CN104332510A - Subwavelength plasmonic microcavity light coupling structure for promoting photoelectric detector response - Google Patents

Abstract

The invention discloses a sub-wavelength plasmon microcavity coupling structure that improves the responsivity of a photodetector. The propagation direction and light field distribution of incident light are modulated by the plasmon microcavity, so that the incident light is limited to a micro Propagation in the cavity reduces the escape of light and improves the utilization rate of photons. The incident light field is concentrated in the microcavity so that the intensity is greatly enhanced, and a photodetector with high responsivity can be formed by clamping the photoelectric conversion material in the microcavity. The coupling structure is composed of a metal grating layer formed by periodic metal strips on the upper layer, a photoelectric conversion active layer and a lower metal reflective layer. The advantage of the present invention is: using the mode selection effect of the electromagnetic wave near-field coupling microcavity formed by the plasmon resonance between the upper metal grating and the lower metal reflective layer, the photons entering the microcavity propagate along the lateral direction and form a standing wave , which not only gathers the energy of the light field but also increases the length of the equivalent light absorption, so that the detector responsivity is greatly improved.

Description

Sub-wavelength plasmonic microcavity optical coupling structure for enhancing photodetector photoresponse

technical field

The invention relates to a semiconductor photodetector, in particular to a photodetector integrated on a picture element and adopting a sub-wavelength plasmon microcavity coupling structure with improved photodetection capability.

Background technique

Semiconductor photodetectors realize the detection of light by absorbing incident photons to form electrons or hole transitions and then changing their conductive state to form photocurrent or photovoltage induced by photogenerated carriers. General photodetectors can directly receive incident light to complete photoelectric detection. However, continuously improving the performance of photodetectors is the goal that people are always pursuing. The figure of merit of a photodetector is the detectivity, and its value directly reflects the performance of the detector. The detectivity is directly proportional to the photoresponsivity, therefore, increasing the photoresponsivity of the photodetector will directly increase the detectivity of the device.

In addition to improving the photoelectric conversion efficiency of the photoelectric conversion material itself, the way to improve the responsivity can also be achieved by improving the efficiency of light coupling. The improvement of optical coupling improves the utilization rate of incident photons, so that as many incident photons as possible can be collected to participate in the excitation process of photogenerated carriers, correspondingly, the responsivity of photodetectors can be improved.

Ordinary photodetectors that do not use an optical coupling structure are limited by the effective conversion area of photoelectric conversion materials (such as the thickness of the junction region of the p-n junction or the thickness of the photoconductive active layer) and the effective transport length of photogenerated carriers (diffusion length and lifetime ), so that the utilization rate of incident photons is low, and a considerable part of incident photons does not participate in the process of photoelectric conversion but escapes from the detector. By designing a sub-wavelength plasmon microcavity coupling structure, the present invention can confine the incident photons in the microcavity, thereby suppressing the escape of the incident photons and improving the utilization rate of the incident photons. Concentrating the photon energy can greatly improve the photoresponsivity of the photodetector.

Contents of the invention

The purpose of the present invention is to propose a structure for improving the photoresponse of a photodetector using a subwavelength plasmon microcavity for photocoupling.

The sub-wavelength plasmonic microcavity used in the present invention is a structure for optical coupling. The structure is in order of the incident light passing through: the metal grating layer 1 formed by the upper metal strip, the photoelectric conversion active layer 2, and the lower metal grating layer. reflective layer3.

