Janet E. Hails

MOVPE of narrow and wide gap II-VI compounds

Abstract The development of high quality II–VI compounds is dependent on advances made in certain key research activities. This review is concerned with these activities which includes precursors such as i Pr 2 Te, t Bu 2 Te, Me(allylTe and (allyl) 2 Te for low temperature growth, substrates including CdZnTe, CdTe, CdTeSe, GaAs on Si for epitaxy, doping for p-n junction formation and photo-assisted growth processes for low temperature growth, photo-patterning and selected area deposition.

High-performance long-wavelength HgCdTe infrared detectors grownon silicon substrates

Long-wavelength HgCdTe heterostructures on silicon (100) substrates have been grown using metal-organic vapor phase epitaxy. Test diodes have been fabricated from this material using mesa technology and flip-chip bonding. We have demonstrated excellent resistance-area product characteristics for diodes with a 10.2μm cutoff wavelength. R0A values approaching 103Ωcm2 at 80K have been measured and the resistance-area product maintained above 102Ωcm2 at 1V reverse bias. Variable temperature R0A values correspond to expected generation-recombination loss mechanisms between 60 and 120K. Current-voltage characteristics of two diodes at opposite sides of an array indicate that a very uniform imaging long-wavelength infrared array could be fabricated from this material.

Evidence for a surface-bound free radical mechanism during the decomposition of iPr2Te in the presence or absence of mercury and/or Me2Cd under MOVPE conditions obtained from deuterium-labelled precursors

Abstract The presence of the cross-over products, d1- and d7-propane, along with d0- and d8-propane, d0- and d6-propene, d0, d7- and d14-2,3-dimethylbutane (DMB) and small amounts of d7-iPr2Te from the co-decomposition of d0- and d14-iPr2Te in helium is interpreted in terms of homolytic fission of the TeC bond to form iPr and iPrTe . The small amount of DMB, as well as its labelling pattern, indicate that the main fate of the iPr formed is to abstract H from intact iPr2Te to give propane, 2 × propene, Te and H . A kinetic isotope effect (kH/kD ∼ 2) is observed for this abstraction reaction. The relative ratios of propene : propane : DMB and the labelling pattern for DMB suggest that the latter arises from coupling reactions of surface bound iPr . iPrTe probably releases more iPr . In the presence of Me2Cd, Me releases mor iPr and MeTeiPr from iPr2Te via MeiPr2Te . The larger amounts of free radicals in this system mean that combination reactions of surface bound radicals dominate over abstraction of H from iPr2Te. Reactions carried out in hydrogen show that many of the free radicals react with H2 at rates comparable with the other available pathways. Finally, decomposition of (Me2CD)2Te in helium provides unequivocal evidence that all hydrogen abstraction reactions occur from the methyl groups rather than from the secondary carbon atom, since d1-propene and d1-propane are the only observable products.

Methyl(allyl)telluride as a Te precursor for growth of (Hg,Cd)Te by metalorganic vapour phase epitaxy

Abstract Methyl(allyl)telluride (Me(allyl)Te) has previously been used as a precursor for the metalorganic vapour phase epitaxy (MOVPE) growth of CdTe and HgTe. The decomposition reactions of Me(allyl)Te alone and in all combinations with Me 2 Cd and Hg and in H 2 and He carrier gases have been studied using the technique of gas chromatography-mass spectrometry. The main Te containing product from the decomposition of Me(allyl)Te alone is Me 2 Te 2 with little Te deposited. When Me(allyl)Te, Me 2 Cd and Hg were co-pyrolysed in H 2 at 350°C the observed products included methane, 1-butene, 1,5-hexadiene and Me 2 Te. A mechanism for the decomposition is proposed. By comparison with Pr i 2 Te we would predict that Me(allyl)Te is not likely to be a good precursor.

Photomultiplication with low excess noise factor in MWIR to optical fiber compatible wavelengths in cooled HgCdTe mesa diodes

Infrared avalanche diodes are key components in diverse applications such as eye-safe burst illumination imaging systems and quantum cryptography systems operating at telecommunications fiber wavelengths. HgCdTe is a mature infrared detector material tunable over all infrared wavelengths longer than ~850nm. HgCdTe has fundamental properties conducive to producing excellent detectors with low noise gain. The huge asymmetry between the conduction and valence bands in HgCdTe is a necessary starting point for producing impact ionization with low excess noise factor. Other factors in the band structure are also favorable. The low bandgap necessitates at least multi-stage thermoelectric cooling. Mesa diode structures with electron initiated multiplication have been designed for gains of up to around 100 at temperatures at or above 80K. Backside illuminated, flip-chip, test diode arrays have been fabricated by MOVPE using a process identical to that required for producing large imaging arrays. Test diode results have been obtained with the following parameters characterized, dark current vs. voltage and temperature, gain vs. voltage, and spectral response as a function of wavelength and bias. The effect of changing active region cadmium composition and active region doping is presented along with an assessment of some of the trade-offs between dark leakage current, gain, operating voltage and temperature of operation.

