| Silicon is the most commonly used material in semiconductor industry because itis easily purified and doped. However, the sensitivity and efficiency of silicon-basedphotodetectors are limited by two factors: silicon performs highly reflection across theelectromagnetic spectrum, and silicon is transparent to wavelengths longer than1100nm. The discovery of black silicon makes it realizable to dramatically enhance thesensitivity and efficiency of silicon-based photodetectors. Black silicon is fabricated byirradiating a silicon surface with ultrahigh energy femtosecond laser pulses. However,the femtosecond laser process is complex, and it is incompatible with mass productiontechnology and silicon-based semiconductor technology. Therefore, new methods forfabricating black silicon have attracted more and more attention recently. In this study,micro/nano two-tier structured black silicon (micro/nanostructured silicon) has beenfabricated using a simple wet chemical etching process. The optical and electricalproperties of micro/nanostructured silicon have been studied.1. The optimum parameters of alkaline anisotropic etching and metal assistedisotropic etching have been obtained from orthogonal test, respectively. The regulardistributed pyramids which are produced through alkaline anisotropic etching with amask layer are advantageous to the application of micro/nanostructured silicon in thearray photodetectors. The nanopores, whose diameters and depth are varied with theincease of etching time in metal assisted isotropic etching, are advantageous to thesuppression of reflection and enhancement of absorption of micro/nanostructuredsilicon.2. The optical properties of unstructured silicon, microstructured silicon,nanostructured silicon, and micro/nanostructured silicon have been studied. Thereflectance from the surface of silicon is dramatically decreased after micro/nanostructured, and the reflectance of micro/nanostructured silicon is the lowest. Themicro/nanostructured silicon absorbs about98%and30%of the incident light atultraviolet-visible wavelengths and near infrared wavelengths, respectively. Theabsorbance of the micro/nanostructured silicon is, on average, higher than that of unstructured silicon by50%and28%at ultraviolet-visible wavelengths and nearinfrared wavelengths, respectively.3. The electrical properties of micro/nanostructured silicon, such as contactresistance, carrier concentrations, carrier mobility, and temperature coefficients ofresistance(TCR) have been studied. The contact performance of micro/nanostructuredsilicon-aluminium are better than that of unstructured silicon-aluminium beforevacuum annealing, and the contact performance has been largely improved for allsmples after vacuum annealing. The carrier concentration is slightly reduced, whereascarrier mobility is drastically decreased after micro/nanostructuring. TCR of siliconchanges from positive value into negative value after micro/nanostructuring, and theabsolute value of TCR increases with nanostructuring duration. The capability ofmodulating TCR is one of the major advantages of micro/nanostructured silicon. HighTCR of-3.01%/°C is obtained after etching for7min, which is larger than that of mostheat sensitive materials.4. Micro/nanostructured silicon photodetectors based on metal-semiconductor-metal (MSM) structure have been fabricated and studied. Interdigital electrodes,produced by lift-off technology, are regular and uniform. The sizes of producedinterdigital electrodes are as same as the designed ones. The photoresponses ofmicro/nanostructured silicon MSM photodetectors with different nanostructuringduration and interdigital electrode sizes have been studied. Negative resistance effectcaused by quantum tunneling effect of nanostructures is observed inmicro/nanostructured silicon. The dark current of micro/nanostructured silicon MSMphotodetector is relative high, although was reduced by at least two orders ofmagnitude after introducing a100nm SiNXthin film between micro/nanostructuredsilicon and aluminium. The dark current density is3.6μA/cm~2with the bias of5V. Theresponsivities of micro/nanostructured silicon MSM photodetectors increase with theincreasing of nanostructuring duration as well as the optimizing of interdigitalelectrode sizes. Responsivity of0.72A/W is obtained after etching for5min whenInterdigital electrode size is5:10.5. Photoresponses of micro/nanostructured silicon detectors are simulated byMEDICI software. Simulation results of current density of micro/nanostructuredsilicon detector are compared with experimental values, respectively. The results indicate that MEDICI software is suitable for the simulation of properties ofmicro/nanostructured silicon photodetectors. The peak response wavelengths ofmicro/nanostructured silicon MSM photodetectors are760nm when interdigitalelectrode sizes are5:10and2:10, but the peak responsivities are0.89A/W and1.35A/W, respectively. Moreover, the peak responsivities raise to1.31A/W and1.61A/W after using transparent electrodes. The photocurrent and responsivity ofunstructured silicon PIN photodetector to incident light with wavelength of800nm is1.9μA and0.337A/W, respectively, and the values raise to2.7μA and0.472A/W afterusing micro/nanostructured silicon with thickness of1μm. The response times inunstructured silicon PIN photodetector and micro/nanostructured silicon PINphotodetectors are the same, and the rising time and dropping time are about20ns and60ns, respectively. Peak response wavelengths and response spectra of variousmicro/nanostructured silicon PIN photodetectors show different red-shifts comparedwith that of unstructured silicon PIN photodetector, and the red-shift is60nm in PINphotodetectors with5μm micro/nanostructured silicon. The responsivity ofmicro/nanostructured silicon PIN photodetector reduces about53%compared with thatof micro/nanostructured silicon MSM photodetector. |
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