Fundamentals of molecular beam epitaxy - Essay Example

?Fundamentals of Molecular Beam Epitaxy (MBE) Molecular beam epitaxy is a process to produce device grade epitaxial films and mutilayers. This is a very advanced and sophisticated process an have evolved gaining clear edge over competing processes like Liquid Phase Epitaxy (LPE), Vapor Phase Epitaxy (VPE) etc. This paper provides basic description of Molecular Beam Epitaxy (MBE) process. Different theoretical concepts of this process and essential ingredients of a MBE system are also briefly discussed. Introduction: Epitaxial growth refers to the situation in which the depositing layer extends the existing crystalline order of the substrate i.e. growth of the new layer does not cause any crystalline discontinuity on the interface between the substrate and the film. This occurs even during industrial processes like welding and cladding; where the initial mode of solidification is planer mode and the growth layer is essentially epitaxial growth of the existing grains. However, there are many grains on which this growth occurs an also this very soon degenerates into cellular and cellular dendritic growth and thus disrupting the crystalline order of the substrate. However, it is the planer mode of solidification coupled with solute partitioning between liquid and solid phase which form the basic underlying principle of Liquid Phase Epitaxy (LPE) [1- 3]. Similarly, epitaxial films can be grown by placing a substrate in a chamber filled with vapors of constituent atoms / molecules through a process known as Vapor Phase Epitaxy (VPE) [4 – 6]. However, it must be realized that the driving force behind the epitaxial growth processes was the demand of high grade semiconductor films and multilayers of very high quality in terms of the control on composition, dopant level, defects and properties. However, these stringent demands on the film quality could not be met by the processes like LPE, VPE etc.; while MBE process could meet the same and hence emerged as the process of choice for epitaxial film growth. In 1960s, there were remarkable advancements in microwave and optoelectronic devices. There was a need of compact semiconductor devices of smaller dimensions with high performance capability to support these advancements. High performance from a semiconductor device requires control on chemistry, dopant level, defects etc. Many thin film technologies such as LPE, VPE, Sputtering, vacuum deposition etc. were developed for producing high quality epitaxial thin films. However, the films produced by these techniques were structurally different from the substrate and hence not useful for device making. Differential vapor pressure of different constituents atoms / molecules was the main problem associated with VPE. GaAs is one such useful film for device making. IN this case vapor pressures of Ga and As differ by two orders of magnitude at about 600 oC. Therefore, these sources will have to be heated at different temperatures to achieve equal vapor pressure and the temperature will have to be controlled very accurately, which is very difficult. [7]. Attempts were made to use different temperatures for different sources [8] and by exploiting angular distribution of the atomic / molecular fluxes [9] for maintaining desired ratio of atomic fluxes. In 1960s it was not possible to perform online monitoring and characterization of the film growth process and the film itself. Even the characterization of the substrate surface condition and vacuum quality level was also not possible. One had to rely on post deposition characterization of the deposited film for the feedback for the subsequent deposition experiments; which was indeed a very slow, laborious and tiring procedure. Development of small mass spectrometers, auger electron spectroscopy and compact electron diffraction instruments made it possible to characterize the films in-situ while it was getting deposited and Molecular Beam Epitaxy (MBE) was discovered as a result of developing a process for surface characterization [10].In-situ characterization of MBE growth process using high energy electron diffraction was reported by Cho A. Y. [11] for the first time. This provided instant feedback on the influence of the film growth conditions on the film structure. It was also demonstrated that MBE could produce atomically flat and ordered layers. Methods to dope impurities [12] and to fabricate layered structures of GaAs-AlxGa1-xAs [13] were also reported. Since then there has been great amount of work has been reported in literature covering different aspects of the MBE process including deposition conditions, deposition procedures for different kinds of films and multilayers, characterization of the different properties of the films etc. [14 - 18]. Important aspects of the MBE process and ingredients of a MBE system are briefly described in the subsequent sections. Epitaxial Growth For epitaxial growth the depositing film must lead to continuation of the crystalline order of the underlying substrate. If chemical composition of the film is identical to that of the substrate