Bulk Crystal Growth

Stability Fields of In-О System

ln₂0₃ has a high melting point (MP) and is highly unstable at high temperatures. A combination of these two characteristics restricts the conversion of ln₂0₃ to the liquid phase. Flux and vapor growth methods, however, offer much lower operating temperatures although the size of such crystals is very small. The stability diagram of the In-0 system shown in Fig. 5.3 was obtained with the FactSage software combined with experimental data of Galazka et al. [95, 96].

At very low oxygen partial pressure p(0₂), roughly below 10'⁹ atm and temperatures up to about 2080°C, only elemental In in the liquid phase In(liq) is stable. The stability of the solid phase of ln₂0₃(s) requires increasing p(0₂) with rising temperature; otherwise, a gas phase (ideal gas) will form. At high temperatures, roughly above 2080°C, only gas is present in the system for p(0₂) values up to 1 atm.

The liquid phase of ln₂0₃(liq) is confined only to very small window limited by a temperature range of about 1950-2080°C and p(0₂) of 0.03-1 atm. This estimated Ellingham diagram points to obtaining ln₂0₃ crystals from the melt as really challenging, although possible, as demonstrated further below. Growth from the gas phase can be practiced at low or high temperatures in a wide range of p(0₂) in the system, while flux growth would require temperatures high enough to melt the flux and a higher p(0₂) at similar operating temperatures as compared with the gas phase growth methods.

Figure 5.3 Diagram of stability fields of ln,0₃ (Ellingham-type diagram).

Historical Events

First bulk ln₂0₃ single crystals were reported in 1954 by Roy and Shafer [131] with the use of the hydrothermal method. Gas phase (PVT) grown bulk crystals were first obtained by Weiher [132] in 1962, while the flux method was first applied by Remeika and Spencer [133] in 1964. A typical chemical vapor transport (CVT) method for growing bulk ln₂0₃ crystals was likely first utilized by De Wit [134] in 1972. Until 2013, ln₂0₃ single crystals have not been grown directly from the melt, when Galazka et al. [94] defined and developed a novel crystal growth technique under the name Levitation-Assisted Self-Seeding Crystal Growth Method (LASSCGM), which enabled to obtain large ln₂0₃ single crystals. That short history of bulk crystal growth of ln₂0₃ is depicted in Fig. 5.4.

Vapor Phase

The growth of ln₂0₃ from the gas phase can be divided into three groups depending on the creation and transportation of In- containing species from a source zone to a growth zone, where the In-containing species is oxidized to solid ln₂0₃ in the form of crystals:

• (i) A direct thermal decomposition of ln₂0₃ starting material that involves creation of volatile ln₂0, its transportation, and re-oxidation to ln₂0₃ solid (crystal). This technique is purely physical vapor transport (PVT) method.
• (ii) Use of a reducing agent (graphite) and elemental In or the reducing agent in a combination with ln₂0₃ or ln₂0₃ + In starting material that facilitates the formation of volatile ln₂0 at lower temperatures. It is also called a vapor reaction method.
• (iii) Use a chemically active transport agent, such as S + I₂ or Cl₂, for transporting In-containing species, and its oxidation to ln₂0₃ solid (crystal). This technique defines the CVT method.

Figure 5.4 Main events in bulk crystal growth of ln₂0₃.

Thermal decomposition of ln2О3 (PVT)

The PVT growth experiments of ln₂0₃ crystals by thermal evaporation (sublimation) were conducted by Galazka et al. [95]. Solid ln₂0₃(s) decomposes at high temperatures (source temperature, Т$) and forms mainly gaseous ln₂0(g) of high partial pressure (plus other species, but at much lower partial pressure):

which is transported to a cold region (T_s - AT) at the substrate (seed) where it oxides into solid ln₂0₃(s):

The growth experiments were performed in a growth furnace consistingof a 40 mm diameter Ir crucible, a sapphire substrate on the top ofthe crucible, and a thermal insulation surrounding the crucible with the substrate from all sides. The crucible was inductively heated by an RF coil. The starting material was dried, pressed, and sintered ln₂0₃ powder of 4N purity. The growth experiments were performed with the use of C0₂ growth atmosphere and about 1600°C growth temperature (at sapphire substrate). The temperature of the crucible was about 1700-1800°C. Such high temperature was sufficient to produce high partial pressure of ln₂0(g) resulting from the ln₂0₃(s) decomposition, which was transported to sapphire and reacted with oxygen from C0₂ and ln₂0₃ decompositions. The distance between the starting material and the substrate was about 2 cm.

