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In a Fe/(Cd,Mg)Te/CdTe quantum well hybrid structure, short-range and long-range ferromagnetic proximity effects are found to coexist. The former is observed for conduction band electrons, while the latter is observed for holes bound to shallow acceptors in the CdTe quantum well. These effects arise from the interaction of charge carriers confined in the quantum well with different ferromagnets, where electrons interact with the Fe film and holes with an interfacial ferromagnet at the Fe/(Cd,Mg)Te interface. The two proximity effects originate from fundamentally different physical mechanisms. The short-range proximity effect for electrons is determined by the overlap of their wave functions with d-electrons of the Fe film. On the contrary, the long-range effect for holes bound to acceptors is not associated with overlapping wave functions and can be mediated by elliptically polarized phonons. The coexistence of the two ferromagnetic proximity effects reveals the presence of a nontrivial spin texture within the same heterostructure.

Reflectance spectra of thin Pb1 –xSnxTe films (~60 nm) with x = 0.25, 0.53, and 0.59, grown by MBE on hybrid GaAs/CdTe substrates, are measured in the frequency range of 20–5500 cm–1 and the temperature range of 5–300 K. The temperature dependence of transverse optical phonon frequencies and structural phase transition temperatures are determined based on a dispersion analysis. An increase in the plasma frequency with decreasing temperature in the range 300‒77 K in all films is detected.

Copper-doped self-assembled CdSe/ZnSe quantum dots (QDs) and (Zn,Cd)Se/ZnSe quantum wells (QWs) are grown by molecular beam epitaxy. Low-temperature luminescence properties of these structures are ...

Thin epitaxial Pb1−xCdxSe films with cadmium content x ≤ 0.36 have been grown by molecular beam epitaxy (MBE) on highly resistive, lattice matched (0 0 1) ZnTe/GaAs hybrid substrates. The High Resolution X-ray Diffraction (HRXRD) indicates that the films crystallize in the rock salt structure. Up to x = 0.12 the Pb1−xCdxSe films are (0 0 1) oriented and monocrystalline with negligible amount of other phases. The lattice constant of the films decreases proportionally to the Cd content. As evidenced by low-temperature photoluminescence (PL) the energy gap of Pb1−xCdxSe films increases with the increasing Cd content. The parameters of optical phonons and plasmons in the material are established by infrared reflectance measurements. The most interesting property of the Pb1−xCdxSe films is revealed by classical transport measurements. The alloying of PbSe with CdSe reduces the hole concentration in the material from a typical value of 3 × 1018 cm−3 in the pure PbSe film down to 3 × 1016 cm−3 in the film containing x = 0.052 of Cd. The reduction of the hole concentration by two orders of magnitude makes the Pb1−xCdxSe films very attractive for infrared detector applications. Simple photoresistors made of Pb1−xCdxSe films exhibit sensitivity to the infrared radiation up to the room temperature.

We study the crystal structure-mismatched, quantum dot-like (QD) composite metamaterial based on rock-salt narrow gap PbTe and zinc-blende wide gap CdTe. In the case of PbTe/CdTe QDs the quantum effects are observed for dots with dimensions of the order of 100 nm even at room temperature making this material system a promising candidate for mid-infrared applications. Using molecular beam epitaxy technique and utilizing difference in crystal structure of both semiconductors a variety of samples containing PbTe nanostructures with complicated morphology embedded in CdTe were obtained. Investigated nanocomposite PbTe/CdTe samples exhibit unusually strong and surprisingly narrow (about 5 meV) mid-infrared photoluminescence emission (250 meV) in contrast to the spectrally wide luminescence of a typical ensemble of PbTe/CdTe quantum dots measured in similar temperature (about 100 K) and excitation power conditions (400 microW). For excitation power exceeding 500 microW additional emission in energy about 15 meV lower than previous one appears, which dominates PL spectrum for excitation higher than 800 microW. This line, not reported for such kind of QDs yet, exhibit non-linear dependence of amplitude on excitation power. We discuss the observed behavior of photoluminescence considering presence of two-dimensional electron gas with high electron mobility and carrier density up to 10^19 cm-3 spontaneously formed close to the polar CdTe/PbTe interfaces. As estimated plasmon energy in our samples (240 meV) matches well the energy of observed emission, non-resonant coupling of photons with interface plasmons mediated by LO phonon is most possible explanation of unusual enhancement of PL from studied PbTe/CdTe metamaterial.

