Excitation spectroscopy on the M-band of red HgI2

~

337—339. Solid State Co~rinunications, ©Pergau~nPress Ltd. 1978. Vol.28, Printedpp. in Great Britain.

0038—1098/78/1022—0337 $02 .00/0

EXCITATION SPECTROSCOPY ON THE M—BAND OF RED MgI 2

C. Kurtze and C. Klingshirn

Institut für angewandte Physik der Universität D 7500 Karlsruhe (Received

17 August

1978 by E.

Moliwo)

The luminescence of red Hg12 is investigated as a function of excitation

intensity and —wavelength. At low excitation,

emission is due to free and bound exciton recombination,

at

high levels a “M—band” is dominating. This M—band is partially ascribed to biexciton decay, this assumption being supported by resonances found

in the excitation

spectra.

The biexciton binding energy is determined to be 6±1 meV.

Introduction Red Hg12 is a tetragonal semiconductor with layer structure, belonging to the point group D4h. Its band structure is similar to that of the Il—VI compounds with wurtzite structure: there is one mating conduction from band Hg with 6s—levels. symmetrySpin—orbit 11, origi— coupling and crystal field give rise to the splitting of the valence band (I 5p—levels) into three bands A, B, C with symmetries r 7—,r6—,r6—, respective— ly. At high excitation intensities, at least two characteristic luminescence lines are to be seen. These two lines have been interpreted b~ recombination processes of biexcitons leaving either a longitudinal or a transverse free exciton (ML, MT—band). From this, Nakao— ka et al. determine the biexciton 2 deduce binding from the evaluation energy E~ie~to of be similar 4.6 meV.spectra the surprisingly high value of 15 rneV. The Il—VI compounds with wurtzite structure show a M—band mostly due to extrinsic processes shoving, however, similar be— haviour and lineshape as biexciton annihilation3. Therefore it will be necessary to collect more information for HgI 2 before the existence of bi— excitons is well established. In the present paper, results of excitation spectroscopy are reported, an experimen— tal technique, which has been used successfully on CuCl~. Experimental

N2—laser pumped tunable dye laser. In the latter case, excitation intensity is kept independent of wavelength by means of two rotatory polaroids. The N2—laser emits pulses of 10 ns FWHM with a repe— tition rate of 10 Hz at a wavelength of 2 337,1 nm (~3,678 eV). The maximum exci— tation when using intensity the N 10 is about 5 MW/cm 2 when using the dye laser.and The1 crystals 2—laser MW/cm are mounted on a sapphire holder inside an evaporation cryostat where they can be kept at temperatures close to the boiling point of helium. For recording experimental data an optical multichannel analyser (OMA from PAR) with a SIT (silicon intensified target) vidicon is used, the latter being coupled to a I—rn— spectrometer (Monospek 1000) with 12009./mm grating. Many differentcrystals have grown been from investigated, the vapor phase5, some of some them from solution in acetone, and others from the vapor phase by dynamic gradient reversal techniques6. In spite of their different origin, most crystals show similar spectra with significant differences only as far as recombinations from impurity centers are concerned. Best results are obtained with as grown or carefully polished sur— faces. All techniques as rough as cleaving create such high densities of lattice defects that the crystal surface is no longer useful. Experimental results Fig.! shows spectra obtained by nitro—

setup

The Hg12 crystals can be excited by light emitted from either a 400 kW nitro— gem laser (Molectron UV—400) or a 4—ky

gen laser excitation with various excitation intensities I at 5K. On the right hand factors are listed with which 337

338

SPECTROSCOPY ON THE M—BAND OF RED Hg1

2

Vol. 28, No.4

the luminescence spectra have been multiplied. An arrow marks the position ~WL of 7.the Twolongitudinal lines are to free be excitonaccording seen, one of them being the bound exciton line at to 2,329 eV. At lower excitation levels a pedestal on the high energy side of this line marks the position of the lower transverse polariton. With increasing

~ i=400KWkm2

intensity, there is an emission called N—band appearing at 2,323 eV showing weakly superlinear growth. The dashed curve

l2OKWIcm2 _________

~

shows the extrapolated high energy de— cay up to 2,327 eV. ofIn the fig.N—band 2a and excitation spectra are b given~ the exciting light being polarized with E H ~ and ~ I ~, respectively. With ii ~, the luminescence intensity decreases

_____

4OKWIcm1 xlO BKW/cm2 ___________

4 2.35

2.36

x30

at energies below level of the hwL, only exciting atis the maximum excitation there a photons sharp re— sonance at 2,3305 eV. With ~ I ~ the spectra show less structure, the luminescence intensity being higher

xlOO

some spectra, a at resonance is toeV,be In seen and decreasing the region of the hw bound exc ‘2excitons. 331 i.e.in

I

233

I

232

231

than at 2,3325 that one eV which with El! is weaker ~. The lineshape and broader of the N—band shows a rather ener~y decay and a high energysteep tail low

2.29 eV

2.30

phOtOn energy

for E1~, with El! E vice versa.

fig.1 Fig.!

