Direct Formation of Fine Structure by Low Energy Focused Ion Beam
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INTRODUCTION Recently carrier confinement in low dimension has been attempted for practical device
application as well as fundamental interests[l]. Especially the quantum dot has been studied
intensively because of its potential for laser application[2]. To fabricate the quantum dots, various type of methods had been proposed[3-5]. As a sophisticated fabrication method, a " Droplet Epitaxy", where GaAs micro crystals grow from Ga droplets on Se or S terminated GaAs surface, was proposed and successful results have been reported[6,7]. However, the position of GaAs micro crystals could not be controlled. In addition,to improve optical properties, a position control of GaAs micro crystal is required. For this purpose, a low energy focused ion beam system ( LEFIB) with new idea was proposed. Here, Ga droplet position is controlled by low energy focused ion beam at first. Subsequently Arsenic molecules are supplied to the droplets to grow GaAs micro crystals. This concept is shown in the figure 1. In general, a serious problem in LE-FIB has been a difficulty in making fine structures due to its expanded beam diameter in retarding. However, a combination of LE-FIB and the "Droplet Epitaxy" brings a great advantage. Namely it is possible to form Ga droplets which have smaller diameter than ion beam size. This leads to an expectation of fine GaAs micro crystal growth from the Ga droplets. By this method, a direct GaAs micro crystal growth on the S-terminated GaAs surface was challenged. LOW ENERGY FOCUSED ION BEAM SYSTEM (LE-FIB) In the past, several types of focused ion beam with retarding system has been proposed for 211
Mat. Res. Soc. Symp. Proc. Vol. 448 ©1997 Materials Research Society
etching and deposition by kinetic energy control [8-11]. The newly proposed LE-FIB system with a liquid Ga ion source had four static lens groups. The first one was used for Ga ion to be picked up from the source to the acceleration column at 7.0-8.0 kV. The second lens was biased up to 10.0 KV for making a fine beam. The third lens was biased for focusing around 4.0 kV. The characteristic point of this system is that the last one had four isotropic electronodes for retarding. The isotropic retarding in the cylindrical column suppressed the ion beam expanding to some extent.
beam 7
As molecule III II
t Ga droplets I)Soft-landing 11)Position control
I
ttt
GaAs
Selective Growth
Fig.] Positioncontrol of GaAs micro crystal by combination of low energy focused ion beam system and "Droplet Epitaxy".
The retarding bias was applied up to 9.97KV. Reflectors in middle of the collum scanned the beam and secondary electrons caused by ion bombardment were detected by multi-channel plate to obtain surface images in the same manner of scanning electron microscopy (SEM).The end of the retarding part was set 2.0 cm apart from the sample in the ultra high vacuum ( UHV) chamber. To keep the sample at the grand level electrically, the LE-FIB system is insulated from the earth level. This LE-FIB system is illustrated schematically in the figur
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