B.E. Weir

Tunneling current noise in thin gate oxides

We have examined fluctuations in the tunneling current of 3.5 nm SiO2 barriers for voltages in the direct tunneling regime. We find a 1/f power law for the spectral density of the fluctuations where f is the frequency. This 1/f noise can be attributed to fluctuations of a trap assisted tunneling current through the oxide that causes current noise but is not evident in the I–V curves. We suggest that this noise may be a more sensitive probe of trap assisted tunneling and degradation in thin oxides than other measures. At voltages above a threshold of 2.5 V, we observe the reversible onset of non‐Gaussian current transients in the noise. The onset of these current transients can be related to a transition in the spacial uniformity of the tunneling current density that may result in eventual breakdown of the oxide.

Understanding the limits of ultrathin SiO 2 and Si-O-N gate dielectrics for sub-50 nm CMOS

Abstract In spite of its many attributes such as nativity to silicon, low interfacial defect density, high melting point, large energy gap, high resistivity, and good dielectric strength, SiO 2 suffers from one disadvantage, low dielectric constant (K=3.9). Thus, ultrathin SiO 2 gate dielectric layers are required to generate the high capacitance and drive current required of sub-50 nm transistors. The silicon industry roadmap dictates 4 nm SiO 2 gate dielectrics for 0.25 μm technology today, and calls for 2 thickness for 0.05 μm technology in 2012. SiO 2 layers in this thickness range may suffer from boron penetration, reduced drive current, reliability degradation, and high gate leakage current. We will argue that none of these problems are limitations for thicknesses greater than about 1.3 nm. Below that thickness, the fundamental problems of high tunneling current and reduced current drive will prevent further scaling, unless alternate gate dielectrics are introduced.

TRANSPARENT CONDUCTIVE METAL-OXIDE CONTACTS IN VERTICAL-INJECTION TOP-EMITTING QUANTUM WELL LASERS

Optically transparent and electrically conductive cadmium tin oxide is employed in vertical cavity surface‐emitting lasers for vertical current injection. Continuous wave lasing at room temperature is achieved in GaAs/AlGaAs quantum well lasers. Devices with a 10 μm optical window, which also serves as a vertical current injection inlet, give lasing threshold currents as low as 3.8 mA. The differential series resistance is 350–450 Ω with a diode voltage of 5.1–5.6 V at lasing threshold. Far‐field pattern of the laser emission is Gaussian‐like with a full width at half maximum of 7°.

Misfit dislocation propagation kinetics in GexSi1−x/Ge(100) heterostructures

We report measurements of misfit dislocation propagation velocities in GexSi1−x epilayers grown upon Ge(100) substrates, as opposed to the more usual Si(100) substrates. This geometry allows us to study structures with high Ge concentration (x≥0.8), and to compare with previous extensive measurements for lower Ge concentration layers (x≤0.35) grown upon Si(100). It is found that all data are well described by a misfit dislocation velocity which is linear with excess stress, and which incorporates a compositionally dependent activation energy with linear interpolation between bulk values for Si and Ge. The combined data sets from structures grown on Si(100) and Ge(100) substrates is analyzed in the framework of the diffusive double kink model for dislocation motion.

Trap assisted tunneling as a mechanism of degradation and noise in 2-5 nm oxides

The mechanism of stress induced leakage current and dielectric breakdown is examined through 1/f noise in the tunneling current of 1.7-5 nm oxides. Before breakdown occurs, we find a linear relationship between low frequency 1/f noise and the increased current due to stress. This behavior can be described by a model of trap assisted tunneling for both phenomena. We develop a quantitative new model for the noise in terms of fluctuations in a trap assisted tunneling current through the oxide and show that the traditional charge-state fluctuation model is inconsistent with the voltage scaling of the noise. Our results demonstrate that noise can be used as a very sensitive measure of interface states, with a higher sensitivity than conventional capacitance-voltage relations. We show that the conduction mechanism in stressed and unstressed oxides is fundamentally different with the tunneling current in the unstressed oxides dominated by the fundamental limit of direct tunneling. Finally, noise in the post-breakdown state is used to understand the softening of breakdown at lower stressing conditions.

