Effect of Dislocations on VLWIR HgCdTe Photodiodes T. PARODOS, 1,3 E.A. FITZGERALD, 2 A. CASTER, 1 S. TOBIN, 1 J. MARCINIEC, 1 J. WELSCH, 1 A. HAIRSTON, 1 P. LAMARRE, 1 J. RIENDEAU, 1 B. WOODWARD, 1 S. HU, 1 M. REINE, 1 and P. LOVECCHIO 1 1.—BAE Systems, 2 Forbes Road, Lexington, MA 02421, USA. 2.—Department of Materials Science Engineering, Massachusetts Institute of Technology, Cambridge, MA 02139, USA. 3.—e-mail: themistoclis.parodos@baesystems.com The effects of dislocations on very-long-wavelength infrared (VLWIR) HgCdTe photodiodes (cutoff wavelength >14 lm at 40 K) have been determined experimentally and analyzed. The photodiodes are in the back-illuminated configuration, fabricated from HgCdTe p-on-n double-layer heterostructure (DLHJ) films grown at BAE Systems by liquid phase epitaxy (LPE) onto lat- tice-matched (111) CdZnTe substrates. Arrays were hybridized to silicon ROICs to form focal plane arrays (FPAs). After characterization for dark current and response, the arrays were dehybridized and stripped of their metals and passivation layers. Dislocations were revealed using a Ha ¨ hnert and Schenk (H&S) etch. Pixel traceability was maintained throughout the analysis, permitting one-to-one correlation between photodiode performance and dislocation density measured within that photodiode. We found that response and dark current were correlated to etch pit density (EPD), which we assumed to be equal to dislocation density. Our results support earlier dislo- cation studies on larger-bandgap HgCdTe, which showed response was only weakly impacted by EPD, while dark current was strongly affected by EPD. Measured EPD values ranged from low 10 5 to low 10 7 cm -2 . Potential causes for this range in EPD are discussed. Key words: HgCdTe, photodiode, dislocations, misfit, LPE, EPD, VLWIR INTRODUCTION AND BACKGROUND It has become well established that dislocations can degrade HgCdTe photodiode performance. 1–9 Dislocations threading from the growth substrate and misfits from epitaxial interfaces can all con- tribute leakage currents that degrade diode perfor- mance. For VLWIR FPAs operating at 40 K, dislocations can be major causes of pixel outages. The photodiodes for this study were fabricated by etching mesas into liquid-phase epitaxy (LPE)- grown double-layer heterostructures (DLHJ). CdTe passivation was applied by molecular-beam epitaxy (MBE). Contact, barrier and bump metallization were photolithographicaly patterned on the mesa tops, providing interconnect to readout electronics. Figure 1 shows a schematic cross section of this structure along with the principal dislocation types arising at the base/substrate and cap/base inter- faces. Each type of dislocation can have threading components and segments traveling in (111) planes. Dislocations are typically formed at regions of high strain, which can be caused by lattice mis- match, impurities, surface stress or other forms of damage. Misfit dislocations form at the HgCdTe/ CdZnTe interface once the critical thickness is exceeded. Those that reach the surface as threads and intercept a junction will degrade performance by acting as conduction paths. 1 The cap/base interface in epitaxial heterostructures can also be a source of misfits. Dislocation segments running along the heterojunction interface also degrade performance and are more difficult to observe. In our hetero- junctions, the cap Dx and thickness are chosen to stay below the critical thickness for formation of dislocations. 10,11 The fabrication process can also (Received October 16, 2006; accepted March 20, 2007; published online July 6, 2007) Journal of ELECTRONIC MATERIALS, Vol. 36, No. 8, 2007 Special Issue Paper DOI: 10.1007/s11664-007-0173-x Ó 2007 TMS 1068