Growth and characterization of large diameter undoped semi-insulating GaAs for direct ion implanted FET technology

The growth of large diameter, semi-insulating GaAs crystals of improved purity by Liquid Encapsulated Czochralski (LEC) pulling from pyrolytic boron nitride (PBN) crucibles and characterization of this material for direct ion implantation technology, is described. Three-inch diameter, 〈100〉-oriented GaAs crystals have been grown in a high pressure Melbourn crystal puller using 3 kg starting charges synthesized in-situ from 6/9s purity elemental gallium and arsenic. Undoped and Cr-doped LEC GaAs crystals pulled from PBN crucibles exhibit bulk resistivities in the 10⁷ and 10⁸ Ω cm range, respectively. High sensitivity secondary ion mass spectrometry (SIMS) demonstrates that GaAs crystals grown from PBN crucibles contain residual silicon concentrations in the mid 10¹⁴ cm^?3 range, compared to concentrations up to the 10¹⁶ cm^?3 range for growths in fused silica containers. The residual chromium content in undoped LEC grown GaAs crystals is below the SIMS detection limit for Cr (4 × 10¹⁴ cm^?3).The achievement of direct ion implanted channel layers of near-theoretical mobilities is further evidence of the improved purity of undoped, semi-insulating GaAs prepared by LEC/PBN crucible techniques. Direct implant FET channels with (1–1.5) × 10¹⁷ cm^?3 peak donor concentrations exhibit channel mobilities of 4,800–5,000 cm²/V sec in undoped, semi-insulating GaAs substrates, compared with mobilities ranging from 3,700 to 4,500 cm²/V sec for various Cr-doped GaAs substrates. The concentration of compensating acceptor impurities in semi-insulating GaAs/PBN substrates is estimated to be 1 × 10¹⁶ cm^?3 or less, and permits the implantation of 2 × 10¹⁶ cm^?3 channels which exhibit mobilities of 5,700 and 12,000 cm²/V sec at 298K and 77K, respectively.Discrete power FET's which exhibit 0.7 watts/mm output and 8 dB associated gain at 8 GHz have been fabricated using these directly implanted semi-insulating GaAs substrates.     

Pushing the limits of concentrated photovoltaic solar cell tunnel junctions in novel high‐efficiency GaAs phototransducers based on a vertical epitaxial heterostructure architecture

A monolithic compound semiconductor phototransducer optimized for narrow‐band light sources was designed for achieving conversion efficiencies exceeding 50%. The III‐V heterostructure was grown by metal‐organic chemical vapor deposition, based on the vertical stacking of 5 partially absorbing GaAs n/p junctions connected in series with tunnel junctions. The thicknesses of the p‐type base layers of the diodes were engineered for optimal absorption and current matching for an optical input with wavelengths centered near 830 nm. Devices with active areas of ~3.4 mm² were fabricated and tested with different emitter gridline spacings. The open circuit voltage (Voc) of the electrical output is five times or more than that of a single GaAs n/p junction under similar illumination. The device architecture allows for improved Voc generation in the individual base segments because of efficient carrier extraction while simultaneously maintaining a complete absorption of the input photons with no needs for complicated fabrication processes or reflecting layers. With illumination powers in the range of a few 100 mW, the measured fill factor (FF) varied between 88 and 89%, and the Voc reached over 5.75 V. The data also demonstrated that a proper combination of highly doped emitter and window layers without gridlines is adequate for sustaining such FF values for optical input powers of several hundred milliwatts. As the optical input power is further increased and approaches 2 W (intensities ~58 W/cm²), the multiple tunnel junctions sequentially exceed their peak current densities in the case for which typical (n++)GaInP/ (p++)AlGaAs concentrated photovoltaic tunnel junctions are used. Lower bandgap tunnel junctions designed with improved peak current densities result in phototransducer devices having high FF and conversion efficiencies for up to 5 W optical input powers (intensities ~144 W/cm²). Measurements at different temperatures revealed a Voc reduction of −6 mV/°C at ~59 W/cm². Copyright © 2015 John Wiley & Sons, Ltd.     

