Showing papers in "Semiconductor Science and Technology in 1993"

Quantum well intermixing

Abstract: Intermixing the wells and barriers of quantum well structures generally results in an increase in the band gap and is accompanied by changes in the refractive index. A range of techniques, based on impurity diffusion, dielectric capping and laser annealing has been developed to enhance the quantum well intermixing (QWI) rate in selected areas of a wafer; such processes offer the prospect of a powerful and relatively simple fabrication route for integrating optoelectronic devices and for forming photonic integrated circuits (PICS). Recent progress in QWI techniques is reviewed, concentrating on processes which are compatible with PIC applications, in particular the achievement of low optical propagation losses.

X-ray diffraction from low-dimensional structures

Abstract: This review paper presents the applications of X-ray diffraction to routine measurements and the procedures for extending its capabilities of analysis to undertake a detailed structural investigation of low-dimensional structures. The uses and limitations of the familiar double-crystal diffractometer are discussed as are the advantages of 'reciprocal space mapping' with a multiple-crystal diffractometer. In general X-ray diffraction has been used for composition and thickness measurement in low-dimensional structures (LDS) and these aspects are covered, as well as the avoidance of the pitfalls associated with their determination. The possibilities for the use of X-ray diffraction methods to determine interface quality, the evolution of lattice relaxation, the detailed microstructure, etc, are discussed, with an indication of the limits of the techniques.

Selective-area low-pressure MOCVD of GaInAsP and related materials on planar InP substrates

Abstract: Low-pressure MOCVD has been used to grow layers of InP, InGaAs, GaInAsP and quantum well material on planar substrates patterned with silica masks. The thicknesses and, where relevant, the compositions of these selectively grown layers were investigated by optical and scanning electron microscopy, surface profiling, energy dispersive X-ray analysis, secondary-ion mass spectroscopy and spatially resolved photoluminescence. The epitaxial layers were found to be both thicker and richer in indium in the vicinity of a mask. The perturbations in the thickness and composition of material grown around a given mask pattern were independent of the orientation of the pattern with respect to the gas flow and the crystallographic axes of the substrate. Lateral movement of material from the masked regions to the surrounding areas was found to take place in the gas above the water surface. A gas-phase diffusion model based on Laplace's equation was used to analyze the thickness and compositional variations caused by selective growth. The emission wavelength of selectively grown InGaAs/GaInAsP MQW material was shifted by over 100 nm without degradation in emission efficiency. The lasing wavelength of Fabry-Perot lasers fabricated on such material was increased by a similar amount without degradation of threshold current.

Minority carrier lifetime in mercury cadmium telluride

Abstract: The authors review the current status of minority carrier lifetime in n-type and p-type (Hg, Cd)Te. This review includes a discussion of the relevant (Hg, Cd)Te recombination mechanisms and measurement techniques. The reported experimentally determined lifetimes were related to (Hg, Cd)Te material properties of carrier concentration, Shockley-Read-Hall centres, non-uniformities and dislocation densities.

Epitaxial lift-off and its applications

Abstract: In this paper the authors will give an overview of the epitaxial lift-off (ELO) technique and its applications. This first part will describe the basic technology, which includes chemical lift-off, handling bonding, stress, alignment, etc. The second part will give an overview of device results obtained with ELO (LED and lasers on Si, MESFETs on InP, OEICs etc).

Edge-state transport and its experimental consequences in high magnetic fields

Abstract: Many magnetotransport experiments in two-dimensional electron systems have been explained in recent years within the so-called edge-state model. Starting from a historical context, the model is described. Experiments in Hall-bar geometries with and without gate barriers are reviewed and discussed in terms of the edge-state picture.

