1. Field of the invention
The present invention relates to a photoelectric transfer device, particularly a photoelectric transfer device utilizing the avalanche effect which multiplies photo-generated carriers by collision ionization.
The present invention also pertains to a photoelectric transfer device with low noise which can be preferably employed for sensors for photometry of cameras, image sensors for image reading devices of facsimiles, copying machines, etc., or light receiving sensors of optical communication devices, etc.
1. Related Background Art
In the art of information transmission which uses light as the medium for information signals, for example, image information system, optical communication, other industries, civilian life, etc., a semiconductor light receiving device which transfers an optical signal into an electric signal is one of the constituent elements which is most important and basic, and a large number of devices have been practically applied. Generally speaking, semicondutor light receiving devices are required to have high signal-to-noise ratio for their photoelectric transfer characteristics.
Among them, avalanche photodiode (hereinafter called APD) utilizing the avalanche effect is high in gain and also rapid in response speed, and therefore it is a promising candidate for a semiconductor light receiving device satisfying such demands.
A large number of such APD, presently, have been already frequently applied as the semiconductor light receiving device in optical communication system, with a compound semiconductor such as InGaAs, etc. as the material. Further, progress has been made for improvement of basic performances such as low noise, high speed response, high gain, etc., and application to other fields such as visible light receiving device, etc. has been also desired.
FIG. 1 is a longitudinal sectional view showing the structure of a conventional APD for optical communication.
In the same Figure, 101 is an n.sup.+ -type InP layer, 102 an n-type InGaAs layer, 103 an n-type InP layer, 104 a p.sup.+ type InP layer. Here, the n-type InGaAs layer 102, the n-type InP layer 103, the p.sup.+ -type InP Iayer 104 are formed in the mesa-form. On the upper surface of the p.sup.+ -type InP layer 104 is formed a p-electrode 106 with the window 105 being left, and on the back surface of the n.sup.+ -type InP layer 101 is formed an n-type electrode 107. 108 is a passivation film. Here, when photoirradiation is effected from the window 105 with the p-electrode 106 and the n-electrode 107 being biased in the opposite directions, light is absorbed at the n-type InGaAs layer 102 (which becomes the light-absorbing layer), light-electricity conversion is effected. More specifically, the electron-positive hole pairs formed at the n-type InGaAs layer 102 will each run toward the n-electrode 107 end the p-electrode 106. Since the n-type InP layer 103 (which becomes the multiplying layer) has a strong electrical field, there occurs the avalanche phenomenon of forming a large number of electron positive hole pairs in the running process of positive holes, thereby creating the multiplying action of forming a plurality of electron-positive hole pairs per one photon. As a result, a signal can be detected even at weak incident light. However, in the structure of the prior art. practical multiplication ratio is small as about 2, and also due to fluctuation inherent in the multiplication process, there have been the two drawbacks that excessive multiplification noise is generated and that the signal-to-noise ratio (S/N ratio) is lowered.
The, noise generated in the avalanche multiplification process has been known to be strongly dependent on the ratio k=.beta./.alpha. wherein .alpha. is the ionization ratio of an electron and .beta.is the ionization ratio of positive hole as described in the essay of a R. J. McIntyre in IEEE Transactions on Electron Devices, 13th Ed. (January, 1966), pp. 164-168.
Here, the ionization ratio of electron refers to a ratio of electron-positive hole pairs formed by collision ionization when electrons are accelerated by an electrical field. The ionization ratio of positive hole refers to a ratio of collision ionization with positive holes. Further, according to this essay, for obtaining an APD with low noise, it has been clarified that k may be small when effecting electron multiplication while large when effecting positive hole multiplication. More specifically, it is important for obtaining high signal-to-noise ratio in APD to effect avalanche multiplication only of the carriers with a larger ionization ratio with materials with large difference in ionization ratio of the carriers (electrons or positive holes). Also, according to this essay, it is said that the excess noise index F becomes 2 as the limit of noise reduction achieved when only one of the carriers are subjected to avalanche multiplication. In the ideal case where no noise is generated at all, F shall be 1, and therefore the limit of F=2 suggests that there exists still some mechanism which will generate noise. As such mechanism, a conceivable mechanism may be that the places where ionization (reverse Auger generation) which is the elementary process of avalanche multiplication is effected in carrying out avalanche multiplication fluctuate are individually, which are in turn integrated to cause fluctuation of the multiplying ratio to occur as a whole.
