analytica chimica acta 615 (2008) 1–9 available at www.sciencedirect.com journal homepage: www.elsevier.com/locate/aca Review Novel semiconductor materials for the development of chemical sensors and biosensors: A review Nikos Chaniotakis ∗ , Nikoletta Sofikiti Laboratory of Analytical Chemistry, Department of Chemistry, University of Crete, Voutes 71003 Iraklion, Crete, Greece article info Article history: Received 15 November 2007 Received in revised form 13 March 2008 Accepted 18 March 2008 Published on line 30 March 2008 Keywords: Chemical sensor Biosensor Semiconductor Gallium nitride Indium nitride Conductive diamond Transduction Surface potential abstract The aim of this manuscript is to provide a condensed overview of the contribution of certain relatively new semiconductor substrates in the development of chemical and biochemical field effect transistors. The silicon era is initially reviewed providing the background onto which the deployment of the new semiconductor materials for the development of bio- chem-FETs is based on. Subsequently emphasis is given to the selective interaction of novel semiconductor surfaces, including doped conductive diamond, gallium nitride, and indium nitride, with the analyte, and how this interaction can be properly transduced using semi- conductor technology. The main advantages and drawbacks of these materials, as well as their future prospects for their applications in the sensor area are also described. © 2008 Elsevier B.V. All rights reserved. Contents 1. Introduction 2 2. The silicon era 4 3. New semiconductor substrates 4 4. Diamond 5 5. GaN and III-nitrides 6 6. Forecasting the future 7 References 8 ∗ Corresponding author. Tel.: +30 810545018; fax: +30 810545165. E-mail address: nchan@chemistry.uoc.gr (N. Chaniotakis). URL: http://www.analytical chemsitry.uoc.gr (N. Chaniotakis). 0003-2670/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.aca.2008.03.046 2 analytica chimica acta 615 (2008) 1–9 1. Introduction The semiconductor technology has boomed since the inven- tion of the transistor in 1948 [1]. Initially the semiconductor- based transistors were based on the physicochemical interfacial properties of mainly two semiconductors, silicon and germanium. The important physical properties of the transistors were related to the actual area and thickness of the active layer, and the final size and shape of the complete device. Most importantly, it has been known early on that the chemical characteristics of the active area of the semicon- ductor play a major role in determining the behavior and the performance of the final device. This is indeed the case since a close look at the interfacial processes involved during the operation on transistors will reveal that it is without a doubt a chemical process technology. Based on these facts it was clear that the precise control of the surface chemistry was manda- tory for the final optimization of the device performance. For this reason there are some basic parameters that need to be taken into account when new semiconductor materials are to be developed and optimized, and which play a decisive role in their applicability to biosensors. The chemical synthe- sis (growth) of the material, the post-material treatment such as doping or ion implantation and the final chemical surface treatment are the three most important ones. The develop- ment and optimization of these semiconductor technology procedures has allowed for the growth or synthesis of materi- als withvery well controlled and unique physical and chemical characteristics, down to the atomic level. The main parameter, which has been shown very early to play a very significant role in the behavior of these mate- rials, especially in their application to bio-FETs as well as all other electrochemically based biosensors, is the-what- is-called “work function”, “contact potential”, or “electrode potential”. Even though there might be some fine differences between these terms, for the purpose of this work we will refer to all these terms as the “surface potential” of the semiconduc- tor. Semiconductor surface potential plays an important role in the performance and characteristics of all devices involving surface chemistry and thus semiconductor-based biosensors. Fundamental studies of the surface potential of surfaces have been vital in understanding the behavior of these materials, as well as their applications in chemical sensors and biosensors in general. The surface potential of a material is of fundamental inter- est to many areas of semiconductor sciences. Both the native and the imposed potential can play a major role in the space charge effect. The induced depletion or inversion layer, and the Fermi energy shift or pinning, are parameters that are directly related to, not only the chemical composition of the bulk material, but also to the chemical equilibria that exists between the surface of the semiconductor and the analyte sensed. The surface potential, and therefore the nature of the space charge double layer associated with the surface, depends on the chemistry