Methods for magnetotransport characterization of IR detector materials

Abstract: Advanced techniques for the magnetotransport characterization of semiconductor IR detector materials are reviewed. Both conventional mixed-conduction and 'mobility spectrum' analyses of the resistivity tensor as a function of magnetic field and temperature are discussed, as well as a hybrid approach which exploits the advantages of both methods. Assuming that the data are sufficiently sensitive, one obtains concentrations and mobilities for each electron and hole species present in a given sample. This is particularly valuable for narrow-gap infrared detector materials such as Hg1-xCdxTe, since the coexistence of multiple species tends to be the rule rather than the exception, and 'anomalous' Hall data are easily misinterpreted if inferences are drawn from measurements at only a single magnetic field. In addition to bulk electron and hole densities and mobilities, one can determine inversion and accumulation layer properties from the anomalous Hall effect, acceptor binding energies and compensation ratios from the low-temperature freeze-out of free holes, and energy gaps from the temperature dependence of the intrinsic carrier concentration. Shubnikov-de Haas and quantum Hall measurements provide additional information about the spatial distribution of the carriers. Electron and hole mobilities in Hg1-xCdxTe detector materials will be briefly reviewed, and theoretical and experimental results compared.

Silicon solar cells: evolution, high-efficiency design and efficiency enhancements

Abstract: With a history dating back over 50 years, silicon solar cells were amongst the first bipolar silicon devices demonstrated. Notwithstanding this long history, the last ten years have seen rapid progress in both the efficiency of experimental devices and in the understanding of basic design constraints. The evolution of silicon cell design over the last 50 years is described and the features of current high-efficiency devices are discussed in some detail. With energy conversion efficiencies above 23%, these devices are now approaching basic limits for conventional homojunction cells. The potential for substantially increasing cell performance above these limits, by taking advantage of concepts such as the impurity photovoltaic effect, the incorporation of alloys and superlattices, and tandem cells based on silicon, is discussed.

The impact of characterization techniques on HgCdTe infrared detector technology

Abstract: The authors review those characterization techniques that have played significant roles in the development of HgCdTe infrared detector technology. They focus on the two specific HgCdTe devices that have achieved widespread application for infrared detection in the LWIR (8-12 mu m) and VLWIR (12-20 mu m) spectral regions: the simple n-type photoconductor and the P-on-n LPE heterojunction photodiode. They review the device physics of these two detectors, relate device performance to starting material properties and processing parameters, and describe the most important characterization techniques that have had a role in their development.

The mechanisms of electronic excitation of rare earth impurities in semiconductors

Abstract: Impact and Auger excitation of threefold ionized rare earth ions by carriers are calculated for silicon and III-V semiconductors. The interaction between strongly localized f-electrons and local vibrations, which makes it possible to achieve energy balance in Auger processes, has been taken into account. It is shown that the probability of Auger excitation increases if there are additional localized electron states in the forbidden gap, which correspond to defects containing rare earth ions.

Migration-enhanced epitaxy of GaAs and AlGaAs

Abstract: The principle and characteristics of migration-enhanced epitaxy are reviewed. Migration of surface adatoms along the surface is very important for growing high quality layers and atomically flat heterojunctions. In the migration-enhanced epitaxy of GaAs and AlGaAs, migration of surface Ga and Al atoms is enhanced even at low substrate temperatures by evaporating them onto a clean GaAs surface under an As-free or low As pressure atmosphere. Thus, high quality GaAs and AlGaAs layers and flat heterojunctions have been grown by this method. Migration-enhanced epitaxy has also proved useful in investigating atomic processes during epitaxial growth.

Narrow-gap semiconductor magnetic-field sensors and applications

Abstract: Narrow-gap semiconductors have been used for decades in the fabrication of magnetic field sensors, such as magnetoresistors and Hall sensors. Magnetic field sensors are, in turn, used in conjunction with permanent magnets to make contactless potentiometers and rotary encoders. This sensing technology offers the most reliable way to convert a mechanical movement into an electrical signal, and is widespread in automotive applications. Recent developments in the growth of thin epitaxial layers of InAs and InSb on semiinsulating GaAs or InP substrates have resulted in the development of magnetoresistors with excellent sensitivity and operating temperatures up to 285 degrees C. Magnetoresistors and Hall sensors require a very thin active semiconductor region, a high carrier density and a high room-temperature mobility. The best materials are narrow-gap III-V compounds. 2DEG layers in InSb and InAs would be ideally suited for these devices. The accumulation layer at the surface of InAs has been used to make magnetoresistors, Hall sensors and magnetotransistors. n-type doped thin InSb films are used to make magnetoresistors that outperform Si-based Hall sensors, even with integrated amplification. The authors describe device design criteria, materials requirements and a direct comparison of the three types of galvanomagnetic devices, magnetoresistors, Hall sensors and magnetotransistors, made from the same material. They compare the output of different magnetic field sensing technologies, such as Si and GaAs Hall sensors, and NiFe-based magnetoresistors, with InSb magnetoresistors.

