Comparative optical studies of semimagnetic II–VI quantum well structures

Superlattices

and Microstructures,

COMPARATIVE

OPTICAL

O.Goede(a), W.Heimbrodt(a),

(a) (b) (c)

351

Vol. 12, No. 3, 1992

STUDIES OF SEMIMAGNETIC WELL STRUCTURES

II-VI QUANTUM

H.-E.Gumlich(b), B.Lunn(c), D.E.Ashenford(c), J.Griesche(a), and N.Hoffmann(a)

K.Jacobs(a),

Humboldt-Universiti zu Berlin, FB Physik, Institut fur Optik und Spektroskopie und MBE Gruppe, InvalidenstraDe 110, D-0-1040 Berlin, Germany Technische Universitilt Berlin, FB Physik, Institut fur Festkarperphysik, HardenbergstraDe 36, D-W-1000 Berlin 12, Germany University of Hull, Department of Engineering Design and Manufacture, Hull, HU6 7RX, UK (Received 5 August 1992)

MBE grown semimagnetic quantum well structures CdTe/(Cd,Mn)Te on InSb, ZnTe/(Zn,Mn)Te on GaSb, and ZnSe/(Zn,Mn)Se on GaAs are studied by optical spectroscopy. Confinement and lattice misfit induced strain effects on the optical spectra, especially the lh-hh well exciton splitting, are discussed. In contrast to CdTe/(Cd,Mn)Te, internal Mn2+(3ds) transitions are observed in ZnTe/(Zn,Mn)Te and ZnSe/(Zn,Mn)Se samples due to the increased band gap. The valence band offset is found to be of the same order of magnitude in the three investigated systems.

The MBE grown semimagnetic II-VI semiconductor quantum well structures are of special interest, due to the wide tunability of the potential heights by an external magnetic field. This opportimity is based on the giant Zeeman splittings of the band gap or exciton energies in the semimagnetic (Cd,Mn)Te, (Zn,Mn)Te or (Zn,Mn)Se barriers, caused by the strong s,p-d exchange interaction between electron and hole band states and the Mn*+ 3d-electron states [1,2,3]. The interest in the broadgap quantum well structures ZnTe/(Zn,Mn)Te and ZnSe/(Zn,Mn)Se, which are much less investigated than CdTe/(Cd,Mn)Te are stimulated by the recent successful preparation of blue emitting II-VI semiconductor injection lasers [4,5]. In the present paper, comparative optical studies are reported of ZnSe/(Zn,Mn)Se, ZnTe/(Zn,Mn)Te, and CdTe/(Cd,Mn)Te single (SQW) and multiple (MQW) quantum well structures and superlattices (SL), grown by MBE on (100) GaAs, GaSb, and InSb substrates, respectively, at a substrate temperature between 240 and 320°C and a growth rate of about 0.7um/h. The substrates

0749-6036/92/070351+04$08.00/0

were selected to be lattice-matched to the well materials ZnSe, ZnTe, or CdTe. Usually, buffer layers with a typical thickness of about lum were used. The CdTe/(Cd,Mn)Te and ZnTe/(Zn,Mn)Te samples were grown at Hull university, whereas the ZnSe/(Zn,Mn)Se structures are produced at Humboldt university. In Fig.1 the photoluminescence spectra of four CdTe/(Cd,Mn)Te MQW structures are shown, demonstrating the pronounced confinement effect in these systems [6]. With decreasing well width a shift of the well exciton peak X, to higher energies is observed. An enhancement of the confinement-induced shift to higher energies up to 160 meV, due to the essentially increased barrier height, is shown in Fig. Id. The bound exciton peak X, from the (Cd,Mn)Te cap barrier indicates the variation of the Mn concentration xi,,“. A more exact xMMn determination for all samples was carried out using reflection measurements of thefree exciton energies in the cap barrier. For comparison, in Fig.2 the luminescence spectra of a ZnTe/(Zn,Mn)Te and a ZnSe/(Zn,Mn)Se SQW

0 1992 Academic

Press Limited

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ZnSe/(Zn,Mn)Se samples, being 8 to 10 meV, does not essentially exceed that of the X, peak in the CdTe/(Cd,Mn)Te structures. Due to the broader band gap of the well material, in the ZnTe/(Zn,Mn)Te and ZnSe/(Zn,Mn)Se SQW structures also the yellow Mn*’ emission band can be seen with slight energetic difference in the two systems due to different crystal fields. In the ZnSe/(Zn,Mn)Se structures also the three lowest-energy internal MI?’ (3d’) excitation transitions were observed in the measured excitation spectra of the yellow emission band. Comparing the three types of quantum well structures, it has to be taken into account that both the band gap offset and the exciton Bohr radius decrease from CdTe/(Cd,Mn)Te over ZnTe/(Zn,Mn)Te to

b

A

and Microstructures,

1.92

E(eV1 -

Fig.1 Photoluminescence spectra of CdTe/(Cd,Mn)Te MQW structures at T = 5 K and cw Arf laser excitation. (a) L,,,= L, = 15 nm, xMn= 0.063, (b) L, = L,= 7.5 nm, xMlo= 0.056, (c) L, = L,= 3.75 nm, x~“= 0.074, (d) L,,, = L,= 2.5 nm, xMo = 0.25