of the adsorbed layers on the elec- trode surface, as it has been known since the early 1930s [2,3]. This idea has been extended to the semiconductor sur- face, especially after the invention of FETs in 1948 by Bardeen and Brattain [1]. In 1954 Brattain and Bardeen actually mea- sured for the first time the effect that different electrolytes, such as HCl, KCl or KOH, had on the half-cell potential of the germanium semiconductor [4]. In the same journal issue, Bardeen and Morisson [5] presented the effect that different electrolytes and gasses had on the properties of the semi- conductor as manifested by the change in the surface space charge barrier. In addition, the effect of both ions and pH on the surface of semiconductors was reviewed a little later by Boddy [6], showing both the dependence of surface potential in germanium and in silicone semiconductors [7]. It was shown in these early works that the surface chemistry of the material is determined by the active chemical functionalities found at the surface, and to a lesser degree by the crystal orientation. At the same time, the type and amount of the surface chemical functionalities depend on both the chemical composition of the material itself, as well as, on any chemical post-treatment of the surface. The surface chemistry, or to be more precise the surface chemical functionalities, can induce specific physicochemi- cal properties of the semiconductor as presented early on by Bardeen and Morrison [5], and proven by many other scientists since then. Those are: 1. Work function or contact potential [8–13]. 2. Rectification [14,15]. 3. Chemical reactions with electron transfer [16,17]. 4. Adsorption [18,19]. 5. Surface recombination–photoconductivity [20,21]. 6. Change in contact potential with light [22]. 7. Surface conductance–channel effect [23]. 8. Change in surface conductance with electrostatic field–field effect [24–26]. 9. Noise. All these properties can be used as the basis for the development of analytically useful devices, including chem- ical sensors, biosensors, and bio-chem-FETs. This is due to the fact that external chemical stimuli can drastically alter these fundamental and easily measurable surface semicon- ductor properties. Monitoring surface current, potential or impedance characteristics can be directly related to the chem- ical stimuli interrogating the semiconductor sensing surface, as shown in Fig. 1. These facts make semiconductors ideal matrices as sensor elements and transducers for the development of a variety of chemical sensor and biosensor systems, especially sur- face active Potentiometric Ion Selective Electrodes (ISEs), Field Effect structures (FETs), amperometric biosensors, Surface Acoustic Wave sensors (SAWs), and Film Bulk Acoustic Res- onators (FBARs). The development of such sensing device is always based on the affect resulting from the specific or selective inter- action of an analyte with the semiconductor surface. Such interaction will usually result in changes of the electrochem- ical characteristics of the surface [27], as shown in Fig. 2. The most important of these parameters are the space charge, the field and the potential. Once this interaction has been characterized, a variety of sensing schemes can be envisaged. Potentiometric ISE are based on the induced potential gen- erated at the semiconductor solution interphase, while the analytica chimica acta 615 (2008) 1–9 3 Fig. 1 – Generic experimental setups employed for the design of chemical sensors and biosensors based on semiconductor surfaces. (A) Surface Acoustic Waves (SAW) biosensor, (B) normal and light addressable electrochemical sensors, (C) CHEMFET sensor, and (D) hall effect sensor. Fig. 2 – Schematic diagram of the electrical parameter distribution for an electrochemically active semiconductor surface. 4 analytica chimica acta 615 (2008) 1–9 current that passes the devise is practically zero. On the other hand, the development of a CHEMFET will depend on the effect this surface potential will have on the characteristics of the underlying semiconductor layers, and specifically on the depletion layer of the gate. Similarly, the selectivity and sensitivity of semiconductor-based FBARs will depend on both the initial physical characteristics of the material, as well as the induced physical changes upon chemical interaction of the analyte with the sensing surface. Finally, amperometric sensors are dependent on changes in either the conductance of the material, or changes in the activity of redox species available within the sensor element. It is thus clear from the above that creating chemical sensors or biosensors from semiconductors requires precise chemical control of the surface chemistry. Only under these conditions the analytical characteristics of the sensor such as selectivity, sensitivity, detection limit, response time, and sig- nal stability can be optimized. Since analyte recognition and detection is a result of the perturbation of the electro optical properties of the semiconductor surface and subsurface layers there must be a specific and reversible chemical interaction of the analyte with the semiconductor sensing element. As the material science community evolved and was able to have complete control of the growth process, more and more these materials were used for the development of sensors. 