Vinay Bhagwat

InGaSb photodetectors using an InGaSb substrate for 2μm applications

Detectors operating in the 2 mm regime are critical for several applications such as atmospheric remote sensing of CO2 and optical communication systems. Current state-ofthe-art detectors based on InGaAs and HgCdTe materials epitaxially grown on binary substrates suffer fabrication complexity and performance deterioration when tuned to the 2 mm wavelength. Beside the existing technology, InGaSb ternary alloy systems show a promising performance for near-infrared detectors. By varying the indium composition, one can tune the wavelength of detection from 1.7 to 5 mm, and specifically to 2 mm by growing In1˛xGaxSb with x value of 0.8. Epitaxial growth of InGaSb layers on different binary substrates has been reported using different techniques. 1‐3 Beside the complexity of these techniques, tuning to the 2 mm regime usually involves performance deterioration mainly due to the lattice mismatch problems. Lattice mismatch in the grown layers increases dark current and noise, limiting both dynamic range and sensitivity of a detector. The availability of bulk ternary substrates may significantly simplify the fabrication process by using simpler and lower cost techniques. In this letter the fabrication and characterization of InGaSb p‐n photodetectors using InGaSb substrates are presented. The device fabrication was carried out at Rensselaer Polytechnic Institute, while the characterization was carried out at NASA Langley Research Center. Tellurium doped n-type In0.17Ga0.83Sb substrates were grown from a high temperature melt using the vertical Bridgman technique. 4 The synthesis of InGaSb was carried out in silica crucible under argon ambient using pre-synthesized GaSb and InSb polycrystals. The compositionally graded InGaSb crystal was grown using a ,100. GaSb seed. The growth rate varied in the range of 0.1‐ 0.5 mm/hr during the course of the experiment. The temperature gradient of the furnace near the melt-solid interface was in the range of 10‐ 15° C/cm. No melt stirring was employed during the growth. After the entire melt solidified, the furnace was cooled down gradually to room temperature (at a rate of 15‐ 20° C/hr ) to avoid thermal cracking of the crystal. Wafers were sliced using a diamond wheel saw (Southbay Technology). The wafers were then polished to mirror shining using a three-step optimized lapping and polishing process. Lapping was done using boron carbide 14 micron size abrasives on a PanW pad and the two step polishing was done using alumina 1 and 0.3 micron slurries on nylon and velvet pads, respectively. To form the p-type layers, Zn diffusion was carried out using the leaky box technique. Zn pellets were used as the source and the diffusion was done at 450° C for 2 h under flowing nitrogen gas. A schematic for the photodiode structure is shown in Fig. 1. Metallization was carried out using electron-beam evaporator by depositing 200 A tin followed by 1000 A gold for the back side contact and 400 A tin followed by 800 A gold for the front side metal pads. The p-layer thickness is about 0.4 mm with 10 19 cm ˛3 doping concentration, while the bulk substrate is 750 mm thick with about 10 17 cm ˛3 doping concentration as determined by C‐V measurements. No antireflective coating or surface passivation was applied. Several diodes and photodiodes with different areas were fabricated. The device characterization was

Effects of substrate temperature on properties of pulsed dc reactively sputtered tantalum oxide films

The effects of substrate heating on the stoichiometry and the electrical properties of pulsed dc reactively sputtered tantalum oxide films over a range of film thickness (0.14to5.4μm) are discussed. The film stoichiometry, and hence the electrical properties, of tantalum oxide films; e.g., breakdown field, leakage current density, dielectric constant, and dielectric loss are compared for two different cases: (a) when no intentional substrate/film cooling is provided, and (b) when the substrate is water cooled during deposition. All other operating conditions are the same, and the film thickness is directly related to deposition time. The tantalum oxide films deposited on the water-cooled substrates are stoichiometric, and exhibit excellent electrical properties over the entire range of film thickness. “Noncooled” tantalum oxide films are stoichiometric up to ∼1μm film thickness, beyond that the deposited oxide is increasingly nonstoichiometric. The presence of partially oxidized Ta in thicker (>∼1μm) nonc...

A Comparison of Dry Plasma and Wet Chemical Etching of GaSb Photodiodes

We report on the performance of GaSb pn junction photodiodes fabricated using electron cyclotron resonance plasma etching using Cl 2 /Ar recipe, a mixed gas recipe consisting of Cl 2 /BCl 3 /CH 4 /Ar/H 2 and wet chemical etching. Diodes fabricated using Cl 2 /BCl 3 /CH 4 /Ar/H 2 recipe show an order of magnitude lower leakage current density and lower ideality factor. The highest value of the zero bias dynamic resistance-area product was obtained for Cl 2 /BCl 3 /CH 4 /Ar/H 2 etched diodes and was equal to 830 Ω cm 2 as compared to 300 Ωcm 2 for Cl 2 /Ar and 330 Ω cm 2 for wet etching. Spectral responsivity of Cl 2 /BCl 3 /CH 4 /Ar/H 2 etched diodes was observed to be three times that of Cl 2 /Ar and wet etched diodes. Overall, the diodes etched using the recently reported Cl 2 /BCl 3 /CH 4 /Ar/H 2 recipe provided the best optical and electrical characteristics.

