Experiment Procedure

Preprocedure

In FTIR reflectance spectrum measurements, the Mylar 6, Mercury lamp and DTGS 201 will be chosen for its beamsplitter, far IR light source and detector that for the InGaAlP/GaAs samples. And the KBr, Globar and DTGS 301 will be chosen for its beamsplitter, middle IR light source and detector that for the InN/sapphire. The pressure of the sample room contains at about 10^-6 torr in the experiment process.

Then, these InAs_1-xN_x/InP were measured by Prof. Hung [53].

When we start to measure these samples, we should confirm the IR background that the MIR and FIR background spectra were presented in Figure 3.5.1 and Figure 3.5.2.

1000 2000 3000 4000 5000 6000 7000 Vacuum 300 K

Wavenumber (cm^-1)

MIR-background

FIGURE 3.5.1: The background of middle infrared spectrum.

100 200 300 400 500 600 700

Vacuum 300 K

Wavenumber (cm^-1)

FIR-background

FIGURE 3.5.2: The background of far infrared spectrum.

Spectra Analysis

After the measurements, the spectra could be fitted by the fitting program which was written by Fortran or Matlab and is showing below with its directions. We used the dielectric response function to analysis the infrared spectra. From the spectral peak positions, the phonon modes frequency all could be fined approximately.

However, it is impossible for us to find the other parameters without a calculating program. Therefore the multi-layer fitting model is employed to analysis the infrared spectra.

The multi-layer fitting model is based on the non-linear least squares fitting. It is the form of least squares analysis which is used to fit a set of m observations with a model that is non-linear in n unknown parameters (m > n). It is used in some forms of non-linear regression. The basis of the method is to approximate the model by a linear one and to refine the parameters by successive iterations. There the parameter ² could be expressed as

 

the theoretical values.

 

[33-35]. All the fitting parameters from eq. (2.5.1) are

_: High-frequency dielectric constant.

S : Phonon oscillator strength.

 : Phonon damping constant.

n: Free carrier concentration.

: Free carrier mobility.

m*: Effective mass of free carrier.

d : Thickness of the film.

The details of the program operation procedures are presented in the appendix.

FIGURE 3.5.3: Fitting program of dielectric response model by Fortran.

FIGURE 3.5.4: Fitting program of dielectric response model by Matlab.

CHAPTER 4

EXPERIMENT RESULTS AND DISCUSSIONS

The results of infrared reflectance spectra measurements were presented in this chapter. At the beginning, let we recall the dielectric response model and its relations with refractive index, extinction coefficient and the reflectivity from chapter 2.

   



 

 

 

 program. After fitting, several parameters will be defined by dielectric response function fitting such as high-frequency dielectric constant, transverse-optical (TO) phonon frequency, strength, damping, free carrier concentration, mobility, effective mass and the thickness of the sample.

However, there are some parameters undefined such as the longitudinal-optical (LO) phonon mode frequency, conductivity and force constant between cations and

anions. We continued to discuss these equations. The conductivity was calculated from the relation with free carrier concentration and mobility as [39]



 ne (4.1) With  is conductivity, n is free carrier concentration, e is the charge of an electron and  is mobility. Furthermore, from the eq. (2.3.12), the phonon strength of TO1 and TO2 can be expressed as [41] TO2 and LO2 phonon modes. It also could be extended to TO3 and LO3.

After fitting and calculation, these optical and transport properties parameters will be defined and their physical meanings also will be discussed.

4.1 MIDDLE INFRARED SPECTRUM ANALYSIS OF InN/Sapphire

These are twelve InN films grown on Sapphire have been measured by Fourier transform far infrared spectrometer Bruker IFS 66 v/S with different grown conditions.

The measured range of middle infrared reflectance spectra is from 400 cm^-1 to 4000 cm^-1 at 300K. We found a one mode behavior of InN but the effects from Sapphire substrate cannot be ignored. And we also found a four mode behavior of sapphire

fitting results and discussions will be presented, too.

