Preparation Of GaN/Si Nanoporous Pillar Array Structures And Their Applications In Light/gas Sensing Devices

The continuous and rapid development of semiconductor materials and technologyhaspromotedtheinformatizationprocess.Asapromising third-generation semiconductor,III-V GaN material has increasingly attracted our research interest because of its remarkable optoelectronic characterizations,demonstrating application advantages in the high-power,high-temperature,high-frequency,high-speed,and optoelectronic integration fields.Therefore,further study on the electronic and optoelectronic device applications of GaN materials is deemed as the imperative work,and also the key of research on the third-generation semiconductor materials and devices.Generally,the GaN materials were often prepared on the Al₂O₃ substrates with an insulation characteristic.By contrast,the GaN-related electronic devices based on Si material have more advantages in device integration.However,the relatively large lattice mismatch and thermal mismatch severely limit the promotion of device performance.Silicon nanoporous pillar array?Si-NPA?is a novel siliconnanometer-micrometer hierarchical structure featured by regularly micron-sized,quasi-identical and nanoporous pillar array,which is convinced to be a potential functional substrate for establishing nanoheterostucture devices.If the typical Si-NPA functional substrate can be used to construct the heterostructured devices,high device performances could be expected because of the exclusive advantages provided by the heterostructures.In this work,GaN/Si-NPA heterostructures were prepared by the chemical vapor deposition?CVD?method by using Si-NPA as the functional template,and we investigated the effects of growth parameters on the morphology,crystalline,and optical properties of GaN/Si-NPA in detail.In device applications,GaN/Si-NPA nanostructure gas sensor was fabricated,and also photoconductive and photovoltaic detectors were constructed to evaluate the photodetection ability of the GaN/Si-NPA to ultraviolet light.1.We systematically investigated the effects of growth parameters on the optical,morphology,and crystalline properties of Si-NPA.The growth of GaN on Si-NPA belongs to the gas-liquid-solid growth mechanism.At a low saturation vapor pressure of metal Ga source,the metal Pt has the ability to achieve the accumulation of meal Ga,and further nucleates and grows up gradually by reaction with N.The experiments indicates that growth temperature has litter influence on the optical properties of GaN/Si-NPA.For three samples prepared at 850,900,and 950°C,their photoluminescence?PL?are characterized by two components,blue and red emissions.In contrast,reaction pressure plays an important role on the optical properties of Si-NPA.At 500 Pa,there exists a dominant near-band-edge emission at the ultraviolet region.The flow rate of NH₃ also has great influence on the optical properties of GaN.We prepared three samples at different NH₃ flow rates?5,10,and15 sccm?,and they shared the similar spectra characteristics,an ultraviolet emission at 380 nm,a blue emission at 430 nm,and a red emission at 630 nm.But,the NH₃flow rate influences the relative intensity of three emission components obviously.Especially,the emission intensity of red emission increases with the NH₃ flow rate.Besides,the effects of processing parameters on the morphology and crystalline properties of GaN/Si-NPA was studied,and the observed trends were analyzed in detail.Finally,the optical properties of the GaN layer were further studied by the temperature-dependent PL measurements,focusing on the dependences of ultraviolet and deep-level emissions on the temperature.As for the shoulder emission peaks at390 nm at low-temperatures,we attributed it to two kinds of donor bound exciton transitions,and the corresponding activation energy in the temperature quenching process can be fitted by the equation.As for the temperature quenching of the near-band-emission ultraviolet emission,the exciton binding energy of the GaN was fitted to be 27.9±0.2 meV.2.Based on the prepared GaN/Si-NPA nano-heterostructures by CVD method,we fabricated a resistance-type gas sensors.Owing to a high specific surface area and massive surface active sites provided by the GaN/Si-NPA,the gas sensors are responsive to conventional gases.After a comparative study,we observed that the gas sensors have a better response to methanol,which maybe related to the difference of relative molecular mass of different gases.The gas with a small molecular mass like methanol interacts and diffuses more quickly than other gases with large molecular masses on the surface of sensitive materials.Under the optimum temperature of350°C,the GaN/Si-NPA gas sensor demonstrated good sensing responses of 1.22,1.92,and 2.50 to 5 ppm,200 ppm,and 500 ppm methanol,respectively.The response and recovery rates to 500 ppm methanol were 8 s and 7 s,respectively.The methanol sensing ability is superior to other reference devices constructed with oxide semiconductors.By comparing the methanol sensing properties of GaN/Si-NPA,Pt/Si-NPA,and Si-NPA structures,we have confirmed that the remarkable sensing behavior originates from the GaN/Si-NPA heterostructure.Moreover,the sensing mechanisms of the resulting GaN/Si-NPA nanostructure sensors were studied by drawing the methanol adsorption and desorption models based on the n-GaN/p-Si heterojunction,including the equilibrium state,oxygen adsorption process,methanol adsorption and reaction process,and the methanol desorption process.3.Photoconductive and photovoltaic detectors were fabricated based on the prepared GaN/Si-NPA nano-heterostructures to evaluate their photodetection ability of ultraviolet light.For the photoconductive detectors,we used an ultraviolet light of325 nm as the excitation source for measurement.At a low incident light excitation power of 0.01 mW,the photoresponsivity and specific detectivity of the devices were obtained to be 10.2 mA/W and 1.3×10⁹Jones.After the temporal photoresponse curves measurement at different voltages,no persistent photoconductivity phenomenon was observed.Also,a stable and reproducible response to on/off cycles were obtained,indicating a good light-switching behavior of the as-prepared device.In addition,according to the photocurrent response profile of one cycle,the rise time?from 10%to 90%of the saturated value?and the fall time?from 90%to 10%of the peak value?of the device are found to be 112 and 75 ms,respectively.Further,a heterojunction device based on ITO/GaN/Si-NPA/Ag structure was fabricated to achieve a self-powered photodetector.At 0 V,the on/off photocurrent ratio reaches a high value of 532,which results from the existence of a built-in electric field at the GaN/Si-NPA interface,by which the device is able to separate the photogenerated electron-hole pairs in an efficient manner.At an incident light power of 0.01 mW?325 nm?,a high photoresponsivity of 90.2 mA/W and a specific detectivity of2.9×10⁹Jones were obtained,and typical rise and fall times of 32 and 26 ms were achieved.Because of the inherent material stability of GaN and the good thermal conductivity of Si materials,the fabricated ITO/GaN/Si-NPA/Ag device possesses a remarkable operation stability,and there is no any photocurrent decay after a continuous operation for 10 hours.At a high working temperature of 100°C,about83%of the original photocurrent of the photodetectors was retained,suggesting the stable device operation ability.The reasons for the photocurrent decay of the photodetecors at high temperature were also provided.It can be anticipated that the above experiment results provide valuable information for the fabrication of novel ultraviolet photodetectors by using GaN/Si-NPA as the building blocks.