Study On Hyperdoping And Photoelectric Properties Of Microstructured Silicon-based Material

Developing Low-dark-current,high-responsivity and low-cost near-infrared photodetector is of great interest for wide applications in the telecommunications,security,and automotive industries.For example,1064 nm Nd:YAG laser,which is relatively common in the fields of laser communication,tracking,guidance,etc.,require low-cost,high-performance detection devices to match it.Silicon?Si?,as the eighth-most-abundant element in the universe,has become the cornerstone of the semiconductor industry with its abundant reserves,low cost,and mature process.However,Si has a wide intrinsic band gap?1.12 eV?,thus its absorption coefficient decreases sharply as the wavelength of incident light exceeds 1000 nm,which makes it difficult for silicon-based photodetectors to be applied in the near-infrared band.As an effective method,femtosecond-laser?fs-laser?irradiation is capable of extending the absorption-edge of silicon to the infrared region.Under ultrafast laser ablation,an array of hyperdoped microstructures can be formed on Si surface.The visible and near-infrared average absorptance of fs-laser microstructured silicon formed in some special background gases is extremely high.Thus,this Si-based materials is also called black silicon.Among a variety of doping elements,sulfur?S?doping has significant advantages in many aspects,such as simple doping process,low doping cost,and good uniformity of the prepared microstructure.For S-doped black silicon,although the role of sulfur hyperdoping layer covering the microstructure in extending the infrared absorption-edge of the material has been widely recognized,clarifying the physical mechanism of the extremely high near-infrared absorption contributed by the interaction between top hyperdoping layer and surface microstructure is greatly desired.And the near-infrared black silicon photodetector still faces some problems that need to be solved urgently.For one thing,the near-infrared response of the device operating at low voltages is too low to match its high absorption.Although the thickness of the hyperdoping layer and the concentration of sulfur impurities are considered to have important effects on it,it is difficult to isolate the surface microstructure and the hyperdoping layer to study them separately due to the special morphology of sulfur-doped black silicon.For another,during fs-laser microstructuring,high-density structural defects are introduced in surface layer of sulfur-doped black silicon,which will cause the dark current of the device to increase sharply.These factors severely limits the application of sulfur-doped black silicon in near-infrared photodetector.On the one hand,when a high bias voltage is applied to the photodetector for higher gain,the dark current of the device is further amplified.On the other hand,one of the main development goals of integrated circuits is low power consumption.At present,the working voltage of some advanced CMOS circuits has been less than 5 V,or even as low as 1 V.Devices capable of operating at the same power supply are required,which places higher requirements on the operating voltage of the photodetector.In this dissertation,sulfur-hyperdoped fs-laser-microstructured silicon have been formed in SF₆.Aiming at the problems of low near-infrared response and high dark current noise of black silicon photodetector,optical and electrical properties of the material and their device applications have been studied.The main contents and results of the dissertation are listed as follows:1.Si-based microstructures coverd with sulfur hyperdoping layer are prepared on the surface of single-crystal Si substrate by using fs-laser irradiation.The average absorptance of S-doped black silicon is as high as 90%in the wavelength range of500²000nm.The influence of the hyperdoping layer and the crystal quality of the surface layer of the material on the optical properties of sulfur-doped black silicon are investigated.Based on this,a physical model of sulfur-doped black silicon is builted.The super absorption characteristics of sulfur-doped black silicon are studied by using the finite-difference time-domain method.The calculational results show that a strong localization of an incident electromagnetic wave was observed around the top sulfur hyperdoping layer due to the effect of sulfur-hyperdoped silicon-based conical microstructures,so that the hyperdoping layer absorbs the incident light in near-infrared band effectively.The interaction between the microstructures and the hyperdoping layer played a critical role in the extremely-high absorption of the broadband near-infrared region of the black silicon material.2.Sulfur-hyperdoped silicon-based microstructures are fabricated on the surface of commercial Si-PIN-diode by using fs-laser irradiation.The effects of the S-dopant depth-concentration distribution and the disordered surface layer of the laser-microstructured silicon-diode on the electrical properties of the photodetector are mainly studied.The measurement results show that,the thickness of the hyperdoping layer and the concentration of S-dopants increase with the rise of the laser fluence,which further enhance the near-infrared absorptance of the fs-laser-microstructured photodiode,but the photoelectric response performance of the device will worsen as a result.The physical mechanism behind this phenomenon is further discussed.And the physical reason why the near-infrared responsivity of the black silicon photodetector is difficult to match with its ultra-high absorption is explored.Futhermore,a short-time continuous etching treatment to improve the electrical performance of the fs-laser-irradiated silicon photodiode working at near-infrared wavelengths is proposed.The experimental results indicate that the etching treatment can peel off the hyperdoped layer by an appropriate amount,control the dopant concentration of the microstructured Si-based photodetector prepared by fs-laser irradiation,and improve its crystal quality.After etching,the near-infrared responsivity of the black silicon photodiode increased from 0.2 A/W@1064 nm to 0.45 A/W@1064 nm at-0.1 V.And the dark current of the device remains in the order of magnitude of nA.3.A double-sided microstructured Si-PIN-diode and a microstructured CMOS image sensor are reported.The influence of laser fluence on the dark current of black silicon devices is studied.The effect of the uniformity of microstructures on the photon response non-uniformity noise of the devices was initially explored.The detector surface microstructured process is further optimized.The responsivity of the double-sided microstructured photodiode is 0.56 A/W@1064 nm at-0.1 V.Compared to a unprocessed commercial silicon-based photodetector?0.22 A/W@1064 nm?,the increase is up to 150%.And the dark current of the device remains less than 10 nA.After treatment,the quantum efficiency of the commercial silicon-based CMOS image sensor are improved by 30%in the near-infrared band from 950 nm to 1100 nm;the near-infrared imaging performance of the device is enhanced significantly.