Dependence Of Barrier Height Research Articles

A novel fabrication process and analytical model for Pt/GaAs Schottky barrier mixer diodes

A new fabrication process for small-area honeycomb structured GaAs Schottky barrier diodes is described. This fabrication process is based on the anodic pulse etching technique of GaAs in combination with an electrolytic in situ Pt deposition. The presented diodes exhibit nearly ideal I/V and noise characteristics. An analytical model has been developed to self-consistently model the fabricated near-ideal Schottky barrier diodes. The analytical model includes the effects of a current dependent recombination velocity, an accurate derivation of the spreading resistance for epitaxial diodes, a n-n + junction voltage dependent on the diode current and a bias dependent barrier height. A closed-form expression for the recombination current in the depletion region is given. This model, compared to the simple thermionic model is able to determine technology dependent parameters such as the bias dependent series resistance and junction capacitance as well as the current dependent noise temperature. These are the most important diode parameters which define the diode mixing performance. Although the simple thermionic model also fits the measured data very well, it is shown that the physical parameters of the Schottky diodes, determined by this approach, are inaccurate. The novel analytical model gives new insight into the physical understanding of forward biased epitaxial Schottky diodes with small junction areas.

Bias dependence of Schottky barrier height in GaAs from internal photoemission and current-voltage characteristics

Bias dependence of Schottky barrier height φB was directly measured using an internal photoemission effect in Au-n-GaAs Schottky barrier diodes under both the forward and reverse applied voltages. φB increased by about 0.015–0.016 eV as the forward bias increased from 0 to 0.3 V. This is probably due to an applied-voltage-induced change in electron population in the interface states. The results were compared with the bias dependence derived from dark forward current-voltage (I-V) characteristics. Both bias dependences were in good agreement, indicating that nonideal forward I-V characteristics (a deviation from the ideal thermionic emission theory) could be fully explained by the bias dependence of φB for diodes of a relatively good quality (an ideality factor, n=1.02–1.07). The effective Richardson constant A** was calculated to be about 1.3–1.4×105 A K−2 m−2 for the diodes of the lower n values using the value of the measured Schottky barrier height at zero bias. These values of A** were higher than the calculated value of about 8×104 A K−2 m−2 and that of 0.41±0.15×104 A K−2 m−2 reported by Missous and Rhoderick [J. Appl. Phys. 69, 7142 (1991)]. This is probably due to an enhancement of an electron tunneling through the high density interface states in our case. φB decreased by about 0.05–0.06 eV when the reverse voltage increased from 0 to 3.0 V. An image force lowering effect could not account for this large reverse bias dependence of the barrier height. The nonideal reverse I-V characteristics could not be interpreted solely by the bias dependence of φB. This discrepancy was attributed to other leakage current mechanisms.

Electrical in situ characterisation of metal/gallium phosphide (110) Schottky contacts

Schottky barrier formation for metals on p-GaP(110) has been systematically investigated by electrical transport measurements, in order to clarify the dependence of Schottky barrier height on metal and interface properties. Metals were deposited onto clean cleaved GaP(110) in ultrahigh vacuum and the temperature dependent current-voltage ( I–V) and capacitance-voltage ( C–V) characteristics were recorded in situ. The Schottky barriers (Ф B) were determined for nine metals (Na, Mg, In, Ti, Sn, Sb, Ag, Au and Pt), covering a wide range of metal work function (Ф M) from 2.75 to 5.65 eV. The results reveal a weak dependence of Ф B on Ф M with S=dФ B/dФ M ≈ 0.14 , which is consistent with a strong Fermi level pinning by induced gap states.

Grain boundary trap distribution in polycrystalline silicon thin-film transistors

The grain boundary trap state density is evaluated in polysilicon thin-film transistors by a method based on the dependence of the grain boundary potential barrier height on the gate voltage. Assuming a Gaussian energy distribution of the grain boundary trap states, the distribution parameters are determined by fitting the grain boundary barrier height experimental data with the theory. In low-pressure chemical vapor deposited polysilicon films, the influence of deposition pressure on the grain boundary trap distribution is examined by using this method. A large number of traps exist at the grain boundaries near the midgap of the material deposited at lower pressure due probably to an increased impurity contamination.

