JPH0366179A - Avalanche photodiode - Google Patents

Abstract

PURPOSE:To reduce dark current, to improve high-frequency characteristics at the same time end to reduce an operating voltage by a method wherein an InxGa1-xAsyP1-y layer or an InxGayAl1-x-yAs layer whose band gap is larger than an energy of photons of signal light is used as a well layer of a simperlattice. CONSTITUTION:An n<+> InP butter layer 2, an undoped In0.52Al0.48As In0.8Ga0.2As0.6 P0.4 superlattice layer 3 and p-type In0.53Ga0.47As layers 4, 5, 6 are laminated on an n<+> InP substrate 1; ohmic electrodes 7, 8 of AuGeNi and AuznNi are formed. Incident light from a hole 9 which is made in the ohmic electrode comes 8, is passed through the n<+> InP substrate 1, the n<+> InP buffer layer 2 and the undoped InAlAs/InGaAs superlattice layer 3 and is absorbed by the p-type InGaAs layers 4, 5, 6 whose band gap is small. Transmitted photons are reflected by the ohmic electrode 7 and are returned to the light-absorbing layers 4, 5, 6.

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Industrial Application] The present invention relates generally to photodetectors for optical communications, and in particular to optical detectors for optical communications.
Aj! As and InGaAsP or InGaA It
The present invention relates to an avalanche photodiode (APD) having a structure in which a superlattice layer made of As is used as a carrier multiplication layer and a light absorption layer is provided in contact with the superlattice layer.

[Conventional technology]

Conventional Ge-APDs are used for avalanche photodiodes (APDs) for optical communications with wavelengths of 1.3 μm or 1.55 μm.
Alternatively, InP/InGaAs heterozygous APDs have been used. In these APDs, Ge or InP is used as a carrier multiplication layer. -APD for boat fishing
The multiplication noise becomes smaller as the ratio of the ionization rate (α and β) of electrons and holes, which are quantities specific to the semiconductor used in the multiplication layer, departs from 1. However, since this ratio is close to 1 in Ge and InP, there is a problem that noise is large.

To solve this problem, an APD structure has been proposed in which one of α or β is increased and the ratio of α/β or β/α is increased by using a superlattice structure in the multiplication layer. Only an InP/InGaAs heterojunction type or an InAl1As/InGaAs heterojunction type has been used for a superlattice APD sensitive to light with a wavelength of 1.3 μm or 1.5 μm. InAl2As/InGa
It has been confirmed that because the discontinuity of the As heterojunction conduction band is large, α becomes large, which is effective in reducing noise.

However, since the bandgap of the InGaAs layer, which is the well layer of the superlattice in this structure, is as small as 0.77 eV, when a high electric field is applied, the tunneling current to the InGaAs layer is large. A problem with superlattice APDs is that the dark current is large. Also, InGaA
Because the bandgap of s1 is small, the multiplication layer is not transparent to signal light, and therefore light has to enter from the side opposite to the multiplication layer. For this reason, the thickness d of the light absorption layer must be sufficiently large compared to the reciprocal of the light absorption coefficient α (d>1/α), and it is possible that the photons absorbed on the side far from the multiplication layer Because of the large number of carriers generated by light absorption, the carriers must travel through a thick light absorption layer, which poses a problem in that the response speed is also limited. Furthermore, since it is necessary to completely deplete this thick light absorption layer, there is a problem in that the operating voltage also increases.

[Problems to be solved by the invention]

The present invention aims to solve the above-mentioned conventional drawbacks, and to reduce dark current, improve high frequency characteristics, and lower operating voltage in an APD using a superlattice as a multiplication layer.

[Means to solve the problem]

The present invention uses an In, 1GaI- A major feature is the use of an As layer. More specifically, the superlattice APD according to the present invention has I no, szA on an n-type InP substrate.
l o, 4aA S and its lattice matching sinter I nX
Ga, -XAs, PI-y or InXGa.

A superlattice made of alternating layers of AL-x-y As (0<x<1.O<y<1) is used as a carrier multiplication layer, and in contact with this is a light absorption layer made of p-type 1no, 53Gao, and 4yAs. It is characterized by having the following.

More specifically, in the above superlattice APD, a carrier multiplication layer is provided between the InP substrate and the light absorption layer, and the InP
The feature may be that a hole is provided in the electrode of the substrate and light is incident through this hole, or the thickness d of the light absorption layer may be set to be smaller than the reciprocal of the absorption coefficient α8v of the light absorption layer (d<1/
α, V), and a metal layer is provided on the side opposite to the carrier multiplication layer via a semiconductor layer that is in contact with the metal layer or has a larger band gap than the metal layer, and reflects light. It is a structure.

