1. Introduction

Commercially available gallium nitride based light emitting diodes (LEDs) are grown along the polar c-axis direction and have strong built-in electric fields, such as spontaneous and piezoelectric polarization fields, which reduce the overlap of the electron and hole wave functions within quantum wells (QWs), resulting in lower device efficiencies [1,2]. In the LEDs, the carrier-induced screening by the polarization-induced electric fields presents a blueshift in peak emission wavelength with increasing injection current [3]. To eliminate these polarization effects in InGaN QWs, several groups have tried to fabricate LEDs grown on nonpolar planes such as the (10-10) m-plane or the (11-20) a-plane [4–7]. Another approach to reduce the polarization effects is to use the inclined planes with respect to c-direction, i.e., semipolar planes, (10-1-1), (11-22), (10-1-3), and (20-21) [8–10]. Besides circumventing the polarization effects, the LEDs grown on the nonpolar and the semipolar planes also emit the polarized light due to optical gain anisotropy [11–14].

The research on the longer wavelength InGaN LEDs with high indium composition is required to expand the LED applications, for example monolithic white light LEDs, full color displays, and so on. Although the InGaN LEDs with various indium compositions fabricated on polar, semipolar and nonpolar planes have been widely reported [9,15–17], there are few reports on the nonpolar a-plane LEDs with long wavelength because high indium InGaN growth is very difficult. In this paper, we demonstrate the orange a-plane InGaN/GaN single quantum well (SQW) LEDs grown on r-plane sapphire substrates by metal organic chemical vapor deposition (MOCVD).

2. Experimental details

High crystalline quality a-plane (11-20) GaN film was directly grown on r-plane (1-102) sapphire substrates by MOCVD. Trimethylgallium (TMGa), trimethylindium (TMIn), bis-magnesium (CP₂Mg), monosilane (SiH₄), and ammonia (NH₃) were used as the Ga, In, Mg, Si, and N precursors, respectively. Hydrogen was used as the carrier gas. Figure 1 shows the schematic structure of a-plane SQW LED with high indium composition used in this study. A 150-nm thick GaN buffer layer was grown in mixed atmosphere of N₂ and H₂ at high temperature to obtain a minimum surface roughness and dislocation density. Subsequently, a 4.5-μm thick a-plane GaN template with a high crystalline quality surface was obtained by the conventional two-step growth method. Next, we grew a 2-μm Si-doped n-type GaN layer, followed by an active region consisting of SQW with 20-nm thick GaN barriers and 18 nm InGaN well, followed by a 120-nm thick p-type GaN:Mg layer. The more detailed growth conditions and the properties of the a-plane GaN films were reported previously [18]. The growth temperature, the reactor pressure, and the V/III ratio of the InGaN layer were 740 °C, 200 mbar, and 230, respectively. To obtain the high indium composition, we maintained the low growth temperature and the low V/III ratio. We could estimate the indium composition to be 42% from the simulation using SiLENSe software. The doping concentration of the n-type GaN layer was ~4.4 × 10¹⁸ cm⁻³, and the hole concentration was estimated to be ~1.4 × 10¹⁸ cm⁻³. For a p-electrode, Ni/Au (3 nm/3 nm) was deposited on the p-GaN layer using an e-beam evaporator, followed by annealing in ambient air to form ohmic contacts. The mesa pattern with a dimension of 200 µm × 500 µm was transferred into the n-GaN layer down to 1 µm by inductively coupled plasma etching. Ti/Al/Ni/Au (20/150/30/100nm) was deposited as the n-electrode.

 

Fig. 1 Schematic view of orange a-plane InGaN/GaN SQW LED structure.

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All measurements of orange a-plane LEDs were carried out on wafer-level under direct current operation at room temperature.

3. Results and discussion

Figure 2(a) shows the change in the electroluminescence (EL) spectra as the driving current is increased from 10 to 100 mA. The LED emitted in the orange spectral range and the EL spectra have a single peak up to 20 mA despite the high indium content in the InGaN layer. Above the 30 mA, there is a shoulder on the shorter wavelength side due to inhomogeneous InGaN layer. The current-voltage (I-V) characteristic of the orange a-plane LED is shown in Fig. 2(b). The forward voltage was 4 V at a forward current of 20 mA. This relatively high operating voltage in comparison to the c-plane LEDs is probably attributed to the unoptimized InGaN SQW, p-GaN layer, and Ni/Au contact.

 

Fig. 2 (a) EL spectra of orange a-plane InGaN/GaN SQW LED. (b) I-V characteristic of orange LED.

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Figure 3(a) shows the EL emission spectrum of an as-grown LED structure with the peak wavelength of 612.2 nm and the full-width at half maximum (FWHM) of 72 nm at 20 mA. To our knowledge, this wavelength is the longest value obtained for the nonpolar LEDs compared with previous reports [6,19]. The peak wavelength and the FWHM of the EL spectra were also plotted with increasing current, as shown in Fig. 3(b). The orange LED had a blue shift in emission wavelength from 614.6 nm at 10 mA to 607.5 nm at 100 mA, representing a net shift of 7.1 nm over a 90 mA range. The blue shift can be attributed to the band-filling of localized states induced by alloy fluctuation in the InGaN SQW.

