| Terahertz waves are defined as electromagnetic waves with frequencies of 0.1-10THz and corresponding wavelengths of 3000?m-30?m strictly,which have higher spatial and temporal resolution than microwave and millimeter waves.Compared with the infrared waves,terahertz waves have higher penetration depths and wider angle of field.With the rapid development and technological breakthroughs of terahertz sources such as terahertz quantum cascade lasers,terahertz detection technology,which is an important part of terahertz technology,has limited the development of terahertz technology due to the lack of effective research methods.Presently,the traditional detectors including Golay cell,silicon bolometers,and Schottky diodes are mainly used for the terahertz wave detection.However,due to the limitations in the working conditions and insufficient detection performance,such as the slow response of the Golay cell,the wide-spectrum-measurement-limited silicon bolometers,and the high noise equivalent power of the Schottky diode,which make it difficult to make breakthroughs in terahertz detection technology,especially in the field of military applications and the fields that have strict requirements on detector performance.The research on terahertz quantum well photodetector（THz QWP）based on solid-state semiconductor GaAs/AlGaAs system brings hope to the breakthrough of terahertz detection technology.The response time of THz QWP can reach sub-picosecond,having the advantage of high response speed,high resolution,mature process,stable performance and easy integration,especially for the application of high-speed detection and imaging in the terahertz range.The research of terahertz quantum well photodetectors is necessary to the wide development of terahertz technology,which provides the necessary basic support for the development of terahertz imaging and terahertz communication,especially in the fields of security detection,medical diagnosis and military detection.With the development of semiconductor micro-nano processing technology such as photolithography,evaporation,etching,etc.It is possible to integrate small-sized photonic coupling structures in terahertz quantum well detectors.The main purpose of this paper is to obtain high-performance terahertz quantum well detectors by the coupling of photonic micro-nano structures.In addition,we have also made effort on how to improve the operating temperature of current terahertz quantum well detectors.Therefore,the main research contents of the paper include:The molecular beam epitaxy technology growth of the standard terahertz quantum well detector material,surface-plasmon-enhanced terahertz quantum well detector,metal-dielectric-metal（MDM）micro Cavity-enhanced terahertz quantum well detector,high-temperature very long-wave quantum well detector based on resonant tunneling effect.The details are listed as follows:1.In order to obtain a terahertz quantum well detector with transition from bound state to quasi-continuous state（B-QC）（the comprehensive performance of detectors with B-QC transition is optimal）,we carry out the energy band calculation（considering the influence of multi-body effect）.The relationships between the detector’s response peak position,quantum well width,barrier layer composition,and transition type are obtained.The the actual growth parameters of the MBE are selected based on the results of the energy band design.The material composition and thickness of the grown material were respectively calibrated by fluorescence spectroscopy（PL）and scanning tunneling microscopy（SEM）,and the standard device of the terahertz quantum well detector（45 degree grinding device）was obtained by semiconductor micro-nano processing.The samples of response peaks between 45-54?m with background temperatures of 12-16 K and the peak responsivities of 0.1-0.24A/W（standard device）have been successfully prepared.2.In order to achieve a high responsivity terahertz quantum well detector and meet the requirements of simplified fabrication process,we fabricate a terahertz quantum well detector based on SPP resonance by integrating metallic reflection gratings on the top.The preparation of the periodic metal grating is obtained by vapor-depositing a layer of gold after eching the grating groove in the top contact.And the substrate is thinned to 180?m by mechanical thinning,to reduce the substrate absorption When the terahertz light is incident from the substrate backside,the reciprocal vector of periodic metal grating compensates the in-plane vector difference between the incident wave and surface plasmon polariton.The incident terahertz wave can couple with the surface plasmon of the metal,and the laterally propagating plasmon polariton can be excited.The quantum well active region is placed right below the metal grating.Due to the near-field effect,the electric filed component parallel to the growth direction of the quantum wells can be strongly enhanced in the quantum well active region.Based on this principle,the one-dimensional-metallic-reflection-grating coupled terahertz