III-Nitride nanowire array monolithic photonic integrated circuit on (001)silicon operating at near-infrared wavelengths

Photonic devices such as semiconductor lasers and photodetectors of various operating wavelengths are grown monolithically on a Silicon substrate, and formed of nanowire structures with quantum structures as active regions. A reduction of strain during fabrication results from the use of these nanowire structures, thereby allowing devices to operate for extended periods of time at elevated temperatures. Monolithic photonic devices and monolithic photonic integrated circuits formed on Silicon substrates are thus provided.

FIELD OF THE DISCLOSURE

The present disclosure relates to solid-state semiconductor devices and, more specifically, to the monolithic growth of nanowire array quantum structures on Silicon (Si) substrate and photonic devices formed thereof.

BACKGROUND

Every year electronic devices become faster. This happens because of Moore's law which states that transistors, which are the major element of all electronic circuits, become smaller every year. This trend results in a higher density of the number of transistors that can be fabricated on a microchip. One of the biggest technological concerns of the 21stcentury is the possible saturation of Moore's law and as a consequence the improvement of device speed with time may come to an end.

Many techniques have been suggested to keep Moore's law relevant. One of them is the incorporation of photonic devices on Silicon (Si) microchips, which would further improve the speed of the next generation devices such as microchips and integrated circuits, with light being much faster than electricity.

Currently, the main high-volume industrial manufacturing technology that is used to produce integrated circuits, such as electronic microchips and computer microprocessors, is Complementary Metal-Oxide-Semiconductor (CMOS) technology. Current CMOS microchips in the microelectronics industry are based on (001) Silicon (Si) substrates. Hence initial photonic devices also have to be compatible with (001) Silicon. Unfortunately, Si itself cannot emit light. To circumnavigate this problem, in conventional systems, lasers made of other materials have been fabricated separately and then placed on the Si substrate.

SUMMARY OF THE INVENTION

The present techniques include methods of fabricating photonic devices, such as semiconductor diode lasers, formed of nanowire structures grown monolithically on a Silicon substrate, and in particular on a (001) Silicon substrate. The photonic devices may be monolithically grown by growing an array of III-V nanowires directly from the (001) Silicon substrate in a single epitaxial growth process. The result is a monolithic structure in which a nanowire array of one type of semiconductor material is grown extending from a substrate of another semiconductor material, in particular Silicon.

In some examples, monolithically grown nanowires are grown as an array of nanowires, where the array may be formed into different photonic devices. In some examples, the photonic devices are edge emitting lasers formed from nanowires monolithically grown in the Silicon substrate. In some examples, the photonic devices are vertical cavity surface emitting lasers formed from nanowires monolithically grown in the Silicon substrate. The lasers may operate for extended periods of time under continuous wave or pulsed operation at elevated temperatures. Other photonic devices formed of these monolithically grown nanowire arrays include photodetectors.

Instead of traditional planar epitaxial layers, in some examples, the present techniques are able to form by monolithically grown nanowires having quantum structures that form the active gain region of a laser or form the absorption region in a photodetector. In these examples, monolithic growth may be achieved through the growth of nanowires, from which monolithic photonic devices are thereby formed from these nanowires. A reduction of strain results from using nanowire structures and that has made it possible for these devices to be formed monolithically on Silicon substrate and for these devices to operate under continuous wave or pulsed mode of operation for extended periods of time at elevated temperatures. In other words, in various examples, photonic devices are thermally stable and their performance does not degrade significantly with increasing temperatures.

In some examples, the present techniques provide a complete monolithic photonic integrated circuit directly grown on (001) silicon. In some examples, the circuit includes a diode laser, dielectric waveguide, and photodetector. The diode laser may be an edge-emitting laser and the photodetector a guided-wave photodiode, where both are fabricated of the same III-nitride nanowire arrays, providing more flexibility to these devices.

The present techniques also include the fabrication of a nitride-based nanowire array photodiode on silicon. For example, in some implementations, a nanowire array photodiode may be formed exhibiting a large responsivity at 1.3 μm, making the photodiodes ideal for silicon photonics and on-chip communication, as described. The photodiodes may be realized with the same monolithic nanowire array as used to form a monolithic semiconductor laser, but the photodiode may be operated under reverse bias, in contrast the semiconductor laser which is operated under forward bias.

The present techniques further provide the fabrication of a monolithic laser, which can be used for coherent light optical communication, inter-chip or intra-chip. The present techniques provide for lasers with emission wavelength of 1.3 μm, which is a desirable wavelength, as this particular wavelength produces low light dispersion in SiO2and is transparent to silicon. This wavelength also allows eye-safe operation.

The present techniques further provide the first monolithic photonic integrated circuit directly grown on (001) silicon substrate. In some examples, a photonic integrated circuit may be formed of a monolithic semiconductor laser having emission wavelength at or around 1.3 μm and a detector having 0.1 A/W responsivity at 1.3 μm, each grown on a (001) silicon subsrate. In some examples, the active material of the laser and detector is an array of InN/InGaN/GaN heterostructure nanowires. In some examples, InN disks have been inserted in GaN nanowires and this enables laser emission and detector absorption at 1.3 μm in the photonic integrated circuit.

