Telephone conversation between I. Ben-Zvi (BNL Collider/Accelerator division) and R.T Jones (Connecticut), G. Yang (Glasgow), August 18, 2006

Notes taken by R.T. Jones, August 21, 2006

1. The requirements for high-gain diamond photocathode amplifiers (DPA) are similar to those for diamond tracking detectors (RD42).
   • large area monocrystals
   • good experience with diamonds grown by the chemical-vapour deposition technique (CVD), manufacturer Element Six (formerly Drukkers, research arm of DeBeers), represented in the U.S. by Harris International (http://www.harrisinternational.com).
   • CVD technique enables production of crystals with very low (ppm) levels of typical impurities (boron, nitrogen, hydrogen, lithium, sodium, phosphorous) -- type IIa diamonds.
   • hydrogen is the most problematic because it is a dominant residual gas in vacuum systems, such as inside the CVD apparatus.
   • driving consideration is conduction electron mobility
   • point defects are a particular concern because they act as traps for electrons and produce internal electron fields that block current
   • mosaic spread is not a particular concern, ie. distributed strains that result from the CVD growth process are not a problem if it does not reduce the electron mobility or produce traps.
   • mosaic spread of these CVD diamonds may not be very close to ideal, but the BNL team has not attempted to measure it.
2. The requirements for coherent bremsstrahlung radiators (CBR) are different from those for DPA crystals, and more similar to the requirements for diamond monochomators.
   • large area monocrystals
   • good experience with diamonds grown from a single seed by the high pressure, high temperature technique (HPHT), manufacturer Element Six (formerly Drukkers, research arm of DeBeers), obtained through an internal contact within Glasgow University.
   • HPHT technique capable of producing crystal wafers of thickness several hundred microns and ~100 mm² area, with near-perfect mosaic spread.
   • mosaic spread is the driving concern, impurity levels at the level 200 ppm are tolerable -- type Ib diamonds.
   • electron mobility in CBR diamonds is probably not very close to ideal because of the impurity levels -- nitrogen is a N-type impurity -- but CBR groups have not measure it, to our knownledge.
3. Both HPHT diamonds and CVD monocrystal diamonds are grown from a single-crystal diamon substrate (or seed).
4. Removal of a large-area slice of grown diamond from the substrate produces a wafer no thinner than 100 - 300 microns thickness.
5. The DPA crystals needed by the Brookhaven group are of optimum thickness 30 microns, which requires that the original crystals be thinned by some process.
   • first attempt to thin by grinding (using diamond grit) failed with the sample breaking.
   • the firm Delaware Diamond Knives (DDK) is now working with the BNL group to perfect the diamond thin-film grinding process, and there is reasonable confidence that they will succeed.
   • the BNL group is considering a second technique, involving ion bombardment, either in a pure Argon plasma or in a Argon plasma with a few-percent admixture of hydrogen.
   • depending on the outcome of the DDK project, the BNL group plans to move ahead with more or less speed on experimenting with ion bombardment thinning.
6. The Brookhaven and Jefferson Lab applications both require a reliable means to thin diamond wafers from thicknesses of order 100 microns to 20-30 microns in a way which maintains a good uniformity of thickness across the surface and does not introduce significant impurities. In the case of CBR diamonds, it is also important that the thinning process not introduce significant crystaline stress that might deform the shape of the crystal under conditions of reduced inherent rigidity.
7. There is considerable common interest between BNL and Jlab groups working on the manufacturing of thin diamond monocrystals. R.T. Jones and G. Yang will propose dates for a possible visit to the BNL laboratory of I. Ben-Zvi where these development is being carried out.