Embedded InN Dot-Like Structure within InGaN Layers Using Gradient-Indium Content in Nitride-Based Solar Cell

Lung-Hsing Hsu1, Chien-Chung Lin2*, Ming-Hsuan Tan2, Yun-Ling Yeh3, Da-Wei Lin3, Hau-Vei Han3, and Hao-Chung Kuo3

1Institute of Lighting and Energy Photonics, National Chiao-Tung University, Tainan 711, Taiwan 2Institute of Photonic System, National Chiao-Tung University, Tainan 711, Taiwan

3Department of Photonic & Institute of Electro-Optical Engineering, National Chiao-Tung University, Hsinchu 300, Taiwan

*E-mail: chienchunglin@faculty.nctu.edu.tw

Abstract — The novel design of embedded InN dot-like

structure within InGaN was useful as an absorption layer in photovoltaic (PV) cells. We constructed the simulation model by employing the commercial software APSYS® and integrating the absorption coefficient of thin InN materials fabricated by metal organic vapor deposition (MOCVD). The model of simulating gradient Indium content of InGaN used as transition interface between InN and GaN was investigated. The results exhibit utilizing the effective variation of Indium content and suitable thickness to approach the optimal characteristic of hybrid InN/InGaN structure within solar cells shall be anticipated to enhance the performance of current nitride-based solar cells.

Index Terms — APSYS®, gradient Indium content, InN materials, photovoltaic cells.

I. INTRODUCTION

In recent years, nitride-based alloys, such as AlN, GaN and InN etc, have been anticipated to achieve the higher conversion-efficiency and better characteristics of devices in light-emitting diodes (LEDs) [1]–[2], laser diodes and photovoltaic cells as solar cell. Most of research groups have believed that the congenital properties of nitride-based alloys were suitable to utilize in optoelectronic devices. One of the great features about this nitride-based materials is the direct bandgap energy for the entire alloy range, which varies from 0.7 eV for InN to 3.4 eV for GaN, and thus provides wide range absorption of solar spectrum from ultraviolet to visible and infrared [3]. Among the materials adapted for solar cells, III-V alloys are the forerunner at present as they hold the record of power conversion efficiency (PCE) [4]. Multi-junction III-V solar cells were the pinnacle of current technologies during several years. However, if we could optimize all the issues we encountered in device fabrication and design, a photovoltaic device with one single band gap material can reach 44% PCE according to the Shockley-Queisser limit (SQ limit) [5]. Furthermore, fabricated in these nitride-based alloys solar cells, the novel structure of nano-scale nitride semiconductor [6]–[7] would be a potential role that leads to increase the light harvesting from more probability of incident light and generate the quantum-effect of

electron-hole pairs coupling and tunneling to enhance the photo-electron conversion. Several groups also research the extra light-absorption through generating intermediate band (IB) effect [8]–[9] of quantum dots.

In our studies, we employed MOCVD to fabricate the InN-dots structure and measure the absorption ratio (%) of it to calculate the absorption coefficient value. Next, this simulation work of APSYS®, which embedded InN dot-like structure within gradient InGaN layers exhibited in, is feasible to achieve the simulation results of the various thickness of InN dot-like layer through integrating the absorption coefficient of experimental results for InN-dots structure fabricated by MOCVD. In addition, the optimization performance using different composition between InN dot-like structure and gradient InGaN layers was exhibited in this work.

II. EXPERIMENT

The nitride-based structures were grown on c-plane sapphire substrate by metal organic chemical vapor deposition. On the bottom sapphire substrate, the nitride-based structure consists of a low-temperature GaN buffer layer, a 2μm-thick undoped GaN layer, followed by 65nm-thick InN-dots layer that was grown to conduct at a temperature of 525 degrees Celsius, and then the thin capping GaN was grown above. In the growth, trimethylgallium (TMGa), trimethylindium (TMIn) and ammonia (NH3) were used as gallium, indium, and nitrogen sources, respectively. For the growth of InN-dots structure, flow rates of TMIn and NH3 were fixed at 130 sccm and 7000 sccm, respectively, during the InN-dots formation process for several minutes. After the growth of InN-dots layer, the samples were characterized using field-emission scanning electron microscopy (SEM), high resolution X-ray diffraction (HRXRD) and the spectrophotometer in ranging from UV to visible wavelength (UV-VIS). In addition to InN epitaxy experiments and material measurement, we employed APSYS® software to setup and adjust the suitable structure of nitride-based solar cell, which was able to be an applied device for experiments. The illustration of the simulating

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nitride-based structure for solar cell was shown in Fig. 1. Next, we also have utilized APSYS® simulation method to construct absorption components of embedded InN dot-like structure within InGaN layers in nitride-based solar cell in order to calculate the characteristics of devices, and then the absorption coefficient of experimental InN-dots layer measured by UV-VIS would be embedded within the APSYS® simulation system. After it has been completed to setup, we have set the similar 65nm-thick of InN layer to compare the APSYS® calculation method of InN absorption with that using the absorption coefficient of experimental InN-dots layer, which was fabricated on undoped GaN by MOCVD, in order to confirm the difference results between them. Afterward utilizing the modified simulation component to research the best embedded InN-dots device in the APSYS® frame has been investigated.

