Anomalous behavior of excess energy curves of InxGa1−xN grown on GaN and InN

We worked out the excess energies for bulk In_xGa_1−xN and In_xGa_1−xN thin films on GaN and InN in order to investigate their thermodynamic stabilities. It has been found that the excess energy maximum shifted toward x0.80 for InGaN/GaN and x0.10 for InGaN/InN due to the lattice constraint in contrast with x0.50 for bulk. Moreover, it has been revealed that the excess energy for InGaN/GaN is larger than that for bulk at x>0.65. This suggests that In-rich films are less stable on GaN than bulk state. These results indicate that the lattice constraint has a significant influence on thermodynamic stabilities of thin films.     

Cd1−xZnxTe: Growth and characterization of crystals for X-ray and gamma-ray detectors

The choice a suitable crystal growth method and a reasonable x value is of profound importance in the preparation of high quality Cd_1−xZn_xTe crystals for x-ray and gamma-ray detectors. The present paper reviews the evolution and development of Cd_1−xZn_xTe crystal growth for x-ray and gamma-ray detectors. At the same time, emphasis is put upon finding the relationship between the x value and the quality of the Cd_1−xZn_xTe. Three sets of Cd_1−xZn_xTe ingots with different x values, specifically 0.10, 0.15, and 0.20 were grown by the vertical Bridgman method (VBM) and characterized. Their x specification was then correlated with their dislocation densities, Te precipitates, inclusions, IR transmission, resistivities, and impurity concentrations, respectively. It was found that VBM Cd_0.85Zn_0.15Te as grown in this paper possessed the best choice of qualities with respect to defects and impurities.     

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.     

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.     

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.     

Laser-induced structural transformation of AsxSe1-x thin amorphous films

The changes in the optical and holographic recording properties of amorphous As_xSe_1-x films with 0 ≤ x ≤ 0.2 under laser irradiation have been measured. The dependence of the transmissivity and diffraction efficiency on the irradiation energy density shows two qualitatively different regions. Below an energy density threshold, E_th, only small changes in the local structure of the system can be detected. In this low-E region, the transmissivity varies reversibly with exposure. Qualitative explanation of the observed behavior may be based on associating such with alteration of deep defect states. Above E_th, the changes were attributed to crystallization transformation. The Raman spectra reveal the progressive transformation of the system from amorphous into the crystalline phase under laser irradiation.     

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.     

Compositional control of CdxZn1−xSe grown by photoassisted organometallic vapor phase epitaxy

The photoassisted OMVPE growth technique is important for the fabrication of blue/green laser diodes based on Cd_xZn_1−xSe quantum wells. Low temperature growth with photoassistance is key to the fabrication of these devices, however, the compositional control of Cd_xZn_1−xSe becomes increasingly difficult as the growth temperature is reduced. We have studied the compositional control of Cd_xZn_1−xSe using the sources DMCd, DMZn, and DMSe, with irradiation from a Hg arc lamp. We studied the dependence of the composition on the growth temperature, irradiation intensity, and source mass flows. The composition x increases with increasing temperature and decreases with increasing irradiation intensity. The solid-phase composition is a non-linear function of the gas-phase composition X. The slope of this characteristic, dx/dX, should be minimized for good compositional control. At 475°C without photoassistance, dx/dX is 1.75 near a composition of 20%, as determined from the data of Parbrook et al. Decreasing the temperature increases dx/dX. At 370°C with 12 mW/cm², dx/dX ≈ 13 and at 350°C with 58 mW/cm² dx/dX ≈ 60. We have investigated this behavior at 370°C with 12 mW/cm² irradiation by studying both the composition and the growth rate as a function of the gas-phase composition. The growth rate is non-monotonic, and is minimum for a gas-phase composition of 0.20. The behavior is quite complex, and is not fully understood at the present time. Nonetheless, our results indicate that the Cd-bearing precursor is adsorbed much more strongly than the Zn-bearing precursor. In addition to this, the introduction of the DMCd strongly inhibits the growth of ZnSe. We have achieved sufficiently good compositional control at 370°C and 12 mW/cm² to grow ZnSe/Cd_xZn_1−xSe/ZnSe multiple quantum well structures. More work is necessary in order to clarify the roles of irradiation intensity and VI/II ratio so that good compositional control can be achieved at lower growth temperatures.     

