Band Gap Of Ge Research Articles

Room temperature infrared photoresponse of self assembled Ge/Si (001) quantum dots grown by molecular beam epitaxy

We report on the observation of intraband near infrared (∼3.1 μm) and mid infrared (∼6.2 μm) photocurrent response at room temperature using Ge/Si self-assembled quantum dots grown by molecular beam epitaxy. Due to the bimodal size distribution and SiGe intermixing, distinguishable photoluminescence transitions are observed at 10 K, below and above the optical band gap of bulk Ge. The observed redshift in photocurrent with increasing temperature has been explained by the excitonic electric field originated due to infrared excitation at low temperatures. A good correlation between the spectral photocurrent response and photoluminescence of the quantum dots has been established.

Preparation and properties of heavily doped and strongly compensated Ge films on GaAs

We present and generalize the preparation conditions and properties of heavily doped and strongly compensated (HDSC) Ge films obtained by deposition in the vacuum onto the semi-insulating GaAs (100) substrates. A possibility of formation of Ge films with various doping levels and compensation degrees (in particular, fully compensated) is demonstrated. Heavily doped and fully compensated Ge single-crystalline thin (∼0.1 μm) films obtained have high resistivity (up to 140 Ω cm), conductance activation energy as high as half the bandgap of Ge, low free charge carrier mobility (∼50 cm2/V s), and concentration (∼1014–1015 cm−3). The electrical and optical properties of the films are explained with allowance made for the presence of large-scale fluctuations of electrostatic potential in Ge. Under certain conditions, a two-dimensional potential relief may exist in thin HDSC Ge films, as well as two-dimensional percolation may occur.

First Principles Study of the Adsorption Structure of Ethylene on Ge(001) Surface

The adsorption of ethylene on the Ge(001) surface is investigated by the first principles density-functional calculations. Our total energy calculations and reaction path investigations clarify the relative importance of various adsorption configurations at 0.5 and 1.0 monolayer adsorption coverage. The results are consistent with the experimental data in the literature in that both the di-σ and paired-end-bridge configurations are the favorable structures on Ge(001). In addition, our calculation results clarify that having di-σ-bound ethylene and end-bridge-bound ethylene next to each other is unfavorable. Such new results imply that although di-σ-bound ethylene and end-bridge-bound ethylene may coexist on Ge(001), phase separation will occur to form adsorbate domains. The electronic structures have also been studied, and the band structure calculations show that both the di-σ and end-bridge models at 0.5 monolayer are semiconducting with a small band gap of ∼0.4 eV, which is slightly larger than the band gap of the virgin Ge(001) p(2 × 2) surface. Increasing the coverage to 1.0 monolayer further widens the band gap in both cases. The results thus rule out the past postulation that ethylene adsorption may turn Ge(001) to metallic.

Low-temperature characterization and modeling of advanced GeOI pMOSFETs: Mobility mechanisms and origin of the parasitic conduction

We present the fabrication and characterization of Fully-Depleted pMOSFETs processed on ultra-thin GeOI (Germanium-On-Insulator) wafers obtained by the Enrichment technique. The fabrication procedures for wafers and pMOSFETs are detailed. The influence of temperature (77K<T<300K) on front (Ge-high-κ) and back (Ge–SiO2) channel properties, such as the hole mobility, the threshold voltage and the substhreshold swing, is reported. Very high values of hole mobility have been measured (350 and 595cm2V−1s−1 at 300K and 77K, respectively). The carrier scattering mechanisms are revealed from the temperature-dependent hole mobility. The role of the interface defects and residual body doping on the parasitic conduction is clarified based on the threshold voltage shift measured at low temperature. An appropriate VT(T) model, providing relevant information on the Dit distribution in the Ge band gap, is proposed.

Characteristics of surface states and charge neutrality level in Ge

The characteristics of surface states on Ge are investigated using metal/insulator/semiconductor structures and the conductance technique. Evidence of acceptor-like and donor-like surface states on the valence band side of the Ge bandgap is shown by trap time constant analysis. The dependency of trap time constants on trap energy separation from band edge, capture cross section, and temperature are studied through conductance measurements and simulations. The effect of surface states on the location of charge neutrality level at the Ge surface and the consequences such as surface conductivity and negative charge buildup at the interface are discussed.

