JPH02246381A - Superlattice avalanche photodiode - Google Patents

Abstract

PURPOSE:To prevent carrier in a superlattice multiplying region from staying by providing a chirp superlattice in the part of a potential barrier transferred from a well region to a barrier region, and gradually varying a band structure from the well region to the barrier region. CONSTITUTION:A superlattice multiplying region 10 for multiplying carrier is formed on a light absorption region 3, and a plurality of laminated structures of a well region 5, a chirp superlattice region 7, and a barrier region 9 are superposed in a direction separated from the region 3. An effective band structure is gradually varied in the region 7. For example, a chirp superlattice is formed of the substance A of the region 9 and the substance B of the region 5, a layer A is thinned and a layer B is thickened in a part adjacent to the region 5, the layer A is thickened and the layer B is thinned in a part adjacent to the region 9, and its intermediate is gradually varied. Then, a distribution of an effective band structure which is gradually varied from a band structure near B to a band structure near A is obtained. Thus, even if a composition gradient layer is not used, the carrier can be prevented from staying.

Description

[Detailed Description of the Invention] [Summary] Regarding a superlattice avalanche photodiode having a superlattice structure in the region where carriers are borrowed by avalanche, the present invention relates to a superlattice avalanche photodiode having a superlattice structure that does not limit the materials used and can prevent carrier retention. The object of the present invention is to provide a superlattice avalanche photodiode having a superlattice multiplication region that performs carrier multiplication using a light absorption region and a superlattice structure, wherein the superlattice multiplication region is separated from the light absorption region. along the direction of moving away,
An effective band structure having at least one set of a well region, a chirped superlattice region, and a barrier region arranged in this order, wherein the chirp superlattice region substantially smoothly connects the band structure of the well region and the band structure of the barrier region. It is configured to have the following.

[Industrial Field of Application] The present invention relates to a semiconductor photodetector, and more particularly to a superlattice avalanche photodiode having a superlattice structure in a region where carriers are borrowed by avalanche.

2. Description of the Related Art In optical communications and the like, a photodetection device with high sensitivity and fast operation speed (with excellent frequency characteristics) is desired. Avalanche photodiodes (A
PD), and in particular, superlattice APD using superlattices is expected.

[Prior Art] An avalanche photodiode according to the prior art will be described with reference to FIGS. 2A, 2B, 2C, and 2D.

FIG. 2(A) shows a conceptual cross-sectional structure.

A p-type light absorption region 55 with a narrow bandgap and a p-type carrier multiplication region 57 with high resistivity are arranged between the n+ type substrate 51 and the P+ type window region 53 to form a pin diode structure. ing. It is used by applying a reverse bias from a reverse bias source 59.

When incident light with photon energy hv enters through the window region 53, the light absorption region 55 with a narrow band gap absorbs the incident light and generates electron-hole pairs. These carriers are accelerated by the applied reverse bias, and the electrons are n
Towards the 10-shaped substrate, holes move towards the P 10-shaped window region. The electrons are accelerated by a strong electric field in the high resistivity carrier multiplication region 57 and reach a high energy state, causing avalanche multiplication, and thus an amplified photodetection signal is obtained from the diode electrode.

Energy above a certain level is required for carriers (for example, electrons) to ionize and generate new carriers. This energy is called the ion hole value energy El,
Although there is a difference between the case where a child causes ionization and the case where a hole causes ionization, the ion-hole value energy is about 3/2 times the band gap energy Eg.

As shown in FIG. 2(B), for example, in order for an electron near the bottom of the conduction band to obtain this ion-hole value energy E1,
It must be accelerated over a certain distance 'MQ' without collision.

FIG. 2(C) shows a mechanism in which ionization is facilitated by band offset (energy step) by the superlattice. Suppose we have an offset of .

The electrons entering the well layer 63 from the barrier layer 61 are instantly given energy of ΔEc. Therefore, the ion hole value energy E1 is effectively El' =
(Ei - ΔEc), and if this energy is obtained through acceleration etc., ionization can occur. By utilizing a superlattice structure having energy steps in this manner, ionization becomes easy and high efficiency can be easily obtained.

It is known that when the ionization rate of electrons is α and the ionization rate of holes is β, the larger the ratio α/β or β/α, the lower the noise and the better the frequency characteristics ( McIntyre's (14cIntyre's theory).

When a superlattice is used for the barrier layer and the well layer, if one of the conduction band energy offset ΔBc and the valence band energy offset ΔEv is large and the other is negligibly small, α/β( or β/α)
〉〉1 condition can be realized.

