Characterization of AlxGa1−xAs layers grown on (100) GaAs by metallic-arsenic-based-MOCVD

We present the electrical and structural characterization of Al_xGa_1−xAs layers grown in a metallic-arsenic-based-MOCVD system. The gallium and aluminium precursors were the metal-organic compounds trimethylgallium (TMGa) and trimethylaluminium (TMAl), respectively. Al_xGa_1−xAs layers that were grown at temperatures less than 750 °C present a high electrical resistivity. Independent of the used III/V ratio the samples that were grown at temperatures greater that 750 °C were n-type with an electron concentration of around 10¹⁷ cm⁻³ and a carrier mobility of 2200 cm²/V-s. Chemical composition studies by SIMS exhibit the presence of silicon, carbon and oxygen as the main residual impurities. Silicon concentration of around of 10¹⁷ cm⁻³ is very close to the free carrier concentration determined by the Hall-van der Pauw measurements. Composition homogeneity and structural quality are demonstrated by Raman measurements. As the growth temperature is increased the layers compensation decreases but the Raman spectra show that the crystalline quality of the layers diminishes.     

Preparation and characterization of in1−xGaxAsyP1−y epilayers by liquid-phase epitaxy

In_1–x Ga _x As _y P_1–y epilayers with three different solid compositions of ln_0.73Ga_0.27As_0.60P_1.40, In_0.59Ga_0.41As_0.87P_0.13 and ln_0.53Ga_0.47As were grown on (1 0 0) InP substrate at 623° C by the step cooling technique of liquid-phase epitaxy. From the optical transmission measurements, the corresponding wavelengths of the InGaAsP epilayers were 1.30, 1.55 and 1.69 m, respectively, which are in good agreement with those obtained from the calculations using Vegard's law. The full widths at half maximum of the photoluminescent spectra at 14 K of these layers were as low as 18.6, 22.5 and 7.9meV, respectively. The electron mobility of the InGaAsP epilayers is a function of the solid composition with the ln_0.53Ga_0.47As epilayer having the highest electron mobility. The mobility and concentration of this layer are 8,873cm²V^–1 sec^–1, 9.7×10¹⁵cm^–3 and 22,900 cm²V^–1 sec^–1, 8.5×10¹⁵cm^–3 at 300 and 77 K, respectively. The compensation ratio is between 2 and 5.     

Investigation of Al/(Al,Ga)As/GaAs metal/insulator/ semiconductor-type structures and application to buried interface field effect transistors

Undoped Al_0.5Ga_0.5As is used as an insulator layer in the fabrication of metal/ insulator/semiconductor- (MIS-) like capacitors with aluminum Schottky gates. Current-voltage and capacitance-voltage characteristics were measured as a function of temperature in the range 300-77 K. At high temperatures current occurs by thermionic emission over the barrier determined by the Schottky contact and the conduction band discontinuity. As the temperature is lowered, Fowler-Nordheim tunneling is observed for sufficiently high gate biases and at 77 K conduction is ohmic. Using an Al_0.5Ga_0.5As layer 100 nm thick MIS-like buried interface field effect transistors were fabricated. When these were operated in an accumulation mode and cooled to 77 K the transconductance increased by a factor of 2 and the current by a factor of 2.5. These preliminary results represent the first observation of enhanced performance at cryogenic temperatures in this type of device as a result of increasing electron saturation velocity. At 77 K and a saturation current density of 138 mA mm^-1 the transconductance was 40 mS mm^-1, compared with a theoretically estimated value of 130 mS mm^-1.     

InGaAsp-i-n photodiodes for fibre-optic communication

High purity layers of In_{1 −x} Ga _x As have been grown by liquid phase epitaxy using a novel impurity gettering technique with rare earth atoms. The electron concentration could thus be decreased from 3 × 10¹⁸ cm⁻³ to 2·4 × 10¹⁵ cm⁻³ and the mobility increased from 7 110 cm²/Vs to 18,981 cm²/Vs (100 K). The excellent quality of the layers has been evidenced by X-ray diffraction and photoluminescence measurements. The fabrication ofp-i-n photodiodes using this technique is described and reliability aspects addressed.     

