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Abstract
This paper studies the influence of low-temperature GaN-cap (LT-Cap) layer thickness on the InGaN/GaN multiple quantum well (MQW) structure and the related luminescence characteristics. The research results show that the thickness variation of LT-Cap layers seems not to have a substantial impact on the structure of MQWs, i.e., the well layer thickness, but strongly affects the indium composition of well layers. The LT-Cap layer can effectively weaken the decomposition of InGaN, however, the increase in the thickness of the LT-Cap layer will lead to an increase in the polarization effect, resulting in a red shift of the emission peak. Different LT-Cap layers will affect the distribution of the tail states, resulting in an energy shift of carrier emission from the local states. In addition, the thickness variation of LT-Cap layers also affects the luminescence characteristics of MQWs. It is found that as the thickness of the LT-Cap layer increases, the internal quantum efficiency (IQE) of the material gradually decreases, which may be due to the introduction of new non-radiative recombination centers.
© 2021 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
Having been attractive materials for around 30 years in optoelectronic devices including light-emitting diodes (LED) and laser diodes (LD) [1–9], the research on group III nitride semiconductor devices have made great progress in material physical understanding, growth control, fabrication and structure design. The key issues for realizing green InGaN light emitting devices are the preparation of high quality high In content InGaN/GaN MQW, as well as the reduction of charge separation in the active region. To date, many ways [10–12] have been reported to improve the crystal quality in InGaN-based LEDs and address to the charge separation. A significant improvement of IQE has been made in green emitting InGaN-based LEDs [13–18]. In this work, we mainly focus on the growth of high quality InGaN/GaN MQW by inserting a LT-cap layer and a growth interruption during the barrier layer growth. Due to the weak bond strength of InN, the growth temperature of InGaN QWs must be lower than 800°C. As a result, the crystal quality of InGaN QWs is lowered in general. In order to improve the crystal quality of MQW region, a ramping up process to an increased growth temperature of GaN quantum barrier (QB) layers is often used in metal–organic chemical vapor deposition (MOCVD) growth [19]. However, indium may lose from QWs during this temperature ramp process [20]. In previous works, many reporters have demonstrated that inserting a thin LT-cap layer after the growth of each InGaN QW layer can effectively protect indium from desorption during the temperature ramp process [21–25] and improve the luminescence efficiency of MQW. However, some research groups have found that increasing the LT-cap layer thickness will result in a decreased room-temperature optical efficiency [26]. The exact reason for this low optical efficiency is still not very clear. This paper will discuss the effects of the LT-Cap layer thickness on the structure and luminescence characteristics of InGaN/GaN MQW.
2. Experiment
InGaN/GaN MQW samples with different thicknesses of LT-cap layers were grown by an AIXTRON 3 × 2 in. close-coupled showerhead reactor on c-plane sapphire substrate. Trimethylgallium, trimethylindium, and ammonia were used as Ga, In, and N precursors, respectively. All the samples were deposited by MOCVD layer by layer with a 20 nm thick buffer layer, a 1 µm thick Si-doped GaN layer, a two-period unintentionally-doped InGaN/InGaN MQW active region and a 120 nm Mg-doped p-GaN layer. During the growth of each well layer of the MQW, the TMIn flow rate was kept constant. Then a GaN layer was grown at a temperature the same as well layer, i.e., 710 °C, which is called as GaN cap layer when it is mentioned in this work. Afterwards, the temperature was ramped up to 830 °C, and stay several seconds, and then the barrier layer was grown at 830 °C. All the investigated samples are grown under the same conditions except the GaN cap layer growth time, i.e., it is 120 s for sample A, 200 s for sample B and 250s for sample C. The schematic diagram of epilayer structure of samples is shown in Fig. 1.
High resolution X-ray diffraction (HRXRD) was used to characterize the structural parameters of MQW. Room-temperature photoluminescence (PL), temperature-dependent photoluminescence (TDPL) and microscopy PL were used to characterize the luminescence properties of MQW. During the TDPL measurement, a 325 nm He-Cd laser with an output power of 8 mW was used as the excitation source and the temperature was controlled by a CTI Cryogenics closed cycle refrigerator, ranging from 30K to 300K. Microscopy photoluminescence image was measured by laser excitation at 405nm through Nikon A1 confocal optical system.
