Jeffrey P. Bender

Transparent p-type conducting BaCu2S2 films

p-type conducting films of α-BuCu2S2 have been deposited onto glass and KBr substrates, yielding a conductivity of 17 S/cm and a Hall mobility of 3.5 cm2/V s. For a 430-nm-thick film, the optical transparency approaches 90% in the visible portion of the spectrum at 650 nm, and a transparency of 40% extends throughout the infrared to the long-wavelength cutoff of the KBr substrate at 23 μm.

Zn2GeO4:Mn alternating-current thin-film electroluminescent devices

Abstract Electrical, electro-optic, temperature, and aging characteristics of green-emitting Zn2GeO4:Mn alternating-current thin-film electroluminescent (ACTFEL) devices are presented. The Zn2GeO4:Mn phosphor layers are prepared by RF sputtering. A maximum luminous efficiency of 0.45 lm/W and luminance of 105 cd/m 2 at 60 Hz and 40 V above threshold are obtained. Field-ionization of impact-excited Mn2+ luminescent impurities is found to give rise to positive space charge within the Zn2GeO4 phosphor, leading to unusual device behaviors such as conduction current delay, anomalous positive polarity transient luminance annihilation, transferred charge being comprised of mainly relaxation charge, and a decrease in the 60 Hz threshold voltage with increasing temperature. Low temperature aging experiments suggest that aging is at least partially due to hot electron-induced degradation. Although most Zn2GeO4:Mn ACTFEL devices exhibit a moderate amount of aging, certain devices are found to exhibit no measurable aging at 1 kHz when aged for 24 h .

Luminescent impurity doping trends in alternating-current thin-film electroluminescent phosphors

Abstract The doping properties of three alternating-current thin-film electroluminescent (ACTFEL) phosphor host/luminescent impurity systems, ZnS:Mn, SrS:Ce, and SrS:Cu, are elucidated, and the ACTFEL device implications of these properties are assessed. Mn is isovalent, Ce is a donor, and Cu is an acceptor. Moreover, Ce is readily ionized in SrS, so that it behaves as a double donor. The distinctly different doping nature of these three luminescent impurities leads to dramatically disparate defect and device physics trends. The donor/acceptor nature of Ce/Cu in SrS results in charge neutrality being achieved in SrS:Ce and SrS:Cu via self-compensation-induced vacancy creation; subsequent defect complexing between oppositely charged luminescent impurities and self-compensation-induced vacancies results in more complex ACTFEL device behaviors such as dynamic space charge, trailing-edge emission, charge collapse, color tuning, and electroluminescence (EL) thermal quenching. In contrast, the isovalent nature of ZnS:Mn leads to more ideal ACTFEL device operation. This suggests that the optimal ACTFEL phosphor luminescent impurity is isovalent.

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.

Transparent electronics and prospects for transparent displays

Transparent electronics is a nascent technology whose objective is the realization of invisible electronic circuits. Part of the impetus for the development of transparent electronics is the recent availability of p-type transparent conductive oxides (TCOs). With the emergence of p-type TCOs, in addition to conventional n-type TCOs such as indium-tin oxide, tin oxide, and zinc oxide, fabrication of transparent bipolar electronic devices becomes feasible. The first part of this paper reviews TCOs and discusses our work in the development of p-TCOs and alternative TC materials (e.g. sulfides). We have recently invented a novel, n-channel, accumulation-mode transparent thin-film transistor (TTFT). This TTFT is highly transparent, has very little light sensitivity, and exhibits electrical characteristics that appear to be suitable for implementation as a transparent select-transistor in each pixel of an active-matrix liquid-crystal display (AMLCD). Moreover, the processing technology used to fabricate this device is relatively simple and appears to be compatible with inexpensive glass substrate technology. The second part of this paper focuses on TTFTs. If transparent electronics is employed to realize transparent back-plane electronic drivers on transparent substrates, fabrication of a transparent display becomes feasible. The third part of this paper offers an approach for realization of a transparent display.

Alternating-current thin-film electroluminescent device modeling via SPICE Fowler-Nordheim diode

A novel SPICE model is proposed for simulating the electrical characteristics of alternating-current thin-film electroluminescent (ACTFEL) devices. The model consists of two capacitors, representing the top and bottom insulators of the ACTFEL device, connected to a Fowler-Nordheim diode shunted by a resistor, which account for conduction and leakage, respectively, in the ACTFEL phosphor layer. A software interface is developed so that the user may conveniently input the two adjustable model parameters (trap barrier height and phosphor shunt resistance) as well as five parameters corresponding to the physical structure of the ACTFEL device (electron effective mass, device area, insulator capacitance, phosphor thickness, and phosphor dielectric constant). Excellent agreement is obtained between measured and simulated electrical characteristics for evaporated ZnS:Mn ACTFEL devices.

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.