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Excitation spectroscopy, Raman scattering and the temperature dependence of the luminescence in highly excited red HgI2
Journal of Luminescence 20 (1979) 151—161 © North-Holland Publishing Company
EXCITATION SPECTROSCOPY, RAMAN SCATFERING AND THE TEMPERATURE DEPENDENCE OF THE LUMINESCENCE IN HIGHLY EXCITED RED Hg!2 G. KURTZE, C. KLINGSHIRN Institut Jllr angewandte Physik der Unlversitdt, D 7500 Karisruhe, Fed. Rep. Germany
B. HONE RLAGE Laboratoire de Spectroscopie et d’Optique du Corps Solide, F 67000 Strassbourg, France
E. TOMZIG Inst itut für We,kstoffwjssenschaften VI der Universitdt, D 8520 Erlangen, Fed. Rep. Germany
and H. SCHOLZ PhilipsForschungslaboratorium, D 5100 Aachen, Fed. Rep. Germany
Received 5 September 1978
The luminescence of red Hg12 is investigated as a function of temperature, excitation intensity and wavelength. At high excitation intensity and low temperature an “M-band” emission dominates. This M-band is assigned to biexciton decay and bound exciton scattering with acoustic phonons (“acoustic wing”), this assumption being supported by the results of excitation spectroscopy. The energy of the biexciton is determined to be (4661 ± 1) meV. From the evaluation of Raman spectra, the phonon energies (1.9, 3.1 and 14.0 ±0.2) meV are found. At higher temperatures two lines are observed, one of which is ascribed to exciton—free carrier scattering. Position and line shape are in good agreement with theoretical results. The other emission line is found to be due to scattering involving excitons or carriers bound to lattice defects.
1. Introduction Red Hg12 is a layered semiconductor with a tetragonal unit cell, its point group being D4h. The band structure is similar to those of the H VI compounds with the wurtzite structure: there is band a conduction band with originating from 2~-6s-1evels. The valence (1, 5p-levels) is splitsymmetry into threeF~, bands, A, B and C, Hg with symmetries I’, r~,r~, respectively, due to spin—orbit coupling and crystal field. 151
152
G. Kurtze et aL
/ Luminescence in highly excited red Hg12
We investigated HgI2 at both low and higher temperatures. At low temperatures and with high excitation intensity, two characteristic bands are to be seen. These have been ascribed to biexciton decay [1,2] leaving either a longitudinal or a transverse free exciton (ML, MT-band) [2]. From this, the biexciton binding energy ‘iex has been determined to be 15 meV [1] or 4.6 meV [2] In the wurtzite II VI compounds a similar M-band is observed, which shows some properties of biexciton decay; however, this is mainly due to impurity transitions [3]. Therefore, it seems necessary to collect more information for Hg!2, too, before the M-band can be attributed to biexciton decay. In the present paper results of excitation spectroscopy are reported. This technique has been used successfully on CuC1 for the identification of biexcitons [4]. At higher terriperatures, i.e. in the region between 40 and 200 K, highly excited Hg!2 shows two bands which have been assigned to free exciton scattering with either electrons or holes [5,61.From this, Goto et al. [5] determined the ratio of electron to hole mass to be 0.09. In the theory used by them [7], polariton effects and the momentum dependence of the matrix element are neglected. Therefore, only qualitative results can be expected [8]. The effective masses are not so well known: values are ranging from mh = 0.4 to 1 .2m0, and from me = 0.2 to 1 .6m0 [9—13].Most probably, the hole mass is mh = 0.58m0 [10] as measured by cyclotron resonance experiments. Hanerlage et al. [8] have carried out a more detailed calculation for the exciton free carrier scattering process. Using their results, we try to decide between the different sets of effective masses mentioned above. Different authors also give different values of the free exciton energies. Our own values are hwL = 2.339 eV (longitudinal free exciton), L~LT= 5 meV corresponding to [14];the position of the triplet exciton relative to the transverse polariton is deduced from [15] .
2. Experimental setup The Hg!2 crystals can be excited by light emitted from either a 400 kW nitrogen laser (Molectron UV-400) or a 4 kW tunable dye laser pumped by the N2-laser. 2 and 1 MW/cm2, respectively. TheThe maximum excitation intensities are 5 MW/cm N 2-laser emits pulses of 10 ns fwhm with a repetition rate of 10 Hz at a wavelength of 337.1 nm (~3.678 eV). Using the dye laser, the excitation intensity is kept independent of wavelength by means of two polaroids. The crystals are mounted on a sapphire holder inside an evaporation cryostat where they can be kept at temperatures ranging from the boiling point of helium up to room temperature. For recording experimental data, an optical multichannel analyser (OMA from PAR) is used together with a SIT vidicon (silicon intensified target) coupled to a 1 -m spectrometer (Monospek 1000) with 1200 1/mm grating. Many different crystals have been investigated: some of them grown from solution in acetone and others from the vapor phase [16], some of these by dynamic gradi-
