Germanium Epitaxial Layers Research Articles

Optimal Regime for Growth of Epitaxial Germanium Layers from the Liquid Phase Based on Thermodynamic Calculations

Thermodynamic calculations were performed to determine the optimal conditions for the growth of germanium epitaxial layers from a Ge-Sn solution (system) to a germanium substrate. The determination of the optimal conditions was based on the change in the Gibbs energy values of the system during the crystallization process and the size of the crystal-forming nanoclusters. Based on the results obtained, we determined the optimal conditions for obtaining low-dislocation, crystalline perfect germanium epitaxial layers from a liquid tin solution, and recommended starting the crystallization process at 923 K and finishing at 800 K. When the temperature drops below 800 K, the formation of Ge1-xSnx epitaxial layers from the Ge-Sn solution was observed.

Impact of source-doping gradient in terms of lateral straggle on the performance of germanium epitaxial layer double-gate TFET

The impact of source doping gradient (SDG) in terms of lateral straggle (σ) on the performance of germanium epitaxial layer double-gate tunnel field effect transistor (ETL-DGTFET) was demonstrated by a simulation study. The non-zero tilt angle used in the ion implantation during the device fabrication process extends the implanted dopant atoms to the channel region from the source and drain which affects the performance of a device during both the ON- and OFF-states. To get a deeper insight of the impact of σ on the device performance, various DC performance parameters like Ion/Ioff ratio, average Subthreshold Slope (SSavg), and analog/RF figure of merits (FOMs) like transconductance (gm), output conductance (gds), intrinsic gain (gm/gds), parasitic capacitances and cutoff frequency (fT) were explored for σ variation from 0 to 5 nm. Circuit-level performance was also studied using an n-type ETL-DGTFET based resistive load inverter. Though the device performance is effected by the strain due to lattice mismatch between Si/Ge, it does not influence the effect of lateral straggle.

Theoretical Calculation and Experimental Verification for Dislocation Reduction in Germanium Epitaxial Layers with Semicylindrical Voids on Silicon.

Reduction of threading dislocation density (TDD) in epitaxial germanium (Ge) on silicon (Si) has been one of the most important challenges for the realization of monolithically integrated photonics circuits. The present paper describes methods of theoretical calculation and experimental verification of a novel model for the reduction of TDD. The method of theoretical calculation describes the bending of threading dislocations (TDs) based on the interaction of TDs and non-planar growth surfaces of selective epitaxial growth (SEG) in terms of dislocation image force. The calculation reveals that the presence of voids on SiO2 masks help to reduce TDD. Experimental verification is described by germanium (Ge) SEG, using an ultra-high vacuum chemical vapor deposition method and TD observations of the grown Ge via etching and cross-sectional transmission electron microscope (TEM). It is strongly suggested that the TDD reduction would be due to the presence of semicylindrical voids over the SiO2 SEG masks and growth temperature. For experimental verification, epitaxial Ge layers with semicylindrical voids are formed as the result of SEG of Ge layers and their coalescence. The experimentally obtained TDDs reproduce the calculated TDDs based on the theoretical model. Cross-sectional TEM observations reveal that both the termination and generation of TDs occur at semicylindrical voids. Plan-view TEM observations reveal a unique behavior of TDs in Ge with semicylindrical voids (i.e., TDs are bent to be parallel to the SEG masks and the Si substrate).

Gallium-doped germanium epitaxial layers grown on silicon substrates by hot wire chemical vapor deposition

We report on growing the p-Ge:Ga/Si(0 0 1) epitaxial layers by hot wire chemical vapor deposition using a solid Ge:Ga sublimation source of Ga. The Ge:Ga layers grown using this doping method were featured by a high crystal quality and full electrical activation of the Ga impurity. The results of the present study demonstrate the prospects of application of the sublimation Ga source for growing the Ge:Ga layers in the Ge/Si(0 0 1) device structures for Si-based electronics and photonics.

