ZnS:Cr and ZnSe:Cr are the focus of studies as media for broadly tunable optically pumped lasers operating in the near- and mid-IR regions. It is of great interest to obtain such lasers electrically pumped. The effective electrical excitation of Cr2+ ions was demonstrated only in the case of the impact mechanism for thin-film structures with insulator layers between an electroluminescent film and electrodes, which prevent avalanche breakdown at high electric field (≥ 1 MV/cm). To obtain lasing, the waveguide electroluminescent structures were used. For the first time, the stimulated emission and the laser oscillation were demonstrated in the ZnS:Cr waveguide structures. Although the lasing is unstable as yet, these results lay the foundations of a new direction in laser physics aimed at creation of electrically pumped lasers by dint of impact excitation. Published results concerning various aspects of ZnS:Cr and ZnSe:Cr thin films and waveguide structures as promising electrically pumped laser media are reviewed. The causes of the instability of the laser oscillation and means of improving their characteristics are also considered.
A new class of optically pumped broadly tunable lasers based on chalcogenides (ZnS, ZnSe, CdSe, Cd1–xMnxTe) doped with transition metals (TM) is under intensive investigation for the last two decades [1-13] due to the operation at room temperature in the near- and mid-infrared (IR) regions. Lasing in these materials results from transitions within a 3d shell of divalent TM ions situated in cation sites of tetrahedral symmetry. The major attention of researchers was focused on ZnSe:Cr lasers performed on the crystals (see references [47-66] in ), hot-pressed ceramics , and polycrystalline powders . Recently, lasing under optical pumping in a planar waveguide structure consisted of ZnSe:Cr thin film deposited on a sapphire substrate has been demonstrated [16, 17]. ZnS:Cr crystals are also considered as a promising laser material . Broadly tunable continuous-wave ZnS:Cr lasers were realized in [18, 19].
It is of great interest to obtain lasing in the above materials by electrical pumping. Some attempts to pump electrically ZnSe:Cr bulk samples met with failure [20-23]. Very weak spontaneous electroluminescence (EL) was observed in both the ZnSe:Cr powders  and an n-type ZnSe:Cr,Al crystal forming p-n junction with a ZnSe:Cr, Ag crystal [21-23]. The impact excitation mechanism of EL was in the former, whereas the injection mechanism was in the latter. The excitation by injection of free carriers is inefficient in the case of intraionic transitions inherent to TM and rare-earth (RE) luminescent centers. The excitation of these centers is possible not directly by trapping of carriers and following recombination, but by resonant energy transfer from some close recombination centers. As to the impact mechanism, the EL with such an excitation is weak in bulk samples due to electrical breakdown taking place at fields lower than the fields required for intense EL (0.5–0.8 and ≥ 1 MV/cm, respectively). The sufficiently high fields can be obtained in thin films, especially, in thin-film electroluminescent structures (TFELS) of the MISIM type, where S is an electroluminescent film, M is an electrode, and I is an insulator layer. I layers serve as a ballast capacity reactance protecting against avalanche breakdown. Intense visible EL of TM and RE ions (Mn2+, Tb3+, Er3+, etc.) takes place in such structures .
Rather strong EL of Cr2+ ions in the range from 1.8 to 2.9 μm resulted from the 5E –> 5T2 transition was observed in ZnS:Cr and ZnSe:Cr TFELS [25-28]. More recently, ZnS:Cr waveguide thin-film electroluminescent structures (Fig. 1) were used to increase the attainable gain. The MISIM structure is a planar optical waveguide if the index of refraction of the I layers is lower than that of the electroluminescent film. In such structures, emission propagates mainly along the waveguide and emerges from its edge. Such TFELS emitting in the visible region were extensively investigated in 1980–1990 as emitters of very high luminance [29-31]. However, the stimulated emission was not observed then because of using nonlaser materials (e.g., ZnS:Mn) as well as of high optical losses in the visible region resulting from light scattering by grains in polycrystalline films and from optical absorption by various lattice defects. So, the emission intensity was shown to decrease by over two-fold in ZnS:Mn waveguide structures of length ≤ 1 mm . The losses should be diminished significantly in the near- and mid-IR regions because light scattering decreases with increasing wavelength, and the absorption bands of lattice defects are absent in these regions [33, 34]. The stimulated emission  and the laser oscillation [28, 35] in ZnS:Cr waveguide structures were demonstrated for the first time. Although the lasing is unstable as yet, these results lay the foundations of a new direction in the laser physics aimed at creation of lasers electrically pumped by dint of impact excitation that are based on waveguide TFELS doped with TM. Published results concerning various aspects of ZnS:Cr and ZnSe:Cr thin films and waveguide structures as promising electrically pumped laser media are reviewed below.
2 Fabrication of ZnS:Cr and ZnSe:Cr thin films and waveguide structures
ZnSe:Cr thin films were grown by molecular beam epitaxy (MBE) with the long-term goal of demonstrating a route for the development of lasers, particularly, of electrically pumped ones [36-38]. The growth was performed in a MBE system with solid sources of elemental Zn and Se on semi-insulating GaAs (001) substrate. For doping with Cr, a crucible-based effusion cell containing chromium biphenyl benzol trycarbonyl was used. The Cr concentration (CCr) ranged from 1014 to 1020 cm−3, however, high structural quality was maintained only up to ∼1019 cm−3. Surface segregation and accumulation of Cr were revealed to occur during growth. Cr was incorporated in the optically active Cr2+ state up to a level of 5×1019 cm−3. Mid-IR photoluminescence (PL) was observed only. Electrical excitation of luminescence in films grown by MBE is improbable. Therefore, their characteristics will not be considered in this review.
