High performance GeSi avalanche photodiode operating beyond Ge bandgap limits

Avalanche photodiodes (APDs) having at least one top stressor layer disposed on a germanium (Ge)-containing absorption layer are described herein. The top stressor layer can increase the tensile strain of the Ge-containing absorption layer, thus extending the absorption of APDs to longer wavelengths beyond 1550 nm. In one embodiment, the top stressor layer has a four-layer structure, including an amorphous silicon (Si) layer disposed on the Ge-containing absorption layer; a first silicon dioxide (SiO2) layer disposed on the amorphous Si layer, a silicon nitride (SiN) layer disposed on the first SiO2 layer, and a second SiO2 layer disposed on the SiN layer. The Ge-containing absorption layer can be further doped by p-type dopants. The doping concentration of p-type dopants is controlled such that a graded doping profile is formed within the Ge-containing absorption layer to decrease the dark currents in APDs.

TECHNICAL FIELD

The present disclosure relates to photosensitive devices. More particularly, the present disclosure relates to an avalanche photodiode.

BACKGROUND

An avalanche photodiode (APD) is a type of photosensitive semiconductor device in which light is converted to electricity due to the photoelectric effect coupled with electric current multiplication as a result of avalanche breakdown. APDs differ from conventional photodiodes in that incoming photons internally trigger a charge avalanche in APDs, thus APDs can measure light of even lower level and are widely used in long-distance optical communications and optical distance measurement where high sensitivity is needed.

Germanium/Silicon (GeSi) APDs combine the characteristic of excellent optical absorption of Ge at telecommunication wavelength with the characteristic of outstanding carrier multiplication properties of Si. The use of Ge allows the extension of the spectral response of GeSi APDs to longer wavelengths, up to 1550 nm. However, the absorption of bulk Ge ceases at 1550 nm at room temperature, which is limited by its bandgap in Gamma band. Since there is a requirement for the optical band in optical communication systems to cover a wide wavelength range, from 1260 nm to 1620 nm, the longer wavelength limitation of optical absorption of Ge is a main reason restricting the wide application of GeSi APDs in optical communication systems. Therefore, there is a need to extend the absorption of Ge to longer wavelengths above 1550 nm.

One of the parameters that impact the applicability and usefulness of APDs is dark current. Dark current is a relatively small electric current that flows through a photosensitive device, such as a photodiode, even when no photons are entering the photosensitive device. Dark current is one of the major sources of noise in photosensitive devices. Consequently, dark current is a limiting factor for GeSi APDs in high-speed optical communication applications. Therefore, there is a need to reduce the dark current to achieve high performance in APDs.

SUMMARY

The present disclosure provides APDs having at least one top stressor layer disposed on the light absorption layer. The top stressor layer can increase the tensile strain of the absorption layer. As a result, the absorption layer can absorb light with wavelengths beyond its optical bandgap. The absorption layer can be further doped with p-type dopants. The doping concentration of the p-type dopants is controlled such that a graded doping profile is formed within the absorption layer to decrease the dark currents of APDs.

In one aspect, an APD may comprise a substrate and a multi-layer structure disposed on a first surface of the substrate. The multi-layer structure may comprise: at least one top stressor layer, the at least one top stressor layer is coupled to at least one metal contact of a first electrical polarity; a germanium (Ge)-containing absorption layer on which the at least one top stressor layer is disposed; a charge layer on which the Ge-containing absorption layer is disposed; a multiplication layer on which the charge layer is disposed; and a contact layer on which the multiplication layer is disposed, the contact layer is coupled to at least one metal contact of a second electrical polarity opposite to the first electrical polarity. The at least one top stressor layer is configured to increase a tensile strain of the Ge-containing absorption layer.

In some embodiments, the at least one top stressor layer may comprise: an amorphous silicon (Si) layer disposed on the Ge-containing absorption layer; a first silicon dioxide (SiO2) layer disposed on the amorphous Si layer; a silicon nitride (SiN) layer disposed on the first SiO2layer; and a second SiO2layer disposed on the SiN layer.

