Selenium doped high-index GaAs epilayers grown by molecular beam epitaxy

Using elementary Se we grew Se-doped GaAs films on GaAs (111), (411), (711) and (100) substrates by molecular beam epitaxy. The films grown on all the high-index substrates showed n-type conduction and the maximum carrier concentration reached 2.1 × 10¹⁹ cm⁻³ for the film grown on the (411)B substrate. The carrier concentration began to saturate at a Se concentration near 10¹⁹ cm⁻³ but continued to increase up to a Se concentration of 2 × 10²⁰ cm⁻³. Above 2 × 10²⁰ cm⁻³ Se concentration, slow reduction of the carrier concentration was observed. We obtained excellent surface morphology when n-type GaAs films were grown on (411)A and (711)B substrates even at a Se concentration of 7 × 10²⁰ cm⁻³.     

GaN p–n junction diode formed by Si ion implantation into p-GaN

²⁸Si⁺ implantation into Mg-doped GaN, followed by thermal annealing in N₂ was performed to achieve n⁺-GaN layers. The carrier concentrations of the films changed from 3×10¹⁷ (p-type) to 5×10¹⁹ cm⁻³ (n-type) when the Si-implanted p-type GaN was properly annealed. Specific contact resistance (ρ_c) of Ti/Al/Pt/Au Ohmic contact to n-GaN, formed by ²⁸Si⁺ implantation into p-type GaN, was also evaluated by transmission line model. It was found that we could achieve a ρ_c value as low as 1.5×10⁻⁶ Ω cm² when the metal contact was alloyed in N₂ ambience at 600 °C. Si-implanted GaN p–n junction light-emitting diodes were also fabricated. Electroluminescence measurements showed that two emission peaks at around 385 and 420 nm were observed, which could be attributed to the near band-edge transition and donor-to-acceptor transition, respectively.     

Optoelectronic properties of Zn0.52Se0.48/Si Schottky diodes

Zn_0.52Se_0.48/Si Schottky diodes are fabricated by depositing zinc selenide (Zn_0.52Se_0.48) thin films onto Si(1 0 0) substrates by vacuum evaporation technique. Rutherford backscattering spectrometry (RBS) analysis shows that the deposited films are nearly stoichiometric in nature. X-ray diffractogram of the films reveals the preferential orientation of the films along (1 1 1) direction. Structural parameters such as crystallite size (D), dislocation density (δ), strain (ε), and the lattice parameter are calculated as 29.13 nm, 1.187 × 10⁻¹⁵ lin/m², 1.354 × 10⁻³ lin⁻² m⁻⁴ and 5.676 × 10⁻¹⁰ m respectively. From the I–V measurements on the Zn_0.52Se_0.48/p-Si Schottky diodes, ideality and diode rectification factors are evaluated, as 1.749 (305 K) and 1.04 × 10⁴ (305 K) respectively. The built-in potential, effective carrier concentration (N_A) and barrier height were also evaluated from C–V measurement, which are found to be 1.02 V, 5.907 × 10¹⁵ cm⁻³ and 1.359 eV respectively.     

High doped MBE Si p–n and n–n heterojunction diodes on 4H-SiC

The physical and electrical properties of heavily doped silicon (5×10¹⁹ cm⁻³) deposited by molecular beam epitaxy (MBE) on 4H-SiC are investigated in this paper. Silicon layers on silicon carbide have a broad number of potential applications including device fabrication or passivation when oxidised. In particular, Si/SiC contacts present several atractive material advantages for the semiconductor industry and especially for SiC processing procedures for avoiding stages such as high temperature contact annealing or SiC etching. Si films of 100 nm thickness have been grown using a MBE system after different cleaning procedures on n-type (0 0 0 1) Si face 8° off 4H-SiC substrates. Isotype (n–n) and an-isotype (p–n) devices were fabricated at both 500 and 900 °C using antimonium (Sb) or boron (B), respectively. X-ray diffraction analysis (XRD) and scanning electronic mircorscope (SEM) have been used to investigate the crystal composition and morphology of the deposited layers. The electrical mesurements were performed to determine the rectifiying contact characteristics and band offsets.     

