PART III: METALORGANIC CHEMICAL VAPOR DEPOSITION Description of the MOCVD equipment Analysis of the MOCVD growth process Growth modes in MOCVD

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PART III: METALORGANIC CHEMICAL VAPOR DEPOSITION Description of the MOCVD equipment Analysis of the MOCVD growth process Growth modes in MOCVD Slide 2 Metalorgenic Chemical Vapor Deposition (MOCVD) [Metalorganic Vapor Phase Epitaxy (MOVPE), OMCVD, OMVPE] One of the premier techniques for epitaxial growth of thin layer structures (semiconductors, oxides, superconductors) Introduced around 25 years ago as the most versatile technique for growing semiconductor films. Wide application for devices such Lasers, LEDs, solar cells, photodetectors, HBTs, FETs. Principle of operation: transport of precursor molecules (group-III metalorganics + group-V hydrides or alkyls) by a carrier gas (H 2, N 2 ) onto a heated substrate; surface chemical reactions. Complex transport phenomena and reactions, complicated models to determine reactor designs,growth modes and rates. In-situ diagnostics less common than in MBE. Slide 3 Description of the MOCVD equipment R. L. Moon and Y.-M. Houng, in Chemical vapor deposition - Principles and applications, edited by M. L. Hitchman and K. F. Jensen (Academic Press, London, 1993). G. B. Stringfellow, Organometallic vapor phase epitaxy: theory and practice (Academic Press, Boston, 1989). Slide 4 MOCVD Facility, horizontal reactor Research system (left): AIX 200 1X2 wafer capacity Production system (right): AIX 2600 Up to 5X10 wafer capacity (AIX 3000) Gas handling Reactor Glove box Slide 5 Schematics of a MOCVD system Carrier gas Material sources Gas handling system Reactor Exhaust system Safety system In-situ diagnostics NO electron beam probes! Reflectance Ellipsometry RAS Slide 6 Gas handling system The function of gas handling system is mixing and metering of the gas that will enter the reactor. Timing and composition of the gas entering the reactor will determine the epilayer structure. Leak-tight of the gas panel is essential, because the oxygen contamination will degrade the growing films properties. Fast switch of valve system is very important for thin film and abrupt interface structure growth, Accurate control of flow rate, pressure and temperature can ensure stability and repeatability. Slide 7 Carrier gas Inert carrier gas constitutes about 90 % of the gas phase stringent purity requirements. H 2 traditionally used, simple to purify by being passed through a palladium foil heated to 400 C. Problem: H 2 is highly explosive in contact with O 2 high safety costs. Alternative precursor : N 2 : safer, recently with similar purity, more effective in cracking precursor molecules (heavier). High flux fast change of vapor phase composition. Regulation: mass flow controller P ~ 5- 800 mbar Mass flow controllers Slide 8 Material sources Volatile precursor molecules transported by the carrier gas Growth of III-V semiconductors: Group III: generally metalorganic molecules (trimethyl- or triethyl- species) Group V: generally toxic hydrides (AsH 3 ; PH 3 flammable as well); alternative: alkyls (TBAs, TBP). Slide 9 Hidrides and dopants Form: gases from high pressure cylinders Mixed into the carrier gas line Flow control: valve + mass flow controller (MFC) Slide 10 Metalorganics Liquid (or finely divided solid TMIn) contained in a stainess steel bubbler. Vapor pressure fixed by constant temperature in a thermal bath; T -20 o C 40 o C; T = 1 o C. Controlled H 2 flow through the bubbler saturated stream; composition depends on H 2 flow rate adjustment through MFC P ressure controller (PC) to keep a fixed pressure in the bubbler and throttles the resulting mixture of H 2 and MO down to the reactor pressure. PC MFC Valve NC Valve NO H 2, N 2 To reactor Bubbler Thermal bath Bubblers Slide 11 Metalorganic compounds Optimal thermal decomposition temperature between 300 and 500C availability of transported reactant at the substrate surface. The vapor pressure of the MO source is an important consideration in MOCVD, since it determines the concentration of source material in the reactor and the deposition rate. Too low a vapor pressure makes it difficult to transport the source into the deposition zone and to achieve reasonable growth rates. Too high a vapor pressure may raise safety concerns if the compound is toxic. Vapor pressures of Metalorganic compounds are calculated in terms of the expression Log(p)=B-A/T Slide 12 Vapor pressure of most common MO compounds CompoundP at 298 K (torr) AB Melt point ( o C) (Al(CH 3 ) 3 ) 2 TMAl14.2278010.4815 Al(C 2 H 5 ) 3 TEAl0.041362510.78-52.5 Ga(CH 3 ) 3 TMGa23818258.50-15.8 Ga(C 2 H 5 ) 3 TEGa4.7925309.19-82.5 In(CH3) 3 TMIn1.7528309.7488 In(C 2 H 5 ) 3 TEIn0.3128158.94-32 Zn(C 2 H 5 ) 2 DEZn8.5321908.28-28 Mg(C 5 H 5 ) 2 Cp2Mg0.05355610.56175 Log(p)=B-A/T Slide 13 Flow rate of MO sources Ideal gas