Ge Buffer Layer Research Articles

Optical Pumped Gesn Waveguides Room Temperature Amplified Spontaneous Emission Observation

The successful epitaxial growth of Ge-on-Si has opened a new gateway for Si-based laser research. Ge is not a direct band gap material, however it possible to induce Ge towards direct band emission due to the small band gap energy differences between the direct and indirect. Recently lasing cavities have been successfully engineered by introducing tensile strain to the Ge lattice or through Sn alloying. The two methods have similar effects, lowering both direct and indirect band gaps of Ge with different rate. For a critical or higher Sn content, the direct band can become the lowest gap. This research is motivated by GeSn alloys with high potential to overcome many difficulties such as integration scale limits, excessive cost, extreme operating condition, etc in terms of engineering Si-based lasers. In this work, we explore the properties of optical emission from optically-pumped waveguides fabricated from GeSn alloys with Sn concentrations below the direct band gap transition. The fabrication process of the GeSn waveguides starts with growing undoped Ge buffer layers on Si (100) substrates using gas source molecular epitaxy with Ge4H10 and the wafers were annealed in situ at 650 °C for 3 min. After aqueous HF solution Ge virtual substrates cleaning, flows 5% Ge2H6 in H2 at 30 mTorr for 5 min for additional surface cleaning. Then the mixtures of Ge3H8, SnD4, and P(GeH3)3 (resulting in a concentration of 4.4% Sn in this research study) grows n-type doped GeSn films around 285-335 °C. On the GeSn surface, strip waveguides with width 3 μm and length 4 mm were BCl3 reactive ion etched by using standard UV contact photolithography. With plasma-enhanced chemical vapor deposition, a thickness of 180nm SiO2 was deposited for surface passivation after downright Ge buffer layer to Si interface etching process. For the experimental procedures, the waveguide with both end mirror polished, and one end is Al deposited to form the cavity. The other open end is aligned with a lens fiber as the collection prosses of the output. A 976 nm laser beam is shaped into a line and focused onto the top surface of the waveguide. The signal is collected by an optical fiber and sent through a grating spectrometer with an extended InGaAs detector for spectrum analysis or direct into the detector for power measurements. For spectral analysis each data points are generated by wavelength corresponded grating reflection angle and its amplitude within still pump level and sets of data are measured at different pump levels. The power curve measurements signal amplitude is mapped directly from different pump levels. Room temperature is maintained by using active cooling for all pumping operations. At 4.4% Sn, the material still has indirect bandgap as the lowest band and direct band energy is slightly above. This results in emission curves due to the indirect band gap and the direct band gap at lower output power from the spectrum analysis. It also shows that with increasing the pump level, the ratio between the emission power of the two processes increases and the two emission peaks separate further as the pump power is increased. Meanwhile the emission power from our waveguide versus pump power shows an exponential dependence, indicating the population inversion and optical gain at targeted spectrum reign. In this research the amplified spontaneous emission and its spectrum from optically pumped GeSn waveguide on Si at room temperature has been successfully observed. GeSn with 4.4% Sn content provides promising results as a gain material at mid-IR range.

High performance Ge0.89Sn0.11 photodiodes for low-cost shortwave infrared imaging

Low-cost shortwave infrared detectors have great potential for emerging civilian night-vision applications. This paper reports the characteristics of Ge0.89Sn0.11 photodiodes monolithically grown on a Si substrate that holds great promise for those applications. At room temperature, the 500 μm diameter active area device demonstrated a longwave cutoff of 2.65 μm and a responsivity of 0.32 A/W at 2 μm, which corresponds to an external quantum efficiency of 20% without any contribution from the Ge buffer layer. The measured peak specific detectivity at 300 K and 77 K is 1.7 × 109 Jones and 4.3 × 109 Jones, respectively. The specific detectivity at 77 K is only one-order-of-magnitude lower than that of the market dominating extended-InGaAs photodiode. The detailed device analysis indicated that the 700-nm thick fully relaxed high-quality GeSn absorbing layer and the modified depletion region lead to the above-mentioned device performance.

