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II-VI Semiconductors for Energy Capture, Photon Generation and Detection

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Geoff Pain
added 4 research items
Indium-doped CdxHg1-xTe grown by low-temperature metal-organic chemical vapour deposition (MOCVD) is highly resistive. Indium is also known to react with HgTe and with CdxHg1-xTe to give InTe. The equilibrium distribution of indium tellurides resulting from indium doping of CdxHg1-xTe has been calculated for a range of temperatures from 373 to 633 K and shows that the major component is the mixed-valence InTe, with a few per cent of In2Te3. These results suggest that self-compensation in In-doped Cdx-Hg1-xTe is due to the formation of InTe, but that this self-compensation will decline with increasing temperature and increasing Hg content.
The Fourth Australian Conference on II-VI Semiconductor Compounds was held in Warburton, Victoria, Australia in April 1990 bringing together 79 researchers in the field. At the time it was thought that Australia would take a leading role in invention, manufacture and export of optoelectronic devices based on these materials. This invited review of proceedings records the mood of the times.
Geoff Pain
added 2 research items
Ultramicrotome techniques are used to prepare thin cross sections of a {111} epilayer of CdTe deposited by metalorganic chemical vapor deposition onto a {100} GaAs substrate. The structure of these samples is investigated by transmission electron microscopy using high‐resolution (HRTEM) and diffraction contrast, and the polarity of the {111} layer by convergent beam electron diffraction (CBED) and characteristic x‐ray emission under various electron channelling conditions, or ALCHEMI. Rutherford backscattering and channelling experiments on the bulk film confirm the presence of a multiply twinned lamellar structure as observed by electron beam techniques. Strong channeling confirms that the crystallinity is good, and that no significant concentration of defects occurs. HRTEM images of the {111} epilayer from the interface across the lamellar twins show few dislocations or crystal defects. Diffraction contrast indicates the presence of a periodic strain in the GaAs and parallel to the interface. CBED and ALCHEMI results confirm that the layer is B type, and that the lamellar twins do not invert phase.
We demonstrate the application of ultramicrotome techniques for the preparation of thin semiconductor interfaces in cross-section, for analysis by electron beam techniques. This is of significance for semiconductor device technology, since a specific area on a particular device may be examined in cross-section. High-resolution transmission electron microscopy studies of twinned structures at the interface of a 1.2 μm metal-organic chemical vapour deposited HgCdTe epilayer on {100} GaAs are reported. {100} epitaxial growth starts to dominate about 40 nm from the interface, and continues to the top surface of the epilayer. Energy-dispersive X-ray analysis shows that the mercury concentration within the epilayer increases as the interface is approached. No evidence of diffusion into the GaAs substrate was obtained.
Geoff Pain
added 2 research items
The anomalous scattering of X-rays has been used to determine the polarity of CdTe epitaxial layers on sapphire. The results for two samples are presented, one of (111) orientation ('A face'), the other of (III) orientation (,B face'). The (III) layer is twinned, the two twin species being related by a 180· rotation about the [1111 axis. The twin fraction shows considerable variation for different positions on this sample, and must be taken into account when analysing the integrated X-ray intensities, in order to get meaningful Bijvoet ratios. The polarities of the two twin species are found to be the same.
The anomalous scattering of x rays has been used to determine the polarity of single‐crystal epitaxial layers, as grown on the substrate material. The method relies on the breakdown of Friedel’s law in noncentrosymmetric structures and is particularly effective when there exists a large difference between the values of the imaginary anomalous dispersion correction, f‘, for the respective atomic species. The method is nondestructive and measurements are taken from only one side of the sample (the side with the epitaxial layer).
Geoff Pain
added 2 research items
Multidirectional RBS channeling analysis is used to identify the crystal orientation of a MOCVD-grown CdTe layer on a (100) GaAs substrate. Results show that the CdTe has a (111) orientation. However, from channeling in different axial directions and a rotational angular scan around the <111> axis, it is deduced that the CdTe layer is multiply twinned about this axis. Cross-sectional electron microscopy has revealed the twins to be 180° rotational twins.
