Use of ZnY as a source material and an oxidizing agent
The use of ZnY (Y = S, Se, and Те) as the source material for growing ZnO crystals was experimentally studied by Park and Reynolds . ZnS or ZnSe powder is placed in a quartz boat, which in turn is located inside a quartz tube inserted into amullite tube (Fig. 3.12).
The quartz tube was about 50 mm in diameter, while the mullite tube was about 65 mm. During the heating up, only an argon flow was maintained at the rate of 4 cm3/min, and when the starting material reached its sublimation temperature, oxygen flow was initiated at the rate of 0.8 cm3/min. Such a procedure was to avoid the formation of an oxide layer on the source material during an early stage of heating. In the case of ZnS, the temperatures were 1375°C and 1300°C for the source material and growth zone, respectively.
For ZnSe, the temperatures were about 100 К lower. The growth duration was 16 h.
Figure 3.12 Furnace for growing ZnO crystals by the CVT method from ZnS/ ZnSe as the source material. Here the temperature values are for the ZnS source material. Redrawn from Ref. [1S9] with some modifications.
From ZnS, prism-type ZnO crystals grew mainly in the quartz boat, while platelet-type ZnO crystals grew mainly in the quartz tube above the quartz boat and further downstream at lower temperatures. From ZnSe, all ZnO prisms and also some platelets occurred in the quartz boat and many smaller ZnO platelets in the quartz tube at lower temperatures. Dimensions of the largest crystals were 30x5x5 mm3 and 10x6xl mm3 for the prisms and platelets, respectively. The ZnO prisms grown from ZnS had the hexagonal geometry and were bounded by the (1010) and (1120) planes. In the case of the plates, the c-axis was always perpendicular to the plane of the plates. On the other hand, the ZnO prisms grown from ZnSe were hollow and the plates had c-axis normal to the plane of the plates. The differences in the growth habit of the ZnO crystals from ZnS and ZnSe likely arise from a greater interaction of Se (Se has a higher boiling point than S) with a growth interface that alters the diffusion process. Growing ZnO crystals from ZnTe as the source material was not successful due to the high boiling point of Те (1390°C).
Use of ZnO as the source material and a chemically active transport agent
ZnO as the source material (typically of4N-5N purity) for growing ZnO single crystals is the most commonly studied option in the CVT configuration. The CVT growth can be performed at a wide spectrum of temperatures, from relatively low temperatures (typically <1000°C) through moderate to high temperatures (up to about 1300°C). At high temperatures, ceramic materials are used, while low/moderate temperatures enable using quartz as ampoule/ furnace materials. At such temperatures, however, p(Zn) is too low for the growth; therefore, a chemically active agent is applied to transport Zn in the gas phase to a growth zone, where it oxidizes to ZnO. Several chemically active transport agents were investigated: halogens (Cl2, Br2, I2), hydrogen halides (HC1, HBr), mercuric chloride (HgCl2), and ammonium halides (NH4C1, NH4Br, and NH4I). The chemically active agent reacts with the ZnO source material at a combustion zone at Tx and transport Zn to a growth zone at T2, where Zn oxidizes to form ZnO crystals, while X2 diffuses back to the combustion zone (T4). This CVT configuration is shown in Fig. 3.13 and expressed by the following reactions:
Figure 3.13 Schematic diagram of the CVT method with ZnO as a source material and a chemically active transport agent. A(X„), where n = 1, 2, symbolizes the chemically active transport agent, P(02, B) symbolizes reaction products containing 02 and possible other species B, while X is an halogen.
wherein in reaction 3.5, there is an intermediate reaction, where ammonium halide decomposes first to hydrogen halide and ammonia:
and the next reaction (3.4) proceeds. Depending on the halogen X, the temperature regime, and the total pressure, competing direct reactions 3.5 and indirect reactions 3.6 and 3.4 may proceed in parallel.
In each case, Zn is transported from the combustion to the growth zone by the halogen ZnX2. Other products of the reaction include 02,
H20, or H20 + NH3, depending on the chemically active transport agent. Here, oxygen is transported either by 02 or by H20. In Section 18.104.22.168, ZnX2 was directly used as the source material, while in these systems, it is produced from the reaction between the chemically active transport agent and ZnO solid.
Reaction 3.3 is endothermic for all X = F, Cl, Br, and I, so the expected transport proceeds from the zone of higher temperature (Tj) to the zone of lower temperature (T2). This system was investigated by Piekarczyk et al. , Widmer , and Ziegler et al. . Piekarczyk et al.  performed ZnO growth experiments in a quartz glass of 80 mm length and 20 mm diameter. The transport rate was investigated in the dependence of an initial concentration of the chemically active transport agent (0.5 - 8x 10~5 mol/ml), the temperature in the growth zone (700-1150°C), and the temperature difference between the combustion and growth zones (20-150°C).
Br2 was found to be the most and I2 the least efficient transport agent (with I2, only very small crystals were obtained in the form of hexagonal prisms of size below 0.5 mm). Under very similar growth conditions, Br2 and Cl2 resulted in transport rates up to 50 and 10 mg/h, respectively. For Br2 and Cl2, the transport rate increases with the initial concentration in a similar way, as well as with the temperature difference. However, for higher growth temperature, the transport rates changed opposite for Br2 (increases) and Cl2 (decreases). The most optimum growth conditions were found as follows:
• for Br2: concentration of the chemically active transport agent = 2 x 10~5 mol/ml, temperature of the growth zone = 850°C, and the temperature difference = 20-50°C.
• for Cl2: concentration of the chemically active transport agent = 2 - 4 x 10~5 mol/ml, temperature of the growth zone = 900- 1000°C, and the temperature difference = 50°C.
