Light-receiving device having avalanche photodiodes of different types

A light-receiving device includes a silicon semiconductor substrate, a plurality of first serial connections each of which includes a first avalanche photodiode (APD) and a first resistor connected in series, and a plurality of second serial connections each of which includes a second avalanche photodiode (APD) and a second resistor connected in series. The first APDs and the first resistors are formed on the silicon semiconductor substrate, and the first APDs is formed of silicon. The second APDs and the second resistors are formed on the silicon semiconductor substrate, and the second APDs is formed of a material having a smaller band gap than silicon. The plurality of first and second serial connections is connected in parallel between an anode terminal and a cathode terminal.

CROSS-REFERENCE TO RELATED APPLICATION

This application is based upon and claims the benefit of priority from Japanese Patent Application No. 2016-165666, filed on Aug. 26, 2016, the entire contents of which are incorporated herein by reference.

FIELD

Embodiments described herein relate generally to a light-receiving device including avalanche photodiodes.

BACKGROUND

A light-receiving device of one type includes a plurality of serial connections, each including a quenching resistor and an avalanche photodiode (APD) connected in parallel. The light-receiving device measures the number of incident photons using the APDs. The light-receiving device is typically referred to as “SiPM (Silicon Photomultiplier)” since the APDs are formed of silicon (Si).

It is expected to employ the light-receiving device having the APDs in an automatic operation (driving) system so that light reflected by an object can be measured for measuring a distance to the object. The APD formed of silicon, however, has low sensitivity to long-wavelength light. On the other hand, the cost of the light-receiving device would increase if the light-receiving device is formed of a semiconductor material so as to detect the long-wavelength light. It is desirable to provide a light-receiving device capable of detecting long-wavelength light with high sensitivity and lower cost.

DETAILED DESCRIPTION

An embodiment provides a light-receiving device capable of detecting long-wavelength light with high sensitivity and lower cost.

In general, according to an embodiment, a light-receiving device includes a silicon semiconductor substrate, a plurality of first serial connections each of which includes a first avalanche photodiode (APD) and a first resistor connected in series, and a plurality of second serial connections each of which includes a second avalanche photodiode (APD) and a second resistor connected in series. The first APDs and the first resistors are formed on the silicon semiconductor substrate, and the first APDs is formed of silicon. The second APDs and the second resistors are formed on the silicon semiconductor substrate, and the second APDs is formed of a material having a smaller band gap than silicon. The plurality of first and second serial connections is connected in parallel between an anode terminal and a cathode terminal.

Light-receiving devices according to embodiments will be described hereinafter in detail with reference to the accompanying drawings. Note that the embodiments given below are not intended to limit the present disclosure.

First Embodiment

FIG. 1is a top plan view of a part of a light-receiving device according to a first embodiment. The light-receiving device according to the first embodiment includes photoelectric transducers10to13that are selectively formed on a principal surface of a silicon semiconductor substrate (not shown). Upon detecting photons, each of the photoelectric transducers10to13outputs a detection signal amplified thereby. The photoelectric transducers10to13are selectively formed on an XY planar surface of the silicon semiconductor substrate in a matrix layout. The photoelectric transducers12and13are formed of Si. In contrast, the photoelectric transducers10and11are formed of a semiconductor material, for example, germanium (Ge) smaller in band gap than Si. An anode region and a cathode region (not shown) forming a PN junction of the photoelectric transducer are formed of either Si or Ge.

InFIG. 1, the photoelectric transducers formed of either Si or Ge are arranged in a staggered pattern. In other words, the Ge photoelectric transducers10and11and the Si photoelectric transducers12and13are alternately arranged in X-axis and Y-axis directions.

Each of the photoelectric transducers10to13is an APD that operates in a Geiger mode. In the Geiger mode, an inverse bias voltage higher than a breakdown voltage is applied between an anode and a cathode of the APD.

Quenching resistors20to23are connected to the photoelectric transducers10to13, respectively. In other words, one end of the quenching resistor20is electrically connected to the anode of the photoelectric transducer10through an interconnect30while the other end thereof is connected to an anode interconnect40. Each of the quenching resistors20to23operates to terminate the amplification action of the APD by voltage drop if the photons are incident on the APD and an electron avalanche occurs. Resistance values of the quenching resistors20to23may be same or different and are set, for example, to approximately several hundred kΩ. The quenching resistors20to23are formed of, for example, polycrystalline silicon.

