Active solid-state devices (e.g. – transistors – solid-state diode – Thin active physical layer which is – Heterojunction
Reexamination Certificate
2001-05-21
2003-03-18
Nelms, David (Department: 2818)
Active solid-state devices (e.g., transistors, solid-state diode
Thin active physical layer which is
Heterojunction
C257S017000, C257S021000, C136S206000, C136S255000
Reexamination Certificate
active
06534784
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates generally to optoelectronic devices and, more particularly, to electron tunneling devices, especially for solar energy conversion.
Recent energy crises have highlighted the growing demands placed on traditional sources of power, such as gas and electricity. With rising energy costs, it is desirable to find alternative power sources to augment traditional power sources such as hydroelectric and thermonuclear. Solar energy conversion provides such an alternative by tapping into the readily available power of the sun.
One of the main obstacles preventing the proliferation of solar energy conversion systems is efficiency. Currently available semiconductor solar cell systems are not able to provide the amount of power for the dollar that is possible by traditional power sources. Especially semiconductor solar cells with high energy conversion efficiency (ratio of incident solar power to electrical power out) are expensive. Most solar cell systems are based on semiconductor technology, which can be difficult to scale to the size required for large solar panels. Using the present technology, it is expensive to fabricate a semiconductor-based solar panel which is large enough to replace the traditional sources of power. Moreover, semiconductor devices are generally single bandgap energy devices. This characteristic of semiconductor devices means that no current is produced when a photon having energy less than the bandgap energy is incident on the semiconductor device and, when a photon having energy greater than the bandgap energy is incident on the semiconductor device, only current corresponding to the bandgap energy is produced in the semiconductor device. In other words, the response of the semiconductor device is limited by the bandgap energy. Thus, the semiconductor device does not respond at all to photons having energy less than the bandgap energy, and incident electromagnetic energy in excess of the bandgap energy is wasted in the energy conversion. Therefore, the energy conversion efficiency of the semiconductor device is low, on the order of 25% or less. Therefore, it would be desirable to achieve effective solar energy conversion using materials other than semiconductors.
One possible alternative to semiconductors is the use of a metal-insulator-metal (MIM) configuration.
1-6
The MIM configuration is relatively inexpensive to manufacture in comparison to semiconductor-based systems. The native oxides of the metals are generally used as the insulator materials, therefore the MIM configuration is straightforward to fabricate. Efforts have been made even as recently as 1998 (See Ref. 6) to improve the characteristics of MIM devices, without substantially modifying the basic MIM configuration. Recent research in this area include efforts to use the MIM configuration to potentially provide devices capable of detecting and mixing signals at optical frequencies at optical communications wavelengths.
Turning now to the drawings, wherein like components are indicated by like reference numbers throughout the various figures, attention is immediately directed to
FIGS. 1A-1E
.
FIGS. 1A-1E
illustrate the operation of an MIM device for reference purposes. As a simplified configuration, an MIM device is illustrated in FIG.
1
A. The MIM device, generally indicated by reference number
10
, includes first and second metal layers
12
and
14
, respectively, with an insulator layer
16
positioned therebetween. A corresponding energy band profile
20
is shown in FIG.
1
B. Energy band profile
20
represents height of the Fermi level in the metals and the height of the conduction band edge in the insulator (y-axis
22
) as a function of distance (x-axis
24
) through MIM device
10
in the absence of provided voltage across the device.
FIG. 1C
illustrates a first modified energy band profile
30
when a voltage is provided in a reverse direction to MIM device
10
. The voltage may be provided by, for example, an applied external voltage or an induced voltage due to the incidence of electromagnetic energy. In this case, tunneling of the electrons (not shown) can occur in a reverse direction, represented by an arrow
36
. In contrast, as shown in
FIG. 1D
, when a voltage is provided in a forward direction to MIM device
10
, a second modified energy band profile
40
results. In the case of the situation shown in
FIG. 1D
, tunneling of the electrons can again occur but in a forward direction, represented by an arrow
46
.
FIG. 1E
illustrates a typical I-V curve
50
of current (y-axis
52
) as a function of voltage (x-axis
54
) for MIM device
10
. I-V curve
50
demonstrates that the MIM device functions as a rectifying element. An MIM device provides rectification and energy detection/conversion by tunneling of electrons between first and second metal layers
12
and
14
.
Continuing to refer to
FIGS. 1A-1E
, in energy conversion applications, it is further desirable to achieve high degrees of asymmetry and nonlinearity and sufficiently high current magnitudes in the current-to-voltage performance (I-V curve). If the current magnitude is too low, the incident electromagnetic energy will not be collected with high efficiency. The required current magnitude is a function of the MIM device geometry, dielectric properties of the oxide, and the size and number of the incident electromagnetic energy quanta. A higher degree of asymmetry in the I-V curve between positive values of V (forward bias voltage) and negative values of V (reverse bias voltage) about the operating point results in better rectification performance of the device. In addition, the differential resistance of the device, which influences the responsivity and coupling efficiency of the device to incoming electromagnetic energy, is directly related to the nonlinearity of the I-V curve. An optimal value of differential resistance is required to impedance match the MIM device to the antenna resulting in maximum power transfer to the device. The differential resistance of MIM devices are often too large for energy conversion applications and, consequently, it is desirable to lower differential resistance values in order to impedance match the antenna. In other words, in solar energy conversion applications, it is preferable to have a higher degree of nonlinearity in the I-V curve and optimal value of differential resistance in the device, thus yielding higher sensitivity of the device to incoming solar energy. As a result, high degrees of asymmetry and nonlinearity in the current-to-voltage characteristics of the device yields high efficiency in the energy conversion process. Currently available MIM devices are not able to provide sufficiently high degrees of asymmetry and nonlinearity with sufficiently low differential resistance in the current-to-voltage performance, hence the energy conversion efficiency of MIM devices is low.
A known alternative to the simple MIM device is a device with additional metal and insulator layers, as demonstrated by Suemasu, el al. (Suemasu)
7
and Asada, et al (Asada).
8
The devices of Suemasu and Asada have the configuration of MIMIMIM, in which the three insulator layers between the outer metal layers act as a triple-barrier structure. The insulator layers are crystalline insulator layers formed by an epitaxial growth procedure detailed in Ref. 7. The presence of the barriers between the outer metal layers result in resonant tunneling of the electrons between the outer metal layers under the appropriate bias voltage conditions, as opposed to simple, tunneling of the MIM device. The resonant tunneling mechanism in the electron transport yields increased asymmetry and nonlinearity and reduced differential resistance values for the MIMIMIM device. The resonance tunneling also results in a characteristic resonance peak in the current-voltage curve of the device, which yields a region of negative differential resistance and leads to the possibility of optical devices with very fast responses and high efficiency.
However, the MIMIM
Eliasson Blake J.
Moddel Garret
Ho Tu-Tu
Morita Yoriko
Nelms David
Shear Stephen C.
The Regents of the University of Colorado
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