Epitaxial Growth
Epitaxial Growth
Introduction Epitaxy comes from Greek words:–Epi: upon–Taxis: orderedThe term epitaxial comes from the Greek word meaning ‗arranged upon‘.Epitaxial growth: single crystal growth of a material in which a substrate serve as a seedIn semiconductor technology, it refers to the single crystalline structure or a film depositedover the silicon wafer.Epitaxy or epitaxial growth is the process of depositing a thin layer (0.5 to 20 microns) ofsingle crystal material over a single crystal substrate usually through chemical vapordeposition (CVD).In the same material as the substrate, and the process is known as homoepitaxy, or simply,epi. An example of this is silicon deposition over a silicon substrate.Two types of epitaxy:1.Homoepitaxy – material is grown epitaxially on a substrate of the same material. E.g.growth of Si on Si substrate2.Heteroepitaxy – a layer grown on a chemically different substrate. E.g. Si growth onsapphireSimilar crystal structures of the layer and the substrate, BUT–The shift of composition causes difference in lattice parameters–Limit the ability to produce epitaxial layers of dissimilar materialsEpitaxy or epitaxial growth is the process of depositing a thin layer (0.5 to 20 microns) of singlecrystal material over a single crystal substrate, usually through chemical vapor deposition (CVD).
Applications of epitaxial layers1.2.3.Discrete and power devicesEpitaxy for MOS devicesIntegrated circuits Silicon Epitaxy is done to improve the performance of bipolar devices.By growing a rightly doped epi layer over a heavily-doped silicon substrate: a higher breakdown voltage across the collector-substrate junction is achieved while maintaining low collector resistance. Lower collector resistance allows a higher operating speed with the same current.Epitaxial technique is used to developed 2 and 3 layers epitaxial structure For lightly doped area of collector Base region was also grown epitaxiallyExample of multilayer structures: Si-Controlled Rectifier (SCR), Triac, high voltage or high power discrete products Epitaxy has also recently been used in CMOS VLSI circuits.Unipolar devices such as junction field-effect transistors (JFETs), VMOS, DRAMs technology also use epitaxialstructures VLSI CMOS (complimentary metal-oxide-semiconductor) devices have been built in thin (3-8 micron) lightlydoped epitaxial layers on heavily doped substrates of the same type (N or P) That epitaxial structure reduces the ―latch up‖ of high density CMOS IC by reducing the unwanted interactionof closely spaced devices Aside from improving the performance of devices, epitaxy also allows better control of doping concentrations ofthe devices The layer can also be made oxygen- and carbon-free.
Epitaxy in Integrated circuit (IC) Development of planar bipolar IC require devices built on the same substrate to be electricallyisolated–The use of opposite type substrate and epitaxial layer met part of the requirement–Device isolation was completed by the diffusion of ―isolation‖ region through the epitaxiallayer to contact the substrate between active areas In planar bipolar circuits, common to employ a heavily doped diffused (or implanted) regionunder the transistor–Usually called ‗buried layer‘ or ‗DUF‘ ( diffusion under film)–The buried layer serves to lower the lateral series resistance between collector area below the emitterand the collector contact produce uniform planar operation of the emitter, avoiding current crowding whichleads to hot spots near edges of the emitterEpitaxy is also used to grow layers of pre-doped silicon on the polished sides of silicon wafers,before they are processed into semiconductor devices. This is typical of power devices, such asthose used in pacemakers, vending machine controllers, automobile computers, etc.