In the case of one-dimensional periodic repetition, the metal grating layer 1 is a metal line grating arranged periodically in one dimension with a period of p, a line width of s, and a thickness of h1; in the case of two-dimensional periodic repetition, It is a two-dimensional periodic metal grating with a period of p in both directions, a line width of s, and a thickness of h1. Its material includes but not limited to highly conductive gold or silver. In order to improve its adhesion, a layer of adhesive metal with a thickness of 5-30 nanometers is added between it and the photoelectric conversion active layer 2 , and its material includes but not limited to titanium. Its period p, line width s, and thickness h1 are determined by the optimization results obtained from theoretical calculations. The goal of the optimization calculations is to enable the incident light wave to collectively oscillate with the electrons in the metal to form the localized surface mode of the plasmon (Localized Surface Plasmon, LSP) undergoes resonant coupling, and enters the coupled microcavity under the induction of the resonant mode, forming a transverse standing wave cavity mode. Finite Difference Time Domain (FDTD) theoretical calculations give the following design ranges for the size parameters of metal gratings: ①The value of the stripe width s is between one-tenth and ten-tenths of the detection wavelength. For the resonant coupling mode, the relationship between the metal stripe width s and the detection wavelength λ satisfies s=kλ/2n, where k is the resonance order, n is the refractive index of the active material in the photoelectric conversion active layer 2, and the general value is 3-5 , depends on the type of photoelectric conversion material and is related to the thickness of layer 2. For k=1, s is at least one tenth of the detection wavelength. When the resonance order k>2n, s can reach a detection wavelength at most. But when k>6, the resonance coupling effect will gradually weaken to almost disappear. Therefore the maximum value of s does not exceed ten tenths of the detection wavelength. ② The value of the period p is one-tenth to thirty-tenths of the detection wavelength. This is because the period p must be greater than the fringe width s, and therefore must be greater than one-tenth of the detection wavelength. But when the stripe width s remains unchanged, increasing the period is equivalent to increasing the slit width. When the slit width is greater than twice the stripe width, theoretical calculations show that the resonant coupling effect will gradually weaken to almost disappear. The value of the period p therefore does not exceed thirty-tenths of the detection wavelength at most. ③ The value of the thickness h1 of the metal grating layer 1 is not less than 0.0096 times the square root of the detection wavelength in microns. This is because the resonant coupling condition requires that there is no interaction between the electromagnetic fields on the upper and lower surfaces of the metal grating layer 1, that is, the value of the thickness h1 is required to be not less than twice the skin depth. In the photoelectric detection band, the theoretically given skin depth of electromagnetic waves in metals is d ~ 0.0048·λ ^1/2 .

The photoelectric conversion active layer 2 refers to a semiconductor photoelectric thin film material capable of absorbing incident light and generating photogenerated carriers, which is grown on the substrate by epitaxial growth technology and obtained by substrate lift-off technology. Under the light, the carriers in the ground state in the film absorb photons and transition to the excited state to form photo-generated carriers, and transport the photo-generated carriers to the external circuit through the electric field between the upper and lower metal electrodes to form Photoelectrically converted signal. Its thickness h2 is determined by the optimization result obtained by theoretical calculation, and the goal of the optimization calculation is to make the transverse standing wave mode formed by the electromagnetic wave coupled into the coupled microcavity structure the strongest. According to the near-field coupling requirements of the plasmonic microcavity, h2 must be smaller than the equivalent light wavelength of the detected incident light, that is, the light wavelength in vacuum divided by the refractive index of the layer material. When the minimum value of the refractive index is 3, h2 should not be greater than one-third of the detection wavelength.

The lower metal reflective layer 3 refers to a complete metal layer with a thickness of h3. The value of h3 must be greater than the skin depth of the probe light in the metal, that is, 0.0048 times the square root of the probe wavelength in microns. The metal layer at this time can serve as a reflective layer for electromagnetic waves. Its material includes but not limited to highly conductive gold or silver. In order to improve its adhesion, a layer of adhesive metal with a thickness of 5-30 nanometers can be added between it and the photoelectric conversion active layer 2 , and its material includes but not limited to titanium.

The working principle of the present invention is: the metal grating designed for a specific photodetection wavelength is arranged periodically, and its period is smaller than the wavelength of the detected light, so that the plasmons formed by the collective oscillation of electrons in the grating metal can be compared with The incident light is resonantly coupled. The upper metal grating and the lower metal reflective layer work together to form a new modulation on the distribution of the light field, so that the light wave coupled into the microcavity propagates along the transverse direction and forms a cavity mode in the form of a standing wave. Its propagation direction changes from the z direction perpendicular to the detector plane in free space to the x direction along the detector plane, and is absorbed by the photoelectric conversion active layer 2 and converted into an electrical signal. This coupling method makes the photons in the microcavity almost unable to escape. Except for a small part of the heat loss at the metal interface, most of the photons will be absorbed by the photoelectric conversion material, which greatly improves the utilization rate of photons and correspondingly improves the photoelectricity. The response rate of the detector.

