Letter pubs.acs.org/NanoLett
On-Chip Optical Interconnects Made with Gallium Nitride Nanowires Matt D. Brubaker,*,†,‡ Paul T. Blanchard,† John B. Schlager,† Aric W. Sanders,† Alexana Roshko,† Shannon M. Duff,† Jason M. Gray,‡ Victor M. Bright,‡ Norman A. Sanford,† and Kris A. Bertness† †
Physical Measurement Laboratory, National Institute of Standards and Technology, Boulder, Colorado 80305, United States Department of Mechanical Engineering, University of Colorado, Boulder, Colorado 80309, United States
‡
ABSTRACT: In this Letter we report on the fabrication, device characteristics, and optical coupling of a two-nanowire device comprising GaN nanowires with light-emitting and photoconductive capabilities. Axial p−n junction GaN nanowires were grown by molecular beam epitaxy, transferred to a non-native substrate, and selectively contacted to form discrete optical source or detector nanowire components. The optical coupling demonstrated for this device may provide new opportunities for integration of optical interconnects between on-chip electrical subsystems. KEYWORDS: Gallium nitride, light-emitting diodes, nanowires, photoconductivity, optical interconnects
G
allium nitride nanowires (GaN NWs) present novel opportunities for integrating optically active nanoscale components with nonphotonic devices. For example, GaN NW devices can be grown directly on silicon substrates1−8 or they can be grown on epitaxially convenient substrates and transferred to separate device substrates as single NWs. While the former approach requires consideration of the NW growth temperature on the thermal budget of the underlying devices, the latter approach bypasses process integration issues and can be implemented for devices with stringent thermal budget constraints. The transfer-based approach has been utilized previously to fabricate single light-emitting diode nanowire (LED NW) devices9−12 and photoconductive nanowire (PC NW) devices13−16 on non-native substrates. Nanowire devices have also been proposed for intrachip optical interconnects,16−19 which can provide key advantages over electrical interconnects including increased bandwidth, immunity from electromagnetic interference, and voltage isolation.20 The primary components of an intrachip optical interconnect are an optical source and a detector that can be integrated on-chip, which can in principle be realized by use of the GaN LED and PC NW devices mentioned above. However, a GaN NW device comprising both LED and PC NWs has yet to be demonstrated and is the objective of this work. Optical coupling in an LED/PC NW pair occurs when a forward-biased LED NW illuminates a PC NW, inducing a photocurrent, as shown in Figure 1a. Unlike conventional planar p−n junction photodetectors, GaN NW photoconductivity results when the surface depletion layer shrinks as photogenerated minority carriers accumulate at the NW surface.13,15 For an axial p−n junction GaN NW that is sufficiently long to accommodate multiple contact locations, either LED or PC functionality can be selected, depending on the contact layout with respect to the junction. Contacts © 2013 American Chemical Society
Figure 1. Coupled LED/PC NW images showing (a) schematic illustration of device concept and operation, (b) layout and testing conventions for fabricated device, and (c) SEM images of LED and PC NWs.
positioned on opposite sides of the p−n junction produce an LED NW, while a PC NW is obtained for contacts positioned on the n-side only. This affords considerable simplification to the device fabrication process by allowing NWs from a single growth run to serve as both source and detector. In this study, we employ the transfer-based approach described above to Received: September 20, 2012 Revised: December 17, 2012 Published: January 16, 2013 374
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fabricate an LED/PC NW pair by use of axial p−n junction NWs with a lightly doped n-region for increased photoconductive sensitivity. The performance of the individual LED and PC NW components were first validated for this hybrid nanowire structure, prior to measuring the overall response of the coupled system. Axial p−n junction GaN NWs were grown by plasma-assisted molecular beam epitaxy, by use of procedures and growth conditions described elsewhere.21 The NWs were grown n-side first, with the p−n junction located approximately at the middle of the NW, and with an overall length of approximately 12 μm. It has been previously shown that band-edge electroluminescence is obtained for axial p−n junction NWs when an AlGaN electron blocking layer (EBL) is incorporated on the p-side of the junction.9 A nominally identical EBL was employed in this study to produce LED NWs with an electroluminescence spectrum near the absorption edge of PC NWs. The NWs were released from the growth substrate into suspension via ultrasonic agitation in isopropanol, and dispersed onto an oxidized silicon substrate. An array of two-terminal LED NW electrodes and PC NW contact pads was created by use of optical photolithography, e-beam evaporation, and lift-off processing. The devices were then prescreened for proper LED NW contact registration and availability of a second proximal PC NW. Local electrodes to the n-region of the PC NW, identifiable as the smaller-diameter section of the NW, were then created by use of e-beam lithography and a second metallization to produce the device shown in Figure 1b and c. Both contact metallizations consisted of a 20 nm Ti/200 nm Al stack, with no postcontact anneal. The electrical and optical characteristics of the LED NW are shown in Figure 2, by use of the testing conventions illustrated in Figure 1b. Rectifying I−V characteristics were obtained with forward-bias conditions corresponding to the expected bias polarity. The origin of the large ideality factor is attributed to a nonideal p-contact and is discussed in greater detail elsewhere.9 The electroluminescence spectra exhibit significant content in
the 365−370 nm range, with intensity increasing monotonically with current injection level. The total optical power is estimated to be approximately 40 nW for 20 uA injection current.9 The electroluminescence is observed to emanate from the junction, approximately 35 μm from the PC NW. Some diffuse scattering from the contact or minor electroluminescence is also observed near the left side of the electrode gap. The PC NW response was characterized with a wavelengthtunable UV light source based on a xenon arc lamp and a monochromator.15 I−V measurements of the initial dark level were less than 10 pA for ±10 V bias, indicating that the PC NW is substantially depleted. As shown in Figure 3a, illumination
Figure 3. PC NW characteristics showing (a) photocurrent as function of optical excitation wavelength (inset shows logarithm scale plot), (b) photoconductive decay measurements, and (c) photocurrent as function of optical excitation intensity.
