Research On Inas-based Nanowire Growth And Their Optoelectronic Devices

As a new material,semiconductor nanowires?NWs?show great potential in revealing the basic theory of physics and new applications of optoelectronic devices.As nanostructures,the quantum confinement effect in the size-limited direction often exhibits peculiar superior performance and shows extraordinary prospects in exploring the physical mechanism of low-dimensional devices.At the same time,NWs have an extremely high Surface area to volume ratio due to the reduced size.As the proportion of surface atoms increases,the influence on device performance becomes more and more important,and is widely used in gas sensing and medical testing.Moreover,InAs-based semiconductor NWs are important candidates for the fabrication of high-speed devices and infrared optoelectronic devices due to their high mobility and narrow bandgap properties.Therefore,the growth of InAs-based narrow-bandgap semiconductor NWs and the research of NW optoelectronic devices are particularly important.In this thesis,the growth of InAs and InGaAs NWs by Au-catalyzed molecular beam epitaxy and the growth of GaAs/InGaAs quantum dots ordered NW array by selective area metal organic vapor phase?MOCVD?were well studied.On the other hand,based on nanowire FET optoelectronic devices,the role of surface states of InAs NWs in photodetection was studied.Ferroelectric materials/InAs NW hybrid mid-wavelength infrared photodetectors,ferroelectric materials/InAs NW hybrid volatile rewritable memory and InGaAs near-infrared detectors were well studied.The main innovations and contents of this thesis are as follows:1.Here we report single MBE-grown high quality InAs NW FET devices that can be converted from unintentionally-doped n-type unipolar to ambipolar when the temperature is reduced from room temperature?RT?to 77 K.The density of carriers can be modulated by the gate voltage.At 77 K and upon light illumination,the InAs NW ambipolar transistor based photodetector shows a novel phenomenon:the device exhibits a PPR behavior when it is p-type(back-gate voltage(V_bg)<0 V);whereas when the device is n-type(V_bg>0 V),its photoconductive behavior transforms from NPR to PPR when the incident light wavelength is increased from visible to short wavelength-infrared.Specifically,we show that there is an electron-trapping level caused by the InAs NW surface defect state above the conduction band,which plays an important role in its photosensitivity.A theoretical model based on charge transfer and electronic transition is proposed to explain this phenomena.Then,high-performance InAs nanowire detectors are well studied for sensitive detection in the near infrared.2.Here we use ferroelectric poly?vinylidene fluoride-trifluoroethylene??P?VDF-TrFE??as the top gate dielectric layer for the NW device.By utilizing the remnant polarization properties of this polymer layer,an ultrahigh local electrostatic field(?10⁹Vm^-1)can be produced to control the surface states.When covered by the P?VDF-TrFE?polymer layer with one specific polarization direction,electrons are trapped at the surface states to produce a built-in electrostatic field in the NW,resulting in carrier depletion in the NW core.Such a condition leads to an ultra-low dark current thus a very good photoresponse in the mid-wave infrared region.More significantly,these trapped electrons at the surface also cause the energy band between the nanowire core and the native oxide layer to bend downward and the Franz-Keldysh effect,allowing sub-bandgap photodetection from InAs NWs extending the photoresponse beyond InAs band edge.In opposite polarization direction,the surface states of the nanowire are devoid of electrons,leading to large dark current and a unusual illumination wavelength dependent photoresponse?positive or negative?.Furthermore,different stable levels of current output can be obtained and durably maintained when illuminated with different wavelength light pulses.Thus the"read"and"write"functions of the device can be achieved with light assistance,which can be potentially employed as an optically modulated nonvolatile memory device.3.Here we grew epitaxial ternary InGaAs nanowires with high In concentration grown on GaAs{111}B substrates.Our detailed electron microscopy characterizations suggest that the grown ternary InGaAs nanowires have an extraordinary core-shell structure with In-rich cores and Ga-enriched shells,in which both nanowire cores and shells showed compositional gradient.It was found that In-rich nanowire cores are formed due to the Ga-limited growth environment,caused by the competition with the spontaneous InGaAs planar layer growth on the substrate that consumes more Ga than the nominal Ga concentration during the growth.Moreover,the composition gradient in the nanowires cores and shells is a result of strain relaxation between them.Our optoelectronic property measurements from prototype nanowire devices show a remarkable photoresponsivity under the near-infrared illumination.This provides a new approach for designing and realizing complex nanowire heterostructures for high-efficiency nanowire-based systems and devices.4.The selected area MOCVD growth of GaAs nanowire arrays,GaAs/InGaAs heterojunction nanowire arrays,and GaAs/InGaAs?QDs?/GaAs nanowire arrays were investigated separately.By optimizing the growth parameter temperature and the V/III ratio,a uniform and periodic distribution of nanowire arrays was obtained.Molecular dynamics analysis of nanowire growth was performed.Detailed microstructure,compositional distribution,and photoluminescence spectrum were then performed.Finally,the growth of the AlGaAs passivation layer was carefully studied in order to further reduce the non-radiative recombination on the NW surface and enhance the optical properties of the NW.