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Four blue-violet light emitting InGaN/GaN multiple quantum well (MQW) structures with different well widths are grown by metal–organic chemical vapor deposition. The carrier localization effect in these samples is investigated mainly by temperature-dependent photoluminescence measurements. It is found that the localization effect is enhanced as the well width increases from 1.8 nm to 3.6 nm in our experiments. The temperature induced PL peak blueshift and linewidth variation increase with increasing well width, implying that a greater amplitude of potential fluctuation as well as more localization states exist in wider wells. In addition, it is noted that the broadening of the PL spectra always occurs mainly on the low-energy side of the PL spectra due to the temperature-induced band-gap shrinkage, while in the case of the widest well, a large extension of the spectral curve also occurs in the high energy sides due to the existence of more shallow localized centers.
InGaN/GaN multiple quantum wells (MQWs) have been extensively used as active regions in high luminescent efficiency light emitting diode (LED) and laser diode (LD) devices.[1–7] Despite the high dislocation densities (
In this work, the dependence of the localization effect on the well width is mainly investigated by the aid of temperature-dependent photoluminescence (PL) measurements. The parameters obtained from the temperature-dependent PL spectra including the peak intensity, the full width at half maximum (FWHM), and the shape are analyzed in detail. In order to study the influence of the InGaN QW width, four InGaN/GaN MQW samples with varying well thickness are grown by using a metal–organic chemical vapor deposition (MOCVD) system. It is found that the potential fluctuation is increased with increasing well thickness. Greater amplitudes of potential fluctuations as well as more localization states exist in wider wells. It is also found that the PL peak intensity has a non-monotonous variation as the well width increases, which is believed due to the comprehensive effects of the localization effect, QCSE, the influence of the material quality of the InGaN layers, and the laser-absorbing active region volume.
The four InGaN/GaN MQW samples with different well widths studied in this work were grown in an AIXTRON close-coupled showerhead 3×2 in low-pressure vertical reactor MOCVD system on c-plane sapphire substrates spinning at 100 rpm. Trimethylindium, trimethylaluminum, and ammonia (NH
Temperature-dependent PL measurements were carried out with the 325-nm line of a He–Cd laser at an excitation density of 0.8 W/cm2 to investigate the localization effect, and the temperature was controlled to change from 30 K to 300 K using a closed-cycle refrigerator of CTI Cryogenics. The detected PL spectral lines were fitted by Gaussian functions to eliminate the influence induced by the Fabry–Perot interference fringes for analyzing the accurate peak energy and FWHM.
In fact, the investigation of the InGaN/GaN MQW structures with only three periods is mainly for the purpose of optimizing the QW growth of GaN based LDs. Unfortunately, because of the small QW number and the additional thick layer on the top of the active region, the intensity of the MQW layer peak signal in high resolution x-ray diffraction (HRXRD) is too low to measure,[18] and the indium mole fraction and the InGaN QW width cannot be exactly determined for these four samples. To obtain the InGaN well layer growth rate, an extra 37.5 nm-thick InGaN layer was grown on a GaN template using the same growth conditions with the InGaN QW of these samples, and taken as a reference sample. The thickness of the reference sample was controlled by an in situ monitoring system installed in the MOCVD system (data not shown here). The growth time of the 37.5 nm-thick reference sample was recorded as 2500 s and thus the average growth rate of QW was calculated to be 0.015 nm/s. According to the growth rate, the estimated QW width of samples S120, S160, S200, and S240 is around 1.8 nm, 2.4 nm, 3 nm, and 3.6 nm, respectively.
The PL spectra of these four samples measured at room temperature are plotted in Fig.
![]() | Fig. 1. (color online) The room temperature PL spectra of the InGaN/GaN MQW samples S120, S160, S200, and S240. |
![]() | Table 1.
The optical parameters extracted from the PL spectra of all samples, including peak energy, peak intensity, and FWHM. . |
The PL spectra of the InGaN/GaN MQW structures of the four samples are measured under the same conditions when the temperature increases from 30 K to 300 K. The internal quantum efficiency (IQE) is often obtained through temperature-dependent PL measurement,[20] and here is defined as the ratio of the integrated PL intensity at 300 K and 30 K (data not shown here). The IQE is calculated as 1.5%, 7.2%, 9.5%, and 3.8% for samples S120, S160, S200, and S240, respectively, which is in accordance with their PL intensity. The PL peak energy as a function of temperature is depicted in Fig.
![]() | Fig. 2. (color online) The PL peak energy as a function of temperature for samples (a) S120, (b) S160, (c) S200, and (d) S240. The red solid lines are the fitting curves according to Eq. ( |
The band-tail model is always employed to investigate the localization degree.[24] In InGaN QWs, band tail states can appear in an electron energy diagram caused by the disorders in InGaN/GaN QWs, introduced by either strong compositional non-uniformity of InGaN or QW thickness fluctuations. The tail states can provide the energy levels lower than the nominal band edge to confine the localized carriers. If a Gaussian-like distribution of the density of band tail states for the conduction and valence bands is assumed, the temperature-dependent emission energy could be described by the following expression:[13]
![]() | (1) |
![](cpb_26_1_017805/cpb_26_1_017805_eqn006.gif)
To further study the relation of potential fluctuations with the well width, the temperature-dependent FWHM of all samples is measured as demonstrated in Fig.
To analyze the broadening behavior of the PL spectra with increasing temperature and clearly check the variation of the spectral shape, the intensity of the temperature-dependent PL spectra of each sample is normalized and depicted in Fig.
Based on the above discussion about the localization states, the PL spectra of these four samples can be better understood. As the well width increases, the number of photo-generated carriers increases due to increasing laser-absorbing active region volume. Also, the carrier localization effect is enhanced, so the carriers can be better confined in the localized states and form bound excitons, suppressing the possible nonradiative recombination at dislocations. Therefore, the PL intensity is increased when the well width increases from 1.8 nm to 3 nm. However, in spite of the increasing laser-absorbing active region volume and localization effect, the PL intensity is reduced after the well width reaches 3.6 nm. The reduced PL intensity may be attributed to the enhanced QCSE and the degraded material quality of the thicker InGaN layers.
In summary, the localization effect in blue-violet light emitting InGaN/GaN MQW structures with varying well width is investigated by temperature-dependent PL measurement. It is found that the carrier localization effect is enhanced with increasing well width from 1.8 nm to 3.6 nm in our experiments, which may be related to the increased thickness fluctuations of the InGaN well layers. By comparing the localization effect and the linewidth variation of the PL peak, the study further shows that a greater amplitude of potential fluctuation as well as more localization states exist in MQW with wider wells. As for the temperature-induced broadening of the PL spectra, it is found that the shift of the spectral curves always mainly occurs on the low-energy side of the PL spectra, while in the case of the widest well, an extension occurs also at the high-energy sides. As a conclusion, due to the comprehensive effects of the localization effect, QCSE, the influence of the material quality of InGaN layers, and the laser-absorbing active region volume, a moderate well width will be favorable in improving PL quality.
[1] | |
[2] | |
[3] | |
[4] | |
[5] | |
[6] | |
[7] | |
[8] | |
[9] | |
[10] | |
[11] | |
[12] | |
[13] | |
[14] | |
[15] | |
[16] | |
[17] | |
[18] | |
[19] | |
[20] | |
[21] | |
[22] | |
[23] | |
[24] | |
[25] |