We observe an enhancement of optical absorption and photocurrent from semi-insulating gallium arsenide (SI-GaAs) irradiated by femtosecond laser pulses. The SI-GaAs wafer is treated by a regeneratively amplified Ti: Sapphire laser of 120 fs laser pulse at 800 nm wavelength. The laser ablation induced 0.74 μm periodic ripples, and its optical absorption-edge is shifted to a longer wavelength. Meanwhile, the steady photocurrent of irradiated SI-GaAs is found to enhance 50%. The electrical properties of samples are calibrated by van der Pauw method. It is found that femtosecond laser ablation causes a microscale anti-reflection coating surface which enhances the absorption and photoconductivity.
© 2014 Optical Society of America
The performance of gallium arsenide (GaAs) devices is subject to the optical and electrical parameters of material. For instance, a shorter carrier lifetime as well as a smaller thickness of GaAs results in a broader response bandwidth of THz devices [1,2]. The highest light-photocurrent (L-I) conversion efficiency of GaAs-based photovoltaic devices attributes to the high optical absorbance as well as the excellent photoconductivity of compounds [3,4]. The infrared light-emitting diodes are mostly made of GaAs due to its direct bandgap . As a consequence, a great deal of work ranging from ion implantation , nanoparticles bury [7,8], to chemical passivation , has being engaged in upgrading the function of above devices towards a cutting-edge level. However, the physical doping methods are normally high-cost as well as low productivity. The chemical treatment needs a toxic or corrosive environment and the homogeneity of treated areas is very unfavorable. There is continuous interest on new approaches to achieve a rapid reproducibility with a precise control of treated area. Recently, the femtosecond laser pulse ablation has been successfully used to change the surface morphology in the precision range from microscale even up to nanoscale . This process is found to broaden the wavelength range of optical absorption in silicon  or enhance the photocurrent of 6H-silicon carbide . As a result, there is much attention on the relation between laser ablation parameters and induced periodic patterns . An investigation on surface periodic structuring caused by femtosecond laser pulses at transparent wavelengths to GaAs is reported .
In this work, we report a photocurrent enhancement in semi-insulating gallium arsenide (SI-GaAs) via femtosecond laser pulses induced surface periodic structures under resonant irradiation condition. The SI-GaAs is selected as the material for laser processing because of its Fermi level to be pinned to near the center of the bandgap . Herein, it has very high resistivity close to an intrinsic GaAs. The ablation process is conducted by a single beam femtosecond amplified system setup. The optical properties of samples are calibrated via a visible-near infrared (Vis-NIR) optical spectrometer. The electrical properties of samples are investigated by homemade I-V measurement platform as well as van der Pauw measurement. Finally, the origin of photocurrent enhancement in the treated samples is discussed.
A 0.5 mm thick SI-GaAs wafer (ATX Inc.) with the diameter of 4 inches is firstly sliced mechanically into small sheets with the size of 10 mm × 10 mm. Then, each sheet is purified in deionized water under supersonic environment for 60 seconds before laser ablation. A commercial regeneratively amplified Ti: Sapphire laser (Spitfire, Spectra-Physics) is used for laser ablation, which delivers a 120 fs pulse train at a repetition rate of 1 kHz with 800 nm central wavelength. The SI-GaAs slice is mounted on a three-dimensional translation stage, which was controlled by a computer. The laser beam was focused onto the samples by a 5 × microscope objective (NA 0.15), which is attenuated from 700 mW to 5 mW by a neutral density (ND) filter. The sample is translated perpendicularly to the laser beam. The laser beam polarization is perpendicular to the direction of scanning track and the scanning speed is 800 μm/s. The single pulse energy irradiated on the surface of SI-GaAs is 5 μJ with a shot-to-shot fluctuation less than 1% rms. The ablation area is 8 mm × 8 mm. A schematic diagram of the laser induced transient grating is shown in Fig. 1.
The optical transmittance as well as reflectance in the range from visible to near-infrared (NIR) wavelength was inspected by a transmission spectrometer (NKD-8000, Aquila Instrument). The incidence angle is 45°. One silicon photodiode is used to measure the transmitted beam while the other one is fixed along the route of reflected beam to monitor the surface reflectance of samples. The photoconductivity of samples is measured using a domicile photocurrent-voltage measurement system, where a SUN 2000 solar simulator (Wavelab Inc.) emitting AM 1.0 solar spectrum is used as a light source. A 2 inch square exposure area is sufficient to cover the whole SI-GaAs sample. The photocurrent as a function of bias voltage is recorded by a Keithley 2400 electrometer, of which the bias range is from −10 V to + 10 V. The carrier transport property is calibrated by a Hall measurement system (Lakeshore 7500). The SI-GaAs sample is located in the center of the prober card. 4 tungsten needle probes contact the sample at the 4 right angle corners of SI-GaAs slice for van der Pauw measurement. The magnetic field is from −5000 G to 5000 G at a step rate of 500 G.
