IV–VI Compound heterostructures grown by molecular beam epitaxy

Structural and optical characterization of some IV–VI superlattices (SL) and multi-quantum wells (MQW) grown by molecular beam epitaxy (MBE) on BaF₂ (111) substrates are shown. Three different types of systems were investigated, namely, PbTe/PbSnTe, PbTe/SnTe and PbTe/PbEuTe. High-resolution X-ray diffraction analysis was performed to determine the strain in the structures. The analysis revealed sharp interfaces and good thickness control. The transition energies between the confined levels in the wells were obtained from the absorption steps observed in infrared transmission measurements. Preliminary results on PbTe/Si heterojunction grown by MBE are also presented.     

Comparison of 3C–SiC, 6H–SiC and 4H–SiC MESFETs performances

A comparative analysis of the main DC and microwave performances of MESFETs made of the commercially available silicon carbide polytypes 3C–SiC, 6H–SiC and 4H–SiC is presented. In this purpose, we have developed an analytical model that takes into account the basic material properties such as field dependent mobility, critical electric field, ionization grade of impurities, and saturation of the charge carrier velocity. For a better precision in appreciating device characteristics in the case of a short gate device, the influences of the gate length and parasitic elements of the structure, e.g. source and drain resistances, are considered too. Cut-off frequency f_T, the corresponding output power P_m and the thermal stability are also evaluated and compared with the available experimental data, revealing the specific electrical performances of MESFETs, when any of the three polytypes is used in device fabrication.     

Defects annealing in 4H–SiC epitaxial layer probed by low temperature photoluminescence

The thermal evolution of defects induced in 4H–SiC by multiple implantation of C ions was investigated by Low Temperature Photoluminescence in the temperature range 450–1000 K. The photoluminescence spectra show sharp luminescent lines (alphabet lines) in the wavelength range 426–440 nm upon irradiation and thermal treatment at 450 K induces the appearance of a new line at 427 nm (D_I centre). The trend shown by the luminescence lines as a function of the temperature is quite complex. The alphabet lines intensity increases up to 850 K, whereas at higher temperature decreases with an activation energy of 2.0 eV, suggesting that the defect, responsible for these lines, is the Si-vacancy. The luminescence yield of D_I centre is always increasing as a function of the temperature, with a higher slope from 750 K, suggesting a correlation to the reconfiguration and to the annealing of point defects.     

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.     

As cluster array formation in GaAs grown by molecular-beam epitaxy at a low temperature and δ-doped with phosphorus

Semiconductors - GaAs films δ-doped with phosphorus (one monolayer) were grown by molecular beam epitaxy at a temperature of 200°C (LT-GaAs), isochronally annealed at 400, 500 or...     

Band structure anisotropy effects on the hole transport transient in 4H–SiC

We study the role of band structure anisotropy on the hole transport in 4H–SiC during the transient regime. For the same strength of the applied electric field, the drift velocity overshoot of the hole is stronger and reaches steady state later when the field is applied perpendicular to the c-axis, than when the field is in the c-axis direction. In both cases, the time for the hole drift velocity and mean energy to reach steady state is under 50 fs, depending on the electric field strength, and are one order of magnitude shorter than the time for the electron drift velocity and mean energy to attain the steady state.     

Growth and microstructure of InxGa1−xN films grown on SiC substrates via low pressure metalorganic vapor phase epitaxy

The growth and microstructures of In_xGa_1−xN films (x≤0.23) grown on α(6H)–SiC(0001) wafer/AIN buffer layer/GaN heterostructures by low pressure metalorganic vapor phase epitaxy have been investigated. The system deposition pressure limited the InN content in these films. The maximum InN contents achievable at the deposition pressures of 45 and 90 torr were ∼13 and ∼23%, respectively. Kinetic phenomena based on the rates of adsorption and desorption of the In growth species off the growth surface are presented to explain the film composition dependence on the system pressure. The surface morphologies and microstructures of the In_xGa_1−xN films were analyzed using several techniques, and the formation of pinhole defects in the films was investigated. Most of the pinhole defects were associated with threading dislocations with a c-component Burgers vector. Edge-type dislocations were never observed to terminate in pinholes in the samples observed here. Indium segregation to areas around the defect areas was observed, as was an In compositional gradient in the growth direction. Based on experimental observations, the strain field around dislocations with a c-component Burgers vector could result in the increase of In atoms at the dislocation sites in the film, which result in a change to the local growth mode of the film and causes the pinhole defects to form.     

4H–SiC BJTs with current gain of 110

4H–SiC BJTs with a common emitter current gain of 110 have been demonstrated. The high current gain was attributed to a thin base of 0.25 μm which reduces the carrier recombination in the base region. The device open base breakdown voltage (BV_CEO) of 270 V was much less than the open emitter breakdown voltage (BV_CBO) of 1560 V due to the emitter leakage current multiplication from the high current gain by “transistor action” of BJTs. The device has shown minimal gain degradation after electrical stress at high current density of >200 A/cm²up to 25 h.     

