Exciton luminescence of compensated SiC-6H

PHYSICA IId

Physica B 185 (1993) 313-318 North-Holland

Exciton luminescence V.V. Evstropov,

of compensated

I.Yu. Linkov, Ya.V. Morozenko

A. F. Ioffe Physico-Technical

Institute,

St. Petersburg

SIC-6H

and F.G. Pikus

194021, Russian

Federation

Photoluminescence of compensated and uncompensated epitaxial Sic-6H layers has been investigated. The layers were grown by container-free LPE and contained different concentrations of nitrogen (donor) and aluminum (acceptor). It is proposed that the high-temperature (500-900 K) photoluminescence (PL) of Sic-6H is mainly due to radiative annihilation of the free exciton. It is found that at high temperature, the shape of the exciton band is gaussian. Factors affecting the exciton band width are discussed. In closely compensated samples, there is a shift of the exciton luminescence band to lower energies. Exciton localization by the potential fluctuations is supposed. The free exciton luminescence intensity is found to depend markedly on the Al concentration. A decay model of the exciton in the vicinity of an Al acceptor in compensated silicon carbide is proposed.

1. Introduction

The degree of compensation is an important semiconductor characteristic. Optical methods are convenient for measuring the compensation level because they do not require the preparation of special samples or ohmic contacts, but are based on luminescence analysis of spectrum features. Compensation leads to band fluctuations and changes the donor-acceptor pair luminescence (DAP luminescence) [I]. In Sic-6H, compensation causes changes in the DAP luminescence. In photoluminescence (PL) spectra of highly compensated epitaxial layers of Sic-6H doped with N and Al, some distinctive phenomena have been observed: (1) a shift of the DAP luminescence by 120 meV to lower energies and smearing of the fine structure in the spectra; (2) growth of the PL quantum efficiency with increasing degree of compensation; (3) shift of the short-wave wing of the luminescence band with photoexcitation intensity. These three effects indicate the presence of Correspondence to: V.V. Evstropov, A.F. Ioffe Physico-Technical Institute, St. Petersburg 194021, Politechnicheskaya 26, Russian Federation.

0921-4526/93/$06.00 0

pronounced potential fluctuations [2] in the compensated Sic-6H epitaxial layers. PL studies in a wide temperature range (up to 900 K) revealed some differences in the free exciton (FE) luminescence from compensated and uncompensated samples. It is found that the FE can be localized in the band fluctuations. The localization occurs in the vicinity of the Al acceptor and causes a radiationless decay of the exciton. The investigation can be used to develop a method for local determination of the degree of compensation in SIC based on the features of the FE luminescence.

2. Sample preparation investigation

technology techniques

and

Luminescence measurements were made on Sic-6H epitaxial layers grown by container-free LPE from a Si-C melt [3] and doped with N and Al (main doping impurities in Sic). Two groups of epitaxial layers were grown, with donor concentrations of 0.5 x 10” cme3 in group 1 and 1.5 X 1018 cme3 in group 2, respectively. The acceptor concentration in both groups was varied

1993 - Elsevier Science Publishers B.V. All rights reserved

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V.V. Evstropov et al. I Exciton luminescence

by introducing different amounts of Al into the Si-C melt, from zero up to 6 wt%. It has been shown [4] that doping in such amounts results in an Al concentration proportional to that in the melt. By increasing the Al concentration, the layer conductivity could be changed from n- to p-type as a result of overcompensation. In group 1 samples, the compensation point was reached at 0.2 wt% of Al in the Si-C melt and in group 2 at 0.6 wt%. Photoluminescence was excited with 3.68 eV photons of a nitrogen laser under pulsed operation.

