Micro‐Raman analysis of GaAs Schottky barrier solar cell

Gallium arsenide (GaAs) cells have been in the race with silicon single‐crystal cells for the highest efficiency photovoltaic devices. The annealed, irradiated Schottky barrier (SB) solar cells were characterised using micro‐Raman spectroscopy at three different regions: namely, at the (1) ohmic contact region, (2) unirradiated region and (3) irradiated region. We also present a micro‐Raman study of the damage process in annealed GaAs SB solar cells bombarded by high‐energy ions. A Gaussian line shape was fitted to the Raman spectra of the longitudinal optical phonon A₁(LO), and parameters such as intensity, full width at half maximum (FWHM) and the area under the peak were obtained for the different annealing temperatures. Biaxial stress (σ), carrier concentration (n), depletion length (L_d), dislocation velocity (ν) and life time of the first‐order optical phonon (τ) of the A₁(LO) mode of the irradiated region of the samples annealed at different temperatures were calculated. Copyright © 2010 John Wiley & Sons, Ltd.     

Raman studies of lattice and local vibrational modes of GaInNAs prepared by molecular beam epitaxy

Polarized Raman spectra have been studied on the lattice-matched Ga_0.94In_0.06N_0.025As_0.975 epitaxial layers grown on (100) GaAs by molecular beam epitaxy. Polarization dependence of TO and LO phonon modes has been examined. The N-related local vibrational mode (LVM) in GaInNAs has been studied with emphasis on the light polarization and the effect of the thermal annealing. The thermal annealing-induced change of the N-related LVM in GaInNAs is discussed in terms of the local atomic arrangement around N atoms.     

Multiphonon resonant Raman scattering in N‐doped ZnO

Multiphonon resonant Raman scattering in N‐doped ZnO films was studied, and an enhancement of the resonant Raman scattering process as well as longitudinal optical (LO) phonon overtones up to the sixth order were observed at room temperature. The resonant Raman scattering intensity of the 1LO phonon in N‐doped ZnO appears three times as strong as that of undoped ZnO, which mainly arises from the defect‐induced Raman scattering caused by N‐doping. The nature of the 1LO phonon at 578 cm⁻¹ is interpreted as a quasimode with mixed A₁ and E₁ symmetry because of the defects formed in the ZnO lattice. In addition, the previously neglected impurity‐induced two‐LO‐phonon scattering process was clearly observed in N‐doped ZnO. Copyright © 2009 John Wiley & Sons, Ltd.     

Phonon freedom and confinement in GaAsAlxGa1−xAs superlattices

Phonon modes in GaAsAl_xGa_1?xAs superlattices simplify when the phonon wavevector q is perpendicular to the plane of the layers. We have studied such modes using a Raman back-scattering technique on SL's grown by MBE. The results are consistent with simple ideas of LA phonon freedom and LO phonon confinement suggested by one-dimensional lattice dynamical calculations. The longitudinal acoustic (LA) modes show zone folding due to mini-zone formation. Their frequencies occur in doublets linearly dependent on q and show little mini-gap formation. This is consistent with a picture of approximately free plane wave propagating through the interfaces with Raman coupling due to SL layering of the photoelastic coefficient. By contrast, Raman data on LO modes in small period GaAsAlAs SL's suggest that these modes are standing waves strongly confined in either GaAs or AlAs.     

Raman scattering studies of (GaP)n/(InP)n (n = 1, 1.7, 2) short‐period superlattice structures

We report Raman scattering from (GaP)_n/(InP)_n (n = 1, 1.7, 2) short‐period superlattice (SPS) structures to study the effect of lateral composition modulation (LCM) on the behavior of optical phonons. Cross‐sectional transmission electron microscope images revealed that LCM was formed with complex pattern in the n = 1.7 and n = 2 samples grown at 490 °C. Interestingly, both the InP‐ and the GaP‐like longitudinal optical (LO) phonon energies increased systematically as the number of monolayers was increased from n = 1 to n = 2. A significant broadening of the phonon line shapes was also observed for the n = 1.7 and n = 2 samples. In contrast, for samples grown at 425 °C, both the increase of the LO phonon energies and the broadening of the phonon line shapes were observed only when n = 1.7. Our results demonstrate that the optical phonons in the (GaP)_n/(InP)_n SPS structures are significantly affected in the occurrence of LCM related to the growth temperature and the number of monolayers.Copyright © 2009 John Wiley & Sons, Ltd.     

