Ishwara B. Bhat

Electrical characteristics of magnesium-doped gallium nitride junction diodes

Electrical characteristics of lateral p+n diodes made from gallium nitride epitaxial layers on sapphire substrates are reported. The current–voltage characteristics are observed to have several distinct regions in which a tunneling current has been identified at low forward bias in addition to the conventional temperature-dependent diffusion current observed at moderate forward bias. A tunneling behavior indicates the presence of deep-level traps at the junction, which alter the electrical behavior of these junctions compared to the conventional behavior. In addition, space-charge-limited currents are found to influence these junctions at large forward and reverse bias.

Electrical characterization of Mg‐doped GaN grown by metalorganic vapor phase epitaxy

We have applied frequency‐dependent capacitance measurements and admittance spectroscopy on GaN:Mg to study the electronic states associated with Mg doping. Metalorganic vapor phase epitaxy GaN:Mg samples with two different Mg doping levels were grown and thermally annealed in nitrogen. Lateral dot‐and‐ring Schottky diodes using Au/Ti were fabricated. Frequency‐dependent measurements on these diodes show that the capacitance is reduced at a higher frequency, most likely due to the inability of a deep center to maintain an equilibrium ionization state under a high‐frequency modulation. Admittance spectroscopy, in which the conductance is monitored as a function of temperature, verifies the existence of one impurity‐related acceptor level in the higher Mg‐doped sample with an activation energy of 136 meV. For the lower Mg‐doped sample, two acceptor levels at 124 and 160 meV were observed. We believe these levels are most probably associated with the Mg acceptor state itself, possessing energy levels which a...

Independently accessed back-to-back HgCdTe photodiodes: a new dual-band infrared detector

We report the first data for a new two-color HgCdTe infrared detector for use in large dual-band infrared focal plane arrays (IRFPAs). Referred to as the independently accessed back-to-back photodiode structure, this novel dual-band HgCdTe detector provides independent electrical access to each of two spatially collocated back-to-back HgCdTe photodiodes so that true simultaneous and independent detection of medium wavelength (MW, 3–5 μm) and long wavelength (LW, 8–12 μm) infrared radiation can be accomplished. This new dual-band detector is directly compatible with standard backside-illuminated bump-interconnected hybrid HgCdTe IRFPA technology. It is capable of high fill factor, and allows high quantum efficiency and BLIP sensitivity to be realized in both the MW and LW photodiodes. We report data that demonstrate experimentally the key features of this new dual-band detector. These arrays have a unit cell size of 100 x 100 μm2, and were fabricated from a four-layer p-n-N-P HgCdTe film grown in situ by metalorganic chemical vapor deposition on a CdZnTe substrate. At 80K, the MW detector cutoff wavelength is 4.5 μm and the LW detector cutoff wavelength is 8.0 μm. Spectral crosstalk is less than 3%. Data confirm that the MW and LW photodiodes are electrically and radiometrically independent.

Real-time monitoring and control during MOVPE growth of CdTe using multiwavelength ellipsometry

Abstract New multi-wavelength in-situ ellipsometer hardware and data analysis software are described. The hardware can simultaneously acquire accurate ellipsometric data at 12 wavelengths in less than 1 s, is simple and compact and is well suited for in-situ monitoring. The data analysis software implements a “virtual interface” approach to determine in real time the characteristics (growth rate and composition) of the near-surface region of the film. These new tools were used to study the metal-organic vapor phase epitaxy (MOVPE) growth of CdTe on GaAs. From a post-deposition analysis of the in-situ data, dielectric constants of CdTe at growth temperature were obtained. Non-uniformity in the CdTe film thickness, and film nucleation during the initial stages of growth, were also observed in the post-deposition analysis. The determined CdTe dielectric constants were utilized in subsequent depositions to determine the growth rate of a CdTe film in real time. Feedback control of the CdTe growth rate was effected by connecting an analog control voltage line from the data acquisition/analysis computer to the Cd mass flow controller.

High quality Hg1−xCdxTe epitaxial layers by the organometallic process

An organometallic process for the epitaxial growth of Hg1−xCdxTe is described in this letter. This process involves the simultaneous pyrolysis of dimethylcadmium and diethyltelluride in mercury vapor at 415 °C, using hydrogen as the carrier gas. It is shown this process results in device quality layers of uniform composition. Layers with x=0.17 exhibited n‐type conduction, with an approximate carrier concentration of 3.8×1015 cm−3, and a Hall mobility of 2.45×105 cm2/Vs at 77 K. Thus, they are comparable to the best grown by liquid phase epitaxy. A p‐type layer, with anomalous electrical characteristics, has also been described in this letter. Reasons for these anomalous characteristics are outlined briefly.

