Influence of surface carbon on the performance of cesiated p-GaN photocathodes with high quantum efficiency

Abstract

This study shows residual surface carbon's influence on photocathodes' quantum efficiency based on p-GaN grown on sapphire by metal organic chemical vapor deposition. An X-ray photoelectron spectrometer (XPS) built in an ultrahigh vacuum system allowed the in-situ monitoring of the photocathode surface beginning immediately after their cleaning and throughout the activation and degradation processes. An atomically clean surface is necessary to achieve a negative electron affinity, which is the main prerequisite for high quantum efficiency. The p-GaN samples were cleaned with ethanol and underwent a sub-sequential thermal vacuum cleaning. Although carbon and oxygen contaminations are expected to be undesired impurities from the metal organic chemical vapor deposition, which remained on the surface, p-GaN could still form a negative electron affinity surface when exclusively activated with cesium. After the activation with cesium, a shift to a higher binding energy of the photoemission peaks was observed, and a new species, a so-called cesium carbide, was formed, growing over time. The XPS data elucidated the critical role of these cesium carbide species in photocathode degradation. The X-ray damage to the p-GaN:Cs photocathodes, especially the influence on the cesium, was additionally discussed.

Figure 1

O 1 s and C 1 s photoelectron spectra for the p-GaN surface cleaned with EtOH (line 0) and after thermal cleaning at 450 °C (line 1). Dashed lines represent the peak fitting of the photoemission peaks.

Figure 2

In-situ photocurrent (blue curve) and vacuum value (black curve, in logarithmic scale) during the Cs activation of the p-GaN surface (sample A).

Figure 3

Ga 3d_3/2, N 1 s, O 1 s, and C 1 s photoelectron spectra for the p-GaN surface after thermal cleaning at 450 °C (line 0) and after Cs activation with 7.7% QE (line 1). Dashed lines represent the peak fitting in the C 1 s spectra.

Figure 4

(a) Cs 3d photoemission spectrum of the activated p-GaN surface, showing the spin–orbit splitting at 14 eV and (b) Cs 3d_5/2 photoemission spectrum of the adsorbed Cs on the p-GaN surface, with the peak fitting showing three components. The dashed lines represent the peak fitting.

Figure 5

The QE decay of the p-GaN:Cs photocathode (sample A) and the Ga 3d_3/2, Cs 3d_5/2, and O 1 s photoemission spectra at different times during its decay.

Figure 6

(a) Stacked C 1 s photoemission spectra showing the shift to lower BE along with the degradation of the p-GaN:Cs photocathode (sample A) and (b) the same C 1 s photoemission spectra (normalized to their background) showing the evolution of the cesium carbide (Cs_xC_y) species with peak fittings (dashed lines) along with the degradation.

Figure 7

(a) The surface model of a freshly prepared p-GaN:Cs photocathode. The surface C attracts a part of the Cs and (b) the surface model of an aged p-GaN:Cs photocathode showing the formation of Cs_xC_y.

Figure 8

The Ga 3d_3/2, Cs 3d_5/2, O 1 s, and C 1 s photoelectron spectra of the p-GaN:Cs surface before (line 0) and after (line 1) renewed thermal cleaning at 500 °C. Dashed lines represent the peak fitting in the C 1 s spectra.

Figure 9

The QE decay curve of the reactivated p-GaN:Cs photocathode (sample A) with the corresponding N 1 s, Ga 3d_3/2, Cs 3d_5/2, and C 1 s photoemission spectra, taken at different points in the decay curve.

Figure 10

(a) Cs 3d photoelectron spectra for p-GaN:Cs surface (sample D) influenced by the X-ray’s irradiation time and (b) the same Cs 3d spectra of sample D, stacked and with peak fittings (dashed lines).

Figure 11

The interior of the UHV preparation chamber (showing a sample holder, a halogen lamp with reflector, a steel ring anode, and Cs dispensers) and the XPS analysis chamber connected to the preparation chamber.