equation MO flux Q MO P MO (T bub ) = equilibrium vapor pressure of the metalorganic component T bub = bubbler temperature Q B = carrier gas flux at standard atmosphere P standard = standard atmosphere P B = regulated bubbler pressure (Rolf Engelhardt, Ph.D. Thesis, TU Berlin, 2000, Slide 14 Partial pressure of MO sources P MO-reactor = partial pressure of the metalorganic components in the reactor P MO (T bub ) = equilibrium vapor pressure of the metalorganic component Q B = carrier gas flux P standard = standard atmosphere P B = regulated bubbler pressure Q tot = total gas flux (Rolf Engelhardt, Ph.D. Thesis, TU Berlin, 2000, Slide 15 MOCVD reactors Different orientations and geometries. Most common: Horizontal reactors: gases inserted laterally with respect to sample standing horizontally on a slowly- rotating (~60RPM) susceptor plate. Vertical reactors: gases enter from top, sample mounted horizontally on a fast-rotating (~500- 1000RPM) susceptor plate. Slide 16 Horizontal reactors Primary vendors: AIXTRON (Germany). The substrate rests on a graphite susceptor heated by RF induction or by IR lamps. Quartz liner tube, generally rectangular Gas flow is horizontal, parallel to the sample. Rotation ~ 60RPM for uniformity by H 2 flux below the sample holder. Slide 17 Horizontal reactors Advantages Common reactor high experience. Uniform epitaxial growth provided the gas velocity is large enough, and attention is paid to hydrodynamic flow. Small height above the wafer the effect of natural convection is minimized. Quite large gas velocity very rapid changes in the gas phase composition. Disadvantages Uniformity can either be achieved by very high gas flow, ( inefficient deposition), or by implementing rotation, which is tricky in this type of design. Throughput: difficult to scale this design up to accommodate large volume production. Slide 18 Planetary reactors Primary vendors: AIXTRON. Derived from horizontal reactor. Material: stainless steel Very widespread now for production, and can achieve very good wafer uniformities. Uniformity: rotation of the main disk + individual satellites. Up to 5X10 wafer capacity (AIX 3000, see photo) Slide 19 Vertical reactors Primary vendors: Veeco (former Emcore (USA)). Gas flow generally normal to the wafer. Temperature gradients buoyancy induced convection high residence time of the gases degradation of heterostructure compositional abruptness. Solution: rotation of susceptor at high angular velocities (centrifugal pumping action to suppress convection and obtain more efficient use of precursors. Simulated streamlines in a vertical spinning cylinder reactor for MOCVD of GaAs from TMGa, AsH 3, H 2. Gases enter at 600K through the top plane and react at the flat top surface of the spinning inside cylinder. The rotation rate is 1000rpm and the deposition surface temperature is 900K ( ) Slide 20 Vertical reactors Features All stainless construction MBE vacuum technology Safety (no glass) Electrical resistance heating Gate valve, and antechamber for minimizing O 2 /H 2 O contamination. Advantages High precursor utilization efficiency Scaling to very large wafers/ multiple wafers. Multiple wafer capacity: Up to 3 x 8", 5 x 6", 12 x 4", and 20 x 3" Disadvantages: Very high speed rotation, up to 1200 rpm. Possible memory effects. Slide 21 Reflectance anisotropy spectroscopy (Reflectance difference spectroscopy) Linear polarized light source directed on the sample. Light is reflected from the sample. The reflection is monochromatized and a spectrum is detected. Only requirement for the system: transparent ambient and a window above the sample. easily fulfilled for MOVPE and MBE Bulk: isotropic signal Surface: reconstruction anisotropy in two directions (with square lattices) RAS signal: normalized change of polarization along two axes. Markus Pristovsek, Ph.D. Thesis, TU Berlin, 2001, http://edocs.tu- Slide 22 Reflectance anisotropy spectroscopy (Reflectance difference spectroscopy) A RAS spectrum can be used to identify a surface, by comparing it to spectra measured on well-ordered reference surfaces with known reconstruction (measured at the same time, e.g., by RHEED in MBE). RAS spectra of a c(4x4) and a 2(2x4) reconstruction on a GaAs (001) surface. Grey spectra are the spectra of a 33% c(4x4) /66% 2(2x4) and 66% c(4x4) /33% 2(2x4). (Markus Pristovsek, Ph.D. Thesis, TU Berlin, 2001, http://edocs.tu- Slide 23 Exhaust system Pump and pressure controller Low pressure growth: mechanic pump and pressure controller control of growth pressure. The pump should be designed to handle large gas load (rotary pump). Waste gas treatment system The treatment of exhaust gas is a matter of safety concern. GaAs and InP: toxic materials like AsH 3 and PH 3. The exhaust gases still contain some not reacted AsH 3 and PH 3, Normally, the toxic gas need to be removed by using chemical scrubber. For GaN system, it is not a problem. AIXTO