High Quality In x Ga 1-x As (x: 0.08 – 0.13) Crystal Growth for Substrates of (cid:540) = 1.3 (cid:541) m Laser Diodes by the Travelling Liquidus-Zone Method

and crystal quality owe to matching of sample translation rate and freezing rate and resulting in a fixed freezing interface position relative to heater segments. In this point of view, constant temperature gradient during translation of a sample was important for obtaining compositional uniformity. Such increase in surface area in platy crystals without deteriorating crystal quality also shows that convection in a melt is suppressed by the limitation of melt thickness (2 mm) and increase in melt width did not cause convection. If convection occurs in a melt, local inhomogeneity will be resulted due to stirring of a melt by convection. Good homogeneity shows that convection was suppressed in the course of crystal growth. In modern research and development, materials manufacturing crystal growth is known as a way to solve a wide range of technological tasks in the fabrication of materials with preset properties. a reader to gain insight into selected aspects of the field, including growth of bulk inorganic crystals, preparation of thin films, low-dimensional structures, crystallization of proteins, and other organic compounds.

growth is carried out by travelling almost saturated liquid-zone. Sample configuration, temperature profile, solute profile in a sample and its relation to a phase diagram in the TLZ method is depicted in Fig. 1 as referring to the growth of In 0.3 Ga 0.7 As. It is needless to say that convection in a thin melt zone is suppressed more effectively than in a long melt even in microgravity. In Fig. 2, solute concentration profile in the diffusion boundary layer in the steady-state growth is compared between directional solidification method and TLZ method (Kinoshita and Yoda, 2011). When temperature gradient is low and zone thickness is thin, solute concentration profile is linearly approximated and it can be related to the liquidus line in the pseudo binary phase diagram as shown in Fig. 1. Liquidus line is also linearly approximated in a narrow temperature range. Then, it should be noticed that solute concentration gradient in a zone can be controlled by temperature gradient in such case as is given by eq. (1), where C L is solute concentration in the zone, T is temperature, Z is a distance measured from the freezing interface and L CT   is reciprocal of the slope of the liquidus line in the phase diagram. Then, freezing rate V is given by eq. (3) if we suppose the diffusion controlled steady-state growth is realized as given by eq. (2) and V is determined by temperature gradient in the zone, where D is interdiffusion coefficient between solute and solvent, C L0 and C S0 are liquidus and solidus concentrations at the freezing interface. If we know interdiffusion coefficient D and phase diagram date, we can calculate the freezing rate V using eq. (3). Then, it is very easy to fix the interface position as shown in Fig. 3. If a sample device is translated toward opposite direction at the same rate of interface travel rate as calculated by eq. (3), the interface position is fixed and the interface temperature is kept constant, resulting in a compositionally uniform alloy crystals. Thus, compositionally uniform alloy crystals are grown by simply translating a sample device at constant rate in the TLZ method. According to our numerical analysis, 1 to 2 hours are required for establishing an almost saturated solute concentration profile in a zone and spontaneous crystal growth starts.   (3) is evaluated by growth experiments. A seed, a zone forming material and a feed were inserted into a boron nitride crucible and the crucible was vacuum sealed in a quartz ampoule at about 1×10 -4 Pa. In the growth of In x Ga 1-x As, the zone forming material is InAs or InAs rich solid solution with GaAs. The ampoule was inserted into a temperature gradient furnace and was heated to about 1100℃ at temperature gradients of 10 -20℃/cm. InAs-GaAs interdiffusion coefficient was measured in microgravity using a sounding rocket (Kinoshita et al. 2000). It was measured to be 1.5±0.2×10 -8 m 2 /s at 1070℃. For In 0.3 Ga 0.7 As growth, C L0 is given by 0.83 from phase diagram. L CT   is also given by phase diagram as -465 K/mol. Then, V is calculated to be 0.22 mm/h for the temperature gradient of 10℃/cm (Nakamura et al., 2003). Crystal growth experiments were performed for crystals with 2 mm diameter in order to suppress convection