Intentional doping and dopant segregation

An important aspect in crystal growth technology is the control of residual impurities or intentional doping, which affect not only optical, electrical, and possible magnetic properties of obtained crystals, but also the growth process itself through growth kinetic, defect formation, heat absorption in a growing crystal, and constitutional supercooling. Intentional doping constitutes an effective tool to expand the functionality of p-Ga203. The Czochralski- grown bulk p-Ga203 single crystals were intentionally doped with a number of mono-, di-, tri-, and tetravalent ions.

An incorporation of the dopant into the crystal lattice is quite complex, and it depends on (i) dopant properties (ionic radii, valency, coordination, and partial pressure), (ii) crystal properties (structure and ionic radius of an ion to be substituted by the dopant), (iii) dopant solubility, (iv) melt properties (kinematic viscosity, diffusion coefficient), (v) growth conditions (temperature gradients across the interface and oxygen partial pressure), (vi) growth kinetic (faceting), and (vii) operating parameters (growth and rotation rates).

For given growth conditions and operating parameters during the growth of bulk p-Ga203 single crystals by the Czochralski method, the amount of incorporated dopant into the crystal structure is mainly governed by two factors: (a) ionic radius of the dopant that supposes to substitute Ga3+ in either at tetrahedral (IV) or octahedral (VI) site; and (b) partial pressure of its volatile species, if any.

(a) Ionic radii of the dopants

Coordination- and valency-dependent ionic radii of various elements that were used for intentional doping bulk p-Ga203 single crystals grown from the melt by the Czochralski method are shown in Table 4.5. The list reveals that all the ions of interest, except Ce, have not too large difference in ionic radii with respect to Ga3+ ions, so we can expect their incorporation into p-Ga203 crystal lattice with certain segregation coefficient. As will be shown further below, Ce, indeed, is very poorly build in the Ga203 crystals, while others more easily. From the point of view of ionic radii, the most efficient dopant incorporation into the crystal lattice is expected for Mg2+, Co2+, Ni2+, Cr3+, Al3+, Ge4+, and Sn4+ ions. However, this picture is disturbed by the thermal stability of the dopants.

Table 4.5 Shannon ionic radii of elements used for doping p-Ga203 crystals at tetrahedral (IV) and octahedral (VI) sites

Element

Ionic radii

IV

VI

Ga3*

0.47

0.62

Li1*

0.59

0.76

Mg2*

0.57

0.66

Cu1*

0.6

0.77

Cu2*

0.57

0.73

Co2*

0.58

0.65/0.745

Ni2*

0.55/0.49

0.69

Cr2*

0.73

0.8

Cr3*

0.615

Cr4*

0.41

0.55

Al3*

0.39

0.535

Ce3*

1.01

Ce4*

0.87

Si4*

0.26

0.4

Ge2*

0.73

Ge4*

0.39

0.53

Sn4*

0.55

0.69

(b) Thermal stability of the dopants

Dopants are added to the Ga203 starting material in the form of metal oxides or carbonates. However, not all the dopant oxides are thermally stable at high temperatures and will decompose during the growth of crystals in a similar manner to decomposition of Ga203. As a result, the dopant in quest will be partially or almost entirely lost from the melt due to evaporation in the form of an oxide, suboxide, or elemental metal, depending on metal oxide or carbonate. This issue is visualized in Fig. 4.27. The most stable dopants are Si02, Ce02, A1203, and MgO (region (I) in Fig. 4.27), and their most volatile species (Si02, CeO, AlO, Mg) have low or very low partial pressures, much lower than those of Ga-containing species (Ga20, GaO). Losses of these dopants during growth will be neglecta- ble. Another group of dopants consists of Cr203, CoO, and NiO