Middle Infrared Reflectance Experiment Results

400 500 600 700 800 900 1000

0.0 0.2 0.4 0.6 0.8 1.0

InN/sapphire 300K

Reflectivity

Wavenumber (cm^-1) CC21 CC28 CC30 CC30 CD2 CD7 CD8 CD9 CD10 CD14 CD15 CD16

FIGURE 4.1.1: Middle Infrared reflectance spectra of InN/sapphire from 400 cm^-1 to 1000 cm^-1.

500 1000 1500 2000 2500 3000 3500 4000

FIGURE 4.1.2: Middle Infrared reflectance spectra of InN/sapphire from 400 cm^-1 to 4000 cm^-1.

400 500 600 700 800 900 1000

0.0

Sapphire (substrate)

300K

Reflectivity

Wavenumber (cm^-1)

Experiment

FIGURE 4.1.3: Middle Infrared reflectance spectra of Sapphire substrate from 400 cm^-1 to 1050 cm^-1.

The reflectance spectra measured at room temperature for the InN/sapphire samples and Sapphire substrate were presented in Figure 4.1.1-3. As shown in Figure 4.1.1-2, the spectra in the figure can be simply divided into two regions, i.e. the region between 400 cm^-1 and 1000 cm^-1 and above 1000 cm^-1. All spectra display four peaks and we can see the position at the frequencies around of 444, 577 and 644 cm^-1 that are the TO-phonon frequencies of the sapphire [56,57], and also at the frequency around of 483 cm^-1 that is the E₁(TO)-mode of the InN. Obviously the all peaks seem no shift and the sapphire substrate structures only appear between 400 cm^-1 and 1000 cm^-1. But M-IR structures of the InN/sapphire extend to 1500cm^-1 even higher. We guess it will be contribution from the carrier concentration [58].

By the way, the characteristic plasma edge also can be seen. From the report of E.

Frayssinet et al [58], the plasma edge will shifted towards higher frequencies with increasing electron concentration. So we could discussion this phenomenon at next section. The thickness of films can be determined by various interferometric methods [59]. In our work, we use the IR measurement and theoretical calculations to obtain the thickness of film.

Fitting Results and Discussions

400 500 600 700 800 900 1000

0.0 0.2 0.4 0.6 0.8 1.0

Sapphire (substrate)

300K

Reflectivity

Wavenumber (cm^-1)

Experiment Fitting

FIGURE 4.1.4: Middle infrared reflectance spectrum and fitting of Sapphire substrate.

TABLE 4.1.1: Classical oscillator parameters for sapphire o-ray. [55]

i S_i ωi

i i

ω γ

(cm^-1) (cm^-1)

1 0.33 384.3 0.011

2 2.788 438.9 0.006

3 2.98 568.2 0.012

4 0.145 633.6 0.010

5 0.0185 809.6 0.157

6 0.65068713 85723.7 0.00001

7 1.4313993 137621.4 0.00001

500 1000 1500 2000 2500 3000 3500 4000 0.0

0.2 0.4 0.6 0.8 1.0

400 600 800 1000 1200 1400

InN/sapphire CC-21

300K

Reflectivity

Wavenumber (cm^-1)

Experiment Fitting

FIGURE 4.1.5: Middle infrared reflectance spectrum and fitting of InN/sapphire CC-21.

500 1000 1500 2000 2500 3000 3500 4000 0.0

0.2 0.4 0.6 0.8 1.0

400 600 800 1000 1200

InN/sapphire CC-28

300K

Reflectivity

Wavenumber (cm^-1)

Experiment Fitting

FIGURE 4.1.6: Middle infrared reflectance spectrum and fitting of InN/sapphire CC-28.

