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Efficiency-droop reduction in blue InGaN light-emitting diodes with low temperature p-type insertion layer
Accepted Manuscript Efficiency-Droop Reduction in Blue InGaN Light-Emitting Diodes with Low Temperature p-type Insertion Layer Jun Zhang, Xiang-Jing Zhuo, Dan-Wei Li, Zhi-Wei Ren, Han-Xiang Yi, JinHui Tong, Xing-Fu Wang, Xin Chen, Bi-Jun Zhao, Shu-Ti Li PII: DOI: Reference:
S0749-6036(14)00176-1 http://dx.doi.org/10.1016/j.spmi.2014.05.017 YSPMI 3271
To appear in:
Superlattices and Microstructures
Received Date: Revised Date: Accepted Date:
6 February 2014 28 March 2014 6 May 2014
Please cite this article as: J. Zhang, X-J. Zhuo, D-W. Li, Z-W. Ren, H-X. Yi, J-H. Tong, X-F. Wang, X. Chen, BJ. Zhao, S-T. Li, Efficiency-Droop Reduction in Blue InGaN Light-Emitting Diodes with Low Temperature p-type Insertion Layer, Superlattices and Microstructures (2014), doi: http://dx.doi.org/10.1016/j.spmi.2014.05.017
This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Efficiency-Droop Reduction in Blue InGaN Light-Emitting Diodes with Low Temperature p-type Insertion Layer Jun Zhang, Xiang-Jing Zhuo, Dan-Wei Li, Zhi-Wei Ren, Han-Xiang Yi, Jin-Hui Tong, Xing-Fu Wang, Xin Chen, Bi-Jun Zhao and Shu-Ti Li* Laboratory of Nano-photonic Functional Materials and Devices, Institute of Opto-electronic Materials and Technology, South China Normal University，Guangzhou 510631, P. R. China *Corresponding author, e-mail: [email protected] Abstract GaN based light emitting diodes with specially designed low temperature (LT) transition layers are investigated. Theoretical simulation shows that the LT-pGaN with LT-InGaN/GaN superlattice composite transition layer structure can effectively alleviate polarization field in the active layer and p-n junction region, consequently enhancing hole injection efficiency and electron blocking capability. Furthermore, the experimental results demonstrate that the low temperature (LT) transition layers can protect the active region from detrimental thermal annealing effect during high temperature p-GaN growth process. Finally, the reduced forward voltage and the enhanced output power about 42.2% were achieved at the injection current of 200mA.
Keywords: light-emitting diodes, low temperature transition layers, polarization field, efficiency droop 1. Introduction Recently, GaN-based light-emitting diodes (LEDs) have attracted much attention due to its employment in general illumination, back lighting, display, and other applications. In spite of significant improvements achieved, there still remain numerous technical challenges for the GaN-based LED to be competitive in terms of high-brightness and high-power applications [1], including the quantum confined Stark effect (QCSE) [2-4], current injection efficiency [5], electron leakage [6, 7], lack of holes injection [8, 9] and Auger recombination [10], et al. For conventional LED, the insertion of an AlGaN as an electron-blocking layer (EBL) between the active region and p-type holes injection layer to suppress electron leakage has been suggested [11]. However, the use of AlGaN EBL can cause some undesired effects. Due to lattice mismatch between the last GaN barrier and AlGaN EBL, the spontaneous and piezoelectric polarization fields go against for electron confinement and hole injection [12, 13]. Furthermore, AlGaN layer may also act as a potential barrier for hole transportation depending on the band-offset ratio of AlGaN/GaN hetero-structure, thus reducing the holes injection efficiency [14-16]. On the other side, the growth temperature for AlGaN EBL layer is much higher (almost 150℃) than the active layers, which would result in crystal quality deterioration in the active region due to the detrimental thermal annealing effect [17, 18]. Recently, several significant research works have been done in the suppression of polarization field induced QCSE and carriers leakage problems via staggered QWs [19], large overlap QWs [20], lattice-matched AlGaInN barriers [21], InAlN EBL [22], etc. In this paper, the new designed structure is proposed to alleviate the drawbacks mentioned above in two ways: On the one hand, from device structure, the introduction of low temperature p-GaN