By the way, as shown in FIG. 2(D), a well layer 63 having a superlattice structure is defined by barrier layers 61 on both sides thereof. Even if the electrons entering from the left barrier layer 61 are ionized and the carriers are multiplied, the electrons in a low energy state move toward the boundary Δ from the well layer 63 to the right barrier layer 61.
Carriers that are blocked by the potential barrier of Ec and remain in this potential barrier are difficult to extract, slowing down the operating speed of the superlattice APD.

In order to operate a superlattice APD at high speed, it is necessary to devise a way to release stationary carriers.

Although the case where electrons cause an avalanche phenomenon has been mainly described above, the same applies to the case where a hole causes an avalanche phenomenon.

If we configure the superlattice so that the effective band offset exists only in either the conduction band or the valence band, as described above,
Retention of carriers #J Retention of either electrons or holes.

FIGS. 3A, 3B, and 3C show prior art superlattice avalanche photodiodes that include compositionally graded layers to prevent carrier retention as described above.
In the figure, (A) schematically shows the laminated structure, (B), (C)
The energy band structure of the conduction band and valence band corresponding to
The diagram schematically shows the non-biased state and the biased state.

Referring to FIG. 3(A), from left to right, AlI
A barrier layer 61 formed of a material with a relatively wide bandgap such as nAs, a well layer 63 formed of a material with a relatively narrow bandgap such as Ga1nAs, for example, a material with a relatively wide bandgap such as Ga1nAs to ^I InAs. A composition gradient layer 65 whose composition is continuously changed from a narrow bandgap composition to a relatively wide bandgap composition is laminated to form one unit.

A superlattice is formed by overlapping a plurality of stacked layers of this unit. As shown in FIG. 3(B), when looking at the conduction band Ec from left to right, the energy level decreases stepwise when moving from the barrier layer 61 to the well layer 63, and due to the presence of the composition gradient layer 65, the energy level decreases in the well layer 63. The energy level gradually increases in the middle from the to the barrier layer 61. The band offset is ΔEc in the conduction band of about 0.5 eV and ΔEv in the valence band of about 0.2 eV.Ionization is mainly performed by electrons, and by biasing the left side of the stacked structure negatively and the right side positively. As shown in FIG. 3(C), if a gradient is given to the potential downward to the right over the whole, electrons moving from left to right can move without being stopped.

In this way, an avalanche photodiode using a superlattice structure and preventing carrier retention can be obtained.

[Problem to be solved by the invention] As mentioned above, it is difficult to increase the operating speed of a simple superlattice avalanche photodiode due to carrier retention, and in order to prevent carrier retention, it is difficult to increase the operating speed. It was necessary to insert a gradient layer into the superlattice. However, in order to form a composition gradient layer, it is necessary to control the composition in addition to controlling the crystallinity and growth rate, and the manufacturing process is not easy. Especially when it comes to mixed crystals with five or more elements, composition control becomes difficult6. For example, AlInAs barrier layer, GaA
In order to create a compositionally gradient layer in a superlattice with an sSb well layer, a quinary mixed crystal of AlGa1nAsAb is required, but there is currently no suitable means for reliably forming a compositionally gradient layer of such a quinary mixed crystal. I can't find it there.

An object of the present invention is to provide a superlattice avalanche photodiode having a superlattice structure that is not limited in the materials used and can prevent carrier retention.

Another object of the present invention is to form a barrier layer of AlInAs and a barrier layer of GaA.
An object of the present invention is to provide a superlattice avalanche photodiode having a mechanism that can effectively perform the same function as a composition gradient layer in a superlattice avalanche photodiode using elements of five or more elements, such as a S5B well layer. .

[Means for solving the problem] In order to prevent carriers from staying in the superlattice, the rapid increase in energy level when moving from the well layer to the barrier layer can be effectively replaced with a gentle one. An effectively gentle energy level can be achieved by a Char-1 superlattice.

FIGS. 1(A) and 1(B) are diagrams explaining the principle of the present invention, and (
If A) uses avalanche by holes, (B)
This shows the case where avalanche by electrons is used.

In FIG. 1(A), for example, on the right side there is a substrate 1 with a wide bandgap and a light absorption region 3 with a narrow bandgap thereon, and the light incident through the substrate 1 is transmitted to the light absorption region 3.
absorb with. A superlattice multiplication region 10 for carrier multiplication is formed above the light absorption region 3 (to the left in the figure). In the superlattice multiplication region 10, a plurality of stacked layers of the well region 5, the Char-1 superlattice region 7, and the barrier region 9 are formed in a stacked manner in the direction away from the light absorption region 3.

