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Both AuGe based alloys and Ti/Au metal layer stacks are widely used as ohmic metal contacts for photoconductive THz antennas made of low temperature grown GaAs. Here, we present the first systematic comparison between these two metallization types. A series of antennas of both kinds is excited by femtosecond laser pulses and by the emission from two diode lasers, i.e. we test the structures as pulsed THz emitters and as photomixers. In both cases, coherent and incoherent detection schemes are employed. We find that the power emitted from the antennas with AuGe metallization is 50% higher than that of antennas with a Ti/Au metal layer. From a comparison with a photomixer model we conclude that the higher output power results from a lower contact resistance of the AuGe contacts leading to an increased current flow. However, Ti/Au contacts have a higher thermal stability which might be advantageous if high system stability is called for.
©2008 Optical Society of America
1. Introduction
Low temperature grown gallium arsenide (LT GaAs) is established as a key material for the generation and detection of THz radiation [1–5]. Consequently, several groups have worked on the improvement of LT GaAs devices [6,7]. A lot of work has been devoted to increase the efficiency of the LT GaAs material and the design of optimal geometries for the device structures [8–10]. However, further enhancement of the performance of such an antenna is expected by the optimization of the device contacts [11,12]. Hence, the question arises: What is the metallization which provides the best properties for THz applications? In 1996 Zhang already presented a study in which he investigated a series of metals like Cu, Al, Au and Ag, and their effect on the performance of THz antennas [13]. Yet, the most widely used contact types for photoconductive THz antennas are metallizations based on AuGe alloys [1,14–16] (e.g. Ni/AuGe/Ni/Au) and Ti/Au metal layer stacks [17–22]. Unfortunately, no systematic comparison between these two metallization types has been reported so far. Here, we present a quantitative study of the influence of both contact types on the performance of photomixers and pulsed THz antennas based on LT GaAs.
The choice of a suitable type of metallization strongly depends on whether the semiconductor material is n-doped, p-doped or even an LT GaAs substrate. For decades both types, the AuGe alloy and Ti/Au layer stack, have been found to be good candidates for contacts to n-doped III–V semiconductors and their properties have been thoroughly studied [23–25]. GaAs grown at low temperatures exhibits an excess of arsenic atoms in comparison to conventional GaAs. From this point of view LT GaAs holds similarities to a highly n-doped semiconductor. Therefore, researchers deemed that the AuGe and Ti/Au contacts are also appropriate for LT GaAs. However, the properties of contacts to LT GaAs are different in comparison to conventional n-doped material. To clearly point out the differences, it is worth giving a brief overview on the effects of these contacts to n-doped GaAs.
The contact based on the AuGe eutectic alloy was first used by J.B. Gunn in 1964 and then investigated by Braslau in detail [26,27]. This contact system has been used for over a quarter century for advanced n-doped GaAs devices [28,29] and significant improvements of the contact properties have been made [30–32]. Its advantages are summarized as follow: i) the contacts are reproducible, ii) it exhibits a high conductivity after an annealing process, and iii) conventional preparation techniques and various annealing methods can be used [23].
The Ti/Au metallization to n-doped GaAs gained in interest as there is an increasing demand for non-annealed contacts. One reason for this is that semiconductor devices continually become smaller, and hence they are more and more susceptible to damage due to overheating through the process of annealing. Besides, Ti/Au contacts are preferred for their thermal stability. As titanium has a much higher melting temperature (1725°C) in comparison to metals like gold (1063°C) or germanium (959°C) [33], the Ti/Au contact is less reactive than the AuGe alloy. Thus, a reliable operation of the device can be ensured. On n-doped GaAs both, Ti/Au and AuGe form Schottky contacts prior to annealing [23]. However, from extensive studies on n-doped GaAs it is known that both Ti/Au and the contact based on the AuGe alloy (e.g. Ni/AuGe) interact with the substrate during annealing and create intermediate layers of TiAs and NiAs(Ge), respectively [23,34–36]. This additional n-doping of the semiconductor caused by the intermediate layers reduces the Schottky barrier for the Ti/Au (0,7 eV) as well as for the AuGe-based metallization (0.2–0.4 eV). In contrast to the Ti/Au contact, the AuGe based contacts show a lower contact resistance, which is due to the high diffusion depth (200nm) of Ge into the bulk of the n-doped semiconductor [23,32]. In essence, AuGe based contacts on n-doped GaAs become ohmic after annealing while the Ti/Au contacts remain Schottky-like due to the lower impact of the annealing process [23,35]. However, we focus our attention on the investigation of the effect of contacts to antennas fabricated on low temperature grown GaAs, since this type of semiconductor is widely used for the generation and detection of THz radiation [1–5,14]. Due to the low growing temperature, LT GaAs exhibits deep arsenide donor states (AsGa) within the entire substrate.
