Terahertz emitters are important for fundamental studies in an interesting frequency regime and for applications ranging
from medical diagnostics to see-through imaging. A simple approach to THz emission from semiconductors is based on
intracenter optical transitions in dopants and impurities in semiconductors. The centers can be excited either electrically
or optically, and the THz emission occurs when carriers in the dopant upper energy states relax toward the ground state.
Both n–type and p–type dopants as well as deep impurities can be used for THz emission from many host
semiconductors including silicon, SiC, and GaN. Unlike with conventional p–n junction devices, the centers for THz
emission must be occupied and not thermally ionized, which suggests the need for deep energy levels and/or low
temperature operation. Significant center occupation at elevated temperatures favors the wide bandgap semiconductors
such as SiC and GaN, in which the dopant ionization energy can greatly exceed the thermal energy kBT at room
temperature. For example, electrically pumped THz emitters fabricated from nitrogen-doped SiC can operate at
temperatures to about 250 K in pulse mode. The SiC emission spectra had peaks from 5 to 12 THz (20 to 50 meV), and
these surface-emitting devices produced a peak power density of 30 milliwatt-cm-2 at 77 K, which is suitable for a wide
range of high power THz applications. We report the characteristics and limitations of electrically pumped dopanttransition
THz emitters, and their performance in several semiconductor systems.
The broadband imaging capabilities of a vanadium oxide microbolometer camera were investigated in the far-infrared for applications in real-time terahertz imaging and analysis. To accomplish this, we used an optical configuration consisting of a broadband terahertz source, terahertz filtering optics, and a modified commercial broadband microbolometer camera. A blackbody radiator was employed as the broadband terahertz source to illuminate the microbolometer array with all components in a nitrogen purged enclosure. Data was taken using several different levels of radiant flux intensity. Optical filtering were necessary to isolate incident radiation frequencies into a band from 1.5 to 7.5 THz. Fourier transform infrared spectroscopy was used to characterize the transmission properties of each optical component. The noise equivalent differential temperature (NEDT) and the noise equivalent power (NEP) were recorded over a range of blackbody intensities. We discuss the relative utility of these two figures of merit for terahertz imaging. For example, at a blackbody temperature of 925°C the NEDT was recorded below 800 mK, and the NEP was calculated to be 136 pW/√Hz. This study provides a complete analysis of a microbolometer as the detector component of a terahertz imaging system in a broadband imaging configuration.
Optimization of the photonic bandgap in finite-height photonic crystal (PhC) slab structures requires high-fill-factor lattices. We present a method for fabrication of high-fill-factor PhC devices in silicon-on-insulator (SOI) substrates using electron-beam lithography and high-aspect-ratio reactive-ion etching (RIE). We achieve 8:1 aspect-ratio PhC structures with 60-nm vertical membrane walls using a custom deep reactive-ion etching process in a conventional low-end RIE with patterned resist as the only etch mask. We present examples of various PhC devices fabricated using this method including a high-efficiency coupling structure for PhC waveguides.
An intense THz emission was observed from strained SiGe/Si quantum-well structures under strong pulsed electric field. The p-type structures were MBE-grown on n-type Si substrate and δ-doped with boron. Lines with wavelengths near 100 microns were observed in the emission spectrum. The modal structure in the spectrum gave evidence for the stimulated nature of the emission. The origin of the THz emission was attributed to intracenter optical transitions between resonant and localized boron levels.
Terahertz electroluminescence was produced by intersubband transitions in silicongermanium quantum wells. The devices were grown by solid-source molecular-beam epitaxy on high-resistivity silicon substrates, and were fabricated by standard photolithography and processing techniques. Using FTIR spectroscopy at at temperature of 5 K, electroluminescence was observed around 9 THz. The emission was attributed to heavy-hole-to-light-hole transitions and demonstrates the potential for SiGe technology as terahertz emitters.
Bandgap tailoring and lattice-matching in SiGeC/Si heterostructures has potential for improving the performance and capabilities of Si based optoelectronics. Although remarkable progress in the molecular beam epitaxy (MBE) of SiGeC/Si heterostructures has been achieved, important questions concerning growth kinetics and thermal stability are still not fully understood. One major obstacle during MBE growth of these heterostructures may be the high surface diffusivity of carbon, which leads to small fractions of substitutional carbon at temperatures necessary for device quality epitaxial growth. We report on the surface kinetic properties of Si0.992C0.008/Si and thermal stability of strained Si0.992C0.008/Si and strain compensated Si0.892Ge0.10C0.008/Si alloys were shown to be more stable then the binary Si0.992C0.008/Si heterostructure alloys.
Ge1-yCy alloys are meta-stable and challenging to produce due to the large disparity between the atomic sizes of Ge and C. However, this same disparity results in an alloy system that potentially spans a wide range of bandgaps, refraction indices, and lattice constants. As such, it has potential as a Si lattice matched material for use in Si based waveguides, detectors, modulators, and other devices. The performance of these devices, however, depends on the refractive indices, which are not well known in these alloys. We present the results of comprehensive measurements of refractive index, energy bandgap, and free carrier absorption versus doping level. In situ B and P doped 550 nm Ge1-yCy alloy films were grown on Si (100) substrates by molecular beam epitaxy. The infrared optical transmission spectra were measured at room temperature. The indices of refraction were obtained from the amplitude of the interference fringes at sub-bandgap photon energies. Hall effect measurements were employed to measure the carrier concentrations. The refractive index of our Ge- 1-y)Cy alloys was nominally 4.01, and decreased with increasing B and P concentration.
We report on the fabrication and characterization of a photodiode made from a heterojunction of epitaxial p-type Ge1-xCx on an n-type Si substrate. Epitaxial Ge1-xCx layers with carbon percentages of 0.2, 0.8, 1.4 and 2% were grown on (100) Si substrates by solid source molecular beam epitaxy. The p-GeC/n-Si junction exhibits diode rectification with low reverse saturation current (2 at -1 volt) and high reverse breakdown voltage in excess of -40 volts. Despite the large number of dislocations and defects at the heterojunction, photoresponsivity was observed from the p- Ge1-xCx/n-Si diodes using laser excitation at a wavelength of 1.3 micrometers . External quantum efficiency was measured between 1.2 and 2.3%.