Distinctive features of the creation of radiation-induced defects in p-Si by photonassisted low-dose ion implantation

Deep-level transient spectroscopy (DLTS) was used to investigate how temperature and in situ photoexcitation affect the creation of radiation-induced defect complexes in p-Si during low-dose ion implantation. Samples of p-Si were simultaneously irradiated by Ar1 ions accelerated to 150 keV in doses of 7×10¹⁰ cm⁻² and photoexcited with ultraviolet light at temperatures of 300 and 600 K. It was found that nonradiative heating of the samples by the implanted ions increases the total concentration of defect complexes while simultaneously changing the nature of the dominant complex. In contrast, ultraviolet illumination of the semiconductor suppresses defect formation. It was observed that in situ photoexcitation has a progressively smaller effect on the formation of radiation-induced defect complexes as the target temperature increases. The dependence of the concentration of secondary defects created as the accelerated ions are incorporated into the p-Si on the UV illumination intensity is found to be nonmonotonic. The results obtained for p-Si were analyzed and compared with previously known data for n-Si. Fiz. Tekh. Poluprovodn. 33, 897–899 (August 1999)     

Optical and Magnetic Properties of ZnO:V Prepared by Ion Implantation

We report on the optical and magnetic properties of the magnetic semiconductor Zn(V)O fabricated by implantation of 195 keV ₅₁V⁺ ions into bulk ZnO:Al grown by a hydrothermal technique. Two sets of the samples, containing N _d –N _a ∼ 10¹⁵ cm⁻³ and 10¹⁸ cm⁻³, were implanted to doses of 1 × 10¹⁵ cm⁻², 3 × 10¹⁵ cm⁻², and 1 × 10¹⁶ cm⁻². The ion implantation was performed at 573 K. To remove irradiation-induced defects, the samples were annealed in air at 1073 K. Photoluminescence (PL) measurements of Zn(V)O films were carried out at temperatures from 10 K to 300 K. The effects of implantation dose and free carrier concentration on the magnetic properties of Zn(V)O were studied using a superconducting quantum interference device magnetometer. Ferromagnetism has been observed in annealed highly conductive samples implanted to 1 × 10¹⁶ cm⁻². The PL studies of ZnO bulk samples implanted with V⁺ have revealed that thermal annealing at 1073 K restores to a large extent the optical quality of the material. A new emission line centered at 3.307 eV has been found in the PL spectrum of the highly conductive samples implanted to the dose of 1 × 10¹⁶ cm⁻², which is most probably due to complexes involving V ions.     

Simultaneous Formation of Ni/Al Ohmic Contacts to Both n- and p-Type 4H-SiC

The fabrication procedure for silicon carbide power metal oxide semiconductor field-effect transistors can be improved through simultaneous formation (i.e., using the same contact materials and a one-step annealing process) of ohmic contacts on both the n-source and p-well regions. We have succeeded in the simultaneous formation of Ni/Al ohmic contacts to n- and p-type SiC after annealing at 1000°C for 5 min in an ultrahigh vacuum. Ohmic contacts to n-type SiC were found when the Al-layer thickness was less than about 6 nm, while ohmic contacts to p-type SiC were observed for an Al-layer thickness greater than about 5 nm. Only the contacts with an Al-layer thickness in the range of 5 nm to 6 nm exhibited ohmic behavior to both n- and p-type SiC, with a specific contact resistance of 1.8 × 10⁻⁴ Ω cm² and 1.2 × 10⁻² Ω cm² for n- and p-type SiC, respectively. An about 100-nm-thick contact layer was uniformly formed on the SiC substrate, and polycrystalline δ-Ni₂Si(Al) grains were formed at the contact/SiC interface. In the samples that exhibited ohmic behavior to both n- and p-type SiC, the distribution of the Al/Ni ratios in the δ-Ni₂Si(Al) grains was larger than that observed for any of the samples that showed ohmic behavior to either n- or p-type SiC. Furthermore, the grain size of the δ-Ni₂Si(Al) grains in the samples showing ohmic behavior to both n- and p-type SiC was smaller than the grains in any of the samples that showed ohmic behavior to either n- or p-type SiC. Thus, the large distribution in the Al/Ni ratios and a fine microstructure were found to be characteristic of the ohmic contacts to both n- and p-type SiC. Grains with a low Al concentration correspond to ohmic contacts to n-type SiC, while grains with a high Al concentration correspond to ohmic contacts to p-type SiC.     

