High Power, Room Temperature Terahertz Emitters Based on Dopant Transitions in 6H-Silicon Carbide

James Kolodzey, Guang-Chi Xuan, Peng-Cheng Lv, Nathan Sustersic, and Xin Ma

High Power, Room Temperature Terahertz Emitters Based on Dopant Transitions in 6H-Silicon Carbide

James Kolodzey, Guang-Chi Xuan, Peng-Cheng Lv, Nathan Sustersic, and Xin Ma

—Electrically pumped high power terahertz (THz) emitters that operated above room temperature in a pulse mode were fabricated from nitrogen-doped n-type 6H-SiC. The emission spectra had peaks centered on 5THz and 12THz (20meV and 50meV) that were attributed to radiative transitions of excitons bound to nitrogen donor impurities. Due to the relatively deep binding energies of the nitrogen donors, above 100meV, and the high thermal conductivity of the SiC substrates, the THz output power and operating temperature were significantly higher than previous dopant based emitters. With peak applied currents of a few amperes, and a top surface area of 1mm2, the device emitted up to 0.5mW at liquid nitrogen temperature (77K), and tens of microwatts up to 333K. This result is the highest temperature of THz emission reported from impuritybased emitters.

Index Terms—Intracenter radiative transitions, semiconductor devices, silicon carbide, terahertz emitting devices, wide band gap semiconductors.

1. Introduction

In recent years, new applications in terahertz (THz) communication, imaging, medicine, remote sensing, and spectroscopy have initiated huge research interests in THz devices[1]-[3]. There are few sources, however, which emit in the THz frequency range (1 THz-10 THz) and are suitable for compact, portable, and low cost applications. Powerful terahertz sources were desired, especially with higher operating temperatures that could, in principle, operate without cryogenics. There has been considerable interest in THz emitters based on doped semiconductors, such as impurity-doped Si[4]-[7], impurity-doped Ge[8]-[10], and impurity-doped SiC[11]-[13]. These impurity-based devices emit THz frequencies by a mechanism of intracenter radiative transitions in hydrogenic dopant states[14]. The operating temperatures of such devices were limited, however, by the relatively low ionization energies of the dopants compared with the thermal energykBT[14]. For example, boron in Si has an ionization energy of 45 meV, and at temperatures above 130 K, most of the holes would be excited into the valence band and unavailable for radiative transitions. For Si-based impurity emitters with boron dopants, the highest reported operating temperature was 118 K[7]. Alternative elements with deeper binding energies, however, may be able to achieve higher operating temperatures. For example, Lvet al.[12]reported THz emission at 150 K from a nitrogen-doped 4H-SiC device. The nitrogen donor in 4H-SiC has ionization energies of 52.1 meV for theh-site (hexagonal) and 91.8 meV for thek-site (cubic)[15]. Nitrogen in 6H-SiC, as described here, has deeper ionization energies of 81 meV for theh-site, 137.6 meV for thek1-site, and 142.4 meV for thek2-site[16]. In addition, the high thermal conductivity of silicon carbide enables it to sustain high drive currents with less heating, which would depopulate the excited states, as explained elsewhere[14]. In this report, we describe THz emission from nitrogen doped 6H-SiC devices that operate at much higher temperature than previous SiC THz emitters.

2. Experiment

The THz devices were fabricated from a 625 μm thick double-sided polished n-type 6H-SiC wafer of 0.1 Ohm resistivity (at room temperature), which was predominantly doped with 1018cm-3Nitrogen donors. Compensating dopants included 1016cm-3Boron and 1015cm-3Aluminum, as indicated in the Secondary Ion Mass Spectrometry (SIMS) profile shown in Fig. 1 (a), measured by the Evans Analytical Group. For device fabrication, wafer pieces were RCA cleaned, followed by contact photolithography to define a mesh-shaped metal contact pattern in the photoresist with 80 μm lines and spaces, for a 50% shading factor, as shown in Fig. 1 (b). The metal contacts weredeposited by the e-beam evaporation of Ti/Au (10 nm/300 nm), on both the front and back of the samples. After photoresist lift-off, the samples were cut into 1×1 mm2and 2×2 mm2dices, and then mounted onto a copper block heat sink using low temperature epoxy with high electrical and thermal conductivity. Fig. 1 (b) shows a close-up photo of two devices fabricated on a die, with one device wire-bonded to a soldering pad. The copper heat sink was attached to the cold finger of a liquid nitrogen-cooled cryostat (forT≥77 K) with a high density polyethylene (HDPE) optical window, transparent to wavelengths longer than about 17 μm. The device temperatures reported here were of the heat sink measured with a platinum resistor. During the current pulses, the temperature of the active portion of the actual device could be as much as 50 K higher than the heat sink, according to our calculations and to reports by Kumaret al.[17]. The emission spectra were measured using a Thermo Nicolet Nexus 870 Fourier transform infra-red (FTIR) spectrometer operated in the step scan mode, and equipped with a liquid helium-cooled silicon bolometer detector made by IRLabs. An Agilent electrical pulse generator was used to drive the samples with sub-microsecond pulse trains. The applied current on the device was measured using an inductive current probe and an oscilloscope. An EG&G Princeton Applied Research Model 5209 lock-in amplifier was used to synchronously detect the signals from the bolometer.

