thicker P(VDF/TrFE) ferroelectric layer was used as a large enough polarisation is needed in order to accumulate enough bound charge at the semiconductor interface in order to deplete the 2DEG of its electrons. The pellets that were dissolved in cylclohexanone were purchased from Solvay-Solexis, its characteristics are summarised in table 4.3. What will be extremely important in the future processing of devices is that the melting temperature is quite low at 154.5◦C, which could impede the use of negative photoresists.
Table 4.3: P(VDF/TrFE) data sheet provided from Solexis.
VF2 70 % mol.
Ohmic/Schottky electrodes were deposited to make contact with the 2DEG in the Al-GaN/GaN heterostructure. Using a Ti/Al/Ti/Au sandwich, Motayed et al. [2003] suc-cessfully had ohmic contact with the 2DEG after annealing at high temperatures. Anneal-ing of the electrodes is not possible in our configuration and might not be necessary. Here 30/100/30/30 nm electrodes, deposited by electron beam, created good quality Schottky contacts without annealing at high temperatures.
The best way to assess the quality of the electrodes to the 2DEG in the AlGaN het-erostructure is to use the transmission line method, TLM, setup as shown in figure 4.15.
This setup allows the contact resistance Rc to be extrapolated after making a series of IV curves, Reeves and Harrison [1982]. A resistance versus electrode separation distance curve is plotted and extrapolated to a distance of zero. The resistance calculated here is equal to twice Rc.
A Hall bar structure was designed without PZT to observe the transport properties of the 2DEG in the AlGaN heterostructure, see section 3.1 for more details on this technique.
That is a Hall bar structure was etched using the electron cyclotron resonance reactive ion etching, ECR-RIE, technique described in section 4.2.4 and Ti/Al/Ti/Au electrodes were deposited and annealed at 700◦C for 30 s. The Hall bar measurements gave values of Rs = 431Ω/, ns = 1.15x1013 electrons/cm2 and µ = 1260cm2/Vs. These values are within experimental error of the data supplied to us from Cree, Inc., therefore since the AlGaN samples are extremely expensive no TLM measurements were done in order to minimise the contact resistance.
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Figure 4.15: Transmission Line Measurements are done with multiple electrodes in series on a rectan-gular etched semiconductor.
The PZT can henceforth be deposited first and then the annealing of the bottom electrodes can be done, see figure 4.16.
4.2.2 Gate Electrode
Gate Electrode for PZT
Since top electrodes for PZT has already been investigated, Baborowski [2004], thus making the choice for the optimal top electrode for this device simple. Standard Pt electrodes usually give the best results, however there are many inconveniences to using them for this device. The main problem is due to the fact that they are deposited using PVD, magnetron sputtering technique after which a high temperature annealing is necessary in order to restore the Pt/PZT interface and to re-crystallize the Pt. It is absolutely critical that all unnecessary annealing steps are reduced since annealing in oxygen will cause the bottom electrodes to oxidise and lose their ohmic contact properties with the 2DEG, also high temperature processes can cause the PZT to further diffuse into the AlGaN and potentially into the GaN reducing the transport characteristics of the 2DEG. Also an additional processing step of dry etching needs to be done in order to pattern the electrodes, further damaging the side walls of the PZT by ion bombardment, making again the post-annealing treatment an important fabrication step. Therefore it was chosen to use Au(100 nm/Cr(10 nm)) electrodes due to the fact that these two metals can be deposited using electron-beam evaporation, making it possible to get high resolution structures in a one processing step by lift-off technique. Most importantly is that during the electron-beam evaporation there is no ion bombardment of our sample making a post-annealing treatment unnecessary.
Gate Electrode to P(VDF/TrFE)
There were different electrodes used as the gate contact to the P(VDF/TrFE) ferroelectric layer. Firstly, chrome electrodes of 100 nm were used but this did not create a good external contact. Therefore both Au/Cr 100/10 nm electrodes deposited by electron beam
4.2. PROCESSING 71
Figure 4.16: a) Vertical amplitude, b) vertical phase, c) lateral amplitude, and d) lateral phase of the PFM images when poled −30 V on the left side and +30 V on the right side with the conducting cantilever. This poling shows retention and concludes that the PZT can sustain nitrogen annealing.
evaporation and 100 nm Au electrodes deposited by joule effect evaporation. The Au electrodes could be easily structured with a wet etching solution of KI, I2 and H2O.
Aluminium electrodes, of 100 nm, deposited by electron beam evaporation were also tried as they are reported to make a chemical reaction with the fluoro-polymer, P(VDF/TrFE), preventing diffusion, Wu et al. [1994], Chen and Mukhopadhyay [1995] and Xia and Zhang [2004]. The aluminium electrodes could easily be patterned with a wet etching solution of ANP. Where ANP is a chemical solution of the ratio 5:3:75 of acetic acid, CH3COOH 100%, nitric acid, HNO3 70%, phosphoric acid, H3PO4 85%.