Strain relaxation mechanisms in Si1−xGex layers grown by solid-phase epitaxy: Influence of the layer composition and growth temperature

The influence of the composition and growth temperature on the strain and defect structure of Si_1−xGe_x layers of 0.21≤x≤0.34 grown on (001) Si wafers by solid phase epitaxy is presented. The strain in the layers was measured by Raman spectroscopy and Rutherford backscattering spectrometry/channeling angular scans. The defects were analyzed using high resolution electron microscopy. Three different relaxation mechanisms have been identified and characterized. The first mechanism occurs at the layer-substrate interface of the samples by the introduction of isolated defects. It is found to be thermally activated with an activation energy of E_a=0.16 eV and a prefactor that depends on the Gecontent of the layer. This mechanism produces partial relaxation of the layers and hinders the growth of fully strained layers. The second relaxation mechanism emerges at a distance from the interface which depends on the stress in the crystallized portion of the layer. In this case, the strain relaxation is caused by stacking faults that nucleate when they are energetically feasible and propagate toward the surface of the sample during growth. At low growth temperatures, the defects are confined to the upper part of the epitaxial layers at a distance from the interface that agrees with the theoretical predictions based on the equilibrium critical layer thickness. The third relaxation mechanism is introduced at high growth temperature and is based on the gliding of the stacking faults toward the layer-substrate interface. As a result of this mechanism, the stress in the layers is reduced compared to the stress in the layers grown at lower temperatures and approaches the equilibrium value corresponding to the total layer thickness. This behavior indicates that the layers grown at low temperature, where the stacking faults are confined to the upper part, are to some extent metastable.     

Inductively coupled plasma etch damage in 4H−SiC investigated by Schottky diode characterization

Ti Schottky diodes have been used to investigate the damage caused by inductively coupled plasma (ICP) etching of silicon carbide. The Schottky diodes were characterized using IV and CV measurements. An oxidation approach was tested in order to anneal the damage, and the diode characterization was used to determine the success of the annealing. The barrier height, leakage current, and ideality factor changed significantly on the sample exposed to the etch. When the etched samples were oxidized the electrical properties were recovered and were similar to the unetched reference sample (with oxidation temperatures ranging from 900°C up to 1250°C). Annealing in nitrogen at 1050°C did not improve the electrical characteristics. A low energy etch showed little influence on the electrical characteristics, but since the etch rate was very low the etched depth may not be sufficient in order to reach a steady state condition for the surface damage.     

Efficient traffic control and lifetime maximization in mobile ad hoc network by using PSO–BAT optimization

Recent developments in dynamic mobile ad-hoc network enhance the network speed and reliability. The nodes in the dynamic ad-hoc network are moving in nature. Due to the increased subscribers in this network, the network traffic has increased to manifold which in turn creating the challenge of maintaining the energy level. In path optimization process in mobile ad-hoc network consumes more energy and the draining of the energy is dependent on network reliability and connectivity. Further, the network also suffers by harmful attacks such as denial of service attack, black hole attack and warm hole attack. The primary focus of this paper is to prevent these attacks with the help of dynamic mobile ad-hoc network on demand protocol and hybrid meta-heuristics methodologies, and also to reduce the energy drain rate. This is achieved by estimating the velocity and fitness value of the nodes. Finally, the empirical simulation results of hybrid particle swarm optimization with bat algorithm (PSO–BAT) shows that the energy drain rate level is reduced 90% as 1 mJ/s than ad-hoc on demand vector. The end-to-end delay minimized to 50% than existing Ad hoc on-demand distance vector routing. The performance metrics routing overhead and execution time has been reduced and throughput is gradually increased in PSO–BAT optimization in dynamic mobile ad hoc network scenario.

     

Growth of SiGe Films by Cold-wall UHV/CVD Using GeH4 and Si2H6

An ultrahigh vacuum chemical vapor deposition (UHV/CVD) system is introduced.SiGe alloys and SiGe/Si multiple quantum wells (MQWs) have been grown by cold-wall UHV/CVD using disilane(Si2H6) and germane (GeH4) as the reactant gases on Si(100) substrates.The growth rate and Ge contents in SiGe alloys are studied at different temperature and different gas flow.The growth rate of SiGe alloy is decreased with the increase of GeH4 flow at high temperature.X-ray diffraction measurement shows that SiGe/Si MQWs have good crystallinity,sharp interface and uniformity.No dislocation is found in the observation of transmission electron microscopy(TEM) of SiGe/Si MQWs.The average deviation of the thickness and the fraction of Ge in single SiGe alloy sample are 3.31% and 2.01%, respectively.     

Influence of inhomogeneous contact in electrical properties of 4H–SiC based Schottky diode

Schottky diodes realized on 4H–SiC n-type wafers with an epitaxial layer and a metal-oxide overlap for electric field termination were studied. The oxide was grown by plasma enhanced chemical vapor deposition (PECVD) and the Schottky barriers were formed by thermal evaporation of titanium or nickel. Diodes, with voltage breakdown as high as 700 V and ideality factor as low as 1.05, were obtained and characterized after packaging in standard commercial package (TO220).The electrical properties such as ideality factor, hight barrier, the series resistance R_s were deduced by current/voltage (I–V) analysis using the least mean square (LMS) method. The temperature effect on break voltage, R_s and saturation current was studied. A model based on two parallel Schottky diodes with two barrier heights is presented for some devices having an inhomogeneous contact. It is shown that the excess current at low voltage can be explained by a lowering of the Schottky barrier in localized regions. We use the two series RC components electrical model in order to study the dynamic behaviour of the Schottky diode in low frequency and to improve the effect of barrier inhomogeneities in electrical properties.     

