Substrate Orientation Research Articles

Exploring the impact of sheet thickness scaling on Nanosheet FET gate electrostatics using k.p based simulations

This work explores the impact of sheet thickness scaling on gate electrostatics of NsFETs using k.p simulation. It is shown that thin channel NsFETs exhibit higher threshold voltage irrespective of the substrate orientation and channel material. However, the influence of geometrical confinement varies among different substrate orientations and channel materials due to variations in carrier quantization mass. It is also shown that thin channel NsFETs deliver higher inversion charges at equivalent gate over-drive voltages, thereby offering enhanced gate electrostatics. However, the advantage of gate electrostatics in thin channel NsFETs is limited by quantum capacitance. Optimizing the sub-band structure through strategic selection of substrate orientations and channel materials is essential to regulate quantum capacitance and to fully exploit the benefits of sheet thickness scaling in NsFETs.

Effects of the grain size and orientation of Cu on the formation and growth behavior of intermetallic compounds in Sn-Ag-Cu solder joints

The rapid development of semiconductor packaging technology has accelerated the applications of flip-chip bonding technology using fine-pitch micro bumps and Cu pillar bumps. These micro-bump systems are composed of metallic Cu and Sn. The properties of the Cu- and Sn-based bonding materials applied in these technologies have a significant effect on the mechanical properties and long-term reliability of micro-joints. This study focuses on the effects of the grain size and orientation of the Cu substrate on the formation and growth of intermetallic compounds (IMCs) at the Sn/Cu interface. The Cu substrate was annealed from 200 to 1000 °C to alter its grain size, which was measured via electron backscatter diffraction analysis. Sn-3.0Ag-0.5Cu (SAC) solder paste was printed onto the Cu substrate, and the Sn/Cu interfacial microstructure was observed by scanning electron microscopy and energy-dispersive X-ray spectroscopy after the reflow soldering process. A thin IMC formed on substrates with small grain sizes, whereas larger grain sizes resulted in the formation of thick and elongated IMCs. In addition, the IMC growth exhibited a specific directional preference in the (21̅1̅0) direction on substrates annealed at 1000 ℃. The effects of the Cu grain size and orientation on the interfacial reactions and growth of IMCs were compared and analyzed by observing IMCs formed at the SAC/Cu interface. Microstructural analysis revealed that controlling the Cu grain size and orientation significantly impacts the reliability of the solder joints. This study shows that the effect of grain size and orientation can be utilized to micro-solder joints, which can significantly enhance their mechanical strength and reliability.

3D Crystal Construction by Single-Crystal 2D Material Supercell Multiplying.

2D stacking presents a promising avenue for creating periodic superstructures that unveil novel physical phenomena. While extensive research has focused on lateral 2D material superstructures formed through composition modulation and twisted moiré structures, the exploration of vertical periodicity in 2D material superstructures remains limited. Although weak van der Waals interfaces enable layer-by-layer vertical stacking, traditional methods struggle to control in-plane crystal orientation over large areas, and the vertical dimension is constrained by unscalable, low-throughput processes, preventing the achievement of global order structures. In this study, a supercell multiplying approach is introduced that enables high-throughput construction of 3D superstructures on a macroscopic scale, utilizing artificially stacked single-crystalline 2D multilayers as foundational repeating units. By employing wafer-scale single-crystalline 2D materials and referencing the crystal orientation of substrates, the method ensures precise alignment of crystal orientation within and across each supercell, thereby achieving controllable periodicity along all three axes. A centimeter-scale 3R-MoS₂ crystal is successfully constructed, comprising over 200 monolayers of single-crystalline MoS₂, through a bottom-up stacking process. Additionally, the approach accommodates the integration of amorphous oxide, enabling the assembly of 3D non-linear optical crystals with quasi-phase matching. This method paves the way for the bottom-up construction of macroscopic artificial 3D crystals with atomic plane precision, enabling tailored optical, electrical, and thermal properties and advancing the development of novel artificial materials and high-performance applications.

