Zero-Bias Broadband Ultraviolet Photoconductor Based on Ultrananocrystalline Diamond Nanowire Arrays


SUMMARY

This article focuses on developing a broadband ultraviolet (UV) photodetector (PD) based on superflat, boron-doped ultrananocrystalline diamond (UNCD) nanowire (NW) arrays functionalized with platinum (Pt) nanoparticles and capable of withstanding high operating temperatures.  This PD exhibits an extremely large responsivity (1,224 A/W) to 300-nm light radiation at zero bias while taking advantage of diamond’s unique stability from its ability to function at temperatures as high as  200 °C.  Additionally, it has a fast response time of 17 ms.

INTRODUCTION

Current needs in applications, such as space exploration and Earth’s surface incident radiation monitoring, have imposed new requirements on the next generation of deep UV detectors. In particular, reduced energy consumption, enhanced resistance to the performance degradation at high temperatures, and high portability appear to be extremely important properties for a growing demand of crucial applications [1]. Recent progress has been made toward achieving these goals. Several nanostructured, wide band-gap, materials-based detectors have been developed [2–4]. However, designing a multifunctional material that simultaneously exhibits desired properties in PDs still remains a challenge.

We aim at creating a cost-effective, high-performance, UNCD-based UV PD that satisfies the market needs of high-temperature and self-power operation. Diamond has attracted significant interest as a promising candidate for UV sensing applications due to its wide band gap, chemical stability, and robustness. Diamond also has excellent mechanical hardness and tunable band gap via doping [5, 6].  The undoped, single-crystal diamond has a band gap of 5.5 eV [7, 8].  Lansley and his group used natural and synthetic diamond to achieve a very fast time response (less than a microsecond) when these detectors were radiated with pulsed laser beams [9].  Pace and De Sio used both single crystalline and polycrystalline diamond films to develop two types of photoconductive detectors [10].  The obtained experimental data suggested that single crystalline, diamond-based detectors had much higher sensitivity than the PD using polycrystalline diamond [11, 12].

However, natural single crystalline diamond is very expensive, whereas either nanocrystalline diamond or micropolycrystalline diamond films have rough surfaces with which no precise nanostructures for electronic devices can be made. In contrast, UNCD has excellent film thickness uniformity with extremely low surface roughness independently of its film thickness. Furthermore, UNCD can be obtained from a reliable and cost-effective production method yielding consistent UNCD material properties.

Our research focuses on the functionalization of superflat UNCD material with Pt nanoparticles from which NW arrays have been fabricated for developing novel, self-powered UV PD devices. We base this research on the fact that Pt nanoparticle-functionalized, UNCD NW-based PDs would drastically improve the response times because of their large surface-to-volume ratios. One-dimensional UNCD NW nanostructures with Pt particles on the surface of the NWs are essential blocks to fabricate a field-effect transistor (FET).

The Pt on the surface of the NWs may change the conductance by changing the surface charges and states. This changes the gate potential, work function, and band alignment, resulting in a gate coupling and change in carrier mobility. All of these will be regarded as a floating gate effect on the conducting channel of the FET.

The strong local electric field at the reversely-biased Schottky barrier area will quickly separate the photon-generated electrons and holes, reducing the electron-hole recombination rates. This reduces the power consumption, increases the portability, and enhances the signal-to-noise ratio [13]. The obtained experimental data clearly indicate that the newly-designed PD exhibits high photocurrent and outstanding responsivity to both UVB and UVC radiations at zero-applied voltage while taking advantage of diamond’s unique stability to function at temperatures as high as 300 °C. Additionally, the investigated PD has fast response times less than 17 ms. Few materials reported so far, if any, possess such a desirable set of UV-sensing performance characteristics.

