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Binary and Multinary II–VI Compounds (CdTe, CdZnTe, CdMnTe) Grown from the Melt
Binary and Multinary II–VI Compounds (CdTe, CdZnTe, CdMnTe) Grown from the Melt$ HL Glass, Honeywell Electronic Materials, Spokane, WA, USA K Strzałkowski, Nicolaus Copernicus University, Torun, Poland r 2016 Elsevier Inc. All rights reserved.
1 2 2.1 2.2 2.3 2.4 2.5 2.6 3 3.1 3.2 3.3 3.4 3.5 4 References
Review of Growth Methods Detailed Descriptions of Bridgman Methods Preparation of Source Materials Choice of Containers Reaction of Elements to Produce Compounds Vertical Bridgman Horizontal Bridgman High Pressure Bridgman Control of Properties (During or After Growth) Purity Stoichiometry Second Phases Crystal Defects Doping Concluding Remarks
1 2 3 3 3 5 6 7 7 7 7 8 9 9 9 10
CdTe and CdZnTe (Cd1–xZnxTe) in bulk crystal form are used as X-ray and gamma-ray detectors, in electrooptic and photorefractive devices and as substrates for epitaxy, especially epitaxial HgCdTe for infrared detector arrays. Cd1–xZnxTe crystals for composition of the order of 10–20 atomic percentage have become one of the most studied material for g-ray detection application (Schlesinger et al., 2001; Bolotnikov et al., 2002; Sordo et al., 2009; Swain et al., 2014; Choi, 2015) and also spectroscopic X-ray imaging (Szeles et al., 2008). Their potential arises from few facts: (1) both, cadmium and telluride possess high atomic numbers; (2) the bandgap is large enough for high resistivity and low leakage current, but on the other hand is small enough so that the electron hole ionization energy is small; (3) high quantum efficiency; and (4) it is possible now to grow good-quality large crystals. Substitution of manganese for part of the cadmium in CdTe matrix produces one of the most widely studied diluted (semimagnetic) semiconductors. Apart from magnetic properties, CdMnTe compound is also considered as material for X-ray and gamma-ray detectors (Zhang et al., 2007; Rafiei et al., 2013; Hossain et al., 2015). Large crystals of all these compositions are commonly produced by various versions of growth from the melt.
1
Review of Growth Methods
CdTe and related compounds have been grown from the melt and from high-temperature solutions. As is readily seen from the phase diagram in Figure 1, CdTe can be grown from liquid compositions spanning a broad range from cadmium-rich to telluriumrich and including the congruently melting composition, which deviates slightly from perfect stoichiometry. In common usage, melt growth involves liquid compositions near stoichiometry for which solidification occurs close to the congruent melting temperature, 1092 1C. Solution growth is generally carried out from liquids far off stoichiometry, allowing crystallization at substantially lower temperatures, for example, 800 1C. There are several trade-offs between melt and solution growth. Melt growth offers higher growth rates and larger crystals. However, at the high temperatures used, it is difficult to maintain purity. Also, control of the growth atmosphere is crucial to regulate stoichiometry and avoid second phases. Solution growth avoids some difficulties of melt growth, but solvent inclusions may limit quality. ☆
Change History: June 2015. K. Strzałkowski made the following changes: a paragraph was added to the introduction section, a paragraph was added to the end of Section 1, some sentences were included in Sections 2.1 and 2.2, a small paragraph was added to Section 3.5, ‘concluding remarks’ section was completely rewritten, and 18 new references were added throughout the manuscript.
Reference Module in Materials Science and Materials Engineering
doi:10.1016/B978-0-12-803581-8.02395-X
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Binary and Multinary II–VI Compounds (CdTe, CdZnTe, CdMnTe) Grown from the Melt
Figure 1 CdTe phase diagram. Near-solidus region is given on an enlarged scale (after Greenberg, 1996).
The Czochralski method of pulling from the melt is preferred for many semiconductors. For CdTe the high vapor pressure of cadmium necessitates special measures (Mullin and Straughan, 1977) such as the liquid encapsulated Czochralski (LEC) method. While suitable for several other compound semiconductors, LEC has not truly succeeded for CdTe. Some work has been reported on growth of CdTe by the floating zone method and by use of a zone refining apparatus. The most frequently used methods for melt growth of CdTe and related compounds are variations of the Bridgman method. These include both vertical (VB) and horizontal (HB) arrangements, either with or without control of the (usually cadmium) vapor pressure. Other variations include vertical and horizontal gradient freeze (VGF and HGF) and the heat exchanger method (HEM). Typically, total pressure at the melting point is in the vicinity of 0.1 MPa. Fully single crystal wafers up to 30 cm2 in area are produced by these Bridgman methods and are commercially available for use as infrared substrates. A high-pressure Bridgman (HPB) method is also used, especially for growth of high resistivity CdZnTe for gamma-ray detectors, although single crystal size is smaller. Solution growth, which will not be discussed in detail, may use a Bridgman arrangement for directional solidification of a highly nonstoichiometric melt. Alternatively, in the traveling solvent (TSM) or traveling heater methods (THM) (Ohmori et al., 1993; El Mokri et al., 1994; Hage-Ali and Siffert, 1995; Chen et al., 2008), the feed material is a solid rod. A heating coil melts a liquid zone, which is traversed along the length of the rod. Among others, vertical High-Pressure Electro-Dynamic Gradient (HP EDG) crystal growth system was proposed (Szeles et al., 2002). This technology improves the CdZnTe detector fabrication yield compared to other methods, mainly due to reduction of thermal and mechanical stress in the ingots, improved yield and electron transport properties. The multi-zone heater system allows dynamic control of heat flow which helps to optimize the shape of the growth interface. Thanks to that it is possible to growth large in diameter ingots (140 mm) of CdZnTe crystals almost free of cracking. The solution growth methods have been used particularly for nuclear detector applications, where control of stoichiometry, purity, and doping is more important than large size.
2
Detailed Descriptions of Bridgman Methods
As outlined in Section 1, the most commonly used methods for crystal growth of CdTe and related compositions are versions of the Bridgman methods.
Binary and Multinary II–VI Compounds (CdTe, CdZnTe, CdMnTe) Grown from the Melt 2.1
3
Preparation of Source Materials
Cadmium, zinc, and tellurium melt at relatively low temperatures, which simplifies their purification. Once a reasonably high degree of purity has been obtained, multiple pass zone refining, typically in hydrogen, is carried out. Some impurities, notably copper in cadmium, cannot be separated adequately by this method (Zanio 1978, pp. 38–51; Kuchar et al., 1996). A process that combines in situ distillation with zone refining reduces copper to below detection limits of a few ppba (parts per billion atomic) and total metallic impurities, excluding zinc and tellurium, to less than 30 ppba (Bollong et al., 1995). Similar purity levels are achieved in zinc and tellurium. These are significant improvements over the 99.9999% purity levels that were the best commercially available in the 1990s. Prior to use, cadmium and zinc are typically etched in dilute nitric acid to remove surface contamination, then thoroughly rinsed and dried. Zone refined ingots of tellurium tend to have voids and cracks, which make etching and rinsing problematical. Also, tellurium is brittle and tends to splinter. Casting tellurium under hydrogen to form a slug surmounts these difficulties and may reduce oxides. The dense slug simplifies loading into the crystal growth container or reaction vessel. Since cleaning of tellurium is somewhat uncertain, avoiding exposure to air during storage is recommended to minimize oxide formation. Unlike cadmium, zinc, and tellurium, manganese has a high melting temperature. It is difficult to purify manganese to the same level as the other elements. A purity of 99.99% with respect to metallic impurities is about the best currently available. Oxide formation is a major problem. Sublimation in a dynamic vacuum at 1000 1C has been found effective to purify manganese for crystal growth (Giriat and Furdyna, 1988). Crystal growth can start with the elements. Alternatively, the elements may first be reacted to form the binary compounds (see Section 2.3) and these used as source materials. Another option is commercially available pure CdTe powder with purity of 99.9999%. When the binaries are used, sublimation is often performed for purification (Kyle, 1971). Prior to sealing the carboncoated quartz (silica) sublimation tube, the binary may be heated under hydrogen to remove oxides. Oxides can react with the carbon coating and the quartz walls, leading to ampoule failure, sticking of the compound to the wall and contamination. A vacuum of 1 108 torr is ordinarily attained prior to sealing. Typical sublimation temperature is 900 1C. While sublimation may not be needed for purification when state-of-the-art high purity elements are used, it probably provides reproducible stoichiometry. This may be beneficial if vapor pressure is not controlled during crystal growth (see Sections 2.4, 2.5, and 3.2).
2.2
Choice of Containers
Obviously containers must withstand the physical and chemical environment of the crystal growth process and not introduce significant impurities. If possible, the container should be a barrier to impurities emerging from furnace components (Triboulet et al., 1995). Most crystal growth of CdTe and related compounds has used quartz (silica) containers, nearly always coated with carbon. Procedures for preparing quartz and depositing carbon pyrolytically are well known. Cleaning typically includes washing, etching in a mixture containing HF, thorough rinsing, and a high temperature bake in a clean vacuum. High purity synthetic quartz, specially developed for electronic applications, is strongly recommended. Carbon coating is accomplished by heating the open tube while organic gas or vapor flows through it. Other coatings, such as boron nitride (Shetty et al., 1995) have been used. The primary function of the coating is to avoid sticking or reaction between the charge and container. Coatings may be effective as barriers to impurities. The quartz container may function as a sealed vacuum ampoule. Alternatively, a separate crucible may be used in the ampoule. There is evidence that dislocation density is lower when growth is in pyrolytic boron nitride crucibles rather than carbon-coated quartz (Glass et al., 1998a). Graphite containers are commonly used in the high pressure and horizontal Bridgman (HB) processes. Recommended source material for crucibles is high density high ordered pyrolytic graphite (HOPG). One reason for choosing graphite is that quartz may be a source of impurities (Kyle, 1971; Butler et al., 1993). Another one is case when higher melting temperature is needed, where quartz containers may be additional source of impurities. In the commonly used vertical Bridgman (VB) method, the container is an axially symmetric cylinder, usually with a slight taper to facilitate removal of the crystal at the end of the process. The same purpose is applying crucibles composed from two parts. Although specially designed tips are often used in Bridgman methods, containers for CdTe frequently have flat, rounded, or shallow conical bottoms. Sometimes there is a well to hold a seed crystal.