Growth and characterization of undoped and N-type (Te) doped MOVPE grown gallium antimonide

The growth of GaSb by MOVPE and itsn-type doping using a dimethyltellurium dopant source are investigated. The results of growth rate, morphology and Te incorporation as a function of growth parameters are given. Increasing growth temperature and V/III reactant ratio were found to reduce the Te incorporation. The lowest Hall carrier concentrations obtained at room-temperature, onp-type andn-type MOVPE GaSb are respectively:p _H= 2.2 × 10¹⁶cm⁻³ with a Hall mobility ofμ _H= 860 cm²/V.s andn _H= 8.5 × 10¹⁵cm⁻³ withμ _H= 3860 cm²/V.s. Furthermore, Hall mobilities as high as 5000 cm²/V.s were measured onn-type GaSb samples.     

How the type of bombarding ion affects the formation of radiation defects in silicon

The method of nonstationary capacitance was used to study how the chemical nature of implanted ions affects the creation of electrically active defects in silicon. Oxygen O⁺ and nitrogen N⁺ ions were implanted into Si at a target temperature of 300 K, in doses of 2×10¹¹cm⁻² with energies of 75 keV, and argon Ar⁺ ions were implanted in a dose of 7×10¹⁰cm⁻² with energies of 150 keV, in such a way that all the samples of n-and p-Si received approximately the same number and spatial distribution of primary radiation defects. It was observed that the spectrum of stable radiation defects depends on the nature of the bombarding ion. Thus, the DLTS spectrum of n-Si irradiated by O⁺ ions has three peaks, whereas the spectrum of n-Si implanted by N⁺ ions exhibited only one of these peaks. The DLTS spectra of samples of n-and p-Si implanted by O⁺ and N⁺ ions revealed peaks of reverse (anomalous) polarity, whose energy positions matched the most clearly defined DLTS peaks of silicon samples with the opposite type of conductivity. Fiz. Tekh. Poluprovodn. 32, 523–526 (May 1998)     

Doping and impurity compensation by ion implantation in a-SiGe films

This paper discusses the electrical properties of a-SiGe films (N _Ge∼2.2 at. %) prepared by co-evaporation of Si and Ge from separate sources and doped by ion implantation of substitutional impurities (B⁺ and P⁺), as well as the results of controlled impurity compensation by ion-beam doping. It was found that B⁺ and P⁺ implantation into a-SiGe films in the dose range 1.3×10¹⁴–1.3×10¹⁷ cm⁻², followed by annealing at 350 °C, increased the conductivity of these films from 10⁻⁹ to 10⁻⁴ and to 10⁻⁵ S/cm for B⁺ and P⁺, respectively. The position of the Fermi level could be varied from (E _v+0.27) to (E _c−0.19) eV. These investigations indicate that compensation of pre-doped a-SiGe films by ion implantation is feasible and reproducible. It is also found that higher doping efficiency of a-SiGe films is obtained by using boron than by using phosphorus. Fiz. Tekh. Poluprovodn. 32, 1260–1262 (October 1998)     

Ohmic contact formation on inductively coupled plasma etched 4H-silicon carbide

We report on the investigation of ohmic contact formation using sputtered titanium-tungsten contacts on an inductively coupled plasma (ICP) etch-damaged 4H-SiC surface. Transfer length method (TLM) measurements were performed to characterize how ICP-etch damage affects the performance of ohmic contacts to silicon carbide. In order to recover etch damage, high-temperature oxidation (1250°C for 1 h) was evaluated for one of the samples. Some of the etch damage was recovered, but it did not fully recover the etch damage for the sample etched with medium platen power (60 W). From our TLM measurements, the specific contact resistance (ρ _C of sputtered titanium tungsten on highly doped n⁺-type 4H-SiC epilayers with a doping of 1.1×10¹⁹ cm⁻³ for the unetched reference sample, 30-W etched, and 60-W etched with and without sacrificial oxidation was as low as 3.8×10⁻⁵ Ωcm², 3.3×10⁻⁵ Ωcm², 2.3×10⁻⁴ Ωcm², and 1.3×10⁻³ Ωcm², respectively. We found that the low-power (30 W) ICP-etching process did not affect the formation of ohmic contacts, and we did not observe any difference between the unetched and the 30-W etched sample from our TLM measurements, having the same value of the ρ _C. However, medium-platen-power (60 W) ICP etching showed significant influence on the ohmic contact formation. We found that the specific contact resistance is highly related to the surface roughness and quality of the metals, and the lower, specific contact resistance is due to smoother and denser ohmic contacts.     

