JP2669040B2 - Avalanche photodiode - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION (Field of Industrial Application) The present invention relates to an avalanche photodiode (APD).
About.

(Prior Art) To configure a high-speed, large-capacity optical communication system, a semiconductor light-receiving element having ultra-high speed, low noise, and high sensitivity characteristics is indispensable. For this reason, in recent years, researches on high-speed and high-sensitivity InP / InGaAs-based avalanche photodiodes (APD) that can be applied to a low-loss wavelength range of 1.0 to 1.6 μm of silica-based fibers have been actively conducted. In this InP / InGaAs APD, the gain bandwidth (GB) is currently achieved by reducing the capacitance by reducing the light receiving diameter, reducing the carrier transit time by optimizing the layer thickness, and suppressing the carrier trap by introducing an intermediate layer at the hetero interface.
Product 75GHz (This is the top data, 40-50GHz on average)
Has been realized. However, in this device structure, the ionization ratio β / α of InP, which is the avalanche multiplication layer, is as small as ２2 (α: electron ionization ratio,
β: the ionization rate of holes) and the excess noise figure x (increased as the ionization rate ratio decreases) as large as 0.7, and there is a limit to low noise and high sensitivity. This is because other III-
The same applies when a group V compound semiconductor is used for the avalanche multiplication layer. Therefore, Capasso et al. (Appl. Phys. Lett.) 40
(1) Volume, Jan., 1982, proposed a structure that artificially increases the ionization rate α / β by using the conduction band energy discontinuity ΔEc due to the superlattice for electron ionization.
It was confirmed that the ionization rate α / β was increased in the GaAs / GaAlAs-based superlattice (８8 in the superlattice layer compared to の 2 in bulk GaAs).
Furthermore, we have improved this structure, and proposed a structure that uses a graded superlattice layer to suppress carrier trapping in the well layer and improve the response speed (IETL-Transactions on Electron Devices (IEEE, TRA).
NS.ELECTRON.DEVICES), ED-30, P.381, 1983). An energy band diagram when the bias is applied is shown in FIG. In the figure, 1 is an n ⁺ -type InP substrate, 2 is an n-type InP buffer layer, 3 is an n ^-- type graded superlattice avalanche multiplication layer, and 4 and 5 are n ^- type graded superlattice avalanche multiplications, respectively. The ternary mixed crystal semiconductor ionized region (InGaAs), which is a part of the layer 3, and the graded obstacle layer (InGa
AlAs). 6 is an n ⁻ type InGaAs light absorption layer, and 7 is a p ⁺ type region (InP). In this structure, the electrons generated in the light absorption layer 6 are injected into the graded superlattice avalanche multiplication layer 3 by the bias electric field. The barrier layer 5 constituting the graded superlattice avalanche multiplication layer 3 has a small band gap (Eg ₁ ) on the side where electrons are injected, and the band gap gradually increases in the direction in which electrons drift. The thickness of this graded layer is such that the bias electric field is not canceled by the internal electric field due to the graded layer. At the point where the band gap becomes Eg ₂ , the band gap is discontinuously reduced to Eg ₁ , and the band gap Eg ₁ continues for 2 to 3 times the thickness of the mean free path in which electrons are ionized. Is a mixed crystal semiconductor ionized region 4 of ternary or more. Such a structure is a structure in which the structure is cyclically manipulated in stages. In this structure, the energy discontinuity is larger on the conduction band side ΔEc than on the valence band side ΔEv, and by obtaining this energy, the electrons get more energy than the energy obtained by acceleration by the electric field, and it becomes easier to ionize. . On the other hand, for holes, ΔEv is smaller than ΔEc, and this energy discontinuity is felt as a barrier. Therefore, holes lose energy here (however, the amount thereof is small) and it is difficult to ionize. The ionization rate α / β is increased based on the above principle. Moreover, despite the large band discontinuity on the conduction band side, the barrier on the electron injection side has a graded structure, so that electrons are not trapped and the response speed does not decrease. Using this structure as a model, K. Brennan calculated the α / β ratio by the Monte Carlo method (I Triple E.
Transactions on Electron Devices (IEEE TRANS. ELECTRON. DEVICES) ED-33 1986)). According to this, theoretically, α / β = ~ 10, excess noise figure x
It was expected that the noise could be reduced to 0.3-0.4.

