R. L. Jiang

Metal–semiconductor–metal GaN ultraviolet photodetectors on Si(111)

GaN metal–semiconductor–metal photoconductive detectors have been fabricated on Si(111) substrates. The GaN epitaxial layers were grown on Si substrates by means of metalorganic chemical-vapor deposition. These detectors exhibited a sharp cutoff at the wavelength of 363 nm and a high responsivity at a wavelength from 360 to 250 nm. A maximum responsivity of 6.9 A/W was achieved at 357 nm with a 5 V bias. The relationship between the responsivity and the bias voltage was measured. The responsivity saturated when the bias voltage reached 5 V. The response time of 4.8 ms was determined by the measurements of photocurrent versus modulation frequency.

Studies of metal–ferroelectric–GaN structures

A GaN-based metal–insulator–semiconductor (MIS) structure has been fabricated by using ferroelectric Pb(Zr0.53Ti0.47)O3 instead of conventional oxides as insulator gate. Because of the polarization field provided by ferroelectric and the high dielectric constant of ferroelectric insulator, the capacitance–voltage characteristics of GaN-based metal–ferroelectric–semiconductor (MFS) structures are markedly improved compared to those of other previously studied GaN MIS structures. The GaN active layer in MFS structures can reach inversion just under the bias of smaller than 5 V, which is the generally applied voltage used in semiconductor-based integrated circuits. The surface carrier concentration of the GaN layer in the MFS structure is decreased by one order compared with the background carrier concentration. The GaN MFS structures look promising for the practical application of GaN-based field effect transistors.

PROPERTIES OF SCHOTTKY CONTACT OF AL ON SIGE ALLOYS

Electrical properties of Schottky contacts of Al on p‐Si1−xGex alloys were investigated. The Si1−xGex strained layers were grown on p‐Si substrates by using rapid thermal process/very low pressure‐chemical vapor deposition. Low reverse currents were obtained. It was found that the Schottky barrier height of Al/p‐Si1−xGex contacts decreased with increasing Ge fraction. The decrement is in accordance with the decrement of the band gap of the strained Si1−xGex. The Fermi level at the interface is pinned at about 0.43 eV below the conduction band. The influence of strain relaxation for SiGe alloy layers and the Si sacrificial cap layers on the properties of Schottky contacts were also investigated.

Temperature dependence of the pyroelectric coefficient and the spontaneous polarization of AlN

Using the Debye model and existing experimental data of the pyroelectric coefficient of AlN, the temperature dependence of the pyroelectric coefficient as well as the spontaneous polarization of AlN is calculated over a wide temperature range from 0to1000K. The pyroelectric coefficient is proportional to T3 at low temperature and increases acutely from 0 to around 400K, and then increases gently from 400to1000K. It makes AlN uniquely suitable for application in high temperature pyroelectric sensors. The spontaneous polarization of AlN changes a little from 0to1000K, which indicates that the features of III-nitrides based devices will hardly be degraded by the change of the spontaneous polarization.

Ti/Al/Pt/Au and Al ohmic contacts on Si-substrated GaN

Al and Ti/Al/Pt/Au ohmic contacts on GaN epitaxial layers were studied. The epilayers were grown on Si (111) substrates by low-pressure metalorganic chemical vapor deposition. Al/GaN contacts achieved a minimum contact resistivity of 7.5×10−3 Ω cm2 after annealing in N2 ambient at 450 °C for 3 min. Further annealing degraded the contacts. Ti/Al/Pt/Au and GaN contacts achieved a minimum contact resistivity of 8.4×10−5 Ω cm2 after annealing in N2 at 650 °C for 20 s. The Ti/Al/Pt/Au contacts on GaN showed a better thermal stability than Al/GaN contacts. After annealing at 600 °C for 30 min. they were still ohmic contacts. The mechanisms for ohmic contact formation of Ti/Al/Pt/Au contacts were also analyzed.

Composition pulling effect and strain relief mechanism in AlGaN/AlN distributed Bragg reflectors

We report on the composition pulling effect and strain relief mechanism in AlGaN/AlN distributed Bragg reflectors (DBRs) grown on GaN template/α-Al2O3(0001) by metal organic chemical vapor deposition. The reciprocal space mapping contours reveal that these DBRs are coherently grown. Cross-section transmission electron microscopy image of the AlGaN/AlN DBRs and the energy-dispersive x-ray analysis indicate that an AlGaN layer with gradient Al composition is located between the Al0.4Ga0.6N and AlN layers along the [0001] direction. It is attributed to the fact that Ga atoms in AlGaN are pulled and segregated to the upper layer by the strain. The density of strain energy is estimated to reduce more than one order by forming this quasi-three-sublayer structure comparing to the designed bi-sublayer structure.

Study on strain and piezoelectric polarization of AlN thin films grown on Si

The strain and piezoelectric polarization of AlN thin films grown on Si(111) substrates by metalorganic chemical-vapor deposition were investigated by using x-ray diffraction and Raman measurements. The stress and piezoelectric polarization of the AlN films were analyzed with E2 (high) phonon-mode frequency shifts in Raman spectra. The result indicates that the biaxial compressive stress is about 4.3GPa, and the piezoelectric polarization is about 1.91×10−2C∕m2. Phonons of Si–N bonds were also observed accompanying phonons of AlN crystal in Raman spectra, which indicate the interdiffusion of Si and N atoms during growth.

Aluminum and GaN contacts on Si(111) and sapphire

Al and GaN contacts on both Si(111) and sapphire substrates were studied in this letter. After annealing at 450, 520, 600, and 650 °C for 35 min under a N2 flowing ambient, the current–voltage characteristics of Al/GaN contacts on Si(111) substrates were different from those on sapphire ones. The interfacial properties were discussed using the Schottky emission model in which the current is proportional to the square of the applied voltage. The metallurgical reactions were analyzed using photoluminescence (PL) spectra and x-ray diffraction (XRD). After annealing, blueshifts in the PL spectra and new peaks in the XRD data indicated that the wide-band gap interfacial phase AlGaN was formed at the Al/GaN interface.

Study on thermal oxidation of Si nanowires

In this paper we reported the results on the low-temperature thermal oxidation of Si nanowires. Various polygon-shaped Si nanowires with linewidths between 100 and 300 nm were fabricated on Si/Si 1-x Ge x /Si heterostructure substrates by using lithography, reactive ion etching, and subsequent selective chemical etching. We find that oxidized Si nanowires following 750 and 775 °C wet oxidation will keep the same shape as the initial unoxidized samples, and all the SiO 2 boundaries of oxidized samples will arrive at a circular shape. These results, which should be due to stress effects, provide useful information for understanding the behavior of non-planar oxidation that is used in modern Si nanoelectronic technology.

Normal-incidence SiGe/Si photodetectors with different buffer layers

Normal-incidence SiGe/Si p–i–n photodetectors with relaxed GeSi alloy layers grown on Si buffer layers at low temperature and fraction-graded Si1−xGex buffers were fabricated by rapid thermal process/very-low-pressure chemical-vapor deposition. The response wavelength of these detectors ranges from 0.7 to 1.55 μm. The peak wavelengths are 0.98 and 1.06 μm, at which the responsivities are 2.7 and 1.8 A/W (−2 V), respectively. The responsivities at 1.3 μm are 0.15 and 0.07 A/W (−5 V), and the dark current densities are 0.05 and 0.03 μA/mm2 (−2 V), respectively. The influences of the Ge fraction, epilayer thickness, and bias voltage on the detectors are discussed.