Staggered AlGaN quantum wells (QWs) are designed to enhance the transverse-electric (TE) polarized optical emission in deep ultraviolet (DUV) light- emitting diodes (LED). The optical polarization properties of the conventional and staggered AlGaN QWs are investigated by a theoretical model based on the k·p method as well as polarized photoluminescence (PL) measurements. Based on an analysis of the valence subbands and momentum matrix elements, it is found that AlGaN QWs with step-function-like Al content in QWs offers much stronger TE polarized emission in comparison to that from conventional AlGaN QWs. Experimental results show that the degree of the PL polarization at room temperature can be enhanced from 20.8% of conventional AlGaN QWs to 40.2% of staggered AlGaN QWs grown by MOCVD, which is in good agreement with the theoretical simulation. It suggests that polarization band engineering via staggered AlGaN QWs can be well applied in high efficiency AlGaN-based DUV LEDs.
© 2016 Optical Society of America
AlGaN-based light-emitting diodes (LED) with high Al content can emit deep ultraviolet (DUV) light for potential applications in sterilization, medicine, and biochemistry [1, 2]. Compared with conventional mercury vapor lamps, AlGaN-based solid-state UV sources have significant advantages in terms of size, operation voltage, spectral tenability, and being environmentally friendly. Despite tremendous research efforts with certain progress, AlGaN DUV-LEDs still suffer from relatively low external quantum efficiencies and consequently low output light power, especially when lowering down the emission wavelength from 365 nm to 210 nm [3,4]. There are a number of factors limiting the performance of AlGaN-based DUV LEDs, such as high defect density in materials with high aluminum contents, low hole density in p-AlGaN alloys caused by high activation energy of the Mg acceptors, and constrained efficiency of light extraction from the LED dice [5, 6]. The last one is related to the dominating emission of transverse-magnetic (TM) polarized light in c-plane AlGaN alloys and relevant quantum well structure with increasing Al content. The TM polarized light is emitted primarily parallel to the LEDs interfaces and so that difficult to extract from the c-plane surface, resulting in a significant reduction of the surface emission. It is therefore very important to obtain AlGaN-based DUV LEDs with dominant TE polarized emission.
It has been reported that strong transverse-electric (TE) polarized emission can be obtained by large in-plane compressive strain or strong quantum confinement for c-plane AlGaN-based quantum wells (QWs) [7, 8]. Furthermore, the use of semipolar and nonpolar substrates are also beneficial to improve the surface emission properties of AlGaN QWs [9, 10]. However, change of quantum confinement (decreasing well width) or growth orientation will lead to crystal quality declining of the AlGaN layers. Many specific designs of nonconventional QWs have hence been proposed, including GaN delta insertion layer , staggered QWs , and triangular QWs , etc. In particular, the changes of the valence subband structure (including energy level position and subbands coupling relation) induced by the variation of the quantum confinement and polarization field are believed to promote the surface emission from staggered QWs. In addition, staggered QWs have an advantage that the growth process of this structure is identical to the conventional ones, so the crystal quality of the QWs will not deteriorate. Therefore, staggered QWs are promising for improving the emission light polarization from AlGaN- based DUV LEDs.
The concept of the staggered structure was first proposed in InGaN QWs by Arif et al. , then various similar designs have been recommended [15–18], most of which are to reduce the strong polarization-induced electrostatic field and further enhance the internal quantum efficiency, but neither much experimental results nor serious theoretical simulations for the polarization properties have been reported. In this letter, we first present a design to enhance the TE polarized emission by using a specific staggered QWs structure with step-function-like Al content profile in the QW layer. The total well width and emission wavelength of the designed staggered AlGaN QWs are equal to that of the conventional AlGaN QWs with peak position at about 285 nm. Then we employ the MOCVD technique to grow this designed staggered QWs, and the experimental results show super agreement with the theoretical prediction that the TE polarized emission from the staggered QWs can be enhanced with the achieved degree of polarization increasing from 20.8% to 40.2%.
