P-I-N Photodiode With Transparent Conductor N+layer

This invention relates to opto-electronic devices and, more particularly, to a photodetector for converting an optical signal to an electrical signal. Specifically, one embodiment of the invention is directed to a p-i-n photodiode which has a uniform detection characteristic in response to radiation that is distributed across the surface of the photodiode and which can be structured to operate over a broad range of optical wavelengths, such as from 0.4 to 1.6 .mu.m. For example, such a p-i-n photodiode has potential use in photonics instrumentation, such as an optical spectrum analyzer.

InP/InGaAs/InP p-i-n photodiodes and avalanche photodiodes are the typical candidates for optical-fiber communications in the 1.0 to 1.6 .mu.m wavelength range. See, for example, Carey, K. W., Wang, S. Y., Hull, R., Turner, J. E., Oertel, D., Bauer, R., and Bimberg, D., "Characterization of InP/InGaAs/InP Heterostructures Grown by Organometallic Vapor Phase Epitaxy For High-Speed p-i-n Photodiodes," Journal of Crystal Growth, 77, 558-563, (1986), and Moseley, A. J., Urquhart, J., Hodson, P. D., Riffat, J. R., and Davies, J. I., "InGaAs/InP SAGM Avalanche Photodiodes Incorporating a Pseudoquaternary Superlattice Graded Heterojunction Grown by Atmospheric-Pressure MOCVD," Electronics Letters, 23, 914-915, (1987). These p-i-n photodiodes are the present photodetectors-of-choice because of their superior bandwidth. Bowers, J. E., and Burrus, C. A., "InGaAs PIN Photodetectors with Modulation Response To Millimeter Wavelengths," Electronics Letters, 21, 812-814, (1985), and Tucker, R. S., Taylor, A. J., Burrus, C. A., Eisenstein, G., Wiesenfeld, J. M., "Coaxially Mounted 67 GHz Bandwidth InGaAs PIN Photodiode," Electronics Letters, 22, 917-918, (1986). Also, these p-i-n photodiodes are considered superior because of their low dark current characteristic. Ohnaka, K., Kubo, M., and Shibata, J., "A Low Dark Current InGaAs/InP p-i-n Photodiode With Covered Mesa Structure," IEEE Transactions On Electron Devices, ED-34, 199-204, (1987).

A window layer, that is, the InP top layer in an InP/InGaAs/InP p-i-n photodiode structure, which is typically an InP:Zn p.sup.+ material, is needed in the structure of these photodiodes in order for them to provide a fast response. If an InGaAs p.sup.+ layer were substituted for the InP:Zn p.sup.+ top layer to provide an InGaAs p.sup.+ /InGaAs n.sup.- /InP:S structure, the resulting p-i-n photodiodes would need to rely on the diffusion characteristic of the InGaAs n.sup.- material to operate, which would result in an unacceptably slow response. However, the resistance of the InP:Zn p+ layer is not low enough to assure uniform response of the p-i-n photodiode when radiation is distributed across the surface of the entire photodiode. Considered in more detail, the known InP/InGaAs/InP p-i-n photodiode structure is shown in FIG. 1. This p-i-n photodiode typically comprises an InP:Zn/InGaAs/InP:S structure 1,2,3 disposed between top Ti/Au/AuZn/Au and bottom Au-Ge/Ni/Au contacts 4, 5, respectively. As shown in FIG. 1, a buffer layer 6, for example, InP, can be incorporated between the InGaAs n.sup.- and InP:S layers 2 and 3, respectively. The InP:Zn p+ layer is typically on the order of 1.0 to 1.1 .mu.m thick. Unfortunately, the InP:Zn p.sup.+ layer 1 is thick enough that a relatively long growing time is needed. This causes diffusion of Zn into the InGaAs n.sup.- layer 2. Consequently, there is a migration of p dopant from the InP:Zn p+ layer 1 into the InGaAs n.sup.- layer 2, that is, the Zn dopes the InGaAs material p.sup.+. This means that the p-i-n photodiode begins to act as though the InP:Zn p.sup.+ window layer 1 is eliminated. Accordingly, carriers move more slowly in the InGaAs n.sup.- layer 2, which is evidenced by the p-i-n photodiode having a slow response.

A single, large area p-i-n photodiode having a fast response could potentially be used to detect light from both single mode and multi-mode optical fibers, which produce spots of different diameters. However, the need to detect light beams of different diameters also mandates not only a fast response, as provided by known p-i-n photodiodes, but also a uniform photodetector response characteristic across the detecting area. Unfortunately, these known InP/InGaAs/InP p-i-n photodiodes do not provide a uniform response.

