20 ps (FWHM) GaAs photodiodes with transparent indium tin oxide Schottky gates

110

20 ps ( F W H M ) G a A s P H O T O D I O D E S SCHOTTKY GATES

Nuclear Instruments and Methods in Physics Research A288 (1990) 110-113 North-Holland

WITH TRANSPARENT

INDIUM TIN OXIDE

M. M I T T E L H O L Z E R , R. L O E P F E , A. S C H A E L I N a n d H. M E L C H I O R Ins'titute of Quantum Electronics, Swiss Federal Institute of Technology, ETH Zurich, CH-8093 Zurich, Switzerland

High-speed (FWHM 20 ps) GaAs photodiodes have been fabricated using highly transparent indium tin oxide (ITO) Schonky gates. The use of ITO results in diodes combining high speed and responsivities of 0.30 A / W (external quantum efficiencies of 45~) at 830 rim. The device has been mounted on a sapphire substrate embedded in coplanar waveguides. Nonlinear au~ocorrelation measurements at 583 nm show an autocorrelation response of FWHM 28 ps. Deconvolution of the autocorrelation signal (assuming Gaussian pulses) leads to a photodiode's intrinsic FWHM of 20 ps corresponding to a - 3 dB bandwidth of 24 GHz.

1. Introduction

2. Device fabrication

With photodiodes based on semitransparent metal Schottky gates it is difficult to achieve high responsivities due to the often poor transmittance of the metal layer. A better solution uses transparent contacts such highly transpe.re~t Schottky gates. We will describe the fabrication of 1~igh-speed GaAs Schottky photodiodes with large rcs~onsivities and high speed based on transparent indium tin oxide (ITO). Thin films of ITO have found wide-ranging applications in numerous optoelectronic devices [1]. They have been extensively used as transparent electrodes in various display devices, as transparent Schottky gates in solar cells and, recently, in high sensitivity photodetectors [2]. The responsivity ( A / W ) of a usual Schottky-barrier photodiode with semitransparent metal gate is limited by both the active layer thickness and the absorption and reflection of the metal gate. The use of ITO as Schottky gate and antireflection coating results in larger responsivities without increasing the active layer thickness, so that large bandwidths can be obtained. For this purpose an ITO deposition process has been developed using reactive rf ion-beam sputtering at low deposition temperatures. In this paper the fabrication of I T O / G a A s photodi.. odes in vertical geometry (mesa type) and the ITO deposition process is described. The responsivity of the I T O / G a A s photodiodes, which have been designed for the 830 nm wavelength, has been measured using a pulsed 830 nm semiconductor laser source. The intrinsic response speed of the device has been determined by nonlinear autocorrelation measurements at 583 nm. The appeal of this method is that neither high-speed electrizal connections nor cross-correlation measurements between two devices are required [3,5].

A schematic representation of the mesa-type device structure is shown in fig. 1. The I T O / G a A s mesa photodiodes consist of a 0.6 _+0.1 ~tm thick n - absorbing layer (N D = 2 × 1016 cm -3) over an underlying 2 ~m thick highly doped n + buffer and contact layer (N o = 3 × 1018 cm-3). In order to minimize parasitic capaci:ance the GaAs material is grown by liquid phase epitaxy (LPE) on a semi-insulating substrate. The diode area is 250 ~m 2 resulting in a capacitance of 46 fF at 10 V reverse bias. The ITO layer (Schottky gate) is deposited by reactive rf ion-beam sputtering. Since ionbeam sputtering, unlike rf diode sputtering, involves mirfimal intrinsic heating and electron bombardment, we used this method for the d~position of ITO films. High-quality films ca~ be reproducibly prepared at deposition temperatures below 100 ° C by carefully controlling the oxygen partial pressure p(O2) (see fig. 2). The low temperatures minimize interdiffusion so that abrupt junctions can be formed. Using an oxide target of composition 90 wt.% In203-10 wt.% SnO2, an argon pressure of 8 × 10 -4 mbar, an oxygen partial pressure of 3.8 × 10 -5 mbar, and applying a rf sputtering power of 250 W, we obtained films with a resistivity of 6.5 × 10 -4 ~ cm and a n-type carrier density of 2.7 × 1020 c m - 3 (measured by the Van der Pauw method). During film deposition, the substrate platform was rotated for film uniformity. The films are 1000 A i~*,thickness and an average visible transmission on glass substrate of Ni/Ge/Au/Ni

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87% was measured while no post-deposition annealing was required. Since the re fractice index of ITO is ~ 2 the ~t000/k thick ITO layer acts as an ideal antireflecdon coating to GaAs at 830 nm wavelength. Five photolithographic process steps follow the ITO deposition process (fig. 3). In a first step, the ITO layer is etched by a H C I : H 2 0 solution and the n - GaAs absorbing layer by a C4H606 : H202 : H 2 0 solution using the same photoresist mask. In a second step, the n + GaAs buffer layer is etched down to the semi-insulating substrate. In a third step, an ohmic n-contact ( N i / G e / A u / N i ) is evaporated, structured using a liftoff mask process and alloyed at 450 o C during 30 s. In a