2. Theoretical simulation
To design the staggered QWs, 6 × 6 block-diagonalized Hamiltonian presented by Chuang and Chang for c-oriented wurtzite III-nitride based on k·p method are modified by considering strain effect, subband coupling and polarization effects [19,20]. The effective-mass equation is then calculated by using finite-difference method to get the subband structure. The parameters for wurtzite GaN and AlN are taken from reference , while the AlGaN parameters are obtained by linear interpolation between GaN and AlN except the energy gap for which a bowing parameter b = 0.98 eV is applied . Finally, the spontaneous emission rate per energy interval per unit volume are given as 
Since the valence band structure determines the hole population in different subbands, it is thus essential to investigate the valence band structure for both conventional and staggered QWs, for the sake of comparison. Generally, as the heavy hole subband (HH) and light hole subband (LH) have the component and the crystal-field split off hole subband (CH) has the component, the transitions from the first conduction subband (C1) to the first HH subband (C1-HH1) and first LH subband (C1-LH1) are mainly TE polarized, while C1 to the first CH subband (C1-CH1) is almost entirely TM polarized. Furthermore, the subband coupling relation at k ≠ 0 would also change the proportion of the components. The energy level of the three valence subbands and subband coupling are thus both primary factors influencing the polarization properties. Figure 1 shows the calculated valence subband structure of a conventional Al0.37Ga0.63N/ Al0.55Ga0.45N QWs with total well width of 3 nm and a 3 nm staggered Al0.33Ga0.67N- Al0.45Ga0.55N/Al0.55Ga0.45N QWs with each part of two compositions in the well being equally about 1.5 nm. For the conventional QWs, the HH1 and LH1 valence subbands are above CH1. Although the energy order is not changed when using the staggered QWs, but the energy spacing increased at the Brillouin zone center because of stronger quantum confinement, and so that CH1 is further separated from HH1 and LH1, and at k≠0 the subband coupling relation is also varied. All these correspond to the spectacular optical polarization properties of the staggered Al0.33Ga0.67N- Al0.45Ga0.55N /Al0.55Ga0.45N QWs.
To further analyze the optical properties of the staggered QWs, the momentum matrix elements for the conduction band and the three top valence subbands transitions are calculated, as shown in Fig. 2. As indicated by the solid black lines that the dominant TE component comes from not only the C1-HH1 and C1-LH1 transitions, but also C1-CH1 transitions, especially when the momentum is away from the Brillouin zone center. While the main TM polarization component shown by red dashed lines originates also from both C1-HH1 and C1-LH1 transitions away from the zone center together with the C1-CH1 transition. Compared to the conventional QWs, the momentum matrix elements for the staggered QWs change not only at Brillouin zone center, but also away from kt = 0. For C1-HH1 and C1-LH1 transition, the MTE increases and the MTM decreases from kt = 0 to kt = 1.Considering that momentum matrix elements directly determine the transition rate from conduction to valence subbands, the increase of MTE would enhance the probability of the TE polarized emission light and the decrease of MTM would reduce the component of the TM polarization. Although C1-CH1 transition shows an inverse trend, whose contribution to the optical transition is much less important as the CH1 energy subband is much lower than that of convention QWs, less holes would populate onto this energy subband, so the increase of MTM and decrease of MTE in the CH1 energy subband play a much less important role.
Based on the specific valence subbands and momentum matrix elements of the staggered QWs, the spontaneous emission rates and related proportion of TE and TM polarization contributed by the transitions from conduction subband C1 to HH1, LH1 and CH1 subbands for conventional Al0.37Ga0.63N/Al0.55Ga0.45N QWs and staggered Al0.33Ga0.67N-Al0.45Ga0.55N/ Al0.55Ga0.45N QWs could be calculated using Eq. (1), and the results are shown in Fig. 3. For the staggered QWs, the TE polarized spontaneous emission associated with HH1 and LH1 increases and the TM polarized spontaneous emission is suppressed. Although the CH1 exhibits an inverse pattern, the overall effect of the staggered QW on polarization properties is an enhancement of the TE-polarized emission and a suppression of the TM-polarized emission. The simulated spontaneous emission spectra are shown in the inset of Fig. 3. If the degree of polarization (DOP) is defined as, whereand are the integrated PL intensity for TE and TM polarized component, respectively, then the DOP is calculated up to 44.7% for the staggered QW, while that of the conventional one is only 21.4%.
