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Experiment

Experimental schemes (Korean)

 * Detailed desciptions are listed in Engligh for each experiment underneath this section

기술 항목

실험 방법론

기본 광학 특성 측정

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ㆍElectroluminescence, photoluminescence, fluorescence, 반사율, 흡수율 등의 기본 광물성 측정과 평가
ㆍ측정 파장 범위: 200~1800 nm (0.01 nm 분해능)
ㆍ측정 파워: 최대 300 mW
ㆍ측정 온도 범위: 10~300 K
ㆍ광원
  - 레이저: 325/532/350-450/700-900 nm
  - 백색 광원: 제논 (UV), 텅스텐 램프(IR)

두 펨토초 레이저의 동기화 기술

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ㆍ독립적인 두 개의 레이저를 연동시켜 실험할 수 있도록 반복률을 정확하게 일치시키는 기술
ㆍ레이저 하나를 pump로, 나머지는 probe로 쓸 경우에 필요한 경로차를 레이저 cavity길이 조절을 통해 일치시키는 기술
ㆍ연구 적용 분야 예시:
  - 전기적 특성 평가를 제외한 대부분의 광 물성 연구에 모두 활용이 되어 실험 시간을 단축시켜 연구의 효율을 극대화 함

이색 펌프-프로브

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ㆍmmWave-IR-UV에 걸친 넓은 파장 대역에서, 독립적으로 튜닝이 가능한 이색의 펌프-프로브 시스템
ㆍ연구 적용 분야 예시:
  - 격자떨림이나 전하의 이동 등의 초고속 현상연구
  - 전자소자의 기판/공정방법에 따른 다양한 에너지 구조분석

다양한 파장 가변 기술

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ㆍIR영역 고체 레이저 (700-1000 nm)의 파장을 1/2, 1/3으로 변조시켜, 가시광-UV에 걸친 넓은 파장 대역에서 다양한 전기적 광학적 특성의 특정과 평가를 가능하게 함.
ㆍ연구 적용 분야 예시:
  - UV-blue-green LED 소자연구
  - Bio-chip등 형광 현상 응용 연구

반복률 변조 기술

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ㆍ기존 초고속 레이저의 반복률 (~76 MHz)를, 1 kHz까지 조절하는 기술.
ㆍ연구 적용 분야 예시:
  - 상대적으로 느린 반응 시간을 가진 소자 연구
  - Bio-Nano물질 상호작용 연구
  - Pulse 형태의 고전압 하에서의 소자 반응 연구

THz 발진과 측정

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ㆍ안테나를 이용한 테라헤르츠를 발진하고, 이를 측정하는 기술: 전자기파의 전기장을 시간상에서 추적함.
ㆍTHz 영역의 반사또는 투과실험에도 응용
ㆍ테라헤르츠 렌즈를 이용한 광 집속 기술
ㆍ연구 적용 분야 예시:
  - 다양한 물질의 THz 광소자 응용 가능성연구
  - 전하 가속 특성을 연구
  - 여기된 전하의 이완및 재결합 특성 연구
  - 온도에 따른 전하 분포 연구 등

Optics &THz 를 동시에 이용한 이색 초고속 현상 연구

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ㆍ연구 적용 분야 예시:
  - GaN-based UV-visible LED 소자의 압전 전기장 특성에 따른 THz 발진 소자 연구
  - 새로운 나노물질의 THz영역 전하 분포 연구

전기적 특성 측정
-DLTS
-Hall 측정법

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ㆍDLTS: 축전 용량의 시간적 변화에 근거한 반도체 구조의 결함 분석
ㆍHall 측정: 반도체 내의 전하의 종류, 이동도, 농도를 측정
ㆍ연구 적용 분야 예시:
  - 축전기 구조 구현과 DLTS 기술을 이용한 결함 상태 측정
  - 전자 소자의 전기적 특성 분석

Photoluminescence

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> Introduction

Photoluminescence spectroscopy is a contactless, nondestructive method of probing the electronic structure of materials. Light is directed onto a sample, where it is absorbed and imparts excess energy into the material in a process called photo-excitation. One way this excess energy can be dissipated by the sample is through the emission of light, or luminescence. In the case of photo-excitation, this luminescence is called photoluminescence. The intensity and spectral content of this photoluminescence is a direct measure of various important material properties.

