Band Gap Of Ge Research Articles

Room temperature photoluminescence of Ge-on-insulator structures formed by rapid melt growth

Room temperature photoluminescence (PL) was observed along 50 μm long Ge strips on insulator on bulk Si substrates fabricated by rapid melt growth. The PL peaks evidently exhibited a redshift from the origin to the end of the Ge strip because of the shrinkage of the direct bandgap of Ge. Moreover, PL intensities increased along the direction of lateral epitaxial growth primarily because of the decrease in the energy difference between the direct and indirect gaps of Ge. The change in the Ge band structure, which facilitated changes in PL peaks and intensities, was found to have resulted from the variation of tensile strain ratios and Si fractions along Ge strips. Furthermore, the PL intensity at the end of the strip was one magnitude higher than that of bulk Ge, which indicates the high quality of Ge-on-insulator structures.

Structural and band alignment properties of Al2O3 on epitaxial Ge grown on (100), (110), and (111)A GaAs substrates by molecular beam epitaxy

Structural and band alignment properties of atomic layer Al2O3 oxide film deposited on crystallographically oriented epitaxial Ge grown in-situ on (100), (110), and (111)A GaAs substrates using two separate molecular beam epitaxy chambers were investigated using cross-sectional transmission microscopy (TEM) and x-ray photoelectron spectroscopy (XPS). High-resolution triple axis x-ray measurement demonstrated pseudomorphic and high-quality Ge epitaxial layer on crystallographically oriented GaAs substrates. The cross-sectional TEM exhibited a sharp interface between the Ge epilayer and each orientation of the GaAs substrate as well as the Al2O3 film and the Ge epilayer. The extracted valence band offset, ΔEv, values of Al2O3 relative to (100), (110), and (111) Ge orientations using XPS measurement were 3.17 eV, 3.34 eV, and 3.10 eV, respectively. Using XPS data, variations in ΔEv related to the crystallographic orientation were ΔEV(110)Ge>ΔEV(100)Ge≥ΔEV(111)Ge and the conduction band offset, ΔEc, related to the crystallographic orientation was ΔEc(111)Ge>ΔEc(110)Ge>ΔEc(100)Ge using the measured ΔEv, bandgap of Al2O3 in each orientation, and well-known Ge bandgap of 0.67 eV. These band offset parameters are important for future application of Ge-based p- and n-channel metal-oxide field-effect transistor design.

Spontaneous formation of aluminum germanate on Ge(100) by atomic layer deposition with trimethylaluminum and microwave-generated atomic oxygen

The application of microwave-generated atomic oxygen as an oxidant is found to change the manner of atomic layer deposition (ALD) of an Al2O3 layer on a Ge substrate, leading to the spontaneous formation of aluminum germanate with a deposition rate higher than that of conventional ALD with water oxidant. Electrical characterization of the Al/aluminum germanate (11 nm)/p-Ge(100) structure indicates that both the bulk and the interface properties of the aluminum germanate are promising with small capacitance-voltage hysteresis of less than 20 mV and interface trap densities ranging from 2×1011 to 6×1011 cm−2 eV−1 in the upper half of the Ge band gap.

Defect energy levels in La and Hf germanates on Ge

We have calculated the energy levels of oxygen vacancies in LaGeOx and HfGeOx using a density functional method which does not need a band gap correction. The defect energy levels are aligned to the band gap of Ge by the calculated band offsets. The defect configurations have more complex bonding than those of simple binary oxides. The defect levels of metal germanates lie deeper in the band gap than the equivalent binary metal oxides, so that there should be less charge trapping in the germanates. The defect levels of HfGeOx are found to be deeper and the defect orbitals are more localized than those in HfSiOx.

Energy band alignment of atomic layer deposited HfO2 oxide film on epitaxial (100)Ge, (110)Ge, and (111)Ge layers

Crystallographically oriented epitaxial Ge layers were grown on (100), (110), and (111)A GaAs substrates by in situ growth process using two separate molecular beam epitaxy chambers. The band alignment properties of atomic layer hafnium oxide (HfO2) film deposited on crystallographically oriented epitaxial Ge were investigated using x-ray photoelectron spectroscopy (XPS). Valence band offset, ΔEv values of HfO2 relative to (100)Ge, (110)Ge, and (111)Ge orientations were 2.8 eV, 2.28 eV, and 2.5 eV, respectively. Using XPS data, variation in valence band offset, ΔEV(100)Ge>ΔEV(111)Ge>ΔEV(110)Ge, was obtained related to Ge orientation. Also, the conduction band offset, ΔEc relation, ΔEc(110)Ge>ΔEc(111)Ge>ΔEc(100)Ge related to Ge orientations was obtained using the measured bandgap of HfO2 on each orientation and with the Ge bandgap of 0.67 eV. These band offset parameters for carrier confinement would offer an important guidance to design Ge-based p- and n-channel metal-oxide field-effect transistor for low-power application.

