Band Gap Of Ge Research Articles

Selective Passivation of GeO2/Ge Interface Defects in Atomic Layer Deposited High-k MOS Structures.

Effective passivation of interface defects in high-k metal oxide/Ge gate stacks is a longstanding goal of research on germanium metal-oxide-semiconductor devices. In this paper, we use photoelectron spectroscopy to probe the formation of a GeO2 interface layer between an atomic layer deposited Al2O3 gate dielectric and a Ge(100) substrate during forming gas anneal (FGA). Capacitance- and conductance-voltage data were used to extract the interface trap density energy distribution. These results show selective passivation of interface traps with energies in the top half of the Ge band gap under annealing conditions that produce GeO2 interface layer growth. First-principles modeling of Ge/GeO2 and Ge/GeO/GeO2 structures and calculations of the resulting partial density of states (PDOS) are in good agreement with the experiment results.

A comparison of random discrete dopant induced variability between Ge and Si junctionless p-FinFETs

In this letter, the random discrete dopant (RDD) induced variability for a Ge junctionless (JL) p-FinFET is reported. A one-to-one comparison of the RDD-induced variability between Ge and Si JL FinFETs for varying device parameters and supply voltage is made using a 3-D numerical device simulator. Results indicate that the Ge JL FinFET shows higher immunity to RDD induced threshold voltage fluctuation than its Si counterpart, which is partially due to the higher dielectric constant of Ge than Si. Because of the lower band gap of Ge than Si, a higher variation in subthreshold swing due to RDD is, however, observed for Ge devices. Technology scaling is found to reduce σSS, although it has almost no effects on σVT for both types of devices. The difference in σSS between Ge and Si devices decreases with technology scaling, which makes Ge very attractive for scaled p-type JL FinFETs.

(Invited) Electrical Properties of Group 4 Metal-Nitride/Ge Contacts and the Application to Ge Optoelectronic Devices

1) Introduction Ge is of great interest as a candidate for the channel material in CMOS devices. In order to integrate Ge-CMOS with current Si platforms, metal source/drain (S/D) MOSFETs are promising candidate because of simple structures and low-temperature processing. To realize high-performance, metal/Ge contacts with low electron barrier height (ΦBN) and low hole barrier height (ΦB P) are needed for n- and p-MOSFETs, respectively. However, one difficulty is the Fermi-level pinning (FLP) where the Fermi level is pinned near the valence band edge at the metal/Ge interface. Thus, the formation of a p-MOSFET is straight forward, but an n-MOSFET is difficult. We found that TiN/Ge contacts can alleviate the FLP position toward the conduction band edge [1-3]. By using this contact technique, optical devices with function of light emission and detection are possible [4], which are useful for optical communication. This is because the direct band gap of Ge is 0.80 eV, corresponding to a wavelength of 1.55 mm. In this presentation, from investigations on electrical properties and interfacial structures of group-4 metal-nitride/Ge contacts such as TiN/Ge, ZrN/Ge, and HfN/Ge, we show that an amorphous interlayer (a-IL) with N plays an important role in the FLP alleviation. We also present direct-bandgap room temperature electroluminescence as well as light detection using bulk-Ge diodes with a fin type lateral PtGe/Ge/TiN structure. 2) Electrical and structural properties of group-4 metal-nitride/Ge contacts Metal nitrides were directly deposited on (100) Ge by rf magnetron sputtering using a nitride target. Then, a 50-nm-thick Al film was deposited by evaporation, and the contact pattern was formed by liftoff technique. Finally, PMA was carried out at temperatures below 600°C. HAADF-STEM was used to clarify the interfacial structures. A TiN/Ge contact with 350°C-PMA showeda high ΦBP of 0.53 eV. In the HAADF-STEM images for samples with no-PMA and 350°C-PMA, an IL was present at the TiN/Ge interface. The IL thicknesses were 1.3 and 1.9 nm for the no-PMA and 350°C-PMA samples, respectively. The IL structure was determined to be amorphous (a). EDX for the no-PMA and 350°C-PMA samples revealed that the ILs consisted of Ge (90 at%)/Ti (10 at%) and Ge (76 at%)/Ti (24 at%), respectively. The N atoms in the a-IL could be also detected by the EDX. The a-IL changed to a crystalline phase after 600°C-PMA, where the TiN was in direct contact with the Ge crystal, resulting in pinned contact property. An a-IL with a thickness of 1.9 nm exists at an interface in a ZrN/Ge contact with 450°C-PMA, leading to strong FLP alleviation (ΦBP=0.56 eV). An a-IL in a HfN/Ge contact is thin (0.9 nm) even for 350°C-PMA samples, leading to small FLP alleviation (ΦBP=0.39 V). Thus, it was found that FLP alleviation for metal nitride/Ge contacts is strongly associated with existence of an a-IL with N. The extrinsic states associated with bonding defects at an a-IL/Ge interface cause the creation of an interfacial dipole. Considering the upward shift of the FLP position, the dipole should be positively charged at the Ge side of the interface. The dipole is likely caused by N-related defects near the a-IL/Ge interface. 3 ) Light e mi ssion and detection Ge devices We fabricated fin type lateral PtGe/Ge/TiN device using above contact techniques. The contacts were passivated by SiO2/GeO2 bilayer. The light emission with a wavelength of 1.55 mm could be observed when forward currents were flowed in the diode. The wavelength corresponded to direct energy gap of Ge (0.80 eV). This is because electron and hole were effectively injected from both contacts. At the wavelength of 1.55 μm, a linear dependence of photo current intensity on laser power was observed with a responsivity of 0.44 A/W at a reverse bias voltage of -1 V. Acknowledgement This work was supported by (JSPS) KAKENHI (grant No. 25249035 & 26289090) and was partially supported by JSPS Core-to-Core Program, A. Advanced Research Networks.