RBS spectra analysis near the (111) planar channeling direction is used for polarity determination of MOCVD-grown epitaxial layers of (111) and (100) CdTe on sapphire and GaAs substrates. Extensive multiple twinning limits the application of RBS channeling analysis for polarity determination. Comparison is made with polarity determination by X-ray diffraction and two electron diffraction techniques.
Geoff Pain
added 6 research items
The fabrication of the first MESFET structures on Hg<sub>1-x</sub>Cd<sub>x</sub>Te is reported using MOCVD grown layers on GaAs substrates. The 6 mu m gate devices exhibited a room temperature transconductance of 1.0 mS/mm and pinch off voltage of -4.0 V. The Schottky barrier characteristics of the devices were critically dependent on the stoichiometric x ratio of the Hg<sub>1-x</sub>Cd<sub>x</sub>Te with diode formation evident only at x >0.5.
CdTe and HgCdTe epitaxial layers were grown on 50 mm diameter (100) GaAs wafers in a Quantax 226 metallo-organic chemical vapour deposition system. Dimethylcadmium, diethyl telluride and elemental mercury were used as starting materials in the thermal growth process which takes place in a flow of hydrogen at 250–350°C. The layers were characterized by (1) an interference contrast microscope for surface morphology; (2) FTIR spectros-copy for compositional and thickness uniformity; (3) x-ray topography and double crystal diffractometry for the non-destructive evaluation of the crystalline quality and microstructure; (4) energy-dispersive x-ray analysis (WDX/EDX) for compositional studies; (5) transmission electron microscopy for interface observations and analysis; and (6) Rutherford backscattering and channelling analysis for surface stoichiometry and crystal quality determinations.
The fabrication of the first metal-semiconductor-metal photodetectors on Hg<sub>1-x</sub>Cd<sub>x</sub>Te is reported using MOCVD grown layers on GaAs substrates. An epitaxial CdTe overlayer has been incorporated in the device structure for the enhancement of Schottky barrier characteristics. The interdigitated devices (2.3 mu m electrode width, 3.3 mu m spacing) exhibited a breakdown voltage of -60 V and responsivities of more than 1.0 A/W at a wavelength of 1.3 mu m and bias voltage of 40 V. Over the range of bias voltage examined, the dark leakage current of the detectors was dependent on the choice of contact metal, with minimum values of 10 nA at <1 V for Pt/CdTd/Hg<sub>1-x</sub>Cd<sub>x</sub>Te.
Geoff Pain
added 2 research items
High-resolution X-ray diffraction studies of MOCVD-grown Cd1−xMnxTe/Cd1−yMnyTe superlattices on a (001) GaAs substrate have been carried out. Topographs and rocking-curve maps were taken from two directions at right angles to each other ([110] and [10], the latter being the gas flow direction). The structural properties and uniformity of the Cd1−xMnxTe/ Cd1−yMnyTe superlattice epilayers have been established from analyses of both rocking-curve maps and topographs. An anisotropic mosaic structure, where the mosaic blocks are well ordered along the [10] direction and disordered along the [110] direction, is proposed in order to explain these observations. It is shown that combinations of rocking-curve maps and topographs give a much more informative characterisation than a single-point measurement. The satellite peaks of the double-crystal rocking curve of Cd1−xMnxTe/Cd1−yMnyTe superlattices have been observed using an asymmetric 335 Bragg reflection. The angular positions of the observed satellites have been investigated and compared with the predictions of a standard theoretical formula, extended to the case of asymmetric reflections. The rocking curves of Cd1−xMnxTe/Cd1−yMnyTe superlattices have also been simulated using the Takagi-Taupin equations. The agreement obtained is quite satisfactory.
The techniques, Particle Induced X-ray Emission (PIXE) and Rutherford Backscattering Spectrometry (RBS) have been used to investigate compositional and thickness uniformity of Hg1-xCdxTe (MCT) grown on GaAs substrates by Metal Organic Chemical Vapour Deposition (MOCVD). Composition and thickness variations are reported for orientations perpendicular and parallel to gas flow in the MOCVD reactor. Crystalline quality of the MCT layer was also determined by RBS channelling analysis.
