Study of optical and electrical properties of AlxGa1−xSb grown by metalorganic chemical vapor deposition

Al_xGa_1−xSb films in the regime 0×0.25 have been grown by metalorganic chemical vapor deposition on GaAs and GaSb substrates using TMAl, TMGa, and TMSb precursors. We report growth conditions and film properties, including the effect of V/III ratio and growth temperature on electrical and optical properties. Growth temperatures in the range of 520°C and 680°C and V/III ratios from 1 to 5 have been investigated. All epilayers grown exhibit p-type behavior. The mobility decreases and the carrier concentration increases sharply when a small amount of Al is incorporated into GaSb. The sharp cutoff and Fabry-Perot oscillations of the transmission spectra of the AlGaSb layers confirm the high quality of the films. The principle photoluminescence features observed are attributed to bound exciton and donor-acceptor transitions with FWHM comparable to the best values reported elsewhere.     

Structural characterization of Zn1−xCdxO (0≤x≤0.20) microrods grown by spray pyrolysis

Zn_1−xCd_xO (x= 0.00, 0.05, 0.10, 0.15 and 0.20) thin films were obtained by spray pyrolysis and characterized by XRD, SEM, EDAX and optical measurements. The Zn_1−xCd_xO microrods are in the wurtzite crystallographic phase with (0 0 2) preferred orientation. A narrowing of the fundamental band gap from 3.30 to 3.10 eV was observed with the increasing nominal Cd content up to 20 at% due to the direct modulation of the band gap caused by Cd substitution. The undoped ZnO film showed two emission bands in the spectra: one sharp UV luminescence at ∼382 nm and one broad visible emission ranging from 430 to 600 nm. The sharp peak at ∼382 nm is split into two at 376 and 400 nm upon Cd doping at levels of 5 and 10 at%. However this splitting is not observed in the doped ZnO samples containing 15 at% Cd and more. It should also be mentioned that the broad peak at the range of 430–600 nm has almost disappeared in the films containing 5, 10 and 15 at% Cd.     

Structural and optical properties of GaxIn1−xP layers grown by chemical beam epitaxy

Chemical beam epitaxial (CBE) Ga_xIn_1?xP layers (x≈0.5) grown on (001) GaAs substrates at temperatures ranging from 490 to 580°C have been investigated using transmission electron diffraction (TED), transmission electron microscopy, and photoluminescence (PL). TED examination revealed the presence of diffuse scattering 1/2{111}_B positions, indicating the occurrence of typical CuPt-type ordering in the GaInP CBE layers. As the growth temperature decreased from 580 to 490°C, maxima in the intensity of the diffuse scattering moved from ½{111}_B to ½{?1+δ,1?δ,0} positions, where δ is a positive value. As the growth temperature increased from 490 to 550°C, the maxima in the diffuse scattering intensity progressively approached positions of $\frac{1}{2}\{\bar 110\} $ , i.e., the value of δ decreased from 0.25 to 0.17. Bandgap reduction (～45 meV) was observed in the CBE GaInP layers and was attributed to the presence of ordered structures.     

High purity InxGa1−xSb single crystals with cutoff wavelength of 7–8 µm grown by melt epitaxy

For the first time, InGaSb single crystals with a cutoff wavelength of 7–8 μm were successfully grown on GaAs substrates by a new growth technique named melt epitaxy. The band gap of InGaSb layers obviously narrowed compared with those with the same compositions grown by ordinary methods and the longest cutoff wavelength reached 8.3 μm. High electron mobility of 8.05×10⁴ cm²/Vs and low carrier density of 1×10¹⁵ cm⁻³ at 77 K were obtained indicating high purity of InGaSb epilayers.     

Fabrication and characterization of Pb1−xYbxTe-based alloy thin-film thermoelectric generators grown by thermal evaporation technique

In this study p-Pb_0.925Yb_0.075Te:Te and n-Pb_0.94Yb_0.06Te powders synthesized by solid-state microwave technique were used to fabricate thermally evaporated thin films. The nanostructure and composition of the films were studied using X-ray diffraction (XRD), field emission scanning electron microscopy (FESEM) and energy dispersive X-ray spectroscopy (EDX). Electrical characterizations of the as-deposited films in terms of the Seebeck coefficient and electrical conductivity and power factor were conducted at a range of 298 K to 523 K. The microthermoelectric devices were composed of 20-pair and 10-pair p-Pb_0.925Yb_0.075Te:Te and n-Pb_0.94Yb_0.06Te thin films on glass substrates. The dimensions of the thin-film thermoelectric generators, which consisted of 20-pair and 10-pair legs connected by aluminum electrodes, were 23 mm×20 mm and 12 mm×10 mm, respectively. The 20-pair p–n thermocouples in series generated a maximum open-circuit voltage output (V_oc) of 0.581 V and a maximum output power of 25.87×10^?8 W at a temperature difference ΔT=164 K, whereas the 10-pair p–n thermocouples generated 0.311 V and 13.71×10^?8 W maximum V_oc and maximum output power, respectively, at ΔT=164 K.     

