Zndiffused backilluminated pin photodiodes in InGaAs/InP ... - Core

Zndiffused backilluminated pin photodiodes in InGaAs/InP grown by molecularbeam epitaxy T. P. Lee, C. A. Burrus, A. Y. Cho, K. Y. Cheng, and D. D. Manchon Citation: Appl. Phys. Lett. 37, 730 (1980); doi: 10.1063/1.92061 View online: http://dx.doi.org/10.1063/1.92061 View Table of Contents: http://apl.aip.org/resource/1/APPLAB/v37/i8 Published by the American Institute of Physics.

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Zn-diffused back-illuminated p-i-n photodiodes in InGaAs/lnP grown by molecular beam epitaxy T. P. Lee and C. A. Burrus Bell Laboratories, Crawford Hill Laboratory, Holmdel, New Jersey 07733

A. Y. Cho and K. Y. Cheng a) Bell Laboratories, Murray Hill, New Jersey 07974

D. D. Manchon, Jr. Bell Laboratories, Reading, Pennsylvania 19604

(Received 23 June 1980; accepted for publication 12 August 1980) Epitaxial films ofIDo.53 GIlo. 47 As lattice-matched to InP substrates have been grown by molecular beam epitaxy. The carrier concentrations were as low as 3 X 1015 cm 3. High-speed backilluminated p-i-n photodiodes for use at 0.95-1.6 pm wavelength were made from the resulting layers. PACS numbers: 8U5.Ef, 68.55.

+ b, 85.60.Dw

Epitaxial films of Ino.53 GIlo. 47 As, lattice-matched to InP substrates and grown by molecular beam epitaxy (MBE) for injection lasers,l.z have been reported previously. For application to photodiode detectors, however, the required material background impurity concentrations must be in the range of 1015 cm - 3 or lower. Only very recently,p-i-n photodiodes in lDo.53 GIlo.47 As/lnP having background impurities (n type) in the 1-2 X 1015 cm - 3range have been made by liquid-phase epitaxy (LPE). 3.4 We wish to report here, for the first time, MBE-grown InGaAs/lnP films with background impurities as low as 3 X 10 15 cm - 3, and to describe the performance of photodiodes fabricated from this material. The basic MBE system used here is similar to that used for the GaAs/AIGaAs growth previously described. 5 However, two different versions of the source effusion cell were investigated. In contrast to the one-cell design using an InGa mixture reported by Miller and McFee, I we chose an arrangement that uses separate In and Ga effusion cells. This avoids the rapid depletion of In experienced in the one-cell apparatus reported in Ref. 1. In our first version the In and Ga cells were separated by about 2 cm and located 10 cm from the substrate. The In and Ga beam currents were controlled independently by the cell temperatures. Because the beam intensity distributions did not exactly overlap the entire substrate (due to the offset of the beam angles relative to the substrate), only the center portion of the substrate yielded a grown layer with an ideal lattice match. An area about 3 mm by 5 mm near the center of the substrate showed a lattice-mismatch factor Aa/a less than 5 X 10- 4 . The atomic incorporation rates of both In and Ga also depended on the substrate temperature. By carefully regulating the substrate temperature throughout the growth period, layers latticematched toAa/a = 5 X 10 -4 in the direction of growth were grown as thick as 5.5 pm. The growth rate was about 1.2 pm/h. The second version of the independent In-Ga effusion cells took the form of a coaxial oven in which the In and Ga

beam intensities were controlled by apertures in the oven. The aperture ratio of the In and Ga reservoirs was approximately 1: 10. Fine adjustment of the In to Ga ratio was achieved by adjusting the oven temperature. Lattice match was obtained at an oven temperature of 910°C and a substrate temperature between 480 and 510 °C, where the As background pressure was about 10 6 Torr. With this coaxial oven arrangement, films grown over a substrate 2.5 cm by 3 cm could be lattice matched to InP. The (100) InP substrates were either Sn- or S-doped. The substrates were first polished to a mirrorlike finish with bromine-methanol on lens cleaning paper. They then were etched in a hot solution of HzS04:H202:H20 (4: 1: 1), and finally, they were treated in a diluted bromine-methanol solution for a few minutes just before loading into the growth chamber. Details of the growth procedures and the effects of growth parameters will be reported in a separate publication. The InGaAs grown layer was nominally undoped. Background impurities were reduced by use of pure starting materials, and the In source was baked in a pure Hz atmosphere for at least 16 h at 700 °C, a procedure previously used to achieve high purity in LPE-grown layers. The background impurity in the grown layers was found to be n type in the range of 3 X 10 15 -2 X 10 16 cm J. To our knowledge, the value at the low end of this range is the lowest so far

