Peicai Wu

Self-powered high performance photodetectors based on CdSe nanobelt/graphene Schottky junctions

Self-powered photodetectors based on CdSe nanobelt (NB)/graphene Schottky junctions are fabricated and investigated. Typically such Schottky junctions exhibit good rectifying behavior without light illumination. The on/off ratio is more than 1 × 105 when the voltage changes from −1 to 1 V. Under zero bias, typically such photodetectors show high photosensitivity (∼3.5 × 105), which is defined as (Iphoto − Idark)/Idark, to above-band-gap irradiation. Under 1000 Hz light switching frequency, the response and recovery times of such photodetector are typically 82 and 179 μs, respectively, and the photoconductive gain is 28, greater than unity. The high photosensitivity and gain, as well as fast response speed, guarantee the feasibility of such self-powered photodetectors.

Fast-speed and high-gain photodetectors of individual single crystalline Zn3P2 nanowires

We report high-performance photodetectors of individual single crystalline Zn3P2 nanowires. A facile catalyst-free physical evaporation process was developed to synthesize high-quality single crystalline Zn3P2 nanowires (NWs) with high yield. A typical individual Zn3P2 NW photodetector exhibited fast speeds (the response and recovery times were less than 25 and 26 μs, respectively) and high gain (∼470), which is an excellent result towards getting a balance in the speed and gain trade-off of the semiconductor nanostructure-based photodetectors. In addition, the device had the advantages of high photosensitivity (Iphoto − Idark/Idark ≈ 100) and small device size (constructed on an individual nanowire). All these merits substantiate that the Zn3P2 nanowires can serve as excellent photoconductive materials, and the Zn3P2 NW photodetectors are very promising for practical applications.

Schottky junction photovoltaic devices based on CdS single nanobelts

Schottky junction photovoltaic (PV) devices were fabricated on single CdS nanobelts (NBs). Au was used as the Schottky contact, and In/Au was used as the ohmic contact to CdS NB. Typically, the Schottky junction exhibits a well-defined rectifying behavior in the dark with a rectification ratio greater than 10(3) at +/- 0.3 V; and the PV device exhibits a clear PV behavior with an open circuit photovoltage of about 0.16 V, a short circuit current of about 23.8 pA, a maximum output power of about 1.6 pW, and a fill factor of 42%. Moreover, the output power can be multiplied by connecting two or more of the Schottky junction PV devices, made on a single CdS NB, in parallel or in series. This study demonstrates that the 1D Schottky junction PV devices, which have the merits of low cost, easy fabrication and material universality, can be an important candidate for power sources in nano-optoelectronic systems.

Impurity-Dependent Photoresponse Properties in Single CdSe Nanobelt Photodetectors

Impurity-dependent photoresponse properties of semiconductor nanostructures are studied for the first time in the photodetectors (PDs) made from intrinsic (i-) and extrinsic n-type single CdSe nanobelts (NBs). Both ohmic and Schottky contact based CdSe NB PDs were studied. The sizes of i- and n- NBs were purposely chosen to be nearly identical to minimize the size-dependent effect. Our experimental results demonstrate that i-CdSe NBs are more suitable for fast and sensitive PD applications, while n-CdSe NBs are more advantageous for high-gain PD applications. The different photoresponse properties result mainly from the impurity induced traps inside the CdSe NBs. This conclusion can be applicable to other semiconductor nanomaterials. Moreover, we have achieved short response/recovery times (∼15/31 μs) and a high photosensitivity (∼100) simultaneously from the i-CdSe NB PDs under a 3500 Hz pulsed illumination. This may be the best reported result so far in the effort of getting fast speed and high photosensitivity simultaneously from nanostructure based PDs for practical applications.

Ultrahigh-Performance Inverters Based on CdS Nanobelts

We report ultrahigh-performance inverters, each consisting of two top-gate metal-oxide-semiconductor field-effect transistors based on n-CdS nanobelts. High-kappa HfO(2) dielectrics are used as the top-gate oxide layers. The inverters have a large supply voltage (V(DD)) range (from 50 mV to 10 V) and very high voltage gain ( approximately 10, 100, and 1000 at V(DD) = 0.2, 1, and 10 V, respectively). Current consumption is less than 7 nA at V(DD) = 1 V, corresponding to a power consumption of less than 7 nW. The high and low output voltages are close to full rail. The inverters also exhibit good dynamic behavior with square wave input at frequencies up to 1 kHz. The operation of the inverters is analyzed in detail. The inverters are promising for future low power high performance logic circuit applications.

