JPH04112582A - Avalanche photodiode - Google Patents

Abstract

PURPOSE:To enable low noises as well as high-speed response characteristics at a high ionization rate ratio by satisfying required conditions by first, second and third semiconductor layers. CONSTITUTION:An n-type InP buffer layer 2, a superlattice layer 3 composed of the repeated structure of n<-> type In0.52Al0.48As4, In0.53Ga0.47As5 and GaxAl1-xSby6, a p<+> type In0.52Al0.48As field-drop layer 7, a p<-> type In0.53Ga0.47As optical absorption layer 8 and p<+> type InP cap layer 9 are grown successively on an n<+> type InP substrate. The composition (x) of the GaxAl1-xSby of the superlattice layer is determined by the dependency of the Al composition ratio of a conduction band end and a valence band end to In0.53Ga0.47As of Ga1-xAlxAs1-ySby6 at a time when a lattice constant is conformed to In0.53Ga0.47 As. Specified conditional formula is satisfied when 0.48<x<0.92 holds. In the structure, the ionization rate of holes is decreased though bulk In0.53Ga0.47As is increased in the ionization rate of electrons, an ion ionization rate ratio is made larger than bulk InGaAs, and an excess noise index is also noise- reduced. The speed of response is not decreased remarkably. According to the constitution, high-speed response characteristics as well as low noises can be realized at a high ionization rate ratio alpha/beta.

Description

DETAILED DESCRIPTION OF THE INVENTION (Field of Industrial Application) The present invention relates to an avalanche photodiode (APD) having high speed and low noise characteristics.

(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/InGaA, which can be applied to the low-loss wavelength range of 1.0 to 166 μm for silica-based fibers, has recently been developed.
Research into increasing the speed and sensitivity of S-based avalanche photodiodes (APDs) is active. This I
Currently, nP/InGaAs APDs are able to improve the gain bandwidth (GB) by reducing the capacitance by reducing the diameter of the vesicle, by reducing the carrier transit time by optimizing the layer thickness, and by 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 βl of InP, which is an avalanche multiplication layer, is
Since α is small at ~2 (a = electron ionization rate, β: hole ionization rate), the excess noise index・There are limits to increasing sensitivity. This is the other bulk I
The same is true when an ILV group compound semiconductor is used in the avalanche multiplication layer, and in order to achieve low noise and high GB stacking (high-speed response characteristics), the ionization rate ratio must be adjusted. It is necessary to artificially increase ψ.

Therefore, Capasso (F, Capasso) et al.
Lett, ), vol. 40(1), p. 38-40.19
In 1982, the conduction band energy discontinuity ΔE due to the superlattice
Using c for electron collision ionization, the ionization rate ratio a/β
proposed a structure that artificially increases GaAs/
An increase in the ionization rate ratio αψ was confirmed in the GaAlAs diameter superlattice (~8 in the superlattice layer compared to ~2 in bulk GaAs). Furthermore, Kagawa et al., Applied Phys. Lett, p.

993-995.55 (10), 1989, a similar structure was developed using an InGaAs/InAlAs superlattice having light-receiving sensitivity in the wavelength band of 1.0 to 1.6 □m, which is used for long-distance optical communications. , and the ionization rate ratio αψ also increases (~1 in the superlattice layer compared to ~2 in bulk InGaAs).
0) was confirmed. FIG. 5 shows an energy band diagram of the avalanche multiplication layer when bias is applied. 41 is n-
Type Ino 5□A1o48As barrier layer, 42 is n-type Ino, 53GaO, 47As well layer, layers 41 and 4
The repetition of 2 constitutes a superlattice avalanche multiplication layer. In addition, 43.44 is the conduction band discontinuity amount ΔEc
, the valence band discontinuity amount ΔEv. Further, 45 and 46 are electrons and holes, respectively. In this structure, the conduction band discontinuity amount ΔEc is 0.5 eV and the valence band discontinuity amount ΔEv is 0.5 eV.
2 eV, the energy acquired by the electron due to the hand discontinuity when entering the well layer is larger, and this makes it easier for the electron to reach the ionization threshold energy, increasing the electron ionization rate, and increasing the ionization rate ratio α /
We are trying to increase β.

(Problems to be Solved by the Invention) However, in the avalanche photodiode with this structure, an energy barrier exists due to multiple band discontinuities ΔEc and ΔEv in the superlattice avalanche multiplication layer, and the valence especially for holes with a large effective mass exists. Due to the electronic band discontinuity ΔEv, multiplication carriers generated in the superlattice during high multiplication (
This has the drawback that the frequency response characteristics of 2 to 3 Gb/s or more deteriorate due to trapping of holes (mainly holes).

