The W/n-GaAs Schottky junctions A and B of area 1.75×l0-2 cm2 were fabricated by deposition of W on the chemically etched polished surfaces of n-GaAs samples by rf sputtering using a rf powers of 300 Watt for 30 min. The W contact B was subjected to a 90 min. thermal anneal at 390 °C. The room temperature I-V and C-V/f (with 200 Hz < f < 1 MHz) measurements were carried out for both the as-deposited and thermally annealed W/n-GaAs Schottky junctions A and B, respectively. From the direct I-V data, the values of 1.09 and 8.1×10-8 A for the ideality factor (n) and the reverse saturation current (Io), respectively, were estimated for the diode B, compared to the values of n=1.70 and Io=6.3×10-6 A for the diode A. The observed frequency dispersion in the zero bias capacitance in the diode B was attributed to fast interface states with a time constant, τ2=6 μs and density, Nss2=5.8×1010 eV-1cm-2, whereas, both the slow interface states (with τ1=4 ms and density, Nss1=7.8×1012 eV-1cm-2) and fast states (with τ2=1 μs and density Nss2=8.6×1010 eV-1cm-2) were responsible for the observed frequency variation of the zero bias capacitance in the diode A. For the forward bias values in the range 20-100 mV, the frequency dispersion in the measured capacitance suggested the presence of both the fast and slow interface states (with time constants differing by three orders of magnitude) in the as-deposited and the heat treated W/n-GaAs interfaces. Thermal anneal at 390 °C for 90 min. lowered the density of states at the W/n-GaAs interface by two orders of magnitude and resulted in the formation of a high quality rectifying W contact to n-GaAs with a rectification ratio of 1.4×104, a low Io and an ideality factor close to unity.

We present an improved method to analyze simultaneously the current–voltage and capacitance–voltage characteristics of metal–insulator–semiconductor (MIS) diodes. We use the method to study the effect of Zn doping concentration on the current transport in Au MIS contacts fabricated on In0.21Ga0.79As layers grown by metalorganic vapor phase epitaxy on highly doped GaAs substrates. At room temperature and for low reverse bias voltage, the generation/recombination process via mid-gap traps is the only dominant mechanism in these MIS diodes. For high reverse bias, both this mechanism and thermionic-field emission control current transport. The generation/recombination current observed is due to donor type mid-gap traps whose density shows an almost linear dependence with Zn concentration. The value of the barrier height at zero bias and at room temperature (φb0=0.73 V±12%) is independent of the Zn concentration. For the procedure used to prepare the In0.21Ga0.79As:Zn surfaces, the thickness of the oxide layer and the transmission coefficient of holes across this layer depend on the Zn doping concentration in the range 7×1014⩽NA⩽5×1018 cm−3. Zn doping seems to inhibit the formation of the unintentional native oxide on the surface of In0.21Ga0.79As epilayers.

A detailed study of the effect of doping density on current transport was undertaken in Au metal-insulator-semiconductor (MIS) contacts fabricated on Zn-doped InP layers grown by metal-organic vapor phase epitaxy. A recently developed method was used for the simultaneous analysis of the current-voltage (I-V) and capacitance-voltage (C-V) characteristics in an epitaxial MIS diode which brings out the contributions of different current transport mechanisms to the total current. I-V and high-frequency C-V measurements were performed on two MIS diodes at different temperatures in the range 220-395 K. The barrier height at zero bias of Au/InP:Zn MIS diodes, phi(o) (1.06 V +/- 10%), was independent both of the Zn-doping density and of the surface preparation. The interface state density distribution N-ss as well as the thickness of the oxide layer (2.2 +/- 15% nm) unintentionally grown before Au deposition were independent of the Zn-doping concentration in the range 10(16) < N-A < 10(17) cm(-3); not so the effective potential barrier chi of the insulator layer and the density of the mid-gap traps. chi was much lower for the highly-doped sample. Our results indicate that at high temperatures, independent of the Zn-doping concentration, the interfacial layer-thermionic (ITE) and interfacial layer-diffusion (ID) mechanisms compete with each other to control the current transport. Ar intermediate temperatures, however, ITE and ID will no longer be the only dominant mechanisms in the MIS diode fabricated on the highly-doped sample. In this case, the assumption of a generation-recombination current permits a better fit to the experimental data. Analysis of the data suggests that the generation-recombination current, observed only in the highly-doped sample, is associated with an increase in the Zn-doping density. From the forward I-V data for this diode we obtained the energy level (0.60 eV from the conduction band) for the most effective recombination centers. (C) 1998 Elsevier Science Ltd. All rights reserved.