Organometallic VPE growth of AlxGa1−xAs

The " hybrid" organometallic VPE process for the growth of Al_xGa_l-xAs has been explored. Six growth parameters have been considered; substrate orientation, substrate temperature during growth, total flow rate and ratios of Al, HC1 and As to the total group III flow rate. The effects of these six growth parameters on the growth process (growth rate, composition and surface morphology) and the materials properties (carrier concentration, photoluminescence intensity and spectrum) have been systematically studied. A technique has been developed for the growth of heterostructures by changing only the H₂ diluent flow rate. This results in high quality heterostructures with x changing from 0.05 to 0.30 in > 100 å.     

Structural characterization and properties of nano-sized Cd1−xCoxO dilute magnetic semiconductors prepared by solvothermal method

Nanoparticles of Co-doped CdO with 4.2, 9.3, and 15.4 weight% were successfully synthesized by a modified solvothermal method through oxalate precursor route. The oxalate served as an excellent precursor for the synthesis of pure and homogeneous oxide nanoparticles. Powder X-ray diffraction studies revealed the monophasic nature of these nanoparticles. The lattice parameter decreased monotonically with increasing Co concentration, confirming Co was doped into the CdO host lattice. Particle size decreased and surface area increased on increasing the dopant ion concentration. Optical absorption measurements showed an energy band gap that increased with Co concentration. Magnetization studies indicated that the samples showed paramagnetic behavior with weak antiferromagnetic interactions.     

High C-doping of MOVPE grown thin AlxGa1−xAs layers for AlGaAs/GaAs interband tunneling devices

High hole concentrations in LP-MOVPE grown GaAs and AlGaAs layers can be achieved by intrinsic C-doping using TMGa and TMAl as carbon sources. Free carrier concentrations exceeding 10²⁰ cm⁻³ were realised at low growth temperatures between 520–540°C and V/III ratios <1.2. The C-concentration increases significantly with the Al-content in Al_xGa_1−xAs layers. We observed an increase in the atom- and free carrier concentration from 5·10¹⁹ cm⁻³ in GaAs to 1.5·10²⁰ cm⁻³ in Al_0.2Ga_0.8As for the same growth conditions. Interband tunneling devices with n-type Si and p-type C-doped AlGaAs layers and barriers made of Al_0.25Ga_0.26In_0.49P have been investigated.     

Ellipsometric study of ultrathin AlxGa1−x As layers

A method for rapid monitoring of the parameters of thin-layer semiconductor structures by ellipsometry is proposed. The results of ellipsometric analysis of the material thickness and composition distribution in Al_xGa_1?x As films grown by low-temperature liquid-phase epitaxy (LPE) are presented. The ellipsometric data are compared to those obtained by the Raman scattering spectroscopy.     

Properties of epitaxial (Al x Ga1 − x As)1 − y C y alloys grown by MOCVD autoepitaxy

The growth of epitaxial Al _x Ga_{1 ? x} As:C alloys by metal-organic chemical vapor deposition (MOCVD) at low temperatures results in the formation of quaternary (Al _x Ga_{1 ? x} As)_{1 ? y} C _y alloys, in which carbon atoms can be concentrated at lattice defects in the epitaxial alloy with the formation of impurity nanoclusters.     

多片LPE生长AlxGa1—xAs／GaAs单晶薄膜

介绍了一种新的“分离三室水平推挤式多片外延舟”，可在国产水平外延炉中实现多片单晶薄膜外延。同时，研究了适合于多片外延的各种工艺条件。用于研制的ｐ－ＡｌｘＧａ１－ｘＡｓ／ｐ－ｎ－ｎ＾＋－ＧａＡｓ太阳通电池，ＡＭ０．２５℃，１２０ｍＷ．ｃｍ＾－２的转换室１９．８％。     

AlxGa1−xN/GaN/AlN heterostructures grown on Si(1 1 1) substrates by MBE for MSM UV photodetector applications

Al_xGa_1−xN/GaN/AlN heterostructures on silicon (Si) substrate was developed by nitrogen plasma-assisted molecular beam epitaxy (MBE) and their properties were investigated by scanning electron microscopy (SEM), electron dispersive X-ray (EDX), atomic force microscopy (AFM), high resolution X-ray diffraction (XRD), Raman spectroscopy and Hall effect measurements. High purity gallium (7N) and aluminum (6N5) were used in the Knudsen cells. High purity nitrogen with 7N purity was supplied to radio frequency (RF) source to generate reactive nitrogen species. The nitrogen pressure and a discharge power were kept at 1.5×10⁻⁵ Torr and 300 W, respectively. From SEM measurements, the surface morphology of samples presented 2- and 3-dimensional growth modes. The EDX measurements showed that there were no foreign elements in the grown samples. The HR-XRD measurement has confirmed that the Al_xGa_1−xN/GaN/AlN heterostructures samples were epitaxially grown on Si substrate. All the dominant E₂ phonon modes were found in Raman spectra results. Lastly, Al_xGa_1−xN/GaN/AlN heterostructures based metal–semiconductor–metal (MSM) UV photodetectors were fabricated and the electrical characteristics of the devices were investigated by using current–voltage (I–V) and photo-conductivity measurements. The devices presented good I–V and photoconductivity characteristics.     

