Raman Peak Shift Research Articles

Raman peak shift and broadening in crystalline nanoparticles with lattice impurities

The effect of point-like lattice impurities on nanoparticle Raman spectra is studied using both numerical and analytical methods. Particular cases of replacement atoms of various masses, vacancies, and disorder in interatomic bonds are considered. It is shown that the disorder leads not only to the broadening of optical phonon lines but also to the shift of the corresponding Raman peak. The latter can be either positive (i.e., blueshift) or negative (redshift) depending on the type of impurities. Thus there is an additional contribution to the well-known redshift that occurs due to the size-quantization (confinement) effect. Considering nanometer-sized diamond particles as a representative example, we show that the broadening and the shift are, as a rule, of the same order of magnitude. The results are discussed in the framework of the self-consistent T-matrix approach. It is argued that both effects should be considered for accurate treatment of experimental Raman spectra. Simple recipes to do so are formulated for several important cases including NV centers in nanodiamonds.

Locally resolved stress measurement in the ultra-hard composites polycrystalline diamond and polycrystalline cubic boron nitride

Cutting tools made of the ultra-hard composites polycrystalline diamond and polycrystalline boron nitride are being used in more and more sectors of machining. Due to the laborious preparation processes such as grinding, brushing, electrical discharge and laser machining, the subsurface of these tools is strongly stressed mechanically and thermally. This also changes the residual stress state in the highly loaded cutting edge area. The measurement of these residual stresses is not possible by established XRD methods due to the highly curved surface of the cutting edge. The measurement method Raman spectroscopy shows high potential for this application, but conversion factors are necessary for the application. These factors enable the conversion of the stress-induced peak shift in the Raman spectrum into absolute residual stress values. Previous conversion factors are mainly based on hydrostatic load cases, which, however, cannot be transferred to the application on cutting tools. In this work, axial load cases were provided by bending and conversion factors were determined by comparing XRD stress measurements and Raman peak shifts. The conversion factors determined were then plotted against existing results from other studies and the causes for the deviations that occurred were determined. By this, for the first time, a conversion factor for an axial load case for cubic boron nitride could be determined and it could be shown that, as for diamond, it differs significantly from the hydrostatic load case.

Raman Study of 532-Nanometer Laser-Induced Degradation of Red Lead.

Red lead is commonly employed as a red pigment in numerous valuable cultural artifacts. Raman spectrometry has been widely employed as the primary tool in many nondestructive studies on red lead. Therefore, it is necessary to evaluate and study the impact of lasers on the pigment. The degradation of red lead induced by a 532 nm laser is investigated using micro-Raman spectroscopy. At room temperature, red lead begins to degrade into β-PbO when the power density of the 532 nm laser reaches approximately 5.1 × 104 W/cm2 (laser: 532 nm, objective: 50×). At this point, the temperature at the focus of the sample is estimated to be at least 500 °C, aided by the Raman peak shift of β-PbO. Furthermore, the power density of the laser-induced degradation decreases as the temperature of the red lead increases. Hence, the degradation of red lead can be attributed to the photothermal effect. The temperature rise can be explained by two factors. First, red lead exhibits a high absorbance of approximately 0.5942 at 532 nm. Second, red lead has significantly low thermal diffusivity and conductivity, measuring 0.039 mm2·s-1 and 0.078 W·m-1·K-1, respectively, which leads to heat accumulation at the focal point of the laser beam. To better preserve cultural heritage, the appropriate laser power should be prioritized when the degradation process is caused by the thermal effect of laser irradiation.

Surface enhanced Raman scattering microtips for microenvironment pH determination of semi-solid preparations.

Semi-solid preparations such as ointments, creams, gels, and pastes are common topical dosage forms with complex compositions and microstructures. pH is a critical quality attribute for semi-solid preparations, affecting properties such as particle size distribution, drug dissociation state, and rheological behavior. Currently, traditional pH electrode methods only provide an "average" pH for large-volume matrices, with low spatial resolution and accuracy. Microenvironment pH detection is crucial for accurately assessing semi-solid preparations. Herein, we developed pH-sensitive surface enhanced Raman scattering (SERS) microtips to achieve localized pH detection in semi-solid preparations. SERS microtips were prepared from glass needles with a tip size of around 1 μm and gold nanoparticles (Au NPs) grown in situ on glass surfaces for SERS enhancement. 4-Mercaptopyridine was selected as a pH sensitive Raman reporter and immobilized on the Au NPs, exhibiting characteristic Raman peak shifts within the pH range of 3-10. The SERS microtips were employed to conduct highly sensitive pH measurements in localized areas of 15 commercial ointments, 8 gels, and 1 laboratory-made ointment, providing higher spatial resolution and microenvironment differentiation compared to pH meters. The SERS microtips were used to monitor pH changes over time in ointment applied to localized wounds on live mice. This work introduces a new tool for pH detection in semi-solid preparations, offering a new method to enhance the prescription process and quality assessment of complex preparations like topical semi-solid preparations.

