Reliability and degradation modes of 280 nm deep UV LEDs on sapphire
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Reliability and degradation modes of 280 nm deep UV LEDs on sapphire Z. Gong, S. Chhajed, M. E. Gaevski, W. H. Sun, V. Adivarahan, M. Shatalov, M. Asif Khan Department of Electrical Engineering, University of South Carolina 301 South Main Street, Columbia, SC 29208 USA ABSTRACT In this paper, we report a study of the degradation of AlGaN-based 280 nm LEDs, which were grown on sapphire substrates using migration-enhanced metalorganic chemical vapor deposition process (MEMOCVD). Electroluminescence (EL), atomic force microscopy (AFM), cathodoluminescence (CL), and scanning electron microscopy (SEM) observations showed that the degradation of deep UV LEDs generally fell into two categories: catastrophic degradation and gradual degradation. The catastrophic degradation was found to be mostly caused by the non-uniformity of surface morphology. The gradual power reduction had two characteristic time constants indicating two possible degradation mechanisms as found from temperature and bias dependent LED power degradation measurements. The faster time constant was bias dependent and virtually constant with temperature whereas the second time constant (slower) varied exponentially with junction temperature. For this temperature dependent part, the activation energies of degradation were determined to be 0.23 eV and 0.27 eV under injected current density of 100 A/cm2 and 200 A/cm2 respectively.
INTRODUCTION Significant progress has been made in the development of deep ultraviolet (UV) light emitting diodes (LEDs). UV LEDs with wavelength shorter than 300nm, considered to be best candidate to replace bulky mercury lamps due to their lower power consumption and compact size, have potential applications in biochemical detection, water purification, solid state lighting, data storage and non-line of sight communications. Several groups including ours have reported deep UV LEDs over sapphire and SiC substrates with emission wavelength from 250 nm to 300 nm [1-3]. Recently we have reported on 280 nm deep UV light emitting diodes with powers as high as 1 mW at 20 mA and wall plug efficiency as high as 1%. In spite of all this progress, only very limited work has been done on the reliability study of deep UV LEDs and the degradation modes are still not well understood. In this paper we present our recently results of reliability study on 280 nm deep UV LEDs.
EXPERIMENTAL DETAILS As before, the device epilayer structure was deposited over basal plane sapphire substrates
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using the MEMOCVD process. The growth pressure and temperature were 50 Torr and 1000 °C respectively. SiH4 and bis-Mg were used as the n- and the p-type dopants. The p-dopant was activated using a 30 min 800 °C anneal. Then the reactive ion etching was employed to access the bottom n+-Al0.55Ga0.45N layer. The n-type ohmic contact was formed by Ti/Al/Ti/Au and annealed at 900 oC. The p-type ohmic contact metal consisted of Ti/Al/Ti/Au and the anneal temperatures was 510 oC. To avoid current crowding primarily due to the high Al-compo
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