The obtained ln₂0₃ crystals of almost 40 mm in diameter and 3 mm thickness were green and transparent in visible light (Fig. 5.5). They consisted of small crystal grains of 0.1-0.2 mm having two different orientations of <111> and <100>. Interestingly, the crystal coloration and grain structure were not affected by any annealing in an oxidizing atmosphere at 400-1400°C for 20 h.

Figure 5.5 ln₂0₃ crystal obtained from the gas phase by the PVT method.

Use of a reducing agent

This technique has a few variations depending on the source material, which can be solid In(s), solid ln₂0₃(s), or a mixture of In(s] + ln₂0₃(s). With the use of a reducing agent and the presence of residual oxygen, volatile ln₂0(g) is created at relatively high temperatures, but much lower as compared with thermal decomposition of solid ln₂0₃. Volatile ln₂0(g) is transported to a cold region, where it is oxidized to solid ln₂0₃ in the form of crystals.

Weiher [132] and Weiher and Ley [135] used as a starting material a mixture of In metal and carbon. The growth experiments were performed in a covered porcelain crucible that was heated in ambient oxygen at 1000°C for 24 h. Needle-shaped, pale-yellow crystals approximately of 0.5 x 0.5 x 5 mm³ or plates with an area of approximately 5 mm² and thickness of 30-100 pm in size grew on the crucible wall and the cover. The cross section of the needle- shaped crystals was either square or hexagonal, corresponding to [100] and [111] growth directions, respectively. The main identified impurities were Si and Mo at concentration of about 1 wt. ppm.

Shimada et al. [136] used different systems for growing ln₂0₃ crystals from the gas phase: graphite-ln₂0₃, active carbon-In₂0₃, graphite-In, graphite-ln₂0₃-ln, and In₂0₃-In.

In the C-In₂0₃ systems, solid ln₂0₃ is first reduced to gaseous ln₂0 at high temperatures which is transported to a lower temperature zone T_s - AT, where it is re-oxidized to solid ln₂0₃. Thermodynamically favorable reactions are as follows:

wherein p(ln₂0) > p(In).

In the systems with metallic In at high temperatures, liquid In with oxygen and solid ln₂0₃ form gaseous ln₂0:

which is next re-oxidized to form ln₂0₃ crystals according to Eq. 5.2.

Powders of these systems of 1.3-4 g were mixed in an appropriate molar ratio, placed in 5 ml porcelain crucibles and heated in air at 800-1300°C for 12 h. Tiny crystals deposited along the junction between the crucible rim and a cover as well as on inner walls of the crucible. That junction is the passage to the interior of the crucible for oxygen, which reacts with ln₂0(g) to form ln₂0₃ crystals. As at the operating temperature p(0₂) is more than 10 orders of magnitude smaller than p(ln₂0), additional oxygen in the growth furnace is likely needed for the formation of ln₂0₃ crystals.

The graphite-ln₂0₃ system produced at temperatures 960 and 1000°C transparent pale-yellow needle-shaped crystals of size 0.1 x 0.1 x 3, 0.3 x 0.3 x 3, and 0.5 x 0.5 x 7/8 mm³, depending on the temperature and molar ratio of graphite/ln₂0₃, which was between 20/1 and 40/1. Above 1040°C, fine crystals were formed between the cover and the crucible rim. In the system, active carbon-In₂0₃ fine yellow crystals were obtained only at 900°C. It was found that the favorable amount of the starting material for the 5 ml crucible was 2.4-2.9 g; otherwise, the crystals became much smaller. When using the graphite-In system, transparent crystals of size 0.3 x 0.3 x 10 mm³ were obtained at 1000°C. They were yellow or blue/ black if metallic In was used in the form of powder or granules, respectively. The In₂0₃-In system produced fine yellow crystals at 1100°C and opaque yellow at 1200°C, but no crystals grew below 1100°C or above 1200°C. When using the graphite-ln₂0₃-ln system as the starting material, the obtained crystals were fine and yellow for the temperature range 1000-1080°C. Above 1080°C, no crystals were obtained. Investigation of the largest crystals obtained from the graphite-ln₂0₃ system showed the growth direction along the o-axis.

In other growth experiments from the gas phase, Shimada et al. [137] used the In₂0₃-Sn0₂-graphite system. The starting material was ln₂0₃ and Sn0₂ in the same molar content, and graphite in the amount of 5.6, 10, and 15 wt.%. The powders were mixed in a mortar and placed in a silica boat of size 40 x 10 x 7 mm³, which was covered by a similar silica boat. The boats with the starting material were placed in a tube furnace of 25 mm diameter, through which N₂ was flowing at the rate of 100 ml/min. The furnace was heated up to 860-1100°C at the rate of about 8 K/min and held at these temperatures for 3-6 h.

