BACKGROUND OF THE INVENTION

The present invention is directed to a semiconductor infrared detector and a method of detecting thermal radiation and more particularly to a semiconductor infrared detector having a plurality of doped quantum wells separated by short period superlattice layers which form a miniband of energy states.

High performance detectors and detector arrays for detecting thermal radiation are used in a wide variety of military and commercial electro-optic system such as night vision, military surveilance and navigation, the tracking of missiles and aircraft and navigational aids for commercial aircrafts.

Detectors in the medium wavelength infrared (MWIR) wavelength band (with wavelengths from 3 to 5 microns) have a number of detector materials that are available to be used. However, detectors in the long wavelength infrared (LWIR) wavelength band (from wavelengths of 8 to 12 microns) have only been able to be fabricated with a narrow band gap semiconductor of mercury cadmium telluride (MCT) . Many problems with respect to the uniformity of optical and electrical properties and mechanical stability are associated with MCT. As a result, routine fabrication of LWIR detector arrays is prevented.

Conventional quantum well infrared photo- detectors (QWIP) detect the presence of thermal

radiation by an internal photoemission of charged carriers from confined states in a quantum well which changes the electrical conductivity of the material for the QWIP detectors. The materials used for the conventional QWIP detectors are III-V compound semiconductors such as gallium arsenide and aluminum gallium arsenide. Although processing technology suggests that III-V compound semiconductors may be used to fabricate detectors with very uniform detectivities, the performance of such detectors are an order of magnitude of less than the performance of the MCT detectors in the LWIR wavelength band.

One example of a conventional QWIP detector is illustrated in Figure 1. The QWIP detector of Figure 1 includes a stack of quantum wells having widths and depths chosen to provide two confined states, the ground state and the excited state as illustrated in Figure la. The energy separation between the ground state and the excited state is equal to the energy of the photon to be detected. The quantum walls are doped with electron donor impurities which partially fill the lowest energy state with electrons. A barrier having- a thickness of approximately one hundred angstroms (100 ) separates each of the quantum walls. When a bias is applied to the QWIP detector as illustrated in Figure lb, electrons are photoexcited into the excited state and tunnel through the barrier separating the quantum wells to be collected as a photocurrent. However, dark current which flows in the absence of thermal radiation in the above described QWIP detectors is unacceptably high due to tunneling of electrons from the ground state through the thin barriers between the quantum wells.

Another example of a conventional QWIP detecto is illustrated in Figure 2. In this QWIP detector , th widths of the quantum wells are chosen so that only on state is bound in the quantum well and the virtual excite state is pushed slightly above the barrier into th continuum as illustrated in Figure 2a. In these QWI detectors, the barriers between the quantum wells are mad to be relatively large, approximately 500 A, which reduce the dark current while still allowing the carriers to b collected because photoexcitation pushes the carrier above the barrier for allowing the carriers to be swep out by an applied electric field. However, in the abov described QWIP detectors, there is only one bound state i the quantum well and the first excited state must be clos in energy to the top of the barrier because the absorptio strength drops rapidly as the excited state moves highe above the barrier which reduces the detectivity. As a result of these requirements, the well width and barrier height are uniquely defined and therefore cannot be varied to optimize detector performance. Additionally, the effective mass of the carriers must be low in order to provide one bound state in the quantum well. As a result, the dopants in the QWIP detector must be n-type (electro donors) which further limits the design parameters. Furthermore, the wavelength range in which the QWI detector is sensitive is primarily a material parameter that is determined by the wavelength dependence of th process of absorption from ^" a discrete state to th continuum, and the wavelength range can only be changed b varying the energy of the excited state above the. barrier which greatly decreases the absorption strength.

SUMMARY OF THE INVENTION

One object of the present invention is to provide an improved semiconductor device for detecting thermal radiation. It is a further object of the present invention to provide a miniband transport quantum well infrared semiconductor detector having a predetermined bandwidth corresponding to the range of wavelengths to which the detector is sensitive that is determined by the thickness and composition of the semiconductor layers of the detector.

Another object of the present invention is to obtain an electrically tunable peak absorption and bandwidth in response to an electric field applied to the detector.

