Chapter 4 Optical properties of InGaN/GaN multiple quantum well light emitting diodes
4.2 Sample preparation
The sample structure in this study is shown in Fig. 4.2.1. The InGaN/GaN MQW LEDs in this work were grown by a commercial LP-MOCVD system with a vertical reactor. The
MO compounds of TMGa, TMIn, TMAl and gaseous NH3 were employed as the reactant source materials for Ga, In, Al and N, respectively, and H2 and N2 were used as the carrier gas.
All samples were grown on c-plane (0001) sapphire substrate with different misorientation angle of 0o, 0.2o, 0.35o and 1o toward [1120] direction, consisting of 50-nm-thick AlN nucleation layer, a 3.5 µm Si-doped n-type GaN, and an unintentionally doped active layer with InxGa1-xN/GaN MQWs, and 0.4 µm Mg-doped p-type GaN. The doped concentration of n- and p-type GaN is nominally 5 x 1018 and 1 x 1019 cm-3, respectively. For conventional MQW LEDs, the MQWs layer comprise six periods In0.2Ga0.8N well (~ 3 nm) and GaN barrier (~ 14 nm).
After epitaxial growth, the fabrication of devices was carried out as follows. First, the top ohmic contacts of In0.2Ga0.8N/GaN MQW LEDs were formed using a metal system of Ni/Au for p+-GaN contact layer. Next, Ti/Ai systems were evaporated onto the exposed n-GaN epilayer. Finally, In0.2Ga0.8N/GaN MQW LEDs were cut into square pieces with the dimension of 300 × 300 µm2.
Material properties of InGaN/GaN MQW LEDs
4.3
4.3.1 Atomic force microscope image
The AFM images (5µm x 5µm) of the p-type GaN are shown in Fig. 4.3.1 and the root mean square (RMS) of surface roughness measured by AFM is shown in Fig. 4.3.2. From
images we can observe that the surface morphology becomes smoother as misorientation angle increases from 0o to 0.2o, but it is deteriorated as misorientation angle increase above 0.2o. Moreover, except the 0.2o sample, we can see some pits formed in p-type GaN and become more deteriorative in the angle above 0.2o. The formation of pits on p-GaN is due to large dislocation density produced from underlying layer. The pits will degrade the surface morphology and causes rough surfaces with higher RMS values.
4.3.2 High resolution transmission electron microscopy
In order to observe the homogeneity of InGaN/GaN MQWs and the dislocation density in the device, the structural analysis was performed by means of cross-section HRTEM. Fig.
4.3.3 is the cross-section HRTEM observation of the LED grown on sapphire substrate with misorientation angle of 0o, 0.2o and 1o. The n-type GaN, p-type GaN and six QWs can be clearly identified in the images. For 0o and 1o sample, we can see that many dislocations produced from underlying GaN layer penetrate through the InGaN/GaN MQWs and reach upper p-type GaN layer. The dislocation density extracted from TEM image is shown in Fig.
4.3.5. By using misorientation angle of 0.2o, the producing of dislocations is suppressed and the quality of device is improved. About its origins, some articles have proposed that the growth mode of GaN will be shifted from spiral-dominated to step-flow as sapphire substrate with slight misorientation angle about 0.2o [16]. From Burton, Cabrera and Frank (BCF)
theory, spiral growth is dislocations controlled, and it competes with step flow growth on a misorientation substrate [53]. The condition for spiral annihilation can be simplified to:
S
MC ω
ω < (4.3.1)
where ωMC is the misorientation terrace width which decrease with increase of misorientation angle; ωS is width of the terrace on the two interlocking spiral ramps that create a spiral hillock in GaN. When the misorientation angle increases to satisfy the above condition, the misorientation step would annihilate the spiral steps faster than the spiral could spread on the surface. Therefore, due to suppression of spiral growth, the GaN grown on misorientation angle of 0.2o can effectively decrease dislocation produced in underlying GaN layer. On the other hand, the incorporation of group III atoms, the epitaxial growth of III-nitrides, is closely related to the density of dangling bonds at step edges [72]. Therefore, when the misorientation angle is too large, the density of step edges is too high and it becomes difficult to grow with smooth surface, and the strain relaxation in the interface increases, resulting in increasing of dislocation density.
