Chapter 1 Time-resolved Electroluminescence Studies of Nonpolar m-plane InGaN/GaN Multiple Quantum Wells
1.1 Introduction
III-nitride semiconductors have become the key technological material for light-emitting diodes and laser diodes. These devices are generally fabricated along the c-axis, where piezoelectric and spontaneous polarizations lead to quantum-confined Stark Effect (QCSE) [1,2]. Nonpolar devices grown along the m- and a-axes have been demonstrated to be polarization-free, so can be used to reduce QCSE [3]. In our previous study, we report the roles of island coalescence rate and strain relaxation in the development of anisotropic in-plane strains and subsequent degree of polarization in m-plane GaN [4].
Figure 1.1 shows PL peak energies of the LEDs grown on m- and c-plane substrates, together with the data of InGaN single quantum well (SQW) with a well thickness of 100 nm.
The following fitting data was obtained for the 100 nm SQW and 8.0 nm MQWs. With In composition in InGaN increasing, the peak position is red-shifted. It is clearly seen that the PL peak energy of InGaN well thickness of 2.5 nm is higher than that of 8.0 nm. The energy difference is estimated to be 0.2 - 0.3 eV, which is reasonable to explain the quantum confinement effect. This trend was also confirmed in m-plane [5] and a-plane InGaN/GaN MQWs [6,7].
Figure 1.2 shows the optical polarization ratios of the LED grown on m-plane substrate as a function of emission wavelength. Degree of polarization is defined as
) /(
)
(IaIc IaIc
, where Ia and Ic denote electroluminescence (EL) intensity at 1 mA with polarization parallel to the a- and c-axis, respectively. The polarization ratio increases from 0.27 to 0.89 with increasing emission wavelength from 383 to 476 nm, corresponding to the indium composition from 0.02 to 0.08. The increase in the polarization
2
ratio is most probably due to energy separation, which is caused by compressively strained InGaN QWs [7].
Figure 1.3 (a) shows the electroluminescence (EL) spectra of the m-plane LED for the DC current density ranging from 1.1 to 330 A/cm2, and the emission peak and full width at half-maxima (FWHM) of EL are shown in Figure 1.3 (b). The emission peak remains constant before suffering from excess heat at high current operation, indicating the absence of the polarization-induced electric fields in the m-plane MQWs. The initial blue-shift in the emission peak for driving current from 1.1 to 11 A/cm2 can be attributed to the band-filling of the localized states induced by alloy fluctuation in the InGaN QWs [8, 9].
Figure 1.4 (a) shows the EL polarization anisotropy [10]. The peak energy shift E and the degree of polarization of the EL intensity were analyzed by rotating a polarizer between the polarization angles 0 and 360°. The E is about 39.7 meV between the electric field perpendicular (E) to and parallel (E//)to the c-axis. The peak energy shift comes from the splitting of the valence bands caused by the in-plane compressive strain of the MQW, and the energy separation is proportional to the In composition of the active layer. Figure 1.4 (b) shows the angular dependence of the polarization ratio under the operation current of 20 mA. The degree of polarization is defined as (I I//)/(I I//), where I and I//
are the intensities of the E and E// components, respectively. The degree of polarization in nonpolar LEDs gradually increases with a larger valance band splitting due to the higher In composition as well as higher compressive strain in the active region InGaN [11]. The large value of the degree of polarization in m-plane LEDs could be contributed from the higher quality of InGaN MQWs because the proper growth condition can prevent the InGaN from phase separation on the m-plane GaN surface and can result in higher compressive strain [9].
Figure 1.5 (a) shows optical micrographs of the Si-doped n-type GaN films. Angle
3
indicates the misorientation towards the [0001] c -direction. For the on-axis free-standing )
0 1 10
( m-plane GaN substrates, MOCVD growth results in surfaces covered with shallow four-sided pyramidal hillocks. The pyramidal hillocks could be eliminated by growing GaN films on misoriented m-plane GaN substrates instead of on-axis m-plane GaN substrates [12].
Figure 1.5 (b) shows the dependence of root-mean-square (RMS) roughness on misorientation angle for Si-doped n-type GaN films [13,14].
Figures 1.6 show Nomarski [(a) and (c)] and fluorescence [(b) and (d)] optical micrographs from samples A (nominally on-axis m-plane GaN substrate) and B (m-plane GaN substrate misoriented 1◦ towards the [0001] c -direction), respectively. Figures 1.6 (e) and (f) show light versus current (L–I) curves from several 2 μm × 500 μm LDs for samples A and B, respectively. The threshold currents for the sample A (400-1000 mA) were higher and showed more variation than the threshold currents for the sample B. Figures 1.6 (g) and (h) show emission spectra for 2 μm × 500 μm LDs for samples A and B with drive currents of 10mA, 0.9 times the threshold current (I 0.9Ith), and just above the threshold current
)
(I Ith , respectively [14,15].
Figure 1.7 shows the dependence of relative output power on EL peak wavelength for a large number of LDs grown on m-plane and (2021) GaN substrates. The m-plane LDs show a drastic decrease in output power between 480 and 520 nm, while the (2021) LDs maintain relatively high output powers beyond 530 nm [14,16].
Figure 1.8 shows peak wavelength as a function of TMI flow for SQW LEDs grown on (a) (2021)-, (2021)-, (3031)-, (3031)-, and m-planes at a growth temperature of 780°C and on (b) (2021)- and (1122)-planes at a growth temperature of 830°C. The relevant nonpolar and semipolar planes in the wurtzite crystal structure are shown in the inset. The different TMI flow can affect EL wavelength position. The EL peak position is red-shifted under higher TMI flow [17].
4
Figure 1.9 shows simulated band diagrams and emission wavelengths for SQW (with 25% indium composition) blue and green LEDs grown on the (a) (2021)-, (b) (2021)-, (c)
) 2 2 11
( -, and (d) m-planes at a current density of 20 mA/cm2. These differences are explained by the magnitudes and directions of polarization-related electric fields in the MQWs. For the
)