Active solid-state devices (e.g. – transistors – solid-state diode – Thin active physical layer which is – Heterojunction
Reexamination Certificate
1999-04-14
2001-05-29
Mintel, William (Department: 2811)
Active solid-state devices (e.g., transistors, solid-state diode
Thin active physical layer which is
Heterojunction
C257S021000, C257S022000, C438S093000, C438S094000
Reexamination Certificate
active
06239449
ABSTRACT:
FIELD OF THE INVENTION
This invention relates to apparatus and methods for converting light signals into electrical signals. More particularly, the invention relates to a semiconductor photodetector device and a method for converting electromagnetic radiation such as infrared signals into electrical signals using self-assembled semiconductor quantum dots.
BACKGROUND TO THE INVENTION
Presently, state-of-the art photodetectors are either based on interband transitions in bulk material or quantum wells, or intersubband transitions in quantum wells. The interband transition devices operate mainly in the visible and near infrared wavelength range due to the energy difference between the conduction and valence band of semiconductors. The intersubband devices operate with multiple layers of quantum wells, in which carriers are confined to quantum dimensions in 1 direction (perpendicular to the surface of the device), and are free to move in the other 2 directions (in the plan of the quantum wells).
Such quantum well infrared photodetectors (QWIPs) are being used successfully for detecting InfraRed (IR) light for detectors and sensors and for imaging purposes. A major limitation of these QWIPs is that due to the transition selection rules they are not sensitive to normally incident light, and only have narrow response range in the IR. Consequently, normal incidence detection over a broad range in the IR can only be achieved with complicated light coupling devices and/or schemes, and by combining layers with different sets of quantum wells each having its own narrow response range.
SUMMARY OF THE INVENTION
Multiple layers of Self-Assembled Quantum Dots (QD) can be grown by epitaxy of highly strained semiconductors embedded in barrier spacers. Due to the small size of the quantum dots, which typically have diameters of about 200 Angstroms, quantum mechanics dictate the energy position of energy levels allowed in the quantum dots. Infrared light detection is achieved by the transitions of charged carriers from confined energy levels in the quantum dots to higher confined energy levels in the quantum dots, and/or from confined energy levels in the quantum dots to higher energy states in the wetting layer which is formed underneath the quantum dots in the self-assembling growth, and/or from confined energy levels in the quantum dots to higher energy states in the barrier material.
In contrast to the QWIPs, QDIPs are sensitive at normal incidence, and have a broad response range. The QDIPs do not suffer from the normal incidence limitation because of the unique symmetry resulting from confining the carriers in all 3 directions, and have a much broader response range in the infrared compared to the present QWIP design because the self-assembled quantum dots naturally grow with an inhomogeneous broadening in the size dispersion and with intersublevel energies which are both suitable for the long wavelength range. Growth and fabrication can be accomplished with well established and simple growth and fabrication techniques.
There has been a real need for photodetectors capable of normal incidence detection over a broad range to efficiently convert infrared light into electrical signals, which is satisfied by the present invention, which provides an apparatus and method capable of converting broad ranges of long wavelength light signals into electrical signals with good efficiencies.
The present invention also provides an apparatus and method capable of converting broad ranges of long wavelength light signals into electrical signals with direct normal incidence sensitivity without the assistance of light coupling devices/schemes.
These and other embodiments are realized by apparatus and a method in which stored charged-carriers are ejected by photons from quantum dots, then flow over the other barrier and quantum dot layers with the help of an electric field produced with a voltage applied to the device, therefore resulting in a detectable photovoltage and photocurrent.
The apparatus takes the form of a photodetector comprising multiple layers of semiconductor materials including a least 1 quantum dot layer between an emitter layer and a collector layer, with a first barrier layer between the quantum dot layer and the emitter layer, and another barrier layer between the quantum dot layer and the collector. In the case where multiple quantum dot layers are used, barriers separate the quantum dot layers. The emitter and collector are preferably doped to act as a reservoir of charge carriers and to conduct the current during detection and for operation under applied bias. The charge carriers are introduced in the quantum dot layer by either doping the barrier and/or the dot layers, and/or after redistribution of the charge carriers and adjustment of the Fermi level. The level of doping in the various regions of the device and the scheme of doping is preferably adjusted to set the Fermi level such that the desired number of energy levels in the quantum dots are occupied with charge carriers to achieve the targeted range of detection wavelengths. Also preferably, the size and the number of quantum dots per unit area is adjusted from the growth parameters in conjunction with the doping to achieve the desired detection range while optimizing the detection efficiency for the wavelengths of interest. Similarly, the choice of the barrier material, height, and thickness is adjusted in conjunction with the quantum dot size to set the detection range, to select a balance between low detector capacitance and low carrier transit time, and to achieve the desired growth mode in the self-assembling growth. For multiple layers of quantum dots very thin barriers will result in coupled zero-dimensional states in vertically self-organized quantum dots, thicker barriers will result in isolated zero-dimensional states in vertically self-organized quantum dots, and thick barriers will result in isolated zero-dimensional states in uncorrelated independent quantum dot layers.
In a preferred form of the invention the layers of the semiconductor materials are grown on a substrate from materials consisting essentially of gallium, indium, aluminum, arsenic, phosphorus, and possibly nitrogen, using known techniques such as molecular beam epitaxy, or metalorganic chemical vapor deposition, or chemical beam epitaxy, with dopant such as silicon, beryllium, or other. The carriers are introduced by n-doping or p-doping the quantum dots and/or the barriers, either continuously, or using modulation doping layers.
On GaAs substrates, the quantum dot material can be InGaAs, AlInAs, InP, or other alloys of AlGaInAsP, with barriers of AlGaAs or AlGaInP. On InP substrate, the quantum dot material can be InGaAs. Alloys with nitrogen can be used with the above group III-V materials in cases where high band gap materials are desirable. On group IV substrates, Si can be used for the barrier, and SiGe or some of the group III-V materials mentioned above for the quantum dot material. The substrate is needed to give structural integrity to the very thin layers of the device and to allow proper crystal growth. The emitter and the collector are preferably doped with the type of carriers which will be used in the quantum dots to detect the infrared light. The type of charged carrier is preferably chosen in conjunction with the size of the quantum dots, and with the thickness and height of the barriers, to obtain the desired spectral response.
The photodetector is exposed to electromagnetic radiation, and in particular infrared radiation, having no required specific orientation of the polarization vector with respect to the surface of the device. Light of substantially all polarizations and angles of incidence can be detected because of the symmetry and shape of the self-assembled quantum dots which preferably take a form resembling an hemispherical cap, a lens shape, a disk shape, a pyramidal or truncated and/or rounded pyramid shape. The self-assembled quantum dots are obtained with the spontaneous island formation during the epitaxy of highly st
Chun Liu Hui
Fafard Simon
Mintel William
National Research Council of Canada
Pascal & Associates
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