Figure 6 is a band energy diagram of a conventional superlattice avalanche multiplication layer, where CI, C2, and C4 are the conduction band edges of the first, second, and fourth semiconductors, respectively') V1,
V2°■4 indicates the respective valence band edges. (The left end of each is negatively biased.) In the band energy diagram of the conventional structure shown in FIG. 6(a), the amount of discontinuity ΔEv at the valence band edge of the first type semiconductor and the second type semiconductor is 1. Ino, 52A1o, 5aAS, /Ino, which is a material system sensitive to 0 to 1.6 pm.
Taking the 53Gao 4-tAS superlattice as an example, it is 0.2eV.
When holes generated by avalanche multiplication travel through a superlattice in which a large number of energy barriers exist, they are trapped by these barriers and the response speed deteriorates to about 3 GHz or less. To avoid such response deterioration, the valence band discontinuity should be changed to the first
It is conceivable to insert a fourth type semiconductor layer having a mixed crystal structure of a first type semiconductor and a second type semiconductor, which have an intermediate value (lattice matching) between the seed semiconductor and the second type semiconductor. However, this I
no, 52AIo,, asAS/In, o 53Gao
, in the 4-tAS system, such valence band discontinuity amount △E
InAlGaAs with vO of 1 eV, the conduction band discontinuity amount is as small as 0.28 eV, so it cannot be used in a superlattice avalanche multiplication layer where the larger the conduction band discontinuity amount is, the more advantageous it is to selectively ionize electrons. I can't.

Therefore, an object of the present invention is to realize an avalanche photodiode that has light receiving sensitivity in the wavelength band of 1.0 to 1.6□m, has a high ionization ratio αψ, low noise, and has high-speed response characteristics. .

(Means for Solving the Problems) The avalanche photodiode of the present invention has a barrier layer (first semiconductor layer), a well layer (second semiconductor layer), and a third semiconductor layer that are repeated in order in the direction of an applied electric field. A semiconductor layer is included in at least a portion of a semiconductor superlattice layer, the semiconductor superlattice layer is an avalanche multiplication layer, and the first, second, and third semiconductor layers satisfy the following condition (1). The special condition (1) is that the electron affinity of the barrier layer which is the first semiconductor layer is χ1, the forbidden band width is Eg1, the electron affinity of the well layer which is the second semiconductor layer is χ2, and the forbidden band width is Eg2. , when the electron affinity of the third semiconductor layer is χ3 and the forbidden band width is Eg3, χ2>χl>χ3 and χ1+Eg1>χ3+Eg3>χ20Eg2 are satisfied.

(Function) FIG. 2 shows a band diagram of the superlattice multiplication layer of the avalanche photodiode of the present invention. The present invention can reduce the amount of discontinuity in the valence band without causing a large decrease in the amount of discontinuity in the conduction band as described above.

The reason for this will be explained using FIGS. 2 and 3.

Ino of Ga1-xAlxAsl-YSbY when the lattice constant is matched to InO, 53GaO, and 4□As shown in Fig. 3 (that is, when InP also has the same lattice constant)
Looking at the diagram of the A1 composition ratio X dependence of the conduction band edge C1 valence band edge V for 53Gao and 47AS, Ga1-xA
IXAsl-YSbY satisfies conditional expression (1) of the present invention when its A1 composition ratio X is 0.48 or more and 0.92 or less.

Then, at X=0.7, the valence band discontinuity ΔEv is exactly Ino 53Gao, 47AS and Ino, 52A1
The value is 0.1 eV, which is an intermediate value between 0.2 eV and 4sAS. Such a semiconductor layer (third type semiconductor) is formed into a superlattice well layer (
When provided on the hole exit side (see FIG. 2, bias is negative on the left side) of a type 2 semiconductor), it has the effect of suppressing trapping of holes into the heterobarrier.

On the other hand, the conduction band edge C increases with increasing A1 composition,
When the composition X=0.7, the amount of conductor discontinuity ΔEc is 0.
．． It becomes 77eV. In the structure of the present invention, as can be seen from FIG. 2, this semiconductor layer is placed on the entrance side to the well layer with respect to electrons, and the conduction band discontinuity amount ΔE
It can be said that the larger c is, the more suitable the superlattice avalanche multiplication layer is, which is advantageous for selective ionization of electrons. The amount of conductive discontinuity ΔEc between the conventional barrier material Ino 5°Alo 4sAs and this semiconductor is as small as 0.27 eV, and this barrier does not hinder the movement of electrons. Also, even if
Even if this conductor discontinuity amount △Ec is large, the third
If the seed semiconductor layer is as thin as about 70 to 100 layers, electrons with a small effective mass can pass through this layer due to the tunnel effect, so no problem arises in the response characteristics.

