Polycrystalline Germanium Films Research Articles

Evaluation of extremely thin polycrystalline germanium films and their TFT performance fabricated at 400 °C by Cu-induced crystallization on a glass substrate

Germanium (Ge) has high mobility and is more suitable for low-temperature device processes than silicon (Si); thus, it has attracted attention as an upper-layer transistor for monolithic three-dimensional (M3D) integration. We evaluated in detail the crystalline quality of extremely thin polycrystalline-Ge (poly-Ge) thin films thinner than 15 nm fabricated by metal-induced crystallization using copper (Cu-MIC) at 400 °C using micro-Raman scattering, in-plane X-ray diffraction, transmission electron microscopy (TEM), TEM energy dispersive X-ray spectroscopy, and TEM electron diffraction. Additionally, the films were applied to p-ch double-gate (DG) poly-Ge thin-film transistors (TFTs), and their performance was evaluated. As a result, a high I on/I off ratio of 2.8 × 103 was realized by crystallization at 400 °C with a ratio equal to that of crystallization at 500 °C. This study demonstrated the feasibility of Cu-MIC DG poly-Ge TFTs for M3D applications.

Single-shot selective femtosecond and picosecond infrared laser crystallization of an amorphous Ge/Si multilayer stack

Pulsed laser crystallization is an efficient annealing technique to obtain polycrystalline silicon or germanium films on non-refractory substrates. This is important for creating “flexible electronics” and can also be used to fabricate thin-film solar cells. In this work, near- and mid-infrared femtosecond and picosecond laser pulses were used to crystallize a Ge/Si multilayer stack consisting of alternating amorphous thin films of silicon and germanium. The use of infrared radiation at wavelengths of 1030 and 1500 nm with photon energies lower than the optical absorption edge in amorphous silicon allowed obtaining selective crystallization of germanium layers with a single laser shot. The phase composition of the irradiated stack was investigated by the Raman scattering technique. Several non-ablative regimes of ultrashort-pulse laser crystallization were found, from partial crystallization of germanium without intermixing the Ge/Si layers to complete intermixing of the layers with formation of GexSi1-x solid alloys. The roles of single- and two-photon absorption, thermal and non-thermal (ultrafast) melting processes, and laser-induced stresses in selective pico- and femtosecond laser annealing are discussed. It is concluded that, due to a mismatch of the thermal expansion coefficients between the adjacent stack layers, efficient explosive solid-phase crystallization of the Ge layers is possible at relatively low temperatures, well below the melting point.

Experimental evidence of impurity-band transport in p-type polycrystalline germanium film

Herein, we report on the Hall transport properties of a p-type polycrystalline germanium film, which is grown by the DC magnetron sputtering method. The average grain size of 85 nm is calculated. The resistivity and Hall coefficient of the germanium film were measured at temperatures between 100 K and 350 K. At room temperature (T = 300 K), the resistivity of 1.06 Ω cm, the hole concentration of 7.6 × 1017 cm−3 and mobility of 7.76 cm2V−1s−1 are obtained. The temperature dependence of Hall coefficient presents a steep maximum at a specific temperature (T☆ ≈ 215 K). At the same time, the slope of the log resistivity versus 1/T curve is changed. The unusually results can be well explained by a two-band conduction model, which includes the valence band with a high mobility, and the acceptor impurity band with a much lower mobility. This work gives an evidence for impurity-band conduction in the polycrystalline germanium film.

Temperature-dependent anomalous dispersion of polycrystalline germanium film near absorption threshold

Temperature-dependent refractive index of semiconductor germanium (Ge) near the absorption threshold has not been well studied theoretically and experimentally up to now. This paper presents a study of the temperature-dependent refractive index of Ge film near its absorption threshold (about 0.8 eV or 1550 nm), whose values deduced from the reflectivity and transmissivity of high crystalline Ge film fabricated on a fiber tip. Different from previous reports, which show a nearly linear relationship between refractive index and photon energy in the temperature range of −20–120 °C, an abnormal drop has been observed in the refractive index spectrum at the wavelengths near the absorption threshold. To theoretically understand this anomalous dispersion, we first bring the HigherCPs model and M0CP model together to provide a good description of inter-band transition and rewrite the dielectric function of Ge near its absorption edge. Then, the abnormal drop is well interpreted by introducing the Wannier exciton model to the combined model. Furthermore, we evaluate the band gap E0 and Lorentzian broadening of the exciton line as the function of temperature with this new model. The experimental and theoretical results allow for the optimized design of the Ge-based optical applications near the C-band of optical fiber communication.

