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A series of samples with varying growth pressure are grown and their optical and structural properties are investigated. It is found that the residual carbon concentration decreases when the reactor pressure increases from 80 to 450 Torr during the InGaN/GaN multiple quantum well growth. It results in an enhanced peak intensity of electroluminescence because carbon impurities can induce deep energy levels and act as non-radiative recombination centers in InGaN layers.
© 2016 Optical Society of America
1. Introduction
GaN based III-V compound materials and their metal organic chemical vapor deposition (MOCVD) growth technology have attracted a great attention for their successful application in light emitting devices [1–3]. In the development of these lighting devices, fabrication of high quality InGaN/GaN multiple quantum wells (MQWs) is a key requirement. Although much research attention has focused on the growth techniques for InGaN based MQWs and contributed to the improvement of InGaN/GaN MQW performance [4–6], this technology still has a significant problem related to material defects. To date, the effect of threading dislocations (TDs) on the optical and electrical properties has been widely studied, which can reduce the radiative efficiency by forming deep energy levels in the band gap and cause non-radiative recombination. However, the effect related to point defects, especially the carbon impurities in InGaN material, is still not completely understood. It is well known that carbon is an important unintentionally doped impurity in GaN films grown by MOCVD which uses C-contained metalorganic compounds as precursors. Carbon related point defects in GaN layers may take different forms and therefore may play different roles in GaN based devices. In past few years, the variation of residual carbon concentration with the change of growth conditions and the effects of carbon on performance of un-doped, Mg-doped GaN films and GaN based photodetectors have been studied by our group [7–9]. In this work, we focus on the influence of growth pressure and residual carbon impurities on optical and structural properties of InGaN/GaN MQWs, and find that reducing the unintentionally doped carbon concentration in InGaN/GaN MQWs can benefit for improving the output power of InGaN/GaN MQW lighting devices.
2. Experiments
Ten light emitting diode (LED) samples with varying growth pressure of InGaN/GaN MQWs were grown on (0001) sapphire substrates by an AIXTRON 3 × 2 in. close-coupled showerhead reactor. The structure of samples is consisted of a 2-μm thick Si-doped n-type GaN layer (n = 3 × 1018 cm−3) grown at 1040 °C, an unintentionally doped InGaN/GaN MQW active region, and a 140-nm thick Mg-doped p-type GaN layer. The MQW contains three periods of 2.5 nm thick InGaN wells and 7.5 nm thick GaN barriers. Both of the InGaN well and GaN barrier layers were grown at temperature of 720 °C. The growth pressure of MQWs were different for 10 samples. It was 80, 150, 270, 300, 350, 400, 450, 500, 550, 600 Torr, respectively. Due to the stronger pre-reaction between TEGa and NH3 at higher growth pressure, the growth rate of InGaN layer was slightly reduced with increasing the growth pressure (It has been verified by the experiments of growing bulk InGaN epilayers), thus the TEGa flow rate was intentionally increased slightly to make the growth rate of InGaN QW and GaN QB layers keep nearly constant at 0.013 and 0.023 nm/s, respectively, for all samples. Then the LEDs were processed into 300 μm × 300 μm mesas through etching down to the n-type GaN by ion beam etching technique. The Ni/Au (5 nm/5 nm) semitransparent Ohmic contact layers were deposited on p-GaN layer by electron beam evaporation, followed by rapid thermal annealing at 500 °C in a forming gas of N2: O2 (2: 1) for 5 min. Finally Ti/Al/Ti/Au (15 nm/250 nm/50 nm/250 nm) layers were deposited on both Ni/Au semitransparent film and n-type GaN to form p-type and n-type electrode pads. For the electroluminescence (EL) measurements, the injection current was ranged from 1 to 300 mA, and the current source was operated in a pulsed mode, with a 0.1% duty cycle. The optical output power of the samples was measured by a calibrated silicon detector. The crystal quality of InGaN/GaN MQW samples was characterized by full width at half maximum (FWHM) of double-crystal X-ray diffraction (XRD) ω-scan rocking curves. The impurity concentration was checked by secondary ion mass spectroscopy (SIMS) measurements.
3. Results and discussion
After the epilayer growth, room temperature (RT) on-wafer EL spectra of all 10 samples are measured firstly. The RT EL spectra of four samples with the growth pressure of 80, 150, 270, 400 Torr, named as samples A, B, C and D respectively, are shown in the inset of Fig. 1. The peak wavelength and intensity of all 10 samples in the series of samples grown under varying pressure are obtained by using Gaussian fitting to the EL spectra. Their dependences on growth pressure are shown in Fig. 1. It is found that the peak wavelength always increases with increasing reactor pressure. This may be mainly attributed to an increase of indium content in InGaN/GaN MQWs when the reactor pressure increases [10, 11]. On the other hand, the peak intensity shows a different behavior of pressure dependence. It increases strongly when the reactor pressure increases from 80 to 450 Torr, and then decreases slightly when the growth pressure increases further. It indicates that a suitably high reactor pressure (400-450 Torr) during InGaN/GaN MQW growth may benefit for improving the performance of LEDs.