with above bandgap light induces a photocurrent several decades above the initial dark level. The wavelength dependence of the photocurrent is characteristic of GaN, decreasing abruptly for wavelengths longer than 365 nm. The post-UV illumination dark level was increased significantly from the initial dark level, indicated by the persistent photoconductivity level on the inset of Figure 3a. Because the switching speed and lower detection limit of the PC NW is related in part to the persistent photoconductivity level, its time dependence was studied in more detail. As shown in Figure 3b, asymmetry with respect to the PC NW bias polarity was observed in the steady-state photocurrent and the photoconductive decay rate. The photoconductive gain is higher for the +10 V bias condition; however, a period of several minutes is required for decay back to the persistent photoconductivity level, which was then stable for a duration in excess of 12 h. In contrast, the −10 V bias condition exhibited a relatively fast decay rate, but at the expense of lower photoconductive gain. Interestingly, a brief bias duration at −10 V was able to restore the initial dark level of the PC NW device. It is possible that the bias-dependent asymmetry in the
Figure 2. LED NW characteristics showing (a) I−V characteristics, (b) electroluminescence spectra, and (c) image of electroluminescence (EL) emission. 375
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Additionally, the right LED NW contact pad partially occludes the line of sight between the LED and PC NWs, with the metallization thickness obscuring approximately half of the full nanowire diameter. Further attenuation may result from transmission losses through off-normal interfaces along the line of sight between the NWs. The switching behavior of the coupled LED/PC NW device was also observed by applying current pulses to the LED NW. As shown in Figure 5a, the PC NW photocurrent was measured
photoconductive decay rate results from trap states or from the proximity of the p−n junction, although further study would be required to clarify this phenomenon. The PC NW photoresponse was established by correlating the steady-state photocurrent to the illumination intensity for several wavelengths. The more sensitive +10 V PC NW bias condition was used for these measurements, starting from the persistent photoconductivity level (represented by the bottom axis in Figure 3c). The photoconductive gain, defined as ratio of electrons passing the electrodes to absorbed photons, ranges from 104−106 for a wavelength range similar to that of the LED NW (370 nm peak intensity). This performance is comparable to that of an avalanche photodiode and other nanowire photodetectors,16 although nanowire photoconductors with higher gain have been reported. These measurements demonstrate that the PC NW is sensitive enough to distinguish an on-state photocurrent against the background persistent photocurrent at irradiance levels of approximately 10−7 W/cm2. Minor sensitivity to subgap illumination is also observed, similar to the wavelength scan shown in Figure 3a. The coupled response between the LED NW and the PC NW was characterized by four-terminal measurements, according to the test configuration shown in Figure 1b. Prior to measurement, the PC NW was reset to the original dark current level by a short bias dwell at −10 V, after which VPC was fixed at +10 V. A constant current bias was then applied to the LED NW, and the induced photocurrent was measured as a function of time after activation, as shown in Figure 4. At an
Figure 5. Coupled four-terminal LED/PC NW measurements showing IPC for 5 s ILED pulses at (a) increasing LED NW current injection levels and (b) offset LED/PC NW bias conditions. The PC NW was allowed to return to the persistent photoconductivity level prior to each measurement.