The band structure in the superlattice multiplication region IO is as shown in the lower part of FIG.
The material of each region is selected so that the area is small, and in the chirp region 7, the part adjacent to the well region 5 has a band structure close to that of the well region, and the part adjacent to the barrier region 9 has a band structure of the barrier region. It has an effective band structure close to , and its properties gradually change in the middle. The light absorption region 3 is of n-type so as to increase the energy of holes generated by photoexcitation so that they can be easily injected into the superlattice multiplication region 10.

In FIG. 1(B), the substrate 1 is on the right and the light absorption region 3 is on the left surface, and the superlattice-enhanced ttI region 10 is between the light absorption region 3 and the substrate 1, Well region 5 and char 1 in the direction away from light absorption region 3
A plurality of stacked layers of superlattice regions 7 and barrier regions 9 are formed one on top of the other.

It is assumed that the band structure within the superlattice multiplication region 10 is as shown in the lower part of FIG. 1(B). The band offset ΔEc of the conduction band is large, and the band offset ΔEv of the valence band is large.
The material of each region is selected so that
The portion adjacent to the barrier region has an effective band structure close to the band structure of the barrier region, and the properties thereof gradually change in the middle. The light absorption region 3 is of p-type so as to increase the energy of electrons generated by photoexcitation so that they can be easily injected into the superlattice multiplication region 10.

The Char-1 superlattice whose properties gradually change as described above is
For example, it can be formed by stacking layers of substances A and B alternately while gradually increasing or decreasing the ratio of their film thicknesses.

For example, a well region is formed by a first ternary mixed crystal with a narrow bandgap, a barrier region is formed by a second ternary mixed crystal with a wide bandgap, and the first ternary mixed crystal layer and the second ternary mixed crystal layer are A chirped superlattice is formed by stacking layers in which ternary mixed crystal layers are alternately arranged and the ratio of their widths changes monotonically.

[Function] Within the chirped superlattice region 7, the effective band structure gradually changes. For example, material A in the barrier region and material B in the well region form a chirified superlattice, and the portion adjacent to the well region Then, the layer A is made thinner and the layer B is made thicker, and in the area adjacent to the barrier area, the layer A is made thicker and the layer B is made thinner.
By gradually changing the middle part, the band structure changes from a band structure close to B to A.
The effective band #RM gradually changes to a band structure close to
The distribution of a is obtained.

In this way, a structure in which the band structure effectively changes smoothly can be obtained without using a composition gradient layer.

In a superlattice avalanche photodiode, carrier retention can be prevented by providing a Char 1 superlattice as described above in a portion that creates an energy barrier when a reverse bias is applied during use.

[Embodiment] FIGS. 4A and 4B show a superlattice avalanche photodiode having a superlattice structure using GaAsSb as a well layer and AlInAs as a barrier layer according to an embodiment of the present invention.

In FIG. 4(A), an n-type light absorption region 13 made of Ga1nAs having a narrow band gap is formed on a substrate 11 of n+ type 1nP. Incident light is InP substrate 1
1 passes through the InP substrate 11 and enters the Ga1nAS light absorption region 13. Ga1nAs light absorption region 1
A superlattice multiplication region 20 is formed on the superlattice multiplication region 20 for multiplying the generated photoexcited carriers, and a p+ type AflnAs contact head region containing an impurity of, for example, about 1018cIl-3 is formed on the superlattice multiplication region 20 for contacting with the electrode. 23 and also 10
A p+ type contact region 22 containing an impurity of about 18Cr#-3 is formed. On the bottom surface of the InP substrate 11, a layer of nflll such as Au-Ge alloy is formed to surround the entrance window.
An electrode 24 is formed, and a p-side electrode 26 such as a Ti/Pt/Au layer is formed on the p+ type Ga1nAs contact region 22.

The superlattice multiplication region 20 uses non-doped GaAsSb as a well layer, non-doped AlInAs as a barrier layer, and non-doped GaAsSb/A I
It includes a chirped superlattice of InAs as a region for preventing carrier retention. When these three layers are considered as one unit, the superlattice multiplication region 20 is constituted by, for example, 20 to 30 units.

FIG. 4(B) schematically shows the band structure of a part of the superlattice multiplication region 20, where the left side is the surface (upper surface) side and the right side is the substrate side, and the band structure generated in the n-type Ga1nAs light absorption region 13. Holes enter from the right to the left.

