Ultra low dark current pin photodetector

A photodetector and a method for fabricating a photodetector. The photodetector may include a substrate, a buffer layer formed on the substrate, and an absorption layer formed on the buffer layer for receiving incident photons and generating charged carriers. An N-doped interface layer may be formed on the absorption layer, an N-doped cap layer may be formed on the N-doped interface layer, and a dielectric passivation layer may be formed above the cap layer. A P+ diffusion region may be formed within the cap layer, the N-doped interface layer and at least a portion of the absorption layer, and at least one contact may be formed on and coupled to the P+ diffusion region.

BACKGROUND INFORMATION

The disclosure relates generally to a photodetector and method and, more particularly, to a photodetector having ultra low dark current and a method for fabricating a photodetector having ultra low dark current.

Dark current is a current that flows in a photodetector when it is not receiving any light, and is a significant source of noise in PIN photodetectors having high sensitivity. Current efforts to reduce dark current in PIN photodetectors focus primarily on reducing diffusion currents in the photodetector (the process of carriers distributing themselves from regions of high concentration to regions of low concentration) which is a significant contributor to dark current.

One known approach to reducing diffusion current is to uniformly dope the “intrinsic absorption” layer of the photodetector (see, for example, U.S. Pat. No. 6,573,581). Although this approach does reduce diffusion current, it fails to address generation current due to defects in the photodetector material or at a surface, which is also a major contributor to and actually dominates the total dark current in small pixels used for imaging arrays at or below room temperature.

There is, accordingly, a need for a photodetector that has very low dark current and to a method for fabricating a photodetector having a very low dark current.

SUMMARY

An embodiment of the disclosure provides a photodetector. The photodetector may include a substrate, a buffer layer formed on the substrate, and an absorption layer formed on the buffer layer for receiving incident photons and generating charged carriers. An N-doped interface layer may be formed on the absorption layer, a cap layer may be formed on the N-doped interface layer, and a dielectric passivation layer may be formed above the cap layer. A P+diffusion region may be formed within the cap layer, the N-doped interface layer and at least a portion of the absorption layer, and at least one contact may be formed on and coupled to the P+diffusion region.

A further embodiment of the disclosure provides a method for fabricating a photodetector. The method may include forming a substrate, forming a buffer layer on the substrate, and forming an absorption layer on the buffer layer for converting incident photons to electrons and holes. An N-doped interface layer may be formed on the absorption layer, a cap layer may be formed on the N-doped interface layer, and a dielectric passivation layer may be formed above the cap layer. A P+diffusion region may be formed within the cap layer, the N-doped interface layer and at least a portion of the absorption layer, and at least one contact may be formed on and coupled to the P+diffusion region.

A further embodiment of the disclosure provides a PIN photodetector. The PIN photodetector may include an N+InP substrate, an N+InP buffer layer formed on the substrate, and an intrinsic InGaAs absorption layer formed on the buffer layer for receiving incident photons and generating charged carriers. An N-doped interface layer may be formed on the absorption layer and a cap layer may be formed on the N-doped interface layer, wherein the N-doped interface layer reduces depletion width at an interface between the intrinsic InGaAs absorption layer and the cap layer. A dielectric passivation layer may be formed above the cap layer, and a P+Zn diffusion region may be formed within the cap layer, the N-doped interface layer and at least a portion of the absorption layer. At least one metal contact may be formed on and coupled to the P+Zn diffusion region.

The features, functions, and advantages can be achieved independently in various embodiments or may be combined in yet other embodiments.

