JP2993010B2 - Semiconductor light receiving element - Google Patents

Description

Description: FIELD OF THE INVENTION The present invention relates to a semiconductor photodetector, and more particularly to an avalanche multiplication semiconductor photodetector excellent in low noise and high-speed response characteristics in a wavelength band of 1 to 1.6 μm. About.

(Prior art) Conventionally, as a semiconductor light receiving element for optical communication of 1 to 1.6 μm band, a pin type semiconductor light receiving element having an In _0.53 Ga _0.47 As layer lattice-matched on an InP substrate as a light absorbing layer (for example, Electronics Letters (Electron. Lett.) 1984, 20, pp653-pp6
54), avalanche multiplication type semiconductor photodetectors (for example, IEE
EE, Electron Device Lett.) 1986, 7, pp257-pp258)
Etc. are known. In particular, among these, avalanche multiplication type semiconductor light receiving elements have attracted attention for long-distance optical communication because they have an internal gain effect due to avalanche multiplication and high-speed response.

The basic principle of the avalanche multiplication type semiconductor light receiving element is that photocarriers generated by light absorption become seeds to cause ionization collision in a high electric field to generate electron-hole pairs. On the other hand, it is phenomenologically known that low-noise and high-speed response characteristics desired as a light-receiving element for optical communication are governed by the process of ionization collision.

That is, since the ionization collision itself behaves as shot noise and further contributes to the multiplication rise time, it is ideally desirable that the ionization process does not last long. For this purpose, it is desirable that there is a difference between the electron ionization rate (α) and the hole ionization rate (β) in the multiplication region.

By the way, the ionization ratio (α / β or β / α) is determined materially, and the above-mentioned device using In _0.53 Ga _0.47 As as a light absorbing layer uses the InP layer as a multiplication region. Therefore, at most β / α = 2. this is,
There is a great difference from α / β = 20 of a Si-based device having low noise characteristics, which is a great barrier in realizing lower noise and higher speed response characteristics.

On the other hand, the Al _x Ga _1-x Sb system has attracted attention as a material system having a resonance ionization phenomenon using a split-off band. The ionization process is equivalent to the inverse process of the Auger effect. That is, holes generated by light absorption obtain energy by electric field acceleration and are excited into a split-off band. Since the effective mass is light in the split-off band, it is easy to reach ionization. When ionization occurs, the split-off band energy Δ
When s ₀ is about the same as the energy gap Eg, electrons are excited from the valence band to generate two holes and one electron. However, such ionization is in competition with ionization in the heavy hole band and the light hole band, and therefore requires that the ionization threshold energy in the split-off band be smaller than other hole bands.

O.Hildebrand et al. (IEJ Journal Quantum Electronics (IEEE.
ectronics), 1981, 17, pp284-pp288), and showed that the ionization threshold energy Ei, th of holes is represented by the following equation.

Thus, when Δ _s0 = Eg, the ionization threshold energy of holes decreases, and ionization of holes selectively occurs.
In the Al _x Ga _1-x Sb system, Δ _s0 / Eg changes by changing the Al composition ratio x as shown in FIG. 5, and Δs ₀ near x = 0.05.
= Eg in some regions. In fact, O. Hildebrand et al. Obtained an element structure by a liquid phase growth method and verified the ionization ratio, and found that a composition near Δ _s0 / Eg = 1 and a multiplication factor M = 1
It is estimated that 0, the ionization ratio β / α = 20, and the noise figure F = 2.3.

However, only a few examples of avalanche multiplication type photodetectors utilizing the above-described resonance ionization phenomenon have been demonstrated since the report of O. Hildebrand et al. (1981). One of the main reasons is that it is difficult to finely control the Al composition ratio in Al _x Ga _{1 -x} Sb growth. In particular, as shown in FIG. 5, _Δs0 / Eg is 1 near x = 0.05.
_{で は s0} / Eg is sensitive to x in the vicinity. Therefore, it is necessary to control the Al composition ratio within ± 0.01, but it is practically difficult to control the liquid phase growth method or the vapor phase growth method.

On the other hand, the present inventor has previously proposed a structure in which a superlattice structure is introduced in the multiplication region in order to solve the above-mentioned problem.
No. 231707.

In other words, the present structure utilizes the fact that the energy gap of the superlattice structure is freely changed depending on the thickness of the well layer. It is known that, in the superlattice structure, the energy gap is increased by the ground quantum level energy ΔE from the forbidden band width Eg ′ of the well layer. In this case Δ
E is represented by equation (2).