Advantages of epitaxy Ability to place a lightly oppositely doped region over a heavily doped regionAbility to contour and tailor the doping profile in ways not possible using diffusion orimplantation aloneProvide a layer of oxygen free material that is also contained low carbonTechniques for silicon epitaxy1.Chemical Vapour Deposition (CVD)2.Molecular Beam Epitaxy (MBE)3.Liquid Phase Epitaxy (LPE)4.Solid phase regrowth
1. Chemical Vapour Deposition (CVD)It is a chemical process for depositing thin films of various materials.The most common technique in Si epitaxyChemical Vapor Deposition is the formation of a non-volatile solid film on a substrate by thereaction of vapor phase chemicals (reactants) that contain the required constituents.The reactant gases are introduced into a reaction chamber and are decomposed and reacted at aheated surface to form the thin film.In the CVD technique– Si substrate is heated in a chamber: sufficient heat to allow the depositing Si atoms to moveinto position– Si is exposed to one or more volatile gases, which react and/or decompose -on the hotsubstrate surface to produce the desired deposit.– Gases react on the substrate and deposit a Si layer– The deposit will take on Si substrate structure if the substrate is atomically clean and thetemperature is sufficient for atoms to have surface mobilityFrequently, volatile byproducts are also produced.Examples of CVD films
CVD systemsSchematic CVD reactor geometriesfor(a)True vertical reactor(b)Classic horizontal flow reactor(c)Modified vertical (or pancake)reactor(d)Downflow cylinder reactorHorizontal APCVD ReactorSchematic drawing of a simplehorizontal flow, cold wall, CVD reactor
CVD Process steps:Pre-cleanDepositionEvaluation Pre-clean: remove particulates and mobile ioniccontaminants Deposition:Load wafer intochamber, inertatmosphereHeatIntroducechemicalvapour Evaluation: thickness, step coverage, purity,cleanliness and compositionRemove vapourFlush excesschemicalvapour source
Explanation of Steps involved in a CVD process1. Transport of reactants by forcedconvection to the depositionregion.2. Transport of reactants bydiffusion from the main gasstream through the boundarylayer to the wafer surface.3. Adsorption of reactants on the wafer surface.4. Surface processes, including chemical decomposition orreaction, surface migration to attachment sites (such as atomiclevel ledges and kinks), site incorporation, and other surfacereactions.5. Desorption of byproducts from the surface.6. Transport of byproducts by diffusion through the boundarylayer and back to the main gas stream.7. Transport of byproducts by forced convection away from thedeposition region.
CVD for silicon devices
CVD reactions1.Pyrolysis: chemical reaction is driven by heat alone, e.g. silane decomposes with heatingSiH4 Si 2H22.Reduction: chemical reaction by reacting a molecule with hydrogen, e.g. silicon tetrachloride-reduction in hydrogen ambient to form solid siliconSiCl4 2H2 Si 4HCl3.Oxidation: chemical reaction of an atom or molecule with oxygen, e.g. SiH4 decomposes at lowertemperatureSiH4 O2 SiO2 2H24.Nitridation: chemical process of forming silicon nitride by exposing Si wafer to nitrogen at hightemperature e.g. SiH2Cl2 readily decomposes at 1050 C3SiH2Cl2 4NH3 Si3N4 pH 6H2
CVD of Si - Epitaxy When SiH4 gas is used in a CVD reactor, a Si layer is deposited on the wafer surface. The size of the crystallites depends on the deposition temperature. At high enough temperature, the ad-atoms have enough kinetic energy to move on the surface andalign themselves with the underlying Si. This is an epitaxial layer, and the process is called Epitaxy instead of CVD. At lower deposition temperatures, the layer is poly-crystalline Si (consisting of small crystallites)Si EpitaxyThe chemical reaction that produces theSi is fairly simple:SiCl4(g) 2H2(g) (1000-1200oC) Si(s) 4HCl(g)The chemical vapor deposition of siliconepitaxy is usually achieved using anepitaxial reactor (Fig. 1) that consistsof a quartz reaction chamber intowhich a susceptor is placed.The susceptor provides two things:1) mechanical support for the wafers2) an environment with uniform thermaldistribution.Epitaxial deposition takes place at a hightemperature as the required processgases flow into the chamber.Instead of SiCl4 you may Si Epitaxy:want to use SiHXCl4-Xcan form very thick doped structures (30-100 um) not(Fig. 1)possible with implantation or diffusion.Such thick, pure layers are often used in power devices whilethinner, 1-5 um, are commonly used for many CMOS andbipolar technology.