Low-Temperature Crystallization of Germanium in the Thin-Film Ge/Al System

The crystallization of thin germanium films during vacuum thermal deposition on an aluminum surface has been investigated. Significant changes in the morphology and crystal structure of films in dependence of the formation temperature are demonstrated using atomic force microscopy and X-ray diffraction. The temperature ranges for growth of amorphous and polycrystalline germanium films are determined. It is shown that the decrease in the germanium crystallization temperature to 300°С is caused by the size effect and is explained within the metal-induced crystallization model.

High-performance n-channel polycrystalline germanium thin-film transistors via continuous-wave laser crystallization and green nanosecond laser annealing for source and drain dopant activation

High-quality polycrystalline germanium (poly-Ge) films have been successfully fabricated via the continuous-wave laser crystallization (CLC) process. Grain sizes as large as 0.8 μm were obtained for the poly-Ge films by CLC at 5.7 W. Furthermore, the source and drain dopants could then be effectively activated by green nanosecond laser annealing (GNS-LA). Consequently, n-channel CLC Ge thin-film transistors (TFTs) with a high field-effect mobility of 576 cm2 V−1 s−1 were demonstrated for an effective channel width of 0.86 μm and a channel length of 0.5 μm. It is shown that CLC combined with GNS-LA is effective for attaining high-performance n-channel poly-Ge TFTs.

The effects of silicon substrate thickness and annealing temperature on surface coverage for aluminum-induced crystallization of germanium films

This work investigates polycrystalline germanium films for photovoltaic applications that were developed using aluminum-induced crystallization on silicon substrates coated with 500 nm of thermally grown silicon dioxide. Five silicon substrate thicknesses were investigated between 200 and 525 µm and the samples were annealed for 50 hours at either 365 °C, 375 °C, or 385 °C. When annealed, the amorphous germanium films deposited on the 200 µm silicon substrate developed into discrete monocrystalline germanium islands with an average grain size of 10.76 µm, but poor overall surface coverage between 50% and 59%. The germanium films deposited on silicon substrates with thickness between 250 and 375 µm and annealed below 385 °C revealed the highest and most uniform polycrystalline germanium surface coverage between 80% and 97% and demonstrated ~300 nm grain size. The 525 µm silicon substrate exhibited dense island formation at 385 °C as well as high residual aluminum content, and cracks in the film at lower temperatures.

Effects of Crystallinity on the Electrical Characteristics of Counter-Doped Polycrystalline Germanium Thin-Film Transistor via Continuous-Wave Laser Crystallization

The crystallinity of polycrystalline germanium (poly-Ge) films were demonstrated through continuous-wave laser crystallization (CLC) with Gaussian-distribution beam profile. The different grain sizes of CLC poly-Ge were observed in their three crystallization regions, which were 2 $\mu \text{m}$ , 680 nm, and 90 nm for the central, transition, and edge regions, respectively. Furthermore, the relation between crystallinity and carrier types in these three regions of counter-doped CLC poly-Ge films were investigated. In the central and transition regions, the CLC poly-Ge films with relatively low hole concentration were easily converted to n-type poly-Ge films through a counter-doping process. In contrast, the edge region with poor crystallinity exhibited p-type behavior due to high defect-generated hole concentration. According to these material properties of counter-doped CLC poly-Ge films, the corresponding transfer characteristics of p-channel poly-Ge thin-film transistor for three crystallization regions were further investigated. Subsequently, high-performance p-channel poly-Ge thin-film transistors in the central region exhibited a superior field-effect mobility of 792.2 cm2/V-s.

Quantum interference magnetoconductance of polycrystalline germanium films in the variable-range hopping regime

Direct evidence of quantum interference magnetotransport in polycrystalline germanium films in the variable-range hopping (VRH) regime is reported. The temperature dependence of the conductivity of germanium films fulfilled the Mott VRH mechanism with the form of in the low-temperature regime (). For the magnetotransport behaviour of our germanium films in the VRH regime, a crossover, from negative magnetoconductance at the low-field to positive magnetoconductance at the high-field, is observed while the zero-field conductivity is higher than the critical value (). In the regime of , the magnetoconductance is positive and quadratic in the field for some germanium films. These features are in agreement with the VRH magnetotransport theory based on the quantum interference effect among random paths in the hopping process.

