Number Of Semiconductors Research Articles

Secondary ion mass spectrometry—first microelectronics, now the rest of the world

AbstractSecondary ion mass spectrometry (SIMS) has achieved great success as a materials characterization tool because of its parts‐per‐million to parts‐per‐billion sensitivity and excellent depth resolution. SIMS is best known for applications in the semiconductor industry, but is increasingly used in many other fields. Despite this growth, three myths persist regarding SIMS: the technique is not quantitative, insulators are difficult or impossible to analyze and good depth resolution cannot be obtained for metals. Recent work by several groups should dispell these misconceptions. Quantification with a reproducibility and accuracy of 10–20% is commonplace with ion implantation calibration methods. Patterns for secondary ion yields of the elements when plotted against ionization potential and electron affinity have been identified in a number of semiconductor and metal matrices. The pattern similarities allow sensitivity factors to be estimated for elements for which an implanted calibration is not available. The Lawrence Livermore project of selected elements into 200 matrices is expected to extend this concept to oxides, minerals, alloys, ceramics and fluorides. The analysis of insulators has been made routine by the use of an electron beam for charge neutralization and an understanding of the placement of the electron beam and the electron to ion current density ratio required. The glass industry provides many applications for this capability. Metals and other polycrystalline materials can be sputtered evenly if the sample is rotated during ion bombardment. This rotation method has been used for matrix‐level species in Auger analysis, and has now been shown to be viable in SIMS at the parts per million level.

Amorphization of elemental and compound semiconductors upon ion implantation

Cross-sectional TEM studies of ion implantation induced amorphization in a large number of semiconductors have been performed. Samples of Si, AlAs, GaAs, GaP, GaSb, InP, InAs, and ZnSe were simultaneously implanted at 77 K with 20 keV Si+ at doses between 1 × 1014/cm2 and 1 × 1016/cm2. A dose of 1 × 1015/cm2 minimized the ion beam induced epitaxial crystallization and sputtering effects. The depth of the amorphous layer at this dose was compared with Monte Carlo damage density distribution calculations (TRIM'90). The threshold damage density (TDD) necessary for amorphization was determined for each compound. The values of the threshold damage density vary from as low as 2.4 × 1019 keV/cm3 for InAs up to 7.3 × 1020 keV/cm3 for AlAs. ZnSe never became amorphous and GaSb exhibited an unusual disordering after the highest dose. The values of the threshold damage density for the various compositions were compared with known thermochemical data and several bond energy estimates. No single calculation explained all of the trends observed.

Optical Properties of Multi‐Layer Structures. III. Raman Scattering in Multi‐Layer Structures and Superlattices

AbstractThe theory of Raman scattering is extended over multi‐layer structures with any number of semiconductor and dielectric layers; the explanation is based on the modulation transfer matrices. The location and configuration of Raman peaks are calculated as functions of the polarization of incident and scattered radiation caused by different types of oscillatory excitations in superlattices by spatially extended interface phonons, spatially decreasing phonons and “confined” bulk phonons.

Infrared and millimeter wave absorber structures for thermal detectors

In a classical (i.e. Type I) pyroelectric detector, a crystal plate is coated with two metal electrodes. In order to achieve an absorbing power as close as possible to 100%, different structures have been used in the past: (i) a metal-black coating on the front electrode and (ii) a very thin front electrode having a square resistance matching the impedance of vacuum, with the back electrode making a quarter wavelength structure. However, the quarter wavelength structure becomes inefficient when the absorption length becomes smaller than the plate thickness. The simpler solution is to use a transparent antireflective layer, so that the whole radiant energy would enter the pyroelectric plate and be absorbed. This can be a perfect solution when the double path through the layer matches the phase shift on reflection at the pyroelectric material, and a nearly perfect one for a broad band centered at that wavelength. Transformation of radiant power into heat occurs directly within the pyroelectric. It is shown that a number of semiconductors can be used to make such antireflective layers, and practical solutions are feasible for many pyroelectrics. The advantages of this simple solution over λ/4 structures stuck to the pyroelectric plate, as proposed by Parsons et al., are a negligible added heat capacity as the antireflective coating can generally have enough electrical conductivity to be used as an electrode, and a negligible lateral heat conductivity. When such a solution is not feasible (i.e. reflectivity cannot be cancelled completely), we can still use a very thin metal electrode. It is shown that the electrical conductivity may be 2 orders of magnitude smaller than for the bulk. This leads to small indexes in the IR (e.g. n 3≅ k 3≅ 6), very convenient to give the structure a null reflectivity, when the granular metal layer is covered by a suitable, experimentally available coating (index of refraction n 2 = √2 n 3. Additionally, the determination of the wavelengths and values of the absorption maxima give useful information on the complex refractive index of the coated crystal at wavelengths where reflectivity and transmission measurements are often inaccurate or impossible ( T0̃).

Photochemical processing of semiconductors: New applications for visible and ultraviolet lasers

A wide variety of semiconductor and metal films have recently been grown, doped, or etched by laser photochemical processes. While still early in its development, laser-driven processing of semiconductor devices appears promising as a supplement to existing processing techniques and particularly in the fabrication of III-V compound and custom devices. A discussion of film growth and etching shows that the laser is capable of performing all of the basic processing steps required in device fabrication without the need for photolithographic techniques. The boundaries of the field are set only by the number of semiconductor and metal-containing compounds having reasonable vapor pressure (~1 torr) at several hundred degrees centigrade or less.

