Bulk Germanium Research Articles

Evidence for spin-to-charge conversion by Rashba coupling in metallic states at the Fe/Ge(111) interface.

The spin–orbit coupling relating the electron spin and momentum allows for spin generation, detection and manipulation. It thus fulfils the three basic functions of the spin field-effect transistor. However, the spin Hall effect in bulk germanium is too weak to produce spin currents, whereas large Rashba effect at Ge(111) surfaces covered with heavy metals could generate spin-polarized currents. The Rashba spin splitting can actually be as large as hundreds of meV. Here we show a giant spin-to-charge conversion in metallic states at the Fe/Ge(111) interface due to the Rashba coupling. We generate very large charge currents by direct spin pumping into the interface states from 20 K to room temperature. The presence of these metallic states at the Fe/Ge(111) interface is demonstrated by first-principles electronic structure calculations. By this, we demonstrate how to take advantage of the spin–orbit coupling for the development of the spin field-effect transistor.

Uniform phosphorus doping of untapered germanium nanowires

One of the major challenges in the growth of vapor–liquid–solid (VLS) nanowires is the control of dopant incorporation in the structures. In this work, we study the n-type doping and morphology of nanowires grown by chemical vapor deposition when HCl is introduced. We obtain fully untapered nanowires with a growth temperature up to 410 °C and measure their resistivity using the 4-probe technique to be 2.0 mΩ cm. We perform energy dispersive x-ray measurements showing a concentration of dopants in the (5–7) × 1018 cm−3 range, being radially and axially uniform. The combination of these two measurements shows that the mobility is the same as for bulk germanium, demonstrating that the VLS mechanism has no detrimental effect for the electron transport in these nanowires.

Differential analysis of band-edge photoluminescence spectra of germanium single crystals with different orientations under biaxial tensile strains

The previously published photoluminescence spectra of bulk germanium single crystals with orientations (100), (110), and (111) under different biaxial tensile strains have been investigated using the differential method proposed by the author for the analysis of luminescence spectra of semiconductors. An increase in the strain for all these orientations of the single crystals leads to a shift in the maxima of the differential spectra in the region of direct radiative transitions toward lower photon energies due to the narrowing of the germanium direct band gap. At the same time, the positions of the maxima of the differential spectra in the region of indirect radiative transitions remain almost unchanged. This indicates that the germanium indirect band gap does not depend on the tensile strains, at least for their values of ∼0.2–0.3%.

High Performance Tri-Gate Germanium-on-insulator Based Junctionless Nanowire Transistors

