CN114823946A - Photoelectric detector for controlling conduction channel by light and preparation method thereof - Google Patents

Photoelectric detector for controlling conduction channel by light and preparation method thereof Download PDF

Info

Publication number
CN114823946A
CN114823946A CN202210236491.4A CN202210236491A CN114823946A CN 114823946 A CN114823946 A CN 114823946A CN 202210236491 A CN202210236491 A CN 202210236491A CN 114823946 A CN114823946 A CN 114823946A
Authority
CN
China
Prior art keywords
electrode
type semiconductor
layer
depletion
depletion region
Prior art date
Legal status (The legal status is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the status listed.)
Pending
Application number
CN202210236491.4A
Other languages
Chinese (zh)
Inventor
崔积适
Current Assignee (The listed assignees may be inaccurate. Google has not performed a legal analysis and makes no representation or warranty as to the accuracy of the list.)
Sanming University
Original Assignee
Sanming University
Priority date (The priority date is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the date listed.)
Filing date
Publication date
Application filed by Sanming University filed Critical Sanming University
Publication of CN114823946A publication Critical patent/CN114823946A/en
Pending legal-status Critical Current

Links

Images

Classifications

    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01L—SEMICONDUCTOR DEVICES NOT COVERED BY CLASS H10
    • H01L31/00—Semiconductor devices sensitive to infrared radiation, light, electromagnetic radiation of shorter wavelength or corpuscular radiation and specially adapted either for the conversion of the energy of such radiation into electrical energy or for the control of electrical energy by such radiation; Processes or apparatus specially adapted for the manufacture or treatment thereof or of parts thereof; Details thereof
    • H01L31/0248—Semiconductor devices sensitive to infrared radiation, light, electromagnetic radiation of shorter wavelength or corpuscular radiation and specially adapted either for the conversion of the energy of such radiation into electrical energy or for the control of electrical energy by such radiation; Processes or apparatus specially adapted for the manufacture or treatment thereof or of parts thereof; Details thereof characterised by their semiconductor bodies
    • H01L31/0352—Semiconductor devices sensitive to infrared radiation, light, electromagnetic radiation of shorter wavelength or corpuscular radiation and specially adapted either for the conversion of the energy of such radiation into electrical energy or for the control of electrical energy by such radiation; Processes or apparatus specially adapted for the manufacture or treatment thereof or of parts thereof; Details thereof characterised by their semiconductor bodies characterised by their shape or by the shapes, relative sizes or disposition of the semiconductor regions
    • H01L31/035272—Semiconductor devices sensitive to infrared radiation, light, electromagnetic radiation of shorter wavelength or corpuscular radiation and specially adapted either for the conversion of the energy of such radiation into electrical energy or for the control of electrical energy by such radiation; Processes or apparatus specially adapted for the manufacture or treatment thereof or of parts thereof; Details thereof characterised by their semiconductor bodies characterised by their shape or by the shapes, relative sizes or disposition of the semiconductor regions characterised by at least one potential jump barrier or surface barrier
    • H01L31/03529—Shape of the potential jump barrier or surface barrier
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01L—SEMICONDUCTOR DEVICES NOT COVERED BY CLASS H10
    • H01L31/00—Semiconductor devices sensitive to infrared radiation, light, electromagnetic radiation of shorter wavelength or corpuscular radiation and specially adapted either for the conversion of the energy of such radiation into electrical energy or for the control of electrical energy by such radiation; Processes or apparatus specially adapted for the manufacture or treatment thereof or of parts thereof; Details thereof
    • H01L31/08—Semiconductor devices sensitive to infrared radiation, light, electromagnetic radiation of shorter wavelength or corpuscular radiation and specially adapted either for the conversion of the energy of such radiation into electrical energy or for the control of electrical energy by such radiation; Processes or apparatus specially adapted for the manufacture or treatment thereof or of parts thereof; Details thereof in which radiation controls flow of current through the device, e.g. photoresistors
    • H01L31/10—Semiconductor devices sensitive to infrared radiation, light, electromagnetic radiation of shorter wavelength or corpuscular radiation and specially adapted either for the conversion of the energy of such radiation into electrical energy or for the control of electrical energy by such radiation; Processes or apparatus specially adapted for the manufacture or treatment thereof or of parts thereof; Details thereof in which radiation controls flow of current through the device, e.g. photoresistors characterised by at least one potential-jump barrier or surface barrier, e.g. phototransistors
    • H01L31/101—Devices sensitive to infrared, visible or ultraviolet radiation
    • H01L31/102—Devices sensitive to infrared, visible or ultraviolet radiation characterised by only one potential barrier or surface barrier
    • H01L31/105—Devices sensitive to infrared, visible or ultraviolet radiation characterised by only one potential barrier or surface barrier the potential barrier being of the PIN type
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01L—SEMICONDUCTOR DEVICES NOT COVERED BY CLASS H10
    • H01L31/00—Semiconductor devices sensitive to infrared radiation, light, electromagnetic radiation of shorter wavelength or corpuscular radiation and specially adapted either for the conversion of the energy of such radiation into electrical energy or for the control of electrical energy by such radiation; Processes or apparatus specially adapted for the manufacture or treatment thereof or of parts thereof; Details thereof
    • H01L31/18—Processes or apparatus specially adapted for the manufacture or treatment of these devices or of parts thereof
    • H01L31/1804—Processes or apparatus specially adapted for the manufacture or treatment of these devices or of parts thereof comprising only elements of Group IV of the Periodic System
    • H01L31/1808—Processes or apparatus specially adapted for the manufacture or treatment of these devices or of parts thereof comprising only elements of Group IV of the Periodic System including only Ge
    • Y—GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
    • Y02—TECHNOLOGIES OR APPLICATIONS FOR MITIGATION OR ADAPTATION AGAINST CLIMATE CHANGE
    • Y02P—CLIMATE CHANGE MITIGATION TECHNOLOGIES IN THE PRODUCTION OR PROCESSING OF GOODS
    • Y02P70/00—Climate change mitigation technologies in the production process for final industrial or consumer products
    • Y02P70/50—Manufacturing or production processes characterised by the final manufactured product

