Short Notes

phys. stat. sol. (b) 180, K87 (1993)

Subject classification: 78.55; S7.15

K87

Laboratoire d'Optique Physique, Ecole Supirieure de Physique et Chimie Industrielles, Paris') (a ) , Ecole Polytechnique, Dhpartement de Ghnie Physique, Montreal') (b ) , Laboratoire de Physique de la MatiPre Condensee de I'Ecole Normale SupPrieure, Paris3) ( c ) , and Equipe de Micro-optohlectronique de Montpellier, Unith de Recherche associh uu C.N.R.S. N o 392, Universitk de Montpellier 114) ( d )

Low Temperature Photoluminescence Spectra

BY P. TRONC (a), B. REID (b), H. MANI (b), R. MACIEJKO (b), A. N. T I T K O V ~ ) (c), J. L. LAZZARI (d), and C. ALIBERT (d)

Introduction We have previously recorded [ 11 the photoluminescence (PL) spectra of non-intentionally doped Ga,.,,In,,2,As,,,,Sb,,81 layers grown by liquid phase epitaxy and nearly lattice matched to p-type GaSb substrates. The laser beam power density was 3 kW cm-2. The spectra consisted of only one broad peak, the full width at half maximum (FWHM) of which increases with temperature from 10 meV at 4.5 K to 15.4 meV at 65 K. A slight enhancement could be noticed on the long-wavelength (low-energy) side of the peak when the temperature is less than 40 K approximately. We have shown [l] that, in addition to the contribution to the width of the peak due to the temperature, another contribution arises from the fluctuations of composition. The size of this last contribution is not so large that it could be calculated from the fluctuations of compositions alone since the local pressure associated with these fluctuations has an effect onto the bandgap opposite to that of the chemical composition: both effects partially compensate to reduce the fluctuations of the bandgap.

In our previous work, the nature of the transition associated with the PL peak has not been considered. It is widely known [2] that the PL spectra of pure GaSb crystals present numerous peaks, the intensities of which depend on the preparation characteristics : the so-called free exciton peak [2] is always very weak, whereas the main peak is located at an energy lower by 15 meV approximately and is generally assigned to a bound exciton. Moreover conduction band to acceptor transitions seem to be involved in the low-energy side of the spectra [2]. Data have been published about the PL spectra of GaInAsSb/GaSb heterostructures [3 to 61. In [6] it is shown that degenerate n-Ga,,8,1n,~,,As,,,sSb,,sl layers present at 2 K a peak near 0.62 eV, which is shifted to higher energy when the laser power is increased. Another peak is present near 0.685 eV, the intensity of which increases very

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K88 physica status solidi (b) 180

rapidly with the laser power. The two peaks have been assigned to transitions between the conduction band and, respectively, a negative and a neutral acceptor. Recently data about the PL spectra at 4.5 K of non-intentionally doped Ga,_,In,As,Sb,-, alloys lattice matched to GaSb have been published by Iyer et al. [7]. The free exciton peak which is weak in pure GaSb spectra [7] disappears rapidly when x and y are increased whereas the relative intensities of the other peaks undergo dramatic changes. For a sample (the y-11 sample, the composition of which is not given in [7]) with a gap close to 0.61 eV, i.e. close to the gap of our sample, they found a main peak, labelled BE;, at 0.6071 eV and two weak peaks, labelled BE; and A, at respectively 0.5941 eV and 0.5781 eV. BE; and BE', have been associated with the recombinations of excitons bound to two different neutral acceptors and A with the radiative recombination of a free exciton with a bound hole.

The plot of the bandgap of the GaInAsSb alloys lattice matched to GaSb versus their compositions has been published by Eglash et al. [8]. This plot results from a calculation based on the bandgaps of the binary alloys and the bowing parameters of the ternary alloys. Nevertheless, to get a more precise value of the bandgap of our samples and of its variation with the composition we used the results by Cheban [9] (Fig. 1) who deduced the bandgap of the alloys from the P1 spectra of heavily p-doped layers according to a procedure described elsewhere [lo].

Samples preparation and characterization The layer used for the measurements reported here is that already used in [l]. It has to be underlined that the X-ray double diffraction spectrum of this thick layer (approximately 4 pm thick) presents two peaks corresponding to two slightly different lattice parameters a, perpendicular to the layers [ 11. The corresponding perpendicular lattice mismatches of the layer with respect to the substrate, defined as AaJa, = (allayer - alsubstrate)/alsubstrale, are, respectively, + 6 x and

I 0.8

I, - L

a 5

t 5 -0

2 0.1

0.6

0 05

0.1 0.2

+ 3 x Castaing microprobe measure- ments have shown that x and y do not vary by more than 0.01 from their central values.