it is known as Homoepitaxial growth else it is known as Heteroepitaxial growth. Examples of homoepitaxy are planer growth of solidification front at the solid liquid interface during autgeneous welding or during welding with filler wire with same composition or epitaxial deposition of Si over Si. Examples of heteroepitaxial growth are planer growth of solidification front at the solid liquid interface welding with filler wire having different composition or epitaxial deposition of an n-type or p-type film on a Si substrate. For epitaxial growth to occur, the substrate must be single crystal and it should be oriented such that deposition takes place at a plane of maximum packing density like (111) set of planes in a cubic structure and therefore, the film grows in [111] direction. Further, the substrate must be very clean to allow atom to atom contact with the incoming flux of the vapors of the depositing species. The film must be grown layer by layer on atomic scale and hence the film is flat on atomic level. The substrate must be heated sufficiently usually in 400 – 800 oC range to ensure mobility to the atoms sticking on the substrate so that these atoms can move to the desired lattice positions. These are some of the conditions which will be revisited under appropriate headings in this report. A typical process leading to epitaxial growth is shown in Fig. 1 [19]. Fig. 1: A typical epitaxial film growth [19] Evaporation Cells These are the source of the depositing atoms / molecules for deposition. Source materials are loaded in the individual cells. These are heated to in highly controlled manner to the desired temperature to ensure sufficient vapor pressure and hence flux of these entities. Hence ovens are cooled by liquid nitrogen to minimize radiative heating of the MBE chamber. Besides, the angular position of these cells with respect to the substrate is maintained to ensure that the axis of the effusion cell opening meet at the centre of the substrate. The crucibles are made of high grade pyrolytic boron nitride (PBN), because this material is least reactive with the substances of interest to the semiconductor industry at elevated temperature. The oven is of conical shape to minimize focused beaming of the atomic / molecular flux and to ensure a reliable angular distribution of the source material in the crucible as the same keeps depleting. The flux of incoming atomic / molecular vapors varies inversely with distance and directly proportional to third power of the cosine of the angle. Hence the flux drops very rapidly in angular direction. These considerations are must to design an MBE system and also to perform necessary calculation for arriving at optimum deposition condition of a film in an MBE system. Substrate Heating For epitaxial film deposition using MBE substrate heating is must. Normally the substrate is heated to a temperature around 400 - 800 oC (actual temperature depends on the system of interest). This is done to ensure sufficient mobility to the sticking atoms so that they can move to the desired lattice sites. If substrate temperature is very low then one may end up in either amorphous or polycrystalline film which is not at all desired. Also excessively high temperature will induce defects in the substrate and the film which is not desired. Substrate is also heated post argon sputtering to anneal out the defects generated during the sputtering process. Sputtering is performed to clean the substrate. Mounting of semiconductor substrate to the metallic surface heater is also very important issue. This is done by using a low melting metal such as Indium or Indium – Gallium eutectic which provides a liquid thermal contact at the deposition temperature. In is also beneficial as it is unreactive and relatively insoluble to the heater surface and the substrate. However, it has been reported that Ga reacts with Mo (used to make heater) at temperatures above 500 oC. Besides, it has fairly low vapor pressure up to ~ 600 oC, which is the deposition temperature. The liquid metal contact has advantage of providing good thermal contact. Besides, one also gets reasonably strain free adhesion of the substrate to the heater. This helps in minimizing clamping requirements and hence the films do not show strain induced slip lines near the clamps. Liquid metal bonding is also of particular benefit for odd shapes substrates. However, liquid metal bonding is not suitable at further higher temperatures of deposition and also there are chances of contamination from Indium. Additionally, the indium needs to be completely removed from the substrate before further processing of the coated wafer. To avoid these problems associated with Indium, nowadays radiative coupling is being used. However, for this to be used the substrate must be of exact dimensions of the sample holder and hence one needs separate sample holder for each size and shape of the wafer. To avoid thermal gradients and hot spot formation due to radiative heating, a thermal diffusing plate made of BN is used between heater and the substrate to even out the radiation flux. Another important issue