Fig. 1. Wafer dopant concentrations and device configuration: (a) SIMS depth profile of dopant concentrations for the 6H-SiC wafer used in this paper and (b) photomicrograph of a typical 1×2 mm2SiC die with two 1×1 mm2devices fabricated on it. One of the devices was wire-bonded to a gold contact pad (at bottom).

3. Results and Discussion

Intense electroluminescence (EL) was observed over the spectral range from 2 THz to 13 THz, which increased in intensity with a peak pumping current as shown in Fig. 2 (a). Two families of spectral peaks were centered around 4.7 THz and 12 THz, with internal fine structures that were resolved at higher currents. Fig. 2 (b) shows the emission spectra over a temperature range from 77 K to 333 K, at the same pumping current of 3 A. As the heat sink temperature increased from 90 K to 150 K, the two emission features around 4.7 THz and 12 THz broadened and merged, and additional peaks appeared at intermediate frequencies. The devices continued to emit in the pulse mode at temperatures up to 333 K (60°C), which is the highest emission temperature reported for any dopant based terahertz emitter of which we are aware.

Fig. 2. Electroluminescent spectra under different pumping conditions: (a) emitted spectra from N-doped 6H-SiC device with indicated current pulse heights at 77 K, and (b) emission from the THz device at different temperatures with fixed peak pumping current of 3 A. The vertical scale has been varied and offset to help identify the emission features.

Terahertz emission from dopant based devices is typically associated with hydrogenic transitions, for example from 2p→1sstates. The THz emission energies from the 6H-SiC devices were observed to be around 20 meV and 46 meV, however, and were unlikely to be from transitions between nitrogen states because the minimum energy separation betweenp-states ands-states is 59 meV from theh-site 2p0→1stransition[16]. This observation suggests that the THz emission might not originate from hydrogenic radiative transitions among impurity states. On the other hand, the THz emission peaks matched very well with the spectral peaks obtained from low-temperature photoluminescence excitation (PLE) spectroscopy[18]on 6H-SiC materials with similar nitrogen doping, which wereassociated with nitrogen-bound excitons. Thus it was reasonable to attribute the observed THz emission to intra-excitonic transitions.

Fig. 3 shows the emitted power versus temperature from 77 K to 333 K for applied current pulses with a 3 A peak value. The emitted power dropped steeply from 526 μW at 77 K to 249 μW at 90 K, and then more gradually decreased to 49 μW at 333 K, implying that two thermal activation energies were involved. The inset to Fig. 3 shows the calculated percentage of neutral (freeze-out) nitrogen donors versus temperature. Below ～90 K, most of the nitrogen donors were occupied by electrons. The reason for the reduction in emitted power above 77 K is uncertain, but is attributed to donor ionization. Due to thermal ionization as the temperature increased, fewer excitons were bound to the donor states, which reduced the output power of the device, and the emitted power tended to follow the carrier occupation as shown in the inset to Fig. 3.

Fig. 3. Emitted power versus temperature at uniform 3 A peak pumping current. The inset shows the calculated neutral (freeze-out) nitrogen donor percentage versus temperature. Above 100 K, the trend of decease in emitted power versus temperature followed the dependence of donor occupation with temperature.

Although the applied current pulse duty cycle was intentionally kept low (＜0.1%) to reduce local heating, the broad spectra above 150 K may contain contributions from blackbody radiation. To determine if the spectra were from electrically pumped dopant based transitions, a series of current and temperature dependent measurements was performed. Fig. 3 shows that the THz emission intensity was observed to decrease with increasing temperatures with fixed 3 A current pulses, whereas the blackbody emission should increase proportionally toT4(Tis the blackbody temperature). In addition, if the emitted power was due to thermal heating, it should increase roughly withI2(Iis the pumping current). As shown in Fig. 4 of the emitted power versus pumping current, however, at 210 K the power exhibited weakI2components (curve bending slightly upwards), which meant that there may be thermal blackbody components. Finally, the total radiated power in the range from 1 THz to 15 THz for a blackbody of the size of our device at 333 K was calculated to be less than 8 μW, whereas our device emitted about 50 μW at 333 K, confirming that the emission peaks were associated with the applied current pulses and not to steady state residual heat from the device. Thus the emission spectra presented in this paper were attributed primarily to dopant-based transitions. It may be however, that the peak current modulated the device temperature, which contributed slightly to blackbody emission.

Fig. 4. Emitted power integrated over the THz spectral range from 2 THz to 15 THz versus peak applied pumping current at the indicated device heat sink temperatures of 77 K, 90 K, and 210 K.

The spectral response of the FTIR was calibrated with an external variable-temperature, recessed-cone blackbody radiator. Fig. 4 shows the emitted power versus pumping current at three different temperatures. At 77 K, the emitted power first increased with the current but then decreased when the current was above 4 A. This trend with the current was similar to that observed for the THz emission from 4H-SiC[12]. It may be that the combined higher applied field, current, and heating increased the probability for the bound carriers to be freed from the impurities, thus reducing the bound exciton population, and hence reducing the emission intensity.

