Vertical Gradient Freeze (VGF) and Vertical Bridgman (VB) Methods

VGF and VB methods were utilized by Galazka et al. [227, 228] to grow highly conducting p-Ga203 crystals as the Czochralski method shows some limitation in crystal volume due to limited heat transfer through a growing crystal by free carrier absorption, which leads to growth instability. In the VGF and VB methods, a crystal is not pulled out of the melt surface, but the melt just crystallizes step by step entirely inside a crucible during cooling down as the result of imposed vertical temperature gradients.

A growth furnace utilizes a 2-inch-diameter Ir crucible having a cylindrical portion, a seed portion in the form of a small-diameter tube at the bottom part to accommodate a crystal seed, and ashoulder portion connecting both the cylindrical and seed portions. The crucible was covered with an Ir lid with a small opening to minimize temperature gradients within the crucible, avoid contamination of the melt from insulation materials, and to create a path for a growth atmosphere communicating with the melt. The crucible with the lid was surrounded from all sides by thermal insulation comprising alumina and zirconia. The crucible was inductively heated by an RF coil disposed around the crucible and the thermal insulation. As the seed and shoulder portions have a smaller diameter than the cylindrical portion, they are inductively heated much less than the cylindrical portion. Additionally, locating the bottom part of the crucible at the bottom part of the induction coil decreases the probability of melting the seed sitting in the small-diameter tube of the crucible. The melting of the Ga203 starting material within the crucible can be detected by a thermocouple contacting the bottom or top part of the crucible or by a pyrometer monitoring a temperature of the lid. The melting is detected by the fast increase in the temperature typically by several degrees. After holding the melt for a predefined time for homogenization and stabilization in terms of melt flow (for about 1-2 h), the crucible with the melt is cooled down to RT. The imposed vertical temperature gradients will force solidification from the seed in the upwards direction. The initial cooling rate until full solidification should not be too high (below 100 K/h) in order to minimize the formation of high defect density, but on the other hand, very slow cooling rate (a few K/h) may enhance the formation of metallic Ga in large melt volumes, as discussed above. In the case of smaller-diameter crucibles (i.e., smaller melt volumes), the cooling rate can be very small as the formation of metallic Ga is minor. Once the melt is solidified, the cooling rate can be higher. In the discussed example, the initial cooling rate until melt solidification was about 75 K/h, while after solidification about 125 K/h, giving an average cooling rate of 100 K/h.

The seed was oriented parallel to the <010> crystallographic direction, and the growth atmosphere consisted of 17 vol.% 02 +

83 vol.% Ar. High electrical conductivity was achieved by adding 0.1 mol.% ofSn02 to the Ga203 starting material. The solidified material was a single crystal in the central part and polycrystal along the rim adjacent to the crucible wall. The obtained crystals of 55 mm diameter and 25 mm length (cylinder) were dark blue with visible twins, which could be formed due to high coolingrate and differences in thermal expansion of Ga203 and Ir crucible.

The VB and VGF methods to grow bulk p-Ga203 crystals were applied by Hoshikawa et al. [233] and Ohba et al. [276]. In the growth furnace, a crucible and heater surrounding the crucible were both made of Pt70%-Rh30% alloy. The heater with the crucible was surrounded from all sides by a zirconia and alumina thermal insulation, around which an RF coil was disposed operating at 15 kHz. The growth furnace was connected with rotation and translation mechanism to rotate and translate the crucible with respect to the coil. Temperature at the bottom of the crucible was monitored by a В-type thermocouple (PtRh30%-PtRh6%). Two types of crucible were used, cylindrical and conical, at the bottom both being 1 inch in diameter and 2 inch in length (cylindrical part). The advantage of the Pt-Rh alloy is the possibility of using high 02 concentration without the risk of oxidation of the crucible and heater material. The MP of the Pt70%-Rh30% alloy is about 1900°C, i.e., 100 К higher than the MP of Ga203. This is sufficient for 1-inch- and perhaps for 2-inch- diameter crucibles, but it can be problematic for larger crucibles due to temperature gradients between the heater and crucible, as well as within the crucible. Additionally, Rh from the Pt-Rh alloy may contaminate the melt and the obtained crystals.