SYNTHESIS 

The boron-doped UNCD thin films were synthesized by Advanced Diamond Technologies, Inc., with the microwave plasma, chemical vapor deposition system (Lambda Technologies Inc.) using a standard deposition procedure. They were grown at a substrate temperature of 760 °C, a microwave power of  2100 W, and an Ar/CH4/H2 gas mixture, with flow rates of 400/1.2/8 sccm, respectively.  Throughout the UNCD deposition, the chamber pressure was maintained at 120 mbar.  The growth procedure was performed on a silicon wafer substrate for 1 hr, resulting in an 80-nm-thick UNCD film deposition.  As a source of boron, trimethylboron was used (i.e., B[CH3]3) for the doping.  (For more details regarding the synthesis process of UNCD growth and the doped process, see references 14–17.)

UNCD NW arrays (70 nm wide and 35 μm long) were then fabricated using top-down-based lithography techniques. (A detailed description of the crystalline structure and surface morphology characterization, electron beam lithography, and NW array fabrication can be found in references 11, 12, and 18.)

Finally, using a sputtering method, 10-nm aluminum (Al)/100-nm Pt pairs were deposited as electrodes (ohmic contacts) onto two sides of the UNCD NW arrays. In the process, Pt nanoparticles functionalized on the surface of UNCD NWs, resulting in individual punctual Schottky contacts in the NW’s surface. The fabricated NW device was then annealed with a LakeShore temperature controller at 150 °C for 5 min in the probe station chamber.

At zero bias, no induced photocurrent could be detected from the two ohmic, contact-based PDs exposed to deep UV light radiation. The carrier transport of UNCD that relied on hopping transport mechanisms resulted in a low carrier velocity. In other words, the UNCD films-based PDs had very poor performance. Therefore, very few groups have worked on them, even though UNCD is much cheaper than single-crystal diamond.

We used Pt nanoparticles to functionalize the surface of UNCD NWs in an attempt to improve the performance of a UNCD-based PD. The prototype formed a nonsymmetrical Schottky contact device containing a Schottky barrier (SB) contact at one side of the device and an ohmic contact at the other side for each Pt nanoparticle on the UNCD wire surface. The entire device could be regarded as an NW connected in series with an SB diode (one for each Pt nanoparticle).

The Schottky contact area in each wire constituted the bottleneck for the current transport in the device. The original SB height φSB was determined by the work-function difference between the metal and Pt nanoparticles on the surface of the UNCD wires and the interface states. The current passing through the Schottky contact was very sensitive to the Schottky barrier height and width. Light irradiation at or near the SB area could change the local electric-field distribution and thus cause the variations of the SB height and width, resulting in a detectable current change [13, 19].

SEM Images

Figure 1a shows the SEM image of the fabricated UNCD NWs, which are nearly 35 μm long and connected to the Al/Pt electrodes.  The gap is approximately 1 μm between the UNCD NWs.  Figure 1b shows an enlarged SEM image of a typical single wire consisting of tiny UNCD nanoparticles, with a diameter of about 50 to 70 nm.  The UNCD NWs have an average width of 70 nm.  (A detailed characterization of the UNCD with a high-resolution transmission, electron microscope can be found in previous reports [11, 12].)


Raman Spectroscopy

Raman scattering is a powerful, semiquantitative method to examine the diamond structure because the sp2 and sp3 carbon bondings are very sensitive to this scattering. Figure 2 shows a typical Raman spectrum of the boron-doped UNCD wire arrays by using a triple monochromator with an excitation wavelength of 514 nm from an Ar+ ion laser and a microscope to focus the laser beam onto the NWs. Two broad bands assigned to the regular D and G bands are clearly visible at 1332 cm-1 and 1595 cm-1, respectively.

Normally, the G band is from the sp2 carbon bonds, indicating the synthesized sample is a carbon-mixed UNCD material. The broad Raman spectral intensity taken at the D band (1332 cm-1) from sp3 carbon bonds proves that the material is indeed diamond. A broadened profile of the D band could be related to the polycrystalline structure of randomly-oriented grains or the nanoscale effect [20].

PROTOTYPE

To understand the electrical properties, the fabricated UNCD NWs were connected to a basic electric circuit to form the prototype. Figure 3 shows a schematic diagram of the prototypical PD. Two external conductive electrodes installed at two ends of UNCD NWs were serially connected to a precision resistor Rprecise, a switcher, and a step-up/step-down voltage regulator Vo (Keysight E3643A power supply). The setup also consisted of two Hewlett-Packard 34401A programmable electronic multimeters (V1 and V2) monitored by a LabVIEW program from which the variations of the two voltages  (Vprototye = V2 – V1) and the current (Iprototype = V1/Rprecise) in the prototype were recorded. A tungsten filament and a thermocouple were used as a controllable heater for obtaining a desired operating temperature.