In some embodiments, the Ge-containing absorption layer may comprise Ge, germanium-silicon (GeSi), or silicon-germanium-carbon (SiGeC). In some embodiment, the charge layer may comprise p-type Si, p-type GeSi, or p-type SiGeC.

In some embodiments, the multiplication layer may comprise intrinsic Si or lightly doped n-type Si.

In some embodiments, the contact layer may comprise n-type Si.

In some embodiments, the substrate may comprise a Si substrate or a silicon-on-insulator (SOI) substrate.

In some embodiments, the APD may additionally comprise at least one anti-reflection layer disposed on a second surface of the substrate opposite to the first surface. The at least one anti-reflection layer may comprise a single SiO2layer or three layers comprising a SiN layer disposed between two SiO2layers.

In some embodiments, the Ge-containing absorption layer may further comprise p-type dopants; a doping concentration of the p-type dopants is controlled such that a graded doping profiled of the p-type dopants is formed inside the Ge-containing absorption layer.

In some embodiments, the p-type dopants may comprise gallium (Ga) or boron (B).

In another aspect, an APD may comprise a substrate and a multi-layer structure disposed on a first surface of the substrate. The multi-layer structure may comprise: at least one top stressor layer, the at least one top stressor layer is coupled to at least one metal contact of a first electrical polarity; a Ge absorption layer on which the at least one top stressor layer is disposed; a charge layer on which the Ge absorption layer is disposed; a multiplication layer on which the charge layer is disposed; and a contact layer on which the multiplication layer is disposed, the contact layer is coupled to at least one metal contact of a second electrical polarity opposite to the first electrical polarity. The at least one top stressor layer is configured to increase a tensile strain of the Ge absorption layer.

In some embodiments, the at least one top stressor layer may comprise: an amorphous Si layer disposed on the Ge absorption layer; a first SiO2layer disposed on the amorphous Si layer; a SiN layer disposed on the first SiO2layer; and a second SiO2layer disposed on the SiN layer.

In some embodiment, the charge layer may comprise p-type Si, p-type GeSi, or p-type SiGeC, the multiplication layer may comprise intrinsic Si or lightly doped n-type Si, and the contact layer may comprise n-type Si.

In some embodiments, the substrate may comprise a Si substrate or a SOI substrate.

In some embodiments, the APD may additionally comprise at least one anti-reflection layer disposed on a second surface of the substrate opposite to the first surface. The at least one anti-reflection layer may comprise a single SiO2layer or three layers comprising a SiN layer disposed between two SiO2layers.

In some embodiments, the Ge absorption layer may further comprise p-type dopants; a doping concentration of the p-type dopants is controlled such that a graded doping profile of the p-type dopants is formed inside the Ge absorption layer.

In some embodiments, the p-type dopants may comprise Ga or B.

In yet another aspect, an APD may comprise a substrate and a multi-layer structure disposed on the substrate. The multi-layer structure may comprise: at least one top stressor layer, the at least one top stressor layer is coupled to at least one metal contact of a first electrical polarity; a Ge-containing absorption layer doped by p-type dopants on which the at least one top stressor layer is disposed, a doping concentration of the p-type dopants is controlled such that a graded doping profile of the p-type dopants is formed within the Ge-containing absorption layer; a charge layer on which the Ge-containing absorption layer is disposed; a multiplication layer on which the charge layer is disposed; and a contact layer on which the multiplication layer is disposed, the contact layer is coupled to at least one metal contact of a second electrical polarity opposite to the first electrical polarity. The at least one top stressor layer is configured to increase a tensile strain of the Ge-containing absorption layer.

In some embodiments, the at least one top stressor layer may comprise: an amorphous Si layer disposed on the absorption layer; a first SiO2layer disposed on the amorphous Si layer; a SiN layer disposed on the first SiO2layer; and a second SiO2layer disposed on the SiN layer.

In some embodiments, the Ge-containing absorption layer may comprise Ge, GeSi, or SiGeC, and the p-type dopants may comprise Ga or B.