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.     

Electronic conduction in vitreous semiconductors in the pseudo-binary system (As2S3)1-x(PbS)x

AC conductivity and dielectric relaxation measurements of the bulk amorphous compositions in the pseudo-binary system (As₂S₃)_1-x(PbS)_x (x = 0, 0.1, 0.4 and 0.5) in the frequency range 500 Hz-10 kHz and in the temperature span 180–450 K are reported. The temperature dependence of the ac conductivity, σ_ac(ω), has a broad structure at all frequencies in compositions with x = 0.1, 0.4 and 0.5 whose peak position is not thermally activated. A similar structure was also observed in the data on the dielectric constant, ε₁, which peaked at a frequency of 1 kHz in the composition with x = 0.5. Analysis of the results using the correlated barrier hopping model revealed that the electronic conduction takes place by single polaron and bipolaron hopping processes at high and low temperatures, respectively, in compositions containing Pb. The microstructure and phase-separation in the glasses containing Pb influence the electrical transport and dielectric dispersion. This study has revealed the possible presence of a phase transformation at around 300 K at a frequency of 1 kHz in the dielectric dispersion behaviour of composition with x = 0.5.     

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.     

The lattice constants of ternary and quaternary alloys in the PbTe–SnTe–MnTe system

The study of structural properties of quaternary Pb_1−x−ySn_xMn_yTe alloys revealed that they crystallise in a cubic structure of NaCl-type for a wide region of manganese content (up to y=0.16). It was found from X-ray measurements and microprobe analysis that Vegard's law is obeyed. The dependencies of the lattice constants on composition for Pb_1−x−ySn_xMn_y, Pb_1−yMn_yTe and Sn_1−yMn_yTe were used to extract the lattice parameter of NaCl-type MnTe phase by extrapolation. The experimental data were also used to obtain an improved estimate of the covalent octahedral radius for Mn.     

New aluminium precursors for MOMBE (CBE): a comparative study

Recently different new Al precursors have been developed to improve the electrical and optical quality of AlGaAs layers grown by MOMBE (CBE), since AlGaAs layers still suffer from the high incorporation of oxygen and carbon. Three approaches are introduced and results obtained from Al_xGa_1−xAs layers (0 < x ≤ 1) are discussed. APAH, a double ring structure molecule, was found to yield AlGaAs layers with high contents of carbon and nitrogen. The use of an Alane-adduct decreases impurity concentrations and improves optical properties. However, TIBAl is superior and provides highest PL response together with carrier concentrations below p = 10¹⁶ cm^-3. Even though the concept of coordinative saturation is promising, results achieved by TIBAl showed that trialkyls could also be well suited for AlGaAs, assuming that they are properly synthesized.     

Physical properties of amorphous Ge20Sb25−xBixSe55 thin films

The influence of substitution of Sb atoms by Bi atoms on the electrical and optical properties of thin films of the Ge₂₀Sb_25−xBi_xSe₅₅ [0x15] system are reported. Results of dc conductivity and thermoelectric power measurements between 150 and 450 K show that the Ge---Sb---Se system is chemically modified by addition of large concentrations of Bi atoms between X=5 and X=10 at.%). A transition from p-type for Sb-doped to n-type for Bi-doped films and a decrease of resistivity is observed. The absorption edge shifts to shorter wavelength, thereby decreasing the optical band gap of the system. Compositional dependences of electrical conductivity, thermoelectric power, and the appearance of n-type conduction are discussed from the stand point of chemical bonds formed in the films and related to the defect states produced due to incorporation of Bi atoms in high concentrations. The coexistence of band and hopping conduction is proposed. The ac conductivity in 0.1–10.0 kHz frequency and 150–450 K temperature range was found to obey a power law σ(ω, T) = Aω^s. The results were interpreted in terms of Elliott's theory, which assumes correlated barrier hopping (CBH) between the charged defect centres. It was found that computed results from the CBH model and experimental one are qualitative agreement for the present materials.     

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.     