It is therefore desirable to provide a p-i-n photodiode having a fast response, which also has a uniform photodetector response characteristic across the detecting area. It is also desirable that the p-i-n photodiode operate over a broad range of optical wavelengths.

One embodiment of the present invention provides a p-i-n photodiode having a fast response which also has a uniform detection characteristic in response to radiation that is distributed across the surface of the photodiode. An InP/InGaAs/InP p-i-n photodiode in accordance with one embodiment of the invention has an improved response uniformity due to the incorporation of a transparent n.sup.+ layer conductor contact. For example, the use of an overlayer of indium tin oxide (ITO) n.sup.+ material as a transparent conductor contact improves the frequency response uniformity of an InP/InGaAs/InP p-i-n photodiode. Preferably, in accordance with a further embodiment of the invention, the p-i-n photodiode is structured so that an ITOn.sup.+ layer is substituted for the top InP layer in an InP/InGaAs/InP p-i-n photodiode to enable detection of a broad range of optical wavelengths, such as from 0.4 to 1.6 .mu.m.

While known p-i-n photodiodes include an InP:Zn p.sup.+ top layer, which has a resistance that is not low enough to assure uniform device response, the added ITOn.sup.+ contact overlayer incorporated into the p-i-n photodiode in accordance with one embodiment of the invention provides a relatively low resistance n.sup.+ layer conductor in parallel with the InP:Zn p.sup.+ layer to reduce the effective resistance of the window layer. In a further embodiment in accordance with the invention, the ITOn.sup.+ contact layer is deposited to provide a p-i-n photodiode that has an operating range from 0.4 to 1.6 .mu.m and therefore detects radiation from the visible region through the near infrared. In this further embodiment of the invention, the InP:Zn p.sup.+ layer replaces the InP:S bottom layer, and the InP:S layer eliminated.

The spatial variation of the frequency response of large area p-i-n photodiodes is therefore eliminated by using ITOn.sup.+ material as a transparent contact over the entire window layer or as the window layer. This result makes such large area p-i-n photodiodes suitable for use with both single and multi-mode optical fibers.

The use of the transparent conductor n.sup.+ layer in a p-i-n photodiode also provides another advantage over the InP:Zn p+layer of known p-i-n photodiodes. In the known InP/InGaAs/InP structure, the InP top layer establishes the short wavelength end of the visible spectrum as the lower limit for the photodiode. Accordingly, one could previously only detect radiation in the range from 1.0 to 1.6 .mu.m. The advantage of the p-i-n photodiode in accordance with the further embodiment of the invention is that the optical characteristic of transparent ITOn.sup.+ conductor material establishes the short wavelength end of the visible spectrum as the lower limit for the photodiode. Accordingly, the p-i-n photodiode in accordance with the further embodiment of the invention extends the range to encompass optical wavelengths from 0.4 to 1.6 .mu.m.

The above and other features of the invention and the concomitant advantages will be better understood and appreciated by persons skilled in the field to which the invention pertains in view of the following description given in conjunction with the accompanying drawings. In the drawings:

FIG. 1 is a cross-sectional view of a known p-i-n photodiode;

FIG. 2 is a cross-sectional view of a p-i-n photodiode in accordance with one embodiment of the invention comprising an InP/InGaAs/InP p-i-n photodiode using an indium tin oxide (ITO) n.sup.+ material overlayer as a transparent conductor contact;

FIG. 3 illustrates typical I-V and C-V characteristics for a known p-i-n photodiode and for the p-i-n photodiode shown in FIG. 2 having an ITO n.sup.+ conductor material overlayer;

FIG. 4, comprising FIGS. 4A and 4B, illustrates the frequency response of 80.0 .mu.m diameter p-i-n photodiodes when illuminated at the center of the detecting area and at the edge of the detecting area next to an ohmic contact, in which FIG. 4A is the response of a known p-i-n photodiode that does not have an ITOn.sup.+ conductor material overlayer; and FIG. 4B is the response of the p-i-n photodiode shown in FIG. 2 having an ITO n.sup.+ conductor material overlayer;

FIG. 5 is a cross-sectional view of a p-i-n photodiode in accordance with a further embodiment of the invention comprising an InGaAs/InP p-i-n photodiode using indium tin oxide (ITO) n.sup.+ material as the window layer;

FIG. 6 illustrates a plot of representative data on the dark current of the p-i-n photodiode shown in FIG. 5 measured after a mesa etching step;

FIG. 7, comprising FIGS. 7A and 7B, illustrates frequency response measurements for the p-i-n photodiode shown in FIG. 5 for modulated 1300 nm and 1550 nm optical sources, respectively; and

FIG. 8, comprising FIGS. 8A and 8B, illustrates frequency response measurements for an alternative embodiment of the p-i-n photodiode shown in FIG. 5 for modulated 1300 nm and 1550 nm optical sources, respectively.

