High-pressure Bridgman Method Research Articles

Correlation of tellurium inclusions and carrier lifetime in detector grade cadmium zinc telluride

Carrier lifetimes and tellurium inclusion densities in detector grade cadmium zinc telluride crystals grown by the high pressure Bridgman method were optically measured using pulsed laser microwave cavity perturbation and infrared microscopy. Excess carriers were produced in the material using a pulsed laser with a wavelength of 1064 nm and pulse width of 7 ns, and the electronic decay was measured at room temperature. Spatial mapping of lifetimes and defect densities in cadmium zinc telluride was performed to determine the relationship between tellurium defect density and trapping. A strong correlation was found between the volume fraction of tellurium inclusions and the carrier trapping time.

Optical characterization ofbulk Zn1−xBexTe crystals

This paper presents an optical characterization of three bulk sphaleriteZn1−xBexTe crystals grown by the modified high pressure Bridgman method. The study wasconducted in the near-band-edge interband transition regime using low temperaturephotoluminescence (PL), temperature-dependent contactless electroreflectance (CER)and/or photoreflectance (PR) in the temperature range of 15–400 K, and surfacephotovoltage spectroscopy (SPS) at room temperature. PL spectra at low temperatures ofthe samples investigated consist of an excitonic line, a band due to recombination of freeelectrons with holes located at shallow acceptors and a broad band related torecombination through deeper level defects. The band-edge excitonic transitionshave been observed in the CER/PR spectra. The fundamental transition energiesE0 are determined via lineshape fits to the CER/PR spectra. The values ofE0 at room temperature obtained from CER/PR spectra correspond wellto that determined from SPS measurements, and the Be contentsx of the samples are determined using a linear equation which describesthe room temperature band gap dependence on composition for theZn1−xBexTe alloy system. The parameters describing the temperature dependence of the band-edgeexcitonic transition energies are evaluated and discussed.

Lattice absorption of Be‐containing semiconductor alloys determined by spectroscopic ellipsometry

AbstractLattice absorption of Zn1–x –y Bex Mgy Se and Cd1–x –y Bex Zny Se mixed crystals grown by a high pressure Bridgman method has been investigated by infrared spectroscopic ellipsometry in the wave number range from 230 cm–1 to 5000 cm–1. Ellipsometric spectra of Cd1–x –y Bex Zny Se crystals display features in the spectral range 350–550 cm–1 associated with BeSe‐type phonon modes. In the optical spectra of Zn1–x –y Bex Mgy Se crystals both BeSe‐type and MgSe‐type lattice absorption bands are detected. The MgSe‐like modes are located at approximately 300 cm–1. The frequencies and oscillator strengths of optical phonon modes depend strongly on the alloy composition. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Nonlinear optical properties of Zn 1− xMg xSe and Cd 1− xMg xSe crystals

Nonlinear optical properties of Zn 1− x Mg x Se and Cd 1− x Mg x Se crystals have been studied using degenerate four wave mixing (DFWM), nonlinear transmission (NLT), second harmonic generation (SHG) and third harmonic generation (THG) methods by Nd:YAG laser operating at 532 nm and 1064 nm, respectively. The studied Zn 1− x Mg x Se and Cd 1− x Mg x Se crystals were grown from the melt by the modified high-pressure Bridgman method. The quadratic nonlinear optical properties of ternary crystals were investigated by second harmonic generation (SHG) technique. We found that the highest value of the second order nonlinear optical susceptibility was obtained for Cd 0.7Mg 0.3Se. In the case of cubic nonlinear optical properties we used third harmonic generation (THG) and degenerate four wave mixing (DFWM) methods. We found that the values of third order nonlinear optical susceptibility for Cd 1− x Mg x Se crystals are higher than for Zn 1− x Mg x Se ones.

Optical characterization of Zn0.96Be0.04Se and Zn0.93Mg0.07Se mixed crystals

A detailed optical characterization of Zn0.96Be0.04Se and Zn0.93Mg0.07Se mixed crystal samples grown by the modified high-pressure Bridgman method has been carried out via photoluminescence (PL), contactless electroreflectance (CER), and photoreflectance (PR) in the temperature range of 15–400 K. With these optical diagnostic tools we observed features originating from exciton emission, an edge emission due to recombination of shallow donor-acceptor pairs (DAPs), and a broad band related to recombination through deep-level defects, as well as band-edge and spin-orbit splitting critical points interband transitions in CER and PR experiments. The optical properties of these two mixed crystals are compared. Our comparative study revealed that the beneficial effect of beryllium (Be) incorporation is superior to that of magnesium (Mg), in the sense that a Be-containing ternary alloy resulted in better crystalline quality with more intense excitonic emission line and weaker DAPs and deep-level defect-related bands. The peak positions of the excitonic emission lines in the PL spectra correspond quite well to the energies of the fundamental transitions obtained by means of electromodulation spectra. The parameters that describe the temperature dependence of the fundamental and spin split-off band gaps, and the broadening function of the band-edge excitons, are evaluated and discussed.

