JPH03237765A - Avalanche photodiode - Google Patents

Abstract

PURPOSE:To obtain an avalanche photodiode of low noise and very high speed, by constituting superlattice of a barrier layer having resonance tunnel characteristics of short period superlattice and a well layer. CONSTITUTION:The title diode is constituted of a barrier layer 21 having resonance tunnel characteristics of short period superlattice and a well layer 22. A region (resonance tunnel barrier layer 21) where electrons 26 turn to ions and a region (well layer 22) where electrons 26 are accelerated are isolated. Since the well layer 22 is a comparatively thick layer where the quantum level is not formed, the increase of band gap corresponding to the energy amount of quantum level is not generated, so that the increase of ionization threshold energy and the decrease of ionization ratio are not caused. Further, since the resonance tunnel barrier 21 is used for the barrier layer, the electrons 26 can easily escape from the well layer 22 through electron conduction of a miniband, and the reduction of noise caused by random emission of electrons can be realized.

Description

DETAILED DESCRIPTION OF THE INVENTION (Field of Industrial Application) The present invention relates to an avalanche photodiode (APD) having low noise characteristics used in optical communications and the like.

(Prior Art) In order to construct a high-speed, large-capacity optical communication system, a semiconductor light-receiving element having ultra-high speed, low noise, and high sensitivity characteristics is essential. For this reason, InP/InGa, which can be applied to the low-loss wavelength range 1.0 to 1.6 pm of silica-based fibers, has recently been developed.
Research into increasing the speed and sensitivity of As-based avalanche photodiodes (APDs) is active. In this InP/InGaAs APD, the gain bandwidth (GB) has been improved by reducing the capacitance by increasing the diameter of the skewer, reducing the Canoa transit time by optimizing the layer thickness, and suppressing carrier traps by introducing an intermediate layer at the Peter interface. A speed increase of 75 GHz has been achieved. However, in this device structure, the ionization rate ratio I of InP, which is the avalanche multiplication layer, is
Since 3/α is small at ~2 (α: electron ionization rate, I
3: Hole ionization rate), excess noise index X (the smaller the ionization rate ratio, the larger it becomes) becomes ~0.7,
There are limits to lower noise and higher sensitivity. This is the other II
The same applies when an IV group compound semiconductor is used for the avalanche multiplication layer. Therefore, Capasso (F, Capa
sso) etc. are applied physics letters (Ap
pl, Phys, Lett,), vol. 40(1), 3
p. 8, Jan, 1982, proposed a structure in which the ionization rate ratio a/p is artificially increased by utilizing the conduction band energy discontinuity amount ΔEc due to the superlattice for electron ionization,
In fact, we confirmed an increase in the ionization rate ratio a/13 in the GaAlAs/GaAs superlattice (~8 in the superlattice layer compared to ~2 in bulk GaAs). FIG. 5 is an energy band diagram when bias is applied. In this figure, 41 is n+
+GaAs substrate, 42 is n-type GaAs buffer layer, 4
3 is an n-type GaAlAs/GaAs superlattice avalanche multiplication layer, 44 n-type GaAlAs barrier layers, 4
It is composed of 5 n-type GaAs well layers. Also, 46
The p+ type region 47 is the conduction band discontinuity amount ΔEc, 48 is the valence band discontinuity amount ΔEv, and 49 is the ionization threshold energy of the well layer. The effect of increasing the ionization rate ratio C/13 due to the superlattice will be explained using FIG. 5. In this structure, electrons and holes acquire energy equal to the conduction band discontinuity and valence band discontinuity at each potential step, respectively, and the band discontinuity is higher than the average energy determined by the internal electric field and various scattering. It is implanted into the GaAs well layer 45 in a high energy state (=a value close to the threshold energy of ionization) by an energy equal to the amount. In a GaAlAs/GaAs system heterojunction, the amount of discontinuity in the conduction band is larger than the amount of discontinuity in the valence band, and holes tend to lose energy due to more frequent scattering than electrons, so electrons and holes There is a difference in the distribution probability of exceeding the ionization threshold energy at , and the ionization rate of electrons increases, but the ionization rate of holes does not. Therefore, C.L.
The /I3 ratio can be improved several times to about 8 in the superlattice compared to about 2 in the bulk case, and the excess noise figure X can be reduced from 0.9 of bulk GaAs to 0.25.