4. Conclusion

In summary, a powerful THz emitter that operated above room temperature in the pulsed mode was fabricated from nitrogen doped n-type 6H-SiC. The emission spectra had peaks in the range from 2 THz to 13 THz, which were attributed to radiative bound exciton transitions associated with the nitrogen donor impurities. A 1×1×0.6 mm3size device was capable of emitting more than 500 μW of power at 77 K, and 49 μW at 333 K. This result is the highest recorded power and highest operating temperature among impurity-based THz emitters. The power density at 77 K corresponded to 50 mW·cm-2, which is suitable for a wide range of THz device applications.

Acknowledgment

Special thanks to Alex Andrianov, James Choyke, R. Chris Clark, Matthew Coppinger, Gregory DeSalvo, Joseph Gigante, Keith Goossen, Tanya Paskova, Dimitris Pavlidis, Adrian Powell, Steven Saddow, John Zavada, and John Zolper for useful discussions and important contributions.

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James Kolodzeywas born in Philadelphia, Pennsylvania in the USA in 1950. He received the Ph.D. degree in electrical engineering from Princeton University in Princeton, New Jersey in 1986 for research on silicon germanium alloys.

From 1986 to 1990, he was an assistant professor of electrical engineering at the University of Illinois at Urbana-Champaign where he established laboratories for the cryogenic studies of high frequency devices and the fabrication of devices by molecular beam epitaxy. Since 1991, he has been in the Department of Electrical and Computer Engineering at the University of Delaware, in Newark, Delaware in the USA, where he is currently the Charles Black Evans Professor of Electrical Engineering. He has over 130 publications in refereed journals and over 100 conference publications. His research interests include: the fabrication and characterization of high frequency optical and electronic devices; the properties of terahertz sources and detectors; silicon-germanium-tin-carbon materials for infrared optoelectronics; quantum dot devices; spintronic devices, alternative gate dielectrics for CMOS; and interfaces between biological materials and semiconductors.

Prof. Kolodzey is a Senior Member of the Institute of Electrical and Electronic Engineers, has several patents, served as chair of conferences, and received awards for research contributions.

Guang-Chi Xuanwas born in Guangdong, China in 1977. He received the B.S. degree from Tsinghua University, Beijing in 2000 and the Ph.D. degree from the University of Delaware, Newark, DE in 2007, both in electrical engineering. He is currently with the Silicon Systems Group at Applied Materials Inc. in Santa Clara, CA working on Etch products.

Peng-Cheng Lvwas born in Hubei, China in 1977. He received the B.S. degree in materials science&engineering from South China University of Technology in 1999, the M.S. degree in advanced materials for micro-&nano-systems from the Singapore-M.I.T. Alliance in 2001, and the Ph.D. degree in electrical engineering from the University of Delaware, USA in 2005. His research interests include novel optoelectronic materials, devices, and sensors.

Nathan Sustersicwas born in Cleveland, Ohio, USA in 1981. He received the B.E.E (2003), M.S. (2005), and Ph.D. (2009) degrees in electrical engineering from the University of Delaware, Newark. He is currently a Process Technology Development Engineer at Intel Corporation, in Hillsboro, Oregon, focusing on the research and development of CVD and ALD thin films for future generation microprocessor fabrication technology. His other areas of expertise include Group IV Molecular Beam Epitaxy for the fabrication of optoelectronic devices as well as low bandgap solar cell structures for multijunction solar cells.

Xin Mawas born in Hebei, China. She received the Ph.D. degree in electrical and computer engineering from the University of Delaware, Newark, DE in 2014 for research of solution processed organic and nano-material based novel light emitting devices. She is currently working as a postdoc researcher at University of Delaware, focusing on germanium-tin optoelectronic device fabrication and characterization. Her research interests include novel semiconductor materials, quantum dot devices, organic electronics, and material characterization.

Dr. Ma has served as a committee member for Women in Engineering at University of Delaware, and received several awards including Graduate Dissertation Fellowship, Graduate Faculty Award, and etc.

Manuscript received September 15 2014; revised September 16, 2014. This work was supported by the NSF Award No. DMR-0601920, and by ONR Contract No. N0001-4-00-1-0834.

J. Kolodzey is with Electrical & Computer Engineering Department, University of Delaware, Newark, DE 19716, USA (Corresponding author e-mail:kolodzey@udel.edu).

G.-C. Xuan is with Applied Materials Inc., Santa Clara, CA 95054, USA (e-mail: gc.xuan@gmail.com).

P.-C. Lv is with AlphaSense, Inc., Wilmington, DE 19809, USA (e-mail: pengcheng@alphasense.net).

N. Sustersic is with Intel Corporation, Hillsboro, OR 97214, USA (e-mail: natrons@gmail.com).

X. Ma is with Electrical & Computer Engineering Department, University of Delaware, Newark, DE 19716, USA (e-mail: xinma@udel.edu).

Digital Object Identifier: 10.3969/j.issn.1674-862X.2014.03.002