ZnSe:Cr films were also fabricated by means of pulse laser deposition (PLD) [16, 17, 39]. The target for laser ablation was made by mixing powders of ZnSe and CrSe pressed to pellets, then sealed in a quartz ampule and annealed at 1000 °C for 10 days. The ablation was performed by a KrF eximer laser on GaAs, sapphire, and Si substrates. The film thickness was of 1–8 μm. PL of Cr2+ ions of PLD ZnSe:Cr films with CCr from 6 × 10 18 to 6 × 1019 cm−3 was reported . With the aim to obtain EL, structures Cu/ZnS (∼3 μm)/ZnSe:Cr (∼6 μm)/ZnS (∼3 μm)/GaAs were made by PLD . However, no EL was observed.
Waveguide structures consisted of a ZnSe:Cr film of 7.5 μm thickness with CCr of 6 × 1019 cm−3 grown by PLD on a double-side polished sapphire substrate were fabricated recently [16, 17]. The structure air/ZnSe:Cr/sapphire is a planar waveguide since the index of refraction of ZnSe is higher than that of sapphire (∼2.44 and 1.75, respectively, in the mid-IR region). Lasing in such structures under optical pumping was demonstrated.
PLD of ZnS:Cr thin films was also performed . The target was made from the mixture of Cr or Cr2O3 and ZnS powders pressed into pellets and annealed at ∼900 °C for 24 h under flowing argon. The films were deposited on Si (111) substrate heated at 550–650 °C. A 248 nm KrF laser was used for ablation. PL of the 11.8-μm thick films was observed merely.
Fabrication of ZnSe:Cr thin films by radio-frequency (rf) magnetron sputtering at room temperature was reported in . Sputtering of a SiO2 target covered by a number of ZnSe chips and various quantities of Cr among these chips was performed. Substrates of monocrystalline GaAs and Si as well as glass substrates were used. The film thickness was ranged from 3 to 45 μm. The Cr2+ concentration in the films could not be determined. PL of the thickest films was reported.
Electroluminescent ZnS:Cr and ZnSe:Cr films were fabricated by thermal evaporation (TE) and electron-beam evaporation (EBE) [25-28, 34, 42-45]. Three methods were sampled: 1) TE of adequately doped crystals [25, 34]; 2) alternate deposition of the basic material and Cr [25, 34]; 3) coevaporation of ZnS or ZnSe and Cr from separate evaporators [25-28, 34, 42-45]. In all cases, the deposition was performed on glass substrates heated to 150–180 °C. Annealing the films was performed in vacuum at 500–600 °C. The film thickness ranged from 0.5 to 1.0 μm. The Cr concentration in the films was varied from 1019 to 5×1020 cm−3. The third of the above methods was shown to be the best. Different electroluminescent structures were fabricated: MISIM [27, 28, 42-45] and MISM [25, 34] with rather thick (250–300 nm) or thin (50–100 nm) I layers. Not only high-quality insulators (SiO2, Al2O3, Y2O3, etc.), but also SiOx, or pure ZnS were used for I layers [25, 34]. The bottom electrode was an ITO film and the upper one was an Al film, which were deposited by magnetron sputtering and TE, respectively. The best electroluminescent characteristics were obtained for MISIM structures with wide-bandgap dielectrics as I layers.
Fabrication of ZnS:Cr waveguide TFELS was reported in [27, 28, 42-45]. These structures were of MISIM type with two-component Al2O3/SiO2 I layers (Fig. 1). They represent a planar waveguide because the index of refraction of ZnS, Al2O3, and SiO2, in near- and mid-IR regions is ∼2.3, ∼1.6 and ∼1.4, respectively. The Al electrode was deposited in the form of a strip of 5×1.5 mm2. The edge on one or both waveguide ends was made by cutting the structure normally to the Al strip. The stimulated emission of Cr2+ ions [27, 42] and the laser oscillation [28, 35, 43-45] in the ZnS:Cr waveguide TFELS were demonstrated.
3 Crystalline quality of luminescent ZnS:Cr and ZnSe:Cr films
Crystalline quality of ZnS:Cr and ZnSe:Cr films, which is characterized by the surface roughness, crystallite size, and crystal structure, was studied by means of atomic force microscopy (AFM), scanning electron microscopy, and X-ray diffraction (XRD). ZnS:Cr  and ZnSe:Cr [16, 39] films grown by PLD at 550–650 °C and 600 °C on Si and sapphire substrates, respectively, were shown to have a rather smooth surface morphology. The root mean square roughness of the ZnSe:Cr film surface was less than 10 nm; the main crystallite size estimated from their AFM image was 300–400 nm . These parameters were not reported for the ZnS:Cr films . XRD patterns measured by Θ–2Θ scanning indicated that all films were polycrystalline [16, 40]. The crystalline phase of the ZnS:Cr films was mainly cubic with a little degree (≤ 3%) of hexagonality . The full width at half-maximum (FWHM) of the most intensive cubic (111) peak was equal to 0.44o. The ZnSe:Cr films were purely cubic with a FWHM of the (111) peak of 0.472o .
The crystalline size of ZnSe:Cr films deposited by rf magnetron sputtering was in the 60–100 nm range . XRD diagrams show that the crystalline phase of the films was cubic with the <111> preferential orientation. No extra peak corresponding to Cr or localized modes of CrZn–Se was observed due to a small quantity of Cr incorporated in the films.
The crystalline quality of ZnS:Cr and ZnSe:Cr films deposited by TE or EBE [25-27, 34, 42-45] was the same as of other electroluminescent thin films fabricated by such methods . They were fine grained (of several tens nanometers size) and polycrystalline with mainly cubic phase.