Indium tin oxide (ITO) has been previously used in photodiode and other opto-electronic devices. D. G. Parker reported using ITO to make progressively faster Schottky barrier photodiodes in GaAs. Parker, D. G., Say, P. G., Hansom, A. M., and Sibbett, W., "110 GHz High-Efficiency Photodiodes Fabricated From Indium Tin Oxide/GaAs," Electronics Letters, 23, 527-528, (1987); Parker, D. G., and Say, P. G., "Indium Tin Oxide/GaAs Photodiodes For Millimetric-Wave Applications," Electronics Letters, 22, 1266-1267, (1986); and Parker, D. G., "Use of Transparent Indium Tin Oxide to Form a Highly Efficient 20 GHz Schottky Barrier Photodiode," Electronics Letters, 21, 778, (1985). However, the Schottky barrier structure is substantially different from a p-i-n photodiode structure in which ITO is not previously known to have been used.

One embodiment of a p-i-n photodiode in accordance with the invention eliminates the spatial variation in the frequency response of large area p-i-n photodiodes by incorporating an ITOn.sup.+ overlayer as a transparent conductor contact. As will be described later, the I-V, C-V, and frequency response characteristics of p-i-n photodiodes fabricated with and without the ITOn.sup.+ contact overlayer demonstrate the superior performance of the indium-tin-oxide p-i-n photodiode.

A front-side illuminated p-i-n photodiode in accordance with one embodiment of the invention, generally designated by the numeral 10, is shown in cross-section in FIG. 2. The p-i-n photodiode 10 comprises a grown InP/InGaAs/InP structure (sandwich).

Referring in more detail to FIG. 2, the p-i-n photodiode 10 comprises an InP:S conducting substrate, a 1.5 .mu.m thick In.sub.0.53 Ga.sub.0.47 As unintentionally doped layer, lattice-matched to the substrate, and a 0.5 .mu.m thick InP:Zn p.sup.+ contact layer. The InP:Zn p.sup.+ layer is a relatively thin layer. In contrast, the InP:Zn p.sup.+ layer is typically on the order of 1.0 to 1.1 .mu.m thick in known p-i-n photodiodes. The In.sub.0.53 Ga.sub.0.47 As n.sup.+ layer is the active layer where radiation that impinges on the surface of the p-i-n photodiode 10 is absorbed. The InP:S layer is a relatively thick layer.

An Au-Ge/Ni/Au bottom contact is evaporated onto the InP:S layer included in the structure. Then, Au-Zn top contacts are evaporated onto the InP:Zn p.sup.+ layer.

A suitable photoresist mask is used to mask the top surface of the structure, except where ITO is to be applied. Next, ITO is RF sputter-deposited onto the top surface of the structure. Thereafter, the photoresist is removed.

Next, another suitable photoresist mask is applied to the ITO/contact area, and the mesa is wet-etched. Polyimide is then preferably spun onto the structure. Next, via holes are opened in the polyimide using a further suitable photoresist mask followed by wet-etching.

Finally, an Au bond pad is plated on. During this final processing, an additional suitable photoresist mask is preferably applied to the polyimide where the Au bond pad is to be plated on, and TiAu is deposited on the polyimide to enable the Au to be plated-up. Then, the photoresist mask is removed.

One implementation of the p-i-n photodiode 10 was fabricated as follows. The In.sub.0.53 Ga.sub.0.47 As n.sup.- layer was unintentionally doped 5E15 cm.sup.-3 n-type, and the InP:Zn p.sup.+ contact layer was doped approximately 1E18 cm.sup.-3 ; the epitaxial layers were grown using atmospheric metallorganic chemical vapor deposition (MOCVD). The n.sup.+ contact metallization was Au-Ge/Ni/Au, and Au-Zn/Au was used for the ring-shaped p.sup.+ contact; the contacts were alloyed at 380.degree. C. and 420.degree. C. respectively The p.sup.+ contact defines the diameter of the detecting area.

