Investigation of dislocation migration in substrate-grade CdZnTe crystals during post-annealing

Journal of Crystal Growth ∎ (∎∎∎∎) ∎∎∎–∎∎∎

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Investigation of dislocation migration in substrate-grade CdZnTe crystals during post-annealing Ningbo Jia a, Yadong Xu a,b,n, Rongrong Guo a, Yaxu Gu a, Xu Fu a, Yuhan Wang a, Wanqi Jie a a b

State Key Laboratory of Solidification Processing, Northwestern Polytechnical University, Xi'an 710072, China Laboratory of Solid State Microstructures, Nanjing University, 210093, China

art ic l e i nf o

a b s t r a c t

Communicated by Andrea Zappettini

The migration of dislocations in substrate-grade CdZnTe (CZT) single crystals during temperature gradient annealing under Cd/Zn vapor has been investigated. The etch pit density (EPD) and configuration of dislocations have been evaluated before and after annealing in CZT crystals with and without Cd-rich second phase (Cd-SP) particles, respectively. After Cd/Zn overpressure annealing, dislocation reduction in CZT crystals was observed. However, dislocation walls with 120° intervals along o 211 4 crystalline direction were observed in the both types of CZT crystals. The formation of these dislocation walls can be attributed to the reaction of o 110 4 dislocations. Moreover, it is considered that the release of the restored stress during annealing act as the domain driving force for dislocation migration, by comparing the variation of dislocation configuration in CZT crystals with and without Cd-SP particles. & 2016 Elsevier B.V. All rights reserved.

Keywords: A1. Line defects A1. Substrates A2. Bridgman technique B1. Cadmium compounds B2. Semiconducting ternary compounds

1. Introduction HgCdTe based infrared (IR) detectors are extensively used in the last decades and expected to continue in future due to the tunable bandgap that makes it a suitable material for IR radiations with multi-color detection possibility [1,2]. And CZT has widely been preferred as the substrate materials for the growth of HgCdTe epilayers for the fact that CZT provides the best lattice match to HgCdTe epilayers [1,2]. However, high densities of defects such as inclusions, precipitates, and dislocations occurred during CZT crystal growth still limit the quality and yield of CZT materials [3]. Moreover, the crystalline quality and properties of HgCdTe epilayers are strongly affected by the substrate which it is grown on [4]. Among the various defects appearing in CZT substrate materials, dislocation has gained a great interest since it can penetrate into the epilayers [5]. It has been reported that the minimum dislocation density achieved in HgCdTe material were grown by liquid phase epitaxy (LPE) and it is close to that in CZT substrates [5,6]. Thus it is important to find the methods to reduce dislocation densities in CZT wafers, in order to orient towards material with lower concentration defects and higher performance of HgCdTe devices. The enhancement of dislocation movement and reaction through high thermal stress has been considered as one of the effective methods to reduce dislocation densities in various n Corresponding author at: State Key Laboratory of Solidification Processing, Northwestern Polytechnical University, Xi'an 710072, China. E-mail address: [email protected] (Y. Xu).

materials, such as Si [7], GaAs/GaSb/Si heterostructure [8], et al. Therefore, annealing process is considered an important way to reduce dislocations in CZT crystals. Vydyanath et al. [9] reported that annealing under Cd overpressure obtained a better quality of CZT substrate for epitaxial growth of HgCdTe. However, the study on the evolution of dislocation during annealing in CZT has not attract much attention. To elucidate the driving forces for dislocation migration, the variation of dislocation densities near CZT surfaces and inner part was investigated after thermal gradient annealing under given Cd/Zn atmospheres in this work. Employing defect-selective etching techniques, the distribution of dislocations on CZT (111)Te surfaces was observed using correlative reflected-light/IR transmission (IRTM) and scanning electron microscopy (SEM). The reasons for the dislocation configuration variations and the dominant driving force for the dislocations migration during annealing are discussed.

2. Experiment Cd0.96Zn0.04Te (CZT) crystals used in this work were grown by the modified vertical Bridgman method (MVB) in our laboratory [10]. The accelerated crucible rotation technique (ACRT) was utilized to improve the mass and heat transfer and smooth the solidliquid interface. To give a convenient comparison, two types of as-grown CZT crystals named CZT-I and CZT-II, with and without second phase (SP) particles respectively, were sliced from the substrate-grade CZT ingot. More than ten wafers of each type were selected to eradicate any discrepancies. All the wafers were

http://dx.doi.org/10.1016/j.jcrysgro.2016.08.008 0022-0248/& 2016 Elsevier B.V. All rights reserved.

