  This paper is the first report describing how the concentrations of silicon
and boron govern the cryogenic scintillation properties of n-type GaAs. It
shows that valence band holes are promptly trapped on radiative centers and
then combine radiatively with silicon donor band electrons at rates that
increase with the density of free carriers. It also presents the range of
silicon and boron concentrations needed for efficient light emission under
X-ray excitation, which along with its low band gap and apparent absence of
afterglow, make scintillating GaAs suitable for the detection of rare,
low-energy electronic excitations from interacting dark matter particles. A
total of 29 samples from four different suppliers were studied. Luminosities
and timing responses were measured for the four principal emission bands
centered at 860, 930, 1070, and 1335 nm, and for the total emissions.
Excitation pulses of 40 kVp X-rays were provided by a light-excited X-ray tube
driven by an ultra-fast laser. Scintillation emissions from 800 to 1350 nm were
measured using an InGaAs photomultiplier. Within the concentration ranges of
free carriers from 2 x 10^16/cm3 to 6 x 10^17/cm3 and boron from 1.5 x
10^18/cm3 to 6 x 10^18/cm3, nine samples have luminosities > 70 photons/keV and
two have luminosities > 110 photons/keV. Other samples in that range have lower
luminosities due to higher concentrations of non-radiative centers. The decay
times decrease by typically a factor of ten with increasing free carrier
concentrations from 10^17/cm3 to 2 x 10^18/cm3.
