Investigation on InSb Epitaxial Growth and Surface Passivation to Improve Device Performances

Abstract

Photodetectors (PDs) detecting mid-wavelength infrared (MWIR) region has been developed for the military applications for the first time by some developed countries, these days they have been widely used in the field of industry, medical and astronomy. Hence the market of IR PDs has been expanded significantly. However, the fabrication technology of IR PDs was selected as a key technology for military application, exportation to abroad or other countries are strictly restricted. On the other hand, the fabrication technology of IR PDs in Korea is positioned at low level to study fundamental technologies hence huge gap are existed.

Indium antimonide (InSb) is one of the suitable materials for MWIR detection due to the band gap of 0.23 eV at 77 K and its high electron mobility. To fabricate IR PDs, diffusion or ion-implantation of p-type dopant into n-type InSb substrate were generally adopted. However, these methods produced the surface defects, acting as the trap centers. Hence, device performances can be degraded due to the leakage current. In order to improve device performances, epitaxial growth of InSb was conducted. Unlike to other As, P and N related material growth, it has lots of difficulties in the epitaxial growth of InSb. Even though there are lots of difficulties, in this thesis, InSb epitaxial layers were grown by low pressure metalorganic chemical vapor deposition to obtain high quality of InSb epitaxial layers and its morphological, structural, electrical and optical properties was systematically investigated. For the device application, surface passivations for InSb were thoroughly investigated using ZnS films.

First prerequisites to growth high quality of InSb epitaxial layers was how to prepare the substrate for the epitaxial growth, called the preparation of epi-ready substrate. Although the preparation of well prepared substrate is very important process, it was not easy due to the low vapor pressure of Sb and no hydride precursors of Sb. Due to the these facts, thermal cleaning without Sb overpressure was reported. I noticed, however, that Sb overpressure during thermal cleaning process is necessary process in order to suppress undesirable byproducts. The thermal cleaning under H2 ambient produces the In droplets surrounded by rectangular etch pits on InSb surface due to the selectively evaporation of Sb. Because, Sb has a much higher vapor pressure than that of In. The formation of rectangular etch pits were reflected by the etching characteristics of zincblende structures. This preparation method of substrates found to influence on the crystal quality of InSb epitaxial layers.

In order to growth high quality of InSb epitaxial layers, growth parameters i.e., V/III ratio and growth temperature, were varied to find optimized growth conditions and its morphological, structural, electrical and optical properties were investigated. Optimized growth condition in this thesis found to be a growth temperature of 490 oC under V/III ratio of 8.8. With decreasing V/III ratio at low growth temperature such as 450 and 470 oC, In droplets were formed on the surface due to the insufficient supply of Sb. They were removed with increasing V/III ratio. At low V/III ratio at higher temperature of 490 or 510 oC, the formation of In droplets were prohibited due to the sufficient Sb supply, implying that TMSb was not fully decomposed in the investigated growth temperature ranges. Based on the growth behaviors of InSb, InSb growth rate was controlled by the surface reaction kinetics, called kinetically limited. It was dependent on the TMIn mole fraction but independent on the TMSb mole fraction. These growth behaviors represented that TMIn was also not fully decomposed in this growth temperature regions. From the optical properties analysis, two dominant phenomena were observed. Growth parameters influenced on the optical quality of InSb epilayers. When the InSb were grown at non-stoichiometric growth condition, indium interstitial defects were incorporated into InSb epitaxial layers, affecting the reduction of band gap energy. Second, the origin of new PL emission, which has not been reported yet, was investigated. It was originated from the carbon due to the uncomplete decomposition of metaloranic precursors.

Properties of ZnS were studied for the InSb surface passivation material. Until now, SiO2 passivation material has been widely used but it showed degradation during the deposition or after deposition at the interface regions. In order to overcome these problems, new material was studied. After deposition of ZnS, interface properties showed the similar quality with interface trap density of 2 x 1011 cm-2eV-1 compared to SiO2. It was found that surface leakage currents were reduced by depositing slightly non-stoichiometric ZnS films due to the charge compensation effects. It produces the flat-band voltage conditions, restricting the formation of surface leakage current paths.

The effectiveness of ZnS passivation deposited under optimized condition was investigated. The dark current of the sample passivated ZnS films were more than one order of magnitude reduction compared to the unpassivated sample. By comparing the sample passivated SiO2, it was also much effective way to reduce surface leakage current. Differential resistance area product at zero bias (RoA) was extracted from the current-voltage measurement. It shows that 1.6 x 104 Ω cm2 was obtained from the ZnS passivated InSb devices. It was decreased to 2 x 103 Ω cm2 and 31 Ω cm2 when the SiO2 and unpassivated films were deposited, respectively. The reduction of surface leakage current also influence on the spectral response, responsivity and detectivity. All devices show the spectral response in the wavelength range between 1 to 5.5 m where maximum intensities were observed near 5.5 m. However, the strong intensity of spectral response when ZnS deposited was observed and it was decreased when SiO2 and unppassivated films were deposited. In case of ZnS deposition sample, spectral response was even observed near 220 K, implying that crystal quality was very high. From the responsivity and detectivity measurements at 77 K, maximum intensity of responsivity was observed near 5.18 m. The corresponding detectivity show the strong intensity when ZnS films were deposited but it was reduced when InSb surface were not passivated.