500 1000 1500 2000 2500 3000 3500 4000 0.0

0.2 0.4 0.6 0.8 1.0

400 600 800 1000

InN/sapphire CC-30

300K

Reflectivity

Wavenumber (cm^-1)

Experiment Fitting

FIGURE 4.1.7: Middle infrared reflectance spectrum and fitting of InN/sapphire CC-30.

500 1000 1500 2000 2500 3000 3500 4000 0.0

0.2 0.4 0.6 0.8 1.0

400 600 800 1000 1200

InN/sapphire CC-40

300K

Reflectivity

Wavenumber (cm^-1)

Experiment Fitting

FIGURE 4.1.8: Middle infrared reflectance spectrum and fitting of InN/sapphire CC-40.

500 1000 1500 2000 2500 3000 3500 4000 0.0

0.2 0.4 0.6 0.8 1.0

400 600 800

InN/sapphire CD-2

Reflectivity 300K

Wavenumber (cm^-1)

Experiment Fitting

FIGURE 4.1.9: Middle infrared reflectance spectrum and fitting of InN/sapphire CD-2.

500 1000 1500 2000 2500 3000 3500 4000 0.0

0.2 0.4 0.6 0.8 1.0

400 600 800 1000

InN/sapphire CD-7 300K

Reflectivity

Wavenumber (cm^-1)

Experiment Fitting

FIGURE 4.1.10: Middle infrared reflectance spectrum and fitting of InN/sapphire CD-7.

500 1000 1500 2000 2500 3000 3500 4000 0.0

0.2 0.4 0.6 0.8 1.0

400 600 800 1000

InN/sapphire CD-8 300K

Reflectivity

Wavenumber (cm^-1)

Experiment Fitting

FIGURE 4.1.11: Middle infrared reflectance spectrum and fitting of InN/sapphire CD-8.

500 1000 1500 2000 2500 3000 3500 4000 0.0

0.2 0.4 0.6 0.8 1.0

400 600 800 1000

InN/sapphire CD-9 300K

Reflectivity

Wavenumber (cm^-1)

Experiment Fitting

FIGURE 4.1.12: Middle infrared reflectance spectrum and fitting of InN/sapphire CD-9.

500 1000 1500 2000 2500 3000 3500 4000 0.0

0.2 0.4 0.6 0.8 1.0

400 600 800

InN/sapphire CD-10

Reflectivity 300K

Wavenumber (cm^-1)

Experiment Fitting

FIGURE 4.1.13: Middle infrared reflectance spectrum and fitting of InN/sapphire CD-10.

500 1000 1500 2000 2500 3000 3500 4000 0.0

0.2 0.4 0.6 0.8 1.0

400 600 800

InN/sapphire CD-14

300K

Reflectivity

Wavenumber (cm^-1)

Experiment Fitting

FIGURE 4.1.14: Middle infrared reflectance spectrum and fitting of InN/sapphire CD-14.

500 1000 1500 2000 2500 3000 3500 4000 0.0

0.2 0.4 0.6 0.8 1.0

400 600 800 1000

InN/sapphire CD-15

300K

Reflectivity

Wavenumber (cm^-1)

Experiment Fitting

FIGURE 4.1.15: Middle infrared reflectance spectrum and fitting of InN/sapphire CD-15.

500 1000 1500 2000 2500 3000 3500 4000 0.0

0.2 0.4 0.6 0.8 1.0

600 900 1200 1500 1800

InN/sapphire CD-16

Reflectivity 300K

Wavenumber (cm^-1)

Experiment Fitting

FIGURE 4.1.16: Middle infrared reflectance spectrum and fitting of InN/sapphire CD-16.

TABLE 4.1.2: Optical constants of InN by dielectric response function fitting.

