Using Dx Centers to Write Erasable Metallic Patterns in AlGaAs
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ABSTRACT DX centers are semiconductor dopants which form deep states due to a large lattice relaxation. At low temperature, the DX centers exhibit persistent photoconductivity. When exposed to light in a spatial pattern, the photocarriers are confined to the illuminated regions by Coulomb interaction with the localised DX centers. The resulting spatial modulation of the free carrier density gives rise to a
modulation of both the electrical conductivity and the dielectric constant. We demonstrate both effects by measurements of the conductance anisotropy and optical diffraction of samples exposed to excitation
in a striped pattern. Erasure is achieved by thermal annealing. The constrast ratio of the conductivity modulation is greater than 108; in our experiment it is limited to -100 by light scattering. We estimate that 100nm resolution is feasible. Optical diffraction efficiencies up to 40% have been demonstrated in a stripe-illuminated thick sample. The persistence of the written patterns at low temperature is potentially useful in high-density data storage applications and the fabrication of erasable submicron
devices. 1. INTRODUCTION. In many III-V and II-VI semiconductors, certain dopants form deep donor states called DX centers, due to a lattice relaxation around the donor site.1 ,2 The DX is a negative-U center: In the ground state it is negatively charged. Exposure to visible light converts the DX into a normal shallow donor with the release of two electrons into the conduction band, the donor impurity atom becoming positively charged. Since a structural relaxation is required to return to the DX state, this creates a barrier to recombination, 3 resulting in persistent photoconductivity (PPC) at low temperature. We demonstrate the writing of a persistent metallic pattern in an epilayer of insulating AlxGaj_xAs containing DX centers (AlxGal_xAs:DX), by exposing the layer to visible light of the desired pattern. The doped layer, grown on an undoped AlxGajxAs buffer layer, is insulating when cooled in the dark. At low temperature, it is exposed to visible light in a striped pattern. The photogenerated electrons move freely in the conduction band; however, they are confined by the Coulomb interaction to the vicinity of the positively charged donor ions which are localized in the exposed regions. The resulting spatial modulation of the free carrier density is persistent, and induces a modulation of both the electrical conductivity a and the dielectric constant E. For striped excitation, the conductivity modulation is evinced by an anisotropic sample conductance, 4 whereas the modulation of F is demonstrated by optical diffraction. 5 The written patterns are erased by thermal annealing. This effect is potentially applicable in the fabrication of submicron devices, switching applications, and in erasable high-density storage media. 91 Mat. Res. Soc. Symp. Proc. Vol. 360 01995 Materials Research Society
2. EXPERIMENTAL DETAILS. AlxGal_xAs samples, doped with Si or Se, were MBE-grown on a buffer layer of undoped
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