4 Cr as impurity in ZnS and ZnSe films
Cr incorporates into ZnS and ZnSe films as Cr2+substitution ion (3d4 configuration) in common with its incorporation into crystals of II–VI compounds. This was confirmed by its spectroscopic properties. However, some number of Cr+ ions (3d5configuration) was shown to be present in the films due to the capture of an electron by Cr2+ ions. The Cr2+ → Cr+ photostimulated recharge was revealed by EPR experiments not only in ZnS:Cr, but also in other II–VI compounds [46-53]. Confirmation of the presence of Cr+ ions even in equilibrium state was obtained by studying the influence of magnetic field on the visible EL of hot electrons observed in ZnS:Cr TFELS  as well as by the spectral dependence of photoconductivity of these structures [55, 56]. It was found that the number of Cr+ ions relative to that of Cr2+ ions increases with the Cr concentration.
Lasing in ZnS:Cr and ZnSe:Cr takes place on the transition from the lowest excited state 5E to the ground state 5T2. These levels are formed from the 5D state of free Cr2+ ion owing to splitting its levels in the tetrahedral crystal field inherent to II–VI semiconductors. Further splitting of triplet 5T2 and duplet 5E takes place under a combined Jahn–Teller effect and spin-orbit interaction [57, 58]. Strong electron–phonon coupling results in broadening of the absorption and emission Cr2+ bands and a strong Stokes shift between them. Jahn–Teller vibronic coupling contributes to the phonon broadening of these bands significantly . The shape of phonon-assisted optical bands is normally Gaussian. However, the asymmetrical form of the Cr2 emission band for ZnS:Cr and ZnSe:Cr crystals and thin films as well as two peaks in this band are often observed [12, 16, 18, 25, 27, 40]. This is stem from the presence of Cr2 ions with different local crystal field symmetry due to occurrence of various structural defects close to them, e.g. hexagonal crystal phase , charge defects of lattice , and others.
It is important to know what energy levels arise in the bandgap of ZnS and ZnSe after doping with Cr. A general energy-level scheme for II–VI and III–V compounds doped with an isovalent TM impurity, in particular, for ZnS:Cr has been proposed in  (Fig. 2a). A donor (D) or acceptor (A) state was concluded to appear in the bandgap after the capture of an electron (e) or hole (h) by the isovalent trap (IVT) formed by a neutral TM impurity center (M0). As was concluded according to EPR experiments (Fig. 7 in ), the energy of A and D levels from the conduction band (CB) for ZnS:Cr is equal to 1–1.1 and 2.8–2.9 eV, respectively. The M0 → M− recharge, i.e. the change from dn to dn+1 configuration (n = 4 for Cr2+ ions) was supposed to stem from the (0/–) transition of e from the valence band (VB) to the A level. The (0/+) transition of e from the D level to the CB results in the M0 → M+ recharge (d4 → d3). This energy-level scheme was subjected to criticism in  on the following grounds. First, the transition (0/–) signifies that e from the VB goes directly into the 3d shell, i.e. a Cr+ ion arises. Therefore, the A level is indeed the level of a Cr+ ion. However, any levels of the 3d shell cannot be placed within the bandgap because these electron states are not described by the band theory. Secondly, the IVT level of a cation isovalent impurity splits out from the VB if its ion radius is larger than the radius of a substituted lattice ion and is located nearby this band. So, the IVT level of Mn2+ ion in ZnS:Mn predicted by the Koster–Slater model  has been revealed experimentally at ∼0.3 eV above the VB [61, 62]. Consequently, the IVT level of the Cr2+ should also be placed not far from the VB. Lastly, levels of lattice A and D defects must arise in the bandgap of ZnS when Cr+ or Cr3+ ions substitute for Zn2+ ions likely due to the appearance of defect levels of another metal impurity with such a valency (e.g. Cu+ or In3+).
Energy levels of Cr-related defects in ZnS and their interpretation have been reported in . The energy of these levels were obtained by employment of photodepolarization spectroscopy methods developed specially for studying deep levels in thin-film MISIM structures based on high-resistance wide-band semiconductors . The main method used consists in the measurement of the spectral dependence of the photocurrent (Iph) arising in a precharged MISIM structure under the action of the probing monochromatic light in a residual polarization field. The charging was performed by application of a rather low dc voltage for a short time. To obtain additional information, the influence of CCr, the charging voltage, and depolarization in darkness or by light of a certain wavelength on the spectral dependence of Iph, was also studied. In addition, the relaxation of the dark current in time after charging was measured and examined theoretically to determine the thermal ionization energy of electron traps.
Typical dependences of the photocurrent on the photon energy for ZnS:Cr MISIM structures with different CCr, and a ZnS structure reported in  are shown in Fig. 3. It is seen that in the case of the pure ZnS, there are the inherent photocurrent with the peak at 3.68 eV and the weak impurity one, which is related to Zn vacancies, singly (at 2.7–3.3 eV) and associated with donors (at 1.2–2.3 eV) [61, 62, 64]. Doping with Cr causes an increase in the impurity Iph relative to the inherent photocurrent and the appearance of new peaks. The peaks of the impurity photocurrent have been examined taking into account known data about the energy of the defect levels for ZnS doped with various one- and two-valent metal impurities  as well as the ionization energy of Cr+ and Cr2+ ions obtained from EPR experiments [46-53]. The results are shown in Fig. 2b. The most intensive peak at 3.53 eV was explained as ionization of IVT arising when Zn2+ ion is substituted by Cr2+ ion. The peak at ∼2.7 eV was attributed to the acceptor formed by Cr+ ion in a cation site because such a spectral location is typical of most A defects in ZnS [61, 62, 64]. The photocurrent in the range from 1.8 to 2.2 eV was attributed to ionization of this defect associated with various D centers. The peak at 2.8 or 3.0 eV observed in the case of lower or higher Cr concentration, respectively, was explained by ionization of Cr2+ ions with different local crystal field symmetry. Such an interpretation agrees with the conclusion drawn in . The 3d levels of Cr2+ and Cr+ ions were shown on the scheme separately (Fig. 2c) with allowance of their ionization energy as these states are not described by the band theory. The scheme of Cr-related energy levels obtained in  is of great importance for correct interpretation of various physical processes in ZnS:Cr, in particular, the photostimulated recharge Cr2+ → Cr+ and excitation of EL via recombination processes. Explanation of these processes reported in [52, 66, 67] is still open to question.