ELECTRICAL PROPERTIES

Even though we focus on the ability of the PD to operate at zero bias (for this case, V2 = 0), it is still necessary to understand how the bias affects the properties of the UNCD NWs-based detector. Figure 4 shows typical responses at different reverse biases when the UNCD NWs-based PD is cycled, with 2 min between the “switch-on” and “switch-off” of the 250-nm UV light radiation at room temperature. In Figure 4a, the fabricated PD displays a quick, well-defined response. A higher-bias voltage yields a higher-induced photocurrent and a larger responsivity ratio between the output photocurrent and the input UV light power.

As seen in Figure 4b, the obtained photocurrents (Iph) at –1, –2, and –2.5 V bias with and without UV radiation are approximately 6, 14, and 29x larger, respectively, than at zero bias. This characteristic was much better than the SiC, diamond, and other oxide semiconductors-based detectors, although the measurements were taken at a higher bias [21–24]. Furthermore, fast response and recovery times were observed, regardless of the applied bias.

A slight variation of the baseline was also observed in Figure 4a, as the reverse bias from the power supply increased. Following an increase of bias magnitude, the dark current unavoidably increased, as well as the noise level. Therefore, the signal-to-noise ratio did not improve significantly as the bias magnitude increased. Figure 4b shows the typical current-voltage reverse bias characteristics in dark and 250-nm UV light at room temperature. A nonlinear current-voltage curve was observed at the applied bias, indicating a Schottky diode behavior of the NWs/Pt contacts.

Characterization Responsivity 

When the PD is exposed to UV light, the photonic energy is absorbed by the valence electrons, leading to the induced photocurrent. Three different UV wavelengths were used for characterizing the responsivity of the boron-doped UNCD NWs. The intensity of the UV light onto the surface of the PD was controlled by shifting the height between the UV light source and the detector. Figure 5 illustrates typical photoresponses of the fabricated UV PD to 250-, 300-, and 350-nm UV light illuminations, respectively, subjected to the on/off cycles with 120 s. Results of the UV PD presented in Figure 5 were tested at room temperature under three different incident radiation intensities.

Figure 5a shows the photoresponses at zero bias and room temperature when the prototype was exposed to 250-nm light illumination cycling with 120 s. When the detector was exposed to UV light radiation, the induced photocurrent quickly increased and reached a stable maximum value. When the UV radiation switched off, the photocurrent decreased to zero value. Excellent features in repeatability and stability are clearly visible. The induced photocurrent can be attributed to the absorption of UV photons by the active UNCD NW’s material.

A decrease in photocurrent magnitude was expected as the intensity of UV light reaching the surface of the PD decreased. However, no significant change in photocurrent was observed when the UV intensity ranged from 1 mW/cm2 to 0.03 mW/cm2, indicating that the detector might be saturated with UV radiation. Under this condition, the generated photocurrent from the UV PD reached 0.94 μA. Since the average length and width of each NW was 35 μm and 70 nm and the total number of NWs in the platform was 100, we estimated a total exposure area of 245 µm2 for the present prototype.  Consequently, we obtained a responsivity around 406 A/W for 300-nm UV light.  This value was at least three orders of magnitude higher than what has been reported so far on zero-bias or self-powered, deep UV PDs made of different materials [3, 4, 15].

Similar phenomena, including good repeatability and stable baseline, were also observed when the PD was exposed to 300 nm of UV light illumination (shown in Figure 5b). The main observed difference was that the 300-nm induced photocurrent was almost 3x larger than that induced with 250-nm light illumination. As a result, an extremely high responsivity up to 1,224 A/W was obtained from the present PD when exposed to 300-nm light.

