Contactless electromodulation and surface photovoltage spectroscopy for the nondestructive, room temperature characterization of wafer-scale III–V semiconductor device structures

This paper will review the use of the contactless methods of photoreflectance, contactless electroreflectance, and surface photovoltage spectroscopy for the nondestructive, room temperature characterization of a wide variety of wafer-scale III–V semiconductor device structures. Some systems that will be discussed include heterojunction bipolar transistors (including determination of collector and emitter doping levels, alloy composition, and dc current gain factor, pseudomorphic GaAlAs/InGaAs/GaAs high electron mobility transistors (including the determination of the composition, width, and two-dimensional electron gas density in the channel), edge emitting lasers [InGaAsP/InP (including the detection of p-dopant interdiffusion), graded index of refraction separate confinement heterostructure GaAlAs/GaAs/InGaAs], and vertical-cavity surface-emitting lasers (determination of fundamental conduction to heavy-hole excitonic transition and cavity mode). These methods are already being used by more than a dozen industries world-wide for the production-line qualification of these device structures.     

Ellipsometric study of metal-organic chemically vapor deposited III–V semiconductor structures

Metal-organic chemical vapor deposition was used to grow epitaxial layers of AlGaAs, GaAs and InGaAs on semi-insulating GaAs substrates. The ternary composition of the thick layers was determined by double-crystal X-ray diffraction (DCXRD). Variable angle spectroscopic ellipsometry was used to characterize several types of structures including relaxed single-component thick films and strained lattice multilayer structures. The thick film characterization included ternary alloy composition as determined by a numerical algorithm and interface quality. The results for the alloy composition were equal to the DCXRD results, to within the experimental errors except for the top layer of a thick AlGaAs film. The strained layer multistructures were analyzed for all layer thicknesses and alloy compositions. For most layers, the thickness was equal to the nominal values, to within the experimental errors. However, in all three In_0.3Ga_0.7As samples, the indium concentration estimated from the relaxed layer's InGaAs algorithm was around 21%, i.e. much lower than the nominal value. This result indicates a shift in the critical points of the dielectric function, owing to strain effects.     

Optimization of III–V compound semiconductor heterostructures for distributed Bragg reflector applications in VCSELs

A selectively oxidized vertical cavity surface emitting laser (VCSEL) has been designed and fabricated for operation at a wavelength of 1.546 μm. The device structure was grown on an InP substrate using III–V quaternary semiconductor alloys for the two mirror stacks and unstrained multi-quantum wells for the active layer. A threshold current as low as 2.2 mA has been achieved. The influence of the intentional and growth-related compositional grading at the heterointerfaces on the mirror reflectivity and laser characteristics has been investigated, and key sensitivities to laser performance have been determined.     

The structure and optical dispersion of the refractive index of tellurite glass

The structure and optical properties of a 80TeO₂-(20−x)Li₂O-xTiO₂ glass system where x = 0, 5, 10, and 15 mol% has been investigated using FTIR spectroscopy and Brewster angle measurements. The sample preparation, linear refractive index and density measurements, and infrared spectroscopic analysis are described. The refractive index and density of the studied tellurite glass samples increase when the amount of Ti in the glass is increased. The dispersion of the phase refractive index was analyzed using Wemple’s model. The dispersion energy E_d is significantly affected by the addition of Ti to TeO₂-based glass. The analysis of FTIR spectra indicate a Te coordination change that is in agreement with the increase of the Te coordination number determined from dispersion data using Wemple’s equation.     

