Luminescence in ZnMnTe/ZnMgTe and CdMnTe/CdMgTe structures with different parameters of quantum wells

The low-temperature luminescence of ZnMnTe/ZnMgTe and CdMnTe/CdMgTe quantum well structures with different quantum well widths and different Mn proportions is studied at optical-excitation power densities ranging from 10⁴ to 10⁶ W cm^?2. Because of saturation of the lowest excited state ⁴ T ₁ of the 3d shell of Mn²⁺ ions, transitions to higher states start to play an important role. As a result, the intracenter luminescence of Mn²⁺ ions deteriorates at high excitation levels. Simultaneously, the temperature-dependent saturation of the main exciton-emission band e1hh1 of the quantum wells occurs, and the band e2hh2 emerges. As the optical excitation is increased, the intracenter luminescence band of Mn²⁺ ions changes its shape. This effect is attributed to the faster saturation of the excited states of interface ions. For CdMnTe/CdMgTe structures, the effect of the quantum well width and Mn content on the relation between the emission intensities corresponding to excitons in quantum wells, excitons in barriers, and the 3d shell of Mn²⁺ ions is established.     

Luminescence of stepped quantum wells in GaAs/GaAlAs and InGaAs/GaAs/GaAlAs structures

Luminescence spectra of doped and undoped GaAs/GaAlAs and InGaAs/GaAs/GaAlAs structures containing several tens of stepped quantum wells (QW) are investigated. The emission bands related to free and bound excitons and impurity states are observed in QW spectra. The luminescence excitation spectra indicate that the relaxation of free excitons to the e1hh1 state proceeds via the exciton mechanism, whereas an independent relaxation of electrons and holes is specific to bound excitons and impurity states. The energy levels for electrons and holes in stepped QWs, calculated in terms of Kane’s model, are compared with the data obtained from the luminescence excitation spectra. The analysis of the relative intensities of emission bands related to e1hh1 excitons and exciton states of higher energy shows that, as the optical excitation intensity increases, the e1hh1 transition is more readily saturated at higher temperature, because the lifetime of excitons increases. Under stronger excitation, the emission band of electron-hole plasma arises and increases in intensity superlinearly. At an excitation level of ～10⁵ W/cm², excitons are screened and the plasma emission band dominates in the QW emission. Nonequilibrium luminescence spectra obtained in a picosecond excitation and recording mode show that the e1hh1 and e2hh2 radiative transitions are 100% polarized in the plane of QWs.     

Electron spin resonance in Cd<Subscript>1−<Emphasis Type="Italic">x</Emphasis></Subscript>Mn<Subscript>x</Subscript>Te and Zn<Subscript>1−<Emphasis Type="Italic">x</Emphasis></Subscript>Mn<Subscript>x</Subscript>Te compounds

Electron spin resonance in semimagnetic Cd_1?xMn_xTe (0<x<0.7) and Zn_1?xMn_xTe (0<x<0.53) compounds was studied at temperatures of 77 and 300 K. It is found that two types of paramagnetic centers exist in Zn_1?xMn_xTe, one of which is related to Mn²⁺ ions and the other is attributed to structural defects in the crystals.     

Determination of the minimum island size for full exciton localization due to thickness fluctuations in Zn1−xCdxSe quantum wells

Fluctuations of the thickness of quantum wells (QWs) of few monolayers are one of the causes of exciton localization. Here, we present the results of the determination of the minimum lateral dimensions of islands produced by thickness fluctuations in Zn_1−xCd_xSe QWs, which cause full exciton localization. We have calculated the localization energy of excitons in the frame of the factorized-envelope approximation. We found that the excitons are well localized in the islands of the QW when their lateral dimensions are larger than ∼15 times the exciton Bohr radius.     

