Epitaxial Perfection Research Articles

Targeted Dehydration As a Route to Site-Selective Atomic Layer Deposition at TiO2 Defects

Crystallographic perfection in epitaxial thin film heterostructures can eliminate interface defects that dilute unique properties and reduce device performance. However, the requirement for epitaxial perfection greatly limits the selection of material candidates and deposition processes. Using selective interface reactions (SIRs), an atomic layer deposition (ALD)-based technique, we target transformation of undesirable defect sites at imperfect surfaces. Defects on the TiO2 surface affect the electronic properties, interfaces, and performance of optoelectronic devices that leverage TiO2 interfaces. We present first principles calculations to predict the difference in hydration/hydroxylation of pristine TiO2 terraces and minority atomic configurations (i.e. “defects”) including step edges and oxygen vacancies. We investigate hydroxylation differences through temperature dependent scanning tunneling microscopy (STM), and ultimately exploit these differences to selectively react ALD precursors at surface defect sites.

Epitaxial Designs for Maximizing Efficiency in Resonant Tunneling Diode Based Terahertz Emitters

We discuss the modelling of high current density InGaAs/AlAs/InP resonant tunneling diodes to maximize their efficiency as THz emitters. A figure of merit which contributes to the wall plug efficiency, the intrinsic resonator efficiency, is used for the development of epitaxial designs. With the contribution of key parameters identified, we analyze the limitations of accumulated stress to assess the manufacturability of such designs. Optimal epitaxial designs are revealed, utilizing thin barriers, with a wide and shallow quantum well that satisfies the strained layer epitaxy constraint. We then assess the advantages to epitaxial perfection and electrical characteristics provided by devices with a narrow InAs sub-well inside a lattice-matched InGaAs alloy. These new structures will assist in the realization of the next-generation submillimeter emitters.

Functional oxide interfaces

Abstract

Depth profile of donor–acceptor pair transition revealing its effect on the efficiency of green LEDs

GaInN/GaN light emitting diodes are the primary choice for efficient green light emitters, yet their performance limiting factors have yet to be identified. Here we perform a low-temperature luminescence study in depth-resolved cathodoluminescence. A series of LEDs with a combination of blue and green light emitting quantum wells exhibiting low and high electroluminescence output power were compared. In all samples, a band of donor-acceptor pair recombination was identified at 77 K. In the samples with lowest performance, such transitions located within the active region of the quantum wells had a particular strong contribution to the spectrum. In LEDs of higher performance, such luminescence was substantially suppressed within those layers. We argue that the higher performance of LEDs without such donor acceptor transition bands may be associated with the absence of corresponding dopant impurities and indicate a higher epitaxial perfection within the active quantum well region.

Discrete Steps in the Capacitance-Voltage Characteristics of GaInN/GaN Light Emitting Diode Structures

ABSTRACTA detailed modeling of the electronic bandstructure of GaInN alloys and GaInN/GaN heterostructures typically used for high efficiency light emitting diodes is of high relevance for future improvements. Here we are exploring opportunities to accurately quantify the carrier dynamics under forward and reverse voltage bias. In GaInN/GaN LED-type heterostructures we observe distinct steps in the junction capacitance as a function of bias voltage within the depletion regime. Up to three individual steps can be identified that correspond to alternating ranges of capacitive and resistive impedances. Our analysis suggests that we are quantitatively monitoring the electron concentration in each individual quantum well. The pronounced clarity of the data reveals a high level of epitaxial perfection and spatial homogeneity across the entire area of the junction.

Optical Micro-Characterization of Complex GaN Structures

Various techniques of pattern-controlled epitaxy like epitaxial lateral overgrowth (ELO) and lateral seeding epitaxy have successfully been used to overcome the lattice mismatch problem, however, resulting in a complex system of self-organized growth domains. For a detailed understanding a correlation of the structural, electronic, and optical properties on a micro-scale is mandatory. Scanning cathodoluminescence (CL) microscopy provides a powerful tool compiling low temperatures, spatial resolution Δx < 45 nm and high spectral resolution. ELO GaN on stripe patterned SiO 2 and W masks were characterized by CL microscopy, directly visualizing the formation of different growth domains: 1. the coherently grown regions between the stripes; 2. the ELO areas coalescing on the center of the masks and showing a blue shifted and strongly broadened CL where μ-Raman evidences the reduction of local strain and probes the free carrier concentration which is dramatically changed over the masks; 3. the coalescence regions with poor epitaxial perfection including voids and strong incorporation of impurities; 4. the transition regions between 1. and 2. at the very edges of the masks characterized by high defect density and strongly blue shifted CL. Plan view CL directly images the local areas of improved crystal perfection only involving part of the total ELO regions. The formation of specific micro-domains in 5 μm thick crack-free Al 0.19 Ga 0.81 N, exhibiting different Al concentrations, during the initial stage of MOVPE grown on patterned GaN substrates and the transition to finally homogeneous growth is directly visualized by cross-sectional CL.