Optical control of electronic spins is the basis for ultrafast spintronics: circularly polarized light in combination with spin-orbit coupling of the electronic states allows for spin manipulation in condensed matter. However, the conventional approach is limited to spin orientation along one particular orientation that is dictated by the direction of photon propagation. Plasmonics opens new capabilities, allowing one to tailor the light polarization at the nanoscale. Here, we demonstrate ultrafast optical excitation of electron spin on femtosecond time scales via plasmon to exciton spin conversion. By time-resolving the THz spin dynamics in a hybrid (Cd,Mn)Te quantum well structure covered with a metallic grating, we unambiguously determine the orientation of the photoexcited electron spins which is locked to the propagation direction of surface plasmon-polaritons. Using the spin of the incident photons as additional degree of freedom, one can orient the photoexcited electron spin at will in a two-dimensional plane., main text 23 pages, 5 figures. Supplementing material with 6 figures

We report the observation of tens of minor lines of the combinational spin-flip Raman scattering in a CdTe:Mn quantum well by means of the high-resolution optical spectroscopy. Classification of this manifold leads to four characteristic values of energy, that correspond to four different types of pair clusters of Mn ions: the nearest, second, third, etc., neighbors. All the four energies show up in a single experiment with a very high precision, providing experimental grounds for a deeper understanding of the $d\ensuremath{-}d$ exchange interactions in a diluted magnetic semiconductor and demonstrating the capacity of the employed method. The major (nearest-neighbor) exchange constant ${J}_{1}=6.15\ifmmode\pm\else\textpm\fi{}0.05$ K was found to consent with its previously reported value. Other detected characteristic energies are as follows ($\ifmmode\pm\else\textpm\fi{}0.05$ K): ${J}_{(2)}=1.80$ K, ${J}_{(3)}=1.39$ K, ${J}_{(4)}=0.81$ K.

We use the two-pulse spin-dependent photon echo technique to study the in-plane hole spin anisotropy in a 20~nm-thick CdTe/Cd$_{0.76}$Mg$_{0.24}$Te single quantum well by exciting the donor-bound exciton resonance. We take advantage of the photon echo sensitivity to the relative phase of the electron and hole spin precession and study various interactions contributing to the hole in-plane spin properties. The main contribution is found to arise from the crystal cubic symmetry described by the Luttinger parameter $q=0.095$, which is substantially larger than the one theoretically expected for CdTe or found in other quantum well structures. Another contribution is induced by the strain within the quantum well. These two contributions manifest as different harmonics of the spin precession frequencies in the photon echo experiment, when strength and orientation of the Voigt magnetic field are varied. The magnitude of the effective in-plane hole $g$ factor is found to vary in the range $|\tilde{g_h}|$=0.125--0.160 in the well plane., 7 figures

Poly (3-hexylthiophene) (P3HT) and phenyl-C61-butyric acid methyl ester (PCBM) are commonly used for the fabrication of organic photovoltaics (OPV). Efficiency limitations of OPVs could be circumvented by incorporation of inorganic nanostructures into organic blends. Again, integration of organic solar cells with well-developed silicon photovoltaic technology is ultimately desirable. In the present work, GaN nanowires with diameters of 25–50 nm and two lengths (200 and 500 nm) have been grown using molecular beam epitaxy technique. Solar-grade monocrystalline silicon wafers were used as substrates for nanowire synthesis. GaN nanostructures were incorporated into P3HT:PCBM photoactive layer in order to facilitate charge transfer between P3HT:PCBM and Si. Samples with and without nanowires were compared. Addition of nanowires led to the improvement in photovoltaic performance. Open circuit voltage has risen by 72% and short circuit current density by 200%. Series resistance has decreased 50 times, and power conversion efficiency has risen 20.7 times. Additional maxima are found in photocurrent spectrum corresponding to carriers being generated near GaN absorption edge. Moreover, external quantum efficiency peaks near GaN absorption edge, indicating the formation of current transfer channel via P3HT/GaN/Si cascade heterojunction. Mechanism explaining source of abovementioned improvement is proposed.

Infrared spectra of reflection and attenuated total reflection of PbTe thin films deposited by molecular beam epitaxy on a GaAs/CdTe substrate have been measured. The high-frequency dielectric constant, frequencies and oscillator strengths of transverse optical phonons, plasma frequencies, and plasma collision frequencies have been determined for all layers using the dispersion analysis. Attenuated total reflection spectra obtained with silicon and diamond prisms demonstrate surface phonon and plasmon–phonon polaritons. The dispersion curves of surface polaritons have been obtained.