can help reduce internal polarization effect, while the introduction of short period superlattice layers can further reduce the electron leakage level and improve hole injection efficiency; On the other hand, from growth technique, the adoption of low temperature p-GaN is mainly to protect crystal quality of active regions yet has bad influence on the following growth, while the adoption of the LT-InGaN/GaN superlattice layers is beneficial to ameliorate crystal quality of the typical p-GaN contact layer. 2. Experiment The structure of conventional LED (labeled structure A) was grown on a c-plane sapphire substrate, followed by a 2-μm-thick undoped GaN buffer layer and a 2-μm-thick Si-doped n-GaN layer. The active region consists of six pairs of 3nm undoped InGaN wells and 10nm GaN barriers. On top of the last QB are a 20nm-thick p-Al0.15GaN EBL and a 200nm thick p-GaN cap layer. Another LED epitaxial structure (labeled structure B) has the same structure except that a 25nm thick LT-pGaN transition layer is inserted between the last QB and EBL. The growth temperature of the interlayer is 760℃, the same as that of QW, and the hole concentration in the Mg doped LT p-GaN is 5×1017cm-3，which is the same as the typical p-GaN. To improve the p-GaN crystal quality and optical performance, we further designed a low temperature composite layer structure (labeled structure C) which composed of a 25nm thick LT-pGaN layer and six periods LT-InGaN/GaN superlattice with 1.5nm thick each layer without an AlGaN EBL. The growth parameters of the LT-pGaN/SL interlayer is almost same as sample B except the growth temperature of p-InGaN/GaN SL is 50℃ higher than the LT p-GaN with Mg doping level of 1×1018cm-3.The detailed structures of the three LEDs are shown in Fig.1. All samples were grown by metal-organic chemical vapor deposition (MOCVD) in a closely coupled showerhead (CCS). The device geometry was designed to be a rectangular shape of 300×300 μm2. LED chips were fabricated using a conventional mesa structure method. Indium tin oxide was used as a transparent conducting layer, and Cr/Au metal was deposited as p- type and n- type electrodes, respectively. 3. Results and discussion The optical and electrical properties of the LEDs were investigated numerically by using the APSYS simulation program [23]. Here, for all the simulation results, we assume the structure characteristics of the three samples, but the Mg activation rate in low temperature p-GaN is usually higher than that at normal temperature. [24] For that reason, we adequately enhance the parameter setting of hole concentration (6×1017cm-3) in the LT p-GaN than that in experiment situation. In the simulation, the light extraction efficiency is assumed to be 0.78, the Shockley- Read- Hall is estimated to be 100ns and the Auger Recombination coefficient is set to be 1.0×10-34 cm6 s-1 which is similar to the reported values. [25] The built- in interface charges due to spontaneous and piezoelectric polarization are calculated by the methods developed by Fiorentini et al. [26] 50% of the theoretical value is used to account for the compensation by fixed defects and other interface charges. [27] Fig. 2(a) shows the calculated electrostatic fields in the active region of the three samples. Strong piezoelectric polarization field was generated nearby the p-n junction region due to the large lattice mismatch between the last QB and AlGaN EBL, particularly
in the last quantum well, as can be seen in sample A. However, this situation gets improved after a 25nm LT-pGaN transition layer is inserted, which can take apart the interface between the last QB and EBL from space, and thus reducing the electrostatic fields shown in sample B. While for sample C, we adopted composite low temperature layers which consist of a LT-pGaN layer and six periods InGaN/GaN superlattice layers, the polarization effect in the active region is further diminished, especially in the last quantum well. Since the original AlGaN EBL is replaced by InGaN/GaN superlattice layers, the lattice mismatch between the MQWs and the following layers would be substantially reduced. Fig. 2(b) and (c) represent the calculated electrons and holes distributions in the active regions of the three samples. It can be seen that a large amount of electrons accumulates in the last QW next to p-type layer for sample A, which is caused by band bending effect due to the strong polarization field. Under the condition of insufficient hole concentration and hole injection efficiency, the overflowed electrons out of the QWs would have more opportunities recombine in the non-radiative recombination centers, therefore results in higher Auger recombination rate. By employing the LT p-GaN transition layer on the last GaN quantum barrier for sample B and C, the electron concentration in the last QW can be effectively decreased，meanwhile enhancing the electron concentration in the other QWs, leading to much more uniform electron distribution in the active regions. Furthermore, the LT insertion layer can lower the effective potential height for holes in the valence band, resulting in higher hole concentration in the active region. In this case, the electron and hole concentration can be better matched, leading to better radiative recombination. The electroluminescence (EL) spectra of the three LED samples, at the injection current of 20mA, are shown in Fig.3. Besides the sharp band-to-band peaks at around 460nm for the three samples, there still appear sub-band parasitic emissions around 320nm for sample A and B, which is