JP3699961B2 - Photodiode - Google Patents

Description

[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a device structure of a long-wavelength broadband photodiode using a III-V compound semiconductor, particularly an InGaAsP-based material.
[0002]
[Prior art]
Wide-band photodiodes in the long wavelength band (1.5 μm band) are generally pin-type diodes (pin-PD) using InGaAs as a light absorption layer, and this light absorption layer shares a carrier traveling region that induces current. To do. Here, the InGaAs light absorption layer is designed to be depleted in the operating state so that the generated carriers are immediately accelerated by the electric field. Structurally, the InGaAs light absorption layer is made of p-type and n-type InP or the like. A so-called double hetero pin structure is adopted. In the examples so far, 110 GHz is reported as a ₃ dB band (f _{3 dB} ) in a surface light incident type element having a thickness of an InGaAs light absorption layer of about 0.2 μm and a normal waveguide type element.
The main factors that limit the bandwidth are a decrease in frequency response and a CR time constant as carriers run (C is the diode junction capacitance, R is the element parasitic resistance + line characteristic impedance). Since the capacity also changes as the light absorption layer thickness changes, the carrier travel time and the CR time constant are in a trade-off relationship. Because of this relationship, in the surface light incident type element, there is a light absorption layer thickness that maximizes the band for a certain diode junction area. Further, when the thickness of the light absorption layer is reduced, the carrier transit time is improved, but the internal quantum efficiency is lowered, so both are in a trade-off relationship.
The waveguide type element introduces light along the waveguide, and therefore has an advantage that the internal quantum efficiency can be increased compared to the surface light incident type element, and the trade-off between the carrier transit time and the internal quantum efficiency can be improved. The trade-off between carrier travel time and CR time constant is basically unchanged.
After all, unless the carrier traveling speed is increased, the frequency response (3 dB bandwidth, f _3dB ) is maintained while maintaining the quantum efficiency and CR time constant of the surface light incident type element (while maintaining the CR time constant in the waveguide type element). It is difficult to stretch significantly. This basically originates from the physical properties of the semiconductor. The direct transition type III-V compound semiconductor has a property of “slow hole drift speed”, and therefore, the effective carrier travel time is controlled by the hole drift speed. In other words, despite the high electron drift speed, “the hole determines the carrier travel time” is a basic problem of this type of photodiode.
On the other hand, increasing the possible output current is also important when applying photodiodes to optical communication receivers and the like. In order to obtain a high output, the carrier concentration in the carrier travel region must be increased. However, the response of the diode deteriorates at high light input due to the influence of electric field modulation accompanying the generation of internal space charge. That is, the remaining hole concentration becomes higher than the electron concentration, and the electric field of the traveling layer is flattened by the positive charge, and the hole extraction becomes worse. Again, the slow hole drift rate is the limiting factor.
[0003]
[Problems to be solved by the invention]
An object of the present invention is to solve the above-described problems in the prior art, and a photodiode having an element structure for improving frequency response and saturation output while ensuring effective internal quantum efficiency and CR time constant Is to provide.
[0004]
[Means for Solving the Problems]
In order to achieve the above-described object of the present invention, the present invention is configured as described in the claims. That is,
According to a first aspect of the present invention, a p-type first semiconductor layer, an n-type second semiconductor layer, and these semiconductor layers are sandwiched between the first semiconductor layer and the second semiconductor layer. A third semiconductor layer having a lower doping concentration than the semiconductor layer, and a band gap energy higher than that of the first semiconductor layer disposed on the opposite side of the third semiconductor layer in contact with the first semiconductor layer. A photodiode having a semiconductor pn junction structure including a large p-type fourth semiconductor layer,
The doping concentration is set so that at least a part of the first semiconductor layer maintains a charge neutral condition, and the band gap energy is set so as to function as a light absorption layer, so that the third semiconductor layer is depleted. The doping concentration is set so as to function as a carrier traveling layer, and the second and third semiconductor layers are set to a larger band gap energy than the first semiconductor layer so as not to function as a light absorption layer,
The photodiode has a structure in which the energy with respect to the hole at the valence band edge of the first semiconductor layer is set lower than the energy with respect to the hole at the valence band edge of the third semiconductor layer.
