ARCHIVESOFMETALLURGYANDMATERIALS Volume 60 2015 Issue 1 DOI: 10.1515/amm-2015-0059 X. LIU∗, D.R. QUEEN∗∗, T.H. METCALF∗, J.E. KAREL∗∗∗, F. HELLMAN∗∗∗;∗∗∗∗ AMORPHOUS DIELECTRIC THIN FILMS WITH EXTREMELY LOW MECHANICAL LOSS AMORFICZNE CIENKIE WARSTWY DIELEKTRYCZNE Z BARDZO MAŁĄ WIELKOŚCIĄ STRAT MECHANICZNYCH The ubiquitous low-energy excitations are one of the universal phenomena of amorphous solids. These excitations dominate the acoustic, dielectric, and thermal properties of structurally disordered solids. One exception has been a type of hydrogenated amorphous silicon (a-Si:H) with 1 at.% H. Using low temperature elastic and thermal measurements of electron-beam evap- orated amorphous silicon (a-Si), we show that TLS can be eliminated in this system as the films become denser and more structurally ordered under certain deposition conditions. Our results demonstrate that TLS are not intrinsic to the glassy state but instead reside in low density regions of the amorphous network. This work obviates the role hydrogen was previously thought to play in removing TLS in a-Si:H and favors an ideal four-fold covalently bonded amorphous structure as the cause for the disappearance of TLS. Our result supports the notion that a-Si can be made a “perfect glass” with “crystal-like” properties, thus offering an encouraging opportunity to use it as a simple crystal dielectric alternative in applications, such as in modern quantum devices where TLS are the source of dissipation, decoherence and 1/f noise. Keywords: Internal friction, amorphous silicon, elastic modulus, speed of sound, tunneling systems Wszechobecne niskoenergetyczne wzbudzenia są jednym z powszechnych zjawisk w amorficznych ciałach stałych. Wzbu- dzenia te dominują akustyczne, dielektryczne i termiczne właściwości strukturalnie nieuporządkowanych ciał stałych. Wyjątkiem jest rodzaj uwodornionego amorficznego krzemu (α-Si:H) o zawartości 1 at.% H. Na podstawie niskotemperaturowych badań własności sprężystych i termicznych krzemu amorficznego (α-Si) naparowanego wiązką elektronów wykazaliśmy, że w pewnych warunkach osadzania można wyeliminować TLS w tym układzie tak, że warstwy stają się gęstsze i strukturalnie bardziej upo- rządkowane. Uzyskane przez nas wyniki wskazują, że TLS nie są nieodłączną cechą stanu szklistego, ale lokują się w regionach o niskim zagęszczeniu sieci amorficznej. Praca niniejsza wyjaśnia, że wodór nie pełni roli w usuwaniu TLS w α-Si:H, jak dotąd sądzono, i wskazuje na idealną czterokrotnie kowalencyjnie związaną amorficzną strukturę jako przyczynę znikania TLS. Nasz wynik potwierdza koncepcję, że z α-Si można wytworzyć ”doskonałe szkło” o ”podobnych do krystalicznych” właściwościach, oferując w ten sposób zachęcającą możliwość wykorzystania go alternatywnie jako prosty krystaliczny dielektryk w takich aplikacjach jak w nowoczesne urządzenia kwantowe, gdzie TLS są źródłem dyssypacji dekoherencji i szumu 1/f. 1. Introduction of two-level tunneling systems (TLS) provides a phenomeno- logical description for the low energy excitations by assuming that a small number of atoms or groups of atoms with energet- Amorphous solids exhibit many universal phenomena that ically similar configurations can tunnel between the minima are absent in crystalline solids. One such example is the low of neighboring double-well potentials in amorphous energy energy excitations [1], where, at low temperatures, only the landscape [3,4]. But the TLS model does not provide a mi- 3 propagating phonons that give rise to the T Debye specific croscopic description of the tunneling entities and the cause heat are expected. While in crystals defects usually have char- of universality. The nature of the low energy excitations of acteristic energies, the low energy excitations have a broad and amorphous solids remains one of the unsolved mysteries in almost energy independent distribution. They are the source of condensed matter physics. anomalous thermal, elastic and dielectric properties of amor- Amorphous Si (a-Si) has been a model system for cova- phous solids [2], such as: a linear T dependent specific heat, lent amorphous solids [5]. In fact, the first amorphous solids 2 T thermal conductivity below 1 K, and an almost T inde- without TLS were found 15 years ago in a type of hydro- pendent plateau in internal friction at a few degrees K. These genated amorphous silicon (a-Si:H) thin films containing 1 properties are thought to be universal due to the quantitative at.