High Density Of Point Defects Research Articles

Raman-scattering and optical studies of argon-etched GaAs surfaces.

We have studied the structual damage in low-energy argon-ion-bombarded (ion-etched) GaAs using Raman scattering and ultraviolet reflectivity. When combined with post-bombardment sequential chemical etching, the Raman results reveal a graded depth profile of the damage layer, with a nearly linear damage dropoff with depth. The total damage-layer thickness is about 600 A for high-fluence bombardment with 3.89-keV Ar{sup +} ions. The spectral effects produced by argon etching are very different from those produced by high-energy ion implantation. The longitudinal-optic Raman line seen for argon-etched GaAs is not shifted and broadened as in ion-implanted GaAs. More striking are the results of the reflectivity measurements. For argon-etched GaAs, the electronic interband peaks are both broadened and strongly red shifted relative to the crystal peaks; for ion-implanted GaAs, only the broadening occurs. Distinct nanocrystals, which account for the effects seen in ion-implanted GaAs, are evidently absent in argon-etched GaAs. Instead, the damage layer caused by argon etching appears to be characterized by a very high density of point defects, which previous work suggests may be arsenic vacancies.

Defect mechanisms in degradation of 1.3-μm wavelength channeled-substrate buried heterostructure lasers

Channeled-substrate buried heterostructure (CSBH) lasers which were purged from populations undergoing high reliability qualification have been studied in detail. Gradual and rapid degradation mechanisms leading to accelerated aging failure modes have been analyzed by transmission electron microscopy, convergent beam electron diffraction, electroluminescence, energy dispersive x-ray analysis, and chemical etching. The gradual degradation mode of CSBH lasers is characterized by (1) a gradual increase in room-temperature threshold current; (2) a decrease in external quantum efficiency, typically a drop in peak value of dL/dI greater than 25%; (3) a drop in forward voltage at low current, indicating a change in junction characteristics; (4) a large peak in I(dV/dI) below threshold (at around 3 mA); and (5) an enhancement in the peak in I2(d2V/dI2) at laser threshold. A defect mechanism associated with the gradual degradation begins with a nucleation of extrinsic dislocation loops along the V-groove {111} p-n–type sidewall interfaces between the Cd-diffused p-InP and liquid-phase-epitaxial-grown n-InP buffer inside the groove. These dislocation loops subsequently grow out of the interfaces into the n-InP buffer region in the direction of minority-carrier injection, indicating a nonradiative recombination-assisted defect growth process. For those loops which enter the quaternary active region near the tip of the active crescent, the growth rate along the (001) and (010) planes is greatly enhanced and the loops eventually cut across the active stripe and become dark-line defects, as confirmed by electroluminescence. Nucleation of dislocation loops is not observed along the {111} p-p–type sidewall interfaces above the active stripe. The fact that the dislocation loops are all extrinsic in nature implies that the {111} sidewall interfaces as well as the quaternary active region contain a high density of interstitials. The possible causes for the generation and growth of the dislocation loops and the high density of point defects are discussed. The rapid degradation mode of the CSBH laser is characterized by a sudden drop in light intensity during the aging process. The associated defect mechanism starts with localized melting at the mirror facet or inside the lasing cavity. A metal-rich droplet subsequently forms which propagates along the center of the active stripe in the direction towards the cavity center via a meltback-regrowth process; i.e., material melts in front of the droplet and regrows after it propagates by. The nonideal condition of regrowth results in the formation of a wormlike defect composed of a cylinder of defective materials bounded by an off-stoichiometric interface. The wormlike defect is dark under electroluminescence. Complicated dislocation structures can also be grown from the wormlike defect under a nonradiative recombination-assisted defect growth process. These phenomena are presented and discussed.

Shock deformation of polycrystalline aluminium

Abstract The deformation microstructures associated with aluminium after shock loading and cold rolling at room temperature and at liquid nitrogen temperature are presented and discussed. A high density of point defects is produced by shock loading aluminium and it is observed that vacancy-produced dislocation loops are more numerous than those produced by interstitial clustering. Comparison is made with the response of copper and nickel to shock and it is shown that aluminium does not form dislocation cell structures as these metals do under similar loading conditions. The temperature of deformation has little effect on the microstructure in the low pressure range, but as pressure increases the temperature of deformation becomes more important. Mechanisms for the production of defects are discussed.