Epitaxial In0.75Ga0.25P/Ag Schottky contacts were fabricated by electron beam evaporation of Ag on an epitaxial thin film of In0.75Ga0.25P grown on highly doped n‐InP substrate by MOCVD. The diode showed non‐ideal behavior with an ideality factor of 1.3 and is thought to have a metal‐interface layer‐semiconductor (MIS) structure. The room temperature small signal ac capacitance (Cm) was measured as a function of applied voltage (Va), in the frequency (f ) range 0.5 KHz and 1 MHz. Under forward bias, the large frequency dispersion in Cm was attributed to the interface states in equilibrium with the semiconductor (In0.75Ga0.25P). From Cm, the interface states capacitance Cp was extracted. The experimental Cp−f data fitted well to Lehovec’s model of an interface state continuum with a single time constant and provided the values of the interface states energy density (Nss), relaxation time (τ) and capture cross‐section (σn) for forward bias (Va) voltages between 0.0 V and 0.4 V. Over the forward bias range 0...

W/n-GaAs/In Schottky contacts of area 1.75 mm2 were fabricated by deposition of W on (100) n-GaAs by rf Sputtering using rf power values in the range 200–400 Watt. The I-V and high frequency C-V measurements at 300 K, in the 200 Watt W/n-GaAs Schottky contact indicated that W formed a good rectifying contact to n-GaAs, with a rectification ratio (r) of 270, ideality factor (n) of 1.39, reverse saturation current (Io) of 1.2×10−6 A and the C-V barrier height (φbo) of 1.6 V. However, n and Io increased, whereas r and φbo decreased for the W/n-GaAs Schottky contacts prepared using higher rf power. For the 300 Watt W/n-GaAs contact, the values of 70, 1.70, 6.3}10−6 A, and 1.2V for r, n, Io and φbo, respectively, were estimated. The low frequency forward bias capacitance (or surface defect density) was an order of magnitude higher in the 300 Watt contact than in the 200 Watt contact. This fact suggested that the degradation in the quality of W/n-GaAs Schottky contacts fabricated by using high rf power was caused by high density surface defects created during sputter deposition of W on n-GaAs.

Au/p-In0.21Ga0.79As Schottky barrier diodes were fabricated by evaporation of Au on chemically etched surfaces of Zn doped In0.21Ga0.79As epitaxial layers grown on highly p-doped GaAs substrates by metalorganic vapor phase epitaxy (MOVPE). Room temperature current-voltage measurements show that Au forms high quality rectifying contacts to P-In0.21Ga0.79As:Zn with an ideality factor of 1.2. High frequency capacitance-voltage (C-V) and capacitance-frequency (C-f) measurements over a wide frequency range (1 kHz < f < 1 MHz) were carried out at room temperature on Au Schottky diodes made on four P-In0.21Ga0.79As:Zn samples with varying acceptor doping concentrations (N(A)) in a range between 5.8 X 10(14) and 4.3 X 10(17) CM-3. Under forward bias, the capacitance showed large frequency dispersion, possibly caused by interface states in thermal equilibrium with the semiconductor. The C-f data was analyzed in terms of Lehovec's model of an interface state continuum with a single time constant. The density and relaxation time of interface states were obtained in an energy range between 0.40 and 0.65 eV from the top of the valence band. The density of interface states varied between 1 X 10(11) and 3.5 X 10(12) eV-1 CM-2, and the relaxation times were in the range of 7 X 10(-6)-6 X 10(-5) s. For samples with N(A) between 1.5 X 10(17) and 4.3 X 10(17) CM-3, the interface state density increased exponentially with interface energy in the range of 0.65 and 0.40 eV, from midgap towards the top of the valence band. The density of interface states in the highly doped samples (N(A) = 4.3 X 10(17) cm-3) was one order of magnitude higher than that in the lightly doped samples (N(A) = 5.8 X 10(14) cm-3).