J-ramp on sub-3 nm dielectrics: noise as a breakdown criterion

An alternate criterion of failure for very thin oxides is proposed that can reliably detect the occurrence of both soft breakdown and hard breakdown during accelerated stress tests. We show that an increase in current noise that occurs at oxide breakdown can be detected rapidly with commercial test equipment, even when no discernable voltage drop can be observed. As an initial test vehicle for implementation of a noise test, we chose the JEDEC standard J-ramp test. This test can be implemented with very minor software changes and can detect both hard and soft breakdowns for 2-6 nm oxides.

Ultra-thin gate oxide reliability projections

Abstract We describe the reliability projection methods currently used and show that 1.6 nm oxides are sufficiently reliable even if soft breakdown is considered the point of failure. We also explore the possibility of using oxides after soft breakdown.

Effect of electronic corrections on the thickness dependence of thin oxide reliability

The thickness dependence of constant voltage lifetime tests are compared for ultrathin oxides in the range of 50–125 A. An apparent factor of 100 enhancement in the lifetime of 50 A oxides relative to the 125 A oxides is observed at a fixed electric field when the field in the oxide is calculated using the physical thickness of the oxide as determined by ellipsometry. When corrections are made for the distribution of electrons at the silicon interface including depletion in the silicon and quantum-mechanical screening effects then this apparent enhancement is reduced and all oxides have similar lifetimes at a fixed field. Such a rescaling of oxide reliability demonstrates the importance of accurate determination of electric field and oxide voltage in thin oxides and that oxide reliability is not significantly affected by thickness down to 50 A, depending only on field. We compare different techniques for determining the effective thickness using current-voltage or capacitance-voltage (C-V) curves. We show...

Vertical cavity top-surface emitting lasers with thin Ag mirrors and hybrid reflectors

Abstract Three types of vertical cavity surface emitting lasers based on GaAs/AlGaAs have been grown by molecular beam epitaxy (MBE). The laser structures with top emission have been evolved from a simple double heterostructure (DH) with n-type distributed Bragg reflectors (DBR) as the bottom mirror and a semitransparent Ag as the top mirror, to a DH structure with a hybrid reflector consisting of a thin metal and a few pairs of p-DBR as the top mirror, and to a 3-QW structure again with a hybrid reflector as the top mirror. MBE growth of these three device structures is discussed, particularly on the in-situ growth of metals on GaAs. Devices have been fabricated and measured. A theoretical model has been used to gain an understanding of the parameters in each design. The MBE growth techniques have been improved to modify the heterostructural interface in the DBR to reduce the series resistance. The laser performance has been improved from a high threshold current in the first structure to a much lower threshold current in the second structure and finally to a cw room temperature operation with low threshold currents in the third structure.

Monolithic integration of a photodetector and a vertical-cavity surface-emitting laser

Summary form only given. Monolithic integration of a photodetector with a vertical-cavity surface-emitting laser (SEL) is reported. The SELs lase at room temperature and emit a 850-nm highly coherent, low-divergence, circular beam directly from the top surface. The integrated photodetector shows a linear response to the laser emission with an effective responsivity of 0.25 A/W. The SEL consists of a GaAs multiquantum-well active region and doped Al/sub 0.15/Ga/sub 0.85/As/AlAs distributed Bragg reflectors (DBRs). Proton implantation was used to fabricate planar gain-guided lasers. Since the GaAs substrate is opaque to the lasing wavelength, the top mirror is designed to have lower reflectivity and annular contacts are used to permit emission from the top surface. After growing the SEL, additional i-GaAs and n-AlGaAs layers were grown on top of the p-doped DBR to form the p-i-n photodetector (PD). >