Spectrometric properties of SiC detectors based on ion-implanted p +-n junctions

Results of spectrometric studies of nuclear radiation detectors based on p ⁺-n junctions formed in 4H-SiC films are presented for the first time. The junctions were fabricated by ion implantation of aluminum into 26-μm-thick CVD-grown epitaxial 4H-SiC layers with an uncompensated donor concentration of (3–5) × 10¹⁵ cm^?3. The detector characteristics were measured in testing with natural-decay alpha particles with energies of 3.35 and 5.4 MeV. The collection efficiency of charge generated by 3.35 MeV alpha particles was as high as 100% at an energy resolution of ? 2%.     

A high-temperature radiation-resistant rectifier based on p<Superscript>+</Superscript>-n junctions in 4H-SiC ion-implanted with aluminum

A combination of a high-dose (5 s- 10¹⁶ cm^-2) implantation of Al ions into epitaxial n-type 4H SiC layers grown by chemical deposition from th e vapor phase and rapid (15 s) thermal annealing at 1700–1750°C has been used to form layers with a rectangular impurity profile according to the mechanism of solid-phase epitaxial crystallization. The combined effects of enhanced diffusion of radiation defects after implantation and gettering of defects during annealing bring about an improvement in the quality of the initial material, which ensures an increase in the diffusion length of the minority charge carriers by several times. Metastable states annealed within different temperature ranges are formed in SiC under the effect of irradiation with various particles. Low-temperature annealing of radiation defects increases the radiation and temporal lifetime of devices under irradiation. High-temperature annealing of radiation defects makes it possible to vary the lifetime of nonequilibrium charge carriers, i.e, vary the frequency range of devices. The radiation resistance of SiC-based devices increases as the operation temperature is increased to 500°C.     

Recombination mechanisms in doped n-type Hg1−x CdxTe crystals and properties of diffusion p +-n junctions based on them

We made p ⁺-n-type photodiodes for the 3–5 and 8–12 μm wavelength regions by diffusing As into single-crystal n-Hg_1−x Cd_xTe substrates, and investigated their electrical and photoelectric properties. Analysis of the temperature dependences of the differential resistance and current-voltage characteristics led us to conclude that charge-carrier transport is predominately due to the generation-recombination mechanism at a temperature of 77 K. As the temperature increases, a contribution from the diffusion component also appears. We obtained values of the product R ₀ A≅0.3–1.0, 1–10, and (1–10)×10⁴ Ω · cm² for diodes with long-wavelength photosensitivity cutoffs λ_c≅11.5, 10.5, and 6.0 μm, respectively, indicating that they could operate in the regime where performance is limited by background radiation fluctuations. Fiz. Tekh. Poluprovodn. 31, 350–354 (March 1997)     

4H-SiC紫外光电探测器光电特性随温度变化的研究

利用光电流谱法研究了300K到60K温度范围内的p-i-n结构4H-SiC紫外光电探测器的暗电流及相对光谱响应特性。研究发现随着温度的降低,探测器的暗电流和相对光谱响应都逐渐减小;而且,反向偏压越高,暗电流减小的速率越大。在零偏压下,随着温度的降低,器件的温度从300K降低到60K时,相对光谱响应的峰值波长先向短波方向移动,后向长波方向移动,在60K时移至从272nm附近移至282nm附近;同时观察到探测器的相对光谱响应范围略有缩小,。此外,我们对器件并讨论了温度变化对器件p、i、n各层产生的光电流随温度变化的机理进行讨论,提出了可以通过减少i层缺陷和适当减小n层的掺杂浓度的方式来提高器件的相对光谱响应。     

High field-effect mobility in n-channel Si face 4H-SiC MOSFETs with gate oxide grown on aluminum ion-implanted material

We report investigations of Si face 4H-SiC MOSFETs with aluminum (Al) ion-implanted gate channels. High-quality SiO/sub 2/-SiC interfaces are obtained both when the gate oxide is grown on p-type epitaxial material and when grown on ion-implanted regions. A peak field-effect mobility of 170 cm/sup 2//V/spl middot/s is extracted from transistors with epitaxially grown channel region of doping 5/spl times/10/sup 15/ cm/sup -3/. Transistors with implanted gate channels with an Al concentration of 1/spl times/10/sup 17/ cm/sup -3/ exhibit peak field-effect mobility of 100 cm/sup 2//V/spl middot/s, while the mobility is 51 cm/sup 2//V/spl middot/s for an Al concentration of 5/spl times/10/sup 17/ cm/sup -3/. The mobility reduction with increasing acceptor density follows the same functional relationship as in n-channel Si MOSFETs.     