Epitaxial Furnace
CVD film growth steps1.2.3.4.Nucleation Dependent on substrate quality Occurs at first few atoms or molecules deposit on a surfaceNuclei growth Atoms or molecules form islands that grow into larger islandsIsland coalescence The islands spread , and coalescing into a continuous film This is the transition stage of the film growth, thicknessseveral hundreds Angstroms Transition region film possesses different chemical andphysical properties for thicker bulk filmBulk growth Bulk growth begins after transition film is formed
CVD film growth stepsTypes of film structureAmorphousPolycrystallineSingle crystalBasic CVD subsystem
Advantages of CVD processesCVD processes are ideally suited for depositing thin layers of materials onsome substrate. In contrast to some other deposition processes which wewill encounter later, CVD layers always follow the contours of thesubstrate: They are conformal to the substrate as shown below.Disadvantages of CVD processesThe two most important ones (and the only ones we willaddress here) are:1. They are not possible for some materials; there simply is nosuitable chemical reaction.2. They are generally not suitable for mixtures of materials.
Tillat 11A number of forms of CVD are in wide use and a‘frequently referenced in the literature Plasma Enhanced CVD (PECVD) CVIprocesses that utilize a plasma to enhanc chemicalreaction rates of the precursorf PECVD processing allowsdeposition t‘Llower tern er res, which Is often critical i I emanufacture of semiconductors. Rapid Thermal CVD (RTCVD) - CVI processes that useheating lamps or oth methods to rapidly heat the wafersubstrat‘ Heating only the substrate rather than th gas orchamber walls helps reduce& unw h e reactiofls that cãii1oc1ii&ma A(mosIieric Pressure CVD (APCVD) CVD processes atatmospheric pressure. Low Pressure CVD (LPCVD) CV processes atsubatmospheric pressure Reduced pressures tend to redLunwanted gas phase reactions and irnpro{ filmuniformity across the wafer. M.t. modern CVD processare either LPCVD UHVCVD. Ultra-High Vacuum CVD (UHVCVD) - C\ processes atvery low pre5sures, typically the range of a few to ahundred millltorr5.-.--- —-- —ii -
Molecular Beam Epitaxy Molecular beam epitaxy (MBE) was developed in the early 1970s as a means of growing high-purity epitaxiallayers of compound semiconductors. Since that time it has evolved into a popular technique for growing Ill-V compound semiconductors as well asseveral other materials. MBE can produce high- quality layers with very abrupt interfaces and good control of thickness, doping, andcomposition. Because of the high degree of control possible with MBE, it is a valuable tool in the development ofsophisticated electronic and optoelectronic devices. The term ‗beam‖ simply means that evaporated atoms do not meet each other or any other gases until theyreach the wafer.Tilat In MBE, a source material is heated to produce an evaporated beam of particles. These particles travel through a very high vacuum (10-8 Pa ; practically free space) to the substrate, wherethey condense. Means that Molecular beam epitaxy takes place in high vacuum or ultra high vacuum (10 8 Pa). The most important aspect of MBE is the slow deposition rate (1 to 300 nm per minute), which allows thefilms to grow epitaxially. However, the slow deposition rates require proportionally better vacuum in order to achieve the sameimpurity levels as other deposition techniques. MBE has lower throughput than other forms of epitaxy. This technique is widely used for growing III-V semiconductor crystals. Wiki In solid source MBE, ultra-pure elements such as gallium and arsenic are heated in separate furnaces until they each slowly begin to evaporate. (The material sources, or effusions cells, are independently heated until the desired material flux is achieved. Changes in the temperature of a cell as small as 0.5 C can lead to flux changes on the order of one percent). The evaporated elements (the arsenic in this case is actually in a molecular form) condense on the wafer, where they react with each other forming, in this case, gallium arsenide.Tilat done this para else where
