Pd/Ge contacts to n-type GaAs

Sintered metal-semiconductor contacts, formed by thin, evaporated layers of Pd and Ge on n-type GaAs, were studied using Auger electron spectroscopy, X-ray diffraction, X-ray photoelectron spectroscopy, secondary ion mass spectroscopy, current-voltage measurements, and capacitance-voltage measurements. Prior to sintering, the as-deposited Pd/Ge/GaAs contacts were rectifying and exhibited a reproducible Schottky barrier energy φ_Bn of 0.67±0.02 eV. Auger analysis indicated the initial behavior of the contact structure, upon sintering, to be an interdiffusion and reaction of Pd and Ge on a non-reacting GaAs substrate. Two germanide phases, Pd₂Ge and PdGe, were identified using X-ray diffraction and Auger analysis. The intervening Ge layer prevented the reaction of Pd with the GaAs substrate at low temperatures. Because of the PdGe reaction, φ_Bn increased to approximately 0.85 eV. Sintering at higher temperatures (i.e. between 300 and 400°C) produced additional reactions between Pd and the GaAs substrate. The electrical properties of the contact remained rectifying and φ_Bn exhibited little change from the value of 0.85 eV with the interdiffusion of Pd, Ga, and As. Sintering above 400°C resulted in the formation of ohmic contacts. The diffusion of Ge to the GaAs interface was found to correlate with the onset of ohmic behavior. Current conduction in the contact was best described by thermionic-field emission theory, and a specific contact resistance of 3.5 × 10^?4Ω-cm² was obtained after sintering above 550°C, independent of the initial impurity concentration in the substrate. Over the entire range of sintering temperatures (i.e. at or below 600°C), the interaction between the thin-film layers appeared to be governed by diffusion-controlled, solid-phase processes with no evidence of the formation of a liquid phase. As a result, the surface of the contact structure remained smooth and uniform during sintering.     

Noise in near-ballistic n+nn+ and n+pn+ gallium arsenide submicron diodes

Direct-current (d.c.) characteristics and noise measurements in the range 1 Hz-25 kHz are reported for n⁺nn⁺ and n⁺pn⁺ near-ballistic devices, with n regions (p regions) of 0.4 μm (0.45 μm), fabricated by molecular beam epitaxy at Cornell. The n⁺nn⁺ mesa structures show very low 1/? noise. indicating a Hooge parameter α_H = 6.0 × 10^?. This very low noise is attributed to the near absence of phonon collisions. The thermal (? like) noise above 1 kHz is equal to Nyquist noise at the lowest currents, rising to slightly above Nyquist noise for high currents, indicating the presence of carrier drag effects. The n⁺pn⁺ noise, on the contrary, is quite high. It seems to be associated with the ambipolar effects occurring for low injection of electrons in the p region. The importance of noise measurements for confirming ballistic or near-ballistic behavior is discussed.     

Effect of heat treatment on electron traps in n-type GaAs

Annealing of bulk and vapor phase epitaxial (VPE) n-type GaAs at about 600°C for one hour in an apparatus of fused quartz (GE 204 or vitrosil) is seen to cause considerable reduction in the free electron concentration (N_S) and the commonly observed 0.85 eV electron traps (N_T) in these materials. Heating at elevated temperatures in the same environment causes an n-type sample to convert to p-type. The reduction in N_S and N_T is found to be considerably smaller for comparable heat treatments in a vacuum system free from quartz. It is suggested that the observed reduction is caused by in-diffusion of impurities like Cu and Li which are present in the types of quartz used in the experiments.     

Studies of n-type GaAs material properties by anodic current behaviour

The carrier concentrations of n-type GaAs material have been measured by a technique using anodisation in the dark. The results are compared with a theoretical model and with other results based on Schottky barrier reverse breakdown. The technique has also been extended to include surfaces containing more than one type of material and n-type materials of different carrier concentrations. Additionally the technique has been used to make an assessment of the surface quality of the material.     

