Mechnism Of THz Wave Emission Crystal Prepared By Ion Irradiation

As the core component of terahertz system, terahertz source determines theperformance of the whole system. The THz wave emission efficiency dependsdecisively on the THz emitting materials which are usually made oflow-temperature-grown gallium arsenide. It is now found that through ion irradiationbetter THz wave emission material can be prepared, which overcomes the poorreproducibility of high quality material prepared by the tradiational method. In order tofind out the best irradiation condition for producing the required photoconductivematerial, it is important to clarify the mechnism of the THz wave emission induced byion implantation.In this work, we used N+ions to implant into GaAs and other semiconductors forproducing THz wave emission materials. The ion energy ranges from several keV toseveral MeV and the ion dose ranges from1010to1015ions/cm2. By using a THztime-domain spectroscopy system the THz emission ability of the photoconductiveantenna made of implanted samples was meausred. It is found that the implantationcannot change the pulse width of the THz wave, but the peak current of THz signal canbe greatly changed. Not matter what the ion energy is, the peak current depends onlyon the defect density of the implanted layer. It implicates that the implanted ions do notinfluence the terahertz emission properties. With the increase of the ion dose (defectdensity), the peak current increases at first, then decrease with further irradiation. It isfound that the best irradiation dose for producing GaAs-based THz emission crystalwith nitrogen ions is around11011/cm3for1.5MeV N+ions and the correspondingdefect density is around31017/cm3. The best irradiation doses for producingInP(Fe)-based THz emission crystal are around11010/cm3for500keV N+ion or11011/cm3for1.5MeV N+ion. The corresponding defect density is around2.21017/cm3.The terahertz emission behavior was simulated under different defect conditionsby Monte Carlo method. It is found that the THz pulse width and peak intensity both changes sensitively with the change of trap density and capture cross section in GaAs.And the scattering rate of charge carriers increases with implantation dose. It istherefore concluded that the defects induced by nitrogen ion implantation may not bethe effective carrier traps and the intrinsic defect introduced during the growth of GaAsis the key defect that is responsible for the THz wave emission. It is proposed that thedefects introduced by ion implantation may only modify the scatter processes ofcarriers and the conduction properties of GaAs, by which it changes the THz power.In addition, applying the Drude model, we calculated the THz wave underdifferent momentum relaxation time and capture time. It is found that the peakintensity increases with the increase of momentum relaxation time which is themultiplicative inverse of scattering rate. This fact verified to some part the conclusionmade before.