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光電工程研究所
碩
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文
氧離子佈植砷化鎵光導天線之 THz 輻射特性
THz radiation emission properties of oxygen-ion-implanted GaAs
photoconductive antennas
研 究 生：王韋文
指導教授：潘犀靈 教授
氧離子佈植砷化鎵光導天線之 THz 輻射特性
THz radiation emission properties of oxygen-ion-implanted GaAs
photoconductive antennas
研 究 生：王韋文 Student：Wei-Wen Wang
指導教授：潘犀靈 教授 Advisor：Prof. Ci-Ling Pan
國 立 交 通 大 學
光電工程研究所
碩 士 論 文
A Thesis
Submitted to Institute of Electro-Optical Engineering College of Electrical Engineering and Computer Science
National Chiao Tung University In Partial Fulfillment of the Requirements
For the Degree of Master of Engineering
In
Electro-Optical Engineering July 2007
Hsinchu, Taiwan, Republic of China
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1. 本授權書請以黑筆撰寫，並列印二份，其中一份影印裝訂於附錄三之 二(博碩士紙本論文著作權授權書)之次頁﹔另一份於辦理離校時繳交 給系所助理，由圖書館彙總寄交國家圖書館。氧離子佈植砷化鎵光導天線之 THz 輻射特性
研究生：王韋文 指導教授：潘犀靈 教授
國立交通大學光電工程研究所
摘 要
本 論 文 討 論 在 兩 組 多 重 氧 離 子 佈 植 砷 化 鎵 ( (500 & 800 ), (1200 ) 和 (500 & 800 ), (1200 ))薄膜材料，以此製作的偶極天線兆赫波的放射特性。 13 2 2.5 10 ions/cm× 2 keV keV keV keV 4 10 ions/cm× 13 2 keV 6 10 ions/cm× 131 1× 014 ions/cm2 keV 利用光脈衝寬約 100 飛秒、中心波長為 800 奈米的鎖模雷射激發氧離子佈植砷化 鎵光導天線，與砷離子佈植砷化鎵光導天線(~60KV/cm)比較，前者可操作在較高的電 場 上 (~110KV/cm) 放 射 出 高 功 率 的 兆 赫 波 訊 號 ， 且 具 有 較 高 的 飽 和 光 激 發 能 量 (~45mW)。此外，改變光導天線的偶極長度也會影響到輻射波強度。該天線最大發射頻 寬達 1.6THz，訊噪比最佳可達到~10000 以上。 最後，我們比較低溫成長砷化鎵與多重氧離子佈植砷化鎵偶極天線在輻射兆赫波上 的差異性。在相同的光激發能量及電場下，我們發現後者可發射出較高的兆赫波訊號， 強度(~ )約比低溫成長砷化鎵(~ 0.09 )大上近一倍。雖然在頻寬上多重氧離子 佈植天線和低溫成長砷化鎵天線還具有一段的差異，但由於多重佈植的方式在製程上比 低溫成長來得更經濟，而且我們可以再去改變離子佈植濃度和退火溫度獲得更好的效 果，因此在兆赫波的應用上還是具有相當的潛力。 0.17 Wμ μW
THz radiation Emission Properties of
Oxygen-ion-implanted GaAs
Photoconductive Antennas
Student: Wei-Wen Wang Advisor: Prof. Ci-Ling Pan
Institute of Electro-Optical Engineering
College of Electrical Engineering and Computer Science
National Chiao Tung University
Abstract
THz wave was generated from dipole antenna-type devices made by using Oxygen-ion implanted GaAs. We compared the emission properties of GaAs:O photoconductive (PC) antennas with different fabricated condition( (500 &
800 ), (1200 ) and 6 (500 & 800 ),
(1200 )) in the pulse mode. The absolute power of THz wave was also measured by a bolometer for comparison of the relative radiation power. High breakdown voltage threshold biasing (>110KV/cm) and large saturation optical pumping power (~45mW) in multi-GaAs:O based PC antennas are reported. THz radiation power is proportional to the effective antenna length and photocurrent.
13 2 keV 13 2 keV 13 2.5 10 ions/cm× 2 ions/cm keV keV keV 4 10 ions/cm× 2 m keV 10 × 14 10 1× ions/c
We also compared the material multi-GaAs:O with low-temperature (LT) grown GaAs based on the same structure of PC antennas. The material multi-GaAs:O can generate higher THz power than LT-GaAs. It is almost 2 times higher for the GaAs:O (~ ) than for the LT-GaAs (~ ) under the same pump power (~35mW) and dc bias (~30V), respectively. Although the bandwidth of GaAs:O antennas is not broader than LT-GaAs, but it could be improved by optimizing the implant dosage and annealing temperature, and adopting other antenna structures for high frequency purpose. And the process of preparation for GaAs:O is easier than LT-GaAs. So it is promising as the substrate of the PC emitter antennas for THz radiation.
0.17 Wμ 0.09 Wμ
Acknowledgement
致謝
本論文得以順利的完成，首先要感謝指導老師潘犀靈教授的悉心教導，提供良好的 研究資源和實驗環境，讓我可以順利完成實驗。在這兩年研究生活中，我真的很感謝黎 宇泰學長在實驗上的協助及理論的指導，使我獲益良多。也很感謝香港中文大學陳錦泰 教授實驗室的陳克堅學長幫忙製作的光導天線和提供許多寶貴的意見及經驗。此外，也 謝謝實驗室的王怡超、陳晉瑋學長們，及同學們：哲睿、彥毓、宜貞、君豪、昀甫因為 有你們的陪伴，我的研究生生活才如此多采多姿，可以在枯燥的研究生活中帶來一些歡 樂和喜悅，還有也謝謝學弟們：孟桓、介暐、育賢、松輝幫忙處理實驗室的事務及在實 驗器材的協助，也因為有你們的鼓勵及幫助，我才能順利畢業。 最後，我要感謝我最親愛的家人：爸爸、媽媽和妹妹，在我求學過程中，一路上 給予我最大的關懷與支持；也謝謝女友學雯，陪伴我這幾年一路走來，給我的支持及 鼓勵，讓我無後顧之憂，順利完成碩士學位！謝謝。 于新竹 交大 2004/08/22Table of Contents
Abstract
...i.
Acknowledgements
...iii.
Table of Contents
...iv.
List of Figures
...vi.
List of Tables
...vii.
Chapter 1 Introduction
...1.
1.1 Background
...1.
1.2 Method of generating THz radiation
...2.
1.2-1 Optical rectification
...2.
1.2-2 Photoconductive switch
...3.
1.3 Method of detecting THz radiation
...5.
1.3-1 Liquid-helium cooled bolometer
...5.
1.3-2 Photoconductive antenna
...6.
1.3-3 Electro-Optical sampling
...6.
1.4 Material of Photoconductor antenna
...7.
1.4-1 Material of Low-temperature (LT) grown GaAs
...7.
1.4-2 Material of ion-implanted GaAs
...7.
1.5 Objective
...8.
1.5-1 Motive
...8.
1.5-2 Organization of this thesis
...9.
Chapter 2 Basic Theory
. ...10.
