Semiconductor Physics and Devices

Transcription

1 Syllabus Advanced Nano Materials Semiconductor Physics and Devices Textbook Donald A. Neamen (McGraw-Hill) Semiconductor Physics and Devices Seong Jun Kang Department of Advanced Materials Engineering for Information and Electronics Laboratory for Advanced Nano Technologies Professor Seong Jun Kang, Lecture Room & Time Room 363, Mon 18:00 ~ 20:50 Evaluation of Grade Mid Ex. (30%) + Final Ex. (30%) + Quiz & H.W. (20%) + Attendance & Attitude (20%) Notifications Time schedule 1 of semiconductor engineering 9 Junction 1 2 Crystal properties and growth of semiconductor 10 Junction 2 3 Electrical properties of solid 1 11 Field effect transistor 1 4 Electrical properties of solid 2 12 Field effect transistor 2 5 Energy band and charge carrier in semiconductor 1 13 Thin film transistor 6 Energy band and charge carrier in semiconductor 2 14 Light emitting diodes 7 Excess carrier in semiconductor 15 Solar cells 8 Midterm Examination 16 Final Examination Semiconductors and the integrated circuit Semiconductors and integrated circuit (IC) are the foundation of analog and digital electronics, including information and wireless technologies. In 1874, Braun discovered the asymmetric nature of electrical conduction between metal contacts and semiconductor. In 1906, Pickard took out a patent for a point contact detector using silicon. In 1907, Pierce published rectification characteristics of diodes made by metals onto semiconductors. In 1935, selenium rectifiers and silicon point contact diodes were used as radio detectors. We are considering electron in materials for any electronics. In 1942, Bethe developed the thermionic-emission theory; the current is determined by the process of emission of electrons into the metal. In 1947, the first transistor was constructed at Bell Lab by William Shockley, John Bardeen, and Walter Brattain. The first transistor was a point contact device and used polycrystalline germanium. This was the most important electronic event, and made possible the integrated circuit and microprocessor that are the basis of modern electronics. The transistor effect was soon demonstrated in silicon as well, and a single-crystal material was used rather than the polycrystalline material at the end of The next significant step in the development of transistor was the use of the diffusion process to form the necessary junction. This process allowed better control of the transistor characteristics and allowed many transistors to be fabricated on a single silicon slices (wafer).

2 The first IC was fabricated by Jack Kilby of Texas Instruments in A planar version IC was independently developed by Robert Noyce of Fairchild. IC made in silicon using SiO 2 as the insulator and Al for the metallic interconnects. Several fabrication technique for IC Thermal Oxidation Several fabrication technique for IC Diffusion - Dopant atoms gradually diffuse into the silicon due to a density gradient. Photomasks and Photolithography Ion Implantation - A beam of dopant ions is accelerated to a high energy and is directed at the surface of semiconductor. Fabrication of a PN junction Semiconductors Analog world Digital world

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4 The crystal structure of solids A solid consists of atoms, ions, and molecules, which are packed closely together. The covalent bonds can hold several atoms to form a molecule. Also, it can hold unlimited number of atoms to form a solid. Ionic, van der Waals, and metallic bonds provide a force to form a solid as well. Semiconductor Physics and Devices Chapter 1. The crystal structure of solids Ionic bond hold a number of ions to make a solid. Van der Waals bond hold a number of molecules to make a solid. Metallic bond provide a force to hold a number of metal atoms to form a solid. Covalent, ionic, van der Waals and metallic bonds are originated from the electric force. Seong Jun Kang Department of Advanced Materials Engineering for Information and Electronics Laboratory for Advanced Nano Technologies Semiconductor materials Semiconductor materials =Resistivity -cm=1/ Conductor Semiconductor Insulator Electrical conductivities of semiconductor materials : - Intermediate between metals & insulators - Variable electrical conductivity by the temp, optical excitation, & impurity content Elemental semiconductors - Single species of atoms : Si, Ge Compound semiconductors - III-V compounds : AlP, AlAs, AlSb, GaN, GaP, GaAs, InP, InAs, InSb - II-VI compounds : ZnS, ZnSe, ZnTe, CdS, CdSe, CdTe, ZnO - IV-IV compounds : SiC, SiGe Semiconductor materials The elemental semiconductor material's vs. The compound semiconductor materials Semiconductor Energy Band Gap (Eg) : - One of the most important characteristics of a semiconductor, distinguishing it from metals and insulators (details later on) - Determinant of the wavelengths of the light absorbed or emitted by semiconductor Magnitude of Eg : λ (um) = 1.24/Eg (ev) (Appendix III!) - GaAs : 1.43 ev (~ wavelength of infrared light) - GaP : 2.3 ev (~ green portion of the spectrum) - Si : 1.1 ev, Ge : 0.67 ev

5 Semiconductor Semiconductor Doping in Semiconductors : - Imply a process of precisely controlled addition of impurities - Strongly affect the electronic/optical properties by varying the conductivities of semiconductors -Electrical properties of semiconductor are critically dependent on the impurity doping (n-type Si, p-type Si..) 1. Electrical conductivity increase with increasing Temperature Conductivity Semiconductor Metal T 2. Light irradiation led to increase in electrical conductivity 3. Temperature difference results in electromotive force 4. Electrical conductivity could be significantly enhanced by dopping n-type Si p-type Si 5. Photoelectric effect, Hall effect Type of solids Type of solids Need to understand the arrangements of atoms in semiconductors to investigate the properties of semiconductors. Polycrystalline Amorphous Amorphous: order only within few atomic or molecule dimension. Polycrystalline: high degree of order over many atoms. Single crystalline: high degree of order throughout the entire volume of materials. Electrical properties of semiconductor are critically dependent on their microstructure. Single Type of solids Space lattice Compare electrical conductivity. Lattice : Periodic arrangement of atoms in crystals Single crystal Polycrystalline Amorphous - Lattice determines the mechanical as well as electrical properties! Unit Cell : a volume which is representative of the entire lattice and is regularly repeated throughout the crystal Basis Vectors : a, b, c r=pa +qb +sc (p,q,s:integers) Primitive Cell : Smallest unit cell that can be repeated to form the lattice

