Strong ultraviolet electroluminescence from porous silicon light-emitting diodes
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Strong ultraviolet electroluminescence from porous silicon light-emitting diodes H. L. Tam 1 , J. Yuan 1 , K. F. Li 1 , W. K. Wong 2 , and K. W. Cheah 1, 3* 1 Department of Physics, 3Center for Surface Analysis and Research, Hong Kong Baptist University, Kowloon Tong, 2 Department of Chemistry, Hong Kong Baptist University, Kowloon Tong, Hong Kong, China ABSTRACT Porous silicon light-emitting diodes were found to emit strong line-shaped ultraviolet under a forward bias driving voltage of about 20 volts. The intensity was sufficiently strong to pump an organic crystal, Tb-dipicolinic acid, producing clear Tb 4f intra-shell transition photoluminescence spectrum. The current-voltage characteristics of the devices also showed negative differential resistance, which was frequency dependent. In addition, purging of the device with various gases could quench the electroluminescence but the intensity recovered partially after each purging, but with no change in emission spectrum. Both results indicate the transport was influenced strongly by local space charge. From the results, the electroluminescence mechanism is tentatively attributed to core recombination in the porous layer, and the spectral characteristics is due to the microcavity effect between the top Au contact and silicon substrate. The present study shows that porous silicon has the potential as UV source in optoelectronics applications. INTRODUCTION The development of information and communication technology depends almost exclusively on the advancement of silicon integrated circuits technology. However, the indirect 1.1 eV band nature of silicon restricts its usage as optoelectronics or photonic material. The discovery of efficient visible photoluminescence from electrochemically etched porous silicon (PS) [1] opened a possible way to exploit Si in optoelectronics applications. Intensive investigations had led to many reports on successful fabrication of light emitting diodes (LEDs) from PS [2-5] and other forms of Si materials [6-8]. However, applicable devices are still not available due to the low efficiency, poor durability, and broad emission. In 1993, Kozlowski et al [9] reported ultraviolet light emission from porous silicon LEDs. The emission was sharp and was attributed to N2 discharging in the microsized cavities inside the PS. The threshold voltage of the LED was in excess of 100V, thus, the device reported by them behaved more like a field emission device. In this report, we demonstrated that it is possible to reproduce the UV emission with lower driving voltage. Our experiment demonstrated that a threshold voltage of as low as 19 volts can be obtained for the devices to work. The emission may not relate to microsized N2 discharging as proposed but originate from core recombination. EXPERIMENT Polished Sb doped CZ n-type (100) silicon wafer with resistivity 0.008~0.02 Ω.cm was used in the experiment. After dipping in HF acid to remove the native oxide, a layer 300 nm of Al was deposited by thermal evaporation on the wafer as back contact. The Al co
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