Metallorganic Vapor Phase Epitaxy Yields InSb Photodiodes for Focal Plane Arrays
Daniel S. Burgess
Researchers at Soreq NRC in Yavne and Semi Conductor Devices in Haifa, both in Israel, have reported the fabrication of InSb photodiodes by metallorganic vapor phase epitaxy. The development promises to enable the production of mid-IR focal plane arrays with superior performance at higher temperatures than detectors manufactured using ion-implantation into bulk N-type InSb.
Mesa-type InSb photodiodes grown by metallorganic vapor phase epitaxy display dark current similar to that of photodiodes grown by molecular beam epitaxy. Courtesy of Semi Conductor Devices.
Focal plane arrays operating in the 3- to 5-µm atmospheric window have military and civilian applications from forward-looking infrared and missile guidance to nondestructive materials testing and tumor detection. Ion-implantation yields detectors of high uniformity, said Yossi Paltiel of Soreq's Electro-Optics Div., but the process limits the quality of the PN junctions, increasing dark current in the instrument and requiring low operating temperatures.
Of interest, therefore, are alternate fabrication techniques, especially molecular beam and metallorganic vapor phase epitaxy, which produce higher-quality junctions and thus enable lower dark currents. Each has its own advantages and disadvantages. Molecular beam epitaxy is a versatile technique that uses elemental sources, but growth takes place under ultrahigh-vacuum conditions. Metallorganic vapor phase epitaxy requires less extreme conditions, making it easier and faster to reload and resume growth in a reactor, but its molecular precursors are more difficult to produce.
Semi Conductor Devices is producing prototype InSb and InAlSb large-format staring focal plane arrays by molecular beam epitaxy, and the researchers sought to investigate whether metallorganic vapor phase epitaxy also is suitable for such work. They fabricated mesa-type photodiodes in a vertical reactor from Thomas Swan Scientific Equipment Ltd. of Cambridge, UK, at a temperature of 465 °C and a reactor pressure of 400 t. They performed the deposition through a photolithographic test mask to yield square diodes between 30 and 300 µm on a side.
The measured current-voltage characteristics at 77 K indicated that the diodes grown by metallorganic vapor phase epitaxy are as good as those grown by molecular beam epitaxy. Diodes fabricated by either process display a dark current of approximately 4 fA/µm2 at a bias of 2100 mV and a temperature of ~90 K, suggesting that it should be possible to begin pilot production of focal plane arrays based on these photodiodes.
Further research will investigate epitaxial growth of lattice-matched InAsSb on GaSb, Paltiel said. The material may enable the production of detectors with a cutoff wavelength of 4.1 µm and an operating temperature above 150 K.
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