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Abstract
In InGaN-based LEDs, an InGaN layer underlying active region has been widely used to improve the luminescence efficiency of the quantum wells (QWs). It has been reported recently that the role of InGaN underlayer (UL) is to block the diffusion of point defects or surface defects in n-GaN into QWs. The type and the source of the point defects need further investigations. In this paper, using temperature-dependent photoluminescence (PL) measurements, we observe emission peak related to nitrogen vacancies (VN) in n-GaN. In combination with secondary ion mass spectroscopy (SIMS) measurement and theoretical calculation, it is found that VN concentration in n-GaN is as high as about 3 × 1018 cm-3 in n-GaN grown with low growth V/III ratio and can be suppressed to about 1.5 × 1016 cm-3 by increasing growth V/III ratio. Luminescence efficiency of QWs grown on n-GaN under high V/III ratio is greatly improved. These results indicate high density of nitrogen vacancies are formed in n-GaN layer grown under low V/III ratio and diffuse into quantum wells during epitaxial growth and reduce the luminescence efficiency of the QWs.
© 2023 Optica Publishing Group under the terms of the Optica Open Access Publishing Agreement
1. Introduction
Since the invention of high-efficiency InGaN-based blue LEDs [1], InGaN-based LEDs and LDs [2–7] have developed rapidly. An InGaN underlayer (UL) inserted between multiple quantum wells (MQWs) and n-GaN templates could improve the luminescence efficiency of MQWs [8–15], whether they are grown on sapphire, silicon, or free-standing substrates. Typically, the indium content of this InGaN UL is lower than the active region, with a thickness of about tens of nanometers. It has been reported that the mechanism by which the InGaN UL enhances the luminescence efficiency of MQWs is to reduce point defects in the MQWs, rather than to reduce their dislocation density [13,14]. Several recent studies have suggested that the type of these defects be mainly VN, which come from n-GaN template and are nonradiative recombination centers in quantum wells [15–18]. However, the cause of VN is still in debate. It is suggested they are formed during cooling process after the end of n-GaN growth [15], or high growth temperature of n-GaN [17] by different researchers. Both the origin and the characterization methods of VN needs further investigation.
In this paper, we characterize VN in n-GaN by temperature dependent photoluminescence (TDPL). In addition to band edge emission of n-GaN, two defect emissions have also been observed. Their relationship with V/III ratio of n-GaN growth is studied. The concentration of VN can be reduced by optimizing V/III ratio of n-GaN growth. Single quantum well (SQW) and double quantum wells (DQWs) were then grown on n-GaN templates with different V/III ratios. It is found that the PL intensity of DQWs grown on n-GaN with least VN can be greatly enhanced regardless of whether the InGaN UL is added.
2. Experimental details
All samples were grown by Metal-organic Chemical Vapor Deposition (MOCVD) on C-plane sapphires sputtered with a 25 nm AlN buffer layer. Trimethylgallium (TMGa) was used as a gallium source for n-GaN, and triethylgallium (TEGa) and trimethylindium (TMIn) were used as gallium and indium sources for SQW, DQWs, and InGaN UL. Hydrogen and nitrogen were used as the carrier gas. The sample structures are shown in Fig. 1. Series A of samples include 1 μm uGaN grown on a sputtered AlN sapphire substrate and 3 μm n-GaN grown at 1040 °C. By varying ammonia and TMGa flow, n-GaN templates with different V/III ratios are fabricated. Series A are five n-GaN template samples with V/III ratios ranging from 174 to 2069. The dislocation density of n-GaN templates is about 5 × 108 cm-2, measured by high-resolution X-ray diffraction (HRXRD). To minimize the effect of intentional doping on PL, low Si doping concentration of 5 × 1016 cm-3 was used. Series B are blue QWs with InGaN UL grown on n-GaN templates with the same growth conditions to Series A. Before DQWs are grown, a 500 nm regrowth n-GaN layer is prepared. Then the temperature is reduced to 745 °C, and SQW or DQWs were grown in a nitrogen environment. The InGaN UL has a thickness of 50 nm and the indium content is 3%. To analyze point defects in n-GaN templates, n-GaN templates grown with different V/III ratios were characterized by temperature-dependent PL. A 325 nm He-Cd laser and a 405 nm semiconductor laser were used for PL characterization of series A and series B, respectively. The excitation power densities of these two lasers are 5.1 W/cm2 and 4.8 W/cm2.
Fig. 1. Schematic diagram of the sample structure. Series A are n-GaN templates with different V/III ratios; Series B are DQWs or SQW grown on the different n-GaN templates with InGaN UL or not. For the n-GaN templates, a 325 nm laser is used for PL, and a 405 nm laser is used for the blue QWs.