probably due to the recombination of overflowed electrons with holes in the Al0.15GaN EBL. It is believed that the holes populating on the Mg-related deep acceptor levels, once recombine with electrons overflow into the cladding layer, would results in the parasitic peak [28]. The parasitic peak for sample B is smaller than the conventional one, indicates the LT p-GaN insert layer is helpful to reduce electrons leakage. While for sample C, because of better carrier limitation effect of the LT p-GaN with p-InGaN/GaN short periods superlattice layers, there is hardly parasitic emission observed, which probably attribute to the optimized energy band situation for the enhanced holes injection efficiency and suppressed electrons leakage. Fig.4 shows the EL peak wavelength and full width at half maximum (FWHM) as a function of forward injection current for the three samples. All the samples demonstrate a blue-shift in the main emission wavelength as the current increases. The smallest blue-shift of ~5.3nm is observed for sample C at 200mA, which is much less than that of sample A（~8.4nm）, sample B also gains an improved blue-shift of ~5.8nm compared with the conventional one. Generally, the peak wavelength blue-shift of LEDs may result from the Coulomb screening of piezoelectric polarization field induced QCSE and band filling effect. The blue shifts of the three samples vary widely under the same injection current, as can be seen from Fig.4. And the smaller blue-shift of sample B and C mainly attribute to the reduction of QCSE, which means the introduction of LT p-GaN insert layer can help alleviate polarization field in MQWs thus resulting in less wavelength shift. While the smallest blue-shift of sample C is mainly due to the extra introduction of low indium-content p-InGaN/GaN short period SL layers instead of
AlGaN EBL, thus alleviating the lattice mismatch between the MQW regions and EBL interface, which may also assist in the reduction of polarization field in the MQWs. It was reported that the band filling effect accompanies the increasing of FWHM [29]. We can observe from Fig.4 that the FWHM of all the samples tend to increase with increasing injection current. The FWHM at 200mA are increased by about 66%, 48.8%, 36% for sample A, B, and C, respectively, as compared to those at 1mA, which indicates that as the injection current increases, the FWHM broadening can be effectively improved by the introduction of LT insert layers. Since the reduction of QCSE of sample B and C will alleviate the band bending in the active region, which may assist in the suppression of band filling effect. Besides, the improved crystalline quality in the active region due to lower temperature growth in LT p-GaN layer also contribute to the smaller broadening of FWHM in sample B and C, and the limited strain energy of the inserted LT p-InGaN/GaN SL for sample C even enhances the crystalline quality of the whole wafer thus exhibiting the smallest FWHM broadening. LT-pGaN growth was in fact an annealing process for active layers, so the crystalline quality of the active region would be optimized by recrystallisation [30]. However, the LT-pGaN itself might suffer from severe crystalline quality deterioration due to improper growth temperature or doping level, which would bring bad effect on the following growth of AlGaN EBL and p-GaN thus degrading the device performance. The crystalline quality was analyzed by the High-resolution X-Ray Diffractometer (HRXRD). XRD omega/2-theta scans taken across (002) reflection for sample B and sample C are depicted in Fig. 5, the corresponding full width at half maximum (FWHM) of GaN (0002) rocking curves are 305 and 243 arcsec for sample B and sample C, respectively. And the substantially improved crystalline quality of sample C is mainly due to the limited strain energy of the inserted short period LT-InGaN/GaN superlattice layers. Besides, the diffraction pattern of structure C with low temperature superlattice layers shows more InGaN satellite peaks and sharper interface than the LT-pGaN sample. Hence, the LT p-GaN/SL interlayer can substantially ameliorate crystalline quality. The measured and simulated light-current (L-I) performances of the three samples with increasing current are shown in Fig. 6(a). The relative ideal simulated L-I curve demonstrate consistent increase with the experimental data. The light output power of sample B represents 15% increase at 200mA due to the reduced QCSE and improved hole injection efficiency. While for sample C, once the extra p-InGaN/GaN SL layers added on the LT p-GaN, the emission power of sample C exhibit 42.2% boost at 200mA compared to the conventional one. The forward voltage (Vf) at 20mA for LT-pGaN/SL LED is 3.25V, which is lower than that of 3.35V for conventional LED. Since the LT p-GaN/SL insert layer allow the alleviation of strain induced piezoelectric field and the reduction of