Стимулированное излучение на длине волны 1.3 mum в метаморфной структуре InGaAs/InGaAsP с квантовыми ямами, выращенной на подложке Ge/Si (001)

AbstractA laser structure comprising metamorphic InGaAsP layer and InGaAs quantum wells on a non-inclined Si(001) substrate with relaxed Ge buffer layer has been grown for the first time by metal-organic vapor phase epitaxy (MOVPE). The optically pumped lasers exhibit stimulated emission at a wavelength of 1.3 μm. At liquid-nitrogen temperature, the threshold power density of pumping at 0.8 μm amounted to 250 kW/cm^2.

Technology of the production of laser diodes based on GaAs/InGaAs/AlGaAs structures grown on a Ge/Si substrate

InGaAs/GaAs/AlGaAs laser diodes with quantum wells are grown by the metal-organic chemical vapor deposition (MOCVD) method on an exact Si (001) substrate with a Ge buffer layer. The diodes generate stimulated emission in the pulsed mode at room temperature in the spectral range from 1.09 to 1.11 μm.

Peculiarities of growing InGaAs/GaAs/AlGaAs laser structures by MOCVD on Ge/Si substrates

The growth of InGaAs/GaAs/AlGaAs laser structures by MOCVD at low pressures on Si(001) substrates with a Ge epitaxial metamorphic buffer layer of various thicknesses is studied. The results of the influence of the growth temperature and incorporation of additional AlAs layers at the boundary with the Ge/Si(001) substrate on the crystalline and optical quality of the formed III–V structures are presented. It is shown that the incorporation of the AlAs/GaAs/AlAs lattice at the initial growth stages of III–V heterostructures on Ge buffer layers grown on exact Si(001) substrates makes it possible to considerably decrease the density of threading defects and, consequently, form effectively emitting laser structures. The possibility of growing strained InGaAs quantum wells on Si(001) substrates allowing stimulated radiation in a wavelength region longer than 1100 nm is shown.

Investigation of hydrogen inductively coupled plasma treatment effect for Ge0.938Sn0.062/Ge/Si film using photoreflectance spectroscopy

Temperature-dependent photoreflectance (PR) measurements for Ge0.938Sn0.062/Ge/Si films treated with a hydrogen inductively coupled plasma (H2-ICP) have been performed. The Ge0.938Sn0.062 film is grown on Ge-buffered Si substrate by ultra-high vacuum chemical vapor deposition method, and the H2-ICP treatment was carried out for 5min for the Ge0.938Sn0.062 epitaxial layer. The high-resolution X-ray diffraction results show that the compressive strain of the Ge0.938Sn0.062 layer decreases and the tensile strain of the Ge buffer layer increases after H2-ICP treatment. The PR spectrum of as-grown Ge0.938Sn0.062/Ge/Si film at 300K consists of three signals at 0.603, 0.782 and 0.814eV, which are assigned to the direct transitions from conduction Γ valley to the valence bands related to the surface of Ge0.938Sn0.062, the Ge/Si and GeSn/Ge interfaces, respectively. After H2-ICP treatment, two PR signals of 0.604 and 0.781eV were obtained at 300K and they are attributed to the direct transition energies of the Ge0.938Sn0.062 and the Ge buffer layer, respectively. As temperature decreases, new weak PR signals appear in the lower energy regions of both PR signals from the H2-ICP treated GeSn and Ge layers at 210K and 130K, respectively, due to the increase of tensile strain in Ge layer while no new signal appears for the as-grown sample. The PR spectrum of the H2-ICP treated sample at 10K shows four signals, and these signals are assigned to the surface of GeSn and GeSn at the interface between GeSn and Ge buffer layers, the Ge at the interface between GeSn and Ge buffer layers, and the Ge at the interface between Ge buffer layer and Si substrate, respectively. These PR results indicate that H2-ICP treatment significantly affects the strain and defects near the interface between GeSn and Ge layers, and thus optical properties of GeSn layer are also altered by H2-ICP treatment.

