Ge Laser Research Articles

Theoretical study of small signal modulation behavior of Fabry-Perot germanium-on-silicon lasers

This work investigated the modulation responses of Fabry–Perot Ge-on-Si lasers by modeling and simulations. The 3 dB bandwidth dependence on the structure parameters such as poly-Si cladding thickness, Ge cavity width and thickness, and minority carrier lifetime were studied. A 3 dB bandwidth of 33.94 GHz at a biasing current of 270.5 mA is predicted after Ge laser structure optimization with a defect limited carrier lifetime of 1 ns. The eye diagrams simulated show a stable eye-opening window at 20 Gb·s−1 NRZ. The improvement to 10 ns minority carrier lifetime would reduce the threshold current to 6.85 mA, and increase the 3 dB bandwidth to 36.89 GHz.

Interplay Between Strain and Thickness on the Effective Carrier Lifetime of Buffer-Mediated Epitaxial Germanium Probed by the Photoconductance Decay Technique

We report contactless effective minority carrier lifetime of epitaxially grown unstrained and in-plane <110> biaxially tensile-strained (001) germanium (ε-Ge) epilayers measured using microwave-reflectance photoconductance decay measurements. Strained Ge epilayers were grown using InxGa1–xAs linearly graded buffers on (001) GaAs substrates. Using homogeneous excitation of unstrained Ge epilayers, thickness-dependent separation of minority carrier lifetime components under low injection conditions yielded a bulk lifetime of 114 ± 2 ns and low surface recombination velocity of 21.3 ± 0.04 cm/s. More notably, an effective minority carrier lifetime of >100 ns obtained from sub-50 nm 1.6% tensile-strained Ge epilayers showed no degradation relative to the unstrained counterpart. Detailed material characterization using X-ray diffractometry revealed successful strain transfer of 0.61 and 0.89% to the Ge epilayers via InxGa1–xAs metamorphic buffers and confirms pseudomorphic growth. Lattice coherence observed at the ε-Ge epilayer and InxGa1–xAs buffer heterointerfaces via transmission electron microscopy substantiates the prime material quality achieved. The relatively high carrier lifetimes achieved are an indicator of excellent material quality and provide a path forward to realize low-threshold Ge laser sources.

N-type doping of germanium epilayer on silicon by ex-situ phosphorus diffusion based on POCl3 phosphosilicate glass

We report an ex-situ phosphorus diffusion doping for germanium integrated photonic devices on silicon chip. Here, phosphorus oxychloride (POCl3)-based phosphosilicate glass is chosen for n-type diffusion. As an alternative process to the in-situ P doping during Ge epitaxy so far reported for Ge laser prototyping, the presented external P-diffusion method demonstrates photoluminescence (PL) emission enhancement of Ge-on-Si. The PL enhancement, along with the P secondary ion mass spectroscopy profile in Ge, clearly indicates that our ex-situ diffusion method to form n-type Ge has a significant potential for Ge active device fabrication as an enabling technology. It should be also noted that PL quenching is observed at high temperature diffusion processes which is induced by intermixing at the Ge and Si interface. The presented ex-situ P-diffusion process can serve as a template to monolithically integrate Ge devices such as not only light sources but modulators and photodetectors on Si complementary metal-oxde-semiconductor platform, as it may tailor device-specific pn junctions.

Room temperature lasing unraveled by a strong resonance between gain and parasitic absorption in uniaxially strained germanium

A complementary metal-oxide semiconductor compatible on-chip light source is the holy grail of silicon photonics and has the potential to alleviate the key scaling issues arising due to electrical interconnects. Despite several theoretical predictions, a sustainable, room temperature laser from a group-IV material is yet to be demonstrated. In this work, we show that a particular loss mechanism, inter-valence-band absorption (IVBA), has been inadequately modeled until now and capturing its effect accurately as a function of strain is crucial to understanding light emission processes from uniaxially strained germanium (Ge). We present a detailed model of light emission in Ge that accurately models IVBA in the presence of strain and other factors such as polarization, doping, and carrier injection, thereby revising the road map toward a room temperature Ge laser. Strikingly, a special resonance between gain and loss mechanisms at 4%--5% $\ensuremath{\langle}100\ensuremath{\rangle}$ uniaxial strain is found resulting in a high net gain of more than $400\phantom{\rule{0.28em}{0ex}}{\text{cm}}^{\ensuremath{-}1}$ at room temperature. It is shown that achieving this resonance should be the goal of experimental work rather than pursuing a direct band gap Ge.

