Rare Earth Oxides Material Technology Translucent
Translucent Inc. Rare Earth Oxides Material Technology Key Concepts Discussed: • General properties of rare-‐earth oxides • Epitaxial growth of rare-‐earth oxides • Application of heterostructures of rare-‐earth oxide with semiconductors Page 1 952 Commercial St., Palo Alto, CA 94303 • For sales, contact Translucent Inc., Tel. (+1) 650 213 9311, Fax (+1) 650 213 9511 or [email protected] Rare Earth Oxides Material Technology Translucent
Introduction / background Translucent’s rare – earth oxide (REO) epitaxial layers are designed to be high quality templates for III-‐V and III-‐N devices on silicon substrates. These crystalline REOs provide an excellent lattice match for GaN and for Silicon with a flexible design space such that pre-‐strained layers can compensate for various thermal processing induced stresses. REOs have optical, electrical, chemical and thermal properties that allow incorporation into existing manufacturing processes for high performance, high breakdown transistors and optically active devices such as sensors and LEDs. . Figure 1. Illustration of some application of rare-‐earth oxides in photonics and electronics Process Translucent’s proprietary Solid Source Epitaxial (SSE) process provides growth of quality layers of oxide on silicon substrates at over 300nm per hour. The process uses effusion cells for the metal source and molecular oxygen. SSE delivers excellent repeatability and controllability with automated in-‐situ analysis methods. Additionally, the photoluminescence of the REOs is used to easily evaluate any drift in process parameters. These measures ensure maximal repeatability of the epitaxial process for fabrication of templates for epitaxy of advanced semiconductor structures. Translucent’s SSE equipment provides several types of REOs on 100, 150, 200mm wafers with highly uniform layers. Using variations of substrate temperature, metal and oxygen flux, engineering of interface between the oxide layer and silicon substrate can range from very abrupt (Figure 2a) to one containing an amorphous silicon dioxide – like interlayer. The latter allows stress relief due to low viscosity of the amorphous silicon dioxide at elevated temperatures typical for further processing in typical III-‐N MOCVD processes. Translucent’s SSE approach makes it possible to form additional interfacial silicon dioxide-‐like inlayers in the single crystal oxide layer (Figure 2b). Page 2 952 Commercial St., Palo Alto, CA 94303 • For sales, contact Translucent Inc., Tel. (+1) 650 213 9311, Fax (+1) 650 213 9511 or [email protected] Rare Earth Oxides Material Technology Translucent
(a) (b) Figure 2. TEM image of abrupt interface between a rare-‐earth oxide layer and a silicon (111) substrate (a), TEM cross-‐section image of a structure with amorphous silicon dioxide-‐like interlayers (b) Figure 3. X-‐ray diffraction measurements of ternary erbium-‐neodymium oxide with different neodymium concentration and comparison with silicon-‐germanium alloy with different germanium concentration Ternary rare-‐earth oxides open the possibility for engineering the lattice structure of a buffer layer which could be used for the growth of germanium-‐silicon layers on silicon substrates. In the case of stable cubic oxide structures, we can apply Vegard’s law to predict the lattice constant of the ternary oxide. For example, the shift Page 3 952 Commercial St., Palo Alto, CA 94303 • For sales, contact Translucent Inc., Tel. (+1) 650 213 9311, Fax (+1) 650 213 9511 or [email protected] Translucent
Rare Earth Oxides Material Technology of the (Er1-‐xNdx)2O3 (222) X-‐ray diffraction peak to lower θ with increasing neodymium content reveals a very good fit with Vegard’s law (see Figure 3). Translucent has shown that these ternary oxides are suitable as a buffer layer for epitaxial growth of silicon germanium layers. Material Properties Optical Rare-‐earth elements are well known additives to phosphors with the characteristic 4F electron shell responsible for optical properties. Likewise when incorporated in high quality epitaxial oxide of the from RE2O3 they similar optical transitions are observed. The efficiency of the photoluminescence is defined by the maximum phonon energy in the host material and quenching processes which is defined by crystal quality of the host. Therefore, photoluminescence can be used for evaluation of crystal structure of rare-‐earth oxides which are used as the host (Er2O3 or Gd2O3) for phosphors like Nd2O3. Figure 3. Photoluminescence spectra of ternary erbium-‐neodymium oxide with different neodymium concentration Additionally, concentration of the phosphor material ions in the host matrix dictates photoluminescence intensity. High concentration of the ions reduces the average spacing between ions, and hence an increase in the resonant energy transfers between adjacent ions. The energy transfer is a relatively fast process, so that as the ion density increases, excited states can quickly migrate to quenching sites, where they relax to ground state via non-‐radiative decay paths. Conversely, as the ion density is reduced, for a given thickness of material, the optical absorption is reduced as shown in Figure 3. Page 4 952 Commercial St., Palo Alto, CA 94303 • For sales, contact Translucent Inc., Tel. (+1) 650 213 9311, Fax (+1) 650 213 9511 or [email protected] Rare Earth Oxides Material Technology Translucent