Effect of O+ implantation on silicon-silicon dioxide interface properties

A detailed investigation has been made by the MOS capacitance method, into the mechanism by which the fixed positive surface state charge, due to silicon rich oxide near SiSiO₂ interface, is controlled by O⁺ implantation into the oxide near the SiSiO₂ interface, and subsequent heat treatment. High dosage implantation of 3 × 10¹³ O⁺ ions cm^?2 results in damage in oxide which is occured by 450°C annealing. However, low dosage implantation of 3 × 10^?2 produces no detectable damage in the oxide, and increases the effective positive charge in the oxide at Si'SiO₂ interface. It is shown that prolonged 450°C heat treatment of 0⁺ ion implanted oxides results in an oxygen-silicon reaction in the silicon enriched oxide layer and reduces the fixed positive surface state charge. Subsequent heat treatments at 838°C increase the positive surface state charge to the original pre-ion implantation values, hence converting the oxide into the original silicon rich condition.     

Temperature behavior and modeling of ohmic contacts to Si implanted n-type GaN

The behavior of an ohmic contact to an implanted Si GaN n-well in the temperature range of 25-300 °C has been investigated. This is the sort of contact one would expect in many GaN based devices such as (source/drain) in a metal-oxide-semiconductor transistor. A low resistivity ohmic contact was achieved using the metal combination of Ti (350 Å)/Al (1150 Å) on a protected (SiO₂ cap) and unprotected samples during the post implantation annealing. Sheet resistance of the implanted layer and metal-semiconductor contact resistance to N⁺ GaN have been extracted at different temperatures. Both, the experimental sheet resistance and the contact resistance decrease with the temperature and their characteristics are fitted by means of physical based models.     

Avalanche multiplication noise characteristics in thin GaAs p+-i-n+ diodes

Avalanche noise measurements have been performed on a range of homojunction GaAs p⁺-i-n⁺ and n⁺-i-p ⁺ diodes with “i” region widths, ω from 2.61 to 0.05 μm. The results show that for ω⩽1 μm the dependence of excess noise factor F on multiplication does not follow the well-established continuous noise theory of McIntyre [1966]. Instead, a decreasing noise factor is observed as ω decreases for a constant multiplication. This reduction in F occurs for both electron and hole initiated multiplication in the thinner ω structures even though the ionization coefficient ratio is close to unity. The dead-space, the minimum distance a carrier must travel to gain the ionization threshold energy, becomes increasingly important in these thinner structures and largely accounts for the reduction in noise     

GaAs planar gunn devices with sulfur-ion implanted n layers

Planar type Gunn effect devices have been fabricated by sulfur-ion implantation into the Cr doped semi-insulating GaAs substrates. The high doping efficiency as 90% was obtained as a result of long heat treatment. The mobility of the sulfur-ion implanted n layers with average carrier concentration of 4 × 10¹⁶cm^?3 was 5200 cm²/Vsec at room temperature and 12,000 cm²/Vsec at 77 K. The minimum gate trigger voltage of the Gunn effect digital devices was 100 mV. Sulfur-ion implanted Gunn effect devices have shown superior current drop ratio dependence on doping-depth product, compared to the devices prepared from the epitaxial layer.     

Stabilizing effects for the supercritical GaAs n+nn+-transferred electron amplifier

In this paper effects of importance for the stabilization of supercritical n⁺nn⁺ GaAs transferred electron devices are considered. By small-signal impedance calculations and measurements it is shown that doping- as well as temperature gradients of correct polarity reduce the device negative resistance and enhance stability. It is also found that an increasing doping density reduces the negative resistance. Finally it is demonstrated that relaxation effects have a profound influence on the impedance, and that such effects have to be included in a small-signal analysis in order to give reasonable agreement with measurements.     

Post-breakdown bulk oscillations in gold-doped silicon p+−i−n+ double-injection diodes

Experimental work on post-breakdown bulk oscillations in n-type gold-doped phosphorus-compensated p⁺?i?n⁺ double-injection diodes is presented. An empirical relationship for the frequency of oscillation in this region is derived for the first time and discussed.     