Multilayer growth of an immiscible material system is an example of a self-ordering process which results in nanostructures of various patterns. PbTe and CdTe produce an excellent example of an immiscible material system because, despite having nearly the same lattice constants, both components crystallize in different crystal structures - rock salt and zinc blende - respectively. In this report we adjust the kinetic Monte Carlo (kMC) model proposed in our previous publication [1] to the case of PbTe/CdTe multilayer structures formed of ultra-thin PbTe and CdTe layers. The morphology of the multilayers is revealed by transmission electron microscopy (TEM) and the morphological patterns are simulated using the kMC model, which takes into account bulk and surface diffusion. An important factor which determines the morphology of the PbTe/CdTe superlattices is a significant anisotropy of both the surface and the bulk diffusion. The simulations show that the shape, size and dimensionality of the emerging nanostructures strongly depend on the amount of deposited components and the growth rate. Since the amount of the deposited material plays an important role, the desorption process from the surface has to be considered. Our kMC model with adjusted parameters correctly reproduces all morphological details observed in real PbTe/CdTe nanostructures grown with different growth rates and composition ratios. Finally, we show that the growth rate may be used to control morphological transitions between different types of structures of given compositions.

The reflectance spectra of the topological insulator Pb1–xSn x Se (x = 0.2, 0.34) films grown by molecular-beam epitaxy on a ZnTe/GaAs substrate are measured in the range of 12–2500 cm–1 at room temperature. Using dispersion analysis, the frequencies of transverse optical phonons, plasma frequencies, highfrequency permittivities, and layer thicknesses are determined. In the quasi-steady-state approximation, the interface mode frequencies of the four-layer structure are calculated as a function of the overlap parameter χ1 (0 ≤ χ1 ≤ 1). The parameter describes the degree of overlap of two interface modes localized at planes bounding the layer to the right and left. The existence of interacting interface modes in the structure makes its spectrum different from the sum of component spectra. These differences manifest themselves in the experiment. The conditions of the interaction of interface modes with IR radiation are discussed.

A specialized heterojunction of p -ZnTe:N/CdTe:Mg/n-CdTe:I/GaAs for varicap and high response signal diode applications was epitaxially grown by using molecular beam epitaxy. Frequency dependence of the capacitance-voltage, conductance-voltage and series -resistance characteristics in the frequency spectrum from 10 kHz to 10 MHz at ambient temperature was investigated. An increase in the capacitance is observed especially at lower frequency range that can be attributed to the predominant effect of interface states. A negative capacitance is observed and is explained on the basis of the loss of interface charges or interface states localized between metal and semiconductor due to impact ionization process. The frequency and voltage dependencies of the series resistance were investigated and characteristic peaks were observed for the voltage dependence of series resistance under low-frequency range. Some important parameters such as barrier height, carrier concentration, cut off voltage were also estimated and discussed. The electrical characteristics of p -ZnTe:N/CdTe:Mg/n-CdTe:I/GaAs show a good candidacy for electronic and optoelectronic devices.

In this work, heterojunctions of p-ZnTe:N/CdTe:Mg/n-CdTe:I/GaAs for photovoltaic and photodiode application were grown epitaxially by using molecular beam system. Current-voltage profile of the heterojunction device was studied in dark and under various illumination intensities. The obtained photocurrent is found to depend on the light intensity. The main heterojunction parameters, such as shunt and series resistances, the barrier height and the ideality factor were extracted from the current-voltage profile using diverse methods at ambient temperature. Values of the barrier height and the series resistance were obtained from Cheung's functions. A large value of the series resistance causes the non-ideal characteristics of current–voltage measurements. The study of the current-voltage characteristics of high voltage region suggests a predominant space charge limited mechanism. Moderate values of short circuit current and open circuit voltage were obtained through a light intensity of 140 mW/cm2, a current and voltages of 0.336 mA and 370 mV, respectively. The high photosensitivity and responsivity for the current under illumination condition suggests that the prepared heterojunction device could be employed as a photodiode sensor.