Samples

Name STO ωTO γTO ωP γP

(cm^-1) (cm^-1) (cm^-1) (cm^-1) (cm^-1)

CC-21 5.47 478.01 5.84 2457.15 169.34

CC-28 4.87 477.65 5.56 1890.96 150.07

CC-30 3.34 475.54 7.15 1425.87 98.90

CC-40 5.11 478.33 5.09 1715.75 109.29

CD-2 4.72 478.35 4.48 1385.37 256.67

CD-7 4.91 478.70 4.53 1564.27 167.31

CD-8 3.68 477.17 5.34 1615.79 169.30

CD-9 6.18 479.21 5.15 1814.30 189.62

CD-10 4.63 474.77 5.55 1108.99 218.04

CD-14 2.65 473.91 6.36 1296.51 144.47

CD-15 3.78 477.68 3.59 1543.39 210.05

CD-16 5.63 475.68 5.08 2302.70 106.15

2 3 4 5 6

2 3 4 5 6

IR Concentration (1019 cm-3 )

Hall Concentration (10¹⁹ cm^-3)

FIGURE 4.1.17: A comparison between the IR carrier concentrations and that obtained from Hall measurements.

TABLE 4.1.3: Transport properties of InN films by Hall measurement and electric response function fitting.

Samples name

Thickness of the films

IR Hall

Carrier

Concentration Mobility Carrier

Concentration Mobility (nm) 10¹⁹ (cm^-3) (cm²/V s) 10¹⁹ (cm^-3) (cm²/V s)

CC-21 272.92 5.68 550.24 6.49 451.2

CC-28 514.17 3.36 620.89 4.13 564.0

CC-30 418.80 1.91 942.11 - -

CC-40 351.01 2.77 852.52 3.32 788.8

CD-2 182.65 1.80 363.02 2.08 361.0

CD-7 347.12 2.30 556.92 2.49 451.4

CD-8 366.93 2.45 550.34 3.29 509.3

CD-9 232.57 3.09 491.39 - -

CD-10 195.31 1.16 427.33 - -

CD-14 362.37 1.58 644.93 1.74 592.2

CD-15 355.08 2.24 443.59 - -

CD-16 728.02 4.98 877.78 - -

Figure 4.1.4-14 shows both the experimental and calculated results of IR reflection spectra of four InN/sapphire samples and Sapphire substrate at 300 K. The fitted curve is based on the two layers model that in agreement with the observed spectrum. Where the high frequency dielectric constant for Sapphire is 1 [56]. We have employed the effective mass of electron m^* of 0.11m0 and the high-frequency dielectric constant ε of 8.4 for InN films [60]. The detailed parameters, such as the _∞ film thickness d, mobility μ, carrier concentration n, transverse-optical phonon frequency ω_TO, strength S_TO, damping constant γ_TO, plasma frequency ω , and _P plasma damping constant γ are listed in Table 4.1.2 and Table 4.1.3 for all twelve _P samples.

The spectra between 400 cm^-1 and 1000 cm^-1 exhibits high reflectivity, due to the lattice vibrations from InN film and sapphire substrate [61]. When we start analyzed the films, we should obtain the parameters of substrate. The all parameters and fitting result of sapphire substrate were shown in Figure 4.1.4 and Table 4.1.1 [55].

According to the eq. (2.5.1), we could see the E₁ (TO) phonon mode were obtained at 473~478 cm⁻¹ by dielectric response function fitting. The InN TO mode seems a little blue shift that have a good agreement with the work by O. Briot et al [62].

Carrier concentration and sheet resistance are critical for device applications. The values of carrier concentration and mobility are dependent on samples growing conditions. From Table 4.1.3 we can see that the values of the free carrier concentration and mobility derived from both methods agree with each other well. All the films have been found to be with similar carrier concentration in the order of 10¹⁹ cm^-3, but different carrier mobilities of 363, 427, 443, 491, 550, 550, 556, 620, 644, 852, 877 and 942 cm²/Vs respectively. And the plasma frequency could be deduced by eq. (2.4.3) and are shown in Table 4.1.2 that the ω_p is in the range of about 1100~2400 cm^-1. So the plasma edge will shift towards higher frequencies with the increase of carrier concentration that also similar with reported in Ref. 59. We guess that InN usually has high carrier concentration and low mobility. The carrier concentration increase caused the defects and impurities increase that affects the carrier mobility.