By adopting the above configuration, the conventional type uses the same depleted semiconductor layer for the light absorption layer and the carrier traveling layer, but in the present invention, the generation of carriers and the traveling can be separated. Therefore, only a carrier having a high traveling speed can be used, and the response speed and output amplitude characteristics can be improved. That is, according to the configuration method of the photodiode of the present invention, it is excluded that holes having a low drift speed are involved in current generation, and as a result, compared to a conventional photodiode using both electron and hole carriers. High speed operation is possible. In particular, the photodiode of the present invention has a remarkable effect of improving response characteristics when applied to a so-called ultrahigh-speed photodiode having a relatively thin absorption layer.
Further, in the present invention, since the generation and traveling of carriers can be separated, the drift rate of holes in the direct transition type III-V group compound semiconductor is slow, which is a limitation in the conventional structure, that is, “effective carrier carrier”. The traveling time is governed by the drift speed of the hole, and the problem that the hole determines the traveling time of the carrier despite the high drift speed of electrons can be solved. In the present invention, since only a carrier having a high traveling speed can be used, there is an effect that a photodiode having a high response speed and excellent output amplitude characteristics can be realized.
A basic structure of the photodiode of the present invention is shown in FIGS. 1A is a schematic diagram showing a cross-sectional structure of a surface light incident photodiode, and FIG. 1B is a band diagram of the element shown in FIG. 11 is a p-type light absorption layer, 12 is an n-type electrode layer, 13 is a carrier traveling layer, and 14 is a p-type carrier block layer. Reference numeral 15 denotes an anode electrode, 16 denotes a cathode electrode, and 17 denotes a semi-insulating substrate.
The light absorption layer is set to a doping concentration of a certain level or more so as not to be depleted in a bias state, and the InP carrier traveling layer is set to a low doping concentration so as to be depleted. The light absorption layer is preferably neutral in most regions, but may be partially depleted. When the photodiode is operated, it is reverse-biased from -0.5V to -2.5V. However, when the output current level is small, 0 bias may be used.
The optical response of the element is performed as follows.
First, light incident from the semi-insulating substrate 17 side passes through the n-type electrode layer 12 and the carrier traveling layer 13 and is absorbed by the p-type light absorption layer 11. Of the generated electrons and holes, the electrons diffuse into the carrier travel layer 13 and generate an induced current in the external circuit. Since the holes flow directly into the anode electrode 15, they hardly contribute to the induced current in the traveling layer.
Here, the difference of the photodiode of the present invention from the prior art will be described. Fig.5 (a) is a schematic diagram which shows the cross-section of the conventional double hetero pin-PD, and the band diagram is shown in FIG.5 (b). In the figure, 51 is a p-type electrode layer, 52 is an n-type electrode layer, 53 is a carrier traveling layer, 55 is an anode electrode, 56 is a cathode electrode, and 57 is a semi-insulating substrate. In this conventional photodiode, since the light absorbing layer and the carrier traveling layer use the same depleted semiconductor layer, an equal number of electrons and holes are generated and reach the n-type electrode layer and the p-type electrode layer, respectively. In both cases, an induced current is generated in the external circuit. Here, the induced current occurs in the form of the sum of two current components, electrons and holes, which determines the frequency response of the diode.
In a compound semiconductor such as InGaAs, the drift speeds of both carriers differ by about 4 to 8 times, and the travel delay time characteristics are almost determined by holes having a low drift speed. The 3 dB bandwidth f _3dB is expressed as follows: τ _h is the hole travel time, v _h is the hole drift velocity, and W _T is the depletion layer width.
f _3dB = 3.5 / (2πτ _h ) = 3.5 v _h / (2πW _T ) (Equation 1)
Can be expressed approximately.
On the other hand, the pin-PD [FIG. 1A] of the present invention has a structure in which carriers having a low drift speed do not directly participate in element operation. Since optical absorption and carrier travel are functionally separated, the overall optical response is a two-step process of carrier injection and induction current generation. When there is no difference in the carrier drift speed, the response is slower than that of the conventional pin-PD. However, when there is a difference of a certain level or more in the carrier drift speed, the response becomes faster by selectively using a carrier having a high speed (generally, electrons).