Photoluminescence of phosphorus atomic layer doped Ge grown on Si

Improvement of the photoluminescence (PL) of Phosphorus (P) doped Ge by P atomic layer doping (ALD) is investigated. Fifty P delta layers of 8 × 1013 cm−2 separated by 4 nm Ge spacer are selectively deposited at 300 °C on a 700 nm thick P-doped Ge buffer layer of 1.4 × 1019 cm−3 on SiO2 structured Si (100) substrate. A high P concentration region of 1.6 × 1020 cm−3 with abrupt P delta profiles is formed by the P-ALD process. Compared to the P-doped Ge buffer layer, a reduced PL intensity is observed, which might be caused by a higher density of point defects in the P delta doped Ge layer. The peak position is shifted by ∼0.1 eV towards lower energy, indicating an increased active carrier concentration in the P-delta doped Ge layer. By introducing annealing at 400 °C to 500 °C after each Ge spacer deposition, P desorption and diffusion is observed resulting in relatively uniform P profiles of ∼2 × 1019 cm−3. Increased PL intensity and red shift of the PL peak are observed due to improved crystallinity and higher active P concentration.

Fabrication and characterization of single junction GaAs solar cells on Si with As-doped Ge buffer

A single junction gallium arsenide (GaAs) solar cell on silicon (Si) substrate with energy conversion efficiency of 11.88% under the AM1.5G spectrum at 1 sun intensity without an anti-reflection coating (ARC) has been developed. This development was enabled by utilizing an intermediate, thin arsenic-doped germanium (As-doped Ge) buffer layer. The thin epitaxial Ge layer (< 2µm) grown on the Si substrate created a virtual Ge-on-Si (Ge/Si) substrate for subsequent growth of the GaAs solar cell, providing low threading dislocation density (TDD) of < 5 × 106cm−2. The energy conversion efficiency could be further improved to 16% upon optimizing the ARC and metal coverage. Hence, a manufacturable III-V photovoltaic on a large-area Si substrate has become possible.

On the stimulated emission of InGaAs/GaAs/AlGaAs laser structures grown by MOCVD on exact and inclined Ge/Si(001) substrates

GaAs/AlGaAs laser structures with InGaAs quantum wells are grown by metal-organic chemical vapor deposition (MOCVD) on exact Si(001) substrates and substrates inclined by 4° to the [011] axis with a relaxed Ge buffer layer, emitting in the transparency region of bulk silicon (the wavelength is longer than 1100 nm at room temperature). The threshold power densities of stimulated emission, observed for the structures grown on exact and inclined substrates are 45 and 37 kW/cm2, respectively.

Dislocation density and strain-relaxation in Ge1−xSnx layers grown on Ge/Si (0 0 1) by low-temperature molecular beam epitaxy

The density and origin of dislocations in a variety of molecular-beam-epitaxy-(MBE)-grown Ge1−xSnx/Ge/Si (001) heterostructures, with Sn-content varying from 0.4 to 5.2%, have been investigated systematically by high resolution X-ray diffraction (HRXRD). In particular, using the approach due to Kaganer et al. (V.M. Kaganer et al., Phys. Rev. B 72, 045423 (2005)) for the first time to the Ge1−xSnx alloy, it is demonstrated that reliable estimates of both edge and screw dislocation densities can be obtained from HRXRD data. Based on the correlations of strain relaxation and dislocation densities of the alloy epilayers and the underlying Ge buffer layers, we observe that dislocations threading from the latter predominantly contribute to the strain relaxation of the former. Thus, Ge1−xSnx epilayers of sub-critical thicknesses can be made to relax significantly by growing them on partially-relaxed, relatively-thin Ge buffer layers. This may be promising for the realization of Ge1−xSnx epilayers with direct electronic band gap.

Simulation of the effects of defects in low temperature Ge buffer layer on dark current of Si-based Ge photodiodes**Project supported by the Key Project of Natural Science Foundation of China (No. 61534005), the National Science Foundation of China (No. 61474081), the National Basic Research Program of China (No. 2013CB632103), the Natural Science Foundation of Fujian Province (No. 2015D020), and the

The influence of defects in low temperature Ge layer on electrical characteristics of p-Ge/i-Ge/n-Si and n-Ge/i-Ge/p-Ge photodiodes (PDs) was studied. Due to a two-step growth method, there are high defect densities in low-temperature buffer Ge layer. It is shown that the defects in low-temperature Ge layer change the band diagrams and the distribution of electric field, leading to the increase of the total dark current for p-Ge/i-Ge/n-Si PDs, whereas these defects have no influence on the dark current for n-Ge/i-Ge/p-Ge PDs. As a complement, a three-dimensional simulation of the total current under illumination was also performed.

