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Review

Defect-Related Etch Pits on Crystals and Their Utilization

by
Dongzhu Lu
1,2,3,4,5,*,
Quantong Jiang
1,2,4,
Xiumin Ma
1,2,4,
Qichao Zhang
6,
Xiaole Fu
7 and
Liang Fan
1,2,4,*
1
CAS Key Laboratory of Marine Environmental Corrosion and Bio-Fouling, Institute of Oceanology, Chinese Academy of Sciences, Qingdao 266071, China
2
Open Studio for Marine Corrosion and Protection, Pilot National Laboratory for Marine Science and Technology (Qingdao), Qingdao 266237, China
3
Guangxi Key Laboratory of Marine Environmental Science, Guangxi Academy of Sciences, Nanning 530007, China
4
Center for Ocean Mega-Science, Chinese Academy of Sciences, Qingdao 266071, China
5
Research and Development Center of Marine Science and Technology, Institute of Oceanology, Nantong 226006, China
6
Navy Submarine Academy, Qingdao 266199, China
7
EastChina Design Institute Co., Ltd., China National Petroleum Corporation, Qingdao 266071, China
*
Authors to whom correspondence should be addressed.
Crystals 2022, 12(11), 1549; https://doi.org/10.3390/cryst12111549
Submission received: 22 September 2022 / Revised: 25 October 2022 / Accepted: 27 October 2022 / Published: 30 October 2022
(This article belongs to the Special Issue Corrosion, Coatings and Crystals)
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Abstract

:
Etch pits could form on an exposed surface of a crystal when the crystal is exposed to an etching environment or chemicals. Due to different dissolution rates along various crystalline directions in a crystal, the dissolution process is anisotropic; hence, etch pits usually have a regular shape. Here, the morphology and origin of the regular-shaped etch pits are discussed firstly; then, factors which could affect the morphology and density of etch pits are shown; finally, the state of the art of etch pit technology and the utilization of etch pits is presented. Traditionally, etch pits are utilized to evaluate the dislocation density and some defect-related properties. Now, in the modern fabrication industries, the relationship between etch pits and defects has been utilized more skillfully. High-quality crystals can be fabricated by controlling dislocations revealed by etch pits. Meanwhile, with the as-revealed dislocation as the diffusion path of atoms, new crystals will emerge in corresponding etch pits.

1. Introduction

Dissolution, chemical or electrochemical reactions, can change the morphology of crystal surfaces. If some points dissolve or react preferentially on the exposed surfaces, etch pits will form.
The dissolution of a crystal has two major morphologic changes, grain rounding and etch pit formation [1]. Dissolution proceeds by both the retreat of cleavage steps and the nucleation and coalescence of etch pits [2].
Etch pits can be obtained on solids with a crystalline structure, not only on metals, alloys, ceramics, and minerals but also on other crystalline structures, such as organic compounds. For example, on an organic molecular crystal, naphthalene, well-defined etch pits will form after etching with fuming sulfuric acid for a period [3].
As the atom arrangement in a crystal is regular, the atom arrangement on a plane varies with the orientation of the plane. Hence, the dissolution rate, or etching rate, on different planes varies. Then, after etching in a certain chemical environment for a period, well-defined, regular-shaped etch pits can be obtained on crystals [4] For example, regular-shaped etch pits formed on an Fe-36 wt% Ni Invar alloy etched with a mixture containing CuSO4, HCl, and H2O [5]. These etch pits had regular shapes. Triangular, quadrilateral, and hexagonal etch pits were observed in different grains. Etch pits in an individual grain share the same shape and orientation. The shape of etch pits changes with the orientation and polarity of the basal plane [6,7].
Utilizing etch pit characterization, the relationship between different planes can be deduced. On an HCl-etched ZnO-Bi2O3 crystal, triangular-shaped etch pits aligned face to face across the inversion boundary [8]. EBSD analysis was performed and confirmed that the grain is twinned with respect to the basal plane.
The origins of etch pits on crystals have been investigated by numerous researchers [9,10,11,12]. At sites of crystal defects, there is a region with locally high surface energy, and etch pits are prone to initiate at such a position [13]. One of the most common and important origins is dislocation [14]. Formation of the dislocation-related etch pits is the result of the interaction of the dislocation stress field and the surface energy. The etching process at dislocation-related etch pits is a reverse process of crystal growth by the screw dislocation mechanism [15]. Upon etching, the crystal growth unit at the dislocation site is in high energy and prone to be etched; hence, a nonstationary nucleus for spontaneously etching begins. As time goes on, the size of the etch pits enlarges.
According to the relationship between dislocations and etch pits, etch pit observation has become one of the most useful methods to investigate dislocations in crystals [16,17,18,19,20,21,22,23] because of its merits of low cost and simple experimental procedure [24].
Etch pits reveal dislocations on an etched surface of a crystal, and then the relationship between dislocations and properties of the crystal can be estimated by etch pit observation indirectly. For example, the etch pit observation method was utilized to investigate the dislocation structure near nano indents and low-angle boundaries after uniaxial compression of the sample [25]. In addition, the type of dislocations can be distinguished by the morphology of etch pits [26,27].
Furthermore, the etch pit observation method is also frequently utilized to count the dislocation density in crystals. For samples with high dislocation density, small etch pits are preferred for an accurate determination of the overall dislocation density, as it is harder for small etch pits to merge [28]. For samples with low dislocation density, larger etch pits are preferred to distinguish different kinds of dislocations.
To count the etch pits accurately, the etching parameters should be optimized, as the morphology and density of etch pits are affected by different factors [29,30,31,32,33]. Generally, the etchant, the etching time, the etching temperature, etc. affect the etch pits on crystals. While the etch pit formation process is not bound to a manmade corrosive environment, etch pits even form on minerals naturally. Conical- or funnel-shaped etch pits were observed on the surface of weathered olivine samples [34]. On dolomite crystals, rhombohedral etch pits formed on exposed surfaces utilizing water as the etchant [2].
As dislocations have been revealed by etch pits, the control of the dislocation density becomes feasible, and this is very important for the modern electronic industry. Hence, examples are shown here to demonstrate the improvements of the application of etch pit technology. By limiting the growth of dislocations revealed by etch pits, the dislocation density of a newly grown crystal can be reduced effectively. Meanwhile, new tiny crystals can grow in etch pits. Utilizing dislocations below etch pits as a channel, some kinds of atoms in the substrate can be transported into corresponding etch pits preferentially and form new crystals. By presenting the new and skillful utilization of etch pits, it is our hope that more ingenious methods can be found to promote the application of etch pit technology as well as related modern fabrication industries.
Pitting corrosion is a typical and hazardous form of corrosion [35,36], and it occurs on some susceptible regions which dissolve faster than the rest [37,38]. An unsteady state, such as passivity breakdown, can trigger the initiation of a pit [39]. Other than ordinary pits, this study only focuses on regular-shaped etch pits, which are caused by uneven dissolution rates along various crystalline directions and initiated at defects in crystals. Meanwhile, in many circumstances, these regular-shaped etch pits are obtained intentionally, not naturally. The morphology, origin, and influencing factors of the regular-shaped etch pits, as well as the state-of-the-art utilization and potential trends of etch pit technology, are discussed separately in the following sections.