2.3
Reaction of Elements to Produce Compounds
Reaction of the elements to form the compounds may be done as the initial part of the crystal growth process or as a separate step. Major concerns are to maintain purity and keep the highly exothermic reactions from proceeding too rapidly, which could produce high vapor pressure. The reaction furnace should be shielded in case of explosion. Separate reaction is usually performed in carboncoated, vacuum-sealed, high-purity quartz ampoules (see Sections 2.1 and 2.2). There are two commonly used procedures. The more common is simply to seal weighed amounts of the elements into the
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Binary and Multinary II–VI Compounds (CdTe, CdZnTe, CdMnTe) Grown from the Melt
ampoule. The weights may be stoichiometric; however, a small excess of tellurium is often used to avoid the possibility of high pressures associated with excess cadmium (Zanio 1978, p. 12). The charge is heated slowly to a temperature sufficient to start the reaction, B800 1C. After reaction is complete, either sublimation is carried out (see Section 2.2) or the temperature is raised above the melting point and held to allow the melt to homogenize. Sometimes the furnace is rocked to facilitate homogenization before cooling to room temperature. The second procedure for synthesizing the compound uses vapor transport of the more volatile cadmium. The tellurium and somewhat more than the stoichiometrically required amount of cadmium, are loaded into separate compartments in a quartz ampoule. The elements are kept apart except for vapor transport. Figures 2 and 3 show crystal growth furnaces that can be used for this purpose. The ampoule is slowly heated, so the cadmium vapors react with the molten tellurium. The cadmium end of the ampoule is heated only enough to achieve cadmium vapor pressure of roughly 0.1–0.2 MPa, B800 1C. The tellurium end is heated above the melting point of the compound, 1092 1C for CdTe.
Figure 2 Schematic of vertical Bridgman growth, showing three-zone furnace and ampoule with cadmium reservoir at the top.
The excess cadmium serves as a reservoir to maintain a desired cadmium vapor pressure. This regulates stoichiometry and prevents sublimation of the compound to the cold region of the ampoule (see Section 3.2; Zanio 1978, p. 7). When CdZnTe is to be synthesized, the zinc may be included with the cadmium for vapor transport to the tellurium. Dopants or substituents of lower volatility, such as manganese, are included with the tellurium.
Binary and Multinary II–VI Compounds (CdTe, CdZnTe, CdMnTe) Grown from the Melt
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Figure 3 Schematic of horizontal Bridgman growth, showing three-zone furnace and separate boat for cadmium source.
If the reaction process includes melting the compound, additional purification can be accomplished by directional solidification followed by discarding of the last-to-freeze end of the ingot, where impurities tend to segregate.
2.4
Vertical Bridgman
The apparatus for VB growth can be as simple as a single zone furnace, or a pair of furnaces constituting the hot and cold zones, plus a mechanism to translate the ampoule or furnace. Translation is not used in the vertical gradient freeze (VGF) technique. More sophisticated furnace designs may include a third furnace for control of vapor pressure, multiple zone furnaces to adjust temperature gradients, and heat pipes to maintain regions at uniform temperature (Kyle, 1971; Sen et al., 1988; Yasuda et al., 1990; Casagrande et al., 1993; Rudolph and Mühlberg, 1993). The reservoir for controlling vapor pressure may be located above (Kyle, 1971; Sen et al., 1988) or below (Rudolph et al., 1994; Asahi et al., 1996) the growth zones. Further discussion of vapor pressure control is found in Section 3. Mixing in the melt may be enhanced by the accelerated crucible rotation technique (ACRT) (Capper et al., 1993, 1996) or vibration (Lu et al., 1990). Slow rotation of the ampoule is sometimes used to compensate thermal asymmetries. The furnaces used in VB methods are rather conventional since the melting point is only about 1100 1C. Furnaces should be stable to 70.2 1C over long time periods (Sen et al., 1990; Asahi et al., 1996). The furnace liner tube may be ceramic or quartz (silica), but should be high purity. Figure 2 is a schematic representation of a simple three-zone furnace with an ampoule containing a cadmium reservoir at the top. If the reservoir is large enough to contain the full quantity of cadmium, then reaction by vapor transport can be effected prior to growth. Once reaction to form the compound has been completed (see Section 2.3), the growth sequence begins with heating the charge to above the melting point and holding, generally several hours, to establish a homogeneous melt. There is evidence that the melt should be heated at least 10 1C above the melting point to break up atomic associations in the liquid (Rudolph and Mühlberg, 1993); however, this requirement may be relaxed if the melt deviates from stoichiometry (Mühlberg et al., 1993). After establishing a uniform melt, the furnace is set to the temperature profile for growth and allowed to equilibrate. Growth is initiated by starting the translation or the gradient freeze program. There is some evidence that an initial rapid translation rate, followed by a pause, improves the chance of obtaining a single dominant grain after spontaneous nucleation (Kuppurao et al., 1996). Once the entire melt has been solidified, the furnace is cooled to room temperature. The ingot obtained typically has a thin surface coating of cadmium (or tellurium) that is removed by etching or sandblasting. There is a lack of agreement about optimum temperature distributions. The divergence of views may be related to several factors, including difficulties in measuring temperatures accurately during growth. Three temperature parameters typically must be specified: the hot and cold zone temperatures and the axial gradient in the region between these zones. The hot zone may be maintained at 1140 1C or 1150 1C, limited mainly by concerns over softening of the quartz (Bruder et al., 1993; Capper et al., 1993; Route et al., 1984). On the other hand, hot zone temperatures more than 10 1C above the melting point may cause the interface (surface of the growing crystal) to become concave (Kyle, 1971; Parfeniuk et al., 1992). A concave interface may lead to growth of additional grains (Tiller, 1963). Axial temperature uniformity in the hot zone may be desirable (Sen et al., 1990) and is commonly used. A temperature that increases monotonically upward will prevent sublimation of the solid during cool-down. Sublimation can also be suppressed by maintaining sufficient cadmium vapor pressure (see Section 3.2). This allows the melt to be cooler at the top, inducing buoyancy (thermally) driven convection, which may minimize formation of inclusions (see Section 3.3; Rudolph et al., 1994). Numerical simulations show the radial nature of the heating produces appreciable circulation in the bulk of the melt (Kuppurao et al., 1996).
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Binary and Multinary II–VI Compounds (CdTe, CdZnTe, CdMnTe) Grown from the Melt
It is difficult, and probably undesirable, to have the hot zone much above the melting point. Therefore, the cold zone plays a major role in establishing the high axial heat flow required for a convex interface. Kyle (1971) used a large metal heat sink on the nose of the crystal to increase axial heat flow. Numerical modeling has shown how such an ampoule support can help induce a favorable temperature distribution (Kuppurao and Derby, 1997). Choice of cold zone temperature is limited by its effect on thermal gradient. There is considerable range in recommended axial temperature gradient. A steep gradient seems necessary to promote the desirable axial heat flow to avoid growth of spurious grains. However, a steep gradient may also induce high stresses that generate defects (see Section 3.4). A very shallow gradient during the initial part of the process can result in a large volume of melt becoming supercooled and then freezing rapidly. This would produce an ingot lacking large single crystal regions. Based on experiment, a sharp gradient of at least 20 1Ccm–1 was recommended by Kyle (1971), who did not use a separate cold zone furnace. The numerical simulations of Parfeniuk et al. (1992) recommended a moderate gradient of 15 1Ccm–1, in combination with low cold zone and hot zone furnace temperatures. These recommendations were followed by Casagrande et al. (1993), who obtained large single crystal CdTe. Bruder et al. (1993) used axial gradients of 10 1Ccm–1 with a temperature of 900 1C in the cold (post anneal) zone. Rudolph et al. (1994) used low gradients of 7–10 1Ccm–1 to minimize thermal stresses. Sen et al. (1990) specified a low axial gradient of 6 1Ccm–1 as a critical parameter established experimentally and through numerical modeling. Yasuda et al. (1990) maintained a gradient of 2.6 1Ccm–1 with a cold zone at 1080 1C for CdTe. Asahi et al. (1996), using vertical gradient freeze, reported a gradient of C 1 1Ccm–1, yet recognized that a much larger gradient should be necessary for good heat flow. Considering the difficulty of actually determining the gradient during growth, the wide variation in reported gradients may be more apparent than real. Moreover, while the temperature gradient in the vicinity of the interface is important, the temperature distributions in the hot and cold zones are also significant as are other factors, such as the geometry of the system and materials used. In contrast to temperatures and gradient, there is fairly good agreement on translation rates of the furnace or ampoule. Rates are slow, especially for larger diameter crystals, because of the low thermal conductivity (5.28 Wm1 K1) of CdTe (Strzałkowski et al., 2014). Typically, translation ranges from 1mmh–1 for 50 mm diameter to 0.5 mmh–1 for 75 mm diameter (Sen et al., 1990). Kyle (1971) found rates of 2.5–5.0 mmh–1 to be optimum for 25 mm diameter. Faster rates led to excessive dislocation densities, while slower rates failed to yield a large single crystal volume. In vertical gradient freeze, reported equivalent growth velocities are 1.8 mmh–1 for 100 mm diameter (Asahi et al., 1996), relatively fast compared to VB. Once solidification is completed, cool-down to room temperature affects crystal quality. Excessive thermal stresses will increase dislocation density and may introduce twins. Cracking also has been reported. Slow cooling rates of about 10 1Ch–1 are often used (Bruder et al., 1993; Casagrande et al., 1993). Capper et al. (1996) cooled 50–75 mm diameter CdTe at 40 1Ch–1, but to avoid cracks added a 100 h annealing at 800 1C for Cd0.96Zn0.04Te. Asahi et al. (1996) reported a cooling rate of about 100 1Ch–1 for 100 mm diameter vertical gradient freeze growth of Cd0.97Zn0.03Te. Precipitation of second phases is affected by cooling rates as well as the atmosphere in the ampoule (see Section 3.3). CdMnTe is grown similarly to CdTe or CdZnTe (Triboulet and Didier, 1981; Giriat and Furdyna, 1988; Azoulay et al., 1993). Cd1–xMnxTe exists as a single phase up to x ¼ 0.77, while Zn1–xMnxTe is known with x up to 0.86. The quaternary Cd1–x– yZnxMnyTe would allow independent selection of bandgap and magnetic properties.
2.5
Horizontal Bridgman
Although the VB process is more commonly used, the HB process, either with translation or as a gradient freeze method, appears attractive in several respects. The melt is contained in a relatively shallow boat and has a large surface area. This geometry facilitates attainment of vapor–solid equilibrium, which should help control stoichiometry and avoid formation of second phases (see Sections 3.2 and 3.3). Typically, the furnaces (Figure 3) resemble those used in VB, but have a small viewing window. If the crystal appears unsatisfactory, growth can be restarted, which should improve yield. Seeded growth may be easier since the seed can be seen through the window. Stresses should be lower than in VB as there is less confinement of the ingot by the container and no massive column of melt or crystal to be supported. The horizontal configuration also is easier to scale up to larger ingot sizes. In fact, ingots of 6–8 kg, about twice the size of large VB ingots, are grown routinely. Typical growth rates are somewhat faster than for VB. Many of these advantages are difficult to realize in practice. The most commonly used boat material, graphite, has high thermal conductivity, which often causes nucleation to occur on both the bottom and top of the melt. The interface may then advance more rapidly along the top and bottom surfaces than in the middle of the melt. This so-called shelf growth (Edwards and Derby, 1997) can result in very low stress near the top surface of the growing ingot. It is not clear that this has resulted in overall lower defect densities than the vertical process. Yield may be below expectations because of the poor crystal quality at the bottom of the ingot and in the region where top and bottom grow together. Avoiding second-phase particles (see Section 3.3) does not yet seem to be as good in HB as in VB. The complex interface shape of shelf growth may trap inclusions. Similarly, the interface shape seems to cause less predictable distributions of zinc in CdZnTe and a less effective sweeping of impurities to the last-to-freeze end of the ingot.