3. Results and discussion
Figure 2(a) shows PL spectra of sample series A. At ambient temperature of 5 k, the band edge emission peaks (about 3.49 eV), the blue luminescence bands (BL) (2.64∼3.35 eV), and the yellow luminescence bands (YL) (1.77∼2.64 eV) can be seen for samples grown with low V/III ratios. The integral intensities of BL and YL showed an inverse relationship with the V/III ratios. BL and YL were no longer noticeable in the two samples with V/III ratios of 1391 and 2069, respectively. At room temperature of 300 K, the YL in all samples is almost dominant in the PL spectra as shown in Fig. 2(b). At the same time, band edge emissions and BL intensities are significantly reduced. From 5 K to 300 K, the YL integral intensity of the five samples do not change significantly (Fig. 2(d)), while the BL intensity decrease significantly till 140 K. The BL intensity was relatively weak over 140 K as shown in Fig. 2(c). The origin of YL has been widely reported to be mainly related to carbon impurities [19], including CGa [20], CN [20,21], Ci [22], and other types, but CN is the main form of carbon impurities in n-type GaN [23]. Some complex defects, such as VGa [24], CN-VGa [24,25], VGa-ON [25,26], etc. are also believed to contribute to YL. However, according to the calculations of first-principle in some reports, the formation energy of these complex defects in n-type GaN is significantly higher than CN [23].
Fig. 2. Characterization of temperature dependent PL of n-GaN samples grown with different V/III ratios (a) T = 5 K, (b) T = 300 K, (c) integral intensity of BL as a function of temperature, (d) integral intensity of YL as a function of temperature.
The correlation coefficient R was introduced to analyze the relationship between YL integration intensity and carbon impurity concentration. The correlation coefficient R is a common indicator in statistics to reflect the closeness of the correlation between variables. It was found that the correlation coefficient R is 0.998 between YL integration intensity and carbon impurity concentration measured by secondary ion mass spectroscopy (SIMS). These are shown in Fig. 3. It can be suggested that the intensity of YL correlates very well with the concentration of carbon impurities in n-GaN. Under low V/III ratio, carbon atoms readily incorporated into the GaN lattice [27]. For BL, in the low-temperature PL spectra of 5 K, the wavelength range from violet to blue light is covered. The BL intensity decreases significantly at the room temperature. According to some studies, BL might be associated with VN and decreases with increasing temperature [16,23,28]. The VN is generally considered 0.2∼0.3 eV under the conduction band [22,26]. With the reduction of the V/III ratio, the probability of VN formation increases theoretically, and it is reasonable to think that the 370nm-410 nm part of the BL corresponds to VN. The part of 410 nm∼470 nm may correspond to CN-VN or other point defects [29,30]. And some reports believe that VN (2+/3+) corresponds to the acceptor level of 0.4∼0.5 eV above the valence band [28,31,32], which may contribute to BL. There are some reports suggesting that the transition between SiGa and SiN [33] may also be one of the sources of BL, etc. Because the concentration of Si doping in n-GaN templates is low, this does not contradict with the inference that the BL integral intensity reflects the VN concentration in n-GaN.
Fig. 3. The BL intensity, the YL intensity, the carbon impurity, and the nitrogen vacancy concentration of n-GaN templates varied with V/III ratios. The carbon impurity concentration is measured by SIMS, and the nitrogen vacancy concentration is calculated according to the Arrhenius formula.
To further confirm the type of defect corresponding to BL, we designed additional experiments. First-principles calculations of defect formation energy in GaN have been reported [23]. As Fermi energy in GaN increases, the formation energy of CN and VGa gradually decreases, while the formation energy of VN gradually increases. Therefore the concentrations of VN will decrease as increasing Si doping concentration due to increasing Fermi energy. The low-temperature PL spectra for GaN samples with various Si concentration are shown in Fig. 4(a). These nGaN samples were grown under conditions of V/III = 348. In Fig. 4(a), it can be seen that the BL intensity gradually decreases as the concentration of Si doping increases. This is consistent with the viewpoint that BL is associated with VN mentioned above. Meanwhile, the SIMS data shown in Fig. 4(b) demonstrate that changes in Si doping concentration do not affect the concentration of C in n-GaN, indicating that C impurity is not associated with BL. Theoretical calculations about VN concentration are carried out based on this assumption.
Fig. 4. (a) Low-temperature PL measurements on n-GaN samples (V/III = 348) with different Si doping concentrations; (b) SIMS measurements shows that changes in Si doping concentration do not affect C concentration in n-GaN.