The advantages of the present invention are:

1 A new design is proposed in the photoelectric coupling structure of the photodetector, which uses the mode selection effect of the electromagnetic wave near-field coupling microcavity formed by the plasmon resonance between the upper metal grating and the lower metal reflective layer, so that it can enter Photons to the microcavity are dominated by those capable of resonating with the probe wavelength.

2 The electric vector direction of the photons entering the microcavity is changed from the x direction to the z direction under the modulation of the microcavity mode, and the propagation direction is changed from the z direction to the x direction (one-dimensional case) or x-y direction (two-dimensional case), It can not only be absorbed by photoelectric materials with isotropic absorption characteristics, but also be absorbed by quantum well subband transitions that only absorb electric fields in the z direction under the constraints of quantum transition selection rules, and form a photoelectric conversion process. Adopting the coupling method of the present invention can greatly improve the responsivity of the photodetector. In the embodiment of the present invention, the responsivity of the quantum well photodetector can be increased by 160 times, which has a very high enhancement capability.

Description of drawings

FIG. 1 is a schematic cross-sectional structure diagram of a subwavelength plasmonic microcavity coupling structure of the present invention. In the figure 1: the metal grating layer formed by the upper metal strips, 2: the photoelectric conversion active layer, 3: the lower metal reflective layer.

Fig. 2 is a schematic diagram of the three-dimensional structure of the photodetector pixel when the metal grating of the subwavelength plasmon microcavity coupling structure of the present invention repeats in one dimension. In the figure 1: the metal grating layer formed by the upper metal lines, 2: the photoelectric conversion active layer, 3: the lower metal reflective layer.

3 is a schematic diagram of the three-dimensional structure of the photodetector pixel when the metal grating of the subwavelength plasmonic microcavity coupling structure of the present invention repeats two-dimensionally. In the figure 1: the two-dimensional metal grating layer formed by the upper metal square, 2: the photoelectric conversion active layer, 3: the lower metal reflective layer.

Fig. 4 is the local surface plasmon of the propagation direction along the x direction and the electric vector along the z direction in the subwavelength plasmon microcavity coupling structure calculated by using the finite difference time domain method in the first embodiment of the present invention. Transverse optical field power distribution of a polariton (LSP) mode. When the total optical power before the incident light reaches the grating is taken as a unit value of 1, the integrated average optical power in the area indicated by the white dashed box in the figure reaches 8, which shows a great light field enhancement capability. The integration area only takes the part of the white frame in the microcavity because it is considered that as a photoelectric conversion device, its structure must inevitably include an electrode layer that does not have a photoelectric conversion function, which is used to form an ohmic contact with the metal electrode and load the light. The current flow is output to the metal electrode. The white dotted box area shows the effective photoelectric conversion area after deducting the electrode layer. In the figure 1: upper metal grating layer, 2: photoelectric conversion active layer, 3: lower metal reflective layer.

Fig. 5 is a ratio curve of the responsivity of the quantum well detector in Embodiment 1 of the present invention. The numerator part of the ratio calculation is the relationship between the responsivity and the wavelength actually measured by the quantum well detector in the subwavelength plasmonic microcavity structure, and the denominator part is the quantum well epitaxy material on the same wafer ground at 45 degrees. The relationship curve of responsivity versus wavelength measured by angular coupling. Since the response shape of photodetection is not modulated by coupling structures such as gratings, the response spectrum obtained by the 45-degree grinding angle structure is considered to reflect the absorption characteristics of the quantum well material's own quantum structure. The physical significance of the responsivity ratio curve in Figure 5 is that since the epitaxial photoelectric absorbing material on the same wafer is used, the response spectrum of the microcavity structure is divided by the response spectrum of the quantum well itself to obtain the mode of the light field in the microcavity structure The variation of intensity with wavelength, that is, the mode distribution of the light field, reflects the concentration and enhancement of the light field in the subwavelength plasmonic microcavity structure. It can be seen from Figure 5 that the enhancement can reach 160 times when the wavelength is near 15.5 microns. Thus, the responsivity of photodetection can be greatly improved.