3. Experimental results In all the Sic-6H samples investigated, the luminescence near the band edge is observed up 900 K. At a low temperature (-80 K) in un-

of compensated

SC-6H

intentionally doped samples of n-type Sic-6H, a few well-resolved luminescence lines of free and bound excitons and a band of DAP N-Al are observed (fig. 1). An increase in temperature quenches the impurity luminescence effectively, while the 2.94 and 2.91 eV emission enhances, and already at T = 150 K, dominates the spectra of unintentionally doped samples. With further increase in temperature, the two lines broaden and at T = 300 K merge into band F. The band maximum hv,,, shifts to lower energy with increase in temperature (fig. 2). At temperatures above 650 K, band F becomes symmetrical with a shape fitting the gaussian curve well (fig. 1). In the range 600-900 K, the band F half-width (AS) increases in proportion to the temperature. It has been found that band F shifts to lower energies with increasing degree of compensation (fig. 3(a)). In closely compensated samples, the band shift is highest and equals 10 and 35 meV for groups 1 and 2, respectively. The DAP

2.8

200 2.4

2.6

2.8 ENERGY

3.0

3.2

400

600

TEMPERATURE

800 (K)

(ev) -

Fig. 1. Photoluminescence spectra of undoped epitaxial layers of Sic-6H at different temperatures T (K): l-83; 2-290; 3-550; 4-720; 5-900. Dashed lines correspond to approximations with gaussian contours.

Fig. 2. Characteristics of the free exciton in Sic-6H as a function of temperature: 1 - exciton bandgap E,, (after ref. [lo]); 2-energy of the maximum of the FE luminescence hv,,,,,; 3 - hu,,,,, - : kT.

V.V. Evstropov et al. I Exciton luminescence of compensated Sic-6H

315

2.86

2.82 300

400

600

500

TEMPERATURE

(K)

RECIPROCAL TEMPERATURE

(K-1)

Fig. 4. Temperature dependence of the exciton luminescence intensities (amplitudes in the spectral maxima) for Sic-6H epitaxial layers with different aluminium content NA, (wt%): 1-O; 2-0.0.5; 3-0.5; 4-0.65; 5-1.0; 6-1.9 (all samples from group 2).

Y

Fig. 3. Effect of the compensation on the (a) temperature variation of the energy of in Sic-6H epitaxial layers mum hv,,, content NA, (wt%): 1-O; 2-0.5; 3-1.0. (b) localization by potential fluctuations.

1000/T

FE luminescence: the spectral maxiwith different Al Scheme

of the FE

4. Discussion 4.1. Interpretation of high-temperature photoluminescence

luminescence in these samples is affected most dramatically, giving evidence for the potential fluctuations in the bands 121. In samples not intentionally compensated or with a low degree of compensation, the band F amplitudes (L,,) are weakly dependent on temperature (in the range 150-900 K), whereas in highly compensated samples with pronounced band fluctuations, the F luminescence is very weak at low temperature (~150 K) and rises with an activation energy of -30 meV for group 2 and -12meV for group 1 samples (fig. 4). At high temperature (750 K) in highly compensated samples, L,, is nearly as large as in uncompensated samples. At still higher temperatures, a weak quenching of the F luminescence takes place.

To interpret the high-temperature PL, we compare the temperature dependence of hv,,, with the width of the exciton forbidden gap EgX [5] (fig. 2). At low temperatures, hv,,, - ;kT is less than E,, by 105 meV which is the LO phonon energy. With increasing temperature, the difference between hv,,,,, - 4 kT and Egx decreases (to 60 meV at 8OOK), which can be related to the increasing share of excited states of the exciton contributing a growing number of lower energy phonons. On the other hand, the contribution from the electron-hole recombination should be small, because of the radiative lifetime value of 1O-5 s. This value is estimated in the same manner as used in ref. [6], whereas the radiative lifetime of the free exciton annihilation derived from data

316

V.V. Evstropov et al.

