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Abstract
InGaAs metamorphic laser power converters (LPCs) have the potential to deliver electrical energy over distances of several kilometers. In this study, metalorganic chemical vapor deposition (MOCVD) was used to grow InGaAs-based LPCs with an absorption wavelength of 1064 nm. At step thicknesses of 2800 nm, overshoot thicknesses of 6000 nm, reverse component and thicknesses of 2.4% and 700 nm, respectively, a surface roughness of 6.0 nm and InGaAs (24%) lattice relaxation of 93.7% of the InGaAs metamorphic buffer were obtained. The I-V characteristics of LPCs with 10 × 10 mm2 apertures were investigated as a function of laser power and temperature. The maximum conversion efficiency of 44.1% and 550 hours of continuous stable operation at 4 W were demonstrated. Under 1064 nm laser illumination of 4 W, the temperature coefficients for the conversion efficiency and open-circuit voltage were -0.1%abs/°C and -1.6 mV/°C, respectively, and the LPC output power fluctuation was less than 0.5% during 216 hours of continuous temperature change from 20 to 100°C.
© 2022 Optica Publishing Group under the terms of the Optica Open Access Publishing Agreement
1. Introduction
Attributed to their inherent immunity to electromagnetic interference and electric sparks [1], laser wireless power transmission (LWPT) systems can be used to supply energy for remote sensors and unmanned aerial vehicles (UAVs) and power beaming in free space [2,3]. Laser power converts (LPCs) are critical components of the LWPT system and have recently attracted increasing attention [4–6]. LPCs can directly convert laser light into electrical energy, and the conversion efficiency with monochromatic lasers is much higher than that with a broad solar spectrum [7,8].
Researchers have conducted numerous studies on LPCs of various wavelengths and materials, such as InGaP/GaAs [9], GaAs/GaAs [10], InGaAs/InP [11,12], and InGaAs/GaAs [13,14]. Because of the better atmospheric transmittance and commercially available high beam quality 1064 nm lasers, the wavelength of 1064 nm is considered one of the candidates for long-distance wireless laser power transmission. The majority of 1064 nm LPCs have been manufactured using InGaAsP [15–17], InAlGaAs lattice matched to InP substrates [18], as well as metamorphic InGaAs on GaAs substrates [19]. An InGaAs/GaAs laser power converter with a monochromatic efficiency of 45.3% at 1064 nm illumination power of 0.7 W (cell area: 0.0784 cm2) has been reported [4].
The InGaAs/GaAs laser power converter has great potential in wireless energy transmission systems. Compared to the 1064 nm LPCs of InP/InGaAsP and InP/InAlGaAs [4], the GaAs/InGaAs LPCs are mechanically stable, less expensive, and commercially available in diameters up to 8 inches, allowing for high power output from a single LPC.
In this paper, step + overshoot + reverse InGaAs metamorphic buffer layer was employed in 1064 nm laser power converts (10 × 10 mm2). An efficiency of 44.1%, and continuous stable operation at 4 W over 550 hours were demonstrated, and the power and temperature dependencies of the LPC parameters were investigated.
2. Materials and methods
Figure 1(a) shows the InGaAs metamorphic LPC structure, which includes the InGaAs contact layer, a Si-doped InGaAs emitter, and the Zn-doped GaAs base sandwiched between the InGaP window layer and the Inx(AlGa)1-xAs back surface field layer, was designed to operate at 1064 nm. Lambert's law was used to simulate the absorption thickness. Trimethylindium (TMIn), trimethylgallium (TMGa), AsH3, and PH3 were used as the group III and group V sources, respectively. The InGaAs metamorphic LPC wafers were grown using an MOCVD (metal-organic chemical vapor deposition) system at a temperature of 670°C and a pressure of 50 mbar, with Zn (DMZn source) and Si (Si2H6 source) were used as p-type and n-type dopants [20–22], respectively. The LPC wafers were processed into chips with a side length of 1 cm, with SiNx single-layer as anti-reflection coatings (ARCs). The measured surface reflectivity of the reference LPCs around 1064 nm was less than 0.8%. LPCs were placed on copper-plated ceramic heat sinks with silver paste between them to improve thermal conductance. A heating plate precisely controlled the LPCs’ temperature with an accuracy of ±0.5°C. I-V measurements were performed using a 1064 nm laser and a Keithley 2601B source meter.
