Design and performance of InAlGaAs/InAlAs superlattice avalanchephotodiodes

InAlGaAs-InAlAs superlattice avalanche photodiodes (SL-APD's) have been designed and experimentally analyzed. The layer structures and the electric field profiles were designed for 10 Gb/s applications. The mesa-structure SL-APD's exhibited a gain-bandwidth product of 120-150 GHz, a top bandwidth of 15 GHz, and a multiplied dark current of 15-20 nA for a mesa-diameter of 30 μm. Other characteristics, such as temperature dependence of dark current and dynamic ranges, were also analyzed. Furthermore, an estimated life-time of longer than 10⁵ h was achieved for the first time. The obtained characteristics, especially their high-speed characteristics with their low dark current, indicate potential for 2.5-10 Gb/s high-sensitivity and small optical receiver applications     

Amorphous silicon/silicon carbide heterojunction bulk unipolar diodes (HEBUD)

A new hydrogenated amorphous silicon/silicon carbide heterojunction bulk unipolar diode (HEBUD) has been successfully fabricated. The sawtooth-shaped composition wave of α-SiC:H between layers of n-type α-Si:H was made on an ITO/glass substrate by the RF glow-discharge deposition method. Rectification produced by an asymmetric potential barrier is demonstrated. Preliminary results showed that the design principle is feasible, and that the device is stable. The capacitance was constant regardless of bias. A switching time of 15 µs and a propagation delay time of 9 µs were obtained.     

Wannier-Stark localization in the natural superlattice of silicon carbide polytypes

Results of a study of silicon carbide polytypes under high electric fields are presented. The presence of a natural superlattice in silicon carbide polytypes is shown to introduce a miniband structure into the conduction band, which leads to a number of effects: negative differential conductivity in the Bloch oscillation mode, electron-phonon resonances, localization in the lower miniband, resonance tunneling between the first and second minibands, prevalence of holes in the impact ionization in a wide range of fields, anomalously high avalanche breakdown fields (with a negative temperature dependence of the breakdown field) and other unusual effects. Notice that all the phenomena mentioned above are parts of a single Wannier-Stark localization process, which sets in as the mean electric field increases from 100 to 2900 kV/cm (the maximal field magnitude achieved in an abrupt p-n junction is twice larger).     

Electrical characteristics of rectifying polycrystalline silicon/silicon carbide heterojunctions

This paper presents a study of the rectifying properties of heavily doped polycrystalline silicon (polysilicon) on 4H silicon carbide (4H-SiC). Current properties and barrier heights were found using analysis of the heterojunction. This revealed that Schottky analysis would be valid for the large barrier height devices. Isotype and an-isotype devices were fabricated on both p-type and n-type SiC and the electrical characteristics were investigated using capacitance vs voltage measurements, current vs voltage measurements (I-V), and temperature I-V measurements. Extraction of the barrier height, built-in potential, and Richardson constant were made and then compared to theoretical values for the heterojunction. Temperature I-V measurements demonstrated that the current transport mechanism is thermionic emission, confirming the validity of the Schottky diode model. The I-V characteristics show near ideal diode rectifying behavior and the capacitance-voltage characteristics show ideal junction space charge modulation for all polysilicon/SiC combinations. These experimental results match well with heterojunction band-offset estimated barrier heights and demonstrate that the barrier height of the polysilicon/4H SiC interface may be controlled by varying the polysilicon doping type.     

Negative differential conductance and the bloch oscillations in the natural superlattice of 8H silicon carbide polytype

Special features of electron transport in the strong electric fields in a hexagonal 8H-SiC polytype were studied; these features are caused by the natural superlattice in the crystal structure and are related to the periodic potential of minibands in the electron spectrum. It is shown that in this polytype, as in the case of 6H-and 4H-SiC polytypes studied previously, negative differential conductance is observed. This conductance is defined by the Bloch oscillation effect.     