(Problems to be Solved by the Invention) However, in the light-receiving element having the above-described structure, since the region 4 where the avalanche doubling occurs is formed of a multi-element mixed crystal, high-energy electrons are easily ionized by mixed crystal scattering. It has the disadvantage of not reaching.

Generally, the mixed crystal scattering of carriers is an important mechanism in the ionization phenomenon of a mixed crystal semiconductor. The scattering rate P of mixed crystal scattering is represented by the following equation according to the Litteljohn's equation.

Here, Ω represents the volume of the unit cell, ΔU represents the degree of potential fluctuation due to the mixed crystal, and γ represents the deviation of the band from the parabolic shape. According to this, it is understood that the scattering ratio of the mixed crystal scattering is proportional to the cube of the energy E of the carrier, and the higher the energy, the larger the ratio. For example, in the case of bulk InGaAs, when the mixed crystal scattering is not considered according to the Monte Carlo method, the ionization rate is up to 6 times for electrons and up to 3 times for holes when the bias is 4.5 × 10 ⁵ V / cm. In the above-described graded superlattice avalanche multiplication layer 3, electrons acquire energy equal to the band discontinuity at the potential step, and energy equal to the band discontinuity than the average energy determined by the internal electric field and various scatterings. Is injected into the ionization region 4 in a high energy state (= value close to the threshold energy of ionization). In the conduction band, there are an L valley and an X valley in addition to the Γ valley, so that the state density at high energy is large, and electrons are easily distributed at such energy values. At this time, if the ionized region is formed of a ternary or higher mixed crystal semiconductor as in the prior art, the electrons have a higher energy than in the low energy state (the energy value before obtaining the energy for the potential step) because of the high energy. High probability of receiving mixed crystal scattering,
The momentum of the carriers is averaged due to the increase of the mixed crystal scattering, and the energy is easily lost, so that it is difficult to reach the ionization threshold.

An object of the present invention is to realize an avalanche photodiode which facilitates ionization, has a high response speed, and has lower noise characteristics.

(Means for Solving the Problems) An APD according to the present invention has an avalanche multiplication region in which a band discontinuity of a semiconductor having a large band gap and a semiconductor having a small band gap are periodically formed on a conduction band side.
In the photodiode, an ionized region including a strained layer of a group III-V binary semiconductor is formed between the semiconductor having a large band gap and the semiconductor having a small band gap.

(Function) When the ionized region is formed of the binary semiconductor strained layer as in the present invention, the mixed crystal scattering is suppressed, so that the electrons are less likely to lose energy, and are synergistically ionized as a potential step effect. The rate is significantly higher than when the ionized region is composed of a mixed crystal semiconductor.
However, the thickness of the binary semiconductor strained layer is set to a value less than or equal to the critical thickness at which dislocation due to lattice mismatch occurs, and close to the mean free path of electrons required for ionization. On the other hand, for holes, the valence band discontinuity ΔEv is small, and since the band discontinuity is a barrier for holes, holes lose energy (however, the amount is small). , Is injected into the ionization region with an average energy that is almost determined only by the internal electric field and various scatterings. In the valence band, holes in the high-energy state close to the ionization threshold have transitioned to the split-off band, but their density of states is originally low, and holes remain in such high-energy state for a long time. However, since the average energy is originally smaller than that of the electrons as described above, holes are less likely to be distributed at high energy. Therefore, the effect of suppressing the mixed crystal scattering, which often occurs at high energy, is not remarkable, and the increase in the ionization rate is the same as or less than that of the bulk, even if it is high. Therefore, the α / β ratio can be improved several times as compared with the conventional case where the ionization region is composed of a mixed crystal.

