Patent Application
20120217475
The present invention relates to optoelectronic devices, and more particularly relates to mid-infrared optoelectronic devices based on type-II band alignments having a compound valence-band quantum well structure.
Optoelectronic devices based on the III-V family of compound semiconductors have assumed an increasingly important role in a wide variety of applications. Such devices include various types of semiconductor lasers, optical modulators, photon detectors and the like. The wavelengths of these optoelectronic devices have reached into the mid-infrared spectral region with the increasing maturity of “6.1-Å” materials including InAs, GaSb, AlSb and their alloys.
The heterojunctions involved in optoelectronic device structures that use the 6.1-Å materials often have type-II or broken-gap band alignments, in which electrons are localized in one set of material layers and holes are localized in another set of material layers. A feature of type-II-based semiconductor optoelectronic devices is that some of the semiconductor layers in such devices are not lattice-matched to the substrates used. These layers are usually grown by a technique such as molecular beam epitaxy. The lattice mismatches of individual layers relative to the substrate introduces another challenge in the design of such devices, namely, the necessity of strain balancing the various layers of such devices.
In the type-II material systems typically used in optoelectronic device applications, the strain balancing problem is complicated by the fact that adjacent materials typically share neither a common cation nor a common anion. Thus, the interfaces separating adjacent materials can have a variety of different types of structures depending on whether a layer of cations, anions, or a mixture of cations and anions terminates the first of a pair of materials. The structures of these interfaces can possibly be altered by using techniques such as deposition of additional group III elements or soaks with group V species in attempts to achieve overall strain balancing of the epitaxial structures. However, the material quality of the epitaxial films is often compromised by these treatments.
It is known to use materials having type-II band alignments in mid-infrared photon detectors. For example, InAs has been used as the conduction-band quantum well and either the alloy (In,Ga)Sb or GaSb have been used as the valence-band quantum well. For these types of devices, the key quantity of interest is the optical matrix element between the highest energy valence-band state of the system and the lowest energy conduction-band state of the system.
Attempts have been made to improve the performance of type-II infrared detectors by introducing a specialized type of active region based on a “W” structure which, rather than using a simple pair of semiconductor layers as the basic absorbing unit, uses a sequence of four layers: InAs; (In,Ga)Sb; InAs; and (In,Al,Ga)Sb. The first three of these layers comprise the W structure and the fourth is a barrier layer that separates adjacent repeats of the basic unit. However, the use of the (In,Al,Ga)Sb, or simply AlSb, inhibits transport of photogenerated carriers through the active region of the structure and thus is detrimental to good detector performance.
A number of different types of electron- and hole-blocking layers have been used to improve the dark current characteristics of type-II superlattice detectors. For example, nBn structures with blocking layers based on InAs/GaSb type-II strained layer superlattices have been proposed.
Another family of optoelectronic devices that can be extended into the mid-infrared spectral region is the optical modulator based on the principle of the Stark shift, which is a shift in the fundamental absorption edge of a quantum well system with application of an electric field. Depending on the difference in energy between the fundamental absorption edge and the photon energy, such a mechanism can produce a change in either the real or the imaginary component of the medium's index of refraction. Consequently, a device based on this mechanism can serve either as an intensity modulator or a phase modulator.
Another type of optoelectronic device that utilizes type-II semiconductor materials is the interband cascade laser. Certain variants of the interband cascade laser utilize the W structure in the active region of the device. The W structure has also been used in type-II-based conventional (non-cascaded) diode lasers, and in optically-pumped type-II lasers.
The performance of semiconductor optoelectronic devices based on type-II material systems is generally a compromise between optimizing the relevant optical properties of the structure and strain balancing the structure. It would be desirable to produce a new type of semiconductor structure for use in the active regions of such devices that would allow, simultaneously and independently, the optimization of the relevant optical properties and the strain balancing of the epitaxial structure. Such a structure would achieve significant improvements in the performance of many different types of optoelectronic devices.
The present invention has been developed in view of the foregoing and to remedy other deficiencies of the prior art.
The present invention provides semiconductor optoelectronic devices based on type-II band alignments comprising a compound valence-band quantum well structure, known as an “H-layer”, that optimizes device performance and material quality. The use of the H-layer structure allows simultaneous optimization of relevant optical properties of the semiconductor structures in which the H-layer structure is contained as well as lattice matching of the device structure to the selected substrate. The unique properties of the H-layer valence-band quantum well enable significant improvements in many optical device applications.