Epitaxial p-InP/Au Schottky diodes were fabricated by evaporation of Au onto Zn doped epitaxial layers of InP grown by MOVPE, on a highly doped InP substrate. The reverse current-voltage (Ir-Vr) and 1 MHz capacitance-voltage (C-V) characteristics of the Au/p-InP diodes were measured in the temperature range 220–393 K. At all temperatures, soft reverse current-voltage characteristics were observed, which may be due to the decrease in the effective Schottky barrier height (ϕbr) with the increase of Vr. The voltage dependence of the reverse current was well described in terms of the interface layer thermionic emission (ITE) model which incorporates the effects of applied reverse voltage drop and the transmission coefficient across the interface layer and image force lowering of the barrier height into the thermionic emission theory. A self consistent iteration and least square fitting technique was used to obtain the zero bias barrier height (ϕbo) and interface layer capacitance (Ci) from the Ir-Vr data. Both, the Ir-Vr, and the C-V data were analyzed under the assumption of reverse bias voltage independence of the charge trapped in the interface states, which was supported by our experimental data. The values of ϕbo obtained from the C-V measurements agreed well with those obtained from the Ir-Vr data for a value of 0.45 AK−2cm−2 for the effective Richardson constant (Aeff).

We have developed a new approach to analyze the current- voltage (I-V) characteristics of a tunnel metal-interface layer - semiconductor (MIS) diode which takes into account the voltage dependence of interface states distribution (Nss) and the barrier lowering due to image force. Our method of analysis uses simultaneously the I-V/T and C-V/T data to determine the characteristic parameters of an MIS diodes and is ideal for new epitaxial materials and devices where the carrier density is not known precisely before hand. The experimental verification of our approach to analyze the nonideal I-V/T and C-V/T characteristics of MIS diodes was done by comparing the values of Nss extracted from the room temperature forward I-V characteristics of a p-InP/Au MIS diodes with those obtained by the multi-frequency admittance method. Excellent agreement between the values of Nss determined by two different techniques strongly support the validity of our theoretical expression for the I-V/T characteristics and the method of analysis. Our results indicate that the interface layer thermionic emission was clearly the dominant mechanism of the forward current transport in epitaxial Au/p-InP MIS diodes over the temperature range 200–393 K. The transmission coefficient of the interface-layer obtained from the reverse I-V characteristics has a value of l.43×l0−1 (±7%).

Epitaxial p-In0.21Ga0.79As/Au Schottky barrier type diodes were fabricated by evaporation of Au on chemically etched surfaces of In0.21Ga0.79As:Zn layers grown on highly doped GaAs substrate by MOVPE. 1 MHz capacitance-voltage (C-V) and Capacitance-frequency (C-f) measurements were performed in the frequency range 1 KHz-1 MHz at room temperature in Au Schottky diodes made on four epitaxial p-ln0.2 Ga0.79As:Zn samples with doping concentrations between 6×l014cm−3 and 4×l017 cm−3. Under forward bias, a large frequency dispersion in the junction capacitance was observed which was attributed to the interface states in thermal equilibrium with the semiconductor. The interface states capacitance extracted from the C-f data was analyzed in terms of Lehovec’s theoretical model of interface state continuum with single time constant, and the characteristic parameters of the interface states (energy density (Nss), relaxation time (τ) and hole capture cross-section (σh)) were determined. In the samples with doping concentration in the range 1.5×l017–4.3×l017 cm−3, Nss was about an order of magnitude higher than in the sample having a doping concentration of 5.8×l014 cm−3. Over the interface states energy range 0.40–0.65 eV, Nss decreased exponentially with energy in the highly doped samples and σh increased with energy in all the samples.