R. Adari

Dual-

A dual-k spacer concept is proposed and evaluated in underlap and nonunderlap n-channel silicon tunnel field-effect transistors (FETs) for the first time using extensive device simulations. The dual-k spacer consists of an inner layer made of a high-k material and an outer layer made of a low-k material. It is shown that the dual-k spacer improves the performance of n-channel tunneling FETs and more so for the underlap structures. Performance improvements are illustrated and explained for SiO2, Al2O3, and HfO2 gate dielectrics. The structure is optimized for the on-state current without degrading the off-state current or the subthreshold slope.

k

We have developed a double-channel high electron mobility transistor with back barriers for carrier confinement. We have observed that the double-channel devices may suffer from the lack of gate control particularly for the lower channel. However, the problem can be contained by using a suitable back barrier for the lower channel. The double-channel back-barrier devices show good current gain and power gain cutoff frequencies. These devices can be operated with excellent gain linearity up to a larger value for input power and frequency.

Spacer Device Architecture for the Improvement of Performance of Silicon n-Channel Tunnel FETs

We have demonstrated electrical spin-injection from GaCrN dilute magnetic semiconductor (DMS) in a GaN-based spin light emitting diode (spin-LED). The remanent in-plane magnetization of the thin-film semiconducting ferromagnet has been used for introducing the spin polarized electrons into the non-magnetic InGaN quantum well. The output circular polarization obtained from the spin-LED closely follows the normalized in-plane magnetization curve of the DMS. A saturation circular polarization of ∼2.5% is obtained at 200 K.

Double-Channel AlGaN/GaN High Electron Mobility Transistor With Back Barriers

We report room temperature ferromagnetism in crystalline GaCrN prepared by Cr deposition and drive-in diffusion with Curie temperature much above 300 K. The Curie temperature increases with increasing active Cr concentration. Cr doped GaN acts as an n-type material with significant increase in electron carrier concentration due to the presence of Cr. Optical property of GaCrN is found to be very similar to GaN with an additional peak at 3.29 eV due to Cr. The hysteresis measurements show that the ferromagnetic ordering is maintained up to 300 K with no significant change in saturation magnetization.

Electrical spin injection using GaCrN in a GaN based spin light emitting diode

The effect of feature sizes on the characteristics of lateral spintronic devices have been investigated experimentally and theoretically. It is demonstrated that confining spin-transport in the active region of a device enhances magnitude of the spin-dependent response substantially. Numerical simulation of spin-transport corroborates the experimental observations. Device characteristics are found to be a strong function of spin-polarizer and analyzer dimensions. The response is observed to attain a peak value for an optimum device feature size, and this is seen to be a function of temperature. Spin dependent effects become weaker for very small and very large devices.

Room temperature ferromagnetism in thermally diffused Cr in GaN

We have studied small signal frequency response of a spin laser. We have shown that the response is characterized by two distinct resonant peaks corresponding to the two polarization modes of the spin laser. It is observed that the modulation bandwidth of a spin laser can be smaller or larger than that of a conventional laser depending upon the current bias and spin relaxation time constant. A small value for spin relaxation constant may not be detrimental for modulation bandwidth. This anomalous observation is explained by considering both the amplitude and phase response of the two polarization modes. A spin laser can act as a combination of low-pass and bandpass filters. The passband frequency range is tunable by external bias. We have also studied the evolution of resonant peaks and modulation bandwidth as a function of spin relaxation time constant.

Enhanced magnetoresistance in lateral spin-valves

We have investigated ferromagnetic (Fe, Co and Ni) Schottky diodes on heavily n-doped GaN. We have grown, fabricated, characterized, and analyzed these diodes to extract junction parameters and ascertain transport mechanism. We have observed simple thermionic emission failed to explain the experimental characteristics. Thermionic emission with Gaussian distribution for barrier inhomogeneity can explain the data for high bias voltages. It is observed other transport mechanisms at low bias voltages are important. Direct tunneling have been found to contribute significantly for Fe/GaN Schottky diodes. However, trap assisted tunneling is found to be dominant for Co/GaN and Ni/GaN devices under same condition.

Modulation bandwidth of a spin laser

A surface emitting spin-polarized light emitting diode (Spin-LED) is considered for the study of the device geometry dependence of the output circular polarization It is found that the output circular polarization is maximum at the outer region of the circular non-FM Ohmic contact to the p-region, which delineates radial extent of the active region in the intrinsic layer. We observe that the output circular polarization increase as a function of the p-contact radius when the contact size is small but for large values of the p-contact radius the reverse trend is noted. An enhancement of the spin diffusion length increases the degree of polarization of the emitted circular light. The enhancement however is observed to fall off for still higher values of spin lifetime.

Characteristics of ferromagnetic Schottky diodes on heavily n-doped GaN semiconductor

We have theoretically designed and experimentally demonstrated a multiterminal lateral magnetoresistive device using electrical spin injection and detection in semiconductor, which can act as a memory array. The memory state is associated with the direction of the magnetization state. The state is read out from clear transition in current both in magnitude and direction depending on the relative magnetization direction. The current shows a plateau in varying magnetic field regions indicating stable magnetization direction due to differences in coercivity. The device characteristics are well explained in the framework of two-dimensional spin transport and spin-dependent tunneling.

Effects of Device Geometry on Output Circular Polarization in a Spin-LED

GaN is one of the most important semiconductor materials, for high-speed and many other applications [1]. It also happens to have small spin-orbit coupling, which suggests that spintronic functionality may be effectively integrated on to it. This would require optimized ferromagnet-GaN Schottky contacts for spin injection or detection [2]. We choose iron (Fe) as a high spin polarization material and investigate for the first time the characteristics of Fe/n-GaN Schottky diodes. We have determined Schottky barrier height, barrier inhomogeneity, and ideality factor from current-voltage (I-V ) and capacitancevoltage (C-V ) characteristics. We have also determined the effect of temperature on these parameters. We have observed barrier height inhomogeneity which is found to match well with a Gaussian Distribution (GD). We have also extracted Richardson’s constant for these diodes. Though thermionic emission is found to be the dominant mode of carrier transport under forward bias condition, electron tunneling may contribute significantly under reverse biased condition. The heterostructure of the sample used in our study is shown in Fig. 1(a). An undoped GaN buffer layer of 7 μm is grown on sapphire (Al2O3) substrate by hydride vapor-phase-epitaxy (HVPE). The Si-doped (n-type) GaN layer (2 μm) is then grown by metal-organic-chemical-vapor-deposition (MOCVD) having a doping density of 1-2×10 cm−3. The sample is cleaned and native oxide is removed before depositing Ti(20 nm)/Al(150 nm)/Ni(50 nm)/Au(125 nm) metal stack by electron-beam (e-beam) evaporation and annealing at 775 C using rapid thermal annealing (RTA) for the annular low resistance ohmic contact. Fe(50 nm)/Ti (6 nm)/Au(125 nm) Schottky contacts 60 μm in diameter are then deposited at the centre by e-beam evaporation. A micro-photograph of the device is shown in Fig. 1(b).