In view of comprehensive considerations of the facts as mentioned above, for performing avalanche multiplication of generating no noise, it is required that (1) the places for causing ionization which is the elementary process to occur should be specified in the device, and (2) the probability of the ionization at the places where the above-mentioned ionization occurs should be specified. Further, in order to effect avalanche multiplication of a high gain, it is important to approximate the probability of ionization infinitely to 1.
In view of the two drawbacks of small extent of multiplication and lowering in signal-to-noise ratio (SN ratio), for example, F. Capasso et al has proposed a low noise APD available for optical communication system prepared by using primarily a compound semiconductor belonging to the group III-V according to molecular beam epitaxy (MBE), etc. as the APD for optical communication, as disclosed in Japanese Laid-open Patent Application No. 58-157179 or IEEE Electron Device Letters, EDL3 ed. (1982), pp. 71-73.
The device has a specific feature such that, by varying the composition ratio of the constituent materials (e.g. when a compound semiconductor belonging to the III-V group is the constituent material, the composition ratio of the semiconductor of the group III to the semiconductor of the group V), multiple semiconductor layers in which the band gap is continuously varied from the narrower side to the broader side, thereby forming a multi-layer heterojunction structure which promotes ionization by utilizing the stepwise transition portions of the energy band formed thereby (hereinafter abbreviated as step-back structure). The schematic structure of the device proposed there is described below by referring to FIG. 2 to FIG. 4.
FIG. 2 is a longitudinal sectional view of the device, having step-back structural layers 201, 203, 205, 207 and 209 including 5 layers which become the multiplying layers sandwiched between the p-type semiconductor layer 211 and the n-type semiconductor layer 215 which become light-absorbing layers, with the electrode 213 being in ohmic contact with the p-type semiconductor layer 211, and the electrode 214 with the n-type semiconductor layer 215, respectively.
FIG. 3 is a structural view of the energy band of the band gap inclined layers during no biasing of the device where three band gap inclined layers are shown. Each layer has a composition in which the band gap is varied linearly from the narrow band gap Eg2 to the wide band gap Eg3.
The sizes of step-backs of the conduction band and the valence electron band are shown by .DELTA.Ec and .DELTA.Ev, respectively. As described below, .DELTA.Ec is taken greater than .DELTA.Ev primarily for the reason of making ionization of electrons easier.
FIG. 4 is a structural view of the energy band when a reverse bias voltage is applied on the device. The reverse bias voltage is not required to be a stronger electrical field as compared with APD shown in FIG. 1 as described above.
Here, when light enters through the p-type semiconductor layer 211, the light absorbed in the p-type semiconductor layer and the respective step-back structural layers is photoelectrically converted similarly as in APD as described above, and the electron-positive hole pairs formed will run toward the n-type semiconductor layer 215 and the p-type semiconductor layer 211. However, the difference from the APD shown in FIG. 1 is that when the energy step difference .DELTA.Ec of each step-back structure (in the case of electrons, while .DELTA.Ev in the case of holes) becomes greater than the ionization energy, electrons are ionized to generate electron-positive hole pairs, thereby giving rise to multiplying action. Of course, for each of the step-back structural layers to have the same action, multiplication occurs at 2.sup.n for its layer number n. For example, ideally, by making .DELTA.Ec&gt;&gt;.DELTA.Ev.apprxeq.0, the ionization ratio of positive holes is suppressed to be much smaller when compared with the ionization ratio of electrons, whereby low noise can be attained as compared with APD as described above.