FIG. 2schematically illustrates a cross-sectional structure of the light-receiving device according to the first embodiment taken along I-I inFIG. 1. A silicon semiconductor substrate1has a principal surface (one surface) on which the photoelectric transducers10-13are formed and has a back surface (the other surface) on which a cathode interconnect50is formed. The Ge photoelectric transducer10includes an N—Ge epitaxial layer210that is selectively formed on the principal surface of the silicon semiconductor substrate1. The Ge epitaxial layer210includes a P—Ge region211. The Ge epitaxial layer210and the Ge region211forma PN junction of the photoelectric transducer10.

The Si photoelectric transducer12includes an N—Si epitaxial layer212that is selectively formed on the principal surface of the silicon semiconductor substrate1. The Si epitaxial layer212includes a P—Si region213. The Si epitaxial layer212and the Si region213form a PN junction of the photoelectric transducer12. A silicon oxide film60defines a region where the Ge epitaxial layer210and the Si epitaxial layer212are formed. A silicon oxide film61is formed, as a protective film, on a surface of each of the photoelectric transducers10and12.

The Ge region211is electrically connected to one end of the quenching resistor20through the interconnect30. One end of the interconnect30is connected to the Ge region211by a connection section31formed in the silicon oxide film61while the other end thereof is connected to one end of the quenching resistor20by a connection section32. The connection sections31and32are formed from, for example, a through-hole into which a metal material is filled. The interconnect30is illustrated such that the interconnect30contains the connection sections31and32. The other end of the quenching resistor20is electrically connected to the anode interconnect40by a connection section33.

The Si region213is electrically connected to one end of the quenching resistor22through an interconnect37. One end of the interconnect37is connected to the Si region213by a connection section35formed in the silicon oxide film61while the other end thereof is connected to one end of the quenching resistor22by a connection section36. The interconnect37is illustrated such that the interconnect37contains the connection sections35and36. The other end of the quenching resistor22is electrically connected to the anode interconnect40by a connection section34. While the quenching resistors20and22are formed in peripheral edge portions of the Ge epitaxial layer210and the Si epitaxial layer212, respectively, the quenching resistors20and22are illustrated to be provided above the epitaxial layers210and212inFIG. 2for the sake of convenience.

FIG. 3is an equivalent circuit diagram of the light-receiving device according to the first embodiment. The light-receiving device is configured such that series-connected pairs of the quenching resistors and the photoelectric transducers are connected in parallel between the anode interconnect40and the cathode interconnect50. Each photoelectric transducer outputs a detection signal when detecting photons. If any of the photoelectric transducers10to13connected in parallel detects photons, the detection signal obtained by the amplification action of the photoelectric transducer can be extracted from either an anode electrode41or a cathode electrode51as an addition-result detection signal.

FIG. 4depicts characteristics of the light-receiving device according to the first embodiment. InFIG. 4, a horizontal axis denotes a wavelength of incident light and a vertical axis denotes a sensitivity R of the light-receiving device. A curve71indicates sensitivity characteristics of the Si photoelectric transducer and a curve72indicates sensitivity characteristics of the Ge photoelectric transducer. Overall sensitivity characteristics of the light-receiving device according to the present embodiment are indicated by a curve70obtained by integrating the curve71with the curve72since the Ge photoelectric transducers and the Si photoelectric transducers are connected in series.

In the present embodiment, the Ge photoelectric transducers and the Si photoelectric transducers are connected in series between the anode electrode41and the cathode electrode51. For that reason, the light-receiving device according to the present embodiment is capable of carrying out light detection in broad wavelength ranges in which the Si photoelectric transducers and the Ge photoelectric transducers detect light. The detection using a long-wavelength light source enables the light-receiving device to carry out long-distance measurement. Furthermore, since the normally used silicon semiconductor substrate is used, it is possible to prevent cost increase. Moreover, as the Si photoelectric transducers and the Ge photoelectric transducers are arranged in a matrix layout and alternatively, it is possible to arrange the photoelectric transducers having different characteristics uniformly. It is, therefore, possible to provide uniform sensitivity characteristics.