Abstract

The invention discloses a photoelectric detector for controlling a conduction channel by light, which comprises a waveguide layer, a depletion layer and an electrode layer. The depletion layer is arranged on the waveguide layer; the depletion layer comprises a P-type semiconductor and an N-type semiconductor which are arranged side by side along the transverse direction, and a depletion region is formed between the P-type semiconductor and the N-type semiconductor; the width of the depletion region is configured to dynamically vary according to the intensity of the signal light incident to the waveguide layer. An electrode layer including a first electrode and a second electrode; the first electrode and the second electrode are arranged on the depletion layer at intervals, and two ends of the first electrode and the second electrode are configured to be just flush with the edge of the depletion region when no signal light is incident. The invention realizes a silicon-based photoelectric detector which controls the conduction channel through light control and thus controls the photocurrent, and the silicon-based photoelectric detector has higher signal-to-noise ratio so as to reduce the bit error rate.

Description

Photoelectric detector for controlling conduction channel by light and preparation method thereof
The application of the patent requires the priority of application date of 2021, 6 and 11 months, application number of CN202110654238.6, original receiving and handling organization, and invention creation title of field effect conduction channel photoelectric detector and preparation method thereof.
Technical Field
The invention relates to the technical field of photoelectric detectors, in particular to a photoelectric detector for controlling a conduction channel in light and a preparation method thereof.
Background
In recent years, with the rapid development of the internet of things, the optical fiber communication system is used as an important support for the internet of things, and the development of the optical fiber communication system is more emphasized. In the field of long-distance backbone networks, with the maturity and development of optical transmission technology, the construction of trunk transmission networks has been hot in the world, and the transmission bandwidth and the transmission capacity are rapidly developed.
With the development of optical fiber communication systems, the development of optical devices also faces opportunities and challenges, and how to develop optical devices with excellent performance and low price has become a primary problem. Silicon-based optoelectronic devices have the advantages of easy integration, low process cost and the like, and have attracted extensive attention of researchers in recent years. Silicon (Si) material is used as a traditional material in the field of microelectronics, has incomparable advantages of other materials in processing technology and manufacturing cost, and a silicon-based photoelectron integration technology is developed. The photodetector, one of the important representative elements in silicon-based optoelectronic integration technology, functions to convert an incident optical signal into an electrical signal for analysis by subsequent signal processing circuitry. The silicon-based germanium photoelectric detector is continuously optimized in structure and further improved in performance after being developed for more than ten years.
In recent years, under continuous innovative efforts in academia and industry, various waveguide-integrated silicon-based germanium photodetectors with high performance indexes are continuously proposed, and part of indexes reach the level of commercial three-five detectors. At present, the photocurrent of the PIN structure silicon-based germanium detector mainly depends on the concentration of a photon-generated carrier, and the numerical value is usually small, so that the signal-to-noise ratio of the PIN structure silicon-based germanium detector is low, and the error rate is further increased.
Disclosure of Invention
The invention aims to solve the technical problem that the photocurrent of the existing PIN structure silicon-based germanium detector mainly depends on the concentration of photon-generated carriers, and the carrier concentration is usually small, so that the signal-to-noise ratio is low, and the error rate is further increased.
In order to solve the above technical problem, the present invention provides a photodetector for controlling a conduction channel optically, comprising
A waveguide layer;
a depletion layer disposed on the waveguide layer; the depletion layer comprises a P-type semiconductor and an N-type semiconductor which are arranged side by side along the transverse direction, and a depletion region is formed between the P-type semiconductor and the N-type semiconductor; the width of the depletion region is configured to dynamically vary according to the intensity of the signal light incident to the waveguide layer;
an electrode layer including a first electrode and a second electrode; the first electrode and the second electrode are arranged on the depletion layer at intervals, and two ends of the first electrode and the second electrode are configured to be just flush with the edge of the depletion region when no signal light is incident.
Preferably, the first electrode and the second electrode are respectively disposed on the depletion region at front and rear ends perpendicular to the lateral direction.
