Epitaxial Deposition

1. Epitaxial Deposition Daniel Lentz EE 518 Penn State University March 29, 2007 Instructor: Dr. J. Ruzyllo

2. Outline • Introduction • Mechanism of epitaxial growth • Methods of epitaxial deposition • Properties of epitaxial layers • Applications of epitaxial layers

3. Epitaxial Growth • Deposition of a layer on a substrate which matches the crystalline order of the substrate • Homoepitaxy • Growth of a layer of the same material as the substrate • Si on Si • Heteroepitaxy • Growth of a layer of a different material than the substrate • GaAs on Si Ordered, crystalline growth; NOT epitaxial Epitaxial growth:

4. Motivation • Epitaxial growth is useful for applications that place stringent demands on a deposited layer: • High purity • Low defect density • Abrupt interfaces • Controlled doping profiles • High repeatability and uniformity • Safe, efficient operation • Can create clean, fresh surface for device fabrication

5. General Epitaxial Deposition Requirements • Surface preparation • Clean surface needed • Defects of surface duplicated in epitaxial layer • Hydrogen passivation of surface with water/HF • Surface mobility • High temperature required heated substrate • Epitaxial temperature exists, above which deposition is ordered • Species need to be able to move into correct crystallographic location • Relatively slow growth rates result • Ex. ~0.4 to 4 nm/min., SiGe on Si

6. General Scheme Modified from http://www.acsu.buffalo.edu/~tjm/MOVPE-GaN-schematic.jpg

7. Thermodynamics • Specific thermodynamics varies by process • Chemical potentials • Driving force • High temperature process is mass transport controlled, not very sensitive to temperature changes • Steady state • Close enough to equilibrium that chemical forces that drive growth are minimized to avoid creation of defects and allow for correct ordering • Sufficient energy and time for adsorbed species to reach their lowest energy state, duplicating the crystal lattice structure • Thermodynamic calculations allow the determination of solid composition based on growth temperature and source composition

8. Kinetics • Growth rate controlled by kinetic considerations • Mass transport of reactants to surface • Reactions in liquid or gas • Reactions at surface • Physical processes on surface • Nature and motion of step growth • Controlling factor in ordering • Specific reactions depend greatly on method employed

9. Kinetics Example • Atoms can bond to flat surface, steps, or kinks. • On surface requires some critical radius • Easier at steps • Easiest at kinks • As-rich GaAs surface • As only forms two bonds to underlying Ga • Very high energy • Reconstructs by forming As dimers • Lowers energy • Causes kinks and steps on surface • Results in motion of steps on surface • If start with flat surface, create step once first group has bonded • Growth continues in same way http://www.bnl.gov/nsls2/sciOps/chemSci/growth.asp

10. Vapor Phase Epitaxy • Specific form of chemical vapor deposition (CVD) • Reactants introduced as gases • Material to be deposited bound to ligands • Ligands dissociate, allowing desired chemistry to reach surface • Some desorption, but most adsorbed atoms find proper crystallographic position • Example: Deposition of silicon • SiCl4 introduced with hydrogen • Forms silicon and HCl gas • Alternatively, SiHCl3, SiH2Cl2 • SiH4 breaks via thermal decomposition

11. Precursors for VPE • Must be sufficiently volatile to allow acceptable growth rates • Heating to desired T must result in pyrolysis • Less hazardous chemicals preferable • Arsine highly toxic; use t-butyl arsine instead • VPE techniques distinguished by precursors used

12. Varieties of VPE • Chloride VPE • Chlorides of group III and V elements • Hydride VPE • Chlorides of group III element • Group III hydrides desirable, but too unstable • Hydrides of group V element • Organometallic VPE • Organometallic group III compound • Hydride or organometallic of group V element • Not quite that simple • Combinations of ligands in order to optimize deposition or improve compound stability • Ex. trimethylaminealane gives less carbon contamination than trimethylalluminum http://upload.wikimedia.org/wikipedia/en/thumb/e/e5/Trimethylaluminum.png/100px-Trimethylaluminum.png, http://pubs.acs.org/cgi-bin/abstract.cgi/jpchax/1995/99/i01/f-pdf/f_j100001a033.pdf?sessid=6006l3

13. Liquid Phase Epitaxy Reactants are dissolved in a molten solvent at high temperature Substrate dipped into solution while the temperature is held constant Example: SiGe on Si Bismuth used as solvent Temperature held at 800°C High quality layer Fast, inexpensive Not ideal for large area layers or abrupt interfaces Thermodynamic driving force relatively very low Molecular Beam Epitaxy Very promising technique Elemental vapor phase method Beams created by evaporating solid source in UHV Other Methods

14. Doping of Epitaxial Layers • Incorporate dopants during deposition • Theoretically abrupt dopant distribution • Add impurities to gas during deposition • Arsine, phosphine, and diborane common • Low thermal budget results • High T treatment results in diffusion of dopant into substrate • Reason abrupt distribution not perfect

15. Properties of Epitaxial Layer • Crystallographic structure of film reproduces that of substrate • Substrate defects reproduced in epi layer • Electrical parameters of epi layer independent of substrate • Dopant concentration of substrate cannot be reduced • Epitaxial layer with less dopant can be deposited • Epitaxial layer can be chemically purer than substrate • Abrupt interfaces with appropriate methods

16. Applications • Engineered wafers • Clean, flat layer on top of less ideal Si substrate • On top of SOI structures • Ex.: Silicon on sapphire • Higher purity layer on lower quality substrate (SiC) • In CMOS structures • Layers of different doping • Ex. p- layer on top of p+ substrate to avoid latch-up

17. More applications • Bipolar Transistor • Needed to produce buried layer • III-V Devices • Interface quality key • Heterojunction Bipolar Transistor • LED • Laser http://www.search.com/reference/Bipolar_junction_transistor http://www.veeco.com/library/elements/images/hbt.jpg

18. Summary • Deposition continues crystal structure • Creates clean, abrupt interfaces and high quality surfaces • High temperature, clean surface required • Vapor phase epitaxy a major method of deposition • Epitaxial layers used in highest quality wafers • Very important in III-V semiconductor production

19. References • P. O. Hansson, J. H. Werner, L. Tapfer, L. P. Tilly, and E. Bauser, Journal of Applied Physics, 68 (5), 2158-2163 (1990). • G. B. Stringfellow, Journal of Crystal Growth, 115, 1-11 (1991). • S. M. Gates, Journal of Physical Chemistry, 96, 10439-10443 (1992). • C. Chatillon and J. Emery, Journal of Crystal Growth, 129, 312-320 (1993). • M. A. Herman, Thin Solid Films, 267, 1-14 (1995). • D. L. Harame et al, IEEE Transactions on Electron Devices, 42 (3), 455-468 (1995). • G. H. Gilmer, H. Huang, and C. Roland, Computational Materials Science, 12, 354-380 (1998). • B. Ferrand, B. Chambaz, and M. Couchaud, Optical Materials, 11, 101-114 (1999). • R. C. Cammarata, K. Sieradzki, and F. Spaepen, Journal of Applied Physics, 87 (3), 1227-1234 (2000). • R. C. Jaeger, Introduction to Microelectronic Fabrication, 141-148 (2002). • R. C. Cammarata and K. Sieradzki, Journal of Applied Mechanics, 69, 415-418 (2002). • A. N. Larsen, Materials Science in Semiconductor Processing, 9, 454-459 (2006).