APPLIED PHYSICS LETTERS 93, 261108 共2008兲
High-quality InGaN / GaN heterojunctions and their photovoltaic effects Xinhe Zheng,1 Ray-Hua Horng,2,a兲 Dong-Sing Wuu,1,b兲 Mu-Tao Chu,2 Wen-Yih Liao,3 Ming-Hsien Wu,3 Ray-Ming Lin,4 and Yuan-Chieh Lu4 1
Department of Materials Science and Engineering, National Chung Hsing University, Taichung 402, Taiwan 2 Institute of Precision Engineering, National Chung Hsing University, Taichung 402, Taiwan 3 Industrial Technology Research Institute, Electronics and Opto-Electronics Research Laboratories, Hsinchu 310, Taiwan 4 Department of Electronic Engineering, Chang-Gung University, Taoyuan 333, Taiwan
共Received 30 October 2008; accepted 4 December 2008; published online 29 December 2008兲 High-quality p-GaN / i-In0.1Ga0.9N / n-GaN heterojunctional epilayers are grown on 共0001兲-oriented sapphire substrates by metal organic chemical vapor deposition. The Pendellösung fringes around the InGaN peak in high-resolution x-ray diffraction 共HRXRD兲 confirm a sharp interface between InGaN and GaN films. The corresponding HRXRD and photoluminescence measurements demonstrate that there is no observable phase separation. The improvement in crystal quality yields high-performance photovoltaic cells with open-circuit voltage of around 2.1 eV and fill factor up to 81% under standard AM 1.5 condition. The dark current-voltage measurements show very large shunt resistance, implying an insignificant leakage current in the devices and therefore achieving the high fill factor in the illuminated case. © 2008 American Institute of Physics. 关DOI: 10.1063/1.3056628兴 The group III-nitride semiconductors are the subject of current investigations due to their potential applications in optoelectronic and electronic devices. Recent studies reveal the revision of small band-gap 共around 0.7 eV兲 of InN,1,2 which implies that the band-gap of ternary alloy InGaN could span from the infrared to the ultraviolet, an optical match to the solar spectrum. The unique band-gap range leads to an increased research interest into developing highefficiency 共50%兲 multijunction solar cells3 and full-spectrum devices using the nitride-based material system. However, because of many difficulties or challenges in growth of InN 共Ref. 4兲 and In-rich InGaN films as well as their p-type doping,5 few reports to date have been focusing on their corresponding photovoltaic devices.6–9 In the Ref. 8, the authors experimentally demonstrated the photovoltaic effect of n-GaN / i-InGaN / p-GaN structure with open-circuit voltages 共Voc兲 of around 2.4 V and reasonably high internal quantum efficiencies. Note that phase separation in the In0.05Ga0.95N / GaN epilayers are easily found, which not only affects the resulting Voc but degrades performance of InGaN / GaN solar cell. Additionally, considering a higher band gap in low In-content InGaN or GaN absorbed layers, the photovoltaic effect of 共In兲GaN / GaN solar cell was measured under an illumination of ultraviolet light or modified solar source.7–9 So, actual conversion efficiency under onesun spectra was not reported. In this paper, a high-quality p-GaN / i-In0.1Ga0.9N / n-GaN double heterojunction with no observable phase separation and relaxation is obtained using metal organic chemical vapor deposition 共MOCVD兲. In terms of the achieved crystal quality, we discuss its effects on photovoltaic performance 共especially shunt resistance兲 of the fabricated p-i-n solar cell under standard air mass 共AM兲 1.5 solar spectra. a兲
