Large Size Crystal Growth, Photoluminescence, Crystal Excellence

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Large size crystal growth, photoluminescence, crystal excellence and hardness properties of In doped Cadmium Zinc telluride Mohd. Shkir, V. Ganesh, Salem AlFaify, Andres Black, Ernesto Dieguez, and K. K. Maurya Cryst. Growth Des., Just Accepted Manuscript • DOI: 10.1021/acs.cgd.7b01483 • Publication Date (Web): 20 Feb 2018 Downloaded from http://pubs.acs.org on February 20, 2018

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Crystal Growth & Design

Large size crystal growth, photoluminescence, crystal excellence and hardness properties of In doped Cadmium Zinc telluride Mohd. Shkir1,4*, V. Ganesh1,4, Salem AlFaify1, Andres Black2,3, Ernesto Dieguez2, K. Maurya5 1

Department of Physics, Faculty of Science, King Khalid University, P.O. Box.9004, Abha 61413, Saudi

Arabia. 2

Crystal Growth Lab, Departamento de Física de Materiales, Universidad Autónoma de Madrid, Madrid

28049, Spain 3

IMDEA Nanocience, C/Farday 9, Madrid, Madrid 28049, Spain

4

Research Center for Advanced Materials Science (RCAMS), King Khalid University, Abha, 61413, P.O.

Box 9004, Saudi Arabia 5

NPL-Council of scientific and industrial research, Dr. K.S. Krishnan road, New Delhi 110012, India

Corresponding author* Dr. Mohd. Shkir E-mail: [email protected], [email protected] 1 ACS Paragon Plus Environment

Crystal Growth & Design 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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Abstract In the current work, a successful growth of In doped CdZnTe (InCZT) bulk size ~8 cm long and 2.5 cm dia. ingot was achieved through Gradient Freeze process. Whole crystal ingot was cut in six small ingots (1 to 6) from these we have selected 1, 2, 4 and 6 which were again cut in to ~ 2 mm thickness (named as 1-1, 1-2, 2-1, 2-2, 4-1, 6-1) and polished for further characterization. The single phase and direction of growth was verified by X-ray diffraction analysis which confirmed its growth along (111) plane. The excellence of grown crystal was analyzed through high-resolution X-ray diffraction (HRXRD) and confirms that the grown crystals is quite perfect and may be used in device fabrications. The Vickers microhardness indentation studies and load dependence studies on different parts of 1-1, 12, 2-1, 2-2, 4-1 and 6-1 InCZT crystals are carried at different loads from 0.098N to 0.49 N. Examination of these samples reveals RISE behavior and it is explained by several models of Mayer’s law, Hays-Kendall’s tactic, Proportional resistance model. Also fracture toughness, brittleness index, yield strength and elastic stiffness values are measured from indentation studies and found very much correlated with HRXRD analysis. Keywords: Crystal growth; VGF technique; optical properties; crystalline perfection; mechanical properties 1. Introduction In earlier eras various kinds of research and development has been done on CdZnTe (CZT) and its related compounds to achieve a good quality RT x-ray and γ-ray detector grade single crystals and also can be applied in key applications

1-4

. The single crystals of CZT

have been grown by different techniques such as high-pressure vertical Bridgman 5, Modified Vertical Bridgman (MVB) technique

6

, traveling heater method

electrodynamic gradient freeze (EDG) furnace 9, Vapor growth 2 ACS Paragon Plus Environment

10

7,

8

,

, oscillatory Bridgman

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technique

11, 12

. The analysis on grown CZT crystal shows that the crystals possess grain

boundaries and other imperfections like: great Te inclusions, twin and dislocations that makes limited use of CZT

4, 13-17

. Amongst all these technique vertical gradient freezing

(VGF) 18-21 is found to have better results in terms of good quality with less defects crystals for detection applications. The single crystals of CZT with different dopants like: Bi, In, Ti, Pb etc. were grown by oscillatory Bridgman technique vertical Bridgman methods

23-28

22

, high-pressure, modified and

. It is well known in the literature that CZT:In is a key

artistic material for nuclear radiation detection at RT. However, there is no/least report available for In doped CZT in which the correlation between growth, optical, quality excellence and strength etc. chattels are described. Reviewed literature shows that VGF technique is an excellent tool to grow CZT crystal and hence we have applied this technique to grow good quality In doped CZT crystal. The crystalline perfection and mechanical properties will provide a clear idea about the defects/grain boundaries, fracture and brittleness in the grown crystals as these properties are directly affected by such defects 29-31

