Organic-GaAs heterostructure diodes for microwave applications

Applied Surface Science 234 (2004) 22–27

Organic-GaAs heterostructure diodes for microwave applications G. Ginev*, T. Riedl, R. Parashkov, H-H. Johannes, W. Kowalsky Institute of High-Frequency Technology, Technical University, Braunschweig, 22 Schleinitzstrasse, 38106 Braunschweig, Germany Available online 20 July 2004

Abstract Heterostructure devices using three different organic materials, namely PTCDA (3,4,9,10-perylenetetracarboxylic dianhydride), DiMe-PTCDI (dimethyl-3,4,9,10-perylene tetracarboxylic diimide) and CuPc (copper phthalocyanine) evaporated on GaAs substrate have been prepared. To accomplish the diode structure a silver metalisation on top of the organic layer (OL) and an ohmic contact to GaAs were fabricated. The lateral geometry of the samples is convenient to contact with a microwave probe in ground-signal-ground configuration for characterization directly on the wafer. Four groups of diodes with different active area diameters 10, 20, 40 and 80 mm have been prepared, respectively. Their characteristics in dependence on the device geometry have been determined and compared in a large frequency range (up to 2 GHz). Reflection parameters have been evaluated in dependence on the bias current. The data have been used for non-linear modeling of the diode equivalent circuit. We have measured frequency conversion ability of the devices in a single diode mixer scheme and discuss the suitability of the devices for microwave applications. The active role, played by the organic material and the organic-inorganic interface is considered in the explanation of the experimental results. # 2004 Published by Elsevier B.V. Keywords: Organic-GaAs heterostructure diodes; Heterostructure devices; Microwave applications

1. Introduction In the last few decades the organic semiconductors and organic–inorganic semiconductor interfaces have been a subject of intensive research. The potential of these materials for photonic and electronic device applications has already been demonstrated. A successful fabrication of organic and organic-on-inorganic heterostructures useful for electroluminescent devices, waveguides and photodetectors has been realized [1–6]. Schottky diodes made by introducing a thin PTCDA organic layer in InP and GaAs based structures have been shown to exhibit promising *

Corresponding author.

0169-4332/$ – see front matter # 2004 Published by Elsevier B.V. doi:10.1016/j.apsusc.2004.05.085

characteristics for microwave applications [2,7]. The investigations on other organic substances in this practical direction are ongoing. The devices studied in this work have been prepared by using three different organic materials—PTCDA, DiMe-PTCDI and CuPc. We evaluated our diodes with regard to their capabilities for frequency conversion, receiving and processing signals and we compared them to reference samples without OL.

2. Devices preparation The planar structure of an organic–inorganic (OI) diode is shown on Fig. 1 (inset). The details of the

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Fig. 1. JV characteristics of GaAs/PTCDA/Ag diodes with different thickness of the organic layers. The inset shows the planar structure of a GaAs/organic/Ag diode.

Fig. 2. JV characteristics of GaAs/organic/Ag diodes with 20 nm thick organic films.

fabrication process are described in Refs. [2,7]. A hard-baked, 1400 nm thick photoresist layer is used instead of SiO2 one and defines the active areas of the devices with diameter d (10, 20, 40 and 80 mm). This substitution simplifies the technology sequence and reduces the parasitic capacitances of the diodes. Improved ohmic contacts to the n-GaAs wafer are achieved by using a combination of Au:Ge, Ni and Au metal layers with respective thickness of 30, 20 and 180 nm. After the deposition of the metals, a rapid thermal annealing at 400 8C for 10 s in nitrogen atmosphere was applied. A variety of cleaning and passivation procedures of the GaAs surface before the evaporation of the organic material have been tested [8,9]. We chose 15 s dip in 1% H as the most suitable one [2,7]. We have no information about the reconstruction of the GaAs surface, but a mechanism similar to other halogen passivation procedures is the most likely one [4]. An explanation of the surface stoichiometry can be found in the literature [10]. Problems with the final lift-off process have been observed only in the case of the CuPc samples.

curves we extracted the device characteristics, e.g. junction resistance RJ and effective barrier height FBn (Fig. 3). The reverse JV characteristics have been used for estimation of the edge-leakage currents and their dependence on the device geometry (Fig. 4) [11–13]. The junction capacitance CJ and serial resistance RS are the important parameters which affect the mixing behavior of the diodes, since they characterize the high-frequency losses through the pseudo Schottky-diode junction. Their values have been determined considering the equivalent circuit of the devices (Fig. 5) and by measuring the reflection

3. Experimental The current density–voltage (JV, Figs. 1 and 2) and capacitance–voltage (CV) characteristics have been obtained by conventional methods. All CV measurements have been taken at 100 kHz. From JV and CV

Fig. 3. Effective barrier height of GaAs/organic/Ag diodes extracted from the JV and CV characteristics. The plots are guided from the average values.

