Optical down-conversion is the most successful commercial technique for THz generation using Low temperature grown GaAs (LT-GaAs). The technique is often known as Terahertz Time-Domain Spectroscopy (THz-TDS). This technique works by optical pulse excitation of a photoconductive switch. Here, a femtosecond laser pulse illuminates a gap between two electrodes (or antenna) printed on a semiconductor substrate, see figure 1. The laser pulse creates electrons and holes which are then accelerated by the applied bias between the electrodes, this transient photocurrent, which is coupled to an antenna, contains frequency components that reflect the pulse duration, hence generating an electromagnetic wave containing THz components. In a THz-TDS setup, the THz radiation is detected using a receiver device which is identical to the photoconductive switch emitter, and it is gated by the same optical pulse.
For Figure 1, please click below:
The main reason behind using LT-GaAs is the attractive properties of this material for ultrafast photoconductive application. LT-GaAs has unique combination of physical properties including: short carrier lifetime (< 200 fs), high resistivity, high electron mobility and high break down field. Low temperature growth of GaAs (between 190-350 C) allows excess arsenic to be incorporated as point defects: arsenic antisite (which represents the majority of the defects), arsenic interstitial and gallium vacancies. Ionised antisite defects which act as deep donors, approximately 0.7 eV bellow the conduction band, provide fast trapping for electrons from the conduction band to mid-gap states (0.7eV). Due to this fast trapping of electrons by arsenic atisite defects, as-grown LT-GaAs may have carrier life time as short as 90 fs. This enhances electron-hole recombination leading to a substantial decrease in the electron lifetime, hence making the LT-GaAs suitable for THz generation.For Figure 2, please click below:
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Remark: Powerway wafer can offer LT-GaAs, size from 2″ to 4″, epi layer can be up to 3um, micro defect density can be <5/cm2,carrier lifetime can be <0.5ps