P-type Ge Research Articles

Fermi-level pinning and charge neutrality level in germanium

The Schottky barrier height in metal/Ge contacts shows weak dependence on the metal work function indicating strong Fermi-level pinning close to the Bardeen limit. The pinning factor S is about 0.05 and the charge neutrality level (CNL) is only about 0.09eV above the top of the valence band. Because of this, the Fermi level in Ge lies higher than CNL in most cases of interest so that unpassivated acceptorlike gap states at the interface are easily filled, building up a net negative fixed charge. This could prevent efficient inversion of a p-type Ge surface in a metal-oxide-semiconductor structure.

Application of voxel phantoms and Monte Carlo method to whole-body counter calibration

Voxel phantoms and Monte Carlo methods are of considerable interest in calibrating in vivo counting applications for radioactive body burden such as whole-body counters. At the Japan Atomic Energy Research Institute (JAERI), a calculation code--UCWBC code--for whole-body counter calibrations using voxel phantoms has been developed as an EGS4 Monte Carlo user code. To validate the UCWBC code for calibrating whole-body counters at JAERI, response functions and counting efficiencies of a p-type high-purity Ge (HPGe) semiconductor detector used for the whole-body counter were evaluated for a water-filled block-shape phantom by use of UCWBC code and were measured by experiments. The voxel version of the water-filled block-shape phantom based on an actual phantom was used for the calculations. Furthermore, counting efficiencies of the Ge semiconductor detector for the male and female voxel phantoms developed at JAERI were evaluated in the photon energy range from 60 to 1836 keV by the UCWBC code in order to examine the differences between the counting efficiencies for voxel phantoms. In conclusion, it was found that the response functions and counting efficiencies of the Ge semiconductor detector found by the UCWBC code for the water-filled block-shape phantom are in good agreement with measured data. The UCWBC code was validated by those comparisons. It was also found that the counting efficiencies of the Ge semiconductor detector depend on the size of the phantoms and the effective distance between phantom and detector. The calibration of whole-body counters using voxel phantoms and Monte Carlo methods would be quite useful for the improvement in the accuracy of measurement results.

DLTS studies of irradiation-induced defects in p-type germanium

In the present investigation, we have made use of our newly produced n +p-mesa diodes on Ga-doped, 2-Ω cm Ge-single crystals to study irradiation-induced defects in p-type Ge. Irradiation with electrons shows five distinct levels introduced. The use of protons and α-particles (and at different doses) for irradiation, reveals four new levels, and allows for the possibility of connecting some of the levels to the divacancy.

Optical characterization of dislocation free Ge and GeOI wafers

Optical properties of dislocations free state-of-the-art Germanium(Ge) and Germanium-on-insulator(GeOI) wafers have been characterized using Fourier transformed infrared spectroscopy at oblique incidence, attenuated total reflectance, laser Raman scattering and optical transmission at room temperature. In n-type Ge, in addition to vibrational modes observed in intrinsic(i) Ge, a band at 535 cm −1, 668 cm −1 and a strong peak at 1590 cm −1 were observed. At oblique incidence, the bands at high energies loose their intensity and those at 500–600 cm −1 become stronger and new bands appear at 840 and 945 cm −1. Thermal annealing reveals the precipitation of GeO as evidenced from the band structure at 845 cm −1 and an absorption dip at 808 cm −1 in p-type Ge. The appearance of new bands and the change in band strengths are in general observed in both type of wafers at oblique incidence. GeOI exhibits a strong disorder induced longitudinal optic (LO)–TO coupling mode which can only be observed at non-normal incidence. Optical absorption at the near band edge reveals the presence of doping related disorder and band shrinkage, which is supported also by Ge–Ge one-phonon line broadening at 301 cm −1.

