The Materials Science of Thin Films 2011 Part 8 pps

w N Q) 7-15. of P. K. Tien, AT&T Bell Laboratories.) Energy gaps and corresponding lattice constants for various compound semiconductors. (Courtesy m m z. X Y 7.4. Epitaxy of Compound Semiconductors 329 Figure 7-1 6. Electron microscope lattice image of GaAs-AAs heterojunction taken with [lo01 illumination. (From Ref. 13). (Courtesy of JOEL USA, Inc.) Therefore, the correct value for EJO.4) = 2.00 eV. In addition, the index of refraction n, required for light-guiding properties, varies as (Ref. 15) .(x) = 3.590 - 0.710~ + 0.091~~. (7-12) In summary, it is possible to design ternary alloys with Eg larger than GaAs, with n smaller than GaAs, while maintaining an acceptable lattice match for high-quality heterojunctions. This unique combination of properties has led to the development of a family of injection lasers, light-emitting diodes, and photodetectors based on the GaAs- AlAs system. 7.4.3. Additional Applications 7.4.3.1. Optical Communications. Optical communication systems are used to transmit information optically. This is done by converting the initial electronic signals into light pulses using laser or light-emitting diode light sources. The light is launched at one end of an optical fiber that may extend over long distances (e.g., 40 km). At the other end of the system, the light pulses are detected by photodiodes or phototransistors and converted back into electronic signals that, in telephone applications, finally generate sound. In such a system it is crucial to transmit the light with minimum attenuation or low optical loss. Great efforts have been made to use the lowest-loss fiber possible and minimize loss at the source and detector ends. If optical losses are high, it means that the optical signals must be reamplified and that additional, 330 Epitaxy costly repeater stations will be necessary. The magnitude of the problem can be appreciated when transoceanic communications systems are involved. In silica-based fibers it has been found that minimum transmission losses occur with light of approximately 1.3-1.5 pm wavelength. The necessity to operate within this infrared wavelength window bears directly on the choice of suitable semiconductors and epitaxial deposition technology required to fabricate the required sources and detectors. Reference to Table 7-1 shows that InP is transparent to 1.3-pm light, and this simplifies the coupling of fibers to devices. A very close lattice match to InP (a, = 5.869 A) can be effected by alloying GaAs and InAs. Through the use of Vegard’s law, it is easily shown that the necessary composition is Ga,,,,In,,,, As. In the same vein, high-performance lasers based on the lattice-matched GaInAsP-InP system have recently emerged for optical com- munications use. 7.4.3.2. Silicon Heteroepitaxy (Ref. 8). Since the early 1960s, Si has been the semiconductor of choice. Its dominance cannot, however, be attributed solely to its electronic properties for it has mediocre carrier mobilities and only average breakdown voltage and carrier saturation velocities. The absence of a direct band gap rules out light emission and severely limits its efficiency as a photodetector. Silicon does, however, possess excellent mechanical and chemi- cal properties. The high modulus of elasticity and high hardness enable Si wafers to withstand the rigors of handling and device processing. Its great natural abundance, the ability to readily purify it and the fact that it possesses a highly inert and passivating oxide have all helped to secure the dominant role for Si in solid-state technology. Nevertheless, Si is being increasingly sup- planted in high-speed and optical applications by compound semiconductors. The idea of combining semiconductors that can be epitaxially grown on low-cost Si wafers is very attractive. Monolithic integration of III-V devices with Si-integrated circuits offers the advantages of higher-speed signal process- ing distributed over larger substrate areas. Furthermore, Si wafers are more robust and dissipate heat more rapidly than GaAs wafers. Unfortunately, there are severe crystallographic, as well as chemical compatibility problems that limit Si-based heteroepitaxy. From data in Table 7-1, it is evident that Si is only closely lattice matched to GaP and ZnS. Furthermore, its small lattice constant limits the possible epitaxial matching to semiconductor alloys. Never- theless, high-quality, lattice-mismatched (strained-layer) heterostructures of AlGaAs-Si and Ge,Si,-,-Si have been prepared and show much promise for new device applications. 