CONTACTS /

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·"On leave from Chung Cheng Institute of Technology, Taiwan, Republic of China. 730

Appl. Phys. Lett. 37(8), 15 October 1980

FIG. I. Cross section of a back-illuminated long-wavelength InGaAs p-i·n photodiode. The InP substrate is transparent at the operating wavelengths.

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achieved in InGaAs by MBE, and it is comparable to the level reported in InGaAs layers grown by LPE. The photodiodes made from the MBE material were fabricated in a back-illuminated configuration (Fig. 1) similar to those reported very recently4 but made from LPE material. Briefly, a single InGaAs layer, 5 pm thick, was grown by MBE on an InP substrate. A p-n junction in the grown layer was formed by Zn diffusion in an evacuated ampule at 500 °C. Ohmic p- and n-contacts were made by alloying electroplated Au-Zn and Au-Sn, respectively, to the appropriate surfaces. Mesas with a diameter of 80 pm ± 10 pm were formed by chemical etching, and windows about 250 pm in diameter were etched in the substrate metallization to permit unobstructed illumination of the junction through the transparent substrate. This back-illuminated configuration takes advantage of the transparency of the InP substrate to minimize absorption loss and surface recombination, and at the same time, to achieve minimum dark current and minimum parasitic capacitance; there is no need to enlarge the junction area to accommodate the contacts or contacting pads. The diodes were operated without antireflection coatings. With diodes biased near one-half of the breakdown voltage the external quantum efficiencies over the spectral region from 0.95-1.6pm are shown in Fig. 2 for two doping concentrations and two junction depths. Maximum quantum efficiency at 1.3 pm was - 55%. For the diode with the 2 X 10 16 cm -3 impurity concentration, the breakdown voltage (at 10 pA) was 10-12 V. For the diode with 3 X 10 15 cm -3 concentration, "punch-through" of the ternary layer occurred at about -20 V and breakdown (reduced by the

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Appl. Phys. Lett., Vol. 37, No.8, 15 October 1980

punch-through effect) was at 35-36 V. In the latter junction, diode capacitance was 1.5 pF at zero bias and 0.53 pF at -lOV. At small bias voltages the diode 1- V characteristic followed the normal diode equation with the ideality factor n = 2, as shown in Fig. 3. This implies that the leakage current was primarily generation-recombination current. At -10 V the leakage current was about 50 nA, somewhat higher than for our previously reported diodes of LPE InGaAs. The response time was S 0.1 ns. In conclusion, we have demonstrated for the first time that high-speed small-area back-illuminated photodiodes fabricated in the InGaAs/lnP system grown by MBE can show performance comparable to those made in LPE-grown wafers. The background impurity concentration in our MBE-grown InGaAs layers was as low as 3 X 10 15 cm -3 and was uniform over a thickness of 5 pm. With a coaxial-oven arrangement, our MBE system can grow uniform layers on large substrates. Thus it provides an alternative growth technique for InGaAs/lnP material to be used in photodiodes and other devices to be operated in the 1-1.6 pm wavelength region. We are grateful to A. G. Dentai for supplying the baked In sources, to W. R. Wagner for determining the lattice constant, to C. Radice for assistance in the MBE growth, to F. Favire for assistance in device fabrication, and to W. B. Sessa for assistance in device evaluation. J. A. Copeland provided the computer programs for the automatic determination of the diode 1- V characteristics and concentration profiles.

I B. L Miller and J. H. McFee, J. Electrochern. Soc. 125, 13 10 (1978). 2B. L Miller, J. H. McFee, R. J. Martin, and P. K. Tien, AppL Phys. Lett.

33, 44 (1978). 3R. F. Leheny, R. E. Nahory, and M. A. Pollack, Electron. Lett. IS, 650 (1979). 'T, P. Lee, C. A. Burrus, A. G. Dentai, and K. Ogawa, Electron. Lett. 16, 155 (1980). SA. Y. Cho and J. R. Arthur, Prog. Solid State Chern. 10, 157 (1975).

Lee etal.


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