(region (II) in Fig. 4.27), the volatile species of which (Cr02, Co, Ni) have similar partial pressures to those of Ga203. They will partially evaporate during growth, but it is not critical. Volatile species of dopants CuO and Li2C03 (region (III) in Fig. 4.27) have much higher partial pressures as compared to Ga- containing species. Their losses (as Cu and Li, respectively) from the melt during growth will be significant; therefore, it should be compensated by higher initial concentration, higher p(02), and possible higher total pressure. And finally, volatile species of Sn02 and Ge02 (region (III) in Fig. 4.27) at the MP of Ga203 are very or extremely high. Even at external pressures of 150 and 1000 atm, the partial pressures of their volatile species (SnO and GeO, respectively) are still very high, which may lead to their high or almost entire loss during growth. These dopants are not well suited for the long growth runs and large growth systems. In addition to losses, their incorporation into the crystal lattice will be highly non-uniform with crystal length.

Partial pressures of volatile species of different metal oxides used as dopants for 3-Ga0 single crystals grown by the Czochralski method

Figure 4.27 Partial pressures of volatile species of different metal oxides used as dopants for 3-Ga203 single crystals grown by the Czochralski method. In all cases, p(02) = 0.02 atm. The circles, squares, and triangles refer to a total pressure of 1,150, and 1000 atm. The dashed line at 10~3 atm indicates the limit above which there is intensive evaporation. Adapted from Ref. [324].

When summarizing possible dopants from the point of view of their thermal stability, the most suited dopants of different valency are Si4+, Al3+, Ce3+, Mg2+, which are added to the Ga203 starting material as Si02, Al203, Ce02, and MgO. This is shown as region (I). Other dopants shown in region (II), i.e., Cr3+, Co2+, and Ni2+, which are used in the form of Cr203, CoO, and NiO, can effectively dope p-Ga203 crystals, but dopant losses through evaporation and dopant concentration gradient along a crystal are expected. Dopants requiring technical attention (e.g., high initial concentration and short growth time) are Sn4+ (Sn02), Cu1+ (Cu20), and Li1+ (Li2C03), which are located in region (III). Moreover, Ge4+ (Ge02) can be used only in very short growth runs; otherwise, it will entirely evaporate.

In such cases, a high dopant concentration gradient will be produced in growing p-Ga203 crystals.

All the elements listed in Table 4.5 and Fig. 4.27 were used for doping bulk p-Ga203 single crystals grown by the Czochralski method, which are shown in Fig. 4.28. As the growth conditions were very similar to each other, particularly p(02) in a growth atmosphere, we can clearly see changes in crystal appearance in terms of coloration, surface roughness, and shape as the result of doping. Undoped crystal is bluish, has a straight cylindrical shape, and convex interface.

The free carrier concentration is not high enough for the interface inversion and spiral formation. Tetravalent ions Si, Sn, and Ge all were demonstrated to enhance the free carrier concentration, which has an impact on growth stability through free carrier absorption in the NIR spectral region (see Section 4.5.6.3). Indeed, this is valid for doping with Si and Sn, which make the crystals blue with a tendency for spiral formation after reaching a certain length. As a dopant, Si02 is thermally stable at operating temperatures during the growth of p-Ga203 by the Czochralski method, so its incorporation into the growing crystal is decent. Sn02 is highly volatile, and less Sn4+ ions are incorporated into the crystal despite higher initial concentration, which is indicated by light blue coloration that almost disappears at the bottom part of the crystal as most of the Sn02 dopant in the melt has evaporated. As shown in Fig. 4.29a, low Sn02 doping level (0.2 mol.%) led to a colorless crystal, indicating very low Sn concentration in the obtained crystal, while higher Sn02 doping concentration (1 mol.%) led to blue crystals due to relatively high free carrier concentration donated by Sn. Different results were obtained by doping with Ge02. The crystal is substantially colorless, and the interface is nicely convex even at high doping level of Ge02

(2 mol.%). This is due to the already discussed very high partial pressure of GeO and corresponding entire loss from the melt (no Ge was detected in the obtained crystals). Tri-, di-, and monovalent dopants do not enhance the free carrier concentration, but rather compensate it (at least some of them). Due to minimized free carrier absorption, the crystals have convex interface and can be grown as straight cylinders with a reasonable length. Doping with Ce, Mg, and Co resulted in yellow crystals, with Ni resulted in brownish, with Cr resulted in green, and with A1 and Li resulted in colorless crystals. In the case of highly volatile Li dopant, the crystal appearance is very similar whether the Li20 doping level was either 1 or 2 mol.% (Fig. 4.29b). A blue coloration of Cu-doped p-Ga203 crystal is associated with high free carrier concentration (lower oxygen concentration in the growth atmosphere) rather than by Cu itself (no Cu was found in the crystals).

