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Chapter 3. Chemical Beam Epitaxy of Compound Semiconductors

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Chapter 3. Chemical Beam Epitaxy of Compound Semiconductors Chapter 3. Chemical Beam Epitaxy of Compound Semiconductors Chapter 3. Chemical Beam Epitaxy of Compound Semiconductors Academic and Research Staff Professor Leslie A. Kolodziejski, Dr. Gale S. Petrich Graduate Students Christopher A. Coronado, Jay N. Damask, Philip A. Fisher, Easen Ho, Jody L. House, Kan Lu Undergraduate Students Kuo-yi Lim, Sang H. Park, Ayca Yuksel Technical and Support Staff Karen Chenausky, Charmaine A. Cudjoe-Flanders, Kelley S. Donovan, David S. Lee, Angela R. Odoardi 3.1 Introduction degree of freedom in device design by providing the ability to integrate the II-VI and Ill-V material fami- Current state-of-the-art epitaxial growth techniques lies in a single device. The Ill-V GSMBE uses employ various metalorganic solid and hydride gases, elemental sources of Ga, In, Al, Si and Be and particularly for high vapor pressure species such as gaseous hydride sources of arsenic and phosphorus and sulfur, to deliver constituent phosphorus. The II-VI reactor, on the other species to the substrate hand, surface. Chemical beam is highly flexible, offering MO gas sources, hydride epitaxy (CBE) utilizes both metalorganic (MO) gas sources, and solid effusion cell type sources. gases and hydride gas sources; metalorganic Various constituent species available in the Il-VI molecular beam epitaxy (MOMBE) uses MO gases; reactor include Zn, S, Se, Te, Cl, N, In, Ga, and As. and gas source molecular beam epitaxy (GSMBE) Figure 1 highlights the many material combines hydride gas systems, sources with the more typical based on II-VI heterostructures, Ill-V heterostruc- solid elemental sources. The more conventional tures, or on a combination of II-VI and III-V semi- growth approach, molecular beam epitaxy (MBE), conductors, which are available for exploration. uses only molecular beams derived from the Many of these are currently being fabricated in the thermal evaporation of elemental or compound solid epitaxy facility. sources. All the research objectives described in this chapter are concerned with layered structures composed of 4 compounds containing As and P, or Se, S, and Te. ZnS The presence of these high vapor pressure species 3.5 suggests that many advantages will be gained 3 through fabrication of the device structures by the gaseous source epitaxy approach. 2.5 AIP ZnTe ' 2 In the chemical beam epitaxy facility at MIT, epi- taxial growth of both Il-VI and Ill-V compound semi- 1.5 GaAs ASb conductors is underway using all of the . 1 sSi 3 aforementioned growth techniques. The chemical Ge 0 GaSb beam epitaxy laboratory consists of two intercon- InAs nected, fully operational gaseous source epitaxy 0 systems along with several smaller chambers used 5.3 5.5 5.7 5.9 6.1 6.3 for sample introduction and in-situ "Lattice Constants" (A) analysis/metalization. The multichamber epitaxy system allows various heterostructures to be grown Figure 1. The shaded area highlights the many different in a continuous ultrahigh vacuum environment. The Il-VI and III-V semiconductors and the various hetero- interconnection feature enables an additional structure configurations which are available for investi- gation by fabrication in the MIT chemical beam epitaxy laboratory. Chapter 3. Chemical Beam Epitaxy of Compound Semiconductors Wide bandgap II-VI materials have important tech- which was occuring that limits the growth when nological potential, for example, as short wave- using particular MO sources. The II-VI effort is length (visible to ultraviolet) optical sources with complemented by a new research program with a applications in optical recording and bright emissive research goal of fabricating lattice-matched epitaxial displays. A recent key advance in the area of Zn buffer layers of (In,Ga)P (section 4.3). The recently chalcogenides involves successful p-type doping of functioning Ill-V GSMBE is also being utilized to ZnSe-based semiconductors using nitrogen as begin the investigation of very sophisticated acceptor species (first reported by Park et al. 1). (In,Ga)(As,P) waveguide devices for application as This led to the achievement of a pn diode injection tunable filters operating at 1.55 m which is the laser operating at blue/blue-green wavelengths by wavelength of interest for optical fiber communi- Haase et al.2 and Jeon et al.3 These structures cation (section 4.4). consisted of (Zn,Cd)Se narrow bandgap well layers with Zn(Se,S) cladding barrier layers, although more recent structures are composed of barrier 3.2 Photo-Assisted Metalorganic 4 layers of (Zn,Mg)(Se,S). To minimize defect gen- Molecular Beam Epitaxy of ZnSe Using eration within the active regions of the device, as well as to maximize the incorporation of the a Combination of Solid and Gaseous nitrogen acceptor species, the substrate temper- Sources atures have been reported to be extremely low (-150-250'C). By employing gas source epitaxy Sponsors technologies for the growth of the aforementioned 3M Company