Here, 1 is the thickness of the well layer.

Further, A.Forchel like regarding split-off band energy delta _s0 (Applied Physics Letters (Appl.Phys.Lett.), 1987,50, pp182 -pp184) by, demonstrating that there is no well layer thickness dependence Have been. Therefore, _Δs0 / Eg may be controllable by the following equation.

Based on this idea, the present inventor has proposed a basic element structure as shown in FIG. This structure has n-type Ga
N-type GaSb light absorbing layer 2 formed on Sb substrate 1, n-type GaSb
/ AlSb superlattice structure layer (avalanche multiplication layer) 3, n-type Al
_It consists of a _0.5 Ga _0.5 Sb cap layer 4. A basic structure is obtained by forming ap ⁺ region 5 in such a layer structure by a selective diffusion method. In the drawing, reference numeral 6 denotes incident light.

Here, the energy gap of GaSb is 0.726 eV, the split-off hand energy is 0.752 eV, and m _e = 0.047 m ₀ , m
It is known that _hh = _0.49m is _0. From this Δs ₀ / E
When g is calculated, it becomes as shown in FIG. Well thickness 1
At 80 °, Δs ₀ / Eg = 1 is satisfied, but the change is relatively insensitive to the thickness of the well layer in the vicinity, and it is considered that controllability of ± 20 ° is sufficient. This is much easier than controlling the Al composition ratio proposed by O. Hildebrand et al.

The basic operating principle is that carriers generated in the GaSb light absorbing layer 2 are injected into the avalanche multiplication layer satisfying the condition of Δs ₀ / Eg = 1, so that only holes are passed through the split-off band. It utilizes the effect of resonance ionization. In this case, as described above, Δs ₀ / Eg = 1 can be relatively easily obtained by the superlattice structure, so that the state of resonance ionization is easily obtained. Therefore, the ionization rate ratio β / α
Is obtained, and an avalanche multiplication type light receiving element is obtained.

(Problem to be Solved by the Invention) The Al _x Ga _{1 -x} Sb-based avalanche multiplication type light receiving element to which the above-described superlattice structure is applied is easy to control Δs ₀ / Eg. However, by forming a superlattice structure, the influence of the band barrier becomes significant. That is, the barrier layer AlSb and the well layer GaSb have 1.16 eV on the conduction band side and 0.232 eV on the valence band side.
It is known that there is a barrier energy difference of about V.
Thus, there is a high possibility that electrons are trapped in the well layer especially when the carriers travel in the multiplication region. In terms of device characteristics, it is conceivable that the trapped electrons contribute as a slow component in response, and this poses a problem in terms of high-speed response.

(Means for Solving the Problems) A semiconductor light receiving element according to the present invention has a second semiconductor having a superlattice structure forming a first semiconductor layer as a light absorption layer and an avalanche multiplication layer on a semiconductor substrate. In a semiconductor light receiving device including a layer and a third semiconductor layer constituting a cap layer, a resonance ion phenomenon is achieved by controlling a split-off band energy and a forbidden band energy of a semiconductor material forming the superlattice structure. And the thickness of the barrier layer in the superlattice structure is a thin film having a thickness sufficient to cause a tunnel effect.

(Operation) The present invention is to generate an electron tunneling phenomenon by sufficiently reducing the thickness of the barrier layer as described above.

2 (a) and 2 (b) are views for explaining the basic principle of the present invention.

FIG. 2B is a band structure diagram of an avalanche multiplication region in a conventional avalanche multiplication semiconductor device having a superlattice structure. The injected holes travel by an electric field (), and by obtaining sufficient kinetic energy, ionization collision occurs to generate a hole-electron pair (). As described above, the generated holes travel in an electric field because the conduction band barrier is only 0.232 eV at the maximum, but electrons are trapped at the hetero interface because the conduction band barrier of 1.16 eV is felt (). Thus, the trapped electrons contribute as a slow carrier component.

On the other hand, FIG. 2A is a band structure diagram of an avalanche multiplication type semiconductor device having a super lattice structure according to the present invention. The injected holes are similar to FIG. 2 (b) described above.
Ionization collisions occur, generating hole-electron pairs. After that, holes and electrons will travel due to the electric field, but in the present invention, since the AlSb layer serving as a barrier layer is sufficiently thin,
The electrons travel without being trapped in the barrier layer by the tunnel effect (). The thinner the barrier layer is, the more effective the tunnel effect is. However, in general, it is sufficient if the barrier layer is less than the Debye length, and therefore it is sufficient if the barrier layer is less than 100 °.