Analysis of threshold current of uniaxially tensile stressed bulk Ge and Ge/SiGe quantum well lasers.

We propose and design uniaxially tensile stressed bulk Ge and Ge/SiGe quantum well lasers with the stress along <100> direction. The micro-bridge structure is adapted for introducing uniaxial stress in Ge/SiGe quantum well. To enhance the fabrication tolerance, full-etched circular gratings with high reflectivity bandwidths of ~500 nm are deployed in laser cavities. We compare and analyze the density of state, the number of states between Γ- and L-points, the carrier injection efficiency, and the threshold current density for the uniaxially tensile stressed bulk Ge and Ge/SiGe quantum well lasers. Simulation results show that the threshold current density of the Ge/SiGe quantum well laser is much higher than that of the bulk Ge laser, even combined with high uniaxial tensile stress owing to the larger number of states between Γ- and L- points and extremely low carrier injection efficiency. Electrical transport simulation reveals that the reduced effective mass of the hole and the small conduction band offset cause the low carrier injection efficiency of the Ge/SiGe quantum well laser. Our theoretical results imply that unlike III-V material, uniaxially tensile stressed bulk Ge outperforms a Ge/SiGe quantum well with the same strain level and is a promising approach for Si-compatible light sources.

Stress Engineering With Silicon Nitride Stressors for Ge-on-Si Lasers

Side and top silicon nitride stressors were proposed and shown to be effective ways to reduce the threshold current Ith and improve the wall-plug efficiency {\eta}wp of Ge-on-Si lasers. Side stressors only turned out to be a more efficient way to increase {\eta}wp than using top and side stressors together. With the side stressors only and geometry optimizations, a {\eta}wp of 30.5% and an Ith of 50 mA (Jth of 37 kA/cm2) can be achieved with the defect limited carrier lifetime of 1 nsec. With the defect limited carrier lifetime of 10 nsec, an Ith of 7.8 mA (Jth of 5.8 kA/cm2) and a wall-plug efficiency of 38.7% can be achieved. These are tremendous improvements from the case without any stressors. These results give strong support to the Ge-on-Si laser technology and provide an effective way to improve the Ge laser performance.

Enabling n-type polycrystalline Ge junctionless FinFET of low thermal budget by in situ doping of channel and visible pulsed laser annealing

A low-thermal-budget n-type polycrystalline Ge (poly-Ge) channel that was prepared by plasma in-situ-doped nanocrystalline Ge (nc-Ge) and visible pulsed laser annealing exhibits a high electrically active concentration of 2 × 1019 cm−3 and a narrow Raman FWHM of 3.9 cm−1. Furthermore, the fabricated n-type poly-Ge junctionless FinFET (JL-FinFET) shows an Ion/Ioff ratio of 6 × 104, Vth of −0.3 V, and a subthreshold swing of 237 mV/dec at Vd of 1 V and DIBL of 101 mV/V. The poly-Ge JL-FinFET with a high-aspect-ratio fin channel is less sensitive to Vth roll-off and subthreshold-swing degradation as the gate length is scaled down to 50 nm. This low-thermal-budget JL-FinFET can be integrated into three-dimensional sequential-layer integration and flexible electronics.

