New Buffer Layers for GaN on Sapphire by Atomic Layer and Molecular Stream Epitaxy
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Mat. Res. Soc. Symp. Proc. Vol. 395 01996 Materials Research Society
stream containing both MO and NH 3 thus depositing a few monolayers per rotation. The deposited layer is being annealed during the rest of the cycle and, for example, for a 30 rpm rotational speed growing by MSE, the growth time is 0.2 seconds followed by an annealing time of 1.8 seconds. For conventional MOCVD, the susceptor is stationary and both reactant gases flow over the substrate simultaneously. Typical rotation rates are 30 to 60 rpm. The design of our reactor allows the growth of 111-nitride thin films by either conventional MOCVD, MSE, or ALE or any combination of the three. EXPERIMENT The nitride films are grown on on-axis (0001) sapphire. Prior to growth, the substrate is cleaned and etched in H 2SO4 :H20 2 (1:1) for 45 minutes at 120 0 C. It is then annealed at 1050TC for 15 minutes inside the reactor and then passivated with ammonia for 1 minute. The layers0 are grown at temperatures ranging from 550 to 900'C. Column 111 reactants are TMAI (+18 C), TMGa (-10 0 C), and EDMIn (+10 0 C) with flows varying from 3-15, 1-8, and 15120 sccm, respectively. The column V reactant is NH 3 with flows in the range of 0.5-2 slm. The carrier and purge gases are purified nitrogen. The pressure in the growth chamber can also be varied from less than 100 torr to atmospheric. All samples grown are characterized by double crystal x-ray diffraction (DCXRD) and transmission electron microscopy (TEM) will also be used for interface and microstructural analysis. Photoluminescence (PL) will be used to assess the optical properties. RESULTS It is well known that a strained layer superlattice (SLS) can be used to suppress threading dislocations in zinc-blende semiconductor epilayers. However, the threading dislocations in GaN films are mostly perfect edge type that have slip planes of {1TOO} type, which require larger resolved shear stress to initiate glide. Furthermore, since the strain field introduced by the SLS is parallel to the (0001) growth plane, the resolved shear stress on the {1100 1 plane is zero. Therefore, it can be expected that, to suppress or eventually eliminate this type of threading dislocation with a SLS could be difficult. The nature of these threading dislocations suggests that it is important to carefully select the thickness, deposition parameters and growth mode of the buffer layer to minimize or avoid formation of the edge type dislocation in the Ill-nitride films. Ammonia Metalorganics + Niroe
Nitrogen
Ammonia + Nitrogenn Nitoger+oitrge
Rotating susceptor
Metalorganics +
en
Nitrogene
Rotating susceptor
Figure la. Schematic drawing of the susceptor and gas flow locations for the ALE growth method.
Figure lb. Schematic drawing of the susceptor and gas flow locations for MSE and MOCVD growth methods. 308
In this work we have studied the effect of the buffer layers grown by MSE and ALE on the crystalline quality and defect densities of the grown GaN films. The objective is to enhance two dimensional growth of a single crysta
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