7.5. Methods for Depositing Epitaxial Semiconductor Films 331 7.4.3.3. Epitaxy in Il-VI Compounds (Ref. 16). Semiconductors based on elements from the second (e.g., Cd, Zn, Hg) and sixth (e.g., S, Se, Te) columns of the periodic table display a rich array of potentially exploitable properties. They have direct energy band gaps ranging from a fraction of an electron volt in Hg compounds to over 3.5 eV in ZnS, and low-temperature carrier mobilities approaching lo6 cm2 /V-sec are available. Interest in the wide-gap 11-VI compounds has been stimulated by the need for electronically addressable flat-panel display devices and for the development of LED and injection lasers operating in the blue portion of the visible spectrum. For these purposes, ZnSe and ZnS have long been the favored candidates. When the group I1 element is substituted by a magnetic transition ion such as Mn, new classes of materials known as diluted magnetic or semimagnetic semiconduc- tors result. Examples are Cd(Mn)Te or Zn(Mn)Se, and these largely retain the semiconducting properties of the pure compound. But the five electrons in the unfilled 3d shell of Mn give rise to localized magnetic moments. As a result, large magneto-optical effects (e.g., Zeeman splitting in magnetic fields, Fara- day rotation, etc.) occur and have been exploited in optical isolator devices. For this, as well as other potential applications in integrated optics, high-qual- ity epitaxial films are essential. 7.5. METHODS FOR DEPOSITING EPITAXIAL SEMICONDUCTOR FILMS 7.5.1. Liquid Phase Epitaxy In this section an account of the processes used to deposit epitaxial semicon- ductor films is given. We start with LPE, a process in which melts rather than vapors are in contact with the growing films. Introduced in the early 1960s, LPE is still used to produce heterojunction devices. However, for greater layer uniformity and atomic abruptness, it has been supplanted by CVD and MBE techniques. LPE involves the precipitation of a crystalline film from a super- saturated melt onto the parent substrate, which serves as both the template for epitaxy and the physical support for the heterostructure. The process can be understood by referring to the GaAs binary-phase diagram on p. 31. Consider a Ga-rich melt containing 10 at% As. When heated above 95OoC, all of the As dissolves. If the melt is cooled below the liquidus temperature into the two-phase field, it becomes supersaturated with respect to As. Only a melt of lower than the original As content can now be in equilibrium with GaAs. The excess As is, therefore, rejected from solution in the form of GaAs that grows epitaxially on a suitably placed substrate. Many readers will appreciate that the 332 Epitaxy crystals they grew as children from supersaturated aqueous solutions essen- tially formed by this mechanism. Through control of the cooling rates, different kinetics of layer growth apply. For example, the melt temperature can either be lowered continuously together with the substrate (equilibrium cooling) or separately reduced some 5-20 "C and then brought into contact with the substrate at the lower temperature (step cooling). Theory backed by experiment has demonstrated that the epitaxial layer thickness increases with time as t3/2 for equilibrium cooling and as t1/2 for step cooling (Ref. 10). Correspondingly, the growth rates or time derivatives vary as t1l2 and t-'/*, respectively. These diffusion- controlled kinetics respectively indicate either an increasing or decreasing film growth rate with time depending on mechanism. Typical growth rates range from - 0.1 to 1 pm/min. A detailed analysis of LPE is extremely compli- cated in ternary systems because it requires knowledge of the thermodynamic equilibria between solid and solutions, nucleation and interface attachment FUSED -SILICA FURNACE TUBE ROWTH SEED RELATIVE POSITION TI ME Figure 7-17. Schematic of LPE reactor. (Courtesy of M. B. Panish, AT&T Bell Laboratories.) 