OPTICAL STUDIES OF TYPE I AND TYPE II RECOMBINATION IN GaAs-AlAs QUANTUM WELLS

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OPTICAL STUDIES OF TYPE I AND TYPE II RECOMBINATION IN GaAs-AlAs QUANTUM

WELLS

K. Moore, P. Dawson, C. Foxon

To cite this version:

K. Moore, P. Dawson, C. Foxon. OPTICAL STUDIES OF TYPE I AND TYPE II RECOMBINA-

TION IN GaAs-AlAs QUANTUM WELLS. Journal de Physique Colloques, 1987, 48 (C5), pp.C5-

525-C5-528. �10.1051/jphyscol:19875112�. �jpa-00226695�

JOURNAL DE PHYSIQUE

Colloque C5, supplkment au n o l l , Tome 48, novembre 1987

OPTICAL STUDIES OF TYPE I AND TYPE I1 RECOMBINATION IN GaAs-AlAs QUANTUM WELLS

K.J. MOORE, P. DAWSON and C.T. FOXON

P h i l i p s R e s e a r c h L a b o r a t o r i e s , GB-Redhill, RH1 5HA, S u r r e y , G r e a t - B r i t a i n

Abstract

I n t h i s paper we report the results of a systematic investigation on the effects of electronic coupling on a series of G&-- rmlt' l e quantum well samples in which the thickness of the G& layers was fixed a t s 2 5 L d the AlAs thickness varied between samples from 392 t o 62. Using the techniques of photoluminescence and p h o t o l ~ s c e n c e excitation spectroscopy we have followed the positions of both the

r-related direct gap and the F-X pseudo-indirect gap a s a function of A&

thickness. A s the A l A s thickness is reduced we observe the system change from a type I1 t o a type I band alignment and measure t h i s crossover a t % l 3 8 of AlAs i n these samples.

Intrcduction

h G&-Al&ai-,As m l t i p l e quantum well (m) structures it is c o m n l y accepted that the fractional valence band offset l i e s i n the region 0.3-0.4 (1). One consequence of t h i s is that when the barrier material i s indirect

> 0.45), the Al&al-xAs X point lies a t lower energy than the GaAs X point. In this case the AlGaAs acts as a well for X-related s t a t e s while the GaAs forms a potential well for

r-related states. The valence levels a r e always preferentially localised i n the G&. Therefore by choosing appropriate quantum well parameters, GaAs-Alfial-,&i quantum well and superlattice structures may display either a direct r-related f u n h n t a l gap or a r-X pseudo-indirect gap. We s h a l l r e f e r t o these as type I and type I1 systems respectively.

Photoluminescence studies have identified type I1 recombination processes in A10.3fiw.6*-A1As heterojunctions (2) and also in GaAs-MO .$W.+ quantum wells under hydrostatic pressure (3). Further evidence for this phenamenm has been provided by studies of narrow G&-& quantum wells (4,5,6,7) . I n such structures it has been shown (6,7) that when the GaAs thickness is reduced below %35% the n=1 electron s t a t e in the GaAs i s pushed above the X minimum in the AZAs and the system becomes type 11. Finkman e t a1 (7) and Dawson e t a1 (6) have identified the lowest confined exciton s t a t e of the type I1 system in the excitation spectrum. Both groups assign the observed features t o exciton absorption inwlwhg electrons a t the

A1As minimum ie. with momentum parallel to the (001) direction, and holes a t the F-point i n the G&. Combining t h i s with the temperature dependence of the type I1 p h o t o l ~ e s c e n c e spectrum mwson et a1 (6) ascribe the emission t o the

recombination of localised excitons involving electrons a t the X, minimum, while Finlrman e t a1 (7) assign the main component of the type I1 emission t o excitons involving electrons a t the AlAs Xv valleys ie. mnoenta perpendicular to the (001) direction. However, the cornnon conclusion from a l l these reports is that in G&-AlAs WW w i t h GaAs thickness <352 the dominant reconbination process a t l o w temperatures involves the r-X pseudo-indirect gap.

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:19875112

(25-526 JOURNAL DE PHYSIQUE

The structures studied by hwson e t a1 had thin G& layers (<30@ but relatively wide AlAs layers $708 thus ensuring that the electrons in neighbouring GaAs layers were uncoupled. Calculations indicate (8,9) t h a t intrcducing coupling between adjacent G& layers results i n a lowering of the r-point subband minima and hence predict that as the AlAs thickness is reduced the system eventually reverts t o a type I band alignment. I n this paper we report photoluminescence (PL) and photoluminescence excitation (PLE) spectroscopy measurements on a s e t of samples w i t h a fixed GaAs thickness b258 in which the AlAs thickness was progressively reduced. I n t h i s way we have made a systematic study of the effects of coupling on a type I1 system.