related to substrate heating is related to the measurement of the temperature of the substrate. Because, the substrate needs to be rotated, therefore, it is not possible to bond the thermocouple directly to the substrate.Therefore, a non-contact stationary thermocouple is used. However, it needs to be calibrated very accurately by using infrared optical pyrometers. Many times optical pyrometers are used directly to measure the temperature of the substrate. However, there is problem in using an optical pyrometer, because emissivity of the substrate changes, when the films are coated onto it. Substrate Rotation Atomic / molecular fluxes from different sources arrive at the substrate from different direction. Also, the flux drops drastically as angle from the axis of the effusion cell increases. Therefore, deposition on a static substrate will leads to thickness variation as well has compositional variation. If this continues for sometime, then these variations will become too high to sustain layer by layer growth of the film and eventually epitaxy itself will breakdown into a polycrystalline film. Substrate heating helps in smoothening of this variation to some extent. However, that is not sufficient and rotation of the substrate becomes essential for growth of homogeneous film in terms of thickness and composition. Mean Free Path Mean free path is the average distance over which molecules / atoms in a vapor / gas phase do not collide with each other. For high value of mean free path number of molecules in a given volume should be low and average inter-atomic / molecular spacing should be large. For this to happen the pressure of the chamber should be low. Thus mean free path is inversely proportional to the chamber pressure. In molecular beam epitaxy process, collision of the atom / molecules is not desired, because that will lead to change the flux profile in the atomic / molecular flux in an unpredictable manner. Therefore, mean free path of the species in the atomic / molecular flux should be much larger than the distance between the effusion cell and the substrate. Mean free path of nitrogen molecule as a function of pressure is shown in Fig. 2 [22]. It can be seen from this figure that at typical MBE deposition pressure range of 10-8 - 10-10 torr mean free path of nitrogen molecules is much larger than a typical chamber size. Hence this requirement is automatically satisfied. Fig. 2: Mean free path of nitrogen atoms at different pressures Cleanliness of the film The film must be very clean because impurity of more than 1 parts per million is not at all acceptable in any case device application. These films should be much cleaner to justify usage of extremely high cost associated with the MBE in depositing these films for device fabrication. Deposition time of a monolayer The incoming flux of depositing species – the main constituents as well as the dopant can be calculated from the vapor pressure data. This is then multiplied by the respective sticking coefficients to calculate the rate of deposition and hence in calculating the time of deposition of a monolayer. Experimentally a typical deposition rate of Si film is 0.01 ?m per minute which implies approximately 2s time for deposition of a monolayer. Usually the monolayer deposition time in a MBE system is in 1 – 5 seconds [22]. Sticking coefficient of residual gases The residual gases in the deposition chamber may contaminate the film if these are also depositing during the film deposition. Any chamber cannot be evacuated to zero pressure, so there are always some residual gases in the chamber. There is certain flux of atoms / molecules from the residual gases arriving at the substrate surface. This rate of influx depends on the temperature and pressure in the chamber. For a typical residual gas of molecular weight of 40; at 25 oC temperature and 10-6 torr pressure the influx from the residual gases will be 3.2*1014 atom-cm-2s-1. This value is comparable to the number of atoms on (111) plane of silicon wafer. If all these molecules stick to the surface then there will be very high level of contamination. Even if the film remains in the chamber it will get contaminated in a few days despite the UHV being there. Fortunately, sticking coefficient for the gases in the background is very low and hence the film does not get contaminated. Ultra High Vacuum (UHV) and Construction Materials Ultra High Vacuum (UHV) is a first and foremost requirement for molecular beam epitaxy process. Without UHV one cannot even conceive of this process as there will be large scale contamination and unpredictable atomic / molecular flux on the substrate surface without UHV system. Therefore, an MBE chamber is constructed with great care and very high engineering precision. First comes the materials selection for construction of a MBE system. All the construction materials should have very low vapor pressure at the deposition temperature. The surface finish should be very high to minimized adsorption of gases, which will get desorbed during operation. The system must be leak tight and should be connected to advance vacuum systems