Some characteristics of electron-beam pumped GaAs lasers

The threshold current density of an electron-beam pumped GaAs laser is sensitively dependent upon the profile of the electron beam. Minimum power threshold current density is obtained with the excitation far above threshold value at the center of the Fabry-Perot cavity and with vanishingly small values at the ends. Threshold current density is also dependent upon beam voltage. For voltages of the order of 30 kV, the peak of the distribution of minority carriers (including the effects of diffusion and surface recombination) is of the order of one diffusion length from the surface, suggesting that nonradiative surface recombination plays an important role. This has been confirmed by measurements on samples with a Schottky barrier. Threshold current density of
n
-type GaAs at 77°K (40-mil Fabry-Perot cavity, 30-kV beam voltage) decreases from 0.69 A/cm
²
at
4 \times 10^{17}
/cm
³
to 0.44 A/cm
²
at
4 \times 10^{18}
/cm
³
. Power output over the same doping range peaks at about
2 \times 10^{18}
/cm
³
with 20 watts obtained at an overall efficiency of 2.5 percent. Peak differential external quantum efficiency of the order of 30 percent occurs at about the same doping density. Using Hunsperger and Ballantyne's values of α, we deduce that the internal quantum efficiency may be as high as 90 percent.