ZnSe Bulk Crystals Research Articles

Comparative Study of Bulk and Nanoengineered Doped ZnSe

Chromium- and cobalt-doped zinc selenide nanoparticles were synthesized using a low-temperature reactive solution growth method. The morphological and optical characteristics were compared to those of doped zinc selenide (ZnSe) bulk crystals grown by the physical vapor transport (PVT) method. We observed agglomeration of particles; however, the thioglycerol capping agent has been shown to limit particle grain growth and agglomeration. This process enables doping by addition of chromium and cobalt salts in the solution. A slightly longer refluxing time was required to achieve cobalt doping as compared with chromium doping due to lower refluxing temperature. The nanoparticle growth process showed an average particle size of approximately 300 nm for both Cr- and Co-doped zinc selenide. The optical characterization of Co:ZnSe is ongoing; however, preliminary results showed a very high bandgap compared to that of pure ZnSe bulk crystal. Additionally, Co:ZnSe has an order of magnitude higher fluorescence intensity compared to bulk Cr:ZnSe samples.

Crystal Defect Investigation in PVT-Grown ZnSe Under Different Seeding Conditions and Growth Configurations Using Synchrotron X-Ray Topography

Zinc Selenide (ZnSe) as a prominent II-VI semiconductor is notable for its excellent performance on high efficiency electronic and optoelectronic devices including blue light-emitting diodes, lasers, and blue/ultraviolet-light-sensitive photodetectors, which are of considerable interest for applications in optical storage, lighting and illumination, biomedical sensing, etc. The effective utilization of ZnSe is owing to its superior properties such as wide bandgap, low optical absorption in the visible and infrared spectral region, and intense edge luminescence in the short-wavelength spectral region. The preliminary requisite to satisfy all those essential properties is the crystalline quality. Therefore, the realization of high-performance devices requires high-quality ZnSe bulk crystals with minimized structural defects. In order to overcome limitations and continuously fulfill the requirement of modern device applications, obtaining a comprehensive understanding on ZnSe crystallographic structure and defect formation mechanism is crucial. In this study, characterization techniques including synchrotron white beam X-ray topography (SWBXT), optical microscopy and high-resolution triple X-ray diffraction (HRTXD) were conducted on seeded and self-seeded ZnSe crystals grown by physical vapor transport (PVT) method in horizontal and vertical growth configurations. The effect of seeding condition and gravity vector orientation with respect to growth geometry on crystalline quality and defect structures is investigated. The presence of grains and the occurrence of twinning is revealed and analyzed. This study provides a clear understanding of factors that causes ZnSe crystal quality variation during the growth process, which contributes to achieving the routine production of high-quality ZnSe crystal and modern device enhancement. Figure 1

Photoluminescence and laser properties of active media based on ZnSe doped with Cr, Al, Na from spray pyrolysis deposited films

The prospects for using multicomponent films produced by aerosol spray pyrolysis for doping bulk ZnSe crystals with optically and electrically active impurities are shown for the first time. Lasing was obtained on ZnSe:Cr active elements doped during hot isostatic pressing treatment from CrS and CdS:Cr films. The lasing efficiency with respect to the absorbed radiation energy at a wavelength of 1.94 μm was about 54% for ZnSe:Cr doped from CrS film and 50% for ZnSe:Cr doped from CdS:Cr film, respectively. The luminescence and lasing characteristics of ZnSe:Cr active elements co-doped with Al and Na impurities were investigated. A high diffusion rate of Cr2+ ions into polycrystalline ZnSe doping process from sulfide films was observed.

Вариативность результирующего поля микродеформаций в кристаллах Zn-=SUB=-1-x-=/SUB=-V-=SUB=-x-=/SUB=-Se (0.01≤ x≤0.10)

AbstractA detailed neutronographic study of the bulk ZnSe crystals doped with vanadium up to the content commensurate with the solubility limit in a semiconductor matrix has been carried out for the first time at room temperature. The data that characterize nonuniformly-deformed states based on the cubic structural modification of the II–VI compounds are obtained. A simplified analysis of the broadening patterns of the diffraction profiles of main Bragg reflexes of the studied crystals shows that the resulting deformation covers macroscopic volumes, and the distribution of vanadium ions in the given cases may significantly deviate from the uniform distribution over volume. Relative to the initial cubic lattice, dominating trends towards symmetry changes preceding the phase stratification in the ZnSe crystals heavily doped with vanadium are revealed.