Measurement of Optical Emission from Gesn Waveguides

The development of Si-based lasers has been a goal of the photonics industry for many years. A number of methods have been explored for achieving lasers on Si platforms, and a few attempts, such as the Si Raman laser, have achieved marginal success, but typically suffer from low efficiency. The silicon photonics problem requires the realization of high-efficiency lasers on CMOS-compatible platforms. One possible solution to this problem is to employ the use of direct band gap materials that can be grown on Si. Of course, the epitaxial growth of such materials has been extremely difficult, and the efforts have mostly concentrated on the use of III-V materials like InGaAs. These materials typically suffer from high defect densities when grown directly on silicon, making them unsuitable for use in lasers. Additionally, the introduction of Group III and Group V materials into a CMOS fabrication facility can cause severe problems in device production due the electronic properties of these atoms, which are typically used as dopants in Si. GeSn alloys present a possible path forward for the realization of Si-based lasers, as these materials are grown directly on Si or Ge-buffered Si and have a band structure that is more suited for efficient optical emission. A laser based on tensile strained Ge grown directly on Si has been demonstrated at room temperature, but the efficiency is limited by the indirect nature of the Ge band gap. More recently, demonstration of lasing at low temperature in GeSn (12.6% Sn) was achieved using optical pumping of a GeSn waveguide. While this represents a first step to the actualization of GeSn lasers, room temperature operation is a necessary condition for widespread adoption and incorporation into everyday devices. The approach for demonstration of lasing in GeSn and Ge-on-Si has typically relied on measuring the optical emission spectrum at very high resolution to see the narrowing of the emission linewidth and the presence of cavity modes while optically-pumping a waveguide with a pulsed laser via topside illumination. We have designed a new system for optically pumping waveguides by coupling into the waveguide edge, which increases the absorption length by orders of magnitude, thereby increasing optical emission intensity and relaxing the constraints under which lasing is achieved. We employ a CW laser and erbium-doped fiber amplifier at 1550 nm as the pump source, which serves to decrease the cost of the experiment and increase the pump power. The edge emission from the waveguide is sent into a Fabry-Perot cavity with an InGaAs detector, and the optical pump power is increased until coherence is achieved, which results in the observance of modes in the Fabry-Perot cavity. This measurement can be performed with much lower pump power than a measurement using a high-resolution spectrometer. In this work, we demonstrate the use of this method for measuring the optical emission from GeSn waveguides with Sn concentrations of 4-7%.

