High-pressure Bridgman Method Research Articles

Advanced Cd1-xMnxTe:Fe2+ semiconductor crystals for IR applications: The role of the composition and sin-band redshift spectra effect

Doped semiconductor crystals of solid solution Cd1-xMnxTe:Fe2+ were grown by the high-pressure Bridgman method, covering the range of their existence as a zinc blende structure (0 < x < 0.76). The concentration of the Fe2+ impurities was approximately 10−3 wt% in all the studied samples. The structural and optical properties of the crystals were investigated, including the case of high concentrations of x > 0.4. The correlations between the composition of solid solution crystals of Cd1-xMnxTe:Fe2+, the lattice constant, the band gap, and the maxima positions of the Fe2+ active ion absorption and emission spectra were found experimentally and explained physically. A theoretical model based on the principle of compositional additivity for solid solution semiconductor materials was first used for explaining the long-wavelength sin-band linear “redshift” of absorption and emission bands in the spectra of Cd1-xMnxTe:Fe2+ crystals with increasing solid solution concentration. A new effect is discovered for the differentiated redshift of the Jahn-Teller components (bands) of the total absorption spectrum. Longwavelength absorption bands have a stronger redshift than shortwavelength absorption bands. The redshifts in the maxima of the total absorption and emission spectra have shift (slope) coefficients Kab≈1.7 nm/at.% and Kem≈6.1 nm/at.%, respectively. The obtained results can be used to predict and design laser media based on Cd1-xMnxTe:Fe2+ solid solution crystals while controlling the lasing range (for all Mn concentrations).

Effect of the surface roughness on the measured thermal diffusivity of the ZnBeMnSe single-crystalline solids

In this work, the influence of the surface treatment of bulk samples of the single-crystals observed by means of the Atomic Force Microscopy on the measured thermal diffusivity of the material is presented and discussed. The mixed Zn1 − x − yBexMnySe crystals were grown from the melt by the modified high-pressure Bridgman method with different Be and Mn contents. The investigated specimens were first ground, then polished, chemically etched and finally annealed in zinc vapor. After each surface technological preparation the AFM images were taken and the thermal diffusivity of Zn1 − x − yBexMnySe compounds was determined by the photopyroelectric technique in the back configuration. The ground sample exhibited the highest roughness, whereas the annealed sample showed the lowest one. It has also been shown that the thermal diffusivity of the ground sample was significantly lower compared to the samples whose surface has been treated by other procedures. It was assumed that the measured thermal diffusivity had an effective value attained through the mediation of the thermal parameters of the bulk sample with those of the damaged surface layer. To analyze the results, a two-layer model proposed by J.J. Alvarado-Gil was employed. It turned out that the effective thermal diffusivity value depends much stronger on the conductivity of the system than on the diffusivity of the bulk.

Correlation between optical and thermal properties in ZnMgSe solid solutions

The correlation between the optical (such as complex refractive index, energy band gap) and thermal (such as diffusivity, effusivity and conductivity) properties of Zn1−xMgxSe (with 0 ≤ x ≤ 0.50) crystals are presented. The Zn1−xMgxSe mixed crystals were grown by the modified high-pressure Bridgman method and were studied using spectroscopic ellipsometry as well as photopyroelectric technique.We found that when Mg content in Zn1−xMgxSe increases the value of Eg increases. Moreover, the values of thermal conductivity and refractive index of studied materials decrease with increasing Mg content in Zn1−xMgxSe. Therefore, from the performed measurements, one can conclude that the incorporation of Mg as constituent into ZnSe crystals plays a crucial role in the observed optical and thermal response.

Thermal characterization of ZnBeMnSe mixed compounds by means of photopyroelectric and lock-in thermography methods

In this work, a thermal characterization (measurement of dynamic thermal parameters) of quaternary Zn1−x−yBexMnySe mixed crystals was carried out. The crystals under investigation were grown from the melt by the modified high-pressure Bridgman method with different Be and Mn contents. The effect of Be and Mn contents on thermal properties of Zn1−x−yBexMnySe compounds was analyzed, by using the photopyroelectric (PPE) method in the back configuration (BPPE) for thermal diffusivity measurements and the PPE technique in the front configuration for thermal effusivity investigations. Infrared lock-in thermography was used in order to validate the BPPE results. The measured thermal effusivity and diffusivity allowed the calculation of thermal conductivity of the investigated materials.

