Masatoshi Shiiki

p-Type conduction of ZnSe highly doped with nitrogen by metalorganic molecular beam epitaxy

Abstract Highly conductive p-type ZnSe layers have been grown reproducibly as a consequence of high nitrogen-doping (≈1019cm-3) by using metalorganic molecular beam epitaxy (MOMBE). The dependence of electrical and photoluminescent properties on substrate temperature (Ts) has been investigated. p-Type conductivity of the ZnSe films at room temperature is enhanced with an increase in Ts; e.g. from a high 0.1 MΩcm resistivity at Ts=300°C to a low 0.57 Ωcm resistivity at 450 °C. By contrast, the dependence of the electrical properties for undoped ZnSe on Ts is similar to that of nitrogen-doped ZnSe except for the type of conductivity. The n-type resistivity of undoped ZnSe also decreases with an increase in Ts (e.g. 20 KΩcm at 300°C to 3.5 Ωcm at 400°C). A nitrogen-doped p-type ZnSe/undoped n-type ZnSe junction grown at 350°C on n-GaAs (100) has been succesfully produced. The diffusion potential of the p-n junction can be evaluated to be between 2.45 and 2.55 eV from the C-2-V plot. Observations of electron beam induced current (EBIC) have also shown formation of the p-n junction. At 77 K bluish emission has been observed from the p-n junction with a 5V forward-bias voltage.

Saturation of ZnS:Ag,Al under cathode‐ray excitation

The decrease in efficiency of ZnS:Ag,Al phosphor powders under high cathode‐ray excitation densities has been investigated in a wide range of Ag concentrations (18–1560 ppm) with the Al/Ag ratio fixed at 1.5. Two distinct regions have been observed in this saturation behavior. For low Ag concentrations up to some 100 ppm (mol) saturation is due to activator ground‐state depletion, which manifests itself in a concentration dependence of saturation. For higher doping levels saturation becomes independent on the Ag concentration. Saturation was found to be temperature independent between 83 and 303 K. Diffuse reflectance measurements revealed an onset of optical absorption near 2.8 eV due to Ag incorporation, which continuously increased with Ag concentration giving no evidence for a change in activator incorporation around 100 ppm. Varying the Al/Ag ratio up to 56.2 had no influence on the saturation behavior, while the efficiency at low excitation densities showed a considerable decrease which could not be...

The preparation of ZnS:Mn electroluminescent layers by MOCVD using new manganese sources

In the preparation of ZnS:Mn polycrystalline films by MOCVD, CPM [(C5H5)2Mn: di-π-cyclopentadienyl manganese] and BCPM [(CH3C5H4)2Mn: bismethylcyclopentadienyl manganese] are employed as new doping sources, and are compared with previously reported TCM [(CH3C5H4)Mn(CO)3: tricarbonyl methylcyclopentadienyl manganese]. In contrast to TCM, which is only partially decomposed even at 400 ∼ 500°C, CPM and BCPM are completely decomposed around the optimum growth temperature of ZnS, i.e. 280–350°C. When thermally decomposed, TCM produces a by-product containing Mn and carbonyl, which does not contribute to the luminescence, but CPM and BCPM do not. By these advantages, the double-insulator type electroluminescent (EL) devices prepared with CPM or BCPM show higher luminance than those prepared with TCM. A 500 nm thick ZnS:Mn layer by CPM gives the maximum efficiency (ηmax) of 4.8 lm/W and a saturated luminance (Lsat) of 4300 cd/m2 at 1 kHz sine wave excitation. By BCPM, Lsat of 3150 cd/m2 was obtained. Meanwhile, the devices prepared with TCM in this investigation show Lsat to be less than 1000 cd/m2 and a low efficiency of less than 1 lm/W. By the combination with red filters, ZnS:Mn by MOCVD with CPM or BCPM can also provide efficient red EL. When a glass filter (the cut-off wavelength is 590 nm) is used, the EL device prepared with CPM gives Lsat=1420 cd/m2 and ηmax=1.6 lm/W (the color coordinates, x=0.626 and y=0.373) at 1 kHz sine wave excitation.

Luminescence properties of ZnS/GaAs grown by gas source MBE

Abstract High purity cubic ZnS films were grown on GaAs(100) by the gas source MBE method using (CH 3 ) 2 Zn and H 2 S as source materials. Photoluminescence studies of these films by the excitation of an N 2 or an Xe-Cl excimer laser have shown that the intensity of self-activated emission depends on the [S]/[Zn] ratio and the substrate temperature. No SA emission was observed for ZnS films grown at 400 °C or more with an [S]/[Zn] ratio of 2.3. Band edge emission consists of four sharp lines. The one at the highest photon energy, 3.797 eV, can be assigned to the recombination of a free exciton.

Electroluminescence of zinc sulfide thin films activated with donors and acceptors

Abstract Thin films of zinc sulfide (ZnS) doped with donors and acceptors are prepared by metalorganic chemical vapor deposition (MOCVD) under reduced pressure. The photoluminescence spectrum of a ZnS: Ag, Cl film shows a peak at 460 nm. This emission is attributed to the donor-acceptor (D-A) pair emission from the time-resolved spectra. A ZnS: Ag, Cl thin film is used in an AC-coupled EL device having a MIS structure, ITO-ZnS: Ag,Cl(600 nm thick)-Ta 2 O 5 -Al. This device has a blue broad band emission with a peak at 467 nm. Luminance of 30 cd/m 2 is obtained under 5 KHz sinusoidal voltage excitation.