The above I nXGa+-x Asy pl-y II or In
The composition ratios x and y of the five layers are applicable within the range of O<x<1 and Q<y<l.

[For production]

The operation of the lattice APD according to the present invention will now be described.

In the superlattice APD of the present invention, light enters from the carrier multiplication layer side, and the carrier multiplication layer is made of InAβA s / 1nGa.
1.3 μm or 1.5 μm due to the superlattice structure consisting of AsP or InA6As/InGaAlAs
Since it is transparent to light, the light passes through and enters the light absorption layer made of InGaAs (p-type) on the opposite side of the carrier multiplication layer. Here, a large amount of light is absorbed at a position close to the carrier multiplication layer in InGaAsM, and therefore many electron-hole pairs are generated. Therefore, carriers are oxidized in the light absorption layer near the carrier multiplication layer, so the electrons among them immediately respond at high speed, are injected into the carrier multiplication layer, are accelerated, and enter the carrier multiplication layer with this superlattice structure. This means that the avalanche is multiplied.

In particular, the light absorption layer is made of high-resistance InGaAs with a predetermined thickness.
A layer is provided in which an electric field due to an applied voltage is concentrated, and the anti-oxidized carriers are accelerated by this electric field and are injected into a superlattice structure serving as a carrier multiplication layer.

A layer with a large band gap (In
GaAsP or InGaAj! The dark current is reduced by using the As layer), resulting in an AP with low dark current noise.
Operation D is realized. In addition, the voltage applied between the main electrodes of the APD with this structure is used to apply an electric field to the layer with low impurity density in the light absorption layer, so the light absorption layer can be made thinner without reducing quantum efficiency. It operates at low voltage. Since many carriers are generated in the light absorption layer near the carrier multiplication layer, the response operation is also faster.

〔Example〕

FIG. 1 shows an embodiment of the present invention, where 1 is n”I
nP substrate, 2 is n”InP buffer layer, 3 is non-doped I no, 5zAAo, 4sAs/I n@, e G
ao, z Aso, 6P0.4 superlattice layer, 4 is impurity density 2×10110l7″, thickness 400 p-type 1no,
saG ao, avA s layer, 5 is impurity density 2
X 10"cm-" 1μm thick p-type 1 no, s:
+G ao, aqA s layer, 6 is impurity density 2X
10 layers 7 cm-', thickness 500 p-type Ino,
s+G a 0.4TA 3rd layer, 7 and 8 are each A
These are uGeNi and AuGeNi oak electrodes. The incident light enters through the hole 9 made in the AuZnNi open electrode 8, and is applied to the n''InP substrate 1゜n''InP buffer layer 2 and non-doped InAj! After passing through the As/InGaAs superlattice layer 3, 4, 5.
It is absorbed by the p-type InGaAs layer of No. 6. This I nG
The thickness d of the aAs layer (4°5.6) is the light absorption coefficient α8v
Since the photons are smaller than the reciprocal of (approximately 2 μm), all the light is not absorbed and some photons are transmitted to the AuGeNi open electrode 7, but these are reflected by the AuGeNi open electrode 7 and become light. absorption layer (4, 5, 6 p-type 1nGaAs≠ layers). Therefore, the effective thickness of the light absorption layer is doubled and almost all photons are absorbed. Pairs of electrons and holes are generated by the collection of these lights c')v&, and the electrons among these electrons travel due to the strong electric field applied in the InGaAs layer 5, and the non-doped 1. nAβA
The light is injected into the superlattice multiplier layer made of s/InGaAsP superlattice layer (multiplier layer), but since the light is incident from the multiplier layer 3 side, many electrons are generated near the multiplier layer 3. , the traveling distance to reach the multiplication layer 3 becomes shorter and the response speed becomes faster.

The most dominant factor determining dark current is tunneling current, which strongly depends on the bandgap and decreases as the bandgap increases. Therefore, the superlattice A
Dark current can be significantly reduced by making the well layer of the PD an InGaAsP layer, which has a larger band gap than In0.53Gao,4yAs.

Also, the voltage applied to the APD is the carrier multiplication layer 3
Non-doped I nAf! As/I nGaAsP superlattice layer and p-type 1nGaAs layer (light absorption layer) (3, 4, 5)
Of these, high resistance I n O, S3G with low impurity density
a O, 4? A 5 Used to apply an electric field to layer 5. Therefore, in this example, 5
Since the light absorption layer can be formed narrow and thin, the operating voltage can be reduced.

Generally, in a superlattice APD, the band discontinuity at the heterojunction interface increases the electron ionization rate, thereby reducing noise. Therefore, by increasing the bandgap of the well layer, this band discontinuity becomes smaller and the noise reduction effect is considered to be reduced.
This can be avoided by using a 4As layer. This will be explained in comparison with the case where the band gap is increased by using the well layer InGaAs layer s layer.

