C. Munasinghe

Spectroscopic and energy transfer studies of Eu3+ centers in GaN

Photoluminescence (PL), photoluminescence excitation (PLE), and time-resolved PL spectroscopies have been carried out at room temperature and 86K on transitions from D25, D15, and D05 excited states to numerous FJ7 ground states of Eu-doped GaN films grown by conventional solid-source molecular beam epitaxy (MBE) and interrupted growth epitaxy MBE. Within the visible spectral range of 1.8–2.7eV, 42 spectral features were observed and assignments were attempted for each transition. PL and PLE indicate that four Eu3+ centers exist in the GaN lattice whose relative concentration can be controlled by the duration of growth interruption. The energy levels for these four sites are self-consistently obtained, and time-resolved photoluminescence measurements reveal details about the radiative and nonradiative relaxations of excitation among these levels. The data indicate a near-resonant cross relaxation among these sites. The D25 and D15 states are observed to decay nonradiatively by filling the D05 state with c...

Optically active centers in Eu implanted, Eu in situ doped GaN, and Eu doped GaN quantum dots

A comparison is presented between Eu implanted and Eu in situ doped GaN thin films showing that two predominant Eu sites are optically active around 620 nm in both types of samples with below and above bandgap excitation. One of these sites, identified as a Ga substitutional site, is common to both types of Eu doped GaN samples despite the difference in the GaN film growth method and in the doping technique. High-resolution photoluminescence (PL) spectra under resonant excitation reveal that in all samples these two host-sensitized sites are in small amount compared to the majority of Eu ions which occupy isolated Ga substitutional sites and thus cannot be excited through the GaN host. The relative concentrations of the two predominant host-sensitized Eu sites are strongly affected by the annealing temperature for Eu implanted samples and by the group III element time opening in the molecular beam epitaxy growth. Red luminescence decay characteristics for the two Eu sites reveal different excitation paths...

Excitation-Wavelength Dependent and Time-Resolved Photoluminescence Studies of Europium Doped GaN Grown by Interrupted Growth Epitaxy (IGE)

Abstract : The emission properties of Eu doped GaN thin films prepared by interrupted growth epitaxy (IGE) were investigated through excitation-wavelength dependent and time-resolved photoluminescence (PL) studies. Under above-gap excitation (333-363 nm) large differences were observed in the Eu3(+) PL intensity and spectral features as a function of Ga shutter cycling time. The overall strongest red Eu(3+) PL intensity was obtained from a sample grown with a Ga-shutter cycling time of 20 minutes. The main Eu(3+) emission line originating from (5)D(sub 0) -> (7)F(sub 2) transition was composed of two peaks located at 620 nm and 622 nm, which varied in relative intensity depending on the growth conditions. The room-temperature emission lifetimes of the samples were non-exponential and varied from ~50 s to ~200 s (1/e lifetimes). Under resonant excitation at 471 nm [(7)F(sub 0) -> (5)D(sub 2)] all samples exhibited nearly identical PL spectra independent of Ga shutter cycling time. Moreover, the Eu(3+) PL intensities and lifetimes varied significantly less compared to above-gap excitation. The excitation wavelengths dependent PL results indicate the existence of different Eu(3+) centers in GaN: Eu, which can be controlled by the Ga shutter cycling time.

High brightness ZnS and GaN electroluminescent devices using PZT thick dielectric layers

An improved thick dielectric (TD) layer for inorganic electroluminescent (EL) display devices has been achieved through a composite high-/spl kappa/ dielectric sol-gel/powder route. This composite TD film results in a luminance improvement (up to 10/spl times/) in these TDEL devices with Eu-doped GaN and Mn-doped ZnS phosphor layers. The use of a composite TD film, composed primarily of lead-zirconate-titanate (PZT), results in a significantly higher charge (>3 /spl mu/C/cm/sup 2/) coupling to the phosphor layer. Furthermore, the reduction in porosity of the TD has improved the homogeneity of electric field applied to the phosphor layer, resulting in a steeper luminance-voltage slope. The reduction in porosity has also decreased the diffuse reflection of the TD, which when pigmented, exhibits a diffuse reflectivity of <2% resulting in high display contrast. High luminance levels of up to 3500 cd/m/sup 2/ have been achieved from the ZnS:Mn TDEL devices and 450 cd/m/sup 2/ from GaN:Eu devices. A detailed analysis of the electrical steady-state time-varying characteristics has shown that the electrical performance of TDELs is very similar to TFELs in spite of the physical asymmetry in the device structure. These results demonstrate that three critical requirements for practicality of the TDEL approach (formation on standard display glass, low reflectivity, and electric field homogeneity) can be obtained by careful selection and design of the device materials, fabrication process and device structure.

