Chapter 3. Molecular Beam Epitaxy of Compound Semiconductors · Chapter 3. Molecular Beam Epitaxy of Compound Semiconductors 0 MgS 4 Aat Q7nS o MgSe 3 U, 0GaP ZnTe 2 SGaAs.t S 1 Si

Chapter 3. Molecular Beam Epitaxy of Compound Semiconductors

Chapter 3. Gas Source Molecular Beam Epitaxy of CompoundSemiconductors

Academic and Research Staff

Professor Leslie A. Kolodziejski, Dr. Irina Mnushkina, Dr. Gale S. Petrich

Graduate Students

Joseph F. Ahadian, Christopher A.Ho, Jody L. House, Kan Lu

Coronado, Jay N. Damask, Sean M. Donovan, Philip A. Fisher, Easen

Undergraduate Students

James R. Geraci, Kuo-Yi Lim

Technical and Support Staff

Charmaine A. Cudjoe-Flanders, Angela R. Odoardi

3.1 Introduction

Current state-of-the-art epitaxial growth techniquesemploy various metalorganic and hydride gases todeliver constituent species to the substrate surface,particularly high vapor pressure species such asphosphorus and sulfur. Gas source molecularbeam epitaxy (GSMBE) utilizes hydride gas sourcesand solid elemental sources. The more conven-tional growth approach, molecular beam epitaxy(MBE), uses only molecular beams derived from thethermal evaporation of elemental or compound solidsources.

All the research objectives described in this chapterare concerned with layered structures composed ofcompounds containing As and P, or Se, S, and Te.The presence of these high vapor pressure speciessuggests that many advantages will be attainedthrough the fabrication of the various multilayereddevice structures using the gaseous source epitaxyapproach.

In the chemical beam epitaxy facility at MIT, theepitaxial growth of both II-VI and Ill-V compoundsemiconductors is underway using GSMBE growthtechniques. The chemical beam epitaxy laboratoryconsists of two gaseous source epitaxy reactorsinterconnected to several smaller chambers whichare used for sample introduction and in-situ surfaceanalysis and metalization. The multichamberepitaxy system allows for the fabrication of a large

number of different heterostructures completelywithin a continuous ultrahigh vacuum environment.The interconnected reactors enable an additionaldegree of freedom in device design by providing theability to integrate the II-VI and Ill-V material fami-lies into a single device. The Ill-V GSMBE reactoruses solid elemental sources of Ga, In, Al, Si andBe and gaseous hydride sources of arsenic andphosphorus. The II-VI reactor currently uses a solidZn source and gaseous hydrogen selenide, in addi-tion to a nitrogen plasma source and a solid ZnCI 2source to achieve n- and p-type doping. Theunique design of the Il-VI reactor also allows theuse of metalorganic sources. Figure 1 highlightsthe many heterostructure systems, based on Il-VImaterials, Ill-V materials, or on a combination ofIl-VI and Ill-V semiconductors, available for explora-tion. Many are currently being fabricated in ourepitaxy facility.

Wide bandgap II-VI heterostructures have thepotential for commercially viable short wavelength(visible to ultraviolet) optical sources for use inoptical recording applications and bright emissivedisplays, for example. A key advance in the areaof Zn chalcogenides involved the successful p-typedoping of ZnSe-based semiconductors usingnitrogen as the acceptor species, as first reportedby Park et al.1 The ability to dope the wide bandgapIl-VIs led to the fabrication of a pn diode injectionlaser which operated at blue/blue-green wave-

1 M. Park, M.B. Troffer, C.M. Rouleau, J.M. DePuydt, and M.A. Haase, "P-Type ZnSe by Nitrogen Atom Beam Doping During Molec-ular Beam Epitaxial Growth," Appl. Phys. Lett. 57: 2127-2129 (1990).


0 MgS

4Aat Q7nS o MgSe

3U,

0GaP ZnTe2

.t SGaAsSiS 1 0

Ge [ GaSb

InAs0 1 a5.3 5.5 5.7 5.9 6.1 6.3

"Lattice Constants" (A)

Figure 1. The shaded area highlights the many different II-VI and Ill-V semiconductors and the various heterostructureconfigurations which are available for investigation in the MIT chemical beam epitaxy laboratory.

lengths by Haase et al.2 and H. Jeon et al.3 Thelaser structures consisted of (Zn,Cd)Se narrowbandgap well layers with Zn(Se,S) cladding barrierlayers, although more recent structures are com-posed of barrier layers of (Zn,Mg)(Se,S).4 To mini-mize defect generation within the active regions ofthe device as well as to maximize the incorporationof the nitrogen acceptor species, the reportedsubstrate temperatures have ranged from 150 0C to2500C. At low growth temperatures, control of theflux ratio of high vapor pressure Group II and GroupVI species is very crucial and extremely difficult. Byemploying gas source epitaxy technologies, controlof the constituent species via precision mass flowcontrollers (as opposed to solid source thermaleffusion ovens) is anticipated to offer a solution to

the difficulties encountered in controlling the compo-sition of ternaries and quaternaries. To addressthese material-related issues, we have embarkedupon a II-VI-based research program which empha-sizes the growth, using gas source molecular beamepitaxy, of various heterostructures which have thepotential for being visible light emitters.

