Stratified Ion Damage and in Situ Regrowth in GaAs/AlAs Heterostructures
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STRATIFIED ION DAMAGE AND IN SITU REGROWTH IN GaAs/AlAs HETEROSTRUCTURES A G CULLIS, D J EAGLESHAM*, D C JACOBSON*, J M POATE*, C R WHITEHOUSE AND P W SMITH Royal Signals & Radar Establishment, St Andrews Rd, Malvern, Worcs WRI4 3PS, UK * AT&T Bell Laboratories, 600 Mountain Avenue, Murray Hill, New Jersey 07974, USA
ABSTRACT The material-dependent manner in which ion damage occurs in AlAs/GaAs heteroepitaxial structures is demonstrated using conventional and high resolution transmission electron microscopy. Both 150keV and 2MeV Si+ ion implants are employed over a wide range of ion doses. Under conditions which yield rapid build-up of lattice damage in GaAs, the AlAs is found to be relatively resistant to structure breakdown. Indeed, the crystalline AlAs For exerts a novel protective effect on immediately adjacent regions of the GaAs layers. high implantation doses amorphous-crystal superlattices are formed in multilayer structures. For the highest ion doses the AlAs lattice begins to be disrupted by a characteristic, These observations suggest that mobile point boundary-dependent, heterogeneous mechanism. defects play a significant r6le in AlAs in situ restructuring processes. INTRODUCTION When crystalline semiconducting materials are subjected to ion bombardment, lattice damage can be introduced ranging from point defect disorder, possibly accompanying The extended defect arrays, to complete lattice breakdown and amorphous layer formation. amorphization process has had considerable scientific and technological impact [1 ] and can be modelled either as simply the build-up and overlap of small amorphous zones created by the collision cascades [2] or as depending upon [3] the migration and agglomeration of point defects. When implantation is carried out into multilayer materials the situation becomes Indeed, for GaAs/AlAs layers it has been shown [4,5] that the initial more complex. implantation damage accumulates inhomogeneously, the crystalline GaAs being damaged and ultimately amorphized much more rapidly than the crystalline AlAs. In the present paper the disorder transitions induced in both single and multilayer Fluences of both GaAs/AlAs structures by ion bombardment are examined in some detail. 150keV and 2MeV ions are employed to give widely different damage depth distributions. The material-dependent damage behaviour is highlighted and interactions between the different compounds are identified.
EXPERIMENTAL DETAILS The initial heteroepitaxial GaAs/AlAs samples were produced by molecular beam epitaxy (MBE) in ultrahigh vacuum. After deposition of GaAs buffer layers on each (001) GaAs substrate, for the single layer samples, a "-90nm thick layer of AlAs was grown followed by For the multilayer samples, 5 repeats of 80nm AlAs a final -70nm capping layer of GaAs. All Samples were cooled to liquid and 80nm GaAs were grown upon the buffer layer. nitrogen temperatures before bombardment with 28Si+ ions from a 1.7MV tandem An ion energy of 150keV was used for the single layer samples, while the accelerator. mul
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