Dual Signature of Chirality Induced Spin Selectivity through Spontaneous Resolution of 2D Metal–Organic Frameworks

AbstractThe Chiral‐Induced Spin Selectivity (CISS) effect has emerged as a fascinating phenomenon within the realm of electron's spin manipulation, showcasing a unique interplay between electron's spin and molecular chirality. Subsequent to its discovery, researchers have been actively involved in exploring the new chiral molecules as effective spin filters. In the realm of observing the CISS effect, the conventional approach has mandated the utilization of two distinct enantiomers of chiral molecules. However, this present study represents a significant advancement by demonstrating the ability to control both spin states of electrons in a single system. In this work, we have demonstrated the preparation of chiral metal–organic frameworks (MOFs) via a “spontaneous resolution” process, obviating the requirement for chiral sources. This resulted in the production of chiral crystals exhibiting opposite handedness (1P and 1M) and these crystals were subsequently employed as a new class of spin filters based on CISS effect. Remarkably, this work signifies the first instance of achieving dual signature of spin selectivity from a single and exclusively achiral system through a spontaneous resolution process. This holds immense potential as it facilitates the production of two distinct spin‐filtering materials from a unified system. Furthermore, we investigated the contact potential differences (CPD) of these chiral crystals and, for the first time, associated it with the preferential spin transport properties. Our findings revealed a correlation between the CPD and the chirality of the crystals, as well as the magnetization orientations of the ferromagnetic substrate, which can be elucidated by the CISS effect. In overall, the significant findings achieved using these robust and easily synthesized MOF crystals without the requirement for chiral medium represent a crucial advancement in enhancing the effectiveness of spin filtering materials to produce spintronic devices.

Insights into surrogate respiratory droplet behaviour on inclined surfaces: Implications for disease transmission

Disease transmission via fluid ejections from infected individuals presents a significant public health challenge. The ejected droplet frequently lands on substrates with varying orientations, including horizontal, vertical, and inclined. However, the behaviour of disease-causing droplets on inclined substrates remains unexplored. Recent research has demonstrated that droplet evaporation, flow, and colloidal deposition are significantly altered when surfaces are inclined. Changes in contact angle impact evaporative flux and induce flow between the top and bottom edges of the droplet. This study examines the behaviour of simulated respiratory droplets containing NaCl, mucin, and micrometer-sized particles as pathogen surrogates, focusing on substrate inclinations of 0°, 45°, and 90° on glass surfaces. As respiratory droplets evaporate, their solute concentration rises, altering both surface tension and viscosity. The study explores the coupled effects of solute concentration variations and asymmetric evaporative flux at varying relative humidity conditions and droplet volumes. The presence of salt and mucin induces Marangoni flow, potentially altering evaporation, flow patterns, and deposition dynamics. Additionally, sedimentation flow influences crystal nucleation sites and precipitation patterns. Results reveal unprecedented crystallization dynamics on inclined substrates, with notable differences in nucleation and growth based on the angle of inclination. Optical profilometry and confocal microscopy further show that surrogate bacterial particles accumulate excessively at the bottom edge of inclined droplets, suggesting an elevated risk of pathogen survival on inclined fomites. These findings highlight the critical importance of considering substrate orientation in understanding the transmission of disease through surface-bound droplets, offering insights that could inform public health strategies.

Crystallographic Orientation of Grains Formed in the Laser Melt-Pool of (CoCuFeZr)17Sm2 Anisotropic Permanent Magnets

Textured microstructures and anisotropic properties are key factors for the optimization of magnetic materials. Only for high texture grades can the remanence Jr and the maximum energy product (BH)max be maximized. In additive manufacturing such as laser powder bed fusion (PBF-LB), methods to achieve texture have to be developed. In this work, anisotropic (CoCuFeZr)17Sm2 sintered magnets have been used as a substrate in experiments featuring single laser tracks to study the relationships between crystallographic orientation of the substrate grains and crystallographic orientation of grain growth in the melt-pool. The <0001> crystal direction (c-axis) of the substrate has been systematically varied with respect to the orientation of the laser scan track on the specimen surface. Crystallographic orientations of the melt-pool and the substrate have been analyzed using electron backscatter diffraction (EBSD). It is found that if the c-axis is oriented perpendicular to the temperature gradient in the melt-pool, grains grow with orientation similar to that of the substrate grain. If the c-axis and the temperature gradient are oriented in the same direction, the grains grow with high misorientation to the substrate. The highest anisotropy in the melt-pool is achieved when the substrate’s c-axis is oriented along the laser scan track. Under these conditions, 98.7% of the melt-pool area shows a misorientation <45° compared to the substrate orientation. The texture grade of the melt-pool area is comparable to that of the substrate magnet, at 91.8% and 92.2%, respectively.

