Photoluminescence studies on AlxGa1−xSb alloys

Journal of Luminescence 17 (1978) 301 o North-Holland Publishing Company

310

PHOTOLUMINESCENCE STUDIES ON M~Gai~~Sb ALLOYS J. ALLEGRE, M. AVEROUS and A. JOULLIE Centre d’Etudes d’Electronique des Solides (LA 21), Université des Sciences et Techniques du Languedoc, P1. E. Bataillon, 34060 Montpellier Cedex, France Received 28 De(ember 1977 Revised manuscript received 30 January 1978

Photoluminescence experiments have been performed on aluminum gallium antimonide alloy (A1~Gai ~Sb)at low temperatures for compositions ranging from x 0 to 0.53. This work permits the determination of the composition dependence of the direct band gap E 0 and gives evidence for the occurrence of two bands L and X minima for indirect energy gaps. We deduce cross-over points at x = 0.21, between the I’ and L bands, and at x — 0.40 between the L and X bands.

1. Introduction

The study of aluminum gallium antimonide is of interest because of the possibility of obtaining solar cells with a good efficiency. The mixed compound GaSh A1Sb possesses a singlilar point which separates the composition region into two parts,namely the GaSb side with a direct gap at k = 0 (F0 = F1~ I’~~~) and the AlSb side with an indirect gap. The preparation of Al~Ga1~Sb involves several problems, particularly that of the oxidation of the aluminum, therefore few studies have been undertaken; only Burdiyan [1] and Miller et al. [2] offer any information. Recently, the concentration at the cross over between the F and X bands has been determined by Mathieu and co-workers [3,4] using piezo-reflectance measurements, and by Ance et al. [5] using absorption and reflectivity measurements. Theoretically van Vechten [6] found the cross over at a value of x 0.325. Experimentally, Mathieu and Ance found x — 0.40 at room temperature and x — 0.44 at 100 K [3]. In contrast, Bedair [7], Cheng et al. [8] and Anderson et al. [9] found two cross over points of the conduction band minima: one between the F and Lbands, indicating the crossing of the direct and indirect gaps, and one between the L and X bands. Cheng [10] made the most extensive study from absorption and luminescence measurements on liquid phase epitaxial films. From the fitting of his experimental points and from the values well known for GaSb and AISb, he obtained the following three 301

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J. Allegre et al. /Photoluminescence studies on Al~Gai ~Sb

parabolic laws for the direct and indirect energy gaps at room temperature: Eo(f)(x) Ej(x)(x)

—

0.722 + 1.129x + 0.368x2, 1.020+0.492x +0.077x2,

and Ej(L)(x)

0.799

+

0.746x+ 0.334x2.

The most significant feature of these laws is the double cross over points along the lowest conduction band minimum: E(x)

EL(x)—O.960eV,

atx0.198,

EL(X)

Ex(x)

atx

—

1.318 eV,

—

0.557.

It follows that Cheng finds the bandgap in Al~Gai ~Sb is direct only forx 0.198. In this work we report photoluminescence experiments at 4.2 K in samples of Al~Gai~Sb in the composition range 0 ~x ~ 0.53. ‘~

2. Experimental results We start with the photoluminescence spectrum of GaSb in order to identify the different emission lines in GaA1Sb as the composition of Al increases. The luminescent spectra of ten samples, the compositions of which are given in table 1, are shown in fig. 1. It is seen that these is a general broadening of the main peak with increasing x. These spectra present generally a continuum with one or two maxima due to recombination levels. The excitonic complex lines are hardly visible and were not studied further. Sample 1 in fig. 1 has an Al composition of 3%. At liquid helium temperature its photoluminescence spectrum is comparable to that of GaSb with a large carrier concentration. The emission extends from 825 to 765 meV with an intense peak at 803 meV attributed to the residual acceptor level, as in GaSb (peak at 778 meV). A second maximum is seen at 818 meV. The spectra at higher temperatures, i.e. 20 40 K (fig. 2), have the same shape with a small blue shift of the principal peak to 805 meV. At the band edge, two shoulders are present at 834 and 829 meV, while the 818 line disappears at these temperatures, in agreement with the behavior of the excitonic line labelled yin GaSb [14,18]. We analyse the spectra of the other samples with larger Al composition in the same way. For sample 2 (5% Al) the energy position of the mean peak is 829 meV, and the highest emission occurs at 869 meV. For samples 3,4 and 5, having respectively an Al composition of 12, 15 and 16%, we note that the spectra in fig 1 are progressively less intense and so more difficult to analyse; the study as a function of temperature

778

The most important

Remarks

757 748

810(20 K) 805(20K) 803 800 (30—77 K) 796

The highest emission

peak (recombination on impurity level)

GaSb[1l,l8] 0

Sample Al composition

779

829

838

819 803

869

2 0.05

834(30 K) 829(30K)

