Oriented Growth And In Situ Resistance Tuning Technology Of GaN Nanowires

As one of the third-generation semiconductor material,GaN has excellent photoelectric properties and chemical stability.GaN nanowire is one-dimensional(1D)nanomaterial,which shows excellent performance in low threshold current lasers,high mobility field-effect transistors,micro-LEDs,nanogenerators,solar cells,and quantum interference devices.However,the application of GaN nanowire devices is far from commercialization.There are still a series of problems in nanowires growth control and device fabrication which need further research.In GaN nanowires preparation,this paper studies the growth method of oriented GaN nanowire arrays.Based on MOCVD method,the influence of preparation parameters on the growth direction and morphology of nanowires is revealed.A two-step growth method of oriented nanowires is obtained.And in GaN nanowires device fabrication,based on the bridging nanowire structure,the real-time online adjustment of nanowire’s resistance is realized by using the controllable oxidation of nanowire’s surface under different bias voltage.The main work and achievements of this paper are as follows:(1)The influence of Ni/Au catalyst thickness,growth temperature and Ⅲ-Ⅴ ratio on growth mode and morphology of GaN nanowires is revealed.The growth mode of GaN nanowires can be gradually changed from VS to VLS with the decrease of the catalyst thickness,the increase of growth temperature or Ⅲ-Ⅴ ratio.When using the thin catalyst(4nm),the nanowires can be grown in VLS mode at low III-V ratio(2:20).Increasing the catalyst thickness(10nm)will turn the nanowires to grow under VS mode;When using the thick catalyst(10nm),the growth mode can be transited to VLS gradually and realize the mixed growth of VLS and VS by using a high III-V ratio(4:20).Meanwhile,nanowire’s morphology can be controlled by combining the temperature and III-V ratio.For example,a higher III-V ratio(2:15)is needed to maintain VLS growth at a lower temperature(750℃);while the growth rate can be adjusted by appropriately reducing the III-V ratio(1:20)at a higher temperature(850℃)to obtain nanowires with good directivity.By adjusting the morphology of nanowires through growth parameters,a "funnel-shaped" GaN nanowire structure with unique morphology was prepared,and a national invention patent(Patent No.202110670495.9)was applied.(2)A two-step growth method for preparing oriented GaN nanowires is proposed.That is,by optimizing the growth parameters such as catalyst thickness,growth temperature and Ⅲ-Ⅴratio,the growth process is divided into nucleation stage and growth stage,and different growth parameters are used in different stages to achieve the oriented nanowires.In the nucleation stage,GaN nanowires are grown at 850℃ for 20 s to obtain well oriented GaN "crystal seed",and then lower the temperature to 750℃ and use the optimized III-V ratio(2:20)to grow 800 s to obtain good morphology in the growth stage,and finally oriented nanowires are obtained.Comparing with the traditional template method for growing oriented nanowires,this two-step method reduces the main growth temperature to 750℃,and has the advantage of low cost,and does not need complex process and expensive equipment.(3)A novel method for in situ tuning the resistance of nanowires is proposed.That is,by applying the bias voltage at both ends of the nanowires,the Joule heat generated can be concentrated on the nanowires(rather than the electrodes),so that the temperature of the nanowires increases and the surface oxidation occurs,which leads to the permanent change of its resistance.This method has the advantages of easy to use,wide adjustable range(the resistance can be increased by about 700 times),on-line control,and also avoiding the electrode contact problems in traditional nanowire devices.