The rotation and translation rates were 3 rpm and 0.5 mm/h, respectively, while the growth atmosphere was air. The obtained crystals were removed from crucibles by destructive peeling off the crucible material from the crystal instead of core drilling as in the case of p-Ga203 grown by the VGF/VB methods from Ir crucibles.

This was possible because of no adhesion between the p-Ga203 crystals and the Pt-Rh crucibles. However, the crucibles are of single use only.

At early attempts, no crystal seed was used, so the nucleation was spontaneous. As a result, some crystals were entirely polycrystals, some partly poly and partly single crystals, while others were entirely single crystals. The obtained crystals of size 2.5 x 2.5 cm (cylindrical part) had yellowish coloration with some color gradient depending on location within the crucible (Fig. 4.33).

(3-Ga0 crystals obtained by the VB method from a cylindrical

Figure 4.33 (3-Ga203 crystals obtained by the VB method from a cylindrical

(left) and conical-bottom (right) crucibles. Reprinted from Ref. [233], Copyright 2016, with permission from Elsevier.

In the VB configuration, due to larger vertical temperature gradients as compared to horizontal gradients, and due to structural and thermal anisotropy of p-Ga203, the growth starts at the bottom with higher growth rate in the horizontal direction as compared with the vertical direction. With no seed, this will initiate a horizontal growth along the [010] direction and a vertical growth in a direction perpendicular to the (100) plane. Due to spontaneous nucleation, differently oriented grains may form, which explain the formation of polycrystalline material at early stage of crystal growth in some of the growth experiments. As the growth proceeds and temperature gradients change, one of the crystal grains may preferentially expand more in favor of others and lead to the formation of a single crystal. If growth conditions are optimized, this polycrystal-single crystal conversion may initiate at the very beginning of the growth. The formation of (001) facets on the crystal periphery, which form an angle of about 104° with the (100) or (100) planes, leads to a zigzag structure (Fig. 4.34).

Facet formation at the crystal periphery during spontaneous growth of (3-Ga0 by the VB method. Reprinted from Ref. [233], Copyright 2016, with permission from Elsevier

Figure 4.34 Facet formation at the crystal periphery during spontaneous growth of (3-Ga203 by the VB method. Reprinted from Ref. [233], Copyright 2016, with permission from Elsevier.

The main impurities found in the crystals were (in wt. ppm) Rh = 3-30, Pt = 0.5-5, both coming from Pt-Rh crucibles, and Si = 1-10 likely from the Ga203 starting material.

In a similar configuration, the crystals were also grown by the VGF method, i.e., instead of the crucible translation, it was located at a fixed position and slowly cooled down to RT. The vertical temperature profile along the centerline was parabolic (lower temperature at the bottom and higher at the top) with the temperature difference between the top and bottom of about 50 K. The Ga203 starting material was hold for 1 h after melting, i.e., after reaching the MP of Ga203 at the bottom of the crucible. In growth experiments, the crucible was rotated at the rate of 5 rpm. Two types of crucibles were used: cylindrical and conical with the diameter and length of 1 and 2 inch, respectively. After melting, the cooling rate was between 0.75 and 1.5 K/h, while after solidification of Ga203, starting from 1650°C, the cooling rate was 45 K/h. The estimated growth rates arising from temperature gradients and cooling rates were between 0.5 and 4 mm/h. (lOO)-oriented samples prepared from different regions of the crystals grown at different rates, 3.3 and 1.1 mm/h, respectively, at corresponding temperature gradients of 4.5 and 7 К/cm, respectively, revealed different defect densities after etching in 30% KOH at 60°C for 2 h. Two types of defects were observed: dislocation-associated etch pits and line-shaped defects. Additionally, stressed areas at the periphery of the crystals were seen by X-ray topography. The samples grown at a higher rate, 3.3 mm/h, and lower temperature gradients (4.5 К/cm) had a mean etch pit density (EPD) of 2.3 x 103 cm'2 and a mean line-shaped defect density of 4.6 x Ю2 cm"2. On the other hand, the sample grown at a lower rate, 1.1 mm/h and higher temperature gradients (7 К/cm), had a higher mean EPD, 2.9 x Ю4 cm'2, and a lower mean line-shaped defect density, 0.5 x Ю2 cm"2. This suggests that dislocation-related pits are preferably formed at higher temperature gradients, while line-shaped defects at a higher growth rate. The line-shaped defects had the size of 20-150 pm extending along the [010] direction, 0.05-1.2 pm along the [001] direction, and <0.04 pm along the [100] direction. The cross section of the line-shaped defects in [100] and [001] directions is a rhomboid, while lengths of all of the defect edges are inversely related with the lengths of edges of the p-Ga203 unit cell. This type of defects are likely microvoids [276], the same as in the case of the EFG method, resulted from a strong growth anisotropy of p-Ga203.