Czochralski-grown p-Ga0 crystals, undoped and doped with Si, Sn, Ge, Ce, Al, Cr, Mg, Co, Ni, Cu, and Li

Figure 4.28 Czochralski-grown p-Ga203 crystals, undoped and doped with Si, Sn, Ge, Ce, Al, Cr, Mg, Co, Ni, Cu, and Li. Indicated dopant concentrations are in mol.% and relate to initial concentration in the Ga203 starting material. In all cases, oxygen concentration in a growth atmosphere was between 2 and 4 vol.%.

Czochralski-grown p-Ga0 crystals doped with Sn and Li at different concentrations

Figure 4.29 Czochralski-grown p-Ga203 crystals doped with Sn4+ and Li1+ at different concentrations (in mol.% in the melt as Sn02 and Li20). Both dopant oxides are thermally unstable at high temperatures (see Fig. 4.27). Reprinted from Ref. [349], Copyright 2020, with permission from Elsevier.

At given growth conditions, all the crystals are clear and transparent, with enhanced absorption in Si- and Cr-doped crystals due to high free carrier absorption (red spectral region) and blue Cr absorption, respectively. The crystals have smooth or a bit rough (undoped, doped with Co, Ni, and Li) surface.

Tetravalent ions Si4+, Sn4+, and Ge4+ aim to increase the free electron concentration in p-Ga203 single crystals. Tested Si02 doping levels in the Ga203 melt for the Czochralski method were between 0.1 and 1 mol.%. Due to good thermal stability of Si02 at the MP of Ga203, low doping level of 0.1-0.2 mol.% is sufficient to achieve the maximum level of the free electron concentration, which is about 1019 cm'3. Sn02 concentrations in the Ga203 melt were between 0.1 and 2 mol.%, wherein high free electron concentrations (>1018 cm"3) could be obtained only for high Sn02 doping level >1 mol.%. As expected from the thermal instability of Ge02 at high temperatures, no Ge was found in the crystals even at high initial doping level of the starting material. Therefore, Ge did not only enhance the free electron concentration for Ge02 doping levels between 0.1 and 2 mol.% but decreased as compared with undoped crystal obtained under substantially the same growth conditions. The reason is decomposition of Ge02, the product of which is GeO and oxygen that is left inside the melt (Ge02 —» GeO(g) + %02(g)). In other words, Ge02 can be used as an effective oxygen source without any doping effect of Ge, which is entirely lost from the melt. The advantage is the oxygen production inside the melt.

Although there is an effective interaction of oxygen from the growth atmosphere and the melt surface, its diffusion to the melt volume is rather slow and limited. Ge02 doping as an oxygen source can be particularly important during the growth of large-diameter p-Ga203 single crystals (>2 inch), which will stabilize the growth through the entire decomposition process of the dopant oxide. Similar effect has also Ce02, which releases an oxygen in the melt, but through another process, i.e., valency change (see further below).

