Growth and characterization of AlGaN/GaN heterostructure using unintentionally doped AlN/GaN superlattices as barrier layer

AlGaN/GaN heterostructure using unintentionally doped AlN/GaN superlattices (SLs) as barrier layer is grown on C-plane sapphire by metal organic vapor deposition (MOCVD). Compared with the conventional Si-doped structure, electrical property is improved. An average sheet resistance of 287.1 Omega/square and high resistance uniformity of 0.82% are obtained across the 2-inch epilayer wafer with an equivalent Al composition of 38%. Hall measurement shows that the mobility of two-dimensional electron gas (2DEG) is 1852 cm(2)/V s with a sheet carrier density of 1.2 x 10(13) cm(-2) at room temperature. The root mean square roughness (RMS) value is 0.159 nm with 5 x 5 mu m(2) scan area and the monolayer steps are clearly observed. The reason for the property improvement is discussed. (c) 2008 Elsevier Ltd. All rights reserved.

Stress and its effect on optical properties of GaN epilayers grown on Si(111), 6H-SiC(0001), and c-plane sapphire

The stress states in unintentionally doped GaN epilayers grown on Si(111), 6H-SiC(0001), and c-plane sapphire, and their effects on optical properties of GaN films were investigated by means of room-temperature confocal micro-Raman scattering and photoluminescence techniques. Relatively large tensile stress exists in GaN epilayers grown on Si and 6H-SiC while a small compressive stress appears in the film grown on sapphire. The latter indicates effective strain relaxation in the GaN buffer layer inserted in the GaN/sapphire sample, while the 50-nm-thick AlN buffer adopted in the GaN/Si sample remains highly strained. The analysis shows that the thermal mismatch between the epilayers and the substrates plays a major role in determining the residual strain in the films. Finally, a linear coefficient of 21.1+/-3.2 meV/GPa characterizing the relationship between the luminescent bandgap and the biaxial stress of the GaN films is obtained. (C) 2003 American Institute of Physics.

In situ doping of 3C-SiC grown on (0001) sapphire substrates by LPCVD

The heteroepitaxial growth of n-type and p-type 3C-SiC on (0001) sapphire substrates has been performed with a supply of SiH4+C2H4+H-2 system by introducing ammonia (NH3) and diborane (B2H6) precursors, respectively, into gas mixtures. Intentionally incorporated nitrogen impurity levels were affected by changing the Si/C ratio within the growth reactor. As an acceptor, boron can be added uniformly into the growing 3C-SiC epilayers. Nitrogen-doped 3C-SiC epilayers were n-type conduction, and boron-doped epilayers were p-type and probably heavily compensated.

Photoluminescence of rapid-thermal annealed Mg-doped GaN films

We report the investigation of temperature and excitation power dependence in photoluminescence spectroscopy measured in Mg-doped GaN epitaxial layers grown on sapphire by metalorganic chemical vapor deposition, The objective is to examine the effects of rapid-thermal annealing on Mg-related emissions. It is observed that the peak position of the 2.7-2.8 eV emission line is a function of the device temperature and annealing conditions, The phenomenon is attributed to Coulomb-potential fluctuations in the conduction and valence band edge and impurity levels due to the Mg-related complex dissociation. The blue shift of the 2.7-2.8 eV emission line with increasing excitation power provides clear evidence that a donor-acceptor recombination process underlies the observed emission spectrum. In addition, quenching of minor peaks at 3.2 and 3.3 eV are observed and their possible origin is discussed. (C) 2001 Elsevier Science Ltd. All rights reserved.

Competition between band gap and yellow luminescence in undoped GaN grown by MOVPE on sapphire substrate

Undoped GaN epilayer on c-face (0 0 0 1) sapphire substrate has been grown by metalorganic vapor-phase epitaxy (MOVPE) in a horizontal-type low-pressure two-channel reactor. Photoluminescence (PL) as a function of temperature and excitation intensity have been systematically studied, and the competition between near band gap ultraviolet (UV) and defect-related yellow luminescence (YL) has been extensively investigated, It is revealed that the ratio of the UV-to-YL peak intensities depends strongly on the excitation intensity and the measurement temperature. The obtained results have been analyzed in comparison with the theoretical predications based on a bimolecular model. (C) 2001 Elsevier Science B.V. All rights reserved.