These and other features, aspects, and advantages of the present disclosure will be explained below with reference to the following figures. It is to be understood that both the foregoing general description and the following detailed description are by examples, and are intended to provide further explanation of the present disclosure as claimed.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Overview

The present disclosure provides avalanche photodiodes (APDs) having top stressor layers disposed on an absorption layer that can increase the tensile strains of the absorption layer. As a result, the optical absorption in wavelengths beyond the optical bandgap of the absorption layer is enhanced to achieve high device performance. Illustrative APDs of the present disclosure are schematically shown in cross-sectional views inFIGS. 1-4.FIGS. 1-4are not drawn to scale and are provided to convey the concept of the various embodiments of the present disclosure.

Exemplary Embodiments

FIG. 1Ais a cross-sectional view of an APD100in accordance with an embodiment of the present disclosure. Referring toFIG. 1, the APD100may comprise a substrate110and a multi-layer structure120disposed on the substrate110. The multi-layer structure120may comprise: a top stressor layer130electrically coupled to one or more one first-type metal contacts135of a first electrical polarity, an absorption layer140on which the top stressor layer130is disposed, a charge layer150on which the absorption layer140is disposed, a multiplication layer160on which the charge layer150is disposed, and a contact layer170on which the multiplication layer160is disposed. One or more second-type metal contacts175of a second electrical polarity are electrically coupled to the contact layer170. The second electrical polarity is opposite to the first electrical polarity. For example, the one or more first-type metal contacts135are p-type and the one or more second-type metal contacts175are n-type, or vice versa. The APD100may further comprise an oxide coating180that covers the multi-layer structure120.

The top stressor layer130increases the tensile strain of the absorption layer140, thus greatly enhancing optical absorption in wavelengths beyond the optical bandgap of the absorption layer140. The top stressor layer130also serves as an anti-reflection layer to improve the quantum efficiency of the APD100. The top stressor layer130can be a single-layer or multi-layer structure. In one embodiment, the top stressor layer130has a multi-layer structure comprising four layers, including an amorphous silicon (Si) layer1301disposed on the absorption layer140; a first silicon dioxide (SiO2) layer1302disposed on the amorphous Si layer1301; a silicon nitride (SiN) layer1303disposed on the first SiO2layer1302; and a second SiO2layer1304disposed on the SiN layer1303. The amorphous Si layer is electrically coupled to the one or more first-type metal contracts135.

In one embodiment, the absorption layer140includes germanium (Ge), germanium-silicon (GeSi), or silicon-germanium-carbon (SiGeC). In one embodiment, the charge layer150includes p-type Si, p-type GeSi, or p-type SiGeC. In one embodiment, the multiplication layer160includes intrinsic Si or lightly doped n-type Si. In one embodiment, the contact layer170includes n-type Si. In one embodiment, the substrate110includes a Si substrate or a silicon-on-insulator (SOI) substrate.

FIG. 1Billustrates an exemplary embodiment of the APD100. In the illustrated embodiment, the top stressor layer130is electrically coupled to two p-type metal contacts135, the absorption layer140is a Ge absorption layer, the charge layer150is a p-type Si layer, the multiplication layer160is a Si multiplication layer, and the contact layer170is an n-type Si layer. The contact layer170is electrically coupled to two n-type metal contacts175.

FIG. 2Ais a cross-sectional view of an APD200in accordance with an embodiment of the present disclosure. Referring toFIG. 2A, the APD200may comprise a substrate210, a multi-layer structure220disposed on a first surface of the substrate210, and an anti-reflection layer290disposed on a second surface of the substrate210opposite to the first surface. The multi-layer structure220may comprise: a top stressor layer230electrically coupled to one or more first-type metal contacts235of a first electrical polarity, an absorption layer240on which the top stressor layer230is disposed, a charge layer250on which the absorption layer240is disposed, a multiplication layer260on which the charge layer250is disposed, and a contact layer270on which the multiplication layer260is disposed. One or more second-type metal contacts275of a second electrical polarity are electrically coupled to the contact layer270. The second electrical polarity is opposite to the first electrical polarity. For example, the one or more first-type metal contacts235are p-type and the one or more second-type metal contacts275are n-type, or vice versa. The APD200may further comprise an oxide coating280that covers the multi-layer structure220.

