K. H. Herrmann

On the nature of the excitonic luminescence in narrow-gap Hg1−xCdxTe (x ≊ 0.3)

Exciton luminescence is studied in a series of annealed narrow-gap Hg1−xCdxTe (0.3 < x < 0.4) crystals grown by the travelling heater method (THM). A number of techniques — photoluminescence (PL), PL excitation, magnetoluminescence — was applied to get insight in the action of many-body effects (band gap renormalization, free excitons and their localization) in this narrow-gap material.

Interband and intraband contributions to refractive index and dispersion in narrow-gap semiconductors

Abstract This review covers experimental methods and results for the determination of refractive index. It discusses various empirical relations between refractive index and energy gap (Moss relation, Ravindra and Gopal formulae and the Wemple DiDomenico approach). Effects of free carriers and temperature are included. Finally, the Kramers-Kronig transformation of absorption or reflection spectra is considered.

Valence band hybridizing in europium-alloyed lead selenide

The quasibinary compounds (Pb,Sr)Se and (Pb,Eu)Se are compared with each other and with PbSe regarding the shape of the interband optical absorption edge and the photoluminescence spectra. No differences in the edge are detected without magnetic field, if scaling with composition of the gap and of the effective masses is taken into account. Thus admixing of d states from the paramagnetic Eu2+ ions should not take place in the composition up to 10 at.% EuSe. In (Pb,Eu)Se an additional luminescence line was detected, and was attributed to transitions from the conduction band to the Eu2+ 4f states. In magnetic fields up to 7 T this line does not shift, whereas the interband line shifts with B. This shift cannot be explained by the Zeeman and spin splitting of the band states alone, but additional spin-spin interaction between the localized 4f and extended valence band states is proposed.

Broadening mechanisms near the E0 transition in narrow-gap Hg1−xCdxTe (0.2 < x < 0.6)

Abstract The near-band-edge optical and photoelectrical properties of Hg 1− x Cd x Te are investigated by transmittance, photoconductive and luminescence spectroscopy. An empirical description of the absorption coefficient below and above the energy gap E g is given. The physical nature of the transitions below E g (Urbach tail) is being discussed. Contributions due to alloy disorder as well as to shallow levels are distinguished.

Identification of the nature of the optical transitions in Hg0.42Cd0.58Te

Abstract Photoluminescence spectra of Hg 0.42 Cd 0.58 Te were measured by exciting the material with intensities from 10 mW cm −2 to 0.5 MW cm −2 and by using different techniques (conventional cw, pulsed with time-dependent detection, FTPL). For the interpretation, additional transmission and photoconductivity measurements are taken into account. It is concluded that the physical concepts for interpreting the luminescence spectra successfully for wide gap materials also apply to Hg 0.42 Cd 0.58 Te. Band-to-band transitions, donor-acceptor pair transitions and recombination via deep levels were clearly identified, whereas the character of the observed excitionic transitions is not completely understood.

Middle infrared photoluminescence (PL) in the Hg1-xCdxTe (0.22≤ x ≤ 0.75) system

Abstract The nature of the mid-IR optical transitions in narrow-gap mercury cadmium telluride (MCT) was investigated. The results were explained within the framework of models developed for wider-gap materials from the analysis of near infrared and visible luminescence. For narrow-gap MCT luminescence was found to be a tool delivering information about gap and lifetime.

Middle infrared photoluminescence mapping of II–VI semiconductor wafers

Abstract Photoluminescence maps of II–VI semiconductor wafers were obtained by scanning the luminescence signal in the 2–10 μm wavelength range. The signals excited by either pulsed or cw YAG laser radiation were measured either in distinct spectral bands or without any spectral decomposition. From time resolved measurements the ratio of the intensities due to different optical transitions was deduced, if those exhibit different time constants. Hg 1- x Cd x Te and Hg 1- x Mn x Te wafers were examined by using the techniques mentioned. Further it was possible to identify tellurium inclusions in wide-gap CdTe and Cd 1- x Zn x Te due to the tellurium luminescence signal.

Magnetoluminescence spectroscopic investigations in Hg0.7Cd0.3Te/Hg0.15Cd0.85Te superlattices

We present the first magnetoluminescence study in the Voigt and Faraday configurations of mercury-containing superlattices (SL). In the Faraday configuration, the photoluminescence (PL) peak positions consist of the lowest allowed transitions between the conduction and heavy-hole valence bands after taking into account the masses as determined by measurements of the temperature-dependent SdH effect. Coulomb enhancement causes a modification of the magnetoluminescence data. The spontaneous emission intensity shows only a weak magnetic field dependence for moderate and high excitation levels. In the Voigt configuration, the PL peak positions are independent of the magnetic field due to the significantly increased masses parallel to the growth direction. The spontaneous emission intensity shows a step-like behaviour in the magnetic field for moderate excitation levels. This is attributed to changes in the spontaneous emission intensity perpendicular to the SL plane when the cyclotron diameter becomes equal to the thickness of one or two coupled wells.

Nature of laser emission in narrow-gap Hg1−xCdxTe

Abstract An analysis of the nature of optical transitions in the (Hg, Cd) Te alloy system is presented. Free, bound and localized excitons are the origin of the main luminescence lines, whereas for very narrow gap material (fitting the 10 μm region) plasma transitions dominate. Special attention is paid to the luminescence at high excitation levels, where a distinction between plasma transitions and exciton-exciton scattering becomes possible for different samples. The results concerning the gain mechanisms are discussed together with experimental results obtained from injection luminescence.

Interband and intraband contributions to refractive index in the new PbSe-based narrow-gap semiconductors

(Hg,Cd)Te was the first mixed crystal which allowed the study of the transition from a wide gap semiconductor (CdTE) to a narrow-gap material without change of the optical transitions forming the interband edge. Now with (Pb,Sr)Se a material with rocksalt structure is available offering the same possibility. As an example, a critical re-examination of interband and intraband contributions to the refractive index is given. In the narrow-gap lead salts a maximum of n(E) near Eg was found earlier and attributed to the rapid change of absorption coefficient at the interband absorption edge. On the basis of measurements of the real absorption edge slope, as determined both by band structure and by broadening, a study was made connecting for the first time the experimental absorption edge parameters with the n(E) maximum.