Patent ID: 12255264

DETAILED DESCRIPTION

Type-II semiconductor (e.g., InGaSb) quantum dots (QDs) can be grown with pn junction architecture. This allows for active plasmonic devices with integrated quantum engineered emitters. Mid-IR light emitting diodes (LEDs) based on type-II QD active regions grown with monolithically integrated semiconductor metal layers are provided. These LEDs comprise layers of type-II semiconductor (e.g., InGaSb) quantum dots integrated into a pn junction diode (e.g., InAs pn junction diode) grown above a highly doped backplane (e.g., an n++ InAs backplane), all in the same epitaxial growth. Aspects described herein significantly increase radiative recombination rates rate, which allows for increased efficiency emitters by controlling the emission of the emitting QDs in the near field of an optical metal.

Growth techniques are described to grow quantum nanostructures (e.g., QDs) of InGaSb in an InAs matrix. These QDs are so-called type-II quantum dots, where electrons in the conduction band of the InAs recombine with holes confined in the InGaSb. Because of the additional quantum confinement offered by the QD structures, a significant decrease in Auger recombination is expected. Longer lifetimes and improved temperature performance for QD emitters is achieved as compared to quantum well (QW) emitters at the same wavelength.

Moreover, QDs are used as the active light emitters as opposed to superlattice LEDs (SLEDs) or interband cascade LEDs (ICLEDs). The uses of QDs in the structures, methods, and devices described herein differ significantly from existing commercial technology.

In some implementations, a semiconductor structure comprises a highly doped (n++) epitaxial layer, and a diode structure disposed above the n++ epitaxial layer. The diode structure comprises an n-doped InAs layer followed by a p-doped InAs layer, with an unintentionally doped layer between the n and p material forming a pn junction which contains a number of QD layers acting as QD emitters. The combination of the n++ and the pn junction creates a cavity and provides a metallic surface in the near field of the QD emitters which increases radiative recombination rates in the QD layers.

FIG.1is a cross-section side view of an example semiconductor structure100according to an embodiment. An n type InAs substrate110is provided and a heavily doped n++ InAs layer120is grown (or otherwise deposited) thereon. As shown, the layer120has a thickness of 1000 nm, though this is not intended to be limiting. The thickness and doping of the layer120may change depending on the particular implementation.

A thin layer130of InAs:Si (e.g., doped at 1018/cm3) is formed above the layer120. The layer130is illustrated as having a thickness of 50 nm, but this is not intended to be limiting. Above the layer130, five alternating layers150of InAs155(each 50 nm thick) and InGaSb157(each 2.25 ML thick) are grown or otherwise deposited.

Above the layers150, a layer160of AlAsSb:Be (doped at 1018/cm3) is grown or deposited (e.g., to a thickness of 500 nm). A layer170of InAs:Be (doped at 1019/cm3) is then grown or deposited on the layer160.

In another implementation, instead of a cavity, emission is into surface plasmon polariton modes, which should be very efficient emission into these modes, and then couple out by a grating or pattern on the surface of the emitter.

FIG.2is a chart200of the electroluminescence of the n+++ diode of the structure100ofFIG.1and a control diode at 77K (the temperature at which the electroluminescence is measured). The electroluminescence curve210of an InGaSb control diode is shown, along with the electroluminescence curve220of the InGaSb n++ diode of the structure100.

FIG.3is a chart300of the electroluminescence of the n++ diode of the structure100ofFIG.1with reflectance at 77K (the temperature at which the electroluminescence is measured). The electroluminescence curve320of the InGaSb n++ diode of the structure100remains unchanged from curve220ofFIG.2. However, the emission is modified by the reflectance of the materials of the structure100as shown in the electroluminescence curve310(the reflectance spectrum) of the InGaSb n++ diode. The emission is modified by the reflectance of the materials of the structure100as shown the measured reflectance curve310overlaid with the measured electroluminescence320.

FIG.4is a chart400of the electroluminescence of the n++ diode of the structure100ofFIG.1and a control diode at 296K (the temperature at which the electroluminescence is measured). The electroluminescence curve410of an InGaSb control diode is shown, along with the electroluminescence curve420of the InGaSb n++ diode of the structure100.

FIG.5is a chart comparing emission from the n++ diode of the structure ofFIG.1at 78K (e.g., low temperature) and 296K (e.g., room temperature), the temperature at which the electroluminescence is measured, as shown in the curves510,520, respectively. While the short wavelength feature of the n++ diode (centered around 4.5 μm) decays in intensity as expected, the long wavelength emission peak from the n++ diode shows only a 13% decrease in emission intensity between low temperature and room temperature.