3. Results and discussion
The surface morphology of the laser processed SI-GaAs sample is shown in Fig. 2. The artificial grooves along the laser track can be distinctly found in Fig. 2(a). The average intervals between the centers of grooves are about 20 μm, which is identical to the step of translation stage. The mean depth of the grooves is about 2.35 μm, as being calibrated by a profilometer (Bruke Stylus). There is a distinct granulation of SI-GaAs appearing at the ridge of grooves, of which the grain size is about 3 μm. Meanwhile, hundreds of periodic ripples are found at the groove face. Figure 2(b) shows the zoom-in image of the ripples, which appear to be a symmetric distribution along the laser tracks. The ripple orientation is perpendicular to the groove direction as well as the laser beam polarization. The geometric character of each ripple is about length of 5.2 μm and width of 0.5 μm. The period of ripple is about 0.74 μm, close to the central wavelength of irradiation laser. The physical origin responsible for the periodic structure formation by femtosecond-laser ablation is able to be identified from the features of ripples . The 0.8 μm incident wavelengths cause a giant number of photo-carriers at the surface area of SI-GaAs, where the real part of the dielectric function changes sign across the interface transiently, which induces formation of surface plasmon (SP). As long as the surface roughness of material is much smaller than the incident optical wavelength, the incident field will coherently couple with the collective oscillations of free electrons to excite the surface plasmon polarity (SPPs). The SPPs have periodic change in the surface charge and resulting in a local field amplitude to periodically initiate the microscale ablation of the surface . The high surface quality of as-received SI-GaAs allows the formation of SPPs. Therefore, one can find that the formation of the ripples is attributed to the interference of laser induced SPPs.
The optical transmission spectra of the treated SI-GaAs and reference samples are presented in Fig. 3(a). Clearly, there is a sudden jump existing at 0.88 μm in the optical transmission spectrum of the reference sample, as corresponds to the bandgap of SI-GaAs (Eg = 1.4 eV) . To the treated sample, however, such a jump seems to be very weak. The inset of Fig. 3(a) shows the transmission below Eg is about 1%. The entire transmittance of treated SI-GaAs decreases at a factor of 35 to the SI-GaAs reference below Eg. Therefore, the surface ripples result distinctly in an optical opaque below Eg of SI-GaAs. The reflection spectra of samples are presented in Fig. 3(b). The SI-GaAs shows a sudden jump behavior in the reflection spectrum at the same point as in transmission spectrum. To the treated sample, however, the slope of entire reflection as a function of wavelength becomes very smooth. It can be found that the reflection ratio decreases at a factor of 10 in the entire spectrum. According to the optics principle, the strong attenuation of transmittance and reflectance implies a dramatically increase of optical absorption in measurement, which obeys the regulation as below :Fig. 4.
Above the intrinsic Eg of SI-GaAs, the laser treatment increases the internal absorption of SI-GaAs from 55% to 99%, while from 10% even high up to 98% below the intrinsic Eg. Comparing the absorption edge of both samples, one can find that the treated SI-GaAs shows an apparent shift of the absorption edge towards a longer wavelength, which is beyond the detectable range of our spectrometer. As a direct bandgap material, the absorption edge refers to the optical bandgap Eg of SI-GaAs. At this point, one can summarize that the laser treatment makes the optical bandgap of SI-GaAs narrow. The origin of absorption enhancement can be revealed from the SEM image. The particles at the groove ridge as well as the ripples enlarge the effective area for optical absorption, which enhances the efficiency of photon trapping. Besides, these microstructures increase the surface roughness inducing a diffusion reflection, which is responsible for the energy loss of specular reflection.
Finally, the steady state photocurrent of as received and treated SI-GaAs are compared. The light source emitting AM 1.0 solar spectrum so as to cover the optical wavelengths illustrated in Figs. 3 and 4. A couple of 50 nm thick electrodes with the gap of 50 μm are evaporated onto the center area of both samples. The electrodes are composed of 5 nm titanium and 45 nm gold in order to achieve ohmic contacts. The images of ohmic contact are shown in the insets of Fig. 5. The photocurrent-voltage (I-V) relation of both samples is revealed in Fig. 5. Both samples show linear dependence of I-V, as attribute to the good ohmic contact. In comparison, the photocurrent of the treated SI-GaAs sample is enhanced about 50% at a bias voltage of ± 10 V. In agreement with the Figs. 1 and 3, the laser ablation causes more defect center and the absorption edge is shifted beyond 1.1 μm. Therefore, the solar light can cause excess photocarriers Δn below the intrinsic Eg of SI-GaAs, as is described in the following equation :17]:Table 1.
It is found that the laser ablation increases σ at 3 orders of magnitude and n at 5 orders of magnitude. Meanwhile, the μ decreases around 2 orders of magnitude. The detect sites tends to increase the recombination process, however, a higher generation rate of electrons per unit volume tends to compensate the annihilation to produce a larger photocurrent. In combination of the optical absorption enhancement and bandgap shift, the total conversion efficiency of light to photocurrent is increased. However, we address that an accurate expression of the relation between the structural period of surface microstructure, the distance of absorption-edge shift, and the amplitude of photocurrent enhancement needs further investigation.
In summary, femtosecond laser pulse induces a microscale periodic structure on the surface of SI-GaAs. These structures increase the optical absorption in a wide wavelength range even below the intrinsic bandgap. A 50% enhancement of photocurrent is achieved. Resonant irradiation laser ablation by femtosecond pulse laser becomes a promise approach to enhance light-current conversion efficiency of SI-GaAs optoelectronic device with the feasibility of rapid reproducibility as well as precise control of treated area.
The authors acknowledge the financial supported by the National Natural Science Foundation of China (Grants Nos. 61307130 and 11374316), Scientific Research Foundation for the Returned Overseas Chinese Scholars, State Education Ministry, Innovation Program of Shanghai Municipal Education Commission (Grant No.14YZ077) and of Shanghai Normal University (Grant No. DXL121).
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