Fabrication and characterization of GaSb based thermophotovoltaic cells using Zn diffusion from a doped spin-on glass source

The GaInSb material system is attractive for application in therm ophotovoltaic (TPV) cells since its band gap can be tuned to match the radiation of the emitter. At present, most of the TPV cells are fabricated using epitaxial layers and hence are expensive. To reduce the cost, Zn diffusion using elemental vapors in a semiclosed diffusion system is being pursued by several laboratories. In this paper, we present studies carried out on Zn diffusion into n-type (Te-doped) GaSb substrates in an open tube diffusion furnace. The dopant precursor was a 2000 ? thick, zinc doped spin-on glass. The diffusion was carried out at temperatures ranging from 550 to 600°C, for times from 1 to 10 h. The diffused layers were characterized by Hall measurements using step-and-repeat etching by anodic oxidation, secondary ion mass spectrometry measurements, and TPV device fabrication. For diffusion carried out at 600°C, the junction depth was 0.3 μm, and the hole concentration near the surface was 5 × 10¹⁹/cm³. The external quantum efficiency, measured without any anti-reflection coating of the TPV cells fabricated using mesa-etching had a maximum value of 38%. Masked diffusion was also carried out by opening windows in a Si₃N₄ coated, GaSb wafer. TPV cells fabricated on these structures had similar quantum efficiency, but lower dark current.     

Deposition of thin refractory MIS structures on InP

In the present investigation, we report on significant increases in the refractory-InP Schottky barrier by process modification of the metal-semiconductor interface. Refractory-InP MIS structures were investigated as pos-sible gate metallizations. These structures consisted of Ta/Ta₂}O₅}/InP and TiW/TiO₂} /InP. A thin layer of Ta (100å) was electron beam vapor aeposited followed by the in situ formation of Ta₂}O₅} at 300?C and an oxygen overpressure. The remainder of the Ta film was deposited at 90?C substrate temperature. The TiW/TiO₂} system was formed by sputter deposition of Ti (100A?), in situ oxidation to form TiO₂} followed by deposition of TiW (88 wt % W, 12 wt % Ti). Effective barrier height for the TiW/TiO /InP structure was measured to be 0.65 eV and for the Ta/Ta₂}O₂} /InP, 0.5 eV. Leakage current at-5V was less than 3 A/cm²} for both refractory-InP MIS structures. The TiW/TiO₂} /InP structure was stable up to 400-450°C, while the Ta/Ta₂ O₅/InP system degraded to 0.2 eV at 300-350?C.     

Interface strain in InGaAs-InP superlattices

Strain has been measured within (001) oriented OMVPE grown multilayer superlattices consisting of thin As-compound layers in InP and thin P-compound layers in GaAs. From the strain behavior, it is interpreted that As rapidly replaces P on an InP surface exposed to AsH₃ and P slowly replaces As on a As-terminated surface exposed to PH₃. This results in incorporation of an InAs-like strain in InP whose magnitude depends on the nature of the As-terminated surface. At growth temperatures above 600°C, the strain is equivalent to about one monolayer of InAs; while below 600°C, it is equivalent to two monolayers of InAs. PH₃ interaction with GaAs surfaces is sufficiently slow that GaP-like strain is observed only when deliberate interrupts under PH₃ are introduced. GaP grown on GaAs at 650°C is found to incorporate enough residual As to sustain a layer composition of GaAs_0.5P_0.5 over the first several monolayers.     

Growth and characterizations of InP self-assembled quantum dots embedded in InAIP grown on GaAs substrates

We report the characteristics of InP self-assembled quantum dots embedded in In_0.5Al_0.5P on GaAs substrates grown by metalorganic chemical vapor deposition. The InP quantum dots show increased average dot sizes and decreased dot densities, as the growth temperature increases from 475°C to 600°C with constant growth time. Above the growth temperature of 600°C, however, dramatically smaller and densely distributed self-assembled InP quantum dots are formed. The small InP quantum dots grown at 650°C are dislocation-free “coherent” regions with an average size of ∼20 nm (height) and a density of ∼1.5 × 10⁸ mm⁻². These InP quantum dots have a broad range of luminescence corresponding to red or organge in the visible spectrum.     

A study on MIS Schottky diode based hydrogen sensor using La2O3 as gate insulator

In this work, a Metal–Insulator–Semiconductor (MIS) based Schottky-diode hydrogen sensor was fabricated with La₂O₃ as a gate insulator. The electrical properties (current–voltage characteristics, change in barrier height and sensitivity) and hydrogen sensing performance (dynamic response and response time) were examined from 25 °C to 300 °C and towards H₂ with different concentrations. The conduction mechanisms were explained in terms of Fowler–Nordheim tunneling (below 120 °C) and the Poole–Frenkel effect at temperatures (above 120 °C). The results show that at an operating temperature of 260 °C, the sensitivity of the device can reach a maximum value of 4.6 with respect to 10,000-ppm hydrogen gas and its response time was 20 s.     