Single‐Crystal Germanium Growth on Amorphous Silicon

AbstractGrowing single‐crystal semiconductors directly on an amorphous substrate without epitaxy or wafer bonding has long been a significant fundamental challenge in materials science. Such technology is especially important for semiconductor devices that require cost‐effective, high‐throughput fabrication, including thin‐film solar cells and transistors on glass substrates as well as large‐scale active photonic circuits on Si using back‐end‐of‐line CMOS technology. This work demonstrates a CMOS‐compatible method of fabricating high‐quality germanium single crystals on amorphous silicon at low temperatures of <450 °C. Grain orientation selection by geometric confinement of polycrystalline germanium films selectively grown on amorphous silicon by chemical vapor deposition is presented, where the confinement selects the fast‐growing grains for extended growth and eventually leads to single crystalline material. Germanium crystals grown using this method exhibit (110) texture and twin‐mediated growth. A model of confined growth is developed to predict the optimal confining channel dimensions for consistent, single‐crystal growth. Germanium films grown from one‐dimensional confinement exhibit a 200% grain size increase at 1 μm film thickness compared to unconfined films, while 2D confinement growth achieved single crystal Ge. The area of single crystalline Ge on amorphous layers is only limited by the growth time. Significant enhancement in room temperature photoluminescence and reduction in residual carrier density have been achieved using confined growth, demonstrating excellent optoelectronic properties. This growth method is readily extensible to any materials system capable of selective non‐epitaxial deposition, thus allowing for the fabrication of devices from high‐quality single crystal material when only an amorphous substrate is available.

Formation of heavily boron-doped hydrogenated polycrystalline germanium thin films by co-sputtering for developing p + emitters of bottom cells

Properties of heavily boron-doped hydrogenated polycrystalline germanium (Ge) films sputter-deposited on glass are investigated for developing p + emitters of the bottom cells of low cost monolithic tandem solar cells. The films were deposited and in-situ doped by co-sputtering Ge with boron at various power levels ( P B) in a mixture of argon and hydrogen at 500 °C, and then followed by a rapid thermal anneal process at 550 °C for 120 s or 600 °C for 60 s, respectively. The dependence of the structural properties of the films on the various boron incorporations as well as annealing conditions was explored by Raman and X-ray diffraction measurements. We find that the films as deposited at 500 °C are polycrystalline with strong (2 2 0) preferential orientation, which are normally of columnar structure, confirmed by cross-sectional transmission electron microscopy. Revealed by Hall measurements, a boron activation level of 6.24×10 19 atoms/cm 3, well above reported maximum solid solubility in crystalline Ge, was obtained in the as deposited film at P B=55 W. Annealing at 600 °C enhanced the concentration to 1.21×10 20 atoms/cm 3. A fact that boron hardly diffuses in Ge at temperatures below 800 °C makes the resultant material an excellent candidate for thin p + emitters of the bottom cells of monolithic tandem solar cells.

Electrical properties for poly-Ge films fabricated by pulsed laser annealing

Electrical properties of 50-nm-thick polycrystalline germanium (poly-Ge) films fabricated by pulsed laser annealing were investigated. Analysis of the electrical conductivity using a numerical calculation program revealed that the density of defect states at grain boundaries in the poly-Ge films was a low of 1.1 × 10 12 cm − 2 in contrast to a high density of 2.6 × 10 13 cm − 2 in the case of laser crystallized poly-Si films. Transient photoconductivity measurements using pulsed laser irradiation revealed that 50-nm-thick poly-Ge films had a long carrier lifetime of 3.0 μs.

A solid-state near infrared spectrum analyzer based on polycrystalline Ge on Si

We report on a novel solid-state spectrum analyzer in the near infrared. The device is an array of six photodetectors based on polycrystalline germanium film evaporated on a silicon substrate and each element is a wavelength-selective detector. We describe the fabrication and characterization of such device and we demonstrate its capability both as a wavelengthmeter for quasi-monochromatic light beams and as a spectrum analyzer.

