Molecular Beam Deposition System Research Articles

Sn Doping Effects in Solid-Phase Crystallized Ge Thin-Film Transistors

Introduction Low-temperature synthesis of high carrier mobility thin films on insulating substrates is actively being conducted to dramatically improve the device performance of thin-film transistors (TFTs). Ge is one of the candidates for next-generation TFTs because of its high carrier mobility and good compatibility with Si. We recently reported that the densification of amorphous (a-) Ge precursor in solid-phase crystallization (SPC) significantly enlarged the grain size of polycrystalline (poly-) Ge and improved Hall effect hole mobility μ Hall [1]. The TFT using this Ge layer exhibited both high field effect mobility μ FE (170 cm2/Vs) and on/off current ratio I on/I off (~102) without minimizing the channel region (< 1 μm) [2]. The key technology was thinning the 100 nm thick Ge film with a large grain size (3.7 μm) enough to fully deplete the channel. Very recently, we further improved μ Hall by combining the precursor densification method with Sn incorporation in SPC [3,4]. In this study, we fabricate TFT using SPC-GeSn and further improve its performance. Experimental Procedure The a-Ge1−x Sn x (0 ≤ x ≤ 4.5%) precursors were deposited on SiO2 glass substrates using the Knudsen cell of a molecular beam deposition system (base pressure of 5 × 10−7 Pa). The deposition rate of Ge was fixed at 1.0 nm/min at room temperature, whereas that of Sn was adjusted to obtain the targeted GeSn composition. The deposition time was 100 min. The precursors were densified by heating the substrate at 125 °C during deposition [1,3,4]. The samples were then loaded into a conventional tube furnace in an N2 atmosphere annealed at 450 °C for 5 h to induce SPC. The grown layers were analyzed by the electron backscattering diffraction (EBSD) measurement and Hall effect measurement (van der Pauw method).We fabricated accumulation-mode metal source/drain (S/D) p-channel TFTs using SPC-GeSn. Pt and TiN were sequentially formed as a metal S/D and a capping layer, respectively. The PtGe/Ge contacts have low hole barrier height and are suitable for p-channel Ge-TFTs. We used Al/SiO2/Al2O3/SiO2/GeO2 for the gate stack with the equivalent oxide thickness of 17.7 nm. The channel width and length (W/L) were 55 and 5–10 μm, respectively. Here, all process including SPC were conducted below 450 °C. Results and Discussion First, we discuss the crystalline quality and electrical properties of the SPC-GeSn layer. Figure 1(a) shows that the crystal grain size dramatically varies with initial Sn concentration x and maximized at x = 1.6%. These results are roughly consistent with the solid solubility of Sn in Ge of approximately 1%–2%. We thinned x = 1.6% sample with the maximum grain size using chemical-mechanical polishing (CMP) to fabricate TFT for full depletion in poly-GeSn layer. As a reference, we also thinned x = 0% sample. Figure 1(b) shows that hole concentration p is constant in-depth direction although hole mobility μ Hall decreases by thinning. The Sn doping in Ge provides lower p.Finally, we discuss the TFT characteristics fabricated on the thinned SPC samples (55 nm for Ge and 50 nm for GeSn). Figure 2(a) shows that the typical p-channel transistor operation, i.e., drain current I D increases with increasing gate voltage V G. Figure 2(b) shows Sn doping reduces the off current I off significantly under keeping the on current I on. This reflects the electrical properties of the GeSn film obtained by the Hall effect measurement (Fig.1 (b)). Figure 3 summarizes the relationship between μ FE and I on/I off for different channel length (L = 5, 10, and 15 μm) in the same substrate. The experimental data show a clear increase in variability as the channel length decreases, which is in good agreement with the trends of poly-Si-TFTs. This is the first result showing such variability reflecting grain boundary at poly-Ge and GeSn. Furthermore, Sn doping improves I on/I off by roughly an order of magnitude although μ FE is almost identical. These results thus demonstrate the usefulness of Sn doping of polycrystalline Ge in TFT application. The current TFT performance will be further improved by the optimization of Sn concentration, film thickness, and channel dimensions (W/L). Reference [1] K. Toko, K. Moto et al., Sci. Rep. 7, 16981 (2017).[2] K. Moto et al., Appl. Phys. Lett. 114, 212107 (2019).[3] K. Moto et al., Sci. Rep. 8, 14832 (2018).[4] K. Moto et al., Appl. Phys. Lett. 114, 112110 (2019). Figure 1

Ultrahigh-vacuum organic molecular-beam deposition system for in situ growth and characterization.