ZnSe/(Zn,Mn)Se (ae(CdTe) = 6 nm, aB(ZnTe) = 5 nm, aB(ZnSe) = 2.8 nm). Therefore, in this series a decreasing confinement effect can be expected for structures with the same well width and Mn concentration. Furthermore, in the Zn containing

structures

the lattice-misfit

induced

strain leads to a red shift of the well exciton energies, in contrast to CdTe/(Cd,Mn)Te structures, as the ZnTe and ZnSe wells are biaxially dilated. In Fig.3 the well exciton peaks of four different ZnSe/(Zn,Mn)Se quantum well the case of the ZnSe/(Zn,Mn)Se the X, peak is found below the result of the strong lattice-misfit

structures are shown. In SQW structure (Fig.3a), ZnSe exciton energy as a induced strain m the well.

The barriers in this sample are nearly unstrained. In the ZnSe/(Zn,Mn)Se superlattice, on the other hand, a mean lattice parameter between wells and barriers results, leading to a comparatively lower strain in the wells. Therefore, a small blue shift of the (Fig.3b). In the case of Fig.3c h4nSe material, leading to somewhat higher peak. As proved by in-situ RHEED MnSe barriers show pure zincblende ENI

-

Fig.2 Luminescence spectra of a) a ZnTe/(Zn,Mn)Te and b) a ZnSe/(Zn,Mn)Se - SQW structure at T = 5 K and UV Ar+ laser / dye laser excitation. (a) L, = 7.5 nm, xlvln= 0.18 (b) L, = 7.0 nm, xMn = 0.19

structure are given, having nearly equal well width and Mn concentration. Again a sharp well exciton peak X, in the 2.4 and 2.8 eV region, respectively, and the barrier exciton X, at correspondingly higher energies are observed. The halfwidth of the Xw peak in the

X, peak is found is used as barrier energy of the X, measurements, the structure up to a

critical thickness of about 4 nm in agreement with earlier results [7,8]. The phase transition of the MnSe barriers in ZnSelMnSe superlattices from zincblende to rocksalt structure with increasing thickness of the h4nSe layers was also studied by measurements of the Mn2+ (3d’) luminescence and excitation spectra [9]. Finally, a ZnSe/(Zn,Mn)Se quantum well structure with extremely narrow wells is considered (see Fig.3d). For this sample the highest net exciton blue shift of nearly 40 meV is observed in comparison to ZnSe bulk material. The three distinct peaks in Fig.3d may tentatively be ascribed to the three wells in the sample, but further evidence is necessary for this interpretation.

Superlattices

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Vol. 72, No. 3, 7992

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hhl-1

hhl-1

1 1.;3

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1.67

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E(eV) -

Fig.4 Luminescence (a) and excitation spectra (b) of the CdTe/(Cd.Mn)Te MQW structure of Fig. l(a) using cw dye laser. T = 4.2 K 2.76

2.86

J

.a

2

EkVI----

Fig.3 Well exciton luminescence spectra of various ZnSe quantum well structures at T = 5 K and W Ar+ laser excitation. (a) ZnSe/(Zn,Mn)Se- SQW: L, = 7 nm, xMvln = 0.22 (b) ZnSe/(Zn,Mn)Se- SL: L,,,= 7nm, L,= 6.5nm, xr,,“= 0.23 (c) ZnSe/MnSe- SL: L,= 5 nm, L,= 2 nm (d) ZnSe/(Zn,Mn)Se quantum well structure with three isolated wells: L, r= 1,L,2=2andL,s=3nm,xM,=0.17

As well known, the lattice misfit induced strain leads to a Ih-hh exciton energy splitting, as shown in Fig.4 for a CdTe/(Cd,Mn)Te MQW structure as an example. The excitation spectrum shows a Ih-hh splitting for the well exciton energies of about 10 meV with the Ih l-l being higher than the hh 1-l exciton energy. The higherenergy peaks n - n‘ correspond to transitions between the n-th excited hole and the n’-th excited electron state. Ih-hh splittings of the same order of magnitude were found for the ZnTe/(Zn,Mn)Te and ZnSe/(Zn,Mn)Se structures using reflection measurements. The sign of the splitting, however, is just opposite as follows from the inverted sign of the strain in the wells. In Fig.5 the results of Kronig-Penney model calculations are shown for a superlattice with the same well and barrier widths of 4 nm and a valence band offset of 30%. The strain is calculated assuming the lateral lattice parameters of wells and barriers to be equal to the mean value. The Ih-hh exciton energy splittings are presented as a function of the Mn concentration xMn for