The energy band structure of Si and Ge nanolayers

First-principles calculation based on density functional theory (DFT) with the generalized gradient approximation (GGA) were carried out to investigate the energy band gap structure of Si and Ge nanofilms. Calculation results show that the band gaps of Si(111) and Ge(110) nanofilms are indirect structures and independent of film thickness, the band gaps of Si(110) and Ge(100) nanofilms could be transfered into the direct structure for nanofilm thickness of less than a certain value, and the band gaps of Si(100) and Ge(111) nanofilms are the direct structures in the present model thickness range (about 7 nm). Moreover, the changes of the band gaps on the Si and Ge nanofilms follow the quantum confinement effects. It will be a good way to obtain direct band gap emission in Si and Ge materials, and to develop Si and Ge laser on Si chip.

Germanium laser with a hybrid surface plasmon mode

The possibility of creating an n++-Ge laser with a hybrid surface plasmon TM mode is theoretically studied. The distribution of electromagnetic fields and the absorbance in the mode under study, the optical confinement factor, the gain, and the threshold current density in the laser under consideration are calculated. It is shown that the threshold current density at optimal layer thicknesses of an n++-Ge laser with a hybrid surface plasmon TM mode can be 2–3 times lower than the experimentally observed threshold current density in an n++-Ge laser with a conventional dielectric waveguide.

Low Threshold Light Emission from Reverse-Rib n+ Ge Cavity Made by P Diffusion

Strain-tensile n+ Ge is an enabler of monolithically integrated light sources on a Si chip. The challenges are (1) high lasing thresholds and (2) n+ doping methods. (1) The threshold currents reported for electrically pumped Ge lasers are 280 and 510 kA/cm2 [1, 2]; much higher than III-V lasers (less than 1 kA/cm2) and even theoretical prediction of the Ge laser (~6 kA/cm2) [3]. (2) Ge can be used as photodetectors (PDs) and modulators (MODs), which need different in-depth dopant profiles. Ge epitaxy cannot control in-plane P concentration locally unless otherwise one performs multiple epitaxy. In the present study we tried to reduce the threshold using specific epi-structures and to diffuse Phosphorus (P) to make n+ Ge, and successfully observed a threshold in light emission using P diffusion for the first time. We have reported that a reverse-rib structure can be made using the epitaxial lateral overgrowth (ELO) technique [4], and can effectively isolate light modes from Si substrate. It further reduces light scattering at the Ge/SiO2 sidewall formed by dry etching as shown in Fig. 1 [5], some of which should decrease the optical net gain. In addition, our group reported that the Ge/Si interfaces if located in the pn junction enhance significantly non-radiative recombination [6]. Thus, n+ Ge was grown on n+ Si. P diffusion was employed to achieve 1 × 1019 P/cm3 both in the reverse-rib and the blanket Ge epilayers using a well-controlled diffusion method [7]. Optical pumping was used to excite the structure. 40-nm thick SiO2 was formed by thermal oxidation on an n+ Si wafer and patterned in line-and-space shape along [110] orientation by electron beam lithography followed by a wet etching. Epitaxial Ge layers were grown on the wafer using ultra high vacuum chemical vapor deposition (UHVCVD) at 700 ˚C. The epilayers were 1 µm thick at reverse-rib areas, while 1.8 µm thick at blanket areas. P diffusion was performed at 850 ˚C for 5 minutes, which achieves uniform P concentration of 1 × 1019 /cm3 through these Ge epilayers. Schematic of the measurement system is shown in Fig. 2. A continuous wave YAG:Nd laser (wavelength: 1.07 µm, power: ~kW/cm2) was used for optical pumping and a Ge PD was used to measure the relation of light emission and optical pumping. The effective input power density should be actually smaller than the set value since the reverse-rib Ge epilayer (300 µm wide and 3 mm long) is wider than the pumping laser (30 µm wide and 4 mm long), causing out-diffusion of photo-excited minority carriers (holes) to non-pumped areas. The measurements were carried out at room temperature. Figure 3 shows a cross sectional TEM image of the as-grown reverse-rib Ge. The air semi-cylinders are observed over the SiO2 masks (invisible in this image) since Ge will not grow on SiO2 in this condition. The air semi-cylinders are 150 nm high and 450 nm wide, which is big enough to isolate light modes from the Si substrate according to a mode solver simulation. Raman scattering spectroscopy reveals 0.17 % tensile strain at the top surface of the reverse-rib Ge while there should be strain relaxation around the air semi-cylinders. The results of the optical pumping measurements are summarized in Fig. 4. Light emission from the reverse-rib Ge shows a sharp threshold at the input power density of 8 kW/cm2 (set value). The results can be understood as either amplified spontaneous emission (ASE) or lasing. We will soon report the spectra to identify the results. Assuming lasing, the threshold of 8 kW/cm2 is very small compared to the value reported (30 kW/cm2) [8].