High performing ITO/Ge heterojunction photodetector for broad wavelength detection

High-performing photodetector was fabricated by the ITO/Ge heterojunction device. Due to a small bandgap Ge semiconductor, ITO/Ge photodetector showed a significantly high photoresponse for long wavelengths. For ITO/Ge photodetector fabrication, an optically transparent and electrically conductive ITO layer was deposited on a Ge substrate and spontaneously formed a rectifying junction. The HRTEM image revealed the presence of GeOx at the interface of the ITO and Ge. The XPS and UPS studies were carried out to realize the band-offset structure of ITO/Ge photodiode. When the p-type Ge and ITO are brought into contact, the Fermi levels of both the materials are lined up. Besides, the energy bands of Ge are pinned to the Fermi level of the ITO layer at the junction. The band bending effect is depicted from the band offset of the ITO/Ge photodetector. We discuss an effective method of the ITO/Ge heterojunction device fabrication and suggest a strong route for high-performing broad range wavelength (600–900 nm) photodetectors.

Electronic properties of manganese impurities in germanium

The electronic properties of manganese in crystalline germanium have been investigated by means of deep level transient spectroscopy (DLTS). Mn was diffused in the material by a thermal treatment at 700 °C. Next to the deep levels of nickel and copper, which are known contaminants in Ge treated at high temperature, three not previously reported levels were observed. These two hole and one electron traps, with apparent energy level positions at EV +0.136 eV, EV +0.342 eV and EC − 0.363 eV, were assigned to substitutional Mn. The analysis of the carrier capture cross-sections, the absence of field-assisted emission and the observation of the Mn2− electron paramagnetic resonance spectrum in n-type Ge:Mn at low temperature are all compatible with Mn introducing two acceptor and one donor levels in the band gap of Ge.

Recent progress in Ge and GeSn light emission on Si

Si-based optical interconnection is expected to solve the problems caused by electric interconnection with increasing the density of integrated circuits, due to its merits of high speed, high bandwidth, and low consumption. So far, all of the key components except light source of Si-based optical interconnection have been demonstrated. Therefore, the light source has been considered as one of the most important components. Ge and GeSn based on Si have emerged as very promising candidates because of their high compatibility with Si CMOS processing, and the pseudo direct-bandgap characteristic. The energy difference between the direct and indirect bandgap of Ge is only 136 meV at room temperature. Under tensile strain or incorporation with Sn, the energy difference becomes smaller, and even less than zero, which means that Ge or GeSn changes into direct bandgap material. What is more, using large n-type doping to increase the fraction of electrons in valley, we can further increase the luminous efficiency of Ge or GeSn. In this paper, we briefly overview the recent progress that has been reported in the study of Ge and GeSn light emitters for silicon photonics, including theoretical models for calculating the optical gain and loss, several common methods of introducing tensile strain into Ge, methods of increasing the n-type doping density, and the method of fabricating luminescent devices of Ge and GeSn. Finally, we discuss the challenges facing us and the development prospects, in order to have a further understanding of Ge and GeSn light sources. Several breakthroughs have been made in past years, especially in the realizing of lasing from GeSn by optically pumping and Ge by optically and electrically pumping, which makes it possible to fabricate a practical laser used in silicon photonics and CMOS technology.

Effects of Ge- and Sb-doping and annealing on the tunable bandgaps of SnS films

Abstract SnS, Ge- and Sb-doped SnS films with single orthorhombic SnS phase were fabricated via solvothermal routes and subsequent spin-coating, respectively. The substitution solubilities of Ge and Sb in SnS are about 6 and 5 at.%, respectively. The bandgaps of Ge- and Sb-doped SnS films can be tuned in the ranges of 1.25–1.35 and 1.30–1.39 eV, respectively. The possible mechanisms for the tunable bandgaps of Ge- and Sb-doped SnS films are discussed. For the Ge- and Sb-doped SnS films subjected to annealing at 200–350 °C in N2, the bandgaps of 200 °C-annealed films remain unchanged, while those of 300 °C- and 350 °C-annealed films decrease with the annealing temperature because of the evaporation of Ge and Sb respectively.

