1. MBE Source Cells Types of cells by Our Cells Manufacturers
Temperature ranges
Material compatibility
Operation principle
Our Cells
Manufacturers

2. Introduction
Source cells are used to generate input molecules for deposition
Needs to produce accurate and constant flux, without much transient
Material of the cell must be compatible with the environment (LV, HV, UHV), as well as with ambient species (O2, NH3)

3. Types of sources Effusion Cells Plasma Sources Cracking Sources
Gaseous Sources
E-beam Sources

4. Solid Effusion Cells Also known as Knudson Cells
Used for all elemental sources
Crucible is heated (from resistively heated wires) to evaporate molecules from the source material
Highly controllable and efficient for constant flux– thermocouples in closed feedback loops ensures stability and reproducibility.
Includes a wide range of cells with different operation temperatures and filament configurations (next few slides)
A single filament effusion cell (In, Ga and Al) used by Yoon’s group in NTU

5. Operation Temperatures
High Temp. Evaporators
Goes up to C
Used for low vapor pressure species such as (Ga)
Low Temp. Evaporators
Up to 800 C
Used for high vapor pressure species such as As, P, Se, S.
Oxygen and Ammonia resistant cells are available in the market

6. Crucibles
Usually made of pyrolytic Boron Nitride (PBN ) – high electrical resistance, good thermal conductivity, chemical inertness, low outgassing
Other materials include pyrolytic graphite, quartz, Al2O3, BeO, MgO, W and Ta (special and high temperature operations)
Crucibles may be double walled

7. Crucibles Continued
A common shape is a tapered cone to allow for a wide angled flux distribution while using only small amounts of material.
Straight-walled crucibles
SUMO crucibles (from Veeco) has a cylindrical shape for even distribution of materials within the crucible, and a small tapered orifice to achieve the wide flux distribution.

8. Filament Usually comprises of Ta wire heaters
Single filament and double filament configuration
“hot-lip” and “cold-lip” type operation
For high vapor pressure materials, it is beneficial to have non-conductive guides to limit the flux geometry

9. Lip temperature
Single filament tips are generally cheaper and easier to use
In double filament tips, two independent heaters are used to produce the desired temperature gradient
Hot tip – prevents recondensation of the material; good for species such as Ga and Zn, which are known to deposit at the tip
Cold tip – reduces upward drift of materials such as Al. The cold tip allows the material to condensate and remain stationary

10. RF Plasma Sources
RF nitrogen plasma source
A gas plasma made by RF is used to convert H2, O2, or N2 to active species for MBE growth.
Used to grow GaN, As/N materials, and doping processes with nitrogen and oxygen
The source needs to be tuned periodically to ensure that the plasma is stable.
Fast growth rate (up to 6μm/hr with GaN), while retaining similar film properties

11. Valved sources
Normal evaporators rely on temperature to change beam fluxes
For materials with high vapor pressures, small temperature fluctuations can result in large deposit rate changes
Valved evaporation sources maintain a constant temperature, and uses calibrated valve positions to control the flux
The beam is more stable and consistent

12. Valved Arsenic Cracker
Cracking sources
For some materials, it is beneficial to crack the source before entering the beam
Ex. For As, dimers produce better films than tetramers.
As is stored as tetramers, and is cracked before it beams off
Cracking is achieved by interactions between tetramers (ex for As) with cracking surfaces at high temperatures -> higher temp, higher percentage dimers
Valved Arsenic Cracker

13. Atomic Hydrogen Source
This design is by Omicron
Replaces the crucible with a tungsten capillary, and the cracking takes place on the H-W surface.
The cooling shroud reduces outgassing by blocking off the W tube

14. Gas Source MBE
Gas injectors are designed for gas source thin film depositions such as Silicon epitaxy, nitride MBE, or laser MBE.
Operational principle is gas cracking with a filament furnace
An integrated gas delivery system is usually implemented to precisely control the source output

15. Gas sources Continued Example of an ozone delivery system
Ozone needs to be generated on site from an oxygen supply
The ozone supply needs to be charged before each application

16. E-beam sources
Electron beam supplies power used for electron emission and flux monitoring
The source material is held at a positive potential, thus attracting electrons from the filament and heated.
Deposits C, Si, and refractory metals

17. Our sources: Omicron E-beam (X2) (in use)
Molecular Evaporator (in use)
OME Cell
Dual Filament Evaporator

18. Organic Material Effusion Cell
Manufacturer: MBE-Komponenten GmbH
Model #: OME
Flange Size: DN40CF
Heating element: Ta wire heater
Operation temperature: 15 – 350 °C
Bakeout temperature: 250 °C
Thermocouple: NiAl/NiCr (type K)
Connections: 2 X 1/8 BSP, Omega Sub-miniature
Cooling: water-cooled, flow >0.5 l/min

19. Dual Filament Cell Make: home-built Connections: (omega miniature) X2,
Flange size: DN40

20. Manufacturers
Veeco – leading manufacturer of MBE systems and components among other thin film technologies. Based in Plainview, NY.
SVT associates – PLD, ALD, and MBE systems and components. Based in Eden Prairie, MN
MBE-Komponenten GmbH – MBE sources and components. Based in Germany
Omicron NanoTechnology GmbH (Oxford Instruments) – Nanoprobing and thin film systems. Based in Germany

21. Summary
Sources supply the flux required for deposition -> looking for stable, controllable beams
Different types of sources are available for different materials -> main factors are vapor pressure, condensation, cracking requirements and phase of source material
Our sources include: 2 Omicron e-beams, a molecular evap, a Knudson Cell (Organic), and a home-built dual filament evap.