The objects of the present invention are fulfilled by providing a semiconductor miniband transport quantum well infrared detector comprising a substrate, a multilayer structure disposed on the substrate, and first and second contact layers disposed on. the top and bottom surfaces of the multilayer structure. The multilayer structure includes a plurality of doped quantum wells and superlattice barrier layers disposed on each side of the quantum wells. The superlattice barrier layers comprise a plurality of alternating first and second layers wherein the first layers have a relatively low band gap and the second layers have a relatively high band gap. The superlattice barrier layers form a miniband of energy states which transport photoexcited carriers from the quantum wells for collection as photocurrent.

The semiconductor detector of the presen invention comprises quantum wells having two or more boun states so that a wide range of materials and carrier type may be used for the semiconductor layers. In on embodiment of the present invention, the semiconducto detector comprises a GaAs/AlGaAs quantum well system. I further embodiments, a wide range of semiconducto materials for the detector may be used such a inGaAs/InAlAs (lattice-matched to InP substrates) o InGaAs/InAlGaAs strained layer systems for example.

The present invention additionally provides method for detecting thermal radiation by a semiconducto device comprising the steps of providing a plurality o doped quantum well layers, forming a miniband from plurality of strongly coupled superlattice barrier layer disposed on each of the quantum well layers, photoexcitin carriers from the quantum wells to the miniband an applying an electric field across the superlattice barrie layers and the quantum well layers to transport th carriers through the miniband for collection as photocurrent corresponding to the thermal radiation.

According to the present invention, semiconductor detector and a method is provided whic allows the quantum wells to be formed with more than on bound state and a wide variety of well widths and barrie heights. The superlattice barrier layers reduce the dar current and the sensitivity to processing variations b using wider quantum wells. By allowing the quantum well to contain two or more bound states, a larger range o materials and carrier types may be used for th detectors. Furthermore, the bandwidth of the detector ma be selected over a wide range by varying the coupling o the layers in the superlattice barrier to provide

broader or narrower miniband as desired which broadens or narrows the absorption range respectively while maintaining the absorption strength.

Further scope of applicability of the present invention will become apparent from the detailed description given herein after. However, it should be understood that the detailed description and specific examples, while indicating preferred embodiments of the present invention, are given by way of illustration only, since various changes and modifications within the spirit and scope of the present invention will become apparent to those skilled in the art from this detailed description.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention will become more fully understood from the detailed description given herein below and the accompanying drawings which are given by way of illustration only and thus are not limitative of the present invention, and wherein:

Figures la and lb illustrate the energy states of a conventional quantum well infrared photodetector where no bias is applied to the detector in Figure la and a bias is applied to the detector in Figure lb;

Figures 2a and 2b illustrate the energy states of another conventional quantum well infrared photodetector where no bias is applied to the detector in Figure 2a and a bias is applied to the detector in Figure 2b;

Figure 3 illustrates the structure for the miniband transport quantum well infrared detector for an embodiment of the present invention;

Figure 4 illustrates a miniband transport quantum well infrared detector for an embodiment of the present invention which utilizes the semiconductor structure of Figure 3; Figures 5a and 5b illustrate the energy states for the miniband transport detector for the present invention where no bias is applied in Figure 5a and a bias is applied in Figure 5b;

Figure 6 illustrates the wavelength dependence of photocurrent obtained from a miniband transport detector of the present invention; and

Figure 7 illustrates the bias dependence of the detectivity of the miniband transport detector of the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Figure 3 is first referred to and illustrates an embodiment of the structure for a miniband transport quantum well infrared detector (hereinafter referred to as MBT detector). The MBT detector is formed of a multilayer structure 40 which has alternating quantum wells 10^, 10,, . . . 10 and superlattice barrier layers 20., 20,, . . . 20 _n* ^A capping layer 60 and a buried contact 70 are disposed on each end of the multilayer structure 40. The above described structure is formed on a substrate 30.