Electrochemical characterization of TiO2 nanotubes formed on Ti6Al4V manufactured by PBF-EB or forging

AbstractThis study introduces the anisotropy effect of Ti6Al4V substrate obtained by electron beam melting (PBF-EB) on the anodizing process, revealing its capacity to induce anisotropic TiO2 nanotubes. Highly organized TiO2 nanotubes are formed on Ti6Al4V substrates produced through PBF-EB or forging, with the PBF-EB cross-orientation displaying superior nanotube growth due to enhanced catalytic activity. Morphological and electrochemical characterizations underscore the significant influence of substrate orientation and anodizing voltage on nanotube growth and corrosion resistance. PBF-EB-cross orientation at 30 V exhibits a thicker and more homogeneous nanotube layer, resulting in improved film resistance and substantially lower corrosion rates compared to forged substrates. The electrochemically calculated nanotube film thickness aligns with microscopic analyses, emphasizing the importance of a homogenous and resistive nanotube coating for effective corrosion control.

Engineering the substrate specificity and regioselectivity of Burkholderia thailandensis lipoxygenase

Lipoxygenases (LOXs) catalyze the regioselective dioxygenation of polyunsaturated fatty acids (PUFAs), generating fatty acid hydroperoxides (FAHPs) with diverse industrial applications. Bacterial LOXs have garnered significant attention in recent years due to their broad activity towards PUFAs, yet knowledge about the structural factors influencing their substrate preferences remains limited. Here, we characterized a bacterial LOX from Burkholderia thailandensis (Bt-LOX), and identified key residues affecting its substrate preference and regioselectivity through site-directed mutagenesis. Bt-LOX preferred ω-6 PUFAs and exhibited regioselectivity at the ω-5 position. Mutations targeting the substrate binding pocket and the oxygen access channel led to the production of three active variants with distinct catalytic properties. The A431G variant bifurcated dioxygenation between the ω-5 and ω-9 positions, while F446V showed reduced regioselectivity with longer PUFAs. Interestingly, L445A displayed altered substrate specificity, favoring ω-3 over ω-6 PUFAs. Furthermore, L445A shifted the regioselectivity of dioxygenation to the ω-2 position in ω-3 PUFAs, and, for some substrates, facilitated dioxygenation closer to the carboxylic acid terminus, suggesting an altered substrate orientation. Among these variants, L445A represents a significant milestone in LOX research, as these alterations in substrate specificity, dioxygenation regioselectivity, and substrate orientation were achieved by a single mutation only. These findings illuminate key residues governing substrate preference and regioselectivity in Bt-LOX, offering opportunities for synthesizing diverse FAHPs and highlighting the potential of bacterial LOXs as biocatalysts with widespread applications.

Arsenic-flux dependence of surface morphology in InAs homoepitaxy

Surface morphology in molecular-beam epitaxy of InAs(001), (111)A, and (111)B has been studied using scanning tunneling microscopy. The surface morphologies of InAs strongly depend on substrate temperature, substrate orientation, and As/In flux ratio. The size and density of two-dimensional InAs islands on the (001) surface decreases and increases, respectively, as the As/In flux ratio is increased. On the other hand, island size (density) is increased (decreased) with the increase in As flux on (111)A and (111)B surfaces. Surface reconstructions on growing surfaces strongly affect the diffusion and incorporation kinetics of In atoms, thereby resulting in the observed surface morphologies.

The Effect of Optimized Substrate Orientation on Layer Step in Laser Metal Deposition of Single-Crystal Nickel-Base Superalloys.

Laser metal deposition is a promising way to repair the surface defects of single-crystal components in turbo engines. Understanding the mechanisms and improving the efficiency of the repair have been long-standing problems. In this study, the influence of the substrate orientation on the laser metal deposition (LMD) was investigated and its effect on repair layer-step was examined. LMD experiments were conducted on single crystal superalloys with a normal substrate orientation (001)/[100] and with an optimized substrate orientation (101)/[101¯]. It reveals that the laser cladding with the optimized orientation leads to a larger height of the [001] dendrite region than that with the normal orientation. The calculated results of the growth velocity, thermal gradient, and susceptibility to CET in the dendrite-preferred growth direction indicate that, for the (101)/[101¯] orientation, the [001]/[100] boundary is located at relative high position in each layer, which not only decreases the formation ability of stray grain significantly, but also eliminates the appearance of the maximum susceptibility. This makes the necessary dilution position much higher, and thus, a large cladding step can be selected. Our findings could find potential applications in laser repair of single-crystal components.