Furthermore, the conductor discontinuity ΔE on the exit side from the electron well layer
c is Ino, 52A1o 4sAS/Ino 53Ga
Since the value remains the same as that of the 47AS system, electrons are not trapped in this energy barrier and the frequency response is not degraded, as in the conventional example (FIG. 6(a)).

The layer thickness of the third type semiconductor layer of the present invention is set to such a value that the hole energy distribution reaches a steady value due to drift in the layer in order to alleviate the valence band discontinuity ΔEv of the conventional example.
In other words, a minimum of 100 people is required. This value is consistent with the above-mentioned structure in which a tunneling effect for electrons is obtained.

As a result of the above effects, the structure of the present invention can avoid trapping and deterioration of response speed due to hole valence band barriers without impairing the effect of increasing electron ionization rate due to reduction in conduction band discontinuity. In this way, it has light receiving sensitivity in the wavelength band of 1.0 to 1.6 μm, and the ionization rate ratio. By improving ψ, it is possible to realize an avalanche photodiode that simultaneously achieves low noise characteristics and high-speed response characteristics.

(Example) Hereinafter, as an example of the present invention,
no5□Alo48As/Ga1-xAIXAsl-8
This will be explained using a Sb/Ino53Gao4°As superlattice avalanche photodiode.

An avalanche photodiode according to the present invention shown in FIGS. 1 and 2 was manufactured by the following steps.

A 1μ thick n-type InP buffer layer 2 is placed on the meter-shaped InP substrate 1.
Ino, 52A1o 4sAS4, Ino 53G, n-type with carrier concentration ~IX1-5cm-3 in m thickness
ao, 4-rAS5, GaxA1□xAS1ySbY
The superlattice layer 3 consisting of a repeating structure of 6 and 6 is 1.2 □m thick, and the p+ type Ino, 52A1o, 4sAs electric field drop layer 7 with a carrier concentration of ~I x 1017 cm-3 is 0.2.
4zm, p-type I with carrier concentration ~2015cm-3
no 5aGao, 4-tAS light absorption layer 8 ~1.7.
zm thickness, carrier concentration ~5X1018cm-3 p+
An InP type cap layer 9 is sequentially grown to a thickness of 1 μm using metal organic vapor phase epitaxy (MOVPE). This superlattice layer consists of a 200-thick Ino53Gao4□AS4 and a 100-thick GaXA11-xAsl-YSbY (X=
0.7) 5, thickness 300 people ■nO, 52A1o48A
It has a structure in which s5 layers are alternately stacked for 20 periods.

In order to suppress the trapping of holes to the hetero interface, the composition Ino 53Ga of Ga1-xAIXAsl-YSbY
o, A1 of conduction band edge C1 valence band edge V for 47AS
From the diagram of composition ratio X dependence, the amount of valence band discontinuity ΔEv is I
no, 53Gao, 47AS and Ino5゜Alo48A
It was determined based on the composition region where the value of s is in the middle of 0.2 eV.

Next, a circular mesa with a diameter of 5011 m was formed using conventional photolithography and wet etching techniques.
An insulating protective film 1.2 is formed. The p-side electrode 11 is made of AuZn.
After forming the n-side electrode 10 by polishing the back surface, the n-side electrode 10 is
It was formed from uGe. In this way, the device of the first example was completed.

In the structure of this example, the electron ionization rate is bulk n
While the hole ionization rate increased to about 1.5 times that of O, 53GaO, and 4°As, the hole ionization rate decreased to about 173 to 115 times that of bulk InGaAs, and the ionization rate ratio was about 10, which is about 10 times higher than that of bulk InGaAs. The noise was increased compared to 2 of InGaAs, and the excess noise figure was also lowered to ~0.3. Moreover, the frequency response characteristics of Ga1-xAlxA of the present invention are
When compared with a conventional example using the same superlattice layer structure as this example except for the absence of the Sl-YSbY layer, the wavelength was 1.55.
When the multiplication factor at /1m incidence is 10, in the conventional example, the trapping of holes in the superlattice heterobarrier causes a 3dB degradation band of 3GH.
(When the multiplication factor was about 1.5, the 3 dB degradation band was about 7 GHz in both the conventional example and this example), whereas the structure of the present invention maintains about 7 GHz. No significant deterioration in response speed was observed.

As a second embodiment of the present invention, I
no, 52A1o, 48AS/Ga t -xAlxA
The Sl-YSby/Ino 53Gao, 17AS superlattice avalanche photodiode will be described.

