Volume 4, number 5,6 ,I MATERIALS LETTERS July 1986

IDENTIFICATION OF TWO BANDS IN THE PL SPECTRA OF SI LEC GaAs ON THE BASIS OF A STRAIN MODEL

Zhongji ZOU

Department of Materials Science, Shanghai University of Science and Technology, Jiading, Shanghai, China

and

Yuanxi ZOU

Shanghai Insrirute of Metallurgy, Academia Sinica, 865 Chang Ning Road, Shanghai 200050, China

Received 1 April 1986

The use of an empirical method is proposed for interpreting the 0.65 and 0.80 eV bands in the photoluminescence spectra of

SI LEC GaAs on the basis of a strain modeLThe EL2 centers and the simple arsenic antisite AsGa are shown to be responsible

for these two bands.

1. Introduction

It has been shown by Tajima [I] that the intensity profiles along the (110) direction of a (100) slice of SI LEC GaAs for the 0.65 and 0.80 eV bands are corre- lated with the EPD profile in a positive and a negative way respectively. Some attempts have been made with

a view to identifying the centers associated with these bands, but no definite conclusion has been reached.

In the present work, we try to attack this problem by means of an empirical method for comparing the intensity of different bands in the PL spectrum and a strain model described elsewhere [2 1.

2. A brief survey of the strain model

A strain model has been proposed by one of the authors [2] for the formation of the midgap level EL2 in GaAs, which is assumed to be the ternary complex

ASGaVGaVAs. Both AsGaVGaVAs and the simple anti-

site AsG, are assumed to originate from a double gal- lium vacancy according to the following reactions:

= AsGaVGaVAs or AsGa ’ ‘GaVAs) (1)

ASGa ’ ‘GaVAs * ASGaVGaVAs’ (2)

In as much as the formation of a defect complex

will result in a lowering of the strain energy and lat- tice distortion should be present in the neighborhood

of dislocations, one can expect that the higher the dis- location density, the more will the ternary complex AsGaVGaVAs be formed. Consequently, there should exist a positive correlation between the EL2 profile and that of the dislocation density (EPD), while a nega- tive correlation is expected between the AsGa and EPD profiles. In other words, a combination of our strain model and the stress model proposed by Holmes et al. [3] is needed to explain the profiles of dislocation den- sity, EL2, and the simple antisite AS,, simultaneously. Our model also explains the much lower concentration and more homogeneous distribution of EL2 in In- doped, dislocation-free SI LEC GaAs observed by Duseaux and Martin [4].

A corollary of the strain model is that the sum of AsGa and AsGaVGaVAs (EL2) along the ( 110) direc-

286 0 167-577x/86/$ 03.50 0 Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)

Volume 4, number 5,6,7 MATERIALS LETTERS July 1986

tion of a (100) slice of SI LEC GaAs crystal should be approximately constant.

3. An empirical method for interpreting PL bands

In order to apply the strain model for interpreting the 0.65 and 0.80 eV bands in the photoluminescence spectra of SI LEC GaAs, a convenient way is to make tentative assignments to the configuration of the radia- tive recombination centers associated with these bands

and then to prove it on the basis of the model. For this purpose, we choose the EL2 level (AsGaVGaVAs) for the 0.65 eV band after Tajima [l] in view of the close resemblance between their radial profiles, and the sim- ple arsenic antisite AsGa for the 0.80 eV band accord-

ing to the suggestion of Kikuta et al. [5] . For the sake of ascertaining whether these centers are actually re-

sponsible for the said bands, an empirical method for the summation of these bands with different photo-

luminescence efficiencies should, however, be found.

As a first approximation, we introduce a constant K to take care of the difference in photoluminescence ef- ficiencies of the two centers involved. Let I, and IB

represent the intensities of the 0.65 and 0.80 eV bands respectively, we have

X = ZA(O.65 eV) t KIB(0.80 eV), (3)

where 2 represents the sum of the concentrations of

the centers concerned, which should be constant along

the (110) direction of a (100) slice of an SI LEC crys- tal. It follows from eq. (3) that a straight line should

be obtained by plotting IA versus I,, if a constant K exists in accord with the abovementioned assumption. Conversely, the existence of a K value may be con- sidered as evidence for the reasonableness of our em- pirical method.

Two sets of experimental data [ 1,6] are available for testing the validity of our method of treatment of

data as well as the applicability of the strain model. It turns out that almost the same K values (0.47,0.49) are found from the two independent sets of data. The

Z radial profiles obtained are shown in fig. 1. It can be seen that the profiles remain substantially flat along

the (110) diameter, although small upward bends occur at the periphery, which might, in our opinion, origi-

nate from a reaction between the B,O, encapsulant

and the crystal 171.

The success of our empirical method as well as the close agreement between the two K values seem to

provide adequate support for not only our method of treatment of data but also our suggestion of the con-

-30 -20 -10

4r 40 . : 3:

I I I I I I 6 0 5 10 15 20 25 30 35

mm

Fig. 1. Radial profiles of ~(0.65/0.67 eV) and X(0.74/0.80 eV) PL bands after Tajima [ 1,6].