. Thus, the study of crystalline perfection and mechanical properties will provide a key

information about the crystal quality and its strength to be applied in device applications. The InCZT detector grade crystals have huge applications in the radiation field like medical and nuclear safe guard’s demands high strength and defect free material during the growth and loading effects. Such materials have to withstand high thermal tolerance and high loads. The failure at the loading effects and defects during the growth process makes huge damage to the detector application material 32. This defects and loading failure also affects various physical properties of the grown materials such as lattice strains, fracture toughness, Brittleness, elastic properties 33, 34. Indentation hardness testing is a micron scale property investigates the microstructural properties of structural parameters 35, 36. Hence, in 3 ACS Paragon Plus Environment


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view of such key applications, in present study we are reporting the various mechanical parameters of the InCZT ingots at different loads using Vicker’s indentation method. Hence, here we are presenting the growth of In doped CCZT (InCZT) crystals of bulk size ~ 8 cm length and 2.5 cm diameter using by VGF technique. The details on crystal growth process are presented and six wafers (1 to 6) were cut from the grown crystals along growth direction and again the wafers 1, 2, 4 and 6 were selected and cut in to thin sheets of thicknesses about 2 mm and named as 1-1, 1-2, 2-1, 2-2, 4-1 and 6-1 and polished with 1 ߤ݉ silica powder. These cut and polished specimens were imperiled to X-ray diffraction to confirm direction of growth and further subjected to perform photoluminescence, crystal quality and hardness characteristics. 2. Experimental details 2.1. Experimental setup and growth process The required materials to grow In doped CZT single crystal were purchased from Sigma Aldrich and Alfa Aesar of 7N and 5N purity. A crucible made up of PBN of dimension: 150 mm length and 25 mm dia. was used. Growth feedstock consisted of 170 g of feedstock CdZnTe, 5% Zn, along with 0.6 mg In, corresponding to a concentration of 1.15 e+17 at/cm3 and loaded in PBN crucible which was further inserted in quartz ampoule and then sealed at high vacuum. The well-sealed ampoule was placed on top of a quartz ampoule with a SiC "cold finger" in the center of vertical gradient freezing furnace

18, 19

. The cold finger

increased heat extraction from the bottom of the ingot, ensuring a more controlled nucleation event and an initially convex solid/melt interface. The monitoring of temperature was done through five thermocouples which were applied along outside of ampoule at different heights: 0.0 cm, 2.2 cm, 4.2 cm, 5.4 cm and 8.1 cm. The feedstock melt was superheated for 3 h 55 °C above the melting point (Tm = 1104° C), to ensure an inclusion 4 ACS Paragon Plus Environment

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free, fully melted feedstock. The melt was then homogenized for 24 h at 30° C above the melting point. A homogeneous temperature profile was applied over the ingot height during these steps. After homogenization, the bottom part of the crucible was cooled rapidly (in 4 hours), applying a “Bell” temperature profile to achieve melt nucleation as reported by us previously [see figure 2]20. Significant under-cooling of the melt required for nucleation, about 10° C, results in a substantial volume of melt at lowermost of crucible rapidly solidifying. To improve crystal quality of this rapidly solidified portion, it was partially melted-back by heating the middle part of ingot, and subsequently re-solidified in more uniform/controlled manner 20. The cold finger helped to ensure about the favorable rounded melt interface silhouette throughout the beginning of the growth

37

. After the melt back, a

bell shaped temperature profile was applied to the growth ingot, as seen in the blue curve, with lower temperatures at the top of the ingot than in the middle. Ingot solidification then took place over 100 hours, during which the temperature profile was shifted in a linear manner from the “Bell” configuration to the “Straight” configuration shown by us previously in Figure 220. The initial bell-shaped temperature profile helped to maintain the convex melt interface, and also resulted in intense fluid convection, improving isolation of surplus Te in melt and thus tumbling the formation of Te inclusions in solid