Energy band alignment of atomic layer deposited HfO2 on epitaxial (110)Ge grown by molecular beam epitaxy

The band alignment properties of atomic layer HfO2 film deposited on epitaxial (110)Ge, grown by molecular beam epitaxy, was investigated using x-ray photoelectron spectroscopy. The cross-sectional transmission electron microscopy exhibited a sharp interface between the (110)Ge epilayer and the HfO2 film. The measured valence band offset value of HfO2 relative to (110)Ge was 2.28 ± 0.05 eV. The extracted conduction band offset value was 2.66 ± 0.1 eV using the bandgaps of HfO2 of 5.61 eV and Ge bandgap of 0.67 eV. These band offset parameters and the interface chemical properties of HfO2/(110)Ge system are of tremendous importance for the design of future high hole mobility and low-power Ge-based metal-oxide transistor devices.

Electronic properties of STM-constructed dangling-bond dimer lines on a Ge(001)-(2×1):H surface

Atomically precise dangling-bond (DB) lines are constructed dimer-by-dimer on a hydrogen-passivated Ge(001)-(2$\ifmmode\times\else\texttimes\fi{}$1):H surface by an efficient scanning tunneling microscope (STM) tip-induced desorption protocol. Due to the smaller surface band gap of the undoped Ge(001) substrate compared to Si(001), states associated with individually created DBs can be characterized spectroscopically by scanning tunneling spectroscopy (STS). Corresponding $dI/dV$ spectra corroborated by first-principle modeling demonstrate that DB dimers introduce states below the Ge(001):H surface conduction band edge. For a DB line parallel to the surface reconstruction rows, the DB-derived states near the conduction band edge shift to lower energies with increasing number of DBs. The coupling between the DB states results in a dispersive band spanning 0.7 eV for an infinite DB line. For a long DB line perpendicular to the surface reconstruction rows, a similar band is not formed since the interdimer coupling is weak. However, for a short DB line (2--3 DBs) perpendicular to the reconstruction rows a significant shift is still observed due to the more flexible dimer buckling.

Monolithic Ge-on-Si lasers for large-scale electronic–photonic integration

A silicon-based monolithic laser source has long been envisioned as a key enabling component for large-scale electronic–photonic integration in future generations of high-performance computation and communication systems. In this paper we present a comprehensive review on the development of monolithic Ge-on-Si lasers for this application. Starting with a historical review of light emission from the direct gap transition of Ge dating back to the 1960s, we focus on the rapid progress in band-engineered Ge-on-Si lasers in the past five years after a nearly 30-year gap in this research field. Ge has become an interesting candidate for active devices in Si photonics in the past decade due to its pseudo-direct gap behavior and compatibility with Si complementary metal oxide semiconductor (CMOS) processing. In 2007, we proposed combing tensile strain with n-type doping to compensate the energy difference between the direct and indirect band gap of Ge, thereby achieving net optical gain for CMOS-compatible diode lasers. Here we systematically present theoretical modeling, material growth methods, spontaneous emission, optical gain, and lasing under optical and electrical pumping from band-engineered Ge-on-Si, culminated by recently demonstrated electrically pumped Ge-on-Si lasers with >1 mW output in the communication wavelength window of 1500–1700 nm. The broad gain spectrum enables on-chip wavelength division multiplexing. A unique feature of band-engineered pseudo-direct gap Ge light emitters is that the emission intensity increases with temperature, exactly opposite to conventional direct gap semiconductor light-emitting devices. This extraordinary thermal anti-quenching behavior greatly facilitates monolithic integration on Si microchips where temperatures can reach up to 80 °C during operation. The same band-engineering approach can be extended to other pseudo-direct gap semiconductors, allowing us to achieve efficient light emission at wavelengths previously considered inaccessible.

Synthesis of Ge1−x Sn x Alloy Thin Films Using Ion Implantation and Pulsed Laser Melting (II-PLM)

Ge1−x Sn x thin films are interesting for all-group-IV optoelectronics because of a crossover to a direct bandgap with dilute Sn alloying. However, Sn has vanishing room-temperature equilibrium solubility in Ge, making their synthesis very challenging. Herein, we report on our attempts to synthesize Ge1−x Sn x films on Ge (001) using ion implantation and pulsed laser melting (II-PLM). A maximum of 2 at.% Sn was incorporated with our experimental conditions in the samples as determined by Rutherford back scattering spectroscopy. A red-shift in the Ge optical phonon branch and increased absorption below the Ge bandgap with increasing Sn concentration indicate Sn-induced lattice- and band-structure changes after II-PLM. However, ion-channeling and electron microscopy show that the films are not of sufficient epitaxial quality for use in devices.