Influence of dislocation density on recombination at grain boundaries in multicrystalline silicon

Abstract: This paper reports a study of the causes of room-temperature recombination activity of grain boundaries in annealed multicrystalline silicon using the methods of electron-beam-induced current (EBIC) and transmission electron microscopy. It is found that the recombination activity is related to the density of dislocations in the boundary, with decoration by precipitates producing additional activity. Grain boundaries without dislocations and precipitates show only very low EBIC contrast.

Dry etching of thin-film InN, AlN and GaN

Abstract: Smooth, anisotropic dry etching of InN, AlN and GaN layers is demonstrated using low-pressure (1-30 mTorr) CH4/H2/Ar or Cl2/H2 ECR discharges with additional DC biasing of the sample. The etch rates are in the range 100-400 AA min-1 at 1 mTorr and -150 V DC for Cl2/H2, while higher biases are needed to initiate etching in CH4/H2/H discharges. The presence of hydrogen in the gas chemistries is necessary to facilitate equi-rate removal of the group III and nitrogen etch products, leading to smooth surface morphologies.

Transparent conducting CdO films formed by chemical bath deposition

Abstract: The formation of transparent conducting CdO films by chemical bath deposition is reported for the first time. These films exhibit an optical transmittance and electrical conductivity much higher than those CdO films prepared by other physical vapour deposition techniques. They exhibit an optical transmittance of more than 90% and electrical conductivity of the order of 103 Omega -1 cm-1. The very high optical transmittance coupled with the high electrical conductivity exhibited by these films makes them potential candidates for application as low-cost large-area transparent conductors.

A review of the plasma oxidation of silicon and its applications

Abstract: In recent years there has been an increasing trend towards 'dry' and 'cold' processing. The oxidation of silicon in an oxygen plasma allows the controlled growth of thin, high-quality films of silicon dioxide at temperatures down to room temperature in a clean vacuum environment. In this review the various experimental systems which have been reported in the literature over the last twenty-eight years are compared and contrasted. Physical properties of the oxides are found to be very similar to thermally grown films. The reported kinetics of plasma oxide growth are reviewed, together with the electrical and electronic properties of the grown oxide layers. Inductively coupled plasma oxidation is shown to have advantages over the other techniques, and some recent results obtained using this method are highlighted. The current and possible future applications of plasma oxidation for novel silicon and silicon-based device processing are discussed.

Infrared reflection and transmission of undoped and Si-doped InAs grown on GaAs by molecular beam epitaxy

Abstract: Nondestructive optical methods, based on measurements of the 'plasma edge' and the Moss-Burstein shift, are investigated as contactless alternatives to Hall measurements for determining carrier concentrations. Infrared reflection and transmission spectra of undoped and Si-doped InAs grown on GaAs by MBE are studied. A curve-fitting procedure is developed to fit the reflectivity spectra with or without phonon-plasmon coupling. The range of carrier concentrations over which these optical methods can provide useful characterization is evaluated. The effective mass determined from 'plasma edge' measurements agrees well with the simple Kane model for n below 2.7*1019 cm-3. For n above 4*1019 cm-3, the sample effective mass deviates considerably from the simple Kane model. Excitonic structure in the absorption edge is reported for high-purity undoped samples.

HgCdTe infrared diode lasers grown by MBE

Abstract: The authors report their latest results on the fabrication and successful operation of HgCdTe infrared diode lasers. The stripe-geometry double-heterostructure lasers were grown by molecular beam epitaxy (MBE). The active layer thickness ranges between 0.9 and 1.4 mu m, and the p+ and n+ confinement layers were in situ doped up to 1018 cm-3 with arsenic and indium, respectively. Five double heterostructures were grown, all of which produced working lasers. The devices were operated under pulsed currents at temperatures between 40 and 90 K. The 77 K stimulated emission wavelengths for these lasers were 2.9, 3.4, 3.9 and 4.4 mu m. Operation at 5.3 mu m was demonstrated at 60 K. The lowest 77 K threshold current density was 419 A cm-2 which is very close to the prediction of a numerical calculation. Characterization of the devices, including, for example, temperature dependence of the threshold currents and spectral analysis, was performed and showed the characteristics of well-behaved, stable devices that operated without failure while being tested.

