Cl-Doped CdTe Crystal Growth for Medical Imaging Applications
Round 1
Reviewer 1 Report
Generally, the paper topic is of interest, however, there are no any news and improved crystal qualities. The chapters on crystal growth and crystal quality characterization are superficial. There are many inadequacies in the material properties and growth method. For instance, the first careful publication on CdTe growth from Te-rich melt-solution is missing (Zanio in: Willardson and Beer (eds.), Semiconductors and Semimetals 13, Academic Press N.Y. 1978). Zano showed (like for al growth experiments from solution) that the segregation coefficient (here of Cl) is markedly reduced from 0.36 at melt growth down to 0.005 at growth from Te-rich melt. This is of decisional importance for the axial and radial distribution inhomogeneity. But there is no any relation in the present manuscript. Also Zhou JCG 483 (2018) 281 on Te inclusions at ACRT growth is missing. What are the authors thinking about the modeling paper of Divecha and Derby in JCG 576 (2021)126386 on ACRT VB/VGF concerning flow instabilities at ACRT ? Maybe there is an important relation to the present results ?
There is only one preference of CT to CZT, i.e. the improved hole transport. But much more favourable qualities are obtaind at Zn addition, such as lattice hardening, higher resistivity, lower lackage currents, lower EPD and dislocation patterning ... The authors omit these arguments. Today, the leading producers of phton counting detectors prefer CZT detectors (see e.g. Schulman, Thesis, Univ. Helsinki 2006 and actual internet reports).
Ch. 1, lines 41-43: the ratio of about 2 of the melt-to-solid thermal conductivity is typical for nearly all semiconductors (Si, GaAs,...). More important is the lowest stacking fault energy of CT evoking twinning (!). Then, the very high ionicity of CT (> 70 %) is responsible for preservation of tetrahedrally coordinated melt leading to high degree of association (not clustrering!) at low superheating values (Gaspard 1996, Dalgic 2007, Godlevsky 1991, Rudolph 1994...). Also in Te-rich melt-solutions the molecular character is still present (Xe JCG 17 (1997)). Lines 49-50: stoichiometry cannot "improved" (its a fixed composition) but may be approached. Lines 56-57: generally, the nucleus size depends on degree of superheating. Due to high degree of association at low superheating only very few nuclei are generated. With increasing superheating multicrystallinity in the tip increases. However, there is a pronounced tendency of self-orientation in CT - no any relation to this internationally well-known fact in the present manuscript. Line 58: not only the Cd partial pressure is reduced but the region of existence (solidus) of solid CT is enlarged with Te excess (maximum 0.0155 at% at about 900°C). V(Cd) must be still dominant due to the lowest formation energy of all point defects (see e.g. Biwas, New J. Phys 14 (2017) 063020). Lines 60-61: wrong ! At 800°C CZT shows 0.008 at% Te excess (please study Ndap in: CdTe and Related Compounds, Vol. Defects, ch. 5., Elsevier 2010, p 230, Fig. 1).
Ch. 2: Generally, the crystal growth passages are very weak. Missing are the discussion of self-seeding process, the overheating degree, the crystal diameter and length, the crystallographic main grain orientation, the reduced segregation coeff. of Cl and its axial distribution, which ampoule is used instead of pBN (CRY-4) ?, the cooling rate to room temperature a.s.o. Line 143: the conclusion that Te excess enhances sub-grain boundaries belongs to VGF/VB only because at THM with seed and much higher Te excess within the traveling zone much better twin- and grain-free crystals are obtained. Ch. 3.1: Many important crystal qualities are missing, such as EPD, Rocking curve FWHM, dislocation patterns. Then, no differenciation is made between Te inclusions (incorporated at the interface) and precipitates (due to redrograde solidus during cooling down). Both particle types are totally mixed. The discussion about decreasing Te particle size vs. Te excess is nebulous. But a more careful study of the phase relations of the system Cd-Te would give a possible answer (see e.g. Greenberg, Thermodynamic Basis of Crystal Growth (Springer 2002) p. 122, Fig. 65 or Landhold-Börnstein, Cd-Zn-Te, eds. Tomashik and Shcherbak). Accordingly, the increasing Te excess decreases the melting point (liquidus) and, at the same time, reduces the existence region of solid CT due to the retrograde solidus character. Thus, high Te excess in the melt-solution meets a smaller deviation from stoichiometry. Line 166-167: what means "Te moves back to its proper stoichiometric position releasing Cl"? That's a non-scientific explanation. Please qualify and quantify which thermodynamic driving force is acting? By the way, why the Te particle diameters in CRY-2 and -3 are so unusual large ? Line 190: what means exactly "annealing causes a change in Te secondary phases"? The whole annealing explanation is weak. The authors should study the much better discussions of Franc in CRT 48 (2013) 214, Kim in IEEE Trans Nucl. Sci. 6X (2017) 1, and He in JCG 402 (2014) 15, for example.