Variability of the resulting microdeformation field in the Zn1 – xV x Se crystals (0.01 ≤ x ≤ 0.10)

A detailed neutronographic study of the bulk ZnSe crystals doped with vanadium up to the content commensurate with the solubility limit in a semiconductor matrix has been carried out for the first time at room temperature. The data that characterize nonuniformly-deformed states based on the cubic structural modification of the II–VI compounds are obtained. A simplified analysis of the broadening patterns of the diffraction profiles of main Bragg reflexes of the studied crystals shows that the resulting deformation covers macroscopic volumes, and the distribution of vanadium ions in the given cases may significantly deviate from the uniform distribution over volume. Relative to the initial cubic lattice, dominating trends towards symmetry changes preceding the phase stratification in the ZnSe crystals heavily doped with vanadium are revealed.

Obtaining of ZnSe nanocrystals from ZnSe bulk crystals by mechanical milling and chemical vapor deposition in silica matrices

Obtaining of ZnSe nanocrystals from ZnSe bulk crystals by mechanical milling and chemical vapor deposition in silica matrices

Photoluminescent tomography of ZnSe bulk crystals

High potential of two-photon confocal microscopy for photoluminescent tomography of bulk semiconductor crystals are demonstrated in this work using ZnSe crystals as an example. The first results of investigation of ZnSe bulk crystals, both undoped and doped by 3d metals, are presented. Appearance of luminescence quenching centres in specific parts of crystals and grain recombination activity are demonstrated by the proposed method; these phenomena cannot be detected by other luminescence-based methods.

Visible nonlinear band-edge luminescence in ZnSe and CdS excited by a mid-infrared free-electron laser

Visible nonlinear band-edge luminescence in ZnSe and CdS bulk crystals was observed upon excitation by a mid-infrared free-electron laser (mid-IR FEL) at approximately 9 mm. The emission intensity is proportional to the 74th and 45th powers of the excitation intensity for ZnSe and CdS, respectively. For ZnSe, the temporal profile of the emission intensity does not follow the profile of the excitation macropulse of the FEL, but sharply rises and decays only at the maximum of the macropulse profile. These features are in marked contrast to those of a previous report, where the emission profile follows that of the macropulse, and the emission intensity scales with the 4th power of the excitation intensity. The experimental observations were reproduced by a numerical simulation based on impact ionization and avalanche ionization by electrons accelerated by the optical electric field of the FEL. The large nonlinearity in the bandedge emission comes from the macropulse temporal structure, which consists of micropulses densely spaced to allow excited carriers to survive when the next micropulse arrives. They work as seed carriers in the next carrier multiplication step.

En route to electrically pumped broadly tunable middle infrared lasers based on transition metal doped II–VI semiconductors

In this work we report the study of Cr 2+:ZnSe photo-luminescence under UV and visible excitation as well as middle-infrared electroluminescence of n-type, Cr doped bulk ZnSe crystals. Photo-conductance studies were performed to verify (2+)→(1+)→(2+)* ionization transitions responsible for Cr 2+ excitation. We report the first to our knowledge, Cr 2+:ZnSe lasing using 532 nm excitation. The first ever room temperature electroluminescence was also achieved in bulk n-type Cr:Al:ZnSe. This electroluminescence over 1800–2800 nm spectral range is in a good agreement with the mid-IR Cr 2+ fluorescence under intra-shell optical excitation. Other spectral bands of electroluminescence were also observed at 600 nm and 8 μm. The visible electroluminescence observed is attributed to V zn–Al complexes in conductive crystals. The nature of the 8 μm electroluminescence requires additional studies. Photo-ionization results are essential for optical pumping of Cr:ZnSe by easily available visible lasers, and, most importantly, both these and the electroluminescence results open a pathway for Cr:ZnSe broadband mid-IR lasing under direct injection of free carriers. Future directions for electrical excitation of low dimensional II–VI structures, where quantum confinement of the atomic impurity is used, could result in a much more efficient transfer of energy from the host to the localized impurity, are discussed.