With feature size of transistors reducing down to sub-20 nm nodes, high mobility channel materials and novel device structures are critical to further improve the performance of transistors. Junctionless nanowire transistor (JNT) is recently proposed and regarded as promising candidate structure due to its simple fabrication process and excellent electronic properties [1]. Germanium-on-insulator (GOI) substrate is regarded as promising substrate due to its advantages of both Ge and on-insulator substrates. Although GOI-based JNTs have been reported [2,3], the performances are far from satisfactory. In this work, we fabricated high performance GOI-based JNTs with much smaller dimensions, where channel length and width are both less than 100 nm, and demonstrated their fabrication process and electrical properties in detail. Fig.1 shows SEM images of tri-gate JNTs with different dimension, (a)W/L=40nm/70nm and (b) W/L=40nm/100nm, respectively. Fig.2 shows TEM image of a GOI sample with Tge=10 nm. The original thickness of Ge film was about 50 nm. We reduced Tge down to 10 nm using a simple wet retching method as reported in our previous work [4]. Fig.3 shows the schematic diagrams of fabrication process. To reduce costs and improve efficiency, we used both optical lithography (OL) and electron beam lithography (EBL). First, we used OL to define active region of the devices. GeO2 channel passivation was carried out by ozone oxidation at 300 °C for 30 min. Then EBL was carried out to define the channel and gate electrode, which had much smaller dimensions. Finally, we used OL and lift-off process to define measuring electrodes. Fig.4 (a) shows Id-Vg characteristics of JNTs with Tge =20nm and Tge=10nm, respectively. It can be observed that Ion/Ioff ratio of the JNT with Tge=10nm is larger than 105 at Vd= -1V. The subthreshold slope at Vd= -0.1V and the drain induced-barrier lowering (DIBL) are estimated to be 110 mV/dec and 140 mV/V, respectively. With decrease of Tge, we get better device performance. Fig.4 (b) shows the Id-Vd characteristics of JNT with W/L=40nm/70nm, Na=1018 cm-3 and Tge=10nm. Liner and saturation regions are clearly exhibited. Fig.5 (a) shows the influence of temperature on Id-Vg characteristics of JNTs with W/L=40nm/70nm, Na=1018cm-3 and Tge =10nm. It can be observed that with increase of temperature leakage current increased dramatically. Fig.5 (b) shows mobility of the same JNT. The reported mobility values of a GOI JNT with Na= 1019 cm−3 [2] and a conventional Si pMOSFET [5] are also plotted for comparison. The peak mobility is larger than 200 cm2V-1S-1, which is close to that of bulk germanium with a doping concentration of 1018 cm−3. In summary, we fabricated high performance GOI-based JNTs using a Si-compatible process which combines OL and EBL. Good Ion/Ioff ratio, subthreshold slope, DIBL and mobility are extracted, which indicating that ultra-thin body GOI-based JNTs are promising for high performance circuits. The device performance can be improved by optimizing the fabrication process and device structure. Acknowledgments: This work was supported in part by the National Natural Science Foundation of China (No. 61306105). [1] J. P. Colinge et.al, Nat. Nanotechnol. 5, 225, 2010. [2] D. D. Zhao et al. Jpn. J. Appl. Phys., Part 1, 51, 04DA03, 2012. [3] R. Yu et al., Phys. Status Solidi RRL 8, 65, 2014. [4] C. Sun et al. ECS Solid State Lett. 4, P43, 2015. [5] S. Takagi et al, IEEE Trans. Electron Devices, 41,2363, 1994. Figure 1

