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J. Semicond. > 2014, Volume 35 > Issue 11 > 114007

SEMICONDUCTOR DEVICES

Advantages of InGaN/GaN light emitting diodes with p-GaN grown under high pressure

Binglei Fu, Naixin Liu, Zhe Liu, Jinmin Li and Junxi Wang

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 Corresponding author: Fu Binglei, Email:fubinglei@semi.ac.cn

DOI: 10.1088/1674-4926/35/11/114007

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Abstract: The advantages of InGaN/GaN light emitting diodes (LEDs) with p-GaN grown under high pressures are studied. It is shown that the high growth pressure could lead to better electronic properties of p-GaN layers due to the eliminated compensation effect. The contact resistivity of p-GaN layers are decreased due to the reduced donor-like defects on the p-GaN surface. The leakage current is also reduced, which may be induced by the better filling of V-defects with p-GaN layers grown under high pressures. The LED efficiency thus could be enhanced with high pressure grown p-GaN layers.

Key words: light emitting diodesV-defectsgrowth pressurep-GaN

The rapid developments of light emitting diode (LED) technology have a significant influence on the traditional lighting industry and our daily lives. Despite fruitful achievements, there are still many problems limiting the further development of LED technology. One of the most concerning issues is the optimization of p-GaN layers in the LED device structure. Due to the high activation energy of Mg acceptors in p-GaN layers, the hole concentrations of p-GaN layers are much lower than the electron concentration of n-GaN. Accompanied by the much smaller hole mobility, the asymmetry of the carrier transport in LED devices is a serious concern, which has recently been described as one of the main reasons for the efficiency droop of LEDs operating at high currents[1]. What is more, the surface of InGaN/GaN multiple quantum wells (MQWs) are usually full of V-defects, which is known as the current leakage path of LED devices[2]. The effects of V-defects could be eliminated when they are filled with the p-GaN capping layers. The growth parameters of p-GaN thus must be optimized to promote this process[3]. In this letter, we put our focus on the effect of growth pressure of p-GaN layers on the LED performance. It is shown that the p-GaN layer with high growth pressures could eliminate the current leakage and reduce the series resistance of LED samples, leading to the enhanced LED efficiency.

The Mg doped GaN layers and LED structures were grown on 2-inch sapphire substrates with a Veeco P125 metalorganic chemical vapor deposition (MOCVD) system. To calibrate the electrical properties of Mg doped GaN layers, 0.5 μm p-GaN layers were grown after the deposition of 2 μm thick undoped GaN layers. The p-GaN layers were grown under a temperature of 950 ℃ and a constant growth rate of 0.4 μm/h. The Cp2Mg/TMGa ratio was 0.3% for all samples. The growth pressures of our samples were varied from 130 to 400 Torr. Further increasing the growth pressure will lead to serious parasite reactions and deteriorate the electrical properties of p-GaN layers. In order to analyze the effect of growth pressures of p-GaN layers on the LED performances, In0.15Ga0.85N/GaN LED structures with p-GaN layers grown under 130 and 400 Torr were fabricated. Except for the growth pressure of p-GaN layers, the other growth parameters for the two sets of LED structures are the same. The LED structures were grown on the c-plane sapphire substrates, followed by 2 μm undoped and Si doped GaN layers. The active region consisted of nine periods of InGaN/GaN quantum wells (QWs). Then the electron blocking layer (EBL) and p-type layer were grown. All LEDs (1 × 1 mm2) were fabricated with the standard mesa structure.

The electrical properties of our p-GaN layers were examined by room temperature Hall effect measurements in Van Der Pauw geometry. For LED samples, the atomic force microscopy (AFM) scans were performed to observe the surface morphologies of samples using a Nanoscope DimensionTM 3100 scanning probe microscope system. High-resolution X-ray diffraction (HRXRD) measurements were carried out using a Bede D1 system to examine the structure qualities of our LED samples.

The electrical properties of our p-GaN layers are shown in Table 1. It is shown that the hole concentrations and the hole mobilities are both increased with the increased growth pressure, resulting in the decreased sample resistivity. The improved electrical properties of our p-GaN layers are induced by the reduced compensation effect. With increased growth pressure, the NH3 over-pressure also increased and reduced the concentration of compensation nitrogen vacancy centers, which is discussed in detail in our previous work[4]. In order to illustrate the advantages of the modified p-GaN layers on the performances of LED devices, we carefully examined the performances of LEDs with p-GaN layers grown under 130 Torr and 400 Torr, which is shown below.