Conclusion

The growth of bulk p-Ga203 crystals from the gas phase and solution (flux) was carried out at an early stage of research, and nowadays it is not practiced, as large crystals can be grown directly from the melt. Although the Verneuil method was demonstrated for p-Ga203 decades ago, it is not practical due to small crystal size and poor structural quality. The OFZ technique (crucible free), due to its simplicity and low operational costs, was extensively utilized for growing bulk p-Ga203 crystals; however, a typical crystal diameter of about 5-6 mm is not sufficient for applications in a larger scale. However, it is enough for studying the impact of growth conditions (atmosphere, doping, orientation) on the crystal properties. Technologically important growth methods are those involving noble metal crucibles, including the Czochralski, EFG, and possible Bridgman/VGF methods. The advantage of the EFG method is a possibility of obtaining slabs with a large surface area (up to 4

Method

Noble metal crucible

Crystal shape

Maximum crystal size [mm]

Intentional

dopant

Industrial

importance

Reference

Gas phase

PVT

No

Fibers, plates

None

No

201

CVT

No

Fibers, bars, platelets, lumps

3x3x0.2, 2x2x2.5, D=2, L< 5

None

No

208, 209,210,211,212,213, 214,215, 234

Solution

Flux

Yes

Needles, plates, prisms

16 at edge

Cr, Er

No

202, 203, 204, 235, 236, 237, 238, 239

Melt

Vemeuil

No

Cylinders

D=10, L=25

Cr, Fe

No

207, 208, 240

OFZ

No

Cylinders

D=5-8, L< 70 also

D=25, Z,=25

Sn, Si, Ge,

In, Ce, Cr, Ti, Mn, Mg, Co, Cu, Ni, Fe

No

217,218,219, 220, 221,222, 223, 241, 242, 243, 244, 245, 246, 247, 253, 248 249, 254, 255, 256, 257, 258, 259, 260, 261, 262, 263, 264, 265

EFG

Yes

Slabs

18x60,6x110

Sn, Si, Fe

Yes

230, 232, 266

Czochralski

Yes

Cylinders

D=52,L=100

Si,Sn, Ge, Ce, Al, Cr, Mg,

Co, Ni, Cu, Li

Yes

216, 224, 225, 226,227,228, 229,8, 271,324,349

VB / VGF

Yes

Cylinders

D=55,L=25;

D=25,L=25

Sn

Yes

227, 228, 233, 276

inch); however, a demanded (010) orientation is limited to a cross section of the slab. The advantages of the Czochralski method is large crystal volume and cylindrical shape enabling preparation of differently oriented wafers, including 2-inch-diameter (010) wafers. Both methods provide high structural quality crystals and allow for doping with a number of elements. More importantly, they enable scaling-up capabilities what is crucial for industrial- scale crystal production. Possibly the Bridgman/VGF method may become an important technique for growing bulk p-Ga203 crystals, but it requires more development. The basic summary of the growth techniques utilized to grow bulk p-Ga203 crystals is included in Table 4.7.

 
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