MOVPE growth of strain-compensated 1300 nm In1−xGaxAsyP1−y quantum well structures

Different concepts for achieving strain-compensated quantum well structures emitting at 1300 nm have been investigated. Structures employing up to eight compressively strained wells with the same x in well and barrier exhibits excellent structural and optical properties, including very high photoluminescence efficiency. Increased number of quantum wells beyond 8 resulted in deteriorated materials quality, most likely due to accumulated strain-induced roughness of the growing surface. Good laser characteristics, including T₀ values of 64 K, were demonstrated for strain-compensated structures with tensile wells.     

Photocurrent measurements on GaAs1−xNx epilayers grown by metalorganic chemical vapor deposition

GaAs_1−xN_x epilayers were grown on a GaAs(0 0 1) substrate by metalorganic chemical vapor deposition. Composition was determined by high resolution X-ray diffraction. Band gap was measured from 77 to 400 K by using photocurrent measurements. The photocurrent spectra show clear near-band-edge peak and their peak energies drastically decrease with increasing nitrogen composition due to band gap bowing in the GaAs_1−xN_x epilayers. Those red shifts were particularly notable for low nitrogen compositions. However, the shifts tended to saturate when the nitrogen composition become higher than 0.98%. When the nitrogen composition is in the range 1.68–3.11%, the measured temperature dependence of the energy band gap was nicely fitted. However, the properties for the nitrogen composition range 0.31–0.98% could not be fitted with a single fitting model. This result indicates that the bowing parameter reaches 25.39 eV for low nitrogen incorporation (x=0.31%), and decreases with increasing nitrogen composition.     

Design and optimisation of AlxGa1−xAs/AlyGa1−yAs multilayer structures for visible wavelength applications

Quantum wells with excitonic features in the visible wavelength range were designed using the Al_xGa_1−xAs/Al_yGa_1−yAs material system, and grown using low pressure metal organic vapour phase epitaxy. Characterisation of these quantum well samples was carried out by using photovoltaic spectroscopy, photoluminescence, differential reflectance or photoreflectance techniques. These measurements showed that excitonic absorption in the 520–630 nm range could be achieved using these AlGaAs quantum wells.     

Compositional dependence of the optical gap in Ge1−xSx, Ge40−xSbxS60 and (As2S3)x(Sb2S3)1−x non-crystalline systems

An attempt is made to calculate the compositional dependence of the optical gap (E_g) in Ge_1−xS_x, Ge_40−xSb_xS₆₀ and (As₂S₃)_x(Sb₂S₃)_1−x non-crystalline systems in an alloy-like approach. From the comparison of both the experimental dependence of E_g and the calculated ones using the Shimakawa relation [E_{g AB} = YE_{g A} + (1−Y)E_{g B}] it is assumed that this equation is useful for such systems or parts of the systems which behave like almost ideal solutions.     

Electrical and structural properties of y(MnxFe3−xO4)−(1 − y)B2O3 glasses between 400 and 1000 K

The structure of silica glass was investigated by high-resolution electron microscopy. Observed dark field images were compared with those calculated by a kinematical electron diffraction theory on the basis of random and crystallite models. Bright small spots with sharp contrast were observed in the dark field image, which were found to originate from crystallites. It was indicated that small crystallites of about 17 Å in size were present in silica glass.     

XPS and electroluminescence studies on SrS1−xSex and ZnS1−xSex thin films deposited by atomic layer deposition technique

SrS_1−xSe_x and ZnS_1−xSe_x thin films were deposited by the atomic layer deposition (ALD) technique using elemental selenium as the Se source, thus avoiding use of H₂Se or organometallic selenium compounds. X-ray diffraction (XRD) analysis showed that the films were solid solutions and X-ray photoelectron spectroscopy (XPS) data showed that the surface of both ZnS_1−xSe_x and SrS_1−xSe_x were covered with an oxide and carbon-containing contaminants from exposure to air. The oxidation of SrS_1−xSe_x extended into the film and peak shifts from sulfate were found on the surface. Luminance measurements showed that emission intensity of the ZnS_1−xSe_x:Mn alternating current thin film electroluminescent (ACTFEL) devices at fixed voltage was almost the same as that of the ZnS:Mn device, while emission intensity of the SrS_1−xSe_x:Ce devices decreased markedly as compared to the SrS:Ce device. Emission colors of the devices were altered only slightly due to selenium addition.     