Experimental The PL measurements re-

sample were performed between 4.5 and 77 K with the 488 nm blue line of an Ar laser. The detector was a PBS cell cooled to

ported On Our Ga0,771n,.,,As0.,,Sb0,8~

- 30 "C.

Results and discussion A difficulty to detect possible weak peaks in the PL spectra of our ~ao,77~no,,,~s,,,,~bo.81 layer arises

Fig. 1. Bandgap versus x for Ga,~,In,As,Sb,~, undoped alloys lattice matched to GaSb; (a) at 2 K, (b) at 300 K

0.3 X-

Short Notes K89

M

I

2.00 2.05 210 2.15 2.20 I. (prnl-

Fig. 2. Photoluminescence spectra, recorded at 4.5 K, with the 488 nm line of an Ar laser, of the Ga,,,,Ino,,,Aso.,,Sbo~~~ alloy. The laser beam power density is (a) 125, (b) 250, (c) 500, (d) 1100, (e) 1400, (f) 1800 W cm-’

from the broadness of the unique peak previously recorded [ 11. This broadness could occult other weak peaks if they are too close to it. When measuring the PL spectrum of our layer, to make possible low energy peaks (when compared to the main peak which will be labelled hereafter M) appear, it is convenient to cool the sample to a low temperature (liquid helium). This makes the quantum statis- tical distributions very sharp since kT is equal to 0.388 meV at 4.5 K. On the other hand, a low level of excitation has to be used since the width of the M peak increases slightly with the level of excitation (Fig. 2). Doing that, a shoulder clearly appears on the long-wavelength (low- energy) side of the M peak when the laser beam power density is equal to or less than 250 W cm-2 (Fig. 2). This shoulder can be resolved in one peak located at 2.14 pm (0.579 eV) at 4.5 K, which will be labelled L. The difference in energy between the M and L peaks is 8 meV. It can be seen on Fig. 2 that the width of the M peak decreases and that the peak shifts toward lower energies when the laser power is decreased; this suggests that, at a low level of excitation, only the low energy side of the density of states corresponding to the M peak is populated.

To make possible high energy peaks appear, it is convenient to increase the temperature to get softer quantum statis- tical distributions and to adjust the excitation at a level high enough to

populate the high energy peaks but not such that the M peak becomes too broad. Thus, at temperatures around 48 K, a peak located at 2.06 pm (0.602 eV) appears (Fig. 3), which will be labelled H. The difference in energy between the H and the M peaks is 12 meV.

K90 physica status solidi (b) 180

2.00 2.05 2.70 2.75 2.20

(jd -

Fig. 3. Photoluminescence spectrum, recorded at 48 K, with the 488 nm line of an Ar laser, of the Gao,,,Ino,,,Aso,,,Sbo,~~ alloy. The laser beam power density is 570 W cm-’

Before going on, one may wonder if one peak in a given spectrum has the same assignment that the other peak in the same spectrum but is associated with the other mean value of the a, lattice parameter within the layer (see above). In [ 11, we have shown that a local variation of Aa,/a, equal to + 3 x lop4 within the layer induces, through the associated variation of pression, an increase of bandgap equal to 7 meV. Such an effect can be achieved by increasing the amount of In and/or Sb. But, on the other hand, an increase of x equal to 0.01 in our layer (which is lattice matched to GaSb) induces, due to the effect

of the chemical composition, a decrease of the bandgap equal to 5 meV (Fig. 1). Both phenomena have opposite effects on the bandgap since Ga versus In and As versus Sb have the smaller atomic radii. It is reasonable to admit that the values of x corresponding to the two mean values of a, do not differ by more than 0.01 (see above). Even if the actual variations of composition within the layer probably correspond neither to perfectly constant lattice parameter alloys nor to a constant bandgap when no pressure is applied, it can therefore be concluded that due to their too large differences in energy, the H, M, and L peaks correspond to transitions with different origins within the layer. The role of the inter- face with the substrate can be neglected because of the thickness of the layer (4 pm).

Coming back to our experimental results, we can conclude that the M peak, which is located 12 meV below the H peak, cannot obvioulsy be associated with free electron-hole recombination. It cannot also be associated with free exciton decay since the binding energy of the free exciton is less than 1.6 meV [l].