In the next section, we will describe our progress inthe growth and doping of ZnSe using gas sourcemolecular beam epitaxy. The II-VI effort is comple-mented by a program having the goal to fabricatelattice-matched epitaxial buffer layers for ZnSe thatare composed of (In,Ga)P. The III-V GSMBEsystem is also utilized for the fabrication of(In,Ga)(As,P) waveguide devices that operate astunable filters at 1.55 pm, which is the wavelength

2 M.A. Haase, J. Qui, J.M. DePuydt, and H. Cheng, "Blue-Green Laser Diodes," Appl. Phys. Leftt. 59: 1272-1274 (1991).

3 Jeon, J. Ding, W. Patterson, A.V. Nurmikko, W. Xie, D.C. Grillo, M. Kobayashi, and R.L. Gunshor, "Blue-green Injection Laser Diodesin (Zn,Cd)Se/ZnSe Quantum Wells," Appl. Phys. Lett. 59: 3619-3621 (1991).

4 T. Okuyama, T. Miyajima, Y. Morinaga, F. Hiei, M. Ozawa, and K. Akimoto, "ZnSe/ZnMgSSe Blue Laser Diode," Electron. Leftt. 28:1798-1799 (1992).

36 RLE Progress Report Number 136


of interest for optical fiber communication. Newprojects which are in the early stages of investi-gation are described at the end of the chapter andtake advantage of the many capabilities available inthe chemical beam epitaxy laboratory.

3.2 Gas Source Molecular BeamEpitaxy of ZnSe, ZnSe:CI and ZnSe:N

Sponsors

Advanced Research Projects AgencySubcontract 284-25041

Joint Services Electronics ProgramContract DAAL03-92-C-0001

National Center for Integrated Photonic TechnologyContract 542-381

Project Staff

Professor Leslie A. Kolodziejski, Dr. IrinaMnushkina, Dr. Gale S. Petrich, Christopher A.Coronado, Philip A. Fisher, Easen Ho, Jody L.House, James R. Geraci, Kuo-Yi Lim

In the case of the growth of the II-VI material family,all the constituent species are high vapor pressureelements necessitating the reproducible control ofeffusion cells which operate at low temperatures.The difficulty in maintaining elemental effusion cellsat very low temperatures (- 180 - 300 0C) is elimin-ated by the use of mass flow controllers which pre-cisely regulate the flow of gaseous anion species.The ZnSe-based heterostructure system has rele-vance to the optoelectronics community for systemapplications requiring optical sources operating inthe green to UV spectral range. In this program,the growth and doping of ZnSe by GSMBE areinvestigated.

ZnSe was grown on semi-insulating, n- and p-type(001) GaAs bulk substrates using elemental Zn andgaseous H2Se. The growth temperature rangedfrom 2400C to 400 0C, although the majority of thefilms were grown at 2700C. The substrate temper-ature was calibrated by observing the eutecticphase transition (356°C) of 500 A of Au that wasdeposited onto Ge and was continuously monitoredusing an optical pyrometer. The H2Se gas flow wasvaried from 0.8 to 4.2 sccm and resulted in achamber pressure of approximately 2 x 10-1 Torrduring growth; the H2Se was thermally decomposedat 10000C. The zinc flux, as measured by a water-

cooled crystal oscillator placed in the substrateposition, corresponded to a zinc deposition rateranging from 0.5 to 2.3 A/s. Under these condi-tions, the Se:Zn flux ratio ranged from 1.3:1 to0.6:1. During the epitaxial growth process, thesurface stoichiometry was monitored by observingthe surface reconstruction using reflection highenergy electron diffraction (RHEED). A radio fre-quency (RF) plasma cell was used for a source ofatomic nitrogen acceptor species. The RF power tothe plasma cell was varied from 100 to 500 Watts.A leak valve regulated the nitrogen gas flow, whilean ionization gauge in the reactor measured theequilibrium nitrogen pressure, which ranged from6 x 10- 7 to 2 x 10- 1Torr. The ZnSe was dopedn-type using a ZnCI2 source having a source tem-perature which ranged from 150'C to 2500C. TheZnSe was grown at rates of 0.3 to 0.7 pm/hour tolayer thicknesses of 1 to 4 pm.

The optical properties of the epitaxial ZnSe layerswere measured by photoluminescence (PL). PLwas obtained by optically exciting the samples withthe 3250 A line of a focused He-Cd laser, providinga power density of approximately 300 mW/cm 2.The ZnSe luminescence was analyzed with a half-meter spectrometer and a photomultiplier. Figure 2shows the low temperature (10K) PL spectraobtained from a series of undoped ZnSe filmsgrown under identical growth conditions, but withdifferent substrate temperatures. For each growthtemperature investigated, the PL spectrum wasdominated by an intense donor-bound excitonfeature having an energy of 2.798 eV and is specu-lated to be due to an unintentionally incorporateddonor originating from the source materials. Thedeep level defect-related luminescence band, whichis broadly centered about 2.25 eV, was at least100-1000 times weaker in intensity than the near-bandedge features. To elucidate the presence ofthe defect-related luminescence, photolu-minescence was also obtained at 77K. As seen infigure 3, as a function of growth temperature, thePL spectra showed that the donor-bound excitonpersisted as the dominant feature. However, at thegrowth temperature extremes, a small amount ofluminescence originating from deep levels wasobserved. As another indication of the high qualityof the ZnSe layers, the integrated PL intensity ofthe near-bandedge feature was found to decreaseto one-sixtieth of its 10K value as the sample tem-perature was increased to room temperature; roomtemperature luminescence was easily observed bythe naked eye.


Figure 2. The 10K photoluminescence intensity as a function of energy for ZnSe epilayers grown at various substratetemperatures. The dominant donor-bound exciton had an energy of 2.798 eV. The ZnSe film that was grown at 2840C,was deposited on an (In,Ga)P buffer layer.