The total size of the platform used was only 0.5 mm2, which was much smaller than those UV detectors reported by others. From the literature, several groups were successful in achieving very high responsivity from nanomaterials- based PDs in the UV region at a high applied bias. This has the disadvantage of increasing dark current level, resulting in a poor signal-to-noise ratio. Furthermore, neither size nor weight of a bias-based detector could be reduced easily due to the power supply constraint. In contrast, the self-power, zero-bias PDs tended to exhibit low, dark current magnitudes [25–29]. In the present case, the obtained dark current was only 2 × 10-7 A. Since the induced current was 3.5 μA, we had a signal-to-noise ratio of 18. Because it did not require an external power supply, the detector could be much smaller. The miniaturization of PD devices was extremely important for a wide number of space applications.

Characterizations of the time-dependent photoresponsivity of the fabricated detector at zero bias and room temperature when exposed to 350-nm light illumination were also achieved.  The results are shown in Figure 5c. Excellent repeatability and stability, as well as quick response time, are still clearly visible.  However, the obtained 350-nm light-induced photocurrent was almost 6x less than that induced by 300-nm light.  Correspondingly, a responsivity of 244 A/W related to 350-nm radiation was obtained.

As shown in Figure 5, the responsivity of the present boron-doped, UNCD-based PD is highly related to the wavelength of UV light radiation. The highest responsivity of 1,224 A/W is related to the 300-nm UV light illumination. Applying Mendoza’s model to the obtained relationship between the responsivity strength and wavelength of UV light radiation, we estimated that the present boron-doped UNCD had a band-gap energy around 4.1 eV (~300 nm) [30]. This agreed with the previous studies of the band-gap shift obtained from the basic characterization of UNCD [31, 32].

Temperature Effect

The photoresponse of the UNCD-based UV sensor was also studied at different operating temperatures. The results, presented in Figure 6, were conducted with a 250-nm wavelength UV light radiation. The vast majority of UV PDs reported were inoperable at temperatures above 100 to 150 °C [23]. The UNCD-based PD continued to perform well above this temperature limit.  Comparing the responses obtained at room temperature in Figure 5 with higher temperatures in Figure 6 revealed that the photocurrent decreased with increasing temperature.  In addition to a decrease in signal-to-noise ratio, this was expected due to the unavoidable increase in dark current and thermal noise as the temperature increased.

However, as shown in Figure 6, the UNCD prototype maintains an excellent repeatability and stability as operating temperature is increased up to 100 °C. The PD under investigation was capable of withstanding operating temperatures as high as 200 °C while retaining a clear, stable, and reproducible signal well above the thermal noise threshold. Further increasing the temperature to 300 °C, the light-induced photocurrent response from the PD is still measurable, even with the large thermal noise contribution. This is a considerable increase in PD heat tolerance, compared to the majority of UV sensors reported, and is directly attributed to diamond’s intrinsic properties.

Similar results for the PD prototype under investigation were obtained upon exposition to 300- and 350-nm radiations.  As seen in Figure 7, at an operating temperature of 100 °C, the UNCD-based PD experiences a slight decrease in photocurrent compared to its room-temperature performance. However, the signal-to-noise ratio is still significantly high, and the fast response times were not affected. Also, at an operating temperature of 100 °C,  the the PD prototype still exhibited high responsivity values when exposed to 250-, 300-, and 350-nm radiations.

Response Time and Recovery Time

The UNCD-based PD exhibited fast response and recovery times regardless of the incident radiation at operating temperatures up to 200 °C. To further investigate the characteristic response and recovery times (tres and trec) of the prototype, high time-resolution measurements were performed. Determining the tres values was based on the time interval for the photocurrent to rise to 90% of its peak value and trec as the time interval for the signal to decay to 10% of its peak value.

Figure 8 shows typical measurements of both tres and trec when the 250-nm light illumination is turned on and off at room temperature.  The PD under investigation exhibits a response time of 17 ms.  On the other hand, trec is around 30 ms, almost twice the response time tres.  This is likely due to the intrinsic radiation intensity decay of the light source as it switches off.  Even though the obtained rise and decay times of the UNCD-based PD cannot compare to those of the reported single-crystalline, diamond-based PD, the present UNCD NW-based PD still shows it advantages, including a cost-effective, self-powered feature and higher responsivity. Nevertheless, the obtained response times are still much faster than most reported PDs with similar responsivity values [33, 34].