Characterization of thermal properties of Cd1−x−yZnxMgySe mixed crystals by means of photopyroelectric and infrared imaging techniques

Abstract In this work a complete thermal characterization of Cd 1− x − y Zn x Mg y Se mixed crystals was carried out. Bulk Cd 1− x − y Zn x Mg y Se semiconductors with different x and y contents were grown from the melt by the modified high pressure Bridgman method. The photopyroelectric (PPE) method in the back configuration (BPPE) and the infrared (IR) lock-in thermography were applied to measure the thermal diffusivity. Values of thermal effusivity of the samples were obtained with the PPE technique in the front configuration (FPPE), coupled with a thickness thermal wave resonator cavity (TWRC) scanning procedure. Measured thermal effusivity together with the thermal diffusivity allowed calculating the thermal conductivity of the investigated materials. For the calculation of the specific heat, the densities of the samples were calculated from their weight and geometry. The effect of Mg/Zn molar ratio on thermal properties of these quaternary Cd 1− x − y Zn x Mg y Se compounds was analyzed and discussed.

Surface investigations of ZnBeMnSe mixed crystals by means of the piezoelectric spectroscopy and the AFM technique

Piezoelectric photoacoustic spectroscopy with a piezoelectric detection has been used for measurements of the amplitude and phase spectra of Zn1−x−yBexMnySe mixed semiconductors. The investigated crystals were grown from the melt by the modified high pressure Bridgman method under the argon overpressure. The preliminary study of the sample's surface of the investigated crystals was carried out using the AFM technique. The influence of a different surface treatment on the amplitude and phase piezoelectric spectra as well as on AFM images is presented and analyzed. The correlations between these two techniques have been found and are discussed. Piezoelectric (PZE) spectra were analyzed using an extended and modified Jackson–Amer theory.

New insight into the 1.1-eV trap level in CdTe-based semiconductor

We investigated trap levels in detector-grade CdZnTe (CZT) material grown by using three different methods, viz, the Bridgman, traveling heater method (THM), and high-pressure Bridgman method (HPB), by current deep-level transient spectroscopy (I-DLTS). All CZT detectors contained deep trap levels located at around 1.1 eV (which we designated DE1), which has been attributed to Te vacancies induced one. However, our crystal growth and characterization results indicated that dislocations induced by Te secondary defects (inclusions/precipitates)were a more probable origin than Te vacancies. Also, a theoretical calculation of the electron de-trapping time associated with DE1 can explain well the abnormal residual current behavior at temperatures slightly above room temperature. Our results show better control of the concentrations and the sizes of Te secondary defects is critical to improving the detector’s performance at room temperature by reducing lagging effects.

Magnetoresistive switching and highly polarized electroluminescence from semimagnetic semiconductor bicrystals Zn1−xMnxTe

The Zn1−xMnxTe bicrystals grown by the high-pressure Bridgman method exhibit magnetic-field-sensitive current-voltage characteristics. The threshold voltage of avalanche breakdown decreases by 40% as the magnetic field increases from 0 to 14 T. In the postbreakdown regime, the Zn1−xMnxTe bicrystals emit an intense red and green electroluminescence. Under magnetic field, the green emission exhibits a high degree of circular polarization which reaches 0.95 and 0.78 at 1.8 K and 4.2 K, respectively. The relation between the photon energy of the green emission and the threshold voltage has been studied.