Fig. 1 Wavelength and intensity of RT EL peak, each as a function of growth pressure. The inset is the EL spectra of samples A, B, C and D
To investigate the phenomenon that EL intensity increases when the growth pressure increases from 80 to 450 Torr, the EL measurement on chips of samples A, B and D under different injection currents is done. Figures 2(a) and (b) shows the EL spectra of samples A and D under pulsed injection currents with a small 0.1% duty cycle, where the thermal effect is weak. Figures 2 (c) and (d) shows the variations of EL peak wavelength and FWHM of samples A, B and D with the increase of the injection current, which are obtained from EL spectra measured under different injection currents. It is noticed that the peak wavelength shifts toward shorter wavelength side (a blue-shift) and FWHM increases with increasing injection current. It is also shown that a larger blue-shift is observed for the sample with a higher growth pressure. As is known, two possible mechanisms may be responsible for a blue-shift of EL peak energy when the injection current increases, i.e., the Coulomb screening of the polarization field and the state-filling effect in InGaN QWs. However, their influences on FWHM of EL spectra are very different [3]. The Coulomb screening of the polarization field causes the reduction of EL FWHM, while FWHM increases when the state-filling effect in InGaN QWs dominates. Therefore, a monotonic increase of EL FWHM for samples A, B and D indicates that the blue-shift of EL peak wavelength is mainly caused by state-filling effect in InGaN QWs when the injection current increases. A smaller blue-shift for sample A compared with samples B and D thus may suggest a less state filling effect in sample A, i.e. the carrier concentration increases slower in InGaN QWs of sample A than in samples B and D when the injection current increases. It means that the consumption rate of injected carriers is larger in sample A.
Fig. 2 Room-temperature EL spectra of samples A (a) and D (b) under various injection currents. The peak position at 10 and 200 mA is indicated by two vertical dash lines. Figures 2(c) and (d) show the peak wavelength and FWHM of samples A, B and D with the increase of injection current, which are derived from the EL spectra measured under different injection currents.
It is known that the carriers recombine mainly through two ways in MQW region, i.e., radiative recombination and non-radiative recombination. In fact, EL intensity of sample A is lower than those of other two samples, indicating that the number of carriers consumed by radiative recombination in sample A is lower and the non-radiative recombination in it is more serious. Therefore, a slower increase of carrier concentration with the increase of injection current in sample A implies that there is a larger defect density in its MQW region.
The output power and external quantum efficiency (EQE) are also derived from the EL measurement made under different injection currents. Figure 3 shows the output power and EQE each as a function of injection current. It can be seen from Fig. 3(a) that sample D grown at the highest pressure of 400 Torr has the highest emission efficiency, while sample A always exhibits the weakest EL intensity at all injection currents within all samples. For example, the emission efficiency of sample D is three times higher than that of sample A when the injection current is 200 mA. Figure 3(b) displays the normalized EQE of three samples A, B, and D. The peak efficiency shifts toward higher current and the droop effect is apparently mitigated with decreasing growth pressure of the InGaN/GaN MQWs, i.e., for samples A, B and D, the maximum EQE occurs at 50 mA, 50 mA, and 220 mA, and EQE is declined by 19%, 15%, and 0% from the onset of EQE droop to a high current value of I = 200 mA, respectively. As is well known, efficiency droop at high injection current is mainly caused by Auger recombination and carrier leakage [12, 13]. In fact, both of these mechanisms are related to carrier concentration in the MQW region. When the injection current is the same for all of these samples, if the carrier concentration in InGaN QWs of sample A is relatively smaller, the Auger recombination and carrier leakage will be smaller. It will lead to a weaker efficiency droop behavior for LED, as what is really shown by the droop curve of sample A in Fig. 3.
Fig. 3 EL output power (a) and EQE (b) of samples A, B and D under pulsed injection current. The inset of Fig. 3 (b) is also EQE spectra, but its horizontal axis changes from injection current to carrier concentration.
