JPH06232443A - Superlattice avalanche photodiode and manufacture thereof - Google Patents

Abstract

PURPOSE:To improve a low dark current and reliability by embedding mesa formed by etching and removing particular layers with a p<-> type InP or the like, thereby reducing a surface leak dark current. CONSTITUTION:Dangling bond causing an interfacial level or a surface defect is terminated at a p<-> type InP embedded layer 110 which has lattice-matching with a substrate 11, so that these interfacial level and surface defect can be considerably reduced. Also, when recrystalization growth of a semiconductor p<-> type InP embedded layer 110 is in progress at the mesa sidewall, a natural oxide film created on mesa sidewall can be removed by substrate heating or irradiation with a hydrogen radical beam in the molecular beam gaseous phase growth method, so that dangling bond even on a semiconductor natural oxide film or a semiconductor surface can be reduced. Because of this, p<-> type InP layer/interfacial defect on mesa sidewall or interfacial level have been reduced, so that a secular increase can be restricted and a semiconductor photodetector with improved element reliability can be realized.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a superlattice avalanche photodiode having low dark current and high reliability.

[0002]

2. Description of the Related Art In order to construct an optical communication system of high speed, high sensitivity and high reliability, a semiconductor light receiving element having high speed response, low dark current and high reliability is indispensable. For this reason,
Recently, low loss wavelength range of silica fiber 1.3-1.6μ
InP / InGaAs avalanche photodiodes (APDs) and pin photodiodes (pinPDs), which can be applied to m, have been actively researched for higher speed and higher sensitivity. InP / InGaAs APDs are currently gaining by reducing the capacity by reducing the light receiving system, reducing the carrier transit time by optimizing the semiconductor layer thickness, suppressing carrier traps by introducing an intermediate layer at the hetero interface, and adopting a planar structure. A high-speed and highly reliable element having a bandwidth (GB) product of about 800 GHz and a maximum bandwidth of about 8 GHz has been put into practical use.

However, in this device structure, the ionization rate ratio β / α of InP, which is the avalanche multiplication layer, is about 2 or less.
Is small (α: electron ionization rate, β: hole ionization rate ratio), the maximum value of the GB product is limited to 80 to 100 GHz, and the excess noise figure X (the smaller the ionization rate ratio, the larger). Becomes as large as ~ 0.7, and there is a limit to speeding up, noise reduction, and high sensitivity. This is the same when other bulk III-V group compound semiconductors are used for the avalanche multiplication layer, and in order to achieve high GB product (fast response characteristics) and low noise, the ionization rate ratio α / It is necessary to artificially increase β.

[0004] Therefore, F. Capasso and the like, Applied Physics Letter (Appl. P
hys. Lett. ), 40 (1), p. 38-4
In 0, 1992, we proposed a structure to artificially increase the ionization rate ratio α / β by utilizing the conduction band energy discontinuity ΔEc due to superlattice for electron impact ionization.
It was confirmed that the ionization rate ratio α / β was increased in the As / GaAlAs-based superlattice (up to 8 in the superlattice compared to ˜2 in bulk GaAs).

Furthermore, Kagawa et al., Journal of Quantum Electronics (J. Quantum.
electron. ), 28 (6), p. 1419-1
423, 1992, InGaAs having a photosensitivity in the wavelength band 1.3 to 1.6 μm used for long-distance optical communication.
A similar structure was formed using a P / InAlAs superlattice, and the ionization rate ratio α / β was also increased (bulk InGa
Reported ~ 10) in superlattice layers for ~ 2 As. The element structure is shown in FIG. In FIG. 2, 21 is an n ⁺ type InP substrate, 22 is an n ⁺ type InP buffer layer, 23 is an n ⁻ type InGaAsP / InAlAs superlattice avalanche multiplication layer, 24 is a p type InP electric field drop layer, and 25 is a p ⁻ type. InGaAs light absorption layer, 26 is p ⁺ type InP cap layer, 27 is p ⁺ type InGaAs contact layer, and 28 is n
A side electrode, 29 is a p-side electrode, and 210 is a SiN passivation film. In this superlattice structure, the conduction band discontinuity Δ
Ec of 0.39 eV and valence band discontinuity ΔEv of 0.0
Electrons are larger than holes, and the energy acquired by the band discontinuity when entering the well layer is larger than 3 eV.
This makes it easier for electrons to reach the ionization threshold energy, thereby increasing the electron ionization rate, and increasing the ionization rate ratio α / β and resulting noise reduction.