The photonic integrated circuit is useful in silicon photonics based applications, i.e., on-chip communication etc. The laser output power, detector responsivity, and overall response of the photonic integrated circuit is sufficiently large for such applications. The lasers show high temperature stability, and good differential gain. The detector photocurrent response follows the laser injection current well, demonstrating a successful photonic integrated circuit on (001) Si. These characteristics can be exploited in a variety of applications where the environment can be challenging, e.g., smart car engine systems.

In some examples, the present techniques provide particular advantages over conventional systems. Graded layer regions have been grown in III-Nitride nanowire laser structures. The growth of InN disks as the active region has been achieved, thus providing for emissions in the near infrared, e.g., at 1.3 μm. A III-nitride nanowire photodiode operating at near-infrared using InN disks has been demonstrated. The external deposition of a dielectric to form a waveguide in between a monolithically grown nanowire laser and a monolithically grown nanowire photodetector to fabricate a complete photonic integrated circuit has been shown.

In accordance with an example, a semiconductor device comprises: a Silicon (Si) substrate; and a III-Nitride nanowire structure having (i) a quantum region formed of one or more layers of InN quantum disks, (ii) a first graded layer region, and (iii) a second graded layer region, wherein the quantum region is located between the first graded layer region and the second graded layer region, and wherein the III-Nitride nanowire structure is monolithically grown from the Si substrate, and wherein the III-Nitride nanowire structure is responsive at or about 1.3 μm.

In accordance with another example, a nanowire array structure comprises: a Silicon (Si) substrate; and a plurality of III-Nitride nanowire structures each having (i) a quantum region formed of one or more layers of InN quantum disks, (ii) a first graded layer region, and (iii) a second graded layer region, wherein the quantum region is located between the first graded layer region and the second graded layer region, wherein the plurality of III-Nitride nanowire structures are monolithically grown from the Si substrate, and wherein the plurality of III-Nitride nanowire structures are responsive at or about 1.3 μm.

In accordance with another example, a photonic integrated circuit comprises: a Silicon (Si) substrate; a first plurality of III-Nitride nanowire structures each having (i) a quantum region formed of one or more layers of InN quantum disks, (ii) a first graded layer region, and (iii) a second graded layer region, wherein the quantum region is located between the first graded layer region and the second graded layer region, wherein the first plurality of III-Nitride nanowire structures are monolithically grown from the Si substrate, and wherein the first plurality of III-Nitride nanowire structures form a nanowire semiconductor laser capable of emitting a photonic output at or about 1.3 μm; and a second plurality of III-Nitride nanowire structures each having (i) a quantum region formed of one or more layers of InN quantum disks, (ii) a first graded layer region, and (iii) a second graded layer region, wherein the quantum region is located between the first graded layer region and the second graded layer region, wherein the second plurality of III-Nitride nanowire structures are monolithically grown from the Si substrate, and wherein the second plurality of III-Nitride nanowire structures form a nanowire semiconductor photodetector capable of absorbing a photon input at or about 1.3 μm.

DETAILED DESCRIPTION

The present techniques include methods of fabricating photonic devices, such as semiconductor lasers and photodiodes, formed of nanowire structures grown monolithically on a Silicon substrate, and in particular on a (001) Silicon substrate. The devices are monolithically grown by growing an array of III-V nanowires directly from the (001) Silicon substrate in a single epitaxial growth process. The result a monolithic structure in which a nanowire array of one type of semiconductor material is grown extending from a substrate of another semiconductor material, in particular Silicon.

In some examples, these monolithically grown nanowires are grown as an array of nanowires, where the array may be formed into different photonic devices. In some examples, the photonic devices are edge emitting lasers formed from nanowires monolithically grown in the Silicon substrate. In some examples, the photonic devices are vertical cavity surface emitting lasers formed from nanowires monolithically grown in the Silicon substrate. The lasers may operate for extended periods of time under continuous wave or pulsed operation at elevated temperatures. Other photonic devices formed of these monolithically grown nanowire arrays include photodetectors.

In various examples, as described, III-nitride nanowire lasers and photodiodes are monolithically grown, having identical heterostructures and constituent material sections. In this way, these structures are able to form a monolithically grown photonic integrated circuit that can be realized by one-step epitaxy on (001) silicon substrates, adding significant flexibility to device and circuit fabrication. While nanowire arrays have been incorporated in the design of lasers emitting in the visible range, the nanowire heterostructure described in various examples herein are different in fundamental ways. Nanowire heterostructures in some examples herein are formed of: (i) InN disks inserted to form the light emission/absorption region of the respective devices; and (ii) graded InGaN regions incorporated for strain balancing in the heterostructure, reduction of defect density and optimal guiding of light in the lasers and detectors.

Further still in various examples herein, the present techniques include the fabrication of a nitride-based nanowire array lasers and photodiodes monolithically grown on silicon. In particular, nanowire array lasers and photodiodes exhibiting large responsivity at 1.3 μm are shown.

From the formation of such elements, the present techniques further provide the monolithic photonic integrated circuit directly grown on (001) silicon substrate. Many applications can use near-infrared (NIR) lasers and photodiodes operating at wavelengths of ˜1.3 μm. These include such electronic applications as on-chip and off-chip communication to design faster processors and computers. Together with a waveguide and detector these lasers can serve as a complete on-chip monolithic photonic link or optical interconnect.FIGS. 1 and 2illustrate an example photonic integrated circuit (PIC) formed monolithically on a Silicon substrate.

