Terahertz radiation from InAs induced by carrier diffusion and drift

Terahertz (THz) radiation from a (100) oriented InAs surfaces is dominated by the photo-Dember effect. The strength of the radiation is influenced by screening the radiation with doped carriers. When irradiated by femtosecond pulses, the wafer with the lowest doping concentration radiates THz power nearly two orders higher than the wafer with highest doping concentration. With identical optical excitation and same doping concentration, a $p$-type InAs generates stronger THz waves than an $n$-type InAs due to the weaker screening effect. The low doping $p$-type InAs ($1\times {10}^{16}{\mathrm{cm}}^{-3}$) sample is the strongest THz wave emitter among all the unbiased semiconductors we have ever tested with a Ti:sapphire laser oscillator. The drift-diffusion equation (DDE) is used in the study of carrier drift and diffusion as well as subsequent THz radiation from InAs wafers. The calculation explains well the experimental observation of the relationship between a THz electric field and the doping properties of InAs. The physical pictures of the carrier drift and diffusion characteristics in InAs surfaces are also clearly provided in this report.

Figure 1

THz waveform generated from $n$-type InAs (doping concentration is $3\times {10}^{16}{\mathrm{cm}}^{-3}$), $p$-type InAs (doping concentration is $1\times {10}^{16}{\mathrm{cm}}^{-3}$), $n$-type GaAs (doping concentration is $3\times {10}^{17}{\mathrm{cm}}^{-3}$), $p$-type GaAs (doping concentration is $3\times {10}^{17}{\mathrm{cm}}^{-3}$), respectively.

Figure 2

A three-dimensional spatial and temporal carrier concentration distribution of photoexcited electrons and holes at the InAs surface. The gray curves represent electron distribution and the dark curves represent the hole distribution.

Figure 3

Experimental and theoretical results of THz wave radiation versus increased doping concentration. The squares represent the THz wave amplitude from $p$-type (100) InAs, and the dots represent THz wave amplitude from $n$-type (100) InAs. The diamond represents the THz wave amplitude from a $p$-type (111) InAs with a carrier concentration at $3\times {10}^{16}{\mathrm{cm}}^{-3}$. The curves are the calculated results.

Figure 4

Calculation of net charge distribution at the InAs surface at $-50$, 50, 250, $500\mathrm{fs}$, top to bottom, respectively. The solid lines represent the $p$-type InAs with a doping concentration at $1\times {10}^{16}{\mathrm{cm}}^{-3}$, and the dotted lines represent the $n$-type InAs with a doping concentration at $1\times {10}^{18}{\mathrm{cm}}^{-3}$.

Figure 5

Calculation of the photo-Dember electric field at the InAs surface as time evolved at $-50$, 50, 250, $500\mathrm{fs}$, and $1\mathrm{ps}$ top to bottom, respectively. The electric field is generated by the spatial difference between the photoexcited electrons and the holes. The solid lines represent the $p$-type InAs with doping concentration at $1\times {10}^{16}{\mathrm{cm}}^{-3}$, and the dotted lines represent the $n$-type InAs with doping concentration at $1\times {10}^{18}{\mathrm{cm}}^{-3}$.

Figure 6

The azimuthal dependence of THz wave generation from multi-oriented InAs surfaces. The squares represent the experimental measurements of THz wave amplitude from a (111) oriented $p$-type InAs with a doping concentration at $3\times {10}^{16}{\mathrm{cm}}^{-3}$, while the dots represent the experimental data of THz wave amplitude from a (100) oriented $p$-type InAs with a doping concentration at $1\times {10}^{16}{\mathrm{cm}}^{-3}$. The curves represent the fitting results by cosine equations.