[0006]

However, in the avalanche photodiode having this structure, the semiconductor 23, 24, 25 / SiN surface passivation film 2 on the side wall of the mesa is formed.
10 interface levels, residual oxide film on semiconductor surface
Leakage current is generated through the defect, and the dark current increases to about 0.8 to several μA in the practical multiplication factor region (10 to 20), and the noise increase due to this dark current is reduced due to the improvement of the ionization ratio. It has the drawback of canceling the noise effect. In addition, this passivation interface is unstable over time under the conditions of a general reliability test (for example, an ambient temperature of 200 ° C. and a bias current of 100 μA in the reverse direction) as previously reported, and is dark. It has a drawback that the reliability of device characteristics due to an increase in current is not sufficient.

On the other hand, as shown in FIG. 3, Nakamura et al.
european Conference on Opt
ical Communication), TuC5-
4, p. 261-264, in the structure of a superlattice APD using the polyimide film 310 as a mesa passivation film, reported in 1991, leak dark current and reliability generated via interface states, residual oxide film on the semiconductor surface, and defects. Is essentially the same as the case of the example of FIG. In FIG. 3, 31 is an n ⁺ type InP substrate, 32 is an n ⁺ type InP buffer layer, and 33 is an n ⁻ type InGaAs / In.
AlAs superlattice avalanche multiplication layer, 34 is p-type InA
lAs electric field drop layer, 35 p ⁻ type InGaAs light absorption layer, 36 p ⁺ type InAlAs cap layer, 37 p ⁺
InGaAs contact layer, 38 is an n-side electrode, 39 is a p-side electrode, and 311 is an antireflection film.

Therefore, an object of the present invention is to realize a superlattice avalanche photodiode having a low dark current and a high reliability by reducing the surface leak dark current which is a problem in the mesa type pn junction photodiode.

Another object of the present invention is to provide a method of manufacturing such a superlattice avalanche photodiode.

[0010]

The present invention is a p ⁺ type InP
On the substrate, p ⁻ type InGaAs light absorption layer, p type InP electric field relaxation layer, undoped n ⁻ type InAlAs / InAlGa
As superlattice multiplication layer, n ⁺ type InP cap layer, n ⁺ type I
In a superlattice avalanche photodiode having a structure in which nGaAs (P) contact layers are sequentially laminated, the superlattice multiplication layer, n ⁺ type InP cap layer, n ⁺ type In
The mesa formed by etching away only the GaAs (P) contact layer is used as p ⁻ type InP or p ⁻ type InAl.
It is characterized by having a structure embedded with As.

A method of manufacturing a superlattice avalanche photodiode according to the present invention comprises a p ⁺ -type InP substrate and a p ⁻ -type InG.
aAs light absorption layer, p-type InP electric field relaxation layer, undoped n
^- type InAlAs / InAlGaAs superlattice multiplication layer, n
^{A +} -type InP cap layer and an n ⁺ -type InGaAs (P) contact layer are sequentially stacked to form the superlattice multiplication layer and the n ⁺ -type In layer.
Only the P cap layer and the n ⁺ type InGaAs (P) contact layer are removed by etching to form a mesa.
^- and wherein the embedding in the type InAlAs ^- -type InP or p.

[0012]

The present invention has improved characteristics as compared with the conventional example by the above-mentioned structure. FIG. 1 is a structural diagram of an element of the present invention.

In FIG. 1, 11 is a p ⁺ type InP substrate,
12 is p ⁺ type InP buffer layer, 13 is p ⁻ type InG
aAs light absorption layer, 14 p-type InP field drop layer, 15 undoped n ⁻ type InAlAs / InAlGaAs superlattice avalanche multiplication layer, 16 n ⁺ type InP cap layer, 17 n ⁺ type InGaAs contact layer, 18 n
A side electrode, 19 is a p-side electrode, and 110 is p ⁻ -type InP
The burying layer 111 is an antireflection film.