20

. The melt-

back, SiC cold finger and bell-shaped temperature profile helped to ensure favorable solidification conditions at the critical beginning stages of the growth, which largely determine the crystal quality throughout the entire ingot 18-20. The shift towards a “Straight” temperature profile is necessary to ensure that the top third of the ingot solidifies in a controlled manner, at the target value of 0.5 to 1 mm/hr. The growth procedure is described in greater detail and verified by experimental and numerical simulation results in our 5 ACS Paragon Plus Environment


previous publication

20

. After the ingot had completely solidified, the ingot was relaxed at

speed of 20° C/hr to 300K. The VGF grown ingot is depicts in Figure 1(a) which was cut in to six ingots according to shown design in Figure 1(b) and finally cut wafers are shown in Figure 1(c). We have selected all the wafers for further cutting in to thin specimens (~2mm) to assess the quality of the whole grown ingot as shown in Figure 1(d). Thin specimens from ingot 1, 2, 4 and 6 are named as 1-1, 1-2, 2-1, 2-2, 4-1 and 6-1 [see figure (d)], respectively. It may be mentioned here that the cut and polished piece for wafer 6 is not shown because it has broken. SEM mapping was carried out to assess the presence of all elements as publicized in Figure 2 which confirms the presence of all metals such as Cd, Zn, Te and In in final ingot and homogeneity is also confirmed.

Figure 1 (a) grown ingot (b) design for cutting wafers and (c) cut wafers and (d) polished specimens of InCZT crystal ingots. 6 ACS Paragon Plus Environment

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Figure 2 SEM mapping image for InCZT grown crystal. 2.2. Characterization techniques Different specimens from the whole VGF grown ingot are selected for Photoluminescence (PL), crystalline perfection and mechanical studies to assess its quality thoroughly. The PL analysis provide the major information about the defects concentrations in the grown crystals. The PL measurement was done on luminescence spectrophotometer (PerkinElmer LS-55) over the range of 650 to 890 nm wavelength in which the specimens from the

7 ACS Paragon Plus Environment


grown crystals were exposed to incident beam and emission spectra were recorded. The crystalline perfection gives a clear idea about the application of the grown crystal in device fabrication by assessing its quality. For crystalline perfection analysis the commercial highresolution X-ray diffractometer (HRXRD) (PANalytical X’Pert PRO MRD System) using CuKα1 radiation (λ = 1.540598 Å) was used. Along with PL and HRXRD analyses, the mechanical studies also have vibrant role to know the strength or stability of grown crystal for fabrication and testing of devices and hence it seems to be necessary to study the detailed mechanical properties. Optically high quality wafers of InCZT crystals of thickness of 2 mm were used for Dynamic indentation Vicker’s hardness testing using Leitz-Wetzlar microhardness tester in the loading effect of 0.998 N to 4.9 N at ambient temperature. The average indentation time of 15 sec kept constant for all the samples and by measuring the indentation profile various mechanical parameters are calculated. 3. Results and discussion 3.1. Photoluminescence (PL) analysis Measured room temperature PL emission spectra for 1-1, 2-1, 4-1 and 6-1 selected crystal specimens are shown in Figure 3 (a to d). The emission spectra were recorded by exciting the grown crystals at 415 nm wavelength. The emission spectra consist only single peak in all the specimens with minute variation in the position, however the PL intensity is showing noticeable change for the lower portion of the crystal. The position and intensity of PL emission peak for 1-1 and 2-1 is observed to be at almost same position (i.e. 836 nm) and intensity. However, the position of emission peaks for 4-1 and 6-1 is found to be minutely change from 836 to 837 nm and the intensity is gradually enhanced for respective crystal specimens. The enhancement in the PL emission peak intensity gives a clear indication of higher defects in the lower part of the grown ingot. 8 ACS Paragon Plus Environment

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Figure 3 PL spectra of (a) 1-1, (b) 2-1, (c) 4-1 and (d) 6-1 crystal specimens of InCZT crystal ingot. 3.2. HRXRD study (1-1 and 1-2) InCZT crystal specimens The HRXRD curve for 1-1 crystal specimen recorded for (111) diffraction planes is depicts in Fig. 4(a). From which it is apparent that there is an extra peak i.e. around 219 arc s away to core peak of FWHM ~ 180 arc s. This extra peak is due to physical low-angle boundary. The representation of structural grain boundary can be seen in previous reports for better understanding

20, 29

. Angular division of both diffraction crests provides angle of tilt α

which is around 219 arc s. The FWHM of low angle boundary is ~ 134 arcs. Such tiny FWHM value and low angle feast (~ 900 arcs) of diffraction curve signify to healthier crystal perfection and the reason of such defects are well explained in literature corporeal chattels of crystals may not be affected to such defects.