Binary and Multinary II–VI Compounds (CdTe, CdZnTe, CdMnTe) Grown from the Melt
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Seeded growth in HB has been found more challenging than first thought. It is difficult to keep the seed from melting. When the seed end of the boat is cool to avoid excessive melt-back, there is an added driving force for Marangoni (surface) convection that transports hot melt toward the seed. These shortcomings represent technical challenges to be solved by further development rather than fundamental limitations. Indeed, CdZnTe suitable for infrared substrates and for X-ray and gamma-ray detectors has been manufactured using the HB process. The growth of very large single crystal substrates, 60 cm2 or larger, has been demonstrated.
2.6
High Pressure Bridgman
The high pressure Bridgman (HPB) method is used especially for growth of CdZnTe for X-ray and gamma-ray detectors, for which very high resistivity is required. As practiced (Butler et al., 1992, 1993; Szeles and Eissler, 1998; Yeckel et al., 1999), it is a VB process with crucible translation. Ingots up to 10 kg mass and 10 cm diameter have been grown and compositions span the full range from CdTe to ZnTe. To accomplish this, the furnace can operate at pressures 410 MPa and temperatures 41600 1C. The covered crucible and heaters are made from high-purity graphite. Porous graphite facilitates evacuation and hightemperature bake-out. Quartz, which may be a source of impurities, is specifically avoided. The high pressure of argon or nitrogen, used inside a steel pressure vessel, reduces, but does not eliminate, preferential evaporation of cadmium from the melt and loss from the crucible. The melt tends to become increasingly enriched in tellurium as growth proceeds. Few details of growth parameters and procedures have been published. Reportedly (Butler et al., 1992), high purity starting materials are weighed out stoichiometrically and high resistivity is achieved without doping. However, it is widely accepted that doping (e.g., with Al) is in fact used to achieve high resistivity consistently. Process time is one month. Assuming the cool-down after growth takes a few days, translation rates for the largest ingot would be about 0.4 mm h–1, in line with conventional Bridgman growth. Although resistivity is high, and regions of the ingot have excellent properties for X-ray and gamma-ray detectors, single crystal regions are generally smaller than in the conventional Bridgman methods. Also, HPB CdZnTe generally has a high incidence of defects such as second-phase particles and twins. Cracks and elongated voids (pipes) have been reported. Progress is being made in improving crystal quality (Szeles and Eissler, 1998).
3
Control of Properties (During or After Growth)
There is a good deal of interaction among the various factors influencing the properties of CdTe and related compounds, for example, control of stoichiometry affects native point defects and formation of second phases. In turn, both of these affect the location of impurities. Second-phase regions may serve as nucleation sites for dislocations. High densities of twins and degradation of crystal quality have been attributed to deviations from stoichiometry.
3.1
Purity
Control of purity has been described in Section 2. The growth process is the primary area of concern. Chemical analysis by glow discharge mass spectroscopy (GDMS) shows the crystals to be much less pure than the high-purity starting materials (Glass et al. 1998a,b). Some of these impurities, including copper, probably originate in the furnace. GDMS, which can sample many elements in one analysis and generally has sensitivity of ppba, appears to be the most useful analytical method (Bollong et al., 1995). If attention is focused on only one, or a few impurities, more sensitive techniques such as sputter initiated resonant ion spectroscopy may be appropriate (Tower et al., 1996; Sen et al., 1996). Although impurities are introduced during growth, the Bridgman process tends to segregate them to the last-to-freeze end. If this end is discarded, the remaining crystal has purity adequate for most applications. There is considerable evidence that copper concentrates in precipitates or inclusions of second phases (Tower et al., 1996; Sen et al., 1996). Other impurities probably do so as well. This effect can be beneficial by inactivating the impurities, provided the second-phase particles can be tolerated. Incorporation of substitutional impurities may be enhanced if there is an appreciable concentration of vacancies (Rudolph et al., 1994). Control of stoichiometry during growth, e.g., by providing a cadmium reservoir, can influence the uptake and activation of impurities through second phases and vacancies.
3.2
Stoichiometry
The most important direct benefits of controlling stoichiometry are on second-phase particles and native point defects (Swain et al., 2014). The second-phase particles may be inclusions or precipitates (Carini et al., 2006), either cadmium-rich or telluriumrich (see Section 3.3). Native defects are vacancies, interstitials, and antisite defects. As far as the growth process is concerned,
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Binary and Multinary II–VI Compounds (CdTe, CdZnTe, CdMnTe) Grown from the Melt
stoichiometry must be controlled over three temperature regimes: a high-temperature regime near the melting point, a moderatetemperature regime down to perhaps 700 1C, and a low-temperature regime in which vacancy diffusion rates remain significant. In the high- and moderate-temperature regimes, second-phase particles may form. In the low-temperature regime, native defects continue to come to equilibrium with the ampoule atmosphere. Since native defects are electrically and optically active, electrical resistivity and optical absorption will be influenced by control of stoichiometry over this range of temperatures as well as by impurities or dopants. Infrared transmission is often used to determine whether CdTe, or a ternary, is close to stoichiometry. If there is appreciable conductivity due to deviation from stoichiometry, free-carrier absorption will cause the transmission to fall off as the wavelength increases over the (typical) spectral range of 2–16 mm (Sen et al., 1990, 1996). There is not complete agreement on the dominant native defects in CdTe (Berding, 1999). Almost certainly, cadmium vacancies dominate CdTe that is deficient in cadmium, but the antisite defect of tellurium on cadmium sites also may be appreciable. For tellurium-deficient CdTe, cadmium on interstitial sites now appears to be the native defect having highest concentration. The requirements for controlling native defects can be met after solidification is completed by annealing during cool-down or in a separate process. If vacancies or antisite defects (see Section 3.1) affect impurity incorporation, control during cool-down may be preferred. Providing a cadmium or tellurium reservoir at a controlled temperature enables the atmosphere to be adjusted over the range from cadmium saturation to tellurium saturation. The required atmosphere depends on the desired carrier concentration and the presence of impurities or dopants (see Section 3.5). Vapor pressure is also an important factor in sublimation. In the absence of a sufficient vapor pressure of one of the elements, CdTe, or a ternary, at the hot end of an ampoule will sublimate and condense at a colder region (Zanio 1978, p. 6; Greenberg, 1996).
3.3
Second Phases
At the melting temperature, since the stoichiometric and congruently melting compositions are not the same, the composition of the crystal differs from that of the melt. The result is a boundary layer in the interfacial region. Compared to the bulk of the melt, this boundary layer is enriched in the element that is in excess. The effect is enhanced if the melt composition is much different from that of the stoichiometric compound, as in high-temperature solution growth. Under these circumstances, inclusions may form. Use of a slow growth rate is important to allow time for the boundary layer composition to adjust by diffusion with the bulk of the melt and avoid formation of inclusions. In addition, control of the vapor pressure can be used to replenish the melt so its composition does not continually shift away from that of the stoichiometric compound. Mixing in the bulk of the melt (see Section 2.4) also is beneficial. As shown in Figure 1, in the moderate-temperature regime (see Section 3.2), CdTe can exist over a significantly wider range of compositions than at lower temperatures (Zanio, 1978, p. 6; Greenberg, 1996; Berding, 1999). If a large deviation from stoichiometry is allowed to exist at moderate temperatures, then precipitation of a second phase tends to occur during subsequent cooling. Even if the precipitates are removed by subsequent annealing, they are likely to leave behind regions of high defect density (Sen et al., 1996). The ternary compositions exhibit effects similar to CdTe. Although inclusions and precipitates have distinctly different mechanisms of formation, they may be indistinguishable after the fact. Both appear as opaque regions when imaged in an infrared microscope. Figure 4 shows examples of large second-phase particles in CdZnTe.
Figure 4 Examples of large (25–50 mm diameter) second-phase particles in {111} CdZnTe wafers observed by infrared microscopy (courtesy of P. Bodie).
A complication in control of stoichiometry is that the crystal is not at a uniform temperature when growth is occurring. The interface is at the melting temperature, while the first-to-freeze end may be in the moderate-temperature regime (see Sections 2.4 and 2.5). There could be conflicting requirements on vapor pressure to avoid inclusions and precipitates. Commonly, control of stoichiometry during crystal growth is accomplished by providing a cadmium reservoir. The reservoir temperature is selected semi-empirically to provide a cadmium pressure of about 1 atm (Kyle, 1971; Asahi et al., 1996), or even higher (Rudolph et al., 1993, 1994), to obtain acceptably low levels of inclusions and precipitates in the final crystal. Simply placing a small excess of cadmium in the ampoule can also provide the required vapor pressure. To avoid the possibility of developing dangerously high pressure, the ampoule should be long enough to keep one end cool. The pressure will not exceed the elemental vapor pressure at the cool-end temperature. This approach can be convenient in the VB method since a special
Binary and Multinary II–VI Compounds (CdTe, CdZnTe, CdMnTe) Grown from the Melt
9
reservoir structure need not be provided. A fairly simple experiment has shown the effectiveness of this method in reducing formation of second-phase particles (Glass et al., 1998b). Since there is preferential vaporization of cadmium from the melt, it may be sufficient to keep the ampoule volume as small as possible. In this case, only a small loss of cadmium occurs before the vapor pressure reaches the equilibrium value. As described in Section 3.1, impurities may segregate at inclusions and precipitates.
3.4
Crystal Defects
The most significant crystal defects are dislocations and twins, which are found in CdTe and related compounds regardless of the crystal growth method. The Nakagawa chemical etch is widely accepted for revealing dislocations (Nakagawa et al., 1979; Mackett, 1994). This etch is effective only on or close to {111}-oriented surfaces. Etch pit densities are about 2 105 cm–2 in good CdTe and about a factor of 10 lower in good CdZnTe. Thermal stresses probably are the major driving force for dislocation multiplication and most likely also induce deformation twins (Rai et al., 1991; Parfeniuk et al., 1992). The yield stress is higher in CdZnTe than in CdTe, consistent with the lower dislocation density in the ternary. However, this hardening in CdZnTe may make deformation twins more numerous (Rai et al., 1991; McDevitt et al., 1986). Frequently, a few lamellar twins run across large, single-crystal regions. Although twins can occur on any of the four distinct sets of {111} planes, within any single-crystal region most of the large twins tend to lie along just one particular set of {111} planes. In VB ingots having one dominant single-crystal grain, this preferred plane often is the one most nearly vertical. If the ingot is sliced parallel to these twins, large, twin free wafers of {111} orientation can be obtained. Twins may form less frequently in the vertical gradient freeze method (Azoulay et al., 1990; Asahi et al., 1996) perhaps because of lower thermal gradients. Thinner, shorter twins (micro-twins) are sometimes seen after a wafer is polished and etched. This has been reported more often in the very large singlecrystal regions obtained in HB ingots.