Luminescence

Discussion The lineshape of an N—band due to bi—

spectra of Hg1

2 at S K for various N2—laser excitation intensities I.

eXciton annihilation can be described by an inverted Boltzmann distribution,

a)

b) -.

EIIC

500.

Hg!2 lMWkm 2

•gioo i~50. 10~ ~

a, IJ

-0

e~

OiHWIcm2

S. 1•

E1C

500

1 Hg!2 i~ OlMWIcm

100

0.5 ~

5K

~3MWkm2 1~(A3~t~WT

0.1~L~0O2MWl~

2.35

0.1— 23~

233

PIWeXCIt

Fig.2

232

231

235

23~ 233 tlWexcit

Excitation spectra of the M—band of HgI~ at various excitation intensities and at 5K, with exciting light polarization ~.H ~ and E~I

232

231 eV

Vol. 28, No.4

SPECTROSCOPY ON THE N—BAND OF RED MgI

2

although with modifications because longitudinal, transverse or mixed—mode 89 The polaritons M—band observed may arise inin the the experiment final state fulfills this condition, so we shall tentatively ascribe it to biexciton annihilation’°. Then the position of the high energy edge of the M—band gives E~jex 6~1 meV relative to two free triplett excitOns (2,3335 eV). There are extrinsic processes such as scattering with acoustic phonons or electron—hole plasma recombinations, however, to which the M—band may be ascribed, but for these, a resonance in the excitation spectra for ~ II ~ cannot be expected. On the other hand, this resonance is consistent with resonant two—photon—excitation of bi— excitons. Its position gives an energy of the biexciton of 4,661 eV indicating also a binding energy E~iex of 6±1 neV. A photon with E II ~ has symmetry r~, two of these can generate a r + 1—biexciton only, whereas, with ~ two r~—photons can generate biexcitons with symmetries rl+,r2+,r3+orrZ.+. This means, that different levels of the biexciton can be reached with ~ and therefore, it seems reasonable that with ~ I~ the resonance is broader than with ~ II ~. In the latter case, the resonance lies on the low energy side of that one with ~ thus

339

indicating, that the rt—state of the bi— one—photon dominating, exciton is excitation the ground is state. With El so c, that the resonance is weak or even die— appearing at certain excitation inten— sities and with certain crystals. The N—band emission is therefore considered as a superposition of different recombi— nation processes, e.g. biexciton decay, bound exciton scattering with phonons (“acoustic wing”), carriers or excitons, or plasma recombi nations. The combination of several different processes also explains the variation of the lineshape depending on polarization. In conclusion we may say:there are certain hints that biexciton annihilation is participating in the H—band, but corres— ponding to the N—band of the II—VI conpounds other processes are involved, too.3 Results for higher temperatures and Raman where. scattering will be published else—

Acknowledgements — For the high quality Hg12 samples, we are very much obliged to E. Tonzig, Erlangen; H.Scholz, Aachen, G.Scheiber, Karlsruhe. This work was supported by the Deutsche Forschungsge— meinschaft.

REFERENCES

1 2

K.NAKAOKA, T.GOTO and Y.NISHINA, II NUOVO CIMENTO 38B 588 (1977) I.Kh.AKOPYAN, B.V.NOVIKOV, M.M.PINONENKO and B.S.RAZBIRIN Soviet Physics JETP Letters 17 299 (1973) 3 H.SCHREY and C.KLINGSHIRN, to be published in physica status saudi (b), 90 (1978) 4 R.LEVY, C.KLINGSHIRN, E.OSTERTAG, VU DUY PHACH and J.B.GRUN, phystEa status solidi b77 38! (1976) 5 E.TOMZIG and G.MULLER, Verhandlungen der Deutschen Physikalischen Gesell— schaft (VI) 13, HL 125 (1978) 6 W.PUSCHERT and H.SCHOLZ, Applied Physics Letters 28 357 (1976) 7 J.AKOPYAN, B.NOVIKOV, S.PERMOCOROV, A.SELKIN and V.TRAVNIKOV physica status solidi b70 353 (197S) 8 R.PLANEL and C.BENOIT a la GUILLAUME, Physical Review BIS 1192 (1977) 9 F.HENNEBERGER, K.HENNEBERGER and J.VOIGT, physica status solidi b 79 K81(1977) 1 thisthat peakthehas been interpreted band. else than 10 There is no indication, maximum at 2,329 eV asa is MT anything a bound exciton. In