WO2001054171A1

WO2001054171A1 - A chemical sensor using chemically induced electron-hole production at a schottky barrier

Info

Publication number: WO2001054171A1
Application number: PCT/US1999/029363
Authority: WO
Inventors: Eric W. Mcfarland; W. Henry Weinberg; Hermann Nienhaus; Howard S. Bergh; Brian Gergen; Arunava Mujumdar
Original assignee: The Regents Of The University Of California
Priority date: 2000-01-19
Filing date: 2000-01-19
Publication date: 2001-07-26
Also published as: AU2000263333A1; EP1254478A4; EP1254478A1; JP2003520351A

Abstract

Electron-hole production at a Schottky barrier has recently been observed experimentally as a result of chemical processes. This conversion of chemical energy to electronic energy may serve as a basic link between chemistry and electronics and offers the potential for generation of unique electronic signatures for chemical reactions and the creation of a new class of solide state chemical sensors. Detection of the following chemical species was established: hydrogen, deuterium, carbon monoxide, molecular oxygen. The detector (1b) consists of a Schottky diode between an Si layer and an ultrathin metal layer with zero force electrical contacts.

Description

Title of the Invention:

A CHEMICAL SENSOR USING CHEMICALLY INDUCED ELECTRON-HOLE PRODUCTION AT A SCHOTTKY BARRIER

Be it known that we, Eric W. McFarland, Howard S. Bergh, Brian Gergen, and W. Henry Weinberg, citizens of the United States, Herman Nienhaus, a citizen of Germany, and Arunava Mujumdar, a citizen of India, have invented a new and useful method and apparatus for a chemical sensor for gas detection using chemically induced electron- hole production at a Schottky barrier of which the following is a specification.

The invention is a demonstrated with atomic hydrogen, H, deuterium, D, carbon monoxide, CO, and molecular oxygen, O₂, chemisorption on Ag, Cu, and Fe thin films. However, many other configurations are possible. When these or other metals are deposited on a semiconductor in a Schottky diode detector structure, a current can be measured when different species are incident on the metal surface. This "chemicurrent" is a result of chemisorption induced excited charge carriers which pass over the Schottky barrier. That energy transfer from chemisorption can proceed by direct electronic excitation was predicted to be possible, however, never directly observed. It is commonly thought that the heat of adsorption is dissipated primarily as phonon excitations. In one embodiment, the process is specific for atomic H or atomic D as opposed to molecular H₂ or D₂ and is the first direct means of measuring specifically atomic H or D, more importantly the sensor can be used to differentiate H from D. In addition, chemical reactions occurring at surfaces can be uniquely identified by their chemicurrent signature. In extensions of the basic idea: 1) sensing of specific chemicals and chemical reactions is possible, 2) sensors sensitive for a variety of specific atoms and molecules in gas or liquid states can be fabricated by incorporation of semipermeable, selective membranes; and 3) "artificial nose" type sensor systems can be fabricated by creating an array of sensors with different metals and semiconductor substrates. The invention is better understood by considering the attached specification and Appendices. A Chemical Sensor Using Chemically Induced

Electron-Hole Production at a Schottky Barrier

Background of the Invention

1. Field of the Invention

The field endeavor of the invention relates to sensors for detecting chemicals

and in particular to a sensor for detecting and distinguishing atomic hydrogen or atomic deuterium oxygen, carbon monoxide, and nitric oxide.

2. Description of the Prior Art

Electron transport through a metal-semiconductor interface is determined largely

by the Schottky barrier between them.

The detailed pathways of energy transfer in exothermic and endothermic

reactions at metal surface is incompletely understood and of fundamental interest.

Bond formation energy of up to several electron volts is transferred into the substrate

during such exothermic reactions. Since bulk phonon energies are typically two orders

of magnitude smaller, it has been appreciated by the prior art that non-adiabatic

excitations of electron-hole pairs may be an alternative to the creation of multiple

phonons as a mechanism for sensor detectors. With surface reactions at thermal

collision energies, there are few examples of energy transferring to the electronic

system accompanied by light emission or chemiluminescence and exoelectron ejection.

Chemiluminescence and exoelectron injection are observed only with exothermic adsorption of electronegative molecules on reactive metal surfaces. In addition, exoelectron emission requires that the metal have a low work function. Heretofore, there has been no direct experimental evidence for adsorption induced electron-hole pair excitations at transition metal surfaces. Therefore, what is needed is some type of sensor design or principal in which adsorption induced electron hole pair excitations at a transition metal surface can be exploited to provide a chemical sensor.