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o111 4-oriented single crystal free from twins and grain boundaries, with the size of 5
5
1 mm3. The damaged layers on crystal surface induced by cutting were removed through lapping, mechanical and finely chemo-mechanical polish with 2% Br-methanol solution consecutively before annealing. To avoid introducing impurities and exothermic reaction between Cd-SP particles and Te vapor similar to the work of reference [11], temperature gradient annealing on CZT wafers were employed under Cd/Zn overpressure, more details are listed in Table 1. Constant heating rate (100 °C/h) and cooling rate (50 °C/h) were adopted during the annealing processes. Everson's solution (HF:HNO3:lactic acid¼ 1:4:20) was used to reveal dislocations on {111}Te surface. As a criterion of the dislocation density, etch pit density (EPD) mappings of individual sample were observed from high-resolution reflected-light microscopy images. The distributions of dislocations in bulk crystals were evaluated by using IR transmission microscopy (IRTM) system, with the optical resolution limit of  1 mm. The Cd-rich second phase (Cd-SP) particles and dislocations in selected areas were characterized by correlative reflected-light/IR transmission microscopy and Field Emission-SEM at 15 kV. Table 1 The annealing parameters under Cd/Zn vapors. Atmosphere

Twafers (°C)

Tsources (°C)

∇T n (°C/ cm)

Annealing time t (h)

Cd/Zn vapor

650

550

7–8

60

n

∇T represents the temperature gradient of wafers during annealing.

a

3. Results and discussions Fig. 1 shows the typical IR images and the corresponding size distribution histogram of SP particles of CZT-I wafers before and after temperature gradient annealing under Cd/Zn overpressure. Star-shaped SP particles were identified in the as-grown CZT samples, which has been certified as Cd-rich SP by Belas et al. [12] according their morphologies. SP particles are randomly distributed and encompass both large and small inclusions with an average size of approximate 5–20 mm, as shown in Fig. 1a and b. After annealing, majority of Cd-SP were eliminated, as seen in Fig. 1c and d, which was attributed to the thermo-migration as described by Anthony [13] and Vydyanath et al. [14]. Similar to the reduction theory of Te-SP in CZT crystals, more details can be seen in reference [11,15]. The thermo-migration theory of SP particles has been fully investigated, therefore we focused on the detailed results on the behavior of dislocations under annealing in this work. To give an insight observation into dislocations distribution, CZT surfaces before and after annealing were observed after etching by Everson's solution. Dislocation clusters surrounding Cd-SP appear in as-grown CZT crystals, as shown in Fig. 2a and c. For convenience, we define the original dislocations in CZT matrix as the grown-in dislocations and the dislocation clusters around SP particles as the induced dislocations, respectively. The dislocation density in the as-grown matrix is in the range of 1–3
105 cm  2 by counting the triangular pyramid etch pits on CZT-I (111)Te surfaces. The EPD are usually counted in five regions, selected randomly on the slices, and the average values are evaluated. After annealing, the EPD in the matrix reduces to 4–7
104 cm  2, with 60%70% reduction. It is suggested that the annihilation among

b

40

Area density/cm2

2

30

20

10

0 0

5

10

15

20

25

30

35

30

35

c

d

40

Area density/cm2

Diameter/µm

30

20

10

0 0

5

10

15

20

25

Diameter/µm Fig. 1. Typical IRTM images and the corresponding size distribution histograms of Cd-SP in CZT-I wafers. (a) and (b) Before annealing, the insert in (a) shows an enlarged CdSP, (c) and (d) After annealing.

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Fig. 2. (a) Typical SEM image of as-grown CZT-I (111)Te face after etching using Everson's solution (b) SEM image of the enlarged dislocation cluster in as-grown CZT-I. (c) SEM image of CZT-I (111)Te face after post-annealing, the insert is an enlarged dislocation wall image. (d) SEM image of an enlarged dislocation cluster in CZT-I after annealing. (e) IRTM image of dislocation configuration in CZT-I after annealing.

