Summary

From our material experimental results, the LED grown on sapphire sapphire substrates with miorientation angle of about 0.2^o 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 0^o, 0.2^o, 0.35^o and 1^o 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 0.2^o. 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 0.2^o. 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 0^o, 0.2^o, 0.35^o and 1^o 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 0^o to 0.2^o, but as the misorientation increases above 0.2^o, 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 0.2^o, therefore the PL FWHM is decreased, but the potential fluctuation would be deteriorated as sapphire substrate misorientation angle increases above 0.2^o, 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 0.2^o have good uniformity of indium composition and QWs thickness. But as misorientation angle increases above 0.2^o, 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 photoluminescence

Several 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 0^o, 0.2^o, 0.35^o and 1^o. 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 0^o, 0.2^o, 0.35^o and 1^o 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 1^o. 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 0^o to 0.2^o, then it increases as misorientation angle increases above 0.2^o, which indicate the degree of carrier localization in InGaN QW is decreased as misorientation angle increases from 0^o to 0.2^o, but carrier localization effect becomes stronger as misorientation increases above 0.2^o.

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 with different misorientation angle, and the solid line in figure is the fitted results. We got Ea

= 46.55, 35.72, 45.27 and 62.02 meV for the sample with misorientation angle of 0^o, 0.2^o,

0.35^o and 1^o, respectively, which are shown in Fig. 4.4.8. The results indicate that the effective potential barrier of localized states decreases as misorientation angle increases from 0^o to 0.2^o, but increases as misorientation angle increases above 0.2^o. And due to smaller effective potential of localized states in InGaN QW, the 0.2^o sample exhibits the weak carrier localization effect observed in temperature dependent emission energy.

4.4.3.2 Temperature dependent time-resolved photoluminescence

The temperature dependence of TRPL was performed for the evaluation of dimensionality of nanostructure (Related information is introduced in Chapter 2). By temperature dependenct PL and TRPL experiment, the temperature dependent radiative and nonradiative recombination lifetimes can be obtained. Fig. 4.4.9 shows the temperature dependent carrier lifetime, radiative and nonradiative recombination lifetime for all samples.

By analyzing the temperature dependent radiative recombination lifetime, the dimensionality of nanostructure in InGaN QW can be examined. Fig. 4.4.10 shows the temperature dependent radiative lifetime for all samples. The τr at lower temperature range exhibits an almost temperature independent behavior for all samples, this indicates that the excitons are confined in the quasi-zero-dimensional (0D) potential, which is related to the indium-rich localized states. But as temperature increases above the transfer temperature (Tr), the τr

increases gradually, and increases proportionally to T^P (P > 0). This indicates that excitons are

delocalized from low dimensional deeper localized states to higher dimensional extended states as temperatue increases above Tr. And the P values are 1.24, 1.46, 1.37, and 0.52 for 0^o, 0.2^o, 0.35^o, and 1^o sample, respectively. From the information introduced in Chapter 2, the P = 3/2 represents the three-dimensional (3D) feature of excitons. Therefore, the excitons are delocalized from localized states to quasi-3D states as temperature increases above Tr for 0^o, 0.2^o and 0.35^o sample. But for 1^o sample, the P exhibits a small value of about 0.5, it indicates the excitons are confined in quasi-one-dimensional (1D) potential, although the temperature increases above Tr. On the other hand, the value of Tr decreases from 140 K to 100 K as misorientation angle increases from 0^o to 0.2^o, but it increases as misorientation angle increases above 0.2^o. This indicates that the effective potential barrier of localized states in InGaN QW for 0.2^o sample is the smallest, so carriers can escape from localized states to higher dimensional states at lower temperature, and the effective potential barrier of localized states in InGaN QW for 1^o sample is the largest, so carriers need higher thermal energy to escape. The results are consistent with the thermal activation energy extracted from temperature dependent PL intensity.

From the analysis of dimensional related parameter P and transfer temperature Tr, we could know the sample with misorientation angle of 0.2^o has best homogeneity of InGaN MQWs and smallest effective potential barrier of localized states, resulting in the weakest carrier confinement, and the sample with misorientation angle of 1^o has worst homogeneity of

InGaN MQWs and largest effective potential barrier of localized states, resulting in the strong carrier localization effect.