Junction potentials of strongly illuminated n+ - p - p+ solar cells

The low-junction (LHJ) model is applied to an n⁺ - p - p⁺ solar cell having finite dimensions, in order to investigate its performance under intense illumination. Ambipolar transport equations are solved in the three sections of the cell using appropriate boundary conditions. Expressions for junction currents are derived, and the junction potentials under open-circuit conditions are computed by the Newton-Raphson method.The theory presented here includes the effects of high level injection. The generalized current density equations which are derived here for an n⁺ - p - p⁺ device are shown to reduce to the ideal Shockley diode equation with appropriate modifications. The effects of p - p⁺ low-high junction on the open-circuit voltage of the cell are explained. The theoretical results of this paper are consistent with the experimental results of others.     

Epitaxial n+ layer GaAs mesa-finger interdigital surfacephotodetectors

Several varieties of interdigital surface photodetectors have been fabricated on semi-insulating GaAs using mesa-etched n⁺ epitaxial layers as the carrier collection electrodes. This structure provides a significant responsivity improvement over conventional metal-semiconductor-metal (MSM) photodiodes by eliminating the surface reflection of the metal fingers and by providing increased photoconductive gain. High-speed testing using 100-ps doubled Nd:YAG pulses gave FWHM (full width at half maximum) responses of less than 500 ps for a 4-μm finger width and spacing and showed minimal degradation from standard MSM detectors of the same surface geometry. By introducing no additional process steps, the mesa-finger detectors are also monolithically compatible with mesa-etched GaAs MESFET technology     

Proton bombardment in n- and p-type Ga0.47In0.53As

Ga_0.47In_0.53As epitaxial layers on InP substrate have been subjected to proton bombardment. The resistivity increases up to 10⁴Ω cm for 10¹⁴H⁺/cm² in p-type and 3 · 10¹⁶H⁺/cm² in n-type a_0.47In_0.53As implanted at 77 K. Proton bombardment at 300 K showed this increase in resistivity only for p-type material. Channeling experiments indicated that the damage of the lattice which seems to be responsible for the resistivity increase of n-material can be produced only at low temperature with doses of the order of 10¹⁶H⁺/cm². Crystalline layers implanted with high dose showed blistering effects after heat treatments.     

Metallurgical and electrical properties of alloyed Ni/AuGe films on n-type GaAs

An experimental study of the alloying characteristics of a composite thin-film structure which is often used as an ohmic contact to GaAs is presented. A AuGe layer of eutectic composition covered by a thin-film of Ni and deposited on n-type epitaxial GaAs is investigated in order to better understand the relationship between the alloying behavior and the electrical properties of the contact. The barrier energy ?_Bn and the specific contact resistance of the Ni/AuGe/GaAs system is measured for a wide range of alloy temperatures and times. The metallurgical properties of the Ni/AuGe/GaAs system are obtained with Auger electron spectroscopy and scanning electron microscopy. Auger spectroscopy combined with in situ sputter etching is used to determine depth-composition profiles for all constituents of both as-deposited and alloyed Ni/AuGe/GaAs contacts. In samples heat-treated below the AuGe eutectic temperature, Ni is found to move rapidly through the intervening AuGe layer to collect at the GaAs interface, and the effective value of ?_Bn rises to the value characteristic of Ni/GaAs Schottky diodes. For heat-treatment above the AuGe eutectic temperature, ohmic contact behavior is observed, and uniform alloyed contact surfaces are found to result from the presence of Ni at the GaAs interface. Ga outdiffusion and surface accumulation resulting from GaAs dissociation occurs for all alloy temperatures and times. The Ga outdiffusion appears to be characterized by a very low activation energy.     

Metal-N-type semiconductor ohmic contact with a shallow N+ surface layer

Most papers covering metal-semiconductor ohmic contact theory which have been published up to date consider systems with homogeneous impurity concentration in the semiconductor. However, there are techniques of ohmic contact formation on nondegenerate semiconductor where only a very shallow surface layer is impurity enriched. In this paper a model of such contacts is proposed and a simple approximate analytical expression for the specific resistivity is derived. If the impurity concentration in the surface layer is very high, the contact specific resistivity is essentially proportional to N_B^?1, N_B being the semiconductor substrate impurity concentration. To make a good ohmic contact, it is sufficient that the width of the heavily doped surface layer be equal to the equilibrium contact depletion region width. Any further enlargement of the enriched layer practically does not influence the total sample resistance due to the dominant share of the semiconductor body resistance. Experimental results confirm these conclusions qualitatively.     