(Problem to be Solved by the Invention) However, since the light receiving element with the above structure does not have photosensitivity in the wavelength range of 1.3 to 1.5 pm used for long-distance optical communication, it cannot be used in this wavelength range. I can't. Therefore, Young-June Yu et al.
Lett.

51 (18) p. 1433, 1987), InAlA was used as an InP-based superlattice sensitive to this wavelength range.
An attempt was made to improve the a/13 ratio by fabricating an s/InGaAs superlattice, but a later report (47th Device Research Conference, DRC) was made.

Extended Abstract, VIA-5)
Noise reduction was not achieved due to the generation of new noise due to the random emission of electrons from the superlattice well layer as described in . Figure 4 shows the InAlAs when bias is applied.
FIG. 3 is an energy band diagram of a part of the /InGaAs superlattice. In this figure, 31.32 n-type InAlAs
It is composed of a barrier layer and 33 n-type InGaAs well layers. 34 is the ionization threshold energy of the well layer 33, and 35 is the conduction band discontinuity amount ΔEc of 0.55 eV. Further, 36 is an electron.

In this figure, the cause of noise newly observed in the InAlAs/InGaAs superlattice will be described. In this structure, if the electrons 36 are not ionized after entering the well layer 33 from the barrier layer 31, they try to enter the next barrier layer 32 from the well layer 33 again. At this time, the electron 36 feels energy equal to the conduction band discontinuity amount ΔEc35 as an energy barrier. It is impossible for the electrons 36 to travel and pass through the well layer 33 in a parisistic manner (without scattering) because the probability of scattering under a strong electric field is on the order of 1014/s.
Electrons 36 are scattered several times in the well layer 33 (especially valley scattering)
, the direction of the momentum vector is disordered, and the kinetic energy in the direction perpendicular to the barrier interface is lost due to energy loss due to several phonon emissions due to phonon scattering. be trapped. If this trapped electron 36 then scatters several times and stochastically acquires kinetic energy in the direction perpendicular to the barrier interface due to re-disordering of the momentum vector, or if it acquires energy due to the electric field, Electrons 36 can overcome barrier 32 and escape the trap. Since this phenomenon occurs stochastically, the noise increases because it behaves like shot noise. Therefore, if noise due to random electron emission from this newly generated superlattice well layer is generated from a large number of superlattice well layers, the a713 ratio is improved and avalanche shot noise is reduced. It is also thought that this effect will be offset. In order to alleviate such electron traps in the well layer, it has been proposed to introduce a gray-loved barrier layer (Williams et al., IEEE Electron Device Letters IEEE).

ELECTRON DEVICE LETTER8,V
OL, EDL-3, No.

3, pp. 71.1982), the problem remains that crystal growth requires precise compositional control, making the growth method complicated and difficult.

Therefore, the present invention investigates the band structure of the semiconductor that constitutes the superlattice, and designs and designs an ultra-high-speed avalanche photodiode with low noise characteristics using a simple semiconductor layer structure.
The purpose is to realize this.

(Means for Solving the Problems) The avalanche photodiode of the present invention has the following features:
The superlattice is characterized in that it is composed of a barrier layer having resonant tunneling characteristics due to a short-period superlattice, and a well layer.