Table  1.  Results of RT-Hall measurement.
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The surface morphologies of our LED samples are shown in Fig. 1. It is shown that the LED with p-GaN grown under relatively low pressure (130 Torr) presented quite a rough surface. In Fig. 1(b), the surface of our LED sample with p-GaN grown under high pressure (400 Torr) shows a flatter surface. The flatter surface of LED samples grown under high pressure may result from the higher surface atom mobilities. Under high growth pressures, the increased NH3 over-pressure will increase the surface atom mobilities and thus result in the flattened surface[4, 5]. With high surface mobilities, the surface adatoms could easily reach the proper lattice sites, this effect could also be achieved by lowering the growth rate of GaN layers, which leads to a flat LED surface and better filling of V-defects[3].

Figure  1.  AFM images with 5 × 5 μm2 scans of LED samples with p-GaN grown at (a) 130 Torr and (b) 400 Torr.

The root-mean-square (RMS) roughness of the 5 × 5 μm2 sample surfaces is shown in Table 2. The RMS value of the sample with p-GaN grown under 130 Torr is 1.641 nm, while that of the sample with p-GaN grown under 400 Torr is reduced to 0.995 nm, indicating a flatter surface. In order to examine the growth condition of the p-GaN layer to the contact resistivity of the p-electrode, measurements of the transmission line method (TLM) were performed to calculate the resistivity of the contacts. The TLM patterns consist of 100 μm2 ohmic pads with gap spacing of 10, 20, 30, 40, 50, and 60 μm to calculate the resistivity of the contacts. The measured values are shown in Table 2. It is shown that the p-type contact resistivity for an LED with p-GaN grown under 400 Torr is much lower than that for an LED with p-GaN grown under 130 Torr. The reduced number of donor-like defects such as the nitrogen vacancies of p-GaN layers, as we have discussed previously, might contribute to this phenomenon[6].

Table  2.  RSM roughness of LED surface and specific contact resistivity of ohmic contact on p-GaN.
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Figure 2 shows the I-V characteristics of our LED samples with p-GaN grown under different pressures. The series resistance of the sample with p-GaN grown under 400 Torr is much lower than that of the sample with p-GaN grown under 130 Torr, as can be seen from the slopes of the I-V curves in Fig. 2(a). The decreased series resistance of the LED sample is a result of the reduced p-GaN resistivity and contact resistivity, as we discussed before. Figures 2(b) and 2(c) show that the low-forward-bias currents and the reverse currents are clearly decreased for LEDs with p-GaN grown under high pressures. It is known that the carrier leakage in LED samples can be caused by the defect-assisted tunneling[7]. The existence of V-defects may provide preferential paths for the leakage current. Thus the reduced leakage current of the LED sample with p-GaN grown under high pressures might result from the better filling of V-defects by p-GaN layers.

Figure  2.  (a) Forward I-V curves, (b) forward log I-V curves and (c) reverse I-V curves of LED samples with p-GaN grown at different pressures (130 Torr in black and 400 Torr in red).

The growth conditions of p-GaN layers might have a strong influence on MQWs structures. It is generally accepted that the growth temperature of p-GaN layers in LED structures could not be too high, as the In atom diffusion caused by the high growth temperature might deteriorate the structure of MQWs[8]. The HRXRD ω/2θ scans for both LEDs are shown in Fig. 3(a) to illustrate the structure qualities of LEDs with p-GaN with different growth pressures. It is presented that the XRD profiles of the LED samples with p-GaN grown with different pressures are almost identical, meaning that the influences of p-GaN growth pressure on the structural qualities of LED samples are not significant. This conclusion was further supported by the XRD reciprocal space maps (RSMs) in Figs. 3(b) and 3(c).

Figure  3.  (a) The HRXRD ω/2θ spectra of LED samples with p-GaN grown at different pressures. The XRD reciprocal space maps of LED samples with p-GaN grown under (b) 130 Torr and (c) 400 Torr.

