No abstract

4f-4f excitation and emission spectra involving various int'erma,nifold transitions are determined for two Tm3+ centres in ZnS crystals by dye-laser site-selective measurements a t helium temperatures. A crystal-field analysis together with polarization data indicates that in both cases Tm is located on lattice sites, probably Zn sites. One of the sites (D3) is thought to be in "unperturbed" environment, whereas in the second case (D2) the four nearest-neighbour atoms are probably replaced by impurity atoms. The excitation and deexcitation mechanisms of the blue lG, -3H, rmission of these centres are studied by site-selective broad-band excitat,ion spectra and kinetical measurements. These studies demonstrate that donor-accept.or pairs involving thc two T m centres as the donors play an important role in the excitation process. Estimates arc given of the relevant pair to 4f state transfer rates.4f4f-Anregungsspektren und -Emissionsspektren von zwei Tm3+-Zentren in ZnS-Kristallen werden fiir verschiedene ionare ubergange durch zentrenselektive Messungen mit Hilfe von Farbstofflasern bei Heliumtemperaturen bestimmt. Eine Kristallfeldanalyse lafit zusammen mit Polarisationsdaten darauf schliefien, dafi in beiden Fallen Tm auf Gitterplatzen, wahrscheinlich Zn-Platzen cingebaut ist. Es ist anzunehmen, da13 einer dieser Platze (D3) eine ,,ungestorte" Umgebung besitzt, wahrend im zweiten Fall (D2) die vier nachsten Nachbaratome wahrscheinlich durch Fremdatome ersetzt sind. Die Anregungs-und Deexzitationsmechanismen der blauen lG, --L 3H,-Emission dieser Zentren werden mit Hilfe zentrenselektiver breitbandiger Anregungsspektren und entsprechender kinet,ischer Messungen untersucht. Diese Untersuchungen zeigen, daB Donator-Akzeptor-Paare, in denen die Tm-Zentren als Donatoren wirken, eine wichtige Rolle im Anregungsprozel3 spielen. Fiir die ubergangsraten Paarzustande --+ 4f-Znstande werden Abschatzangen gegeben.

EPR and optical ( 4 f 4 photoluminescence and photoluminescence excihdon) speara due to Er centres are studied on bulk &Se crystals, which were gmwn by the high-pressure Bridgman technique and doped with ErF? (and pMly. in addition. with Liz-) during crystal growth. Besides the well known. almost isotropic signal with 8 = 5.94, which has been assignedto isolated E*+ on Zn lattice sires. we observe three strongly anisotropic EPR s p c h a (A, B and B' ) which are due to haositions in non-cubic Er'+ (spectrum A) and @+ (specua B and B') centres. The symmetry axes of these centres have directions close to (111) (A) and parallel to (I 10) @ and B'). The angular dependences of the signals are influenced by twinning effects.In the erystals doped with ErF3 alone. only a single type of centre, which is obviously identical with the type A EPR cenho, manifests itself in photoluminescence. A crystal-field analysis of the companding BPR and optical spectra shows that this antre has a rs-type ground level and is chamtenzed by a clystal-field parameter ratio A 6 ( r 6 ) / A j ( / ) = -0.22 We thhk that this centre is a complex consisling of E?+ on a Zn site and F on a nearest-neighbour interstitial site.The g-factors found for the EPR signals B and B' can be explained on the basis of non-Kramers doublet ground levels of E?+ 4fL2 which result from the splitting of cubic rs triplets due to non-cubic crystal-field components. These two signals are ascribed (0 E?+ on the two zincblende-type interstitial sites, respectively, each forming a complex with some other kind of atom on its next-nearest-neighbour interstitial site.A discussion is given of the result that the EPR spectra (unlike the optical spectra) were detected only in lhe case of Li2C0, codoping.

Optical absorption spectra due to intrinsic defects in CdS were measured between 0.13 and 2.5 eV, at 80 O K . The results indicate that Cd interstitials (Cdi) and Cd vacancies (Vcd) are the dominant electronically active defects, which act as double donors and double acceptors, respectively. A detailed discussion of the properties of the observed absorption bands, in particular their shape, polarization, and the effects of Cd and S heat treatment, leads to assignments to optical transitions a t isolated Cdi and VCd and a t nearest-neighbour donor-acceptor associates between these intrinsic defects and impurity ions.Durch Eigenstorstellen hervorgerufene optische Absorptionsspektren in CdS wurden zwischen 0,13 und 2,5 eV bei 80 O K gemessen. Die Ergebnisse zeigen, daD Cd auf Zwischengitterplatz (Cdi) und Cd-Lucken (Vcd) die dominierenden elektronisch aktiven Eigenstorstellen sind und als Doppeldonatoren bzw. Doppelakzeptoren wirken. Eine detaillierte Diskussion der Eigenschaften der beobachteten Absorptionsbanden, insbesondere ihrer Form, Polarisation und des Einflusses von Cd-und S-Temperung, fuhrt zu Zuordnungen zu optischen Ubergiingen an isolierten Cdi und VCd sowie an Donator-Akzeptor-Assoziaten, die aus derartigen Eigenstorstellen und niichstbenachbarten Fremdstorstellen bestehen. 20,

Room‐temperature infrared extinction spectra measured on a large number of as‐grown p‐CdTe crystals are decomposed into three contributions: (i) intra‐valence‐band absorption, (ii) absorption due to a characteristic impurity center, and (iii) scattering from Te precipitates and/or inclusions. The separation of the scattering contribution is based on calculations yielding power‐law dependences on photon energy. Proportionality between the intra‐valence‐band absorption coefficient and the hole concentration obtained from Hall measurements is observed over nearly four orders of magnitude, giving an absorption cross‐section of 2.9 × 10−16 cm2 at λ = 10 μm. The scattering coefficients are found to be correlated with the concentrations of Te particles determined by transimission infrared microscopy. The radial and axial distributions of the hole concentration and of the scattering coefficient within a vertical Bridgman grown boule are determined, and these can be explained on the basis of the growth conditions and the segregation behaviour of point defects.