The effect of interface bond type on the structural and optical properties of GaSb/InAs superlattices

Abstract: The effect of interface bond configuration on the structural and optical properties of short-period ( approximately=50 AA) GaSb/InAs superlattices has been examined. Structures consisting of eight monolayers of GaSb and seven monolayers of InAs with either 'GaAs-like' or 'InSb-like' interface bonds were grown by MBE. Evidence for differences in the structural properties, vibrational properties and band structure of the two types of material was obtained using X-ray diffraction, Raman scattering, interband magneto-absorption and photoconductivity measurements.

Recombination processes in erbium-doped MBE silicon

Abstract: The incorporation of erbium from a solid source into molecular beam epitaxy (MBE) Si and Si/Ge alloys grown at substrate temperatures of 500 degrees C and 700 degrees C has been studied by photoluminescence, electrical measurements, secondary-ion mass spectrometry (SIMS), Rutherford backscattering (RBS) and transmission electron microscopy (TEM). Erbium concentrations between 1018 and 1022 cm-3 were obtained but the maximum photoluminescence intensity was from samples with an erbium concentration of 2*1018 cm-3. Above this concentration the onset of erbium precipitation could just be observed by TEM. The authors found no shallow donors or acceptors attributable to erbium but they observed a high concentration of deep acceptors with an activation energy of 360 meV; these may be due to impurities in the erbium source rather than being directly related to the rare earth. Implantation with oxygen is found to enhance the Er3+-related photoluminescence signal when measured at temperatures greater than 77 K but to have little effect on the low-temperature luminescence. A detailed study of the temperature dependence of the luminescence reveals tree quenching mechanisms with average activation energies of approximately 5, 20 and 130 meV. The authors attribute the first two to de-excitation effects in the matrix, and the last to processes competing with the internal 4f transition.

Overview of compositional measurement techniques for HgCdTe with emphasis on IR transmission, energy dispersive X-ray analysis and optical reflectance

Abstract: Three techniques for determining the composition (x identical to mole fraction of CdTe) of Hg1-xCdxTe are reviewed. These three techniques are infrared transmission (often called FTIR, for Fourier transform infrared) spectroscopy, energy dispersive X-ray analysis (EDX) and optical reflectance (OR). A brief summary of several methods for determining composition in Hg1-xCdxTe is included.

Structural studies of natural superlattices in group III-V alloy epitaxial layers

Abstract: Structural studies have been performed using transmission electron microscopy and diffraction on III-V ternary and quaternary alloy epitaxial layers. The results revealed that 'natural superlattice' structures had spontaneously formed in some of the layers by phase separation and/or atomic ordering which occurred at the surface during growth. The structure of these natural superlattices and the growth conditions for their occurrence are described and their effect on the electrical and optical properties of the layers, e.g. bandgap narrowing, and possible mechanisms of formation are briefly discussed.

Spatially resolved characterization of HgCdTe materials and devices by scanning laser microscopy

Abstract: Spatially resolved characterization of HgCdTe materials and p-n junction diodes using scanning laser microscopy is reviewed. Several techniques that yield spatial maps of electrical inhomogeneities in HgCdTe material and non-uniformities in various performance parameters of p-n junctions fabricated using these materials have been developed. Many of the techniques are non-destructive, or can be made such with minor changes in sample preparation, and are scalable to large full wafers. A high-resolution and non-destructive technique called 'laser beam induced current (LBIC)' has been developed for spatial mapping of electrically active regions in HgCdTe. When applied to unprocessed HgCdTe material, LBIC images represent spatial distributions of electrically active defects including inclusions, strain, damage, precipitates, stacking faults, twin boundaries, dislocation clusters, bandgap and doping variations. Device structures such as p-n junctions are special cases of electrically active regions, therefore the LBIC technique lends itself to a non-destructive study of p-n junction arrays without requiring any direct electrical contact to the individual elements of the array. The remote contacting, especially using pressure contacts, makes the application of the LBIC technique non-destructive, allowing testing at various stages of device processing to identify particular processing procedures that need optimization. LBIC has also been used to spatially map electrical non-uniformities at the HgCdTe surface near its interface with an insulating passivation layer. Besides LBIC imaging, scanning laser microscopy has been used for several other applications. For HgCdTe p-n junctions, applications include photoresponse mapping (uniformity, active area and diffusion length determination, contact bonding effects), spatially resolved light-induced degradation, and avalanche photodiode properties (ionization coefficients, localized breakdown). A key technique currently in development is the non-contact diode R0A determination based on trapping mode photoconductivity. Examples of scanning laser microscopy on HgCdTe materials are infrared transmission mapping (thickness and bandgap variations), photoluminescence mapping, and minority carrier lifetime mapping (distribution of recombination centres).

