Epitaxial Ge Films Research Articles

Strain Field Mapping of Dislocations in a Ge/Si Heterostructure

Ge/Si heterostructure with fully strain-relaxed Ge film was grown on a Si (001) substrate by using a two-step process by ultra-high vacuum chemical vapor deposition. The dislocations in the Ge/Si heterostructure were experimentally investigated by high-resolution transmission electron microscopy (HRTEM). The dislocations at the Ge/Si interface were identified to be 90° full-edge dislocations, which are the most efficient way for obtaining a fully relaxed Ge film. The only defect found in the Ge epitaxial film was a 60° dislocation. The nanoscale strain field of the dislocations was mapped by geometric phase analysis technique from the HRTEM image. The strain field around the edge component of the 60° dislocation core was compared with those of the Peierls–Nabarro and Foreman dislocation models. Comparison results show that the Foreman model with a = 1.5 can describe appropriately the strain field around the edge component of a 60° dislocation core in a relaxed Ge film on a Si substrate.

Heteroepitaxy of Ge on Si(001) with pits and windows transferred from free-standing porous alumina mask

This paper reports the use of ultrathin free-standing porous alumina membrane (PAM) in pattern transferring for selective epitaxial growth (SEG) of Ge dots and films on Si. PAM, as a large-scale, controllable and lithography-free mask, can transfer nanopatterns onto Si without introducing any contaminants. High-density Ge dots are achievable with Ge adatoms confined in Si pits transferred from PAM. High-quality Ge films can also be grown on Si substrates through SiO2 nano-windows. In this work, 80 and 60 nm pore sizes of PAM were transferred to 70, 50 and 20 nm windows for comparison. For the former two sizes, over-etching of Si beneath every SiO2 window forms epi-seeds to improve intermixing of Ge–Si. No threading dislocations can be observed emanating from the epi-seeds due to the decreased lattice mismatch. An innovative shadow-etching technique utilizing the aspect ratio of PAM further decreased the lateral dimension of patterns from 60 to 20 nm. Cross-sectional transmission electron microscopy images show that the selective epitaxial Ge films grown from a 20 nm-width interface are defect free, which is attributed to the exponential decay of strain energy as well as Ge–Si intermixing.

Comparative Studies of the Growth and Characterization of Germanium Epitaxial Film on Silicon (001) with 0° and 6° Offcut

The quality of germanium (Ge) epitaxial films grown directly on silicon (Si) (001) with 0° and 6° offcut orientation using a reduced-pressure chemical vapor deposition system is studied and compared. Ge film grown on Si (001) with 6° offcut presents ∼65% higher threading dislocation density and higher root-mean-square (RMS) surface roughness (1.92 nm versus 0.98 nm) than Ge film grown on Si (001) with 0° offcut. Plan-view transmission electron microscopy also reveals that threading dislocations are more severe (in terms of contrast and density) for the 6° offcut. In addition, both high-resolution x-ray diffraction and Raman spectroscopy analyses show that the Ge epilayer on 6° offcut wafer presents higher tensile strain. The poorer quality of the Ge film on Si (001) with 6° offcut is a result of an imbalance in Burgers vectors that favors dislocation nucleation over annihilation.

Ge epitaxial films on GaAs (100), (110), and (111) substrates for applications of CMOS heterostructural integrations

Epitaxial Ge films were grown on GaAs (100), (110), and (111) substrates by using ultra-high vacuum chemical vapor deposition and studied with various methods. The incubation times and growth rates were quite different for these three GaAs substrates because the surface arsenic coverage on GaAs and hydrogen desorption energy on Ge are different for each orientation. High-resolution x-ray diffraction measurements, direct band-gap emission of photoluminescence measurements, and cross-sectional transmission electron microscopy showed that the Ge films had high crystal quality, low defect density, and sharp Ge/GaAs interfaces. In this study, atomic force microscopy analysis found that the Ge films grow on GaAs (100) and (111) via the Frank van der Merwe mode, while the Ge film grows on GaAs (110) via the Volmer-Weber mode at the initial growth stage, which can be explained by the thermodynamic theory of capillarity. Interestingly, when the thickness of the Ge film on the GaAs (110) substrate increases to ∼220 nm, the 3D Ge islands merge and form a smooth surface (rms roughness of 0.3 nm), which is useful for devices. The authors also fabricated Ge metal-oxide-semiconductor capacitors (MOSCAPs) on GaAs (100) and (110) substrates. Both Ge/GaAs (100) and Ge/GaAs (110) MOSCAPs exhibit good capacitance–voltage responses with strong inversion behaviors, which means the grown material has reached device quality. The Ge/GaAs (110) structure especially offers optimal integration of Ge pMOSFETs on GaAs substrates because Ge (110) has a high hole mobility compared with Ge (100) and (111).