FIG. 1depicts a monolithically grown nanowire quantum semiconductor laser and a monolithically grown nanowire quantum semiconductor detector both grown on a Silicon substrate and both combing to form a monolithically fabricated photonic integrated circuit100. In the illustrated example, the integrated circuit100is formed of a curved waveguide in addition to a laser and a detector.

More specifically, in the illustrate example, photonic integrated circuit100includes a semiconductor laser110formed of a nanowire array structure112at its core, wherein the laser110is monolithically grown on a (001) Silicon substrate102. In operation, with an electric voltage applied to the laser110, an electric current is injected into the laser110through a p-contact metal electrode114and a n-contact metal electrode116. In the illustrated example, the laser110is configured as an edge-emitting laser, such that light134that is emitted from the laser110and travels along a waveguide130(also part of the circuit100) from which the light exists (as light136) and is absorbed by a photodetector120(also part of the circuit100). The light136absorbed by the detector120may be the same light134emitted by the laser110. Note that throughout this disclosure the terms detector, photodetector, and photodiode are used interchangeably.

The photodetector120is formed of a nanowire array structure122. In the illustrated example, the nanowire array structure112of the laser110and the nanowire array structure122of the photodetector120are identical. However, in other implementations, the nanowire array structure122may be a different structure with different compositions and different design than the nanowire array structure112. Depicted are also a p-type metal electrode124and an n-type metal electrode126of the photodetector120.

The photonic integrated circuit100is a monolithically grown circuit, where the semiconductor laser110and the semiconductor photodetector120have been monolithically grown on (001) Si substrate. As described further, the semiconductor laser110may operate under a continuous wave (CW) mode of operation for extended periods of time (˜1000 hours or more). Both the laser110and the photodetector120are thermally stable and their functionality and performance are stable with increasing temperatures. The semiconductor laser100may be monolithically grown having different epitaxial crystal layers forming the heterostructure, which depending on their composition and structure will emit (absorb) at different wavelengths, including the desirable wavelength of ˜1.3 μm. In this way, the present techniques provide a first of its kind and tunable design photonic devices and photonic integrated circuit grown monolithically from a Silicon substrate.

FIG. 2depicts a more detailed illustration of the monolithic nanowire array quantum semiconductor laser110monolithically grown on a (001) Silicon substrate. An array210(also termed herein a cluster) of four individual nanowire structures220that are part of the densely packed nanowire array structure112, where these nanowire structures are grown monolithically on the Si substrate102, e.g., using a single step epitaxial growth process or in some examples using a multiple step epitaxial growth process.

In the illustrated example, the semiconductor laser110is an edge-emitting type of laser device. This type of edge-emitting semiconductor laser110typically has two mirror facets. InFIG. 2, according to one possible example, a mirror facet242(not visible in the drawing) of the laser device110points in the direction of the waveguide structure130and another mirror facet244of the laser device110points in the opposite direction which could be pointing into another waveguide structure or, as depicted in this particular embodiment, emit light into the air in the form of a beam of light202. The nanowire array structure112emitting in the NIR, e.g., at or around 1.3 μm, may have unique nanowire heterostructures that include (i) InN disks forming the quantum active region and (ii) graded InGaN regions for strain balancing in the heterostructure, reduction of defect density and optimal guiding of light in the lasers and detectors.

In telecommunication applications a semiconductor laser with an emission wavelength at or around 1.3 μm can be used in single-mode or multi-mode communication. The laser devices which are described in this disclosure demonstrate a wavelength of emission at or around this important 1.3 μm wavelength. However, this same technology that is described in this disclosure can be used to fabricate semiconductor lasers, grown monolithically on Si substrate, that have a wavelength of emission at or around other wavelengths including 1.55 μm. Such devices operating at the wavelength of ˜1.55 μm, and grown monolithically on Silicon substrate, may be useful in long-haul fiber-optic links within the data communication and telecommunication industries.

More broadly, the techniques described herein may be used for any type of photonic integrated circuit. Silicon Complementary Metal-Oxide-Semiconductor (CMOS) microchip applications can now be achieved using epitaxial growth and monolithic growth and integration of semiconductor lasers and optical detectors with guided wave components on a (001) Si wafer, with components preferably operating in the wavelength range of 1.3 μm to 1.55 μm at room temperature. Techniques demonstrated in the past for having optically pumped or electrically pumped GaAs and InP based semiconductor lasers on Silicon included wafer bonding, selective area epitaxy, epitaxy on tilted substrates, and use of quantum dot or planar buffer layers. The present techniques, however, provide a monolithic optical interconnect on a (001) Si substrate comprising of a nanowire array edge emitting electrically pumped semiconductor laser and guided wave photodetector, with a planar dielectric waveguide in between the laser and the photodetector. The laser and the photodiode devices are realized with the same nanowire heterostructure by one-step epitaxial crystal growth process, as further described below. An example structure is a III-Nitride dot-in-nanowire array edge emitting semiconductor laser and guided wave photodetector, with a planar SiO2/Si3N4dielectric waveguide in between the laser and the photodetector.