折光率法测定四氢呋喃溶液的组成

应用阿贝折射仪测量THF(四氢呋喃)溶液的折光率,以此确定THF溶液的组成。实验研究表明,测试数据与文献值吻合良好;THF与水、乙醇及甲醇的两组分溶液折光率与组成之间有很好的规律性,前者可以用抛物线关系式描述,后两者可以用线性关系式描述,其相关系数R在0.99以上,标准偏差SD在0.0015以下,完全适合用于相关溶液组成的测定。     

Origin of crystallization-induced refractive index changes in photo-thermo-refractive glass

Photo-thermo-refractive (PTR) glass is a multi-component silicate that undergoes localized refractive index decrease after UV-exposure and thermal treatment for partial crystallization. Based on this refractive index change, high efficiency volume Bragg gratings have been developed in PTR glass and have been successfully used for laser beam control. However, despite the fact that this type of glass has been widely studied and used over the last 20 years, the origin of the refractive index change upon crystallization is poorly understood. In this paper, we introduce three possible mechanisms (the precipitation of nano-sized NaF crystals and the associated local chemical changes of the glass matrix, the volumetric changes due to relaxation, and the local residual stresses) for the refractive index decrement in PTR glass and estimate the partial refractive index change due to each mechanism. Refractive index measurements are compared with high temperature XRD experiments and a general approach for the simulation of the refractive index change in PTR glass is proposed. We show that among the studied variables the residual stresses surrounding the crystals are the main responsible for the local refractive index decrement in this glass.     

Understanding the mechanism of refractive index modulation in materials undergoing photo-Fries rearrangement

Molecules and polymers that undergo a photo-Fries rearrangement are studied by using DFT calculation in order to predict the refractive index modulation which accompanies this light induced process. In particular, monomeric units of polystyrene derivatives (such as poly(p-formyloxystyrene) and poly(4-acetoxystyrene)) are considered and the refractive index calculated starting from the molecular polarizability and following the Lorentz–Lorenz approach. The results demonstrate that the conversion of ester groups to ketone/alcohol provides just a small modulation of the refractive index that does not fit the experimental results reported in the literature. A change in material density is therefore considered as main source of the modulation of the refractive index. Theoretical and experimental evidences are reported to support the picture.     

Determination of the refractive index of n- and p-type porous Si samples

Photochemical etching of porous Si layers has been shown to be able to create micrometer or submicrometer-scale lateral gratings very promising for photonic applications. However, the reduced size of this lateral periodicity hinders standard measurements of refractive index variations. Therefore accurate characterizations of such gratings are usually difficult. In this paper we address this problem by reproducing on a larger scale (millimeter) the micrometer scale light-induced refractive index variations associated to the lateral periodicity. Using this procedure we perform standard X-ray and optical reflectivity measurements on our samples. One can then proceed to the determination of light-induced variations of porosity and refractive index. We present results for p-type samples, where the photo-dissolution can only be realized after the formation of the porous layer, as well as for n⁺-type samples, where light action can only be effective during the formation of the porous layer.     

New Er-doped phosphate glass for ion-exchange active waveguides: accurate determination of the refractive index

The optical properties of a new Er-doped glass for ion-exchange active waveguides have been characterized, including fluorescence lifetime, Ag–Na diffusion coefficient, and refractive index. In particular, the Sellmeier curve was determined from the visible to the near infrared by means of a non-conventional method based on the measurement of the output angle of radiation modes propagating within the substrate. An accuracy of about 1×10⁻⁴ for the determination of the refractive index is obtained.     

Infrared non-planar plasmonic perfect absorber for enhanced sensitive refractive index sensing

We present a non-planar all-metal plasmonic perfect absorber (PA) with response polarization independent in infrared region, which can be served as a sensor for enhanced refractive index sensing. Distinct from previous designs, the proposed PA consisted of all metal structured film constructed with an assembly of four-tined rod resonators (FRRs). The PA with a high quality-factor (Q-factor) of 41.2 and an absorbance of 99.9% at 142.6 THz has been demonstrated numerically. The resonance behavior occurs in the space between the rods of the FRRs, which is remarkable different conventional sandwiched structural PAs. Based on equivalent LC circuit theory, the absorption peak can be finely tuned by varying the geometrical dimensions of the FRRs. Furthermore, the resonance frequency shows highly sensitive response to the change of refractive index in the surrounding medium. A careful design for refractive index sensor can yield a sensitivity of 1445 nm/refractive index unit (RIU) and a figure of merit (FOM) of 28.8. The demonstrated design of the plasmonic PA for sensing provides great potential application in enhancing refractive index sensors and the enhanced infrared spectroscopy.     