Electrical, structural, and optical characterization of free-standing GaN template grown by hydride vapor phase epitaxy

Electrical, structural, and optical properties of a free-standing 200 μm thick n-type GaN template grown by hydride vapor phase epitaxy have been investigated. Hall mobilities of 1100 and 6800 cm²/V s have been obtained at room temperature and 50 K, respectively. Quantitative analysis of acceptor concentration, donor concentration and donor activation energy has been conducted through simultaneous fitting of the temperature dependent Hall mobility and carrier concentration data which led to a donor concentration of 2.10×10¹⁶ cm⁻³ and an acceptor concentration of 4.9×10¹⁵ cm⁻³. The resultant donor activation energy is 18 meV. The analysis indicates that the dominant scattering mechanism at low temperatures is by ionized impurities. The extended defect concentrations on Ga- and N-faces were about 5×10⁵ cm⁻² for the former and about 1×10⁷ cm⁻² for the latter, as revealed by a chemical etch. The full width at half maximum of the symmetric (0 0 0 2) X-ray diffraction peak was 69″ and 160″ for the Ga- and N-faces, respectively. That for the asymmetric (10–14) peak was 103″ and 140″ for Ga- and N-faces, respectively. The donor bound exciton linewidth as measured on the Ga- and N-face (after a chemical etch to remove the damage) is about 1 meV each at 10 K. Instead of the commonly observed yellow band, this sample displayed a green band, which is centered at about 2.45 eV.     

Estimation for the capture cross-sections of surface state on p-Si surface by photovoltaic method

A new method which can nondestructively measure the surface-state density (SSD) D_s and estimate the capture cross-sections (CCS) of surface state σ⁰_n and σ⁻_p on surface of p-type semiconductor crystals is proposed. This method is based on the photovoltage measurements at various temperatures. The photovoltage experiment was carried out with a (1 1 1) p-type Si single crystal (N_A=4.8×10¹⁴ cm ⁻³). Owing to that the surface barrier height φ_BP=0.6421 V and the surface-recombination velocity s_n=9.6×10³ cm s⁻¹ of this sample can be determined, the SSD D_s=1.2×10¹¹ cm⁻² eV⁻¹ can therefore be obtained, furthermore CCS σ⁰_n≈5×10⁻¹⁴ cm² and σ⁻_p≈2×10⁻¹⁰ cm² can also be estimated. These results are consistent with that of related reports obtained by other methods.     

Characterization of phosphorus implanted low pressure chemical vapor deposited polycrystalline silicon

Thin (3000–5000Å) low pressure chemically vapor deposited (LPCVD) films of polycrystalline silicon suitable for microelectronics applications have been deposited from silane at 600°C and at a pressure of 0.25 Torr. The films were phosphorus implanted at 150 KeV and electrically characterized with the annealing conditions and film thickness as parameters, over a resistivity range of four orders of magnitude (10³–10⁷Ω/□). Annealing during silox deposition was found to result in a lower film resistivity than annealing done in nitrogen atmosphere. Resistivity measurements as a function of temperature indicate that the electrical activation energy is a linear function of 1/N(N is the doping concentration), changing from 0.056 eV for a doping concentration of 8.9 × 10¹⁸ cm⁻³ to 0.310 eV for doping concentration of 3.3 × 10¹⁸ cm⁻³. The grain boundary trap density was found to have a logarithmically decreasing dependence on the polysilicon thickness, decreasing from 1.3 × 10¹³ cm⁻² for 2850Å polysilicon film to 8.3 × 10¹² cm⁻² for 4500Å polysilicon film.     