The Fig. 4.4.11 shows the carrier lifetime detected at peak energy as a function of misorientation angle at 15 K. The carrier lifetime increases as misorientation angle increases from 0^o to 0.2^o, but it decreases as misorientation angle increases above 0.2^o. The carrier lifetime at 15 K approximately equals to radiative recombination lifetime. This indicates that the sample with misorientation angle of 0.2^o has lowest radiative recombination rate, it may due to the weaker carrier localized effect, resulting in decreasing of radiative recombination efficiency.

4.4.3.3 Power dependent photoluminescence

The Fig. 4.4.12 shows the PL intensity as a function of excitation power at 15 K. The PL intensity is linear with excitation power at lower excitation power range, which indicates that the radiative recombination dominates the recombination process at 15 K for all samples in this excitation power range [56][57][58]. But as excitation power increases above about 20 mW, the PL intensity exhibits a tendency of saturation, this phenomenon will be discussed later. The Fig. 4.4.13 shows the emission energy and FWHM as a function of excitation power at 15 K for all samples. The emission energy gradually increases with increasing excitation power. On the other hand, the linewidth of spectra shrinks with increasing excitation power

from 0.05 mW to about 1 mW, and if injected power increase continually, then broadening of spectra is observed. In generally, the left side of red dash line in Fig. 4.4.13 is dominated by the coulomb screening of the QCSE and the right side of red dash line is dominated by the band filling effect of localized states [33][34][35]. (More detailed explanation will be stated in Chapter 5) The emission energy blue-shift resulted by bang filling effect is calculated to examine the degree of carrier localization, and the values are 55.5, 45.2, 47.2 and 133.3 meV for 0^o, 0.2^o, 0.35^o and 1^o sample, respectively. In general, the degree of emission energy blue-shift with increasing excitation power is proportion to the effective potential barrier of localized states, more deep of localized states will result in more blue shift of emission energy with increasing excitation power. The results indicate that the effective potential barrier of localized states decreases as misorientation angle increases from 0^o to 0.2^o, but it increases as misorientation angle increases above 0.2^o. And the results are good agree with the analysis of TRPL and thermal activation energy extracted from temperature dependent PL.

On the other hand, the PL efficiency (will be defined in Chapter 5) as a function of excitation power at 15 K was studied, which is shown in Fig. 4.4.14. The PL efficiency curve in Fig. 4.4.14 reflects the IQE, because the light extraction efficiency (LEE) does not depend on excitation power of laser light. From figure we can see the IQE increases with increasing excitation power under lower excitation power range, when the excitation power further increases, then the IQE decreases. From the discussion in Chapter 5, we know that the

increasing of PL efficiency at 15 K is due to the coulomb screening of the QCSE, and the decreasing of IQE is due to band filling effect of localized states. The former effect will increase the electron-hole wavefunction overlap, resulting in increasing of IQE. The latter effect will make the carriers more easily escape from localized states to extended states, which deteriorates the IQE. From the figure we can observe two phenomena: (i) The more pronounced decreasing of IQE for 0.2^o and 0.35^o. (ii) The IQE for 0.2^o and 0.35^o sample are more easy saturate than it for 0^o and 1^o sample

From the analysis of previous section we can know this phenomenon is due to that the 0.2^o and 0.35^o sample has smaller effective potential barrier of localized states, making the carriers more easily escape from localized states to extended states, resulting in more easy saturation and more pronounced decreasing of IQE. On the contrary, the 0^o and 1^o sample has larger effective potential barrier of localized states, making the carriers more difficultly escape from localized states to extended states, resulting in the peak of efficiency curve shifts to higher excitation power and PL efficiency decreases more weakly at higher excitation power.

4.5 Electroluminescence intensity as a function of injected current

Fig. 4.5.1 shows the output power as a function of injected current for the LED with different misorientation angle. The light output powers at 20 mA are 10.3, 13.3, 12.1 and 7.1

mW for the LED with misorientation angle of 0^o, 0.2^o, 0.35^o and 1^o, respectively, i.e., an improvement factor of approximate to 1.29 was achieved by increasing misorientation angle from 0^o to 0.2^o. But as the misorientation angle increases above 0.2^o, the degradation of output power is observed. The improvement of luminescence efficiency in 0.2^o sample could be due to the good surface morphology and the low dislocation density in the device, lowering the leakage current and contact resistant. On the other hand, p-type doping of GaN is a very important parameter in achieving high quality devices. The smoother surface morphology and the low dislocation density will improve the p-type doping level [56][57]. Although the InGaN/GaN MQWs with misorientation angle of 0^o, 0.35^o, and 1^o has larger effective potential barrier of localized states, but due to the increasing in dislocation density, current leakage and contact resistant, and the decreasing of p-type doping level, the performance of device is degraded.