CONCLUSIONS

Based on the superflat surface, synthesized UNCD boron-doped film, a precise UNCD NW array was designed and fabricated. To the best of our knowledge, this was the first demonstration of the zero-bias, solar-blind PDs based on Pt nanoparticle-functionalized, boron-doped UNCD NW arrays providing ultra-high responsivities up to 406, 1,224, and 244 A/W at 250-, 300-, and 350-nm UV wavelengths, respectively. 

The newly-developed PD possesses outstanding features in quick response (less than 17 ms), with excellent repeatability and stability.  Experimental data also showed that an extremely large responsivity could be achieved with a high reverse bias.  The obtained photocurrents were almost 6, 14, and 29x larger at –1, –2, and –2.5 V bias, respectively, than at zero bias.  These characteristics were much better than those reported for SiC, diamond, and other oxide semiconductors-based, zero-bias, solar-blind PDs.  In addition, the PD based on UNCD NW arrays performed extremely well, even at temperatures as high as 200 °C, making the UNCD NW array arrangement an ideal candidate for UV sensing applications in harsh environments.

ACKNOWLEDGMENTS

This work was financially supported by the National Science Foundation’s Centers of Research Excellence in Science and Technology for Innovation, Research, and Education in Environmental Nanotechnology, grant no. HRD-736093. The authors also want to acknowledge the help of Ali Aldalbahi, Xinpeng Wang, and the Naval Air Warfare Center Aircraft Division (NAWCAD) 4.5.14 for their support.

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BIOGRAPHIES

RAFAEL VELÁZQUEZ works at NAWCAD, Airborne Anti-Submarine Warfare Systems Engineering Division, Patuxent River, MD. He has performed fundamental research in nanomaterials for light harvesting, photovoltaics, energy storage, microcrystalline diamond, ultrananocrystalline diamond nanowire arrays in deep UV photodetection, and nanomaterials for biomedical applications in bacteria and cancer cells. He was also associate professor at the Molecular Sciences Research Center for the Biomedical Applications of Nanomaterials Research Lab. He has written more than a dozen publications. Dr. Velázquez holds a Ph.D. in chemical physics from the University of Puerto Rico.

MANUEL RIVERA is the leading investigator in a National Institutes of Health Small Business Innovation Research grant with Sil Technologies in collaboration with the University of Puerto Rico Mayaguez Campus. He has performed fundamental research in molecular dynamics of liquid crystals and polymers subjected to nanoconfinement and contributed to the advancement in novel applications of tailored nanomaterials. He has investigated the use of polymer-carbon nanotube metacomposites in gas detection and boron nitride nanosheets in deep UV photodetection and written more than a dozen publications. Dr. Rivera received his Ph.D. in chemical physics.

ANDREW F. ZHOU is a professor at the Indiana University of Pennsylvania, Indiana, PA. His research interests include solid-state lasers and nanophotonics. He has held postdoctoral positions at the University of Strathclyde, Glasgow University, and Imperial College in the United Kingdom. He has worked as a research scientist in the United States and assistant professor at Nanyang Technological University in Singapore. Dr. Zhou holds a Ph.D. in optics.

DAVID BROMLEY works at NAWCAD, Airborne Anti-Submarine Warfare Systems Engineering Division, Patuxent River, MD. He has 25 years of experience in underwater acoustics, magnetics, and sonobuoys and is a Naval Air Systems Command Associate Fellow. Dr. Bromley holds a Ph.D. in physics from Drexel University, with a physics specialty in laser dynamics.

PETER X. FENG is a professor in the Department of Physics, University of Puerto Rico. His research interests include semiconducting and ceramic materials and various types of electronic devices, including field emission displays; gas, humidity, and thermal sensors; Schottky diodes; photodetectors; sensor calibration systems; and digitally-controlled pulse deposition systems. He has authored more than 190 articles. Dr. Feng holds a Ph.D. in physics from LaTrobe University.


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