Experimental Details

The samples reported on in this study were grown by MBE in a Varian Gen I1 system (10). The layers w e r e deposited on (001) orientated semi-insulating G& substrates a t a temperature of 630'~. The growth sequence was a s follows, (a) 1.0 um of G&

buffer material, (b) 60 periods of G& and AlAs where the G& thickness was fixed and fx AUS Mckness varied in the different samples, and (c) a capping layer of 1000 of G&. Five samples were gram for t h i s work and the thicknesses of the constituent layers were measured by X-ray diffraction techniques (11). The GaAs thickness was determined t o be 25*2g for a l l s l e s while the thicknesses of the

AUs layers were measured a s 39% 242, 168, 8 . 3 a n d 5.78.

The photoluminescence and excitation spectra were recorded with the samples mxlnted on the cold finger of a variable temperature (4-300K) continuous flow cryostat. The samples were excited by either an A r ion laser or a lamp and uxmochromator. The luminescence was collected and analysed by a double-grating Spex 1404 spectrometer and detected w i t h a cooled G& photormltiplier (C31034) and associated photon- counting electronics. The PLE experiments were performed by exciting the sample with radiation from either a lamp and scanning spectrometer or a tunable dye (IXX) laser, and monitoring the emission intensity as a function of excitation energy.

rimental Results and Discussion

Z t l y , consider the quantum well structure which has AUS layers b398. ~k low temperature PL spectrum from this sample is s h i n figure 1. The nature of the emission has already been discussed (6) and we identify no-phonon and

phonon-assisted exciton transitions h a l v i n g a hole a t the r-point in the G& and an electron a t the X-point in the adjacent AUS layer. In the same sample we can observe the energy transitions associated with the type I subbands by m i t o r i n g the intensity of one component of the type I1 emission and recording the PLE spectrum.

As shuwn i n figure 2 we observe strong type I exciton peaks involving n=l electrons confined a t the direct subband mininann (Elr) of the G& and either n=l heavy holes (HHl) o r n=1 light holes (W). Conbining these experimental techniques enables us t o follow the positions of both the r-related direct gap and the r-X indirect gap a s we reduce the AlAs thickness.

Consider now samples in which the AlAs thickness is progressively reduced t o 242 and 168. The law temperature PL spectra of these samples are illustrated in figure 1 and the corresponding PLE spectra are shown in figure 2. In each case the

l d n e s c e n c e can again be ascribed t o type 11 recombination. Notice however that the emission s h i f t s t o higher energy with decreasing A U s thickness. This r e f l e c t s the increase in the electron confinement at the X - m i n i m m in the AlAs. ^{I n the PLE} spectra (figure 2) we can identify the type I exciton transitions (Elr-HH1) and (El r-LH1). A s the MAs thickness is reduced we observe both a s h i f t of the type I transitions t o lower energy and also a decrease in the s p l i t t i n g of heavy and light-hole peaks. W e believe t h i s arises as a result of coupling adjacent GaAs layers and because the effects are most dramatic on the particles of smaller mass ie. the electrons and light-holes, the light-hole subband mves to lower energy more rapidly than the heavy-hole subband.

When the AlAs thickness is reduced s t i l l further to$8.48

observe a dramatic

s h i f t of the luminescence peak t o lower energy (figure 1). In this sample we assign

the emission as due t o type I recombination of electrons and holes both confined i n

the G&. lhis assignment has been confirmed by recording the PLE spectrum. The

(Elr-HH1) exciton peak i n the PLE spectrum and the emission peak are coincident within lmeV. A further decrease i n the AlAs thickness t o

5.7% results in a s h i f t

of the type I emission spectrum t o s t i l l lower energy; figure 1. This reflects a continued lowering of the r-point subband mknimm with increased coupling.

1.70 1.75 1.80 1.85 1.90 Emission energy (eV)

I ^I I J

1.80 1.90 2.00 2 .l0

Excitation energy (eV)

Fiwe 1 - ^Low temperature (7K) PL -e 2 - ^Low temperature (7K) spectra. The emission from samples w i t h PLE=tra showing the type I AlAs thicknesses of 39x, 242 and 16% is transitions for the samples associated w i t h a ^type 1 1 process. When described in figure l. 'lhe y axis the AlAs thickness is reduced t o 8.4g and has been displaced for clarity.

5.7% the emission becomes type _{I. The} y axis has been displaced for clarity.

1.94-

5 _{of the} (ElP-HHl) exciton peak as

measured by PLE spectroscopy.