Second Embodiment

FIG. 5is a plan view of a part of a light-receiving device according to a second embodiment. The same signs are attached to elements corresponding to those in the first embodiment, and repetitive description will be made only as needed. In the present embodiment, the Ge photoelectric transducers10and11and the Si photoelectric transducers12and13are alternately arranged in X direction. The photoelectric transducers formed of the same semiconductor material Si or Ge are arranged in Y direction. The light-receiving device also includes a first anode interconnect42and a second anode interconnect44that connect the photoelectric transducers formed of the same semiconductor material in Y direction. The anodes of the Ge photoelectric transducers10and11are electrically connected to the first anode interconnect42through the quenching resistors20and21, respectively. The anodes of the Si photoelectric transducers12and13are electrically connected to the second anode interconnect44through the quenching resistors22and23, respectively.

FIG. 6is an equivalent circuit diagram of the light-receiving device according to the second embodiment. The cathodes of the photoelectric transducers10to13are connected to the cathode electrode51through the cathode interconnect50. The anodes of the Ge photoelectric transducers10and11are connected to a first anode interconnect42through the quenching resistors20and21, respectively. The first anode interconnect42is electrically connected to a first anode electrode43. The anodes of the Si photoelectric transducers12and13are connected to a second anode interconnect44through the quenching resistors22and23, respectively. The second anode interconnect44is electrically connected to a second anode electrode45.

FIG. 7depicts characteristics of the light-receiving device according to the second embodiment. A curve74denotes sensitivity characteristics of the Ge photoelectric transducers10and11. The curve71denotes sensitivity characteristics of the Si photoelectric transducers12and13. The sensitivity characteristics of the light-receiving device can be changed by selecting the first anode electrode43or the second anode electrode45as appropriate. For example, if the first anode electrode43is selected, the light-receiving device can be activated as a light-receiving device that exhibits the sensitivity characteristics denoted by the curve74.

In the present embodiment, the photoelectric transducers can be activated selectively by switching the anode electrodes, so that it is possible to switch the sensitivity characteristics of the light-receiving device. In other words, it is possible to select the photoelectric transducers that exhibit desired sensitivity characteristics depending on a wavelength of a light source to be used. Furthermore, it is possible to achieve the saving of power consumption by not activating all photoelectric transducers but adjusting the number of photoelectric transducers to be activated depending on the light source to be used.

FIG. 8is a block diagram of an optical detection system including the light-receiving device according to the second embodiment. The optical detection system includes a central processing unit (CPU)100that controls optical detection operation, a timing adjustment circuit101, an emission pulse control circuit102, an LD drive circuit103, a switch104, light emission sources120,121, a measurement circuit107, and a light-receiving unit200. The CPU100controls the timing adjustment circuit101. The timing adjustment circuit101controls the emission pulse control circuit102. The LD drive circuit103outputs a drive signal in response to an output signal from the emission pulse control circuit102.

An output signal from the LD drive circuit103is selectively supplied to the first light emission source120or the second light emission source121through the switch104. A connection terminal of the switch104is switched between switching terminals110and111in response to a switching signal from the CPU100, thereby selecting either the first light emission source120or the second light emission source121. The first light emission source120is, for example, an infrared laser diode. The second light emission source121is, for example, a red laser diode. By selecting either the first light emission source120or the second light emission source121, it is possible to emit light at different wavelengths.

The light is emitted from the first light emission source120or the second light emission source121, and reflected light reflected by an object to be measured is detected by the light-receiving unit200. If the light-receiving unit200detects photons, the photons are amplified by each of the photoelectric transducers10to13operating in the Geiger mode and output as the detection signal. For example, the light-receiving unit can detect a voltage drop generated in a resistor (not shown) connected to the cathode electrode51or an anode electrode46, and output the detection signal. The detection signal is supplied to the measurement circuit107.