To describe in more detail, the bias voltage is applied so that at least the step-back structural layers (band-gap inclined layers) 201, 203, 205, 207 and 209 are depleted and an electrical field of such extent that drifting of carriers occurs within the band gap inclined layers but no ionization occurs is generated (drift electric field). The light h.nu. is absorbed in the depleted region next to the p-type semiconductor layer 211, namely the band gap inclined layer 201 to generate electrons in the conduction band and positive boles in the valence electron band, respectively. The electrons generated will be drifted through the layer 201 toward the step-back of the first conduction band. In the step-back, there is already an energy step difference of .DELTA.Ec, and the electrons can compensate the energy necessary for causing ionization to occur with the energy step difference .DELTA.Ec, and therefore the probability of ionization by electrons becomes higher immediately after the step-back. Here, when the energy step difference .DELTA.Ec is equal to or larger than the ionization energy of electrons, or even if smaller than the ionization energy of electrons, when the energy which is required can be supplied from the drift electric field, the probability of causing ionization to occur immediately after the step-back can be approximated to 1. When ionization occurs, one electron becomes two electrons and one positive hole. Two electrons will be drifted through the band gap inclined layer 203 toward the second step-back, and the same phenomenon is caused to occur in the second step-back. On the other hand. The positive holes generated ahead of within the band gap inclined layer 203 by ionization will be drifted ahead opposite to the electrons until they reach the first step-back. If there were previously present the energy step difference .DELTA.Ev in the valence electron band of the first step-back to the extent such that the positive holes will not cause ionization to occur, the positive holes drifted will further proceed ideally ahead. If there is a positive energy step difference ahead as seen from the positive holes as shown in FIG. 4, the positive holes will be scattered or accumulated at the step-back, but never cause ionization to occur. Thus, drifting and ionization of electrons, drifting of positive holes occur repeatedly at the respective band gap inclined layers and the step-backs, whereby the number of carriers will be multiplied. Finally, the electrons multiplied by ionization reach the N-type semiconductor layer and are taken out as the electron current from the layer in ohmic contact with the N-type semiconductor layer, while, the positive holes reach the P-type semiconductor layer, and are taken out as the positive hole current from the layer in ohmic contact with the P-type semiconductor layer.
By superposing a large number of semiconductor layers wherein the band gap is continuously varied from the narrower side to the broader side by varying the composition ratio of the constituent materials as described above, and utilizing the step-back formed thereby to form a multi-layer hetero-junction structure for promoting ionization, to specify the places where ionization occurs and approximate the probability of ionization infinitely to 1, whereby an APD with a lower noise can be constituted.
The device structure as described above is a means for realizing an APD with a lower noise, but preparation of the device having such structure will practically encounter various restrictions.
First, for obtaining the device having the step-back structure which can promote ionization only by varying the compositional ratio of the constituent materials, the constituent materials and preparation methods are limited. For example, as the material capable of constituting the device having such structure, there may be included GaSb which is a group III-V compound semiconductor substrate having AlGaAsSb/GaSb grown thereon, an InP substrate having InGaAlAs/InGaAs grown thereon, a GaSb substrate having InGaAsSb/GaSb grown thereon, and a lattice matched substrate having HgCdTe as a II-VI group compound semiconductor grown thereon, etc.
However, Ga, As, Hg, Cd, etc. used here are strongly toxic, and also rare and expensive elements, thus posing many problems in industrial handling.
All of these have been prepared by the molecular beam epitaxy method (MBE method), but the MBE method requires ultra-vacuum, and also the growth speed of a semiconductor is slow, unsuitable for enlargement of area, whereby bulk production can be done with difficulty. Further, in the MBE method, the growth temperature of a semiconductor is typically as high as 500.degree. C. to 650.degree. C., and preparation of such a light receiving device by lamination on a semiconductor device having already an integrated circuit prepared thereon also has the problem that some damages may be given to the already existing semiconductor device.
Further, for preparation of such an APD with a lower noise, the compositional ratio of these materials must be varied so that ionization is necessarily effected at the step-back, and for that purpose, it becomes necessary to determine the compositional ratio of the materials in view of the electron affinity so as to have a step-back energy step difference .DELTA.E of about the ionization energy or higher and a lattice matching which will not give rise to a trap level of the hetero-junction interface. As the result, the band gap of the APD which can be practically prepared will be restricted.