5 Photoluminescence of ZnS:Cr and ZnSe:Cr thin films
PL was observed in ZnS:Cr and ZnSe:Cr thin films grown by PLD if their thickness was rather large (∼10 μm) [16, 17, 39, 40]. The excitation of PL was performed by a 1.532-μm CW Er-doped fiber laser. All PL spectra were typical of the 5T2 → 5E transition of Cr2+ ions and the shape of the band was similar to that for the same crystals with the exception of some blueshift in the case of ZnS:Cr films . Distortion of PL spectra taken place due to interference in the microcavity formed by film interfaces . The PL intensity decreased significantly at CCr > 1×1019 cm−3 . This was attributed to concentration quenching resulting also in shortening the lifetime in the excited state by a factor four and more [16, 40]. In addition, the lifetime was less than for the crystals at the same CCr.
PL of the 45-μm thick ZnSe:Cr films deposited by rf magnetron sputtering was studied at temperatures from 10 to 300 K using direct pumping into the absorption band of Cr2+ ion with a CW Tm3+: KY3F10 laser (1.85 μm) . As distinct from PL of ZnSe:Cr crystals, the PL intensity of the films decreased more than one order of magnitude when the sample temperature increased. At room temperature, the intensity was ∼40 times less than that of the crystals, but the shape of both spectra was the same with the peak at ∼2.2 μm.
6 Electroluminescence of ZnS:Cr thin-film MISIM structures: characterization and excitation mechanism
For the first time, EL of ZnS:Cr and ZnSe:Cr films in the range from 1.3 to 2.8 μm was obtained in . Structures with thin (60–80 nm) semi-insulator layers (ZnS or SiOx) between the electroluminescent film and electrodes were used initially. However, EL of these structures was unstable due to breakdown, particularly, in the case of ZnSe:Cr films. Therefore, ZnS:Cr TFELS of the MISIM type with high-quality I layers were studied later [27, 28, 34, 45].
At the outset, some principal features of EL common to all TFELS should be recalled . The average threshold electric field is ≥1.0 MV/cm and the field ≥ 2 MV/cm is attainable in the case of MISIM structures. Under ac voltage drive (pulsed or sinusoidal), the field is due not only to the applied voltage dropped across an electroluminescent film, but also the polarization field stemming from electrons accumulated during the preceding voltage pulse (or half-cycle) on the interface states at the anode film side. These electrons remain at the interface for a rather long period of time even after the voltage is removed, and hence the polarization is fairly persistent. When the polarity of the voltage is altered, the field in the film is enhanced by this polarization. The enhancement of the field can range from 1.2 to 1.8 times depending on the number of accumulated electrons and parameters of the interface states. For the first time, the effect of the electrical polarization on EL of ZnS:Mn thin films has been considered in .
It was shown [26, 33] that the typical dependence of the EL intensity (IEL) on the applied sinusoidal voltage for a ZnS:Cr TFELS with CCr of ∼5 × 1019 cm−3 when the Cr2+ emission was detected through the ITO electrode is analogous to that of other TFELS of the MISIM type (Fig. 4a). The intensity increases with V exponentially above the EL threshold. However, growth of IEL becomes far less at some higher voltages as a result of a decrease of the voltage dropped on the electroluminescent film due to a diminution of its impedance. The charge (Q) transferred through the structure for the voltage cycle also increases with the voltage almost exponentially, but beginning from some higher V and without a noticeable saturation (Fig. 4b). The increasing Q is slower in comparison with that of IEL. Averaged absorbed power before the saturation of the IEL(V) dependence ranges from 5 to 7 W/cm2 and their energy efficiency is equal to (2–3) × 10−3. The energy efficiency is nearly the same as for the best ZnS:Mn TFELS emitting in the visible region that was deposited by EBE .
The EL spectra of the ZnS:Cr TFELS reported in [27, 55] for the emission detected through the ITO electrode are shown in Fig. 5. The spectrum of the TFELS with the lower Cr concentration (∼5 × 1019 cm−3) consists of two bands (Fig. 5a). The peak of the main band at 1.75 μm is somewhat blueshifted against the peak in the PL spectrum of ZnS:Cr crystals  due to, probably, the Stark effect. The lesser band is located in the long-wavelength tail of the spectrum with the peak at ∼2.5 μm. This band becomes dominant when CCr in the ZnS:Cr films increases up to ∼4 × 1020 cm−3 (Fig. 5b). The long-wavelength shift of Cr2+ has been observed earlier when there is some degree of hexagonality in ZnS:Cr crystals . Therefore, this band was attributed to the emission of Cr2+ ions with lower local crystal-field symmetry [27, 55].
It should be noted that a weak EL of hot electrons was also observed for the ZnS:Cr and ZnSe:Cr TFELS [25, 26]. This EL is due to intraband transitions of free electrons. Its spectrum is continuous and located from the edge of the ZnS fundamental absorption band to the Cr2+ emission band (Fig. 6) that confirms the occurrence of hot electrons with energy up to 3.5 eV. The spectrum is distorted by interference extrema. The emission intensity in the spectrum increases with wavelength superlinearly.