Optical spectra of Zn1-xBexTe mixed crystals determined by IR–VIS–UV ellipsometry and photoluminescence measurements

Spectroscopic ellipsometry in the photon energy range from 0.04eV to 6.50eV is used for investigation of the optical response of Zn1-xBexTe crystals grown by a high-pressure Bridgman method in the composition range x≤0.12. Infrared spectra display absorption bands centred between 411cm−1 and 420cm−1 associated with BeTe-type optical phonon modes. The positions of the transverse-optical and longitudinal-optical phonon modes have been found by modelling the line shape of the complex dielectric functions, ε˜ and Im(−ε˜−1), using a classical damped Lorentzian oscillator approach. Ellipsometric measurements in the VIS–UV range allow determination of the fundamental energy-gap (E0) and the higher threshold energies (E1, E1+Δ1, E2) originating from the band edge and spin-orbit splitting critical points. We have found that the Be content x=0.12 causes an increase of the fundamental energy gap about 0.15eV at room temperature when compared to the E0=2.23eV of ZnTe crystal at the same temperature. Photoluminescence spectra were measured in the temperature range from 30K to room temperature. Luminescence at temperature T>200K is very weak. The peak positions of the exciton emission lines agree well with the E0 band-gaps derived from ellipsometric data if corrected for their temperature dependence.

Temperature dependence of energies and broadening parameters of near band edge interband excitonic transitions in wurtzite ZnCdMgSe

An optical characterization of two wurtzite ZnCdMgSe crystalline alloys grown by the modified high-pressure Bridgman method was carried out by temperature-dependent photoluminescence (PL) and contactless electroreflectance (CER) in the temperature range of 10–300 K and photoreflectance (PR) measurements between 300–400 K. Low temperature PL spectra of the investigated samples consisted of the excitonic line, the “edge-emission” due to radiative recombination of shallow donor-acceptor pairs and a broad band related to recombination through deep level defects. Three excitonic features, A, B, and C, in the vicinity of band edge were observed in the CER and PR spectra. The peak positions of band edge excitonic features in the PL spectra were shifted slightly toward lower energies as compared to the lowest corresponding transition energies of A exciton determined from CER and PR data. The increase in the CER-PL shift with the increasing of Mg content in the investigated crystals was explained by the rising of compositional disorder causing the smearing of the band edge energies. In addition, the parameters that describe the temperature dependence of the transition energies and broadening parameters of the band edge excitonic transitions were evaluated and discussed.

Optical and photothermal investigations of Zn1−x−yBexMnySe solid solutions

AbstractQuaternary Zn1−x−yBexMnySe solid solutions were grown from the melt by the high pressure Bridgman method in the range of composition 0 &lt; x &lt; 0.24, 0 &lt; y &lt; 0.45. Luminescence and photothermal properties of this material were investigated. From the spectral dependence of the amplitude and phase of photoacoustic signal as well as from photoluminescence (PL) spectra the variation of energy gap with composition was determined. The photoluminescence‐excitation (PLE) spectra were measured and the origin of luminescence in Zn1−x−yBexMnySe was discussed.

Growth and characterization of Zn1–x–yBexMgySe solid solutions with luminescence and photoacoustic methods

AbstractZn1–x–yBexMgySe mixed crystals were grown from the melt by the high pressure Bridgman method. Photoluminescence and photoluminescence‐excitation spectra were measured and the origin of luminescence in Zn1–x–yBexMgySe was discussed. Photothermal properties were investigated with photoacoustic method. From the spectral dependence of the amplitude and phase of photoacoustic signal as well as from photoluminescence spectra the values of energy gap were determined. The influence of surface preparation on photoacoustics data was investigated and discussed. (© 2010 WILEY‐VCH Verlag GmbH &amp; Co. KGaA, Weinheim)

Characterization of Zn0.95‐xBex Mn0.05Se mixed crystals by photoluminescence and contactless electroreflectance