The Fig. 4.3.4 shows the HRTEM images for three samples, the six pairs InGaN/GaN QWs are clear observed, and the well and barrier layer thickness are estimated to be about 3 nm and 14 nm, respectively. In the detailed structure of the MQWs, a lot of dark spots are observed in the wells, the diameters of these dark spots are about 5 nm. This dark spots have
been observed for several articles [26][68], which is the indium-rich clusters due to large discrepancy in atomic size between indium and gallium and the large lattice mismatch of 11%
between InN and GaN. Besides, we also could see that thickness fluctuation existing in MQWs. And from figure we can see the composition and thickness homogeneity of MQWs is improved as miorientation angle increases from 0o to 0.2o, but it deteriorated as misorientation increases above 0.2o.
There are many reports said that indium incorporation is dislocation sensitive [54][71].
From Fig. 4.3.3, the dislocations penetrating through the InGaN/GaN MQWs structures could significantly increase the indium compositional fluctuation around the dislocation. The presence of dislocation in the underlying MQW layer strongly effects the indium incorporation into the upper MQW layers.
Moreover, the indium incorporation is also significantly sensitive to the surface morphology of an underlying layer. The article indicates that the indium incorporation efficiency is drastically higher on the facets than on a smooth surface [55]. The deposition of the indium atoms is attracted at facets rather on a smooth surface to reduce the surface energy due to many dangling bonds at facets. Therefore, the indium phase separation in InGaN QWs layer can easily occur on the rough surface of epilayer. The dislocations will also degrade the surface morphology and cause rough surface with higher RMS values. Therefore, due to fewer dislocation and smoother surface of underlying GaN layer for 0.2o sample, the
homogeneity of InGaN/GaN MQWs is improved.
On the other hand, the dislocations penetrate into p-type GaN will degrade the surface morphology and cause rough surfaces with higher RMS values, which may result in a depletion of Mg from the surface and cause a reduced doping level, deteriorating the contact resistant for InGaN/GaN MQWs LED [56][57]. Moreover, the high dislocation density will induce anomalous high leakage current, then, the performance of LED is degraded [69].
4.3.3 High resolution X-ray diffraction
HRXRD rocking curve (XRC) measurement for symmetrical GaN (002) reflections was applied to evaluate the crystal quality of GaN thin films. The FWHM of XRC for GaN (002) was plotted as shown in Fig. 4.3.6. We can observe that the FWHM is decreased as misorientation angle increases from 0o to 0.2o, but it increases as misorientation angle increase above 0.2o. The FWHM of the symmetrical GaN (002) is correlated to the threading dislocations density [49]. Therefore, it is ascertained that dislocation density tends to decrease as sapphire substrate with slight misorientation angle about 0.2o, but the crystal quality is deteriorated as misorientation angle increases above 0.2o, which is agree with our HRTEM analysis results.
Fig. 4.3.7 shows the HRXRD ω-2θ diffraction pattern for the LEDs with misorientation of 0o, 0.2o, 0.35o, and 1o, respectively. As can be seen, the HRXRD diffraction pattern shows
one periodical structures, which can be attributed to the In0.2Ga0.8N/GaN MQWs. In addition, the third order satellite peak of the diffraction pattern for all samples can be clearly observed, suggesting that the grown structure has sharp interfaces and good periodicity between GaN barrier and InGaN well. Additionally, the inset shows the variation of MQWs interface roughness (IRN) with misorientation angle of sapphire substrate. The interface roughness value of MQWs can be obtained from the following equation [63].