There were several viewpoints on the mechanism of EL in ZnS thin films initially. The impact ionization or excitation was assumed in . However, injection of minority carriers in p-n junctions within the film bulk or at its interfaces was also concluded to be responsible for the EL [70, 71]. In addition, the possibility of direct ionization or excitation by the electric field was suggested . Later, some confirmations of the impact mechanism were obtained for ZnS thin films [73-75] and MISIM TFELS [76-81]. The generally accepted mechanism of the EL is illustrated by a simple diagram shown in Fig. 7. Above the threshold voltage, electrons are tunnel-injected from the states on the cathode interface between the electroluminescent film and I layer into the film. They are accelerated and directly excite luminescent centers (Mn2+, Cr2+, Tb3+, etc.) by impact when their energy is large enough. Each electron can cause several excitations traveling along the film. Finally, the electrons are captured by the interface states on the anode side. When the polarity of the voltage is reversed, the same processes take place in the opposite direction. The EL intensity is proportional to the number of transferred electrons and luminescent centers, as well as to the possibilities of electron acceleration, the impact excitation of the centers and radiative transitions in them.
The principal process of the impact EL is hot-electron transport. The aim of the transport theories is to predict the fraction of electrons, which can obtain the energy required to the center excitation (e.g., this energy for Mn2+ ions is of 2.12 eV ). There are two approaches for theoretical consideration of the hot-electron transport in ZnS. The first takes into account the structure of all conduction bands (Г, L, X) for Monte-Carlo simulation [82-87]. The other proceeds from a mean free path for high-energy electrons determined by electron–phonon interactions (“lucky-drift model”) [88-92]. A fairly complete review of the transport theories has been given in . Therefore, the more essential results of the theoretical considerations will only be cited here. The steady-state electron energy distribution for the different electric fields (F) according to the calculation based on a nonparabolic multivalley model  is shown in Fig. 8. The Mn impact excitation rate is also given here . It follows from this modeling that the fraction of hot electrons with the energy above 2.1 eV, which is necessary to excite ion Mn2+, is 26% at F = 1 MV/cm and 65% at 2 MV/cm, whereas the other model (“full-band model” ) gives only 1% and 50%, respectively. According to the lucky-drift model of Bringuier  this fraction is equal to 27% at F = 1 MV/cm and 72% at 2 MV/cm. Consequently, the excitation efficiency of centers with the excitation energy of ∼2 eV increases with the field significantly, but the impact excitation rate of Mn2+ ions decreases to zero when the electron energy is above 3 eV (Fig. 8). One can expected that the electron energy of ∼0.7 or ∼1.1 eV is sufficient for the excitation of Cr2+ ion into the first or second excited state. However, it is seen from Fig. 8 that only a low fraction of hot electrons have such energy, especially at F > 1 MV/cm. The lucky-drift model  predicts an even lower fraction of such electrons. However, the intense EL of Cr2+ ions takes place in ZnS:Cr TFELS at fields above 1.5 MV/cm. Consequently, the most effective excitation of these ions takes place, evidently, into higher excited levels, in which the energy ranges from 1.5 to 2.0 eV, with subsequent relaxation to the lowest excited level.
A different excitation mechanism of TM2+ ions, in particular, Cr2+ ions via recombination processes, which does not require high electric fields, was considered in [12, 67, 68]. It was suggested that the following processes can result in the excitation (Fig. 9): i) the energy transfer from the excitons (1) or an adjacent D–A pair (3) to the excited Cr2+ level directly; ii) Auger-type recombination processes on the initial stage followed by ionization of Cr2+ ion due to the hole capture, formation of excited (Cr2+)* ions and their intrashell relaxation to the lowest excited level (2, 4); iii) trapping carriers by Cr ions with subsequent relaxation to the lowest excited level (5). The above processes raise doubts on the following grounds. First, it was noted above that the 3d levels of Cr2+ ions cannot be placed in the bandgap. Secondly, the direct capture of a free carrier on the 3d levels of Cr2+ ion is only possible by resonance tunneling from the band states isoenergetic with Cr2+ excited states . However, such states are only present in the CB, but not in the VB (see Figs. 5b and c). The resonance tunneling of an electron from the CB on the isoenergetic Cr2+ levels, but not the direct transition of e from the VB to the 3d Cr2+ shell (as was concluded in ) is, evidently, responsible for photostimulated recharge Cr2+ → Cr+ . Lastly, the energy transfer from the excited level of a neighboring recombination center to the excited level of Cr2+ ion is only possible in the case of the resonance transfer, i.e. if both levels are isoenergetic. It is difficult to realize conditions necessary for this.
7 Laser oscillation in optically pumped ZnSe:Cr waveguide structures
The laser oscillation in the waveguide structures consisting of air/ZnSe:Cr thin film/sapphire substrate was demonstrated in [16, 17]. Fabrication of these structures was described above (see Section 'Fabrication of ZnS:Cr and ZnSe:Cr thin films and waveguide structures'). The structures were pumped by 1560-nm D2–Raman-shifted Nd-YAG laser radiation with the pulse duration of 5 ns. The pump beam of diameter ∼3 mm was directed normally to the front film facet. The emission along the waveguide direction was collected with a CaF2 lens on the slit of a spectrometer and detected by a liquid-nitrogen-cooled InSb detector. The Cr concentration in the ZnSe:Cr film was 6 × 1019 cm−3, although the critical CCr for beginning concentration quenching is equal to ∼1 × 1019 cm−3. The PL intensity of the structures at low pumping energies was about ten times lower than that of the structures with the Cr concentration of 6 × 1018 cm−3. The lifetime was significantly shorter than for low concentration doped ZnSe:Cr bulk samples (∼500 ns and ∼5 μs, respectively). This was explained by concentration quenching. Pumping above some threshold energy density (∼0.11 J/cm2) resulted in the appearance of the narrow band with the half-width of ∼100 nm and the peak around 2.6 μm instead of the broad band (∼400 nm) with the peak at ∼2.1 μm before the threshold. The output–input characteristic above the threshold was superlinear. These results were accounted for by the laser oscillation arising. However, it is more evident that only the stimulated emission taken place, because the half-width of the Cr2+ band after the narrowing is larger than that of the oscillation line for ZnSe:Cr and ZnS:Cr crystal lasers (40–50 nm) [18, 19], and it is near to the half-width of the band in the case of the stimulated emission in ZnS:Cr waveguide TFELS (∼80 nm)  (see below).
