Optical characterization of heavily doped silicon

Out of a variety of optical techniques used to characterize heavily doped semiconductors photoluminescence and Raman spectroscopy will be discussed as tools to study heavy doping effects. Photoluminescence spectroscopy is sensitive to electronic transitions between the conduction and valence band whereas electronic Raman scattering probes transitions within either band. Parameters relevant to device physics such as the band gap shrinkage due to heavy doping are extracted from these measurements. It is further shown that both techniques are applicable to the characterization of thin heavily doped implanted or epitaxial layers.     

应力层多量子阱InGaAs/GaAs的光电流谱研究(英文)

The band configurations of the undoped strained-layer InxGa1-xAs(8 or 15nm)-GaAs(15nm) MQW with x = 0.1,0.15 and 0.2, respectively, have been investigated by photocurrent spectroscopy at the temperature range of 10- 300K. Both intersubband transitions and transitions between confined level and continuum are observed. The photocurrent peak related to the 2s or other excited states of heavy-hole exciton is also observed and the exci-tonic binding energy thus obtained is about 8meV. The valence band offset △Ev is determined to be 0.38 and 0.40 by means of two different methods. Owing to the strain effect, both electrons and heavy holes are confined in the InGaAs layers while light holes in the GaAs layers.     

Terahertz time-domain-spectroscopy system using a 1 micron wavelength laser and photoconductive components made from low-temperature-grown GaAs

A terahertz time-domain spectroscopy (TDS) system based on a femtosecond Yb:KGW laser, photoconductive emitters and detectors made from as-grown and from annealed at moderate temperatures (~400°C) low-temperature-grown GaAs (LTG GaAs) layers was demonstrated. The measured photoconductivity of these layers increased linearly with the optical power, showing that transitions from the defect band to the conduction band are dominant. The largest amplitude THz pulse with a useful signal bandwidth reaching 3 THz and its signal-to-noise ratio exceeding 50 dB was emitted by the device made from the LTG GaAs layer annealed at 420°C temperature. The detector made from this material was by an order of magnitude less sensitive than conventional GaBiAs detectors.     

Influence of substoichiometry,hydrogen content and crystallinity on the optical and electrical properties of HxWOy thin films

We have prepared H_xWO_y amorphous thin films both by evaporation of tungsten trioxide powder and by cathodic sputtering of a tungsten target in an argon/oxygen/hydrogen reactive gas mixture. The evaporated layers have the composition H_xWO_2.7 (0.2 < × < 0.5). Their oxygen content seems rather insensitive to the evaporation parameters. We do not observe any correlation between x and these parameters. Evaporated virgin layers are nearly transparent. Annealing,under vacuum leaves y unchanged, under oxygen increases y to 3. Annealing of the virgin layer under vacuum induces the growth of the 1.38 eV absorption band (giving blue coloration) and a decrease of the activation energy for conduction. Annealing the blue layers in oxygen destroys the 1.38 eV band and increases the activation energy for conduction. In both cases annealing at high temperature induces a microcrystalline phase with an absorption band centered about 0.72 eV (giving also a blue coloration) and a jump in electronic conductivity. As in the case of the 1.38 eV band, an increase of the intensity of the 0.72 eV band induces a decrease of the activation energy for conduction. The two bands are interpreted as polaronic like. They can be induced in transparent layers without any change in global composition by excitation of the hydrogen atoms from a “ passive” state to an “ active” state. In addition to the hydrogen content, the existence of the 1.38 eV band requires some substoichiometry. The study of the optical and electrical properties of amorphous H_xWO_y sputtered layers, supports our previous conclusions about the composition range (C) for the coloration capability of transparent thin films. In addition there is a composition range (B) where the virgin layers are blue, and a composition range (M) where they have a metallic like behavior. On the other side of (C), there is a range (C’) where uv illumination only induces a decrease in the activation energy for conduction, then a range (T) where the layers are completely nonresponsive. One can pass from (T) to (B) through (C') and (C) either at constant hydrogen content by increasing the departure from stochiometry, or at constant substoichiometry by increasing the hydrogen content. A part of this work was presented at EMC Cornell, New York July 1, 1977.     

Intersubband Raman scattering in InAs/AlSb quantum wells

Raman spectroscopy has been used to study intersubband transitions in InAs/AlSb single quantum wells grown by molecular-beam epitaxy on (100) GaAs substrates using strain-relaxed AlSb or GaSb buffer layers. From the measured energies of the coupled longitudinal optical phonon-intersubband plasmon modes the single particle transition energies between the first and second confined electron subbands were deduced as a function of the width of the pseudomorphically strained InAs well. Subband spacings calculated including the effects of strain and nonparabolicity were found to be in agreement with the experimental transition energies. For a given well width, the two-dimensional electron concentration deduced from the Raman measurements was found to be lower than the concentration measured in the dark by Hall effect, but showed a significant increase with increasing optical excitation intensity.     