2. Molecular Bean Epitaxy (MBE) MBE is a non-CVD epitaxial process that uses an evaporation method.MBE is carried out at a lower temperature than 1000-1200 C (typical CVD temperature)– Reduces outdiffusion of local areas of dopant diffused into substrates and reduce auto-doping which isunintentionally transfer of dopant into epitaxial layerMBE is favourable in– preparation of sub-micron thickness epitaxial layers or– high frequency devices requiring hyper-abrupt transition in the doping concentration between theepitaxial layer and the substratePrinciple: In MBE,– Si and dopant(s) are evaporated in an ultra high vacuum (UHV) chamber– The evaporated atoms are transported at relatively high velocity in a straight line from the source to thesubstrate– They condense on the low temperature substrate– The condensed atoms of Si or dopant will diffuse on the surface until they reach a low energy site that theyfit well the atomic structure of the surface– The ―adatom‖ then bonds in that low energy site, extending the underlying crystal by a vapour to solidphase crystal growth– Usual temperature range of the substrate is 400-800 C.– Higher than 800 C is possible but it will increase outdiffusion or lateral diffusion of dopants in thesubstrateOffers the highest purity material (due to UHV conditions) and the best layer control (almost any fraction of anatomic layer can be deposited and layers can be sequenced one layer at a time (for example Ga then As then Gaetc ).
Molecular Beam Epitaxial SystemImportant Parameters: Conventional temperature range– for MBE is from 400-800oC– Higher temperature are feasible, but the advantages of reduced outdiffusion and auto-doping arelost. Growth rates in the range 0.01 to 0.3µm/min have been reported. In-situ cleaning of the substrate is done in two ways– First technique: by high temperature bake at 1000-1250 C for several(30) minutes under highvacuum to decompose the native surface oxide and to remove other surface contaminants such ascarbon– Other technique: is by using a low energy beam of inert gas to sputter clean the substrate– Difficult to remove carbon but short anneal at 800-900 C will reorder the surface– Wider range of dopants for MBE than CVD epitaxy:– Typical dopants: Antimony, Sb (N-type), aluminium, Al or gallium (Ga) for P-type– N-type dopant: As and P, evaporate rapidly even at 200 C. Difficult to control– P-type dopant: Boron, evaporates slowly even at 1300 C
Molecular Beam Epitaxial System In contrast to CVD processes, MBE is not complicated by boundary-layer transport effects, nor are therechemical reactions to consider. The essence of the process is an evaporation of silicon and one or more dopants, as depicted in Fig. The evaporated species are transported at a relatively high velocity in a vacuum to the substrate. The relatively low vapor pressure of silicon and the dopants ensures condensation on a low-temperaturesubstrate. Usually, silicon MBE is performed under ultra-high vacuum (UHV) conditions of 10-8 to 10-10 Torr, where themean free path of the atoms is given bywhere L is the mean free path in cm, and P is the system pressure in Torr.At a system pressure of 10-9 Torr, L would be 5 x 106 cm. The mean free path is very long (can be hundreds of meters) because ultra high purity materials are evaporated in an UHVchamber and because of the very low pressure. Thus, the evaporated material travels in a straight line (amolecular beam) toward a hot substrate. Once on the substrate, the atom or molecule moves arounduntil it finds an atomic site to chemically bond to.System Equipment: An elementary MBE system is shown in Fig.1. It is, in essence, a UHV chamber where furnaces holdingelectronic-grade silicon and dopant direct a flux ofmaterial to the heated surface.Fig.1. Schematic of an elementary MBE System
Sze83 Fig. 2 illustrates the many components of a comprehensive system. A distinguishing feature of MBE is the ability to use sophisticatedanalytical techniques in situ to monitor the process. In contrast to the CVD process, MBE does not require extensive safetyprecautions, although solid arsenic dopant must be handled carefully. The vacuum system is the heart of the apparatus. To consistently attain a vacuum level in the 10-10 Torr range, materialsand construction choices must be carefully considered. Materials should have low vapor pressure and low sticking coefficients.Fig.2 -Schematic of practical MBE system. Repeated exposure to air is detrimental to a UHV system because of the long bakes needed to desorbatmospheric species from the system walls. A load lock system minimizes this problem. Consistently low base pressure is needed to ensure overall film perfection and purity. These needs are best met with an oil-free pump design, such as a cryogenic pump. Because of its high melting point, silicon is volatilized not by heating in the furnace, but by electron-beamheating. Dopants are heated in a furnace. A constant flux is assured by the use of closed-loop temperature control. Baffles and shutters shape and control the flux, so uniformity of doping and deposition can he attainedwithout boundary layer effects. Substrates are best heated when they are placed in proximity to a resistance heater with closed-looptemperature control. Resistance heating generates temperatures over the range of 400 to 1100 C. A wide choice of temperaturesensing methods is available, including thermocouples, optical pyrometry, and infrared detection.