In the present system, the light-receiving device according to the second embodiment is used as the light-receiving unit200. A high-potential voltage Vp is applied to the cathode electrode51. Switches105and106are provided between the first anode electrode43and the anode electrode46and between the second anode electrode45and the anode electrode46, respectively. The first anode electrode43or the second anode electrode45is selected by switching between the switches105and106and the selected anode electrode is connected to the anode electrode46. In other words, by selecting either the first anode electrode43or the second anode electrode45, it is possible to selectively activate the Ge photoelectric transducers10and11and the Si photoelectric transducers12and13. The first anode electrode43or the second anode electrode45is selected under control of the switching signal from the CPU100.

The present system switches between the first light emission source120and the second light emission source121depending on, for example, a distance to be measured, and selects the photoelectric transducers to be activated in the light-receiving unit200in response to the switchover. In other words, the sensitivity characteristics of the light-receiving unit200changes depending on the wavelength of the light emission source to be used, so that it is possible to ensure the measurement with high sensitivity. Furthermore, by selectively activating the photoelectric transducers in the light-receiving unit200, it is possible to achieve the saving of power consumption. Alternatively, the system can be configured such that both the first anode electrode43and the second anode electrode45are connected to the anode electrode46by the switches105and106, and both of the Ge photoelectric transducers10and11and the Si photoelectric transducers12and13are activated.

FIGS. 9A through 9Fillustrate a method of manufacturing the light-receiving device according to the embodiments. On the principal surface of the silicon semiconductor substrate1, the silicon oxide film60is formed by, for example, Chemical Vapor Deposition (CVD) (FIG. 9A). On the principal surface of the silicon semiconductor substrate1, an isolation region (not shown) mutually isolating the photoelectric transducers may be formed by, for example, Local Oxidation of Silicon (LOCOS) before the silicon oxide film60is formed. An opening300is selectively formed in the silicon oxide film60(FIG. 9B). The opening300is formed in regions where the photoelectric transducers are to be formed.

The N—Ge epitaxial layer210is selectively formed on the principal surface of the silicon semiconductor substrate1exposed in the opening300by, for example, the CVD method (FIG. 9C). For example, it is possible to use hydrogen as carrier gas, germane (GeH4), and arsine (AsH3) or phosphine (PH3) as N-doping gas.

After a silicon oxide film301is formed on a surface of the selectively-formed N—Ge epitaxial layer210, an opening302is formed in the silicon oxide film60and a silicon oxide film301(FIG. 9D). The opening302is formed in regions where the Si photoelectric transducers are to be formed.

The N—Si epitaxial layer212is selectively formed on the principal surface of the silicon semiconductor substrate1exposed by the opening302by, for example, the CVD method (FIG. 9E). It is possible to use hydrogen (H2) as carrier gas, dichlorosilane (SiH2Cl2) and hydrogen chloride (HCl), for example, as gas species, and arsine (AsH3) as N-doping gas.

A silicon oxide film303is formed on surfaces of the N—Ge epitaxial layer210and the N—Si epitaxial layer212by, for example, the CVD method (FIG. 9F). Note that the silicon oxide film303is illustrated such that the silicon oxide film303contains the silicon oxide film301.

Subsequently, processes of implantation of P-impurity ions, for example, boron ions into the N—Ge epitaxial layer210and the N—Si epitaxial layer212, heat treatment for activating ion-implanted impurity regions, formation of the quenching resistors20to23, and formation of electrode interconnects, and the like are carried out.

Semiconductor materials such as SiGe or InGaAs may be used as the semiconductor material smaller in band gap than Si. Alternatively, photoelectric transducers formed of a combination of these semiconductor materials may be combined as appropriate. Regions of these semiconductor materials can be similarly formed on the surface of the silicon semiconductor substrate by selective epitaxial growth. Alternatively, a silicon semiconductor substrate, on a surface on which Si epitaxial layers are formed, may be used. In this case, photoelectric transducers formed of Si may be formed by selectively exposing the Si epitaxial layers formed on the surface of the silicon semiconductor substrate and implanting predetermined impurities into the exposed epitaxial layers. Furthermore, a P-silicon semiconductor substrate may be used as the silicon semiconductor substrate. In this case, photoelectric transducers can be similarly configured by changing a conductivity type of each of the selectively formed photoelectric transducers from a P type to an N type. In this case, an anode electrode is connected to the silicon semiconductor substrate as a common electrode.