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Ultraviolet light-emitting diodes operating at 280 nm, grown by gas source molecular-beam epitaxy with ammonia, are described. The device is composed of n- and p-type superlattices of AlN(1.2 nm thick)/AlGaInN(0.5 nm thick) doped with Si and Mg, respectively. With these superlattices, and despite the high average Al content, we obtain hole concentrations of (0.7-1.1) × 10[sup 18] cm[sup -3], with the mobility of 3-4 cm²/V s and electron concentrations of 3 × 10[sup 19] cm[sup -3], with the mobility of 10-20 cm²/V s, at room temperature. These carrier concentrations are sufficient to form effective p-n junctions needed in UV light sources. [ABSTRACT FROM AUTHOR]

With the help of a light-emitting diode with a radiation maximum at 407 nm and an S-2000 UV-VIS spectrometer connected with a computer, the spectral changes in the fluorescence of chlorophyll from acacia leaves ( Acacia sp.) preliminarily subjected to a dark adaptation are studied. It is found that, in the slow induction phase, the Kautsky effect manifests itself in the “compression” of the intensity of the chlorophyll fluorescence spectrum, with this spectrum exponentially approaching the steady-state shape with a time constant of 10–20 s. Once the steady-state fluorescence spectrum of chlorophyll was established, the amount of energy delivered to the upper singlet level is one and a half times greater than at 735 nm. The ratio between the energies spent for photochemical processes of photosynthesis and for fluorescence depends on the wavelength and the instant of time of the induction period. In the steady-state state of the chlorophyll fluorescence, the values of this ratio at 685 and 735 nm are equal to 5.9 and 3.4, respectively. [ABSTRACT FROM AUTHOR]

Low contact resistance to digital alloys of n-type AlGaN/AlN with high average AI concentration is described. Low-energy electron diffraction was used to evaluate surface precleaning with HCI and buffered HF. The contact metallization consisting of a stack of Ti/Al/Ti/Au, 20/100/45/60 nm in thickness, was e-beam deposited and etch-patterned. The lowest specific contact resistance of .5.6 ⊗ 10-5 Ω · cm² was obtained after annealing in N2 ambient at 700 °C. [ABSTRACT FROM AUTHOR]

High-quality short-period superlattices of AlN/Al0.08Ga0.92N have been grown by gas-source molecular-beam epitaxy with ammonia on Al face of AlN (0001) substrates. A significant reduction was achieved in the dislocation density, down to 3×108 cm-2. Complete removal of residual Al2O3 surface oxide is needed in order to obtain low dislocation density in homoepitaxy on AlN. We show that the presence of Al2O3 islands with the surface coverage as low as 0.2% results in increased dislocation density. [ABSTRACT FROM AUTHOR]

Ultraviolet light-emitting diodes grown on Si(111) by gas-source molecular-beam epitaxy with ammonia are described. The layers are composed of superlattices of A1GaN/GaN and A1N/A1GalnN. The layers are doped n and p type with Si and Mg, respectively. Hole concentration of 4 x 10[sub 17] cm[sup -3], with a mobility of 8 cm[sub 2]/Vs, is measured in Al[sub 0.4]Ga[sub 0.]6N/GaN. We demonstrate effective n- and p-type doping of structures based on A1N/A1GalnN. Light-emitting diodes based on these structures show light emission between 290 and 334 nm. [ABSTRACT FROM AUTHOR]