(Operation) The present invention clarifies the physical mechanism that causes the drawbacks of the conventional example (InAlAs/InGaAs superlattice) by theoretically examining it, and reduces noise due to random electron emission from the superlattice well layer. The purpose is to effectively reduce noise by increasing the ionization rate ratio a/13. and,
The above configuration improves the conventional characteristics. Figure 1 shows
1 is a structural cross-sectional view showing an example of an avalanche photodiode of the present invention, in which 1 is an n++ semiconductor substrate, 2 is an n-type buffer layer, and 3 is an n''-type superlattice avalanche multiplication layer of the present invention; It consists of four n-type resonant tunnel barrier layers and five n-type well layers, which are constituent elements. Also, 6
7 is an n-type light absorption layer, 7 is an n-type cap layer, 8 is a p+ type region 9 is an n-side electrode, 1° is a p-side electrode, and 11 is an insulating protective film.

FIG. 2(a) shows an energy band diagram of a part of the superlattice of the present invention when a bias is applied. In this figure, 21.
22 are resonant tunnel barrier layers each forming a superlattice;
It is a well layer, 23 conduction band discontinuity amount ΔEc, 24 and 25
represents a quantum level ΔEq, and 26 represents an electron. Also, Figure 2 (
b) shows an energy band diagram of a part of a conventional superlattice different from the one described above when a bias is applied.

In this Fig. 2(b), 29 and 22 are barrier layers and well layers that constitute a superlattice, respectively, and the superlattice formed by the combination of these has a period so short that it forms a mini band 27 and 28. .

The present invention will be explained in detail using FIGS. 2(a), (b) and FIG. 4. Conventional example InAlAs/InG
In the aAs-based superlattice, shot noise due to random electron emission from the superlattice well layer mentioned above is ignored. The reason for this will be explained below.

In the superlattice shown in FIG. 4, as in the conventional example, the well layer 33 is Ino, 47Gao, and the 53As1 barrier layer 32 is In□.
The conduction band discontinuity amount ΔEc35 when configured with 52A10,4BAs is as large as 0.55 eV;
InAlGa which is lattice matched to InP to selectively increase the ionization of electrons 36 by energy donation by
This is because ΔEc was made as large as possible in the As system.

On the other hand, since the electrons 36 sense ΔEc as a barrier to escape from the well layer 33 and are trapped in the well layer 33 before being randomly emitted, the larger ΔEc is, the more likely noise is generated. That is, in order to use the superlattice layer as an avalanche multiplication layer, ΔEc needs to be large, but in order to reduce noise, it is necessary to simultaneously suppress the trapping of electrons 36 in the well layer 33. It can be said that it is necessary. In order to satisfy the above conditions, a structure should be adopted in which the electrons 36 can easily escape from the well layer 33 while keeping ΔEc35 as large as possible.

Therefore, the first consideration is to use a short-period superlattice as the avalanche multiplication layer, as shown in the conventional example shown in FIG. 2(b). In this figure, since the mini band 27 is formed, there is electron conduction through this band,
In addition, the practical amount of band discontinuity is the value ΔEc−ΔE obtained by subtracting the quantum level energy ΔEq24 from ΔEc23.
q, so the electrons are closer to well 2 than in the case of Figure 4.
It will be easier to escape from 2. However, in this example, since the entire multiplication layer is formed of one short-period superlattice, the region where the electrons 26 are ionized and the region where they are accelerated are separated, and the quantum levels of the electrons and holes are separated. The band gap increases by the amount of energy, resulting in an increase in the ionization threshold energy (approximately proportional to the band gap), resulting in a disadvantage that ionization becomes difficult.

The structure of the present invention shown in FIG. 2(a) was devised to solve these drawbacks of the conventional example.

In the present invention, the barrier layer 21 and the well layer 22 have resonant tunneling characteristics (based on a short-period superlattice).
It is characterized in that the accelerating region (well layer 22) is separated from the accelerating region (well layer 22). The well layer 22 has a relatively thick layer thickness to the extent that no quantum level is formed (and the layer thickness of the well layer 22 is a length such that the coherence of electron waves is strongly preserved at room temperature (several 10
0 people), the interference between the resonant tunnel barriers is reduced. ), the band gap does not increase by the energy of the quantum level, and therefore the ionization threshold energy does not increase and the ionization rate does not decrease. Moreover, since the resonant tunnel barrier 21 is used as the barrier layer, electrons 26 are transferred to the well layer 22 by electron conduction by mini-bands.
Escape from the electron beam becomes easier, and noise reduction due to random electron emission can be realized.