In the bulk of the semiconductor these traps are mostly filled by electrons. In contrast to that, vacancies in the contact region remain unoccupied because of the band bending. This reduces the effective barrier height between the metal and the semiconductor [38]. Thus, carriers at the Fermi energy level only need to pass through a low barrier to fill these states. This effect has been reported by H. Yamamoto et al. [38] for AuGe alloys and by M. Griebel for Ti/Au and AuGe contacts [39]. Thus, a contact metallization such as Ti/Au, that creates Schottky-contacts on conventional n-doped GaAs, shows ohmic behavior on LT GaAs even without being annealed [38–46]. As Titanium is less reactive, a noticeable change in the contact resistance of the Ti/Au contact is expected at higher temperatures far beyond 600°C, but higher temperatures can cause subsequent annealing and a drastic change of the properties of the LT GaAs material itself. Below this temperature, the resistance alteration is negligible [39,47–48]. In contrast, the contact resistance decreases for the AuGe based metallization to LT GaAs at temperatures around 420°C [12]. This is similar to the behavior of such a contact to n-doped GaAs. Therefore, enhanced THz emission is expected from antennas with AuGe based contacts, whereas the antennas with Ti/Au contacts possess a distinct thermal stability.
In summary, both contact types are widely used for photoconductive THz antennas because of their ohmic behavior on low temperature grown GaAs. While the metallization based on the AuGe eutectic alloy needs to be annealed for an optimal performance, no annealing is required for the Ti/Au contacts [39,47–48]. Although annealing is also possible for Ti/Au metallization it has no considerable effect. Moreover, the Ti/Au contacts are more stable under laser illumination. In the following we perform a systematic comparison on the device efficiency mediated by these two metallization types.
2. Experimental setups and fabrication details
To investigate the performance of the AuGe type and Ti/Au contacts we use structures of both metallization types as THz emitter antennas. We use the antennas for the generation of broadband THz pulses and for the generation of continuous wave (cw) THz radiation. In the pulsed case the antennas are gated by femtosecond laser pulses [1–3,8,49], for cw THz generation the antennas are driven by the emission from a two-diode laser system [9,10,50– 52].
2.1 Semiconductor material:
A 1.5 µm thick undoped LT GaAs layer is deposited onto a semi insulating (SI) GaAs wafer. The growth process is performed at 250 °C monitored by thermocouple reading from the substrate side and followed by in-situ isothermal annealing at 600°C for 10 min. We deposit the two types of metallic structures onto the LT GaAs using a conventional photolithography and lift-off technique. Exact growing parameters are reported in detail elsewhere [9,11].
2.2 Antenna structures and metallization:
Antennas with different kinds of geometries are fabricated depending on whether they are used as photomixers or for the generation of THz pulses. Two antenna designs are produced i) the typical stripline-fed dipole structure with a photoconductive gap of 5 µm and a dipole length of 100 µm for photomixing experiments [50] and ii) a coplanar stripline (CPS) with a gap of 10 µm for experiments under pulsed excitation [1,49]. To ensure a fair comparison between the two types of metallization the antenna geometry is identical within the same experimental configuration.
The contacts based on the AuGe alloy are fabricated using a Ni/AuGe/Ni/Au layer stack with 5/90/25/50 nm thickness and subsequent annealing at 420°C for 90 seconds. Non-annealed ohmic Ti/Au contacts are fabricated using Ti/Au with a thickness of 10/160 nm.