Growth of In. 53Ga. 47as layers on InP substrates for I.R. detectors by MBE

The increasing interest in the 1.3–1.55 μm region for optical fibre communications assignes to In_{. 53}Ga_{. 47}As a prominent role for detectors operating at these wavelengths. In this contribute we mainly discuss the origin of several kinds of morphological defects on this material and the technical solutions used to reduce the defect density to less than 1×10⁴/cm². Using chemical etching techniques combined with transmission cathodoluminescence and optical microscope observations, we show that there is no relation between single dislocations in the substrate and morphological defects in the layer. Most of defects originate at the interface between the substrate and the layer, and are strongly dependent on the substrate cleaning procedures. Moreover we discuss the influence of growth conditions (growth rate, substrate temperatures and As pressure) on the evolution of these defects.Hall measurements on InGaAs single layers give a residual doping concentration as low as n = 1.10¹⁴cm^?3 and a mobility (at n = 2 10¹⁵cm^?3) as high as 10000 cm²·V^?1·sec^?1 at room temperature.InGaAs PIN detectors were fabbricated, and dark currents as low as 3 nA at 10V were obtained over 65 μm mesa diameter.     

Epitaxy of In0.01Ga0.99As on Ge/offcut Si (001) virtual substrate

In_0.01Ga_0.99As thin films free of anti-phase domains were grown on 7° offcut Si (001) substrates using Ge as buffer layers. The Ge layers were grown by ultrahigh vacuum chemical vapor deposition using ‘low/high temperature’ two-step strategy, while the In_0.01Ga_0.99As layers were grown by metal-organic chemical vapor deposition. The etch-pit counting, cross-section and plane-view transmission electron microscopy, room temperature photoluminescence measurements are performed to study the dependence of In_0.01Ga_0.99As quality on the thickness of Ge buffer. The threading dislocation density of Ge layer was found to be inversely proportional to the square root of its thickness. The threading dislocation density of In_0.01Ga_0.99As on 300 nm thick Ge/offcut Si was about 4 × 10⁸ cm^− 2. Higher quality In_0.01Ga_0.99As can be obtained on thicker Ge/offcut Si virtual substrate. We found that the threading dislocations acted as non-radiative recombination centers and deteriorated the luminescence of In_0.01Ga_0.99As remarkably. Secondary ion mass spectrometry measurement indicated as low as 10¹⁶ cm^− 3 Ge unintended doping in In_0.01Ga_0.99As.     

Electrical properties of polycrystalline GaInAs thin films

Polycrystalline Ga_xIn_1 − xAs films with x ranging from 0 to 1 were deposited on glass substrates by molecular-beam deposition at 240 or 350 °C. Room temperature Hall-effect measurements showed that the Ga_xIn_1 − xAs films deposited at either temperature exhibit high electron concentrations in the range of 10¹⁸ cm^− 3 for x ≤ 0.21 while the electron concentration decreases with increasing Ga content for x ≥ 0.29 to be < 10¹⁵ cm^− 3 at x = 0.64. Even at the low deposition temperature of 240 °C, the electron mobility remains > 400 cm²/(V s) at x ~ 0.2 and then decreases with Ga content to be ~ 40 cm²/(V s) at x = 0.64. Temperature-varying Hall-effect measurements in the range of 100-390 K revealed that both the electron concentration and mobility of the samples with x ≤ 0.21 are almost independent of the measurement temperature, while those of the samples with x ≥ 0.30 decrease with decreasing measurement temperature. The concentrations and ionization energies of donor levels were deduced from the temperature dependence of the electron concentration with the non-parabolicity of the conduction band taken into account. The temperature dependences of electron mobility in the samples with x ≥ 0.30 are well explained in terms of thermionic electron emission across the grain-boundary barriers assuming fluctuation in potential barrier height, while the almost temperature-independent high electron mobilities in the samples with x ≤ 0.21 are attributed to the absence of potential barrier at the grain boundaries.     