The hydrogenated amorphous silicon reach-through avalanchephotodiodes (a-Si:H RAPDs)

The RAPD (reach-through avalanche photodiode) structure is adopted to improve the electrical and optical performance of photosensing devices made of a-Si:H. Both the electron-injection n⁺ -i-δp-i-p⁺ and hole-injection p⁺-i-δn-i-n⁺ a-Si:H RAPDs are fabricated on the indium-tin-oxide-coated glass substrates by plasma-enhanced chemical vapor deposition (PECVD). The photocurrent multiplication method is employed to study the multiplication factors and the impact ionization coefficients of the RAPDs. Since the electron-injection models have better performance, the relationships between the device dimensions and characteristics, such as I-V curves, optical gains, impact ionization rates, and excess noise factors, are further studied. The results indicate that the a-Si:H RAPD is a promising device for photosensing applications     

Amorphous silicon solar cells

Amorphous silicon solar cells have been fabricated in several different structures: heterojunctions, p-i-n junctions, and Schottky barrier devices. The procedures used in constructing the various solar cells are discussed, and their photovoltaic properties are compared. At present, the highest conversion efficiency (5.5 percent) has been obtained with a Schottky barrier cell, and this structure appears to offer the best promise of approaching the estimated efficiency limit of ∼ 15 percent.     

Amorphous silicon buried-channel thin-film transistors

We demonstrate a buried-channel thin-film field effect transistor (TFT) based on deposited silicon nitride and hydrogenated amorphous silicon with the conducting channel recessed approximately 50 Å from the interface. We fabricate transistors and capacitors by DC reactive magnetron sputtering of a silicon target in a plasma of (Ar+H ₂+N₂) or (Ar+H₂) for the nitride and silicon layers, respectively. To create a step in the conduction band, and thus a buried-channel, we vary the hydrogen partial pressure which varies the hydrogen content and the bandgap of amorphous silicon. Capacitance-voltage and current-voltage measurements on these devices present strong evidence for the existence of the buried-channel. We achieve a record field effect mobility in saturation of 1.68 cm² /V-s with amorphous silicon deposited at 230°C, and an acceptable mobility of 0.44 cm²/V-s with amorphous silicon deposited at 125°C     

Photoluminescence of anodized silicon carbide

The luminescence of single-crystalline 6H-SiC plates after electrochemical etching has been investigated. The photoluminescence spectrum was found to change strongly after etching; the decay times of separate bands were determined. Just as in the case of silicon, the change in the photoluminescence could be due to the formation of nanostructures. Fiz. Tekh. Poluprovodn. 31, 315–317 (February 1997)     

High reliability planar-type GaInAs/InP heterostructure avalanchephotodiodes

High-temperature, long-term life tests of GaInAs/InP heterostructure avalanche photodiodes have been carried out to establish criteria for high reliability photodetectors in 1.55 μm-wavelength optical submarine cable systems. A failure rate of less than 0.2 FIT at 10°C was predicted for the first time with an activation energy of 0.7 eV     

Multiplication noise of wide-bandwidth InP/InGaAsP/InGaAs avalanchephotodiodes

Measurements of InP/InGaAsP/InGaAs separate absorption, grading, and multiplication avalanche photodiode multiplication indicate that at high gains the excess noise factors approach values predicted by the conventional continuum theory. However, at lower gains the noise is suppressed. This is probably an artifact of the very thin multiplication layers which have been used to increase the gain-bandwidth product. From the frequency response of the noise power, a gain-bandwidth product of 60 GHz, which is consistent with the value of 57 GHz obtained directly from bandwidth measurements, is deduced     

Amorphous silicon as a resist material

Lithographic properties of amorphous silicon films exposed to glow-discharge hydrogen plasma and ion beams have been investigated. The rate of film etching by a CF₄ plasma is lowered by exposure, giving rise to a negative resist behavior of the material. The sensitivity and contrast are ~10¹⁸ ions/cm² and 1.1, respectively. The effect of exposure time on etching characteristics was also studied     

Time-dependent degradation of AlGaN/GaN heterostructures grown on silicon carbide

The AlGaN/GaN heterostructure field-effect transistors (HFETs) were grown on 4H-SiC substrates by metal-organic chemical-vapor deposition (MOCVD) with a range of Al compositions (30–35%) and AlGaN barrier thicknesses. Films with higher strains exhibited a time-dependent degradation of the two-dimensional electron gas (2DEG) that varied from days to weeks. Atomic force microscopy (AFM) measurements of the degraded films revealed a hexagonal cracking pattern with an increase in the medium-scale surface roughness. The localized strain relaxation of AlGaN barriers and increased roughness of the AlGaN/GaN interface and AlGaN surface result in a broad shoulder at the lower angle of the AlGaN peak and a loss of satellite fringes in the (0006) reflection x-ray diffraction (XRD) curve. This degradation raises serious questions with regard to reliability and survivability of AlGaN HFETs and may complicate device fabrication.     

