N. K. Morozova

A study of luminescence centers related to copper and oxygen in ZnSe

The effects of deviation from stoichiometry and the copper and oxygen concentrations on cathodoluminescence spectra were studied in ZnSe condensates obtained by chemical vapor deposition and doped with Cu during growth. The results were supplemented with the study of the microstructure and microcomposition using scanning electron microscopy, measurements of electrical conductivity, and calculations of the equilibrium between the native point defects. It is shown that three types of Cu-related centers are always accompanied with self-activated centers that include oxygen at the Se lattice site (OSe). The paired centers SA(I)-Cu(I), SAL(II)-Cu(II), and III-Cu(III) are typical of all II-VI compounds. All observed I-III Cu-containing centers are associative. Models of emission centers are suggested. A change of the emission type I-III is related to the recharging of the same group of defects that include OSe, Zn(Cu), and VZn. New phenomena that occurred in the region of I-III bands and were related to the profound purification of the material were observed.

Specific features of luminescence spectra of ZnS:O and ZnS:Cu(O) crystals in the context of the band anticrossing theory

The anticrossing band theory, which specifies the change induced in the band structure by an isoelectronic impurity substantially distorting the lattice, is used to interpret the nature and specific features of the complex self-activated luminescence spectra of ZnS:O and ZnS:Cu(O). The light emission involving the self-activated luminescence centers SA and SAL, the spectral dependence of the emission components in relation to the content of dissolved oxygen, and the effects of the centers on the formation of bound excitons are considered. It is found that some absorption bands in the near-infrared spectral region are associated with the transitions between sublevels of the conduction band split in the presence of oxygen. On the basis of the experimental data, a heretofore unknown band model of ZnS:O is developed, and the Cu-induced modifications of the model are discussed. The effect of oxygen-containing agglomerates on the spectra is established. It is suggested that these agglomerates can be responsible for green emission from ZnS:Cu. The results complement the previously reported data of similar studies of ZnS:O and ZnS:Cu(O).

Transformation of luminescence centers in CVD ZnS films subjected to a high hydrostatic pressure

The cathodoluminescence and optical-transmission spectra of ZnS were analyzed to study the effect of a high hydrostatic gas pressure (1500 atm at 1000°C) on the equilibrium between intrinsic point defects in zinc sulfide grown by chemical vapor deposition (CVD) with an excess of zinc. The cathodoluminescence spectra were measured at 80–300 K and excitation levels of 1022 and 1026 cm−3 s−1; the optical-transmission spectra were measured at 300 K in the wavelength range 4–12 µm. It is found that exposure to a high hydrostatic gas pressure transforms the self-activated emission in the cathodoluminescence spectrum: (i) a new short-wave-length band appears at 415 nm with its intensity increasing by one to three orders of magnitude; and (ii) the long-wavelength band that peaks at 445 nm and is observed in as-grown crystals becomes quenched. Simultaneously, the cathodoluminescence band peaked at 850 nm and related to vacancies VS is no longer observed after high-pressure treatment. These effects are attributed to a partial escape of excess zinc (Zni) from crystals and additional incorporation of oxygen into lattice sites (OS). A doublet band I1, which peaked at ∼331–332 nm at 80 K and at ∼342–343 nm at 300 K and is related to excitons bound to acceptor levels of oxygen centers, was observed. This band is found to be dominant in the cathodoluminescence spectrum at an excitation level of 1026 cm−3 s−1. Traces of the ZnO phase are apparent after the high-pressure treatment in both the cathodolumi-nescence spectra (the bands at 730 and 370 nm) and the transmission spectra (narrow bands in the region of 6–7 µm).

Effect of oxygen doping on the IR transmission and cathodoluminescence of ZnSe

The effect of oxygen doping (0.9 and 4.3 vol % O2in the gas phase) on the transmission and cathodoluminescence of CVD ZnSe was studied. The incorporation of oxygen was found to reduce the transmission in the spectral range 700–1900 cm–1. Examination by scanning electron microscopy shows that Se excesses and high O2concentrations lead to tabular growth. ZnO precipitation at structural defects during cooling was observed only in the end portion of the deposit. The dimensions of microinhomogeneities are shown to have a significant effect on the transmission of ZnSe. The oxygen-containing species present in the deposits were identified using cathodoluminescence spectra. The CL spectrum of Se-enriched p-type ZnSe is dominated by the band at 490 nm (SAL) at 80 K and the band at 640 nm (SA) at 300 K. The 640-nm band is attributed to recharging of the SALcenters upon a variation of the Fermi energy with temperature. Decreasing the Se excess leads to the quenching of the SALemission and appearance of the shorter wavelength component of the SAband at 600 nm (80–300 K), characteristic of slightly Zn-enriched ZnSe. The intensity of the self-activated bands is shown to increase as the concentration of dissolved oxygen increases.