⋅Λ
where n is the order of the satellite; Λ and σ/Λ are thickness of the period and IRN, respectively; ∆θM is the angle distance between adjacent satellite peaks; W0 and Wn are the FWHM of zeroth- and nth-order peaks, respectively. The results are shown in Fig. 4.3.7. From figure we can found that IRN of InGaN/GaN MQWs decreases with increasing misorientation angle of substrate, and reaches minimum which is about 1.35% for 0.2° misorientation sapphire substrate was used. Increasing the misorientation angle to 1° increases the IRN of MQWs. The previous reports indicated that the IRN of MQWs is affected by defect, microstructure and phase separation in MQWs [63]. Based on x-ray analysis, we can prove that the defect density of 0.2o sample is lower than other samples, and can also concluded that defect density of 0.2o sample was reduced dramatically, which are consistent with the results observed from HRTEM images.
Fig. 4.3.9 shows the RSM results for InGaN/GaN MQW LEDs grown on (a) 0°, (b) 0.2°,
(c) 0.35° and (d) 1° misorientation sapphire substrate, respectively. The diffraction pattern was obtained from the asymmetric (1015) reflection of the GaN epilayer. Compared with sample 0o, 0.35o and 1o, the reciprocal lattice point (RLP) of GaN layer of 0.2o sample is circular, indicating that the spread of RSM intensity for 0.2o sample is narrower than that of RSM intensity for other samples. In general, the spread of diffraction intensity is related to the orientation distribution and decrease in the coherency of the scattering along the structure [64].
The asymmetrical feature for RLP of GaN indicates the out-of-plane lattice mismatch formed in GaN layer. This means that the GaN epilayer grown on 0.2° substrate has better crystallization quality. Besides, the RLP of satellite peaks can be clearly observed for 0.2o sample and the satellite peaks up to four orders, while the reciprocal lattice point of satellite peaks became relatively weak for 0o sample and 0.35o sample or vanished for 1o sample, suggesting that the InGaN/GaN MQWs of good quality were grown. Therefore, based on the above-mentioned results, the crystalline quality of 0.2o sample is relatively good in contrast to the quality of other samples, and the noticeable improvement in the quality of GaN epilayer could be responsible for achievement of high quality InGaN/GaN MQWs.
On the other hand, the overall strain state of MQWs with respect to the GaN epilayer can be examined by RSM. S. Pereira et al. proposed the method to investigate the strain and composition distributions within wurtzite InGaN/GaN layers [50]. The information can be extracted from the elongation of broadened RLP in asymmetric x-ray reflections. The
schematic diagram illustrating the effect of strain and composition gradients in the symmetric and asymmetric RLP of InxGa1-xN is shown in Fig. 4.3.10.
The RSM pattern in the Fig. 4.3.9 represent the Qx and Qy position of GaN and MQWs satellite peaks. The distance between GaN epilayer and MQWs satellite peaks in the Qx
direction indicates the degree of lattice relaxation, εxx, and the relationship between εxx and Qx
for all samples can be described by the following equation [73]:
−1 layer and InGaN layer, respectively. Hence, the degree of lattice relaxation for 0
GaN
Qx QxInGaNs
o, 0.2o, 0.35o
and 1o sample is about 1.28×10-4, 7.4×10-5, 8×10-5 and 3.64×10-4, respectively. We can find that the 0.2o sample have the smallest degree of lattice relaxation. For heteroepitaxial structure, the defect density, such as threading dislocation, V defect and so on, of MQWs will increases with increasing the lattice relaxation degree of MQWs. In other words, the InGaN/GaN MQWs of 0.2o sample have the lowest defect density compared with other samples. This think can be proved by our AFM and HRTEM images, the dislocation density decreases as misorientation agnle increases from 0o to 0.2o, but it increases as misorientation angle increases above 0.2o.