At 48 K, kT is equal to 4.1 meV; the H peak cannot therefore be associated with free exciton decay since the free exciton is dissociated at this temperature due to its weak binding energy. The H peak seems to be a candidate for free electron-hole recombination or bound exciton decay.

For the L peak, one can think of bound exciton decay or perhaps acceptor to conduction band transition.

It is interesting to compare the energy of the H peak with the values of bandgap proposed by Cheban [9]. For the nominal composition of our alloys (x = 0.23 and y = 0.19), he proposed, at 2 K, 0.606 eV with an accuracy equal to 0.005 eV (Fig. 1). We could not see the H peak at 4.5 K, probably because of the sharpness of the quantum statistical distributions. Nevertheless, if one assumes that the difference in energy between, respectively, the H and M peaks does not appreciably vary between 4.5 and 48 K, one gets an energy equal to

Short Notes K9 1

0.603 eV for the H peak at 4.5 K. (We refer, to determine the energy of the H peak at 4.5 K to curve (c) of Fig. 2, which corresponds to a laser beam power density equal to 500 W cm-2, therefore close that used to record the spectrum of Fig. 3, i.e. 570 W cm-2. It can be seen that the energy of the M peak does not appreciably vary between 4.5 and 48 K.) Keeping in mind ‘the accuracy of the results of Cheban [9], which is equal to 0.005 eV, one sees that the energy of the H peak at 4.5 K, i.e. 0.603 eV, can fit that of the bandgap, but can also correspond to a bound exciton with a binding energy equal to a few meV.

If we compare our results with those of Iyer et al. [7] for their y-11 sample, we can notice that they did not found any high energy peak, perhaps because they recorded all their spectra at 4.5 K. Their BE; peak corresponds to our M one. On the low energy side of the spectrum, the closest peak with respect to BEl they found is BE;, located at 13 meV below BE;, whereas in our sample, the L peak is located at 8 meV below the M peak. Such a difference has to be connected with the difference in the growth processes, liquid-phase epitaxy for our sample [l] and liquid-phase electroepitaxy for Iyer et al.’s samples [7].

Conclusion By varying the power density of the laser beam and the temperature of a Ga,,,,In,,,,As,.,,Sb,,~~ alloy, we have put in evidence in its PL spectra a weak peak on both sides of the main peak, which is located at 2.1 pm at 4.5 K. The two possible assignments for the high energy weak peak, the existence of which is reported for the first time in a compound with such a narrow gap, seem to be, respectively, free carrier recombination and bound exciton decay.

References

[ 11 P. TRONC, B. REID, R. MACIEJKO, J. L. LECLERCQ, and J . L. LAZZARI, Solid Sate Commun. 85, 177 (1993).

[2] E. T. R. CHIDLEY, S. K. HAYWOOD, A. B. HENRIQUES, N. J. MASON, R. J . NICHOLAS, and P. J. WALKER, Semicond. Sci. Technol. 6 , 45 (1991).

[3] A. N. TITKOV, V. N. CHEBAN, A. N. BARANOV, A. A. GWSEINOV, and Yu. P. YAKOVLEV, Soviet Phys. - Semicond. 24, 666 (1990).

[4] A. N. BARANOV, A. N. IMENKOV, M. P. MIKHAILOVA, A. A. ROGACHEV, A. N. TrrKov, and Y u. P. YAKOVLEV, Superlattice Microstructure 8, 375 (1990).

[5] A. N. TITKOV, Yu. P. YAKOVLEV, A. N. BARANOV, and V. N. CHEBAN, in: Proc. Twentieth Inter- nat. Conf. Phys. Semicond., Thessaloniki 1990; Ed. E. M. ANASTASSAKIS and J. D. JOANNOPOULOS, World Scientific, Singapore 1990 (p. 985).

[6] N. S. BRESLER, A. N. TITKOV, V. N. CHEBAN, and 0. B. GUSEV, Fiz. Tekh. Poluprov. 27 (1993), to be published.

[7] S. TYER, S. HEGDE, Ar.1 ABUL-FADL, K. K . BAJAJ, and W. MITCHEL, Phys. Rev. B 47, 1329 (1993). [8] S. J. EGLASH, H. K. CHOI, and G. W. TURNER, Proc. Sixth Internat. Conf. Molecular Beam

[9] V. N. CHEBAN, PhD Thesis, St. Petersburg 1990 (unpublished). Epitaxy, J. Crystal Growth 111, 669 (1991).

[lo] A. N. TITKOV, E. I. CHAIKINA, E. M. KOMOVA, and N. G. ERMAKOVA, Soviet Phys. - Semicond.

(Received August 23, 1993)

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