Metalorganic vapor phase epitaxial growth and structural characterization of self-assembled InAs nanometer-sized Islands on InP(001)

Self-assembled InAs islands were grown by metalorganic vapor phase epitaxy on InP(001) and characterized by atomic force microscopy and transmission electron microscopy. The growth temperature (450–600°C), the InAs deposition time (3–12 s, using a growth rate of ～2.3Å/s), and the growth interruption time (8–240 s) were varied systematically in order to investigate the effect of thermodynamic and kinetic factors on the structural properties of InAs/InP and InP/InAs/InP structures. It is found that the structural properties of islands vary widely with the growth conditions, ranging from very small (4–5 nm height, ～30–60 nm in diameter) coherent islands at low temperature (450–500°C) to large (～350 nm wide) plastically relaxed islands at high temperature (600°C). For a given deposition time, the height of the coherent islands increases markedly with the growth temperature while their diameter shows only a moderate increase. The growth interruption time also affects the formation and the evolution of islands, which clearly shows that these processes are kinetically limited. Coherent islands with structural properties suitable for use in optoelectronic devices are obtained from ～2.4–4.8 monolayer thick InAs layers using a growth temperature of 500°C and a 30 s interruption time.     

The effect of annealing temperature on the electrical properties of metal-ferroelectric (PbZr0.53Ti0.47O3)-insulator (ZrO2)-semiconductor (MFIS) thin-film capacitors

Metal-ferroelectric-insulator-semiconductor (MFIS) thin-film structures using PbZr_0.53Ti_0.47O₃ (PZT) as the ferroelectric layer and zirconium oxide (ZrO₂) as the insulator layer were fabricated in this work. The leakage current and the C-V memory window were measured for MFIS capacitors with the PZT layer annealed at temperatures of 400 °C, 500 °C, 600 °C, 700 °C. The dominant conduction mechanism of Al/PZT(290 nm)/ZrO₂(15 nm)/Si structure is Poole-Frenkel emission in the temperature range of 300-425 K. Under a sweep voltage of 6 V, the largest memory window of 1.31 V was obtained for 500 °C-annealed samples. The memory window as a function of insulator thickness was also discussed. More serious charge injection is observed when the voltage was swept from negative to positive.     

Improved Schottky Contacts on <Emphasis Type="Italic">n</Emphasis>-Type 4H-SiC Using ZrB<Subscript>2</Subscript> Deposited at High Temperatures

We report on improved electrical properties and thermal stability of ZrB₂ Schottky contacts deposited on n-type 4H-SiC at temperatures between 20°C and 800°C. The Schottky barrier heights (SBHs) determined by current-voltage measurements increased with deposition temperature, from 0.87 eV for contacts deposited at 20°C to 1.07 eV for those deposited at 600°C. The Rutherford backscattering spectroscopy (RBS) spectra of these contacts revealed a decrease in oxygen peak with an increase in the deposition temperature and showed no reaction at the ZrB₂/SiC interface. These results indicate improved electrical and thermal properties of ZrB₂/SiC Schottky contacts, making them attractive for high-temperature applications.     

Evaluation of encapsulation and passivation of InGaAs/InP DHBT devices for long-term reliability

Device encapsulation and passivation are critical for long-term reliability and stability. Several encapsulation techniques were evaluated in terms of degradation of electrical characteristics, gap filling under the mesa structures, and adhesion to the semiconductor and metal surfaces. These included plasma enhanced chemical vapor deposited (PECVD) SiO₂, electron cyclotron resonance CVD SiN_x, spin-on glass, benzocyclobutene, and polyimide. Damage from plasma exposure caused gain degradation in the devices. Spin-on coatings cause little to no gain degradation, provided that there is minimal stress in the cured film. SOG and BCB films have acceptable adhesion properties and were excellent for gap filling. Polyimide films have excellent adhesion properties, however, they were poor at gap filling and had a great deal of shrinkage during curing. Device passivation was evaluated using double heterojunction bipolar transistor structures with either an abrupt or graded emitter-base junction. Abrupt junction devices had the self-aligned base metal directly on the p⁺ InGaAs base. Graded junction devices had the base metal on top of graded InGaAsP layers, which the metal was diffused through, to make contact to the base region. Abrupt junction devices stressed at an initial J_E of 90 kA/cm² at a V_CE of 2V at 25°C degraded 20% within 70 h of operation, whereas, the graded junction devices show no degradation in dc characteristics after operation for over 500 h. Typical common emitter current gain was 50. An f_t of 80 and f_max of 155 GHz were achieved for 2×4 μm² emitter size devices.     

