JP2590817B2 - Photo Detector - Google Patents

Description

Description: TECHNICAL FIELD The present invention relates to a photodetector used in an optical communication device or the like.

[Conventional technology]

A modulation-doped photodetector (hereinafter abbreviated as MDPD) is a photodetector having a high-speed response characteristic, and is a semiconductor device having a heterojunction.

Applied Physics Letters (Appl. Phys. Lett. 43 (1983) 308) is an example of a previously reported MDPD.
2 (a) is a sectional view of the structure of the MDPD shown in FIG. 2, and FIG. 2 (b) is an energy band diagram of a main part thereof. This MD
The PD structure is formed on a semi-insulating InP substrate 11 by undoped Al _{0 . 48} In _{0 . 52} As buffer layer 12, undoped (n-type) Ga _{0 . 47} In _{0 . 53} As light absorbing layer 13, undoped Al _{0 . 48} In _{0 . 52} As
Spacer layer 14 and n-type Al _{0 . 48} In _{0 . 52} As charge supply layer 15,
And n-type Ga _{0 . 47} In _{0 . 53} As contact layer 16
Having. The drain electrode 17 and the source electrode 18 are gold-germanium (Au-Ge) alloy electrodes. As shown by the broken lines, the alloy goes deeply into the two-dimensional electron gas region.

In this structure, electrons excited by incident photons
Of the hole pairs, the electrons are Ga _{0 . 47} In _{0 . 53} As layer 13 and Al _{0 .
48} In _{0 .} 2-dimensional electron gas by the internal electric field which can be the interface ₅₂ As layer _{14 (Ga 0. 47 In 0} . 53 As layer 13 and the _{Al 0. 48 In 0. 52} As layer
It travels to the region 10 (formed on the layer 13 side at the interface 14), and can be taken out as a current flowing by the voltage applied between the source and the drain. Since this device has a high electron mobility in the two-dimensional electron gas region, it is expected to respond quickly, and in fact, the rise time of the signal is very short in the above-mentioned paper. The reason why the mobility of electrons in the two-dimensional electron gas region is large is that electrons are spatially separated from ionized impurities in the charge supply layer 15 by the spacer layer 14, so that scattering due to these ionized impurities is reduced. Undoped Ga with few impurities
_{0 . 47} In _{0 . This} is because it exists in the ₅₃ As light absorption layer 13. Further, Ga _{0 . 47} In _{0 .} The substance ₅₃ As has an effective electron mass of 0 _. 04m. And GaAs 0 _. 067
m. Therefore, a large electron mobility can be expected by this alone.

[Problems to be solved by the invention]

However, Ga ₀ .2 in which the two-dimensional electron gas region 10 is formed _{. 47} In _{0 . Since the 53} As light absorption layer 13 is a ternary mixed crystal, it undergoes alloy scattering peculiar to the mixed crystal, and a sufficiently large electron mobility cannot be obtained.

In addition, in order to obtain a light absorption layer 13 having good crystallinity, it is necessary to strictly match the lattice with the InP substrate 11, but (G
a, In, As) in a ternary mixed crystal consisting of Ga _{0 . 47} In _{0 . 53} As is the only composition for this. Therefore, the basic absorption edge of the light absorption layer 13 is Ga _{0 . 47} In _{0 .} Value for ₅₃ As, ie 0
_. 75eV _(1. 65μm, but room temperature) only there is no choice,
1 _. It becomes transparent and does not respond to light having a wavelength longer than 65 μm.

An object of the present invention is to provide a photodetector that solves the above problems.

[Means for solving the problem]

A photodetector according to the present invention includes a superlattice layer for generating at least a two-dimensional electron gas region on a semiconductor substrate, and a spacer in contact with an upper portion of the superlattice layer and having a smaller electron affinity and a larger bandgap than the superlattice layer. A layer, an n-type charge supply layer having a smaller electron affinity than the superlattice layer and a larger forbidden band width, and first and second n-type charge supply layers formed by alloying from the charge supply layer to a region where the two-dimensional electron gas is generated. Wherein the superlattice layer is formed by alternately laminating a plurality of semiconductor layers having different forbidden band widths for at least two periods.

[Action]

By using the superlattice layer as the light absorbing layer, regularity is generated in the crystal lattice and alloy scattering is reduced as compared with the bulk mixed crystal. Assuming that two types of semiconductor layers constituting the superlattice layer are used, a well layer having a small forbidden band width and a barrier layer having a large forbidden band width. If the thickness lb is sufficiently small, the two-dimensional electron gas will ooze out not only in the well layer but also in the barrier layer. When the barrier layer thickness lb is large, electrons are confined in each well layer, but the point is a two-dimensional electron gas, and anyway, a large electron mobility can be obtained.

In addition, the materials used for the well layers and the barrier layers and the layer thicknesses lz, l
By selecting b, it is possible to appropriately design the basic absorption edge of light absorption. Therefore, the wavelength range that can respond to light can be expanded as compared with the conventional MDPD. On this occasion,
Even if the lattice constants of the well layer and the barrier layer are not equal, the layer thickness l
If z and lb are properly selected, the crystal will be distorted in accordance with the lattice constant of the semiconductor substrate and form a so-called “strained superlattice” that is stacked without any transition at the interface due to lattice mismatch. If this “strained superlattice” is used as the superlattice layer, the designable optical response wavelength range is greatly expanded. In addition, the semiconductor substrate that can be used can use not only InP but also GaAs.
The photodetector of the present invention can also be mounted on an electro-optical IC integrated on a substrate.

