High-Performance Spin-Polarized Photocathodes Using a GaAs/GaAsP Strain-Compensated Superlattice

Abstract

We have successfully developed a transmission-type GaAs/GaAsP strained superlattice (SL) photocathode, and a high spin-polarization (SP) (90%) with a super-high brightness (~10 7 A⋅cm −2 ⋅sr −1 ) of electron beam was achieved [1]. In this study, we report the design and fabrication of an optimized transmission-type photocathode with strain-compensated SL for higher quantum efficiency (QE).In the GaAs/GaAsP strained SL, a compressive strain was introduced in the GaAs well layers to obtain a large band-splitting between heavy-hole and light-hole mini-bands. The increasing SL pair-number causes strain relaxation with resultant SP degradation. A smaller SL layer thickness is one reason behind the limited value of the QE. To overcome this problem by increasing the SL layer thickness without degradation, the use of strain-compensated SL was proposed [2]. In this structure, a strain is introduced in the SL barrier layers to the opposite direction to compensate the strain in the SL well layers. Figure 1 shows the GaAs/GaAsP strain-compensated SL structure. The maximum pair of the prepared SL is 90.X-ray diffraction revealed that the strain relaxation by thickness increase was effectively controlled. Figure 2 shows the change of maximal spin-polarization with the SL pair number. The superlattice photocathodes up to 36-pair maintain high SP of about 90%. Then, the SP obviously decreased. During the transport, the spin-polarized electrons should flip by scattering with holes. The scattering effect becomes stronger in the thicker SL photocathodes. The thickness effect on the QE and transport time will be investigated.

Comparison of quantum and collection efficiency of field‐assisted uniform‐doping and exponential‐doping GaN nanowire cathodes

“…Figure depicts comparison of QE and polarization. All results show that the polarization is inversely proportional to QE, in accordance with the results of Jin et al Thus, it can be simply stated that the increase in polarization is always achieved at the expense of QE. However, it must be pointed out that reducing the polarization does not necessarily increase the QE.…”

Strain mapping at nanometer resolution using advanced nano-beam electron diffraction

“…3 Strain compensated super-lattice structures are used to improve crystal growth and achieve higher quantum efficiencies in photocathode diodes. 4 The continuous scaling of the active regions of these devices requires characterization techniques with nanometer scale resolution to assist the development of new material processing methods.…”

“…It has been reported that a GaAs/GaAsP strain-compensated SL could provide a high spin-polarization of over 90%. 4 We first investigated the strain state of the heterostructure using atomic-resolution HAADF-STEM coupled with geometric phase analysis (GPA). 14,21 Figure 1(b) displays the HRSTEM image of the bottom part of the active device region.…”

Mean transverse energy and response time measurements of GaInP based photocathodes

“…Using a strained layer superlattice structure and circularly polarized light irradiation with a near band gap energy, highly spin-polarized electrons can be selectively excited up to the conduction band with a low kinetic energy, and extracted through the NEA surface to the vacuum. GaAs/GaAsP superlattice photocathodes with NEA surfaces provide a high spin-polarization of over 90%, 1,2 and are thus being applied to spin-polarized low-energy electron microscopy (LEEM) 3 and spin-polarized transmission electron microscopy (TEM), 4 and are expected to be employed for the international linear collider.…”