QCSE in the active region, the carrier injection efficiency and recombination rate are enhanced. Furthermore, the substantial improved crystalline quality and reduced electron leakage level are mainly due to the new interlayer by the alternative p-InGaN/GaN short period SL layers instead AlGaN EBL, which may also play a key role in the emission power. Fig. 6(b) shows the relative external quantum efficiency (EQE) of the three samples as a function of injection current. Efficiency droop behaviors are demonstrated in all the samples. The maximum EQE values of the three samples coincide with the light output power performance. Sample C shows the highest EQE values in the measured range because of the largest EL emission power. In addition, sample C shows a reduced efficiency droop from 65% to 52% with the increasing injection current, indicating enhanced holes injection and electron
confinement. It has been reported that the efficiency droop is mainly due to the Auger recombination in the active MQWs, as well as holes recombine with the overflowed electrons outside the active region. Our experimental and simulation results also support that the reduced polarization field in the active region and the improved holes injection efficiency can reduce the carrier recombination outside the active region, therefore improving efficiency and suppressing droop effect. 4. Conclusions GaN-based LEDs with special designed low temperature transition layers (LT-pGaN/SL) was studied both numerically and experimentally. The simulated results show that the electrostatic fields in the active region, especially in the last QW near p-side, can be greatly relieved through the insertion of LT-pGaN/SL interlayer, and the reduction of QCSE is helpful to improve carrier distribution as well as enhance the hole injection efficiency. The simulated light output power ameliorates substantially owing to the improved carrier transportation and enhanced radiative recombination. The experimental results show that the crystalline quality of both active regions and typical p-GaN has been improved due to the reduced detrimental thermal annealing effect during low temperature growth and the insertion of InGaN/GaN superlattice layers. The output power of the sample with new designed interlayer is enhanced by 42.2% at the injection current of 200mA compared to the conventional LED, which can be attributed to the improved crystalline quality, ameliorated electron leakage level and enhanced hole-injection efficiency.
Acknowledgments This work was supported by the National Nature Science Foundation of China (Grant No. 51172079), the Science and Technology Program of Guangdong province, China (Grant Nos. 2010B090400456, and 2010A081002002), the Science and Technology Program of Guangzhou (Grant No. 2011J4300018)
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Highlights (1) LT-pGaN/SL is inserted to alleviate the polarization field in the active region.
(2) Crystalline quality of epitaxial wafer is improved when LT-pGaN/SL is adopted. (2) Experiment results show that the peak wavelength is more stable when LT-pGaN/SL is adopted. (3) Our results show that photoelectric characteristic is improved when LT-pGaN/SL is adopted. (4) Our results reveal that polarization field and hole injection efficiency are the main reasons for efficiency droop.
Figure caption:
Fig. 1. Schematic plot of the LEDs under study
Fig. 2(a). The electrostatic fields (b) Electrons and (c) holes concentrations of sample A, B, and C in the active region at 200 A/cm2. (There is a small location shift on horizontal axis for better observation.)
Fig. 3. EL spectra of the three LED samples at the injection current of 20mA. Fig. 4. Wavelength shifts and FWHMs of the EL spectra for the three LED samples as a function of the injection current. Fig. 5. XRD ω-2θ scans taken across the (0002) reflection for both samples. Fig.6. (a) Output power and (b) normalized EQE for the three LED samples as a function of the injection current.
Fig.1 Schematic plot of the LEDs under study.
Fig. 2(a) The electrostatic fields (b) Electrons and (c) holes concentrations of sample A, B, and C in the active region at 200 A/cm2. (There is a small location shift on horizontal axis for better observation.)
Fig. 3 EL spectra of the three samples at the injection current of 20mA.
Fig. 4 Wavelength shifts and FWHMs of the EL spectra for the three LED samples as a function if the injection current.
Fig. 5 XRD ω-2θ scans taken across the (0002) reflection for both samples.
Fig.6. (a) Output power and (b) normalized EQE for the three LED samples as a function of the injection current.
Highlights (1) LT-pGaN/SL is inserted to alleviate the polarization field in the active region. (2) Crystalline quality of epitaxial wafer is improved when LT-pGaN/SL is adopted. (2) Experiment results show that the peak wavelength is more stable when LT-pGaN/SL is
adopted.