Geoff Pain
added 8 research items
Ion beam analysis has been used to investigate the epitaxial quality and compositional and thickness uniformity of epitaxial Hg1-xCdxTe (MCT) layers on GaAs substrates grown by low temperature MOCVD. To determine the effect of the ion beam on the MCT layer during a typical measurement, damage studies have been performed by channeling analysis using a nuclear microprobe. Microprobe channeling enabled high flux damage to be investigated. Areas of crystal ranging from approximately 30 x 30-mu-m2 to 260 x 260-mu-m2 were scanned using both 2.0 MeV H+ and He+ ions and the chi(min) measured as a function of ion beam dose. Macrobeam channeling was also performed with an unscanned beam of 1 mm spot size. The results indicate the acceptable range of dose for ion beam analysis.
Hg1-xCdxTe epitaxial layers on GaAs substrates grown by Metal Organic Chemical Vapour Deposition (MOCVD) display growth defects resembling pyramidal faceted hillocks which appear to originate from defects originally present on the substrate. For <100> oriented GaAs substrates and normal growth conditions, these growth defects have an areal density of 1–1000 mm-2. The size of the hillocks depends on the layer thickness and they have the potential to degrade performance of optoelectronic devices fabricated in the epitaxial layers. Nuclear microprobe analysis, performed with a 2 MeV He+ beam focused to less than 5 μm in diameter, has allowed the hillocks to be imaged with the technique of Channeling Contrast Microscopy (CCM). Channeling spectra, obtained by Rutherford Backseat tering Spectrometry (RBS) of the hillocks themselves, showed that the χmin was 13 %. This was similar to the χmin of the high quality single crystal surrounding material. The CCM images also revealed extensive regions of poor channeling, with shapes that suggested that the regions originally arose from scratches in the substrate. These poor channeling regions were not readily observable by other techniques.
Epitaxial layers of Hg1-xCdxTe grown by metal organic chemical vapour deposition (MOCVD) are being developed for infrared optoelectronic devices. These layers can be very non-uniform in stoichiometry particularly in the direction of gas flow. PIXE analysis has been successfully applied to the determination of stoichiometry over a wafer. However, due to cost in accelerator time and ion beam damage, PIXE analysis is unsuitable for the routine stoichiometric analyses of these layers in production control. An energy dispersive XRF system using Am-241 as an excitation source has been developed. The results obtained from EDXRF are indistinguishable from those of PIXE after correction for secondary fluorescence. EDXRF is suitable for routine stoichiometric analysis of wafers as may be required in a production environment.
Geoff Pain
added a research item
Near‐surface regions of Cd 1-x Mn x Te and Hg 1-x Cd x Te epilayers (down to tens of angstroms) on (001) GaAs substrates have been characterized by extremely asymmetric Bragg reflection topography (EABRT) with the laboratory x‐ray source condition and a Lang camera, using x‐ray grazing incidence angles less than the critical angle for total external reflection. The experimental topographs obtained in the present work illustrate the potential of the EABRT technique for nondestructive characterization of near‐surface regions of crystals. The resolution of an image in EABRT is discussed in detail.
Geoff Pain
added a research item
We report on progress in the monolithic integration of a metal-semiconductor-metal (MSM) detector and transimpedence amplifier and of a photoconductive detector (PCD) with a metal-semiconductor field effect transistor (MESFET) in Hg1-xCdxTe. The layers of CdTe/n-type Hg1-xCdxTe were grown by MOCVD on semi-insulating GaAs substrates (2° misoriented 100). Fabrication of the devices was by an FET planar process; with a standard lift-off used to form Schottky metallization on both the interdigitated electrodes of the MSM detector (2μm width, 2μm spacing) and the gate of the MESFETs (5μm length, 100μm width). The MSM photodetectors exhibited breakdown voltages in the range 60 to 80V, a dark current of 1 Ona at 5V bias, and responsivities of > 1.0 A/W measured at 40V using CW 1.3um illumination. The integrated devices have been characterised by electrical and micro RBS techniques; the results were found to be strongly dependent on the stoichiometric x ratio of the Hg1-xCdxTe. This initial work demonstrates the suitability of Hg1-xCdxTe/GaAs structures in the fabrication of integrated optoelectronic circuits.
