B. Pathangey

The measurement of deep level states caused by misfit dislocations in InGaAs/GaAs grown on patterned GaAs substrates

Rectangular Schottky diodes were fabricated on In0.06Ga0.94As grown by organometallic vapor phase epitaxy on GaAs substrates patterned with mesas. The density of α and β misfit dislocations at the strained‐layer interface changed with the size of the rectangular mesas. Since all mesas (four sizes and two orientations) are processed simultaneously, all other defect concentrations are expected to remain constant in each diode. Scanning cathodoluminescence showed that the misfit dislocation density varied linearly with rectangle size. Deep‐level transient spectroscopy showed that an n‐type majority‐carrier trap is present at 0.58 eV below the conduction band with a concentration that increases with increasing α‐type misfit dislocation density. The β misfit dislocation density had no influence the deep level spectra, indicating that this trap is related to the cores of only α‐type misfit dislocations. The capture rate trend corroborates the view that the trap is associated with the dislocation cores and not w...

MOCVD growth of non-epitaxial and epitaxial ZnS thin films

Thin films of ZnS have been deposited by MOCVD on both BaTa2O6/ITO/glass and Si substrates. Diethylzinc (DEZn) and H2S are used for deposition on substrates heated to the 250–400°C temperature range. The microstructure and properties of ZnS films were studied by X-ray diffractometry (XRD), ultraviolet/visible spectrophotometry (UVS) and scanning electron microscopy (SEM). Films prepared on BaTa2O6/ITO/glass were polycrystalline with mixed cubic and hexagonal phases. The hexagonal structure dominated for film thicknesses > 700 nm, while the cubic structure dominated for film thicknesses > 700 nm. The bandgap energy of ZnS films decreased and the surface morphology became worse with the increasing film thickness. Upon annealing 200–600 nm films at temperatures of 450–550°C in H2S for 1 h, the ZnS films developed an even stronger cubic (111) or hexagonal (002) preferred orientation. Polycrystalline ZnS films grown on Si(100) and (111) substrates tended to be cubic. Nucleation and growth rates on Si(100) substrates were initially faster than on Si(111) substrates. At 400°C, epitaxial growth of ZnS on Si(100) was achieved.

Evaluation of Zn{N[Si(CH 3 ) 3 ] 2 } 2 as a p -type dopant in OMVPE growth of ZnSe

We have grown nominally undoped ZnSe on GaAs from the precursors H2Se and Et2Zn. Replacement of Et2Zn by Zn[N(TMS)2]2 produced crystalline ZnSe of a lesser quality. Data indicate incorporation of nitrogen into the films when Et2Zn is utilized as the main zinc source with Zn[N(TMS)2]2 being introduced at dopant levels. Characterization techniques employed include NMR, XRD, SIMS, SEM, PL, RGA, GC/MS, and Raman spectroscopy.

F INDUCED LAYER DISORDERING OF GAAS/INGAP QUANTUM WELLS

F implantation (80–175 keV) induced GaAs/InGaP quantum well disordering was performed in a conventional furnace at 600–750 °C and in lamp annealing at 850 °C. Group V intermixing is found to be substantially enhanced for certain implantation and anneal conditions. Either the group III intermixing leading to lower band gaps or group V intermixing leading to higher band gaps may be made to dominate by choosing the process conditions. Only 50% reduction in integrated luminescence intensities from the as-grown layer makes this quantum well disordering process suitable for device fabrication.

Passivation of deep level states caused by misfit dislocations in InGaAs on patterned GaAs

Deep level transient spectroscopy (DLTS) and cathodoluminescence (CL) were used to study the hydrogen passivation of misfit dislocations in In0.06Ga0.94As/GaAs heterostructures. The CL observations indicate that hydrogen plasma exposure passivates most, but not all, of the dark line defects existing in the specimen prior to hydrogenation. The concentration of deep level defect states that cannot be passivated is below the detection limit of the DLTS instrument (approximately 4×1012 cm−3). We find the passivation is stable after anneals at temperatures as high as 600 °C, indicating that hydrogen passivation of misfit dislocations is at least as stable as that of the isolated point defect studied previously with DLTS [W. C. Dautremont‐Smith, J. C. Nabity, V. Swaminathan, M. Stavola, J. Chevalier, C. W. Tu, and S. J. Pearton, Appl. Phys. Lett. 49 1098 (1986)].

