Lipei Zhang

Gamma‐rays irradiation: An effective method for improving light emission stability of porous silicon

We report a study on gamma irradiated porous Si. The electron paramagnetic resonance study on porous Si irradiated by gamma rays shows that the observed signals come from an intrinsic defect, a Si dangling bond, at the interface of Si/SiOx in porous Si. The photoluminescence measurements show that the gamma irradiation not only increases the intensity of the photoluminescence but also greatly improves its stability. The spectra of the Fourier transform infrared absorption show that the gamma irradiation is an effective method for accelerating oxidation of porous Si. All experimental results can be explained by the increase of the oxidation layer thickness which decreases the nonradiative recombination probability of electron‐hole pairs.

Electron paramagnetic resonance observation of trigonally symmetric Si dangling bonds in porous silicon layers: Evidence for crystalline Si phase

In photoluminescent porous Si layers is observed a dominant intrinsic EPR signal of trigonal symmetry with g∥=2.0023±0.0003 and g⊥=2.0086±0.0003 as principal g values. This EPR signal can be identified with Si dangling bonds by its symmetry and characteristic g values. The rotation pattern of the EPR signal indicates that the axial directions of the dangling bonds are distributed in all the four 〈111〉 crystal axes of the original silicon lattice. These results can be exclusively explained by the existence of the crystalline Si phase with retention of the original crystal orientation in porous Si. The dangling bond formation is found to be closely related to the surface oxidation.

Electroluminescence from Au/native oxide/p-Si and its correlation to that from Au/porous Si

Abstract Visible electroluminescence (EL) has been observed from Au/native oxide/p-Si diodes and has been studied in comparison to the EL from Au/porous Si (PS) diodes. Both kinds of diode share similarities in voltage-current characteristic, EL spectra, and the bias dependence of EL peak intensity. However, Au/PS exhibits quite strong photoluminescence (PL) while Au/native oxide/p-Si does not show any measurable PL. The native oxide layer plays a key role in light emission of Au/native oxide/p-Si; when the oxide layer was removed away in HF, no EL can be observed. We discuss the EL mechanisms for both kinds of the light-emitting diode and suggest a physics model for their EL.

Two‐band structure in photoluminescence spectra from porous silicon and its dependence on excitation wavelength

We have systematically measured the room‐temperature photoluminescence spectra with a two‐band structure from porous silicon (PS), as a function of excitation wavelengths in a wavelength range from 260 to 460 nm. Each spectrum can be fitted by two Gaussian functions centered at about 610 and 700 nm, the intensities of the two bands change with excitation wavelength and the intensity maxima occur when the excitation wavelength is about 340 and 400 nm, respectively. When the excitation wavelength exceeds 420 nm, the band at 610 nm is very weak. The above phenomena can be accounted for in the quantum confinement/luminescence centers model [G. G. Qin and Y. Q. Jia, Solid State Commun. 86, 559 (1993)], where it is supposed that there are two kinds of luminescence centers in SiOx layers covering the nanoscale silicon units in PS.

PHOTOLUMINESCENCE OF SM DOPED POROUS SILICON - EVIDENCE FOR LIGHT-EMISSION THROUGH LUMINESCENCE-CENTERS IN SIO2 LAYERS

After oxidation promoted by gamma‐ray irradiation, in the photoluminescence (PL) spectra of Sm doped porous silicon (PS), there are three sharp peaks, superimposed on a broad band, with wavelengths near to those of the Sm doped SiO2 [R. Morimo, T. Mizushima, and H. Okumura, J. Electrochem. Soc. 137, 2340 (1990)]. The experimental results indicate that Sm‐related luminescence centers can be created within the oxide of porous silicon, and only in porous silicon with high porosity can the Sm‐related luminescence be found in the SiO2 layer. This experimental result can be explained by the fact that the excitation of electron‐hole pairs occurs in nanoscale silicon, and the recombination occurs at the Sm‐related luminescence centers in SiO2 layers covering nanoscale silicon.