2.1
The theory of generated THz radiation
...10.
2.1-1 Current-surge model
...11.
2.1-2 Drude-Lorentz model
...16.
2.1-3 Photoconductor antenna with different structure
....18.
2.2 The properties of the material
...19.
2.2-1 Choosing and analysis the material
...19.
2.2-2 Dark current
...20.
Chapter 3
Experiment Method and Setup
...23.
3.1 Sample preparation
...23.
3.1-1 Preparation of the PC antenna
...24.
3.1-2 The setup of PC antenna
...25.
3.2 Electric characteristics measurement
...27.
3.2-1 Experiment setup
...27.
3.2-2 The calibration of the Si bolometer
...28.
3.2-3 Measurement result
...30.
3.3 The time & frequency domain of THz pulse
...30.
3.3-1 Martin-Puplett polarization interfered Spectrometer
(FTIR)
...30.
3.3-2 Terahertz Time-Domain Spectroscopy (THz-TDS)
...33.
Chapter 4 Result and Analysis
...35.
4.1 The result of Electric characteristics
...35.
4.1-1 Current-voltage (I-V) curve
...35.
4.1-2 THz power-voltage curve
...36.
4.1-3 Analysis and Discussion
...37.
4.2 THz radiation of GaAs:O antennas
...40.
4.2-1 Measurement Result
...40.
4.2-2 Different systems (FTIR & TDS)
...46.
4.2-3 Analysis and Discussions
...48.
4.3 Compare with LT-GaAs PC antenna
...50.
4.3-1 Electric characteristics
...51.
4.3-2 THz wave form & Spectrum
...53.
Chapter 5 Conclusion
...57.
5.1 Summary
...57.
5.2 Future work
...58.
List of Figures
1.1-1 Electromagnetic spectrum
...1.
1.2-1
Optical rectification
...3.
1.2-2
Photoconductive switch
...4.
2.1-1
Energy band structure of GaAs
...10.
3.1-1
Schematic diagram of the PC antenna
...23.
3.1-2
Schematic of focused the pump laser on the gap of dipole
antennas
...26.
3.2-1
Schematic of experiment setup for measure the electric
characteristics of PC antenna
...28.
3.3-1
Schematic of a Martin-Puplett-type Fourier Transform Infrared
Spectrometer (FTIR) system
...31.
3.3-2
The reflectivity of the electric field dependent with frequency
u n d e r ( a ) p a r a l l e l a n d ( b ) p e r p e n d i c u l a r
component
...33.
3.3-3
Terahertz Time-Domain Spectroscopy system
...34.
4.1-1
I-V curve of oxygen ion-implant PC antennas with
differentdosage concentration and annealing temperature
...36.
4.1-2
THz power-voltage curve of oxygen ion-implant PC antennas
w i t h d i ff e r e n t d o s a g e c o n c e n t r a t i o n a n d a n n e a l i n g
temperature
...37.
4.1-3
Intensity of THz radiation measured as a function of the bias
voltage for the GaAs:O PC antenna with different conditions.
The pump power is fixed at 35mW
...38.
4.1-4
Intensity of THz radiation measured as a function of the laser
pump power for the GaAs:O PC antenna under the same
antenna length. The bias is fixed at 15V
...39.
4.2-1
The spectrum of different PC antennas (a) dipole 1 (b) dipole 3.
The inset is the interfere waveform measured by FTIR
system
...41.
4.2-2
THz wave forms and spectrums of dipole 1 under (a) &(c) the
d i ff e r e n t p u mp p o w e r a n d ( b ) & ( d ) t h e d i ff e r e n t
bias
...42.
4.2-3
The bias dependence of amplitude at the positive main peak in
Dipole 3 under the same pump power
...43.
4.2-4
The spectrum of different PC antennas (a) dipole 1 (b) dipole 3.
The inset is waveform of electric pulse measured by
TDS
...45.
4.2-5
The comparison of spectrum with different measurement system
in antenna of (a) Dipole 1 and (b) Dipole 3
...47.
4.2-6
The THz wave form of the GaAs:O PC antenna under the
different conditions and compared with LT-GaAs PC
antenna
...49.
4.2-7
The spectrum of the GaAs:O PC antenna under the different
conditions
...50.
4.3-1
The structure of PC antenna fabricated on GaAs:O and LT-GaAs
material
...51.
4.3-2 (a)
Current-voltage
curve
measurements for GaAs:O and
LT-GaAs PC antenna with pump power(~35mW) and without
laser irradiation
...52.
4.3-2
(b) THz power generated from GaAs:O and LT-GaAs PC
antenna versus bias voltage under the same pump power
(~35mW)
...53.
4.3-3
THz wave form of GaAs:O PC antenna under (a) the different
pump power and (b) the different bias
...54.
4.3-4
The spectrum of dipole antenna with different substrate GaAs:O
and LT-GaAs
...55.
List of Tables
Chapter 1 Introduction
1.1 Background
“Terahertz radiation” is the unit of electromagnetic radiation, which the frequency is near one trillion hertz ( Hz) as shown in Figure 1.1-1. THz radiation having a frequency of 1 THz has a wavelength of 0.3 millimeter (mm), or 300 micrometer (μm). It is between microwave radiation and infrared radiation. Wireless transmissions and computer clock speed are at frequencies far below 1THz. In the past twenty years, terahertz radiation has already been researched in many domains. The THz radiation is not commonly used in computer and wireless technology, although it is possible that a microprocessor with a clock speed on 1 THz might be developed in the future. From some literatures, the THz radiation has been primarily developed on physicists and astronomers [01]. The imaging technology and spectroscopic technology are
12 10
Figure 1.1-1 Electromagnetic spectrum.
studied in the Terahertz range [02]. Besides, it can be applied to measure not only medical but also materials inspection. There is great potential for its application in many other sciential fields as well. In order to generate THz radiation, the femto-second pulse laser is the usual optical source, such as Ti-sapphire laser, which is expensive and large size. This kind of laser is not usually available in laboratories. Besides, there is a different source to generate THz radiation. Using semiconductor laser diode can generate the continuous wave THz radiation.
1.2 Method of generating THz radiation
Due to the resent development of pico-second and femto-second laser system, there are various ways to generate the ultrashort pulsed millimeter and submiliimeter radiation [03], such as optical rectification and resonant THz radiation in photoconductor generated by a pulse laser or a pair of continuous wave laser.
1.2-1 Optical rectification
Optical rectification is the nonlinear effect of the second order characteristic. In the electro-optic medium, the low frequency electric field is produced under intense optical illumination. Optical rectification in a non-absorbing medium is a process in which a laser pulse creates a time-dependent polarization that radiates an electric field which can be written as
2 2 P E (t) t ∂ ∝ ∂ JK JK
polarization P follows the pulse intensity envelope. It is called rectification because the rapid oscillations of the electric field of the pulse laser are “rectified” and only the envelope of the oscillations remains. There is the schematic setup in Figure 1.2-1. Since the medium is nonabsorbing, the polarization instantaneously follows the pulse envelope implying that there is practically no limit on the speed at which the polarization can be switched on and off. The incident of polarization wave radiates a transient electric field which consists of one or one-and-a-half oscillation and therefore has a broadband THz frequency bandwidth. Bandwidth as large as 30 THz has been obtained using this generation mechanism.