The thermal stability of lattice mismatched InGaAs grown on patterned GaAs

Patterning and etching substrates into mesas separated by trenches before the growth of mismatched (by about 1% or less) epitaxial layers considerably reduces the interface misfit dislocation density when the layer thickness exceeds the critical thickness. Such films are in a metastable state, since misfit dislocations allow the epitaxial layers to relax to an in-plane lattice parameter closer to its strain-free value. Thermal annealing (from 600 to 850° C) has been used to study the stability of these structures to explore the properties of the misfit dislocations and their formation. The misfit dislocation density was determined by counting the dark line defects at the InGaAs/GaAs interface, imaged by scanning cathodoluminescence. InGaAs epitaxial layers grown on patterned GaAs substrates by organometallic chemical vapor deposition possess a very small as-grown misfit dislocation density, and even after severe annealing for up to 300 sec at 800° C the defect density is less than 1500 cm−1 for a In0.04Ga0.96As, 300 nm thick layer (about 25% of the dislocation density found in unpatterned material that has not been annealed). The misfit dislocation nucleation properties of the material are found to depend on the trench depth; samples made with deeper (greater than 0.5 μm) trenches are more stable. Molecular beam epitaxially grown layers are much less stable than the above material; misfit dislocations nucleate in much greater numbers than in comparable organo-metallic chemical vapor deposited material at all of the temperatures studied.

Comparison of plasma chemistries for dry etching thin film electroluminescent display materials

Chlorine (Cl2 or BCl3, with additions of Ar or N2), fluorine (SF6/Ar) and methane/hydrogen (CH4/H2/Ar or CH4/H2/N2) based plasmas have been examined as a function of composition, source and sample chuck power, and pressure, for dry etching of the typical luminescent sulphide phosphors (ZnS, SrS), conductive electrode materials [indium tin oxide, (ITO) and TiW] and insulators (Al2O3, alumina/titania-ATO) used in thin film electroluminescent displays. It is necessary to have both a high ion flux and an ion energy above a particular threshold (typically ⩾125 eV) in order to achieve practical etch rates for the high bond strength materials such as Al2O3, alumina/titania and SrS. The fastest etch rates for ZnS, Al2O3 and aluminum tin oxide are obtained with Cl2/Ar for SrS with SF6/Ar and for ITO with CH4/H2/Ar.

The barrier to misfit dislocation glide in continuous, strained, epitaxial layers on patterned substrates

In a previous report [G. P. Watson, D. G. Ast, T. J. Anderson, and Y. Hayakawa, Appl. Phys. Lett. 58, 2517 (1991)] we demonstrated that the motion of misfit dislocations in InGaAs, grown by organometallic vapor phase epitaxy on patterned GaAs substrates, can be impeded even if the strained epitaxial layer is continuous. Trenches etched into GaAs before growth are known to act as a barrier to misfit dislocation propagation [E. A. Fitzgerald, G. P. Watson, R. E. Proano, D. G. Ast, P. D. Kirchner, G. D. Pettit, and J. M. Woodall, J. Appl. Phys. 65, 2220 (1989)] when those trenches create discontinuities in the epitaxial layers; but even shallow trenches, with continuous strained layers following the surface features, can act as barriers. By considering the strain energy required to change the length of the dislocation glide segments that stretch from the interface to the free surface, a simple model is developed that explains the major features of the unique blocking action observed at the trench edges. The ...

Electron cyclotron resonance plasma etching of oxides and SrS and ZnS-based electroluminescent materials for flat panel displays

A number of different plasma chemistries have been investigated for the etching of oxides (indium tin oxide for conductive electrodes; alumina/titania and Al2O3 for insulators) and phosphors (SrS, ZnS) used in thin film electroluminescent displays. Under high ion density conditions, such as in an electron cyclotron resonance source, maximum etch rates above 1500 A/min are obtained for ZnS in Cl2/Ar, BCl3/Ar, and SF6/Ar, for SrS in SF6/Ar and CH4/H2/Ar, for ITO in CH4/H2/Ar and for ATO in SF6/Ar. The etching is ion activated under most conditions, producing good feature anisotropy. Near-surface stoichiometry could generally be maintained on the etched surfaces of all materials except SrS where we invariably detected strong preferential loss of S. An optimized process for etching a typical metal-insulators-semiconductor-insulator-metal stack would involve switching plasma chemistries for each individual layer, but we have successfully patterned such a stack using only the CH4/H2/Ar chemistry.

Segregation of Ce in Ca0.55Sr0.45Ga2S4:Ce thin films during annealing

Abstract Surface and interfacial segregation of cerium in sputter-deposited Ca0.55Sr0.45Ga2S4:Ce (thiogallate) films during annealing between 550°C and 750°C has been studied. Composition vs. depth of the thiogallate thin films was determined using secondary ion mass spectrometry (SIMS) and microstructure was examined using transmission electron microscopy (TEM). Cerium segregation to the ambient/thiogallate surface was driven by oxide formation, while segregation to the thiogallate/insulator was driven by formation of a Ce–Zn–O–S quaternary phase.