An avalanche photodiode according to a second embodiment of the present invention shown in FIG. 4 was manufactured by the following steps.

A p+ type InP layer 3 is formed on a p+ type InP substrate 31.
2 to 1/, m thickness, carrier concentration ~2"cm-3 p-type Ino53Gao, <?As light absorption layer 8 ~L7
p+ type ■nO, 52A10.48"" electric field drop layer 7 of 0.2 μm thickness with carrier concentration ~1017 cm-3
m, carrier concentration ~ 1"cm-3 n-type work n.5
□A1o48As4/GaxA11-xAsl-YSb
Superlattice layer 3 made of Y61Ino53Gao4°As5
An n+ type 1nP capping layer 33 with a carrier concentration of ~5 cm and a thickness of 1 μm is sequentially grown using metal organic vapor phase epitaxy (MOVPE). Layers are 300 thick Ino, 52A1o, 4sA
s4, thickness 100 people GaxAll-xASl-YSb
Y(X=0.7)6, thickness 200 people Ino 5sGa
It has a structure in which 20 cycles of 47As5 are alternately stacked.

The composition X of GaxAll-XAs1-YSbY was determined for the same reason as in the first example.

Next, a circular mesa having a diameter of 50 μm is formed using conventional photolithography and wet etching techniques, and an insulating protective film 12 is formed on the substrate surface. The n-side electrode 10 is
After forming with uGe, the back surface is polished, and then an insulating protective film 12 with a diameter of 150 mm is coated on the mesa portion of the back surface of the substrate as a non-reflective film.
After forming a μml circular shape by photolithography, p
The mold electrode 11 was formed of Au and Zn.

In this embodiment, the structure is such that the incident light 34 enters from the back surface of the substrate.

The avalanche photodiode of this second embodiment also has a high-speed response characteristic of about 7 GHz when the multiplication factor is 10.
The response characteristics were improved by more than twice compared to the conventional example.

(Effects of the Invention) According to the present invention, an avalanche photodiode is realized which has light receiving sensitivity at a wavelength of 1.0 to 1.6 μm, has a high ionization rate ratio a/β, has low noise, and has high-speed response characteristics. It can be done, and the effect is great.

[Brief explanation of the drawing]

FIG. 1 is a structural sectional view of one embodiment of an avalanche photodiode of the present invention. FIG. 2 is an energy band diagram of the superlattice avalanche multiplication layer of the present invention. (bias the left end to negative)
. Figure 3 shows Ga1-xAlxAsl-YS when the lattice constant is matched to Ino, 53Gao 4□AS, and InP.
A diagram showing the dependence of the conduction band edge C1 valence band edge V on the A1 composition ratio X for Ino 53Gao 4-tAS of bY. FIG. 4 is a structural sectional view of an avalanche photodiode according to a second embodiment of the present invention. FIG. 5 is an energy band diagram of a conventional superlattice avalanche multiplication layer when a bias is applied. FIG. 6 is an energy band diagram of a conventional superlattice avalanche multiplication layer. In each figure, 1...n+ type semiconductor substrate, 2...n type buffer layer,
3... N-type superlattice avalanche multiplication layer, 4... First type semiconductor layer, 5... Second type semiconductor layer, 6... Third type semiconductor layer.
Seed semiconductor layer, 7...p+ type wide gap electric field drop layer, 8...p
- type light absorption layer, 9...p+ type cap layer, 10...
n-side electrode, 11... p-side electrode, 12... insulating protective film, 21.45... electron, 22.46... hole, 41
. . . barrier layer, 42 . . . well layer, 43 . . . conduction band discontinuity amount ΔEc, 441.・Valence band discontinuity amount ΔEv,

Claims (1)

[Claims] At least a portion of a semiconductor superlattice layer includes semiconductor layers in which a barrier layer (first semiconductor layer), a well layer (second semiconductor layer), and a third semiconductor layer are repeated in the direction of an applied electric field. , the semiconductor superlattice layer is an avalanche multiplication layer, and the first, second,
and an avalanche photodiode, wherein the third semiconductor layer satisfies the following condition (1). Condition (1) The electron affinity of the barrier layer which is the first semiconductor layer is χ_1, the forbidden band width is Eg_1, the electron affinity of the well layer which is the second semiconductor layer is χ_2, the forbidden band width is Eg_2, and the electron of the third semiconductor layer is When the affinity is χ_3 and the forbidden band width is Eg_3, χ_
2>χ_1>χ_3 and χ_1+Eg_1>χ_3+Eg_3>χ_2+Eg_
2 must be satisfied.