Figure 2 shows I nPs I no, 2 Gao, e As
o, 6 Po, 4, no, s: +G a o, ntA
S \I no, szG ao, 36A lo,
+zA S % I no, s, zA I 0.4
An energy position diagram of the conduction band and valence band of the BA S structure is shown. The conduction band of InA6As is 0.55 eV higher than that of InGaAs, while the conduction band of InP is higher than that of InGaAs.
o, szA j! o, aaA The difference is only 0.15 eV compared to S. InGaAsP and InGaAs layers having an intermediate composition ratio of the respective semiconductors
The conduction band energies of I nP and I nGa are respectively
As, In is expressed by linear approximation connecting InGaAs and InGaAs with a straight line. As is clear from FIG. 2, InGaAsP has a band gap of 0 1.3 eV and InAj! The discontinuity in the conduction band between As is the discontinuity in the conduction band between InGaAs and InAlAs. There is almost no difference from the l-continuation, and the discontinuity in the valence band almost disappears. Therefore, the electron is In/j
! While holes can obtain kinetic energy from discontinuities in the conduction band when passing through the heterointerface between As and InGaAsP, holes can hardly obtain kinetic energy. Therefore, the ratio of α/β is I nGaAs/I
It is less degraded than the nA#As superlattice.

Therefore, by using InGaAsP for the well layer, it is possible to achieve high speed and low dark current without sacrificing noise characteristics, which is the most ideal. However, due to the circumstances of the crystal growth equipment, InGaAsP and InAlAs In some cases, the growth cannot be performed using the same device. In this case, the well layer can be made of InGaAffAs, and it is possible to achieve high speed and low noise while sacrificing the noise characteristics to some extent. In other words, in this case, the first In the example shown in the figure, non-doped I n6.5zAllo, 4eAs/ I no, a
Gao, z Aso, bPo, 4 superlattice layer 3 is non-doped I no, szA I20.48A S /
I no, szG a 0.36A It o, +z
It has a structure in which the A S superlattice layer is substituted with ".

Embodiments of the invention are as follows. That is, the present invention provides I no, 5zAj2o,
asAs and lattice matched l nXGa+-x A
syP l-1 or Inx Ga, ke, -x-, A
A superlattice layer (3) in which s (Q<x<1.0<, y<1) are alternately stacked is used as a carrier multiplication layer, and in contact with this is a p-type 1nO, S3G a 0.47A! ! It is a light balanchef odd diode characterized by having a light absorption layer made of InP substrate (1) and a light absorption layer (4, 5, 6), or further provided with a carrier multiplication layer between InP substrate (1) and light absorption layers (4, 5, 6), and
An avalanche photo die characterized by providing a hole in the electrode (8) of the P substrate (1) and allowing light to enter through the hole. The present invention relates to an avalanche photodiode characterized in that a metal layer (7) 1 2 is provided on the opposite side of the carrier multiplication layer via a semiconductor layer with a large band gap to reflect light.

〔Effect of the invention〕

As explained above, in the avalanche photodiode according to the present invention, high speed and low dark current can be achieved without sacrificing noise characteristics, and the operating voltage can be reduced.

[Brief explanation of drawings]

FIG. 1 is a schematic cross-sectional structure diagram as an example of the superlattice APD according to the present invention, and FIG.
o, 6 Po, 4I no, s+Ga6.47Ass
I no, 5zGao, saA'o, tzAs% I
It is an energy position diagram of a conduction band and a valence band of no, 5zAlo, and 4sAs. 1...n4 ■n P group (anti-2...n"lnP buffer layer 3...non-doped InO, S2^9ao, 4sA S
/ I no, aGao, z Aso, b Po, S
Superlattice layer 4... impurity density 2 X I 017cm-
”, thickness 400 p-type 1 n(,, s3G ao, 4
tA s layer 5... impurity density 2 x 10''cm
-', thickness 1. crm p-type I n O, S3G a
0.4fA 3 layers 6... impurity density 2 x 101'
cm-', thickness 500 p-type I no, s3c a
0.47A S7...AuGeNi ohmic electrode 8...AuZnNi ohmic electrode 9...Light incidence window

Claims (1)

[Claims] In_0_. on an n-type InP substrate. _5_2Al_0_.
_4_8As and In_xGa_1_ lattice matched to this
−_xAs_yP_1_-_y or In_xGa_y
Al_1_-_x_-_yAs(0<x<1, 0<y<
1) is used as a carrier multiplication layer, and p-type In_0_. _5_3Ga_0. ＿４＿
An avalanche photodiode characterized by having a light absorption layer made of 7As.