The anti-reflection layer290can be a single-layer or multi-layer structure. In one embodiment, the anti-reflection layer290is a single SiO2layer. In another embodiment, the anti-reflection layer290has three layers, including a SiN layer disposed between two SiO2layers.

The top stressor layer230increases the tensile strain of the absorption layer240, thus greatly enhancing optical absorption in wavelengths beyond the optical bandgap of the absorption layer240. The top stressor layer230also serves as an anti-reflection layer to improve the quantum efficiency of the APD200. The top stressor layer230can be a single-layer or multi-layer structure. In one embodiment, the top stressor layer230has a multi-layer structure comprising four layers, including an amorphous Si layer2301disposed on the absorption layer240; a first SiO2layer2302disposed on the amorphous Si layer2301; a SiN layer2303disposed on the first SiO2layer2302; and a second SiO2layer2304disposed on the SiN layer2303. The amorphous Si layer is electrically coupled to the one or more first-type metal contracts235.

In one embodiment, the absorption layer240includes Ge, GeSi, or SiGeC. In one embodiment, the charge layer250includes p-type Si p-type GeSi, or p-type SiGeC. In one embodiment, the multiplication layer260includes intrinsic Si, or lightly doped n-type Si. In one embodiment, the contact layer270includes n-type Si. In one embodiment, the substrate210includes a Si substrate or an SOI substrate.

FIG. 2Billustrates an exemplary embodiment of the APD200. In the illustrated embodiment, the top stressor layer230is electrically coupled to two p-type metal contacts235, the absorption layer240is a Ge absorption layer, the charge layer250is a p-type Si layer, the multiplication layer260is a Si multiplication layer, and the contact layer270is an n-type Si layer. The contact layer270is electrically coupled to two n-type metal contacts275.

In comparison with the APD100, the APD200in accordance withFIGS. 2A-2Bfurther comprises the anti-reflection layer290. Incoming Light of optical signals may be illuminated from the side of the anti-reflection layer290to enter into the APD200. Thus, the anti-reflection layer290helps avoid optical loss at the incident surface295. Moreover, when operating under this bottom illumination condition, the optical absorption of the APD200can be further increased. Due to the presence of the highly reflective top stressor layer230, a major portion of the optical signals that has already passed through the absorption layer240will be reflected back into the absorption layer240, thus effectively increasing optical absorptions of the absorption layer240, especially for those wavelengths beyond the bandgap limits of the absorption layer240.

FIG. 3Ais a cross-sectional view of an APD300in accordance with an embodiment of the present disclosure. Referring toFIG. 3A, the APD300may comprise a substrate310and a multi-layer structure320disposed on the substrate310. The multi-layer structure320may comprise: a top stressor layer330electrically coupled to one or more first-type metal contacts335of a first electrical polarity, an absorption layer340doped with first-type dopants and on which the top stressor layer330is disposed, a charge layer350on which the absorption layer340is disposed, a multiplication layer360on which the charge layer350is disposed, and a contact layer370on which the multiplication layer360is disposed. One or more second-type metal contacts375of a second electrical polarity are electrically coupled to the contact layer370. The second electrical polarity is opposite to the first electrical polarity. For example, the one or more first-type metal contacts335are p-type and the one or more second-type metal contacts375are n-type, or vice versa. The APD300may further comprise an oxide coating380that covers the multi-layer structure320. The doping concentration of the first-type dopants in the absorption layer340is controlled such that a graded doping profile of the first-type dopants is formed within the absorption layer340. The graded doping profile of the first-type dopants is shown inFIG. 3A. For example, the first-type dopants are p-type dopants.