Hydrogen solubility in silicon and hydrogen defects present after quenching

Abstract: Boron-doped silicon ((B) approximately 1017 cm-3) was heated in H2 gas at a temperature in the range 900

Molecular beam epitaxy growth and characterization of thin (<2 mu m) GaSb layers on GaAs(100) substrates

Abstract: This paper reports the effect of growth parameters and buffer structure on the luminescence electrical and structural characteristics of thin (<2 mu m) GaSb layers grown by molecular beam epitaxy (MBE) on GaAs(100) substrates. A two-stage substrate temperature growth regime has been developed on the basis of a thermodynamic consideration of the growth process, taking into account the additional Gibbs free energy due to the strain at the initial growth. This regime permits the growth of GaSb epilayers with a small Sb/Ga ratio at high substrate temperatures; it provides a greater exciton photoluminescence (PL) intensity and Hall mobility (over 5000 cm2 V-1 s-1 at 77 K) and improves the layer quality. The appearance of a free exciton luminescence line (809 meV) in the PL spectra of MBE GaSb/GaAs is reported for the first time. It is also shown that a (50 AA AlSb/50 AA GaSb)10 short-period superlattice prevents non-radiative recombination centres and carrier scattering centres from propagating in the top GaSb layer but does not bend the misfit dislocation growing through the layer from the GaSb/GaAs interface.

Prospects for the future of narrow bandgap materials

Abstract: Recently there has been greatly expended interest in narrow bandgap materials. Modern epitaxial techniques and the growing interest in nanostructures have provided areas of application for some of the unique properties of the narrow bandgap material. As always, one of the primary sources of interest is the small bandgap which makes them the material of choice for many applications in the infrared. However, in recent years their other unique properties have been the basis for a broader set of interests in narrow bandgap semiconductors. The type II band offsets (InAs/GaSb) have been the basis for novel tunnel devices and infrared superlattices. The very small effective masses inherent in small bandgap materials make them the obvious candidates in which to observe quantum confinement effects at larger dimensions than in materials of larger effective mass and wider gap. The ease of making electrical contact to some of the materials (ohmic contact to n-InAs) has made them the material of choice for electrical nanostructures. The ability to put in large amounts of magnetic ions to make magnetic semiconductors has led to a number of novel properties. The technical importance of a narrow bandgap and the unique applications promised by some of the other properties of these materials bode well for substantial research in narrow bandgap semiconductors well into the next decade.

The fabrication of a back-gated high electron mobility transistor - a novel approach using MBE regrowth on an in situ ion beam patterned epilayer