Negative capacitance (impedance of the inductive type) of silicon p +-n junctions irradiated with fast electrons

Silicon p ⁺-n junction diodes irradiated with 3.5-MeV electrons (with the dose of 4 × 10¹⁶ cm^?2) are studied. The diodes’ inductance (L) was measured at a frequency f = 1 MHz with the amplitude of alternating current equal to 0.25 mA. Simultaneously with measurements of L at alternating current, a direct current was passed through the forward-biased diode, which brought about the injection of minority charge carriers into the base. In order to identify both of the mechanisms that give rise to the inductive-type impedance in irradiated diodes with the p ⁺-n junction and the main radiation defects that are directly involved in the formation of this impedance, irradiated samples were annealed isochronously in the temperature range T _a = 225–375°C with sub-sequent study of the main characteristics of the defects by deep-level transient spectroscopy. It is shown that the inductive-type impedance in irradiated diodes is caused by the processes of capture and retention of charge carriers injected into the base at the trapping centers for a time ～1/2f, i.e., for a half-period of oscillations. It is also shown that the trapping centers are the vacancy-oxygen complexes introduced by irradiation with electrons.     

Ultrashallow p +-n junctions in silicon (100): Electron-beam diagnostics of the surface zone

Ultrashallow p ⁺-n junctions formed in silicon (100) under nonequilibrium impurity diffusion conditions are analyzed by electron-beam diagnostics of the surface zone using a probe of low-to medium-energy electrons. The energy dependence of the radiation conductivity is investigated, along with its distribution over the area of the p ⁺-n junction. This procedure can be used to determine the depth distribution (in the crystal) of the probability of separation of electron-hole pairs by the field of the p-n junction; the experimental results show that this distribution differs according to whether the kick-out mechanism or the dissociative vacancy mechanism of impurity diffusion is predominant as the basis of formation of the ultrashallow p ⁺-n junctions. Also reported here for the first time are the results of investigations of the distribution of secondary point centers formed near the boundary of the ultrashallow diffusion profile, which exert a major influence on the transport of nonequilibrium carriers. The data obtained in the study demonstrate the possibility of improving the efficiency of photodetectors, α-particle detectors, and solar batteries constructed on the basis of ultrashallow p-n junctions. Fiz. Tekh. Poluprovodn. 32, 137–144 (February 1998)     

Ultrashallow p +-n junctions in Si(111): Electron-beam diagnostics of the surface region

Ultrashallow p ⁺-n junctions fabricated in Si(111) are investigated by low-and intermediateenergy electron-beam probing of the surface region in order to determine how the crystallographic orientation of the silicon films affects the mechanisms for nonequilibrium diffusion of boron. A comparative study is made of p ⁺-n junctions made on both (111) and (100) silicon with regard to how the irradiation-induced conductivity depends on the energy of the primary electron beam, and also its distribution with area. Using this method, it is possible to determine how the probability of an electron-hole pair being separated by the electric field of the Si(111) and Si(100) p ⁺-n junctions varies with depth into the crystal, which experiments show is different, depending on whether diffusive motion of impurities is dominated by the kick-out or dissociative-vacancy mechanisms. It was found that for boron in silicon the kick-out type of diffusion mechanism is strongly enhanced in the [111] crystallographic direction, whereas diffusion in the [100] direction is primarily driven by vacancy mechanisms. It is shown that collection of nonequilibrium carriers in the p ⁺-n junction field is strongly enhanced when the diffusion profile consists of certain combinations of longitudinal and transverse quantum wells. Fiz. Tekh. Poluprovodn. 33, 58–63 (January 1999)     