Optimization of the growth of epitaxial SrBi 2Ta 2O 9 thin films by pulsed laser deposition

The growth of epitaxial (001)-oriented SrBi 2Ta 2O 9 thin films by pulsed laser deposition (PLD) has been investigated. The effects of target composition, substrate temperature, oxidant pressure, laser pulse rate, laser fluence, and sample cooling have been studied. Variation of deposition parameters significantly affects the phase purity and epitaxial perfection of these ferroelectrics, and individual and combinatorial effects of the aforementioned growth variables as related to the adsorption-controlled growth regime are discussed. X-Ray diffraction and Rutherford backscattering spectrometry (χ min=12%) analyses indicate that the (001)-oriented epitaxial films grown under the optimized conditions identified in this study have the highest structural perfection reported to date.

X-ray photoelectron and Auger electron diffraction probing of Ge heteroepitaxy on Si (001) 2×1

Epitaxial molecular beam epitaxy growth of nanometric Ge layers on Si (001) 2×1 has been investigated, in situ, by x-ray Si 2p photoelectron diffraction and Auger Ge LMM electron diffraction which consist essentially in preferential scattering of electrons in the direction of interatomic axes. Particular attention was paid to measuring the contrasts of this anisotropic emission in the (11̄0) plane as a function of deposition parameters. It can thus be determined how crystalline material quality and epitaxial perfection are affected by the residual pressure below 5×10−8 mbar, the substrate temperature decrease to room temperature, the deposition rate, and the Ge overlayer thickness.

Influence of the BaF 2 substrate preparation on the structural perfection of epitaxially grown IV–VI compounds

The influence of preheating time and temperature of (111) oriented BaF 2 substrates on the crystalline epitaxial perfection of IV–VI film growth is investigated by in situ reflection high energy electron diffraction, and after growth, by X-ray diffraction and electron channelling patterns. A preheating temperature of 500°C for 10 min yields excellent epitaxial film relation ship and perfect overgrowth.

Reflection high energy electron diffraction study of CoSi2/Si multilayer structures

Abstract The epitaxial perfection and microstructure of Si/CoSi2/Si heterostructures have been examined by various techniques, such as transmission electron microscopy and Rutherford backscattering spectroscopy. In this paper, an in situ reflection high energy electron diffraction (RHEED) pattern investigation of Si/CoSi2/Si double heterostructures grown on Si(111) by solid phase epitaxy is presented. During the thermal annealing of a Co/Si film deposited in the stoichiometric ratio of CoSi2 at room temperature, RHEED patterns show directly the formation stages of CoSi2 as well as the crystallinity of the films. A very sharp (1 × 1) RHEED pattern of a CoSi2 single layer grown on Si(111) has been obtained in our experiment. Ultrathin silicon layers grown on top of CoSi2 showed a combination of the Si(7 × 7) pattern and the CoSi2(1 × 1) pattern. From this study, additional information on the process of formation of the Si/CoSi2/Si multilayer structure is obtained.

Growth of single crystal CoSi 2 layers at room temperature

We will describe recent results concerning the growth of high quality single crystal CoSi 2 thin layers on Si(111) by MBE. The use of unusually low MBE temperature (near room temperature) and the use of template layers with low defect densities are the keys in the improved epitaxial perfection. Attention is paid to the relationship between the surface step density of the substrate and the dislocation density in the silicide thin film. Procedures which reduce the dislocation density will be introduced. The change of the silicide structure upon annealing will be mentioned.

Schottky-barrier heights of single-crystal NiSi2 on Si(111): The effect of a surface p-n junction.

