1. Field
This invention relates broadly to an avalanche photodiode structure, to a method of fabricating an avalanche photodiode structure, and to devices incorporating an avalanche photodiode structure. In particular, embodiments of the present invention relate to a bandgap engineered low noise Ge—AlxGa1-xAs heterojunction avalanche photodiode structure.
2. Description of Related Art
Avalanche photodiodes (APDs) are used to convert optical signal to electrical signal and at the same time provide internal gain (amplification of the electrical signal) via an impact ionization process. Avalanche multiplication in APD enables the APD to be used in low light level applications, for example, in active optical cables (AOCs) with light signals in the range of microwatts, and helps to compensate for the signal losses in the conventional optical fiber communication systems. In contrast, pin photodiodes with unity multiplication factor require external pre-amplifiers to strengthen the signal.
Low noise in APDs can be obtained if one type of carrier has a significantly larger ionization coefficient (α for electrons and β for holes) than the other. Si was found to exhibit large α/β ratio and hence produces low multiplication (excess) noise. High speed and high capacity optical fiber telecommunication systems need to operate at long wavelengths because of their low fiber attenuation (wavelength, λ=1550 nm) and low chromatic dispersion (λ=1310 nm). The emerging AOC and optical interconnect market requires detection at 850 and 1310 nm. Unfortunately, existing Si homojunction detectors do not respond at wavelengths>1100 nm, and Ge homojunction detectors were found to suffer from an intolerably high amplification noise. Stimulated by market demand a considerable amount of work has been carried out in APD research since the development of these telecommunication systems. In seeking an alternative, III-V compound semiconductors have received much attention. However, most semiconductors have nearly equal ionization coefficients.
A need therefore exists to provide an avalanche photodiode, a method of fabricating an avalanche photodiode, and devices incorporating an avalanche photodiode that seek to address at least one of the above mentioned problems.
According to a first aspect of the present invention there is provided an avalanche photodiode structure comprising a Ge doped region having a first polarity; a GaAs doped region having a second polarity opposite to the first polarity; and an undoped region between the Ge doped region and the GaAs doped region forming a heterojunction; wherein the undoped region comprises Ge and AlxGa1-xAs.
The undoped region may comprise Ge, GaAs and AlxGa1-xAs.
The undoped region may comprise a GaAs—AlxGa1-xAs graded composition portion.
The GaAs—AlxGa1-xAs graded composition portion may extend between a Ge layer of the undoped region and a AlxGa1-xAs layer of the undoped region.
The GaAs—AlxGa1-xAs graded composition portion may extend between a Ge layer of the undoped region and a AlxGa1-xAs portion of the GaAs doped region.
The GaAs doped region may comprise an n-type light absorption layer for 850 nm and the Ge doped region comprises a p-type absorption layer for 1310 and 1550 nm.
The heterojunction may have a p-i-n configuration disposed on one of a group consisting of a Ge p-type substrate, a GaAs n-type substrate, a Ge-on-insulator p-type substrate or GaAs-on-insulator n-type substrate.
The undoped region may be smaller than 0.3 μm thick.
The undoped region may be smaller than 0.2 μm thick.
In example embodiments, 1≧x≧0.2, preferably 1≧x≧0.4.
An AlxGa1-xAs undoped layer thickness (tAlxGa1-xAs) may be greater than an Ge undoped layer thickness (tGe).
An AlxGa1-xAs undoped layer thickness (tAlxGa1-xA)s may be greater than a combined thickness of a Ge undoped layer and a GaAs undoped layer (tGe+tGaAs).
An AlxGa1-xAs undoped layer thickness (tAlxGa1-xA) may be greater than a combined thickness of a Ge undoped layer plus a GaAs—AlxGa1-xAs undoped graded layer (tGe+tGaAs—AlGaAs).
An GaAs—AlxGa1-xAs undoped graded composition layer thickness (tGaAs—AlxGa1-xAs) may be greater than a Ge undoped layer thickness (tGe).
The avalanche photodiode structure may have a detection range covering about 400-1700 nm wavelength.
The GaAs doped region may comprise a graded AlxGa1-xAs/GaAs n+ layer at an undoped-n+ interface.
The avalanche photodiode structure may further comprise ohmic contacts for the Ge doped region and the GaAs doped region respectively.
The ohmic contacts to the Ge doped region and the GaAs doped region respectively may be formed from substantially the same material.
A GaAs layer interfacing to a Ge layer of the undoped region may comprise indium.
The GaAs layer interfacing to the Ge layer of the undoped region may comprise 1% indium (In0.01Ga0.99As).
According to a second aspect of the present invention there is provided device comprising an avalanche photodiode structure as defined in the first aspect.
According to a third aspect of the present invention there is provided method of fabricating an avalanche photodiode structure, the method comprising the steps of providing a Ge doped region having a first polarity; providing a GaAs doped region having a second polarity opposite to the first polarity; and providing an undoped region between the Ge doped region and the GaAs doped region forming a heterojunction; wherein the undoped region comprises Ge and AlxGa1-xAs.