Spin-flip Raman scattering studies of ZnSe bulk crystals doped with antimony

We have used spin-flip Raman scattering and photoluminescence to investigate ZnSe crystals doped with antimony, which is a potential p-type dopant. The experiments provide evidence for the existence of a shallow acceptor level at about 70 meV above the valence band and for the presence of a previously unreported deep donor level at about 55 meV below the conduction band. As in ZnSe doped with nitrogen, it is possible that the formation of such deep-donor states may be an inevitable consequence of high concentrations of p-type dopants and that the resulting compensation may be one of the reasons why attempts to obtain p-type conductivity continue to be difficult.

Chemical etching‐induced defects in n‐type ZnSe crystal grown by physical vapor transport

We have found with deep level transient spectroscopy that chemical etching induced two isolated point defects, E1(0.26–0.31) and E2(0.20), in the near-surface region of n-type ZnSe single crystals. The etching effect on the traps in Br-doped ZnSe bulk crystals was studied by using 5% Br-methanol solution, and then the E1 trap density increased with the chemical etching time, and reached to maximum value of 5.3 × 1013 cm–3. The E1 trap was obtained in the samples etched in some kinds of acid solutions, Br-methanol solution, HCl, and HBr, but did hardly in NaOH alkali etchant. These results suggest that the E1 trap is induced during the chemical etching and diffuse into the interior of the crystals. From above etching effects, we propose a tentative identification that E1 and E2 traps arise from two kinds of hydrogen related defects.

Identification and quantitative evaluation of compensating Zn-vacancy–donor complexes in ZnSe by positron annihilation

We investigate defects in n-type ZnSe bulk crystals by positron annihilation. With positron lifetime spectroscopy, vacancies with a positron lifetime of $(280\ifmmode\pm\else\textpm\fi{}1)$ and $(275\ifmmode\pm\else\textpm\fi{}10)\mathrm{ps}$ were found in ZnS:I and ZnSe:Al, respectively. The positron bulk lifetime was 241 ps at 300 K, determined in an undoped semi-insulating reference. The vacancies were identified to be Zn-vacancy--donor complexes by combining the positron lifetime measurements with that of the positron-electron annihilation momentum distribution. This finding is supported by previous electron-paramagnetic-resonance measurements. The samples exhibited different degrees of electrical compensation, adjustable by thermal treatment under defined Zn vapor pressure. The vacancy concentration measured by positron annihilation could explain the deactivation of donors quantitatively. Thus, compensation of n-type conductivity in ZnSe is due to self-compensation through native Zn-vacancy--donor complexes.

Spectroscopic evidence and control of compensating native defects in doped ZnSe

Electron paramagnetic resonance spectroscopy combined with photoexcitation is used to identify and characterise the defects responsible for the compensation in iodine or aluminium doped ZnSe bulk crystals. The principal g-values of the Al Zn V Zn complex are reported for the first time. The acceptor levels of both A-centres observed are determined at 0.50 and 0.54 eV above the valence band edge for I Se V Zn and Al Zn V Zn, respectively. Annealing of these samples in Zn vapour is shown to be suitable for reproducible adjustments of the electron concentration at room temperature up to a few 10 18 cm −3. The tight coupling of these vapour phase equilibrations with the electrical and spectroscopic analysis allows to propose a simple defect-chemical model.

Reversible conductivity control and quantitative identification of compensating defects in ZnSe bulk crystals

Donor compensation by associates of donor impurities and cation vacancies is investigated in vapor grown ZnSe : I and Al diffused melt grown bulk crystals. Both the (I SeV Zn) and the (Al ZnV Zn) A-centers could be identified by electron paramagnetic resonance (EPR) spectroscopy. The concentrations of the compensating defects can be reversibly changed by vapor-phase equilibration and a reproducible adjustment of room temperature (RT) carrier concentrations n e up to 1.4×10 18 cm −3 (ZnSe : I) and 2.7×10 18 cm −3 (ZnSe : Al) is possible. The results are explained in terms of a defect chemical model. The supposed high concentrations of one type of vacancy associated defects are confirmed using positron annihilation (PA). Lowered compensation leads to an increase of carrier mobility μ e with increasing n e. In low doped systems mobilities of 500 cm 2/Vs can be obtained. A similarly drastic shift of the absorption edge to lower photon energies is observed for both I- and Al-doped crystals.