Effects of crystallinity on the luminescence of ZnS:Tb,P powder and film

Abstract To investigate the possibility of applying donor (D)-acceptor (A) pair emission in ZnS crystals to electroluminescence (EL) devices, the luminescence properties of ZnS:Tb,P are examined. In the cathodoluminescence spectra of ZnS:Tb,P powder, f-f emission of Tb 3+ ions and D-A pair emission related to Tb donors and P acceptors are observed. It is found that D-A pair emission decreases more easily than f-f emission with degradation in crystallinity. In the cathodo- and electroluminescence of ZnS:Tb,P film, however, only f-f emissions, not D-A pair emissions, are observed. This is because of the lack of good crystallinity in the film. Based on these findings, it is proposed that not only less distortion but also a critical size are required for the observation of D-A pair emission in ZnS films.

Electronic energy states in Si-doped MgO for exoelectron emission

A generalized analytical method to determine the density of energy states of electron emission source (EES) is devised by using a thermal excitation and emission model for an exoelectron in the MgO layer and the emission time constants of the exoelectron extracted from experimental stochastic distributions of discharge delay time. When applied to Si-doped MgO, the emission time constant of the exoelectron from the Si EES becomes shorter at high temperature and at short time intervals due to thermal excitation. The density of energy states of the Si EES DSi(E) shows the main peak at 736 meV, a satellite peak at 601 meV, and broad energy structures over the range of 586–896 meV. The effective number of Si EES is 5.5 times larger than that in purified MgO. The excitation energy in a Si-doped MgO cluster with a crystal structure is obtained to be 0.83 eV by using the symmetry-adapted-cluster configuration interaction method and the Si EES contributes to exoelectron emission. The thermal excitation is governed...

Numerical analysis of density of energy states for electron‐emission sources in MgO

— An analytical method to determine the density of energy states of electron-emission sources (EESs) in chemical-doped MgO is described using a discharge probability model and a thermal excitation and emission model. The density of energy states for multiple types of EESs is represented by using a linear combination of Gaussian functions of which parameters are determined by the theoretical emission time constant of an exoelectron and statistical delay time ts extracted from experimental stochastic distributions of discharge delay time in plasma-display panels. When applied to Si-doped MgO, the effective number of Si EES is calculated to be 1.8 × 106 per cell. The average and standard deviations of activation energy have an energy level of 770 meV and a large value of 55 meV. In Si and H co-doped MgO, the high peak density of [H2−]0 appears at 550 meV. ts at the short time interval of 1 msec decreases and is independent of temperature due to exoelectron emission from the [H2−]0. The dependence of ts at a time interval of 10 msec on temperature becomes weak because the energy structure of the Si EES broadens significantly attributed to the electrostatic effects of the doped H atoms.

Emission Time Constant of Exoelectron and Formative Delay Time Analyzed by Using Discharge Probability Distribution

A discharge probability model is proposed to analyze the stochastic distribution of the discharge delay time. The distribution is described as a hybrid function between the exponential and Gaussian distributions and their characteristic properties, such as the emission time constant of an exoelectron and the average and standard deviations of the formative delay time. The calculated results of the probability of a successful discharge show a good agreement with the experimental results measured in plasma display panels. The analytical protocol allows the discharge delay time to be accurately separated into the statistical and formative delay times. A thermal excitation and emission model is devised to analyze the effective number and the activation energy of electron emission sources (EESs) in a MgO layer using the emission time constant of an exoelectron. The effective number of the EES, i.e., 3.79 × 105 per cell, decreases after a long time interval because of the thermal excitation; thus, the emission time constant increases significantly. The effective number of the EES after 1000 h of sustain discharge decreases to 2.07 × 104 per cell, which is 0.055 times that before the sustain discharge. This degradation is explained by 2.6-4.3 times of increase in the density of electron traps due to the ion sputtering of noble gases against the MgO surface. The emission time constant is found to decrease to 0.45 times when the wall voltage near the MgO surface is increased by 11 V, which demonstrates that the exoelectron emission can be controlled by the electric field.

Improvement on the linearity of blue phosphor: Evaluation of hexagonal ZnS:Ag,Al

ZnS:Ag,Al phosphors were prepared in an H 2 S flow without using flux at various temperatures. A cubic structure (a zinc-blende structure) was obtained for the sample prepared at 900°C, and a hexagonal structure (a wurzite structure) was obtained at 1050-1200°C. The life was improved simply with increasing preparation temperature. The luminous efficacy and the current coefficient had their optimum temperature at 1050°C. They showed similar tendencies related to the preparation temperature. The diffuse reflectance also showed the similar tendency of the preparation temperature dependence. The thermoluminescence measurement showed that the traps at a depths of 1.3 eV (observed below 1050°C) and 1.15 eV (observed above 1050°C) disappeared at a preparation temperature of 1050°C.