Crystal growth did not occur if a flowing air or uncovered boat was used, or at temperatures below 960°C. At temperatures above

960°C, needle-shaped or dendrite crystals were obtained on the walls and the bottom of the boat. Typically, white, cotton-like Sn0₂ crystals were grown, which were overlapped by ln₂0₃ crystals. The size and coloration of ln₂0₃ crystals were strongly correlated with the growth temperature: at temperatures up to 1000°C, the ln₂0₃ crystals were yellowish-green of size 1 x 0.05 x 0.05 mm³, which became green and of larger size, 10 x 0.1 x 0.1 mm³, when grown at 1045°C. The needle-shaped ln₂0₃ crystals were square in a cross section, which contained, the same as dendrites, a varying concentration of Sn, between about 7 and 30%. The most contaminated ln₂0₃ crystals with Sn were located at the bottom of the silica boat in the proximity of cotton-like Sn0₂ crystals.

The growth of ln₂0₃ in the ln₂0₃-graphite system (i.e., without Sn0₂) occurred only at 1045°C, with the resulted very small crystals.

This indicates an important role of Sn0₂ in the transport mechanism.

Here, solid Sn0₂ is reduced by graphite to gaseous SnO in the same way as ln₂0₃:

Both ln₂0(g) and SnO(g) react with each other to form oxides in the solid phase:

Analysis of the products after growth experiments revealed, indeed, Sn metal and Sn-In alloy, indicating the formation of both Sn and In metals.

Use of chemically active transport agent (CVT)

In this type of vapor growth, the transport agent aims to bind In and transport it to a growth zone, where it is oxidized to solid ln₂0₃. De Wit [134] used HC1 as the chemically active transport agent. One gram of solid ln₂0₃ was placed in one end of a quartz ampoule of 1 cm inner diameter and 20 cm length. The ampoule was evacuated and then filled with HC1 vapor at a pressure of 2000 Pa at room temperature (RT). The ampoule was next sealed off and horizontally placed within a furnace in such a way that its end with ln₂0₃ was in the hot (reaction) zone at 950°C, while the other end was in a cold (growth) zone at 680-720°C. The transport rate was 4 mg/h. The growth took place at the growth zone for several days. The obtained small crystals of 1 mm³ in volume were green and octahedrally shaped. At lower HC1 pressure, the growth habit was different, and the crystals were in the shape of cubes.

The transport reaction at high temperatures in the In₂0₃-HCl system and a reverse reaction at lower temperatures can be written as follows:

Cl₂ as the chemically active transport agent was used by Jozefowicz and Piekarczyk [138]. The experiments were made in a closed quartz ampoule with an inner diameter of 19 mm and length of 110 mm. Three grams of ln₂0₃ was loaded into the ampoule, which was then evacuated to about 10'³ Pa. Next the ampoule was filled with about 0.15 g of chlorine. The ampoule was sealed off and placed in a two-zone furnace. For 24 h, the ampoule was kept in a reverse temperature gradient (850°C in the reaction zone and 950°C in the growth zone) to clean the ampoule in the growth zone. After that the temperature was changed to 950°C in the reaction zone and 900°C in the growth zone. The growth runs lasted for about 100 h. The transport did not start if the amount of Cl₂ was smaller than 0.12 g and the temperature in the reaction zone was smaller than 950°C. The temperature difference between the reaction and growth zones, AT, should be larger than 30 K; however, the larger the AT, the smaller the crystals obtained. Temperature in the growth zone larger than 900°C resulted in a strong crystal adherence to the ampoule walls.

The obtained crystals were cubes in average size of 3 x 3 x 3 mm³. They were yellow and transparent. The transport rate was about 9 mg/h. The obtained crystals were much larger than those grown with HCl or I₂ + Cl₂.

Although different species of indium chlorides are formed, the dominating reaction is:

At the operating temperatures (<1000°C), the highest partial pressure in the In₂0₃-Cl₂ system has InCl₃, 0₂, and Cl₂, as shown in Fig. 5.6. The thermodynamic calculations clearly show that In is transported by InCl₃(g), which has the highest partial pressure in the system above 650°C. Comparable p(0₂) and p(InCl₃) enable reoxidation of In to form ln₂0₃ single crystals. The transport proceeds from the hot (reaction) zone to the cold (growth) zone. In the case of using HC1 instead of Cl₂, p(0₂) is smaller by about three orders of magnitude than p(InCl₃) at 850°C at the same precursor amounts and pressure. This would explain larger crystals in the case of ln₂0₃- Cl₂ as compared with the In₂0₃-HCl system. When considering the In₂0₃-Cl₂-I₂ system, the ratio of p(InCl₃)/p(0₂) is higher than that in the In₂0₃-Cl₂ system. From the thermodynamic point of view, the In₂0₃-Cl₂ system seems to be very effective in growing ln₂0₃ single crystals from the gas phase.

