More than 90% of the photons emitted from forward-biased GaAs diodes have energies hv higher than the applied voltage V. Thus, a portion of the energy of these photons must come from lattice heat. This portion is 3% of hv for photons at the peak of the incoherent emission spectrum. An upper limit of (hv-qV) is estimated from thermodynamics. This difference is larger at 78 than 27°K, in agreement with theory. Also, the dependence of the effect on voltage and current is in fair agreement with expectations. At high forward currents, near the threshold for stimulated emission, hv is about equal to qV. The removal of heat, in the form of photon energy from the crystal, should in principle lead to refrigeration. The main requirement for net cooling is a quantum efficiency (photons/electron) higher than 0.97. At 10 mA, with an assumed quantum efficiency of 0.99, the heat removal rate (per diode) is estimated as 3X10 -4 W compared to a Joule heating rateof 5X10-5 W.

We use the surface forces apparatus to investigate the transmission of a Fabry-PCrot interferometer consisting of a thin dye layer (a few nanometers Rhodamine B) adsorbed from solution and sandwiched between two mica sheets of equal thickness. After adding the dye, we consider not only the shift of the resonance wavelength but also the height and width of the transmission peak; thus, we can deduce the cavity parameters. Knowing the cavity parameters at a resonance wavelength, we can calculate the complex dielectric constant of the dye layer with simple approximate formulas, as we demonstrate by comparison to calculations based on the more complete matrix formulation. The absorption spectrum of a few molecular layers was measured, and based on the integrated absorption, the dye concentration in this thin layer was determined. The spectrum shows a pronounced shift compared to monomers in solution, suggesting that many dye molecules at the interface are arranged as dimers. We are now in the position to investigate the interaction of dye molecules within the surface forces apparatus.

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A method was developed for depositing silicon films by the pyrolytic decomposition of SiH4 on single crystal sapphire. Electron dif€raction and Laue reflection examinations of the films shows singlecrystal patterns. The silicon 6lm has a H a l l mobility of 135 cmt/volt-second at a hole density of 1017/cma. Insulated-gate field-effect transistors with a transconductance of l o 0 0 pmho at 5 ma were made with dimensions of 10 p source-to-drain spacing and an active distance of 120 p. This value compares favorably with similar units made on bulk silicon and is very encouraging for the possibilities of thin-film silicon devices.

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