Efficiency improvement of quantum well solar cell with the AuGeNi metallization and Si3N4 ARC design

This study demonstrates the power conversion efficiency enhancement on In_0.19Ga_0.81As/GaAs quantum well solar cells (QWSC). The solar cell structure was grown on n-type (100)-oriented GaAs substrate by using solid-source molecular beam epitaxy technique and divided into square pieces. In order to understand whether the eight quantum wells were grown or not, scanning electron microscopy (SEM), and secondary ion mass spectrometry characterizations were done at room temperature. After that, the Si₃N₄ antireflection layers were coated onto both two square pieces of In_0.19Ga_0.81As/GaAs QWSC structure and p-GaAs substrate at different temperatures by the radio frequency magnetron sputtering system. The optical properties of the Si₃N₄ coated and uncoated p-GaAs samples have been evaluated by means of ultraviolet-visible spectrometry measurements at room temperature. According to ultraviolet-visible spectrometry results, the best Si₃N₄ antireflection coated one was obtained at 100 °C substrate temperature. Thus, the In_0.19Ga_0.81As/GaAs QWSC structure with and without Si₃N₄ layer, which was coated at 100 °C substrate temperature, was selected for other measurements and processes. Moreover, the In_0.19Ga_0.81As/GaAs QWSC samples with and without Si₃N₄ antireflection coating were separately fabricated with different metallization materials for obtaining the solar cell electrical output parameters. AuGe and AuGeNi metallization materials were used for the fabrication processes. After fabrication, the electrical output parameters were extracted from the current-voltage measurements at room temperature both in dark and under AM1.5 – 1 Sun illumination. The proposed design which includes the AuGeNi metallization and Si₃N₄ antireflection layer enhanced the power conversion efficiency by 44.40%.     

InGaN薄膜的紫外共振喇曼散射

利用325nm紫外光激发,对不同组分的In_xGa_1-xN薄膜的喇曼散射谱进行了研究.在光子能量大于带隙的情况下,观察到显著增强的二阶A₁(LO)声子散射峰.二阶LO声子峰都从一阶LO声子的二倍处向高能方向移动,移动量随样品In组分的增加而增大,认为是带内Frhlich相互作用决定的多共振效应引起的.分析了一阶LO声子散射频率和峰型与In组分的关系.在喇曼谱中观察到样品存在相分离现象,并与X射线衍射的实验结果进行 关键词： xGa_1-xN合金')" href="#">In_xGa_1-xN合金 紫外共振喇曼散射 二阶声子 相分离     

Morphology of InN nanorods using spectroscopic Raman imaging

We report the vibrational properties of vertical and oblique InN nanorods (NRs) grown by molecular beam epitaxy (MBE). Surface optical (SO) Raman mode at 561 cm⁻¹, belonging to E₁ symmetry [SO(E₁)], is identified along with symmetry allowed Raman modes of E₂(low), E₂(high), and E₁(LO) at 87, 489, and 589 cm⁻¹, respectively, corresponding to wurtzite InN phase. Usually, SO phonon modes arise due to breakdown of translational symmetry of surface potential at surface defects, which are attributed by the surface roughness. Intensity distribution of E₁(LO) and SO(E₁) phonon modes over a specified area have been analysed using Raman area mapping with an optical resolution of 400 nm. Imaging with E₁(LO) phonon mode, originating from the bulk of the sample, distinguishes the vertical NRs alone. We are able to resolve NR morphologies in both vertical and oblique cases with additional Raman mapping analysis of SO(E₁) phonon mode, emerging from the surface irregularities, which are confined to the tip of MBE grown NRs. Copyright © 2013 John Wiley & Sons, Ltd.     

Dopant diffusion and segregation in semiconductor heterostructures: Part III, diffusion of Si into GaAs

We have mentioned previously that in the third part of the present series of papers, a variety of n-doping associated phenomena will be treated. Instead, we have decided that this paper, in which the subject treated is diffusion of Si into GaAs, shall be the third paper of the series. This choice is arrived at because this subject is a most relevent heterostructure problem, and also because of space and timing considerations. The main n-type dopant Si in GaAs is amphoteric which may be incorporated as shallow donor species Si_Ga ⁺ and as shallow acceptor species Si_As ^-. The solubility of Si_As ^- is much lower than that of Si_Ga ⁺ except at very high Si concentration levels. Hence, a severe electrical self-compensation occurs at very high Si concentrations. In this study we have modeled the Si distribution process in GaAs by assuming that the diffusing species is Si_Ga ⁺ which will convert into Si_As ^- in accordance with their solubilities and that the point defect species governing the diffusion of Si_Ga ⁺ are triply-negatively-charged Ga vacancies V_Ga ^3-. The outstanding features of the Si indiffusion profiles near the Si/GaAs interface have been quantitatively explained for the first time. Deposited on the GaAs crystal surface, the Si source material is a polycrystalline Si layer which may be undoped or n⁺-doped using As or P. Without the use of an As vapor phase in the ambient, the As- and P-doped source materials effectively render the GaAs crystals into an As-rich composition, which leads to a much more efficient Si indiffusion process than for the case of using undoped source materials which maintains the GaAs crystals in a relatively As-poor condition. The source material and the GaAs crystal together form a heterostructure with its junction influencing the electron distribution in the region, which, in turn, affects the Si indiffusion process prominently. Received: 19 April 1999 / Accepted: 3 May 1999 / Published online: 4 August 1999     