(3) Our results show that photoelectric characteristic is improved when LT-pGaN/SL is adopted. (4) Our results reveal that polarization field and hole injection efficiency are the main reasons for efficiency droop.
S0749-6036(14)00176-1 http://dx.doi.org/10.1016/j.spmi.2014.05.017 YSPMI 3271
To appear in:
Superlattices and Microstructures
Received Date: Revised Date: Accepted Date:
6 February 2014 28 March 2014 6 May 2014
Please cite this article as: J. Zhang, X-J. Zhuo, D-W. Li, Z-W. Ren, H-X. Yi, J-H. Tong, X-F. Wang, X. Chen, BJ. Zhao, S-T. Li, Efficiency-Droop Reduction in Blue InGaN Light-Emitting Diodes with Low Temperature p-type Insertion Layer, Superlattices and Microstructures (2014), doi: http://dx.doi.org/10.1016/j.spmi.2014.05.017
This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Efficiency-Droop Reduction in Blue InGaN Light-Emitting Diodes with Low Temperature p-type Insertion Layer Jun Zhang, Xiang-Jing Zhuo, Dan-Wei Li, Zhi-Wei Ren, Han-Xiang Yi, Jin-Hui Tong, Xing-Fu Wang, Xin Chen, Bi-Jun Zhao and Shu-Ti Li* Laboratory of Nano-photonic Functional Materials and Devices, Institute of Opto-electronic Materials and Technology, South China Normal University，Guangzhou 510631, P. R. China *Corresponding author, e-mail: [email protected] Abstract GaN based light emitting diodes with specially designed low temperature (LT) transition layers are investigated. Theoretical simulation shows that the LT-pGaN with LT-InGaN/GaN superlattice composite transition layer structure can effectively alleviate polarization field in the active layer and p-n junction region, consequently enhancing hole injection efficiency and electron blocking capability. Furthermore, the experimental results demonstrate that the low temperature (LT) transition layers can protect the active region from detrimental thermal annealing effect during high temperature p-GaN growth process. Finally, the reduced forward voltage and the enhanced output power about 42.2% were achieved at the injection current of 200mA.
Keywords: light-emitting diodes, low temperature transition layers, polarization field, efficiency droop 1. Introduction Recently, GaN-based light-emitting diodes (LEDs) have attracted much attention due to its employment in general illumination, back lighting, display, and other applications. In spite of significant improvements achieved, there still remain numerous technical challenges for the GaN-based LED to be competitive in terms of high-brightness and high-power applications [1], including the quantum confined Stark effect (QCSE) [2-4], current injection efficiency [5], electron leakage [6, 7], lack of holes injection [8, 9] and Auger recombination [10], et al. For conventional LED, the insertion of an AlGaN as an electron-blocking layer (EBL) between the active region and p-type holes injection layer to suppress electron leakage has been suggested [11]. However, the use of AlGaN EBL can cause some undesired effects. Due to lattice mismatch between the last GaN barrier and AlGaN EBL, the spontaneous and piezoelectric polarization fields go against for electron confinement and hole injection [12, 13]. Furthermore, AlGaN layer may also act as a potential barrier for hole transportation depending on the band-offset ratio of AlGaN/GaN hetero-structure, thus reducing the holes injection efficiency [14-16]. On the other side, the growth temperature for AlGaN EBL layer is much higher (almost 150℃) than the active layers, which would result in crystal quality deterioration in the active region due to the detrimental thermal annealing effect [17, 18]. Recently, several significant research works have been done in the suppression of polarization field induced QCSE and carriers leakage problems via staggered QWs [19], large overlap QWs [20], lattice-matched AlGaInN barriers [21], InAlN EBL [22], etc. In this paper, the new designed structure is proposed to alleviate the drawbacks mentioned above in two ways: On the one hand, from device structure, the introduction of low temperature p-GaN
can help reduce internal polarization effect, while the introduction of short period superlattice layers can further reduce the electron leakage level and improve hole injection efficiency; On the other hand, from growth technique, the adoption of low temperature p-GaN is mainly to protect crystal quality of active regions yet has bad influence on the following growth, while the adoption of the LT-InGaN/GaN superlattice layers is beneficial to ameliorate crystal quality of the typical p-GaN contact layer. 