The operation of the pin-PD of the present invention will be described below. First, of the electrons and holes generated in the light absorption layer, the electrons diffuse into the carrier traveling layer. The hole responds electrically according to the behavior of the electrons so as to maintain the charge neutral condition of the light absorption layer. The response time of this hole is that of the dielectric relaxation time and is extremely short. The response time τ _Abs of carrier injection into the traveling layer is determined by the diffusion time of electrons in the light absorbing layer. This is the thickness of the total light absorbing layer W _Abs , the traveling time τ _eAbs , and the electron mobility. Assuming μ _e , 1/2 of τ estimated as the base travel time from the analog base travel time of the bipolar transistor, that is,
τ _A = τ _eAbs / 2 = [W _Abs ² / ( ² kTμ _e / q)] / 2 = [W _Abs ² / ( ² kTμ _e / q)] / 2 = W _Abs ² / (4 kTμ _e / q) ∝W _Abs ² ( _Equation ² )
Can be approximated by Here, the response time of the carrier generation layer is proportional to the square of the layer thickness. The f _3dB band is a characteristic of the diffusion transfer function: 1 / (1 + jωτ)
f _3dB = 4 / (2πτ _eAbs ) (Equation 3)
It becomes. Comparing the above equation with (Equation 1), when τ _h and τ _eAbs are equal, the difference between the conventional type and the present invention is small, but in the structure in which the electron diffusion rate is larger than the hole drift velocity, In the present invention, f _{3 dB} is much larger.
The response time τ _T of the traveling layer is
Transfer function of traveling carrier in capacitor: From the characteristic of {1-exp (−jωτ)} / (jωτ), the electron drift velocity is v _e , the electron traveling time is τ _eT , and the traveling layer width is W _T.
τ _T = W _T / 2v _e = 1 / 2τ _eT ( _Expression 4)
The corresponding f _3dB band is
f _3dB = 2.4 / (2πτ _eT ) (Equation 5)
It becomes. Compared to the above equation (Equation 1), v _h = v in the case of _e is towards conventional 3.5 / 2.4 times the bandwidth is wide, the three electron velocity relative to the hole velocity. When it becomes 5 / 2.4 times or more, the relationship of f _{3 dB} is reversed.
Along with the frequency response, the output amplitude characteristic is also an important characteristic index of pin-PD. Output saturation is caused by electric field modulation accompanying the generation of internal space charge, and by using only electrons as carriers, a higher current density is allowed by a higher speed for a certain carrier concentration. Thus allowing for higher output amplitudes.
After all, the difference between the conventional type and the pin-PD of the present invention is that the conventional type “uses the same depleted semiconductor layer for the light absorption layer and the carrier traveling layer”, whereas the present invention “ And running ". As a result, the response speed and output amplitude characteristics are improved by “using only a carrier having a high traveling speed”.
[0005]
DETAILED DESCRIPTION OF THE INVENTION
<First Embodiment>
FIG. 2 shows a band diagram of a surface light incident photodiode illustrated in this embodiment. In the figure, 21 is a p-type In _0.53 Ga _0.47 As light absorption layer, 22 is an n-type InP electrode layer, 23 is an undoped InP carrier running layer, and 24 is a p-type In _0.73 Ga _0.27 As _0.6 P _0.4 carrier block. Layer 25 is the anode electrode.
The In _0.53 Ga _0.47 As light absorption layer 21 is set to a certain doping concentration or more so as not to be depleted in a bias state, and the InP carrier running layer 23 is set to a low doping concentration so as to be depleted. Since the band gap energy of the In _0.73 Ga _0.27 As _0.6 P _0.4 carrier blocking layer 24 is 200 meV larger than that of the In _0.53 Ga _0.47 As light absorbing layer 21, it blocks the diffusion of electrons as minority carriers to the electrode side.
As an example, when considering a p-type In _0.53 Ga _0.47 As light absorption layer with a doping concentration p = 2 × 10 ¹⁸ / cm ² , the response time τ _A in the approximation of uniform light absorption is the electron mobility μ _e = 4000 cm ^2. / Vs, W _Abs = 0.2 μm, τ _A = 1 ps from (Equation 2), and the band is f _3dB = 320 GHz from (Equation 3). Also, assuming that the electron traveling speed in the InP carrier traveling layer is v _e = 4 × 10 ⁷ cm / s and W _T = 0.2 μm, the response time is τ _T = 0.25 ps and the band is (Equation 4) From equation (5), f _3dB = 764 GHz. The total bandwidth is calculated to be 295 GHz from the relationship of (1 / f _3dB ² ) _total = Σ (1 / f _3dB ² ). In the conventional pin-PD, for the same running layer thickness (= same quantum efficiency) W _T = 0.2 μm, the hole velocity in InGaAs is v _h = 5 × 10 ⁶ cm / s, and the band is f _3dB = 140 GHz is calculated. The structure of the present invention is twice as large as this.