Along the direction of movement of the holes, the GaAsSb well region 15, the GaAsSb/AlInAs chirped superlattice region 17, and the AIInAS barrier region 19 are arranged in the following order: 3W! layers are repeatedly stacked. The thickness of the well region 15 and the barrier region 19 is, for example, 100°-200°. The band offset is that the conduction band is ΔEc=0°054e
V, and the valence band is ΔEv=0.656 eV. In the chirped superlattice region 17, the thickness of the GaAsSb layer and the thickness of the AlInAs layer are selected so that the effective conduction band and valence band gradually change as shown by the broken lines, and the bands of the well region 15 and barrier region 19 are adjusted. Connect smoothly and with a gentle slope.

An example of such a chirped superlattice will be explained with reference to FIG. Layer A1 of material A and material B as a char-1 superlattice
layers Bi are stacked alternately, one set of layers AI, the thickness of Bi is I
The sum of I (Ai) and Q (B1) is constant, and the thickness of one, for example, II (AI>) gradually increases, and the thickness of the other, for example, II
Consider the case where (Bi) gradually decreases.

In FIG. 6A, a continuous layer Bo of material B is on the left and a continuous layer Ao of material A is on the right.

For example, nine pairs of ABIAI, Bl, A2, B2
..., As and B9 are connected. A layer At, A2
. . . As gradually becomes thicker from left to right, and B layers B1, B2, . . . B9 become gradually thinner from left to right.

Substance A forms a barrier layer with a wide band gap of 1, and substance B
creates a well layer with a narrow bandgap, then the sixth
The band structure of the Char-1 superlattice shown in Figure (A) is the 61st N
The result is as shown in (B). Furthermore, according to the layered structure of the chirped superlattice, which is shown in Fig. 6 (C) by enlarging a part of the band structure, the conduction band Ec and valence band Ev in each layer form a step with band offsets ΔEc and ΔEv. It is connected.

Substance A has a bandgap Eg (A) and the thickness of its layer is 1! (A), and substance B is Bandgiyang E
g (B) and the layer thickness is e (B), the effective band structure within the superlattice can be approximated by smoothing the irregularities as shown by the broken line, The effective bandgap Eg' is Eg' = [Eg(A) a
(A) 10Eg(B) e (B) ]/[e(A)
+1l(B)] It is known that it can be approximated as follows. (F, Capas
so, Appl, Phys, Lett, 45.1t93
．． (1984).

Therefore, by gradually changing @e (A) of the layer of material A from a small value to a large value and the width 1) (B) of the layer of material B from a large value to a small value, the effective band gap Eg ' is from a value close to Eg (B) to Eg (A)
It changes to a value close to . That is, by changing the thickness of the layer, a structure in which the band structure changes gradually, similar to the composition gradient layer, can be obtained. Although a chirped superlattice having nine pairs of layers is illustrated, the number of layer pairs may be arbitrarily selected depending on the purpose, for example, several tens of pairs.

Returning to FIG. 4(B), GaA with the above-mentioned properties
Due to the s5b/A11nAs Churg superlattice region 17, the band gap offset from the GaAsSb well region 15 to the AlInAs barrier region 19 is connected by a smooth change. When the holes carried from the right enter the well tank 15 from the barrier layer 19, they have a large energy corresponding to the band offset ΔEv, thereby improving the ionization efficiency. *By applying a suitable electric field, the energy barrier in the chirped superlattice region 17 disappears, so holes that have lost energy due to ionization etc. easily migrate from the well region 15 to the barrier region 19. Since no stagnant carriers are generated, the operating speed can be increased. Since the accumulated carriers can be used for ionization without being retained, ionization can be performed with high efficiency.

In this way, a superlattice avalanche photodiode with high efficiency and excellent frequency characteristics can be obtained.

The configuration described above uses five elements in the carrier multiplication region, and if you try to create a composition gradient layer and gradually connect the band offset, it will be necessary to control the five elements.
Although it has been very difficult to produce a composition gradient layer, according to this embodiment, by controlling the layer thickness of the ternary layer, it is possible to obtain the same effect as a quinary composition gradient layer.

FIG. 5 shows another embodiment of the present invention using the same material combination AlInAs/Ga1nAs used in the conventional example shown in FIG.

A carrier multiplication region 40 having a p- type superlattice structure is formed on an n+ type 1nP substrate 31, and a p-type Ga1nAS layer is formed thereon.
A light absorption region 33 and a P+ type 1nP contact/window region 42 are formed. On the bottom surface of the InP substrate 31, n
A ρ side electrode 46 is formed on the upper surface of the side electrode 44 and the p+ type 1nP contact region 42 so as to surround the incident region.