THz Radiation Laser Pulse Non-linear optical crystal THz Radiation Laser Pulse Non-linear optical crystal
Figure 1.2-1 Optical rectification
1.2-2 Photoconductive switch
This mechanism of generated THz radiation from the biased photoconductive switch is generally based on the theory, which is called “current surge model” [04]. We will discuss the basic theory of this model in detail in next chapter. According to this model, as the energy of the input laser is higher than the energy of bandgap of photoconductor, then electron-hole pairs are excited and the
mechanism of generated THz radiation can occur.
In briefly, the electromagnetic field of THz radiation is generated from a transient current which is generated on the surface of the photoconductor. The pulse laser generates carriers instantaneously. We add a bias to accelerate the carriers. Then, the resultant transient current, or be called current surge, produces an electric field on the surface of the photoconductor. This surface electric field is regarded as the source of the THz radiation. Figure 1.2-2 shows the setup for this method. THz Radiation Laser Pulse Bias GaAs wafer Electrodes THz Radiation Laser Pulse Bias GaAs wafer Electrodes
Figure 1.2-2 Photoconductive switch
We put two electrodes on the top of substrate which can use different materials, such as GaAs 、 low temperature grown GaAs (LT-GaAs). This photoconductor behaved as an isolation without illuminate optical source. When we give the voltage across two electrodes and illuminate the region between two electrodes, the substrate becomes conducting. Under the influence of the applied electric field, current starts to flow. A time dependent current can also radiate transient electric field with a higher efficiency than optical rectification. The
frequency contents of this transient are determined by the transient behavior of the current and have 3dB bandwidth of 1 THz typically and roughly.
1.3 Method of detecting THz radiation
In the traditional method, THz radiation is generated by heat source and detected by liquid-helium cooled bolometer. In addition, there are different ways which are developed in recent years to detect THz radiation, such as photoconductive antenna and electro-optic sampling (EO sampling).
1.3-1 Liquid-helium cooled bolometer
This kind of detectors has been usually used for far-infrared (FIR), submillimeter wave and millimeter wave at astronomy in the past 40years. A hot electron bolometer is a device. When it absorbs the incident radiation, the resistance would responds to the change of electron’s temperature. A traditional bolometer consists of a heat-sensitive detection element mounted inside a heat sink and physically supported by a thermally conductive physical supporter. The most common systems are helium-cooled Si, Ge and InSb bolometer. It can measure the radiation power lower to nanowatt grade, but it losses the information of phase and frequency. Because of it, using bolometer to detect the power of THz radiation usually accompanies Martin-Puplett interferometer [05]. By using this system, we can obtain the interferogram of THz radiation [06].We will detail this system in next chapter.
1.3-2 Photoconductive antenna
The photoconductive detector is using photoconductive antenna as the detector [07]. When the incident THz radiation illuminates the photoconductive antenna, it induces transient current and then accelerate electron by probe beam. In this kind of detectors, two factors determine the spectrum bandwidth. One of them is the photocurrent response (i.e. carrier lifetime) and the other one is the frequency dependence of the antenna structure [08-09]. In general, the low frequency cutoff of the detectors results from the collection efficiency of the dipole, while the upper frequency limit is determined by the photocarrier response. The photocurrent response is the convolution of the transient photoconductivity and the THz electric field across the photoconductive antenna.
1.3-3 Electro-Optical sampling
The detector of Electro-Optical (EO) sampling has broad bandwidth spectrum and is easy to implement. Recently, technique of EO sampling has become an alternative to PC detection [10]. The Zinc-Blende crystal was used to measure THz radiation based on the Pockels effect [10]. When we vary the temporal delay between pump and probe beam, the synchronous probe beam will probe the transient change of refractive index result from THz radiation changing the refractive index. There is a trade-off in this method, so thickness of the crystal plays an important role. Thick crystal will introduce longer interaction length but reduce the frequency response.
1.4 Material of Photoconductor antenna
1.4-1 Material of Low-temperature (LT) grown GaAs
Low-temperature (LT) grown GaAs is widely used as the photoconductive substrate of PC antennas for generation and detection of THz radiation because of the high resistivity ( ) [11] and good mobility (100-300 ) [12-13], in addition to short carrier lifetime (
7
10 Ωcm cm2/Vs
1ps
< ) [14-15]. These excellent characteristics are however difficult to reproduce from sample because the quality of the material depends critically on both the growth temperature and the post growth thermal annealing conditions. An alternative material was reported to be promising as the substrate material of PC antennas, which is the arsenic ion-implanted GaAs [16]. These materials exhibit good structural and electrical properties and show ultrafast optoelectronic response. It is again possible to improve the carrier mobility of these ion-bombarded materials using postimplantation thermal annealing process. Good control over the overall fabrication process allows studies of the influenceof parameters such as the ion-implantation dose, the ion energy, and the thermal annealing conditions on the PC antenna characteristics.
1.4-2 Material of ion-implanted GaAs
The arsenic (As) ion implanted GaAs PC antennas have shown to be better THz emitters than those made on semi-insulating (Si) GaAs substrates [17-20]. It
has demonstrated that the terahertz emission property of arsenic ion-implanted GaAs has almost the same characteristics with Low-temperature (LT) grown GaAs. Such enhancement results from ultrafast carrier recombination associated with the presence of the implantation-induced defects. On the other hand, several groups have shown good characteristics of THz emitters with the use of PC antennas made on GaAs substrates grown by the Czochralski method when these devices are photoexcited near the anode. Although defects seem to play a crucial role in the characteristics of THz PC antenna emitters, there are very few studies that investigate the role of defects on these characteristics. Ion-implantation using other types of ions has already been used successfully to reduce the carrier lifetime in Si GaAs. For example, subpico-second lifetime can be obtained in GaAs:H GaAs:N and GaAs:O [21-26].
1.5 Objective
1.5-1 Motive
In this thesis, we used the pulse laser to measure the characteristics of the terahertz emitter fabricated on the substrates of Oxygen ion-implanted GaAs which have the different multi-implants dosage concentrations and annealing temperatures. We compared the emission properties of GaAs:O PC antennas under the different bias and pump power. And we measure the spectrums of our samples to compare with other material.
1.5-2 Organization of this thesis
There are mainly five chapters in the thesis, consisting of introduction (Ch.1), basic theory (Ch.2), experiment setup and method (Ch.3) result and analysis (Ch.4) and conclusion (Ch.5).
In chapter 1, we introduce the background of THz radiation and describe the motivation of the researching work. The basic theories about THz radiation generated from dipole antenna are mentioned in chapter 2. The experiment setup and measuring procedure are shown in chapter 3. And in chapter 4, the results of the researching and analysis are discussed. Finally, we will give a summary about the researching work from our experiment and list some future works which can be studied continuously in chapter 5.
Chapter 2 Basic Theory
In this chapter, the most important part is to introduce the basic theory of generated and detected THz radiation using photoconductor antenna. And then, we introduce the properties of material which will be the substrates of PC antennas.