G. Kurtze et al.
/ Luminescence in highly excited red Hg12
153
ent reversal techniques [17] In spite of their different origins, most crystals show similar spectra with significant differences mainly as far as recombinations from impurity centers are concerned. The best results are obtained when surfaces are used which are as-grown, or carefully polished with a solution of K! in water. All techniques as rough as cleaving create such high densities of lattice defects that the crystal surface is no longer useful for luminescence measurements, as will be seen later. -
3. Experimental results
—
low temperatures
Several spectra with various N2-laser excitation intensities I~are given in fig. 1. With the factors listed on the right-hand side the luminescence intensities have been multiplied. An arrow marks the position of the longitudinal free exciton flWL according to [14]. Two lines are to be seen, one of them being the bound exciton line at 2.329 eV. At low excitation levels a pedestal on the high energy side of this line marks the position of the lower transverse polariton I~WT.With increasing intensity, there is an emission called the M-band appearing at 2.323 eV showing weakly superlinear growth. The dashed curve shows the extrapolated high energy
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1111111
232
231
2.30
219 eV
photon energy Fig. 1. Luminescence spectra of Hg!2 at 5 K for various N2 -laser excitation intensities jo~
154
G. Kurtze et al.
/ Luminescence in highly excited red HgI~
decay up ro 2.327 eV. A second band, called MT [2], or a P-band [7] cannot be found. The line shape of the M-band depends on the polarization of the exciting light: with El c, it shows a rather steep low energy decay and a high energy tail, and with E II c the opposite is true. (In the following E always refers to the polarization of the exciting laser.) Fig. 2 shows an example of stimulated emission. At the lowest excitation level, two bound excitons (2330 and 2.332 eV) can be seen, also the M-band. At the next highest level there is one peak at 2.323 eV dominating every other line. At the highest level the broad emission at 2.3 19 eV domintates. This line has a steep low energy decay and a high energy tail. In fig. 3a, b excitation spectra of the M-band are given, the exciting light being polarized with E II c and El c, respectively. With E II c the luminescence intensity decreases at exciting photon energies flWexcjt below hWL, only at the maximum excitation level, there is a sharp resonance at 2.3305 eV. With El c, the spectra show less structure, the luminescence intensity being higher and decreasing at ~~~excit ~ 2.33 1 eV, i.e. in the region of the bound excitons. In some spectra a resonance is to be seen at 2.3325 eV which is less pronounced and broader than that one with E II c. When varying llWexcit we do not only observe structures in the excitation spectra of the luminescence bands, but also several narrow Raman lines, which shift linearly with J~~0excit.The more intense ones are equidistant lines grows more than linearly smaller emission structures are separated from the main lines by integer multiples of 1 .9 and 3.1 meV. The intensity ofthe equidistant lines grows more than linearly with excitation intensity. Slopes up to 4 have been observed. Fig. 5 shows excitation spectra of the first Raman line. There are two resonances at 2.326 and 2.321 eV: The Raman lines disappear rapidly for T~30K and can be observed for El C only, the electric vector of the Raman emission being polarized .Lc, too.
Hg12
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2.32
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2.31
photon energy Fig. 2. Stimulated emission of Hg! 2 observed at 5 K with N2-laser excitation.
G. Kurtze et al. / Luminescence in highly excited red Hg!
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500
155
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.
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I 2.33 tiWexcit
I II 2.32 2.31
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I 232
231 eV
Fig. 3. Excitation spectra of the M-band of Hg! 2 at various excitation intensities and at 5 K, with exciting light polarized E I
C
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.2 I 234
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Laser and Raman lines of Hg!2 at
2.33 2.32 hWexcit
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5 K with dye laser excitation.
Fig. 5. Excitation spectra of the Raman line at lIwexcit —14 meV for two excitation intensities, 5 K.
156
G. Kurtze et aL
4. Experimental results
—
/ Luminescence in highly excited red Hg12
high temperatures
In the temperature region between 40 and 200 K two lines (1 and 2, elsewhere [5] called XH and XE) have been observed. Line 1 can be seen with any crystal, whereas the existence of line 2 depends on the surface quality. Fig. 6 shows three spectra at the same excitation intensity and temperature, but with different surfaces (the luminescence intensities are not comparable). The crystal named e2 has been excited on both a cleaved surface and an as-grown surface. Only the cleaved surface shows the emission line 2. The lowest spectrum shows the emission of a crystal with a very bad-looking surface: once being carefully polished, the surface is now covered with yellow HgO, which always arises from UV light excitation even under high vacuum. This spectrum also does not show any line 2, but the luminescence intensity has strongly decreased. The spectral positions of lines 1 and 2 at different temperatures are given in fig. 7 together with theoretical results (see discussion) and the position of the longitudinal exciton according to [18]. In fig. 8 the excitation spectra of lines 1 and 2 are given. The luminescence intensity of line 1 decreases at ~Wexcit below 2.31 eV, whereas line 2 decreases below 2.30 eV only. Line 2 shows a weak resonance around 2.305 eV, and line 1 has its maximum intensity in the region of the free excitons.
5. Discussion
—
low temperatures
The line shape at an M-band due to biexciton annihilation can be described by an inverted Boltzmann distribution, although with modifications because longitu80K 2
Hg!
60KW/cm 2
cleuved surfuce (e 2)
I/N ~III~I
233
2.32
2.31
230
229
2.28
2.27
226 eV
photon energy Fig. 6. Luminescence spectra of Hg!2 at 80K under N2-laser excitation for different crystal surfaces.
G. Kurtze et al.
/ Luminescence in highly excited red Hg!2
157
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0
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I
I
I
50
100
150
‘S
200
K
250
temperature Fig. 7. Temperature dependence of the spectral positions of lines 1 and 2 at Hgl2,theoretically and experimentally, and the position of the longitudinal free exciton according to [181.
in 4-
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2I 100KW/cm 2.36 2.35 234
233
2.32
2.31
2.30 eV
excit Fig. 8. Excitation spectra of line 1 and line 2 of the Hg1 2 at 70 K.