Experiments have been conducted to compare the optical polarization properties of the designed staggered Al0.33Ga0.67N-Al0.45Ga0.55N/Al0.55Ga0.45N QWs with that of conventional Al0.37Ga0.63N /Al0.55Ga0.45N QWs. Both the conventional and staggered QWs samples were grown by metal organic chemical vapor deposition (MOCVD). Deposition was initiated from a 2.2-μm-thick AlN layer, followed by a 20-periods AlN/AlGaN superlattice layer and a 2-μm-thick Si-doped Al0.55Ga0.45N layer. Then the QWs were grown on the Al0.55Ga0.45N buffer layer with barrier width about 10 nm and QW periods about ten. Finally, the structure was completed with a 25-nm-thick Al0.55Ga0.45N barrier layer. The composition and growth rate of the AlGaN alloy were calibrated individually for a particular composition using high-resolution x-ray diffraction measurements. The staggered AlGaN QWs structure was then realized by controlling the growth conditions (gas flows, V/III ratio, growth rate and duration, temperature, pressure) according to the calibration. In order to confirm the success growth of the staggered QWs structure, transmission electron microscope energy dispersive spectroscopy (TEM-EDS) line scan was performed to provide high-accuracy and high- space-resolution Al or Ga composition information. Figure 4 shows the cross-sectional morphology and qualitative distribution of Al, Ga, and N elements in the selected area (red line with the arrow) for our staggered QWs. Evidently, the sharp interfaces between the well layer and the barrier can be seen and the Al and Ga composition distribution exhibit step shapes.
Room temperature polarization photoluminescence (PL) measurements were performed using a 4th harmonic of Q-switched YAG:Nd laser with wavelength 266nm and pulse width 7 ns as the excitation light source. The in-plane emitted light was collected with a lens and analyzed by a Glan-Taylor prism, and then focused by a second lens into the spectrometer. The entire setup was carefully calibrated using an unpolarized light source with a second polarizer. The sample's coordinate diagram is as follows: X-axis and Y-axis are in the c-plane, with Z-axis perpendicular to c-plane, and the polarized angle of 0 degree is defined to be parallel to the c axis (E//C, TM), while 90 degree is perpendicular to the c axis (E⊥C,TE).
The room temperature polarized PL results of the conventional Al0.37Ga0.63N/ /Al0.55Ga0.45N QWs and staggered Al0.33Ga0.67N-Al0.45Ga0.55N/ Al0.55Ga0.45N QWs samples are shown in Fig. 5. The staggered QWs are designed to have the same peak wavelength emission at about 285 nm with the conventional QWs by modifying the well width and Al content of each step layer. The integrated intensity distribution of the edge emission from the conventional and staggered QWs samples against the prism angle is also depicted in the inset of Fig. 5. It is clear that both of the two samples exhibit dominant TE polarized light emission, but for the staggered QWs, the relative TE emission is obviously stronger than that of the conventional QWs while the TM emission is much lower, so the obtained DOP of the staggered QWs increases to 40.2%, nearly twice of that of conventional QWs (20.8%). In comparison with the above theoretical simulation results (staggered about 44.7% and conventional about 21.4%), our experimental data show great agreement with the theory calculation.
In summary, the optical polarization properties of staggered AlGaN QWs structure have been investigated using a theoretical model based on the k·p method. In order to realize the same wavelength, the Al content and well width were modified in the staggered QWs compared to the conventional QWs. Because of the changes of the valence subband structure including energy level and subband coupling relation, and hence the momentum matrix elements, the intensity of the surface emission (i.e. the TE mode) is predicated to increase. Along with the theoretical prediction, polarized PL experiments were carried out to verify the simulation results. As we expected, the staggered QWs exhibit stronger TE polarized emission and much smaller TM polarized emission. The DOP of the staggered QW at room temperature increased to 40.2% from 20.8% of the conventional QWs. Therefore, it is reasonable to conclude that staggered QWs structure can be applied to enhance the surface emission and further realize high EQE of AlGaN-based DUV LEDs.
This work was supported by the National Basic Research Program of China (Nos. 2012CB619306, 2012CB921304, and 2013CB921901), the National Natural Science Foundation of China (Nos. 61504005, 61376095,61522401, 61574006,61521004, 11174008, and 61361166007), the National High-Tech Research and Development Program of China (No. 2014AA032606), and the Beijing Higher Education Young Elite Teacher Project (No. YETP0006).
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