2. The silicon era In the late 1960s, the use of silicon as a matrix for integrated sensor–transducer systems had begun [28,29]. Silicon-based devices for the in vivo measurement of electrophysiological measurements had already been developed. The revolution came from a publication of Bregveld [23], in which he showed that Si-based devices, the so-call pH-FETs, can be used to mea- sure pH in very small volumes, and with good accuracy. This Si technology came to maturation with the commercialization of these pH sensors in the mid-80’s, while they were the platform for the development of other ion-selective FETs and bio-FETs up to date. When SiO 2 , or other metal oxides or nitrides such as Al 2 O 3 ,Ta 2 O 5 , SnO 2 , and Si 3 N 4 are used as the chemical recognition element, the resulting sensor is highly selective for the hydrogen ion, due to the very strong hydrogen bond- ing that exists with the oxide layer. The oxides can coordinate reversibly with the hydrogen ion in solution, affecting the sur- face potential of the sensing element. These surface potential changes affect the gate potential in the same way as a metal gate field effect transistor (MEGFET) works, altering the signal of the pH-FETs. The Si technology offered considerable advan- tages in the microsensing area, due to the ability to integrate the sensing element directly with the readout circuit to obtain, self-contained microsensor devices with high sensitivities and signal to noise ratios, thus allowing for the development of large sensor arrays highly useful in the area of biochips. One of the major obstacles to overcome during the design of a continuous sensing system is the long-term storage and operational stability of the sensing element. Even though SiO 2 and its related metal tin oxide substrates are very selective substrates for the detection of hydrogen ions, their stability in harsh environments is limited. Treatment with very acidic or basic solution, or fluoride containing solutions should thus be avoided since it can be detrimental to the analytical behav- ior of these systems. Surface etching and oxidation in these solutions will result in the drastic decrease in the sensitivity, while the response time increases considerably. In addition the proper isolation between the devices and the chemical solutions, as well as the sensitivity to light are still issues to be completely resolved. It is therefore a challenge to develop inert semiconductor electrodes. The bio-chemical sensors developed up until very recently were based on the pH sensitive FETs. Any chemical or bio- logical process that can result in changes of the pH can be combined with a pH-FET transducer, resulting in what is called CHEMFET of BIOFET. In all of these devices, the surface poten- tial developed at the surface of the semiconductor is based on the direct interaction of the ligand with the exposed atoms of the semiconductor, as shown in Fig. 5 [30]. This chemical interaction (chemisorption or coordination) of the charged or polarized analytes with the semiconductor surface induces a surface potential. It is important to recognize this fact, which is much more pronounced and important for the design of chemical sensors and biosensors than the inherent band bending due to layered structure or to crystal structure end. The development of novel semiconductor matrices for application is the area of chemical sensors and biosensors is based on the fact that the surface of the new materials must possess certain chemical and physical properties that can deal with the drawbacks of the Si technology, and which have been extensively analyzed in the last decades. Those are the selectivity to species other than hydrogen ion, the chemical stability of the surface to extreme chemical envi- ronments, the ability for surface functionalization, increased signal sensitivity and stability, and the biocompatibility of the final device. New semiconductor materials with well-defined surface chemistry, which are stable in aqueous solutions and can selectively interact with analytes other than the hydro- gen ion, can thus be very valuable tools in the design of novel chemical sensors. 