K. L. Wang

Experimental proof-of-principle investigation of enhanced Z[sub 3D]T in (001) oriented Si/Ge superlattices

An experimental proof-of-principle of an enhanced Z3DT (thermoelectric figure of merit) is demonstrated using (001) oriented Si/Ge superlattices. The highest value of the experimental Z3DT at 300 K for a (001) oriented Si(20 A)/Ge(20 A) superlattice is 0.1 using κ=5u200aWm−1u200aK−1, for the in-plane thermal conductivity, which is a factor of seven enhancement relative to the estimated value of Z3DT=0.014 for bulk Si. The good agreement between experiment and theory validates our modeling approach (denoted as “carrier pocket engineering”) to design superlattices with enhanced values of Z3DT. Proposals are made to enhance the experimental values of Z3DT for Si/Ge superlattices even further.

Changes in luminescence emission induced by proton irradiation: InGaAs/GaAs quantum wells and quantum dots

The photoluminescence (PL) emission from InGaAs/GaAs quantum-well and quantum-dot (QD) structures are compared after controlled irradiation with 1.5 MeV proton fluxes. Results presented here show a significant enhancement in radiation tolerance with three-dimensional quantum confinement. Some additional radiation-induced changes in photocarrier recombination from QDs, which include a slight increase in PL emission with low and intermediate proton doses, are also examined.

Normal incidence infrared detector using p‐type SiGe/Si multiple quantum wells

The photoresponse of p‐type SiGe/Si multiple quantum well infrared detectors is measured as a function of incidence beam polarization. With the infrared (IR) beam polarized in the growth direction, a photoresponse peak is observed at near 8.6 μm with a full width at half maximum (FWHM) of about 80 meV. On the other hand, with the beam polarized parallel to the growth plane (normal incidence), a peak is observed at near 7.2 μm with nearly the same FWHM. The photoresponse at peak positions is about 0.3 A/W for both cases. With an unpolarized beam, the peak photoresponse of about 0.6 A/W is observed for the present unoptimized structure. The results of the detection of normal incidence IR suggest possible applications of SiGe/Si multiple quantum wells for normal incidence IR detection.

Nanofabrication of thin chromium film deposited on Si(100) surfaces by tip induced anodization in atomic force microscopy

Writing of nanostructures on thin metal films using atomic force microscopy (AFM) was demonstrated. The writing experiments were done in a nitrogen ambient having variable humidity. Using a p‐type heavily doped silicon AFM tip, a bias voltage was independently applied between the tip and the surface of a thin chromium layer deposited on a Si(100) substrate. Protruded patterns of various shapes were formed only on the water‐adsorbed chromium surface when applying a negative bias on the tip. Their sizes were found to be dependent on the writing time, the bias voltage, and the humidity. The smallest feature size obtained is about 20 nm. From Auger electron spectroscopy (AES) analysis, the products are shown to be Cr oxides. The surface modification mechanism appears to be tip‐induced local oxidation, i.e., anodization.

Towards Si1−xGex quantum-well resonant-state terahertz laser

We report on the experimental evidence for terahertz (THz) lasing of boron-doped strained Si1−xGex quantum-well structures. The lasing arises under strong electric fields (300–1500 V/cm) applied parallel to interfaces. The spectrum of THz stimulated emission is presented showing the lasing wavelength near 100 μm and the modal structure caused by a resonator. The mechanism of population inversion is based on the formation of resonant acceptor states in strained SiGe layer.

A SURFACTANT-MEDIATED RELAXED SI0.5GE0.5 GRADED LAYER WITH A VERY LOW THREADING DISLOCATION DENSITY AND SMOOTH SURFACE

A method to grow a relaxed Si0.5Ge0.5 graded layer with a very smooth surface and a very low threading dislocation density using solid-source molecular-beam epitaxy is reported. This method included the use of Sb as a surfactant for the growth of a 2 μm compositionally graded SiGe buffer with the Ge concentration linearly graded from 0% to 50% followed by a 0.3 μm constant Si0.5Ge0.5 layer. The substrate temperature was kept at 510u200a°C during the growth. Both Raman scattering and x-ray diffraction were used to determine the Ge mole fraction and the degree of strain relaxation. Both x-ray reflectivity and atomic force microscopy measurements show a surface root mean square roughness of only 20 A. The threading dislocation density was determined to be as low as 1.5×104u200acm−2 as obtained by the Schimmel etch method. This study shows that the use of a Sb surfactant and low temperature growth is an effective method to fabricate high-quality graded buffer layers.

Intervalence‐subband transition in SiGe/Si multiple quantum wells−normal incident detection

Normal incident infrared absorption is observed in intervalence‐subband transition of Si1−xGex/Si multiple quantum wells for the first time. The observed absorption peak wavelength, strength, and broadness are shown to be strongly dependent on the Ge content in the well. The intervalence‐subband absorption peak shows up at a shorter wavelength with the beam polarized parallel to the growth plane (normal incidence), compared with the previously reported intersubband transition between two heavy hole subbands, which occurs when the optical field is polarized along the growth direction. The dependence of the absorption peak height with the Ge composition and thus the direct gap energy in the quantum well indicates that the origin of this transition is due to the intervalence‐subband transition from the heavy hole ground state to another valence subband. The normal incident detection suggests the convenient fabrication of focal plane infrared detector arrays using Si1−xGex/Si multiple quantum wells.

Normal‐incidence strained‐layer superlattice Ge0.5Si0.5/Si photodiodes near 1.3 μm

Ge0.5Si0.5 strained‐layer pin photodiodes, in which multiple strained layers serve as the absorption region, have been fabricated. These devices exhibit an optical response at wavelengths beyond 1.3 μm at normal incidence. The measured external quantum efficiencies at an applied bias of 4 V are 17% at 0.85 μm and 1% at 1.3 μm, respectively. Excellent electrical characteristics evidenced by the avalanche breakdown at 20 V have also been demonstrated.

Nanometer scale patterning of silicon (100) surfaces by an atomic force microscope operating in air

Modification on silicon (100) surfaces was demonstrated by using an atomic force microscope operating in air. Field‐enhanced oxidation on silicon surfaces with protection oxide was done locally by biasing a p‐type heavily doped silicon tip between −3 and −10 V. Oxide lines of width as small as ∼10 nm were achieved. After a dip in aqueous HF solution, the oxide was etched away; the modification depth, ∼1 nm, was characterized by the same atomic force microscope. Other field induced reactions for patterning are possible.

Electron mobility enhancement from coupled wells in delta‐doped GaAs

It is found that when two Si delta (δ) doped wells in GaAs are placed in close proximity to one another, the electron Hall mobility is enhanced two to five times over that of the single well case. The temperature dependence of the mobility is also reported. Samples with a variety of spacer widths have been studied. Our theoretical and experimental data show that the excited subbands in the double well structure have significant carrier densities located in the undoped region where Coulombic scattering is reduced, and are thus found to play an important role in the observed mobility behavior.