Nickel impurity excitons and photoinduced lattice distortion in ZnSe1−y Sy:Ni and Zn1−x CdxSe:Ni solid solutions

A heavy dependence of the zero-phonon line of the donor Ni excitons and of a series of its phonon replicas on the composition of ZnSe_1?y S_y:Ni and Zn_1?x Cd_xSe:Ni solid solutions was studied. A model of nonradiative recombination of impurity excitons with allowance made for an intermediate virtual state {d ⁸} is discussed in detail. It is shown that this state depends on the lattice distortion induced by the Ni³⁺ impurity center whose charge is positive in reference to the lattice.     

Optical band gap of Cd1−x MnxTe and Zn1−x MnxTe semiconductors

The dependence of the optical band gap for Zn_1?x Mn_xTe and Cd_1?x Mn_xTe semiconductor compounds was investigated by the methods of cathodoluminescence and optical reflection. It was found that, for Zn_1?x Mn_xTe compounds in the region x?0.2, the band gap is additionally broadened by a magnitude of about 0.08 eV, which is related to the high density of interstitial-type defects in single crystals. For x?0.3, the probability of the existence of these defects decreases substantially, which is related to the distortion of tetrahedra of the crystal lattice of Zn_1?x Mn_xTe by Mn atoms, which are incorporated into each tetrahedron.     

A study of polycrystalline Cd(Zn,Mn)Te/Cds films and interfaces

Polycrystalline films of Cd_1-x Zn _x Te (x = 0–0.4) and Cd_1-x Mn _x Te (x = 0–0.25) were grown by MBE and MOCVD, respectively, on CdS/SnO₂/glass substrates to investigate their feasibility for solar cell applications. The compositional uniformity and interface quality of the films were analyzed by x-ray diffraction, surface photovoltage, and Auger depth profile measurements to establish a correlation between growth conditions and lattice constant, atomic concentration, and bandgap of the ternary films. MBE-grown polycrystalline Cd_1-x Zn _x Te films showed a linear dependence between Zn/(Cd + Zn) beam flux ratio, Zn concentration in the film, and the bandgap. Polycrystalline Cd_1-x Zn _x Te films grown at 300° C showed good compositional uniformity in contrast to compositionally non-uniform Cd_1-x Mn _x Te films grown by MOCVD in the temperature range of 420–450° C. The MBE-grown Cd_1-x Zn _x Te interface also showed significantly less interdiffusion compared to the MOCVD-grown Cd_1-x Mn _x Te/CdS interface, where preferential exchange between Cd from the CdS layer and Mn from the Cd_1-x Mn _x Te film was observed. The compositional uniformity of MOCVD-grown polycrystalline Cd_1-x Mn _x Te films grown on CdS/SnO₂/glass substrates was found to be a strong function of the growth conditions as well as the Mn source.     

Thermodynamic stability of bulk and epitaxial CdHgTe,ZnHgTe, and MnHgTe alloys

The thermodynamic stability of Cd_1?xHg_xTe, Mn_xHg_1?xTe, and Zn_xHg_1?xTe alloys is studied. Calculations performed in the context of the δ lattice-parameter model indicate that CdHgTe and ZnHgTe alloys are stable over the entire range of compositions at typical growth temperatures. At the same time, a miscibility gap is found in Mn_xHg_1?xTe at 0.33 < x < 1 at T = 950 K, which is consistent with the known experimental data. It is shown that the biaxial strains observed in Mn_xHg_1?xTe/CdTe and Mn_xHg_1?xTe/Cd_0.96Zn_0.04Te thin epitaxial films lead to a narrowing of the miscibility gap and to insignificant lowering of critical temperatures.     