1.43 eV 1.43 eV
Figure 2.1-1 Energy band structure of GaAs
2.1 The theory of generated THz radiation
As we illuminated the photoconductor antenna and the optical power is greater than the energy gap of semiconductor which is the substrate of the photoconductor antenna. The Energy band structure of GaAs has is shown in Figural 2.1-1. Then, this semiconductor will absorb the energy of incident wave and generate free electron-hole pair at the same time. We give bias to the electrode
of the photoconductor antenna; it can accelerate the carrier which is generated by incident wave. The brief surface current will be generated. This process can find expression in the Current-surge model.
2.1-1 Current-surge model
The first step, there are some time-average parameters which we need to define the THz radiation [22].
Charge density⇒ρ(x,y,z,t) Current density⇒J(x,y,z,t)K
Electric field intensity⇒E(x,y,z,t)JK Magnetic flux density⇒B(x,y,z,t)JK
Then, it is necessary to construct Maxwell’s equation for Current-surge model. From Maxwell’s equation [28]:
B E=-t ∂ ∇× ∂ JK JK (Faraday’s Law) (2.1-1) E=ρ ε ∇ •JK (Gauss Law) (2.1-2) D H=J+ t ∂ ∇× ∂ JK JK K (Ampere’s Law) (2.1-3) B=0 ∇ •JK (2.1-4) And we know B=JK ∇×AJK (2.1-5) From Equation (2.1-1) and (2.1-5), we get:
(
)
B A E=- =- A = - E+ =0 t t t t ⎛ ⎞ ⎛ ⎞ ∂ ∂ ⎜ ∂ ⎟⎟ ⎜ ∂ ∇× ∇× ∇×⎜⎜ ⎟⎟ ⇒ ∇×⎜⎜ ⎜ ⎜ ∂ ∂ ⎝ ∂ ⎠ ⎝ ∂A⎟⎟⎠⎟⎟ JK JK JK JK JK JK (2.1-6)Then, we set a non-vector value V and employ that to substitute the Equation (2.1-6). V=0 ∇×∇ A - V=E+ t ∂ ∇ ∂ JK JK A E=- V-t ∂ ⇒ ∇ ∂ JK JK (2.1-7)
From Equation (2.1-3)、(2.1-5) and H=B
μ JK JK 、D= EJK εJK,we obtain B E =J+ t ε μ ∇× ∂ ∂ JK JK K
(
A = J+)
E t μ⎛⎜ ε∂ ⎟⎞⎟ ⇒ ∇× ∇× ⎜⎜ ⎟⎟ ⎜ ∂ ⎝ ⎠ JK JK K (2.1-8)Substitute Equation (2.1-7) into Equation (2.1-8)
(
)
(
)
2 2 2 A A = J+ - V-t t V A A - A= J- -t t μ ε μ με με ⎡ ∂⎛⎜ ∂ ⎟⎞⎤ ⎢ ⎥⎟ ∇× ∇× ⎢ ⎜⎜ ∇ ⎟⎟⎥ ⎜ ∂ ⎝ ∂ ⎠ ⎢ ⎥ ⎣ ⎦ ∂ ∂ ∇ ∇• ∇ ∇ ∂ ∂ JK JK K JK JK JK K 2 2 2 A A- =- J+ A+ t t με∂ μ ⎛⎜ με ⎞⎟ ⇒ ∇ ∇ ∇•⎜⎜⎝ ⎟⎟ ∂ ∂ V ∂ ⎠ JK JK K JK (2.1-9) From Equation (2.1-2) and D= EJK εJK( )
E =- V+ A = t ε ⎡⎢ε⎛⎜ ∂ ⎟⎞⎤⎥⎟ ∇• ∇•⎢ ⎜⎜∇ ⎟⎟⎥ ⎜ ∂ ⎝ ⎠ ⎢ ⎥ ⎣ ⎦ ρ JK JK(
)
2V+ A =-t ρ ε ∂ ⇒ ∇ ∇• ∂ JK (2.1-10)Because of Lorentz gauge A+ V=0 t εμ ∂ ∇• ∂ JK , Equation (2.1-9) becomes as 2 2 2 A A- =- J t με∂ μ ∇ ∂ JK JK K (2.1-11)
And, Equation (2.1-10) can be written as 2 2 2 V V- =-t ρ με ε ∂ ∇ ∂ (2.1-12) Equation (2.1-11) and (2.1-12) are the two inhomogeneous wave equations written in terms of AJKand V. The two wave equations are used to determine a functional
and time dependent of the radiated electric field in the far field.
From Equation (2.1-3), the continuity equation of the free carriers is obtained.
(
H =)
J+ D = J+ =0 t t ρ ⎛ ∂ ⎞⎟ ∂ ⎜ ⎟ ∇• ∇× ∇•⎜⎜ ⎟⎟ ∇• ⎜ ∂ ∂ ⎝ ⎠ JK JK K K (2.1-13)In the fact, the current density of the photoconductor antenna which adds the bias is the transverse current, parallel the surface of the photoconductor antenna and perpendicular the direction of propagation. So that
J=0
∇•K (2.1-14) From Equation (2.1-13) and (2.1-14), we can deduce that the charge density does not vary with time and not contribute the time dependent radiated electric field. As the result, Equation (2.1-7) becomes
( )
E r,t =- A(t) t ∂ ∂ JK K JK (2.1-15)The solution of the vector potential AJK in Equation (2.1-11) leads into the Equation (2.1-15) to express the time dependent radiated electric field E (r,t)rad
JK K at the displacement rKfrom the center of the photoconductor antenna.
s rad 2 0 r-r' J r',t-c 1 E (r,t)=- da' 4πεc t r-r' ⎛ ⎞⎟ ⎜ ⎟ ⎜ ⎟ ⎜ ⎟ ⎜ ⎟ ⎜ ⎟ ⎜ ∂ ⎝ ⎠ ∂
∫
K K K K JK K K K (2.1-16)where is the permittivity of free space, c is the speed of light in vacuum, is the surface current of the photoconductor antenna in the retarded time, and is the increment of surface area at the displacement
0
ε Js
K
da'
r'K from the center of the photoconductor antenna. The integration is covered with whole the optically
illuminated area of the photoconductor antenna. In the far field, n r' r-r' =r 1- r r ⎛ • ⎟⎞ ⎜ ⎟ ⎜ ⎟≈ ⎜ ⎟⎟ ⎜⎝ ⎠ K K K (2.1-17)
The gap between the electrodes of the photoconductor antenna is assumed to be uniformly illuminated by the optical source. Therefore, the surface current can be set as a constant in space. Then, the radiated electric field in Equation (2.1-16) can be written as s J K rad 2 s 0 1 A r E (r,t)=- J t-4πεc r t c ⎛ ∂ ⎜ ⎟ ⎟ ⎜ ⎟ ⎜⎝ ⎠ ∂ JK K K ⎞ (2.1-18)
where A is the illuminated area of the photoconductor antenna and 2 2 2
r= x +y +z .We considered that the THz radiation is generated on Z axis (i.e. x
= y = 0 ), and let t t- z c ⎛ ⎞⎟ ⎜
→ ⎜ ⎟⎜⎝ ⎠⎟. Thus, The Equation (2.1-18) can be written as
( )
rad 2 s 0 1 A E (r,t)=- J t 4πεc z t ∂ ∂ JK K K (2.1-19)Beside, we can know the equation (2.1-20) from surface current [29].