Fabrication of planar optical waveguides in LiB/sub 3/O/sub 5/ by 2 MeV He/sup +/ ion implantation

The fabrication of planar optical waveguides in LiB/sub 3/O/sub 5/ is discussed. Using 2-MeV /sup 4/He/sup +/ implantation with a dose of 1.5*10/sup 16/ ions/cm/sup 2/ at 300 K, the refractive indexes of a 0.2- mu m-thick layer 5.1 mu m below the crystal surface are reduced to form optical barrier guides. For this ion dose the maximum change from the bulk values of refractive index at a wavelength of 0.488 mu m are 1.5%, 5.25%, and 4% for n/sub x/, n/sub y/, and n/sub z/, respectively. The refractive indexes of the guiding region change by less than 0.02% from the bulk values. The dose dependence of the optical barrier height has been measured. A threshold ion dose of about 0.75*10/sup 16/ ions/cm/sup 2/ is required to form a refractive index barrier and ion doses higher than about 2.5*10/sup 16/ ions/cm/sup 2/. saturate the refractive index decrease. Waveguide propagation losses for annealed single energy implants of dose 1.5*10/sup 16/ ions/cm/sup 2/ are dominated by tunneling and are estimated to be approximately 8.9 dB/cm for the z-cut waveguides used. Multiple energy implants broaden the optical barrier, and losses of <4 dB/cm have been observed.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Temperature dependence of Schottky barrier heights on silicon

We investigate the temperature dependence of Schottky barrier heights on silicon. The analysis of a large variety of polycrystalline diodes shows that the temperature coefficient of the barrier height depends on the chemical nature of the metal. This observation is in contradiction with models suggesting Fermi-level pinning at the center of the semiconductor’s indirect band gap. From the analysis of epitaxial NiSi2/Si Schottky contacts, we conclude that there is a direct influence of interface crystallography on both the barrier height and its temperature dependence. Finally, we present some new results on the pressure coefficient of barrier heights. Pressure and temperature coefficients of polycrystalline Schottky contacts are correlated similarly to the pressure and temperature coefficients of the band gap.

Relationship Between Interface Structure and Schottky Barrier Height

ABSTRACTThe clear dependence of the Schottky barrier height (SBH) on the structure of a metalsemiconductor (MS) interface is described for a few high-quality epitaxial MS systems. Polycrystalline MS interfaces, for which such a dependence is often absent, are shown to display clear evidence for SBH inhomogeneity, in direct conflict with the Fermi level (FL) pinning concept they helped to create. Recent theoretical calculations show that many basic tenets of the interface state pinning models are unfounded. The redistribution of charge as a result of bonding at the MS interface seems to be the most important contribution to the interface dipole and, hence, the SBH.

Contactless electromodulation for in situ characterization of semiconductor processing

We present results of a new contactless electroreflectance (CER) mode which has considerable potential for in situ monitoring. This method utilizes a condenser-like system, one electrode consisting of a transparent conductive coating on a transparent substrate which is separated from the sample surface by a thin layer of air (or other ambient). In order to demonstrate the utility of this approach we have measured the CER spectra from (a) bulk Hg 0.8Cd 0.2Te (80 and 300 K) sputtered with Xe ions as well as (b) (001) n- and p-type GaAs structures with large, uniform electric fields. The latter configurations can be used to investigate Fermi level pinning effects and metallization. Studies were carried out as a function of temperature (10 < T < 600 K) yielding a temperature dependence of surface barrier height. The relative merits of CER and photoreflectance will be discussed.

D- centers in spherical quantum dots.