According to the temperature dependent PL measurement data of n-GaN templates, and the carbon impurity concentrations measured by SIMS, the nitrogen vacancy concentrations in n-GaN templates grown with different V/III ratios can be roughly estimated [32]. The normalized integral intensity of BL and YL as a function of temperature can be fitted using the Arrhenius formula [32,34,35]:
Among them, IiPL represents luminescence intensity of different types of defects (BL and YL), Ci and Ei are the corresponding capture coefficient and activation energy. The fitting of the normalized integral luminescence intensity of the defects with temperature may correspond to multiple Ci and Ei, because the luminescence of defects may correspond to multiple defect energy levels. Through fitting, three values of Ci can be obtained, of which only one value is significantly greater than the other two values by more than two orders of magnitude. These maximum values are selected as CBL or CYL. CBL and CYL represent the capture coefficients of the corresponding defect energy levels. By comparing the integral luminescence intensity of different defects, the concentrations of VN can be roughly estimated [32]:
To further investigate the effect of VN on luminescence of QWs, the samples of seriers B (as shown in Fig. 1) are measured by PL. As shown in Fig. 5(a), the PL integral intensities of SQW increase with increasing V/III ratios of n-GaN. It should be noted that the sample structure in Fig. 5(a) does not add InGaN UL. This is because VN concentration in n-GaN decreases with increasing V/III ratios of n-GaN, and these VN diffuse into the QW during epitaxial growth and act as nonradiative recombination centers [36] in SQW. For the DQWs, the situation is a litte different. The PL integral intensities are almost constant until the V/III ratio of n-GaN is 2096. An InGaN underlying layer can block the diffusion of VN into QW, so does the QWs, which are even more effective due to higher indium content [37]. This is the reason that the integral intensities of DQWs are much higher than that of SQW. It should be noted that the PL intensity scale for DQWs is one order of magnitude higher than that SQW. When VN concentration in n-GaN is high, the QW close to the n-GaN becomes inefficient because it traps VN that diffuse from n-GaN, whereas the upper QW is efficient due to less effect by VN that diffuse from n-GaN. The upper QW dominates the overall luminescence of DQWs and results in almost constant PL intensity up to V/III ratio of 1391. In contrast, in the sample with highest V/III ratio, its luminescence intensity is significantly higher than other samples due to the lower concentration of VN in both quantum wells.
Because PL measurements with lower excitation power density are used, the recombination of carriers in QW with high VN concentration is dominated by nonradiative recombination. If the VN concentration in QW can be reduced, the difference in luminescence intensities of DQWs in different samples can be revealed. So, a 50 nm In0.03GaN UL between DQWs and n-GaN is applied to block VN diffusion. The insertion of InGaN UL significantly enhances the luminescence intensity of DQWs. As shown in Fig. 5(b), the luminescence intensities of the DQWs and lifetimes of the photogenerated carriers are positively correlated to the V/III ratios of n-GaN. In the sample structure with an InGaN UL, when the V/III ratio of n-GaN is 174, the effective carrier lifetime of DQWs is 50.5 ns obtained by time-resolved photoluminescence (TRPL). When the V/III ratio of n-GaN rises to 2069, the effective carrier lifetime of DQWs increases to 107.7 ns. TRPL is measured at room temperature and the laser source is a 405 nm laser.
Fig. 5. (a) Changes in the PL luminescence intensities of SQW and DQWs above the n-GaN templates grown with different V/III ratios. (b) A 50 nm In0.03GaN is inserted between the DQWs and n-GaN templates, and the luminescence intensity of DQWs also becomes more robust with increasing of the V/III ratio of n-GaN.
Because the excitation power density for TRPL measurement is low, the effective carrier lifetime obtained by TRPL is approximately equal to the nonradiative recombination lifetime of carriers. The longer the nonradiative recombination lifetime, the lower the density of the nonradiative recombination centers. The InGaN UL can blocks the diffusion of VN. However, since the thickness is only 50 nm, it does not completely block VN from entering DQWs, so the concentrations of VN in DQWs above InGaN are still related to the V/III ratios of n-GaN templates. The “filtering” effect of InGaN UL also allows difference in the luminescence intensities of DQWs to be observed at a low excitation power density, which is different from the samples without InGaN UL.
4. Conclusion
In summary, temperature dependent PL measurement in combination with SIMS measurement and theoretical calculation are used to investigate VN in n-GaN template grown under different V/III ratio. It is found that VN concentration decreases with increasing V/III ration. The luminescence efficiency of SQW/DQWs is greatly affected by the growth V/III ratio of n-GaN templates, which is because VN in n-GaN diffuse into SQW/DQWs during epitaxial growth. Even with the insertion of an InGaN UL that has been reported recently to inhibit the diffusion of VN into active region, using of the n-GaN template with a higher V/III ratio can further enhance the luminescence efficiency of InGaN QWs.
Funding
National Natural Science Foundation of China (61834008, U21A20493); National Key Research and Development Program of China (2022YFB2802801, 2017YFE0131500); Key Research and Development Program of Jiangsu Province (BE2020004, BE2021008-1); Special Project for Research and Development in Key areas of Guangdong Province (2020B090922001); Basic and Applied Basic Research Foundation of Guangdong Province (2019B1515120091).
Acknowledgments
The authors are grateful for the technical support for Nano-X from Suzhou Institute of Nano-Tech and Nano-Bionics, Chinese Academy of Sciences (SINANO). The authors are grateful for the technical support for the Nano Fabrication Facility from Suzhou Institute of Nano-Tech and Nano-Bionics, Chinese Academy of Sciences (SINANO).
Disclosures
The authors declare no conflicts of interest.
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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