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Epitaxial layers of GaN and InGaN were grown on c-plane sapphire substrates by MOCVD using the conventional two-step growth process. The p-i-n junction for photovoltaic effect consists of a 3 m thick bottom Si-doped n-type GaN 共n-GaN兲, 0.15 m thick intrinsic In0.1Ga0.9N layer 共i-InGaN兲, and 0.15 m thick top p-GaN. The epilayers are characterized by high-resolution x-ray diffraction 共HRXRD兲 and transmission electron microscopy 共TEM兲. To investigate photoluminescence 共PL兲 properties of InGaN layer, an InGaN / GaN heterostructure without p-GaN capping layer is intentionally grown using the same deposition conditions described above. The photovoltaic device with a size of 1 ⫻ 1 mm2 was designed and fabricated based on a style of lateral lightemitting diodes 共LEDs兲. First, a Ni/ Au 共3 nm/ 3 nm兲 metal system was deposited to form a semitransparent current spreading layer on p-GaN using thermal evaporation. Then a mesa structure to expose the n-GaN for contact was defined using standard photolithography and dry etching techniques. Finally, Cr/ Au 共25 nm/ 200 nm兲 metal systems were deposited to form p- and n-type Ohmic contacts by thermal evaporation. No antireflection layers are coated on the fabricated solar cells. The measurement of photovoltaic effect was performed using an AM 1.5 solar simulator light source 共100 mW/ cm2兲. The measurement system has been calibrated using the standard cell based on the technology requirements from the National Institute of Standards and Technology 共NIST兲. Current-Voltage 共I-V兲 characteristics under dark and illumination conditions were measured for all the p-i-n solar cells. Figure 1 shows the -2 scanning curve measured in the vicinity of InGaN and GaN 共0002兲 reflections from the p-GaN / i-InGaN / n-GaN solar cell. Note that the clearly resolved Pendellösung fringes or interference patterns appear on either side of the main GaN epilayer peak, which offers convincing evidence of high structural quality for the InGaN / GaN heterojunction. This is one of the reported ob-
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FIG. 1. High-resolution x-ray -2 scanning profile of the 共0002兲 reflection from GaN / InGaN / GaN heterojunctions grown on c-plane sapphire. Pendellösung fringes are clearly observed around the main GaN and InGaN diffraction peaks.
servations of Pendellösung fringes in the x-ray rocking curve. From the fringe spacing, the thickness or coherent length of InGaN is accurately determined to be 0.15 m, which is very close to the nominated value. However, the deduced thickness of InGaN layer remarkably exceeds its critical thickness.10 This should imply that a relaxation which introduces misfit dislocations could occur in the InGaN layer. To further confirm this, reciprocal space mapping measurements of around the asymmetrical 共10.5兲 reflection are carried out, as shown in Fig. 2. In general, if a relaxation occurs by dislocation within the interfaces of the heteroepitaxial layers, a poor interface quality and nonuniform thickness will result in no appearance of the Pendellösung fringes. While for the case of InGaN-based epilayers on GaN, a lack of lattice-matched substrates and growth challenges of InGaN films limit the observation of the Pendellösung fringes. Therefore, this phenomenon is frequently used to judge heterojunction quality as the figure-of-merit. Here, an inspection of the figure reveals that the diffraction peaks of GaN and InGaN layers are aligned in a vertical line parallel to the Q共0002兲 or Qz axis, indicating that the InGaN epilayer is coherently strained to the GaN template which implies no re-
FIG. 2. 共Color online兲 Reciprocal space mapping of x-ray diffraction intensity from the 共10.5兲 reflection for the p-GaN / i-InGaN / n-GaN solar cell on the u-GaN buffer layer. Inset is cross-sectional TEM image obtained from GaN / InGaN / GaN epilayers. The dark background within the up and down interfaces of InGaN / GaN exhibits a strain contrast due to the stored misfit strain energy.
Appl. Phys. Lett. 93, 261108 共2008兲
FIG. 3. PL spectrum of InGaN epilayer measured at room temperature.