Brief Summary of the Invention

The invention is a silicon device structure, or more specifically a metal- semiconductor Schottky diode, which exploits the current-voltage characteristics of the diode for separation of charge and the interaction of the surface adsorbates on the metal to produce electrons or holes of sufficient energy to transverse the ultrathin metal film and cross the Schottky barrier. The structure allows reliable, zero force electrical contacts to be made to metal films less than 100 Angstroms thick. In one embodiment two metalized contacts are deposited using photolithographic techniques on a 4000 Angstrom oxide layer prepared on Si (111). The oxide is etched from between the contacts and the exposed 6mm x 6mm Si (111) surface is wet chemically treated. Under vacuum conditions ultrathin metal is deposited onto the device to form a diode under well defined conditions. The sensor device may be microfabricated on n- or p-doped semiconductor

wafers. In the illustrated embodiment p_n = 5-10 Ω cm, p_p = 1-20 Ω cm), in an ohmic

contact is provided on the back of wafer by means of by As⁺ and B⁺ ion implantation,

respectively. Isolated from the silicon, the thick gold contact pads are evaporated on a

4000 angstrom thermal oxide layer on the opposing or front side of the device. A 0.3

cm² window is chemically wet etched through the oxide layer between isolated the gold

pads through the use of buffered hydrofluoric acid leaving a clean, passivated silicon

surface. The device is then transferred into an ultrahigh vacuum chamber (p « 10⁸ Pa)

for metal deposition and measurement.

Copper and silver films, for example, are deposited by e-beam evaporation at

substrate temperatures of 135° K. The nominal thickness is measured by a quartz

microbalance. The etching of the oxide produces an angle of inclination between the

oxide and the top surface of the silicon substrate with typically 25°. The evaporated

thin metal films are connected to the thick gold pads across the small inclination angle

to provide a zero force front contact to the device. This contact design allows electrical

contact for the current/voltage measurements between the front contacts and back

contact even with film thicknesses below 80 angstroms.

In preliminary experiments investigating the energy transfer during

chemisorption, a new process has been discovered associated with chemisorption of

atomic hydrogen or atomic deuterium on Ag and Cu ultrathin films. When these metals

are deposited (30 Angstroms -150 Angstroms) onto Si(111) in a Schottky diode

detector structure, a current is generated associated with an incident atomic H or D beam on the film. It is hypothesized that this "chemicurrent" is a result of chemisorption

induced excited charge carriers which traverse the Schottky barrier. That energy

transfer from chemisorption can proceed by direct electronic excitation is a significant

departure from the conventional dogma which holds that multiple phonon excitation is

the means through which the heat of adsorption is dissipated.

The implications of this observation for the study of surface catalyzed reactions

are many. In addition, this process serves as a basic link between chemical processes

and electronics and offers the potential for the generation of unique electronic

signatures for chemical reactions and the creation of a new class of solid-state chemical

sensor. The first direct means of measuring atomic H or atomic D separate from the

diatomic molecule is demonstrated below. More importantly, it may also be possible to

differentiate H from D on the basis of the signal. It is expected that there are unique

chemicurrent signals associated with many types of surface reactions.

Hot electrons and holes created at a transition metal surface, such as a silver or

copper surface by adsorption of thermal hydrogen and deuterium atoms can be

measured directly with ultrathin-metal film Schottky diode detectors on silicon (111)

according to the invention. When the metal surface is exposed to these atoms, charge

carriers at the surface and travel ballistically toward the interface. The charge carriers

are detected as a chemicurrent in the diode. The current decreases with increasing

exposure and eventually reaches a constant value at a steady state response. The

invention uses the first discovery of a non-adiabatic energy dissipation during adsorption at a transition metal surface as a means of providing a chemical sensor or thin film "nose" able to sniff out the presence of chemicals.

The mechanism of the invention is based on the speculation that although the

maximum energy of any hot charge carriers are smaller than the metal work function of

the transition metal surface thereby precluding exoelectron emission, the energy of the

hot-charged carriers may be sufficiently large enough to enable the charge carriers to

be collected by crossing a smaller potential barrier. As will be described below the

direct detection of chemisorption-induced electron-hole pairs is feasible using a

Schottky barrier by transition metal-semiconductor diode detector. The invention shall

be described in terms of an atomic hydrogen adsorption on copper and silver film

surfaces, however, it is to be expressly understood that many other chemical molecules

or elements may be detectable on these and other different thin film metal surfaces

according to the teachings of the invention. Silver and copper film surfaces exhibit high

reactivity to atomic hydrogen, but negligible dissassociative adsorption of molecular

hydrogen, H₂. The formation energy of the hydrogen-metal bond is large, about 2.5

electron volts in both cases. To detect the hot charged carriers, a sensor is provided

which is comprised of a large area of metal-semiconductor contact with an ultrathin

metal film.

The device structure allows current-voltage curves to be measured from which

Schottky barrier heights and ideality factors as a function of metal film thickness can be

determined. It is observed that barrier heights increases and ideality factors decreases with increasing metal film thickness (10 Angstroms to 100 Angstroms). Room temperature annealing of diodes produced with a low temperature metalization

increases the measured barrier heights and lowers the ideality factors. The magnitude

of these effects depends on the metal used. Results for iron and copper on silicon

(111) substrates are among the embodiments described below.