Energy band structures of group IV compound semiconductors for solar cells

The impact of isotropic lattice strain and the composition ratios of mono and binary systems of group IV compounds with diamond structures on energy band structures were analyzed with ab initio calculations by covering wider ranges of strain (−30 to 30%) and compositions (0–100%) than those in actual cases. The bandgap of C crystal increased (decreased) under isotropic compressive (tensile) strain while the bandgap of Si (Ge) crystal reached maximum under tensile strain of 5% (0%). The bandgaps of Ge–Sn (Si–Ge and Si–Sn) at 1:1 stoichiometry reached maximum under compressive strain of 5% (0%). These values slightly differed from the values predicted by Vegard's law. The bandgaps of 1:1 compounds Si–C (Ge–C and Sn–C) reached maximum under tensile strain of 5% (0%). The bandgaps remarkably differed from the values predicted by Vegard's law when the compounds contained C atoms.

Near-infrared emission from mesoporous crystalline germanium

Mesoporous crystalline germanium was fabricated by bipolar electrochemical etching of Ge wafer in HF-based electrolyte. It yields uniform mesoporous germanium layers composed of high density of crystallites with an average size 5-7 nm. Subsequent extended chemical etching allows tuning of crystallites size while preserving the same chemical composition. This highly controllable nanostructure exhibits photoluminescence emission above the bulk Ge bandgap, in the near-infrared range (1095-1360nm) with strong evidence of quantum confinement within the crystallites.

Fabrication of Relaxed Germanium on Insulator via Room Temperature Wafer Bonding

Germanium with higher electron and hole mobilities has been considered as a potential material for replacement of Si in CMOS transistors. However, due to the lower band gap of Ge compare to Si, bulk Ge devices suffer a high leakage current, which could be suppressed by employing a germanium on insulator (GeOI) substrate. High-κ-all-around asymmetrically-strained germanium nanowire trigate p-MOSFETs have exhibited the highest hole mobility recorded (1490 cm2/V.s) [1]. There are several approaches for the fabrication of GeOI substrates including separation-by-implanted-oxygen (SIMOX) [2], Ge condensation [3], liquid phase epitaxy [4], and wafer bonding [5]. Wafer bonding technique is advantageous among the other techniques due to its simplicity and potential for achieving high quality crystalline Ge layer.In this work we present a method to fabricate relaxed and strained GeOI substrates using heteroepitaxy and layer transfer techniques. The goal is to produce high quality wafer scale GeOI platform for relaxed and strained Ge nanowire CMOS fabrication. GeOI substrates are designed and fabricated via wafer bonding and etch back process without intermediary chemical mechanical polishing (CMP) step. Low temperature bonding avoids strain relaxation, Ge diffusion and detaching due to thermal expansion coefficient differences. In this design, GeOI structure is achieved by epitaxial growth and conventional dry and wet-etch techniques. All the epitaxial layers in the process are grown using RPCVD Epsilon 2000 ASM reactor. The fabrication process for both structures starts with the epitaxial growth of a roughly 1 µm Si0.96Ge0.04 etch stop layer. In the strained GeOI design the process is followed by epitaxial growth of an intermediate relaxed SiGe buffer with a Ge content of 40-50 percent, then a strained Si etch-stop layer and finally pseudomorphic growth of a thin strained Ge layer on top (Fig. 1). In relaxed GeOI structure on 1 µm Si0.96Ge0.04 etch stop layer a rather thick Ge layer (~300 nm) is grown in two steps, first a low temperature (LT) Ge is grown with high defect density and then a high temperature (HT) high quality Ge layer is grown on top (Fig. 2). Once the epitaxial growth of the structures is finished, 10 nm Al2O3 is deposited on top the Ge layer using atomic layer deposition (ALD). Then the wafers are brought together in the atmosphere pressure for room temperature bonding. The etch-back process begins by thinning down the donor wafer using Applied Materials Centura deep trench Si etch chamber (etch rate ~7 µm/min ). The etching process stops on the Si0.96Ge0.04 etch stop layer using endpoint detection system. At that point, the Si0.96Ge0.04etch-stop layer is selectively etched in TMAH solution. Then in relaxed GeOI structure the 300 nm Ge layer is oxidized and chemically thinned, up to the desired thickness (Fig. 2). In the strained GeOI structure SiGe relaxed buffer layer is selectively wet etched using dHF(100H2O:1HF):HNO3:CH3COOH (0.15:0.20:0.30) solution, and the strained Si layer is selectively removed in TMAH solution, leaving the strained Ge layer on Al2O3 (Fig. 1). Infrared (IR) image of the bonded wafers shows few voids at the interface and the bonding can withstand fast deep Si dry etching. X-ray analysis shows high quality well separated peaks for both structures and demonstrates the strain in top Ge layer (for the strained GeOI). The bonding strength will be measured using stress tester. The bonded interfaces will be observed by high resolution transmission electron microscopy (HRTEM). Topology of both Ge surfaces will be shown by atomic force microscopy (AFM).In summary, processes for the fabrication of high quality, low defect, monocrystalline, relaxed and strained germanium on insulator substrates are developed and optimized. This technique can serve as a platform for the fabrication of high performance Ge devices. AcknowledgmentThis work was supported by European Union ERC Advanced Grant 228229 OSIRIS.Fig. 1) schematic image of the bond and etch back process for strained GeOIFig. 2) schematic image of the bond and etch back process for relaxed GeOI[1] J. L. H. et al., IEEE, p. 4, 2012.[2] T. Mizuno, et al., IEEE Electron Device Lett., no. 5, pp. 230–232, May 2000.[3] T. Tezuka, et al., Appl. Phys. Lett., no. 12, p. 1798, 2001.[4] M. Miyao, et al., Appl. Phys. Express, p. 045503, Apr. 2009.[5] G. Taraschi, et al., Solid. State. Electron., no. 8, pp. 1297–1305, Aug. 2004.