Detailed ways

Taking the sub-wavelength plasmon microcavity coupling structure in which the photoelectric conversion material is GaAs/ _AlxGalxAs quantum wells as an example, the specific implementation of _the present invention will be further described in detail in conjunction with the accompanying drawings.

See Figure 1, Figure 2 and Figure 3. The sub-wavelength plasmon microcavity coupling structure for improving the photoresponse of photodetectors involved in the present invention includes: a metal grating layer 1, which is a metal grating periodically arranged with a period of p, a line width of s, and a thickness of h1, Periodically arranged in one-dimensional (Figure 2) or two-dimensional (Figure 3). The metal used in this embodiment is gold. In order to improve its adhesion, a layer of metal titanium is added between it and the photoelectric conversion active layer 2 . The period p, line width s and thickness h1 of the metal grating layer 1 are calculated and determined by the finite difference time domain (FDTD) method. The value is between one-tenth and ten-tenths of the detection wavelength in microns, and the value of the thickness h1 is not less than 0.0096 times the square root of the detection wavelength in microns. The metal grating layer 1 is prepared by thin film deposition, and a grating pattern is formed by photolithography and etching.

The photoelectric conversion active layer 2 refers to a semiconductor thin film material capable of absorbing incident light and generating photogenerated carriers. It can be a quantum well material sandwiched by a single or multiple quantum wells in a barrier layer, or it can be made of a photoelectric A p-n junction formed by absorbing semiconductor materials. Under the light, the carriers in the ground state absorb photons and transition to the excited state to form photo-generated carriers, and transport the photo-generated carriers to the external circuit through the upper and lower metal electrodes to form photoelectric conversion signals. The thickness h2 of the photoelectric conversion active layer is determined by the optimization results obtained from FDTD theoretical calculations. The goal of the optimization calculations is to maximize the transverse standing wave mode formed by the electromagnetic wave coupled into the subwavelength plasmonic microcavity. According to the near-field coupling requirements of the plasmonic microcavity, h2 must be smaller than the equivalent light wavelength of the detected incident light, that is, the light wavelength in vacuum divided by the refractive index of the layer material. The photoelectric conversion active layer can be prepared on the substrate by molecular beam epitaxy (MBE), metal organic chemical vapor phase epitaxy (MOCVD) or other thin film epitaxy methods, and then a separate epitaxial layer film is formed by substrate lift-off method.

The lower metal reflective layer 3 is a complete metal layer with a thickness h3 not less than the skin depth of the detection light wave in the metal, that is, the thickness h3 is not less than 0.0048 times the square root of the detection wavelength in microns. The metal reflective layer is prepared by thin film deposition method.

In the above size range, strict FDTD theoretical calculations show that due to the common coupling effect of the metal periodic grating and the metal reflective layer, the incident light is coupled and resonantly modulated, so that the incident light can enter the coupling microcavity whose thickness is much smaller than the wavelength scale. In , its propagation direction changes from the z direction perpendicular to the detector surface to the x direction or x-y direction parallel to the detector surface, and a transversely oscillating column wave is formed. The electric vector of the electromagnetic wave in the microcavity is along the z direction, which can be absorbed not only by quantum well subband transitions, but also by other isotropic photoelectric conversion materials.

The detection pixel table area of the coupling structure used in this embodiment is 230×200 μm ² , which can be adjusted according to actual needs. The manufacturing process of the sub-wavelength plasmonic microcavity coupling structure in this embodiment can follow the steps in the invention patent (patent number: 201110082811.7, patent name: process for metal waveguide microcavity optical coupling structure for optoelectronic functional devices) method, and can also be realized through other micromachining processes.

This embodiment uses GaAs/ _AlxGalxAs quantum wells with a response wavelength range of 10-16 microns as _the photoelectric conversion active material, and proves the feasibility and effectiveness of the present invention through three coupling structure embodiments using different size parameters . The thickness dimension parameters h1, h2 and h3 are fixed, and the period p, line width s and thickness of the viscous metal grating are changed.

The thickness dimension parameter h1 is taken as 0.1 micron in this embodiment, which satisfies the condition that it is not less than 0.0096 times the square root of the detection wavelength in micron.