Absorption Spectrum of ZnO Precipitates in ZnSe

The absorption spectrum of ZnO precipitates in ZnSe saturated with oxygen is studied in the spectral range 500–2000 cm–1 and is shown to correlate with the transmission spectrum of single-crystal ZnO. Saturation of ZnSe with oxygen in the course of growth leads to ZnO precipitation during cooling. The precipitates give rise to narrow absorption bands in the range 5.8–7.1 μm, which correlate with the components of the multiphonon absorption spectrum of ZnO, formed by combinations of LO and TO phonons. The 2LO mode defines the long-wavelength transmission edge of ZnO. In addition, the spectrum shows strong absorptions due to the LO + TO(10.2 μm) and 2TO (∼11.3 and 12.7 μm) modes. The possible role of SeO2 is discussed. It is suggested that ZnO with a high carrier concentration may act as a “gray” filter in the spectral range 2–9 μm.

Absorption, luminescence excitation, and infrared transmittance spectra of ZnS(O)-ZnSe(O) crystals in the context of the band anticrossing theory

The paper reports the results of optical studies of the absorption, luminescence excitation, and infrared transmittance spectra for the set of ZnS(O)-ZnSe(O) alloys with highly inconsistent properties of anions. It is shown that the band anticrossing theory applied to the alloys provides a general interpretation of their specific optical properties not properly understood previously. A band model of transitions with absorption in a complex multiband formed due to oxygen is presented. The specific features of the absorption and luminescence excitation spectra are interpreted to gain an insight into the distribution and states of oxygen in the crystals. The effect of oxygen on the transmittance band of the ZnS-ZnSe alloys in the near-infrared spectral region is considered. A new approach to the interpretation of the infrared absorption bands associated with oxygen is developed. The calculation algorithms that provide a means for determining the spectral position of the bands in relation to the dissolved oxygen content are suggested.

Studies of the infrared luminescence of ZnSe doped with copper and oxygen

The results of studying the poorly understood 700-to 2000-nm spectral region in ZnSe cathodoluminescence of condensates deposited from the vapor phase with deviations from stoichiometry and Cu and O introduction into a pure matrix are reported. The nature of cathodoluminescence in the region of 1300–1400 nm is refined: the conclusion is drawn that the emission is caused by isolated VZnI(II) vacancies. The behavior of VSe-related luminescence bands at 830 and 960 nm, when the matrix composition changes, is considered.

Optical properties of oxygen-implanted CdS:O layers in terms of band anticrossing theory

The microcathodoluminescence (MCL) and photoreflection spectra of CdS:O layers implanted with oxygen ions to 4 × 1020 cm−3 are investigated. Used method of MCL spectroscopy yields information only about the implanted-layer volume. Exciton MCL spectra, which allow one to determine the concentration of dissolved oxygen in the CdS:O layers and the influence of deviation of the substrates from stoichiometry, are recorded. The homogeneity of the ion-implanted layers is studied by cathodoluminescence (CL) scanning electron microscopy. The relationship between light-emitting areas and the luminescence band at ∼630 nm is established. The reason for enhancement of this band upon radiation annealing is revealed and its nature as the luminescence of F+ centers in CdS is confirmed. New photoreflection spectroscopy data are obtained, which describe the specific behavioral features of oxygen on the layer surface as an isoelectronic impurity in highly mismatched alloys (HMAs). It is shown that sulfur completely bonds and removes oxygen from CdS:O. Oxygen-free CdS remains on the surface in the form of nanoparticles, the size of which depends on the oxygen concentration in the CdS:O layer bulk. The results obtained are in agreement with the predictions of band anticrossing theory.

Role of background O and Cu impurities in the optics of ZnSe crystals in the context of the band anticrossing model

A new interpretation of self-activated luminescence spectra of II–VI compounds is suggested. This interpretation is based on the theory of the conduction-band splitting induced by oxygen isoelectronic impurity and makes it possible to interpret, from a general viewpoint, the nature of spectral bands whose origin was not clear up to now. The discussion is based on extensive previously obtained experimental material and is illustrated by specific examples necessary for the statement of the problem.

The effect of oxygen on the ZnS electronic energy-band structure

Experimental data indicating that the band gap of Zn-O-S solid solutions decreases appreciably in accordance with the theory of noncrossing energy bands are reported for the first time. It is shown that this effect is mainly characteristic of ZnS with an excess of Zn. The concentration of dissolved oxygen [OS] has been determined from data taken using precision X-ray structure analysis and chemical phase analysis. The decrease in the band gap determined from the cathodoluminescence spectra is equal to 75 meV for sphalerite ZnS (s) and 90 meV for wurtzite ZnS (w) per 1 mol % and depends virtually linearly on the oxygen concentration [OS]. An increase in [OS], in addition to an intensification and shift of the free-exciton (FE) band, is also conducive to the formation of SA oxygen-containing complexes in ZnS. These complexes are responsible for emission in the visible region of the spectrum and for the band I1 of excitons bound to these complexes. The binding energy is equal to ∼61 and ∼104 meV for ZnS (s) and ZnS (w), respectively. The band I1 shifts as [OS] varies, similarly to the shift of the FE band. The obtained dependences define the position of the FE band in oxygen-free ZnS and make it possible to assess the oxygen concentration in the compound from the shift of the FE band.