Direct-bandgap electroluminescence from a horizontal Ge p-i-n ridge waveguide on Si(001) substrate

Horizontal injection Ge p-i-n ridge waveguide light emitting diodes (LEDs) were fabricated on n−-Si(001) substrates by ultrahigh vacuum chemical vapor deposition. The direct-bandgap electroluminescence (EL) of Ge waveguide LEDs under a continuous/pulse electrical pump was studied. The heating effect from a continuous electrical pump was found to significantly enhance the emission of devices. The top surface EL intensity of the Ge waveguide LEDs significantly depended on the position. Most direct-bandgap radiative recombination of Ge p-i-n waveguide LEDs occurred near the N+ region of the junction. This interesting phenomenon could be explained by the carrier distribution in the junction and the pseudo-direct bandgap of Ge.

Infrared photoresponse of GeSn/n-Ge heterojunctions grown by molecular beam epitaxy.

Heterojunction devices of Ge(1-x)Sn(x) / n-Ge were grown by solid source molecular beam epitaxy (MBE), and the mid-infrared (IR) photocurrent response was measured. With increasing Sn composition from 4% to 12%, the photocurrent spectra became red-shifted, suggesting that the bandgap of Ge(1-x)Sn(x) alloys was lowered compared to pure Ge. At a temperature of 100 K, the wavelengths of peak photocurrent were shifted from 1.42 µm for pure Ge (0% Sn) to 2.0 µm for 12% Sn. The bias dependence of the device response showed that the optimum reverse bias was > 0.5 volts for saturated photocurrent. The responsivity of the Ge(1-x)Sn(x) devices was estimated to be 0.17 A/W for 4% Sn. These results suggest that Ge(1-x)Sn(x) photodetectors may have practical applications in the near/mid IR wavelength regime.

Direct bandgap germanium-on-silicon inferred from 57% 〈100〉 uniaxial tensile strain [Invited

We report uniaxial tensile strains up to 5.7% along 〈100〉 in suspended germanium (Ge) wires on a silicon substrate, measured using Raman spectroscopy. This strain is sufficient to make Ge a direct bandgap semiconductor. Theoretical calculations show that a significant fraction of electrons remain in the indirect conduction valley despite the direct bandgap due to the much larger density of states; however, recombination can nevertheless be dominated by radiative direct bandgap transitions if defects are minimized. We then calculate the theoretical efficiency of direct bandgap Ge LEDs and lasers. These strained Ge wires represent a direct bandgap Group IV semiconductor integrated directly on a silicon platform.