Electrical properties of He+ ion-implanted GaInP

The electrical properties of high resistivity GaInP layers produced by He⁺ ion implantation have been studied. Thick high-resistivity layers (ρ > 10⁷ Ω-cm) were obtained using multi-energy implants (80 keV, 120 keV, and 150 keV). Current-voltage (I-V) measurements of mesa diodes with two ohmic contacts indicate that in the temperature range from 200 to 300K, the dominant current flow mechanism in both n-type and p-type implanted materials is Poole-Frenkel emission, especially in the range of high electric field (>10⁵ V/cm). The thermal activation energy E_a and the potential barrier height Φ_o of the generated deep levels are 0.16±0.005 eV and 0.33±0.005 eV, respectively. At low temperature, the hopping current dominates at low and moderate applied electric fields, and the I-V curves show an ohmic characteristic. The high-temperature annealing behavior of the implanted GaInP indicates that the compensation of free carriers in the material is dominated by damage-related levels, which are annealed out at high temperatures. In regard to typical alloying cycles of metal contacts in device fabrication, it is worth noting that the resistivity is still as high as 2 × 10⁸ Ω-cm for n-type samples (5 × 10⁷ Ω-cm for p-type) after 350°C annealing, which suggests that multi-energy He⁺ implantation is suitable for implant isolation in the GaInP device technology.     

Properties of p +-n structures with a buried layer of radiation-induced defects

The p ⁺-n structures based on n-type Si with dopant density 1.7×10¹³–1.2×10¹⁴ cm⁻³ were irradiated with ²³⁸Pu α particles. A layer containing radiation-induced defects with a density of the order of 3×10¹³ cm⁻³ was produced at a depth of 20 μm. This defect density gave rise to intense draining of nonequilibrium carriers in the injection-extraction regime with stationary injection as well as with pulsed generation by single particles. This makes it possible to treat the damaged layer as a plane, introduced into the bulk, with an infinite surface recombination rate. The radiation-induced defects also participated in decreasing the conductivity. A characteristic space charge distribution and, correspondingly, a bias dependence of the capacitance are observed in the structure under reverse bias. Despite the presence of formally three charge regions, four sections appear on the capacitance curve. This latter effect is due to the “additional” charge step arising in the contact potential difference field and is characteristic of compensated deep levels in semiconductors. Fiz. Tekh. Poluprovodn. 32, 359–365 (March 1998)     

4H-SiC MESFET的特性研究

对4H-SiC MESFET的特性研究发现,在室温下4H-SiC MESFET饱和漏电流的值为0.75A/mm,随着温度的上升,器件的饱和漏电流和跨导一直下降;栅长越短,沟道层掺杂浓度越高,饱和漏电流就越大.300K时器件的击穿电压为209V,计算出来的最大功率密度可达19.22W/mm.这些结果显示了4H-SiC在高温、高压、大功率器件应用中的优势.     

Some properties of ion-implanted p−n junctions in silicon

Diodes have been made by implantation of boron or gallium ions in n-type, and phosphorus ions in p-type silicon. The doses range from 5 × 10¹² to 10¹⁵ ions/cm², and the energies from 20 to 70 keV. In all diodes the reverse current shows a sharp recovery step upon annealing at 500–600°C. The reverse current after this annealing is typically of the order of 1 nA/cm² at 1 V reverse bias. To overcome the problem of low breakdown voltages usually found for implanted junctions, methods have been developed to enlarge the effective radius of curvature at the edge of the implanted junction. In a planar process with oxide masking, breakdown voltages of 150 V for 3 Ωcm or 1500 V for 300 Ωcm silicon are obtained. This is done by implanting the ions through a tapered oxide, where the oxide walls make an angle of only 3–5° with the silicon surface. The junction depth in this case is 0.4 μm.Another method uses a mask, placed free in front of the slice. Slice and mask rotate during implantation. In this way, a breakdown voltage of 2700 V is obtained with 300 Ωcm silicon.     

Voltage dependence of the differential capacitance of a p +-n junction

The dependences of the differential capacitance and current of a p ⁺-n junction with a uniformly doped n region on the voltage in the junction region are calculated. The p ⁺-n junction capacitance controls the charge change in the junction region taking into account a change in the electric field of the quasi-neutral n region and a change in its bipolar drift mobility with increasing excess charge-carrier concentration. It is shown that the change in the sign of the p ⁺-n junction capacitance with increasing injection level is caused by a decrease in the bipolar drift mobility as the electron-hole pair concentration in the n region increases. It is shown that the p ⁺-n junction capacitance decreases with increasing reverse voltage and tends to a constant positive value.     