Determination of the refractive index of single crystal bulk samples and micro-structures

We present comparative studies for the exact determination of the refractive index of single crystals using spectroscopic ellipsometry and photonic-mode-structure investigations by means of spatially resolved photoluminescence spectroscopy, especially in the near band-gap spectral range. By applying such complementary methods we can overcome the uncertainties in the determination of the bulk refractive index introduced by surface properties. The physical effects used are the electromagnetic field reflection used by spectroscopic ellipsometry at large scale planar single crystals and the whispering-gallery-mode formation by total internal reflection in confined micro-structures. We demonstrate the applicability of such studies using the example of uniaxial ZnO bulk samples and micro-wires. By assuming a surface near region with electronic properties different from the bulk material, the method presented here gives the refractive index dispersion for both types of samples in an energy range from 1 to 3.4 eV.     

Determination of refractive index increments of PMMA particles dispersed in liquid phase

The refractive index increments of poly(methyl methacrylate) (PMMA) particles with sizes ranging from submicron to several microns were determined by measuring different concentrations of the solution and using a refractometer and microbalance calibrated by accurate traceability source. Conventionally, it was common to measure only the difference in refractive index between solvent and solution using a Bryce-type cell system, but it is difficult to perform traceable calibration since there is no reference material for refractive index increment. In this paper, we accurately quantify the refractive index increment by making individual measurements of the solvent and solution refractive indices, respectively, using Abbe-type refractometer. In addition, an uncertainty analysis was performed, and the results clarified the main factor of the variance in the refractive index increment measurements. The most important factor of variation of refractive index increment measurement is the measured apparent values at dilute concentration region including fluctuation of the system. This method was able to be applied to the determination of water content existing in particles. The percentage of the water content was determined using two different solvents, and the water content values were in good agreement within their uncertainties.     

Organometallic molecular precursors for low-temperature MOCVD of III–V semiconductors

Metal-organic chemical vapor deposition (MOCVD) is a suitable technique for the preparation of III–V epitaxial layers which are used in the fabrication of microelectronic and optoelectronic devices. The usual Ga and As sources for GaAs are Ga(CH₃)₃ and AsH₃, respectively. However, the use of these precursors has some disadvantages including: The toxicity and storage of arsine, stoichiometry control, carbon incorporation and unwanted side reactions. Several groups of researchers have investigated alternative sources for both the group–III and group-V elements. A review of these new organometallic precursors is presented in this paper. However, because group-III and group-V elements form Lewis-acid/base adducts in the MOCVD reactor, we have especially investigated the use of this class of compounds as single starting molecules. Several adducts have been successfully used for the epitaxial growth of GaAs. Moreover, to avoid any stoichiometry loss due to dissociation of the adduct, the properties of organometallic molecules which feature a covalent bond between the group-III and group-V elements have also been investigated. These covalent compounds are probably formed in the MOCVD reactor when alkyl group-V compounds containing acidic hydrogen R_3?nMH_n (M = As, P; n = 1,2) are used. Such new precursors are also briefly reviewed.     

Gold-based ohmic contacts on III–V compounds: Thermally induced reactions between metallization and the semiconductor compound

Au/Zn/Au and Au---Be layers were used to prepare ohmic contacts on p-type GaAs, GaP and InP. Rutherford backscattering spectrometry with 2 MeV He⁺ ions, secondary ion mass spectrometry, X-ray diffraction analysis and scanning electron microscopy were used to study the thermally induced reactions accompanying the formation of ohmic contacts. A change in the ohmic behaviour in the sequence GaAs, InP, GaP occurred upon heating. A marked ability of the group III element in the semiconductor substrate to react with gold appeared to be a common feature of the systems investigated. No reaction product involving the group V element was observed in contacts on GaAs and GaP, whereas the Au₂P₃ phase was identified in contacts on InP. The zinc dopant was found to form the intermetallic phases AuZn and Au₃Zn with gold, and an increase in the content of the beryllium dopant resulted in the retardation of the metal-semiconductor reaction.     