7.5. Methods for Depositing Epitaxial Semiconductor Films 333 kinetics, solute partitioning, diffusion, and heat transfer. LPE offers several advantages over other epitaxial deposition methods, including low-cost appara- tus capable of yielding films of controlled composition and thickness, with lower dislocation densities than the parent substrates. To grow multiple GaAs- AlGaAs heterostructures, one translates the seed substrate sequentially past a series of crucibles holding melts containing various amounts of Ga and As together with such dopants as Zn, Ge, Sn, and Se as shown in Fig. 7-17. Each film grown requires a separate melt. Growth is typically carried out at temperatures of - 800 "C with maximum cooling rates of a few degrees Celsius per minute. Limitations of LPE growth include poor thickness uniformity and rough surface morphology particularly in thin layers. The CVD and MBE techniques are distinctly superior to LPE in these regards. 7.5.2. Seeded Lateral Epitaxial Film Growth over Insulators The methods we describe here briefly have been successfully implemented in Si but not in GaAs or other compound semiconductors. The use of melts suggests the inclusion of this subject at this point. Technological needs for three-dimensional VLSI and isolation of high-voltage devices have spurred the development of techniques to grow epitaxial Si layers over such insulators as SiO, or sapphire. In the recently proposed LEG0 (lateral epitaxial growth over oxide) process (Ref. 17), the intent is to form isolated tubs of high-quality Si surrounded on all sides by a moat of SiO,. Devices fabricated within the tubs require the electrical insulation provided by the SO,. As a result they are also radiation-hardened or immune from radiation-induced charge effects originating in the underlying bulk substrate. The process shown schematically in Fig. 7-18 starts with patterning and masking a Si wafer to define the tub regions followed by etching of deep-slanted wall troughs. A thick SiO, film is grown and seed windows are opened down to the substrate by etching away the SiO, . Then a thick polycrystalline Si layer (- 100 pm thick) is deposited by CVD methods. This surface layer is melted by the unidirectional radiant heat flux from incoherent light emitted by tungsten halogen arc lamps (lamp furnace). The underlying wafer protected by the thermally insulating SiO, film does not melt except in the seed windows. Crystalline Si nucleates at each seed, grows vertically, and then laterally across the SO,, leaving a single- crystal layer in its wake upon solidification. Lastly, mechanical grinding and lapping of the solidified layer prepares the structure for further microdevice processing. Conventional dielectric isolation processing also employs a thick CVD Si layer. But the latter merely serves as the mechanical handle enabling the bulk of the Si wafer to be ground away. 334 Epitaxy V-GROOVE FORMATION OXIDATION TUBS DEFINED BY KOH ETCHING ISOLATION OXIDE AND SEEDING WINDOWS FORMED POLY-Si & Si0 CAP DEPOSITED POLY-SI MELTED & RECRYSTALLIZED 1 SURFACE POLISHED Figure 7-1 8. Schematics of methods employed to isolate single-crystal Si tubs. (left) conventional dielectric isolation process; (right) LEG0 process. (Courtesy of G. K. Celler, AT&T Bell Laboratories.) An alternative process for broad-area lateral epitaxial growth over SiO, employs a strip heat source in the form of a hot graphite or tungsten wire, scanned laser, or electron beam. After patterning the exposed polycrystalline or amorphous Si above the surrounding oxide, the strip sweeps laterally across the wafer surface. Local zones of the surface then successively melt and recrystallize to yield, under ideal conditions, one large epitaxial Si film layer. Analogous processes involving seeded lateral growth and selective deposition from the vapor phase also show much promise. 7.5.3. Vapor Phase Epitaxy (VPE) An account of the most widely used VPE methods-chloride, hydride, and organometallic CVD processes-has been given in Chapter 4. Here we briefly address a couple of novel VPE concepts that have emerged in recent years. The first is known as vapor levitation epitaxy (VLE), and the geometry is shown in Fig. 7-19. The heated substrate is levitated above a nitrogen track close to a porous frit through which the hot gaseous reactants pass. Upon impingement on the substrate, chemical reactions and film deposition occur while product gases escape into the effluent stream. As a function of radial distance from the center of the circular substrate, the gas velocity increases 7.5. Methods for Deposlting Epltaxial Semiconductor Films 335 VLE GEOMETRY EFFLUENT /STY NITROGEN GROWTH CHAMBER Figure 7-19. (Top) Schematic of VLE process; (bottom) schematic of RTCVD process. (Courtesy of M. L. Green, AT&T Bell Laboratories.) while the gas concentration profile exhibits depletion. These effects cancel one another, and uniform films are deposited. The VLE process was designed for the growth of epitaxial III-V semiconductor films and has certain advantages worth noting: 1. There is no physical contact between substrate and reactor. 