Trivalent dopants Ce3+, Al3+, Cr3+ rather do not have a direct impact on electrical properties of bulk Ga203 single crystals obtained by the Czochralski method but may have an indirect impact. Such ions influence optical properties of bulk Ga203 crystals. In particular, Ce3+ modifies scintillation properties under gamma quanta, Al3+ enlarges the bandgap, while Cr3+ induces electroluminescence [273] and could potentially constitute an active element in the Ga203 host for solid-state lasers. Ce02 doping level in Ga203 melt was practiced between 0.1 and 2.5 mol.%. Due to very low segregation coefficient of Ce arising from a very large ionic radius (see further below), rather high Ce02 concentration (>0.5 mol.%) is required to obtain a reasonable Ce concentration in the crystals (>20 wt. ppm). A concentration of Al203 in the melt was practiced at high levels, between 2 and 5 mol.%. Doping Ga203 crystals with 5 mol.% A1203 increases the bandgap by about 0.2 eV. Higher doping with Al203 is still possible (up to about 10 mol.%); however, it will be occupied by deteriorated structural quality, mainly due to constitutional supercooling. In the case of Cr203, its initial concentration in the melt was practiced in the range of 0.04-1 mol.%. Cr incorporates into the p-Ga203 crystal lattice easily (high segregation coefficient— see further below) and even the lowest initial Cr203 concentration makes the crystals greenish, i.e., it is sufficient to introduce absorption bands in the blue spectral region.

Divalent ions Mg2+, Co2+, and Ni2+ aim to compensate the electrical conductivity of bulk p-Ga203 crystals and make them electrically insulating or semi-insulating, what is often required for epitaxial purposes (as substrates), e.g., to fabricate horizontal electronic devices thereon. Indeed, Mg2+ was found to be a very efficient compensator of shallow donors. MgO concentration in the Ga203 melt of 0.1-0.2 mol.% was already sufficient to turn the crystals into electrically insulating (or semi-insulating) state. CoO and NiO at concentrations of 0.1-0.25 mol.% and 1 mol.% in the melt, respectively, also compensated the electrical conductivity in bulk p-Ga203 single crystals obtained by the Czochralski method, as the obtained crystals were electrical insulator or semi-insulators. Therefore, all the Mg2+, Co2+, Ni2+ dopants are practical dopants for bulk p-Ga203 crystals that can effectively be utilized as electrical conductivity compensators.

And finally, both Cu1+ and Li1+ are supposed to be the most effective shallow donor compensators in bulk (3-Ga203. Due to the very high partial pressure of Li and Cu and their associated high losses, the initial Li20 concentration in the melt (added to the Ga203 starting material as Li2C03) was also high, 1 and 2 mol.%. Li2C03 added to the starting material releases upon heating C02, which is an additional oxygen source. Here, indeed, the obtained crystals were electrically insulating. However, Cu20 concentration of 0.2 mol.% in the melt did not compensate the shallow donors due to too high Cu losses by evaporation and possible marginal (below detection limit), if any, amount incorporated into the crystals, which was not sufficient to compensate the electrical conductivity of p-Ga203.

A concentration of the dopant/impurity (whether intentional or not) in the growing crystal is described by the Scheil-Gulliver equation for non-equilibrium solidification with the assumption of a complete mixing of the dopant in the melt and no diffusion of the dopant in the crystal (which are typically met in the growth of oxide crystals by the Czochralski method):

where Cs is the dopant concentration in the crystal, C0 is the initial dopant concentration in the melt, keff is the effective segregation coefficient, and g is the solidified fraction of the melt.

The effective segregation coefficient keff in the steady-state case is described by the Burton, Prim, and Slichter (BPS) model [274], while its more realistic description was given by Ostrogorsky and Muller [275], who have taken into account a convective dopant transport next to the solid-liquid growth interface.

Knowing the segregation coefficient, it is easy to select a proper initial dopant concentration in the melt to obtain a desired dopant concentration in the crystal at given growth conditions and operating parameters. This can be carried out by measurements of the dopant concentration at different crystal locations corresponding to the crystallization fraction and fitting the measured values with the Scheil-Gulliver equation. This works well for many oxides, such as Y3A15012 (YAG) or sapphire. In the case of (3-Ga203, which decomposes and evaporates, the fitting is less accurate, as the initial dopant concentration dynamically changes with growth time. Also, some of the dopant elements, their oxides or suboxides have a high vapor pressure and evaporate as well (see Fig. 4.26 and corresponding discussion). Moreover, some dopants may change their oxidation state depending on p(02) and form different oxides having different partial pressures, which, due to different ionic radii, may change the effective segregation coefficient. In such cases, the segregation coefficient is not constant but may dynamically change with the crystallization fraction, causing a deviation from the Scheil- Gulliver equation.