3.5
Doping
Undoped CdTe and Cd1–xZnxTe (at least up to x B 0.25) can be produced as either p-type or n-type semiconductors. Growth or annealing under cadmium-rich conditions gives n-type crystals, while tellurium-rich conditions give p-type. The most commonly used crystal growth conditions result in p-type material. Even with doping, electrical properties may be difficult to predict due to interactions with native defects and variations in background impurities (Hage-Ali and Siffert, 1995). Gettering by second phases (see Sections 3.1 and 3.3) is a further complication, which can lead to an apparent low level of activation and make a determination of segregation coefficient difficult. Segregation coefficients are generally much less than unity (Astles, 1994), which facilitates purification but leads to nonuniform dopant concentration in the crystal. n-type crystals also are grown by adding a shallow donor dopant to the melt. The most commonly used donors are indium (from group III), which substitutes on the cadmium site, and chlorine (from group VII), which substitutes for tellurium. The compensation effect between indium defects and Cd vacancies in CdZnTe:In ingots was observed and lead to low concentration of free electrons. Doping with indium is often used to move Fermi level to the middle of the energy gap. Consequently, high resistivity with n-type conduction can be achieved (Yang et al., 2005; Xu et al., 2015). Other elements from these groups, aluminum, gallium, and iodine, also have been used. There is a good deal of variability in reported segregation coefficients for growth from the melt. The behavior of aluminum, which appears to have a segregation coefficient 4 1, may be complex. Doping with shallow donors has been used to obtain high resistivity, apparently because compensating deep acceptors are also present. These deep acceptors may be A-centers formed by association of the shallow dopant with a metal vacancy. There has been less interest in p-type dopants since the crystals usually are p-type as grown. The most frequently studied acceptor dopants are lithium and the group IB elements, copper, silver, and gold. All of these are rapid diffusers and have real or potential problems. They can be incorporated substitutionally for cadmium as well as interstitially, especially at higher concentrations. This results in some degree of compensation. The high diffusion rates facilitate formation of complexes and segregation to defects such as second-phase particles (see Section 3.3). Lithium may diffuse out of the crystal. When shallow acceptors are compensated by a deep donor, high resistivity may be achieved (Fiederle et al., 1998). Deep lying impurity states are of interest for obtaining semi-insulating crystals or photorefractive behavior for optical devices. Vanadium is most commonly used to enhance photorefractive effects. Other transition metals have been used to obtain high resistivity (Moravec et al., 1993; Kreissl and Schulz, 1996).
4
Concluding Remarks
Nowadays one can get commercially available large, high-quality CdZnTe crystals and many kinds of X-ray and g-ray detectors manufactured from them. The unique properties of CdZnTe at room temperature make it an ideal detector solution for Medical, Industrial, Homeland Security and Laboratory applications.
10
Binary and Multinary II–VI Compounds (CdTe, CdZnTe, CdMnTe) Grown from the Melt
However, there are still some shortcomings which should be considered. The main reason why production of large and homogenous crystals is still problematic is the segregation coefficient of zinc in CdTe matrix is about 1.35 (Zhang et al., 2011). This is why CdMnTe (Rafiei et al., 2013), CdMgTe (Hossain et al., 2013), and also CdTeSe (Roy et al., 2015) compounds are also being considered as promising materials for gamma radiation detection. CdTe and CdZnTe have a fairly long history and are used in a variety of applications. Many other applications and even more widespread use will develop as further improvements are made in growth of large, high-quality single crystals and in control of optical and electronic properties.
References Asahi, T., Oda, O., Taniguchi, Y., Koyama, A., 1996. Growth and characterization of 100mm diameter CdZnTe single crystals by the vertical gradient freeze method. J. Cryst. Growth 161, 20–27. Astles, M.G., 1994. In: Capper, P. (Ed.), Properties of Narrow Gap Cadmium-based Compounds. London: INSPEC, Institution of Electrical Engineers. Azoulay, M., Raizman, A., Gafni, G., Roth, M., 1990. Crystalline perfection of melt-grown CdTe. J. Cryst. Growth 101, 256–260. Azoulay, M., Raizman, A., Weingarten, R., Shacham, H., Feldstein, H., 1993. The growth of a cubic, single phase, Cd0.6Mn0.4Te single crystal by the vertical gradient freeze method. J. Cryst. Growth 128, 588–592. Berding, M.A., 1999. Native defects in cadmium telluride. Phys. Rev. B 60, 8943–8951. Bollong, A.B., Feldewerth, G., Tower, J.P., et al., 1995. Impurity study of CdZnTe substrates used for LPE HgCdTe. Adv. Mater. Opt. Electron. 5, 87–99. Bolotnikov, A.E., Boggs, S.E., Chen, C.M.H., et al., 2002. Properties of Pt Schottky type contacts on high-resistivity CdZnTe detectors. Nucl. Instrum. Methods Phys. Res. Sect. A 482, 395–407. Bruder, M., Figgermeier, H., Schmitt, R., Maier, H., 1993. Cd1yZnyTe substrates for Hg1xCdxTe liquid-phase epitaxy. Mater. Sci. Eng. B 16, 40–43. Butler, J.F., Doty, F.P., Apotovsky, B., Lajzerowicz, J., Verger, L., 1993. Gamma- and X-ray detectors manufactured from Cd1xZnxTe grown by a high pressure Bridgman method. Mater. Sci. Eng. B 16, 291–295. Butler, J.F., Doty, F.P., Lingren, C., 1992. Recent developments in CdZnTe gamma ray detector technology. SPIE Gamma-Ray Detectors 1734, 131–139. Capper, P., Gosney, J., Harris, J.E., O’Keefe, E., Maxey, C.D., 1996. Infra-red materials activities at GEC-Marconi Infra-red Limited: Part I—Bulk growth techniques. GEC-J. Res. 13, 164–174. Capper, P., Harris, J.E., O’Keefe, E., et al., 1993. Bridgman growth and assessment of CdTe and CdZnTe using the accelerated crucible rotation technique. Mater. Sci. Eng. B 16, 29–39. Carini, G.A., Bolotnikov, A.E., Camarda, G.S., et al., 2006. Effect of Te precipitates on the performance of CdZnTe detectors. Appl. Phys. Lett. 88, 143515–143518. Casagrande, L.G., Di Marzio, D., Lee, M.B., Larson Jr, D.J., 1993. Vertical Bridgman growth and characterization of large-diameter single-crystal CdTe. J. Cryst. Growth 128, 576–581. Chen, H., Awadalla, S.A., Iniewski, K., et al., 2008. Characterization of large cadmium zinc telluride crystals grown by traveling heater method. J. Appl. Phys. 103, 014903–014908. Choi, H., 2015. Growth and Fabrication Method of CdTe and Its Performance As a Radiation Detector. J. Korean Phys. Soc. 66, 27–30. Edwards, K., Derby, J.J., 1997. Understanding horizontal Bridgman shelf growth of cadmium telluride and cadmium zinc telluride. J. Cryst. Growth 179, 120–143. El Mokri, A., Triboulet, R., Lusson, A., Tromson-Carli, A., Didier, G., 1994. Growth of large, high-purity, low cost, uniform CdZnTe crystals by the “cold travelling heater method.”. J. Cryst. Growth 138, 168–174. Fiederle, M., Eiche, C., Salk, M., et al., 1998. Modified compensation model of CdTe. J. Appl. Phys. 84, 6689–6692. Giriat, W., Furdyna, J.K., 1988. Dilute magnetic semiconductors. In: Furdyna, J.K., Kossut, J. (Eds.), Semiconductors and Semimetals, Vol. 25. New York: Academic Press. Glass, H.L., Socha, A.J., Bakken, D.W., Speziale, V.M., Flint, J.P., 1998a. Control of defects and impurities in production of CdZnTe crystals by the Bridgman method. Mater. Res. Soc. Symp. Proc. 487, 27–32. Glass, H.L., Socha, A.J., Parfeniuk, C.L., Bakken, D.W., 1998b. Improvements in production of CdZnTe crystals grown by the Bridgman method. J. Cryst. Growth 184/185, 1035–1038. Greenberg, J.H., 1996. P-T-X phase equilibrium and vapor pressure scanning of non-stoichiometry in CdTe. J. Cryst. Growth 161, 1–11. Hage-Ali, M., Siffert, P., 1995. Semiconductors for room temperature nuclear detector applications. In: Schlesinger, T.E., James, R.B. (Eds.), Semiconductors and Semimetals, Vol. 43. San Diego, CA: Academic Press. Hossain, A., Gu, G.D., Bolotnikov, A.E., et al., 2015. Material and detector properties of cadmium manganese telluride (Cd1-xMnxTe) crystals grown by the modified floatingzone method. Nucl. Instrum. Methods Phys. Res. Sect. A 784, 33–36. Hossain, A., Yakimovich, V., Bolotnikov, A.E., et al., 2013. Development of Cadmium Magnesium Telluride (Cd1-xMgxTe) for room temperature X- and gamma-ray detectors. J. Cryst. Growth 379, 34–40. Kreissl, J., Schulz, H.-J., 1996. Transition-metal impurities in II−VI semiconductors: characterization and switching of charge states. J. Cryst. Growth 161, 239–249. Kuchar, L., Drapala, J., Lunacek., 1996. Purification methods of Cd, Te and CdTe and periodicity of segregation coefficients of admixtures. J. Cryst. Growth 161, 94–103. Kuppurao, S., Brandon, S., Derby, J.J., 1996. Analysis of interrupted growth strategies for cadmium telluride in an unseeded vertical Bridgman system. J. Cryst. Growth 158, 459–470. Kuppurao, S., Derby, J.J., 1997. Designing thermal environments to promote convex interface shapes during the vertical Bridgman growth of cadmium zinc telluride. J. Cryst. Growth 172, 350–360. Kyle, N.R., 1971. Growth of semi-insulating cadmium telluride. J. Electrochem. Soc. 118, 1790–1797. Lu, Y.-C., Shiau, J.-J., Feigelson, R.S., Route, R.K., 1990. Effect of vibrational stirring on the quality of Bridgman-grown CdTe. J. Cryst. Growth 102, 807–813. Mackett, P., 1994. In: Capper, P. (Ed.), Properties of Narrow Gap Cadmium-based Compounds. London: INSPEC, Institution of Electrical Engineers. McDevitt, S., Dean, B.E., Ryding, D.G., Scheltens, F.J., Mahajan, S., 1986. Characterization of CdTe and (Cd,Zn) Te single-crystal substrates. Mater. Lett. 4, 451–454. Moravec, P., Hage-Ali, M., Chibani, L., Siffert, P., 1993. Deep levels in semi-insulating CdTe. Mater. Sci. Eng. B 16, 223–227. Mühlberg, M., Rudolph, P., Laasch, M., Treser, E., 1993. The correlation between superheating and supercooling in CdTe melts during unseeded Bridgman growth. J. Cryst. Growth 128, 571–575. Mullin, J.B., Straughan, B.W., 1977. The melt growth and characterization of cadmium telluride. Rev. Phys. Appl. 12, 105–115. Nakagawa, K., Maeda, K., Takeuchi, S., 1979. Observation of dislocations in cadmium telluride by cathodoluminescence microscopy. Appl. Phys. Lett. 34, 574–575. Ohmori, M., Iwase, Y., Ohno, R., 1993. High quality CdTe and its application to radiation detectors. Mater. Sci. Eng. B 16, 283–290. Parfeniuk, C., Weinberg, F., Samarasekera, I.V., Schvezov, C., Li, L., 1992. Measured critical resolved shear stress and calculated temperature and stress field during growth of CdZnTe. J. Cryst. Growth 119, 261–270. Rafiei, R., Reinhard, M.I., Sarbutt, A., et al., 2013. Characterization of CdMnTe radiation detectors using current and charge transients. J. Semicond. 34, 073001–073008.
