1. Introduction

III-nitride-based optoelectronic devices such as light emitting diodes (LEDs), and laser diodes (LDs) have been experiencing a rapid technology progress in recent years, and are now beginning to be applied for many commercial areas such as full-color displays, traffic lights, optical storage, solid-state lighting, and medical applications [1–3]. These optoelectronic devices normally employ an InGaN/GaN multiple quantum wells (MQWs) as the active layer, however, its electron capture rate is low due to the much smaller effective mass and higher corresponding thermal velocity of electrons than of holes [4], this results in the leakage of electrons to the p-GaN and harms the radiative recombination efficiency. Besides, a large lattice and thermal mismatch exits between InGaN and GaN, so there is large strain between the InGaN/GaN active layer and underlying n-GaN template. The strain can generate a large piezoelectric field in the MQW region [5], and this piezoelectric field-induced quantum-confined stark effect (QCSE) leads to a spatial separation of electrons and holes. For this reason, the radiative recombination efficiency of the MQWs will be significantly reduced [6]. Furthermore, strain in MQWs is believed to act as a driving force for the formation of the V-pits [7–9], which can make interface loss abruptness between periods within the MQW growth and form a rough surface.

Several attempts have been made to enhance the electron capture rate of the active region and release the residual strain in InGaN/GaN MQWs. The electron blocking layers (EBL), stepwise-stage electron injection layer (EIL) [10, 11] and nanostructure insert layers [12, 13] have been induced and studied for improving the emission of InGaN/GaN LEDs. Although nanostructures have been approved to be effective in strain relaxation, complicated processes need to improve the compatibility in LED production. One of the practical approaches is to introduce an additional layer such as InGaN/GaN short period superlattices (SPS) [14–16], InGaN layer and InGaN/GaN MQW layer [17,18], or low-temperature (LT) n-GaN layer [19], between the n-type GaN and the InGaN/GaN active layer. Studies have revealed that such insertion layers can be regarded as the lattice-mismatch buffer between the underlying n-GaN and overlying QWs, which can release the residual strain in MQWs layer, reduce the V-pits density in MQWs, improve the crystal quality of InGaN/GaN MQWs, enhance electron capture rate of the active layer and improve the current spreading in LED. As a result, the quantum efficiency and the output power of LED can be enhanced. However, a comprehensive mechanism involved in every well of the strain relief layer (SRL) has not been discussed in detail, nor has the structure been optimized. In this work, we managed to reduce the strain of InGaN/GaN MQWs by inserting a step stage SRL between n-GaN and MQWs, in which the well nearer the n-type GaN has shallower depths. The strain effect in MQW were evaluated by comparing calculated emission spectra using effective mass model based on k⋅p method and measured luminescence properties. Compared with a fixed indium concentration InGaN/GaN SRL, this step stage SRL further improved the crystal quality and enhanced the electron capture rate of the active region because of the gradual change of lattice constant in the emitter layer, more greatly modified the luminescence properties of InGaN/GaN MQWs.