Study of the third order nonlinear optical properties of Zn1−xMgxSe and Cd1−xMgxSe crystals

AbstractThird order nonlinear optical susceptibilities χ<3> of ternary Zn1−xMgxSe and Cd1−xMgxSe crystals have been measured using standard degenerate four-wave mixing (DFWM) method at 532 nm. The nonlinear transmission technique has been applied to check if our crystals exhibit two-photon absorption. The studied Zn1−xMgxSe and Cd1−xMgxSe solid solutions were grown from the melt by the modified high-pressure Bridgman method. For both crystals the energy gap increases with increasing Mg content.In the case of Zn1−xMgxSe, it was found that the value of third order nonlinear optical susceptibility χ<3> decreases with increasing Mg content. An explanation of this behaviour results from the dependence of optical nonlinearities on the energy band gap Eg of the studied crystals. In the case of Cd1txMgxSe with low content of Mg, no response was observed for the studied wavelength since the energy gap in such crystals is smaller than the photon energy of the used laser radiation.It was also found that the value of third order nonlinear optical susceptibility χ<3> for Cd0.70Mg0.30Se is higher than for Zn0.67Mg0.33Se. This behaviour can be understood if one take into consideration that the free carrier concentration in Cd1−xMgxSe samples is about four orders of magnitude higher than that in Zn1txMgxSe ones with comparable Mg content respectively. It is commonly known that when the electric conductivity increases, the values of nonlinear optical properties increase.From the performed measurements one can conclude that the incorporation of Mg as constituent into ZnSe and CdSe crystals leads to a change of the third order nonlinear optical susceptibilities.

Photoelectric and photothermal investigations of Zn1‐x‐yBexMnySe solid solutions

AbstractZn1‐x‐yBexMnySe solid solutions were grown from the melt by the high pressure Bridgman method. Optical, luminescence, photoelectric and photothermal properties of this material were investigated. Interpretation of the piezoelectrically detected photoacoustic spectra was performed using Jackson and Amer theory. From the spectral dependence of the amplitude and phase of photoacoustic signal as well as from photoluminescence spectra the variation of energy gap with composition was determined. The values of thermal diffusivity were derived from the dependence of photoacoustic signal on modulation frequency of the laser radiation illuminating the sample. The photoluminescence‐excitation spectra and photoconductivity spectra were measured. The origin of luminescence in Zn1‐x‐yBexMnySe was discussed. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Detailed studies of pixelated CZT detectors grown with the modified horizontal Bridgman method

The detector material Cadmium Zinc Telluride (CZT) achieves excellent spatial resolution and good energy resolution over a broad energy range, several keV up to some MeV. Presently, there are two main methods to grow CZT crystals, the Modified High-Pressure Bridgman (MHB) and the High-Pressure Bridgman (HPB) process. The study presented in this paper is based on MHB CZT substrates from the company Orbotech Medical Solutions Ltd. [Orbotech Medical Solutions Ltd., 10 Plaut St., Park Rabin, P.O. Box 2489, Rehovot, Israel, 76124]. Former studies have shown that high-work-function materials on the cathode side reduce the leakage current and, therefore, improve the energy resolution at lower energies. None of the studies have emphasized on the anode contact material. Therefore, we present in this paper the result of a detailed study in which for the first time the cathode material was kept constant and the anode material was varied. We used four different anode materials: Indium, Titanium, Chromium and Gold, metals with work-functions between 4.1eV and 5.1eV. The detector size was 2.0×2.0×0.5cm3 with 8×8 pixels and a pitch of 2.46mm. The best performance was achieved with the low-work-function materials Indium and Titanium with energy resolutions of 2.0keV (at 59keV) and 1.9keV (at 122keV) for Titanium and 2.1keV (at 59keV) and 2.9keV (at 122keV) for Indium. Taking into account the large pixel pitch of 2.46mm, these resolutions are very competitive in comparison to those achieved with detectors made of material produced with the more expensive conventional HPB method. We present a detailed comparison of our detector response with 3D simulations. The latter comparisons allow us to determine the mobility-lifetime-products (μτ-products) for electrons and holes. Finally, we evaluated the temperature dependency of the detector performance and μτ-products. For many applications temperature dependence is important, therefore, we extended the scope of our study to temperatures as low as −30°C. There are two important results. The breakdown voltage increases with decreasing temperature, and electron mobility-lifetime-product decreases by about 30% over a range from 20°C to −30°C. The latter effect causes the energy resolution to deteriorate, but the concomitantly increasing breakdown voltage makes it possible to increase the applied bias voltage and restore the full performance.