Further still, the present techniques may be used to form lasers in inter-chip or intra-chip communication applications, such as those related to optical communication applications. Semiconductor lasers with emission wavelength at 1.3 μm are desirable as this particular wavelength produces least dispersion in SiO2and is transparent to Silicon. This wavelength also allows eye-safe operation. Hence the emitted light has negligible attenuation in Si-based devices and more signals (or channels) can be accompanied if the light is guided using SiO2based waveguides. Making such an electrically pumped semiconductor laser directly and monolithically on Silicon has proven to be a great challenge to the photonic industry.

While the primary examples described are that of NIR emissions and photodetection, the techniques may be used to form monolithically grown nanowire semiconductor lasers emitting over a range of frequencies, including in a green region of the spectrum, a red region of the spectrum, and infra-red region of the spectrum. Particular examples include emissions at or about 1.3 μm, at or about 560 nm, at or about 610 nm, at or about 630 nm.

We now turn to describing monolithic growth techniques and further example photonic devices that can be formed of nanowire structures grown in accordance with the techniques herein.

Since Silicon (Si) with an indirect bandgap is an inefficient light-emitting semiconductor. The common technique of incorporating an electrically-pumped laser on a Silicon platform has been the integration of III-V based devices on it, either by direct epitaxy or by wafer bonding techniques. Direct epitaxial growth of III-V materials and heterostructures on Silicon presents three challenges. A usually large lattice mismatch leads to a high density of threading dislocations. There is also a thermal mismatch due to unequal thermal expansion coefficients. Finally, the epitaxy of polar III-V materials such as GaAs on non-polar Si leads to the formation of antiphase domains (APDs). This is usually alleviated by growing the III-V heterostructure on a (001) Si substrate offcut by 4° toward the [011] plane. It is unlikely that CMOS and related Si-based technologies will be developed on such tilted platforms. Selective area epitaxy and growth on special buffer layers have led to some success.

A different approach to solving the Si and III-V mismatch problem is to use entirely different semiconductors, the III-nitride compounds, but not in their usual planar form. (Al, Ga, In)N nanowires and nanowire heterostructures grown catalyst-free on (001) Si substrates have shown extraordinary promise as crystalline (wurtzite) nanostructures for the realization of visible light-emitting diodes (LEDs) and diode lasers. We demonstrate the application of such nanowires, grown on silicon, to near-infrared (1.3 μm) lasers and photodetectors. Without any patterning on the substrates the nanowires grow as a random array along the c-axis and are relatively free of extended defects due to the large surface-to-volume ratio and the formation of a thin SiNxlayer at the nanowire-Silicon interface.

The SiNxlayer reduces the ˜13.5% stress and reduces the defect density at the interface. Compared to planar heterostructures, the nanowires have reduced polarization field due to radial relaxation of strain during epitaxy. Consequently, the radiative recombination times are smaller than in quantum wells. Thin (2 nm to 3 nm thick) single or multiple InGaN disks can be incorporated along the length of the nanowires and the alloy composition in the disk region can be varied to yield optical emission ranging from the ultraviolet (UV) to near-infrared (near-IR). It has been established that a quantum dot is formed in the disk region, possibly due to strain relaxation along the surface of the nanowire during epitaxy. It has also been reported that the surface recombination velocity of GaN nanowires is small and about ˜103cm/s. In contrast to conventional techniques, the self-organized random array of nanowires can be grown on any size of Si substrate, depending on the growth facility, and the process is therefore scalable. The nanowire area density can be varied in the range of 107cm−2to 1011cm−2by tuning the growth parameters. Sections of the nanowires can be doped n-type and p-type and thereby diodes can be realized.

The unique properties of structures such as III-Nitride nanowire and their heterostructures make possible the realization of nanowire-based photonic integrated circuits with active devices on a (001) Si substrate platform. Of particular interest is a monolithic optical interconnect consisting of a diode laser, a passive waveguide or other guided-wave elements, and a photodiode. As used herein, the terms “diode laser”, “laser”, “laser diode”, and “semiconductor laser” are used interchangeably. With modulation of the semiconductor laser, this example photonic integrated circuit would constitute an optical communication system

FIG. 3Adepicts a SEM image of a ridge waveguide semiconductor laser310that is grown monolithically on a Si substrate312.FIG. 3Bdepicts a SEM image of GaN nanowire array structure depicting multiple nanowire structures clustered together to form a densely packed nanowire array structure. The laser310in this example is approximately about 50 μm wide. A p-type metallic contact314and an n-side metallic contact316, similar to that ofFIG. 1, are also shown in addition to a portion of a front mirror facet344.

The laser310is formed for a nanowire array structure320includes densely packaged array of individual nanowires340monolithically grown on the Si substrate312. An interface region322is shown between the nanowire array320and the Si substrate312.

FIG. 4illustrates the epitaxial crystal structure400of a nanowire semiconductor laser designed for emission at a wavelength at or about 1.3 μm. As used herein, references to emissions (or absorptions) “at or about” or “approximately” or “˜” 1.3 μm refers to emissions (or absorptions) at 1.3 μm±0.05 μm. Emissions (absorptions) at 1.3 μm are preferred for telecommunication applications and Si CMOS fabrication. The various crystal layers within the nanowire structure are depicted, showing the different layer compositions. What is depicted in the diagram is the epitaxial crystal structure of an individual InN/In0.4Ga0.6N/GaN nanowire structure, which may be included within an array of densely packaged nanowire structures monolithically grown on (001) Si substrate, such as substrate402. The first epitaxial crystal layer that is grown directly on top of the (001) Si substrate402is a 10 nm thick n+-GaN layer404. This is followed by a 250 nm thick n-GaN layer406. Ten (10) semiconductor crystal layers408, in this example comprising of InxGa1−xN material of various compositions in which the Indium (In) content portion, are grown. As indicated, for the InxGa1−xN layers x varies from about 0.04 to 0.4. The layers of408form a graded layer region of the heterostructure, where each of the 10 layers shown has different In and Ga concentrations, where in the illustrated example those concentrations increase/decrease with each successive layer. The thickness of each of these ten layers of InxGa1−xN is about 15 nm.