0 ( ) J ( ) ( ) 1 1 s b s s t E t t n σ σ η = + + K K (2.1-20)
where n is the refractive index of photoconductor antenna under the wavelength of . is the impedance in free space, and is the surface conductivity which is shown in the equation (2.1-21).
m μ η0 σs( )t (1 ) ( ') ( ) t ' ( , ') ( ') exp[ ] s opt car e R t t t dt m t t I t w σ τ −∞ − ≡
∫
= − − (2.1-21)where e is the electric charge, R is the reflectivity of photoconductor antenna, is the photon energy, is the time-dependent carrier mobility at time t from created carrier at time ,
w =
( , ')
m t t
'
t Iopt is the time-dependent of optical intensity, and
is the carrier lifetime of excited carriers. For the present derivation, we
assume that carrier mobility is a constant. ( , ')
m t t = (2.1-22) m
And assume that the carrier lifetime is long enough, . A Gaussian intensity profile of the optical beam is assumed:
car τ → ∞ 2 0 2 ' ( ') exp( ) opt t I t I τ − = (2.1-23) From these assumptions, the surface conductivity becomes
2 0 2 (1 ) ' ( ) t ' exp( ) s e R t t I dt m w σ τ −∞ − ≡
∫
= − (2.1-24) From the equation (2.1-20) and (2.1-24) lead to the equation (2.1-19).2 2 2 0 0 rad 2 0 2 0 (1 ) 1 A (1 ) E (r,t)=- exp( ) 1 exp( ) 4 c z ( 1) t e R I m e R t I m x w n w τ η τ πε τ − −∞ dx ⎡ − ⎤ − − ⎢ ⎥ × +⎢ − ⎥ + ⎣
∫
⎦ JK K = = (2.1-25)Comparing with the result from experiment, it is necessary to rewrite the equation (2.1-25) in term of the experimental parameters which is the bias electric field applied across the photoconductor, and which is the incident optical intensity. b E opt F 2 0exp( 2 ) 0 opt opt E t F I dt I A π τ τ ∞ −∞ − =
∫
= ≡ (2.1-26) where is the average optical energy and A is the area of the incident optical beam. Then we define the parameter B and D to simplify the equation.opt E 2 0 (1 ) 4 Ae R m B c z w πε π − = = (2.1-27) 0 (1 ) ( 1) e R m D n w η π − = + = (2.1-28) And then, the electric field in the far field can be written as
2 2
2 2
( ) opt exp( ) 1 t exp( )
rad b opt F t E t BE DF x dx τ τ τ − −∞ − ⎡ ⎤ = − × +⎢⎢ − ⎥⎥ ⎣
∫
⎦ (2.1-29)dynamics in semiconductor to analyze the THz generation by Drude-Lorentz model when we need to discuss the factor of material in photoconductive antenna [30-31].
2.1-2 Drude-Lorentz model
For the calculation of carrier transport and THz radiation in a biased semiconductor, the one-dimensional Drude-Lorentz model is used. When a biased semiconductor is pumped by a laser pulse with photon energies greater than the band gap of the semiconductor, electrons and holes will be created in the conduction band and valence band, respectively. The carrier pumped by ultrashort laser pulse is trapped in the mid-gap states with the time constant of the carrier trapping time. The time-dependence of carrier density is given by the following equation. c (t) (t) (t) dn n G dt = − τ + (2.1-30)
where n(t) is the density of the carrier, G(t) is the generation rate of the carrier by the laser pulse, and τ is the carrier trapping time. The generated carriers will be c
accelerated by the bias electric field. The acceleration of electrons (holes) in the electric field is given by
, , , (t) (t) e h e h e h s e h d q E dt m ν ν τ = − + , (2.1-31) where is the average velocity of the carrier, is the charge of an electron (a hole), is the effective mass of the electron (hole), is the momentum relaxation time, and E is the local electric field. The subscript e and h
, (t) e h ν qe h, , e h m τe h,
represent electron and hole, respectively. The local electric field E is smaller than the applied bias electric field due to the screening effects of the space charges, b E b P E E αε = − (2.1 -32)
where P is the polarization induced by the spatial separation of the electron and hole, is the dielectric constant of the substrate and is the geometrical factor of the photoconductive material. The geometrical factor is equal to three for an isotropic dielectric material. It is noted that both, the free and trapped carriers contribute to the screening of the electric field. The time dependence of polarization P can be written as
ε α α dP dt r P J τ − + ＝ (2.1-33) where is the recombination time between an electron and hole. In the equation (2.1-33), J is the density of the current contributed by an electron and hole,
r
τ
h
J=enν −enν (2.1-34) e
where e is the charge of a proton. The change of electric currents leads to electromagnetic radiation according to Maxwell’s equations. In a simple Hertzian dipole theory, the far-field of the radiation ETHz is given by
THz J E t ∂ ∝ ∂ (2.1-35) To simplify the following calculations, we introduce a relative speed ν between an electron and hole,
h =
ν ν −νe (2.1-36) Then the electric field of THz radiation can be expressed as
THz n E e +en t t ν ν∂ ∂ ∝ ∂ ∂ (2.1-37)
The first term on the right hand side of the equation (2.1-37) represents the electromagnetic radiation due to the carrier density change, and the second term represents the electromagnetic radiation which is proportional the acceleration of the carrier under the electric field.
2.1-3 Photoconductor antenna with different structure
In our experiment, we used the different structure of PC dipole antennas to radiate THz wave. According to the literature [32] and equation (2.1-35), the radiation ETHz can be written as:
THz J(t) E (t) t e l ∂ ∝ ∂ (2.1-38) where l is the effective length of the dipole. From Equation (2.1-38), the e
amplitude of THz radiation is influenced by the effect length and time-dependence photocurrent. The effect of current will be discussed on next section. The effect length relate to the structure of PC antenna. The larger effective length of the dipole can get the larger amplitude of THz radiation.
2.2 The properties of the material
2.2-1 Choosing and analysis the material
In order to generate high power and broadband THz radiation, two properties are necessary for the material. One of properties is short carrier lifetime, and another is high carrier mobility. Because of them, PC antenna could be generated the shorter pulse. However, it can’t give consideration to two conditions. When we create the more recombination centers in semiconductor, it can trap carriers by using recombination centers to decrease the carrier lifetime. To create the more recombination centers, we implant the more defect in substrate and from deep level recombination. However, implanting the more defect, the structure of the crystal will be destroyed and decrease the carrier mobility.