The rectifying properties of the Schottky diode formed are improved by annealing

the devices to room temperature and cooling back to 135° K. The measured l-V curves

can then be analyzed using thermionic emission theory. Effective barrier heights of 0.6

- 0.65 electron volts and 0.5 - 0.55 electron volts were determined for copper and silver

films of 75 angstrom thickness on n-silicon (111), respectively. On p-silicon (111), silver

and copper diodes showed barriers of 0.5 - 0.6 electron volts. Ideality factors between

1.05 and 1.5 indicate that large-area diodes are laterally nonuniform and exhibit a

barrier height distribution.

The invention now having been briefly summarized turn to the following drawings

wherein like elements are referenced by like numerals.

Brief Description of the Drawings

Fig. 1(a) is a Fermi diagram of the chemicurrent detection. Hydrogen atoms

react with the metal surface creating electron-hole pairs . The hot electrons travel

ballistically through the film into the semiconductor where they are detected.

Fig. 1(b) is a schematic side cross-sectional view through a hydrogen sensing

Schottky diode made according to the invention as described by the Fermi diagram of

Fig. 1(a). The ultrathin metal film is connected to the gold pad during evaporation. Fig. 1(c) is a plan elevational view of the device of Figs. 1(a) and 1(b).

Figs. 2(a) and 2(b) are graphs of the chemicurrent as a function of hydrogen

exposure time for diodes with thin silver and copper films respectively in a device

shown in Figs. 1(a) and 1(b). The transients correspond to the filling of empty

adsorption sites by atomic hydrogen on the metal surfaces. The steady-state currents

are explained by a balance of abstraction and re-adsorption of atomic hydrogen.

Fig. 3 is a graph of the chemicurrent, I, as a function of time, t, recorded from

silver/n-Si (1 11) diodes of the type shown in Figs. 1 (a) and 1(b) exposed to atomic

hydrogen and deuterium. The chemicurrent due to atomic hydrogen adsorption is

multiplied by a factor of 0.3.

Fig. 4 is a graph of the chemisorption current for a 60 Angstrom Ag/Si (111)

sensor at 135K as a function of the time of exposure to CO.

Fig. 5 is a graph of the chemisorption current for an 80 Angstrom Ag/Si (111)

sensor at 135K as a function of the time of exposure to CO.

Fig. 6 is a diagrammatic side view of a sensor used for catalytic chemisorption

detection.

Fig. 7 is an array of sensors of the type shown in Fig. 10 in which each one of

the sensors has a different catalytic layer so the corresponding sensor detects a

different reactant. Fig. 8 is a graph of the chemisorption current for molecular oxygen on a 75

Angstrom Ag/Si (111) sensor at 130K as a function of the time of exposure to O₂. The invention now having been illustrated in the foregoing drawing the invention

and its various embodiments now may be understood in context in the following

detailed description.

Detailed Description of the Preferred Embodiments

Electron-hole production at a Schottky barrier has recently been observed

experimentally as a result of chemical processes. This conversion of chemical energy

to electronic energy may serve as a basic link between chemistry and electronics and

offers the potential for the generation of unique electronic signatures for chemical

reactions and the creation of a new class of solid state chemical sensors. The initial

results have been for a atomic and molecular adsorption, however, it also expected that

bimolecular surface catalyzed reactions may also cause direct excitation of charge

carriers during the formation of bonds between surface adsorbed species. Therefore,

in addition to the demonstrated detection of hydrogen, deuterium and oxygen,

sensitivity for chemisoprtion for carbon monoxide, carbon dioxide, molecular and atomic

oxygen, molecular and atomic nitrogen, nitrogen monoxide and organic hydrocarbons

and other species is expected. Detector responses to surface catalyzed reactions of

several different combinations of these species following adsorption are expected to

produce a chemicurrent including reactions with the combinations of carbon monoxide

and molecular oxygen, carbon monoxide with nitrogen oxide and molecular hydrogen

and oxygen. The basic configuration of the detector can be extended to include

selective coatings, mult-juncition arrays, and tunnel junctions.

The mechanism of current production in a sensor is best illustrated in Fig. 1 (a) in

the case of hot electrons. Fig. 1(a) is an energy diagram of a charge carriers across the

metal film to silicon interface with the position in the interface being shown on the horizontal axis and energy on the vertical axis. Fig. 1(b) is a corresponding side cross-

sectional view in an enlarged scale of the junction which is graft in Fig. 1 (a). Fig. 1(c) is

a plan elevational view of the device of Figs. 1(a) and 1(b). Transition metal film 10 is

evaporated on an n-type silicon 12 forming a diode at their interface 14 with a Schottky

barrier Φ illustrated in Fig. 1(a). Fig. 1(a) shows the Fermi level, E_F, also denoted by

reference numeral 16, the conduction band minimum, denoted by reference numeral 18

and the valence band minimum denoted by reference numeral 20. If the exothermic

chemisorption of hydrogen atoms creates electron-hole pairs, hot electrons may travel

ballistically through film 10 and across the potential barrier of the Schottky diode Φ .