Fig. 1. (a) Coherent detection setup: a second photoconductive antenna is used as detector. The excitation is performed either with the emission of two diode lasers or with femtosecond laser pulses. b) Incoherent detection scheme: a Golay cell is used as detector.
2.3 Experimental setups:
We perform four different kinds of measurements. In all cases, antennas with both types of metallization are used as emitters. Firstly, to generate cw THz radiation, the antennas are gated by the emission from two Toptica DFB diode lasers (λ=860 nm) in a coherent detection setup (Fig. 1(a)). A second antenna with a photoconductive gap of 5 µm, a dipole length of 100 µm and AuGe type contacts is used as a detector. The incident optical power is Popt=42 mW for each photomixer and the bipolar chopped bias voltage is 40 V at 4.03 kHz [50]. The current flowing in the receiver is measured via lock-in detection. The focusing spot size of the laser on the photoconductive gap is approximately 3.4 µm.
Secondly, to generate short THz pulses, we use a mode locked Ti:sapphire laser from Femtolasers with 20 fs pulse duration, a central wavelength of 800 nm and a repetition rate of 80 MHz. As mentioned above we use coplanar stripline antennas for these experiments. Detection is performed coherently using a second antenna with a photoconductive gap of 5 µm, a dipole length of 20 µm and AuGe type contacts (Fig. 1(a)). Each antenna is gated with an optical power of P̄opt=30 mW and the bipolar chopped bias voltage was 30 V at 1.01 kHz. The focusing spot size of the laser on the photoconductive gap is approximately 4 µm.
Thirdly, to ensure a reproducible experimental comparison between the two types of photomixers, which are excited with two diode lasers, we also use a Golay cell for incoherent detection (Fig. 1 (b)). A Golay cell is a comparatively large power detector, which is far less prone to misalignment than coherent detection schemes. The 20 V bias voltage applied to the antennas is unipolar chopped at 12 Hz for the use of a lock-in amplifier.
Fourthly and finally, we also use a Golay cell to detect the THz pulses generated with the striplines and the femtosecond laser (Fig. 1(b)).
3. Results and discussion
3.1 Photomixing:
To compare the radiated power of the photomixers with annealed AuGe based contacts and the Ti/Au metallization we simulated and measured its spectral distribution. The model is based on a finite integration technique (FIT) simulation (used software: CST microwave studio) of the antenna impedance RL (Fig. 2) and an equivalent electronic circuit reported by Brown et al. [51]. Note, that this model only holds true for photomixing and not for pulsed excitation. The equivalent electronic circuit, which is shown in Fig. 3 includes the applied bias VB, the antenna impedance RL, the photomixers capacitance C, the contact resistance RS and the photoconductance G0.
G0 is given by
where µe and µh are the carrier mobilities, η is the quantum efficiency, Topt is the transmission coefficient, e is the electron charge, h is Planck’s constant, τ is the photocarrier lifetime, ν is the frequency, A is the photoconductive area illuminated by the laser spot and P;-opt is the average optical input power. In contrast to Brown et al., who neglected the contact resistance RS, we take it into account. Therefore, an effective photoconductance Geff is derived.
To obtain the contact resistance RS, we developed formula (3) from the equivalent electronic circuit.
However, at ω=0 the admittance of the capacitance C vanishes and the expression can be simplified:
The values for the total resistance Rtot are achieved experimentally. For this we measured the photocurrent under bias and laser illumination. The voltage-current ratio leads to the total resistance of the electronic circuit. The values for the contact resistances are determined to be RSTi/Au=47.5 kΩ and RSAuGe=20.2 kΩ.
After including Geff into Browns expression of the radiated power, the formula is modified into
The ratio of the two, the power radiated by the antenna with AuGe based contacts and the Ti/Au layer stack, results in
According to Eq. (1), the photoconductance G0 depends on the pump power. On the one hand, for large values of the pump power the product of RS and G0 becomes large. Hence, Eq. 6 is simplified to
which is not dependent on G0 in this case. On the other hand, for small values of the pump power the ratio of the output power becomes unity. The value of the photoconductance used in the experiments is G0=9.87µS. In this case, the ratio of the output power is 1.5.