Preferably, the photodetector further comprises a silicon dioxide epitaxial layer and a substrate silicon layer which are connected with each other; the waveguide layer, the depletion layer and the electrode layer are all arranged in the silicon dioxide epitaxial layer.
Preferably, the P-type semiconductor is a P-type germanium-based semiconductor, and the N-type semiconductor is an N-type germanium-based semiconductor.
Preferably, the P-type semiconductor is a P-type silicon-based semiconductor, and the N-type semiconductor is an N-type silicon-based semiconductor.
Preferably, the photodetector is a three-five family detector. The three-five group detector can be a semiconductor detector of gallium arsenide, gallium phosphide, indium phosphide and the like. Arsenic and phosphorus are group five elements, and gallium and indium are group three elements.
The invention further provides a preparation method of the photoelectric detector for controlling the conduction channel by light, which comprises the following preparation steps:
firstly, preparing a semiconductor device with a PN junction by an alloy method, a diffusion method, an ion implantation method or an epitaxial growth method, and forming the P-type semiconductor, the depletion region and the N-type semiconductor; the width of the PN junction is set according to the maximum value of the change of the depletion region width caused by the same intensity signal light energy;
then depositing the first electrode and the second electrode on the PN junction at intervals respectively, wherein two ends of the first electrode and two ends of the second electrode are connected to the P-type semiconductor and the N-type semiconductor respectively;
and finally, depositing the waveguide layer on the surfaces, far away from the first electrode and the second electrode, of the P-type semiconductor and the N-type semiconductor.
The light of the present invention controls the photodetector of the conduction channel: (1) when no signal light exists, the width of a depletion region is just equal to the width of an electrode, a depletion region is arranged between two electrodes (the depletion region has no carriers and is equivalent to an insulator), a conductive path cannot be formed, and the theoretical current value is 0; (2) when signal light exists, the signal light is excited to generate carriers, a depletion region is narrowed, a conduction channel is formed between the two electrodes, and further photocurrent is formed. The stronger the signal light intensity, the narrower the depletion region, and the wider the conduction channel, the smaller the resistance between the two electrodes, and the larger the photocurrent. The invention realizes a silicon-based photoelectric detector which controls the conduction channel through light control so as to control the photocurrent, and the silicon-based photoelectric detector has the advantages of large signal current, high speed and higher signal-to-noise ratio, thereby being capable of reducing the bit error rate.
Drawings
Fig. 1 is a schematic cross-sectional view of a photodetector for controlling a conduction channel according to a first embodiment of the present invention.
Fig. 2 is a schematic top view of the photodetector of fig. 1 in an environment without signal light.
Fig. 3 is a schematic top view of the photodetector of fig. 1 in an environment with signal light.
Reference numerals: the semiconductor device comprises a waveguide layer 1, a depletion layer 2, an electrode layer 3, a P-type semiconductor 21, an N-type semiconductor 22, a depletion region 23, a first electrode 31, a second electrode 32, a silicon dioxide epitaxial layer 4 and a substrate silicon layer 5.
Detailed Description
To make the objects, technical solutions and advantages of the embodiments of the present invention clearer and more fully described below with reference to the accompanying drawings of the embodiments of the present invention, it is to be understood that the following detailed description of the embodiments of the present invention provided in the drawings is not intended to limit the scope of the claimed invention, but is merely representative of selected embodiments of the present invention. All other embodiments, which can be obtained by a person skilled in the art without any inventive step based on the embodiments of the present invention, are within the scope of the present invention.
Referring to fig. 1, a cross-sectional structure of a photo detector for controlling a conduction channel according to a first embodiment of the present invention is shown. The photodetector includes a waveguide layer 1, a depletion layer 2, and an electrode layer 3.
Wherein, the depletion layer 2 is arranged on the waveguide layer 1. The depletion layer 2 includes a P-type semiconductor 21 and an N-type semiconductor 22 arranged side by side in a lateral direction. The P-type semiconductor 21 has holes therein, which are illustrated as open circles in fig. 1. The N-type semiconductor 22 has electrons therein, such as the round black dots in fig. 1. A depletion region 23 is formed between the P-type semiconductor 21 and the N-type semiconductor 22. The width of the depletion region 23 is configured to dynamically vary according to the intensity of the signal light incident to the waveguide layer 1.