1 0.03

n type

Te-doped

834

881

922

940

3 0.12

Table 1 Energies (meV) of various emissions noted on our spectra at 4.2 K (excep

304

J. Allegre et al. /Photoluininescence studies on Al~Gai ~Sh energy 1,20

( eV

110

0,90

0,80 GaSb

Al~ Ge 1

Sb

42K

D

3 54

lao

1.~0

120

130

140

150

150

~ t~tm)

Fig. 1. Al~Gai ~Sb photoluminescence spectra at 4.2 K for various compositions between 0 and 0.53, numbered in table 1. 085

080

375 1eV)

20K

1.44

1.52

t60

1.58

2’.(uml—.~

J. Alle~reeta!. /Photoluminescence studies on Al~Gai ~Sb 100

0.90

305

leVI

sample 4 115%Al I

15O~l< I

~0K

1.24

132

1.40 A (sm)—,.

Fig. 3. Photoluminescence spectra of sample 4 (x

=

0.15) for different temperatures.

is therefore necessary to characterize the various lines, such as the results shown in fig. 3. For the next samples, 6, 7,8 and 9, the intensity of the photoluminescence spectra continues to decrease drastically when x increases. More emission lines corresponding to excitonic complexes do not appear when the temperature increased. This indicates that their formation is really attenuated in these alloy compositions. The presence of two impurity levels at 35 40 meV and 55 60 meV from the edge band seen in GaSb is confirmed in GaAlSb alloys with a small shift. When x increases (fig. 1, samples 6, 7 and 8), the second impurity level becomes the most efficient recombination center. The emission tail shows several shoulders due to phonon replicas. Lostly, for sample 10 (53% Al), the emission intensity shows only a very weak continuurn between 1.24 and 1.13 eV, so that the position of the recombination center could not be deduced from this spectrum.

3. Interpretation Our interpretation is based on the analogy with GaSb, and the continuous change between two spectra when x slowly varies. The main emission line, as in GaSb, is

306

J. Allegre et al. /Photoluminescence studies on .4l~Gai ~Sh

Al~Ga

1~Sb

4.2 K

uJ

1,500

/

~ t~i.-.” —

1.250

m.”

_.-

-

.....

~

-

-.

‘

.7’S

-

i

‘~

1.00

r

)~ 0.368)

,

2 U to rO,69) 3

5 Ic = 0,077)

4

X (cr0 0, 48) 334)

—4’,

0.750

5 L

0 GaSb

0.10

0.20

0.30

cr

0,40 41

concentration

0.50 —~.

Fig. 4. Plot versus composition of the shortest wavelength peak Cv) and of the most intense peak (o).

attributed to an electron hole recombination on the acceptor level. Since the acceptor level is associated with the F 8 level, as is established in GaInSb [11], it is supposed that its ionization energy is rather constant. Here two remarks can be made, when the minimum of the conduction band is not at the center of the Brillouin zone. First, the oscillator strength for a F~ X1~or F15 L1~transition is lower than the one of a direct transition, so a radiative recombination is not attenuated when k * 0. Secondly the probability of having an excitonic complex is lower. Nelson et al. [22] has shown that the ratio between Auger’s recombination and radiation recombination is about iü~in indirect band gap semiconductors. This fact explain why the excitonic lines disappear in GaAlSb when x increases. In fig. 4 we have plotted the points deduced from our experimental results versus

composition. Two points are shown on the figure: 0 and y•0 is the most intense point of the spectrum, while vis the beginning of the emission, i.e. the point of the highest energy where an emission exists (the nearest to the gap). The iiitespretation

J. Allegre et al.

/ Photoluminescence studies on A1~Gai ~Sb

307

of GaA1Sb spectra in the range of x between 0 and 20% is made by comparison with GaSb [11] where there is no problem because the gap is direct. The highest energy emission is seen at 20 K generally and corresponds to the free exciton annihilation: we deduce the direct band gap energy which is higher, by about 2 or 3 meV, than hpexc. It is possible within this composition range, with the help of the well-known values for GaSh and A1Sb [12], to determine the parabolic law for F. We found Ep(x)

0.813 + 1.097x

+

0.400x2.

The value of the bowing parameter c of 0.40 is in very good agreement with the value deduced by Cheng who obtained c 0.368 at room temperature. When x >20%, we observe a break: the emission energies of the band edge do not follow the variation law of F. We must therefore study the significance of the photonic emissions. The selection rules of the interaction hamiltonian indicate the existence of a phonon with a specific (X 1 or X3) symmetry. The transition X1~ F~Sv is allowed with participation of an LO phonon, and the transition L1~ F15~~, is allowed with participation of an LO(X1) or LA(X3) phonon by taking as an intermediate state F1~,the state which has the highest probability. At k constant, the recombination energy is =

h~ E~ E~

hP(LOorLAphonon).

In the case where the recombination occurs via an acceptor level, hv—E~ EA

hP(LO0rLA).