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Rai, R.S., Mahajan, S., Michel, D.J., et al., 1991. Deformation behavior of CdTe and (Cd,Zn)Te single crystals between 200 and 6001 C. Mater. Sci. Eng. B 10, 219–225. Route, R.K., Wolf, M., Feigelson, R.S., 1984. Interface studies during vertical Bridgman CdTe crystal growth. J. Cryst. Growth 70, 379–385. Roy, U.N., Bolotnikov, A.E., Camarda, G.S., et al., 2015. High compositional homogeneity of CdTexSe1 x crystals grown by the Bridgman method. APL Mat. 3, 026102–026109. Rudolph, P., Mühlberg, M., 1993. Basic problems of vertical Bridgman growth of CdTe. Mater. Sci. Eng. B 16, 8–16. Rudolph, P., Rinas, U., Jacobs, K., 1994. Systematic steps towards exactly stoichiometric and uncompensated CdTe Bridgman crystals. J. Cryst. Growth 138, 249–254. Schlesinger, T.E., Toney, J.E., Yoon, H., et al., 2001. Cadmium zinc telluride and its use as a nuclear radiation detector material. Mater. Sci. Eng. R 32, 103–189. Sen, S., Johnson, S.M., Kiele, J.A., Konkel, W.H., Stannard, J.E., 1990. Mater. Res. Soc. Symp. Proc. 161, 3–14. Sen, S., Konkel, W.H., Tighe, S.J., et al., 1988. Crystal growth of large-area single-crystal CdTe and CdZnTe by the computer-controlled vertical modified Bridgman process. J. Cryst. Growth 86, 111–117. Sen, S., Liang, C.S., Rhiger, D.R., Stannard, J.E., Arlinghaus, H.F., 1996. Reduction of CdZnTe substrate defects and relation to epitaxial HgCdTe quality. J. Electron. Mater. 25, 1188–1195. Shetty, R., Wilcox, W.R., Regel, L.L., 1995. Influence of ampoule coatings on cadmium telluride solidification. J. Cryst. Growth 153, 103–109. Sordo, S.D., Abbene, L., Caroli, E., et al., 2009. Progress in the development of CdTe and CdZnTe semiconductor radiation detectors for astrophysical and medical applications. Sensors 9, 3491–3526. Strzałkowski, K., Firszt, F., Marasek, A., 2014. Thermal diffusivity, effusivity and conductivity of CdMnTe mixed crystals. Int. J. Thermophys. 35, 2140–2149. Swain, S.K., Cui, Y., Datta, A., et al., 2014. Bulk growth of uniform and near stoichiometric cadmium telluride. J. Cryst. Growth 389, 134–138. Szeles, C., Eissler, E.E., 1998. Current issues of high-pressure Bridgman growth of semi-insulating CdZnTe. Mater. Res. Soc. Symp. Proc. 487, 3–12. Szeles, C., Cameron, S.E., Ndap, J.-O., Chalmers, W.C., 2002. Advances in the high-pressure crystal growth technology of semi-insulating CdZnTe for radiation detector applications. IEEE Trans. Nucl. Sci. 49, 2535–2540. Szeles, C., Soldner, S.A., Vydrin, S., Graves, J., Bale, D.S., 2008. CdZnTe semiconductor detectors for spectroscopic X-ray imaging. IEEE Trans. Nucl. Sci. 55, 572–582. Tiller, W.A., 1963. In: Gilman, J.J. (Ed.), The Art and Science of Growing Crystals. New York: Wiley. Tower, J.P., Tobin, S.P., Norton, P.W., et al., 1996. Trace copper measurements and electrical effects in LPE HgCdTe. J. Electron. Mater. 25, 1183–1187. Triboulet, R., Aoudia, A., Lusson, A., 1995. Assessment of the purity of cadmium and tellurium as components of the CdTe-based substrates. J. Electron. Mater. 24, 1061–1065. Triboulet, R., Didier, G., 1981. Growth and characterization of Cd1xMnxTe and MnTe crystals; contribution to the CdTe−MnTe pseudo-binary phase diagram determination. J. Cryst. Growth 52, 614–618. Xu, L., Jie, W., Fu, X., et al., 2015. Axial distribution of deep-level defects in as-grown CdZnTe:In ingots and their effects on the material's electrical properties. J. Cryst. Growth 409, 71–74. Yang, G., Jie, W., Li, Q., Wang, T., Li, G., Hua, H., 2005. Effects of In doping on the properties of CdZnTe single crystals. J. Cryst. Growth 283, 431–437. Yasuda, K., Iwakami, Y., Saji, M., 1990. Crystal growth of CdTe in a multi-zone vertical Bridgman furnace with a PBN crucible. J. Cryst. Growth 99, 727–730. Yeckel, A., Doty, F.P., Derby, J.J., 1999. Effect of steady crucible rotation on segregation in high-pressure vertical Bridgman growth of cadmium zinc telluride. J. Cryst. Growth 203, 87–102. Zanio, K., 1978. In: Willardson, R.K., Beer, A.C. (Eds.), Semiconductors and Semimetals, Vol. 13. New York: Academic Press. Zhang, J., Jie, W., Wang, T., Zeng, D., Yang, B., 2007. Growth and characterization of In doped Cd0.8Mn0.2Te single crystal. J. Cryst. Growth 306, 33–38. Zhang, N., Yeckel, A., Burger, A., et al., 2011. Anomalous segregation during electrodynamic gradient freeze growth of cadmium zinc telluride. J. Cryst. Growth 325, 10–19.
1 2 2.1 2.2 2.3 2.4 2.5 2.6 3 3.1 3.2 3.3 3.4 3.5 4 References
Review of Growth Methods Detailed Descriptions of Bridgman Methods Preparation of Source Materials Choice of Containers Reaction of Elements to Produce Compounds Vertical Bridgman Horizontal Bridgman High Pressure Bridgman Control of Properties (During or After Growth) Purity Stoichiometry Second Phases Crystal Defects Doping Concluding Remarks
1 2 3 3 3 5 6 7 7 7 7 8 9 9 9 10
CdTe and CdZnTe (Cd1–xZnxTe) in bulk crystal form are used as X-ray and gamma-ray detectors, in electrooptic and photorefractive devices and as substrates for epitaxy, especially epitaxial HgCdTe for infrared detector arrays. Cd1–xZnxTe crystals for composition of the order of 10–20 atomic percentage have become one of the most studied material for g-ray detection application (Schlesinger et al., 2001; Bolotnikov et al., 2002; Sordo et al., 2009; Swain et al., 2014; Choi, 2015) and also spectroscopic X-ray imaging (Szeles et al., 2008). Their potential arises from few facts: (1) both, cadmium and telluride possess high atomic numbers; (2) the bandgap is large enough for high resistivity and low leakage current, but on the other hand is small enough so that the electron hole ionization energy is small; (3) high quantum efficiency; and (4) it is possible now to grow good-quality large crystals. Substitution of manganese for part of the cadmium in CdTe matrix produces one of the most widely studied diluted (semimagnetic) semiconductors. Apart from magnetic properties, CdMnTe compound is also considered as material for X-ray and gamma-ray detectors (Zhang et al., 2007; Rafiei et al., 2013; Hossain et al., 2015). Large crystals of all these compositions are commonly produced by various versions of growth from the melt.
1
Review of Growth Methods
CdTe and related compounds have been grown from the melt and from high-temperature solutions. As is readily seen from the phase diagram in Figure 1, CdTe can be grown from liquid compositions spanning a broad range from cadmium-rich to telluriumrich and including the congruently melting composition, which deviates slightly from perfect stoichiometry. In common usage, melt growth involves liquid compositions near stoichiometry for which solidification occurs close to the congruent melting temperature, 1092 1C. Solution growth is generally carried out from liquids far off stoichiometry, allowing crystallization at substantially lower temperatures, for example, 800 1C. There are several trade-offs between melt and solution growth. Melt growth offers higher growth rates and larger crystals. However, at the high temperatures used, it is difficult to maintain purity. Also, control of the growth atmosphere is crucial to regulate stoichiometry and avoid second phases. Solution growth avoids some difficulties of melt growth, but solvent inclusions may limit quality. ☆
Change History: June 2015. K. Strzałkowski made the following changes: a paragraph was added to the introduction section, a paragraph was added to the end of Section 1, some sentences were included in Sections 2.1 and 2.2, a small paragraph was added to Section 3.5, ‘concluding remarks’ section was completely rewritten, and 18 new references were added throughout the manuscript.
Reference Module in Materials Science and Materials Engineering
doi:10.1016/B978-0-12-803581-8.02395-X
1
2
Binary and Multinary II–VI Compounds (CdTe, CdZnTe, CdMnTe) Grown from the Melt
Figure 1 CdTe phase diagram. Near-solidus region is given on an enlarged scale (after Greenberg, 1996).