like a series of turbo-molecular pumps, cryo pumps and ion pumps. Diffusion and rotary pumps are ruled out in any UHV system because these pumps use oil which will contaminate the vacuum. Air lock mechanism is used to load and unload the substrate to minimize the pumping time. For whatever reason, whenever the chamber is exposed to atmosphere it must be evacuated for days at slightly elevated temperature (150 oC) to restore UHV. Shutter Operation Atomic flux of the dopant atoms / molecules is controlled by means of a shutter. These shutters are operated by a microprocessor based system. These are opened for ~0.1 s which is much shorter duration as compared to the monolayer deposition time. This is because the concentration of dopants is much smaller than that of the matrix. A typical doping level is 10 ppm or so. PID Controllers All the controls are very crucial in an MBE system. The most important is the control of the source temperature. This temperature determines the partial pressure and hence relative flux of the different constituent atoms / molecules. Therefore, the temperature is controlled by means of a feedback based controller. The controller employs a logic based on the difference in the set temperature and the measured temperature. Based on this difference the current in the heating coil is changed. The quantum of this change is calculated by using PID logic. PID stands for proportional, integrator and Differentiator and this controller is very accurate in controlling the parameter of interest. Reflection High Energy Electron Diffraction (RHEED) In-situ characterization of the film in terms of smoothness of the layer, crystallinity and chemistry is very important for producing a quality film. Many characterization instruments are employed for this purpose. Reflection High Energy Electron Diffraction is one such instrument. This is attached to an MBE system. A grazing angle (~1o) electron beam of high energy ~ 5 – 40 keV is used in this instrument. The normal component of momentum of the electron beam is low (due to grazing incidence); therefore, these electrons penetrate no more than two layers of the film. The resulting diffraction pattern gives information about smoothness of the film and crystallinity of the film. This system is also used for monitoring thermal / sputtering cleaning of the substrate prior to deposition. Conclusions: MBE system is the state of art in the epitaxial film deposition. This system is routinely used to deposit novel materials and hence aids greatly in development of new and efficient devices. The MBE system employs very advanced materials for construction and very advanced tools for characterization of the films. A multidisciplinary approach is needed for effective utilization of this system for advanced research and industrial applications. References: [1] G?nther K. G. in “The use of thin films in physical investigations”, ed. J. C. Anderson (Academic, London, 1966), pp. 213 - 232 [2] Freller H. and G?nther K. G., Thin Solid Films, 88, 291 (1982) [3] Cho. A. Y., J. Vac. Sci. Technol., 8, S31 (1971) [4] Foxon C. T. and Joyce B. A., Surf. Sci. 50, 434 (1975) [5] Foxon C. T. and Joyce B. A., Surf. Sci. 64, 293 (1977) [6] Cho. A. Y. and Arthur J. R., Jr., Progr. Solid State Chem. 10, 157 (1975) [7] Arthur J. R. “Molecular beam epitaxy”, Surface Science 500 (2002) 189 – 217 [8] G?nter K. G. “Evaporated layers of semiconducting III-V compounds”, Naturwissenschaften 45 (1958) 415 [9] Collins R. J. Reynolds F. W., Stilwell G. R. “Electrical and optical properties of GaSb films”, Phys. Rev. 98 (1955) 227 [10] Arthur J. R. “Interaction of Ga and As2 molecular beams with GaAs surfaces”, J. of Appl. Phys. 39 (1968) [11] Cho. A. Y. “Morphology of epitaxial growth of GaAs by a molecular beam method: the observation of surface structures”, J. of Appl. Phys. 41 (1970) 782 [12] Cho A. Y. and Hayashi I. “Epitaxy of silicon doped gallium arsenide by molecular beam method”, Metall. Trans. 2 (1971) 4422 [13] Cho A. Y. and Casey H. C. “GaAs-AlxGa1-xAs double-hetero-structure lasers prepared by molecular beam epitaxy”, Appl. Phys. Lett. 25 (1974) 288 [14] Dingle R., Weigmann W. and Henry C. H. “Quantum states of confined particles in very thin AlxGa1-xAs-GaAs-AlxGa1-xAs heterostructures”, Phys. Rev. Lett. 33(1974) 827 [15] Cho A. Y. and Arthur J. R. “Molecular Beam Epitaxy”, Prog. Solid-State Chem. 10 (1975) 157 [16] Tsao J. Y. “Materials Fundamentals of Molecular Beam Epitaxy”, Academic Press., Boston, 1993 [17] Sitter M. A. and Sitter S. “Molecular Beam Epitaxy, Fundamentals and Current Status”, 2nd Ed., Springer, Berlin 1996 [18] Joyce B. A. “Kinetic and Surface Aspects of MBE” in Chang L. L. and Ploog K. (Eds.), “Molecular Beam Epitaxy and Heterostructures”, MartinusNijhoff Publishers, [19]http://www-llb.cea.fr/en/Phocea/Vie_des_labos/Ast/ast_service.php?id_unit=0&voir=technique [20] http://hyperphysics.phy-astr.gsu.edu/hbase/kinetic/menfre.html [21] http://www.practicalphysics.org/go/Guidance_98.html [22] Rinaldi F. “Basics of Molecular BeamEpitaxy (MBE)”, Annual Report 2002, Optoelectronics Department, University of Ulm Read More