Dopant concentrations of Cr

Figure 4.30 Dopant concentrations of Cr (a), Al (b), Mg (c), and Ce (d) of doped p-Ga203 crystals obtained by the Czochralski method. The dots are measured values, while solid lines represent fittings with the Scheil-Gulliver equation. Figures (a), (b), and (d) reprinted from Ref. [229], Copyright 2018, with permission from Elsevier.

If the segregation coefficient is greater than unity (e.g., Cr), the dopant concentration will decrease from the top to the bottom part over the crystal length. For segregation coefficients below unity (e.g., Mg, Sn, Si, Ce), the situation is reversed, while for those close to unity (e.g., Al) the dopant will be distributed in the crystal relatively uniformly. The segregation phenomenon with the segregation coefficients deviating from unity will cause a non-uniform dopant distribution along the crystal length. In addition to the longitudinal segregation, there is also a lateral dopant segregation (across the crystal) due to the shape of the growth interface that is different from a planar one and usually changes as the growth proceeds.

Examples of measured dopant concentrations in Czochralski- grown p-Ga203 crystals as a function of the crystallization fraction, compared with the fitting of the Scheil-Gulliver equation, are shown in Fig. 4.30 for Cr (a), A1 (b), Mg (c), and Ce (d), which have the segregation coefficient greater than unity, close to unity, smaller than unity, and much smaller than unity, respectively.

The deviation of the Cr concentration in (3-Ga203 crystals from the Scheil-Gulliver equation (Fig. 4.30a) arises from the change in the oxidation state of Cr that forms different oxides: Cr02 and Cr03, which have different partial pressure and evaporate at different rates. In addition, there is also evaporation of the main constituent forming the melt, Ga20. The formation and partial pressures of high- valency Cr oxides and Ga20 depend on p(02) in such a way that both p(Cr02) and p(Cr03) increase, while the p(Ga20) decreases with p(02). At the MP of Ga203, the p(Cr02) and p(Cr03) are higher than p(Ga203) at high 02 concentrations (e.g., 20 vol.%), but at lower 02 concentrations (e.g., 7 vol.%), the situation is reversed, leading to different losses of Cr- and Ga-containing species. The formation of Cr02 and Cr03 in the Ga203 melt leads to a different incorporation rate into the p-Ga203 crystal lattice due to different ionic radii, as shown in Table 4.5. The mixture of Cr203, Cr02, and Cr03 in the Ga203 melt will, therefore, affect the effective segregation coefficient as less and less Cr3+ ions, which have the closest ionic radii to Ga3+, are available as the growth proceeds. As a result, the effective segregation coefficient will change with the 02 concentration in a growth atmosphere, as shown in Fig. 4.31a.

The segregation coefficient of Al3+ in p-Ga203 is close to unity and fits the Scheil-Gulliver equation well. This is the result of the thermal and oxidation state stability of Al, and low losses of Ga at relatively high 02 concentration in the growth atmosphere. Moreover, Al3+ has the ionic radius very close to that of Ga3+; therefore, the expected segregation coefficient is close to unity. As the result, Al distribution in p-Ga203 crystals is relatively uniform.

Segregation coefficient of Cr (a) and Ce concentration (b) in Czochralski-grown (3-Ga0 crystals as a function of 0 concentration in the growth atmosphere. Adapted from Ref. [229]

Figure 4.31 Segregation coefficient of Cr (a) and Ce concentration (b) in Czochralski-grown (3-Ga203 crystals as a function of 02 concentration in the growth atmosphere. Adapted from Ref. [229].

Mg2+ is stable in the same way as Al3+, so we can expect a small deviation from the Scheil-Gulliver equation, as shown in Fig. 4.30c. A larger ionic radius of Mg2+ as compared with Ga3+ results in keff < 1, which is about 0.4.

