Effect of KI/LiF/CdCl2 on photoluminescent and electroluminescent properties of nanocrystalline (Zn-Cd)S: Cu films

The effect of KI/LiF/CdCl₂ on photoluminescent and electroluminescent (EL) spectra have been reported for (Zn-Cd)S:Cu films. Nanocrystalline films of (Zn-Cd)S:Cu have been prepared using chemical deposition technique in aqueous alkaline bath and their subsequent condensation on substrates. Important results in terms of XRD, SEM, absorption spectra, PL and EL spectra, voltage and frequency dependence of EL brightness are presented. Also, EL brightness waves, EL decay and dependence of EL brightness on nature of electrode material are presented and discussed. SEM studies show best growth conditions in the presence of CdCl₂. Results of XRD studies are associated to ZnS and CdS. Both the studies show average particle sizes to be in the nano order. PL and EL emissions from different films show emission peaks in the blue–green region. Results of absorption spectra show a slight change in band gaps owing to the addition of impurities. Voltage dependence of EL brightness shows effectiveness of acceleration–collision mechanism. Frequency dependence of EL brightness first shows an increase in brightness in the lower frequency range, followed by saturation at higher frequencies. Brightness waves consist of primary and secondary waves, which depend on voltage and frequency of excitation. EL cells with Al electrode give better brightness compared with cells with Ag electrodes. The lifetimes of EL emission are found to be of the order of microseconds.     

Surface and edge emission in organic light emitting devices

We report the studies on photoluminescence (PL) and electroluminescence (EL) spectra of organic light emitting diodes (OLED) based on most widely used light emitting material, i.e., Tris-(8-hydroxyquinoline) aluminum (Alq₃). PL studies from the edges and top surface were carried out on different thicknesses of single layer of Alq₃ coated on glass substrate and the PL intensity from the edges was found to be more than that from the top surface. On the other hand EL emission intensity from the surface was found to be larger than that from the edges of the OLED device. The discrepancy in the PL and EL emission from edge and the top surfaces is discussed. The effect of the thickness of Alq₃ layer on the PL intensity and the emission spectra are also investigated.     

Spin polarization dependent optical transition rates observed in the integer quantum Hall region

We report on a field-dependent photoluminescence (PL) emission rate for the transitions between band states in modulation-doped CdTe/Cd_1−xMg_xTe single quantum wells in the integer quantum Hall region. The recombination time observed for the magneto-PL spectra varies in concomitance with the integer quantum Hall plateaus. Furthermore, different PL decay times were observed for the two circular polarizations, i.e. for the transitions between the Zeeman split subbands of the Landau levels. We analyzed the data in comparison with the experimentally determined spin polarization of the conduction electrons and the Zeeman splitting of the valence band. Furthermore, we discuss the relevance of the spin polarization of the conduction electrons, the electron–hole exchange interaction and the spin-flip processes of the hole states for the PL decay time.     

Organic LED device based on PtOEP phosphor without doping in host material

Electroluminescence (EL) and photoluminescence (PL) have been studied on multi-layer organic light-emitting diode (OLED) devices based on phosphorescent platinum octaethyl porphine (PtOEP) molecule. A multi-layer OLED (called Pt5) which has 100% PtOEP without doping in host as the emitting layer is investigated and compared its EL and PL characteristics with those of the other OLEDs (Pt2 and Pt3) with emitting layer of PtOEP doped in 4,4′-N,N′-dicarbazole-biphenyl (CBP) host material. It is observed that Pt5 shows a lower EL efficiency than Pt2 and Pt3. Three broad EL bands are observed at 500, 527 and 570 nm in the multi-layer device in addition to red sharp EL band due to PtOEP in Pt5, while only the red PtOEP EL is observed in Pt2 and Pt3. The 500, 527 and 570 nm EL peaks arise from absorption of the broad 525 nm Alq₃ emission band by PtOEP layer. The emission from the Alq₃ electron-transport layer is caused by the carrier leakage from the hole-blocking BAlq layer. The intensity of red EL due to PtOEP is much weaker in Pt5 than in Pt2. Taking into account the result of PL, it is suggested that highly efficient energy transfer from CBP host to PtOEP guest occurs in Pt2 and Pt3, giving rise to higher PtOEP luminance, while concentration quenching occurs in PtOEP layer in Pt5.     

Photoluminescence probed minority processes in La2/3Sr1/3MnO3 thin films

La_2/3Sr_1/3MnO₃ thin films are studied with temperature variable photoluminescence (PL) spectroscopy. Two emission peaks are assigned to the minority carriers related transition processes. The temperature independent 2.526 eV peak is attributed to the charge transfer type inter-band transition, while the redshifted doublet peak around 1.686 eV to the spin flip process. Band structures are obtained within the density functional theory, which show the consistent band gaps with the PL data. The temperature dependence of the intensity of PL emission suggests that these minority carrier processes are relevant to polaron formation.     