Next, a photoresist liftoff mask was processed, leaving exposed the p.sup.+ contact and the detecting area. Using RF-bias sputtering in a reactive environment, 1950 Angstroms of ITO were deposited onto the wafer. The resulting ITO film composition was approximately, by weight, 85% indium oxide and 15% tin oxide with a film resistivity equal to 2.9.times.10.sup.-4 .OMEGA.-cm. Removing the liftoff mask left the ITO covering the detecting area of the p-i-n photodiode 10 and the p.sup.+ contact. Conventional wet chemical processing was used to form the mesa. A 3.0 .mu.m thick layer of Dupont 2556 polyimide served as passivation for the mesa and isolated the plated Au bond pad from the substrate.

Typical I-V and C-V characteristics of p-i-n photodiodes with and without an ITOn.sup.+ conductor material layer with 25.0 .mu.m diameter for the detecting area are shown in FIG. 3. The corresponding current density for these devices is approximately 5.0 .mu.A/cm.sup.2.

For frequency response measurements, the p-i-n photodiodes with and without an ITOn.sup.+ conductor material layer were attached to a coplanar test circuit with conductive epoxy. The photodiode active area was illuminated with the output of a 1.3 .mu.m continuous wave (CW) laser diode modulated by a Mach-Zender interferometer optical modulator and microwave synthesizer. The frequency of modulation was from 1.0 to 26.5 GHz. Radiation was introduced to the p-i-n photodiodes using optical fibers and a modified Leitz Laborlux 12 HL microscope creating a final spot size of approximately 10.0 .mu.m diameter.

The electrical response of the combined modulator and p-i-n photodiodes were measured with a Hewlett-Packard Company HP 8510 microwave network analyzer. For absolute measurements of the p-i-n photodiodes alone, the optical modulator would have to be calibrated separately, and its frequency response subtracted from the overall frequency response. However, absolute accuracy was not necessary for a relative measurement, where a comparison of the frequency response of the p-i-n photodiodes when illuminated in the center of the active area to the response of the photodiodes when illuminated near an edge is desired. For this measurement, a "response" calibration was made with an uncalibrated high-speed reference p-i-n photodiode, providing an approximate calibration of the system.

The results of the frequency response measurements for 80.0 .mu.m diameter p-i-n photodiodes at 4.5 volts reverse bias are shown in FIG. 4. While the measurements illustrated in FIG. 4 extend to 26.5 GHz, the data are only valid to about 20 GHz due to degradation of signal-to-noise above this frequency. For p-i-n photodiodes with no ITO covering the detecting area, there is a significant difference between the frequency response for illumination at the center of the active area and for illumination at the edge. The center response is worse because the carriers created there are the farthest from the ohmic contacts. Generally, the photodetector bandwidth decreased the farther away from the ohmic contacts that the radiation illuminated the p-i-n photodiode. This accords with Degani, J., Wilt, D. P., and Besomi, P., "Effect of Photocarrier Spreading on the Photoluminescence of Double Heterostructure Material," Journal Of Applied Physics, 56, 468-476, (1984). There was no difference between the center and edge responses for p-i-n photodiodes having the ITOn.sup.+ conductor material overlayer.

The advantages of the p-i-n photodiode 10 are that the frequency response of the photodiode is now uniform across the entire detecting area and there is no longer a need for a thick p.sup.+ layer to reduce the resistivity of the p.sup.+ layer, thereby reducing the growth time of the material layers. Additionally, the use of the ITO transparent conductor contact potentially removes the need for a separate anti-reflection coating.

In accordance with a further embodiment of the invention, a p-i-n photodiode is structured to have a broad operating range, such as from 0.4 to 1.6 .mu.m. As described above, the p.sup.+ layer resistance is reduced on an InP/InGaAs/InP p-i-n photodiode by the use of an ITOn.sup.+ contact overlayer. This allows a contact layer that is not a material that absorbs radiation at these optical wavelengths.

It is apparent that the ITOn.sup.+ contact layer also provides a portion of the window layer with the retained InP:Zn p.sup.+ material in the structure shown in FIG. 2. Therefore, in the InP/InGaAs/InP material structure, the InP establishes the short wavelength end of the visible spectrum as the lower limit for the p-i-n photodiode, which can only detect radiation in the range from 1.0 to 1.6 .mu.m.