158
G. Kurtze et al.
/ Luminescence in highly excited red Hg!2
dinal, transverse or mixed-mode polaritons may arise in the final state [19,20]. The M-band observed in our experiment may possibly be described in this way, therefore we shall tentatively ascribe it to biexciton decay. The position of the high energy edge of the M-band gives E~iex= 6 ±1 meV relative to two free triplet excitons at 2.3335 eV. As for the other line in the spectrum at 2.329 eV (fig. 1), there is no indication that it is anything other than a bound exciton line. Nakaoka et a!. [2] have interpreted this peak as an MT band. But when taking the mixed-mode polaritons into consideration, it most probably cannot be expected that the M-band splits into an ML and a rather sharp MT-band. There are, however, extrinsic processes such as scattering with acoustic phonons or electron hole plasma recombinations, to which the M-band may be ascribed, but these will not show a resonance in the excitation spectra with E II c. On the other hand, this resonance is consistent with resonant two-photon excitation of biexcitons. Its position gives an energy of the biexciton of 4.66 1 eV indicating also a binding energy E~iexof(6 ±1) meV. The energy of the biexciton with (4661 ±1) meV can be deduced from the excitation spectrum directly. The binding energy depends on the energy of the free triplet exciton. The value of 2.3335 eV used here is consistent with our luminescence measurements at low excitation. Some authors give values up to 2.335 eV [15] which would result in an biexciton binding energy of 9 meV. A photon with E II c has symmetry F~,two of these can generate a Ft-biexciton only. whereas with E I c two Fi-photons can reach different excitation levels of biexcitons with symmetries Ft, F~,F~,or F~.Therefore, it seems reasonable that with E I c the resonance is broader than with E U c. In the latter case the resonance lies on the low energy side of that one with E I c, thus indicating that the Ft-state of the biexciton is the ground state. Since with El c, one-photon excitation competes with two-photon excitation, the two-photon resonance in this case is weak or may even disappear. The M-band emission for El c therefore must be due to a superposition of different recombination processes. We will take into consideration biexciton decay, bound exciton scattering with phonons (“acoustic wing”), carriers or excitons, and plasma recombinations. A combination of different processes also explains the variation of line shape depending on polarization, and corresponds to the interpretation of the M-band of the Il—VI compounds with wurtzite structure [3]. Free or bound exciton scattering with either carriers or excitons should not occur at hWexcit below the free exciton region, however. In addition, the most probable exciton—exciton scattering processes (generally called P2 and P~)would give rise to emission lines, the position and temperature variation of which are not consistent with those of the M-band. An M-band due to plasma recombinations could show similar excitation spectra as does our M-band but it shouldsince not be 2 likeforinEl ourc,experiments, Hg!visible at excitation intensities as low as 1 kW/cm 2 is a direct semiconductor with dipole allowed interband transition. When increasing the excitation intensity, a
G. Kurtze et al.
/ Luminescence in highly excited red HgJ’2
159
plasma emission may possibly broaden and shift to the high energy side if a gaseous plasma phase is assumed. Such a behaviour also has not been observed. The only process fully consistent with both luminescence and excitation spectra apart from biexciton decay is bound exciton scattering with acoustic phonons, so that in conclusion we may say that the M-band in Hg12 most probably is partly due to biexciton annihilation, and partly is a so-called acoustic wing. The stimulated emission spectra (fig. 2) may also be interpreted by2means of eV, is 2.323 biexciton annihilation: the spectral position of the peak at 40 kW/cm and this is exactly the position of a biexciton decay into a photon and a longitudinal exciton. This process is more favorable for stimulated emission, since the reabsorption probability by the reverse process is lower in the ML than in the MT-band. At 120 kW/cm2, position and line shape indicate that the emission is due to a superposition of the phonon replicas of free and bound excitons. From the Raman spectra (fig. 4) we deduce energies of Raman-active phonons at (1.9, 3.1 and 14.0 ± 0.2) meV. Our results are thus almost consistent with [21]. The steep increase of the Raman intensity with increasing 1o at constant hwexcit is due to stimulation of the Raman process. The position and shape of the Raman excitation spectra (fig. 5) are mainly determined by the increase of one-photon absorption with increasing ~~~excit, and the increase of the resonance denominator for decreasing h(~.)excit.These two facts bring about the maximum in the excitation spectrum, while its dependence on 4 is due to the stimulation of the Raman lines. —
6. Discussion
—
—
high temperatures
Line 2 is obviously due to lattice defects created by cleaving. The emission process may be scattering with excitons or charge carriers bound to these defects. The binding energy possibly is about 20 meV as indicated by the weak resonance in the excitation spectrum (fig. 8). Accordingly, line 2 disappears at temperatures above 200 K, and there are certain hints that the extrapolated spectral position of line 2 at 5 K is 2.3 1—2.32 eV, i.e. below the free exciton. Our interpretation does not affect the possibility of finding an optical gain in line 2, as did Catalano et a!. [6]. Furthermore, the different excitation spectra of lines I and 2 are a proofthat they are not both due to free exciton—free carrier scattering. Since line I is present in all crystals investigated, we assume that it originates from an intrinsic process, and we tentatively ascribe line 1 to exciton—free carrier scattering (EFCS). A detailed description of the EFCS-theory used here is given in [8]. As stated above, rnh is rather well known ( 0.58rn 0). Based on this value we vary the electron effective mass. A good agreement of the theoretically calculated spectral position and of the line shape with the experimental results (figs. 7 and 9) is obtained forM~m, i.e. when the exciton massMis nearly equal to the mass of the scattered carrier, which is equivalent to a high ratio rn/rn’ ofthe two carrier
160
/ Luminescence in
G. Kurtze et al.
Hg!2
2
line 1
~
~
\~
//‘.. II.
