Wafer holders. Mo- or Ta- made holders Bonding: In (Ga), or In-free (clamped) Quick and easy transfer

Transcription

1 Wafer holders Mo- or Ta- made holders Bonding: In (Ga), or In-free (clamped) Quick and easy transfer Image: In-free, 3-inch sample holder fitting a quarter of a 2- inch wafer

2 Reflection High Energy Electron Diffraction Cells substrate RHEED // substrate Control of the crystallographic structure of the growing epitaxial surface Pattern for 2D surface: series of // lines Si(001) RHEED patterns sputter-cleaned surface perfect surface high density of step rough surface

3 Surface sensitivity of RHEED E k = 5-40KeV λ = [ Å] E K Θ = 1-3 o d(penetration) = λ e sin θ Λ e 10ML d = 0.5ML Surface sensitivity

4 Diffraction: : 3D vs 2D 3D 2D (1st layer of perfectly flat surface) ΔK = G G // rods a=5.65å G=2π/a=1.1 Å -1 E k =5KeV k 1/λ=36.5Å -1 k >> G

5 Ideal RHEED Pattern Ewald sphere Projected image on screen Sample Perfect 2D crystalline surface Perfectly monochromatic, collimated beam Intersection of Ewald sphere with G vectors Ideal pattern: series of points on a half circle (for each nthorder Laue zone)

6 Diffraction in real case Thermal vibrations, lattice imperfections finite thickness of reciprocal lattice rods Divergence and dispersion of e-beam finite thickness of Ewald sphere Diffraction spots streaks with modulated intensity even for 2D surfaces

7 Growth dynamics: : RHEED Oscillations RHEED maximum spot intensity indicate completion of growing layers layer-by-layer control of the growing crystal surface # of deposited atomic layers = # of maxima growth rate = 1ML / τ

8 Doping in III-V materials υ Doping necessary for carrier transport in electronic or optoelectronic devices Doping by group II (p-type), IV (p- or n- type) and VI atoms (n-type) Group II II-b atoms (Zn, Cd): too high vapour pressure at usual growth temperatures not used in MBE II-a atoms (Be in particular): the universal choice Group-VI atoms uncommon (surface segregation, re-evaporation)

9 Doping for GaAs Si cm -3 Be cm -3 Sn high surface segregation C operating at 2000 o C Ge difficult to control

10 Metalorganic Chemical Vapor Deposition (MOCVD)

11 Metalorganic Chemical Vapor Deposition (MOCVD) 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.

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

13 Schematics of a MOCVD system Carrier gas Materia l sources Gas handlin g In-situ systemdiagnostics NO electron beam probes! Reflectance Ellipsometry RAS Exhaust system Safety system Reactor

14 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.

15 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 ~ mbar Mass flow controllers

16 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(tertiarybutilarsene), TBP).

17 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 (~ RPM) susceptor plate.

18 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.

19 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:

20 Safety issues Concerns: Flammable gases (H 2 ) Toxic gases (AsH 3, PH 3 ) 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 N 2 carrier TBAs, TBP (toxic but liquid low vapor pressure)

21 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 (Partially decomposed) precursor diffusion to the surface reaction to form the desired material. [Ga(CH3)3]gas+[AsH3] gas ->[GaAs]solid +[3CH3] gas Simultaneous desorption of reaction products (hydrocarbons), surface diffusion of material to lattice sites.

22 MBE versus MOCVD growth rate MBE T cell P v (T) Ballistic transport Sticking coefficient = 1 r = r (T) Flow rate f (total flow F, total pressure P, vapor pressure P v ) MOCVD r = r (F, P, P v, mass transport, (reaction kinetics) Diffusive mass transport Chemical reaction kinetics

23 Comparison of MBE and MOCVD Feature Source materials Evaporation MBE Elemental Thermal, e-beam MOCVD Gas-liquid compounds Vapor pressure, Carrier gas Flux control Switching Environment Molecular transport Surface reactions Cell temperature Mechanical shutters UHV Ballistic (mol. beams) Physi-chemisorption Mass flow controllers Valves H 2 -N 2 ( mbar) Diffusive Chemical reactions

24 Hybrid techniques Gas source MBE, Metal Organic Molecular Beam Epitaxy (MOMBE), Chemical Beam Epitaxy (CBE). Principle: using group V or/and group III gas sources in a UHV MBE environment. Developed in order to combine advantages (but also disadvantages!) of MBE and MOCVD.