8 Stimulated emission in electrically pumped ZnS:Cr waveguide thin-film structures
For the first time, the stimulated emission in ZnS:Cr under electrical pumping was obtained for thin-film waveguide structures of the MISIM type with the Cr concentration of ∼5 × 1019 cm−3 . Their schematic view is given in Fig. 1. Such samples enable detection of the EL emission of one and the same cell either through the ITO electrode (“face” emission) or from the structure edge (“edge” emission). This allows the peculiarities of the emission propagating along the waveguide to be established.
It was shown that the EL spectrum of the face emission did not change with voltage increasing except for some redistribution of the intensity of both bands and a little narrowing of the main band (Fig. 10a). On the contrary, the behavior of the edge emission spectrum was revealed to be essentially different (Fig. 10b). At the threshold voltage, there were three equally spaced peaks (Fig. 10b), which are inherent to waveguide modes. The most intense of these peaks was located not far from the main peak in the spectrum of the face emission. Its intensity in the range near by the Cr2+ main emission peak increased most strongly with voltage increasing, whereas other waveguide-mode peaks decreased initially and then disappeared. With the further increase of voltage the emission within the central part of the Cr2+ band remained only that was followed by a significant narrowing of the band. The difference in the voltage dependence of the band width for the face and edge emission reported in  is shown in Fig. 11a. When the voltage increases from 151 to 157 V, the bandwidth for the former decreases slightly (by ∼20%) while the decrease is more than five times for the latter. The least bandwidth is of 80 nm in comparison with 50 nm for the oscillation line of ZnS:Cr crystal laser . The band narrowing takes place mainly at the voltages corresponding to the steepest section of the voltage dependence of the EL intensity (Fig. 11b). This dependence was significantly stronger than that of the face emission.
It was concluded from the above results that a rather high gain is attainable in the ZnS:Cr waveguide structures under the impact excitation for the edge emission. This emission became stimulated. For the first time, the stimulated emission under electrical pumping was demonstrated not only in the ZnS:Cr, but in all solid states apart from the weak stimulated emission in ZnS:Er waveguide TFELS observed formerly in .
9 Lasing in ZnS:Cr waveguide thin-film structures electrically pumped by dint of impact excitation
The further effort for obtaining the laser oscillation in ZnS:Cr waveguide TFELS with somewhat improved properties specific to laser media was reported in [28, 42-45]. The structures were the same as the structures used earlier (see Section 'Stimulated emission in electrically pumped ZnS:Cr waveguide thin-film structures') with the exception of the higher Cr concentration (∼4 × 1020 cm−3) and the two edges made on both the waveguide ends instead of the one. These changes provided higher gain and the lower lasing threshold. It was shown that the EL spectra of these structures for the emission detected from the face of an operating cell and its edge changed with increasing voltage differently (Fig. 12). The spectra were corrected on absorption of air existing in this spectral region. The spectrum of the face emission was identical at all voltages and consisted of one asymmetrical band with the peak shifted to the long-wavelength side as comparison with the peak in the case of the lower CCr (Fig. 5). As has been reported in [54, 55], this shift is due to the lower local crystal-field symmetry of most Cr2+ ions that is resulted from the presence of many charged defects, in particular, Cr-related acceptors in the high-doped ZnS:Cr films. Contrary to the face-emission spectrum, the edge-emission spectrum changed with the voltage increase as follows. There were three peaks specific to waveguide modes at the threshold voltage (∼160 V). The relative intensity of the peak close to the peak in the face-emission spectrum increased with the voltage. Only one band with the peak at ∼2.6 μm remained at some voltage (∼170 V). It was (2.5–3) times narrower than the face-emission band. Simultaneously with these changes in the spectrum, the stronger intensification of the edge emission with voltage increasing than of the face emission was observed (Fig. 13). In addition, the dependence of the intensity on the charge was also sharper for the edge emission (Fig. 14). These peculiarities indicated that the emission propagated along the waveguide became stimulated. The voltage increase up to ∼180 V resulted in the fast (for a minute) and very strong (∼100 times) intensification of the edge emission at the fixed V only (Fig. 15) and lower increase (∼2 times) of the charge (Fig. 14). The energy efficiency for this emission became almost two orders of magnitude higher than that of the face emission, which was equal to ∼10−3. On account of such an enhancement of the intensity and efficiency of the edge emission without noticeable change of these values for the face emission, it was concluded that the laser oscillation arose. Unfortunately, the lasing spectrum and other characteristics could not be measured because the lasing soon ceased (see inset in Fig. 13). Moreover, the edge emission became very weak, whereas the rather intense face emission persisted. Consequently, irreversible changes arisen during the laser oscillation did not essentially affect the excitation of Cr2+ ions. It was revealed that the changes consisted in the appearance of strong light scattering because the surface of operated cell mirror initially became mat. The light scattering resulted in a significant increase of optical losses in the waveguide. This is the main cause of the lasing cessation and edge emission weakening. These findings were confirmed by the topographical study of the structure surface by means of AFM.
