{'fraction_non_alphanumeric': 0.04299928926794598, 'fraction_numerical': 0.019782042170101873, 'mean_word_length': 4.655056932350972, 'pattern_counts': {'":': 0, '<': 0, '<?xml version=': 0, '>': 0, 'https://': 0, 'lorem ipsum': 0, 'www.': 0, 'xml': 0}, 'pii_count': 0, 'substrings_counts': 0, 'word_list_counts': {'cursed_substrings.json': 0, 'profanity_word_list.json': 0, 'sexual_word_list.json': 0, 'zh_pornsignals.json': 0}}
Fabrication of ohmic contact based on platinum to p-type compositionally graded AlGaAs layers

Novel metallization scheme was proposed for ohmic contact formation to compositionally graded p-type AlGaAs. A metal multilayers of Ti/Pt/Au, Pt/Ti/Pt/Au and Pt/Ti/Ni/Au were deposited by thermal evaporation using electron gun and resistance heater. The contacts were sequentially annealed by rapid thermall annealing system in N2 atmosphere at various temperatures (in the range from 350°C to 550°C). The duration of annealing step was 2 minutes. The as-deposited Pt/Ti/Pt/Au and Pt/Ti/Ni/Au multilayer metallizations had resistivities of 1.4·10-5 Ω·cm2 which have been gradually deteriorated after each subsequent annealing. The current-voltage characteristics of the ohmic contacts to compositionally graded p-type AlGaAs epitaxial layers were studied and discussed.

Therefore, in the present work, we focused on the development of low resistance ohmic contact based on platinum to p-type compositionally graded AlGaAs layer [10]. Application of GaAs cap layer was unavoidable because it prohibited the oxidation of AlGaAs layer oxidation and improved the properties of the metallic contact to semiconductor. Previous works indicated that the use of low doped GaAs cap layer did not fulfil requirements for good quality contact fabrication. Thus, the heavy doped GaAs cap layers, with thickness of 20 nm, have to be applied.

Experimental details
A cross-sectional diagrams of the AlGaAs/GaAs structure is shown in figure 1  The p-type GaAs layer of 20 nm thick was applied as a cap layer to prevent oxidation of aluminum. The cap layer protected the active layer by inhibition of point defects generation at the subsurface of AlGaAs. It results in the improvement of electrical characteristics of the device [11].
The transfer length method (TLM), originally proposed by Shockley [12,13] was used to determine the specific-contact resistivity. Deposited metallization were examined by TLM technique for which mesas were etched using H 3 PO 4 :H 2 O 2 :H 2 O (1:1:40). Next, rectangular pads (80×90 µm) were patterned by lift-off method on these electrically isolated mesas. Further, the Ti/Pt/Au (5/30/150 nm), Pt/Ti/Pt/Au (5/5/30/150 nm) and Pt/Ti/Ni/Au (5/5/30/150 nm) multilayer contacts were evaporated on the mesas. Prior to metal deposition, the native oxide was removed by etching of the samples in HCl:H 2 O (1:1) solution for 30 seconds, followed by a deionized water rinsing and drying in N 2 flow. The metallic contact was deposited in sequence on the substrate under a vacuum conditions with a base pressure lower than 10 -6 mbar. Metals stack and layers thicknesses of the multilayer metallizations of ohmic contact were selected on the basis of our previous study. Electron-beam evaporation was used to deposit titanium, nickel and platinum, while gold was deposited by thermal evaporation. The influence of annealing process of the contact on the properties of investigated metallization stacks was evaluated. Rapid thermal annealing of the samples was preformed in an nitrogen ambient. The temperature of the annealing was changed in the range from 350 to 410°C. The duration of annealing step was 2 minutes. On-wafer current-voltage measurements were performed at room temperature.
The titanium layer in multilayer metallization was introduced to improve the adhesion of ohmic contact while the gold layer allowed us to reduce the contact resistance. The nickel layer in Pt/Ti/Ni/Au multilayer metallization was inserted to check the role of second platinum layer in Pt/Ti/Pt/Au multilayer metallization.