>5.
highly excited red Hg!
105K
.
/1
2) ———theory M — m(J =400KW/cm theory M = 2m — experiment I I I I I 0 10 20 30 60 50 60 70
Fig. 9. Line shape of line 1 of Hg! 2 at 50, 77 and 105 K, theoretically dependence of the distance ~E to the transverse lower polariton.
and experimentally, in
masses. The values used here are m’ = 0.58m0, m = 1 .6m0 and M = m + rn’ = 2.18m0. To show the sensitivity of the theory on rn/rn’ we performed similar calculations with rn = m’, i.e. M = 2rn, and we find significant deviations concerning both spectral position and line shape (figs. 7 and 9). So we may say that line 1 is due to EFCS processes with a high rn/rn’ ratio and scattering of the exciton with the heavier carrier. Since in Hg!2 the electron mobility is much higher than the hole mobility [9] and since Hg!2 is said to be an n-type
>..
Hg!2
232
72K
231
230
2.29
2.28
2.27
2.26 eV
photon energy Fig. 10. Line shape of line 1 and line 2 of Hg!2 at 72K as compared to exciton electron and exciton—hole scattering theory respectively.
G. Kurtze et al.
/ Luminescence in highly excited red Hg!2
161
semiconductor at higher temperatures by the majority of the authors (e.g. [14]), the scattering process is probably exciton—electron scattering, and this would support the mass values given by Anedda [Ii] The spectral position of line 2 can be fitted theoretically forM~m’, but for the line shape no agreement can be found for any mass ratio (fig. 10). In fact, the superposition of the theoretical line shapes for exciton—electron and exciton—hole scattering does not lead to two separated maxima but only to a broad emission, the maximum of which lies between both extreme cases [22]. -
Acknowledgement This work was supported by the Deutsche Forschungsgemeinschaft (DFG).
References [1] 1. Akopyan, B.V. Novikov, M.M. Pimonenko and B.S. Razbirin, Soy. Phys. Sem. Lett. 17 (1973) 299. [2] K. Nakaoka, T. Goto and Y. Nishina, Nuovo Cimento 38B (1977) 588. [3] H. Schrey and C. Klingshirn, Phys. Stat. Sol. (b) 90 (1978) 67. [4] R. Levy, C. Klingshirn, E. Ostertag, Vu Duy Phach and J.B. Grun, Phys. Stat. Sol. (b) 77 (1976) 381. [5] T. Goto, K. Nakaoka andY. Nishina, J. Luminescence 12113 (1976) 599. [6] !.M. Catalano, A. Cingolani, M. Ferrara, M. Lugara and A. Minafra, Appi. Phys. Lett. 32 (1) (1978) 36. [71 J. Bllle, Festkorperprobleme 13(1973)111. [8] B. Honerlage, C. Klingshirn and J.B. Grun, Phys. Stat. Sol. (b) 78 (1976) 599. [9] R. Minder, G. Ottaviani and C. Canall, J. Phys. Chem. Solids 37 (1976) 417. [10] P.D. Bloch, J.W. Hodby, T.E. Jenkins, D.W. Stacey and C. Schwab, Nuovo Cimento 38B (1977) 337. [11] A. Anedda and F. Raga, Nuovo Cimento 38B (1977) 439. [12] J.H. Yee, J.W. Sherohman and G.A. Armantrout, IEEE Trans Nucl. Sci. NS-23 (1976) 117. [13] M. Möller. Diplomarbeit Erlangen (1978). [14] B.V. Novikov and MM. Pimonenko, Soy. Phys. Scm. 4, 11(1971)1785. [15] 1. Akopyan, B. Novikov, S. Permogorov, A. Selkin and V. Travnikov, Phys. Stat. Sol. (b) 76(1975) 353. [16] E. Tomzig and G. MUller, Verhandl. der Deutschen Physikalischen Gesellschaft (VI) 13, HL (1978) 125. [17] W. Puschert and H. Scholz, Appi. Phys. Lett. 28 (6) (1970) 357; H. Scholz, Solid State Cornmun. 20 (1976) 447. [18] A. Anedda and E. Fortin, Phys. Stat. Sol. (b) 84 (1977) K 87. [19] R. Plane! and C. Benoit ala Guilaume, Phys. Rev. B15 (1977) K81. [20] F. Henneberger, K. Henneberger and J. Voigt, Phys. Stat. Sol. (b) 79 (1977) K81. [21] Y. Marqueton, E.A. Decamps and M.A. Nusimovici, Proc. 2nd mt. Conf. on Light scattering in solids, Paris, 1971. [22] G. Kurtze, Diplomarbeit, Karlsruhe (1978).
EXCITATION SPECTROSCOPY, RAMAN SCATFERING AND THE TEMPERATURE DEPENDENCE OF THE LUMINESCENCE IN HIGHLY EXCITED RED Hg!2 G. KURTZE, C. KLINGSHIRN Institut Jllr angewandte Physik der Unlversitdt, D 7500 Karisruhe, Fed. Rep. Germany
B. HONE RLAGE Laboratoire de Spectroscopie et d’Optique du Corps Solide, F 67000 Strassbourg, France
E. TOMZIG Inst itut für We,kstoffwjssenschaften VI der Universitdt, D 8520 Erlangen, Fed. Rep. Germany
and H. SCHOLZ PhilipsForschungslaboratorium, D 5100 Aachen, Fed. Rep. Germany
Received 5 September 1978
The luminescence of red Hg12 is investigated as a function of temperature, excitation intensity and wavelength. At high excitation intensity and low temperature an “M-band” emission dominates. This M-band is assigned to biexciton decay and bound exciton scattering with acoustic phonons (“acoustic wing”), this assumption being supported by the results of excitation spectroscopy. The energy of the biexciton is determined to be (4661 ± 1) meV. From the evaluation of Raman spectra, the phonon energies (1.9, 3.1 and 14.0 ±0.2) meV are found. At higher temperatures two lines are observed, one of which is ascribed to exciton—free carrier scattering. Position and line shape are in good agreement with theoretical results. The other emission line is found to be due to scattering involving excitons or carriers bound to lattice defects.