2. The Morphology of Etch Pits

It is well known that the atom arrangement on different crystallographic planes varies. Meanwhile, in a crystal that contains at least two kinds of elements, the relative content and distribution of different kinds of atoms could result in polar surfaces enriched in some kinds of atoms. Hence, the dissolution rate along various crystalline directions in a crystal is anisotropic, as shown in Figure 1.
On a β-FeSi2 crystal, the etch rates of the {111}, {100}, and {001} faces were measured to be 1.57, 1.43, and 1.09 μm/min, respectively [40]. Planes with the lowest etch rate are more likely to define the shape of etch pits. For example, on a Ge substrate, the (111) plane has more available bonds per unit area than the (110) and (100) planes; hence, the etch rate is slower for the (111) plane [41]. Truncated square- and triangular-shaped etch pits formed on the (100) plane and the (111) plane, respectively, as shown in Figure 2.
As the orientation of a crystal and the crystallographic plane of the exposed surface of the crystal are determined, etch pits on the exposed surface of the crystal share a unique shape. That is, the shape of etch pits is determined by the orientation of the basal plane as well as the crystalline structure of the crystal. For example, on a GdFeO3 crystal, triangular etch pits are on the ( 1 ¯ 1 ¯ 0) face, while hexagonal etch pits are on the (001) face [42]. The density of etch pits may vary with the orientation of the basal plane. On Al crystals, the etch pit density of the {111} plane is higher than that on the {110} plane and the {100} plane after etching [43].
Furthermore, the polarity of the basal plane could also affect the morphology of the etched surface. On an etched AlN single crystal, hexagonal pyramids/hillocks were observed on the N polar basal plane, while hexagonal etch pits were observed on the Al polar plane [44]. On natural dark mica, hexagonal-shaped etch pits are related to the oxygen plane of the SiO4 tetrahedron, and triangular-shaped etch pits are related to the hydroxyl plane of the octahedral sheet [45]. On sapphire crystals, triangular or hexagonal pyramidal etch pits were observed on the c-plane sapphire, and rhombic pyramidal etch pits were observed on the a-plane sapphire [46].
The orientation of facets of regular-shaped etch pits can also be determined. The X-ray rocking method can be utilized to identify the orientation of the side facets of etch pits [47]. On a hot water-etched (001) surface of a LiAlO2 crystal, the facets of the inverted pyramidal etch pits were identified to be the (11 2 ¯ ), (1 1 ¯ 2 ¯ ), ( 1 ¯ 1 2 ¯ ), and ( 1 ¯ 1 ¯ 2 ¯ ) planes [48]. Tetragonal etch pits formed on the (001) face of YBa2Cu3O7-δ have {100}, {110}, and {111} facets [49].
As etch pits could initiate at different kinds of defects in a crystal, the type of defects also affects the shape of etch pits, and this will be discussed in the following section.

3. The Origin of Etch Pits

As regular-shaped etch pits form on crystals after etching, to utilize these etch pits skillfully, the origin of etch pits should be clarified. Usually, there are various kinds of defects, such as dislocations, in an ordinary crystal. The integrity and continuity of the crystalline structure will be interrupted at such defects. The coordinate numbers of atoms located at dislocations will be relatively smaller, and the local stress is also different compared with the stress around the atoms in the bulk crystal. Hence, dislocations are inferred to be preferential sites for etch pits to initiate. It has been confirmed that dislocations are the main origin of regular-shaped etch pits. Dislocations give rise to the local mechanical stress in a crystal lattice; hence, the area near dislocations is etched slightly faster than the surrounding area, and then etch pits form [50]. In addition to dislocations, other defects, such as nano pipes, voids, and inclusions in the crystal, are preferred sites for etch pits to initiate and grow up as well.