An InN/InGaN active region410is then formed. In the illustrated example, the active region410includes 4 layers of InN each about 6 nm thick. These InN layers are surrounded by In0.4Ga0.6N barrier layers of about 12 nm thickness. The InN layers comprise the quantum structures which in this embodiment, and according to example, are quantum disk. Grown on top of the active region410are ten semiconductor crystal layers412, which form a graded layer region. In the illustrated layers412are formed of InxGa1−xN material of various compositions in which the Indium (In) content portion, as indicated, x, in InxGa1−xN, varies from about 0.04 to 0.4. The thickness of each of these ten layers of InxGa1−xN material is about 15 nm. At the top of the nanowire structure are grown the p-type GaN layers414which, in this embodiment and according to an example, comprise of a 40 nm thick p-GaN layer followed, at the top of the nanowire structure, by a 10 nm thick p+-GaN layer.

An example fabrication of a graded refractive index separate confinement heterostructure (GRIN-SCH) nanowire structure400is as follows. The nanowire structure400was monolithically grown by plasma-assisted molecular beam epitaxy (PAMBE) on (001) Si substrates in a Veeco GEN II system. In this fabrication example, the entire nanowire structure400was grown with a nitrogen plasma flow rate of 1 sccm. GaN sections404,406, and414were grown at a substrate temperature of 820° C., except the top p+-GaN region of414, which was grown at 800° C. The graded InxGa1−xN regions (0≤×≤0.4)408,410, and412forming the surrounding graded layers and active region were grown in 10 equal steps of 15 nm on both sides of the gain region consisting of 4 InN disks of thickness 6 nm surrounded by 12 nm In0.4Ga0.6N barriers410. The graded regions were grown at substrate temperatures varying from 631° C. (In0.4Ga0.6N) to 819° C. (GaN) and the entire InN-disk and In0.4Ga0.6N-barrier region was grown at 489° C. The Gallium (Ga) and Indium (In) fluxes were in the range of 1.1×10−8to 1.2×10−7Torr and 2×10−8to 1×10−7Torr, respectively, depending on the composition of the material being grown. The height, diameter and density of the nanowires are estimated to be ˜400 nm, ˜60 nm, and ˜3.2×1010cm−2respectively, and the fill factor is estimated to be 0.91.

In examples herein, the 1.3 μm semiconductor laser may have an active area (laser gain region) that includes a particular type of quantum structure referred to as quantum dot structure. The particular type of quantum dot structure may be a quantum disk structure, which is one variety of quantum dot structure. These quantum disks, in some implementations, are comprised of InN material surrounded by InGaN barriers. Such active InN quantum disks have been incorporated in nanowire structures for the first time, as a result of the present techniques.

Instead of traditional planar epitaxial layers, nanowire structures with quantum disks were used for the laser gain material. Use of nanowire structures reduces strain in the heterostructure layers of the laser, which has enabled the inventors to incorporate Indium Nitride (InN) quantum disks that can emit light at near-infrared wavelengths including at the wavelength of, at or around, 1.3 μm. The reduction of strain, through the use of the nanowire structures, also has made it possible for these devices to operate under CW mode of operation for extended periods of time (e.g. ˜1000 hours or more). At the same time these lasers can operate at elevated temperatures. Indicative of this is the relatively high characteristic temperature, also referred to as the T-zero (T0) parameter, of these lasers. A high T0value indicates that the performance of the laser decreases less rapidly with increasing temperatures. These nanowire structure laser devices are thermally stable devices and their performance does not degrade significantly with increasing temperatures, another feature heretofore unattainable with conventional techniques.

Example quantum structures that may be used to form the laser gain regions include quantum disks, as well as other quantum dot structures, including quantum spheres, quantum disks, core-shell quantum structures, or other similar forms of quantum elements and/or quantum structures. The term quantum dot is herein used to refer to all these various possible shapes of quantum structure within the nanowire structure, one particular type of which, according to an example, is the quantum disk. As such, any of the techniques and devices herein may be implemented using any of a variety of quantum structures, and are not limited only to quantum disks.

The quantum structures herein may be disk-in-nanowire (DINW) structures grown using state-of-the-art plasma assisted molecular beam epitaxy (MBE). The term DINW refers to nanowire structures which have quantum disks embedded within them. From transmission electron microscopy (TEM) images it has been demonstrated that these quantum disks form quantum dot type of structures, which further improve the device characteristics. The nanowire structures were grown in a nitrogen plasma rich environment on a (001) Silicon substrate. Typical nanowire heterostructures consist of a graded cladding layer that reduces the strain and improves the light confinement. As stated before, to achieve near-infrared emission while keeping the advantages of quantum confinement, InN disks were grown between In0.4Ga0.6N barriers. Such demonstration of InN disk-in-nanowires is the first and only one of its kind. The InN/InGaN quantum disks have excellent optical properties, which are exploited in the lasers.