9 ACS Paragon Plus Environment

38

. The


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HRXRD curve for (1-2) has been presented in Figure 4(b). From figure it can be recognize that diffraction pattern contains two peaks which were observed through its deconvolution in which an extra peaks is around 244 arc s away from core peak. It is similar to previous one. FWHM of core and additional diffraction peaks are ~240 and ~155 arcs, respectively. Such low FWHM values and angular feast (~ 900 arcs) signify better crystal perfection which is much better and comparable to previous reports on CZT crystal

37, 39-41

. Though,

the broad FWHM (i.e. ~ 134 arc s) along with low intensity peak signify about few sections of grown ingot are unideal 20.

Figure 4 HRXRD curves recorded along (111) planes for (a) 1-1 and (b) 1-2, InCZT single crystal specimens. (2-1) and (2-2) CZT single crystal specimen Similar explanation can be drawn for crystal specimen (2-1) as of (1-2) explained above for which the recorded DC is shown in Figure 5(a). The DC for (2-2) InCZT single crystal specimen depicts in Figure 5(b). From this it can be observe that the DC possess only a single diffraction peak with FWHM ~ 56 arc s only, which give a clear indication about the perfection of the crystal specimen as there is no additional peak or boundary. However, it is relatively bigger compared to likely from plane 10 ACS Paragon Plus Environment

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wave theory for model crystals 42,20. Though, such defects of very low concentration barely offer any cause to fabricated devices performance and if density of defects is higher, then FWHM will be ample larger that habitually leads to the formation of boundaries

38, 43

.

These results for crystal specimen 2-1 are much better than previous reports on CZT crystal 20, 37, 39-41, 44

.

Figure 5 HRXRD curves recorded along (111) planes for (a) 2-1 and (b) 2-1, InCZT single crystal specimens. (4-1) and (6-1) InCZT single crystal specimen The recorded DC for (4-1) crystal specimen has been depicted in Figure 6 (a) and it can be seen that DC does not contain a single diffraction peak. In DC there is an extra peak of FWHM ~ 190 arc s, that is ~ 547 arc s after core diffraction peak along with a main diffraction peak of FWHM ~ 244 arc s and this extra peak is well-explained in previous reports

20, 29

. Tilt angle α was observed ~ 547 arc s from angular impartiality of two

diffraction peaks and this low FWHM value and angle feast (~ 2000 arc s) of DC imply good crystal perfectionism. Creation of such defects depends on many parameters 38. Figure 6(b) depicts the DC for (6-1) crystal specimen. The deconvolution of the DC was done which shows that there around 5 number of peaks and the convoluted curves/solid lines is well fitted with experimental points. Such number of diffraction peaks shows that 11 ACS Paragon Plus Environment


the 6-1 crystal specimen which is from the almost lower part of the crystal ingot contains a number of low angle structural boundaries whose tilt angles are mentioned in the DC [figure 6(b)]. The more explanation and reasons about the grain boundaries, sub grain mosaic blocks can be seen elsewhere

20

. The ratio of fineness crystals can be considered

higher because the crystal quality of 1-2 from ingot 1, 2-1, 2-2 from ingot 2, 4-1 from ingot 4 and 6-1 from last ingot 6 (see figure 1) was assessed. These results suggests that the ingot 2 is better than others, however the other ingots are also convincingly good. The crystalline perfection confirms that the currently grown In doped CZT single crystal possess better perfection than as well as comparable to earlier reported pure and doped CZT crystals 20, 37, 39-41, 44-46

.

Figure 6 HRXRD curves recorded along (111) planes for (a) 4-1 and (b) 6-1, InCZT single crystal specimens. 3.3. Mechanical studies A Leitz-Wetzlar microhardness tester is chosen for present study at diverse loading conditions varying from the 0.098N to 0.49 N on Vertical Gradient Freeze grown crystals. To measure various mechanical parameters, the grown crystals of InCZT ingot was cut in to different wafers [termed as 1-1, 1-2, 2-1, 2-2, 4-1 and 6-1] of thicknesses 2 mm using the 12 ACS Paragon Plus Environment

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well diamond wire saw cutting machine (model 3032) and polished with mechanical polishing Logitech instrument. To understand the complete profile and loading effect, the load variation of hardness have been determined and analyzed by different theoretical models and detail information is discussed in concerned sections. Indentations are applied on the cut and polished wafers of the 1-1, 1-2, 2-1, 2-2, 4-1 and 6-1 of InCZT crystals at different loading conditions (P in N) and evident of the Vicker’s indenter impression lengths (d is in µm) are measured using the following relation 47: Hv = 1.854 P/d2