0

I Cl2

I

I

C!L

I

I

I

0.6

I I Cl8

Xrln-

Fig.5 Calculated hh - Ih well exciton splitting as a function of the Mn concentration for strained (--- ) and unstrained (- - - -) CdTe/(Cd,Mn)Te (curvesl,l’), ZnTel (Zn,Mn)Te (curves 2,2‘) and ZnSe/(Zn,Mn)Se (curves 3,3’)superlattices. L,= L,,= 4nm, valence band offset 30 %

the three systems CdTe/(Cd,Mn)Te, ZnTe/(Zn,Mn)Te, and ZnSe/(Zn,Mn)Se, using the known deformation potential and elastic constants, the effective electron and hole masses, and the gap-variation as a function of xhln [2,10,11]. In the case of CdTe/(Cd,Mn)Te one obtains E,, ’ Ehh as the CdTe wells are compressed, whereas the ZnTe and ZnSe wells are dilated yielding E,, < E,, For the same Mn concentrations the Ih - hh exciton energy splitting is expected to be considerably larger for

354

Superlattices

CdTe/(Cd,Mn)Te than for the Zn containing structures. The dashed curves in Fig.5 show the Ih - hh exciton splitting in the hypothetical case of unstrained wells and barriers to demonstrate the effect of the Ih-hh mass differences only. Whereas in the case of CdTe/(Cd,Mn)Te a valence band offset of 20 to 30% obviously is accepted [3,12,13], the corresponding value for the Zn containing systems is rather uncertain. From the observed high efficiency of the well exciton luminescence it can be concluded that the three considered systems are of type I. The well exciton peak positions and splittings of our ZnTe/(Zn,Mn)Te and ZnSe/(Zn,Mn)Se samples can sufficiently be fitted on the basis of Kronig-Penney calculations assuming a valence band offset of 20 to 40%. The fitting procedure, however, is rather insensitive with respect to the value of the valence - band offset. Therefore, measurements of the exciton Zeeman splitting have to be taken into account besides the zero field data. Corresponding experiments are in progress. Furthermore, a detailed knowledge of the strain field in the samples from X-ray diffraction measurements is desirable for a more exact determination of the valence-band ZnSe/(Zn.Mn)Se

offset

in ZnTel(Zn,Mn)Te

and

quantum well structures.

and Microstructures,

Vol. 12, No. 3, 1992

[2]

O.Goede, W.Heimbrodt, physica stafus solidi (b) 146 (1988) 11

[3]

O.Goede, W.Heimbrodt, Fesrkiirperprobleme (Advances in Solid Slate Physrcs) 32 (1992) W.Xie, D.C.Grillo, M.Kobayashi, R.L.Gunshor, G.C.Hua, N.Otsuka, H.Jeon, J.Ding, A.V.Nurmikko,

[4]

Journal of Crystal Growth 117 (1992) 1079 [5] [6]

[7]

[8]

[9]

J.M.DePuydt,M.A.Haase, J.Qiu,H.Cheng, Journalof Crystal Growrh 117 (1992) 1078 W.Heimbrodt, O.Goede, H.-E.Gumlich, H.Hoffmam, UStutenblumer, B.Lunn, D.E.Ashenford, Journal of Luminescence 48/49 (1991) 750 L.A.Kolodziejski, R.L.Gunshor, N.Otsuka, B.P.Gu, Y.Hefetz, A.V.Nurmikko Applied Physics Letters 48 (1986) 1482 L.A.Kolodziejski, R.L.Gunshor, N.Otsuka, B.P.Gu, Y.Hefetz, A.V.Nurmikko Journal of Crystal Growth 81 (1987) 491 W.Heimbrodt, O.Goede, I.Tschentscher, V.Weinhold, A.Klimakow, U.Pohl, K.Jacobs, N.Hoffmann; 7th

Trieste Semiconductor ‘Symposium on Wide-BandGap Semiconductors’; Physica B (1992) to be published [lo] A.Blacha, H.Presting, M.Cardona, physica status solidi (b) 126 (1984) 1 I [ll] P.Lawaetz, Physical Review B4 (1971) 3460 [12] B.Kuhn, W.Ossau, A.Waag, R.N.Bicknell-Tassius, G.Landwehr, Journal of Crystal Growth 117 (1992)

References

[l]

J.K.Furdyna, Journal R29

ofApplied Physics 65 (1988)

871 [13] A.Wasiele, P.Peyla, Y.Merle d' Aubigne, JE.Nicholls, D.E.Ashenford, B.Lunn, Solid State Communications 76 (1990) 263