The coherent spin dynamics of electrons in tunnel-coupled CdTe and (Cd,Mn)Te quantum wells (QWs) is studied by time-resolved pump-probe Kerr rotation. The coupled QWs have different thicknesses; the narrow one is doped by ${\mathrm{Mn}}^{2+}$ magnetic ions. A short range proximity effect between them is observed: the Zeeman splitting of electrons in the wide QW is given in addition to the intrinsic electron $g$ factor by the exchange interaction with the ${\mathrm{Mn}}^{2+}$ ions mediated by electron tunneling into the narrow QW. The exchange interaction strength scales with the ${\mathrm{Cd}}_{0.88}{\mathrm{Mg}}_{0.12}\mathrm{Te}$ barrier thickness separating the QWs. The Kerr rotation signal measured on the wide QW shows two close frequencies of electron spin Larmor precession in a transverse magnetic field. These components have very different spin dephasing times, 50 ps and 1 ns. The two frequencies originate from electrons in the wide QW being either part of an exciton or being resident. The proximity effect of the exciton electron is smaller due to the binding by Coulomb interaction, which decreases the tunneling to the narrow well. The experimental data are in good agreement with model calculations.

Herein, the properties of ZnO:N/n‐SiC heterojunctions (HJs) and light‐emitting diodes based on them are studied. The HJs are grown by molecular beam epitaxy. Active nitrogen generated by a radio frequency plasma source is used for p‐type doping. The location of the space charge area on the ZnO:N/n‐SiC interface is revealed by electron‐beam‐induced current (EBIC) scans. The diffusion lengths of holes and electrons are calculated. This article presents the characterization of ZnO:N/n‐SiC HJs and reveals the presence of tunneling‐related current transport in them as well as the contribution of exponentially distributed traps at large voltage bias. Electroluminescence (EL) is measured at ambient pressure by a standard EL system and also at 77 K in vacuum by a system utilizing EBIC in a scanning electron microscope. Analysis of the light output power at higher current level indicates the limited effect of nonradiative defects in this structure. EL results are compared with cathodoluminescence spectra. Color temperature for HJs based on the EL spectra is also calculated.

We study the quantum beats in the polarization of the photon echo from donor-bound exciton ensembles in semiconductor quantum wells. To induce these quantum beats, a sequence composed of a circularly polarized and a linearly polarized picosecond laser pulse in combination with an external transverse magnetic field is used. This results in an oscillatory behavior of the photon echo amplitude, detected in the $\sigma^+$ and $\sigma^-$ circular polarizations, occurring with opposite phases relative to each other. The beating frequency is the sum of the Larmor frequencies of the resident electron and the heavy hole when the second pulse is polarized along the magnetic field. The beating frequency is, on the other hand, the difference of these Larmor frequencies when the second pulse is polarized orthogonal to the magnetic field. The measurement of both beating frequencies serves as a method to determine precisely the in-plane hole $g$ factor, including its sign. We apply this technique to observe the quantum beats in the polarization of the photon echo from the donor-bound excitons in a 20-nm-thick CdTe/Cd$_{0.76}$Mg$_{0.24}$Te quantum well. From these quantum beats we obtain the in-plane heavy hole $g$ factor $g_h=-0.143\pm0.005$., Comment: 4 pages, 4 figures

Spin-dependent photon echoes in combination with pump-probe Kerr rotation are used to study the microscopic electron spin transport in a CdTe/(Cd,Mg)Te quantum well in the hopping regime. We demonstrate that independent of the particular spin relaxation mechanism, hopping of resident electrons leads to a shortening of the photon echo decay time, while the transverse spin relaxation time evaluated from pump-probe transients increases due to motional narrowing of spin dynamics in the fluctuating effective magnetic field of the lattice nuclei., 11 pages, 8 figures

Double CdTe/CdMgTe quantum wells modulation doped with Iodine were etched to fabricate grids with a period of $8 \mu \mathrm{m}$ and a geometrical aspect ratio ($\delta$) between 0.2 and 0.7. Etching removed only the well closer to the sample's surface. Transmission of terahertz radiation as a function of magnetic field (B) up to 10 T was measured at 2 K. The spectra show a single magnetoplasmon resonance (MPR) which position in B depends on $\delta$. A huge amplitude of MPR, comparable to that of the cyclotron resonance, allows us to propose such structures as tunable plasmonic terahertz detectors.

