Jae Mook Kim

Thermomigration of tellurium precipitates in CdZnTe crystals grown by vertical Bridgman method

Te precipitates in CdZnTe have been characterized by x-ray diffraction at room and higher temperatures. From the x-ray results at room temperature, it has been confirmed that Te precipitates in CdZnTe have the same structural phase as observed in elemental Te under high pressure. The x-ray results at higher temperature indicate that Te precipitates melt around 440°C. CdZnTe samples containing Te precipitates have been annealed at temperatures below and above 440°C with thermal gradient of ∼70°C/cm. Results of the observation with infrared microscope before and after the annealings indicate distinct occurrence of thermomigration of Te precipitates in samples annealed at temperature above 440°C compared with ones annealed at temperature below 440°C. Thermomigration velocity obtained from these results is ∼50 μm/h. The average value for the effective diffusion coefficient of the metallic atoms in Te precipitates calculated by using the thermomigration velocity is ∼3 x 10−5 cm2/s.

Vertical Bridgman techniques to homogenize zinc composition of CdZn Te substrates

In order to improve the Zn homogeneity along the axial direction of CdZnTe boule, we have employed a modified Bridgman technique using a (Cd, Zn) alloy source in communication with the melt, whose temperature has been gradually changed from 800 to 840°C during growth. Electron probe microanalysis (EPMA) measurements of Zn composition in the boule shows an excellent homogeneity of Zn along the axis of the CdZnTe boule compared with results in a boule grown by using a fixed source temperature. We have performed a numerical simulation to obtain the approximate temperatures of additional heating and cooling needed to improve the radial Zn homogeneity. CdZnTe boule has been grown by seeded vertical Bridgman furnace with two zones of heater and cooler. Ultraviolet/visible spectroscopic measurements of Zn composition over the length of the boule indicate that the radial distribution of Zn composition is very homogeneous in the body region of the boule, where the radial variation of Zn composition is ±0.0005.

Rapid Thermal Diffusion of Indium in p-HgCdTe/CdTe

In this study, the rapid thermal diffusion (RTD) of indium and its activation as a donor in p-Hg0.70Cd0.30Te/CdTe were investigated using atomic indium and electron concentration profiles obtained by secondary ion mass spectroscopy (SIMS) and differential Hall measurements, respectively. Photodiodes with a junction formed by the RTD of indium were also demonstrated. There are two diffusion components, a complementary error function fitted atomic diffusion component and an exponentially fitted fast diffusion component. The diffusion coefficients of atomic indium into p-Hg0.70Cd0.30Te by RTD can be fitted as D(T)( cm2/s)=1.45×10-2exp (-0.772(eV)/kT) in the temperature range of 100–200°C. It is observed that diffused indium atoms are partially activated as donors and that the ratio of electron to diffused indium concentration is increased with increased diffusion depth from the surface. For photodiodes with an indium RTD junction, trap-assisted tunneling current is markedly suppressed. The origin of suppression of trap-assisted tunneling current can be explained by suppression of defect generation in the depletion region during junction formation by the RTD process.

Electrical properties of MIS devices on CdZnTe/HgCdTe

In this paper, we report the capacitance-voltage (C-V) properties of metal-insulator-semiconductor (MIS) devices on CdTe/HgCdTe by the metalorganic chemical vapor deposition (MOCVD) and CdZnTe/HgCdTe by thermal evaporation. In MOCVD, CdTe layers are directly grown on HgCdTe using the metal organic sources of DMCd and DiPTe. HgCdTe layers are converted to n-type and the carrier concentration, ND is low 1015 cm-3 after Hg-vacancy annealing at 260 degrees Celsius. In thermal evaporation, CdZnTe passivation layers were deposited on HgCdTe surfaces after the surfaces were etched with 0.5 - 2.0% bromine in methanol solution. To investigate the electrical properties of the MIS devices, the C-V measurement is conducted at 80 K and 1 MHz. C-V curve of MIS devices on CdTe/HgCdTe by MOCVD has shown nearly flat band condition and large hysteresis, which is inferred to result from many defects in CdTe layer induced during Hg-vacancy annealing process. A negative flat band voltage (VFB approximately equals -2 V) and a small hysteresis have been observed for MIS devices on CdZnTe/HgCdTe by thermal evaporation. It is inferred that the negative flat band voltage results from residual Te4+ on the surface after etching with bromine in methanol solution.

Dependence of the optical properties of p‐CdTe and p‐Cd0.96Zn0.04Te on the bromine concentration in a bromine‐methanol etching solution

Photoluminescence measurements were carried out to investigate the dependence of the optical properties of p‐CdTe and p‐Cd0.96Zn0.04Te on various relative concentrations of bromine in a mixture of methanol and bromine. As the bromine concentration increased, the intensity of the luminescence increased; additionally, an exciton bound to a neutral acceptor (A°X) peak at 1.588 eV in p‐CdTe was resolved into an exciton bound to neutral donor (D°X) peak at 1.592 eV and an A°X peak. The mixed bands of the 1.574 and 1.546 eV peaks were well resolved by using an etching bromine concentration of 2%; in particular, the intensities of the D°X and 1.574 eV peaks increased as much as five and seven times, respectively. The intensity of the peak at 1.48 eV related to defects also increased. The intensity of A°X at 1.609 eV in p‐Cd0.96Zn0.04Te changed slightly with the bromine concentration. The intensities of the luminescence peaks due to the recombination of the electrons in conduction band with acceptors and to the defect relation did not vary. These results indicated that the number of Cd vacancies could be reduced due to the addition of Zn.