. Ihe broken l i n e shows the

K

--- variation of the type I1 emission

a--*

l i n e w i t h AlAs thickness. The type 11-type I crossover occurs a t %138.

1.70 ^I

0 10 20 30 40

50

Thickness of ALAS

(a)

JOURNAL DE PHYSIQUE

surmazy

The r e s u l t s are s~mrarised in figure 3; the error bars represent the change in transition energy which arises as a result of the deviation i n G a A s thickness as measured by X-ray diffraction, from the nominal thickness of 258. W e observe that

for a fixed G& thickness of %252 we can reduce the A l A s thickness to %252 before any significant s h i f t in the I'-point subband energies is observed. Below 255( the result of coupling neigNxxlring GaAs layers is t o decrease the subband energies and we observe a rapid s h i f t of the type I exciton peaks t o lower energy. This is in agreement with the predictions of Bastard (8) and Duggan (9) but contrary t o the work of J.L. de Miguel e t a1 (12) who observe high energy s h i f t s of exciton peaks in G&-AlAs I%jW as the AlAs thickness is reduced. S h l t a n e o u s l y , as the AlAs

thickness is reduced the confinement of the electron a t the XmiTlirmrm i n the AlAs i s increased and we observe the type I f emission s h i f t t o higher energy.

In order t o plot the energy dependence of the r-related direct gap and the

r-X indirect gap in a precisely quantative mmer one should measure the exciton absorption peaks associated w i t h the transitions and take into account exciton binding energies. It should be pointed out that the data presented here for the type I1 systems maps the energy s h i f t of the emission l i n e rather than the

absorption peak, although we do not expect t h i s t o have a significant effect on the shape of the plotted curve.

W e can conclude, therefore, that in type I1 GeAs-AlAs the system reverts to a type I band alignment when there is sufficient coupling between the G& layers t o pull the lowest mnfined electron s t a t e in the GaAs belaw the X minimum in the A&. For a GaAs thickness of ' ~ 2 5 2 t h i s Cype 11-type I crossover occurs a t % l 3 1 of AlAs. Further analysis of the data w i l l allow us t o define precisely which electron s t a t e (either o r X- at the AlAs X point) is the lowest f o r the structures studied. Tnis is particularly important i n light of the work of Ihm (13) who has proposed that Xv s t a t e s may be below X, s t a t e s for very thin AlAs samples.

Ackmwledpement

The authors are grateful t o C.J. Curling and P.F. Fewster for the X-Ray diffraction measurements and t o D. Hilton for assistance with the MBE grawth.

References

1. WGGAN G., J. Vac. Sci. Technol. B3, (1985) 1224.

2. MWSON P, WlLSON B.A., TU C.W. anFMILZ.FR R.C., Appl. Ebys. Lett . ^{48, (1986)}

541.

3. WOLFDRD D.J.. HEUCH T.F.. BRADLEY J.R., GFLL M.A. and D., J. Vac. Sci.

Technol . ^B4, - (1986) 1043 1

4. BRFi L., m E M J R C., DE MIGLEL J .L., BRIONES F. and PLOOG K., presented a t the 18th I n t . ^{Conf. m the} Physics of Semiconductors, Stockholtn (1986).

5. DANAN G., JEAN-LOUIS A.M., ALMANDRE F., ETCFNNE B., JUSSERAM) B., LWOUX G., W I N J.Y., MOLUrr F., PLANEL R. ,and SERMAGE B., presented a t the 18th Int.

Conf. on the Physics of Semiconductors, S t o c k (1986).

6 . DAWSON P., MOORE K.J. and FOXON C.T., Proc. Int. Soc. Optical Ehgineering, Advances in Semiconductors and Semiconductor Structures (1987) t o be published.

7. FINKt.IAN E., SIURGE M.D. and TAMARCO M., Appl. Phys. Lett. 49,(1986) 1299.

8. BASTARD G., Proc. 4th Course of the I n t . School of Solid-SGte Device Research, Erice I t a l y (1987), t o be published.

9. WGGAN G. and RALPH H.I., Proc. Int. Soc. Optical Engineering, Advances i n Semiconductors and Semiconductor Structures (1987) , t o be published.

10. FOXON C.T. and HARRIS J.J., Philips J. Res. 41 (1986) 313.

11. FEWSTER P.F., Philips J. Res., 41, (1986) 2 6 8 7

12. DE MIGUEL J.L., FWIGIARA K., TAPFER L. and PLOOG K., Appl. Phys. Lett. 47,

(1985) 836.

13. MM J., &l. Phys. Lett. x,(1987) 1068.