I

320300

Growth 2 0 3 3.2Temperature 2.6 2.8

(Celsius) 240 1.6 1.8 2 2.2

Energy (eV)

Figure 3. The 77K photoluminescence intensity as a function of energy for ZnSe epilayers grown at various substratetemperatures. The dominant donor-bound exciton has an energy of 2.787 eV. The ZnSe film that was grown at 2840C,was deposited on an (In,Ga)P buffer layer.


77 K


2.798 10K

FWHM (a)m 5 meV

2.72 2.74 2.76 2.78 2.8

2.798

FWHM (b)5 meV 2.803

C _

2.72 2.74 2.76 2.78 2.8

1.8 2 2.2 2.4 2.6 2.8 3 3.2Energy (eV)

Figure 4. The 10K photoluminescence intensity as a function of energy (the inset shows a higher resolution energyscan). The spectrum in (a) reflects the PL obtained from ZnSe grown under Se-rich surface stoichiometry conditions,whereas (b) was obtained from a sample grown under conditions of Zn-rich surface stoichiometry. All other growthparameters were held constant.

During the growth, the surface stoichiometry wasvaried to provide either a Zn-rich or Se-rich surfaceby modifying the H2Se gas flow. Changes in theRHEED pattern from a (2x1) reconstructed pattern,suggesting a Se-rich surface, to a (2x2) recon-structed pattern, suggesting a Zn-rich surface, veri-fied the presence of a particular surfacestoichiometry. The surface stoichiometry, whichwas maintained throughout the growth, was foundto influence the near-bandedge PL features, as wellas the unintentional impurity incorporation. Figure 4compares the PL spectrum, obtained from anundoped ZnSe film grown under Se-rich surfacestoichiometry (figure 4a) to a sample grown underZn-rich surface stoichiometry (figure 4b). All othergrowth parameters were identical.

Regardless of the surface stoichiometry, the near-bandedge luminescence was dominated by adonor-bound exciton transition. However, anincrease in the intensity of the free exciton feature(at an energy of 2.803 eV) was apparent in theundoped films grown under conditions whichprovide a Zn-rich surface stoichiometry. Differ-ences in the undoped carrier concentration werealso observed depending on the surfacestoichiometry. Room temperature Hall effect mea-surements were performed on the undoped ZnSesamples with indium contacts in the Van der Paawgeometry. Films grown with a Se-rich surface

stoichiometry had free electron concentrations inthe mid 1016 to low 1017cm- 3 range, while filmsgrown with a Zn-rich surface stoichiometry had freeelectron concentrations from the mid 1015 to low1016 cm- 3.

The RF plasma cell provided a source of bothatomic and molecular nitrogen species, while aZnCI2 effusion cell provided a source of Cl forinvestigation of the p- and n-type doping, respec-tively, of ZnSe using the GSMBE growth technique.The nitrogen pressure was monitored by an ioniza-tion gauge within the reactor (prior to insertion ofthe H2Se gas). The CI doping level was controlledby the temperature of the ZnCI2 effusion cell. Char-acterization of the doped ZnSe layers was per-formed using photoluminescence, Nomarskimicroscopy, double crystal x-ray diffraction rockingcurves, secondary ion mass spectroscopy (SIMS),and capacitance-voltage (C-V) measurements. Theobserved structural characteristics were similar tothe undoped layers. Specifically, the surfaceremained featureless as examined by Nomarskimicroscopy for all doping conditions explored thusfar; the (400) peak of the x-ray rocking curvesexhibited typical full width at half maximum (FWHM)ranging from 175 to 200 arc seconds.

Low temperature PL was used to optically charac-terize both the nitrogen- and chlorine-doped ZnSe


2.5 2.55 2.6 2.65 2.7 2.75 2.8 2.85 2.9 2.95 3Energy (eV)

Figure 5. The 10K photoluminescence intensity as a function of energy for various nitrogen gas flows. (b)-(d) Thenitrogen flow was increased to vary the incorporation of nitrogen acceptor species. The PL obtained from undopedZnSe (a) is shown for comparison.

epilayers. The photoluminescence obtained fromZnSe:N layers are shown in figure 5. In this figure,the 10K spectra are compared for samples grownunder identical growth conditions; however, thenitrogen gas flow was systematically increased toenhance the incorporation of acceptors into thelattice. Figure 5 illustrates that the characteristic PLfeatures of the ZnSe:N films were strongly depen-dent on the degree of nitrogen incorporation. Thegrowth conditions for the series of films in figure 5were as follows: a 2700C calibrated substrate tem-perature, a Se-rich surface stoichiometry, and aconstant RF power of 100 Watts for the plasmasource. From the comparisons of the dominanttransitions observed in the 10K PL, we deducedthat the nitrogen concentration increased withhigher nitrogen gas flow, as expected. For thesample grown with the lowest nitrogen flow, shownin figure 5b, the near-bandedge transitions weredominated by the neutral N acceptor peak at 2.793

s P.J. Dean, D.C. Herbert, C.J. Werkhoven, B.J. Fitzpatrick, andSelenide," Phys. Rev. B 23: 4888-4901 (1981).

eV. (PL obtained from an undoped sample isshown for reference in figure 5a.)