Geoff Pain
added 6 research items
Epitaxial layers of Mercury Cadmium Telluride have been grown on GaAs using the MOCVD process, and used in the fabrication of photoconductive detector devices. Results are presented and a process for the fabrication of integrated optic fibre receiver devices is described. (
Geoff Pain
added a research item
Organometallic feedstocks of cadmium, mercury and tellurium, as well as dopants to create n-type, p-type or diluted magnetic semiconductors, can impose limits on the range of programmable parameters in the MOCVD reactor. The influence of the chosen organometallics on the crystal growth process is critically reviewed.
Geoff Pain
added 2 research items
The availability of new organomanganese feedstocks RMn(CO)5 (R = CH3 [methyl] or C6H5CH2 [benzyl]) has enabled growth of diluted magnetic semiconductor superlattices Cd1-xMnxTe/Cd1-yTe by low temperature metal organic chemical vapour deposition (MOCVD) for the first time. Secondary ion mass spectrometry (SIMS) was used to measure composition modulation and there was no evidence for carbon contamination from the new sources. X-ray topography, double crystal X-ray diffraction and Nomarski interference microscopy showed preferential facetting and different mosaic spread for some substrate orientations. Particle-induced X-ray emission (PIXE), Rutherford backscattering spectroscopy (RBS), high resolution transmission electron microscopy (HRTEM), and analytical electron microscopy (AEM) were used to evaluate material composition and quality.
The availability of new organomanganese feedstocks RMn(CO)5 (R = CH3 [methyl] or C6H5CH2 [benzyl]) has enabled growth of diluted magnetic semiconductor superlattices Cd1-xMnxTe/Cd1-yTe by low temperature metal organic chemical vapour deposition (MOCVD) for the first time. Secondary ion mass spectrometry (SIMS) was used to measure composition modulation and there was no evidence for carbon contamination from the new sources. X-ray topography, double crystal X-ray diffraction and Nomarski interference microscopy showed preferential facetting and different mosaic spread for some substrate orientations. Particle-induced X-ray emission (PIXE), Rutherford backscattering spectroscopy (RBS), high resolution transmission electron microscopy (HRTEM), and analytical electron microscopy (AEM) were used to evaluate material composition and quality.
Geoff Pain
added 4 research items
Specular Cd1−yMnyTe epilayers of (100) and (111)B orientation have been grown on 5 cm GaAs (100) and GaAs/Si (100) substrates using dimethyl cadmium, diethyl tellurium and the new organomanganese sources RMn(CO)5 (R = C6H5CH2 or CH3). The manganese compounds are volatile, air and water stable solids which are prepared in high yield and are readily purified by sublimation. Growth was performed at 320–350°C in a horizontal reactor at atmospheric pressure. The feasibility of low temperature interdiffused multilayer process (IMP) MOCVD of the quaternary alloy Hg1−x−yMnyCdxTe is demonstrated. Cati on interdiffusion profiles, including the first reported for Mn in II–VI compounds, are obtained by analytical electron microscopy of ultramicrotomed cross sectional samples using a fine 10 nm electron beam probe. Interdiffusion of Mn is comparable to that of Cd. The Cd1−yMnyTe (111)B layers exhibit fine scale 180°C rotational twinning. Growth of untwinned HgTe on Cd1−yMnyTe (111)B buffer layers is observed, with the HgTe layer adopting the orientation of the surface of the buffer layer. Interdiffusion during growth and subsequent annealing yields untwinned (111)B quaternary alloy on the HgTe side of the interface. The structural quality of (111)B and (100) epilayers is compared by lattice imaging and selected area electron diffraction patterns. No voids or precipitates are observed, in contrast to material grown by techniques employing higher temperatures.