Stackable nonvolatile memory with ultra thin polysilicon film and low-leakage (Ti, Dy)xOy for low processing temperature and low operating voltages

We report the fabrication process as well as material and electrical characterization of ultra thin body (UTB) thin film transistors (TFTs) for stackable nonvolatile memories by using in situ phosphorous doped low-temperature polysilicon followed by the chemical mechanical polishing (CMP) process. The resulting polysilicon film is about 13 nm thick with approximately 10¹⁹ cm⁻³ doping. Root mean square surface roughness below 1 nm is achieved. Metal nanocrystals and high-k dielectric are selected for storage nodes and tunneling barriers to achieve low operating voltages. The number density and average diameter of nanocrystals embedded in the gate stack are 7.5 × 10¹¹ cm⁻² and 5.8 nm, respectively. Furthermore, scanning transmission electron microscopy (STEM), convergent beam electron diffraction (CBED) and electron energy loss spectroscopy (EELS) are performed for material characterization. The dielectric constant of the (Ti, Dy)_xO_y film is 35, and the off-state leakage current at −1 V bias and 2.8 nm equivalent oxide thickness is 5 × 10⁻⁷ A/cm². We obtain a memory window of about 0.95 V with ±6 V program/erase voltages. Our results show that UTB TFT is a promising candidate for the three-dimensional integration in high-density nonvolatile memory applications.     

Effect of dislocations on 1/f noise of long wavelength infrared photodiodes fabricated with HgCdTe layers grown on GaAs by metalorganic vapor phase epitaxy

We studied the effect of dislocations on the 1/f noise current of long wavelength infrared photodiodes fabricated with HgCdTe layers grown on GaAs by metalorganic vapor phase epitaxy. N-on-p junctions were formed by boron ion implantation into Hg-vacancy doped epilayers. The 1/f noise dominated from 0.5 to 100 Hz, and shot noise caused by photocurrent (√2eI_p) dominated at higher frequencies. We observed two types of 1/f noise. One is caused by the leakage current generated at dislocations, and the other is induced by the photocurrent. The 1/f noise current increased with the photon flux in the low-etch pit density (EPD) range independently of EPD. It increased with EPD in the high-EPD range. The 1/f noise current measured at zero field of view increased with EPD. This suggests that the 1/f noise generated by the photocurrent dominated in the low-EPD range, and that the 1/f noise current caused by dislocations dominated in the high-EPD range. In order to obtain a thermal image of a room-temperature object, the 1/f noise current induced by background photon flux is as high as that caused by dislocations of more than 10⁷ cm⁻². Therefore, the 1/f noise current induced by the photocurrent is dominant in photodiodes fabricated with HgCdTe layers on GaAs, since the EPD is less than 2 x 10⁶ cm⁻². We expect the detectivity to be as high as with LPE-layers. We fabricated 64 x 64 photodiode arrays, and obtained a thermal image.     

Properties of Be-implanted planar GaAs p-n junctions

The fabrication and characteristics of planar junctions in GaAs formed by Be ion implantation are discussed. The critical processing step is shown to be the use of a carefully deposited oxygen-free Si/SUB 3/N/SUB 4/ encapsulation during post-implantation annealing. Forward and reverse characteristics are presented for Be-implanted junctions formed by encapsulating with SiO/SUB 2/, Si/SUB x/O/SUB y/N/SUB z/, or Si/SUB 3/N/SUB 4/ layers prior to annealing at 900/spl deg/C. Junctions which exhibit leakage current density of ~2/spl times/10/SUP -7/ A/cm/SUP 2/ at 80 V reverse bias and breakdown voltage >200 V have been fabricated using RF-plasma deposited Si/SUB 3/N/SUB 4/ layers as the encapsulant.     

Measurement of As diffusivity in Ni2Si thin films

The lattice and grain boundary diffusion coefficients of As in 260 nm-thick Ni₂Si films were measured. The Ni₂Si layers were prepared via the reaction between a Si layer deposited by low pressure chemical vapor deposition and a Ni layer deposited by magnetron sputtering on a Si substrate covered with a SiO₂ film. As was implanted in the silicide. Its concentration profiles were measured using secondary ion mass spectroscopy before and after annealing (550-700 °C). 2D finite element diffusion simulations taking into account lattice diffusion and grain boundary (GB) diffusion were performed based on the microstructure of the samples. They were found to fit accurately the measured profiles and allowed to measure the diffusion coefficients for each temperature. Lattice diffusion is characterized by a pre-exponential factor D⁰v ∼ 1.5 × 10⁻¹ cm² s⁻¹ and an activation energy Qv ∼ 2.72 eV. In the case of GB diffusion P⁰ = sδD⁰gb = 9.0 × 10⁻³ cm³ s⁻¹ and the activation energy was found to be higher than for lattice diffusion with Qgb ∼ 3.07 eV. Existing data concerning diffusion in silicides and other materials is used to discuss these results. The diffusion of As in Ni₂Si could be reduced due to impurity segregation in GBs.     