1 c t c th N τ σ ν = < > (2.2-1) 22 0 0 * t m 1.4 10 N m ε μ ε × ≈ (2.2-2) 0
m : the original mass of carrier
*
m : the effective mass of carrier
0
ε : quasi-static dielectric constant
r
ε : relative dielectric constant
0 = r ε ε ε t N : defect density μ : carrier mobility
c
τ : carrier lifetime
c
σ : capture cross section
th
<ν > : mean thermal velocity
According to the Equation (2.1-1) and (2.1-2), we can know that the shorter carrier lifetime the less carrier mobility. It means that we decrease the carrier lifetime by implanting defects and the carrier mobility also decrease at the same time. Because of it, we use the way which is thermal annealing process to solve this problem. The thermal annealing processes can rearrangement the crystal of material which implanted defects. It can increase some of the carrier mobility. However, the carrier lifetime also increase. So we need to do the balance between the concentration of defects and the time of the thermal annealing process.
2.2-2 Dark current
In theory, there is no current generated from PC antenna which applies voltage, when it does not illuminate leaser beam yet. However, we can measure a few current in PC antenna, which we call dark current. In general, the theory of current transport of the metal-semiconductor junction potential barrier is mainly drift, diffusion, and tunneling effect. It’s contribution is in relation to intrinsic potential barrier , built-in electric potential and applied voltage. In order to avoid generating rectification effect and junction resistance on the metal-semiconductor junction, it is usually using the process of implant high concentration of defect in semiconductor to increase built-in electric potential and
B
decrease the width of depletion region. So the carrier can easily through the junction potential as applying voltage. It is the process of Ohm contact.
2.2-3 Photocurrent
Comparing with dark current, Photocurrent means that it generated from PC antenna which applies voltage, when it illuminates leaser beam. We use the ultra-pulse laser to incident into the gap on the photoconductor antenna, and generate the transient photocurrent to radiate THz electromagnetic pulse. The value of photocurrent is in relation to the bias value and the power of the leaser beam. It can express as:
0 V V I= ( ) ( ) ( ) gap bias Vbias R +R t =R t ≈ R t (2.2-3) 0
R is time-independence value of resistance. It can divide into two parts, one is
contact resistance and the other is load resistance. R(t) is time-dependence value of resistance which photoconductor illuminates leaser beam. We set that there is an incident beam on the photoconductor and using the photon energy generate the electron-hole pair. And them, the photoconductor parameter σ can be expressed as:
n p =q( + ) n
σ μ μ Δ (2.2-4) where q is the charge of electron, μnand μ are the mobility of electron and hole. p
is the density of carrier. If the average power of incident optical pulse is and photon energy is . The carrier generation rate G per unit volume can express as:
n Δ
avg
avg pw 0 P G= h QV η ντ (2.2-5)
where is the quantum effect, η τpw
0 V
is the bandwidth of leaser pulse, Q is the repetition rate of leaser pulse and is the total volume of activity region.
Under the steady state, the carrier generation rate G is:
pw n G= τ Δ (2.2-6) n p n p pw =q( + ) n=q( + )G σ μ μ μ μ τ ∴ Δ (2.2-7) From Equation (2.2-7), we can get the optical resistanceR . p
2 p n p avg L h QL R = = A q ( + )P ν σ η μ μ (2.2-8)
where A is the cross-section area of activity region and L is the length of activity region.
From Equation (2.2-3) and Equation (2.2-8), we can express the photocurrent as: n p bias avg bias 2 p q ( + )V P V I= = R h QL η μ μ ν (2.2-9)
Chapter 3 Experiment Method and Setup
In this chapter, we introduce the samples which are fabricated as our substrates of PC antenna. And then, we introduce the experiment methods and setups for measure some emission properties of THz radiation.
3.1 Sample preparation
In our experiment, we employed the dipole antenna as our antenna structure. This kind of structure has been demonstrated to obtain a large signal and high bandwidth [31]. The detail structure of the PC antenna has been shown in Figure 3.1-1. And, Knon et al. proved that the spectral response of the PC antenna for broadband detection is mainly determined by the temporal behavior of the number of photo-excited carriers [15].
Gap
Antenna length Transmission line width
Antenna width
Gap
Antenna length Transmission line width
Antenna width
Recently, Salem et al. [25] introduce oxygen ion as the dose to optimize the characteristics of GaAs substrate and generated THz waves from it. Normally, oxygen ion is looked as dust (not good thing) for electrical devices fabrication during the process, as its existence should make the contacts be not Ohmic-contact. But, in this case (material for THz wave generation), it looks its advantages are more than its disadvantages. As the electrical level formed by oxygen ion in GaAs is close to Femi-level, it makes the oxygen ion implanted GaAs close to electrically neutral and has comparative high resistance. This kind of material also can have high breakdown voltage and considerable short carrier lifetime by implanted under certain high dosage and annealed under suitable annealing temperature.
3.1-1 Preparation of the PC antenna
We used multi-implanted GaAs:O as the substrate and then fabricated dipole antenna on it. From some papers [22-23, 25], the GaAs:O is further studied. They show that this kind of material really has some advantages for THz waves generation, and can generate higher power and higher frequency if we can optimize the implant dosage, annealing temperature and introduce some antenna design for high frequency purpose.
We optimize the dosage and the implanted energy to match the requirements. Considering the cost of time for RBS facility, we prepare two kinds of sample which have different implanted dosage concentration and annealing temperature
as the dipole antenna structure. In implanting process, one of samples multi-implants dosage concentrations is (500Kev & 800Kev) and (1200kev) and the other sample multi-implants the higher dosage concentrations is 6 (500Kev & 800Kev) and
(1200kev). After implantation process, we do the thermal annealing by RTA (Rapid Temperature Annealing) under N2 gas environment and
use a GaAs cap to prevent As ion desorption. In our experiment, one of the parameter of RTA is 550℃ for 60s and the other is 500℃ for 60s. (For reference, under 500℃ for 60s annealing, the resistance of this GaAs:O substrate is close to
13 2 2.5 10 ions/cm× 2 ions/cm 13 2 4 10 ions/cm× 14 2 0 ions/cm 13 10 × 1 1× 7
1.26 10 (× Ω sq)) And we prepared different structure of antenna length for measure THz radiation. The detail sample list is shown in the Table 3.1.
Table 3.1 The antennas list
Substrate Dosage Condition Annealing Temperature Antenna length and width (L/W) Photo- conductive Gap(G) Transmission Line width (D) Device Type 200/25μm 20μm 25μm Dipole 1 500KeV/6e13 800KeV/6e13 1200KeV/1e14 300℃/5s 550℃/60s 100/25μm 20μm 25μm Dipole 2 100/25μm 20μm 25μm Dipole 3 GaAs:O 500KeV/2.5e13 800KeV/2.5e13 1200KeV/4e13 300℃/5s 500℃/60s 50/25μm 20μm 25μm Dipole 4
3.1-2 The setup of PC antenna
design. The pump laser could be focused on the dipole by the objective lens. And then, we use the silicon lens to decrease the angle of the THz radiation outgoing the substrate. The type of our silicon lens is hyper-hemispherical lens, the radius is 6.75mm and the total thickness is 8.35mm. There are two steps to make sure the pump laser is focused on the gap of the dipole antenna. The first step, we use a thin glass and put it before the objective lens. Then, we can see the image on the screen which is reflected from the dipole antenna and check the location of focused laser beam on the sample. The detail experiment method is shown in the Figure 3.1-2. After check the correct location, the next step is contact the multi-meter to the dipole antenna for measure the resistance, and optimizes the objective lens and incident pump beam to search the minimum value of resistance as possible as we can.
