Buried Quantum Well Structure Fabricated by in Situ EB Lithography
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BURIED QUANTUM WELL STRUCTURE FABRICATED BY IN SITU EB LITHOGRAPHY H.KAWANISHI, Y.SUGIMOTO, T.ISHIKAWA, N.TANAKA, and H.HIDAKA* Optoelectronics Technology Research Laboratory, 5-5, Tohkodai, Tsukuba, Ibaraki 300-26, Japan *Fujikura Ltd., Optical Device Section, Advanced Technology R&D Center, 1440, Mutsuzaki Sakura, Chiba 285, Japan ABSTRACT Buried quantum well structures have been fabricated in GaAs/AIGaAs system using an in situ lithography process. The process utilizes an ultrathin oxide layer formed in situ on a GaAs surface as a mask against C12 gas etching. An electron beam (EB)-induced C12 gas etching is used to locally remove the oxide mask for positive-type lithography. For negativetype lithography, the oxide mask is selectively formed on a GaAs surface by EB-stimulated oxidation. Subsequent C12 gas etching results in the formation of isolated quantum wells. After removing the oxide mask, overgrowth using molecular beam epitaxy is successfully carried out on the patterned surface. The cathodoluminescence image of the buried quantum well demonstrates the high quality of the resulting structure formed by this "in situ EB lithography" process. The photoluminescence intensity from the quantum well of the processed sample is proved to be the same order of magnitude compared with that from a successively grown sample, showing that the use of the oxide mask causes no serious degradation in the processed interface. INTRODUCTION Lateral patterning of quantum wells in nanometer scale has recently been attracting a lot of attention toward the fabrication of quantum wires and quantum dots. In the fabrication of such fine structures, surface passivation is necessary because the ratio of the surface to the entire volume becomes very high and the effect of the surface state density becomes very serious. Fabrication of buried structures is one of the solutions to this problem. However, there exists an interface-related problem which is certain to degrade the performance of such fine structure devices. Conventional photolithography processes and overgrowth need air exposure of the surface which causes the creation of interface states. In situ processing in an ultrahigh vacuum (UHV) environment is a hopeful technique, because it can create fully buried structures in a controlled atmosphere.[l] In the in situ fabrication process, lithography is followed by crystal overgrowth without exposing the wafer surface to air so as not to degrade the quality of the overgrowth interface. Fabrication of buried structures have been demonstrated in the InGaAs/InP system using in situ focused ion beam (FIB) lithography process.[2] In the case of GaAs/AlGaAs system, however, the sample processed using a FIB contains an ion-induced damage extending deeply into the sample.[3,4] In this paper, the fabrication of buried single quantum well (SQW) structures in the GaAs/AlGaAs system using "in situ electron beam (EB) lithography" process[5-7] is described. In this process, an electron beam was used for the pattern formation. The use of electron b
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