The crystal growth habit was highly influenced by the transport in the case of Br2, but less in the case of Cl2. The obtained ZnO crystals were colorless and sometimes had a yellow-green tint. They were in the form of bars, prisms, or sometimes hexagonal plates. If the transport rate was higher than 30 mg/h, usually polycrystalline materials were obtained. The number of crystals obtained with Cl2 was smaller than that obtained with Br2, but they were 2-3 times larger. The crystal dimensions were 5 mm long and 2 mm2 in cross section, and 5-15 mm long and a few mm2 in cross section for Br2 (growth duration 140-170 h) and Cl2 (growth duration 250-350 h), respectively.
Widmer  used Br2 as the chemically active transport agent within a sealed silica ampoule enabling a vacuum of 10~5 Torr. The ampoule consisted of two integrated ampoules one within other, wherein the source material was located within the inner ampoule having a capillary or conical section acting as a seeding section. To avoid a reaction of the source material with silica, the inner ampoule could be coated with alumina. At operating temperature of 990- 1010°C, the obtained ZnO crystals were hexagonal plates of the size 4 x 3 x 0.5 mm3.
Ziegler et al. , who performed ZnO growth experiments in closed quartz ampoules, pointed out Br2 as an effective transport agent. With an optimum growth temperature between 900 and 1000°C, the obtained ZnO crystals were mainly rod shaped with a hexagonal cross section. The crystals were typically colorless and clear, but some of them were opaque as well.
Hydrogen halides (HCl, HBr) defined by reaction 3.4 are not effective chemically active transport agents according to several experimental works. Piekarczyk et al.  mentioned that experiments with HCl led to polycrystalline precipitates at similar growth conditions to those with Br2 and Cl2 that produced single crystals. Also, Ziegler et al.  and Shiloh and Gutman  stressed the lowest transport with HCl as compared with halogens or ammonium halides at similar growth temperatures. Reaction between ZnO and HBr is exothermic [AH = -13.3 kj/mol at 900°C and 1 atm) and the transport proceeds from the cold to the hot zone. Ziegler et al.  performed growth experiments with HBr and extra elemental Zn in the ZnO source material. At growth temperature between 900 and 1000°C, the obtained ZnO crystals were orange to brown with an increase in the color brightness with growth temperature. In such a system, often Zn particles adhered to the crystals.
Better results were achieved with the use HgCl2 as compared to HCl or HBr. This chemically active transportagentwas experimentally tested by Shiloh and Gutman . Growth experiments were performed in sealed quartz ampoules having an inner diameter of 11 mm and length of 150 mm. Before sealing the ampoules, ZnO was heated at 350°C in vacuum. The temperatures of the source material in the combustion zone and in the growth zone were 1295°C and 1000°C, respectively. Such high temperature difference (about 300 K) was achieved by separation of the combustion and growth zones with a channel for an effective vapor transport. The crystals grew on a ZnO seed fused to a quartz rod in the growth zone. ZnO crystals were yellowish in color and of 2 mm diameter and 8 mm length, wherein they were less colored when obtained with low initial pressure of the chemically active transport agent (i.e., growth at lower transport rate). The crystals grew typically along the c-axis.
Experimental studies of ammonium halides (NH4C1, NH4Br, and NH4I) as chemically active transport agents for growing ZnO crystals were conducted by Shiloh and Gutman , Ziegler et al. , and Matsumoto and Shimaoka . The results of ZnO growth with NH4X in terms of growth rate and crystal size were better as compared to the growth with the use of HX. Matsumoto and Shimaoka  used closed silica ampoules of 9.5 mm diameter and 90 mm length. The temperature of the source material was 1000°C, and the temperature difference between the combustion and growth zones was between 20 and 200 K. The transport rate was almost proportional to the temperature difference between the combustion and growth zones, and increased from 0.6 to 5 mg/cm3 with the increase in the temperature difference from 20 to 200 K.
The obtained crystals were typically rod shaped and of 1.5 mm in diameter and 8 mm in length or prismatic, if a small amount of the transport agent and large temperature difference were applied. The largest ZnO crystals were 1.5 mm in diameter and 8 mm in length. Some of the crystals were colorless, but others were brownish.
The crystals were brown if elemental Zn was used in addition to NH4C1 as the transport agent. Ziegler et al. , who carried out growth experiments in closed quartz ampoules at 900-1000°C, noticed a relatively high transport rate when using NH4Br and NH4C1 as chemically active transport agents. The obtained crystals were typically rod shaped with orange to brown coloration when, additionally, an excess of elemental Zn was used in addition to ZnO as the source material.
Yet other combinations of the ZnO source material and chemically active transport agents can be used. For instance, Matsumoto et al.  used a ZnCl2 transport agent and also elemental Zn in the powdered ZnO source material to increase the transport rate. The ZnO source material was first heated up in air at 600°C for 1 h and then elemental Zn was added. ZnCl2 was added to the ZnO powder as a water solution, which was then dried at 100°C. The experiments were performed in a closed fused silica ampoule of
12.5 mm diameter and 180 mm length. The ampoule with the source material and the transport agent was evacuated to about 10'3 Pa at room temperature and then sealed off. The ampoule was inserted into a vertical Kanthal furnace in such a way that the source material was kept at the temperature of 1000°C while the growth zone at a temperature of 900°C. At such growth conditions, the transport rate was 10-20 mg/h. When using only ZnCl2 as the transport agent, the obtained ZnO crystals were typically needle-shaped; however, after adding elemental Zn, prismatic and substantially colorless crystals of about 1.1 mm diameter and 4.5 mm length were obtained after 100 h growth duration. With a larger excess of elemental Zn, the crystals were yellow to orange.