(Example) Hereinafter, an example of the present invention will be described using an InAlAs / InGaAs-based APD, but other semiconductor systems, for example, In _1-x Ga _x As
_The same applies to the _Y P _1-Y / GaSb system and the like. The semiconductor light receiving element shown in FIG. 2 was manufactured by the following steps.

n ⁺ on the InP substrate 1, an n-type InP buffer layer 2 to 1μm thick, n carrier concentration ~1 × 10 ¹⁵ cm ^{-3 -} type In _0.53 Ga _0.47 As
−In _0.52 (Ga _1−x Al _x ) _0.48 As−In _0.52 Al _0.48 As The graded superlattice avalanche multiplication layer 13 is 2 μm thick, and the n ^- type In has a carrier concentration of 1 × 10 ¹⁵ cm ^−3. _0.53 Ga _0.47 As
The thickness of the light absorption layer 6 is about 2 μm, and the carrier concentration is about 1 × 10 ¹⁶ cm.
^-3 n ⁻ -type InP cap layer 8 is sequentially grown to a thickness of 1 μm by using metal organic vapor phase epitaxy (MOVPE). This multiplication layer 13
Is less than the critical thickness for dislocation generation due to lattice misfit ~ 70
Å GaSb binary semiconductor strained layer ionization region 14 and thickness ~ 500 Å
The graded barrier layer 15 is a quaternary mixed crystal.
Since the thickness of the multiplication layer 13 is 2 μm, the periodic structure of the barrier layer 15 and the ionization region 14 is about 35 periods. Next, using a SiO ₂ diffusion mask, Zn selective diffusion is performed to a depth of 1 μm in a circular region having a diameter of 30 μm to form ap ⁺ type region 7. After polishing the substrate, an insulating protective film 10 was formed, and the n-side electrode 20 was formed of AuGe and the p-side electrode 9 was formed of AuZn.

In the above embodiment, the case where the barrier layer is graded is described. However, in the present invention, whether or not the barrier layer is graded is not essential to the effect of the ionization region of the binary semiconductor strain layer, and the barrier layer is uniform. Obviously, any composition may be used.

(Effects of the Invention) The results of comparison of the ionization ratio of electrons and holes between the case of the above embodiment and the case of a conventional mixed crystal semiconductor (here, the structure other than the ionized region is the same) In the conventional example as well, the ionization rate of electrons increases due to the effect of the potential step, and the ionization rate ratio α / β
Is about 10 and the excess noise figure x is about 0.3, while in the present embodiment, the effect of the potential step and 2
Due to the synergistic effect of the former semiconductor strained layer ionization region, the ionization rate of electrons is increased more than the conventional structure, and the ionization rate ratio α / β
Was about 20 to 30, and the excess noise figure x was about 0.2. Since the thickness of the strained layer was less than the critical film for dislocation generation due to lattice misfit, no increase in dark current or deterioration in reliability was observed. As described above, according to the present invention, a low noise APD having sensitivity to a wavelength of 1.0 to 1.6 μm can be realized. In addition, the GB product is ~ 150 GHz, which is 2 to 4 times that of the conventional product, and the response speed can be increased.

[Brief description of the drawings]

FIG. 1 and FIG. 2 are diagrams showing an example of the APD of the present invention, and are an energy band diagram and a cross-sectional diagram when a bias is applied, respectively. FIG. 3 is an energy band diagram when a bias is applied, showing an example of a conventional superlattice APD. In FIG, 1 ...... n ⁺ -type InP substrate, 2 ...... n-type InP buffer layer, 3,13 ...... n ^- -type graded superlattice avalanche multiplication layer, 4 ...... n ^- -type ternary mixed crystal semiconductor ionization Area, 5 ...
n ^- type graded barrier layer, 6 ... n ^- type InGaAs light absorption layer,
14 …… Binary semiconductor strained layer ionization region, 15 …… Graded barrier layer, 16 …… n ^- type light absorption layer.

Claims (1)

(57) [Claims] 1. An avalanche photodiode having an avalanche multiplication region in which band discontinuity due to a semiconductor having a large band gap and a semiconductor having a small band gap is periodically formed on a conduction band side. An avalanche photodiode, characterized in that an ionized region composed of a strained layer of a III-V group binary semiconductor is formed therebetween.