High-performance CdSe nanobelt based MESFETs and their application in photodetection

High performance metal–semiconductor field-effect transistors (MESFETs) based on single n-CdSe nanobelts (NBs) have been fabricated and applied as photodetectors. Au is used as the gate metal, which formed a good Schottky contact with the CdSe NB with a rectification ratio of about 2 × 108. The CdSe NB MESFETs exhibit a near-zero threshold voltage (−0.55 V), low subthreshold swing (60.4 mV per dec), no clearly observed current hysteresis, and the highest on/off current ratio (5 × 108) reported so far for NW/NB MESFETs. We have also investigated the photoresponse properties of these MESFETs. Typical CdSe NB MESFET based photodetectors have high current responsivity (∼1.4 × 103 A W−1), high gain (∼2.7 × 103), and fast photoresponse speed (the rise time and the decay time are about 35 and 60 μs, respectively.) under a gate voltage of −1 V. All these results show that the CdSe NB based MESFETs can be promising candidates for both electronic and opto-electronic nanodevices.

High-performance CdS nanobelt field-effect transistors with high-κ HfO2 top-gate dielectrics

We have fabricated high-performance enhancement and depletion mode (E- and D-mode) top-gate metal-insulator-semiconductor field-effect transistors (T-G MISFETs) using two kinds of CdS nanobelts (NBs), labeled as NB A and NB B, respectively. High-κ HfO2 dielectric is used as the insulator layer. The thicknesses of NBs A and B are about 60 and 180 nm, respectively. The threshold voltage and subthreshold swing of the CdS NB A T-G MISFET are about 0.15 V and 62 mV/dec, respectively. The on/off ratio is about 6 × 104, which is the best result for E-mode CdS nanoFETs reported so far. The threshold voltage, on/off ratio and peak transconductance of the CdS NW B T-G MISFET are about −3.4 V, 2 × 109, and 11 µS, respectively. The on/off ratio, to the best of our knowledge, is the highest reported for nanoFETs so far. Both of the two kinds of T-G MISFETs have quite small hysteresis in their transfer characteristics. The mechanisms for the different gate transfer characteristics are discussed. Corresponding Si back-gate CdS NB MISFETs each with a 600 nm SiO2 film as the insulator layer are also measured for comparison.

Logic gates constructed on CdS nanobelt field-effect transistors with high-κ HfO2 top-gate dielectrics

A high-performance NOT logic gate (inverter) was constructed by combining two identical metal-insulator-semiconductor field-effect transistors (MISFETs) made on an individual CdS nanobelt (NB). The MISFETs, which used high-κ HfO2 dielectrics as the top-gate insulator layer, show excellent performance, such as low threshold voltage (∼ −0.1 V), a high on/off ratio (∼108), small subthreshold swing (∼65 mV dec−1) and threshold voltage hysteresis (∼30 mV). The supply voltage for the inverter can be as low as 1 V with a voltage gain as high as 14. When the supply voltage is 7 V, the voltage gain is 72, which is the best reported value, as far as we know, for the inverters based on NB/NW n-channel MISFETs. Besides, the inverter can work in a larger input range with highly stable output voltages. Their high output voltages can make full use of the supply voltages, and their low output voltages can be close to zero. NAND and NOR logic gates have been constructed by assembling three such CdS NB MISFETs, which show high performances as well. The operating principle of the inverter is discussed in detail.

High-performance non-volatile CdS nanobelt-based floating nanodot gate memory

High-performance, non-volatile, floating nanodot gate memories (FNGMs) based on single CdS nanobelts (NBs) are reported. Their structure consists of a CdS NB field-effect transistor and Au nanodots embedded in high-κ HfO2 top-gate dielectrics. Direct tunnelling of charges between the CdS NB and the Au nanodots causes a shift of the threshold. A simple thermal evaporation method was employed to fabricate high-density, uniformly distributed Au nanodots (∼3 × 1012 cm−2) in between a 5 nm HfO2 tunnelling layer and a 15 nm HfO2 control oxide layer. Under a low operation voltage of 5 V, a typical as-fabricated FNGM has a large memory window of 3.2 V, long retention time of up to 105 s, and good stress endurance of more than 104 write/erase cycles. The working principle of the CdS nanobelt-based FNGM is discussed in detail in this paper.

Enhancement-mode nanowire (nanobelt) field-effect-transistors with Schottky-contact source and drain electrodes

Enhancement-mode (E-mode) field-effect-transistors (FETs) have advantages in making high-speed and low power consumption devices. However, most reported nano-FETs work in the depletion-mode, because E-mode nano-FETs are usually difficult to be implemented. We suggest a device structure, based on which high-performance E-mode nanowire (NW) or nanobelt based FETs can be reliably fabricated. In this device structure, both source and drain electrodes form Schottky contact with the NW, and a top gate is long enough to control the entire conductive channel. The working principle is discussed in detail. This device structure is universal to semiconductor materials and has diverse application prospects.