GaN and AlGaN high-voltage rectifiers grown by metal-organic chemical-vapor deposition

In this paper, we report the study of the electrical characteristics of GaN and AlGaN vertical p-i-n junctions and Schottky rectifiers grown on both sapphire and SiC substrates by metal-organic chemical-vapor deposition. For GaN p-i-n rectifiers grown on SiC with a relatively thin “i” region of 2 μm, a breakdown voltage over 400 V, and forward voltage as low as 4.5 V at 100 A/cm² are exhibited for a 60-μm-diameter device. A GaN Schottky diode with a 2-μm-thick undoped layer exhibits a blocking voltage in excess of ∼230 V at a reverse-leakage current density below 1 mA/cm², and a forward-voltage drop of 3.5 V at a current density of 100 A/cm². It has been found that with the same device structure and process approach, the leakage current of a device grown on a SiC substrate is much lower than a device grown on a sapphire substrate. The use of Mg ion implantation for p-guard rings as planar-edge terminations in mesageometry GaN Schottky rectifiers has also been studied.     

The influence of assisted ion beam bombardment on structure and electrical characteristics of HfSiO dielectric synthesized by DIBSD

We have investigated the influence of assisted ion beam bombardment on structure and electrical properties of HfSiO dielectrics deposited on Si (1 0 0) substrate by dual-ion beam sputtering deposition (DIBSD). The X-ray photoelectron spectroscopy (XPS) analysis indicates that assisted ion beam bombardment could suppress the formation of Si clusters and partial SiO bonds. The excellent electrical properties with maximum dielectric constant (18.6) and the smaller oxide-charge density (7.2 × 10¹¹ cm⁻²) and leakage current (2.8 × 10⁻⁷ A/cm² at (V_fb−1) V) were obtained for HfSiO film by assisted ion beam bombardment at AIE = 100 eV, which provide a initial energy for the formation of film, activate the substrate surface atoms, enhance the polarization rate and improve the film surface compact and adhesion.     

Sidewall defects of AlGaN/GaN HEMTs evaluated by low frequency noise analysis

The traditional dry etching isolation process in AlGaN/GaN HEMTs causes the gate metal to contact the mesa sidewalls region, forming a parasitic gate leakage path. In this paper, we suppress the gate leakage current from the mesa-sidewall to increase the gate-to-drain breakdown voltage and thereby reduce the interface trap density by using the ion implantation (I/I) isolation technology. According to the capacitance–voltage (C–V) measured curve, the hysteresis voltage was 9.3 mV and the interface state density was 5.26 × 10¹² cm⁻² for the I/I isolation sample. The 1/f noise phenomena and Schottky characteristics are particularly studied to indicate device linearity, which is sensitive to the semiconductor surface. The fluctuation that causes trapping/detrapping of free carriers near the gate interface can be reduced because side-wall plasma-induced damages were eliminated. The reduced DC and flicker noise variation of I/I isolation HEMTs is beneficial for high power transistor applications.     

The growth of Magnesium-doped GaAs by the Om-Vpe process

Magnesium-doped GaAs has been grown by organometallic vapor phase epitaxy (OM-VPE). Bis (cyclopentadienyl) mag-nesium (Cp₂Mg) is used as the organometallic precursor to Mg. The epitaxial layers have been characterized by resis-tivity and Hall measurements, photoluminescence spectro-scopy and optical microscopy. The material is of high electrical and optical quality; controllable doping over the range 10¹⁵ to 10¹⁹cm^-3 is reproducibly attained. The ionization energy of the Mg acceptor is determined to be 30 ± 2.5 meV at 77K. Negligible compensation is observed, consistent with clean thermolysis of the Cp₂Mg under growth conditions. GaAs diodes have been fabricated using Mg as the p-dopant and either Se, Si, or Sn as the n-dopant. The diodes show very low leakage currents under reverse bias, even at relatively high doping levels. Degenerately-doped junctions for interconnecting monolithic cascade concentrator solar cells have also been successfully grown, displaying forward conductivities as high as 19 amps V⁻¹ cm^-2 at 0.05V forward bias.     

