Cathodoluminescence from In x Ga 1-x as Layers Grown on GaAs Using a Transmission Electron Microscope
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present address: Sincrotrone Trieste, Basovizza, Trieste, Italy.
245 Mat. Res. Soc. Symp. Proc. Vol. 588 ©2000 Materials Research Society
EXPERIMENTAL The ternary layers of ZnSe/In•Ga 1 .,As/GaAs were grown by molecular beam epitaxy. The substrates were GaAs(001) wafers, on which 150nm-thick GaAs(001)2x4 buffer layers were grown at 6000 C. In.Gal.As epilayers were subsequently grown at 490*C with nominal x-0.05 composition and 2 pm nominal thickness, and ZnSe overlayers were deposited at 290'C with 100 nm thickness. In these conditions the ternary layers are expected to be largely relaxed, through the formation of a network of misfit dislocations oriented along perpendicular [110] and [1-10] directions.
The plan-view samples, i.e. an as-grown bulk sample and a thin sample prepared by Ar ion milling to remove the GaAs substrate, were examined by a CL detection system combined with a TEM [4]. In this system CL light emitted from a sample is collected by an
ellipsoidal mirror located above the sample. All the CL measurements were performed at a sample temperature of 35 K, an accelerating voltage of 80 kV, and a spectral resolution of 0.8 nm. RESULTS Figure 1(a) shows a CL spectrum from a bulk sample taken at 35 K Three peaks appear at 820 nm, 830 nm and 863 am, respectively. The emission of the peaks at 820 am and 830 nm come from GaAs buffer layer and substrate, and that at 863 am comes from the In.Gal.,As layer. Figure 1(b) shows a SEM image of the sample, where the dark region at the upper-left is the vacuum.
(e)
(a)
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(d) (C)
_j
800
820
840
060
880
wavelength (nm)
901
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Fig.1 (a) CL spectrum from a bulk sample of ZnSefln 0 Gaj.,Ws/GaAs taken at 35 K, (b) an SEM image, and monochromatic CL images taken at various wavelengths of (c)820 am, (d) 830 nm, (e) 863 nm, and (f)867 am. Size of the observed area is 50oim x 50jim.
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Figure l(c) to (f)are monochromatic CL images taken at various wavelengths, ie., (c)820 nm, (d) 830 am, (e) 863 rm and (f) 867 rm, respectively. The CL intensity is rather uniform in (c) and (d). A broad distribution in CL intensity exists due to the focusing effect of the ellipsoidal mirror. Linear features appear in the CL image of (e) taken at the In,,Ga , .,As peak wavelength, and their contrast becomes stronger in (f) taken at the longer wavelength with respect to the peak. The linear features run along the [110] and [-110] directions. These line features did not appear in the monochromatic CL image taken by using the emission from the ZnSe layer (not shown). Figure 2 shows (a) a panchromatic CL image, and monochromatic CL images taken at various wavelengths. The CL spectrum in Fig.2(f) was taken from a wide area of the sample, showing two peaks at 820 nm and 867.5 nm, while the 830 nm peak disappeared because the GaAs substrate was removed. The emission of a 820 nm peak comes from the GaAs buffer layer. In the panchromatic CL image of Fig.2(a), dark lines are seen to run in the [110] and [1-10] directions. This contrast is due to misfit dislocations formed
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