Characteristics of polycrystalline films grown by ultrahigh vacuum chemical vapor deposition system

In situ boron-doped polycrystalline Si_1−xGe_x (poly-Si_1−xGe_x) films deposited by ultrahigh vacuum chemical vapor deposition (UHV/CVD) system were characterized. Optimum fitted values of grain boundary trap state densities, 4.0 × 10¹² cm⁻² and 4.9 × 10¹² cm⁻² were obtained for poly-Si and poly-Si_0.79Ge_0.21, respectively. The extracted average carrier concentration in the grain agrees with secondary ion mass spectroscopy (SIMS) analysis. In turn, we found that these films are suitable Hall elements to sense magnetic field. Experimental results show that the sensitivity decreased with the increasing input current, which can be well explained using the thermionic emission theory. Finally, we use these films to fabricate thin film transistors.     

n-Channel AlSb/GaSb modulation-doped field-effect transistors

We report the first fabrication of a GaSb n-channel modulation-doped field-effect transistor (MODFET) grown by molecular beam epitaxy. The modulation-doped structure exhibits a room temperature Hall mobility of 3140 cm² V⁻¹ s⁻¹ and 77 K value of 16000 cm² V⁻¹ s⁻¹, with corresponding sheet carrier densities of 1.3 × 10¹² cm⁻² and 1.2 × 10¹² cm⁻². Devices with 1 μm gate length yield transconductances of 180 mS mm⁻¹ and output of 5 mS mm⁻¹ at 85 K. The device characteristics indicate that electron transport in the channel occurs primarily via the L-valley of GaSb above 85 K. The effective electron saturation velocity is estimated to be 0.9 × 10⁷ cm s⁻¹. Calculations show that a complementary circuit consisting of GaSb n- and p-channel MODFETs can provide at least two times improvement in performance over AlGaAs/GaAs complementary circuits.     

The physics and fabrication of in situ back-gated (311)A hole gas heterojunctions

This paper reports the fabrication of an in situ back-gated hole gas on the (311)A surface of GaAs. The hole density can be varied from fully depleted to p_s = 2.1 × 10¹¹ cm⁻² with mobilities of up to μ = 1.1 × 10⁶ cm²V⁻¹ s⁻¹. It is seen that for carrier densities down to p_s = 4 × 10¹⁰ cm⁻² the mobility in the [ ] direction is greater than that in the [ ] direction. Using a combination of front- and back-gates we are able to keep the carrier density constant and deform the hole gas wavefunction such that the holes are pushed up against or moved further away from the heterointerface. Thus we are able to separately investigate the various scattering mechanisms that determine the mobility, and compare the experimental data with theoretical calculations based on the shape of the wavefunction.     

Lateral tunneling transistors fabricated by plane-dependent Si-doping in nonplanar epitaxy on GaAs (311)A and (411)A substrates

We have demonstrated the lateral tunneling transistors on GaAs (311)A and (411)A patterned substrates by using the plane-dependent Si-doping technique. Lateral p⁺-n⁺ tunneling junctions are formed by growing heavily Si-doped layers on patterned substrates. Current—voltage curves for both transistors show gate-controlled negative differential resistance characteristics. Furthermore, the peak current density of the lateral tunneling diodes fabricated on the (311)A patterned substrates increases as buffer layer thickness is increased, and a typical peak current density of 58 A/cm² for p = 6 × 10¹⁹ cm⁻³ and n = 7 × 10¹⁸ cm⁻³ is obtained when the buffer layer thickness is 1.2 μm. This study shows that plane-dependent Si-doping in non-planar epitaxy is a promising technique for fabricating tunneling transistors.     

Process and device characterization of high voltage gallium arsenide P-i-N layers grown by an improved liquid phase epitaxy method

GaAs P-i-N layers with an i-region net doping of less than 10¹² cm⁻³ were grown on P⁺ and N⁺ substrates by a modified liquid phase epitaxy (LPE) method. Doping profiles and structural data obtained by varius characterization techniques are presented and discussed. A P⁺-P-i-N-N⁺ diode with a 25 μm-wide i-region exhibits a breakdown voltage of 1000 V, a t_rr of 50 ns, and reverse current densities (at V_R = 800 V) of − 3 × 10⁻⁶ A/cm² at 25°C and 10⁻² A/cm² at 260° C.     