Fig. 1. The schematic diagram of embedded InN dot-like structure within InGaN layers in nitride-based solar cell.

III. RESULTS AND DISCUSSIONS

Fig. 2 shows measurement results of InN-dots layer fabricated by MOCVD. The morphology of InN-dots through top-view of SEM image reveals that the diameter of dots is between 300 nm and 400 nm and the estimation height is about 65 nm. The measurement results were shown in Fig. 2(a). Furthermore, two results of XRD patterns demonstrated that the crystal category of InN-dots is hexagonal-type, as shown in Fig. 2(b) and (d). Through confirming the type of InN-dots, we could be accurate to adjust the option of APSYS® components. Fig. 2(c) shows the experimental absorption coefficient of InN-dots layer. In order to obtain the results, the calculation method of absorption coefficient is through subtracting the absorption ratio of capping layer on undoped GaN template from the absorption ratio of InN-dots layer without capping layer. It could eliminate the complicated optical-absorption of capping layer above and undoped GaN

below and then advantageously provide the accurate case of InN-dots absorption in order to embed in the APSYS® frame.

Fig. 2. (a) the top view of SEM image; (b) and (d) XRD patterns; (c) the experimental absorption coefficient for InN-dots layer on undoped GaN template.

Fig. 3. The band diagram of (a) 5nm-thick InN dot-like layer between 70nm-thick gradient InGaN layers, (b) 65nm-thick InN dot-like between 40nm-thick gradient InGaN layers, and (c) 5nm-thick InN dot-like layer between 10nm-thick In0.5GaN0.5N inside 60nm-thick gradient InGaN layers; all with solar irradiation condition.

Through APSYS® commercial software, Fig. 3 shows the band diagram of three types of structure that exhibit the two designs as fixing 145nm-thick InN/all-gradient InGaN layer with various thickness of InN dot-like layer and fixing 5nm-thick InN dot-like layer between 10nm-thick various Indium-content InGaN inside 60nm-thick gradient InGaN layers. Compared to all-gradient type, it was defined that FG is

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Indium-content ratio x of 10nm-thick InxGa1-xN, with the exception of FG is 1 defined as all-gradient type. All structure of InN/InGaN layers were fixed at the total thickness of 145 nm. Afterward we utilized the APSYS® software simulation that employed the experimental absorption coefficient of InN-dots layer, which fabricated by MOCVD, to calculate the characteristics of solar cells with InN dot-like structure. Fig. 4 shows the simulation result of I-V characteristic and external quantum efficiency (EQE). As the thickness of InN dot-like layer varying from 65 to 5 nm, PCE increases from 2.32 to 4.87 %; similarly, open current voltage (Voc) increases from 0.22 to 0.37 V. Due to the maximum of experimental absorption coefficient at Energy of 1.68 eV, EQE reveals that various thickness of InN dot-like layer at wavelength of 740 nm divide absorption-variation tendency in this case. As less than wavelength 740 nm, EQE of the sample of 5nm-thick InN dot-like layer is better than others. On the other hand, the factor FG value varying from 0.5 to 1, PCE increases from 1.02 to 4.87 %, and EQE results follow the similar tendency. It’s clear to observe the enhancement of short current density (Jsc) due to the minority carriers of photo-electron generation transferring in the less barrier of all-gradient InGaN, as shown in Fig. 3(a).

Fig. 4. Simulated I-V characteristic of (a) various thickness of InN dot-like layer and (b) various Indium-content of 10nm-thick InGaN inside 60nm-thick gradient InGaN layers; EQE of (c) various thickness of InN dot-like layer and (d) various Indium-content of 10nm-thick InGaN inside 60nm-thick gradient InGaN layers.

Furthermore, we tried to optimize the structure of different thickness InN dot-like layer within different thickness gradient InGaN layers in nitride-based solar cell to search the modified model limit that employed the experimental absorption coefficient in APSYS® frame. This case was similar to the prior case, which was an InN dot-like structure within gradient InGaN layers as an absorption layer. Fig. 5 shows the PCE results exhibit that the optimization thickness of InN dot-like

layer is 5 nm, and optimizing gradient InGaN thickness of this sample indicated the best thickness ~360 nm, as shown at dotted line. The optimal characteristic of devices that we demonstrated shows Voc of 0.303 V, Jsc of 39.72 mA/cm2, fill-factor (F.F.) of 67.97 %, and PCE of 8.2 %. Fig. 5. Simulated PCE correlated thickness of gradient InGaN layers for different thickness of InN dot-like layer.

IV. CONCLUSION

In conclusion, we successfully have fabricated InN-dots layer by MOCVD and utilized the absorption coefficient of experimental InN-dots to construct a simulation model by APSYS® components. The optimization composition of InN dot-like structure within single junction nitride-based solar cell has been completed by APSYS® employing variable parameter of experimental InN-dots. Finally, we show the best performance of devices can reach PCE of 8.2 %. We believe the novel structure shall be potential to improve the current complication of nitride-based solar cells.

ACKNOWLEDGEMENT

The authors would like to thank the financial supports from Nation Science Council of Taiwan through the contract numbers of NSC 101-2221-E-009-046-MY3 and NSC 102-3113-E-110 -002.

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