Silver-Functionalized AlGaN/GaN Heterostructure Diode for Ethanol Sensing

The use of Ag in the gate region of AlGaN/GaN heterostructure diodes is shown to provide stable, reversible changes in barrier height and thus current during exposure to ethanol at 250 ◦ C. The exposed ethanol molecules are adsorbed on the silver and oxidized, resulting in the increase of Schottky barrier height. The detection limit of ethanol at this temperature was 58 ppm, and the sensor response was linear over the range 58–58700 ppm. For the high end of this concentration range (5.87%), ethanol exposure at 250 ◦ C caused the Schottky barrier height to change from 0.604 eV to 0.656 eV, which produced a forward current relative change of 45.4% at 0.9V forward bias. The results are competitive with detection of ethanol by oxide thin ﬁlms or nanostructures of SnO 2 , Fe 2 O 3 , CuO, and ZnO. ©

There are many applications in which detection of the common aliphatic alcohol, ethanol, is needed, including the chemical, biomedical, and food industries, where the rate of ethanol production during fermentation needs to be monitored. 1As an example, the change in alcohol content during fermentation of grapes is of great importance in the winemaking industry. 2Ethanol gas sensors can detect early spoilage of carbohydrate-rich foods, and these sensors are now employed in consumer electronics including refrigerators and mobile devices. 1 Another common application is monitoring blood alcohol content related to consumption of alcoholic beverages.Most of the research into solid state sensors has focused on metal oxide semiconductors such as SnO 2 , Fe 2 O 3 , CuO, and ZnO.  The8][39][40][41] Many types of GaN based devices have been reported for gas and chemical sensing, including Schottky diodes, metal oxide semiconductor (MOS) diodes, and Al-GaN/GaN high electron mobility transistors (HEMTs).The specificity of the sensors can be tailored by the choice of the gate material, which might include antibody layers for biological species detection or catalytic metals for gas sensing. 42,43In particular, the AlGaN/GaN HEMT structure with its two dimensional electron gas (2-DEG) channel induced by piezoelectric and spontaneous polarization at the interface between the AlGaN and GaN layers shows highly sensitive current changes to surface charges created by catalytic reaction of target gases on the specific active sensing layer.With 30% Al concentration in the AlGaN layer, 5∼10 times higher channel sheet electron densities are obtained compared to GaAs or InP HEMTs.
In this study, AlGaN/GaN HEMT based Schottky diode ethanol sensors using silver as a sensing material were fabricated, and the response of the devices to ethanol gas was investigated as a function of temperature and ethanol concentration.The diodes showed a current reduction in ethanol-containing ambients due to the increase of Schottky barrier height.This process was most efficient at 250 • C, and rapid responses to cyclic exposures of various concentrations of ethanol gas were obtained.

Experimental
AlGaN/GaN HEMT layer structures were grown on c-plane sapphire by metal organic chemical vapor deposition (MOCVD).The epilayer structures were composed of 2 μm thick undoped GaN buffer layer followed by a 35 nm unintentionally doped Al 0.3 Ga 0.7 N layer.Sheet resistances of 350 ohm/square, sheet carrier concentrations of 1.06 × 10 13 cm 2 , and mobilities of 1900 cm 2 /Vs were obtained from Hall measurements.ohmic contacts of Ti/Al/Ni/Au were formed by E-beam evaporator and lift-off, and annealed at 900 • C for 60 s under a N 2 ambient.A 200 nm SiN x layer was deposited for diode isolation by plasma enhanced chemical vapor deposition (PECVD).The windows for active area opening were achieved by buffered oxide etchant (BOE) etching.A 10 nm Ag film was deposited on the diode Schottky contact area by E-beam evaporation.Finally, Ti/Au based contact pads were deposited for probing and wire bonding.Figure 1 shows the layer structure and geometry, and the microscopic image of the sensor.Current-voltage (I-V) characteristics of the Schottky diodes were measured at 25-300 • C using an Agilent 4156C parameter analyzer with the diodes in a gas test chamber in ambients of N 2 bubbled through ethanol to produce controlled concentrations of 58-58700 ppm (0.00578-5.87%by volume).Figure 2 shows a schematic of the test setup.The temperature of the sensors could be controlled from 25-300 • C and dry N 2 was used to transport ethanol vapor to the sensor surface.