Dopant activation energy and hole effective mass in heavily Zn-Doped InP

Heavily doped InP epitaxial layers were investigated using variable-temperature van der Pauw and photoluminescence spectroscopy measurements. A quantitative analysis of the electronic properties of the p-type layers was performed to determine the activation energy of Zn as an acceptor as well as total acceptor and donor concentrations. The doping-concentration dependence of the activation energy was discussed in terms of merging of the excited acceptor states with the valence band. The activation energy of the acceptor state for dilute Zn concentrations was determined to be 52 ± 1 meV. A temperature and concentration-dependent density-of-states hole effective mass is proposed to fit the measured hole-concentration curves in the temperature range between 77 and 300K. To explain a hole-concentration saturation effect, the model of incorporation of Zn as an interstitial donor is discussed.     

Characterization of Hg1−xCdxTe heterostructures by thermoelectric measurements

P-on-n mercury cadmium telluride (MCT) heterostructures grown by MOCVD with As and In as n- and p-type dopants, respectively, are examined by measuring the Seebeck and Hall coefficients between 20 and 320K. The results are analyzed regarding doping and composition of the layers by least squares fitting the experimental profiles with the calculated temperature dependencies. The electron and hole densities of the layers are calculated taking into account Fermi-Dirac statistics, a nonparabolic conduction band, a parabolic valence band, a discrete acceptor level, and fully ionized donors. For the Seebeck coefficient, the relation we previously showed to be valid for p-type MCT¹ is used. This relation relies on the thermoelectric effect in a temperature gradient resulting from the diffusion of nondegenerate carriers scattered by LO-phonons. It also fits the observed thermoelectric properties of n-type MCT in a wide temperature range. The doping and structural parameters determined from the thermoelectric measurements agreed very well with As and In profiles obtained from secondary ion mass spectroscopy measurements and the data obtained from analyses of infrared transmission measurements.     

Blue/green luminescence based on Zn(S)Se/GaAs heterostructures

Substantial advances have been realized in the aim to achieve blue–green light emitting devices based on Zn(S)Se wide band gap II–VI semi-conductor materials. Two light emitting diodes p on n and n on p heterostructures were grown on GaAs substrate by molecular beam epitaxy. The active layer was a single ZnCdSe quantum well, with ZnSSe guiding layers and ZnSe cladding layers. p-GaInP, p-AlGaAs and p-CdZnSe buffer layers were deposited at the p-ZnSe/GaAs interface to reduce the valence band offset in the case of n on p heterostructures. Electrical and optical properties were investigated using current voltage, capacitance voltage, electroluminescence, photoluminescence and photocurrent measurements at room temperature. Blue–green luminescence centered at 516.7 nm is observed. The highest luminescence intensity is observed under 7 V forward bias. Photoluminescence spectrum shows two wide peaks at 2.2 and 1.9 eV energies. These energies are attributed to the transitions between ZnSe and GaAs conduction bands and the deep level at E_v−0.6 eV. Absorption process from ZnSe and ZnSSe conduction bands to the shallow nitrogen acceptor level (2.6 and 2.8 eV, respectively) have been observed using photocurrent measurements. From these results we present a band alignment diagram which confirms the presence of the two levels at 0.1 and 0.6 eV from the valence band of ZnSe.     

Atomic layer deposited zirconium oxide electron injection layer for efficient organic light emitting diodes

In this work, we demonstrate efficient polyfluorene-based light emitting diodes on which conformal, thin ZrO₂ layers, formed by atomic layer deposition at a relatively low temperature (175 °C), in order to avoid introducing any damage in the organic under layer, efficiently inject electrons from their high lying conduction band to the polymer’s LUMO. An optimal thickness of 2 nm for ZrO₂ results in a threefold improvement in luminous current efficiency compared to the reference device. The relationship between the thickness of the ZrO₂ layer and the device operational characteristics is further investigated and the possible reasons for the improved device performance are discussed based on the experimental results obtained by a combination of photoemission spectroscopy and electrical/optical measurements.     