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The electrical diode characteristics for a typical device show excellent rectifying characteristics with a forward bias turn-on voltage of approximately 0.5 V and a reverse-bias breakdown voltage of 23 V. The leakage current is less than 1 nA at 10 V reverse bias, the ideality factor is 1.13 + 0.1, and the built-in potential was measurod to be - 1 . 3 7 V. The measured value of the diode capacitance of 46 fF (totally depleted) corresponds well to the calculated value of 48 fF. The flip-chip-mounted I T O / G a A s photodiodes are optically controlled through the sapphire substrate. Due to surface reflection the optical transmission of the sapphire substrate is 85%. The photocurrent characteristics of the photodiodes have been measured by means of a pulsed semiconductor laser of 830 nm wavelength with a repetition rate of 100 MHz and a pulse width of less than 25 ps. The pulsed laser beam has been mechanically chopped, which allows the accurate measuring of the average photogenI1. NEW DETECTOR TYPES

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Fig. 5. Photogenerated current versus optical power at a w~velength of 830 nm and at different bias voltages. Responsivities: 0.27 A / W ( - 5 V bias) and 0.30 A / W ( - 1 2 V bias) respectively. erated current whereas the leakage current is not measured. As a result, the photodiodes e:2fibit responsivities of 0.27 A / W ( - 5 V bias) and 0.30 A / W ( - 12 V bias) respectively at 830 nm (fig. 5). Its values are limited by the thickness of the active layer.

4. Nonlinear autocorrelation measurements

The intrinsic response speed of the I T O / G a A s photodiode has been determined by nonlinear autocorrelation measurements (fig. 6). This methode combines overlapping optical pulses on the photodetector and relies on inherent detector nonlinearities to arrive at an autocorrelation function [3,5]. In particular a beam of 4 ps ( F W H M ) optical pulses from a synchronously pumped mode-locked dye laser at a wavelength of 583 nm is split in two, and one beam is delayed a variable time ¢ with respect to the other. The optical pulses have a repetition rate of 76 MHz, and the laser pulse average power is around 1 mW. The two beams ate mechanically chopped at different frequencies fl and f2 and are focused to overlapping spots onto the photodiode, which is reverse biased at 8 V. Signal components at the sum fl +/'2 of the chopping

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TIME DELAY [ p s ] Fig. 7. Autocorrelation signal of the ITO/OaAs photodiode's impulse response at 583 nm with FWHM 28 ps, measured at the sum f] + f2 of the two chopping frequencies, as a function of the time delay ~. frequencies are detected using a lock-in amplifier (fig. 7). Components at f~ +/'2 are present only if the photodetector's response is nonlinear with respect to the incident optical er.~ergies. The magnitude of ~he nonlinear response depends upon the arrival time difference between the two trains of pulses, ranging from a maximum mutual effect for simultaneously arriving pulses to no interaction for pulses widely separated in time. Such measurements yield directly the autocorrelation function of the temporal response of the device. If the electrical output signal l ( t ) of the photodiode is assumed to depend upon the instantaneous number of incident photons N(t), the detected signal Q(~) at ./1 + f2 has been shown [3] to be, t o lowest order,

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M. Mittelholzer et a L / 20 ps (FWHM) GaAs photodiodes

between the optical pulses. The F W H M of the photodiode's autocorrelation signal is 28 ps. Assuming Gaussian pulses, deconvolution of the optical pulse width leads to a photodiode's intrinsic F W H M of 20 ps, which corresponds to a - 3 dB bandwidth of 24 GHz. The: appeal of this technique is that the only highspeed part of the experimental system need to be the optical pulse themselves. It gives the intrinsic response of the diode. For practical use the photodiodes have nonetheless to be mounted into broadband microwave structures.

5. Conclusions A high-speed 250 ~m 2 GaAs Schottky barrier photodiode for the wavelength range of 0.8 lxm has been designed, fabricated and its impulse response measured by a nonlinear autocorrelation method. The device exhibits a quantum efficiency of 45% (0.30 A / W ) at 830 nm due to the use of a transparent layer of indium tin oxide (ITO) to form the rectifying contact. Assuming Gaussian impulse responses, the - 3 dB bandwidth has been determined to be 24 GHz (20 ps FWHM). The broadband and high sensitivity I T O / G a A s photodiodes

113

may find applications as monitor diodes for characterizing optical pulses and as detector diodes in 0.8-~mwavelength fiber optic communication systems.

Acknowledgements The authors would like to thank H. Jaeckei, IBM Rueschlikon (Zurich), for permission to use the short pulse laser facilities for the high speed measurements. They also thank W. Herrmann (ETH Zurich) for assistance in ITO deposition and P. Brack (ETH Zurich) for maintenance of the vacuum systems.

References [1] K.L. Chopra, S. Major and D.K. Pandya, Thin Solid Films 102 (1983) 1. [2] D.G. Parker, Electron. Lett. 21 (1985) 778. [3] T.F. Carmthers and J.F. Weller, Appl. Phys. Lett. 48 (1986) 460. [4] P. Schmid and H. Melchior, Rev. Sci. Instr. 55 (1984) 1854. [5] R. Loepfe, A. Schaelin, H. Melchior and M. Blaser, Appl. Phys. Lett. 52 (1988) 2130.

11. NEW DETECTOR TYPES