Elastic constant anomaly in Fe80−xMoxB20 metallic glasses

The extensional sound velocities V_E of Fe_80−xMo_xB₂₀(x = 6, 7, 8) metallic glasses have been measured between 100 and 600 K in a saturating magnetic field. Young's modulus (E = _E², = density) shows anomalous temperature dependence at and below the magnetic transition temperature of each glass studied. These anomalies are interpreted as arising from the change of long-range spin coupling energy at the Curie temperature in a molecular field approximation. Comparison is made with similar effects in other crystalline and non-crystalline magnetic materials.     

Heavily carbon-doped p-type (In)GaAs grown by gas-source molecular beam epitaxy using diiodomethane

Heavily carbon-doped p-type In_xGa_1−xAs (0≤x<0.49) was successfully grown by gas-source molecular beam epitaxy using diiodomethane (CH₂I₂), triethylindium (TEIn), triethylgallium (TEGa) and AsH₃. Hole concentrations as high as 2.1×10²⁰ cm⁻³ were achieved in GaAs at an electrical activation efficiency of 100%. For In_xGa_1−xAs, both the hole and the atomic carbon concentrations gradually decreased as the InAs mole fraction, x, increased from 0.41 to 0.49. Hole concentrations of 5.1×10¹⁸ and 1.5×10¹⁹ cm⁻³ for x = 0.49 and x = 0.41, respectively, were obtained by a preliminary experiment. After post-growth annealing (500°C, 5 min under As₄ pressure), the hole concentration increased to 6.2×10¹⁸ cm⁻³ for x = 0.49, probably due to the activation of hydrogen-passivated carbon accepters.     

Low-temperature growth of epitaxial layers of ZnSe1−xTex solid solutions

The layers of ZnSe_1−xTe_x (0 < x < 1.0) solid solutions have been grown by liquid-phase epitaxy in a closed tube at 620–680 °C. Zinc chloride served as a solvent. ZnTe and ZnSe crystals were used as sources and substrates with orienting surfaces (110) and (111) for ZnSe and (110) for ZnTe. The composition of the grown layer was specified by the relative content of the ZnSe and the ZnTe in the solution and was controlled by X-ray analysis. The position of the exciton bands in the photoluminescence spectra of ZnSe_1−xTe_x over the interval 0.3 < x < 1.0 is in agreement with the free exciton energies calculated for these compositions. Relatively low-ohmic (of about 10² Ω cm) epitaxial layers of ZnSe_1−xTe_x solid solutions were grown.     

Characterization and growth of ZnSTe epilayers by hot-wall epitaxy

ZnS_1−xTe_x epilayers were grown on GaAs(1 0 0) substrates by hot-wall epitaxy in a wide range of Te composition. The Te composition was determined by Rutherford backscattering spectrometry and the lattice constant was measured by double-crystal rocking curve. It was found that the lattice of the epilayer matches well with that of the substrate at x=0.37 as expected by Vegard's rule, and the energy gap was also determined as a function of Te composition by spectrophotometer. It showed that a quadratic relation with the composition: E_g(x)=3.71−5.27x+3.83x². Photoluminescence characteristics were also studied.     

Indium content determination related with structural and optical properties of InGaN layers

We studied the structural and optical properties of a set of nominally undoped epitaxial single layers of In_xGa_1−xN (0<x0.2) grown by MOCVD on top of GaN/Al₂O₃ substrates. A comparison of composition values obtained for thin (tens of nanometers) and thick (≈0.5 μm) layers by different analytical methods was performed. It is shown that the indium mole fraction determined by X-ray diffraction, measuring only one lattice parameter strongly depend on the assumptions made about strain, usually full relaxation or pseudomorphic growth. The results attained under such approximations are compared with the value of indium content derived from Rutherford backscattering spectrometry (RBS). It is shown that significant inaccuracies may arise when strain in In_xGa_1−xN/GaN heterostructures is not properly taken into account. Interpretation of these findings, together with the different criteria used to define the optical bandgap of In_xGa_1−xN layers, may explain the wide dispersion of bowing parameters found in the literature. Our results indicate a linear, E_g(x)=3.42−3.86x eV (x0.2), “anomalous” dependence of the optical bandgap at room temperature with In content for In_xGa_1−xN single layers.