Faculty Development Grant quantum well structures, control of the constituent Defense Advanced Research Projects Agency species via precision mass flow controllers is antic- Subcontracts 216-25013 and 542383 ipated to offer a solution to the difficulties encount- Joint Services Electronics Program ered in the MBE of materials containing one or Contract DAAL03-92-C-0001 more high vapor pressure species. In addition, as National Science Foundation the growth temperature is lowered, control of the Grant ECS 88-46919 flux ratio will become even more crucial. Grant ECS 89-05909 The use of a beam-assisted growth approach in The use of photon illumination during the epitaxial conjunction with metalorganic molecular beam growth of compound semiconductors enhances the epitaxy provides us with the capability of lateral pat- capabilities of conventional growth techniques by terning as well as composition modulation in the modifying the surface chemistry and thus providing growth direction, which cannot be achieved by con- a potential tool necessary for in-situ selective area ventional MBE. To address some of these epitaxy. Photo-assisted epitaxy has been reported materials-related issues, we have embarked on a for the growth of both narrow and wide bandgap II-VI-based research program which emphasizes II-VI materials, as well as for a variety of Ill-V semi- growth by the gas source epitaxy approach of conductors. The effect of photons on the growth is various heterostructures which can be used for dependent on the precursor selected for constituent visible light emitters. materials (and/or the type of growth technology In section 4.2 we describe the progress that we employed), as well as the energy and flux of illumi- have made in understanding the photo-assisted nating photons. MOMBE growth of ZnSe using a combination of The various physical mechanisms which have been solid and gaseous MO sources with various ener- either reported or verified experimentally to explain gies of impinging photons. The laser-assisted the effect of photon illumination include: (1) selec- growth technique was instrumental in aiding our tive desorption of adsorbed surface species, (2) understanding of the complex surface chemistry pyrolysis of source precursors at the surface via 1 R.M. Park, M.B. Troffer, C.M. Rouleau, J.M. Depuydt, and M.A. Hasse, "P-Type ZnSe by Nitrogen Atom Beam Doping during Molec- ular Beam Epitaxial Growth," Appl. Phys. Lett. 57: 2127-2129 (1! 2 M.A. Hasse, J. Qui, J.M. Depuydt, and H. Cheng, "Blue-Green Laser Diodes," Appl. Phys. Lett. 59: 1272-1274 (1991). 3 H. Jeon, J. Ding, W. Patterson, A.V. Nurmikko, W. Xie, D.C. Grillo, M. Kobayashi, and R.L. Gunshor, "Blue-green Injection Laser Diodes in (Zn,Cd)Se/ZnSe Quantum Wells," Appl. Phys. Lett. 59: 3619-3621 (1991). 4 H. Okuyama, T. Miyajima, Y. Morinaga, F. Hiei, M. Ozawa, and K. Akimoto, "ZnSe/ZnMgSSe Blue Laser Diode," Electron. Lett. 28: 1798-1799 (1992). 26 RLE Progress Report Number 135 Chapter 3. Chemical Beam Epitaxy of Compound Semiconductors increases in the substrate temperature, (3) direct addition of hydrogen supplied by a gas injector, and photolysis of source precursors which are present the thermal pyrolysis of each gaseous species in the gas phase, and (4) photo-catalysis of molec- using a "cracking" gas injector. The growth ular species through the creation of free carriers parameter-independent low growth rate suggested present at the surface. Similar effects on the epi- that the growth rate was limited not by mass trans- taxial growth process have been reported when the port to the substrate surface, but rather that the Zn surface is illuminated with an electron beam. In the and Se atoms were unable to incorporate into the case of an electron beam, the physical mechanisms crystalline lattice. However, we found that illumi- which have been postulated include kinetically- nation with photons having energy greater than the induced decomposition of adsorbed precursor mole- energy bandgap of the ZnSe, was found to signif- cules and/or the presence of charge carriers which icantly increase the growth rate under certain condi- interact with surface species. tions. To verify that the resultant low growth rate and high illuminated growth rate was unique to the In this program, the effect of "beam-assisted" use of diethyl-based metalorganic sources, ZnSe growth during the metalorganic molecular beam was grown with all combinations of DEZn, DMZn, epitaxy (MOMBE) of ZnSe is under investigation. DESe, and elemental Zn and Se. Growth rates By utilizing the wavelength-dependent laser- obtained using atomic Zn or DMZn and elemental assisted growth technique and a combination of Se were typical of that achievable with molecular both gaseous metalorganic and solid elemental beam epitaxy and limited only by mass transport to sources during the growth, we were able to identify the substrate surface; however, when at least one the dominant surface reactions which result in sig- diethyl metalorganic was used, the resultant unillu- nificant enhancements in the growth rate, as well as minated growth rate was very low.
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