Nature of E c −0.37 eV centers and the formation of high-resistivity layers in n-type silicon

The DLTS and Van der Pauw methods are used to investigate the production of E _c −0.37 eV centers responsible for the formation of high-resistivity layers in n-type Si irradiated with electrons and annealed in the temperature range 80–320 °C. An analysis of the experimental data leads to a conclusion as to the composition of the E _c −0.37 eV centers ([V-O-C]) and to the conclusion that their formation is stimulated by a flux of interstitial atoms away from the interface into the interior of the semiconductor during annealing accompanied by the reactions: 1) I+C_s→C_i,C_i+[V-O]→[V-O-C] (dominant reaction); 2) I+V ₂→V,V+[C-O]→[V-O-C]. Fiz. Tekh. Poluprovodn. 31, 993–997 (August 1997)     

Effect of ion irradiation of amorphous-silicon films on their crystallization

The change in the structure of amorphous Si films implanted with inert-gas ions and chemically active impurity was investigated by transmission electron microscopy and electron diffraction methods. It was shown that as a result of radiation-induced formation of thermally stable vacancy complexes, Si films irradiated with Ar⁺ and P⁺ ions with doses above 7×10¹⁵ cm⁻² do not crystallize up to temperature 680 °C. It was established that crystallization of Si films after implantation of lower doses of P⁺ ions accelerates the growth of grains in the films as a compared with the unirradiated films. A model of the mechanism by which the ion irradiation influences the crystallization of Si films is discussed. Fiz. Tekh. Poluprovodn. 32, 349–352 (March 1998)     

Low-frequency noise in n-GaN

Low-frequency noise has been investigated in the hexagonal polytype of n-type gallium nitride (GaN) with equilibrium electron concentration at 300 K n ₀⋍7×10¹⁷ cm⁻³. The frequency and temperature dependence of the noise spectral density S _I/I² was studied in the range of analysis frequencies f from 20 Hz to 20 kHz in the temperature range from 80 to 400 K. Over the entire temperature range the frequency dependence of the dark noise is close to S _I/I²∼1/f (flicker noise). The rather weak temperature dependence of the noise level is characterized by very high values of the Hooge constant α⋍5–7. These large α values indicate a rather low level of structural quality of the material. The effects of infrared and band-to-band illumination on low-frequency noise in GaN are studied here for the first time. The noise level is unaffected by illumination with photon energy E _ph<E _g (E _g is the band gap) even for a relatively high value of the photoconductivity Δσ⋍50%. Band-to-band illumination (E _ph⩾E _g) influences the low-frequency noise level over the entire investigated temperature range. At relatively high temperatures the influence of illumination is qualitatively similar to that of band-to-band illumination on low-frequency noise in Si and GaAs. At relatively low temperatures the influence of illumination on the noise in GaN is qualitatively different from the results obtained earlier for Si and GaAs. Fiz. Tekh. Poluprovodn. 32, 285–289 (March 1998)     

Hot-implantation of nitrogen donors into p- type α-SiC and characterization of n+-p junction