ECR Ar/CH4/H2 plasma damage in HgCdTe

The capacitance-voltage (C-V) and the Hall effect measurements were used, in order to study electron cyclotron resonance (ECR) plasma damage in HgCdTe (MCT). In this study using ECR treatments of MCT and C-V measurements, we observed that the type conversion of MCT surface largely depended on the ECR etching conditions, when MCT was etched by ECR plasma as a function of the ECR power and dc bias. The n-type conversion was not observed when the p-type MCT was etched under the condition of ECR power 150 W and dc bias -20 V. As dc bias of ECR increased over -40 V at the constant ECR power 150 W, the p-type MCT was converted to n-type. The p-type MCT was also converted to n-type when ECR power increased to 500 W at the constant dc bias -20 V. These results probably were due to the inter-diffusion of a large amount of excess mercury, liberated during the ECR treatment, into MCT, which were similar to the results of ion milling process. Another interesting result, observed in C-V measurements, was the p- type conversion from n-type MCT when the n-type MCT was etched under the condition of ECR power 150 W and dc bias -20 V. As dc bias of ECR increased over -40 V, the C-V curve was the results of n-type MCT characteristics. We considered that a low dc bias of -20 V, the hydrogen passivation and the deficiency of mercury in the etched surface were dominant and resulted in conversion to p-type. As dc bias increased over -40 V, the inter-diffusion of excess mercury into MCT was dominant and associated with keeping the n-type characteristics.

Current status and technology of the HgCdTe IR detector in Korea

The progress and current status of HgCdTe infrared detector in Korea during the last ten years is reviewed and future perspectives of infrared detector research and development are also given. The research and development of HgCdTe infrared detector was started in 1987. In the first five years, we had focused on the material growth, especially liquid phase epitaxy (LPE) by slider method and single element MWIR photovoltaic detector with large active area was realized with this LPE material. After that, the development of the linear array infrared detectors including photoconductive and photovoltaic devices was initiated and will be finished very soon. During this period we developed the travelling heater method (THM) for the use of the linear arrays. On the other hand MBE growth of HgCdTe was started for a specific applications and MOVPE process was employed for the two-color infrared development. Focal plane array program will be initiated very soon.

Single-element photoconductive Hg0.79Cd0.21Te IR detector fabrications and their characteristics

Long wavelength infrared (LWIR) photoconductive (PC) detectors of single element Hg0.79Cd0.21Te (MCT) on sapphire substrate were fabricated, using three kinds of MCT etching processes, such as wet only, wet & dry mixed, and dry only process. The ohmic contact metals, which were used to the first contact layer in the IR detector fabrication, were Au, Ni, and Ti. The performance test of the fabricated IR detectors showed the good results in the wet etched MCT IR detectors with the detectivities (D*) of (1-3) X 1010 cmHz1/2W-1 and the responsivities of (2-3) X 104 VW-1 at field of view (FOV) of 180 degrees.

Monolithic two-color detector for short- and middle-wavelength IR using p-HgCdTe/N-HgCdTe/CdTe/GaAs

A new device concept and implementation procedure of a monolithic two-color IR detector using MOVPE grown p-HgCdTe/N- HgCdTe/CdTe/GaAs is discussed. Newly introduced two-color IR detector consists of simple n-p-N structure, which can be realized using simple p-N double layer HgCdTe material. Formation of potential barrier in the conduction band of p-N heterojunction is a key to the successful operation of monolithic two-color IR detector. It prevents photogenerated minority carriers in small band gap region (p-HgCdTe) from diffusing to N-HgCdTe. The monolithic two-color IR detector was firstly fabricated using MOVPE grown p-Hg0.69Cd0.31Te/N-Hg0.64Cd0.36Te/CdTe/GaAs for SW/MWIR. SWIR diode shows RoA value of 752 (Omega) cm2, while MWIR diode shows RoA value of 140 (Omega) cm2.

Suppression of reverse bias tunneling current in HgCdTe photodiodes formed by rapid thermal diffusion

In this paper, we report electrical and optical properties of the rapid thermal diffused (RTD) p-n junction photodiodes fabricated on LPE-grown p-type Hg0.70Cd0.30Te/CdZnTe substrate. In comparison with the ion implanted p-n junction on the same substrate, the reverse bias tunneling current is drastically suppressed in the RTD junction. The spectral photo-response of indium diffused HgCdTe photodiode shows the high quantum efficiency and the detectivity of 1.3 by 1011 cm/Hz1/2W. The suppression of the reverse bias leakage current, high quantum efficiency and low noise of the RTD photodiode could be explained by the suppression of the electrical active defects generation in the depletion region during the junction formation. The extracted carrier lifetime in the junction depletion region of the RTD HgCdTe photodiodes is larger than that of the ion-implanted one.