Influence of Growth Temperature and Phosphine Flow on CuPt Type Ordering in InGaP Grown by Chemical Beam Epitaxy
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from 15 to 35 results in a small reduction in band gap. TEM-dark field shows that the ordered regions become larger, elongated and inclined. EXPERIMENT In this study, 0.5 ýim thick InGai-xP layers were grown lattice matched (x.-0.49) on GaAs by CBE (Riber 32 system). The GaAs substrates were semi-insulating, oriented in the [001]+0.5' direction. Triethylgallium (TEGa) and Trimethylindium (TMIn) were the organometallic sources for group III atoms. Phosphine (PH 3) and Arsine (AsH3 ), introduced into the growth chamber through a cracker cell kept at 1050 0C, were the hydrides sources for group V atoms. The substrate oxide was desorbed at 590 0 C under AsH 3 flow. The growth temperatures were 500 0C, 520 0C, 540 0 C and 560 0C (with V/il ratio fixed in 15), measured through a pyrometer calibrated using the InSb melting point. The V/il1 ratio was varied from 15 to 35 with growth temperature fixed at 5600 C. The growth rate was approximately l im/h. A 0.3 gtm thick GaAs buffer layer was grown on the substrate prior to the growth of the InGaP layers. The lattice mismatch and composition were calculated by X-ray diffraction. Photoluminescence measurements were carried out at 77K using the 488nm wavelength from an Ar+ laser as the excitation source. For TED and TEM measurements, two orthogonal [110] and [il 0] thin film cross-sections were prepared by mechanical polishing and Ar+ ion milling using a liquid N 2 cold stage. TED and TEM-dark field examinations were performed in a JEM 3010 instrument operated at 300kV. RESULTS AND DISCUSSIONS The PL peak energy value is dependent on the composition. The X-ray diffraction shows a difference on the composition (above 1%) for these samples. In order to eliminate this effect, the PL peak energy was calculated and corrected to same composition. Fig. la shows the photoluminescence spectra at 77K for InGaP samples grown at different temperatures and fig. I b the ordering degree calculated [19]. b) a)
77K
._•
0.39--
.
m=500°C T"
4)0.6
0.36--
T°=20•4)~
"--
T T=5200C
1.915eV
o.
1.75
1.80
1.85 1.9
0.3303
1.939 eV
(
0.27 0.24
1.95 2.00 2.05
Band GaP (eV)
*
500
520
54o
560
Temperature (C)
Fig. 1 -(a) Photoluminescence spectra at 77K of the InGaP layers grown at different growth temperatures and (b) calculated ordering degree for the same samples.
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The sample grown at 500PC presents two emission peaks at 1.915eV and for 1.963eV. Photoluminescence measurements as a function of temperature show that the lower energy peak decreases when the temperature rises. This behavior indicates that the lower energy emission peak is related to the transition involving an acceptor impurity, probably Carbon [9], while the higher energy peak corresponds to the InGaP band gap energy. For the other samples, similar analysis also indicates the emission peak corresponding to the InGaP band gap energy. Thus, we observe that the increase in growth temperature results in a 48meV reduction in the band gap of the material. Even for the 5000C sample a reduction in the band gap energy
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