Kissel, H.

Knauer, A.

Krispin, P.

Weyers, M.

Zeimer, U.

English

Università del Salento: ESE - Salento University Publishing

Interlayer formation due to arsine stabilisation during interruptions ofMOVPE−growth of InGaPA. Knauer 1) ,  P. Krispin 2) ,  U. Zeimer 1) ,  H. Kissel 1) ,  M. Weyers 1)1) Ferdinand−Braun−Institut für Höchstfrequenztechnik (FBH), Albert−Einstein−Str. 11, D−12489 Berlin, Germany,  2) Paul−Drude−Institut für Festkörperelektronik (PDI), Hausvogteiplatz 5−7, D−10117 Berlin, GermanyIntroductionHeterointerfaces are the basis of band−gap engineering used for electronic and optoelectronic devices. Theoptimisation of these interfaces is a challenge, especially when switching between As and P occurs. Unintentionalinterlayer formation during metalorganic vapor phase epitaxy (MOVPE) of GaAs on InGaP has often beenreported /1−5/. Such interlayers can drastically change the electrical and optical properties of device structures.The growth of GaAs on InGaP is usually preceded by switching from PH3 to AsH3 and a stabilization of thesurface under group V−hydrides. Depending on the time of purging with group V−hydrides and their concentrationduring growth interruptions the formation of interfacial layer can be modified. Beside the As−P exchange at theGaAs−on−InGaP interfaces several studies discuss the influence of indium−carry−over on the formation ofunintentional interlayer. These interlayers give often rise for an additional low temperature photoluminescence(PL) peak around 1.4 eV /1−4/. Thus, additional interlayers are often deposited /1,2,4/ or the gas switching/purgingsequences /3/ are optimised to avoid these additional PL peaks. Capacitance−voltage (C−V) studies withmetal−semiconductor contacts on isotype heterojunctions are known to provide reliable values for the band offset.Therefore we use in addition to PL also C−V measurements to detect the inadvertent interlayers at theGaAs−on−InGaP or InGaP−on−InGaP interfaces by their conduction band offsets DEC /5,6/.Experimental procedureThe InGaP/GaAs layers were grown in a horizontal MOVPE reactor (Aix 200) at 70 hPa on (100) n−GaAssubstrate using TMGa, TMIn, Si2H6, PH3, and AsH3 as precursors. Nearly disordered InGaP was grown at580°C with a V/III input ratio of 70 and a growth rate of 2.5 µm/h. The InGaP growth was interrupted by switchingoff TMGa, TMIn and PH3 and replacing them by AsH3 (10 sccm/min in 7 l/min total hydrogen flow) for 60 s. TheInGaP growth was restarted by replacing the AsH3 directly by PH3 for 0.5 s and then switching on TMGa andTMIn again at the same time. Thickness, composition and strain of the interlayers as well as of the InGaP layerwere determined by high resolution X−ray diffraction it the symmetric (004) reflection. The lattice mismatch ofthe InGaP layer was smaller than 5*10−4 . The 514.5 nm line of a cw Ar+ laser was used for excitation in lowtemperature photoluminescence (PL). In order to determine the electronic properties of the interfaces by the C−Vmethod (measurement frequency: 1 MHz), the layer structures were Si−doped with carrier concentrations in the1017 cm−3 range (cf. Ref. /7/). The As, In, Ga and P concentrations were measured by SIMS /8/.Results and discussionFig. 1 shows the depth profile of the electron concentration n of an InGaP layer with growth interrupts underAsH3 flux. The horizontal dashed line marks the Si doping level. Remarkable peaks accompanied by strongdepletion in the adjacent InGaP are formed as the results of interrupts. Fig. 2 shows the corresponding SIMS depthprofiles. It is evident that the As−related signal is drastically enhanced at the position of the growth interruptions,in contrast to the In−related one. As the result of the growth interruption under AsH3, an As−enriched quaternary(In,Ga)(As,P) interlayer is formed. The positions of the As−related peaks agree with the positions of the maxima ofthe electron concentration in Fig. 1. The thickness of the quantum well (QW) formed as the results of interruptscan be estimated from the SIMS depth profile in Fig. 2 to be below 7 nm with an As concentration y of about 0.08.