The influence of six new GaAs substrate orientations on the surface morphology and defect hillocks of epitaxial Hg(1-x)Cd(x)Te (MCT) layers, grown by low temperature MOCVD, is investigated using Nomarski interference contrast microscopy and SEM. Smoother epitaxial layers are obtained on (311), (511) and (711) substrates of A or B polarity compared with growth on (100) or (100) 2-degrees off toward (110) substrates. The quality and orientation of the layers was monitored by X-ray techniques, Rutherford backscattering/channeling and selected area electron channeling patterns. Absolute polarity determination by X-ray diffraction confirmed that the epilayers adopted the polarity of the substrate. Conformal growth on contoured substrates revealed preferential facet formation and growth rate variation which leads to local composition changes in the interdiffused multilayer process. The growth hillocks commonly observed in epitaxy of MCT are related to the oval defects in III-V compounds, and dissolve on annealing.
Geoff Pain
added 2 research items
New organomanganese sources RMn(CO)5 (e.g. R = CH3 or C6H5CH2) have enabled low-temperature MOCVD of Cd1-xMn(x)Te (0 < x < 0.8) thin films on GaAs, GaAs/Si, InP, sapphire and glass substrates over an area > 50 cm2 in a horizontal reactor at atmospheric pressure. Diluted magnetic Cd1-yMn(y)Te/Cd1-xMn(x)Te strained layer superlattices have been characterized by SIMS, PIXE, RBS, HRTEM of ultramicrotomed cross-sectional samples, double-crystal XRD rocking curve and satellite analysis and simulation, X-ray topography, WDX/EPMA, optical transmission and FTIR. Local thickness mapping by simultaneous PIXE and RBS, profilometry, SIMS sputter times and FTIR has provided uniformity information required for improved reactor design. Preferential (011) growth on (100) GaAs leads to anisotropic micrograting-like X-ray diffraction at glancing angles. Smoother material is obtained on (111B), (311A), (311B), and (100) 2-degrees off toward (110) GaAs.
Geoff Pain
added 2 research items
A new intrinsic defect, the mercurous ion (Hg–Hg)<sup>2+</sup>, is proposed to account for n‐type doping of Hg 1-x Cd x Te by excess mercury. The double cations can be formed through capture of neutral interstitial mercury atoms by mercuric ions. This effectively decouples the mercuric ion from the four tellurium atoms previously coordinated, releasing two electrons for each mercury atom captured. This model can explain the stability of metal‐rich MCT and recent reports of intrinsic acceptor–donor pairs correlated within a lattice constant. Formation of mercurous ions could greatly enhance self‐diffusion of Hg through a knock‐on effect. Thermodynamic calculations support the proposition that self‐compensation of indium in MCT can occur through the presence of nearly equal populations of In<sup>3+</sup> (donor) and In<sup>+</sup> (acceptor), as occurs in the stable phase InTe. For low or metastable doping levels, the solid phase can be considered as a solid solution of InTe, In 2 Te 3 , and MCT. Any excess trivalent In is perfectly compensated by introduction of a cation vacancy for every two In<sup>3+</sup> ions. On subsequent mercury‐rich annealing, only the excess In<sup>3+</sup> will be ‘‘activated’’ through filling of cation vacancies, imposing a limit to the maximum n‐type doping achievable.
Geoff Pain
added 2 research items
A general method is described for production of uniform thin films from the vapour phase. A brief review of the prior art reveals that various approaches to this problem, and in particular substrate rotation, fail to achieve uniformity of thickness and composition in such technologies as molecular beam epitaxy (MBE) or chemical vapour deposition (CVD). The new technique, for which patent protection has been obtained, involves establishment of uniform growth conditions in one direction, and translation of the substrate at a constant rate in a direction perpendicular to the first. The effect is to integrate and average any property in the direction of translation, so that every point on the substrate experiences an identical history, ensuring complete uniformity of the deposited film.