2. Thin layer growth is possible. 3. Sharp transitions can be produced between film layers of multilayer stacks. 4. Commercial scale-up appears to be feasible. 336 Epitaxy The second method, known as rapid thermal CVD processing (RTCVD), is an elaboration on conventional VPE. Epitaxial deposition is influenced through rapid, controlled variations of substrate temperature. Source gases (e.g., halides, hydrides, metalorganics) react on low-thermal-mass substrates heated by the radiation from external high-intensity lamps (Fig. 7-19). The latter enable rapid temperature excursions, and heating rates of hundreds of degrees Celsius per second are possible. For III-V semiconductors, high-quality epitax- ial films have been deposited by first desorbing substrate impurities at elevated temperatures followed by immediate lower temperature growth (Ref. 18). Very high quality lattice-matched heteroepitaxial films can be grown by CVD methods. This is particularly true of OMVPE techniques where atomi- cally abrupt heterojunction interfaces have been demonstrated in alternating AlAs-GaAs (superlattice) structures. Only molecular-beam epitaxy, which is considered next, can match or exceed these capabilities. 7.5.4. Molecular-Beam Epitaxy (Refs. 19 - 21) Molecular-beam epitaxy is conceptually a rather simple single-crystal film growth technique that, however, represents the state-of-the-art attainable in deposition processing from the vapor phase. It essentially involves highly controlled evaporation in an ultrahigh-vacuum ( - lo-'' torr) system. Interac- tion of one or more evaporated beams of atoms or molecules with the single-crystal substrate yields the desired epitaxial film. The clean environment coupled with the slow growth rate and independent control of the beam sources enable the precise fabrication of semiconductor heterostructures at an atomic level. Deposition of thin layers from a fraction of a micron thick down to a single monolayer is possible. In general, MBE growth rates are quite low, and for GaAs materials a value of 1 pm/h is typical. A modem MBE system is displayed in the photograph of Fig. 7-20. Representing the ultimate in film deposition control, cleanliness and real-time structural and chemical characterization capability, such systems typically cost more than $1 million. The heart of a deposition facility is shown schematically in Fig. 7-21a. Arrayed around the substrate are semiconductor and dopant sources, which usually consist of so-called effusion cells or electron-beam guns. The latter are employed for the high-melting Si and Ge materials. On the other hand, effusion cells consisting of an isothermal cavity with a hole through which the evaporant exits are used for compound semiconductor elements and their dopants. Effusion cells behave like small-area sources and exhibit a cos 4 emission. Vapor pressures of important compound semiconduc- tor species are displayed in Fig. 3-2. 7.5. Methods for Depositing Epitaxial Semiconductor Films 337 Figure 7-20. Photograph of multichamber MBE system. (Courtesy of Riber Divi- sion, Inc. Instruments SA). Consider now a substrate positioned a distance I from a source aperture of area A, with q5 = 0. An expression for the number of evaporant species striking the substrate is . 3.51 x 1022PA R= ?rI2 (MT) 1'2 As an example, consider a Ga source molecules/cm2-sec. (7-13) in a system where A = 5 cm2 and I = 12 cm. At T = 900 "C the vapor pressure PGa = 1 x torr, and substituting MGa = 70, the arrival rate of Ga at the substrate is calculated to be 1.35 x 1014 atoms/cm2-sec. The As arrival rate is usually much higher, and, therefore, film deposition is controlled by the Ga flux. An average monolayer of GaAs is 2.83 i thick and contains - 6.3 x 1014Ga atoms/cm*. Hence, the growth rate is calculated to be (1.35 x 1014)/(6.3 x 1014) x 2.83 x 60 = 36 i/min, a rather low rate when compared with VPE. One of the recent advances in MBE technology incorporates a gas source to supply As and P, as shown in Fig. 7-21b. Organometallics used for this purpose are thermally cracked, releasing the group V element as a molecular beam into the system. Excellent epitaxial film quality has been obtained by this [...]... context of the following subjects: 8. 2 Fundamentals of Diffusion 8. 3 Interdiffusion in Metal Alloy Films 8. 4 Electromigration in Thin Films 8. 