Fig. 1. (a) Schematic structure of the 1064 nm InGaAs metamorphic LPC. (b) InGaAs metamorphic buffer structure: step (S), step + overshoot (S + O), step + reverse (S + R), step + overshoot + reverse (S + O + R).
The structure of InGaAs metamorphic is designed as shown in Fig. 1(b), where step (S) is the way to increment the In component by arithmetic with the same thickness, step + overshoot (S + O) is the structure of adding a layer of In component overshoot on top of step, and step + reverse (S + R) is the structure of step with the In component and thickness reverse at each step, which combines step with overshoot and reverse, which constitutes the step + overshoot + reverse (S + O + R) structure.
3. Result and discussion
Table 1 and Fig. 2 show the structural design and experimental results of InGaAs metamorphic. The components and thickness of step (S) were set to 3.6% and 2800 nm, respectively, resulting in surface roughness and lattice relaxation of 4.9 and 79.7%, which do not satisfy the lattice-matched growth of materials in the absorption region. In order to increase the lattice relaxation, the step + overshoot (S + O) scheme was proposed. The F0062 and F0063 overshoot thicknesses were set to 4000 nm and 6000 nm, respectively, and the lattice relaxation was 91.7% and 94.2%, respectively. However, the surface roughness has degraded to 5.4 and 8.2 nm, respectively. In order to decrease the surface roughness, the step + reverse (S + R) scheme was proposed. The reverse components of F0052 and F0053 were both set at 2.4%, and the reverse thicknesses were 700 nm and 850 nm, respectively, resulting in surface roughness of 4.1 nm and 4.1 nm, as well as lattice relaxation of 80.4% and 79.6%, respectively, and the F0055 had a 1.2% reverse component and 700 nm reverse thickness, resulting in surface roughness and lattice relaxation of 3.9 nm. Combining step with overshoot and reverse, as shown in F0056 and F0058, yields surface roughness and lattice relaxation of 7.4 nm and 94.5% at a reverse component of 2.4%. At a reverse component of 2.4%, the surface roughness and lattice relaxation were 6.0 nm and 93.7%, respectively.
Fig. 2. (a) AFM measured the surface roughness of the InGaAs metamorphic buffer. (b) X-ray measured the lattice relaxation of the InGaAs metamorphic buffer.
Table 1. Abbreviation of experiment
With a [110] projection direction, the TEM image of the F0058 InGaAs metamorphic buffer structure demonstrates the existence of a large number of dislocations, as shown in Fig. 3(a). The misfit and treading dislocations were identified in the TEM image as indicated by the white arrows. The thick interfaces of metamorphic buffer layers were due to the accumulation of dislocations at each reverse layer, where the strain could be effectively relieved. The metamorphic buffer layers significantly limit the density and propagation of threading dislocations and misfit dislocations. The dislocations were not detected at the top InGaAs buffer layer, where a dislocation density was estimated to be less than 1 × 106 cm-2 based on the TEM resolvability [23]. As shown in Fig. 3(b), a high-resolution TEM (HR-TEM) image was taken from the top InGaAs layer, and the d-spacing of 0.33 nm corresponds to {111} plane. The fast Fourier transform (FFT) image was taken from a square region of the HR-TEM image shown in Fig. 3(c), and the {111} lattice plane (the most compact atomic plane in zinc blended crystal structure) on the FFT image was selected for acquiring an Inverse fast Fourier transform (IFFT) image for evaluating the dislocations in the region shown in Fig. 3(d). A few dislocations were detected, as indicated by a red square, which indicates the top InGaAs layer has a relatively low concentration of dislocations. The F0058 growth scheme was selected in this paper to meet the subsequent demand for lattice matching growth of LPC absorption region materials, and other metamorphic buffer strategies were not selected because higher surface roughness or lower relaxation would lead to degradation of the conversion efficiency of the LPC.
Fig. 3. TEM image of the F0058 InGaAs metamorphic buffer. (a) Cross-sectional TEM bright field image. (b) High-resolution TEM (HR-TEM) image of the top InGaAs layer. (c) A fast Fourier transform (FFT) image was taken from the white square region of the HR-TEM image. (d) Inverse fast Fourier transform (IFFT) image for evaluating the dislocations in the region.