Measurements of stresses in β‐(Y,Yb)2Si2O7 environmental barrier coating layers using Raman spectroscopy

AbstractResidual stress measurements in environmental barrier coatings (EBCs) can help in understanding coating durability and potential damage mechanisms. We demonstrate the capability of Raman spectroscopy to measure average residual stresses in two different β‐RE2Si2O7 (RE = rare earth or Y3+, denoted as REDS) EBCs with a nominal (Yb,Y)2Si2O7 composition, focusing on macroscopic residual stresses acting at macroscopic length scales. Uniaxial compression measurements of dense β‐(Yb,Y)2Si2O7 ceramics are used for calibration curves for applied stress versus Raman peak shift. We use these calibration curves to evaluate the macroscopic thermoelastic residual stress in the (Yb,Y)2Si2O7 layers from coefficient of thermal expansion (CTE) mismatch of two different (Yb,Y)2Si2O7‐based EBCs on SiC‐based substrates. Microscopic stress contributions to Raman peak shifts are subtracted using reference free‐standing coatings and analytic estimates for inclusion stresses when (Yb,Y)2Si2O7 coatings have a high, variable volume fraction of (Yb,Y)2SiO5. The macroscopic thermal residual stress in these (Yb,Y)2Si2O7 EBCs is compressive, as expected from the smaller CTE of β‐REDS versus SiC. These thermal residual stresses can vary between different YbYDS EBC coatings by more than a factor of three with a measured layer residual stress of approximately –180 MPa (compressive) for a high modulus, low CTE coating, and our measured values generally agree with calculated estimates.

Vibrational Properties and Raman Peak Shift Relationships in Si1−xGex Epilayers Grown on Annealed Double Porous Silicon

Vibrational Properties and Raman Peak Shift Relationships in Si1−xGex Epilayers Grown on Annealed Double Porous Silicon

Graphene on SiO2/Si and Al2O3 under thermal annealing and electric current: Competition of dopant desorption and conformation to substrate

In this work, we study phonon and electronic properties of graphene on SiO2/Si and Al2O3 by simultaneous Raman and electrical measurements in the temperature range from room temperature to 550 °C, or at voltages from 20 to −20 V. The dependencies of G and 2D peak parameters and electrical resistance on temperature and voltage made it possible to observe in situ a competition between the p-type adsorbate removal from graphene surface and substrate-induced doping due to graphene-substrate conformality increase, both stimulated by either ambient or Joule heating. The analyzed parameters were dominated by the conformality increase, with the hole density increasing significantly and unidirectionally, while resistance and I-V curves fluctuated due to the competition. Having calculated Raman peak shift temperature coefficients, resistance temperature coefficients, total variations of carrier density, resistance and strain, we show that Al2O3 substrate can be used to reduce the desorption barrier, the overall doping, the impact on graphene resistance and on phonon anharmonicity – however, it should be used with regard to the possibility of introducing strain or stronger doping after longer treatments. The conformality effects should be taken into account when performing annealing, as well as graphene applications for sensors or strong electric currents.

A two-step in situ measurement method for temperature and thermal stress of power device based on a single Raman peak

Temperature and thermal stress are primary factors leading to the performance failure of power devices. Thus, simultaneously determining the temperature and stress variation of the working devices has become the premise of revealing the failure mechanism. However, the existing methods are still unsatisfactory for in-situ temperature-stress decoupling measurement. To solve this problem, a two-step decoupling method based on a single Raman peak is proposed. In the calibration step, the effective temperature - Raman shift coefficients at different positions of the power devices were measured in the OFF state, and the contribution ratio of temperature and thermal stress to the single peak shift can be determined by analyzing the difference of effective temperature - Raman shift coefficients with intrinsic temperature - Raman shift coefficient. In the in-situ measurement step, the temperature rise and thermal stress variation can be calculated based on the Raman peak shift of the target semiconductor layer and the calibrated contribution ratio. A measurement of the GaN high electron mobility transistor was used to further illustrate the advantages of the proposed method. Compared with the previous multi-peak decoupling method, the temperature and thermal stress distributions of the working device obtained by the two-step decoupling method make more physical sense, and the measurement uncertainty of temperature and stress can be reduced to 1/4 and 1/3 of the previous method respectively. The consistency with the thermo-stress simulation further verifies the reliability and accuracy of the proposed method.

