The growth of compositionally uniform alloy crystals is promising for variety of applications because lattice parameters as well as electrical and optical properties can be controlled by composition. Among them, In x Ga 1-x As bulk crystals are expected as substrates o f l a s e r d i o d e s w i t h e m i t t i n g w a v e l e n g t h o f 1 . 3 µ m . H i g h o p t i c a l g a i n w i t h s m a l l temperature dependence was demonstrated for strained quantum well grown on In x Ga 1-x As

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... es (Ishikawa, 1993; Ishikawa & Suemune, 1994) . However, growth of homogeneous In x Ga 1-x As bulk single crystals is very difficult due to large separation of liquidus and solidus lines in the pseudobinary phase diagram (Bublik & Leikin, 1978) . In x Ga 1-x As bulk crystals were grown by the liquid encapsulated Czochralski (LEC) method with supplying GaAs (Nakajima et al., 1991) , zone levelling method (Sell, 1991) and multicomponent zone melting method (Nishijima et al. 2005) . InAs mole fraction was limited to 0.2 in LEC method and zone levelling method due to temperature fluctuation in the melt and large separation of liquidus and solidus lines. Multicomponent zone growth (MCZG) using a seed with graded InAs concentration produced a single crystal with InAs mole fraction of 0.3 and length of about 17 mm (Nishijima et al. 2005) . However, MCZG requires complicated growth technique and no good reproducible results were obtained. In all of the methods, the most difficult point is to keep the freezing interface temperature constant for growing compositionally uniform crystals since interface travelling rate depends on temperature gradient, mass transport in a melt and solute concentration gradient ahead the interface. As a result, no device quality In x Ga 1-x As crystals with uniform composition have been obtained so far. The travelling liquidus-zone (TLZ) method was invented for keeping the interface temperature constant and growing compositionally uniform alloy crystals (Kinoshita et al., 2001 . In the TLZ method, the interface travelling rate can be determined exactly if temperature gradient in the zone is known and then the interface position can be fixed in relation to the heater position by translating a sample device in accordance with the interface travelling rate (Kinoshita et al. 2002) . Principle of the TLZ method is proven by the precise measurement of temperature gradient in the melt zone,