Compositionally Graded Interface for Passivation of HgCdTe Photodiodes R. PAL, 1,3 A. MALIK, 1 V. SRIVASTAV, 1 B.L. SHARMA, 1 V. DHAR, 1 B. SREEDHAR, 2 and H.P. VYAS 1 1.—Solid State Physics Laboratory, Delhi 110054, India. 2.—Indian Institute of Chemical Technology, Hyderabad, India. 3.—Email: ravindra_pal/sspl@ssplnet.org A compositionally graded CdTe-Hg 1x Cd x Te interface was created by deposi- tion of CdTe on p-HgCdTe and subsequent annealing. The compositionally graded layer between CdTe and HgCdTe was formed by an interdiffusion process and was used for passivation. The composition gradient (Dx) in the interfacial region and the width of the graded region were tailored by adopting a suitable annealing procedure. The effect of process conditions on the inter- facial profile and photoelectric properties such as lifetime and surface recom- bination velocity was studied in detail. Surface recombination velocity of the p-HgCdTe could be reduced to the level of 3,000 cm/s at 77 K, which represents very good passivation characteristics. The passivation layer formed by this method can be used for the fabrication of high performance and stable modern infrared detectors. Thus, a passivation process is developed, which is simple, effective, reproducible, and compatible with the HgCdTe device fabrication and packaging processes. Key words: HgCdTe, CdTe, passivation, surface recombination, x-ray photoelectron spectroscopy (XPS) INTRODUCTION Third generation infrared detectors such as multi- color photodiodes, high operating temperature detectors, avalanche photodiodes, very long wave- length infrared (VLWIR) photodiodes, and mono- lithic focal plane arrays (FPAs) are currently being researched and fabricated with considerable improvement in the quality of HgCdTe epitaxy by molecular beam epitaxy (MBE). The surface leakage currents normally limit performance of photodiodes fabricated on these HgCdTe epitaxial layers. This is ascribed to the problem of defect formation in the interface region due to the nonstoichiometric, con- taminated, or damaged surface prior to or during the passivation process. These defects induce a high density of fixed charges and interface traps, which are usually responsible for the excessive dark cur- rent, low signal, and high noise level in the photo- detectors. Creation of optimum surface conditions is essential to minimize surface recombination. CdTe has proven to be the most suitable for pas- sivation of narrow gap HgCdTe third generation detectors, since it is transparent to infrared light, nearly lattice matched, and chemically compatible with HgCdTe. It is known to yield low defect den- sities and hence low density of fixed surface charges (Q f ), interface traps (D it ), low surface recombination velocities, and long effective lifetimes. 1–3 However, conductivity type, resistance, and band structure of the CdTe-HgCdTe heterostructure influence the interface characteristics, thermal stability, and hence the detector performance. Sher et al. 4 made energy band structure calculations of CdTe-HgCdTe heterostructures for a variety of conditions and demonstrated that a compositionally graded CdTe- HgCdTe interface shows better passivation charac- teristics for LWIR photodiodes. It has been reported that formation of a graded interface improves the defective interface between CdTe and HgCdTe. This compositionally graded layer acts as a repulsive bar- rier for both electrons and holes, thus keeping them away from the original ‘‘defective’’ interface, and reduces the surface recombination velocity. This barrier also prevents electrical conduction through the CdTe-passivant layer, thus reducing the leakage current. The realization of heterostructures is feasi- ble by the epitaxy techniques such as metal organic chemical vapor deposition (MOCVD) or MBE. 5,6 These techniques enable in-situ growth of high-quality (Received February 17, 2006; accepted May 3, 2006) Journal of ELECTRONIC MATERIALS, Vol. 35, No. 10, 2006 Regular Issue Paper 1793