Blue photo- and electroluminescence of silicon dioxide layers ion-implanted with group IV elements

The microstructural, optical and electrical properties of Si-, Ge- and Sn-implanted silicon dioxide layers were investigated. It was found, that these layers exhibit strong photoluminescence (PL) around 2.7 eV (Si) and between 3 and 3.2 eV (Ge, Sn) at room temperature (RT), which is accompanied by an UV emission around 4.3 eV. This PL is compared with that of Ar-implanted silicon dioxide and that of Si- and Ge-rich oxide made by rf magnetron sputtering. Based on PL and PL excitation (PLE) spectra we tentatively interpret the blue–violet PL as due to a T₁→S₀ transition of the neutral oxygen vacancy typical for Si-rich SiO₂ and similar Ge- or Sn-related defects in Ge- and Sn-implanted silicon dioxide. The differences between Si, Ge and Sn will be explained by means of the heavy atom effect. For Ge-implanted silicon dioxide layers a strong electroluminescence (EL) well visible with the naked eye and with a power efficiency up to 5×10^-4 was achieved. The EL spectrum correlates very well with the PL one. Whereas the EL intensity shows a linear dependence on the injection current over three orders of magnitude, the shape of the EL spectrum remains unchanged. The I-V dependence exhibiting the typical behavior of Fowler–Nordheim tunneling shows an increase of the breakdown voltage and the tunnel current in comparison to the unimplanted material. Finally, the suitability of Ge-implanted silicon dioxide layers for optoelectronic applications is briefly discussed. Received: 9 March 2000 / Published online: 30 June 2000     

Spin injection in the multiple quantum-well LED structure with the Fe/AlOx injector

A spin-injection/-detection device has been fabricated based on the multiple quantum well light emitting diode (LED) structure. It is found that only a broad electroluminescence (EL) peak of a full width at half maximum of 8.6 nm appears at the wavelength of 801 nm in EL spectra with a circular luminescence polarization degree of 18%, despite PL spectra always show three well resolved peaks. The kinetic energy gained by injected electrons and holes in their drift along opposite directions broadens the EL pe...     

Influence of injection current and temperature on electroluminescence in InGaN/GaN multiple quantum wells

Electroluminescence (EL) spectra of blue InGaN/GaN multiple-quantum-well light-emitting diode (LED) have been investigated over a wide range of injection current (0.001–200 mA) and at various temperatures (6–300 K). Surprisingly, with increasing the injection current the EL peak energy shows an initial blueshift accompanied by a broadening of the EL linewidth at low temperatures (below 30 K). This trend differs from the usual photoluminescence (PL) measurement results, which have shown that with increasing the optical excitation power the PL peak energy gave an initial blueshift accompanied by a narrowing of the PL linewidth at low temperatures. The anomalous current behavior of the EL spectra may be attributed to electron leakage results in the failure of Coulomb screening effect and the relative enhancement of the low-energetic localized state filling at low temperatures and low currents. The electron leakage for the LED is further confirmed by both the current dependence of the EL intensity and the temperature dependence of the EL efficiency.     

Photo and electroluminescence of ZnO : Er and ZnO : Ag,Er electroluminors

A number of ZnO : Er and ZnO : Ag, Er electroluminors have been prepared and their photo (PL) and electroluminescent (EL) properties investigated. While the addition of Ag slightly shifts the PL spectra towards longer wavelength side, the EL spectra not only shift but consist of some new transitions. In ZnO : Er electroluminors, additional transitions also exist at higher frequencies of excitations. Brightness waves for this system consist of two secondary peaks during each half cycle of exciting field. Temperature dependence shows two broad peaks. While voltage dependence of ZnO : Er satisfies the relationB=B ₀ exp(−b/V ^1/2), the relationB=B ₀ V exp(−b/V ^1/2) is found to be suitable for ZnO : Ag, Er electroluminor. Possible mechanisms for these phenomena have been proposed. A preliminary account of this work was presented at the International Symposium on Solid State Physics held at the Indian Association for the Cultivation of Science, Calcutta in January 1977.     

Thermal quenching of electroluminescence assisted by an electric field in thin films of tris-(8-hydroxyquinolinato) aluminum (III)

Temperature dependence of the electroluminescence (EL)-current efficiency of tris-(8-hydroxyquinolinato) aluminum (III) (Alq₃)-based organic light-emitting diodes (OLEDs) operated at a constant current density was investigated. The effects of temperature and electric field on photoluminescence (PL) efficiency of Alq₃ thin layers were also investigated. On the basis of these results, it was found that the EL efficiency decreases more markedly with increasing temperature than does PL efficiency. The temperature dependence of the EL efficiency can be interpreted in terms of the thermal dissociation of excitons that is assisted by the electric field.     