The free electron-to-acceptor transition (FA) waspresent at 2.716 eV indicating a nitrogen acceptorbinding energy of 109 meV, assuming a 10 Kbandgap energy of 2.825 eV.5 The donor-to-acceptor-pair (DAP) transition peak at 2.698 eV andthe associated LO phonon replicas were alsodetected. Further increases in the nitrogen flowresulted in a spectrum dominated by the FA andDAP transitions (and their LO phonon replicas), asshown in figure 5c. At the highest nitrogen gasflows investigated (figure 5d), the FA transition dis-appeared, while the DAP and phonon replicasmerged into a single broad feature. Similar PLspectra for variations in the nitrogen incorporationfor ZnSe:N grown by MBE were observed byOhkawa et al.6 In addition, for a given doping condi-tion, a Zn-rich surface stoichiometry appeared to

R.N. Bhargava, "Donor Bound Exciton Excited States in Zinc

6 K. Ohkawa, T. Karasawa and T. Mitsuyu, "Characteristics of p-type ZnSe Layers Grown by Molecular Beam Epitaxy with RadicalDoping," Jpn. J. Appl. Phys. 30: L152-L155 (1991).



incorporate the nitrogen species more effectively,since the PL spectrum from a Zn-rich surfacestoichiometry exhibits a single broad DAP transition,while the spectrum from a Se-rich surfacestoichiometry consisted of multiple, easily distin-guished DAP (and LO phonon replicas) transitions.The PL spectra from 2 pm Cl-doped ZnSe epilayersconsisted of a single donor-bound exciton transitionat 2.795 eV. By comparing various samples, theintensity of the donor-bound exciton transition wasfound to increase linearly as the Cl concentrationwas increased from 2 x 1016 to 2 x 1017 cm-3. Thefree electron carrier concentration was determinedby room temperature Hall measurements and corre-lated to atomic incorporation levels obtained bySIMS measurements.7

The incorporation level of nitrogen in the ZnSe:Nsamples was also determined by secondary ionmass spectroscopy. A nitrogen-implanted ZnSestandard was used to calibrate the nitrogen concen-tration, resulting in a detection limit that was esti-mated to be approximately 1017 cm- 3. For growthunder Se-rich surface stoichiometry, the measurednitrogen concentration ranged from below thedetection limit of the SIMS apparatus (< 1017 cm-3)to 4.0 x 1018 cm- 3 . In comparison, samples grownunder Zn-rich conditions had nitrogen concen-trations ranging from 2 x 1017 to 5.0 x 1018 cm- 3.These results agreed with the qualitative compar-isons of the PL features obtained from the samplesthat were grown under Zn-rich and Se-rich surfacestoichiometries.

The ZnSe:N films were electrically characterized byC-V measurements due to the difficulty in formingan ohmic contact to p-type ZnSe. The measure-ments were made using Cr/Au Schottky contacts ina ring-dot electrode configuration at a measurementfrequency of 1 MHz. The devices were typically200 pm dots with a 20 pm wide separationbetween the center dot and the outer ground plane.Thus far, net acceptor concentrations (NA-ND) Of1.0 x 1017 cm- 3 have been measured and corre-spond to a nitrogen concentration of 1.5 x 1018 cm-3

(as measured by SIMS).

3.2.1 Publications

Coronado, C.A., E. Ho, and L.A. Kolodziejski."Effect of Laser on MOMBE of ZnSe UsingGaseous and Solid Sources." J. Crystal Growth127: 323-326 (1993).

Coronado, C.A., E. Ho, P.A. Fisher, J.L. House, K.Lu, G.S. Petrich, and L.A. Kolodziejski. "P-TypeDoping of ZnSe Using a Nitrogen Plasma duringGSMBE." Paper presented at the ElectronicMaterials Conference, Santa Barbara, California,June 21-23, 1993.

Coronado, C.A., E. Ho, P.A. Fisher, J.L. House, K.Lu, G.S. Petrich, and L.A. Kolodziejski. "GasSource Molecular Beam Epitaxy of ZnSe andZnSe:N." J. Electron. Mater. 23: 269-274(1994).

Ho, E., C.A. Coronado, and L.A. Kolodziejski."Elimination of Surface Site Blockage Due toEthyl Species in MOMBE of ZnSe." J. Electron.Mater. 22(5): 473-478 (1993).

Ho, E., C.A. Coronado, and L.A. Kolodziejski."Photo-Assisted Chemical Beam Epitaxy of II-VISemiconductors." In Beam-Solid Interactions:Fundamentals and Applications. Eds. M.Nastasi, L.R. Harriott, N. Herbots, and R.S.Averback. Pittsburgh: Materials ResearchSociety, 1993, vol. 279, pp. 635-644.

Ho, E., P.A. Fisher, J.L. House, C.A. Coronado,G.S. Petrich, L.A. Kolodziejski, M.S. Brandt,and N.M. Johnson. "P- and N-Type Doping ofZnSe Using GSMBE." Spring Meeting of theMaterials Research Society, San Fransisco, Cal-ifornia, April 4-8, 1994.

Ho, E., C.A. Coronado, P.A. Fisher, J.L. House, K.Lu, G.S. Petrich, and L.A. Kolodziejski."Gaseous Source Epitaxy of ZnSe: MOMBE andGSMBE." Paper presented at the InternationalConference on Chemical Beam Epitaxy andRelated Growth Techniques, Nara, Japan, July23-25, 1993.

Thesis

House, J.L. Optical Characterization of ZnSe byPhotoluminescence. S.M. thesis, Dept. ofElectr. Eng. and Comput. Sci., MIT, 1994.

7 SIMS performed in collaboration with Dr. Noble Johnson, Xerox Palo Alto Research Center, Palo Alto, California.


Figure 6. (a) (400) x-ray rocking curve obtained from a 1.1 pm ZnSe film grown on 4.3 pm (In,Ga)P. The residualstrain between the GaAs substrate and the (In,Ga)P buffer layer is Aa/a = 0.15 percent. (b) The (In,Ga)P (400) x-rayrocking curve obtained after the ZnSe had been selectively etched. The peak of the GaAs substrate feature has beenshifted to 0 arc seconds for clarity.