For the thickness dimension parameter h2, in the embodiment of the present invention, the direction of the incident light is taken as the upper direction, then the layer consists of 5 sub-layers from bottom to top, which are respectively: the n-type doped GaAs lower electrode with a sub-layer thickness of 490 nanometers layer, with a doping concentration of 5.0×10 ¹⁷ cm ^-3 ; an Al _x Ga _lx As lower barrier layer with a sublayer thickness of 100 nanometers, where x=0.15; an n-type doped GaAs potential well with a sublayer thickness of 7 nanometers layer with a doping concentration of 2.0×10 ¹⁷ cm ^-3 ; an Al _x Ga _lx As upper barrier layer with a sublayer thickness of 100 nm, where x=0.15; an n-type doped GaAs upper electrode with a sublayer thickness of 190 nm layer with a doping concentration of 5.0×10 ¹⁷ cm ^-3 . The total thickness of the 5 sublayers constitutes h2, which has a value of 887 nanometers, or 0.887 micrometers. This value satisfies the condition that the thickness is smaller than the equivalent wavelength (for the detection wavelength range of 10-16 microns in this embodiment, if the refractive index of the quantum well is 3.3, the equivalent wavelength is 3-4.5 microns).

The thickness dimension parameter h3 is taken as 0.1 micron in this embodiment, which satisfies the condition that it is not less than 0.0048 times the square root of the detection wavelength in micron.

Embodiment 1: The upper metal is a one-dimensional grating, the line width s is 5.5 microns, the period p is 9.6 microns, and the thickness of the viscous titanium metal is 20 nanometers.

Embodiment 2: The upper metal layer is a one-dimensional grating, and the line width s is taken as 1.36 microns, which is one-tenth of the detection wavelength. The period p is taken as 2.6 microns, which satisfies the condition of being greater than the line width s. The sticky metal titanium is 5 nanometers thick.

Embodiment 3: The upper metal layer is a one-dimensional grating, and the line width s is taken as 13.6 microns, which is ten tenths of the detection wavelength. The period p is taken as 40 microns, which is thirty-tenths of the detection wavelength. The sticky metal titanium is 30 nanometers thick.

Embodiment 4: The upper metal is a two-dimensional square grating, the line width s is 5.5 microns, the period p is 10.6 microns, and the thickness of the viscous metal titanium is 20 nanometers.

The results obtained in the above four embodiments are similar, and the calculation and test results of the first embodiment are shown in accompanying drawings 4 and 5 .

Fig. 4 shows the distribution of the electric vector of the light field along the z direction inside the sub-wavelength plasmonic microcavity of the sub-wavelength plasmonic microcavity with a wavelength of 15.5 microns calculated by FDTD in the first embodiment of the present invention, and provides Take the integral average value of the optical power enhancement factor within the effective photoelectric conversion thickness range (as shown by the white dashed box in the figure) when the incident optical power is the unit value 1. It can be clearly seen that the incident light incident along the z direction is modulated by the microcavity plasmons after entering the microcavity, and the propagation direction is changed to be along the x direction. Because there is a sudden change in the overall equivalent dielectric function between the double-layer metal region and the single-layer metal region in the plasmonic microcavity, an impedance mismatch is formed at the interface. For light waves, an equivalent abrupt interface is formed, so the interfaces at both ends of the double-layer metal region form reflection interfaces for light waves, forming an F-P resonant cavity [see Y. Todorov, et al., Strong Light-Matter Coupling for details. in Subwavelength Metal-Dielectric Microcavities at Terahertz Frequencies, Physical Review Letters 102, 186402 (2009)]. The light wave is reflected back and forth in the F-P resonant cavity to form a standing wave. On the one hand, it gathers the energy of the light field, and on the other hand, the light wave is continuously absorbed by the photoelectric conversion material during the back and forth oscillation process, which greatly increases the utilization rate of photons. The energy of the light field is mainly concentrated in the microcavity range of the double-layer metal under the metal layer of the grating, and the highest power increase factor can reach 32. However, the integral average value of the increase factor within the entire range of the photoelectric conversion material (the range of the white dotted line box in FIG. 4 ) is 8.