Electroluminescence from thin SiO2 layers after Si- and C-coimplantation

In this paper we explore the electroluminescence (EL) properties of thermally grown 350 nm thick SiO₂ layers co-implanted with Si⁺ and C⁺ ions. The implanted fluences were chosen in such a way that peak concentrations of excess Si and C of 5–10 at% were achieved. The devices show a broad photoluminescence (PL) between 2.0 and 3.2 eV with a main peak around 2.7 eV. The broad EL spectra show additional peaks around 3.3 eV and between 2.1 and 2.5 eV which are decreased with increasing Si/C concentration. The shape of the EL spectra does not change with increasing injection currents which implies that various types of defects occur for the different concentrations. The device stability is improved in comparison to Ge or Sn implanted oxide layers.     

Demonstration of electrical spin injection into a semiconductor using a semimagnetic spin aligner

Spin injection into semiconductors has been a field of growing interest during recent years, because of the large possibilities in basic physics and for device applications that a controlled manipulation of the electrons spin would enable. However, it has proven very difficult to realize such a spin injector experimentally. Here we demonstrate electrical spin injection and detection in a GaAs/AlGaAs p-i-n diode using a semimagnetic II–VI semiconductor (Zn_{1 − x − y}Be_xMn_ySe) as a spin aligner. The degree of circular polarization of the electroluminescence from the diode is related to the spin polarization of the conduction electrons. Thus, it may be used as a detector for injected spin-polarized carriers. Our experimental results indicate a spin polarization of the injected electrons of up to 90% and are reproduced for several samples. The degree of optical polarization depends strongly on the Mn concentration and the thickness of the spin aligner. Electroluminescence from a reference sample without spin aligner as well as photoluminescence after unpolarized excitation in the spin aligner sample show only the intrinsic polarization in an external magnetic field due to the GaAs bandstructure. We can thus exclude side effects from Faraday effect or magnetic circular dichroism in the semimagnetic layer as the origin of the observed circularly polarized electroluminescence.     

Circularly polarized lasing over wide wavelength range in spin-controlled (1 1 0) vertical-cavity surface-emitting laser

We investigated the emission wavelength dependence of the lasing polarization in a (1 1 0)-oriented vertical-cavity surface-emitting laser (VCSEL) with GaAs/AlGaAs quantum wells under optical spin injection at room temperature. Lasing was observed in the one circularly polarized mode over a wide wavelength range from 838 to 857 nm, in which a degree of circular polarization higher than 0.8 was maintained. The optical gain spectrum that contributed to the circularly polarized lasing is discussed based on the optical selection rules and the measured polarization-resolved photoluminescence spectra of the active layers.     

衬底温度对PLD法生长的Mg0.05Zn0.95O薄膜结构和发光特性的影响

用脉冲激光沉积(PLD)法在不同温度的Si(111)衬底上成功制备了c轴择优取向的Mg_005Zn_095O薄膜.通过X射线衍射(XRD)和光致发光谱(PL)研究了衬底温度对Mg_005Zn_095O薄膜结构和发光特性的影响,探讨了薄膜的结晶质量与发光特性之间的关系.结果表明,在衬底温度为450℃时生长的Mg_005Zn_095O薄膜具有很好的c轴取向和较强的光致发光峰.室温下分别用激发波长为240,300和325nm的氙灯作为激发光源得到不同样品的PL谱,分析表明紫外发光峰和紫峰来源于自由激子的复合辐射且发光强度与薄膜的结晶质量密切相关,蓝绿发光峰与氧空位有关.此外,探讨了衬底温度影响紫外光致发光峰红移和蓝移的可能机理. 关键词： 005Zn_095O薄膜')" href="#">Mg_005Zn_095O薄膜 PLD 衬底温度 光致发光     

Enhancement of spin polarization in asymmetrically coupled CdSe and CdZnMnSe quantum dots in ZnSe matrix

An asymmetrically coupled double quantum dot (QD) system consisting of adjacent CdSe and CdZnMnSe QD layers in a ZnSe matrix was investigated using polarization-selective magneto-photoluminescence (PL). Two well-resolved PL peaks are observed corresponding, respectively, to the CdSe and the CdZnMnSe QDs. The peaks exhibit significant change in the intensity and energy position when a magnetic field is applied. The enhancement of the degree of σ⁻ circular polarization emitted by the non-magnetic CdSe QDs is observed in the double layer system, as compared to that observed in CdSe QDs without the influence of neighboring CdZnMnSe QDs. This behavior was discussed in terms of antiferromagnetic interaction between carrier spins localized in pairs of CdSe and CdZnMnSe QDs that are electronically coupled.     