用SIPOS-SiO_2复合层对4H-SiCn~+pp~+结构钝化的分析

用半绝缘多晶硅(SIPOS)-SiO2复合层作为4H-SiCn+pp+结构的钝化层,克服了用多孔碳化硅或单纯用SiO2钝化的不足.在LPCVD淀积SIPOS层后,用900℃氧气氛退火代替了平常的热氧化,在SIPOS层上生长了一层SiO2.实际测量证实了这种新方法的合理性.分析了各主要工艺对钝化效果的影响,综合优化指出:在淀积SIPOS层时,掺氧量要高,而淀积温度不应太高.用此方法钝化的4H-SiCn+pp+结构,击穿电压接近理想值,反向漏电流明显降低     

Simultaneous Formation of Ohmic Contacts on p +- and n +-4H-SiC Using a Ti/Ni Bilayer

In this work, Ti/Ni bilayer contacts were fabricated on both p ⁺- and n ⁺-4H-SiC formed by ion implantation, and the effects of the Ti interlayer on the contact resistance and interfacial microstructure were studied. Adoption of a thin (10 nm) Ti interlayer resulted in specific contact resistance of 4.8 μΩ cm² and 1.3 mΩ cm² on n ⁺- and p ⁺-4H-SiC, respectively, comparable to the values for contacts using only Ni. Moreover, contacts using Ti/Ni provide a flat and uniform interface between Ni₂Si and SiC, whereas discontinuous, agglomerated Ni₂Si islands are formed without the use of a Ti interlayer. In addition, the Ti interlayer was demonstrated to effectively dissociate the thin oxide film on SiC, which is advantageous for low-resistance, reliable ohmic contact formation. In summary, use of a Ti/Ni bilayer is a promising solution for one-step formation of ohmic contacts on both p ⁺- and n ⁺-4H-SiC, being especially suitable for SiC n-channel metal-oxide-semiconductor field-effect transistor (nMOSFET) fabrication.     

用SIPOS-SiO2复合层对4H-SiC n+pp+结构钝化的分析

用半绝缘多晶硅（ＳＩＰＯＳ）－ＳｉＯ２复合层作为４Ｈ－ＳｉＣｎ＾＋ｐｐ＾＋结构的钝化层,克服了用多孔碳化硅或单纯用ＳｉＯ２钝化的不足。在ＬＰＣＶＤ淀积ＳＩＰＯＳ层后,用９００℃氧气氛退火代替了平常的热氧化,在ＳＩＰＯＳ层上生长了一层ＳｉＯ２。实际测量证实了这种新方法的合理性,分析了各主要工艺对钝化效果的影响,综合优化指出：在淀积ＳＬＰＯＳ层时,掺氧量要高,而淀积温度不应太高,用此方法钝化的４Ｈ－Ｓ     

用SIPOS-SiO2复合层对4H-SiC n+pp+结构钝化的分析

用半绝缘多晶硅(SIPOS)-SiO2复合层作为4H-SiCn+pp+结构的钝化层,克服了用多孔碳化硅或单纯用SiO2钝化的不足.在LPCVD淀积SIPOS层后,用900℃氧气氛退火代替了平常的热氧化,在SIPOS层上生长了一层SiO2·实际测量证实了这种新方法的合理性.分析了各主要工艺对钝化效果的影响,综合优化指出:在淀积SIPOS层时,掺氧量要高,而淀积温度不应太高.用此方法钝化的4H-SiCn+pp+结构,击穿电压接近理想值,反向漏电流明显降低.     

Optical and electrical properties of 4H-SiC irradiated with Xe ions

Structures with aluminum-ion-implanted p ⁺-n junctions formed in 26-μm-thick chemicalvapor-deposited-epitaxial 4H-SiC layers with an uncompensated donor concentration N _d ?N _a = (1–3) × 10¹⁵ cm^?3 are irradiated with 167-MeV Xe ions at fluences of 4 × 10⁹ to 1 × 10¹¹ cm^?2 and temperatures of 25 and 500°C. Then as-grown and irradiated structures are thermally annealed at a temperature of 500°C for 30 min. The as-grown, irradiated, and annealed samples are analyzed by means of cathodoluminescence, including the cross-sectional local cathodoluminescence technique, and electrical methods. According to the experimental data, radiation defects penetrate to a depth in excess of several tens of times the range of Xe ions. Irradiation of the structures at 500°C is accompanied by “dynamic annealing” of some low-temperature radiation defects, which increases the radiation resource of 4H-SiC devices operating at elevated temperatures.