The operation of the present invention will be described with reference to FIG. 1 and FIGS. 2 and 3 showing a conventional example. In the conventional structure, the passivation film SiN film 210 or the polyimide film 31
0 and many interface states (2
X10 ¹² cm ^-2 / eV or more) is present. This interface level is a normal semiconductor / SiN film (or polyimide film)
Examples include dangling bonds at the interface, dangling bonds at the semiconductor natural oxide film / semiconductor interface formed after mesa formation, and those caused by surface defects. In particular, semiconductor layers (23 to 25,
33-35), p ^- type In having a relatively small forbidden band width
The former is also present in the GaAs light absorption layers 25 and 35, and the superlattice multiplication layer 2 containing aluminum atoms which are easily oxidized naturally.
It is considered that the latter is abundant among 3,33. Therefore, in the conventional structure, under the reverse bias, a surface leak dark current is generated via these interface states, and it becomes on the order of μA for increasing the electric field to obtain a practical multiplication factor. Further, dark current deterioration occurs due to an increase in these interface states and surface defects over time, and it is difficult to obtain sufficient reliability of the device.

On the other hand, in the structure of the present invention shown in FIG. 1, the dangling bonds that cause the above-mentioned interface states and surface defects are terminated by the p ^-- type InP buried layer which is lattice-matched with the substrate. , These interface states and surface defects are significantly reduced. Further, when the semiconductor p ⁻ type InP buried layer is recrystallized on the mesa side wall, the molecular beam vapor deposition method (M
In the BE method), by heating the substrate or irradiating a hydrogen radical beam, the natural oxide film formed on the side wall of the mesa can be removed, so that dangling bonds at the semiconductor natural oxide film / semiconductor interface can be reduced. At this time, p ^- type I
The nP buried layer forms a pn junction with the n ⁺ type contact layer 17, the n ⁺ type cap layer 16, and the undoped n ⁻ type superlattice multiplication layer 15, and a depletion layer is formed by applying a reverse bias. , P concentration is set to about 1 × 10 ¹⁶ cm ⁻³ , the electric field strength of InP tunnel breakdown voltage or avalanche breakdown voltage (both are multiplication of superlattice multiplication layer) even when a reverse bias electric field is applied. The leakage current can be suppressed to about several tens of nA up to the electric field strength).

Due to the above effects, the surface leakage dark current is reduced in the structure of the present invention as compared with the conventional mesa type photodiode structure in which the surface protection film is the SiN film or the polyimide film. Furthermore, since the defects and interface states of the p ⁻ -type InP layer / mesa side wall interface are originally significantly reduced, their increase over time is suppressed as compared with the conventional case.
A semiconductor light receiving element having improved element reliability can be realized.

[0017]

EXAMPLE An InAlAs / InAlGaAs superlattice avalanche photodiode which is lattice-matched to InP will be described below as an example of the present invention.

The semiconductor light receiving element of the present invention shown in FIG. 1 was manufactured by the following steps. On the p ⁺ type InP substrate 11, the p ⁺ type InP buffer layer 12 having a thickness of 0.5 μm,
Carrier concentration ~ 2 x 10 ¹⁵ cm ^-3 p ^- type InGaAs
The thickness of the light absorption layer 13 is 1 μm, and the carrier concentration is up to 5 × 10 ¹⁷
cm ⁻³ p ⁺ type InP electric field drop layer 14 is 0.2 μm, carrier concentration is up to 1 × 10 ¹⁵ cm ⁻³ undoped n ⁻ type In.
The AlGaAs / InAlAs superlattice multiplication layer 15 is 0.2
The n ⁺ -type InP carrier layer 16 having a carrier concentration of 5 × 10 ¹⁸ cm ^{−3 has} a thickness of 0.5 μm, and the n ⁺ -type InGaAs contact layer 17 having a carrier concentration of 1 × 10 ¹⁹ cm ⁻³ has a thickness of 0.3 μm. The film is sequentially grown to a thickness of 1 μm using a gas source molecular beam growth method (gas source MBE).

Next, a circular mesa having a diameter of 50 μm is formed by using ordinary photolithography and wet etching techniques. The etching depth of this mesa etching is
The depth at which the upper surface of the p ⁺ type InP field drop layer 14 is exposed, that is, 0.83 μm is set. A p ⁻ -type InP burying layer 110 is regrown to about 0.5 μm on this wafer by using a gas source MBE. Before re-growth, the native oxide film on the semiconductor surface is removed by a method such as substrate heating or hydrogen radical beam treatment in the growth apparatus.

As described above, the antireflection film 111 is formed of a SiN film on the surface of the obtained wafer, and the p-side electrode 19 is AuZ.
n. Finally, after backside polishing, n-side electrode 1
8 is formed of AuGeNi.