Band gap and dispersive behavior of Ge alloyed a-SbSe thin films using single transmission spectrum

Abstract The transparency of SbSeGe glasses in the IR region makes them attractive candidates for low transmission loss applications. The samples of Sb 10 Se 90−x Ge x ( x = 0, 19, 21, 23, 25, 27) glasses have been prepared by melt quench technique. The thin films of these glasses have been deposited by vacuum evaporation technique. The optical study of thin films has been carried out. The refractive index, oscillator parameters, optical band gap and dielectric parameters have been calculated from optical measurements. The optical study reveals that the variation in the density of localized defect states on Ge addition affects the optical parameters of the system. The variation in concentration of localized defect states has been interpreted in terms of the change in structural network of the system.

Direct and Indirect Band-to-Band Tunneling in Germanium-Based TFETs

Germanium is a widely used material for tunnel FETs because of its small band gap and compatibility with silicon. Typically, only the indirect band gap of Ge at 0.66 eV is considered. However, direct band-to-band tunneling (BTBT) in Ge should be included in tunnel FET modeling and simulations since the energy difference between the Ge conduction band edges at the <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">L</i> and Γ valleys is only 0.14 eV at room temperature. In this paper, we theoretically calculate the parameters <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">A</i> and <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">B</i> of Kane's direct and indirect BTBT models at different tunneling directions ([100], [110], and [111]) for Si, Ge and unstrained Si <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">1-x</sub> Ge <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">x</sub> . We highlight how the direct BTBT component becomes more important as the Ge mole fraction increases. The calculation of the band-to-band generation rate in the uniform electric field limit reveals that direct tunneling always dominates over indirect tunneling in Ge. The impact of the direct transition in Ge on the performance of two realistic tunnel field-effect transistor configurations is illustrated with TCAD simulations. The influence of field-induced quantum confinement is included in the analysis based on a back-of-the-envelope calculation.

Studies on electrical switching behavior and optical band gap of amorphous Ge–Te–Sn thin films

Amorphous thin film Ge15Te85-xSnx (1 <= x <= 5) and Ge17Te83-xSnx (1 <= x <= 4) switching devices have been deposited in sandwich geometry using a flash evaporation technique, with aluminum as the top and bottom electrodes. Electrical switching studies indicate that these films exhibit memory type electrical switching behavior. The switching fields for both the series of samples have been found to decrease with increase in Sn concentration, which confirms that the metallicity effect on switching fields/voltages, commonly seen in bulk glassy chalcogenides, is valid in amorphous chalcogenide thin films also. In addition, there is no manifestation of rigidity percolation in the composition dependence of switching fields of Ge15Te85-xSnx and Ge17Te83-xSnx amorphous thin film samples. The observed composition dependence of switching fields of amorphous Ge15Te85-xSnx and Ge17Te83-xSnx thin films has been understood on the basis of Chemically Ordered Network model. The optical band gap for these samples, calculated from the absorption spectra, has been found to exhibit a decreasing trend with increasing Sn concentration, which is consistent with the composition dependence of switching fields.

Tailoring the electrical properties of Ge/GaAs by film deposition rate and preparation of fully compensated Ge films

We investigate the electronic, optical, and structural properties of thin Ge/GaAs(100) films for a variety of growth rates. All of the films have a granular, single-crystal structure, but the electronic properties vary dramatically, with resistivity and carrier concentration changing by more than three orders of magnitude. For high deposition rates, the films are heavily doped and of $n$-type, with relatively high carrier concentration and low resistivity. The temperature dependence of the resistivity indicates metalliclike transport with degenerate charge carriers. For low deposition rates, the films are $p$-type, with lower carrier concentrations and higher resistivity whose temperature dependence indicates semiconducting, activation-type transport with an activation energy that can reach up to half of the Ge band gap. Such films are heavily doped and strongly (sometimes fully) compensated Ge. At moderate deposition rates, a metal--insulator transition occurs. The electrical properties of strongly compensated Ge films are analyzed in terms of the two-dimensional percolation of charge carriers through a fluctuating electrostatic potential.

Trap density of GeNx/Ge interface fabricated by electron-cyclotron-resonance plasma nitridation

We have investigated GeNx/Ge interface properties using Si3N4(7 nm)/GeNx(2 nm)/Ge metal-insulator-semiconductor structures fabricated by the plasma nitridation of Ge substrates using an electron-cyclotron-resonance-generated nitrogen plasma. The interface trap density (Dit) measured by the conductance method is found to be distributed symmetrically in the Ge band gap with a minimum Dit value lower than 3 × 1011 cm−2eV−1 near the midgap. This result may lead to the development of processes for the fabrication of p- and n-Ge Schottky-barrier (SB) source/drain metal-insulator-semiconductor field-effect transistors using chemically and thermally robust GeNx dielectrics as interlayers for SB source/drain contacts and high-κ gate dielectrics.

