THE INVESTIGATION OF CUSTOM GROWN VERTICAL ZONE MELT SEMIINSULATING BULK GALLIUM-ARSENIDE AS A RADIATION SPECTROMETER

Vertical zone melt (VZM) bulk GaAs boules have been zone refined (ZR) and zone leveled (ZL) to reduce EL2 deep donor levels and impurity con centrations with the intent of improving properties for gamma ray dete ctors. ZR and ZL GaAs boules had background impurity levels and deep d onor EL2 concentrations near or below detectable limits. The crystal m osaic of the material at locations near the seed end was slightly supe rior to commercial liquid encapsulated Czochralski (LEC) material, and nearly equivalent to commercial vertical gradient freeze (VGF) materi al. The crystal mosaic in ZL material degraded towards the tail end. T he homogeneity of the electrical properties for the ZL and ZR VZM mate rial was inferior compared to commercially available bulk GaAs materia l. Post growth annealing may help to homogenize some electrical proper ties of the material. The charge collection efficiency of the ZR GaAs detectors was only 30% maximum, and only 25% maximum for the ZL GaAs d etectors. Resulting gamma ray spectra was poor from detectors fabricat ed with the ZL or ZR VZM material. Detectors fabricated from material that was both ZR and ZL did not demonstrate gamma ray resolution, and operated mainly as counters. The poor spectroscopic performance is pre sently attributed to the inhomogeneity of the electrical properties of the ZR and ZL GaAs materials. Comparisons are made with detectors fab ricated from VGF SI bulk GaAs.