Tilat pp.10 In solid source MBE: ultra-pure elements such as gallium and arsenic are heated in separatefurnaces until they each slowly begin to evaporate. (The material sources, or effusions cells, are independently heated until thedesired material flux is achieved. Changes in the temperature of a cell as small as 0.5 C can lead to fluxchanges on the order of one percent). The evaporated elements (the arsenic in this case is actually in a molecularform) condense on the wafer, where they react with each other forming, inthis case, gallium arsenide. Shutters can be used to turn the beam flux on and off The flux of atoms/molecules is controlled by the temperature of the ―effusion cell‖ (evaporation source). A computer controls the shutter in front of each furnace, allowing precise control of the thickness of each layer,Tilatpp. 9 more to from pp.10down to a single layer of atoms. During operation, RHEED (Reflection High Energy Electron Diffraction) is often used for monitoring thegrowth of the crystal layers. The ultra-high vacuum environment within the growth camber- is—maintained by a system of cryopumps andcryopanels, chilled using liquid nitrogen to a temperature of 77 Kelvin ( 196 degrees Celsius). The wafers on which the crystals are grown are mounted on a rotating platter which can be heated to severalhundred degrees C during operation. For improved layer uniformity, the sample holder is designed for continual azimuthal rotation of the sample,and is thus commonly termed the ‗CAR‘. The ‗CAR‘ also has an ion gauge mounted on the side opposite the sample which can read the chamberpressure, or be placed facing the sources to measure beam equivalent pressure (BEP) of the material sources.toroidal (azimuthal) field
RHEED (Reflection High Energy Electron Diffraction): One of the most useful tools for in-situ monitoring of the growth is reflection high-energy electron diffraction(RHEED). It can be used to calibrate growth rates, observe removal of oxides from the surface. calibrate the substrate temperature, monitor the arrangement of the surface atoms, determine the proper arsenicoverpressure and provide information about growth kinetics. The RHEED gun emits 1OKeV electrons which strike the surface at a shallow angle ( 0. 5-2 degrees), making it asensitive probe of the semiconductor surface. Electrons reflect from the surface and strike a phosphor screen forming a pattern consisting of a spectral reflectionand a diffraction pattern which is indicative of the surface crystallography. A camera monitors the screen and can record instantaneous pictures or measure the intensity of a given pixel as afunction of time.
Advantages and Disadvantages of MBESze-pp80 Present since 1960 but was not in use due to absence of industrial equipment and quality was not suitable fordevice needs. Equipment is now commercially available, but the process has low throughput and is expensive. MBE, however, does have a number of inherent advantages over CVD techniques. Its main advantage for VLSI use is low- temperature processing. Low-temperature processing minimizes outdiffusion and auto-doping, a limitation in thin layers prepared byconventional CVD. Another advantage is the precise control of doping that MBE allows. Because doping in MBE is not affected by time-constant considerations as is CVD epitaxy, complicated dopingprofiles can be generated. Presently, these advantages are not being exploited for IC fabrication, but they have found application indiscrete microwave and photonic devices. For example, the C-V characteristic of a diode with uniform doping is nonlinear with respect to reverse bias. Varactor diodes used as FM modulators could advantageously employ a linear dependence of capacitance onvoltage. This linear voltage—capacitance relationship can be achieved with a linear doping profile, which is easilyobtained with MBE.