GaAs/Ge/Si epitaxial substrates: Development and characteristics

We developed high quality 2-inch GaAs/Ge/Si (100) epitaxial substrates, which may be used instead of GaAs monolithic substrates for fabrication of solar cells, photodetectors, LEDs, lasers, etc. A 200–300 nm Ge buffer layer was grown on Si substrates using the HW-CVD technique at 300°C, a tantalum strip heated to 1400°C was used as the “hotwire”. The MOCVD method was used to grow a 1 μ GaAs layer on a Ge buffer. The TDD in the GaAs layers did not exceed (1–2)∙105 cm-2 and the surface RMS roughness value was under 1 nm.

Silicon and III-V Solar Cells: From Modus Vivendi to Modus Operandi

In the present paper, some novel opportunities for the development of high-efficient Si and III-V-based solar cells are considered: energy-saving environment friendly low-temperature technology of forming p-n junctions in Si (1), elaboration of structurally perfect GaAs/Ge/Si epitaxial substrates (2) and application of protective antireflecting coatings based on cubic zirconia (3). As a result: 1) New technique of forming p-n junctions in silicon has been elaborated. The technique provided easy and comparatively cheap process of production of semiconductor devices such as solar cells. The essence of the technique under the study is comprised in formation p-n junctions in silicon by a change of conductivity in the bulk of the sample occurring as a result of redistribution of the impurities, which already exists in the sample before its processing by ions. It differs from the techniques of diffusion and ion doping where change of conductivity and formation of p-n junction in the sample occur as a result of introduction of atoms of the other dopants from the outside; 2) The conditions for synthesis of GaAs/Ge/Si epitaxial substrates with a thin (200 nm) Ge buffer layer featured with (1 - 2) × 105 cm-2 density of the threading dislocation in the GaAs layer. Ge buffer was obtained by chemical vapor deposition with a hot wire and GaAs layer of 1 μm thick was grown by the metal organic chemical vapor deposition. Root mean square surface roughness of GaAs layers of the less than 1 nm and good photoluminescence properties along with their high uniformity were obtained; 3) The conditions ensuring the synthesis of uniform functional (buffer, insulating and protective) fianite layers on Si and GaAs substrates by means of magnetron and electron-beam sputtering have been determined. Fianite films have been shown to be suitable for the use as an ideal anti-reflecting material with high protective and anticorrosive properties.

Fabrication and characteristics of high performance SOI-based Ge PIN waveguide photodetector

Silicon-based photonics has aroused an increasing interest in the recent year, mainly for optical telecommunications or optical interconnects in microelectronic circuits. The waveguide photodetector is one of the building blocks needed for the implementation of fast silicon photonics integrated circuits. The main considerations for designing such a device are the bandwidth, the power consumption and the responsivity. Germanium is now considered as an ideal candidate for fully integrated receivers based on silicon-on-insulator (SOI) substrates and complementary metal oxide semiconductor (CMOS)-like process because of its large optical absorption coefficient at the wavelength for optical communication. Therefore, the study of high speed and high responsivity Ge waveguide photodetectors is necessary. In this paper, high concentration phosphor doped SOI substrate is achieved by using solid-state source diffusion at first. Secondly, the high quality epitaxial germanium (Ge) is grown on phosphor doped SOI substrate by using low temperature Ge buffer layer technique based on the UHV/CVD system. The surface profile, crystal quality and strain of epitaxial Ge film are characterized by using atomic force microscopy, X-ray diffraction (XRD), and Raman scattering spectrum. The results show that the Ge film has a smooth surface of 1.12 nm roughness and about 0.2% tensile strain, which is verified by XRD characterization result. Thirdly, ptype Ge region is formed by BF2+ implantation, and rapid thermal annealing to repair the implantation damages and activate impurity. Finally, the highperformance Ge PIN waveguide photodetectors with different sizes are fabricated by standard COMS technology. Moreover, the device performances, in terms of dark current versus voltage characteristics, photocurrent responsivity and 3 dB bandwidth, are well studied. The results show that the detector with a size of 4 m20 m demonstrates a dark current density of 75 mA/cm2 at -1 V and a photocurrent responsivity of 0.58 A/W for 1.55 m optical wavelength. In addition, an optical band width of 5.3 GHz at -2 V for 1.55 m is also demonstrated, which is far below theortical value of about 40 GHz. This can mainly be attributed to two aspects. On the one hand, Ge PIN structure contains low temperature Ge buffer layer, which has highdensity dislocation because of large lattice mismatch between Si and Ge. Those dislocations or defects can trap and release the photo-generated carrier, which increases the transit time. On the other hand, the contact characteristics of Al with n+-Si and p+-Ge are not very good, leading to a large contact resistance and RC delay. Through improving the above two aspects, the performance of Ge PIN waveguide photodetector will be further enhanced.

