Laser Annealing of Narrow Gap HgTe-Based Alloys
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LASER ANNEALING OF NARROW GAP HgTe-BASED ALLOYS*
R. E. KREMER,
F. G. MOORE, M. R. TAMJIDI,
Oregon Graduate Center,
AND Y. TANG
19600 NW Von Neumann Dr.,
Beaverton,
OR
97006
ABSTRACT
We have used a cv CO2 laser to study the effects of rapidly annealing HgTe-based alloys. Both as-grown and thermally annealed samples of HgCdTe, HgMnTe, HgZnTe, and HgMgTe have been examined for mercury loss and surface optical reflectivity damage using energy-dispersive x-ray analysis and Small, but systematic differences were found between the asmeasurements. grown and the thermally annealed samples and among the various materials studied. No degradation of the material at all was observed when the samples were cooled to 77 K and exposed to the laser. INTRODUCTION for fabricating of choice the material HgCdTe has long been In photodetectors for the 3 - 5 micron and 8 - 14 micron atmospheric windows. order to make a photovoltaic detector, a p-n junction must be created in the One way to do this is to anneal p-type samples in a saturated material. mercury vapor atmosphere. The mercury diffuses into the material, annihilate By mercury vacancy acceptors, and forming a thin n-type layer at the surface. the carefully monitoring the temperature and time of the anneal, some control More recently, efforts over the depth and abruptness of the is achieved [1]. have been made to form the junction by implanting dopant ions into the material [2]. The major problem with this technique is that ion implantation always produces large amounts of damage. Unlike other semiconductors, such as the volatility of mercury in HgCdTe prevents long-term silicon and GaAs, If mercury out-diffuses thermal anneals to reduce or eliminate the damage. from the sample during annealing, the bandgap of the material will change. by which the sample can be annealed rapidly We are exploring techniques This paper reports on the use of enough to prevent significant mercury loss. short exposures to relatively low power radiation from a cw CO2 laser for this The use of a laser allows precise control of the annealing time, purpose. A CO 2 laser was chosen although at the expense of temperature control. to the quite close (about 0.12 eV) is because the energy of these photons Thus, the absorpbandgap of most of the materials that were to be examined. tion should be relatively small, and concentrated in the implant damage layer, [3). If absorption and subsequent heating can be rather than the bulk compared to greatly reduced also be minimized, mercury loss should
conventional thermal or laser annealing techniques. The mercury loss problem stems from the weakness of the Hg - Te bond. is added, the Hg - Te bond may change When another element, e.g. cadmium, tends to weaken the bond, while that of [4]. The presence of some elements
Laser annealing experiments other elements can act to strengthen the bond. can also be used to directly measure the stability of alloys made by substituting other elements into the mercury sublattice. The exposure time required to cause significan
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