Extended performance GeSn/Si(100) p-i-n photodetectors for full spectral range telecommunication applications

First-generation n-i-GeSn/p-Si(100) photodiode detectors with Ge0.98Sn0.02 active layers were fabricated under complementary metal oxide semiconductor compatible conditions. It is found that, even at this low Sn concentration, the detector quantum efficiencies are higher than those in comparable pure-Ge device designs processed at low temperature. Most significantly, the spectral range of the GeSn device responsivity is dramatically increased—to at least 1750 nm—well beyond the direct band gap of Ge (1550 nm). This allows coverage of all telecommunication bands using entirely group IV materials.

Ge (100) and (111) N- and P-FETs With High Mobility and Low-$T$ Mobility Characterization

In this paper, we demonstrate high-mobility bulk Ge N- and P-FETs with GeON gate dielectric. The highest electron mobility to date in Ge is reported, and two times improvement over universal hole mobility is achieved for Ge P-FETs. For the first time, the effect of surface orientation on Ge mobility is investigated experimentally. A 50% improvement in electron mobility is shown for the (111) substrate orientation compared to the (100) orientation. Carrier scattering mechanisms are studied through low-temperature mobility measurements and interface characterization. The conductance technique is applied at low temperatures for complete mapping of the density of interface traps (Dit) across the Ge bandgap and also close to the band edges. Carrier scattering mechanisms and the distribution of Dit are compared for Ge NMOS and PMOS.

First-Principles Analysis of Indirect-to-Direct Band Gap Transition of Ge under Tensile Strain

Ge is a promising material not only for its high carrier mobility but also for its unique optical property. The optical transition properties of Ge under six types of tensile strain, which was hardly investigated using a conventional semi-empirical method, have been analyzed using first-principles calculation. It has been found that the direct band gap of Ge shrinks faster than the indirect band gap under five types of tensile strain. In particular, Ge becomes a direct-transition semiconductor under 2% (001) in-plane biaxial tensile strain. Therefore, the use of Ge and strain technology is an excellent approach to the construction of Si-compatible photonic devices and to the monolithic integration of such devices with Si electronics. Furthermore, it has been predicted that direct-transition tensile-strained Ge can be constructed using only Si and Ge by a finite-element method.

High-Quality Schottky Contacts for Limiting Leakage Currents in Ge-Based Schottky Barrier MOSFETs

Schottky barrier (SB) Ge channel MOSFETs suffer from high drain-body leakage at the required elevated substrate doping concentrations to suppress source-drain leakage. Here, we show that electrodeposited Ni-Ge and NiGe/Ge Schottky diodes on highly doped Ge show low off current, which might make them suitable for SB p-MOSFETs. The Schottky diodes showed rectification of up to five orders of magnitude. At low forward biases, the overlap of the forward current density curves for the as-deposited Ni/n-Ge and NiGe/n-Ge Schottky diodes indicates Fermi-level pinning in the Ge bandgap. The SB height for electrons remains virtually constant at 0.52 eV (indicating a hole barrier height of 0.14 eV) under various annealing temperatures. The series resistance decreases with increasing annealing temperature in agreement with four-point probe measurements indicating the lower specific resistance of NiGe as compared to Ni, which is crucial for high drive current in SB p-MOSFETs. We show by numerical simulation that by incorporating such high-quality Schottky diodes in the source/drain of a Ge channel PMOS, a highly doped substrate could be used to minimize the source-to-drain subthreshold leakage current.

Modeling of negatively charged states at the Ge surface and interfaces

Modeling based on surface charge neutrality predicts that the Ge surface tends to be p-type, irrespective of the bulk conductivity. This is a consequence of the fact that the Ge band gap is small and the charge neutrality level lies low in the gap very close to the valence band, probably determined by low-lying unpassivated surface dangling bond acceptors or other defects. According to the model, the acceptor defects build negative charge, inverting the surface of n-type Ge at no gate bias for low doping concentration (&amp;lt;1016 cm−3) and moderate or high interface state densities (&amp;gt;5×1011 eV−1 cm−2). This is predicted to cause undesired positive threshold voltage shift in the range of 0.2–0.4 V in Ge p-channel field effect transistors. The model also predicts that inversion in n-channel field effect transistors is inhibited, which could be related to the observed poor performance of these devices.