3.1. Relationship between Etch Pits and Dislocations

3.1.1. Identification of the Relationship between Dislocations and Etch Pits

To investigate the relationship between dislocations and etch pits, scanning electron microscopy and/or atomic force microscopy are often employed to characterize the morphology of etch pits. The focused ion beam can be utilized to obtain cross sections of the matrix just under the pointed bottom of an etch pit; then, a transmission electron microscope and/or X-ray topography [51] are frequently adopted to identify the dislocation beneath the etch pit.
With the above instruments, it has been proven that dislocations are the main origin of regular-shaped etch pits on crystals. Dislocations are usually found on treading dislocations [52,53], including edge dislocations, screw dislocations or mixed dislocations. For example, on a heteroepitaxial diamond etched with hydrogen plasma [54], it was found that the origin of etch pits was mainly [001] edge dislocations, as shown in Figure 3, while etch pits could also be found on a mixed dislocation [55] or a screw dislocation [51].

3.1.2. Identification of Dislocation Types

The type of threading dislocations can be identified by observing the size of the corresponding etch pits. Generally, etch pits originating from screw dislocations or mixed dislocations are larger than those originating from edge dislocations [9]. This is related to the number of dangling bonds on the surface around the center of dislocations. For a screw or mixed dislocation, there is an edge of atomic step at the core of the dislocation, which means that there would be more dangling bonds. For example, on n+-4H-SiC, a precise one-to-one correlation was found between large hexagonal etch pits and screw dislocations as well as between small dot-like etch pits and edge dislocations [56]. It was observed on a single crystal diamond that etch pits formed on mixed dislocations are wider and deeper than those formed on edge dislocations [57].
Dislocations on the basal plane can be distinguished according to their shapes. A dislocation that extends nearly parallel to the exposed plane could result in an oval-shaped etch pit, while an etch pit perpendicular to the exposed plane usually has a polygonal shape [58]. Meanwhile, the shapes of the bottoms of etch pits are also utilized to identify dislocations. An etch pit with a flat bottom is more likely related to a dislocation or a microfracture near the surface, while an etch pit with a pointed bottom is related to an intrinsic dislocation in the crystal [59,60].
Furthermore, the inclination angles of the etch pit sides can also be utilized to identify the type of defects [61].

3.2. Other Defects

Local stress can be introduced by other defects, such as voids, nano-pipes, and inclusions, and thus preferential dissolution initiates at these defects as well. Hence, the origin of etch pits is not limited to dislocations.
On the surface of a ( 2 ¯ 01) β-Ga2O3 single crystal, the line-shaped etch pits were attributed to void defects [62]. On a LiTaO3 single crystal, inverted domains were observed at the tips of the hillocks and the bottoms of the etch pits [63]. In a CdZnTe crystal containing Te inclusions, the results show that Te inclusions with triangular, hexagonal, and irregular shapes led to the formation of flat-bottomed triangular etch pits [64]. Hexagonal etch pits on (0001) GaN films were found to be related to nano pipes, such as open core screw dislocations [65]. On β-Ga2O3 single crystals, the parallelogram-shaped etch pits contained brighter facets, and a parallelogram-shaped dark region was found to be related to plate-like nano pipes [66].

4. Influencing Factors for the Formation of Etch Pits

The morphology, size, and density of etch pits can be affected by various factors, including the chemical composition of the etchant, etching time, etching temperature, status of the matrix, and so on. It is important to investigate the effect of different factors on etch pits to obtain well-defined etch pits that can be utilized to reveal defects in crystals precisely.

4.1. The Etchant

The selection of proper etchants is critical to obtain well-defined etch pits on crystals. The dissolution rate of surface atoms on crystals is closely related to the type of etchants. The morphology and density of etch pits can also be affected significantly by the etchant.

4.1.1. Effect on the Morphology of Etch Pits

The etchant could affect the size of the etch pits. The Sopori etchant and the Secco etchant were utilized to etch a multi-crystalline silicon sample. Etch pits obtained with 60 s Secco etching have a size of 0.5~1 μm, while etch pits produced by Sopori have a size of 2 μm after etching for only 5 s [67].
The types of the as-revealed etch pits could vary with etchants. Some etchants are more suitable to reveal more kinds of etch pits. On a lateral epitaxial overgrowth GaN film, molten KOH revealed threading screw dislocations, while mixed HCl vapor revealed threading screw dislocations, mixed dislocations, and threading edge dislocations [68].
The nucleation and growth process of etch pits are closely related to the composition of etchants. After etching with a saturated calcite solution containing 1 mM EDTA, symmetric rhombic etch pits were observed on calcite crystals. When the etchant was changed to a flowing saturated calcite solution, etch pit nucleation and growth halted immediately. When the etchant was changed to an undersaturated calcite solution, the growth of etch pits reinitiated, and the shape of etch pits gradually changed to an asymmetric rhombus [69].
The morphology of etch pits can be affected by adding other chemicals into the etchant. After the addition of SO42− and S2O32− ions in the 1 M NaCl solution, macro etch pits with rough walls appeared, and after the addition of HSO4− ions in the 1 M NaCl solution, strip-like etch pits appeared on the Al substrate [70]. Conventional KOH etchants are not applicable to SiC with high electron concentrations, while the addition of Na2O2 to the etchant is effective in revealing threading screw dislocations, threading edge dislocations, and basal plane dislocations [71]. The addition of sulfate ions to HCl leads to the passivation of the existing etch pits and results in a more uniform etch pit structure on the aluminum foil after alternating current etching [72].
The relative content of chemicals in an etchant can affect etch pits. On GaN, pure H3PO4 results in stripe-like etch pits; a mixture that contains H3PO4 and H2SO4 results in etch pits composed of two long {10 1 ¯ 0} facets, and pure H2SO4 results in rhombus-like etch pits [73]. On the (100) surface of InAs, the etchant contains HCl, H2SO4, and H2O with a volume ratio of 10:m:1 (0 ≤ m ≤ 20). When m = 0, elongated hexagonal etch pits with a long axis parallel to the [1 1 ¯ 0] direction were observed; as m increased from 1 to 10, the shape of the etch pits changed to a quasi-rectangle, and when m = 17, rectangular etch pits were observed [74].

