Hongjia He

Effect of acceptor on efficiencies of exciplex-type organic light emitting diodes

The relationship between the electroluminescent (EL) efficiencies of interfacial exciplex emission and the lowest unoccupied molecular orbital (LUMO) of the acceptors systematically investigated. A nearly linear relationship was observed between LUMOs of acceptors and exciplex efficiency for a fixed donor of 4,4′,4″-tris[3-methyl-phenyl(phenyl)amino]-triphenylamine in exciplex-type devices. This result indicates that the lower the LUMO of the acceptor is, the higher the EL efficiency of the exciplex is. The effect of acceptor on the efficiencies of exciplex-type devices is attributable to the interactions between the donor and acceptor molecules, which were closely related to the matched LUMOs and intermolecular conformations overlap between donor and acceptor molecules.

Broad wavelength modulating and design of organic white diode based on lighting by using exciplex emission from mixed acceptors

Modulating electroluminescent (EL) spectra from interfacial exciplex emissions were observed by varying the ratios of two acceptors of exciplex-type devices in which the emissive wavelengths were tuned from 530to656nm. In the devices 4,4′,4″-tris[3-methyl-pheny(phenyl)amino]triphenylamine and (bathocuproine: scdolinium-(dibenzoyl-methane)3bathophenyl-phenathroline) mixtures were used as donor and acceptor materials, respectively. In terms of the exciplex broad band emission a white organic light emitting diode was demonstrated by skillfully designing the structure when blue subband was subjoined in the white spectrum. The white device behaves the Commission Internationale de l’Eclairage coordinates of (0.32, 0.35) with higher color stability at various biases, a color rendering index of 90.4, and a maximum luminance of 425cd∕m2, respectively, although the EL efficiency needs to be further improved. The emission mechanism of the broad exciplex band formed by two mixed acceptors was also discussed.

Stoichiometric defects in semi-insulating GaAs

The influences of arsenic interstitials and dislocations on the lattice parameters of undoped semi-insulating (SI) GaAs single crystals were analyzed. It was shown that the dislocations in such crystals serve as effective gettering sites for arsenic interstitials due to the deformation energy of dislocations. The average excess arsenic in GaAs epilayers grown by molecular-beam epitaxy (MBE) at low temperatures (LT) is about 1%, and the lattice parameters of these epilayers are larger than those of liquid-encapsulated Czochralski-grown (LEC) SI-GaAs by about 0.1%. The atomic ratio, [As]/([Ga] + [As]), in SI-GaAs grown by low-pressure (LP) LEC is the nearest to the strict stoichiometry compared with those grown by high-pressure (HP) LEC and vertical gradient freeze (VGF). After multiple wafer annealing (MWA), the crystals grown by HPLEC become closer to be strictly stoichiometric.

Nondestructive measurements of stoichiometry in undoped semi‐insulating gallium arsenide by x‐ray bond method

The influences of microdefects and dislocations on the lattice parameters of undoped semi‐insulating GaAs single crystals were analyzed, and a novel nondestructive method for measuring stoichiometry in undoped semi‐insulating GaAs was established in this letter. The comparison of this method with coulometric titration indicates that the method of nondestructive measurements is indeed convenient and reliable.

(Ga,Mn,As) compounds grown on semi-insulating GaAs with mass-analyzed low energy dual ion beam deposition

The (Ga,Mn,As) compounds were obtained by the implantation of Mn ions into semi-insulating GaAs substrate with mass-analyzed low energy dual ion beam deposition technique. Auger electron spectroscopy depth profile of a typical sample grown at the substrate temperature of 250°C showed that the Mn ions were successfully implanted into GaAs substrate with the implantation depth of 160 nm. X-ray diffraction was employed for the structural analyses of all samples. The experimental results were greatly affected by the substrate temperature. Ga5.2Mn was obtained in the sample grown at the substrate temperature of 250°C. Ga5.2Mn, Ga5Mn8 and Mn3Ga were obtained in the sample grown at the substrate temperature of 400°C. However, there is no new phase in the sample grown at the substrate temperature of 200°C. The sample grown at 400°C was annealed at 840°C. In this annealed sample Mn3Ga disappeared, Ga5Mn8 tended to disappear, Ga5.2Mn crystallized better and a new phase of Mn2As was generated.

Relationship between deep-level centers and stoichiometry in semi-insulating gallium arsenide

Experimental results have shown the fact that the deep-level centers in semi-insulating GaAs decrease with the improvement in stoichiometry. The electrical resistivity doubles when the concentration of EL2 centers decreases to a half. The microgravity-growth experiments also show that improved crystal stoichiometry results in a decrease of deep-level centers.

Excess arsenic in GaAs grown at low temperatures by molecular beam epitaxy

The structural properties of GaAs grown at low temperatures by molecular beam epitaxy (LTMBE GaAs) were studied. The excess arsenic atoms in LTMBE GaAs exist in the form of arsenic interstitial couples (i,e, two ns atoms share the one host site), and cause an increase in the lattice parameter of LTMBE GaAs. Annealing at above 300 degrees C, the arsenic interstitial couples decomposed, and As precipitates formed, resulting in a decrease in the lattice parameter.

Dislocations and precipitates in semi-insulating gallium arsenide revealed by ultrasonic Abrahams-Buiocchi etching

The dislocations and precipitates in SI-GaAs single crystals are revealed by ultrasonic-aided Abrahams-Buiocchi etching (USAB), and the etch pits are observed and measured by metalloscope and scanning electron microscope (SEM) equipped with an energy dispersive X-ray spectrometer (EDS), respectively. The size of etch pit revealed by USAB etching is about 1 order of magnitude smaller than that revealed by molten KOH. The amount of arsenic atoms in the dislocation-dense zone is about 1% larger than that in an adjacent dislocation-free zone measured by EDS attached to SEM, which indicates that the excess arsenic atoms adjacent to the dislocation-dense zone are attracted to the dislocations and precipitate there due to the deformation energy.

Properties of GaAs single crystals grown by molecular beam epitaxy at low temperatures

Properties of GaAs single crystals grown at low temperatures by molecular beam epitaxy (LTMBE GaAs) have been studied. The results show that excessive arsenic atoms of about 1020 cm−3 exist in LTMBE GaAs in the form of arsenic interstitial couples, and cause the dilation in lattice parameter of LTMBE GaAs. The arsenic interstitial couples will be decomposed, and the excessive arsenic atoms will precipitate during the annealing above 300°C. Arsenic precipitates accumulate in the junctions of epilayers with the increase in the temperature of annealing. The depletion regions caused by arsenic precipitates overlap each other in LTMBE GaAs, taking on the character of high resistivity, and the effects of backgating or sidegating are effectively restrained.

Effects of stoichiometry in semi-insulating GaAs on optoelectronic devices

The influences of point defects, dislocations, and precipitates on the lattice parameter of undoped semi- insulating GaAs single crystals were analyzed. It was shown that dislocations in such crystals serve as effective gettering sites for As interstitials due to the deformation energy of dislocations. The lattice parameters of these dislocated regions remain relatively constant due to the counterbalance between lattice compression and dilation around the dislocation. Regions away from dislocations show a linear dependence of lattice parameter with As interstitial concentration. Measurements of the lattice parameter in these latter regions by the nondestructive measurement of stoichiometry technique can be used to determine As interstitial concentrations. The nonuniformity in semi-insulating GaAs results in the variation in the threshold voltages of corresponding devices.