GaAs quantum wells

Solid State Conmwnications,Vo1.61,No.7, pp.423-426, 1987. Printed in Great Britain.

003%1098/87 $3.00 + .OO Pergamon Journals Ltd.

MANGANESE LUMINESCENCE IN AlGaAs-ALLOYS AND AlGaAs/GaAs QUANTUM WELLS F. Bantien and J. Weber Max-Planck-Institut fiirFestkGrperforschung Heisenbergstr.1; 7000 Stuttgart 00 Federal Republic of Germany (Received: Oct. 20; 1986 by M. Cardona)

Our low-temperature photoluminescence studies of Mn-doped Al,Gal_xAs-alloys reveal a linear shift of the Mn-acceptor level with increasing Al-content x. Using the Mn-acceptor as a reference level; we determine a valence-band offset between GaAs and AlAs of AEv=U.33eV. At x-O.25 the Mn-related donor-to-acceptor pair-transitions interchange with the internal d-shell transitions within the Mn-acceptor. Characteristic luminescence transitions in GaAs quantum wells doped with Mn are identified with the recombination of electrons with the Mn-acceptor bound holes. In these quantum wells, the Mn-level shifts with decreasing wellwidth parallel to the heavy-hole band.

Transition metal (TM) impurities III-V compound semiconductors have been the subject of intense studies during the last decade.' Most of the TM-ions substitute the metal site and give rise to deep acceptors. The recent suggestion by Langer and Heinrich,2 that the deep TM-levels can be used as a common reference energy in all III-V materials, renewed interest about the behavior of TMs in ternary alloys.3-5 According to this statement, it is possible to determine the band offsets of the alloys from the change in the binding energies of the TM-acceptors with alloy-composition. In the present work we confirm the data2 on the band offset in AlGaAs alloys by using the Mn-acceptor as a probe, and present new results on the Mn-acceptor level in AlGaAs/GaAs:Mn quantum well (QW) structures. Manganese-doped Al,Gal_xAs-layers are grown by liquid phase epitaxy (LPE) in the composition range Otx (0.45 on semi-insulating undoped GaAssubstrates. Quantum well structures comparable to those reported in Ref.6, but with an addition of metallic Mn to the GaAs-solution, are also grown by LPE. The heterostructure consists of a GaAs:Mn buffer layer, an AlGaAs cladding layer, a thin manganese-doped GaAs QW and an AlGaAs cladding layer at the surface of the structure. The aluminum content of the cladding layers differs from sample to sample and is in the range from 15% to 20%. The samples are investigated by in binary

low-temperature photoluminescence (PL) measurements. The PL spectra are recorded with the samples immersed in a liquid helium bath cryostat and cooled down to 1.8K. Excitation above the AlGaAs bandgaps is performed by the use of various lines of a Krypton-ion laser. The luminescence is dispersed by a 3/4m Spex spectrometer and detected by either a cooled photomultiplier with Sl-cathode or a cooled optical multichannel analyser (Semiconductor Industries). .A typical near-bandgap PL-spectrum of a Mn-doped GaAs sample is shown in Fig.la. Three characteristic features can be distinguished: (1) the excitonic decay at shallow impurities is found around 820nm, (2) donor-acceptor pair transitions (D",Ao) and the free electron to bound hole transitions (e,A") at the shallow acceptors in the wavelength regime from 830nm to 840nm, (3) the PL-structure at about 88Onm associated with the shallow donor-deep Mn-acceptor pair transitions (D",Ao)Mn and their LO-phonon replicas.7 Manganese forms a well-known deep acceptor in GaAs with a binding energy of -1lOmeV. However, the nature of this center is still under debate.8 Photoluminescence spectra of different AlGaAs samples with varying Alcompositions are shown in Fig.1. With increasing x, a shift to hisher energies is found for the excitonic transitions, the (D",Ao) and the (e.AO). The shift corresponds-directlyg.to the increase of the band gap energy AE = 1.247x (eV), and we determine fromgthe 423

424

A~G&s-ALLOYSAND AlGaAs/GaAsQUANTUMwELLs ENERGY [eVl 21 2.0 1.9 1.8 1.7 1.6 I I I I I I

01

600

1.4 I

02 03 04 05 X 650

700

750

WAVELENGTH

Fig.1:

1.5 I

800

850

900

t nm 1

Photoluminescence spectra of AlGaAs:Mn with varying Al-composition x. The arrows indicate the luminescence energy of the Mn-band EM,,. The inset shows EMn as a function of x.

position of the PL-lines the Al-content of our samples. The Mn-luminescence shifts also to higher energies with increasing Al-content. A pronounced increase in the linewidth of this transition is presumably due to different Al-Ga configurations within the second neighbor shell of the Mn-ions. The luminescence energy of the Mn-band (E,,) as a function of the Al-content x is?hown in the inset of Fig.1. The (DotAo) -band shifts linearly with x up to x= M8 .25. The increase of the transition energy AEMn can be described by AEMn

=

0.93 x

(eV).