〔Example〕

FIG. 1A is a sectional view of a photodetector according to an embodiment of the present invention, and FIG. 1B is an energy band diagram of a main part thereof.

On a semi-insulating GaAs substrate 1 by molecular beam epitaxy,
An undoped GaAs. (P ^- -type) buffer layer 2 (thickness ₁ 0Myu
m), a strained superlattice layer 3 in which an undoped InAs well layer 31 (thickness lz = 100 °) and an undoped GaAs barrier layer 32 (thickness lb = 50 °) are stacked for at least two periods, in this embodiment, 67 periods.
(Thickness _{1. 005μm),} Adopu Al _{0. 4} Ga _{0 . 6} As spacer layer 4 (thickness 50 °), n-type Al _{0 . 4} Ga _{0 . 6} As charge supply layer 5
(Thickness _1. 0 .mu.m, Si-doped ^{5 × 10 17 cm -3),} n -type GaAs
Contact layer 6 (thickness: _0. 5 [mu] m, Si-doped 2 × 10 ¹⁸ cm
^-3 ) is sequentially laminated, the contact layer 6 is further removed except for the light incident portion, Au-Ge is adhered on the contact layer 6 and alloyed to form a drain electrode 7 and a source electrode 8 to form a photo-electrode of the present invention. A detector has been obtained.

In the case of the present embodiment, in the GaAs / InAs strained superlattice layer 3, the lattice constants of the InAs well layer and the GaAs barrier layer are 6 _. 058Å, 5 _. Although the lattice constant differs by about 7% at 653 °, defects such as dislocations are not introduced because the lattice of InAs is distorted and almost lattice-matched to GaAs.

On the other hand, in the strained superlattice layer 3, the forbidden band width of InAs is _. About 0 from 36 eV _. Increase by 17 eV _. 53eV
Although (wavelength: about _2. 3 [mu] m) becomes, Ga conventional MDPD shown in Figure 2 _{0. 47} In _{0 .} 0 in case of ₅₃ As light absorption layer 13 _. 75
eV (wavelength _1. 65 .mu.m) smaller than, and therefore responds to longer wavelengths of light. Since the forbidden bandwidth increases and decreases due to the inherent distortion, the above value is not always accurate. However, it is still possible to receive light with a longer wavelength than before. It is also possible to moderate the strain in the strained superlattice layer 3 by appropriately selecting the layer thicknesses lz and lb. In this case, however, the GaAs substrate 1 or Al _{0 . 4} Ga _{0 . This} is not always desirable because lattice matching with the 6As layer is lost.

As can be seen from the above description, according to this embodiment, the incident light is absorbed by the super-strained superlattice layer 3 and the two-dimensional electron gas region 10 is generated in the strained superlattice layer 3 with little alloy scattering. It has high mobility and responds to light at high speed.

In the above embodiment, the spacer layer 4 and the charge supply layer 5 are made of Al _{0 . 4} Ga _{0 . Although 6} As was used, other semiconductors, such as InAlAs, may be used as long as they conform to the purpose of the present invention. The well layer 31 and the barrier layer 32 constituting the superlattice layer 3 are G
It goes without saying that a mixed crystal (0 <x <1) such as axIn ₁ -xAs and AlxIn ₁ -xAs may be used, or another compound semiconductor satisfying the purpose of the present invention.

〔The invention's effect〕

As described above, according to the present invention, the superlattice layer is used as the light absorbing layer, and at the same time, the two-dimensional electron gas region is formed at the interface between the superlattice layer and the spacer layer, so that the photoresponse characteristics are faster than before. In addition, a photodetector having a sensitive wavelength range extended to a longer wavelength side can be obtained. Further, since a photodetector having such excellent characteristics can be formed on a substrate other than InP, it can be applied to a GaAs opto-electric IC or the like.

[Brief description of the drawings]

FIG. 1 is a sectional view of a photodetector of the present invention and an energy band diagram of a main part thereof, and FIG. 2 is a sectional view of a conventional photodetector and an energy band diagram of a main part thereof. 1,11 semiconductor substrate 2,12 buffer layer 3,13 superlattice layer (light absorbing layer) 4,14 spacer layer 5,15 charge supply layer 6,16 contact layer 7 , 17 …… Drain electrode 8,18 …… Source electrode 10 …… 2D electron gas region

Claims (1)

(57) [Claims] A superlattice layer for generating at least a two-dimensional electron gas region on a semiconductor substrate; and a spacer layer in contact with an upper portion of the superlattice layer and having a smaller electron affinity and a larger forbidden band width than the superlattice layer. An n-type charge supply layer having a smaller electron affinity and a larger bandgap than the superlattice layer, and first and second alloys formed by alloying from the charge supply layer to a region where the two-dimensional electron gas is generated. An electrode, wherein the superlattice layer is formed by alternately laminating a plurality of semiconductor layers having different forbidden band widths for at least two periods.