Interface-Engineered High-Mobility High-$k$/Ge pMOSFETs With 1-nm Equivalent Oxide Thickness

High-k/germanium (Ge) interfaces are significantly improved through a new interface engineering scheme of using both effective pregate surface GeO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> passivation and postgate dielectric (postgate) treatment incorporating fluorine (F) into a high-k/Ge gate stack. Capacitance-voltage (C-V) characteristics are significantly improved with minimum density of interface states (D <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">it</sub> ) of 2 times 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">11</sup> cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-2</sup> ldr eV <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-1</sup> for Ge MOS capacitors. A hole mobility up to 396 cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> /V ldr s is achieved for Ge p-metal-oxide-semiconductor field-effect transistors (pMOSFETs) with equivalent oxide thickness that is ~10 Aring and gate leakage current density that is less than 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-3</sup> A/cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> at V <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">t</sub> plusmn 1 V. A high drain current of 37.8 muA/mum at V <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">g</sub> - V <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">t</sub> = V <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">d</sub> = -1.2 V is presented for a channel length of 10 mum. The Ge MOSFET interface properties are further investigated using the variable-rise-and-fall-time charge-pumping method. Over three times D <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">it</sub> reduction in both upper and lower halves of the Ge bandgap is observed with F incorporation, which is consistent with the observation that frequency-dependent flat voltage shift is much less for samples with F incorporation in the C-V characteristics of Ge MOS capacitors.

Applicability of Charge Pumping on Germanium MOSFETs

In this letter, the charge-pumping (CP) technique is validated for germanium MOSFETs. Effects of the smaller Ge bandgap on CP are discussed through both experiments and simulations. The standard CP setup with ~ 100-ns transition times at room temperature tuned for Si/SiO2 MOS evaluates the Ge interface-trap density only near midgap, and the total density is thus strongly underestimated. We show two CP methods which can be used to correctly reflect the actual complete interface-trap density by probing closer to the band edges. The use of low-temperature measurements to probe traps near the band edges with CP is discussed. CP measurements are demonstrated with transition times down to 6-ns at 300-K without making use of RF structures. Using these fast measurements, it is possible to obtain an interface state density closer to the band edges for Ge MOSFETs at 300-K.

Ge dangling bonds at the (100)Ge/GeO2 interface and the viscoelastic properties of GeO2

The electronic energy band structure of a (100)Ge/GeO2 interface with a Ge3≡Ge• center (Ge dangling bond) is calculated using the density functional theory. The defect level associated with this dangling bond is found to lie near the middle of the Ge band gap. Consequently, the Ge dangling bond at this interface should be paramagnetic when the Fermi level is near the midgap, and it should thus be observed by electron spin resonance (ESR), which appears to be in contradiction with the results reported by [Afanas’ev et al., Appl. Phys. Lett. 87, 032107 (2005)]. We point out that the density of Ge3≡Ge• centers at the (100)Ge/GeO2 interface is possibly at/below the ESR detection limit due to the viscoelastic properties of GeO2, owing to a better stress relaxation at/near the interface, compared to its silicon counterpart.

Asymmetric Energy Distribution of Interface Traps in Germanium MOSFETs with HfO2 Gate Dielectric

In this work, a simple model, with a higher interface trap density in the upper half of Ge bandgap than that in the lower half, is proposed to explain the abnormal behaviors in Capacitance-Voltage characteristics of both p- and n-MOS capacitors on Ge substrate using different surface passivation. Variable rise/fall-time charging pumping measurement is used to study the energy distribution of interface trap density in HfO2 gated Ge MOSFETs. Our results reveal that Dit is higher in the upper half of the Ge bandgap than that in the lower half of the bandgap. These results are also consistent with the observation that n-channel mobility is more severely degraded compared to p-channel for Ge MOSFET.

High-quality NiGe/Ge diodes for Schottky barrier MOSFETs

Schottky barrier (SB) Ge channel MOSFETs suffer from high drain-body leakage at the required elevated substrate doping concentrations to suppress source–drain leakage. Here, we show that electrodeposited Ni–Ge and NiGe/Ge Schottky diodes on highly doped Ge show low off current, which might make them suitable for SB-MOSFETs. The Schottky diodes showed rectification of up to five orders in magnitude. At low forward biases, the overlap of the forward current density curves for the as-deposited Ni/ n-Ge and NiGe/ n-Ge Schottky diodes indicates Fermi-level pinning in the Ge bandgap. The SB height for electrons remains virtually constant at ∼0.52 eV (indicating a hole barrier height of ∼0.14 eV) under various annealing temperatures. The series resistance decreases with increasing annealing temperature in agreement with four-point probe measurements indicating the lower specific resistance of NiGe as compared with Ni, which is crucial for high drive current in SB-MOSFETs. We show by numerical simulation that by incorporating such high-quality Schottky diodes in the source/drain of a Ge channel PMOS, highly doped substrate could be used to minimize the subthreshold source to drain leakage current.