The top stressor layer330increases the tensile strain of the absorption layer340, thus greatly enhancing optical absorption in wavelengths beyond the optical bandgap of the absorption layer340. The top stressor layer330also serves as an anti-reflection layer to improve the quantum efficiency of the APD300. The top stressor layer330can be a single-layer or multi-layer structure. In one embodiment, the top stressor layer330has a multi-layer structure comprising four layers, including an amorphous Si layer3301disposed on the absorption layer340; a first SiO2layer3302disposed on the amorphous Si layer3301; a SiN layer3303disposed on the first SiO2layer3302; and a second SiO2layer3304disposed on the SiN layer3303. The amorphous Si layer is electrically coupled to the one or more first-type metal contracts335.

In one embodiment, the absorption layer340includes Ge, GeSi, or SiGeC. In one embodiment, the charge layer350includes p-type Si, p-type GeSi, or p-type SiGeC. In one embodiment, the multiplication layer360includes intrinsic Si, or lightly doped n-type Si. In one embodiment, the contact layer370includes n-type Si. In one embodiment, the substrate310includes a Si substrate or an SOI substrate. In one embodiment, the p-type dopants include gallium (Ga) or boron (B).

FIG. 3Billustrates an exemplary embodiment of the APD300. In the illustrated embodiment, the top stressor layer330is electrically coupled to two p-type metal contacts335, the absorption layer340is a p-type Ge absorption layer, the charge layer350is a p-type Si layer, the multiplication layer360is a Si multiplication layer, and the contact layer370is an n-type Si layer. The contact layer370is electrically coupled to two n-type metal contacts375.

In comparison with the APD100, the APD300in accordance withFIGS. 3A-3Bhas an undepleted absorption layer340with a graded doping profile for reducing the electrical field and dark current within the absorption layer340. The graded first-type doping of the absorption layer340can be achieved by in-situ doping or ion implantation. The graded doping profile of the first-type dopants formed in the absorption layer340can generate a built-in electrical field. This electrical field is mainly dependent on doping gradients and is independent on external applied bias. For example, as shown inFIG. 3B, with a proper design of the doping profile in the p-type Ge absorption layer340, the built-in electrical field can reach several kV/cm in the p-type Ge absorption layer340, thus ensuring carriers drift velocities close to saturation velocities. As a result, with extremely low dark current, GeSi APDs with an undepleted absorption layer can operate at a high speed condition like conventional GeSi APDs.

Moreover, considering the electrical field inside the p-type Ge absorption layer340, the built-in electrical field (˜several kV/cm) in the APD300is much weaker than that of the conventional GeSi APDs (˜100 kV/cm). Since dark currents in GeSi APDs are mainly depended on the electrical field inside the Ge absorption layer, GeSi APDs with an undepleted absorption layer can significantly reduce dark currents in GeSi APDs.

FIG. 4Ais a cross-sectional view of an APD400in accordance with an embodiment of the present disclosure. Referring toFIG. 4A, the APD400may comprise a substrate410and a multi-layer structure420disposed on the substrate410. The substrate410is a SOI substrate with a buried oxide (BOX) layer415. The multi-layer structure420may comprise: a top stressor layer430electrically coupled to one or more first-type metal contacts435of a first electrical polarity, an absorption layer440on which the top stressor layer430is disposed, a charge layer450on which the absorption layer440is disposed, a multiplication layer460on which the charge layer450is disposed, and a contact layer470on which the multiplication layer460is disposed. One or more second-type metal contacts475of a second electrical polarity are electrically coupled to the contact layer470. The second electrical polarity is opposite to the first electrical polarity. For example, the one or more first-type metal contacts435are p-type and the one or more second-type metal contacts475are n-type, or vice versa. The APD400may further comprise an oxide coating480that covers the multi-layer structure420.