DETAILED DESCRIPTION

With reference now to the figures, and, in particular, with reference toFIG. 1, an illustration of a photodetector that is known in the art is depicted to assist in explaining advantageous embodiments of the disclosure. More particularly,FIG. 1illustrates a PIN photodetector100that was designed to reduce dark current in the photodetector by reducing diffusion current, which is known to be a significant source of dark current in PIN photodetectors. PIN photodetector100generally includes N+InP substrate102, N+InP buffer layer104formed on substrate102, intrinsic InGaAs absorption layer106formed on buffer layer104, InP cap layer108formed on absorption layer106, P+Zn diffusion region110formed in cap layer108and extending into absorption layer106, and metal contacts112formed on and coupled to diffusion region110. A dielectric passivation layer114is formed above the cap layer108.

PIN photodetector100is fabricated in the following manner. Substrate102is an N+InP substrate typically doped with sulfur between 1×1018·5×1018/cm3, and buffer layer104is an N+InP layer of about the same doping concentration as substrate102. Buffer layer104is grown on substrate102to a thickness of about 0.5-1.0 μm. Either sulfur or silicon may be used as the dopant species for the substrate and buffer layer. A Metal Organic Vapor Phase Epitaxy (MOVPE) reactor or Molecular Beam Epitaxy (MBE) reactor may be used to grow the buffer layer and all subsequent semiconductor layers of PIN photodetector100.

Intrinsic InGaAs absorption layer106is then grown on top of buffer layer104. Absorption layer106comprises an unintentionally-doped In0.53Ga0.47As semiconductor layer grown to a thickness of about 3.5 μm on top of buffer layer104. Intrinsic InGaAs absorption layer106functions to convert photons to electrons and holes through the photoelectric effect. The InGaAs absorption layer alloy composition is typically chosen to provide a layer that is lattice-matched to InP.

InP cap layer108is then grown over intrinsic InGaAs absorption layer106. Cap layer108is grown to a thickness of about 0.5-1.0 μm, and can be unintentionally doped or doped to about 1×1016/cm3using either sulfur or silicon as the dopant species. Cap layer108is used to provide a wide bandgap semiconductor over narrow bandgap absorption layer106in order to reduce surface generation current.

Although not illustrated inFIG. 1, an optional contact layer of heavily doped In0.53Ga0.47As may then be grown over InP cap layer108if desired, to provide a low electrical resistance to a subsequently applied metal layer.

After growth of the epitaxial layers as described above, P+Zn diffusion region110is formed. In particular, a thin (approximately 1000 Å-2000 Å) layer of silicon nitride is deposited on cap layer108using plasma enhanced chemical vapor deposition. The layer is then patterned using photolithographic techniques in conjunction with either reactive ion etching or wet chemical etching to form openings in the silicon nitride.

The photodetector structure is then put back into the MOVPE or MBE reactor and exposed to di-methyl zinc which provides a diffusion source of the P-type zinc dopant to the semiconductor surface in those areas where the silicon nitride has been etched away. The exposure time and temperature are controlled to insure that the zinc diffuses through InP cap layer108and extends a small distance (for example, about 1000 Å-1500 Å) into the underlying absorption layer106.

After formation of Zn diffusion region110, metal contacts112are formed on region110using photolithographic techniques. Light can be incident from either the metal (contact) side or the substrate side of the photodetector, and an anti-reflection coating may be applied to either side to enhance optical sensitivity, depending on the particular application in which the photodetector is to be used.

Although the design of PIN photodetector100is effective in reducing dark current in the photodetector, it has been found that intrinsic InGaAs absorption layer106and InP cap layer108provide a wide depletion width at interface120between the layers, and this wide depletion width has been found to be a significant source of dark current that reduces the effectiveness of the overall photodetector design.

FIG. 2is an illustration of a photodetector in accordance with an advantageous embodiment of the disclosure. More particularly,FIG. 2illustrates a PIN photodetector200that provides a significantly reduced depletion width at the interface220between intrinsic InGaAs absorption layer206and cap layer208of PIN photodetector200so as to significantly reduce dark current in PIN photodetector200relative to PIN photodetector100illustrated inFIG. 1.