A weak electron transporting material with high triplet energy and thermal stability via a super twisted structure for high efficient blue electrophosphorescent devices

A high triplet energy (ET = 3.2 eV) electron transporting/hole blocking (ET/HB) material, 1,2,4,5-tetra(3-pyrid-3-yl-phenyl)benzene (TemPPB) with a super twisted structure and high thermal stability has been synthesized. An external quantum efficiency (EQE) of 19.6% was achieved by using TemPPB as the ET/HB material in a blue electrophosphorescent device, much higher than the EQE of 12.5% for the device using the conventional ET material, 3-(4-biphenyl)-4-phenyl-5-(4-tert-butylphenyl)-1,2,4-triazole (TAZ). In addition, the weak ET property of TemPPB resulting from its super twisted structure can be enhanced via n-type doping with LiF. An EQE of 24.5% was achieved by combining n-type doping and a double-emission layer. This shows an alternative way to design ET/HB materials with high ET and improved thermal stability for blue electrophosphorescent devices.

Photoluminescence peak energy evolution for porous silicon during photo-oxidation and gamma -ray oxidation

The evolution of photoluminescence (PL) from a series of as-anodized porous silicon (PS) samples with peaks in a wide energy range has been investigated systematically during oxidation enhanced by either laser illumination or gamma -ray irradiation. Under both oxidation conditions, PL spectra initially located in the infrared-red region undergo a blue shift, while those initially located in the orange-yellow region experience a red shift. Eventually, all the peaks are pinned at fixed energies which are in an energy range narrower than that for as-anodized PS samples. These experimental results can be explained by the quantum confinement-luminescence centres model, which argues that photoexcitation proceeds in nanoscale silicon and photoemission through the luminescence centres outside nanoscale silicon, and also by assuming that during oxidation the main luminescence centres change from one kind to another and/or the relative contributions to the PL intensity from various kinds of luminescence centres vary.

Stable blue light emission from oxidized porous silicon

We have achieved production of porous silicon (PS) that emits blue light at a peak wavelength of 460 nm. On storing it in air for three months, or illuminating it with the 365 nm line of a uv lamp, its photoluminescence intensity and peak wavelength remain stable. With Fourier-transform infrared (MR) measurements, we have studied the surface chemical bonds of the PS emitting blue light, and compared its FTIR spectrum with those of the as-prepared PS emitting red-orange light, and those of the processed PS emitting no light. We consider that the blue light emission originates from the SiO(x) layers covering nanoscale silicon units in PS.

Characterization of stain etched porous Si with photoluminescence, electron paramagnetic resonance, and infrared absorption spectroscopy

Porous Si (PS) layers are prepared by stain etching in a HF/NaNO2 solution on both p- and n-type crystal Si substrates, and are characterized by photoluminescence (PL), electron paramagnetic resonance (EPR), and infrared absorption (IR) spectroscopy. The PL spectra under 488 nm laser excitation exhibit a strong peak at 680-720 nm for various samples of different substrate parameters and remain stable upon aging in air or gamma irradiation; as-etched (approximately 20 min in air before measurement) and aged (for up to six months) samples show no detectable EPR signal but the gamma-irradiated samples show an isotropic g = 2.006 signal of peak-to-peak linewidth of 1.1 mT supporting an amorphous Si structure; the IR spectra show both hydrogen and oxygen related IR modes in the as-etched samples and the former decreases with aging time in air while the latter increases. Comparing our results with those of anodically etched PS samples we conclude that: (1) the PL peak position of the stain PS seems to be unique and stable as compared with that of the anodic PS varying in 620-830 nm; (2) the isotropic EPR signal of the stain PS reflects no crystallinity, in contrast with the anisotropic signal of the anodic PS; and (3) obvious oxidation in the as-etched stain PS is also in contrast with the nonobservation of oxygen-related IR modes in the as-etched anodic PS. We discuss the results in terms of structural properties and PL mechanism of PS.

Abnormal selection rules of interface modes in ultrathin GaAs/AlAs superlattice

We observed a violation of the normal Raman selection rule in the resonant Raman spectra of interface (IF) phonon modes of the ultrathin (GaAs)4/(AlAs)2 superlattice. Contrary to the prediction of conventional theories, all four IF modes were observed in both (XX) and (XY) geometries. The result can be interpreted as a consequence of the deep penetration of the electron wave function in the GaAs wells into the AlAs barriers and a lack of definite parity of the electron wave function. Furthermore, our result indicates that conventional theory for bulk (thicker) systems may need to be modified and further developed to be applicable to ultrathin systems.