Electrodepositing ZnxMnyOz alloys from zinc oxide to manganese oxide

Electrodeposition has emerged as a practical and simple method to synthesise semiconductor materials under different forms, thin films or nanostructured layers. This work reports on the cathodic electrodeposition of ZnMnO thin layers using both zinc and manganese chlorides as precursors. The composition of thin films can be varied from binary zinc oxide to manganese oxide varying the Mn/(Mn+Zn) ratio between 0 and 1. The composition of Zn_xMn_yO_z films was obtained by energy dispersive spectroscopy. Zn_1−xMn_xO films with Mn/Zn ratio less than 10% exhibit a crystalline wurtzite structure typical of ZnO fully oriented in the (0 0 2) direction. Higher Mn content leads to deformation of the ZnO lattice and the wurtzite structure is no longer maintained. X-ray photoelectron spectroscopy points out that Mn₃O₄ tends to be deposited when a high Mn/Zn ratio is used in the starting solution. Magnetic measurements on films with Mn/(Zn+Mn) ratio near 1 reveal magnetic characteristics similar to Mn₃O₄ compounds. The transmission spectra of Zn_xMn_yO_z show the typical absorption edge of crystalline ZnO while the wurtzite structure is maintained and it shifts to higher wavelengths when Mn content increases.     

On the origin of the 1-eV band in photoluminescence from Cd1−x Zn x Te

Photoluminescence (PL) at 77 K from Cd_1?x Zn _x Te samples (x = 0, 0.005 and 0.01) annealed at 900°C and cadmium vapor pressure P _Cd = 3 × 10⁴?2 × 10⁵ Pa has been studied. It was found that the contribution of the 1-eV band to the spectrum-integrated PL from these samples is independent of P _Cd, in contrast to Cd_0.95Zn_0.05Te samples in which this contribution increases up to ～90% as P _Cd grows. The band is not shifted to shorter wavelengths as x becomes larger. The conclusion that Zn vacancies are involved in the formation of Cd_1?x Zn _x Te properties is confirmed. The 1-eV band is attributed to capture of free holes to acceptor levels related to vacancies of both cadmium and zinc. These levels are closely spaced and, therefore, are difficult to resolve.     

Band offsets in Zn 1−x Cd x Te/ZnTe single-quantum-well structures grown by molecular-beam epitaxy on GaAs(001) substrates

ZnTe heteroepitaxial layers and ZnTe/Zn _1?x Cd _x Te/ZnTe strained quantum-confinement structures grown by molecular-beam epitaxy on GaAs(001) were studied by low-temperature cathodoluminescence spectroscopy and current-relaxation deep-level transient spectroscopy (DLTS). A peak related to electron emission from the ground size-quantization level in the conduction-band was observed in the DLTS spectra of quantum-confinement structures. The conduction-band offset parameter Q _C was determined from the DLTS and cathodoluminescence data. For Zn _1?x Cd _x Te/ZnTe single-quantum-well structures with x=0.2–0.22, Q _C equals 0.82 ± 0.05. The effect of internal elastic strain on the band offsets and Q _C at the Cd _x Zn _1?x Te quantum well interfaces was calculated; the results of calculations agree well with experimental data.     

Growth of HgTe Quantum Wells for IR to THz Detectors

We zone-engineered HgCdTe/HgTe/HgCdTe quantum wells (QWs) using the molecular-beam epitaxy (MBE) method with in situ high-precision ellipsometric control of composition and thickness. The variations of ellipsometric parameters in the ψ–Δ plane were represented by smooth broken curves during HgTe QW growth with abrupt composition changes. The form of the spiral fragments and their extensions from fracture to fracture revealed the growing layer composition and its thickness. Single and multiple (up to 30) Cd _x Hg_1−x Te/HgTe/Cd _x Hg_1−x Te QWs with abrupt changes of composition were grown reproducibly on (013) GaAs substrates. HgTe thickness was in the range of 16 nm to 22 nm, with the central portion of Cd _x Hg_1−x Te spacers doped by In to a concentration of 10¹⁴ cm⁻³ to 10¹⁷ cm⁻³. Based on this research, high-quality (013)-grown HgTe QW structures can be used for all-electric detection of radiation ellipticity in a wide spectral range, from far-infrared (terahertz radiation) to mid-infrared wavelengths. Detection was demonstrated for various low-power continuous-wave (CW) lasers and high-power THz pulsed laser systems.     

