Hetero-interface avalance photodetector

Details Hetero-interface avalance photodetector Hetero-interface avalance photodetector

2001-10-03

2003-06-24

Wilson, Allan R. (Department: 2815)

Active solid-state devices (e.g., transistors, solid-state diode

Responsive to non-electrical signal

Electromagnetic or particle radiation

C257S461000, C257S464000

Reexamination Certificate

active

06583482

ABSTRACT:

FIELD OF THE INVENTION
This invention is directed toward avalanche photodetectors (APDs) useful in high-speed optical fiber applications. More particularly, the invention is related to avalanche photodetectors having high and controllable electric fields in both their absorption and multiplication regions.
BACKGROUND OF THE INVENTION
Optical fiber transmission systems are being increasingly deployed to meet the demand for high throughput telecommunication and data signal transmission. With the increasing use of optical fiber transmission systems there is a concomitant need for cost-effective high performance (i.e., low noise, high speed) components to interface between the optical fiber used for signal transmission and receiver electronics.
Interface components such as optical detectors used with the optical fiber systems are often based on the InGaAs/InP material system. InGaAs exhibits material properties such as high absorption of light in the wavelength range between 1.3 &mgr;m and 1.5 &mgr;m, high carrier mobilities and saturation velocities. These properties make InGaAs a material of choice for making efficient, high-speed photodiodes. InGaAs layers are conveniently grown on InP substrates using common epitaxial growth techniques. Most of today's APDs use InGaAs as the material for the absorption region, and use InP as the material for the multiplication region. The avalanche multiplication gain and hence the overall gain-bandwidth performance of an APD depends on the difference between the electron and hole ionization coefficients of the material used for the multiplication region. For InP material these coefficients are nearly the same. Therefore, the gain-bandwidth product of InGaAs/InP APDs is limited.
Materials such as silicon which have dissimilar electron and hole ionization coefficients are more suitable than InP for obtaining high multiplication gain. Silicon is indeed used for making low-noise and high-speed operation APDs, for example, high-performance reach-through APDs. However, silicon does not absorb light in the optical wavelength regions used for optical fiber signal transmission and cannot be used as the absorption region material for light in the wavelength range between 1.3 &mgr;m and 1.5 &mgr;m.
Recently, attempts have been made to combine the desirable optical properties of InGaAs material with the desirable low-noise and high-speed properties of silicon. Using wafer fusion techniques, composite InGaAs/silicon mesa-type APDs have been demonstrated. In these APDs an InGaAs/InP substrate wafer is fused on top of a silicon wafer. The InP substrate is etched away to leave a thin InGaAs layer on top of the silicon wafer. Portions of the silicon substrate wafer are used as the multiplication region while the InGaAs layer is used as the absorption region. Mesa type device structures are formed to limit the active area of the device. Electrical contacts are formed at the top of the InGaAs mesa and at the bottom of the silicon substrate. In the operation of an APD, a reverse bias voltage is applied across these contacts. The reverse bias voltage creates electric fields between the contacts across the multiplication and the absorption regions.
For the proper operation of these composite InGaAs/silicon APD devices, it is essential that the electric field in the InGaAs absorption regions and the silicon multiplication regions be well controlled. The two regions have contradictory electric field requirements for their proper operation. For an InGaAs/silicon APD to operate at high frequencies, the electric field in its InGaAs absorption region should be sufficiently high to impart high velocities to the photogenerated charge carriers, but should be smaller than about 150 kV/cm to suppress carrier tunneling in the InGaAs material. However, the electric field in the silicon multiplication region preferably should be above 300 kV/cm for efficient avalanche multiplication.
In an attempt to separately set the electric fields in the absorption region and the multiplication region, the prior art composite APDs use a scheme in which a thin p
+
doped silicon layer separates the two regions. The p
+
layer may be a few nanometers thick. The thin p
+
layer may be formed, for example, by shallow ion-implantation techniques. A portion of the reverse bias voltage applied to an APD is used in depleting the p
+
layer. This reduces the voltage drop across the absorption region itself and may allow the electric fields in the absorption region to be set within a different range of values than the range of electric field values in the silicon multiplication region.
However, for this scheme to work the specifications on thickness and doping concentration of the p
+
doped silicon layer are stringent. If either the doping concentration or the thickness of the p
+
layer is on the high side, a larger portion of the applied reverse bias voltage is used to deplete the p
+
layer. The consequently smaller voltage drop across the absorption region results in low values of electric field in the absorption region. These low values of electric field impair the bandwidth performance of the device. The reverse bias voltage required to completely deplete the p
+
layer may even exceed the breakdown voltage of the device if either the doping or the thickness of the p
+
layer is excessively on the high side. Similarly, if either the doping or the thickness of the p
+
layer is on the low side, a smaller portion of the applied reverse bias voltage is used to deplete the p
+
layer. Consequently, the p
+
layer may not prevent the electric fields in the InGaAs layer from attaining high values at which the undesirable carrier tunneling occurs.
Unfortunately, the thicknesses and doping concentrations of thin implanted p
+
layers are parameters that are susceptible to fabrication process variations. Conventional shallow ion-implantation techniques are not yet sufficiently developed to reliably or reproducibly control these parameters.
It would therefore be desirable to have composite APD structures in which the electric fields do not depend on sensitive process parameters and which can be fabricated with a wider processing latitude.
Another disadvantage of prior-art composite APD structures is that the electric field values in the absorption regions also are sensitive to changes in the applied reverse bias voltage. In APDs with p
+
doped layers, the electric fields are at a maximum at the edges of the absorption regions. With increasing reverse bias voltages the electric fields increasingly penetrate into the multiplication regions in a direction away from the absorption layers. This feature of the APD structures with p
+
doped layers is not conducive to controlling electric fields at high values within the absorption regions. Slight changes in the reverse bias voltages may cause large changes in the electric fields in the absorption regions. It is therefore further desirable to have composite APD structures in which the electric fields in the absorption regions change gradually in response to changes in the applied reverse bias voltages.
SUMMARY OF THE INVENTION
The invention is directed toward composite APD devices in which the multiplication regions and absorption regions are made of different semiconductor materials. The APD device structures of the present invention are designed to achieve control of electric fields in the absorption regions and the multiplication regions without use of intervening p
+
doped layers. The composite APD devices may be fabricated using wafer fusion, bonding or any other suitable techniques for making composite structures.
Control over the electrical field values in an APD device is obtained by placing the p-n junction diode that is used to separate charge carriers in the APD device at some suitable distance away from the absorption region in the device structure. The electrical fields values in the absorption region and the multiplication region may be controllably chosen for

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