laxation or misfit dislocations in the InGaN layer. On the other hand, the bright-field TEM image can support this point as shown in the inset of Fig. 2. The dark background within the up and down interfaces of InGaN / GaN exhibits a strain contrast due to the stored misfit strain energy. Nevertheless, the image reveals that no structural defects such as dislocations are observed within the intrinsic In0.1Ga0.9N and the p-GaN capping layer in spite of the InGaN layer of more than the critical thickness. Moreover, the i-InGaN appears to have mainly uniform thickness which is in agreement with the inferred values via the fringe spacing from the HRXRD rocking curve. Based on a complete strain state extracted from the experimental results, the In molar fraction in the i-InGaN is determined to 0.1 from the -2 scanning curve. Figure 3 shows the room-temperature PL spectrum of the InGaN / GaN epilayers. A sharp single PL peak corresponding to near-band edge transition in the epitaxially grown InGaN phase appears at 393 nm 共⬃3.15 eV兲. We observe that no additional peaks appear in either lower energy side or higher energy side of the observable PL peak of InGaN. An induced broadening of the primary InGaN peak which implies phase separation is not observed. These results confirm a good suppression of phase separation in the InGaN epilayers. Additionally, based on the band gap dependence on composition for InGaN on GaN,11 the In concentration agrees well with the obtained value elucidated by the HRXRD method. Figure 4 shows light and dark current-voltage 共I-V兲 measurements of the p-i-n solar cell. The device displays conversion efficiency of 0.5%, reasonably high Voc of around 2.1 V and very high fill factor of up to 81% confirming excellent photovoltaic performance. It should be noted, however, that the measured Voc of the device is substantially less than the optical band gap of i-InGaN 共2.1 V compared to 3.15 eV兲. Furthermore, it is still lower than that 共2.4 V兲 of p-GaN / i-In0.05Ga0.95N / n-GaN solar cell despite the existence of the lower band-gap phase-separated InGaN 共2.8 eV兲, which is one of dominant factors causing a reduction of Voc of the solar cell.6 Actually, due to the good crystal quality and interface property in InGaN / GaN heterojunction, our devices may exhibit the low reversed saturation current density. The limited Voc in this study could be ascribed to the relatively smaller built-in potential caused by the lower doping concentration in GaN and/or less incident light due to the
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FIG. 4. 共Color online兲 Typical I-V curves of p-GaN / i-InGaN / n-GaN solar cell under the illuminated and dark conditions, along with calculated P-V characteristic in the illuminated case. The inset graph shows the dark I-V curve of the same photovoltaic cells using a logarithm scale.
adsorption in the Ni/ Au contact layer. A detailed study of a dependence of Voc on doping levels and contact schemes for the same device structures is in progress. It is worth noting that in spite of relatively lower open-circuit voltage the solar cell demonstrates remarkably high fill factor of up to 81%, which is much larger than that of the reported GaN solar cell.7,9 The relatively high fill factor could result from the negligible leakage current or large shunt resistance in the fabricated solar cells. The inset in Fig. 4 shows the dark I-V characteristic of the same photovoltaic cells using a logarithm scale. Notice that current increases linearly with the voltage at low voltage values 共⬍0.5 V兲, marked by a blue line. From a linear fit to the data, we could infer a very large shunt resistance, indicating the insignificant leakage current. In general, the cause of the improved shunt resistance or leakage current could be thought of as the reduction of defects such as dislocations in the bulk portion or interface of the heterojunctions, which is recently reported in high In-conent InGaN p-i-n solar cell.9 The authors observed that the leakage current density increases with the area/periphery ratio of the diode, reavealing leakage current as one of the main components degrading photovolatic performance of the devices. Such a discussion is also quite consistent with high-quality InGaN / GaN heterojunction clearly verified by HRXRD and PL measure-
ments. Additionally, we measured the properties of the p-i-n solar cells with a thicker In0.1Ga0.9N intrinsic layer 共⬃250 nm兲. It is found that the devices display the poorer photovoltaic effect 共especially the low fill factor, data are not shown here兲, which could be attributed to worse crystal quality caused by a relaxation of InGaN layer which generates dislocations 共increasing leakage current and recombination兲 because the growth thickness exceeds the critical thickness of InGaN on GaN. In summary, high-quality GaN / In0.1Ga0.9N / GaN double heterojunctions are obtained using MOCVD. The corresponding p-i-n solar cells show reasonably high Voc and high fill factor of up to 81% under an illumination of AM 1.5 solar light. The good photovoltaic effect could be attributed to the negligible leakage current obtained from dark-current– voltage characteristics. These findings are in good agreement with the results characterized by HRXRD and PL techniques, which could be a significant step toward understanding solar cell properties of InGaN-based solar cells. This work has been financially supported by the Industrial Technology Research Institute and ministry of education 共Taiwan兲 ATU plane. 1
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