Papers by Keyword: III-Nitrides

Abstract: The growth of AlN buffer layer at extremely high temperature (1100-1150oC) in ammonia MBE STE3N2 system is shown to be the key step to obtain high quality GaN layers for DHFET channels. The buffer layer sequence from c-sapphire substrate involved AlN, AlGaN/AlN superlattice and AlGaN transition layers. TEM study showed gradual decrease of threading dislocation density from (2-4)×10¹⁰ cm^-2 in AlN to (9-10)×10⁸ cm^-2 in the top GaN active layer. The improvement of structural quality resulted in substantial increase in electron mobility up to 600-650 сm²/Vs in a 1.5-μm-thick GaN top layer lightly doped with silicon up to n=(3-5)×10¹⁶ cm^-3. These results correspond to a good quality MOCVD GaN grown on sapphire and several times better than in conventional MBE. Employing such a GaN layer in a double heterostructure (DH) with the cap Al_xGa_1-xN barrier layer (x=0.25-0.4) allows to change the electron sheet density, mobility and sheet resistance in a two dimensional electron gas in the range of 1300-1700 cm²/V^.s, (1.0-1.8)×10¹³ cm^-2 and 230-400 Ω/, respectively. Application of this technology and DH design for growing on SiC substrates enabled one to manufacture a DHFET with a gate length of 0.5 μm for 0.03-4.0 GHz extra-broadband power amplifiers having P_out=2.5 W, gain 17-25 dB and efficiency 30%.

Abstract: Al0.35Ga0.65N/GaN- and Al0.2Ga0.8N/AlN/GaN-heterostructures high electron mobility transistors (HEMTs) with a gate length (LG) varying from 1.2 to 0.08 µm were fabricated on silicon Si(111) substrates using a 3C-SiC transition layer. Metal organic chemical vapour deposition (MOCVD) was used to growth the AlGaN-heterostructures and a low pressure chemical vapour deposition (LPCVD) to create the 3C-SiC(111) transition layer preventing Ga-induced melt back etching and Si-out diffusion. Reduced Al content and an AlN interlayer improved the device performance. The HEMTs with LG=0.08µm had a maximum drain current density of 1.25 A/mm and a peak extrinsic transconductance of 400 mS/mm. A unity current gain cut-off frequency (ƒT) of 180 GHz and maximum frequency (ƒmax) of 70 GHz were measured on these devices.

Abstract: In this paper, we studied growth of AlN/GaN/AlN on Si (111) by using plasma assisted molecular beam epitaxy (PA-MBE) system. The structural and optical characteristics of the sample have been investigated by using high resolution X-ray diffraction (HR-XRD), Raman spectroscopy and photoluminescence (PL). PL spectrum of the sample has shown sharp and intense band edge emission of GaN with the absence of yellow emission band, indicating good crystal quality of the sample. The silver (Ag) metal contact was then deposited on the sample followed by thermal treatment at 500°C and 700°C, respectively. Treated sample at 700°C showed good spherical Ag islands on sample compared to the treated sample at 500°C. The effect of Ag islands on the electrical characteristics of sample was also examined by using I-V measurement. The results showed that the treated sample at 700°C has decreased the photo-current of Schottky diode.

Abstract: High quality undoped Al_xGa_1-xN with high Ga composition was grown on Si (111) substrate, using GaN/AlN as the buffer layer, by plasma-assisted molecular beam epitaxy (PAMBE). The present work reports on the photoluminescence (PL) studies of porous Al_xGa_1-xN prepared by ultraviolet (UV) assisted electrochemical etching in a solution of 2 % concentration of KOH electrolyte under illumination of an UV lamp with 500 W power for 30 min. The optical properties of porous Al_xGa_1-xN sample was compared to the corresponding as-grown GaN. PL studies suggested that the porosity was capable of improving the lattice mismatch induced strain. Porosity induced PL intensity enhancement was found in nanoporous sample. The resulting nanoporous Al_xGa_1-xN display red-shifted PL spectra compared to the as-grown Al_xGa_1-xN. Appearance of the red-shifted emission is correlated with the development of highly anisotropic structures in the morphology.

Abstract: In this report, the growth of GaN p-n junction on Si (111) substrate by plasma-assisted molecular beam epitaxy (PAMBE) is demonstrated. Doping of the GaN p-n junction has been carried out using Si and Mg as n-type and p-type dopants, respectively. Silicon substrate is used to grow the GaNpn-junction. In order to improve the crystalline quality of the nitride based junction, AlN is used as a buffer layer. The optical properties of the sample have been characterized by photoluminescence (PL) and Raman spectroscopy.PL spectrum shows a strong band edge emission of GaN at ~364nm, indicating good quality of the sample.The presence of peak ~657cm^-1 in Raman measurement has exhibited asuccessful doping of Mg in the sample. The structural properties are measured by high-resolution x-ray diffraction (HR-XRD) and scanning electron microscopy (SEM). The cross section of the SEM image of the sample has shown sharp interfaces.

Abstract: In the paper, a model is developed to discribe the band gap energy of Ⅲ nitride alloys. A new parameter A is used to discribe the band gap bowing. The new bowing parameter A is obtained by fitting the experimental values of the band gap energy. AAlGaN =0.46, AInGaN =0.59 and AInAlN =1.90 are obtained by fitting the experimental values of the band gap energy for AlGaN, InGaN and InAlN, respectively. The model is also suitable to discribe the band gap energy of other Ⅲ-Ⅴ ternary alloys.

Abstract: Stimulated emission and optical gain problems are reviewed on the basis of analysis of highly photoexcited III-Nitride and ZnO wide-gap materials. Spontaneous and stimulated emission along with optical gain of these materials in UV region is analyzed. The possible mechanisms of inverted population in In-containing materials and ZnO semiconductors are discussed involving mechanisms of dense electron-hole plasma, carrier (exciton) localization, as well as various interaction processes in dense exciton gas.

Abstract: This chapter serves as an introduction to the chapters on III-nitrides in this book. It gives a brief review of the development of relevant III-nitride materials for light emitters since the late 1960´s, when single crystalline GaN layers grown on sapphire were first demonstrated. The first wave of scientific work died out in the late 1970´s, since low-ohmic p-GaN could not be made at the time. After another 10 years several important breakthroughs were made, using the technology of metal organic vapor phase epitaxy (MOVPE). Smooth thin epilayers could be made, and ways to dope the materials n-type as well as p-type were invented. In the period 1986-1997 high brightness violet and blue double heterostructure (DH) LEDs, narrow quantum well (QW) LEDs, and QW based violet laser diodes with a long operating lifetime of 10000 hours were demonstrated, mainly by Japanese groups. Since then the development efforts have spread worldwide, and a large spectrum of novel applications based on nitride emitters are already in practical use. Perhaps the most important one is the future possibility of using nitride LEDs for general lighting purposes.