N⁺ implantation into p-type a-SiC (6H-SiC, 4H-SiC) epilayers at elevated temperatures was investigated and compared with implantation at room temperature (RT). When the implant dose exceeded 4 × 10₁₅ cm₋₂, a complete amorphous layer was formed in RT implantation and severe damage remained even after post implantation annealing at 1500°C. By employing hot implantation at 500~800°C, the formation of a complete amorphous layer was suppressed and the residual damage after annealing was significantly reduced. For implant doses higher than 10¹⁵ cm⁻², the sheet resistance of implanted layers was much reduced by hot implantation. The lowest sheet resistance of 542Ω/ was obtained by implantation at 500 ~ 800°C with a 4 × 10¹⁵ cm⁻² dose. Characterization of n⁺-p junctions fabricated by N⁺ implantation into p-type epilayers was carried out in detail. The net doping concentration in the region close to the junction showed a linearly graded profile. The forward current was clearly divided into two components of diffusion and recombination. A high breakdown voltage of 615 ∼ 810V, that is almost an ideal value, was obtained, even if the implant dose exceeded 10¹⁵ cm⁻². By employing hot implantation at 800°C, the reverse leakage current was significantly reduced.     

Spatial Localization of Carrier Traps in 4H-SiC MOSFET Devices Using Thermally Stimulated Current

Carrier traps in 4H-SiC metal–oxide–semiconductor (MOS) capacitor and transistor devices were studied using the thermally stimulated current (TSC) method. TSC spectra from p-type MOS capacitors and n-channel MOS field-effect transistors (MOSFETs) indicated the presence of oxide traps with peak emission around 55 K. An additional peak near 80 K was observed due to acceptor activation and hole traps near the interface. The physical location of the traps in the devices was deduced using a localized electric field approach. The density of hole traps contributing to the 80-K peak was separated from the acceptor trap density using a gamma-ray irradiation method. As a result, hole trap density of N _t,hole = 2.08 × 10¹⁵ cm⁻³ at 2 MV/cm gate field and N _t,hole = 2.5 × 10¹⁶ cm⁻³ at 4.5 MV/cm gate field was extracted from the 80-K TSC spectra. Measurements of the source-body n ⁺–p junction suggested the presence of implantation damage in the space-charge region, as well as defect states near the n ⁺ SiC substrate.     

Hybrid Inorganic–Organic Heterojunction Solar Cell

In this study, solar cells were fabricated by spin-coating polyaniline (PANI) base (EB) over an n-type Si substrate. The final heterojunction’s device structure was Al/n-type Si/EB/Au. The electrical properties of the resultant device were investigated by measuring the current density–voltage (J–V), capacitance–voltage (C–V), and impedance characteristics in the dark and under illumination. N-methyl-2-pyrrolidone (NMP), dimethylformamide (DMF), and tetrahydrofuran (THF) were used as solvents for EB. The effects of these solvents on the photovoltaic cell parameters were investigated, and the open-circuit voltage (V _oc), short-circuit current density (J _sc), fill factor (FF), and energy conversion efficiency (η) were determined. It was found that heterojunctions fabricated using EB dissolved in NMP, DMF, and THF produced J _sc of 10 mA/cm², 5.123 mA/cm², and 2.78 mA/cm², respectively. Rollover and crossover phenomena in the J–V curves under illumination were explained based on the back-contact barrier and surface recombination of electrons at the back contact. The linearity of Mott–Schottky plots indicated the formation of a heterojunction between EB and n-type Si, and the slope of 1/C ² versus voltage changed under illumination. The high values of shunt resistance were decreased under illumination, indicating that the efficiency of this type of heterojunction solar cell was limited by shunt resistance and the narrow absorption range of the solar spectrum by EB.     

New low contact resistance triple capping layer enabling very high Gm InAIAs/lnGaAs HEMTs

It has been demonstrated that a highly doped (Si:3 × 10¹⁹ cm^-3) triple capping layer consisting of n⁺−In_0.53Ga_0.47As, n⁺−In_0.52Al_0.48As, and n⁺-In_0.53Ga_0.47As can remarkably reduce the parasitic source resistance in InP-based high electron mobility transistors (HEMTs). The analysis of the source resistance revealed that the resistance element at the n⁺−In_0.53Ga_0.47As/un−In_0.52Al_0.48As/un-In_0.53Ga_0.47As channel heterointerfaces was as large as 70% of the source resis-tance when nonalloyed ohmic electrodes were used. The highly doped triple capping layer reduces the resistance contribution of vertical conduction between the capping layer and 2DEG channel. A low source resistance of 0.57 Ωmm and a low contact resistivity of 3 × 10⁻⁵ Ωcm² were obtained for the HEMTs with the highly doped triple capping layer, which were 60% lower and one order of magnitude smaller than those for the HEMTs with a conventional single capping layer doped 5 × 10¹⁸ cm⁻³, respectively. These values were also 70 and 30% lower than those for the HEMTs with a highly doped (3 × 10¹⁹ cm⁻³) single capping layer, respectively. The low source resistance brings high peak extrinsic transconduc-tance of 1 S/mm for a device with 0.4 μm long gate, which was 42% higher than the previously reported HEMTs with the same gate length.     