3.3 Novel Epitaxial III-V Buffer Layersfor Wide Bandgap II-VI Visible Sources

Sponsors

Advanced Research Projects AgencySubcontract 284-25041


National Science FoundationGrant DMR 92-02957

Project Staff

Professor Leslie A. Kolodziejski, Dr. Gale S.Petrich, Christopher A. Coronado, Philip A. Fisher,Easen Ho, Jody L. House, Kan Lu, Kuo-Yi Lim

The ZnSe-based material system, in which(Zn,Mg)(S,Se) barrier layers and (Zn,Cd)Se welllayers are used, offers the ability to fabricate a mul-titude of heterostructure devices capable of emitting

8 T. Okuyama, T. Miyajima, Y. Morinaga, F. Hiei, M. Ozawa, and1798-1799 (1992).

light throughout the entire visible region. A signif-icant device bottleneck, however, is the lack oflarge area, high quality bulk II-VI substrates. As aresult, the II-VI devices are grown on Ill-Vsubstrates which can potentially lead to the pres-ence of appreciable lattice-mismatch strain withinthe II-VI layer. The detrimental effects of lattice-mismatch constrain the present state-of-the-artblue-green semiconductor lasers to be designed tobe fully strained, such that the structures arepseudomorphic to bulk GaAs. 8 The use of (In,Ga)Pepitaxial buffer layers as an "alternate substrate"enables the (In,Ga)P lattice constant to be contin-uously varied from that of ZnS (GaP) to roughly thatof Zn.s5Cd. 5Se (InP) (see figure 1). By matchingthe lattice constant of the Ill-V "alternativesubstrate" to the II-VI structure, the strain is mini-mized in the II-VI device layers, and the misfit dislo-cations can be engineered to remain primarily nearthe (In,Ga)P/GaAs interface, as opposed to theZnSe/(In,Ga)P interface. The use of these novel

K. Akimoto, "ZnSe/ZnMgSSe Blue Laser Diode," Electron. Lett. 28:


500-1000 -500Arcseconds


buffer layers is anticipated to lead to an improve-ment in the device's performance, as well as toprovide greater flexibility in II-VI laser design toachieve shorter wavelengths of blue emission.Additional benefits are expected to be derived (1)from the chemical differences which exist betweenthe ZnSe and (In,Ga)P, as compared to GaAs, forfuture device processing, and (2) by utilizing variousband structure engineering techniques to provide agraded transition in the valence band discontinuitypresent between GaAs and ZnSe which is neces-sary for optimized electronic transport.

The Ill-V layers were grown using GSMBE in adedicated reactor. The (In,Ga)P buffer layers weregrown at 4500C (on a 0.5 pm homoepitaxial GaAsbuffer layer), where the substrate temperature wascalibrated using the melting point of InSb (5250C).Elemental effusions cells containing In and Ga, inaddition to PH 3, which is cracked at 9000C, wereused as sources. The (In,Ga)P surface exhibited asharp (2x1) surface reconstruction during growth,as observed by RHEED. During the initial minutesfollowing nucleation, RHEED intensity oscillationswere observed and were used to estimate the alloycomposition in real time. The precise alloy compo-sition was determined by (400) and (511) reflectionsof high resolution x-ray diffraction rocking curves.The thicknesses of the (In,Ga)P buffer layers were(1) estimated from the Group III fluxes, (2) obtainedfrom RHEED oscillations which indicate the growthrate, and (3) subsequently measured by aprofilometer following the selective removal of theZnSe and (In,Ga)P by chemical etching. At thecompletion of the (In,Ga)P buffer layer, the surfacewas coated in situ with amorphous As by loweringthe substrate temperature to below 1000C in anarsenic-rich environment (obtained by using acracked AsH 3 flux). The samples were then trans-ferred ex situ to the Il-VI GSMBE reactor. Theamorphous As was thermally desorbed from the(In,Ga)P buffer layers at approximately 2700C.During the nucleation of ZnSe on the (In,Ga)Pepitaxial surface, a streaky RHEED pattern wasobserved in most cases, and persisted throughoutthe entire growth of the ZnSe layer. However, abulk diffraction pattern was observed from the ZnSeduring the initial stages of growth, if the temper-ature of the (In,Ga)P buffer layer was increased totemperatures nearing 4000C during the Asdesorption process. Excessive phosphorus evapo-ration is speculated to have occurred at the higherAs desorption temperatures and contributed to thethree dimensional nucleation behavior of ZnSe on(In,Ga)P.

The ZnSe/(In,Ga)P/GaAs heterostructures wereexamined by high resolution (four and doublecrystal) x-ray diffraction. The (In,Ga)P compositionswere calculated using both the (400) and the (511)rocking curves and correlated with the RHEEDintensity oscillations. The information derived fromthe (511) rocking curves indicated that the (In,Ga)Pfilms having thicknesses of roughly 1 pm werepseudomorphic to the GaAs substrate, and thateven films having thicknesses greater than 4 pmwere only partially relaxed. Figure 6a shows the(400) x-ray rocking curve obtained from theZnSe/(In,Ga)P/GaAs heterostructure containing a4.3 pm Ino.52Ga. 48P buffer layer. Figure 6b showsthe identical structure following the removal of the1.1 pm ZnSe layer by selective etching. TheFWHM for each of the peaks from the Ill-V layersare 30 arc seconds for GaAs and 40 arc secondsfor the (In,Ga)P. The (511) rocking curves for thisstructure indicated that the (In,Ga)P was not fullyrelaxed, but still contained approximately 0.11percent lattice mismatch between the ZnSe and the(In,Ga)P. According to the (511) rocking curves,the composition of the (In,Ga)P film was such thatwhen fully relaxed, the in-plane lattice constantwould be the same as that of ZnSe. The width ofthe (400) ZnSe peak reflects the presence of dislo-cations at the ZnSe/Ill-V heterointerface due to thepresence of the lattice-mismatch which still remains.

