UC San Diego

This dissertation concerns with fundamental aspects of organo-metallic vapor phase epitaxy (OMVPE) of III-V semiconductor nanowires (NWs), and their structural and electrical properties inferred from a variety of device schemes. An historical perspective on the NW growth techniques and mechanisms, and an overview of demonstrated NW devices and their performance is summarized in chapter 1. In part I of the dissertation, OMVPE synthesis of InAs NWs on SiO₂/Si and InAs (111)B surfaces is discussed and their growth mechanism is resolved. Nucleation, evolution, and the role of Au nanoparticles in the growth of InAs NWs on SiO₂/Si surfaces are presented in chapter 2. Our results indicate that In droplets can lead to InAs NW growth and that Au nanoparticles are necessary for efficient AsH₃ pyrolysis. Chapter 3 discusses the key thermodynamic and kinetic processes that contribute to the InAs NW growth on InAs (111)B surfaces. Controversy in the interpretation of III-V NW growth is overviewed. Experimental evidence on the nucleation of InAs NWs from In droplets as well as the catalytic effect of Au nanoparticles on the InAs (111)B surfaces are described. NW cessation at high growth temperatures or at increased input molar V/III ratios is explained via a switch-over from vapor-liquid-solid (VLS) NW growth to vapor-solid thin film growth, in contrast to previous interpretation of vapor-solid-solid growth of III-V NWs. The substrate-NW adatom exchange is also treated, and experimental distinction of two NW growth regimes depending on this exchange is demonstrated for the first time. Our results indicate that when growing extremely uniform InAs NWs, solid-phase diffusion of In adatoms on the NW sidewalls is the dominant material incorporation process with surface diffusion lengths of ̃ 1 [mu]m. This understanding was further utilized for the growth of axial and radial InAs- InP heterostructure NWs. Polymorphism in III-V NW crystal structure is also discussed and growth conditions that lead to its observation are summarized. In part II of the dissertation, transport coefficient extraction, field-, diameter-, and surface state-dependent transport properties, and their correlation with crystal structure in InAs NWs is presented. Chapter 4 overviews the fabrication of top-gate InAs NW field-effect transistors (NWFETs), presents a model for accurate extraction of carrier mobility and carrier concentration from NWFETs, and demonstration of high electron mobility values in InAs NWs is illustrated. Chapter 5 describes the effects of surface states on transport properties and parameter extraction from InAs NWFETs. Mobility values in excess of 10000 cm²/V·s are obtained from measurements at slow gate voltage sweep rates at which charge balance in carrier capture and emission from interface states is achieved. Chapter 6 discusses scaling effects on the NW transport properties and provides experimental evidence of ballistic electron transport over length scales of ̃ 200 nm in InAs NWs at room temperature. Diameter-dependent mobility and free carrier concentration is observed and is attributed to Fermi energy pinning in the conduction band that leads to surface electron accumulation and enhanced surface scattering. Chapter 7 discusses direct correlation of InAs NW microstructures with their transport properties. Our results show that the distinct difference observed in the subthreshold characteristics between wurtzite and zinc blende InAs NWFETs is due to the presence of spontaneous polarization charges at the WZ }0001} plane interfaces with ZB segments. Numerical simulations point out that a polarization charge density of ̃ 10¹³ cm⁻² is required to surpass surface state induced electron accumulation and result in high Ion/Ioff ratios for the WZ NWFETs. Chapter 8 presents detailed experimental studies on the gate and source-drain field-dependent transport properties in InAs NWFETs. Mobility degradation at high injection fields is observed and is attributed to enhanced phonon scattering, which was verified through electro-thermal simulations and ex-situ transmission electron microscopy (TEM) and scanning TEM compositional studies on NWs exposed to high injection fields. Chapter 9 presents a novel scheme for III-V NW integration to the standard Si mainstream utilizing ion-cut induced transferred III-V layers to SiOv(2) /Si. Vertically integrated and electrically isolated III-V NWs on Si are achieved for the first time. Key challenges related to growth and implementation of vertical devices in future technology nodes are also summarized