Screen
Thin Glass
Objective
lens
Silicon lens
Reflected image of
dipole antenna
Screen
Thin Glass
Objective
lens
Silicon lens
Reflected image of
dipole antenna
Figure 3.1-2 Schematic of focused the pump laser on the gap of dipole antennas.
3.2 Electric characteristics measurement
3.2-1 Experiment setup
In order to measuring the performance, we directly excited our PC antenna by using femto-second mode-locked Ti:sapphire laser as our source. The excitation pump pulse has a center wavelength at ~800 nm, ~50 fs (FWHM) pulse width and the repetition rate is about 82MHz. The femto-second pump pulse was focused by an objective lens on the biased gap of the PC antenna, which has put on the mount we design. The emitted THz radiation was collimated and focused by a pair of off-axis parabolic mirrors onto the detector. We used a instrument, Model 2410 High-Voltage SourceMeter, giving bias to the PC antenna and measure the current-voltage curve. To measure total THz power, we used a 4.2 K liquid-helium-cooled Si bolometer as our detector, which was carefully calibrated with a blackbody radiation source. The experiment setup for generation and detection of THz radiation is shown in Figure 3.2-1.
Bolometer
Bolometer
THz Wave
Chopper
Chopper
Emitter
Emitter
Source meter
Source meter
Lock in amplifier
Lock in amplifier
Pump pulse
Pump pulse
Bolometer
Bolometer
THz Wave
Chopper
Chopper
Emitter
Emitter
Source meter
Source meter
Source meter
Source meter
Lock in amplifier
Lock in amplifier
Lock in amplifier
Lock in amplifier
Pump pulse
Pump pulse
Figure 3.2-1 Schematic of experiment setup for measure the electric characteristics of PC antenna.
3.2-2 The calibration of the Si bolometer
Before using bolometer to detect THz radiation, we need to calibrate our meter firstly. The output of the bolometer is a voltage signal; it is not directly the power of the THz radiation. We use a blackbody radiation as the thermal source to radiate electric wave for the bolometer which we need to calibrate. According to the literatures [33-34], we can get the following equation (3.2-1).
2 1 output peak BB d F 2 V R = A A M ( ,T ) F R ( ) d R λ λ λ λ λ π
∫
(3.2-1)Where λ and 1 λ are the cut-on and cot-off wavelength, which we set form 2
100μmto 3000μm, in the response spectrum of our Si bolometer limited by the window filter. Rpeak is the peak response in the response spectrum. R is the distance between the bolometer and the blackbody radiation source. A and BB
d
A are the area of aperture in the blackbody radiation source and the detective window of bolometer, respectively. R( )λ is the response spectrum of the bolometer which is assumed to have a rectangle shape due to that the response of the Si bolometer is nearly independent of the frequencies from cut-on to cut-off wavelength, although the response of the Si bolometer actually decrease with the decrease of the frequency slightly. Voutput is the voltage value which we obtain in lock-in amplifier. F is the modulation parameter of chopper, which is about 0.5. F There is the experiment setup in Figure 3.2-2. Then, M( ,T)λ is the absolute power spectrum of the blackbody radiation source, which is the function of wavelengths and temperatures. The formula is given as following equation (3.2-2).
4 1 4 3 8 8 5 T 3 .7 3 1 0 M ( ,T )= (e λ -1 ) λ λ × (3.2-2)
where the unit is . There is the experiment setup show after calibrate Bolometer, we know that the obtained response is about 50 mV per with the preamplifier gain set to 180 chopping frequency. So we can use the information of the voltage value we measure in experiment to calculate the energy power of THz radiation.
-1 -1 Wcm mμ
W μ
3.3 The time & frequency domain of THz pulse
In order to measure the time and frequency domain of THz pulse generated from PC antenna, we use two different systems for measurement; one of them is Martin-Puplett-type Fourier Transform Infrared Spectrometer (FTIR) system and the other is Terahertz Time-Domain Spectroscopy system (THz-TDS). In this section, we will introduce the setup of two different systems in detail.
3.3-1 Martin-Puplett-type Fourier Transform Infrared
Spectrometer (FTIR)
Method & Setup
The design of the Martin-Puplett polarization interferometer which is based on a concept originally produced by Martin and Puplett in 1969 resembles the well-know Michelson interferometer. The Martin-Puplett polarization interferometer takes advantage of the polarization of electromagnetic radiation. In the field of THz frequency range, it is the most interferometer system is used. It has also been choice as the spectrometer in sub-millimeter wavelength range. Comparing the classical Michelson interferometer, it offers several advantages. Such as, the modulation efficiency of the polarizing beam splitter is higher and more uniform under wide spectral range. The simplified schematic is shown in Figure 3.3-1.
Lock
Lock
-
-
in Amplifier
in Amplifier
Bolometer
Bolometer
Parabolic mirror
Parabolic mirror
wire grid
wire grid
Polarizer 2
Polarizer 2
Fixed retro
Fixed retro
-
-
reflector
reflector
Movable retro
Movable retro
-
-
reflector
reflector
wire grid
wire grid
Polarizer 1
Polarizer 1
THz Wave
Chopper
Chopper
Computer
Computer
Emitter
Emitter
Lock
Lock
-
-
in Amplifier
in Amplifier
Bolometer
Bolometer
Parabolic mirror
Parabolic mirror
wire grid
wire grid
Polarizer 2
Polarizer 2
Fixed retro
Fixed retro
-
-
reflector
reflector
Movable retro
Movable retro
-
-
reflector
reflector
wire grid
wire grid
Polarizer 1
Polarizer 1
THz Wave
Chopper
Chopper
Computer
Computer
Computer
Computer
Emitter
Emitter
Figure 3.3-1 Schematic of a Martin-Puplett-type Fourier Transform Infrared Spectrometer (FTIR) system
We use the parabolic mirror to collect THz radiation and become the parallel beam, then we incident the parallel beam into the Martin-Puplett polarization interferometer system. The spectrometer utilized two wire grid polarizes which will discuss clearly later, used also as the polarizing beam splitters. The reflected and transmitted waves in two arms of the spectrometer have equal intensities while their directions of polarization are orthogonal to each other. The retro-reflector in each arm rotates the polarization of the incident light 180°. The
orthogonal transmitted and reflected waves are recombined at wire grid polarizer #2, then propagated through the wire-grid polarizer #1 and be collected by a parabolic mirror. By scanning the movable retro-reflector, the interferogram can be measured by the bolometer and stored by the computer.
