Basics of Molecular Beam Epitaxy - Assignment Example

Basics of Molecular Beam Epitaxy (MBE) This paper presents the basic of molecular beam epitaxy (MBE) process. Besides, different concepts and essential elements of this process and MBE system are also briefly discussed. Introduction: Molecular Beam Epitaxy (MBE) is a method to grow epitaxial thin film of highly controlled chemistry and purity level over a substrate. The driving force behind development of Molecular Beam Epitaxy process was essentially the breakthroughs in microwave and optoelectronic devices way back in 1960s. These developments required semiconductor devices of smaller dimensions with high performance capability; which in turn required stringent control on the structure and chemistry of the devices on atomic level. Many thin film technologies like liquid phase epitaxy, chemical vapor phase deposition, sputtering, vacuum deposition etc. were developed and used to meet the stringent demands on these semiconductor devices. However, the semiconductor films produced by these processes were not structurally equivalent t the bulk material and hence were not of device grade. The problem associated with vapor phase deposition processes was differential vapor pressure of different constituents. For example vapor pressure of Ga and As differs by as much as two orders of magnitude at temperatures useful for thin film deposition. Therefore, to achieve equal vapor pressure and hence equal flux of arrival of Ga and As on the substrate by thermal evaporation of separate As and Ga sources will require too stringent control on the temperature to be achieved [1]. Many research groups tried to overcome this limitation by innovative approaches. One remarkable approach by Collins et al [2] involved using two separate crucibles containing Ga and Sb to deposit films of GaSb onto a glass substrate by evaporating these substances. His basic idea was to exploit decrease in angular decrease in the flux and hence to place the substrate at an angular location with respect to the individual crucibles where the two atomic fluxes will be equal and hence a stoichiometric film will be produced. However, the films he could produce were polycrystalline. Gϋnter K. G. [3] tried to arrive at appropriate vapor ratio by control of the temperatures of the evaporating species and the substrate separately. However, he could also not produce quality ordered films. During those days (1960 s) it was not easy to characterize the condition of the substrate, the quality of the vacuum and the composition and crystallinity of the films. The researchers had to do electron-diffraction after growing the films and this was taking much longer. It was development of small mass spectrometers, auger electron spectroscopy and compact electron diffraction instruments which made it possible to characterize the films in-situ while it was getting deposited. In fact, the very discovery of the MBE process was a result of developing a process to study surface – vapor interaction using compact mass – spectrometer [4]. While studying the reflection of pulsed molecular beams of Ga and As2 from GaAs surfaces Arthur J. R. et al [5] to measure the energy of adsorption they observed that presence of Ga layer immensely increased the bond strength of As on Ga as compared to the energy of As on GaAs. It can be concluded from this observation that only arrival fluxes of the depositing species do not control the composition of the film as some species may get preferably adsorbed while others may get desorbed also like in case of GaAs, Arthur J. R. et al could produce stoichiometric films of GaAs and GaP at approximately 500 oC by maintain a slightly over pressure of As and P respectively. For the first time in-situ characterization of MBE growth process using high energy electron diffraction was reported by Cho A. Y. [6]. This was a real breakthrough in the field of MBE as it provided instant feedback on the influence of the film growth conditions on the film structure. He also demonstrated that MBE could produce atomically flat and ordered layers and hence it was a potential candidate for device fabrication. He also reported many work on methods to dope impurities [7] and also how to fabricate layered structures of GaAs-AlxGa1-xAs [8]. Since then there has been great amount of work in different aspects of the MBE process like deposition conditions, deposition of layered structures, characterization of the deposited films for transverse conductance, optical properties etc. [9 -15]. Some important aspects of the MBE process are briefly discussed in the following sections. Epitaxial Growth Epitaxial growth refers to deposition of a film that matches the crystalline order of the underlying substrate material. This is of two types – Homoepitaxial and