For example, first, when the materials initially mentioned are used, according to experiments, in the case of the lattice matched structure, the band gap of the material with the narrowest band gap (GaSb) is 0.73 eV. while the band gap of the material with the broadest band gap (Al.sub.1.0 Ga.sub.0.0 As.sub.0.08 Sb.sub.0.92) is 1.58 eV, with the maximum band gap difference being 0.72 eV at the conduction band side, 0.13 eV at the valence electron band side, and it has been confirmed that the electron ionization energy is 0.80 eV (GaSb). The deficiency 0.08 eV relative to the ionization energy of electrons at the step-back will be supplied from the drift electrical field of electrons. Whereas, in such a device, leak current (dark current) signal generated when no light is irradiated is liable to occur, which will increase noise components, thus having a great problem that a lower noise can not be ultimately effected. As the causes for generating dark current, there may be included carriers injected from the layer in ohmic contact (external electrode), carriers thermally levels within the device, etc. In such a device, first, the effect of preventing the injected carriers is consequently drawn out by setting of a P-type semiconductor layer and an N-type semiconductor layer, but neither conscious nor sufficient care has been fully paid on this point, and therefore such effect cannot be said to be satisfactory. The amount of the carriers thermally generated depends on defective level density, interface level density, eto., but it depends essentially on the size of the band gap, and generally speaking, it been known that the amount of carriers becomes small as the band gap is larger. However, in such a device, there is also the drawback that the minimum band gap is too narrow to inhibit thermally generated carriers. The semiconductor light receiving device having such a band gap may be suitable for receiving a light in the wavelength region from 1.0 .mu.m to 1.6 .mu.m, but it can hardly be said to be suitable for devices for receiving light of other wavelength, for example, a visible light receiving device, and its application field is limited.
Next, for example, in the combination of the materials mentioned secondly, in spite of large ionization energy of about 1 eV, the conduction band energy step difference in the step-back is as small as about 0.6 eV, and therefore it is not promising at all.
Other materials as mentioned above also have the same drawbacks as the first material. Particularly, in the combination of materials finally mentioned, for example, according to the essay by T. P. Pearsall described in Electronics Letters, 18th ed., No. 12 (June, 1982), pp. 512-514, a device has been proposed having the minimum band gap of 0.5 eV and the maximum band gap of 1.3 eV by varying the compositional ratio of Hg and Cd. but in such a device the minimum band gap is very narrow and therefore the device has become susceptible to the dark current thermally generated.
Therefore for effective practical application of an APD with a lower noise having a structure for enlarging the ionization ratio of carriers, it is necessary to consider degree of freedom of choice of materials and preparation method, inhibition of dark current, a band structure having a broad light-receiving wavelength region, etc.
Also, by incidence of light on the above-mentioned step-back structure, carriers may be sometimes generated also within the above-mentioned step-back structure, whereby the multiplying ratio may be sometimes changed depending on the wavelength of the incident light.
Thus, to summarize the technical tasks to be solved for the APD as described above, there may be included the technical tasks in performance and preparation as shown below.
The technical tasks in performance of the device are as follows.
(1) Because the incident light is absorbed in the p-type semiconductor layer and the multiplying layer, the multiplying ratio changes depending on the wavelength of the incident light and the device is therefore not suitable as a reading device.
(2) Because the band gaps of the light-absorbing layer, the multiplying layer are small, dark current during actuation is high and noise is great.
(3) Because the device is intended to be used for optical communication, the material is limited, and the light to which it can correspond is about 800 to 1600 nm. and it cannot correspond to other wavelength light such as visible light.
The technical tasks in preparation of the device are as follows.
(1) For preparation of a step-back structure with a compound semiconductor, the composition modulation is difficult, and the magnitudes of the energy step differences .DELTA.Ec and .DELTA.Ev are limited, whereby a lower noise can be effected only limitedly.
(2) Because compound semiconductors belonging to the groups III-V, II-VI, etc., the problems are involved as the industrial material with respect to toxicity, cost, etc. of the materials.
(3) The formation process of the compound semiconductor involves such problems as requiring ultra-high vacuum and film formation at a high temperature (about 500.degree. to 650.degree. C.), difficulty in enlargement of area, etc., thus being unsuitable as the preparation process of a reading device.
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