$\hbox{Al}_{2}\hbox{O}_{3}$ Interface Engineering of Germanium Epitaxial Layer Grown Directly on Silicon

The quality of germanium (Ge) epitaxial film grown directly on silicon (Si) substrate is investigated based on the electrical properties of a metal-oxide-semiconductor capacitor (MOSCAP). Different thermal cycling temperatures are used in this study to investigate the effect of temperature on the Ge film quality. Prior to high-k dielectric deposition, various surface treatments are applied on the Ge film to determine the leakage current density using scanning tunneling microscopy. The interface trap density (Dit) and leakage current obtained from the C-V and I-V measurements on the MOSCAP, as well as the threading dislocation density (TDD), show a linear relationship with the thermal cycling temperature. It is found that the Ge epitaxial film that undergoes the highest thermal cycling temperature of 825°C and surface treatment in ultraviolet ozone, followed by germanium oxynitride (GeOxNy) formation, demonstrates the lowest leakage current of ~ 2.3×10-8 A/cm2 (at -2 V), Dit ~ 3.5 × 1011 cm-2/V, and TDD <; 107 cm-2.

Atomically Controlled Epitaxial Growth of Single-Crystalline Germanium Films on a Metallic Silicide

We demonstrate high-quality epitaxial germanium (Ge) films on a metallic silicide, Fe3Si, grown directly on a Ge(111) substrate. Using molecular beam epitaxy techniques, we can obtain an artificially controlled arrangement of silicon (Si) or iron (Fe) atoms at the surface on Fe3Si(111). The Si-terminated Fe3Si(111) surface enables us to grow two-dimensional epitaxial Ge films, whereas the Fe-terminated one causes the three-dimensional epitaxial growth of Ge films. The high-quality Ge grown on the Si-terminated surface has almost no strain, meaning that the Ge films are not grown on the low-temperature-grown Si buffer layer but on the lattice matched metallic Fe3Si. This study will open a new way for vertical-type Ge-channel transistors with metallic source/drain contacts.

Dependences of photoluminescence from P-implanted epitaxial Ge

A systematic investigation has been carried out to study the influence of various annealings and implantations on the photoluminescence (PL) properties of phosphorus (P)-implanted Ge epitaxial films on Si substrate. For un-capped Ge samples, rapid thermal annealing (RTA) at 700 °C for 300 seconds yields the strongest PL emission peaked at 1550 nm. The influence of employing various capping layers (i.e., SiO(2), Si(3)N(4), and α-Si ) on the PL properties has been investigated. The capping layers are found to effectively decrease the dopant loss, leading to a significant PL enhancement. Si(3)N(4) is found to be the most efficient capping layer to prevent dopant out-diffusion and thus lead to strongest PL. Furthermore, it has been found that capping layers not only enhance the PL intensities but also make PL emission peak red- and blue- shift, depending on the stress type of the capping films. The effect of implantation dose on the PL has been also investigated.

Direct Bonding of Ge-Ge Using Epitaxially Grown Ge-on-Si Wafers

High quality germanium (Ge) epitaxial film grown directly on silicon (001) substrate is used to study the characteristics of direct Ge-Ge bonding. The Ge epilayer on Si(100) is found to be hydrophilic in nature and storage test for seven days in ambient has no affect on its surface contact angle. Analysis shows that the Ge-Ge bond strength (mJ/m2) is directly dependent on the annealing temperature (°C). At room ambient, the bond strength of the bonded wafer pair is ∼55 mJ/m2 and increases to ∼2460 mJ/m2 subjected to annealing at 300°C in N2 ambient for 3 hours. Also, the transmission electron microscopy (TEM) cross-sectional view at the bonding interface indicates that the bond is seamless and no micro-void is observed.

Investigation of Growth Kinetics, Inter-Diffusion and Quality of Ge-on-Si Epitaxial Film by a Three-Step Growth Approach

The growth kinetics and inter-diffusion of germanium (Ge) epitaxial film on (100) silicon (Si) are investigated. The first part of this work consists of the study on the Ge growth kinetics in the temperature range from 325 to 600°C. Next, secondary ion mass spectrometry (SIMS) is used to characterize the inter-diffusion property of Ge film and Si substrate as a result of post-growth thermal cycling with a higher bound annealing temperature ranging from 725 to 825°C. The experimental raw data are then fitted with theoretical model to obtain the diffusion constant of Ge diffusion in Si and vice versa. This is completed by determining both the activation energy, Ea, and the diffusion pre-factor, D0, based on error function.