Figure 4 illustrates an embodiment of the present invention which includes an array of detector elements (pixels) . Each pixel is delineated and the buried contact is exposed by etching away the material surrounding the pixels. Each pixel includes a layer 80 having an optical diffraction grating etched into the capping layer 60 and a

metal contact deposited on the surface of the optical diffraction grating. Radiation is incident through the substrate 30 and is coupled into the detector layers by the optical diffraction grating. The quantum wells of the MBT detector contain two or more bound states which include at least a ground state and an excited state. A miniband of energy states is formed by strongly coupling the superlattice layers of the structure. The superlattice barrier layers include a plurality of thin quantum wells separated by thin barrier layers. These plurality of layers correspond to alternating low band gap layers and high band gap layers which are chosen so that the lowest energy miniband is in resonance with the excited state of the quantum wells. An example of this structure is illustrated in

Figure 5a where the quantum well includes a ground state and an excited state which falls between the energy levels of the miniband 100. Figure 5b illustrates the application of an electric field to the structure wherein the minibahd is tilted and breaks up into a series of quasi-discrete states (not shown in the figures) extending through several periods of the superlattice layers. The quantum wells are doped so that the ground states are partially filled with carriers. The carriers in the ground states of the quantum wells may thereby be excited into the next higher energy state (or any odd order energy state as dictated by selection rules for optical transition) by infrared photons. As a result, the carriers are placed in the miniband which allows the carriers to move through the barrier layers with relatively ease for collection as photocurrent in the contact layers.

The superlattice barrier layers may be made of a thickness so that the probability of carriers tunneling directly from the ground state into the next quantum well or into the continuum is substantially zero except when very high biases are applied to the structure. In other words, the tunneling components of the dark current is approximately zero. Also, the energy of the superlattice barrier layers may be chosen to be sufficiently large for limiting the thermionic emission of carriers into the continuum at the desired operating temperature of 77 Kelvin. The precise quantum well and superlattice barrier layer geometry is determined by solving the Schroedinger equation for the system in order to obtain the desired wavelength peak and range in the responsitivity of the detector. The number of layers in the superlattice barrier layers are chosen based on the crystal growth time or other similar considerations. The quantum well layers are doped so that the ground states are partially filled with carriers where the layers are usually doped with electron donors.

The energy and the spatial extents of the quasi-discrete states in the miniband are strong functions of the applied electric field. As a result, properties of the MBT detector such as the peak absorption and the responsitivity bandwidth may be electrically tuned.

One example of an MBT detector that was formed by molecular beam epitaxy includes forty GaAs quantum wells, which have a thickness of approximately 78 and are doped with silicon to a level of 4x10 17/cm3, separated by superlattice barrier layers having nine GaAs wells, approximately 20 A thick, and ten AlGaAs barriers, approximately 40 & thick. Doped contact layers having a thickness of 1 μm were formed above and below the active

quantum well regions. Detector elements having various areas were thereafter defined by chemical etching in order to form ohmic metalization areas. A conventional 4 μ m pitch triangular grating is etched into the back of the wafer in order to collect light with a polarization component perpendicular to the quantum well layers.

The MBT detector is not restricted to this specific GaAs-AlGaAs quantum well system. A wide variety of layers, layer thicknesses and materials may be used for the MBT detector. For example, InGaAs/InAlAs (lattice matched to InP substrates) or InGaAs/InAlGaAs strained layer systems may be used. The widths of the quantum wells and the thicknesses of the superlattice barrier layers may range from one monolayer to several hundred Angstroms. The wavelength sensitivity is determined by material parameters such as the effective mass, band offsets and the well depths. The structure of the present invention allows the use of p-type acceptor doping in the quantum wells. As a result, the higher effective mass of the p-type carriers reduces the dark current of the device by reducing the Fermi level and the corresponding thermionic emission.

Figures 6 and 7 illustrate the improved characteristics of the MBT detector of the present invention. Figure 6 illustrates the wavelength dependence of the photocurrent which has a peak response of about 10.5 _/ um and a long wavelength cutoff (a half power point) of 11.1Am. Figure 7 illustrates the bias dependence of the peak detectivity at a temperature of 77 Kelvin which obtains a value of 1x10 cm J HzVw.

The embodiments of the present invention provide an infrared detector for collecting photocurrent by quantum wells having a wide range of depths and widths.

Because the MBT detectors allow materials having more than one bound state for the quantum wells to be used, the range of materials and carrier types are greater. Furthermore, the bandwidth of the MBT detector may be selected over a wide range by strongly coupling the superlattice barriers layers to provide a broader miniband which increases the absorption range while maintaining the strength of absorption in the quasi-bound state. Accordingly, an enhanced semiconductor device and a method for detecting thermal radiation is provided in the present invention.

The invention being thus described, it will be obvious that the same may be varied in many ways. Such variations are not to be regarded as a departure from the spirit and scope of the invention, and all such modifications as would be obvious to one skilled in the art are intended to be included within the scope of the following claims.