Impurity conduction in ion beam synthesized β-FeSi2/Si

Carrier transport in ion-beam synthesized (IBS) β-FeSi₂ was investigated by using Hall effect measurement at low temperatures (15-300 K). The measurement showed p-type conduction in the temperature range of 15-300 K. The Hall coefficient increased with increasing temperature up to 25 K, and then it decreased, which suggested the two carrier conduction, i.e., the impurity conduction, as well as the conduction in the valence band, play an important role in the carrier transport. Based on the two carrier model, the hole concentration and mobility for the impurity conduction at 25 K were evaluated to be 9.9×10¹⁷ cm⁻³ and 0.85 cm² V⁻¹ s⁻¹, respectively, which suggested that the acceptors were isolated and did not form the impurity band at the impurity concentration of 9.9×10¹⁷ cm⁻³. Thus, the threshold concentration for the impurity band formation was more than three orders of magnitude higher than that for GaAs (2×10¹⁶ cm⁻³), which could be explained on the basis of the Mott criterion.     

Characterization of GaN and In x Ga1−x N films grown by MOCVD and MBE on free-standing GaN templates and quantum well structures

Structural and optical studies have been performed on GaN, InGaN layers, In_0.08Ga_0.92N/GaN heterostructures, In_0.08Ga_0.92N/In_0.02Ga_0.98N single and multiquantum wells grown by metal organic chemical vapor deposition (MOCVD) and GaN by molecular beam epitaxy (MBE) on GaN templates by using transmission electron microscopy (TEM), X-ray diffraction (XRD), and photoluminescence (PL). The layers are found to be high quality with low defect density, on the order of 10⁶ cm^?2, which are mainly related to the threading dislocations originating/propagating from the hydride vapor phase epitaxy (HVPE) GaN template. The interface between the layers and substrate could not be detected by TEM and was therefore deemed to be of high quality. Convergent beam electron diffraction studies revealed that the polarity of the films is Ga-polarity, which is the same as that of the substrate. A dual structure with different compositions and having thicknesses of 10 and 25 nm was observed in InGaN layers grown on GaN in one of the heterostructure samples. The full width at half maximum (FWHM) of the XRD rocking curves of (0 0 0 2) for heterostructures and quantum wells were found to be in the range of 15–28 arcmin for a slit width of 2 mm. PL studies on GaN layers grown by MBE and MOCVD on GaN templates are reasonably similar. The PL spectra from all the MBE and MOCVD epilayers and the substrate contain a plethora of sharp peaks related to excitonic transitions. With the presence of donor-bound exciton peaks and their associated two-electron satellites, the binding energies of two distinct shallow donors (28.8 and 32.6 meV), which are attributed to Si and O, respectively, were determined. PL measurements revealed that the FWHM of the main donor bound exciton peak increased from 0.6 to 2.9 meV but no change in peak position (3.472 eV) was observed in GaN when doping with Si (5×10¹⁷ cm^?3). However, the intensities of the yellow band and the shallow donor–acceptor pair band increased 10 times as compared to that in the undoped GaN samples. In the case of InGaN/GaN heterostructures, a similar trend was observed when compared to the doped samples. In the multiquantum well In_0.08Ga_0.92N/In_0.02Ga_0.98N heterostructures, the activation energy of the exciton emission, found to be 18 meV, was the lowest in the samples studied. The peak at 3.02 eV related to the InGaN was strongly pronounced in the In_0.08Ga_0.92N/In_0.02Ga_0.98N multiquantum well structure. In the In_0.08Ga_0.92N/In_0.02Ga_0.98N quantum well structures, the change in peak position with variation of temperature from 15 to 300 K in PL spectra is “S”-shaped. The cause for the “S” shape, i.e., a red–blue–red shift, is discussed.     

