J. B. Posthill

Single‐crystal diamond plate liftoff achieved by ion implantation and subsequent annealing

We describe a new method for removing thin, large area sheets of diamond from bulk or homoepitaxial diamond crystals. This method consists of an ion implantation step, followed by a selective etching procedure. High energy (4–5 MeV) implantation of carbon or oxygen ions creates a well‐defined layer of damaged diamond that is buried at a controlled depth below the surface. For C implantations, this layer is graphitized by annealing in vacuum, and then etched in either an acid solution, or by heating at 550–600 °C in oxygen. This process successfully lifts off the diamond plate above the graphite layer. For O implantations of a suitable dose (3×1017 cm−2 or greater), the liftoff is achieved by annealing in vacuum or flowing oxygen. In this case, the O required for etching of the graphitic layer is also supplied internally by the implantation. This liftoff method, combined with well‐established homoepitaxial growth processes, has considerable potential for the fabrication of large area single crystalline dia...

On the feasibility of growing dilute CxSi1−x epitaxial alloys

Dilute CxSi1−x epitaxial films have been grown on Si(100) by remote plasma‐enhanced chemical vapor deposition. Carbon concentrations of ∼3 at.% have been achieved at a growth temperature of 725 °C. No evidence for the formation or precipitation of SiC was found using x‐ray diffraction and transmission electron microscopy.

Direct deposition of polycrystalline diamond films on Si(100) without surface pretreatment

Dense nucleation of small‐grain polycrystalline diamond films on Si(100) substrates has been accomplished without the use of any surface pretreatment such as abrasive diamond scratching, surface oil treatments, or diamond‐like carbon predeposition. Diamond depositions occurred in a low‐pressure rf plasma‐assisted chemical vapor deposition system using mixtures of CF4 and H2. Films deposited at 5 Torr and 850 °C on as‐received silicon wafers show dense nucleation, well‐defined facets, and crystallites which ranged in size from 500 to 10 000 A. X‐ray photoelectron spectroscopy and electron energy loss show the films to be diamond with no major impurity and no detectable graphitic component. Raman spectroscopy shows a pronounced 1332 cm−1 line accompanied with a broad band centered about 1500 cm−1.

Energy Harvesting for Electronics with Thermoelectric Devices using Nanoscale Materials

Significant developments have occurred in the last few years in the area of nanoscale thermoelectric materials using superlattices and self-assembled quantum-dots. Thin-film thermoelectric (TE) devices employing these materials have been developed for many applications including energy harvesting. Thin-film TE devices, for a 1 mm3 of converter volume, are available that can produce well over 775 muW/mm3 with an external DeltaT of 9 K. Such modules can be packaged within conventional chip packages, unobtrusively, and provide valuable DC electric power in the range of 100 mW without the need for any DC-DC conversion using heat produced by ~10 to 20 Watt chips. Even larger power levels are harvestable in high power electronics such as IGBTs. It appears that the advanced TE modules can provide sufficient power, over the background requirements, to directly power electronics with temperature differentials as little as 1degC. Near-term candidate applications are in bio-implants, sensors, robotics and energy-limited electronics in thermally active environments. Miniature thermoelectric power harvesters can be integrated with other energy harvesting technologies such as photovoltaics and vibration energy harvesters to provide universal energy harvesting for autonomous systems.

In situ cleaning of GaAs surfaces using hydrogen dissociated with a remote noble-gas discharge

In situ cleaning of GaAs surfaces has been achieved at 350 °C with a novel technique employing hydrogen that is excited and dissociated using a remote Ar discharge. Reconstructed surfaces characteristic of clean, As‐stabilized GaAs surfaces have been observed with reflection high‐energy electron diffraction following the cleaning treatment. Auger electron spectroscopy analyses confirm that such a treatment removes both carbon and oxygen contamination from the surface. X‐ray photoelectron spectroscopy shows the removal of oxygen bonded to both Ga and As on the surface. Emission spectroscopy shows evidence of excited molecular and atomic hydrogen with the downstream‐excitation process.