(3.2 nm steps between measurement points). The As carry−over into the succeeding InGaP layer after each10th European Workshop on MOVPE, Lecce (Italy) 8−11 June 2003 PS.IV.02A. Knauer et al interruption extends over more than 20 nm with y > 0.01.Fig. 1: C−V depth profile of the electron concentration n for an InGaP layer deposited with several growthinterruptions under AsH3 flux, (measured at 100 kHz and 295 K). Solid line: simulated best fit, horizontal dashedline: doping level in InGaP, vertical dotted lines: marker for the interfaces of the interlayers.Fig. 2: SIMS depth profiles of the In and As concentrations for the InGaP sample (caped by GaAs) shown in Fig. 1.Vertical dotted lines: marker for the interfaces of the interlayers.10th European Workshop on MOVPE, Lecce (Italy) 8−11 June 2003 PS.IV.02A. Knauer et al The compressive stress of the interlayers is so high, that they are clearly visible also in X−ray rocking curves.Their extension can be well fitted with 5−6 nm width and an As concentration between 0.15 and 0.08.Further information on the interlayer is obtained by comparing measured and calculated electron distributions.With the same set of parameters for all interruptions, the fit to the measured electron distribution n shown in Fig. 1as a line is perfect. The effective conduction band offset DEC for the interface between interlayer and InGaP isfound to be about 28 meV. The effective QW thickness is found to be about 20 nm, in reasonable agreement withthe value estimated from SIMS measurements. The vertical dotted lines in Figs. 1 and 2 indicate the thickness ofthe interfacial layers as determined by simulation of the C−V data.The 10−K−PL spectra of the InGaP layer are shown in Fig. 3. Additional peaks around 1.76 eV appear in additionto the typical emission of the InGaP band−to−band transitions of ordered (1.937 eV) and disordered component(1.973 eV). The additional peaks can be assigned to the unintentional interlayer formed by the growthinterruptions. The (In,Ga)(As,P) QW thickness can be estimated to be 5.1 nm by band−gap calculations assumingrectangular potential wells for the electrons and holes using the determined band−gap shift of 217 meV and aDEC of 28 meV from the C−V measurement. The valence band offset used in the calculation is 189 meV. Forweakly ordered In0.48Ga0.52AsyP1−y layers, the band gap difference with respect to (In,Ga)P is given by /9/: Eg (eV)= 1.19y−0.09y2  (1).The As content would then be about y= 0.185 for a 5 nm thick As−containing interlayer. This is a bit higher thanthe results determined from SIMS and X−ray simulations but still in reasonable accordance.Fig. 3: Low−temperature PL spectra of the InGaP sample shown in Figs. 1 and 2 for several excitation densities.The surprisingly high thickness of the interlayers suggests that, besides the As−P exchange, an additionalinterlayer growth takes place during or/and after the growth interruptions. The conduction band offset of 28 meV10th European Workshop on MOVPE, Lecce (Italy) 8−11 June 2003 PS.IV.02A. Knauer et al detected by the C−V measurement is too small for possible InGaAs or InAsP interlayers, which could be grownduring the arsine stabilised growth interruption with excessive indium on the InGaP surface and/or from hotreactor parts.ConclusionsInterlayer formation as a result of prolonged stabilisation under arsine instead of phosphine during interruption ofInGaP MOVPE growth was found. The measurements support that the unintentionally formed interlayer is an Asrich (In,Ga)(As,P) layer with a thickness of about 5 +− 2 nm and an As content of about y= 0.10 +− 0.05. TheSIMS depth profile shows that As carry−over occurs into the succeeding InGaP layer over an extension of up to 20nm with y > 0.01 after each arsine stabilized growth interruption.References/1/  F.E.G. Guimarães, B. Elsner, R. Westphalen, B. Spangenberg, H. J. Geelen, P. Balk, J. Cryst. Growth 124(1992) 199./2/ R. Bhat, M.A. Koza, M.J.S.P. Brasil, R.E. Nahory, C.J. Palmstrom, B.J. Wilkens, J. Cryst. Growth 124 (1992)576./3/ R.C.Y. Tsai, M. Moser, C. Geng, V. Härle, T. Forner, P. Michler, A. Hangleiter, F. Scholz, J. Cryst. Growth145 (1994) 786./4/ Yong−Hwan Kwon, W.G. Jeong, Yong−Hoon Cho, Byung−Doo Choe, Appl. Phys. Lett. 76 (2000) 2379./5/ A. Knauer,  P. Krispin, V.R. Balakrishnan, M. Weyers, J. Cryst. Growth 248 (2003) 364./6/ P. Krispin, A. Knauer, S. Gramlich, Appl. Phys. Lett. 80 (2002) 2493./7/ P. Krispin, A. Knauer, S. Gramlich, Mater. Sci. Eng. B 88 (2002) 129./8/ RTG Mikroanalyse GmbH Berlin, Schwarzschildstr. 1, D−12489 Berlin, email: rtg@bbtt.de/9/ E. Kuphal, J. Cryst. Growth 67 (1984) 441.10th European Workshop on MOVPE, Lecce (Italy) 8−11 June 2003 PS.IV.02A. Knauer et al 

Interlayer formation due to arsine stabilisation during interruptions of MOVPE-growth of InGaP

ESE - Salento University Publishing

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Interlayer formation due to arsine stabilisation during interruptions of MOVPE-growth of InGaP

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