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Fig. 4. Edge-leakage currents of GaAs/Ag and GaAs/organic/Ag diodes.

parameter S11 as function of the frequency [2,7,10– 13]. Conversion of signals from high to low frequencies is one of the major applications of Schottky diodes. If a constant bias voltage and two high-frequency signals named radio frequency (RF) and local oscillator (LO) are applied, the amplitude of the resulting mixed signal is linearly dependent of the LO amplitude and the frequency is fIF ¼ fLO  fRf, where fLO and fRf are the LO and RF signal frequencies. The mixing efficiency is measured by the conversion gain (CG) which is defined as the ratio CG ¼ PIF/ PRf where PRf is the power of the RF signal and PIF is

Fig. 5. Junction capacitances of GaAs/organic/Ag diodes obtained from reflection measurements. Setup and equivalent circuit of the devices are shown in the inset.

the power of the output at the intermediate frequency. When PIF and PRf are measured in dBm: CG ½dB ¼ PIF ½dBm  PRF ½dBm

(1)

The dependence of CG on applied to the device external dc voltage characterize the mixing properties of the diode. It has a maximal value for optimum bias conditions. For higher voltages the signal distortion is present and to lowering of the CG values [12]. The mixing ability of the devices was determined using the electrical scheme shown on Fig. 6. LO and RF are sources generating the signals with fLO and fRf

Fig. 6. Electrical scheme for the measurement of the mixing characteristics of GaAs/organic/Ag diodes.

G. Ginev et al. / Applied Surface Science 234 (2004) 22–27

frequencies. The narrow band pass yttrium–iron garnet (YIG) filter cuts unwanted additional harmonics which avoid losses generated by them. A micro strip filter network is used for the separation of the high frequency parts from intermediate the frequency (IF) and supplies the diodes with bias voltage. For adjustment of the output a double-stub-tuner and a matching network are included in the circuit. All relevant parameters were tuned for each device individually. By all ac characterization methods the diodes were contacted with a microwave probe directly on the wafer [2,7,12,13].

4. Results and discussion The results presented in this paper are mainly for the devices with d ¼ 40 mm. Like in previous investigations they show an optimum geometry for mixer application [2,7]. In some cases the diodes with the other diameters of the active area will be discussed. In Fig. 1 we show a typical JV plots for GaAs/Ag, GaAs/PTCDA/Ag and in Fig. 2 for GaAs/20 nm OL/ Ag diodes. For voltages below 0.5 V the samples with OL have higher current densities in comparison with the reference samples. It indicates a lowering of the barrier FBn with the introduction of the additional organic layer. However, higher tunneling currents and space charge limited currents through the organic film cause a part of this effect. This suggestion is supported also by the larger than unity values obtained for the ideality factors and a slight lowering of RJ with increasing the thickness of the organic film. For voltages higher than 0.5 the devices with OL have lower current densities due to additional resistance introduced by the organic material. Similar results for GaAs/PTCDA/Ag diodes have been reported in Refs. [6,7]. In Fig. 3 we summarize the values of the effective barrier height for samples with different thicknesses of the organic layer. Because of the tunneling currents, the values extracted from the JV characteristics are taken as a lower limit. The diffusion nature of the device capacitance increases up the values of effective energy barrier obtained from the CV characteristics and thus they are considered as a higher limit. The outlined minimum of FBn for devices with 20 nm thick PTCDA and DiMe-PTCDI is related to the