(Invited) The (R)Evolution of the Junctionless Transistor

Trends in the electronic industry require smaller and smaller components resulting in transistor sizes down to the nano-scale. This is starting to pose significant manufacturing problems. In classical scaled transistors one has to form two junctions, since source and drain regions are separated by channel area with opposite doping polarity. The diffusion of source and drain doping atoms is difficult to control in very short-channel transistors as shown in Figure 1. In all transistors, the diffusion of source and drain impurities into the channel region becomes a bottleneck to the fabrication of very short-channel devices, and very low thermal budget processing techniques must be used [1]. An electrical junction refers to a thermoelectricity junction, a metal-semiconductor junction or a p-n junction (p-type semiconductor to n-type semiconductor junction). Typically, Metal Oxide Semiconductor (MOS) transistors are made using two p-n junctions: the source junction and the drain junction. An n-channel transistor is an N-P-N structure. A p-channel transistor is a P-N-P structure. Very costly techniques are used to minimize this diffusion, but even in the absence of diffusion the statistical variation of the impurity concentration due to ion implantation or other doping techniques can cause device parameter variation problems. There arises therefore, the need to provide a transistor device structure that overcomes the above-mentioned problems. Ideally, it should be possible to completely deplete the semiconductor film of carriers, in which case the resistance of the device becomes quasi-infinite. In a multigate FET (MugFET) the gate electrode is wrapped around a silicon wire, called “finger” or “fin”, forming a wrapping gate structure with excellent control of the channel electrostatics. The excellent gate-to channel coupling allows one to fully deplete the channel region even if it is heavily doped. The junctionless transistor (JNT) is fabricated without source and drain formation process, as the doping type and concentration in the channel region is essentially equal to that in the source and drain, or at least to that in the source and drain extensions [2-9]. These devices are essentially junction-free as shown in Figure 1. JNT is basically a fully depleted accumulation-mode device, consisting of a heavily-doped SOI nanowire resistor with a MOS gate to control current flow. Doping concentration is constant and uniform throughout the device and typically ranges from 1018 and 1020 cm-3. The JNT device can be tuned to normal-off state when the gate work function is properly chosen and the highly doped channel can be fully depleted with no gate bias. As gate voltage is increased, the JNT enters into partially depletion state, and current conducts in the center of the nanowire when VD is supplied, and then at flatband voltage, the depletion region is completely gone. The accumulation starts at the nanowire surface when further raises the VD, which additionally offers an accumulation current, in spite of the bulk current. This talk will first focus on the revolution of the junctionless transistor, focusing on the initial reports and studies in 2010. Then the evolution of the junctionless transistor will be discussed since the early reports in 2010. The variety of studies will be reviewed, as junctionless transistors are being reported in many more materials other than Si, such as Ge and InGaAs [10-11], and very recently in MoS2 [12] and in other transition-metal-dichalcogenide semiconductors. References : [1} Jain SH, et al. Low resistance, low-leakage ultrashallow p+ junction formation using millisecond flash anneals. Electron Devices, IEEE Transactions on. 2005;52:1610-5. [2] Colinge J-P, et al. Nanowire transistors without junctions. Nat Nanotechnol. 2010;5:225-9. [3] Kranti A, et al. Junctionless nanowire transistor (JNT): Properties and design guidelines. Solid-State Device Research Conference (ESSDERC), 2010 Proceedings of the European: IEEE; 2010. p. 357-60.[4] Lee C-W, et al., Junctionless multigate field-effect transistor. Appl Phys Lett. 2009;94:053511--2. [5] Lee C-W, et al. Performance estimation of junctionless multigate transistors. Solid State Electron. 2010;54:97-103. [8] Colinge J-P, et al. Reduced electric field in junctionless transistors. Appl Phys Lett. 2010;96:073510--3. [9] Colinge J, et al. Junctionless nanowire transistor (JNT): Properties and design guidelines. Solid State Electron. 2011;65:33-7. [10] Yu, R., et al., “Impact ionization induced dynamic floating body effect in junctionless transistors”, (2013) Solid State Electronics, 90, pp. 28-33. [11] Yu, R., et al.. “Device design and estimated performance for p-type junctionless transistors on bulk germanium substrates” (2012) IEEE Transactions on Electron Devices, 59 (9), art. no. 6226451, pp. 2308-2313 [12] Das, S., et al.., “Nb-doped single crystalline MoS2 field effect transistor”, Appl. Phys. Lett. 106, 173506 (2015). Figure 1

Effects of metal/Ge contact and surface passivation on direct band gap light emission and detection for asymmetric metal/Ge/metal diodes

Direct band gap electroluminescence (EL) and light detection were studied at room temperature for n-type bulk germanium (Ge) by using fin-type asymmetric lateral metal/Ge/metal diodes. HfGe/Ge and PtGe/Ge contacts were used for injecting holes. Electron cyclotron resonance plasma oxidation and physical vapor deposition bilayer passivation (BLP) methods were employed for passivating the surface of the active region. A high EL intensity and a low dark current intensity were observed for the sample with PtGe/Ge contact and BLP, owing to the small/large barrier height of holes/electrons for PtGe/Ge contact, respectively, and the low density of interface states for the active region with BLP. The local-heating-induced redshift of the EL peak for the sample with PtGe/Ge contact is smaller than that for the sample with HfGe/Ge contact, owing to the lower parasitic resistance of PtGe/Ge contact. The diode with PtGe/Ge contact and BLP shows an on/off ratio of ∼104 and a responsivity of 0.70 A/W, corresponding to an external quantum efficiency of 56.0% under a wavelength of 1.55 µm.