AbstractTemperature‐dependent photoluminescence (PL) and contactless electroreflectance (CER) measurements of Zn0.95‐xBexMn0.05Se mixed crystals with different Be content (0 ≤ x ≤ 0.26) grown by the modified high pressure Bridgman method has been carried out in the temperature range 15–300 K. The peak positions of band‐edge exciton features in the PL spectra are slightly shifted towards lower energies as compared to the corresponding transition energies obtained from CER data. The observed increases of the CER‐PL shift with the increasing of Be content are explained by the increasing compositional disorder causing the smearing of the band‐edge energies. The excitonic line broadening for the samples with larger Be/Zn ratio are attributed in part to the alloy scattering effects and also to the poorer crystalline quality of the samples with higher content of Be. (© 2010 WILEY‐VCH Verlag GmbH &amp; Co. KGaA, Weinheim)

Multimode phonon structure of Be‐containing II ‐ VI mixed crystals determined by IR spectroscopic ellipsometry

AbstractSpectroscopic ellipsometry in the infrared spectral range 250‐5000 cm‐1 is used for analysis of the dielectric response of Zn1‐x‐yBexMgySe and Zn1‐x‐yBexMnySe crystals grown by a high‐pressure Bridgman method. Ellipsometric spectra display features in the spectral range 390‐500 cm‐1 associated with BeSe‐type phonon modes. In the optical spectra of Zn1‐x‐yBexMgySe crystals both BeSe‐type and MgSe‐type lattice absorption bands are detected. The MgSe‐like modes are located at approximately 300 cm‐1. The complex dielectric functions can be reproduced using a model with two or three and one or two classical damped oscillators corresponding to the BeSe‐like and the MgSe‐like transverse‐optical phonon modes, respectively. The frequencies of longitudinal‐optical phonons have been derived from the dielectric loss functions. A red‐shift of the BeSe‐like phonons frequencies with a mean rate 0.42 cm‐1 (0.50 cm‐1) per mole percent of Mg (Mn) incorporated to the alloy has been found for examined concentration range x, y ≤ 0.25. A noticeable damping the intensities of BeSe‐type modes with increasing fraction of Mg and Mn dopant is observed in comparison to the strengths of BeSe‐type modes in Zn1‐xBexSe crystals. (© 2010 WILEY‐VCH Verlag GmbH &amp; Co. KGaA, Weinheim)

High-pressure vertical Bridgman growth of lead magnesium niobate–lead titanate single crystal

A modified high-pressure vertical Bridgman (HPVB) method has been used to grow single crystals of lead magnesium niobate–lead titanate (PMN–PT) using self-seeding. The starting charge was prepared close to stoichiometric composition of (1− x)Pb(Mg 1/3Nb 2/3)O 3– xPbTiO 3, ranging from x=0.29 to x=0.32. The ingots grown had a top diameter of ∼114 mm and weighing ∼1.9 kg with a volume of ∼230 cc. Standard form off-the-shelf platinum crucibles from vendors with a wall thickness ∼0.34±0.01 mm were used. The thinner platinum helped to attain a larger radial thermal gradient as well as enhancing the cost effectiveness of the process by reducing usage of precious metals. An oxygen partial pressure of 8000–10 000 ppm was used and controlled during the entire growth process. Self-seeding technique was used in order to attempt to grow large diameter single crystals of PMN–PT. The composition variation in the ingot is dependent on the growth rate, thermal gradients and oxygen partial pressure. The segregation coefficient of Ti was 0.86±0.008 for the starting composition of x=0.32.

Dependence of nonlinear refractive index of ZnSe on Be and Mg content

AbstractThe values of the nonlinear refractive index n2 and the two-photon absorption coefficient β of ternary and quaternary ZnSe-based mixed crystals were extracted from the standard backward degenerate four wave mixing (DFWM) and nonlinear transmission measurements at 532 nm, respectively. Studied crystals were grown by the modified high-pressure Bridgman method. We found that the value of the nonlinear refractive index n2 for Zn0.79Be0.21Se is higher than that for Zn0.80Mg0.20Se. However, the opposite behaviour was found in the case of two-photon absorption coefficient β for these compounds. We also found that the values of the nonlinear refractive index n2 and the two-photon absorption coefficient β for Zn0.83Be0.04Mg0.13Se are about five times lower and three times higher than that for Zn0.80Mg0.13Se, respectively. In the case of ternary ZnSe-based crystals we noticed that the value of the nonlinear refractive index n2 decreases with increasing Mg or Be content. However, the value of the two-photon absorption coefficient β increases with increasing Mg or Be content.

