Strain Induced Compositional Modulations in AlGaAs Overlayers Induced by Lateral Surface Gratings
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Mat. Res. Soc. Symp. Proc. Vol. 618 © 2000 Materials Research Society
already reported a periodic strain modulation accompanied by a periodic Ga-segregation, where the strain amplitude did proceed up the completely smooth sample surface [9]. SAMPLE PREPARATION AND SEM CHARACTERIZATION Surface gratings with ridges and valleys running parallel to [1-10] and a period of D = 314 nm were prepared on exactly oriented GaAs-substrates by holographic photolithography and subsequent wet chemical etching (for details see [9]). To make sure to have the same grating shape for the overgrowth with AlxGal-xAs at both temperatures TG used (630 and 770'C), the gratings were first heated up to 730'C under arsine overpressure and then cooled down or heated up further to the required growth temperature. The growth process was carried out using trimethygallium, trimethylaluminum and arsine as precursors and hydrogen as carrier gas in a horizontal MOVPE reactor with a rotating susceptor. The gratings were overgrown with a single AlxGal.xAs-layer using two diffent Al-concentrations (x = 0.2 and x = 0.7). To prevent oxidation the AlxGa1 .xAs-layer was capped with a 10 nm thick GaAs layer. A compilation of sample descriptions is given in Tab. 1.
Sample Sample Sample Sample Tablel:
1 2 3 4
target Al-content x
TG (0 C)
0.2 0.2 0.7 0.7
630 770 630 770
Top/ bottom overlayer thickness (nm) 80/210 180/315 70/200 170/300
VQW thickness w (nm) 23 43 14 14
Growth parameters and SEM results of the investigated samples.
The cleavage planes of the samples were inspected after growth by a SEM Gemini-DSM 982 using the composition-sensitive backscattered electron mode. All samples exhibit periodic modulations of the Al concentration. The VQWs are visible as bright stripes of low Al concentration reaching from the bottom of the valleys up to the planar surface. At a growth temperature of 770'C the VQWs appear as broad stripes for the sample with x = 0.2, whereas a uniform small stripe of about 14 rum width is found for the sample with x = 0.7 (Fig. la & b).
Fig. Ia sample 2 (x = 0.2, TG = 770*C)
Fig. lb sample 4 (x = 0.7, TG = 770'C)
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PL MEASUREMENTS Photoluminescence (PL) measurements were carried out at 10 K using a HeNe laser. The excitation direction was perpendicular to the sample surface. Since the excitation spot was much larger than the grating period an integral information from all regions with different Alconcentrations was obtained. For depth resolution a sample bevelled at a very small angle ((p = 0.00P1) was used. The measurement could only be carried out for samples 1 and 2 due to the indirect band gap of the other samples. For the estimation of the Al-concentration the exciton transition was used, which appears at higher excitation intensity (4 mW) as a shoulder shifted about 7 nm to shorter wavelengths from the main peak, which is attributed to a band-acceptor transition. The target Al-concentration x0 was determined on a non-patterned area overgrown in the same growth run. In fig. 2 the PL spectra of sample 2 are shown.
n
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