Recent investigations have shown that cycloaddition reactions, widely used in organic chemistry to form ring compounds, can also be applied to link organic molecules to the (001) surfaces of crystalline silicon, germanium, and diamond. While these surfaces are comprised of SiSi, GeGe, and CC structural units that resemble the CC bonds of organic alkenes, the rates and mechanisms of the surface reactions show some distinct differences from those of their organic counterparts This article reviews recent studies of [2 + 2], [4 + 2] Diels−Alder, and other cycloaddition reactions of organic molecules with semiconductor surfaces and summarizes the current understanding of the reaction pathways.

Cycloaddition chemistry of organic molecules with semiconductor surfaces.

A two-step procedure, involving radical-initiated chlorination of the Si surface with PCl5 followed by reaction of the chlorinated surface with alkyl-Grignard or alkyl-lithium reagents, has been developed to functionalize crystalline (111)-oriented H-terminated Si surfaces. The surface chemistry that accompanies these reaction steps has been investigated using X-ray photoelectron spectroscopy (XPS), Auger electron spectroscopy (AES), temperature programmed desorption spectroscopy (TPDS), high-resolution electron energy loss spectroscopy (HREELS), infrared (IR) spectroscopy in both glancing transmission (TIR) and attenuated total multiple internal reflection (ATR) modes, ellipsometry, and contact angle goniometry. The XPS data show the appearance of the Cl signal after exposure to PCl5 and show its removal, and concomitant appearance of a C 1s signal, after the alkylation step. Auger electron spectra, in combination with TPD spectroscopy, demonstrate the presence of Cl after the chlorination process and it...

Spectroscopic Studies of the Modification of Crystalline Si(111) Surfaces with Covalently-Attached Alkyl Chains Using a Chlorination/Alkylation Method

Light emitting diodes include a diode region having first and second opposing faces that include therein an n-type layer and a p-type layer, an anode contact that ohmically contacts the p-type layer and extends on the first face, and a cathode contact that ohmically contacts the n-type layer and also extends on the first face. The anode contact and/or the cathode contact may further provide a hybrid reflective structure on the first face that is configured to reflect substantially all light that emerges from the first face back into the first face. Related fabrication methods are also described.

Semiconductor light emitting diodes having reflective structures and methods of fabricating same

We demonstrated that nanoimprint lithography (NIL) can create three-dimensional patterns, sub-40 nm T-gates, and air-bridge structures, in a single step imprint in polymer and metal by lift-off. A method based on electron beam lithography and reactive ion etching was developed to fabricate NIL molds with three-dimensional protrusions. The low-cost and high-throughput nanoimprint lithography for three-dimensional nanostructures has many significant applications such as monolithic microwave integrated circuits and nanoelectromechanical system.

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Direct three-dimensional patterning using nanoimprint lithography

The adsorption of pyrrole, aniline, 3-pyrroline, and pyrrolidine on the Si(001)−(2 × 1) surface has been studied using Fourier transform infrared (FTIR) spectroscopy and X-ray photoelectron spectroscopy (XPS). Both pyrrole and aniline retain their aromatic character after bonding to the surface. Spectroscopic evidence indicates that each of these aromatic molecules can attach to the Si(001) surface via cleavage of one N−H bond, linking the molecule to the surface through a Si−N tether. Isotopic studies of pyrrole show evidence for additional cleavage of C−H bonds. While strong selectivity favoring bonding through the nitrogen atom is observed for the aromatic molecules, the unsaturated molecule 3-pyrroline shows evidence for at least two bonding configurations. XPS and FTIR data show that 3-pyrroline can bond either through the nitrogen atom with cleavage of an N−H bond, or through the CC bond via the surface equivalent of a [2 + 2] cycloaddition reaction. Pyrrolidine appears to bond only through the nitr...