A negatively charged donor center ${\mathit{D}}^{\mathrm{\ensuremath{-}}}$ (i.e., a neutral shallow donor ${\mathit{D}}^{0}$ that binds a second electron) in a spherical quantum dot is studied by use of a variational approach. A trial function which includes electron-correlation effects and approaches the Chandrasekhar-type function in the limit of zero barrier height is used. The well-radius and barrier-height dependence of the ``binding energy'' of the ${\mathit{D}}^{\mathrm{\ensuremath{-}}}$ center is obtained. The dimensional characteristics are clearly demonstrated not only for the ``binding energy'' and its maximum of the ${\mathit{D}}^{\mathrm{\ensuremath{-}}}$ center but also for the ratio of ${\mathit{D}}^{\mathrm{\ensuremath{-}}}$ to ${\mathit{D}}^{0}$ ``binding energy'' and the electron-correlation effect.

Internal barriers in GaAs epitaxial layers produced by delta-doped dipoles

Using a closely spaced pair of δ‐doped layers, one n‐ and one p‐type, at a semiconductor homojunction with different isotype doping densities on each side of the junction, controlled internal Schottky‐type barriers may be produced. We have studied such prototype GaAs diodes with Be and Si δ doping. Good agreement has been obtained between the experimentally determined barrier and that from a self‐consistent Poisson calculation of the zero bias band profile. The temperature dependence of the I–V characteristics has been measured in detail. The results are consistent with the weak temperature dependence of barrier height predicted by the theoretical model. The value of the Richardson constant was also calculated. It has been found that in general the current transport behavior is close to the thermonic limit. A preliminary study of the thermal stability of such diodes has also been carried out.

Barrier Inhomogeneities at Schottky Contacts: Curved Richardson Plots, Idealities, and Flat Band Barriers

ABSTRACTEvaluating either curved Richardson plots or temperature-dependent ideality data allows for a quantitative characterization of spatial inhomogeneities at Schottky contacts. Applying the two independent methods to PtSi/Si diodes we obtain a standard deviation around 70mV for the barrier fluctuations. These results agree with those from the comparison of temperature-dependent current and capacitance barriers. We discuss also flat band barrier heights which should be used if one investigates the temperature dependence of fundamental Schottky barrier heights. Their temperature coefficients depend on metallization. For epitaxial NiSi2/Si contacts on (100) oriented Si we find a strong influence of interface crystallography on the temperature coefficients.

Orientation and perpendicular energy dependence of dissociative scattering of hydrogen from Cu(111)

We report measurements of the final molecular axis orientation and kinetic energy released in the dissociative neutralization of 2.98 keV H 2 + incident at glancing angles to a Cu(111) surface. We observe two distinct structures which we correlate with scattering on two distinct regions of the potential hypersurface (PES) describing the interaction. The relative contribution of each of these structures to the total spectrum exhibits a threshold-like behaviour as a function of the perpendicular energy of the beam in the (nominal) range 1–2 eV. The perpendicular energy at which this threshold appears is dependent on the molecular axis orientation. We correlate this threshold with a barrier on the PES, and its dependence on orientation with an orientation dependent barrier height. The height variation is of order 0.9 eV, and the barrier is lowest for the surface-parallel axis orientation.

Pinning, creep and possible melting in the mixed state of NbZr films

Current-Voltage (I–V) characteristics and resistivity measurements on Nb xZr 1−x (x ∼ 0.15) thin films are reported over a broad range of temperature and magnetic field. In the vicinity of the critical temperature (field) the I–V curves show a transition from the Ohmic to a strongly non-linear behavior on increasing the excitation current density. The linear resistivity is thermally activated with a field dependent barrier height. Although the Ohmic and non-linear data can be described in the framework of the glass model for the mixed state in hard superconductors, it does not yield a universal set of exponents for the glass to fluid transition. The overall response of the mixed state can be understood in the framework of the standard flux-creep theory with inter-vortex interaction.

Is scanning tunneling microscopy a useful tool for probing the surface potential?

Abstract Here, we examine two methods for determining the effective surface potential using the scanning tunneling microscope (STM) as a probe. One is based on measurements of the differential conductance and the other on the apparent barrier height dependence with the tip-sample separation. We simulate theoretically both methods and compare the results when several models for the interface potential are used. Our findings are: (i) under several conditions the STM can determine the surface potential; (ii) those conditions are better achieved when the apparent barrier height is measured.