Results
The current-voltage characteristics for the as-deposited and annealed contacts were measured ( Fig. 2 in Fig. 5-6, respectively. The as deposited Ti/Pt/Au contact was not linear (table 1). Annealing of that contact at 350ºC assures its linearity (Fig. 2).   As-deposited Pt/Ti/Pt/Au and Pt/Ti/Ni/Au multilayer metallizations were ohmics ( Fig. 3 and 4), with specific resistance of 1.4·10 -5 Ω·cm 2 ( Table 1). The role of platinum layer deposited on the semiconductor surface in the Pt/Ti/Pt/Au and Pt/Ti/Ni/Au system was found to be essential in obtaining better electrical properties of the metallic contact compared to non-annealed Ti/Pt/Au contact. During annealing the Pt layer diffused into the titanium layer in Ti/Pt/Au metallization and formed the ohmic contact with specific contact resistance of 2.8·10 -5 Ω·cm 2 (Table 1) The surface of Pt/Ti/Pt/Au contact after annealing at 500ºC was still smooth (Fig. 6(c)) compared to Ti/Pt/Au contact, annealed at the same temperature (Fig. 5(c)). It meant, that annealing at this temperature started to degrade the Ti/Pt/Au and Pt/Ti/Pt/Au ohmic contacts multilayer structure. Also, the specific contact resistivity increased steadily with the increase of annealing temperature above 450ºC (table 1), what corresponded with escape of arsine from GaAs. Further increase of the annealing temperature above 500ºC caused the nonlinearity of I-V characteristics in each contact. The role of the second Pt layer in the Pt/Ti/Pt/Au system was also investigated. It was observed that platinum played also essential role in obtaining better electrical performance of the contact in comparison with Pt/Ti/Ni/Au contact. In this case, platinum in Pt/Ti/Pt/Au and nickel in Pt/Ti/Ni/Au multilayers, operated as a barriers for diffusion of Au layer. The as-deposited Pt/Ti/Ni/Au multilayer contact to compositionally graded p-type AlGaAs had ohmic character. The annealing of this contact, above 350ºC, dramatically deteriorated its electrical parameters -the current-voltage characteristics were not linear (Fig. 4). Pt and Ti layers in Pt/Ti/Ni/Au multilayer metallization had the thickness of 5 nm. This is the reason of dramatic deterioration of Pt/Ti/Ni/Au contact characteristics. During the annealing step, the Ni layer diffused into the Pt/Ti bilayer in Pt/Ti/Pt/Au metallization and decreased the electrical parameters (Table 1) of the contacts.
Additionally, the optical image of the Pt/Ti/Ni/Au metallization (Fig. 7), annealed above 400ºC, showed that Au has been soluted in Ni stop diffusion layer. In general, higher-melting-point metals exhibit lower-bulk diffusivities [14] what explained well the superiority of Pt for the application as diffusion barrier in comparison to Ni.

Conclusions
We have investigated the electrical properties of Ti/Pt/Au, Pt/Ti/Pt/Au and Pt/Ti/Ni/Au ohmic contacts to p-type compositionally graded AlGaAs layers by current-voltage (I-V) measurement performed at the function of annealing temperature ranging from 350ºC to 550ºC. A specific contact resistivities as low as 1.4·10 -5 Ω·cm 2 have been obtained for ohmic contacts Pt/Ti/Pt/Au and Pt/Ti/Ni/Au without annealing. The specific contact resistivity of 1.4·10 -5 Ω·cm 2 of Pt/Ti/Pt/Au contact remained constant up to 400ºC, next, decreased steadily to 4.8·10 -5 Ω·cm 2 with the increase of annealing temperature to 475ºC. Further growth of annealing temperature resulted in nonlinear I-V characteristics of the contact. The role of the second Pt stop diffusion layer in the Pt/Ti/Pt/Au system was also investigated. It was found that its presence was essential in obtaining better electrical performance in comparison with the Pt/Ti/Ni/Au contacts containing the Ni stop diffusion layer. The specific contact resistivity of 1.4·10 -5 Ω·cm 2 of Pt/Ti/Ni/Au contact was constant up to 400ºC, increase of the annealing temperature dramatically deteriorated the electrical parameters of the contact -the current-voltage characteristics became nonlinear (Fig. 4).
The improvement of the electrical properties of Ti/Pt/Au contacts after annealing was observed, compared to as-deposited ones. The Ti/Pt/Au contacts were found to be relatively stable during annealing up to 500ºC but as-deposited contacts were nonlinear. At an elevated temperature the Pt diffused into the Ti layer what assured the formation of the ohmic contact to compositionally graded p-type AlGaAs layer.
Results indicated that Ti/Pt/Au and Pt/Ti/Pt/Au contacts are suitable for the high temperature applications. Using elaborated multilayer metallization, photodetectors with compositionally graded active area were fabricated by utilizing standard photolithography and lift-off processes of metallization. Examined ohmic contacts have also excellent adhesion to the compositionally graded AlGaAs layer.