at a fixed PC NW bias of +10 V, while pulsing the LED NW at 5 s on and 10 s off cycles. These waveforms show the initial three pulses starting from the persistent photoconductivity level. The PC NW photocurrent tracks the output state of the LED NW, indicating optical coupling between the NWs. Increasing the on-state current level and optical emission from the LED NW induces a larger photocurrent in the PC NW, similar to the results shown in Figure 4. The magnitude of the induced photocurrent pulses increases with iteration, as the offstate duration is shorter than the period required for full decay to the initial persistent photoconductivity level. To eliminate the possibility of coupling through parasitic leakage pathways, measurements were taken with the LED NW ground pad disconnected. In this scenario, bias pulses were applied to the device, but without optical emission from the LED NW. No pulsed response was observed in the PC NW photocurrent, indicating that the coupling was not due to parasitic leakage pathways. The coupled LED/PC NW switching speed is admittedly slow and results from the combined effects of turn-on and turnoff transients. The turn-on transient in this device is primarily related to the low-intensity illumination incident at the PC
Figure 4. Coupled four-terminal LED/PC NW measurements illustrating time evolution of IPC at constant ILED current biases. The PC NW was reset to its initial dark level by a brief bias dwell at −10 V prior to each measurement.
LED current bias of 5 μA the induced photocurrent exceeds the noise floor approximately 50 s after activating the LED and then increases slowly with measurement duration. With increasing LED current bias the induced photocurrent emerges from the noise floor in a shorter period and then increases at a faster rate, as expected for higher irradiance by the LED NW. At an LED current injection level of 20 μA the induced photocurrent stabilizes at a steady-state value of ∼3 nA. This corresponds to an optical intensity at the PC NW in the range of mid 10−7 W/cm2, as correlated to the experimental data in Figure 3c. This is well below the estimated intensity supplied by the LED NW (mid 10−4 W/cm2), assuming the LED emission pattern is uniform in all directions. More likely, the far-field emission pattern is nonuniform,22 resulting in low-intensity EL regions that potentially coincide with the PC NW location. 376
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ACKNOWLEDGMENTS Financial support for this research was provided by the National Institute of Standards and Technology (NIST). Partial support was provided by the DARPA Center on Nanoscale Science and Technology for Integrated Micro/ Nano-Electromechanical Transducers (iMINT) and the DARPA N/MEMS S&T Fundamentals program under grant no. N66001-10-1-4007 issued by the Space and Naval Warfare Systems Center Pacific (SPAWAR).
NW. Several avenues exist for improving the optical coupling including decreased distance between LED and PC NWs, direct coupling with optical waveguides, and increasing the spectral absorption range of the PC NW. Integration with waveguides could be accomplished in future refinements by using a combination of dielectrophoresis and photoresist trench patterning.23 Alternatively, a single axial p−n junction NW could be sectioned using a focused ion beam technique24 to produce two closely spaced and end-aligned LED and PC NWs. An InGaN section could also be incorporated into the PC NW region such that a greater fraction of the LED NW spectral output is absorbed, although this would pose a greater growth challenge. The turn-off transient is related to the surface detrapping time, and in general reflects the inherent trade-off between sensitivity and switching speed in nanowire photoconductors.13,16 While the high sensitivity of the nanowire photoconductor is enabling for this demonstration-level device, it may be possible to utilize a faster p−n photodiode structure to improve the switching speed in devices with stronger optical coupling. Some applications may be envisioned for the coupled LED/ PC NW device with the current performance, particularly where the benefits of galvanic isolation would outweigh the drawbacks of slow switching speed. For instance, the coupled LED/PC NW could function as a solid state relay that uses lowvoltage CMOS signals to activate high-voltage MEMS actuators.25 A key requirement of the coupled LED/PC NW relay is the ability to maintain optical coupling while floating the LED and PC NW bias levels relative to one another, provided that the potential drop on the individual components remains constant. Figure 5b shows pulsed operation under measurement conditions similar to those shown in Figure 5a with 20 μA LED NW injection current. In addition to the standard bias configuration (indicated as VLED = +31 V), an additional measurement is shown with the LED NW terminals switched and the bias polarity inverted (VLED = −31 V). This maintains the forward-bias condition of the LED NW, while offsetting the relative potential between the LED and PC NWs by 31 V. As expected, the magnitude and direction of the measured photocurrent pulses are invariant to the bias offset, due to the inherent isolation provided by the optical coupling. In conclusion, we have demonstrated a coupled LED/PC NW device in which LED NW electroluminescence is used to induce a PC NW photocurrent. This device makes use of axial p−n junction GaN NWs, with selectable LED or PC functionality depending on the contact registration. The NWs incorporate a lightly doped n-region for PC NW sensitivity and an electron blocking layer for LED NW electroluminescence efficiency and have been shown capable of providing source optical power of ∼40 nW and detector sensitivity down to ∼10−7 W/cm2. This type of device could potentially be integrated with nonphotonic materials to provide on-chip optical interconnects or isolation.
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. Notes
The authors declare no competing financial interest. 377
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