Eur. Phys. J. B 27, 449–458 (2002) DOI: 10.1140/epjb/e2002-00177-x T HE EUROPEAN P HYSICAL JOURNAL B Optical functions of InGaP/GaAs epitaxial layers from 0.01 to 5.5 eV R. Ferrini 1 , G. Guizzetti 1 , a , M. Patrini 1 , A. Parisini 2 , L. Tarricone 2 , and B. Valenti 2 1 INFM-Dipartimento di Fisica “A. Volta” dell’Universit`a, Via Bassi 6, 27100 Pavia, Italy 2 INFM-Dipartimento di Fisica dell’Universit`a, Viale delle Scienze, 43100 Parma, Italy Received 13 December 2001 Published online 25 June 2002 – c  EDP Sciences, Societ`a Italiana di Fisica, Springer-Verlag 2002 Abstract. In0.49Ga0.51 P films, both undoped and doped n- and p-type (up to 10 18 cm −3 ), were grown lat- tice matched on GaAs substrates, with different miscut angles, by Metal-Organic Vapour Phase Epitaxy (MOVPE) at different temperatures. The shift of the fundamental gap E0, caused by “ordering effect” was measured as a function of temperature by photoluminescence. The complex refractive index ˜ n = n +ik and the dielectric function ˜ ε = ε1 +iε2 at room temperature were determined from 0.01 to 5.5 eV by us- ing complementary data from fast-Fourier-transform far-infrared (FFT-FIR), dispersive, and ellipsometric spectroscopies. The effect of the native oxide was accounted for and the self-consistency of the optical func- tions was checked in the framework of the Kramers-Kronig causality relations. In the restrahlen region the dielectric function was well fitted by classical Lorentz oscillators; in the transparent region below E0, the refractive index was modelled by a Sellmeier dispersion relation; in the interband region the dielectric func- tion was well reproduced by analytical lineshapes associated to seven critical points. Thus parametrized analytical expressions were obtained for the optical functions all over the spectral range, without dis- continuities, to be used in the modelling and characterization of multi-layer structures, also on opaque substrates. PACS. 78.20.Ci Optical constants (including refractive index, complex dielectric constant, absorption, reflection and transmission coefficients, emissivity) – 78.66.Fd III-V semiconductors – 78.30.Fs III-V and II-VI semiconductors – 78.40.Fy Semiconductors 1 Introduction In x Ga 1−x P mixed crystals have recently gained a consid- erable technological importance as wide band-gap semi- conductors for devices applications [1,2]. In particular In 0.484 Ga 0.516 P lattice matched on GaAs has been recog- nised as an attractive alternative to the AlGaAs/GaAs system to realize Al-free high-quality and high-reliability heterostructure devices [3]. In fact, it has a number of superior features such as: lower oxidation and surface re- combination rate; low propagation velocity of dislocations; minor presence of deep level centres (DX-like); strong etch selectivity and a unique opportunity of designing the band gap line-up. Thus a successful fabrication of high perfor- mance LED, lasers [4], photonic devices [5], solar cells [6], amplifiers, HBT [7] etc., has been recently reported. However, depending on growth conditions (e.g. V/III ratio, growth temperature, growth rate, doping) and GaAs substrate misorientation, In 0.484 Ga 0.516 P shows the for- mation of domains, with different size and statistical dis- tribution [8], where a spontaneous “ordering” appears in the cationic sublattice: it consists in the alternate se- quence of cationic layers along the [111] direction with a e-mail: guizzetti@fisav.unipv.it prevalence of Ga atoms and of In atoms [9]. The strongest fingerprints of “ordering” formation are [10,11] a band gap reduction (BGR) and a splitting of the degenerate valence bands (VBS), caused by the reduced crystal symmetry from T d (zinc-blende) to the C 3v - (CuPt B ). Consequently the presence of “ordering” is expected to strongly influ- ence optical and electrical properties of the material, as well as the performances of InGaP-based devices. The effects of ordering and, at minor extent, of doping on the optical properties of InGaP have been intensively investigated during the last decade. The studies concerned mainly the effects on the energies of phonons, fundamen- tal gap and interband critical points, and reported the measured dielectric functions in the corresponding spec- tral ranges. However, the differences in the examined sam- ples (growth conditions and doping) and in the experimen- tal techniques make difficult to compare the spectra and merge them to obtain the optical functions on extended spectral regions, which are important for the design and engineering of optoelectronic InGaP-based devices. More specifically, optical functions at critical points (CPs) have been determined by spectroscopic ellipsome- try (SE) on undoped epitaxial layers [12–14]; the disor- der or doping effects on interband critical points has been