In this example, the Ga1nAS light absorption region 33 with a narrow bandgap is provided on the surface side, so the layer above it must be made of a material with a wide bandgap that transmits incident light, such as the above-mentioned InP. flhAIInAs
When light enters the Ga1nAs light absorption region 33 and electron-hole pairs are generated, the electrons are accelerated toward the substrate by the applied reverse bias electric field, and the superlattice multiplication region 40 enters and causes avalanche multiplication.

As shown in FIG. 5(B), the superlattice multiplication region 40 is arranged from the left to the right by the GaJnAS well region 35 and the Ga1n
Char1 superlattice region 37 of As/^lInAs and Al1
In the Ga1nAs/^1InAs chirb superlattice region 37, the band structure close to that of Ga1nAs changes to AlI.
The effective band structure gradually changes to a band structure close to that of nAs. The band offset is ΔE c ”r 0
, 5 e V, ΔE v"; 0.2 e V.

By positively biasing the substrate-side electrode on the inner cloth side in FIG. 5(B), it is possible to eliminate a potential barrier within the chirped superlattice 37. Electrons entering from the left are Ga1nAs
When entering the well region 35, a large amount of energy is obtained by the band offset ΔEc, resulting in high ionization efficiency. Ga1nA
Even in the s/^1InAs chirp superlattice region 37, it proceeds to the right without stopping and reaches the Al1n^S barrier region.

In this way, a superlattice APD that prevents carrier retention can be obtained without using a composition gradient layer.

[Effects of the Invention] In the superlattice avalanche photodiode, a Char-1 superlattice is provided in the part of the potential barrier that moves from the well region to the barrier region, so that the band structure gradually changes from the well region to the barrier region. , it is possible to prevent carrier retention in the superlattice multiplication region and obtain a superlattice avalanche photodiode with excellent frequency characteristics without using a compositionally gradient mixed crystal region.

[Brief explanation of drawings]

Figures 1 (A) and (B) are diagrams explaining the principle of the present invention; (A)
2 shows a case in which avalanche by holes is used, and (B) shows a case in which avalanche by electrons is used. FIGS. 2A to 2D show avalanche photodiodes according to the prior art, and FIG. (B), (C)
, (D) is a diadem for explaining the characteristics, FIGS. 3(A), (B), and (C) show other avalanche photodiodes according to the prior art, and (A) is a schematic cross-sectional view showing a laminated structure. (B) and (C) are diadems showing a band structure.
is a cross-sectional view, (B) is a diadarum showing a band structure, FIGS. 5(A) and (B) are another embodiment of the present invention, and (
A) is a cross-sectional view, (B) is a diadarum showing a band structure, and FIGS. 6(A), (B), and (C) are Char-1 superlattice used for carrier retention prevention in the embodiment of the present invention,
(A) is a cross-sectional view schematically showing a lamination concept, (B) is a diadem that schematically shows a band structure, and (C) is a partially enlarged view thereof. In the figure, 24.26 Substrate light absorption region Well region Chirp superlattice region Barrier region Superlattice multiplication region InP substrate Ga1nAs Light absorption region GaAsSb Well region GaAsSb/AlInAs Char 1 superlattice region ^11nAS Barrier region Superlattice multiplication region Ga1nAS Contact region ^ flnAs contact region electrode 44.46 c ΔEc v ΔBv g g InP substrate Ga1nAs light absorption region GaInAS twist region Ga1nAs/AlInAs Char 1 superlattice region ^11nAs barrier region superlattice multiplication region InP contact/window region Electrode Conduction band energy Offset of conduction band energy Valance band energy Offset of valence band energy Bandgap Effective bandgap distance Ionization amount value Energy n+ type substrate P+ type Window region Light absorption region Carrier multiplication region Reverse bias source Barrier layer Well Layer (A) Avalanche by holes (B) L avalanche by electrons Figure 1 (A) Cross-sectional structure (B) Noonization! i21 value engineered wood (C) Improvement by superlattice (D) Carrier retention by electric pi barrier Avalanche photodanoode by conventional technology Band structure (at no bias) (C) Band structure (at bias) Other avalanches by conventional technology; Photodiode hso (A>Cross-sectional structure alnAs alnAs (B) Band structure

Claims (1)

[Claims] (1) A superlattice avalanche photodiode having a light absorption region (3) and a superlattice multiplication region (10) that performs carrier multiplication using a superlattice structure, wherein the superlattice multiplication region (10) Along the direction away from the light absorption region (3), a well region (5), a chirped superlattice region (
7), and at least one set of barrier regions (9) arranged in this order.
1. A superlattice avalanche photodiode having a chirped superlattice region having an effective band structure that substantially smoothly connects a band structure of a well region and a band structure of a barrier region.