4.3.4 Raman spectroscopy
To determine the stress for the LED grown on different misorientation angle, the Raman spectroscopy was performed. In this study, we chose z(y,y)z configuration to probe Raman spectra. For this scattering configuration, the A1(LO) and the E2(high) phonon modes are observed, which correspond to the lattice vibration paralleling and perpendicularing to the c-axis [0001] direction, respectively. Fig 4.3.11 shows the Raman spectra for the InGaN/GaN MQW LEDs grown on different misorientation angle. The two clear active phonon bands are observed at around 568 cm-1 and 735 cm-1, corresponding to the E2(high) and A1(LO) phonon modes of GaN [51], and no InGaN relatived phonon bands are observed in the figure. The E2(high) mode is sensitive to changes in the elastic properties of the material, which is generally observed to estimate the biaxial stress in material. From inset of Fig. 4.3.12, we can observe the position of E2(high) peak shifts slightly with increasing misorientation angle. To extract the accurate position of E2(high) peak, the Lorentzian function was used to fit E2(high) mode peak, the results are shown in Fig. 4.3.12. We can observe that the Raman shift increases as misorientation angle increases from 0o to 0.2o, but it decreases as misorientation angle increase above 0.2o. In general, the Raman shift as a function of the biaxial stress can be expressed as:
σ ω
ω
ωE = E − =C
∆ 2 2 0 (4.3.4)
where ωE2 and ω0 are the Ramsn shift of E2(high) mode for strained sample and strain free
sample, respectively, C is the constant that the shift of the E2(high) with biaxial stress, and σ is the stress. The reports show that the E2(high) phonon of a strain free sample has an ω0 of 568 cm-1 and the shift of the E2(high) mode with biaxial stress is 4.1 cm-1/GPa [52]. From above parameter, the calculated stress as a function of misorientation angle is shown in Fig.
4.3.13. We can observe that the compressive stress increases as misorientation angle increases from 0o to 0.2o, but it decreases as misorientation angle increase above 0.2o, which is agree with previous analysis, due to lowest dislocation density in material, the LED grown on misorientation angle of 0.2o has the smallest degree of lattice relaxation. But as misorientation angle increases above 0.2o, the dislocation density increases, resulting in increasing of lattice relaxation degree of materials.
4.3.5 Summary
From our material experimental results, the LED grown on sapphire sapphire substrates with miorientation angle of about 0.2o can reduce the dislocation density and improve the surface morphology of GaN film, moreover, due to the fewer dislocation density, the best homogeneity and sharper interface of InGaN/GaN MQWs was observed. And the origins are related to the competition between spiral and step growth mode of GaN. But when the misorientation angle is too large, the density of step edges is too high and it becomes difficult
to grow with smooth surface, and the strain relaxation in the interface increases, resulting in increasing of dislocation density, deteriorating the crystal quality and surface morphology of upper layer, and homogeneity of InGaN/GaN MQWs.
Optical properties of InGaN/GaN MQW LEDs
4.4
4.4.1 Room temperature photoluminescence
The Fig. 4.4.1 shows the PL spectra for the LED grown on different misorientation angle.
We observed the InGaN-related emission with peak energy of 2.63, 2.6241, 2.6337 and 2.6848 eV for 0o, 0.2o, 0.35o and 1o sample, respectively. The inset of Fig. 4.4.1 shows the FWHM of PL spectra as a function of misorientation angle. It can be seen that the PL FWHM varies as using different misorientation angle of sapphire substrate, and the minimum is at the misorientation angle of 0.2o. For InGaN based LED, the broadening of PL FWHM may relate to the potential fluctuation of InGaN MQWs, which could be induced by (i) thickness fluctuation, (ii) spatial indium compositional fluctuation [34]. Therefore, by using the sapphire substrate with slight misorientation angle from c-plane may alter the degree of thickness and/or spatial indium compositional fluctuation of InGaN QWs.
4.4.2 Emission energy mapping of micro-photoluminescence
From room temperature PL analysis, we find that the thickness and/or spatial indium compositional fluctuation may be decreased by using the sapphire substrate with
misorientation angle about 0.2o. To direct examine the homogeneity of InGaN MQWs, the emission energy mapping of µ-PL was performed, and the scanning size is 20 x 20 μm2 with the step size of 1μm. Fig. 4.4.2 shows emission energy mapping of µ-PL for InGaN/GaN MQWs grown on sapphire substrate with different misorientation angle. Here, the fluctuation of emission energy was denoted as ∆E and the unit is meV. The fluctuation of emission energy for all samples is summarized in Fig. 4.4.3. As can been seen, the ∆E of 0o, 0.2o, 0.35o and 1o is about 25, 5.6, 11 and 71.2 meV, respectively. It can be seen that the homogeneity of emission energy can be improved as the misorientation angle increases from 0o to 0.2o, but as the misorientation increases above 0.2o, the potential fluctuation becomes larger. These results indicate that degree of potential fluctuation of InGaN MQWs could be decreased by using sapphire substrate misorientation angle about 0.2o, therefore the PL FWHM is decreased, but the potential fluctuation would be deteriorated as sapphire substrate misorientation angle increases above 0.2o, resulting in broadening of PL FWHM.