Judging from the above reasons, it is possible to increase the electron ionization rate by preventing noise caused by random capture and release of electrons from the well layer of the superlattice, and there is no need for precise control of the graded composition of the quaternary mixed crystal. Therefore, using a rectangular superlattice that is easy to fabricate,
In order to form the superlattice multiplication layer, a multiplication layer composed of a barrier layer and a well layer having resonant tunneling characteristics due to a short-period superlattice is used, that is, a region where electrons are ionized is separated from the resonant tunnel barrier layer 21. ) and the accelerating region (well layer 22) are separated from each other, which is newer than the conventional concept.

(Example) In the following, as an example of the present invention, an InAlGaAs/InGaAs superlattice will be used as a semiconductor material having light-receiving sensitivity in a wavelength range of 1.0 to 1.6 pm, which is an important wavelength range in optical communication, and an avalanche photodiode will be described. was created.

Using the above materials, the semiconductor light receiving element shown in FIG. 1 was manufactured through the following steps.

n+ pear-shaped InP substrate l, n-type InP buffer-rN2 to lpm thickness, carrier concentration ~2 x 1015 cm-3 n
-type In0.52AI0.4BAs/In□, 53Ga
o, superlattice avalanche multiplication layer 3 made of 47As ~
2 pm thick, carrier concentration ~2 x 1015 cm-3 n
- type In□, 53Gao, 47AS light absorption layer 6 ~ 21
With a thickness of 1 m, the carrier concentration is ~l x 1016 cm-3 of n-
An InP type cap layer 7 is sequentially grown to a thickness of lpm using metal organic vapor phase epitaxy (MOVPE).

The superlattice avalanche multiplication layer 3 consists of two layers of n-type In□, 52A10.48As layer 19 with a thickness of 70A, which satisfies the conditions of the present invention, and an n-type layer with a layer thickness of 20A. InO
, 53Ga□, 47As layer 5 In FIG. 2(a), 20 is a double resonant tunnel barrier layer 4 consisting of one layer;
1 and 400A thick n-type In□, 53Ga□, 47
The As well layer 5, 22 in FIG. 2(a), is repeatedly laminated. FIG. 3 shows the calculation results of the energy dependence of the tunnel transmission probability of this double resonant tunnel barrier layer 4. From this figure, the energy values are approximately 0.1 eV and approximately 0.4 eV.
A resonant tunnel level is formed in the InGaAs well layer 5, which facilitates electron escape from the InGaAs well layer 5.

Based on the above crystal, Zn is selectively diffused into a circular region having a diameter of 30 pm to a depth of 1 pm using a 5i02 diffusion mask to form a p+ type region 8. Insulating protective film 1 after substrate polishing
Further, the n-side electrode 9 was formed of AuGe, and the p-side electrode 10 was formed of AuZn by vacuum evaporation.

The above embodiment that satisfies the conditions of the present invention, that is, the avalanche multiplication layer has a thickness of 70 mm In0.52A10.48
A double resonant tunnel barrier consisting of an As1 layer 20 and a thickness of 4
00A n-type In0053Gao, 47As well layer 2
The case where a superlattice composed of
Ionization rate ratio a/p (α: electron ionization rate, p: hole ionization rate) and noise characteristics (excess noise figure measured at a frequency of 30 MHz and a bandwidth of IMHz) in the case of a 47As superlattice structure Effective ionization rate ratio ke found from
rr=13/α), assuming that the structure other than the superlattice layer is the same, the conventional examples In□, 52A1o, 4BAS
/Ino, 47Ga□, 53 As superlattice APD, the ionization rate ratio c/13 measured by photocurrent multiplication measurement method is ~20, which is improved from the ionization rate ratio of 2 for bulk InGaAs, but the noise characteristics are low electric field and low gain. In the double region, the effective ionization rate ratio ke (in this case 131α) is higher than that of conventional Matsukin tires.
(R,J.

