PART III: METALORGANIC CHEMICAL VAPOR DEPOSITION

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

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 (H2, N2) 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.

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: theoryand practice (Academic Press, Boston, 1989).

4. Gas handling Glove box Reactor MOCVD Facility, horizontal reactor • Research system (left): AIX 200 • 1X2” wafer capacity • Production system (right): AIX 2600 • Up to 5X10” wafer capacity (AIX 3000)

5. Gas handling system Exhaust system Schematics of a MOCVD system Material sources Carrier gas In-situ diagnostics NO electron beam probes! • Reflectance • Ellipsometry • RAS Reactor Safety system

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.

7. Carrier gas • “Inert” carrier gas constitutes about 90 % of the gas phase  stringent purity requirements. • H2 traditionally used, simple to purify by being passed through a palladium foil heated to 400 °C. Problem: H2 is highly explosive in contact with O2 high safety costs. • Alternative precursor : N2: 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

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 (AsH3; PH3 flammable as well); alternative: alkyls (TBAs, TBP).

9. Hidrides and dopants • Form: gases from high pressure cylinders • Mixed into the carrier gas line • Flow control: valve + mass flow controller (MFC)

10. Bubblers PC MFC Valve NC Valve NO Metalorganics • Liquid (or finely divided solid – TMIn) contained in a stainess steel bubbler. • Vapor pressure fixed by constant temperature in a thermal bath; T ≈ -20oC ÷ 40oC; DT = ±1oC. • Controlled H2 flow through the bubbler  saturated stream; composition depends on H2 flow rate  adjustment through MFC • P ressure controller (PC) to keep a fixed pressure in the bubbler and throttles the resulting mixture of H2 and MO down to the reactor pressure. H2, N2 To reactor Bubbler Thermal bath To reactor

11. Metalorganic compounds • Optimal thermal decomposition temperature between 300 and 500°C  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

12. Vapor pressure of most common MO compounds Log(p)=B-A/T

13. Flow rate of MO sources Ideal gas equation  MO flux QMO • PMO(Tbub) = equilibrium vapor pressure of the metalorganic component • Tbub = bubbler temperature • QB = carrier gas flux at standard atmosphere • Pstandard = standard atmosphere • PB = regulated bubbler pressure (Rolf Engelhardt, Ph.D. Thesis, TU Berlin,2000,http://edocs.tu-berlin.de/diss/2000/engelhardt_rolf.pdf)

14. Partial pressure of MO sources • PMO-reactor = partial pressure of the metalorganic components in the reactor • PMO(Tbub) = equilibrium vapor pressure of the metalorganic component • QB = carrier gas flux • Pstandard = standard atmosphere • PB = regulated bubbler pressure • Qtot = total gas flux (Rolf Engelhardt, Ph.D. Thesis, TU Berlin,2000,http://edocs.tu-berlin.de/diss/2000/engelhardt_rolf.pdf)

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.

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 H2 flux below the sample holder.

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.

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)

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, AsH3, H2. 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 (http://www.cs.sandia.gov/CRF/MPSalsa/ )

20. Vertical reactors • Features • All stainless construction • MBE vacuum technology • Safety (no glass) • Electrical resistance heating • Gate valve, and antechamber forminimizing O2/H2O 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.

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-berlin.de/diss/2000/pristovsek_markus.pdf)

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 ß2(2x4) reconstruction on a GaAs (001) surface. Grey spectraare the spectra of a 33%c(4x4) /66%ß2(2x4) and 66%c(4x4) /33%ß2(2x4). (Markus Pristovsek, Ph.D. Thesis, TU Berlin,2001, http://edocs.tu-berlin.de/diss/2000/pristovsek_markus.pdf)

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 AsH3 and PH3. The exhaust gases still contain some not reacted AsH3 and PH3, Normally, the toxic gas need to be removed by using chemical scrubber. • For GaN system, it is not a problem. AIXTOX system

24. Safety issues • Concerns: • Flammable gases (H2) • Toxic gases (AsH3, PH3) • Safety measures: • Lab underpressurization. • Design of hydrides cylinders. • Extensive gas monitoring systems placed in different locations, able to detect the presence of gas as small as parts per billion. • Alarms located in different parts of the buildings + beeper calls to operators. • Immediate shut down of the system to a failsafe condition in case of leakages and other severe failures. • Alternatives: use of alternate gases • N2 carrier • TBAs, TBP (toxic but liquid  low vaopr pressure)

25. Analysis of the MOCVD growth process

26. MBE versus MOCVD growth rate MBE Ballistictransport Sticking coefficient = 1 Tcell Pv(T) r = r (T) MOCVD Flow rate f (total flow F, total pressure P, vapor pressure Pv) r = r (F, P, Pv, mass transport, reaction kinetics) Diffusive masstransport Chemical reaction kinetics