The electrode layer 3 includes a first electrode 31 and a second electrode 32. The first electrode 31 and the second electrode 32 are provided above the depletion layer 2 with a gap therebetween. And both ends of the first electrode 31 and the second electrode 32 are configured to be just flush with the edge of the depletion region 23 when no signal light is incident.
The light-controlling on-channel photodetector of the present embodiment: (1) referring to fig. 2, when there is no signal light, carriers are guided out by the first electrode 31 and the second electrode 32, the width of the depletion region 23 is just the width of the electrodes, and a depletion region 23 is formed between the two electrodes (the depletion region 23 has no carriers and is equivalent to an insulator), so that a conductive path cannot be formed, and the theoretical current value is 0 (actually, there is dark current due to process defects); (2) referring to fig. 3, when there is signal light, the signal light excites to generate carriers, the depletion region 23 narrows, and a conduction channel is formed between the two electrodes, thereby forming a photocurrent. The stronger the signal light intensity, the narrower the depletion region 23, and the wider the on channel, the smaller the resistance between the two electrodes, and the larger the photocurrent. That is, the embodiment implements a silicon-based photodetector for controlling a conduction channel by light control to control a photocurrent, which has a large signal current, a fast speed, and a high signal-to-noise ratio, thereby reducing an error rate.
The first electrode 31 and the second electrode 32 may be respectively disposed on the depletion region 23 at front and rear ends perpendicular to the lateral direction, so as to increase the length of the depletion region 23, reduce the dark current of the device, and improve the accuracy of the photoelectric conversion signal processing.
Optionally, the photodetector may be a silicon-based germanium detector, a silicon detector, a three-five-family detector, or the like, which is not specifically limited in this embodiment, and the solutions of the first embodiment can be applied thereto.
In the silicon-based germanium detector, the P-type semiconductor 21 is a P-type germanium-based semiconductor, and the N-type semiconductor 22 is an N-type germanium-based semiconductor.
In the silicon detector, the P-type semiconductor 21 is a P-type silicon-based semiconductor, and the N-type semiconductor 22 is an N-type silicon-based semiconductor.
The three-five group detector can be a semiconductor detector of gallium arsenide, gallium phosphide, indium phosphide and the like.
In a third embodiment of the present invention, a method for manufacturing a photo-detector for controlling a conduction channel as described above is provided, including the following steps:
(1) firstly, preparing a semiconductor device with a PN junction by an alloy method, a diffusion method, an ion implantation method or an epitaxial growth method, and forming the P-type semiconductor 21, the depletion region 23 and the N-type semiconductor 22; for example, one end of the silicon substrate is doped with phosphorus, and the other end of the silicon substrate is doped with boron to form a PN junction; the width of the PN junction is set to the maximum value at which the same intensity of signal light can cause the width of the depletion region 23 to vary.
(2) Then depositing the first electrode 31 and the second electrode 32 on the PN junction at intervals, wherein two ends of the first electrode 31 and two ends of the second electrode 32 are connected to the P-type semiconductor 21 and the N-type semiconductor 22, respectively; the material of the first electrode 31 and the second electrode 32 may be aluminum.
(3) Finally, the waveguide layer 1 is deposited on the sides of the P-type semiconductor 21 and the N-type semiconductor 22 remote from the first electrode 31 and the second electrode 32. The waveguide layer 1 may be a silicon dioxide waveguide layer or a gallium arsenide waveguide layer.
In the depletion layer 2, the absorption region is a PN junction, and the depletion region 23 is located in the middle of the PN junction. A first electrode 31 and a second electrode 32 are grown at both ends of the absorption region, and the width of the electrodes is the width of the depletion region 23 in the absence of signal light. The width of the PN junction depends on the distribution of the signal optical field in the PN junction, and it is preferable that the same intensity of signal light can cause the maximum value of the width variation of the depletion region 23. The length increase of the PN junction can improve the resistance of the device, on one hand, the dark current of the device is reduced, on the other hand, the responsivity of the device is reduced, the dark current and the responsivity are mutually restricted, and the PN junction can be designed according to specific requirements.
While the above is directed to some, but not all embodiments of the invention, the detailed description of the embodiments of the invention is not intended to limit the scope of the invention, which is claimed, but is merely representative of selected embodiments of the invention. All other embodiments, which can be obtained by a person skilled in the art without inventive step based on the embodiments of the present invention, are within the scope of protection of the present invention.