In summary, the reviewer cannot find a decisiv step forward in crystal growth technique, quality and better scientific understanding compared to numerous already published papers. The present passages of crystal growth and crystal characterization should be rewritten by better inclusion of the current degree of scientific knowledge.
Author Response
Generally, the paper topic is of interest, however, there are no any news and improved crystal qualities. The chapters on crystal growth and crystal quality characterization are superficial. There are many inadequacies in the material properties and growth method. For instance, the first careful publication on CdTe growth from Te-rich melt-solution is missing (Zanio in: Willardson and Beer (eds.), Semiconductors and Semimetals 13, Academic Press N.Y. 1978). Zano showed (like for al growth experiments from solution) that the segregation coefficient (here of Cl) is markedly reduced from 0.36 at melt growth down to 0.005 at growth from Te-rich melt. This is of decisional importance for the axial and radial distribution inhomogeneity. But there is no any relation in the present manuscript. Also Zhou JCG 483 (2018) 281 on Te inclusions at ACRT growth is missing. What are the authors thinking about the modeling paper of Divecha and Derby in JCG 576 (2021)126386 on ACRT VB/VGF concerning flow instabilities at ACRT? Maybe there is an important relation to the present results?
The text in the section.3.1. says, “ The grains are becoming smaller with the increasing Te excess due to more nucleation [24].” In results, small grains are reported due to the excess of Te.
It’s a valid point but we don’t have data to show in results that Cl segregation is reduced due to Te excess. It was not the focus of the research.
ACRT Vs. Te- inclusions are already published, its not the scope of the paper. Main focus is Te-excess Vs. crystallinity and its performance.
There is only one preference of CT to CZT, i.e. the improved hole transport. But much more favourable qualities are obtaind at Zn addition, such as lattice hardening, higher resistivity, lower lackage currents, lower EPD and dislocation patterning ... The authors omit these arguments. Today, the leading producers of phton counting detectors prefer CZT detectors (see e.g. Schulman, Thesis, Univ. Helsinki 2006 and actual internet reports).
CdTe here, is presented as an alternative to CZT in medical applications. In CZT detectors, where electrons are majority carriers, the polarization are observed at high voltages/ fluxes.
Ch. 1, lines 41-43: the ratio of about 2 of the melt-to-solid thermal conductivity is typical for nearly all semiconductors (Si, GaAs,...). More important is the lowest stacking fault energy of CT evoking twinning (!). Then, the very high ionicity of CT (> 70 %) is responsible for preservation of tetrahedrally coordinated melt leading to high degree of association (not clustrering!) at low superheating values (Gaspard 1996, Dalgic 2007, Godlevsky 1991, Rudolph 1994...). Also in Te-rich melt-solutions the molecular character is still present (Xe JCG 17 (1997)). Lines 49-50: stoichiometry cannot "improved" (its a fixed composition) but may be approached. Lines 56-57: generally, the nucleus size depends on degree of superheating. Due to high degree of association at low superheating only very few nuclei are generated. With increasing superheating multicrystallinity in the tip increases. However, there is a pronounced tendency of self-orientation in CT - no any relation to this internationally well-known fact in the present manuscript.
Lines 49-50: The term “improved stoichiometry” changed to “highly achieved stoichiometry”
Lines 56-57: Word changed from super- cooling to cooling. Reference is already provided.
The references are provided for each statement. Te excess caused lower temperature crystal growth than without the Te excess. Hence caused more nucleation/ small grains/ low crystallinity. This is what we got and is in literature.