The electrodes can be detected as a current which is defined as the "chemicurrent."

Similarly, hot holes may be measured with a p-type electrode as well as an n-type as

shown in the illustration of Figs. 1(a) and (b). The charge carrier energies lie between

the barrier height and the adsorption energy, i.e., between 0.5 and 2.5 electron volts

above E_F 16. The mean free path (mfp) of electrons and holes in this energy range is

typically on the order of 100 angstrom, as determined by thermal and field emission,

internal photoemission and ballistic electron emission microscopy. The film thickness is

in the range of the mean free path of the charge carriers (electrons or holes).

A silicon based device 22 was developed to facilitate contacting extremely thin

metalization layers 10 during the initial Schottky barrier formation. Devices 22 were

prepared on silicon (111) substrates 12 and processed using conventional silicon

microfabrication techniques to produce the device depicted in Figs. 1(a), 1(b) and 1(c).

Microfabricated substrates 12 were made from 3" diameter 5 Ω-cm phosphorous doped n-type Si (111) wafers. Before processing, the wafers were backside ion implanted with

10¹⁵ cm ² arsenic at 150 keV. After implantation the wafers were diced into rectangular

samples 0.45-x0.70". The samples were then cleaned by sonication in water, acetone

and isopropanol and were wet oxidized in a tube furnace to grow between 3000 and

4000 Angstrom thermal oxide. For the processing of the substrates, AZ5214 image

reversal photoresist was used as a positive resist. Two photolithographic masks were

used, one for front metal pads 26 and one for oxide window 30 between pads 26. The

first step of the processing was to metalize front contact pads 26. An oxidized

substrate 12 was spin coated with photoresist and patterned using a UV mask aligner.

Metal pads 26 were deposited in a thermal evaporator using an initial adhesion layer of

100 Angstroms chromium followed by 2000 Angstroms of gold. After metalization, the

excess metal was removed in an isopropanol sonication lift-off. This completed front

contacts 26 and the next step was to make back ohmic contacts 24. The ion

implantation was activated during the thermal oxidation so that under backside oxide 25

of silicon substrate 12 was n+. Front side 32 of substrate 12 was coated with a

protective photoresist layer and backside oxide 25 was removed with buffered

hydrofluoric acid, HF. The backside metalization was done through an aluminum

shadow mask. Back contacts 24 were Cr (100 Angstroms) / Au (3000 Angstroms)

deposited in a thermal evaporator. To complete the backside metalization, the frontside

photoresist was removed in an isopropanol sonication. The final step of the processing

was to etch a window 30 in the SiO₂ layer 28 between front contact pads 26. The

sample was recoated with photoresist and patterned with the mask aligner. The photoresist was developed and the exposed oxide region was removed with a six

minute buffered HF dip. After this step the photoresist was removed by 85°C H₂O₂:

H₂SO₄ solution. The sample was subsequently cleaned and chemically oxidized in a

fresh H₂O₂: H₂SO₄ solution at 110°C. The final step was to prepare the silicon surface.

After removal from the sulfuric acid, the sample was dipped in buffered HF for 15

seconds, which was just long enough to ensure removal of the chemical oxide off the

silicon surface. The sample was finally rinsed in deionized water and blown dry with

nitrogen to complete the processing. Because of the etching properties of the buffered

HF solution and the photoresist, the resultant oxide had a gentle slope of 15-20

degrees from the unetched SiO₂ down to the silicon substrate. This angle was

independently measured by a scanning electron microscope (SEM) and an atomic force

microscope (AFM). Sloping oxide sidewall 34 allows thin Schottky metalization layer 10

to connect continuously from one gold contact pad 26 to the other.

After the final buffered HF dip to prepare a hydrogen terminated and passivated

surface, microfabricated substrate 12 was quickly indium bonded to a molybdenum

sample holder and loaded into an ultrahigh vacuum chamber onto a sample

manipulator (not shown). The manipulator has four independent electrical contacts, two

front and two back contacts. The two front contacts can be actuated from outside the

vacuum chamber and were used to electrically contact gold pads 26 on the right and

left sides of substrate 12 while the back contacts 24 contact the molybdenum sample

holder. After a sample was in place on the manipulator, it was checked for contact-to-

contact current leakage. All samples used for experiments had room temperature left- front-contact to right-front-contact resistance greater than 100 MΩ and front-to-back

resistances greater than 10 MΩ .

Metal films 10 were evaporated by shuttered electron-beam wire evaporators.

The evaporation rate depended on the metal used. In the embodiment where iron was

used, iron was evaporated at 10 watts with a rate of 10 Angstrom min^"1, and copper was

evaporated with a heating power of 16 watts and a rate of 1.5 Angstrom min ¹. The

evaporator produced a collimated flux that deposited a rectangular area of metal onto

substrate 12. Evaporated metal film 10 spanned metal contact pads 26 on either side

of substrate 12, but did not extend out to the edge of substrate 12.

