Md. Tanvir Hasan

Phase separation of thick (∼1 µm) InxGa1−xN (x ∼ 0.3) grown on AlN/Si(111): Simultaneous emergence of metallic In–Ga and GaN-rich InGaN

0.3–2-µm-thick InxGa1−xN (x ~ 0.3) films are grown at 650 °C on AlN/Si(111) substrates by metal organic vapor phase epitaxy. When the thickness of an epitaxial InGaN film exceeds ~1 µm, peaks of GaN-rich InGaN(0002) and metallic In(101) appear in the X-ray diffraction profiles. The InN composition ~0.03 in the GaN-rich InGaN film is in agreement with the solubility of InN in GaN at 650 °C. The metallic In contains a small amount (~0.03 at. %) of Ga. These results clearly show that the epitaxial InGaN film is phase-separated into GaN-rich and InN-rich InGaN. The latter is changed into metallic In–Ga owing to its thermal instability at 650 °C.

Surface barrier height lowering at above 540 K in AlInN/AlN/GaN heterostructures

Hall mobility (lH) and two dimensional electron gas density (ns) have been measured from 77 up to 973 K in AlInN/AlN/GaN heterostructures, where the atmospheric condition is changed as measured in vacuum and air. The lH decreases monotonically with increasing the temperature. The characteristic feature is observed in ns that it is almost constant up to around 540 K and shows sudden increase at higher temperatures when measured in the vacuum, while it is almost constant measured in the air. The surface barrier lowering originated from the decomposition of the surface oxide layer on AlInN is proposed as the most probable mechanism for the increase in ns. VC 2011 American Institute of Physics. [doi:10.1063/1.3644161]

Performance evaluation of various vocoders in mobile ad hoc network (MANET)

Realtime voice communication is an essential requirement in emergency scenarios. Mobile ad hoc network (MANET) is a best suitable option for premises like conference or convention centre. The performance of VoIP services depends on voice encoder scheme. In this paper, the performance of different voice codec i.e.G.728, G.711, G.723, G.729, G.726, GSM-EFR, GSM-HR under similar load condition for Interactive voice over MANET with and without speech activity detection(SAD) is analysed. In a series of simulation study it has been found that G.711 is the best optimal performance in small scale scenario while GSM-EFR performs best at large scale scenario.

Effect of symmetrical underlap length on device performance of a GaN-based double gate MOSFET

GaN-based double gate DG-MOSFETs have been designed and simulated in nano-scale regime for future logic-switching applications. To minimize the short-channel effects (SCEs), both gate-to-source (G-S) and gate-to-drain (G-D) lengths, symmetrical underlap length, LUN, have been extended. The underlap architectured-devices exhibit better performance due to reduction of coupling capacitance between the contacts (S-G and G-D). The value of subthreshold slope (SS) and drain induced barrier lowering (DIBL) are 62.897 mV/decade and 33.59 mV/V, respectively for an underlap length, LUN = 8 nm with a gate length, LG = 12 nm.

MOVPE growth of thick (∼1 µm) InGaN on AlN/Si substrates for InGaN/Si tandem solar cells

In order to develop key technologies for InGaN/Si two-junction tandem solar cells, MOVPE growth of thick InGaN on Si p-on-n cell structures has been studied. By clarifying the phase separation behavior of MOVPE InGaN, a thick (~1 µm) InxGa1−xN (x ~ 0.5) without phase separation is successfully grown on AlN/Si(111) wafers. A sufficient current flow for the operation of the tandem cell is obtained through n-InGaN/AlN/p-Si structures by employing the annealing of AlN/Si wafer at around 1000 °C in NH3 flow just before InGaN growth. The annealing of AlN/Si wafers also brings about the degradation of the underlying Si pn junction. The optimization of the annealing conditions is required to balance such favorable and unfavorable effects.

GaN-based Double Gate MOSFETs: Effect of gate length

The performance of GaN-based Double Gate (DG) MOSFETs have been explored with different gate lengths, LG for future high performance switching device applications. Devices have been designed according to the international technology roadmap for semiconductor (ITRS) - 2013. The ON-state currents are found to be 11.38 mA/µm and 9.67 mA/µm for LG = 11.6 nm (year 2019) and LG =8 nm (year 2023), respectively. These values are higher than that of conventional Si-based DG-MOSFETs. Simulation results show that the proposed GaN-based DG MOSFETs provide lesser values of subthreshold slope (SS) and drain induced barrier lowering (DIBL). These results indicate that proposed devices are plausible candidate for fabricating of future high-speed, high performance (HP) logic devices.

Effects of cap layer on 2DEGs in InN-based heterostructures

This paper describes the effects of cap layer on sheet carrier concentration and mobility of two dimensional electron gases (2DEGs) in InN-based InN/InGa(Al)N/InN heterostructures. Addition of a InN cap layer in InGa(Al)N/InN heterostructure leads to a very interesting dependence of 2DEGs. The sheet carrier concentration decreases and mobility increases with increase of the InN cap layer thickness. The calculated values of sheet carrier concentration are found to be decreased from 6×10¹³ to 1.2×10¹³ cm⁻² with increase of the cap layer thickness from 1 to 60 nm for a 10 nm barrier. The 2DEGs mobilities are found to be around 3.3×10³, 3.5×10³, and 4.8×10⁴ cm²V⁻¹sce⁻¹, respectively, without, with 10 nm and 100 nm cap layer. The cap layer also enhances the peak drift velocity. The above calculated results indicate that addition of cap layer is very promising for the fabrication of high performance HEMTs.

The Effects of Interdot Spacing and Dot Size on the Performance of InGaAs/GaAs QDIBSC

In0.53Ga0.47As/GaAs-based quantum dot intermediate band solar cells (QDIBSCs) have been designed and optimized for the next generation photovoltaic technology. The wave behavior of charge carriers inside the dot and their barrier have been analyzed with different dot sizes and interdot spacing. The device characteristics such as short circuit current density, Jsc, open circuit voltage, Voc, and conversion efficiency, η, have been evaluated. Based on the behavior of electron wave function, it is found that varying the dot spacing leads to a change in the IB width and in the density of states, whereas varying the size of dots leads to a formation of a second IB. For a fixed dot spacing, two ranges of dot sizes vary the number of IBs in In0.53Ga0.47As/GaAs QDIBSC. Smaller dots of a size ranging from 2 nm to 5 nm form a single IB while larger dots of a size ranging from 6 nm to 9 nm can produce 2 IBs. The efficiency of 2 IBs close to 1 IB suggests that formation of multiple IBs can possibly enhance the device efficiency.

In x Ga 1−x As/GaAs-based intermediate band solar cell: Effects of quantum dots

This paper describes the influence of In0.53Ga0.47As/GaAs quantum dots (QDs) in the intermediate band solar cell (IBSC). The In0.53Ga0.49As QDs have been inserted in the intrinsic region of a p-i-n GaAs solar cell in order to increase the absorption range as well as cell efficiency. The cell structure has been optimized with respect to the horizontal and vertical dot-to-dot spacing. The optimum efficiency is enhanced from 27.1% to 32.09% for increasing the number of QD layers.

A dual triangular cut resonator patch antenna for WLAN applications

A dual triangular cut resonator (DTCR) design of a patch antenna is proposed. The antenna is able to perform at 2.44 GHz band applicable for wireless local area networks. The gain and directivity are 7.75 dB and 8.29 dBi, respectively. Proposed cut resonator develops stored capacitive charge and boosts the antenna radiation and electromagnetic field which enhances the antenna performance.