Collector-up HBT's fabricated by Be+and O+ion implantations

A novel collector-up (C-up) GaAs/AlGaAs heterojunction bipolar transistor (HBT) has been developed and initial device results presented. This device has its extrinsic base defined by Be + and O + implantations. The combination of Be + / O + implantations and heat-pulse annealing is leading to a considerable reduction in the parasitic current and capacitance in the extrinsic base region. Improved device structure and performance expected from this device are discussed in detail.     

Carrier removal profiles from oxygen implanted GaAs

Carrier removal profiles have been measured in annealed, oxygen implanted, n-type GaAs samples. The implants were performed at room temperature with an ion energy of 400 keV. A dose of 1015 O+/cm2 produced a resistivity of about 108 ?/? over a layer 0.6 ?m thick, but no significant electrical compensation was observed from doses less than 1013 O+/cm2. However, doses of 1011 to 5×1012 O+/cm2 produced resistivities of 108 to 109 ?/? without subsequent annealing.     

Deep trapping of implanted Na+ and Li+ ions near the Si/SiO2 interface in metal-oxide-silicon structures

Na⁺ and Li⁺ ions have been implanted in the oxide layer of MOS structures with doses ranging from 3 × 10¹¹ to 3 × 10¹³ ions/cm². Part of the implanted ions can be retraced as mobile ions: this fraction decreased with increasing dose. The trapping of the mobile ions near the Si/SiO₂ interface has been investigated by means of the thermally stimulated ionic current (TSIC) technique. The average energy depth of the ionic traps appeared to increase with increasing dose. Moreover, we found that Li⁺ ions are trapped deeper than Na⁺ ions under equivalent experimental conditions. The influence of the applied electric field on the detrapping has been studied. In the case of 3 × 10¹³ Na⁺ implantation, the barrier lowering corresponds with the Poole-Frenkel theory. We have also paid attention to the effects of bias-temperature stress treatments on the trapping kinetics. We observed a decrease of the mobile ion current after long BTS treatments.     

Damaged-induced isolation in n-type InP by light-ion implantation

A detailed study of the insulating properties of ion-implantation induced damage in InP has been carried out for H, He, B and Be implantation. For each ion, there was found to be an optimal implantation fluence for the formation of resistive layers. At this fluence, a maximum resistivity of 10³ to 10⁴Ω·cm was observed. Lower resistivities were observed for higher and lower implantation fluences. The primary anneal stage for the maximum resistivity layers was between 250 and 300°C.Anomalous results were observed for H implantation in that the resistivity observed depends on the test structure geometry. Measurements carried out by contacting the front and back of the damage layer gave resistivity values two orders of magnitude greater than those measured by contacting adjacent points on an epitaxial structure. For all other ions, the results obtained for the two geometries were in good agreement. It has been shown that a conductive layer produced by the proton bombardment of the underlying Fe-doped substrate gives rise to a low resistance shunt in the epitaxial study.     

A Monte Carlo investigation of multiplication noise in thin p+-i-n+ GaAs avalanche photodiodes

A Monte Carlo (MC) model has been used to estimate the excess noise factor in thin p⁺-i-n⁺ GaAs avalanche photodiodes (APD's). Multiplication initiated both by pure electron and hole injection is studied for different lengths of multiplication region and for a range of electric fields. In each ease a reduction in excess noise factor is observed as the multiplication length decreases, in good agreement with recent experimental measurements. This low noise behavior results from the higher operating electric field needed in short devices, which causes the probability distribution function for both electron and hole ionization path lengths to change from the conventionally assumed exponential shape and to exhibit a strong dead space effect. In turn this reduces the probability of higher order ionization events and narrows the probability distribution for multiplication. In addition, our simulations suggest that fur a given overall multiplication, electron initiated multiplication in short devices has inherently reduced noise, despite the higher feedback from hole ionization, compared to long devices