3. New semiconductor substrates In recent years there is an intense effort in the design of new semiconductor materials other than Si, for use in power elec- tronics and other microelectronic applications [31,32] in order to deal with the problems associated with the use of the Si- based electronic devices. Among these materials, emphasis has been given to those with relatively large band gap (Wide- Bandgap Semiconductors, WBSs) due to their application in UV lasers and photonics. But besides that, some of these materials, such as silicon carbide (SiC), gallium nitride (GaN), and diamond are proven to be unique materials for a variety of applications mainly due to the fact that they are overall more efficient in many electronic processes, since they can withstand larger voltages, they have higher thermal conduc- tivity, and they are more stable over time and thus are more reliable. Moreover, the aforementioned WBG materials have excellent reverse recovery characteristics, and for this reason they require less time and energy to return to the base line signal. In addition they are less susceptible to electromag- analytica chimica acta 615 (2008) 1–9 5 Table 1 – Summary of the analytical applications of novel semiconductor materials Gas sensing (H 2 ,NH 3 ,NOx,O 2 , CO, H 2 O, combustion gases, ethanol, organic vapours, hydrocarbons, fluorocarbons) GaN [33–42] InN [43] AlN [44–46] SiC [47–53] Ion sensing GaN [54–60] InN [61] Diamond [62,63] Other electroanalytical applications Diamond [64–74] Bio-electrochemical applications Diamond [75–77] GaN [78–80] Electrocatalysis Diamond [81–83] Shear mode acoustic wave biosensors AlN [84–88] GaAs [89] netic interference (EMI), while the devices based on them can operate at higher frequencies. This unique chemical, physi- cal and mechanical stability make these new semiconductor materials ideal for the development of specific chemical sen- sor and biosensor systems. Table 1 summarizes the analytical applications of novel semiconductor materials. 4. Diamond Diamond is a unique WBS (E g = 5.45 eV) since it possesses several distinct properties including extreme hardness, high electrical resistance, chemical inertness, high thermal con- ductivity, high electron and hole mobility, and optical transparency. These properties appoint this material ideal for various highly demanding applications [90,91]. Since diamond is one of nature’s best insulators doping is required for its use in electrochemical studies. Chemical Vapor Deposition (CVD) methods [89,90] can produce highly conductive boron doped diamond films [92]. The surface of as-grown undoped and boron-doped diamond films is relatively nonpolar, with the surface carbon atoms terminated by hydrogens [93]. This fact, along with the sp 3 -hybridization of carbon atoms in dia- mond and the steric hindrance of such surfaces, are the main reasons for the chemical inertness of diamond. Despite that, there are several notable exceptions to the generally low reactivity of diamond. First of all diamond sur- faces can be oxidized by several post-growth treatments, such as oxygen-ambient annealing [94], oxygen-plasma treatment [93] or anodic polarization [95]. All these oxidizing techniques result in an increase of surface O/C ratio and the presence of carbon–oxygen bonds. An important characteristic of this sur- face is that this oxygen termination can partially regenerated by subsequent acid washing and hydrogen-plasma treatment [93]. Another very useful modification of diamond surface is the halogenation using atomic and molecular chlorine and fluorine [96,97]. Although molecular Cl 2 and F 2 have been used as reagents, the reaction conditions are such that atomic radical species are produced. Since the reaction conditions required are very extreme (for example, Cl 2 /400–500 ◦ C), and thus unsuitable for large-scale implementation, the photo- chemical radical production is usually preferred [98]. Except from these two small atomic radicals, larger organic radicals have been also photochemically introduced onto dia- mond surfaces. Such an example is the perfluorobutyl moiety which has been successfully attached to diamond surfaces by irradiating perfluorobutyl iodide (C 4 F 4 I) either using UV light or X-rays [99,100]. Moreover, a quite big variety of long-chain organic moieties have been also introduced on diamond sur- faces by photochemical reactions. Although such compounds are either functionalized alkanes or alkenes, it is proven that alkenes significantly increase the attachment efficiency and are thus used preferencially [101]. The main purpose of all the aforementioned chemical modification methods is to provide diamond surface with the appropriate binding groups (mainly primary amine and carboxylic acid groups). These groups are required for fur- ther functionalization of diamond surface with more complex molecules, such as DNA or proteins, with the altimate goal of diamond-based biosensor development. At the same time diamond has attracted a lot of atten- tion due to its unique electrochemical properties. In particular, boron-doped, hydrogen-terminated, polycrystalline diamond has a very wide working potential window (+3.0 to 3.5 V) in aqueous and non-aqueous media, and low overpotential for several redox analytes. In addition this material has low and stable background current, leading to improved signal-to- noise ratios [102]. Finally adsorption of polar molecules on its surface is insignificant, leading to improved resistance to sur- face deactivation and fouling. It should be mentioned though that the surface chemistry of the diamond is strongly influ- enced by the amount of boron doping [103,104]. All the above-mentioned properties, along with the fact that diamond is