6 Lattice A crystal is a solid in which the atoms are arranged in such a way as to be periodic. The most basic structure associated with this periodic geometry is a mathematical construction called the crystal lattice or a space lattice. Lattice Usually, crystals in solids can consist of a mixture of different type of atom. Lattice points are points in space at which the atomic arrangement is identical in any one particular direction. Alternatively, we can say that when one translates one s position from one lattice point to another, the arrangement of atoms remains unchanged. A crystal lattice is a set of points in space at which the atomic arrangement of atoms is the same no matter which point is chosen. In the simplest case, the lattice points are identical to the atom positions, when all the atoms are of the same type. The repeating array of atoms is called the basis. The basis, when superimposed upon the crystal lattice, provides a mathematical framework for a description of the crystal structure of the solid. Lattice and basis I Lattice and basis II a 1 = a 2 a 1 a 2 60 a 2 a 1 atomic position relative to lattice point 0 0 atom 1 basis replication of basis a 1 = a 2 a 1 a 2 60 a 2 a 1 Basis 0 0 : atom : atom 2 more replication of basis Lattice and basis II a 1 = a 2 a 1 a 2 60 Basic crystal structure By knowing the crystal structure of a materials and its lattice dimensions, we can determined several characteristics of the crystal, such as volume density of atoms. a 2 a 1 Basis 0 0 : atom : atom 2 Graphene

7 Bravais lattice Bravais lattice EM phenomena could be Index system for crystal planes Directions in crystals The orientation of a crystal plane is defined by three points in the plane, which are not collinear. Rule for determination of Miller index. 1. Choose the origin as a lattice point. 2. Find the intercepts on the axes of the lattice constant a 1, a 2, a Take the reciprocals of these numbers. 4. Reduce to three integers having the same ratio. 5. (hkl) is the index of crystal plane. Diamond structure Silicon and Germanium have the same diamond structure. (single element) An important characteristic of the diamond lattice is that any atom within the diamond structure will have four nearest neighboring atoms. Diamond lattice can be thought of as an FCC structure with an extra atom placed at a/4+b/4+ c/4 from each of the FCC atoms (Si, Ge). Zincblende structure The zincblende structure differs from the diamond structure only in that there are two different types of atoms in the lattice. Compound semiconductors, such as gallium arsenide, have the zincblende structure. The important feature of both the diamond and the zincblende structures is that the atoms are joined together to form a tetrahedron.

8 Diamond vs. Zincblende structures Atomic bonding Diamond Structure Zinc blend Structure Materials Si GaAs Total number of atoms 8 atoms 4 Ga 4 As Application Electronic devices Optoelectronic devices Imperfections in solids Point defect Impurities in solids Impurity atoms may be located at normal lattice sites (substitutional impurities), and located between normal sites (interstitial impurities). The technique of adding impurity atoms to a semiconductor material in order to change its conductivity is called doping. There are two general methods of doping: impurity diffusion and ion implantation. Line defect Growth of semiconductor materials Growth from a melt Growth of semiconductor materials Silicon ingot Jan Czochralski Pulling of a Si crystal from the melt (Czochralski method): (an 8-in. diameter, (100) oriented Si crystal being pulled from the melt. Silicon crystal grown by the Czochralski method. This large single-crystal ingot provides 300 mm (12-in.) diameter wafers when sliced using a saw. The ingot is about 1.5 m long (excluding the tapered regions), and weighs about 275 kg. (Photograph courtesy of MEMC Electronics Intl.)

9 Growth of semiconductor materials Silicon wafer Growth of semiconductor materials Epitaxial growth A technique of growing an oriented single-crystal layer on a substrate wafer where the growing crystal layer maintains the crystal structure and orientation of the substrate [Growing Methods] - CVD (Chemical vapor deposition) - LPE (Liquid-phase epitaxy) - MBE (Molecular beam epitaxy) - ALD (Atomic Layer Deposition) Molecular beam epitaxy MBE can be considered as a special case of evaporation for single crystal film growth, with highly controlled evaporation of a variety of sources in ultrahigh vacuum of typically ~ torr. Usually, besides the MBE system, realtime structural and chemical characterization system are included inside the system. The mean free path of atoms or molecules is around 100 m, which is far enough the distance between the source and the substrates. The extremely clean environment, slow growth rate, and independent control of the evaporation of individual sources enable the precise fabrication of nanostructures and nanomaterials. Molecular beam epitaxy The main attributes of MBE 1. A low growth temperature that limits diffusion, which is very important in fabricating two dimensional nanostructures or multilayer structures. 2. A slow growth rate that ensures a well controlled two dimensional growth. A very smooth surface and interface is achievable through controlling the growth rate. 3. A simple growth mechanism compared to other film growth techniques ensures better understanding due to the ability of individually controlled evaporation of sources. 4. A variety of in-situ analysis capabilities provide valuable information during the process. Molecular beam epitaxy Example of MBE films