Correlation and Morphology of Dopant Decomposition in Mn and Co Codoped Ge Epitaxial Films

Atom probe tomography (APT) was used to quantify inhomogeneities in the distribution of Mn and Co in doped epitaxial Ge thin films for which X-ray diffraction (XRD) studies indicate single phase material. The segregation of dopants into Co and Mn-rich regions with characteristic sizes was evident upon visual inspection of the APT reconstruction and a frequency distribution analysis of the concentration of Co, Mn, and Ge atoms verified that the composition fluctuations exceeded those of a random alloy. Isoconcentration surfaces were generated to establish the connectedness of regions enriched in Mn that have been proposed to enhance the Curie temperature in dilute magnetic semiconductors. The analysis demonstrates important contributions that APT can make to the understanding of magnetism in these materials.

Growth and characterization of germanium epitaxial film on silicon (001) using reduced pressure chemical vapor deposition

High quality germanium (Ge) epitaxial film is grown directly on silicon (001) substrate using a “three-step growth” approach in a reduced pressure chemical vapor deposition system. The growth steps consist of sequential low temperature (LT) at 400°C, intermediate temperature ramp (LT-HT) of ~6.5°C/min and high temperature (HT) at 600°C. This is followed by post-growth anneal in hydrogen at temperature ranging from 680 to 825°C. Analytical characterizations have shown that the Ge epitaxial film of thickness ~1μm experiences thermally induced tensile strain of 0.20% with a threading dislocation density of <107cm−2 under optical microscope and root mean square roughness of ~0.9nm. Further analysis has shown that the annealing time at high temperature has an impact on the surface morphology of the Ge epitaxial film. Further reduction in the RMS roughness can be achieved either through chemical mechanical polishing or to insert an annealing step between the LT–HT ramp and HT steps.

Epitaxial films of GeSi, AlGaN, and GaSb and GaSb/InAs superlattices on substrates of fianite

Fianite is a single crystal of cubic solid solutions based on zirconium dioxide (ZrO2) or hafnium dioxide (HfO2) with stabilizing oxides of yttrium, scandium, and lanthanides. It is characterized by a unique combination of physical and chemical properties, making it a promising material for wide use in electronics. In this work, we consider new uses of fianite as a monolith substrate for obtaining Ge, GeSi, AlGaN, and GaSb epitaxial films and GaSb/InAs superlattices.

Ferromagnetism in Mn-implanted epitaxially grown Ge on Si(100)

We have studied ferromagnetism of Mn-implanted epitaxial Ge films on silicon. The Ge films were grown by ultrahigh vacuum chemical vapor deposition using a mixture of germane (GeH${}_{4}$) and methylgermane (CH${}_{3}$GeH${}_{3}$) gases with a carbon concentration of less than 1 at. %, and observed surface rms roughness of \ensuremath{\sim}0.5 nm, as measured by atomic force microscopy. Manganese ions were implanted in epitaxial Ge films grown on Si (100) wafers to an effective concentration of \ensuremath{\sim}16, 12, 6, and 2 at. %. Superconducting quantum interference device measurements showed that only the three highest Mn concentration samples are ferromagnetic, while the fourth sample, with [Mn] $=$ 2 at. %, is paramagnetic. X-ray absorption spectroscopy and x-ray magnetic circular dichroism measurements indicate that localized Mn moments are ferromagnetically coupled below the Curie temperature. Isothermal annealing of Mn-implanted Ge films with [Mn] $=$ 16 at. % at 300 \ifmmode^\circ\else\textdegree\fi{}C for up to 1200 s decreases the magnetization but does not change the Curie temperature, suggesting that the amount of the magnetic phase slowly decreases with time at this anneal temperature. Furthermore, transmission electron microscopy and synchrotron grazing incidence x-ray diffraction experiments show that the Mn-implanted region is amorphous, and we believe that it is this phase that is responsible for the ferromagnetism. This is supported by our observation that high-temperature annealing leads to recrystallization and transformation of the material into a paramagnetic phase.

One-Step Ge/Si Epitaxial Growth

Fabricating a low-cost virtual germanium (Ge) template by epitaxial growth of Ge films on silicon wafer with a Ge(x)Si(1-x) (0 < x < 1) graded buffer layer was demonstrated through a facile chemical vapor deposition method in one step by decomposing a hazardousless GeO(2) powder under hydrogen atmosphere without ultra-high vacuum condition and then depositing in a low-temperature region. X-ray diffraction analysis shows that the Ge film with an epitaxial relationship is along the in-plane direction of Si. The successful growth of epitaxial Ge films on Si substrate demonstrates the feasibility of integrating various functional devices on the Ge/Si substrates.

