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LUMINESCENCE STUDY OF A SET OF
Al0.27Ga0.73As/GaAs QUANTUM WELLS COUPLED TO A CONTINUUM
P. Brechet, C. Hermann, G. Lampel, A. Mourchid, F. Alexandre
To cite this version:
P. Brechet, C. Hermann, G. Lampel, A. Mourchid, F. Alexandre. LUMINESCENCE STUDY OF A SET OF Al0.27Ga0.73As/GaAs QUANTUM WELLS COUPLED TO A CONTINUUM. Journal de Physique Colloques, 1987, 48 (C5), pp.C5-475-C5-478. �10.1051/jphyscol:19875100�. �jpa-00226682�
Colloque C5, supplement au nall, Tome 48, novembre 1987
LUMINESCENCE STUDY OF A SET O F A l o ~ 2 7 G a o ~ 7 3 A ~ / G a A ~ QUANTUM WELLS
COUPLED TO A CONTINUUM
P. BRECHET, C. HERMANN, G. LAMPEL, A. M O U R C H I D ( ~ )
and F. ALEXANDRE*
Laboratoire de Physique de la Matiere ond den see'^), Ecole Polytechnique, F-91128 Palaiseau Cedex, France
"centre National D'Etudes des TBlBcomunications, 196, Rue Henri Ravera, F-92220 Bagneux, France
On a mesurd I'intensitd de la photoluminescence d'une sdrie d'6chantillons cornportant un nombre rn de puits quantiques A10,27Ga0.73A~IGaAs (rn = 1, 2, 10) couplds entre eux et h un continuum par une barrihre de largeur d variable. L'6tude en fonction de I'dnergie et de I'intensit6 d'excitation montre I'imporlance des effets d'interfaces.
We have measured the photoluminescence intensity of a series of samples with a number m of A10.27Ga0.73AsIGaAs quantum wells (m = 1, 2, 10) coupled to each other and to a continuum through a barrier with variable width d. The study versus the excitation energy and intensity emphasizes the importance of interface effects.
INTRODUCTION
We have studied the photoluminescence intensity of MBE-grown samples which are described in fig. 1 ; they differ by the number of periods (m) and the width of the barrier (d) which couples the quantum wells (QW's). This configuration, with the GaAs continuum, the Be doping on the surface and the narrow barriers, is adapted to negative samples ~ t h bwier electron affinity (NEA) photoemission experiments (1) and
\Ilidth
*
(A) and the number addresses the problem of vertical transport towards thewells mi I I
I I I I I I
conduction band Ga As
semiinsulating (ae) vacuum
valence band
fg.1 schematied band profile of the samples I
("~tagiaire du DEA de Physique des Solides. UniversitCs de Paris V1. VII et Orsay ("~roupe de recherche du Centre National de la Recherche Scientifique
Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:19875100
C5-476 JOURNAL DE PHYSIQUE
1.60 1.55 1.50
ENERGY (eVf
fig.2 Typical luminescence spectrum. The lines positions, in eV,are:
1/=1.618,lh=1.596,e~=1.573, und=1.507 and Be-b1.484
surface. The samples are labelled (m-d) according to their wells number m and their barrier width d.
The sample is cooled by contact with liquid nitrogen and illuminated by a He-Ne, Kr+ or dye laser (DCM). The photon energy hv, 1.65st-1~ 12.34 eV, allows to excite either the QWs, or the whole sample (~~AIGaAs=l.84eV).The exciting power P ranges from a fradion of mW to 70 mW ; foarssed on a cm2spot, it corresponds to a photon flux ranging from 10' to 1 02' cm9.s-'for the W s used in our experiments.
The luminescence spectrum of sample 10-72, under 1.65 eV excitation, is shown in fig. 2. It presents several lines : i) Recombination between the quantized n = 1 conduction and heavy (h)or light (I) hole states of the QW ( I h and 11 ). From the intensity ratio of these two lines we deduce a temperature of 90-100 K. The lower energy structure (ex) is probably of extrinsic origine (2) ; ii) Band-to-band recombination of free carriers in the undoped GaAs layers (und) ; iii) Band-to-acceptor recombination in the Be doped overlayer (Be-u).
The luminescence spectra of the other samples are similar to the one in fig. 2.For samples 2-50 and 10-50 the I h peak is 6 meV lower than the one for sample 1-50. This difference is understood in a simple calculation done for a single QW lying between infinite barriers and for a double well in the same conditions Note also that, in all samples, we were not able to observe the AlGaAs luminescence ,with our maximum exciting power,at any excitation wavelength .