PIN photodetector200may be fabricated in the following manner. Similar to PIN photodetector100illustrated inFIG. 1, PIN photodetector200may also include N+ InP substrate202, and N+InP buffer layer204which may be formed of the same materials and in the same manner as substrate102and buffer layer104in PIN photodetector100. Intrinsic InGaAs absorption layer206may also be formed in the same manner as intrinsic InGaAs absorption layer106in photodetector100. Specifically, absorption layer206may be an unintentionally-doped In0.53Ga0.47As semiconductor layer grown on top of buffer layer204to a thickness of about 3.5 μm.

PIN photodetector200differs from PIN photodetector100in that following forming of intrinsic InGaAs absorption layer206, the topmost region of absorption layer206may then be doped to between about 1×1017/cm3-5×1017/cm3N-type using either sulfur or silicon as the dopant species. This topmost region, which may be less than about 1000 Å thick, defines an interface layer or region230between absorption layer206and cap layer208as illustrated inFIG. 2. Interface layer230functions to decrease the depletion width and generation current volume at the interface220between absorption layer206and cap layer208, and results in a significant reduction in dark current in PIN photodetector200.

After interface layer230is formed, cap layer208may then be grown over the interface layer. Cap layer208may be an InP layer grown to a thickness of about 0.5-1.0 μm, and may be doped to between about 1×1017-5×1017/cm3using either sulfur or silicon as the dopant species. Cap layer208is used to provide a wide bandgap semiconductor over narrow bandgap absorption layer206in order to reduce surface generation current. Although not illustrated inFIG. 2, an optional contact layer of heavily doped In0.53Ga0.47As may then be grown over cap layer208if desired, to provide a low electrical resistance to a subsequently applied metal layer. A dielectric passivation layer214, similar to dielectric passivation layer114in PIN photodetector100illustrated inFIG. 1, may then be deposited over cap layer208or over the contact layer if the contact layer is provided.

P+Zn diffusion region210may then be formed within the cap layer208, the N-doped interface layer230and a portion of the absorption layer206(for example, to a depth of about 1000 Å-1500 Å into absorption layer206). Diffusion region210may be similar to and grown in a similar manner as described with respect to diffusion region110in photodetector100illustrated inFIG. 1. Metal contacts212may be formed on and coupled to P+ Zn diffusion region210and may be formed in the same manner as described above with respect to metal contacts112in photodetector100illustrated inFIG. 1.

FIG. 3is a graph that illustrates dark current density versus absolute bias for the photodetectors illustrated inFIGS. 1 and 2. In particular, trace310illustrates dark current density (A/cm2) versus absolute bias (V) for a PIN photodetector such as PIN photodetector100shown inFIG. 1, and trace320illustrates dark current density (A/cm2) versus absolute bias (V) for a PIN photodetector such as PIN photodetector200shown inFIG. 2having interface layer230formed between absorption layer206and cap layer208. Both measurements were made at room temperature. As is clearly shown inFIG. 3, dark current is significantly less in photodetector200as compared to photodetector100.

In general, interfaces between various semiconductor material layers are typically the greatest source of generation current due to localized defects that either induce generation currents or shunt leakage paths to ground points. By choosing appropriate doping levels and thicknesses, it has been found that these sources of dark current can be suppressed without degrading other photodetector figures of merit such as responsivity or capacitance both of which must be maintained if the signal-to-noise ratio is to be improved.

A PIN photodetector designed as illustrated inFIG. 2has demonstrated a reduction in dark current density by a factor of about nine or a reduction in overall noise by a factor of about three (as compared to a photodetector such as photodetector100inFIG. 1) with no decrease in responsivity and with no significant increase in capacitance.