Heteroepitaxial. In case of homoepitaxy, the chemical composition of the deposited layer is the same as that of the underlying substrate for example deposition of Si over Si. In case of heteroepitaxy the chemical composition of the deposited layer is different from that of the underlying substrate for example deposition of a n-type or p-type film on a Si substrate. It is not difficult to realize that for epitaxial growth the substrate must be single crystal and oriented such that deposition takes place at a plane of maximum packing density like (111) set of planes in a cubic structure. Further, the substrate must be ultraclean to ensure atom to atom contact with the incoming flux of the vapors of the depositing species. The growth of the film must be layer by layer i.e. flat on atomic scale otherwise the film will very soon degenerate into a polycrystalline film. Also, there should be sufficient temperature on the substrate 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. Fig. 1: Schematic Diagram Showing an Epitaxial Film on a Substrate Evaporation Cells Evaporation cells are the source of the depositing atoms forming the film. Individual species of very high purity are loaded in the individual cells. These are placed in ovens to heat them in highly controlled manner to the desired temperature to ensure sufficient vapor pressure and hence flux of these entities. Hese ovens are mounted such that each of them is cooled by liquid nitrogen to minimize radiative heating of the MBE chamber. Also, the angular position of these cells with respect to the substrate is maintained to ensure desired flux of these species on the substrate surface. The crucibles containing these materials 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 [1]. 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 [1]. 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 600 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. 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 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. 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 Incoming flux from different effusion cells have an angular profile and therefore, uneven chemistry and thickness in the film will occur if the substrate is not rotated during the deposition process. The variation in chemical composition of the dopant atoms becomes very critical for the properties and performance of the deposited film. To some extent, temperature of the substrate helps in equalizing the composition gradient on the layer by facilitating movement of the atoms. However, to get homogeneous composition, rotation of the substrate becomes very important. Fig. 2 shows a schematic deposition profile on a stationary substrate [1]. From this figure the necessity of substrate rotation becomes very obvious. Fig. 2: A typical deposition profile on a stationary substrate Mean Free Path In MBE process it is not desirable that atoms or molecules collide with each – other in their travel from effusion cell to the substrate. Because, this will cause alteration in the energy profile and most importantly angular distribution of the atomic and molecular flux arriving at the substrate. Hence the mean free path of the molecules in the chamber should be much larger than the chamber dimensions. As mean free path of gaseous species is inversely proportional to the pressure, a MBE chamber will require low operating pressure. Mean free path () of a molecule is given by the following equation [16] Where, R = 8.2*10-2 (lit-atm)/(mol-K) = 8.2*10-5 (m3-atm)/(mol-K) is Gas Constant T = 300 K is absolute temperature of the air molecules in TEM column NA = 6.023*1023 molecules per mole is Avogadro’s Number d = 3*10-10 m is average molecule diameter of nitrogen molecules [17] P = 10-6 torr = 1.316*10-9 Atm is pressure in the TEM column Mean free path of nitrogen molecule as a function of pressure is shown in Fig. 3 [18]. It can be seen from this figure that at typical MBE deposition pressure of 10-10 torr mean free path of nitrogen molecules is much larger than a typical chamber size. Hence this requirement is automatically satisfied. Fig. 3: Mean free path of nitrogen atoms at different pressures Cleanliness of the film Cleanliness of the film is very important, because in any case device application requires purity level of less than 1 in 106 atoms. Besides, the films are expected to perform better to justify the cost involved in deposition and for that the films must be cleaner than the substrate. Deposition time of a monolayer Deposition time of a monolayer in a MBE system is typically 1 – 5 seconds [18]. Sticking coefficient of residual gases There are residual gases in any vacuum system as any chamber cannot be evacuated to zero pressure. If a typical residual gas of molecular weight of 40 is