Electrochemical pore etching in Ge

While electrochemical pore etching in semiconductors has become a thriving field for research and applications in the past 15 years or so, little work has been done in Ge. Besides Si, Ge is the only semiconductor with a minority carrier diffusion length large enough to enable the use of backside illumination, which has proved to be the decisive “trick” for the formation of excellent macropores in Si. First experiments in Ge have been performed, but Ge proved to behave in rather unexpected ways—the large body of pore etching knowledge obtained with Si and the III–V semiconductors was not directly applicable to Ge. While no good pores could be produced in most previous endeavors including our own, we finally succeeded in producing deep pores in n-type but also in p-type Ge of various doping levels and crystal orientations. In particular, it is now possible to create “good” macropores in a uniform distribution. A host of new and not yet totally understood phenomena was discovered, including pores of various kinds and crystallography. The paper will shortly outline the results obtained and relate it to what is known from other semiconductors under comparable circumstances.

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.

Electrochemical pore etching in germanium

Abstract Nucleation and growth of electrochemically etched pores in Germanium (Ge) was investigated for n- and p-type Ge single crystals with {1 0 0}, {1 1 0}, and {1 1 1} orientations and doping concentrations of (10 14 –10 18 ) cm −3 . Various types of electrolytes, illumination conditions (front side, back side or none), and pre-treatments for optimizing nucleation were used. Several kinds of macropores could be obtained, mostly for the first time. In particular, pores could be obtained in p-type Ge samples. Pore geometries, morphologies, and growth peculiarities were found to be quite different from other semiconductors. Nucleation is generally difficult, the preferred growth direction is 〈1 0 0〉 or 〈1 1 1〉, stop planes are of {1 1 0} type, and there is always a strong electropolishing component compromising pore geometry and stability. Porous membranes have been produced showing electrocapillarity effects.

Electrical Behavior of Germanium Oxide/Germanium Interface Prepared by Electron-Cyclotron-Resonance Plasma Oxidation in Capacitance and Conductance Measurements

In this letter, we describe the electrical behavior of the GeOx and p-type Ge interface on the basis of capacitance and conductance measurements. The primary conclusion of this work is that when the Ge surface is biased in the depletion region, the frequency response of interface traps in the lower half of the Ge band-gap is so fast that there is no significant frequency dispersion observed in the capacitance measurements over a typical frequency range of ≤ 1 MHz. As a result, the density and energy distributions of the interface traps cannot be determined by a conventional method of combined high-frequency and low-frequency capacitance measurements. Instead, a comparison of the measured capacitance with a theoretical capacitance calculated for a system with no interface traps must be conducted to obtain information on the interface traps. Furthermore, the conductance method provides information on the interface traps in the upper half of the p-type Ge band-gap.

The antimony-vacancy defect in p-type germanium

Ge - n + p mesa diodes have been produced in 2-Ωcm single crystals using a molecular-beam epitaxy (MBE) process to grow the Sb-doped epitaxial Ge n+-top layer. The diodes are characterized by a leakage current at room temperature of 8×10−4A∕cm2 at a reverse bias of 3 V. The diodes have been used to study irradiation-induced defects in p-type Ge, in particular Sb-related defects, where Sb stems from in-diffusion during the MBE growth. Two lines in the deep level transient spectroscopy (DLTS) spectra are related to the presence of Sb. One of these lines originates from the single-acceptor state of the SbV pair with an enthalpy of ionization of ΔHp=(0.309±0.007)eV, the other from a state with an enthalpy of ionization of ΔHp=(0.095±0.006)eV, which is concluded to be the single donor-charge state of the SbV pair.

Electron dynamics in Ge crystals studied by terahertz emission from photoexcited surfaces

A systematic study of terahertz (THz) emission from Ge surfaces excited by femtosecond laser pulses was accomplished. THz transients emitted by differently doped Ge crystals were investigated both experimentally and theoretically by using Monte Carlo modelling. The dependences of the radiation spectra on the crystal doping level have indicated that the main mechanism leading to the THz emission is cold plasma oscillation; the same polarity of the transients emitted from n- and p-type Ge suggests that this oscillation was activated by the photo-Dember effect at the semiconductor surface. The optical pump–THz probe experiment and corresponding numerical calculation had revealed the details of the photoexcited electron redistribution among different conduction band valleys.