FIG. 4is an illustration of a photodetector in accordance with a further advantageous embodiment of the disclosure. More particularly,FIG. 4illustrates a PIN photodetector400that is generally similar to PIN photodetector200illustrated inFIG. 2, and uses similar reference numbers to identify similar components. PIN photodetector400differs from PIN photodetector200in that cap layer408in PIN photodetector400comprises a bandgap-graded cap layer. More particularly, after forming interface layer230as described with reference toFIG. 2, an InGaAsP cap layer of about 0.5-1.0 μm may then be grown over the interface layer. The InGaAsP cap layer alloy composition is typically chosen to provide a layer that is lattice matched to InP. This cap layer is doped to about 1×1017/cm3-5×1017/cm3using either sulfur or silicon as the dopant species.

In this advantageous embodiment illustrated inFIG. 4, the bandgap-graded cap layer408provides a wide bandgap semiconductor over the narrow bandgap In0.53Ga0.47As absorption layer in order to reduce surface generation current. By tailoring the composition of the quaternary InGaAsP cap layer, phosphorus replacement of arsenic atoms at the interface with the absorption layer can be minimized. Replacement of arsenic with phosphorus is the principal source of defects at interface120in PIN photodetector100illustrated inFIG. 1.

FIG. 5is an illustration of a photodetector in accordance with a further advantageous embodiment of the disclosure. More particularly,FIG. 5illustrates a PIN photodetector500that is generally similar to PIN photodetector200illustrated inFIG. 2, and uses similar reference numbers to identify similar components. PIN photodetector500differs from PIN photodetector200in that it has a thinner cap layer508, i.e. about 0.01-0.05 μm as compared to a thickness of about 0.5-1.0 μm of cap layer208in PIN photodetector200.

More particularly, cap layer508may comprise an InP or InGaAsP cap layer grown over interface layer230to a thickness of about 0.01-0.05 μm. This layer may be doped to about 1×1017/cm3-5×1017/cm3using either sulfur or silicon as the dopant species. In this advantageous embodiment the thin cap layer provides a wide bandgap semiconductor over the narrow bandgap absorption layer to reduce surface generation current. By reducing the time at elevated temperature, phosphorus replacement of arsenic atoms at the interface with the In0.53Ga0.47As absorption layer can be minimized.

FIG. 6is an illustration of a photodetector in accordance with a further advantageous embodiment of the disclosure. More particularly,FIG. 6illustrates a PIN photodetector600that is generally similar to PIN photodetector200illustrated inFIG. 2, and uses similar reference numbers to identify similar components. PIN photodetector600differs from PIN photodetector200in that it includes a wide bandgap cap layer608.

In particular, after forming interface layer230as described with reference toFIG. 2, an InAlAs cap layer of about 0.5-1.0 μm thick may then be grown over the interface layer. This cap layer may be doped to about 1×1017/cm3-5×1017/cm3using either sulfur or silicon as the dopant species. In this advantageous embodiment, the InAlAs cap layer is used to provide a wide bandgap semiconductor over the narrow bandgap absorption layer to reduce surface generation current. The InAlAs cap layer alloy composition is typically chosen to provide a layer that is lattice matched to InP.

The wider bandgap InAlAs cap layer may provide at least two important advantages over an InP cap layer. First, there is no issue of intermixing of phosphorous at the interface between the absorption layer and cap layer. Also, it provides a wider bandgap semiconductor (1.49 eV versus 1.35 eV for InP). As a result, it has better passivation properties. A quaternary layer InGaAlAs can also be used for cap layer608. The InGaAlAs cap layer alloy composition is typically chosen to provide a layer that is lattice matched to InP. In general, the choice of the alloy system to be used for cap layer608(InAlAs) is intended to be exemplary only as other wide bandgap alloys may also be used, and it is not intended to limit advantageous embodiments to any particular alloy.

FIG. 7is an illustration of a photodetector in accordance with a further advantageous embodiment of the disclosure. More particularly,FIG. 7illustrates a PIN photodetector700that is generally similar to PIN photodetector200illustrated inFIG. 2, and uses similar reference numbers to identify similar components. PIN photodetector700differs from PIN photodetector200in that a counterdoped interface layer740is formed between N+InP buffer layer204and intrinsic InGaAs absorption layer706of PIN photodetector700. Counterdoped interface layer740enables an internal electric field profile to be created that minimizes dark current from the interface.