Thin-film transistors with sol-gel deposited Mg0.1Zn0.9O films as active channel layer

We report on field-effect transistors using Mg_0.1Zn_0.9O thin film as channel layer. The effect of Mg is to increase the band gap and decrease the electron carrier concentration. The Mg_0.1Zn_0.9O film deposited by sol-gel method has a highly c-axis orientation and excellent optical properties. The devices display a channel mobility of 0.76 cm² V^− 1 s^− 1 and an on/off ratio of 400.     

Graphoepitaxy of High‐Quality GaN Layers on Graphene/6H–SiC

The implementation of graphene layers in gallium nitride (GaN) heterostructure growth can solve self‐heating problems in nitride‐based high‐power electronic and light‐emitting optoelectronic devices. In the present study, high‐quality GaN layers are grown on patterned graphene layers and 6H–SiC by metalorganic chemical vapor deposition. A periodic pattern of graphene layers is fabricated on 6H–SiC by using polymethyl methacrylate deposition and electron beam lithography, followed by etching using an Ar/O₂ gas atmosphere. Prior to GaN growth, an AlN buffer layer and an Al_0.2Ga_0.8N transition layer are deposited. The atomic structures of the interfaces between the 6H–SiC and graphene, as well as between the graphene and AlN, are studied using scanning transmission electron microscopy. Phase separation of the Al_0.2Ga_0.8N transition layer into an AlN and GaN superlattice is observed. Above the continuous graphene layers, polycrystalline defective GaN is rapidly overgrown by better quality single‐crystalline GaN from the etched regions. The lateral overgrowth of GaN results in the presence of a low density of dislocations (≈10⁹ cm⁻²) and inversion domains and the formation of a smooth GaN surface.     

Simulation of doping levels and deep levels in InGaN-based single-junction solar cell

Doping levels and deep levels in In_0.65Ga_0.35N single junction solar cells are studied theoretically, and simulation of cell properties is performed. Effective-mass approximation (EMA) is used to calculate the ionization energies and the radius of ground-state orbit for donors and acceptors in wurtzite In_0.65Ga_0.35N. The ionization energies of donors and acceptors are estimated to be about 15.5 and 92.9 meV, respectively. The validity of EMA to wurtzite InGaN alloy has also been discussed. AMPS-1D software is used to simulate the doping levels and deep levels in In_0.65Ga_0.35N single junction solar cells with assumption that the deep level is located at the middle of In_0.65Ga_0.35N band gap where the recombination is maximum. Band structure and concentration distributions of equilibrium carriers are obtained. The influence of deep level recombination on efficiency is estimated to be about 9.6% while recombination center concentration is 5 × 10¹⁵ cm⁻³, and capture cross section is 10⁻¹³ cm². The simulated results show that the increase of reverse saturation current and the decrease of open-circuit voltages (V _oc) and fill factor (FF) are mainly responsible for the decrease of the efficiency. Short-circuit current density (J _sc) is found to be not sensitive to deep level concentrations and capture cross sections. As the crystal quality of InGaN and p-type doping of In-rich InGaN may be the most important challenges for InGaN solar cells, this study is useful for the study of InGaN-based super-high efficiency solar cells.     