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Volume 4, number 5,6,7 MATERIALS LEl-l-ERS July 1986

figurations AsCaVGaVA, (EL2) and Asc, as the origin Terashima et al. from the same laboratory as Ishida et of the 0.65 and 0.80 eV bands respectively. al. has been found to exhibit a dominant 0.80 eV peak,

4. Effects of variations in crystal growth technique 5. Summary

4.1. Effects of ambient gas, crucible material and magnetic field

From the above analysis, it readily follows that the

details of the crystal growth process, such as ambient gas, crucible material and the introduction of a mag- netic field, can affect the relative intensity of the 0.65 and 0.80 eV bands through their influence on disloca-

tion density only. It is gratifying to note that this is ac- tually so from an examination of the data of Emori et

al. [7], Kikuta et al. [5] and Terashima et al. [8].

An empirical method has been devised for the sum- mation of two bands in a photoluminescence spectrum by introducing a constant K to take care of the dif- ference in radiation efficiency of the corresponding

recombination centers. This method has been applied to the identification of the 0.65 and 0.80 eV bands in

the PL spectra of SI LEC GaAs on the basis of a strain model. It is found that the 0.65 and 0.80 eV bands should originate from the EL2 level (As,-.,V,,V,,)

and the simple antisite AsCa respectively in agreement with the postulates of previous investigators.

4.2. Effect of water content of B203 encapsulant

Another complication comes from the effect of wet B,03 in increasing the peak height of the 0.65 eV band

[5,9]. This problem may be solved by assuming the existence of two bands which are close to each other

after Tajima [6].

The effects of crystal growth conditions on the

dominance of either the 0.65 eV or the 0.80 eV band are discussed in terms of their influences on dislocation density. The recent observation of Ishida et al. of the

apparent inconsistency between the optically deter-

mined EL2 concentrations and the “EL2” concentra- tions measured by the DLTS method may be resolved

in the same way.

4.3. Apparent inconsistency between DLTS and optical absorption methods for measuring “EL2” Acknowledgement

Ishida et al. [lo] have recently pointed out that the

optically determined concentrations of EL2 in n-CaAs are different from those measured by the DLTS meth- od, and have, therefore, doubted the optical method for the determination of EL2. In our opinion, this ap- parent contradiction can be easily resolved by assign- ing the simple antisite AsGa to the DLTS-measured “EL2” and AsGaVGaVAs to the optically determined EL2 respectively. It is gratifying to note that the op-

tically determined EL2 displays a W-shaped radial profile, while the DLTS-measured profile exhibits an M shape. Although these investigators have not re- ported the EPD data, we can expect that the crystals should have a low dislocation density, as they were grown by the LEC method from SiO, crucibles using dry B,O, in an Ar ambient gas. From an examination of the data mentioned in 4.1, low dislocation density may be expected under these crystal growth condi- tions. Anyway, the majority of the crystals grown by

We would like to thank Professor Tajima for help-

ful discussions.

Note added

Just after this article had been prepared, we ob- tained some new experimental results suggesting the presence of a new trap, probably AsGaVAs, in LPE

GaAs grown under As-deficient conditions. It has an energy level practically the same as that estimated by Yu et al. [ 1 I] for the 0.77 eV band. It is believed, therefore, that this center, which is presumably formed from AsGa and VA, during further cooling, or its com- bination with Aka, could be the origin of the 0.74/ 0.80 eV band. Further work is underway with a view to clarifying this point.

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Volume 4, number 5,6,7 MATERIALS LETTERS July 1986

References

[l] M. Tajima, Japan. J. Appl. Phys. 21 (1982) L227. [2] Y. Zou, to be published. [3 ] D.E. Holmes, R.T. Chen, K.R. Elliott and C.G.

Kirkpatrick, Appl. Phys. Letters 43 (1983) 305. /4] M. Duseaux and S. Martin, in: Semi-insulating 111-V

materials, eds. D.C. Look and J.S. Blakemore Ghiva, Nantwich, 1984) p. 118.

[ 5 ] T. Kikuta, K. Terashhna and K. I&da, Japan. 3. Appl. Phys. 22 (1983) L409.

[6] M. Tajima, Appl. Phys. Letters 46 (1985) 484. [7] H. Emori, K. Terashbna. F. Orito, T. Kikuta and T.

Fukuda, in: Semi-insulating III-V materials, eds. DC. Look and J.S. Blakemore (Shiva, Nantwich, 1984) p, I1 1.

[ 81 K. Terashima, T. Katsumata, F. Orito, T. Kikuta and T. Fukuda, Japan. J. Appl. Phys. 22 (1983) L325.

[9] J.R. Oliver, R.E. Faiman, R.T. Chen and P,W. Yu, Elec- tron, Letters 29 (1981) 839.

[IO] K. Ishida, A. Yahata and T. Kikuta, Japan. J. Appl. Pbys. 24 (1985) L250.

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