M. I. Nathan

High barrier height GaN Schottky diodes: Pt/GaN and Pd/GaN

Platinum (Pt) and palladium (Pd) Schottky diodes on n‐type GaN grown by metalorganic chemical vapor deposition were achieved and investigated. Aluminum was used for ohmic contacts. Barrier heights were determined to be as high as ΦB=1.13 eV by the current–voltage (I–V) method and ΦB=1.27 eV by the capacitance–voltage (C–V) method for the Pt/GaN diode, and ΦB=1.11 eV, ΦB=0.96 eV, and ΦB=1.24 eV by I–V, activation energy (I–V–T), and C–V methods for the Pd/GaN diode, respectively. The ideality factors were obtained to be n∼1.10.

Spectroscopic applications and frequency locking of THz photomixing with distributed-Bragg-reflector diode lasers in low-temperature-grown GaAs

A compact, narrow-linewidth, tunable source of THz radiation has been developed for spectroscopy and other high-resolution applications. Distributed-Bragg-reflector (DBR) diode lasers at 850 nm are used to pump a low-temperature-grown GaAs photomixer. Resonant optical feedback is employed to stabilize the center frequencies and narrow the linewidths of the DBR lasers. The heterodyne linewidth full-width at half-maximum of two optically locked DBR lasers is 50 kHz on the 20 ms time scale and 2 MHz over 10 s; free-running DBR lasers have linewidths of 40 and 90 MHz on such time scales. This instrument has been used to obtain rotational spectra of acetonitrile (CH3CN) at 313 GHz. Detection limits of 1 × 10^–4 Hz^1/2 (noise/total power) have been achieved, with the noise floor dominated by the detectors noise equivalent power.

Barrier height variation in Al/GaAs Schottky diodes with a thin silicon interfacial layer

Al/n‐GaAs and Al/Si/n‐GaAs structures with thin silicon interfacial layers were grown in situ by molecular beam epitaxy and their electrical characteristics were measured. Effective barrier heights between 0.30 and 1.04 eV were determined through I‐V and C‐V measurements in the Al/Si/n‐GaAs structures under varying conditions of deposition of the silicon layer, in contrast to a barrier height of 0.78 eV without the silicon layer. The conduction‐band offset between Si and GaAs is estimated to be of the order of 0.3±0.05 eV. The results indicate that the Fermi level at the interface of GaAs on Si in the Al/Si/n‐GaAs structure is unpinned from its midgap value.

Barrier height change in GaAs Schottky diodes induced by piezoelectric effect

A novel manifestation of piezoelectric effects in GaAs has been observed. The change of barrier height, φB, of Schottky diodes induced by uniaxial stresses, S, along 〈100〉, 〈011〉, 〈011〉, and 〈111〉 has been measured. Shifts in φB due to the appearance of piezoelectric polarization charges at the semiconductor‐metal interface for directions other than 〈100〉 are observed.

Effects of hydrostatic and uniaxial stress on the Schottky barrier heights of Ga-polarity and N-polarity n-GaN

We report measurements of the Schottky barrier heights of Ni/Au contacts on Ga-polarity and N-polarity n-GaN under hydrostatic pressure and applied in-plane uniaxial stress. Under hydrostatic pressure the two different polarities of GaN yield significantly different rates of Schottky barrier height increase with increasing pressure. Uniaxial stress parallel to the surface affects the Schottky barrier height only minimally. The observed changes in barrier height under stress are attributed to a combination of band structure and piezoelectric effects.

Unpinned GaAs Schottky barriers with an epitaxial silicon layer

We present experimental results of Al/n‐ and Al/p‐type GaAs Schottky barrier structures grown in situ by molecular‐beam epitaxy with thin Si epitaxial interfacial layers. The barrier heights are measured by the I‐V, thermal activation, C‐V, and photoresponse methods. Barrier heights in the range of 0.3<φbN<1.04 eV for n‐type GaAs and 0.28 < φbP<1.01 eV for p‐type GaAs were obtained for Si layer thicknesses between 6 and 100 A. Annealing studies conducted on the samples indicate that the structures are thermally stable to temperatures up to 450u2009°C. These results imply that the GaAs surface Fermi level at the Si/GaAs interface is unpinned from its customary near‐midgap value. A model which involves the energy‐band discontinuities ΔEC and ΔEV between GaAs and Si, the thickness, and the doping of the Si layer is suggested to account for the different barrier‐height values obtained.

Bias-assisted photoelectrochemical etching of p-GaN at 300 K

Photoelectrochemical (PEC) etching of p-type GaN has been realized in room temperature, 0.1 M KOH solutions. PEC etching of GaN was achieved by applying a positive bias to the surface of the p-GaN layer through a deposited titanium mask. The applied bias reduces the field at the semiconductor surface, which induced the dissolution of the GaN. The effect of bias on etch rate and morphology was examined. It was found that insulating the Ti mask from the KOH solution with Si3N4 significantly increases the etch rate. The rms roughness of the etched region decreased as the bias voltage increased. Etch rates as high as 4.4 nm/min were recorded for films etched at 2 V.

Direct measurement of piezoelectric field in a [111]B grown InGaAs/GaAs heterostructure by Franz–Keldysh oscillations

We report the first photoreflectance measurement of strain‐induced piezoelectric field in a (111)B InGaAs/GaAs structure. The InGaAs quantum well was pseudomorphically grown in the undoped regions of a GaAs undoped–heavily doped structure. Four structures, two each with the same layer structures but different orientation, (111)B and (100), were used in this study. The electric fields in the undoped GaAs region were measured by Franz‐Keldysh oscillations in photoreflectance. All the samples have a surface barrier height of about 0.7 eV. However, the measured electric field is 30% stronger in the (111)B sample compared to the (100) sample. We attribute this difference to the strain‐induced electric field in the (111)B sample. The piezoelectric field in (111)B strained In0.15Ga0.85As obtained in this measurement is 2.2±0.5×105 V/cm, which agrees very well with theory.

Temperature dependence of minority‐carrier mobility and recombination time in p‐type GaAs

The electron mobility in p‐type GaAs, μpe, has been determined as a function of temperature by measuring the common‐emitter cutoff frequency, fT, of an AlGaAs/GaAs n‐p‐n heterojunction bipolar transistor. The base was 0.6 μm thick and it was doped with 4×1018 cm−3 Be. The 300 K value of 1055 cm2/Vu2009s and 79 K value of 5000 cm2/Vu2009s for μpe are comparable to the previously measured values. The discrepancy with the calculated values is pointed out. The recombination lifetime is also measured as a function of temperature for minority carriers. The results agree reasonably well with the calculated radiative recominbation time.

Minority electron mobility and lifetime in the p+GaAs base of AlGaAs/GaAs heterojunction bipolar transistors

AlGaAs/GaAs heterojunction bipolar transistors with different base doping concentrations grown by gas source molecular beam epitaxy were fabricated and characterized at dc and high frequency. Three different base doping concentrations; 5×1018, 1×1019, and 5×1019 cm−3 doped with Be were used for the characterization with the same structural and process parameters, including 0.2 μm base width. Minority electron mobilities in heavily doped p type were measured as 1.1×103, 1.3×103, and 3×103 cm2 V/s for 5×1018, 1×1019, and 5×1019 cm−3, respectively, by using the current gain cutoff frequency (fT) which agrees well with theoretical predictions in heavily doped p‐type GaAs. Combining dc and high frequency measurements, electron lifetimes of 1.2×10−9, 5.5×10−10, and 2×10−11 s have been obtained for these three dopings, respectively.