Optical characterization of Cd1−x−yBexZnySe mixed crystals

A detailed optical characterization of a series of wurtzite-type Cd1−x−yBexZnySe mixed crystals grown by the modified high pressure Bridgman method has been carried out via photoluminescence (PL) and contactless electroreflectance (CER) in the temperature range of 15–300 K. Room temperature surface photovoltage spectroscopy has been used as a diagnostic technique for checking the surface condition of the sample. For the mechanically polished sample, the surface photovoltage spectrum at room temperature recorded a below band edge broad feature which is most likely related to the surface states induced by mechanical defect. This feature can be eliminated via mechanical polishing and followed by chemical etching, and hence, only the exciton line was observed for the damage-removed sample. Typical PL spectrum at low temperature consists of an exciton line, an edge emission due to recombination of shallow donor-acceptor pairs, and a broadband related to recombination through deep level defects. Various interband transitions, originating from the band edge and spin-orbit splitting critical points, of the samples have been observed in the CER spectra. Comparison of PL and CER data for the Cd1−x−yBexZnySe samples shows that for a low Zn and Be content the peak positions of the excitonic emission lines in the PL spectra correspond quite well to the transition energies of the A exciton from the CER data. For samples with higher Be and Zn contents, the results indicate that localization of excitons has taken place at low temperatures. The parameters that describe the temperature dependence of the transition energy and broadening parameter of the fundamental band edge exciton have been evaluated and discussed.

Growth and properties of Zn1–x –yBex Mny Se crystals

AbstractPhotoluminescence spectra, photoluminescence‐excitation spectra, Raman and EPR studies were performed in bulk Zn1–x –y Bex Mny Se solid solutions grown by the high pressure Bridgman method in the range of composition 0 < x < 0.2, 0 < y < 0.2. For low Mn content photoluminescence spectra at low temperatures consist of exciton, “edge” and deep levels emission bands. The main yellow band (2.05 eV) is due to internal transition in Mn2+. The exciton energy gap versus Mn content was determined for Zn1–x –y Bex Mny Se samples with Be content x = 0.05. EPR studies show that the g ‐factor is noticeably larger than the gfree value indicating a large exchange interaction. The decreasing of g ‐factor with increasing Mn concentration in the investigated samples can be explained as due to increasing of spin ordering. Applying the DPPH as a standard, the concentration of unpaired spins per unit weight of the crystal was estimated. Measured at room temperature Raman spectra consist of ZnSe‐like, MnSe‐like and BeSe‐like transverse (TO), longitudinal (LO), transverse (TA) and longitudinal (LA) phonon modes. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Temperature-dependent photoluminescence characterization ofCd1−x−yBexZnySe mixed crystals

Temperature-dependent photoluminescence (PL) characterization of a series of wurtzite-typeCd1−x−yBexZnySe mixed crystals has been carried out in the temperature range between 15 and 300 K. Thesamples were grown by the modified high pressure Bridgman method. A typical PLspectrum at low temperature consists of an exciton line, an edge emission due torecombination of shallow donor–acceptor pairs and a broad band related to recombinationthrough deep level defects. The peak positions of the excitonic emission line in the PLspectra correspond quite well to the transition energies of exciton A from previouslyreported electromodulation reflectance data (Liu et al 2005 J. Appl. Phys. 98 083519). Theparameters that describe the temperature dependence of the transition energy andbroadening parameter of the band-edge excitonic emission are evaluated and discussed.

Investigation of degradation in beryllium chalcogenide II-VI semiconductors

The wurtzite-type Cd1−x−yZnxBeySe crystals grown by the high-pressure Bridgman method were investigated to obtain the suitable beryllium composition for improving the lifetime of II-VI based optoelectronic devices. A method based on the continuous exposure of the electron beam of cathodoluminescence measurement was introduced to probe the aging characteristics of optoelectronic materials. The results show that in order to improve the degradation problem, the incorporation of beryllium content in II-VI compounds should be less than 10%. In addition, the authors demonstrate that cathodoluminescence image is a very powerful tool to reveal the generation of defects of an optoelectronic solid under external perturbation.