From the fit to the experiment curves, carrier concentration and mobility can be determined. Their values are shown along with the Hall measurements in Figure 4.1.17 which shows that the carrier concentrations determined by the both methods are in good agreement.

The thickness of films also will be obtained by the fitting. We obtained these films thickness all under 1μm even only 200nm [59,61].

4.2 FAR ( &MIDDLE ) INFRARED SPECTRUM ANALYSIS OF InAs

1-x

N

x

/InP

A series of InAs1-xNx films grown on InP have been measured by Fourier transform far infrared spectrometer Bruker IFS 120 v/S with different composition x = 0.002, 0.022, 0.024 and 0.03 [53]. The measured range of far infrared reflectance spectra is from 150 cm^-1 to 1200 cm^-1 at 300K. We found a two mode behavior of InAs1-xNx but the effects from InP substrate cannot be ignored. And we also found a one mode behavior of InP substrate at this range [63]. The experiment results are displaying below and the fitting results and discussions will be presented, too.

Far ( &Middle ) Infrared Reflectance Experiment Results

200 400 600 800 1000 1200

0.0 0.2 0.4 0.6 0.8 1.0

InAs_1-xN_x/InP 300K

x value 0.002 0.022 0.024 0.030

Reflectivity

Wavenumber (cm^-1)

FIGURE 4.2.1: Far infrared reflectance spectra of different N composition of InAs1-xNx/InP.

200 300 400 500 0.0

0.2 0.4 0.6 0.8 1.0

Experiment InP (substrate)

300K

Reflectivity

Wavenumber (cm^-1)

FIGURE 4.2.2: Far infrared reflectance spectrum of InP substrate.

The far infrared reflectance spectra of InAs_1-xN_x films grown on InP and InP substrate are displayed in Figure 4.2.1 and Figure 4.2.2. As shown in Figure 4.2.1, the spectra in the figure also can be simply divided into two regions, i.e. the region between 200 cm^-1 and 500 cm^-1 and above 500 cm^-1. There are apparently two main peaks that the position at the frequencies around of 218 and 438 cm^-1 between 200 cm^-1 and 500 cm^-1 in the Figure 4.2.1. With N composition increasing, the lower frequency peak decreases and the higher frequency peak increases. So we could define the lower frequency peak as InAs-like phonon mode and the higher frequency peak as InN-like phonon mode. By the way, the effects from InP substrate at about 303cm^-1 cannot be ignored. With a high composition of N, the InN-like phonon mode has a defect which was called a clustering in mixed crystal by Verleur and Barker [64].

According to the previous experience, the characteristic plasma frequency also

can be seen that the ω_p is in the range of about 500~700 cm^-1. The plasma frequency shifted towards higher frequencies with N composition increasing. It is also said that the carrier concentration increase with N composition increasing.

Fitting Results and Discussions

200 300 400 500

0.0 0.2 0.4 0.6 0.8 1.0

InP(substrate)

Reflectivity 300K

Wavenumber (cm^-1)

Experiment Fitting

FIGURE 4.2.3: Far infrared reflectance spectrum and fitting of InP substrate.

TABLE 4.2.1: Optical parameters and high-frequency dielectric constant of InP substrate at 300K by fitting. [63]

ε∞ S ω_TO γ

(cm^-1) (cm^-1)

9.61 3.22 303.28 2.25

200 400 600 800 1000 1200 0.0

0.2 0.4 0.6 0.8 1.0

InP

InN-like

InAs-like InAs_1-xN_x/InP

x = 0.002

Reflectivity 300K

Wavenumber (cm^-1)

Experiment Fitting

FIGURE 4.2.4: Far infrared reflectance spectrum and fitting of InAs0.998N0.002/InP.