Model relating process variables to film electrical properties for reactively sputtered tantalum oxide thin films

An experimentally verified analytical model for pulsed dc reactive sputtering of tantalum oxide films is described. The influences of important process variables, like oxygen flow rate and sputtering ion current, on the oxygen partial pressure in the chamber, deposition rate, as well as film breakdown and leakage characteristics, are predicted using this model. The experimentally established existence of multiple oxygen partial pressures at a given oxygen flow rate (hysteresis loop) is theoretically explained using steady state analysis. The partial pressure of oxygen during deposition can be used to correlate and predict the critical oxygen flow rate required to achieve insulating dielectric films (with no metallic tantalum) for a specified sputtering ion current. The experimental results suggest that in order to ensure the electrical reliability of tantalum oxide films, deposition should be done at oxygen flow rates more than that required to go beyond the hysteresis region in partial pressure versus fl...

Dry etching, surface passivation and capping processes for antimonide based photodetectors

III-V antimonide based devices suffer from leakage currents. Surface passivation and subsequent capping of the surfaces are absolutely essential for any practical applicability of antimonide based devices. The quest for a suitable surface passivation technology is still on. In this paper, we will present some of the promising recent developments in this area based on dry etching of GaSb based homojunction photodiodes structures followed by various passivation and capping schemes. We have developed a damage-free, universal dry etching recipe based on unique ratios of Cl2/BCl3/CH4/Ar/H2 in ECR plasma. This novel dry plasma process etches all III-V compounds at different rates with minimal damage to the side walls. In GaSb based photodiodes, an order of magnitude lower leakage current, improved ideality factor and higher responsivity has been demonstrated using this recipe compared to widely used Cl2/Ar and wet chemical etch recipes. The dynamic zero bias resistance-area product of the Cl2/BCl3/CH4/Ar/H2 etched diodes (830 Ω cm2) is higher than the Cl2/Ar (300 Ω cm2) and wet etched (330 Ω cm2) diodes. Ammonium sulfide has been known to passivate surfaces of III-V compounds. In GaSb photodiodes, the leakage current density reduces by a factor of 3 upon sulfur passivation using ammonium sulfide. However, device performance degrades over a period of time in the absence of any capping or protective layer. Silicon Nitride has been used as a cap layer by various researchers. We have found that by using silicon nitride caps, the devices exhibit higher leakage than unpassivated devices probably due to plasma damage during SiNx deposition. We have experimented with various polymers for capping material. It has been observed that ammonium sulfide passivation when combined with parylene capping layer (150 Å), devices retain their improved performance for over 4 months.

Characterization of InGaSb detectors for 1.0- to 2.4-μm applications

Near infrared detectors in the 1 to 2.4 μm spectral range are important for many applications such as atmospheric remote sensing, where several species have strong absorption spectra in that range. Antimonide-based III-V compound semiconductor materials are good candidates for developing detectors in that spectral range. Electrical and optical characteristics of In1-xGaxSb p-n photodetectors at different temperatures are presented. The devices were fabricated either on bulk InGaSb substrates by zinc diffusion or InGaSb epitaxial layers grown on GaSb substrates by organo-metallic vapor phase epitaxy (OMVPE). Variable area devices were fabricated. Current-voltage measurements indicated higher dark current in InGaSb devices grown on GaSb substrate, due to defects generated by the lattice-mismatch. Spectral response measurements were obtained in the 1 to 2.4 μm wavelength range at different temperatures. At room temperature, the cut-off wavelengths were observed at 2.3 and 2.1 μm for InGaSb devices grown on GaSb and for devices fabricated on bulk InGaSb substrates respectively. Reducing the operating temperature shifts the cut-off wavelength to shorter values and increases the responsivity. Noise calculations indicated a room temperature detectivities of 3.3x1010 and 5.5x1010 cmHz1/2/W at 2 μm for the GaSb and InGaSb respectively. Detectivity variation with wavelength will be presented and compared to the background limited performance.

Dark current reduction of GaInAsSb based photodetectors by surface treatment with octadecylthiol

In this work, we report dark current reduction, which is achieved by surface treatment with Octadecylthiol (OTD), for GaInAsSb based photodetectors. Epitaxial layers of the GaInAsSb photodetector were grown on n-type GaSb substrates with a horizontal MOCVD reactor, and the devices were fabricated by wet chemical etching. Surface treatment was carried out by immersing fresh-prepared detector samples in molten ODT solution maintained at 100 °C for 5 hours. The ODT treated devices show an order of magnitude reduction in the leakage current density in comparison with the untreated devices. The inverse of the dynamic zero bias resistance area product (1/R0A) is also lower for ODT treated devices. XPS analyses indicate the formation of Ga-S (20.1 eV) and In-S (445.3 eV) bonds at the surface and reduction in the formation of native oxide on ODT treated GaInAsSb surface. This means that surface treatment with ODT can effectively passivate dangling bonds and also reduce the native oxide. These results indicate that ODT can be used for an effective passivation technique when more sophisticated processing steps are further developed.

A comparison of dry plasma and wet chemical passivation of GaSb photodiodes

In this paper, the effect of ammonium sulfide passivation on the wet etched sample is reported. The device performance in terms of leakage current breakdown voltage, zero bias resistance area product and responsivity for the diodes is reviewed.

Comparison of GaSb p-n junction photodiodes fabricated using Cl/sub 2//Ar and Cl/sub 2//BCl/sub 3//CH/sub 4//Ar/H/sub 2/ plasma

GaSb p-n junction mesa photodiodes were fabricated using plasma etching with chlorine/argon and an improved mixed gas recipe. The performance of the diodes is compared.