 

Fig. 3 (a) EL spectrum of orange a-plane LED at 20mA. (b) The peak wavelength and the FWHM of the orange LED as a function of the injection current.

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The FWHMs of the EL spectra exhibited narrowing followed by broadening with increasing current. A narrowing FWHM was observed in the low current range, and this is because the band-filling effects of the localized energy states with lower energy were saturated with increasing current. A broadening of the EL FWHM over 40 mA was shown; this was caused by heat generation due to non-radiative processes. The broadening of the EL FWHM in the higher current range is accompanied by redshift of the peak wavelength in the blue LED [20]. Our results show the absence of the redshift of the peak wavelength and the presence of the broadening of the FWHM. This indicates that the blueshift due to the state-filling effect surpasses the redshift due to the heat generation. Funato et al. demonstrated the amber InGaN/GaN LEDs on semipolar (11-22) bulk GaN substrates [9]. Our orange nonpolar (11-20) LED in comparison to an amber semipolar (11-22) LED has a smaller wavelength shift and broader FWHM. The smaller wavelength shift despite the longer wavelength indicates the absence of polarization-related internal electric fields in the active region of the device. The broader FWHM suggests the stronger state-filling effect in the orange nonpolar (11-20) LED due to compositional fluctuations by phase separation in InGaN alloys.

When an InGaN QW is fabricated on the nonpolar a-plane GaN, the in-plane strain will be anisotropic. Due to the anisotropic strain, original |X ± iY> valence band (VB) states are broken into |X>- and |Y>-like ones [21]. The VBs are reconstituted to |Y>-like, |Z>-like, and |X>-like ones, which are in order of decreasing electron energy in an a-plane InGaN/GaN QW system [22]. The transition from conduction band to |Y>-like state is related to the light polarization E perpendicular to the c-axis and the transition involving the |Z>-like state occurs at the higher energy under E parallel to the c-axis.

The energy separation between |Y>-like and |Z>-like states increases as compressive strain increases and the optical polarization becomes stronger as the splitting energy becomes larger [22]. Figure 4(a) shows the polarized emission spectra for the orange a-plane LED measured at 40mA. The solid line is light polarized along the m-axis and the dotted line is light polarized along the c-axis. The solid line with a longer wavelength has a stronger emission compared to the dotted line, which is a general phenomenon observed in InGaN/GaN QW systems on nonpolar or semi-polar planes [19,23]. This indicates that the optical transition from conduction band and the highest VB is related to the light polarization perpendicular to the c-axis. The solid line has a single peak at 609.2 nm, while the dotted line has a peak at 605.3 nm with 39% of the intensity of the solid line. The peak separation and the relative intensities reflect the splitting of the VB states.

 

Fig. 4 (a) Polarization resolved EL spectra of orange a-plane LED. (b) Optical polarization ratio of the orange LED as a function of the injection current.

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The polarization ratio as a function of forward currents for the orange a-plane LED was measured, as shown in Fig. 4(b). The data were obtained by observation from the top a-plane LED using on-wafer measurements. The polarization characteristics of light emission from the top surface of the LEDs were evaluated by placing a rotational polarizer between the LEDs and a spectrometer. The polarization ratio, ρ, is defined as (I_⊥– I_∥)/(I_⊥ + I_∥), where I_⊥ and I_∥ are EL intensities for polarizations perpendicular and parallel to the c-axis, respectively. The polarization ratio values obtained from the orange LED were between 0.36 and 0.44, and the optical polarization ratio is not strongly affected by the current injection from 10 to 100 mA; this tendency is consistent with previous reports [10,24]. In our previous report [25], the polarization ratio of the blue a-plane LED was 0.4 at 20mA. According to the theoretical results [22], the orange a-plane LED should have a higher polarization ratio value than the blue a-plane LED because the in-plane strain increases with more indium composition in an InGaN QW, and further splits the VBs, which results in a higher polarization ratio. The low polarization ratio of the orange LED might be attributed to the zero-dimensional nature of the localizing radiative centers by the dotlike indium-rich region in SQW [26]. Another possible cause is the strain relaxation by stacking faults and misfit dislocations [27] which reduce the separation of the VBs. Regarding the number of QWs on the polarization ratio, the SQW is beneficial for a higher polarization ratio because the a-plane green LED with four periodic quantum wells in our experiments had a 0.24 polarization ratio. We believe that the low polarization ratio is due to strain relaxation by the multiple QWs which increases the generation of stacking faults compared to the SQW. We expect that a higher polarization ratio value can be achieved with improved crystal quality, a homogeneous InGaN single layer, and reduced random polarization.

4. Conclusion

We have demonstrated the orange a-plane LEDs with InGaN SQW grown on r-plane sapphire substrates. The device shows a blue shift in emission wavelength from 614.6 nm at 10 mA to 607.5 nm at 100 mA, representing a net shift of 7.1 nm over a 90 mA range. Our orange nonpolar (11-20) LED in comparison to an amber semipolar (11-22) LED has a smaller wavelength shift. This smaller wavelength shift, despite the longer wavelength, indicates the absence of polarization-related internal electric fields in the active region of the device. The polarization ratio values obtained from the orange LED were between 0.36 and 0.44, and the optical polarization ratio is not strongly affected by the current injection. These results suggest that nonpolar a-plane LEDs on r-plane sapphire substrates could be a possible approach for the longer wavelength.