Combined Excitation Emission Spectroscopy of Europium ions in GaN and AlGaN films

Site-selective combined excitation emission spectroscopy studies have been performed on Eu-doped GaN and numerous sites have been identified. Relative numbers and broadening of these peaks has been investigated for different growth conditions and for increasing AL content of the Al x Ga 1-x N alloy.

Inorganic electroluminescent displays: the impact of new materials

An overview of inorganic EL (iEL) flat panel displays (FPD) is presented. This includes a summary of the key points of iEL device operation and on overview of the impact of new phosphors and dielectric materials critical in EL device operation. This is followed by in-depth review of iEL devices and displays utilizing rare-earth-doped GaN and transition-metal-doped ZnS, in conjunction with high dielectric constant insulating layers. Next, a brief status of companies in the iEL field is given. Finally, speculative concepts on combining inorganic and organic light emitting materials are discussed.

Combined excitation emission spectroscopy of Eu-doped GaN

In this contribution we address the first task by applying to the Eu:GaN material system the site-selective technique of combined excitation emission spectroscopy, in which a large number of emission spectra are recorded and a 2D data set of emission intensities as function of excitation and emission intensity is obtained.

GaN:Eu Interrupted Growth Epitaxy (IGE): Thin Film Growth and Electroluminescent Devices

Abstract : The GaN:RE phosphor development plays a major role in the GaN:RE AC thick dielectric electroluminescent (TDEL) device optimization. In this paper we report on EL devices fabricated using Eu-doped GaN red phosphors films grown by interrupted growth epitaxy (IGE). IGE consists of a sequence of ON/OFF cycles of the Ga and Eu beams, while the N2 plasma is kept constant during the entire growth time. IGE growth of GaN:Eu resulted in significant enhancement in the Eu emission intensity based primarily at 620.5nm. The increase in the material crystallinity observed with the IGE phosphors appears to be the dominant cause of the emission enhancement. Thick dielectric EL devices fabricated on glass substrates using IGE-grown GaN:Eu have resulted in luminance of ~1000 cd/m2.

1.5 /spl mu/m Zn/sub 2/Si/sub 0.5/Ge/sub 0.5/O/sub 4/:Er electroluminescent waveguide amplifiers

In this paper, we report on Er-doped ZSG waveguide amplifiers and electroluminescent devices (ELDs). A 3.5 /spl mu/m wide ZSG:Er optical amplifier with a core thickness of 1.1 /spl mu/m and a length of 4.7 cm was fabricated. The refractive index variation with wavelength was obtained using a variable wavelength spectroscopic ellipsometer with a 0.27 /spl mu/m ZSG:Er sample. The results indicate that electroluminescent optical amplifiers in ZSG:Er films are possible.

Improved luminance and efficiency of ZnS:Mn and GaN:Eu TDEL devices using PZT thick dielectric films

In this paper, we report on the optimization of TDEL devices in both the phosphor material and the device structure. The TDEL device consists of a metal-insulator-semiconductor-insulator-metal (MISIM) stacked film structure built upon a transparent glass substrate. The high dielectric constant and break down field of PZT thick dielectric film along with the other thin film stacks has enabled a significantly higher charge (>3 /spl mu/C/cm/sup 2/) transport across the phosphor layer. Furthermore, the nano-porous PZT film has reduced the intensity of high field points in the device, resulting in a steeper luminance-voltage slope after device turn-on. We have also found that the phosphor electric field of the TDEL surpasses that of a thin film electroluminescent (TFEL) device, resulting in higher efficiencies under same biasing conditions.