Crystallization and Characterization of GeSn Deposited on Si with Ge Buffer Layer by Low-temperature Sputter Epitaxy

Recently, GeSn is drawing great deal of interests as one of the candidates for group-IV-driven optical interconnect for integration with the Si complementary metal-oxide-semiconductor (CMOS) owing to its pseudo-direct band structure and high electron and hole mobilities. However, the large lattice mismatch between GeSn and Si as well as the Sn segregation have been considered to be issues in preparing GeSn on Si. In this work, we deposit the GeSn films on Si by DC magnetron sputtering at a low temperature of <TEX>$250^{\circ}C$</TEX> and characterize the thin films. To reduce the stresses by GeSn onto Si, Ge buffer deposited under different processing conditions were inserted between Si and GeSn. As the result, polycrystalline GeSn domains with Sn atomic fraction of 6.51% on Si were successfully obtained and it has been demonstrated that the Ge buffer layer deposited at a higher sputtering power can relax the stress induced by the large lattice mismatch between Si substrate and GeSn thin films.

Performance improvement of flexible CZTSSe thin film solar cell by adding a Ge buffer layer

Flexible CZTSSe thin film solar cells were prepared on Ti foil substrate. With adding a Ge buffer layer between the Ti foil substrate and the Mo back contact, CZTSSe thin film showed a better crystallization and less secondary phases. It was confirmed that Ge diffused into CZTSSe layer and had a promoting effect on Sn+4 state formation and stress release. The flexible CZTSSe solar cell with Ge buffer layer showed a conversion efficiency of 2.00%, which presented a 200% enhancement comparing to the solar cell without Ge buffer layer.

Growth of uniform CaGe2 films by alternating layer molecular beam epitaxy

Layered Zintl phase van der Waals (vdW) materials are of interest due to their strong spin-orbit coupling and potential for high mobility. Here, we report the successful growth of large area CaGe2 films, as a model of layered Zintl phase materials, on atomically flat Ge(111) substrates by molecular beam epitaxy (MBE) using an alternating layer growth (ALG) protocol. Reflection high energy electron diffraction (RHEED) patterns of the Ge buffer layer and CaGe2 indicate high quality two dimensional surfaces, which is further confirmed by atomic force microscopy (AFM), showing atomically flat and uniform CaGe2 films. The appearance of Laue oscillations in X-ray diffraction (XRD) and Kiessig fringes in the X-ray reflectivity (XRR), which are absent in co-deposited CaGe2, confirms the uniformity of the CaGe2 film and the smoothness of the interface. These results demonstrate a novel method of deposition of CaGe2 that could be also applied to other layered Zintl phase vdW materials. Also, the high quality of the CaGe2 film is promising for the exploration of novel properties of germanane.

Stimulated emission in heterostructures with double InGaAs/GaAsSb/GaAs quantum wells, grown on GaAs and Ge/Si(001) substrates

We report the observation of stimulated emission in heterostructures with double InGaAs/GaAsSb/GaAs quantum wells, grown on Si(001) substrates with the application of a relaxed Ge buffer layer. Stimulated emission is observed at 77 K under pulsed optical pumping at a wavelength of 1.11 μm, i.e., in the transparency range of bulk silicon. In similar InGaAs/GaAsSb/GaAs structures grown on GaAs substrates, room-temperature stimulated emission is observed at 1.17 μm. The results obtained are promising for integration of the structures into silicon-based optoelectronics.

