Search

Photoreflectance measurements have been carried out in order to determine the electric field strength F within the topmost layers of Ga-face polarity Al x Ga 1-x N/GaN and N-face polarity GaN/Al x Ga 1-x N/GaN heterostructures containing high-mobile polarisation induced 2DEGs. For both types of samples F decreased from 400 kV/cm at room temperature up to 200 kV/cm when cooling down the structure to T = 5K. Our results strongly emphasise the existence of surface donor and surface acceptor states of the Ga- and N-face samples, respectively. The temperature dependence of F is explained by the change of the piezoelectric and spontaneous polarization. From self-consistent conduction band calculations the bare surface potential was obtained.

Electronic transport in semiconductors that possess high internal spontaneous and piezoelectric polarization opens up a new field of pyroelectronics and pyrosensors. The pyroelectric character of group-III-nitrides with wurtzite crystal structure yields a novel degree of freedom in designing and tailoring devices for modern micro- and nanoelectronic applications. Furthermore, spontaneous and piezoelectric polarization induced surface and interface charges can be used to develop very sensitive but robust sensors for the detection of ions, gases and polar liquids. We present a review of both theoretical and experimental studies of spontaneous and piezoelectric polarization present in AlGaN/GaN heterostructures as well as the electronic transport properties of polarization induced two-dimensional electron gases which are formed at the AlGaN/GaN interface due to the difference in the total polarization of two adjacent III-nitride layers. We demonstrate that the two-dimensional electron gases (2DEGs) achieved without modulation doping are very suitable as channel of high electron mobility transistors optimally suited for high power and high frequency applications (PART A) as well as for various kinds of sensors which can be operated in harsh environments (PART B). (© 2003 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

DC current-switching and power-switching transients of various GaN-based FET structures are investigated. Two different characteristics are compared, namely, thermal and electronic transients. While the thermal transients are mainly reflected in changes in channel carrier mobility, the electronic transients are dominated by charge instabilities caused by the polar nature of the material. The discussion of the electronic transients focuses, therefore, on instabilities caused by polarization-induced image charges. Three structures are discussed, which are: 1) a conventional AlGaN/GaN heterostructure FET; 2) an InGaN-channel FET; and 3) an AlGaN/GaN double-barrier structure. In structures 2) and 3), field-induced image charges are substituted by doping impurities, eliminating this source of related instability. This is indeed observed.

A study of aging effects on the electron transport properties of AlGaN/GaN heterostructures grown on sapphire and silicon carbide substrates using temperature dependent Hall effect measurements is presented with the focus on the variations in the mobility, carrier concentration, and resistivity of these structures over time. The transport parameters for several of these structures were measured at various times after the initial measurement, including times exceeding one year. These studies show that these parameters are not stable in most of the samples. The most common effects are an increase in carrier concentration and a decrease in mobility. Changes on the order of several tens of percent were common. Since surface oxidation and other variations in surface states were suspected as possible sources of the observed changes, one sample was treated with HCl and KOH and measured again after each treatment. However, we were unable to recover the initial results although further changes in the electrical pr...

We have carried out photoreflectance measurements in order to determine the electric field strength F B within the unintentionally doped barriers of Ga-face polarity Al x Ga 1-x N/GaN heterostructures containing high-mobile polarisation induced 2DEGs. For the sample with both the lowest alloy composition (x = 0.06) and lowest sheet carrier density (n s = 1 × 10 12 cm -2 ) the analysis of the FKO yields F B = 335 kV/cm at RT. F B decreases by 130 kV/cm when cooling down the structure to T = 5 K. In comparison, samples with larger x and n S values show lower field strength at RT but a similar temperature dependence. The results can be explained by a change of piezoelectric polarisation.

To study the electronic transport properties of two-dimensional electron gases confined at the interfaces of AlGaN/GaN heterostructures, Shubnikov-de Haas (SdH) and Hall measurements were performed with structures covering a wide range of sheet carrier concentrations from 2.25 × 10 12 to 1.83 × 10 13 cm -2 . For samples with sheet carrier concentrations above 1.7 × 10 13 cm -2 , the occupation of a second subband was observed. Fourier transformation was used to separate the contributions of both occupied subbands to the electronic transport. Quite similar quantum scattering times and effective masses of the electrons in the first and second subband were determined. In samples with lower sheet carrier concentration traces of a spin splitting could be found by investigating the angle dependence of the SdH-oscillations.

We have performed Shubnikov-de Haas (SdH) and Hall effect measurements to investigate the electronic transport properties of polarisation induced 2DEGs in Al x Ga 1-x N/GaN heterostructures with alloy compositions between x = 0.10 and 0.35 and sheet carrier concentrations of up to n s = 1.05 × 10 13 cm -2 . From SdH measurements of 2DEGs with sheet carrier concentrations of 2.1 × 10 12 and 4.6 × 10 12 cm -2 , effective electron masses were determined to be 0.24 and 0.207m 0 , respectively. In addition, angle resolved SdH measurements were performed to evaluate the effective g-factor from the angle of zero oscillation amplitude. Including the measured electron masses, the g-factors were calculated to be 2.11 and 2.47, respectively. In order to identify the main electron scattering mechanism we determined the ratio between transport- and quantum scattering times as a function of sheet carrier concentration. We observe a significant decrease of the τ t /τ q ratio with increasing 2DEG carrier concentration, indicating a transition from a dominant small angle to large angle scattering mechanism when n s exceeds 7 × 10 12 cm -2 .

Hole capture time in p-type GaN was measured by using a femtosecond pump-probe technique. The capture time constant that holes are trapped by Mg-related states was found to be shorter than 10 ps. The hole capture time increases with decreased hole excess energy. By comparing two samples with different doping concentrations, it was found that the hole capture time also decreases with increased doping concentrations.