More particularly, after forming N+InP buffer layer204as described with reference toFIG. 2, an unintentionally-doped In0.53Ga0.47As semiconductor layer of about 3.5 μm thickness may then be grown on top of the InP buffer layer to form intrinsic absorption layer706. The bottommost region of absorption layer706may be doped, typically with sulfur, which diffuses out of the InP substrate during epitaxy growth of subsequent layers to form counterdoped interface layer740. The sulfur profile results in a built-in electric field that reduces diffusion dark current.

In this advantageous embodiment, the sulfur profile is intentionally counter-doped with a P-type dopant, preferably carbon, to tailor the resultant carrier profile to further reduce dark current.

After forming absorption layer706, interface layer230is formed as described with reference toFIG. 2, and a cap layer708is then formed on the interface layer230. Cap layer708may be formed of InP, InGaAsP or InAlAs or other wide bandgap semiconductor material as described previously.

FIG. 8is a flowchart that illustrates a method for fabricating a photodetector in accordance with an advantageous embodiment of the disclosure. The method is generally designated by reference number800, and begins by forming a substrate (Step810) and forming a buffer layer on the substrate (Step820). Steps810and820may be implemented by forming an N+InP substrate, typically doped with sulfur between 0.5×1018-5×1018/cm3, and by forming a buffer layer that also comprises an N+InP layer and that is of about the same doping concentration as the substrate. The buffer layer may be grown to a thickness of about 0.5-1.0 μm. Either sulfur or silicon may be used as the dopant species. A Metal-Organic Vapor Phase Epitaxy (MOVPE) reactor or a Molecular Beam Epitaxy (MBE) reactor may be used to grow the buffer layer and all subsequent semiconductor layers.

An absorber layer may then be formed on the buffer layer (Step830). The absorber layer may be an intrinsic InGaAs absorbent layer designed and grown as described with reference to any of FIGS.2and4-7.

An interface layer may then be formed in the absorbent layer (Step840). The interface layer may, for example, be formed as interface layer230as described with reference to FIGS.2and4-7.

A cap layer may then be formed on the interface layer (Step850). The cap layer may, for example, be formed as one of cap layers208,408508,608and708described with reference to FIGS.2and4-7.

A contact layer may then be optionally formed on the interface layer, if desired (Step860) as described, for example, with reference toFIG. 2.

A dielectric passivation layer may then be formed above the cap layer, i.e., either on the cap layer or on the contact layer if a contact layer is provided, (Step870). The dielectric passivation layer may, for example, be formed as dielectric passivation layer214in FIGS.2and4-7.

A diffusion region may then be formed in the cap layer, the interface layer and in at least a portion of the absorption layer (Step880), and contacts may be formed on and coupled to the diffusion layer (Step890). The diffusion region and contacts may, for example, be formed as diffusion region210and contacts212in FIGS.2and4-7.

PIN photodetectors according to advantageous embodiments of the disclosure can be effectively used in numerous applications. For example, advantageous embodiments can provide very sensitive photodetector arrays for night vision cameras that are capable of operating at room temperature without thermo-electric cooling. The photodetectors can be of great utility when used by soldiers or other individuals having limited stored electrical power. PIN photodetectors according to advantageous embodiments can also be used for astronomical imaging where very high sensitivity photodetectors are desirable.

The description of advantageous embodiments has been presented for purposes of illustration and description, and is not intended to be exhaustive or limited to the form disclosed. Many modifications and variations will be apparent to those of ordinary skill in the art. Further, different advantageous embodiments may provide different advantages as compared to other advantageous embodiments. The embodiments selected were chosen and described in order to best explain features and practical applications, and to enable others of ordinary skill in the art to understand various embodiments with various modifications as are suited to particular uses contemplated.