(Invited) Interface Engineering Routes for a Future CMOS Ge-Based Technology

An alternative route to continue the trend of scaling of CMOS technology is to implement novel channel materials with superior transport properties. Germanium has high intrinsic mobilities for both electrons (3900 cm2/Vs) and holes (1900 cm2/Vs), and is compatible with Si process technology, making it a suitable channel material for low power, high performance devices. The smaller band gap of Ge (0.67 eV) potentially allows for lower contact resistances compared to Si due to a reduced barrier height, and hence is more suitable for voltage scaling. A challenging task is the formation of a high-quality gate stack with low interface trap density and sub-nm equivalent oxide thickness (EOT) to maintain the intrinsically high performance of Ge. The peak electron mobility has been dramatically improved for Ge MOSFETs in recent years [1,2], largely achieved by improving the quality of GeO2/Ge interface. Although thermally grown GeO2 is the most natural choice, its inherent shortcomings still remain, such as: high water solubility, low desorption temperature (~400°C), low dielectric constant (~6); unconventional growth methods. Since the dielectric constant of GeO2 is low it cannot be used as the gate dielectric for aggressively scaled devices. The focus is on high-k dielectric (k>20) for sub-nm EOT scaling. The Liverpool group has been interested in two possible routes for Ge interface engineering: (i) using high-k materials that are intimate with Ge, such as La2O3 and Y2O3, and (ii) introducing a robust ultra-thin high-k interfacial layer (IL) barrier, such as Al2O3 and Tm2O3. Concerning the first route, high reactivity of Ge with high-k allows for germanate IL formation, which role is two-fold: to reduce the interface states, and to suppress the GeO desorption at the interface. The second route involves the use of ultra-thin barrier layers, Al2O3 and Tm2O3, as oxides highly resistant to oxygen diffusion and to reaction with Ge. The rare-earth metals (La, Y, Tm) tend to possess multiple valency, such as +2 and +3 oxidation states, that can provide effective passivation of electrically active defects. Both routes lead to achieving a GeOx-free gate stack with effective Ge surface passivation.We will present an overview study of physical and electrical properties of La2O3/Ge, Y2O3/Ge, Tm2O3/Ge, Al2O3/Ge and HfO2/Al2O3/GeO2/Ge gate stacks. The interfacial composition, valence band offset, uniformity, thickness, band gap, crystallinity, dielectric function and absorption features will be shown, as ascertained using X-ray photoelectron spectroscopy, X-ray diffraction, high resolution transmission electron microscopy, medium energy ion scattering and ultra violet variable angle spectroscopic ellipsometry. The correlation of these results with electrical characterization data, will make a case for Ge interface engineering with rare-earth inclusion as a viable route to achieve high performance Ge CMOS.Acknowledgement: the authors thank the EPSRC (UK) and the EU (OSIRIS and ESTEEM2 projects) for funding.[1] S. Takagi et al., Microelectron. Eng. 109, 389 (2013); [2] A. Toriumi et al. IEEE IEDM Tech. Dig., 646 (2011).

Material Characterization of Ge1−x Sn x Alloys Grown by a Commercial CVD System for Optoelectronic Device Applications

High-quality compressive-strained Ge1−xSnx/Ge films have been deposited on Si(001) substrate using a mainstream commercial chemical vapor deposition reactor. The growth temperature was kept below 450°C to be compatible with Si complementary metal–oxide–semiconductor processes. Germanium tin (Ge1−xSnx) layers were grown with different Sn composition ranging from 0.9% to 7%. Material characterizations, such as secondary-ion mass spectrometry, Rutherford backscattering spectrometry, and x-ray diffraction analysis, show stable Sn incorporation in the Ge lattice. Comparison of the Sn mole fractions obtained using these methods shows that the bowing factor of 0.166 nm (in Vegard’s law) is in close agreement with other experimental data. High-resolution transmission electron microscopy and atomic force microscopy results show that the films have started to relax through the formation of misfit and threading dislocations. Raman spectroscopy, ellipsometry, and photoluminescence (PL) techniques are used to study the structural and optical properties of the films. Room-temperature PL of the films shows that 7% Sn incorporation in the Ge lattice results in a decrease in the direct bandgap of Ge from 0.8 eV to 0.56 eV.