4.1.2. Effect on the Density of Etch Pits

The density of dislocations revealed by different etchants can be different. This is related to the fact that some etchants can reveal more kinds of dislocations than other etchants [75,76]. On a GaN film, etch pit densities revealed by molten KOH and a mixture of H3PO4 and H2SO4 were 4 × 107/cm2 and 5 × 108/cm2, respectively [77].

4.1.3. Effect on the Etching Rate

The etching rate could be affected by the composition of etchants [78]. The etching rate of Ga-polar on GaN crystals was 30 nm/min with H3PO4 as the etchant, while the etch rate dropped to 8 nm/min when KOH was used as the etchant [79].

4.2. The Etching Time

As the etching time increases, the size, depth, and shape of etch pits can be changed. Meanwhile, the density of etch pits can also be affected.

4.2.1. Effect on the Morphology of Etch Pits

Generally, etch pits will enlarge and deepen with increasing etching time due to the continuous dissolution of atoms on the exposed surface of the substrate, and neighboring etching pits will merge into a larger one after a period of etching [80,81]. On an organic single crystal of 2-amino 4,6-dimethyl pyrimidine 4-nitrophenol, the size of the etch pits enlarged with increasing etching time, and then the etch pits overlapped [82]. On single crystals of stearic acid, small flat-bottomed etch pits at an etching time of 5 s became large flat-bottomed etch pits at 13 s, while pyramidal etch pits became deeper and larger, and the distance between the pyramidal etch pits varied because of the inclination of dislocations [19].
The shape of the etch pits also varies with the etching time [83,84]. On the (111) face of a diamond etched with molten NaNO3, after 40 min of etching, the conical steep-sided etch pits became flat-bottomed [85]. On the (100) face of Al under an anodic pulse current in a mixture of HCl and H2SO4, only circular etch pits were observed before reaching the steady-state potential, while after dissolution at the steady-state potential, circular etch pits transformed into square etch pits [86].
Molten potassium was chosen to etch a diamond at 600 °C, and the exposed surface was determined to be {100} [87]. The etch pit size gradually increased with increasing etching time. Initially, shallow, square etch pits were observed. When the etching time reached 1 h, the corners of some etch pits were blunted. When the etching time reached 1.5 h, octagon-shaped etch pits were observed, as shown in Figure 4.

4.2.2. Effect on the Density of Etch Pits

The density of etch pits can increase with increasing etching time. On 4H-SiC crystals etched with KOH, as the etching time increased, etch pits increased in number, and the original etch pits increased in size, while the increased number of etch pits could be related to the fact that more dislocations were revealed gradually as some buried dislocations emerged at the continuously lowering surface [88]. On the CR-39 surface etched with a NaOH solution, etch pits could hardly be observed when the etching time was shorter than 45 min, and the etch pit density increased slightly and then saturated with the etching time; a longer etching time resulted in over-etched pits [89].

4.3. The Etching Temperature

The size, depth, shape, and density of etch pits can be affected by the etching temperature as well [90]. Hence, the etching temperature should be confined in a reasonable region to obtain well-defined etch pits.
The morphology of the etch pits formed on crystals can be affected by the etching temperature. On ScN crystals etched with a eutectic mixture of KOH and NaOH and then rinsed in HCl solution for 5 min, it was observed that after etching at 220 °C for 10 min, hexagonal and distorted square etch pits were observed; after etching at 230~240 °C for 10 min, octagon-shaped etch pits were observed; after etching at 330 °C for 5 min, square-shaped etch pits were apparent [91]. On ZnO, well-defined hexagonal etch pits were observed on the oxygen terminated face after etching at 1050 °C for 4 h in air; when the etching temperature increased to 1100 °C, gutter-shaped hollows were observed; after etching at 1150 °C, a distortion of atomic terraces emerged [92].
The etching temperature should be maintained in a reasonable region in the etching process, as under an etching temperature that is too low or too high, no etch pits could be observed. On the C-face (0001) 4H-SiC etched with chlorine trifluoride, at 573 K, no etch pit were observed, while at 623 K, 973 K, and 1073 K, etch pits were observed [93]. On the (001) face of GaAs etched with H2 plasma, when the etching temperature was 640 K, distinct etch pits with 20~50 nm diameters and 5~10 nm depths were observed; when the temperature reached 800 K, individual etch pits could hardly be seen [94]. On the {100} face of a diamond etched with molten potassium for 15 min, when the etching temperature was 600 °C, only some small and shallow etch pits were observed; when the etching temperature increased to 700 °C, large and deep etch pits with <100>-oriented edges were observed; when the etching temperature reached 800 °C, the outline of the diamond crystal was dissolved due to the high etching rate [87].