2. Experiment

All the InGaN-LED samples in this work were grown on (0001) sapphire substrate using a low pressure metal organic chemical vapor deposition (LP-MOCVD) system. Trimethylgallium (TMGa), trimethylindium (TMIn), and ammonia (NH3) were used as precursors. Silane (SiH4) and biscyclopentadienymagnesium (Cp2Mg) were used as the n-dopant and p-dopant source. For reference, InGaN/GaN LED without SRL were also grown. All the LED structures were consisted of a 30-nm-thick GaN nucleation layer grown at 550°C, followed by a 2-μm-thick undoped GaN layer grown at 1050°C, a 4-μm-thick Si doped n-type GaN layer with a carrier concentration of 5 × 10¹⁸cm^-3 grown at 1050°C, five pairs of In_0.15Ga_0.85N/GaN MQWs active layer grown at 750°C, 5 periods of p-Al_0.15Ga_0.85N(2nm)/GaN(2nm) superlattices EBL and a 150-nm-thick Mg-doped p-GaN layer grown at 980°C, as shown in Figs. 1(a) and 1(b). SRLs were sandwiched between n-GaN and InGaN/GaN MQWs. First, a kind of fixed In content SRL which has 5 periods of 2nm In_0.08Ga_0.92N well and 3nm GaN barrier were grown and shown as Fig. 1(c). Another short period superlattice-like SRL structure with step stage indium content consists of 5 periods of 2nm thick InGaN well and 3nm thick n-GaN barrier. Stepwise indium doped MQW structures are obtained by controlling the flow rate of TMIn during growth, the In content in each well layer is set to be 0.01, 0.04, 0.07, 0.1, 0.13 as illustrated in Fig. 1(d). After growth, the LED chips were fabricated by regular chip process with indium tin oxide current spreading layer and Ni/Au contact metal, and the size of mesa is 350 × 350μm². In order to investigate the surface morphology of MQWs by atom force microscopy (AFM), three samples without the p-AlGaN/GaN EBL and the p-GaN layer were also grown, denoted as single stage MQW, dual stage MQW and step stage MQW, and LED samples correspondingly called as single stage LED, dual stage LED and step stage LED. Room temperature electro-luminescence (EL) characteristics, current-voltage (I-V), and output power-current (L-I) characteristics of LEDs were evaluated at various injection currents.

 

Fig. 1 (a) The schematic diagram of LED with InGaN/GaN SRL; Illustration of TMIn flow rate for controlling the indium composition during the growth of (b) single stage MQW, (c) dual stage MQW and (d) step stage MQW.

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3. Results and discussion

The piezoelectric polarization induced by strain effect can change directly the emission properties of GaN-based MQW LEDs. So, the strain effect in MQW can be evaluated by comparing calculated emission spectra using effective mass model based on k⋅p method and measured luminescence properties [20, 21]. Figure 2(a) gives the polarization field in well layers and corresponding carrier overlap percentage varying with in-plane compressive strain for In_0.15Ga_0.85N/GaN MQWs grown along the (0001) direction. The polarization filed increases as in-plane compressive strain that leads to the decline of overlap percentage of electrons and holes. As a result, the emission from MQWs enhances when compressive strain in well layers reduces. Simultaneously, the emission peak shifts toward shorter wavelength due to the decrease of polarization field. Figure 2(b) gives the measured PL spectra of the single stage LED, dual stage LED and step stage LED. In order to evaluate the strain effect in such different MQWs, the spontaneous emission spectra of MQWs with different in-plane compressive strain are calculated and shown in Fig. 2(c). By comparing the calculated and measured emission peak, it is concluded that the in-plane compressive strain is about 0.016 for conventional single stage In_0.15Ga_0.85N/GaN without SRL that results in 2.27MV/cm polarization field and 14% carrier overlap from Fig. 2(a). The overlap percentage of electrons and holes increases to 19% when the strain reduces to 0.014 by using step stage SRL. As a result, the spontaneous emission intensity can be enhanced by inserting InGaN/GaN SRL, especially the step stage SRL. However, the peak intensity step stage MQWs increases more than 100% as compared to single stage MQW in measured results, more obviously than that in calculated results. Except for the decrease of polarization field, the improvement of crystal quality from reduction of the strain in MQWs with InGaN/GaN SRL should be also responsible for the improvement of the PL properties.

 

Fig. 2 (a) Strain effect on polarization field and overlap percentage of electrons and holes (b) measured PL spectra at room temperature and (c) calculated spontaneous emission spectra for InGaN/GaN MQW

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Figure 3 shows the HRXRD 2theta/omega scans of (0002) plane in single stage LED, dual stage LED and step stage LED for comparison. Single stage MQW structure has well defined satellite diffraction peaks up to the fourth order, while both the dual stage MQW and the step stage MQW structures give satellite peaks up to the fifth order. This means good structure quality of the MQWs and the abrupt interfaces between the barriers and wells. Furthermore, it is observed that the satellite peaks in step stage LED and dual stage LED is clearer than that in single stage LED, indicating that the inserted InGaN/GaN SRL acts as a buffer modulating the strain in the active region and the underlying GaN and overlaying layers are of better quality.