As described above, the problem of the electron trap due to the introduction of the superlattice structure, which was a problem in the prior art, is solved, and an avalanche multiplication type semiconductor light receiving element excellent in high-speed response utilizing the resonance ionization phenomenon can be obtained. become.

(Example) Next, an example of the present invention will be described in detail with reference to the drawings.

FIG. 1 shows a basic structure diagram of an avalanche multiplication type semiconductor light receiving element according to an embodiment of the present invention.

An n-type GaSb light absorbing layer 2 and an n-type GaSb
/ AlSb superlattice layer (avalanche multiplication layer) 7, n-type Al _0.5 Ga
_It consists of a _0.5 Sb cap layer 4. The basic structure can be obtained by forming the p ⁺ region 5 as the light receiving portion in such a layer structure. Here, in the GaSb / AlAb superlattice structure, the thickness of the well layer GaSb is 180 ° necessary to satisfy Δs ₀ / Eg = 1 (see FIG. 4), and the thickness of the barrier layer AlSb is shown by the action. The angle is set to 50Å which is enough to cause a tunnel phenomenon.

Hereinafter, the operation principle of the semiconductor light receiving element of the present invention having the above structure will be described.

In the photocarriers generated in the GaSb light absorbing layer 2, only holes are injected into the superlattice structure 7, which is an avalanche multiplication layer, by a reverse electric field. Here, the superlattice multiplication region is Δs ₀
Since the condition of / Eg = 1 is satisfied, only holes cause resonance ionization via the split-off band. Each of the electrons and holes generated by ionization travels by an electric field.Especially, electrons pass through the barrier layer by tunnel effect because the barrier layer is sufficiently thin, and the avalanche multiplication has excellent high-speed response. Thus, a semiconductor light receiving device of the type can be obtained.

(Effects of the Invention) As described above, the avalanche multiplication type light-receiving element of the present invention uses a superlattice structure in an Al _x Ga _1-x Sb system and has a sufficient barrier layer compared to the Debye length. The thickness is so small that a resonance ionization phenomenon can be realized without trapping electrons.

As a result, an avalanche multiplication type semiconductor photodetector having an improved ionization rate ratio, low noise and excellent high-speed response can be realized, and surpasses conventional characteristics as a photodetector for optical communication in a wavelength band of 1 to 1.6 μm. It becomes possible.

[Brief description of the drawings]

FIG. 1 is a diagram showing the basic structure of an avalanche multiplication type semiconductor photodetector having a GaSb / AlSb superlattice structure as a multiplication layer according to an embodiment of the present invention, and FIGS. 2 (a) and 2 (b) show the present invention. FIGS. 3A and 3B are diagrams for explaining the basic principle, wherein FIG. 3A is a band structure diagram of an avalanche multiplication layer having a superlattice structure according to the present invention, FIG.
GaSb / AlSb structure diagram of avalanche multiplication semiconductor photodetector, which is multiplication layer superlattice structure, Figure 4 shows a GaSb well layer thickness dependence of the Delta] s ₀ / Eg drawing, Fig. 5 Al _x Ga ₁ Δ at _-x Sb
FIG. 3 is a graph showing the dependence of s ₀ / Eg on the Al composition ratio. 1 .... n-type GaSb substrate, 2 .... n-type GaSb layer (light absorbing layer),
3... N-type GaSb / AlSb superlattice structure layer (avalanche multiplication layer according to the prior art) 4... N-type Al _x Ga _{1 -x} Sb (x = 0.5)
Layer (cap layer), 5... P ⁺ region, 6... Incident light, 7.
... n-type GaSb / AlSb superlattice structure layer (avalanche multiplication layer according to the present invention).

Claims (1)

(57) [Claims] 1. A semiconductor substrate comprising: a first semiconductor layer serving as a light absorbing layer; a second semiconductor layer having a superlattice structure forming an avalanche multiplication layer; and a third semiconductor forming a cap layer. A semiconductor layer forming the superlattice structure, wherein the semiconductor material forming the superlattice structure controls a split-off band energy and a forbidden band energy to cause a resonance ion phenomenon, and a barrier layer in the superlattice structure. A semiconductor light receiving element characterized in that the thickness is a thin film having a thickness sufficient to cause a tunnel effect.