considered, then at 25 oC and 10-6 torr it will have a flux of 3.2*1014 atom-cm-2s-1; which is roughly equal to number of atoms on (111) plane of silicon substrate. This means if all the molecules stick to the surface then it will spoil the film in no time. Even at the typical deposition pressure of 10-10 torr this will amount to 1 impurity in 104 atoms which is also too high. Fortunately all the atoms which arrive do not stick and also these are not reactive to the substrate and the film being deposited. Ultra High Vacuum (UHV) and Construction Materials From our discussion so far it is needless to say that ultra high vacuum (UHV) is required for molecular beam epitaxy (MBE). Construction and maintenance of an UHV system is not an easy task. All the construction materials should have very low vapor pressure at the deposition temperature. Besides, the surface should be in high finish (mirror finish) polished condition to minimized adsorption and desorption of gases. The system must be leak tight and should be connected to advance vacuum systems like a series of turbo-molecular pumps, diffusion pump and rotary pump. The chamber should be loaded and unloaded using air locks. Whenever the system is exposed to the open atmosphere it must be evacuated while baking it at 150 oC for at least 24 hours. Shutter Operation Shutters are used to regulate atomic / molecular flux of certain constituents like dopant etc. Because concentration of these elements is much smaller usually 1 in 106; therefore, time of shutter opening and closing must be very small. Normally it is 0.1 second which is much smaller than the time to deposit a monolayer. PID Controllers It is very crucial to regulate temperature of the atom / molecule sources. This is done by means of a feedback based PID controller. PID stands for proportional, integrator and differentiator; which controls the temperature of the source ovens very accurately. Reflection High Energy Electron Diffraction (RHEED) Many characterization tools are used to monitor the film characteristics being produced by MBE process. It is better if these characterizations can be carried out in-situ and preferably in online mode. Reflection High Energy Electron Diffraction (RHEED) is one such instrument. This instrument is an attachment to an MBE system. This uses glancing angle electron beams of 10 – 50 eV. As the normal momentum of these electron is very small these electrons penetrate no more than 1 to 2 atomic layer and the diffraction wave functions contain information about essentially the surface. Thus RHEED helps in monitoring smoothness of the film on atomic level during the process. Conclusions: It can be concluded that MBE is very advance method of thin film deposition on atomic scale and very useful for novel device fabrication. This is an state of art deposition process as on date and it requires many advanced technologies to make a MBE system. References: [1] Arthur J. R. “Molecular beam epitaxy”, Surface Science 500 (2002) 189 – 217 [2] Collins R. J. Reynolds F. W., Stilwell G. R. “Electrical and optical properties of GaSb films”, Phys. Rev. 98 (1955) 227 [3] Gϋnter K. G. “Evaporated layers of semiconducting III-V compounds”, Naturwissenschaften 45 (1958) 415 [4] Arthur J. R. “Interaction of Ga and As2 molecular beams with GaAs surfaces”, J. of Appl. Phys. 39 (1968) [5] Arthur J. R. and LePore J. J. “GaAs, GaP and GaAsxP1-x epitaxial films grown by molecular beam depositions”, J. of Vac. Sci. Technol. 6 (1969) 545 [6] 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 [7] Cho A. Y. and Hayashi I. “Epitaxy of silicon doped gallium arsenide by molecular beam method”, Metall. Trans. 2 (1971) 4422 [8] 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 [9] Chang L. L, Esaki L., Howard W. E. and Ludeke R. “The growth of a GaAs-GaAlAs superlattice”, J. of Vac. Sci. Technol. 10 (1973) 11 [10] Chang L. L, Esaki L. and Tsu R. “Resonant tunneling in semiconductor double barriers”, Appl. Phys. Lett. 24 (1974) 593 [11] 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 [12] Cho A. Y. and Arthur J. R. “Molecular Beam Epitaxy”, Prog. Solid-State Chem. 10 (1975) 157 [13] Sitter M. A. and Sitter S. “Molecular Beam Epitaxy, Fundamentals and Current Status”, 2nd Ed., Springer, Berlin 1996 [14] Tsao J. Y. “Materials Fundamentals of Molecular Beam Epitaxy”, Academic Press., Boston, 1993 [15] Joyce B. A. “Kinetic and Surface Aspects of MBE” in Chang L. L. and Ploog K. (Eds.), “Molecular Beam Epitaxy and Heterostructures”, Martinus Nijhoff Publishers, Dordretch, 1985, p 191 [16] http://hyperphysics.phy-astr.gsu.edu/hbase/kinetic/menfre.html [17] http://www.practicalphysics.org/go/Guidance_98.html [18] Rinaldi F. “Basics of Molecular Beam Epitaxy (MBE)”, Annual Report 2002, Optoelectronics Department, University of Ulm Read More