From previous material analysis, we can know the compositional homogeneity of InGaN MQWs is influenced by the dislocation density produced in the underlying GaN layer. Due to lowest dislocation density in material, the MQWs grown on misorientation angle of 0.2o have good uniformity of indium composition and QWs thickness. But as misorientation angle increases above 0.2o, the dislocation density increases, resulting in increasing of indium composition and thickness fluctuation in MQWs.
4.4.3 The localization effect of InGaN/GaN MQW LEDs
4.4.3.1 Temperature dependent photoluminescenceSeveral authors have reported that the potential fluctuation may induce the carrier localization effect such as anomaly S-shaped temperature dependent emission energy shift [18][19]. So, by measuring the temperature dependent PL we can characterize the degree of potential fluctuation for sapphire substrate with misorientation angle of 0o, 0.2o, 0.35o and 1o. To direct probe the optical properties of InGaN MQWs and avoid the absorption in GaN layer, the frequency doubled (2w) Ti : sapphire laser with resonant wavelength of 390 nm was used to excite sample. The temperature dependent PL spectra over a temperature range from 15 K to 300 K for all samples are shown in Fig. 4.4.4. We observed InGaN-related emission with peak energy of 2.6811, 2.682, 2.6747 and 2.7314 eV for 0o, 0.2o, 0.35o and 1o sample at 15 K, respectively. The anomalous emission behavior, so-called S-shaped (red-blue-red) temperature dependence of the peak energy for InGaN-related emission with increasing temperature is observed, especially for sample with misorientation angle of 1o. The anomalous emission behavior is generally observed in InGaN material system, which is attributed to excitons localized in potential minimum resulted from indium composition fluctuation and/or thickness fluctuation of QW, the more detailed statement is introduced in Chapter 2. The emission energies as a function of temperature for InGaN-related emission for all samples are shown in Fig. 4.4.5. To quantify the degree of potential fluctuation of InGaN
as function of misorientation angle, we used Eliseev et. al. proposed model mentioned in chapter 2 to fit our experimental data. They assumed that the DOS of excitons induced by potential fluctuation is of Gaussian form having dispersions of σ2, then the emission peak can be expressed as E(T) = E(0) - αT2/(T+β) - σ2/kBT.
By using above equation to fit our temperature dependent results (solid line in 4.4.5) we can extract the broadening parameter σ, which are shown in Fig. 4.4.6. The broadening parameter σ is the parameter related to the degree of carrier localization in MQWs, it increases as the degree of carrier localization in MQWs increases. We can observe that the σ decreases as misorientation angle increases from 0o to 0.2o, then it increases as misorientation angle increases above 0.2o, which indicate the degree of carrier localization in InGaN QW is decreased as misorientation angle increases from 0o to 0.2o, but carrier localization effect becomes stronger as misorientation increases above 0.2o.
On the other hand, the thermal activation energy Ea is considered to be related to the effective potential barrier of localized states, assuming that the radiatively recombining excitons are localized and escape by thermal activation to recombination nonradiatively. The thermal activation energy Ea can be obtained by fitting the data with I(T) = I(0)/(1+A*exp(-Ea/kT)). Fig. 4.4.7 shows the Arrhenius plot for PL intensity of the sample
On the other hand, the thermal activation energy Ea is considered to be related to the effective potential barrier of localized states, assuming that the radiatively recombining excitons are localized and escape by thermal activation to recombination nonradiatively. The thermal activation energy Ea can be obtained by fitting the data with I(T) = I(0)/(1+A*exp(-Ea/kT)). Fig. 4.4.7 shows the Arrhenius plot for PL intensity of the sample