The Valved GaP Compound Source VGCS is a high purity P2-source, based on the decomposition of high purity GaP. The concept is derived from our DECO effusion cells to which a mechanical valve mechanism is added for rapid beam flux control.
Full MBE compatibility is reached by the integrated Gallium-Trapping-System, the integrated water cooling as well as the use of high purity inert materials such as pyrolytic boron nitride for all parts in the direct phosphorus path.
In contrast to sources using elemental phosphorus only minor amounts of white phosphorus are formed within the cell and the formation of white phosphorus within the MBE chamber is significantly reduced due to the direct formation of P2 species from the decomposition of single crystal GaP chunks with purity 6N-7N. Operation of the mechanical valve unit shows fast, stable and reproducible flux control. Together with our valve controller an easy handling and integration to your MBE system is provided.
The design of the valve mechanism is schematically illustrated on the right side. It provides a large cross section opening which allows very good pumping of the GaP reservoir. The robust wide angle valve design effectively elimitates the chance of locking, which is a frequently observed problem for needle valves. The VGCS has proven to be a very reliable P2 source in the field and is compatible to commenly used industrial and research MBE systems.
Schematic illustration of the VGCS valve design
The figure on the right hand side shows the beam equivalent pressure (BEP) at the substrate position measured as a function of the valve position. The Motorized Valve Control Unit MVCU is designed for operating the valve of a valved source with a servo motor drive. Manual or remote control with 0-10 V analogue input signal is possible. The display indicates the linear position of the valve from 0 - 7.99 mm from fully closed to fully open position. The servo motor drive has a resolution of 0.01 mm per step and a high motor speed of 10 mm/s. The automatic zero calibration guarantees a highly reliable and reproducible operation of the valve unit.
High purity of GaInP layers grown on GaAs is demonstrated by SIMS measurements (see figure on the left). The contamination of the GaInP layer with oxygen and carbon is below the detection limit of about 5x1016 cm-3 using highest purity single crystal GaP source material. The high performance of the phosphorus source allows growth of high quality GaInP/AlGaInP laser diodes or other P-compounds.
BEP vs. time, with the valve on and off. The P2 pressure can be switched by about two orders of magnitude.
BEP during ramp up of the reservoir temperature from 800 to 1000°C. The flux can be adjusted within minutes, due to the high reservoir temperature.
Comparison with other Elemental Phosphorus Cells
Valved Elemental Phosphorus Cracker
Valved GaP Compound Source (VGCS)
complicated 2-reservoir system
convertion of red P to white P
separate high temperature cracker for P2 forming
overpressure in reservoir possible and boost problem
single reservoir GaP sublimation
natural P2 source
no high temperature cracking needed
efficient Ga trapping
no pressure boost due to recombination of excess P back to GaP in the reservoir
mechanical valve control
instanteous shut off
mechanical valve control
instanteous shut off
red P chunk, 6N-7N purity
crystalline GaP chunk, 6N-7N purity
handling + safety
reload delicate with lots of white P everywhere
very high risk of fire
stable semiconductor crystals
easy to clean and handle
no white P in the reservoir
reloading easy with much lower risk of fire
The VGCS is designed for growth of phosphide compounds in III-V-MBE. It has been readily approved in industrial applications. The fast and reproducible flux control using a valve allows the growth of phosphide-arsenide heterostructures with very sharp interfaces like quantum wells and superlattices. It is perfectly suited for applications in HEMTs, HBTs, GaAlInP laser diodes and other devices. Also GaInP/InP quantum dot lasers have been prepared. Moreover, very good results for pseudomorphic high electron mobility transistors have been achieved. The large crucible size makes the VGCS well suited for MBE research and production systems.
The right picture shows red light emission from a GaAs/GaInP/AlInP laser diode grown and processed at the Max Planck Institute for Solid State Research in Stuttgart
Very sharp transitions of P are achieved by application of VGCS and VACS sources at one deposition run. 300nm GaInP are deposited on a GaAs buffer layer on a (100) GaAs substrate. Due to the fast flux control properties of the valve unit P is sharply reduced on the GaInP/GaAs interface as demonstrated by the SIMS measurement on the right. In return, the As concentration in the GaInP layer is at its detection limit and three orders of magnitude below the GaAs level.
other mounting flange diameters feasible in combination with VADP adapter flanges (on request)
other capacities on request
specify UHV length L with order
e.g. VGCS 100-420-L300D57
is a valved GaP compound souce on DN100 CF flange with 420cm³ crucible and UHV-length 300mm.