The Czochralski method of pulling from the melt is preferred for many semiconductors. For CdTe the high vapor pressure of cadmium necessitates special measures (Mullin and Straughan, 1977) such as the liquid encapsulated Czochralski (LEC) method. While suitable for several other compound semiconductors, LEC has not truly succeeded for CdTe. Some work has been reported on growth of CdTe by the floating zone method and by use of a zone refining apparatus. The most frequently used methods for melt growth of CdTe and related compounds are variations of the Bridgman method. These include both vertical (VB) and horizontal (HB) arrangements, either with or without control of the (usually cadmium) vapor pressure. Other variations include vertical and horizontal gradient freeze (VGF and HGF) and the heat exchanger method (HEM). Typically, total pressure at the melting point is in the vicinity of 0.1 MPa. Fully single crystal wafers up to 30 cm2 in area are produced by these Bridgman methods and are commercially available for use as infrared substrates. A high-pressure Bridgman (HPB) method is also used, especially for growth of high resistivity CdZnTe for gamma-ray detectors, although single crystal size is smaller. Solution growth, which will not be discussed in detail, may use a Bridgman arrangement for directional solidification of a highly nonstoichiometric melt. Alternatively, in the traveling solvent (TSM) or traveling heater methods (THM) (Ohmori et al., 1993; El Mokri et al., 1994; Hage-Ali and Siffert, 1995; Chen et al., 2008), the feed material is a solid rod. A heating coil melts a liquid zone, which is traversed along the length of the rod. Among others, vertical High-Pressure Electro-Dynamic Gradient (HP EDG) crystal growth system was proposed (Szeles et al., 2002). This technology improves the CdZnTe detector fabrication yield compared to other methods, mainly due to reduction of thermal and mechanical stress in the ingots, improved yield and electron transport properties. The multi-zone heater system allows dynamic control of heat flow which helps to optimize the shape of the growth interface. Thanks to that it is possible to growth large in diameter ingots (140 mm) of CdZnTe crystals almost free of cracking. The solution growth methods have been used particularly for nuclear detector applications, where control of stoichiometry, purity, and doping is more important than large size.
2
Detailed Descriptions of Bridgman Methods
As outlined in Section 1, the most commonly used methods for crystal growth of CdTe and related compositions are versions of the Bridgman methods.
Binary and Multinary II–VI Compounds (CdTe, CdZnTe, CdMnTe) Grown from the Melt 2.1
3
Preparation of Source Materials
Cadmium, zinc, and tellurium melt at relatively low temperatures, which simplifies their purification. Once a reasonably high degree of purity has been obtained, multiple pass zone refining, typically in hydrogen, is carried out. Some impurities, notably copper in cadmium, cannot be separated adequately by this method (Zanio 1978, pp. 38–51; Kuchar et al., 1996). A process that combines in situ distillation with zone refining reduces copper to below detection limits of a few ppba (parts per billion atomic) and total metallic impurities, excluding zinc and tellurium, to less than 30 ppba (Bollong et al., 1995). Similar purity levels are achieved in zinc and tellurium. These are significant improvements over the 99.9999% purity levels that were the best commercially available in the 1990s. Prior to use, cadmium and zinc are typically etched in dilute nitric acid to remove surface contamination, then thoroughly rinsed and dried. Zone refined ingots of tellurium tend to have voids and cracks, which make etching and rinsing problematical. Also, tellurium is brittle and tends to splinter. Casting tellurium under hydrogen to form a slug surmounts these difficulties and may reduce oxides. The dense slug simplifies loading into the crystal growth container or reaction vessel. Since cleaning of tellurium is somewhat uncertain, avoiding exposure to air during storage is recommended to minimize oxide formation. Unlike cadmium, zinc, and tellurium, manganese has a high melting temperature. It is difficult to purify manganese to the same level as the other elements. A purity of 99.99% with respect to metallic impurities is about the best currently available. Oxide formation is a major problem. Sublimation in a dynamic vacuum at 1000 1C has been found effective to purify manganese for crystal growth (Giriat and Furdyna, 1988). Crystal growth can start with the elements. Alternatively, the elements may first be reacted to form the binary compounds (see Section 2.3) and these used as source materials. Another option is commercially available pure CdTe powder with purity of 99.9999%. When the binaries are used, sublimation is often performed for purification (Kyle, 1971). Prior to sealing the carboncoated quartz (silica) sublimation tube, the binary may be heated under hydrogen to remove oxides. Oxides can react with the carbon coating and the quartz walls, leading to ampoule failure, sticking of the compound to the wall and contamination. A vacuum of 1 108 torr is ordinarily attained prior to sealing. Typical sublimation temperature is 900 1C. While sublimation may not be needed for purification when state-of-the-art high purity elements are used, it probably provides reproducible stoichiometry. This may be beneficial if vapor pressure is not controlled during crystal growth (see Sections 2.4, 2.5, and 3.2).
2.2
Choice of Containers
Obviously containers must withstand the physical and chemical environment of the crystal growth process and not introduce significant impurities. If possible, the container should be a barrier to impurities emerging from furnace components (Triboulet et al., 1995). Most crystal growth of CdTe and related compounds has used quartz (silica) containers, nearly always coated with carbon. Procedures for preparing quartz and depositing carbon pyrolytically are well known. Cleaning typically includes washing, etching in a mixture containing HF, thorough rinsing, and a high temperature bake in a clean vacuum. High purity synthetic quartz, specially developed for electronic applications, is strongly recommended. Carbon coating is accomplished by heating the open tube while organic gas or vapor flows through it. Other coatings, such as boron nitride (Shetty et al., 1995) have been used. The primary function of the coating is to avoid sticking or reaction between the charge and container. Coatings may be effective as barriers to impurities. The quartz container may function as a sealed vacuum ampoule. Alternatively, a separate crucible may be used in the ampoule. There is evidence that dislocation density is lower when growth is in pyrolytic boron nitride crucibles rather than carbon-coated quartz (Glass et al., 1998a). Graphite containers are commonly used in the high pressure and horizontal Bridgman (HB) processes. Recommended source material for crucibles is high density high ordered pyrolytic graphite (HOPG). One reason for choosing graphite is that quartz may be a source of impurities (Kyle, 1971; Butler et al., 1993). Another one is case when higher melting temperature is needed, where quartz containers may be additional source of impurities. In the commonly used vertical Bridgman (VB) method, the container is an axially symmetric cylinder, usually with a slight taper to facilitate removal of the crystal at the end of the process. The same purpose is applying crucibles composed from two parts. Although specially designed tips are often used in Bridgman methods, containers for CdTe frequently have flat, rounded, or shallow conical bottoms. Sometimes there is a well to hold a seed crystal.
2.3
Reaction of Elements to Produce Compounds
Reaction of the elements to form the compounds may be done as the initial part of the crystal growth process or as a separate step. Major concerns are to maintain purity and keep the highly exothermic reactions from proceeding too rapidly, which could produce high vapor pressure. The reaction furnace should be shielded in case of explosion. Separate reaction is usually performed in carboncoated, vacuum-sealed, high-purity quartz ampoules (see Sections 2.1 and 2.2). There are two commonly used procedures. The more common is simply to seal weighed amounts of the elements into the
4
Binary and Multinary II–VI Compounds (CdTe, CdZnTe, CdMnTe) Grown from the Melt
ampoule. The weights may be stoichiometric; however, a small excess of tellurium is often used to avoid the possibility of high pressures associated with excess cadmium (Zanio 1978, p. 12). The charge is heated slowly to a temperature sufficient to start the reaction, B800 1C. After reaction is complete, either sublimation is carried out (see Section 2.2) or the temperature is raised above the melting point and held to allow the melt to homogenize. Sometimes the furnace is rocked to facilitate homogenization before cooling to room temperature. The second procedure for synthesizing the compound uses vapor transport of the more volatile cadmium. The tellurium and somewhat more than the stoichiometrically required amount of cadmium, are loaded into separate compartments in a quartz ampoule. The elements are kept apart except for vapor transport. Figures 2 and 3 show crystal growth furnaces that can be used for this purpose. The ampoule is slowly heated, so the cadmium vapors react with the molten tellurium. The cadmium end of the ampoule is heated only enough to achieve cadmium vapor pressure of roughly 0.1–0.2 MPa, B800 1C. The tellurium end is heated above the melting point of the compound, 1092 1C for CdTe.
Figure 2 Schematic of vertical Bridgman growth, showing three-zone furnace and ampoule with cadmium reservoir at the top.
The excess cadmium serves as a reservoir to maintain a desired cadmium vapor pressure. This regulates stoichiometry and prevents sublimation of the compound to the cold region of the ampoule (see Section 3.2; Zanio 1978, p. 7). When CdZnTe is to be synthesized, the zinc may be included with the cadmium for vapor transport to the tellurium. Dopants or substituents of lower volatility, such as manganese, are included with the tellurium.
Binary and Multinary II–VI Compounds (CdTe, CdZnTe, CdMnTe) Grown from the Melt
5
Figure 3 Schematic of horizontal Bridgman growth, showing three-zone furnace and separate boat for cadmium source.
If the reaction process includes melting the compound, additional purification can be accomplished by directional solidification followed by discarding of the last-to-freeze end of the ingot, where impurities tend to segregate.
2.4
Vertical Bridgman
The apparatus for VB growth can be as simple as a single zone furnace, or a pair of furnaces constituting the hot and cold zones, plus a mechanism to translate the ampoule or furnace. Translation is not used in the vertical gradient freeze (VGF) technique. More sophisticated furnace designs may include a third furnace for control of vapor pressure, multiple zone furnaces to adjust temperature gradients, and heat pipes to maintain regions at uniform temperature (Kyle, 1971; Sen et al., 1988; Yasuda et al., 1990; Casagrande et al., 1993; Rudolph and Mühlberg, 1993). The reservoir for controlling vapor pressure may be located above (Kyle, 1971; Sen et al., 1988) or below (Rudolph et al., 1994; Asahi et al., 1996) the growth zones. Further discussion of vapor pressure control is found in Section 3. Mixing in the melt may be enhanced by the accelerated crucible rotation technique (ACRT) (Capper et al., 1993, 1996) or vibration (Lu et al., 1990). Slow rotation of the ampoule is sometimes used to compensate thermal asymmetries. The furnaces used in VB methods are rather conventional since the melting point is only about 1100 1C. Furnaces should be stable to 70.2 1C over long time periods (Sen et al., 1990; Asahi et al., 1996). The furnace liner tube may be ceramic or quartz (silica), but should be high purity. Figure 2 is a schematic representation of a simple three-zone furnace with an ampoule containing a cadmium reservoir at the top. If the reservoir is large enough to contain the full quantity of cadmium, then reaction by vapor transport can be effected prior to growth. Once reaction to form the compound has been completed (see Section 2.3), the growth sequence begins with heating the charge to above the melting point and holding, generally several hours, to establish a homogeneous melt. There is evidence that the melt should be heated at least 10 1C above the melting point to break up atomic associations in the liquid (Rudolph and Mühlberg, 1993); however, this requirement may be relaxed if the melt deviates from stoichiometry (Mühlberg et al., 1993). After establishing a uniform melt, the furnace is set to the temperature profile for growth and allowed to equilibrate. Growth is initiated by starting the translation or the gradient freeze program. There is some evidence that an initial rapid translation rate, followed by a pause, improves the chance of obtaining a single dominant grain after spontaneous nucleation (Kuppurao et al., 1996). Once the entire melt has been solidified, the furnace is cooled to room temperature. The ingot obtained typically has a thin surface coating of cadmium (or tellurium) that is removed by etching or sandblasting. There is a lack of agreement about optimum temperature distributions. The divergence of views may be related to several factors, including difficulties in measuring temperatures accurately during growth. Three temperature parameters typically must be specified: the hot and cold zone temperatures and the axial gradient in the region between these zones. The hot zone may be maintained at 1140 1C or 1150 1C, limited mainly by concerns over softening of the quartz (Bruder et al., 1993; Capper et al., 1993; Route et al., 1984). On the other hand, hot zone temperatures more than 10 1C above the melting point may cause the interface (surface of the growing crystal) to become concave (Kyle, 1971; Parfeniuk et al., 1992). A concave interface may lead to growth of additional grains (Tiller, 1963). Axial temperature uniformity in the hot zone may be desirable (Sen et al., 1990) and is commonly used. A temperature that increases monotonically upward will prevent sublimation of the solid during cool-down. Sublimation can also be suppressed by maintaining sufficient cadmium vapor pressure (see Section 3.2). This allows the melt to be cooler at the top, inducing buoyancy (thermally) driven convection, which may minimize formation of inclusions (see Section 3.3; Rudolph et al., 1994). Numerical simulations show the radial nature of the heating produces appreciable circulation in the bulk of the melt (Kuppurao et al., 1996).