1. Introduction Red Hg12 is a layered semiconductor with a tetragonal unit cell, its point group being D4h. The band structure is similar to those of the H VI compounds with the wurtzite structure: there is band a conduction band with originating from 2~-6s-1evels. The valence (1, 5p-levels) is splitsymmetry into threeF~, bands, A, B and C, Hg with symmetries I’, r~,r~, respectively, due to spin—orbit coupling and crystal field. 151
152
G. Kurtze et aL
/ Luminescence in highly excited red Hg12
We investigated HgI2 at both low and higher temperatures. At low temperatures and with high excitation intensity, two characteristic bands are to be seen. These have been ascribed to biexciton decay [1,2] leaving either a longitudinal or a transverse free exciton (ML, MT-band) [2]. From this, the biexciton binding energy ‘iex has been determined to be 15 meV [1] or 4.6 meV [2] In the wurtzite II VI compounds a similar M-band is observed, which shows some properties of biexciton decay; however, this is mainly due to impurity transitions [3]. Therefore, it seems necessary to collect more information for Hg!2, too, before the M-band can be attributed to biexciton decay. In the present paper results of excitation spectroscopy are reported. This technique has been used successfully on CuC1 for the identification of biexcitons [4]. At higher terriperatures, i.e. in the region between 40 and 200 K, highly excited Hg!2 shows two bands which have been assigned to free exciton scattering with either electrons or holes [5,61.From this, Goto et al. [5] determined the ratio of electron to hole mass to be 0.09. In the theory used by them [7], polariton effects and the momentum dependence of the matrix element are neglected. Therefore, only qualitative results can be expected [8]. The effective masses are not so well known: values are ranging from mh = 0.4 to 1 .2m0, and from me = 0.2 to 1 .6m0 [9—13].Most probably, the hole mass is mh = 0.58m0 [10] as measured by cyclotron resonance experiments. Hanerlage et al. [8] have carried out a more detailed calculation for the exciton free carrier scattering process. Using their results, we try to decide between the different sets of effective masses mentioned above. Different authors also give different values of the free exciton energies. Our own values are hwL = 2.339 eV (longitudinal free exciton), L~LT= 5 meV corresponding to [14];the position of the triplet exciton relative to the transverse polariton is deduced from [15] .
2. Experimental setup The Hg!2 crystals can be excited by light emitted from either a 400 kW nitrogen laser (Molectron UV-400) or a 4 kW tunable dye laser pumped by the N2-laser. 2 and 1 MW/cm2, respectively. TheThe maximum excitation intensities are 5 MW/cm N 2-laser emits pulses of 10 ns fwhm with a repetition rate of 10 Hz at a wavelength of 337.1 nm (~3.678 eV). Using the dye laser, the excitation intensity is kept independent of wavelength by means of two polaroids. The crystals are mounted on a sapphire holder inside an evaporation cryostat where they can be kept at temperatures ranging from the boiling point of helium up to room temperature. For recording experimental data, an optical multichannel analyser (OMA from PAR) is used together with a SIT vidicon (silicon intensified target) coupled to a 1 -m spectrometer (Monospek 1000) with 1200 1/mm grating. Many different crystals have been investigated: some of them grown from solution in acetone and others from the vapor phase [16], some of these by dynamic gradi-
G. Kurtze et al.
/ Luminescence in highly excited red Hg12
153
ent reversal techniques [17] In spite of their different origins, most crystals show similar spectra with significant differences mainly as far as recombinations from impurity centers are concerned. The best results are obtained when surfaces are used which are as-grown, or carefully polished with a solution of K! in water. All techniques as rough as cleaving create such high densities of lattice defects that the crystal surface is no longer useful for luminescence measurements, as will be seen later. -
3. Experimental results
—
low temperatures
Several spectra with various N2-laser excitation intensities I~are given in fig. 1. With the factors listed on the right-hand side the luminescence intensities have been multiplied. An arrow marks the position of the longitudinal free exciton flWL according to [14]. Two lines are to be seen, one of them being the bound exciton line at 2.329 eV. At low excitation levels a pedestal on the high energy side of this line marks the position of the lower transverse polariton I~WT.With increasing intensity, there is an emission called the M-band appearing at 2.323 eV showing weakly superlinear growth. The dashed curve shows the extrapolated high energy
~~OOKWkm2~_
8~2
__
t~W~tiW3 II
2.35
g,iL~,i 234 2.33
1111111
232
231
2.30
219 eV
photon energy Fig. 1. Luminescence spectra of Hg!2 at 5 K for various N2 -laser excitation intensities jo~