Characterisation of Cd1–x–yZnxBeySe crystals by spectroscopic ellipsometry and luminescence

AbstractOptical and luminescence properties of Cd1–x–yZnxBeySe solid solutions grown in the composition range 0 < x < 0.15 and 0 < y < 0.30 by the high pressure Bridgman method are reported. Spectroscopic ellipsometry was employed for determination of spectral dependence of the complex dielectric function ε(E), refractive index n(E) and absorption coefficient α(E) in the photon energy range 0.75–6.0 eV. The dispersion of the refractive index of the alloys was modelled using a Sellmeier‐type relation. The excitonic band‐gap energies were estimated from the ellipsometric and photoluminescence measurements. (© 2006 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

CdTe native point defects under Cd saturation

The high temperature point defect (PD) structure of undoped CdTe under well-defined Cd vapour pressure was studied. The single crystals were grown by different techniques (vapor phase, THM, classical and high pressure Bridgman methods). By using in situ Hall effect measurements it was established that the native double charged PDs Cd 2+ i and V 2+ Te are the dominant donors at t > 500 °C. The formation enthalpies of these PDs were determined from [e∼] temperature dependencies (ΔHv 2+ Te =1.3 eV; ΔH Cd 2+ i =2.5 eV). The PD structure modelling demonstrated satisfactory agreement with experimental results both for temperature and Cd vapor pressure dependencies. It showed that V 2+ Te dominates at low temperatures and Cd 2 i begins to prevail at t >930 K.

Characterization of Cd1–x–yBexZnySe mixed crystals via electromodulation spectroscopy

AbstractA detailed optical characterization of three wurtzite‐type Cd1–x–yBexZnySe mixed crystals has been carried out by using contactless electroreflectance (CER) and photoreflectance (PR) in the temperature range of 15 to 400 K. The samples were grown by the modified high pressure Bridgman method. The transition energies of the A, B and C features of Cd1–x–yBexZnySe are determined via a lineshape fit to the CER and PR spectra. The parameters that describe the temperature dependence of the interband transition energies of the features A and C are evaluated and discussed. (© 2006 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Simulation and experimental results on monolithic CdZnTe gamma-ray detectors

Monolithic CdZnTe detectors are promising for medical and small animal imaging thanks to their good energy resolution that allows both multidrug diagnostics and scatter rejection. Using the ULYSSE simulator, electrode geometry and detector thickness are optimized to improve detection efficiency and energy resolution for various material transport properties. As an example, for a 5 mm thick detector with (mutau) <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">e</sub> =3times10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-3</sup> cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> /V, a 1.8 mm electrode pitch gives the best-simulated energy resolution on the conventional spectrum at 122 keV. Once the electrode geometry and thickness are optimized, electronic correction methods are shown to further reduce spectrum tailing. Corrections using cathode and anode signals have been tested with the high pressure Bridgman method (HPBM) CdZnTe and THM CdTe:Cl detectors. The best method for electronic correction appears to depend on the material

Growth and optical characterization of Cd1‐xBexSe and Cd1‐xMgxSe crystals

AbstractCd1‐xBexSe and Cd1‐xMgxSe solid solutions were grown from the melt by the high pressure Bridgman method. Optical, luminescence and photothermal properties of these materials were investigated. Spectroscopic ellipsometry was applied for determination of the spectral dependence of the complex dielectric function $ \tilde \varepsilon $(E) and refractive index n(E) at room temperature in the photon energy range 0.75‐6.5 eV for samples with optic axis (c‐axis) perpendicular to the air‐sample interface. The critical point (CP) parameters for E0 and E1 transitions were determined using a standard excitonic CP function to fit the numerically calculated differential spectra ∂2ε2/∂E2. The dispersion of the refractive index of the alloys was modelled using a Sellmeier‐type relation. The values of fundamental and exciton band‐gap energies were estimated from the ellipsometric and photoluminescence measurements. The origin of luminescence in Cd1‐xBexSe and Cd1‐xMgxSe was discussed. (© 2005 WILEY‐VCH Verlag GmbH &amp; Co. KGaA, Weinheim)

Photoacoustic Studies of Cd1-xBexSe Mixed Crystals

This paper reports the results of photoacoustic measurements of Cd1-xBexSe mixed crystals grown by the high pressure Bridgman method with varying concentrations of Be (0.1 < x < 0.2). For examining continuous wave photoacoustic spectra, a piezoelectric transducer (PZT) and an open cell were used. An increase of the energy gap with increasing x has been observed. The thermal diffusivity values were estimated using the dependence of the amplitude and phase of the PA signal on the light modulation frequency.

Persistent photoconductivity and optically stimulated luminescence in Zn 0.8 Mg 0.2 Se mixed crystals

Photoelectric properties of Zn0.8Mg0.2Se mixed crystals grown by high pressure Bridgman method were investigated. Spectral distributions and temperature characteristics of photoconductivity as well as thermal and optical stimulation and quenching of luminescence and photoconductivity were measured. Relaxation of stored carriers were discussed in terms of random local potential fluctuation (RLPF) due to statistical fluctuations of composition. Based on the observed photoinduced phenomena in ZnxMg1−xSe, the potential applications of II–VI solid solutions for construction of optoelectronic devices were discussed. (© 2004 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Growth and characterization of single-crystal lead magnesium niobate–lead titanate via high-pressure vertical Bridgman method

Growth and characterization of single-crystal lead magnesium niobate–lead titanate via high-pressure vertical Bridgman method