Claims (6)

1. A photodetector for controlling conduction channel light, comprising:
a waveguide layer;
a depletion layer disposed on the waveguide layer; the depletion layer comprises a P-type semiconductor and an N-type semiconductor which are arranged side by side along the transverse direction, and a depletion region is formed between the P-type semiconductor and the N-type semiconductor; the width of the depletion region is configured to dynamically vary according to the intensity of the signal light incident to the waveguide layer;
an electrode layer including a first electrode and a second electrode; the first electrode and the second electrode are arranged on the depletion layer at intervals, and two ends of the first electrode and the second electrode are configured to be just flush with the edge of the depletion region when no signal light is incident.
2. A light-controlling conduction channel photodetector as claimed in claim 1, wherein: the first electrode and the second electrode are respectively arranged at the front end and the rear end of the depletion region, and the front end and the rear end of the depletion region are perpendicular to the transverse direction.
3. A light-controlling conduction channel photodetector as claimed in claim 1, wherein: the photoelectric detector also comprises a silicon dioxide epitaxial layer and a substrate silicon layer which are connected with each other; the waveguide layer, the depletion layer and the electrode layer are all arranged in the silicon dioxide epitaxial layer.
4. A light-controlling conduction channel photodetector as claimed in claim 1, wherein: the P-type semiconductor is a P-type germanium-based semiconductor, and the N-type semiconductor is an N-type germanium-based semiconductor.
5. A light-controlling conduction channel photodetector as claimed in claim 1, wherein: the P-type semiconductor is a P-type silicon-based semiconductor, and the N-type semiconductor is an N-type silicon-based semiconductor.
6. A method for manufacturing a photo-detector for controlling a conduction channel according to any one of claims 1 to 5, comprising the steps of:
firstly, preparing a semiconductor device with a PN junction by an alloy method, a diffusion method, an ion implantation method or an epitaxial growth method, and forming the P-type semiconductor, the depletion region and the N-type semiconductor; the width of the PN junction is set according to the maximum value of the change of the depletion region width caused by the same intensity signal light energy;
then depositing the first electrode and the second electrode on the PN junction at intervals respectively, wherein two ends of the first electrode and two ends of the second electrode are connected to the P-type semiconductor and the N-type semiconductor respectively;
and finally, depositing the waveguide layer on the surfaces, far away from the first electrode and the second electrode, of the P-type semiconductor and the N-type semiconductor.
CN202210236491.4A 2021-06-11 2022-03-10 Photoelectric detector for controlling conduction channel by light and preparation method thereof Pending CN114823946A (en)