2. Experiment The structure of conventional LED (labeled structure A) was grown on a c-plane sapphire substrate, followed by a 2-μm-thick undoped GaN buffer layer and a 2-μm-thick Si-doped n-GaN layer. The active region consists of six pairs of 3nm undoped InGaN wells and 10nm GaN barriers. On top of the last QB are a 20nm-thick p-Al0.15GaN EBL and a 200nm thick p-GaN cap layer. Another LED epitaxial structure (labeled structure B) has the same structure except that a 25nm thick LT-pGaN transition layer is inserted between the last QB and EBL. The growth temperature of the interlayer is 760℃, the same as that of QW, and the hole concentration in the Mg doped LT p-GaN is 5×1017cm-3，which is the same as the typical p-GaN. To improve the p-GaN crystal quality and optical performance, we further designed a low temperature composite layer structure (labeled structure C) which composed of a 25nm thick LT-pGaN layer and six periods LT-InGaN/GaN superlattice with 1.5nm thick each layer without an AlGaN EBL. The growth parameters of the LT-pGaN/SL interlayer is almost same as sample B except the growth temperature of p-InGaN/GaN SL is 50℃ higher than the LT p-GaN with Mg doping level of 1×1018cm-3.The detailed structures of the three LEDs are shown in Fig.1. All samples were grown by metal-organic chemical vapor deposition (MOCVD) in a closely coupled showerhead (CCS). The device geometry was designed to be a rectangular shape of 300×300 μm2. LED chips were fabricated using a conventional mesa structure method. Indium tin oxide was used as a transparent conducting layer, and Cr/Au metal was deposited as p- type and n- type electrodes, respectively. 3. Results and discussion The optical and electrical properties of the LEDs were investigated numerically by using the APSYS simulation program [23]. Here, for all the simulation results, we assume the structure characteristics of the three samples, but the Mg activation rate in low temperature p-GaN is usually higher than that at normal temperature. [24] For that reason, we adequately enhance the parameter setting of hole concentration (6×1017cm-3) in the LT p-GaN than that in experiment situation. In the simulation, the light extraction efficiency is assumed to be 0.78, the Shockley- Read- Hall is estimated to be 100ns and the Auger Recombination coefficient is set to be 1.0×10-34 cm6 s-1 which is similar to the reported values. [25] The built- in interface charges due to spontaneous and piezoelectric polarization are calculated by the methods developed by Fiorentini et al. [26] 50% of the theoretical value is used to account for the compensation by fixed defects and other interface charges. [27] Fig. 2(a) shows the calculated electrostatic fields in the active region of the three samples. Strong piezoelectric polarization field was generated nearby the p-n junction region due to the large lattice mismatch between the last QB and AlGaN EBL, particularly
in the last quantum well, as can be seen in sample A. However, this situation gets improved after a 25nm LT-pGaN transition layer is inserted, which can take apart the interface between the last QB and EBL from space, and thus reducing the electrostatic fields shown in sample B. While for sample C, we adopted composite low temperature layers which consist of a LT-pGaN layer and six periods InGaN/GaN superlattice layers, the polarization effect in the active region is further diminished, especially in the last quantum well. Since the original AlGaN EBL is replaced by InGaN/GaN superlattice layers, the lattice mismatch between the MQWs and the following layers would be substantially reduced. Fig. 2(b) and (c) represent the calculated electrons and holes distributions in the active regions of the three samples. It can be seen that a large amount of electrons accumulates in the last QW next to p-type layer for sample A, which is caused by band bending effect due to the strong polarization field. Under the condition of insufficient hole concentration and hole injection efficiency, the overflowed electrons out of the QWs would have more opportunities recombine in the non-radiative recombination centers, therefore results in higher Auger recombination rate. By employing the LT p-GaN transition layer on the last GaN quantum barrier for sample B and C, the electron concentration in the last QW can be effectively decreased，meanwhile enhancing the electron concentration in the other QWs, leading to much more uniform electron distribution in the active regions. Furthermore, the LT insertion layer can lower the effective potential height for holes in the valence band, resulting in higher hole concentration in the active region. In this case, the electron and hole concentration can be better matched, leading to better radiative recombination. The electroluminescence (EL) spectra of the three LED samples, at the injection current of 20mA, are shown in Fig.3. Besides the sharp band-to-band peaks at around 460nm for the three samples, there still appear sub-band parasitic emissions around 320nm for sample A and B, which is probably due to the recombination of overflowed electrons with holes in the Al0.15GaN EBL. It is believed that the holes populating on the Mg-related deep acceptor levels, once recombine with electrons overflow into the