considered highly biocompatible, make this material ideal for the development of completely integrated bioelectronic sensing systems. This seems to be true since already a large number of diamond’s electroanalytical appli- cations have been reported, in flow-injection analysis (FIA) systems or ion and high-performance liquid chromatogra- phy (IC & HPLC), for the detection of azide [63,64], metal ions [63,65], nitrite [63], dopamine [63,66,67], chlorpromazine [63], hydrazine, biogenic aliphatic polyamines [68,69], NADH [70], uric acid [71], histamine and serotonin [72], and carba- mate pesticides [73]. It is worth to be mentioned that, in all the above cases, diamond demonstrated superior electrode performance in terms of linear dynamic range, sensitivity, limit of detection, response stability and long-term activity, as compared with glassy carbon. In the field of electrocataly- sis some interesting applications have also been reported, for the oxidation of methanol and the reduction of oxygen, both using a conductive, dimensionally stable diamond electrode containing Pt nanoparticles [80–82]. Another very attractive application of diamond, coming from the area of spectro- electrochemistry, was reported in 2001 [105], concerning a free-standing boron-doped diamond disc (0.38mm thick and 8 mm in diameter) used for the oxidation of ferrocyanide, or the reduction of methyl viologen, and the simultaneous spec- troscopic monitoring of the products through the disc. In the same year, the construction of an electrolyte-solution-gate diamond field-effect transistor (SGFET) was reported for the first time [106], and after 2 years, in 2003, the first anion- sensitive diamond SGFET was reported from the same group 6 analytica chimica acta 615 (2008) 1–9 Fig.3– TheGaN(0001)wurtzite crystal. The outer most atomic layer of the material (Ga) is theoretically non-bonded, allowing for a strong interaction with overlaying coordinating ligands. as well [61,62]. The first bio-electrochemical application of diamond was reported in 2002, concerning the direct elec- trochemistry of cytochrome c at nanocrystalline boron-doped diamond [74]. Since then, many other bio-electrochemical applications have been reported, such as the direct obser- vation of DNA hybridization via simple measurement of interfacial impedance, using DNA-modified diamond thin films [75], or the in vitro measurement of norepinephrine (NE)-release from a test animal’s mesenteric artery, using a Pt-microelectrode coated with a thin film of boron-doped dia- mond [76]. 5. GaN and III-nitrides GaN, AlN and InN are the so-called III-nitride semiconductor materials. Some of these semiconductors have been the sub- ject of intense research lately, due to their very high electron mobility, high energy band gap, and biocompatibility. These properties are very important in the design of chemical sen- sors and biosensors for remote, in vivo, and low detection level measurements. III-Nitrides prefer to crystallize in the wurtzite crystal structure (Fig. 3). The important feature to understand, in wurtzite crystal structures and in particular the (0001)ori- entation is the fact that the outer most atomic layer has three bonds to the underlying nitrogen atomic plane while the forth unoccupied bond (tangling bond) is available for interaction with ligands that exist within the close proximity test envi- ronment. The type of ligands that can interact chemically with this surface will thus depend on the chemistry of the surface layer of the material. Extensive studies [107] have proven that there is an induce polarity in these bonds, with the more elec- tropositive atoms being electron deficient relative to nitrogen atoms, as shown in the case of GaN in Fig. 4 [108]. Up until very recently, the growth of these materials was not very well controlled, and thus their availability for sen- sor applications was very limited. Of these, the polar GaN c-plane is the first of the III-nitrides to be available at high crystal quality and because of this it has been more exten- sively studied for sensor applications. In particular, of the two possible orientations of the c-plane GaN, the Ga-face is the one almost exclusively used. This is due to the fact that this material is very robust, inert to etching, while at the same time it has available free bonding for coordination with Lewis base-type ligands. In addition it can be chemically function- alized, thus allowing the possibility to generate multi-layer chemical systems [78]. On the other hand the N-face struc- ture is not chemically stable; it etches easily, while at the same time it cannot coordinate with bases due to the unfavorable electronic charge density distribution. Since most of the published work is based on the c-plane GaN Ga-face, we will concentrate on this particular substrate for the remaining of this section. Based on theoretical results the outer most layer of the GaN, and in particular the Gal- lium atoms, will be partially electron deficient, and will thus interact preferentially with Lewis bases, such as thiols, organic alcohols [109] and anions [59]. This surface chemical inter- action will have considerable effect on the physicochemical characteristics of the GaN substrate. To start with, it will develop an interfacial layer which will be negatively charged (Fig. 5). As a result, potentiometric or impedometric sensors selec- tive to the specific ligand can be developed. The calibration curve of such a pair of sensors to chloride ion is shown in Fig. 6 [59]. In the case of a GaN-based CHEMFET, the interaction of Lewis bases with the surface will drastically influence the internal band structure of the semiconductor. As a result, the carrier density in the surface-near two-Dimensional Electron Gas (2DEG) of GaN will be determined by this band bend- ing. Since, under normal growth conditions, GaN acts as a p-type semiconductor, it is expected that there is going to be a decrease in the current upon increasing of the nega- tive surface charge. The V–I curves of a GaN-based CHEMFET, as it is shown in Fig. 7 [110], prove that indeed this is the case. Fig. 4 – Gallium nitride (GaN) electronic charge density distribution. The numbers on the contour line are indicated in equiv./atomic volume units. Reproduced with permission from Ref. [107]. analytica chimica acta 615 (2008) 1–9 7 Fig. 5 – Schematic diagram of the Ga-face GaN-solution interface. The potential and impedance changes appear between the semiconductor and the solution, and across the Helmholtz layers. It should be pointed out that the exact nature of the sur- face chemistry of GaN is very important since any changes will drastically influence the behavior of the final device. For example, it is known that oxidation of the surface will gen- erate a surface layer saturated with hydroxyl groups. In this case, the behavior of the sensor will be reversed, since it will now be sensitive to cations, and not to anions as in its origi- nal state. Additionally, care must be taken so that the studies used for the evaluation of these sensors do not involve any pH changes, since this can interfere with the measurement of the analyte ions. These studies of the GaN surface indicate that the unique selectivity of the GaN surface is very impor- tant for the future development of not only electrochemical sensors and biosensors, but also optical fluorescent sensors. Indium nitride (InN) is a semiconductor for which there has been done very little work in the area of chemical sen- Fig. 6 – Correlation between the activity of KCl and the induced potential and interfacial capacitance of the Ga-face GaN-solution interface. Reproduced with permission from Ref. [59]. Fig. 7 – IDS–VDS characteristics of a GaN EGHEMT with Lg=80␮m and Wg= 100 ␮m, measured in air (— black solid line) and within aqueous solutions with pH 3.35 (··· red dotted line), pH 6.84 (–·– blue dashed-dotted line) and pH 12.45 (– – magenta dashed line). Reproduced with permission from Ref. [109]. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of the article.) sors and biosensors. InN is a chemically stable wurtzite crystal which, as in the case of GaN, also has an induced polar surface. The unique property though of this material is the fact that it has very high surface electron concentrations [111]. This phe- nomenon has been utilized for the development of a series of gas sensors, and only in a few instances in the development of solution chemical sensors. It has been found for example that the InN surface shows a pronounced response to certain solvents as shown by Hall mobility and sheet carrier density measurements [110]. It is suggested that the near to surface accumulated electrons, contribute considerably to the lateral conductivity of thin InN films. Based on this, upon interac- tion of a substance with the surface of InN, this will affect the surface charge and thus it will modulate the current, or alter the surface potential, providing the grounds for the develop- ment of highly sensitive sensors. Up until now there are some results indicating that this indeed is the case for gas sensing [110,112]. Despite that, this material is relatively unexplored as a matrix for chemical sensor and biosensor applications. On the other hand, AlN is a material that even thought it has not been used extensively as a substrate for chemical sen- sor and biosensor development, it has been utilized for the development of shear mode acoustic biosensors [86,87]. 6. Forecasting the future As the area of semiconductor synthesis evolves there is a very significant opportunity for the development of a new era of biosensors. While III-nitrites and doped diamond have already shown that they can provide specific advantages for their use in sensor science, at the same time new surfaces such as SiC and other semiconductor materials are becoming widely available, and might be useful for sensor applications. The 8 analytica chimica acta 615 (2008) 1–9 major problem of material availability and suitability for elec- trochemical and optical transduction is being undertaken via multidisciplinary research efforts. 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