I Exciton luminescence of compensated SC-6H

on the kinetics of electroluminescence [7] is of the order of lo-” s. The correlation between the temperature variations of hv,,, and E,, and the small contribution of the electron-hole recombination give reasons to believe that the F band is mainly due to FE luminescence. 4.2. Factors causing broadening of the free exciton luminescence

band

We found that the FE band halfwidth, AS, increases with temperature more quickly in SiC6H than in Sic-3C. At T = 300 K, the AS values are 90 meV for 3C [S] and 115 meV for 6H, but at 900 K: they correspond to AS,, = 200 meV [8] and AS,, = 350 meV. Let us consider the factors that cause broadening of the FE luminescence band. One is the thermal distribution of the excitons over available energy states in the excitonic band ( 5kT = 120 meV at 900 K), the second is the involvement of different types of phonons in the exciton annihilation. At higher temperatures, different phonon lines merge into a broad band. In a study [9] of the shape of the FE luminescence band in GaP in the temperature range 80-300 K, it was demonstrated that the band is asymmetric and that it can be fitted well with a calculated curve taking into account the energy distribution of excitons and the involvement of different types of phonons. We have carried out investigation of the FE an experimental luminescence in undoped epitaxial GaP layers over a wide temperature range (up to 680 K) and also found the experimental and calculated band shapes to be in good agreement. The maximum broadening of the excitonic band for SIC due to the involvement of TA, LA, TO and LO phonons cannot exceed 60 meV. AS of the FE luminescence in 3C-Sic at high temperatures does not exceed the contributions of the above two factors. In Sic-6H, however, the band broadening in the temperature range 650-900 K is almost twice the sum of the contributions from thermal distribution of the FE energy and from phonon participation. Raman scattering studies of the

vibrational spectrum of Sic-6H [lo] have shown the presence of an additional mode at 235 cm-’ with an anomalously high damping (r = 450 cm-‘) that is not found in the 3C polytype. This low-frequency mode might be a final disintegration state of phonons taking part in excitonic luminescence and a cause of the phonon spectrum broadening in Sic-6H. This disintegration scheme appears highly probable for two reasons. Firstly, phonons taking part in luminescence might turn into two highly damped vibrational modes; secondly, anomalously high damping of the final state alleviates the selection rules for the momentum and symmetry. At low temperatures, damping of phonon modes is low, hence the small bandwidth of FE exciton luminescence. At high temperatures, the possibility of phonon disintegration in Sic-6H can be very high considering the large number of final states available for disintegration. Broadening due to a number of factors of a different nature is certain to result in a gaussian shape of the emission line. The broadening mechanisms of the FE band considered above give a linear variation with the temperature which is consistent with the experiment 4.3. An approach to modeling the exciton in compensated

Sic-6H

We assume that reducing the FE band maximum in compensated samples is related to the emergence of potential fluctuations of the bands caused by fluctuations of donor and acceptor concentrations. Free carriers become localized in the potential wells of the fluctuations. When free carriers form excitons as a result of Coulomb attraction, they localize between the extrema of the bands, in areas of the highest electric field strength (fig. 3(b)). It is estimated that the excitons are most likely to localize by small-scale potential fluctuations in areas where an average fluctuation scale of donor or acceptor concentration is only slightly in excess of the exciton Bohr radius [ 111. This means that the exciton would be localized in the

V.V. Evstropov et al. I Exciton luminescence

neighborhood of ionized acceptors (in n-type crystals). Then follows a decay of the exciton as a result of the hole capture by the ionized acceptor. The difference between the hole binding energy (240meV) and that of the exciton (78 meV) may be given over to what remained of the exciton, the electron, in the form of kinetic causes deenergy*. Raising the temperature localization of the excitons trapped by the band fluctuations and thus enhances the exciton luminescence. Strong damping of the FE luminescence is also observed in p-type (fig. 4), signifying that the small-size fluctuations persist in the overcompensated material. In addition, DAP spectra show features which indicate the presence of largescale fluctuations [2]. Thus, the luminescent properties of overcompensated p-type layers provide evidence for the presence of closely compensated inclusions. From the observation that damping of the exciton luminescence in p-layers l.Or

8

I

O

I

1

I

2 ALUMINUM

3

4

5

of compensated

Sic-6H

317

progresses with increasing concentration of both aluminum and nitrogen (fig. 5), it may be suggested that the total volume of compensated inclusions increases with an increase in the net concentration of dopants. It appears that the small-scale fluctuations are formed during growth of compensated epitaxial layers at low temperatures as a result of correlated incorporation of impurity atoms. In group 1 samples the luminescence is less affected by the fluctuations than in group 2. This is due to the fact that in group 1 the total impurity concentration is less than in group 2 by a factor of 3 and the fluctuations are less pronounced [ 121.

