r-how molecular beam epitaxy (mbe) began and its projection.pdf

*Tel.: #1 908 582 2093; fax: #1 908 582 2043; e-mail:[email protected].

Journal of Crystal Growth 201/202 (1999) 1}7

How molecular beam epitaxy (MBE) began and its projectioninto the future

A.Y. Cho*Bell Laboratories, Lucent Technologies, Murray Hill, New Jersey, USA

Abstrat

This is an extended abstract of a talk given at the International Conference on Molecular Beam Epitaxy in Cannes,France, August 31, 1998. It describes some critical turning points of the development of MBE from basic research to highvolume production technology. ( 1999 Elsevier Science B.V. All rights reserved.

I would like to thank the committee for invitingme here to give this talk. In fact, the committee gavethe title of this talk to me. Because of the limitedtime, my discussion will only focus on III}V com-pounds where as Group IV, II}VI, III}IV, metaland insulating materials by MBE will be coveredby other speakers.

There were many attempts to grow compoundsemiconductors in vacuum before the developmentof molecular beam epitaxy. For III}V compounds,Gunther [1] proposed the use of a `three temper-aturea technique where the substrate, Group III,and Group V sources had di!erent temperatures toindependently control their vapor pressures. How-ever, he did not use single crystal substrates and,therefore, `epitaxya could not even be de"ned.Later Davey and Pankey [2] prepared epitaxialGaAs on single crystalline GaAs substrates; but,

most of the "lms showed texture or twinning, andthey could only report crystal structures with elec-tron and X-ray di!ractions. No transport or opticalproperties such as photoluminescence were re-ported because the "lms were, in general, too poorto measure.

In the late 1960s, as devices were getting smallerand smaller, there was a great demand for a crystalgrowth technology that could prepare single cry-stalline "lms as thin as 500}1000 A_ . There wasa need to invent a new process. Invention some-times happens when one combines the knowledgeof two established technologies and applies them toa third to create a new technology. Such is the casefor the development of molecular beam epitaxy.Molecular beam epitaxy borrowed the knowledgeof `surface physicsa and `ion propulsion techno-logya to create a new crystal growth technology.

The term molecular beam epitaxy (MBE) was"rst used in 1970 [3] after several years of extensivestudies of atomic and molecular beams interactingwith solid surfaces [4,5]. A great deal of outstand-ing surface physics work was carried out by Arthur,

0022-0248/99/$ } see front matter ( 1999 Elsevier Science B.V. All rights reserved.PII: S 0 0 2 2 - 0 2 4 8 ( 9 8 ) 0 1 2 6 5 - 2

Fig. 1. High energy electron di!raction (HEED) patterns of (1 0 0) GaAs and the corresponding electron micrographs (38400X) of Pt}Creplicas of the same surface.

2 A.Y. Cho / Journal of Crystal Growth 201/202 (1999) 1}7

Fig. 2. As2/Ga molecular beam #ux ratio as a function of sub-

strate temperature when the transition between As- and Ga-stabilized structure occurs. Since there is hysteresis in thetransitions, two sets of curves are shown.

Fig. 3. E!usion cell used for Cs ion beam experiment for ionpropulsion in 1964.

Foxon, Harvey, and Joyce on measurements of theadsorption lifetime, sticking coe$cient, and reac-tion order, giving insight into the interaction of Gaand As

4beams on GaAs surfaces [5,6]. However,

the construction of the surface phase diagram ofGaAs, by observing the As-stabilized and Ga-sta-bilized surface structures with high energy electrondi!raction (HEED), was the beginning of control-led epitaxial growth of GaAs thin "lms [7}9] asshown in Fig. 1. Fig. 2 shows that the surface struc-ture changes as a function of the substrate temper-ature and relative #uxes of As and Ga incidentupon the surface. The conversion of surface struc-tures implied a change in surface composition whichwas used to construct the `surface phase diagramsa.When I "rst presented the reconstruction of surfacestructures in 1970, I met tremendous oppositionand resistance in the surface physics community. Atthat time, surface reconstruction was considered interms of impurity atoms sitting on the surface of thecrystal. It was very di$cult for the community toaccept the concept that intrinsic atoms on the sur-face will reconstruct themselves into a di!erentsurface net.