in a melt on the ground. Concentration profiles were measured by EPMA. Results are shown in Fig. 4. In the figure, crystal growth started at the length of about 8 mm and the sample was quenched at the length of about 32 mm. Therefore, about 24 mm long crystal was grown. It should be noted that expected InAs concentration (15 at%) is realized without initial transient region. This is one unique point typical to the TLZ method when compared with diffusion controlled steady-state growth in the directional solidification. This is because steady-state solute concentration profile can be established without transient region in the case of zone melting method under temperature gradient. Compositional homogeneity is excellent; In concentration 15±0.5 at% is achieved for the length of more than 20 mm. Scattered concentration region was liquidus-zone before quenching. Dendrite crystals were grown during quenching and dendrite growth resulted in concentration scattering. It is astonishing that excellent compositional homogeneity was achieved at just calculated sample translation rate using eq. (3). Since temperature gradient in the zone was measured precisely by sophisticated method namely by measuring solidus compositions at two interfaces (freezing and dissolving interfaces), model evaluation was possible. Solidus compositions measured at two interfaces are shown in Fig. 5 together with a determined temperature gradient in the zone (Nakamura et al., 2003, Kinoshita et al., 2005. At the freezing interface, In concentration was 15.1 at% and this concentration gives solidus temperature of 1083.1℃ from the phase diagram, while In concentration at dissolving interface 12.3 at% gives solidus temperature of 1099.8℃. Distance between two interfaces is 16.8 mm and temperature gradient 10℃/cm was determined. In the temperature gradient measurement, a special sample whose In concentration is gradually decreasing towards a feed and solid-liquid coexistence at the dissolving interface was used. This enabled us to measure solidus composition at the dissolving interface.  Numerical analyses show that InAs concentration increases when sample translation rate is faster than the predicted one from eq. (3), while it decreases when sample translation rate is slower than the predicted one ( Fig. 6). It should be noted that spontaneous growth occurs even if sample is stationary. This is because solute concentration difference is established between the two interfaces and interdiffusion occurs in a melt zone. At the freezing interface, In concentration is higher and In is transported towards the dissolving interface by diffusion. Indium concentration decrease causes solidification at the freezing interface because equilibrium temperature gets higher. However solidification segregates In at the interface. The segregated In is diffused away towards the dissolving interface. At the dissolving interface, a feed is dissolved by transported In because In concentration gets higher than the equilibrium one. Thus, crystal grows at the freezing interface and a feed is dissolved at the dissolving interface; crystal growth proceeds spontaneously towards lower In concentration side (dissolving interface side, which is a higher temperature side). Sample translation towards a lower temperature side plays a role of keeping the interface temperature constant and it is not a driving force for crystal growth. The driving force in the TLZ growth is interdiffusion between solute and solvent in a zone and segregation on solidification. This should be noted clearly as one of features of the TLZ growth. Figures 7 (a) and 7 (b) show experimental results when sample translation rates do not match the freezing rate of 0.22 mm/h at a temperature gradient of 10℃/cm (Kinoshita et al., 2005). Fig.  7 (a) is a case of R = 0 (stationary) and Fig. 7 (b) is a case where R is faster than 0.22 mm/h. Both results agree well with the numerical analyses. As described later, higher sample translation rate than 0.3 mm/h caused constitutional supercooling and resulted in fluctuation of composition. Therefore, in Fig. 7 (b) an experimental result at R = 0.27 mm/h is compared with a numerical analysis. Thus, TLZ growth model was confirmed by growth experiments at various sample translation rates and comparison of compositional profiles with those obtained by numerical analyses. Latent heat effect on growth rate was also investigated by numerical analysis and the effect is shown to be negligibly small except for end menber region (Adachi et al., 2004b).