Electrical characteristics of AlGaN/GaN metal-insulator semiconductor heterostructure field-effect transistors on sapphire substrates

The electrical performance of AlGaN/GaN metal-insulator semiconductor, heterostructure field-effect transistors (MISHFETs) were studied and compared to passivated and unpassivated HFETs. Record MISHFET current densities up to 1,010 mA/mm were achieved, and the devices exhibited stable operation at elevated temperatures up to 200°C. Higher maximum-drain current, breakdown voltage, and a lower gate-leakage current were obtained in the MISHFETs compared to unpassivated HFETs. The breakdown voltage of these devices exhibited a negative temperature coefficient of 0.14 VK⁻¹, suggesting that a mechanism other than impact ionization may be responsible. Different structures of MIS diodes also reveal that the high-field region at the gate edge dominates the breakdown mechanism of these devices. Gate-pulse measurements indicate the presence of current collapse in the MISHFETs, despite the expected passivation effect of the insulator. However, a striking feature observed was the mitigation of these effects upon annealing the devices at 385°C for 5 min under N₂ ambient.     

Formation of thermally stable Ni monosilicide using an inductively coupled plasma process

The effect of argon plasma treatment on the structural and electrical properties of Ni silicides has been investigated. Electron-beam-evaporated Ni films on Si substrates are Ar plasma-treated by an inductively coupled plasma (ICP) reactor. For silicidation reactions, all the samples with and without the Ar plasma treatment are rapid-thermal annealed (RTA) at temperatures of 500–750°C in a nitrogen ambient. It is shown that the Ar plasma-treated samples produce better electrical and structural properties across the whole temperature range as compared with the untreated samples. It is further shown that the Ar plasma-treated samples contain nitrogen, which plays an important role in improving the morphological stability and the electrical properties of the silicide films.     

Research of SiO2/InP structure prepared by photo-CVD

The SiO₂ film as an insulator in InP MOS structure was grown by mercury-sensitized photo induced chemical-vapor deposition (photo-CVD) utilizing gaseous mixture of monosilane (SiH₄) and nitrous oxide (N₄O) under 253.7 nm ultraviolet light irradiation. The PHOTOX SiO₂ film (i.e., SiO₂ film prepared by photo-CVD system) deposited at 250° C has a refractive index of 1.46 and breakdown field strength of 7.0 MV/cm. The 1 MHz capacitance-voltage characteristics of the InP MOS diode was measured to study the interface state densities. The minimum value is 1.2 × 10¹¹ cm⁻²eV⁻¹ for the sample prepared at a substrate temperature of 250° C.     

Current multiplication in MIS structures

In this paper, the current multiplication mechanism in MIS structures is studied theoretically by employing a parameter, φ, which indicates the degree of inversion of the MIS system. Current expressions for majority carriers as well as minority carriers are derived in terms of this parameter. V_ao, the voltage drop across the sandwiched insulator is also calculated by solving Poisson's equation for the system, in terms of this parameter. The calculated V_ao is then used to compute the multiplication factor for the system. Because of the use of this parameter, a clear view of the physical process of the multiplication mechanism is obtained from the analysis.The following results are obtained for MIS structures through this study: (1) For MIS with a thin insulator (γ 50 Å), V_ao is negligibly small before the MIS is in inversion and increases rapidly after the MIS is in inversion. (2) For MIS with a thin insulator, φ_(inv) is independent of the applied voltage. (3) An MIS with a thicker insulator has a larger current multiplication than that of an MIS with a thinner insulator. (4) An MIS with a lower φ_Bn, the electron barrier height, has a larger current multiplication.     

InP and Si metal-oxide semiconductor structures fabricated using oxygen plasma assisted wafer bonding

In this paper, InP metal-oxide-semiconductor (MOS) structures are fabricated by transferring thermally grown SiO₂ to InP from oxidized Si wafers using oxygen plasma assisted wafer bonding followed by annealing at either 125°C or at 400°C. Well-defined accumulation and inversion regions in recorded capacitance-voltage (C-V) curves were obtained. The long-term stability was comparable to what has been previously reported. The structures exhibited high breakdown fields, equivalent to thermally grown SiO₂-Si MOS structures. The transferring process was also used to fabricate bonded Si MOS structures.     