Results and Discussion
Figure 3a shows the I-V characteristics from the Ag-AlGaN/GaN diode in dry N 2 or during exposure to 5.87% ethanol in N 2 at 250 • C. The current decreases, consistent with an increase in the effective Schottky barrier height and this is more obvious at forward bias voltages, as is normal for gas sensing with nitride-based sensors. 42,43The relative current change as a function of bias is shown in Figure 3b.The response is largest (45.4%) at a forward bias of 0.9 V.
The barrier height could be measured from the I-V characteristic during the period that ethanol was present on the sensor surface and compared to its value when N 2 was the ambient in the test chamber.The barrier height ( b ) was then extracted from the relationship between current density J, temperature T and applied voltage V where A * is Richardson's constant, k is Boltzmann's constant, e is the electronic charge, and n is the diode ideality factor. 36,40,41Figure 4 shows the value of barrier height as 5.87% ethanol is introduced and nitrogen is switched back to the test chamber with the diode held at 250 • C. The barrier height increases by 0.053 eV to 0.656 eV.
7]40,41,[44][45][46] The possible mechanism for the current decrease due to barrier height increase can be described as follows.Firstly, there is adsorption of ethanol molecules onto the silver surface on the gate of the diode.The adsorption of ethanol molecules can be enhanced by adsorbed oxygen ions on silver, which are induced from the decomposition of silver oxide (Ag 2 O) formed during the device fabrication process. 47,48It is notable that both the formation of oxygen ion and decomposition   of Ag 2 O occur above 200 • C. [49][50][51] The silver on the semiconductor surface enhances the catalytic oxidation of alcohol gas. 47,49,52The adsorbed ethanol molecules react with oxygen ions to produce C 2 H 4 O, H 2 O, and electrons.Hence, this reaction causes negative charging of silver electrode, resulting in the Schottky barrier height increase.Finally, this increase of Schottky barrier height induces a decrease in current at both forward and reverse bias.The reaction and the energy band diagram of Ag Schottky barrier on AlGaN/GaN structure upon ethanol exposure are shown schematically in Figure 5.The overall reaction is: The current change was measured as a function of ethanol concentration at 250 • C. Figure 6 shows that the current response was linear over the range 58-58700 ppm at this temperature.We should point out that at temperatures below 250 • C, there was not a significant change in diode current upon exposure to ethanol, while at 300 • C, the response was slightly lower than at 250 • C, which may be due to additional reactions of the Ag with the semiconductor.Many country's legal limit for blood alcohol content of automobile drivers is 0.05%, corresponding to 0.01% (100 ppm) breath alcohol content.Any measurable alcohol in the blood is mostly illegal in U. S.These sensors are easily able to detect these types of concentrations.
1][12][13][14][15][16][17] Note that the operating temperature and sensitivity of the Ag-AlGaN/GaN diodes is competitive with most of the other technologies.Among the potential advantages of the nitridebased diodes is the ease of array fabrication to integrate other types of sensors or use of diode pairs in which one diode is sensitive to temperature changes but not ethanol. 42Finally, these types of sensors are easily integrated with wireless data transmission circuits for remote monitoring applications. 42

Conclusions
The Ag-AlGaN/GaN diodes showed maximum sensitivity to ethanol at 250 • C, and stable and recoverable response to ethanol vapor resulting from reversible changes in barrier height.The barrier height increase for ethanol vapor exposure at 250 • C was 0.053 eV for the thin silver film Schottky diode.The diodes showed a rapid sensing response to ethanol, as well as a full recovery to their initial current level after removing the ethanol from the ambient.This study shows the possibility of AlGaN/GaN HEMT with active silver layer to ethanol sensing to monitor food spoilage, drunk driving, and alcohol levels in many industrial applications.

Figure 1 .
Figure 1.(a) Layer structure and geometry of the Ag-AlGaN/GaN diode sensor of the completed device.The circular Ag Schottky contact is surrounded by the ohmic contact.(b) Top-view microscopic image of the device with dimensions.

Figure 3 .
Figure 3. (a) I-V characteristics from the Ag-AlGaN/GaN diode in dry N 2 or during exposure to 5.87% ethanol in N 2 at 250 • C. The inset is the semi-log plot of I-V.(b) Relative current change as a function of bias.

Figure 4 .
Figure 4. Change in effective barrier height as a function of time for an injection of ethanol gas into the test chamber, followed by a return to dry N 2 .

Figure 6 .
Figure 6.Normalized current change in Ag-AlGaN/GaN diodes as a function of ethanol concentration at 250 • C.

Table I . Summary of ethanol detection limits and operating temperatures for Ag-AlGaN/GaN and different oxide-based materials. 9-17
* Sensitivity has been re-calculated with the definition of r ref −r ethanol r ref × 100% from the literature.The r re f and r ethanol are resistances in the reference and ethanol gases respectively.