Wei-Chuan Chen

The effect of thermal annealing on the Ni/Au contact of p-type GaN

In this study, the Ni/Au layers prepared by electron beam evaporation and thermal alloying were used to form Ohmic contacts on p-type GaN films. Before thermal alloying, the current–voltage (I–V) characteristic of Ni/Au contact on p-type GaN film shows non-Ohmic behavior. As the alloying temperature increases to 700 °C, the I–V curve shows a characteristic of Ohmic contact. The Schottky barrier height reduction may be attributed to the presence of Ga–Ni and Ga–Au compounds, such as Ga4Ni3, Ga3Ni2, GaAu, and GaAu2, at the metal-semiconductor interface. The diffusing behavior of both Ni and Au have been studied by using Auger electron spectroscopy and Rutherford backscattering spectrometry. In addition, x-ray diffraction measurements indicate that the Ni3N and Ga4Ni3 compounds were formed at the metal-semiconductor interface.

Reduction in critical current density for spin torque transfer switching with composite free layer

A composite free layer (CFL) consisting of a soft layer and a hard layer exchange coupled in parallel is proposed. The experimental results showed that the critical current density (Jc) can be reduced from 7.05×106A∕cm2 of strong coupled CoFeB(12.5A)∕Ru(4A)∕NiFe(17.5A) CFL to 2.65×106A∕cm2 of weak coupled CoFeB(12.5A)∕Ru(15A)∕NiFe(17.5A) CFL. The macrospin simulations showed that the soft layer of CFL can assist the hard layer of CFL to switch at a lower Jc. These results suggest that by using CFL, it is possible to reduce the Jc of spin torque transfer switching without compromising the thermal stability.

PHOTOLUMINESCENCE SPECTROSCOPY OF MG-DOPED GAN

We have grown Mg-doped GaN films by metalorganic chemical vapor deposition with various CP2Mg flow rates. After 750 °C postgrowth annealing, p-type GaN films with carrier concentrations and mobilities about 2×1017/cm3 and 10 cm2/V s, respectively, have been achieved. A dominant photoluminescence (PL) line around 2.9 eV was observed at room temperature. By studying the dependence of PL on excitation density at 20 K, the emission line around 2.95 eV can be attributed to a donor-to-acceptor pair transition rather than to a conduction band-to-impurity transition involving the Mg-related deep level. We suggest that the DAP transition line is caused by a Mg related deep level at about 510 meV above the valence band. It is much deeper than the acceptor level at 250 meV commonly produced by the Mg dopants.

Interfacial and annealing effects on magnetic properties of CoFeB thin films

The interfacial and annealing effects on magnetic properties of CoFeB thin films with Ta and Ru interfaces were investigated. It is found that the thickness of magnetically dead layer of as-deposited CoFeB film strongly depends on the interfaces. After annealing above 150°C boron atoms seem to diffuse out of CoFeB film and the effective saturation magnetization of CoFeB film increases with annealing temperature and annealing time. In addition, CoFeB film with Ta interfaces remains amorphous while that with Ru interfaces starts to crystallize during the annealing above 250°C. Furthermore, the annealing-induced anisotropy field of CoFeB film can be modified by using different seed and capping materials.

Reduction in critical current density by tuning damping constants of CoFeB for spin-torque-transfer switching

Different capping layers were deposited on 2.5 nm Co72Fe8B20 to investigate their effects on damping constants and critical current density (JC0) for spin-torque-transfer switching. The damping constant of CoFeB is affected by the interfacial conditions and the spin-pumping effects. The Cu capping layer significantly suppresses intermixing and possesses the lowest damping constant of 0.009. The micromagnetic simulations reveal that the low damping constant of CoFeB may result in the nucleation of domains at lower current density, and thus reduces JC0. A reduction of 27% in JC0 can be achieved by replacing the conventional Ta capping layer with a Cu layer.