An example ofa large deviation of measured dopant concentration in the crystals from the Scheil-Gulliver equation is Ce (Fig. 4.30d). Generally, Ce3+ has a very large ionic radius as compared with the Ga3+ ionic radius, so the expected segregation coefficient is very low. Indeed, the measured Ce concentration in the crystals is very low as compared with the initial dopant concentration in the melt, but additionally we can observe an enormous deviation from the Scheil-Gulliver equation. The main reason is the tendency of Ce incorporation into structural defects (dislocations, twin boundaries) instead of substituting Ga3+ ions in the crystal lattice. Additionally, at high temperatures, Ce is stable in both Ce3+ and Ce4+ oxidation states depending on the 02 concentration, which results in different ionic radii and a dynamic ke{{ depending on the 02 concentration in the growth atmosphere, as illustrated in Fig. 4.31b. Segregation coefficients of elements used as intentional dopants for p-Ga203 crystals grown by the Czochralski method, determined mainly by ICP-OES, but also by XRF and SIMS, are shown in Table 4.6.

Some of the thermally stable intentional dopants may have a positive impact on the growth stability. For example, it was found that the presence of A1203 and Ce02 in the melt decreases the decomposition rate of Ga203 as compared with undoped melts even by a factor of 3 and makes the growth process more stable (Galazka et al. [229]). This is the entropy-stabilizing effect, on one hand, and improved heat transfer through the growing crystal, on the other hand, which keeps the growth interface convex toward the melt. Further, Ce02 may provide extra oxygen by changing its oxidation state from 4+ to 3+ (2Ce02 -» Ce203 + Уг02), while Al expands the bandgap of p-Ga203, which may lead to make the donor level deeper and consequently lower free carrier concentration at high temperatures (lower free carrier absorption). These dopants can be used as growth stabilizers for conducting crystals by doping with Si4+ or Sn4+ ions. Indeed, co-doping (3-Ga203 single crystals with Al3+ and Si4+, or with Al3+, Ce3+, and Si4+, keeps the growth much more stable as compared to doping with Si4+ only, while still keeping similar electrical properties despite larger bandgap. Such triple- doped bulk ()-Ga203 single crystals obtained by the Czochralski method are shown in Fig. 4.32. Stabilising effect on the growth have also divalent (Mg, Co, Ni) dopants by a decrease of heat absorption by free carriers during growth (see Section 4.5.6.4). In the same way as some of other highly unstable dopant oxides (Sn02, Ge02, Cu20), Li2C03 also provides at high temperatures an extra oxygen inside the melt through its thermal decomposition: Li2C03 —» 2Li(g) + CO(g) + 02(g)Table 4.6 Segregation coefficients used as intentional doping in Czochralski- grown bulk single crystals of 3-Ga203 Some of the data are from Ref. [324]. Note that for dopants with keff = 0 the Scheil—Gulliver equation does not apply, however, they are included here for a general overview

Dopant

Dopant concentration in the melt [mol.%]

02 concentration [vol.%]

Effective segregation coefficient, fceff

Si4*

0.1-1.0

1-2

0.07-0.12

Sn4*

0.1-2.0

2-17

0.00 2-0.06W

Ge4*

0.1

2-4

0(0

Ce3*

0.25-2.5

1.5-35

2xl0'4-10"2

Al3*

2.0-5.0

1.5-21

1.11

Cr3*

0.04-1.0

7-24

1.45-3.11

Mg2*

0.05-0.45

1-35

0.4

Co2*

0.1-0.25

4

0.05-0.0600

Ni2*

1

4

0.0300

Cu1*

0.2

1

0»)

Li1*

1-2

2

oco

Муегу high partial pressure at the melting point of Ga203 (keff governed by dopant evaporation), Mhigh partial pressure at the melting point of Ga203 (ke(f governed by dopant evaporation to a large extent)