Abstract: A new technique for the fabrication of GaAs/AlGaAs back-gated high electron mobility transistors (HEMTs) is described. First the authors demonstrate that a dose of >2*1013 cm-2 Ga ions at an energy of 10 keV can be used to damage a 67 nm n+ GaAs layer, rendering the implanted regions non-conducting. After implantation the epilayer has a 4 K sheet resistivity which is increased by a factor of approximately=107 when compared with the original unimplanted value. This isolation procedure is then used to form a patterned back-gated HEMT by MBE regrowth on top of an in situ ion-implanted n+ GaAs layer. The resulting structure is designed so that the back gate is rendered highly resistive under the regions where the ohmic contacts to the two-dimensional electron gas (2DEG) are formed, thus making shallow ohmic contacts unnecessary. Using this fabrication technique, the authors obtain a high yield of working devices in which it is possible to alter the carrier concentration (n) in the 2DEG from 0 to 3.6*1011 cm-2 using the back gate. Typical leakage currents at 1.2 K are <0.5 nA over the whole of this carrier concentration range-an improvement over devices of a similar design which have been fabricated using shallow ohmic contacts. At zero back-gate voltage the 1.2 K mobility ( mu ) of the 2DEG is approximately=400000 cm2 V-1 s-1 with n approximately=2.8*1011 cm-2, whilst on altering the back-gate voltage it is found that mu varies as nx where x approximately=1.5. These results are characteristic of a high-quality 2DEG with mobility limited by remote ionized impurity scattering. This technique can therefore be used as a means of controlling the 2DEG carrier concentration, whilst leaving the surface of the HEMT structure free for conventional lithographic processing. Further, it is possible to pattern the back gate beneath the conducting channel. This would allow the shape of the 2DEG to be defined, and its width altered, using an in-grown patterned back gate a feature which is impossible using conventional techniques.

Doping studies in MOVPE-grown CdxHg1-xTe

Abstract: In two recent review papers on metal organic vapour phase epitaxy (MOVPE) of cadmium mercury telluride (CMT) particular emphasis was placed on the crucial importance of doping studies to the realization of future device structures Irvine et al. (1991) and Tribowet (1991). If the full potential of MOVPE growth of CMT is to be realized then extrinsic doping of heterostructures is required. If the doping and composition junctions can be grown with the correct degree of grading then this will create the potential for the production of device structures leading to either improved performance and/or increased operating temperatures. This papers reviews published doping studies and also presents some recent results on both acceptor and donor doping studies carried out by the authors. In the latter studies, interdiffused multilayer process (IMP) growth of CMT has been performed at approximately=360 degrees C using dimethyl cadmium (DMC) and di-isopropyl tellurium (DIPT) as the MO precursors while the Hg overpressure was provided by a heated elemental source. Alternative acceptor doping sources to arsine have been investigated including phosphine, triphenyl arsenic, and phenyl arsenic of which the latter appears to be most suitable. Iodine has continued to show the donor dopant potential in CMT that it exhibited with higher-temperature ( approximately=400 degrees C) MOVPE growth using di-ethyl tellurium (DET). Characterization of fully doped structures is described.

Theory of some effects of photon recycling in semiconductors

Abstract: The effect of photon recycling (or self-excitation) in semiconductors is established theoretically for the minority carrier lifetime, the minority carrier diffusion coefficient and an electric field parameter, for a wide range of physical situations. These include absorption coefficients, reflection coefficients at the boundary of the semiconductor (and therefore the refractive indices), semiconductor layer thickness and internal quantum efficiency. The new features of this work are: the inclusion of the dependence on the incidence angle of the reflection coefficients, an improved but fairly simple derivation of the corrections to the observable quantities due to photon recycling, and the effect of photon recycling on the electric field. As already known, the most important effects are on the lifetime, whereas the diffusion coefficient is relatively insensitive to photon recycling. It is pointed out here that the effect can, however, be significant if the absorption length exceeds the diffusion length. In this case the effect on the electric field is also important and increases with the reflection coefficient.

Optical and electrical investigation of subband populations, mobilities and Fermi level pinning in delta-doped quantum wells

Abstract: Measurements are reported on the electrical and optical properties of a series of GaAs/Al0.33Ga0.67As quantum well structures in which a Si delta-doped plane has been placed at the centre of the well. By using a range of well widths for a given planar doping level, it has been possible to control the populations of the electron subbands, as evidenced by Hall, Shubnikov-de Haas, photoluminescence and photoluminescence excitation measurements. Combination of these techniques with the results of a self-consistent Poisson-Schrodinger model has enabled the bandgap renormalization to be determined as a function of electron density, and has also demonstrated that the Fermi energy is pinned at 190 meV above the Gamma conduction band minimum at the delta-doped plane. There is, however, no evidence for a Fermi-edge singularity in the optical spectra of these layers. The transport and the quantum mobilities of the individual subbands have been measured at low temperature, and were found to be in the ratio of approximately 2:1.