Study of δ-doped GaAs layers by micro-Raman spectroscopy on bevelled samples

Si δ-doped GaAs layers were studied by micro-Raman spectroscopy. The spectra were recorded along the bevelled structure using the light of Ar⁺-ion laser (514.5 nm line) with high power density. The observed changes in the Raman spectra are discussed in the sense of coupling present between LO phonon and photoexcited electron–hole plasma and plasma of electrons arising from ionised Si atoms. Plasmon-LO-Phonon modes of the coupling of photoexcited plasma in δ-doped GaAs layers were observed for the first time. The minimal thickness of cap layer was estimated in the range of 10–19 nm depending on the doping concentration.     

Strong resonant intersubband magnetopolaron effect in heavily modulation-doped GaAs/AlGaAs single quantum wells at high magnetic fields

Electron cyclotron resonance (CR) has been studied in magnetic fields up to 32 T in two heavily modulation-δ-doped GaAs/Al_0.3Ga_0.7As single quantum well samples. Little effect on electron CR is observed in either sample in the region of resonance with the GaAs LO phonons. However, above the LO-phonon frequency energy E_LO at B>27 T, electron CR exhibits a strong avoided-level-crossing splitting for both samples at energies close to E_LO+(E₂−E₁), where E₂, and E₁ are the energies of the bottoms of the second and the first subbands, respectively. The energy separation between the two branches is large, reaching a minimum of about 40 cm⁻¹ around 30.5 T for both samples. This splitting is due to a three-level resonance between the second LL of the first electron subband and the lowest LL of the second subband plus an LO phonon. The large splitting in the presence of high electron densities is due to the absence of occupation (Pauli-principle) effects in the final states and weak screening for this three-level process.     

Chemical composition of matrix-embedded ternary II–VI nanocrystals derived from first- and second-order Raman spectra

A one- and multiphonon Raman scattering study is performed for an extensive set of CdS_1–xSe_x, Cd_1–yZn_yS, Cd_1–yZn_ySe, and CdSe_1–xTe_x nanocrystals to investigate the applicability of first- and second-order Raman spectra for the determination of the matrix-embedded ternary nanocrystal composition. For one-mode ternary systems both the LO and 2LO phonon frequencies in the Raman spectra are shown to be a good measure of the nanocrystal composition. For two-mode systems, the approaches based on the difference of the LO phonon frequencies (first-order Raman spectra) or double LO overtone and combination tone frequencies (second-order Raman spectra) as well as on the LO phonon band intensity ratios are analysed. The weak electron–phonon coupling in the II–VI nanocrystals and the polaron constant values for the nanocrystal sublattices are discussed.     

Inter- and intra-subband relaxation of hot electrons in GaAs/AlGaAs quantum wells

In this work, we have studied the inter- and intra-subband scattering of hot electrons in quantum wells using the hot electron-neutral acceptor luminescence technique. We have observed direct evidence of the emission of confined optical phonons by hot electrons excited slightly above the n=2 subband in GaAs/Al_0.37Ga_0.63As quantum wells. Scattering rates of photoexcited electrons via inter- and intra-subband LO phonon emission were calculated based on the dielectric continuum model. We found that, for wide wells with the Al composition of our experiments, both the calculated and experimental results suggest that the scattering of the electrons is dominated by the confined LO phonon mode. In the calculations, scatterings among higher subbands are also dominated by the same type of phonon at well width of 10 nm.     

Residual strain and alloying effects on the vibrational properties of step-graded InxAl1−xAs layers grown on GaAs

Lattice-relaxed In_xAl_1−xAs-graded buffer layers grown by MBE on GaAs substrate have been studied by micro-Raman scattering and photoluminescence (PL). In these heterostructures, the indium composition was gradually increased in six or four intermediate layers each of 100 nm thickness. The alloying effect in the In_xAl_1−xAs layers has been interpreted using the modified random element isodisplacement (MREI) formalism. The dependence in the MREI model of the longitudinal optical (LO) phonon energy and the In composition in the InAlAs alloy with PL measurements and Raman analysis allow the evaluation of disorder degree and give the In composition in the InAlAs active layers. The obtained InAs- and AlAs-like phonon frequencies from the fitting of Raman spectra are in reasonable agreement with those calculated according to the MREI model. Raman spectra show that InAs-like phonon frequencies are not strongly dependent either on the buffer structure or on the residual strain in the active layers. Using the AlAs-like LO phonon frequency shifts, we have calculated the residual strain in the In_xAl_1−xAs active layer. Raman results show that the slope of the grade is an important parameter that allows the growth of samples with good quality. The lower residual strain value was obtained by thick buffer with a smaller grading rate. PL measurements show that In compositions of active layers of the different studied samples are slightly higher than those measured during growth.     