Diodes were made on room temperate substrates as well as substrates cooled

to -130K with liquid nitrogen. A Labview virtual instrument (VI) was used to automate

current-voltage measurements. The voltage source was a digital-to-analog board

controlled by the computer and the current was measured with a Kiethley picoammeter

under GPIB control from the computer.

In the present demonstration of device 22, device 22 was maintained at 135° K

and exposed to a modulated, thermal hydrogen beam produced by a microwave

plasma. Photons are extracted from the beam to avoid photoexcitation which can be

orders of magnitude stronger than the chemicurrent. A light blocking fixture was

developed for the plasma tube which prevents photon transmission and thermalized the

beam particles as is described in H. Nienhaus, et al., " ," (to be

published). The content of atomic hydrogen, C_H, i.e. the number of atoms relative to the

total number of particles in the beam, was measured with an in-line mass spectrometer. It varied typically between 7 - 25% where the variations were associated with the

plasma fluctuations. The kinetic energy of the atomic hydrogen was also measured

between 300 and 350 K. The total atomic and molecular hydrogen impinging upon

diode 22 was approximately constant, e.g., about 1013 particles per second. Hence,

with a sensor area of 0.3 cm², the atomic flux varied between 3 and 10 x10¹² hydrogen

atoms per cm²-second. The reaction-induced chemicurrent was detected between the

front contact 26 and back contact 24 using standard lock-in techniques. No bias was

applied to detector 22 during measurement. Due to the low temperature, the noise

level was less than 0.5 picoamps.

Detector current responses as a function of time of device 22 in response to

atomic hydrogen are shown in Fig. 2 in which the chemicurrent is mapped against

exposure times. Fig. 2(a) is a graph showing the chemicurrent in silver/n-silicon,

interface and a silver/p-silicon interface. Fig. 2(b) shows the chemicurrent for a

copper/n-silicon diode. The atomic impingement rate, q_H, was 7.5 ± 2.5 x 10¹¹ atoms

per second. At t = 0, the beam shutter was opened. Current increases instantaneously

upon exposure and decays exponentially, and eventually reaches a steady state of

value as shown in Figs. 2(a) and 2(b) at each of the diode embodiments. The dip

observed in the l/t curve for copper at about 2,000 seconds is due to a decrease in

atomic hydrogen flux due to plasma instabilities. The atomic hydrogen content, C_H,

decreases from 15% at t = 1600 seconds to below the detection limit of 2% at t = 2,100

seconds in Fig. 2(b) . The chemicurrent then recovers its original value. The total

beam intensity, atomic and molecular hydrogen remained constant during this time. Thus, chemicurrent is only detected if atomic hydrogen is present. Fig. 2(a) shows that

chemicurrents were detected for both p- and n-type silver/silicon diodes, thereby

implying that both hot electrons and hot holes are created by the reaction.

The chemicurrent transient shown in Figs. 2(a) and 2(b) represents the

occupation of empty adsorption sites by atomic hydrogen on metal film 10. The

hydrogen coverage, Θ , increases and the adsorption probability decreases with the

decreasing availability of empty sites. The steady-state chemicurrent observed in the

long time limit in Figs. 2(a) and 2(b) is a consequence of a balance between hydrogen

removal from the surface by abstraction and re-adsorption. The chemicurrent, I, is

expected to be proportional to the hydrogen atom flux and the fraction of unoccupied

adsorption sites, i.e., I = a q_H (Θ _s, - Θ ), where Θ _s is the saturation coverage if no

abstraction occurs and is a constant.

If the adsorption of atomic hydrogen and its abstraction by atomic hydrogen in

the gas phase are governed by the Langmuirian and an Eley-Rideal mechanism,

respectively, the time rate equation for I and Θ obey a first-order kinetics described by:

d l /df oc - dΘ/df = - (q_H/A) [σ _a (Θ _s, - Θ ) - σ_r Θ ],

where A is the active diode area, and σ _a and σ.are the cross sections for adsorption

and abstraction, respectively. By considering the time limits for t = 0 and t → infinity,

the ratio of the cross sections may be determined from the maximum value, l_max, and

the steady state value, l_s of the chemicurrent via σ _a/σ_r= l_s/ (l_max -l_s). Cross section

ratio is calculated from the data in Figs. 2(a) and 2(b) are 0.2 for the silver/n-silicon

diode and 0.4 for the two other diodes. Equation (1) above predicts an exponential decay of the chemicurrent with a time constant of A/q_H (σ _a + σ _r ). Single exponential

fits to the data in Fig. 2 result in decay constants of 480 seconds for the silver/p-silicon

diode, 670 seconds for the copper/n-silicon diode, and 750 seconds for silver/n-silicon

diode. The observed variation is within the range of uncertainties of the beam flux. The

cross section ratio and decay constant allow the calculation of an absolute cross

section if the active diode area, A, and the hydrogen atom rate, q_H are known. With an

active area A = to about 0.3 cm², the analysis gives values for σ _a of approximately