Low‐defect‐density germanium on silicon obtained by a novel growth phenomenon

Heteroepitaxial Ge on Si has been grown using molecular beam epitaxy at a Si substrate temperature of 900 °C. Electron microscopy results reveal a highly faceted interface, indicating localized Ge melting and subsequent local alloying with Si. Furthermore, this phenomenon is associated with extensive threading dislocation confinement near the Ge/Si interface. Etch pit density measurements obtained on Ge heteroepitaxial films that have undergone interfacial melting are as low as 105 cm−2.

The role of atomic hydrogen and its influence on the enhancement of secondary electron emission from C(001) surfaces

The role of chemisorbed hydrogen in the enhancement of low-energy electron emission from natural type IIb C(001) diamond surfaces has been investigated. A hydrogen induced low-energy emission peak, whose intensity was found to be a linear function of surface coverage, was observed. The direct observation of emission from vacuum level states in the photoemission spectra has determined a negative electron affinity of ∼0.4 eV for the hydrogenated C(001)-1×1 surface. Constant initial states photoemission has unambiguously identified the electron emission process with the escape of electrons from bulk electron states at the conduction-band minimum.

SECONDARY ELECTRON EMISSION ENHANCEMENT AND DEFECT CONTRAST FROM DIAMOND FOLLOWING EXPOSURE TO ATOMIC HYDROGEN

Polished nominal (100) surfaces of four types of diamonds were exposed to atomic hydrogen by hot filament cracking of H2 gas or by immersion in a H2 plasma discharge. Both types IIa and IIb (100) diamond surfaces exhibited the following characteristic changes: (a) secondary electron (SE) yield increased by a factor of ∼30 as measured in a scanning electron microscope (SEM), (b) near‐surface, nontopographical defects were observable directly using the conventional SE mode of the SEM, (c) surface conductance increased by up to 10 orders of magnitude. These changes were observed only weakly in nitrogen‐containing types Ia and Ib diamonds.

High quality GaAs on Si using Si0.04Ge0.96/Ge buffer layers

Abstract High-quality epitaxial growth of GaAs on Si has been achieved using Si0.04Ge0.96/Ge buffer layers. GaAs layers, approximately 1.3 μm thick, have been grown on Si using interfaces of Ge/Si0.04Ge0.96 layers to confine the majority of misfit dislocations that are generated by the 4% lattice mismatch between Ge and Si. The GaAs layers, grown by organometallic vapor phase epitaxy (OMVPE), have background carrier concentrations of ∼2×1015 cm-3. Transmission electron microscopy (TEM) indicates dislocation densities as low as 107 cm-2 in the GaAs layers. A photoluminescence-decay measurement on an AlGaAs/GaAs double heterojunction (DH), grown on a (100)-oriented Si substrate and the Si0.04Ge0.96/Ge buffers, yields a minority-carrier hole lifetime of ∼2.5 ns which is state-of-the-art for heteroepitaxial GaAs on Si. This value represents a significant development for a novel approach in the heteroepitaxy of GaAs on Si.

Organometallic vapor‐phase‐epitaxial growth and characterization of ZnGeAs2 on GaAs

We report the epitaxial growth of single‐crystal stoichiometric ZnGeAs2. The (001) ZnGeAs2 layers were deposited by organometallic vapor‐phase epitaxy on (100) GaAs. The epitaxy has specular surface morphology. The stoichiometric chemical composition has been confirmed by x‐ray diffraction, electron microprobe, and Auger electron spectroscopy. Selected‐area electron diffraction patterns clearly indicate the chalcopyrite structure and that the [001] lattice direction is the growth direction. X‐ray diffraction indicates that the c‐direction lattice constant is 11.192 A for our epitaxial material, which is an elongation of 0.35% from the bulk material value of 11.153 A. When stiffness constants for ZnGeAs2 are approximated by those of GaAs, this c‐axis elongation can be explained by a contraction in the a direction induced by the 3.4×10−3 lattice mismatch between the ZnGeAs2 epitaxy and the GaAs substrate. Absorptance and transmittance measurements indicate that this material has a direct band gap of approxi...