cladding layer, would results in the parasitic peak [28]. The parasitic peak for sample B is smaller than the conventional one, indicates the LT p-GaN insert layer is helpful to reduce electrons leakage. While for sample C, because of better carrier limitation effect of the LT p-GaN with p-InGaN/GaN short periods superlattice layers, there is hardly parasitic emission observed, which probably attribute to the optimized energy band situation for the enhanced holes injection efficiency and suppressed electrons leakage. Fig.4 shows the EL peak wavelength and full width at half maximum (FWHM) as a function of forward injection current for the three samples. All the samples demonstrate a blue-shift in the main emission wavelength as the current increases. The smallest blue-shift of ~5.3nm is observed for sample C at 200mA, which is much less than that of sample A（~8.4nm）, sample B also gains an improved blue-shift of ~5.8nm compared with the conventional one. Generally, the peak wavelength blue-shift of LEDs may result from the Coulomb screening of piezoelectric polarization field induced QCSE and band filling effect. The blue shifts of the three samples vary widely under the same injection current, as can be seen from Fig.4. And the smaller blue-shift of sample B and C mainly attribute to the reduction of QCSE, which means the introduction of LT p-GaN insert layer can help alleviate polarization field in MQWs thus resulting in less wavelength shift. While the smallest blue-shift of sample C is mainly due to the extra introduction of low indium-content p-InGaN/GaN short period SL layers instead of
AlGaN EBL, thus alleviating the lattice mismatch between the MQW regions and EBL interface, which may also assist in the reduction of polarization field in the MQWs. It was reported that the band filling effect accompanies the increasing of FWHM [29]. We can observe from Fig.4 that the FWHM of all the samples tend to increase with increasing injection current. The FWHM at 200mA are increased by about 66%, 48.8%, 36% for sample A, B, and C, respectively, as compared to those at 1mA, which indicates that as the injection current increases, the FWHM broadening can be effectively improved by the introduction of LT insert layers. Since the reduction of QCSE of sample B and C will alleviate the band bending in the active region, which may assist in the suppression of band filling effect. Besides, the improved crystalline quality in the active region due to lower temperature growth in LT p-GaN layer also contribute to the smaller broadening of FWHM in sample B and C, and the limited strain energy of the inserted LT p-InGaN/GaN SL for sample C even enhances the crystalline quality of the whole wafer thus exhibiting the smallest FWHM broadening. LT-pGaN growth was in fact an annealing process for active layers, so the crystalline quality of the active region would be optimized by recrystallisation [30]. However, the LT-pGaN itself might suffer from severe crystalline quality deterioration due to improper growth temperature or doping level, which would bring bad effect on the following growth of AlGaN EBL and p-GaN thus degrading the device performance. The crystalline quality was analyzed by the High-resolution X-Ray Diffractometer (HRXRD). XRD omega/2-theta scans taken across (002) reflection for sample B and sample C are depicted in Fig. 5, the corresponding full width at half maximum (FWHM) of GaN (0002) rocking curves are 305 and 243 arcsec for sample B and sample C, respectively. And the substantially improved crystalline quality of sample C is mainly due to the limited strain energy of the inserted short period LT-InGaN/GaN superlattice layers. Besides, the diffraction pattern of structure C with low temperature superlattice layers shows more InGaN satellite peaks and sharper interface than the LT-pGaN sample. Hence, the LT p-GaN/SL interlayer can substantially ameliorate crystalline quality. The measured and simulated light-current (L-I) performances of the three samples with increasing current are shown in Fig. 6(a). The relative ideal simulated L-I curve demonstrate consistent increase with the experimental data. The light output power of sample B represents 15% increase at 200mA due to the reduced QCSE and improved hole injection efficiency. While for sample C, once the extra p-InGaN/GaN SL layers added on the LT p-GaN, the emission power of sample C exhibit 42.2% boost at 200mA compared to the conventional one. The forward voltage (Vf) at 20mA for LT-pGaN/SL LED is 3.25V, which is lower than that of 3.35V for conventional LED. Since the LT p-GaN/SL insert layer allow the alleviation of strain induced piezoelectric field and the reduction of QCSE in the active region, the carrier injection efficiency and recombination rate are enhanced. Furthermore, the substantial improved crystalline quality and reduced electron leakage level are mainly due to the new interlayer by the alternative p-InGaN/GaN short period SL layers instead AlGaN EBL, which may also play a key role in the emission power. Fig. 6(b) shows the relative external quantum efficiency (EQE) of the three samples as a function of injection current. Efficiency droop behaviors are demonstrated in all the samples. The maximum EQE values of the three samples coincide with the light output power performance. Sample C shows the highest EQE values in the measured range because of the largest EL emission power. In addition, sample C shows a reduced efficiency droop from 65% to 52% with the increasing injection current, indicating enhanced holes injection and electron