dislocations with inverse Burgers vectors may happen during annealing, i.e. the glide and climb systems are activated and dislocations begin to move during the annealing. Once become mobile, dislocations can be removed from the material via several mechanisms, including pairwise annihilation and out-diffusion to surfaces [16]. As shown in Fig. 2a, dislocations are randomly distributed in the asgrown CZT wafers. During the annealing process, the glide and climb systems are activated, and adjacent dislocations with inverse Burgers vectors attract each other. With the annealing proceeding, the dislocations with opposite Burgers vectors at some point approach close enough to each other for annihilate pairwise to occur, and lead to the dislocation reduction. In addition, diffusion of point defects, such as Cdi, VTe is enhanced with the temperature rising during the annealing process, which may promote the dislocation out-diffusion to CZT surface. The typical induced dislocation clusters with size of 20–40 mm around Cd-SP particles are presented in the yellow circle of Fig. 2a. The corresponding enlarged image of that is given in Fig. 2b intentionally, which are comprised by plenty of pyramid etch pits. However, the average region of induced dislocation clusters on CZT (111)Te increases after 650 °C annealing, with typical diameters in the range of 50–80 mm, as shown in the yellow circle of Fig. 2d and e. A model for the dislocation cluster configuration is proposed as

illustrated in Fig. 3, the induced dislocations in as-grown CZT wafers overlap layer by layer (Fig. 3I), and it is unfavorable to be distinguished individually through etch pits. During the annealing, when the glide and climb systems are activated, dislocations move from the cluster core (surrounding Cd-SP) to the edge. And some of the dislocations with opposite Burgers vectors react with each other. Thus, the diameter of dislocation clusters increases, and the EPD of dislocations decreased slightly, as seen in Fig. 3II. With the annealing time prolongation, the movement of induced dislocations from the core to the edge and the dislocation reaction in the induced dislocation cluster proceed (Fig. 3III). The increase of dislocation clusters diameters from 20–40 mm to 50–80 mm in the experiments is in coincidence with the model. In particular, the variations in size of induced dislocation clusters confirm the assumption of dislocation migration. In addition, dislocation walls at 120° intervals are observed after annealing, as indicated by the red arrows in Fig. 2c and e. According to the Thompson tetrahedron [17], the glide directions in (111) crystalline plane are [ 110], [ 101], [ 011], and dislocation reactions possibly happen because the energy of dislocations with 1 Burgers vector 2 110 are higher than the combined dislocations with Burgers vector

1 6

211 . The dislocation reaction equations are:

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Fig. 3. Diagram of the evolution of Cd-SP and the induced cluster on CZT {111}Te. The black core of the cluster is Cd-SP particle and the blue triangles represent dislocation etch pits. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

1⎡ 1 1 ⎣ 110⎤⎦ → ⎡⎣ 121⎤⎦ + ⎡⎣ 211⎤⎦ 2 6 6

(1)

1⎡ 1 1 ⎣ 101⎤⎦ → ⎡⎣ 112⎤⎦ + ⎡⎣ 211⎤⎦ 2 6 6

(2)

1⎡ 1 1 ⎣ 011⎤⎦ → ⎡⎣ 112⎤⎦ + ⎡⎣ 121⎤⎦ 2 6 6

(3)

According to the above equations, it can be seen that the directions of dislocation walls in Fig. 2c are ⎡⎣ 112⎤⎦, ⎡⎣ 121⎤⎦, and ⎡⎣ 211⎤⎦, respectively. The angular of these three directions are 120° in theory, which are in good agreement with the experiment results. By comparing the reactions shown by Eqs. (1)–(3) with the experiment results, dislocations interaction actually happens during the annealing process, and the dislocations line up along ⎡⎣ 112⎤⎦, ⎡⎣ 121⎤⎦, and ⎡⎣ 211⎤⎦ directions, respectively. Moreover, the formation of the dislocation walls consumes part of initial dislocations in the as-grown crystals and the EPD is reduced in CZT matrix. Meanwhile, the formation of dislocation walls also implies the migration of dislocation in CZT-I wafers during annealing. The above changes of CZT-I wafers after annealing can attribute to the migration of dislocations during annealing. And the stresses in the CZT-I wafers during annealing should be also taken into account. Firstly, annealing is a process that relieves restored stress through the whole wafer. CZT wafers usually attain a more stable state with the reduction of the overall energy after annealing. Then, since annealing temperature is higher than Cd melting point (321.07 °C), Cd-SP particles will be melted and dissociated, which results in the variation of deformation stress between Cd-SP and CZT matrix during annealing. Meanwhile, there exists thermal stress between Cd-SP and CZT wafers since the expansion coefficients of both the solid and liquid Cd exceed that of the solid CZT [12] and the corresponding force induces shrinkage of the SP particles. The above reasons may drive the dislocations to migrate. Once the dislocations are able to move, one dislocation can interact with another, and hence dislocation pairwise annihilation occurs. To give an evaluation of dislocation distribution in depth, the surface layer of about 0.2 mm is removed on CZT-I (111)Te, as shown in Fig. 4. The EPD in the inner part (Fig. 4) decreases significantly by comparing with that on the surface (Fig. 2c). Besides, dislocation walls vanish after removing the surface layer. It indicates that dislocation walls mainly form in the surface layer, which may be related with the high dislocation density in these regions. Dislocations closed to the surface tend to move to outside due to the image forces [18,19]. The image force can be assumed to