Energy distribution of interface traps in germanium metal-oxide-semiconductor field effect transistors with HfO2 gate dielectric and its impact on mobility

The energy distribution of interface trap density (Dit) in HfO2 gated germanium metal-oxide-semiconductor field effect transistors (MOSFETs) is investigated by using charge pumping method with variable rise/fall-time measurement. Our results reveal that a high density of interface traps is present in the upper half of the Ge bandgap. As a result, the inversion-layer electron mobility of Ge n-channel MOSFETs was significantly degraded by the Coulomb scatterings. These results are also consistent with the abnormal capacitance-voltage (C-V) characteristics of Ge MOS capacitors.

Effects of fluorine incorporation and forming gas annealing on high-k gated germanium metal-oxide-semiconductor with GeO2 surface passivation

Effective pregate surface passivation and postgate treatments are very important to optimize the germanium metal-oxide-semiconductor (MOS) interface quality. In this work, pregate GeO2 surface passivation and postgate treatments including CF4-plasma treatment and forming gas annealing are employed to make high quality HfO2 gated germanium MOS capacitors. Excellent electrical characteristics with negligible capacitance-voltage stretch-out and frequency dispersion are achieved. The interface trap density of TaN∕HfO2∕GeOx∕Ge MOS structure is as low as 2.02×1011cm−2eV−1 at the minimum. Comparing to the forming gas annealing, it is found that CF4-plasma treatment is more effective to passivate interface states located in the upper half of Ge bandgap.

Heavily doped and fully compensated Ge single-crystalline films on GaAs

Heavily doped and fully compensated Ge single-crystalline thin (∼0.1μm) films were epitaxially grown in vacuum on semi-insulating GaAs substrates. Such films have high resistivity (up to 140Ω∕cm), conductance activation energy as high as half the band gap of Ge, low free charge carrier mobility (∼50cm2∕V⋅s) and concentration (∼1014cm−3). The transport properties of the films were studied and explained involving the theory of two-dimensional potential fluctuations and percolation.

Electrochemical Impedance Study of the Germanium/Electrolyte Interface

In this paper, an electrochemical impedance study of the Ge/electrolyte interface has been carried out. At n-type Ge(100) and Ge(111) electrodes with different donor densities, linear Mott–Schottky plots were observed over a wide potential region, showing that the depletion region is conserved up to a band bending exceeding the bandgap of Ge. In most cases, the Mott–Schottky plots show some frequency dependence. A more elaborate investigation of the total interfacial impedance using electrochemical impedance spectroscopy shows that this frequency dependence may be caused by an additional impedance in parallel with the depletion layer capacity in the equivalent circuit describing the Ge/electrolyte interface. Under very weak depletion conditions, i.e., close to the flatband potential, an additional impedance is also observed, which may be related to electron–hole recombination at the semiconductor surface. For p-type Ge electrodes, linear Mott–Schottky plots are much more difficult to obtain, probably because electrochemical reactions and/or surface recombination strongly complicate the total impedance of the Ge/electrolyte interface.

Phase segregation in Pb:GeSbTe chalcogenide system

Effect of Pb substitution on the amorphous-crystalline transformation temperature, optical band gap and crystalline structure of Ge 2 Sb 2 Te 5 has been studied. In Pb:GeSbTe chalcogenide films prepared by thermal evaporation, an amorphous to crystallization transition is observed at 124, 129, 136 and 138 °C in Pb 0 Ge 20 Sb 24 Te 56 , Pb 1.6 Ge 19 Sb 26 Te 54 , Pb 3 Ge 17 Sb 28 Te 53 and Pb 5 Ge 12 Sb 28 Te 55 respectively. XRD investigations of annealed samples reveal that Pb substitution retains NaCl type crystalline structure of GST but expands the lattice due to large atomic radii. The increase in amorphous-crystalline transformation temperature is followed with the increase in phase segregation. The optical gap shows marginal variations with composition.