Comparisons of the surface morphology usingoptical Nomarski microscopy and scanning electronmicroscopy (SEM), indicated that the ZnSe filmsgrown on the (In,Ga)P buffer layers (and on thebulk GaAs substrates) were featureless at SEMmagnifications as high as 12,000 times. Forepitaxial buffer layers of (In,Ga)As lattice-matchedto ZnSe, the Ill-V surface was reported to have sig-nificant cross hatching due to lattice relaxation.9

Similar surface features have been observed forInxGa-lP films which have been grown at In com-positions (x = 0.55), which when fully relaxed,would have a lattice constant greater than the in-plane lattice constant of ZnSe. For In compositionsof x = 0.55, the layers were only partially relaxed,but exhibited in-plane lattice constants lattice-matched to ZnSe; in this case, the (In,Ga)Psurface appeared cross-hatched. In addition, for agiven In composition, growth conditions using ahigher growth temperature (5000C) resulted in across-hatched surface.

The optical properties of the ZnSe films were meas-ured using PL spectroscopy. The 10K PL spectra

9 Private communication with R.L. Gunshor, Purdue University, LaFayette, Indiana.


of the ZnSe on the pseudomorphic (In,Ga)P bufferlayers, or on bulk GaAs substrates, were dominatedby the luminescence attributed to transitions associ-ated with donor-bound excitons. Defect-relatedluminescence originating from mid-bandgap deeplevels was not observed. However, the photolu-minescence from ZnSe grown on the partially-relaxed (In,Ga)P buffer layers contained featuresidentified as those due to both donor-bound exc-itons (2.795 eV) and free excitons (2.803 eV)having roughly the same intensity (as can be seenin figure 7). For the samples examined, which havea very high degree of purity, the Yo (2.603 eV) and

Iv (2.775 eV) transitions were also detected. Boththe Yo and Iv transitions have been reported to bedue to extended defects l o which suggested that theZnSe layer still contained defects due to latticerelaxation. The observation of the Yo and I, transi-tions served as additional evidence that these(In,Ga)P buffer layers were only partially relaxed,whereas the ZnSe was completely relaxed (andconfirmed by cross-sectional transmission electronmicroscopy). The transition occurring at 1.938 eVis attributed to the PL originating from the (In,Ga)Pbuffer layer.

Figure 7. 10 K photoluminescence spectrum of the 1.1 pm ZnSe film grown on a partially relaxed 4.3 pm (In,Ga)P film.The feature at 1.938 eV is attributed to the photoluminescence from the (In,Ga)P buffer layer. Features at 2.803, 2.795,2.776 and 2.603 are identified as transitions due to free excitons, donor-bound excitons, Ivo, and Yo, respectively. Theorigin of the feature at 2.717 is unknown and under investigation. (The feature at 1.907 eV is the second harmonic ofthe 3250 A line from the HeCd laser.)

1o J. Saraie, M. Matsumura, M. Tsubokura, K. Miyagawa, and N. Nakamura, "Y-line Emission and Lattice Relaxation in MBE-ZnSe and-ZnSSe on GaAs," Jpn. J. Appl. Phys. 28: L108-L111 (1989); K. Shahzad, J. Petruzzello, D.J. Olego, D.A. Cammack, and J.M.Gaines, "Correlation between Radiative Transitions and Structural Defects in Zinc Selenide Epitaxial Layers," Appl. Phys. Lett. 57:2452-2454 (1990); J.O. Williams, A. Wright, and H.M. Yates, "High Resolution and Conventional Transmission Electron Microcopy inthe Characterization of Thin Films and Interfaces Involving II-VI Materials," J. Cryst. Growth 117: 441-453 (1992).



Characterization by photoluminescence indicatesthat high quality ZnSe epilayers have been grownon (In,Ga)P buffer layers by GSMBE. However, thecritical thickness of the (In,Ga)P epilayers greatlyexceeded the value predicted by the Matthews andBlakeslee force balancing model (47 nm), 11 as wellas by a model which uses an assumption of energybalance (3.2 pm). 12 Various techniques that inducelattice relaxation are under investigation so that wecan control the selection of a specific in-planelattice constant for heteroepitaxy. Thus far,(In,Ga)P is a promising epitaxial substrate materialfor the realization of devices based on ZnSe andrelated II-VI compounds.

3.3.1 Publications

Lu, K., J.L. House, P.A. Fisher, C.A. Coronado, E.Ho, G.S. Petrich and L.A. Kolodziejski."GSMBE Growth of ZnSe on Novel BufferLayers." J. Vac. Sci. Tech. B (1994). Forth-coming.

Lu, K., P.A. Fisher, E. Ho, J.L. House, C.A.Coronado, G.S. Petrich, and L.A. Kolodziejski."GSMBE of ZnSe on (In,Ga)P." Spring Meetingof the Materials Research Society, SanFransisco, California, April 4-8, 1994.

Lu, K., P.A. Fisher, J.L. House, E. Ho, C.A.Coronado, G.S. Petrich, and L.A. Kolodziejski."(ln,Ga)P Buffer Layers for ZnSe-Based VisibleEmitters." J. Cryst. Growth (1994). Forth-coming.

Thesis

Lu, K. Lattice-Matched (In,Ga)P Buffer Layers forZnSe Based Visible Emitters. S.M. thesis, Dept.of Electr. Eng. and Comput. Sci., MIT, 1994.

