Investigations On The Performance And High Temperature Detection Of Terahertz Quantum Well Photodetectors

Terahertz?THz?radiation consists of electromagnetic waves within the band of frequencies from 0.1 to 10 THz(1 THz=10¹² Hz).THz radiation occupies a middle region known as the terahertz gap between microwaves and infrared waves.THz wave is transient with wide band width and has a lot of properties such as low energy and coherence.THz technology can be used in the field of medical imaging,security,scientific imaging,communication and Manufacturing.Thus,it is called?one of the most important ten technologies that change the world?.The rapid development of terahertz quantum cascade lasers?THz QCLs?as broadly tunable terahertz sources and their continuous performance improvements impose urgent needs for high-performance THz detectors.Recently,THz quantum-well photodetectors?QWPs?have emerged as a promising candidate for compact detection systems in the THz region originating from the characteristics of intersubband transition and unipolar transport.Compared with other THz detectors,for example,Si bolometers,thermal detector Golay cells,pyroelectric detectors made from LiTaO crystals,nano metric FETs,or antenna-coupled field-effect transistors for THz imaging,THz QWPs show their special features in the detection performance:?i?THz QWPs are narrow band and enable wide wavelength coverage by adjusting the intersubband transition energy.?ii?The intrinsic high speed due to the inherent short carrier lifetime helps THz QWPs to realize high-speed and high-frequency detection.?iii?The availability of a mature material and processing technology based on GaAs makes it possible to fabricate large-scale uniform and long-term stable THz QWPs.THz QWPs are designed as a natural extension of quantum well infrared photodetectors and extensive research has been conducted to achieve better performance after their first experimental demonstration.However,THz QWPs still have a major limitation to their widespread use:they usually require cooling to low temperatures to realize background noise limited performance?BLIP?detection due to its lower barrier height in comparison with mid-infrared QWP.The BLIP temperatures for THz QWPs are generally lower than 20K.It is therefore crucial to develop optimized THz QWPs that can be operated at higher temperature.However,there is no systematical investigation that has been reported on high-temperature detection with THz QWPs.In this paper,we present a systematic research on THz QWPs and propose several ways for high temperature detection.In the first part,all parameters such as dark current,photoresponse,BLIP temperatures and detectivity has been investigated in detail especially for the dark current.Detailed physicasl mechanisms of darkcurrent for THz QWPs are investigated experimentally and theoretically by measuring two newly designed structures combined with samples reported previously.In contrast to previous investigations,scattering-assisted tunneling dark current is found to cause significant contributions to total dark current.Also,we give the calculated transition field to provide a fundamental criterion to determine which mechanism is dominant in total dark current for THz QWPs with different peak response frequencies.From the breakdown fields and transition fields,we can conclude that thermionic emission dark current is the major dark current mechanism for THz QWPs with very low peak response frequencies?below about 4 THz?in their working biases,while scattering-assisted tunneling dark current can be much larger than thermionic emission dark current for higher frequency THz QWPs in their normal operation biases We further determine BLIP temperatures,which decrease both experimentally and theoretically as the electric field increases.This work gives good description of dark current mechanism for QWPs in the THz region and is extended to determine the transition fields and BLIP temperatures with response peaks from 3 to 12 THz.Then,we have investigated the effects of Hartree potential,exchange-correlation,and depolarization interactions on the photon response spectra of THz QWPs.Due to the decrease of energy difference between the ground subband and the first excited subband,large relative errors are introduced without including the above many-body effects in the theoretical design.The exchange-correlation potential deepens the quantum well.As a result,the ground subband and the first excited subband shift to the lower energy region,and the energy difference between them increases.Because the expression for exchange-correlation potential is inadequate in the low electron density region and the leakage of wave function into the barrier layer.The discrepancy between the theoretical and experimental photoresponse peak positions decreases evidently with including the exchangecorrelation and depolarization effects.Our numerical results show that it is very important to consider the many-body interactions for designing THz QWPs.In order to obtain high-temperture detection for THz QWPs,we made a deep investigation on different optical coupling structures for THz QWPs including 45°polishing structure,1D and 2D grating structure and antenna coupling structures.For the antenna coupling structures,we propose to incorporate an optical antenna which focuses incident THz wave.Numerical results show that the QWP with peak frequency higher than 5.5 THz is expected to achieve background-noise-limited performance at 77K or above when employing a 10⁶ times enhancement antenna.Lastly,we also propose using a THz QWP in combination with a terahertz source to realize a detection system with photon-noise limited performance?PLIP?at high temperatures.Systematical investigations on the high-temperature performances of THz QWPs,including required signal power density for PLIP,detectivity,and the signal to-noise ratio,have been carried out by elaborating their dark current mechanism and photocurrent response both experimentally and theoretically.We also present the optimal doping concentration of THz QWPs designed for different peak wavelengths and the resulting optimum performance regarding the above three key parameters.Numerical results show that optimal designed QWP with peak response frequency of 5.5 THz is expected to achieve PLIP at 77 K at signal power density at 819 W/cm² and above.This work gives aprecise description of PLIP performance of THz QWPs and will open ways for new applications for high-temperature detection in the THz regime.