Coherent acoustic phonon oscillations were generated and studied in strained GaN thin films. Inside the bulk GaN film, the longitudinal interference of an ultraviolet femtosecond pump pulse created periodic carrier distribution that screened out the strain-induced piezoelectric field and initiated the coherent longitudinal acoustic phonon oscillations corresponding to the carrier periods. The created coherent phonon oscillation modulated the piezoelectric field thus modified the absorption property of the GaN thin film through Franz–Keldysh effect. This time-dependent absorption modulation was reflected in the transmission variation of the followed probe pulses, resulting a long decay time ∼300 ps for the initiated coherent phonon oscillations.

The hole-transport properties of Mg-doped AlGaN/GaN superlattices are carefully examined. Variable-temperature Hall-effect measurements indicate that the use of such superlattices enhances the average hole concentration at a temperature of 120 K by over five orders of magnitude compared to a bulk GaN film (the enhancement at room temperature is a factor of 9). An unusual modulation-doping scheme, which has been realized using molecular-beam epitaxy, has yielded high-hole-mobility superlattices and conclusively demonstrated the pivotal role of piezoelectric and spontaneous polarization in determining the band structure of the superlattices.

Summary form only given. Progress in femtosecond lasers and ultrafast spectroscopy technology has enabled us to generate and directly observe the coherent oscillation of phonon modes. Coherent optical phonon oscillations have been observed in bulk GaAs, bulk Ge, and a number of other bulk materials. For acoustic phonons, it is much harder to selectively excite a specific coherent acoustic phonon mode in bulk materials. Higher frequency coherent acoustic phonon oscillations were recently observed in AlAs/GaAs superlattices, InGaN/GaN multiple-quantum-wells, and PbTe/PbS quantum dots. In this presentation, we report our observation of high frequency coherent acoustic phonon oscillations in a bulk material, a highly strained bulk GaN film. The longitudinal interference of an ultraviolet femtosecond pump pulse was used to create periodic carrier distribution in the bulk GaN thin film. The periodic carrier distribution screened out the strain-induced piezoelectric field and initiated the coherent acoustic phonon oscillations corresponding to the carrier periods. The decay time of the initiated coherent phonon oscillation is longer than 250 ps. This long decay time and the traveling wave nature of the generated acoustic phonons provide the hope for the realization of the first phonon laser.

Dramatic progress in GaN electronics has led to increased interest in bipolar transistors. Although there have been reports of GaN bipolars from several groups, the development of the GaN bipolar transistor is still in its fundamental stages. In the case of GaN, the usual correlation between common base, Gummel, and common emitter characteristics does not exist due to significant collector-emitter leakage, leaving only the common emitter characteristic as a reliable measure of DC device performance. We identify the source of this leakage as threading dislocations and clarify the effect of this leakage on the transistor DC characteristics. Furthermore, we conclude from various growth structures and methods of device fabrication that the electron lifetime in the neutral base is currently the limiting factor in GaN NPN transistor performance. Typical GaN material has high threading dislocation densities, 10/sup 7/-10/sup 9/ cm/sup -2/, due to lattice mismatch with the substrate, typically sapphire or SiC. To study the effects of threading dislocations on GaN bipolar transistors, we have fabricated devices on material grown using the lateral epitaxial overgrowth technique, LEO. To the authors' knowledge, this is the first demonstration of GaN bipolar transistors grown on nondislocated material. The LEO substrate allows us to compare devices grown on material with a negligible dislocation density with those grown on a standard template.

Summary form only given. The AlGaN-GaN based material system offers the three most important materials properties required for efficient microwave power generation: very high breakdown electric field (/spl sim/2 MV/cm), high electron mobility and velocity (/spl mu//sub c//spl sim/1000 cm/sup 2//spl middot/V/sup -1/s/sup -1/ and v/sub s//spl sim/2/spl times/10/sup 7/ cm/spl middot/s/sup -1/ in bulk materials) and heterojunction technology to optimize device design. This has led to the rapid development of AlGaN-GaN HEMTs, grown heteroepitaxially on either sapphire or SiC substrates. AlGaN-GaN HBTs also show promise for power amplifier and switching applications. Inherent strengths of the HBT structure include high current densities and low phase noise, as well as improved linearity and threshold control. The progress in HEMT DC and RF performance has been spectacular and the possibility of insertion into high power systems is a distinct possibility. The HBTs continue to have to battle problems with p-type doping and associated processing difficulties. However, sustained progress in engineering around this problem holds promise for the future.

AlGaN/GaN HBTs using regrown emitter AlGaN/GaN HBTs have been realised using the regrown emitter method. The device structure consists of an n-GaN collector, p-GaN base, and selectively grown AlGaN emitter. The HBTs were grown using metal organic chemical vapour deposition on sapphire substrate. The emitter was grown selectively on a pn junction diode after it was patterned with SiN. A common emitter curve showing low-leakage has been obtained.

The AlGaN/GaN heterostructure is important for both electronic and optoelectronic devices. In addition to having a large band gap, the heterostructure has a strong piezoelectric effect and a large spontaneous polarization. This allows one to incorporate a large electric field (>106 V/cm) and high sheet charge (>1013 cm−2) without doping. Theoretical studies are done to examine how polarization effects controls the sheet charge density. The studies also focus on how interface roughness, aluminum mole fraction in the barrier, impurity scattering, sheet charge density, and phonon scattering influence mobility. Results are compared with experimental studies on samples grown by both molecular beam epitaxy (MBE) and metalorganic chemical vapor deposition (MOCVD). We find that interface roughness is a dominant source of scattering in the samples reported. Due to the variation in growth techniques we find that the MBE samples have a smoother interface quality compared to the MOCVD samples. By carefully fitting th...