In this paper we report on the observation of very high and narrow magnetoconductance peaks which we attribute to the transition to quantum Hall ferromagnet (QHFM) state occurring at the edges of the sample. We show that the expected spatial degeneracy of chiral edge channels is spontaneously lifted in agreement with theoretical studies performed within the Hartree-Fock approximation. We indicate also that separated edge currents which flow in parallel, may nevertheless cross at certain points, giving rise to the formation of topological defects or one-dimensional magnetic domains. Furthermore, we find that such local crossing of chiral channels can be induced on demand by coupling spin states of a Landau level to different current terminals and applying a DC source-drain voltage., Comment: a detailed discussion of the results has been added

Structural defects in the p-ZnTe/i-CdTe/n-CdTe single-crystalline heterojunctions designed for photovoltaic applications have been investigated by transmission electron microscopy (TEM) and deep-level transient spectroscopy (DLTS). Lattice parameters and misfit strain in the undoped cadmium telluride (CdTe) absorber layers of the heterojunctions, grown by the molecular-beam epitaxy technique on two different substrates, GaAs and CdTe, have been determined with high-resolution x-ray diffractometry. A dense network of misfit dislocations at the lattice-mismatched CdTe/GaAs and ZnTe/CdTe interfaces and numerous threading dislocations and stacking faults have been shown by the cross-sectional TEM imaging of the heterojunctions. The DLTS measurements revealed five deep-level traps in the heterojunctions grown on GaAs and only three of them in the heterojunctions grown on CdTe. Four of the traps have been attributed to the electronic states of extended defects, presumably dislocations, on the grounds of their logarithmic capture kinetics of charge carriers. Two of these traps, displaying the largest values of their capture cross-section and the properties characteristic of bandlike electronic states, have been ascribed to the core states of dislocations. It is argued that they are most likely responsible for decreased lifetime of photo-excited carriers resulting in a low energy conversion efficiency of solar cells based on similarly grown heterojunctions.

We show that by subsequent sensitization of nanostructured ZnO photoanodes with N-719 and Rose Bengal dyes an improvement or worsening of the cell efficiency may be obtained, relating to photoanodes sensitized with N-719 dye only (JSC = 2.97 mA/cm2, VOC = 0.68 V, η = 0.99%) depending on the order in which the dyes are applied. We observe that for the case when the N-719 dye is followed by Rose Bengal an increase in efficiency, short circuit current and open circuit voltage is observed (JSC = 3.95 mA/cm2, VOC = 0.71 V, η = 1.26%), which we relate to the cascade band alignment of the ZnO and the dyes. In the case when Rose Bengal is first on ZnO followed by N-719, a lowering of all parameters is observed (JSC = 2.86 mA/cm2, VOC = 0.64 V, η = 0.94%) due to the trap band alignment. Electrochemical impedance spectroscopy measurements and modelling confirmed this theory showing longer electron lifetimes in the photoanode for the cascade band alignment, enhancing electron-hole separation, than for the trap alignment, facilitating electron-hole recombination.

We demonstrate that a GaN nanowire array can be used for efficient charge transfer between the organic photovoltaic layer and silicon in a Si/GaN/P3HT:PC71BM inverted hybrid heterostructure. The band alignment of such a material combination is favorable to facilitate exciton dissociation, carrier separation and electron transport into Si. The ordered nature of the GaN array helps to mitigate the intrinsic performance limitations of the organic active layer. The dependence of photovoltaic performance enhancement on the morphology of the nanostructure with nanowire diameters 30, 50, 60, 100 and 150 nm was studied in detail. The short circuit current was enhanced by a factor of 4.25, while an open circuit voltage increase by 0.32 volts was achieved compared to similar planar layers.

We report on fabrication and characterization of infrared detectors made of a composite material—PbTe/CdTe multilayer (ML). The multilayer consists of 10 repetitions of 35 nm thick PbTe layers and 75 nm thick CdTe layers grown by molecular beam epitaxy on GaAs (100) semi-insulating substrates. Simple technological methods were used to manufacture photoresistors from the structure containing the PbTe/CdTe composite. The front-side illuminated photodetectors show a cut-off wavelength of 3.57 μm and a peak current responsivity of 127 mA/W at a bias voltage of 10 V, a frequency of 730 Hz, and a temperature of 300 K. The specific detectivity of photoresistors at the peak wavelength of 2.9 μm equals 2.7 × 1010 cm·Hz1/2/W and 6.1 × 109 cm·Hz1/2/W for 77 and 300 K, respectively. Although the ML photoresistors were not optimized, neither antireflection coated nor lithographically defined, their detectivity, especially at room temperature, is highly comparable to that of photoconducting infrared detectors available in the market. Possible mechanisms causing the relatively high performance of PbTe/CdTe ML detectors have been discussed in detail. These are a decrease in the electron concentration in the conducting PbTe layers caused by capturing some mobile electrons by dangling bonds present at the PbTe/CdTe interfaces and the effective suppression of the Auger recombination in nanostructures made of narrow and wide bandgap semiconductors.

