Humidity-sensing Properties Of ?-? Compound Semiconductors Based On Silicon Nanoporous Pillar Array

Many researches have showed that the humidity-sensing properties of a sensor are related to two aspects,the micro-structure and the humidity-sensing properties of the humidity-sensing materials.Humidity sensor with larger specific surface area often has higher humidity response.So it is reasonable that a sensor with high humidity-sensing properties is fabricated with high humidity-sensing material which has large specific surface area,or constructed with large specific surface area material composited/loaded with high humidity-sensing material.Silicon nanoporous pillar array(Si-NPA)is a silicon micro/nanometer hierarchical structural system with large quantities of well separated,micron-sized and semi-identical nanoporous silicon pillars and the nanoporous substrate beneath.It has been proved to be a good substrate or sensing material for humidity sensor because of the channel network,composed of silicon pillar and the valley around the pillars,is beneficial to the transportation of water molecules.As a member of ?-? compound semiconductor,ZnO(zinc oxide)nanomaterial has been studied extensively for its stable property,abundant surface morphologies,and can be used to fabricate microstructure with large specific surface area.In addition,ZnO has been studied widely as a sensing material for humidity sensor.As another member of ?-? compound semiconductor,CdS(cadmium sulfide)is a promising humidity-sensing material with high hygroscopic property.In order to fabricate a humidity sensor with large response and good response/recovery properties,we select Si-NPA as the substrate,and ?-? compound semiconductor as the humidity-sensing materials.ZnO nanomaterial can be used to enhance the specific surface area,or just as a humidity-sensing material.Then CdS nanoparticles are deposited on the surface of ZnO nanorods by SILAR method to form a sesame-seed-candy bundle array(CdS/ZnO/Si-NPA)on Si-NPA.CdS nanoparticles and ZnO nanorods were prepared on Si-NPA separately to investigate the humidity-sensing mechanism of CdS/ZnO/Si-NPA.After the composition of a small amount of lithium chloride(LiCl)respectively,the humidity properties of CdS/Si-NPA and ZnO/Si-NPA were promoted significantly.The detailed work is listed as below.1.The humidity-sensing properties of CdS/ZnO/Si-NPA humidity sensor with a sesame-seed-candy bundle array structure.ZnO was deposited on Si-NPA by CVD method to form a ZnO nanorods bundle array which can enlarge the specific surface area greatly.The humidity-sensing property of the sensor was promoted by the humidity-sensing properties of ZnO.After the deposition of CdS nanoparticles on ZnO nanorods,a sesame-seed-candy bundle array structure of CdS/ZnO on Si-NPA was formed,ZnO nanorods acted as the candy,and CdS acted as the sesame-seed.As a hygroscopic material,CdS nanoparticle not only enlarged the specific surface area,but also promoted the humidity adsorption property of the CdS/ZnO/Si-NPA humidity sensor.The measurements results of the humidity sensor showed extremely large response of 201530%at 20 Hz,a relative short response/recovery time of 110/32 s from 11-95%RH,a relative low hysteresis of 2.67%at 75%RH.It is concluded that the extreme large capacitance response should be attributed to the large specific surface area of the sensor and the humidity-sensing properties of CdS,the quick response/recovery property should owed to the unique channel network of the Si-NPA and the combination property of CdS and ZnO.So CdS/ZnO/Si-NPA can be a promising humidity-sensing material system,and it could be a path for preparing a high performance sensor and a guide for the following work.2.The humidity-sensing properties of CdS/Si-NPA and its humidity-sensing properties enhanced by LiCl.The 0D CdS nanoparticles are deposited on Si-NPA by SILAR method to uncover the humidity-sensing properties and the mechanism of CdS/ZnO/Si-NPA to the next step.The measurements exhibit that CdS/Si-NPA has big capacitance response,which is much smaller than the response of CdS/ZnO/Si-NPA.It is believed that the sesame-seed-candy structure of CdS/ZnO/Si-NPA may be has larger specific surface area in comparison with the volcano-like structure of CdS/Si-NPA,this may be the reason that CdS/ZnO/Si-NPA has larger capacitance response.Additionally,CdS/Si-NPA has relative long response/recovery time,so CdS may be the reason for the poor response/recovery property of CdS/ZnO/Si-NPA humidity sensor.The relative poor response/recovery property of CdS/Si-NPA may cause big hysteresis and poor reproducibility.In summary,it can be concluded that there are may be two reasons for the extreme large capacitance response of CdS/ZnO/Si-NPA humidity sensor.For reason one,the high hygroscopic property of CdS.For reason two,the huge specific surface area of CdS/ZnO/Si-NPA itself.The relative slow response and recovery time of CdS/ZnO/Si-NPA humidity sensor may due to the humidity-sensing property of CdS nanoparticle.The composition of LiCl was used to enhance the humidity-sensing properties of CdS/Si-NPA.The measurements showed that the response/recovery time of CdS/Si-NPA has been reduced significantly after the composition of LiCl.The LiCl:CdS/Si-NPA has negligible hysteresis of 1.36%at 11%RH,good reproducibility(can recover to the original state after 3 cycles between 11-54%RH),good stability after 7 h stability test in 54%RH.3.The humidity-sensing properties of ZnO/Si-NPA and its humidity-sensing properties promoted by LiCl.In order to reveal the effect of ZnO in the CdS/ZnO/Si-NPA humidity sensor,ZnO nanorods were grown on Si-NPA via CVD method,the humidity performance of ZnO/Si-NPA was measured and analyzed eventually.The test results showed that the ZnO/Si-NPA humidity sensor has a quick response/recovery property,a wide humidity detection range of 11-75%RH,and a low hysteresis.It can be deduced that the wide detection range of CdS/ZnO/Si-NPA may be due to the performance of ZnO/Si-NPA in a great extent.As far as the response/recovery performance concerned,ZnO/Si-NPA has the shortest ones,CdS/ZnO/Si-NPA has a shorter ones than CdS/Si-NPA gets.So the response/recovery performance of CdS/ZnO/Si-NPA is the combinational function of CdS and ZnO.The capacitance response of CdS/Si-NPA is one order of magnitude bigger than that of ZnO/Si-NPA.So the huge capacitance response of CdS/ZnO/Si-NPA is due to the hygroscopic performance of CdS and the huge specific surface area of CdS/ZnO/Si-NPA itself.In view of the good performance of LiCl-composited CdS/Si-NPA,ZnO/Si-NPA is composited with LiCl as well.The results show that the performance of the sensor after LiCl-composition is much better than that of without LiCl-treatment,the response/recovery speed is accelerated obviously,the stability is improved greatly,and the linearity is promoted significantly.So LiCl-treatment on ZnO/Si-NPA can improve its humidity-sensing performance for sure.