confinement. It has been reported that the efficiency droop is mainly due to the Auger recombination in the active MQWs, as well as holes recombine with the overflowed electrons outside the active region. Our experimental and simulation results also support that the reduced polarization field in the active region and the improved holes injection efficiency can reduce the carrier recombination outside the active region, therefore improving efficiency and suppressing droop effect. 4. Conclusions GaN-based LEDs with special designed low temperature transition layers (LT-pGaN/SL) was studied both numerically and experimentally. The simulated results show that the electrostatic fields in the active region, especially in the last QW near p-side, can be greatly relieved through the insertion of LT-pGaN/SL interlayer, and the reduction of QCSE is helpful to improve carrier distribution as well as enhance the hole injection efficiency. The simulated light output power ameliorates substantially owing to the improved carrier transportation and enhanced radiative recombination. The experimental results show that the crystalline quality of both active regions and typical p-GaN has been improved due to the reduced detrimental thermal annealing effect during low temperature growth and the insertion of InGaN/GaN superlattice layers. The output power of the sample with new designed interlayer is enhanced by 42.2% at the injection current of 200mA compared to the conventional LED, which can be attributed to the improved crystalline quality, ameliorated electron leakage level and enhanced hole-injection efficiency.
Acknowledgments This work was supported by the National Nature Science Foundation of China (Grant No. 51172079), the Science and Technology Program of Guangdong province, China (Grant Nos. 2010B090400456, and 2010A081002002), the Science and Technology Program of Guangzhou (Grant No. 2011J4300018)
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Highlights (1) LT-pGaN/SL is inserted to alleviate the polarization field in the active region.
(2) Crystalline quality of epitaxial wafer is improved when LT-pGaN/SL is adopted. (2) Experiment results show that the peak wavelength is more stable when LT-pGaN/SL is adopted. (3) Our results show that photoelectric characteristic is improved when LT-pGaN/SL is adopted. (4) Our results reveal that polarization field and hole injection efficiency are the main reasons for efficiency droop.
Figure caption:
Fig. 1. Schematic plot of the LEDs under study
Fig. 2(a). The electrostatic fields (b) Electrons and (c) holes concentrations of sample A, B, and C in the active region at 200 A/cm2. (There is a small location shift on horizontal axis for better observation.)
Fig. 3. EL spectra of the three LED samples at the injection current of 20mA. Fig. 4. Wavelength shifts and FWHMs of the EL spectra for the three LED samples as a function of the injection current. Fig. 5. XRD ω-2θ scans taken across the (0002) reflection for both samples. Fig.6. (a) Output power and (b) normalized EQE for the three LED samples as a function of the injection current.
Fig.1 Schematic plot of the LEDs under study.
Fig. 2(a) The electrostatic fields (b) Electrons and (c) holes concentrations of sample A, B, and C in the active region at 200 A/cm2. (There is a small location shift on horizontal axis for better observation.)
Fig. 3 EL spectra of the three samples at the injection current of 20mA.
Fig. 4 Wavelength shifts and FWHMs of the EL spectra for the three LED samples as a function if the injection current.
Fig. 5 XRD ω-2θ scans taken across the (0002) reflection for both samples.
Fig.6. (a) Output power and (b) normalized EQE for the three LED samples as a function of the injection current.
Highlights (1) LT-pGaN/SL is inserted to alleviate the polarization field in the active region. (2) Crystalline quality of epitaxial wafer is improved when LT-pGaN/SL is adopted. (2) Experiment results show that the peak wavelength is more stable when LT-pGaN/SL is
adopted.
(3) Our results show that photoelectric characteristic is improved when LT-pGaN/SL is adopted. (4) Our results reveal that polarization field and hole injection efficiency are the main reasons for efficiency droop.