Properties of Cu in GaAs
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PROPERTIES OF Cu IN GaAs
Rosa Leon, Maria Kaminska, Kin Man Yu*, and Eicke Weber Department of Materials Science, University of California, Berkeley, CA 94720 *Materials Science Division, Lawrence Berkeley Laboratory, Berkeley, CA 94720
ABSTRACT: The electrical properties and preferred lattice sites of Cu in GaAs were studied combining electrical and optical measurements with Particle Induced X-ray Emission (PIXE) and Channeling. For electronic characterization, Deep Level Transient Spectroscopy (DLTS), Hall
Effect measurements, and Photoluminescence (PL) were used. From this comprehensive study it was determined that Cu introduces two levels in the bandgap, that the concentration of electrically active copper is considerably smaller than the total copper concentration, and that most of the Cu in GaAs is not of purely substitutional character.
INTRODUCTION: Cu is a ubiquitous impurity in 111-V semiconductors, and it appears in significant concentrations in much commercially grown GaAs. Modification of the electrical behavior of GaAs after Cu diffusion has been reported, 1 "6 but neither the exact role of Cu nor the lattice position of this impurity is fully understood. Even though most studies agree that Cu introduces acceptor level(s) in GaAs, it has not been shown whether Cu is a single or double acceptor. Also, it is believed that most of the Cu that diffuses into GaAs occupies substitutional positions, taking the Ga sites: Here it is shown that whereas the electrically active Cu is likely to-occupy substitutional sites, most of the Cu present in the lattice is neither electrically active nor substitutional. These conclusions were reached after.Cu diffusion experiments into GaAs. Electrical and optical measurements gave the concentration of electrically active defects and the energy levels introduced by Cu. PIXE and Channeling were used to determine the most common lattice site of copper in the GaAs lattice. EXPERIMENTAL AND SAMPLE PREPARATION Semi-insulating GaAs crystals were used as starting material for all of the diffusion experiments described herein. Prior to Cu evaporation, the GaAs samples of typical dimension 10x4x4 mm were cleaned in baths of hot acetone and methanol and were etched for 2 minutes in a room temperature H 2 S0 4 :H 2 0 :H2 02 solution in a 5:1:1 ratio. Pure copper (99.999%) was etched first in nitric acid and then in hydrochloric acid, to remove native oxides.
Cu was evaporated on the GaAs surface using a resistive evaporator with a base pressure of 5
x 107-Torr. The samples were then placed in quartz ampules, evacuated to low 10-7 Torr, and
sealed using an acethylene torch. A sufficient amount of pure Arsenic was introduced in the ampule with the sample to maintain the equilibrium arsenic overpressure at the chosen diffusion temperature. Samples were placed in a vertical furnace, and diffused for 12 to 36 hours at temperatures ranging from 6700 C to 11000 C. After diffusion, some of the samples were rapidly quenched
to room temperature by dropping them in oil. The other samples
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