4.4. The Matrix

Etch pits preferentially initiate at sites with unevenly distributed atoms or stress. If the crystal is doped with other elements or the atom arrangement or stress distribution in the crystal is affected by factors, such as pretreatments and/or heat treatments, the morphology, distribution, or density of etch pits would be affected accordingly.
The size and shape of the etch pits and the etching rate can be affected by doping in crystals [95]. On the {111} faces of synthetic diamond crystals, the etching rate increased as the N impurity concentration increased from 1 ppm to 200 ppm, and then the etching rate subsequently decreased. Meanwhile, the etch pits with a low N content have a convex triangle shape, while etch pits on nitrogen-doped crystals were found to be regular triangular [96].
Some etching processes contain multiple steps, including a pretreatment process. The effect of chemical pretreatment of the Al substrate on the electrochemically etched surface of Al was investigated [97]. The results show that a two-step pretreatment with H3PO4 and H2SiF6 led to a high density of pre-etch pits on the Al surface. During the subsequent electrochemical etching, a high density of etch pits was observed.
Etch pits can be affected by the heat treatments of the matrix. As it is well known that heat treatment can affect dislocations in the matrix, the variation in the etch pits before and after heat treatment can be related to the variation in dislocations in the matrix. The etch pit variation before and after a heat treatment of a HgCdTe matrix was investigated [98]. After annealing for 4 cycles at 494 °C, there was a nonuniform reduction in the dislocation density identified by etch pit observation; meanwhile, the fish-shaped etch pits were completely absent.

4.5. Electrochemical Parameters

Etch pits can also be obtained by electrochemical etching; hence, electrochemical parameters also affect the etch pits on crystals. Anodic etching was carried out to obtain etch pits on GaN films [99]. When the electrolysis voltage was 50 V and 30 V, the surface was over-etched; when the voltage was 20 V, the etch pits were observed; when the voltage was 10 V, the etch pits could also be observed, but the density was lower. An Al substrate was etched in a HCl-H2SO4 solution with different current profiles including the direct current, anodic pulse, and square wave, only square waves produced cubic etch pits [100].

4.6. The Atmosphere

As the formation of etch pits is determined by the dissolution or chemical reactions of the surface atoms of crystals and the dissolution and chemical reactions can be affected by the chemical environment; hence, etch pits can also be affected by the chemical environment, including the atmosphere. Many gases are effective etchants to obtain etch pits on crystals.
Atomic hydrogen was utilized to etch epitaxial diamond (111) films [101]. After exposure to atomic hydrogen for 12 min at 500 °C, square pits with (111)-oriented facets were observed. After exposure to oxygen at 1 × 10−3 Torr for 1 min at room temperature and then exposure to atomic hydrogen at 500 °C for 5 min again, new etch pits were formed, and these new pits were inferred to be related to oxidation.
After exposure to ambient humidity, etch pits formed on potassium dihydrogen phosphate crystals with porous coatings [102]. The shapes of the etch pits were regular. At 10% RH, the etch pits were 0.5~1 μm in length; at 55% RH, the average length of the etch pits was 3.5 ± 0.5 μm; at 75% RH, the etch pits grew to 20 μm. When the RH increased from 10% to 75%, the etch pit density decreased by two orders of magnitude.

4.7. Other Factors

There are other factors that can affect the formation of etch pits on crystals, such as catalysts [103], magnetic fields [104], and illumination [105]. Catalysts can affect chemical reactions on the surface of crystals. The magnetic field can act on the ions in the liquid in the etch pit and affect the transport of reactants and products of chemical reactions. In the n-type material, light could not only speed up the reaction rate but also reveal the defects as crystallographic defects are effective sites for the recombination of photogenerated carriers [105].
A highly {110}-oriented diamond coating was etched with hydrogen gas with Fe, Co, Ni and Pt nanoparticles as catalysts [103]. The metal atoms were vacuum-evaporated onto the diamond coating and formed nanoparticles by themselves at an elevated temperature. The samples were kept in a flowing gas mixed with H2 (10%) and N2 (90%) between 700 °C and 1000 °C for a specific duration. Co was preferred by the etching process carried out at 800 °C for 2 h, as dense etch pits were observed. When Pt was utilized as the catalyst, it was difficult to identify the etch pits after etching at 900 °C.

5. Applications of Etch Pit Technology

In the beginning of the 19th century, etch pit technology had already been utilized to investigate crystals [106]. It is well known that the properties of a crystal and/or a multi-crystalline substrate are closely related to the intrinsic defects in the crystal and/or the multi-crystalline substrate. For example, the dislocation slip and the deformation twinning are the most common deformation mechanisms in coarse-grained materials. Dislocations can nucleate and slip under stress to accommodate the applied plastic deformation, while dislocation entanglement and accumulation will consequently lead to strain hardening [107]. As defects can be revealed by etch pits, it is meaningful to utilize etch pits to evaluate and even improve some key properties of the matrix, as shown in Figure 5.

5.1. Investigation of Dislocations

Due to the relationship between dislocations and etch pits, etch pit observation is frequently employed to characterize, evaluate, and count dislocations in crystals [18,31]. In the middle of the 20th century, researchers had already revealed dislocations with etch pits deliberately [108].
The distribution of dislocations can be revealed by etch pits [109,110]. Polishing and etching could be carried out several times to trace the dislocation lines at different depths in an identical specimen [111]. As a relatively larger area can be etched with etchants, the distribution of dislocations can be identified in a relatively macroscopic manner [112] compared to those revealed by transmission electron microscope.
Etch pit observation can be utilized to investigate an individual dislocation or compare dislocations in different substrates. On the (111) face of CdZnTe, the orientations of dislocations related to the triangular etch pits were determined by measuring the depths of the etch pits, the orientations of the etch pit tips, and the side lengths of the triangular etch pits [113]. The density of non-basal dislocations on the (001) face of anthracene was found to be larger in an annealed crystal than in an unannealed crystal [114].