Applications Claiming Priority (2)

Application Number Priority Date Filing Date Title
CN202110654238.6A CN113437168A (en) 2021-06-11 2021-06-11 Field-effect conduction channel photoelectric detector and preparation method thereof
CN2021106542386 2021-06-11

Publications (1)

Publication Number Publication Date
CN114823946A true CN114823946A (en) 2022-07-29

Family

ID=77755731

Family Applications (2)

Application Number Title Priority Date Filing Date
CN202110654238.6A Withdrawn CN113437168A (en) 2021-06-11 2021-06-11 Field-effect conduction channel photoelectric detector and preparation method thereof
CN202210236491.4A Pending CN114823946A (en) 2021-06-11 2022-03-10 Photoelectric detector for controlling conduction channel by light and preparation method thereof

Family Applications Before (1)

Application Number Title Priority Date Filing Date
CN202110654238.6A Withdrawn CN113437168A (en) 2021-06-11 2021-06-11 Field-effect conduction channel photoelectric detector and preparation method thereof

Country Status (1)

Country Link
CN (2) CN113437168A (en)

Also Published As

Publication number Publication date
CN113437168A (en) 2021-09-24

Similar Documents

Publication Publication Date Title
Benedikovic et al. Silicon–germanium receivers for short-wave-infrared optoelectronics and communications: High-speed silicon–germanium receivers (invited review)
US6515315B1 (en) Avalanche photodiode for high-speed applications
EP3349252B1 (en) Optical waveguide detector and optical module
Masini et al. A four-channel, 10Gbps monolithic optical receiver in 130nm CMOS with integrated Ge waveguide photodetectors
CA2985057C (en) Light-receiving element and optical integrated circuit
CN113035982B (en) All-silicon-doped multi-junction electric field enhanced germanium optical waveguide detector
EP1498960A2 (en) Increased responsivity photodetector
CN105720130A (en) Photoelectric detector based on quantum well inter-band
Achouche et al. High performance evanescent edge coupled waveguide unitraveling-carrier photodiodes for> 40-Gb/s optical receivers
Duan et al. High speed waveguide-integrated Ge/Si avalanche photodetector
Forrest Optical detectors: Three contenders: Depending on the application, the photoeonductor, pin diode, or avalanche photodiode may prove the best choice
CN108447940B (en) Back-to-back double-absorption silicon-based photoelectric detector and preparation method thereof
JP3675223B2 (en) Avalanche photodiode and manufacturing method thereof
US9406832B2 (en) Waveguide-coupled MSM-type photodiode
CN210467859U (en) High-speed photoelectric detector
CN115224138B (en) Horizontal pull-through germanium-silicon avalanche photodetector
CN114823946A (en) Photoelectric detector for controlling conduction channel by light and preparation method thereof
CN115295646A (en) High-performance light detector chip epitaxial wafer
Kim et al. Improvement of dark current using InP/InGaAsP transition layer in large-area InGaAs MSM photodetectors
CN108987530B (en) Method for manufacturing photoelectric detector
JPS6262564A (en) Semiconductor photodetector
CN220829969U (en) Visible light and near infrared sheet on-chip sensing structure
WO2024027359A1 (en) Photodetector and manufacturing method therefor and optical receiver
CN115274910A (en) Silicon-based MSM photoelectric detector
JPH0750429A (en) Photodetector and manufacture thereof

Legal Events

Date Code Title Description
PB01 Publication
PB01 Publication
SE01 Entry into force of request for substantive examination
SE01 Entry into force of request for substantive examination