Photoluminescence of phosphorous doped Ge on Si (100)

Photoluminescence (PL) of selectively grown phosphorus (P) doped germanium (Ge) is investigated. 350–600nm thick P-doped Ge is grown on 100nm thick P-doped Ge buffer layer, which is annealed at 800°C before the main part of Ge deposition. In the case of Ge deposited at 325°C, approximately two times higher PL intensity is observed by P doping of ~3.2×1019cm−3. Further increase of PL intensity by a factor of 1.5 is observed by increasing the growth temperature from 325°C to 400°C due to improved crystal quality. Varying PH3 partial pressure at 400°C, red shift of the PL occurred with increasing P concentration due to higher bandgap narrowing. With increasing P concentration up to ~1.4×1019cm−3 at 400°C the PL peak intensity increases by filling electrons into the L valley and decreases due to enhanced point defect concentration and degraded crystallinity. By post-annealing at 500–800°C, the PL intensity is further increased by a factor of 2.5 because of increased active P concentration and improved crystal quality. Reduced direct bandgap energy by introducing tensile strain is also observed.

(Invited) Fabrication of Ge Waveguides by Epitaxial Lateral Overgrowth toward Monolithic Integration of Light Sources

The Ge laser is one of the most promising devices as a monolithic light source for high-speed optical interconnections due to its compatibility with Si processes. Although optical gain has been observed [1, 2], further improvements of crystallinity are required [3, 4] to ensure continuous wave operation of Ge lasers. In this work, we fabricated high-quality Ge waveguides using epitaxial lateral overgrowth on a SiO2 layer and chemical mechanical polishing [5] , and we investigated its crystallographic and optical properties. An eight-inch Si wafer was used as a substrate, and a SiO2 window was fabricated as a mask for Ge selective epitaxial growth (SEG). A Ge layer was selectively grown by using low-pressure chemical vapor deposition along with germane (GeH4) as a source of Ge with H2 carrier gas. To prevent indirect transition by filling electrons into the L-valley in the conduction band of Ge [6], we also conducted in-situ phosphorus (P) doping by supplying phosphine (PH3). First, a Ge buffer layer was deposited within the SiO2 window at 400°C and annealed at 750°C, then an additional Ge layer was selectively grown only on the Ge buffer layer at relatively high temperature. Finally, rapid thermal annealing was carried out at 850°C. By optimizing the growth pressure, the length of the epitaxial lateral overgrowth (ELO) on the SiO2 layer was increased to more than 5 mm. Although the dislocation and stacking faults were observed around a region of the Ge on the Si substrate by transmission electron microscopy, no dislocations were evident on the ELO-Ge region grown on the SiO2 layer. Since the thickness of the SEG-Ge layer increased as the length of the ELO increased, chemical mechanical polishing (CMP) was applied to remove the top part of the SEG-Ge layer. By using measurements of micro-Raman spectroscopy, it was confirmed that the tensile strain was remained in the ELO-Ge on SiO2 layer even after the CMP process. Then, additional P doping was carried out by spin-on-dopant (SOD) process. The SOD solution [Filmtronics, P8545SF] was coated on the CMP-Ge layer, and the annealed at 750°C, for 10min. The maximum P concentration of 3.2×1019cm-3 was achieved, which was measured by secondary ion mass spectroscopy. Finally, Ge waveguides (Ge-WGs) were fabricated by reactive ion etching of the CMP-Ge layer to remove a part of the Ge layer that contained a lot of crystal defects due to the lattice mismatch between Ge and Si. Figure 1(a) shows a bird's-eye SEM image of the Ge-WG after dry etching with an offset (Dx) of 3.1 mm to the SiO2 window. Photoluminescence (PL) spectra from the Ge-WGs with various Dx are shown in Fig. 1(b). Obvious PL spectra were observed from the Ge-WGs with peak wavelength around 1600 nm. The PL peak intensity from the Ge-WG with Dx = 3.1 mm was four-times higher than that corresponding to Dx = 0.1 mm. This result indicates that the better crystallinity of the Ge-WG was obtained at the position apart from the Ge/Si interface. This work was supported by Project for Developing Innovation Systems of the Ministry of Education, Culture, Sports, Science and Technology (MEXT), Japan.