Thermal expansion and mechanical strains The thermal expansion coefficient of the rare earth oxides is higher than that of silicon. Asymmetric X-‐ray diffraction results in temperature interval between 25 and 1000°C derived thermal expansion coefficient of gadolinium oxide and erbium oxide was respectively 7.77×10-‐6 K-‐1 and 7.25×10-‐6 K-‐1 (silicon thermal expansion coefficient 3.9x10-‐6 K-‐1). In combination with very strong crystallographic coupling between silicon substrate and the oxide layer, the mismatch in thermal coefficient results stronger thermal expansion of the oxide crystal lattice in out-‐of plane direction which you can see in the shift of gadolinium oxide X-‐ray diffraction peak relative to silicon ( Figure 5.) The cubic bixbyite structure of rare-‐earth oxides is based on calcium flouride fcc lattice with one ¼ of the oxygen atoms removed. This results in compressively pre-‐strained wafers after cooling down as shown in Figure 6. (a) (b) Figure 4. X-‐ray diffraction measurements of Gd2O3 on Si(111) during heating at different temperatures (a), bow measurement results of structure with 300 nm thick rare-‐earth oxide on silicon (111) (b) Thermal conductivity The thermal conductivity of rare-‐earth oxides was measured using reflectance and 3ω methods. The results show 6.5 W/(m・K) for erbium oxide and 6.2 W/(m・K) for gadolinium oxide. These films are approximately five times higher than thermal conductivity of silicon dioxide films. Higher thermal conductivity of the rare earth oxides compared to silicon dioxide makes them a superior candidate as a buried insulator layer for semiconductor on insulator structures. Electrical properties Rare-‐earth oxides are well known for their excellent dielectric characteristics. They exhibit moderate dielectric permittivity between 12 and 16 as well as a band gap of approximately 5.6 eV. The symmetrical band off-‐set of more than 1 eV when compared to wide band-‐gap semiconductors like SiC and GaN delivers low tunneling current and makes the oxides a reliable gate dielectric material for transistors. Additionally, Translucent’s crystalline REOs show typical breakdown electric fields of greater than 4 MV/cm . Transistors can be designed to take advantage of the REO as an embedded dielectric buffer layer to improve Page 5 952 Commercial St., Palo Alto, CA 94303 • For sales, contact Translucent Inc., Tel. (+1) 650 213 9311, Fax (+1) 650 213 9511 or [email protected] Rare Earth Oxides Material Technology Translucent
vertical breakdown targets. Using the REO this way can also reduce cost by reducing the semiconductor layer thickness (e.g. GaN-‐on-‐Si) and thereby process and cycle time, without compromising transistor performance. Template technology Growth of a Si/REO/Si structure opens-‐up a path for realization of a non-‐bonded silicon-‐on-‐insulator (SOI) structure grown in a one step process. The performance of microelectronic devices fabricated on SOI wafers is superior to those on bulk silicon due to the elimination of parasitic capacitances, current leakage paths to the substrate and prevention of latch up. Since quality of the silicon layer improves with increasing thickness, growth of a thick layer can be performed in two steps by initial epitaxy of Si template by e-‐beam evaporation (Figure 5a) and subsequent growth of thick (several micrometers) Si by standard industrial CVD techniques (a) (b) Figure 5. Bright field TEM cross-‐section image of an epitaxial silicon-‐on-‐insulator structure on Si(111) substrate (a), dark field TEM cross-‐section image of a distributed Bragg’s reflector based on Si/REO heterostructure with GaN grown by MOCVD (b) Rare-‐earth oxide – silicon heterostructures have the advantage of a large difference in refractive index of the materials – this is excellent for the fabrication of distributed Bragg reflectors (DBR). For example, the refractive index ratio of Si and Gd2O3 is larger than 2 for GaN based light emitting devices operating with the characteristic 450 nm wavelength. Calculations for a three period reflector indicate a reflectivity of higher than 80% could be achieved by adjusting the thickness of the layers. These calculations were verified with growth of reflector structures (three periods 1/4λ oxide, 3/4λ silicon) which demonstrated 82% reflectivity for a wavelength of λ=450nm. As can be seen from XRTEM pictures (Figure 5b), the layers have very sharp interfaces without any trace of chemical reaction between the oxide and silicon. Page 6 952 Commercial St., Palo Alto, CA 94303 • For sales, contact Translucent Inc., Tel. (+1) 650 213 9311, Fax (+1) 650 213 9511 or [email protected] Rare Earth Oxides Material Technology Translucent
(a) (b) Figure 6. RHEED pattern in <110> azimuth (a) and AFM 1μm x 1μm surface area scan (b) of Si0.6Ge0.4 grown on (Er0.1Nd0.9) 2O3 layer on Si(111). A ternary oxide of this type can be used as a template for growth of an active semiconductor layer with a high charge carrier mobility. For example, the lattice constant of ternary erbium neodymium oxide with neodymium content of 90% is twice the lattice constant of silicon-‐germanium with germanium content of 40%. A 100 nm thick SiGe layer grown on the top of an 100 nm thick (Er0.1Nd0.9) 2O3 shows SiGe’s distinctive 5x5 reconstructed surface (Figure 6a). The good surface morphology is also confirmed by AFM measurement -‐ root mean square of the surface roughness is only 1.7 nm for 1μm x 1μm surface area. The surface is smooth with very few defects (Figure 6b). For the MOCVD growth of GaN a template consisting of a single layer of REO on silicon has been shown to be very successful. The templates produced in wafer sizes to 200mm can be used with either a GaN or AlN first buffer layer thereby affording the MOCVD engineer full design compatibility for the III-‐N process. See the GaN on Silicon whitepaper for more details Conclusion Translucent’s advanced Rare Earth Oxide epitaxial layers can be applied to many kinds of devices based on multilayer heterostructures. Engineered REOs make lattice engineering of virtual substrates possible with design of high charge carrier mobility, wide band-‐gap semiconductors for advanced electronic and photonic devices. Additionally, REO’s excellent dielectric properties enables engineering of dielectric buffer layer for high-‐
breakdown, high-‐power devices at lower costs and new gate dielectric layers for metal-‐oxide-‐semiconductor structures. Page 7 952 Commercial St., Palo Alto, CA 94303 • For sales, contact Translucent Inc., Tel. (+1) 650 213 9311, Fax (+1) 650 213 9511 or [email protected]