200 400 600 800 1000 1200

0.0 0.2 0.4 0.6 0.8 1.0

InP

InN-like InAs-like

InAs_1-xN_x/InP x = 0.022

300K

Reflectivity

Wavenumber (cm^-1)

Experiment Fitting

FIGURE 4.2.5: Far infrared reflectance spectrum and fitting of InAs0.978N0.022/InP.

200 400 600 800 1000 1200 0.0

0.2 0.4 0.6 0.8 1.0

InP InAs-like

InN-like InAs_1-xN_x/InP x = 0.024

300K

Reflectivity

Wavenumber (cm^-1)

Experiment Fitting

FIGURE 4.2.6: Far infrared reflectance spectrum and fitting of InAs0.976N0.024/InP.

200 400 600 800 1000 1200

0.0 0.2 0.4 0.6 0.8 1.0

InP

InN-like InAs-like

InAs_1-xN_x/InP x = 0.030

300K

Reflectivity

Wavenumber (cm^-1)

Experiment Fitting

FIGURE 4.2.7: Far infrared reflectance spectrum and fitting of InAs0.97N0.03/InP.

200 400 600 800 1000 1200 303.28

438.10 218.66

438.57 219.04

439.03 220.09

220.60

439.61

InAs

_1-x

N

_x

/InP 300K

x = 0.030

x = 0.024

x = 0.022

x = 0.002

InP ( substrate )

Wavenumber ( cm

^-1

)

FIGURE 4.2.8: Far infrared reflectance spectra and TO frequencies of different N composition of InAs1-xNx/InP.

0.000 0.005 0.010 0.015 0.020 0.025 0.030

FIGURE 4.2.9: TO and LO frequencies versus N composition x of InAs1-xNx/InP by fittings and calculations.

0.000 0.005 0.010 0.015 0.020 0.025 0.030 0.00

FIGURE 4.2.10: Phonon oscillator strength versus N composition x of InAs_1-xN_x/InP.

TABLE 4.2.2: Optical parameters and high-frequency dielectric constant of InAs1-xNx/InP at 300K by fitting.

InAs_1-xN_x 300 K InAs-like mode InN-like mode

x ε∞ S1 ωTO1 γ1 S2 ωTO2 γ2

(cm^-1) (cm^-1) (cm^-1) (cm^-1) 0.002 12.76 4.02 218.66 7.88 0.02 438.10 5.46 0.022 12.00 3.55 219.04 15.14 0.06 438.57 8.61 0.024 11.32 3.11 220.09 12.66 0.11 439.03 13.05 0.030 11.87 2.91 220.60 13.47 0.13 440.61 11.85

0.000 0.005 0.010 0.015 0.020 0.025 0.030 0.00

0.05 0.10 0.15 0.20 0.25 0.30 0.35

InAs_1-xN_x/InP 300K

Effective Mass(m/m e)

N composition x

FIGURE 4.2.11: Effective mass versus N composition x of InAs1-xNx/InP.

0.0000 0.005 0.010 0.015 0.020 0.025 0.030 3

6 9 12 15 18

InAs_1-xN_x/InP 300K

N composition x Carrier Concentraition( 1018 cm-3 )

0 200 400 600 800 1000 1200 1400 1600 1800

Mobility( cm2 /V s)

FIGURE 4.2.12: Carrier concentration and mobility versus N composition x of InAs1-xNx/InP

0.000 0.006 0.012 0.018 0.024 0.030 300

350 400 450 500

InAs_1-xN_x/InP 300K

Conductivity (ohm-cm)-1

N composition x

FIGURE 4.2.13: Conductivity versus N composition x of InAs_1-xN_x/InP.

TABLE 4.2.3: Transport and electrical parameters of InAs1-xNx/InP at 300K by fitting.