(1)

In order to measure the various other mechanical parameters of fracture toughness, brittleness index, yield strength and elastic stiffness, radial cracks generated at the indenter impression 20. Measurement of hardness of rock-hard purely dependent on loading effect of the sample, which will provide the information about the indentation size effect (ISE). Usually, the ISE is detected in two models, one is normal indentation size effect (NISE) where with increasing the loading effect the hardness is decreasing and where as in Reverse indentation size effect (RISE) it decreased with loads

48

. Figure 7(a) presents the disparity

of Hv with practical loading effect for different wafers of InCZT, from this graph, it can be seen that strength increased with loads indicating the load dependent hardness up to 1.0 N. further, increase in hardness up to 0.49 N, all the samples are showing the load independent hardness region with RISE behavior. From figure 7(a) it is clearly appeared that the ingot 22 of the grown crystal is showing higher hardness compare to rest of the ingots and is well correlated with the crystalline perfection of this ingot obtained through HRXRD as explained in the respective section above. The phenomena of load dependent and independent hardness with RISE, is explained by many authors in different way

48-50

, but

suitable and acceptable explanation is deformation of the sample surface with indentation 13 ACS Paragon Plus Environment


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process. At lower loads, the indenter penetrate to the upper surface may cause easy glide of slip planes and at higher loads the interaction and pinning of the dislocations may decrease the gliding of the planes during indentation process

49, 50

. There are different models are

available in literature to explain the present behavior of RISE which gives the complete portrait of the different wafers of InCZT.

Figure 7 Disparity of (a) Hv vs P, (b) cracks, (c) l vs P and (d) lnP vs lnd. The indentations patterns of InCZT wafers at different loading effect have been recorded using an optical microscope fitted with hardness tester and are shown in Figure 7(b). From figure it was observed that the crack profile is linear [see figure 7(c)]. From this micrograph, each impression is consisting well-defined cracks at the edges of indenter and is measured using the following relation 20: l = C1Pm1

(2)

and l = C2dm2

(3) 14 ACS Paragon Plus Environment

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Where C1, C2 and m1, m2 are coefficients, values of m1, m2 and obtained from the slope of plots between lnl vs ln P and lnl vs lnd. From these m1, m2 values the work hardening index is calculated by using the relation and obtained values are given in Table 1. n = m2/m1

(4)

The relation between the applied loading effect (P) and impression size (d) is relates to 51: P = A dn(5) here A termed as coefficient of indenter and n is a work hardening index calculated from slope of ln P, ln d Plots [Figure 7(d)]. From this plots, usually the samples shows NISE possess n2 values 51. The value of n for different wavers of InCZT ingot was found to be n>2 values are suggesting that the present samples are showing RISE behavior. Further, the validity of this law was explained by the matching of the n values calculated by using the eq. (4) and it is found that the Meyer’s law is showing quite near n values which are calculated from the cracking phenomenon (eqn. 4). The RISE behavior of the sample is explained on the basis of P and d by Hays-Kendall's approach 20: P = W+A1d2

(6)

here W is smallest load which pledge plastic distortion and A1 is a load sovereign coefficient. Value of W is calculated through plots of P vs. d2 [Fig. 8(a)] and the obtained values are quantified in Table 1. As per Hays-Kendall's law the material which exhibit negative values of plastic deformation is shows RISE and positive values of W indicates the NISE 20. Hence, in the present indentation study all these wafers are showing negative and high values W, suggest that they are belong to RISE behavior 48. The corrected indentation size independent hardness H0 is also calculated from same approach using the relation 52: H0 = 1854.4 A1

(7) 15 ACS Paragon Plus Environment


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The values of the W and H0 for all the wafers of InCZT were given in Table 1. The load dependence and independent hardness are two important parameters to understanding the complete micro-structural property of the loading effect which is indicated by P/d vs. d [Fig. 8(b)] graphs. Hence, to determine the micro-structural behavior of the present samples Proportional specimen resistance (PSR) model is applied using following relation 47: P = ad+bd2

(8)

Where a, b are linked to elastic and plastic behavior, which are attributed as frictional resistance constant and load sovereign constant, respectively

53

and ad is accredited to

specimen surface energy. The value of b is calculated through P/d vs. d plot and a is determined as 53: a= P/d-bd

(9)

From the above relation it should be noted that a>0 indicates the NISE and a

Large Size Crystal Growth, Photoluminescence, Crystal Excellence

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