Aaron Vallett EE 518 April 5 th, 2007 Principles and Applications of Molecular Beam Epitaxy Instructor: Dr. J. Ruzyllo

Outline n Introduction n Review of epitaxial growth n MBE Process lChamber construction lBeam sources lCharacterization n MBE Applications lDevices lR&D/Commercial n Summary

Introduction n Invented in late 1960s at Bell Labs by J. R. Arthur and A. Y. Cho n An epitaxial growth process involving one or more molecular beams of atoms or molecules physically arranging themselves on a crystalline surface under ultrahigh-vacuum conditions n Growth is tightly controlled – layer compositions and thickness can be adjusted at an atomic scale

Epitaxy Review n Growth of thin, high quality, single-crystal layers on a similar-type crystal substrate n Molecules are adsorbed on the surface n Diffuse across the surface until finding a suitable crystal site Image from

MBE Process Overview n Beam impinges on heated substrate (600°C) n Incident molecules diffuse around the surface to the proper crystal sites and form crystalline layers n Characterization tools allow growth to be monitored in-situ Image modified from n Very similar to thermal evaporation with one big difference - UHV ( torr) n Solid source materials are heated to melting point in effusion cells n UHV gives source molecules a large mean free path, forming a straight beam

MBE Chamber n Stainless steel chamber and seals reduce leaks n After servicing, chamber must be baked and outgassed at ~200°C for 2-5 days n UHV achieved through use of cryo, Ti-sublimation and ion pumps – no oil n Cryo-shroud promotes condensation of contaminants and stray particles Image from 772Spring2003/B5D923F5-9B4C-4436-A1F1-0343B35E1928/0/lect8_part1.pdf

Sample Preparation and Loading n Starting substrate must be ultra clean and flat lWafer usually comes “epi- ready” with a protective oxide n Substrate loaded in load-lock and heated for outgassing for several hours n Substrate may then move to a buffer chamber and be outgassed again n Growth substrate then loaded onto holder in growth chamber n Protective oxide desorbed by heating substrate on the chuck in UHV n Goal is to keep the chamber and sample as pure as possible Image from

Effusion Sources and the Molecular Beam n Effusion: the process where individual molecules flow through a hole without collisions n Source material is heated to vapor phase n Ultra-low pressure in UHV leads to molecules with mean free paths of hundreds of meters n Opening in effusion cell is small – molecules travel straight out of it with no collisions, forming a beam Images from and

Effusion Cell Construction n A typical MBE system may feature 8 effusion cells n Crucible is constructed of pyrolytic boron nitride (PBN) to withstand temperatures up to 1400°C n Thermocouple must accurately measure crucible temperature lChange in T of.5°C changes flux by 1% lDuring the day flux variations of <1%, day-to-day <5% lT must be controlled within a half-degree at 1000°C Images from and n Sources seated in a cooling shroud to maintain flux and eliminate thermal crosstalk between cells n Mechanical shutters in front of sources control the beam

In-situ Characterization n Deposition in UHV allows unique in-situ measurements to be taken n RHEED – reflection-high-energy-electron-diffraction lElectrons from a gun strike the growing surface at a shallow angle lThe crystal reflects electrons into a diffraction pattern lDiffraction pattern and intensity can provide information on the state of the surface n Mass spectrometry lUsed to measure surface and chamber composition n Ionization gage lUsed to measure chamber pressure or molecular beam flux Images from and

MBE Abilities n Deposition rate is ~ 1 μm/hr or 1 monolayer/sec n Computer controlled shutter can be opened or closed in 100 mS n Defect free, super abrupt, single- atom layers can be grown – only MBE allows this precision n Multiple beams can impinge the surface at once to create III-V materials or dope a layer during growth Images from and 15 monolayers AlGaAs/GaAs alternating layers

Device Applications n Traditionally used for very specific, commonly compound- semiconductor, applications lHBTs, MESFETs and HEMTs lQuantum wells lSemiconductor lasers lSilicon-on-sapphire growth Images from and Thompson et. al. IEEE Trans. On Semicon. Manufacturing, Vol. 18, No.1, February 2005 n Also being considered for use in commercial production of SiGe MOSFETs

MBE in Industry n By nature MBE has a very low throughput n If it is needed for future CMOS processing, manufacturers will install clustered MBE chambers to increase throughput Images from CVD%20and%20Epitaxy.pdf

Summary n MBE creates near-perfect crystalline layers n MBE is a purely physical process, so blocking the beam can stop layer growth n Slow growth time allows atomically thin and super abrupt layers to be grown n Mixing of beams permits growth of compound semiconductor and doped layers n MBE is a costly and time consuming technique, but its high level of precision may drive it into the commercial CMOS world

References Parker, E. “Technology and Physics of Molecular Beam Epitaxy” 1985 Chang, L. and K. Ploog “Molecular Beam Epitaxy and Heterostructures” 1985 Liu, W. “Fundamentals of III-V Devices” R M Sidek et al 2000 Semicond. Sci. Technol Thompson et. al. IEEE Trans. On Semicon. Manufacturing, Vol. 18, No.1, February %20Epitaxy.pdf