(Invited) On the Manipulation of Phosphorus Diffusion as Well as the Reduction of Specific Contact Resistivity in Ge by Carbon Co-Doping

Thanks to its intrinsic high mobilities for carriers i.e. 3900 and 1900 cm2V-1s-1 for holes and electrons respectively, Ge has resurged as an alternative channel material for metal-oxide-semiconductor field-effect transistors (MOSFETs) [1]. To realize the-state-of-the-art high-performance Ge devices, the formation of highly activated shallow junctions especially n-type junction as well as the large contact specific contact resistivity (ρc), however, are still great challenges remained to be resolved, in spite of numerous efforts spent in the past decade [2, 3]. Impurity species such as carbon (C), nitrogen (N) and fluorine (F) are utilized to suppress the rapid diffusion of phosphorus in Ge by trapping vacancies [4, 5], while at the expense of a reduced activation level usually. Specifically, in this paper, the impact of C implantation parameters (energy and dose) on the diffusion and activation of P in Ge pre-amorphized (PAI) germanium epitaxial layers on Si (100), is systematically explored. All samples received a Ge PAI at 20 keV, 6×1014 at/cm2, followed by C implantation at different energies ranging from 3 keV to 20 keV and doses varying from 5×1014 to 2×101 5 at/cm2. P was implanted with a fixed condition using an energy of 5 keV and dose of 2×101 5 at/cm2 and was completely contained in the amorphous Ge layer. During annealing, to prevent out-diffusion of the implanted dopants, all samples were capped with a 20 nm thick SiO2 deposited by plasma enhanced chemical vapor deposition (PECVD) at 280 oC. The annealing process was carried out in N2 ambient at 600 oC for 60 s. It is illustrated in Fig. 1a that the shallowest N-type junction occurs for C implantation at 8 keV with a dose of 1×1015 cm-2. This optimum condition can be ascribed to the carbon distribution shown in Fig. 1b that carbon atoms straddles the amorphous/crystal (a/c) interface. As a result, both the diffusion of P in the amorphous layer and in the Ge virtual substrate beyond the a/c interface can be effectively suppressed. Except the control of P diffusion, the specific contact resistivity (ρc) between NiGe and n-Ge by the presence of C is also investigated. As illustrated in Fig. 2, the thermal stability of NiGe films is obviously enhanced by the presence of C and this can be inferred by the decreasing sheet resistance till 600 oC and the pretty smooth NiGe/Ge interface for samples with C. At 500 oC germanidation temperature, the rc values are reduced from 1.1×10-4 Ω-cm2 and 2.9×10-5 Ω-cm2 for NiGe/n- and p-Ge contacts without carbon to 7.3×10-5 Ω-cm2 and 1.4×10-5 Ω-cm2 for their counterparts with carbon, respectively, as illustrated in Fig. 3 and 4.

Damage accumulation during cryogenic and room temperature implantations in strained SiGe alloys

The amorphization by implantation of strained silicon–germanium epitaxial layers is investigated by experiments and simulation according to the germanium content in the film and the implantation conditions. Experimental results are used to calibrate a numerical model based on a combined approach between a Binary Collision Approximation (BCA) module and a kinetic Monte Carlo (kMC) module. The calibration is implemented in the Synopsys Sentaurus Process simulator and a close agreement between simulations and experiments is shown. Experimental results show that by increasing the germanium concentration, amorphous thickness is reduced for high energy implantations. Germanium content seems to have less impact at low energy implantations.

(Invited) Silicon Germanium FinFET Device Physics, Process Integration and Modeling Considerations

We introduce SiGe MOSFET process integration and device impact from a simulation perspective. Germanium is know to have a higher channel hole mobility. Enhancement of hole velocity due to lattice mismatch strain in shilicon germanium epitaxial layers is significant. Processing challenges include the elimination of interface trap states at the dielectric interface, fast diffusion of n-type dopants, defects in stress relaxed buffer and critical thickness limitations. Uniaxial stress is beneficial for device performance. Transformation of biaxial stress to uniaxial is natural when etching silicon germanium into stripes. From a device perspective, silicon germanium is useful for shifting MOSFET device Vth.

Effective surface modification of a germanium layer by using an atmospheric pressure plasma jet with a round ended nozzle

We investigate the surface modification of a germanium epitaxial layer grown on a silicon substrate by means of an atmospheric-pressure plasma jet. The plasma jet contains a large density of highly reactive radicals, as confirmed by optical emission spectra. The plasma jet with a nozzle with a round end has a higher current level compared to cylindrical and pencil-shaped nozzles, indicating the usefulness of this plasma jet for semiconductor processing. Raman spectra reveal a significant modification of the germanium surface under exposure to an atmospheric pressure plasma jet even for 1 min. The first-order Raman peak at 320 cm−1 has an asymmetric shoulder on the low-frequency side, which can be attributed to amorphization/oxidation of the germanium layer due to highly reactive radicals. The rapid surface change under short-time exposure indicates the practicality of the plasma jet for semiconductor processing.