Arsenic Diffusion Study in HgCdTe for Low <Emphasis Type="Italic">p</Emphasis>-Type Doping in Auger-Suppressed Photodiodes

Controllable p-type doping at low concentrations is desired for multilayer HgCdTe samples in a P ⁺/π/N ⁺ structure due to the promise of suppressing Auger processes, and ultimately reduced dark current for infrared detectors operating at a given temperature. In this study, a series of arsenic implantation and annealing experiments have been conducted to study diffusion at low Hg partial pressure with the goal of achieving effective control over dopant profiles at low concentration. Arsenic dopant profiles were measured by secondary ion mass spectroscopy (SIMS), where diffusion coefficients were extracted with values ranging between 3.35 × 10⁻¹⁶ cm² s⁻¹ and 6 × 10⁻¹⁴ cm² s⁻¹. Arsenic diffusion coefficients were found to vary strongly with Hg partial pressure and HgCdTe alloy composition, corresponding to variations in Hg vacancy concentration.     

Temperature-Dependent Studies of Si-Implanted Al<Subscript>0.33</Subscript>Ga<Subscript>0.67</Subscript>N with Different Annealing Temperatures and Times

Electrical activation studies were carried out on Si-implanted Al_0.33Ga_0.67N as a function of ion dose, annealing temperature, and annealing time. The samples were implanted at room temperature with Si ions at 200 keV in doses ranging from 1 × 10¹⁴ cm⁻² to 1 × 10¹⁵ cm⁻², and subsequently proximity-cap annealed from 1150°C to 1350°C for 20 min to 60 min in a nitrogen environment. One hundred percent electrical activation efficiency was obtained for Al_0.33Ga_0.67N samples implanted with a dose of 1 × 10¹⁵ cm⁻² after annealing at either 1200°C for 40 min or at 1300°C for 20 min. The samples implanted with doses of 1 × 10¹⁴ cm⁻² and 5 × 10¹⁴ cm⁻² exhibited significant activations of 74% and 90% after annealing for 20 min at 1300°C and 1350°C, respectively. The mobility increased as the annealing temperature increased from 1150°C to 1350°C, showing peak mobilities of 80 cm²/V s, 64 cm²/V s, and 61 cm²/V s for doses of 1 × 10¹⁴ cm⁻², 5 × 10¹⁴ cm⁻², and 1 × 10¹⁵ cm⁻², respectively. Temperature-dependent Hall-effect measurements showed that most of the implanted layers were degenerately doped. Cathodoluminescence measurements for all samples exhibited a sharp neutral donor-bound exciton peak at 4.08 eV, indicating excellent recovery of damage caused by ion implantation.     

Avalanche Mechanism in p +-n −-n + and p +-n Mid-Wavelength Infrared Hg1−x Cd x Te Diodes on Si Substrates

Hg_1−x Cd _x Te mid-wavelength infrared (MWIR) p ⁺-n ⁻-n ⁺ and p ⁺-n ⁻ avalanche photodiodes (APDs) with a cut-off of 4.9 μm at 80 K were fabricated on Si substrates. Diode characteristics, avalanche characteristics, and excess noise characteristics were measured on two devices. Temperature-dependent diode and avalanche characterization was performed. Maximum 3 × 10⁶ Ω cm² and 9 × 10⁵ Ω cm² zero-bias resistance times active area (R ₀ A) products were measured for the p ⁺-n ⁻-n and p ⁺-n devices at 77 K, respectively. Multiplication gains of 1250 and 410 were measured at −10 and −4 V for the p ⁺-n ⁻-n ⁺ and p ⁺-n APDs at 77 K, respectively, in the front-illumination mode with the help of a laser with an incident wavelength of 632 nm. The gains reduce to 200 and 50 at 120 K, respectively. The excess noise factor in all APDs was measured to be in the range of 1 to 1.2.     