The method may further comprise forming ohmic contacts to the Ge doped region and the GaAs doped region respectively from substantially the same material.
The following description is presented to enable a person of ordinary skill in the art to make and use the various embodiments. Descriptions of specific devices, techniques, and applications are provided only as examples. Various modifications to the examples described herein will be readily apparent to those of ordinary skill in the art, and the general principles defined herein may be applied to other examples and applications without departing from the spirit and scope of the various embodiments. Thus, the various embodiments are not intended to be limited to the examples described herein and shown, but are to be accorded the scope consistent with the claims.
Example embodiments of the present invention provide a bandgap engineered low noise Ge—AlxGa1-xAs heterojunction avalanche photodiode (APD) with a large difference in the electron and hole ionization coefficients. The heterojunction APD can be used for a wide detection range of 400 nm-1600 nm, with low excess noise, at 850, 1310 and 1550 nm, owing to the suppressed ionization of the opposite carrier and dead space effects.
It has been found by the inventors that an enhanced difference in the hole and electron multiplication factors respectively can be realized due to the incorporation of a Ge layer in the undoped region of heterojunction APDs in example embodiments.
Example embodiments of the present invention incorporate an AlxGa1-xAs layer into the undoped region to suppress the device leakage current, and at the same time enhance the difference in the electron and hole impact ionization coefficients in the heterojunction APD.
In one example embodiments of the present invention, a device structure with a n(GaAs/AlxGa1-xAs)-i(AlxGa1-xAs/GaAs/Ge)-p(Ge) configuration is provided, thereby preferably minimizing carrier trapping issues.
Example embodiments of the present invention advantageously suppress the feedback of electron ionization in the heterojunction APD, thereby reducing the overall carrier transit time and increasing the operation speed.
In example embodiments, x is preferably in the range 1≧x≧0.2, and more preferably x is in the range 1≧x≧0.4.
The undoped region of example embodiments is preferably <0.3 μm thick, and more preferably <0.2 μm thick.
In the example embodiment of
In example embodiments, indium can be added into the GaAs layers interfacing to the Ge layer of the undoped region for a better lattice matching with Ge, for example about 1% (In0.01Ga0.99As).
A Ge—GaAs heterojunction structure 500 is provided. The heterojunction structure can for example be in the forms described above with reference to
A dielectric film 504 (for example SiO2) with a thickness of about 200-400 nm is deposited using plasma-enhanced chemical vapor deposition (PECVD).
A resist 506 is applied to the surface of the dielectric layer 504 using e.g. a spin-coating machine. The resist 506 is then pre-baked, where the sample is gently heated in a convection oven and then on a hotplate to evaporate the resist solvent and to partially solidify the resist 506.
The photomask 508 is brought into contact or in proximity contact with the resist 506. During a UV exposure process, the positive resist 506 undergoes a chemical reaction and leaves a negative image of the mask 508 pattern after being immersed in a developer. The sample can undergo a post-bake process to further harden the resist 506 and to remove any residue of the developer.
Using a wet chemical etch, e.g. a buffered hydrofluoric (BHF) etch, the dielectric layer 504 is selectively removed. The remaining resist is then removed using a resist solvent.
The dielectric layer 504 pattern is used as a hard mask for etching of the semiconductor materials 510 to form mesa structures 512. This can be done using dry etching, e.g. inductively coupled plasma (ICP) etching, or wet (chemical) etching. After the mesa structures 512 have been formed, the dielectric layer 504 is removed,
A dielectric (eg. SiO2 or SiNx) film and/or an antireflection coating (eg. SiO2/SiNx), indicated at numeral 514, are then deposited using PECVD for isolation and to minimize the light reflection at the wavelengths of interest.
Using a photomask (not shown) designed for contact windows, patterns for metal contacts on the mesa structures 512 can be formed by repeating the patterning steps as described above with reference to
In order to form and isolate the p- and n-type contacts 520, 522, a photomask for lift off is used by repeating the photoresist patterning steps as described above with reference to
The characteristics of the APDs according to example embodiment are demonstrated in the following using a Monte Carlo (MC) simulation program.
The MC method has been used widely in semiconductor device simulation to study hot carrier motion in a crystal under the influence of an applied electric field and subject to scattering processes according to given probabilities describing the microscopic scattering mechanisms [Chen Shiyu, Xu Kunyuan, and Wang Gang, “Monte Carlo Investigation of Size-Dependent Impact Ionization Properties in InP Under Submicron Scale”, Journal of Lightwave Technology, Vol. 27, No. 10, May 15 (2009)]. The method relies on the generation of random numbers to determine the occurrence of random events such as the termination of a carrier's free flight, selection of a scattering event, the change in flight angle and in momentum. Essential fitting parameters in the program such as phonon scattering and impact ionization rates, are obtained by fitting to the experimental data for bulk materials, whereas ionization threshold energy Eth and phonon energy hw are obtained from the literature [Adachi S., “GaAs, AlAs, and AlxGa1-xAs: Material parameters for use in research and device application”, J. Appl. Phys., 58, R1 (1985); Dargys A. and J. Kundrotas, “Handbook on Physical Properties of Ge, Si, GaAs and InP”, Vilnius, Science and Encyclopedia Publishers, (1994); Vorobyev L. E. Handbook Series on Semiconductor Parameters, vol. 1, M. Levinshtein, S. Rumyantsev and M. Shur, ed., World Scientific, London, pp. 33-57 (1996)].