5.2. Reduction of Dislocations

As etch pits have already revealed many emergences of dislocations, in the following growth process of a new crystal upon a pre-etched crystal, it is important to confine the extension of the as-revealed dislocations, so that a low dislocation density crystal can be obtained.
Pits can be utilized to reduce the dislocation density in a newly formed crystal; as in the growing process of crystals on pits, dislocations tend to merge [115]. A recess-patterned sapphire substrate and a protruding sapphire substrate were prepared utilizing a SiO2 mask [116], and the recess-patterned sapphire substrate was proven to be better since the density of dislocations was relatively smaller, as shown in Figure 6.
Then, as a typical kind of pit, etch pits were utilized directly to reduce dislocations [117]. On a GaN substrate with several large and deep etch pits, a new bulk GaN with low dislocation density was obtained by hydride vapor phase epitaxy [118]. A Ge film with a threading dislocation density of 2.6 × 108 cm−2 was etched to produce etch pits, and then a 15 nm thick SiO2 film was utilized to cover the etch pits. The subsequent growth resulted in a Ge epilayer with a threading dislocation density of 1.7 × 106 cm−2 [119].

5.3. Count of Precipitates

The areal defect densities caused by the precipitates on the surface of CdZnTe could be measured by etch pit observation [120]. Etch pits caused by the Te-rich precipitate and Cd-rich precipitate were different. The former etch pits were crater-like, and the latter etch pits were always enwrapped by many other etch pits. The areal densities obtained by etch pit observation were consistent with those obtained by IR transmission microscopy, while the etch pit observation method saves time, and only one side of the specimen needs to be polished.

5.4. Polytype Identification

Polytypes of SiC could be identified by etch pit observation [121]. SiC crystals with perfect hexagonal etch pits were 6H-SiC along the (0001) direction, and the SiC crystals with three long-edge and three short-edge hexagonal etch pits were 15R-SiC along the (0001) direction. This relationship was verified by the Raman spectra.

5.5. Polarity Detemination

The polarity of InN can be identified by etch pit observation [122]. After etching with KOH, hexagonal pyramids surrounded by {10 1 ¯ 1 ¯ } facets were observed on the N-polar surface, while hexagonal and dot-shaped etch pits were observed on the In-polar surface. Meanwhile, the dislocation density identified by the etch pit density on the In-polar surface almost corresponded to the threading dislocation density determined by transmission electron microscopy.

5.6. Detection of Leakage Current

The relationship between crystal defects and leakage current can be determined via etch pit observation. On β-Ga2O3 Schottky barrier diodes, dislocations and voids were revealed by etch pits after etching, and a comparison between the leakage current and the crystal defects indicated that dislocations along [010] acted as paths for the leakage current [123]. In a GaN-based light-emitting device, the origin of the leakage current was also investigated by etch pit observation, and the leakage current was attributed to open core dislocations [124].

6. Trends Related to Etch Pit Technology

6.1. Characterization of Etch Pits

Usually, etch pits are characterized by an optical microscope, a scanning electron microscope, or an atomic force microscope, and these etch pits are usually analyzed by manually interpreting the photographs of etch pits. To improve the efficiency of the interpretation process, new methods have been developed.
The etch pit density can be counted and/or analyzed utilizing a software [125]. Kanik Palodhi et al. [126] utilized an image processing technique based on convolution to analyze etch pits on nuclear track detectors, and this approach substantially sped up the task of track identification and analysis. M. Wzorek et al. [127] proposed a method that implements a shape-from-shading method to estimate the depths of etch pits by analyzing the image brightness dependence on the slopes of etch pits, and the depth distribution revealed etch pits attributed to different kinds of dislocations in a GaN substrate. The ratio of different kinds of dislocations estimated by this method is comparable to that obtained by transmission electron microscopy.
Furthermore, a dissolution process can be measured by the phase shifting interferometry (PSI) approach. A.E.S. Van Driessche et al. utilized a white beam PSI [128] to characterize the variation in the morphology of etch pits on a gypsum crystal [129].
As the size, morphology, and density of etch pits vary with etching time, it is important to characterize the etch pits in-situ to further understand the growth process of the etch pits. Fuminobu Sato et al. [130] recorded time-lapse images of etch pits formed on a nuclear track detector by an in-situ observation system, and then the pit evolution images were constructed by digital image processing of the time-lapse images.

6.2. Simulation of Disolution

As it is difficult to characterize the initiation and expansion of an etch pit in-situ on the atomic scale, it is critical to simulate these processes for a deeper understanding. Inna Kurganskaya et al. [131] utilized a stochastic Kinetic Monte Carlo approach to describe the dissolution of phyllosilicates and predicted the surface evolution from the kinetics of elementary surface reactions and found that the dissolution of minerals proceeds via the formation of etch pits at opened screw dislocation cores through movement and coalescence of step waves. Konstanze Stübner et al. [132] simulated the evolution of etch pits with an atomistic Monte Carlo simulation, and the results suggested that there are four stages in the evolution of etch pits on the (001) surface of an idealized regular lattice. During the first stage, an etch pit was an inverted pyramid. Then, the pyramid apex was truncated in the second stage. During the third stage, an etch pit consisted of a single concave up bottom plane. During the fourth stage, it was inferred that the etch pits would shrink until they disappeared altogether.

6.3. Etching on Macro Patterns

Usually, to statistically obtain the distribution of etch pits or other defects on an exposed surface, a sufficiently large surface area should be etched. It is also possible to etch partial areas or patterned areas of an exposed surface of a crystal for some purposes. Sachiko Ono et al. [133] utilized a honeycomb Al2O3 mask to directly control the initial sites of etch pits; hence, the distribution, density, and homogeneity of the etch pits were adjusted. Dmitri A Brevnov et al. [134] utilized Al2O3 as the etching mask and carried out electrochemical etching on Al (100) foils. Then, crystallographic cubic etch pits were observed at the unmasked area, and the Al substrate was preserved at the masked region.