5x10^"16 cm² for silver and 4x10^"16 cm² for the copper film. With assumed initial

adsorption probability of unity, the reciprocal of the cross section σ_a is equal to the

adsorption site density. From the data, in Figs. 2(a) and 2(b), the adsorption site

densities of 2-3x 10¹⁵ cm^"2 for both, silver and copper films obtained. These values are

in excellent agreement with the number of metal atoms per unit area which is about

2.4x 10¹⁵ cm² for silver on (111 silicon) and 3.1x10¹⁵ cm² copper on silicon (111)

surfaces. This data supports the interpretation of the l/t curves of Figs. 2(a) and 2(b)

given above. Furthermore, the data shows a new way of measuring selfabstraction

rates with only one atomic species. In the prior art, the abstraction of atomic hydrogen

at surfaces is studied by deuterium-hydrogen or hydrogen-deuterium exchange

reactions.

The chemicurrent is attenuated exponentially with increasing metal thickness in

the silver/n-silicon diodes. The attenuation length correlates well with the mfp of

electrons in silver, which further supports the idea that the charge carriers are created

at the metal surface and travel ballistically through the metal film into semiconductor 12. The quantitative difference between the n-type silver and copper Schottky diodes

shown in Figs. 2(a) and 2(b) is striking. The sensitivity may be defined by dividing the

initial chemicurrent at t = 0 by the hydrogen atom impingement rate. This gives the

number of electrons detected per adsorption event as 4.5x10³ for silver and 1.5x10^"4 for

copper, an order of magnitude difference. On p-type diodes, the same sensitivity ratio

is found. Thus, the difference does not correspond to a barrier height difference, which

is only observed with n-type Schottky diodes. The sensitivity difference is the standard

attributed to two effects. First, the mfp electrons and copper films has been measured

to be half that of mfp and silver films. Second, the interface properties of silver/silicon

and copper/silicon are very different, e.g., the copper reacts with silicon and may form a

suicide while similar reactions are not known for silver on silicon. Since the diodes are

annealed, copper/silicon interfaces are expected to be rougher and have more

scattering centers than silver/silicon interfaces. The enhanced roughness may reduce

the transmission probability considerably, in agreement with reported results on mfp in

suicides which are smaller than in metals.

The p-type silver/silicon diodes seen in Fig. 2(a) is approximately 3.5 times less

sensitive than n-type device. These might be explained by differences in the mfp path

of holes and electrons in silver, as observed previously in gold and in platinum silicon

thin films. In these prior art observations, the attenuation lengths of holes were a factor

of approximately 1.5 smaller than for electrons. Additionally, sensitivity differences may

be related to the energy spectra of holes and electrons excited by the surface reactions.

The d-bands of bulk silver cannot contribute to the ballistic current, since they are more than 2.7 electron volts below the Fermi energy. The ballistic charge carriers thus have

nearly a free SP character. The probability of exciting an electron-hole pairs is assumed

to depend on the joint density of states of occupied and empty electronic states. Since

the density of states of silver increases slightly with energy in the range of ± 3 electron

volts around the Fermi energy, electrons closer to the Fermi energy are excited more

effectively. Consequently, the energy distributions of ballistic holes and electrons are

not symmetric around the Fermi energy and on the average the ballistic electrons are

expected to have higher kinetic energies than hot holes. Such an asymmetry would

lead to a significant sensitivity difference between p- and n-type diodes.

Fig. 3 is a graph of the chemicurrent as a function of time for atomic hydrogen

and deuterium reacting with a 75 angstrom silver film on n-silicon (111). The

oscillations in the decay curve for deuterium are due to plasma fluctuations. Although

for the exposure graph of Fig. 3 the impingement rate of atomic deuterium was

approximately twice as large as that for atomic hydrogen, the measured chemicurrent

with deuterium exposure was smaller by a factor 3, i.e., a sensitivity to atomic

deuterium is six times smaller than that to atomic hydrogen. The slight differences in

the strengths of hydrogen and deuterium metal bonds cannot explain this observed

isotope effect. A reduced adsorption probability for deuterium on silver would also not

account for this observation, since this would affect the decay rate as well. The decay

rates in Fig. 3 differ by a factor of approximately 1.8 which may be exclusively attributed

to the flux difference between hydrogen and deuterium. The isotope effect implies

different velocities and interaction times of the incoming hydrogen and deuterium by a factor of v². The interaction time, however, is still in the 10^"13 second range which is at

least an order of magnitude longer than time constants of electron transfer between the

substrate and the impinging atoms. For the same reason, we exclude internal

exoelectron emissions which requires quenching of resonant charge transfer into the

affinity level of the approaching atom accompanied by a drastic change of the surface

oxidation state.

It is believed that the more relevant mechanism behind the isotope effect is likely

to be the de-excitation of highly excited vibrational states formed under chemisorption.