(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.

Enhanced emission in the three-level system of Si and Ge nanostructures

Enhanced mission peak near 700nm is observed on the silicon quantum dots (QDs) embedded in Si amorphous film and the peak near 1100nm occurs on the silicon nanolayer, which have the emission characteristics of direct band-gap, such as the thresholds effect and the supper-line increasing effect in intensity with pumping in our experiment. It is interesting that the Si QDs embedded in nanosilicon layer are prepared by using pulsed laser deposition (PLD) method after annealing. In the same way, the peak near 900nm on the Ge QDs and the peak near 1500nm on the Ge nanolayer are measured in the PL spectra. It is very interesting that the sharper peaks with multi-longitudinal-mode occur in the Si and Ge nanolayers with the super-lattice on SOI in which the QDs are embedded. An emission model for Si and Ge laser on silicon chip with QDs pumping has been provided to explain the experimental results.

Low Threshold Light Emission from Reverse-Rib n+ Ge Cavity Made by P Diffusion

Ge laser is expected as a solution for a key issue in electronic-photonic integrations, CMOS compatible on-chip light source. Two important issues remain in Ge laser technologies; the large threshold current (Ith) and the multiple growth of Ge epi-structures to co-integrate Ge devices such as photodetectors, whose pn junction profiles are much different from the Ge lasers. The reported Ith values are larger than those for III-V lasers by two orders of magnitude, and an in-situ heavily n-type doping in Ge prevents to form various pn structures unless the multiple growth with different doping profiles are performed. To solve these issues, the present paper proposes a "reverse-rib" Ge epi-structure in combination with an ex-situ P diffusion for heavily n-type doping, instead of in-situ doping. P-diffused n+-Ge reverse-rib structures first verify a clear threshold behavior of light emission at a low power of optical pumping.

Junction-less poly-Ge FinFET and charge-trap NVM fabricated by laser-enabled low thermal budget processes

A doping-free poly-Ge film as channel material was implemented by CVD-deposited nano-crystalline Ge and visible-light laser crystallization, which behaves as a p-type semiconductor, exhibiting holes concentration of 1.8 × 1018 cm−3 and high crystallinity (Raman FWHM ∼ 4.54 cm−1). The fabricated junctionless 7 nm-poly-Ge FinFET performs at an Ion/Ioff ratio over 105 and drain-induced barrier lowering of 168 mV/V. Moreover, the fast programming speed of 100 μs–1 ms and reliable retention can be obtained from the junctionless poly-Ge nonvolatile-memory. Such junctionless poly-Ge devices with low thermal budget are compatible with the conventional CMOS technology and are favorable for 3D sequential-layer integration and flexible electronics.

Anomalous threshold reduction from uniaxial strain for a low-threshold Ge laser

We theoretically investigate the effect of <100> uniaxial strain on a Ge-on-Si laser. We predict a dramatic ~200x threshold reduction upon applying sufficient uniaxial tensile strain to Ge. This anomalous reduction is explained by how the topmost valence bands split and become anisotropic with uniaxial tensile strain. Approximately 3.2% uniaxial strain is required to achieve this anomalous threshold reduction for 1×1019cm−3 n-type doping, and a complex interaction between strain and n-type doping is observed. Achieving this critical uniaxial strain level for the anomalous threshold reduction is dramatically more relevant to practical devices than realizing a direct band gap.