High quality Ge thin film grown by ultrahigh vacuum chemical vapor deposition on GaAs substrate

High-quality epitaxial Ge films were grown on GaAs substrates by ultrahigh vacuum chemical vapor deposition. High crystallinity and smooth surface were observed for these films by x-ray diffraction, transmission electron microscopy, and atomic force microscopy. Direct band gap emission (1550 nm) from this structure was detected by photoluminescence. Valence band offset of 0.16 eV at the Ge/GaAs interface was measured by x-ray photoelectron spectroscopy. N-type arsenic self-doping of 1018/cm−3 in the grown Ge layers was determined using electrochemical capacitance voltage measurement. This structure can be used to fabricate p-channel metal-oxide-semiconductor field-effect transistor for the integration of Ge p-channel device with GaAs n-channel electronic device.

Normal incidence p–i–n Ge heterojunction photodiodes on Si substrate grown by ultrahigh vacuum chemical vapor deposition

We report on normal incidence p–i–n heterojunction photodiodes operating in the near-infrared region and realized in pure germanium on planar silicon substrate. The diodes were fabricated by ultrahigh vacuum chemical vapor deposition at 600 °C without thermal annealing and allowing the integration with standard silicon processes. Due to the 0.14% residual tensile strain generated by the thermal expansion mismatch between Ge and Si, an efficiency enhancement of nearly 3-fold at 1.55 μm and the absorption edge shifting to longer wavelength of about 40 nm are achieved in the epitaxial Ge films. The diode with a responsivity of 0.23 A/W at 1.55 μm wavelength and a bulk dark current density of 10 mA/cm 2 is demonstrated. These diodes with high performances and full compatibility with the CMOS processes enable monolithically integrating microphotonics and microelectronics on the same chip.

Study of the transition of the epitaxial Ge film from layer-to-layer to three-dimensional growth in heterostructures with strained SiGe sublayers

The results of the study of features of the Ge film transition from two- to three-dimensional growth in different SiGe/Si(001) heterostructures with buried stressed layers are presented. It is shown that deposition of a stressed planar SiGe layer results in a significant decrease in the critical thickness of two-dimensional Ge growth. It was detected that the effect of SiGe layers on Ge film growth is very significant not only in the case of growth directly on the strained SiGe layer, but also if this layer is capped by an unstrained Si layer to thicknesses of ∼3.5 nm. It is shown that the model in which the effect of buried strained SiGe layers is taken into account by introducing the phenomenological parameter “decay length of the effective elastic energy” allows description of experimental result data with good accuracy.

Effect of isochronal hydrogen annealing on surface roughness and threading dislocation density of epitaxial Ge films grown on Si

We report the effect of hydrogen annealing on the surface roughness and threading dislocation density (TDD) of germanium (Ge) films grown on silicon (Si) substrates by reduced-pressure chemical vapor deposition (RPCVD). The surface roughness initially decreased with an increase in the annealing temperature. At annealing temperatures greater than 650 °C the film thickness varied owing to surface undulations, leading to an increase in the surface roughness. Although high-temperature annealing at 850 °C is effective for reducing TDD, the surface roughness of a 150-nm-thick Ge film annealed at 650 °C reaches a minimum value (~ 0.7 nm).

Germanium films with strong in-plane and out-of-plane texture on flexible, randomly textured metal substrates

High efficiencies have been achieved in photovoltaic cells based on III–V compounds grown on single crystal germanium substrates. Since the size of these substrates is limited and their cost is very high, such III–V photovoltaics have not found widespread terrestrial use. The objective of this work is to develop highly textured, epitaxial germanium thin films on inexpensive substrates suitable for roll-to-roll continuous processing to serve as templates for III–V compounds. Germanium films with a high degree of in-plane and out-of plane texture have been demonstrated on randomly textured, flexible nickel alloy substrates by epitaxial growth on template films made by ion beam-assisted deposition (IBAD). In order to achieve epitaxial growth, an intermediate layer of CeO 2 was found to be required between the IBAD MgO template and the Ge film. Our study shows that structural match between Ge and the underlying oxide layer is the key to epitaxial growth. Room temperature optical bandgap of the Ge films was identified at 0.67 eV suggesting minimal residual strain in the film. Refraction index and extinction coefficient values of the epitaxial Ge film were found to match well with that measured from a reference Ge single crystal.

A model of threading dislocation density in strain-relaxed Ge and GaAs epitaxial films on Si (100)

Strain relaxation in large lattice-mismatched epitaxial films, such as Ge and III-V materials on Si, introduces high threading dislocation densities (TDDs). A thermodynamic model of TDD dependence on film thickness is developed. According to this model, the quasiequilibrium TDD of a given strain-relaxed film scales down with the inverse square of its thickness. The quasiequilibrium TDDs in both Ge and GaAs films follow this model consistently. Our model predicts the lowest possible TDD of a large lattice-mismatched film on Si (100), which is determined by the dislocation glide activation energy and the film thickness.