154
G. Kurtze et al.
/ Luminescence in highly excited red HgI~
decay up ro 2.327 eV. A second band, called MT [2], or a P-band [7] cannot be found. The line shape of the M-band depends on the polarization of the exciting light: with El c, it shows a rather steep low energy decay and a high energy tail, and with E II c the opposite is true. (In the following E always refers to the polarization of the exciting laser.) Fig. 2 shows an example of stimulated emission. At the lowest excitation level, two bound excitons (2330 and 2.332 eV) can be seen, also the M-band. At the next highest level there is one peak at 2.323 eV dominating every other line. At the highest level the broad emission at 2.3 19 eV domintates. This line has a steep low energy decay and a high energy tail. In fig. 3a, b excitation spectra of the M-band are given, the exciting light being polarized with E II c and El c, respectively. With E II c the luminescence intensity decreases at exciting photon energies flWexcjt below hWL, only at the maximum excitation level, there is a sharp resonance at 2.3305 eV. With El c, the spectra show less structure, the luminescence intensity being higher and decreasing at ~~~excit ~ 2.33 1 eV, i.e. in the region of the bound excitons. In some spectra a resonance is to be seen at 2.3325 eV which is less pronounced and broader than that one with E II c. When varying llWexcit we do not only observe structures in the excitation spectra of the luminescence bands, but also several narrow Raman lines, which shift linearly with J~~0excit.The more intense ones are equidistant lines grows more than linearly smaller emission structures are separated from the main lines by integer multiples of 1 .9 and 3.1 meV. The intensity ofthe equidistant lines grows more than linearly with excitation intensity. Slopes up to 4 have been observed. Fig. 5 shows excitation spectra of the first Raman line. There are two resonances at 2.326 and 2.321 eV: The Raman lines disappear rapidly for T~30K and can be observed for El C only, the electric vector of the Raman emission being polarized .Lc, too.
Hg12
I ~
5K 2
~,,/
J=1ZOKW/cm
I~ ~
U
QJ
~
I~OKW/cm2
2.34
xlO
233
2.32
eV
2.31
photon energy Fig. 2. Stimulated emission of Hg! 2 observed at 5 K with N2-laser excitation.
G. Kurtze et al. / Luminescence in highly excited red Hg!
2
a)
b) -.
500
155
-.
-ø
EIIC Hg12
.
B
e~
5K
.~10O. 50 - ~
100 50
1
_,
EIC Hg12
500-
K63
5K 2 01MW/cm
-
MW/cm2 10-
4-
‘:
~3MW/cm2
U C 0) U)
C •ET
1
0.02MW/cm2
-
.20.5P~WL 0.1-I~MW/cm2 I I ~ 2.35 234
I 2.33 tiWexcit
I II 2.32 2.31
0.1-
I 2.34 233 hwexcit
235
I 232
231 eV
Fig. 3. Excitation spectra of the M-band of Hg! 2 at various excitation intensities and at 5 K, with exciting light polarized E I
C
and El
C.
-
—
E
C.-
o I ~
a,
II
4...
U)
I
+
-
1~
2 \100KW/cm
UI
I
0.5
a)(n a)
-~
~i
a) C
I
C
C
+/ +
.1-
C ‘
LI
5K
K63 Elc
_______________ (user Hg12, 5K
-I—I
C U)
Hg!2
30KWIcm~I bound ex. \n
.2 I 234
I
232
I
2.3
0.1-
I
228 eV 226
H 234
photon energy Fig. 4.
Laser and Raman lines of Hg!2 at
2.33 2.32 hWexcit
231 eV
5 K with dye laser excitation.
Fig. 5. Excitation spectra of the Raman line at lIwexcit —14 meV for two excitation intensities, 5 K.
156
G. Kurtze et aL
4. Experimental results
—
/ Luminescence in highly excited red Hg12
high temperatures
In the temperature region between 40 and 200 K two lines (1 and 2, elsewhere [5] called XH and XE) have been observed. Line 1 can be seen with any crystal, whereas the existence of line 2 depends on the surface quality. Fig. 6 shows three spectra at the same excitation intensity and temperature, but with different surfaces (the luminescence intensities are not comparable). The crystal named e2 has been excited on both a cleaved surface and an as-grown surface. Only the cleaved surface shows the emission line 2. The lowest spectrum shows the emission of a crystal with a very bad-looking surface: once being carefully polished, the surface is now covered with yellow HgO, which always arises from UV light excitation even under high vacuum. This spectrum also does not show any line 2, but the luminescence intensity has strongly decreased. The spectral positions of lines 1 and 2 at different temperatures are given in fig. 7 together with theoretical results (see discussion) and the position of the longitudinal exciton according to [18]. In fig. 8 the excitation spectra of lines 1 and 2 are given. The luminescence intensity of line 1 decreases at ~Wexcit below 2.31 eV, whereas line 2 decreases below 2.30 eV only. Line 2 shows a weak resonance around 2.305 eV, and line 1 has its maximum intensity in the region of the free excitons.
5. Discussion
—
low temperatures
The line shape at an M-band due to biexciton annihilation can be described by an inverted Boltzmann distribution, although with modifications because longitu80K 2
Hg!
60KW/cm 2
cleuved surfuce (e 2)
I/N ~III~I
233
2.32
2.31
230
229
2.28
2.27
226 eV
photon energy Fig. 6. Luminescence spectra of Hg!2 at 80K under N2-laser excitation for different crystal surfaces.
G. Kurtze et al.
/ Luminescence in highly excited red Hg!2
157
2.34-—.~ eV
Hg!2
.... S.
2.32
—
.~
-.5 .5.
free exciton ~WL
S. ~
~2.30
-
Q
E
3
-
‘S
‘S .5
‘t~ C..