Limitation in the TLZ method
TLZ growth model is established when solute concentration gradient in the melt zone is linearly approximated as shown in Fig. 2. When temperature gradient is low and zone thickness is thin, such approximation is valid. However, freezing rates deviate from those predicted by eq. (3) at high temperature gradient and for long zone length. Amount of solute in a zone also limits the length of a grown crystal. Moreover, TLZ growth model is based on the diffusion controlled steady-state growth and compositional uniformity is affected by convection in a melt. The TLZ method also lies side-by-side with constitutional supercooling since almost saturated melt is utilized. In this section, such limitations will be described in more detail.

Limitation by temperature gradient
TLZ method is classified as diffusion controlled steady-state growth method as well as a zone method. Solute concentration profile in the diffusion boundary layer is expressed as eq. (4) (Tiller et al., 1953). When VZ/D is small, concentration gradient is approximated by linear relation as eq. (5). Such situation has already been described in Fig. 2. Low temperature gradient gives small value of V and gives a small error in growth rate calculation using eq. (3). However, when temperature gradient is high, V gets large and an error in growth rate calculation becomes large (Adachi et al. 2004a). Figure 8 shows deviation of growth rate from linear relation to temperature gradient. From Fig. 8, it can be said that TLZ growth rate defined by eq. (3) is valid within 10% error when temperature gradient is smaller than 40℃/cm for the zone thickness of 20 mm. Therefore, TLZ growth should be carried out at low temperature gradient like 10 -20℃/cm at zone thickness of 20 mm.

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High Quality In x Ga 1-x As (x: 0.08 -0.13) Crystal Growth for Substrates of = 1.3 m Laser Diodes by the Travelling Liquidus-Zone Method (3) as a function of temperature gradient.

Limitation by zone length
Distance Z is also a variable in eq. (4). Therefore, zone length gives similar effect in growth rate calculation. When the temperature gradient is 20 o C/cm, an error in the growth rate calculation using eq. (3) is about 7 % for the zone length of 40 mm but it beyonds 12 % when the zone length is 50 mm. If growth rate deviation from linearity is desired less than 10%, zone length should be less than 45 mm. Limitation of zone length is natural since long zone length cannot be distinguished from directional solidification.

Limitation of growth length
In the In x Ga 1-x As crystal growth, we usually start from a combination of a GaAs seed, an InAs zone forming material and a GaAs feed. GaAs saturated InAs rich zone is formed and this zone travel towards a GaAs feed. Therefore, InAs is consumed according as crystal growth proceeds. If an InAs length is 10 mm, length of a grown crystal is limited to 10/3 mm for an In 0.3 Ga 0.7 As crystal. In order to increase the length of grown crystal longer zone or InAs contained feed should be used. However, zone length is limited from the point of zone growth as described above and in an InAs contained feed InAs content is restricted to low for preventing melting of a feed. In this meaning, the length of a TLZ-grown crystal is limited.

Limitation by convection in a melt
The TLZ method is possible in the diffusion limited regime. If convection occurs in a melt, solute concentration profile ahead the interface is disturbed and the growth rate does not obey eq. (3). We grew 2 mm diameter In x Ga 1-x As crystals by suppressing convection in a melt and confirmed the TLZ growth model. However, 2 mm diameter crystals cannot be used for opt-electronic devices and we tried increase of crystal diameter. Result of a growth experiment for 10 mm diameter crystal is shown in Fig. 9. Axial compositional homogeneity is achieved for the length of more than 30 mm but radial concentration inhomogeneity exists in a 10 mm diameter crystal. According to numerical analysis, convective vortices occur at both freezing and dissolving interfaces as shown in Fig. 10. When such convective vortices are taken into consideration, radial concentration difference is understood; In-rich melt is transported towards the centre by convection and InAs concentration increases in the centre of a grown crystal. As for the strength of convective driving force, Grashof number G defined by eq. (6) gives order of magnitude where g is gravity acceleration, β is volume expansion coefficient of a melt, ΔT is a temperature difference between the two ends of a melt, L is characteristic length of a melt and ν is kinematic viscosity of a melt. Comparison of convective driving force between 2 and 10 mm diameter melts, convection in 10 mm diameter melt gives 2 orders of magnitude higher than that in 2 mm diameter melt. Result in Fig. 9 shows importance of suppressing convection in a melt for compositionally homogeneous alloy crystals in radial direction as well as in axial direction (Kinoshita et al., 2006).