4.2.3 Wet Etching of PZT
The techniques of both dry and wet etching of the PZT grown on AlGaN/GaN were investigated. Both techniques are well developed and successfully used in the laboratory of ceramics. Wet etching produced problems in lateral over-etching as the solution is isotropic; this is difficult to compensate in mask design. Wet etching of PZT is used to etch out holes in the PZT layer to create a window in order to deposit ohmic electrodes to the 2DEG. For the mesa structure a new technique was developed to simultaneously etch both PZT and AlGaN/GaN in one step, see the below sub-section for further details.
The wet etching solution used is 30 mL of 32 % hydrochloric acid, HCl, 5 drops of 40 % hydrofluoric acid, HF, and 70 mL of water, H2O. It can also be said that the final solution
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has a concentration of 9.4 % HCl, and 1 % HF and 89.6 % H2O.
4.2.4 Dry Etching of PZT/AlGaN/GaN
AlGaN/GaN dry etching has been done successfully by Kao et al. [2004] using inductively coupled plasma reactive ion etching. In this study it was shown that it is possible to anisotropically etch AlGaN/GaN with an etched surface root mean square roughness value of about 0.2 nm. Also shown was that there was an increase of surface roughness (due to ion bombardment) above the saturated bias power of 200 W where the dislocation density and the number of pits increased.
Figure 4.17: A SEM top view of the AlGaN/GaN heterostructrure after having been etched and PZT removed, where a clear step is visibly etched into the semiconductor.
Although the above method might give high etching rates and small aspect ratios, it is necessary also to optimise our device design. Therefore, the same dry etching elec-tron cycloelec-tron resonance reactive ion etching, ECR-RIE, setup, as previously used for PZT by Baborowski et al. [2000], was tested in its etching capabilities of AlGaN/GaN.
Previous attempts to etch GaN with ECR-RIE were done with plasma chemistries of:
Cl2/CH4/H2/Ar, BCl3/Ar, Cl2/H2, Cl2/H2, Cl2/SF6/ HBr/H2 and HI/H2, see Pearton et al. [1997] for a review of such methods. These methods have etching rates from 40 − 310nm/min.
Table 4.4 gives a summary of multiple etching conditions used to etch a 300 nm PZT/AlGaN structure. The best etching conditions with which to etch the PZT/AlGaN/GaN struc-ture, in the ECR-RIE, is with a plasma chemistry of Ar:CCl4 : CF4 at a ratio of 2:3:3 and a total flow rate of 20 sccm. The overall etching rate of this structure is approxi-mately 30nm/min. The main advantage of this technique is one step processing (i.e. one mask/alignment) improving the alignment of the sidewalls of the PZT layer to the Al-GaN/GaN structure. The main drawback of this technique is a large aspect ratio of 1:1, this could imply that PZT does not cover all of the 2DEG which could create channels for leakage current.
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Table 4.4: Dry etching conditions for PZT/AlGaN/GaN structure at an RF power of 50 W and an accelerating voltage of 200 V and a beam voltage of 200 V.
Ar CCl4 CF4 O2 Etch Rate Side Walls [sccm] [sccm] [sccm] [sccm] [nm/min] [ ◦]
3.75 7.5 7.5 0 27.4 no
5 7.5 7.5 1 30 45.9
5 15 15 0 26.7 45
5 15 0 0 30.6 60.2
5 0 15 0 26.7 64.8
5 0 0 0 0 0
15 15 15 1 26 36.9
10 15 15 0 33.7 38.7
0 15 15 1 24.4 31.3
4.2.5 Focused Ion Beam, FIB
Preliminary experiments showed that it is possible to finely structure PZT/AlGaN/GaN devices through focused ion beam with a resolution of approximately 130 nm, see figure 4.18. This is an option to create a pattern of nano-resolution with sidewalls of good aspect ratio. Previous experiments done by Stanishevsky et al. [2002] investigated the radiation damage in PZT after using FIB. For their experiments a focused beam of 50 keV Ga+ ions was used to pattern 100 nm square capacitors. The surface layer was significantly damaged due to the loss of lead and oxygen. A substantial increase in leakage current was found after FIB patterning, this leakage current could be reduced by annealing in oxygen for 30 min at 400◦C. Unfortunately, the ferroelectric hysteresis loops were significantly degraded with the FIB; these properties could not be restored with annealing but only slightly improved, see figure 4.19. To continue using the FIB technique to nano-structure devices, further experiments to reduce surface degradation need to be done.
Figure 4.18: FIB cut in the PZT/AlGaN/GaN structure.
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Figure 4.19: Showing the hysteresis loops of a PZT layer as deposited, after annealing, after FIB and after a final annealing, Stanishevsky et al. [2002]. Permanent degradation of the hysteresis characteristics occur after FIB processing.