Extended defects in the p -ZnTe/n -CdTe heterojunctions grown by the molecular-beam epitaxy technique on two different substrates, GaAs and CdTe, have been investigated by deep-level transient spectroscopy (DLTS) and transmission electron microscopy (TEM). Four hole traps, called H1 to H4, and one electron trap, called E3, have been revealed in the DLTS spectra measured for the heterojunctions grown on the GaAs substrates. The H1, H3, H4 and E3 traps have been attributed to the electronic states of dislocations on the ground of their logarithmic capture kinetics. The DLTS peaks associated with the H1 and E3 traps were not observed in the DLTS spectra measured for the heterojunction grown on the CdTe substrate. They are most likely associated with threading dislocations generated at the mismatched interface with the GaAs substrate. Cross-sectional TEM images point out that they are dislocations of the 60°-type. In both the types of heterojunctions the H4 trap was observed only under forward-bias filling pulse, suggesting that this trap is associated with the CdTe/ZnTe interface. In addition, TEM images revealed also the presence of intrinsic and extrinsic stacking faults in the CdTe layers, which may considerably affect their electronic properties. (© 2015 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

n-CdTe/p-CdMnTe/p-GaAs diluted magnetic diode was grown by molecular beam epitaxy technology. The forward- and reverse-bias capacitance–voltage (C–V), conductance–voltage (G–V), series resistance–voltage (R s–V), impedance–voltage (Z–V), and phase–voltage (θ–V) characteristics of this diode have been analyzed in the frequency range of 5 kHz to 1 MHz at room temperature. Both the density of interface states (N ss) and series resistance (R s) are strongly frequency dependent and decrease with increasing frequency. The values of the built-in potential ( $$ V_{\rm{bi}} $$ ), the doping concentration ( $$ N_{\rm{d}} $$ ), and the barrier height ( $$ \varphi_{{{\rm{b}}(C - V)}} $$ ) of the diode were calculated at different frequencies. Our experimental results revealed that both the series resistance and interface state density must be taken into account in studying the impedance spectroscopy on heterojunction diodes to understand their performance for electronic and optoelectronic applications. Diluted magnetic semiconductors are promising materials for future technology.

Dye sensitized solar cells were fabricated using ZnO nanostructured photoelectrodes sensitized with N-719 or Rose Bengal dyes. We assessed the device performance as a function of the sensitization time and found a dependence on time for the N-719 and no significant changes for the Rose Bengal. Furthermore, we observe that the structure of the N-719 molecule beneficial for sensitization of TiO2 may lead to the degradation of the ZnO crystals and a growth of an amorphous shell limiting dye performance in the cells.

High-resolution X-ray di ractometer was used to study structural quality, lattice parameters and mis t strain in p-ZnTe/n-CdTe heterojunctions grown by the molecular-beam epitaxy technique on two di erent (001)-oriented substrates of GaAs and CdTe. The X-ray di ractometer results indicate that the CdTe layers, grown on lattice mismatched GaAs substrate, are partially relaxed, by the formation of mis t dislocations at the interface, and display residual vertical strain of the order of 10−4. The presence of threading dislocations in the layers e ectively limits the e ciency of solar energy conversion in the investigated heterojunctions. Homoepitaxially grown CdTe layers, of much better structural quality, display unexpected compressive strain in the layers and the relaxed lattice parameter larger than that of the substrate. Possible reasons for the formation of that unusual strain are discussed.