3. Result and discussion
In order to study the structural characteristics of the three samples, ω-2θ symmetric (0002) scans have been carried out, as shown in Fig. 2, where the substrate peak originates from GaN (002) plane, and the satellite peaks are from MQW. For all samples satellite peaks up to the −3th can be clearly observed, which indicates that the MQW has a good layer periodicity. The detailed parameters of MQW are shown in Table 1. The results show that the thickness of the well layer and the thickness of the barrier layer of the three samples are basically the same, but the In content in InGaN is different. The In content in InGaN of samples A, B, and C are 13.0%, 14.1%, and 14.6%, respectively. This shows that as the thickness of the LT-Cap layer increases with increasing GaN LT-Cap layer growth time, the In content in InGaN also increases. This indicates that the LT-Cap layer can weaken the decomposition of InGaN.
Table 1. Structural parameters of InGaN/GaN MQW of sample A, B and C determined by HRXRD.
Figure 3(a) shows the room-temperature PL spectra of sample A, sample B and sample C. They are excited by a 325nm He-Cd laser with an excitation power of 8mW. It can be seen that the emission intensity of sample A is the strongest. The red line in the figure is Gaussian fitting to the tested data. From the fitting result, the peak wavelengths of samples A, B, and C are 514 nm, 545 nm, and 552nm, respectively, which means that the change in thickness of the LT-Cap layer will cause changes in the quantum well structure of the sample, and these changes will lead to a different emission intensity of the sample and a red shift of the emission wavelength. We think this is the result of the polarization effect. We have carried out TDPL test on the samples. Figure 3(b) and Fig. 3(c) are the PL spectra measured at 30K and 300K, respectively. At 30K, the FWHM of samples A, B, and C are 38.74 nm, 24.44 nm, 43.11 nm, respectively, at 300K, the FWHM of samples A, B, and C are 40.34 nm, 66.11 nm, 92.07 nm, respectively. These data are obtained by Gaussian fitting. The FWHM variations of these samples between 30K and 300K are 1.6 nm, 41.67 nm, 48.96 nm. It is noted that the FWHM variation of samples A, B, and C is getting larger and larger, which makes us believe that as the thickness of the LT-Cap layer increases, the polarization effect is enhanced.
Fig. 3. (a) The normalized PL spectra measured at room-temperature for sample A, B and C, (b) the normalized PL spectra measured at 30k for sample A, B and C, (c) the normalized PL spectra measured at 300k for sample A, B and C.
Figure 4 shows the temperature-dependence of luminescence peak energy. From the figure, we can see that as the temperature increases, the peak energy of sample A increases rapidly. When the temperature rises to about 150K, the peak energy will fluctuate and decrease as the temperature further rises. As the temperature rises, the peak energy of sample B rises rapidly. When the temperature rises to approximate 170K, the peak energy of sample B begins to fall rapidly. Sample C also has a behavior similar to samples A and B at low temperatures, that is, as the temperature increases, the peak energy rises rapidly, but after 170K, the peak energy does not decrease, but tends to stabilize. As we all know, the competition between the temperature-dependent band gap shrinkage effect and the carrier redistribution leads to the changing direction of the shift of the emission peak. For samples A and B, the peak energy first increases and then decreases, that is, the luminescence peak first shifts blue and then red shifts. This means that in the competition between the band gap shrinkage effect and the redistribution of carriers by heating, the advantage changes gradually from the latter to the former. For sample C, the heat distribution of carriers has always been dominant, but as the temperature increases, this advantage gradually becomes smaller. In addition, we can also find that the temperatures at which samples A, B, and C begin to accelerate blue-shift are different. They are 50K, 45K, and 70K, respectively. This means that the energy required to excite carriers from the lower localized state is different, and the sample B needs minimal energy to complete the energy shift.
Fig. 4. Samples excited by 325 nm He-Gd laser at 8 mW incident optical power the relationship between the PL emitted peak energy of A, B, and C and temperature, and the solid line (red) is a fitting result obtained using formula (1).