6
Binary and Multinary II–VI Compounds (CdTe, CdZnTe, CdMnTe) Grown from the Melt
It is difficult, and probably undesirable, to have the hot zone much above the melting point. Therefore, the cold zone plays a major role in establishing the high axial heat flow required for a convex interface. Kyle (1971) used a large metal heat sink on the nose of the crystal to increase axial heat flow. Numerical modeling has shown how such an ampoule support can help induce a favorable temperature distribution (Kuppurao and Derby, 1997). Choice of cold zone temperature is limited by its effect on thermal gradient. There is considerable range in recommended axial temperature gradient. A steep gradient seems necessary to promote the desirable axial heat flow to avoid growth of spurious grains. However, a steep gradient may also induce high stresses that generate defects (see Section 3.4). A very shallow gradient during the initial part of the process can result in a large volume of melt becoming supercooled and then freezing rapidly. This would produce an ingot lacking large single crystal regions. Based on experiment, a sharp gradient of at least 20 1Ccm–1 was recommended by Kyle (1971), who did not use a separate cold zone furnace. The numerical simulations of Parfeniuk et al. (1992) recommended a moderate gradient of 15 1Ccm–1, in combination with low cold zone and hot zone furnace temperatures. These recommendations were followed by Casagrande et al. (1993), who obtained large single crystal CdTe. Bruder et al. (1993) used axial gradients of 10 1Ccm–1 with a temperature of 900 1C in the cold (post anneal) zone. Rudolph et al. (1994) used low gradients of 7–10 1Ccm–1 to minimize thermal stresses. Sen et al. (1990) specified a low axial gradient of 6 1Ccm–1 as a critical parameter established experimentally and through numerical modeling. Yasuda et al. (1990) maintained a gradient of 2.6 1Ccm–1 with a cold zone at 1080 1C for CdTe. Asahi et al. (1996), using vertical gradient freeze, reported a gradient of C 1 1Ccm–1, yet recognized that a much larger gradient should be necessary for good heat flow. Considering the difficulty of actually determining the gradient during growth, the wide variation in reported gradients may be more apparent than real. Moreover, while the temperature gradient in the vicinity of the interface is important, the temperature distributions in the hot and cold zones are also significant as are other factors, such as the geometry of the system and materials used. In contrast to temperatures and gradient, there is fairly good agreement on translation rates of the furnace or ampoule. Rates are slow, especially for larger diameter crystals, because of the low thermal conductivity (5.28 Wm1 K1) of CdTe (Strzałkowski et al., 2014). Typically, translation ranges from 1mmh–1 for 50 mm diameter to 0.5 mmh–1 for 75 mm diameter (Sen et al., 1990). Kyle (1971) found rates of 2.5–5.0 mmh–1 to be optimum for 25 mm diameter. Faster rates led to excessive dislocation densities, while slower rates failed to yield a large single crystal volume. In vertical gradient freeze, reported equivalent growth velocities are 1.8 mmh–1 for 100 mm diameter (Asahi et al., 1996), relatively fast compared to VB. Once solidification is completed, cool-down to room temperature affects crystal quality. Excessive thermal stresses will increase dislocation density and may introduce twins. Cracking also has been reported. Slow cooling rates of about 10 1Ch–1 are often used (Bruder et al., 1993; Casagrande et al., 1993). Capper et al. (1996) cooled 50–75 mm diameter CdTe at 40 1Ch–1, but to avoid cracks added a 100 h annealing at 800 1C for Cd0.96Zn0.04Te. Asahi et al. (1996) reported a cooling rate of about 100 1Ch–1 for 100 mm diameter vertical gradient freeze growth of Cd0.97Zn0.03Te. Precipitation of second phases is affected by cooling rates as well as the atmosphere in the ampoule (see Section 3.3). CdMnTe is grown similarly to CdTe or CdZnTe (Triboulet and Didier, 1981; Giriat and Furdyna, 1988; Azoulay et al., 1993). Cd1–xMnxTe exists as a single phase up to x ¼ 0.77, while Zn1–xMnxTe is known with x up to 0.86. The quaternary Cd1–x– yZnxMnyTe would allow independent selection of bandgap and magnetic properties.
2.5
Horizontal Bridgman
Although the VB process is more commonly used, the HB process, either with translation or as a gradient freeze method, appears attractive in several respects. The melt is contained in a relatively shallow boat and has a large surface area. This geometry facilitates attainment of vapor–solid equilibrium, which should help control stoichiometry and avoid formation of second phases (see Sections 3.2 and 3.3). Typically, the furnaces (Figure 3) resemble those used in VB, but have a small viewing window. If the crystal appears unsatisfactory, growth can be restarted, which should improve yield. Seeded growth may be easier since the seed can be seen through the window. Stresses should be lower than in VB as there is less confinement of the ingot by the container and no massive column of melt or crystal to be supported. The horizontal configuration also is easier to scale up to larger ingot sizes. In fact, ingots of 6–8 kg, about twice the size of large VB ingots, are grown routinely. Typical growth rates are somewhat faster than for VB. Many of these advantages are difficult to realize in practice. The most commonly used boat material, graphite, has high thermal conductivity, which often causes nucleation to occur on both the bottom and top of the melt. The interface may then advance more rapidly along the top and bottom surfaces than in the middle of the melt. This so-called shelf growth (Edwards and Derby, 1997) can result in very low stress near the top surface of the growing ingot. It is not clear that this has resulted in overall lower defect densities than the vertical process. Yield may be below expectations because of the poor crystal quality at the bottom of the ingot and in the region where top and bottom grow together. Avoiding second-phase particles (see Section 3.3) does not yet seem to be as good in HB as in VB. The complex interface shape of shelf growth may trap inclusions. Similarly, the interface shape seems to cause less predictable distributions of zinc in CdZnTe and a less effective sweeping of impurities to the last-to-freeze end of the ingot.
Binary and Multinary II–VI Compounds (CdTe, CdZnTe, CdMnTe) Grown from the Melt
7
Seeded growth in HB has been found more challenging than first thought. It is difficult to keep the seed from melting. When the seed end of the boat is cool to avoid excessive melt-back, there is an added driving force for Marangoni (surface) convection that transports hot melt toward the seed. These shortcomings represent technical challenges to be solved by further development rather than fundamental limitations. Indeed, CdZnTe suitable for infrared substrates and for X-ray and gamma-ray detectors has been manufactured using the HB process. The growth of very large single crystal substrates, 60 cm2 or larger, has been demonstrated.
2.6
High Pressure Bridgman
The high pressure Bridgman (HPB) method is used especially for growth of CdZnTe for X-ray and gamma-ray detectors, for which very high resistivity is required. As practiced (Butler et al., 1992, 1993; Szeles and Eissler, 1998; Yeckel et al., 1999), it is a VB process with crucible translation. Ingots up to 10 kg mass and 10 cm diameter have been grown and compositions span the full range from CdTe to ZnTe. To accomplish this, the furnace can operate at pressures 410 MPa and temperatures 41600 1C. The covered crucible and heaters are made from high-purity graphite. Porous graphite facilitates evacuation and hightemperature bake-out. Quartz, which may be a source of impurities, is specifically avoided. The high pressure of argon or nitrogen, used inside a steel pressure vessel, reduces, but does not eliminate, preferential evaporation of cadmium from the melt and loss from the crucible. The melt tends to become increasingly enriched in tellurium as growth proceeds. Few details of growth parameters and procedures have been published. Reportedly (Butler et al., 1992), high purity starting materials are weighed out stoichiometrically and high resistivity is achieved without doping. However, it is widely accepted that doping (e.g., with Al) is in fact used to achieve high resistivity consistently. Process time is one month. Assuming the cool-down after growth takes a few days, translation rates for the largest ingot would be about 0.4 mm h–1, in line with conventional Bridgman growth. Although resistivity is high, and regions of the ingot have excellent properties for X-ray and gamma-ray detectors, single crystal regions are generally smaller than in the conventional Bridgman methods. Also, HPB CdZnTe generally has a high incidence of defects such as second-phase particles and twins. Cracks and elongated voids (pipes) have been reported. Progress is being made in improving crystal quality (Szeles and Eissler, 1998).
3
Control of Properties (During or After Growth)
There is a good deal of interaction among the various factors influencing the properties of CdTe and related compounds, for example, control of stoichiometry affects native point defects and formation of second phases. In turn, both of these affect the location of impurities. Second-phase regions may serve as nucleation sites for dislocations. High densities of twins and degradation of crystal quality have been attributed to deviations from stoichiometry.
3.1
Purity
Control of purity has been described in Section 2. The growth process is the primary area of concern. Chemical analysis by glow discharge mass spectroscopy (GDMS) shows the crystals to be much less pure than the high-purity starting materials (Glass et al. 1998a,b). Some of these impurities, including copper, probably originate in the furnace. GDMS, which can sample many elements in one analysis and generally has sensitivity of ppba, appears to be the most useful analytical method (Bollong et al., 1995). If attention is focused on only one, or a few impurities, more sensitive techniques such as sputter initiated resonant ion spectroscopy may be appropriate (Tower et al., 1996; Sen et al., 1996). Although impurities are introduced during growth, the Bridgman process tends to segregate them to the last-to-freeze end. If this end is discarded, the remaining crystal has purity adequate for most applications. There is considerable evidence that copper concentrates in precipitates or inclusions of second phases (Tower et al., 1996; Sen et al., 1996). Other impurities probably do so as well. This effect can be beneficial by inactivating the impurities, provided the second-phase particles can be tolerated. Incorporation of substitutional impurities may be enhanced if there is an appreciable concentration of vacancies (Rudolph et al., 1994). Control of stoichiometry during growth, e.g., by providing a cadmium reservoir, can influence the uptake and activation of impurities through second phases and vacancies.