The power and temperature dependencies of the InGaAs metamorphic LPCs were investigated. Figure 4(a) depicts the measured I-V curves for various laser input powers between Pin = 0.5 W and Pin = 3.0 W. The dashed (purple) curve in Fig. 4(a) is an ideal diode model fitted to the 1.0 W data, a good fit is obtained here by using single diodes with the ideality factor of n = 1.11 and the photocurrent ratio corresponding to a quantum efficiency of EQE = 85.5% (where the EQE = R × hc/λ, R = Isc/Pin, when R in A/W, λ (the input wavelength) is in nm, h and c are the Plank and the speed of light constants, respectively) [10]. The fit curve accurately reproduces the data when the overall series resistance is set at 0 Ohm. Furthermore, as shown in Fig. 4(b), we investigated the relationship between external loads and output power of the LPCs. There is a power exponential relationship between the output power and external loads of LPCs, with Pout = 0.41*Rm-0.95 for InGaAs metamorphic LPC.
Fig. 4. (a) Measured I-V curve at different laser illuminate for an InGaAs metamorphic LPC, and the 1.0 W curve is also fitted with an ideal diode model (purple dashed line). (b) Measured output power versus external load value for InGaAs metamorphic LPC, the optical input power varied from 0.5 to 3.0 W for InGaAs metamorphic LPCs, and the optimal external load versus output power meets the power exponential relationship.
Figure 5 depicts a more detailed analysis of the laser input power dependence of the LPC parameters shown in Fig. 4(a). As the input laser power increases from 0.5 to 4.0 W, the conversion efficiency (η) of 44.1% was achieved at 1.0 W, the output power Pm is linear to the input power up to 4.0 W, and has a measured slope efficiency of 39.9%, the fill factor (FF) consistently reduced from 80.6% at 0.5 W to 71.0% at 4 W. The output voltage and current are shown in the bottom panel of Fig. 5. The open-circuit voltage (Voc) reaches a maximum value of 0.8 V at 4 W, while the voltage at the maximum power point (Vm) is 0.7 V at 1.5 W. The short-circuit current (Isc) yields a responsivity of 0.7 A/W, corresponding to an EQE of 84.5%.
Fig. 5. Input power (Pin) dependence of the power conversion efficiency (η), fill factor (FF), output power (Pmax), short-circuit current (Isc), open circuit voltage (Voc), and voltage at maximum power point (Vm).
The reliability and stability of LPCs devices are investigated. As shown in Fig. 6(a), the incident laser power varied continuously, and each irradiation was for 24 hours with a steady output. Continuous stable operation at 4 W has been verified with an output steady at 1.5 W for over 550 hours within 1% fluctuation.
Fig. 6. Stability measurement. (a) The output power was continuously measured under various laser powers. (b) The output power was measured at the laser power of 4 W.
Figure 7(a) depicts the temperature-dependence I-V characteristic of the LPCs. The temperature increases from 10 to 100°C for the input laser power of 4 W. A more detailed analysis of the temperature dependence of the LPCs’ parameters is shown in Fig. 7(b), the temperature coefficients can be assessed from the linear region of the data. It found that the conversion efficiency with a temperature coefficient of -0.1%abs/°C (upper panel), and the temperature coefficient of the open-circuit voltage with -1.6 mV/°C (middle panel). The temperature coefficient of the output power is shown in Fig. 7(b) (bottom panel), the output power decreases with a slope of 5.1 mW/°C.
Fig. 7. (a) Measured I-V curve of the LPCs under different temperatures (10-100°C) at an input laser power of 4 W. (b) The temperature dependence of the LPC parameters.
Fig. 8. Stability measurement. The output power is continuously measured under various temperatures.
With an incident laser power of 4 W, the LPC output power fluctuated less than 0.5% over 216 hours when the temperature was continuously increased from 20°C to 100°C and irradiated for 24 hours at each tempe(rature condition .
4. Conclusion
1064 nm InGaAs metamorphic laser power converts (10 × 10 mm2) are fabricated and characterized. The step + overshoot + reverse (S + O + R) InGaAs metamorphic buffer with surface roughness and lattice relaxation of 6.0 nm and 93.7% were obtained, respectively. An efficiency of 44.1% and continuous stable operation at 4 W for over 550 hours were realized. The efficiency and open-circuit voltage temperature coefficient with -0.1%abs/°C and -1.6 mV/°C, respectively, were obtained under 1064 nm laser illumination of 4 W, and 216 hours of operation under 4 W laser irradiation and continuous temperature change from 10-100°C with no degradation in output power (Fig. 8).
Disclosures
The authors declare that there are no conflicts of interest related to this article.
Data availability
Data underlying the results presented in this paper are not publicly available at this time but may be obtained from the authors upon reasonable request.
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