Strain Assisted Enhanced THz Radiation from InAs layers grown on AlAs0.32Sb0.68 meta-morphic buffer layer/GaAs

Strain Assisted Enhanced THz Radiation from InAs layers

grown on AlAs0.32Sb0.68 meta-morphic buffer layer/GaAs

Jihoon Jeong, Hoonill Jeong, Jin-Dong Song*, and Young-Dahl Jho Dep. of Info. & Comm., Gwangju Institute of Science and Technology,

*

Nano-Science Research Division, Korea Institute of Science and Technology [email protected]

Terahertz (THz) wave is a kind of electromagnetic wave featured by the photon energy that has not been available with existing laser sources. Variety of organic materials shows strong dispersion and/or absorption spectra within THz range wave, and it is known as non-ionizing radiation, i.e. it is not harmful for biological tissues. These features make it as a prominent candidate for applications in research, security, and medical area such as THz time domain spectrometer, explosive material detector, and bio-imaging application(1). To develop and support such applications, there are still many studies are conducting.

Among the methods that can be used to generate pulsed THz radiation, the one that using semiconductor surface illuminated by femto-second (fs) laser pulse was found by X. C. Zhang et al.(2) After it is known that InAs is an efficient THz emitter and the efficiency is even more enhanced under the magnetic field(3), intensive studies toward radiation mechanisms and comparisons with other semiconductors are followed(4-9). For now, THz radiation from semiconductor surface is explained by two main reasons: acceleration of electrons excited by fs optical pulse, and second order nonlinear frequency mixing (optical rectification). The acceleration is accounted by intrinsic surface field which is formed by Fermi level pining for large band-gap materials, e.g. GaAs(∼ 1.43eV) or InP(∼1.35eV), and difference of diffusion speed between electrons and holes (photo-Dember effect) for narrow band-gap materials such as InAs(∼0.36eV) or InSb(∼0.18eV)(8). Therefore, doping density and carrier type have a non-negligible influence on radiation intensity. The effect of optical rectification, the other reason of THz wave generation, is largely depends upon crystalline direction. For (111)-oriented InAs, both photo-Dember effect and optical rectification play a major role, whereas optical rectification has little influence in (100)-oriented case(9).

In this work, we measured THz radiation from optically excited InAs layers, which have various thicknesses and grown upon GaAs substrate with AlAsSb buffer layer. For the experiments, a mode-locked Ti:sapphire laser with repetition rate of 76 MHz was used to excite InAs layers, whereas a photo-conductive antenna (PCA) was employed to measure THz radiation from the sample. Radiated waves were collimated and focused on to the PCA by a pair of the off-axis parabolic mirrors. The excitation pulse width was 150fs and the measurements were performed at room temperature.

0.014 0.027 0.109 0.438 0.875 1.750 0 10 20 30 40 50 60

Thickness [μm]

T

H

z Am

p.

[

nA]

Fig.1. Thickness dependent THz radiation intensity. (Note that the horizontal axis is not linear.) Measured radiation intensity was slowly increased with InAs layers thickness, until the thickness became comparable to the absorption depth (∼150nm)(9) of our excitation laser pulse (800 nm), and increased rapidly once the thickness exceeds the absorption depth. Though the increase seemed to be saturated nearby 0.9 micron it even decreased on thicker sample.

It is obvious that the THz radiation from samples thinner than absorption depth is very weak, since both carrier generation and diffusion are restricted in these samples. Also, the rapid increase of radiation intensity in samples thicker than absorption depth could be accounted by the activation of the photo-Dember effect, the major radiation mechanism of InAs. However, the origin of declined radiation is rather seems unclear. It could be related with the length which photo-Dember field would propagate in about 1ps(9). In this talk, we will present our hypothesis for the likely process of strain assisted THz radiation.

1. “Opportunities in THz Science”, Report of a DOE-NSF-NIH Workshop (2004). 2. X.-C. Zhang et al., Appl. Phys. Lett. 56, 1011 (1990).

3. Nobuhiko Sarukura et al., J. Appl. Phys. 84, 654 (1998). 4. Dong-Feng Liu et al., Appl. Opt. 46, 789 (2007).

5. R. Adomavièius et al., Appl. Phys. Lett. 85, 2463 (2004). 6. R. Mendis et al., J. Appl. Phys. 98, 126104 (2005).

7. Volker Cimalla et al., Phys. Stat. Sol. (b) 244, 1829 (2007).

8. Edited by K. Sakai., “Terahertz Optoelectronics”, Berlin: Springer, (2005). 9. Kai Liu et. al., Phys. Rev. B 73, 155330 (2006).