A compact ultrahigh-vacuum molecular-beam deposition system has been developed for the in situ synthesis of organic thin films and multilayers. The system incorporates all the features (heater, thickness monitor, evaporators) necessary for controlled organic thin-film growth. It can be used independently, or it can be docked to the in situ growth system and transferred to other instruments of the PGM beamline, thus allowing extensive film preparation and characterization. A manipulator dedicated to specimen preparation and organic-film deposition with temperature control between 200 K and ∼800 K has been developed. The design and performance of the system are reported with emphasis on a novel solution of masks developed to achieve position-dependent film deposition. To demonstrate the enhanced capabilities of the PGM beamline in the growth and in the characterization of electronic-structure studies of organic molecular films and their heterostructures through synchrotron-based spectroscopies, this paper presents some preliminary results of a study of Fe-phthalocyanine growth on Si substrates and on in situ prepared La0.67Sr0.33MnO3 buffer layers on SrTiO3 single crystal.

Characterisation of Organic Semiconductor Growth using Real-time Electron Spectroscopy

ABSTRACTAn organic molecular beam deposition system coupled to a soft x-ray excitation source has been developed to monitor the growth of organic semiconductor thin films in-situ and in real-time. Rapid collection of photoelectron spectra has been enabled using a multichannel array detector coupled to a hemispherical analyzer. The organic semiconductor tin phthalocyanine (SnPc) exhibits a Stranski-Krastanov growth mode on a polycrystalline gold substrate where the transition thickness between layered and clustered growth has been determined to be comparable to the thickness of a single molecular layer within which the molecules are standing on edge relative to the substrate plane.

Red organic light-emitting devices with dotted-line doped emitting layers

High efficiency red organic light-emitting devices (OLEDs) with several dotted-line doped layers (DLDLs) were fabricated by using an ultra-high vacuum organic molecular-beam deposition system. The red OLEDs consisted of indium-tin-oxide (ITO)/N, N′-diphenyl-N, N′-bis(1-naphthyl)-(1, 1′-biphenyl)-4, 4′-diamine ( α -NPD): 40 nm/tris(8-hydroxyquinoline)aluminum (Alq 3)+4-(dicyanomethylene)-2-t-butyl-6-(1,1,7,7-tetra-methyljuloldyl-9-enyl)-4H-pyran (DCJTB); 3%wt.: x nm/(Alq 3+DCJTB; 3%wt./ Alq 3) n−1 : ( 30 − x ) nm/ Alq 3: 30 nm/Mg:Ag with n of 2, 4, 6, or 8, and x = 30 / ( 2 n − 1 ) . The luminance yield of the device with 8 DLDLs was 75% higher than that of the device with a common doped layer. This was attributed to more formation of the excitons formed in a wider region resulting from the existence of the DLDLs. The dominant mechanisms of the dopant emission for the devices with DLDLs were described on the basis of the sequential carrier trapping process.

Organic molecular beam deposition system and initial studies of organic layer growth

This work describes an organic molecular beam deposition system with substrate entry/exit chamber, buffer chamber and with the possibility to transfer substrate from a III–V molecular beam deposition system. Flux calibrations of organic molecules and the initial growth of organic layers are described. For this purpose, the molecules 3,4,9,10 perylene tetra carboxylic dianhydride and copper phtalocyanine were used. Layers were grown on oxidized and hydrogen passivated Si(100), Indium tin oxide and glass respectively. The growth was investigated with atomic force microscopy, reflection high energy electron diffraction and ultraviolet photoemission spectroscopy. An investigation with x-ray photoelectron and Raman spectroscopy on the effect of atmospheric exposure is also included, showing little effect of surface pollution when the samples were handled carefully. The initial formation (monolayers) of copper phtalocyanine thin films was studied by ultraviolet photoemission spectroscopy.

Deposition of Thermally Unstable Molecules with the Spray-Jet Technique on Au(111) Surface

A porphyrin derivative (5,15-bis(4-ethynylphenyl)-10,20-bis(3,5-di-tert-butylphenyl)porphyrin: trans-BETBPP) possessing chemically reactive substituents was successfully deposited on an Au(111) surface with a new molecular beam deposition system with use of a spray-jet technique (Spray-jet-MBD) without denaturing the molecules. The deposited molecular overlayers were observed at 77 K under ultrahigh vacuum condition by scanning tunneling microscopy (STM). They form two different overlayer structures: a linear arrangement and a square lattice structure. In these overlayers, some molecules were accidentally moved by STM tip agitation, which indicates that the molecules were not polymerized during the deposition process.