Submonolayer barium passivation study for germanium(100)/molecular beam epitaxial Al2O3

The passivation effect of a Ba submonolayer with (2×1) and (4×1) surface reconstructions are investigated for Ge(001)/Al2O3 interfaces. The interface characteristics are evaluated by capacitance-voltage and ac conductance techniques at various temperatures. It is found that the Ba passivating effect on Ge is dose dependent below one monolayer. At the same time, Ge oxides are suppressed by the presence of a Ba submonolayer. The interface is thermally stable, even at a temperature higher than the desorption temperature of Ge oxides. Forming gas annealing at this temperature yields a relatively symmetric distribution of interface traps in the Ge band gap.

Nature of interface traps in Ge metal-insulator-semiconductor structures with GeO2 interfacial layers

The nature of interface traps in Ge metal-insulator-semiconductor (MIS) structures with GeO2 interfacial layers have been systematically investigated at various temperatures by C-V method and the conductance method including surface potential fluctuations. The nature of interface traps was found to depend on the oxidation temperature. Furthermore, the charged center density evaluated from the magnitude of the surface potential fluctuations (which includes charged interface traps, charged slow traps, and fixed oxide charges) increases from the valence bandedge (VBE) to the conduction bandedge (CBE) of Ge in the case of high-temperature oxidation when the Fermi level moves from the VBE to the CBE. This indicates that acceptorlike traps are distributed across the Ge bandgap.

The Effect of Donor/Acceptor Nature of Interface Traps on Ge MOSFET Characteristics

In this paper, the acceptor and donor nature of interface traps are investigated using conductance and interface trap time constant measurements on Ge n- and p-type metal-oxide-semiconductor field-effect transistors (N-and PMOSFETs). The presence of acceptor-type interface trap states in the valence-band side of Ge band gap is confirmed by these measurements. Electron trapping by the acceptor-type interface states and their effect on Ge N- and PMOS performance are discussed. The high density of the acceptor-type interface traps found to be degrading Ge NMOSFET performance, while it is not a concern for Ge PMOSFETs because of the position of charge neutrality level in Ge. Trapped charge calculations show that reducing the interface trap density by the ozone oxidation mitigates the electron trapping by the acceptor-type traps, which otherwise degrade Ge NMOSFET performance. By engineering the gate dielectric interface of Ge NMOSFETs, 40% improvement in inversion electron mobility is reported. Improvement of 2.5× over universal hole mobility is achieved for Ge PMOSFETs.

(Invited) Past, Present and Future: SiGe and CMOS Transistor Scaling

This paper discusses the historical role that SiGe has played in driving the CMOS scaling roadmap, including discussion of NMOS biaxial strain and PMOS uniaxial strain. The paper also discusses the potential future role that Ge or SiGe may play in CMOS scaling as a high mobility replacement for the Si channel. Challenges such as poor quality germanium oxide and the small Ge bandgap are reviewed in light of recent developments (high-k metal gate, and ultra-thin body devices) in MOSFET scaling.

Intense infrared emission of Er^3+ in Ca_8Mg(SiO_4)_4Cl_2 phosphor from energy transfer of Eu^2+ by broadband down-conversion

We report on conversion of near-ultraviolet and visible radiation ranging from 250 to 500 nm into near-infrared emission by a Ca(8)Mg(SiO(4))(4)Cl(2): Eu(2+), Er(3+) phosphor. Efficient 1530-1560 nm Er(3+) emission ((4)I(13/2)-->(4)I(15/2)) was detected under the excitation of Eu(2+) (4f?5d) absorption band as a result of energy transfer from Eu(2+) to Er(3+), which is confirmed by both steady state and time-resolved emission spectra. The laser power dependent emission intensity changes were investigated to analysis the energy transfer mechanism. Energy transfer from Eu(2+) to Er(3+) followed by a multi-photon quantum cutting of Er(3+) is proposed. The result indicates that the phosphor has potential application in enhancement of conversion efficient of germanium solar cells because the energy difference of Er(3+): (4)I(13/2)-->(4)I(15/2) transition matches well with the bandgap of Ge (Eg~0.785 eV).

Electron trapping in substoichiometric germanium oxide

Model structures of substoichiometric germanium oxide generated by ab initio molecular dynamics reveal a rich variety of bonding configurations, including threefold coordinated Ge and O atoms which correspond to valence alternation pairs. A sizable fraction of twofold Ge atoms carrying unoccupied dangling bonds is also found. The present structures are prone to electron capture. The electrons are trapped in additionally formed Ge–Ge bonds. The associated defect levels fall within the Ge band gap and could explain the poor performance of n-type Ge devices.