Zn doping of InP, InAsP/InP, and InAsP/InGaAs heterostructures through metalorganic vapor phase diffusion (MOVPD)

Zinc incorporation by post-growth metalorganic vapor phase diffusion (MOVPD) is used to achieve high p-doping, which is desirable for the fabrication of photodiodes. Diethylzinc (DEZ) is used as precursor and Zn is diffused into InP and InAs_0.6P epitaxial layers grown by low pressure metalorganic vapor phase epitaxy (MOVPE) on different substrate orientations, enabling the investigation of the dislocation density on the Zn incorporation. Diffusion depths are measured using cleave-and-stain techniques, resistivity measurements, electrochemical profiling, and secondary ion mass spectroscopy. High hole concentrations of, respectively, 1.7 10¹⁹ and 6 10¹⁸ cm⁻³, are obtained for, respectively, InAs_0.60P and InP. The diffusion coefficients are derived and the Zn diffusion is used for the fabrication of lattice-mismatched planar PIN InAsP/InGaAs photodiodes.     

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.     

Photolithographic patterning of organic photodetectors with a non-fluorinated photoresist system

We report on the fabrication of organic photodetectors (OPD) based on isolated islands of P3HT:PCBM. Pattern transfer to the active material was done with photolithography based on non-fluorinated solvents and the excessive organic semiconductor was removed with oxygen plasma reactive ion etching. The photoresist system used was found to be benign to the P3HT:PCBM layer as confirmed by absorption, thickness and roughness measurements. Current–voltage characteristics and external quantum efficiency (EQE) remained unchanged after the patterning process. It was demonstrated that it is possible to photolithographically pattern isolated islands with 200 μm edge length with the same dark current density (<10⁻⁵ A/cm² at −2 V bias voltage) and photocurrent density (>5 × 10⁻³ A/cm² at −2 V). Furthermore, concerning the solar cell performance, the patterned, small-area devices showed power conversion efficiency of 2.1% and fill-factor of 60%. Dark current was observed to depend on the size of the remaining semiconductor island, which was demonstrated on OPDs with diameter of 50 μm. The presented results show the feasibility of fabrication of isolated devices based on organic semiconductors patterned with non-fluorinated photolithography.     

In depth study of the compensation in annealed heavily carbon doped GaAs

Heavily C-doped GaAs epitaxial layers with holes concentrations ranging from 10¹⁹ to 1.6×10²⁰ cm⁻³ have been grown by metal organic vapour phase epitaxy (MOVPE) using CCl₄ as C-growth precursor. The carbon doping characteristics of GaAs epilayers have been investigated by optimizing the V/III ratio and the growth temperature. Additional informations have been extracted from the evolution of the in situ reflectivity signal during the growth of GaAs:C. The appearance of discernible oscillations in the reflectivity response indicates the high carbon incorporation and the good surface quality in spite of the CCl₄ etching effect. The hole concentration tends to saturate at about 1.5×10²⁰ cm⁻³. The comparison between Hall effect measurements realized on sets of as grown and annealed layers, and theoretical calculations of the mobility lead to the determination of the compensation ratio of the samples.The lattice matching conditions were systematically investigated by using high X-ray diffraction measurements from (004) and (115) planes. A comparison between the experimental mismatch and the one calculated with the Vegard's law allows the estimation of the possible origin of the compensation. Secondary ion mass spectrometry, scanning electron microscopy and atomic force microscopy have been used as complementary tools to characterize the films.     

Pentacene thin-film transistors with sol-gel derived amorphous Ba0.6Sr0.4TiO3 gate dielectric

An amorphous Ba_0.6Sr_0.4TiO₃ (BST) film with the thickness of 200 nm was deposited on indium-tin-oxide (ITO)-coated glass substrate through sol-gel route and post-annealing at 500 °C. The dielectric constant of the BST film was determined to be 20.6 at 100 kHz by measuring the Ag/BST/ITO parallel plate capacitor, and no dielectric tunability was observed with the bias voltage varying from −5 to 5 V. The BST film shows a dense and uniform microstructure as well as a smooth surface with the root-mean-square (RMS) roughness of about 1.4 nm. The leakage current density was found to be 3.5 × 10⁻⁸ A/cm² at an applied voltage of −5 V. The transmittance of the BST/ITO/glass structure is more than 70% in the visible region. Pentacene based transistor using the as-prepared BST film as gate insulator exhibits a low threshold voltage of −1.3 V, the saturation field-effect mobility of 0.68 cm²/Vs, and the current on/off ratio of 3.6 × 10⁵. The results indicate that the sol-gel derived BST film is a promising high-k gate dielectric for large-area transparent organic transistor arrays on glass substrate.     