Followin the suggestion Heinrich s , we try to use tor level as a reference termine the valence band

of Langer and the Mn-acceplevel to deoffset AE,:

AE,= AEg - AEMn + hEI

= 0.33 x (eV).

hEI is the increase of the shallow donor ionisation energy with x given in Ref.9. Our band offset is smaller than the value of -0.4x(eV) derived for the system AlGaAs:Fe in Ref.2 and is in disagreement with the recent calculations of heterostructure band offsets (see e.g. the recent paper by Cardona and Christensen1o and references therein). We interpret our smaller value as

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due to the still shallow character of the acceptor bound hole*. Recently Plot et al5 performed similar experiments on Mn-diffused AlGaAs. These authors reported also a smaller valence band offset of roughly l/3 of the established value of -0.4x(eV). Their result is even smaller than our value. To clarify this discrepancy, we are presently determining the binding energy of the Mn-acceptor by photoconductivity measurements on our samples. For Al-contents higher than 25%, however, the Mn-luminescence does not follow the linear dependence between x and EM,, as shown in-the inset of Fia.1. The line width increases and the PLlpeak energy remains at a fixed value of -1.55eV. The determination of the exact peak position is rather difficult, because of the overlap of the broad AlGaAs:Mn-luminescence with the near-bandgap transitions from the GaAssubstrate material. Plot et al.5 have also observed a broad Mn-luminescence band in the same wavelength regime for samples having an Al-concentration larger than 34%. The transition energy of this PL-band leads to the assumption that it is due to an internal optical transition within the Mn-ion. PL-measurements on Mn-doped GaPI revealed a broad luminescence band (halfwidth -0.16eV) peaking at -1.34eV with a zero-phonon transition at 1.534eV. This PL-band was explained by2$he internal transition 4T1+6A1 of Mn . Due to the slightly different crystal field parameters and the reduced bandgap in Al,Gal_,As for x(0.25 the excited Mnstate seems to be deaenerate with the conduction band. The-internal transition is quenched in these alloys and onlv _ the (e,AO) or (D",Ao) transitions appear. With an increase of the AlGaAs bandgap by raising x, the excited Mn state slides into the gap and gives rise to the internal transition. In GaP:Mn the internal spin-forbidden transition has a long radiative lifetime (T-lms) compared to the (DO,AO) transiti0ns.l' In AlGaAs:Mn time-resolved PL-measurements are therefore performed at liquid helium temperatures to clarifz+the interpretation of an internal Mn -transition. The measured lifetime of the observed (DO,AO) transitions in AlGaAs with ~~0.25 varies slightly from sample to sample (1-5~s) and depends on the manganese doping concentration and the Alcontent of the layers. The lifetime increases with decreasing Mn-concentration and increasing Al-concentration, due to the change in the Mn-binding energy. l2 Samples with Al-concentrations larger than 25%, however, give a much longer radiative lifetime of -0.6ms for the Mn-related transition. This value is comparable to the measured lifetime in GaP:Mn (~=lms).ll The

425

AlGaAs-ALLOYS AND AlGaAs/GaAs QUANTUM WELLS

Vol. 61, No. 7

luminescence energy, the broad halfwidth, and the long radiative lifetime of the Mn-transition in AlGaAs lead US to the assumption that the observed luminescence band in our samples is due to-the internal '+T,j6A, transition of Mn”+ as it is for daP:&i.II The shift of the Mn-level in AlGaAs layers with different bandgap energies is typical for deep levels. Also in QW structures the band edge exhibits a drastic shift with the layer thickness L. We study the energy variation of &e Mn-luminescence in QWs with different Lx. As is shown in Fig.2 the in-

--iLJL FWHM = 4mev GoAs

I

side of these GaAs:Mn-transitions (marked by arrows) appears only in QWstructures and only if these are doped with Mn. A similar PL-peak was also detected by Petrou et al.lb in GaAs/ AlGaAs multiple QW-structures and attributed to the (CB+AO) recombination of an electron from the n=l conduction band state with a hole bound to the Mn-acceptor. the With decreasing well-width L (CB+AO)-transition shifts to higg;r energies. In Table 1 the peak positions of the band-to-band transitions and the Mn-luminescence of the QW are compared for the samples with different wellwidths. Within the experimental error of about lmeV, the binding energy Ei(Mn) of the Mn acceptor-level determined from both transition energies is not affected by Lx. According to our results the Mn-level shifts parallel to the heavy-hole subband of the QW with decreasing Ls. Our results are different from those reported in Refs. 5 and 15. These authors determined a 17% increase in the binding energy already at a well-width of around 7nm, while in our samples no change of the binding energy is found down to our smallest well-width of 8nm.