In order to quantitatively analyze the test results of the samples, we use the tail state model to describe the local state effects in the quantum well. According to the research results of Eliseev et al., [27] the relationship between temperature and peak luminous energy can be described by the formula (1), the red solid line in Fig. 4 is the fitted curve.
Among them, E0 is the band gap energy at 0K, T is the temperature, KB is the Boltzmann constant, α and β are the fitting parameters in the classical Varshni formula. The σ value indicates the strength of the local effect, that is, the larger the σ value, the deeper the distribution of the band tail state in the forbidden band, and the stronger the local effect. The calculated σ values of the sample A, B, C fitted by the above formula are respectively 19.16 meV, 10.85 meV, 33.98 meV. From the fitting results, the σ value of sample B is the smallest, which means that the localized effect of sample B is the smallest, and the distribution of the tail state in the forbidden band is the shallowest. Therefore, the carriers in the localized state at a relatively low temperature can be launched. The σ value of sample C is the largest, so the localized state effect of sample C is the largest, and the distribution of the tail states in the forbidden band is the deepest. At low temperatures, more energy is needed to emit carriers. The σ value of sample A is in the middle. That is consistent with the above analysis. From the analysis results, we believe that LT-Cap layers of different thicknesses will cause the strength of the localization effect of these samples to be different. According to our experimental results, as the thickness of the LT-Cap layer increases, the local effect weakens first, then becomes to have a strengthening trend.
Figure 5 depicts the temperature-dependences of the integral intensity of the PL spectra of the three samples. From the figure, we can see that the integrated intensity of sample A decreases slowly as the temperature rises at low temperatures. At 50K, the integrated intensity of the sample rises shortly at first and then drops rapidly, forming a small hump in the temperature dependence. The integrated intensity of sample B remains basically unchanged at low temperature, which indicates that the localized state of sample B is very shallow, and the localized state effect is very weak, so that at low temperature, as the temperature increases, there is no obvious change in the luminous intensity of the sample. As the temperature continues to rise, like what happens for sample A, sample B also has a short rise, and the rise is weaker than that of sample A. As the temperature continues to increase, the integrated intensity of sample B continues to decrease. The behavior of sample C is different from samples A and B in that the integrated intensity does not increase with increasing temperature during the whole process, but keeps decreasing. We know that there may be two reasons why the integrated intensity becomes stronger. The first is the increase of injection of external photons, and the second is the increase in the number of carrier radiation recombinations in the quantum well. In order to prove whether the integrated intensity was increased due to the injection of external photons, we used a 405nm laser instead of a 325 nm laser to excite the sample and it was found that in the temperature-dependent curve the upward trend of the hump at nearly 50K decreased, as shown in Fig. 6. Therefore, we believe that this short hump-like ascending process is related to the injection of 325nm excited photons. And we come to the judgment that as the thickness of the LT-Cap layer increases, it becomes more difficult for external 325 nm photons to be injected into the quantum well.
Fig. 5. The temperature dependencies of PL emission integral intensities of samples A, B, and C, excited by 325 nm He-Cd laser at an incident light power of 8 mW.
Fig. 6. The temperature dependences of integral PL emission intensity of samples A and B excited by 405 nm (solid triangles) and 325 nm lasers at an incident light power of 8 mW.
We already know that in normal as the temperature increases, the integral intensity of sample’s PL emission is continuously reduced due to the increasing effect of non-radiative recombination. We take the internal quantum efficiency (IQE) at 30K as 100%, then we can calculate that the IQE of samples A, B, and C at room-temperature are 21%, 5%, and 7.6% respectively. It can be seen that the IQE of sample A at room-temperature is the highest. After the above analysis, we have known that the localized state of sample B is the shallowest and the localized effect is the weakest, and it’s MQW should be the most uniform. However, its room temperature IQE is not the highest, which shows that the localization effect of appropriate degree is beneficial to increase the room-temperature IQE and improve the luminescence of the sample.
