Porous Silicon-Based Optoelectronic Devices: Processing and Characterization
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POROUS SILICON-BASED OPTOELECTRONIC DEVICES: PROCESSING AND CHARACTERIZATION NADER M. KALKHORAN Spire Corporation, One Patriots Park, Bedford, MA 01730-2396 ABSTRACT A patterning process compatible with conventional Si electronics technology, which has resolution better than 5 gm, has been developed in order to perform selected-area anodic etching for producing luminescent porous Si layers (PSL). Correlations between the anodic etching and photolithographic parameters have been identified, and their effects on the resolution and luminescence of porous Si layers have been studied. Finally, the first monolithic processing, i.e., true wafer-scale integration, of a Si-based visible light-emitting diode (LED) and a photodetector using conventional Si technology has been demonstrated. INTRODUCTION Recently, researchers demonstrated that highly efficient visible light emission (photoluminescence, PL) from silicon is possible if the material is properly prepared in a microporous form, typically by anodic etching and/or chemical etching.' As a direct consequence of the etch procedure, such porous Si materials are composed of nanometer-sized crystalline Si structures which show quantum confinement effects and, hence, broadening of their effective energy gap, allowing them to luminesce in the visible wavelength range. Although visible PL in porous silicon is a very important accomplishment, the main challenge toward using the material for practical device applications is to produce luminescence through electrical excitation (electroluminescence, EL). Few research groups have demonstrated EL in simple porous Si devices.2' Kalkhoran, et al.,2 demonstrated the fabrication of the first porous Si-based surface-emitting np heterojunction visible LED. The device, as shown in Figure Ia, was configured as a heterojunction between a wide band gap n-type semiconductor (indium-tinoxide, ITO) deposited by rf sputtering on the porous surface of a p-type Si wafer. The EL spectrum from the LED is shown in Figure lb. A model for the energy band diagram and a theory of operation for the device have been discussed elsewhere. 67 922481
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WAVELENGTH (nm) Figure 1
a) Schematic structure and b) EL spectrum of the PSUITO heterojunction LED.
Since the comprehensive research of optical properties of porous Si is just beginning, devices such as the one described above remain underdeveloped. Many fabrication and design Mat. Res. Soc. Symp. Proc. Vol. 283. ©1993 Materials Research Society
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parameters must be optimized before furthering practical applications, such as their potential use for ultra high speed optical chip-to-chip or board-to-board communication or display panels. The key to the success of the research on porous Si-based devices lies in the anticipated compatibility of their fabrication processing with bulk Si electronics technology. This compatibility may enable true wafer-scale integration of electronic and
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