Towards the Realization of a INP/CDS/LAS Cold Cathode
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Department of Electrical Engineering, University of Cincinnati, Cincinnati, Ohio 45221 Force Research Laboratory, Sensors Directorate, WPAFB, Dayton, Ohio 45433
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Multi Area Research in Science (MARS) Consultants, Fairborn, Ohio 45324 ABSTRACT
In the past, we have proposed a new cold cathode emitter which consists of a thin region of CdS (Cadmium Sulfide) sandwiched between a heavily doped InP (Indium Phosphide) substrate and a low work function LaS (Lanthanum Sulfide) semimetallic thin film. In this paper, we briefly review the principle of operation of the cathode and discuss the preliminary experimental steps undertaken to realize prototypes of the device. More specifically, we describe the growth of bulk samples of LaS which is used to achieve Negative Electron Affinity of the CdS/LaS surface. X-ray diffraction and micro-Raman experiments show the successful growth of the fcc cubic phase of LaS samples. INTRODUCTION
During the last few years, there have been several proposals for new cold cathode emitters based on the concept of Negative Electron Affinity (NEA) [1]. These cold cathodes will eventually compete with the field emitter devices in the development of a wide range of devices including flat panel displays, pressure sensors, high-temperature and radiation tolerant sensors, vacuum transistors, and high brightness microwave tube sources [2]. Recently, we have proposed a new cold cathode emitter based on a InP/CdS/LaS heterostructure [3, 4, 5, 6]. The main elements in the design and operation of the proposed cold cathode consist of a wide bandgap semiconductor slab (CdS, Cadmium sulfide) sandwiched between a heavily doped semiconductor (n++ - InP) that supplies electrons at a sufficient rate into the conduction band of CdS and a thin semimetallic film (LaS) that facilitates the coherent transport (tunneling) of electrons from the semiconductor conduction band into vacuum (Fig.i). The choice of InP as a substrate is particularly attractive since the lattice constant of InP (5.86 A) closely matches the lattice constant of the zincblende cubic CdS (5.83 A). Furthermore, there have been recent reports on the deposition of crystalline layers of CdS on InP by molecular beam epitaxy [7], chemical bath deposition [8], and pulsed laser deposition [9]. The proposed cold cathode should therefore be realizable with present day technology. As shown in Fig. 1, an array of Au fingers is defined on the surface of the LaS thin film to bias the structure. The bias is applied between the InP substrate and the metal grid with emission occuring from the exposed LaS surface. For the InP/CdS/LaS cold cathode, the choice of a LaS semimetallic thin film grown on nominally undoped CdS is quite appropriate since the lattice constant of CdS (5.83 A) is very close to the lattice constant of LaS (5.85 A) in its cubic crystalline form. Furthermore, electrical resistivity of LaS bulk samples as low as 25MO - cm has been reported [10]. As discussed in ref. [4], this allows to reduce the effects of current crowding, even at large
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