27. Growth steps in (MO)CVD • Flow of reactant (precursors) to reactor tube, either by: • Mixing in gas handling manifold, then enter the reactor • Separate until the reactor (no premature side reactions) • In the reactor: establishment of gas layers governing transport of mass, energy and momentum: entry effects and possibly achievement of steady-state condition. At the same time: chemical reactions  homogeneous, heterogeneous (parasitic deposit)  reduction of reactant concentration, shift in alloy composition, reduced growth rate, epitaxial surface roughening. (Partially decomposed) precursor diffusion to the surface  reaction to form the desired material. Simultaneous desorption of reaction products (hydrocarbons), surface diffusion of material to lattice sites. 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).

28. Reactive-flow conservation equations(Crosslight Procom User’s manual) The state of the gas phase in a reactor can be completely described by the continuum mass density r, the individual chemical species number density ni, the momentum density rv, and the energy density E. The basic partial differential conservationequations are: total mass (continuity equation) individual species (precursors,intermediate species…) momentum (Navier-Stokes equation) energy (heat conduction equation)

29. Simplified model of (MO)CVD reaction kinetics • Simplified deposition process of a film, starting from a molecule AB in the gas phase(L. Vescan, in Handbook of thin film process technology, edited by D. A. Glocker and S. I. Shah (Institute of Physics Publishing, Bristol, 1995), p. B1.4:1)AB(g)  A(s) + B(g) • J1: molecular flux from the gas phase to the substrate surface,J2: consumption flux of AB corresponding to the surface reaction:J1≈hG (CG – CS) (~supersaturation)J2≈kSCSwithhG = gas diffusion rate constant,CG = gas-phase concentration of AB,CS = surface concentration of AB,kS = heterogeneous rate constant J1 J2

30. Simplified model of (MO)CVD reaction kinetics • Steady-state conditions: • Growth rate r = J0 (with 0 = unit volume of the crystal)  r  mole fraction of the species AB in the gas phase, and determined by the smaller of the rate constants hG, kS. • Limiting cases:r≈ kS CGW0 surface kinetics controlr≈ hG CGW0 mass transport control

31. Input gas Boundary Interface Solid phase layer Reaction kinetics Chemical potential Mass transport Reaction coordinate Interpretation in terms of supersaturation • Driving force: supersaturation (chemical potential difference between gas phase and solid)  out-of-equilibrium process; equilibrium at the vapor-solid interface • The relative importance of surface kinetics and mass transport can be interpreted as a function of the chemical potential dependence on the reaction coordinate. If most of the chemical potential drop occurs in the boundary layer (red line), the growth is controlled by mass transport; if it occurs at the interface (green line), the growth is kinetically limited 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).

32. Mass transport • Fundamental and very complex aspect in reactor design • Factors influencing gas flow in a reactor: • temperature • concentration and momentum gradients • gravity ( convection) • homogeneous, heterogeneous chemical reactions ( parasitic nucleation) • Simplified (2 regions) picture in a horizontal reactor: • Upper region: turbulence or vorticity good mixing and heat transfer • Close to the susceptor: region of laminar flow (boundary or stagnant layer)  molecular diffusionaltransport to the hotsubstrate, where thetransverse velocity is zero.

33. Mass transport • Assuming a gas velocity U = U in the bulk gasphase, and U = 0 at the growth surface calculation of boundary layer width (D. W. Kisker and T. F. Kuech, in Handbook of crystal growth, edited by D. T. J. Hurle (Elsevier Science, Amsterdam, 1994), Vol. 3, p. 93)d ~ (PU)-1/2, where P is the total reactor pressure. • If the molecular transport in the boundary layer proceeds by diffusion alone, the rate constant hG can be written aswhere D ~ P-1 is the diffusion coefficient •  mass-transport-limited growth ratewhere CG~ pAB= AB partial pressure • growth rate is practically independent of the growth temperature, and depends linearly on the species partial pressure.

34. Reaction kinetics • Two kinds of thermally-activated reactions • Reactions in the gas phase (homogeneous reactions) • Reactions at the surface (heterogeneous reactions) • Forward and reverse rates are characterized by rate constants that can be expressed in an Arrhenius form:k = A exp (-E/kBT),where E is the activation energy for the process. • Surface kinetics are poorly known processes, in which a number of sub-processes can be identified. Among them: • adsorption of reactant species, • heterogeneous decomposition reactions, • surface migration, • incorporation and desorption of products.