Once the material is grown and characterized, lasers were fabricated using a series of steps including Parylene planarization, photolithography, plasma etch, and metallization. These ridge waveguide lasers have 5 μm wide to 50 μm wide laser ridge widths and variable lengths. The laser facets were formed by focused ion beam (FIB) etching technique and subsequent deposition of ZnSe/MgF2distributed Bragg reflector (DBR) mirrors.

The lasers were characterized in a state-of-the-art optoelectronics laboratory. Maximum output power was found to be 7 mW. Characteristic temperature of these laser devices was found to be 220 K. The differential gain parameter was found to be 3×10−16cm2. The differential gain was measured using high speed measurement techniques from which the bandwidth of these lasers were also found to be ˜3 GHz. Such characteristic properties of these lasers make them an ideal candidate for silicon photonics based applications. A liquid nitrogen cooled Ge detector was used to measure the electroluminescence properties of the lasers and the peak emission wavelength at stimulated emission was found to be ˜1.3 μm which is again ideal for on-chip photonic applications. The novel active (gain) material, straight forward fabrication process and favorable characteristics can make these nanowire structure lasers that are grown monolithically on Silicon substrate one of the most important elements in silicon photonics.

The technology, the devices, and the methods that are described herein in this disclosure have several advantages that are apparent. For example, as the lasers are grown on (001) Silicon, they are CMOS-technology compatible. Hence the technology can be transferred to the microelectronics industry. In addition, the III-nitride based lasers demonstrate high characteristic temperature making them suitable for challenging environments (e.g. computer servers, automobiles, etc.). Also, the 1.3 μm emission wavelength is ideal for multi-mode communication applications with the data communication and telecommunication industries.

In addition to fabricating the entire monolithic photonic integrated circuit, the present techniques provide for fabricating discrete nanowire lasers, detectors, and dielectric waveguides.

In some implementations, fabrication of discrete edge emitting semiconductor laser devices, which are also referred to as laser diodes or diode lasers, was initiated by planarizing the nanowire array with Parylene, which was deposited by physical vapor deposition (PVD) at room temperature. It has been reported that Parylene is transparent at the wavelength of about 1.3 μm. Furthermore, Parylene helps to passivate the nanowire surfaces and enhances the internal quantum efficiency by about 10% to 12%.

Excess Parylene is etched to expose the nanowire tips, which are treated with ammonium sulfide to reduce the p-contact resistance. Ridge waveguide devices were fabricated by a combination of reactive ion etching (RIE), photolithography and contact metal deposition. The Aluminum (Al) n-ohmic contact was formed on the Si substrate surface and the Nickel/Gold (Ni/Au) p-ohmic contact was formed on the top to the exposed p+-GaN nanowire tips. Ridge widths of 5 μm to 50 μm (for example as depicted inFIG. 3A) were defined by etching and cavity lengths of 0.5 mm to 2 mm were defined by dicing the substrate. This was followed by planarization with SiO2and interconnect and contact pad deposition. The cleaved facets were further polished by focused ion beam (FIB) etching using a Ga source and 3 pairs of MgF2/ZnSe (237 nm/132 nm) distributed Bragg reflectors (DBR) were deposited on both facets to attain a reflectivity of 88%. The contact geometry was arranged in a ground-signal-ground configuration to facilitate high frequency probing. The laser diodes are characterized by a forward turn-on voltage of ˜3 V, a series resistance of 10Ωto 25Ω, and reverse breakdown voltage of 8 V to 12 V.

FIG. 5depicts an individual nanowire structure500and the epitaxial composition of a nanowire structure within the structure of a semiconductor quantum laser grown monolithically on Silicon substrate and emitting at near infra-red wavelength of at or about 1.3 μm. In the illustrated example, grown on a Si substrate502is a 260 nm n-GaN layer510. On top of this layer510, grown in ten steps, is a group512of 150 nm thick graded cladding layers from n-GaN to n-In0.4Ga0.6N. On top of this group512of layers are the active region514layers. The active region514of this structure includes 4 layers of InN each about 6 nm thick. These InN layers are surrounded by In0.4Ga0.6N barrier layers of about 12 nm thickness. The InN layers form quantum structures which in the illustrated example are quantum disks. On top of the active region516of the nanowire structure grown is a 150 nm p-GaN layer.

FIG. 6provides a plot600of the experimentally measured Light-Current (L-I) characteristic plots602and604of broad area near infra-red (NIR) semiconductor laser devices606and608, respectively, each having emission wavelength of about 1.3 μm. The inset610shows the output wavelength characteristic plot for an injection current of 810 mA. The plot600depicts the steady-state L-I characteristics at room temperature of a 50 μm×2 mm ridge waveguide laser device operating under pulsed (5% duty cycle) mode of operation. Output powers up to ˜10 mW were measured at room temperature without any heat sinking or facet cooling. The output spectrum612depicted in the inset610confirms 1.3 μm peak wavelength of emission. The slope efficiency was 0.14 W/A. A low value of threshold current Ith=673 mA was measured. The wall plug efficiency parameter was ˜0.81%.