Study on intrinsic carrier concentration of direct bandgap Ge1-xSnx

Indirect bandgap Ge can be turned to a direct bandgap semiconductor by the alloy-modified technique, which can be applied to advanced photonic devices and electronic devices. Based on 8 bands Kronig-Penny Hamilton, this paper focuses on the physical parameters of direct bandgap Ge1-xSnx, such as conduction band effective density of states, valence band effective density of states and the intrinsic carrier concentration, and aims to provide valuable references for understanding the direct bandgap modified Ge materials and device physics as well as their applications. Results show that: conduction band effective density of states in direct bandgap Ge1-xSnx alloy decreases obviously with increasing Sn fraction, while its valence band effective density of states almost does not change with increasing Sn fraction. Compared with bulk Ge, the conduction band effective density of states and valence band effective density of states in direct bandgap Ge1-xSnx alloy are lower by two and one orders of magnitude respectively; the intrinsic carrier concentration in direct bandgap Ge1-xSnx alloy increases with increasing Sn fraction, and its value is an order of magnitude higher than that of bulk Ge.

GeSn-based p-i-n photodiodes with strained active layer on a Si wafer

We report an investigation of GeSn-based p-i-n photodiodes with an active GeSn layer that is almost fully strained. The results show that (a) the response of the Ge/GeSn/Ge heterojunction photodiodes is stronger than that of the reference Ge-based photodiodes at photon energies above the 0.8 eV direct bandgap of bulk Ge (<1.55 μm), and (b) the optical response extends to lower energy regions (1.55–1.80 μm wavelengths) as characterized by the strained GeSn bandgap. A cusp-like spectral characteristic is observed for samples with high Sn contents, which is attributed to the significant strain-induced energy splitting of heavy and light hole bands. This work represents a step forward in developing GeSn-based infrared photodetectors.

Polymer-embedded colloidal lead-sulfide nanocrystals integrated to vertically slotted silicon-based ring resonators for telecom applications

The main drawback of the rapidly evolving field of silicon photonics lies in the absence of efficient monolithically integrated radiation sources as a consequence of the indirect bandgap of Si and Ge. Relevant alternatives based on the hybrid combination of Si with optically active materials have to be technologically simple, temporally stable, and provide efficient coupling to the Si waveguides. Lead-sulfide nanocrystals (NCs) were blended into a polymer resist suitable for deep-UV- and electron-beam lithography and integrated into Si-based vertically slotted waveguides and ring resonators. The polymer both stabilizes the NC’s photoluminescence emission against degradation under ambient conditions and allows lithographic patterning of this compound material. After integration into the optoelectronic structures and upon optical pumping, intense photoluminescence emission from ring resonators was recorded at the output of bus-waveguides. The resonator quality factors were investigated for polymer-NC compounds with NC concentrations in the range between 0.1 and 8 vol%. The spontaneous emission rate enhancement for vertically slotted resonators was estimated to be a factor of two higher as compared to unslotted ones. The stable integration of colloidal NCs as well as the improved light coupling to silicon circuits is an important step in the development of silicon-based hybrid photonics.

Radiation Impact of EUV on High-Performance Ge MOSFETs

High-energy extremely ultraviolet (EUV)-induced Ge MOSFETs degradation is investigated. The degradation of threshold voltage, subthreshold swing (SS), and channel mobility is attributed to the generation of interface traps and oxide fixed charges. Much more severe degradation of SS and VT on n-FETs compared to p-FETs suggests that more interface defects in the upper half of Ge bandgap are generated by EUV radiation than in the lower half bandgap. The increase of interface trap is responsible for the mobility degradation of n-FETs due to Coulomb scattering.