In the early experiments, it was thought thatequilibrium evaporation was important. Knudsen

cell design with a pinhole aperture of less than1 mm in diameter was used. The cells were quartzampules with tungsten wire wound over the am-pules and heated by a variac. This con"gurationwas satisfactory for surface physics studies, but notdesirable as a "lm growth e!usion cell because thedeposition rate from the pinhole was limited to lessthan one atomic layer per minute. A major ad-vancement occurred in 1969 when I applied theprevious knowledge of `ion propulsion techno-logya to our experiment. The e!usion cells werechanged to large aperture graphite and aluminaconstruction with heat shielding consisting oflayers of corrugated tantalum foil to reduce theheat loss and temperature cross talk with adjacentcells. All cells were surrounded with a liquid nitro-gen cooled shroud to reduce the background pres-sure. The cell temperatures were also regulated bynegative electronic feedback systems to assureprecise e!usion #uxes. These concepts were allknowledge borrowed from the `ion propulsiontechnologya. Fig. 3 shows a tantalum heat shieldedcesium ion emitter mounted on a 2-3/4 in stainlesssteel #ange used in the 1964 ion beam experiment[10]. This is to be compared to today's standardMBE e!usion cell, for example, by EPI shown inFig. 4. An arsenic cracker cell was "rst used in 1971for the production of As

2to improve the photo-

luminescence e$ciency [11]. High energy electrondi!raction (HEED) became a routine real-time

A.Y. Cho / Journal of Crystal Growth 201/202 (1999) 1}7 3

Fig. 4. E!usion cell used for molecular beam epitaxy (photograph courtesy of EPI).

monitoring tool for the initial cleaning and success-ive growth of epitaxial "lms. We were then able togrow high-quality GaAs layers.

A theoretical paper published in 1970 on superla-ttices and negative di!erential conductivity insemiconductors by Esaki and Tsu [12] openeda whole new dimension for MBE growth for thestudy of quantum mechanical e!ects on a newphysical scale. Chang and Esaki later added com-puter control to the MBE system and observed thenegative di!erential conductivity in superlatticestructures [13]. By the mid 1970s, MBE becamemore visible after publications of the "rst varactor[14], IMPATT diode [15], room temperaturecw semiconductor laser [16], and microwaveFETs [17]. Exciting results on quantum states ofcon"ned carriers in periodic GaAs/Al

xGa

1~xAs

structures, in which di!erent bound-electron and

bound-hole states were reported by Dingle, Wieg-mann, and Henry [18]. The subsequent develop-ment lead to modulation doping [19] and thediscovery of the fractional quantum Hall ef-fect [20]. Fig. 5 shows a transmission electronmicrograph of a cross sectional view of aGaAs/Al

xGa

1~xAs superlattice. Each period con-

sists of four atomic layers of GaAs and four atomiclayers of Al

xGa

1~xAs. This demonstrated the pre-

cise control of MBE growth. Even with all thesegood results, we received many unfavorable com-ments from competitors using well-establishedtechnologies. Comments such as, MBE was tooslow (one micron per hour growth rate), MBE wastoo hard to use (need ultra high vacuum), and MBEwas too expensive (it stands for mega buck equip-ment). It was viewed as a threat rather than anopportunity.


Fig. 5. Scanning electron micrograph of the cross-sectional view of alternating layers of GaAs (dark lines) and AlxGa

1~xAs (light lines)

grown by MBE.

Molecular beam epitaxy continued to developand became a high volume production technologyin the late 1980s. A series of developments enablinglarger substrates, higher uniformity, faster turn-around, and real-time monitoring were designedinto the MBE system for high yield and cost e!ec-tiveness [21}24]. More recently, commerciallyavailable systems such as the Riber MBE 49 canload 15 platens with four 4A wafers on each platen(Fig. 6), or the VG Semicon model V150 can mountnine 4A wafers on each platen (Fig. 7). MBE isbroadly used today for advanced multilayer crystalgrowth and has led to radically new devices includ-ing high-speed transistors, microwave devices, laserdiodes and detectors. Most of the semiconductorlasers used in today's compact disc players andCD-ROM's are manufactured using MBE-grownmaterial [23]. Presently, the Hall sensors used asdisk drive speed controllers for computers andVCR's are produced by MBE [24]. High electron

mobility transistors (HEMT) which are utilized asa high speed circuit components and in high fre-quency, low noise direct broadcast satellite andwireless communications are manufactured byMBE [22].