‘S
a,
\M.,m
‘S ‘S
‘S .5
2.26
5•5
tine 2~’~
aw
‘5
—
“0
0~ 2.24
-
2.22
-
\M
———experiment —theory
0
2m
I
I
I
50
100
150
‘S
200
K
250
temperature Fig. 7. Temperature dependence of the spectral positions of lines 1 and 2 at Hgl2,theoretically and experimentally, and the position of the longitudinal free exciton according to [181.
in 4-
E 100
—
Hg!2
-o /line 1
.1-
C
a)
a
-I-
C
aj5 I-)
in C
a) .2
I I I I I I I I I I~\S’Ss\i~
50 =
a) LI
-
2I 100KW/cm 2.36 2.35 234
233
2.32
2.31
2.30 eV
excit Fig. 8. Excitation spectra of line 1 and line 2 of the Hg1 2 at 70 K.
158
G. Kurtze et al.
/ Luminescence in highly excited red Hg!2
dinal, transverse or mixed-mode polaritons may arise in the final state [19,20]. The M-band observed in our experiment may possibly be described in this way, therefore we shall tentatively ascribe it to biexciton decay. The position of the high energy edge of the M-band gives E~iex= 6 ±1 meV relative to two free triplet excitons at 2.3335 eV. As for the other line in the spectrum at 2.329 eV (fig. 1), there is no indication that it is anything other than a bound exciton line. Nakaoka et a!. [2] have interpreted this peak as an MT band. But when taking the mixed-mode polaritons into consideration, it most probably cannot be expected that the M-band splits into an ML and a rather sharp MT-band. There are, however, extrinsic processes such as scattering with acoustic phonons or electron hole plasma recombinations, to which the M-band may be ascribed, but these will not show a resonance in the excitation spectra with E II c. On the other hand, this resonance is consistent with resonant two-photon excitation of biexcitons. Its position gives an energy of the biexciton of 4.66 1 eV indicating also a binding energy E~iexof(6 ±1) meV. The energy of the biexciton with (4661 ±1) meV can be deduced from the excitation spectrum directly. The binding energy depends on the energy of the free triplet exciton. The value of 2.3335 eV used here is consistent with our luminescence measurements at low excitation. Some authors give values up to 2.335 eV [15] which would result in an biexciton binding energy of 9 meV. A photon with E II c has symmetry F~,two of these can generate a Ft-biexciton only. whereas with E I c two Fi-photons can reach different excitation levels of biexcitons with symmetries Ft, F~,F~,or F~.Therefore, it seems reasonable that with E I c the resonance is broader than with E U c. In the latter case the resonance lies on the low energy side of that one with E I c, thus indicating that the Ft-state of the biexciton is the ground state. Since with El c, one-photon excitation competes with two-photon excitation, the two-photon resonance in this case is weak or may even disappear. The M-band emission for El c therefore must be due to a superposition of different recombination processes. We will take into consideration biexciton decay, bound exciton scattering with phonons (“acoustic wing”), carriers or excitons, and plasma recombinations. A combination of different processes also explains the variation of line shape depending on polarization, and corresponds to the interpretation of the M-band of the Il—VI compounds with wurtzite structure [3]. Free or bound exciton scattering with either carriers or excitons should not occur at hWexcit below the free exciton region, however. In addition, the most probable exciton—exciton scattering processes (generally called P2 and P~)would give rise to emission lines, the position and temperature variation of which are not consistent with those of the M-band. An M-band due to plasma recombinations could show similar excitation spectra as does our M-band but it shouldsince not be 2 likeforinEl ourc,experiments, Hg!visible at excitation intensities as low as 1 kW/cm 2 is a direct semiconductor with dipole allowed interband transition. When increasing the excitation intensity, a
G. Kurtze et al.
/ Luminescence in highly excited red HgJ’2
159
plasma emission may possibly broaden and shift to the high energy side if a gaseous plasma phase is assumed. Such a behaviour also has not been observed. The only process fully consistent with both luminescence and excitation spectra apart from biexciton decay is bound exciton scattering with acoustic phonons, so that in conclusion we may say that the M-band in Hg12 most probably is partly due to biexciton annihilation, and partly is a so-called acoustic wing. The stimulated emission spectra (fig. 2) may also be interpreted by2means of eV, is 2.323 biexciton annihilation: the spectral position of the peak at 40 kW/cm and this is exactly the position of a biexciton decay into a photon and a longitudinal exciton. This process is more favorable for stimulated emission, since the reabsorption probability by the reverse process is lower in the ML than in the MT-band. At 120 kW/cm2, position and line shape indicate that the emission is due to a superposition of the phonon replicas of free and bound excitons. From the Raman spectra (fig. 4) we deduce energies of Raman-active phonons at (1.9, 3.1 and 14.0 ± 0.2) meV. Our results are thus almost consistent with [21]. The steep increase of the Raman intensity with increasing 1o at constant hwexcit is due to stimulation of the Raman process. The position and shape of the Raman excitation spectra (fig. 5) are mainly determined by the increase of one-photon absorption with increasing ~~~excit, and the increase of the resonance denominator for decreasing h(~.)excit.These two facts bring about the maximum in the excitation spectrum, while its dependence on 4 is due to the stimulation of the Raman lines. —
6. Discussion
—
—
high temperatures
Line 2 is obviously due to lattice defects created by cleaving. The emission process may be scattering with excitons or charge carriers bound to these defects. The binding energy possibly is about 20 meV as indicated by the weak resonance in the excitation spectrum (fig. 8). Accordingly, line 2 disappears at temperatures above 200 K, and there are certain hints that the extrapolated spectral position of line 2 at 5 K is 2.3 1—2.32 eV, i.e. below the free exciton. Our interpretation does not affect the possibility of finding an optical gain in line 2, as did Catalano et a!. [6]. Furthermore, the different excitation spectra of lines I and 2 are a proofthat they are not both due to free exciton—free carrier scattering. Since line I is present in all crystals investigated, we assume that it originates from an intrinsic process, and we tentatively ascribe line 1 to exciton—free carrier scattering (EFCS). A detailed description of the EFCS-theory used here is given in [8]. As stated above, rnh is rather well known ( 0.58rn 0). Based on this value we vary the electron effective mass. A good agreement of the theoretically calculated spectral position and of the line shape with the experimental results (figs. 7 and 9) is obtained forM~m, i.e. when the exciton massMis nearly equal to the mass of the scattered carrier, which is equivalent to a high ratio rn/rn’ ofthe two carrier