Bonding of Nitrogen-Containing Organic Molecules to the Silicon(001) Surface: The Role of Aromaticity†

An asymmetric double-recessed gate process achieved through a single lithography step and a combination of wet and dry etching techniques is presented. The double-recessed gate process is beneficial in the fabrication of InGaAs/AlGaAs/GaAs pseudomorphic high electron mobility transistors (PHEMTs) because breakdown voltage is enhanced while surface effects on the drain side of the gate are minimized. In contrast to conventional processes that require two lithography steps, the current process requires only a single lithography step for the asymmetric placement of a T-gate in a wide recess trench. The process utilizes a four-layer resist of polymethylmethacrylate (PMMA) and P(MMA-MAA) exposed by electron beam lithography. Upon development of the resist, a wet selective etch (citric acid:H2O2) is used to define the wide recess trench and then a dry selective etch (SiCl4/SiF4) is used to recess a narrow trench (within the wide recess trench) in which the gate foot rests. This technique can achieve gate length...

Single step lithography for double-recessed gate pseudomorphic high electron mobility transistors

The DC and microwave characteristics of two sets of AlGaAs/InGaAs PHEMTs having a gate length of 0.2 /spl mu/m are compared. The first set is composed of devices fabricated using a trilayer electron beam resist process for T-gate recess and metallization. The second set is composed of devices fabricated using a new four-layer electron beam resist process which enables the asymmetric placement of a T-gate in a wide recess trench. Devices fabricated using the four-layer resist process showed improved breakdown voltage, lower gate-drain feedback capacitance, lower output conductance, and higher f/sub max/ with only slight reduction of drain current and transconductance. For example, the off-state drain-source breakdown voltage increased from 5.2 to 12.5 V, and the f/sub max/, increased from 133 to 158 GHz as the drain side cap recess, L/sub ud/, was increased from 0 to 0.55 /spl mu/m.

Utilization of an electron beam resist process to examine the effects of asymmetric gate recess on the device characteristics of AlGaAs/InGaAs PHEMTs

A procedure utilizing AlAs etch stop layers coupled with wet etch processes has been developed to accurately control etch depth in the fabrication of uniform parallel quantum wires (QWs) in GaAs/AlGaAs heterostructures. Wires are defined in poly(methylmethacrylate) by electron beam lithography and then transferred into the heterostructure using citric acid/hydrogen peroxide solutions. The use of the etch stop layers permits wires to be accurately patterned by shallow or deep etching and allows for a comparative study of the effect of etch depth on the active dimensions of the wires. It is shown that shallow‐etched QWs have active electron channels wider than deep‐etched ones due to minimization of sidewall depletion. Also, it is demonstrated that sidewall passivation is achieved with silicon nitride and this aids in making the active electron channel width closer to the physical width of the QWs. Gated QW devices with wires as long as 10 μm have exhibited quantum conductance at 4.2 K which shows that the ...

Fabrication of parallel quantum wires in GaAs/AlGaAs heterostructures using AlAs etch stop layers

Disorder-induced resonant tunneling in planar quantum-dot nanostructures.

We have successfully observed negative differential conductance in an electrostatically confined double-bend quantum dot device at low temperatures under different source-drain bias conditions. The structure fabricated on a modulation-doped GaAs/AlGaAs layer consists of a zero-dimensional (0D) submicrometre dot that is connected to wide source and drain regions via one-dimensional (1D) leads. By changing the external gate voltage, the size of the dot and 1D leads can be altered. Conductance oscillations have been observed for temperatures as high as 7 K. To our knowledge such oscillations have not previously been reported in similar laterally confined quantum dot devices at such high temperatures.

R. Grundbacher

Papers

Single step lithography for double-recessed gate pseudomorphic high electron mobility transistors

Utilization of an electron beam resist process to examine the effects of asymmetric gate recess on the device characteristics of AlGaAs/InGaAs PHEMTs

Fabrication of parallel quantum wires in GaAs/AlGaAs heterostructures using AlAs etch stop layers

Disorder-induced resonant tunneling in planar quantum-dot nanostructures.

Oscillatory conductance in a double-bend quantum dot device