In order to exam the argument that the weak efficiency droop of sample A is due to the less carrier concentration in its InGaN QWs, the horizontal axis in the inset of Fig. 3 is changed from the injection current to carrier concentration in InGaN QWs according to the equation of [14], where P and n represent the EL output power and carrier concentration in MQW region, respectively. Just like what we expect, the droop of EQE for sample A is actually larger than that of other samples when the value of the carrier concentration is the same. It indicates that the smaller efficiency droop of sample A under the same injection current is due to the lower carrier concentration in its MQW. Combining with the fact that its output power is lower, we are aware that the lower carrier concentration in the MQW of sample A should be attributed to the larger defect density in it, and which leads to a faster non-radiative recombination rate.
It is known that the non-radiative recombination is largely related to the quality of InGaN/GaN MQW region. Therefore, to investigate the reason for the change of optical properties when the growth pressure increases from 80 to 450 Torr, the structural characteristics of samples grown with different reactor pressures must be studied. The threading dislocations (TDs) i.e., the screw and edge dislocations are the usual defects in GaN based material with a high density, and their density can be checked by the FWHM of X-ray diffraction (XRD) rocking curves [15]. Therefore, XRD rocking curves at (002) and (102) reflections for samples A, B, C and D are measured. It is found that all the rocking curves of these four samples are similar to each other, thus only the curves of sample A are shown here (Fig. 4). The FWHMs for samples A, B, C, and D are listed in Table 1. The FWHMs of the (002) and (102) reflections in these LED samples are approximately the same. It implies that the densities of threading dislocations (TDs) in these samples are nearly the same. This result indicates that the difference in TDs should not be the reason for the deterioration of EL output power for samples grown at low reactor pressure.
Table 1. FWHM of (002) and (102), and carbon, oxygen, hydrogen and silicon concentration for samples A, B, C and D.
Apart from the TDs, point defects such as impurities also can induce deep energy levels and deteriorate the optical properties of GaN based materials [16]. In addition, the incorporation rate of impurities largely affects by the growth conditions [7]. Therefore, depth profiles of carbon, oxygen, hydrogen and silicon for samples A, B and D are checked by SIMS and the results are shown in Fig. 5. The average impurity concentrations in InGaN/GaN MQWs are listed in Table 1. It can be seen that the silicon, oxygen and hydrogen concentrations in the MQW region of these samples are nearly the same, but the carbon concentration decreases from 1 × 1017 to 4 × 1016 cm−3 when the growth pressure increases from 80 to 400 Torr. It is well known that carbon is an important unintentionally doped impurity in GaN films grown by MOCVD, and the related point defects may take different forms [17,18], such as substitutional defects (CN, CGa), interstitial defects (Ci), or some species of complexes. They can induce energy levels in the near-mid-bandgap and act as non-radiative recombination centers to quench the band-edge luminescence in GaN films [19]. It is reasonable to expect the similar trends and effects for InGaN QW films. Thus, compared with other samples, a higher carbon concentration in sample A grown with lower reactor pressure may be responsible for the lower emission efficiency. Based on the experimental results and discussions in this work, it is concluded that increasing the growth pressure of InGaN/GaN MQW region should be benefit for improving the material quality and therefore enhancing the output power of InGaN based LEDs.
Fig. 5 Depth profiles of carbon, hydrogen, oxygen and silicon concentrations for samples A, B and D measured by SIMS. The MQW region of these samples is marked by pink line segment.
However, we must keep in mind that it the growth pressure of InGaN/GaN MQWs is not the higher the better. As shown in Fig. 1, the EL intensity starts to decrease slightly when the growth pressure increases higher than 450 Torr. This may be attributed to the fact that the adatom diffusion length on the MOCVD growing surface is reduced as the growth pressure increases [20, 21], it may result in a variation of growth mode from the two dimensional to three dimensional growth (AFM results are not shown here), and hence the interfaces in the MQWs grown at high pressure may become rough [22]. It is in turn to deteriorate the optical properties of InGaN/GaN MQWs when the growth pressure is too high even though the carbon concentration may be lowered in these samples.
4. Conclusion
A series of samples are grown with different growth pressures and their optical and structural properties are investigated. It is found that the peak intensity increases strongly and the peak wavelength redshifts when the reactor pressure increases from 80 to 450 Torr. Combining with the results of XRD, SIMS, and EL measured under different injection currents, we are aware that an enhanced EL peak intensity of samples grown with a relatively high reactor pressure may be attributed to the lower non-radiative recombination rate due to the lower residual carbon concentration in InGaN/GaN MQW region.
Acknowledgments
The authors acknowledge the support from the National Natural Science Foundation of China (Grant Nos. 61574135, 61574134, 61474142, 61474110, 61377020, 61376089, 61223005, and 61321063), One Hundred Person Project of the Chinese Academy of Sciences, and Basic Research Project of Jiangsu Province (Grant No.BK20130362).
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