Nien-Po Chen

Electronic Properties of Metallic Nanoclusters on Semiconductor Surfaces: Implications for Nanoelectronic Device Applications

We review current research on the electronic properties of nanoscale metallic islands and clusters deposited on semiconductor substrates. Reported results for a number of nanoscale metal-semiconductor systems are summarized in terms of their fabrication and characterization. In addition to the issues faced in large-area metal-semiconductor systems, nano-systems present unique challenges in both the realization of well-controlled interfaces at the nanoscale and the ability to adequately characterize their electrical properties. Imaging by scanning tunneling microscopy as well as electrical characterization by current-voltage spectroscopy enable the study of the electrical properties of nanoclusters/semiconductor systems at the nanoscale. As an example of the low-resistance interfaces that can be realized, low-resistance nanocontacts consisting of metal nanoclusters deposited on specially designed ohmic contact structures are described. To illustrate a possible path to employing metal/semiconductor nanostructures in nanoelectronic applications, we also describe the fabrication and performance of uniform 2-D arrays of such metallic clusters on semiconductor substrates. Using self-assembly techniques involving conjugated organic tether molecules, arrays of nanoclusters have been formed in both unpatterned and patterned regions on semiconductor surfaces. Imaging and electrical characterization via scanning tunneling microscopy/spectroscopy indicate that high quality local ordering has been achieved within the arrays and that the clusters are electronically coupled to the semiconductor substrate via the low-resistance metal/semiconductor interface.

Ohmic nanocontacts to GaAs using undoped and p-doped layers of low-temperature-grown GaAs

The development and characterization of high-performance nanocontacts to n-GaAs are reported. The nanocontacts can be made to both undoped and p-doped low-temperature-grown GaAs (LTG:GaAs) cap layers. The geometry of the nanocontact is well characterized and requires the deposition of a 4 nm single-crystalline Au cluster onto an ohmic contact structure which features a chemically stable LTG:GaAs surface layer prepared using an ex situ chemical self-assembly technique. A self-assembled monolayer of xylyl dithiol (HS–CH2–C6H4–CH2–SH) is required to provide mechanical and electronic tethering of the Au cluster to the LTG:GaAs surface. For the case of an undoped LTG:GaAs cap layer, a specific contact resistance of 1×10−6 Ω cm2 and a current density of 1×106 A/cm2 have been measured from scanning tunneling microscopy. When a p-doped LTG:GaAs cap layer is used, the corresponding values are 1×10−7 Ω cm2 and 1×107 A/cm2, respectively. Improved surface stability as evidenced by a lower oxidation rate for p-doped L...

Fermi level unpinning in ex situ Schottky contacts on n-GaAs capped with low-temperature-grown GaAs

The Schottky barrier behavior of a GaAs layer structure consisting of a thick n-GaAs layer, capped by a thin (3.5 nm) layer of as-grown unintentionally or Be-doped low-temperature-grown GaAs (LTG:GaAs), both grown by molecular beam epitaxy, has been studied. Nonalloyed, ex situ Schottky contacts using three different metals were fabricated on the LTG:GaAs-capped layers and on n-GaAs control samples, in order to study the interface barrier height (φb) versus the metal work function (φm). High frequency capacitance–voltage measurements, along with simulations that incorporate a complete description of the defect states in LTG:GaAs, were used to extract the φb values. The variation in φb with the metal work function is nearly six times greater in the LTG:GaAs capped contacts than in uncapped n-GaAs samples. This direct observation of Fermi level unpinning is consistent with earlier measurements that indicated the inhibited oxidation of LTG:GaAs layers in comparable structures for short air exposure times.

A quantitative conduction model for a low-resistance nonalloyed ohmic contact structure utilizing low-temperature-grown GaAs

We present a quantitative conduction model for nonalloyed ohmic contacts to n-type GaAs (n:GaAs) which employ a surface layer of low-temperature-grown GaAs (LTG:GaAs). The energy band edge profile for the contact structure is calculated by solving Poisson’s equation and invoking Fermi statistics using deep donor band and acceptor state parameters for the LTG:GaAs which are consistent with measured bulk and surface electrical properties of this material. The specific contact resistance is then calculated using an analytic expression for tunneling conduction through an equivalent uniformly doped Schottky barrier. The model has been used to fit measured specific contact resistances versus LTG:GaAs layer thickness and versus measurement temperature. These comparisons provide insights into the contact mechanism (electron tunneling between metal states and conduction band states in n:GaAs) and indicate that low barrier heights (0.3–0.5 V) and high activated donor densities (∼1×1020 cm−3) have been achieved in t...

Unpinned interface Fermi-level in Schottky contacts to n-GaAs capped with low-temperature-grown GaAs; experiments and modeling using defect state distributions

Low-temperature-grown GaAs (LTG:GaAs) has been used as a cap layer in ex situ, low-resistance contact structures to n-GaAs, indicating that a chemically stable surface with well-controlled electrical properties can be realized using this cap. Recently, capacitance–voltage (C–V) measurements on Schottky contacts have provided a direct indication of an unpinned interface Fermi-level in structures consisting of n-GaAs layers capped with thin layers of LTG:GaAs. This article describes experimental and modeling efforts to describe the near-interface energy band and Fermi-level behavior at metal/LTG:GaAs/n-GaAs interfaces. The C–V data are summarized, and the results of current–voltage measurements are presented to corroborate the initial observations. In simulation of the energy barrier, the defect bands in as-grown LTG:GaAs, for both unintentionally and Be-doped material, were described by models obtained by fitting to experimental data from scanning tunneling spectroscopy and other techniques. The near-inter...

Electroabsorption field imaging between coplanar metal contacts on semi‐insulating semiconductor epilayers

We use excitonic electroabsorption field imaging to study the electric field distribution between coplanar gold Schottky contacts on semi‐insulating photorefractive AlGaAs epilayers. The field imaging shows consistently large localized enhancements of the electric field adjacent to the anode, followed by a region of reduced field. Complex behavior occurs at the cathode, with high‐field regions extending far from the contact. These inhomogeneous near‐contact field profiles are determined by the superposition of both diffused and drifted charge which affect the performance of many optoelectronic devices that use planar contacts or striplines on semi‐insulating substrates.

Temperature-dependent behavior of low-temperature-grown GaAs nonalloyed ohmic contacts

A study of nonalloyed ohmic contact structures consisting of Au/Ti metallization deposited on a thin (3.5–5 nm) layer of low-temperature-grown GaAs (LTG:GaAs) on a thin (10 nm) layer of heavily doped n-type GaAs is summarized. We demonstrate that this Au/Ti:LTG:GaAs/n+GaAs contact structure has a stable specific contact resistance between 40 and 300 K, with measured contact resistance as low as 2×10−6 Ω cm2 at 40 K. Based on comparisons of the measured data with calculations using a uniformly doped Schottky model, we infer that the activation doping density in these structures is higher than 5×1018 cm−3, and that the surface potential barrier height is lower than 0.7 eV (midgap). The characteristic current–voltage curves of the nonalloyed contact show that tunneling is the primary conduction mechanism.