Photoluminescence excitation spectroscopy of GaP1−xNx alloys: conduction-band-edge formation by nitrogen incorporation

We have investigated the conduction-band-edge formation in GaP_1−xN_x alloys by nitrogen incorporation into GaP using photoluminescence excitation (PLE) spectroscopy. The PLE spectra of GaP_1−xN_x alloys with various nitrogen concentrations show that the absorption edge shifts to lower energies with increasing nitrogen concentration. From the nitrogen concentration dependence of the absorption edge energy we found that the conduction-band-edge formation in GaP_1−xN_x alloys originates from the formation of the A line.     

InxGa1−xAs/GaAs quantum wire structures grown on GaAs (100) patterned substrates with [001] ridges

In_xGa_1−xAs/GaAs (x = 0.12-0.23) quantum well (QW) structures were grown by molecular beam epitaxy (MBE) on [001] ridges with various widths (1.1-12 μm) of patterned GaAs (100) substrate. The smallest lateral width of the InGaAs/GaAs quantum wire (QWR) structures was estimated to be about 0.1 μm by high-resolution scanning electron microscope (SEM). The In contents of the grown InGaAs/GaAs QWs on the ridges were studied as a function of ridge top width (ridge width of the MBE grown layer) by cathodoluminescence (CL) measurements at 78 K. Compared to the InGaAs QW grown on a flat substrate, the In content of the InGaAs/GaAs QW on the ridge increases from 0.22 to 0.23 when the ridge top width decreases to about 2.9 μm, but it decreases steeply from 0.23 down to 0.12 with a further decrease of the ridge width from 2.9 to 0.05 μm. A simulation of MBE growth of InGaAs on the [001] ridges shows that this reduced In content for narrow ridges is due to a large migration of Ga atoms to the (100) ridge top region from {110} side facets.     

Formation mechanism of InxGa1−xAs bridge layers on patterned GaAs substrates

The formation mechanism of In_xGa_1−xAs (x=0.06) bridge layers on patterned GaAs (1 1 1)B substrates using liquid-phase epitaxy has been investigated. For this (i) the effect of density gradient in the solution on the formation of bridge layer and (ii) growth of bridge layer on 1 1 0 line-seed-substrates were studied. The convection induced by destabilizing density gradient in the solution led to an increase of lateral growth rate of the InGaAs bridge layers on a substrate mounted on the upper portion of the solution. However, it did not have any significant effect on the formation of the bridge layers. The formation of bridge layer on 1 1 0 line-seed-substrate took place only for the {1 1 1}B growth fronts, which indicated that “Berg effect” is responsible for the formation of bridge layers.     

Effect of Asi on the optical properties of Ga1−xInxAs/InP grown by molecular beam epitaxy

This report focuses on the effect of the As species and the V/III ratio on the optical properties of Ga_1−xIn_xAs/InP grown by molecular beam epitaxy (MBE). The band gap energies of the layers were measured by low temperature photoluminescence (PL) while the indium contents x were determined by X-ray for samples in the investigated range of 0.50<x<0.56. For the analysis of these data we considered the model of Kuo et al. which we confined with a correction for the different measurement temperatures in PL (4.2 K) and X-ray (300 K). Using As₄ with an effective V/III ratio higher than 1.3, we find the best agreement of the band gap energies and predictions of the theory. A lower V/III ratio always implies a reduction of the band gap energy to values 5-15 meV lower than expected. In contrast, using As₂ the PL data fit quite well independent of the effective V/III ratio above unity.     