Limitation by constitutional supercooling
The TLZ growth utilizes almost saturated melt. In this point of view, constitutional supercooling tends to occur. Constitutional supercooling in the directional solidification of alloy crystals in the diffusion limited growth was studied extensively by Tiller et al. (1953). According to them, the growth rate should be high and should fulfil conditions defined by eq. (7) where G is temperature gradient, R is growth rate, m is the slope of the liquidus line. The TLZ method utilizes solute concentration gradient in diffusion boundary layer ahead the freezing interface. Therefore, growth rate should fulfil the conditions proposed by Tiller et al. When we change notation of eq. (3) to similar ones given by Tiller et al. for comparison, equation (8) is given. Equation (8) is further modified into eq. (9). When we compare eq. (7) with eq. (9), we notice that the growth rate for realizing compositional uniformity in the TLZ method is the critical growth rate for avoiding constitutional supercooling. If growth rate is higher than this, constitutional supercooling will occur. From this discussion, it is clearly shown that homogeneous crystal growth in the TLZ method is side-by-side the constitutional supercooling.
www.intechopen.com Fig. 11a. Comparison of constitutionally supercooled area during TLZ growth at low and high temperature gradients. At high temperature gradient, non-linearity of solute concentration profile aheat the freezing interface is large and gives larger supercooled area Constitutional supercooling is inevitable at high temperature gradient (Kinoshita and Yoda, 2011). This may be peculiar from the requirement of high G/R ratio shown by eq. (7) but this requirement comes from constant G/R for compositional uniformity and solute saturation at two interfaces (freezing and dissolving interfaces) in the TLZ method. At high temperature gradient, non linearity of solute concentration profile increases as shown in Fig. 11(a). In the figure, the straight line shows equilibrium concentration at the given temperature gradient and dashed region is constitutionally supercooled region. It should be noted that constitutionally supercooled area is larger at higher temperature gradient. This is unique point in the TLZ method. It is obvious that degree of constitutional supercooling is greater at higher temperature gradient because deviation from linearity in solute concentration profile is greater at higher temperature gradient (Kinoshita et al., 2011). From this point of view, temperature gradient is limited and growth rate in turn is also limited. In the In x Ga 1-x As crystal growth experiments, it is shown that temperature gradient that exceeds 30℃/cm resulted in constitutional supercooling. This means that growth rate is limited to 0.66 mm/h. In general, compositional fluctuation increases in constitutionally supercooled region. An example of compositional profile of constitutionally supercooled sample is shown in platy crystal growth in later section (an In 0.15 Ga 0.85 As platy crystal grown at a temperature gradient of 37℃/cm showed large compositional fluctuation as shown in 3.2.3).

Platy crystal growth
For device application, large surface area is required. However, increase in crystal size increases convection in a melt and compositional uniformity of TLZ-grown crystals is deteriorated. Suppressing convection in a melt and large surface area is fulfilled by the growth of platy crystals if thickness of the platy crystal is sufficiently thin (Kinoshita et al., 2008(Kinoshita et al., , 2010. Such situation is schematically depicted in Fig. 11(b). We have experimentally confirmed that convection in a 2 mm diameter melt is suppressed and TLZ mode crystal growth is possible. Then, we determined thickness of platy crystals as 2 mm. In this section, growth and characterization of platy crystals for substrates of laser diodes with emitting wavelength λ = 1.3 µm are described. Fig. 11b. Schematic view of a platy crystal growth.