Impact of c- and m- sapphire plane orientations on the structural and electrical properties of β-Ga2O3 thin films grown by metal-organic chemical vapor deposition

Abstract This work presents a comprehensive investigation into the structural and electrical properties of Ga2O3 thin films grown via metal-organic chemical vapor deposition on both c- and m-plane sapphire substrates. Structural characterization revealed the β-Ga2O3 phase formation in both substrate orientations, with strong epitaxial ( 2 ¯ 01 ) preferential growth on c-plane substrates and polycrystalline films on m-plane substrates. Results show that Ga2O3/m-sapphire exhibits the lower electrical resistivity than its counterpart grown on c-sapphire. Activation energies of acceptor levels were estimated at ~1.4 eV and ~0.7 eV , for Ga2O3 films grown on c- and m-plane, respectively. This result shows that growing Ga2O3 on m-plane sapphire is beneficial to reach a weakly compensated sample. Cathodoluminescence analysis suggests that the additional low activation energy of ~0.18 eV observed in Ga2O3 grown with the highest oxygen flow on m-plane sapphire can be associated to thermally-induced migration of self-trapped hole states.

Growth Characteristics and Orientation Tuning of Superconducting Thin Films of Bi2212 Based on Substrate Orientation

Growth Characteristics and Orientation Tuning of Superconducting Thin Films of Bi2212 Based on Substrate Orientation

Exploring the role of the residues into catalytic cavity of inulosucrase from Leuconostoc citreum CW28

Inulosucrases are enzymes capable of synthesizing inulin polymers using sucrose as the main substrate. The enzymatic activity relies on the catalytic triad within the active site and residues responsible for substrate recognition and orientation, termed carbohydrate-binding subsites. This study investigates the role of specific residues within the catalytic cavity of a truncated version of IslA4 in enzymatic catalysis. Mutants at residues S425, L499, A602, R618, F619, Y676, Y692, and R696 were constructed and characterized. Characterization results, and in silico structural comparison with other fructansucrases, reveal these residues' functional significance in catalysis. Residue S425 belongs to subsite −1; residues R618 and Y692 are part of subsite +1, and residue R696 belongs to subsites +1 and +2. Residues L499 and A602 are support residues; the former favors the formation of the fructosyl-enzyme intermediate, while the latter stabilizes the acid/base catalyst during catalysis. Residues Y676 and F619 may participate in stabilizing residues at −1/+1 subsites. This study represents the first comprehensive exploration of the structural determinants essential for enzymatic function in the inulosucrase of Leuconostoc citreum, and proposes the identity of residues involved in the −1 to +2 subsites.

Optical Characterization of the Interband Cascade LWIR Detectors with Type-II InAs/InAsSb Superlattice Absorber.

The long-wave infrared (LWIR) interband cascade detector with type-II superlattices (T2SLs) and a gallium-free ("Ga-free") InAs/InAsSb (x = 0.39) absorber was characterized by photoluminescence (PL) and spectral response (SR) methods. Heterostructures were grown by molecular beam epitaxy (MBE) on a GaAs substrate (001) orientation. The crystallographic quality was confirmed by high-resolution X-Ray diffraction (HRXRD). Two independent methods, combined with theoretical calculations, were able to determine the transitions between the superlattice minibands. Moreover, transitions from the trap states were determined. Studies of the PL intensity as a function of the excitation laser power allowed the identification of optical transitions. The determined effective energy gap (Eg) of the tested absorber layer was 116 meV at 300 K. The transition from the first light hole miniband to the first electron miniband was red-shifted by 76 meV. The detected defects' energy states were constant versus temperature. Their values were 85 meV and 112 meV, respectively. Moreover, two additional transitions from acceptor levels in cryogenic temperature were determined by being shifted from blue to Eg by 6 meV and 16 meV, respectively.

