Nonlinear and Ultra-fast Behaviors of LTG-GaAs Material andDevices for Long Telecommunication Wavelength Applications

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In this thesis, we demonstrated the ultra-fast performances of LTG-GaAs (low-temperature-grown-GaAs) based traveling-wave photodetectors (TWPDs) and verified dominant mechanisms of nonlinear behaviors in LTG-GaAs based devices and material at long telecommunication wavelength (~1300nm) based on the electrical sampling, electro-optic (EO) sampling, and the transient pump-probe technique. The electrical and electro-optic (EO) sampling measurements can characterize the D.C. photocurrent performances and directly investigate transient responses of our novel ultra-high speed/power TWPDs, including LTG-GaAs based p-i-n/n-i-n and metal-semiconductor-metal (MSM) TWPDs. Combined with the transient pump-probe measurements on LTG-GaAs thin film, we raised some dominant possibilities and proposed an integrated model to describe the carrier behaviors in LTG-GaAs based TWPDs and material at both belowband-gap (~1300nm) and aboveband-gap (800nm) excitations. A large bias-dependent broadening behavior in transient response combined with a drastic increase in D.C. photocurrent observed in n-i-n structure at long telecommunication wavelength (~1300nm) may be attributed to the increase of electron trapping time under very high electric field, and this observed nonlinear performance can be applied to high speed/power optoelectronic mixer devices in microwave photonic communication systems. The ultra-high speed/bandwidth performance of LTG-GaAs based p-i-n TWPD was demonstrated at ~1300nm. Furthermore, the record peak-output-voltage-bandwidth product of 568 GHz-V was achieved with a long-absorption-length (70mm) MSM device at long telecommunication wavelength (1300nm~1550nm) due to the improved microwave property in the MSM TWPDs and their high velocity-mismatch bandwidth. We also observed the very different nonlinear behaviors in these LTG-GaAs based devices under short (800nm) and long (~1300nm) wavelength excitations by E-O sampling technique. The measured E-O traces at ~1300nm showed much significant broadening of transient response and serious bandwidth degradation than which at wavelength of 800nm when increasing the generated carrier density and bias voltage. Combined with the same observed phenomena of broadening in the pump-probe measured traces at both wavelengths, the nonlinear behavior at ~1300nm can be attributed to the prolonged electron lifetime including slow recombination process, inter-valley scattering effect, and hot phonon effect, which should be the dominant mechanisms responsible for the broadening with increasing applied bias voltage/generated carrier density. We also introduced a integrated model containing these dominant factors under subband-gap excitation to completely describe the carrier behaviors in LTG-GaAs material and devices at ~1300nm. These results of this study give guidance for further clarification of carrier dynamics in LTG-GaAs and the optimization of the ultra-fast optoelectronic devices for telecommunication wavelength applications.