非晶硅集成型色敏元件及其传感器

研究了一种新型的非晶硅ＰＩＮ异质结集成型色敏元件及其传感器的制备工艺和结构，详细讨论了色敏元件的优化设计以及响应特性、暗电流等性能。     

Surface chemistry of porous silicon carbide

The anodization reaction of SiC using HF solution makes a porous silicon carbide (PSC) layer develop. The luminescence behavior of PSC, however, is somewhat different from that of porous Si in that the so-called blue shift is not observed. Though the quantum confinement effect is said to be responsible for light emission in porous SiC, the surface state of PSC plays an important role. The effects of thermal annealing under various atmospheres on the luminescence properties were studied. Some spectroscopic analyses were adopted to elucidate the surface chemistry of PSC. The surface of PSC, which seems to be an origin of the luminescence, had C-H termination but Si-H or Si-O bonds were not detected. X-ray photoelectron spectroscopy analysis also showed that the Si-O bond that usually exists on the surface of bulk SiC was depressed and a strong peak assigned to -CH- appeared. The oxidation treatment reconstructed the Si-O bonds on the PSC surface, and this surface depressed the luminescence. Two other thermal treatments also depressed the PL spectra from the higher energy region, which is due to alternation from C-H to C-C on the surface.     

Metal oxide-semiconductor capacitors on silicon carbide

The purpose of the work discussed in this paper is to determine the feasibility of using Silicon Carbide for making surface field effect devices such as MOSFET's. The device used for making such a determination is the MOS capacitor.This paper discusses briefly the oxidation of Silicon Carbide. The techniques used to make MOS capacitors are outlined, and experimental data are presented which show that it is possible to use Silicon Carbide for constructing surface field effect devices.     

Carcinogenic properties of silicon carbide whiskers

In experiments with intrapleural injections of silicon carbide whiskers (20 mg X 3, with one month interval) to random-inbred rats their carcinogenic activity close to that of chrysotile B UICC has been established. The pleural mesotheliomas were induced in 47.7 and 34.1% of rats, respectively.     

Polycrystalline cubic silicon carbide photoconductive switch

The first cubic silicon carbide (3C-SiC) photoconductive switches were fabricated from polycrystalline 3C-SiC. The switches had a dark resistivity of 10⁶ Ω/cm. A breakdown field of 250 kV/cm and a peak photocurrent density of 10 kA/cm² through the switch were obtained. The ratio of off-resistance to on-resistance of the switch reached up to 10⁵. The photocurrent had a pulse width as narrow as 15 ns. The trigger gain of the switch was 4.7     

Electro-chemical mechanical polishing of silicon carbide

In an effort to improve the silicon carbide (SiC) substrate surface, a new electro-chemical mechanical polishing (ECMP) technique was developed. This work focused on the Si-terminated 4H-SiC (0001) substrates cut 8° off-axis toward 〈1120〉. Hydrogen peroxide (H₂O₂) and potassium nitrate (KNO₃) were used as the electrolytes while using colloidal silica slurry as the polishing medium for removal of the oxide. The current density during the polishing was varied from 10 μA/cm² to over 20 mA/cm². Even though a high polishing rate can be achieved using high current density, the oxidation rate and the oxide removal rate need to be properly balanced to get a smooth surface after polishing. A two-step ECMP process was developed, which allows us to separately control the anodic oxidation and removal of formed oxide. The optimum surface can be achieved by properly controlling the anodic oxidation current as well as the polishing rate. At higher current flow (>20 mA/cm²), the final surface was rough, whereas a smoother surface was obtained when the current density was in the vicinity of 1 mA/cm². The surface morphology of the as-received wafer, fine diamond slurry (0.1 μm) polished wafer, and EMCP polished wafer were studied by high-resolution atomic force microscopy (AFM).