High-performance MW and LW IRFPAs made from HgCdTe grown by MOVPE

Abstract

This paper describes the fabrication and performance of MW and LW infrared focal plane arrays (IRFPAs) made from HgCdTe (MCT) grown by Metal Organic Vapour Phase Epitaxy (MOVPE) bump bonded to silicon read-out integrated circuits (ROICs). MOVPE of HgCdTe is possible on CdTe, CdTe:Si, GaAs or GaAs:Si substrates. When choosing the substrate an important factor is the difference in thermal expansion coefficient between the array and the ROIC; if it is large the hybrid will delaminate when cooled to its operating temperature. GaAs:Si substrates provide a simple solution to the thermal stress problem so these were used initially and several hundred MW 640x512 arrays were made. The NETDs were in the range 10 to 14 mK and the defect levels could be as low as 0.1%. However, HgCdTe grown on GaAs:Si suffers to varying degrees from short-range non-uniformity in cut-off wavelength and the ability of these devices to withstand storage at elevated temperatures is also variable. Recently, the thermal stress problem for arrays on GaAs substrates has been solved and small quantities of MW and LW arrays have been made; they have excellent uniformity and bake stability. For MW 384x288 arrays with a cut-off wavelength of 4.95 μm the NETD is in the range 15 to 18 mK and the defect level can be as low as 0.05%. For LW 320x256 arrays with a cut-off wavelength of 10.0 μm the NETD is in the range 20 to 25 mK and the defect level can be as low as 1.3%. These devices will withstand temperature excursions up to 70°C and higher while in storage. The ability of the devices to withstand temperature cycling is being assessed. A 384x288 array has survived 1800 cycles between room temperature and 80 K with no change in performance. Thus GaAs is the preferred low cost substrate for MOVPE growth of HgCdTe.