The absorption coefficient α for GaAs at room temperature was determined in the spectral range from 1.3 to 1.6 eV by transmission measurements for 10⩽α⩽103 cm−1 and by a Kramers−Kronig analysis of the reflectance for α≳103 cm−1. Measurements were made on high−purity n−type samples, n−type samples with free−electron concentrations from 5×1016 to 6.7×1018 cm−3, p−type samples with free−hole concentrations from 1.5×1016 to 1.6×1019 cm−3, and p−type samples heavily doped with the amphoteric impurity Si. These data show that near the direct energy gap Eg the shape of the α−vs−photon−energy curve is strongly dependent on the impurity concentration.

Standing wave excitation of two-photon fluorescence in solutions or organic molecules is reported. The observations allow the direct display and measurement of optical pulses as short as 1−2 × 10−2 sec.

No abstract

We report the first observation of the modulation doping effect in Si/Ge0.2Si0.8 heterojunctions grown by molecular beam epitaxy. Peak hole mobilities of ∼3300 cm2 V−1 s−1 have been observed at 4.2 K. These values, although nonoptimum, are comparable to the best reported values for holes in Si/SiO2 inversion layers. Low temperature, angular dependent, Shubnikov–de Haas measurements have demonstrated the two-dimensional nature of the hole gas and yield a surface carrier density of 3.5×1011 cm−2. From the temperature dependence of the Shubnikov–de Haas amplitudes a hole effective mass of 0.30±0.02mo has been derived. Identical measurements on n-type heterojunctions having the same Ge content (x=0.2) have failed to show a sustained enhancement of mobility at low temperatures, indicating that ΔEv≫ΔEc.

The refractive indices of GaAs at room temperature were determined from accurate double-beam reflectance measurements. The uncertainty in the refractive indices obtained by this technique is ±0.005. Measurements were made on high-purity n -type samples, n -type samples with free-electron concentrations from 5×1016 to 6.7×1018 cm−3, p -type samples with free-hole concentrations from 1.5×1016 to 1.6×1019 cm−3, and p -type samples heavily doped with the amphoteric impurity Si. These data agree with Marple's prism refractive data for both of his samples of the same impurity concentration. Analysis of the reflectance for the high-purity samples permitted assignment of the room-temperature energy gap as 1.424±0.001 eV. The shape of the refractive-index-vs-energy curve was found to be strongly dependent on the carrier concentration at energies near the direct energy gap.