Direct Band Gap Germanium Microdisks Obtained with Silicon Nitride Stressor Layers

Germanium is an ideal candidate to achieve a monolithically integrated laser source on silicon. Unfortunately bulk germanium is an indirect band gap semiconductor. Here, we demonstrate that a thick germanium layer can be transformed from an indirect into a direct band gap semiconductor by using silicon nitride stressor layers. We achieve 1.75% (1.67%) biaxial tensile strain in 6 (9) μm diameter microdisks as measured from photoluminescence. The modeling of the photoluminescence amplitude vs temperature indicates that the zone-center Γ valley has the same energy as the L valley for a 9 μm diameter strained microdisk and is even less for the 6 μm diameter microdisk, thus demonstrating that a direct band gap is indeed obtained. We deduce that the crossover in germanium from indirect to direct gap occurs for a 1.67% ± 0.05% biaxial strain at room temperature, the value of this parameter varying between 1.55% and 2% in the literature.

The effects of uniaxial and biaxial strain on the electronic structure of germanium

The effects of uniaxial and biaxial strain on the electronic structure of bulk germanium are investigated using density functional theory in conjunction with four approximations for the exchange correlation interaction: the local density approximation (LDA) and generalized gradient approximation (GGA) with on-site Hubbard corrections (LDA+U, GGA+U), the meta-GGA (MGGA), and the screened hybrid functional (HSE06). The band structure and, especially, the band gap of unstrained Ge are well reproduced by these methods. The results of LDA+U/GGA+U and MGGA show that a biaxial tensile strain above 1.5% turns Ge into a direct-gap (Γ–Γ) semiconductor, whereas the indirect Γ–L gap is maintained for uniaxial strain up to 3%. The HSE06 results confirm a similar trend, although the predicted critical strain is lower. The effective masses were also calculated and they were found to be in good agreement with experiments for bulk Ge. It is predicted that the masses at Γ can be tuned to be smaller/larger by tensile/compressive strain in all directions.

Direct band gap light emission and detection at room temperature in bulk germanium diodes with HfGe/Ge/TiN structure

Direct band gap (DBG) electroluminescence (EL) and photo detection were studied at room temperature for n-type bulk germanium (Ge) diodes with a fin type lateral HfGe/Ge/TiN structure. DBG EL spectra peaked at 1.55μm were clearly observed due to small hole and electron barrier heights of HfGe/Ge and TiN/Ge contacts. DBG EL peak intensity increased with increasing doping level of Ge substrate due to increased electron population in direct conduction band. The integrated intensity of DBG EL spectrum is proportional to the area of active region, implying a good surface-uniformity of EL efficiency. Small dark current intensity was measured as 2.4×10−7A under a reverse bias voltage of −1V, corresponding to dark current densities of 5.3×10−10A/μm or 3.2×10−10A/μm2. At the wavelength of 1.55μm, a linear dependence of photo current intensity on laser power was observed with a responsivity of 0.44A/W at a reverse bias voltage of −1V.

Optical orientation of electron spins and valence-band spectroscopy in germanium

We have investigated optical orientation in the vicinity of the direct gap of bulk germanium. The electron spin polarization is studied via polarization-resolved photoluminescence excitation spectroscopy unfolding the interplay between doping and ultrafast electron transfer from the center of the Brillouin zone towards its edge. As a result, the direct-gap photoluminescence circular polarization can vary from 30% to $\ensuremath{-}60%$ when the excitation laser energy increases. This study provides also simultaneous access to the resonant electronic Raman scattering due to inter-valence-band excitations of spin-polarized holes, yielding a fast spectroscopic approach for the determination of the energy spectrum of holes in semiconducting materials.

Direct band gap electroluminescence from bulk germanium at room temperature using an asymmetric fin type metal/germanium/metal structure

We demonstrated direct band gap (DBG) electroluminescence (EL) at room temperature from n-type bulk germanium (Ge) using a fin type asymmetric lateral metal/Ge/metal structure with TiN/Ge and HfGe/Ge contacts, which was fabricated using a low temperature (<400 °C) process. Small electron and hole barrier heights were obtained for TiN/Ge and HfGe/Ge contacts, respectively. DBG EL spectrum peaked at 1.55 μm was clearly observed even at a small current density of 2.2 μA/μm. Superlinear increase in EL intensity was also observed with increasing current density, due to superlinear increase in population of elections in direct conduction band. The efficiency of hole injection was also clarified.