Photoluminescence and photoacoustic spectra of ZnSe bulk crystals and epitaxial layers

Temperature dependence of the piezoelectric photoacoustic (PPA) and photoluminescence (PL) spectra are clearly observed between liquid helium and room temperatures for nondoped ZnSe bulk crystals and epitaxial layers. In both samples, the PPA signals are clearly observed at room temperature. However, any PL emissions are not observed at room temperature. Two kinds of recombination processes, i.e., both radiative and nonradiative processes are observed at 80 K in the bulk crystals.

AFM and photoluminescence characterization of defects in the mirror-polished ZnSe bulk crystals and MBE-grown homoepitaxial layers

We have described the structural and optical characterization on the lapped and polished (1 0 0) surface of ZnSe bulk crystals using atomic force microscope (AFM) and 4.2 K photoluminescence (PL) measurements. The colloidal silica solution with a particle size of 10–20 μm for polishing with a surface roughness of about 0.24 nm can be obtained, corresponding to the fluctuation of a monolayer. The PL properties indicate that the linewidth of the I 2 line (an exciton bound to a neutral donor) is about 3.3 meV, which is comparable to that of the cleaved surface. The homoepitaxial films have been grown at 325°C on the polished (1 0 0)ZnSe surface and their defect-related PL properties have been studied as a function of the surface-cleaning temperature.

X-ray scattering topographic observation of ZnSe and ZnTe bulk crystals

ZnSe and ZnTe bulk crystals were studied using X-ray scattering topography, which enabled us to take topographic observations for any kind of materials. We presented not only X-ray scattering topographs but also orientation topographs and two-dimensional reconstruction orientation topographs. The results showed the degree of orientation distribution and domain size and so on. Comparing the topographs of them, in ZnTe specimen some smaller grains were included and the spread of orientation distribution was wider than ZnSe one.

Characterizations of ZnSe single crystals grown by physical vapor transport

ZnSe bulk crystals were grown by self-seeded physical vapor transport technique in horizontal and vertical configurations. The impurities and defects in the grown crystals were studied by glow discharge mass spectroscopy (GDMS) and low-temperature photoluminescence (PL) measurements. The PL results on the starting material and the grown crystals are consistent with the low impurity levels measured by GDMS. The crystalline quality of the grown crystals was examined by synchrotron white beam X-ray topography (SWBXT) and high-resolution triple X-ray diffraction (HRTXD). The SWBXT shows that, aside from twins, the overall crystalline quality of the vapor-grown ZnSe crystals, especially in the contactless grown region, was quite high. The HRTXD results are in line with the SWBXT findings. The comparison between the HRTXD on a chemical-mechanically polished and a cleaved surface seems to indicate that polishing damage can obscure the true microstructure in the as-grown ZnSe crystals.

Optical absorption and photoluminescence of ZnSe microcrystals in a Pyrex glass matrix

ZnSe microcrystals have been embedded into Pyrex glass matrix for the first time. The optical extinction and photoluminescence spectra of these ZnSe-doped (-embedded) glasses are measured at various temperatures. The specimens exhibit a clear shift of the optical extinction (absorption) edge due to the spatial confinement of carriers and excitons in three dimensions. Also, the photoluminescence spectra show a weak edge emission band at their absorption edges and an intense broad emission band at longer wavelength. The broad emission band shifts to the longer-wavelength side with increasing time elapsed after excitation. Taking into account some imperfections in the crystallite and impurities in the matrix glass, the results obtained are discussed and are compared with the properties of ZnSe bulk and film crystals.

Determination of deformation potentials in ZnSe/GaAs strained-layer heterostructures.

A method to determine the deformation potentials in strained-layer heterostructures is introduced. This is done by measuring the piezoreflectance of biaxially strained ZnSe/GaAs layers under additional application of an external uniaxial stress along the (110) axis. The resulting lowered crystal symmetry yields four dipole-allowed eigenstates for the 1S exciton. By using a model that includes strain as well as excitonic spatial dispersion and exchange effects we are able to determine the internal biaxial strain, the exchange splitting, and the deformation potentials. The numerical results obtained on ZnSe/GaAs heterostructures as a model system are compared with the deformation potentials given by measurements at bulk ZnSe crystals. \textcopyright{} 1996 The American Physical Society.