Terahertz emission from photoexcited surfaces of Ge crystals

The mechanism of terahertz emission from the surfaces of n - and p-type Ge crystals illuminated by femtosecond Ti:sapphire laser pulses was investigated. It has been concluded that this emission is caused by the cold plasma oscillation activated by a transient polarization arising due to different diffusion rates for photoexcited electrons and holes. The dynamics of those carriers was also investigated with subpicosecond temporal resolution by using an optical pump–terahertz probe technique.

Doping Effects on Thermoelectric Properties of Higher Manganese Silicides (HMSs, MnSi1.74) and Characterization of Thermoelectric Generating Module using p-Type (Al, Ge and Mo)-doped HMSs and n-Type Mg2Si0.4Sn0.6 Legs

The effect of Al Doping on the thermoelectric properties of higher manganese silicides, MnSi1.74 (HMSs) parallel and perpendicular to the c-axis was investigated. It was found that Al doping increased electrical conductivity and decreased thermoelectric power parallel to the c-axis, because Al doping of the Si site increased carrier density according to the valence control rule. Al doping was also effective for lowering thermal conductivity and raising the figure of merit parallel to the c-axis. After optimizing the Al content (y=0.0035), the thermoelectric properties of (Al, Ge and Mo)-doped HMSs parallel and perpendicular to the c-axis were evaluated and compared to those of nondoped and Al-doped HMSs. Two types of thermoelectric module consisting of n-type Mg2Si0.4Sn0.6 and p-type (Al, Ge and Mo)-doped HMSs legs parallel (or perpendicular) to the c-axis were fabricated, and their thermoelectric performances were compared to each other. The discrepancy between the measured maximum output power and the value estimated from the thermoelectric data of constituent materials was well explained by the resistance of a porous Al electrode. The measured heat flux of the module was also compared to the estimated heat flux. The maximum energy conversion efficiency of the module parallel to the c-axis was clearly higher than that perpendicular to the c-axis, because of a higher output power and a lower heat flux, and was higher than 7% at ΔT=520 K (303–823 K).

The evolution of electroluminescence in Ge quantum-dot diodes with the fold number

The electroluminescence of the light-emitting diodes with five-, ten and 30-fold p-type Ge quantum dots grown on n+ Si substrates is studied. The enhanced integral electroluminescence intensity and blueshift of the 30-fold one at high temperature (>200K) act contrary to those in five- and ten-fold ones. It is attributed to the emission in the higher-fold quantum dots enabled by the injected electrons diffusing the farther at the higher temperature. Transmission electron microscopy shows that the size of the Ge quantum dots and the Si component in them, both increase with increasing the fold number. Due to the strain-induced intermixing at the high-fold quantum dots, those dots hence have large band gap and result in the intensity increment and blueshift at the high temperature.

Low-temperature magnetoresistance due to weak localization in lightly doped semiconductors

The first observation of low-temperature magnetoresistance (MR) of interference nature in the case of a light doping is reported. The MR occurs in n- and p-type Ge samples at a frequency of 10 GHz at temperatures below 30 K in weak magnetic fields on the background of the classical MR effect associated with electrons in different valleys (n-Ge) and with heavy and light holes (p-Ge).

Advances in surfactant-mediated growth of germanium on silicon: high-quality p-type Ge films on Si

The use of antimony (Sb) as a surfactant allows the growth of relaxed and smooth germanium films with low-defect density on silicon (1 1 1) substrates. We extend the possibilities of surfactant mediated epitaxy (SME) by introducing in situ boron (B) doping, thus enabling the growth of p-type Ge layers on Si(1 1 1). In Sb-SME, a lower p-doping limit in the order of 1×10 16 cm −3 for B doping is set by the compensating incorporation of the surfactant Sb during growth. Above this limit, high hole Hall mobilities are reached, up to 1600 cm 2/V s for a net p-doping of 1.9×10 16 cm −3. Temperature-dependent Hall measurements show that the hole mobility in p-type Ge layers grown by SME is only limited by ionized impurity scattering of the dopants pointing to high material quality. The structural properties of Ge films grown by SME are characterized by cross-sectional transmission electron microscopy and high-resolution X-ray diffraction. This reveals a complete relaxation and low defect densities. A more detailed study of the Ge/Si(1 1 1) interface shows a regular dislocation array with dislocation lines oriented along the in-plane 〈 1 ¯ 1 0 〉 directions with a spacing of 8.0 nm. This indicates a practically complete relaxation of the Ge layer by a triangular network of interface misfit dislocations.