Relaxation processes in conductivity of Cd1 − x Mn x Te crystals (0.02 < x < 0.55)

The results of studying the temperature dependences of resistivity and the Hall coefficient in crystals of Cd_{1 ? x} Mn _x Te alloys (0.02 < x < 0.55) are used to gain insight into the conductivity relaxation processes observed in the temperature range 200–420 K in Cd_{1 ? x} Mn _x Te crystals with the manganese content x > 0.06. An anomalous isothermal variation in the hole concentration by an amount from half to three orders of magnitude occurs in these crystals. It is shown that relaxation of resistivity in the crystals under consideration can be provided by quasi-chemical reactions with involvement of uncontrolled Cu impurity and defects of CdTe crystal structure.     

Emission sensitization and mechanisms of electron-excitation migration in structures based on III-nitrides doped with rare-earth elements (Eu,Er, Sm)

The effect of doping with Eu, Er, and Sm rare-earth ions on the shape of the luminescence spectrum for heterostructures with GaN/In _x Ga_{1 ? x} N (0.1 < x < 0.4) quantum wells and from p-GaN〈Mg〉/n-GaN and p-AlGaN/n-GaN junctions is investigated. The results of measurements of the electroluminescence of these structures correlate with the previous data on photoluminescence and Mössbauer spectroscopy. It is shown that it is the GaN “yellow” (5000–6000 Å) band that plays the important role in the excitation of intracenter states in the structures with several GaN/InGaN quantum wells doped with Eu and Sm. In this case, Eu is most likely the sensitizer for Sm. Additional introduction of 3d metal (Fe⁵⁷) in p-GaN〈Mg〉/n-GaN:Eu results in the realization of intracenter transitions in Eu³⁺: ⁵ D ₀ → ⁷ F ₁ (6006 Å), ⁵ D ₀ → ⁷ F ₂ (6195 Å), ⁵ D ₀ → ⁷ F ₃ (6627 Å), and ⁵ D ₁ → ⁷ F ₄ (6327 Å) due to the occurrence of new, efficient channels of excitation transfer to intracenter states and in the effect of Fe on the local environment of rare-earth ions including due to the f–d hybridization enhancement.     

Origin of satellite structures of high field epr in Cd1−xMnxTe

The electron paramagnetic resonance (EPR) of Mn²⁺ in Cd_1?x Mn _x Te was examined at low temperatures and high magnetic fields with the Mn concentration ranging fromx=0.01 tox=0.65. A far infrared laser was used as a radiation source. The structure of the resonance is strongly influenced by electromagnetic propagation effects. By variation of the sample thickness it was clearly shown that the satellite structures besides the main EPR peak, formerly explained as originating from transitions in the Mn²⁺ pair energy level scheme, are solely due to interference effects.     

Energy spectrum of acceptors in the semimagnetic semiconductors p-Hg1−x MnxTe in the spin-glass region

The far-IR transmission and photoconductivity in the semimagnetic alloys p-Hg_1−x Mn_xTe (x=0.20–0.22) at temperatures 2–7 K were investigated at fixed frequencies of optically pumped molecular lasers in the region 49–311 μm. We report the observation of photoexcitation of acceptors from the ground into excited states under conditions when the direct interaction of the magnetic moments of the manganese ions becomes substantial and results in the formation of a spin-glass phase. It was found that the internal field produced by the spontaneous and external polarizations of the magnetic moments of the Mn²⁺ ions in the spin-glass temperature range influences the energy spectrum of acceptors in a magnetic field. Fiz. Tekh. Poluprovodn. 32, 450–452 (April 1998)     