The α(semiconductor) ag β(metal) transition in tin

The principal features of the grey tin (α) ag white tin (β) transition in tin are reviewed. In particular it is noted that this is not an isolated transition but one of the general type observed more recently in a number of semiconductors at high pressure. The relatively uncommon feature is that this transition occurs in a readily accessible experimental range, namely room temperature and pressure. Thus the results of a detailed examination of the mechanism and characteristics of the transition observed in tin may be used to develop a more general description of the semiconductor ag metal transitions observed in a wide variety of materials at high pressures where broad experimental analysis is not possible.

Excitons in semi-infinite crystals: Microscopic calculation of optical reflectivity

Expressions for exciton wavefunctions in semi-infinite crystals are derived in closed form within the effective mass approximation. From them the non-homogeneous non-local dielectric susceptibility is obtained showing the existence of the dead-layer and a new additional boundary condition. The results are compared with normal incidence reflectivity experiments in a number of semiconductors and the experimental lineshapes are reproduced.

Efficiency of tandem solar cell systems as a function of temperature and solar energy concentration ratio

This paper presents the results of a comprehensive theoretical analysis of tandem photovoltaic solar cells as a function of temperature and solar concentration ratio. The I– V characteristics of the solar cells were assumed to be governed by the relation I = I 0( e qV/AkT − 1) with I 0 = K e −E G /BkT and A = B The overall efficiencies of tandem cell stacks consisting of as many as 24 cells having energy gaps in the 0.7 to 3.6 eV range were calculated for temperatures of 200, 300, 400 and 500 K and for illumination by an AMO solar spectrum having concentration ratios C = 1, 100, 500 and 1000 suns. For ideal diodes ( A = B = 1), the calculations show that the optimized overall efficiency has a limiting value η opt ≈ 70% for T = 200 K and C = 1000. For T = 300 K and C = 1000, this limiting efficiency approaches 60%. Most of the gain in efficiency occurs with the number of semiconductors in the tandem system between six and ten, e.g. for T = 300 K and C = 1000, an optimized six cell system has a theoretical limit efficiency of about 53%. Calculations were also conducted for the A = B = 2 case (non-ideal diode behavior); in this case the limiting value of η for a 24 cell system is about 65% at 200 K and 55% at 300 K.

The importance of the electron affinity of oxide-semiconductors as used in solar cells

The electron affinity of oxide semiconductors (In2O3,SnO2, etc.) is a topic of current interest. These materials are capable of forming barrier devices on a number of semiconductors which exhibit good photovoltaic conversion efficiency. We will show that simple methods based on the electrical measurements of oxide-semiconductor/base-semiconductor systems often lead to incorrect results, due to the uncertainty in the Fermi-level position in the oxide-semiconductors and other interface parameters. To determine the true potential of a particular oxide-semiconductor, other methods should be used to find the electron affinity.

Fourier analysis of magnetophonon and two-dimensional Shubnikov-de Haas magnetoresistance structure

The advantages of Fourier analysis of quantum magnetoresistance peaks are demonstrated in the first application of these techniques to a study of the magnetophonon effect in a number of semiconductors and also to the subnikov-de Haas effect in MOS devices. The change in amplitude of these series is investigated as a function of applied electric field. The phonons involved are identified and are all shown to be intervalley with the exception of the highest energy phonon. In contrast Fourier analysis of the magnetophonon spectrum for n-Ge shows that intervalley scattering in this material is weaker by at least a factor of 20 than intravalley scattering by high energy phonons. With both n-Ge and n-GaAs considerable sharpening of the magnetophonon peaks is detected and a strong second harmonic of the fundamental magnetophonon frequency is observed in each case on the Fourier analysis. With n-GaAs as the temperature dependence of the amplitude and damping of the magnetophonon series is compared with the theory developed by Barker (1972).

Two-photon transitions at saddle points in semiconductors

Sharp peaks in the frequency dependence of the two-photon absorption probability are shown to occur in correspondence to saddle-points of the electronic band structure. They should be observed in a number of semiconductors with standard two-photon experiments performed with high power laser radiation. It is suggested that this effect could be used to detect new saddle-point transitions which are forbidden in first order.

Effect of Phonons on Optical Transitions near Critical Points in the Band Structure

The effect of phonons on the density of states near a critical point in the band structure of an insulator or semiconductor is investigated. Dyson's equation is solved assuming that the electron-phonon coupling parameter is independent of wave vector. Electron phonon-vertex and central-cell corrections are negligible. It is concluded that so long as the critical point is not near threshold, the phonons broaden the structure associated with the critical point, but introduce no structure of their own. Therefore, electron-phonon self-energies cannot explain the anomalous structure observed in the vicinity of saddle points in a number of semiconductors.

Semiconductors as electrooptic modulators for infrared radiation

The electrooptic properties of a number of semiconductors were investigated. We were particularly interested in the possibility of using these materials for modulation of infrared radiation (λ > 3µ), since many of the efficient modulation materials for the shorter wavelengths, such as KTN and KDP, are opaque in this region.

Photoelectromagnetic Effect in "Many-Valleyed" Solids

The photoelectromagnetic (PEM) effect occurs in a solid when the diffusion currents of excess carriers away from an illuminated surface are deflected by a magnetic field to give a net current parallel to the surface. The effect has been studied in a number of semiconductors, providing information about carrier recombination and transport; recently, PEM responses were obtained in some semimetals. The present paper extends earlier theories of the effect in order to treat the cases of many-valleyed solids, and at the same time the transfer of excess carriers between electron or between hole valleys (intervalley scattering) is taken into account in addition to the transitions from electron to hole valleys (recombination). The possibilities of experimental determination of recombination and intervalley scattering times for various crystal orientations are discussed. Apparently it is often possible to choose an orientation which provides a determination of recombination time independent of the intervalley scattering time, but the reverse is not true, and even for extremely simple band models a reasonably precise determination of scattering time through the PEM effect seems difficult.