1. Introduction

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.

2. Experimental procedures

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 $\mu$m p-GaN layers were grown after the deposition of 2 $\mu$m thick undoped GaN layers. The p-GaN layers were grown under a temperature of $950$ ℃ and a constant growth rate of 0.4 $\mu$m/h. The Cp$_2$Mg/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, In$_{0.15}$Ga$_{0.85}$N/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 $\mu$m 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 $\times$ 1 mm$^{2})$ 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.

3. Results and discussion

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 NH$_{3}$ 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.

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 NH$_{3}$ 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].

The root-mean-square (RMS) roughness of the 5 $\times$ 5 $\mu$m$^{2}$ 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 $\mu$m$^{2}$ ohmic pads with gap spacing of 10, 20, 30, 40, 50, and 60 $\mu$m 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].

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.

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 $\omega$/2$\theta$ 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).

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.

4. Conclusion

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.