3.2
Stoichiometry
The most important direct benefits of controlling stoichiometry are on second-phase particles and native point defects (Swain et al., 2014). The second-phase particles may be inclusions or precipitates (Carini et al., 2006), either cadmium-rich or telluriumrich (see Section 3.3). Native defects are vacancies, interstitials, and antisite defects. As far as the growth process is concerned,
8
Binary and Multinary II–VI Compounds (CdTe, CdZnTe, CdMnTe) Grown from the Melt
stoichiometry must be controlled over three temperature regimes: a high-temperature regime near the melting point, a moderatetemperature regime down to perhaps 700 1C, and a low-temperature regime in which vacancy diffusion rates remain significant. In the high- and moderate-temperature regimes, second-phase particles may form. In the low-temperature regime, native defects continue to come to equilibrium with the ampoule atmosphere. Since native defects are electrically and optically active, electrical resistivity and optical absorption will be influenced by control of stoichiometry over this range of temperatures as well as by impurities or dopants. Infrared transmission is often used to determine whether CdTe, or a ternary, is close to stoichiometry. If there is appreciable conductivity due to deviation from stoichiometry, free-carrier absorption will cause the transmission to fall off as the wavelength increases over the (typical) spectral range of 2–16 mm (Sen et al., 1990, 1996). There is not complete agreement on the dominant native defects in CdTe (Berding, 1999). Almost certainly, cadmium vacancies dominate CdTe that is deficient in cadmium, but the antisite defect of tellurium on cadmium sites also may be appreciable. For tellurium-deficient CdTe, cadmium on interstitial sites now appears to be the native defect having highest concentration. The requirements for controlling native defects can be met after solidification is completed by annealing during cool-down or in a separate process. If vacancies or antisite defects (see Section 3.1) affect impurity incorporation, control during cool-down may be preferred. Providing a cadmium or tellurium reservoir at a controlled temperature enables the atmosphere to be adjusted over the range from cadmium saturation to tellurium saturation. The required atmosphere depends on the desired carrier concentration and the presence of impurities or dopants (see Section 3.5). Vapor pressure is also an important factor in sublimation. In the absence of a sufficient vapor pressure of one of the elements, CdTe, or a ternary, at the hot end of an ampoule will sublimate and condense at a colder region (Zanio 1978, p. 6; Greenberg, 1996).
3.3
Second Phases
At the melting temperature, since the stoichiometric and congruently melting compositions are not the same, the composition of the crystal differs from that of the melt. The result is a boundary layer in the interfacial region. Compared to the bulk of the melt, this boundary layer is enriched in the element that is in excess. The effect is enhanced if the melt composition is much different from that of the stoichiometric compound, as in high-temperature solution growth. Under these circumstances, inclusions may form. Use of a slow growth rate is important to allow time for the boundary layer composition to adjust by diffusion with the bulk of the melt and avoid formation of inclusions. In addition, control of the vapor pressure can be used to replenish the melt so its composition does not continually shift away from that of the stoichiometric compound. Mixing in the bulk of the melt (see Section 2.4) also is beneficial. As shown in Figure 1, in the moderate-temperature regime (see Section 3.2), CdTe can exist over a significantly wider range of compositions than at lower temperatures (Zanio, 1978, p. 6; Greenberg, 1996; Berding, 1999). If a large deviation from stoichiometry is allowed to exist at moderate temperatures, then precipitation of a second phase tends to occur during subsequent cooling. Even if the precipitates are removed by subsequent annealing, they are likely to leave behind regions of high defect density (Sen et al., 1996). The ternary compositions exhibit effects similar to CdTe. Although inclusions and precipitates have distinctly different mechanisms of formation, they may be indistinguishable after the fact. Both appear as opaque regions when imaged in an infrared microscope. Figure 4 shows examples of large second-phase particles in CdZnTe.
Figure 4 Examples of large (25–50 mm diameter) second-phase particles in {111} CdZnTe wafers observed by infrared microscopy (courtesy of P. Bodie).
A complication in control of stoichiometry is that the crystal is not at a uniform temperature when growth is occurring. The interface is at the melting temperature, while the first-to-freeze end may be in the moderate-temperature regime (see Sections 2.4 and 2.5). There could be conflicting requirements on vapor pressure to avoid inclusions and precipitates. Commonly, control of stoichiometry during crystal growth is accomplished by providing a cadmium reservoir. The reservoir temperature is selected semi-empirically to provide a cadmium pressure of about 1 atm (Kyle, 1971; Asahi et al., 1996), or even higher (Rudolph et al., 1993, 1994), to obtain acceptably low levels of inclusions and precipitates in the final crystal. Simply placing a small excess of cadmium in the ampoule can also provide the required vapor pressure. To avoid the possibility of developing dangerously high pressure, the ampoule should be long enough to keep one end cool. The pressure will not exceed the elemental vapor pressure at the cool-end temperature. This approach can be convenient in the VB method since a special
Binary and Multinary II–VI Compounds (CdTe, CdZnTe, CdMnTe) Grown from the Melt
9
reservoir structure need not be provided. A fairly simple experiment has shown the effectiveness of this method in reducing formation of second-phase particles (Glass et al., 1998b). Since there is preferential vaporization of cadmium from the melt, it may be sufficient to keep the ampoule volume as small as possible. In this case, only a small loss of cadmium occurs before the vapor pressure reaches the equilibrium value. As described in Section 3.1, impurities may segregate at inclusions and precipitates.
3.4
Crystal Defects
The most significant crystal defects are dislocations and twins, which are found in CdTe and related compounds regardless of the crystal growth method. The Nakagawa chemical etch is widely accepted for revealing dislocations (Nakagawa et al., 1979; Mackett, 1994). This etch is effective only on or close to {111}-oriented surfaces. Etch pit densities are about 2 105 cm–2 in good CdTe and about a factor of 10 lower in good CdZnTe. Thermal stresses probably are the major driving force for dislocation multiplication and most likely also induce deformation twins (Rai et al., 1991; Parfeniuk et al., 1992). The yield stress is higher in CdZnTe than in CdTe, consistent with the lower dislocation density in the ternary. However, this hardening in CdZnTe may make deformation twins more numerous (Rai et al., 1991; McDevitt et al., 1986). Frequently, a few lamellar twins run across large, single-crystal regions. Although twins can occur on any of the four distinct sets of {111} planes, within any single-crystal region most of the large twins tend to lie along just one particular set of {111} planes. In VB ingots having one dominant single-crystal grain, this preferred plane often is the one most nearly vertical. If the ingot is sliced parallel to these twins, large, twin free wafers of {111} orientation can be obtained. Twins may form less frequently in the vertical gradient freeze method (Azoulay et al., 1990; Asahi et al., 1996) perhaps because of lower thermal gradients. Thinner, shorter twins (micro-twins) are sometimes seen after a wafer is polished and etched. This has been reported more often in the very large singlecrystal regions obtained in HB ingots.
3.5
Doping
Undoped CdTe and Cd1–xZnxTe (at least up to x B 0.25) can be produced as either p-type or n-type semiconductors. Growth or annealing under cadmium-rich conditions gives n-type crystals, while tellurium-rich conditions give p-type. The most commonly used crystal growth conditions result in p-type material. Even with doping, electrical properties may be difficult to predict due to interactions with native defects and variations in background impurities (Hage-Ali and Siffert, 1995). Gettering by second phases (see Sections 3.1 and 3.3) is a further complication, which can lead to an apparent low level of activation and make a determination of segregation coefficient difficult. Segregation coefficients are generally much less than unity (Astles, 1994), which facilitates purification but leads to nonuniform dopant concentration in the crystal. n-type crystals also are grown by adding a shallow donor dopant to the melt. The most commonly used donors are indium (from group III), which substitutes on the cadmium site, and chlorine (from group VII), which substitutes for tellurium. The compensation effect between indium defects and Cd vacancies in CdZnTe:In ingots was observed and lead to low concentration of free electrons. Doping with indium is often used to move Fermi level to the middle of the energy gap. Consequently, high resistivity with n-type conduction can be achieved (Yang et al., 2005; Xu et al., 2015). Other elements from these groups, aluminum, gallium, and iodine, also have been used. There is a good deal of variability in reported segregation coefficients for growth from the melt. The behavior of aluminum, which appears to have a segregation coefficient 4 1, may be complex. Doping with shallow donors has been used to obtain high resistivity, apparently because compensating deep acceptors are also present. These deep acceptors may be A-centers formed by association of the shallow dopant with a metal vacancy. There has been less interest in p-type dopants since the crystals usually are p-type as grown. The most frequently studied acceptor dopants are lithium and the group IB elements, copper, silver, and gold. All of these are rapid diffusers and have real or potential problems. They can be incorporated substitutionally for cadmium as well as interstitially, especially at higher concentrations. This results in some degree of compensation. The high diffusion rates facilitate formation of complexes and segregation to defects such as second-phase particles (see Section 3.3). Lithium may diffuse out of the crystal. When shallow acceptors are compensated by a deep donor, high resistivity may be achieved (Fiederle et al., 1998). Deep lying impurity states are of interest for obtaining semi-insulating crystals or photorefractive behavior for optical devices. Vanadium is most commonly used to enhance photorefractive effects. Other transition metals have been used to obtain high resistivity (Moravec et al., 1993; Kreissl and Schulz, 1996).
4
Concluding Remarks
Nowadays one can get commercially available large, high-quality CdZnTe crystals and many kinds of X-ray and g-ray detectors manufactured from them. The unique properties of CdZnTe at room temperature make it an ideal detector solution for Medical, Industrial, Homeland Security and Laboratory applications.
10
Binary and Multinary II–VI Compounds (CdTe, CdZnTe, CdMnTe) Grown from the Melt
However, there are still some shortcomings which should be considered. The main reason why production of large and homogenous crystals is still problematic is the segregation coefficient of zinc in CdTe matrix is about 1.35 (Zhang et al., 2011). This is why CdMnTe (Rafiei et al., 2013), CdMgTe (Hossain et al., 2013), and also CdTeSe (Roy et al., 2015) compounds are also being considered as promising materials for gamma radiation detection. CdTe and CdZnTe have a fairly long history and are used in a variety of applications. Many other applications and even more widespread use will develop as further improvements are made in growth of large, high-quality single crystals and in control of optical and electronic properties.
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