On the Mechanism of Growth of CdTe by Organometallic Vapor‐Phase Epitaxy

In this paper are presented some experimental results to explain mechanisms involved in the growth of CdTe by organometallic vapor-phase epitaxy (OMVPE). A pyrolysis study of dimethylcadmium (DMCd) was conducted in an OMVPE reactor, in the temperature range 230/sup 0/-400/sup 0/C. It was found that dimethylcadmium decomposes above approximately 230/sup 0/C and the reaction is heterogeneous from 230/sup 0/ to 370/sup 0/C. CdTe growth was also studied over a range of temperature from 300/sup 0/ to 375/sup 0/C and for various reactor parameters. In all cases, the CdTe deposition rate was found to be closely related to the decomposition of dimethylcadmium. A model is presented to explain the CdTe growth at low temperatures where diethyltelluride is very stable. The growth of CdTe, using diethyltelluride and elemental cadmium, was demonstrated and supports the model. The growth rate of CdTe was studied as a function of the partial pressure of DMCd and DETe, and the results were explained in light of this growth model.

6kV 4H-SiC BJTs with Specific On-resistance Below the Unipolar Limit using a Selectively Grown Base Contact Process

High-voltage (6 kV) 4H-SiC NPN BJTs are demonstrated using a novel self-aligned selectively grown base contact process. The devices exhibit specific on-resistance values (Ron,sp) as low as 28 mOmegacm2, the lowest reported to date and below the unipolar limit. The current gain in the active region is found to be 3 and closely related to the depth of the isolation trench. The open-base turn-off curves exhibit a storage time of 0.4 mus, providing evidence for conductivity modulation in SiC high-voltage BJTs for the first time. Blanket growth devices fabricated on the same wafer as the selective growth devices show higher Ron,sp and current gain values as a result of a deeper isolation trench.

Epitaxial growth of AlN and Al0.5Ga0.5N layers on aluminum nitride substrates

High quality epitaxial AlN and AlxGa1−xN layers have been grown by organo-metallic vapor-phase epitaxy on single crystal a-face AlN substrates. Here we report the characterization of these layers using Rutherford backscattering/ion channeling spectroscopy, atomic force microscopy, double crystal x-ray diffraction, and preliminary electrical results. Ion channeling along the [1120] axis gave a minimum yield of 1.5% for an AlN layer and 2.2% for an Al0.5Ga0.5N, indicating excellent crystal quality. A resistivity of 20 Ω cm and a mobility of 20 cm2/V s was measured in a Si-doped, 1-μm-thick Al0.5Ga0.5N grown epitaxially on the AlN substrates.

Elastic strains in CdTe‐GaAs heterostructures grown by metalorganic chemical vapor deposition

The elastic response associated with the lattice mismatch in (100)CdTe∥(100)GaAs heterostructures was investigated by performing photoluminescence measurements as a function of CdTe layer thicknesses. The heterostructures were grown by metalorganic chemical vapor deposition. Estimates of strains, stresses, and lattice constants were obtained from shifts in near‐band‐edge photoluminescence features. Biaxial compressive strains are present in CdTe layers thinner than 1 μm. The magnitudes of the strains are larger than those expected from equilibrium models and from transmission electron microscopy results. With increasing CdTe layer thicknesses above 0.1 μm biaxial tensile strains affect the GaAs surfaces.

Arsenic‐doped p‐CdTe layers grown by organometallic vapor phase epitaxy

Arsenic‐doped CdTe layers have been grown by organometallic vapor phase epitaxy in an atmospheric pressure reactor using arsine as the dopant gas. Doping levels above 2×1017 cm−3 have been reproducibly obtained for the first time in an epitaxial growth system, with a doping uniformity of ±20% over 1.5×1.5 cm. This is a much higher level of doping than usually possible in bulk growth systems. The layers were characterized by photoluminescence measurements at 12 K and by Hall measurements as a function of temperature. The ionization energy of the As acceptor was found to be about 62±4 meV from transport measurements. It was also shown that the electronic activity of the As incorporated is a function of the dimethylcadmium to diethyltelluride partial pressure ratio in the gas phase.