In accordance a further embodiment of the invention, an ITOn.sup.+ layer is deposited to provide a p-i-n photodiode that has an operating range from 0.4 to 1.6 .mu.m and therefore detects radiation from the visible region through the near infrared, since ITO is transparent at these optical wavelengths. Additionally, the InP:Zn p.sup.+ layer is substituted for the InP:S layer. Accordingly, an inverted structure results, as shown in FIG. 5.

Referring in more detail to FIG. 5, a p-i-n photodiode 10' comprises a grown InGaAs/InP structure (laminate). The p-i-n photodiode 10' comprises an InP:Zn p.sup.+ conducting substrate, a 1.8 .mu.m thick In.sub.0.53 Ga.sub.0.47 As unintentionally doped, layer, lattice-matched to the substrate, and a 0.163 .mu.m thick ITOn.sup.+ window layer. The In.sub.0.53 Ga.sub.0.47 As n.sup.+ layer is the active layer where radiation that impinges on the surface of the p-i-n photodiode 10' is absorbed. The InP:Zn p.sup.+ layer is a relatively thick layer.

As shown in FIG. 5, a 0.5 .mu.m thick InP:Zn p.sup.+ buffer layer is preferably grown between the InP:Zn p.sup.+ conducting substrate and the In.sub.0.53 Ga.sub.0.74 As n.sup.- layer. Also, a 0.03 .mu.m thick InP:Te n.sup.+ material layer is preferably grown on the In.sub.0.53 Ga.sub.0.47 As n.sup.- layer to prevent oxidation of the InGaAs material while the laminate structure is transferred to the semiconductor processing station where the ITO n.sup.+ material is deposited.

An Au-Zn bottom contact is evaporated onto the InP:Zn p.sup.+ layer included in the structure. A suitable photoresist mask is then used to mask the top surface of the structure, except where ITO is to be applied. Next, ITO is RF sputter-deposited onto the top surface of the structure. Thereafter, the photoresist is removed. Next, another suitable photoresist mask is applied to the ITO/contact area, and the mesa is wet-etched. Polyimide is then preferably spun onto the structure. Next, via holes are opened in the polyimide using a further suitable photoresist mask followed by wet-etching.

Finally, an Au bond pad is plated on. During this final processing, an additional suitable photoresist mask is preferably applied to the polyimide where the Au bond pad is to be plated on, and TiAu is deposited on the polyimide to enable the Au to be plated-up. Then, the photoresist mask is removed.

As shown in FIG. 5, Au-Zn top contacts can be evaporated onto the InP:Zn p.sup.+ layer before polyimide is applied to the structure. Also, an antireflection coating can be applied to the exposed ITOn.sup.+ window layer.

In an alternative embodiment of the p-i-n photodiode 10' the InGaAs layer is a 15 .mu.m thick In.sub.0.53 Ga.sub.0.47 As layer Also, a 0.5 .mu.m thick unintentionally doped InP layer is substituted for the InP:Te n.sup.+ layer. Finally, the InP:Zn p.sup.+ buffer layer is eliminated.

Representative data on the dark current of p-i-n photodiodes having the structure shown in FIG. 5 are shown in FIG. 6. These data were measured after the mesa etching step. The magnitude of the dark current is comparable to that of known InP/InGaAs/InP p-i-n photodiodes at this process step.

Measured responsivity data for extended wavelength p-i-n photodiodes 10' that have been fabricated are shown in Tables I and II below. Table I shows the responsivity for the p-i-n photodiode 10' shown in FIG. 5. Table II shows the responsivity for the alternative embodiment of the p-i-n photodiode 10' described above. The responsivity at an optical wavelength of 0.85 .mu.m is vastly improved over that of a known InP/InGaAs/InP p-i-n photodiode, while maintaining acceptable responsivity at the other wavelengths of interest.

Also, frequency response measurements for the p-i-n photodiode 10' shown in FIG. 5 for modulated 1300 nm and 1550 nm optical sources appear in FIGS. 7A and 7B, respectively. Similarly, frequency response measurements for the alternative embodiment of the p-i-n photodiode 10' described above for modulated 1300 nm and 1550 nm optical sources appear in FIGS. 8A and 8B, respectively.

The foregoing description is offered primarily for purposes of illustration. While a variety of embodiments has been disclosed, it will be readily apparent to those skilled in the art that numerous other modifications and variations not mentioned above can still be made without departing from the spirit and scope of the invention as claimed below.