ECE236A: Semiconductor Heterostructure Materials Fall 2017, Sept. 25 Dec. 16 class website:

Transcription

1 ECE236A: Semiconductor Heterostructure Materials Fall 2017, Sept. 25 Dec. 16 class website: Instructor: Shadi A. Dayeh Lectures: Tuesday, Thursday, 3:30-4:50 pm Center Hall, Room 207. Office Hours: Monday 1:30-2:30 pm; Wednesday 1:30 am 2:30 pm or by appointment/drop in. Grading: Problem Sets 20% (4 homework sets) Midterm 30% Nov 16 th, in class (tentative). Paper 50% due Monday, Dec 14 th at 3:30 pm. Password: heterointerface Required Textbooks: None. We will use multiple textbooks and journal articles in this class that will be referenced accordingly in the lecture slides. Additional References: Jasprit Singh, Physics of Semiconductors and Their Heterostructures (McGraw Hill 1993) P. Y. Yu and M. Cardona, Fundamentals of Semiconductors, 3rd Edition (Springer-Verlag 2001). 1

2 Course Topics 1. Basic Properties of Semiconductor Heterostructures (Lectures 2 & 3): - Definitions and types of Band Alignments. - Band alignments and Offsets (Anderson, common anion (60:40 rule)/cation, midgap alignment. - Determination of band-offsets (XPS, internal photoemission, CV profiling). 2. Crystal Growth and Material Aspects of Heterostructures (Lectures 4&5): - Introduction to epitaxial growth techniques (LPE, VPE, MBE, OMVPE/MOCVD, CBE). - Thermodynamic analysis (phase diagrams, supersaturation, dopant incorporation, Langmuir isotherm for adsorption). - Kinetic analysis (reaction rates, homo/heterogeneous pyrolysis) 3. Defects in Semiconductor Crystals (Lecture 6): - Stacking sequence in simple crystals. - Defects (stacking faults, twin boundaries, dislocations and Burgers vectors, Shockley and Franck dislocations). - Thompson tetrahedron and dislocations in hexagonal, covalent, and polar/non polar epitaxial crystals.

3 Course Topics 4. Lattice Mismatch and Strain Effects (Lecture 7 & 8): - Lattice match and lattice mismatch materials. - Description of stress and strain - Elastic energy density; strained layer heterostructures, and strain of screw, edge, and mixed dislocation. - Critical thickness calculations, strain sharing and coherency limits in nanoribbons and nanowires. 5. Influence of Strain on Electronic Band Structure (Lecture 8): - Calculation of stress-induced band-edge shifts. - Band-edge splittings. 6. Quantum Wells and Superlattices (Lecture 10 & 11): - Envelope function. - Conduction and valence band quantum wells. - Multiple quantum wells and superlattices. - Modulation doped heterostructures: Energy levels and 2D electron gas.

4 7. Current Transport in Heterojunctions (Lectures 12, 13, & 14) Electrostatics in pn heterojunction Inclusion of minority carriers Forward/reverse bias currents (including junction recombination, tunneling, and surface recombination) Junction breakdown Esaki Tunnel Diodes 8. Group III Nitride Semiconductors (Lectures 15 & 16). Spontaneous and piezoelectric polarization 2DEG in GaN/AlGaN heterostructures Course Topics Polarization and internal electric field in InGaN/GaN quantum wells Growth on semipolar and nonpolar surfaces

5 Course Policy Exams: Midterm is open notes and homework only, no electronic devices such as laptops, cell phones, PDAs, etc Other Allowed Items: Pen/Pencil, Calculator & Blue Book. Rescheduling of midterm exam under extreme circumstances with well-documented reasons will be allowed with notice to the instructor in advance of the exam date. Grading: Grades will be assigned based on your overall performance in the class. Please see Instructor for exam regrades. The whole exam may be regarded and the overall exam grade may increase or decrease. Homework: Discussion of ECE236A course material and homework are allowed and encouraged. However, every student should write his/her own homework. Use of previous exams, homeworks, copying from classmates, allowing others to copy, working out the homeworks together are forbidden. Homeworks are to be handed in class and extensions are not allowed except for extremely exceptional reasons. Integrity Policy: Academic integrity is a serious and an important matter that you should practice throughout your education at UCSD and thereafter. You are expected to complete the homework set and exams based on the course standards defined above. Any attempt to receive a grade by means other than your own individual and honest effort and any kind of unauthorized aid will be considered a violation for the course Integrity Policy.