In this work we discuss experimental data which can explain reasons for the limited energy conversion efficiencies observed in monocrystalline, thin-film CdTe-based solar cells grown molecular beam epitaxy. Two kinds of samples has been investigated and discussed – p- and n-type doped CdTe layers with metallic Schottky barriers and p-i-n heterojunctions (p-ZnTe/i-CdTe/n-CdTe solar cells). Both kinds of junctions exhibit strong rectifying properties with the rectifying coefficient exceeding 105 at 300 K and +/– 1 V. Photovoltaic properties of the solar cells have been determined by I–V measurements under illumination and by spectral characteristics of photocurrent. Based on the spectral measurements it was found that the p-ZnTe/i-CdTe/n-CdTe diodes are sensitive to illumination within the 550–850 nm wavelength range confirming their applicability for solar energy conversion. I–V characteristics measured at 1-sun illumination yield relatively low energy conversion efficiencies below 4.9%. In order to determine the origin of the limited efficiency in the CdTe-based solar cells (theoretical limit for n-CdTe/p-ZnTe solar cells is as high as 20%) the deep level transient spectroscopy (DLTS) technique has been applied. The results of these DLTS studies revealed the presence of deep electron and hole traps. The low-temperature DLTS peaks can be ascribed to the hole traps in the CdTe absorber. The high-temperature DLTS peak changes its sign depending on the sign of the filling pulses acting as a majority trap for negative filling pulses and minority trap for positive filling pulses. The estimated activation energies of the majority- and minority trap level are equal to 0.74–0.78 eV and 1.05 eV, respectively. The sum of these energies is close to the value of the CdTe energy gap, suggesting that this energy level can be assigned to the recombination center present in the CdTe absorber material. Basing on the logarithmic capture kinetics of the traps detected in the high-temperature region we assign the recombination center to electronic states of extended defects, probably threading dislocations. Very similar DLTS results obtained for CdTe Schottky barriers confirm the conclusion. Thus, we conclude that not the point defects but rather extended defect are responsible for the limited energy conversion efficiency in CdTe-based solar cells.

A CdTe/CdMnTe heterojunction magnetic diode for photovoltaic applications was fabricated by using molecular beam epitaxy (MBE). The ideality factor and the potential barrier height of the diode were determined to be 1.25 and 0.836 eV, respectively. Photovoltaic parameters of the studied device were determined at various illumination intensities. The highest open circuit voltage of the CdTe/CdMnTe heterostructure was equal to 0.56 V at the illumination intensity of 130 mW/cm2. The reverse current of the n-CdTe/p-CdMnTe/GaAs diode increases with the increasing illumination intensities. The obtained results suggest that n-CdTe/p-CdMnTe/GaAs diode can be used as a photodiode in photovoltaic and photodetector applications.

High quality n-(CdMgTe:I/n-CdTe:I)/(p-CdTe:N/p-ZnCdTe:N/p-ZnTe:N)/p-GaAs heterojunction diodes have been fabricated by molecular beam epitaxial growth. The illumination effect on the complex impedance and conductivity of heterostructure diode was investigated. The illumination intensities were taken up to the 200 mW/cm2 with frequency range of 42 Hz to 1 MHz. The observed real and imaginary parts of the complex impedance were strongly dependent on the illumination frequency. The inverse relation was observed between the illumination intensity and the complex impedance. The relaxation mechanism of the diode was analyzed by the Cole–Cole plots. The radius of the Cole–Cole curve decreases with increasing illumination intensity. This suggests a mechanism of illumination dependent on the relaxation process. It is also found that the conductivity increases linearly with increasing the illumination intensity. We can conclude that the new design heterostructure diode in our work is a good candidate in photodetector and optoelectronic applications.

AbstractThe reflectance spectra of the topological insulator Pb_1– x Sn_ x Se ( x = 0.2, 0.34) films grown by molecular-beam epitaxy on a ZnTe/GaAs substrate are measured in the range of 12–2500 cm^–1 at room temperature. Using dispersion analysis, the frequencies of transverse optical phonons, plasma frequencies, highfrequency permittivities, and layer thicknesses are determined. In the quasi-steady-state approximation, the interface mode frequencies of the four-layer structure are calculated as a function of the overlap parameter χ_1 (0 ≤ χ_1 ≤ 1). The parameter describes the degree of overlap of two interface modes localized at planes bounding the layer to the right and left. The existence of interacting interface modes in the structure makes its spectrum different from the sum of component spectra. These differences manifest themselves in the experiment. The conditions of the interaction of interface modes with IR radiation are discussed.

We report on the fabrication and characterization of a new type of mid-infrared photodetector. The infrared sensitive element of the detector is a PbSe single quantum well (SQW) embedded in an intrinsic region of a CdTe p-i-n diode. Electron-beam-induced current (EBIC) measurements confirm the location of the PbSe SQW near the middle of the intrinsic CdTe layer. The measured diffusion length of minority carriers of about 0.5 μm allows effective collecting of the photoexcited carriers in the highly doped regions of the diodes. The PbSe/CdTe heterostructures exhibit a strong photosensitivity in the mid-infrared spectral region. Photo-response measurements show abnormal temperature dependence of the photosensitivity. The photo-response signal increases with decreasing temperature reaching a maximum at about 170 K and upon a further decrease in temperature the signal weakens and vanishes at about 100 K. Current-voltage measurements demonstrate very good junction characteristics with a rectifying ratio of 585 at ± 0.5 V and an ideality factor of 1.05 at room temperature. Possible mechanisms of carrier transport trough the junction are discussed.