Fig. 4. Distribution of dislocation etch pits in CZT-I after removing about 0.2 mm surface layer. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

be equivalent to the attractive glide force that would exist between the dislocation and its image dislocation, which located at the same distance from the surface, and with opposite Burgers vector. As dislocations get closer to the surface, the larger the image forces will be. Image forces of dislocations make them glide to the surfaces, where some can be removed by pairwise annihilation among dislocations with reverse Burgers vectors. And the rest dislocations attracted by image forces may be restored in the wafer surface layer and form dislocation walls. To clarify the dominated driving force of dislocations movement, and whether the image forces are related with Cd-SP particles, another type CZT-II wafers without Cd-SP particles are investigated. Fig. 5 shows typical etch pits image and the typical distribution of dislocation etch pits in CZT-II wafers before and after temperature gradient annealing. The grown-in dislocation density in the matrix is 1–3
105 cm  2 on CZT (111)Te before annealing, and after annealing the EPD in the matrix reduces to 6–10
104 cm  2, with 40%  50% reduction. In addition, similar to the above CZT-I wafers with Cd-SP particles, dislocation walls at 120° intervals along o 211 4 crystalline directions are also observed after annealing, as indicated by red arrows in Fig. 5c. There are no deformation stress and thermal stress between Cd-SP and CZT matrix in the CZT-II wafers, but the configuration of dislocation is similar to CZT-I wafers. Thus the restored stress in the wafers released by the annealing process is the decisive driving force, which dominates the glide and climb of dislocations. In

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Fig. 5. Typical SEM image of CZT-II wafers. (a) Before Everson's solution etching. (b) Distribution of dislocation etch pits before annealing. (c) Distribution of dislocation etch pits after annealing in surface layer. (d) Distribution of dislocation etch pits after removing surface layer about 0.2 mm. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

CZT-I wafers, there exist stress due to the mismatch between CdSP and CZT matrix. Besides, Cd-SP may dissociate into Cdi, and that enhance the out-diffusion mechanism. Thus it is reasonable that EPD reduction in the CZT-I wafers is more significant than in CZT-II wafers. Fig. 5d shows the typical dislocation etch pits distribution of CZT-II wafers after removing about 0.2 mm on (111)Te surface, which is less than that on the surface layer, i.e. dislocations in CZTII wafers also suffer image forces similar to CZT-I wafers. Thus, it is suggested that image forces are independent of SP particles. The above results and discussion will help the understanding of the behavior of dislocations in CZT crystals during annealing and improve the annealing technology.

4. Conclusion In this paper, the driving forces for the dislocations migration in substrate-grade CZT single crystals during gradient temperature annealing under Cd/Zn vapor have been investigated. After annealing, dislocation density in CZT crystals with and without Cd-SP particles is reduced owing to the annihilation among dislocations with opposite Burgers vectors and dislocation out-diffusion. However, three kinds of dislocation walls at 120° intervals along o211 4 crystalline direction were clearly observed after annealing in both types of CZT crystals. The formation of dislocation walls can attribute to the reaction of o110 4 dislocation according to the Thompson tetrahedron. Moreover, image forces can account for the variation of dislocation configuration in different layers and are independent of SP particles. By comparing the variation of

dislocation configuration in CZT crystals with and without Cd-SP particles, the restored stress released during annealing is identified as the dominant driving force for dislocation migration.

Acknowledgments This work has been financially supported by National Natural Science Foundations of China (51202197 and 51372205) and the National “973” Program (2011CB610406). And it is supported by the Natural Science Basic Research Plan in Shaanxi Province of China (2016KJXX-09). It is also supported by the Fundamental Research Funds for the Central Universities (3102015BJ(II)ZS014).

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