Concentrations of residual impurities in undoped crystals are also governed by the segregation phenomenon in the same way as intentional dopants. Residual impurity concentrations may differ between crystals depending on the purity of the starting material and Ir crucible, as well as on an insulation type, its composition, and reuse. The starting materials and Ir crucibles, although the same purity of 5N and 3N, respectively, may have different levels of residual impurities even from the same supplier. As a result, the obtained crystals may also have slightly different levels of residual impurities. Residual impurities may also come from insulation surrounding the crucible and a crystal pulling zone above the crucible. Typically used insulation are zirconia, alumina, and quartz (the most outer part of the furnace), which may contaminate the melt with trace concentrations, usually at the level of a few wt. ppm or below. The following residual impurities were identified in different undoped crystals (in wt. ppm): Si < 2, Al < 20, Zr < 6, Ti < 3, Fe < 2, Mo < 2, and occasionally traces of other elements, such as Co, Cr, Mg, and Ca. With larger crystal diameter, some of the impurities appear sometimes at larger concentrations, especially Fe and Al.

Triple-doped (Al, Ce, and Si) bulk p-Ga0 single crystal obtained by the Czochralski method. Al0 = 5 mol.%, Ce0 = 0.1%, and Si0 = 0.2 mol.% in the melt

Figure 4.32 Triple-doped (Al3+, Ce3+, and Si4+) bulk p-Ga203 single crystal obtained by the Czochralski method. Al203 = 5 mol.%, Ce02 = 0.1%, and Si02 = 0.2 mol.% in the melt.

Examples

  • (i) Galazka et al. [225, 226, 324] used for undoped 20 mm diameter p-Ga203 single crystals 40 mm diameter and 40 mm high Ir crucibles, and an active afterheater above the crucible, which were surrounded by zirconia and alumina thermal insulation. Additionally, Ir lids were used inside the growth furnace to further decrease temperature gradients in a pulling zone. The growth atmosphere comprised (1 - x)Ar + C02 with x = 0.3-1. The growth and rotation rates were 1-2 mm/h and 8-15 rpm, respectively. The starting material was 5N purity Ga203 powder, which was dried, pressed, and sintered in air. The heating and cooling-down rates were 180-225 and 120-150 k/h, respectively. The obtained crystals were typically light blue with the length 50-70 mm.
  • (ii) To grow Mg, Si, Cr, Al, and Ce-doped 20 mm diameter p-Ga203 single crystals, Galazka etal. [226, 228, 229, 324] used similar furnace design and operating parameters as in the case of undoped crystals. Here, the growth atmosphere consisted of (1 -y)Ar + y02 withy = 0.01-0.35, or C02 in some of the growth experiments with Mg doping. The crystal coloration and length strongly depended on the dopant type and 02 concentration in the growth atmosphere, as discussed in Sections 4.5.6.4 and 4.5.6.5.
  • (iii) Two-inch-diameter p-Ga203 single crystals obtained by Galazka et al. [228, 229] were grown from 100 mm diameter crucibles with the height smaller than 100 mm. The growth furnace also included an active Ir afterheater and Ir lids. The thermal insulation was composed of different types of the thermal insulation, such as zirconia, alumina, and quartz. The growth atmosphere was (1 -y)Ar + y02 withy = 0.08-0.35, preferably у = 0.2-0.35. The starting material was either pure or Mg-doped Ga203 5N purity powder, which was dried, pressed, and sintered prior loading into the crucible. The growth and rotation rates were 1-2 mm/h and 4-8 rpm, respectively. The heating and cooling rates were usually 120-150 K/h and 70-90 K/h. Undoped crystals grown at high 02 concentration (y > 0.15) were yellowish, while at lower 02 concentration, they were greenish. Mg-doped (0.1-0.2 mol.% in the melt) crystals were yellowish for the whole 02 concentration range. Al-doped [2 mol.% in the melt) crystals at high 02 concentration were yellowish as well. The length of the crystals varied from 50 to about 100 mm, depending on the 02 concentration, dopant, and furnace design.
 
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