5 Metal-Semiconductor Reactions 8. 6 Silicides and Diffusion Barriers 8. 7 Diffusion During Film Growth Before proceeding, the reader may find the survey of diffusion phenomena given in Chapter 1 useful and wish to review it 8. 2 FUNDAMENTALS OF DIFFUSION 8. 2.1 Comparative... propagating in the direction of the incident radiation and terminating at the origin of the reciprocal lattice Following Ewald, we draw of sphere of radius 2 n /X about the center A property of this construction is that the only possible directions of the diffracted rays are those that intersect the reflecting sphere at reciprocal lattice points as shown To prove this, we note that the normal to the reflecting... case the new layer does not grow until the prior one is atomically complete One can also imagine the simultaneous coupled growth of both the new and underlying layers In this section we explore the interactions of molecular beams with the surface and the steps leading to the incorporation of atoms into the growing epitaxial film Although MBE is the focus, the results are, of course, applicable to other... Grain-Boundary Diffusion Of all the mass-transport mechanisms in films, grain-boundary (GB) diffusion has probably received the greatest attention This is a consequence of the rather small grain size and high density of boundaries in deposited films Rapid diffusion within individual GBs coupled with their great profusion make them the pathways through which the major amount of mass is transported Low... pulse of incident Ga atoms, the detected desorption flux closely follows the dependence of Eq 7-16 Similarly, when the Ga beam is abruptly shut off, the desorption rate decays as exp - t/T,(Ga) The exponential rise and decay of the signal is shown schematically in Fig 7-23a In the case of As, molecules incident on a GaAs surface, the lifetime is extremely short (7,(As2) = 0), so the desorption pulse profile... the wavelength is relatively large, yielding a small Ewald sphere A sharp spot diffraction pattern is the result The intense hexagonal spot array of Fig 7-26a reflects the sixfold symmetry of the (111) plane, and the six fainter spots in between are the result of the (7 x 7) surface reconstruction In RHEED, on the other hand, the high electron energies lead to a very large Ewald sphere (Fig 7-27) The. .. out of the Bragg condition The resulting rocking curve diffraction pattern contains the very intense substrate peak that serves as the internal standard against which the position of the low-intensity epitaxial film peak is measured The following example (Ref 26) involving ZnSe, a potential blue laser material, illustrates the power and importance of the technique A rocking curve of an 1100-A film of. .. of the (7 x 7) structure of the Si(ll1) surface are shown in Fig 7-26 To obtain some feel for the nature of these diffraction patterns, we think in terms of reciprocal space Arrays of reciprocal lattice points form rods or columns of reciprocal lattice planes shown as vertical lines pointing normal to the real surface They are indexed as (lo), (20), etc., in Fig 7-27 Consider now an electron wave of. .. 12.5TM/Tcm4/sec (8- 3a) (8- 3b) (8- 3c) 8. 2 359 Fundamentals of Diffusion These approximate expressions represent average data for a variety of FCC metals normalized to the reduced temperature T / T,, where T, is the melting point As an example, the activation energy for lattice self-diffusion in Au is easily estimated through comparison of Eqs 8- 1 and 8- 3a, which gives E L / R T = 17.0TM/T.Therefore, EL... Eq 8- 2 The equations of the boundary lines separating the operative transport mechanisms are thus These are plotted as In 1 / I versus pd in Fig 8- 2 at four levels of T / T , -4 I s 0 0 I I I 2 4 \ I I -6 -2 E a 3 I I I L -4 d I I -6 I 0 2 4 6 8 1 I I I I I I L I I I I 012 0 2 4 6 8 1 0 1 2 L0Ppdh-q Log pd(cm-2) Figure 8- 2 Regimes of dominant diffusion mechanisms in FCC metal films as a function of . propagating in the direction of the incident radiation and terminating at the origin of the reciprocal lattice. Following Ewald, we draw of sphere of radius 2 n /X about the center. A property of this. array of Fig. 7-26a reflects the sixfold symmetry of the (111) plane, and the six fainter spots in between are the result of the (7 x 7) surface reconstruction. In RHEED, on the other. beams with the surface and the steps leading to the incorporation of atoms into the growing epitaxial film. Although MBE is the focus, the results are, of course, applica- ble to other epitaxial