It is well known that for LO phonons hPLO(GaSb) 26 meV [21], hvLo(AISb)

=

36 meV [19],

and for LA phonons hVLA(GaSb) = 19 meV [20] hVLA(AlSb) = 26.6 meV [19] In the virtual crystal approximation for a crystal with one phonon mode, we suppose that the energy variation of the longitudinal phonons is linear with the composition. From the band edge emission of samples 6, 7 and 8, with the molecular composition 24, 27 and 30%, a slope of 28.5 meV per % is deduced. This is in fairly good agreement with the variation of the L band proposed by Cheng [10] in the same range composition, but at higher temperature. Let us recall that when k * 0 both LO and LA phonons are participating in the

308

J. Allegre eta!. /Photolurninescence studies on Al~Gai ~Sb

emission process. Thus, the difference between the law 1 in fig. 4 and the shortest wavelength peaks (v) is 15 20 meV. This value corresponds to the LA phonons, and confirms the symmetry of this band.

We do not suggest a general formulation for the law of variation of the indirect band gap L with x, since we have only three points and because the values for the L band in GaSb and AlSb at 4.2 K are not wellknown. Nevertheless in the range 0.2
0.891 + 0.746x + O.334x

For x higher then 0.32 we cannot derive a conclusion because we have only a few The spectral spreading is due to the structural disorder in the alloys. The emission edges at x 0.39 and 0.53 do not follow the previous law. These points indicate a slope of 5.5 meV per %, similar to the X band variation (curves 3 or 4 in fig. 4). spectra.

—

L

Al, Ca 1,Sb

4. 2 ‘K

2.40

U Ic

040)

2.20 200

L (coO 33

1 80 )cO 50

1 60

140 120 100

080 060 0

Ga Sb

01

02

03

04

0,5

0.6

0.7

08

4) concentration

0,9

1 4) Sb

Fig. 5. Variation of the band extrema with composition that we have deduced from our results (•).

J. Allegre eta!. /Photoluminescence studies on Al~Gai ~Sb

309

Ifthe longitudinal optical phonon is taken into account with hPLO = 30 meV at 40%, the X band edge is situated at 1.200 eV. For x = 50% this value becomes 1 .250 eV. These results are lower than those found by using the law given by Cheng which is almost linear (c 0.077). The difference is about 120 meV. The bowing parameter proposed by Mathieur et al. [3] of c 0.48 is in better agreement with our results. The difference is again about 50 meV, but this can be explained by the fact that the b~’ndedge emission due to excitonic complexes has disappeared, and the “usual” acceptor pair recombinations which are characteristic of pure GaSb are strongly

quenched. Finally the band structure of the Al~Gai ~Sb that we propose is indicated in fig. 5. The corresponding laws are: Er(x) —0.8 13 + 1.097x + 0.400x2, EL(x) —0.891 +O.746x+0.334x2, Ex(x)’~1.11 +0.07x +O.50x2. 4. Conclusion This photoluminescence study of GaA1Sb has enabled us to determine the bowing of the F and L subbands. The bowing parameters are small probably due to the small differences between the two lattice constants (ilGaSb = 6.09 A. aA)Sb = 6.13 A). Hence.

Vegard’s law, which is a linear approximation, is still essentially valid in this case. From the free exciton annihilation energy with band variation the composition x is determined: Eo(r)=0.813+ l.097x+0.400x2. In the range 20 30% the results can be explained by taking into account the electron phonon interaction and the presence of the L point as a conduction minimum. The bowing parameter of the L band parabolic law is similar to that given by Cheng (c 0.3 34). The cross over between the F and L subbands occurs at x 21% corresponding to a value of Eg = 1.046 eV. We cannot make definitive conclusions when x is higher than 30%. Nevertheless, it can be seen that the bowing parameter of the X subband is roughly 0.50, so that a second cross over between the L and X subbands is expected at about x = 0.40 and E= 1.24eV. —

References [1] 1.1. Burdiyan, Soy. Phys. Solid State 42 (1960) 3012. [2] J.F. Miller, H.L. Goering and R.C. Himes, J. Electrochem. Soc. 107 (1960) 527.

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J. Allegre eta!. /Photoluminescence studies on A1~Gai ~Sb

[3] H. Mathieu, D. Auvergne, P. Merle and K.C. Rustagi, Phys. Rev. Bl2 (1975) 5846. [4] D. Auvergne, P. Merle, A. Zein-Eddine, H. Mathieu and A. Nguyen van Mau? Solid State Commun. 17 (1975) 511. [5] C. Ance, J. Robin, A. Nguyen van Mau and G. Bougnot, Solid State Commun. 15 (1974) 1295. [6] J.A. van Vechten, J. AppI. Phys. B! (1970) 3351. [7] S.M. Bedair, J. Appl. Phys. 47 (1976) 4145.

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