Is scanning tunneling microscopy a useful tool for probing the surface potential?

Here, we examine two methods for determining the effective surface potential using the scanning tunneling microscope (STM) as a probe. One is based on measurements of the differential conductance and the other on the apparent barrier height dependence with the tip-sample separation. We simulate theoretically both methods and compare the results when several models for the interface potential are used. Our findings are: (i) under several conditions the STM can determine the surface potential; (ii) those conditions are better achieved when the apparent barrier height is measured.

Electron transport of inhomogeneous Schottky barriers

A novel approach is presented which leads to analytic solutions to the potential and the electron transport through inhomogeneous Schottky barriers. The existence of barrier height nonuniformities is shown to provide a simple explanation of the following abnormal experimental results, routinely observed from various Schottky barriers: greater-than-unity ideality factors, the T0 effect, the ‘‘soft’’ reverse characteristics, and the dependence of barrier height on the technique of measurement.

Temperature dependence of barrier heights in spin glasses

One of the characteristics of spin glasses (SG) below the glass temperature Tg is aging. It is a direct consequence of the ‘‘rugged’’ and highly degenerate nature of the free-energy landscape in configuration space. We have determined the temperature dependence of the height of a given barrier Δ(T) in the ‘‘canonical’’ spin glass AgMn2.6%. Our results show that the rate of change, ‖δΔ/δT‖, increases linearly with increasing Δ in the range of Δ we explore experimentally.

Metal reactivity effects on the surface recombination velocity at InP interfaces

Direct measurements of the surface recombination velocity (SRV) on etched InP(110) and at its interfaces with various metals deposited by thermal evaporation have been performed using ultrafast time-resolved photoluminescence. The results show that the original InP low SRV is retained when these surfaces are coverd with metals which tend to react with the semiconductor’s anion, such as Al, Cr, and Zn. On the other hand, the SRV increases sharply as a function of unreactive metal coverage, such as Cu, Au, and Ag. The SRV results are explained in terms of metal-induced interface states, whose position in the band gap and thus their cross section for recombination depends on the metal reactivity. The similarity of the SRV and the reported Schottky barrier height dependence on the metal-phosphorus heat of reaction is noted.

Dependence of the grain boundary potential barrier height of BaTiO3 ceramics on donor dopant concentration

Barium titanate ceramics exhibiting the positive temperature coefficient of resistance (PTCR) effect were prepared with donor concentrations ranging from 0.05-1.8 at.%. The resistance of the various samples (normalised to give the specific resistance of a unit area of grain boundary rho L in order to eliminate the effect of the observed reduction in grain size with increased donor concentration) was measured in the vicinity of the ferroelectric transition temperature Tc. Arrhenius plots of rho L against (T- theta )/T, where T is the absolute temperature and theta is the Curie point, were then obtained for T>Tc. The plots were found to be linear up to a few degrees below the temperature corresponding to the maximum value of rho L, in agreement with the well known Heywang model. The potential barrier heights, calculated from the slopes of the Arrhenius plots, were found to be similar for samples containing 0.1-1.4 at.% ( approximately 0.2 eV at 150 degrees C) and greater in more heavily doped specimens (0.4 eV at the same temperature). However, the Arrhenius plots belonging to samples containing 0.05 at.% were linear for a small temperature range since the PTCR jump was very small and consequently it was not possible to calculate the potential barrier height with any accuracy.

Temperature dependent barrier heights in bulk unipolar diodes leading to improved temperature stable performance

Surface states in a Schottky diode lead to a pinning of the Fermi level and a negative temperature coefficient for the barrier height Calculations show that the barrier height of a epitaxially grown, bulk, unipolar diode actually increases with temperature, typically by about 0.4 meVK−1 for GaAs between −40°C and +80°C. This novel effect can be used to explain the improved temperature stability of devices employing these diodes as direct retrofit replacements for conventional Schottky diodes.