Controllable Preparation And Photovoltaic Effect Of GaN/Si-NPA Nanoheterojunction With Multi-interface

Gallium nitride（GaN）is a wide and direct band gap semiconductor material.It has superior physical properties,such as high breakdown voltage,high electron transfer rate and so on.Therefore,GaN is widely used in LED,laser diode,field effect transistor and other optoelectronic fields.It is considered to be one of the most important materials in modern electronics and optoelectronic devices.Silicon nanoporous pillar array（Si-NPA）is prepared by hydrothermal etching of single crystal silicon（sc-Si）.It has the following structural characteristics:（1）regular structure array composed of silicon pillar array with micron size perpendicular to the surface;（2）all silicon pillar array have nanoporous structure;（3）the nanoporous wall are composed of silicon nanocrystals.The unique morphology and structure of Si-NPA make it have large specific surface area,very low integral reflectivity and wide spectral absorption characteristics.In this paper,the GaN/Si-NPA heterojunction is prepared by chemical vapor deposition（CVD）on the substrate of Si-NPA,and the AZO/GaN/Si-NPA/sc-Si/Ag structure of the solar cell is fabricated based on GaN/Si-NPA nanoheterojunction.This paper mainly study the controllable preparation and photovoltaic effect of GaN/Si-NPA multi-interface nanoheterojunction.The specific results are as follows:（1）Due to the unique morphology and wide spectral absorption characteristics of Si-NPA,Si-NPA is selected as the substrate for GaN deposition.The Si-NPA substrate prepared by hydrothermal corrosion method has nanometer-micron array structure.After the substrate is dried,one side of Si-NPA is polished with saturated NaOH solution.A layer of metal Pt（³-5 nm）is deposited on the unpolished surface by vacuum ion splashing as the catalyst for the preparation of GaN.GaN/Si-NPA is prepared in a high temperature tube furnace by CVD method.（2）In order to prepare GaN thin films with moderate thickness and low defect concentration,the growth conditions and growth methods of GaN are improved.Firstly,the effects of ammonia（NH₃）flow rate on the morphology,optical and electrical properties of GaN/Si-NPA heterojunction and the photovoltaic effect of GaN/Si-NPA solar cells are studied by one-step method.After the optimal NH₃ flow rate is determined,improving the growth method of GaN and the GaN/Si-NPA heterojunction is prepared by two-step method.By using this method,the effects of seed layer growth temperature on the structure,morphology,optical and electrical properties of GaN/Si-NPA and the photovoltaic effect of GaN/Si-NPA solar cells are studied.Through the continuous improvement of the experimental conditions and methods,the regulation of GaN thin films is realized.（3）The GaN/Si-NPA heterojunction is prepared by one-step method.The growth temperature and pressure are the same,and the NH₃ flow rate is changed to 10 sccm,20 sccm,30 sccm,40 sccm,50 sccm.By adjusting the NH₃ flow rate,it is found that the NH₃ flow has an obvious effect on the morphology,structure,optical and electrical properties of GaN.Firstly,with the increase of NH₃ flow rate,the morphology of GaN/Si-NPA changed obviously.Combined with the XRD diffraction spectrum of GaN/Si-NPA,the grain size of GaN increased with NH₃ flow rate increasing.Secondly,according to the absorption spectrum and photoluminescence spectrum of GaN/Si-NPA,there has two kinds of point defects in GaN/Si-NPA heterojunction,namely gallium vacancy(V_Ga)and nitrogen interstitial（N_i）.With the NH₃ flow rate increasing,the concentration of the two defects also increased gradually.Thirdly,according to the I-V characteristic curve of GaN/Si-NPA in dark state,a good rectification characteristics formed between GaN and Si-NPA,and when the NH₃ flow rate is 10 sccm,the rectifier ratio of GaN/Si-NPA heterojunction is the largest.Analyzing the electric transport characteristics of GaN/Si-NPA prepared at different NH₃ flow rates,with the increase of forward bias voltage,the carrier injection mechanism at the interface of GaN/Si-NPA heterojunction has changed from hot electron injection mechanism at low forward bias voltage to space charge limiting current（SCLC）mechanism at high forward bias voltage.Finally,the photovoltaic effect of solar cell device AZO/GaN/Si-NPA/sc-Si/Ag is tested,the high open-circuit voltage(V_oc)is realized and the device has obvious photovoltaic effect.When the NH₃ flow rate is 10 sccm,the maximum photoelectric conversion efficiency of GaN/Si-NPA solar cells is⁸.57×10^-2%.（4）Further regulate NH₃ flow rate to 15 sccm.According to the SEM,XRD of GaN/Si-NPA heterojunction,the deposition amount of GaN on Si-NPA substrate is the least,and the thickness of GaN thin films is regulated.According to the absorption spectrum and photoluminescence spectrum of GaN/Si-NPA and compared with other samples prepared under different NH₃ flow rates,the defect concentration in GaN/Si-NPA heterojunction is the lowest when NH₃ flow rate is 15 sccm,and the control of defect concentration in GaN thin films is realized.The photovoltaic effect of solar cell AZO/GaN/Si-NPA/sc-Si/Ag is tested.Under this condition,the V_oc of solar cell is¹.31 V,and the photoelectric conversion efficiency is⁰.16%.（5）In order to effectively regulate the thickness of GaN thin films,changing the deposition method of GaN,GaN/Si-NPA heterojunction is prepared by two-step method.The effects of different seed layer growth temperature on the morphology,structure and photoelectric properties of GaN/Si-NPA heterojunction are studied.According to the SEM,GaN films can be effectively controlled.Through the absorption spectrum of GaN/Si-NPA,an obvious GaN absorption edge is observed in the wavelength range of300-500 nm,and with the increase of seed layer growth temperature,the absorption edge of GaN showed obvious blue shift.The integral reflection spectrum of GaN/Si-NPA is measured.When the seed layer growth temperature is 800℃,the integral reflectivity of GaN/Si-NPA is⁹%.By analyzing the I-V characteristic curve of GaN/Si-NPA heterojunction in dark state,the rectification effect is formed between GaN and Si-NPA,and the carrier injection mechanism at the interface of GaN/Si-NPA heterojunction is changed,from the hot electron injection mechanism at low forward bias to the SCLC mechanism at high forward bias.By analyzing the photoelectric conversion efficiency of GaN/Si-NPA solar cells,a relatively large V_oc is obtained.When the seed layer growth temperature is 700℃,the V_oc,filling factor and photoelectric conversion efficiency of solar cells are the highest.