InAs1-xNx

300 K Thickness Carrier

Concentration Mobility Effective

Mass Conductivity x (nm) 10¹⁸ (cm^-3) (cm²/Vs) m^*/me (Ohm-cm)^-1

0.002 2290.00 1.45 1687.46 0.051 390.68

0.022 2670.00 2.76 719.07 0.054 318.00

0.024 2460.00 3.65 713.73 0.088 416.70

0.030 2580.00 16.88 176.93 0.320 477.86

The far infrared spectra are fitted by the dielectric response function (eq. 2.4.2) and the fitting graphs are shown in Figure 4.2.4-8. Where the high frequency dielectric constant for InAsN is near 12.25 [53] and the high frequency dielectric constant of InP is 9.61 [63]. Here InP is semi-insulating substrate, so the ω of InP _P substrate is 0. So we could calculate results (dotted line) agree well with the experimental data (full line). The detailed parameters, such as the film thickness d, mobility μ, carrier concentration n, transverse-optical phonon frequency ω , _TO strength S_TO, damping constant γ_TO, plasma frequency ω , and plasma damping _P constant γ are listed in Table 4.1.1-3 all four samples and InP substrate. _P

When we start analyzed the films, we should obtain the parameters of substrate.

The all parameters and fitting result of InP substrate were shown in Figure 4.2.3 and Table 4.2.1 [63]. According to the eq. (4.2) and eq. (4.3), where _T1 is the InAs-like TO phonon frequency with strength S and ₁ _L1 is the InAs-like LO phonon frequency. Similarly, the subscript "2" means the frequency and strength of InN-like phonon mode. Thus, the longitudinal optical (LO) phonon frequencies of InAs-like and InN-like phonon modes could be calculated from eq. (4.2) and eq. (4.3) and were shown in Figure 4.2.9.

The Figure 4.2.1 also shows a good quality of reflectance spectra from the regular variations of two phonon modes. The reflectance spectra are expanded and the TO frequencies from fitting are labeled at Figure 4.2.3. From the figure, the InAs-like phonon mode frequency has a small blue shift from 218 cm^-1 to 220 cm^-1 with an increase of N composition from 0.002 to 0.03. And the InN-like phonon mode frequency also has a small blue shift from 438 cm^-1 to 439 cm^-1 with an increase of N composition from 0.002 to 0.03.

Both of the TO phonon frequencies from InAs-like TO mode and InN-like TO mode are increase with the increase of N composition x and the expansion of the series reflectance spectra are shown in Figure 4.2.3 with the TO phonon frequencies by fitting labeled on it. With an increase in the N content, the InAs-like TO phonon mode frequency and damping factor increase, but the oscillator strength decreases, whereas the InN-like TO phonon mode frequency, strength increase, and the damping factor increases with an increase in x. And the InAs-like LO mode decrease, but the InN-like LO mode increase with an increase of N composition x. The InP mode which seems no changes is contributed from the substrate InP.

Figure 4.2.12 shows the free carrier concentration versus N composition x and the special trend of it which could be compared to the report of W. K. Hung et al [53]

that were measured by Hall-effect measurements in the Van der Pauw configuration were listed in Table 4.2.4. The free carrier concentration increases from 1.91×10¹⁸ (cm^-3) to 1.69×10¹⁹ (cm^-3) with the N composition x increasing from 0.002 to 0.030.

The mobility decreases from 1740 (cm²/Vs) to 38 (cm²/Vs) with N composition x from 0.002 to 0.030. In our fitting result, the free carrier concentration increases from 1.45×10¹⁸ (cm^-3) to 1.688×10¹⁹ (cm^-3) and the mobility decreases from 1687.46 (cm²/Vs) to 176.93 (cm²/Vs) with the N composition x increasing from 0.002 to 0.030.

Figure 4.2.12, too. With increasing x, the effective mass, carrier concentration and conductivity increase greatly but mobility decrease greatly. From Table 4.2.3 we can see that the values of the free carrier concentration and mobility have a good agreement with the reported in Ref. 53. From the IR or Hall result could find that the more the N composition, the higher the carrier concentration.