MBE growth and photoluminescent properties of InAsSb/AlSbAs quantum wells

Heterostructures with a single InAs_1−x Sb_x/AlSb_1−y As_y quantum well (QW) on (001) GaSb substrates have been grown by MBE and studied using X-ray diffraction, transmission electron microscopy, and photoluminescence (PL) spectroscopy. High-intensity PL was observed at a temperature of 80 K, with a peak half-width of 30–50 meV and a peak wavelength in the range from 2 to 4.5 μm, depending on the QW width, which varied between 4 and 20 nm, respectively. The fundamental absorption edge of such QWs was calculated for a wide range of alloy compositions, x and y. Good correlation between the experimental and calculated dependences of the band gap on the InAsSb/AlSbAs QW thickness was obtained. __________ Translated from Fizika i Tekhnika Poluprovodnikov, Vol. 36, No. 12, 2002, pp. 1470–1474. Original Russian Text Copyright ? 2002 by Solov’ev, Terent’ev, Toropov, Meltser, Semenov, Sitnikova, Ivanov, Meyer, Kop’ev.     

Specific features of conductivity of Cd<Subscript>1−<Emphasis Type="Italic">x</Emphasis></Subscript>Zn<Subscript>x</Subscript>Te and Cd<Subscript>1−<Emphasis Type="Italic">x</Emphasis></Subscript>Mn<Subscript>x</Subscript>Te single crystals

The electrical characteristics of p-type Cd_1?xZn_xTe (x=0.05) and Cd_1?xMn_xTe (x=0.04) single crystals with a resistivity of 10³–10¹⁰ Ω cm at 300 K are studied. The conductivity and its variation with temperature are interpreted on the basis of statistics of electrons and holes in a semiconductor with deep acceptor impurities (defects), with regard to their compensation by donors. The depth of acceptor levels and the degree of their compensation are determined. The problems of attaining near intrinsic conductivity close to intrinsic are discussed.     

Electron Paramagnetic Resonance and Photoluminescence Studies of LaMgAl11O19:Mn2+ Green Phosphors

Manganese-doped LaMgAl₁₁O₁₉ powder has been prepared by an easy combustion method. Powder x-ray diffraction and scanning electron microscopy have been used to characterize the as-prepared phosphor. The electron paramagnetic resonance (EPR) spectrum of LaMgAl₁₁O₁₉:Mn²⁺ phosphor exhibits six-line hyperfine structure centered at g ≈ 1.973. The number of spins participating in resonance (N) and the paramagnetic susceptibility (χ) for the resonance signal at g ≈ 1.973 have been calculated as a function of temperature. The photoluminescence spectrum exhibits green emission at 516 nm, which is attributed to ⁴T₁ → ⁶A₁ transition of Mn²⁺ ions. From EPR and luminescence studies, it is observed that Mn²⁺ ions occupy Mg²⁺ sites and Mn²⁺ ions are located at tetrahedral sites in the prepared phosphors.     

Luminescence Properties of Cobalt-Doped ZnO Films Prepared by Sol–Gel Method

Pure ZnO and Co-doped ZnO films have been deposited on coverslip substrates by sol–gel spin coating. The morphological, structural, and optical properties of the films were investigated. The microstructure of the ZnO films became increasingly fine and the crystalline size decreased with Co doping. Analysis of x-ray diffraction (XRD) and Raman spectra reveals that Co²⁺ ions are substituted for Zn²⁺ ions in the ZnO lattice without changing its wurtzite structure. Co doping induces a decrease of the band-gap energy and fluorescence quenching of the emission bands. The spectra related to transitions within the tetrahedral Co²⁺ ions in the ZnO host crystal were observed in absorption and luminescence spectra. Photoluminescence (PL) spectra under different excitation energies and PL excitation spectra for the visible emissions suggest that the orange–red emission and green emission could be related to interstitial zinc (Zn_i) shallow donors and oxygen vacancy (V _O) deep donors, respectively. The red emission of Co-doped ZnO film could be assigned to the radiative transitions within the tetrahedral Co²⁺ ions in the ZnO host crystal after band-to-band excitation. A consistent explanation for the pure and Co-doped ZnO films is that the red emission under the excitation energy below the band gap is probably associated with extended Zn_i states.