(Invited) Advanced Electronic Characterization of Wide Band Gap Semiconductor Defects at the Nanoscale

Wide bandgap materials such as gallium nitride (GaN) and gallium oxide (Ga2O3) are currently under development for realizing next-generation high power electronic devices beyond silicon. Vertical structures produced through homoepitaxial growth are preferred for scaling high-voltage devices, which places strict demands on the quality of bulk substrates. In particular, extended defects can propagate during epitaxial growth into the active areas of devices. Therefore, there is a need for identifying defects and determining their electronic properties at relevant nano- to micro- scale dimensions in order to disentangle their combined effects on macroscopic devices. Here, we use a combination of laser-based photoemission electron microscopy (PEEM) and electrical atomic force microscopy (AFM) methodologies to investigate the local electronic properties of surface defects in epitaxially grown GaN and β-Ga2O3. PEEM is a non-scanning electron microscopy technique where photoelectrons emitted by a surface illuminated by deep-ultraviolet light are imaged with nanometer-scale spatial resolution. The emitted photoelectrons are sensitive to the local electronic structure, making PEEM a powerful tool for evaluating electronic changes at length scales relevant for devices, and provides valuable information for identifying “killer” defects which must be minimized during device growth and processing.The difficulty in producing large, low dislocation density bulk GaN substrates has led to longstanding issues with leakage current in diodes and transistor devices. We identify surface defects with different electronic properties in GaN epitaxy on different substrates using PEEM. We observe star-shaped defects on strain-patterned GaN substrates and elongated dark patches on growth ridges on ammonothermally grown GaN substrates. In both cases, the observed electronic changes in PEEM correlate directly with local electrical AFM measurements, revealing that local changes in the electronic structure at these defects results in altered electrical conduction.In contrast, the development of large bulk Ga2O3 substrates has been more successful, yet the identification and influence of various defects on device performance is still largely unknown at this point. The topography of β-Ga2O3 epitaxy varies widely between growth methods and substrate orientations. For example, β-Ga2O3 by hydride vapor physical epitaxy lacks fine structure, yet has surface defects that regularly appear in parallel orientations. In β-Ga2O3 grown on different substrate orientations, we observe variations in the surface electronic properties using PEEM. For certain substrate orientations and growths, we find nanoscale variations in doping, while other orientations display micron-scale electronic inhomogeneity and defects. Our results demonstrate that laser-based PEEM can be a powerful tool for detecting electrically active defects, as well as evaluating the nanoscale electronic homogeneity of wide bandgap semiconductors.

Large-scale synthesis of defect-free phosphorene on nickel substrates: enabling atomistic thickness devices

Addressing the main challenges of defect-free, large-scale synthesis of low-dimensional materials composed of phosphorus atoms is essential for advancing promising phosphorene-based technologies. Using molecular dynamics simulation, we demonstrate the large-scale and defect-free synthesis of phosphorene on Nickel (Ni) substrates. We showed that substrate orientation is crucial in the controllable synthesis of different phosphorene allotropes. Specifically, blue phosphorene was successfully grown on Ni (111) and Ni (100) surfaces, while γ-phosphorene, referred to here as Navy phosphorene, was grown on Ni (110). In addition, temperature control (high temperature) and cooling rate (slow cooling) are also crucial in the formation of P6 hexagons. Finally, we report that the phosphorus pentamers (P5) are the essential precursor for phosphorene synthesis. This work provides a robust framework for understanding and controlling the synthesis of large-area, single-crystalline monolayer phosphorene.

Geometrically Controlled Microscale Patterning and Epitaxial Lateral Overgrowth of Nitrogen-Polar GaN.

In this Letter, we report on a novel two-step epitaxial growth technique that enables a significant improvement of the crystal quality of nitrogen-polar GaN. The starting material is grown on 4° vicinal sapphire substrates by metal-organic vapor-phase epitaxy, with an initial high-temperature sapphire nitridation to control polarity. The material is then converted to a regular array of hexagonal pyramids by wet-etching in a KOH solution and subsequently regrown to coalesce the pyramids back into a smooth layer of improved crystal quality. The key points that enable this technique are the control of the array geometry, obtained by exploiting the anisotropic behavior of the wet-etch step, and the use of regrowth conditions that preserve the orientation of the pyramids' sidewalls. In contrast, growth conditions that cause an excessive expansion of the residual (0001̅) facets on the pyramids' tops cause the onset of a very rough surface morphology upon full coalescence. An X-ray diffraction study confirms the reduction of the threading dislocation density as the regrowth step develops. The analysis of the relative position of the 0002̅ GaN peak, with respect to the 0006 sapphire peak, reveals a macroscopic tilt of the pyramids, probably induced by the large off-axis substrate orientation. This tilt correlates very well with an anomalous broadening of the 0002̅ diffraction peaks at the beginning of the regrowth step.