FIG. 7is a plot700of the temperature dependence of the threshold current density Jth. Linear fit line702indicates a characteristic temperature T0=241 K. The data in this diagram are associated with nanowire structures of dimensions as shown. The nanowire structure was operated under a 5% pulsed mode of operation. In this example, the nanowire structure was formed as a laser having an emission wavelength in the near infra-red range of the spectrum at a wavelength of at or about 1.3 μm.

FIG. 8illustrates a plot800of the measured output power of a 1.3 μm wavelength disk-in-nanowire semiconductor laser as a function of time with constant Continuous Wave (CW) current injection. These measurements were made without any heat sinking or active cooling. The plot800indicates a lifetime of ˜1000 hours. Nanowire semiconductor lasers grown monolithically on Silicon substrate have the potential for much higher lifetimes than what is depicted here. In this example, the nanowire structure or array based laser has dimensions of 5 μm width by 500 μm cavity length. The device is operating at a temperature of 300 K and at a CW current of 20 mA.

The fabrication of the photonic integrated circuit follows similar process steps to the fabrication of discrete devices. The details of an example photonic integrated circuit fabrication process are described further below. A SEM image of the entire photonic circuit900is depicted inFIG. 9wherein a laser902, a waveguide904, a detector906, a p-contact908and a n-contact910for current injection are indicated. In the fabrication of the photonic integrated circuit900, the dielectric waveguide904which is in between the nanowire laser902and detector906is formed by selective etching of the nanowires and the deposition of 400 nm SiO2followed by 400 nm of Si3N4. For the laser902, the mirror facet away from the waveguide (not visible) was made reflective by FIB etching and subsequent deposition of MgF2/ZnSe DBR layers and the mirror facet912coupled to the waveguide904was made reflective with 4 pairs of air/nanowire-Parylene DBR layers914(see inset image916), also formed by FIB etching. For the detector906, ˜220 nm of anti-reflective SiO2was deposited on the facet of the detector that is not coupled to the waveguide.

FIGS. 10A and 10Bdepict a SEM image (top imageFIG. 10A) and an optical microscope image (bottom imageFIG. 10B) of a photonic integrated circuit structure1000, respectively. The photonic integrated circuit1000may be like that of device900shown inFIG. 9. The SEM image ofFIG. 10Adepicts a planar top view SEM image of the device.

In the illustrated example of the photonic integrated circuit1000, there is a semiconductor laser1002, a linear waveguide structure1004, and a semiconductor photodetector1006fabricated in a linearly aligned arrangement. Also indicated are two p-contact metallic pads1008and1010and two n-contact metallic pads1012and1014. Also indicated is the mirror facet1016of the laser1002. The semiconductor laser1002and the semiconductor photodetector1006are both III-GaN nanowire array structures that are formed monolithically on a Silicon substrate.

FIG. 11depicts a process flow chart showing a fabrication process1100of fabricating a photonic integrated circuit. The method that is depicted is according to an example implementation. At a block1102, the epitaxial deposition of the first GaN template layer onto the Silicon (Si) substrate is performed. At a block1104, the epitaxial deposition of the additional n-type layers of the crystal structure onto the GaN coated Si substrate is performed. At a block1106, epitaxial deposition of the first group of graded InxGa1−xN layers forming the waveguide is performed. At a block1108, the epitaxial deposition of the quantum structure layers of the nanowire structure is performed. At a block1110, the epitaxial deposition of the second group of graded InxGa1−xN layers forming the waveguide is performed. At a block1112, the epitaxial deposition of the additional p-type layers of the nanowire structure is performed. At blocks1102to1112the processes associated with the epitaxial crystal growth process phase of the fabrication process are shown. Below are described examples associated with the device fabrication phase of the fabrication process1800.

At a block1114, the planarization of the nanowire array structure, by deposition of Parylene material and subsequent etching of excess Parylene material is performed. At a block1116, the deposition of the p-metal contact layer over selected areas of the nanowire structure array is performed. At a block1118, the selective etching of the array to form ridge waveguide structure of the devices is performed. At a block1120, another etching step to expose the part of the Si between the ridges and etch the nanowires between the laser and detector is performed. At a block1122, the deposition of the n-metal contact over selected areas of the exposed n-Silicon is performed. At a block1124, the deposition of waveguide layers in the region between the laser device and the photodetector device is performed. At a block1126, the formation of one mirror facet of the laser through Focused Ion Beam (FIB) etching and subsequent deposition of Distributed Bragg Reflector (DBR) layers on the mirror facet is performed. At a block1128, formation of anti-reflective layer on one side of the detector facet through the deposition of SiO2is performed. At block1130, the formation of the other mirror facet of laser by FIB etching of the nanowire-Parylene composite and as a result forming air-semiconductor DBR layers are performed. The end result of the above mentioned processes of is the realization of the photonic integrated circuit.

FIGS. 12A and 12Bdepict part of the fabrication process1100of forming a photonic integrated circuit1200(see,FIG. 15). The figures show the growth of a nanowire array structure1202, formed of individual nanowire structures1204, grown monolithically on Silicon substrate1206.FIG. 12Adepicts the crystal growth of the nanowire structures on n-type (001) Si substrate using plasma assisted molecular beam epitaxy (PA-MBE) technique.FIG. 12Bdepicts a Parylene planarization of the nanowire array structure1902, in which a Parylene material layer1208fills the empty space between the individual nanowire structures1204.

