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

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)

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

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

Operation Temperatures High Temp. Evaporators Goes up to 1800-2000 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

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

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.

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

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

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

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

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

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

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

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

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

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

Organic Material Effusion Cell Manufacturer: MBE-Komponenten GmbH Model #: OME 40-10-35-5-2103841 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

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

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

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.