6.4. Self-Assemble of Particles at Etch Pits

An etch pit could be a container for other materials. Because of the regular shape of etch pits, extrinsic substances could gather at positions preferentially in the etch pits. Hung-Ming Chen et al. [135] investigated the effect of etch pits on the alignment of Ge dots grown on a pit-patterned Si (001) substrate. When the spacing between the etch pits was less than 100 nm and the depth of the etch pits was greater than 60 nm, the topology and the surface chemical distribution were favored by Ge dots to nucleate inside the pits and completely align with each other.

6.5. Crystal Growth at Etch Pits

A dislocation can be revealed by an etch pit, and the as-revealed dislocation is a shortcut for some kinds of atoms to transport. The aggregation of the transported atoms in the etch pits can result in a new crystal. Che-Ming Liu et al. [136] pointed out that etch pits are preferred sites for crystal growth. One side of a sapphire crystal was etched with Na2B4O7, and etch pits were obtained. Then, a Mg film was deposited on the unpolished surface. After heating and aging of the specimen, precipitation crystals were found at the etch pits of the sapphire as shown in Figure 7. These newly formed crystals are Mg-Al spinel microcrystals.

7. Summary

To conclude, the following remarks can be drawn:
Regular-shaped etch pits result from the anisotropic dissolution of atoms on the exposed surface of a crystal. Planes with the lowest etch rate are prone to define the shape of the etch pits. The shape of the etch pits is closely related to the orientation of the basal plane as well as the crystalline structure of the matrix. The polarity of the basal plane is another factor that affects the shape of the etch pits.
Etch pits tend to emerge at defects, such as dislocations, that break the continuity of the crystalline structure locally. Generally, an etch pit originating from a screw dislocation would be larger than an etch pit originating from an edge dislocation on the same basal plane. Furthermore, other defects, such as voids, inclusions, and nano-pipes, could also be preferred sites for etch pits to initiate.
The morphology, size, and density of etch pits can be affected by factors including etchant, etching time, etching temperature, status of the matrix, and so on. It is crucial to find a suitable etchant to fabricate well-defined etch pits on crystals at a moderate dissolution rate. All the factors that can affect the chemical reactions or transport between the surface atoms of the crystals and the etchant inside or near the etch pit have the potential to change the morphology, density, expansion, combination, or existence of the etch pits.
According to the relationship between defects and etch pits, the defect density and/or defect-related properties can be evaluated by etch pit observation. It is easier to determine the distribution of dislocations on a crystal surface by the etch pit observation method, which facilitates the investigation of dislocation-related properties. Furthermore, the extension of dislocations in a growth process of the pre-etched crystal can be inhibited by etch pits. By taking advantage of the special morphology of etch pits, in a subsequent growth process utilizing the pre-etched crystal as a seed, dislocations in an etch pit tend to merge, and a new crystal with a lower dislocation density can be fabricated.
Image processing technologies were utilized to improve the efficiency of the identification of etch pits. To thoroughly understand the initiation and growth of etch pits, in-situ observations were carried out, while the results related to atom-level in-situ observations are still lacking. Simulations were also proven to be useful for understanding the mechanisms of dissolution and the formation process of etch pits.
The interaction of etch pits and defects can be utilized to fabricate new crystals or obtain well-aligned nano- or microparticles. The as-revealed dislocation could be a channel. Heterogeneous atoms can be transported through the dislocations into the etch pits, and the precipitation and aggregation of these heterogeneous atoms would lead to a new crystal. Meanwhile, the shape of the newly grown crystal fits the regular-shaped etch pit.

8. Outlook

Etch pit observation is traditional; to characterize a commonly formed etch pit, only an optical microscope is needed, while the formation mechanisms, influencing factors, and skillful utilization methods of etch pits are still under investigation.
To study etch pits more precisely and effectively, new methods related to atomic characterization, numerical simulation and post possessing techniques for etch pit images should be developed. Meanwhile, the relationship between defects and etch pits should be understood more deeply and etch pits can be utilized more ingeniously.
  • Atomic Characterization
As dislocations are preferential sites for the formation of etch pits and the emergence of a threading edge dislocation on the surface is on the atomic scale, it is critical to characterize the formation process of an etch pit on the site on the atomic scale. The substrate and etchant should be chosen carefully to facilitate the characterization. Gaseous phases may be chosen as the etchants.
  • In-situ Characterization
It has been demonstrated that the shape of some etch pits may change as the etching time is prolonged; therefore, in-situ characterization is needed. In addition to the shape of etch pits, the chemical composition of the solution in the etch pit and the status of the exposed surface of the etch pit, etc., should also be characterized to analyze the influencing factors of etch pits.
  • Dissolution Mechanism
The formation of etch pits could be affected by the etchant. Chemical reactions occurring on facets of etch pits should be clarified further. As the chemical composition of the solution in etch pits could change due to the formation of reaction products, the thermal dynamic and kinetic processes should be investigated. This would be helpful for the selection and optimization of effective etchants.
  • Numerical Simulation
Numerical simulations on the atomic and/or molecular scale are needed to better understand the initiation, expansion, and variation of the etch pits. For example, some etchants can only reveal some kinds of dislocations, and what is the limiting factor by which a kind of dislocation can be revealed by a particular etchant? How do the molecules in the etchant act on the atoms on facets of etch pits?
  • Etch Pit Cluster
It is well known that metals and alloys are easily corroded in corrosive environments, such as marine atmospheres. After corrosion, corrosion pits are frequently found on the matrix. Could these corrosion pits originate from the etch pits on defects? The multiplication and combination mechanism of etch pits should be clarified. The synergetic actions of a series of etch pits or clusters of etch pits in a larger pit should be investigated to further understand the properties and degradation of the matrix.
  • Micro- and Nanofabrication
Given the relationship between dislocations and etch pits, new methods were proposed to reduce the dislocation density to fabricate high-quality crystals, which is preferred in the electronic industry. Meanwhile, tiny regular-shaped crystals can be fabricated in etch pits, which provides a new method for micro- and nanocrystal fabrication. New methods are continuously emerging in fabrication industries, and some of them have the possibility to be combined with etch pit technology. Along with the development of precise fabrication and nanostructure characterization, etch pits will exhibit more potential in the future.

