Electrical Transport Properties Of Nanocontacts Between Epitaxial Nanostructures And Si/Ge Substrates

As microelectronic circuits and devices shrink into submicron or nanometer scale,the concept of nanoelectronic devices has come into being.However,the traditional lithographical and ion implantation method are reaching their precision limit,so they can't meet with the requirement of smaller size and lower power consumption.Epitaxially grown nanostructures with low dimension,fine single crystal structures,good interfacial quality and multiple electrical properties are expected to be the ideal candidate for the building blocks in nanomicroelectronics,such as Schottky contacts,Ohmic contacts and conductive interconnect wires.As electrical transport properties are crucial for device design and operation,it's of great significance to study the transport properties through the nanostructure and the substrate interface.So we deposite Fe and Mn on heated Si and Ge substrates to form epitaxial nanostructures such as three dimensional islands,nanowires and tabular islands.Then we carry out the electrical property measurement by contacting the Scanning Tunneling Microscope tip on the islands.Finally we have a discussion on the factors that dominant the electrical transport properties across nanoscale contact.The main findings and significance of this work are summarized as follows:1.We grow FeSi₂ tabular islands on n-type and p-type Si(111)substrates at⁶50?.The atomic resolution image taken on the island surface shows clear(2×2)reconstruction,which demonstrates that the tabular islands are metallic?-FeSi₂.The measured current-voltage(I-V)curves of islands on both types of substrate show rectifying behavior and opposite conducting directions.This means the islands form metal-semiconductor Schottky contacts with n-type and p-type substrates.After fitting the I-V curves into the standard Schottky equation,we find the effective Schottky barrier heights of these contacts are around 0.35 V,which is much lower than that of macroscopic ones.Meanwhile,the ideality factors of the contacts are far larger than unit.So we consider that the main current by thermionic emission in macroscopic contacts no longer dominant the current transport in nanoscale and the nonideal current prevails instead.We analyzed the tunneling current,surface minority carrier recombination-generation(R-G)current and surface states conduction,and find that the tunneling current and the surface R-G current are the possible dominant component of the total current.2.By elaborately controlling the growth parameters during the deposition of Fe,we make trench structures around the FeSi₂ islands,which clearly changes the surface properties surrounding the contacts and provides a special candidate for the study of surface current.By comparing the I-V curves on tabular islands in and out of trenches,we find clear differences between the current magnitude and effective Schottky Barrier Height.The current on islands in trench are about one magnitude order lower than that out of trench,which proves the significant influence on electrical transport properties by the change of the surrounding surface.The calculated surface conduction of the Si(111)-7×7is¹0^-9?^-(16),while that in trench is¹0^-10?^-(16),which implies that the presence of trench structure almost eliminates the surface states conduction path.By comparing the surface states conduction current and the total surface current,we find that the surface R-G current should be the dominant factor in nanoscale contacts,which could contributes tens of nA to the total current.3.We grow MnSi_1.7 nanowires(NWs)on n-type and p-type Si(111)and Si(110)substrates.Then we demonstrate the formation of contact barriers at the interfaces between MnSi_1.7 NWs and Si substrates by the I-V curves measured by scanning tunneling microscope with tip contacting the NWs.The NWs on Si(110)exhibit linear reverse bias I-V curves,which suggests a parallel Ohmic surface state conductance of the Si(110)surface.The NWs on Si(111)exhibit nonlinear reverse bias I-V behavior which indicates a considerable amount of minority carrier recombination-generation current.The NW length dependence study of the forward bias current clearly shows that the quantitative change in NW length leads to a qualitative change in electrical transport properties.We derive a characteristic length L_C?200 nm and the corresponding aspect ratio of¹2-18 for MnSi_1.7NWs according to the variation of current density with the NW length.4.We grow three dimensional(3D)islands on Si(111)and Ge(111)substrates.By comparing the I-V curves measured on 3D islands in and out of trenches on Si(111),we find that the surface current is no longer the dominant component,though it prevails in tabular island contact to Si.We attribute this to the enhanced current induced by the tunneling effect through the contact interface,since the high concentration of defects and dislocations present at the interface could decrease the effective Schottky Barrier Height(SBH)and the thickness of the depletion region,which lead to significant tunneling current.By fitting the I-V curves into the Schottky equation,we find the calculated effective SBH of the contact is⁰.28 V,which is much lower than that of tabular islands.So we consider that the interfacial configuration of nanoscale contacts can make a difference to the transport properties of the contact.The measured I-V curves of 3D islands on Ge(111)exhibit perfect linear characteristic,which mean the 3D islands form Ohmic contacts to Ge(111)substrates and the contact resistance is around 2.6 M?.We attribute the formation of Ohmic contact to the narrow band gap of Ge in comparison with Si.Because of the band gap of only 0.66 eV,the majority carriers are more easily excited into the conduction band under similar doping concentration.This lead to the decrease of the depletion region thickness on semiconductor side and lead to the enhanced tunneling.