Strain engineering in 2D hBN and graphene with evaporated thin film stressors

We demonstrate a technique to strain two-dimensional hexagonal boron nitride (hBN) and graphene by depositing stressed thin films to encapsulate exfoliated flakes. We choose optically transparent stressors to be able to analyze strain in 2D flakes through Raman spectroscopy. Combining thickness-dependent analyses of Raman peak shifts with atomistic simulations of hBN and graphene, we can explore layer-by-layer strain transfer in these materials. hBN and graphene show strain transfer into the top four and two layers of multilayer flakes, respectively. hBN has been widely used as a protective capping layer for other 2D materials, while graphene has been used as a top gate layer in various applications. Findings of this work suggest that straining 2D heterostructures with evaporated stressed thin films through the hBN capping layer or graphene top contact is possible since strain is not limited to a single layer.

Unveiling localized electronic properties of ReS2 thin layers at nanoscale using Kelvin force probe microscopy combined with tip-enhanced Raman spectroscopy

Electronic properties of two-dimensional (2D) materials can be strongly modulated by localized strain. The typical spatial resolution of conventional Kelvin probe force microscopy (KPFM) is usually limited in a few hundreds of nanometers, and it is difficult to characterize localized electronic properties of 2D materials at nanoscales. Herein, tip-enhanced Raman spectroscopy (TERS) is proposed to combine with KPFM to break this restriction. TERS scan is conducted on ReS2 bubbles deposited on a rough Au thin film to obtain strain distribution by using the Raman peak shift. The localized contact potential difference (CPD) is inversely calculated with a higher spatial resolution by using strain measured by TERS and CPD-strain working curve obtained using conventional KPFM and atomic force microscopy. This method enhances the spatial resolution of CPD measurements and can be potentially used to characterize localized electronic properties of 2D materials.

High yield synthesis and study of Cu substitution on characteristics and dielectric properties of MgO nanostructures

High-permittivity materials are becoming more popular due to the electrical and electronic industrial development. Single composition materials are well known to fall short of high permittivity requirements. So, in this article, we report pure and Cu (5%, 7%, 9%, and 11%) doped MgO nanostructures synthesized by reverse micelle method and study of their dielectric properties. Energy Dispersive Analysis of X-rays (EDAX), Powder X-ray Diffraction (PXRD), diffused reflectance UV–Vis spectroscopy (DR-UV-Vis), Dynamic Light Scattering (DLS), photoluminescence (PL), Raman spectroscopy and Broadband Dielectric Spectroscopy (BDS) are among the characterization methods used. Each synthesized sample crystallizes in cubic crystal system. As Cu doping increases from 5% to 11%, the average crystallite size dwindles from 22 to 10 nm. The bandgap is reduced from 5.71 eV to 2.58 eV as per the optical results from DR-UV-VIS spectroscopy. The intensity and surface defects are correlated by PL spectra. Raman peak shifts to the higher angle side, which is related to XRD and associated with a reduction in average crystallite size. For pure and all doping concentrations, the dielectric constant and the dissipation factor decreases with the increment in frequency which is a peculiar behaviour of dielectric materials.

Isotopic Signatures of Lithium Carbonate and Lithium Hydroxide Monohydrate Measured Using Raman Spectroscopy.