Growth procedures and characterization methods
Growth method for platy crystals is similar to that of the cylindrical crystal. A GaAs seed, an InAs zone forming material, and a GaAs feed were cut into plates with 2 mm thickness and were inserted into a boron nitride crucible with a rectangular bore and were sealed in vacuum in a quartz ampoule and then heated in a gradient heating furnace. Growth interface temperature was adjusted around 1100℃ so that In x Ga 1-x As (x: 0.1 -0.15) crystals were grown. Temperature gradient was set around 20℃ so that constitutional supercooling at high temperature gradient was avoided. Samples were translated at the rate calculated using eq. (3). Seed orientation was <100> or <110>. Sn was doped as n-type dopant. At the beginning of crystal growth, we thought that In 0.3 Ga 0.7 As composition was required as substrates for λ = 1.3 µm laser diodes. However, it turned out that composition around In 0.1 Ga 0.9 As can be used as substrates since thin film growth technology has been developed and large lattice mismatch between the substrate is conquered. As for crystal size, we started from the growth of 10 mm width and increased crystal width. At the present, we are successful in growing 50 mm wide platy crystals. Grown crystals were polished and crystalline nature was investigated. Compositional profiles of grown crystals were measured by EPMA on polished surfaces. Twodimensional semi-quantitative mapping analysis was performed for measuring overall compositional distribution. Quantitative analysis was performed along growth axis with precision of 0.1 at% for each constituent element. Crystal quality was evaluated by measuring X-ray rocking curves. Electrical properties of crystals were measured by Hall measurements. Etch pit density was measured after KOH etching.

Results and discussion
Experimental results for platy crystal growth are summarized and factors that effect on crystal quality are discussed in this section. Factors for affecting crystal quality are common for the TLZ method except for seeding in rectangular crucible.

Single crystalline nature
Typical two examples of roughly polished surface of two platy crystals are shown in Fig. 12. Composition of Fig. 12 (a) is In 0.10 Ga 0.90 As and that of Fig. 12 (b) is In 0.13 Ga 0.87 As. When InAs mole fraction in a grown crystal is less than 0.1, a single crystal that takes over the seed orientation was grown. However, when InAs mole fraction is more than 0.13, single crystal growth that takes over the seed orientation became very difficult. This may be due to increase in lattice mismatch between a GaAs seed and a grown crystal. Difficulty in single crystallization was settled by using a feed having the same orientation of a seed. In Fig. 12 (b), polycrystallization is observed at the seed and grown crystal interface. However, a single crystal was grown again as crystal growth proceeded. The mechanism of single crystal growth is not clear but it may be related to a crucible shape having rectangular bore and nucleation of the same orientation as a feed might occur in a narrow gap between the crucible wall and feed surface. In wider crystal growth too, single crystals were grown by the same mechanism. Reproducibility of single crystal growth was very good and single crystal length more than 50 mm was obtained.

Growth temperature
Freezing interface temperature determines composition of grown crystals. In this point of view, growth temperature is very important. In the TLZ method, solute in a melt is almost saturated and it is very easy to control interface temperature. We set 8 thermocouples around a growth ampoule and one of them is set at seed/zone interface. Only procedure is to monitor temperatures measured by these thermocouples and to change the interface temperature. It is usual that surface temperatures of a quartz ampoule are not equal to inner temperatures of a crucible but difference between the two was less than 5℃ and minor adjustment gave target compositions. Figure 13(a) is an example of In 0.2 Ga 0.8 As crystal and Fig. 13(b) is an example of In 0.13 Ga 0.87 As crystal. In In 0.2 Ga 0.8 As, interface temperature was set at 1057℃ and in In 0.13 Ga 0.87 As it was set at 1095℃. In both cases, target compositions were obtained at the seed/crystal interface and no initial transient regions were found as is the case of In 0.3 Ga 0.7 As (Fig. 4). Compositional uniformity was excellent for In 0.13 Ga 0.87 As; InAs mole fraction was 0.13±0.005, but uniformity was degraded for In 0.2 Ga 0.8 As. Single crystallinity was also better for lower In concentration crystals. Since we succeeded in laser operation at the wavelength of 1.3 µm for laser diodes on In 0.13 Ga 0.87 As substrates (Arai et al, 2007), we improved crystal quality for the composition of around In 0.13 Ga 0.87 As.