High-power broad-area InGaNAs/GaAs quantum-well lasers in the 1200 nm range

High-power broad-area InGaNAs/GaAs quantum-well (QW) edge-emitting lasers on GaAs substrates in the 1200 nm range are reported. The epitaxial layers of the InGaNAs/GaAs QW laser wafers were grown on n⁺-GaAs substrates by using metal-organic chemical vapor deposition (MOCVD). The thickness of the InGaNAs/GaAs QW layers is 70 Å/1200 Å. The indium content (x) of the In_xGa_1−xN_yAs_1−y QW layers is estimated to be 0.35-0.36, while the nitrogen content (y) is estimated to be 0.006-0.009. More indium content (In) and nitrogen content (N) in the InGaNAs QW layer enables the laser emission up to 1300 nm range. The epitaxial layer quality, however, is limited by the strain in the grown layer. The devices were made with different ridge widths from 5 to 50 μm. A very low threshold current density (J_th) of 80 A/cm² has been obtained for the 50 μm × 500 μm LD. A number of InGaNAs/GaAs epi-wafers were made into broad-area LDs. A maximum output power of 95 mW was measured for the broad-area InGaNAs/GaAs QW LDs. The variations in the output powers of the broad-area LDs are mainly due to strain-induced defects the InGaNAs QW layers.     

Tellurium-implanted N+ layers in GaAs

Ion implantation of Te was investigated as a doping process for the fabrication of submicron n-type layers in GaAs. The implantation was performed with substrates held at 350°C. After implantation, a protective overcoat of AIN or Si₃N₄ was sputtered on the samples to prevent the GaAs from disassociating during anneal (900°C). The electrical characteristics of the n-type implants were then measured. Current-voltage and capacitance-voltage characteristics of implanted diodes indicated that the junctions were linearly graded and that there was no intrinsic layer present after anneal. Sheet resistivity and Hall effect measurements were used to determine the surface carrier concentration and effective mobility in the implanted layers. Ionized impurity profiles extending beyond the implanted junction depth were calculated by matching differential Hall effect data with junction capacitance-voltage data. A peak electron concentration of 7 × 10¹⁸ electrons/cm³ was observed. However, the profiles exhibited penetrating tails that resulted in junction depths being much deeper than the LSS range theory would predict.     

Ion-implanted microwave field-effect transistors in GaAs

Microwave field effect transistors have been fabricated in gallium arsenide by using sulfur ion implantation directly into semi insulating Cr doped substrates to produce the channel region, eliminating the need for growth of an epitaxial layer. This implantation method has been used to produce 0·25 μm thick, n-type layers with uniform thickness and carrier concentration, and carrier mobility ranging from 2410 to 3620 cm²/V sec in different samples. Because of the uniformity, FET's fabricated in these layers have exhibited reproducibility of transconductance and pinchoff voltage from device to device on a wafer to better than ±10 per cent. Cr doped GaAs of commonly available quality was found to be satisfactory for FET fabrication, although minimum Cr compensation is desirable to obtain highest mobility. S parameter measurements of microwave characteristics indicated a projected f_max = 20 GHz but transducer gain cutoff occurred at approximately 7 GHz because of impedance mismatch and package parasitics.     

Ion-implanted GaAs/AlGaAs heterojunction FET's grown by MOCVD

The successful fabrication of an ion-implanted GaAs/AlGaAs heterojunction FET device is discussed. Half-micrometer gate-length FET devices are fabricated by ion implantation into GaAs/AlGa heterostructures grown by metalorganic chemical vapor deposition (MOCVD) on 3-in-diameter GaAs substrates. The FET device exhibits a maximum extrinsic transconductance of 280 mS/mm with reduced transconductance variation over 2 V of gate bias. Excellent microwave performance is achieved with an f_t of 40 GHz, which is comparable to results obtained from 0.25-μm gate GaAs MESFETs. The effects of ion implantation on the heterojunction and corresponding device characteristics are also discussed     

Ultra-shallow np junction formed by PH3 and AsH3 plasma immersion ion implantation

Ultra-shallow 28–88 nm n⁺p junctions formed by PH₃ and AsH₃ plasma immersion ion implantation (PIII) have been studied. The reverse leakage current density and intrinsic bulk leakage current density of the diodes are found to be as low as 4.2 nA cm⁻² and 2.4 nA cm⁻², respectively. The influences of pre-annealing condition and the carrier gas on the junction depth and the sheet resistance are also studied. It is found that the increase of H or He content in the PH₃ PIII can slow down the phosphorus diffusion and shallower junction can been obtained. Annealing conditions have pronounced effect on the sheet resistance. It was found that sample annealed at 850 °C for 20 s has reverse results to that annealed at 900 °C for 6 s. For AsH₃ PIII samples, it is observed that two-step annealing is more effective to activate the dopant and a lower reverse current density resulted.