Contains an introduction and report on one research project.Joint Services Electronics Program (Contract DAAL03-86-K-0002)Joint Services Electronics Program (Contract DAALO3-89-C-0001)U.S. Navy - Office of Naval Research (Contract N00014-88-K-0564

Kolodziejski, Leslie A.

Lee, Dana I.

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Chapter 6. Chemical Beam Epitaxy of Compound SemiconductorsChapter 6. Chemical Beam Epitaxy of CompoundSemiconductorsSponsorsJoint Services Electronics Program (Contracts DAAL03-86-K-0002 and DAAL03-89-C-0001)U.S. Navy - Office of Naval Research (Contract N00014-88-K-0564)Academic and Research StaffProfessor Leslie A. KolodziejskiGraduate StudentDana I. Lee6.1 IntroductionThe new facility for the chemical beamepitaxy of compound semiconductors (bothIl-VI and III-V materials) represents a majoraddition to RLE and is currently undergoingextensive laboratory construction. A spe-cially designed, one-of-a-kind multichamberchemical beam epitaxy (CBE) system isunder development by EMCORE Corporationand is scheduled to be completed in August1989.A schematic drawing of the new CBE facilityis shown in figure 1. In addition to thegrowth chamber, the system has 1) an intro-duction chamber, which is capable ofaccepting five separate 2 inch wafers, 2) abakeout chamber, which is required to cleanthe sample holders prior to mounting thesubstrate, and 3) an ultrahigh vacuumtransfer module. The entire process will becomputer controlled via a sophisticatedMASSCOMP computer, which is completewith customized software. The transfermodule contains a high temperature stage forthermal processing of substrates, an elevatorfor multiple wafer storage, and has additionalporting such that five separate ultrahighvacuum chambers may be added in thefuture; the expansion could include growthchambers, a metalization chamber, or in situprocessing chambers. Porting is also avail-able to add various sample stages, whichwould allow any necessary manipulation ofeach wafer between various processing steps(such as transferring the wafer from onesample holder to another, etc.). Transfer ofeach wafer from one ultrahigh vacuumprocess chamber to another will be carriedout by a computer-controlled, rotatingrobotic arm. Addition of an analyticalchamber, having Auger electron spectroscopy(AES), is currently in the proposal reviewstage, with plans to add electron spectro-scopy for chemical analysis (ESCA) in thenear future. The necessary funds to acquirethe CBE system originated from support fromthe Sloan Foundation, AT&T Bell Laborato-ries, Office of Naval Research, NationalScience Foundation, Army Research Office,and Defense Advanced Research ProjectsAgency, in addition to a substantial donationfrom EMCORE Corporation.The CBE growth chamber is equipped withfour individual gas injectors for metalorganicsof Zn, Ga, In, and Se, and for the hydridearsine. (Each gas injector has the capabilityof accepting four separate gases.) Threetypical effusion cells, such as those used inmolecular beam epitaxy, are available for usewith dopant or alloying elements. In situanalysis facilities include reflection highenergy electron diffraction, a residual gasanalyzer and a quartz crystal flux monitor.Specially designed heated quartz viewports,have been placed so that a laser beam can beboth incident to, and reflected from, thegrowing surface front. An argon ion laser isavailable to perform visible and ultravioletlaser-assisted epitaxial growth experiments.As an extra safety precaution, the entiregrowth chamber, pumping system and gasChapter 6. Chemical Beam Epitaxy of Compound SemiconductorsFigure 1. Schematic drawing of the layout of thechemical beam epitaxy system. The growth chamber,transfer module, and introduction chamber are shownwith considerable detail, whereas the future expansionmodules are only outlined. The approximate dimen-sions are 17 feet x 25 feet.manifold panel is housed in a fullyexhausted, laminar flow bench, providing a"clean room" environment for the CBEfacility. The introduction chamber will alsobe located in an exhausted, laminar flowhood to minimize any contamination fromthe laboratory and to protect the samplesboth before and after growth.A 1200 square feet laboratory will house theepitaxy facility and is currently in preparationat MIT. Plans for the laboratory include achanging room to keep the environment rela-tively clean, in addition to a completesubstrate preparation facility.6.2 Controlled SubstitutionalDoping of ZnSe Grown byChemical Beam EpitaxyThe II-VI compound semiconductor family,and the potential of these materials foroptical device applications, have remainedrelatively untapped due to the substantial dif-44 RLE Progress Report Number 131ficulties in preparing the II-Vis, by anygrowth technique, to contain both p-typeand n-type carriers. Bulk, liquid phase, orvapor phase growth techniques of the pastused very high temperatures for the fabri-cation of ZnSe, and consequently control ofthe carrier type could not be achieved. Theresultant ZnSe always exhibited n-type trans-port properties, or the as-grown material wasso heavily compensated that the material washighly resistive. Newer growth technologies,such as molecular