Carrier multiplication in germanium nanocrystals

Carrier multiplication is demonstrated in a solid-state dispersion of germanium nanocrystals in a silicon–dioxide matrix. This is performed by comparing ultrafast photo-induced absorption transients at different pump photon energies below and above the threshold energy for this process. The average germanium nanocrystal size is approximately 5–6 nm, as inferred from photoluminescence and Raman spectra. A carrier multiplication efficiency of approximately 190% is measured for photo-excitation at 2.8 times the optical bandgap of germanium nanocrystals, deduced from their photoluminescence spectra. Germanium nanocrystals can efficiently generate more than one electron–hole pair following absorption of a single photon of light. That is the finding of a study by scientists from the Netherlands and Portugal. Carrier multiplication — the ability of a semiconductor to create several electron–hole pairs from the absorption of a single photon whose energy exceeds the material’s bandgap — is potentially important for improving the efficiencies of devices like solar cells and photodetectors. In the present study, 5–6 nm diameter germanium nanocrystals inside a silicon-dioxide matrix offer carrier multiplication with an efficiency of 190% for a photon energy of 3.5 eV, which implies that 1.9 electron electron–hole pairs are created for each absorbed photon with 2.8 times the bandgap energy. Importantly, this efficiency is significantly better than that of bulk germanium.

Silicon and germanium nanocrystals: properties and characterization.

Group-IV nanocrystals have emerged as a promising group of materials that extends the realm of application of bulk diamond, silicon, germanium and related materials beyond their traditional boundaries. Over the last two decades of research, their potential for application in areas such as optoelectronic applications and memory devices has been progressively unraveled. Nevertheless, new challenges with no parallel in the respective bulk material counterparts have arisen. In this review, we consider what has been achieved and what are the current limitations with regard to growth, characterization and modeling of silicon and germanium nanocrystals and related materials.

Optical properties of germanium nanoparticles synthesized by pulsed laser ablation in acetone

Germanium (Ge) nanoparticles (NPs) are synthesized by means of pulsed laser ablation of bulk germanium target immersed in acetone with ns laser pulses at different pulse energies. The fabricated NPs are characterized by employing different techniques such as UV-visible absorption spectroscopy, photoluminescence, micro-Raman spectroscopy, transmission electron microscopy (TEM) and field emission scanning electron microscopy (FESEM). The mean size of the Ge NPs is found to vary from few nm to 40 nm with the increase in laser pulse energy. Shift in the position of the absorption spectra is observed and also the photoluminescence peak shift is observed due to quantum confinement effects. High resolution TEM combined with micro-Raman spectroscopy confirms the crystalline nature of the generated germanium nanoparticles. The formation of various sizes of germanium NPs at different laser pulse energies is evident from the asymmetry in the Raman spectra and the shift in its peak position towards the lower wavenumber side. The FESEM micrographs confirm the formation of germanium micro/nanostructures at the laser ablated position of the bulk germanium. In particular, the measured NP sizes from the micro-Raman phonon quantum confinement model are found in good agreement with TEM measurements of Ge NPs.

Spontaneous breaking of time-reversal symmetry in strongly interacting two-dimensional electron layers in silicon and germanium.

We report experimental evidence of a remarkable spontaneous time-reversal symmetry breaking in two-dimensional electron systems formed by atomically confined doping of phosphorus (P) atoms inside bulk crystalline silicon (Si) and germanium (Ge). Weak localization corrections to the conductivity and the universal conductance fluctuations were both found to decrease rapidly with decreasing doping in the Si:P and Ge:P delta layers, suggesting an effect driven by Coulomb interactions. In-plane magnetotransport measurements indicate the presence of intrinsic local spin fluctuations at low doping, providing a microscopic mechanism for spontaneous lifting of the time-reversal symmetry. Our experiments suggest the emergence of a new many-body quantum state when two-dimensional electrons are confined to narrow half-filled impurity bands.