Effect of rapid thermal annealing on Ti-AlN interfaces

The interface diffusion, reaction, and adherence of rapid thermal annealed Ti/ALN were investigated by RES, AES, SIMS, XRD and a scratch test. The experimental results show that diffusion and reaction occurs at the interface of Ti/AlN when the sample is rapidly annealed. During annealing, both the O adsorbed on the surface and doped in the AlN substrate diffuse into the Ti film. At low temperature TiO2 is produced. At higher temperature O reacts with the diffused Al in the Ti film and produces an Al2O3 layer in the middle of the film. N diffuses into the Ti film and produces TiN with an interface reaction. Ti oxide is produced at the interface between the film and the substrate. Scratch test results show that interface adherence is distinctly improved by rapid annealing at low temperature and decreases at higher temperature. (C) 1999 Elsevier Science B.V. All rights reserved.

Fermi-edge singularity observed in a modulation-doped AlGaN/GaN heterostructure

In this letter, we report on the observation of Fermi-edge singularity in a modulation-doped AlGaN/GaN heterostructure grown on a c-face sapphire substrate by NH3 source molecular beam epitaxy. The two-dimensional electron gas (2DEG) characteristic of the structure is manifested by variable temperature Hall effect measurements down to 7 K. Low-temperature photoluminescence (PL) spectra show a broad emission band originating from the recombination of the 2DEG and localized holes. The enhancement in PL intensity in the high-energy side approaching Fermi level was observed at temperatures below 20 K. At higher temperatures, the enhancement disappears because of the thermal broadening of the Fermi edge. (C) 1998 American Institute of Physics. [S0003-6951(98)02543-1].

The growth of an AlGaN/GaN modulation-doped heterostructure by NH3 source molecular beam epitaxy

Using NH3 cracked on the growing surface as the nitrogen precursor, an AlGaN/GaN modulation-doped (MD) heterostructure without a buffer layer was grown on a nitridated sapphire substrate in a home-made molecular beam epitaxy (MBE) system. Though the Al composition is as low as 0.036, as deduced from photoluminescence (PL) measurements, the AlGaN barrier layer can be an efficient carrier supplier for the formation of a two-dimensional electron gas (2DEG) at the heterointerface. The 2DEG characteristics are verified by the variable temperature Hall measurements down to 7 K. Using a parallel conduction model, we estimate the actual mobility of the 2DEG to be 1100 cm(2)/V s as the sheet carrier density to be 1.0 x 10(12) cm(-2). Our results show that the AlGaN/GaN system is very suitable for the fabrication of high electron mobility transistors (HEMTs). (C) 1998 Elsevier Science B.V. All rights reserved.

Influence of Al content on electrical and structural properties of Si-doped AlxGa1-xN/GaN HEMT structures

Improved electrical properties of AlxGa1-xN/GaN high electron mobility transistor (HEMT) structures grown by metalorganic chemical vapor deposition (MOCVD) were achieved through increasing the Al mole fraction in the AlGaN barrier layers. An average sheet resistance of 326.6 Omega/sq and a good resistance uniformity of 98% were obtained for a 2-inch Al0.38Ga0 62N/GaN HEMT structure. The surface morphology of AlxGa1-xN/GaN HEMT structures strongly correlates with the Al content. More defects were formed with increasing Al content due to the increase of tensile strain, which limits further reduction of the sheet resistance. (c) 2006 WILEY-VCH Verlag GmbH & Co KGaA, Weinheim.

X-Band GaN Power HEMTs with Power Density of 2.23W/mm Grown on Sapphire by MOCVD

The growth,fabrication,and characterization of 0.2μm gate-length AlGaN/GaN HEMTs,with a high mobility GaN thin layer as a channel,grown on (0001) sapphire substrates by MOCVD,are described.The unintentionally doped 2.5μm thick GaN epilayers grown with the same conditions as the GaN channel have a room temperature electron mobility of 741cm2/(V·s) at an electron concentration of 1.52×1016 cm-3.The resistivity of the thick GaN buffer layer is greater than 108Ω·cm at room temperature.The 50mm HEMT wafers grown on sapphire substrates show an average sheet resistance of 440.9Ω/□ with uniformity better than 96%.Devices of 0.2μm×40μm gate periphery exhibit a maximum extrinsic transconductance of 250mS/mm and a current gain cutoff frequency of77GHz.The AlGaN/GaN HEMTs with 0.8mm gate width display a total output power of 1.78W (2.23W/mm) and a linear gain of 13.3dB at 8GHz.The power devices also show a saturated current density as high as 1.07A/mm at a gate bias of 0.5V.