The transition probability between two vibrational levels in an nonharmonic potential

decreases the larger the difference of the two respective quantum numbers. Hence,

de-excitation most likely occurs in multiple steps. The spacing between the vibrational

levels, i.e., the density of states of vibrational states, determines the released energy in

each step, and the states in the enharmonic deuterium-silver potential are closer to

each other than for the hydrogen-silver bond. Since the formation energies of

deuterium-silver and hydrogen-silver bonds are almost identical, the deuterium-silver

vibrational energy may be relaxed in more steps of smaller energy quanta compared to

the hydrogen-silver case. This would result in ballistic charge carriers of lower energies

and explain the smaller sensitivity to deuterium.

In summary, the foregoing disclosure is the first direct detection reported of hot

electrons and holes excited by adsorption of atomic hydrogen deuterium on ultrathin

silver and copper films as a chemicurrent. The current is measured in the large-area

Schottky diode formed from these metals on oriented silicon (111). The devices are unique sensors that can discriminate atomic from molecular hydrogen as well as

deuterium from hydrogen atoms. The chemicurrents decay exponentially with exposure

time and reach a steady-state value. This behavior corresponds to occupation of free

adsorption sites by hydrogen atoms and a balance between adsorption and abstraction.

The currents are smaller if p-type semiconductors are used and if the devices are

exposed to deuterium rather than hydrogen. This isotope effect opens a new way of

monitoring reactions on metal surfaces and will certainly initiate further investigations to

clarify the mechanism of the excitation. We have developed a reliable device structure

for the fabrication of ultrathin Schottky diodes.

Many alterations and modifications may be made by those having ordinary skill

in the art without departing from the spirit and scope of the invention. Therefore, it must

be understood that the illustrated embodiment has been set forth only for the purposes

of example and that it should not be taken as limiting the invention which could be more

broadly or narrowly defined later by patent claims.

For example, it is expected that the chemoelectric phenomena associated

with atomic and molecular interactions at metal surfaces will be found to show that

chemical reactions at metal surfaces can directly transfer reaction energy to electrons in

the metal. The phenomena can thus be utilized as the basis of a new class of solid

state sensors. The adsorption induced current of different transition metal-

semiconductor combinations will provide a means of systematically varying the

relationships between the adsorbate and the metal surface and the electronic

environment in the metal at the metal-semiconductor interface, and within the semiconductor. New sensor structures will have improved device sensitivity and allow

discrimination of the electron energy with operation at room temperature and above.

Bimolecular surface catalyzed reactions in addition to chemisorption is usable for direct

excitation of charge carriers during formation of bonds between surface adsorbed

species. In addition to the sensor performance and sensitivity for detection of

hydrogen, several important adsorbates are possible expressly including CO, CO₂,

O₂(0), N₂(N), NO, C₂H₂, C₂H₄, and C₂H₆. Fig. 4 shows the chemisorption current as a

function of time for CO with a 60 Angstrom Ag/n-silicon (111) sensor of the invention at

135K.. Fig. 5 shows the chemisorption current as a function of time for CO with an 80

Angstrom Ag/n-silicon (111) sensor of the invention at 135K. Fig. 8 shows the

response to molecular oxygen. Each adsorbate will have a unique current intensity and

rate of signal decay which will allow differentiation of adsorbates.

The sensor response to surface catalyzed reactions of several combinations of

these species following absorption are with the scope of the invention expressly

including the reactions of CO + O₂, CO + NO, and H₂ + O₂. In the sensor 40 of Fig. 6 a

catalytic layer 46 is added on top of metal layer 44 disposed on doped silicon layer 42

fabricated in a manner consistent with the teachings of the invention. The

chemisorption current is measured by an integrating voltage amplifier 48. Catalytic

layer 46 is chosen specifically to catalyze a selected reaction which then directly

interacts with metal layer 44 to create a measurable chemicurrent. As shown

diagrammatically in Fig. 7 a plurality of sensors 40 of the type shown in Fig. 6 can then

be combined in an array, each one of which plurality of sensors 40 has a different catalytic layer 46 to detect a corresponding plurality of different adsorbates through x and y-addressing circuits 50 and current detector 52. In this manner an electronic nose is realized.

The words used in this specification to describe the invention and its various embodiments are to be understood not only in the sense of their commonly defined meanings, but to include by special definition in this specification structure, material or acts beyond the scope of the commonly defined meanings. Thus if an element can be understood in the context of this specification as including more than one meaning, then its use in later in a claim must be understood as being generic to all possible meanings supported by the specification and by the word itself.

The definitions of the words or elements of the following claims are, therefore, defined in this specification to include not only the combination of elements which are literally set forth, but all equivalent structure, material or acts for performing substantially the same function in substantially the same way to obtain substantially the same result. In this sense it is therefore contemplated that an equivalent substitution of two or more elements may be made for any one of the elements in later defined claims or that a single element may be substituted for two or more elements in later defined claims.

Insubstantial changes from the claimed subject matter as viewed by a person with ordinary skill in the art, now known or later devised, are expressly contemplated as being equivalently within the scope of the invention. Therefore, obvious substitutions now or later known to one with ordinary skill in the art are defined to be within the scope

of the defined elements.