MBE growth of cubic AlyGa1−yN/GaN heterostructures—structural, vibrational and optical properties

Cubic Al_yGa_1−yN/GaN heterostructures on GaAs(0 0 1) substrates were grown by radio-frequency plasma-assisted molecular beam epitaxy. High resolution X-ray diffraction, micro-Raman, spectroscopic ellipsometry, and cathodoluminescence measurements were used to characterize the structural, optical and vibrational properties of the Al_yGa_1−yN epilayers. The AlN mole fraction y of the alloy was varied between 0.07<y<0.20. X-ray diffraction reciprocal space maps demonstrate the good crystal quality of the cubic Al_yGa_1−yN films. The measured Raman shift of the phonon modes of the Al_yGa_1−yN alloy was in excellent agreement with theoretical calculations. Both SE and CL of the Al_yGa_1−yN epilayer showed a linear increase of the band gap with increasing Al-content.     

Photoluminescence characterization of GaxIn1−xAs (0 ≤ x ≤ 0.32) strained quantum wells grown on InP by chemical beam epitaxy

We have investigated some characteristics of Ga_xIn_1−xAs/InP (x = 0, 0.2, 0.32) strained materials by growing multi-quantum wells with various well widths from 3 to 60 Å by chemical beam epitaxy. Growth conditions such as valve sequences and growth interruptions were optimised to have single-peaked photoluminescence spectra for all strained samples. Measured room temperature emission wavelengths were compared with theoretically estimated values. Photoluminescence linewidths were around 16 meV for most samples at 77 K, but broadened for well widths of less than 20 Å. Photoluminescence intensity also peaked at well width of 20 Å for all compositions. Performances of optical devices may be closely related to this “critical thickness” determined probably by the growth system limitation.     

Raman scattering and photodarkening of amorphous Ge1−XSX (0X0.62) films

Amorphous Ge_1−XS_X films with Ge-rich composition of 0X0.62 were prepared by the laser ablation method to investigate the effect of sulfur on optical properties. From Raman spectra, it is found that the structure changes gradually with composition X. The change of properties from tetrahedral to chalcogenide semiconductors was examined in terms of photodarkening phenomena. As X increased, the photodarkening effect began to be observed at X=0.2 and enhanced significantly at X=0.5.     

Annealing effects of highly homogeneous a-Si1−xCx:H

We report the effect of annealing on the properties of amorphous hydrogenated silicon carbide thin films. The samples were deposited onto different substrates by plasma enhanced chemical vapor deposition at temperatures between 300 and 350 °C. The gaseous mixture was formed by silane and methane, at the ‘silane starving plasma regime’, and diluted with hydrogen. Rutherford backscattering and Fourier transform infrared spectrometry were used, respectively, to determine the atomic composition and chemical bonds of the samples. The film’s structure was analyzed by means of X-ray absorption fine structure and X-ray diffraction. For temperatures higher than 600 °C, amorphous silicon carbide films annealed under inert atmosphere (Ar or N₂) clearly changed their structural and compositional properties due to carbon loss and oxidation, caused by the presence of some oxygen in the annealing system. At 1000 °C, crystallization of the films becomes evident but only stoichiometric films deposited on single crystalline Si[1 0 0] substrates presented epitaxial formation of SiC crystals, showing that the crystallization process is substrate dependent. Films annealed in high-vacuum also changed their structural properties for annealing temperatures higher than 600 °C, but no traces of oxidation were observed or variations in their silicon or carbon content. At 1200 °C the stoichiometric films are fully polycrystalline, showing the existence of only a SiC phase. The XANES signal of samples deposited onto different substrates and annealed under high-vacuum also show that crystallization is highly substrate dependent.     