Photo-excitation causes electrons within the material to move into permissible excited states. When these electrons return to their equilibrium states, the excess energy is released and may include the emission of light (a radiative process) or may not (a nonradiative process). The energy of the emitted light (photoluminescence) relates to the difference in energy levels between the two electron states involved in the transition between the excited state and the equilibrium state. The quantity of the emitted light is related to the relative contribution of the radiative process.

> Practical uses for photoluminescence spectroscopy

:: Band Gap Determination
The most common radiative transition in semiconductors is between states in the conduction and valence bands, with the energy difference being known as the band gap. Band gap determination is particularly useful when working with new compound semiconductors.

:: Impurity Levels and Defect Detection
Radiative transitions in semiconductors also involve localized defect levels. The photoluminescence energy associated with these levels can be used to identify specific defects, and the amount of photoluminescence can be used to determine their concentration.

:: Recombination Mechanisms
The return to equilibrium, also known as "recombination," can involve both radiative and nonradiative processes. The amount of photoluminescence and its dependence on the level of photo-excitation and temperature are directly related to the dominant recombination process. Analysis of photoluminescence helps to understand the underlying physics of the recombination mechanism.

:: Material Quality
In general, nonradiative processes are associated with localized defect levels, whose presence is detrimental to material quality and subsequent device performance. Thus, material quality can be measured by quantifying the amount of radiative recombination.

[http://inventors.about.com/od/pstartinventions/a/Photoluminescen.htm]

Pump-probe

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> Introduction

If you are going to take a peek at events that happen in nanoscale materials, then you need to be ultrafast. That is where the pump/probe technique comes into play.The pump-probe experiment as sketched in the figure is our framework for probing what is going on inside the materials in ultrafast time scale.

> The Pump

First you "pump" or create the event by applying a laser pulse that induces in some way an excitation of the sample. The sample is disturbed from it’s equilibrium position and returns after some time to it’s initial position. You keep pumping the sample at regular intervals that are longer than the response of the sample to ensure that there is no overlap of excitation events from the current pump and the previous pump. In this case, we are using laser pulses to create a transient magnetic field at the sample. The transient magnetic field in-turn changes the magnetization of the sample. This is achieved by using a laser pulse to optically close an switch and allow a current to flow. The current in turn creates a magnetic field in close proximity to the sample. The sample is ferromagnetic and seeks to align the magnetic moments of the material parallel to the applied magnetic field. There are all kinds of interesting interactions that can occur, so looking at what happens is a good start to sorting out the details of those interactions. If we can pump the sample and get it to behave the same way over and over again, then we can probe it and find out what is happening over those short intervals.

> The Probe

While the sample is changing, you want to "probe" the response of the system to the excitation event that you created. To come up with a complete picture, you want to probe the system over the entire cycle of the systems response to the event. The probe comes from the same source as the pump. Part of the beam from the laser is split off by a beam splitter and sent off to travel down a different path. This path includes a “delay line” which is a device that lengthens the path the light has to travel relative to the pump beam.

The way the delay line works is fairly simple. There is a retro-reflector on a rail system. The retroreflector can be positioned along the rail by computer control of a stepper motor. If the retroreflector moves backwards and lengthens the path, it takes that much longer for the light to reach the sample than the pump beam. For example: If you moved the retro-reflector back 10mm then the path is lengthened 20mm which takes the beam 20 x 10-3 m /3.0 x 108 m/s = 70 picoseconds longer to reach the sample. This means you can probe the sample 70 picoseconds after it has been pumped. If you keep doing that you can probe it 0, 70, 140, …etc picoseconds after it has been pumped. This gives you the time evolution of some property of the system at one point on the surface of the sample as a function of time. Further, if you move the beam (or the sample) you can probe a different location. If you do this in a raster style fashion, you can build up a time resolved “image” of some property of the materials surface.