3.4 Integrated Photonic Devices: TheChannel-Dropping Filter

SponsorNational Center for Integrated Photonic Technology

Contract 542-381

Project Staff

Professor Hermann A. Haus, Professor Leslie A.Kolodziejski, Professor Henry I. Smith, Dr. Gale S.Petrich, Jay N. Damask

The channel-dropping filter, first proposed by Pro-fessor Hermann Haus, 13 is a member of a family ofintegrated photonic wavelength-division multiplexers(WDM) and demultiplexers. Unlike other multi-plexers which terminate and resolve an entire bitstream within one device, the channel-droppingfilter can selectively add (or remove) only onewavelength channel, while the other remainingchannels are left undisturbed. The property ofsingle channel filtering offers a new degree offreedom to the WDM network architecture. Ourobjective is to fabricate the integrated resonantchannel-dropping filter using the (In,Ga)(As,P) com-pound semiconductor material system.

The (In,Ga)(As,P) semiconductor system meetsseveral of the material requirements for the fabri-cation and eventual implementation of the channel-dropping filter. The (In,Ga)(As,P) heterostructuresystem is compatible with optical communicationsystems operating at 1.55 pm and is lattice-matched to InP over a wide range of direct energybandgaps (Ag, - 0.87 pm for InP throughAg - 1.67 pm for Ino.53Gao.47As). The quaternarymaterial system also possesses sufficiently largedifferences in the index of refraction for variousalloy compositions, which is necessary to confinean optical mode, and the material has beenreported to exhibit waveguide losses of s 1 dB/cm.

Our current research has focused on the devicedesign, epitaxial growth, and fabrication of passivelow-loss waveguides. Vertical optical mode con-finement in the waveguides is achieved by control-ling the index of refraction, via the correct selectionof the energy bandgap for each epitaxial layer. Theenergy bandgap is determined by the mole fractionof the constituent alloy elements: In, Ga, As, and P.

11 J.W. Matthews and A.E. Blakeslee, "Defects in Epitaxial Multilayers: I. Misfit Dislocations," J. Crystal Growth 27: 118-125 (1974).

12 R. People and J.C. Bean, "Calculation of Critical Layer Thickness Versus Lattice Mismatch for Ge,Se, JSi Strained-layer Heterostruc-tures," Appl. Phys. Lett. 47: 322-324 (1985).

13 H.A. Haus and Y. Lai, "Narrow-Band Optical Channel-Dropping Filter," J. Lightwave Technol. 10: 57-62 (1992).


The Ill-V-dedicated gas source molecular beamepitaxy reactor uses elemental sources for In andGa, and gaseous hydride sources for As and P.The ratio of Group III and Group V elements deter-mines the energy bandgap and lattice constant fora particular quaternary alloy.

Control of the alloy composition for In_-xGaxAsyP,_yis difficult due to the vastly different sticking coeffi-cients of the Group III and Group V species. For anarrow range of growth temperature, the stickingcoefficients for the indium and gallium atoms arenear unity, whereas the sticking coefficients for thearsenic dimers and phosphorus dimers are lessthan unity, varying strongly with substrate temper-ature. The In/Ga flux ratio, and hence composition,is selected by the appropriate choice of the two ele-mental source temperatures. However, the As/Pcomposition ratio cannot be similarly established. Incompetition for surface sites, As will displace P by a50 to 1 ratio. Therefore, the epitaxial surface musthave an arsenic surface coverage depleted of As,so as to allow for P incorporation. Thus, theAs/(In+Ga) ratio is controlled, and phosphorus isthe remaining constituent species which is in abun-dance and primarily sets the Group V to Group IIIoverpressure.

The control algorithm for the alloy composition isinitiated by establishing the In/Ga ratio which willdetermine the bandgap of the alloy. By modifyingthe As content, the In_-xGaxAsyP 1_y alloy is lattice-matched to the InP substrate. This control algo-rithm allows for variation of the energy bandgap,while maintaining the lattice match to InP.

Figure 8a shows an (In,Ga)(As,P)/InP rib wave-guide design that has been optimized for bothmaximum coupling to a Bragg grating (not shown)and for minimum fabrication-induced waveguideloss. (The Bragg grating will be etched on the topof the rib.) The 200 A quaternary layer provides anetch stop for the fabrication process, to enable theheight of the waveguide rib to be well defined andrepeatable from sample to sample. Figure 8b is aSEM micrograph of a waveguide facet that wasstained to enhance the contrast between epitaxiallayers. In a parallel effort, the masks necessary tocharacterize the waveguides have been designedand manufactured with emphasis on the study ofwaveguide loss, as measured by cutback andFabry-Perot techniques, waveguide-waveguide cou-pling, and facet reflectivity of waveguides angledwith respect to the cleaved end.

Thesis

Damask, J.N. A New Photonic Device: The Inte-grated Resonant Channel-Dropping Filter. S.M.thesis, Dept. of Electr. Eng. and Comput. Sci.,MIT, 1993.

3.5 Heterovalent Interfaces Composedof II-VI/III-V Heterostructures

SponsorNational Science Foundation

Contract DMR 92-02957Grant DMR 90-22933

Project StaffProfessor Leslie A. Kolodziejski,Petrich, Jody L. House, Kuo-Yi Lim

Dr. Gale S.

