JPH07202252A - Superlattice avalanche photodiode - Google Patents

Abstract

PURPOSE:To realize a superlattice avalanche photodiode having low dark current and high reliability by reducing surface leakage dark current which becomes a problem in a mesa type superlattice avalanche photodiode. CONSTITUTION:An n-type InP cap layer 16 of a mesa structure in which a p<-> type InGaAs light absorption layer 13, a p<+> type InP field-relief layer 14, an undoped i-type or n-type InAlGaAs/InAlAs superlattice multiplying layer 15, an n-type InP cap layer 16 and an n-type InGaAs contact layer 17 are sequentially laminated via a p<+> type InP buffer layer 12 and an n<+> type diffused region 110 on part of the layer 17 are selectively formed on a p<+> type InP board 11.

Description

Detailed Description of the Invention

[0001]

FIELD OF THE INVENTION The present invention relates to a superlattice avalanche photodiode for optical communication.

[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 optical fiber 1.3 to 1.6
Research on high-speed and high-sensitivity InP / InGaAs avalanche photodiodes (APDs) and pin photodiodes (pinPDs) applicable to μm is active. At present, InP / InGaAs APDs have a gain bandwidth (GB) product of about 80 GHz and a maximum bandwidth of 8
High-speed and high-reliability elements of about GHz have been put into practical use.

However, in this device structure, the ionization ratio of InP, which is the avalanche multiplication layer, is β / α (α:
Since the electron ionization rate and β: hole ionization rate ratio are as small as about 2, the maximum value of the GB product is limited to about 80 to 100 GHz, and the excess noise figure X (the smaller the ionization rate ratio is, the larger it is. ) Becomes as large as about 0.7, and there is a limit to speeding up and low noise and high sensitivity. This is the same when other bulk 3-5 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 increase β intentionally.

[0004] Therefore, F. Capasso and the like, Applied Physics Letters (Appli
ed Physics Letters), 1982,
In Vol. 40, No. 1, pp. 38-40, we proposed a structure that artificially increases the ionization rate ratio α / β by utilizing the conduction band energy discontinuity ΔEc by superlattice for electron impact ionization. It was confirmed that the ionization rate ratio α / β was increased in the GaAs / GaAlAs superlattice (about 8 in the superlattice compared to about 2 in the bulk GaAs).

In addition, Kagawa et al., Journal of Quantum Electronics (Journal of
Quantum Electronics) 1992
, Vol. 28, No. 6, pp. 1419-1423, a similar structure is formed using an InGaAsP / InAlAs superlattice having a photosensitivity in the wavelength 1.3-1.6 .mu.m band used for long-distance optical communication. However, it also reported an increase in the ionization ratio α / β (about 10 for the superlattice layer compared to about 2 for bulk InGaAs). The device structure is shown in FIG. In this superlattice structure, the conduction band discontinuity ΔEc is 0.39 eV, which is larger than the valence band discontinuity ΔEv of 0.03 eV, and the energy acquired by the band discontinuity when entering the well layer is that the electron is a hole. The electron ionization rate is increased by increasing the value, which makes it easier for electrons to reach the ionization threshold energy, thereby increasing the ionization rate ratio α / β and thereby reducing noise.

[0006]

However, in this conventional superlattice avalanche photodiode, the interface order at the semiconductor 33, 34, 35 / SiN surface passivation film 310 interface on the mesa side wall, the residual oxide film / defects on the semiconductor surface are eliminated. Through which leakage current will increase and increase,
In a practical multiplication factor region (10 to 20), the dark current is on the order of 0.8 to several μA, and the noise increase due to this dark current has a drawback that the low noise effect due to the improvement of the ionization ratio is canceled.

Further, as previously reported, this passivation interface is unstable over time under general reliability test conditions (for example, an ambient temperature of 200 ° C. and a reverse current of 100 μA bias condition). However, there is a drawback that the reliability of device characteristics due to an increase in dark current is not sufficient.

On the other hand, as shown in FIG. 4, Nakamura et al.
On Optical Fiber Communication (P
rosecedings of European Co
nference on Optical-fiber
Communication) TuC5-4, 261
-Page 264, polyimide film 410 reported in 1991
Even in the structure of a superlattice APD using a mesa passivation film, the problem of leak dark current / reliability caused by the interfacial interface order, residual oxide film / defects on the semiconductor surface at high electric field, and the conventional example of FIG. It is essentially the same as the case of.

On the other hand, as shown in FIG.
In the planar type element described in Japanese Patent No. 478, the superlattice multiplication layer 55 must be formed by p ⁻ type doping with a carrier concentration of 10 ¹⁶ cm ⁻³ or less, which is currently used. Then, there is a problem that it is difficult to form such a mixed crystal containing aluminum that has been subjected to such low concentration p-type doping with good reproducibility without impairing the crystallinity.