Line 58: not only the Cd partial pressure is reduced but the region of existence (solidus) of solid CT is enlarged with Te excess (maximum 0.0155 at% at about 900°C). V(Cd) must be still dominant due to the lowest formation energy of all point defects (see e.g. Biwas, New J. Phys 14 (2017) 063020). Lines 60-61: wrong ! At 800°C CZT shows 0.008 at% Te excess (please study Ndap in: CdTe and Related Compounds, Vol. Defects, ch. 5., Elsevier 2010, p 230, Fig. 1).
Lines 60-61: Seems no need to mention it, so sentence is deleted.
Ch. 2: Generally, the crystal growth passages are very weak. Missing are the discussion of self-seeding process, the overheating degree, the crystal diameter and length, the crystallographic main grain orientation, the reduced and its axial distribution, which ampoule is used instead of pBN (CRY-4) ?, the cooling rate to room temperature a.s.o. Line 143: the conclusion that Te excess enhances sub-grain boundaries belongs to VGF/VB only because at THM with seed and much higher Te excess within the traveling zone much better twin- and grain-free crystals are obtained.
Agree.
The paragraph is re-written with the addition of points mentioned by the reviewer.
As reviewer claimed all the question are valid. However, we have been working on more than 20 years on CdTe/CdZnTe and Accelerated crucible rotation Technique (ACRT) was successfully implemented for excess of Te based CZT growth, Tellurium inclusion was drastically reduced which can be seen through IR imaging microscope. This work was published in Springer (https://link.springer.com/chapter/10.1007/978-3-030-76461-6_12). The forced melt convection techniques such as vibration and ACRT is helping to reduce the inclusion size in the growth. Datta et al. reported a decrease in mean diameter to ~ 6 μm, and the distribution was reported to change from a bimodal, in the case of static growth, to a single distribution, in the case of ACRT growths, with the same composition of the melt and recent optimization of ACRT parameters have led to significantly smaller mean diameter. https://link.springer.com/article/10.1007/s11664-013-2782-x. https://doi.org/10.1016/j.jcrysgro.2020.125542.
Ch. 3.1: Many important crystal qualities are missing, such as EPD, Rocking curve FWHM, dislocation patterns. Then, no differenciation is made between Te inclusions (incorporated at the interface) and precipitates (due to redrograde solidus during cooling down). Both particle types are totally mixed. The discussion about decreasing Te particle size vs. Te excess is nebulous. But a more careful study of the phase relations of the system Cd-Te would give a possible answer (see e.g. Greenberg, Thermodynamic Basis of Crystal Growth (Springer 2002) p. 122, Fig. 65 or Landhold-Börnstein, Cd-Zn-Te, eds. Tomashik and Shcherbak). Accordingly, the increasing Te excess decreases the melting point (liquidus) and, at the same time, reduces the existence region of solid CT due to the retrograde solidus character. Thus, high Te excess in the melt-solution meets a smaller deviation from stoichiometry. Line 166-167: what means "Te moves back to its proper stoichiometric position releasing Cl"? That's a non-scientific explanation. Please qualify and quantify which thermodynamic driving force is acting? By the way, why the Te particle diameters in CRY-2 and -3 are so unusual large ? Line 190: what means exactly "annealing causes a change in Te secondary phases"? The whole annealing explanation is weak. The authors should study the much better discussions of Franc in CRT 48 (2013) 214, Kim in IEEE Trans Nucl. Sci. 6X (2017) 1, and He in JCG 402 (2014) 15, for example.
Edited accordingly. Rewritten and new references added.
Reviewer 2 Report
In the paper Cl- doped CdTe Crystal Growth for Medical Imaging Applications authors presented characterization results for 3 Cl doped CdTe crystals and compare it with In doped reference sample. CdTe crystals are investigated because of potential application in medical imaging.
Current version of manuscript does not qualify for publication in the journal. In my opinion the paper is written in chaotic way and is difficult to understand. There is a lot o typo errors (missing superscripts, missing space between an number and unit, missing unit in line 102), some results are missed (I-V characteristics for CRY-3 and CRY-4 sample), authors used different labels for the same samples: CRY-1 and CG224 AS1 for result shown in Figure 10. It is difficult to follow the manuscript and author idea. Moreover in my opinion some fundamental research is missing: what about Hall measurement of fabricated samples and ECV verification of dopant contribution from sample to sample. Authors performed annealing of crystals but did not mentioned about optimization of annealing time and temperature, what about annealing atmosphere, it influence on the crystal properties can be significant. Some sentences are incomprehensible for me, see lines 153-154.