Influence of Surface Preparation for Different Groups of A2B6 Mixed Crystals

Piezoelectric photothermal spectroscopy has been used for measurements of the optical and thermal parameters of semiconductors. The investigated crystals were grown by the high-pressure Bridgman method under argon overpressure. The obtained photoacoustic (PA) spectra show the complexity of the effects observed for the different groups of selected A2B6 crystals. These effects comprise ideal samples and samples with damaged surfaces. The spectra show the influence of the surface treatment on the PA amplitude and phase spectra.

Optical characterization ofZn0.95−xBexMn0.05Semixed crystals

A systematic study of a series of ${\text{Zn}}_{0.95\ensuremath{-}x}{\text{Be}}_{x}{\text{Mn}}_{0.05}\text{Se}$ mixed crystals with different Be content $(0.05\ensuremath{\le}X\ensuremath{\le}0.20)$ grown by the modified high-pressure Bridgman method were carried out by room-temperature surface photovoltage spectroscopy (SPS), temperature-dependent photoluminescence (PL), and contactless electroreflectance (CER). A typical PL spectrum at low temperature consists of free exciton line, an edge emission due to recombination of shallow donor-acceptor pairs, and the ${\text{Mn}}^{2+}$-related intraionic emission. The near band-edge transition energies determined by analyzing the CER and SPS spectra showed a blueshift with the increase in Be content. The peak positions of band-edge exciton features in the PL spectra shifted slightly toward lower energies as compared to the corresponding transition energies obtained from CER and SPS data. The observed increases in the CER-PL shift with the increasing of Be content are explained by the increasing compositional disorder causing the smearing of the band-edge energies. The excitonic line broadening for the samples with larger Be/Zn ratio are attributed in part to the alloy-scattering effects and also to the poorer crystalline quality of the samples with higher content of Be. In addition, the parameters that describe the temperature dependence of the transition energies and broadening parameters of the band-edge excitonic transitions were evaluated and discussed.

Characterisation of Materials by Raman Scattering, ADDENDA

Reference [1] presents a review of the results obtained for some materials by the Raman scattering method at the Chair of Optical Spectroscopy, Poznan University of Technology. The materials included: zinc selenide (ZnSe) [2] and mixed crystals (Zn1−xMgxSe and Zn1−xBexSe) [3] obtained by the high-pressure Bridgman method, the crystals of strontium lanthanum gallate (SrLaGaO4 — SLG) obtained by the Czochralski method [4] and the crystals of triammonium hydrogen diseleniate ((NH4)3H(SeO4)2 — TAHSe) grown from water solutions [5]. Some of these results are presented and discussed in detail, among others, in the Ph.D. theses [6–8], prepared under my promotion. Moreover, Ref. [1] contains the unpublished Fig. 8, taken from the doctoral dissertation [7], which has not been expressed in the figure caption. I would like to apologize the author of the figure for this oversight. The author wishes once again to thank Dr. eng. M. Muhle, Dr. T. Runka and Dr. M. Szybowicz from the Chair of Optical Spectroscopy, Poznan University of Technology for cooperation in realization of experiments and Prof. Dr. hab. Z. BAlaszczak from the Faculty of Physics, Adam Mickiewicz University for fruitful discussions. ∗e-mail: markoz@phys.put.poznan.pl

Temperature dependent electromodulation characterization of Zn1−x−yBexMgySe mixed crystals

We report a detailed investigation of the temperature dependence of the band-edge excitonic transitions of three Zn1−x−yBexMgySe mixed crystals using contactless electroreflectance (CER) and photoreflectance (PR) in the temperature range of 15–400 K. The samples were grown by the modified high pressure Bridgman methods. The fundamental transition energy E0 and broadening parameters are determined via a lineshape function fit to the CER and PR spectra. The parameters that describe the temperature dependence of the band-edge excitonic transition energy and broadening parameters are evaluated and discussed.