Wide operation margin of toggle mode switching for magnetic random access memory with preceding negative pulse writing scheme

In this work, a writing scheme with preceding negative pulse wave form for toggle magnetic random access memory (MRAM) is proposed to enhance the switching yield and enable a low current switching. The failure mechanism of toggle switching is studied by micromagnetic analysis. As a result of broadened operation window and reduced switching current, the scalability of MRAM is feasible with the robust toggle operation.

Improvement of Transport Properties in Magnetic Tunneling Junctions by Capping Materials

The capping effects on the properties of magnetic tunneling junctions (MTJs) have been investigated to improve the magnetoresistive (MR) ratio [Nagamine , Applied Physics, vol. 99, p. 08K703, 2006] and thermal stability [Fukumoto , Japanese Journal of Applied Physics, vol. 45, p. 3829, 2006. In this paper, MgO/Mg and MgO capping layers for MTJs were used to replace our previously used Ta and Ru capping layer [Yen , IEEE Transactions on Magnetics, vol. 42, no. 10, p. 862, 2004]. The MTJs with NiFe and CoFeB free layers were patterned to ellipses of 0.36 mum times 0.72 mum. The MR ratios of the MTJs with an NiFe free layer were enhanced from 26% to about 33% by capping MgO/Mg bilayers. A great improvement was also found on the MTJs with the CoFeB free layer, where the MR ratio was 50.4% with the MgO cap layer enhanced from 44.7% of Ru-capped MTJs. In addition, the influences of those capping layers included Ta, Ru, MgO, and MgO/Mg on thermal stability were also discussed

Improvement in Linearity of Novel InGaAsN-Based High Electron Mobility Transistors

We have fabricated InGaAsN-based high electron mobility transistors (HEMTs) using InGaAsN as the channel layer. An extremely large gate-voltage swing (GVS) up to 4.2 V can be achieved by utilizing the large conduction band offset between the GaAs spacer layer and the InGaAsN channel layer. However, the poor channel mobility and current density as a result of nitrogen-induced electrically active defects limit the transconductance (gm) performance. Attempts using various annealing temperatures have demonstrated that better device characteristics can be obtained via rapid thermal annealing at 700 °C. In this study, we investigate the effect of nitrogen-induced traps on the basis of Hall measurements and device characterizations of HEMTs. The improvement in GVS in the annealed samples is also discussed. Despite the relatively poor gain, InGaAsN HEMTs with excellent linearity performance after proper thermal annealing are expected to be compatible for novel InGaAsN-based optoelectronics integral circuits (OEICs).

Investigation of the optical properties of InGaAs(N):(Sb) quantum wells grown by metal organic vapor phase epitaxy

Dilute-nitride materials have attracted much attention due to their capability of operating in the wavelength range of optical communication. However, their optical properties degrade with the nitrogen incorporation into the host material such as InGaAs or GaAs, which can be recovered slightly by performing thermal treatment. More recently, the surfactant effect of Sb was verified by molecular beam epitaxy grown samples. Adding Sb into dilute-nitride materials can help to maintain the luminescence efficiency while increasing the emission wavelength toward 1.55μm. But its effect on the metal organic vapor phase epitaxy grown samples is not very clear. In this article, we performed a series of experiments on the InGaAsN(Sb) quantum wells to clarify the role of Sb. The photoluminescence (PL) intensities of InGaAsN:Sb were higher than those of the undoped samples and the extent of blueshift after annealing was slighter than that of the InGaAsN quantum wells. However, the PL intensities were still low and thus...

A 6-F/sup 2/ bit cell design based on one transistor and two uneven magnetic tunnel junctions structure and low power design for MRAM

Novel cell structures based on one transistor and two uneven magnetic tunnel junction cell and pillar write word line architecture are proposed to shrink the bit size with a potential down to 6 F2 by a so-called extended via process, and to reduce the writing current by a factor of 2, combined with the nature of nonvolatility and high speed, making the magnetoresistive random access memory suitable for universal memory applications