The influence of substrate surface preparation on the microstructure of CdTe grown on (001) GaAs by metalorganic chemical vapour deposition

Abstract A cross-sectional transmission electron microscopy study has been performed on the microstructure of CdTe epitaxial buffer layers, which were grown upon 2°-off (001) GaAs substrates by metalorganic chemical vapour deposition (MOCVD). The substrates were prebaked at various temperatures instead of the usual acid etching treatment. Ut was shown that single crystal films were only obtained for baking temperatures of 550 and 590°C. These single crystal deposits exhibited a distinctive anisotropic defect structure with subgrain boundaries and 90°-type misfit dislocations perpendicular to the off-cut axis and stacking faults and a mixture of 60°-type and 90°-type misfit dislocations parallel to the off-cut axis. All of the stacking faults and 60°-type dislocations lie on the plane (111) producing a 6° rotation between the substrate and deposit lattices about the offcut axis. The development of this microstructure has been ascribed to the formation of high aspect ratio island nuclei and differences in Schmid factors due to the offcut. This microstructure was compared to that produced on an etched substrate and it appears that fine scale substrate surface roughness eliminates both of these influences.

Homolytic reactions of diorganotellurium and diorganoditellurium compounds in solution; an EPR study

Abstract The thermal and photochemical decompositions of dialkyltelluriums and dialkylditelluriums in t-butylbenzene solvent have been studied by, EPR spectroscopy. On photolysis, di-t-butyl, diallyl, allyl methyl, and dibenzyl tellurium showed EPR spectra of t-butyl, allyl, allyl, and benzyl radicals respectively. Alkyl radicals were not detected directly on thermolysis in the range 300–370 K but, primary, secondary, and tertiary alkyl, benzyl and allyl telluriums gave spin adducts of the corresponding alkyl radical with nitrosodurene. Dimethyl tellurium was unreactive in the same temperature range. The rates of thermal decomposition of the diorgano telluriums at 315 K were found to be in the following order, as judged by the intensity of the spin adduct spectra: t Bu 2 Te ∼ t BuTeMe > (allyl) 2 Te ∼ allylTeMe > s Bu 2 Te > (benzyl) 2 Te > i Pr 2 Te > Et 2 Te > Me 2 Te. For dihex-5-enyltellurium the results were consistent with formation and cyclisation of hex-5-enyl and hex-5-enetelluryl radicals. In the case of dipent-4-enyltellurium, the results suggested that both exo - and endo -cyclisation of the pentene telluryl radicals were important. Di-isopropyl- and di-t-butyl-ditelluriums gave spin adducts of the corresponding alkyl radicals on thermolysis in the same temperature range as the monoalkyltelluriums.

Dual-waveband infrared focal plane arrays using MCT grown by MOVPE on silicon substrates (Invited Paper)

Dual-waveband, Focal Plane Arrays (FPAs) based on Hg1-xCdxTe multi-layer structures have previously been produced by the Molecular Beam Epitaxy (MBE) growth technique. It is shown that the multi-layer structures required for dual-waveband devices can also be grown by Metal Organic Vapor Phase Epitaxy (MOVPE). The MOVPE growth process allows excellent control of both the composition and doping profiles and has the advantage of allowing growth on a range of substrates including silicon. Previous research on back-to-back diodes for dual-waveband has concentrated on npn structures. The design of the alternative pnp structures is discussed and a model is developed which gives a good fit to the measured spectra. We report on the design and characterization of dual-waveband detectors including current-voltage and spectral cross talk for the case of two close sub-bands within the 3-5 μm mid-wave infrared (MWIR) spectral range. The mechanisms for spectral cross talk are discussed including incomplete absorption, transistor action and radiative coupling. A custom readout circuit (ROIC) has been designed. This allows the capture of data from the two bands which is spatially aligned but sequential in time.

The role of surface adsorbates in the metalorganic vapor phase epitaxial growth of (Hg, Cd) Te onto (100) GaAs substrates

It has been established that a compound present as an impurity in the propan-2-ol used in the preparation of GaAs (100) substrates for the metalorganic vapor phase epitaxy growth of (Hg,Cd)Te has a marked effect on the crystalline perfection and surface morphology of the resulting layers. In particular, the presence of this species, which contains Na, ensures that (i) the epitaxial overgrowth is of (100) orientation without the need for ZnTe nucleation layers, and (ii) the density of pyramidal hillocks on the surface can be reproducibly < 10 cm−2.

Long-wavelength HgCdTe on silicon negative luminescent devices

We have investigated the negative luminescent properties of a device fabricated from metalorganic vapor phase epitaxy grown HgCdTe on a Si substrate. The peak emission was at 7.2μm, and the intrinsic Auger processes were found to be very well suppressed. The low currents (minimum current density, Jmin, of 0.84A∕cm2 at 295K) needed to drive these devices makes them suitable for a range of device applications.