THz Spectroscopy

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> Introduction

The terahertz (THz) and sub-THz frequency region (100 GHz – 10 THz) of electromagnetic spectrum bridges the gap between microwaves and infrared. The “THz gap” is very attractive because of many possible applications in spectroscopy and imaging of biological objects, and in novel communication systems as well.

> THz Time Domain Spectroscopy

Subpicosecond pulses of THz radiation are detected after propagating through a sample and an identical length of a free space. A comparison of Fourier transforms of these pulse shapes gives the spectra of absorption and dispersion of the sample under investigation. Such measurements can be successfully performed for investigation of gases and organic materials.

> Pump-Probe THz Experiments

Femtosecond lasers make it possible to investigate ultrafast nonequilibrium dynamics in semiconductors. For this aim, optical-pump-optical-probe techniques are usually employed. An intense optical pump pulse is used to excite free carriers in a sample, while a weaker probe beam monitors changes in sample optical properties. In contrary to the optical probe, terahertz pulses are non-resonant with the semiconductor band gap and therefore can be used to directly probe free-carrier dynamics, avoiding numerous experimental artefacts typical for optical-pump-optical-probe systems.

> THz Imaging

THz radiation has the ability to penetrate deep into many organic materials, what makes THz imaging attractive when dealing with biological samples. An image of the sample can be obtained by raster-scanning with a focused THz beam. Sub-millimeter resolution has been reported in scientific literature.

[http://www.ekspla.com/en/main/products/84?pid=525]

> The Detector : Photoconductive THz Antenna

 

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A photoconductive antenna (PCA) for terahertz (THz) waves consists of a highly resistive direct semiconductor thin film with two electric contact pads. The film is made in most cases using a III-V compound semiconductor like GaAs. It is epitaxially grown on a semi-insulating GaAs substrate (SI-GaAs), which is also a highly resistive material. The important difference between the SI-GaAs substrate and the film is the relaxation time for excited carriers. In a SI-substrate the carrier lifetime is about 500 ps, but in the film shorter than 1 ps. A short laser puls with puls width < 1 ps is focused between the electric contacts of the PCA. The photons of the laser pulse have a photon energy larger than the energy gap Eg and are absorbed in the film. Each absorbed photon creates a free electron in the conduction band and a hole in the valence band of the film and makes them for a short time electrical conducting until the carriers are recombined. In case of a receiver a current amplifier is connected on the electrical contacts. During the optical pulse the excited carriers are accelerated by the electric field component of the incident terahertz pulse with the time-dependent electrical field E(t). This leads to a measurable current signal in the outer circuit. To get the needed short carrier lifetime, the film must include crystal defects. These defects can be created by ion implantation after the film growth or alternatively by a low temperature growth. Low temperature grown GaAs (LT-GaAs) between 200 and 400 °C contains excess arsenic clusters. These clusters create defect levels within the band gap Eg and lead to a fast non-radiative recombination of the electron-hole pairs within a time interval < 1 ps.

[http://www.batop.com/informations/PCA_infos.html]

> The Emitter : Zn dopped InAs(100) : 6E17~9E17

It was found by Zhang et al. in 1990 that ultrashort electromagnetic radiation with terahertz (THz) bandwidth can be generated by illuminating semiconductor surfaces with femtosecond laser pulses. Soon, the investigations of THz radiation from various semiconductors followed with study into the emission efficiency as well as the emission mechanism. Currently, it is commonly understood that the emission of THz radiation from semiconductor surfaces is primarily due to the surge current normal to the surface and/or the second-order nonlinear optical processes in the semiconductors. The radiation intensity is generally low compared to the other THz radiation sources, such as biased photoconductive (PC) antennas, or phase-matched nonlinear optical crystals. However, after it was revealed that the InAs surface is an efficient THz emitter and that the efficiency is further enhanced by applying an external magnetic field, THz radiation from an InAs surface attracted much interest because of the potential as a simple and powerful THz radiation source in practical applications, such as spectroscopy and imaging.

[Terahertz Optoelectronics, Sakai, Kiyomi (Ed.), Springer, 2005]

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