Initial devices grown (i.e., fabricated) show significant improvements in temperature performance and emitted power when compared to commercial devices. Thus, the thin mid-IR LEDS described herein provide improved temperature performance and power efficiency including minimal degradation in emitted power as a temperature is increased from 77K to 300K and higher output power then commercial mid-IR LEDs. For example, the structure may be configured so there is about no temperature degradation of emission from 77K to 300K, as well as output powers of greater than 1.45 μW. There is also a lowered cost of epitaxial growth, and the devices and structures described herein are compatible with LED fabrication processes and have the flexibility to adjust enhancement wavelength by control of an n++ (or other highly doped such as p++ depending on the implementation) layer.

In some implementations, the highly doped (e.g., n++) epitaxial layer is about 500 to 1500 nm thick and is doped to have a plasma wavelength at <7 μm. The highly doped (n++) epitaxial layer may have a plasma wavelength of about 4 to 4.8 μm. The highly doped (n++, for example) epitaxial layer is doped at about 1019/cm3, although this is not intended to be limiting as other doping amounts may be used depending on the implementation. Moreover, depending on the implementation, p++ doping may be used instead of n++ doping. Thus, the device structure may be considered agnostic in terms of which emitter architecture to use, as the architecture depends on the implementation. The doping controls a plasma wavelength and the permittivity. Moreover, the doping of the highly doped (n++) epitaxial layer is high enough so that a portion of the structure behaves like an optical metal.

In some implementations, a semiconductor structure comprises an n++ (i.e., highly doped) InAs as a backplane, and a mid-IR light emitting diode (LED) emitter. The doping is not to improve electrical conductor properties, but instead such high doping is performed in order to use its optical properties. The doping is so high (e.g., 1019/cm3) that it behaves like an optical metal. Any n-type dopant can be used. It is contemplated that p-type dopants may also be used. In this manner, III-V materials are turned into an optical metal. This allows for reflecting light or forming a cavity, for example.

The mid-IR LED emitter may be based on type-II QD active regions grown with monolithically integrated semiconductor metal layers. More particularly, the mid-IR LED emitter may comprise layers of type-II semiconductor (InGaSb) quantum dots integrated into an InAs pn junction diode grown above the n++ InAs backplane, all in the same epitaxial growth. It is contemplated that other types of mid-IR emitters also work and can be implemented, as long as they are incorporated into the highly doped semiconductor described herein.

The structure is configured to minimize increase radiative recombination rate, which allows for increased efficiency emitters by controlling the emission of the emitting QDs in the near field of an optical metal.

The backplane is a highly doped (n++) epitaxial layer in a range between about 500 to 1500 nm thick and is doped to have a plasma wavelength at less than 7 μm. In some implementations, the backplane has a plasma wavelength of about 4 to 4.8 μm. The backplane is doped at about 1019/cm3, although this is not intended to be limiting as other doping amounts may be used depending on the implementation.

FIG.6is an operational flow of an implementation of a method600for fabricating a semiconductor structure. In epitaxial growth, the growth of the InAs and the doping occur simultaneously.

At610, a highly doped semiconductor plasmonic layer (e.g., n++ InAs) is grown (or otherwise deposited) on a substrate to a predetermined thickness (e.g., the InAs is highly doped (e.g., n++ doped at about 1019/cm3)). The layer of InAs may be doped high enough so that it becomes optically metallic in the mid-IR range.

At620, an emitter region structure, such as one or more quantum dot (QD) emitters, is grown directly above (i.e., on) the plasmonic layer (the layer of highly doped (n++) InAs). The QD emitter structure encompasses the entire undoped region between the p and n regions ofFIG.1, for example (e.g., layers157,155). In some implementations, the QD emitter is grown directly above (i.e., on) the layer of highly doped (n++) InAs. In this manner, the plasmonic layer (e.g., the n++ InAs) is used as a backplane to an emitter structure.

At630, the remaining portion of the device is grown (e.g., a p-doped region is grown to form a pn diode). At640, a diode may be fabricated using traditional (conventional) fabrication techniques (e.g., III-V semiconductor fabrication techniques). At650, the QD emitter may be used as an active light emitter.

A number of unique effects associated with dipole emitters in the near field of such plasmonic layers (e.g., the n++ InAs material) are leveraged in order to strongly enhance the radiative lifetimes of the QD emitters. The devices are the first examples of quantum engineered emitters integrated monolithically with plasmonic materials (in the same material system, and the same epitaxial growth). Using these techniques, electrically driven plasmonic quantum engineered emitters have been grown and characterized with performance better than commercially available mid-IR LEDs operating at the same wavelength. The results suggest almost no temperature degradation of emission from 77K to 300K, as well as output powers of greater than 1.45 μW, comparable to commercial mid-IR LEDs operating at the same wavelength.