[0006]
<Second Embodiment>
FIG. 3 shows a band diagram of a surface light incident photodiode illustrated in this embodiment. 31 is a p-type inclined band gap InGaAsP light absorption layer, 32 is an n-type InP electrode layer, 33 is an undoped InP carrier traveling layer, 34 is a p-type In _0.73 Ga _0.27 As _0.6 P _0.4 carrier blocking layer, 35 It is an anode electrode. The InGaAsP light absorption layer is set to a doping concentration of a certain level or more so as not to be depleted in a bias state, and the InP carrier traveling layer is set to a low doping concentration so as to be depleted.
[0007]
<Third Embodiment>
FIG. 4 shows a band diagram of a surface light incident photodiode illustrated in this embodiment. 41 is an In _0.53 Ga _0.47 As light absorption layer in which the p-type doping concentration is inclined toward the traveling layer side, 42 is an n-type InP electrode layer, 43 is an undoped InP carrier traveling layer, and 44 is a p-type In _0.73 Ga _0.27 As _0.6 P _0.4 carrier block layer, 45 is an anode electrode. The In _0.53 Ga _0.47 As light absorption layer is set to a doping concentration of a certain level or more so as not to be depleted in the bias state, and the InP carrier traveling layer is set to a low doping concentration so as to be depleted.
Each of the embodiments shown in FIGS. 3 and 4 has a structure for generating a pseudo electric field (an electric field that works only for minority carriers in a neutral region) in the light absorption layer. For example, in order to induce a pseudo electric field of 5 kV / cm in the structure of FIG. 3, a band gap inclination of 100 meV is required over the absorption layer thickness of W _Abs = 0.2 μm, specifically, the composition of InGaAsP is changed. It can be realized by changing. In order to induce an electric field of 5 kV / cm in the structure of FIG. 4, a Fermi level gradient of 100 meV is required over the absorption layer thickness of W _Abs = 0.2 μm. Specifically, the doping concentration of InGaAs is changed. It can be realized by changing it 47 times.
The pseudo electric field has an effect of accelerating the electrons in the light absorption layer by electric field drift and reducing the response time τ _eAbs . In the approximation in which the drift effect dominates the diffusion, the electron velocity in the absorption layer is constant, the thickness of the light absorption layer is W _Abs , the electric field strength is E, the potential change is ΔV _G , and the electron mobility. Where μ _e is the travel time
_{τ eAbs = W Abs / (μ} e E) = W Abs / (μ e EΔV G / W Abs) αW Abs 2 ...... ( number 6)
It becomes. Here, the response time of the carrier generation layer is proportional to the square of the layer thickness. The transfer function in the approximation of uniform light absorption is omitted here for details of derivation,
{1-exp (−jωτ)} / (jωτ), and the response time is
τ _A = τ _eAbs / 2 …… ( _Expression 7)
Furthermore, the bandwidth f _3dB is
f _3dB = 2.4 / (2πτ _eAbs ) (Equation 8)
It becomes.
As an example, when the light absorption layer thickness W _Abs = 0.2 μm and the internal electric field E = 5 kV / cm, the electron mobility μ _e = 4000 cm ² / Vs is calculated.
τ _A = τ _{eAbs /2=0.5} ps. The bandwidth is calculated as f _3dB = 2.4 / (2πτ _eAbs ) = 382 GHz. It can be seen that the response time is improved compared to the case of the first embodiment of FIG. 2 (carrier diffusion case). When the internal electric field effect is about the same as the diffusion effect (for example, E = 5 kV / cm), both effects work synergistically, and in reality, it is predicted to be larger than this f _{3 dB} band value. The effect becomes even more remarkable when the absorption layer thickness is reduced. For W _Abs = 0.14 μm,
τ _A = 0.25 ps, f _3dB = 2.4 / (2πτ _eAbs ) = 764 GHz, and significant improvement can be expected. In this case, assuming that the response of the traveling layer is the same as that of the first embodiment, the entire band is f _3dB = 540 GHz.
