M. V. Dolguikh

Neutron transmutation doped far-infrared p-Ge laser

A far-infrared p-type germanium laser with active crystal prepared from ultra pure single-crystal Ge by neutron transmutation doping (NTD) is demonstrated. Calculations show that the high uniformity of Ga acceptor distribution achieved by NTD significantly improves average gain. The stronger ionized impurity scattering due to high compensation in NTD Ge is shown to have insignificant negative impact on the gain at the moderate doping concentrations sufficient for laser operation. Experimentally, this first NTD laser is found to have lower current-density lasing threshold than the best of a number of melt-doped laser crystals studied for comparison.

Terahertz gain on intersubband transitions in multilayer delta-doped p-Ge structures

A far-infrared laser concept based on intersubband transitions of holes in p-type periodically delta-doped semiconductor films is studied using numerical Monte Carlo simulation of hot-hole dynamics. The considered device consists of monocrystalline pure Ge layers periodically interleaved with delta-doped layers and operates with vertical hole transport in the presence of an in-plane magnetic field. Population inversion on intersubband transitions arises due to light-hole accumulation in E⊥B fields, as in the bulk p-Ge laser. However, the considered structure achieves spatial separation of hole accumulation regions from the doped layers, which reduces ionized-impurity and carrier-carrier scattering for the majority of light holes. This allows a remarkable increase of the gain in comparison with bulk p-Ge lasers. Population inversion and gain sufficient for laser operation are expected up to 77K. Test structures grown by chemical-vapor deposition demonstrate feasibility of producing the device with sufficie...

Intervalence-band THz laser in selectively-doped semiconductor structure

Monte Carlo simulation of carrier dynamics and far-infrared absorption in a selectively-doped p-type multi-layer Ge structure with vertical transport was performed to test a novel terahertz laser concept. The design exploits the known mechanism of THz amplification on intersubband transitions in p-Ge, but with spatial separation of light hole accumulation regions from doped regions, which allows remarkable enhancement of the gain. The structure consists of doped layers separated by 300-500 nm gaps of pure-Ge. Vertical electric field (~ 1-2 kV/cm) and perpendicular magnetic field (~ 1T) provide inversion population on direct intersubband light- to heavy-hole transitions. Heavy holes are found to transit the undoped layers quickly and to congregate mainly around the doped layers. Light holes, due to tighter magnetic confinement, are preferably accumulated within the undoped layers. There the relatively small ionized impurity and electron-electron scattering rates allow higher total carrier concentrations, and therefore higher gain, than in bulk crystal p-Ge lasers. In contrast to GaAs-based THz quantum cascade lasers (QCL), the robust design and large structure period suggest that the proposed Ge structures might be grown by the technologically-advantageous chemical vapor deposition (CVD) method. The ability of CVD to grow relatively thick structures will simplify the electrodynamic cavity design and reduce electrodynamic losses in future THz lasers based on the presented scheme.

Scanning Fabry-Perot filter for terahertz spectroscopy based on silicon dielectric mirrors

A scanning Fabry-Perot transmission filter composed of a pair of dielectric mirrors has been demonstrated at millimeter and sub-millimeter wavelengths. The mirrors are formed by alternating quarter-wave optical thicknesses of silicon and air in the usual Bragg configuration. Detailed theoretical considerations are presented for determining the optimum design. Characterization was performed at sub-mm wavelengths using a gas laser together with a Golay cell detector and at mm-wavelengths using a backward wave oscillator and microwave power meter. High resistivity in the silicon layers was found important for achieving high transmittance and finesse, especially at the longer wavelengths. A finesse value of 411 for a scanning Fabry-Perot cavity composed of three-period Bragg mirrors was experimentally demonstrated. Finesse values of several thousand are considered to be within reach. This suggests the possibility of a compact terahertz Fabry-Perot spectrometer that can operate in low resonance order to realize high free spectral range while simultaneously achieving a high spectral resolution. Such a device is directly suitable for airborne/satellite and man-portable sensing instrumentation.

Toward hot-hole THz lasers in homoepitaxial Si and GaAs with layered doping

A recently proposed THz laser concept in homoepitaxially grown p-Ge with layered doping is reviewed. Prospects for realizing a similar design in Si or GaAs are considered.