160
/ Luminescence in
G. Kurtze et al.
Hg!2
2
line 1
~
~
\~
//‘.. II.
>5.
highly excited red Hg!
105K
.
/1
2) ———theory M — m(J =400KW/cm theory M = 2m — experiment I I I I I 0 10 20 30 60 50 60 70
Fig. 9. Line shape of line 1 of Hg! 2 at 50, 77 and 105 K, theoretically dependence of the distance ~E to the transverse lower polariton.
and experimentally, in
masses. The values used here are m’ = 0.58m0, m = 1 .6m0 and M = m + rn’ = 2.18m0. To show the sensitivity of the theory on rn/rn’ we performed similar calculations with rn = m’, i.e. M = 2rn, and we find significant deviations concerning both spectral position and line shape (figs. 7 and 9). So we may say that line 1 is due to EFCS processes with a high rn/rn’ ratio and scattering of the exciton with the heavier carrier. Since in Hg!2 the electron mobility is much higher than the hole mobility [9] and since Hg!2 is said to be an n-type
>..
Hg!2
232
72K
231
230
2.29
2.28
2.27
2.26 eV
photon energy Fig. 10. Line shape of line 1 and line 2 of Hg!2 at 72K as compared to exciton electron and exciton—hole scattering theory respectively.
G. Kurtze et al.
/ Luminescence in highly excited red Hg!2
161
semiconductor at higher temperatures by the majority of the authors (e.g. [14]), the scattering process is probably exciton—electron scattering, and this would support the mass values given by Anedda [Ii] The spectral position of line 2 can be fitted theoretically forM~m’, but for the line shape no agreement can be found for any mass ratio (fig. 10). In fact, the superposition of the theoretical line shapes for exciton—electron and exciton—hole scattering does not lead to two separated maxima but only to a broad emission, the maximum of which lies between both extreme cases [22]. -
Acknowledgement This work was supported by the Deutsche Forschungsgemeinschaft (DFG).
References [1] 1. Akopyan, B.V. Novikov, M.M. Pimonenko and B.S. Razbirin, Soy. Phys. Sem. Lett. 17 (1973) 299. [2] K. Nakaoka, T. Goto and Y. Nishina, Nuovo Cimento 38B (1977) 588. [3] H. Schrey and C. Klingshirn, Phys. Stat. Sol. (b) 90 (1978) 67. [4] R. Levy, C. Klingshirn, E. Ostertag, Vu Duy Phach and J.B. Grun, Phys. Stat. Sol. (b) 77 (1976) 381. [5] T. Goto, K. Nakaoka andY. Nishina, J. Luminescence 12113 (1976) 599. [6] !.M. Catalano, A. Cingolani, M. Ferrara, M. Lugara and A. Minafra, Appi. Phys. Lett. 32 (1) (1978) 36. [71 J. Bllle, Festkorperprobleme 13(1973)111. [8] B. Honerlage, C. Klingshirn and J.B. Grun, Phys. Stat. Sol. (b) 78 (1976) 599. [9] R. Minder, G. Ottaviani and C. Canall, J. Phys. Chem. Solids 37 (1976) 417. [10] P.D. Bloch, J.W. Hodby, T.E. Jenkins, D.W. Stacey and C. Schwab, Nuovo Cimento 38B (1977) 337. [11] A. Anedda and F. Raga, Nuovo Cimento 38B (1977) 439. [12] J.H. Yee, J.W. Sherohman and G.A. Armantrout, IEEE Trans Nucl. Sci. NS-23 (1976) 117. [13] M. Möller. Diplomarbeit Erlangen (1978). [14] B.V. Novikov and MM. Pimonenko, Soy. Phys. Scm. 4, 11(1971)1785. [15] 1. Akopyan, B. Novikov, S. Permogorov, A. Selkin and V. Travnikov, Phys. Stat. Sol. (b) 76(1975) 353. [16] E. Tomzig and G. MUller, Verhandl. der Deutschen Physikalischen Gesellschaft (VI) 13, HL (1978) 125. [17] W. Puschert and H. Scholz, Appi. Phys. Lett. 28 (6) (1970) 357; H. Scholz, Solid State Cornmun. 20 (1976) 447. [18] A. Anedda and E. Fortin, Phys. Stat. Sol. (b) 84 (1977) K 87. [19] R. Plane! and C. Benoit ala Guilaume, Phys. Rev. B15 (1977) K81. [20] F. Henneberger, K. Henneberger and J. Voigt, Phys. Stat. Sol. (b) 79 (1977) K81. [21] Y. Marqueton, E.A. Decamps and M.A. Nusimovici, Proc. 2nd mt. Conf. on Light scattering in solids, Paris, 1971. [22] G. Kurtze, Diplomarbeit, Karlsruhe (1978).