Molecular beam epitaxy of strained Si1−xGex layers on patterned substrates

Epitaxial Si_1-xGe_x layers have been grown on patterned Si(001) substrates. Mesa-like structures of 1.4 μm height on the surface limited by inclined {111} planes were used. Structure dimensions between 10 and 0.2 μm were chosen to allow the mesas to relieve elastically under the strained layer. The film growth, the crystallographic perfection and the relaxation of strained Si_1-xGe_x layers were investigated by transmission electron microscopy (TEM) including in situ annealing. The relaxation mechanism and the control of dislocation generation on top of the mesas are discussed.     

Electronic traps in mixed Si1−xGexO2 films

Thermally stimulated luminescence (TSL) and infrared (IR) spectroscopy were measured in plasma grown Si_1−xGe_xO₂ (x=0, 0.08, 0.15, 0.25, 0.5) with different thicknesses (12–40 nm). A comparison with the TSL properties of thermally grown SiO₂ and GeO₂ was also performed. A main IR absorption structure was detected, due to the superposition of the peaks related to the asymmetric O stretching modes of (i) Si–O–Si (at ≈1060 cm⁻¹) and (ii) Si–O–Ge (at 1001 cm⁻¹). Another peak at ≈860 cm⁻¹ was observed only for Ge concentrations, x>0.15, corresponding to the asymmetric O stretching mode in Ge–O–Ge bonds. A TSL peak was observed at 70°C, and a smaller structure at around 200°C. The 70°C peak was more intense in all Ge rich layers than in plasma grown SiO₂. Based on the thickness dependence of the signal intensity we propose that at Ge concentrations 0.25x0.5 TSL active defects are localised at interfacial regions (oxide/semiconductor, Ge poor/Ge rich internal interface, oxide external surface/atmosphere). Based on similarities between TSL glow curves in plasma grown Si_1−xGe_xO₂, thermally grown GeO₂ and SiO₂ we propose that oxygen vacancy related defects are trapping states in Si_1−xGe_xO₂ and GeO₂.     

Small-angle X-ray scattering studies of a-Si1−xGex: H alloys

Small-angle X-ray scattering (SAXS) is used to study a-Si:H, a-Ge:H, and a-Si_1−xGe_x: H alloys prepared by plasma-enhanced CVD methods. These amorphous semiconductors show various types of scattering characteristics related to their composition, deposition method, and deposition conditions. For poor quality alloys at low scattering vectors, h, the scattering curve shows an I(h)h⁻² dependence, because of the fractal nature of the inhomogeneities present. Also, a-Ge:H shows these features. Therefore, it is concluded that the inhomogeneities arise principally from clustering of Ge atoms.     

Elastic properties of GexAsySe100−x−y glasses

The elastic properties of Ge_xAs_ySe_100−x−y (0x30; 10y40) glasses have been studied. The results were analyzed in terms of the dependence on the theoretical mean coordination number (mean number of covalent bonds per atom) m (m=2+(2x+y)×0.01). Three ranges of m (2.1m2.51, 2.51<m2.78, 2.78<m3) were revealed, where different dependencies of elastic moduli (Young’s modulus, shear modulus) and Poisson’s ratio of glasses on m were observed.     

Effects of alloy composition on the As desorption from and adsorption on strained InxGa1−xAs surfaces

The specular reflectivity of strained In_xGa_1−xAs surfaces grown by molecular beam epitaxy on InAs (100) substrates is measured with reflection high-energy electron diffraction (RHEED). A discontinuous change in the surface reflectivity is observed as the substrate temperature is increased above the transition point where As desorbs from the surface. A clear hysteresis loop is revealed as the substrate temperature is decreased. The substrate temperature required for desorption of surface As increases with Ga composition. A comparison between experimental results and theoretical calculations based on a Monte Carlo simulation shows that the average vertical interaction is increasing with Ga fraction. Fluctuations in alloy composition across the surface result in In-rich domains from which As is preferentially desorbed. The sudden loss of As, corresponding to a first order phase transition, occurs when the As desorbed domains attain a critical size. The metastability of the phase transition is shown to be a minimum for In_0.5Ga_0.5As layers.