An object of the present invention is to provide a superlattice avalanche photodiode having a low dark current and a high reliability, which reduces a surface leak dark current which is a problem in a mesa type pn junction photodiode.

[0011]

A superlattice avalanche photodiode according to the present invention comprises a high-concentration one-conductivity type buffer layer and a high-concentration one-conductivity type buffer layer sequentially formed on a high-concentration one-conductivity type InP substrate. A mesa-shaped laminated structure consisting of a light absorption layer, a one conductivity type electric field relaxation layer, an undoped or low concentration reverse conductivity type superlattice layer, and a low concentration reverse conductivity type cap layer, and formed in the reverse conductivity type cap layer It has a high-concentration opposite conductivity type diffusion region.

[0012]

DESCRIPTION OF THE PREFERRED EMBODIMENTS Next, the present invention will be described with reference to the drawings.

FIG. 1 is a sectional view showing an embodiment of the present invention.

As shown in FIG. 1, p ⁺ type InP substrate 11
On top of the p ⁺ -type Inp buffer layer 12 having a thickness of 0.5 μm
(Band gap Eg ₁ ) and p ⁻ type InGaAs light absorption layer 1 having a thickness of 1 μm and a carrier concentration of about 2 × 10 ¹⁵ cm ^−3.
3 (bandgap Eg ₂ ), a p ⁺ type InP electric field relaxation layer 14 (bandgap Eg ₃ ) having a thickness of 0.2 μm and a carrier concentration of about 5 × 10 ¹⁷ cm ⁻³ , and a carrier concentration of 0.23 μm. An undoped i-type or n ^- type InAlGaAs / InAlAs superlattice multiplication layer 15 of about 1 × 10 ¹⁵ cm ^-3 and a carrier concentration of about 5 × 10 ¹⁵ at a thickness of 0.5 μm.
cm ⁻³ n-type InP cap layer 16 (band gap E
g ₄ ), with a thickness of 0.1 μm and a carrier concentration of about 1 × 10 ¹⁶
A cm ⁻³ n-type InGaAs contact layer 17 (bandgap Eg ₅ ) is sequentially formed by gas source molecular beam growth method (gas source MBE method).

Next, the n-type InGaAs contact layer 17
N-type I by selectively implanting Si ions from the surface of
An n ⁺ type diffusion region 110 and an n ⁺ type contact layer 17a having a diameter of about 20 μm and a carrier concentration of about 10 ¹⁹ cm ⁻³ are formed in the nP cap layer 16. Here, the bottom of the n ⁺ -type diffusion region 110 may be formed in contact with the surface of the superlattice multiplication layer 15 or may be formed close to 0.1 μm or less via the n-type InP cap layer 16.

Next, the n-type InGaAs contact layer 17
To p ⁺ -type InP buffer layer 12 are selectively etched sequentially to form a mesa shape having a diameter of 30 μm with the n ⁺ -type diffusion region 110 as a concentric circle, and a passivation film 112 is formed on the surface including the side surface of the mesa. Next, the passivation film 112 is selectively etched to form an opening, and the n-side electrode 18 made of AuGeNi and the p ⁺ -type InP connected to the n ⁺ -type contact layer 17a in the opening are formed.
A p-side electrode 19 made of AuZn connected to the substrate 11 is formed. Here, the buffer layer 12 and the cap layer 17
Alternatively, an InAlAs layer may be used.

The band gap of each layer has the following relationship.

Eg ₁ > Eg ₂ , Eg ₃ > Eg ₂ , E
g ₄ > Eg ₂ , Eg ₅ <Eg _{4 Further} , the n-type InGaAs contact layer 17 may be omitted.

In the conventional example shown in FIG. 4, a polyimide film 410 for passivation and semiconductors 43, 4 on the side walls of the mesa are formed.
A large number of interface orders (2 × 10 ¹² cm ⁻²
/ EV or more), this interface order is caused by a normal semiconductor / polyimide film interface dangling bond, a semiconductor natural oxide film formed after mesa formation / a semiconductor interface tangling bond, and surface defects. The thing etc. are mentioned. In particular, in the semiconductor layers (43, 44, 45) depleted during reverse bias, the former is contained in the p − -type InGaAs light absorption layer 43 having a relatively small forbidden band, and
It is considered that the latter is abundant in the superlattice multiplication layers 43 and 44 containing aluminum atoms which are easily oxidized naturally.
Therefore, in the conventional structure, a high electric field capable of obtaining avalanche multiplication (in the conventional structure, the electric field distribution becomes the electric field intensity distribution as shown in FIG. 2A at both the center and the end of the mesa,
The value is about 500 to 600 kV / cm) in the multiplication layer. Occasionally, a surface leak dark current is generated through these interface orders, and is in the order of μA. Further, even with time, the interface order and surface defects increase due to the effect of hot carrier injection into the passivation film due to the high electric field, and the dark current increases, so that the reliability of the device becomes insufficient.