The external quantum efficiency (EQE) as a function of currents for samples with p-GaN grown under different pressures is shown in Fig. 4. The LEDs with p-GaN grown under high pressures showed better peak efficiency than the LEDs with low pressure grown p-GaN layers. The reduced series resistance and current leakage path caused by the V-defects might lead to the enhanced LED performances with p-GaN grown under higher pressures. However, the enhancement of LED efficiency is relatively small compared with the electrical properties of LED samples. Note that as an EBL layer was included in our LED structures, the hole injection into MQWs might be blocked by the EBL layer[9]. As a result, the optical efficiency of LEDs was not enhanced as much as the electrical properties of p-GaN layers.

Figure  4.  The EQE as a function of current for LED samples with p-GaN grown at different pressures.

In conclusion, the advantages of InGaN/GaN LED with p-GaN layers grown under relatively high pressures are investigated. The electrical properties of p-GaN layers are increased with the increased growth pressure due to the reduced compensation effect. The contact resistivities of p-GaN layers are reduced due to the decreased donor-like defects on the p-GaN surface. The series resistance of LED samples thus could be reduced. The high growth pressure of p-GaN layers also lead to a flatter LED surface and better filling of V-defects, leading to the reduced current leakage path in LED samples. The LED performances thus could be improved.



[1]
Meyaard D S, Lin G B, Shan Q, et al. Asymmetry of carrier transport leading to efficiency droop in GaInN based light-emitting diodes. Appl Phys Lett, 2011, 99(25):251115 doi: 10.1063/1.3671395
[2]
Le L C, Zhao D G, Jiang D S, et al. Carriers capturing of V-defect and its effect on leakage current and electroluminescence in InGaN-based light-emitting diodes. Appl Phys Lett, 2012, 101(25):252110 doi: 10.1063/1.4772548
[3]
Lee W, Limb J, Ryou J H, et al. Influence of growth temperature and growth rate of p-GaN layers on the characteristics of green light emitting diodes. J Electron Mater, 2006, 35(4):587 doi: 10.1007/s11664-006-0104-2
[4]
Fu B, Liu N, Zhang N, et al. The effect of growth pressure and growth rate on the properties of Mg-doped GaN. J Electron Mater, 2014, DOI: 10.1007/s11664-014-3005-9
[5]
Ratsch C, Garcia J, Caflisch R E. Influence of edge diffusion on the growth mode on vicinal surfaces. Appl Phys Lett, 2005, 87(14):141901 doi: 10.1063/1.2077851
[6]
Wang L C, Guo E Q, Liu Z Q, et al. Electrical characteristics of a vertical light emitting diode with n-type contacts on a selectively wet-etching roughened surface. Journal of Semiconductors, 2011, 32(2):024009 doi: 10.1088/1674-4926/32/2/024009
[7]
Liu L, Ling M, Yang J, et al. Efficiency degradation behaviors of current/thermal co-stressed GaN-based blue light emitting diodes with vertical-structure. J Appl Phys, 2012, 111(9):093110 doi: 10.1063/1.4712030
[8]
Oh M S, Kwon M K, Park I K, et al. Improvement of green LED by growing p-GaN on In0.25GaN/GaN MQWs at low temperature. J Cryst Growth, 2006, 289(1):107 doi: 10.1016/j.jcrysgro.2005.10.129
[9]
Ji Y, Zhang Z H, Kyaw Z, et al. Influence of n-type versus p-type AlGaN electron-blocking layer on InGaN/GaN multiple quantum wells light-emitting diodes. Appl Phys Lett, 2013, 103(5):053512 doi: 10.1063/1.4817381
Fig. 1.  AFM images with 5 × 5 μm2 scans of LED samples with p-GaN grown at (a) 130 Torr and (b) 400 Torr.

Fig. 2.  (a) Forward I-V curves, (b) forward log I-V curves and (c) reverse I-V curves of LED samples with p-GaN grown at different pressures (130 Torr in black and 400 Torr in red).

Fig. 3.  (a) The HRXRD ω/2θ spectra of LED samples with p-GaN grown at different pressures. The XRD reciprocal space maps of LED samples with p-GaN grown under (b) 130 Torr and (c) 400 Torr.

Fig. 4.  The EQE as a function of current for LED samples with p-GaN grown at different pressures.

Table 1.   Results of RT-Hall measurement.

Table 2.   RSM roughness of LED surface and specific contact resistivity of ohmic contact on p-GaN.