In situ doping of 3C-SiC grown on (0001) sapphire substrates by LPCVD

The heteroepitaxial growth of n-type and p-type 3C-SiC on (0001) sapphire substrates has been performed with a supply of SiH4+C2H4+H-2 system by introducing ammonia (NH3) and diborane (B2H6) precursors, respectively, into gas mixtures. Intentionally incorporated nitrogen impurity levels were affected by changing the Si/C ratio within the growth reactor. As an acceptor, boron can be added uniformly into the growing 3C-SiC epilayers. Nitrogen-doped 3C-SiC epilayers were n-type conduction, and boron-doped epilayers were p-type and probably heavily compensated.

V5+-doped Bi3.4Yb0.6Ti3O12 fatigue resistant ferroelectric thin films

In this paper, a serial of Bi3.4Yb0.6Ti3-xVxO12 (BYTV) thin film with different V5+ contents were deposited on Pt/Ti/SiO2/Si substrates by chemical solution deposition (CSD). The crystallized phase and electrical properties of the films were investigated using X-ray diffraction, polarization hysteresis loops, leakage current-voltage, and fatigue test. From our experimental results, it can be found that the ferroelectric properties can be improved greatly using V5+-doped in Bi3.4Yb0.6Ti3O12 (BYT) thin film, compared with the reported BYT thin film. The remanent polarization was enhanced and excellent leakage current characteristic with 10(-11)A at the bias voltage of 4V, which is much lower than the BYT thin film or some reported bismuth layer-structure ferroelectric films. Fatigue test shows that the fabricated films have good anti-fatigue characteristic after 10(10) switching cycles. (c) 2008 Published by Elsevier B.V.

Comparative study on the doping effect of 3d elements in Bi(1.5)Pb(0.2)Sr(2)Ca(2)Cu(2.8)M(0.2)O(y) (M=Sc,Ti,V,Cr,Mn,Fe,Co,Ni, or Zn)

With XRD, R-T, and ac chi measurements a comparative study on the doping effects of 3d elements in Bi(1.5)Pb(0.2)Sr(2)Ca(2)Cu(2.8)M(0.2)O(y) (M = Sc, Ti, V, Cr, Mn, Fe, Co, Ni, or Zn) has been carried out. The effects of the former five members are significantly different, both on phase formed and on T-c, from the latter four. It seems that the effect on phase stabilization correlates with the valency of the doped cation. In connection with the instability of the 2223 phase, the correlation has been discussed.

The phase transitions of hetero atoms substituted molecular sieves Me-VPI-5(Me=Mg, Ti, Sn, Si) and their influence on spectra structures of Eu(III) ion

The hetero atom substituted aluminophosphate molecular sieves Me-VPI-5(Me = Mgt Ti, Sn, Si) were synthesized hydrothermally. Rare earth ions are originally doped into these microporous materials by aqueous solution ion exchange procedures. The phase transitions of the microporous materials are investigated by high-temperature and high-pressure experimental techniques. The influence of the phase transitions on the rare earth ions' spectral structures is discussed, With the increase of temperature, Eu(II)Mg-VPI-5 is converted into Eu(II)Mg-AIPO(4)-8, then into tridymite phase. The pressure has a notable influence on Eu(II) ion's spectral structures. The spectral structures have changed regularly with the increase of pressure.

Enhanced He-α emission from ‘smoked’ Ti targets irradiated with 400nm 45fs laser pulses

We present a study of He-like 1s(2)-1s2p line emission from solid and low-density Ti targets under similar or equal to 45 fs laser pulse irradiation with a frequency doubled Ti: Sapphire laser. By varying the beam spot, the intensity on target was varied from 10(15) W/cm(2) to 10(19) W/cm(2). At best focus, low density "smoked" Ti targets yield similar to 20 times more He-alpha than the foil targets when irradiated at an angle of 45 degrees with s-polarized pulses. The duration of He-alpha emission from smoked targets, measured with a fast streak camera, was similar to that from Ti foils.