Structural and Thermoelectric Properties of Bi1−x Sb x Nanoalloys Prepared by Mechanical Alloying

Bi_1?x Sb _x nanoparticles were prepared by mechanical alloying and compacted using different techniques. The influence of the composition as well as the pressing conditions on the thermoelectric performance was investigated. A strong dependence of the thermoelectric properties on the composition was found, which deviates from the behavior of single crystals. The results indicate a significant change in the band structure of the material induced by the reduced size. The influence of the pressing conditions on the thermoelectric properties also showed composition dependence. The results show that the compacting method has to be chosen carefully.     

Structural and optical properties of ZrB2 and HfxZr1−xB2 films grown by vicinal surface epitaxy on Si(1 1 1) substrates

ZrB₂ and Hf_xZr_1?xB₂ films were grown on 4° miscut Si(1 1 1) substrates by chemical vapor deposition of gaseous Hf(BH₄)₄ and Zr(BH₄)₄. The films display superior structural and optical properties when compared with ZrB₂ films grown on on-axis Si(1 1 1). The observed improvements include an optically featureless surface with rms roughness of ～2.5–3.5 nm, a ～50% reduction in the amount of residual strain, and a ～50% lower resistivity. These properties should promote the use of diboride films as buffer layers for nitride semiconductor epitaxy on large-area Si substrates.     

Temperature and hydrostatic pressure dependence of the electronic structure of AlxGa1−xAs alloys

The study of the electronic band structure of Al_xGa_1−-xAs alloy is calculated within the local empirical pseudo-potential method including the effective disorder potential into the virtual crystal approximation. Monotonic decreasing and increasing functions are obtained for the temperature and pressure dependent form factors, respectively. Some physical quantities as band gaps, bowing parameters, refractive indices, and dielectric constants of the considered alloy with different Al concentration are calculated under the effects of temperature and hydrostatic pressure. The obtained results have been found in good agreement with the experimental and published data.     

Effects of mesa area and orientation on defects in strained InxGa1−xAs films grown by LPOMCVD

We have selectively grown In_xGa_1?xAs (0.04 <x < 0.20, 200 ? 7000Å) on rectangular growth areas (100 μm by 200 mil) patterned on GaAs substrates with very low etch pit densities (～200 cm^?2). The edge orientations of the growth areas were varied on the substrate resulting in distinct facet formation on the deposited mesa structures. Layers were grown using Low-Pressure Organometallic Chemical Vapor Deposition (LPOMCVD) and all samples were annealed for 1 hr at 700° C. Indium content and film thicknesses were very uniform across a wafer so that only the facet shape was varied. Observation of Crosshatch defect densities as a function of growth window orientation showed that 2900Å In_0.08Ga_0.92As mesas with [010] edge orientations exhibited no Crosshatch defects. Identical mesas grown in windows with other edge orientations exhibited varying crosshatch defect densities. Large unpatterned areas of growth were heavily crosshatched. This variation in Crosshatch defect density as a function of mesa orientation appears to be associated with non-area related dislocation nucleation mechanisms at the mesa edges. Proper choice of window orientation for patterned substrate epitaxy will allow the mesa facets to be controlled thereby reducing misfit dislocations in the heterostructure interface.     

Design of AlxGa1−xAs/GaAs/InyGa1−yAs triple junction solar cells with anti-reflective coating

Multi-junction solar cells (SC) made from III–V compound semiconductors are still in the development phase. Here, we perform calculations for multi-junction cells: Al_xGa_1−xAs top junction, GaAs middle junction and In_yGa_1−yAs bottom junction (all of these materials with band-gaps between 2.1 and 0.8 eV) in order to obtain the optimal band gap and thickness for each junction under the AM1.5 solar radiation spectrum. The ideal photo-current density is around 15.5 mA/cm². In order to reduce the natural reflectivity, an anti-reflective coating (ARC) was chosen, based on a MgF₂/ZnS double layer, allowing for a significant increase of the current density with respect to a cell without it. Calculations of external quantum efficiency (QE) were also performed for the three cases mentioned above: ideal one, taking into account the total reflection and with the ARC double layer. Finally, when more realistic calculations are done, taking into account the carrier recombination at each sub-cell, and the light reflection for a tandem cell with the designed ARC on top, the expected conversion efficiency (η), under the AM 1.5 spectrum (without concentration), was determined to be around 38.5%, making this an attractive III–V compound tandem cell to be investigated in the near future.