FIGS. 13A and 13Bdepict further processes of the growth of the nanowire structures on Silicon substrate1206and the formation of a monolithically grown photonic integrated circuit on a Silicon substrate.FIG. 13Adepicts the process of p-metal deposition over selected areas of the nanowire structure array. A p-metal contact1210selectively covers some regions of the nanowire array1202while leaving exposed other regions1212, where the exposed regions are exposed regions of Parylene in the illustrated example.FIG. 13Bdepicts the process of etching of the nanowire structure array1202in order to form channels on the sample between the regions where there are located a laser1214and a photodetector1216. An etched channel1218is shown.FIG. 13Billustrates two p-contact metallic electrode1210, one electrode for the laser1214and the other for the photodetector1216.

FIGS. 14A and 14Bdepict further processes for the monolithic growth of the nanowire structures on Silicon substrate and the formation of a monolithically grown photonic integrated circuit on a Silicon substrate.FIG. 14Adepicts the process of n-metal deposition over selected areas of the exposed n-Silicon substrate1206. An exposed area1220was created by the FIB etching process ofFIG. 13B. The Parylene material1208that fills the empty area between the individual nanowire structures of the array1202is shown. A n-metal contact1222depicted here is Aluminum (Al) according to an example. The p-metal contact1210that is deposited over the detector1216is comprised of Nickel (Ni) and Gold (Au) layers according to an example. The n-metal contact1222is comprised of Aluminum (Al).FIG. 14Bdepicts the process of waveguide layers deposition in the region between the laser1214and the photodetector1216. In this example, a waveguide structure1224is formed and comprises 400 nm thick SiO2bottom layer1226and 400 nm of Si3N4top layer1228.FIG. 15provides a final schematic of the photonic integrated circuit1200formed by these processes. The laser1214produces an output emission1230at or about 1.3 μm. Output light1230is emitted from the laser1214, travels through the waveguide structure1224, and upon reaching the detector1216is absorbed as the input light.

As shown, the inventors have formed both laser and photodetector semiconductor photonic devices on (001) Si substrate in a monolithic fashion. The present techniques successfully demonstrate the coupling of the edge emitting laser emission into a monolithic dielectric waveguide and a subsequent coupling of the guided light into an in-plane guided wave photodiode (also referred to as photodetector). Either passive waveguides, such as those described, or other guided wave components can be used to form parts of an optical interconnect on a Silicon chip. The bottom-up monolithic approach demonstrated here allows optoelectronic integration with Si-based electronic circuits for laser biasing, modulation, and other controls. By virtue of the low growth temperature at which the nanowire heterostructures are grown it is expected that the integration will be compatible with CMOS processing of the electronics.

The present techniques show III-nitride nanowire lasers and photodiodes designed with nanowires having identical heterostructures and constituent materials section. Hence they can be realized by one-step epitaxy on (001) Si, adding significant flexibility to device and circuit fabrication. The nanowire heterostructure may include InN disks quantum regions and graded InGaN regions. Indeed, the present techniques successfully demonstrated the first use of pure InN quantum disks as the active region of III-nitride nanowire devices. While the large In flux required during epitaxy can substantially increase radial growth rate, inducing coalescence of adjacent nanowires, and such coalescence can deteriorate the optical quality (luminescence efficiency) of the nanowire array, the present techniques include all new growth parameters, e.g., compared to the ones used in nanowire growth for visible lasers. The results, including the formation of pure InN quantum disks, was unexpected. The In and Ga metal fluxes have been reduced by almost one order of magnitude compared to metal fluxes used in Ga-rich growths. To ensure low desorption of the metals, the growth temperature was also been reduced accordingly. The growth temperatures are almost 50 to 100 C. lower than the temperatures used in visible (Ga-rich) nanowire growth. The result has been the monolithic fabrication of a nanowire array based photonic integrated circuit having a laser, waveguide and a photodiode.

As will be appreciated, the present techniques provide considerable uniqueness and advantage over conventional concepts of nanowire fabrication, where some have proposed nanowire structures but with limited success and mostly applying what appears to be theoretical approaches, unsupported by actual fabrication. U.S. Pat. Nos. 8,212,235, 8,932,940, and 7,474,811, for example, provide merely general, and in places vague, concepts of nanowire optoelectronic devices, without any real examples. Their proposed nanowires are in a bridging configuration impractical to fabrication, and in configurations that suggest that monolithic nanowire structures are not even possible. Moreover, in most of the proposed devices, charged carriers (electrons and holes) and photons propagate along the axis of the nanowires and hence the optoelectronic devices are vertical and top-emitting or top-absorbing, which means the devices were confined to placement on (111) and (110) silicon and not compatible with CMOS technology and silicon photonics and (001) monolithic fabrication.

While examples herein are provided showing nanowires structures monolithically growth as dot-in-nanowire structures, other quantum active region configurations may be used such as core-shell nanowires. Further still, the present techniques may be used to fabricate the nanowire arrays and individual nanowire structures using a metallic mask with patterned holes used on the substrate in order to define the shape and the dimensions of the nanowire structures. It is possible to adopt a method of precisely controlling the formation of nanowire structures with precise predetermined diameter, height, spacing, and separation.