The invention is thus to be understood to include what is specifically

illustrated and described above, what is conceptionally equivalent, what can be

obviously substituted and also what essentially incorporates the essential idea of the

invention.

Claims

1. An apparatus comprising: a metal-semiconductor Schottky diode, characterized by an ultrathin metal film and Schottky barrier with current-voltage characteristics of a diode for separation of charge and the interaction of the surface adsorbates on said metal film to produce electrons or holes of sufficient energy to transverse said ultrathin metal film and cross said Schottky barrier; and zero force electrical contacts coupled to said metal film, said zero force electrical contacts being less than 100 Angstroms thick.

2. The apparatus of claim 1 wherein said metal-semiconductor Schottky diode is formed in a wafer of silicon substrate and wherein said zero force electrical contacts comprise two metalized contacts and a 4000 Angstrom oxide layer which are deposited using photolithographic techniques on said 4000 Angstrom oxide layer prepared on said silicon substrate.

3. The apparatus of claim 2 wherein said oxide is etched from between said contacts to expose said silicon substrate which is wet chemically treated.

4. The apparatus of claim 3 wherein said ultrathin metal is deposited under vacuum conditions onto said silicon substrate to form a diode.

5. The apparatus of claim 2 wherein said silicon substrate is microfabricated on n- or p-doped semiconductor wafers.

6. The apparatus of claim 5 wherein n-doped semiconductor wafers have p_n = 5-10 Ω cm, and wherein p-doped semiconductor wafers

have p_p = 1-20 Ω cm, and said silicon substrate is (111) silicon.

7. The apparatus of claim 2 wherein said wafer has an upper surface on which said zero force contacts are disposed and a back on which an ohmic contact is provided by means of by As^* and B⁺ ion implantation.

8. The apparatus of claim 7 further comprising thick gold contact pads are evaporated on said 4000 angstrom thermal oxide layer and thus isolated from said silicon substrate, and a window which is chemically wet etched through said 4000 angstrom thermal oxide layer between said isolated the gold pads through the use of buffered hydrofluoric acid leaving a clean, passivated silicon surface in said window.

9. The apparatus of claim 8 further comprising metal depositions formed in an ultrahigh vacuum chamber (p « 10^"8 Pa).

10. The apparatus of claim 1 wherein said ultrathin film is comprised of metal films deposited by e-beam evaporation.

11. The apparatus of claim 10 wherein said ultrathin film is comprised of copper deposited at substrate temperatures of 135° K.

12. The apparatus of claim 10 wherein said ultrathin film is comprised of silver deposited at substrate temperatures of 135° K.

13. The apparatus of claim 10 wherein said ultrathin film is comprised of iron.

14. The apparatus of 7 wherein said oxide is etched from between said contacts to expose said silicon substrate which is wet chemically treated and wherein said etched oxide produces an angle of inclination between the oxide and a top surface of said silicon substrate, said evaporated thin metal films being connected to said thick gold pads across said angle of inclination to provide said zero force contact to allow electrical contact for the current/voltage measurements between front and back contacts on said silicon substrate even with metal film thicknesses below 80 angstroms.

15. A method associated with chemisorption of atomic hydrogen or atomic deuterium on Ag and Cu ultrathin films comprising: providing Ag and Cu ultrathin films deposited onto a silicon surface in a Schottky diode detector; generating an incident atomic H or D beam; and generating a current associated with said incident atomic H or D beam on said Ag and Cu ultrathin films so that a chemicurrent results from chemisorption of induced excited charge carriers which traverse a Schottky barrier in said Schottky diode detector.

16. The method of claim 15 further comprising directly measuring atomic H or atomic D separately from the diatomic H₂.

17. The method of claim 15 further comprising differentiating H from D on the basis of said current, an unique chemicurrent being associated with a type of surface reaction.

18. A method associated with chemisorption of a reactant on a ultrathin transition metal film comprising: providing said ultrathin transition metal film deposited onto a silicon surface in a Schottky diode detector; generating an incident stream of said reactant; and generating a current associated with said incident stream on said ultrathin films so that a chemicurrent results from chemisorption of induced excited charge carriers which traverse a Schottky barrier in said Schottky diode detector.

19. The apparatus of claim 1 further comprising a catalytic layer disposed on top of said ultrathin metal film layer which is disposed on said silicon substrate, said catalytic layer being chosen specifically to catalyze a selected reaction which then directly interacts with said ultrathin metal film to create a measurable chemicurrent, said apparatus being defined as a catalytic sensor.

20. The apparatus of claim 19 wherein said catalytic layer catalyzes reactions which include at least one of the group, CO + O₂, CO + NO, and H₂ + O₂.

21. The apparatus of claim 19 further comprising a plurality of catalytic sensors combined in an array, an x and y-addressing circuit and a current detector, each one of which plurality of sensors having a different catalytic layer to detect a corresponding plurality of different adsorbates through said x and y-addressing circuits and said current detector.