I) INFLUENCE OF THE NUMBER OF WELLS (SAMPLES 10-50,2-50,l-50)
The curves in fig. 3 present the luminescence signal as a function of the excitation photon flux. The measurements are shown for hv=1.86 eV . We estimate a corresponding electron density of 3.10~5 e7cm3 for a lifetime of -10-9s and N-3.10'6 pNs. Also plotted is the variation of the und line in sample 10-50 which is in fact strict[v to the one observed in the other two samples. Versus the excitation flux the single-well sample 1-50 shows up an unexpected non-monotonic variation (see 9 Ill), the double-well sample 2-50 a step (which is.
for hv=1.86eV , at larger photon flux than the fig.3 range) and in sample 10-50 the signal grows continuously. The signal per well strongly increases with the number of wells. This is probably due to the accumulation of impurities at the first AIGaAsJGaAs growth interface (2,3) which kills the photoluminescence through non-radiative recombination. For 1-50, the QW is colle~ting all the AlGaAs arowth impurities. For 10-50 most wells should be free of such centers (sample 2-50 is an intermediate case). To get rid of this possible effect, we shall study the influence of the coupling of the wells on the (10-d) samples.
PHOTON FLUX (cm-2.s-I)
fig.3 Luminescence intensity as a function of the photon flux when m varies (hv=1.86eV and T=90K). The und curve is independent of m. Linear and quadratic variations are superimposed on the und experimental points.
fig.4 Luminescence intensity in sample 10-50 as a function of the photon flux for different hv 's (T=90K).
11) STUDY OF SAMPLES 10-27, 10-50, 10-72
As a function of N, the und line first increases linearly, then quadratically(see fig.3). We attribute the low flux behavior to the recombination with residual impurities : from the kink of the curve in fig.3 we estimate their number to crn-3 in agreement with the data obtained by Hall measurements. At higher fluxes, the recombination is intrinsic. The I h line intensity grows quadratically for two orders of magnitude variation of the flux intensity, as expected for intrinsic excitonic recombination. However at very low flux the signal increases faster with N .
The coupling through barriers of different width (4) influences the und and Ih lines (The B e d line, arising from the surface overlayer, is weakty affected. )For each value of N, the undline and the Ih line increase with the barrier width (decrease with coupling). At high power, the und line is only multiplied by -10 when d varies from 27.8 to 72.4 A ; but the 1h lines present a ratio -100 in the same conditions. At low power the und lines have a similar behavior in all samples, but a ratio -20 is observed on the I h peak by comparing samples 10-50 and 10-72. The decrease of luminescence signal when the coupling between the wells is increased can be explained by considering the propoilion of wavefunction in the barriers (5) ; according to this model, the interface recombination rate increases with the probability of presence of the electron in the barrier. This is the case for narrow wells or, like in the present case, narrow barriers.
C5-478 JOURNAL DE PHYSIQUE
Ill) VARIATION OF THE LUMINESCENCE livi tlJSi TY I C ~ SHMPLES 1-30 AND 2-50 WITH hv For sample 2-50, at any hv, we observe a step in the variation of the 1h line intensity . The
position of this step ,corresponding to N-Ns, occurs at kg@&when hv- but at very b w N, the 1h intensity is the same at any hv (1.77 Shv 5 1.86 eV): for example, Ns=3.75 l o 1 9 cm-2.s-I for hv-1.77eV , Ns-7.5 1 0 ~ ~ c m - ~ . s ' ~ for hv=1.84eV and Ns>2 1021cm-2-s-l for
The variation of luminescence intensity with N for different hv's in sample 1-50 is shown in Fig. 4. We verified that this unexpected variation with N is not due to a trivial artifact (sample inhomogeneity, heating, ...). The maximum and minimumshift towards photon fluxes when hv decreases ;the overall signal intensity seems to reach a maximum for hv close to the transition between the 2h and 2c quantized levels. A possible cause of intensity variation is the combination of growth and residual impurity effects in the well (6), together with tunneling towards the continuum near the surface. In this single well, both effects occur in the same place, whereas in the other samples containing several wells, these effects are separated in different regions. One may attribute this non-monotonic variation of luminescence intensity to an internal electric fiekl,depending on N. Due to the weak incident light flux, this electric field cannot build up from an unbalance between photoexcited carriers, but rather through impurity effects ; the number of centers can be estimated to -1015 cm-3 from the value of N at the maximum of the curve.
In addition to these effects, we observe a decrease of the Ih line on a few months scale whereas the und line is almost constant. This also occurs on samples which stayed on a shelf before study and were never illuminated. (Of course the comparisons presented in this paper have been made on a short time period). This long time evolution could be due to a migration of the growth impurities.
To conclude,the complexity of the behavior of samples containing several QW's shows the need of a careful study of the variation of luminescence signal with intensity and energy of the photon flux, before any comparison between different samples with variable m or d can be made.
The samples studied in the present paper by photoluminescence were designed for photoemission and the combination of both techniques should help to clarify the behaviors described here.
REFERENCES:
+Groupe de recherche du Centre National de la Recherche Scientifique
(a) Stagiaire du DEA de Physique des Solides, Universiths de ParisVI, VII et Orsay
(1) R.Houdr6, C.Hermann, G.Lampel, P.M.Frijlink and A.C.Gossard, Phys.Rev.Lett. 55, 17 (1 985)
(2) R.C.Miller, W.T.Tsang and O.Monteanu, Appl.Phys.Lett. ql, 374 (1982)
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(4) W.T.Masselink, P.J.Pearah, J.Klem, C.K.Peng, H.Morkoq, G.D.Sanders and Yia-Chung Chang, Phys.Rev. 8027 (1985)
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