Epitaxial growth of the wide bandgap semicon-ductor ZnSe onto GaAs buffer layers is complicatedby the disparity between the electronic configurationwhich exists during the formation of theheterointerface. Due to the change in valencyoccurring from one monolayer to the next as theII-VI/Ill-V interface is formed, interface states aregenerated at the heterointerface. The interfacestates affect the efficiency of the carriers beinginjected from the GaAs to the ZnSe (for example ina pn diode configuration). One method for investi-gating the nature of the interface states in theZnSe-on-GaAs "noninverted" heterostructure isthrough the examination of the electrical character-istics of depletion-mode MISFET devices, whereZnSe is the insulator for the GaAs channel. Modifi-cation of the density of interface states will bestudied by engineering the stoichiometry of eachlayer during the interface formation. The fabricationof the ZnSe/GaAs MISFET devices involves thedevelopment of the wet etch and ohmicmetallization techniques, in addition to the creationof an appropriately designed device mask set. Themasks have been designed so that the gate lengthsrange from 2 to 50 microns. Currently, the growthparameters and the processing steps for theZnSe/GaAs heterostructure are under development.

The fabrication of the inverted interface, i.e.,GaAs-on-ZnSe, for the fabrication of multilayeredstructures composed of ZnSe wide bandgap bar-riers and GaAs quantum well layers, represents achallenge in materials science. The growthapproach must address issues regarding materialswhich are fabricated at vastly different growth tem-peratures (the optimum growth temperatures differby almost 300 0C). The first effort toward fabricationof the inverted structure has been to examine the



a)

b)

Oxide

Etch Stop

5000 A

200A3000 A

2000 AT

Top Clad

Bottom Clad

Core

Lower Buffer

I I

Figure 8. A cross sectional view of the (In,Ga)(As,P)/InP rib waveguide. (a) The designed structure showing the indi-vidual layers and thicknesses. The 200 A quaternary layer provides an etch stop for the waveguide. (b) A SEM micro-graph of the InP/(In,Ga)(As,P)/InP waveguide structure.

r _


effects of very low growth temperatures on theGaAs layers, as characterized by high resolutionx-ray diffraction and photoluminescence. Only atthe very lowest growth temperature (- 2700C) isexcess As incorporation observed. In addition,ZnSe layers (grown on GaAs) have been subjectedto various temperature extremes, as would be likelyto occur during the growth of the GaAs well layer.As determined by photoluminescence, the opticalproperties of the ZnSe layers appear to remainunaffected by rapid thermal anneals, for times typi-cally encountered during growth of the well mate-rial. With these two encouraging results,experiments are underway to begin the growth ofthe inverted GaAs-on-ZnSe interface.

3.6 Optoelectronic Very Large ScaleIntegrated Circuits

Sponsor


Project Staff

Professor Clifton G. Fonstad,A. Kolodziejski, Dr. Gale S.Ahadian, Kan Lu

Jr., Professor LesliePetrich, Joseph F.

The project described herein has as an objectivethe development of Al-free compound semicon-ductor lasers for commercially viable OptoElectronicVLSI circuit (OE-VLSI) technology. The laserswhich are being fabricated will eventually beregrown onto commerially fabricated GaAsMESFET VLSI circuits. These fully metallized VLSIcircuits have previously been found to withstandtemperatures of up to 500 0C for several hourswithout degradation. Conventionally-grown(AI,Ga)As-based lasers require growth temperaturesgreater than 600 0C to achieve high quality material,and therefore will not allow for regrowth onto thecompleted VLSI circuitry. In contrast, however, the(In,Ga)(As,P) heterostructure system is normally

grown at substrate temperatures near or below5000C. At present, multiple quantum well(In,Ga)As/GaAs/(In,Ga)P laser structures have beendesigned. High quality (In,Ga)P epilayers on GaAssubstrates have been deposited, and both n- andp-type doping studies are underway to calibrate thedopant sources and growth parameters.

3.7 Heteroepitaxy of GaAs ontoCorrugated Surfaces of Si

Project Staff

Professor Leslie A. Kolodziejski, Professor Henry I.Smith, Professor Carl V. Thompson, Dr. Gale S.Petrich, Sean M. Donovan, Kan Lu

GaAs-based devices provide unique optoelectroniccapabilities which are not available with Si-baseddevices. However, the processing technology forGaAs is not as mature and well characterized asthat for Si VLSI processing technology. A naturalstep would be to combine the two device systemsvia monolithic integration, which requires the heter-oepitaxial growth of GaAs onto Si. A significantobstacle, however, occurs due to the 4.1 percentlattice mismatch, and the presence of large differ-ences in thermal expansion coefficients betweenthe two materials giving rise to unacceptably highdislocation densities in the GaAs overlayer. Inearlier work by Professor Smith and coworkers, 14 itwas observed that by etching a silicon substrate tocontain a saw-tooth-like pattern that the threadingdislocation density in the surface regions of aheteroepitaxially-grown GaAs film was greatlyreduced. A detailed investigation is underway inorder to understand the mechanism responsible forthe reduction in the dislocation density, which willresult in the successful integration of GaAs and Sifunctionalities. The study will also aid in the under-standing of polar on non-polar epitaxy usingGaAs-on-Si as the test structure. Our work thus farhas primarily emphasized refinement of the processfor producing the saw-tooth patterned Si substratesand preparing the surface for GaAs epitaxy.

Karam, J. Carter, and H.I. Smith, "High-quality GaAs on Sawtooth-Patterned Si Substrates," Appl. Phys.


14 K. Ismail, F. Legoues, N.H.Lett. 59: 2418-2420 (1991).

Documents

Chapter 3. Molecular Beam Epitaxy of Compound Semiconductors · Chapter 3. Molecular Beam Epitaxy of Compound Semiconductors 0 MgS 4 Aat Q7nS o MgSe 3 U, 0GaP ZnTe 2 SGaAs.t S 1 Si