Design considerations of biaxially tensile-strained germanium-on-silicon lasers

Physical models of Ge energy band structure and material loss were implemented in LASTIPTM, a 2D simulation tool for edge emitting laser diodes. This model is able to match available experimental data. Important design parameters of a Fabry–Perot Ge laser, such as the cavity length, thickness, width, polycrystalline Si cladding layer thickness were studied and optimized. The laser structure optimizations alone were shown to reduce Ith by 22-fold and increase ηd and ηi by 11 and 6 times. The simulations also showed that improving the defect limited carrier lifetime is critical for achieving an efficient and low-threshold Ge laser. With the optimized structure design (300 μm for the cavity length, 0.4 μm for the cavity width, 0.3 μm for the cavity thickness, and 0.6 μm for the polycrystalline Si cladding layer thickness) and a defect limited carrier lifetime of 100 ns, a wall-plug efficiency of 14.6% at 1 mW output is predicted, where Jth of 2.8 kA cm−2, Ith of 3.3 mA, I1mW of 9 mA, and ηd of 23.6% can be achieved. These are tremendous improvements from the available experimental values at 280 kA cm−2, 756 mA, 837 mA and 1.9%, respectively.

Ultimate limits of biaxial tensile strain and n-type doping for realizing an efficient low-threshold Ge laser

We theoretically investigate the methodology involved in the minimization of the threshold of a Ge-on-Si laser and maximization of the slope efficiency in the presence of both biaxial tensile strain and n-type doping. Our findings suggest that there exist ultimate limits beyond which no further benefit can be realized through increased tensile strain or n-type doping. In this study, we quantify these limits, showing that the optimal design for minimizing threshold involves approximately 3.7% biaxial tensile strain and 2 × 1018 cm−3 n-type doping, whereas the optimal design for maximum slope efficiency involves approximately 2.3% biaxial tensile strain with 1 × 1019 cm−3 n-type doping. Increasing the strain and doping beyond these limits will degrade the threshold and slope efficiency, respectively.

(Invited) Light Emission from Highly-Strained Germanium for On-Chip Optical Interconnects

In this paper, we focus on developing an efficient silicon compatible light emitter based on highly-strained germanium technology. We present various experimental results showing enhanced light emission from strained germanium. First, we describe a thin film membrane technique in which a large residual stress in a tungsten layer is used to induce a biaxial tensile strain in germanium membranes. Second, we introduce an approach to induce sufficiently large uniaxial strain to create a direct band gap in germanium (Ge) nanowires using geometrical amplification of a small pre-existing strain. Lastly, we present a novel way to mimic double-heterostructure behavior within a single material, further enhancing light emission from Ge by capturing photo-generated carriers in a strain-induced potential well. Throughout this paper we discuss the implications of these experimental achievements toward creating an efficient Ge laser for silicon-compatible optical interconnects.

Study of Carrier Statistics in Uniaxially Strained Ge for a Low-Threshold Ge Laser

In this paper, we present a comprehensive study of carrier statistics in germanium with high uniaxial strain along the [100] direction. Several types of PL experiments were conducted to investigate polarization-, temperature- and excitation-dependent carrier statistics in germanium under various amounts of uniaxial strain. With the ability to clearly resolve multiple photoluminescence peaks originating from strain-induced valence band splitting, we experimentally observed strongly polarized light emission from direct band gap transitions. Our experiments also confirm that uniaxial strain increases the hole population in the highest valence band as well as the electron population in the direct conduction band. Based upon our experimental results, we present theoretical modeling showing that the lasing threshold of a germanium laser can be reduced by >100× with 2.5% strain.

Tensile-strained germanium microdisks

We show that a strong tensile strain can be applied to germanium microdisks using silicon nitride stressors. The transferred strain allows one to control the direct band gap emission that is shifted from 1550 nm up to 2000 nm, corresponding to a biaxial tensile strain around 1%. Both Fabry-Perot and whispering gallery modes are evidenced by room temperature photoluminescence measurements. Quality factors up to 1350 and limited by free carrier absorption of the doped layer are observed for the whispering gallery modes. We discuss the strain profile in the microdisks as a function of the disk geometry. These tensile-strained microdisks are promising candidates to achieve Ge laser emission in compact microresonators.