The new type of a p-i-n structure consists of a single acceptor doped ZnO:N film grown by plasma assisted molecular beam epitaxy, a thin isolating Al2O3 layer grown by atomic layer deposition method and n-type GaN template. The maximum forward-to-reverse current ratio I F/I R in the obtained diode is about 8 × 106 at ±7 V. Electron-beam-induced current measurements confirmed the presence of a p-i-n junction and the presence of the isolating layer located at the p-ZnO/n-GaN interface. The diffusion length of carriers was calculated. The heterostructure exhibits an absorption in the UV range and the ratio between light (UV) and dark current is above four orders of magnitude.

We study nanoscale morphology of PbTe/CdTe multilayer heterostuctures grown by molecular beam epitaxy on hybrid GaAs/CdTe (100) substrates. Nominally, the structures consist of 25 repetitions of subsequently deposited CdTe and PbTe layers with comparable thicknesses of 21 and 8 nm, respectively. However, the morphology of the resulting structures crucially depends on the growth temperature. The two-dimensional layered, superlattice-like character of the structures remains preserved only when grown at low substrate temperatures, such as 230 °C. The samples grown at the slightly elevated temperature of 270 °C undergo a morphological transformation to structures consisting of CdTe and PbTe pillars and columns oriented perpendicular to the substrate. Although the pillar-like objects are of various shapes and dimensions these structures exhibit exceptionally strong photoluminescence in the near infrared spectral region. At the higher growth temperature of 310 °C, PbTe and CdTe separate completely forming thick layers oriented longitudinally to the substrate plane. The observed topological transformations are driven by thermally activated atomic diffusion in the solid state phase. The solid state phase remains fully coherent during the processes. The observed topological transitions leading to the material separation in PbTe/CdTe system could be regarded as an analog of spinodal decomposition of an immiscible solid state solution and thus they can be qualitatively described by the Cahn-Hillard model as proposed by Groiss et al (2014 APL Mater. 2 012105).

The high quality p-n structures studied consist of nitrogen doped ZnO:N films grown by plasma assisted molecular beam epitaxy on n-type GaN templates. The nitrogen concentration, determined by secondary ion mass spectroscopy, is about 1 × 1020 cm−3. Temperature dependent photoluminescence studies confirm the presence of acceptor centers with an energy level lying approximately 130 meV above the valence band. The maximum forward-to-reverse current ratio IF/IR in the obtained p-n diodes is about 107 at ±5 V, which is 2–5 orders of magnitude higher than previously reported for this type of heterojunctions. Electron-beam-induced current measurements confirm the presence of a p–n junction, located at the p-ZnO/n-GaN interface. The calculated diffusion length and activation energy of minority carriers are presented. The heterostructures exhibit strong absorption in the UV range with a four orders of magnitude high bright-to-dark current ratio.

Preliminary studies have been performed on photoelectrical properties of CdTe/ZnTe n-i-p junctions grown using the molecular beam epitaxy technique. Photovoltaic properties of the cells have been investigated by the measurements of current-voltage (I-V) characteristics under 1-sun illumination. I-V characteristics yield efficiencies of the cells varying from 3.4% to 4.9%. The low efficiency can be due to the presence of electrically active defects. In order to study the origin of defects in CdTe/ZnTe photovoltaic junctions, space charge techniques (C-V and deep level transient spectroscopy (DLTS)) have been applied. From the C-V measurements, a doping profile was calculated confirming charge accumulation in the i-CdTe layer. The results of the DLTS studies revealed the presence of four traps within a temperature range from 77–420 K. Three of them with activation energies equal to 0.22 eV, 0.45 eV, and 0.78 eV have been ascribed to the hole traps present in the i-CdTe material and their possible origin has been discussed. The fourth, high-temperature DLTS peak observed at ∼350 K has been attributed to extended defects as its amplitude and temperature position depends on the value of the filling pulse width. It is assumed that the defects related to the trap are either located in the i-CdTe layer or at the i-CdTe/ZnTe interface. However, it was found that the trap exhibits twofold nature: it behaves as a majority or as a minority trap, depending on the filling pulse height, which is a characteristic feature of recombination centers. This trap is presumably responsible for the low efficiency of the cells.