Temperature gradient
Temperature gradient determines spontaneous growth rate in the TLZ method as given by eq. (3). So long as the sample translation rate matches this spontaneous growth rate, compositionally uniform alloy crystals can be grown. However, high temperature gradient causes constitutional supercooling and results in compositional fluctuation. Figure 14 compares compositional profiles of crystals grown at different temperature gradient (a) 25℃/m and (b) 37℃/cm. Note that the crystal grown at lower temperature gradient shows lower compositional fluctuation. As described in the section 2.3.5, maximum temperature gradient for suppressing constitutional supercooling may be 30℃/cm. It is concluded that temperature gradient should be as low as possible for suppressing compositional fluctuation but the lowest limit of temperature gradient should be determined from the point of mass productivity of crystals.

Temperature stability
Effect of temperature stability on compositional uniformity was investigated with average composition of In 0.13 Ga 0.87 As (Kinoshita et al., 2008). The most stable temperature was achieved when air flow in a furnace tube was shut. Temperature stability was ±0.1℃ as shown in Fig. 15 (a). With air flow conditions, temperature stability got worse to ±0.2℃ as shown in Fig. 15 (b). The stability difference was small but this small difference gave a considerable effect on the compositional uniformity as shown in Fig. 16. When the temperature stability was ±0.1℃, InAs mole fraction was 0.13 with σ of 0.0005 where σ is the standard deviation in the distance between 20 and 40mm. When the temperature stability was ±0.2℃, InAs concentration uniformity was 0.13 with σ of 0.006 in the distance between 20 and 40mm. The reason why the temperature fluctuation influence on the compositional uniformity may be related to the crystallization from the almost saturated melt, namely, temperature fluctuation gives rise to the formation of constitutionally supercooled region in a melt and such region crystallizes earlier, then it is not strange that earlier crystallized region has lower In concentration. Improvement in compositional stability resulted in higher crystal quality. X-ray rocking curve measurements showed that full width at half maximum (FWHM) of rocking curve ranges from 0.03 to 0.04 degrees for crystals grown at temperature stability ±0.1℃. Such small FWHM shows good crystallineity which can be used as a substrate of laser diodes.

Characterization as substrates of laser diodes
Improvement in compositional stability resulted in higher crystal quality. Figure 17 shows X-ray rocking curves in the distance between 26 mm and 56 mm at an interval of 5mm for the crystal grown at the temperature stability of ±0.1℃. The ω-scan around the (004) diffraction peak was detected by an open detector. Cu k α1 radiation from an X-ray tube (loaded power; 40 kV, 30 mA) was monochromated by a four crystal Ge(220) monochromator and was incident on a crystal. The illuminated area was limited to about 1×1 mm 2 by a divergence and a scatter slit. Full width at half maximum (FWHM) of rocking curve ranges from 0.03 to 0.04 degrees (from 108 to 144 arc seconds). Such small FWHM shows good crystallineity which can be used as a substrate of laser diodes. High quality region extended to 10 × 30 mm 2 . When InAs mole fraction increases, FWHM increased. This phenomenon is usual in many alloy crystals. When solute concentration increases, compositional uniformity is deteriorated and strain increases in the grown crystal. The increased strain degrades crystalline quality. Therefore, the lower InAs mole fraction, the higher is the quality of grown crystals. In the course of our study, we found that In 0.3 Ga 0.7 As could be used as substrates of λ = 1.3 µm laser diodes instead of In0 .3 Ga 0.7 As. This is beneficial for ternary crystal growth since higher quality can be expected. Sn was doped as n-type dopant for substrate use. Carrier concentration was measured to be 5 -8×10 18 cm -3 . Etch pit densities were in the range between 1×10 3 and 3×10 4 cm -2 , which is sufficiently low for a substrate.