Structural,Electrical, and Optical Properties of 4H–SiC for Ultraviolet Photodetectors

Semiconductors - The results of investigations of initial n-4H–SiC structures by various methods are presented. The structures represent a highly doped n+ substrate with epitaxial layers 5...     

Improved multi-recessed 4H–SiC MESFETs with double-recessed p-buffer layer

An improved multi-recessed 4H–SiC metal semiconductor field effect transistor (MRD-MESFET) with double-recessed p-buffer layer (DRB-MESFET) is proposed in this paper. By introducing a double-recessed p-buffer layer, the gate depletion layer is further modulated, and higher drain saturation current and DC transconductance are obtained compared with the MRD-MESFET. The simulations show that the drain saturation current of the DRB-MESFET is about 42.4% larger than that of the MRD-MESFET. The DC transconductance of the DRB-MESFET is almost 15% higher than that of the MRD-MESFET and very close to that of double-recessed structure (DR-MESFET) at the bias conditions of V_gs=0 V and V_ds=40 V. The proposed structure has an improvement of 26.1% and 74.2% in the output maximum power density compared with that of the MRD-MESFET and DR-MESFET, respectively. In the meanwhile, the proposed structure possesses smaller gate-source capacitance, which results in better RF characteristics.     

Morphology Associated Positive Correlation between Carrier Mobility and Carrier Density in Highly Doped Donor–Acceptor Conjugated Polymers

Molecular doping in conjugated polymers (CPs) has recently received intensive attention for its potential to achieve high electrical conductivity in organic thermoelectric materials. In particular, it affects not only the carrier density n but also the carrier mobility µ because high degree of molecular doping changes the morphological properties. Herein, the effect of molecular doping in CP thin films on the pathways and mechanisms of charge transport is investigated, which govern the µ-n relationship. Two representative donor–acceptor type CPs with similar µ but different molecular assembly in an undoped state, that is poly[2,5-(2-octyldodecyl)-3,6-diketopyrrolopyrrole-alt-5,5-(2,5-di(thien-2-yl)thieno[3,2-b]thiophene)] (DPPDTT) and indacenodithiophene-co-benzothiadiazole (IDTBT), are prepared. Heavy doping with iron chloride (FeCl₃) induced DPPDTT with highly crystalline edge-on orientation to increase its µ up to 19.6 cm² V⁻¹ s⁻¹, whereas IDTBT with irregular intermolecular stacking showed little change in µ. It is revealed that this different µ-n relationship is highly attributed to the initial molecular ordering of CP films. The charge transport mechanism also becomes significantly different: both coherent and incoherent transports are observed in the doped DPPDTT, whereas incoherent transport is only found in the doped IDTBT. This study suggests guidelines for enhancing charge transport of CPs under doping in terms of structural disorder.     

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.     

The emission from the structures with arrays of coupled quantum dots grown by the submonolayer epitaxy in the spectral range of 1.3–1.4 µm

Optical properties of structures with vertically coupled quantum dots grown by the combined submonolayer molecular-beam epitaxy were investigated. It is shown that the formation of the laterally coupled conglomerates of quantum dots are possible in upper rows for certain parameters of growth, with the corresponding photoluminescence emission being in the wavelength range of 1.3–1.4 μm at room temperature. __________ Translated from Fizika i Tekhnika Poluprovodnikov, Vol. 34, No. 11, 2000, pp. 1368–1372. Original Russian Text Copyright ? 2000 by Volovik, Sizov, Tsatsul’nikov, Musikhin, Ledentsov, Ustinov, Egorov, Petrov, Polyakov, Tsyrlin.     

High temperature Hall effect measurements of semi-insulating 4H–SiC substrates

High temperature Hall effect and resistivity measurements have been made on semi-insulating 4H–SiC samples. Both vanadium doped and undoped materials have been studied. Resistivity measurements before and after annealing up to 1800 °C are also reported. The thermal activation energy of the resistivity in vanadium doped samples has one of two values, 1.5 and 1.1 eV, due, respectively, to the vanadium donor level and an as yet unidentified defect. The activation energies for high purity semi-insulating material (HPSI) varied from 0.9 to 1.5 eV. Hall effect measurements were made on several HPSI and 1.1 eV V-doped samples. In all cases the material was found to be n-type. Mixed conduction analysis of the data suggests that the hole concentration is negligible in all samples studied. This suggests that the defects responsible for the semi-insulating properties have deep levels located in the upper half of the bandgap. The resistivity of V-doped samples were unaffected by anneals up to 1800 °C. The annealing results for HPSI samples were mixed.