Evidence from spectral emissometry for conduction intraband transitions in the intrinsic regime for silicon

From emissometry measurements in lightly doped Si at elevated temperatures, we have observed an anomalous absorption band in the wavelength range of 1–5 μm. The wavelength at which the band peaks, λ≈2.3 μm, shows a negligible dependence on temperature while the peak intensity increases with temperature presumably as a result of the increasing intrinsic carrier concentration. Spitzer and Fan reported a similar absorption band in direct absorption measurements at room temperature for n-type Si with extrinsic electron concentrations of 10¹⁴ to 10¹⁹cm⁻³. No such structure was found in extrinsic p-type Si. Spitzer and Fan were unable to identify the mechanism for this anomalous absorption. In both the experiments, this absorption of free electrons is due to intraband transitions in the conduction band from the Δ₁ conduction band edge across an energy gap of E ∼ 0.5 eV to a higher lying Δ₂′ conduction band.     

Near-edge conduction band electronic states in SiGe alloys

Spatially resolved electron energy loss spectroscopy (EELS) measurements in GeSi alloys illustrate the relationship of atomic structure to local electronic structure. Extending earlier measurements, where electronic structure was found to be controlled by composition in relaxed alloys, measurements in anisotropically strained alloys show splitting of normally degenerate band edges into two components. In a strained Si quantum well, this allows the engineered band offset to be followed from the GeSi substrate through the well to the alloy-capping layer. In the high-mobility conduction channel, the band edge is found to be very sharp, in spite of obvious composition roughness. Near a misfit dislocation under the Si well, the band edge can shift by as much as 0.25 eV due to local strain. Within the core of the defect, however, strictly local behavior dominates the observations. Local conduction band splitting and in-gap states are both observed.     

Analysis of defect related optical transitions in biased AlGaN/GaN heterostructures

The optical transitions in AlGaN/GaN heterostructures that are grown by metalorganic chemical vapor deposition (MOCVD) have been investigated in detail by using Hall and room temperature (RT) photoluminescence (PL) measurements. The Hall measurements show that there is two-dimensional electron gas (2DEG) conduction at the AlGaN/GaN heterointerface. PL measurements show that in addition to the characteristic near-band edge (BE) transition, there are blue (BL) and yellow luminescence (YL) bands, free-exciton transition (FE), and a neighboring emission band (NEB). To analyze these transitions in detail, the PL measurements were taken under bias where the applied electric field changed from 0 to 50 V/cm. Due to the applied electric field, band bending occurs and NEB separates into two different peaks as an ultraviolet luminescence (UVL) and Y₄ band. Among these bands, only the yellow band is unaffected with the applied electric field. The luminescence intensity change of these bands with an electric field is investigated in detail. As a result, the most probable candidate of the intensity decrease with an increasing electric field is the reduction in the radiative lifetime.     

First direct observation of EL2-like defect levels in annealed LT-GaAS

Nonstoichiometric arsenic-rich GaAs grown at low temperatures by molecular beam epitaxy (LT-GaAs) has been found to be semi-insulating after high-temperature annealing. The origin of this technologically important conversion is not yet fully understood. In order to study this effect, we performed photocurrent measurements on p-LT GaAs-n diodes in the spectral range between 0.75 and 1.5eV at 8K. The photocurrent spectra revealed the following features which are unique to the EL2 level: photoquenching, characteristic photoionization transitions to conduction band minima and a presence of a broad band due to the effect of auto-ionization from the excited state. Moreover, modeling of the optical excitation process using realistic band structure demonstrates that these features cannot be explained by “internal photoemission” originating from As precipitates, as the “buried Schottky barrier model” predicts. This is the first direct experimental evidence for the existence of EL2-like defect levels and their importance for understanding the optical and electronic properties of annealed LT-GaAs.     

Power broadening and nonlinear FIR magneto-photoconductivity in n-GaAs

The saturation of the photoconductivity due to 1s-2p₊ shallow donor transitions in n-GaAs has been investigated using a high power cw-FIR-laser. Magnetic field strengths were chosen in such a way that 2p₊ energy levels were either below or above the N=0 conduction band Landau level. In the former case 1s-p₊ transitions are found to be inhomogeneously broadened with a saturation intensity as low as 0.84 mW/cm², giving an effective lifetime of the 2p₊ state of 1.5 μs. Above the band edge the integrated photoconductivity does not saturate though the intensity-normalized peak photosignal decreases and the linewidth increases with raising intensity. This strange behaviour is tentatively attributed to optical excitations of 2p₊ electrons to higher lying electron Landau states.     