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High Quality In x Ga 1-x As (x: 0.08 -0.13) Crystal Growth for Substrates of =

Increase of a crystal width
Convection in a melt is suppressed in a thin melt. In platy crystal growth, width is considered to be independent of convection strength when thickness of crystals is limited. From this point of view, width of platy crystals was increased step by step from 10 mm to 20, 30, and 50 mm (Kinoshita et al., 2010). In all cases, thickness of platy crystals was set to be 2 mm. The most difficult point was to keep high temperatures in a feed area as growth ampoule was translated towards low temperature side. When width of platy crystals was increased, diameter of boron nitride (BN) crucible increased. BN has higher thermal conductivity than quartz and large diameter BN crucible transferred more heat to lower temperature side. For keeping high temperatures in a feed area, length of BN crucible was increased to receive more radiation from a heater. Figure 18 shows an example of 30 mm wide platy crystal. Outlook of roughly polished surface is shown in Fig. 18 (a) and InAs mole fractions measured along a centre line and 10 mm away from the centre line are shown in Fig, 18 (b). Single crystalline area larger than 30×30 mm 2 was obtained. InAs mole fractions were constant and 0.10±0.01 was achieved for the distance of about 40 mm. Good crystallinity with the FWHM of less than 0.04° in X-ray rocking curves was obtained. Such excellent compositional uniformity and crystal quality owe to matching of sample translation rate and freezing rate and resulting in a fixed freezing interface position relative to heater segments. In this point of view, constant temperature gradient during translation of a sample was important for obtaining compositional uniformity. Such increase in surface area in platy crystals without deteriorating crystal quality also shows that convection in a melt is suppressed by the limitation of melt thickness (2 mm) and increase in melt width did not cause convection. If convection occurs in a melt, local inhomogeneity will be resulted due to stirring of a melt by convection. Good homogeneity shows that convection was suppressed in the course of crystal growth.
www.intechopen.com  Now we have succeeded in growing 50 mm wide platy crystals. Figure 19 shows an example of 50 mm wide crystal. Single crystalline area of almost 50 ×50 mm 2 is obtained. For mass production of laser diodes, large platy crystals with good reproducibility are required. As described earlier, crystals do not take over seed orientation but take over feed orientation. In this case, similar single crystallization mechanism works, too. Reproducibility is now more than 90%.

Laser diodes fabrication
Laser diodes with emitting wavelength λ = 1.3 µm were fabricated on In x Ga 1-x As (x: 0.1 -0.13) substrates. Ishikawa (1993) demonstrated merits of a ternary substrate for high optical gain with small temperature dependence. In the course of study, we found that In x Ga 1-x As (x: 0.1 -0.13) crystals can be used as substrates owing to development of thin film growth technology. Low InAs content crystals are beneficial to substrates because they have better thermal conductivity than high InAs content crystals.

