Hot‐exciton luminescence in CdSe crystals

Abstract

The emission spectra of CdSe crystals under the excitation in the region of fundamental absorption with the He-Ke laser line 3, = 6328 A have been studied at temperatures 4.2 t o 300 OK. A set of narrow emission lines shifted by the energies of an integer number of LO phonons to lower energies from the exciting line has been observed in the region of fundamental absorption. No emission line has been observed a t the posit,ion shifted by 1 LO. The intensity of LO-shifted lines drastically decreases with increasing crystal temperature. It is shown that this temperature dependence and the comparable intensity of multiphonon lines cannot be explained on the basis of a resonant ltaman scattering model. The observed line emission was assigned to a luminescence of hot excitons. The latter model can explain both thermal quenching of emission and intensity distribution in the spectrum. I n the emission spectra of mechanically polished samples a defect activated 1 LO emission line has been observed. This line is due to the indirect exciton transitions without phonon participation in the partially disordered surface layer. Besides the line structure, the continuous emission background has been observed in the region of fundamental absorption. This background forms t,he high energy exponential tail of the quasi-equilibrium exciton emission line n = 1 A and can be considered as Urbach region of t,he exciton emission spectrum.Es wurden die Emissionsspektren von CdSe-Kristallen bei Anregung im Bereich der Grundgitterabsorption mit der He-Nc-Laserlinie 1 = 6328 A im Temperatnrbereich 4,2 bis 500 OK untersucht. Eine Anzahl von schmalen Emissionslinien, deren Energie um ganzzahlige Vielfache der LO-Phononen yon der Anregungslinie zu niedrigen Energien verschoben sind, wurden im Gebiet de,r Crundgitterabsorption heobachtet. Am Ort der Verschiebung urn 1 LO wurde keine Emissionslinie beobachtet. Die Intensitat der LOverschobenen Linien nimmt stark mit steigcnder Kristalltemperatur ab. Es wird gezeigt, dall die Temperaturabhangigkeit und die relat,ive Intensitit der Multiphononlinien auf der Basis eines resonanten Itaman-Streuungsmodells nicht erkliirt werden konnen. Die beobachtete Linienemission wird als Lumineszenz heiBer Exzitonen angesehen. Das letztere Model1 kann sowohl die thermische Tilgung der Emission als auch die Intensitiitsverteilung des Spektrums erklaren. I m Emissionsspektrum von mechanisch polierten Proben wurde eine storstellenaktivierttc 1 LO-Emissionslinie beobachtet. Diese Linie riihrt von indirekten Exzitonenubergiingen ohne Phononenbeteiligung in der teilweise fehlgeordneten Oberflachenschicht her. Xeben der Linienstruktur wurde der kontinuierliche Untergrund im Bereich der Grundgitterabsorption untersucht. Dieser Untergrund bildet den hochenergetischen exponentiellen Auslaufer der Quasigleichgewichts-Exzitonenemissionslinie n = 1 A und kann als Urbach-Bereich des Exzitonen-Emissionsspcktrums angesehen werden.

Multiple‐Phonon Resonant Raman Scattering in Zn1–x CdxSe Crystals

“…Note that we have compared the experimental and theoretical results over the wide frequency range while in the case of CdS such a comparison was made only for one point A x -0. 3 and, what is more, this point lies on the edge of the frequency range (1) over which expressions ( 2 ) and (3) are valid. When analysing the obtained ratios 13/12, we cannot exclude completely the possible effect of absorption on their values.…”

“…Under resonant conditions multiplephonon Ranian scattering spectra may be observed. Such measurements have been previously described for many binary compounds (CdS [2], CdSe [3], ZnTe 141, ZnSe [5], ZnO [GI,GaP [7]) as well as for some mixed crystals such as CdS,Sel-, [8] and Zn,Cdl-,Te [9]. I n this paper the features of multiple-phonon resonant Raman scattering spectra in mixed Znl-,Cd,Se crystals under conditions of excitation above and below t,he band gap are presented.…”

Scattering and Lifetime of Hot Excitons Interacting with LO-Phonons in InBr and InI Polar Semiconductors

“…In this case a great number of MPRRS lines with alternate intensity (N-even lines brighter than N-odd ones) were observed [13 to 20]. This alternation cannot be explained with the model proposed by Gross et al [5] because, as it was shown in [6] the MPRRS cross-section is directly proportional to the exciton±pho-non coupling constant a for all lines, and this means that all lines should have approximately the same intensity, out of resonance. Yoshida et al [13,14] proposed the relaxation model to account for the inter-band excitonic transitions (see Figs.…”

“…Gross et al [5] explained the MPRRS by introducing a model in which secondary radiation occurs through the indirect creation of hot Wannier-Mott excitons in the state with principal quantum number n 1 and momentum " hk T 0. In the latter, the scat-tered photon is produced by relaxation with successive LO-phonons and a subsequent indirect LO-phonon-assisted annihilation.…”

Finite-Difference Time-Domain Analysis of Scanning Tunneling Microscope Light Emission Spectra

“…That is,hω s =hω l − Nhω L O , wherehω s is the energy of the scattered photon andhω L O is that of a LO (longitudinal optical) phonon. These results have been interpreted as multiphonon Raman resonant scattering [6][7][8][9]. Recently, MPRRS in III-VII semiconductors, such as InBr and InI, has been studied.…”