Negative photoinduced current and negative differential characteristics of new optoelectronic sensors with InAs/GaAs nanostructure for visual recognition

Negative photo-induced currents and negative differential characteristics of photoinduced current have been obtained successfully under weak photoexcitations at 300 K for a new optoelectronic sensor including a carrier-storage layer of InAs/GaAs short period superlattice. The phenomena are useful to imitate the selectivity functions in visual recognition, which is based on the difference-of-Gaussian functionlike distribution of light sensitivity in the receptive fields in visual cortex. The phenomena are strongly dependent on the material of the carrier-storage layer, where the photogenerated carriers are separated spatially due to the Schottky forward voltage. The carrier separation is more enhanced as the ratio of electron mobility to hole mobility becomes higher for the material of the carrier-storage layer. This means the In-rich InGaAs material is superior to the Ga-rich one. In addition, compared with In _x Ga_1-x As alloy, the improved surface morphology, the high electron mobility at room temperature and the narrow energy bandgap are observed in the case of InAs/GaAs short period superlattice. That is the InAs/GaAs short period superlattice is superior to In _x Ga_1-x As alloy as the material for the carrier-storage layer.     

Specific features of Hall effect in capped multilayer pHEMT structure of AlxGa1 − xAs/InxGa1−xAs/GaAs

Electrical properties of capped multilayer pHEMT (pseudomorphic High Electron Mobility Transistor) structure of Al_xGa_1−xAs/In_xGa_1−xAs/GaAs have been studied using a Hall effect method. In the device structure In_xGa_1−xAs layer was prepared as a channel layer and Al_xGa_1−xAs as the supply and spacer layers. Mole fraction, x, for the alloys varied from 19% to 20% (for In_xGa_1−xAs) and 20% to 24% (for Al_xGa_1−xAs). From the measurement carried out at room temperature, it was found that increasing In content in the channel layer will be followed by decreasing electron mobility of multilayer properties of pHEMT structure. This phenomenon is found to be in contradiction with the calculation for the alloy concerned. This phenomenon supports the consideration that electrical properties of supply and spacer layer affect the channel layer properties. From the Hall measurement, relationships between Al content in the supply and spacer with respect to the electron mobility and sheet resistance were obtained.     

Far infrared spatial probe of heteroepitaxial indium arsenide

Far infrared (16–320 cm^?1) reflection and transmission data for approximately 15 μm thick epitaxial InAs on high resistivity GaAs are analyzed for surface and interface effects. The results show the presence of a front surface high concentration (about 3×10¹⁷ carriers cm^?3) accumulation layer, a thin (about 0.7 μm) interface layer of In_xGa_1-x As and a damaged back surface in addition to the epitaxial and substrate regions. The layer parameters derived from the analysis are in reasonable agreement with independent determinations.     

Infrared and Structural Properties of Y1−xNdxBa2Cu3O7−δ (0 ≤ x ≤ 1)

Infrared and structural properties of Y_1?x Nd _x Ba₂Cu₃O_7?δ (0 ≤ x ≤ 1) were investigated using infrared absorption spectroscopy and X-ray powder diffraction. The unit cell parameters of the samples were defined using X-ray diffraction data. The resistance measurements showed that the samples revealed superconductivity in the temperature range of 80–100 K. It was observed that by the substitution of Nd to Y in YBa₂Cu₃O_{7 ? δ} IR band at 573 cm^?1 that is assigned as Cu–O axial antisymmetric stretching mode shifts to 533 cm^?1 while the band at 620 cm^?1 that is due to Cu–O symmetric stretching mode in YBa₂Cu₃O_7?δ shifts to 588 cm^?1.     

Molecular beam epitaxial growth of high-quality InP layers from solid phosphorus using a valved phosphorus cracker cell

We report the molecular beam epitaxial (MBE) growth of epitaxial InP using a valved phosphorus cracker cell at a range of cracking-zone temperatures (T_cr = 875–950°C), V/III flux ratios (V/III = 1.2–9.3) and substrate temperatures (T_s = 360–500°C). The as-grown epitaxial InP on an InP (100) substrate is found to be n-type from Hall measurements. The background electron concentration and mobility exhibit a pronounced dependence on the cracking-zone temperature, V/III flux ratio and substrate temperature. Using a T_cr of 850°C, the highest 77 K electron mobility of 40 900 cm² (V s)⁻¹ is achieved at a V/III ratio of 2.3 and a T_s of 440°C. The correponding background electron concentration is 1.74 × 10¹⁵ cm⁻³. The photoluminescence (PL) spectra show two prominent peaks at 1.384 and 1.415 eV, with the intensity of the low-energy peak becoming stronger at higher cracking-zone temperatures. The lowest PL FWHM achieved at 5 K is 5.2 meV. Within the range of substrate temperatures investigated, the effect on the crystalline quality determined from X-ray diffraction (XRD) measurements is not significant.     