% H prepared by hot-wire chemical-vapor deposition [6]. similarities for a wide variety of amorphous solids. The model ∗ NAVAL RESEARCH LABORATORY, CODE 7130, WASHINGTON, DC 20375, USA ∗∗ NRC POSTDOCTORAL ASSOCIATE, CODE 7130, WASHINGTON, DC 20375, USA ∗∗∗ DEPARTMENT OF MATERIALS SCIENCE AND ENGINEERING, UNIVERSITY OF CALIFORNIA, BERKELEY, BERKELEY, CALIFORNIA 94720, USA ∗∗∗∗ DEPARTMENT OF PHYSICS, UNIVERSITY OF CALIFORNIA, BERKELEY, BERKELEY, CALIFORNIA 94720, USA Brought to you by | University of California - Berkeley Authenticated Download Date | 7/7/16 9:22 PM 360 Subsequent research was interpreted as showing that meticu- to extract elastic properties of thin films. The finite-element lous incorporation of a small amount of H eliminated TLS by simulated mode shape of the DPO is illustrated in the in- passivating dangling bonds [7]. Now, we show that a-Si thin set of Fig. 1(a). The DPOs were coupled to two electrodes films prepared by electron-beam (e-beam) evaporation without behinds the wings. They were driven and detected electro- ◦ H but at a growth temperature (Tsub) of 400 C, comparable to statically. The resonance was phase-locked in a self-exciting that of a-Si:H with 1 at.% H, also contain few TLS, despite loop, from which the resonance frequency is determined by still containing significant numbers of dangling bonds. This a high resolution frequency counter. The internal friction Q−1 result supports Phillips original suggestion that TLS are un- was calculated from the amplitude-over-time free ring-down likely to form in overconstrained amorphous systems [4] and measurement, regarded as the classical “wave-height” methods suggests that the structure of a-Si is heterogeneous with TLS [13]. occurring in the low density regions [8]. This finding gives Addition of a film on the neck area, see the insert of new insight into the microscopic origin of TLS. It also comes Fig. 1(a), leads to a measurable shift of a DPO’s resonance −1 at a time when TLS in dielectric amorphous thin films have frequency, from fsub to fosc, and the internal friction, from Qsub −1 become a bottleneck in an array of cutting edge technologies to Qosc. From the shift, the shear modulus, Gfilm, the internal as they cause both elastic and dielectric losses. TLS limit the friction, Q−1 , and the relative change of speed of sound, film elastic quality factor in both nanomechanical and quantum ∆ν νfilm−νfilm(T0) ν = ν (T ) of the film can be calculated through, resonators [9], and are the dominant decoherence source in 0 film film 0 superconducting quantum bits [10]. f − f 3G t osc sub = film film ; (1) fsub 2Gsubtsub 2. Experimental and results −1 Gsubtsub −1 −1 −1 Qfilm = Qosc − Qsub + Qosc; (2) 3Gfilmtfilm ! " ! ! # ! The a-Si thin films studied in this work were grown by ∆ν Gsubtsub ∆f ∆ f ∆ν ◦ = − + ; (3) e-beam evaporation at Tsub =45, 200, 300, 350, and 400 C. ν0 3Gfilmtfilm f0 f0 ν0 All films were approximately 300 nm thick, measured by an film osc sub sub Alpha-Step IQ profilometer. Companion a-Si films deposited where t; G, and ∆ f = f (T)− f (T0) are thicknesses, shear mod- f0 f (T0) at the same time or in identical conditions were examined by uli, and relative change of resonant frequency, respectively of Raman spectroscopy, electron and X-ray diffraction, and trans- the oscillators, the substrates, and the films. T0 is a reference mission electron microscopy [8]. Mass density was measured temperature, usually taken as the lowest temperature point of with Rutherford backscattering: ρ =2.02, 2.14, 2.18 g/cm3 the experiment. The shear modulus of silicon along the neck ◦ for films with Tsub =45, 200, and 350 C, respectively, and of DPOs (<110>) is Gsub = 62 GPa, and tsub = 300 µm. 3 ◦ ρ =2.17 and 2.22 g/cm for the two films with Tsub =400 C. The representative temperature dependent resonant fre- All films are found to be fully amorphous and exhibit in- quency of two DPOs before and after e-beam a-Si evapora- ◦ creased structural order and mass density with the increase of tion at Tsub =45 and 400 C is shown in Figs. 1(a) and (b), Tsub. We measured the complex elastic properties, including respectively. The frequency shift is proportional to Gfilm. We the real (shear modulus, Gfilm, and relative change of speed of calculate the average Gfilm by subtracting resonant frequen- ∆ν −1 sound, ν ) and the imaginary (internal friction, Q ) parts, cy with films and its background at low temperatures (below 0 −1 of e-beam a-Si films from 0.4 K to room temperature by 30 K) where Qosc the lowest and f is relatively temperature using the double-paddle oscillator (DPO) technique [11,12]. independent. These low temperature Gfilm values are about Due to its excellent vibration isolation, the low background 5% higher than those measured at room temperature. The low Q−1 (∼1×10−8) of the torsional vibration mode (∼5500 kHz) temperature data for all the films studied in this work are used in this work ensures high sensitivity of the technique shown in Fig. 1(c). Gfilm increases with Tsub, but even for the Fig. 1. The resonant frequency of DPOs before and after e-beam deposition of a-Si at 45◦C (a) and 400◦C (b). The inset in (a) shows the finite-element simulated torsional mode used in this work. Films are deposited in the gray-colored neck area where most of the shear strain is concentrated.