[1]
Meyaard D S, Lin G B, Shan Q, et al. Asymmetry of carrier transport leading to efficiency droop in GaInN based light-emitting diodes. Appl Phys Lett, 2011, 99(25):251115 doi: 10.1063/1.3671395
[2]
Le L C, Zhao D G, Jiang D S, et al. Carriers capturing of V-defect and its effect on leakage current and electroluminescence in InGaN-based light-emitting diodes. Appl Phys Lett, 2012, 101(25):252110 doi: 10.1063/1.4772548
[3]
Lee W, Limb J, Ryou J H, et al. Influence of growth temperature and growth rate of p-GaN layers on the characteristics of green light emitting diodes. J Electron Mater, 2006, 35(4):587 doi: 10.1007/s11664-006-0104-2
[4]
Fu B, Liu N, Zhang N, et al. The effect of growth pressure and growth rate on the properties of Mg-doped GaN. J Electron Mater, 2014, DOI: 10.1007/s11664-014-3005-9
[5]
Ratsch C, Garcia J, Caflisch R E. Influence of edge diffusion on the growth mode on vicinal surfaces. Appl Phys Lett, 2005, 87(14):141901 doi: 10.1063/1.2077851
[6]
Wang L C, Guo E Q, Liu Z Q, et al. Electrical characteristics of a vertical light emitting diode with n-type contacts on a selectively wet-etching roughened surface. Journal of Semiconductors, 2011, 32(2):024009 doi: 10.1088/1674-4926/32/2/024009
[7]
Liu L, Ling M, Yang J, et al. Efficiency degradation behaviors of current/thermal co-stressed GaN-based blue light emitting diodes with vertical-structure. J Appl Phys, 2012, 111(9):093110 doi: 10.1063/1.4712030
[8]
Oh M S, Kwon M K, Park I K, et al. Improvement of green LED by growing p-GaN on In0.25GaN/GaN MQWs at low temperature. J Cryst Growth, 2006, 289(1):107 doi: 10.1016/j.jcrysgro.2005.10.129
[9]
Ji Y, Zhang Z H, Kyaw Z, et al. Influence of n-type versus p-type AlGaN electron-blocking layer on InGaN/GaN multiple quantum wells light-emitting diodes. Appl Phys Lett, 2013, 103(5):053512 doi: 10.1063/1.4817381
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    Binglei Fu, Naixin Liu, Zhe Liu, Jinmin Li, Junxi Wang. Advantages of InGaN/GaN light emitting diodes with p-GaN grown under high pressure[J]. Journal of Semiconductors, 2014, 35(11): 114007. doi: 10.1088/1674-4926/35/11/114007
    B L Fu, N X Liu, Z Liu, J M Li, J X Wang. Advantages of InGaN/GaN light emitting diodes with p-GaN grown under high pressure[J]. J. Semicond., 2014, 35(11): 114007. doi: 10.1088/1674-4926/35/11/114007.
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    Received: 21 March 2014 Revised: 06 May 2014 Online: Published: 01 November 2014

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      Binglei Fu, Naixin Liu, Zhe Liu, Jinmin Li, Junxi Wang. Advantages of InGaN/GaN light emitting diodes with p-GaN grown under high pressure[J]. Journal of Semiconductors, 2014, 35(11): 114007. doi: 10.1088/1674-4926/35/11/114007 ****B L Fu, N X Liu, Z Liu, J M Li, J X Wang. Advantages of InGaN/GaN light emitting diodes with p-GaN grown under high pressure[J]. J. Semicond., 2014, 35(11): 114007. doi: 10.1088/1674-4926/35/11/114007.
      Citation:
      Binglei Fu, Naixin Liu, Zhe Liu, Jinmin Li, Junxi Wang. Advantages of InGaN/GaN light emitting diodes with p-GaN grown under high pressure[J]. Journal of Semiconductors, 2014, 35(11): 114007. doi: 10.1088/1674-4926/35/11/114007 ****
      B L Fu, N X Liu, Z Liu, J M Li, J X Wang. Advantages of InGaN/GaN light emitting diodes with p-GaN grown under high pressure[J]. J. Semicond., 2014, 35(11): 114007. doi: 10.1088/1674-4926/35/11/114007.

      Advantages of InGaN/GaN light emitting diodes with p-GaN grown under high pressure

      DOI: 10.1088/1674-4926/35/11/114007
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      • Corresponding author: Fu Binglei, Email:fubinglei@semi.ac.cn
      • Received Date: 2014-03-21
      • Revised Date: 2014-05-06
      • Published Date: 2014-11-01

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