Wire grids
For far-infrared spectroscopy, an effective polarizer can be made from an array of closely spaced parallel metallic wires, and it is the so-called wire grid. When an incident electric field passes through the wire grid, it can be divided into one component parallel and one perpendicular to the wire. The parallel component induces a counteracting current in the metal and is thus reflected. In another part, the normal component can pass through pass the wire grid with a little attenuation. If the thickness d of the wires and the spacing s are small compared to the wavelength of the incident wave, the modulus of the reflection coefficients for the electric field components parallel and perpendicular to the wires can be calculated by equation (3.3-1) and (3.3-2) [35]. 1 2 2 2 2 1 s ln s r d λ π − ⎡ ⎛ ⎞ ⎛ ⎞ ⎤ ⎢ ⎜ ⎟ ⎜ ⎟ ⎥ = ⎢ + ⎜⎜⎝ ⎟⎟⎠ ⎜⎜⎝ ⎟⎟⎠ ⎥ ⎢⎣ & ⎥⎦ (3.3-1)
(
)
2 12 4 4 2 1 s r d λ π − ⊥ ⎡ ⎤ ⎢ = +⎢ ⎢ ⎥ ⎣ ⎦ ⎥ ⎥ (3.3-2)In our experiment setup, the wire grid we used made of 10μm thick tungsten wire wound on a circular frame placed at a distance of 45μm. The reflectivity of the electric field for these parameters is plotted in Figure 3.2-2.While the parallel
component is reflected nearly perfectly over the whole spectral range, the transmission of the normal component decreases towards higher frequencies. According the experiment result, the frequency of PC antenna we measure would not be higher than 1.5THz. So we can ignore the THz power decrease from the wire grid. 0.0 0.5 1.0 1.5 2.0 2.5 3.0 0.9990 0.9992 0.9994 0.9996 0.9998 1.0000 Pa ra lle l Reflec tivity Frequency (THz) 0.0 0.5 1.0 1.5 2.0 2.5 3.0 0.00 0.05 0.10 0.15 0.20 0.25 Perpendi cular Ref lec ti vi ty Frequency (THz) 0.0 0.5 1.0 1.5 2.0 2.5 3.0 0.9990 0.9992 0.9994 0.9996 0.9998 1.0000 Pa ra lle l Reflec tivity Frequency (THz) 0.0 0.5 1.0 1.5 2.0 2.5 3.0 0.00 0.05 0.10 0.15 0.20 0.25 Perpendi cular Ref lec ti vi ty Frequency (THz)
Figure 3.3-2 The reflectivity of the electric field dependent with frequency under (a) parallel and (b) perpendicular component.
3.3-2 Terahertz Time-Domain Spectroscopy (THz-TDS)
Setup
The THz time-domain spectroscopy system is shown in Figure 3.3-3. The incident pump pulse was focused by an objective lens on the biased gap of the PC antenna to generate THz radiation. The THz radiation was collimated and focused by a pair of off-axis parabolic mirror on a PC sampling detector. Which was also a PC antenna mounted on the back of a Si hemispherical lens. The PC detector was
gated by femto-second probe beam pulses that were separated from the pump beam pulses by a beam splitter, and the DC photocurrent was induced by the incident electric field of THz radiation on the PC detector. Using delaying the time of the probe pulse to the pump pulse, the time-domain waveform of the electromagnetic pulse was obtained. The time resolution was limited by the carrier lifetime of the LT-GaAs used for the PC detector. To increase the signal-to-noise ratio, the pump beam was modulated with a mechanical chopper at 1 KHz, and output signal from the PC detector was measured with a lock-in amplifier and stored by the computer.
Emitter
Emitter
Detector
Detector
BS
BS
Chopper
Chopper
Lock in amplifier
Lock in amplifier
Source meter
Source meter
Computer
Computer
Emitter
Emitter
Detector
Detector
BS
BS
Chopper
Chopper
Lock in amplifier
Lock in amplifier
Lock in amplifier
Lock in amplifier
Source meter
Source meter
Source meter
Source meter
Computer
Computer
Computer
Computer
Chapter 4 Result and Analysis
In chapter 4, we show the experiment results of emission properties of GaAs:O PC antennas. According to the results, we analysis and discuss the results under the different conditions.
4.1 Electric characteristics result
4.1-1 Current-voltage (I-V) curve
Figure 4.1-1 shows the current-voltage characteristic result of the PC dipole antenna under different oxygen ion-implant concentration and annealing temperature, which is listed on table 3.1. We pump different incident power (25mW, 35mW, 45mW) to the PC antenna and also measure the dark current which is without pump power. It can easily confirm from Figure 4.1-1 that the I-V curve is a linear curve, the current is proportion to the bias we gave. Under the higher pump power, it can generate the more photocurrent from PC antenna. Besides, the shorter antenna length of PC antennas generates the more current than the longer antenna length of PC antennas under the same pump power and bias. We observe that the dark current of the PC antennas which have higher dosage concentration (e14) and annealing temperature (550℃) is lower than the PC antennas which have lower dosage concentration (e13) and annealing temperature (500℃).
0 10 20 30 40 50 0 5 10 15 20 25 30
Current(uA)
Bias(V)
Darkcurrent 25mW 35mW 45mW4.1-2 THz power-voltage curve
Figure 4.1-2 shows the bias voltage dependence of the THz absolute intensity generated from our different conditions of PC antennas with the leaser beam focused near its anode under the different pump power. They are detected by the bolometer. According the Figure 4.1-2, we can clearly obtain that the THz power increases quadratically with the bias. The higher power of THz radiation generated from oxygen ion-implant GaAs PC antenna has been demonstrated in Figure 4.1-2 (c). It is almost up to 3.5V under the high bias (70V) and pump
Figure 4.1-1 I-V curve of oxygen ion-implant PC antennas with different dosage concentration and annealing temperature
(a) Dipole 1
(b) Dipole 2
0 2 4 6 8 10 12 14 16 0 20 40 60 80 100
Current(uA)
Bias(V)
Darkcurrent 25mW 35mW 45mW 0 10 20 30 40 50 60 70 0 20 40 60 80 100 120 140Current
(uA)
Bias(V)
Darkcurrent 25mW 35mW 45mW(c) Dipole 3
0 10 20 30 40 50 0 20 40 60 80 100Current(uA)
Bias(V)
Darkcurrent 25mW 35mW 45mW(d) Dipole 4
power (45mW).
4.1-3 Analysis and Discussion
Figure 4.1-3 shows the bias voltage dependence of the THz intensity measured for our PC antennas using a constant pump power of 35mW. According to the result, we can observe that the lower dosage concentration of PC antenna in the same antenna length (100um) can get the higher THz power. Under the same substrate, the longer antenna length emitted higher power as expected from
0 10 20 30 40 50 60 70 0 250 500 750 1000 1250 1500
THz power (mV)
Bias(V)
15mW 25mW 35mW(a) Dipole 1
0 10 20 30 40 50 60 70 0 500 1000 1500 2000 2500 3000 3500 4000THz po
we
r (m
V)
Bias(V)
25mW 35mW 45mW(c) Dipole 3
0 10 20 30 40 50 0 10 20 30 40 50THz
power (mV)
Bias(V)
25mW 35mW 45mW(d) Dipole 4
0 2 4 6 8 10 12 14 16 18 0 5 10 15 20 25 30THz power (mV)
Bias(V)
(b) Dipole 2
25mW 35mW 45mWFigure 4.1-2 THz power-voltage curve of oxygen ion-implant PC antennas with different dosage concentration and annealing temperature