McIntyre's noise theory, it appears that 10
~50, and it got bigger. This is because the noise caused by the random capture and release of electrons from the wells of the superlattice offset the reduction in noise due to the improvement of the ionization rate ratio. On the other hand, the superlattice AP consisting of the double resonant barrier l-well layer of the present invention
In D, the ionization rate ratio a/13 by photocurrent multiplication measurement method.
is ~10, which is improved from the ionization rate ratio of 2 for bulk InGaAs, and in terms of noise characteristics, the effective ionization rate ratio k''8bo=a/p is maintained from the low electric field/low multiplication region to the high electric field/high multiplication region. ) was as small as 0.1 to 0.2, noise caused by random capture and release of electrons from the superlattice wells was reduced, and noise reduction was achieved by improving the ionization rate ratio.

As described above, in this embodiment of the present invention, a low-noise APD having light receiving sensitivity at a wavelength of 1.0 to 1.6 pm can be realized, and the value of the present invention is extremely large.

In the above embodiment, InAlGaAs system was used, but the invention is not limited to this, and InGaAlAs/AlGaAsSb system, I
A structure using the nGaAsP system also works in the same way and has an outstanding effect.

(Effects of the Invention) According to the present invention, noise can be reduced by improving the ionization rate ratio and noise due to random capture and release of electrons from the well layer can be reduced, and a low-noise and ultra-high-speed avalanche photodiode can be obtained.

[Brief explanation of drawings]

FIG. 1 is a structural sectional view showing one embodiment of an avalanche photodiode of the present invention. FIG. 2(a) shows an energy band diagram of a part of the superlattice of the present invention when a bias is applied. FIG. 2(b) shows the energy band elements of a part of the conventional superlattice when a bias is applied. FIG. 3 is a diagram showing calculation results of a double resonant tunnel barrier and the energy dependence of its tunnel transmission probability in an embodiment of the present invention. FIG. 4 is an energy band diagram of a part of the superlattice multiplication layer of a conventional InAlAs/InGaAs superlattice APD when a bias is applied. FIG. 5 shows a conventional example of GaAlAs/GaAs superlattice A.
FIG. 3 is an energy band diagram of a PD when bias is applied. In each figure, 1... n++ semiconductor substrate, 2... n-type buffer layer, 3, Tl- type superlattice avalanche multiplication layer, 4... n
- type resonant tunnel barrier layer, 5... n-type well layer, 6...
... n-type light absorption layer, 7... n-type cap layer, 8.
・, p+ type region 9..., n side electrode, 10... p side electrode, 11... insulating protective film, 19... n- type InAl
As layer, 2O-n-type InGaAs layer, 21... Resonant tunnel barrier layer, 22... Well layer, 23... Conduction band discontinuity amount, 24.25... Quantum level, 26.36・・・
・Electron, 27.28... Mini band, 29... Barrier layer, 31.32... N-type InAlAs barrier layer, 33
... n-type InGaAs well layer, 34 ... threshold energy of well layer, 35 ... conduction band discontinuity amount, 41
...n++GaAs substrate, 42...n-type GaAs buffer layer, 43...n-type GaAlAs/GaAs
Superlattice avalanche multiplication layer, 44...n-type GaAlA
s barrier layer, 45-n-type GaAs well layer, 46.p+ type region 47... conduction band discontinuity amount, 48... valence band discontinuity amount, 49... well layer threshold value It's energy.

Claims (1)

[Claims] An avalanche photodiode having a superlattice layer as a multiplication layer, characterized in that the superlattice is composed of a barrier layer having resonant tunneling characteristics due to a short-period superlattice, and a well layer.