[0021]

The structure of the above embodiment and the conventional SiN
We compared the high-speed response characteristics, dark current characteristics, and device reliability of the structure in which a mesa side wall passivation film was applied to either a film or a polyimide film. Regarding the high-speed response characteristics, there was no difference between the two, and a high-speed property of a GB product of about 120 GHz was confirmed, while the initial value of the dark current was smaller in the device of this example than in any of the conventional examples. Values were obtained (compare with same mesa diameter). Furthermore, regarding the element reliability, a reliability test (for example, an ambient temperature of 200
Even after 1000 hours under a bias condition of a reverse current of 100 μA and a reverse current of 100 ° C., in the conventional example, an increase in dark current was often observed, whereas in the present invention, deterioration of device characteristics due to an increase in dark current was almost observed. There wasn't.

Therefore, according to the present invention, the wavelength of 1.3 to
Has a photosensitivity in the 1.6 μm band and a high ionization ratio α / β
Thus, it is possible to realize a highly reliable avalanche photodiode having a low surface leak dark current as well as low noise and high speed response characteristics, and its effect is great.

[Brief description of drawings]

FIG. 1 is a structural diagram of a superlattice avalanche photodiode of the present invention.

FIG. 2 is a structural diagram of a conventional superlattice multiplication layer.

FIG. 3 is a structural diagram of a conventional superlattice multiplication layer.

[Explanation of symbols]

11 p ⁺ type InP substrate 12 p ⁺ type InP buffer layer 13 p ⁻ type InGaAs light absorption layer 14 p type InP wide gap field drop layer 15 undoped n ⁻ type InAlAs / InAlGaA
s Superlattice avalanche multiplication layer 16 n ⁺ type InP cap layer 17 n ⁺ type InGaAs contact layer 18 n side electrode 19 p side electrode 110 p ⁻ type InP buried layer 111 antireflection film 21 n ⁺ type InP substrate 22 n ⁺ type InP buffer layer 23 n ^- type InGaAsP / InAlAs superlattice avalanche multiplication layer 24 p-type InP field drop layer 25 p ^- type InGaAs light absorption layer 26 p ⁺ type InP cap layer 27 p ⁺ type InGaAs contact layer 28 n-side electrode 29 p-side electrode 210 SiN passivation film 31 n ⁺ type InP substrate 32 n ⁺ type InAlAs buffer layer 33 n ⁻ type InGaAs / InAlAs superlattice avalanche multiplication layer 34 p type InAlAs electric field drop layer 35 p ⁻ type InGaAs light absorption layer 36 p ⁺ Type InAlAs cap layer 37 p ⁺ Type InGaAs contact layer 38 n-side electrode 39 p-side electrode 310 polyimide passivation film 311 antireflection film

Claims (4)

[Claims] 1. In a mesa-type superlattice avalanche photodiode, a superlattice multiplication layer constituting a mesa, an n ⁺ -type I
A superlattice avalanche photodiode characterized in that an nP cap layer and an n ⁺ -type InGaAs (P) contact layer are filled with p ⁻ -type InP or p ⁻ -type InAlAs. 2. A p ⁺ -type InP substrate and a p ⁻ -type InGaAs
Light absorption layer, p-type InP electric field relaxation layer, undoped n ^- type I
nAlAs / InAlGaAs superlattice multiplication layer, n ⁺ type I
A superlattice avalanche photodiode having a structure in which an nP cap layer and an n ⁺ -type InGaAs (P) contact layer are sequentially stacked, wherein the superlattice multiplication layer, the n ⁺ -type InP cap layer, and the n ⁺ -type I
The mesa formed by etching away only the nGaAs (P) contact layer is used as p ⁻ type InP or p ⁻ type InA.
A superlattice avalanche photodiode characterized by having a structure embedded with lAs. 3. A p ⁺ -type InP substrate on which p ⁻ -type InGaAs is formed.
Light absorption layer, p-type InP electric field relaxation layer, undoped n ^- type I
nAlAs / InAlGaAs superlattice multiplication layer, n ⁺ type I
An nP cap layer and an n ⁺ -type InGaAs (P) contact layer are sequentially stacked, and only the superlattice multiplication layer, the n ⁺ -type InP cap layer and the n ⁺ -type InGaAs (P) contact layer are removed by etching to form a mesa. And use this mesa as p ^- type I
A method for manufacturing a superlattice avalanche photodiode, which comprises burying with nP or p ⁻ type InAlAs. 4. The method of manufacturing a superlattice avalanche photodiode according to claim 3, wherein the natural oxide film formed on the side surface of the mesa is removed before the mesa is buried.