Measurement of low densities of surface states at the Si---SiO2-interface

By a careful process Si---SiO₂-interfaces can be made with a low oxide charge Q_ox and with a low surface states density N_ss. For dry oxides on (100) N_ss-values as low as a few 10⁹ cm⁻² eV⁻¹ are found on samples with an oxide charge density of 3·0 × 10¹⁰ cm⁻². Only the Nicollian-Goetzberger conductance method is proved to give reasonable results on these structures. The quasi-static low frequency C-V-technique is in good agreement with the conductance technique for samples with N_ss-values higher than 1·0 × 10¹⁰ cm⁻² eV⁻¹. The spatial fluctuation of surface potential, mainly caused by oxide charge fluctuations, is an important parameter when studying the high or low frequency C-V-characteristics. Some irregularities in the experimental N_ss-φ_s-curves are explained.     

Electrical and structural properties of W-In based ohmic contacts to GaAs

A low resistance, nonalloyed, nongold ohmic contact to n-GaAs is fabricated by cosputtering W and In targets with one of the dopant elements like Si, Ge or Te. The as-deposited rectifying contacts become ohmic when annealed to 500°C, and show an average specific contact resistance of 5 × 10⁻⁶ cm². A contact resistance as low as 3 × 10⁻⁶ is achieved in W-In-Si/n-GaAs system. The Auger and Rutherford back scattering analysis of the interface reveals an InGaAs phase formation prior to ohmic behavior. The contacts are stable up to 500°C and surface morphology is much superior to presently used AuNiGe contacts. The contact pads can be patterned by dry plasma etching without affecting the GaAs substrate.     

CoSi2 ohmic contacts to n-type 6H---SiC

Cobalt disilicide (CoSi₂) ohmic contacts possessing low specific contact resistivity (_c < 3.0 ± 0.4 × 10⁻⁵ ωcm²) to n-type 6H---SiC are reported. The contacts were fabricated via sequential electron-beam evaporation of Co and Si layers followed by a two-step vacuum anealing process at 500 and 900°C. Stochiometry of the contact so formed was confirmed by Rutherford backscattering spectrometry and X-ray diffraction. Specific contact resistivities were obtained via current-voltage (I-V) analysis at temperatures ranging from 25 to 500°C. _c is compared as a function of carrier concentration, current density, temperature and time at elevated temperature.     

High temperature ohmic contacts to 3C–silicon carbide films

Currently, large-area 3C–SiC films are available from a number of sources and it is imperative that stable high temperature contacts be developed for high power devices on these films. By comparing the existing data in the literature, we demonstrate that the contact behavior on each of the different polytypes of SiC will vary significantly. In particular, we demonstrate this for 6H–SiC and 3C–SiC. The interface slope parameter, S, which is a measure of the Fermi-level pinning in each system varies between 0.4–0.5 on 6H–SiC, while it is 0.6 on 3C–SiC. This implies that the barrier heights of contacts to 3C–SiC will vary more significantly with the choice of metal than for 6H–SiC. Aluminum, nickel and tungsten were deposited on 3C–SiC films and their specific contact resistance measured using the circular TLM method. High temperature measurements (up to 400°C) were performed to determine the behavior of these contacts at operational temperatures. Aluminum was used primarily as a baseline for comparison since it melts at 660°C and cannot be used for very high temperature contacts. The specific contact resistance (ρ_c) for nickel at room temperature was 5×10⁻⁴ Ω cm², but increased with temperature to a value of 1.5×10⁻³ Ω cm² at 400°C. Tungsten had a higher room temperature ρ_c of 2×10⁻³ Ω cm², which remained relatively constant with increasing temperature up to 400°C. This is related to the fact that there is hardly any reaction between tungsten and silicon carbide even up to 900°C, whereas nickel almost completely reacts with SiC by that temperature. Contact resistance measurements were also performed on samples that were annealed at 500°C.     