35. Reaction kinetics • In the most simplified picture, the chemistry of heterogeneous reactions can be modeled by taking into account only adsorption and desorption: • where  is a vacant surface site, A is an adsorbed state,kads and kdes are the adsorption and desorption rate constants • Assumptions: no interaction between absorbed species; equivalence among all the adsorption sites. G. B. Stringfellow, Organometallic vapor phase epitaxy: theoryand practice (Academic Press, Boston, 1989).

36. Reaction kinetics • Steady state (adsorption rate = desorption rate):adsorption coefficient with Q = fraction of occupied lattice sites • Q assumes the form of a Langmuir isotherm: G. B. Stringfellow, Organometallic vapor phase epitaxy: theoryand practice (Academic Press, Boston, 1989).

37. Reaction kinetics • MOCVD of binary compound semiconductors: two molecules AB and CD are transported to the surface, and are adsorbed on cation and anion sites, respectively. • For this noncompetitive process, the growth rate of the bimolecular reaction is proportional to the anion and cation surface coverages (Langmuir-Hinshelwood isotherm): • III-V semiconductors:tipically V/III ratio ~ 100  QAB << 1; QCD ≈ 1 •  r  K’ pAB,with K’ a typical rate constant for the process, temperature-dependent. •  growth rate depends only on temperature and on the group-III precursor partial pressure, and not on the group-V one.

38. Reaction kinetics for GaAs Overall reactions: • TMGa + AsH3: AsH3 + Ga (CH3)3→ GaAs + 3CH4↑ • TEGa + AsH3: AsH3 + Ga (C2H5)3→ GaAs + 3C2H4↑ + 3H2 ↑ Lower activation energies for decomposition for TEGa than for TMGa  ~200K lower temperature for 50% decomposition. (Markus Pristovsek, Ph.D. Thesis, TU Berlin,2001, http://edocs.tu-berlin.de/diss/2000/pristovsek_markus.pdf)

39. Reaction kinetics for GaAs Overall reactions: • TMGa + AsH3: AsH3 + Ga (CH3)3→ GaAs + 3CH4↑ • TEGa + AsH3: AsH3 + Ga (C2H5)3→ GaAs + 3C2H4↑ + 3H2 ↑ TMGa decomposition is strongly enhanced at the onset of the AsH3 de-composition. This is most likely due to hydrogen radicals produced by AsH3 decomposition. (Markus Pristovsek, Ph.D. Thesis, TU Berlin,2001, http://edocs.tu-berlin.de/diss/2000/pristovsek_markus.pdf)

40. Reaction kinetics for GaAs Proposed mechanisms (TMGa + AsH3): Complex series of decomposition steps in the gas phase and on the surface, each with its own characteristic reaction constant and activation energy. K. F. Jensen, Adv. Chem. Ser. 245, 397 (1995)

41. Growth modes in MOCVD

42. Growth mode: studies on GaAs from TMGa and AsH3 Effect of substrate temperature • Studies for atmospheric pressure (AP = 105Pa = 1000mbar) and for low pressure (LP = 104Pa = 100mbar), and different surface orientations. • Three regimes: • Low T: kinetically limited growth strong T dependence, low P dependence (r  K’ pTMGa), with K’ dependent on T. • Mid T: mass transport-limitedgrowth r does not dependappreciably on T and surfaceorientation, but increases withdecreasing P (rpTMGa P-1/2). • High T: increasingly low growthrates, probably due to homogeneousreactions in the gas phase, causinga depletion of reactants, or surfacere-evaporation. G. B. Stringfellow, Organometallic vapor phase epitaxy: theoryand practice (Academic Press, Boston, 1989).

43. Growth mode: studies on GaAs from TMGa and AsH3 Effect of reactor pressure • Studies for T = 650°C and V/III ratio ≈ 100 • Two regimes: • P > 100mbar, growth is limited by mass transport, and r ~ P-1/2 • After a transition region, at P < 20mbar, the growth rate becomes independent on P, and growth becomes kinetically limited. G. B. Stringfellow, Organometallic vapor phase epitaxy: theoryand practice (Academic Press, Boston, 1989).

44. 500°C 700°C 1000°C Growth mode: studies on GaAs from TMGa and AsH3 Effect of TMGa partial pressure • Studies for different T and substrate orientations • Three regimes: • T = 700oC: rpTMGa at all TMGa pressures and substrate orientations (mass transport limited) • T = 500oC: r saturates for high TMGa pressures and depends on orientation (kinetically limited). Evidence for (orientation-dependent) incomplete AsH3 decomposition (with TMGa completely pyrolized). • T = 1000oC: decreased growth rate: gas-phase reactions ( reduction of gas-phase nutrients) and surface desorption ( orientation dependence) 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).