Lithium isotopic ratios have wide ranging applications as chemical signatures, including improved understanding of geochemical processes and battery development. Measurement of isotope ratios using optical spectroscopies would provide an alternative to traditional mass spectrometric methods, which are expensive and often limited to a chemical laboratory. Raman spectra of 7Li2CO3, 6Li2CO3, 7LiOH*H2O, and 6LiOH*H2O have been measured to determine the effect of lithium isotope substitution on the Raman molecular vibrations. Thirteen peaks were observed in the spectrum of lithium carbonate, with discernable isotopic shifts occurring in eleven of the 13 vibrations, two of which have not been previously reported in the literature. The spectrum of lithium hydroxide monohydrate contained nine peaks, with discernable isotopic shifts occurring in eight of the nine vibrations, four of which have not been previously reported in the literature. The Raman spectral data reported here for lithium carbonate and lithium hydroxide monohydrate are in agreement with the previously reported works in the literature, in which the Raman active modes of these molecules were first identified and assigned. However, due to the stability and resolution of the detection system used in this work, isotopic shifts with a magnitude less than one wavenumber have been identified. Principal component regression was used to evaluate the sensitivity to isotopic content of small Raman peak shifts in Li2CO3 and indicates differences greater than 2 atom% could be reliably determined. These measurements add to the body of work on lithium isotope Raman spectroscopy for these two compounds and increases the number of Raman bands which could be used for lithium isotope content analysis.

Effects of hot phonons and thermal stress in micro-Raman spectra of molybdenum disulfide

Micro-Raman spectroscopy has become an important tool in probing thermophysical properties in functional materials. Localized heating by the focused Raman excitation laser beam can produce both stress and local nonequilibrium phonons in the material. Here, we investigate the effects of hot optical phonons in the Raman spectra of molybdenum disulfide and distinguish them from those caused by thermally induced compressive stress, which causes a Raman frequency blue shift. We use a thermomechanical analysis to correct for this stress effect in the equivalent lattice temperature extracted from the measured Raman peak shift. When the heating Gaussian laser beam is reduced to 0.71 μm, the corrected peak shift temperature rise is 17% and 8%, respectively, higher than those determined from the measured peak shift and linewidth without the stress correction, and 32% smaller than the optical phonon temperature rise obtained from the anti-Stokes to Stokes intensity ratio. This nonequilibrium between the hot optical phonons and the lattice vanishes as the beam width increases to 1.53 μm. Much less pronounced than those reported in prior micro-Raman measurements of suspended graphene, this observed hot phonon behavior agrees with a first-principles based multitemperature model of overpopulated zone-center optical phonons compared to other optical phonons in the Brillouin zone and acoustic phonons of this prototypical transition metal dichalcogenide. The findings provide detailed insight into the energy relaxation processes in this emerging electronic and optoelectronic material and clarify an important question in micro-Raman measurements of thermal transport in this and other two-dimensional materials.

Origin of Blue-Water Jadeite Jades from Myanmar and Guatemala: Differentiation by Non-Destructive Spectroscopic Techniques

Identifying the origin of jadeite jades has become increasingly important from both mineral resource and metamorphic geology perspectives. In this study, we differentiate Myanmar gem-quality blue-water jadeite jades from their Guatemala counterparts via integrating various non-destructive spectrographic techniques, including X-ray fluorescence (XRF), Ultraviolet-Visible spectroscopy (UV-Vis), Fourier transform infrared spectroscopy (FTIR), and Raman spectroscopy. Our results show that the Myanmar blue-water jadeite jades are structurally homogenous with very few impurities, while their Guatemala counterparts commonly have a yellowish margin with scattered white albite and disseminated greenish inclusions of omphacite and (minor) aegirine-augite. Geochemically, the UV absorption spectral data indicate that the Guatemala samples have higher total Fe and Fe2+ contents, but lower Fe3+ content than the Myanmar samples. The Guatemala samples also have higher omphacite content (lower molar Na/(Na+Ca) ratio, as reflected by the lower IR absorption peak wavenumber) and higher heterogeneity (as reflected by the ~680 cm−1 Raman peak shift difference) than that from Myanmar. Major differences are also discovered in the blue series (Myanmar: 0–0.7 cm−1; Guatemala: 1.7–3.2 cm−1) and blue-green series (Myanmar: 6.9 cm−1; Guatemala 13.7 cm−1) of the Raman peak shift difference, which altogether can provide a novel, nondestructive method for distinguishing blue-water jadeite jades from different origins.

