Creating gallium arsenide photocathodes with high quantum efficiencies

Biswas et al. report a method for creating activated gallium arsenide (GaAs) photocathodes with both improved lifetimes and high quantum efficiencies (QE). The activation process used cesium (Cs), tellurium (Te), and molecular oxygen (O₂).

This advance has implications for high energy physics and advanced electron microscopy. GaAs photocathodes are an important component of future electron accelerators that rely on polarized beams.

To extend the lifetime of GaAs photocathodes, previous investigators developed methods to activate the surface with low electron affinity materials, such as Cs₂Te. While this improves the lifetime, the resulting material often has a low QE.

The scientists addressed this by heat cleaning the surface at an optimal temperature, then depositing cesium and tellurium in alternating cycles. The maximum QE observed in this first study was 6.6% at 532 nm laser illumination.

“Although this is the highest QE reported so far for Cs-Te activated GaAs photocathodes, it is still not as high as the QE of typical Cs-O activated GaAs photocathodes, typically about 10%,” said author Jyoti Biswas.

To increase the QE, the scientists activated a second heat-cleaned surface by codepositing cesium and oxygen, followed by alternating deposition cycles with tellurium. A 532 nm laser diode was used to follow changes in QE during this process. The maximum QE obtained by this second attempt was 8.8% at 532 nm laser illumination.

Photoemission of polarized electrons requires a negative electron affinity. To achieve this, the work function in a doped semiconductor should be lower than the semiconductor’s bandgap.

“In the Cs-O-Te activation process, both the GaAs-Cs and Cs-O dipoles contribute to the work function reduction,” said author Erdong Wang. This brings the work function below the bandgap value.

Source: “High quantum efficiency GaAs photocathodes activated with Cs, O₂, and Te,” by Jyoti Biswas, Erdong Wang, Mengjia Gaowei, Wei Liu, Omer Rahman, and Jerzy T. Sadowski, AIP Advances (2021) The article can be accessed at https://doi.org/10.1063/5.0026839 .