InxGa1-xAs ohmic contacts to n-type GaAs prepared by sputter deposition

Thermally stable, low contact resistance InAs/Ni/W contacts were previously prepared by sputter depositing InAs, Ni, and W targets in our laboratory. However, the optimum annealing temperature to provide low contact resistance (R_c) was high, resulting in rough contact surface. In the present experiment, the effects of the In concentrations of In_xGa_1-x As targets on the optimum annealing temperature to prepare low R_cand the surface morphology of the_xGa_1-x/ W contacts were studied. In addition, the electrical properties and the interfacial microstructure were correlated to search the optimum In concentration to provide the minimum R_c, where the interfacial microstructure was analyzed by x-ray diffraction and transmission electron microscopy and the contact resistances (R_cc) were measured by the transmission line method. The optimum annealing temperature to provide minimum R_c was reduced by 150°C by using the In_0.7Ga_0.3As targets instead of the previous targets. The contact resistance of 0.4 Ω-mm was obtained for the In_0.7Ga_0.3As/Ni/W contacts after annealing at temperatures of around 600°C. The R_c values did not deteriorate after annealing at 400°C for 2 h. Also, the surface of this contact was smooth and no evidence of In outdiffusion on the contact surface was seen. Finally, the effect of the In concentrations at the metal/GaAs interfaces on the electrical properties will be discussed.     

Growth and crystal quality of inp crystals by the liquid encapsulated czochralski technique

The growth of InP single crystals by the liquid encapsulated Czochralski (LEC) technique has been studied from the standpoint of improving crystal quality. Twin-free crystals have been grown reproducibly in the <lll>P direction under the following conditions; (1) using starting material which does not contain fine InP particles, (2) controlling the cone shape of the crystals such that the angle with the growth axis is less than 19.68°, (3) arranging the.hot zone to produce a temperature at the top surface of the B₂O₃ encapsulant layer below 550°C. It has been confirmed that electrical properties of nominally undoped crystals are dominated by the impurity, Si, and the concentration of Si in an LEC crystal corresponds to that of the starting material. The dislocation densities of undoped LEC InP crystals depend on thermal stresses during the growth process. This knowledge has led to the growth of dislocation-free crystals.     

Fabrication of undoped semi-insulating InP by multiple-step wafer annealing

Recently, it was found that undoped semi-insulating InP can be reproducibly obtained by wafer annealing at 950°C for 40 h under phosphorus vapor pressure of 1 atm. Resistivity variation across the 50 mm diameter wafer after this annealing process, however, was in the range of 22.5–53.7%. In order to realize the fabrication of undoped semi-insulating (SI) InP with uniform electrical properties, multiple-step wafer annealing (MWA) procedure has been applied for the first time. It was found that two-step wafer annealing at 950°C for 40 h under phosphorus vapor pressure of 1 atm and at 807°C for 40 h under phosphorus vapor pressure ranging from 30 to 50 atm, was effective in improvement of the uniformity of the electrical properties of undoped SI InP. By the present MWA, the resistivity variation of 8–12% and the mobility variation of 2–4% could be obtained for 50 mm diameter wafers.     

Reactions between Pd thin films and InP

The reactions between Pd thin films and (001) oriented InP have been studied in detail. Palladium was deposited on InP by electron beam evaporation to a thickness of 60 nm. Specimens were then annealed in vacuum at temperatures up to 500° C for as long as several days. An amorphous ternary phase (Pd≈₃InP) formed during deposition. During annealing, several crystalline ternary phases were detected. Pd₂InP and Pd₅InP were detected at lower annealing temperatures,i.e. from 225–275° C. Pd₂InP grew first, exhibiting an epitaxial relationship with InP, followed by preferred growth of Pd₅InP within the Pd₂InP layer. Both phases later decomposed (≈400° C) producing Pd₂InP(II), which also grew epitaxially on InP. At temperatures greater than 400° C, Pd₂InP(II) decomposed to Pdln and PdP₂, which were thermodynamically stable in contact with InP.     

The electrical and optical properties of InP irradiated with high integrated fluxes of neutrons

The results of studying the electrical properties and optical-absorption spectra of InP irradiated with fast neutrons (E > 0.1 MeV and D_f.n ≤ 10¹⁹ cm^?2) and full-spectrum reactor neutrons (D_th.n ≤ 2.1 × 10¹⁹ cm^?2; the ratio of the fluxes was ?_th.n/?_f.n ≈ 1) are reported. The variations in these properties resulting from postirradiation annealing at temperatures as high as 900°C are also studied. The results of the optical studies indicate that, in InP irradiated heavily with neutrons, free charge carriers appear only after annealing at temperatures higher than 500°C. The efficiency of neutron-initiated transmutational doping and the quality of transmutationdoped InP are assessed.