The simulated data obtained from expression (5) are shown in Fig. 4. These results are in good agreement with the experimental photomixer data obtained with a Golay cell as detector (Fig. 5). It can be seen that the structure with AuGe based contacts surpass the structure with Ti/Au contacts by a factor about 1.5, which can be easily confirmed with the power ratio calculated from expression (6). The comb of resonances, arising from the antenna impedance (Fig. 2), is identical for the antennas with both types of metallization. For our calculation we used the parameters µ=(µe+µh)=250 cm2V-1s-1, η=0.5, τ=250 fs, P̄opt=42 mW, G0=9.87µS and VB=20 V.
Figure 6 shows the dependence of the radiated power on the bias field. At a bias electric field of 40 kV/cm the AuGe type antennas outperform the Ti/Au type antennas by a factor of 1.61. Since the THz power is proportional to the radiated electric field squared we estimate that the THz field emitted from AuGe type structure is about 1.26 times larger than that from the antenna with Ti/Au contacts. Below 30 kV/cm both antennas show the typical quadratic dependence on the bias voltage [52]. At higher bias fields the output power saturates for both devices which we attribute to an increase in the response time of LT GaAs switches at high voltage bias [21].
Finally, we measure the amplitude of the electric field via the coherent detection scheme (Fig. 1(a)) at frequencies of 125 GHz and 290 GHz (Fig. 7). At both frequencies the peak value of the electric field is 1.28 times higher for the structure with AuGe based contacts, which corresponds to a power ratio of 1.63. Hence, these results are in good agreement with the above results obtained with a Golay cell.
3.2 Experiments under pulsed laser excitation:
In the following we present results for pulsed optical excitation. As discussed above these experiments are performed on coplanar striplines. Fig. 8 shows the THz power detected with a Golay cell as a function of the applied electric field. Again, we observe the typical quadratic dependence on the bias. To avoid device damage, the antennas are only driven at electric fields below 30 kV/cm under femtosecond illumination.
Fig. 8. Output power of the coplanar striplines under femtosecond illumination. The power is measured with the Golay cell.
To ensure reproducibility, two samples per contact type are measured. The emitters with AuGe based contacts outperform the Ti/Au-type emitters by a factor of almost two in this case. Since the THz power is proportional to the electric field squared, we estimate that the THz field emitted from the AuGe structures is about 1.4 times larger than that from the Ti/Au type antennas.
Fig. 9. THz waveforms obtained in a THz time-domain spectrometer. A second photoconductive antenna serves as detector.
Finally, we present in Fig. 9 data obtained in a THz time-domain spectrometer. The emitter antennas are gated by femtosecond laser pulses and the emission is coherently detected using a second photoconductive antenna. The peak to peak value of the THz pulse generated by the AuGe type antenna is 1.4 times larger than the value achieved with the Ti/Au type antenna. Fig. 10 shows the corresponding spectra. Fig. 10 shows the corresponding spectra, which is comparable for both types of antennas. At frequencies below 4.5 THz a weak modulation can be seen, which originates from reflection inside the antenna structur“.
4. Summary
In conclusion, we have performed a systematic comparison between two metallization types which are widely used as contacts in photoconductive THz antennas. Antennas with AuGe based contacts reproducibly exhibit a two times higher THz output power than antennas with Ti/Au contacts. However, Ti/Au contacts might be useful if a good thermal stability is called for. Our experimental findings are in agreement with an FIT (CST Microwave Studio) simulation and an equivalent electronic circuit photomixer model.
Acknowledgments
This work was supported in part by the European Commission through the ProFIT programme of the Investitionsbank Berlin and the Bundesministerium für Bildung und Forschung (FKZ 13N9406). Nico Vieweg and Maik Scheller acknowledge financial support from the Studienstiftung des Deutschen Volkes and from the Braunschweig International School of Metrology.
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