太赫兹光电导天线的失效机理

Failure mechanism of THz GaAs photoconductive antenna Syed B.Qadri,Dong H.Wu,Benjamin D.Graber,Nadeemullah A.Mahadik,and

Anthony Garzarella

U.S.Naval Research Laboratory,Washington,DC20375,USA

(Received30March2012;accepted18June2012;published online5July2012)

We investigated the failure mechanism of THz GaAs photoconductive antenna using high resolution x-ray diffraction topography.From these studies,it was found that grain boundaries are formed during the high frequency device operation.This results in the segregation of gold at the boundaries causing electromigration of the metal between the gold micro-strips.This disrupts the photocurrents from being produced by femtosecond laser thus preventing terahertz beam generation from the photoconductive antennae leading to device failure.[/10.1063/1.4733476]

Time domain terahertz spectroscopy is currently being widely used for various applications.1–6A photoconductive antenna,which is also known as Auston-switch,is popular choice to produce a wideband terahertz beam for the time domain terahertz spectrometer.The device consists of two gold micro-strips deposited on GaAs(001)substrate and is designed to produce a terahertz beam over broad-band spec-trum with a relatively high conversion efficiency.However, the photoconductive antenna often experiences noticeable degradation and failure,especially when a high voltage bias is applied to it or when it is exposed to a strong laser beam whether the laser beam is a femtosecond laser pulse to gener-ate terahertz beam or from other lasers.

High resolution x-ray topography is a very surface sensi-tive non-destructive tool to probe the real surface structure of the GaAs substrate and any damage that may occur due to bias applied to the deposited gold micro-strips.To under-stand the cause of the degradation and failure of the photo-conductive antenna we have performed x-ray diffraction topography on a pristine and two failed photoconductive antennae.In addition,we have measured the rocking curves and compared the full width at half maximum(FWHM)of GaAs(004)reflection for the pristine sample and the one that resulted in failure.These two analysis techniques pro-vide insight into the cause of the device failure upon operat-ing at high voltages.

All antennae used in this study are of the same type, which are fabricated by depositing two parallel gold micro-strips on0.4mm thick GaAs(001).The gap width of micro-strips is100l m.Wideband terahertz pulses are produced by illuminating femtosecond laser pulses on the gap,between the microstrips,while applying11kHz AC bias voltage up to 120V to the contacts on the microstrips.Terahertz output increases with the femtosecond laser power and the AC bias voltage while the output changes non-monotonically with the frequency of the AC bias.To produce the maximum tera-hertz output with our femtosecond laser(IMRA C-20), which has a power limitation of19mW,we set the AC bias at120V.This maximum bias voltage setting reduces the lifetime of photoconductive antenna over extended operating time.Within a month the antenna exhibited noticeable degra-dation,producing weaker terahertz output.Then eventually the antenna(device1)failed.

The second failed antenna was from another attempt to increase terahertz output,in which a780nm CW diode laser was used in addition to the femtosecond laser(pulse width 120fs,wavelength$77960.3nm),as shown in Fig.1.The power of CW laser beam was varied by rotating the wave-plate WP1over the range from0to50mW.

The devices were then characterized using high resolu-tion x-ray topography.Figure2shows the schematic diagram of a double crystal diffractometer with a(þ,À)arrange-ment.7Asymmetric(115)diffraction peak of a perfect refer-ence GaAs crystal was used to get a broad beam with a very small lateral angular divergence from a point source of CuK a1radiation.Further diffraction by the sample crystal of the same diffraction plane was captured on nuclear emulsion plates with a sub-micron spatial resolution.Optimal sensitiv-ity with minimum vertical divergence in the x-ray topograph was obtained by placing the recordingfilm as close to the sample as possible.The rocking curves were also measured on a high resolution4-circle diffractometer using18kW rotating generator with a Cu target.In this instrument,two channel-cut Ge(110)crystals were used on the incident beam side,with four Ge(220)reflections,we get an incident focused within a few arc-seconds resolution.The FWHM of the incident beam was$20arcseconds.

We performed high resolution x-ray topography on these two failed antennae devices(device1and device2) along with a pristine antenna device.Figure3shows the topograph of the pristine device taken using(115)reflection of the GaAs substrate using the experimental set-up

shown FIG.1.A schematic diagram for an enhanced THz output.WP1and WP2: half waveplate.M:Mirror,PBS:polarization dependent beam splitter.EO detector:electo-optic THz detector.

0003-6951/2012/101(1)/011910/3/$30.00101,011910-1

APPLIED PHYSICS LETTERS101,011910(2012)