Fig. 5 is a ratio curve of the responsivity of the quantum well detector actually measured in the first embodiment of the present invention. The numerator part of the ratio is the actual measurement of the responsivity of the quantum well detector in the subwavelength plasmonic microcavity structure as a function of wavelength, and the denominator part is the coupling of the quantum well epitaxial material on the same wafer at a grinding angle of 45 degrees. The relationship curve of responsivity versus wavelength measured by the method. Since the response shape of the photodetector is not modulated by the coupling structure such as a grating, the response spectrum obtained by the 45-degree grinding angle structure is considered to reflect the absorption characteristics of the quantum well material’s own quantum structure [see B.F.Levine, Quantum-well infrared photodetectors for details. , Journal of Applied Physics 74(8), R1(1993)]. The physical significance of the responsivity ratio curve in Figure 5 is that since the epitaxial photoelectric absorbing material on the same wafer is used, the response spectrum of the microcavity structure is divided by the response spectrum of the quantum well itself to obtain the change of the intensity of the light field mode with the wavelength The relationship, that is, the mode distribution of the light field in the microcavity structure, reflects the concentration and enhancement of the light field in the subwavelength plasmonic microcavity structure. It can be seen from Figure 5 that the responsivity of the detector at a wavelength of 15.5 microns is increased by about 160 times, which means that the equivalent absorption length of the quantum well is extended by 160 times, because the comparison is from the quantum well on the same wafer material, both have the same absorption coefficient and carrier transport length. Considering the 8-fold effect of light field concentration, the increase in the actual equivalent absorption length is 20 times. The superposition of these two effects, the light field concentration and the increase of the equivalent absorption length, greatly improves the responsivity of the photoelectric response device, reflecting the effectiveness of the sub-wavelength plasmonic microcavity coupling structure of the present invention in improving the photoelectric responsivity. sex.

Claims (3)

1. one kind promotes the subwavelength plasmon microcavity optical coupling structure of photodetector photoresponse, structure is pass through successively for sequence is the metal grating layer (1) that upper strata metal stick is formed successively with incident light, opto-electronic conversion active coating (2) and lower metal reflector (3), is characterized in that:

The metal grating layer (1) that described upper strata metal stick is formed, when One Dimension Periodic repeats, is the metal wire grating that the cycle is p, live width is s, thickness is h1 One Dimension Periodic arranges; When two-dimensional and periodic repeats, be the metal squares shape grating of the two-dimension periodic arrangement that the cycle in both direction is p, live width is s, thickness is h1, this layer of material includes but not limited to gold or the silver of high conductivity; The numerical value of the period p of the metal grating layer (1) that upper strata metal stick is formed is 30/10 to ten/10ths of detection wavelength, the numerical value of live width s be detection wavelength 10/10 to ten/10ths between, the value of thickness h 1 is not less than subduplicate 0.0096 times of the detection wavelength in units of micron;

Described opto-electronic conversion active coating (2) grows for utilizing epitaxially grown method the single or multiple lift semiconductor optoelectronic converting function material formed on backing material and by substrate desquamation technique, composite material includes but not limited to GaAs and AlGaAs, Si, Ge, HgCdTe, GaN, InGaAs, InAlAs and InP or InGaAs and GaAs, and the numerical value of its thickness h 2 is not more than 1/3rd of detection wavelength;

Described lower metal reflector (3) refers to that thickness is the metal level that one deck of h3 is complete, and the value of h3 is not less than subduplicate 0.0048 times of the detection wavelength in units of micron, and its material includes but not limited to gold or the silver of high conductivity.

2. a kind of subwavelength plasmon microcavity optical coupling structure promoting photodetector photoresponse according to claim 1, it is characterized in that: the stickiness metal level being attached with the adhesion improving the metal grating layer (1) that upper strata metal stick is formed between the metal grating layer (1) and opto-electronic conversion active coating (2) of described upper strata metal stick formation, its thickness is 5 ~ 30 nanometers, and material includes but not limited to titanium.

3. a kind of subwavelength plasmon microcavity optical coupling structure promoting photodetector photoresponse according to claim 1, it is characterized in that: between described opto-electronic conversion active coating (2) and lower metal reflector (3), be attached with the stickiness metal level improving lower metal reflector (3) adhesion, its thickness is the stickiness metal of 5 ~ 30 nanometers, and its material includes but not limited to titanium.