The integrated intensity curve of PL spectrum changing with temperature can be well fitted by the Arrhenius formula [28]:
Where Ei is the activation energy of the corresponding non-radiative centers, Ci is a constant related to the center density, and KB is Boltzmann's constant. The solid line in Fig. 5 is the fitting curve of samples A, B, and C, and the fitting values are shown in Table 2.
Table 2. The fitting results of the relationship between the temperature-dependent PL integrated intensity of samples A, B, C.
It can be seen from the calculation results that sample A has two non-radiative recombination centers, the activation energies of which are 41.8 meV and 1.06 meV respectively. Among them, the non-radiative recombination center with an activation energy of 41.meV has a higher density and plays a more important role. Sample B has three non-radiative recombination centers with activation energies of 43 meV, 26.47 meV, and 7.66 meV, respectively. Among them, the density of non-radiative recombination centers with an activation energy of 43 meV is significantly higher than the other two types. Sample C has two non-radiative recombination centers with activation energies of 92.03 meV and 43.82 meV, respectively. Among them, the non-radiative recombination center with an activation energy of 92.03 meV has a higher density and plays a major role. From the calculation results, the type and density of non-radiative recombination centers of sample A are the lowest, so its IQE should be the highest, which is consistent with the above-mentioned analysis. The non-radiative recombination center with activation energy in sample A of 41.8 meV, the non-radiative recombination center with activation energy of 43 meV in sample B, and the non-radiative recombination center with activation energy of 43.82 meV in sample C have very close activation energy, so they may have the same source. It is reported [17,28–30] that this type of non-radiative recombination center originates from dislocation. We found that as the thickness of the LT-Cap layer increases, the density of the non-radiative recombination center first increases and then decreases. It is reported that the non-radiative recombination center with activation energy of 1.06 meV in sample A and the non-radiative center with activation energy of 7.66 meV in sample B may be from the same source. We found that as the thickness of the LT-Cap layer increases, the density of this type of non-radiative recombination center gradually decreases, and finally the non-radiative recombination center disappears. We also found that as the thickness of the LT-Cap layer increases, a new type of non-radiative recombination center will appear. That is, the activation energy of a non-radiative recombination center in sample B is 96.47meV, and the activation energy of another in sample C is 92.03meV. Their sources should be the same, and their density gradually increases with increasing LT- Cap layer. Combined with the IQE of the samples at room temperature, we believe that the non-radiative recombination centers formed during the thin LT-Cap layer growth has an important influence on the luminescence of the material.
Microscopy PL imaging of samples A, B, and C was performed at room-temperature, and the results are shown in Fig. 7. It can be seen that there are a little small dark spots with diameter up to 2.63 nm in the figure, corresponding to the non-radiative recombination centers in QWs sample A which has the thinnest LT-cap layer thickness. However, more and bigger dark areas are observed in Figs. 3(b) and 3(c). From the above-mentioned analysis, we know that the non-radiative recombination centers of sample A are mainly caused by dislocations, so the black areas in sample A are mainly caused by dislocations. The black areas of sample B and sample C are caused by multiple factors. Among the possible sources, the large-size black area of sample B is most likely to be derived from the non-radiative recombination center with activation energy of 26.47meV, and the large-size black area of sample C is most likely to be derived from the non-radiative recombination center with activation energy of 92.03meV.
4. Result
In this work, the effects of the thickness of LT-Cap layer on the structure and luminescence characteristics of InGaN/GaN MQW have been studied. The results show that the thickness variation of LT-Cap layers seem not to have substantial impact on the structure of MQWs, i.e., the well layer thickness, but strongly affect the indium composition of well layers. We found that LT-Cap layer can effectively weaken the decomposition of InGaN. However, the increase of LT-Cap layer thickness will lead to the enhancement of polarization effect. In addition, the thickness variation of LT-Cap layers also affects the luminescence characteristics of MQWs. We have found that with the increase of LT-cap layer thickness, the material IQE gradually decreases, which may be caused by the introduction of new non-radiative composite centers.
Funding
National Natural Science Foundation of China (61474142, 61974162).
Acknowledgments
The authors acknowledge the support from the National Natural Science Foundation of China (Grant Nos.61474142 and 61974162).
Disclosures
The authors declare no conflicts of interest.
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