The top stressor layer430increases the tensile strain of the absorption layer440, thus greatly enhancing optical absorption in wavelengths beyond the optical bandgap of the absorption layer440. The top stressor layer430also serves as an anti-reflection layer to improve the quantum efficiency of the APD400. The top stressor layer430can be a single-layer or multi-layer structure. In one embodiment, the top stressor layer430has a multi-layer structure comprising four layers, including an amorphous Si layer4301disposed on the absorption layer440; a first SiO2layer4302disposed on the amorphous Si layer4301; a SiN layer4303disposed on the first SiO2layer4302; and a second SiO2layer4304disposed on the SiN layer4303. The amorphous Si layer is electrically coupled to the one or more first-type metal contracts435.

The absorption layer440can be an intrinsic semiconductor layer or a semiconductor layer doped with first-type dopants. The doping concentration of the first-type dopants is controlled such that a graded doping profile of the first-type dopants is formed within the absorption layer440. For example, the first-type dopants are p-type dopants.

In one embodiment, the absorption layer440includes Ge, GeSi, or SiGeC. In one embodiment, the charge layer450includes p-type Si, p-type GeSi, or p-type SiGeC. In one embodiment, the multiplication layer460includes intrinsic Si, or lightly doped n-type Si. In one embodiment, the contact layer470includes n-type Si. In one embodiment, the substrate410includes a Si substrate or an SOI substrate. In one embodiment, the p-type dopants include gallium (Ga) or boron (B).

FIG. 4Billustrates an exemplary embodiment of the APD400. In the illustrated embodiment, the substrate410is a SOI substrate with BOX415, the top stressor layer430is electrically coupled to two p-type metal contacts435, the absorption layer440is a Ge absorption layer, the charge layer450is a p-type Si layer, the multiplication layer460is a Si multiplication layer, and the contact layer470is an n-type Si layer. The contact layer470is electrically coupled to two n-type metal contacts475.

The APD400in accordance withFIGS. 4A-4Bcan operate under lateral incident illumination condition like a waveguide device. The light beam is incident laterally at the junction of the absorption layer440and the charge layer450of the APD400. Normally, the dark currents in APDs are proportional to the size of the area of the absorption layer. Since a waveguide device typically has a much smaller size than a normal incident device, the design of the present disclosure can reduce dark currents in GeSi APDs. In addition, a waveguide device according to the present disclosure also has a broader absorption coverage resulted from its lateral incident illumination and a better bandwidth resulted from its smaller capacitance. As a result, the device performance can be greatly enhanced.

Exemplary Test Results

Raman spectra and absorption spectra of a bulk Ge layer and a Ge layer having top stressor layers in accordance with the present disclosure were measured to study the effects of the top stressor layer on the optical properties of Ge. In this study, the top stressor layer has a four-layer structure, including an amorphous Si layer disposed on the Ge absorption layer; a first SiO2layer disposed on the amorphous Si layer; a SiN layer disposed on the first SiO2layer; and a second SiO2layer disposed on the SiN layer.

FIG. 5shows graphs comparing Raman spectra of a bulk Ge layer and a Ge layer having top stressor layers in accordance with the present disclosure. The Ge Raman spectra peaks are at 300.4 cm−1and 299.3 cm−1for the bulk Ge layer and the Ge layer having top stressor layers, respectively. The difference in Raman spectra peaks indicates that the top stressor layers can increase tensile strain inside the Ge layer.

FIG. 6shows graphs comparing absorption spectra of a bulk Ge layer and a Ge layer having top stressor layers in accordance with the present disclosure. The Ge layer having top stressor layers has much higher absorption coefficient between 1500 nm to 1600 nm than those of the bulk Ge layer. The absorption spectra clearly show that the bulk Ge layer cannot efficiently absorb the light with wavelengths beyond 1550 nm, while the Ge layer with top stressor layers not only extends the absorption edge to ˜1600 nm but also greatly increase the absorption coefficient at 1550 nm.

Although some embodiments are disclosed above, they are not intended to limit the scope of the present disclosure. It will be apparent to those skilled in the art that various modifications and variations can be made to the disclosed embodiments of the present disclosure without departing from the scope or spirit of the present disclosure. In view of the foregoing, the scope of the present disclosure shall be defined by the following claims and their equivalents.