Enhancement of mercuric iodide detector performance through crystal growth in microgravity: The roles of lattice order

The hole-mobility·carrier-lifetime product of α mercuric iodide high energy radiation detectors has been enhanced through vapor crystal growth in microgravity. This improvement is closely correlated with specific characteristics of the crystal lattice, which have been identified by high resolution synchrotron X-ray diffraction imaging. These structural features and the associated performance are now being approached in terrestrial growth of α mercuric iodide. Gravity may affect the uniformity of this crystal lattice in two distinct ways: (1) directly through deformation that it imposes on the lattice during growth and (2) indirectly through convection, which mixes any extraneous material. Inclusions associated with these processes harden the lattice and facilitate lattice folding. These changes affect the electronic parameters of detectors made from the crystals. As purification procedures are optimized, the incorporation of extraneous material is curtailed, enhancing electronic properties in spite of lattice flexing through loss of precipitation hardening. These studies provide insight into the contribution of various aspects of crystalline order in α-mercuric iodide crystals to property improvement. This knowledge has led to modification of requirements for starting materials, adjustment of physical vapor growth procedures, and change in crystal handling procedures. As a result, the electronic performance of terrestrially grown radiation detectors has been improved, and the authors provide evidence that further enhancement is still possible