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Alonso, M. I.

Garriga, M.

Hasenöhrl, S.

Novák, J.

Università del Salento: ESE - Salento University Publishing

Influence of  ordered and random part on properties of InGaP alloygrown by MOVPE.J. Novák 1) ,  S. Hasenöhrl 1) ,  M. I. Alonso  2) ,  M. Garriga 2)1) Department of Optoelectronics, Institute of Electrical Engineering, Slovak Academy of Sciences, 841 04 Bratislava, Slovakia ,  2) Institutde Ciencia de Materials de Barcelona, CSIC, Campus de la UAB, 08193 Bellaterra, SpainMOVPE grown InGaP epitaxial layers order spontaneously in CuPt(B)−like structures consisting of alternatingGa− and In−rich planes along the (−111) and (1−11) directions. On exactly oriented GaAs substrates the bothvariants are equally  distributed in platelike domains. Complete ordering is usually not achieved. Recent studieshave shown that ordering has a marked influence on the reduction of band energy as well as on the splitting of thevalence band, which results in the anisotropy of optical and electrical properties of the ternary. Strain produced bylattice mismatch between an InGaP layer and GaAs substrate can also lead to the valence band splitting and to anincrease (if compressive) or decrease (if tensile) of the band−gap energy. It is quite surprisingly,  that role of therandom part of  epitaxial layer and  their influence on optical and electrical properties of alloy was not so carefullystudied as in case of ordered domains. Probably it follows from fact that  photoluminescence spectra of partiallyordered InGaP  epitaxial layers are fully ruled by ordered material and no signal from random part is to observe.On the other hand, spectral ellipsometry allows to show not only optical anisotropy in partially ordered samplesbut also to reveal signal from random part of the epitaxial layer.In our contribution we will show that the band gap energy of InGaP follows closely the compositional dependenceproposed by Wei and Zunger [1]. The Eg value measured in tensile strained epitaxial layers is significantly lowercompared with the one that follows from the generally used parabolic interpolation between the Eg of InP andGaP. The additional transition caused by the random part of the InGaP alloy E0   (random) lies at higher energydepending on the composition of the ternary. A difference between band gap of ordered and random part varies with composition of  ternary and its value lies in interval between 20 − 65 meV.It was previously suggested that the higher band gap associated with the boundaries between domains could act asa barrier and spatially separate the carriers [2]. We studied  dependence of both band gap values on the InP molefraction for two sets of InGaP samples grown at 640 o C and 20 mbar reactor pressure. At this temperature amassive evolution of the ordering effect can usually be achieved. Also, the reactor pressure of 20 mbar is, to ourexperience, the most appropriate as it allows for the best control of ordering as well as the highest reproducibilityof its development. The growth conditions for each growth run were maintained the same, except for the In/Garatio in order to prepare a set of samples with nearly the same degree of order but with various tensile orcompressive strain induced by different lattice mismatch. The source materials were trimethylgallium (TMGa),trimethylindium (TMIn), arsine (AsH3 ), and phosphine (PH3 ). All epitaxial layers were grown on semiinsulatingexactly (001)−oriented GaAs substrates. This procedure led to the obtention of InGaP samples where the InP molefraction ranged from x=0.35 to x=0.565, i.e., in a similar interval for which Wei and Zunger's calculations weremade. The ternary composition of epitaxial layers was determined using X−ray diffraction data along withVegaard's law.Optical measurements were performed using a rotating polarizer ellipsometer with a 1.4 − 5.1 eV spectral energyrange.  Spectra were taken at room temperature after removing surface oxide layers by etching in a solution of 5%hydrofluoric acid in deionized water. During the measurement, samples were kept under a dry N2  flux to preventoxidation. The experimental energy step in the fundamental band gap region analysed was generally 5 meV, butwe used finer steps of 2 meV to resolve the lowest−energy sharp features. For each sample, two differentorientations were measured that approximately gave the two different components of the dielectric tensor.While the band gap of ordered part of samples follows closely the compositional dependence proposed in theory,the random part shows a different dependence with maximum  close to composition lattice matched to GaAssubstrate at the growth temperature  (see Fig.1). Random band gap energy value obtained for exactly lattice10th European Workshop on MOVPE, Lecce (Italy) 8−11 June 2003 PS.IV.03J. Novák et al matched sample is in very good agreement with Eg  value of 1.910 eV published for completely disordered InGaPin [3]. At all other samples, the band gap energy behaves on another maner as the ordered part band gap energyvalue. For samples with lower value of x the random part bandgap energy is lower as it follws from theory, forsamples with higher value of x it is on the contrary.Electrical parameters were measured using van der Pauw configuration. Fig. 2 shows a dependence of Hallconstant and ratio of resistances R1  (measured in the [0−11] direction) and R2 (measured in the [011] direction) independence on the ternary composition. Obtained results have shown that the both electrical parameters  ( (i) Hallconstant and (ii) ratio R1 /R2  behave in relation to  alloy composition similar as the band gap of random part of theepitaxial layer. Consequently, an electrical transport in such complicated structure is influenced by both materials.This influence of various band gaps on electrical properties may be realised either due to type II junction bandoffset between ordered and random part or due to creation of quantum wells in ordered  domains close to interfacesbetween ordered and random alloy. In both cases the random matrix acts as barrier and supplies carriers. [1] J. Novák, S. Hasenöhrl, M. I. Alonso and M. Garriga: Influence of tensile and compressive strain on the bandgap energy of ordered  InGaP, Appl. Phys. Lett. 79 (2001) 2758[2] J.E. Fouquet, M.S. Minsky and S.J. Rosner, Appl. Phys. Lett. 63, 3212 (1993)[3] M.C DeLong, D.J. Mowbray. R.A. Hogg, M.S. Skolnick, J.E. Wiliams, K. Meehan, Sarah R. Kurtz, J.M.Olson, R.P. Schneider, M.C. Wu and M. Hopkison, Appl. Phys. Lett. 66 (1995) 318510th European Workshop on MOVPE, Lecce (Italy) 8−11 June 2003 PS.IV.03J. Novák et al 

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