Thus, epitaxial semiconductor metals (highly doped semiconductors) may be integrated with an active region in order to enhance mid-IR emission, thereby improving temperature performance and emitted power. Additionally, a device may be fabricated epitaxially such that an emitter is grown on an n++ backplane. The plasma wavelength is controlled based on the doping. The permittivity is also controlled in this manner. Depending on the implementation, a 4 to 4.8 μm range device may be fabricated in some implementations. Other implementations may extend the range up to 7 μm.

As explained above, existing mid-IR LEDs suffer from poor temperature performance and generally weak emission due to significant non-radiative recombination effects at higher temperatures (room temperature and above). This means that these sources are extremely power inefficient and require significant input power to achieve miniscule optical emission powers. The devices described herein have minimal degradation as a function of increasing temperature and emit more power than existing commercial products. This is a key improvement in performance. In addition, the optical properties of the underlying highly doped semiconductor metal are controlled, which offers the opportunity to control the spectral location of the performance improvement. In addition, the modulation bandwidth of any emitter is tied to the lifetimes of the excited charge carriers in the emitter. The device structure gives significantly faster radiative recombination times, which opens the door to extremely high modulation rates, with potential impact in on-chip optical interconnects and potentially short distance optical communication, for instance in data servers.

Advantages include: lowered cost of epitaxial growth (thinner devices or at least thinner active regions); significant improvement in temperature performance; and significantly higher modulation bandwidth potential, due to increased radiative recombination rate. It is noted that the fabrication method(s) used may be a limiting factor of device performance. More advanced fabrication techniques such as surface texturing, flip chipping, and improving packaging may improve device performance.

Currently, the primary limitations of the devices described and contemplated herein are the limitation of how high the InAs can be doped and how long of a wavelength the QDs can be made to emit. Other mid-IR emitters such as T2SLs may also be used.

It is contemplated that cheap and power efficient mid-IR LEDs are useful for compact sensor systems for gas sensing.

In an implementation, a semiconductor structure comprises: a highly doped (n++) epitaxial layer; and a diode structure disposed above the n++ epitaxial layer.

Implementations may include some or all of the following features. The diode structure comprises an n-doped InAs layer followed by a p-doped InAs layer, with an unintentionally doped layer between the n and p material forming a pn junction which contains a number quantum dot (QD) layers acting as QD emitters. The combination of the n++ and the pn junction creates a cavity, and provides a metallic surface in the near field of the QD emitters which increases radiative recombination rates in the QD layers. The highly doped (n++) epitaxial layer is in the range of about 500 nm to 1500 nm thick, and is doped to have a plasma wavelength of less than 7 μm. The highly doped (n++) epitaxial layer has a plasma wavelength in the range of about 4 μm to 4.8 μm. The doping controls a plasma wavelength and the permittivity. The structure provides improved temperature performance and power efficiency including minimal degradation in emitted power as temperature is increased from 77K to 300K. The highly doped (n++) epitaxial layer is doped at about 1019/cm3. The doping of the highly doped (n++) epitaxial layer is high enough so that a portion of the structure behaves like an optical metal. The structure is configured so there is about no temperature degradation of emission from 77K to 300K, and outputs power greater than 1.45 μW.

In an implementation, a semiconductor structure comprises: a plasmonic layer as a backplane; and a mid-IR light emitting diode (LED) emitter.

Implementations may include some or all of the following features. The mid-IR LED emitter is based on type-II quantum dot active regions grown with monolithically integrated semiconductor metal layers. The mid-IR LED emitter comprises layers of type-II semiconductor quantum dots integrated into a pn junction diode grown above the backplane, all in the same epitaxial growth. The plasmonic layer comprises an n++ InAs layer, wherein the type-II semiconductor is InGaSb, and the pn junction diode comprises InAs. The structure is configured to increase radiative recombination rate, which allows for increased efficiency emitters by controlling the emission of the emitting QDs in the near field of an optical metal. The backplane is a highly doped (n++) epitaxial layer in the range of about 500 nm to 1500 nm thick. The backplane is doped at about 1019/cm3. The backplane has a plasma wavelength in the range of about 4 μm to 4.8 μm. The backplane is doped to have a plasma wavelength of less than 7 μm.

In an implementation, a method of fabricating a semiconductor structure comprises: depositing a plasmonic layer on a substrate; and growing a quantum dot (QD) emitter above the plasmonic layer.

Implementations may include some or all of the following features. The method further comprises doping the plasmonic layer so that it becomes optically metallic in the mid-IR range. The plasmonic layer is a layer of highly doped (n++) InAs. The InAs layer is doped at about 1019/cm3. The method further comprises using the QD emitter as an active light emitter.

Although the subject matter has been described in language specific to structural features and/or methodological acts, it is to be understood that the subject matter defined in the appended claims is not necessarily limited to the specific features or acts described above. Rather, the specific features and acts described above are disclosed as example forms of implementing the claims.