On the other hand, in the embodiment of the present invention shown in FIG. 1, as shown in FIGS. 2A and 2B, the n-type cap layer 16 is formed at the end of the mesa as compared with the center of the mesa. Since the depletion layer extends, the maximum electric field strength is about 600 kV / cm to 3
It can be significantly reduced to about 00 kV / cm. Further, if the carrier concentration of the cap layer 16 is set to an appropriate value (10 ¹⁶ cm ⁻³ or less), the depletion layer spreads only to the superlattice multiplication layer and the cap layer at the mesa end, and the light absorption layer side Can be prevented from spreading, and p ^- InGa with a narrow forbidden band can be
Even if the conductivity type inversion occurs on the mesa surface of the As light absorption layer 13, it is possible to prevent the reverse bias dark current characteristic from being affected.

As described above, as compared with the conventional mesa type photodiode structure, the structure of the present invention reduces the leak dark current by relaxing the electric field strength on the surface of the mesa, and the change with time is also different from the conventional one. It is possible to realize a semiconductor light receiving element that is suppressed in comparison and has improved element reliability.

Here, InAlAs / InAlGaAs
The case where a superlattice is used as a multiplication layer has been described,
InAlAs / InGaAs superlattice or InAl
The same effect can be obtained when an As / InGaAsP superlattice is used as a multiplication layer.

[0023]

As described above, according to the present invention, by selectively forming the high concentration diffusion region in the cap layer,
Realizes a highly reliable avalanche photodiode with high photosensitivity in the wavelength range 1.3 to 1.6 μm, high ionization ratio α / β, low noise and high speed response characteristics, and small surface leak dark current. It has the effect of being able to do.

[Brief description of drawings]

FIG. 1 is a schematic sectional view showing an embodiment of the present invention.

FIG. 2 is a diagram showing electric field intensity distributions at a central portion of a mesa and an end portion of a mesa according to an embodiment of the present invention.

FIG. 3 is a schematic cross-sectional view showing a first example of a conventional superlattice avalanche photodiode.

FIG. 4 is a schematic cross-sectional view showing a second example of a conventional superlattice avalanche photodiode.

FIG. 5 is a schematic cross-sectional view showing a third example of a conventional superlattice avalanche photodiode.

[Explanation of symbols]

11,51 p ⁺ type InP substrate 12,52 p ⁺ type InP buffer layer 13,53 p ⁻ type InGaAs light absorption layer 14 p ⁺ type InP electric field relaxation layer 15 andove i type or n ⁻ type InAlGaAs /
InAlAs superlattice multiplication layer 16 n-type InP cap layer 17 n-type InGaAs contact layer 17a n ⁺ -type contact layer 18, 38, 48 n-side electrode 19, 39, 49 p-side electrode 110 n ⁺ -type diffusion layer 111 antireflection film 112 surface passivation film 31, 41 n ⁺ type InP substrate 32 n ⁺ type InP buffer layer 33 n ⁻ type InGaAsP / InAlAs superlattice avalanche multiplication layer 34 p type InP electric field relaxation layer 35, 45 p ⁻ type InGaAs light absorption layer 36 p ⁺ type InP cap layer 37, 47 P ⁺ type InGaAs contact layer 310 SiN passivation film 311 polyimide film 42 n ⁺ type InAlAs buffer layer 43 n ⁻ type InGaAs / InAlAs lattice avalanche multiplication layer 44 p type InAlAs electric field relaxation layer 46 p ⁺ Type InAlA s Cap layer 410 Polyimide passivation film 411 Antireflection film 54 p-type InGaAs electric field relaxation layer 55 p-type InGaAs / InAlAs superlattice avalanche multiplication layer 56 p-type InP cap layer 57 high-concentration n-type InP region 58 low-concentration n-type InP region 59 AuGeNi ohmic electrode 510 AuZnNi ohmic electrode 511 Light incident window

Claims (2)

[Claims] 1. A high-concentration one-conductivity type buffer layer, a high-concentration one-conductivity type light absorption layer, a one-conductivity electric field relaxation layer, an undoped or It is characterized by having a mesa-shaped laminated structure including a low-concentration reverse-conductivity type superlattice layer and a low-concentration reverse-conductivity type cap layer, and a high-concentration reverse-conductivity type diffusion region formed in the reverse-conductivity type cap layer. A superlattice avalanche photodiode. 2. The superlattice avalanche photodiode according to claim 1, further comprising a reverse conductivity type contact layer formed on a surface including a high concentration reverse conductivity type diffusion region.