Activation of silicon ion-implanted gallium nitride by furnace annealing

Ion implantation into III–V nitride materials is animportant technology for high-power and high-temperature digital and monolithic microwave integrated circuits. We report the results of the electrical, optical, and surface morphology of Si ion-implanted GaN films using furnace annealing. We demonstrate high sheet-carrier densities for relatively low-dose (n_atoms=5×10¹⁴ cm⁻²) Si implants into AlN/GaN/sapphire heteroepitaxial films. The samples that were annealed at 1150°C in N₂ for 5 min exhibited a smooth surface morphology and a sheet electron concentration n_s ∼9.0×10¹³ cm⁻², corresponding to an estimated 19% electrical activation and a 38% Si donor activation in GaN films grown on sapphire substrates. Variable-temperature Hall-effect measurem entsindicate a Si donor ionization energy ∼15 meV.     

Electrical Activation Studies of Silicon-Implanted Al<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript>Ga<Subscript>1−<Emphasis Type="Italic">x</Emphasis></Subscript>N with Aluminum Mole Fraction of 11% to 51%

Electrical activation studies of Si-implanted Al _x Ga_1−x N with an Al mole fraction of 11% to 51% have been carried out as a function of ion dose and annealing temperature. The Al _x Ga_1−x N samples were implanted at room temperature with Si ions at 200 keV in doses ranging from 1 × 10¹⁴ cm⁻² to 1 × 10¹⁵ cm⁻², and subsequently annealed from 1100°C to 1350°C for 20 min in a nitrogen environment. The maximum electrical activation efficiencies for the Al _x Ga_1−x N samples with an Al mole fraction less than 40% were obtained for samples implanted with the highest Si dose of 1 × 10¹⁵ cm⁻². On the other hand, for the Al _x Ga_1−x N samples with an Al mole fraction more than 40%, nearly perfect activation efficiencies of 99% and 100% were obtained for the samples implanted with the lowest Si dose of 1 × 10¹⁴ cm⁻². The mobility of the Si-implanted Al _x Ga_1−x N samples increased with increasing annealing temperature in spite of the increased number of ionized donors and thus increased impurity scattering, indicating that a greater amount of lattice damage is being repaired with each successive increase in annealing temperature. These results provide suitable annealing conditions for Si-implanted Al _x Ga_1−x N-based devices with an Al mole fraction from 11% to 51%.     

Nearly Perfect Electrical Activation Efficiencies from Silicon-Implanted Al<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript>Ga<Subscript>1−<Emphasis Type="Italic">x</Emphasis></Subscript>N with High Aluminum Mole Fraction

Electrical activation studies of Al _x Ga_1−x N (x = 0.45 and 0.51) implanted with Si for n-type conductivity have been made as a function of ion dose and anneal temperature. Silicon ions were implanted at 200 keV with doses ranging from 1 × 10¹⁴ cm⁻² to 1 × 10¹⁵ cm⁻² at room temperature. The samples were subsequently annealed from 1150°C to 1350°C for 20 min in a nitrogen environment. Nearly 100% electrical activation efficiency was successfully obtained for the Si-implanted Al_0.45Ga_0.55N samples after annealing at 1350°C for doses of 1 × 10¹⁴ cm⁻² and 5 × 10¹⁴ cm⁻² and at 1200°C for a dose of 1 × 10¹⁵ cm⁻², and for the Al_0.51Ga_0.49N implanted with silicon doses of 1 × 10¹⁴ cm⁻² and 5 × 10¹⁴ cm⁻² after annealing at 1300°C. The highest room-temperature mobility obtained was 61 cm²/V s and 55 cm²/V s for the low-dose implanted Al_0.45Ga_0.55N and Al_0.51Ga_0.49N, respectively, after annealing at 1350°C for 20 min. These results show unprecedented activation efficiencies for Al _x Ga_1−x N with high Al mole fractions and provide suitable annealing conditions for Al _x Ga_1−x N-based device applications.