One embodiment of the invention provides a type-II quantum well structure containing the H-layer valence-band quantum well permitting strain balancing of the structure to an appropriate substrate. In another embodiment of the invention, an infrared detector that utilizes the type-II quantum well structure containing the H-layer valence-band quantum well is provided. The infrared detector may optionally include blocking layers. In a further embodiment, an infrared optical modulator that utilizes the type-II quantum well structure containing the H-layer valence-band quantum well is provided. In another embodiment, an interband cascade laser that utilizes the type-II quantum well structure containing the H-layer valence-band quantum well is provided. In a further embodiment, a W diode laser that utilizes the type-II quantum well structure containing the H-layer valence-band quantum well is provided. In another embodiment, an optically pumped laser that utilizes the type-II quantum well structure containing the H-layer valence-band quantum well is provided.
An aspect of the present invention is to provide a type-II optoelectronic device including one or more compound valence-band quantum well structures. The compound valence-band quantum well structures comprise a central layer and a pair of shoulder layers located on opposite sides of the central layer, wherein each shoulder layer has a valence-band-edge energy larger than a valence-band-edge energy of the central layer.
Another aspect of the present invention is to provide a type-II optoelectronic device including one or more compound valence-band quantum well structures. The compound valence-band quantum well structures comprise a central layer and a pair of shoulder layers located on opposite sides of the central layer, wherein at least one of the shoulder layers reduces strain caused by a lattice mismatch between a substrate of the device and other layers of the device.
These and other aspects of the present invention will be more apparent from the following description.
The use of the H-layer structure allows simultaneous optimization of the relevant optical properties of an optoelectronic device in which the H-layer structure is contained, as well as lattice matching of that device to the substrate upon which it is grown without the necessity of interface treatments referred to earlier. In so doing, the present invention simultaneously solves two different problems that had previously been considered to be independent of one another and that also previously entailed compromises to device performance.
To illustrate the benefits of the H-layer structure in terms of lattice matching, we consider the design of an infrared detector with a cutoff wavelength of 10 μm. Using a conventional design based on an epitaxial structure consisting of sequential layers of InAs and GaSb, grown on a GaSb substrate, a structure comprised of multiple repeats of 45.1 Å of InAs and 27.4 Å of GaSb results in the desired cutoff wavelength. However, because the GaSb layers have the same lattice parameter as the substrate (6.09593 Å) and the InAs layers have a smaller lattice parameter (6.0584 Å), the epitaxial structure is not lattice matched to the GaSb substrate. The mismatch, defined as the relative deviation of the epitaxial structure's in-plane lattice parameter from the substrate's lattice parameter, is calculated as −0.00313, which is large enough to cause dislocations during the structure's growth and thereby degrade its performance. However, an H-layer epitaxial structure can be designed for the same cutoff wavelength that incorporates In0.3Ga0.7Sb shoulder layers that serve as lattice-matching layers. In this case, the structure consists of multiple repeats of 42.12 Å of InAs, 5.24 Å of In0.3Ga0.7Sb, 13.87 Å of GaSb, and another 5.24 Å of In0.3Ga0.7Sb. The GaSb layers still have the same lattice parameters as the substrate (6.09593 Å) and the InAs layers have a smaller lattice parameter (6.0584 Å), but the added In0.3Ga0.7Sb layers have a larger lattice parameter (6.21097) than the substrate. The mismatch of the structure relative to the substrate is zero, and the epitaxial structure is lattice matched to the substrate. Thus, the shoulder layers eliminate the lattice mismatch that would otherwise occur between the substrate and the other layers of the device.
Optoelectronic devices that are based on type-II band alignments rely on a transition between a valence-band state that is localized in one type of material and a conduction-band state that is localized in a different material. Consequently, the optical transition is sensitive to conditions in the vicinity of the interface between the two materials, and the strength of that transition is susceptible to imperfections at the interface. In accordance with the present invention, by freeing the device designer from attempting to alter the interface in order to achieve lattice matching, the H-layer structure improves type-II device performance. Furthermore, since one characteristic of the H-layer structure is the “pulling” of the wave function of the highest-energy hole state toward these interfaces, the optical matrix element between this state and the lowest-energy conduction-band state is made larger, thereby intrinsically improving the performance of a wide variety of optoelectronic devices into which the H-layer valence-band quantum well can be incorporated. In the case of a detector or an optical modulator, improvements in device performance are achieved because the increased optical matrix element results in a larger value of the device's absorption coefficient. In the case of a laser, improvements in device performance are achieved because the increased optical matrix element results in a larger value of the device's gain.
The central layer 220 may be made of GaSb, InxGa1-xSb, InxGa1-xAsySb1-y and the like having a typical thickness of from about 15 to about 100 Å. The shoulder layer 240 may be made of InxGa1-xSb, InxGa1-xAsySb1-y and the like having a typical thickness of from about 2 to about 15 Å. The conduction-band quantum well layers 260 may be made of InAs, InSbxAs1-x and the like having a typical thickness of from about 20 to about 100 Å.