1

-

GaAs: Mn

Table 1:

1 I

_h;

& 35

./“-

810 m5

Fig.2:

870 875 880 885 890 WAVELENGTH 1 nm I

Peak energies of t e band-toband transitions E the Mn-luminescenc?EBRdof the quantum well cornpEed to the binding energy Ei(Mn) of the manganese acceptor

EQw/eV

E$i/eV

1.5155 1.5173 1.5188 1.5215 1.5221 1.5226 1.5243 1.5255 1.5580

1.4118 1.4137 1.4147 1.4169 1.4179 1.4186 1.4198 1.4210 1.4532

Ei(Mn)/meV

820

Photoluminescence spectra of AlGaAs/GaAs:Mn heterostructures with varying quantum well quantum well.

vestigated samples with L,>8nm exhibit a strong n=l band-to-band transition with a half-width (FWBM) smaller than 10meV. This transition appears at higher energies than the excitonic transitions of GaAs. The energy belonging to these near-bandgap lines is used to determine the well-width of the QWs according to Ref.13, but with the value of 0.4x(eV) for the valence band offset. In addition to the QW- and GaAstransitions in the wavelength regime around 815nm, another PL-structure around 880nm is found in the samples (Fig.2). The GaAs:Mn luminescence at about 881nm with the TA-phonon replica at 888nm originates from the Mn-doped GaAs buffer layer of the heterostructure. The shoulder at the high-energy

103.7 103.6 104.1 104.6 104.2 104.0 104.5 104.5 104.8

The valence band offset AE, in AlGaAs-alloys is determined from the shift of the Mn-acceptor level with increasing alloy composition. From our measurement we determine AEv=0.33x(eV). This offset is smaller than the value found for the system AlGaAs:Fe and is due to the partly shallow character of the Mn-acceptor bound hole. A new optical transition is found for higher Al-contents of the alloys. The luminescence energy, the broad half-width, and the long radiative lifetime of -0.6msec ;:E,e;y"+:f

:~:,T~et~~t~~sa',~~~n~~-

QW-stru&ur&s, the Mn-acceptor pinned to the heavy-hole band. binding energy shows no change decreasing well-width (Lz<8nm) QWs.

level is The with of the

426

AlGaAs-ALLOYSAND AlGaAs/GaAs QUANTUMWELLS

Acknowledgments - We thank H.J. Queisser and E. Bauser for their encouragement and interest. The help of K. Kijhler and the technical assistance of W. Heinz are

Vol. 61, No. 7

gratefully acknowledged. We like to thank B. Plot for sending us a preprint of Ref.15.

References

1. For a review, see B. Clerjaud, J. Phys. C18, 3615 (1985) 2. J-M. LaGer and H. Heinrich, Phys. 1414 (1985) and in Rev. Lett. E, Festkoerperprobleme (Advances in Solid State-Phsics, Vieweg, 19861, Vo1.26, pp.251, Ed. P.Grosse 3. L. Samuelson, M.-E. Pistol, and S. Nilsson, Phys. Rev. BG, 8776 (1986) 4. S. Nilsson and L. Samuelson, in Proceedings of the 14th International Conference on Defects in Semiconductors, Paris, 1986, to be published 5. B. Plot, B. Deveaud, B. Lambert, A. Chomette, and A. Regreny, J. Phys. Cx, 4279 (1986) 6. K. Keltins, K. Kahler. and P. Zwicknagl, Appi; Phys. Let;. 48, 157 (1986) 7. W. Schairer and M. Schmidt, Phys. Rev. BE, 2501 (1974) 8. A. Zunger, in Solid State Physics (Academic Press, 1986), Vol. 39, pp. 275, Eds F. Seitz and D. Turnbull

9. S. Adachi, J. Appl. Phys. 58; Rl (1985) 10. M. Cardona and N.E. Christensen; Phys. Rev. B, to be published 11. A.T. Vink and G.G.P. van Gorkom; J. Lumin. 2, 379 (1972) 12. D.G. Thomas, J.J. Hopfield, and W.M. Augustyniak, Phys. Rev. l&, A202 (1965) 13. R. Dingle, W. Wiegmann, C.H. Henry, Phys. Rev. Lett. 13, 827 (1974) 14. A. Petrou, M.C. Smith, C.H. Perry, J.M. Warlock, J. Warnock, and R.L. Aggarwal, Solid State Commun. 55, 865 (1985) 15. B. Plot, B. Deveaud, B. Lambert, A. Chomette, and A. Regreny, in Proceedings of the 14th International Conference on Defects in Semiconductors, Paris, 1986, to be published