Structure and defects in epitaxial layer –––– –––Surface morphology of Silicon epitaxial deposits is affected by growth and substrateparametersGrowth parameters:TemperaturePressureConcentration of Si containing gasCl : H2 ratioSubstrates parametersSubstrate orientationDefects in the substrateContaminants on the surface of the substrateTypical defects in epitaxial layers1.2.3.4.5.Ref:3-epitaxy growth-2 USMalaysiaSubstrate orientation effectsSpikes and epitaxial stacking faultsHillocks and pyramids in epitaxial layersDislocations and slipMicroprecipitates (S-pits)Details in Ref:3-epitaxy growth-2 USMalaysia
1. Chemical Vapour Deposition (CVD) It is a chemical process for depositing thin films of various materials. The most common technique in Si epitaxy Chemical Vapor Deposition is the formation of a non-volatile solid film on a substrate by the reaction of vapor phase chemicals (reactants) that contain the required constituents. The reactant gases are introduced into a reaction .
The growth of high-quality organic semiconductors on sup-porting substrates with minimal disorder is the key require-ment for realizing high-performance organic electronics. [ 1,2 ] However, the absence of an epitaxial relation and the presence of disorder make it particularly challenging to grow high-quality
next-generation solar cell concepts [1-4]. Furthermore, re-cent advances in epitaxial growth and processing of III-V thin-film solar cells, including approaches for separation of active device layers from epitaxial growth substrates, have made III-V solar cells increasingly attractive for electricity generation strategies such as .
Supporting Information S1 Reversible Oxidation Quantified by Optical Properties in Epitaxial Fe2CrO4 δ Films on (001) MgAl2O4 Mark D. Scafetta,1# Tiffany C. Kaspar,1 Mark E. Bowden,2 Steven R. Spurgeon,3 Bethany Matthews,3 Scott A. Chambers1* 1) Physical and Compu
Large-Area Dry Transfer of Single-Crystalline Epitaxial Bismuth Thin Films Emily S. Walker,† Seung Ryul Na,‡ Daehwan Jung,§ Stephen D. March,† Joon-Seok Kim,† Tanuj Trivedi,† Wei Li,† Li Tao,† Minjoo L. Lee,§, Kenneth M. Liechti,‡ Deji Akinwande,*,† and Seth R. Bank*,† †Microelectronics Research Center and Department of Electrical and Computer Engineering, The .
Landau Theory of Free Standing and Epitaxial Ferroelectric Film by Ahmad M. A. Musleh Thesis submitted in fulfillment of the requirements for the degree of Doctor of Philosophy May 2009 . ACKNOWLEDGEMENTS I gratefully acknowledge my supervisors Dr Ong Lye Hock and Prof. Junaidah
base current shows maximum small signal current gain 13 of 36 (Figure 5 (b)). From S-parameter measurements we deduce maximum Ft and F„,„ of 110 and 150 GHz, respectively, at an emitter current density of 94 kA cm- (Figure 5 (c)). Through improvements in epitaxial designs, epitaxial material quality,
having a very high driving force, in this case, due to the instability of elemental Ga and As in the vapor at typical growth temperatures. These high driving forces for formation of the solid have prompted many researchers to dub OMVPE, MBE, and CBE as "highly non-equilibrium growth processes [1,6], On the other hand, hydride
Our work opens new avenues for using highl y oriented pyrolytic graphite as a substrate to fabricate transferable optoelectronic devices. Keywords: Mechanical exfoliation graphite, GaN, Growth mechanism, Highly oriented pyrolytic graphite (HOPG) . Fig. 2 a FE-SEM image of annealed islands.