Fabrication procedures
In 0.13 Ga 0.87 As platy crystals were polished mechano-chemically to 0.5 mm thickness. Surface roughness of mirror polished substrates was measured to be less than several nano-meters. Strained multiple-quantum-wells (MQWs) having the combination of In 0.12 Ga 0.88 As and In 0.38 Ga 0.62 As as shown in Fig. 20 (a) were prepared by metal-organic vapor phase epitaxy (MOVPE) on the cleaned substrate (Arai et al., 2009). The active region was consisted of four In 0.38 Ga 0.62 As wells and five In 0.12 Ga 0.88 As barriers sandwitched by two seperate-confinement heterostructure (SCH) layers. The stripe mesa type laser was fabricated by chemical etching. Cross sectional view of a fabricated mesa stripe is shown in Fig. 20 (b). The stripe mesa was in the [011] direction, forming a reverse mesa. Bottom ridge width of 1.7 µm enabled single mode lasing. Selective wet etching was utilized for such narrow ridge formation.  Figure 21 (a) shows the bias current versus output power (I-L) characteristics of a fabricated laser diode The threshold current is 7.2 mA. Figure 21 (b) is the lasing spectrum at a bias of 20 mA. The lasing wavelength is 1.31µm. The laser exhibits a maximum operating temperature of 80 and 150℃ under CW and pulsed operation, respectively. Therefore, the limiting factor was self-heating. An effective way of overcoming this thermal problem is to reduce the indium content of the In x Ga 1-x As substrate and to introduce InAlGaAs barrier layers. We therefore tried to realize 10 Gbps modulation over 85℃ by eliminating the thermal problems and we prepared an In 0.08 Ga 0.92 As substrate. Laser diodes on an In 0.08 Ga 0.92 As substrate showed improved lasing characteristics. Self heating problem due to bad thermal conductivity of In 0.13 Ga 0.87 As substrates was settled by reducing InAs content in the substrate. However, emitting wavelength was 1.26 µm due allowance limit of InAs content in active layers. Figure 22 shows continuous wave (CW) lasing characteristics of this laser diode at various temperatures. CW operation was www.intechopen.com

Characterization of laser diodes
Modern Aspects of Bulk Crystal and Thin Film Preparation 182 possible up to 150℃ and was much improved. Characteristic temperature of the threshold current density between 25 and 125℃ was about 90 K and showed a higher value compared with that of 1.31 µm laser (about 70 K). Such high temperature stability in output power is also expected in 1.31 µm laser diodes by optimizing heat sink structure and heat sink materials.  A 10-Gbps direct modulation and transmission tests through a single mode fiber up to 20 km at 25˚C were successfully performed using a fabricated laser diode with λ = 1.31 µm as shown in Fig. 23. Bit error rate (BER) for back-to-back configuration after 20 km transmission was less than 10 -7 at -18 dBm. We also tested 1.26 µm wavelength laser diodes. In this laser diodes, 10 Gbps modulation at 85˚C was confirmed by an electrically filtered back-to-back (BB) eye diagram as shown in Fig. 24. The extinction ratio and the mask margin of the synchronous optical network (SONET) mask were 7 dB and 9%, respectively. These results show feasibility of fabricated laser diodes.

Conclusions
The TLZ method has been invented for growing compositionally uniform alloy crystals. The solute concentration gradient is controlled by applied temperature gradient utilizing saturation of solute in a zone. Owing to this merit, the spontaneous growth rate is calculated from the diffusion controlled steady-state growth conditions. Compositionally uniform alloy crystals are obtained merely by translating samples at the calculated growth rate relatively to a heater. Principle of the TLZ method was proved by the growth of 2 mm diameter crystals since convection in a melt is suppressed and diffusion limited growth is possible in capillary tubes even on the ground. As predicted, compositionally uniform crystals were grown without initial transient region which is typical to directional solidification method. Although the TLZ method is a superior method, it has limitations and they were also studied theoretically and experimentally. Limitation by temperature gradient, limitation by zone length, limitation by constitutional supercooling, and limitation by convection in a melt were made clear. In microgravity, TLZ growth is free from limitation by convection in a melt. In this point, microgravity is beneficial for TLZ growth. We grew platy InxGa1-xAs (x: 0.08 -0.13) crystals by the TLZ method on the ground for substrates of 1.3 µm wavelength laser diodes. In platy crystal growth, convection in a melt was suppressed by limiting thickness of platy crystals to 2 mm and large surface area was obtained by increasing width of platy crystals. Laser diodes on In x Ga 1-x As (x: 0.08 -0.13) substrates showed excellent temperature stability in output power as predicted by Ishikawa et al. (1993). Now 50×50 mm 2 platy crystals were reproducibly grown and mass production of laser diodes with emitting wavelength of 1.3 µm is expected.