The effective mass and conductivity were also calculated and plotted versus N composition x in Figure 4.2.11 and Figure 4.2.13. The effective mass and conductivity both increase with N composition x from 0.002 to 0.030. The InAs has a very narrow band gap and low-nitrogen doping cause bowing effect that affect the energy-gap becomes smaller that caused the effective mass and carrier concentration increase greatly [19]. The thickness of films also will be obtained by the fitting. We obtained these films thickness all near 2.5 μm that have a good agreement with the reported in Ref. 53.

4.3 FAR INFRARED SPECTRUM ANALYSIS OF In

_0.5

(Ga

_1-x

Al

_x

)

_0.5

P/GaAs

These are ten In0.5(Ga1-xAlx)0.5P films grown on GaAs have been measured by Fourier transform far infrared spectrometer Bruker IFS 66 v/S with different composition x ~ 0.24 and 0.18 [54]. The measured range of far infrared reflectance spectra is from 80 cm^-1 to 500 cm^-1 at 300K. We found a three mode behavior of InGaAlP but the effects from GaAs substrate cannot be ignored. And we also found a one mode behavior of GaAs substrate at this range. The experiment results are displaying below and the fitting results and discussions will be presented, too.

Far Infrared Reflectance Experiment Results

100 150 200 250 300 350 400 450 500 0.0

0.2 0.4 0.6 0.8 1.0

In_0.5(Ga_1-xAl_x)_0.5P /GaAs (x~0.24) 300K

Reflectivity

Wavenumber (cm^-1)

T382 T383 T453 T454

FIGURE 4.3.1: Far infrared reflectance spectra of InGaAlP/GaAs for T-series at 300K.

100 150 200 250 300 350 400 450 500 0.0

0.2 0.4 0.6 0.8 1.0

In_0.5(Ga_1-xAl_x)_0.5P /GaAs (x~0.18) 300K

Reflectivity

Wavenumber (cm^-1) E260

E261 E264 E269 E270 E271

FIGURE 4.3.2: Far infrared reflectance spectra of InGaAlP/GaAs for E-series at

There are two series of the In_0.5(Ga_1-xAl_x)_0.5P/GaAs samples with difference composition of Al(x~0.24 and 0.18). The far infrared reflectance spectra of In_0.5(Ga_1-xAl_x)_0.5P on GaAs are displayed in Figure 4.3.1 and Figure 4.3.2. As shown in Figure 4.3.1-2, the spectra in the figure also can be simply divided into two regions, i.e. the region below 230 cm^-1 and above 230 cm^-1. There are apparently four main peaks that the position at the frequencies around of 270, 330, 370 and 420 cm^-1. And we also could define the four TO frequency peak from low frequency to higher frequency are the GaAs phonon mode, InP-like phonon mode, GaP-like phonon mode and AlP-like phonon mode [63]. But there are only ‘three’ or ‘five’ peaks in some spectra that we could observe. It is interest that we could discussion this phenomenon at next section.

The sample with higher carrier concentration has higher absolute reflectance intensity in the long wavelength range as shown in Figure 4.3.1-2. The higher reflection intensity in the long wavelength range is due to the plasma effect and high carrier concentration.

Fitting Results and Discussions

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0.0

FIGURE 4.3.3: Far infrared reflectance spectrum and fitting of InGaAlP/GaAs T382.

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0.0

FIGURE 4.3.4: Far infrared reflectance spectrum and fitting of InGaAlP/GaAs T383.

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FIGURE 4.3.5: Far infrared reflectance spectrum and fitting of InGaAlP/GaAs T453.

FIGURE 4.3.5: Far infrared reflectance spectrum and fitting of InGaAlP/GaAs T453.

在文檔中 氮化銦、氮砷化銦及磷化銦鎵鋁薄膜的紅外線光譜研究 (頁 47-0)