Optical and electronic functionality arising from controlled defect formation in nanoscale complex oxide lateral epitaxy.

Epitaxial crystallization of complex oxides provides the means to create materials with precisely selected composition, strain, and orientation, thereby controlling their functionalities. Extending this control to nanoscale three-dimensional geometries can be accomplished via a three-dimensional analog of oxide solid-phase epitaxy, lateral epitaxial crystallization. The orientation of crystals within laterally crystallized SrTiO3 systematically changes from the orientation of the SrTiO3 substrate. This evolution occurs as a function of lateral crystallization distance, with a rate of approximately 50° μm-1. The mechanism of the rotation is consistent with a steady-state stress of tens of megapascal over a 100-nanometer scale region near the moving amorphous/crystalline interface arising from the amorphous-crystalline density difference. Second harmonic generation and piezoelectric force microscopy reveal that the laterally crystallized SrTiO3 is noncentrosymmetric and develops a switchable piezoelectric response at room temperature, illustrating the potential to use lateral crystallization to control the functionality of complex oxides.

Modulating growth of graphene on sapphire by chemical vapor deposition

The direct growth of graphene on insulating substrates presents significant promise for the advancement of next-generation semiconductor technologies. Here, the synthesis of graphene directly on sapphire substrates via chemical vapor deposition (CVD) is reported. The CVD growth parameters are systematically manipulated to optimize the crystal quality of graphene. Furthermore, the dependency of graphene growth on the crystallographic orientation of the sapphire substrate is explored to elucidate the underlying mechanisms. It is found that robust C-O chemical bonds are established between the oxygen atoms on the sapphire surface and the carbon atoms in the graphene, predominantly influencing the nucleation process. The oxygen-rich nature of the c-plane sapphire surface contributes to a higher graphene nucleation density and the formation of smaller grains comparing to r-plane sapphire. Accordingly, high quality and uniform monolayer graphene is successfully obtained on both c-plane and r-plane sapphire.

Structural Comparison of Substrate Binding Sites in Dehaloperoxidase A and B.

Dehalperoxidase (DHP) has diverse catalytic activities depending on the substrate binding conformation, pH, and dynamics in the distal pocket above the heme. According to our hypothesis, the molecular structure of the substrate and binding orientation in DHP guide enzymatic function. Enzyme kinetic studies have shown that the catalytic activity of DHP B is significantly higher than that of DHP A despite 96% sequence homology. There are more than 30 substrate-bound structures with DHP B, each providing insight into the nature of enzymatic binding at the active site. By contrast, the only X-ray crystallographic structures of small molecules in a complex with DHP A are phenols. This study is focused on investigating substrate binding in DHP A to compare with DHP B structures. Fifteen substrates were selected that were known to bind to DHP B in the crystal to test whether soaking substrates into DHP A would yield similar structures. Five of these substrates yielded X-ray crystal structures of substrate-bound DHP A, namely, 2,4-dichlorophenol (1.48 Å, PDB: 8EJN), 2,4-dibromophenol (1.52 Å, PDB: 8VSK), 4-nitrophenol (2.03 Å, PDB: 8VKC), 4-nitrocatechol (1.40 Å, PDB: 8VKD), and 4-bromo-o-cresol (1.64 Å, PDB: 8VZR). For the remaining substrates that bind to DHP B, such as cresols, 5-bromoindole, benzimidazole, 4,4-biphenol, 4.4-ethylidenebisphenol, 2,4-dimethoxyphenol, and guaiacol, the electron density maps in DHP A are not sufficient to determine the presence of the substrates, much less their orientation. In our hands, only phenols, 4-Br-o-cresol, and 4-nitrocatechol can be soaked into crystalline DHP A. None of the larger substrates were observed to bind. A minimum of seven hanging drops were selected for soaking with more than 50 crystals screened for each substrate. The five high-quality examples of direct comparison of modes of binding in DHP A and B for the same substrate provide further support for the hypothesis that the substrate-binding conformation determines the enzyme function of DHP.