4.6 Conclusion

From our material analysis results and some references, when the GaN grown on sapphire substrate with slight misorientation angle of 0.2^o, the condition for spiral annihilation can be satisfied, then the growth mode of GaN will be shifted from spiral-dominated to step-flow. Therefore, due to suppression of spiral growth, the dislocation density in underlying GaN layer can be effectively decreased, making upper layer with better crystal

quality and smoother surface, and better homogeneity of InGaN/GaN MQWs. 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. 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.

And due to the dislocation density and surface morphology of underlying GaN layer varies as a function of misorientation angle of sapphire substrate, the homogeneity of upper InGaN/GaN MQWs is influenced, because the indium incorporation rate in InGaN is strong depended on the dislocation density and surface morphology in underlying GaN layer. So from our optical properties analysis results, we observed that the degree of carrier localization in InGaN MQWs varies with the misorientation angle of sapphire substrate. And the InGaN MQWs with misorientation angle of 0.2^o has the smallest effective potential barrier of localized states, which have weaker carrier localization effect and lager dimension of nanostructure.

Moreover, due to decreasing of dislocation density and better surface morphology of sample with misorientation angle of 0.2^o, it may improve the p-type doping level, lower the contact resistant, and reduce the current leakage in the device, therefore enhance the

luminescence efficiency of LED. Although the InGaN/GaN MQWs with misorientation angle of 0^o, 0.35^o, and 1^o has larger effective potential barrier of localized states, but due to poor surface morphology and high dislocation density in the device, the performance of device is deteriorated.

Fig. 4.1.1 Diagram of sapphire substrate with misorientation angle toward [1120] direction.

Fig. 4.2.1 Sample structure.

Fig. 4.3.1 The AFM images of p-type GaN grown on sapphire substrate with different misorientation angle.

0.0 0.2 0.4 0.6 0.8 1.0 0.0

0.5 1.0 1.5 2.0 2.5 3.0

Rms (nm) 5 µ m x 5 µ m

Mis - orientation angle (degree)

Fig. 4.3.2 The surface roughness of p-type GaN as a function of misorientation angle.

0 ^o

p - GaN

MQWs

n - GaN

0.2 ^o

p - GaN

MQWs

n - GaN

1 ^o

p - GaN

MQWs

n - GaN

Fig. 4.3.3 HRTEM images for the LED grown on sapphire substrate with different misorientation angle.

0 ^o 0 ^o

0.2 ^o 0.2 ^o

1 ^o 1 ^o

Fig. 4.3.4 HRTEM images for the MQWs grown on sapphire substrate with different misorientation angle.

0.0 0.2 0.4 0.6 0.8 1.0 1

2 3 4 5 6 7

Dislocation density (cm-2 ) / 108

Mis - orientation angle (degree)

Fig. 4.3.5 The dislocation density as a function of misorientation angle.

0.0 0.2 0.4 0.6 0.8 1.0 240

250 260 270 280 290 300

(0 02) FWHM (arcsec)

Mis - orientation angle (degree)

Fig. 4.3.6 FWHM of XRC for GaN (002) reflections plotted as a function of misorientation angle.

10

-4000 -3000 -2000 -1000 0 1000 2000 3000 4000

ω

Fig. 4.3.7 The ω-2θ scan (0002) for the samples grown on sapphire substrate with different misorientation angle.

Fig. 4.3.8 The IRN of InGaN/GaN MQWs.

(a)

+2

GaN

-1

(b) +2

GaN

0

-1

-2

-3

-4

(c) +2 +1 GaN

-1

-2

(d)

+2

GaN

Fig. 4.3.9 Reciprocal space mapping measured around the (1015) reflection (a) 0^o (b) 0.2^o (c) 0.35^o (d) 1^o.

Fig. 4.3.10 The schematic diagram illustrating the effect of strain and composition gradients in the symmetric and asymmetric RLPs of InxGa1-xN.

500 550 600 650 700 750 800

A1(LO)

Raman intens ity (a.u.)

