Figure 2(c)) reflects the high surface conductivity of this surface. At higher temperatures, it has been reported that the conductivity of the (111) surfaces is due to termination by a graphitic 9 or graphene 16,17 layer. The SPV accounts for the spread in C 1s binding energy reported for the (1 Â 1):H surface including that measured by Cui et al, 9 who studied a diamond with a higher B concentration.…”

“…The binding energy of the main C 1s peak (284.0 eV) for the (1 Â 1):H surface is lower than that reported by Pate (284.3 eV) 14 and is significantly lower than that reported by Cui et al (284.8 eV). 9 This value was however found to be process dependent, showing variation from sample to sample. Our lowest measured value was 283.87 eV and this is taken as the flat-band binding energy for our diamond (111) sample.…”

“…For example, a difference of 0.74 eV has been reported between H-terminated and adsorbate-free diamond (111) surfaces. 9 For metal contacts to diamond surfaces, there is a considerable spread in Fermi level position relative to the band edge, clustered around the charge neutrality level in the lower half of the band gap. 10 Although this can be attributed to surface preparation and metal-induced states, the measurement process, in particular, using photoelectron-based methods, must also be considered.…”