Temperature Dependence of Raman Peak Shift in Single-Crystalline Si-Rich Sige

Background and Objectives Silicon-germanium (SiGe) is used for thermoelectric devices since its thermal conductivity is lower than either pure Si or Ge. Indeed, the material was used in components of the Voyager and New Horizons deep-space craft because of its superior nanoscale heat conductivity. Improving SiGe for better thermoelectric devices requires ever more precise measurements of nanoscale heat conductivity and a comprehensive study on its phonon-scattering mechanism. Especially, a relationship between Raman shift ω and temperature T (dω/dT) is required through the highly precise measurements of its temperature-dependence. Burke et al. have reported the Ge fraction dependence of dω/dT for Si-Si, Si-Ge, and Ge-Ge vibration modes of polycrystalline bulk SiGe samples with high laser power excitation [1]. However, the reported values of dω/dT may be affected by phonon scattering at grain boundaries and the local heating caused by high laser power excitation. Here, we investigated dω/dT of single-crystalline bulk SiGe and its dependence on the Ge fraction by using variable-temperature Raman spectroscopy. Experiments The (001)-oriented single-crystalline Si-rich SiGe bulk samples were prepared by two different growth methods; the Czochralski (Ge fraction: 16%) [2] and traveling liquidus zone (TLZ) methods (Ge fraction: 32 and 45%) [3]. All samples were confirmed to be strain-free by X-ray diffraction (XRD). We also measured Raman spectrum of single-crystalline Si as a reference. For Raman spectroscopy measurements, the wavelength of the excitation source was 532 nm, the focal length of the spectrometer was 2,000 mm, the laser power at the sample surface was kept to less than 1 mW. The sample temperatures were set between 50 and 300°C in 50°C steps, and the Raman signal was measured twice at each temperature (i.e., during heating and cooling). Results and Discussion Raman spectra of Si-Si mode in SiGe (Ge fraction: 16%) are shown in Fig. 1(a). With increasing temperature, the full width at half maximum (FWHM) of the Raman spectra increased and shifted toward the lower side of wavenumber. This wavenumber shift is considered to be caused by the anharmonic vibrations becoming dominant as the temperature increases, and the phonon frequency decrease due to the thermal expansion of the crystal. Figure 1(b) shows the temperature dependences of the Si-Si Raman peak shifts. We experimentally clarified the dω/dT are approximately constant in each bulk SiGe sample, as shown in Fig. 1(b). Figure 1(c) shows the Ge fraction dependence of -(dω/dT) for Si-Si mode. As a result, we found that dω/dT for Si-Si mode is independent of Ge fraction unlike the study of Burke et al. It was reported that the dω/dT depends on the coefficient of linear thermal expansion and anharmonic constants [4]. From the obtained dω/dT values in the investigated range of Ge fraction, we consider that the anharmonic constants of SiGe alloy do not change by Ge fraction. Thus, the Si-Si anharmonic vibration is supposed to be not affected by the Ge fraction in SiGe alloy. In conclusion, we determined dω/dT= -0.023 cm-1/℃ for Si-Si mode from experiments with single-crystalline bulk SiGe samples and low laser power excitation. Acknowledgement This work was supported by Grant-in-Aid for Scientific Research (21K14201).

Roughness Suppression in Electrochemical Nanoimprinting of Si for Applications in Silicon Photonics

Metal-assisted electrochemical nanoimprinting (Mac-Imprint) scales the fabrication of micro- and nanoscale 3D freeform geometries in silicon and holds the promise to enable novel chip-scale optics operating at the near-infrared spectrum. However, Mac-Imprint of silicon concomitantly generates mesoscale roughness (e.g., protrusion size≈45nm) creating prohibitive levels of light scattering. This arises from the requirement to coat stamps with nanoporous gold catalyst that, while sustaining etchant diffusion, imprints its pores (e.g., average diameter≈42nm) onto silicon. In this work, roughness is reduced to sub-10nm levels, which is in par with plasma etching, by decreasing pore size of the catalyst via dealloying in far-from equilibrium conditions. At this level, single-digit nanometric details such as grain-boundary grooves of the catalyst are imprinted and attributed to the resolution limit of Mac-Imprint, which is argued to be twice the Debye length (i.e., 1.7nm)-a finding that broadly applies to metal-assisted chemical etching. Last, Mac-Imprint is employed to produce single-mode rib-waveguides on pre-patterned silicon-on-insulator wafers with root-mean-square line-edge roughness less than 10nm while providing depth uniformity (i.e., 42.9±5.5nm), and limited levels of silicon defect formation (e.g., Raman peak shift<0.1cm-1 ) and sidewall scattering.

