Stack Layer SiO[sub 2]/Gd[sub 2]O[sub 3]/SiO[sub 2]/GaAs pH-Sensitive Electrolyte Insulator Semiconductor Capacitors and Their Anneal Temperature Dependency

Abstract

The pH sensing capacitors with stacked SiO 2 /Gd 2 O 3 /SiO 2 oxide layers prepared on compound semiconductor GaAs substrate were fabricated and until recently had been reported on seldom. In this work, the proposed GaAs-based stack layer capacitors with a so-called electrolyte insulator semiconductor ͑EIS͒ structure are demonstrated and are easily further integrated with other GaAs-based radio-frequency or light emitting devices to construct an integrated multifunction sensor in the future. Among this work, three different EIS capacitors are fabricated for verification, which are Gd 2 O 3 /GaAs, Gd 2 O 3 /SiO 2 /GaAs, and SiO 2 /Gd 2 O 3 /SiO 2 /GaAs structures. Their correspondent linear sensing range and sensitivity are pH 6-10 and 29.9 mV/pH, pH 4-8 and 52.9 mV/pH, and pH 2-8 and 57.9 mV/pH, respectively. Different post oxide annealing ͑POA͒ treatments are also carried out. The correspondent trilayer EIS devices' sensing ranges are then modified effectively from a grown pH 2-8 to POA-treated pH 8-12 with a sensitivity of 57.4 mV/pH. Since Bergveld proposed the first ion-sensitive field effect transistor ͑ISFET͒ in 1970, 1 it has been applied widely over the past several decades. This ISFET removes the metal gate from traditional Metal-Oxide-Semiconductor Field Effect Transistor ͑MOSFET͒ and is immersed in an electrolyte directly so as to create a reaction between the electrolyte and the insulating gate oxide film to form a phase boundary potential. This phase boundary potential varies with the ionic activity of the solution, and then the pH value of the solution or the concentration of other ions based on the change can be measured. During the period 1983-1986, a type of enzyme sensor based on a simple electrolyte insulator semiconductor ͑EIS͒ capacitance structure was reported, 2,3 which is marked by a very simple structure. These EIS chips consisted of a pH-sensitive dielectric layer ͑on the top͒ and are deposited on a silicon substrate. These EIS structures were dipped into the sample solutions to form a capacitor configuration with a variable capacitance that depends on the pH value of the contacting liquid. The flatband capacitance of the EIS chip shifted along the voltage axis as the pH value was changed. By capacitance variation, the correspondent voltage change can be measured as a function of the pH value, the voltage capacitance method. In 1988, a semiconductor-based potential metric sensing system, light addressable potential metric sensor ͑LAPS͒, was proposed 4-6 with a structure similar to that of EIS but using a different measuring method, which is the voltage-current method. The advantage of the LAPS technique is that an arbitrary position on the sensing surface of the LAPS can be independently accessed with a light probe. [7][8][9] If we can integrate the back light source ͓light emitting diode ͑LED͒ or laser diode directly on top of the LAPS͔, then its size and addressing resolution can both be enhanced. Furthermore, no wiring is required, which results in a simple structure of the whole syst...

Characterization on pH sensing performance and structural properties of gadolinium oxide post-treated by nitrogen rapid thermal annealing

“…8 Gd 2 O 3 thin films can be prepared by metal-organic chemical vapor deposition, 9 electron-beam evaporation, 10 or molecular-beam epitaxy. However, some small drawbacks including complicated process, small pH application range (pH, 2-8), 12 lack of well-explained mechanism on surface roughness by atomic force microscopy (AFM), 13 or high drift coefficient (1.37 mV/h) 14 is still needed to be improved. Some relative studies on Gd 2 O 3 sensing membrane of different structure including Gd 2 O 3 /SiO 2 /GaAs, 12 Gd 2 O 3 /Si, 13 and Ti-doped Gd 2 O 3 /Si (Ref.…”

GaN Thin Film Based Light Addressable Potentiometric Sensor for pH Sensing Application

“…Wide band gap GaN-based material systems intensively studied over the last few decades [2][3][4] are alternative options to overcome the above mentioned limitations because of GaN's remarkable properties including excellent chemical stability and biocompatibility. 5) Silicon-based ionsensitive field effect transistors (ISFETs) sensors, electrolyte-insulator-semiconductor (EIS) capacitive sensors [6][7][8][9] and III-V nitride-based ion sensitive sensors [10][11][12] have been widely studied. Among the variants of ISFET sensors, the measured ion concentration is an average over the entire sensing surface in contact with the whole solution.…”