Table 1 shows the results of calculations for a type-II infrared detector structure in accordance with an embodiment of the present invention.
Table 1 compares calculated quantities for a conventional type-II detector made with InAs CB quantum wells and GaSb VB quantum wells with the corresponding calculated quantities for a type-II infrared detector with the same calculated cutoff wavelength (λc˜10 μm) that uses the H-layer valence-band quantum well of the present invention. The other quantities shown in the table are the square of the magnitude of the matrix element, MHH1-CB1, between the highest energy heavy-hole state HH1 and the lowest energy conduction-band state, CB1; the relative absorption coefficient, αrel; and the miniband widths of the lowest energy conduction miniband and the highest energy heavy-hole and light-hole minibands. The calculations show that the value of |MHH1-CB1|2 for the H-layer-based structure is improved by 22.5% over the value calculated for the conventional type-II structure; the value of αrel is improved by 33%; and the values of the CB, HH and LH miniband widths are improved by 13%, 130% and 5.5%, respectively.
The H-layer structure used for the calculations in Table 1 consists of 14 mL of InAs and 8 mL of combined In0.3Ga0.7Sb and GaSb forming the H-layer valence-band quantum well. The conventional type-II detector consists of 15 mL of InAs and 9 mL of GaSb. The improvement in the calculated value of |MHH1-CB1|2 is attributed to the increased overlap between the electron and hole wave functions arising as a result of the valence-band structure of the H-layer valence-band quantum well. The improvement in the calculated value of αrel is attributed both to the increase in |MHH1-CB1|2 and also to the fact that the thickness of a single period of the H layer structure is smaller than that of the conventional structure (αrel is proportional to |MHH1-CB1|2 divided by the period thickness). This smaller period thickness also increases the overlap between wave functions corresponding to adjacent periods and is largely responsible for the improved miniband widths in the H-layer structure, which in turn improves carrier transport in the structure, a critical factor in detector performance.
The improvements shown in Table 1 for a type-II infrared detector based on the H-layer valence-band quantum well structure apply as well to the other optoelectronic devices that are embodiments of the instant invention. For example, the absorption coefficient plays a role in the performance of optical modulators that is similar to its role in type-II infrared detectors. Consequently, concomitant improvements in the performance of optical intensity and phase modulators can be achieved. Furthermore, the square of the magnitude of the optical matrix element |MHH1-CB1|2 appears in the calculation of the gain of a type-II laser active region. Therefore, the above-cited improvement in this matrix element translates directly into improved performance of various type-II based infrared lasers including interband cascade lasers, diode lasers, and optically pumped lasers.
The following example is intended to illustrate various aspects of the invention, and is not intended to limit the scope of the invention.
To confirm the performance of the H-layer valence-band quantum well, molecular beam epitaxy was used to grow a detector structure using a structure similar to that shown in
The detector structure containing the H-layer valence-band quantum wells was characterized using high-resolution x-ray diffraction, which is typically used in connection with molecular beam epitaxial growth of strained structures to quantitatively assess the degree of lattice mismatch, to measure the thickness of a superlattice period, and as an overall measure of material quality.
Detector diodes were fabricated from the epitaxial wafer containing the type-II H-layer valence-band quantum wells using methods that are conventional to those skilled in the art including an inductively coupled plasma dry etch based on BCl3/Ar chemistry to form individual detector mesa diodes as well as SU-8 photoresist deposited upon the mesa side walls to passivate their surfaces and thereby reduce parasitic surface current pathways.
One of the methods used to evaluate the performance of type-II infrared detectors is to measure the dark current of the detector diodes at a temperature at which the detector will be used.
Another method used to evaluate the performance of type-II infrared detectors is to measure the external quantum efficiency of a detector diode at a temperature at which the detector will be used. The external quantum efficiency is the ratio of the number of electron-hole pairs collected at the detector's contacts to the number of photons that are incident on the detector's active area.
A photon detector's detectivity D* is related to the signal-to-noise ratio achieved in a detector with a given incident photon flux, normalized to unit detector area and detection bandwidth. The detectivity is calculated from the measured dark-current density and quantum efficiency results that are shown in
Whereas particular embodiments of this invention have been described above for purposes of illustration, it will be evident to those skilled in the art that numerous variations of the details of the present invention may be made without departing from the invention as defined in the appended claims.
This invention was made with United States government support under Contract Number HQ006-10-C-7355 awarded by the U.S. Missile Defense Agency. The United States government has certain rights in this invention.