Terahertz emission upon the interband excitation of GaN layers

The experimental observation and study of terahertz photoluminescence upon the steady-state interband excitation of epitaxial n-GaN(Si) layers are reported. The properties of the terahertz emission spectrum and the dependence of the spectrum on temperature and photoexcitation intensity suggest that emission occurs due to the trapping of nonequilibrium electrons at charged donors. In the n-type material at low temperatures, charged donors can be formed as a result of the recombination of nonequilibrium holes with electrons localized at donor centers. The main contribution to terahertz photoluminescence is made by 2P ?? 1S optical transitions between the first excited state and ground state of the donors. In addition, optical transitions of electrons from the conduction band states to the ground state as well as to the excited states of donors are evident in the terahertz emission spectrum.     

Influence of deposition rate on the structural,optical and electrical properties of electron beam evaporated SnO2 thin films for transparent conducting electrode applications

The role of deposition rate in the structural,optical and electrical properties of SnO2 thin films deposited by electron beam evaporation method is investigated by varying the deposition powers viz.50,75,and 100 W.The structural characterization of the films is done by X-ray diffraction (XRD) technique.The surface morphology of the films is studied by scanning electron microscopy (SEM).Rutherford back scattering (RBS) measurements revealed the thickness of the films ranging from 200 nm to 400 and also a change in the concentration of oxygen vacancies which is found to be the maximum in the film deposited at the lowest deposition rate.Optical absorption spectrum is recorded using the UV-V is spectroscopy and the films are found to be transparent in nature.A shift in the absorption edge is observed and is attributed to a different level of allowed energy states in conduction band minimum.The Hall effect and electrical measurements show a variation in the carrier concentrations,mobility and resistivity of the films.In order to explore a better compromise in electrical and optical properties for transparent electrode applications,skin depths calculations are also done to find the optimized values of carrier concentration and mobility.     

Deep level defects in unintentionally doped 4H-SiC homoepitaxial layer

Unintentionally doped 4H-SiC homoepitaxial layers grown by hot-wall chemical vapor deposition (HWCVD) have been studied using photoluminescence (PL) technique in the temperature range of 10 to 240 K. A broadband green luminescence has been observed. Vacancies of carbon (Vc) are revealed by electron spin resonance (ESR) technique at 110 K. The results strongly suggest that the green band luminescence, as shallow donor-deep accepter emission, is attributed to the vacancies of C and the extended defects. The broadband green luminescence spectrum can be fitted by the two Gauss-type spectra using nonlinear optimization technique. It shows that the broad-band green luminescence originates from the combination of two independent radiative transitions. The centers of two energy levels are located 2.378 and 2.130 eV below the conduction band, respectively, and the ends of two energy levels are expanded and superimposed each other.     

非刻意掺杂4H-SiC同质外延中的深能级缺陷

Unintentionally doped 4H-SiC homoepitaxial layers grown by hot-wall chemical vapor deposition （HWCVD） have been studied using photoluminescence （PL） technique in the temperature range of 10 to 240 K. A broadband green luminescence has been observed. Vacancies of carbon （Vc） are revealed by electron spin resonance （ESR） technique at 110 K. The results strongly suggest that the green band luminescence, as shallow donor-deep accepter emission, is attributed to the vacancies of C and the extended defects. The broadband green luminescence spectrum can be fitted by the two Gauss-type spectra using nonlinear optimization technique. It shows that the broad-band green luminescence originates from the combination of two independent radiative transitions. The centers of two energy levels are located 2.378 and 2.130 eV below the conduction band, respectively, and the ends of two energy levels are expanded and superimposed each other.     

HgCdTe受主能态的非线性吸收光谱

本文介绍用可选支的连续CO_2激光器在低于禁带宽度的谱线范围内研究100K的Hg_(0.785)Cd_(0.215)Te线性与非线性吸收光谱,由受主能态到导带能态的直接跃迁机理成功地解释了实验现象.井由电子和空穴的速率方程导出饱和吸收的表达式.     

Hopping conduction in heavily doped bulk n-type SiC

The electronic properties of heavily doped n-type 4H, 6H, and 15R SiC have been studied with temperature dependent Hall effect, resistivity measurements, and thermal admittance spectroscopy experiments. Hopping conduction was observed in the resistivity experiments for samples with electron concentrations of 10¹⁷ cm⁻³ or higher. Both band and hopping conduction were observed in all three polytypes in resistivity and Hall effect experiments. The hopping conduction activation energy ε₃ obtained from the resistivity measurements varied from 0.003 to 0.013 eV. The ε₃ value obtained from thermal admittance spectroscopy measurements were slightly lower. The nitrogen ionization levels were observed by thermal admittance spectroscopy only in those samples where hopping conduction was not detected by this experiment. Free carrier activation energy E_a for nitrogen was difficult to determine from temperature dependent Hall effect measurements because of the effects of hopping conduction. A new feature in the apparent carrier concentration vs inverse temperature data in the hopping regime was observed.