Transport of the photoexcited electron-hole plasma in InP

Transport properties of the photoexcited electron-hole plasma in n-type InP have been studied by the spatial-imaged, time-resolved Raman scattering technique with 30μm and 0.1μm spatial resolution for lateral and perpendicular transport, respectively, and on a picosecond time scale. The plasma density ranging from 1 × 10¹⁶ to 2 × 10¹⁷ cm⁻³ was deduced from fitting of the Raman spectra with the plasmon-LO phonon scattering theory which took into account the contributions from free holes. In contrast to the experimental results of Young and Wan who found that ordinary diffusion equation was sufficient to fit their transient plasma density-time profiles in semi-insulating InP, our experimental measurements have shown that perpendicular transport (i.e., expansion into the bulk crystal) of the plasma in n-type InP can be very well described by a modified diffusion equation including the effect of drifting away from the surface based on a hydrodynamic model. The transient plasma density-time profiles were studied at T = 300K and for an initial injected plasma density n 2 × 10¹⁷ cm⁻³. The plasma has been found to expand laterally at a velocity V 5 × 10⁴ cm/sec and perpendicularly into the crystal at a velocity Vp 1.5 × 10⁵ cm/sec.     

Pt-based metallization of PMOS devices for a compatible monolithic integration of semiconducting/Yba2Cu3O7−δ superconducting devices on silicon

Mo, Pt, Pt/Mo and Pt/Ti thin films have been deposited onto Si and SiO₂ substrates by RF sputtering and annealed in the YBa₂Cu₃O_7−δ (YBCO) growth conditions. The effect of annealing on the sheet resistance of unpatterned layers was measured. A Pt-based multilayered metallization for the PMOS devices was proposed and tested for a compatible monolithic integration of semiconducting devices and YBCO sensors on the same silicon substrate. The best results were obtained with a Pt/Ti/Mo-silicide structure showing 0.472 Ω_□ interconnect sheet resistivity and 2×10⁻⁴ Ω cm² specific contact resistivity after annealing for 60 min at 700 °C in 0.5 mbar O₂ pressure.     

On the behaviour of buried oxygen implanted layers in highly doped GaAs

Highly doped GaAs substrate material (doping level 10¹⁸ cm⁻³) has been implanted with 350 keV O⁺ ions with doses of 10¹⁴ – 10¹⁶ cm⁻² to produce high resistivity layers which are stable at high temperatures. LPE growth of flat GaAs epilayers onto the implanted wafers was achieved up to doses of about 1 × 10¹⁵ O⁺/cm² and 5 × 10¹⁵O⁺/cm² for RT and 200°C implants, respectively. N-o-n and p-o-n structures (o: oxygen implanted) were fabricated in which breakdown voltages of up to 15 V were obtained. Examples for application of this isolation technique are shown.     

Avalanche breakdown voltage of GaAs hyperabrupt junctions

The avalanche breakdown voltage of a GaAs hyperabrupt junction diode is calculated by using unequal ionization rates for electrons and holes, and shown graphically as a function of the parameters which characterize the impurity profile of the diode. The breakdown voltage decreases abruptly at the critical point of the characteristic length L_c which varies in accordance with the impurity concentration N₀ at X = 0. For example, the critical length L_c is 7.7 × 10⁻⁶ cm and 3.3 × 10⁻⁵ cm for N₀ = 1 × 10¹⁸ cm⁻³ and 1 × 10¹⁷ cm⁻³, respectively. The breakdown voltage of a diode with extremely short or long characteristic length can be estimated from the results for corresponding abrupt junctions. The experimental results agree well with the calculated ones.