Figure 5.6 Calculated partial pressures of the most volatile species in the ln₂0₃-CI₂ system with ln₂0₃ = 3 g and Cl₂ = 0.15 g, the same as in Ref. [138], and p = 5 atm.

Werner et al. [139] and Behr et al. [140] used 4N purity of Sn0₂ and ln₂0₃ powders as a source material to grow Sn-doped ln₂0₃ and In-doped Sn0₂ crystals and S +1₂ or Cl₂ as chemically active transport agent. The source material in the amount of about 1.5 g was placed, together with the transport agent, in closed silica ampoules of 1.5 cm diameter and 15 cm length. The ampoules with the S + I₂ transport agent were evacuated at low temperature to about 10~³ bar and then sealed off. The ampoules with gaseous Cl₂ were filled up to 450-550 mbar at RT. The total concentration of the transport medium was 20 pmol/ml, i.e., 10 S + 10 I₂, and 20 Cl₂ pmol/ml. The experiments were performed in a horizontal configuration for about 10 days, where the source material was kept at a temperature of82 7- 1027°C, while the growth zone at a lower temperature by 50,100, and 150 K.

For the S + I₂ transport agent, which has a reducing character, I₂ is the transport medium for metals, while S is the transport medium for oxygen:

No sulfides were grown, unless the ratio of S/I₂ was 2, when very small platelets of SnS₂ were found. The residual impurity level of the transport agent in the crystals was less than 10 wt. ppm.

For the Cl₂ transport medium, Cl is the transport agent for metals with the release of free oxygen:

p(SnCl₄) is about 3 orders of magnitude smaller than p(InCl₃) when Cl₂ is used as the chemically active transport agent, so Sn is almost not transported and the obtained crystals are ln₂0₃ doped with Sn. This is visible in Fig. 5.7, showing p(InCl₃), p(SnCl₄), and p(0₂), as the p(InCl₃)/p(SnCl₄) ratio is almost 10³ above 600°C. In₂0₃ crystals doped with Sn (4 mol.%) were up to 5 mm in size.

Figure 5.7 Calculated partial pressures of lnCI₃, SnCI₄, and 0₂ in the ln₂0₃- Sn0₂-Cl₂ system with 1 mol of each of ln₂0₃, Sn0₂, and Cl₂, and p = 5 atm.

In the case of the S + I₂ chemically active transport agent, In is transported by Inl₃ and Ini, while Sn by Snl₂. The ratio ofpflnl, Inl₃)/ p(SnI₂] is about 160 and 43 at 850°C and 1000°C, respectively, which enables a higher doping of ln₂0₃ crystals with Sn at higher growth temperatures as compared with the Cl₂. Crystal stoichiometry in terms of oxygen vacancies seems to be better for the Cl₂ as compared with the S +1₂. This is due to the much higher p(0₂) in the case of using Cl₂, by several orders of magnitude, because oxygen is consumed by sulfur in the case of S + I₂.

In₂0₃ and Sn0₂ solid solutions always lead to an ln₂0₃-rich phase. The Sn0₂ phase can be obtained only with the use of the S + I₂ transport agent.

The phase boundaries have been determined to be 8.2 ± 0.2 mol.% Sn0₂ in ln₂0₃ and 2.4 ± 0.1 mol.% ln₂0₃ in Sn0₂. In the case of liquid phase epitaxy on sapphire grown from Sn-In melts, the solubility of Sn in ln₂0₃ and solubility of In in Sn0₂ of 7 ± 2 at.% and 4 ± 2 at.% at 600-800°C was determined [141].

Figure 5.8 ln₂0₃ single crystals obtained by the CVT method. Reprinted from Ref. [121], Copyright 2012, with permission from AIP Publishing.

The I₂ + S transport medium for growing ln₂0₃ crystals from the gas phase was also used by Scherer et al. [142]. As the source material, 5N purity ln₂0₃ powder was used. A four-zone horizontal furnace was utilized, which was tilted to the horizontal axis by 7° to facilitate vapor transport via gas convection. Two outer zones were buffer zones to minimize the influence of the surrounding on the temperature distribution in the reaction and growth zones.

The obtained crystals, as shown in Fig. 5.8, were transparent and green plates or prisms with the maximum size of 3 x 3 x 1 cm³.