Amplification of terahertz radiation in delta-doped germanium thin films

A new geometry for the intersubband THz laser on delta-doped multi-layer Ge thin films with in-plane transport of carriers in crossed electric and magnetic fields is proposed. A remarkable increase of the gain compared to existing bulk p-Ge lasers is based on spatial separation of light and heavy hole streams, which helps to eliminate scattering of light holes on ionized impurities and the majority of heavy holes. Inversion population and the gain have been studied using Monte-Carlo simulation. The terahertz transparency of a CVD-grown delta-doped Ge test structure has been experimentally studied by intracavity laser absorption spectroscopy using a bulk p-Ge laser. A practical goal of this study is development of a widely tunable (2-4 THz) laser based on intersubband hole transitions in thin germanium films with the gain sufficient to operate at liquid nitrogen temperatures.

Anisotropic optical phonon scattering of holes in cubic semiconductors

The formula for the nonpolar optical phonon scattering rate of holes in cubic semiconductors is obtained in the case of strong valence band anisotropy. The deformation potential approximation is used. A three-band, 6×6, k∙p Luttinger-Kohn representation includes states belonging to the heavy, light, and split-off bands. Mixing with the latter causes strong anisotropy in the transition matrix elements as well as in the density of final states. The derived formula is recommended for silicon, where inter- and intravalence-band scattering rates are much more strongly anisotropic and have significantly different values than those estimated from the usual two-band 4×4, “warped spheres” approximation that neglects the split-off band. Results for the more isotropic case of germanium are presented for comparison.

Terahertz amplification in delta-doped germanium films with in-plane transport

Amplification of terahertz radiation on intersubband transitions has been analyzed by numerical Monte Carlo simulation for p-type delta-doped Ge films with in-plane transport configuration of applied electric and magnetic fields. A significant increase of the gain is found, compared to existing bulk p-Ge lasers, due to spatial separation of light and heavy hole streams, which reduces scattering of light holes on ionized impurities and heavy holes. The considered device has potential as a widely tunable (2–4THz) laser with high duty cycle and operating temperatures up to 50K.

Gain comparison for periodically delta-doped p-Ge structures with vertical and in-plane transport

Calculated terahertz gain for periodically delta-doped p-Ge films with vertical and in-plane transport and an orthogonal magnetic field are compared. Gain as a function of structure period, doping concentration, field strength, and temperature is calculated using distributions determined from Monte Carlo simulations. Both transport schemes achieve spatial separation of light holes from impurity layers and the majority of heavy holes, which significantly increases light hole lifetime and gain compared with bulk p-Ge lasers. For in-plane transport, an optimum doping period of 1-2 μm and a 10-fold increase in gain over bulk p-Ge are found. For vertical transport, the optimum period is 300-400 nm, and the gain increase found of 3-5 times bulk values is more modest. However, it is found that gain can persist to higher temperatures (up to 77 K) for vertical transport, while the in-plane transport scheme appears limited to 30-40 K.

Monte Carlo simulation of multilayer delta doped germanium THz laser

Numerical simulation of carrier dynamics and the amplification of THz radiation in selectively doped multi-layer germanium (Ge/p-Ge) structures has been performed by Monte Carlo technique. The laser design exploits the known widely tunable mechanism of THz amplification on inter-subband transitions in p-Ge, but with spatial separation of carrier accumulation and relaxation regions, which allows remarkable enhancement of the gain. Vertical electric field (/spl sim/ 1 - 2.5 kV/cm) and perpendicular magnetic field (/spl sim/ 1 - 2 T) provide population inversion on direct inter-subband light-to-heavy hole transitions. Heavy holes are found to transit the undoped layers quickly and to congregate mainly around the doped layers. Light holes, due to tighter magnetic confinement, are preferably accumulated within the undoped layers, whose reduced ionized impurity scattering rates allow higher total carrier concentrations, and therefore higher gain, in comparison to bulk p-Ge lasers. Preliminary results of the calculations show a possibility of laser operation at liquid nitrogen temperatures.