Correlation Of INGAP(001) Surface Structure During Growth and CUPTb-Type Bulk Ordering
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ABSTRACT The mechanism causing the CuPtB-type ordering of InGaP grown lattice matched to GaAs was investigated by in-situ reflectance anisotropy spectroscopy (RAS/RDS). Experiments were performed during InGaP growth in metal-organic vapour phase epitaxy (MOVPE). From the experiments it can be concluded that bulk ordering only occurs when InGaP growth is performed under phosphorus-rich (2x 1)-like surface conditions. Bulk ordering completely disappears under growth conditions which cause a less-phosphorus-rich (2x4)-like surface dimer configuration.
INTRODUCTION InGaP is an important semiconductor for the fabrication of opto-electronic and electronic semiconductor devices such as heterojunction bipolar transistors (HBTs) [1] and diode lasers [2]. Influenced by the growth conditions InGaP shows CuPtB-type ordering on the group III sublattice [3,4]. This ordering effect results in a reduction of the band gap Eo [5] and a reduced bulk symmetry. Ordering can be understood as a superlattice of indium and gallium rich B planes (Ino. 5(1l 7)Gao.5(l+)P/Ino.5(l+,1)Gao.5(b-,,)P) quantified by an ordering parameter 17.
This ordering parameter can have values between zero (for completely disordered material) and one (for completely ordered material). In practice ordering parameters up to 0.5 are typical. CuPtB-type ordering was related to the surface structure during growth and it has been proposed that phosphorus dimers along the [hol direction promote the ordering process [6,7,8]. This assumption is based on investigations using surface photoabsorption (SPA) [8] and theoretical calculations within the valence force field approach [6,7]. Philips et al. [61 predicted a model which is based on the assumption that the phosphorus dimers of a (2xX) surface reconstruction produce alternating dilated and compressed lattice rows in the subsurface layers. Since the different atoms which form the ordering sublattice (in this case the group III atoms gallium (small) and indium (big)) differ in their tetrahedral radii, the occupation of certain sites is biased beneath the strained (2xl) dimer structure [6]. To clarify which of the surface processes is responsible for the ordering we performed systematic reflectance anisotropy spectroscopy (RAS/RDS) [9,10] studies under gas phase (metal-organic vapour phase epitaxy, MOVPE) conditions. The dependence on growth parameters like temperature, V/Ill-ratio, sample off-orientation and doping was studied. A clear and unambiguous correlation between the occurrence of the ordering and the presence of a (2x1) surface reconstruction during-growth could be established. 217 Mat. Res. Soc. Symp. Proc. Vol. 583 © 2000 Materials Research Society
EXPERIMENTAL InGaP layers were grown in an Aixtron 200 MOVPE system monitored by a RAS spectrometer [11] attached to the growth chamber via a low-strain optical window. Phosphine (PH3 ), trimethylgallium (TMGa), trimethylindium (TMIn), disilane (H2Si6) and hydrogen selenide (H 2Se) were used as precursors. InGaP layers were grown on undoped, epi-ready GaAs(001)
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