5. Conclusion

It has been experimentally shown that the potential fluctuations due to compensation are revealed in the properties of both the DAP and FE luminescence. The FE luminescence features in compensated Sic-6H provide evidence for exciton localization by potential fluctuations. It is suggested that the free excitons in the vicinity of ionized acceptors (Al) cause radiationless decay of the exciton. The data obtained on FE luminescence can be used to develop a method for local determination of the degree of compensation in Sic not requiring electrical contacts.

I

6

CONCENTRATION

melt) Fig. 5. Amplitude of the exciton luminescence L,, at 300 K as a function of aluminium concentration for samples with different contents of nitrogen and aluminum: (1) group 1 samples with nitrogen concentration NN = 0.5 x 10” cm-3; (2) group 2 samples with nitrogen concentration N, = 1.5 X 10” cm-3.

Acknowledgments

(in wt% added to

* Obviously, this process is attractive from the viewpoint of energy considerations. Yet, the relevant cross-section seems extremely small because an intermediate state of the exciton bound to an ionized acceptor is unstable. Compensated Sic-6H excitons localized at the extrema of band fluctuations happen to be close to ionized acceptors. therefore the probability for the exciton to be destroyed by the ionized acceptor is very high.

The authors are grateful to VA. Dmitriev and A.E. Cherenkov for providing them with SiC6H structures and their assistance with the work. The GaP epitaxial layers were kindly supplied by L.M. Fiodorov. Fruitful discussions with W.J. Choyke, Yu.A. Vodakov, Yu.M. Altayskii, V.V. Rodionov are gratefully acknowledged.

References [I] T. Kamiya 1928.

and

E. Wagner,

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48 (1977)

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[2] L.M. Kogan and Ya.V. Morozenko, in: Amorphous and Crystalline Silicon Carbide 111, Springer Proceedings in Physics (Springer-Verlag, Berlin, 1992). [3] V.A. Dmitriev, P.A. Ivanov, Ya.V. Morozenko, I.V. Popov and V.E. Chelnokov, Sov. Pis’ma Zh. Tekh. Fiz. 12 (1985) 240 (in Russian). [4] V.A. Dmitriev, S.V. Kazakov, V.V. Tretjakov et al. in: Extended Abstracts of the 176 Meeting of the Electrochemical Society, Hollywood, FL (1989) p. 711. [5] W.J. Choyke, Silicon Carbide - 1968 (Pergamon Press, Oxford, 1969). [6] P.J. Dean, M. Gershenzon and G. Kaminsky, J. Appl. Phys. 38 (1967) 5332. [7] M. Ikeda, T. Hayakawa, S. Yamagiwa, H. Matsunamy and T. Tanaka, J. Appl. Phys. 50 (1979) 8215.

of compensated

Sic-6H

[8] V.N. Rodionov, Zh. Pr. Spect. 48 (1988) 847 (in Russian). (91 R.Z. Bachrach and O.G. Lorimor Phys. Rev. B. 7 (1973) 700. [lo] L.I. Berezhinsky, V.A. Klimenko, P.A. Korotkov and M.P. Lisitsa, Sov. Phys. Tverd. Tel. 33 (1991) 134 (in Russian). [ll] V.V. Evstropov, I.Yu. Linkov, Ya.V. Morozenko and F.G. Pikus, Sov. Fiz. Tekh. Polupr. 26 (1992) 969 (in Russian). [12] B.I. Shklovsky and A.L. Efros, Electronic Properties of Doped Semiconductors (Nauka, Moscow, 1979) (in Russian)