[0008]
【The invention's effect】
As explained in detail above, according to the photodiode construction method of the present invention, it is excluded that holes having a low drift velocity are involved in current generation, and as a result, the conventional “both carriers of electrons and holes are used. Compared to a “photo diode”, it can operate at a higher speed. In particular, the photodiode of the present invention has a remarkable effect of improving response characteristics when applied to a so-called ultrahigh-speed photodiode having a relatively thin absorption layer. In the case of a typical absorption layer (thickness 0.2 μm) with a constant bandgap, 295 GHz is expected with the present invention versus the conventional band limit of 140 GHz. Further, in the structure (thickness 0.14 μm) in which the pseudo electric field is built in the light absorption layer, the band limit reaches 540 GHz.
Furthermore, since the electric field modulation of the traveling layer due to the carrier space charge can be suppressed in inverse proportion to the electron velocity / hole velocity, a higher current density is allowed and a higher saturation output is provided.
After all, the basic advantage of the photodiode of the present invention is high-speed operation, which can be effectively used for detecting optical signals of 100 Gb / s and higher. The advantage of high saturation output can contribute to improvement of bit error rate in a receiver for optical communication.
In the embodiment of the present invention, a surface light incident type photodiode mainly using an InGaAsP-based material has been described. However, a similar configuration method can be applied to a waveguide type photodiode, and other III- The present invention can also be applied to a photodiode using a group V compound semiconductor material.
[Brief description of the drawings]
FIG. 1 shows a basic configuration and a band diagram of a photodiode of the present invention.
FIG. 2 is a diagram showing a band diagram of the photodiode exemplified in the first embodiment of the present invention.
FIG. 3 is a diagram showing a band diagram of the photodiode exemplified in the second embodiment of the present invention.
FIG. 4 is a diagram showing a band diagram of the photodiode exemplified in the third embodiment of the present invention.
FIG. 5 is a diagram showing a configuration and a band diagram of a conventional photodiode.
[Explanation of symbols]
DESCRIPTION OF SYMBOLS 11 ... p-type light absorption layer 12 ... n-type electrode layer 13 ... carrier running layer 14 ... p-type carrier block layer 15 ... anode electrode 16 ... cathode electrode 17 ... semi-insulating substrate 21 ... p-type In _0.53 Ga _0.47 As light Absorption layer 22 ... n-type InP electrode layer 23 ... undoped InP carrier transit layer 24 ... p-type In _0.73 Ga _0.27 As _0.6 P _0.4 carrier block layer 25 ... anode electrode 31 ... p-type inclined band gap InGaAsP light absorption layer 32 ... n-type InP electrode layer 33 ... undoped InP carrier running layer 34 ... p-type In _0.73 Ga _0.27 As _0.6 P _0.4 carrier blocking layer 35 ... anode electrode 41 ... p-type doping concentration toward the running layer side Tilted In _0.53 Ga _0.47 As light absorption layer 42 ... n-type InP electrode layer 43 ... undoped InP carrier transit layer 44 ... p-type In _{0.7 3} Ga _0.27 As _0.6 P _0.4 Carrier block layer 45 ... anode electrode 51 ... p-type electrode layer 52 ... n-type electrode layer 53 ... carrier running layer 55 ... anode electrode 56 ... cathode electrode 57 ... semi-insulating substrate

Claims (1)

A p-type first semiconductor layer, an n-type second semiconductor layer, and a third semiconductor layer sandwiched between these semiconductor layers and having a lower doping concentration than the first semiconductor layer and the second semiconductor layer. A semiconductor layer and a p-type fourth semiconductor layer having a band gap energy larger than that of the first semiconductor layer disposed on the opposite side of the third semiconductor layer in contact with the first semiconductor layer. A photodiode having a semiconductor pn junction structure comprising:
The doping concentration is set so that at least a part of the first semiconductor layer maintains a charge neutral condition, and the band gap energy is set so as to function as a light absorption layer, so that the third semiconductor layer is depleted. The doping concentration is set so as to function as a carrier traveling layer, and the second and third semiconductor layers are set to a larger band gap energy than the first semiconductor layer so as not to function as a light absorption layer,
The photodiode characterized in that the energy for the hole at the valence band edge of the first semiconductor layer is set lower than the energy for the hole at the valence band edge of the third semiconductor layer.

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