Resonant spin amplification in intrinsic bulk germanium: Evidence for electron spin lifetimes exceeding 50 ns

Recent experiments have revealed the possibility of optically orienting electron spins in bulk germanium via indirect optical transitions. However, the temporal limitations to both the spin lifetime and the coherence of photogenerated electrons have remained unexplored so far. Here we demonstrate resonant spin amplification in intrinsic bulk germanium using a 90 MHz femtosecond pulse train at 0.8 eV central photon energy. Most importantly, we find remarkably long spin lifetimes exceeding 50 ns at temperatures of up to 60 K, limited by Elliott-Yafet type processes. Consistent with model simulations we also find pronounced signatures of the $g$-factor anisotropy in germanium in the resonant spin amplification data.

Tailoring nanostructures in micrometer size germanium particles to improve their performance as an anode for lithium ion batteries

A facile and scalable single-step approach is employed to synthesize a bulk germanium electrode, which consists of nanoscale Ge-grains in ∼5 μm porous powders. This three-dimensional Ge electrode exhibits superior specific capacity (∼1500 mA h g(-1)) and cyclic performance, attributed to its unique lithiation/delithiation processes.

Silicon-Based Thermoelectrics Made from a Boron-Doped Silicon Dioxide Nanocomposite

We report a method for preparing p-type silicon germanium bulk alloys directly from a boron-doped silica germania nanocomposite. This is the first successful attempt to produce and characterize the thermoelectric properties of SiGe-based thermoelectric materials prepared at temperatures below the alloy’s melting point through a magnesiothermic reduction of the silica-germania nanocomposite. We observe a thermoelectric power factor that is competitive with the literature record obtained for high energy ball milled nanocomposites. The large grain size in our hot pressed samples limits the thermoelectric figure of merit to 0.5 at 800 °C for an optimally doped Si80Ge20 alloy.

Analysis of Threshold Current Behavior for Bulk and Quantum-Well Germanium Laser Structures

We analyze the optical gain of tensile-strained, n-germanium (n-Ge) material taking bandgap narrowing (BGN) for heavily doped Ge into account. Both the direct bandgap and indirect bandgap are narrowed by 60 meV. Our new modeling explains the wide lasing spectrum of 1520-1700 nm in electrically pumped Ge lasers. The calculated materials gain can reach 1000 cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-1</sup> when the injected carrier density is ~mid-10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">19</sup> cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-3</sup> with a combination of 0.25% tensile strain and 4.5×10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">19</sup> cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-3</sup> n-type doping. The threshold current density is estimated to be 0.53 kA/cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> for an optimized edge-emitting double-heterojunction Ge device, comparable to bulk III-V lasers. We also review current progress on Ge quantum-well (QW) structures. The threshold current density of most Ge QW structures is similar to bulk Ge. Only tensile-strained QWs show reduced threshold currents.

Analysis of Lasing From Direct Transition in Ge-on-Si

This paper describes a theoretical investigation of lasing from the direct, Γ-point transition in bulk germanium grown on silicon substrate. Relationships between desired gain and required current density are computed using a screened Hartree- Fock gain model for structures of different tensile strains and n-doping densities. The calculations indicate that for unstrained Ge, high free-carrier absorption and gain saturation may lead to no positive net gain regardless of excitation. With 0.2% tensile strain, the theory predicts possible lasing under laboratory conditions, but uncertainties in free-carrier absorption and Auger losses make difficult definitive predictions for a practical device. Results for 0.6% tensile strain and > 1019 cm-3 n-doping gives a more definitive prediction of constant wave lasing with threshold current density of ~1 kA/cm2.