Efficiency Improvement of Cu(InGa)Se2 Thin Film Solar Cells with a High Ga Composition Using Rapid Thermal Annealing

Cu(InGa)Se2 (CIGS) thin films were grown by three-stage growth process using a molecular beam deposition system on a Mo/soda lime glass (SLG) substrate. The raman scattering spectroscopy of as-grown samples with a high Ga composition showed the presence of Cu2-xSe compounds, which are thought to be responsible for the degradation of the open circuit voltage of solar cells. Rapid thermal annealing (RTA) using an infrared lamp in forming gas (N2 95% + H2 5%) ambient under atmospheric pressure was examined in order to remove the Cu2-xSe compounds, and it was found that hydrogen is necessary to remove the Cu2-xSe compounds. The influence of RTA conditions on device performance was investigated. A Cu(In0.4Ga0.6)Se2 (energy band gap: Eg=1.37 eV) solar cell with an efficiency of 11.7%, particularly open circuit voltage (Voc) increased from 640 to 750 mV, was obtained by RTA at 400°C.

A portable ultrahigh vacuum organic molecular beam deposition system for in situ x-ray diffraction measurements

A portable UHV molecular beam deposition system has been developed for synthesis, in situ, and real-time x-ray diffraction measurements of organic thin films, multilayers, and superlattices. The system has been optimized for small size, while still incorporating full features necessary to achieve thin film growth under molecular beam epitaxy (MBE) conditions. It can be used independently for thin film growth, or it can be transported and mounted on standard diffractometers. Additionally, it can be docked to a stationary multipurpose MBE growth system for sample transfer, thus permitting more extensive growth and characterization. The design and performance of this system are reported, with emphasis on modifications required to deposit organic materials. To demonstrate the capabilities for real-time x-ray scattering experiments, some preliminary results of a study of epitaxial growth of 3,4,9,10-perylene-tetracarboxylic dianhydride on Ag(111) substrates are given.

Exciton confinement in organic multiple quantum well structures

High-quality organic multiple quantum well (MQW) structures based on tris(8-hydroxyquinoline) aluminium and 2-(4-biphenylyl)-5-(4-tert-butyl-phenyl)-1,3,4-oxadiazole (PBD) have been prepared by use of a multisource-type high-vacuum organic molecular beam deposition system. Small-angle x-ray diffraction measurements indicate that the MQW structures have a very uniform layered structure throughout the entire stack. Both optical absorption and photoluminescence (PL) measurements reveal the evidence for exciton confinement in the /PBD MQW structures. The thickness-dependent blue shift of the PL peak observed upon decreasing the well width is attributed to the change of exciton energy as a result of quantum confinement. Theoretical calculations have been performed and the results agree with the experimental results. Light-emitting diodes (LEDs) based on the /PBD MQW structure with greater efficiency, a narrower electroluminescence spectrum and less of a blue shift than the single heterostructure LEDs have also been fabricated.

Lithium–aluminum contacts for organic light-emitting devices

Organic light-emitting devices have been prepared with multilayer Al–Li–Al cathodes in an ultrahigh vacuum molecular beam deposition system. The optimum device characteristics are obtained when there is a single Al layer separating the Li layer from the organic materials. Ab initio molecular dynamics calculations of the Al–Li interaction clarify the role of Al as a blocking layer to Li diffusion into the organic films as well as the behavior of the device when the thickness of this Al interfacial layer is changed.

Van der Waals Epitaxy of GaSe on GaAs(111)

ABSTRACTAlthough heteroepitaxy of lattice-matched and lattice-mismatched materials leading to artificially structured materials has resulted in impressive performance in various electronics devices, material combinations are usually limited by lattice matching constraints. A new concept for fabricating material systems using the atomically abrupt and low dimensional nature of layered materials, called van der Waals epitaxy (VDWE), has been developed. GaSe (Eg = 2.1 eV) has been deposited on the three dimensional surface of GaAs (111) using a molecular beam deposition system. GaSe was evaporated from a single Knudsen source, impinging on a heated substrate. Even with a lattice mismatch of 6% between the substrate and the growing film, good quality single crystal films were grown as determined by RHEED. The films have further been analyzed using a complementary combination of XPS and X-ray reflectivity.

Molecular beam deposition of Dy1Ba2Cu3O7−δ(001) high‐temperature superconductor thin films

Epitaxial Dy1Ba2Cu3O7−δ(001) high‐temperature superconductor thin films on LaAlO3 substrates have been prepared by coevaporation of Dy, BaF2, and Cu and postannealing. The vapors in desired ratio were evaporated from effusion cells in a miniature molecular beam deposition system. The films show critical transition temperature (Tc,0) of 89.5±0.5 K. During process development it was found that single phase Dy1+xBa2−xCu3Oy(001) thin films can be grown (x=0 to 0.3). Their electrical properties were useful in calibration of quartz crystal thin film thickness monitor (FTM) for determination of relationships between the actual vapor arrival rate (flux) and FTM reading.