beam epitaxy (MBE) andchemical beam epitaxy (CBE), providepromise of circumventing the doping prob-lems of the II-VIs, and ZnSe in particular,due to the non-equilibrium nature of thegrowth, and by the ability to affect thegrowth, with photon illumination to cite oneexample. Once ZnSe is controllably dopedas p-type and n-type material, many deviceapplications are foreseen taking advantage ofthe wide, direct bandgap (2.67 eV at roomtemperature) which provides for the emissionof blue light. Blue light emitting diodes, bluesemiconductor lasers, and flat panelelectroluminescent display devices are just asampling of the many possible opticaldevices. The blue wavelength range will beuseful in 1) visible holography and 2) laserdisc recording, where a shorter wavelengthwould allow for a ten-fold increase in thedensity of stored information, and 3) forshort range communications in which newpolymer fibers can be utilized.Studies of dopant incorporation reported inthe literature for ZnSe have been complicatedby the presence of unintentional impuritiesfound in both elemental and compoundsource materials used in the MBE growthprocess. In most cases undoped ZnSe grownby MBE, using commercially available sourcematerial, has been reported to be n-type withlow resistivity (- 1 Q -cm). The lowresistivity of the "undoped" ZnSe materialimplied that the ZnSe had goodstoichiometry, as a relatively small deviationfrom a Zn-to-Se flux ratio of one-to-one,toward either Zn-rich or Se-rich conditions,was found to result in high resistivitymaterial. (The defects generated duringgrowth under non-stoichiometric conditionsappeared to compensate for the non-intentionally incorporated impurities givingrise to the high resistivity.)Chapter 6. Chemical Beam Epitaxy of Compound SemiconductorsAlthough major progress has recently beenmade in the molecular beam epitaxy of ZnSe,significant limitations still exist for the con-trolled substitutional n- and p-type doping.The many advantages of the growth tech-niques of molecular beam epitaxy andmetalorganic chemical vapor deposition(MOCVD) are combined in CBE, while theirmany disadvantages are eliminated. Theultrahigh vacuum environment of the MBEtechnique remains in tact in CBE, but thesolid source material is replaced by gaseoussource material and is similar to MOCVD.The gas sources permit constant flux ratios atreasonable growth rates. By employingmetalorganic gaseous sources for the Zn andSe species, a greater degree of control of theflux intensities will be obtainable than thatpossible by employing low temperature highvapor pressure elemental sources of Zn andSe, as in MBE. Because the elements origi-nate from a gas bottle, long-term operation ispossible, since very little gaseous material isconsumed. Many hardware problemsencountered during the growth of II-VI com-pounds will be eliminated by employingchemical beam epitaxy. It is anticipated thatthe use of chemical beam epitaxy will allowthe crystal growth problems to be overcome,so that scientists and device engineers cancapitalize on the many attractive materialproperties of the Il-VI compound semicon-ductors. With research support from theOffice of Naval Research, the CBE of ZnSewill be explored. The optimum growthparameters will be determined followingmicrostructural, optical, and electrical evalu-ation of the epitaxial layers. Once highquality, ultrapure, undoped ZnSe is achieved,the objective of the program supported underJSEP, is to address the incorporation ofimpurity species in ZnSe.In addition, chemical beam epitaxy isnaturally suited for many experiments whichare designed to modify or affect the epitaxialgrowth process. For example, impingementof either coherent or incoherent light ofvarious wavelengths can effect the "nature"of the incident atomic or molecular species;the actual bonding configuration of the mol-ecules can determine their sticking coefficientand their incorporation. The effects of illumi-nating the growing surface front with a lowpower laser beam will be investigated.The use of gaseous sources to replace highpressure solid sources will allow greater flexi-bility for utilizing the vacuum system for avariety of materials. Historically, the II-VIand Ill-V, or group IV compounds have beengrown in separate vacuum systems to elimi-nate problems associated with unintentionalauto-doping of each material. Since eachelement originates from a gas tank external tothe vacuum in CBE, the source oven andmaterial will not be contaminated with ele-ments of the opposite species. It is envi-sioned that both ultrapure II-VI and III-Vmaterials can be fabricated in the samevacuum system, since the II-VIs and Ill-Vsare not expected to form a compound untilthe metalorganics interact with the heatedsingle crystalline substrate. Studies ofvarious II-VI/Ill-V heterostructures ofcomplex design and dimensions are now fea-sible and will be pursued.Professor Jesus A. del Alamo (center) and students Sandeep Bahl (left) and Walid Azzam46 RLE Progress Report Number 131

Chemical Beam Epitaxy of Compound Semiconductors

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Chemical Beam Epitaxy of Compound Semiconductors

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