The topography maps for an unexcited cell and an operated cell after the lasing reported in  are shown in Figs. 15a and b (the Al electrode was etched in both). It is seen that the lasing results in an enlargement of the size and height of the inhomogeneities on the surface. The enlargement is much greater for the latter than the former (15 and 4.5 times, respectively). The fraction of the largest inhomogeneities does not exceed 10%. These peculiarities indicate that processes resulting in the topology changes during the lasing proceed more intensively in the direction normal to the sample surface and mostly in some its places. The above changes were concluded to stem from recrystallization processes taking place in the ZnS:Cr film under action of the laser emission, but not the electric field. This was confirmed by the following. The analogous topology changes are also observed outside the operated cell (Fig. 15c), but not far (∼0.5 mm) from it. There is no strong field here, but the laser emission emerges into this region because the waveguide is planar. In the waveguide, the laser emission propagates within the ZnS:Cr film and only its little fraction to come up to the I layers and cannot produce any changes in the wide-band Al2O3/SiO2 amorphous thin films. Recrystallization in the ITO film is also unlikely because still less laser emission reaches it. In addition, a high heating of it is impossible due to heat exchange with the thick substrate. Consequently, the topology changes on the structure surface display the growth of grains in the ZnS:Cr film. Recrystallization in this film was concluded to stem from local heating owing to laser emission absorption because noticeable heating the whole operating cell was not detected. However, there is no absorption band of ZnS in the spectral range around 2.6 μm. The Cr2+ absorption band is weak and blueshifted. Therefore, it was supposed [28, 35] that the laser emission is absorbed by Cr clusters that form in high-doped ZnS:Cr films due to chromium segregation.
This suggestion and the above inference that the changes during the lasing take place in the ZnS:Cr film were confirmed by studying a cell of the ZnS:Cr structure before and after the lasing by means of X-ray defectoscopy . XRD patterns were measured by the conventional Θ–2Θ scanning method. The XRD patterns of the cell in the original state (# 1) and the cell after the lasing (# 2) are shown in Fig. 16. It is seen that there are dominant peaks of cubic ZnS and small peaks of hexagonal ZnS phase and ITO in the patterns of the two. The distinctions of the XRD pattern of the cell # 2 relative to that of the cell # 1 consist in the following. The main peak of cubic ZnS (111) at 28.6o becomes somewhat narrower. Its FWHM is 0.410o in comparison with 0.435 for the cell # 1. Other peaks of the cubic phase at 47.4o and 56o are absent. There are only the small peaks of the hexagonal phase on their place. Three peaks at 35.3o, 50.08o, and 60.4o, which are close to the ITO peaks, intensified significantly. Lastly, new small peaks at 37.4o, 45.7o, and 74.5o appeared. The narrowing of the (111) peak and disappearance of other cubic peaks indicate that crystalline quality of the ZnS:Cr films was improved after the lasing, namely, the size of crystallites enlarged and they became textured. The appearance of the texture was explained in that recrystallization occurs in the presence of the very high field, resulting in rapid drift of ions arisen under action of the laser emission in the field direction. Three peaks intensified cannot be attributed to the ITO peaks located nearly because the intensity of other ITO peaks, in particular, of the main peak at 30.27o was smaller than in the XRD pattern of the cell # 1. Moreover, recrystallization in the ITO film is unlikely under the laser emission, as was noted above. To establish the nature of these peaks as well as the new peaks, the known XRD patterns of all materials that could be formed during the lasing (Cr, Zn, Al, CrS, ZnO) were examined by the authors of . It was revealed that the peaks at 50.08 and 74.5o are inherent to Cr. The former is also present in the XRD pattern of the cell # 1, but it is smaller here than after the lasing. This denotes that really there are Cr clusters in ZnS:Cr films with high Cr content and their number increases after the lasing. The peak at 45.7o was found in the CrS XRD pattern. The small CrS inclusions can be formed around the Cr clusters due to the local heating. The very intense peak at 35.3o turned out to coincide with the main peak of hexagonal ZnO. The possibility of the formation of this phase during the lasing was explained by processes taking place on the boundaries between the ZnS:Cr film and Al2O3 layers. Zn ions resulting from the heating drift rapidly into the Al2O3 layers and react with oxygen forming ZnO. The presence of only one main peak of ZnO indicates that the ZnO inclusions were grown highly oriented due the presence of the field. The nature of the peaks at 37.4 and 60.4o was not established. It was supposed that they appertain to some ZnSxOy phase that can form between the ZnS:Cr film and ZnO phase. It should be noted that all the above distinctions in the XRD pattern of the cell # 2 were not observed in the XRD pattern of the cell # 3 after the voltage increasing at great rate up to electrical breakdown (∼195 V), but without the appearance of lasing (Fig. 16) . Therefore, the distinctions were concluded to stem from the laser emission in the presence of a high field, but not only the field. The textural enlargement of the ZnS grains and the formation of the highly oriented ZnO phase in the field direction during the lasing cause deformation in the upper I layer. This explains the above-described larger increase of the height of surface homogeneities than their size.
It follows from the results obtained in [28, 94] that the use of a high Cr concentration, which gives rise to chromium segregation, is not a good way to increase the gain in ZnS:Cr TFELS. Primarily, it is necessary to improve their waveguide properties, particularly replacing the planar waveguide by a channel waveguide and decreasing optical losses.