The ionization rate Ri at an energy E which exceeds the threshold energy, Eth, is described by a modified Keldysh expression, given as
R
i
=C
i[(E/Eth)−1]3, (1)
where Ci is a fitted scattering rate which includes the softness factor. A power 3 was used in this model after [Beattie A. R., “Impact ionisation rate and soft energy thresholds for anisotropic parabolic band structures”, Semicond. Sci. Technol., 3, 48 (1988)] to introduce additional softness in the impact ionization probability rate, accounting for the dependence of threshold energy on direction in k space. Eth is related to the band gaps of the material by the weighted average
where EgΓ, EgX and EgL are the band gaps between the Γ, X and L-valleys and the valence band maximum at Γ. This expression for threshold energy was first used to predict the hydrostatic pressure dependence of breakdown in GaAs [Allam J., Adams A. R., Pate M. A. and Roberts J. S., “Impact ionisation in GaAs—distribution of final electron-states determined from hydrostatic-pressure measurements”, Appl. Phys. Lett., 67, 3304 (1995)] and was able to model accurately the breakdown voltages in many wide-gap III-V materials including AlxGa1-xAs [Allam J., “Universal dependence of avalanche breakdown on bandstructure: Choosing materials for high-power devices”, Jpn. J. Appl. Phys., Part 1, 36, 1529 (1997).]. Ge was also found to follow the Brillouin-zone averaged energy gap in eq. (2) [Allam J. and Adams A. R.: “High Pressure Measurements and the “Universal” Scaling of Impact Ionization with Bandstructure”, phys. stat. sol. (b) 211, 335 (1999).].
The strength of phonon scattering for electrons and holes is obtained by fitting to the multiplication characteristics of the bulk structures and is represented by energy-independent phonon scattering mean free paths, λe and λh, respectively.
Table 1 summarizes the material parameters used in the simulation.
For MC simulation in heterogeneous structures, the conduction (ΔEC) and valence (ΔEV) bandedge discontinuities of the Ge—GaAs interface were determined by the XPS technique, as shown in
Generally, the structure of the avalanche photodiodes of example embodiments can have the configurations comprising regions of (p-doped-undoped/intrinsic-n-doped) or (n-doped-undoped/intrinsic-p-doped), depending on the choice of materials and/or dopants. The undoped region is also known as the multiplication region, where multiplication occurs.
Monte Carlo simulations have been conducted for embodiments according to
The position dependent ionization coefficients, a(z) and b(z), are defined as
where n(z) and p(z) are the concentrations of electrons and holes at position z and dne(z)/dz and dph(z)/dz are the rates of increase of these quantities due to electron and hole ionization, respectively, in the directions of transport of those carriers, as obtained from the simulation.
The simulated results are presented in FIGS. 11(A)-(J) for different heterojunction photodiodes according to example embodiments, where the i-region thickness for the respective devices is 0.1 μm.
From the results in
Example embodiments of the present invention can provide a low noise Ge—AlxGa1-xAs—GaAs heterojunctions APDs covering a wide detection range of about 400 nm-1700 nm. The Ge—GaAs—AlxGa1-xAs APDs have low excess noise, at 850, 1310 and 1550 nm, owing to the suppressed ionization of the opposite carrier. Example embodiments of the present invention advantageously provide a low noise APD operating at a wavelength longer than 1100 nm, in particular at technologically important wavelengths of 1310 and 1550 nm. The APDs of example embodiments exhibit similar low noise characteristic as in Si APD.
Example embodiments of the present invention can also provide photodetectors that are sensitive to both 850 and 1310 nm wavelengths, for use in optical interconnects for consumer application (active optical cables, Intel's light peak) or fiber to the home communication systems. These applications deal with weak optical signals in the range of μW and hence low noise APDs of example embodiments operating at these wavelengths will be very useful to detect and amplify the signals.
Example embodiments of the present invention advantageously provide avalanche photodiodes that can detect and have high responsivity at 850 nm and 1100-1600 nm and with excess noise similar to that of Si.
Example embodiments of the present invention allow the same Ohmic contact for p-type Ge and n-type GaAs in the designed structure, and thereby can provide simplified processing steps and significant cost savings.
Example embodiments of the present invention can be used in applications for data- and tele-communications, single photon detection in secure communication system/Single photon avalanche diode (SPAD) and as detectors in active optical cables/optical USB.
This application claims the benefit, under 25 U.S.C. §119(e), of U.S. Provisional Application No. 61/395,887, filed May 18, 2010, which is hereby incorporated by reference in the present disclosure in its entirety.
Number | Date | Country | |
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61395887 | May 2010 | US |