 

Fig. 3 The HRXRD 2theta/omega scans of (0002) plane in single stage LED, dual stage LED and step stage LED.

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5 × 5μm² AFM images of InGaN/GaN MQWs without SRL, with dual stage SRL and step stage SRL are shown in Fig. 4. The large and dense V-shape pits caused by the threading dislocation or stacking faults as reported in previous studies are observed in the sample without SRL as Fig. 4(a). The density and root mean square (RMS) roughness of pits are in the range of 4-5 × 10⁸cm^-2 and 0.53 nm, respectively. In contrast, the values for dual stage SRL sample in Fig. 4(b) are smaller in the range of 1-2 × 10⁸cm^-2 and 0.6 nm, respectively, and further reduced to 2-4 × 10⁷cm^-2 and 0.23 nm for step stage SRL sample in Fig. 4(c). It has been reported that the strain is a primary cause of the formation of V-pits [7–9]. Then, obvious decrease of V-pits in step stage MQW should be due to the reduced strain in InGaN active layers. Less V-pits in the MQW also benefit good interface abruptness between periods within the MQW stack, so that step stage MQW shows better crystal quality as compared with dual stage MQW and MQW without SRL, and improved performance of step stage LED can be expected.

 

Fig. 4 5 × 5μm² AFM images of InGaN/GaN MQWs (a) without SRL, (b) with dual stage SRL and (c) with step stage SRL.

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The EL spectra of InGaN/GaN MQW LEDs without SRL, with step stage SRL were measured with increasing injection current. A large blue shift of the emission peak about 6.4nm from 452.8nm to 446.4nm was observed for LEDs without SRL when the injection current increased from 1 to 20 mA, shown in Fig. 5(a). A smaller blue shift of the emission peak about 4.2nm from 451.6nm to 447.4nm for dual stage LEDs, and much smaller shift 2.2nm from 444.6nm to 442.4nm for step stage LEDs are shown in Figs. 5(b) and 5(c). Blue-shift could be caused by the band-filling and QCSE [22]. However, the shift due to band-filling effect can be excluded because the highest injection current is only 20mA in this EL spectra measurement. Then, it seems as Leem et al. concluded that blue-shift of emission is due to the strain between the GaN epilayer and MQW [16]. Then, a larger blue-shift is the clear evidence that LEDs without SRL suffer the stronger QCSE than those with step stage SRL.

 

Fig. 5 The EL spectra of InGaN/GaN MQW LEDs (a) without SRL, (b) with dual stage SRL and (c) with step stage SRL, the spectra were measured with increasing injection current from 1mA to 20mA.

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Output powers as functions of injection current for LEDs are shown in Fig. 6. The 20mA output power of the single stage LED, dual stage LED and step stage LED are 7.3mW, 12.7mW and 15.2mW, respectively. The step stage InGaN/GaN underlying layer helped LEDs performance improvement about 108%, and delivered further improvement approximately 20% compared to dual stage MQW LEDs at 20 mA, and such output power enhancement was even enlarged to 112% and 30% at high injection 300mA in our other experiments. Carrier involved in radiative recombination centers and the carrier injection efficiency could be markedly improved by inserting the short period InGaN/GaN superlattice-like SRL structure.

 

Fig. 6 Output powers as functions of injection current for single stage, the dual stage and step stage MQW LED.

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4. Conclusion

The performance of nitride-based LEDs was improved by inserting dual stage and step stage InGaN/GaN SRL between the active layer and n-GaN template as compared with MQWs without SRL. The influences of step stage and dual stage InGaN/GaN SRL on the structure, electrical and optical characteristics of GaN-based LEDs were investigated. With the analysis of strain effect on recombination rate based k⋅p method, it is estimated that the strain in InGaN/GaN MQWs was reduced about 12.5% by inserting step stage InGaN/GaN superlattice-like SRL. The surface morphology was improved and a smaller blue shift in the electroluminescence (EL) spectral with increasing injection current was also observed. The output power of LEDs with SRL at 20mA was increased by more than 108%. The asymmetric step stage InGaN/GaN SRL is believed to be effective to modulate strain and improve the luminous properties of InGaN/GaN MQWs LEDs.