Measurement of the refractive index and thickness of a transparent film from the shift of the interference pattern due to the sample rotation

A method is described for the simultaneous measurement of the refractive index and thickness of a transparent film. The method is based on the rotational shift of the interference pattern caused by the change of the light incidence angle. The refractive index is evaluated without any prior information about film thickness or about the substrate and its refractive index. In addition, the roughness of the interfaces and/or the presence of an unidentified thin layer are not important. In two experimental examples, the refractive index and thickness are measured for a GaN thin film and a cling-film.     

Impact of radio frequency source power-induced ion energy on a refractive index of SiN film deposited by a pulsed-PECVD at room temperature

Using a pulsed-plasma enhanced chemical vapor deposition system, silicon nitride films were deposited at room temperature. A refractive index was examined in the range of bias power and duty ratio, 200-800 W and 40-90%, respectively. Ion energy diagnostics was related to the refractive index. The refractive index decreased with decreasing the duty ratio at all powers but 800 W. For all the duty variations at all powers but 200 W, the refractive index was strongly correlated with the ion energy flux. The refractive index varied between 1.662 and 1.817. Using a neural network model, the refractive index was optimized.     

Temperature dependence of the band gap, refractive index and single-oscillator parameters of amorphous indium selenide thin films

InSe thin films are obtained by evaporating InSe crystal onto ultrasonically cleaned glass substrates under pressure of 10⁻⁵ Torr. The structural and compositional analysis revealed that these films are of amorphous nature and are atomically composed of 51% In and 49% Se. The reflectance and transmittance of the films are measured at various temperatures (300–450 K) in the incident photon energy range of 1.1–2.1 eV. The direct allowed transitions band gap – calculated at various temperatures – show a linear dependence on temperature. The absolute zero value band gap and the rate of change of the band gap with temperature are found to be (1.62 ± 0.01) eV and −(4.27 ± 0.02) × 10⁻⁴ eV/K, respectively. The room temperature refractive index is estimated from the transmittance spectrum. The later analysis allowed the identification of the static refractive index, static dielectric constant, oscillator strength and oscillator energy.     

Extraction of complex refractive index of absorbing films from ellipsometry measurement

Numerical extraction of complex refractive index of an unknown absorbing layer inside a multilayer sample from ellipsometry measurement is discussed. The approach of point by point extraction considering all points of spectroscopic data as independent data points is investigated. This problem has typically multiple solutions and the standard method consisting in fitting calculated to experimental point is likely to converge to a wrong solution if a precise guess value is not given. An alternate method is proposed, based on the determination of contours of the ellipsometric function, to provide all solutions in an as extended as wanted range of complex refractive index values. The method is tested through different kinds of sample examples. Errors relative to any of the parameters used in the sample model are calculated and discussed. This method should be helpful in many practical cases of ellipsometry data interpretation.     

Influence of gamma irradiation on the refractive index of Fe-doped barium titanate thin films

Polycrystalline Fe-doped barium titanate (Fe-doped BaTiO₃) thin films were grown by thermal decomposition of the precursors deposited from a sol-gel system onto quartz substrates. The changes in the transmittance spectra induced by gamma irradiation on the Fe-doped BaTiO₃ thin films were quantified. The values for the optical energy band gap were in the range of 3.42-3.95 eV depending on the annealing time. The refractive index of the film, as measured in the 350-750 nm wavelength range was in the 2.17-1.88 range for the as prepared film, and this increased to 2.34-1.95 after gamma irradiation at 15 kGy. The extinction coefficient of the film was in the order of 10⁻² and increased after gamma irradiation. We obtained tuneable complex refractive index of the films by exposure to various gamma rays doses.