Current-voltage, capacitance-voltage, and activation-energy measurements obtained for epitaxial nickel silicides grown on Si(111) have exposed the importance of the temperature used to flash-evaporate oxide off the surface prior to metal deposition. Near-ideal behavior is found for Schottky barriers grown on substrates cleaned at \ensuremath{\sim}820 \ifmmode^\circ\else\textdegree\fi{}C in ultrahigh vacuum. The Fermi-level positions at the interfaces of single-crystal type-A and type-B ${\mathrm{NiSi}}_{2}$ are shown to differ by greater than 100 meV. Transmission electron microscopy demonstrated the epitaxial perfection of these silicide layers. At a cleaning temperature of 1050 \ifmmode^\circ\else\textdegree\fi{}C, the apparent Schottky-barrier heights increased substantially for substrates with a doping concentration of ${N}_{D}$ less than approximately ${10}^{15}$ ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}3}$. This increase is due to the conversion of the n-type semiconductor surface region to p type during the 1050 \ifmmode^\circ\else\textdegree\fi{}C anneal. The presence of this p-n junction results in a high apparent Schottky-barrier height (\ensuremath{\gtrsim}0.75 eV) which no longer bears an immediate relationship to the interface Fermi-level position. Recent discrepancies reported by different groups concerning the barrier heights of ${\mathrm{NiSi}}_{2}$ on Si(111) are attributed to this effect.

Electrical Properties of Epitaxial Silicide-Silicon Interfaces

ABSTRACTThe Schottky barrier heights of single crystal NiSi2 layers on Si(111) have been studied by current-voltage, capacitance-voltage and activation energy techniques. Near ideal behavior is found for Schottky barriers grown on substrates cleaned at ∼820°C in ultrahigh vacuum. The Fermi level positions at the interfaces of single crystal type A and type B NiSi2 are shown to differ by ∼0.14 eV. Transmission electron microscopy demonstrated the epitaxial perfection of these suicide layers. At a cleaning temperature of 1050° C, the near surface region of lightly doped n-type Si was converted to p-type. The presence of a p-n junction was directly revealed by spreading resistance measurements and resulted in a high apparent Schottky barrier height (≥0.75 eV) which no longer bears immediate relationship to the interface Fermi level position.

The effects of nucleation and growth on epitaxy in the CoSi 2/Si system

The epitaxial perfection and microstructure in thin CoSi 2(111)(and NiSi 2) films on silicon has been examined using transmission electron microscopy, including high resolution cross section techniques. We find that, under ultrahigh vacuum preparation conditions only, genuine single-crystal epitaxy can occur for both reacted and codeposited CoSi 2 films. A unique interfacial defect structure is associated with this epitaxy, in which the silicide films are rotated through 180° about (111) with respect to the silicon substrates. Under less than perfect clean conditions epitaxial perfection is never obtained for CoSi 2. Moreover such perfection has never been attained under any conditions for NiSi 2 on Si(111). By studying the evolution of epitaxy and defect structure in thin and thick films we have proposed a model to explain the unique quality of ultrahigh vacuum CoSi 2 films. The model invokes a defect pinning mechanism to explain the dominance of the 180°-rotated epitaxy during silicide growth.

The Effects of Nucleation and Growth on Epitaxy in the CoSi2/Si System

The epitaxial perfection and microstructure in thin CoSi2 (111) (and NiSi2) films on silicon has been examined using transmission electron microscopy, including high resolution cross section techniques. We find that, under ultrahigh vacuum preparation conditions only, genuine single-crystal epitaxy can occur for bothreacted and codeposited CoSi2 films. A unique interfacial defect structure is associated with this epitaxy, in which the silicide films are rotated through 180° about (111) with respect to the silicon substrates. Under less than perfectly clean conditions epitaxial perfection is never obtained for CoSi2. Moreover such perfection has never been attained under any conditions for NiSi2 on Si(111). By studying the evolution of epitaxy and defect structure in thin and thick films we have proposed a model to explain the unique quality of ultrahigh vacuum CoSi2 films. The model invokes a defect pinning mechanism to explain the dominance of the 180°-rotated epitaxy during silicide growth.

Silicon‐on‐Sapphire Crystalline Perfection and MOS Transistor Mobility

Experimental data show that there is a correlation between the crystalline perfection of a (100) silicon‐on‐sapphire epitaxial film and the magnitude and direction of misorientation of the sapphire substrate from the {11̅02} normal. The perfection is ascertained by measuring the half‐width of Si {400} θ:2θ x‐ray diffraction profiles. Data obtained from SOS/MOS transistors show a direct relationship between epitaxial perfection and FET mobility. Other factors which influence the perfection of the SOS film are growth temperature and film thickness.

The epitaxial growth of sphalerite and wurzite ZnSe on a (111)Ge surface

Epitaxial ZnSe/(111)Ge has been obtained for substrate temperatures from about 180°C to beyond 450°C by the sublimation of a ZnSe source in a vacuum of 1×10−6 Torr during growth. The predominantly sphaleriteor wurzite-structure epitaxial films obtained were found to be dependent on the substrate temperature and their epitaxial perfection to the deposition rate.