quantum well detector has a peak responsivity of 0.82 A/W at 46μm,which is 6 times higher than that of the standard device,and is close to the theoretical calculation result of 8.1 times.In addition,the measured polarization extinction ratio reached 56,and the spectral intensity at each polarization angle satisfies Malus’s law.The two-dimensional-metallic-grating-coupled terahertz quantum well detector is polarization independent.It has a peak responsivity of 1.2A/W,which is more than 9 times higher than that of standard devices.The preparation process of the device is simple.And performance improvement is remarkable,which is far superior to that of the traditional diffraction grating.The results provide a new idea for the application of surface plasmons in terahertz detection.3.To further realize the high responsivity of terahertz quantum well detector and meet the compatibility of focal plane array process,we have studied the metal-dielectric-metal（MDM）coupled microcavity in terahertz quantum well detection.A thicker dielectric can be used as sandwiched layer in the application of the MDM structure in terahertz region than in infrared region（the dielectric layer thickness is about 2?m in the detector operating at 47μm）.The one-dimensional-MDM-microcavity-coupled terahertz quantum well detector has a peak responsivity of 1.07 A/W at the response peak of 47μm,which is 22.3 times higher than that of a standard terahertz quantum well detector of the same structure.And the polarization extinction ratio reached 173,which is the highest value reported in the current literature.The structure preparation is compatible with the focal plane array process.The position of the response peak is less affected by the incident angle of the terahertz wave.The responsivity enhancement does not depend on the periodic structure of the metal grating.The results make it possible to prepare small-sized mesa and focal plane arrays.4.According to the calculation of the theoretical model,we know that heavily doped n-GaAs exhibits metal-like optical properties in the terahertz band.In order to avoid excessive loss of incident light in the metal microcavity structure during terahertz detection,we have studied the application of the MDM-like structures with dielectric waveguide microcavities in terahertz quantum well detectors.The structure uses the upper and lower heavily doped electrode layers of the quantum well to replace the two metal layers of the MDM structure.The upper electrode heavily doped layer is etched into a periodic grating groove,and the quantum well active region is sandwiched between the upper and lower heavily doped electrodes.A microcavity similar to the metal-dielectric-metal structure is formed.It is found that when the doping concentration of the electrode layer reaches 3.2×1018 cm-3（the highest doping concentration of current GaAs material grown by MBE）,the intersubband absorption of quantum well active region can reach 0.25,which is 20 times higher than that of the standard device with the electrode doping concentration of 1×1017 cm-3.And the structure has properties similar to those of the MDM structure,and the resonance wavelength is insensitive to the incident angle of incident light.The preparation process of the structure is extremely simplified.The structure contains no metal,and the substrate doesn’t need to removed.Only the grating groove in the upper electrode needs to be etched.The results are of great significance for the performance improvement of the terahertz quantum well unit and the focal plane detector.5.In order to improve the operating temperature of the quantum well detector,we have designed a quantum well detector based on the resonant tunneling structure to suppress the dark current of the quantum well detector by modulating the energy band.A resonant tunneling diode is grown after the conventional quantum well.When the bias voltage is applied to make the excited state level in the quantum well resonate with the ground state level in the resonant tunneling structure,the photocurrent can be hardly affected.The detection loop is formed,and the thermal excitation current of the broadband distribution is mostly blocked by the high barrier of the resonant tunneling structure.The dark current of the device can be reduced without affecting the photocurrent response.The experimental results obtained are as follows:the black body response of the structure with a peak position of 13μm is reduced by about 5times compared with the conventional quantum well detector without the resonant tunneling structure（the device structure needs to be further optimized）,but the dark current can be reduced by nearly three orders of magnitude.The background temperature has been increased from 35K to 45K.Generally,the detector performance has improved.The results show that the quantum well detector based on the resonant tunneling structure has lower the dark current and higher operating temperature,which provides a possibility to study the high temperature performance of the terahertz quantum well detector. |
