Applied Science and Convergence Technology 2019; 28(3): 46-50
Published online May 31, 2019
Copyright © The Korean Vacuum Society.
Sang Jo Leea, Sanam SaeidNahaiea, Jun Oh Kimb, Sang Jun Leeb, and Jong Su Kima,*
aDepartment of Physics, Yeungnam University, Gyeongsan 38541, Republic of Korea, bKorea Research Institute of Standards and Science, Daejeon 34113, Republic of Korea
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The purpose of this study is twofold: (1) to grow undoped-GaSb epitaxial structure on a high concentration
Keywords: Molecular beam epitaxy, Photoreflectance, GaSb
Modulation spectroscopy is the application of external modulation to measure the optical spectrum of a sample. It is based on changes in the internal or surface electric field of the sample. The optical spectrum measured by modulation spectroscopy has differential characteristics of the perturbation parameter and the reflectivity of the sample. Therefore, the reflectance is highly sensitive to the variation of the perturbation parameter. Accordingly, modulation techniques can be divided into electro-reflectance (ER) spectroscopy (direct field modulation) and light-reflectance or photoreflectance (PR) modulation (indirect field modulation) . ER was first studied by Seraphin , and Wang et al.  developed PR as a non-destructive method.
In the case of PR, the energy of the pump light that is periodically incident is higher than the bandgap energy of the sample in order to generate a carrier. After absorbing the light, the electron-hole pairs in the sample are separated by the internal electric field, which is then reduced . The variation of the electric field causes the band bending of the sample. The optical properties of the material, such as the reflectance, change with the dielectric function. Therefore, the characteristics of the sample can be analyzed by PR measurements.
In this study, undoped-GaSb epitaxial layers were grown on a high concentration
The optical properties of the medium are indicated by the complex dielectric function. When light modulation is applied to the medium, the change in reflectance is related to the real and imaginary parts of the complex dielectric function. Seraphin and Bottka  express the relative change of reflectance as:
Comparing the phenomenological lifetime broadening parameter
Assuming a uniform broadening parameter, the dielectric function has a generalized Lorentzian shape, and the change in reflectance is expressed in the low electric field region as :
In the intermediate field region, the dielectric function represents the Franz-Keldysh oscillation (FKO) . The Franz-Keldysh effect is represented by Airy functions and the change in the dielectric function can be expressed as 
where the value of
Figure 1 shows the theoretical PR spectra in the intermediate electric field region in the GaSb sample structure. Because the transitions in the valence band assume heavy holes and light holes, the PR equation requires two Airy functions. GaSb has
LEIO is the interference of two beams. One is reflected from the surface (air/thin film) and the other is the PR signal reflected from the interface (thin film/substrate). Because the electric field is formed between the thin film layer and the substrate, we can measure the thin film layer thickness from the LEIO PR signals. The signal related to the variation in the refractive index of the substrate can be expressed as :
Figure 2 shows the theoretical LEIO PR signal in the single GaSb layer. Using the theoretical model of
In this study, undoped-GaSb thin films were grown on n+-type GaSb (100) substrates doped with Te at a high concentration (~1018 cm−3). The growth temperature was approximately 485 °C. The GaSb thin film was grown to a thickness of 1 μm at a given substrate temperature with a Ga beam equivalent pressure (BEP) of 7 × 10−7 Torr and a Sb4 BEP of 5 × 10−7 Torr. The samples after growth showed a stable Sb surface with a reflection high-energy electron diffraction pattern (1 × 3) at a given growth temperature. The crystal structure of the sample was evaluated by X-ray diffraction and the optical properties were evaluated by PR measurements at room temperature. For the PR measurements, a 1.3-μm laser diode was used as the modulation light source, with a modulation frequency of 800 Hz. The probe light was a beam obtained from a tungsten-halogen lamp (250 W) dispersed by a monochromator. The probe beam was incident on the sample surface, and the reflected beam was measured using an InGaAs detector. A closed-cycle He refrigerator was used to control the sample’s temperature.
Figure 3 shows the PR signal and simulation results of the sample measured at room temperature. Owing to the decrease in the carrier concentration induced by increasing the growth temperature, the relative magnitude of the electric field is reduced.
In the case of a thin film, the carrier concentration inside the sample is uniform and is depleted in the surface; therefore, the potential is changed in the inward direction. In this case, mainly the surface electric field is measured in the PR of the thin film sample, because the electric fields is related to the magnitude of the electric potential and the thickness of the surface depletion layer.
However, the experimental and simulation results show that the PR signals in the low and intermediate electric field region coexist. In general, the undoped-GaSb thin film grown on
The low-field PR signal is accompanied by the LEIO PR signal. The simulation results show that, with a growth temperature of 485 °C, the intermediate electric field is 70 kV/cm. Undoped-thin films and high doped substrates can be analyzed in the same way as p-n junctions. The thickness of the samples measured from the LEIO signals was d = 1040 nm. Due to the incident angle of the probe beam and the dispersion function of the medium, the measured thicknesses (1040 nm) and the production thicknesses (1000 nm) are slightly different.
In this study, undoped-GaSb layers were grown on high-concentration
This work was supported by the National Research Foundation of Korea (NRF) grant funded by the Korea government (NRF-2018R1D1A3B07050824 and NRF-2018M3A7B4069996).
Theoretical parameters (
|Ahh/Alh||F [kV/cm]||Eg [eV]||mhh/mlh|