Raman shift (cm

^-1

)

0^o 0.2^o 0.35^o 1^o E2(High)

Fig. 4.3.11 The Raman spectra for the InGaN/GaN MQW LEDs grown on sapphire substrate with different misorientation angle.

0.0 0.2 0.4 0.6 0.8 1.0

540 550 560 570 580 590 600

Raman intensity (a.u.)

Fig. 4.3.12 The Raman shift of E2(high) mode as a function of misorientation angle. The inset shows the the Raman spectra for the LED grown on sapphire substrate with different

misorientation angle.

Fig. 4.3.13 The calculated compressive stress as a function of misorientation angle.

2.0 2.2 2.4 2.6 2.8 3.0 3.2 3.4 3.6

PL In ten sity (a .u .)

Emission energy (eV)

R.T.

0^o 0.2^o 0.35^o

1^o 0.0 0.2 0.4 0.6 0.8 1.0

128 132 136 140

FWHM (meV)

Mis-orientation angle (degree)

Fig. 4.4.1 The PL spectra for the LED grown on sapphire substrate with different misorientation angle at room temperature, and the inset shows the FWHM of spectra as a

function of misorientation angle.

Fig. 4.4.2 Normalized emission energy mapping of µ-PL from InGaN/GaN MQWs grown on sapphire substrate with different misorientation angle.

0.0 0.2 0.4 0.6 0.8 1.0 0

10 20 30 40 50 60 70

Fluctuation of emission energy (meV)

Mis - orientation angle (degree)

Fig. 4.4.3 The fluctuation of emission energy as a function of misorientation angle.

2.4 2.6 2.8 3.0

Fig. 4.4.4 The temperature dependent PL spectra over a temperature range from 15 K to 300 K for the LED grown on sapphire substrate with different misorientation angle.

0 50 100 150 200 250 300 350

Fig. 4.4.5 The emission energies as a function of temperature for InGaN-related emission for the LED grown on sapphire substrate with different misorientation angle.

0.0 0.2 0.4 0.6 0.8 1.0

Fig. 4.4.6 The broadening parameter as a function of misorientation angle.

0 10 20 30 40 50 60 70 80

Fig. 4.4.7 The temperature dependent PL intensity for the LED grown on sapphire substrate with different misorientation angle.

Fig. 4.4.8 The thermal activation energy as a function of misorientation angle.

0 50 100 150 200 250 300 350

Fig. 4.4.9 The temperature dependent radiative recombination lifetime, nonradiative recombination and carrier lifetime for the LED grown on sapphire substrate with different misorientation angle.

Fig. 4.4.10 The temperature dependent radiative recombination lifetime for the LED grown on sapphire substrate with different misorientation angle.

0.0 0.2 0.4 0.6 0.8 1.0 12

14 16 18 20 22

Carrier lifetime (ns)

Mis - orientation angle (degree)

Fig. 4.4.11 The carrier lifetime detected at peak energy as a function of misorientation angle at 15 K.

0.01 0.1 1 10 100

PL inte ns ity (a.u.)

Excitation power (mW)

0^o 0.2^o 0.35^o 1^o

Fig. 4.4.12 The PL intensity as a function of excitation power at 15 K for the LED grown on sapphire substrate with different misorientation angle

1E-3 0.01 0.1 1 10 100

Fig. 4.4.13 The emission energy and FWHM as a function of excitation power for the sample grown on sapphire substrate with different misorientation angle.

1E-3 0.01 0.1 1 10 100

Fig. 4.4.14 The PL efficiency as a function of excitation power at 15 K.

0 20 40 60 80 100 0

10 20 30 40 50 60

Output power (mW)

Current (mA)

0^o 0.2^o 0.35^o 1^o

Fig. 4.5.1 Output power as a function of current for the LED grown on sapphire substrate with different misorientation angle.

Chapter 5 Physical mechanisms of excitation power dependent internal quantum efficiency in InGaN/GaN multiple quantum well light emitting diodes

5.1 Introduction

The InGaN/GaN material system has attracted much attention due to their tremendous potential for fabricating light emitting diodes operated from visable to ultraviolet energy

The InGaN/GaN material system has attracted much attention due to their tremendous potential for fabricating light emitting diodes operated from visable to ultraviolet energy

在文檔中 氮化銦鎵/氮化鎵多重量子井發光二極體之光學特性與內部量子效率研究 (頁 57-0)