Tunable normal incidence Ge quantum dot midinfrared detectors

Midinfrared photodetectors in the 3–5 μm region were demonstrated by using molecular beam epitaxy grown self-assembled Ge quantum dots at normal incidence. The structure was a p-i-p with p-type doped Ge dots embedded in the intrinsic layer sandwiched in the two heavily p-doped regions. The dark current density at 77 K is 6.4 mA/cm2 at 1 V. The as-grown sample has a response at normal incidence in the wavelength range of 2.2 to 3.2 μm and peaked at 2.7 μm. Thermal annealing at 900 °C for 10 min shifted the peak response to 3.6 μm. Annealing effect was simulated with the interdiffusion behavior of Ge and Si atoms to explain the shift of the response wavelength.

Growth and transport properties of complementary germanium nanowire field-effect transistors

n- and p-type Ge nanowires were synthesized by a multistep process in which axial elongation, via vapor–liquid–solid (VLS) growth, and doping were accomplished in separate chemical vapor deposition steps. Intrinsic, single-crystal, Ge nanowires prepared by Au nanocluster-mediated VLS growth were surface-doped in situ using diborane or phosphine, and then radial growth of an epitaxial Ge shell was used to cap the dopant layer. Field-effect transistors prepared from these Ge nanowires exhibited on currents and transconductances up to 850 μA/μm and 4.9 μA/V, respectively, with device yields of >85%.

Hole confinement and dynamics in δ-doped Ge quantum dots

We report picosecond pump–probe studies of the dynamics of inter-level transitions in p-type Ge quantum dot structures using a free electron laser as a source of intense mid-infrared pulses. The wavelength-independent lifetime of around 210 ps is much longer than have been recently reported in SiGe/Si multiple quantum-well structures. The observed pump–probe signal is ascribed to photo-excitation within the confined hole state. The presence of the dot potential reduces the phonon emission and the lifetime is strongly temperature dependent falling to 50 ps at 100 K. The origin of the relaxation process is unknown but is probably by acoustic phonon emission.

Isotope effect on the phonon-drag component of the thermoelectric power of germanium

We have measured the thermoelectric power of several germanium single crystals with different isotopic compositions: natural, 96.3%, and 99.99% enriched ${}^{70}\mathrm{Ge}.$ N- and p-type Ge samples with the net concentration of electrically active impurities $<2\ifmmode\times\else\texttimes\fi{}{10}^{13}{\mathrm{cm}}^{3}$ were studied. Our experimental data show a decrease of thermopower with an increase in isotopic mass disorder. This isotope effect is evident at temperatures below approximately 70 K where a phonon-drag part of the thermopower dominates. At high temperatures, in the diffusion regime, the thermopower does not correlate with the isotopic composition within experimental error. The observed isotope effect can be explained with a special phonon-drag contribution induced by a momentum transfer from thermal phonons to low-energy phonons in the enriched samples.

Ge quantum dots sandwiched between two thick Si blocking layers to block the dark current and tune the responsivity spectrum

A p-type Ge quantum dot infrared photodetector is investigated in this paper. The intrinsic thick Si layers are utilized to block the dark current under low bias and adjust the tunneling probability by controlling applied field to tune the responsivity spectrum. The 30 K responsivity spectrum shows broadband from 2 to 9 μm at zero bias with photovoltaic effect and extends its range to 20 μm when the applied bias is increased. For long wavelength detection, our detector is tunable with Si blocking layer. The detectivity in this sample has maximum value toward 2.1 × 10 10 cm (Hz 0.5)/W under 0.2 V for the 6 μm wavelength. That for the long wavelength is ∼10 9 cm (Hz 0.5)/W.