Growth of Very Low Arsenic-Doped HgCdTe

Journal of Electronic Materials, Jun 2005 by Chandra, D, Weirauch, D F, Schaake, H F, Kinch, M A, Et al

Arsenic is known to be an amphoteric impurity that may occupy either sublattice in HgCdTe depending upon sample annealing. As an acceptor in low concentrations, it offers several features that are attractive for the fabrication of certain n -on-p detector diode structures. The epitaxial growth of arsenic-doped HgCdTe from a Te-rich melt can fulfill the requirements for application in a variety of devices where low vacancy concentrations and low defect densities are critical requirements in minimizing dark currents. These devices may include the high operating temperature (HOT) detectors operated in a strong nonequilibrium and reverse bias mode to suppress the Auger-generated dark currents. For the materials' growth process to be effective, the segregation coefficient determining the incorporation of arsenic from the Te-rich melt needs to be established. This coefficient was measured during these investigations and was observed to vary with arsenic concentration. Within the range of interest, this parameter varied between 8 × 10^sup -6^ and 1 × 10^sup -4^. These extremely small values limit the doping that can be achieved to

Key words: HgCdTe, As, doping

INTRODUCTION

Incorporation of arsenic as an extrinsic acceptor dopant in mercury cadmium telluride offers several powerful advantages recognized and exploited in a wide variety of HgCdTe (MCT)-based infrared devices for a significant period of time. This dopant is stable, with a very low diffusion coefficient under metal-saturated conditions,1,2 and associated with some of the best device performance parameters, with minority carrier lifetimes among the highest among extrinsic dopants.3 Of particular importance is the final equilibrium location of arsenic. Unlike a spectrum of acceptor dopants from group IB or group IA, which occupy sites in the metal sublattice and which are thereby rendered unstable from a very high concentration of vacancies in this sublattice, arsenic, belonging to group VA, occupies sites in the tellurium sublattice, with the site vacancy concentrations at levels infinitely lower.

However, incorporation of arsenic at very low levels, ranging to 1 × 10^sup 14^ cm^sup -3^ or lower, reproducibly, and without incorporating material damages, required for high operating temperature (HOT) applications remains a challenging proposition. Segregation coefficients for incorporating arsenic employing Hgrich liquid-phase epitaxy (LPE) vary between 0.1 and 10, depending on the growth temperature and the Te atomic fraction in the melt.4 This renders arsenic doping employing Hg-rich LPE progressively easier with increasing arsenic levels, particularly above 1 × 10^sup 16^ cm^sup -3^. With gradually lower targeted doping levels, however, arsenic incorporation employing Hgrich LPE, while still possible, becomes progressively more difficult, since the level of arsenic in the melt itself has to be maintained at a comparable level as in the grown MCT epifilm. Incorporation of very low levels of arsenic in MCT, employing either of the vapor growth techniques, metal organic chemical vapor deposition (MOCVD) or molecular beam epitaxy (MBE), also has been demonstrated,5,6 but with severe potential and real limitations in reproducibility. Indirect introduction of arsenic, employing ion implantation, required for specific applications, is also associated with specific limitations. In particular, most of these methods of introducing arsenic are associated with very high defect concentration levels, ranging to significantly above 1 × 10^sup 6^ cm^sup -2^.

EXPERIMENTAL PROCEDURES

A highly reproducible method of introducing arsenic in mercury cadmium telluride has been developed using Te-rich LPE. The segregation coefficient for incorporating arsenic from a Te-rich melt has been measured during the present investigations. The actual levels of arsenic incorporated in the film, required to determine the segregation coefficient, were determined from the secondary ion mass spectrometry (SIMS) measurements. These were performed by Charles Evans and Associates using a Cs^sup ^ primary ion beam and employing CsAs^sup ^ positive ions. The sensitivity of this technique permitted a determination of arsenic levels present at or above 2 × 10^sup 14^ cm^sup -3^. The segregation coefficient appears to slowly vary with arsenic concentration measured in the epifilms grown. The magnitude of the coefficient appears to increase with increasing arsenic concentration, as measured for an arsenic level at slightly above 2 × 10^sup 14^ cm^sup -3^ in the epitaxial film to 1.1 × 10^sup 15^ cm^sup -3^ in the epitaxial film. These magnitudes are shown in Fig. 1. With increasing segregation coefficients with increasing concentrations of arsenic, as apparent from the measurements during the present investigations, the rate at which the arsenic concentration is required to increase in the nutrient melt to lead to increasing levels of arsenic in the grown epifilm progressively decreases. However, the increase still appears to be insufficient to enable the use of this method to incorporate arsenic for ranges significantly above ~5 × 10^sup 16^ cm^sup -3^ in HgCdTe. This process however appears to become increasingly powerful with progressively lower arsenic levels, ranging downward from arsenic concentrations at 5 × 10^sup 15^ cm^sup -3^ to levels as low as 1 × 10^sup 13^ cm^sup -3^ in the grown epifilm.