Pressure sensor using polycrystalline germanium films prepared by plasma-assisted chemical vapour deposition

Abstract Pressure sensors in the form of polycrystalline semiconducting films on a stainless steel diaphragm covered with an insulating layer have been fabricated. Phosphorus-doped polycrystalline germanium films are prepared by plasma-assisted chemical vapour deposition (plasma-CVD) and used as the sensing part of these devices. The deposition temperature is between 200 and 400 °C and the PH 3 /GeH 4 ratio is ∼3.6 × 10 −2 . The gauge factor of these films is larger than 30, and is more than twice that of polycrystalline silicon films prepared at 800 °C. The sensitivity of the pressure sensor is more than 1.5 times higher than that of devices using polycrystalline silicon thin films.

CO2 Laser Detection Using a Warm Carrier Device Fabricated by Evaporated Ge Films

Polycrystalline germanium films with a thickness of 600 nm were deposited on glass substrates by vacuum evaporation and used for the first time as antenna-coupled point-contact warm carrier devices. Devices fabricated by using a Ge film deposited at a substrate temperature of 640°C were successfully used to detect a CO2 laser.

Dislocation and Surface Scattering in Polycrystalline Germanium Films

A computer-aided analysis of the transport mechanism for holes in polycrystalline germanium films indicates that the density of dislocation lines obeys the relation N=NC[1+βD exp(−a/δD)],where NC, βD, and δD are constants determined by experimental growth conditions. The film thickness is a. The energy bands are bent down at each surface with uB≃−10 and uS≃−7 in accordance with the surface scattering theory developed by Schrieffer and Greene, Frankl, and Zemel.

Piezoresistance in polycrystalline germanium films

Piezoresistive measurements have been performed on polycrystalline germanium films with different mean crystallite sizes in the temperature range from 115° to 625°K. The results are consistent with a structural model where the crystallites are supposed to be separated by thin layers of disordered material about 10 Å thick. Comparison of these results with those obtained on amorphous Ge films contradict sharply the concept of amorphous Ge being a microcrystalline material.

Germanium piezoresistive element on a flexible substrate

Thin film strain gauge elements have been made from polycrystalline germanium films deposited upon a very flexible substrate. Devices with gauge factors up to about 100 and with a frequecny response to at least 80 kHz and with reasonable long term stability have been obtained in devices deposited upon a 0·001 inch thick Kapton film.

Anomalous Optical Effects in Germanium Films

The surface of a polycrystalline germanium film, vacuum-deposited onto a heated substrate in a narrow range of temperatures, takes on a regular structure exhibiting optical interference effects whose nature is quite sensitive to the angle at which the film is deposited. For all films deposited between 550° and 620°C, Fraunhofer diffraction effects are observed which are independent of crystalline texture. In the case of perpendicular deposition, the diffraction is nearly isotropic about the normal to the film, while in the case of deposition at an acute angle, the diffraction effects are nonisotropic and the observed optical behavior is analogous to that observed with blazed gratings; electron-microscope observations confirm these particular surfaces to be “blazed” in character. The “blue–grey” character of germanium films and the “blue haze” observed on other surfaces is explained on the basis of these results.

Effect of Surface Damage on the Reflectance of Germanium in the 2650–10 000-Å Region

With the recent development of an electropolishing technique for producing optically smooth and flat surfaces with a minimum amount of surface damage, a quantitative study of the influence of surface damage and lattice disorder on the optical properties of semiconductors has become possible. In this investigation precise absolute specular reflectance measurements at normal incidence are reported for electropolished and mechanically polished germanium single crystals and for amorphous and polycrystalline germanium films in the 2650–10 000-A wavelength range. The highest reflectance maxima and considerable structure were exhibited by the electropolished sample. Splitting of the 2.1-eV peak, which had previously been reported only for etched samples, was observed, and shoulders at 3.25 and 2.47-eV were found. The latter shoulder has apparently not been reported previously. The wavelength at which the 2.1-eV peak occurred shifted for the mechanically polished single crystal and for the polycrystalline film and disappeared entirely for the amorphous film. However the 4.4-eV peak, although changing greatly in magnitude, occurred at the same wavelength for all the samples tested. The results of the experiment show that the optical properties of this material are a strong function of the lattice disorder present and emphasize the importance of adequate sample preparation techniques if meaningful optical measurements are to be made.