Pressure-tuned resonance Raman scattering and photoluminescence studies on MBE grown bulk GaAs at theE 0 gap

Hydrostatic pressure has been used to tune in resonance Raman scattering (RRS) in bulk GaAs. Using a diamond anvil cell, both the photoluminescence peak (PL) and the 2 LO and LO-phonon Raman scattered intensities have been monitored, to establish RRS conditions. When theE ₀ gap of GaAs matchesħω _S orħω _L, the 2 LO and LO-phonon intensity, respectively, exhibit resonance Raman scattering maxima, at pressures determined byħω _L. With 647.1 nm radiation (ħω _L = 1.916 eV), a sharp and narrow resonance peak at 3.75 GPa is observed for the 2 LO-phonon. At this pressure the 2 LO-phonon goes through its maximum intensity, and falls right on top of the PL peak, revealing thatħω _S(2 LO) =E ₀. This is the condition for “outgoing” resonance. Experiments with other excitation energies (ħω _L) show, that the 2 LO resonance peak-pressure moves to higher pressure with increasingħω _L, and the shift follows precisely theE ₀ gap. Thus, the 2 LO RRS is an excellent probe to follow theE ₀ gap, far beyond the Γ-X cross-over point. A brief discussion of the theoretical expression for resonance Raman cross section is given, and from this the possibility of a double resonance condition for the observed 2 LO resonance is suggested. The LO-phonon resonance occurs at a pressure whenħω _L ≈E ₀, but the pressure-induced transparency of the GaAs masks the true resonance profile.     

Optical Characterization of Donor Doped Ternary Alloy Ga1-xNxAs and GaN-Ga1-xNxAs MQW by Using Infrared Fourier Transform Spectroscopy

In this work polarized infrared Fourier transform spectroscopy is employed to study the electronic and optical properties of doped Ga_1-xN_xAs ternary alloy and GaN-Ga_1-xN_xAs MQW. We have analyzed the far infrared spectra of GaN-GaNAs MQW by using a simple macroscopic theory base on effective medium approximation model. The dispersion curve of coupled LO phonon- plasmon modes were calculated from the polarized infrared reflectivity data. The GaNAs layer shows two-mode behavior in the infrared spectral range, a GaAs-like and a GaN-like sublattices. We detect the transverse optic phonon of GaN sublattice around 475 cm^-1. The origin of the sharp feature in p-polarization reflectivity about 300 cm^-1 as well as the dip at LO phonon frequency of GaAs sublattice are due to Brewster mode. The Brewster mode is couple strongly to plasmon mode. Attenuated total reflection spectroscopy has been used to excite and investigate surface plasmon and surface polariton.     

Electrical field analysis of metal‐surface plasmon resonance using a biaxially strained Si substrate

Electrical field components of metal‐surface plasmon resonance were analyzed in detail. Both longitudinal optical (LO) and transverse optical (TO) phonon modes of a biaxially strained Si layer can be excited by surface‐enhanced Raman spectroscopy (SERS). The z to y polarization ratio in SERS measurements was calculated to be 0.78 using the intensity ratio of TO to LO phonon modes. The electrical field components of SERS were also calculated by the finite‐difference time‐domain method. Copyright © 2014 John Wiley & Sons, Ltd.     

Raman scattering in the depletion region of GaAs

We have obtained spectra for the LO phonon mode by Raman scattering associated with surface quantization of the hole states in the depletion layer of highly doped n-type GaAs samples. When semi-transparent metal contacts are employed, the zone-center LO peak shifts to higher frequency due to plasmon-phonon interaction. The observed effects are not sensitive to the types of metal. The presence of the plasmon mode is thought to be due to the steady-state carrier injection from the incident laser light. The position of the peak is used to estimate the surface recombination rate of the excited electron-hole pairs.     

The evolution of magnetic,electrical and structural properties of nanogranular [FeCoBN/SiN]×n thin films

Some results concerning the magnetic, electrical and microstructural properties of multilayer [FeCoBN/Si₃N₄]×n films in view of their utilization for manufacturing thin film magnetic inductors are presented. A comparison between the magnetic, electrical and structural properties of FeCoBN and [FeCoBN/Si₃N₄]×n thin films is also reported. The [FeCoBN/Si₃N₄]×n thin films with the thickness of the FeCoBN layers varied from 10 to 30 nm, exhibit good soft magnetic characteristics and high values for electrical resistivity such as Ms of 172–185 A m²/kg, Hc of 318–1433 A/m and ρ of 82–48×10⁻⁷ Ω m, respectively. These physical properties of the samples are discussed in relation with the microstructure of the multilayer system.