Looking into the future, MBE will continue todevelop into a multi-disciplinary, environmentallysafe, method to create new materials and study newphysics with atomic dimension precision. Someexamples are, microwave devices for GHz to THzoperation, CMOS with GaAs (GdGaO

xon GaAs),

band structure engineering for quantum cascade(QC) lasers, multi-wavelength lasers and detectors,information storage of more than 100 Gb per sq.in., environmentally safe processes with solidsource MBE and nitrides, in situ real time process-ing with new knowledge of surface physics and sur-face chemistry, and new discoveries of physicalphenomenon and new material science. In produc-tion, my dream is to have an integrated device


Fig. 6. A cassette for Riber MBE49 system can load 15 platens with four 4A wafers on each platen (photograph courtesy of Riber).


Fig. 7. An MBE multi-wafer system V150 by VG Semicon can grow nine 4A wafers for each run (photograph courtesy of VG Semicon).

processing MBE system where we can load a waferin one end and devices come out the other end.Finally, I would like to conclude by saying that thefuture of MBE will depend on you, all of you, tomake it more useful to explore the frontiers ofscience and more powerful to manufacture the de-vices of the next generation.

References

[1] K.G. Gunther, Z. Naturforsch. 13a (1958) 1081.[2] J.E. Davey, T. Pankey, J. Appl. Phys. 39 (1968) 1941.[3] A.Y. Cho, M.B. Panish, I. Hayashi, 3rd Int. Symp. on

Gallium Arsenide and Related Compounds, The Insti-tute of Physics, Conference Series Number 9, 1970,pp. 18}29.

[4] H. Shelton, A.Y. Cho, J. Appl. Phys. 37 (1966) 3544.[5] J.R. Arthur Jr., J. Appl. Phys. 39 (1968) 4032.[6] C.T. Foxon, J.A. Harvey, B.A. Joyce, J. Phys. Chem. Solids

34 (1973) 1693.[7] A.Y. Cho, J. Appl. Phys. 41 (1970) 2780.[8] A.Y. Cho, J. Appl. Phys. 42 (1971) 2074.[9] A.Y. Cho, J. Vac. Sci. Technol. 8 (1971) S31.

[10] A.Y. Cho, H. Shelton, Ion Emitter Studies, NASA ReportCR-54045, 1964.

[11] A.Y. Cho, I. Hayashi, Solid State Electron. 14 (1971) 125.[12] L. Esaki, R. Tsu, IBM J. Res. Dev. 14 (1970) 61.[13] L.L. Chang, L. Esaki, W.E. Howard, R. Ludeke, J. Vac. Sci.

Technol. 10 (1973) 11.[14] A.Y. Cho, F.K. Reinhart, J. Appl. Phys. 45 (1974) 1812.[15] A.Y. Cho, C.N. Dunn, R.L. Kuvas, W.E. Schroeder, Appl.

Phys. Lett. 25 (1974) 224.[16] A.Y. Cho, R.W. Dixon, H.C. Casey Jr., R.L. Hartman,

Appl. Phys. Lett. 28 (1976) 501.[17] A.Y. Cho, J.V. DiLorenzo, B.S. Hewitt, W.C. Niehaus,

W.O. Schlosser, C. Radice, J. Appl. Phys. 48 (1977) 346.[18] R. Dingle, W. Wiegmann, C.H. Henry, Phys. Rev. Lett. 33

(1974) 827.[19] R. Dingle, H.L. Stormer, A.C. Gossard, W. Wiegmann,

Appl. Phys. Lett. 33 (1978) 665.[20] D.C. Tsui, H.L. Stromer, A.C. Gossard, Phys. Rev. Lett. 48

(1982) 1559.[21] T. Sonoda, M. Ito, M. Kobiki, K. Hayashi, S. Takamiya,

S. Mitsui, J. Crystal Growth 95 (1989) 317.[22] K. Kondo, J. Saito, T. Igarashi, I. Nanbu, T. Ishikawa,

J. Crystal Growth 95 (1989) 309.[23] H. Tanaka, M. Mushiage, J. Crystal Growth 111 (1991) 1043.[24] I. Shibasaki, 9th Int. Conf. on Molecular Beam Epitaxy,

Malibu, CA, August 5}9, 1996.


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