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organic–inorganic interface rather than to the introduced organic layer itself. For diodes with more than 20 nm thick OL the overall device performance is affected by the transport properties of the organic material [6]. The plot for the samples with CuPc layer has a different trace and can be explained with the differences in the molecular structure of the materials and their different crystal organization on the GaAs surface [14]. CV data for the CuPc diodes are not taken into account. Edge-leakage currents appear under reverse bias and they disturb the normal operation of the Schottky-diodes [11]. To illustrate this behavior Fig. 4 shows the reverse current as a function of d for devices with a 10 nm PTCDA layer compared to a device without OL. The slope of the plot for OI diodes is smaller than the corresponding one for the sample without OL. This shows that the edge-leakage currents of heterostructure diodes are strongly more independent on device geometry and they might be a dominant component. To avoid this negative effect on the current transport through the OI interface a guard ring can be introduced in a more optimized device structure [11]. Reflection measurements have been performed for frequencies between 50 and 2000 MHz. The setup is shown in Fig. 5 (inset). In order to achieve near flat band conditions a positive bias of 0.4 V was applied to all devices. The junction capacitances and serial resistance were obtained considering the equivalent circuit of the device. The junction resistance RJ is already determined from dJ/dV plots. The parasitic capacitance CP introduced by the photoresist layer was calculated from the device geometry and has values around 0.3 pF. C0 and R0 represent the capacitance and resistance of the organic layer. R0 is insignificant compared to the 1/(2pfC0). Generally, the serial connection of C0 and CJ of the reference sample explains the decrease of the overall device capacitance but gives two times higher values for the dielectric constant of the organic material. An additional capacitance of the OI interface and the defects existing in the organic film has to be considered. The capacitances obtained from the CV measurements at 0 V are approximately two times higher and have an similar dependence on the OL thickness. The correlation between the conversion gain with the type of organic material and its thickness are

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5. Conclusions

Fig. 7. Mixing efficiency of GaAs/organic/Ag diodes with 20 nm thick organic layers. The lower and upper insets show the data for devices with 10 and 30 nm organic layers, respectively.

shown on Fig. 7. A typical maximum appears when the CG is plotted versus applied positive bias and denotes the optimum working condition for every device. The introduction of a 10 nm OL (Fig. 7, lower inset) lowers the optimum bias by 0.1 V. The maximum values of CG for the OI diodes are reduced, but for some applications DiMe-PTCDI and CuPc have sufficient CG conversion gains at 0.2 V. The samples with 20 nm PTCDA and DiMe-PTCDI layers show the lowest optimum bias 0.2 and 0.3 V, respectively. The previous values reported for GaAs/20 nm PTCDA/Ag are around 0.7 V [7]. The improvement is a result from the optimized technology sequence and the higher doping concentrations of the used GaAs wafers. The lowered values of the optimal bias for GaAs/organic/Ag diodes are a combination of reduced FBn and lower junction capacitances. For the samples with 30 nm OL (Fig. 7, upper inset), the increase of the barrier height and the added resistance of the organic material moderate this effect. The diodes with DiMe-PTCDI and CuPc are mixing in an extended range of bias voltages compared to the PTCDA samples. A reason for that is different qualities of the organic films. In contrast, the diodes with 30 nm CuPc have shown the optimal bias voltage. This can be explained with the different type of conductivity of this organic semiconductor and the different absorption of the organic molecules on the passivated surface [14].

A successful fabrication of GaAs/organic/Ag devices has been presented and their capabilities for mixer application have been discussed. For a thickness of OL up to 20 nm the OI diode characteristics are affected by the change in the Fermi level position relative to the GaAs band edges, which leads to a lower FBn. This effect disappears for samples with 30 nm thick OL due to the added bulk resistance of the organic material. The Schottky diodes with an introduced OL have enhanced mixing characteristics compared to the reference samples without OL. This is achieved by the lowering of the effective energy barrier height and junction capacitance. The samples with 20 nm PTCDA or DiMe-PTCDI layer have optimum bias voltage for mixer application is lower by 0.4 V lower compared to the reference diodes. The devices with 20 nm PTCDA and 30 nm CuPc film show acceptable mixing levels even close to 0 V. This makes them suitable for non-dc-biased mixer circuits with essentially reduced conversion losses at low local oscillator power levels. The use of different organic materials and different OL thickness offers a possibility to control the basic OI interface properties and thus the overall device performance.

Acknowledgements The research was supported by the EU funded Human Potential Research Training Network DIODE (Contract no. HPRN-CT-1999-00164).

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