ZnS:Cr and ZnSe:Cr are the focus of attention as media for broadly tunable lasers operating in the near- and mid-IR regions. To date, only the optically pumped lasers were created. It is of great interest to obtain electrically pumped lasers based on these materials because they could possess some advantages in comparison with other known lasers operating in these regions. However, attempts to electrically pump bulk ZnSe:Cr samples met with failure. Different electrical excitation mechanisms of TM ions, in particular, of Cr2+ ions are reviewed in this work. The injection mechanism is inefficient because the capture of free carriers directly on the 3d levels is impossible. Only resonant energy transfer from some adjacent recombination center can result in the excitation, but the conditions necessary for this are difficult to realize. The mechanism of the excitation via recombination processes raises doubts as it is based on the presence of the 3d levels of Cr2+ ions in the bandgap of ZnS, but these electron states are not described by the band theory. The effective electrical excitation of TM and RE ions with the impact mechanism only was demonstrated so far. However, this mechanism is possible at very high electric fields (≥1 MV/cm), which cannot be achieved in bulk samples due to the high probability of breakdown. The high fields required are attainable in thin-film structures with insulator layers between an electroluminescent film and electrodes, which serve as a ballast capacity reactance preventing avalanche breakdown. Intense EL takes place in TFELS of the MISIM type doped with various TM and RE, in particular in ZnS:Cr and ZnSe:Cr TFELS. To obtain the stimulated emission and lasing, waveguide structures are the most promising. MISIM structures represent a planar waveguide when the refractive index of I layers is lower than that of the S film.
Techniques for fabrication of ZnS:Cr and ZnSe;Cr films and waveguide structures are considered in this review. These films were fabricated by MBE growth, PLD, rf magnetron sputtering, EBE, and TE. EL was only obtained in the films deposited by the latter two methods. First, EL of TFELS based on these films was studied by detection of the emission through the ITO electrode. Recently, ZnS:Cr waveguide TFELS with Al2O3/SiO2 I layers were used to increase gain.
For the first time, the stimulated emission in the ZnS:Cr waveguide structures with the Cr content of ∼5 × 1019 cm−3 was demonstrated if the emission is detected from the structure edge. The induced nature of this emission was confirmed by changes in the EL spectrum with voltage increasing (disappearance of waveguide modes and large narrowing of the Cr2+ band) as well as by the stronger voltage dependence of the edge emission intensity than that of the face emission.
After increasing the Cr concentration up to ∼4 × 1020 cm−3 and making the edges on both waveguide ends of the ZnS:Cr structures, the laser oscillation was obtained, which is followed by strong increase (∼100 times) in the EL intensity and efficiency. However, the laser oscillation turned out to be unstable. The cessation of the lasing was concluded to stem from increased optical losses in the waveguide due to the light scattering by large inhomogeneities formed under the action of the laser emission. The principal cause of these changes is local heating in the ZnS:Cr film owing to absorption of the laser emission by Cr clusters formed due to chromium segregation at high Cr content. The above conclusions were confirmed by AFM and XRD experiments.
The following inferences concerning ways of improving the stability of the laser oscillation and the improvement of other lasing characteristics of the ZnS:Cr waveguide TFELS should be drawn from the above-considered published results. It is necessary not to use the high Cr concentration resulting in Cr segregation. Primarily, the waveguide properties must be improved by: the substitution of a channel waveguide for the planar waveguide, the diminution of the part of the emission flowing out through the waveguide walls due to thin I layers, the decrease of optical losses, and the enhancement of the feedback on the waveguide ends. In addition, more efficient conditions for the impact excitation should be selected, in particular, by using drive rectangular voltage pulses of alternating polarity instead of sinusoidal voltage. New theoretical considerations of the impact excitation of Cr2+ ions and hot-electron transport in ZnS:Cr and ZnSe:Cr are highly desirable. Obtaining stable lasing in the ZnS:Cr waveguide structures will be an essential contribution to the new direction of the laser physics aimed at development of lasers electrically pumped by dint of impact excitation, which are based on TM-doped waveguide TFELS. Such lasers operating in the near- and mid-IR regions will be widely applicable regardless of, obviously, their lower output power against that of the optically pumped lasers because they are compact, inexpensive, and simply fabricated. One more advantage of these lasers is the waveguide structure that is very useful for applications in optoelectronic devices.
Nataliya A. Vlasenko received the Ph.D. degree in optics from the Kharkov State University in 1960, the Dr. Sci. degree in physics of semiconductors from the Institute of Semiconductor Physics, NAS of Ukraine in 1977 and the title of Professor in 1983. Since 1960 she has been working at this Institute as senior research scientist, then as head of the department and now as leading research scientist. Her research focuses on electroluminescence of thin-film structures, in particular, of waveguide ones as a laser media.
Pavel Ph. Oleksenko received the Ph.D. degree in solid state electronics from the Institute of Semiconductor Physics, NAS of Ukraine in 1968, the Dr. Sci. degree from the Institute for Cybernetics, NAS of Ukraine in 1986, and the title of Professor in 1986. Since 1961 he has been working at the Institute of Semiconductor Physics (as head of department since 1968). His research focuses on optoelectronics, integrated optics, and physics of optoelectronic devices.
Miroslav A. Mukhlio received the Ph.D. degree in physics of semiconductors and dielectrics from the Institute of Semiconductor Physics, NAS of Ukraine in 2012 for his work on electroluminescence of thin-film planar and waveguide ZnS:Er and ZnS:Cr structures. Since 2005 he has been working as a research scientist at this Institute.
Zinaida L. Denisova received the Ph.D. degree in physics of semiconductors and dielectrics from the Institute of Semiconductor Physics, NAS of Ukraine in 1986 for her work on luminescence of CdS thin films. She has been working at this Institute since 1965 and as senior research scientist since 1986. Her research focuses on the energy of defect levels, particularly of ZnS:Cr studied by photodepolarization spectroscopy methods.
Lyudmila I. Veligura after graduation from Kyiv State University, she worked in the Research Laboratory of the Plant “Arsenal” from 1968 to 1981. Since 1981 she has been working as research scientist in the Institute of Semiconductor Physics, NAS of Ukraine. She developed methods of deposition of thin-film electroluminescent structures, fabricated them and studied their optical properties. In the recent years her work concentrates on fabrication of TFELS based on ZnS:Cr and ZnSe:Cr.