Author Contributions

Conceptualization, formal analysis, original draft preparation, writing, D.L.; review and editing, D.L., Q.J., X.M., Q.Z., X.F. and L.F.; resources, D.L. and L.F. All authors have read and agreed to the published version of the manuscript.

Funding

This research was supported by the National Natural Science Foundation of China for Exploring Key Scientific Instruments [No. 41827805], the Wenhai Program of the S&T Fund of Shandong Province for Pilot National Laboratory for Marine Science and Technology (Qingdao) [No. 2021WHZZB2302], the Guangxi Key Laboratory of Marine Environmental Science, Guangxi Academy of Sciences [No. GXKLHY22-01], the Nantong Science and Technology Development Funds, China [No. JC2021022], and the Central Government Guiding Funds for Local Science and Technology Development, China [No. YDZX20203700002831].

Acknowledgments

The authors would like to thank Baorong Hou, Yanliang Huang and all the anonymous reviewers for their kind support and useful advice.

Conflicts of Interest

The authors declare no conflict of interests.

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Figure 1. Schematic diagram of isotropic dissolution and anisotropic dissolution.
Figure 1. Schematic diagram of isotropic dissolution and anisotropic dissolution.
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Figure 2. SEM images of the etch pits for different orientations of single-crystal Ge. (a–c) Morphology of the anisotropically etched surface. Top ((d–f)) and side ((g–i)) views of SEM images showing a single etch pit on etch orientation. (a,d,g) Ge (100) crystal plane. (b,e,h) Ge (111) crystal plane. (c,f,i) Ge (110) crystal plane [41]. Reproduced with permission from American Vacuum Society.
Figure 2. SEM images of the etch pits for different orientations of single-crystal Ge. (a–c) Morphology of the anisotropically etched surface. Top ((d–f)) and side ((g–i)) views of SEM images showing a single etch pit on etch orientation. (a,d,g) Ge (100) crystal plane. (b,e,h) Ge (111) crystal plane. (c,f,i) Ge (110) crystal plane [41]. Reproduced with permission from American Vacuum Society.
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Figure 3. (a) Scanning ion microscopy (SIM) image of an etch pit and marker fabricated along the bottom located at the center of the etch pit for TEM sample preparation. (b) Cross-sectional TEM image (bright field) of an etch pit taken along the <110> zone axis. The inset is the TED pattern used for imaging. (c) Plan-view SEM image of etch pits formed at 900 °C for 6 h by hydrogen plasma [54]. Reproduced with permission from Elsevier.
Figure 3. (a) Scanning ion microscopy (SIM) image of an etch pit and marker fabricated along the bottom located at the center of the etch pit for TEM sample preparation. (b) Cross-sectional TEM image (bright field) of an etch pit taken along the <110> zone axis. The inset is the TED pattern used for imaging. (c) Plan-view SEM image of etch pits formed at 900 °C for 6 h by hydrogen plasma [54]. Reproduced with permission from Elsevier.
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Figure 4. Morphology of the diamond {100} face after etching at 600 °C for different times: (a) 30 min; (b) 1 h; and (c) 1.5 h [87]. Reproduced with permission from Elsevier.
Figure 4. Morphology of the diamond {100} face after etching at 600 °C for different times: (a) 30 min; (b) 1 h; and (c) 1.5 h [87]. Reproduced with permission from Elsevier.
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Figure 5. Schematic diagram of the relationship of etch pits, defects, and properties.
Figure 5. Schematic diagram of the relationship of etch pits, defects, and properties.
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Figure 6. Reduction in dislocations by pits (a) and the combination of pits and an oxide mask (b) [116]. Reproduced with permission from Elsevier.
Figure 6. Reduction in dislocations by pits (a) and the combination of pits and an oxide mask (b) [116]. Reproduced with permission from Elsevier.
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Figure 7. Schematic diagram of the fabrication process (a) and the as-fabricated spinels (b) [136]. Reproduced with permission from AIP Publishing.
Figure 7. Schematic diagram of the fabrication process (a) and the as-fabricated spinels (b) [136]. Reproduced with permission from AIP Publishing.
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Lu, D.; Jiang, Q.; Ma, X.; Zhang, Q.; Fu, X.; Fan, L. Defect-Related Etch Pits on Crystals and Their Utilization. Crystals 2022, 12, 1549. https://doi.org/10.3390/cryst12111549

AMA Style

Lu D, Jiang Q, Ma X, Zhang Q, Fu X, Fan L. Defect-Related Etch Pits on Crystals and Their Utilization. Crystals. 2022; 12(11):1549. https://doi.org/10.3390/cryst12111549

Chicago/Turabian Style

Lu, Dongzhu, Quantong Jiang, Xiumin Ma, Qichao Zhang, Xiaole Fu, and Liang Fan. 2022. "Defect-Related Etch Pits on Crystals and Their Utilization" Crystals 12, no. 11: 1549. https://doi.org/10.3390/cryst12111549

APA Style

Lu, D., Jiang, Q., Ma, X., Zhang, Q., Fu, X., & Fan, L. (2022). Defect-Related Etch Pits on Crystals and Their Utilization. Crystals, 12(11), 1549. https://doi.org/10.3390/cryst12111549

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