6 Tentative Schedule September 2017 Sunday Monday Tuesday Wednesday Thursday Friday Saturday Labor Day Introduction

7 Tentative Schedule October 2017 Sunday Monday Tuesday Wednesday Thursday Friday Saturday No Office Hour No Lecture Lecture Columbus Day Lecture 3 Lecture No Office Hour No Lecture Lecture 6 No Office Hour Lecture 5 Lecture Lecture 8 Halloween

8 Tentative Schedule November 2017 Sunday Monday Tuesday Wednesday Thursday Friday Saturday Lecture 9 No Office Hour 8 9 No Lecture Lecture Veterans Day Lecture Midterm Lecture Thanksgiving Day Lecture Lecture 14

9 Tentative Schedule December 2017 Sunday Monday Tuesday Wednesday Thursday Friday Saturday 1 2 Paper due date (hard copy and electronic copy) No Lecture Lecture 15 No Lecture 6 7 Lecture Final Paper due Christmas 31

10 Some Limitations of Homojunction Devices 1. BJTs But high emitter doping for enhanced emitter efficiency leads to bandgap narrowing: Van Overstraeten et al. Solid State Electron. 30, 1077, qv BE% qv BC% Emitter Injection Efficiency: γ = 1 for 1+ N D B E x B N E D B x E Increase N E and reduce x B. ( x B << L B ),( x E << L ) E Emitter Injection Efficiency: 1 γ = for ( x B << L B ), x E << L E 1+ n 2 e ΔE g /kt D i0 E L B γ decreases N ' E n B0 D B L E ( )

11 Solution Offered by Heterojunctions 1. HBT P. M. Asbeck et al. Semicond. Sci. Tech. 17, 898, Wisseman & Frensley in VLSI Electronics: Microstructure Science, vol. 11, Ed. Einspruch & Wisseman, Academic Press

12 Some Limitations of Homojunction Devices 2. FETs High doping & surface channel: μ decreases. In GaAs microwave devices, thin channel body also leads to lower breakdown voltages. Electron transport near channel surface

13 Solution Offered by Heterojunctions 2. HFET x Charge confinement and separation of dopants from conduction channel à very high electron mobility. Chao et al. IEDM pp , Ketterson et al. IEEE Trans. Elect. Dec. 33, 564, x

14 Some Limitations of Homojunction Devices 3. Laser p + Junction n + E c E v E F p E g Holes in V B Electro ns ev o Electrons in C B E F n E c E c E g p + Inv ers ion reg io n n + E c ev E F n E F p (a) E v (b) The energy band diagram of a degenerately doped p-n with no bias. (b) Band diagram with a sufficiently large forward bias to cause population inversion and hence stimulated emission S.O. Kasap, Optoelectronics (Prentice Hall) V Wilson & Hawks, Optoelectronics, Prentice Hall

15 Solution Offered by Heterojunctions 3. Double Heterojunction Laser (a) (b) Electrons in CB E c E v 2 ev n AlGaAs p GaAs (~0.1 µm) 1.4 ev Holes in VB AlGaAs ΔE c p 2 ev E c E v (a) A double heterostructure diode has two junctions which are between two different bandgap semiconductors (GaAs and AlGaAs). (b) Simplified energy band diagram under a large forward bias. Lasing recombination takes place in the p- GaAs layer, the active layer Refractive index Photon density (c) (d) Active region Δn ~ 5% (c) Higher bandgap materials have a lower refractive index (d) AlGaAs layers provide lateral optical confinement S.O. Kasap, Optoelectronics (Prentice Hall)

16 Unique Properties of Heterostructures Conventional homojunction: Doping to control current. E c e Heterojunction: Composition change provides additional degree of freedom to control current. e E c E v h h E v Abrupt composition: Quantum wells and superlattices: new confined energy states. E c E v

17 Unique Properties of III-V Materials filled: electron mobility direct indirect hollow: hole mobility R. Pillarisetty, Nature 479, 324, Cotal et al. Energy Enc. Sci. 2, 174, Geisz et al. Semicond. Sci. Tech. 17, 769, Very high electron mobility in bulk. à a better transistor. Tunable bandgap with composition over a wide range à bandgap engineering. Direct bandgap/high absorption coefficients. à a better light emitter and absorber. à a better solar cell.

18 InSb: Some III-V Energy Band-egde Structures InP: InAs: GaAs:

19 Chemical Trends of Bandgap 5.463A 5.653A 6.479A Isoelectronic substitution Lighter elements à higher bandgap Indirect bandgap (dashed line) Direct bandgap (solid line) Chemical composition affects G-valley only. pveducation.org Wisseman and Frensley, VLSI Electronics Microstructure Sciences 11, p. 13 (Academic Press, 1985)

20 Doping Grown-in or implanted Small segregation coefficient à not diffusion GaAs InP p: Be, C, Zn n: Si, Sn p: Be, Zn n: Si GaN p: Mg n: Si

21 Ternary Compounds A x B 1-x C Property P(A x B 1-x C) = xp AC + (1-x)P BC + x(1-x)p AB = a + bx +cx 2 where a = P BC b = P AC P BC + P AB c = - P AB = bowing parameter, due to lattice disorder from cation intermixing of A and B c = 0 à Vegard s Law; lattice constant, index of refraction Two most important parameters in heterostructures: ΔE g and Δn.