Enhancement of microelectronic device performances by photothermal annealing under SiCl4 ambient

The use of low cost silicon wafers seems to be very attractive for photovoltaic and microelectronic devices. However, this material is widely contaminated by different impurities particularly transitions metals, which deteriorate the lifetimes and the bulk diffusion lengths of the minority charge carriers. One possible way to overcome this undesirable behavior is to include an efficient purification technique in the process of device fabrication. In this work, we present the effect of photothermal treatments of monocrystalline Czochralski silicon substrates under SiCl₄/N₂ atmosphere using a thin sacrificial porous silicon layer. The main results show a decrease of the resistivity over 40 μm depth. The Hall mobility of the majority charge carriers is improved from 300 to 1417 cm² V^− 1 s^− 1. The capacitance–voltage (C–V) characteristics of metal/SiO₂/Si (MIS) structures indicate a decrease of carrier concentration which confirms the results obtained by Hall Effect and Van Der Pauw method. The reduction of boron concentration in Czochralski silicon may reduce boron- and oxygen related metastable defect centers.     

Effect of Ga doping concentrations,substrate temperatures and working pressures on the electrical and optical properties of ZnO thin films

We fabricated Ga-doped ZnO (GZO) thin films on glass substrate by RF magnetron sputtering method with different conditions of Ga₂O₃ concentration, substrate temperature and working pressure. Next we investigated the electrical, optical and structural properties of the GZO thin films. At a substrate temperature of 300 °C, a working pressure of 1 mTorr, and a Ga₂O₃ concentration of 3 wt%, the GZO thin films showed the lowest resistivity of 3.16 × 10^?4 Ω cm, a carrier concentration of 7.64 × 10²⁰ cm^?3 and a Hall mobility of 25.8 cm²/Vs. Moreover, the GZO thin films exhibited the highest (002) orientation under the same conditions and the full width at half maximum of X-ray peak was 0.34°. All GZO thin films showed the optical transmittance of more than 80 % in the visible range regardless of working conditions. The Burstein–Moss effect was observed by the change of doping concentration of Ga₂O₃. The GZO thin films were fabricated to have the good electrical and optical properties through optimizing doping concentration of Ga₂O₃, substrate temperature, working pressure. Therefore, we confirmed the possibility of application of GZO thin film as transparent conductive oxide used in flat panel display and solar cell.     

Defect Equilibria in In- and Ga-Doped CdTe

The Hall coefficient of four CdTe samples with [In] = 5 × 10¹⁶ to 10¹⁹ cm^–3 and two CdTe samples with [Ga] = 4 × 10¹⁷ and 1.4 × 10¹⁸ cm^–3 was measured between 700 and 1300 K at a controlled Cd vapor pressure. The equilibrium constant of In incorporation into CdTe from In precipitates was calculated. Kröger's quasi-chemical equations were used to evaluate the concentrations of point defects as a function of temperature and dopant activity. The results agree well with experimental data. The maximum in the temperature dependence of electron concentration is interpreted in terms of the competition between the increase in the concentration of D _Cd ⁺ donors as a result of the increase in dopant solubility and the compensation of the donors by native acceptors (V ^2- _Cd) and impurity-containing acceptor complexes (A _In ^- or A _Ga ^-). Modeling results confirm that the formation of these complexes is due primarily to Coulombic interaction.