Investigation of temperature effect on pressure measurement using Raman peak shift by variable temperature and pressure Raman experiment

AbstractThe variable temperature and pressure Raman experiments of triamino trinitrobenzene (TATB) have been performed to investigate the temperature effect on the pressure measurement using the Raman peak shift. The experimental results indicate that the temperature has an approximately linear relationship with the Raman peak shift but no significative effect on the pressure dependence of the Raman peak shift. And then, the results of TATB under laser‐driven shock wave are chosen to illustrate the temperature effect in the shock pressure measurement. The absolute differences of the pressure values ignoring and considering the temperature are relatively large in the period of the shock loading, but the percent changes of these two values are relatively large in the period of the shock unloading. Thus, in the pressure measurement using Raman peak shifts, the temperature effect on the Raman peak shift needs to be considered, especially for the case of the shock unloading.

Thermal conductivity estimation of silicon bulk material based on modified Raman thermal technique

The thermal conductivity of single-crystal silicon was investigated by using Raman spectroscopy. The laser simultaneously acted as an excitation source and a heating source. The correlations between Raman spectra with both temperature and laser power for single-crystal silicon, which has a potential relationship with the thermal conductivity, were estimated. The results showed the localized compressive stress caused by laser heating would underestimate Raman peak shift. So, the temperature was determined based on the variation of linewidth. The predicted thermal conductivity of single-crystal silicon is 125 W/m ⋅ K, which is comparable to the theoretical value.

Strain engineering 2D MoS2 with thin film stress capping layers

We demonstrate a method to induce tensile and compressive strain into two-dimensional transition metal dichalcogenide (TMDC) MoS2 via the deposition of stressed thin films to encapsulate exfoliated flakes. With this technique we can directly engineer MoS2 strain magnitude by changing deposited thin film stress, therefore allowing variable strain to be applied on a flake-to-flake level. These thin film stressors are analogous to SiN x based stressors implemented in industrial complementary metal-oxide-semiconductor (CMOS) processes to enhance Si mobility, suggesting that our concept is highly scalable and may be applied for large-scale integration of strain engineered TMDC devices. We choose optically transparent stressors to allow us to probe MoS2 strain through Raman spectroscopy. Combining thickness dependent analyses of Raman peak shifts in MoS2 with atomistic simulations, we can explore layer-by-layer strain transfer. MoS2 on conventional substrates (SiO2, MgO) show strain transfer into the top two layers of multilayer flakes with limited strain transfer to monolayers due to substrate adhesion. To mitigate this limitation, we also explore stressors on van der Waals heterostructures constructed of monolayer (1L) MoS2 on hexagonal boron nitride (h-BN). This concept frees the 1L-MoS2 allowing for a 0.85% strain to be applied to the monolayer with a corresponding strain induced bandgap change of 75 meV. By using thin films with higher stress, strain may be engineered to be even higher. Various stressors and deposition methods are considered, showing a stressor material independent transfer of strain that only depends on stressor film force with negligible defects induced into MoS2 when thermal evaporation is used.

Phonons and Valence-band Splitting in Strained GaAs&lt;SUB&gt;1-x&lt;/SUB&gt;N&lt;SUB&gt;x&lt;/SUB&gt;/GaAs Epilayers

The strain effects in strained GaAs<SUB>1-x</SUB>N<SUB>x</SUB> epilayers are characterized by Raman spectroscopy and photocurrent spectra at various nitrogen composition. In addition, the nitrogen composition and the strain were determined by using high-resolution X-ray diffraction (HR-XRD). The Raman spectra are observed to be dominated by the GaAs-like longitudinal optical (LO) phonon mode as the strongest peaks show up around 289~294 cm<SUP>-1</SUP>. Moreover, the weak and broad peaks features in the range of 255~276 cm<SUP>-1</SUP> originate from the peaks of GaAs-like transverse optical (TO) phonon mode and disorder induced, GaN-like LO. And the Raman peak shifts toward lower wave number with increasing nitrogen compositions, which is indicates the presence of tensile strain in the strained GaAs<SUB>1-x</SUB>N<SUB>x</SUB> epilayers. The valence-band splitting of GaAs<SUB>1-x</SUB>N<SUB>x</SUB> are obtained from photocurrent spectra. As the nitrogen concentration increases, the tensile strain in strained GaAs<SUB>1-x</SUB>N<SUB>x</SUB> epilayers increases while the valence-band splitting increases.