Spectral studies of thin films based on poly(N-vinylcarzole) and red dopant

Organic light-emitting diodes were fabricated with a structure of indium-tin-oxide (ITO)/poly(N-vinylcarzole)(PVK):4-(dicyanom-ethylene)-2-t-butyl-6-(1,1,7,7-tetramethyljulolidyl-9-enyl)-4H-pyran (DCJTB)/8-tris-hydroxyquinoline aluminum (Alq₃)/lithium fluoride (LiF)/Al. The energy transfer from PVK to Alq₃ then to DCJTB and the charge trapping processes were investigated by employing the photoluminescence (PL) and electroluminescence (EL) spectra. With increasing thickness of the Alq₃ layer, the PL and EL emission from PVK were decreased gradually, which indicated that the effective energy transfer occurred from PVK to Alq₃ and then from Alq₃ to DCJTB. At the same time, we found that the exciton recombination zone could be adjusted by controlling the Alq₃ layer thickness and the applied voltages. The effects of different DCJTB concentrations on the optical and electrical characteristics of the devices were investigated, and an obvious red-shift was observed with the DCJTB dopant concentrations increasing in the PL and EL spectra.     

Photoluminescence and Raman spectra of the ordered vacancy compound CuGa5Se8

We studied the photoluminescence (PL) and Raman properties of the ordered defect compound CuGa₅Se₈. Twelve peaks were detected from the room-temperature Raman spectra with the A₁ mode around 160 cm⁻¹. Due to the stress in the polycrystalline thin film the corresponding frequencies of the Raman modes of a CuGa₅Se₈ single crystal were slightly shifted. One broad asymmetric PL band at 1.788 and 1.765 eV was observed at 10 K in the PL spectra of CuGa₅Se₈ single crystal and polycrystalline layer, respectively. The temperature and laser power dependencies of the PL spectra were also studied. The shape and properties of the PL band assure the presence of potential fluctuations and the analyses of the PL data suggest that the emission is due to band-to-tail (BT) or band-to-impurity (BI) recombination.     

调制掺杂的应变In0.60Ga0.40As/In0.52Al0.48As多量子阱结构的光致发光谱研究

报道了调制掺杂的应变In_0.60Ga_0.40As/In_0.52Al_0.48As多量子阱中室温光致发光光谱.观察到n=1和2电子子带到n=1重空穴子带的强发光峰.在低温下可以观察到n=1电子子带到n=1轻空穴弱发光肩胛.通过对发光强度随激发功率及温度依赖关系以及理论模型的分析研究,认为该调制掺杂量子阱中辐射复合效率降低的主要机制是应变失配位错对载流子的陷阱作用.界面上的失配位错是陷阱的主要来源.并用静态的光致发光理论模型 关键词：     

Dynamics of dense spin ensemble excited in a barrier layer and detected in a well

Photoluminescence (PL) polarization of a spin ensemble was examined over a wide excitation wavelength range from 520 nm to 700 nm and a temperature range from 3.5 K to 300 K after it transfers from a (AlGa)As barrier layer and eventually quenches irradiatively in a GaAs quantum well (QW).A highest PL circular polarization of 30% can be kept at temperatures up to 120 K,while its room temperature value reaches about 17%.It is found that the main features of the optical spin orientation in bulk Al 0.27 Ga 0.73...     

Multi-channel magnetotransport in Co2FeSi/(Al,Ga)As spin-LEDs

We have performed Hall effect measurements on Co₂FeSi/(Al,Ga)As spin light emitting diodes and have found unique field dependencies that differ strongly from the expected behaviors for both the ferromagnetic Co₂FeSi layer and the underlying semiconductor structure. To understand such unique field dependencies, we have developed a multi-channel transport model for parallel transport through a ferromagnet and a semiconductor. By applying this model to our data for the Hall and sheet resistance, we extract values for the carrier density and mobility in the semiconductor layer. We find that these values decrease with increasing growth temperature of the Co₂FeSi layer, presumably due to stronger in-diffusion of Co and Fe impurities, which compensate the n-type dopants in the underlying n-(Al, Ga) As layer. Despite such compensation, spin-LEDs with the Co₂FeSi layer grown at the relatively high temperature of 280 °C exhibit the highest spin injection efficiencies of more than 50%, hence calling into question the requirement of electron tunneling through the ferromagnet/semiconductor Schottky barrier for efficient spin injection.