Author Response
In the paper Cl- doped CdTe Crystal Growth for Medical Imaging Applications authors presented characterization results for 3 Cl doped CdTe crystals and compare it with In doped reference sample. CdTe crystals are investigated because of potential application in medical imaging.
Current version of manuscript does not qualify for publication in the journal. In my opinion the paper is written in chaotic way and is difficult to understand. There is a lot o typo errors (missing superscripts, missing space between an number and unit, missing unit in line 102), some results are missed (I-V characteristics for CRY-3 and CRY-4 sample), authors used different labels for the same samples: CRY-1 and CG224 AS1 for result shown in Figure 10. It is difficult to follow the manuscript and author idea. Moreover, in my opinion some fundamental research is missing: what about Hall measurement of fabricated samples and ECV verification of dopant contribution from sample to sample. Authors performed annealing of crystals but did not mentioned about optimization of annealing time and temperature, what about annealing atmosphere, it influence on the crystal properties can be significant. Some sentences are incomprehensible for me, see lines 153-154.
In the title of the figure description is provided for the samples. The resistivities are recorded in the table 2, only a representation of the IV characteristic plots is provided in the draft. The optimum annealing conditions are mentioned in line 191.
line 102: Unit (micron) is mentioned in original word-draft, but missing in pdf draft.
Fig.10 and annealing para are improved and edited accordingly.
Line 153: Period was missing. Correction made.
Reviewer 3 Report
The work presented here addresses an interesting topic, however some points should be improved. The introduction of relevant background and research progress was not comprehensive enough. The doping can improve several material properties and the choice for doping depends on several factors. With respect to this, the authors do not justify the doping concentrations used in this system. Due to the importance for this system, a reasoned explanation must be attached as part of the motivation and justification of the work. As I mentioned, it is interesting that the authors have studied: Cl-doped CdTe Crystal Growth for Medical Imaging Applications, but some results presented must be discussed in depth, few references are used to justify the results, leaving the work qualitative and in some sentences hypothetical. Some figures are loaded with information (Fig 17, 18), is it necessary? Figure 15, please rescale the y-axis to the current figure format. More recent references must be included in the manuscript
Author Response
The work presented here addresses an interesting topic, however some points should be improved. The introduction of relevant background and research progress was not comprehensive enough. The doping can improve several material properties and the choice for doping depends on several factors. With respect to this, the authors do not justify the doping concentrations used in this system. Due to the importance for this system, a reasoned explanation must be attached as part of the motivation and justification of the work. As I mentioned, it is interesting that the authors have studied: Cl-doped CdTe Crystal Growth for Medical Imaging Applications, but some results presented must be discussed in depth, few references are used to justify the results, leaving the work qualitative and in some sentences hypothetical. Some figures are loaded with information (Fig 17, 18), is it necessary? Figure 15, please rescale the y-axis to the current figure format. More recent references must be included in the manuscript
The populated figures got some editing, to reduce the information on the plots in figure 17 and 18.
Y-axis in figure 15 is corrected and edited for simplification.
Recent references are added.
Round 2
Reviewer 1 Report
I am sure the authors consider all recommendations well.
Therefore, I recommend its acceptance.
Author Response
Thank you for all points cleared - certification.
Reviewer 2 Report
Authors in their response do not answered on all my questions (what about Hall or ECV measurements for instance). The improvement made during revision is in my opinion insufficient, therefore I must reject the paper.
Author Response
Hi
I made the changes according to the suggestions, where we thought are important and are needed to improve the quality of the paper. Some suggestions like Hall data (mobility, free charge carrier density and resistivity) for example is not priority for the CdTe- detectors application. Main focus is gamma response for CdTe. Resistivity and mobility-life time we measured with other techniques.
Reviewer 3 Report
This revised version of the manuscript may be accepted for publication
Author Response
Thank you for all points cleared - certification.
