Improved Single-Crystal Cathodoluminescent Lamp

Publishing Venue

IBM

Related People

Feigenblatt, RI: AUTHOR [+2][-]

Yang, KH: AUTHOR

Abstract

Single-crystal phosphors like Ce:YAG offer much improved "coulomb lifetime" and thermal performance over conventional powder phosphors. Assuming quench-temperature-limited operation of the phosphor, the limits on flux and brightness are set by thermal resistances. While YAG itself has a large thermal conductivity, N0.13 W/CMŒK, copper is 30 times as good. The best thermal geometry is one in which the electron-bombarded single crystal surface lies as close to the copper heatsink as possible, such as when the phosphor crystal is a thin plate bonded onto a massive copper block and irradiated from above (Fig. 1). The bottom of the garnet is used to remove heat, leaving the top to accept electrons from the beam and emit the cathodoluminescent light.

Improved Single-Crystal Cathodoluminescent Lamp

Single-crystal phosphors like Ce:YAG offer much improved "coulomb lifetime" and thermal performance over conventional powder phosphors. Assuming quench-temperature-limited operation of the phosphor, the limits on flux and brightness are set by thermal resistances. While YAG itself has a large thermal conductivity, N0.13 W/CMOEK, copper is 30 times as good. The best thermal geometry is one in which the electron-bombarded single crystal surface lies as close to the copper heatsink as possible, such as when the phosphor crystal is a thin plate bonded onto a massive copper block and irradiated from above (Fig.
1). The bottom of the garnet is used to remove heat, leaving the top to accept electrons from the beam and emit the cathodoluminescent light. While use of total internal reflection to produce light trapping would in principle lead to greatly enhanced brightness at the edges of the phosphor [*], the reflection losses of the metal substrate would severely limit the aspect ratio which causes this enhancement, and the high aspect ratio of the emitting edge surface would not allow true exploitation of the brightness enhancement in a projection lamp application. (In a projector the lamp is imaged into the projection lens. For a given required flux, a brighter lamp allows a smaller lamp image and therefore a smaller projection lens. But since cylindrical optics are difficult to make, little advantage accrues to a lamp whose increased brightness is obtained merely by contracting its size in one dimension.) The problem with the geometry of Fig. 1 is that the phosphor is an insulator. This limits the potential difference between it and the electron gun cathode and makes the lamp susceptible to manufacturing and operational variations in light output which prevents its simple use as a projection lamp. (Perhaps beam modulation could actively compensate for fluctuations during operation.) The traditional solution to this problem is to coat the electron- bombarded phosphor surface with a thin (N100~) layer of metallization, largely transparent to the electron beam. Here, however, an optically opaque coating of this type would prevent light emission from the bombarded face. To combat this, it is proposed to use a metallization coating which is structured to provide continuous metal paths while at the same time leaving significant top areas of the crystal free o...