Experimental and theoretical investigation of phosphorus in-situ doping of germanium epitaxial layers

We investigate phosphorus in-situ doping characteristics in germanium (Ge) during epitaxial growth by spreading resistance profiling analysis. In addition, we present an accurate model for the kinetics of the diffusion in the in-situ process, modeling combined growth and diffusion events. The activation energy and pre-exponential factor for phosphorus (P) diffusion are determined to be 1.91 eV and 3.75 × 10−5 cm2/s. These results show that P in-situ doping diffusivity is low enough to form shallow junctions for high performance Ge devices.

A study of highly crystalline novel beryllium oxide film using atomic layer deposition

Beryllium oxide (BeO), which has excellent electrical insulating characteristics and high thermal stability, is a promising gate dielectric and interface passivation layer (IPL), because of its high energy bandgap (10.6eV) and short bond distance between Be and O atoms. In a previous study, we demonstrated the excellent electrical and physical characteristics of BeO grown after atomic layer deposition (ALD) on Si and GaAs substrates. Here we report, for the first time, ALD growth of crystalline BeO as a potential high-k gate dielectric and IPL. From TEM, SAD, RHEED, and XRD, we have found that highly crystalline BeO thin film may be grown in a wurtzite structure as a (101) plane on a Si (100) oriented surface. We have also investigated a germanium epitaxial layer grown on BeO as a semiconductor-on-insulator (SOI) application, and the crystallinity of BeO on a GaAs (100) substrate for III–V MOS device applications.

PULSED LASER ANNEALING OF ULTRA-SHALLOW JUNCTIONS IN SILICON–GERMANIUM

The effect of laser energy fluence and substrate heating on the annealing of boron-implanted silicon–germanium epitaxial layers on silicon was investigated. By making use of the difference in the melting points of silicon–germanium and silicon, a process window in the laser energy fluence can be found such that the meltdepth was confined within the silicon–germanium. Pre-heating of the substrate to 300°C was done to reduce the laser fluence required and improve the surface morphology. Cross-sectional transmission electron microscopy showed that there were no end-of-range defects due to ion implantation at the silicon–germanium/silicon interface after the laser annealing.

Process control of Si/SiGe heterostructures by X-ray diffraction

The introduction of a silicon–germanium epitaxial layer in the base of a bipolar transistor brings about significant gains in speed. SiGe heterojunction bipolar transistors (HBTs) are now challenging GaAs in its traditional stronghold of wireless communications. But this new process step introduces new requirements for control of the Ge content and profile. X-ray diffraction (XRD) has been extensively used to characterize SiGe during it's research and development phase but, until now, it was considered too difficult and time consuming for use in a production environment. Recent advances in analytical software, however, allow fast and reliable extraction of layer thickness, germanium content and grading profile. XRD process control tools are currently being installed in a number of SiGe fab lines.

Epitaxial growth of Si–Ge layers on Si substrates by plasma dissociation of SiH4 and GeH4 mixture

Silicon–germanium epitaxial layers were formed on (111) oriented Si substrates by the plasma dissociation of a mixture of SiH4 and GeH4. A growth rate of 33 Å/sec was obtained at a substrate temperature of 750 °C for Si–Ge epitaxial growth using 1×10−2 Torr of SiH4 and 1×10−4 Torr of GeH4 mixture as source materials. The introduction of GeH4 to SiH4 provided a good nucleation for epitaxy. Less than 1% of GeH4 is sufficient to reduce the concentration of the crystalline defects such as voids which form in the epitaxial layers at low substrate temperature with a high growth rate. The electron Hall mobility of layers coincided with that of bulk Si in the temperature range between 77 °K and room temperature.

Mechanical stresses in the heterosystem germanium- gallium arsenide

Mechanical stresses in thin germanium epitaxial layers deposited by vacuum deposition on monocrystalline gallium arsenide substrates have been examined. The stresses arising are attributed to two factors: the pseudomorphic character of the growth mechanism and the gradient of structural defects in the transition layer. The experimental stress dependence has been shown to be in good agreement with the theory of the dynamic properties of dislocations.

93. Formation of germanium epitaxial layers in vacuum: V V Postnikov et al,Kristallografiya, 10 (4), 1965, 585, (in Russian)

93. Formation of germanium epitaxial layers in vacuum: V V Postnikov et al,Kristallografiya, 10 (4), 1965, 585, (in Russian)

Microwave measurement of resistivity

A mathematical description is given of the possible use of pulse modulation of millimetre e.m. waves for extending the range and flexibility of a previously described method for the simultaneous measurement of resistivity and thickness of germanium epitaxial layers.